CA1309540C

CA1309540C - Heat curable silicone compositions

Info

Publication number: CA1309540C
Application number: CA000511050A
Authority: CA
Inventors: Edwin Robert Evans
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1985-06-13
Filing date: 1986-06-06
Publication date: 1992-10-27
Anticipated expiration: 2009-10-27
Also published as: JPS6230151A; DE3619206A1; JPH0482024B2

Abstract

60SI-725, IMPROVED HEAT CURABLE
SILICONE COMPOSITIONS
ABSTRACT OF THE DISCLOSURE

In a heat curable silicone rubber composition comprising a polydiorganosiloxane base polymer and a curing catalyst, the improvement comprising an effective amount of co-curing agent.
Preferably, the co-curing agent has the general formula or

Description

i3~95~

IMPROVED HEAT CURABLE
SILICONE COMPOSITIONS- --Background of the Invention The present invention relates to heat curable silicone rubber compositions. More particularly, the present invention relates to heat curable silicone rubber compositions containing an amount of certain co-curing agents effective to allow the use of less reinforcing filler while obtaining improved tear strength, suitable durometer and lower compression set.

Heat curable silicone rubber compositions comprising ta) a polydiorganosiloxane base polymer, (b) an effective amount of reinforcing filler, and (c) an effective amount of free radical generating catalyst are well known in the art. It is appreciated by those skilled in the art that the amount of rein~orcing filler employed is directly related to many of the physical properties of the resultant silicone rubber. For example, if the amount of reinforcing filler in a silicone rubber composition is increased, the durometer, tear strength and compression set will all generally increase somewhat. It is also recognized by those skilled in the art that if the filler loading is too high, the rubber becomes very difficult to process.

For most applications the artisan can obtain suitable properties simply by adjusting the filler level. However, in some applications, such as calendering solvent resistant 1~09~0 605I-725~GLL:mz/0195p silicone rubber to fabric, when sufficient filler is added to obtain the necessary durometer, the green strength or bulk mDdulus of the rubber is detrimental to calendering to fabric since it does not form the desired intimate, void-free compound/fabric interface. Moreover, the filler loading necessary to obtain a suitable durometer also increases the compression set beyond acceptable limits.

It has now been discovered that certain co-curing agents when added to heat curable silicone rubber compositions allow the use of less reinforcing filler while obtaining improved tear strength, substantially unchanged durometer, and lower compression set.

Konkle, U.S. Pat. No. 2,684,957, teaches that low compression set heat curable silicone rubber compositions can be made from (a) an organopolysiloxane, (b) a vulcanizing agent, and (c) from .75 to 8 parts by weight per 100 parts by weight of the organopolysiloxane of a cadmium compound selected from cadmium oxide, cadmium peroxide and cadmium carbonate.

Konkle and Talcott, U.S. Pat. No. 2,927,908, relates to the use of polytetrafluoroethylene to improve the tensile strength and tear resistance of fluorinated organopolysiloxane rubber.

Modic~ U.S. Pat. No. 2,979,479, discloses that heat curable silicone rubber compositions having improved tear strength can be prepared from (a) an organopolysiloxane, (b) a finely divided silica filler, (c) a curing agent, and (d) a copolymer composed essentiall~ of trimethylsiloxy groups and SiO2 groups, there being present from about 1.0 to 1.5 methyl groups per silicon atom.

-13Q9~0 60SI-725/GLL:mz/0195p Grabicki et al., U.S. Pat. No. 3,027,3449 provides heat curable silicone rubber compositions characterized by improved tenSjle and tear strength comprising (a) an organopolysiloxane, (b) silica filler, (c) 1 to 20 parts of organic amine, and (d) 1 to 20 parts of a hydroxy organosilane selected from diphenylsilanediol and triphenylsilanol.

Wada et al., U.S. Pat. No. 3,652,475, teaches heat curable silicone rubber compositions comprising (a) 100 parts by weight of polydiorganosiloxane having z degree of polymerization of at least 3000 and containing at most 0.3 mole percent vinyl groups, (b) from 1 to 10 parts by weight of polydiorganosilox-ane having a degree of polymerization of at least 3000 and containing from 5 to 20 mole percent vinyl groups, (c) from 0.5 to 5 parts by weight of polydiorganosiloxane having a degree of polymerization of from 10 to 1000 and containing from 5 to 90 mole percent vinyl units, (d) 20 to 200 parts by weight of silica filler, and (e) a catalytic amount of organic peroxide;
said compositions having improved tear strength and superior compression set.

None of the foregoing references teach or suggest that improved tear strength, acceptable durometer -and lower compression set can be obtained by using less reinforcing filler, but including a co-curing agent such as trimethylol propane trimethacrylate or triallyl trimellitate.

13095~0 6051-725/GLL:mz/01~5p Summary of the Invention It is an ob~ect of the present invention to provide heat curable silicone rubber compositions which penmit the use of less reinforcing filler while obtaining improved tear strength, substantially unchanged durometer, and lower compression set.

It is another object ~f the present invention to provide heat curable silicone rubber compositions suitable for calendering to fabric.

Still another object of the present invention is to provide a method for making the compositions of the present invention.

In accordance with one aspect of the present invention there are provided heat curable silicone rubber compositions, comprising:

(a) a polydiorganosiloxane base polymer;

(b) an effective amount of catalyst; and (c) an effective amount of co-curing agent.

Description of the Invention According to the present invention there are provided heat curable silicone rubber compositions~ comprising:

~o~

60SI -725/GLL :mz/01 95p (a) a polydiorganosiloxane base polymer;

~b) an effective amount of catalyst;

(c~ an effective amount of co-curing agent; and (d) optionally, but preferably, an effective amount of reinforcing filler.

Polydiorganosiloxane (a) can be any polymer or mixture of polymers known by those of ordinary skill in the art to be suitable for use in heat curable silicone rubber compositions.
Generally, such polydiorganosiloxanes have the general formula Ra SiO4 a where R is selected from monovalent substituted and unsubstitu-ted organic radicals, a can vary from about 1.95 to about 2.02, inclusive, and the viscosity is at least about 100,000 centi-poise at 25~C.

Illustrative of suitable R radicals are aryl radicals and halogenated aryl radicals such as phenyl, chlorophenyl, xylyl, and tolyl; aralkyl radicals such as benzyl and phenylethyl;
aliphatic, haloaliphatic and cycloaliphatic radicals such as allkyl, alkenyl, cycloalkyl and haloalkyl, including methyl, ethyl, propyl, vinyl, allyl9 cyclhexyl, chloromethyl, chloro-ethyl and 3,3,3-trifluoropropyl; and cyanoalkyl radicals such as cyanoethyl.

130~5~0 Preferably, R is selected from methyl, vinyl, phenyl and 3,3,3-trifluoropropyl radicals. It is also preferred that the viscosity of polydiorganosiloxane (a) be from about 1,000,000 centipoise to 200,000,000 centipoise at 25C.
Also within the scope of polydiorganosiloxane (a) are copolymers containing two or more different diorganosiloxane units, such as, for example, copolymers of dimethylsiloxane units and methylphenylsiloxane units; copolymers of dimethylsiloxane units, diphenylsiloxane units, and methylphenylsiloxane units; and copolymers of dimethylsiloxane units, methylvinylsiloxane units and diphenylsiloxane units. There is also included within the scope of polydiorganosiloxane (a) mixtures or blends of polymers and/or copolymers.
If a solvent resistant heat curable silicone rubber is to be prepared, polydiorganosiloxane (a) should be a fluorosilicone polymer, for example, of 20 the type described in U.S. Patent No. 4,355,121, issued October 19, 1982 to Evans, U.S. Patent 4,492,786, issued January 8, 1985 to Evans, or U.S.
Patent No. 4,529,774, issued July 16, 1985 to Evans.
Other suitable fluorosilicone polymers are well known in the art. Such fluorosilicone compositions have been found to be especially suitable for practicing the present invention.
Catalyst (b) can be any of the conventionally used means for promoting curing of the silicone rubber and includes both chemical vulcanizing agents and high energy radiation sources such as gamma radiation from a cobalt-60 source. Preferably, 13~9~

60SI-725/GLL:mz/0195p catalyst (b) is one of the organic peroxides coTmonly used in the silicone art such as 2,4-dichlorobenzoyl peroxide, benzoyl peroxide, di-t-butyl pero~ide, 2,5-bis(t-butylperoxy)2~-di-methylhexane or dicumyl peroxide.

Organic peroxides for use as a catalyst for heat curable silicone rubber are commercially available, usually mixed with an inert carrier to form a powder or paste.

The amount of peroxide used in heat curable silicone rubber compositions is not critical, but generally is from about 0.1 to 5 parts by weight per 100 parts by weight of uncatalyzed silicone polymer. The peroxide catalyst is initiated by heating the composition tn a temperature sufficient to activate (e.g. decompose~ the organic peroxide used.

Co-curing agent (c) can be any silicone soluble (i.e.
somewhat polar) monomer having at least three olefinic functional groups. These monomers are called co-curing agents because they do not initiate curing, however, once curing has begun said co-curing agents rapidly form free radicals, thereby increasing the efficiency of the catalyst and, because they are polyfunctional, take part in establishing a tight cure matrix.

In practicing the present invention it is preferable to include from about 0.1 to about 3 parts by weight of co-curing agent per 100 parts by weight of polydiorganosiloxane (a).
More preferably, cD-curing agent (c) is utilized in an amount ranging from about 0.2 to about 0.8 parts by weight or, more preferably, from about 0.2 to about 0.4 parts by weight per 100 parts by weight of uncatalyzed silicone polymer.

13~954~
60SI-725/6LL:mz/0195p 6enerally, the co-curing agents utili~ed in the practice of the present invention have the general fonmula I. C ~ R3 ) or II. ~C - O - R - C = I R ~

where R is hydrogen or a lower alkyl radical having from 1 to 5 carbon atoms, Rl is a lower alkylene radical having from 1 to 5 carbon atoms, each R2 is independently selected from hydrogen and methyl, R3 is hydrogen or a methyl radical, n equals 0 or 1, and m is an integer from 3 to 6, inclusive.
Preferably, R is ethyl, R is methylene, R2 is hydrogen, R3 is methyl, n equals 1, and m equals 3. Thus, the preferred co-curing agents of the present invention are trimethylol propane trimethacrylate, ~ 3 ~ 0 6051-725/GLL:mz/0195p CH2 = C - C - O
Il ~

ICH3 l H2 CH2 = C - C - 0 - CH2 - C - CH2 CH3 I

fH3 1 2 and triallyl trimellitate, O O

CH2 = CH - CH20 - C ~ C - OCH2 - ~H = CH2 ~ ll OCH2 CH CH2 .

- i3~9~

60SI-725~GLL:mz/0195p It should be noted that the olefinic functional groups of the co-curing agent need not all be the same and that, in formula II, it is not critical to which carbon of the benzene ring the olefinic functional groups are bonded. Moreover, it should be understood that the foregoing formulas are merely illustrative of suitable co-curing agents and that other variations will be obvious to the artisan and which are include within the scope of the invention as well as the appended claims.

The use of such co-curing agents in heat curable silicone rubber compositions allows the artisan to use less reinforcing filler than heretofore required, yet provides a cured composition having improved tear strength, substantially unchanged durometer, and lower compression set. Furthermore, I5 the use of co-curing agents disclosed hereinabove may allow the artisan to completely eliminate the need for reinforcing filler in some applications. ~owever, it is most preferable that reinforcing filler be included in the heat curable silico~e rubber composition, only in a smaller amount.

Illustrative of suitable reinforcing filler (d) are included, for example, fumed silica, treated fumed silica, precipitated silica and treated precipitated silica. Of course, mixtures of such reinforcing fillers are also within the scope of the present inYention.

Preferably, the reinforcing filler is a treated silica filler and, most preferably, is treated fumed silica.
Treatment of the filler can be effected by any method known in the art, such as, for example, as taught in U.S. Pat. No.

~ 3~95~Q

60SI-725/GLI:mz/0195p 2,938,009 to Lucas, U.S. Pat. No. 3,635,743 to 5mith, United States Patent Number 3,837,878 to Beers or United States Patent Num~er 4,529,774 to Evans et al, issued July 16, 1~85.

In the case of fluorosilicone heat curable silicone rubbers suitable for calendering to fabric, it is especially preferable to employ the fluorosilicone treated fillers disclosed in Evans, IJnited States Patent Number 4,529,774.
Generally, the amount of reinforcing filler included in the heat curable silicone rubber composition will range from about 5 to about 100 parts by weight and, preferably, from about 30 to 50 parts by weight.

In addition to the ingredients listed above, the heat curable silicone compositions of the present invention may contain additives known in the art, such as antioxidants, pigments, heat stability additives, and the like.

The compositions of the present invention are prepared by mixing the various ingredients in any manner known in the art, for example, in a doughmixer.

In order to better enable the artisan to practice the present invention the following examples are provided by way of illustration and not by way of limitation. All parts are by weight unless otherwise noted.

i3~

60SI-725/GLL:mz/019Sp EXAMPLES
. . . = . = =
Example 1 A base fluorosilicone composition was prepared by mixing 100 parts by weight of methylvinyl tenminated polydiorgano-S siloxane ha~ing a viscosity of 100,000,000 centipoise at 25C;

3 parts by weight of a polydimethylsiloxane diol process aid having the formula H0 ( Si (Me)20 )5 H; 1 part by weight of methylvinyl tenminated polydimethylsiloxane process aid; 0.25 parts by weight bis-dimethylvinylsilazane; 0.65 parts by weight cerium hydroxide and 0.8 parts by weight 2,5 bis-(t-butyl peroxy)-2,5-dimethylhexane.

4764 grams of this base composition was placed in a 3 1/2 quart doughmixer along with 429 grams of fumed silica treated with fluorosilicone in accordance with United States Patent Number, 4,529,774 issued July 16, 1~5. The amount of filler was sufficient to provide a filler loading of 40 parts by weight.

After efflecting uniform dispersion of the filler, 47.7 grams red colorant and amounts of trimethylol propane 20 ~ ~ trimethacrylate (available as Sartomer~r~ 50 from Sartomer Company, West Chester, Pa.~ ranging from 0.25 to 2.0 parts by weight of polymer was added (see Table I). The ingredients were thoroughly mixed for 30 minutes and then strained through a 100 mesh 316 SS screen. The strained compound was then catalyzed on a mill with 2,4-dichlorobenzoyl peroxide (50 weight percent in SF-96 ~ 1000 centistoke fluiJ containing 12.5 weight percent dibutylphthalate~ at a le~el of 1.6 parts by weight per 100 parts by weight of compound. See Table I, compositions B, C, E, F and G.

~309~4 605I-725/GLL:mz/019Sp Example 2 The same procedure as in Ex~mple 1 was followed except that the trimethylol propane trimethacrylate was replaced with 0.25 parts triallyl trimellitate per 100 parts by weight of polymer. See Table I, composition D.

Example 3 For purposes of comparison, FSE-7140, available from General Electric Company, was evaluated since it is an example of a composition that is ineffective for calendering to fabric. Also for purposes of comparison there was prepared a composition as in Example 1 except the co-curing agent was not included. See Table I, Composition A. Lastly, a composition manufactured by Dow Corning Corporation (LS-63) and believed to be used for calendering to fabric was also tested.

. _ _ _ _ _ ~ 13~9~

60SI-725/GLL:mzlO195p TABLE I

FSE

Base Compound 100 100 100 100 100 100 100lQO U
Total Filler Level28 46 46 40 40 39 4040 N
Trimethylol propane K
trimethacrylate -- -- 0.25 0.25 --0.30 0.25 2.0 N
Triallyl trimelliate -- -- -- -- 0.25 -- -- -- O
Peroxide/100 pt. cpd. 1.6 1.6 1.61.6 1.6 1.6 1.6 1.6 W
N

Post Cure 15 min/350F, Post Bake 4 hrs/400F

Physical Properties Shore A 4661 66 64 61 60 62 68 62 Tensile, psi13951485 1285 1440 1150 1460 1455 1210 1163 Elongation, ~400330 320 330 410 430 380 460 240 Tear, Die B, pi140145 190 165 190 175 160 190 115 Specific gravity 1.42 1.45 1.46 1.45 1.46 1.45 1.45 1.43 1.48 Comprsssion Set B
22 hrs/350F27.929.1 28.0 23.0 21.0 12.2 17.7 16.3 14.9 Fuel Immersion - Fuel B (22 hours, room temperature) % Vol. Change 24.5 29.0 29.0 22.1 22.8 23 23 24.9 24.8 Calender erformance Exc. Some No Good Exc. Exc. Exc. Exc. Good Voids 600d ~3~9~

60SI-725/GLL~mz/0195p The data in Table I clearly shows the superior perfor~ance of compositions C through G, relative to A and LS-63, which are prepared with lower filler loadings and the co-curing agents of the present invention. The silicone elastomers prepared S therefrom have a substantially unchanged durometer (relative to A) and exhibit better resistance to tear, compression set and fuel B solvent swell while maintaining good processability.

Claims

1. A heat curable silicone rubber composition, comprising:
(a) a polydiorganosiloxane base polymer having the general formula:

wherein R is selected from monovalent substituted or unsubstituted organic radicals, a varies from about 1.95 to about 2.02, inclusive, and the viscosity is at least about l00,000 centipoise at 25°C:
(b) an effective amount of catalyst to promote curing;
(c) from 0.2 to 0.8 parts by weight of co-curing agent per 100 parts by weight polydiorganosiloxane (a), said co-curing agent comprising a silicone soluble monomer having at least three olefinic functional groups to improve tear strength and lower compression set: and (d) reinforcing filler in an amount from 5 to 40 parts by weight silica per 100 parts by weight said polydiorganosiloxane (a).

2. A composition as in claim 1 wherein the co-curing agent is present in an amount of from 0.2 to 0.4 parts by weight per 100 parts by weight of polydiorganosiloxane (a).

3. A composition as in claim 1 wherein the co-curing agent has the general formula where R is hydrogen or a lower alkyl radical having from 1 to 5 carbon atoms, R1 is a lower alkylene radical having from l to 5 carbon atoms, R2 is hydrogen or a methyl radical, R3 is hydrogen or a methyl radical, and n equals 0 or 1.

4. A composition as in claim 3, wherein the co-curing agent is trimethylol propane trimethacrylate.

5. A composition as in claim 1, wherein the co-curing agent has the general formula where R1 is a lower alkylene radical having from 1 to 5 carbon atoms, each R2 is independently selected from hydrogen or methyl, and m is an integer from 3 to 6, inclusive.

6. A composition as in claim 5, wherein the co-curing agent is triallyl trimellitate.

7. A heat curable silicone rubber composition, comprising:
(a) 100 parts by weight of polydiorgano-siloxane base polymer having the general formula- where R is selected from monovalent substituted or unsubstituted organic radicals, a varies from about 1.95 to about 2.02, inclusive, and the viscosity is at least about 100,000 centipoise at 25°C:
(b) from 0.1 to 5 parts by weight of organic peroxide catalyst, (c) from 5 to 40 parts by weight of reinforcing filler, and (d) from 0.2 to 0.8 parts by weight of co-curing agent per 100 parts by weight of polydiorganosiloxane (a) to improve tear strength and lower compression set, said co-curing agent selected from compounds having the general formula:

or where R is hydrogen or a lower alkyl radical having from 1 to 5 carbon atoms, R1 is a lower alkylene radical having from 1 to 5 carbon atoms, each R2 is independently selected from hydrogen or methyl, R3 is hydrogen or methyl, n equals 0 or 1, and m is an integer from 3 to 6, inclusive.

8. A composition as in claim 7, wherein poly-diorganosiloxane (a) is a fluorosilicone polymer and reinforcing filler (c) is fluorosilicone treated fumed silica present in an amount ranging from 30 to 50 parts by weight per 100 parts by weight of polydiorganosiloxane (a).

9. The cured composition of claim 1.

10. A method for making a heat curable silicone rubber composition, comprising mixing:
(a) a polydiorganosiloxane base polymer having the general formula:

where R is selected from monovalent substituted or unsubstituted organic radicals, a varies from about 1.95 to about 2.02, inclusive, and the viscosity is at least about 100,000 centipoise at 25°C;
(b) an effective amount of catalyst to promote curing;
(c) from 0.2 to 0.8 parts by weight of co-curing agent per 100 parts by weight polydiorganosiloxane (a), said co-curing agent comprising a silicone soluble monomer having at least three olefinic functional groups to improve tear strength and lower compression set; and (d) reinforcing filler in an amount from 5 to 40 parts by weight silica per 100 parts by weight said polydiorganosiloxane (a);

11. A method as in claim 10, wherein the co-curing agent has the general formula or where R is hydrogen or a lower alkyl radical having from 1 to 5 carbon atoms, R1 is a lower alkylene radical having from 1 to 5 carbon atoms, R2 is hydrogen or a methyl radical, R3 is hydrogen or a methyl radical, and n equals o to 1.

12. A method as in claim 10, wherein the co-curing agent is trimethylol propane trimethacrylate.

13. A method as in claim 10, wherein the co-curing agent has the general formula where R1 is a lower alkylene radical having from 1 to 5 carbon atoms, each R2 is independently selected from hydrogen or methyl, and m is an integer from 3 to 6, inclusive.

14. A method as in claim 13, wherein the co-curing agent is triallyl trimellitate.

15. A method for making a heat curable silicone rubber composition, comprising mixing:
(a) 100 parts by weight of polydiorgano-siloxane base polymer having the general formula:

where R is selected from monovalent substituted or unsubstituted organic radicals, a varies from about 1.95 to about 2.02, inclusive, and the viscosity is at least about 100,000 centipoise at 25°C;

(b) from 0.1 to 5 parts by weight of organic peroxide catalyst, (c) from 5 to 40 parts by weight of reinforcing filler, and (d) from 0.2 to 0.8 parts by weight of co-curing agent per 100 parts by weight of polydiorqanosiloxane (a) to improve tear strength and lower compression set, said co-curing agent selected from compounds having the general formula:

or where R is hydrogen or a lower alkyl radical having from 1 to 5 carbon atoms, R1 is a lower alkylene radical having from 1 to 5 carbon atoms, each R2 is independently selected from hydrogen or methyl, R3 is hydrogen or methyl, n equals 0 or 1, and m is an integer from 3 to 6, inclusive.

16. A method as in claim 15, wherein polydiorganosiloxane (a) is a fluorosilicone polymer and reinforcing filler (c) is fluorosilicone treated fumed silica present in an amount ranging from about 30 to about 50 parts by weight per 100 parts by weight of polydiorganosiloxane (a).