I
M. L. DUNHAM, D. L. BAILEY, and R. Y. MIXER Silicones Division, Union Carbide Corp., Tonawanda, N. Y.
New Curing System for Silicone Rubber This system improves processing techniques and product quality and opens up possible new uses for silicone rubber goods
SILICONE
RUBBER properties are dependent on many factors-the more important of which include amount and type of fillers, base polymer composition and molecular weight, curing cycle, special additives, and the type and amount of cross linking. This last factor has been the subject of considerable investigation in these laboratories because it is related to polymer structure and is best studied b y the gum manufacturer. Past commercial practice in the silicone field has involved formulation of compounds designed to be vulcanized by the only known method that permitted utilization of the many inherent, desirable properties of silicones. This method used a highly reactive peroxide (as indicated by effective decomposition temperatures and free-radical activity) in combination with highly unreactive silanic methyl groups in the base polymer. A typical example of such a curing combination is benzoyl peroxide with dimethylsiloxane gum stocks (8). In such a system the degree of cross linking obtained must be controlled by careful measurement and dispersion' of the peroxide. These highly reactive peroxides used with the stable silicone methyl groups represent a very inefficient reaction-the ratio of free radicals necessary to give an acceptable state of cure to the number of cross links actually established is about 5 to 1 (7). Such a curing system is bound to give poor reproducibility and a high concentration of harmful catalyst residues (such as benzoic acid). Furthermore, poor cures result when these active peroxides are used with fillers such as carbon blacks (6) and high PH silicates which have reactive surfaces. Tendency to gas and depolymerization of the base polymer during oven-postcures, particularly with thick sections, are also characteristic of the above curing system. By reversing the activity of the curing system by introducing reactive, unsatu-
rated, pendant groups in the polymer and using a relatively unreactive peroxide exhibiting specificity toward the unsaturated groups, the state of cure can be reproducibly controlled by the composition of the polymer. Furthermore, the proper selection of the less reactive peroxide minimizes harmful curing residues, eliminates the need for long, stepwise oven-postcures required for thick sections, and allows practical curing of stocks compounded with fillers having reactive surfaces. A further dividend is the unpredicted, extremely low compression set, making obsolete the use of special compression set additives which in many cases are toxic and limit the elongation properties. These features are obtained without compromis-
ing other desirable properties of silicone elastomers. Although this new curing method is applicable to many unsaturated groups, such as allyl and cyclohexenyl, the vinyl group was selected for intensive study. The synthesis and chemistry of vinylsilicon compounds have been reported, ranging from chlorosilane monomers through the vinylalkoxysilanes to the vinylpolysiloxanes (2-5, 7). While the main interest in such materials centered around vinyl-to-vinyl polymerization reactions, it was apparent that because of the vinyl reactivity very small quantities in combination with proper catalysts might be used to control selectively cross-linking reactions in high polymers.
NEW CURING SYSTEM
REACTIVITY INCREASING
DECREASING
Di-=butyl
Benzoyl
2, 4 Dichlorobenzoyl
and other D i - E a lky I
'
\\
PRESENT CURING SYSTEM
Controlling silicone rubber curing b y polymer and peroxide reactivity VOL. 49, NO. 9
SEPTEMBER 1957
1373
The activity of the vinyl or other unsaturated groups is not greatly affected by the structure of the second organic group attached to the same silicon atom. The vinyl group has been introduced into the base polymer in such forms as ethylvinylsiloxane, methylvinylsiloxane, or phenylvinylsiloxane. The structure of the polymer is indicated by :
Table 1.
Selective Di-tert-alkyl Peroxides
tert-butyl tert-triptyl peroxide
c c
C
I
I 1
c-c-0-0-c-c-c
I
I
c c
C
teii-butyl triethylmethyl peroxide C
I
C
C
I
I
c-c-0-0-c-c-c , I
I C
I C
I
c Di-tert-butyl peroxide C
C
C
C
1 c-c-0-0-c-c I
Table II. Vinyl Contenta 0.00 0.10 0.20 0.30 0.40
1 I
CH3
R
CH3
I
I
I
0.20 0.30 0.40
I
or
I
AH
CH3
CHI
The general class of di-tert-alkyl peroxides was suitable for completing this curing system. Such peroxides as shown in Table I have been found to operate selectively in cross linking through the unsaturated pendant groups in the polymer. These materials have a very low order of reactivity with methyl and phenyl groups in the silicone but promote a high order of reaction with vinyl silicones. For practical reasons di-tert-butyl peroxide (DTBP) has been used throughout this investigation. Thus, this reported work is concerned mainly with a dimethylsiloxane gum containing very small amounts of vinylalkylsiloxanes, compounded with various fillers, and cured with di-tertbutyl peroxide. The role of the vinyl group is not limited to a di-tert-butyl peroxide cure. Improved elastomers can also be obtained by using the more conventional reactive peroxides-including benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and tert-butyl perbenzoate. With vinyl groups present in the polymer, much
Effect of Vinyl Concentration on Cured Elastomer Properties Mole Ratio Vinyl Silicon
...
Tensile, Elongation, Lbs./Sq. In. %
1
...
No cure 710 820 900 930
250 810 850 850 860 800
e . .
.s.
1 1 1 1
...
Compression Setb
Di-tert-butyl peroxide curedC 1350 675 450 338
1 1 1
Hardness, Shore A
330 270 230 210
45 48 52 55
1350 675 450 338
380 320 270 230 220 240
30 43 46 49 52 52
...
15 9
5 3
... 27 18
10 5
0.00" 78 a Ethylvinylsiloxane, wt. %. Method B, % of original deflection, 22 hours at 360' F. All compounds were silica-filled, cured with 0.4 part DTBP/100 oarts of zum, and ovenpostcured 24 hours a t 480° F. All compounds were silica-filled, cured with 0.4 part Bz202/100 parts of gum, and ovenpostcured 24 hours a t 480° F. e Cured with 2.0 parts Bz202/100 parts of gum.
*
I
Table 111. C yclohexenyl Ethyl Siloxane, Mole % 0.0
Cyclohexenyl as the Cross-Linking Group"
Mole Ratio Cyclohexenyl Silicon
...
...
Tensile, Lb./Sq. In.
Elongation,
%
Hardness, Shore A
No cure 1 667 550 475 68 1 200 700 125 85 a All compounds silica-filled, cured with 0.7 part DTBP/100 parts gum, and oven-postcured 24 hours at 480" F. 0.15 0.50
1 374
INDUSTRIAL AND ENGINEERING CHEMISTRY
[
-Si-0-
1H
I1
CH z
&I,
Benzoyl peroxide curedd 0.00 0.10
CH3
-Si-0-Si-O-Si-O-Si-0-
lXL1: -Si-0-
>
smaller amounts of these peroxides are required than for equivalent cures in an all dimethylsilicone. Again, compression sets considerably lower than those obtained with conventional stocks are realized, although somewhat higher than when di-tert-butyl peroxide is used. Since less catalyst is required because of the high efficiency of reaction, proportionately lower quantities of harmful curing residues are formed. The role of both the polymer composition and curing agent for obtaining a satisfactory cure is summarized. The portion of the chart to the left of the slanted line, in general, represents the previously known curing system with the peroxide controlling the reaction. To the right, the new curing system is depicted with polymer composition controlling the cure. The diagonal arrow indicates the permissible mixing of curing systems. The reaction of stable methyl groups with the relatively stable peroxide will not permit a practical state of cure. Effect of Vinyl and Peroxide Concentrations
In this new curing system, the concentration of vinyl groups built into the polymer obviously controls the state of cure and thus the final elastomer properties (Table 11). The elongations particularly indicate the degree of cross linking obtained. Also as indicated from the physical properties, the most useful range of vinyl concentration is between a 131350 and a 1 :340 vinyl-tosilicon ratio. Lower concentrations gave poor tensiles and high sets a t break indicating definite undercures. Higher vinyl contents decrease elongations to an impractical level. However, low compression sets were obtained over the entire range. The same effect of vinyl concentrations occurs when a small amount of the highly reactive benzoyl peroxide is used as catalyst-again indicating the same useful range of vinyl content and exhibiting the same loiv compression set properties (Table 11). These are to be compared with the typical high compression set (787') of an all dimethyl gum cured with the proper loading of benzoyl peroxide. The benzoyl peroxide loading operable for the binyl gums failed to give a satisfactory cure in the dimethyl gums. Further confirmation of the effect of unsaturation on physical properties is shown in Table 111. I n these experiments the cyclohexenyl group was used to introduce unsaturation. The data
CURING SILICONE RUBBER Nature of Cross-linking Reaction Table IV.
Effect of Catalyst Concentration at Constant Vinyl Concentration"
Vinyl Ethyl Siloxane Gum, Wt. %
Catalyst
0.20
DTBPC
0.75
a
b 0
Catalyst Concn., PHGb 0.20 0.40 . 0.60
.
Tensile, Lb./Sq. In.
Elongation,
%
Hardness, Shore A
750 740 720
220 230 220
42 44 46
BzzOzd
0.20 0.40 0.60
490 750 820
270 230 180
30 38 46
DTBP'
0.20 0.40 0.60
770 725 740
100 80 80
60 62 62
BzzOzd
0.20 0.40 0.60
540 750 720
220 170 120
37 48 55
All compounds silica-filled and oven-postcured 24 hours a t 480' F. Parts/100 parts of gum. Di-tert-butyl peroxide. Benzoyl peroxide.
The nature of the cross-linking reaction is considered as the mechanism by which di-tert-alkyl peroxides react to cure gums containing small amounts of unsaturation. Three obvious possibilities involving the action of free-radicals produced by the decomposition of peroxides are : A. The first possibility is the methylto-methyl cross link that has become generally recognized as the result of benzoyl peroxide cures with dimethylsilicone gums. Here benzoyl freeradicals abstract hydrogen atoms from methyl groups along the polydimethylsiloxane chain to form substituted methyl free-radicals. Then these combine to establish a dimethylene bridge as the cross link. CHa 2 -Si-0-
indicate again the same practical concentration range of cross-linking sites. The insensitivity of the state of cure to di-tert-butyl peroxide concentration is the second factor which characterizes this new curing system. Table IV illustrates this insensitivity a t two levels of vinyl concentration. The tensiles, elongations, and hardnesses remain the same regardless of a threefold increase in di-tert-butyl peroxide concentrations. For comparison, data based on benzoyl peroxide cures of the same compounds show increased cross linking as the amount of peroxide is increased. The effect of catalyst concentration on compression set is also of significance. Table V summarizes the influence of three different catalysts using a 0.15 weight yo ethylvinylsiloxane containing gum. The di-tert-butyl peroxide cured stocks show no change in compression set since the number of cross links remains unchanged. However, with benzoyl peroxide the sets increase with increased catalyst and, thus, with increased cross linking. This appears contrary to com-
RO .
I I
-2 ROH
CHI CHs -Si-0-
I I
CHz.
+
CHz.
I
- % - O w CH3 I
-Si-0Methyl-to-methyl cross linking
mon experience which dictates that compression sets can be improved by further cross linking. This apparent anomaly can be explained in terms of mixtures of two cross-link types because, as the benzoyl peroxide content in a vinyl gum is increased the ratio of methylene-to-vinyl cross links increases, the compression sets approach the high
This possibility can be eliminated with the new curing system on the basis that the di-tert-alkyl peroxides will not cure a dimethylsilicone gum, although a t excessively high peroxide concentrations a slight degree of cure can be detected. B. The second possibility is a vinylto-vinyl reaction as indicated by one such mechanism.
R -Si-O-
R
I I
RO .
CH=CHz
-Si-0-
---+
Table V.
Catalyst Di-tert-Butyl peroxide
Concn., CompresPHGb sion SetC 0.6 1.0
23 22
tert-butyl perbenzoate
0.6 1.0
28 35
Benzoyl peroxide
0.6 1.0
33 47
OAll compounds based on 0.16 wt. yo ethylvinylsiloxane gums, silica-filled, postcured 24 hours a t 480' F. Parts/100 parts of gum. ' Method B, 96 hours a t 350' F., % of original deflection.
R
I I
-Si-0-
. CH-CHzOR
Effects of Catalyst Type on Compression Set"
I
I I CH 2 I
CH-CH20R
CH=CHg -Si-0-
I 1
. CH-CH20R
R -Si-O-
I CH3
I
. CH
I R
-Si-0-
Vinyl-to-vinyl cross linking values obtained with dimethylsilicone gums. The results obtained with tertbutyl perbenzoate indicate the intermediate values. The conservative nature of this peroxide evidently stems from the fact that both the relatively inactive tert-butoxy free-radical and the active benzoyl free-radical are produced.
1
I R
The following facts, however, rule out the possibility of vinyl-to-vinyl cross links as contributing significantly to the curing reaction :
1. The vinyl content of the gums is at such a low level that the vinyl groups are far apart, and a cross link would not VOL. 49, NO. 9
SEPTEMBER 1957
1375
~~
Table VI.
Polymer
Filler
Dimethyl Zeolex 23 Vinyl
Cures with Fillers Having Reactive Surfaces Properties of Postcured Filler Catalyst Tensile, Elonga- HardLoading, Loading lb:/sq. tion, ness, PHGD pH PHGn in. % Shore A 50 10.0 Benzoylper2.0 No cure
Zeolex 23
50
10.0
Dimethyl Philblack 0
40
9.1
Vinyl
40
9.1
a
Philblack 0
Parts/100 parts of gum.
be expected to occur between vinyl groups. 2. By blending a nonvinyl-containing gum with a gum containing proper leveG of unsaturation, satisfactory cures are obtained with di-tert-butyl peroxide. Similarly, by adding monomeric diolefin materials, such as divinyl-substituted cyclic siloxanes, to a dimethylsilicone gum, satisfactory cures with di-tertbutyl peroxide can be obtained. 3. Curing of gum stocks containing relatively high concentrations of vinyl groups is an extremely difficult task because of the very small concentrations of peroxide needed to give a satisfactory cure. Not only does the concentration of the peroxide need to be low, but also
oxide Di-tert-butyl peroxide Benzoylperoxide Di-tert-butyl peroxide
-Si-O-
CH=CHz CH3 -Si-O-
I !
750
2.0
52
I
-Si--O-
I
.CH-CHzOR
2RO.
+
-ROH
Literature Cited (1) Bueche, A. hl., J . Polymer Sci. 15, 105
(1955). Burkhard, C. A., Rrieble, R. H., J . Am. Chem. SOC. 69,2687 (1947). Hurd, D. T., Zbid., 67, 1813 (1945). Hurd, D. T., Roedel, G. F., IND. ENG.CHEM.40,2078 ( I 948). Marsden, J. (to General Electric Co.), U. S. Patent 2,445,794 (July 27, 1948).
K
,
Si-O/-aJ I
CH 2-CHzOR
CiI 2 I
-Si-0-
I
-Si-0-
CH 3
Trimethylene linkage R RO .
I I
I ! I
CH z
CH 3
-Si-0-
I
280
1,2-Propylenelinkage R
R I CH=CH*
46
No cure
4.0
CHI
-SiO-
150
dence for the nature of the cross link, the fact that blends of nonvinyl containing gums with vinyl containing gums can be satisfactorily cured and that divinylsubstituted cyclic siloxanes when added to a dimethylsilicone gum permit curing with di-tert-butyl peroxide points to the third possibility where the cross link involves both a vinyl and a methyl group. Because of this evidence and the fact that the methyl-to-methyl and vinyl-to-vinyl reactions have been eliminated, two speculative mechanisms are proposed for a methyl-vinyl cross link involving the formation of either a 1,Zpropylene linkage or a trimethylene bridge.
R
I I
500
0.8
di-tert-butyl peroxide and vinyl containing gum the carbon black-filled stocks give tensile and elongation properties similar to the widely used silica-filled compounds. In addition, the blackfilled stocks exhibit excellent electrical conductivities that are insensitive to handling and processing techniques. This property combined with thermal stability, inertness, and nonadhesiveness promises to be very useful. This new curing system, involving a silicone polymer containing controlled amounts of unsaturated, pendant groups coupled with either a peroxide exhibiting specificity for this unsaturation or small quantities of conventional peroxides, provides the means for fabricators to improve processing techniques and quality of products as well as to promote new and unique applications for silicone rubber goods.
K ,----S ' i-O-
CH=CHZ
1 I
. CH
--P
-ROH
CH z
-Si--O-
I I
CH a
CH z CM2 -Si-0-
I
1
CH 3 Vinyl to methyl cross linking it must be controlled within very narrow limits. For example, to obtain a satisfactory cure a t a 4% vinylalkylsiloxy concentration, the ratio of the maximum amounts of possible free-radical fragments from the peroxide to the number of vinyl groups is only 1 to 100, indicating a chain reaction through the vinyl groups. The vinyl content is much lower in the gums normally used and the required amount of peroxide is much higher.
C. Although there is no direct evi1 376
If these proposed reactions do occur, it appears likely that the presence of vinyl groups and/or vinyl radicals are necessary to activate the formation of substituted methyl radicals with di-tertbutyl peroxide. The activity of this new curing system was not curtailed by the inclusion of fillers with reactive surfaces such as carbon blacks and some silicates. T h e success of such cures is shown in Table VI. The benzoyl peroxide catalyzed stocks failed to give any cure, while with
INDUSTRIAL A N D ENGINEERING CHEMISTRY
(6) Pfeifer, C. FV., Savage, R. M., White, B. B., India Rubber World 129, 483 (1954). ( 7 ) Wagner, G. H., Bailey, D. L., Pines, A. N., Dunham, M. L., McIntire, D. B., IND.ENG. CHEM.45, 367 (1953). (8) Wright, J. G. E., Oliver, C. S. (to General Electric Co.), U. S. Patent ' 2,448,565 (Sept. 7, 1948).
RECEIVED for review May 31, 1956 ACCEPTEDMarch 11, 1957
Division of Rubber Chemistry, ACS, Cleveland, Ohio, May 1956.
