CA1092278A - Hydraulic elastomer of cross-linked dimethylsiloxane polymers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A hydraulic elastomer having a viscosity of 1,000 to 1,000,000 centi-poises at 25°C. is provided herein which is composed of a polydimethylsilox-ane cross-linked by three-carbon bridges, having a relatively high cross-link density of from 0.25 to 0.50 cross-links per 100 silicon atoms, and in which there are from 0.074 to 0.74 free chain ends per 100 silicon atoms, the free ends having an average of from 50 to 100 silicon atoms each, the elastomer being characterized by a Shore A hardness of from 9 to 15, and being further characterized by its ability to absorb energy by flowing under pressure like a fluid. It is made by heating a particularly reacted silicone fluid, containing approximately 0.45 mole percent of methylvinylsiloxane units, with a vinyl-specific peroxide. This elastomer has a high cross-link density and a high proportion of free chain ends. Because of these char-acteristics it crumbles to a powder under high ahear stress, but has the unique property of flowing like a viscous fluid through a narrow orifice.
It is useful in hydraulic impact absorbers and other hydraulic systems.

Description

- lO~ZZ78 Thls invention relates to hydraulic silicone elastomers, and to processes for preparing the same.
This applicatlon is a division of application Serial No. 208,631 filed Sept. 6, 1974.
Silicone fluids have been used in hydraulic shock absorbers be-cause of their ability to dissipate energy by flowing through an orifice.
They have the disadvantage ~hat must be used in closed systems to avoid loss by gravity. Even the smallest leak will permit deterioration over a period of time. Nevertheless, up till now no other materials have been found suitable in such application.
It is true that elastomers and other materials have some impact-absorbing ability because of their ability to absorb energy by elastic te-formation. It is also true that most elastomers can be forced through an orifice if sub~ected to enough pressure. In the process, however, pre-viously known elastomers become so thoroughly degraded that they cannot be used a second time in the same system. The degradation is to ~ome extent - -a mechanical breakdown from the high shear stress involved, but mainly it is chemical degradation caused by the high temperatures generated.
Silicone elastomers are desirable materials for such an appli-cation because of their high thermal stability. Also they have a high com-pressibility, which tends to smooth out peaks in the stress-strain curve.
However, none of those known heretofore have been suitable. They can be forced through an orifice. thereby bein8 broken down into small particles, but these are relatively hard and do not easily flow back into their origin-al position. Oil has been added as a plasticizer to overcome these dis-advantages; however, it has not been very successful, since the oil bleeds from the elastomer and eventually leaks out of the system.
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It i8 therefore an ob~ect of a broad aspect of this invention as now provided by the present dlvisional application to provide a novel by-draulic elastomer.
An ob~ect of another aspect of this invention as now provided by the present divisional applicatlon to provide a cross-llnked hydraulic sili-cone elastomer that is easily deformed und~r pressure and that breaks down into soft particles under high shear, such particles having the property of flowing under pressure, but not under the influence of gravity alone.
An ob~ect of a further aspect of this invention as now provided by the present divisional application is to provide a process for prepar-ing such novel hydraulic elastomer.
In accordance with a broad aspect of this invention as now pro-vided by the present divisional application a hydraulic elastomer is now provided having a viscosity of 1000 to 1,000,000 centipoises at 25C and composed of a polydimethylsiloxane cross-linked by three-carbon bridges, having a relatively high cross-link density of from 0.25 to 0.50 cross-links per 100 silicon atoms, and in which there are from 0.074 to 0.74 free chain ends per lO0 silicon atoms, the free ends having an average of from 50 to 100 silicon atoms each, the elastomer being characterized by its ability to absorb energy by flowing under pressure like a fluid.
By a variant thereof, there are from 0.18 to 0.34 free ends per 100 silicon atoms, and the Shore A hardness is from ll to 14.
By another variant the hydraulic elastomer is in the form of fine particles which are predominantly less than 1 millimeter in diameter.
By ano~her broad aspect of this invention as now provided by the present divisional application process is provided for preparing ~he above described hydraulic elastomer which ComprifieS heating a silicone fluid com-prising a copolymer of dimethylsiloxane units with 0.1 to 0.9 mole percent of methylvinylsiloxane units; the mole percent of methylvinylslloxane units haiing a volume of

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(4.8/log V) - C, where V is the viscosity in centipoises and C ls a number having a value of from 0.53 to 0.83; such copolymer having end groups selected from the class consisting of trimethylslloxy, hydroxy and alkoxy with 0.1 to 1.5 percent of a curing agent selected rom the class consist-ing of tertiary alkyl peroxides and diperoxy ketals at a temperature from130C. to 210C. for a time equal to at least one half-life of the curing agent.
By a variant thereof, the curing time is equal to at least 5 half-lives of the curing agent.
By another variant, the curing agent is a tertiary alkyl peroxide, the curing temperatures is at least 150C., and the during time is at least one minute.
By a further variant, the curing agent is a diperoxy ketal, the curing temperature is at least 130C. and the curing time is at least one minute.
By a variant thereof, the amount of curlng agent is from 0.3 to 0.8 percent.
By other variants the curing agent is a tertiary alkyl peroxide, e.g., is dicumyl peroxide, or is bis-(t-butylperoxyisopropyl)-ethane, or is bis-(t-butylperoxyisorpopyl)-benzene, or is a diperoxy ketal, e.g., is 1, l-di-t-butylperoxy-3,3,5-trimethylcyclohexane.
By another variant, the curing agent has an ultraviolet absor-bance at 240 nanometers that is no greater than at 260 nanometers.
By a variant thereof, the curing agent is bis-(t-butylperoxyiso-propyl)-ethane.

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10~278 By yet another variant, the copolymer is represented by the for-mula-RO[(CH3)2SiO]x[CH3(C2H3)SiO]yR where R i~ selected from the class con-sisting of alkyl radicals of from 1 to 4 carbon atoms, hydrogen and tri-methylsilyl radicals, x is a number of from 270 to 2,7000 and y is a num-ber of from O.OOlx to O.OO9x.
By another variant, the copolymer contains trimethylsiloxy end groups.
By another variant, the value of C is from 0.63 to 0.78.
By a variation thereof, the mole percent of methylvinylsiloxane units i8 approximately 0.45~
Examples of silicone fluids which may be used in the preparation of hydraulic elastomers of an aspect of this invention as now provided by the present divisional application are linear siloxane copolymers having the general formula RO[(CH3)2SiO]x[CH3(C2H3)SiO]yR
in which R is an alkyl radical from from 1 to 4 carbon atoms, hydrogen or a trimethylsilyl radical, x is a number of from 270 to 2,700 and y is a number of from O.OOlx to O.OO9x. Generally these silicone fluids contain predominantly, dimethylsiloxane units with small amounts of methylvinyl-siloxane units. The end groups may be trimethylsiloxy, hydroxy or alkoxygroups; however, for optimum viscosity control, the trimethylsiloxy groups are preferred.
The molecular weight may vary between 20,000 and 200,000, corresponding with viscosities between 1000 and 1,000,000 centipoises (cp) at 25C.
The amount of methylvinylsiloxane units may vary from 0.1 to 0.9 mole percent. The optimum amount varies inversely with the chain length. Specifically, the optimum methylvinylsiloxane content i8 given . .

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by the relation:
"Vinyl" = lOOY _ 4 8 _ 0 73 (1) Here "vinyl" is mole percent of methylvinylsiloxane, and V i8 the viscosity in cp. "Vinyl" may be as much as 0.05 mole percent lower or 0.10 mole per-cent higher without departing from the optimum range. In mathematical terms, then, the optimum range is given by the relation "Vinyl" = 41 8 V ~ C (2) where C may vary between 0.63 and 0.78. Small departures from the optimum range are permissible, but in any case C should be between 0.53 and 0.83.
As indicated above, the viscosity of the fluid may be as low as 1000 cp. However, even with the optimum vinyl content it is found that the ultimate properties of the cured elastomers are not as good as when the viscosity of the fluid is at least 5000 cp. That is, the hardness of the elastomer and its resistance to flow are below the desired range.
Good physical properties in the hydraulic elastomer as now pro-vided by the present divisional application are obtained if the viscosity of the fluid approaches 1,000,000 cp. However, the vinyl content has to be so low that control of the degree of cross-linking becomes difficult.
Furthermore, fluids with viscosities sbove 30,000 cp are difficult to handle. They are too viscous to~pour easily and too fluid to be handled like a silicona gum, which generally has a viscosity of around 30,000,000 cp .
The preferred range is thus between a viscosity of from 5000 cp, with 0.57 ~ole percent "Vinyl", to 30,000 cp, with 0.34 mole percent ~VIDY1~'. ~De optl=um raDge lc fror. 10,000 t~ 15,000 cp with 0.45 mDle ~ .

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percent of methylvinylsiloxane and a molecular welght of 60,000.
The fluids used in the preparation of the hydraulic elastomers of an aspect of this invention as now provided by the present divisional application may be prepared by any conventional process kno~m in the art : for preparing silicon polymers, e.g., condensation of short-chain hydroxy-terminal polymers, acid-catalyzed equilibration and base-catalyzed equili-bration. In a base-catalyzed equilibration a mixture of cyclic oligomers of dimethylsiloxane, cyclic oligomers containing methylvinylsiloxane, alone or in combination with dimethylsiloxane, and a short-chain siloxane con-taining trimethylsiloxy end group~ is heated to a temperature of from 80 to 90C. with a fugitive catalyst e.g., tetramethylammonium hydroxide.
After 1 to 2 hours the temperature is increased to 130 to 150C. to destroy the catalyst. If desired, volatile by-products can then be removed by fur-ther heating under vacuum. The tetramethylammonium siloxanolate described in United States Patent No. 3,433,765 issued March 18, 1969 to L.E. Geipel is an excellent fugitive catalyst for the equilibration.
The choice of peroxide used for curing is important. Peroxides that generate acyloxy radicals, especially diacyl peroxides, e.g., benzoyl -~ peroxide, are relatively undesirable because they are strong hydrogen ab-stractors. The degree of cross-linking is determined largely by the amount of peroxide and the temperature employed in the vulcanization.
Vinyl-specific peroxides, on the other hand, generate cross-llnks through the vinyl groups, and the degree of cross-linking depends primarily on the number of vinyl groups. Vinyl-specific peroxides are characterized by the fact that their initial decomposition products are principally tertiary alkoxy radicals.
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One class of vlnyl-specific peroxides consista of tertiary alkyl peroxides. The simplest members of this elass, e.g., tertiary butyl perox-ide and tertiary amyl peroxide, are chemically satisfactory but too vola-tile for long-term storage. Thus i~ is desirable to use peroxides having very low volatility at room temperature. For this reason it is preferred that the molecules have at least 14 carbon atoms, for example, as in dicumyl peroxide.
Peroxides with two or more peroxy groups are often preferred.
These include peroxides e.g., bis-(t-butylperoxyisopropyl)-benzene, bis-(t-butylperoxisopropyl)-ethane, and bis-(t-butylperoxyisopropyl)-acetylene.
These are tertiary alkyl peroxides in the sen6e thatevery peroxidic oxygen atom is attached to a tertiary carbon atom.
Another class of suitable vinyl-specific peroxides may be des- -cribed as diperoxy ~etals. Suitable examples of these include l,l-di-t-butylperoxy-3,3,5-trimethylcyclohexane and n-butyl 4,4-di-t-butylperoxy-valerate.
All the specific peroxides enumerated above, as well as other tertiary alkyl peroxides and disperoxy ketals, may be used in the process for preparing hydraulic elastomers of aspects of this invention as now pro-vided by the present divisional application. Bis-(t-butylperoxyisopropyl)-benzene (mixed metal and para isomers) has a particular advantage in that its activity is easily monitored by ultraviolet absorption. It has a characteristic absorption peak at 260 nm, (nanometers) with a minimum 240 nm. Its decomposition products have an intense absorption at 240 nm, so that it is possible to detect a very small amount of decomposition.
Some commercial materials that are not visibly decomposed contain enough of these products to obscure completely all peaks in the range of 230 to 260 nm.

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'- i,O~ZZ7~ , Impure samples o thig peroxide are eagily purified by recry~tal-lization. Any of several solvents may be used, particularly short-chain alcohols. Methanol i8 preferred as it permits recrystallization at tem-peratures at or above 0C. Still higher temperatureY may be used if up to 10 percent of water is pregent. Generally one recrystallization is sufficient. The product is satisfactorily pure if the absorbance at 240 nm is no stronger than the absorbance at 260 nm.
As indicated above the amount of peroxide used is not critical.
Depending somewhat on the equivalent weight of the peraxide, as little as 0.1 percent or as much as 1.5 percent may be used. The preferred range is from 0.3 to 0.8 percent and more preferably 0.5 percent.
These peroxides are all readily soluble in vinyl-containing silicone fluids and are stable enough that the solutions may be stored for many months at room temperature without decomposition. In the case of nor-mally solid peroxides it is sometimes desirable to preheat the silicone fluid to 50C. to facilitate dissolution.
In order to cure the silicone fluid in the process of preparing the hydraulic elastomer of aspects of this invention as now provided by the present divisional application, lt is heated with the peroxide for a length of time and at a temperature appropriate to the peroxide. Minimum cures require a time equal to at least one half-life of the peroxide. Better results are obtained after 2 or 3 half-lives, and full cures require at least 5 to 10 half-lives. Longer heating will not cause any bad effects because the silicones are stable at temperatures above 200C., and the preferred peroxides do not generage acidic by-products. To avoid excessive-ly long cure times, however, it is desirable to choose a temperature such that the half-life is of the order of one to ten minutes. For example, temperature~ of 150 to 180C. for , . - : . .

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- lO~ZZ78 the tertiary alkyl peroxideg and 130 to 150C. for the diperoxy ketals i~
satisfactory The table below illustrateY suitable cure times.

Approx.
Peroxide Temp. Half-life, Time, C. Minutes Minutes l,l,-Di-t-butylperoxy-; 3,3,5-trimethylcyclohexane 140 1 30 Dicumyl peroxide 170 1.5 15 Bis-(t-butylperoxyisopropyl)-benzene 170 3 30 Bis-(t-butylperoxyisopropyl)-benzene 180 1 10 Bis-(t-butylperoxyisopropyl)-ethane 175 1 15 It should be noted that the terms "preferred range" and "optimum range", in reference to the viscosity and vinyl content of the fluid, are used with a particular application in mind, i.e. an impact absorber simi-lar to those described in Unlted States Patents 3,053,526 issued September 11, 1962 and 3,178,037 issued April 13, 1965 to Kendall. It should be pointed out, however, that the hydraullc elastomers of various aspects of this invention as now provided by the present divisionsl application are useful in a great variety of hydraulic devices. Examples include shock absorbers, fluid couplings, braking systems, vibration dampers, rate-con-trol devices and many others. The same elastomers work well in each of these. In some applications the optimum range of usable fluids may be slightly different from those described above. In almost all cases, how-ever, it will be found that the optimum range will lie within the boundar-ies of the preferred range given above. That is, the viscosity of the fluid will lie between 5000 and 30,000 cp and the value of C in equation (2) will lie between 0.53 and 0.83.

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10~;~Z78 Ideally each elastomer whould be tested in the device for which it is designed. In cases where this ls impractical the following labora-tory test was devised, based on a Brabender (PLAgTI-CORDER the Trade Mark of an instrument of C.S. Prabender Instruments, Inc., 50 East Uesley Street, South Hackensack, New Jersey). The measuring head used is Type 6/115 volts/
114 amp., No. 105, with roller blades. It~is preheated to the curing tem-perature, 350F., with the follers turning at 50 rpm, and the uncured fluid-containing peroxide is introduced by means of a hand extruder known by the Trade Mark of SEMCO. A small excess is used to make sure that the cavity is full. The cure is followed by means of a torquemeter. The torque starts to increase noticeable after 3 minutes. Very shortly thereafter a gel point is reached and the material turns into a fine powder. The torque continues to rise, however, and finally levels off after 10 to 20 minutes.
During this time the powder flows around the blades like an extremely viscous liquid. The shear stress is believed to be very similar to that in the actual hydraulic impact absorber. At any rate an elastomer with the rlght hardness that shows a torque of 1000 to 1550 meter-grams will perform well in an impact absorber.
Regardless of the nature of the shearing mechanism the fluid-like properties develop as the hydraulic elastomer of aspects of this in-vention a6 now provided by the present divisional application is broken dcwn into small particles. The size of the particles is not critical, but for optimum reproducibility they should be less than one millimeter in dia-meter.
Hardness is determined with a Shore A durometer on 1/4 inch but-tons cured, typically, for 25 minutes at 350~. (AS~-395 Method B).
Optimum results are obtained if the Shore A hardness is between 11 and 14.
Under certain conditions fairly good results may be obtained with elasto-mers that have a Shore A hardness as low as 9 or as high as 16.
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-- 10~2Z78 In additlon to the physical characteristlcs of hardness and flow behaviour, the cured hydraulic elastomer of aspects of this inventlon as now provided by the present div$sional application may be characterized chemically. It contains an unusually large number of free ends, i.e., terminal segments that are not involved in the cross-linking process. Given a fluid in the molecular weight range of 20,000 to 200,000 and given that there are two end groups per molecule, the number of end groups can be calculated as lying between o.n74 and 0.74 per 100 silicon atoms. In the preferred range the number of free end lies between 0.18 and 0.34 per lO0 silicon at~ms. The optimum fluid molecule, with a molecular weight of 60,000 has 0.25 end groups per 100 silicon atoms; this figure remains essentially unchanged on curing. In the cured elastomer the average length of the free ends os between 50 and 100 silicon atoms.
By contrast, a typical silicone gum molecule, with a molecular weight of around 500,000 has only 0.03 end groups per 100 silicon atoms.
The free end groups in the hydraulic elastomers of aspects of this inven-tion as now provided by the present divisional application are believed to have a plasticizing effect that is important in detérmining the physical properties of the elastomer.
The cured hydraulic elastomer, of aspects of this invention as now provided ~y the present divisional application, particularly one made ` from the preferred range of fluids, is further characterized by a relative-ly high cross-link density. There is normally at least one three-carbon cross-link per original vinyl group. This is true whether curing takes : place by repeated free-radical-initiated vinyl addition reactions or by alternating chain-transfer-to-methyl and vinyl addition. More cross-links form, especially in fluids of low vinyl content, by chain-terminating coupling .

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reactions. Neglecting those formed by coupling, the effectlve cross-llnk density ls given by the expresslon C.C. ~ "Vlnyl" - ~ G./2 (3) where C.D. i8 the number of cross-links per 100 silicon atoms, "Vinyl" is the original number of vlnyl groups per 100 sllicon atoms, and ~.G. is the number of end groups per 100 silicon atoms.~ The reason for the last term is that two end groups in effect counteract one cross-link.
The hydraulic elastomer of aspects of this lnventlon as provlded by the present divisional application made from the optimum fluid is thus calculated to have a minimum cross-link denslty of 0.45 - 0.25/2 or 0.325 cross-llnks per 100 silicon atoms. The preferred range of fluids gives minimum cross-link densities of from 0.25 to 0.40 cross-llnks per 100 silicon atoms. Allowing for some cross-links due to coupling, the preferred range of cross-link density is from 0.25 to 0.50 cross-links per 100 silicon atoms.
A typical "prior art" sllicone elastomer made from~ a gum having 0.1 to 0.2 mole percent vinyl has a calculated minimum cross-link density of between 0.098 and 0.198. The novelty of the hydraulic elastomers of aspects of this invention as now provided by the present divisional appli-cation does not reside solely in their high cross-link density, however, but in the combination of hi8h cross-link density and high free end density.
Various embodiments of this invention are further illustrated in the following examples, in which all parts are by weight unless otherwise specifled.

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A mixture containing 100 parts of octamethylcyclotetrasiloxane, 0.52 part of mixed cyclic methylvinylsiloxanes obtained from the hydrolysis product of methylvinyldichlorosilane, and 1.46 parts of a short-chain tri-methylsilyl-endblocked polydimethylsiloxane having an average of 8.15 sili-con atoms per molecule (endblocked), is heated to 85C. To this is added 0.21 part of a tetramethylammonium siloxanolate solution containing the equivalent of 6.2 percent of tetramethylammonium hydroxide. The viscosity of the mixture begins to increase in a few minutes and equilibration is complete in one hour at 85C. The temperature is then raised to 140C. to destroy the catalyst. The product is then stripped for an hour under vacuum to remove a small amount of volatile matter, consisting primarily of an equilibrium ~uantity of cyclic siloxanes. The product contains 0.45 mole percent of methylvinylsiloxane units, which is essentially the same as in the initial mixture. It is a clear fluid with a viscosity of 12,000 cp at 25C.

An impure lot of bis-(t-butylperoxyisopropyl)-benzene, mixed meta and para isomers, having almost complete ultraviolet absorption in the range of 230 to 260 nm at a concentration of 0.08 percent in heptane, is purified in the following manner. It is dissolved in warm methanol and -water to give a solution containing 15 parts of the peroxide, 80 parts of methanol and 5 parts of water. The solution is cooled slowly to room tem-perature and then to 0C., and held at 0C. to complete the recrystalliza-tion. It is then filtered through a Buchner funnel. The filter cake thus obtained i8 substantially dry, containing no more than 10 percent of water and methanol. It is crushed and finally dried in a stream of nitrogen, giving a ylelt of recovered peroxide of 75 percent. The recrystallized material, when diluted to 0.08 percent in heptane, shows an 80 percent .

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absorbance at 260 nm and 65 percent abgorbance at 240 nm. Although the per-oxide is of a pale buff color, the U.V. sbsorption i8 not significantly different from that of a pure white material obtained by - 11 b 2 -,: , : ' ', ~ ' ' . ~ ', ' ' ' lO9;~Z7~

repeated crystallization. This once-recrystalllzed material will be re-ferred to as "the peroxide of Example 2".
EXAMPL~ 3 One hundred parts of the fluid of Example 1 ls heated to 55C.
.To this is added one half part of the peroxide of Example 2. It dissolves quickly and does not recrystallize on cooling. U.V. absorption shows no decomposition of the peroxide. A portion of the mixture is poured into the cavity of an impact absorber. The assembly is then heated for 30 minuteR in an oven at 450F., the internal temperature reaching a maximum of 410F. This is sufficient for complete cure of the fluid to a soft elastomer. The assembly is then placed in a dynamometer and subjected to repeated compression-retraction cycles, in which the test sample is allowed to cool to room temperature between cycles. The amplitude is such that virtually all of the elastomer is forced through the orifice in the first stroke, thereby being broken into fine particles. These particles will flow back during the retraction phase. The stress-strain curve is virtually the same for all compression-retraction cycles except the first, indicating that there is no further chemical or mechanical breakdown of the elastomer.
In each cycle, the energy absorption is sufficient to absorb an impact of 2000 ~oules without "buttoming out" or transmitting an excessive force at any time during the cycle.

Another portion of the mixture is placed in the mixing head of the Brabender PLASTI-CORDER described above, which has been preheated to 350F. A sharp rise in torque occurs after 3 minutes, reaching 1200 meter-grams in 7 minutes, and finally levelling off at 1350 meter-grams in less than 20 minutes. ~hen cool, the elastomer is found to be in the form of a soft white powder. This can be compressed by hand into a transparent mass with the appearance of a liquid, but on release of pressure it reverts to the powder, thus proving it to be fully cured.

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A third portion i8 cured for 25 minute~ at 350~. in the form of a l-inch by l/4-inch button and te~ted with the Shore A durometer. It .

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---" 10~J~278 is found to have a Shore A hardness of 12.
EXAMPLES 4 to 15 Fluids are prepared in accordance with the procedure of Example 1, except that the amounts of cyclic methylvinylsiloxane and endblocker are ;varied to give different viscosities and vinyl contents. After stripping samples are heated in a vacuum oven and found, by weight 1088, to contain between 2.3 and 4.2 percent of residual volatile matter, which is an acceptable range. The fluid of Example 15 is the most difficult to strip, because of its relatively high viscosity, and has the highest residual volatiles. Each fluid is mixed with 0.5 percent of the peroxide of Example 2 and cured at 350F. in accordance with Example 3. The results of the tests are illustrated in the following table.

Mole Brabender ExampleViscosity, Percent Test, Shore A
No. cp "Vinyl" Meter-gramsHardness 4 1,100 0.75* 900 10 5,400 0.50 1250 11 6 5,230 0.55 1275 12 7 5,100 0.65 1250 14 8 8,800 0.35* 1175 8 9 10,500 0.40 1300 11

3 12,000 0.45 1350 12 12,400 0.55 1350 15 11 10,700 0.65* 1400 19 12 11,000 0.80* 1400 22 13 13,600 l.00* 1300 26 lb l9,000 0.45 1450 13 25,700 0.45 1300 13 .

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~0~7fl It can be seen that all flulds that fall within the optimum range of vinyl content as defined above, glve satisfactory results, i,e.
Brabender values between 1000 and 1550, and Shore A hardness between 9-and 15; all but one, NO. 10, fall in the optimum range of hardness, 11 to 14.
All those that are outside the optimum range of vinyl content, . - 12 a -- . : . ,. ... , . ,-. - : .
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indicated by asteriskY (*), are generally outside the acceptable range of hardness even though some are satisfactory in the Brabender tests.

A fluid having a viscosity of 87,000 cp and 0.20 le percent "Vinyl" is cured in the Brabender Plasti-corder at 350F. in accordance with Example 3 and gives a final torque of~l200 meter-grams which is acceptable.
EXAMPLES 17 to 22 A fluid having a viscosity of 140,000 cp and 0.14 le percent "Vinyl" is cured in the Brabender Plasti-Corder with various amounts of peroxide and at various temperatures as shown in the following table.

Example Peroxide, Cure Temperature Torque, No. Percent F. Meter-grams 17 0.5 350 1300 18 0.75 350 1350 19 0.95 350 1300 -`
1.15 350 1300 21 0.5 400 1425 ; 22 0.5 450 1600 It ig apparent that increasing the peroxide level has little effect on the final torque. On the other hand very high curing tempera-tures are undesirable, as indicated by the high torque value in Example 22.

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f~ 78 EXAMPLES 23 to 26 Example 3 is repeated except that 0.5 percent of the following catalysts are used:
Impure bis-(t-butylperoxyisopropyl)-benzene (20 miinutes at 350F) 1,1-Di-t-butylperoxy-3,3~5-trimethylcyclohexane (30 minutes at 285F.) Dicumyl peroxide (15 minutes at 340F.) Bis-(t-butylperoxyisopropyl)-ethane (20 minutes at 350F.) In all cases the Brabender torque is between 1300-1400 meter-grams, and the cured elastomers are indistinguishable in appearance and mechanical properties from that obtained in Example 3 - 13 a -., ' ' , . : : '-.
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Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A hydraulic elastomer having a viscosity of 1,000 to 1,000,000 centipoises at 25°C and composed of a polydimethylsiloxane cross-linked by three-carbon bridges; having a relatively high cross-link density of from 0.25 to 0.50 cross-links per 100 silicon atoms, and in which there are from 0.074 to 0.74 free chain ends per 100 silicon atoms, said free ends having an average of from 50 to 100 silicon atoms each, said elastomer being characterized by a Shore A hardness of from 9 to 15, and being further characterized by its ability to absorb energy by flowing under pressure like a fluid.

2. The hydraulic elastomer of claim 1 in which there are from 0.18 to 0.34 free ends per 100 silicon atoms, and the Shore A hardness is from 11 to 14.

3. The hydraulic elastomer of claim 1 in the form of fine particles which are predominantly less than 1 millimeter in diameter.

4. A process for preparing a hydraulic elastomer as claimed in claim 1 which comprises: heating a silicone fluid comprising a copolymer of dimethylsiloxane units with 0.1 to 0.9 mole percent of methylvinylsiloxane units, the mole percent of methylvinylsiloxane units having a value of (4.8/log V) - C, where V is the viscosity in centipoises and C is a number hav-ing a value of from 0.53 to 0.83, said copolymer having end groups selected from the class consisting of trimethylsiloxy, hydroxy and alkoxy with 0.1 to 1.5 percent of a curing agent selected from the class consisting of tertiary alkyl peroxides and diperoxy ketals at a temperature from 130°C. to 210°C. for a time equal to at least one half-life of the curing agent.

5. The process of claim 4 wherein the curing time is equal to at least 5 half-lives of the curing agent.

6. The process of claim 4 wherein the curing agent is a tertiary alkyl peroxide, wherein the curing temperature is at least 150°C., and wherein the curing time is at least one minute.

7. The process of claim 4 wherein the curing agent is a diperoxy ketal, wherein the curing temperature is at least 130°C. and wherein the curing time is at least one minute.

8. The process of claim 4 wherein the amount of curing agent is from 0.3 to 0.8 percent.

9. The process of claim 4 wherein the curing agent is a tertiary alkyl peroxide.

10. The process of claim 4 wherein the curing agent is dicumyl peroxide.

11. The process of claim 4 wherein the curing agent is bis-(t-butylperoxyisopropyl)-ethane.

12. The process of claim 4 wherein the curing agent is bis-(t-butylperoxyisopropyl)-benzene.

13. The process of claim 4 wherein the curing agent has an ultra-violet absorbance at 240 nanometers that is no greater than 260 nanometers.

14. The process of claim 4 wherein the curing agent is 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane.

15. The process of claim 4 wherein the copolymer is represented by the formula RO[CH3)2SiO]x[CH3(C2H3)SiO]yR
wherein R is selected from the class consisting of alkyl radicals of from 1 to 4 carbon atoms, hydrogen and trimethylsilyl radicals, x is a number of from 270 to 2,700 and y is a number of from 0.001x to 0.009.

16. The process of claim 4, wherein the copolymer contains trimethyl-siloxy end groups and hag a viscosity of from 5000 to 30,000 centipoises.

17. The process of claim 4 wherein the value of C is from 0,63 to 0.78.

18. The process of claim 4 wherein the mole percent of methylvinyl-siloxane units is approximately 0.45.