U.S. patent application number 10/275639 was filed with the patent office on 2003-11-13 for foam and method of making.
Invention is credited to Gehlsen, Mark D., Haas, Christopher K., Mortenson, Sara B., Strobel, Joan M..
Application Number | 20030211310 10/275639 |
Document ID | / |
Family ID | 29401122 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030211310 |
Kind Code |
A1 |
Haas, Christopher K. ; et
al. |
November 13, 2003 |
Foam and method of making
Abstract
The present invention is directed to a process for producing a
polypropylene foam by mixing a high melt strength polypropylene and
a blowing agent to form a melt mixture, reducing the temperature of
the melt mixture at the exit of the apparatus to an exit
temperature that is no more than 30.degree. C. above the melt
temperature of the neat polypropylene while maintaining the melt
mixture at a pressure sufficient to prevent foaming; passing the
mixture through said exit shaping orifice and exposing the mixture
to atmospheric pressure, whereby the blowing agent expands causing
cell formation resulting in foam formation, and orienting said
foam. The foam is useful as tape backings, thermal and acoustical
insulation and as a diffuse reflector for use in optical
applications such as computer displays.
Inventors: |
Haas, Christopher K.; (St.
Paul, MN) ; Gehlsen, Mark D.; (St. Paul, MN) ;
Mortenson, Sara B.; (St. Paul, MN) ; Strobel, Joan
M.; (St. Paul, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Family ID: |
29401122 |
Appl. No.: |
10/275639 |
Filed: |
November 7, 2002 |
PCT Filed: |
June 21, 2001 |
PCT NO: |
PCT/US01/19989 |
Current U.S.
Class: |
428/318.6 ;
428/317.1; 428/319.3 |
Current CPC
Class: |
Y10T 428/249992
20150401; B29C 44/352 20130101; C08J 9/36 20130101; Y10T 428/249988
20150401; C08J 2205/044 20130101; C08J 9/0061 20130101; C08J
2423/12 20130101; C08J 2205/052 20130101; Y10T 428/249981 20150401;
C08J 9/08 20130101; Y10T 428/249979 20150401; C08J 9/103 20130101;
B29C 44/505 20161101; Y10T 428/249978 20150401; C08J 2201/03
20130101; Y10T 428/249991 20150401; B32B 5/18 20130101; C08J
2323/14 20130101; B29C 44/5627 20130101; C08J 2323/12 20130101;
B29K 2023/12 20130101; Y10T 428/249982 20150401 |
Class at
Publication: |
428/318.6 ;
428/319.3; 428/317.1 |
International
Class: |
B32B 003/26 |
Claims
1. A method for making a foamed article comprising: (1) mixing at
least one high melt strength polypropylene and at least one blowing
agent in an apparatus having an exit shaping orifice at a
temperature and pressure sufficient to form a melt mixture wherein
the blowing agent is uniformly distributed throughout the
polypropylene; (2) reducing the temperature of the melt mixture at
the exit of the apparatus to an exit temperature that is no more
than 30.degree. C. above the melt temperature of the neat
polypropylene while maintaining the melt mixture at a pressure
sufficient to prevent foaming; (3) passing the mixture through said
exit shaping orifice and exposing the mixture to atmospheric
pressure, whereby the blowing agent expands causing cell formation
resulting in foam formation, and (4) orienting said foam.
2. The process of claim 1 wherein said foam is oriented under
conditions such that the density of the extruded foam is
decreased.
3. The process of claim 2 wherein said orientation is at or above
the alpha transition temperature and below the melt temperature of
the polypropylene.
4. The process of claim 1 wherein said orientation is uniaxial.
5. The process of claim 1 wherein said orientation is biaxial.
6. The process of claim 5 wherein said orientation is simultaneous
biaxial.
7. The process of claim 1 wherein said high melt-strength
polypropylene comprises homo- and copolymers containing 50 weight
percent or more propylene monomer units, and having a melt strength
in the range of 25 to 60 cN at 190.degree. C.
8. The process of claim 7 wherein said polypropylene copolymers are
selected from random, block, and grafted copolymers of propylene
and an .alpha.-olefin selected from the group consisting of C3-C8
.alpha.-olefins and C4-C10 dienes.
9. The process of claim 1 wherein said mixture comprises a blowing
agent and a blend of a major amount of a high melt strength
polypropylene and a minor amount of a semicrystalline or amorphous
polymer.
10. The process of claim 1 wherein said extruding step comprises
extruding said mixture at a pressure .gtoreq.2500 psi (17.2
Mpa).
11. The process of claim 4 wherein said orientation is at least
3.times..
12. The process of claim 5 wherein said orientation is 3 to 50
total draw ratio.
13. The process of claim 1 wherein said blowing agent is selected
from physical blowing agents and chemical blowing agents.
14. The process of claim 13 further comprising the step of
elevating the temperature of the melt mixture to a temperature
sufficient to activate said chemical blowing agent prior to step
(2).
15. The process of claim 1 wherein said foam comprises 70% or
greater closed cells prior to orientation.
16. The process of claim 1 wherein said foam, prior to orientation,
has an average cell dimension of 50 micrometers or less.
17. A multilayer article comprising at least one oriented high melt
strength polypropylene foam layer.
18. The multilayer article of claim 17 further comprising at least
one thermoplastic film layer.
19. The multilayer article of claim 18 wherein said film layer
comprises polypropylene.
20. The multilayer article of claim 17 further comprising an
adhesive layer.
21. An oriented high melt strength polypropylene foam wherein the
tensile strength of the foam in the cross direction is within 30%
of the tensile strength in the machine direction.
22. A multilayer article comprising the foam of claim 21.
23. The multilayer article of claim 22 further comprising an
adhesive layer.
24. A foamed article prepared by the process of claim 1.
Description
[0001] The present invention is directed to a process for producing
a polypropylene foam. The foam is useful as tape backings, thermal
and acoustical insulation, low dielectric substrates and as a
diffuse reflector for use in optical applications such as computer
displays.
SUMMARY OF THE INVENTION
[0002] The present invention is directed to a process for preparing
a foamed article, the process comprising the steps of:
[0003] (1) mixing at least one high melt strength polypropylene and
at least one blowing agent in an apparatus having an exit shaping
orifice at a temperature and pressure sufficient, to form a melt
mixture wherein the blowing agent is uniformly distributed
throughout the polypropylene;
[0004] (2) reducing the temperature of the melt mixture at the exit
of the apparatus to an exit temperature that is no more than
30.degree. C. above the melt temperature of the neat polypropylene
while maintaining the melt mixture at a pressure sufficient to
prevent foaming;
[0005] (3) passing the mixture through said exit shaping orifice
and exposing the mixture to atmospheric pressure, whereby the
blowing agent expands causing cell formation resulting in foam
formation, and
[0006] (4) orienting said foam.
[0007] In another aspect, the present invention provides a process
for preparing a foamed article, using a foamable mixture comprising
a major amount of a high melt-strength polypropylene and a minor
amount of second polymer component comprising a semicrystalline or
amorphous thermoplastic polymer. Polymer mixtures comprising a high
melt-strength polypropylene and two or more added polymers are also
within the scope of the invention.
[0008] In another aspect, the present invention provides a process
for preparing a multilayer foamed article, comprising at least one
high-melt strength polypropylene foam
[0009] As used in this invention:
[0010] Alpha-transition temperature, T.alpha.c, means to the
temperature at which crystallite subunits of a polymer are capable
of being moved within the larger lamellar crystal unit. Above this
temperature lamellar slip can occur, and extended chain crystals
form, with the effect that the degree of crystallinity is increased
as amorphous regions of the polymer are drawn into the lamellar
crystal structure.
[0011] "Small-cell foam" means a foam having average cell
dimensions of less than 100 micrometers (.mu.m), preferably 5 to 50
.mu.m (prior to orientation);
[0012] "closed-cell" means a foam that contains substantially no
connected cell pathways that extend from one outer surface through
the material to another outer surface;
[0013] "operating temperature" means the temperature that must be
achieved in the extrusion process to melt all of the polymeric
materials in the melt mix;
[0014] "exit temperature" and "exit pressure" mean the temperature
and pressure of the extrudate in the final zone or zones of the
extruder and preferably in the die;
[0015] "melt solution" or "melt mixture" or "melt mix" means a
melt-blended mixture of polymeric material(s), any desired
additives, and blowing agent(s) wherein the mixture is sufficiently
fluid to be processed through an extruder;
[0016] "neat polymer" means a polymer that contains small amounts
of typical heat-stabilizing additives, but contains no fillers,
pigments or other colorants, blowing agents, slip agents,
anti-blocking agents, lubricants, plasticizers, processing aids,
antistatic agents, ultraviolet-light stabilizing agents, or other
property modifiers;
[0017] "foam density" means the weight of a given volume of
foam;
[0018] "density reduction" refers to a way of measuring the void
volume of a foam based on the following formula: 1 R = 1 - f o
.times. 100 %
[0019] where .rho..sub.R is the density reduction, .rho..sub.f is
the foam density, and .rho..sub.o is the density of the original
material;
[0020] "polydispersity" means the weight average cell diameter
divided by the number average cell diameter for a particular foam
sample; it is a means of measuring the uniformity of cell sizes in
the sample;
[0021] "uniform" means that the cell size distribution has a
polydispersity of 1.0 to 2.0;
[0022] "spherical" means generally rounded; it may include
spherical, oval, or circular structure;
[0023] "polymer matrix" means the polymeric, or "non-cell," areas
of a foam;
[0024] ".alpha.-olefin" means an olefin having three or more carbon
atoms and having a --CH.dbd.CH.sub.2 group.
[0025] "total draw ratio" means the product of the draw ratios in
the machine and transverse directions, i.e=MD.times.CD.
BRIEF DESCRIPTION OF THE FIGURES
[0026] FIGS. 1 and 2 are schematics of processes for preparing the
foams prepared according to the present invention.
[0027] FIG. 3 is a digital image of a scanning electron micrograph
(SEM) of a front elevation of the foam of Comparative Example
1.
[0028] FIG. 4 is a digital image of a SEM of a side elevation of
the foam of Comparative Example 1.
[0029] FIG. 5 is a digital image of a SEM of a side elevation of
the foam of Comparative Example 3.
[0030] FIG. 6 is a digital image of a SEM of a side elevation of
the foam of Comparative Example 4.
[0031] FIG. 7 is a digital image of a SEM of a side elevation of
the foam of Example 5.
[0032] FIG. 8 is a digital image of a SEM of a side elevation of
the foam of Example 6.
[0033] FIG. 9 is a digital image of a SEM of a side elevation of
the unoriented foam of Example 9.
[0034] FIG. 10 is a digital image of a SEM of a side elevation of
the oriented foam of Example 9.
[0035] FIG. 11 is a digital image of a SEM of a side elevation of
the unoriented foam of Example 10.
[0036] FIG. 12 is a digital image of a SEM of a side elevation of
the oriented foam of Example 10.
DETAILED DESCRIPTION
[0037] The high melt strength polypropylene useful in the present
invention includes homo- and copolymers containing 50 weight
percent or more propylene monomer units, preferably at least 70
weight percent, and has a melt strength in the range of 25 to 60 cN
at 190.degree. C. Melt strength may be conveniently measured using
an extensional rheometer by extruding the polymer through a 2.1 mm
diameter capillary having a length of 41.9 mm at 190.degree. C. and
at a rate of 0.030 cc/sec; the strand is then stretched at a
constant rate while measuring the force to stretch at a particular
elongation. Preferably the melt strength of the polypropylene is in
the range of 30 to 55 cN, as described in WO 99/61520.
[0038] The melt strength of linear or straight chain polymers, such
as conventional isotactic polypropylene, decreases rapidly with
temperature. In contrast, the melt strength of highly branched
polypropylenes does not decrease rapidly with temperature. It is
generally believed that the differences in melt strengths and
extensional viscosity is attributable to the presence of long chain
branching. Useful polypropylene resins are those that are branched
or crosslinked. Such high melt strength polypropylenes may be
prepared by methods generally known in the art. Reference may be
made to U.S. Pat. No. 4,916,198 (Scheve et al) which describes a
high melt strength polypropylene having a chain-hardening
elongational viscosity prepared by irradiation of linear propylene
in a controlled oxygen environment. Other useful methods include
those in which compounds are added to the molten polypropylene to
introduce branching and/or crosslinking such as those methods
described in U.S. Pat. No. 4,714,716 (Park), WO 99/36466 (Moad, et
al.) and WO 00/00520 (Borve et al.). High melt strength
polypropylene may also be prepared by irradiation of the resin as
described in U.S. Pat. No. 5,605,936 (Denicola et al.). Still other
useful methods include forming a bipolar molecular weight
distribution as described in J. I. Raukola, A New Technology To
Manufacture Polypropylene Foam Sheet And Biaxially Oriented Foam
Film, VTT Publications 361, Technical Research Center of Finland,
1998 and in U.S. Pat. No. 4,940,736 (Alteepping and Nebe),
incorporated herein by reference.
[0039] The foamable polypropylene may be comprised solely of
propylene homopolymer or may comprise a copolymer having 50 wt % or
more propylene monomer content. Further, the foamable propylene may
comprise a mixture or blend of propylene homopolymers or copolymers
with a homo- or copolymer other than propylene homo- or
copolymers.
[0040] Particularly useful propylene copolymers are those of
propylene and one or more non-propylenic monomers. Propylene
copolymers include random, block, and grafted copolymers of
propylene and olefin monomers selected from the group consisting of
ethylene, C3-C8 .alpha.-olefins and C4-C10 dienes. Propylene
copolymers may also include terpolymers of propylene and
.alpha.-olefins selected from the group consisting of C3-C8
.alpha.-olefins, wherein the .alpha.-olefin content of such
terpolymers is preferably less than 45 wt %. The C3-C8
.alpha.-olefins include 1-butene, isobutylene, 1-pentene,
3-methyl-1-butene, 1-hexene, 3,4-dimethyl-1-butene, 1-heptene,
3-methyl-1-hexene, and the like. Examples of C4-C10 dienes include
1,3-butadiene, 1,4-pentadiene, isoprene, 1,5-hexadiene,
2,3-dimethyl hexadiene and the like.
[0041] Minor amounts (50 percent or less by weight) of other
semicrystalline polymers that may be added to the high melt
strength polypropylene in the foamable composition include high,
medium, low and linear low density polyethylene, fluoropolymers,
poly(1-butene), ethylene/acrylic acid copolymer, ethylene/vinyl
acetate copolymer, ethylene/propylene copolymer, styrene/butadiene
copolymer, ethylene/styrene copolymer, ethylene/ethyl acrylate
copolymer, ionomers and thermoplastic elastomers such as
styrene/ethylene/butylene/styrene (SEBS), and
ethylene/propylene/diene copolymer (EPDM).
[0042] Minor amounts (50 percent or less by weight) of amorphous
polymers may be added to the high melt strength polypropylene.
Suitable amorphous polymers include, e.g., polystyrenes,
polycarbonates, polyacrylics, polymethacrylics, elastomers, such as
styrenic block copolymers, e.g., styrene-isoprene-styrene (SIS),
styrene-ethylene/butylene-styrene block copolymers (SEBS),
polybutadiene, polyisoprene, polychloroprene, random and block
copolymers of styrene and dienes (e.g.,styrene-butadiene rubber
(SBR)), ethylene-propylene-diene monomer rubber, natural rubber,
ethylene propylene rubber, polyethylene-terephthalate (PETG). Other
examples of amorphous polymers include, e.g.,
polystyrene-polyethylene copolymers, polyvinylcyclohexane,
polyacrylonitrile, polyvinyl chloride, thermoplastic polyurethanes,
aromatic epoxies, amorphous polyesters, amorphous polyamides,
acrylonitrile-butadiene-styrene (ABS) copolymers, polyphenylene
oxide alloys, high impact polystyrene, polystyrene copolymers,
polymethylmethacrylate (PMMA), fluorinated elastomers, polydimethyl
siloxane, polyetherimides, amorphous fluoropolymers, amorphous
polyolefins, polyphenylene oxide, polyphenylene oxide--polystyrene
alloys, copolymers containing at least one amorphous component, and
mixtures thereof.
[0043] An extrusion process using a single-screw, double-screw or
tandem extrusion system may prepare the foams of the present
invention. This process involves mixing one or more high melt
strength propylene polymers (and any optional polymers to form a
propylene polymer blend) with a blowing agent, e.g., a physical or
chemical blowing agent, and heating to form a melt mixture. The
temperature and pressure conditions in the extrusion system are
preferably sufficient to maintain the polymeric material and
blowing agent as a homogeneous solution or dispersion. Preferably,
the polymeric materials are foamed at no more than 30.degree. C.
above the melting temperature of the neat polypropylene thereby
producing desirable properties such as uniform and/or small cell
sizes.
[0044] When a physical blowing agent, such as CO.sub.2 is used, the
neat polymer is initially maintained above the melting temperature.
The physical blowing agent is injected (or otherwise mixed) with
the molten polymer and the melt mixture is cooled in the extruder
to an exit temperature that is less than 30.degree. C. above the
melting temperature of the neat polymer
(T.ltoreq.T.sub.m+30.degree. C.) while the pressure is maintained
at or above 2000 psi (13.8 MPa). Under these conditions the
polymer/blowing agent remains a single phase. As the melt mixture
passes through the exit/shaping die the melt rapidly foams and
expands, generating foams with small, uniform cell sizes. It has
been found that by adding a physical blowing agent, the
polypropylene may be processed and foamed at temperatures
considerably lower than otherwise might be required. The blowing
agent may plasticize, i.e., lower the T.sub.m of, the polymeric
material. The lower temperature can allow the foam to cool and
stabilize soon after it exits the die, thereby making it easier to
arrest cell growth and coalescence while the cells are smaller and
more uniform.
[0045] When a chemical blowing agent is used, the blowing agent is
added to the neat polymer, mixed, heated to a temperature above the
T.sub.m of the polypropylene to ensure intimate mixing and further
heated to the activation temperature of the chemical blowing agent,
resulting in decomposition of the blowing agent. The temperature
and pressure of the system are controlled to maintain substantially
a single phase. The gas formed on activation is substantially
dissolved or dispersed in the melt mixture. The resulting single
phase mixture is cooled to a temperature no more than 30.degree. C.
above the melting temperature of the neat polymer, while the
pressure is maintained at or above 2000 psi (13.8 MPa), by passing
the mixture through a cooling zone(s) in the extruder prior to the
exit/shaping die. Generally the chemical blowing agent is dry
blended with the neat polymer prior to introduction to the
extruder, such as in a mixing hopper.
[0046] With either a chemical or physical blowing agent, as the
melt mixture exits the extruder through a shaping die, it is
exposed to the much lower atmospheric pressure causing the blowing
agent (or its decomposition products) to expand. This causes cell
formation resulting in foaming of the melt mixture. When the melt
mixture exit temperature is at or below 30.degree. C. above the
T.sub.m of the neat polypropylene, the increase in T.sub.m of the
polymer as the blowing agent comes out of the solution causes
crystallization of the polypropylene, which in turn arrests the
growth and coalescense of the foam cells within seconds or, most
typically, a fraction of a second. This preferably results in the
formation of small and uniform voids in the polymeric material.
When the exit temperature is no more than 30.degree. C. above the
T.sub.m of the neat polypropylene, the extensional viscosity of the
polymer increases as the blowing agent comes out of the solution
and the polypropylene rapidly crystallizes. These factors arrest
the growth and coalescense of the foam cells within seconds or,
most typically, a fraction of a second. Preferably, under these
conditions, the formation of small and uniform cells in the
polymeric material occurs. When exit temperatures are in excess of
30.degree. C. above the T.sub.m of the neat polymer, cooling of the
polymeric material may take longer, resulting in non-uniform,
unarrested cell growth. In addition to the increase in T.sub.m,
adiabatic cooling of the foam may occur as the blowing agent
expands.
[0047] Either a physical or chemical blowing agent may plasticize,
i.e., lower the T.sub.m and T.sub.g of, the polymeric material.
With the addition of a blowing agent, the melt mixture may be
processed and foamed at temperatures considerably lower than
otherwise might be required, and in some cases may be processed
below the melt temperature of the polypropylene. The lower
temperature can allow the foam to cool and stabilize (i.e., reach a
point of sufficient solidification to arrest further cell growth
and produce smaller and more uniform cell sizes.
[0048] Physical blowing agents useful in the present invention may
be any materials that are a vapor at the temperature and pressure
at which the foam exits the die. The physical blowing agent may be
introduced, i.e., injected into the polymeric material as a gas, a
supercritical fluid, or liquid, preferably as a supercritical fluid
or liquid, most preferably as a liquid. The physical blowing agents
used will depend on the properties sought in the resulting foam
articles. Other factors considered in choosing a blowing agent are
its toxicity, vapor pressure profile, ease of handling, and
solubility with regard to the polymeric materials used. Flammable
blowing agents such as pentane, butane and other organic materials
may be used, but non-flammable, non-toxic, non-ozone depleting
blowing agents such as carbon dioxide, nitrogen, water, SF.sub.6,
nitrous oxide, argon, helium, noble gases, such as xenon, air
(nitrogen and oxygen blend), and blends of these materials are
preferred because they are easier to use, e.g., fewer environmental
and safety concerns. Other suitable physical blowing agents
include, e.g., hydrofluorocarbons (HFC), hydrochlorofluorocarbons
(HCFC), and fully- or partially fluorinated ethers.
[0049] Chemical blowing agents are added to the polymer at a
temperature below that of the decomposition temperature of the
blowing agent, and are typically added to the polymer feed at room
temperature prior to introduction to the extruder. The blowing
agent is then mixed to distribute it throughout the polymer in
undecomposed form, above the melt temperature of the polypropylene,
but below the activation temperature of the chemical blowing agent.
Once dispersed, the chemical blowing agent may be activated by
heating the mixture to a temperature above its decomposition
temperature of the agent. Decomposition of the blowing agent
liberates gas, such as N.sub.2, CO.sub.2 and/or H.sub.2O, yet cell
formation is restrained by the temperature and pressure of the
system. Useful chemical blowing agents typically decompose at a
temperature of 140.degree. C. or above.
[0050] Examples of such materials include synthetic azo-,
carbonate-, and hydrazide-based molecules, including
azodicarbonamide, azodiisobutyronitrile, benzenesulfonhydrazide,
4,4-oxybenzene sulfonyl-semicarbazide, p-toluene sulfonyl
semi-carbazide, barium azodicarboxylate,
N,N'-dimethyl-N,N'-dinitrosoterephthalamide and trihydrazino
triazine. Specific examples of these materials are Celogen OT (4,4'
oxybis (benzenesulfonylhydrazide), Hydrocerol BIF (preparations of
carbonate compounds and polycarbonic acids), Celogen AZ
(azodicarbonamide) and Celogen RA (p-toluenesulfonyl
semicarbazide).
[0051] The amount of blowing agent incorporated into the foamable
polymer mixture is chosen to yield a foam having a void content in
excess of 10%, more preferably in excess of 20%, as measured by
density reduction; [1-the ratio of the density of the foam to that
of the neat polymer].times.100. Generally, greater foam void
content reduces the foam density, weight and material costs for
subsequent end uses.
[0052] FIG. 1 illustrates a tandem extrusion apparatus 10 that may
be used to make the foams of the present invention, and is a
preferred process for use with a physical blowing agent. To form a
melt mixture, polymeric material is initially fed from hopper 12
into a first extruder 14 that melts and conveys the polymeric
material. The polymeric material may be added to extruder 14 in any
convenient form. Additives are typically added with the polymer
material but may be added further downstream. The blowing agent,
typically in a liquid or supercritical form, is injected near the
exit of the first extruder. Due to the conditions in the extruder,
the blowing agent is typically in a supercritical state while in
the extruder.
[0053] The polymers, additives, and blowing agent are melt-mixed in
first extruder 14. The physical blowing agent is typically
introduced by injection at some intermediate stage of extruder 14
by means of fluid handling equipment 16. The purpose of the
melt-mixing step is to prepare a foamable, extrudable composition
in which the blowing agent and other additives, to the extent
present, are distributed homogeneously throughout the molten
polymeric material. Specific operating conditions are selected to
achieve such homogeneous distribution based upon the properties and
characteristics of the particular composition being processed. The
operating and exit pressures in extruder 14 should be sufficient to
prevent the blowing agent from expanding in the extruder. The
operating temperature in the extruder 14 should be sufficient to
melt and/or soften all of the polymers in the melt mixture.
[0054] Next, the melt mix is fed to a second extruder 20 (typically
a single screw extruder) by means of conduit 18. Extruder 20 is
generally operated at conditions (e.g., screw speed, screw length,
pressure, and temperature) selected to achieve optimum mixing, and
to keep the blowing agent in solution. Extruder 20 typically has a
decreasing temperature profile wherein the temperature of the last
zone or zones will bring the melt solution to the desired exit
temperature.
[0055] At the exit end of extruder 20, the foamable, extrudable
composition is metered into die 22 which has a shaping/exit orifice
(e.g., an annular, rod, slit die, or shaped profile die). The
temperature within die 22 is preferably maintained at substantially
the same temperature as the last zone of extruder 20; i.e., at the
exit temperature. The relatively high pressure within extruder 20
and die 22 prevents cell formation and foaming of the melt mix
composition in the extruder and die. Exit pressure is dependent
upon die orifice size, exit temperature, blowing agent
concentration, polymer flowrate, polymer viscosity, screw speed and
polymer type. Exit pressure is typically controlled by adjusting
the die orifice size, but can also be adjusted by altering the exit
temperature, blowing agent concentration, and other variables.
Reducing the size of the die orifice will generally increase exit
pressure. As the composition exits die 22 through the die's shaping
orifice, it is exposed to ambient pressure. The pressure drop
causes the blowing agent to expand, leading to cell formation. Foam
24 is typically quenched, i.e., brought to a temperature below the
T.sub.m of the polypropylene, within two to five centimeters of the
die exit, more typically and preferably less than two centimeters,
as the foamable material exits the die and is exposed to ambient
pressure.
[0056] The shape of the die exit orifice dictates the shape of foam
24. A variety of shapes may be produced, including a continuous
sheet, including sheets with patterned profiles, a tube, a rope,
etc.
[0057] In general, as the blowing agent separates from the melt
mixture, its plasticizing effect on the polymeric material
decreases and the extensional viscosity of the polymeric material
increases. The viscosity increase is much sharper at the T.sub.m
than at the T.sub.g, making the choice of foaming temperatures for
semicrystalline polymers much more stringent than for amorphous
polymers. As the temperature of the polymeric material approaches
the T.sub.m of the neat polymer and becomes more viscous, the cells
cannot as easily expand or coalesce. As the foam material cools
further, it solidifies in the general shape of the exit shaping
orifice of die 22.
[0058] FIG. 2 illustrates a single stage extrusion apparatus 40
that can be used to make the foams of the present invention, and is
the preferred process for use with chemical blowing agents. A twin
screw extruder 44 (as depicted) may be used to form a melt mixture
of the polypropylene and blowing agent, although it will be
understood that a single screw extruder may also be used. The
polypropylene is introduced into zone 1 of extruder 44 by means of
hopper 42. Chemical blowing agents are typically added with the
polymer but may be added further downstream. A physical blowing
agent may be added using fluid handling means 46 at a location
downstream from a point at which the polymer has melted.
[0059] When a physical blowing agent is used, the extruder 44 may
be operated with a generally decreasing temperature profile. If a
chemical blowing agent is used, an intermediate zone is generally
maintained at an elevated temperature sufficient to initiate the
chemical blowing agent, followed by subsequent cooler zones. The
temperature of the initial zone(s) of the extruder must be
sufficient to melt the polypropylene and provide a homogenous melt
mixture with the blowing agent. The final zone or zones of the
extruder are set to achieve the desired extrudate exit
temperature.
[0060] Using a single stage extrusion process, as compared to using
a tandem process, to produce a homogeneous foamable mixture
requires mixing and transitioning from an operating temperature and
pressure to an exit temperature and pressure over a shorter
distance. To achieve a suitable melt mix, approximately the first
half of the extruder screw may have mixing and conveying elements
which knead the polymer and move it through the extruder. The
second half of the screw may have distributive mixing elements to
mix the polymer material and blowing agent into a homogeneous
mixture while cooling.
[0061] As with the tandem process, the operating and exit pressures
(and temperatures) should be sufficient to prevent the blowing
agent from causing cell formation in the extruder. The operating
temperature is preferably sufficient to melt the polymer materials,
while the last zone or zones of the extruder are preferably at a
temperature that will bring the extrudate to the exit
temperature.
[0062] At the exit end of the extruder, the foamable, extrudable
composition is metered into a die 48 having a shaping exit orifice.
The foam is generated in the same manner as with the tandem
system.
[0063] The blowing agent concentrations, exit pressure, and exit
temperature can have a significant effect on the properties of the
resulting foams including foam density, cell size, and distribution
of cell sizes. In general, the lower the exit temperature, the more
uniform, and smaller, the cell sizes of the foamed material. This
is because at lower exit temperatures, the extensional viscosity is
higher, yielding slower cell growth. Extruding the material at
lower than normal extrusion temperatures, i.e. no more than
30.degree. C. above the T.sub.m of the neat polymeric material,
produces foams with small, uniform cell sizes.
[0064] In general, as the melt mixture exits the die, it is
preferable to have a large pressure drop over a short distance.
Keeping the solution at a relatively high pressure until it exits
the die helps to form uniform cell sizes. Maintaining a large
pressure drop between the exit pressure and ambient pressure can
also contribute to the quick foaming of a melt mixture. The lower
limit for forming a foam with uniform cells will depend on the
critical pressure of the blowing agent being used. In general, for
the high melt strength polypropylene useful in the invention, the
lower exit pressure limit for forming acceptably uniform cells is
approximately 7 MPa (1000 psi), preferably 10 MPa (1500 psi), more
preferably 14 MPa (2000 psi). The smallest cell sizes may be
produced at low exit temperatures and high blowing agent
concentrations. However at any given temperature and pressure,
there is a blowing agent concentration at and above which
polydispersity will increase because the polymer becomes
supersaturated with blowing agent and a two phase system is
formed.
[0065] The optimum exit temperature, exit pressure, and blowing
agent concentration for a particular melt mixture will depend on a
number of factors such as the type and amount of polymer(s) used;
the physical properties of the polymers, including viscosity; the
mutual solubility of the polymer(s) and the blowing agent; the type
and amount of additives used; the thickness of the foam to be
produced; the desired density and cell size; whether the foam will
be coextruded with another foam or an unfoamed material; and the
die gap and die orifice design.
[0066] The present invention provides foams having average cell
sizes less than 100 micrometers, and advantageously may provide
foams having average cell sizes less than 50 micrometers.
Additionally the foams produced have a closed cell content of 70
percent or greater. As result of extrusion, the cells may be
elongated in the machine direction.
[0067] In order to optimize the physical properties of the foam,
the polymer chains need to be oriented along at least one major
axis (uniaxial), and may further be oriented along two major axes
(biaxial). The degree of molecular orientation is generally defined
by the draw ratio, that is, the ratio of the final length to the
original length.
[0068] Upon orientation, greater crystallinity is imparted to the
polypropylene component of the foam and the dimensions of the foam
cells change. Typical cells have major directions X and Y,
proportional to the degree of orientation in the machine and
transverse direction respectively. A minor direction Z, normal to
the plane of the foam, remains substantially the same as (or may be
moderately less than) the cross-sectional dimension of the cell
prior to orientation.
[0069] The conditions for orientation are chosen such that the
integrity of the foam is maintained. Thus when stretching in the
machine and/or transverse directions, the orientation temperature
is chosen such that substantial tearing or fragmentation of the
continuous phase is avoided and foam integrity is maintained. The
foam is particularly vulnerable to tearing, cell rupture or even
catastrophic failure if the orientation temperature is too low or
the orientation ratio(s) is/are excessively high. Generally the
foam is oriented at a temperature between the glass transition
temperature and the melting temperature of the neat polypropylene.
Preferably, the orientation temperature is above the alpha
transition temperature of the neat polymer. Such temperature
conditions permit optimum orientation in the X and Y directions
without loss of foam integrity.
[0070] Unexpectedly, it has been found that orienting the foam
reduces the density of the foam, thus enabling the production of
lower density foams than are achievable using blowing agents alone.
Up to a 60% reduction in density has been observed. There is a
practical limit to the amount of blowing agent that can be used to
prepare foams, particularly chemical blowing agents that leave an
organic residue in the polymer matrix on decomposition. The instant
invention provides the additional benefits to the foamed article
such as lower manufacturing and materials costs, better insulation
properties, greater surface area, ease of manufacturing, enhanced
diffuse reflectivity, reduced dielectric constant, controlled
tearability and increased tensile strength on a weight basis.
[0071] After orientation the cells are relatively planar in shape
and have distinct boundaries. Cells are generally coplanar with the
major surfaces of the foam, with major axes in the machine (X) and
transverse (Y) directions (directions of orientation). The sizes of
the cells are uniform and proportional to concentration of blowing
agent, extrusion conditions and degree of orientation. The
percentage of closed cells does not change significantly after
orientation when using high melt strength polypropylene. In
contrast, orientation of conventional polypropylene foam results in
cell collapse and tearing of the foam, reducing the percentage of
closed cells. Cell size, distribution and amount in the foam matrix
may be determined by techniques such as scanning electron
microscopy.
[0072] In the orienting step, the foam is stretched in the machine
direction and may be simultaneously or sequentially stretched in
the transverse direction. When first stretched in the machine
direction, the individual fibrils of the spherulites of the
polypropylene are drawn substantially parallel to the machine
direction (direction of orientation) of the film and in the plane
of the film. The oriented fibrils can be visualized as having a
rope-like appearance. Subsequent or further orientation of the film
in the transverse direction results in reorientation of the
fibrils, again in the plane of the film, with varying populations
along the X, Y and intermediate axes, depending on the degree of
orientation in the machine and transverse directions.
[0073] The stretching conditions are chosen to increase the
crystallinity of the polymer matrix and the void volume of the
foam. It has been found that an oriented foam has significantly
enhanced tensile strength, even with a relatively low void content
when compared to unoriented foams.
[0074] The foam may be biaxially oriented by stretching in mutually
perpendicular directions at a temperature above the alpha
transition temperature and below the melting temperature of the
polypropylene. Generally, the film is stretched in one direction
first and then in a second direction perpendicular to the first.
However, stretching may be effected in both directions
simultaneously if desired. If biaxial orientation is desired, it is
preferable to simultaneously orient the foam, rather than
sequentially orient the foam along the two major axes. It has been
found that simultaneous biaxial orientation provides greater
density reduction as well as improved physical properties such as
tensile strength as compared to sequential biaxial orientation.
Simultaneous biaxial orientation also provides more isotropic
tensile tear properties. Advantageously, simultaneous orientation
provides an oriented high melt strength polypropylene foam wherein
the tensile strength of the foam in the cross direction is within
30% of the tensile strength in the machine direction. Multilayer
articles comprising the simultaneous biaxially oriented foam are
also within the scope of the invention.
[0075] In a typical sequential orientation process, the film is
stretched first in the direction of extrusion over a set of
rotating rollers is then stretched in the direction transverse
thereto by means of a tenter apparatus. Alternatively, foams may be
stretched in both the machine and transverse directions in a tenter
apparatus. Foams may be stretched in one or both directions 3 to 50
times total draw ratio (MD.times.CD). Generally greater orientation
is achievable using foams of small cell size; foams having cell
size of greater than 100 micrometers are not readily oriented more
than 20 times, while foams having a cell size of 50 micrometers or
less could be stretched up to 50 times total draw ratio. In
addition foams with small average cell size exhibit greater tensile
strength and elongation to break after stretching.
[0076] The temperature of the polymer foam during the first
orientation (or stretching) step affects foam properties.
Generally, the first orientation step is in the machine direction.
Orientation temperature may be controlled by the temperature of
heated rolls or by the addition of radiant energy, e.g., by
infrared lamps, as is known in the art. A combination of
temperature control methods may be utilized. Too low an orientation
temperature may result in tearing the foam and rupturing of the
cells. Orientation is generally conducted at temperatures between
the glass transition temperature and the melting temperature of the
neat polypropylene, or at about 110-170.degree. C., preferably
110-140.degree. C. A second orientation, in a direction
perpendicular to the first orientation may be desired. The
temperature of such second orientation is generally similar to or
higher than the temperature of the first orientation.
[0077] After the foam has been stretched it may be further
processed. For example, the foam may be annealed or heat-set by
subjecting the foam to a temperature sufficient to further
crystallize the polypropylene while restraining the foam against
retraction in both directions of stretching.
[0078] The final thickness of the foam will be determined in part
by the extrusion thickness, the degree of orientation, and any
additional processing. The present invention provides thinner foams
than are generally achievable by prior art processes. Most foams
are limited in thickness by the cell size. In the present
invention, the small cell sizes (<50 micrometers) in combination
with the orientation allows the thickness of 2 to 100 mils (0.05 to
0.25 mm) or less, and foams of 10 to 80 mils (0.025 to 0.2 mm) are
readily prepared.
[0079] The present invention may be used to produce multilayer
articles comprising at least one high melt strength foam layer. The
foams of the present invention may be coextruded with materials
having substantially higher or lower processing temperatures from
that of the foam, while still obtaining the desired structures and
cell sizes. It would be expected that exposing the foam to an
adjacent hot polymer as it is extruded, might cause the foam cells,
especially those in direct contact with the hotter material, to
continue to grow and coalesce beyond their desired sizes or might
cause the foam material to melt or collapse.
[0080] The coextrusion process of the present invention may be used
to make a foam material comprising two layers or more. A layered
material or article may be produced by equipping die 22 or 50 with
an appropriate feed block, e.g., a multilayer feedblock, or by
using a multi-vaned or multi-manifold die such as a 3-layer vane
die available from Cloeren, Orange, Tex. Materials or articles
having multiple adjacent foam layers may be made with foam layers
comprising the same or different materials. Foam articles of the
present invention may comprise one or more interior and/or exterior
foam layer(s). In such a case, each extrudable material, including
the high melt strength polypropylene foamable material, may be
processed using one of the above-described extrusion methods
wherein melt mixtures are fed to different inlets on a multi-layer
feedblock, or multi-manifold die, and are brought together prior to
exiting the die. The layers foam in generally the same manner as
described above for the extrusion process. The multi-layer process
can also be used to extrude the foam of this invention with other
types of materials such as thermoplastic films and adhesives. When
a multi-layered article is produced, it is preferable to form
adjacent layers using materials having similar viscosities and
which provide interlayer adhesion. When the multilayer article
comprises a foam layer and a film layer (on one or both surfaces),
greater degrees of orientation, improved tensile properties, and
smaller cell size are possible than with single layer foam.
[0081] If adjacent layers of materials are heated to substantially
different temperatures, a die can be used that thermally isolates
the different materials until just prior to their exiting the die,
for example the die disclosed in FIG. 4 of U.S. Pat. No. 5,599,602,
incorporated by reference. This can diminish or eliminate negative
effects of contacting the different materials such as melting or
collapsing the foam or causing continued cell expansion
coalescense.
[0082] Multilayer foam articles can also be prepared by laminating
polymer or nonpolymer layers to a foam core, or by layering
extruded foams as they exit their respective shaping orifices, with
the use of some affixing means such as an adhesive. Useful
laminated constructions include the high melt strength
polypropylene foam layer with a thermoplastic film layer or a scrim
layer, such as a non-woven layer. Other techniques that can be used
include extrusion coating and inclusion coextrusion, which is
described in U.S. Pat. No. 5,429,856, incorporated by reference.
The multilayer article may be oriented as previously described.
[0083] Oriented foams are especially useful as tape backings or
straps to yield an extremely strong tape due to the high modulus
and tensile strength of the polymer matrix. When used as a tape
backing, the film can be coated with any conventional hot melt,
solvent coated, or like adhesive suitable for application to films.
Either or both surfaces may be coated. Advantageously, when using a
biaxially oriented foam of the present invention, the adhesive
tapes prepared therefrom may be easily torn in either the
longitudinal or transverse direction.
[0084] Many types of adhesives can be used. The adhesive can
include hot melt-coated formulations, transfer-coated formulations,
solvent-coated formulations, water-based, and latex formulations,
as well as laminating, thermally-activated, and water-activated
adhesives. These adhesives can be applied by conventional
techniques, such as solvent coating by methods such as reverse
roll, knife-over-roll, gravure, wire wound rod, floating knife or
air knife, hot-melt coating such as; by slot orifice coaters, roll
coaters or extrusion coaters, at appropriate coating weights.
[0085] Examples of adhesives useful in the invention include those
based on general compositions of polyacrylate; polyvinyl ether;
diene-containing rubber such as natural rubber, polyisoprene, and
polyisobutylene; polychloroprene; butyl rubber;
butadiene-acrylonitrile polymer; thermoplastic elastomer; block
copolymers such as styrene-isoprene and styrene-isoprene-styrene
block copolymers, ethylene-propylene-diene polymers, and
styrene-butadiene polymer; poly-alpha-olefin; amorphous polyolefin;
silicone; ethylene-containing copolymer such as ethylene vinyl
acetate, ethylacrylate, and ethyl methacrylate; polyurethane;
polyamide; epoxy; polyvinylpyrrolidone and vinylpyrrolidone
copolymers; polyesters; and mixtures of the above. Additionally,
the adhesives can contain additives such as tackifiers,
plasticizers, fillers, antioxidants, stabilizers, pigments,
diffusing particles, curatives, and solvents.
[0086] Useful adhesives according to the present invention can be
pressure sensitive adhesives. Pressure sensitive adhesives are
normally tacky at room temperature and can be adhered to a surface
by application of, at most, light finger pressure. A general
description of useful pressure sensitive adhesives may be found in
Encyclopedia of Polymer Science and Engineering, Vol. 13,
Wiley-Interscience Publishers (New York, 1988). Additional
description of useful pressure sensitive adhesives may be found in
Encyclopedia of Polymer Science and Technology, Vol. 1,
Interscience Publishers (New York, 1964).
[0087] When a pressure sensitive adhesive is coated onto one side
of the backing and a release coating (a low adhesion backsize (LAB)
coating) is optionally coated on the opposite side to allow the
resultant tape to unwind from itself when wound in a roll or
release when in a pad form.
[0088] When utilized, the release coating composition should be
compatible with the adhesive composition and not degrade the
adhesive properties of the tape, such as by being transferred to
the adhesive composition.
[0089] Release coating compositions for the LAB layer of tapes may
include silicone, alkyl, or fluorochemical constituents, or
combinations as the release imparting component. Useful release
coating compositions for the invention include silicone containing
polymers, such as silicone polyurethanes, silicone polyureas and
silicone polyurethane/ureas, such as those described in U.S. Pat.
Nos. 5,214,119, 5,290,615, 5,750,630, and 5,356,706, and silicone
acrylate grafted copolymers described in U.S. Pat. Nos. 5,032,460,
5,202,190, and 4,728,571. Other useful release coating compositions
include fluorochemical containing polymers such as those described
in U.S. Pat. No. 3,318,852, and polymers containing long alkyl side
chains such as polyvinyl N-alkyl carbamates (e.g., polyvinyl
N-octadecyl carbamates) as described in U.S. Pat. No. 2,532,011,
and copolymers containing higher alkyl acrylates (e.g., octadecyl
acrylate or behenyl acrylate), such as those described in U.S. Pat.
No. 2,607,711, or alkyl methacrylates (e.g., stearyl methacrylate)
such as those described in U.S. Pat. Nos. 3,502,497 and 4,241,198,
where the alkyl side chain includes from about 16 to 22 carbon
atoms.
[0090] These release polymers can be blended with each other and
with thermosetting resins or thermoplastic film forming polymers to
form the release coating composition. In addition, other additives
may be used in the release coating compositions such as fillers,
pigments, wetting agents, viscosity modifiers, stabilizers,
anti-oxidants, and cross-linking agents.
[0091] Numerous other layers can be added to the tape, such as
primers to increase adhesive layer adhesion to the backing layer.
Also, the release properties of the backing can be modified such
that the backing and the adhesive cooperate to achieve desired
unwind characteristics. The release properties of the backing can
be modified by applying a low surface energy composition, priming,
corona discharge, flame treatment, roughening, etching, and
combinations.
EXAMPLES
Test Methods
[0092] Foam Density (ASTM D792-86)
[0093] Foam samples were cut into 12.5 mm.times.12.5 mm specimens
and weighed on a high precision balance available as Model AG245
from Mettler-Toledo, Greifensee, Switzerland. The volume of each
sample was obtained by measuring the mass of water displaced at
room temperature (25.+-.1.degree. C.). Assuming the density of
water at 25.degree. C. to be 1 g/cm.sup.3, the volume of each
sample was calculated using Archimedes principle. The density of
the foam was obtained by the quotient of the mass and volume.
Accuracy of this measurement is .+-.0.01 g/cm.sup.3.
[0094] Foam Cell Size
[0095] Scanning electron microscopy was performed on all the foam
samples using a scanning electron microscope available as model
JSM-35C from JEOL USA, Inc., Peabody, Mass., operated at 5 and 10
kV. The samples were prepared by freezing in liquid nitrogen for
2-5 minutes and fracturing. A thin palladium-gold coating was
evaporated on the samples to develop a conductive surface. The
diameters of the foam cells were measured using the digital SEM
micrographs and UTHSCSA Image Tool for Windows Software (Version
1.28, University of Texas, San Antonio, Tex.). The diameters of
over 100 cells were measured and recorded. The average cell
dimension was calculated using the Image Tool Software.
[0096] Mechanical Properties
[0097] Mechanical properties of the foams were measured in tensile
mode at approximately 23.degree. C. using an Instron Testing Device
(Instron Corp., Canton, Mass.). The samples were first conditioned
at 23.degree. C. and 50% humidity for two days. The samples were
then made using a dogbone die with a testing region dimensions of
2.54 cm.times.0.32 cm and the thickness of the specimens was
measured and recorded. 3M fiber tape was used around the
non-testing region of the dogbone to provide better grip in the
clip. The samples were tested at tensile rates of 25.4 cm/min until
failure and the stress was measured as a function of elongation.
Average values are reported.
Example 1
[0098] A melt mixture of high melt strength polypropylene
(PF814.TM., Montell North America, Inc., Wilmington, Del.; melt
flow of approximately 2 to 4 dg/min.) and 2% by weight of a
chemical blowing agent (RIC-50.TM., citric acid/sodium bicarbonate
mixture from Reedy International Corp., Keyport, N.J.) was prepared
in a 1.25" (3.2 cm) single screw extruder (Killion Extruders Div.
of Davis-Standard Corp., Cedar Grove, N.J.) equipped with a Saxton
single stage screw at 40 rpm and a temperature profile from 160 to
221 to 182.degree. C. The exit temperature was 180.degree. C.,
creating an exit pressure of 13.8 MPa. The melt mixture was
extruded through a 15.2 cm foam die (Extrusion Dies Inc., Canfield,
Ohio) and the resulting foam sheet was cooled on a chrome cast roll
at 150.degree. F. (65.5.degree. C.), then collected at a draw rate
of 1.5 m/min. The foam was white and opaque, and had a density of
0.56 g/cc at a thickness of 0.6 mm. As shown in FIGS. 3 and 4,
cells of the foam were slightly elongated in the machine direction
(MD) and range in average dimension between 50 and 200 .mu.m,
generally greater than 100 .mu.m. Simultaneous biaxial orientation
of the foam (3.5.times. by 3.5.times.) in a laboratory-scale batch
orienter at approximately 135-140.degree. C. provided a silvery,
diffusely reflective material with a density of 0.37 g/cc.
Example 2 (Comparative)
[0099] A foam sample was prepared from a lower melt strength
isotactic polypropylene having a melt flow of approximately 2 to 4
dg/min (PP3374.TM., Fina Inc., Dallas, Tex.). The polymer was mixed
in a 25 mm diameter twin screw extruder (Berstorff Corp., Florence,
Ky.) at 80 rpm with 2% by weight of azodicarbonamide blowing agent
(FM1307H.TM., available from Ampacet Co., Cincinnati, Ohio).
Temperature and pressure profiles and casting conditions were
essentially identical to those described in Example 1. The
resultant yellowish opaque foam had a thickness of 0.7 mm and a
density of 0.66 g/cc. Simultaneous biaxial orientation of the foam
(4.times. by 4.times.) in a laboratory-scale batch orienter at
approximately 135-140.degree. C. provided a much less opaque
material with a density of 0.62 g/cc. Significant cell collapse was
observed after orientation. When oriented at 2.5.times. by
2.5.times., the foam density was 0.54 g/cc.
Example 3 (Comparative)
[0100] A melt mixture of high melt strength polypropylene
(PF814.TM.) and 3% by weight of FM1307H.TM. chemical blowing agent
was prepared in a 2.5 inch (6.3 cm) single screw extruder
(Davis-Standard) equipped with a Saxton single stage screw at 45
rpm and a temperature profile from 127 to 216 to 204.degree. C. The
exit temperature was 188.degree. C., creating an exit pressure of
9.0 MPa. The melt mixture was extruded through a 25.4 cm foam die
(Extrusion Dies Inc., Canfield, Ohio) and the resulting foam sheet
was cooled on a chrome cast roll at 18.degree. C., then collected
at a draw rate of 9 m/min. The foam had a density of 0.37 g/cc at a
thickness of 0.9 mm. As shown in FIG. 5, cells of the foam were
noticeably elongated in the machine direction, approximately
100-150 .mu.m, and large in general, measuring 60-80 .mu.m in CD.
Attempts at orienting this low-density, large cell foam were
unsuccessful.
Example 4 (Comparative)
[0101] The procedure of Example 3 was repeated using only 1% by
weight chemical blowing agent, in an attempt to increase foam
density. The foam was drawn at 4.5 m/min. The resulting foam had a
density of 0.60 g/cc and a thickness of 1.13 mm. As shown in FIG.
6, cells of the foam were non-uniform in size and shape, were not
evenly distributed, and measured approximately 60 -100 .mu.m in
size.
Example 5
[0102] A foam was prepared as described in Example 3, except that
the exit temperature was lowered to 141.degree. C., creating an
exit pressure of 12.4 MPa. The foam was drawn at 3.1 m/min. The
unoriented foam, shown in FIG. 7, had a density of 0.68 g/cc and a
thickness of 1.1 mm, with cells of uniform size, approximately
30-50 .mu.m. The foam was oriented at 130.degree. C. by 3.5.times.
stretching in the machine direction using a multi-roll length
orienter (LO) followed by 3.times. stretching in the transverse
direction in a tenter oven at about 145.degree. C. The oriented
foam was found to have a tensile strength at break of approximately
5.0 kpsi at 27% elongation in the MD and a tensile strength at
break of approximately 2.1 kpsi at 12% elongation in the CD. The
oriented foam had a density of 0.46 g/cc.
Example 6
[0103] A foam was prepared from high melt strength polypropylene
(Montell PF814.TM.) using 3% by weight FM1307H chemical blowing
agent in a 25 mm Berstorff twin screw extruder operated at 82 rpm.
The temperature profile in the extruder was from 160 to 235 to
180.degree. C. (175.degree. C. exit temperature), creating an exit
pressure of 13.8 MPa. The melt mixed polymer was extruded through a
15 cm foam die and the extrudate was nipped between two chrome cast
rolls, each at 77.degree. C., at 276 kPa, then collected at a draw
rate of 3 m/min. The resulting foam, shown in FIG. 8, had a density
of 0.56 g/cc and a thickness of 0.7 mm, with cells that were
uniform in size (30-50 .mu.m) and significantly elongated in the
MD.
[0104] The foam was subjected to orientation of three types:
sequential biaxial, simultaneous biaxial, and uniaxial. Properties
of the foams resulting from equal simultaneous biaxial and uniaxial
draw are shown in Table 1. In the Table, draw ratios are reported
as Total Draw (TD), calculated as the product of (MD.times.CD).
1TABLE 1 Simultaneous density, density, Sample Biaxial TD g/cc
Sample Uniaxial TD g/cc 6-1 1 0.56 6-1 1 0.56 6-2 4 0.38 6-7 3.5
0.49 6-3 6.25 0.36 6-8 4 0.48 6-4 9 0.31 6-9 5 0.43 6-5 12.25 0.27
6-10 6 0.38 6-6 20.25 0.25
[0105] The data show that foams of the present invention exhibit
the unusual property of decreased density with increased draw or
orientation. Foams can withstand larger total draw when
simultaneous biaxially drawn, and, therefore, greater density
reduction on drawing. Note for Sample 6-4 even though the density
is significantly lower than that of the 2.5.times. by 2.5.times.
sample of Comparative Example 2, the mechanical properties are
improved. The densities of foam samples 6-11 and 6-12 were 0.24 and
0.22 g/cc, respectively.
[0106] Properties of the oriented foams are shown in Table 2.
2TABLE 2 MD tensile MD tensile CD tensile stress, strain, % CD
tensile strain, % Sample Orientation kpsi elongation stress, kpsi
elongation 6-4 Sim 3 .times. 3 5.0 42 4.8 30 6-9 Uni 5 .times. 1 11
26 1.4 10 6-11 Seq 3 .times. 3.5 6.5 37 4.7 24 6-12 Sim 4 .times. 4
6.4 33 6.0 28 Sim = simultaneous biaxial orientation Uni = uniaxial
orientation Seq = sequential biaxial orientation: MD followed by
CD
[0107] The data of Table 2 show that tensile strength increases
slightly as degree of orientation increases. Simultaneous biaxial
orientation provides a more isotropic foam than does sequential
biaxial orientation. Uniaxially oriented foams are quite strong in
the machine direction and are noticeably weaker in the cross
direction.
Example 7
[0108] Foams were prepared as described in Example 6, except that
6% by weight of FM1307H blowing agent was used, producing foams
having a lower density, 0.43 g/cc, and greater thickness, 0.9 mm.
The foam was subjected to either simultaneous biaxial orientation
or uniaxial orientation. Results are shown in Table 3.
3TABLE 3 Simultaneous density, density, Sample Biaxial TD g/cc
Sample Uniaxial TD g/cc 7-1 1 0.43 7-1 1 0.43 7-2 4 0.27 7-8 3 0.29
7-3 6.25 0.25 7-9 3.8 0.29 7-4 9 0.19 7-10 4.5 0.32 7-5 12.25 0.17
7-11 5.5 0.34 7-6 16 0.17 7-7 20.25 0.16
[0109] The data of Table 3 show that, while foam density reaches an
apparent minimum due to uniaxial orientation, no minimum was
reached due to biaxial orientation. Foams that have densities of
less than 0.2 g/cc resulting from the use of chemical blowing
agents (activated during extrusion) are very rare, but were easily
achieved by the method of the invention.
[0110] Tensile properties for two foams prepared by the method of
this example were obtained, shown in Table 4. The density of foam
sample 7-12 was 0.37 g/cc.
4TABLE 4 MD tensile MD tensile CD tensile stress, strain, % CD
tensile strain, % Sample Orientation kpsi elongation stress, kpsi
elongation 7-4 Sim 3 .times. 3 3.5 25 2.8 20 7-12 Uni 4 .times. 1
4.5 26 0.9 10 Sim = simultaneous biaxial orientation Uni = uniaxial
orientation
[0111] The data of Table 4 show that, in comparison to the data of
Table 2, foams with an initial lower density showed generally lower
tensile stress properties.
Example 8
[0112] A melt mixture consisting of 67% by weight of high melt
strength polypropylene (Montell PF814.TM.), 30% by weight of a
metallocene-polymerized semicrystalline polyethylene/polyoctene
copolymer (Engage.TM. 8200, DuPont Dow Elastomers LLC, Wilmington,
Del.), and 3% by weight chemical blowing agent (FM1307H) was
prepared in a 25 mm Berstorrf twin screw extruder operated at 82
rpm with a temperature profile from 160 to 235 to 162.degree. C.
(174.degree. C. exit temperature). The resulting foam was 0.75 mm
thick and had a density of 0.56 g/cc, and was observed to be much
softer and more pliable than previous unblended foams as described,
for example, in Example 6.
[0113] Orientation of the foamed, blended materials gave films
having properties shown in Table 5. The density of foam sample 8-2
was 0.41 g/cc.
5TABLE 5 MD tensile MD tensile CD tensile stress, strain, % CD
tensile strain, % Sample Orientation kpsi elongation stress, kpsi
elongation 8-1 Sim 3.5 .times. 4.0 32 2.8 18 3.5 8-2 Uni 5 .times.
1 6.7 25 0.77 23 Sim = simultaneous biaxial orientation Uni =
uniaxial orientation
[0114] The data of Table 5 can be compared to that of Table 2 to
show that foams prepared from polymer blends show significantly
lower tensile properties than those from homogeneous
compositions.
Example 9
[0115] A melt mixture identical in composition to that described in
Example 8 was prepared in a Davis-Standard 2.5" (6.3 cm) single
screw extruder operated at 40 rpm with a temperature profile of
from 127 to 216 to 204.degree. C. (175.degree. C. exit temperature)
creating an exit pressure of 13.8 MPa. Extrusion through a 25.4 cm
foam die onto a chrome cast roll chilled to 49.degree. C.
(collection draw rate of 3 m/min) produced a foam of 0.66 g/cc
density and 1.1 mm thickness. FIG. 9 represents an electron
micrograph of the unoriented foam, showing uniform cells of
approximately 30-50 .mu.m in size that are slightly elongated in
the MD.
[0116] The foam was oriented continuously as described in Example 5
at a rate of 3.times. in the MD followed by 2.3.times. in the CD.
FIG. 10 represents a micrograph of the oriented foam, showing
elongated cells in the MD (bottom to top) after orientation. The
oriented foam had density of 0.56 g/cc and a thickness of 0.1 mm.
Properties of the foam are presented in Table 6.
6TABLE 6 MD tensile MD tensile CD tensile stress, strain, % CD
tensile strain, % Sample Orientation kpsi elongation stress, kpsi
elongation 9-1 Seq 3 .times. 2.3 4.8 24 2.5 21 Seq = sequential
biaxial orientation: MD followed by CD
[0117] The data of Table 6 can be compared to those presented in
Example 5 to show that a foam of this particular blend has tensile
properties essentially identical to those of pure polypropylene
because the density of the oriented foam in this Example is higher
than that of the oriented foam of Example 5 (0.56 to 0.47
g/cc).
Example 10
[0118] A foam material was prepared having non-foam, coextruded
polymeric skins on each surface of the foam. A melt mixture of
48.9% by weight high melt strength polypropylene (Montell
PF814.TM.), 48.9% by weight isotactic polypropylene (Fina
PP3374.TM.) and 2.2% by weight chemical blowing agent (FM1307H) was
prepared in a Killion single screw extruder operated at 80 rpm with
a temperature profile of from 171 to 221 to 185.degree. C. (exit
temperature 204.degree. C.) to create an exit pressure of 18.6 MPa.
A single-component skin of isotactic polypropylene (Fina
PP3374.TM.) was coextruded on each face of the foam melt mixture by
means of a Killion single screw extruder operated at 171 rpm and
243.degree. C. The foam melt mixture was extruded through a 15 cm
foam die, chilled on a chrome cast roll at 46.degree. C. and
collected at a drawing rate of 0.6 m/min to provide a foam
construction having a thickness of 2.13 mm and a density of 0.53
g/cc. A micrograph of the foam is shown in FIG. 11, in which cells
of approximately 50-100 .mu.m size are seen.
[0119] Sequential 5.times.5 biaxial orientation (multi-roll LO
followed by tenter) of the foam construction provided the foam
shown in FIG. 12, having a density of 0.45 g/cc and a thickness of
0.28 mm. Sequential 5.times.5 biaxial orientation provided a foam
having the properties shown in Table 7.
7TABLE 7 MD tensile MD tensile CD tensile stress, strain, % CD
tensile strain, % Sample Orientation kpsi elongation stress, kpsi
elongation 10-1 Seq 4 .times. 3 7.0 22 4.0 15 Seq = sequential
biaxial orientation: MD followed by CD
[0120] The coextruded, oriented foams of this Example can be
compared to the non-coextruded foam of Example 5, showing that the
coextruded foams were stronger, i.e., had a higher tensile stress
at break, even though the coextruded foams had larger cell
sizes.
Example 11
[0121] A foam material was prepared having non-foam, coextruded
polymeric skins on each surface of the foam. A melt mixture of 98%
by weight high melt strength polypropylene (Montell PF814.TM.) and
2.2% by weight chemical blowing agent (FM1307H) was prepared in a
65 mm Berstorff twin screw extruder operated at 84 rpm with a
temperature profile of from 180 to 230 to 150.degree. C. (exit
temperature 167.degree. C.) to create an exit pressure of 116 bar.
A single-component skin of isotactic polypropylene (Fina
PP3571.TM.) was coextruded on each face of the foam melt mixture by
means of two Davis Standard 2.5" and 2" single screw extruders
operated at 41 and 75 rpm, respectively, and 240.degree. C. The
foam melt mixture was extruded through a 14" three-layer Cloeren
die, chilled on a chrome cast roll at 20.1.degree. C. and collected
at a drawing rate of 3.1 m/min to provide a foam contruction having
a thickness of 2.6 mm and a density of 0.63 g/cc. A micrograph of
the foam is similar to that of FIG. 11 except the cell sizes range
from 30-50 .mu.m.
[0122] Simultaneous 5.4.times.6 biaxial orientation (Berstorff
LISIM tenter) of the foam construction provided a foam having a
density of 0.5 g/cc and a thickness of 0.13 mm with the properties
shown in Table 8.
8TABLE 8 MD tensile MD tensile CD tensile stress, strain, % CD
tensile strain, % Sample Orientation MPa elongation stress, MPa
elongation 11-1 Simo 5.4 .times. 6 74 57 68 N/mm.sup.2 43
N/mm.sup.2
[0123] The coextruded, oriented foams of this Example can be
compared to the coextruded foam of Example 10, showing that the
simultaneous biaxially oriented foams have a more balanced set of
properties in the two directions. In addition, the simultaneously
oriented samples exhibited considerably lower shrinkage.
* * * * *