CA1326099C

CA1326099C - Process of producing a fibrillated semi-interpenetrating polymer network of polytetrafluoroethylene and silicone elastomer and shaped products thereof

Info

Publication number: CA1326099C
Application number: CA000555735A
Authority: CA
Inventors: Mark E. Dillon; Joseph A. Dillon
Original assignee: Tetratec Corp
Current assignee: Donaldson Co Inc
Priority date: 1987-01-05
Filing date: 1988-01-04
Publication date: 1994-01-11
Anticipated expiration: 2011-01-11
Also published as: AU1220988A; DK174355B1; JPH0726311B2; NZ223097A; JPH01501876A; US4945125A; EP0296240A1; AR244743A1; FI91644C; ZA8842B; DK492988D0; NO883930D0; AU599207B2; NO883930L; EP0296240A4; CN88100643A; KR890700446A; CN1014795B; EP0296240B1; DK492988A

Abstract

ABSTRACT
A process for producing a semi-interpenetrating polymer network of polytetrafluoroethylene and silicone elastomers is described which comprises the steps of (1) intimately blending a mixture of a major amount of unsintered and unfibrillated parti-culate polytetrafluoroethylene dispersion resin and minor amounts of (A) a hydrocarbon liquid and (B) an addition curable silicone composition consisting essentially of a polydiorganosiloxane having alkenyl unsaturation, an organohydrogenpolysiloxane cross-linking agent a catalyst for promoting crosslinking of said polysiloxane, and an inhibitor for the catalytic reaction; (2) forming said blend into an extrudable shape; (3) biaxially extru-ding said blend through a die into a shaped extrudate product having a randomly fibrillated structure; (4) evaporating said hydrocarbon liquid, and activating said catalyst so as to generate a cured silicone elastomer and polytetrafluoroethylene semi-inter-penetrating polymer network comprising said fibrillated extrudate structure. Products produced by this process have improved physi-cal properties as compared to extruded fibrillated polytetra-fluoroethylene dispersion resin alone.

Description

1 3 2 6 ~ ~ 9 60973-636 PROCESS OF PRODUCING A FIBRILLATED SEMI-INTERPENETRATING POLYMER
NETWORK OF POLYTETRAFLUOROETHYLENE AND SILICONE
ELASTOMER AND SHAPED PRODUCTS THEREOF

Field of the Invention Thls lnventlon relates to novel composltlons comprislng seml-lnterpenetratlng polymer networks of polytetrafluoroethylene and sillcone elastomers, a process for maklng such composltlons, and shaped products formed from such composltlons. More partlcu-larly, it relates to the compositlons produced by (1) lntlmately blendlng a mixture of a ma~or amount of unsintered and unfibril-lated partlculate polytetrafluoroethylene dlspersion resln and minor amounts of (A) a hydrocarbon liquid and tB) an addition curable silicone composition conslstlng essentlally of a polydl-organoslloxane havlng alkenyl unsaturatlon, an organohydrogen-polysiloxane crossllnklng agent, a catalyst for promoting cross-llnking of said poly~lloxane, and an lnhlbltor for the catalytlc reactlon~ (2) formlng sald blend lnto an extrudable shape~ (3) blaxlally extrudlng sald blend through a dle into a shaped product having a randomly fibrlllated structure; (4) evaporatlng said hydrocarbon llquld, and actlvatlng sald catalyst so as to generate a cured slllcone elastomer and polytetrafluoroethylene seml-lnter-penetratlng polymer network comprlslng sald flbrlllated structure.

DescrlPtlon of the Prior Art In U.S. Patent No. 3,315,020 there is dlsclosed a pro-ces~ for preparing sheet-llke artlcles of biaxially flbrlllated polytetrafluoroethylene whlch, though ln the unslntered state, IX `

1 3 2 6 0 9 ~ 60973-636 have hlgh elongatlon and strength ln all dlrectlon~ ln the ma~Jor plane of the sheet. In thls process, a relatlvely large cyllnder of compacted unflbrlllated dlsperslon grade polytetrafluoro-ethylene ~PTFE) partlcles ls passed through a dle havlng two orlflces ln serles. The flrst orlflce ls a round, square or rectangular oriflce and the second orlflce ls a long sllt-shaped orlflce. Thls extruslon of partlculate PTFE 18 alded by the pre-blendlng wlth the compacted partlcles of an organlc fluld lubrl-cant such as for example kerosene, VM&P naptha and Isobars. The resultant extruslon produces a contlnuous sheet of blaxlally orlented flbrlllated structure of PTFE partlally saturated wlth volatlle organlc llquld. Under certaln processlng condltlons, the hydrocarbon 18 evaporated before further processlng of the PTFE
~heet, such fas sinterlng. For the purposes of thls lnventlon, blaxlal flbrlllatlon refers to the extruslon method dlsclosed in U S. Patefnt No. 3,315,020. Although unslntered (i.e. not havlng been heated to above 327C.), PTFE dlsperslon grade resln 18 hlgh-ly crystalllne (approx. 95%) and has a very hlgh melt vl3coslty, when heated to temperatures above 327C, some of the PTFE crystal-2~f lltes deform, thereby lncreaslng the amorphous content of thepolymer. Such heatlng and subsequent coollng of the polymer to temperatures below 327C produces slntered PTFE. The slnterlng of shffAped unslntered PTFE structures whlle malntalnlng the shape produces a thermosettlng effect upon the polymer, thus enabllng i shape retentlon.
In the process of blaxlal flbrlllatlon of dlsperslon ' grade PTFE resln, lower extruslon pressures are deslrable for `:
~ X 2 .

.

1326~99 60973-636 economlc as well as qualltatlve reasons. The conflguratlons of the dle, the ratlo of cross-sectlonal preform area to dle orlflce area (reductlon ratlo), extruslon speed and the amount of lubricant determlne the pressure necessary to force a glven PTFE
resin through a dle. However, the obvious approach of achlevlng lower pressures by over lubrlcatlon ls detrlmental. The extruded materlal may become overly orlented ln the machlne dlrectlon and suffer a slgnlflcant loss ln transverse directlon strength. Also, when excesslve lubrlcant ls evaporated, volds are created wlthout leavlng any remalnlng structure havlng orlentatlon, thereby caus-lng loss of strength. Perhaps the most slgnlflcant problem of over lubrlcatlon ls merely the softness of the extruslon that occurs, thereby severely restrlctlng further processlng, such as calendering and stretchlng.
Calenderlng of contlnuous blaxlally flbrlllated PTFE
extrudate i8 usually accompllshed whlle the extrudate stlll con-talns the hydrocarbon lubrlcant, and lnvolves compresslon between rolls spaced apart by a predetermlned dlstance and consequent elongatlon. Normally, the extrudate ls then heated to a tempera-ture at whlch the hydrocarbon lubrlcant wlll be safely evaporatedln a reasonable perlod of tlme. In the manufacture of plpe-thread sealant, further orlentatlon of the extrudate ls accompllshed by llnear stretchlng uslng dlfferentlal speed rollers on the drled materlal or on materlal stlll contalnlng hydrocarbon lubrlcant.
In the productlon of mlcro-porous PTFE membranes, llnearly orl-ented extrudate of the blaxlal flbrlllatlon process ls glven fur-ther transverse orientatlon by use of equlpment such as tenter X

~. .

frames or the llke. The membranes thus produced are normally heated to above 327C and subsequently cooled to effect slnterlng.
In the prlor art processe~ of modlfylng the character-lstics of blaxlally flbrlllated PTFE extrudate, dlfflcultles arise ln attempts to produce unlform extrudate havlng satlsfactory orl-entatlon of flbrlllated materlal to facllltate adequate transverse stretchlng. One reason for thls dlfflculty ls the low transverse elongatlon lnherent ln llnearly orlented materlal. Examples of these dlfflcultles are found ln U.S. Patent No. 4,187,390.

3rlef Summarv of the Inventlon We have unexpectedly dlscovered that the prlor art pro-cess of biaxlal flbrlllatlon of PTFE may be modlfled 80 a~ to produce novel products comprlslng a blaxlally flbrlllated seml-lnter-~ir 3a 13~6~9~

penetrating polymer networX of PTF~ and cured silicone elastomers havlng increased strength and unlformity. Such products facilltate the production of stretched and sintered end products having functions comparable to those of microporous PTFE tape and film. The products of thi~ invention have distinctly different properties than the starting materlals from whlch they are formulated.
Thus, according to one aspect, the invention provldes a process for producing a seml-lnterpenetrating polymer network of polyetrafluoroethylene and silicone elastomers which comprlses the steps of (1) intlmately blendlng a mixture of a ma~or amount of unsintered and unfibrillated particulate polytetrafluoroethylene di~per~ion resin and minor amount of (A) a hydrocarbon liquid and (B) an addltion curable ~lllcone compositlon conslstlng es~entlally of a polydlorganosiloxane havlng alkenyl un~aturation, an organohydrogenpoly~iloxane crossllnklng agent, a cataly~t for promotlng cro~slinklng of sald poly~lloxane, and an lnhlbltor for the catalytic reactlon; (2) formlng sald blend lnto an extrudable shape; (3) biaxially extrudlng sald blend through a dle lnto a shaped extrudate product having a randomly flbrlllated structure;
~4) evaporating ~aid hydrocarbon liquld, and activating said catalyst so a~ to generate a cured sillcone elastomer and polytetrafluoroethylene semi-interpenetrating polymer network comprlslng sald flbrlllated extrudate structure.
Accordlng to another a~pect, the lnvention provldes a semi-interpenetrating polymer network of polytetrafluoroethylene polymer, said polytetrafluoroethylene polymer being an un~lntered B

~ .32~099 60973-536 and unfibrillated particulate dispersion resin, and a ~ilicone elastomer, said slllcone elastomer comprislng the crossllnked reaction product of a polydiorgano~lloxane having alkenyl unsaturatlon and an organohydrogenpolysiloxane, said organohydrogenpolyslloxane being a crosslinking agent for the slllcone elastomer, ~ald seml-lnterpenetratlng polymer network being a ~haped extrudate and having randomly fibrillated ~tructure.
Brief Descrition of the Drawlna The drawlng is a scannlng electron mlcrophotograph of a shaped product produced by the proces~ of this lnventlon.
Detailed DescriPtion of the Invention The first ~tep in the process of thl~ lnventlon i8 to lntlmately blend a mlxture of a ma~or amount of un~lntered and unflbrlllated partlculate PTFB dlsperslon resln and mlnor amounts of (A) a hydrocarbon llquld and (B) an addltion curable slllcone composltlon con~l~tlng essentlally of a polydlorgano~lloxane havlng alkenyl unsaturatlon, an organohydrogenpoly~lloxane cross-llnklng agent, a catalyst for promotlng cro3sllnklng of saldpolyslloxane, and an lnhibitor for the catalytic reaction. The mlxture 18 necessarlly blended by mean~ of a llquid-solid3 blender or by ~ar tumbllng to avold any ~ubstantlal shearlng of the PTFE
dlsper~lon re~in.
5ultable unslntered and unflbrlllated PTFE dlsperslon reBln 18 manufactured by E.I. du Pont de Nemours & Co., Inc. under the de~lgnatlon3 TBFL0 ~ 6 and 6~; and by Imperlal Chenlcal -4a-_~ `
~S

`` 132~9~

Industrles as FLU0 ~ CDl, CD123 and CD525.
Alkenyl-containlng polydiorganosiloxanes typically employed in the practice of the present invention can have vi cosities up to loo,ooo,oO0 centipoise or more at 25C, for example, ln accordance with the teaching of U.S. Patent No.
4,061,609 to Bobear. It has been found that excellent results -4b-B

1326~9~

60973-~36 are obtalned when the vlsco~ity of the alkenyl-contalnlng poly-siloxane 18 from about 500 centlpol~e to 50,000 centlpolse at 25C, and especlally when the vlscoslty ls from about 3000 centl-polse to 6000 centipolse at 25C.
Organohydrogenpolysiloxanes that can be utlllzed in the present lnventlon may be llnear or reslnous and have vlscositles of between about 25 centlpolse and 10,000 centlpolse at 25C, wlth the pre~erred range belng from about 100 centlpolse to about lQ00 centlpolse at 25C.
The curlng catalyst can be elther an organlc peroxlde or a preclous metal contalning material. Sultable organlc peroxldes lnclude dlbenzoyl peroxlde, bls-2,4-dlchlorobenzol peroxlde, dl-t-butyl peroxlde, 2,5-dlmethyl-2,5-dl(t-butylperoxy) hexane, and dlcumyl peroxlde. Precious metal contalnlng catalysts can be based on the metals rhodlum, ruthenlum, palladlum, osmlum, lrrld-lum and platlnum. It 18 partlcularly preferred that a platlnum metal complex be employed as the catalyst, for example, as taught by Ashby ln U.S. Patent Nos. 3,159,601 and 3,159,662, Lamoreaux ln U.S. Patent No. 3,220,970, Karstedt ln U.S. Patent No. 3,814,730, and Modlc ln U.S. Patent No. 3,516,946.
In an especlally preferred embodlment, the addltlon curable sillcone composltlon ~urther lncludes a relnforclng organopolyslloxane resln o~ the type dlsclosed ln U.S. Patent No.

3,284,406 to Nelson or U.S. Patent No. 3,436,366 to Modlc. Brlef-ly, such reslns are copolymers of SlO2 unlts, (CH3)3SlOo 5 unlts and (CH3)~CH2-CH)SlOo 5 unlts, and SlO2 unlts, (CH3)3SlOo 5 unlts and (CH3)(CH2-CH)SlO unlts, respectlvely. Partlcularly preferred ~:, 1326~

oranopolyslloxane reslns are MDQ reslns havlng vlnyl unsaturatlon on mono-functlonal slloxane unlts, dlfunctlonal ~lloxane unlts, or both. The use of such relnforclng organopolyslloxane reslns ls especlally deslrable when the vlscoslty of the alkenyl contalnlng polydlorgano~lloxane ls less than about 5000 centlpolse at 25C.
It ls also contemplated that there may be lncluded any conventlonal extendlng and/or relnforclng flllers. Fumed slllca has been found to be partlcularly effectlve as a relnforclng flller.
In another partlcularly preferred embodlment of the pre-sent lnventlon, the addltlon curable slllcone composltlon also contalns a sllane or poly~lloxane whlch functlons both as an lnhl-bltor and as an adheslon promoter. One such composltlon 18 de8-crlbed ln u.æ. Patent No. 3,759,968 to Berqer et al. as a maleate or fumarate functlon sllane or polyslloxane. Composltlons effec-tlve only as an lnhlbltor are dlsclosed ln U.S. Patent No.

4,256,870 to Eckberg and 4,061,609 to Bo~ear. Other sultable lnhlbltors wlll be obvlous to those skllled ln the art.
It 18 further contemplated that the slllcone can be a condensatlon curable sllicone composltlon. Generally, condensa-tlon curable slllcone composltlons are avallable ln elther one or two packaqe~ and comprlse (1) a polydlorganoslloxane havlng terml-nal hydrolyzable ~roups, e.g., hydroxyl or alkoxyl, and t2) a catalyst whlch promotes condensatlon curlng. Such composltlons are well known ln the art, for example, as descrlbed ln U.S.
Patent No. 3,888,815 to Bessmer et al.
Alternatlvely, the polyslloxane network can be prepared by the hydrolytlc polycondensatlon of sllanes havlng the general formula Y - Si - (OX)3 where each X ls lndependently selected from the group conslstlng of hydrogen, alkyl radicals, hydroxyalkyl radlcals, alkoxyalkyl radlcals, and hydroxyalkoxyalkyl radlcals, and Y ls an alkyl radl-cal, OX, where X ls as prevlously deflned, or an amlno or substl-tuted amlno radlcal. The use of sllanes havlng hydrolyzable groups to form a polyslloxane network of an lnterpenetratlng 10 polymer network ls dlscussed ln greater detall ln U.S. Patent No.
4,250,074 to Foscante et al.
The hydrocarbon llquld may sultably be a solvent such as VM&P naptha, Isobars and kero~ene.
The mlnor amounts of hydrocarbon llquld and addltlon curable slllcone composltlon both functlon ln the flrst step of the process of thls lnventlon as a lubrlcant for the PTFE partl-cles, and therefore care must be used to avold the known problem of over lubrlcation. However, when amounts as small as 2% by welght of addltlon curable slllcone composltlon are lncorporated 20 ln the blend, extruslon pressures are caused to be lower than ln the case of a blend of hydrocarbon llquld and PTFE dlsperslon resln alone. At the level of 796 by welght incorporatlon of addl-tlon curable sillcone composltlon lnto the blend, extruslon pres-sures have been reduced as much as 56%.
In the second step of the process of thls lnventlon, the blend ls compacted lnto a preform shape adapted to the conflgura-tlon necessary for the process of blaxlal flbrlllatlon as V"
~ ' .~

132~99 descrlbed ln U.S. Patent No. 3,315,020.
In the third step of the process of thls inventlon, paste extruslon of the preformed blend is carrled out ln the known manner of blaxlal flbrlllatlon as descrlbed ln U.S. Patent No.
3,315,020.
In the fourth step of the process of thls lnventlon, the hydrocarbon ll~uld contalned ln the blend ls evaporated, and slmultaneously therewith or later the catalyst for the slloxane crossllnklng reactlon ls actlvated thereby generatlng a cured slllcone elastomer and polytetrafluoroethylene seml-lnterpenetra-tlng polymer network ln the form of the blaxlally flbrlllated extrudate.

Exam~le The followlng curable slllcone composltlon ls prepared.
Parts bY Welqht Vlnyl N-stopped polydlmethylslloxane 68.2 (3500 cps at Z5C) MDQ slllcone resln blend 22.7 Dlmethyl vlnylsiloxane resln blend 8.2 bls (trlmethoxysllylpropyl) maleate 0.9 Lamoreaux platlnum catalyst 10 p.p.m.

Thls curable slllcone composltlon was comblned wlth FLUO~ CD123 grade polytetrafluoroethylene dlsperslon resln and VM&P naptha as follows, Flve ~lllcone/VM&P naptha/PTFE dlsperslon grade resin blends and one VM&P naptha/PTFE blend were prepared uslng a ~r . 8 1 3 2 ~ ~ 9 ~ 60973-636 llquld-sollds blender. The resultant blends were compacted lnto cylindrical preforms and extruded through a die having a clrcular orlflce and a reductlon ratlo of 900:1. The re~ultant bead extru-dates required the extrusion pressures shown in Table 1. It can be seen from Table 1 that extrusion pressure is substantlally decreased by the addltlon of a small amount of the curable slli-cone polymer, yet all of the extrudate samples appeared to be the same to the eye and had the normal feel of PTFE disperslon grade resin extrusions. Samples 5 and 6 show an increase in extrusion pressure when compared to Samples 2 and 3, respectively, due to a reductlon ln organlc lubrlcant content.

Sample No. PTFESillcone VM&P naptha Extruslon ~g.)(g.) (g.)Pressure (p81) 1-1 3178 0 699.2 8,750 1-2 3114.463.6 699.2 7,110 1-3 3019.1158.9 699.Z 5,770 1-4 2955.5222.5 699.2 3,800 1-5 3114.463.6 607.011,700 1-6 3019.115~.9 562.5 7,520 Exam~le 2 Three kerosene/PTFE di~perslon grade resln blends and four slllcone/kerosene/PTFE dlsperslon grade resln blends were prepared uslng a llquld-sollds blender and the slllcone composl-tlon and PTF~ resln of Example 1. The resultant blends were compacted lnto cyllndrlcal preforms and extruded through a dle constructed to achieve blaxlal flbrlllatlon as descrlbed ln U.S.
Patent No. 3,315,020. The extrudates of Samples 2-l, 2-2, 2-3 and 2-4 were calendered to 5 mll thlcknesses and samples 2-5, 2-6 and G X

1326~9~ 60973-636 2-7 were calendered to 4 mll thlcknesses. Each extrudate was heated at between 310F and 320F to evaporate the kerosene and to cure the sillcone elastomer. The extruslon condltlons and physl-cal propertles of the extrudates are shown ln Table 2. By compar-lng Sample Nos. 2-1, 2-2 and 2-5 wlth Sample Nos. 2-3, 2-4 and 2-7, lt can be seen that the denslty of the extrudates lncorporatlng slllcone elastomer was lncreased by as much as 28% over that of the comparable PTF~ extrudate. It can also be seen that lncorpor-atlon of slllcone elastomer ln the blaxlally flbrlllated extru-dates lmproved the transverse elongatlon before break property asmuch as 37.5% of the value wlthout ~lllcone elastomer lncorpora-tlon.

ExamPle 3 The extrudates obtalned accordlng to Example 2 were transversely stretched uslng a tenter frame. The samples lncor-poratlng slllcone elastomer were heated at about 120C durlng stretchlng whlle the PTFE extrudates were heated at 175C durlng stretchlng. Physlcal propertles of the resultant mlcroporous fllms obtalned are shown ln Table 3. It can be seen ln each case that the lncorporatlon of slllcone elastomer lncreased tenslle strengths of the stretched products ln both machlne dlrectlon and transverse dlrectlon. A scannlng electron mlcro-photograph of stretched Sample 2-3 was taken at 5,000X magnlflcatlon and 18 ~hown ln the drawlng. The random flbrlllatlons ~hown have thlcknesses as large as 1.0 mlcron.

X 9a . ' ` 1326~99 -- 87. 12/31 12: 5 1 3 ~051 ~PRU .~PR.U~ ~1 --P 5~ o o o o ~ æ ~

~â 1 ' _ .
3 3!-- ~ ~ ~ o ~ ~ ~ ~ ~ d ~i I~t i~3~

a~ O . ~ ~ O ~ ~

~ S~ , :!t'~',, l .._. , . -.. ~ _ ~ ~ , . .

1326~9~

8~. 12/31 12 ~ 6 XPRUL~PRUL PHILR
f _ . _ 3 ~oo~oooooo .
. ,~, ~ r .,.,aa 111 C ~4 h ~rl O O O O O O O ~ O O O
X ~ ~ Y~ ~1 0 ~ O~
l _ . , _ ~ _ ~ .
, ~ ~ ~ ~ ~ 3 .
. . .' ' ',.
~ ~o .~ . , . , I
~ ~ IO~ ' I ~ j ~ ~ - '~1 1 1 _ !
' ooor~ or~r.r., ' ..
. . .
l .
3.
i~~1 ~ N 1~ ~ ~ r r~ ~ ~ ' _ _ . - - . , .

~ ~ ~I N 1~
. I .~ - , ~ ' 1326~9~

Example 4 Five portlons of the microporous fllm Sample No. 3-6, produced in accordance wlth Example 3, were sprayed contlnuously for 15 seconds wlth a low-odor kerosene solutlon of the curable slllcone composltion set forth ln Example 1 at lncreaslng slllcone concentratlons (2.5%, 5.0%, 10.0%, 15.0%, 20.0%). After spraylng and evaporatlon of the solvent, the samples were cured at 150C
for 15 mlnutes. Table 4 shows the surprlslng effect of this cured slllcone elastomer lmpregnatlon of mlcroporous fllm products form-ed from the polymer network composltlons of thls lnventlon on theopaclty of the fllm as compared to the untreated fllm. Other physlcal characteristlcs of these products are likewise shown in Table 4.

X i 12 132~99 87.;2/3l 12:~1?38 XPRUL~PRUL PHILFl ' ', ~, ji,! ~ y . . . _ ^ ~ o ,~
, O
~ ~ 3 ~ o ~ ~ N

~ ~ ~ ~ In .j ~ ~ ~ ~no~9 ~ . r _ I ~
i ~ r ~ ' 1326~9~

ExamPle 5 The followlng lngredlents were blended together uslng a liquid-sollds blender.

238.4 g Slllcone composltlon of Example 1 238.4 g Calcium carbonate 2701.3 g PTFE dlsperslon grade resln 730.9 g Kerosene The resultant blend was then compacted lnto cyllndrlcal preforms and extruded through the dle utlllzed ln the extruslon of Samples 2-1 through 2-4. An extruslon pressure of 3,000 psl was observed. The extrudate was calendered to a 5 mll thlckness and then heated at between 310F and 320F to evaporate the kerosene and to cure the slllcone elastomer. It 18 apparent from thls example that the process of thls lnventlon can be utlllzed to produce fllled seml-lnterpenetratlng polymer networks of PTFE and slllcone elastomers.

f V 14 . ' ~,

Claims

1. A process for producing a semi-interpenetrating polymer network of polytetrafluoroethylene and silicone elastomers which comprises the steps of (1) intimately blending a mixture of a major amount of unsintered and unfibrillated particulate polytetrafluoroethylene dispersion resin and minor amounts of (A) a hydrocarbon liquid and (B) an addition curable silicone composition consisting essentially of a polydiorganosiloxane having alkenyl unsaturation, an organohydrogenpolysiloxane crosslinking agent, a catalyst for promoting crosslinking of said polysiloxane, and an inhibitor for the catalytic reaction; (2) forming said blend into an extrudable shape; (3) biaxially extruding said blend through a die into a shaped extrudate product having a randomly fibrillated structure; (4) evaporating said hydrocarbon liquid, and activating said catalyst so as to generate a cured silicone elastomer and polytetrafluoroethylene semi-interpenetrating polymer network comprising said fibrillated extrudate structure.

2. The process of claim 1 further characterized by calendering the shaped extrudate product to a uniform thickness.

3. The process of claim 1 wherein the amount of addition curable silicone composition constitutes at least two percent by weight of the mixture of polytetrafluoroethylene, hydrocarbon liquid and addition curable silicone composition.

4. The process of claim 1 wherein the mixutre of step (1) includes additionally a minor amount of calcium carbonate.

5. The product of the process of claim 1.

6. A semi-interpenetrating polymer network of polytetrafluoroethylene polymer, said polytetrafluoroethylene polymer being an unsintered and unfibrillated particulate dispersion resin, and a silicone elastomer, said silicone elastomer comprising the crosslinked reaction product of a polydiorganosiloxane having alkenyl unsaturation and an organohydrogenpolysiloxane, said organohydrogenpolysiloxane being a crosslinking agent for the silicone elastomer, said semi-interpenetrating polymer network being a shaped extrudate and having randomly fibrillated structure.