Elastomeric sponge

An expandable composition of a solid synthetic vulcanizable elastomer of ethylene/higher alpha-olefin/nonconjugated diene copolymer or polychloroprene, a blowing agent, and a curing agent for the elastomer, said composition containing from about 5-25% based on the weight of total polymer of an ionomer resin which is an ethylene polymer containing at least about 50 mole percent ethylene and bearing from about 0.2-25 mole percent acid functional groups that are at least about 50% neutralized by metal ions.

BACKGROUND OF THE INVENTION 
This invention is directed to an expandable composition of an 
ethylene/higher alpha-olefin/nonconjugated diene copolymer or 
polychloroprene and an ionomer. 
Synthetic elastomers have been expanded by the action of blowing agents to 
manufacture products having a cellular structure. Cellular elastomers 
having closed cells are especially useful because they are flexible while, 
at the same time, they have high compressive strengths. Such cellular 
products are used as automotive and constructive gaskets and pipe 
insulation. Although synthetic elastomers including 
ethylene/propylene/nonconjugated diene copolymers (EPDM) and 
polychloroprene can be expanded to form cellular products, it is a 
somewhat complicated procedure. For example, ethylene/higher 
alpha-olefin/nonconjugated diene copolymers have been expanded by a 
procedure in which the copolymer is partially cured below the 
decomposition temperature of the blowing agent, and then the copolymer is 
blown while curing is completed. Polychloroprene can be blown by a similar 
procedure to form closed cell expanded articles. Usually, therefore, when 
EPDM or polychloroprene compositions are expanded a careful balancing of 
compound viscoelasticity, cure rate and blowing rate is required. This 
necessitates the selection of a particular blowing agent and a particular 
curing agent so that decomposition of the blowing agent is carefully 
coordinated with the rate of cure of the elastomer. Accordingly, there is 
a need for an expandable composition of vulcanizable ethylene/higher 
alpha-olefin/nonconjugated diene copolymer or polychloroprene that can be 
easily processed and that can form a closed cell sponge having uniform 
cell size with little, if any, concern given to careful balancing of the 
cure rate with the rate of blowing. 
SUMMARY OF THE INVENTION 
It has now been discovered that certain synthetic vulcanizable amorphous 
elastomers can be cured and blown simultaneously, with ease, if bubble 
growth is controlled during the blowing step by the presence of a minor 
amount of an ionomer. More specifically, the expandable composition of 
this invention comprises a solid synthetic vulcanizable elastomer selected 
from the group consisting of ethylene/higher alpha-olefin/nonconjugated 
diene copolymer and polychloroprene, a blowing agent and a curing agent 
for the elastomer wherein said composition contains from about 5-25%, 
preferably 5-15%, based on the weight of total polymer, of an ionomer 
resin which is an ethylene polymer containing at least about 50 mole 
percent ethylene and bearing from about 0.2-25 mole percent acid 
functional groups that are at least about 50% neutralized, preferably 
about 85-100%, by metal ions. The expandable composition forms an 
elastomeric closed cellular sponge when it is heated to a temperature 
sufficient to activate the blowing agent and curing agent. The expandable 
composition is much less sensitive to curing conditions than prior art 
compositions, and improved melt rheology permits the composition of this 
invention to be extruded and cured at higher temperatures than normal, 
thus requiring shorter curing times which, of course, is more economical. 
These expanded compositions are particularly useful for automotive and 
construction gaskets. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The vulcanizable synthetic elastomer that is expanded is preferably a solid 
ethylene/higher alpha-olefin/nonconjugated diene copolymer. The higher 
alpha-olefin usually has 3-6 carbon atoms and can be propylene, 1-butene, 
4-methylene-1-pentene, 1-pentene, 1-hexene, and the like. Preferably, 
because of its commercial availability and the resulting properties of 
such elastomers, propylene is the higher alpha-olefin of choice and the 
elastomer is the well known EPDM rubber. Such rubber generally contains 
about 15-50 weight percent propylene and about 1-10 weight percent diene. 
The nonconjugated diene can contain 5-24 carbon atoms in either straight 
or branched chain or cyclic structure. Representative nonconjugated dienes 
include aliphatic dienes such as 1,4-hexadiene, 1,9-octadecadiene, 
11-ethyl-1,11-tridecadiene, 6-methyl-1,5-heptadiene or cyclic dienes such 
as dicyclopentadiene, 5-alkenyl substituted norbornenes, e.g., 
5(2-butenyl)-2-norbornene, 1,5-cyclooctadiene, 5-alkylidene-2-norbornene, 
e.g., 5-methylene-2-norbornene and 5-ethylidene-2-norbornene. Particularly 
preferred EPDM elastomers contain, as the nonconjugated diene, either 
1,4-hexadiene or ethylidene norbornene. 
Another solid synthetic vulcanizable amorphous elastomer that can be used 
as a component of the expandable compositions of this invention is 
polychloroprene rubber (CR), commonly known as neoprene. CR is 
commercially available in a number of grades or modifications, all of 
which are suitable for use in the compositions of this invention. 
The expandable compositions of this invention contain from about 5-25 
weight percent, preferably about 5-15 weight percent, based on the total 
amount of polymers in the composition, of an ionomer that is formed by at 
least partial neutralization, i.e., at least 50%, or complete 
neutralization of a polymer of ethylene containing at least about 50 mole 
% ethylene units and bearing from about 0.2-25 mole percent, usually about 
1-10 mole percent, acid functional groups. By "neutralization" is meant 
that the acid groups of the polymer have been reacted with a basic metal 
compound to convert them to ionic salt groups. Suitable basic metal 
compounds that can be used to neutralize the ethylene polymer to form the 
ionomer include the hydroxides, oxides, carbonates, bicarbonates, and 
organic carboxylates such as formates, acetates or acetal acetonates of 
the metals of Groups I, II and III of the Periodic Table of the Elements. 
The neutralization of the acid functional groups can be effected either 
before mixing with the synthetic elastomer or during the mixing step by 
the addition of a basic metal compound, such as zinc oxide, to the 
formulation. In most cases sufficient basic metal compound is added to the 
composition to neutralize substantially all of the acid groups in the 
acid-bearing polymer, and often an amount in excess of that required for 
complete neutralization of the acid groups is used. 
The polymers of ethylene bearing acid functional groups can be formed in 
several ways, the best known being the copolymerization of ethylene with 
copolymerizable alpha-beta unsaturated carboxylic acids, either alone or 
with additional copolymerizable comonomers. Acid groups may also be 
attached to preformed ethylenic polymers or copolymers by grafting 
unsaturated acids in so-called "carboxylation" reactions or by sulfonation 
with a sulfonating agent such as sulfur trioxide. Such ethylene polymers 
bearing acid functional groups are well known compounds and are further 
described below. 
Copolymers of ethylene with alpha-beta unsaturated carboxylic acids and 
their conversion to ionomers by neutralization have been described, for 
example, in U.S. Pat. No. 3,264,272. The carboxylic acids used in such 
polymers with ethylene may be mono- or dicarboxylic acids - either as free 
acids or their anhydrides - and monoesters of dicarboxylic acids. Usually 
such carboxylic acids contain from 3-8 carbon atoms. Representative 
carboxylic acid monomers used in the copolymers include acrylic acid, 
methacrylic acid, itaconic acid, maleic acid, fumaric acid, methyl 
hydrogen maleate, methyl hydrogen fumarate, and maleic anhydride. Although 
anhydrides such as maleic anhydride, are not carboxylic acids per se 
because they have no hydrogen attached to the carboxyl group, polymers 
containing them are readily converted to ionomers by the action of the 
basic neutralizing agent. Representative copolymers include 
ethylene/acrylic acid copolymers, ethylene/methacrylic acid copolymers, 
ethylene/itaconic acid copolymers, ethylene/methyl hydrogen maleate 
copolymers and ethylene/maleic acid copolymers. In addition to ethylene 
and an alpha-beta unsaturated carboxylic acid, the copolymer can contain 
other copolymerizable monoethylenically unsaturated monomers in amounts up 
to about 50 mole percent of the copolymer. Such monomers include alkyl 
acrylates, usually having from 1-6 carbon atoms in the alkyl group, vinyl 
carboxylates and vinyl esters of organic acids in which the acid usually 
contains 2-8 carbon atoms, acrylic acid derivatives such as acrylonitrile 
or methacrylonitrile and alpha-olefins having 3-8 carbon atoms. 
Representative copolymers of this type include ethylene/methacrylic 
acid/ethyl acrylate copolymers, ethylene/methyl hydrogen maleate/methyl 
acrylate copolymers, ethylene/methacrylic acid/vinyl acetate copolymers, 
ethylene/isobutyl acrylate/methacrylic acid, ethylene/propylene/acrylic 
acid copolymers, ethylene/methacrylic acid/acrylonitrile copolymers, and 
ethylene/vinyl chloride/acrylic acid copolymers. 
Acid groups may be attached to preformed ethylenic polymers such as 
polyethylene or copolymers such as ethylene/propylene, optionally 
containing a nonconjugated diene, by grafting unsaturated organic acids 
containing at least one double bond and at least one functional acid group 
onto the ethylene polymer in amounts of usually from about 0.5-9% by 
weight. Especially suitable acids that can be grafted onto the ethylene 
polymer include fumaric acid, maleic acid, or maleic anhydride, in 
so-called "carboxylation" reactions. These carboxylated polymers are well 
known in the art and can be prepared by heating the polymer with the acid 
at elevated temperatures in the presence of a free radical generator such 
as a peroxide. Such carboxylated hydrocarbon polymers are described in, 
for example, U.S. Pat. Nos. 3,236,917, 3,427,183 and 3,862,265. If the 
hydrocarbon base polymer contains olefinic unsaturated groups, such as 
those present in EPDM rubber, it is possible to attach unsaturated acids, 
particularly maleic acid or anhydride or fumaric acid, to the polymer 
without the use of a free radical generator as described in U.S. Pat. No. 
4,010,223. Acid groups can also be attached to ethylene polymers, such as 
ethylene/propylene copolymers, having olefinic unsaturation by sulfonation 
with a sulfonating agent such as sulfur trioxide complexed with a Lewis 
base so that the polymer usually has a sulfonic acid content of from about 
0.2-8 mole percent. Sulfonated ethylene/propylene copolymers are described 
in U.S. Pat. No. 3,642,728. 
As mentioned above, the ethylene polymer bearing acid functional groups is 
neutralized to the ionomer by using basic metal compounds to convert the 
acid groups to ionic salt groups and thus form a salt of the copolymer of 
ethylene. Neutralization can take place before the ethylene polymer is 
mixed with the elastomer or neutralization to the ionomer can take place 
during the mixing operation. At least 50% of the acid groups are 
neutralized and preferably 80-100%. Usually, an excess of basic metal is 
added to the polymer so that substantially all the acid groups are 
neutralized with metal ions. Representative basic metal compounds that can 
be used to neutralize the ethylene polymer to form the ionomer include the 
hydroxides, oxides, carbonates, bicarbonates, formates, and acetates of 
the metals of Groups I, II and III of the Periodic Table of the Elements. 
Representative basic metal compounds include sodium hydroxide, chromium 
octoate, lithium acetate, or magnesium acetal acetonate, but preferably 
zinc oxide is used because it also functions as a vulcanizing agent or an 
accelerator. 
The blowing agents used in the compositions of this invention are those 
which generate a gas at temperatures used to cure the elastomers. The 
blowing agent can be a solid chemical compound that decomposes to produce 
the gas or an inert liquid that vaporizes at curing temperatures; 
preferably, solid chemical compounds are used. Generally, the amount of 
blowing agent incorporated in the expandable composition is from about 
1-15 parts, preferably 3-8 parts, per 100 parts total polymer. Usually, 
the blowing agents are activated at temperatures of from about 
150.degree.-220.degree. C., most frequently about 170.degree.-200.degree. 
C. Chemical blowing agents that decompose to release an inert gas, such as 
carbon dioxide or nitrogen, at elevated temperatures are especially 
suitable. Representative chemical blowing agents include azodicarbonamide, 
dinitrosopentamethylenetetramine, 
N,N'-dimethyl-N,N'-dinitrosoterephthalamide, azo-bis-isobutyronitrile, 
benzenesulfonyl hydrazide, 4,4'-oxybis(benzenesulfonyl hydrazide), 
p-toluenesulfonyl hydrazide and inorganic compounds such as sodium 
carbonate or bicarbonate. Liquid blowing agents that release inert gases 
can also be employed. Representative liquid blowing agents that volatilize 
at curing temperatures to expand the composition include pentane, hexane, 
heptane, benzene, toluene, methyl chloride, trichloroethane, 
1,2-dichloroethane and similar compounds. Optionally, conventional 
activators for these blowing agents such as pentaerythritol, ethylene 
glycol, salicylic acid, stearic acid, urea, and the like can be added to 
the composition along with the blowing agent. 
Any conventional vulcanizing system for the ethylene/higher 
alpha-olefin/nonconjugated diene rubber or the polychloroprene rubber (CR) 
can be used to cure the expandable composition. Generally, conventional 
vulcanizing temperatures of from about 150.degree.-220.degree. C. are used 
to cure the elastomers. Vulcanizing agents incorporated in the expandable 
composition to cure ethylene/higher alpha-olefin/nonconjugated diene 
elastomers are elemental sulfur or a compound that releases sulfur at 
vulcanization temperatures, or mixtures thereof, which compounds are well 
known in the industry. Representative vulcanizing agents that release 
sulfur at vulcanization temperatures include thiuram polysulfides, amine 
disulfides, and sodium polysulfide. Usually about 0.2-5 parts per 100 
parts ethylene/higher alpha-olefin/nonconjugated diene elastomer of sulfur 
or about 0.6-15 parts per 100 parts ethylene/higher 
alpha-olefin/nonconjugated diene elastomer of compounds that release 
sulfur are incorporated in the foamable composition. Although it is less 
preferred, peroxide curing agents, such as aromatic or aliphatic 
peroxides, can be used to cure the hydrocarbon elastomer. Representative 
peroxide curing agents include aromatic diacyl peroxides and aliphatic 
diacyl peroxides, dibasic acid peroxides, ketone peroxides, alkyl 
peroxyesters, alkyl hydroperoxides, dibenzoyl peroxide, di-tert.-butyl 
peroxide, dicumyl peroxide, tert.-butylperbenzoate, tert.-butyl cumyl 
peroxide, and the like. The polychloroprene can be vulcanized by 
conventional means employing any of the well known vulcanizing systems 
used for this purpose. Generally, such vulcanizing systems include a metal 
oxide, e.g., magnesium or zinc oxide, and an organic accelerator or curing 
agent which can be an amine, phenol, sulfinamide, thiazole, thiuram, 
thiourea or sulfur. 
The expandable compositions of this invention can also include conventional 
fillers, dessicants, plasticizers and lubricants that are normally used in 
compounded elastomeric compositions. 
The expandable composition of this invention is best prepared in a two step 
procedure by mixing, in a first step, about 75-95%, based on the weight of 
total polymer, of the synthetic elastomer, preferably an EPDM rubber, with 
about 5-25% based on the weight of total polymer of an ethylene polymer 
bearing acid functional groups, preferably ethylene and methacrylic acid 
copolymers, either in the free acid or neutralized form, on a two-roll 
mill at about 45.degree.-64.degree. C. or in an internal mixer at a 
temperature of from about 140.degree.-180.degree. C. for 4-8 minutes. If 
the ethylene polymer is added in the free acid form, an amount of a basic 
metal compound such as zinc oxide sufficient to neutralize, preferably, 
substantially all of acid functional groups is added to the mixer. The 
expandable composition can also contain other conventional ingredients, 
such as processing aids, e.g., stearic acid or oleic acid, accelerators 
and fillers such as carbon black or calcium carbonate. In a second mixing 
step the composition described above is mixed on a mill or in an extruder 
with from about 1-15 parts per 100 parts total polymer of a blowing agent 
and sufficient curing agent to vulcanize the synthetic elastomer. The 
resulting composition is heated to activate both the blowing agent and the 
curing agent, resulting in simultaneously curing and blowing the 
composition to form a closed cell sponge. Preferably, the composition is 
mixed in an internal mixer and extruded. It passes out of the extruder and 
is passed into a hot fluid, such as an inert liquid or gas, where it 
expands and cures simultaneously.

The following examples illustrate the invention. All parts and percentages 
are by weight unless stated otherwise. 
EXAMPLE 1 
An EPDM rubber, which is a copolymer of ethylene, 32 wt % propylene, and 4 
wt % 1,4-hexadiene having a Mooney viscosity (ML.sub.4 at 121.degree. C.) 
of 60, is mixed with various ionomer resins and the following ingredients 
in an internal mixer (Brabender Plasticorder) at 175.degree. C. for 5 
minutes at 65 rpm. 
______________________________________ 
Ingredient Parts by Wt. 
______________________________________ 
EPDM rubber (described above) 
16 
Ionomer resin (see table) 
4 
Zinc Oxide 2 
Stearic Acid 0.2 
SRF Carbon Black 16 
Calcium Carbonate (Atomite Whiting) 
16 
Paraffinic Process Oil (Sunpar 2280) 
14 
______________________________________ 
The mixed composition is removed from the mixer and both mixture and mixer 
are cooled to 90.degree. C. The mixture is then returned to the mixer and 
combined with the following curing and blowing agents while mixing for 3.5 
minutes at 90.degree. C. and 45 rpm. 
______________________________________ 
Curing and Blowing Ingredients 
Parts by Wt. 
______________________________________ 
Calcium Oxide Dispersion (Desical P) 
1.2 
Sulfur 0.4 
Methyl Zimate 0.3 
Ethyl Tellurac 0.1 
Mercaptobenzthiazole 0.2 
Thiocarbanilide 0.1 
Ethylene Thiourea (75% in EPM) 
0.06 
Azodicarbonamide - blowing agent 
2.0 
______________________________________ 
The composition is removed from the mixer and pressed into 3 mm thick 
placques in a platen press (5 min at 90.degree. C.). Samples from these 
placques (50 mm.times.75 mm) are placed in a circulating air oven for the 
times and at the temperatures indicated below to effect simultaneous 
blowing and curing, resulting in a closed cell sponge. The density of the 
cooled samples is determined using a water displacement pycnometer. 
______________________________________ 
Ionomers Used 
______________________________________ 
Ionomer A 93.8 mole % ethylene, 3.8 mole % meth- 
acrylic acid, 2.6 mole % isobutyl 
acrylate terpolymer, Zn salt. 
Ionomer B 96.5 mole % ethylene, 3.5 mole % meth- 
acrylic acid, zinc salt. 
Ionomer 88.2 mole % ethylene, 11.4 mole % 
Precursor* C 
vinyl acetate, 0.4 mole % methacrylic 
acid copolymer. 
Ionomer 88 mole % ethylene, 11.4 mole % vinyl 
Precursor* D 
acetate, 2.0 mole % methacrylic acid. 
______________________________________ 
*100% neutralized during mixing by excess ZnO. 
______________________________________ 
Results 
Sponge density (kg/m.sup.3) after 
Ionomers Used 
7 min/195.degree. C. 
10 min/195.degree. C. 
5 min/210.degree. C. 
______________________________________ 
None* 350 350 290 
A 260 260 240 
B 220 240 210 
C 230 210 200 
D 190 210 210 
______________________________________ 
*Amount of EPDM increased to 20 parts to compensate for absence of ionome 
resin. 
All of the resulting expanded compositions, i.e., closed cell sponges 
containing ionomer resin, have relatively small cells (average 100 .mu.m 
or less) of nearly uniform size. 
EXAMPLE 2 
The procedure described in Example 1 to form closed cell sponge is 
repeated, except that the EPDM rubber used is a copolymer of ethylene, 40 
wt % propylene and 4.7 wt % 1,4-hexadiene having a Mooney viscosity 
(ML.sub.4 at 121.degree. C.) of 70, and the ionomer resins used are: 
Ionomer E--Polyethylene grafted with fumaric acid, zinc salt. 
Ionomer F--69.4 mole % ethylene, 29.2 mole % methyl acrylate, 1.4 mole % 
methyl hydrogen maleate copolymer, chromium salt. 
______________________________________ 
Results 
Sponge density (kg/m.sup.3) after 
Ionomer Used 
7 min/195.degree. C. 
10 min/195.degree. C. 
5 min/210 .degree. C. 
______________________________________ 
None* 340 360 290 
E 320 310 260 
F 250 270 230 
B 250 250 230 
______________________________________ 
*See footnote of Example 1 
EXAMPLE 3 
The procedure described in Example 1 to form closed cell sponge is 
repeated, except that the EPDM rubber used is a copolymer of ethylene, 29 
wt % propylene and 3.1 wt % 5-ethylidene-2-norbornene, and the following 
ionomer precursor is used. 
Ionomer Precursor* G--77.1 mole % ethylene, 20.6 mole % propylene, 1.6 mole 
% 1,4-hexadiene, 0.06 mole % norbornadiene copolymer grafted with 0.6 mole 
% fumaric acid. 
FNT *100% neutralized during mixing with excess ZnO. 
______________________________________ 
Results 
Sponge density (kg/m.sup.3) after 
Ionomers Used 
7 min/195.degree. C. 
10 min/195.degree. C. 
5 min/210.degree. C. 
______________________________________ 
None* 1170 680 1270 
G 870 650 650 
A 310 270 830 
C 1020 560 1050 
______________________________________ 
*See footnote of Example 1. 
EXAMPLE 4 
The procedure described in Example 2 to make closed cell sponge is 
repeated, except that the ionomer resins used are: 
Ionomer H--Sulfonated ethylene, 59.2 mole % ethylene, 39.7 mole % 
propylene, 1.2 mole % 2-ethylidene-5-norbornene copolymer, zinc salt. 
Ionomer I--Sulfonated EPDM, Zn salt, containing plasticizer. 
______________________________________ 
Results 
Sponge density (kg/m.sup.3) after 
Ionomers Used 
7 min/195.degree. C. 
10 min/195.degree. C. 
7 min/210.degree. C. 
______________________________________ 
None* 400 390 350 
H 300 310 270 
I 300 310 270 
______________________________________ 
*See footnote of Example 1. 
EXAMPLE 5 
A polychloroprene rubber (Neoprene W) is mixed with various ionomer resins 
given below and the following ingredients on a two-roll mill at 
45.degree.-95.degree. C. 
______________________________________ 
Ingredients Parts by Wt. 
______________________________________ 
Polychloroprene rubber 
40 
Ionomer resin (see table) 
10 
Carbon black, MT 17 
Carbon black, FEF 7.5 
Aromatic Process Oil (Sundex 790) 
10 
Magnesium Oxide 2 
Zinc Oxide 2.5 
Stearic Acid 0.5 
______________________________________ 
When the above ingredients are thoroughly mixed, the following curing 
agents are added: 
Octamine--1 
Heliozone--1.5 
Sodium Carbonate blowing agent--2.5 
Surface Coated Urea (Activator DN, Du Pont)--0.8 
Diethylene Thiourea--1.0 
Ethylene Thiourea (75% in EPM)--1.4 
The mixtures are pressed into sheets, cured and blown as described in 
Example 1 to form closed cell sponge. 
______________________________________ 
Results 
Sponge density (kg/m.sup.3) after 
Ionomers Used 
7 min/195.degree. C. 
10 min/195.degree. C. 
8 min/205.degree. C. 
______________________________________ 
None* 620 620 590 
A 510 500 460 
F 280 290 240 
C 380 370 340 
______________________________________ 
*Polychloroprene increased to 50 parts 
EXAMPLE 6 
The following masterbatch is mixed in a Banbury internal mixer for 2 min at 
177.degree.-190.degree. C., removed and cooled. 
______________________________________ 
Parts by Wt. 
______________________________________ 
EPDM of Example 2 72.4 
EPDM of Example 1 17.6 
Ethylene/10% methacrylic acid copolymer 
10 
Zinc Oxide 3 
Stearic Acid 0.2 
Carbon Black (N 650) 6.8 
______________________________________ 
When it is desired to make extruded sponge, the masterbatch is mixed with 
curing ingredients as follows in a Banbury mixer until the stock 
temperature reaches 90.degree. C. 
______________________________________ 
Parts by Wt. 
______________________________________ 
Above masterbatch 110 
Stearic acid 0.6 
Carbon black (N 650) 40 
Carbon black (N 762) 110 
Paraffinic Process Oil (Sunpar 2280) 
95 
Ethyl Tellurac 0.5 
Butyl Zimate 2 
Mercaptobenzthiazole (MBT) 
1 
Calcium Oxide Dispersion in oil (Desical P) 
3 
Sulfur 2 
Thiocarbanilide (Accelerator A-1) 
0.5 
Azodicarbamide (blowing agent) 
6 
Pentaaerythritol 2 
______________________________________ 
The mixture is removed from the mixer, formed into strips on a two-roll 
mill, fed to an extruder and extruded through a die of the desired shape. 
The extrudate is passed through an air oven maintained at 218.degree. C. 
where it stays three minutes to effect blowing and curing. The product 
closed cell sponge has a density of 300 kg/m.sup.3 and a small uniform 
cell structure and is useful as a gasket for automobile doors.