Uniform small cell foams and a continuous process for making...

Details Uniform small cell foams and a continuous process for making... Uniform small cell foams and a continuous process for making...

1999-04-28

2002-05-21

Seidleck, James J. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S134000, C521S079000, C521S081000

Reexamination Certificate

active

06391931

ABSTRACT:

FIELD OF INVENTION
This invention relates to thermoplastic foams and foam articles having one or more of the following properties: small cells, uniform cell sizes, pressure sensitive adhesive compositions, blended immiscible thermoplastic polymer compositions. The invention further relates to a method for making the foams and a method for coextruding the foams with other materials.
SUMMARY OF INVENTION
In one aspect, the present invention relates to continuous processes for producing foams. The processes can be used to produce foams comprised of amorphous thermoplastic polymers, including pressure sensitive adhesives, and blends of immiscible polymers. Another aspect of the invention is a process to coextrude the foams with other polymeric materials.
In one aspect, the present invention provides a continuous method for producing a foam material comprising:
(1) mixing at least one amorphous thermoplastic polymeric material and at least one physical blowing agent in an apparatus having an exit shaping orifice at a temperature and pressure sufficient to form a melt solution wherein the blowing agent is uniformly distributed throughout the polymeric material;
(2) reducing the temperature of the melt solution at the exit of the apparatus to an exit temperature that is equal to or less than 30° C. above the glass transition temperature of the neat polymeric material while maintaining the melt solution at a pressure sufficient to keep the blowing agent in solution; and
(3) passing the solution through the exit shaping orifice and exposing the solution to atmospheric pressure, thereby causing the blowing agent to expand resulting in nucleation and cell formation, which causes the melt solution to foam at or about the time it exits the shaping orifice.
In another aspect, the invention provides foam-containing articles that can be designed to exhibit a wide range of properties for a myriad of applications. The polymeric materials used in making the articles may comprise amorphous thermoplastic polymers including pressure sensitive adhesives, and blends of immiscible thermoplastic polymers. A range of suitable exit temperatures may be determined based on the polymeric material used to make the foam.
In another aspect, the invention further provides a way to control the cell size and cell size distribution of a foam by adjusting, manipulating, or controlling the blowing agent concentration, the exit temperature, and/or the exit pressure of the foamable melt solution.
In another aspect, the invention features articles comprising a foam having cell sizes of 2 to 200 micrometers, preferably 5 to 50 micrometers. The foam may alternatively, or additionally, have a cell size distribution with a polydispersity from 1.0 to 2.0, preferably from 1.0 to 1.5, more preferably from 1.0 to 1.2.
In another aspect, the invention features articles wherein the foam of the invention comprises at least one layer in a multi-layer construction.
The invention further features a coextrusion process whereby a foam is coextruded with at least one other material, which may be a foamed or unfoamed material.
As used in this invention:
“small-cell foam” means a foam having cell sizes of 2 to 200 micrometers (&mgr;m), preferably 5 to 50 &mgr;m;
“closed-cell” means a foam material that contains substantially no connected cell pathways that extend from one outer surface through the material to another outer surface;
“operating temperature” means the temperature that must be achieved in the extrusion process, prior to the addition of the physical blowing agent, to melt all of the polymeric materials in the melt mix;
“T
g
” means the glass transition temperature, i.e., the temperature at which a polymer changes from a fluid to a solid state;
“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;
“average” means the arithmetic average, i.e., mean;
“standard deviation” means the “typical” deviation in cell size from the mean cell size; it is calculated using the following formula:
σ
=
∑
i
=
1
n
⁢
(
x
i
-
x
_
)
2
n
-
1
 where &sgr; is the standard deviation, x
i
is an observed cell size, {overscore (x)} is the arithmetic average cell size, and n is the total number of cell size observations;
“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;
“neat polymer” means a polymeric material having no additives, and at standard temperature and pressure;
“nucleation” means a process by which a homogeneous solution of polymeric material and dissolved molecules of a species that is a gas under ambient conditions undergoes formations of clusters of molecules of the species that define “nucleation sites” from which cells will grow; i.e., it is a change from a homogeneous solution to a multi-phase mixture in which, throughout the polymeric material, sites of aggregation of at least several molecules of physical blowing agent are formed (if immiscible polymeric materials are used, the physical blowing agent will typically form single-phase solutions with one or more of the polymer materials, but the polymers will typically not combine to form a single phase);
“supercritical fluid” means a substance, which is typically a gas at ambient temperature and pressure, compressed to a state where it has the density and solvation characteristics of a liquid, but the viscosity, permeability, and diffusivity of a gas; a supercritical fluid is a single phase material that exists above a critical point, which point is determined by a critical temperature, T
c
, and critical pressure, P
c
, which T
c
and P
c
depend on the particular gas (for example, the T
c
and P
c
for carbon dioxide are approximately 31° C. and 7.4 MPa (1078 psia), respectively);
“foam density” means the weight of a given volume of foam;
“inversion temperature” means the temperature at which a minimum foam density is obtained for a given polymeric foam; at temperatures above and below the inversion temperature, a higher foam density will typically be obtained;
“density reduction” refers to a way of measuring the void volume of a foam based on the following formula:
ρ
R
=
⌊
1
-
ρ
f
ρ
o
⌋
×
100
⁢
%
 where &rgr;
R
is the density reduction, &rgr;
f
is the foam density, and &rgr;
o
is the density of the original material;
“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;
“uniform” means that the cell size distribution has a polydispersity of 1.0 to 2.0;
“spherical” means generally rounded; it may include spherical, oval, or circular structure;
“fibrillose” means having elongated filament-like or thread-like structures;
“schistose” means having parallel plate-like ribbons;
“polymer matrix” means the polymeric, or “non-cell,” areas of a foam;
“blend matrix” means the polymeric material having the highest volume fraction in a melt mixture comprising at least two immiscible materials;
“immiscible” refers to thermoplastic polymers that will not mix or remain mixed with each other, although at certain conditions, such as high temperatures, they might mix, but any such mixture will typically be thermodynamically unstable and will typically separate into distinct- phases at lower temperatures;
“miscible” refers to two or more thermoplastic materials that will form a homogeneous mixture, that is, dissolve in each other;
“anisotropic” means having different properties or degrees of properties in different directions parallel to a major surface; and
“straight line tear” means a tear not deviating more than 20°, preferably not more than 10°, from the direction in which the tear is initiated.
An advantage of at least one embodiment of the present invention is the ability to alter, adjust, or control the foam

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