CA2087751A1 - Heat curable silicone rubber compositions employing vinyl mq resin and a single vinyl chain stopped polymer - Google Patents

Abstract

PATENTS

ABSTRACT OF THE DISCLOSURE
The combination of a substantially vinyl-oil-free composition comprising a vinyl-containing diorganopolysiloxane gum and a vinyl-containing organosiloxane resin will cure in the presence of a catalyst and a cross-linking agent to form a heat cured silicone elastomer having high tear and tensile strength and low compression set. In a particular embodiment, the composition comprises (A) 100 parts by weight of a vinyl-containing organopolysiloxane or blend having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C and being substantially free of vinyl-containing organosiloxane oil; and (B) from about 1 to about 30 parts by weight, based upon Component (A), of an organopolysiloxane resin or resins having MQ or MDQ units and up to about 10 weight percent vinyl groups. The composition also includes (C) up to about 200 parts by weight, based upon Component (A), of a filler and (D) up to about 2 parts by weight, based upon Component (A), of an organohydrogensiloxane cross-linking agent having a hydrogen content ranging from about .05 to about 2 weight percent and a viscosity from about 5 to about 500 centipoise at 25°C. The composition may further include a process aid, a coupling agent, a surface treatment for the filler, a catalyst, an acid acceptor, and a heat-age additive.

Description

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PATENTS

IMPROVED ~EAT CU ~ LE 5ILICONE
R~BBER CO~POSITIONS EMPLOYING VINYL
~Q RESIN AMD A SIN~LE VINYL CHAIN STOPPED POLYMER

~A~R~ROUND~5~F TH~ INVENTI~N . .
~ he pres~nt invention relate3 to hast cur~hle silicone rubber ~ompo~itions. More partlcularly, the invention r~lates to hest cuxable ~ilicone rubber comp~ition~ whi~h ~re ~ubst~ntially free o~ vinyl containing organo~iloxan~ oLls and have lmproved tear and ten~ile strength, and compres3ion set propertie~.
- He~t curable silicone rubber compo~itions are k~own in the art. Such composition~ are known for their ability to r~sist oh~nge at elevated t0mperature and exposure to adver~e conditions over extended period~ of time. In general, m~terials are needed which have no~ only ~he ou~tandin~ properties of organopoly8iloxane elastomer~ but al80 have a hi~h dsgrea of tear and tensile strength. In particular - . . , ~ . :

, . -20~77~1 _~_ P~TE~S

application~, æuch a~ gasketing applications, a~ati~factory degree o~ r~ n~y a~ mea ured by compre~sion 8et i8 al~o required.
Efforts have been mad~ in the pa~t t~ produce heat 5 curable silicone 21astomer compositions haYing increasad tear strength and reduced compre~sion set propertias.
U.5. Paterlt No. 4,539,357 (~obe~r '357) disclo~es an exemplary he~t-cur~bl~ one composition 10 comprising a vinyl-terminated line~r diorganopoly~iloxane gum h~ving a vi~co~ity in the range of lx106 to 20x107 cen~lpoi~e at 25C, a vinyl-con~aining diorg~nopoly~ilo~ano ~um having a vi8c081ty varying from lx106 to 20x107 centlpoise a~ 25C, a fill0r, ~ hydride-containing polysiloxane, and an org~nic peroxid~ or organic hydroperoxide curing agent.
The compo~ition may furthex compri e a vinyl-containing or~anopolysiloxane r~sin copoly~er. It i~ ~tated in 3 2 0 ~ 7 7~.1PATENTS
~Q~so Bobear ~357 at column 6, lines 22-29 that ~here ~hould be no vinyl-containing fluid in the composition since it has been found that composit~ons containing vinyl-containing fluids of a viscosity of 500,000 centipoise S or less rQsult in elastomer with good tear strength initially, but after the composition ha~ been ~ubjected to po~t-cur~, it~ tear strength properties degrade dramatically. After post-cure baking, the Bobear '357 composition ha~ a tear strength of above 200 pi.
U.S. Patent No. 3,6S~,475 to Wada et al. (Wada '475) di~clo~e~ heat curabl elastomeric silicone compositions report~dly having high tear strength, excellent compression set and resiliency, comprising two vinyl-unsaturated polydiorganopoly~iloxanes each having a degree of pol~mexization of at lea~t 3000; a vinyl-un~aturated polydiorganopolysiloxane having a degree of polymerization of from 10 to 1000; a ~ilica filler; and an organic peroxide catalyst. The sample~
prepared in the Wada '475 examplaæ had tear strength values of about 40 kg/cm, i.e., about 225 pi.
U.S. Patent No. 3,671,480 to Wada et ~ Wada '480) disclo~e~ a heat curable elastomeric ~ilicone compositîon having Lmproved tear ~trength and ~uperior compre~sion set, comprising a mixtuxe of two vinyl-unsaturated polydi.organopolysiloxanes, one o~ whichappear~ to have a hi~h molecular weight and the other o~ which appears to have a low molecular weight; a polyorganohydrogensiloxane; ~ilica fill~r; and a platinum co~pound. The examples in Wada ~4B0 report tear strength values of about S0 kgJcm, i.e.~ about ~80 pi. Compre~sion 8et value~ are not givsn.
U.S. Patent No. ~,061,609 to Rob~ar (Bobear '609) di~cusces a platinum catalyzed silicone rubber ~ . . .
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~ P~EN~
Ç~a~Q

composition comprising a vinyl-containing polysiloxane or a blend of such polysiloxanes; platinum; and ~
hydrogen-containing polysiloxane. Preferably, the vinyl-containing polysiloxane has a vi~cosity in the range of 1000 to 300,000,000. A low viscosity vinyl-containing polysiloxane may be added to the basic composition a~ a rein~orcing agent to give the final composition good physical strength. ClaLm 4 in Bobear '609 recites a first polysiloxane having a viscosity of l,000,000 to 200,000,900 centipoise at 25C and a second polysiloxane having a viscosity of 50,000 ~o 500,000 centipoise at 25C. The Bobear patent is directed to Lmproving the work life of silicone rubber co~positions, rather than tear strQngth. In the examples, tear strength value~ of greater than 200 pi were obtained, but the compositions having these values contained two high viscosity vinylpolysilo~anes rather than one high ~i~co~ity vinylpolysiloxane and one low viscosity vinylpolysiloxane.
U.S. Patent No. 3,660,345 to Bobear (Bobear '345) disclose~ oganopolysiloxane compositions which are convertlble to elastomers having high tear strength and resiliency, compri3ing a two component blend of vinyl-contalning orgar.opolysiloxan~s; ~ilica ~iller, and a ~5 process aid which can be, for example, a methoxy-termin~ted polysiloxane, a ~ilanol-terminated polydLmethyl~iloxane, or hexamethyldisilazane. A
peroxide is used a~ the cataly~t. The visco~i~y of one of the two vinyl-containing organopolysiloxanes can be in the range of 1 to 1 billion centipoise at 25C, while the viscosity o~ the other can be between lOO,OOD
to 2,000,000,000 centipoise at 25C. The examples in , . : .

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_ S ~ PMENTS
60S~-149Q

E~obear '345 report tear ~trength ~ralue8 of greater than 200 pi.
11.S. Patent No. 3,696,068 to Creamer disclose~
heat-curable silicone gumstock composition~ ha~ing high "B" tear strength ~s well as low compression set values, comprising a vinyl-containing diorg~nopolysiloxane gum and a linear vinyl-containing fluid having a vi5c08ity of from 10 to 150,000 centistokes at 25C. The cross-linkable vinyl siloxane gum has a viscosity in exce~s of 200,000 centistoke~ at 25C. The Creamer composition i~ cured by a peroxide ca~alyst. Some of the samples prepared in Creamer had tear strength values in exc2ss of 150 pi. Tear strength after post cure was not mea~ured in any of the examples, which i~ significant since, as pointed out pre~i.ously herein, it has been found that, in composition~ containing low ~isc08ity vinyl-containing fluids, tear strength drop~ dr~matically after po8t cure.
~O U.S. Patent No. 3,884,866 to Jeram et al.
di~closes a high 3trength or~anopoly~iloxane compo~ition suited for low pre0~ure in~ec~ion molding, comprising ~A) a first oomponen~ containin~ (i) a vinyl-containing high vi8c08ity oxgsnopolysilo~ane having a vi~c08ity of 5000 to 1,000,000 centipoise at 25C; and (ii) a low visco~ity o~ 50 to 5000 centipoise at 25~C; (iii) a ~iller; and (iv) a platinum catalyst;
and (B) a hydrogen ~ilicone composition. The highest t~ar strength reported in the example~ was 250 pi.
~lthough heat curable silicone rubber compositions having good tear and ten~ile strength, and low compression set properties are known in the art, it is de~irable to provide heat curable silicone rubber , , :, . : , .

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-6- PAT~NTS

compo~ition~ which have good Rtrength and compre~sion set properties rQsulting from formulations which are more ea~ily proces~ed, have more repeatable and con~istent results, and have stable properties.
It is therefore an ob~ect of the inv~ntion to provide a heat curable silicone rubber composition having good tear and tensile strength and low compre~sion set employing more ea~ily proc~ssed components.

SUMMARY OF THE I~e~lrg~
rhe present invention is based upon the discovery that an oil-fre~ combination of a vinyl containing diorganopoly~iloxane gum and a vinyl containing organosiloxane resin will cure in the pr~ence of a catalyst and a cross-linking agent to fonm a heat cured silicone elastomer having high tear and ten~ile strength and low compression 80t.
In contrast to some pre~ious compositions, the pr~ent invention utilizes a single lower molecular weight gum inste~d of a higher weight gum and a vinyl-containing oil. A high cxoss~link density in the final cured polymer i~ khu~ achieved which exhibits a mono~
modal molecular weight distribution o the ~inyl cha~n-~opped PO1YmQr~
In a particular embodiment, the invention comprises (A) 100 parts by woight of a vinyl oontaining organopoly~iloxane or blend ha~ing a vi8c08ity of about 3,000,000 to about 100,000,000 centipoi~e at 25C and being ~ub~tantially free of vinyl containing organo~iloxane oil; and (B) from abou~ 1 to about 30 par~s by weight, based upon Component (A), of an organopoly~iloxane resin or resins having ~Q or NDQ

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_,_ PATENTS

units where each M unit can represent M or M' e.g. M-vinyl units and where each D unit can repres2nt D or D' e.g. D-vinyl units, and up to about 10 weight percent ~ -vinyl groups. The composi~ion al80 include~ (C~ up to 5 about 200 part~ by waight, ba~ed upon Component (A), of a filler, and (D) up to about 29 p~rts by weight, based upon Component (A), of an organohydrogensiloxane cro8s-linking agent having a hydrogen content ranging from about 0.05 to about 2 weight percent and a viscosity from about 5 to about 500 centipoise at 25C. The composition may further include a processing aid~ a ~urface treat~ent for the filler, a coupling agent, catalyst, an acid ~cceptor, ~nd a heat age additive.
The heat curable composition~ of the invention have good tensile and tear ~trength and low compres~ion set properties without sacrificing other beneficial properties including oil resistance, hardness and elongationO

DESCRIPTION OF TEE IN~VENTIQN: , The present in~ontion is directed ~o a heat cursble silicone composition having improved compre~sion ~et and toar ~tren~th propertie~, containing (A) a vinyl con~aining diorganopolysiloxane gum or blands of vinyl containing diorganopolysiloxane gum8 having a visco~ity fro~ about 3,000,000 to about 100,000,000 centipoise (Cp8) at 25C, (B) an organopolysiloxane re8in copolymer, (c) a fill~r, and (D) a cross linking agen~ curable in the presence of a catalyst.
Component (A) is a vinyl containing dio~ganosilosane gum or blend of such gums ha~ing a visco~ity fro~ about 3,000,000 to ~bout 100,000,000 Cp8 ~ .

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60~-14~Q

at 25C. Preferably, the gum has a visco~ity of between about 3,000,000 and 85~ noo, ~oo cps, more preferably, about 13,000,000 Cp8 at 25C~ The gum may have a weight percent vinyl concentration of from about 5xlO-s to about 1. The gum preferably has a weight percent vinyl concentration in the range from about 6.5xlO-s to about O.U3, more prefsrably from about 8x10-5 to about 1.5xlO-', and more preferably yet from about 8xlO-s to about 1.2x10-4. The organo groups in the vinyl polymer or polym~rs of the gum should all be monovalent hydrocarbon radicals.
The vinyl poly~er or polymers of Compon~nt (A) may contain vinyl-on-chain groups and vinyl end-groups.
Preferably, the polymer i8 vinyl terminated. More pr~ferably the polymer is free of vi~yl substitution except at the chain ends. In other pxeferxed embodLments, Component (A) ha~ the formula:

Vi5iORl2 (SiORl2)x(SiOR22)y SiRl2Vi wherein Vi i~ vinyl; R1 is independently choMen from monovalent hydrocarbon radicals, ree o~ ~liphatic unsaturation, and containing 1 to about 8 carbon atom~;
RZ i8 independen~ly cho~en from monovalent hydrocarbon radical~ containing 1 to about 8 carbon atoms; and x and y are int~gers cho~en Ruch that Component ~A) has a vi~c08ity which ranges from about 3,000,000 to about 85,000,000 Cp8 at 25C and a weight percent vinyl concentra~ion in the range from about 5x10-5 ~o about 2xlO-', preferahly from about 8x10-5 to about 1.5x10-4, and more pxeferably from about 8x10-5 to about 1.2x10-4.
The vinyl containing polymers of Component (A~ can be made by a process well known in the art, or ,.

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example, by reacting cyclotetrasiloxanes in the pr~sence of low molecular weight, linear vinyl chain stoppers at high temperatures in the pre~ence of ba~ic catalysts so as to yield a poly~er of the d~sired S molecular weight. When the reaction is over, the catalyst i~ neutralized and tho exces~ cyclics are vented off to result in the desixed polymer. By controlling the amount of chain stopper and the temperature of the rea~tion, the molecular weight of the des~red vinyl-containing polymer end product can be controlled. For more detail~ of the process by which ~uch vinyl containing polymers are produced, reference i8 made, for 2xample, to U.S. Patent No. 3,660,345, which is incorporated herein by reference.
Component (A) pre~erably has a pen~tration of about 1000. In Exampl~ V and VI below the gum u~ed ha~ a penetration of 1600 as measured using Universal Penetrometer by Precision Scientific Model No. 73510 with a sub~tantially air free sample. The sample penetration is measured at 25C~lnC using ~ lO0 gm weight and a 1/4" diameter by 3~16" needle ~oot with rounded edge~. The needle is lowered until it, ~ust touche~ the sur~ace o~ the polymer. Then, the ~ime to achieve up to 300 mm penotration i8 detonmined or the amount of penetxa~ion a~ex 60 3eC. Penetration gum is de~ined as:

DeDth of Penetration X 60 s~c. at 25C
Time Penetration may be controlled by controlling the molecular weight and the vi~co~ity of the gum. In the present invention, the molecular weight o f Component (A) is preferably between about 300,000 and about .
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850,000, and the vi8~08ity i8 preferably b~tween 3,000 t 000 and about 85,000,OD9 cps at 25C.
In accordance with the invention, the amount of gum present in the final product may vary. However, for purposes of explanation herein, it is assum~d that 100 part~ by waight of the g~m is combined with varying amounts of the other components. In some example~, the formulation has 80 part~ of Component (A). In each cas~, the ~mount of other components in the final product may be calculated or otherwise r~adily inferred. In the example~ below Component ~Al) i~ a vinyl-stopped polydiorganosiloxane of about 9000 D
units in length. CQmponent ~A2) is a vinyl-stopped polydiorgano~iloxane of about 6000 D unit~ in length.
Component (B) is (1) an organopolysiloxane resin copolymer. The re~in copol~mer may contain R3SiOl~2 monofunctional unit~ (~ units) and SiO2 quadrifunctio~al unit~ ~Q units), where each R is independently selected from the ~roup consisting of 2G vinyl radicals and mono~alent hydrocarbon radiGals free of aliphatic un~aturation. The ratio of M units to Q
units range~ from about 0.5:1 to about 1.5:1, wh~rein the re~in copolymer contains about 0.5 to 10.0 weight percent vinyl groups. Alternatively, Component (B) m~y comprise (2) an organopolysiloxane resin copolrmer con~aining M and Q uni~ as aforesaid and ~2SiO2~2 difunctional (D units) where each R i~ independently selected from the group consisting of vinyl radical~
and monovalent hydrocarbon radical~ free of aliphatic unsaturation. Each M unit may represent ~ or M' e.g.
M-vinyl units, and each D unit may repreYent D or D' e.g. D-vinyl units. The ratio of M units ~o Q units is from 0.5:1 to about 1.5:1 and the D units are present ,~, ": :
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60SI~14~0 in an amount of rom about 1 to 70 mol percent based upon the total number of mols of siloxy unit~ in the copolymer. The resinous copolym~r contains from about 0.5 to about 10.0 weight percent vinyl groups. The organopoly~iloxan~ r~sin copolymer of Component (B) may contain mixtures of the MQ and the M D Q, ra~ins.
~ h~ organopoly~iloxane resin copolymer~ of Component (B) in th~ pre~ent compo~ition and their m~nufacture are well known in the art. Such resin~ are usually produced by hydrolysi~ of chlorosilane~ in a process de~cribed in U.SO Patent No~ 3,436,366 which i~
incorporated herein by reference.
Component ~B) i~ pre3ent in the compo~itio~ of the present invention in an amount ranging ~rom about 1.0 to about 30, preferably from 1.0 to about 20 and more preferably from about 4 to about 12 psrts by weight, based upon Component (A).
Component tB) in ~he examples below compri~es (B2) dispersed in xylene. ~uch of the xylene has been vacuum stripped ~o it rQsults in a composition ~omprising 60% by weight solid~ and 40~ by weight xylene. The process for m~nufacturing the compo~ition include~ a cook and strip ~tep to ~acilitate the remo~l o~ khe residual xylene from the ~ini~hed compound.
Component (C) compri3e~ ~rom about 5 to about 200, pr~erably from about 10 to abou~ 100 and more pre~erably from about 20 to a~out 50 parts by weight, bs~ed on Component ~A), of reinforcing filler~ su~h as SiO2. The filler i8 needed in the compo~ition to provide a high tear stren~th. Examples of r0inforcing filler~ that may be used include fumed ~ilica and precipitated silica, with fumed ~ilica being preferred.

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-12- PATE~T5 The filler preferably has a surface area of between about 100 and about 30Q m2Jgm. More preferably, the ~urfac~ area is between 160 and 240 m2/gm The ~iller may be treated with various a~ents 80 as to pre~ent the compo~ition from ~tructuring, for example cyclopolyGiloxanes as di~closed in U~S. Patent NQ .
2,938,009 to Luca~ and sila~anes a~ disclosed in U.S.
Patenk No. 3,635,743 to Smith, or both. The cyclopolysiloxanes present may b~ for example, a cyclotetramethyl siloxane present in the amount of about 15 to 20 weight per~ent of the filler. In the examples below, Component (C) i8 fum~d silica treated as in Lucas above having a ~urf acs area~ in the untreated state, of 240 mZ/gm or 160 m2/gm. A further surface treatment with a linear vinyl contalning silazane may be provided in ~itu.
In addition to the foregoing, Component (D) in the form of a hydride cross-lînking agent may be employed.
In the preferred embodiment, Component (D) may be a random copolymer formed of a polydimethylsiloxane (PD~S) and a polymethylhydrogen~iloxane (PMHS) ha~ing the fonm:

R3SiO(SiORH5iOR2~XSiR

wherein each R i5 independently chosen from a hydrogen or monovalent hydrocarbon radical free of aliphatic unsaturation containing 1 to about 8 carbon atoms, and x ranges from about 2 to about 100. In the pr~sent invention, the hydride iB present in an amount of about .1 to 10 parts by weight based upon Component (A3, pr~ferably 0.625 to 2.5 parts by weight based upon Component (A). A~ mentioned above, when a hydride :'. , ' . :
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20;~
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Q~l~Q ' agent is employed, a platinum c~talyst may be sub~tituted for the peroxide catalyst to cure the co~position. In Component ~D), x may vary ~o that (D) has a vi5c05ity ranging from about 5 to 500 cps, preferably from about 10 to about 100 Cp8 and most preferably from about 10 to 50 cp3 at 25C. In a preferred embodLment, Component (D) has a hydrogen content ranging from about .5 to ~bout 2 weight percent. In the example~ below, Component tD) has a 0 vi~CQ. ity of ~bout 30 centi~tokes, a hydride content of about 0.8 percent by weight, and a chain length of about 100 units.
The linear hydrid~ descri~sd above can be made by many procedures which are known in the art and particularly by the hydrolysi~ c~ appropriate chloro~ilane~. SQe for example, U.S. Patent No.

4,041,101 which is incorporated herein by reference.
Other components may al80 be employed as di3cussed h~reinafter. For example, in order to allow for easier incorporation o~ the filler in the gum, Component (E), a processing aid or plasti~izQr, may be employed. In the prefer~ed emhodiment, Component (E) i~ a low vi8'08ity sLl~nol ~toppod ~iloxane fluid having a vi8c08ity ranging from About 3-50 Cp8 and preferably 3 to 30 cp~ at 2SC. The ~iloxane fluid i3 an equilibrium mix o~ low molecular weight oligomers of from about 4 to about 10 but pr~ferably between 4 and 6 repeating units in length with a minimum ~mount of cyclics in equilibrium with the oligcmer~. The 39 pro~e3~ing aid ~E) may have the fo~m:

Ho~R2sio)xH

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2~7~1 ~14~ PATENT5 where each R is CH3 ~ and x i8 between 4 and 10, preferably between 4 and 6, with re~ulting cyclics of the same number of units in e~uilibrium.
In the present invention, the proeessing aid may be pre~ent in amounts up to 20 parts by weight based upon Component (A), And preferably is present in ~mounts between 2 and 10 parts by weight based upon Component (A) and moxe preferably 3 parts by weight.
Up to about 5 weight percent of Component (E) is preferred for many application~. It shoulcl be underætood that typicslly the moxe filler th~t i used the greater amount of processing aid is employed.
Another Compon~nt (F) may ~e vinyl ~topped linear silazane such a3 tetramethyl-diYinyl ~ilazane. The material may be added a~ a coupling agent in-~itu for enhancing the bonding of filler to gum. Component tF) i~ preferably pr~ent in an amount up to 2 percent by weight, ba~ed upon the weight of Component (A). More prefer~bly, Component ~F) i3 present in an amount of only up to about .3 part~ by weight, based upon 100 parts by weight of Component ~A).
In order ~o form a hea~ curable rubber, an orgsnic p~roxide free radical initiator or curing catalyst i~
providbd. The pre~erred peroxide curing ayent~ are thormal decompo~i~ion organic peroxide~ conveniently u~ed to cur~ ~ilicone elastomers. Exæmples of suitable organic peroxide free radi~al initiators for u~e in the present invention are di~clo~ed, for ex2mple, in U.S.
Patent No. 4,539,357 to ~obear which is incorporated herein by reference. 5uitable peroxide ~ataly~ts include dialkyl peroxide ~uch as di-ter~iary-butyl peroxide, t~rtiary-butyl-triethylmethyl peraxide, tertiary-butyl-tertiary-butyl tertLary-triphenyl ~,; : ;
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60SI-~4_0 peroxide, t-butyl perbenzoate and a di-tertiary alkyl peroxide such as dicumyl peroxide. Vnder certain condition~ hereinafter de~cribed, such as when a hydrid~ is used, a platinum catal~st may be employed 5 instead of an initiator. In the example~ below, the preferred catalyst is a vinyl specific catalYst such a~ 2,5 dimethyl-2,5-di(t-butyl peroxy) hexane e.g., ~Lupersol ~101).
Other agent~ may also be added including an acid acceptor, such a ~gO or ZnO. Acid would otherwise cause cleavage of the product chains. The acid acceptor may be present, in smalI amvunts of about 1 to about 10 parts by weight, bas2d upon Component (A).
In addition to th~ foregoing, a he t aging component such as fumed TiO2, iron octoate, CelOH)4, or mixtures thereof, may be pre~ent in ralatively small ~ounts, e.g., up to 2 psrt by weight ba~ed upon Component ~A). In the example~ below the heat aging component consists of 33 weight percent TiOz, 5 weight percent iron octoate tl2% iron in mineral spirits), 10 weight percent treated ~umed silica ~160 mZ/gm), and 50 weight percent of an 8~0 penetration vin~ opped, vinyl-on-chain gum~
Copending U.S. patent application serial no.
07~587,876, filed 9/25/90, attorney docket no. 60SI-1336, which is incorporated herein by ref~rence, discloses heat curable silicone rubber compositions containing blends of vinyl containing organopolysiloxane gum8, a vinyl chain stopped PDMS, oil, MQ and MDQ re~ins, filler and a process aid cured with a peroxide initiator. The composition3 ar~
formulated for increa~ed tear strength and reduced compression set properties. ~odified versions, . _ - , .. . . ........ .

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substantially free of vinyl-containing silicone oil, of one or more compositions di~clo~ed therein are useful materials for uæe in a heat curable silicone rubber of the present invention.

~XAMPLES I YI
The tahle below lists a number of ~ormulations and their respective properties illuætrative of the present invention. Examples I and III contain a low m~lecular weight vinyl chain stopped oil of the type di~closed in Patent Application Attorney Dockat No: 60SI-1336. This vinyl containing oil appears to positively affect compression Bet but is detrLmental to other propertie~.
Examples II and IV do not have the low molecular weight vinyl containin~ oil component. The oil contributes ~o the overall ~inyl level in the formulation and thus contributes to the cros~link den~ity, leading to the positive effect on compression sat. However, it produces a bi-modal molecular weight distrihution to the vinyl chain-qtopped pol~ner. A similar ~inyl level, and therefore cro3~1ink den~ity, can be achie~ed with a lower vi c08ity polymer hs~ing an intermediate molecular wei~ht, which give~ a mono-modal molecular weight distribution in the vinyl chain stopped pol~ner.
According to the present inventio~, the ~inal gum ~5 pol~ner composition ~hould be substantially free o~
vinyl containing oil and have a mono-modal molecular weight di~tribution. Examples V and VI illustrate formulations r~sulting in surprisingly high ~ensile strength with excellent compression set and tear strength using a high penetration gum. In Ex~mples V
and VI, Component (A) has a molecular weight of about ,"~
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25 ~C
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~A~ ~3 P~n-tr~tlan ~ 0 ~0 eo 8D
16 P~tnDt~tion ~ -- -- 95 0d oll 15 -- 15 --- -- -- :
~B) R~ 301u~c10n ~0 10 ~0 10 10 12 0 ~} ~ Prot0l1~ Ald ~ ~ ~ 6 ~ 3 5 (D) Cro~J-Llnl~ing ~9 int 0 6 0 6 ~ 0 l a 1 ~ 1 Coupllng J~g~nt ~F~ 0 ~5 0 15 0 15 0 15 0 15 0 1 O --- 0.~ 0 1 ---- -- 0 1 (C) ~2~0 ~2~ 30 30 30 30 3C -_ ~c1 ~160 ~2~gn~) - -- -- 30 ~MgO) A~d A~ptor ~G~ -- -- -- 1 0 'c-A~o Add~tlv- __ __ __ __ __ 0 0 P~OP~
ll~ qtr-ngeh ( p-l ) t377 1537 16~2 1059 16~2 1953 2û ~long-tlon ~ 661 657 ~07 8~0 903 733 'r~r 9éronqth (pl) 1~ 1139 l~g 214 152 170 ~-rd~ 55 31 ~ S0 50 Ca-pr,~-~lon 8-t 20.9 24.3 Z0.~ 25 5 20 6 20.7 Al~hough the pre~ent invention has been described 25 with reference to specific embodiments, it is to be understood ~ha~ substitutiono, addi'cion~, deletion~ and modi Fications wS thin the ~kill o~ an o~dinary ar~i~an can be made without depsrting from the scope of the invention a~ defined in the appended claim~.

Test data were ob~ained from the following ASTM methods and procedures: Shore A - D-2240, Tensile, Elongation and modulus -D-412 (Die D~9 Tear - D-624 (Die B); and Compression Set - D-395 (method B).

~,, . ', , ! , , : ': ,, ., ' ' ,.,' ~ :

Claims (30)

1. A heat curable silicone composition having improved processibility, tear strength, tensile strength and reduced compression set comprising:
(A) 100 parts of a vinyl-containing organopolysiloxane or vinyl-containing organopolysiloxane blends wherein the vinyl-containing organopolysiloxane or blend has a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C and a vinyl concentration in the range of from about 5x10-5 to about 1 weight percent;
(B) from about 1 to about 30 parts based upon Component (A) of an organopolysiloxane resin copolymer comprising:
(1) R3SiO1/2 units (M units) and SiO2 units (Q
units) where R is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation, the ratio of M units to Q
units ranging from about 0.5:1 to about 1.5:1, wherein the copolymer contains from about 0.5 to 10 weight percent vinyl groups; or (2) M and Q units as defined above and in the proportions set forth and R2SiO2/2 units (D units ) present in an amount from about 1 to about 70 mol percent based upon the total number of mols of siloxy units in the copolymer, and wherein the resinous copolymer contains from about 0.5 to 10.0 weight percent vinyl groups, or (3) mixtures of (1) and (2);
(C) from about 5 to about 200 parts based upon Component (A) of a filler; and (D) up to about 10 parts based upon Component (A) of an organohydrogensiloxane having a hydrogen content ranging from about 0.05 to about 2 percent by weight and having a viscosity ranging from about 5 to about 500 centipoise at 25°C, said composition being substantially free of vinyl-containing silicone oil.

2. The composition of claim 1, wherein Component (A) has the formula:
ViSiOR12(SiOR12)x(SiOR22)ySiR12Vi wherein Vi is vinyl, R1 is independently chosen from a monovalent hydrocarbon radical, free of aliphatic unsaturation, containing 1 to about 8 carbon atoms, R2 is an independently chosen monovalent hydrocarbon radical, free of aliphatic unsaturation, having up to about 8 carbon atoms, and x and y vary such that the viscosity of Component (A) ranges from about 3,000,000 to 100,000,000 centipoise at 25°C.

3. The composition according to claim 1, wherein the composition has a mono-modal molecular weight distribution and is substantially free of vinyl-containing organosiloxane oil.

4. The composition according to claim 1, wherein the gum has a penetration of at least about 100.

5. The composition of claim 1, wherein the gum has a penetration of about 1600.

6. The composition of claim 1, wherein Component (B) is present in an amount ranging from about 1.0 to about 20 parts by weight based upon (A).

7. The composition of claim 3, wherein (B) is present in an amount ranging from about 10 to 14 parts by weight based upon Component (A).

8. The composition of claim 1, wherein Component (C) is a silica filler and is present in an amount from about 10 to 100 parts by weight, based upon Component (A).

9. The composition of claim 8, wherein Component (C) is present in an amount ranging from about 20 to about 50 parts by weight, based upon Component (A).

10. The composition of claim 8, wherein Component (C) has a surface area ranging from about 100 to above 300 m2/gm.

11. The composition of claim 10, wherein the filler has a surface area ranging from about 160 to about 240 m2/gm.

12. The composition of claim 1, wherein Component (D) comprises a polydimethylsiloxane, polymethylhydrogensiloxane having the formula:
R3SiO(SiORHSiOR2)xSiR3 wherein R is hydrogen or a monovalent hydrocarbon radical free of aliphatic unsaturation containing 1 to about 8 carbon atoms, x varies such that the viscosity of the organohydrogenpolysiloxane varies from about 10 to about 100 centipoise at 25°C and the hydrogen content of the polymer varies from about .5 to about 2 percent by weight.

13. The composition of claim 1, wherein the tear strength of the composition is at least 150 pi.

14. The composition of claim 13, wherein the tear strength ranges from about 160 to about 215 pi.

15. The composition of claim 1, wherein the composition has a minimum elongation of about 450 percent.

16. The composition of claim 15, where the elongation ranges from about 650 to about 900 percent.

17. The composition of claim 1, wherein the composition has a compression set of no more than about 25.5.

18. The composition of claim 1, wherein the composition has a tensile strength ranging from about 1350 to about 2000 psi.

19. The composition of claim 1, further including Component (E) up to about 5 weight percent of a silanol stopped siloxane fluid having a viscosity in a range from about 3 to about 500 cps at 25°C.

20. The composition of claim 19, wherein Component (E) has a viscosity ranging from about 3 to about 30 cps 25 °C.

21. The composition of claim 20, wherein Component (E) is present in a range of about 2 to about 10 parts by weight.

22. The composition of claim 1, further including (F) up to about 2 percent by weight based upon 100 parts by weight of Component (A), of a vinyl-stopped silazane.

23. The composition of claim 22, wherein Component (F) is present in an amount up to about 0.3 parts by weight based upon 100 parts by weight of Component (A).

24. The composition of claim 1, further including (I) up to about 2 parts by weight, based upon 100 parts of Component (A), of a heat aging additive.

25. The composition of claim 24, wherein (I) comprises TiO2, Ce(OH)4, iron octoate, or mixtures thereof.

26. The composition of claim 1, further including up to about 10 parts by weight based upon Component (A), of (G) an acid acceptor.

27. The composition of claim 27, wherein the acid acceptor comprises MgO or ZnO.

28. The composition of claim 1, wherein Component (B) includes M or M' units or both and D or D' units or both.

29. The composition of claim 28, wherein the M' and D' units include vinyl units.

30. The invention as defined in any of the preceding claims including any further features of novelty disclosed.