U.S. patent application number 10/641542 was filed with the patent office on 2004-02-19 for reduced density foam articles and process for making.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Albrecht, Bonnie Weiskopf, Gehlsen, Mark David, Vall, David Loren.
Application Number | 20040033350 10/641542 |
Document ID | / |
Family ID | 24869922 |
Filed Date | 2004-02-19 |
United States Patent
Application |
20040033350 |
Kind Code |
A1 |
Gehlsen, Mark David ; et
al. |
February 19, 2004 |
Reduced density foam articles and process for making
Abstract
The invention discloses reduced density foams and methods of
making the foams by applying or creating a nonfoaming barrier layer
on a foamable layer, which barrier layer inhibits the escape of
fugitive gases during the foaming process.
Inventors: |
Gehlsen, Mark David; (Eagan,
MN) ; Vall, David Loren; (Woodbury, MN) ;
Albrecht, Bonnie Weiskopf; (Lake Elmo, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
24869922 |
Appl. No.: |
10/641542 |
Filed: |
August 15, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10641542 |
Aug 15, 2003 |
|
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09714408 |
Nov 16, 2000 |
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6623674 |
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Current U.S.
Class: |
428/304.4 ;
428/343 |
Current CPC
Class: |
Y10T 428/249953
20150401; C08J 2201/03 20130101; C09J 7/29 20180101; Y10T 428/28
20150115; C09J 2400/243 20130101; C09J 2201/606 20130101; B32B 5/20
20130101; B29C 44/065 20130101; B29C 44/0407 20130101; C08J 9/34
20130101 |
Class at
Publication: |
428/304.4 ;
428/343 |
International
Class: |
B32B 007/12; B32B
015/04 |
Claims
We claim:
1. A foam article comprising a foam layer having at least one major
surface and a barrier layer on said at least one major surface
wherein the foam layer of the article has a density at least 20%
lower than would be the density of a foam layer of a similar foam
article made without a barrier layer.
2. A foam article comprising a foam layer having at least one major
surface and a barrier layer on said at least one major surface
wherein the foam article has a density at least 20% lower than
would be the density of a similar foam article made without a
barrier layer.
3. The foam article of claim 2 comprising at least four layers
wherein at least one foam layer is between two unfoamed layers.
4. The foam article of claim 2 wherein said article is a pressure
sensitive adhesive article.
5. The pressure sensitive adhesive article of claim 4 comprising a
multilayer article wherein at least one interior layer comprises a
nonfoaming barrier layer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No.: 09/714,408, filed Nov. 16, 2000, now allowed, the
disclosure of which is herein incorporated by reference.
TECHNICAL FIELD
[0002] This invention relates to reduced density foam articles and
methods of making same.
BACKGROUND
[0003] Methods for producing reduced density thermoplastic foam
articles often employ flammable hydrocarbon gases as a physical
blowing agent or employ complicated and costly crosslinking
technologies. Nonvolatile gases such as carbon dioxide (CO.sub.2)
and nitrogen are generally not preferred as a blowing agent for
thermoplastics due to their low solubility in the polymer.
SUMMARY OF INVENTION
[0004] The present invention provides reduced density foam
articles. These articles can be made with environmentally-friendly
gases.
[0005] In one aspect of the present invention, an unfoamed barrier
layer on a foam layer is employed to produce reduced density foam
articles. It is believed that incorporating an unfoamed layer onto
a major surface of a layer of foamable material acts as a barrier
and dramatically changes the diffusional characteristics of a
fugitive gas in the foamable material. This provides the ability to
foam materials that may otherwise be difficult to foam.
[0006] One aspect of the invention provides a method of making a
multilayer reduced density foamed article comprising:
[0007] (1) mixing at least one thermoplastic polymer and at least
one blowing agent that is, or that produces, a fugitive gas to form
a foamable melt mixture,
[0008] (2) shaping the melt mixture such that it has at least one
major surface,
[0009] (3) affixing or creating a barrier layer of nonfoaming
material on one or more of said major surfaces, wherein the barrier
layer inhibits diffusion of the fugitive gas out of the foamable
melt mixture, and
[0010] (4) causing the melt mixture to foam.
[0011] Another aspect of the invention provides a foam article
comprising a foam layer with a barrier layer on at least one major
surface of the foam layer wherein the foam layer of the article has
a lower density than a foam layer of a similar article without a
barrier layer.
[0012] Another aspect of the invention provides a foam article
comprising a foam layer with a barrier layer on at least one major
surface wherein the article has a density lower than that of a foam
article without a barrier layer.
[0013] Another aspect of the invention provides a foam article
comprising at least four layers wherein the article comprises at
least one foam layer between two unfoamed layers and wherein the
article has a density lower than a similar article without unfoamed
layers.
[0014] Another aspect of the invention provides a method of
producing a reduced density foam article comprising:
[0015] 1) mixing at least one thermoplastic polymer and at least
one blowing agent that is, or that produces, a fugitive gas to form
a foamable mixture,
[0016] 2) shaping the melt mixture such that it has at least one
major surface,
[0017] 3) affixing or creating a barrier layer of nonfoaming
material on one or more of said major surfaces of the melt mixture
wherein the composition and thickness of the barrier layer cause it
to inhibit gas diffusion such that the time it takes for the
fugitive gas to diffuse out of the melt mixture into the atmosphere
is substantially greater than the time it takes for the fugitive
gas to nucleate and expand, and
[0018] 4) causing the melt mixture to foam.
[0019] Another aspect of the invention provides a method of varying
the density of a foam article comprising:
[0020] 1) varying the amount of fugitive gas in a foamable polymer
melt mixture, and
[0021] 2) during the shaping stage of making a foam article,
varying one or both of the thickness and composition of a
non-foaming barrier layer applied to or created on at least one
major surface of the shaped foamable melt mixture to control the
difference between diffusion time and foaming time.
[0022] Various aspects of the invention employ carbon dioxide as
the fugitive gas. Carbon dioxide may be provided as carbon dioxide
in the form of a physical blowing and/or carbon dioxide produced by
a chemical blowing agent. Other gases are also suitable for use as
fugitive gases, as is explained further in the Detailed Description
section.
[0023] The methods of the invention are especially effective when
high concentrations of fugitive gas are used in combination with an
unfoamed barrier layer. The fact that incorporating unfoamed
barrier layers on a foam layer causes a reduction in foam density
is counterintuitive. Intuitively, one would expect the unfoamed
barrier layers to increase the density of a foam article. However,
it is believed that the presence of the barrier layers changes the
gas diffusion characteristics of the fugitive gas, causing more of
the gas to nucleate and grows cells in the polymer matrix as
opposed to escaping from the polymer matrix of the melt mixture
into the atmosphere.
[0024] One aspect of the invention discloses a technique whereby as
the thickness of the unfoamed barrier layer increases for a given
combination of polymer matrix and fugitive gas concentration and
type, the density of the foam structure decreases. In some
instances, the decrease is about 200%.
[0025] As used in the present application:
[0026] "fugitive gas" means a gas that has a propensity to diffuse
out of a polymer into the atmosphere when exposed to atmospheric
pressure, preferably a gas having a vapor pressure of greater than
0.689 MPa at 0.degree. C.
[0027] An advantage of at least one embodiment of the present
invention is that using unfoamed barrier layers to control the
diffusional properties of fugitive gas(es) alleviates the need to
use flammable hydrocarbons or complicated cross-linking technology
to produce high performance foams.
[0028] Another advantage of at least one embodiment of the present
invention is that it enables low cost production of thermoplastic
foams such as polyolefin foams using gases such as CO.sub.2 as the
sole blowing agent.
[0029] Other features and advantages of the invention will be
apparent from the following figures, detailed description, and
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0030] FIG. 1 shows an illustration of how increasing unfoamed
barrier layer (A layer) thickness in the present invention can
decrease foam density for an ABA structure wherein B is a foamed
layer.
[0031] FIG. 2 shows foam density as a function of casting speed for
comparative ABA foam articles having different barrier layer
thicknesses wherein the barrier and foam layers both comprise a low
density polyethylene. The foam layers were made using a fugitive
gas concentration of 47.4 vol % CO.sub.2 at Standard Temperature
and Pressure (STP). A difference in barrier layer thickness is
indicated by the different revolutions per minute (RPM) of the
barrier layer extruder screw.
[0032] FIG. 3 shows foam density as a function of casting speed for
ABA foam articles having different barrier layer thicknesses
wherein the barrier layer and foam layers both comprise a low
density polyethylene and the foam layers were made using a fugitive
gas concentration of 73.0 vol % of CO.sub.2 at STP. A difference in
barrier layer thickness is indicated by the different revolutions
per minute (RPM) of the barrier layer extruder screw.
[0033] FIG. 4 shows foam density as a function of fugitive gas
concentration for ABA foam articles having different barrier layer
thicknesses wherein the barrier and foam layers each comprise a low
density polyethylene. A difference in barrier layer thickness is
indicated by the different revolutions per minute (RPM) of the
barrier layer extruder screw.
[0034] FIG. 5 shows foam density as a function of casting speed for
ABA foam articles having different barrier layer thicknesses
wherein the barrier and foam layers each comprise a low density
polyethylene. The foam layer was made using a fugitive gas
concentration of 90 vol % CO.sub.2 at STP. A difference in barrier
layer thickness is indicated by the different revolutions per
minute (RPM) of the barrier layer extruder screw.
[0035] FIG. 6 shows foam density as a function of casting speed for
ABA foam articles having different barrier layer thicknesses
wherein the barrier layer comprises an unfoamed pressure sensitive
adhesive and the foam layer comprised a low density polyethylene.
The foam layers were made using a fugitive gas concentration of
91.5 vol. % CO.sub.2 at STP. A difference in barrier layer
thickness is indicated by the different revolutions per minute
(RPM) of the barrier layer extruder screw.
[0036] FIG. 7 shows normalized tensile at break data as a function
of normalized density data for a variety of low density
polyethylene (LDPE) foam articles having unfoamed barrier layers
(of the same thickness) as compared to data representing
comparative foam articles.
[0037] FIG. 8 shows an illustration of a tandem foam coextrusion
apparatus that can be used to make some embodiments of foam
articles of the present invention.
[0038] The present invention is susceptible to various
modifications and alternative forms, and specifics thereof have
been shown by way of example in the drawings and will be described
in detail. It should be understood, however, that the intention is
not to limit the invention to the particular embodiments described.
On the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the spirit and scope
of the invention as described by the following detailed description
and as defined by the appended claims.
DETAILED DESCRIPTION
[0039] The present invention discloses a method for reducing the
density of a foam article (and the foam layer(s) in the foam
articles) while maintaining the mechanical properties of the foam
article. This is achieved in part by applying or creating an
unfoamed barrier layer on one or more major surfaces of a foamable
material. Barrier layers may be the outer, i.e., "skin," layer or
may be an interior layer in a multi-layer structure having more
than one foam layer. The resulting foam article may be in various
shapes such as rods, fibers, sheets, etc.
[0040] Reducing foam density may be achieved by controlling and
coordinating the type of polymer used for the foamable layer, the
type and amount of fugitive gas in the foamable layer, and the type
of material and thickness of the barrier layer.
[0041] To achieve polymer foaming, a gaseous component must
solubilize in the polymer. When the polymer-gas mixture is exposed
to a reduced pressure, the gas nucleates, further diffuses from the
polymer matrix to nucleation sites, and expands to form cells. The
resulting material may then be brought into contact with a cooled
surface, typically a chilled casting drum that stabilizes and
solidifies the foamed article. The resulting material comprises a
polymer foam article comprising gas voids inside a polymer matrix.
To obtain such a structure, the compatibility of the gas and
polymer must be considered. If the gas is highly soluble in the
polymer, it may remain in the polymer instead of nucleating and
forming cells. Conversely, if the gas is not sufficiently soluble
in the polymer, the gas may migrate out of the polymer into the
atmosphere without substantially contributing to the formation of
cells. Optimally, the gas will have high solubility in the polymer
when the gas is in a non-gaseous phase (e.g., under pressurized
conditions), and low solubility in the polymer when it is in a
gaseous phase. Under these conditions, substantially all of the gas
should form cells with minimal losses of gas to the atmosphere
prior to solidification of the foam article. A mass balance on the
system would be as follows:
Total gas added=gas in cells+gas in cell walls+gas lost to
environment
[0042] Polymer foam density can be deduced from this gas material
balance. Making a reduced density foam article requires optimizing
the amount of gas that expands in the polymer. In other words,
maximizing the amount of gas residing in the cells will optimize
the reduction in foam density. Accordingly, gas and polymer
combinations may be selected to minimize the amount of gas
remaining in the cell walls and lost to the environment and to
maximize the amount of gas that forms and expands cells.
[0043] In the past, gases such as chlorofluorocarbons and
hydrocarbons were used to produce foams. These materials are
typically highly soluble in polymers, which minimizes the amount of
gas lost to the atmosphere. Foaming systems that used
chlorofluorocarbons were formulated so that upon exiting an
extruder, the gas would slowly nucleate and expand cells. These
systems were designed with gases having vapor pressures of less
than 0.689 MPa (100 psia) (at 0.degree. C.) and boiling points
between -20.degree. C. and 20.degree. C. For these systems, the
diffusion coefficients (i.e., rates at which the gas diffuses from
the polymer) are about 10.sup.-7 to 10.sup.-8 cm.sup.2/sec (at
200.degree. C.) as shown in Crank, J. and Park, G. S., (eds)
"Diffusion in Polymers," Academic Press, London, 1968.
[0044] Environmentally-friendly gases such as carbon dioxide, air,
nitrogen, helium, etc. are more difficult to use in foaming
processes because they are less soluble in polymers and diffuse
into the atmosphere quickly. Environmentally-friendly gases
typically have vapor pressures that are much greater than 0.689 MPa
(100 psia) (at 0.degree. C.). For example, the vapor pressure of
carbon dioxide is 2.07 MPa (300 psia). With a high vapor pressure
gas, reduction of pressure at the extruder die exit causes the gas
to vaporize quickly and causes pressure in the cells to increase
rapidly. The difference in pressure between the cell interior and
atmosphere acts as a driving force that causes the carbon dioxide
to readily diffuse from the foam into the atmosphere. The diffusion
coefficients for CO.sub.2 are about 10.sup.31 5 to 10.sup.-6
cm.sup.2/sec (at 200.degree. C.) as shown in Crank, J. and Park, G.
S., (eds) "Diffusion in Polymers," Academic Press, London, 1968, an
order of magnitude higher than for CFCs. Because the
environmentally-friendly gases prefer to migrate out of the polymer
prior to cell formation, these "fugitive" gases generally do not
effectively expand in the polymer to make a foam.
[0045] The present invention takes advantage of the propensity of
fugitive gases to vaporize and form cells within a polymer matrix,
while counteracting the tendency of the fugitive gases to diffuse
from the polymer matrix into the atmosphere. The present invention
uses an unfoamed barrier layer adjacent to a gas-containing polymer
layer to decrease the diffusion rate of the gas out of the polymer
matrix and to increase the gas's formation and expansion of cells.
This technique effectively minimizes the amount of gas escaping
into the atmosphere so that more gas is used to form and expand
cells in the polymer. The fugitive gas being used may have some
solubility in the thermoplastic being foamed. However, the present
invention allows foams to be successfully made with polymers and
gases that are not particularly compatible, e.g., polyolefins and
CO.sub.2.
[0046] The inventors have found that non-foaming barrier layer
materials and thicknesses can be chosen such that the time it takes
the fugitive gas to diffuse out of the polymer matrix and through
the barrier layer (diffusion time) is greater than the time it
takes the gas to form and expand cells in the polymer matrix
(foaming time). The diffusion characteristics of gases in polymers
are known and are described in more detail in Crank, J. and Park,
G. S. (eds), "Diffusion in Polymers", Academic Press, London, 1968.
The following equation describes gas diffusion in a polymer, 1 M t
M .infin. = 1 - 4 D t l 2 ( 1 )
[0047] where M.sub.t is the amount of gas in a polymer at time t,
M.sub..infin. is the amount of gas in a polymer at saturation, D is
the diffusion coefficient, and 1 is the sample thickness. This
expression can be simplified to capture the important scaling
relationship of the time required for a gas molecule to diffuse a
length, 1, 2 t l 2 D ( 2 )
[0048] where t is time, 1 is the diffusion length, and D is the
diffusion coefficient. The diffusion time of a gas molecule in a
polymer depends on the polymer-gas combination, which determines
the diffusion coefficient (D), and the diffusion length (1). In the
present invention, each polymer-gas-length combination must be
optimized to control the diffusion of gas for a given length, i.e.,
thickness, of polymer.
[0049] The diffusion coefficients for gases in various polymers are
known. See Durrill, P. L. and Griskey, R. G., AIChE Journal, 12(6);
1147, (1966): Durrill, P. L. and Griskey, R. G., AIChE Journal,
15(1), 106;,(1969); Bonner, D. C., Polym. Eng. Sci., 17(2), 65,
(1977); Wissinger, R. G., and Paulaitis, M. E., J. Polym.
Sci.:Polym. Phys., 25, 2497, (1987); Shim, J. J. and Johnston, K.
P., AIChE Journal, 35(7), 1097, (1989); Kramer, E. J. et al., J.
Polym. Sci.: Polym. Phys., 20, 1371, (1982); Koros, W. J. and Paul,
D. R., Polym. Eng. Sci., 20(1), 14, (1980); Chiou, J. S., Barlow,
J. W., and Paul, D. R., J. Appl. Polym. Sci., 30, 2633, (1985); and
Crank, J. and Park, G. S. (eds); "Diffusion in Polymers", Academic
Press, London, 1968. For example, the value of D for carbon dioxide
in polyethylene is about 10.sup.-6 cm.sup.2/sec. Therefore,
addition of a 25 .mu.m low density polyethylene (LDPE) layer on a
carbon dioxide/polyethylene foam would result in a diffusion time
through the LDPE barrier layer of about 1 second. A 51 .mu.m
polyethylene barrier layer would result in a diffusion time of
about 25 seconds. Because the time required to nucleate and expand
gas voids in the polyethylene polymer is less than one second,
these barrier layers hinder gas diffusion into the atmosphere,
thereby more effectively using the gas to form a foam.
[0050] As an illustration, the data from the inventors' experiments
show that when LDPE barrier layers were added to an LDPE foamable
layer in conjunction with a CO.sub.2 fugitive gas concentration of
greater than about 65 volume % in the LDPE foamable layer, the
density of the foam article decreased compared to LDPE foam
articles in which the foam layer was made with less fugitive gas or
the foam article was made with thinner barrier layers. A similarly
unexpected decrease in density was observed when the barrier layers
were made with pressure sensitive adhesives. The inventors also
found that foam article density further decreased with an increase
in barrier layer thickness. However, below a CO.sub.2 fugitive gas
concentration of about 65 volume % in LDPE, the inventors found
that density increased as expected when barrier layer thicknesses
were increased. This was expected because of additive effect of the
higher density unfoamed layers to the overall density of the foam
article.
[0051] As the foregoing indicates, the type and amount of fugitive
gas mixed into the polymer matrix, the composition of the matrix,
and the composition and thickness of the barrier layer(s) should
all be considered in decreasing density. Selection of these
variables will influence diffusion time and foam density. One of
skill in the art would be able to make judicious sections without
undue experimentation.
[0052] The data in FIG. 4 show that for foam articles having a low
density polyethylene (LDPE) foam layer and CO.sub.2 fugitive gas
concentrations below about 65 vol % (e.g., about 47 vol. % and
about 64 vol. %), increasing an LDPE barrier layer thickness (by
increasing the RPM of the barrier layer extruder screw) increased
foam density. In contrast, for the same type of foam having a
fugitive gas concentration above about 65 vol % (e.g., about 73
vol. %), increasing an LDPE barrier layer thickness (by increasing
the RPM of the barrier layer extruder screw) decreased foam
density. All the samples in FIG. 4 were collected at the same
casting speed (3 m/min).
[0053] Foam layers made per the present invention generally also
have reduced densities as compared to similarly made foam layers
having no barrier layers. For example, the density of a single
layer polyethylene foam article with no barrier layers, made with a
CO.sub.2 fugitive gas concentration of 90 vol. %, was found to be
0.50 g/cm.sup.3. When the same foam layer was made with unfoamed
polyethylene barrier layers extruded from an extruder with a screw
operating at 70 RPM, the article density decreased to about 0.24
g/cm.sup.3. It was also found that the tensile properties of the
two articles were similar even though the foam article with barrier
layers used less than half the material of the single layer foam
article.
[0054] Pressure sensitive adhesives may also be used in barrier
layers. As shown by FIG. 5, for an LDPE foam layer made with 90
vol. % CO.sub.2 fugitive gas, as the thickness of an unfoamed
pressure sensitive adhesive (KRATON) barrier layer increased, the
density of the foam article decreased dramatically. FIG. 5 further
shows that the combination of 90 vol. % fugitive gas concentration
and a thick barrier layer (produced by an extruder screw RPM of 70)
provides a foam article having a density independent of casting
speed. The density for this foam was below 0.27 g/cm.sup.3 and
substantially constant over a range of casting speeds (3 to 9
m/min).
[0055] The data in FIG. 6 were obtained from LDPE foam articles
made with pressure sensitive adhesive (KRATON) barrier layers. The
foam layers of the articles were made with 91.6 vol % CO.sub.2
fugitive gas concentrations. Similar to the data in FIG. 5, FIG. 6
shows that as the barrier layer thickness increased, foam article
density decreased.
[0056] The mechanical, e.g., tensile, properties of polymer foams
depend on several variables including polymeric material, density,
cell size and shape, and pressure inside the foam cells. In
general, the modulus of a foam can be described by the following
equation, 3 S f S o = 2 ( f o ) 2 + ( 1 - ) ( f o ) + p o ( 1 - 2 v
f ) S o ( 1 - f / o ) ( 3 )
[0057] where S.sub.f, S.sub.o are the mechanical properties of the
foam and the original material, respectively. .PHI. is the fraction
of material in the cell edges, .rho..sub.f, .rho..sub.o are the
densities of the foam and polymer, respectively, p.sub.o is the
pressure inside the foam cells, and .nu..sub.f is the polymer
material Poisson's ratio. The first two terms on the right hand
side of (3) are related to the stress distribution on the cell
edges and cell walls, respectively. The p.sub.o term is the
contribution due to the pressure of the gas inside the cells. Most
of the foams produced with atmospheric gases possess a small
p.sub.o term because the internal cell pressure is equivalent to
atmospheric pressure. In addition, since most foams are a
combination of open and closed cells, .PHI. is difficult to
characterize. As a result, (3) can be simplified to the following
expression, 4 S f S o = ( f o ) n ( 4 )
[0058] where n is a power law exponent that is dependent on the
combination of open and closed cells in the polymeric foam. This
equation can be used in a very general sense and the exponent can
be communicated easily, for example if polymer foams are made when
n equals 1, the strength of the material decreases linearly with
density reduction. As n approaches 2, the mechanical properties
decrease significantly faster as the density decreases.
[0059] FIG. 7 shows the mechanical properties of some materials of
the invention and comparative materials. At fugitive gas
concentrations of less than 64 vol. % the slope of (Sf/So)/(Pf/Po)
was greater than 1 whereas at concentrations of greater than 64
vol. % the slope was less than 0.5. These examples demonstrate that
at high fugitive gas concentrations the tensile properties of the
resulting foam articles are significantly independent of
density.
[0060] Another surprising observation from FIG. 7 is that at high
concentrations, e.g., greater than about 65 vol. %, of fugitive
gas, tensile properties of the same polymer matrix material having
the same barrier layer thicknesses remained nearly constant as the
densities of the foam articles decreased. This shows that reduced
density foam articles having the same tensile properties as higher
density foam articles (but using less material) could be produced.
This indicates that it is possible to manufacture foam articles
having barrier layers using 50% less material than a foam article
with no barrier layers without sacrificing mechanical
properties.
[0061] Process
[0062] The foam articles of the present invention may be produced
in any manner so long as at least one foam layer is adjacent to an
unfoamed barrier layer that will inhibit the diffusion of fugitive
gas from the foamable layer. The unfoamed barrier layer may be on
one or more major surfaces of a foam layer, i.e., as a skin, or may
be sandwiched between foam layers when a multiple-layer foam
article is produced.
[0063] The foam articles of the present invention may be made by a
pressurized melt processing method such as an extrusion method. The
extruder may be a tandem system, a single screw extruder, a twin
screw extruder, etc. The extruder may be equipped with multilayer
annular dies, flat film dies and feedblocks, multi-layer feedblocks
such as those disclosed in U.S. Pat. No. 4,908,278 (Bland et al.),
multi-vaned or multi-manifold dies such as a 3-layer vane die
available from Cloeren, Orange, Tex.
[0064] A foamable layer may also be made by combining a chemical
blowing agent and polymer at a temperature below the decomposition
temperature of the chemical blowing agent then later foamed. The
barrier layer can also be applied by lamination to a foamable
layer, which is subsequently foamed.
[0065] One method of producing the foam material of the invention
is by using an extruder with a two (or more) layer feedblock. In
this case, the foamable mixture is extruded as the B layer in an
ABA construction with the barrier layers being extruded as the A
layers. The ABA construction exits an extruder die and upon
exposure to reduced pressure, the fugitive gas immediately
nucleates and forms cells within the polymer to create a foam
article. The resulting foam article is then deposited onto a
temperature-controlled casting drum. The casting drum speed (i.e.,
as produced by the drum RPM) can affect the overall thickness of
the foam article. As the casting roll speed increases, the overall
thickness of the foam article (including barrier and foam layers)
can decrease. However, the barrier layer thickness at the die exit,
which is where foaming occurs, is the diffusion length, l, for the
system. As the foam article is stretched and quenched on the
casting drum, the barrier layer thickness may decrease until the
foam article solidifies. In other words, it is the barrier layer
diffusion length (i.e., thickness) at the die exit that is the
important factor in controlling the diffusion of the fugitive
gas.
[0066] A surface barrier layer on a foamable layer may also be
produced by submerging the exit of an extruder die under water. In
this case, as the foamable material is extruded, the water can cool
the outer portion of the extruded material causing it to form an
unfoamed skin layer. This outer unfoamed layer can inhibit
diffusion of the fugitive gas from the interior foamable material
thereby facilitating the foaming process.
[0067] Foamable Materials
[0068] Polymer materials that may be used for the foamable layer
matrix of the present invention include any thermoplastic material.
Suitable materials include, e.g., thermoplastics that are
amorphous, semi-crystalline, and pressure sensitive adhesives.
Suitable materials may comprise blends of two or more polymers.
[0069] The polymers may be homopolymers or copolymers, including
random and block copolymers. It may be desirable to use two or more
miscible (or immiscible) polymers having different compositions to
achieve unique foam properties. A wide range of foam physical
properties can be obtained by selectively choosing the polymer
component types and concentrations. A particular polymer may be
selected based upon the desired properties of a final
foam-containing article.
[0070] 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.
[0071] Suitable semi-crystalline materials include polyethylene,
polypropylene, polymethylpentene, polyisobutylene, polyolefin
copolymers, Nylon 6, Nylon 66, polyester, polyester copolymers,
fluoropolymers, poly vinyl acetate, poly vinyl alcohol, poly
ethylene oxide, functionalized polyolefins, ethylene vinyl acetate
copolymers, metal neutralized polyolefin ionomers available under
the trade designation SURLYN from E.I. DuPont de Nemours,
Wilmington, Del., polyvinylidene fluoride, polytetrafluorcethylene,
polyformaldehyde, polyvinyl butyral, and copolymers having at least
one semi-crystalline compound.
[0072] Suitable pressure sensitive adhesive (PSA) polymers can be
adhesive polymers (i.e., polymers that are inherently adhesive), or
polymers that are not inherently adhesive but are capable of
forming adhesive compositions when compounded with tackifiers.
Tackifiers that may be used include, for example, those listed in
the additives section below. Pressure Sensitive Adhesives can be
quantitatively described using the "Dahlquist criteria" which
maintains that the elastic modulus of these materials is less than
10.sup.6 dynes/cm.sup.2 at room temperature. See Pocius, A.V.,
Adhesion & Adhesives: An Introduction, Hanser Publishers, New
York, N.Y., First Edition, 1997. Examples of suitable PSA polymers
(as long as they have an appropriate Dahlquist numbers, either
inherently or after being tackified) include acrylics, acrylic
copolymers (e.g., isooctylacrylate-acrylic acid), amorphous
poly-alpha-olefins (e.g., polyoctene, polyhexene, and atactic
polypropylene), block copolymer-based adhesives, natural and
synthetic rubbers, styrene-butadiene rubber (SBR), silicone
adhesives, ethylene-vinyl acetate, siloxanes, and epoxy-containing
structural adhesive blends (e.g., epoxy-acrylate and
epoxy-polyester blends), acrylic copolymers such as those described
in U.S. Pat. No. 5,804,610, incorporated by reference, tackified
styrenic block copolymers, polyolefin copolymers, polyureas,
polyurethanes, vinyl ethers,polyisobutylene/butyl
rubber,ethylene-propylene-diene rubber (EPDM), as well as pressure
sensitive adhesives disclosed in copending application Ser. No.
09/091,683, incorporated by reference, and mixtures of any of the
foregoing pressure sensitive adhesives.
[0073] Immiscible thermoplastic polymer blends may also be used for
the polymer matrices of the foams of this invention as long as the
polymeric materials are suitable for melt extrusion processing and
provide appropriate diffusion characteristics with the fugitive gas
being used. It may be desirable to blend two or more immiscible
polymers having different compositions to achieve unique foam
properties. A wide range of foam physical properties can be
obtained by selectively choosing the blend component types and
concentrations. A particular polymer may be selected based upon the
desired properties of a final foam-containing article.
[0074] Any single component of a blend may comprise greater than
zero, but less than 100 weight % of the foamable matrix. Suitable
immiscible blends may comprise any two or more amorphous
thermoplastic polymers, or semi-crystalline polymers. Pressure
sensitive adhesives may also be used to form immiscible blend
foams. Combinations of one or more immiscible PSAs with one or more
immiscible non-PSA may be used.
[0075] Blowing Agents
[0076] Blowing agents suitable for the present invention may be
physical blowing agents, which are typically the same material as
the fugitive gas, e.g., CO.sub.2, or a chemical blowing agent,
which will produces the fugitive gas. More than one physical or
chemical blowing agent may be used and physical and chemical
blowing agents may be used together.
[0077] Physical blowing agents useful in the present invention
include any naturally occurring atmospheric material which is 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. Non-flammable,
non-toxic, non-ozone depleting blowing are preferred because they
are easier to use, e.g., fewer environmental and safety concerns,
and are generally less soluble in thermoplastic polymers. Suitable
physical blowing agents include, e.g., carbon dioxide, nitrogen,
SF.sub.6, nitrous oxide, perfluorinated fluids, such as
C.sub.2F.sub.6, argon, helium, noble gases, such as xenon, air
(nitrogen and oxygen blend), and blends of these materials.
[0078] Chemical blowing agents that may be used in the present
invention include, e.g., a sodium bicarbonate and citric acid
blend, dinitrosopentamethylenetetramine, p-toluenesulfonyl
hydrazide, 4-4'-oxybis(benzenesulfonyl hydrazide, azodicarbonamide
(1,1'-azobisformamide), p-toluenesulfonyl semicarbazide,
5-phenyltetrazole, 5-phenyltetrazole analogues,
diisopropylhydrazodicarbo- xylate,
5-phenyl-3,6-dihydro-1,3,4-oxadiazin-2-one, and sodium
borohydride.
[0079] Preferably, the blowing agents are, or produce, one or more
fugitive gas(es) having a vapor pressure of greater than 0.689 MPa
at 0.degree. C.
[0080] Barrier Materials
[0081] Barrier layers may be added to the foamable layer by any
method that creates a barrier to gas diffusion prior to the
expansion of the gas in the foamable layer. Suitable methods of
incorporating barrier layers include coextrusion methods.
Alternatively, the outer portion of the foamable layer may be made
into a skin layer, e.g., by cooling such that the outer layer
solidifies before it can foam. Other methods, such as lamination
and extrusion coating, may also be used to apply barrier layers.
The barrier layers may be comprised of a variety of materials,
e.g., thermoplastics, thermosets, foils, or anything else that
inhibits or prevents diffusion of a particular fugitive gas from
the foamable layer. A thermoplastic barrier material may be the
same as, or different from, the foamable layer material. Suitable
thermoplastic barrier layer materials include all of the materials
listed for the foamable layer.
[0082] The inventors found that increasing the thickness of the
barrier layers caused more fugitive gas to remain in the foamable
layer and expand thereby causing the density of a foam article to
significantly decrease. Thicker barrier layers also provide
mechanical support to the resulting foam article.
[0083] Additives
[0084] The foamable melt mixture may also include additives.
However, it should be noted that additives could change the
properties of the melt mixture such that, upon exposure to a
reduced pressure, the diffusion rate of the fugitive gas could
increase or decrease over the rate of the same material without the
additive. The effect of an additive could be easily determined
through routine experimentation by one skilled in the art. Examples
of suitable additives include tackifiers (e.g., rosin esters,
terpenes, phenols, and aliphatic, aromatic, or mixtures of
aliphatic and aromatic synthetic hydrocarbon resins), plasticizers
(other than physical blowing agents), nucleating agents (e.g.,
talc, silicon, or TiO.sub.2), pigments, dyes, reinforcing agents,
solid fillers, hydrophobic or hydrophilic silica, calcium
carbonate, toughening agents, flame retardants, antioxidants,
finely ground polymeric particles (e.g., polyester, nylon, or
polypropylene), expandable microspheres, glass beads, stabilizers
(e.g., UV stabilizers), and combinations thereof.
EXAMPLES
[0085] This invention may be illustrated by way of the following
examples, including the test methods used to evaluate and
characterize the foam articles produced in the examples.
Test Methods
[0086] Foam Density (ASTM D792-86)
[0087] Foam article samples were cut into 12.5 mm.times.25.4 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.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 Archimede's principle. The density of
the foam article was obtained by the quotient of the mass and
volume. Accuracy of this measurement is .+-.0.005 g/cm.sup.3.
[0088] Gas Concentration
[0089] The gas concentration produced by the blowing agent was
calculated for each sample on a volume percent basis. The gas
concentration was based on the gas volume at Standard Temperature
and Pressure (STP), i.e., 298 K, 1 atm, for a given volume of
foamable melt mixture. The percent volume of gas, V.sub.g, in the
system can be calculated using the following expression: 5 V g = (
Q CBA + Q PBA ) * RT M w * P Q p p + ( Q CBA + Q PBA ) * RT M w * P
* 100
[0090] where Q.sub.CBA is the mass flow rate of gas generated using
a chemical blowing agent (CBA), Q.sub.PBA is the mass flowrate of
gas (Physical Blowing Agent (PBA)) injected into the process,
M.sub.W is the molar mass of the gas, R is the gas constant, T is
temperature, P is pressure, Q.sub.p is the mass flowrate of
polymer, and .rho..sub.p is the density of the polymer. For all the
calculations reported in this application the gas volume was
calculated at STP. The volume of gas generated by the CBA was
calculated using the manufacturer's data information. For
additional information on gas volume generated by various CBAs see
Encyclopedia of Polymer Science & Engineering, "Blowing
Agents", volume 2, p. 434-446, John Wiley & Sons, 1985.
[0091] Tensile Strength and Elongation
[0092] The foam article tensile and elongation properties, as
defined in ASTM D638-95, were measured at room temperature using a
testing device available as Model 55R1122, from Instron, Canton,
Mass. The samples were first conditioned at 21.degree. C. and 50%
humidity for 5 days. The samples were then cut into 130
mm.times.12.5 mm specimens. The thickness of each specimen was
measured using a digital linear gauge available as Model EG-233
from Ono Sokki, Tokyo, Japan, and recorded. The samples were tested
using gauge lengths of 51 mm (2 in.) at a rate of 254 mm/min (10
in/min) until failure. The strength (.sigma.) was measured as a
function of elongation (.epsilon.). The maximum values of .sigma.
and .epsilon. are reported as .sigma..sub.max and
p.epsilon..sub.max, respectively.
[0093] Tandem Single Screw Extrusion Process 10
[0094] FIG. 8 illustrates a coextrusion process used to make an ABA
foam article wherein the B layer was foamed and the A layers were
not. To form the B layer, polymer pellets were fed into a
gravimetric batch blender 12 having 4 zones available as Model
ACW-T from ConAir-Franklin, Media, Pa., at a rate of about 10 to
about 21 kg/hr. If a chemical blowing agent (CBA) was used, it was
added in the first zone of the blender at a rate of between 2 and 6
parts per 100 parts of polymer. The blender fed the components to
first single screw extruder 14, an NRM single screw extruder
available from Davis-Standard, Pawcatuck, Conn. Extruder 14 had 6
barrel zones, a 64 mm (2.5 in.) diameter, a length to diameter
ratio of 36:1 and a two-stage screw having a compression ratio of
3:1, available as Model PS-31 from Plastic Engineering Associates,
Inc., Boca Raton, Fla. Extruder 14 was operated at 25 RPM with an
increasing temperature profile from zone 1 to zone 6 of about 166
to about 232.degree. C. and a temperature of about 216.degree. C.
in zones 5 and 6 to form a melt mixture. Temperature was set for
each zone of extruder 14 to create increasing operating pressures
from zone 1 to zone 4 of from about 1.5 to about 27.2 MPa (1667 to
3942 psi) and a decreased operating pressure in zones 5 and 6.
[0095] If used, a physical blowing agent (PBA), carbon dioxide
(CO.sub.2), was injected into extruder 14 between zones 5 and 6,
between two blister rings on the screw, by laboratory injection
system 16 available as Model 567 from Sencorp Systems Inc.,
Hyannis, Mass. The carbon dioxide injection rates were controlled
to a concentration of about 1.6 weight % of the total polymer
flowrate by injecting the carbon dioxide at a flowrate of about
0.23 kg/hr (0.5 lb/hr).
[0096] The polymer and blowing agent mixture was mixed to form a
melt mixture. This melt mixture containing polymer and blowing
agent was conveyed through 25 mm diameter transier pipe 18 to
second single screw extruder 20, an 89 mm diameter (3.5") NRM
Davis-Standard single screw extruder. Extruder 20 had 6 zones, a
length to diameter ratio of 30:1, and a screw having distributive
mixing elements along substantially the entire length of the screw,
available as Model SFS-43 from Plastic Engineering Associates, Inc.
Extruder 20 was operated at 5 RPM with a decreasing temperature
profile from zone 1 to zone 6 typically wherein the temperature of
zone 1 was about 204.degree. C. (although for some examples the
temperature was about 182.degree. C.) and the melt mixture reached
a melt temperature (Tm) of about 173.degree. C. (although for some
examples the melt temperature was about 144.degree. C.) as measured
with a probe in zone 6. The melt pressure (Pm) of the carbon
dioxide entering extruder 14 from injection system 16 was adjusted
to maintain the desired carbon dioxide concentration as the
downstream pressures in extruder 20 changed. The pressure in
extruder 20 was maintained at a level that would prevent nucleation
of the carbon dioxide until the melt solution exited die 22. The
melt pressure was between about 10 and about 15 MPa. The flow rate
of the B layer material was varied from about 13.9 to about 21.0
kg/hr.
[0097] To form the A layers, polymeric material was fed into
extruder 24, a 32 mm (1.25 in.) Killion Single Screw Extruder
(Pawcatuck, Conn., Model KTS 125) with a length to diameter ratio
of 24:1, and 3 barrel zones. The screw had a Saxton mixing element
with a compression ratio of 3:1. The RPM of extruder 24 (0, 30, 50,
and 70 RPM) controlled the polymer flow rates. Extruder 24 had an
increasing temperature profile from zone 1 to zone 3 of about 138
to about 216.degree. C. The flow rate of the A layers were between
0 and 7.1 kg/hr.
[0098] The materials for both the A and B layers were conveyed from
their respective extruders through a 1.27 cm (0.5 in.) OD stainless
steel tubing to multilayer feedblock 26, a three layer Cloeren
feedblock (Cloeren Company, Orange, Tex., Model 96-1501) with an
ABA selector plug.
[0099] After the layers were combined in the feedblock the
polymeric materials were formed into a planar sheet using a 10"
(25.4 cm) wide EDI Ultraflex 40 Drop Die (Extrusion Dies
Incorporated, Chippawa Falls, Wis.). The die gap was set at about
510 .mu.m. Feedblock 26 and die 22 were both operated at
temperatures of about 182.degree. C. The sheets of polymeric
material was cast from die 22 onto temperature-controlled stainless
steel casting drum 28, maintained at about 7.degree. C., as a flat
film shape and collected at speeds between about 3.0 to about 18.3
m/m.
[0100] As the polymeric materials exited die 22 and were exposed to
atmospheric pressure of approximately 0.104 MPa (15 psi) the carbon
dioxide expanded and nucleation and cell growth occurred, forming
foamed material 30. The foamed material was collected on winder
34.
[0101] The thickness of the A layers at the die exit was varied by
changing the screw RPM of extruder 24. The final thicknesses of the
A layers were typically further changed by changing the speed of
casting drum 28. When applied, the A layer typically had a nominal
thickness at the die exit of 25 or 51 micrometers. This initial
barrier layer thickness was then changed, in most cases, by the
casting drum speed.
[0102] Foamed material 30 optionally could have been passed through
nip roll 32.
[0103] Thirteen layer constructions were also made in an
(AB).sub.6A arrangement by replacing feedblock 26 with a feedblock
having a flow channel multiplier function capable of splitting and
recombining flow streams to form the desired multilayer
configuration. This latter feedblock is described in U.S. Pat. No.
4,908,278 (Bland et al.), hereby incorporated by reference. The die
gap was set at about 510 .mu.m.
Comparative Example 1
[0104] The foam articles of this example were made with a foamable
layer having a gas concentration of about 47.4 volume percent.
[0105] For this example both the A and B layers of the ABA foam
articles comprised a low density polyethylene (Tenite LDPE1550P,
Eastman Chemical Co., Kingsport, Tenn.). The A layers had 2 weight
% of a blue dye (50 wt % dye in a LDPE carrier, C.B. Edwards,
Minneapolis, Minn.) added to the polymer to aid foam structure
analysis. Samples were made with different A layer thicknesses (but
both A layers on an individual sample had the same thickness). The
thicknesses of the unfoamed A layers were controlled by varying the
screw RPM of extruder 24 and the speed of casting drum 28. The foam
layer (B layer) was produced using 2 wt % of a chemical blowing
agent (per 100 wt % B layer polymer) comprising 50 wt % sodium
bicarbonate/citric acid in a LDPE carrier (RIC-50, Reedy Chemical
Company, Keyport, N.J.). The melt temperature of the polymer in
zone 6 of extruder 20 was about 173.degree. C. The volume of gas in
the foamable B layer, the final thickness of the A layers, and the
density and thickness of the total film construction were
calculated or measured. Operating conditions and test results are
shown in Table 1. Flowrate refers to the total flowrate of the A
and B layers. Tm and Pm refer to the temperature and pressure in
zone 6 of extruder 20. The A layer Final Thickness was made by
measuring the final material.
[0106] As the data in Table I show, for these ABA foam articles
made with a 47.4vol. % CO.sub.2 fugitive gas concentration in the B
layer, at a given casting speed, density increased (or stayed the
same) as barrier layer thickness increased. The data in Table I is
shown graphically in FIG. 2.
1 TABLE 1 A Layer Final Material Final Total B Layer Thick- Flow-
Casting Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed
Density ness Example wt % wt % .degree. C. MPa vol % Mat'l RPM
.mu.m kg/hr m/min g/cm.sup.3 .mu.m 1A 2.0 none 173 13.9 47.4 none 0
0 13.9 3.0 0.49 780 1B 2.0 none 173 13.9 47.4 none 0 0 13.9 6.1
0.54 380 1C 2.0 none 173 13.9 47.4 none 0 0 13.9 9.1 0.55 260 1D
2.0 none 174 14.4 47.4 LDPE 30 28.2 16.3 3.0 0.49 850 1E 2.0 none
174 14.4 47.4 LDPE 30 14.1 16.3 6.1 0.53 400 1F 2.0 none 174 14.4
47.4 LDPE 30 9.4 16.3 9.1 0.54 270 1G 2.0 none 174 14.6 47.4 LDPE
50 58.6 18.8 3.0 0.52 360 1H 2.0 none 174 14.6 47.4 LDPE 50 29.3
18.8 6.1 0.56 810 1I 2.0 none 174 14.6 47.4 LDPE 50 19.5 18.8 9.1
0.58 790
Comparative Example 2
[0107] The foam articles of this example were made with a foamable
layer having a gas concentration of about 64.3 volume percent.
[0108] The foam articles of Comparative Example 2 were made in the
same manner as those in Comparative Example 1 except that 4 wt %
RIC-50 was used, and the screw RPM of extruder 24 and the speed of
casting drum 28 were different. Operating conditions and test
results are shown in Table 2.
[0109] As the data in Table 2 shows, for these ABA foam articles
made with a 64.3 vol. % CO.sub.2 fugitive gas concentration in the
B layer, at a given casting speed, density increased as barrier
layer thickness increased.
2 TABLE 2 A Layer Final Material Final Total B Layer Thick- Flow-
Casting Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed
Density ness Example wt % wt % .degree. C. MPa vol % Mat'l RPM
.mu.m kg/hr m/min g/cm.sup.3 .mu.m 2A 4.0 none 174 13.9 64.3 LDPE
30 28.2 14.6 3.0 0.37 1260 2B 4.0 none 174 13.9 64.3 LDPE 30 14.1
14.6 6.1 0.39 590 2C 4.0 none 174 13.9 64.3 LDPE 30 9.4 14.6 9.1
0.41 370 2D 4.0 none 174 13.9 64.3 LDPE 70 56.2 14.6 3.0 0.39 930
2E 4.0 none 174 13.9 64.3 LDPE 70 42.1 14.6 6.1 0.41 640 2F 4.0
none 174 13.9 64.3 LDPE 70 28.1 14.6 9.1 0.44 420
Example 3
[0110] The foam articles of this example were made with a foamable
layer having a gas concentration of about 73.0 volume percent.
[0111] The foam articles of Example 3 were made and tested in the
same manner as those in Comparative Example 1 except that 6 wt %
RIC-50 was used as a blowing agent, and the screw RPM of extruder
24 and the line speed were different. Operating conditions and test
results are shown in Table 3.
[0112] Surprisingly, the data in Table 3 show that, for these ABA
foam articles made with a 73.0% vol. CO.sub.2 fugitive gas
concentration in the B layer, at a given casting speed, density
decreased as barrier layer thickness increased. The data in Table 3
are shown graphically in FIG. 3.
3 TABLE 3 A Layer Final Material Final Total B Layer Thick- Flow-
Casting Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed
Density ness Example Wt % wt % .degree. C. MPa vol % Mat'l RPM
.mu.m kg/hr m/min g/cm.sup.3 .mu.m 3A 6.0 none 173 14.9 73.0 None 0
0 13.9 3.0 0.43 830 3B 6.0 none 173 14.9 73.0 None 0 0 13.9 4.6
0.46 690 3C 6.0 none 173 14.9 73.0 None 0 0 13.9 6.1 0.45 550 3D
6.0 none 173 14.9 73.0 None 0 0 13.9 9.1 0.47 420 3E 6.0 none 173
14.9 73.0 LDPE 30 28.2 16.3 3.0 0.38 1300 3F 6.0 none 173 14.9 73.0
LDPE 30 18.8 16.3 4.6 0.38 970 3G 6.0 none 173 14.9 73.0 LDPE 30
14.1 16.3 6.1 0.39 710 3H 6.0 none 173 14.9 73.0 LDPE 30 9.4 16.3
9.1 0.40 470 3I 6.0 none 173 14.9 73.0 LDPE 70 84.3 21.0 3.0 0.31
2000 3J 6.0 none 173 14.9 73.0 LDPE 70 56.2 21.0 4.6 0.31 1400 3K
6.0 none 173 14.9 73.0 LDPE 70 42.1 21.0 6.1 0.35 950 3L 6.0 none
173 14.9 73.0 LDPE 70 28.1 21.0 9.1 0.41 590
Example 4
[0113] The foam articles of this example were made with a foamable
layer having a gas concentration of about 90.0 volume percent.
[0114] The foam articles of Example 4 were made in the same manner
as those in Comparative Example 1 except that a combination of 1.6
wt % carbon dioxide and 2 wt % RIC-50 were used as blowing agents,
and the screw RPM of extruder 24 and the speed of casting drum 28
were different. The carbon dioxide gas (99.9%, Oxygen Services, St.
Paul, Minn.) was introduced into extruder 14 at 0.23 kg/hr (0.5
lb/hr). Operating conditions and test results are shown in Table
4.
[0115] As the data in Table 4 show, for these ABA foam articles
made with a 90.0% vol. CO.sub.2 fugitive gas concentration in the B
layer, at a given casting speed, as barrier layer thickness
increased foam density decreased by between 5 and 60%. For example,
comparison of Examples 4A and 41 shows that adding a barrier layer
having a final thickness of 84.3 .mu.m decreased foam density by
almost 60%. The data in Table 4 are shown graphically in FIG.
5.
[0116] These data demonstrate one aspect of the invention by
showing that at high blowing agent concentrations, increasing the
barrier layer thicknesses caused density to significantly decrease
as compared to a foam article made with no, or thinner, barrier
layers.
4 TABLE 4 Final Material Total B Layer A Layer Flow- Casting Thick-
RIC-50 CO.sub.2 T.sub.m P.sub.m Gas Thick rate speed Density ness
Example wt % wt % .degree. C. MPa vol % Mat'l RPM .mu.m kg/hr m/min
g/cm.sup.3 .mu.m 4A 2.0 1.6 173 14.9 90.0 None 0 0 13.9 3.0 0.57
970 4B 2.0 1.6 173 14.9 90.0 None 0 0 13.9 4.6 0.53 670 4C 2.0 1.6
173 14.9 90.0 None 0 0 13.9 6.1 0.50 540 4D 2.0 1.6 173 14.9 90.0
None 0 0 13.9 9.1 0.48 390 4E 2.0 1.6 173 14.9 90.0 LDPE 30 28.2
13.9 3.0 0.54 1160 4F 2.0 1.6 173 14.9 90.0 LDPE 30 18.8 16.3 4.6
0.48 810 4G 2.0 1.6 173 14.9 90.0 LDPE 30 14.1 16.3 6.1 0.44 620 4H
2.0 1.6 173 14.9 90.0 LDPE 30 9.4 16.3 9.1 0.41 430 4I 2.0 1.6 173
14.9 90.0 LDPE 70 84.3 21.0 3.0 0.23 2150 4J 2.0 1.6 173 14.9 90.0
LDPE 70 56.2 21.0 4.6 0.24 1660 4K 2.0 1.6 173 14.9 90.0 LDPE 70
42.1 21.0 6.1 0.24 1250 4L 2.0 1.6 173 14.9 90.0 LDPE 70 28.1 21.0
9.1 0.22 750
Comparative Example 5
[0117] The foam articles of this example were made with barrier
layers comprising a pressure sensitive adhesive KRATON available as
HL2642X from H.B. Fuller, St. Paul, Minn., and a foamable layer
having a gas volume concentrations of 47.4 vol. %.
[0118] The foam articles of Comparative Example 5 were made in the
same manner as those in Comparative Example 1 except that a
different barrier layer composition was used. The blowing agent
used was 2.0 weight % RIC-50. Operating conditions and test results
are shown in Table 5. Data for Comparative Examples 1A, 1B, and 1C
are also shown in Table 5 for comparative purposes.
[0119] As the data in Table 5 shows, for this ABA foam made with a
47.4 vol. % CO.sub.2 fugitive gas concentration in the B layer, at
a given casting speed, density stayed relatively constant as
barrier layer thickness increased.
5 TABLE 5 A Layer Final Material Final Total B Layer Thick- Flow-
Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed Density
ness Example wt % wt % .degree. C. MPa vol % Mat'l RPM .mu.m kg/hr
m/m g/cm.sup.3 .mu.m 1A 2.0 none 173 13.9 47.4 none 0 0 13.9 3.0
0.49 780 1B 2.0 none 173 13.9 47.4 none 0 0 13.9 6.1 0.54 380 1C
2.0 none 173 13.9 47.4 none 0 0 13.9 9.1 0.55 260 5A 2.0 none 173
11.5 47.4 PSA 15 14.1 15.1 3.0 0.53 780 5B 2.0 none 173 11.5 47.4
PSA 15 7.1 15.1 6.1 0.58 370 5C 2.0 none 173 11.5 47.4 PSA 15 4.7
15.1 9.1 0.59 240 5D 2.0 none 173 11.5 47.4 PSA 30 28.2 16.3 3.0
0.53 860 5E 2.0 none 173 11.5 47.4 PSA 30 14.1 16.3 6.1 0.57 410 5F
2.0 none 173 11.5 47.4 PSA 30 9.4 16.3 9.1 0.60 260 5G 2.0 none 173
11.5 47.4 PSA 70 56.2 21.0 4.6 0.55 660 5H 2.0 none 173 11.5 47.4
PSA 70 42.1 21.0 6.1 0.57 480 5I 2.0 none 173 11.5 47.4 PSA 70 28.1
21.0 9.1 0.59 310
Example 6
[0120] The foam articles of this example were made with barrier
layers comprising a pressure sensitive adhesive KRATON available as
HL2642X from H.B. Fuller, St. Paul, Minn., and a foamable layer
having a gas volume concentrations of 90 vol. %.
[0121] The foam articles of Example 6 were made in the same manner
as those in Comparative Example 5 except that a different blowing
agent formulation was used. The blowing agent used was 2 weight %
RIC-50 and 1.6 weight % CO.sub.2. Operating conditions and test
results are shown in Table 6. Some of the data in Table 6 are shown
in FIG. 6. Data for Examples 4B, 4C, and 4D, which were made using
a similar gas volume percent are also shown in Table 6 for
comparative purposes.
[0122] As the data in Table 6 shows, for these ABA foam articles
made with a 90.0% vol. CO.sub.2 fugitive gas concentration in the B
layer, at a given casting speed, foam density decreased as barrier
layer thickness increased. For example, as shown by comparing 4D
and 6F, increasing the RPM of the extruder screw for the unfoamed
barrier layer from 0 to 70 decreased the density of the foam
construction made at a line speed of 9.1 m/min from 0.48 to 0.26
g/cm.sup.3, respectively.
6 TABLE 6 A Layer Final Material Final Total B Layer Thick- Flow-
Casting Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed
Density ness wt % wt % .degree. C. MPa vol % Mat'l RPM .mu.m kg/hr
m/min g/cm.sup.3 .mu.m 4B 2.0 1.6 173 14.9 90.0 None 0 0 13.9 4.6
0.53 670 4C 2.0 1.6 173 14.9 90.0 None 0 0 13.9 6.1 0.50 540 4D 2.0
1.6 173 14.9 90.0 None 0 0 13.9 9.1 0.48 390 6A 2.0 1.6 173 11.5
91.6 PSA 30 18.8 16.3 4.6 0.34 900 6B 2.0 1.6 173 11.5 91.6 PSA 30
14.1 16.3 6.1 0.29 690 6C 2.0 1.6 173 11.5 91.6 PSA 30 9.4 16.3 9.1
0.34 430 6D 2.0 1.6 173 11.5 91.6 PSA 70 56.2 21.0 4.6 0.27 1300 6E
2.0 1.6 173 11.5 91.6 PSA 70 42.1 21.0 6.1 0.26 940 6F 2.0 1.6 173
11.5 91.6 PSA 70 28.1 21.0 9.1 0.26 590
Example 7
[0123] Table 7 reports measured mechanical properties of some of
the foam articles described in the previous examples (as identified
in Table 7). The tensile strength of the samples at failure is
reported in Table 7.
[0124] Some of the data in Table 7 are shown in FIG. 7. FIG. 7,
shows the normalized tensile strength (N Tensile) as a function of
the normalized density (N Density) for foam articles made with
LDPE1550 foamable layers having fugitive gas concentrations of 47.4
vol. % (from 2 wt % RIC-50), 64.3 vol. % (from 4 wt % RIC-50), 73.0
vol. % (from 4 wt % RIC-50), and 90.0 vol. % (from 1.6 wt % carbon
dioxide with 2 wt % RIC-50), unfoamed barrier layers applied with
the extruder screw set at 0 to 70 RPM, and casting drum speeds of
about 6.1 m/min. The solid line in FIG. 7 represents the
density-to-strength relationship of typical polymer foam articles
as taught in Gibson, L. J. and Ashby, M. F., Cellular Solids, pp.
175-234, Cambridge University Press, 1997. As seen, the LDPE foam
articles of the present invention made with 73.0 vol. % and 90 vol.
% CO.sub.2 fugitive gas have a nearly zero-angle slope indicating
that the tensile strengths of these foam articles are substantially
independent of density.
7 TABLE 7 A Layer Final Material B Layer Final Casting RIC-50
CO.sub.2 Thickness Speed Tensile Elongation Density Example wt % wt
% RPM micron m/min MPa (psi) % N Tensile g/cm.sup.3 N Density 1A
2.0 none 0 0 3.0 5.78 (839) 580 0.56 0.49 0.53 1C 2.0 none 0 0 6.1
6.21 (901) 395 0.60 0.54 0.58 1D 2.0 none 0 0 9.1 6.25 (907) 167
0.60 0.55 0.61 1F 2.0 none 0 0 15.2 6.83 (990) 93 0.66 0.57 0.62 1H
2.0 none 30 28.2 3.0 6.60 (957) 645 0.64 0.49 0.54 1I 2.0 none 30
14.1 6.1 7.80 (1132) 520 0.75 0.53 0.57 1J 2.0 none 30 9.4 9.1 8.20
(1189) 383 0.79 0.54 0.59 1L 2.0 none 50 29.3 6.1 8.47 (1229) 552
0.82 0.56 0.61 2B 4.0 none 30 14.1 6.1 4.01 (582) 315 0.39 0.39
0.42 2E 4.0 none 70 42.1 6.1 4.95 (718) 481 0.48 0.41 0.45 3C 6.0
none 0 0 6.1 3.00 (435) 131 0.29 0.45 0.49 3H 6.0 none 30 14.1 6.1
2.71 (393) 130 0.26 0.39 0.43 3L 6.0 none 70 42.1 6.1 2.59 (376)
220 0.25 0.35 0.38 4C 2.0 1.6 0 0 6.1 2.90 (420) 86 0.28 0.50 0.55
4G 2.0 1.6 30 14.1 6.1 3.03 (440) 162 0.29 0.44 0.48 4K 2.0 1.6 70
42.1 6.1 2.65 (385) 338 0.26 0.24 0.26 4M 2.0 1.6 70 21.1 12.2 2.85
(413) 260 -- 0.24 --
Comparative Example 8
[0125] The foam articles of this example had 13 layers and were
made with foamable layers having a 47.4 vol. % fugitive gas
concentration.
[0126] The foam articles of Comparative Example 8 were made in the
same manner as those in Comparative Example 1 except that a
multi-layer feedblock was used to form a thirteen layer (AB).sub.6A
construction and the screw RPM of extruder 24 and the speed of
casting drum 28 were different. Operating conditions and test
results are shown in Table 8.
[0127] As the data in Table 8 show, for these 13 layer foam
articles made with a 47.4 vol. % CO.sub.2 fugitive gas
concentration in the B layers, at a given casting speed, as the
thickness of the unfoamed A layers of the film construction
increased, the density of the samples increased in a manner similar
to that of Comparative Example 1.
8 TABLE 8 A Layers Final Material Final Total B Layers Thick- Flow-
Casting Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed
Density ness Example wt % wt % .degree. C. MPa vol % Mat'l RPM
.mu.m kg/hr m/min g/cm.sup.3 .mu.m 8A 2.0 none 173 20.7 47.4 None 0
0 11.6 3.0 0.50 1030 8B 2.0 none 173 20.7 47.4 None 0 0 11.6 6.1
0.52 340 8C 2.0 none 173 20.7 47.4 None 0 0 11.6 9.1 0.54 230 8D
2.0 none 173 20.7 47.4 LDPE 30 9.3 14.3 3.0 0.53 1200 8E 2.0 none
173 20.7 47.4 LDPE 30 4.7 14.3 6.1 0.55 360 8F 2.0 none 173 20.7
47.4 LDPE 30 3.1 14.3 9.1 0.56 250 8G 2.0 none 173 20.7 47.4 LDPE
70 22.5 18.2 3.0 0.57 1350 8H 2.0 none 173 20.7 47.4 LDPE 70 11.2
18.2 6.1 0.58 430 8I 2.0 none 173 20.7 47.4 LDPE 70 7.5 18.2 9.1
0.60 280
Example 9
[0128] The foam articles of this example had 13 layers and were
made with a foamable layer having a 73.0 vol. % fugitive gas
concentration.
[0129] The foams of Example 9 were made in the same manner as those
in Comparative Example 8 except that 6 wt % RIC-50 was used as a
blowing agent instead of 2 wt % RIC-50. Operating conditions and
test results are shown in Table 9. Data for Examples 3A, 3C, and 3D
are also shown in Table 9 for comparative purposes.
[0130] As the data in Table 9 show, for these 13 layer foam
articles made with a 73.0 vol. % CO.sub.2 fugitive gas
concentration in the B layers, at a given casting speed, even at a
thickness greater than that of a similar single layer foam article
with no unfoamed A layers, the density of the multilayer sample was
lower than that of the single foam layer.
9 TABLE 9 A Layers Final Material Final Total B Layers Thick- Flow-
Casting Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed
Density ness Example wt % wt % .degree. C. MPa vol % Mat'l RPM
.mu.m kg/hr m/min g/cm.sup.3 .mu.m 3A 6.0 none 173 14.9 73.0 None 0
0 13.9 3.0 0.43 830 3C 6.0 none 173 14.9 73.0 None 0 0 13.9 6.1
0.45 550 3D 6.0 none 173 14.9 73.0 None 0 0 13.9 9.1 0.47 420 9D
6.0 none 173 20.7 73.0 LDPE 30 9.3 14.3 3.0 0.40 1130 9E 6.0 none
173 20.7 73.0 LDPE 30 4.7 14.3 6.1 0.45 650 9F 6.0 none 173 20.7
73.0 LDPE 30 3.1 14.3 9.1 0.52 420 9G 6.0 none 173 20.7 73.0 LDPE
70 22.5 18.2 3.0 0.37 1380 9H 6.0 none 173 20.7 73.0 LDPE 70 11.2
18.2 6.1 0.40 900 9I 6.0 none 173 20.7 73.0 LDPE 70 7.5 18.2 9.1
0.48 520
Example 10
[0131] The foam articles of this example had 13 layers and were
made with a foamable layer having a 91.5 vol. % fugitive gas
concentration.
[0132] The foam articles of Example 10 were made in a manner
similar to those in Comparative Example 8 except that a combination
of 2 wt % RIC-50 and 1.6 wt % carbon dioxide was used as blowing
agents, the melt temperature of the foamable mixture was changed,
and the screw RPM of extruder 24 and speed of casting drum 28 were
different. The melt temperature of the polymer solution in zone 6
of extruder 20 was about 144.degree. C. Operating conditions and
test results are shown in Table 10. Data for Example 4A are also
shown in Table 10 for comparative purposes.
[0133] The data in Table 10 show that for these 13 layer foam
articles made with a 91.5 vol. % CO.sub.2 fugitive gas
concentration in the B layers, at a given casting speed, even at a
thickness greater than that of a similar single layer foam article
with no unfoamed A layers, the density of the multilayer sample was
lower than that of the single foam layer.
10 TABLE 10 A Layer Final Material Final Total B Layer Thick- Flow-
Thick- RIC-50 CO.sub.2 T.sub.m P.sub.m Gas ness rate Speed Density
ness Example wt % wt % .degree. C. MPa vol % Mat'l RPM .mu.m kg/hr
m/min g/cm.sup.3 .mu.m 4A 2.0 1.6 173 14.9 90.0 None 0 0 13.9 3.0
0.57 970 10C 2.0 1.6 144 27.2 91.5 LDPE 30 18.6 14.3 1.5 0.31 2700
10D 2.0 1.6 144 27.2 91.5 LDPE 30 9.3 14.3 3.0 0.29 2999 10E 2.0
1.6 144 27.2 91.5 LDPE 70 45.0 18.2 1.5 0.31 3800 10F 2.0 1.6 144
27.2 91.5 LDPE 70 22.5 18.2 3.0 0.31 3130
[0134] Having now described the features, discoveries and
principles of the invention, the manner in which the process and
apparatus is constructed and used, the characteristics of the
construction, and the advantageous, new and useful results
obtained, the new and useful structures, devices, elements,
arrangements, parts, and combinations, are set forth in the
appended claims.
* * * * *