U.S. patent application number 10/942339 was filed with the patent office on 2005-06-02 for void-containing polyester shrink film.
Invention is credited to Helton, Tony Wayne, Sharpe, Emerson Eston JR., Shelby, Marcus David.
Application Number | 20050119359 10/942339 |
Document ID | / |
Family ID | 37579130 |
Filed Date | 2005-06-02 |
United States Patent
Application |
20050119359 |
Kind Code |
A1 |
Shelby, Marcus David ; et
al. |
June 2, 2005 |
Void-containing polyester shrink film
Abstract
Disclosed are polyester shrink films comprising a voiding agent
dispersed within a continuous polyester phase. The voiding agent
comprises at least one first polymer and at least one second
polymer, in which the polymer components have selected physical
properties such as glass transition temperature, melting point,
tensile modulus, surface tension, and melt viscosity. The resulting
shrink films have high opacity, a low coefficient of friction,
lower density, low shrink force, and good printability. The films
are useful for sleeve label and other shrink film applications, and
their lower density allows them to be readily separated from soft
drink bottles, food containers and the like during recycling
operations. Also disclosed is a process for separating a
void-containing polyester from a mixture of polymers.
Inventors: |
Shelby, Marcus David;
(Kingsport, TN) ; Helton, Tony Wayne; (Kingsport,
TN) ; Sharpe, Emerson Eston JR.; (Kingsport,
TN) |
Correspondence
Address: |
ERIC D. MIDDLEMAS
EASTMAN CHEMICAL COMPANY
P. O. BOX 511
KINGSPORT
TN
37662-5075
US
|
Family ID: |
37579130 |
Appl. No.: |
10/942339 |
Filed: |
September 16, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60526305 |
Dec 2, 2003 |
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Current U.S.
Class: |
521/50 |
Current CPC
Class: |
B32B 2307/736 20130101;
C08J 5/18 20130101; C08L 23/0869 20130101; C08L 67/02 20130101;
B32B 2553/00 20130101; B32B 2519/00 20130101; C08L 2666/02
20130101; C08L 1/14 20130101; Y10T 428/249953 20150401; C08L 25/06
20130101; Y10T 428/249958 20150401; Y10T 428/249986 20150401; B32B
2307/75 20130101; C08L 1/12 20130101; B32B 2307/518 20130101; B32B
2307/516 20130101; B32B 27/205 20130101; C08L 23/12 20130101; B32B
27/08 20130101; C08J 3/005 20130101; B32B 2307/41 20130101; C08L
67/02 20130101 |
Class at
Publication: |
521/050 |
International
Class: |
C08J 009/00 |
Claims
We claim:
1. A void-containing shrink film comprising an oriented, continuous
polyester phase having dispersed therein a voiding agent comprising
at least one first polymer, and at least one second polymer,
wherein said first polymer has a glass transition temperature (Tg)
or a melting point temperature (Tm) greater than the Tg of said
polyester, a tensile modulus of at least 1 GPa, and a surface
tension that differs from the surface tension of said polyester by
an absolute value of 5 dynes/cm or less; and said second polymer
has a surface tension that differs from the surface tension of said
polyester by an absolute value of at least 5 dynes/cm, and a melt
viscosity wherein the ratio of melt viscosity of said second
polymer to melt viscosity of said polyester is about 0.1 to about
3.5.
2. The shrink film of claim 1 wherein said polyester comprises (i)
diacid residues comprising at least 80 mole percent, based on the
total moles of diacid residues, of one or more residues of:
terephthalic acid, naphthalenedicarboxylic acid,
1,4-cyclohexanedicarboxylic acid, or isophthalic acid; and (ii)
diol residues comprising 10 to 100 mole percent, based on the total
moles of diol residues, of one or more residues of
1,4-cyclohexanedimethanol, neopentyl glycol, or diethylene glycol;
and 0 to 90 mole percent of one or more residues of: ethylene
glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol,
1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol,
2,2,4-trimethyl-1,3-pent- anediol,
2,2,4,4-tetramethyl-1,3-cyclobutanediol, 1,3-cyclohexanedimethano-
l, bisphenol A, or polyalkylene glycol.
3. The shrink film of claim 2 wherein said diol residues comprise
about 10 to about 99 mole percent of residues of
1,4-cyclohexanedimethanol, 0 to about 90 mole percent of residues
of ethylene glycol, and about 1 to about 25 mole percent of
residues of diethylene glycol.
4. The shrink film of claim 2 wherein said diacid residues further
comprise 0 to about 20 mole percent of one or more residues of a
modifying diacid containing 4 to 40 carbon atoms.
5. The shrink film of 4 wherein said modifying diacid comprises one
or more of: succinic acid, glutaric acid, adipic acid, suberic
acid, sebacic acid, azelaic acid, dimer acid, or sulfoisophthalic
acid.
6. The shrink film of claim 5 wherein said polyester further
comprises one or more antioxidants, melt strength enhancers,
branching agents, chain extenders, flame retardants, fillers, dyes,
colorants, pigments, nanoclays, antiblocking agents, flow
enhancers, impact modifiers, antistatic agents, processing aids,
mold release additives, or plasticizers.
7. The shrink film of claim 3 wherein said first polymer comprises
one or more polymers selected from the group consisting of:
cellulosic polymers, starch, esterified starch, polyketones,
polyester, polyamides, polysulfones, polyimides, polycarbonates,
olefinic polymers, and copolymers thereof; and said second polymer
comprises one or more polymers selected from the group consisting
of polyamides, polyketones, polysulfones, polyesters,
polycarbonates, olefinic polymers, and copolymers thereof.
8. The shrink film of claim 7 wherein said first polymer comprises
one or more of: microcrystalline cellulose, a cellulose ester, or a
cellulose ether.
9. The shrink film of claim 8 wherein said cellulose ester
comprises one or more of: cellulose acetate, cellulose triacetate,
cellulose acetate proprionate, or cellulose acetate butyrate and
said cellulose ether comprises one or more of: hydroxypropyl
cellulose, methyl ethyl cellulose, or carboxymethyl cellulose.
10. The shrink film of claim 2 wherein said second polymer
comprises one or more polymers selected from the group consisting
of polyethylene, polystyrene, polypropylene, and copolymers
thereof.
11. The shrink film of claim 10 wherein said second polymer
comprises one or more of: ethylene vinyl acetate, ethylene vinyl
alcohol copolymer, ethylene methyl acrylate copolymer, ethylene
butyl acrylate copolymer, ethylene acrylic acid copolymer, or
ionomer.
12. The shrink film of claim 11 wherein said voiding agent
comprises about 5 to about 95 weight percent of said first polymer,
based of the total weight of said voiding agent.
13. The shrink film of claim 10 wherein said first polymer
comprises one or more of cellulose acetate or cellulose acetate
propionate and said second polymer comprises one or more of:
polystyrene, polypropylene, or ethylene methyl methacrylate
copolymer.
14. The shrink film of claim 13 wherein said diacid residues
comprise at least 95 mole percent of the residues of terephthalic
acid; said the diol residues comprise about 10 to about 40 mole
percent of the residues of 1,4-cyclohexanedimethanol, about 1 to
about 25 mole percent of the residues of diethylene glycol, and
about 35 to about 89 mole percent of the residues ethylene glycol;
said first polymer comprises cellulose acetate; and said second
polymer comprises polypropylene and ethylene methyl acrylate
copolymer.
15. The shrink film of claim 2 wherein said second polymer has a Tg
or a Tm greater than the Tg of said polymer matrix and wherein said
voiding agent further comprises a third polymer having a refractive
index that differs from the refractive index of said continuous
polyester phase by an absolute value at least 0.04, a surface
tension that is between the surface tension of said polyester and
said second polymer, and a density of 1.1 g/cc or less.
16. The shrink film of claim 2 wherein said film has a density of
1.0 g/cc or less and comprises at least 1 layer.
17. The shrink film of claim 2 wherein said film is stretched in at
least one direction and has a shrinkage along the principal axis at
least 5% after 10 seconds in a water bath at 70.degree. C. and at
least 30% after 10 seconds in a water bath at 95.degree. C.
18. A void-containing shrink film comprising an oriented,
continuous polyester phase comprising (i) diacid residues
comprising at least 80 mole percent, based on the total moles of
diacid residues, of one or more residues of: terephthalic acid or
isophthalic acid; and (ii) diol residues comprising 15 to 55 mole
percent, based on the total moles of diol residues, of one or more
residues of 1,4-cyclohexane-dimethanol or diethylene glycol; and 45
to 85 mole percent of ethylene glycol, having dispersed therein a
voiding agent comprising at least one first polymer and at least
one second polymer, wherein said first polymer comprises one or
more of: microcrystalline cellulose, a cellulose ester, or a
cellulose ether, has a Tg or a Tm greater than the Tg of said
polyester, and a surface tension that differs from the surface
tension of said polyester by an absolute value of 5 dynes/cm or
less; and said second polymer comprises one or more polymers
selected from the group consisting of polyethylene, polystyrene,
polypropylene, and copolymers thereof, has a surface tension that
differs from the surface tension of said polyester by an absolute
value of at least 5 dynes/cm, and a melt viscosity wherein the
ratio of melt viscosity of said second polymer to the melt
viscosity of said polyester is about 0.1 to about 3.5.
19. The shrink film of claim 18 wherein said first polymer
comprises one or more of cellulose acetate or cellulose acetate
propionate and said second polymer comprises one or more of:
polystyrene, polypropylene, or ethylene methyl acrylate
copolymer.
20. The shrink film of claim 19 wherein said film is stretched in
at least one direction and has a shrinkage along the principal axis
at least 10% after 10 seconds in a water bath at 70.degree. C. and
at least 40% after 10 seconds in a water bath at 95.degree. C.
21. The shrink film of claim 19 wherein said film has a density of
0.9 g/cc or less.
22. The shrink film of claim 20 wherein said film is stretched in
one direction and has a shrinkage in a direction perpendicular to
the principal axis of 10% or less after 10 seconds in a water bath
at 70.degree. C. to 95.degree. C.
23. The film of claim 19 where said film has an optical
absorptivity of 200 cm.sup.-1 or greater.
24. The film of claim 19 wherein said film is produced by
extrusion, calendering, casting, drafting, tentering, or
blowing.
25. A sleeve or roll-fed label comprising said shrink film of claim
19.
26. The sleeve or label of claim 25, wherein said sleeve or label
is seamed by solvent bonding, hot-melt glue, UV-curable adhesive,
radio frequency sealing, heat sealing, or ultrasonic bonding.
27. A process for a void-containing shrink film, comprising: (i)
mixing a polyester and a voiding agent at a temperature at or above
the Tg of said polyester to form a uniform dispersion of said
voiding agent within said polyester, wherein said voiding agent
comprises at least one first polymer and at least one second
polymer, wherein said first polymer has a Tg or a Tm greater than
the Tg of said polyester, a tensile modulus of at least 1 GPa, and
a surface tension that differs from the surface tension of said
polymer matrix by an absolute value of 5 dynes/cm or less; and said
second polymer has a surface tension that differs from the surface
tension of said polyester by an absolute value of at least 5
dynes/cm, and a melt viscosity wherein the ratio of melt viscosity
of said second polymer to the melt viscosity of said polyester is
about 0.1 to about 3.5; (ii) forming a sheet or film; and (iii)
orienting said sheet or film of step (ii) in one or more
directions.
28. The process of claim 27 wherein said sheet or film formation
step (ii) is by extrusion, calendering, casting, or blowing.
29. A void-containing, shrink film prepared by the process of claim
28.
30. A process for separating a void-containing polyester from a
mixture of different polymers, comprising: (i) shredding, chopping,
or grinding a mixture of polymers comprising said void-containing
polyester and at least one other polymer to produce particles of
said mixture; (ii) dispersing said mixture into an aqueous or
gaseous medium; (iii) allowing said particles to partition into a
higher density fraction and a lower density fraction; and (iv)
separating said lower density fraction from said higher density
fraction; wherein said void containing polyester comprises a
continuous polyester phase having dispersed therein a voiding agent
comprising at least one first polymer, and at least one second
polymer, wherein said first polymer has a Tg or a Tm greater than
the Tg of said polyester, a tensile modulus of at least 1 GPa, and
a surface tension that differs from the surface tension of said
polyester by an absolute value of about 5 dynes/cm or less; and
said second polymer has a surface tension that differs from the
surface tension of said polyester by an absolute value of at least
5 dynes/cm, and a melt viscosity wherein the ratio of melt
viscosity of said second polymer to the melt viscosity of said
polyester is about 0.1 to about 3.5.
31. The process of claim 30 wherein said mixture comprises one or
more of: poly(ethylene terephthalate), poly(vinyl chloride),
polypropylene, polycarbonate, poly(butylene terephthalate),
polystyrene, or polyethylene in addition to said void-containing
polyester.
32. The process of claim 31 wherein said mixture comprises
polyethylene or polypropylene and said higher density fraction
comprises substantially said void-containing polyester.
33. The process of claim 31 wherein said mixture comprises
poly(ethylene terephthalate) or poly(vinyl chloride) and said lower
density fraction comprises substantially said void-containing
polyester.
34. The process of claim 31 wherein said poly(ethylene
terephthalate) is obtained from bottles or packaging materials and
said void-containing polyester is obtained from shrink film.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 60/526,305, filed Dec. 2, 2003.
FIELD OF THE INVENTION
[0002] This invention pertains to void-containing, polyester shrink
film. More specifically, this invention pertains to void-containing
shrink films comprising an oriented, continuous polyester phase
having dispersed therein a voiding agent comprising at least one
first polymer and at least one second polymer, in which the polymer
components have selected physical properties such as glass
transition temperature, tensile modulus, melting point, surface
tension, and melt viscosity. The invention further pertains to a
process for a void-containing shrink film and to a process for
separating a void-containing polyester from a mixture of different
polymers.
BACKGROUND OF THE INVENTION
[0003] Shaped articles such as, for example, sheet, film, tubes,
bottles, sleeves, and labels, are commonly used in various
packaging applications. For example, film and sheet produced from
polymers such as polyolefins, polystyrene, poly(vinyl chloride),
polyesters and the like, are used frequently for the manufacture of
shrink labels for plastic beverage or food containers. It is
desirable in many packaging applications that the shaped article
exhibit properties such as, for example, good printability, high
opacity, low density, low shrink force, good texture,
recyclability, and high stiffness. For example, during recycling of
the plastic material, labels are often separated from the rest of
the container because of the presence of inks, glues, and other
substances which can contaminant and discolor the recycled polymer.
If the density of the label polymer is sustantially different from
that of the container polymer, the separation of the label polymer
may be carried out by a simple and economical flotation process in
which the label polymer floats or sinks away from the other
polymers. Unfortunately, the label and container materials used in
packaging often have similar densities that prevents the use of
such flotation processes.
[0004] One approach for reducing the density is to introduce many
small voids or holes into the shaped article. This process is
called "voiding" and may also be referred to as "cavitating" or
"microvoiding". Voids are obtained by incorporating about 5 to
about 50 weight % of small organic or inorganic particles or
"inclusions" (referred in the art as "voiding" or "cavitation"
agents) into a matrix polymer and orienting the polymer by
stretching in at least one direction. During stretching, small
cavities or voids are formed around the voiding agent. When voids
are introduced into polymer films, the resulting voided film not
only has a lower density than the non-voided film, but also becomes
opaque and develops a a paper-like surface. This surface also has
the advantage of increased printability; that is, the surface is
capable of accepting many inks with a substantially greater
capacity over a non-voided film. Typical examples of voided films
are described in U.S. Pat. Nos. 3,426,754; 3,944,699; 4,138,459;
4,582,752; 4,632,869; 4,770,931; 5,176,954; 5,435,955; 5,843,578;
6,004,664; 6,287,680; 6,500,533; 6,720,085; U.S. Patent Application
Publication No.'s 2001/0036545; 2003/0068453; 2003/0165671;
2003/0170427; Japan Patent Application No.'s 61-037827; 63-193822;
2004-181863; European Patent No. 0 581 970 B1, and European Patent
Application No. 0 214 859 A2.
[0005] Although voided films are known, they frequently suffer from
a number of shortcomings and often show inferior properties to the
corresponding non-voided counterparts such as, for example, poor
stiffness, insufficient opacity, high shrink force, and high
surface roughness which make them less desirable for many packaging
applications. For packaging labels, for example, it is often
desirable for aesthetic purposes to have a high concentration of
voids such that the voided film is opaque. Increasing the number of
voids, however, can increase the surface roughness of the film to
the point that the printing quality, texture and feel, and
seamability of the label are reduced. To address this problem, many
voided films have multiple layers in which a non-voided surface
layer is affixed to a void-containing core layer (by adhesion or
coextrusion). The non-voided layer is applied because it provides a
smoother surface than the voided layer. While this approach solves
many of the above problems, production of such multilayer films is
expensive and requires additional coextrusion or lamination
equipment. Multilayered films also typically have a higher overall
film density because of the lack of or decreased voiding on the
surface and are not as desirable as monolayer films. It is also
possible to introduce voids into containers such as, for example, a
bottle or thermoformed tray. Voided containers are lightweight,
require less polymer, and can be printed upon directly, thus
eliminating the need for a label.
[0006] Conventional voiding agents suffer from several
disadvantages. Inorganic agents like calcium carbonate, talc,
silica, and the like may be used as voiding agents but, because
inorganic substances are typically dense materials, the final
density of the shaped article is often too high. In the case of
voided films, for example, the reduction in density imparted by
voiding is frequently offset by the weight of the inorganic
agents.
[0007] Polyolefins such as, for example, polypropylene may be used
as a voiding agents. Polyolefins, however, often do not disperse
well and may require a compatibilizer such as, for example, a
carboxylated polyethylene to obtain a uniform distribution of
voids. When used with polyester polymers to produce voided films,
polyolefins also tend to lower the polyester film surface tension
and thereby reduce the printability of the film. Polyolefins are
softer than the polyester at room temperature which sometimes
lowers the overall film modulus to unacceptable levels. Finally,
polyolefins are relatively inefficient voiding agents and large
amounts are required to achieve the necessary density reduction. As
discussed earlier, this leads to poor surface roughness and
printing problems, thus making it difficult to use in single layer
films.
[0008] Other polymeric voiding agents such as, for example,
styrenics, polymethyl-pentene, polycarbonate, nylons, cellulosics,
and the like, suffer from some of the same voiding efficiency
problems as polyolefins. High modulus styrenics, like atactic
polystyrene, are efficient voiding agents, but suffer from
outgassing problems when mixed and processed at higher temperatures
and, therefore, are useful only at low levels. Styrenics also tend
to embrittle the film. Crosslinked styrene beads may be used to
circumvent this problem, although these beads tend to be expensive.
Cellulosics tend to be hygroscopic and require a separate drying
and moisture removal step before incorporation into the polymer
matrix. For voided shrink films, cellulosics also tend to produce
undesirably high shrink forces.
[0009] There is a need, therefore, for a composition that would
enable the production of voided, shaped articles such as, for
example, film, sheet, bottles, tubes, fibers, and rods, having a
lower density, good printability, lower shrink force, high opacity,
and other desirable physical properties such as high stiffness and
good texture and feel. In the case of films, there is a also need
for a composition that would permit the preparation of single
layer, void-containing films with acceptable printability,
stiffness, and lower densities. Such a composition would have
utility in the beverage and food packaging industry for the
production of voided-containing shrink labels.
SUMMARY OF THE INVENTION
[0010] We have discovered a composition comprising a polymer matrix
and a voiding agent that is useful for the preparation of
void-containing articles. Our composition is useful for the
production of void-containing films in which a polyester comprises
the polymer matrix and which may be biaxially or uniaxially
oriented to provide high quality, void-containing shrink films.
Thus, the present invention provides a void-containing shrink film
comprising an oriented, continuous polyester phase having dispersed
therein a a voiding agent comprising at least one first polymer and
at least one second polymer, wherein the first polymer has a glass
transition temperature (Tg) or a melting point temperature (Tm)
greater than the Tg of the polyester, a tensile modulus of at least
1 GPa, and a surface tension that differs from the surface tension
of the polyester by an absolute value of 5 dynes/cm or less; and
the second polymer has a surface tension that differs from the
surface tension of the polyester by an absolute value of at least 5
dynes/cm, and a melt viscosity wherein the ratio of melt viscosity
of the second polymer to the melt viscosity of the polyester is
about 0.1 to about 3.5.
[0011] The void-containing, polyester shrink films of the present
invention may comprise polyesters of various compositions. For
example, amorphous or semicrystalline polyesters may be used which
comprise one or more diacid residues of terephthalic acid,
naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, or
isophthalic acid, and one or more diol residues of
1,4-cyclohexanedimethanol, neopentyl glycol, or diethylene glycol.
Additional modifying acids and diols may be used to vary the
properties of the film as desired.
[0012] The film of our invention includes a voiding agent dispersed
therein that comprises at least one first polymer and at least one
second polymer that are selected on the basis of certain physical
properties and the relationship of some of these properties to
corresponding properties of the polymer matrix. These parameters
include the glass transition temperature (abbreviated herein as
"Tg") or their melting point temperature (abbreviated herein as
"Tm"), surface tension, tensile modulus, and melt viscosity. This
combination of a first and second polymer provides a voiding agent
with superior performance over either polymer component alone and,
in particular, gives a shrink film with a higher opacity, reduces
cost, improves processability, and reduces shrink forces. Typical
polymers that may be used as the first polymer include, but are not
limited to, cellulosic polymers, starch, esterified starch,
polyketones, polyester, polyamides, polysulfones, polyimides,
polycarbonates, olefinic polymers, and copolymers thereof.
Similarly, second polymer may comprise one or more polymers
selected from polyamides, polyketones, polysulfones, polyesters,
polycarbonates, olefinic polymers, and copolymers thereof. In
another aspect of the invention, the first polymer comprises one or
more of cellulose acetate or cellulose acetate propionate and the
second polymer comprises one or more of: polystyrene,
polypropylene, or ethylene methyl methacrylate copolymer.
[0013] Our invention also includes a process for a void-containing
shrink film, comprising: (i) mixing a polyester and a voiding agent
at a temperature at or above the Tg of the polyester to form a
uniform dispersion of the voiding agent within the polyester,
wherein the voiding agent comprises at least one first polymer and
at least one second polymer, wherein the first polymer has a Tg or
a Tm greater than the Tg of the polyester, a tensile modulus of at
least 1 GPa, and a surface tension that differs from the surface
tension of the polymer matrix by an absolute value of 5 dynes/cm or
less; and the second polymer has a surface tension that differs
from the surface tension of the polyester by an absolute value of
at least 5 dynes/cm and a melt viscosity wherein the ratio of melt
viscosity of the second polymer to the melt viscosity of the
polyester is about 0.1 to about 3.5; (ii) forming a sheet or film;
and (iii) orienting the sheet or film of step (ii) in one or more
directions. The voided shrink films of our invention may be
stretched in one or more directions and may comprise a one or more
layers. The films have excellent opacity, good film stiffness, a
low shrink force, improved seamability, high surface tension (for
printing), and high shrinkage.
[0014] Our void-containing, polyester shrink film may be readily
separated from mixtures of polymers and, thus, may be easily
recovered and recycled from commercial waste. Thus, another aspect
of the instant invention is process for separating a
void-containing polyester from a mixture of different polymers,
comprising:
[0015] (i) shredding, chopping, or grinding a mixture of polymers
comprising the void-containing polyester and at least one other
polymer to produce particles of the mixture;
[0016] (ii) dispersing the mixture into an aqueous or gaseous
medium;
[0017] (iii) allowing the particles to partition into a higher
density fraction and a lower density fraction; and
[0018] (iv) separating the lower density fraction from the higher
density fraction; wherein the void containing polyester comprises a
continuous polyester phase having dispersed therein a voiding agent
comprising at least one first polymer, and at least one second
polymer, wherein the first polymer has a Tg or a Tm greater than
the Tg of the polyester, a tensile modulus of at least 1 GPa, and a
surface tension that differs from the surface tension of the
polyester by an absolute value of 5 dynes/cm or less; and the
second polymer has a surface tension that differs from the surface
tension of the polyester by an absolute value of at least 5
dynes/cm, and a melt viscosity wherein the ratio of melt viscosity
of said second polymer to the melt viscosity of said polyester is
about 0.1 to about 3.5. The recycleability of our shrink film in
combination with their excellent physical properties make them
particularly useful as labels and in other packaging
applications.
DETAILED DESCRIPTION
[0019] The present invention provides a void-containing shrink film
comprising an oriented, continuous polyester phase having dispersed
therein a voiding agent comprising at least one first polymer and
at least one second polymer, wherein the first polymer has a glass
transition temperature (Tg) or a melting point temperature (Tm)
greater than the Tg of the polyester, a tensile modulus of at least
1 GPa, and a surface tension that differs from the surface tension
of the polyester by an absolute value of 5 dynes/cm or less; and
the second polymer has a surface tension that differs from the
surface tension of the polyester by an absolute value of at least 5
dynes/cm and a melt viscosity wherein the ratio of melt viscosity
of the second polymer to the melt viscosity of the polyester is
about 0.1 to about 3.5. Our shrink film may be biaxially or
uniaxially oriented and may be a single or multilayed layered. Our
invention, therefore, is understood to include films in which the
single layered film may be incorporated as one or more layers of a
multilayered structure such as, for example, a laminate or a
coextrusion such as, for example, in roll-fed labels where the
printed label is adhered or laminated to the void-containing
substrate. The shrink films of our invention have a high shrinkage
at lower temperatures, and a lower density. Theses film are useful
for sleeve label applications, roll-fed labels, and other shrink
film applications.
[0020] Unless otherwise indicated, all numbers expressing
quantities of ingredients, properties such as molecular weight,
reaction conditions, and so forth used in the specification and
claims are to be understood as being modified in all instances by
the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the following specification
and attached claims are approximations that may vary depending upon
the desired properties sought to be obtained by the present
invention. At the very least, each numerical parameter should at
least be construed in light of the number of reported significant
digits and by applying ordinary rounding techniques. Further, the
ranges stated in this disclosure and the claims are intended to
include the entire range specifically and not just the endpoint(s).
For example, a range stated to be 0 to 10 is intended to disclose
all whole numbers between 0 and 10 such as, for example 1, 2, 3, 4,
etc., all fractional numbers between 0 and 10, for example 1.5,
2.3, 4.57, 6.1113, etc., and the endpoints 0 and 10. Also, a range
associated with chemical substituent groups such as, for example,
"C.sub.1 to C.sub.5 hydrocarbons", is intended to specifically
include and disclose C.sub.1 and C.sub.5 hydrocarbons as well as
C.sub.2, C.sub.3, and C.sub.4 hydrocarbons.
[0021] Notwithstanding that the numerical ranges and parameters
setting forth the broad scope of the invention are approximations,
the numerical values set forth in the specific examples are
reported as precisely as possible. Any numerical value, however,
inherently contains certain errors necessarily resulting from the
standard deviation found in their respective testing
measurements.
[0022] The composition of the present invention are useful for the
preparation of void-containing articles and comprises a polymer
matrix and a voiding agent dispersed within the polymer matrix. The
terms "voids", "microvoids", and "microporous", as used herein, are
intended to be synonymous and are well-understood by persons
skilled in the art to mean tiny, discrete voids or pores contained
within the polymer below the surface of the article that are
intentionally created during the manufacture of the article.
Similarly, the terms "voided", "microvoided", "cavitated" and
"void-containing", as used herein in reference to the compositions,
polymers, and shaped articles of the invention, are intended to be
synonymous and mean "containing tiny, discrete voids or pores". The
composition of the invention includes a "voiding agent" dispersed
within the polymer matrix. The term "voiding agent", as used
herein, is synonomous with the terms "voiding composition",
"microvoiding agent", and "cavitation agent" and is understood to
mean a substance dispersed within a polymer matrix that is useful
to bring about or cause the formation voids within the polymer
matrix" upon orientation or stretching of the polymer matrix. The
term "polymer matrix", as used herein, is synonymous with the term
"matrix polymer" and refers to one or more polymers providing a
continuous phase in which the voiding again may be dispersed such
that the particles of the voiding agent are surrounded and
contained by the continuous phase.
[0023] The polymer matrix of the composition may be selected from a
range of polymers and may comprises a single polymer or a blend of
one or more polymers. To produce shaped articles with adequate
stiffness, the polymer matrix typically has a glass transition
temperature (abbreviated herein as "Tg") of at least 50.degree. C.
Non-limiting examples of polymers which may comprise the polymer
matrix of our composition include one or more polyesters,
polylactic acid, polyketones, polyamides, olefinic polymers,
fluoropolymers, polyacetals, polysulfones, polyimides,
polycarbonates, or copolymers thereof. The term "olefinic polymer",
as used herein is intended to mean a polymer resulting from the
addition polymerization of ethylenically unsaturated monomers such
as, for example, polyethylene, polypropylene, polystyrene,
poly(acrylonitrile)s, poly(acrylamide), acrylic polymers,
poly(vinyl acetate), poly(vinyl chloride), and copolymers of these
polymers. The composition of the instant invention forms voids on
orientation or stretching at a temperature at or above the Tg of
the matrix polymer. Stretching may be carried out in one or more
directions at a stretch ratio of at least 1.5. The composition,
thus, may be "uniaxially stretched", meaning the polymer matrix is
stretched in one direction or "biaxially stretched," meaning the
polymer matrix has been stretched in two different directions.
[0024] The voiding agent comprises a first and second polymer that
may be selected in accordance with specified physical properties of
glass transition temperature or melting point, surface tension,
modulus, and melt viscosity, and their relationship to the
corresponding properties of the polymer matrix. Therefore, to
generate voids efficiently within the polymer matrix, it is
desirable that the first polymer have a hardness that is greater
than the matrix polymer at the stretch temperature. In other words,
the first polymer should have a tensile modulus at room temperature
that is at least 1 GPa and a melt transition temperature, also
known as "melting point" (abbreviated hereinafter as "Tm") or a Tg
that is higher than the Tg of the polymer matrix. For crystalline
polymers, Tm is the temperature at which crystalline domains lose
their structure, or melt, and the modulus drops precipitously. For
amorphous polymers, Tg is the temperature below which amorphous
domains lose the structural mobility of the polymer chains and
become rigid glasses. Some polymers, particularly many elastomeric
block copolymers such as, for example, polyesterethers, have a high
melting point in one phase, but still have too low of a modulus to
induce any significant amount of voiding.
[0025] Thus, as an example, if the first polymer is a crystalline
polymer, its Tm should be higher than the Tg of the matrix polymer.
Similarly, if the first polymer is amorphous then its Tg should be
higher than the Tg of the polymer matrix. When the Tg or Tm of the
first polymer is greater than the Tg of the polymer matrix, the
particles of the voiding agent tend to be "harder" than the polymer
matrix which enables the efficient formation of voids within the
polymer matrix during stretching. For example, if the polymer
matrix is a copolyester with a Tg of about 74.degree. C. to about
77.degree. C. (165 to 170.degree. F.), the first polymer of our
voiding agent may be a styrenic polymer (Tg=100.degree. C. or
higher). Amorphous polymers typically exhibit only a glass
transition temperature as measured by well-known techniques such
as, for example, by differential scanning calorimetry using ASTM
Method D3418. In one embodiment of the invention, the first and
second polymers of the voiding agent are not crosslinked
polymers.
[0026] The dispersion of the voiding agent within the polymer
matrix is improved if the surface tension of the first and second
polymer components are maintained within certain values relative to
the polymer matrix. Thus, in accordance with the invention, the
first polymer component of the voiding agent has a surface tension
that differs from the surface tension of the polymer matrix by an
absolute value of 5 dynes/cm or less and the surface tension the
second polymer component differs from the surface tension of the
polymer matrix by an absolute value of at least 5 dynes/cm. Further
examples of the difference in surface tension which may be
exhibited between the first polymer and the polymer matrix are an
absolute value of 4 dynes/cm or less and an absolute value of 3
dynes/cm or less. Further examples of the difference in surface
tension which may be exhibited between the second polymer and the
polymer matrix are an absolute value of at least 6 dynes/cm and an
absolute value of at least 7 dynes/cm. Surface tension (also
referred to as "critical surface tension") may be determined from
the published literature or measured according to procedures
well-known in the art such as, for example, by sessile drop methods
or by the Zisman critical surface method (for example, by using
Accudyne.TM. dyne marker pens). The latter method gives the
"critical surface tension" and is effectively the fluid surface
tension where full wetting occurs based on a series of test fluids
each having a different known surface tension. Critical surface
tension and total surface tension by other methods are
approximately the same, but when different, the critical surface
tension values should be used for consistency. To further impart
good dispersion of the voiding agent within the polymer matrix, the
ratio of melt viscosity of the second polymer to the melt viscosity
of the polymer matrix may be about 0.1 to about 3.5. This ratio is
defined as the melt viscosity of the second polymer, divided by the
melt viscosity of the matrix at the temperature of mixing.
Typically, a value at a shear rate of 1 s.sup.-1 is used as
determined from dynamic or steady state parallel plate or cone and
plate viscometry. This number is approximately the "zero shear"
viscosity. In another example, the ratio of melt viscosity of the
second polymer to the melt viscosity of the polymer matrix may be
about 0.5 to about 2.0. Typically, the melt viscosities of the
polymers of the present invention show Newtonian behavior (i.e.,
non-shear thinning) over a broad shear range (including that seen
in an extruder).
[0027] The first polymer typically has an average particle size of
about 0.01 to about 50 .mu.m after dispersion in the polymer
matrix. This particle size range permits the voiding agent to be
uniformly dispersed throughout the matrix polymer. Additional
examples of average particle sizes for the first polymer of the
voiding agent are about 0.01 to about 40 and about 0.1 to about 10
.mu.m. The term "average particle size", as used herein, means the
sum of the diameters of all the particles divided by the total
number of particles. The average particle size of the first polymer
may be measured by optical or electron micros-copy using techniques
known to persons skilled in the art. Typically, the microscopy
measurement is conducted by measuring the diameters of a small,
representative sample of particles containing, typically, 100 to
500 particles, and then calculating the average diameter by
dividing the sum of the diameters by the number of particles. The
microscopy measurement of particle diameters may be carried out
manually or by using automated instrumentation and procedures well
known to persons skilled in the art.
[0028] It may be desirable in many applications that the voiding
agent impart a high level of opacity to the matrix polymer. Opacity
may be enhanced by increasing the absolute value of the difference
in the refractive indices of each of the first and second polymers
and the polymer matrix. Thus, it is advantageous that the first
polymer and the second polymer have a refractive index that differs
from the refractive index of the polymer matrix by an absolute
value at least 0.02. Other examples of differences in the absolute
value between the refractive index of the polymer matrix and the
first polymer or the second polymer of the voiding agent are 0.04
and 0.06. The refractive index of the polymer matrix and the first
polymer may be determined with an Abbe.TM. or Metricon.TM.
refractometer using techniques well known in the art or may be
obtained from the published literature. For example, the refractive
index may be measured using a Metricon.TM. prism coupler with a 633
nm wavelength laser.
[0029] The first and second polymer components of the voiding agent
may be selected from a wide range of polymers. The first polymer
may be a single polymer or blend of one or more polymers. For
example, the first polymer may comprise one or more polymers
selected from cellulosic polymers, starch, esterified starch,
polyketones, fluoropolymers, polyacetals, polyesters, polyamides,
polysulfones, polyimides, polycarbonates, olefinic polymers, and
copolymers of these polymers with other monomers such as, for
example, copolymers of ethylene with acrylic acid and its esters.
Cellulosic polymers are particularly efficient voiding agents and
have surface tensions in the desired range for many typical matrix
polymers. Thus, in one example, the first polymer of our novel
voiding agent may be a cellulosic polymer and may comprise one or
more of microcrystalline cellulose, a cellulose ester, or a
cellulose ether. In another embodiment, the first polymer may be a
cellulose ester such as, for example, cellulose acetate, cellulose
triacetate, cellulose acetate propionate, or cellulose acetate
butyrate. In yet another example, the first polymer may be a
cellulose ether which may include, but is not limited to, one or
more of hydroxypropyl cellulose, methyl ethyl cellulose, or
carboxymethyl cellulose.
[0030] For the voiding agents of the present invention, we have
found that cellulosic polymers such as cellulose acetate, cellulose
triacetate, cellulose acetate propionate, cellulose acetate
butyrate, cellulose ethers, carboxymethyl cellulose, and the like,
are useful as the first polymer component of our voiding agent and
are more efficient at creating voids than other standard voiding
agents like polypropylene, polystyrene, and the like. Typically,
cellulosic polymers have a high Tg and can be used in a large
number of polymer matrices while maintaining sufficient hardness at
stretching. Cellulosics also disperse well in the polymer matrix
and provide an opaque film with uniform hole size distribution.
Cellulosics are produced in either powder or pelletized form, and
either form may be used in the voiding agent of the invention. For
example, our voiding agent may comprise cellulose acetate in
powdered form, having an acetyl content from about 28 to 45 weight
percent and a falling ball viscosity of between 0.01 and 90
seconds.
[0031] Highly crystalline polyester homopolymer powders such as
PET, poly(1,3-trimethylene terephthalate), poly(cyclohexylene
terephthalate), or poly(1,4-butylene terephthalate) also may be
used as the first polymer provided that the Tm of the crystalline
polyester is greater than the Tg of the continuous polyester phase.
In this embodiment, it is preferred that the polymer matrix have a
low enough melt viscosity at a temperature below the crystalline
polyester melt temperature. In the case where the polymer matrix is
a polyester, it is preferable that the polymer matrix is melt
processable at a temperature below the Tm of the crystalline
polyester voiding agent; otherwise, the voiding agent may melt and
transesterify with the polyester matrix and lose its voiding
capability.
[0032] The second polymer may comprise one or more polymers
selected from polyamides, polyketones, polysulfones,
fluoropolymers, polyacetals, polyesters, polycarbonates, olefinic
polymers, and copolymers thereof. For example, the second polymer
may include, but is not limited to, one or more olefinic polymers
such as, for example, polyethylene, polystyrene, polypropylene, and
copolymers thereof. Further non-limiting examples of olefinic
copolymers include ethylene vinyl acetate, ethylene vinyl alcohol
copolymer, ethylene methyl acrylate copolymer, ethylene butyl
acrylate copolymer, ethylene acrylic acid copolymer, and ionomer.
In a preferred embodiment, the first polymer comprises one or more
of cellulose acetate or cellulose acetate propionate and the second
polymer comprises one or more of polystyrene, polypropylene, or
ethylene methyl acrylate copolymer. Typically, the voiding agent
comprises about 5 to about 95 weight percent of the first polymer,
based on the total weight of the voiding agent. Other weight
percent ranges for the first polymer within the voiding agent are
about 30 to about 60 weight percent and about 50 to about 60 weight
percent.
[0033] The polymers that may be used as the first polymer, second
polymer, and polymer matrix of the present invention may be
prepared according to methods well-known in the art or obtained
commercially. Examples of commercially available polymers which may
be used in the invention include EASTAR.TM., EASTAPAK.TM.,
SPECTAR.TM., and EMBRACE.TM. polyesters and copolyesters available
from Eastman Chemical Co,; LUCITE.TM. acrylics available from
Dupont; TENITE.TM. cellulose esters available from Eastman Chemical
Co.; LEXAN.TM. (available from GE Plastics) or MAKROLON.TM.
(available from Bayer) polycarbonates; DELRIN.TM. polyacetals
available from Dupont; K-RESIN.TM. (available from Phillips) and
FINACLEAR.TM. /FINACRYSTAL.TM. (available from Atofina) styrenics
and styrenic copolymers; FINATHENE.TM. (available from Atofina) and
HIFOR.TM./TENITE.TM. (available from Eastman) polyethylenes;
ZYTEL.TM. nylons available from Dupont; ULTRAPEK.TM. PEEK available
from BASF; KAPTON.TM. polyimides available from Dupont; and
TEDLAR.TM. and KYNAR.TM. fluoropolymers available from Dupont and
Atofina, respectively.
[0034] Our invention also provides a composition capable of forming
voids that comprises a voiding agent dispersed within a polymer
matrix. Thus, the present invention provides a composition capable
of forming voids, comprising: a polymer matrix and at least one
first polymer and at least one second polymer each dispersed within
the polymer matrix, wherein the first polymer comprises one or more
of: microcrystalline cellulose, a cellulose ester, or a cellulose
ether, has a Tg or a Tm greater than the Tg of the polymer matrix,
and a surface tension that differs from the surface tension of the
polymer matrix by an absolute value of 3 dynes/cm or less; and the
second polymer comprises one or more polymers selected from the
group consisting of polyethylene, polystyrene, polypropylene, and
copolymers thereof, has a surface tension that differs from the
surface tension of the polymer matrix by an absolute value of at
least 6 dynes/cm, and a melt viscosity wherein the ratio of melt
viscosity of the second polymer to the melt viscosity of the
polymer matrix is about 0.1 to about 3.5. The first polymer may be
cellulosic polymer including, but not limited to microcrystalline
cellulose, a cellulose ester, or a cellulose ether. It is desirable
but not critical to our invention that the refractive index of the
first and second polymers differ from the refractive index of the
polymer matrix by an absolute value of at least 0.04. The surface
tension of the first polymer differs from that of the polymer
matrix by an absolute value of 3 dynes/cm or less while the surface
tension of the second polymer differs from that of the polymer
matrix by an absolute value of at least 6 dynes/cm. The first and
second polymers, Tg's, Tm's, and surface tensions, also include
their various embodiments as described hereinabove in accordance
with the invention.
[0035] The polymer matrix may be selected from a range of polymers
and may comprises a single polymer or a blend of one or more
polymers. Non-limiting examples of polymers which may comprise the
polymer matrix of our composition include one or more polyesters,
polylactic acid, polyketones, polyamides, fluoropolymers,
polyacetals, polysulfones, polyimides, polycarbonates, olefinic
polymers, or copolymers thereof. Typically, the polymer matrix is
oriented. The term "oriented", as used herein, means that the
polymer matrix is stretched to impart direction or orientation in
the polymer chains. The polymer matrix, thus, may be "uniaxially
stretched", meaning the polymer matrix is stretched in one
direction or "biaxially stretched," meaning the polymer matrix has
been stretched in two different directions. Typically, but not
always, the two directions are substantially perpendicular. For
example, in the case of a film, the two directions are in the
longitudinal or machine direction ("MD") of the film (the direction
in which the film is produced on a film-making machine) and the
transverse direction ("TD") of the film (the direction
perpendicular to the MD of the film). Biaxially stretched articles
may be sequentially stretched, simultaneously stretched, or
stretched by some combination of simultaneous and sequential
stretching.
[0036] Although not critical, the second polymer may have a Tg or a
Tm greater than the Tg of the polymer matrix to improve the voiding
efficiency of our invention. The inclusion of a third polymer
component, in addition to a first and second polymer in some
instances, has been found to increase opacity and, in particular,
enhance the dispersion of the first and second polymer components
within the polymer matrix. Thus our novel composition further may
comprise a third polymer dispersed within the polymer matrix having
a a surface tension that is between the surface tension of the
polymer matrix and the second polymer, and a density of 1.1 g/cc or
less. A difference in refractive index between the third polymer
and the polymer matrix by an absolute value of at least 0.04, as
discussed above for the first and second polymer components, is
desirable but not critical to achieve good opacity for the
composition. To enable good dispersion and mixing of the first,
second, and third polymer components with the polymer matrix, it is
advantageous for the third polymer to have a surface tension that
is between that of the polymer matrix and the second polymer. It is
also desirable that the density of the third polymer be about 1.1
g/cc or less to reduce the overall density of the composition.
[0037] In one embodiment, the first polymer comprises one or more
of cellulose acetate or cellulose acetate propionate and the second
polymer comprises one or more of polystyrene, polypropylene, or
ethylene methyl acrylate copolymer. In another example, the first
polymer comprises cellulose acetate, the second polymer comprises
polypropylene, and the third polymer comprises ethylene methyl
acrylate copolymer.
[0038] The composition of our invention may be used to produce
shaped articles which contain voids. Thus, another aspect of our
invention is a void-containing, shaped article comprising an
oriented polymer matrix having dispersed therein a voiding agent
wherein the voiding agent comprises at least one first polymer, and
at least one second polymer, wherein the first polymer comprises
one or more of: microcrystalline cellulose, a cellulose ester, or a
cellulose ether, has a Tg or a Tm greater than the Tg of the
polymer matrix, and a surface tension that differs from the surface
tension of the polymer matrix by an absolute value of 5 dynes/cm or
less; and the second polymer comprises one or more polymers
selected from the group consisting of polyethylene, polystyrene,
polypropylene, and copolymers thereof, has a surface tension that
differs from the surface tension of the polymer matrix by an
absolute value of at least 5 dynes/cm, and a melt viscosity wherein
the ratio of melt viscosity of the second polymer to the melt
viscosity of the polymer matrix is about 0.1 to about 3.5. The
first and second polymers, polymer matrix, Tg's, Tm's, and surface
tensions, also include their various other embodiments as described
hereinabove in accordance with the invention.
[0039] Typically, the shaped article will comprise at least 50
weight percent polymer matrix based the total weight of said
article. The polymer matrix may contain one or more polymers
including, but not limited to, polyesters, polylactic acid,
polyketones, polyamides, polyacetals, fluoropolymers, polysulfones,
polyimides, polycarbonates, olefinic polymers, and copolymers
thereof. In one example, the polymer matrix may comprise one or
more polyesters such as, for example, poly(ethylene terephthalate),
poly(butylene terephthalate), poly(propylene terephthalate),
poly(cyclohexylene terephthalate). In addition to the polyesters
noted above, the shaped articles of the invention may include
polyesters from the condensation one of more aromatic diacids with
one or more diols. Examples of polyesters which may be used as the
polymer matrix include those comprising (i) diacid residues
comprising at least 80 mole percent, based on the total moles of
diacid residues, of one or more residues of: terephthalic acid,
naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, or
isophthalic acid; and (ii) diol residues comprising 10 to 100 mole
percent, based on the total moles of diol residues, of one or more
residues of 1,4-cyclohexanedimethanol, 1,3-cyclohexane-dimethanol,
neopentyl glycol, or diethylene glycol; and 0 to 90 mole percent of
one or more residues of: ethylene glycol, 1,2-propanediol,
1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,
1,8-octanediol, 2,2,4-trimethyl-1,3-pentanediol,
2,2,4,4-tetramethyl-1,3-cyclobutanediol, 1,3-cyclohexanedimethanol,
bisphenol A, or polyalkylene glycol. Typically, these copolyesters
have a glass transition temperature between about 35.degree. C. and
about 150.degree. C.
[0040] The voiding agent will comprise at least one first polymer
and at least one second polymer. Although, the first polymer may
comprise any of the polymers described hereinabove, cellulose
acetate, cellulose acetate propionate, or a mixture thereof are
preferred. Similarly, the second polymer may comprise any of the
polymers described previously but, preferably comprises one or more
of polystyrene, polypropylene, or ethylene methyl acrylate
copolymer. In yet another example of the shaped article of the
invention, the polymer matrix comprises a polyester in which the
diacid residues comprise at least 95 mole percent of the residues
of terephthalic acid, and the diol residues comprise about 10 to
about 40 mole percent of the residues of 1,4-cyclohexanedimethanol,
about 1 to about 25 mole percent of the residues of diethylene
glycol, and about 35 to about 89 mole percent of the residues of
ethylene glycol; the first polymer comprises cellulose acetate; and
the second polymer comprises polypropylene and ethylene methyl
acrylate copolymer.
[0041] Although not critical to our invention, the efficiency of
the voiding agent is increased if the second polymer has a Tg or a
Tm greater than the Tg of the polymer matrix. In addition, the
voiding agent may further comprise a third polymer having a surface
tension that is between the surface tension of the polymer matrix
and the second polymer and a density of 1.1 g/cc or less.
[0042] Typical shaped articles of our invention include fibers,
sheets, films, tubes, bottles, and profiles. The shaped article may
be produced by any means well known to persons skilled in the art,
for example, by extrusion, calendering, thermoforming,
blow-molding, casting, spinning, drafting, tentering, or blowing.
For example, the shaped article may be shrink film. An example of a
shrink film of the invention is one that has one or more layers and
a shrinkage of at least 5 percent after 10 seconds in a water bath
at 70.degree. C.
[0043] Our invention also provides a process for a void-containing,
shaped article, comprising: (i) mixing a polymer matrix and a
voiding agent at a temperature at or above the Tg of the polymer
matrix to form a uniform dispersion of the voiding agent within the
polymer matrix, wherein the voiding agent comprises at least one
first polymer and at least one second polymer, wherein the first
polymer has a Tg or a Tm greater than the Tg of the polymer matrix,
a tensile modulus of at least 1 GPa, and a surface tension that
differs from the surface tension of the polymer matrix by an
absolute value of 5 dynes/cm or less; and the second polymer has a
surface tension that differs from the surface tension of the
polymer matrix by an absolute value of at least 5 dynes/cm, and a
melt viscosity wherein the ratio of melt viscosity of the second
polymer to the melt viscosity of the polymer matrix is about 0.1 to
about 3.5; (ii) forming a shaped article; (iii) orienting the
article; and, (iv) optionally, heatsetting the article of step
(iii).
[0044] The mixture may be formed by forming a melt of the polymer
matrix and mixing therein the voiding agent. The voiding agent may
be in a solid, semi-solid, or molten form. It is advantageous that
the voiding agent is a solid or semi-solid to allow for rapid and
uniform dispersion within the polymer matrix upon mixing.
[0045] When the voiding agent is uniformly dispersed in the polymer
matrix, a shaped article is formed by processes well known in the
art such as, for example, extrusion, calendering, thermoforming,
blow-molding, casting, spinning, drafting, tentering, or blowing.
For example, if the shaped article is a cast or extruded film, the
article may be oriented by stretching in one or more directions
using a tenter frame, drafter, or double bubble blown film line.
Methods of unilaterally or bilaterally orienting sheet or film are
well known in the art. Typically, such methods involve stretching
the sheet or film at least in the machine or longitudinal direction
after its formation in an amount of about 1.5 to about 10, usually
about 3 to about 6, times its original dimension. Such sheet or
film may also be stretched in the transverse or cross-machine
direction by apparatus and methods well known in the art in amounts
of generally about 1.5 to about 10, usually about 3 to about 6,
times the original dimension. The oriented films of the invention
also may be heatset to control shrinkage or to provide a
dimensionally stable film. For example, the polymer matrix may
comprise a crystallizable polymer, such as poly(ethylene
terephthalate), which after voiding and orientation, is heatset at
about 170.degree. C. to about 220.degree. C. to reduce shrinkage
and to impart dimensional stability.
[0046] If the shaped article is in the form of a bottle,
orientation is generally biaxial as the bottle is stretched in all
directions as it is blow-molded. Such formation of bottles is also
well known in the art and are described, for example, in U.S. Pat.
No. 3,849,530.
[0047] The voids are formed around the voiding agent as the polymer
matrix is stretched at or near the glass transition temperature,
Tg, of the polymer. Because the particles of the void-forming
compositon are relatively hard compared to the polymer matrix, the
polymer matrix separates from and slides over the voiding agent as
it is stretched, causing voids to be formed in the direction or
directions of stretch in which the voids elongate as the matrix
polymer continues to be stretched. Thus, the final size and shape
of the voids depends on the direction(s) and amount of stretching.
For example, if stretching is only in one direction, voids will
form at the sides of the voiding agent in the direction of
stretching.
[0048] Typically, the stretching operation simultaneously forms the
voids and orients the matrix polymer. The properties of the final
product depend on and can be controlled by manipulating the
stretching time and temperature and the type and degree of stretch.
The stretching typically is done just above the glass transition
temperature (e.g., Tg+5.degree. C. to Tg+60.degree. C.) of the
polymer matrix.
[0049] The voiding agent of the present invention is may be used
for the preparation voided, shrink films in which the matrix
polymer comprises a polyester. Our invention is thus further
described and illustrated herein with particular reference to
void-containing polyester, shrink films. It is understood that the
embodiments decribed for the polyester shrink films also apply to
the shaped articles described hereinabove.
[0050] The present invention also provides a void-containing shrink
film comprising an oriented, continuous polyester phase having
dispersed therein a voiding agent comprising at least one first
polymer and at least one second polymer, wherein the first polymer
has a glass transition temperature (Tg) or a melting point
temperature (Tm) greater than the Tg of the polyester, a tensile
modulus of at least 1 GPa, and a surface tension that differs from
the surface tension of the polyester by an absolute value of 5
dynes/cm or less; and the second polymer has a surface tension that
differs from the surface tension of the polyester by an absolute
value of at least 5 dynes/cm, and a melt viscosity wherein the
ratio of melt viscosity of said second polymer to the melt
viscosity of said polyester is about 0.1 to about 3.5. The first
and second polymers, Tg's, Tm's, tensile modulus, and surface
tensions, also include their respective embodiments in accordance
with the invention as described hereinabove.
[0051] The void-containing shrink films comprise a polyester as the
matrix polymer. The term "polyester", as used herein, is intended
to include "copolyesters" and is understood to mean a synthetic
polymer prepared by the polycondensation of one or more
difunctional carboxylic acids with one or more difunctional
hydroxyl compounds. Typically the difunctional carboxylic acid is a
dicarboxylic acid and the difunctional hydroxyl compound is a
dihydric alcohol such as, for example, glycols and diols.
Alternatively, the difunctional carboxylic acid may be a hydroxy
carboxylic acid such as, for example, p-hydroxybenzoic acid, and
the difunctional hydroxyl compound may be an aromatic nucleus
bearing 2 hydroxyl substituents such as, for example, hydroquinone.
The term "residue", as used herein, means any organic structure
incorporated into a polymer or plasticizer through a
polycondensation reaction involving the corresponding monomer. The
term "repeating unit", as used herein, means an organic structure
having a dicarboxylic acid residue and a diol residue bonded
through a carbonyloxy group. Thus, the dicarboxylic acid residues
may be derived from a dicarboxylic acid monomer or its associated
acid halides, esters, salts, anhydrides, or mixtures thereof. As
used herein, therefore, the term dicarboxylic acid is intended to
include dicarboxylic acids and any derivative of a dicarboxylic
acid, including its associated acid halides, esters, half-esters,
salts, half-salts, anhydrides, mixed anhydrides, or mixtures
thereof, useful in a polycondensation process with a diol to make a
high molecular weight polyester.
[0052] The polyesters used in the present invention typically are
prepared from dicarboxylic acids and diols which react in
substantially equal proportions and are incorporated into the
polyester polymer as their corresponding residues. The polyesters
of the present invention, therefore, contain substantially equal
molar proportions of acid residues (100 mole %) and diol residues
(100 mole %) such that the total moles of repeating units is equal
to 100 mole %. The mole percentages provided in the present
disclosure, therefore, may be based on the total moles of acid
residues, the total moles of diol residues, or the total moles of
repeating units. For example, a polyester containing 30 mole %
isophthalic acid, based on the total acid residues, means the
polyester contains 30 mole % isophthalic acid residues out of a
total of 100 mole % acid residues. Thus, there are 30 moles of
isophthalic acid residues among every 100 moles of acid residues.
In another example, a polyester containing 30 mole % ethylene
glycol, based on the total diol residues, means the polyester
contains 30 mole % ethylene glycol residues out of a total of 100
mole % diol residues. Thus, there are 30 moles of ethylene glycol
residues among every 100 moles of diol residues.
[0053] The preferred polyesters for shrink film are amorphous or
semicrystalline polymers, or blends, with relatively low
crystallinity. Preferably, the polyesters have a substantially
amorphous morphology, meaning that the polyesters comprise
substantially unordered regions of polymer.
[0054] The polyesters that may be used in the films of the present
invention comprise (i) diacid residues comprising at least 80 mole
percent, based on the total moles of diacid residues, of one or
more residues of: terephthalic acid, naphthalenedicarboxylic acid,
1,4-cyclohexanedicarboxylic acid, or isophthalic acid; and (ii)
diol residues comprising 10 to 100 mole percent, based on the total
moles of diol residues, of one or more residues of
1,4-cyclohexanedimethanol, neopentyl glycol, or diethylene glycol;
and 0 to 90 mole percent of one or more residues of: ethylene
glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol,
1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol,
2,2,4-trimethyl-1,3-pentanediol,
2,2,4,4-tetramethyl-1,3-cyclobutanediol, 1,3-cyclohexanedimethanol,
bisphenol A, or polyalkylene glycol. The 1,4-cyclohexanedimethanol
("CHDM") and 1,4-cyclohexanedicarboxylic acid ("CHDA") may be used
as the pure cis, trans or mixtures of cis/trans isomers. Any of the
naphthalenedicarboxylic acid isomers may be used but the 1,4-,
1,5-, 2,6-, and 2,7-isomers or mixtures of these isomers are
preferred. Examples of polyalkylene glycols include
polytetramethylene glycol (PTMG) and polyethylene glycol (PEG)
having molecular weights up to about 2,000. In another example, the
diol residues may comprise about 10 to about 99 mole percent of
residues of 1,4-cyclohexane-dimethanol, 0 to about 90 mole percent
of residues of ethylene glycol, and about 1 to about 25 mole
percent of residues of diethylene glycol.
[0055] In yet another example, the diacid residues may comprise at
least 95 mole percent of the residues of terephthalic acid and the
diol residues may comprise about 10 to about 40 mole percent of the
residues of 1,4-cyclohexanedimethanol, about 1 to about 25 mole
percent of the residues of diethylene glycol, and about 35 to about
89 mole percent of the residues of ethylene glycol.
[0056] The diacid residues further comprise 0 to about 20 mole
percent of one or more residues of a modifying diacid containing 4
to 40 carbon atoms if desired. For example, from 0 to about 30 mole
% of other aromatic dicarboxylic acids containing 8 to about 16
carbon atoms, cycloaliphatic dicarboxylic acids containing 8 to
about 16 carbon atoms, aliphatic dicarboxylic acids containing
about 2 to about 16 carbon atoms or mixtures thereof may be used.
Examples of modifying carboxylic acids include, but are not limited
to, one or more of succinic acid, glutaric acid,
1,3-cyclohexanedicarboxylic, adipic acid, suberic acid, sebacic
acid, azelaic acid, dimer acid, or sulfoisophthalic acid. It is
understood by persons skilled in the art that the final composition
can be arrived at by blending various resins or by direct reactor
copolymerization. The latter is desirable to minimize compositional
variability but economic necessities often make blending more cost
effective.
[0057] Other examples of polyesters that may comprise the
continuous polyester phase are those based on poly(ethylene
terephthalate) containing about 15 to about 55 mole percent of 1,3-
or 1,4-cyclohexanedimethanol, and from 1 to about 25 mole % of
diethylene glycol and poly(ethylene terephthalate) containing about
15 to about 35 mole % of 1,3- or 1,4-cyclohexanedimethanol, and
about 5 to about 15 mole % of diethylene glycol. Additionally, from
0 to about 5% of other dicarboxylic acids or other modifying
glycols as set forth above may be used if desired.
[0058] The polyesters generally will have inherent viscosity (I.V.)
values in the range of about 0.5 dL/g to about 1.4 dL/g. Additional
examples of I.V. ranges include about 0.65 dL/g to about 1.0 dL/g
and about 0.65 dL/g to about 0.85 dL/g.
[0059] The polyesters of the instant invention are readily prepared
from the appropriate dicarboxylic acids, esters, anhydrides, or
salts, and the appropriate diol or diol mixtures using typical
polycondensation reaction conditions. They may be made by
continuous, semi-continuous, and batch modes of operation and may
utilize a variety of reactor types. Examples of suitable reactor
types include, but are not limited to, stirred tank, continuous
stirred tank, slurry, tubular, wiped-film, falling film, or
extrusion reactors. The term "continuous" as used herein means a
process wherein reactants are introduced and products withdrawn
simultaneously in an uninterrupted manner. By "continuous" it is
meant that the process is substantially or completely continuous in
operation in contrast to a "batch" process. "Continuous" is not
meant in any way to prohibit normal interruptions in the continuity
of the process due to, for example, start-up, reactor maintenance,
or scheduled shut down periods. The term "batch" process as used
herein means a process wherein all the reactants are added to the
reactor and then processed according to a predetermined course of
reaction during which no material is fed or removed into the
reactor. The term "semicontinuous" means a process where some of
the reactants are charged at the beginning of the process and the
remaining reactants are fed continuously as the reaction
progresses. Alternatively, a semicontinuous process may also
include a process similar to a batch process in which all the
reactants are added at the beginning of the process except that one
or more of the products are removed continuously as the reaction
progresses. The process is operated advantageously as a continuous
process for economic reasons and to produce superior coloration of
the polymer as the polyester may deteriorate in appearance if
allowed to reside in a reactor at an elevated temperature for too
long a duration.
[0060] The polyesters of the present invention are prepared by
procedures known to persons skilled in the art. The reaction of the
diol and dicarboxylic acid may be carried out using conventional
polyester polymerization conditions or by melt phase processes, but
those with sufficient crystallinity may be made by melt phase
followed by solid phase polycondensation techniques. For example,
when preparing the polyester by means of an ester interchange
reaction, i.e., from the ester form of the dicarboxylic acid
components, the reaction process may comprise two steps. In the
first step, the diol component and the dicarboxylic acid component,
such as, for example, dimethyl terephthalate, are reacted at
elevated temperatures, typically, about 150.degree. C. to about
250.degree. C. for about 0.5 to about 8 hours at pressures ranging
from about 0.0 kPa gauge to about 414 kPa gauge (60 pounds per
square inch, "psig"). Preferably, the temperature for the ester
interchange reaction ranges from about 180.degree. C. to about
230.degree. C. for about 1 to about 4 hours while the preferred
pressure ranges from about 103 kPa gauge (15 psig) to about 276 kPa
gauge (40 psig). Thereafter, the reaction product is heated under
higher temperatures and under reduced pressure to form the
polyester with the elimination of diol, which is readily
volatilized under these conditions and removed from the system.
This second step, or poly-condensation step, is continued under
higher vacuum and a temperature which generally ranges from about
230.degree. C. to about 350.degree. C., preferably about
250.degree. C. to about 310.degree. C. and, most preferably, about
260.degree. C. to about 290.degree. C. for about 0.1 to about 6
hours, or preferably, for about 0.2 to about 2 hours, until a
polymer having the desired degree of polymerization, as determined
by inherent viscosity, is obtained. The polycondensation step may
be conducted under reduced pressure which ranges from about 53 kPa
(400 torr) to about 0.013 kPa (0.1 torr). Stirring or appropriate
conditions are used in both stages to ensure adequate heat transfer
and surface renewal of the reaction mixture. The reaction rates of
both stages are increased by appropriate catalysts such as, for
example, alkoxy titanium compounds, alkali metal hydroxides and
alcoholates, salts of organic carboxylic acids, alkyl tin
compounds, metal oxides, and the like. A three-stage manufacturing
procedure, similar to that described in U.S. Pat. No. 5,290,631,
may also be used, particularly when a mixed monomer feed of acids
and esters is employed.
[0061] To ensure that the reaction of the diol component and
dicarboxylic acid component by an ester interchange reaction is
driven to completion, it is sometimes desirable to employ about
1.05 to about 2.5 moles of diol component to one mole dicarboxylic
acid component. Persons of skill in the art will understand,
however, that the ratio of diol component to dicarboxylic acid
component is generally determined by the design of the reactor in
which the reaction process occurs.
[0062] In the preparation of polyester by direct esterification,
i.e., from the acid form of the dicarboxylic acid component,
polyesters are produced by reacting the dicarboxylic acid or a
mixture of dicarboxylic acids with the diol component or a mixture
of diol components. The reaction is conducted at a pressure of from
about 7 kPa gauge (1 psig) to about 1379 kPa gauge (200 psig),
preferably less than 689 kPa (100 psig) to produce a low molecular
weight polyester product having an average degree of polymerization
of from about 1.4 to about 10. The temperatures employed during the
direct esterification reaction typically range from about
180.degree. C. to about 280.degree. C., more preferably ranging
from about 220.degree. C. to about 270.degree. C. This low
molecular weight polymer may then be polymerized by a
polycondensation reaction.
[0063] In addition, the polyester may further comprise one or more
of the following: antioxidants, melt strength enhancers, branching
agents (e.g., glycerol, trimellitic acid and anhydride), chain
extenders, flame retardants, fillers, acid scavengers, dyes,
colorants, pigments, antiblocking agents, flow enhancers, impact
modifiers, antistatic agents, processing aids, mold release
additives, plasticizers, slips, stabilizers, waxes, UV absorbers,
optical brighteners, lubricants, pinning additives, foaming agents,
antistats, nucleators, glass beads, metal spheres, ceramic beads,
carbon black, crosslinked polystyrene beads, and the like.
Colorants, sometimes referred to as toners, may be added to impart
a desired neutral hue and/or brightness to the polyester and the
calendered product. Preferably, the polyester compositions may
comprise 0 to about 30 weight percent of one or more processing
aids to alter the surface properties of the composition and/or to
enhance flow. Representative examples of processing aids include
calcium carbonate, talc, clay, mica, zeolites, wollastonite,
kaolin, diatomaceous earth, TiO.sub.2, NH.sub.4Cl, silica, calcium
oxide, sodium sulfate, and calcium phosphate. Use of titanium
dioxide and other pigments or dyes, might be included, for example,
to control whiteness of the film, or to make a colored film. An
antistat or other coating may also be applied to one or both sides
of the film. Corona and/or flame treatment is also an option
although not typically necessary due to the high surface tension of
the void-containing films. For certain combinations of polymers it
may also be necessary to add acid scavengers and stabilizers to
prevent degradation/browning of the cellulose esters.
[0064] The shrink film comprises a voiding agent that comprises a
first and second polymer which may be selected from a wide range of
polymers. The first polymer may be a single polymer or blend of one
or more polymers. For example, the first polymer may comprise one
or more polymers selected from cellulosic polymers, starch,
esterified starch, polyketones, polyester, polyamides,
polysulfones, polyimides, polycarbonates, fluoropolymers,
polyacetals, olefinic polymers, and copolymers of these polymers
with other monomers such as, for example, copolymers of ethylene
with acrylic acid and its esters.
[0065] Cellulosic polymers are efficient voiding agents because of
their high glass transition temperature and good surface tension
match with many typical polymer matrices. Thus, in one example, the
first polymer of our novel voiding agent may a cellulosic polymer
and may comprise one or more of microcrystalline cellulose, a
cellulose ester, or a cellulose ether. In another embodiment, the
first polymer may be a cellulose ester such as, for example,
cellulose acetate, cellulose triacetate, cellulose acetate
propionate, or cellulose acetate butyrate. In yet another example,
the first polymer may be a cellulose ether which may include, but
is not limited to one or more of: hydroxypropyl cellulose, methyl
ethyl cellulose, or carboxymethyl cellulose.
[0066] The second polymer may comprise one or more polymers
selected from polyamides, polyketones, polysulfones, polyesters,
fluoropolymers, polyacetals, polycarbonates, olefinic polymers, and
copolymers thereof. For example, the second polymer may include,
but is not limited to, one or more olefinic polymers such as, for
example, polyethylene, polystyrene, polypropylene, and copolymers
thereof. Further non-limiting examples olefinic copolymers include
ethylene vinyl acetate, ethylene vinyl alcohol copolymer, ethylene
methyl acrylate copolymer, ethylene butyl acrylate copolymer,
ethylene acrylic acid copolymer, or ionomer. In one example, the
first polymer comprises one or more of cellulose acetate or
cellulose acetate propionate and the second polymer comprises one
or more of: polystyrene, polypropylene, or ethylene methyl acrylate
copolymer.
[0067] Typically, the voiding agent comprises about 5 to about 95
weight percent of the first polymer, based on the total weight of
the voiding agent. Other ranges for the first polymer within the
voiding agent are about 40 to about 60 weight percent and about 50
to about 60 weight percent. When the first polymer is a cellulosic,
one benefit of the presence of the second polymer, among other
things, is to improve the processibilty and dispersion of the first
polymer. For example, cellulosic polymer powders can be difficult
to compound directly using a single screw extruder because of
difficulties in handling or feeding. In addition, the high moisture
uptake of the cellulosic can lead to unacceptable hydrolysis and
moisture volatilization in a polyester matrix. Thus, unless already
pre-pelletized, the cellulosic typically requires the use of a
twin-screw, calender, or similar system that can adequately mix and
vent these powders during its dispersion in the polymer matrix.
[0068] As described above, the second polymer may comprise one or
more olefinic polymers and copolymers well-known in the art, such
as, for example, ethylenes, propylenes, styrenics, vinyls,
acrylics, poly(acrylonitrile), and the like. The selection of the
olefinic polymer will depend on the desired properties of the
shrink film. For example, the addition of styrenics such as, for
example, polystyrene, methyl methacrylate butadiene styrene,
styrene acrylonitrile, and the like, can help to modify and improve
solvent seaming (as they are more easily solubilized by many common
solvents), and improve opacity, although they also make the film
more brittle and can tend to cause off-gassing. Their refractive
index is closer to polyesters and their opacifying capacity is
lower than cellulosics. By contrast, voiding agents such as, for
example, polypropylene are less effective than styrenics, but are
softer and more easy to blend with the cellulosics. They also
enhance opacity significantly when used in conjunction with the
cellulosic because of a greater difference in refractive index
although they can affect printing and seaming if used at too high a
level.
[0069] It is advantageous but not critical that the second polymer
has a refractive index that differs from the refractive index the
polyester by an absolute value of at least 0.02. For example,
polyesters typically have a refractive index of around 1.57, thus
polyethylene and polypropylene, with a refractive index of around
1.50, and polystyrene, with a refractive index of around 1.59 may
be selected as the second polymer of the voiding agent because the
difference in their refractive indices from that of the polyester
is greater than 0.02. This difference in refractive indices
enhances the opacity of the shrink film. It is also possible to
increase opacity by adding inorganic fillers or pigments such as,
for example, TiO.sub.2, carbon black, and the like; however, the
addition of inorganic materials often will increase the density of
the final film and are less desirable. For the present invention,
the opacity of the film is high enough that these inorganic fillers
are not required. For example, the films, sheet, and shaped
articles of the present invention typically have an absorptivity of
about 200 cm.sup.-1 or higher. Thus, a 2 mil (50 um) film will have
a total light transmission of about 35% or less at this
absorptivity. Most the transmitted light is diffuse or scattered
light. The presence of voids and any additives that may be used in
the film also serve to block the transmission of UV light for
applications with UV sensitive products. For films made by a blown
film process, the combination of cellulosic with olefin also is
beneficial in that both typically improve the melt strength of the
polyester continuous phase. Higher melt strength improves the
control of bubble shapes and stretch conditions during double
bubbling.
[0070] A certain degree of stiffness is desirable for certain film
applications such as, for example, sleeve labels. Some of the
olefins such as, for example, EMAC, provide a more flexible film
and have a "soft touch" that may be useful where sleeving is not as
critical (e.g. roll-fed labels). This soft touch also may improve
the compatibility of the film with other olefins to make the label
more conducive to gluing with ethylene(vinyl acetate)-based hot
melt adhesives or for heat sealing. It may be useful, therefore, to
combine softer and harder olefins to balance the properties of the
shrink film such as, for example, softness and stiffness.
[0071] We have found that olefinic copolymers such as, for example,
ethylene methyl acrylate copolymer (abbreviated herein as "EMAC"),
ethylene butyl acrylate (abbreviated herein as "EBAC"), ethylene
acrylic acid (abbreviated herein as "EAA") copolymer, maleated,
oxidized or carbyoxylated PE, and ionomers may be used
advantageously with the cellulosic polymers described above as the
second polymer to increase the opacity and improve the overall
aesthetics and feel of the film. These olefins also may aid the
compounding and dispersion of the cellulosic. Thus, for example,
the second polymer may comprise one or more of EMAC or EBAC.
[0072] In another embodiment, the first polymer may comprise one or
more of cellulose acetate or cellulose acetate propionate and the
second polymer may comprise one or more of polystyrene,
polypropylene, or ethylene methyl methacrylate copolymer. Although
not critical, a second polymer having Tg or a Tm greater than the
Tg of the polyester continuous phase may help to improve the
voiding efficiency of the voiding agent. The inclusion of a third
polymer component in addition to a first and second polymer, in
some instances, has been found to increase opacity and, in
particular, enhance the dispersion of the first and second polymer
components within the continuous polyester phase. Thus, our novel
composition further may include a third polymer dispersed within
the continuous polyester phase having a surface tension that is
between the surface tension of the continuous polyester phase and
the second polymer, and a density of 1.1 g/cc or less. A difference
in refractive index between the third polymer and the continuous
polyester phase by an absolute value of at least 0.04, as discussed
above for the first and second polymer components, is desirable to
achieve good opacity for the composition. To enable good dispersion
and mixing of the first, second, and third polymer components with
the continuous polyester phase, it is advantageous for the third
polymer to have a surface tension that is between the surface
tension of the polymer matrix and that of the second polymer. It is
also desirable that the density of the third polymer is about 1.1
g/cc or less to reduce the overall density of the composition.
[0073] When the voiding agent comprises a cellulosic polymer and an
olefinic polymer, the voiding agent typically will comprise at
least 5 weight percent or more of the cellulosic polymer, based on
the total weight of the composition. Preferably, the voiding agent
will comprise at least 30 weight percent of the cellulosic polymer.
The components of the voiding agent can compounded together on a
mixing device such as, for example, a twin screw extruder,
planetary mixer, or Banbury mixer, or the components can be added
separately during film formation. Small amounts of inorganic
voiding agents can also be included. It may be desirable to
precompound the cellulosic polymer and the olefin, in which the
olefin may be used as part of the carrier resin in which the
cellulosic is dispersed. Precompounding the olefin and the
cellulosic polymer provides the added advantage that the olefin
serves as a vehicle for dispersing the cellulosic polymer, and
provides an efficient moisture barrier to prevent uptake of
moisture prior into the cellulosic polymer to final extrusion. In
addition, the voiding agent is easier to handle and dry. It is also
possible to use blends of polymers as voiding agents as long as
sufficient shearing, for example, by the use of a twin screw or
high shear single screw extruder, is used to adequately disperse
the components of the voiding agent.
[0074] The void-containing, shrink film of the present invention
may comprise a single layer or may be incorporated as one or more
layers of a multilayered structure such as, for example, a laminate
used in a roll-fed label. Typically, the density of film is about
1.2 grams/cubic centimeter (abbreviated herein as "g/cc") or less.
Examples of densities of the void-containing film of the present
invention are about 1.20 g/cc or less, about 1.1 g/cc or less,
about 1.0 g/cc or less, about 0.9 g/cc or less, about 0.8 g/cc or
less, about 0.7 g/cc or less, and about 0.6 g/cc or less. For some
applications, for example, recycleable shrink labels, the density
of the film preferably is about 1.0 g/cc or less, more preferably
about 0.90 g/cc or less, in order to achieve true flotation during
recycle and flake washing in a water bath. The final density of the
film is a function of the level and density of the filler, the
degree of voiding, the stretch ratio, and the stretch temperature,
and may be tailored as appropriate, for example, to improve the
separation of the film from the various other polymer components
present within the packaging materials.
[0075] The void-containing shrink film of this invention is
typically prepared by methods well-known to persons skilled in the
art such as, for example, extrusion, calendering, casting,
drafting, tentering, or blowing. These methods initially create an
unoriented or "cast" film that is subsequently stretched in at
least one direction to impart orientation and to create the voids.
In generally, stretch ratios of about 3.times. to about 8.times.
are imparted in one or more directions to create uniaxially or
biaxially oriented films. More typically, stretch ratios are from
4.times. to about 6.times.. The stretching can be performed, for
example, using a double-bubble blown film tower, a tenter frame, or
a machine direction drafter. Stretching is preferably performed at
or near the glass transition temperature (Tg) of the polymer. For
polyesters, for example, this range is typically Tg+5.degree. C.
(Tg+10.degree. F.) to about Tg+33.degree. C. (Tg+60.degree. F.),
although the range may vary slightly depending on additives. A
lower stretch temperature will impart more orientation and voiding
with less relaxation (and hence more shrinkage), but may increase
film tearing. To balance these effects, an optimum temperature in
the mid-range is often chosen. Typically, a stretch ratio of 4.5 to
5.5.times. may be used to optimize the shrinkage performance and
improve gauge uniformity.
[0076] For the present invention, the film typically has a
shrinkage along the principal axis of at least 5% at about
70.degree. C. (167.degree. F.) and at least 30% at about 95.degree.
C. (200.degree. F.). Shrinkage is measured by immersing a
premeasured piece of film into a water bath for 10 seconds. The
term "shrinkage", as used herein, is defined as the change in
length divided by the original length (times 100%). In another
example, the shrinkage is at least 10% at 70.degree. C.
(167.degree. F.) and at least 40% at 95.degree. C. In yet another
example, the shrinkage is 15% or more at 70.degree. C. and 50% or
more at 95.degree. C. Greater amounts of shrinkage may be desirable
for commercial applications such as, for example, shrink labels, as
the fitting of the label over more highly contoured bottles may be
improved.
[0077] It is understood that the present invention also encompasses
various modifications to control and improve shrink properties as
well known to those skilled in the art. For example, to improve
shrinkage at lower temperatures, a polyester or polyester monomer,
or alternate polymer with a low softening point (e.g., DEG or
butanediol) may be incorporated to lower the overall Tg of the
polymer film. Soft segments based on polytetramethylene glycol,
PEG, and similar monomers, may be added to flatten the shrink
curve, lower the shrink onset, control the rate of shrinkage or
improve tear properties. The shrink properties are dependent on the
stretching conditions which may be modifed as appropriate to
provide variations in properties such as, for example, controlled
shrink force, shrink force ratios in each direction, controlled
shrinkage, and property retention after shrinkage. The various
factors that control the shrinkage properties of polyester films
are discussed extensively in several journal articles such as, for
example, in Shih, Polym. Eng. Sci., 34, 1121 (1994).
[0078] The void-containing shrink film of our invention may
comprise polyesters of various compositions. For example, one
embodiment of the present invention is a void-containing, shrink
film comprising an oriented, continuous polyester phase comprising
(i) diacid residues comprising at least 80 mole percent, based on
the total moles of diacid residues, of one or more residues of
terephthalic acid or isophthalic acid; and (ii) diol residues
comprising 15 to 55 mole percent, based on the total moles of diol
residues, of one or more residues of 1,4-cyclohexanedimethanol or
diethylene glycol; and 45 to 85 mole percent of ethylene glycol,
having dispersed therein a voiding agent comprising at least one
first polymer, and at least one second polymer, wherein the first
polymer comprises one or more of microcrystalline cellulose, a
cellulose ester, or a cellulose ether, has a Tg or a Tm greater
than the Tg of the polyester and a surface tension that differs
from the surface tension of the polyester by an absolute value of 5
dynes/cm or less; and the second polymer comprises one or more
polymers selected from the group consisting of polyethylene,
polystyrene, polypropylene, and copolymers thereof, has a surface
tension that differs from the surface tension of the polyester by
an absolute value of at least 5 dynes/cm, and a melt viscosity
wherein the ratio of melt viscosity of the second polymer to the
melt viscosity of the polyester is about 0.1 to about 3.5. The
first polymer may comprise one or more of cellulose acetate or
cellulose acetate propionate and the second polymer may comprise
one or more of: polystyrene, polypropylene, or ethylene methyl
acrylate copolymer. In a further example, the diacid residues may
comprise at least 95 mole percent of the residues of terephthalic
acid; the diol residues comprise about 10 to about 40 mole percent
of the residues of 1,4-cyclohexanedimethanol, about 1 to about 25
mole percent of the residues of diethylene glycol, and about 35 to
about 89 mole percent of the residues of ethylene glycol; the first
polymer comprises cellulose acetate; and the second polymer
comprises polypropylene and ethylene methyl acrylate copolymer.
[0079] As described previously, the film of the present invention
may be produced by extrusion, calendering, casting, drafting,
tentering, or blowing. The film may be stretched in at least one
direction and has a shrinkage along the principal axis at least 10%
after 10 seconds in a water bath at 70.degree. C. and at least 40%
after 10 seconds in a water bath at 95.degree. C. If the film is
stretched in one direction, it may have a low shrinkage in the
direction perpendicular to the principal axis or main shrinkage
direction. For example, the film may be stretched in one direction
and have a shrinkage in a direction perpendicular to the principal
axis of 10% or less after 10 seconds in a water bath over a
temperature range of 70.degree. C. to 95.degree. C. After
orientation and void formation, the film will typically have a
lower density and a higher optical absorptivity than the
non-oriented film. Examples of densities that may be exhibited by
the void-containing film of the invention are about 1.20 g/cc or
less, about 1.1 g/cc or less, about 1.0 g/cc or less, about 0.9
g/cc or less, about 0.8 g/cc or less, about 0.7 g/cc or less, and
about 0.6 g/cc or less. In addition, the void-containing shrink
film will have typically have an absorptivity of about 200
cm.sup.-1 or greater.
[0080] Sleeves and labels may be prepared from the void-containing
shrink film of the present invention according to methods well
known in the art. These sleeves and labels are useful for packaging
applications such as, for example, labels for plastic bottles
comprising poly(ethylene terephthalate). Our invention, therefore,
provides a sleeve or roll-fed label comprising the void-containing
shrink films described hereinabove. These sleeves and labels may be
conveniently seamed by methods well-known in the art such as, for
example, by solvent bonding, hot-melt glue, UV-curable adhesive,
radio frequency sealing, heat sealing, or ultrasonic bonding. For
traditional shrink sleeves involving transverse oriented film (via
tentering or double bubble), the label is first printed and then
seamed along one edge to make a tube. Solvent seaming can be
performed using any of a number of solvents or solvent combinations
known in the art such as, for example, THF, dioxylane, acetone,
cyclohexanone, methylene chloride, n-methylpyrrilidone, and MEK.
These solvents have solubility parameters close to that of the film
and serve to dissolve the film sufficiently for welding. Other
methods such as RF sealing, adhesive gluing, UV curable adhesives,
and ultrasonic bonding can also be applied. The resulting seamed
tube is then cut and applied over the bottle prior to shrinking in
a steam, infrared or hot air type tunnel. During the application of
the sleeve with certain types of sleeving equipment, it is
important that the film have enough stiffness to pass over the
bottle without crushing or collapsing as the sleeve tends to stick
to or "grab" against the side of the bottle because of friction.
The void-containing sleeves of the present invention have a
coefficient of friction (COF) that typically is about 20 to 30%
lower than that of the unvoided film. This lower COF helps to
prevent label hanging and make sleeve application easier and is an
unexpected benefit of the present invention.
[0081] For roll-fed labels, the void-containing film is
traditionally oriented in the machine direction using, for example,
a drafter. These labels are wrapped around the bottle and typically
glued in place online. As production line speeds increase, however,
faster seaming methods are needed, and UV curable, RF sealable, and
hot melt adhesives are becoming more preferred over solvent
seaming. For example, a hot melt polyester might be useful to seam
a polyester-based void-containing film.
[0082] Our invention also provides a process for a void-containing
shrink film, comprising: (i) mixing a polyester and a voiding agent
at a temperature at or above the Tg of the polyester to form a
uniform dispersion of the voiding agent within the polyester,
wherein the voiding agent comprises at least one first polymer and
at least one second polymer, wherein the first polymer has a Tg or
a Tm greater than the Tg of the polyester, a tensile modulus of at
least 1 GPa, and a surface tension that differs from the surface
tension of the polymer matrix by an absolute value of 5 dynes/cm or
less; the second polymer has a surface tension that differs from
the surface tension of the polyester by an absolute value of at
least 5 dynes/cm and a melt viscosity wherein the ratio of melt
viscosity of the second polymer to the melt viscosity of the
polyester is about 0.1 to about 3.5; (ii) forming a sheet or film;
(iii) orienting the sheet or film in one or more directions; and
(iv) optionally, heatsetting the film of step (iii). Further, our
invention provides a film or sheet prepared by the above process.
The various embodiments of the voiding agent and the polyester are
as described previously.
[0083] The formation of the sheet or film may be carried by any
method known to persons having ordinary skill in the art such as,
for example, by extrusion, calendering, casting, or blowing. The
voiding agent and the polyester may be dry blended or melt mixed at
a temperature at or above the Tg of the polyester in a single or
twin screw extruder, roll mill or in a Banbury Mixer to form a
uniform dispersion of the voiding agent in the polyester. In a
typical procedure for preparing film such as, for example, using a
voiding agent comprising a cellulosic polymer and an olefin, and a
polyester as the polymer matrix, the melt is extruded through a
slotted die using melt temperatures in the range of about
200.degree. C. (400.degree. F.) to about 280.degree. C.
(540.degree. F.) and cast onto a chill roll maintained at about
-1.degree. C. (30.degree. F.) to about 82.degree. C. (180.degree.
F.). The film or sheet thus formed will generally have a thickness
of about 5 to about 50 mils, although a more typical range is 5 to
15 mils. The film or sheet is then uniaxally or biaxially stretched
in amounts ranging from about 200 to about 700% to provide an
oriented film having a thickness of about 1 to about 10 mils, more
typically about 1 to about 3 mils. Higher final thicknesses might
be desirable, for example, to take advantage of the insulative
properties or cushioning properties of the void-containing film.
The voids created during the stretching operation can act as
insulators much like the pores of a foamed film. Thus, the
thickness of the film can be increased as appropriate to achieve
the desired level of insulation. It is also possible to combine
void-containing layers with foamed layers in a layered or laminated
structure. For example, a foamed center layer can be encapsulated
by two void-containing layers to maximize density reduction and
improve printing performance.
[0084] The stretching processes may be done in line or in
subsequent operations. For the shrink film of the present
invention, the film typically is not heatset significantly to
provide maximum shrinkage. Subsequently, the void-containing film
may be printed and used, for example, as labels on beverage or food
containers. Because of the presence of voids, the density of the
film is reduced and the effective surface tension of the film is
incresased giving it a more paper-like texture. Accordingly, the
film will readily accept most printing inksand, hence, may be
considered a "synthetic paper". Our shrink film may also be used as
part of a multilayer or coextruded film, or as a component of a
laminated article.
[0085] Post-stretch annealing or heatsetting is also advantageous
for maintaining low density and reducing shrink force. Normally
annealing is carried out in the heatset zone of the tenter frame
but at much lower temperatures than with traditional heatsetting.
The film is constrained while being heated to an annealing
temperature close to Tg of the film. Optionally, the tenter clips
can be brought together slightly (e.g. from about 1 to about 10%)
to facilitate the process and to help relax the film slightly,
which better establishes the voids and reduces shrink stresses.
High shrink stresses may cause the film to shrink prematurely and
may close some of the voids thereby offsetting any density
reduction. Annealing times and temperatures will vary from machine
to machine and with each formulation, but typically will range from
about Tg-20.degree. C. to about Tg+20.degree. C. for about 1 to
about 15 seconds. Higher temperatures usually require shorter
annealing times and are preferred for higher line speeds.
Additional stretching after annealing can be performed, although
not required. The annealing process typically will reduce the
maximum shrinkage slightly (e.g. a few percent); however reduction
is sometimes useful to maintain the void cells and to maintain the
dimensions of the film.
[0086] Through variations in the level of voiding agent and stretch
conditions, composition and process parameters, the the
void-containing polyester compositions, shaped articles, and shrink
film of the instant invention may be prepared in a range of
densities that may advantageously tailored for the separation of
the void-containing polyester from other polymers for recovery and
recycling. Another aspect of our invention, therefore, is a process
for separating a void-containing polyester from a mixture of
different polymers, comprising:
[0087] (i) shredding, chopping, or grinding a mixture of polymers
comprising the void-containing polyester and at least one other
polymer to produce particles of the mixture;
[0088] (ii) dispersing the mixture into an aqueous or gaseous
medium;
[0089] (iii) allowing the particles to partition into a higher
density fraction and a lower density fraction; and
[0090] (iv) separating the lower density fraction from the higher
density fraction;
[0091] wherein the void-containing polyester comprises a continuous
polyester phase having dispersed therein a voiding agent comprising
at least one first polymer and at least one second polymer, wherein
the first polymer has a Tg or a Tm greater than the Tg of the
polyester, a tensile modulus of at least 1 GPa, and a surface
tension that differs from the surface tension of the polyester by
an absolute value of 5 dynes/cm or less; and the second polymer has
a surface tension that differs from the surface tension of the
polyester by an absolute value of at least 5 dynes/cm, and a melt
viscosity wherein the ratio of melt viscosity of the second polymer
to the melt viscosity of the polyester is about 0.1 to about 3.5.
Our process is understood to include the various additional
embodiments of the voiding agent and the polyester as described
previously.
[0092] In addition to the void-containing polyester, the mixture of
polymers may include, but is not limited to, one or more polymers
commonly used in commercial packaging applications such as, for
example, poly(ethylene terephthalate), poly(vinyl chloride),
polypropylene, polycarbonate, poly(butylene terephthalate),
polystyrene, or polyethylene. The mixture of polymers containing
the void-containing polyester is shredded, chopped, or ground to
produce particles of the mixture, for example, in the form of
flakes. The particles are then dispersed into a gaseous or aqueous
medium such as, for example, air (e.g., air elutriation) or water.
The mixture is then allowed to partition into a higher density
fraction and a lower density fraction which are then separated.
Because of the diffence in density between the void-containing
polyester and the other polymers in the mixture, the
void-containing polyester will comprise substantially either the
lower density layer or the higher density layer, depending on the
density of the remaining polymers in the mixture. For example, the
polymer mixture may comprise low density polymers such as, for
example, polyethylene or polypropylene wherein the higher density
fraction will comprise substantially the void-containing polyester.
For example, the void-containing shrink film could be used to
produce a label for a polyethylene bottle in which it is desirable
that the label have a density of about 1.0 g/cc or higher after
shrinkage. With this density, the void-containing label would sink
in a aqueous flotation tank and, thus, enable separation from the
lower density, floating polyethylene polymer. In another example,
the polymer mixture may comprise higher density polymers such as,
for example, poly(ethylene terephthalate) or poly(vinyl chloride),
wherein the lower density fraction will comprise substantially the
void-containing polyester. For example, the void-containing shrink
film could be used to produce a label for a PET bottle. The
void-containing label would preferably have a density of less than
1.0 g/cc after shrinkage, thus permitting it to float in a
separation tank. The PET flake, having a density greater than 1.0,
would sink and, therefore, could be easily separated and collected.
Thus, the optimal density range for the microvoided films depends
on the type of package material to which it is applied.
[0093] Our process may be illustrated and described further with
particular reference to the separation of void-containing polyester
shrink labels from poly(ethylene terephthalate) obtained from
bottles or packaging materials. The recycled bottles and labels
typically are first ground and passed through an air elutriation
step. Air is blown up through the flakes and labels, which forces
the lighter density materials up and out of the tank. This step
removes a substantial portion of the label. The remainining polymer
mixture is then washed in a caustic solution at a temperature of
about 85.degree. C. The void-containing shrink labels having a
density less than 1 g/cc (the approximate density of the bath),
float to the top and are skimmed off. The poly(ethylene
terephthalate) bottle flake, which has a density greater than 1
g/cc, drops to the bottom of the tank where it may be separated
and/or recycled. Similarly, the label and the bottle flake may be
partitioned into a high density fraction and a low density fraction
by air elutriation and subsequently separated. For polymer mixtures
containing lower density polymers such as, for example,
high-density polyethylene or polypropylene, the recycle process is
reversed. In these examples, polymer mixture will comprise
substantially the lower density fraction and the void-containing
polyester will comprise substantially the higher density fraction.
For example, if water is used to disperse the mixture of polymer
particles, the void-containing film advantageously should have a
density greater than 1.0 g/cc in the wash bath.
[0094] The density of the void-containing polyester films may vary
as a result of shrinkage and, thus, should be taken into account
for the separation process of the invention. Typically, the density
increases by ca. 0.05 g/cc after shrinkage. Furthermore, if the
film is printed, the inks and any overcoat may add to the effective
density. Thus, the combined effects of these variables may require
that the starting density be preferably be about 0.90 g/cc or lower
in order to ensure good flotation after shrinkage. It is possible
to control this amount of density reduction during shrinkage by
proper annealing. With greater post-stretch annealing, the voids
become more stabilized but with lower maximum shrinkage.
[0095] The following examples will further illustrate the
invention.
EXAMPLES
[0096] General--Test methods followed standard ASTM procedures
wherever possible. Because of the small size of some of the samples
stretched on the T.M. Long film stretcher, however, some minor
modifications to the ASTM procedures were required.
[0097] Density measurements were performed using a gradient column
made from ethanol and water. The density range of the column was
nominally 0.82 to 1.00 g/cc. In the case of tentered film where the
quantity of film was sufficient, density also was measured by
cutting out and weighing 10 sheets, 10.times.10 cm in area,
measuring the thickness of the sheets at multiple points across the
sheets, and averaging the measurements. The average density was
then calculated from the mass divided by the volume.
[0098] General film quality and aesthetics were based on subjective
evaluation and are shown in Table II. An excellent film was one
with uniform dispersion of voids/additives, high opacity, no
high/low spots, and no streaking from poor mixing. Film tactile
qualities (i.e. "hand") were also included with stiff, noisy films
being rated more poorly, and softer "low-noise" films being
preferred. Poor films generally exhibited a combination of high
noise and poor surface/opacity uniformity (e.g. "streakiness").
[0099] Tensile properties were measured by ASTM Method D-638 to
allow for small sample dimensions of the T.M. Long stretched film.
Some testing also was performed using the more common ASTM Method
D882 where longer gauge lengths were available. All modulus data
were obtained at room temperature and refer to the nominal,
unoriented state of the material unless otherwise noted. Some of
the modulus data in Table I was also obtained from the general
product literature.
[0100] Total light transmittance was measured by ASTM Method D1003.
In addition, absorptivity was determined as a way to normalize the
effects of thickness variation on the transmittance and to provide
a truer measure of opacity. The absorptivity (units of 1/cm) is
based on Beer's Law and is calculated as
Absorptivity=-ln(internal transmittance)/thickness
[0101] where in is the natural log (base e), thickness refers to
the film thickness (in cm) and internal transmittance is the
transmittance corrected for surface reflective losses. For a clear,
non-voided polyester film, there is typically 4% reflective loss on
the front surface and 4% on the back surface because of refractive
index mismatch between film and air. For the opaque,
void-containing films there is typically only the 4% reflected loss
on the front surface as insufficient light penetrates the film to
reach the back surface.
[0102] Film shrinkage was measured by immersing a sample of known
initial length into a water bath at a given temperature from
65.degree. C. to 95.degree. C. for either 5, 10, or 30 seconds, and
then measuring the change in length in each direction. Shrinkage is
reported as change in length divided by original length times 100%.
The shrinkage at 95.degree. C. after 10 s is reported as the
"ultimate" shrinkage. Nominal sample size was 10 cm by 10 cm
although for the T.M. Long stretched films, the off-axis width was
reduced.
[0103] Shrink force was measured on a 1/2 inch wide strip of film,
mounted in a tensile rig with a force transducer. Gauge length
between grips was 2 inches. The sample was rapidly heated with a
hot air gun and the maximum force registered on the force
transducer. Although shrink force can be reported directly in units
of pounds or Newtons, shrink stress is more common and was obtained
by dividing the force by the intial cross-sectional area.
[0104] Surface energy, sometimes referred to "critical surface
tension", was measured using Accudyne.TM. dyne marker pens or
obtained from the literature (Polymer Handbook by J. Brandrup, and
E. H. Immergut, 3.sup.rd Ed. John Wiley and Sons, pages 411-426
(1989), and Properties of Polymers, by D. W. Van Krevelin,
Elsevier, Amsterdam, (1990)) and are given in Table I, in addition
to the refractive indices for various polymers. Values are shown
for amorphous versions of polymers where possible.
[0105] Surface tension may vary with crystallinity and orientation
(e.g. surface tension decreases for polyesters like PET and PBT
when crystallized). During melt blending, the polymers are in their
amorphous state; thus, the unoriented, non-crystalline surface
tension values are reported in Table I wherever possible. For
crystalline particles that do not completely melt during mixing,
however, the crystalline value was deemed more appropriate and
should be used instead. Critical surface tension measured by this
method is approximately the same as the total surface tension
measured by other methods; however, when the values were different,
reference was made to the critical surface tension for consistency
with the data.
[0106] The coefficient of friction (COF) was measured using ASTM
Method D1894 for the film against itself. Melt viscosity was
measured using a parallel plate rheometer with a nitrogen
atmosphere in dynamic mode. The viscosity at 1 rad/s (equivalent to
1 s.sup.-1 shear rate) was taken as the nominal zero shear
viscosity and used for the calculations in the examples because the
polymers of interest were typically Newtonian over a broad shear
range (including that seen in the extruder). Refractive index was
measured using a Metricon.TM. prism coupler with a 633 nm
wavelength laser or obtained from the literature. The refractive
index was calculated as the average value of all three directions
for oriented materials. Nominally the refractive index should be
measured based on the form of the material in the final film (i.e.
crystalline or amorphous).
[0107] Melting points and glass transition temperatures were
determined by either DSC (using ASTM Method D3418), dynamic
mechanical analysis (16 rad/s freq) or obtained from product and
general literature. The property data for the compositions of the
examples are given in Table II and the data for the oriented films
are given in Table III. Table IV provides modulus and density data
for some of the films of the examples.
Comparative Examples 1-3
[0108] Comparison of Cellulosic versus Olefinic Voiding
Efficiency--An amorphous polyester comprising 100 mole percent
terephthalic acid, 10 to 40 mole percent 1,4-cyclohexanedimethanol,
35 to 89 mole percent ethylene glycol, and 1 to 25 mole percent
diethylene glycol (neat density=1.30 g/cc, refractive index 1.565,
surface energy=42 dyne/cm, Tg=75.degree. C.), was combined with
various voiding agents using a twin screw extruder. The voiding
agents included either cellulose acetate ("CA") powder (CA398-30,
available from Eastman Chemical Co.), polypropylene (P4G3Z-039,
available from Huntsman Chemical Co., 5 melt index), or linear low
density polyethylene ("LLDPE") (HIFOR.TM. polyethylene available
from Eastman Chemical Co., 2 melt index) and are shown in Tables II
and III. The CA had a refractive index (RI) of 1.473, a surface
tension of 44 dyne/cm, and glass transition temperature of about
185.degree. C. The polypropylene had a refractive index of 1.490, a
surface tension of 30 dyne/cm, and a melting point of 160.degree.
C.; and the polyethylene had a refractive index of 1.49, a surface
tension of 32 dyne/cm, and a melting point of 120.degree. C. The
cellulosic was within 5 dyne/cm or less of the polyester surface
tension whereas the olefins were not, thus the latter were
classified as "the second polymer" in Table II.
[0109] The compounded products were pelletized for ease of
handling, and then combined with the neat copolyester in a 50/50
ratio resulting in a 10% final loading of voiding agent. All
samples were dried at 54.4.degree. C. (130.degree. F.) for 8 hours
prior to extrusion. This second blending was performed on a 2.5
inch single screw extruder (L/D=30:1) equipped with a film die at a
nominal melt temperature of 260.degree. C. Viscosities of the
resins at this temperature are as follows: polyester: 11000 poise;
PP:6500 poise; LLDPE: 17400 poise. Film was cast from this blend
having a final thickness of 10 mils. The two olefinic concentrates
also contained 1 weight percent of Eastman ethylene methyl acrylate
copolymer (EMAC 2260, available from Eastman Chemical Co.) as a
compatibilizer, based on total film weight.
[0110] Stretching of the cast film was performed on a commercial
tenter frame (Marshall and Williams Co, Providence, R1). The
stretching conditions varied from material to material, but
linespeeds were nominally 15 to 35 feet per minute ("fpm"). The
annealing section of the tenter was set at 70.degree. C. with the
clips retracted about 5% to help reduce shrinkage force for all
samples.
[0111] For the PP voiding agent (Comparative Example 2), densities
of 1.12 to 1.15 g/cc were achieved by stretching the film
5.2.times. at 80.degree. C. (ca. 180.degree. F.). Resulting films
had 36% shrinkage at 75.degree. C. (167.degree. F.) and 70% at
95.degree. C. Total transmittance was 82% because of poor voiding
efficiency.
[0112] For the LLDPE voiding agent (Comparative Example 3), a
density of only 1.19 g/cc was obtained even after stretching
6.times. at 85.degree. C. at 15 fpm linespeed. Film shrinkage was
21% at 75.degree. C. and 70% at 95.degree. C. The softer nature of
LLDPE made it much less effective than PP, even though the stretch
ratio was higher. Transmittance was 89%.
[0113] For the CA voiding agent (Comparative Example 1), densities
of 1.03 to 1.05 g/cc were achieved using the same 5.2.times.stretch
ratio as for PP, and a stretch temperature of 85.degree. C.
Increasing the stretch ratio up to 6.4.times. at 85.degree. C.
(185.degree. F.) resulted in specific gravities of 0.99 g/cc. Film
shrinkages at 5.2.times. were 23% at 75.degree. C. and 72% at
95.degree. C. (200.degree. F.). Transmittance was only 27%, which
was much more opaque than the PP or LDPE voiding agents.
[0114] The CA films were significantly better in appearance than
either of the olefin based films, having a smoother, more uniform
matte finish and much greater opacity (as seen with the
transmittance values) because of the better dispersion (i.e.
surface tension matched with matrix resin). Both olefin films were
transluscent by comparison and exhibited significant "contact
clarity" which would not be of benefit for most packaging.
[0115] Although all of the films used the same weight percent of
voiding agent, the CA based films had a significantly lower density
and higher surface tension. This is true even though the olefins,
which are inherently lower in density (ca. 0.90 g/cc for each)
should have a greater concentration of voiding agent on a
volumetric basis. Thus, the CA was clearly more efficient and
better dispersed.
Comparative Examples 4-5
[0116] Commercial Drafting of Microvoiding Agents--In this example,
a commercial machine direction ("MD") drafting unit (Marshall and
Williams Company) was used in place of the tenter frame for the
stretching to impart machine direction ("MD") orientation as
opposed to transverse direction ("TD") orientation. Drafting is
used as part of biaxial orientation, or by itself for MD shrink
labels (e.g. roll-fed labels). It has the advantage of much higher
shear rates than tentering and the effective orientation level for
a given stretch ratio is higher. Insufficient film was available
for significant testing, although the LLDPE film (Comparative
Example 3) was retested using the drafter and shown as Comparative
Example 4. It was run at a 6.times. stretch ratio and a nominal
stretch temperature of 82.degree. C. (180.degree. F.) and a density
of 1.02 g/cc was achieved. Shrinkage at 95.degree. C. was only 59%.
The film was more nearly opaque (transmittance of 39%) than the
same composition stretched on the tenter frame, but overall was
very streaky and poor in appearance.
[0117] Comparative Example 5 consisted of the same polyester as in
Comparative Examples 1-3, except that the polyester was blended
with 15 weight percent of K-RESIN.TM. KR05
styrene-butadiene-styrene copolymer ("SBS", available from Phillips
Chemical Co.) and 5% Terlux.TM. 2812
methacrylate-acrylonitrile-butadiene-styrene copolymer ("MABS",
available from BASF Corporation), and was stretched on the drafter
using the same conditions described above. Pellet/pellet blending
of the resins at the extruder was performed as a lower cost
alternative to precompounding the additives. The surface tension of
the SBS and MABS were estimated to be 34 and 35-36 dyne/cm
respectively. The Tg of the SBS and MABS are approximately
90.degree. C. and 100.degree. C. respectively (both very broad
transitions due to block structure). The refractive indices of the
SBS and MABS were 1.57 and 1.54 respectively. The SBS viscosity was
estimated to be approximately 100,000 poise at the nominal
processing temperature of 200.degree. C. whereas the polyester was
75,000 poise giving a ratio of 1.3 with respect to the copolyester.
The copolyester/styrenic blends had a final density 0.98 g/cc and
shrinkage of 33% and 70% at 85.degree. C. and 95.degree. C.
respectively. Total transmittance was 32%. It was of higher
aesthetic quality (fewer streaks) and stiffer than the LLDPE, but
was inferior to the CA film produced previously (even with the
greater loading of voiding agents and higher strain rate of the
drafter). The film also was more brittle, which was believed to be
the result of the surface tension mismatch of the MABS with the
copolyester. Although there was insufficient CA resin for a full
trial on the drafter, indications were that it would be similar to
the tenter frame, having even lower density and better aesthetics
than the olefin and styrene based systems due to the higher strain
rate and stretch ratio.
Example 1 and Comparative Examples 6 and 7
[0118] Comparison of Cellulosic with Cellulosic-EMAC and
Cellulosic-EBAC Voiding Agents on T.M. Long Film Stretcher--Various
polymers were combined using a twin screw extruder and pelletized
in a manner similar to Comparative Examples 1-3 above (and using
the same matrix copolyester). These concentrates were then blended
into the copolyester at a 25 weight percent loading using a single
screw extruder (1.5" Killion) and then cast into 10 mil thick film
at a nominal 260.degree. C. melt temperature. Example 1 used a
concentrate consisting of a 60/40 blend of CA-398-3 cellulose
acetate (refractive index RI=1.473, surface tension=44 dyne/cm,
Tg=185.degree. C.) and ethylene methyl acrylate copolymer 2260
("EMAC", available from Eastman Chemical Co., 2 melt index,
RI=1.496, surface tension=34 dyne/cm, viscosity=9900 poise, melting
point=77.degree. C.). Comparative Sample 6 was a 60/40 blend of
CA-398-3 cellulose acetate and ethylene butyl acrylate copolymer
("EBAC", available from Eastman 0.5 melt index, RI=1.495, surface
tension=33 dyne/cm, Tm=86.degree. C.). The EBAC has approximately
4.times. higher viscosity than the EMAC sample because of higher
molecular weight. Comparative Example 7 was a 60/40 blend of CA
398-3 and PETG 6763 copolyester (available from Eastman Chemical
Co., Tg=80.degree. C.). In this latter example, only the CA is
considered part of the "voiding agent" in contrast to the previous
two. Similarly, the PETG 6763 is part of the matrix polyester (and
is similar in properties to the main copolyester). Comparative
Example 8 was similar to Comparative Example 1 except for the
differences in stretch method.
[0119] Film was stretched uniaxially 5.times. (5.times.1 planar
stretch ratio) at 85.degree. C. using a T.M. Long laboratory film
stretcher. After stretching, film is normally air cooled to freeze
in orientation. Densities were all nominally around 1.0 g/cc or
slightly greater and could not be resolved on the column. Light
absorptivity values for the films with the addition of the EMAC or
EBAC were significantly improved as compared with the CA only
(Comparative Example 7). The shrink force was reduced by as much as
25% over the CA sample. The EBAC sample quality was not as good as
the EMAC, and had less overall opacity as indicated by its lower
absorptivity. This example illustrates that the addition of the
2.sup.nd voiding agent with a substantially different surface
tension but within the desired viscosity ratio range (for the EMAC)
helped to enhance the opacity, shrink force, and voiding
characteristics further.
Examples 2-3
[0120] Tentering of Cellulosic/Olefin Blend--The 60/40 concentrate
of Eastman CA 398-3 and EMAC (described above) was used to make
void-containing film using a tenter frame. The concentrate was
blended at 15 and 25 weight percent loadings into the same
copolyester as Comparative Examples 1-3 and cast into film on a 2.5
inch extruder. Film was then stretched on the tenter frame
described in Comparative Example 1. Stretch ratios were nominally
5.5.times. at 89.degree. C.
[0121] Comparison of Example 2 and Example 3 with Comparative
Example 1 (the CA control) shows the effect of the added olefin
under similar stretching conditions. As with Example 3, the olefin
was found to increase the absorptivity (or opacity) and soft feel
over the CA by itself. Furthermore the shrink force also was
greatly reduced (ca. 50% reduction) with no significant loss of
ultimate shrinkage. Surface tensions remained high for all films
thereby maintaining ease of printing.
Comparative Example 8
[0122] Neat Polyester Film--The same amorphous polyester as
described abovefor Comparative Examples 1-3 was stretched as
described in Example 1, using a T.M. Long film stretcher. This film
had no voiding agents present.
Examples 4-15
[0123] Comparison of Various CA/Olefin Voiding Blends on T.M. Long
Stretched Film--A number of concentrates were made on a twin screw
extruder as described in the previous example, and then added at
either a 25 weight percent or 35 weight percent loading into the
polyester. Films were extruded on a 1 inch Killion extruder with a
6 inch die at a nominal temperature of 260.degree. C., and
stretched using T. M. Long film stretcher; film properties are
listed in Tables II, III, and IV. Included in the blending were
EMAC (as described above), styrene acrylonitrile copolymer ("SAN"),
methacrylic-acrylonitrile-butadiene-styrene copolymer "MABS",
atactic polystyrene PS (all available from BASF Corporation,
RI=1.59, surface tension=36 dyne/cm, viscosity=3300 poise,
Tg=105.degree. C.) and PP (melt index=5, RI=1.49, surface
tension=30 dyne/cm, viscosity=6500 poise) although the MBS and SAN
samples are not listed in the Tables because of catalyst
interactions with the cellulosic that led to excessive
discoloration. The samples containing 35% loading were only
stretched 5.times. instead of the 5.5.times. used for the 25 weight
percent loading of voiding concentrate.
[0124] From the table, it is observed that the PS and PP helped
improve the density reduction as compared with the EMAC and also
significantly improved the opacity (and shrink force) as compared
with the CA control (Comparative Example 1 and Example 7). Both
have a large surface tension mismatch with polyester and the
viscosity ratios were within the desired range, thereby
contributing to their excellent performance. PP was better with
regards to opacity which is in line with its greater refractive
index mismatch with the copolyester. Ultimate shrinkage remained
high while shrink force remained low, for all of these blends.
Comparative Example 9
[0125] Use of CA/Olefin Blend to Void oPS Film--A 25 weight percent
loading of the 60/40 CA/EMAC concentrate as described in
Comparative Example 6 was blended in with SBS and crystal styrene
(60 weight percent SBS with 40 wt % crystalline polystyrene) to
make a void-containing oriented polystyrene ("oPS") film. The RI of
the styrene matrix was approximately 1.57 and the surface tension
was approximately 36 dyne/cm. The glass transition temperature of
the styrenic matrix was 90 to 95.degree. C. Extrusion conditions
were colder with the nominal melt temperature around 210.degree. C.
and with stretching being performed at a nominal 105.degree. C. The
styrene blend viscosity at this temperature was nominally about
50,000 poise; however the CA was solid at this temperature with a
viscosity approaching infinity. Thus, the viscosity ratio is much
greater than 3.5. No attempts at optimization were made, but the
density was reduced from 1.05 g/cc for the neat oPS to below 1.0
with the microvoiding. The quality of the voiding was poor and
inferior to previous examples. Samples also were yellow in
appearance, possibly as a result of an acid interaction between the
SBS catalysts and cellulosic. It is expected that the color problem
could be remedied easily with an acid scavenger and/or
stabilizer.
Example 16
[0126] Cellulose Acetate Propionate--In this example, the cellulose
acetate was replaced with cellulose acetate propionate (Eastman CAP
482-20) (surface tension=40 dyne/cm, RI=1.476, Tg
(nominal)=120.degree. C.) and films were prepared as described in
Comparative Example 1 using a 60/30/10 CAP/PP/EMAC concentrate and
the same copolyester as the polymer matrix. CAP was easily melt
processed on a single screw extruder and the dispersion was found
to be excellent. Although the CAP used in this example was a
powder, it is also readily available in pellet form and could be
mixed directly at the extruder rather than precompounding on a
twin-screw extruder.
Example 17-18
[0127] Microvoiding of Nylon and Polycarbonate Films--In order to
verify the effectiveness of the voiding agent on a variety of
polymer matrixes, additional samples were prepared from nylon MXD-6
6007.TM. (available from Mitsubishi Gas Chemical, refractive
index=1.582, surface tension=42 to 47 dyne/cm, Tg=85.degree. C.)
and bisphenol-A polycarbonate (6=Melt Index, refractive
index=1.585, surface tension=45 dyne/cm, Tg=150.degree. C.) as the
base resins. Into each sample was blended 25 weight percent of a
voiding agent comprising 50 weight percent cellulose acetate, 30
weight percent polypropylene, 10 weight percent EMAC, and 10 weight
percent PS using components described previously. Film was extruded
using a 1 inch Killion extruder with a 6 inch film die. Nominal
extrusion temperature was 280.degree. C. and the film thickness was
10 mils (0.0254 cm). The viscosities of the components at this
temperature are as follows: nylon=7300 poise, PC=14000 poise,
PP=5200 poise, EMAC=7000 poise, PS=1500 poise. All were well
matched for good mixing with nylon, although the viscosity ratio of
the polystyrene with respect to polycarbonate was marginal. The Tg
of the CA (ca. 180.degree. C.) was above the Tg of the
polycarbonate matrix (150.degree. C.) and the Tg of the nylon
(85.degree. C.). The surface tensions were within the desired range
for all components. The resulting cast film quality for the nylon
blends was excellent with the exception of yellowing because of
degradation and/or residual catalyst activity. By contrast, the
film quality for the PC blend was acceptable, although not as good
as for nylon.
[0128] Samples of each film were then cut into 1/2 inch strips (1.3
cm) and mounted on a Chatillon LTCM-6 tensile rig with a nominal 2
inch gauge length between grips. The film was then heated using a
hot air gun until soft, and then stretched uniaxially at 15
inches/minute (38 cm/min) to induce voiding. Stretch ratios varied
from about 3.times. to 6.times. depending on resin and heating
uniformity. Voiding was apparent from the whitening of the film
during necking. Pieces of the voided film were then cut from the
strips and immersed in water. Both floated indicating a density
less than 1 g/cc and thereby verifying the effectiveness of the
voiding package.
Comparative Example 10
[0129] This example was taken from Example 13 of U.S. Pat. No.
4,770,931 and is provided as a comparative example with additional
data on melt viscosities. A void-containing blend was made on a 3.2
cm extruder using cellulose acetate, 4.7 melt index polypropylene
(PP 4230), and a 0.70 IV PET. Cellulose acetate loading was 20
weight percent, whereas the PP loading was varied at 3%, 5% and
10%. The nominal melt processing temperature was 280.degree. C.,
and the melt viscosities at this temperature were 4000 poise and
20,000 poise for PET and PP respectively, resulting in a viscosity
ratio of approximately 5. This is outside the range for good
mixing. Films were stretched and void-containing on a T.M. Long
film stretcher. White, opaque, paper-like films were made; however
film strength and quality decreased as the level of polypropylene
increased.
Comparative Example 11
[0130] In this prophetic example, a blend is made using the
polyester of Comparative Examples 1-3 as the matrix resin and a mix
of polypropylene, polystyrene, and/or polymethylpentene as the
voiding agent. A comparison of the surface tensions show that none
of the matrix resins are within the acceptable 5 dyne/cm range of
the polyester (which is ca. 42-43 dyne/cm). Because of this poor
surface tension match, these voiding agents must rely on optimal
mixing conditions, such as high shear and proper viscosity ratio,
and mixing will not be as efficient due to reagglomeration. As a
result, particle size distributions are not as good, the film
surface is rough, and require an additional smooth layer on top of
the film so it can be printed.
Example 19
[0131] Use of Crystalline Polyester as Voiding Agent--In this
prophetic example, PET powder is produced both for commercial use,
and in the form of "fines" filtered from a reaction. These fines
are an undesirable byproduct of the manufacturing process of PET
that are removed during filtration. They are highly crystalline and
have a high melting temperature (Tm=240.degree. C. up to
280.degree. C.) and are difficult to melt completely during normal
processing, thus the values of the crystalline phase are more
applicable. Surface tension of the crystalline PET tends to be
lower since the particles do not melt significantly (crystalline
surface tension estimated at 36 to 40 dyne/cm depending on local
degree of melting). The polyester fines and/or powdered PET powder
are mixed into a copolyester along with EMAC resin using a twin
screw extruder. The extrusion conditions are at 220.degree. C.,
which is below the melting point of PET to minimize melting. Film
can be stretched and void-containing as previously described, but
with the PET particles serving as a surface tension matched voiding
agent. Other crystalline polymers like PBT may be used in place of
the PET particles.
Example 20
[0132] Void Size Analysis--Film samples from Comparative Example 1
(copolyester and CA) and Example 3 (copolyester with CA and EMAC)
were analyzed using electron microscopy to determine void size and
particle size. Comparative Example 1 had an average void size of
1.398 microns, with a standard deviation of 1.30 microns. The
largest detectable hole size was 17.6 microns. By contrast, Example
3 had an average hole size of 0.286 microns and a standard
deviation of 0.183 microns. The largest detectable hole size was
2.38 microns. Example 3, which contained both the CA and the EMAC,
had a finer hole size distribution than the sample with only CA,
thereby yielding a better quality film. The particle size of the
individual voiding agents was not determined because of the
difficulty of separating the voids from particles.
Example 21
[0133] Solvent Seaming of Films--The films from Examples 1-15 and
Comparative Examples 1-8 were tested for solvent seaming using
commercial seaming solvents (Flexcraft 460.TM. or Flexcraft 331.TM.
(available from Flexcraft Industries) and the results reported in
Table III. Films were cut into strips and a small amount of solvent
applied to one side using a pipette or swab, followed by
application of the 2.sup.nd strip of film and "rolling" with a
wallpaper roller. Films were allowed to sit either 1 minute or for
2.5 hours (long term) after which a pull test was performed (only
data for 1 minute test with Flexcraft 460 is shown in Table I). A
strong bond strength indicates that the film broke or deformed
before the bond did. A medium bond strength indicates some
deformation before bond breakage, whereas a poor strength indicates
easy bond breakage. Most of the bond strengths were strong after
2.5 hrs (i.e., the film tore or deformed before the bond broke)
except for the films containing high levels of PP (Example 15).
After 1 minute, most of the films exhibited sufficient bond
strength for a typical label application. It is thus evident that
any of these labels can be seamed using traditional methods
although some tailoring of solvents may be needed based on the
components of the film.
Example 22
[0134] Coefficient of Friction--A sample of void-containing film
(Comparative Example 8) was compared with a neat copolyester shrink
film control. The static COF for the neat copolyester was 0.35
whereas the value for Example 3 was 0.24. The void-containing films
also had a much slicker feel in agreement with the data.
Examples 23-24
[0135] Shrinkage Properties--All of the films described in Table II
and III had shrinkage of greater than 5% at 70.degree. C. after 10
seconds. Most were in the range of 15 to 50% shrinkage under these
conditions. All had off-axis shrinkage values of less than 10% over
the entire temperature range. The onset of shrinkage is primarily a
function of the matrix polymer and nominally starts about 5 to
10.degree. C. below the glass transition temperature (for the
polyester used in Examples 1-15 and Comparative Examples 1-8, the
Tg was 75.degree. C.).
[0136] Shrinkage properties were changed considerably by the
voiding agents and stretch conditions as is well known in the art.
For this example, concentrates were produced by twin-screw extruder
as described previously. Example 23 used a concentrate based on 60
wt % CA and 40 wt % EMAC. Example 24 used a concentrate of 60 wt %
CA, 20% EMAC, 10 wt % PP and 10 wt % PS. Both concentrates were
blended at 25 wt % loading into the polyester described in
Comparative Examples 1-3. Surface tensions and viscosity ratios
were within the preferred range for both samples. Film was extruded
and tentered at a nominal 5.times. stretch ratio and a stretch
temperature of 85.degree. C. For Example 23, the anneal temperature
after stretching was approximately 70.degree. C. and the anneal
temperature for Example 24 was approximately 80.degree. C. Film
shrinkage for each was measured at different temperatures and
times.
[0137] In Example 23, the shrinkage in the principal direction at
70.degree. C. after 5 and 10 seconds was 33% and 46% respectively.
At 80.degree. C., the shrinkages at 5 and 10 seconds were 65 and
68%. Off-axis shrinkage never exceeded 5%. Thus, Example 23
illustrates that a microvoided film can be made with very rapid and
high shrinkage. This type of film is useful in, for example,
steam-type shrink tunnels where very precise temperature control
allow for significant, but uniform shrinkage around a highly
contoured container.
[0138] By contrast, in Example 24, the shrinkage in the principal
direction at 70C after 5 and 10 seconds was 9 and 15% respectively.
At 80.degree. C. the shrinkages were 41 and 45% respectively.
Maximum off-axis shrinkage was 3%. This example illustrates a
shrink film with a more gradual increase in shrinkage with
temperature and which would be more useful in, for example, a hot
air type shrink tunnel where temperature control is not
precise.
Example 25
[0139] Low Temperature Onset Shrink Film--In this prophetic
example, the polyester matrix of the previous examples is replaced
with Eastman EASTOBOND.TM. copolyester, a high DEG, low Tg
copolyester (Tg=55.degree. C.). EASTOBOND.TM. has approximately the
same surface tension, refractive index, and viscosity as the
previous copolyester, thus good mixing will occur using the voiding
agents described previously (e.g. the 60 wt % CA/40 wt % EMAC
additive). Thus, for example, the shrinkage will begin at
approximately 50.degree. C. and reaching a maximum shrinkage by
approximately 65.degree. C. The ultimate shrinkage will depend on
stretch temperature and stretch ratio. For a nominal stretch
temperature of 65.degree. C. and a stretch ratio of 5.times., the
ultimate shrinkage will be approximately 60 to 75%. A film of this
type is useful, for example in applications where very low
shrinkage temperatures are required, such as with film that could
be activated for home use, via a standard hair dryer.
Example 26
[0140] Stiffness Properties of the Oriented Film--The film modulus
in the off-axis (i.e. non-shrink direction) is particularly
important for allowing a shrink sleeve to be applied over a bottle
without crushing or buckling. Off-axis moduli were determined for
Examples 4 through 15 and Examples 23 and 24 and are Tabulated in
Table IV. A direct correlation between modulus and density was
found based on linear regression.
E=2.04*density-1.15
[0141] where E is the modulus (GPa) and the density is in units of
g/cc. While this correlation was matrix dependent, it is believed
to be reasonably accurate for most of the typical polyesters used
in shrink film because most polyesters have approximately the same
neat modulus. The correlation also illustrates the strong
dependence of the modulus on the density (voids have no stiffness).
Slight variations from this line resulted from variations in the
stiffness of the voiding agents. For example, Examples 9 and 15,
which contained none of the softer EMAC material, had higher moduli
at constant density than the other films that contained EMAC. Thus
for a given density target, it was possible to tweak the modulus
slightly through composition, although stiffness was primarily
density dependent.
Example 38
[0142] Sleeve Buckling Strength--The correlation between modulus
and density was used to correlate the density to the crush strength
of the sleeve. The buckling strength of a thin wall tube with an
applied topload is well documented in the engineering literature
(see for example "Roark's Formulas for Stress and Strain" by W. C.
Young, 6.sup.th Edition, McGraw-Hill, New York, page 689 (1989)).
For the geometry of a typical sleeve, the topload stress .sigma. at
which point buckling/collapsing will occur is
.sigma.=0.3Et/R
[0143] where E is the modulus (in Pascals), R is the radius and t
is the thickness (in meters). Topload stress can be converted to a
top load force G (in Newtons) at the onset of buckling:
G=0.6.pi.Et.sup.2
[0144] By substituting for E as a function of density, the
following formula is obtained:
G=0.6.pi.(2.04*density-1.15)t.sup.2
[0145] This expression gives the top load strength as a function of
density and thickness. As the degree of microvoiding increases and
the density decreases, the thickness has to be increased to
compensate. Notice that ring crush/buckling strength is a function
of thickness squared but only a linear function of the
density/modulus. It is also dependent on the uniformity of film
gauge with highly uneven thicknesses showing lower strengths.
[0146] As an example, the top load strength was measured on
Comparative Example 8 and Examples 23 and 24 using a procedure
similar to that outlined in Tappi 822 for paperboard. A hoop of
film 50 mm in diameter, and 12 mm wide was tested with a crosshead
load rate of 13 mm/min. The load direction was such that the
non-shrink direction of the film was along the axial direction. The
onset of buckling was obtained from the force vs. time plot during
loading (average of five measurements). For unvoided the film from
Comparative Example 8, with a thickness of 55 um (55.sup.-6
meters), the measured buckling strength was 11.2 Newtons and the
predicted value was 9.2 Newtons, which are in good agreement. The
films from Examples 23 and 24 had thicknesses 45 and 57 microns
respectively. For the film from Example 23, the measured crush
strength was 3.9 Newtons and the predicted value was 3.6 Newtons.
For the film from Example 24, the measured value was 7.1 Newtons
and the predicted value was 5.9 Newtons. All of the experimental
and predicted values are in good agreement and within experimental
error thereby verifying the equations. Thus, if there is a minimum
crush strength requirement for a given label applicator, one can
use the above equations to give the required thickness of the film
as a function of density so as to meet the crush strength.
1TABLE I Nominal Property Data at Room Temperature Refractive Index
Surface Tension Modulus Polymer (n) (dynes/cm) (GPa) cellulose
1.540 36-42 1.5 cellulose acetate 1.473 39-46 1.8 cellulose acetate
1.480 34 1.3 butyrate cellulose acetate 1.476 36-42 1.7 propionate
nitrocellulose 1.510 38 15 nylon 6, 6 1.530 42-47 2.1 MXD-6 nylon
1.582 42-47 4.7 polyethylene 1.490 31-37 0.5 polyethylene 1.510
41-60 0.08 methacrylic acid copolymer (ionomer) polypropylene 1.490
29-30 1.5 poly(ethylene methyl 1.496 34 0.03 acrylate) copolymer
(EMAC) poly(butylene methyl 1.495 33 0.03 acrylate) copolymer
(EBAC) polystyrene 1.591 33-36 3 styrene-butadiene- 1.570 33-36 1.8
styrene (SBS) polymethylpentene 1.460 25 1.9 poly(ethylene vinyl
1.460 30-36 0.08 acetate) poly vinyl alcohol 1.500 37 2.7 PVC 1.539
42 2.7 polycarbonate 1.585 45 2.4 PET 1.571 43 2.6 PBT 1.57-1.60
38-40 2.1 polyester-ether -- 38-42 0.16-0.17 (HYTREL .TM., ECDEL
.TM.) PETG 1.563 41-43 1.9 Embrace .TM. 1.565 41-42 1.6 copolyester
PMMA 1.479 39 3.4 PTFE 1.350 24 0.61 poly(acrylonitrile) 1.514 50
>1 GPa polyimides 1.640-1.670 38-41 2.5
[0147]
2TABLE II Surface Tension Surface Tension Difference Difference
Between Between Viscosity Ratio Polymer 1 and Polymer 2 and
(Polymer 2 over Example # Matrix Polymer Polymer 1 Polymer 2 Matrix
Matrix Matrix Film Quality C1 copolyester cellulose acetate -- 2
dyne/cm -- -- good C2 copolyester -- PP 12 dyne/cm 0.6 poor C3
copolyester -- LLDPE 10 dyne/cm 1.6 poor C4 copolyester -- LLDPE 10
dyne/cm 1.6 poor C5 copolyester MBS SBS 6 dyne/cm 10 dyne/cm 1.3
fair 1 copolyester cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9
excellent C6 copolyester cellulose acetate EBAC 2 dyne/cm 9 dyne/cm
3.6 good C7 copolyesters cellulose acetate -- 2 dyne/cm -- -- good
2 copolyester cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9
excellent 3 copolyester cellulose acetate EMAC 2 dyne/cm 8 dyne/cm
0.9 excellent C8 copolyester -- -- -- -- -- N/A 4 copolyester
cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9 excellent 5
copolyester cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9
excellent PS 6 dynes/cm 0.3 6 copolyester cellulose acetate EMAC 2
dyne/cm 8 dyne/cm 0.9 excellent PS 6 dynes/cm 0.3 7 copolyester
cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9 excellent PP 12
dyne/cm 0.6 8 copolyester cellulose acetate EMAC 2 dyne/cm 8
dyne/cm 0.9 excellent PP 12 dyne/cm 0.6 9 copolyester cellulose
acetate PP 2 dyne/cm 12 dyne/cm 0.6 excellent 10 copolyester
cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9 excellent 11
copolyester cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9 good PS
6 dynes/cm 0.3 12 copolyester cellulose acetate EMAC 2 dyne/cm 8
dyne/cm 0.9 good PS 6 dynes/cm 0.3 13 copolyester cellulose acetate
EMAC 2 dyne/cm 8 dyne/cm 0.9 excellent PP 12 dyne/cm 0.6 14
copolyester cellulose acetate EMAC 2 dyne/cm 8 dyne/cm 0.9
excellent PP 12 dyne/cm 0.6 15 copolyester cellulose acetate PP 2
dyne/cm 12 dyne/cm 0.6 excellent C9 60/40 SBS/PS EMAC cellulose
acetate 2 dyne/cm 8 dyne/cm >3.5 poor 16 copolyester CAP EMAC 2
dyne/cm 8 dyne/cm 0.9 excellent PP 12 dyne/cm 0.6 17 nylon
cellulose acetate PP 1 dyne/cm 15 dyne/cm 0.7 good EMAC 11 dyne/cm
1 PS 9 dyne/cm 0.2 18 PC cellulose acetate PP 1 dyne/cm 15 dyne/cm
0.4 fair EMAC 11 dyne/cm 0.5 PS 9 dyne/cm 0.1 C10 PET polyester
cellulose acetate PP 3 dyne/cm 13 dyne/cm 5 fair C11 copolyester PS
PP 6 dyne/cm 12 dyne/cm N/A N/A PMP 17 dyne/cm 19 copolyester PET
polyester EMAC 1-3 dyne/cm 8 dyne/cm 0.9 N/A
[0148]
3TABLE III Data for Oriented Films (Comparative Examples C1-C8 and
Examples 1-15) Measured Shrink Surface Ex- Stretch Density Film
Tot. Absorp- Ultimate Stress Energy am- Stretch Stretch Temp.
(Gradient), Thickness, Trans- tivity Shrinkage, (100%) Solvent
(dyn/ ple # Description Method Ratio .degree. C. Film mils mit.
(1/cm) % Mpa Seaming cm) C1 90/10 polyester/CA tenter 5.2 .times. 1
85 1.03-1.05 2.5 27 200 72 16.1 strong 59 C2 90/10 polyester/PP
tenter 5.2 .times. 1 80 1.12-1.15 2.2 82 28 36 14.4 medium 45 C3
90/10 polyester/LLDPE tenter 6 .times. 1 80 1.19 1.95 89 15 21 15.2
strong 43 C4 90/10 polyester/LLDPE drafter 6 .times. 1 82 1.02 2.5
39 142 59 17.8 poor 53 C5 80/15/5 drafter 6 .times. 1 82 0.98 2.2
32 197 70 17.2 medium 55 polyester/SBS/MBS 1 80/15/5 TM 5 .times. 1
85 .about.1 2.9 24 188 62 10.6 strong 40 polyester/CA/EMAC Long C6
80/15/5 TM 5 .times. 1 85 .about.1 2.9 37 129 62 12.4 medium 43
polyester/CA/EBAC Long C7 85/15 polyester/CA TM 5 .times. 1 85
.about.1 2.3 47 122 63 16.5 strong 44 Long 2 84/10/6 tenter 5.5
.times. 1 89 1.01 2.3 29 205 71 9.8 medium 45 polyester/CA/EMAC 3
75/15/10 tenter 5.5 .times. 1 89 0.9 2.5 26 206 64 7.4 strong 57
polyester/CA/EMAC C8 100% polyester (no TM 5.5 .times. 1 85 1.3 2.5
92 7 75 14.3 strong 41 voids) Long 4 75/15/10 TM 5.5 .times. 1 80
0.86 3.2 12 256 73 8.40 strong 46 polyester/CA/EMAC Long 5
75/15/5/5 TM 5.5 .times. 1 80 <0.82 4 8 245 71 6.89 poor 47
polyester/CA/EMAC/PS Long 6 75/10/10/5 TM 5.5 .times. 1 80 0.86 3.8
13 207 73 6.71 strong 51 polyester/CA/EMAC/PS Long 7 75/15/5/5 TM
5.5 .times. 1 80 0.86 3.6 12 227 73 6.51 medium 47
polyester/CA/EMAC/PP Long 8 75/10/10/5 TM 5.5 .times. 1 80 0.91 3
13 262 72 7.35 strong 41 polyester/CA/EMAC/PP Long 9 75/12.5/12.5
TM 5.5 .times. 1 80 0.83 3.2 9 291 66 8.61 medium 37
polyester/CA/PP Long 10 65/20/15 TM 5 .times. 1 85 <0.82 4 8 245
68 5.17 medium 51 polyester/CA/EMAC Long 11 65/20/7.5/7.5 TM 5
.times. 1 85 <0.82 4.8 7 215 58 4.16 poor 59
polyester/CA/EMAC/PS Long 12 65/15/10/10 TM 5 .times. 1 85 0.87 4
10 223 63 5.34 strong 57 polyester/CA/EMAC/PS Long 13 65/20/7.5/7.5
TM 5 .times. 1 85 0.83 4 12 205 71 5.00 strong 45
polyester/CA/EMAC/PP Long 14 65/15/10/10 TM 5 .times. 1 85 0.9 3.6
17 189 68 4.98 Poor 39 polyester/CA/EMAC/PP Long 15 65/17.5/17.5 TM
5 .times. 1 85 <0.82 4 8 245 68 5.17 Poor 36 to polyester/CA/PP
Long 38
[0149]
4TABLE IV Off Axis Modulus vs. Density Data for Example 26 Example
# Density (g/cc) E (GPa) 4 0.86 0.57 5 0.82 0.46 6 0.86 0.54 7 0.86
0.52 8 0.91 0.65 9 0.83 0.69 10 0.82 0.46 11 0.82 0.49 12 0.87 0.63
13 0.83 0.49 14 0.90 0.68 15 0.82 0.58 23 0.99 0.94 24 0.97 0.97 C8
1.30 1.61 C8 (repeat) 1.30 1.33
* * * * *