GB1577894A

GB1577894A - Method of curing thick section elastomers

Info

Publication number: GB1577894A
Application number: GB7803/77A
Authority: GB
Original assignee: Dow Corning Corp
Current assignee: Dow Silicones Corp
Priority date: 1976-03-03
Filing date: 1977-02-24
Publication date: 1980-10-29
Also published as: FR2343028B1; DE2709372A1; CA1078325A; DE2709372B2; FR2343028A1; JPS52105998A; JPS5310103B2; DE2709372C3

Description

(54) METHOD OF CURING THICK SECTION ELASTOMERS (71) We, DOW CORNING CORPORATION, of Midland, Michigan, United States of America, a corporation organised under the laws of the state of Michigan, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of curing silicone elastomers with ultraviolet light and electron beam and silicone elastomers so formed.

Compositions containing mercaptoorgano functional siloxanes and aliphatically unsaturated organosiloxanes have been suggested in the prior art. For example, U.S. Patent No. 3,816,282 discloses silicone rubber compositions which cure at room temperature in the presence of electromagnetic and particulate radiation.The compositions exposed to the radiation for curing comprise an organopolysiloxane having a viscosity of from 100 to 100,000 centipoise at 250C from 0.1 to 2.0 mole percent of the organic groups as silicon-bonded vinyl radicals and from 1.98 to 2.05 organic groups per silicon atom, an organopolysiloxane fluid having the general unit formula R38[HS(CH2)c]bSiO4-a-b where R3 is an alkyl or aryl radical containing up to 18 carbon atoms, c is from 1 to 25, a is from 0.088 to 2.08 b is from 0.009 to 0.97 and the sum of a+b is from 2.019 to 2.16 and the number of mercapto siloxane units equal or exceed the number of non-mercapto siloxane units, and from 0.025van to 1 /" by weight of the composition of a free radical source which includes certain organic peroxides.

German Offenlegungsschrift No. 2,008,426 suggests five different possibilities of making three dimensionally crosslinked silicone polymers. These reactions were found to proceed in the presence of radical reaction initiators at a temperature of from 30C to 110 C or by UV-light initiation.The five possibilities are defined as follows: One possibility is a reaction between an alkenylsiloxane of the general formula

where the sum of m+k is greater than 30, the ratio of k:m is from 1:10 to 1:60, x is from 0 to 4 and R4 is an alkyl group containing from 1 to 6 carbon atoms, a cycloalkyl group containing from 5 to 8 carbon atoms, an aryl group or a siloxy group, and a dithiol of the general formula

where p is from 0 to 30, x is from 1 to 4 and R5 is an alkyl group containing from 1 to 6 carbon atoms, a cycloalkyl group containing from 5 to 8 carbon atoms, or an aryl group.

The second possibility is a reaction between an alkenylsiloxane of the general formula I and a high molecular weight thioalkylpolysiloxane of the general formula

where the sum of p and q is greater than 30, the ratio of q:p is from 1:10 to 1:60, R5 is as defined above and R6 is an alkylene group containing from 1 to 6 carbon atoms, a cycloalkylene group containing from 5 to 8 carbon atoms or an arylene group.

The third possibility is a reaction between a thioalkylpolysiloxane of the general formula III and a low molecular weight alkenylsiloxane of the general formula

where r is from 0 to 30, x is from 0 to 4 and R5 is as defined above.

The fourth possibility is a reaction between a high molecular weight thioalkyl siloxane and an alkenyl substituted siloxane of the general formula

where the sum of m+k+p is greater than 30, the ratio of m:p is from I to 1:60, and R5 and R6 are as defined above.

The fifth possibility is a reaction between a thioalkylpolysiloxane and an organic diisocyanate.

British Patents Specification No. 1,409,223 discloses and claims a composition curable upon exposure to ultraviolet light, which comprises (A) a siloxane consisting essentially of from 0.5 to 100 mole percent of vinyl containing siloxane units of the general formula (CH2=CH)RnSiO3-n 2 wherein R is a monovalent hydrocarbon or monovalent halogenated hydrocarbon radical containing 1--30 carbon atoms and n has a value from 0 to 2, any other siloxane units present being any non-vinyl containing siloxane units having the general formula RtmSiO4~m T wherein R' is a monovalent hydrocarbon other than CH2=CH- or a non vinyl containing monovalent halogenated hydrocarbon radical containing 1--30 carbon atoms and m has a value from 0 to 3, (B) a siloxane containing at least one silicon bonded hydrogen atom, the ratio of silicon bonded vinyl groups in (A) to the silicon bonded hydr gen atoms in (B) being in the range of 1:100 to 100:1, (C) a photosensitizing amount of photosensitizer, and (D) a mercaptofunctional siloxane or silane, as herein defined, in an amount sufficient to accelerate the cure of the composition.

U.S Patent No. 3,873,499 suggests that resin compositions containing a copolymer of from 10 to 30 mol percent HSR'SiO312 from 10 to 75 mol percent monopropylsiloxane, from 5 to 35 mol percent dimethylsiloxane and from 5 to 43 mol percent monomethylsiloxane where R' is ethylene or propylene, a methylvinylpolysiloxane and a gelation inhibitor can be cured to resins by exposure to ultraviolet light or electron radiation. The ratio of vinyl per SH is from 0.8:1 to 1.2:1.

Although the prior art suggests that compositions containing mercapto functional siloxanes and vinyl containing siloxanes can be cured and that some compositions cure to elastomeric products, it is not suggested or obvious that certain compositions containing fillers can be cured in thick section by ultraviolet light or electron beam radiation. For example, U.S.Specification No. 3,816,282 requires specific mercapto-containing siloxanes, a free radical source and radiation for curing the composition; German Offenlegungsschrift No. 2,008,426 place strict limitations on the type of alkenyl siloxanes and mercapto siloxanes which can be combined to provide a three dimensional cross-linked siloxane and describes no fillers; British Patent Specification No. 1,409,223 requires the presence of a photosensitizer and ultraviolet light to cure the compositions and finally US Specification No. 3,873,499 suggests only resins with high mercapto content which require a gelation inhibitor to get useful products. Thus, it was unexpected that polydiorganosiloxanes having aliphatically unsaturated radicals, mercaptoorganopolysiloxanes and a filler could be cured to an elastomer with ultraviolet light or electron beam radiation.

Accordingly the present invention provides a method of forming a thick section elastomer which comprises (I) mixing to form a composition (A) an aliphatically unsaturated polydiorganosiloxane having at least three diorganosiloxane units per molecule wherein each unit of a combination forming the molecule is selected from dimethylsiloxane units, methylvinylsiloxane units and units of the formula

and terminal siloxane units present being selected from trimethylsiloxane units, methylphenylvinylsiloxane units, dimethylvinylsiloxane units and units of the formula

wherein R is an alkyl radical containing from 1 to 3 carbon atoms or a phenyl radical, there being at least two aliphatically unsaturated siloxane units per molecule, (B) a mercaptoorganopolysiloxane comprising a combination of units selected from dimethylsiloxaile units, trimethylsiloxane units, units of the formula

units of the formula

and units of the formula

wherein R is an alkyl radical containing from 1 to 3 carbon atoms inclusive or a phenyl radical, R' is a methyl or ethyl radical and n has a value of from 1 to 4 inclusive, there being in the mercaptoorganopolysiloxane an average of at least two mercapto-containing siloxane units per molecule and no more than 10 mol percent -SH containing siloxane units based on the total number of siloxane units in the mercaptoorganopolysiloxane, (A) and (B) being combined in a weight ratio sufficient to provide a molar ratio of moles of -SH group in (B) per mole of vinyl radical in (A) of from 0.2:1 to 10::1, at least one of (A) and (B) having at least 100 siloxane units per molecule and (C) a filler in an amount sufficient to alter the physical characteristics of the final cured elastomer compared with a cured elastomer without filler (II) forming the composition from (I) into a shape having a thickness of at least 0.25 mm, and thereafter (III) exposing the shaped composition (II) to ultraviolet radiation or electron beam until the composition is cured into an elastomer.

This method results in elastomeric articles which are cured rapidly and without the need for other ingredients to activate the composition even though a filler is present.

Before the present method was discovered films or webs or sheets which were pigmented or contained filler, could only be cured practically in thicknesses up to about 0.25 mm with electron beam radiation and in thicknesses much less than 0.25 mm with ultraviolet radiation. The present method which involves using a filler containing or pigmented composition can cure compositions in thicknesses of at least 0.25 mm up to 5 mm or higher. Even high radiation absorbing fillers, such as carbon black, can be used and cured, by the present method, to elastomers.

The composition to be cured can be prepared by mixing (A), (B) and (C) in any order. Preferably, the ingredients are not heated to achieve mixing since undesirable reaction may occur prematurely and the working time may be reduced.

Although (A), (B) and (C) can be mixed and stored for sufficiently long times, it is preferred to mix the components within a period of less than six months before use and preferably within a month of the time of the curing operation. Conventionally available mixing equipment can be used to prepare the composition, such as blade mixers, 2 or 3 roll rubber mills, static mixers or extruder mixers. The composition can be mixed immediately before the forming and exposure steps or it can be mixed in advance of these steps.

After the composition is formulated, it is then formed into a shape which has a thickness of at least 0.25 mm. As used herein the term "thick section" means a section having a thickness of 0.25 mm or greater. Thick section is also to be understood as the thickness of the composition or cured elastomer in the direction which is parallel to the direction of the radiation directed at the composition. The shape of the composition can be a film, sheet, web, coating, or any other form which has a thickness greater than 0.25 mm. It is not necessary that the shape be flat or plate-like. It can be curved or irregular. Also it is not necessary for the composition to cure all the way through if only a thick cured surface is desired.

After the shape has been formed, the composition is exposed to ultraviolet radiation or electron beam radiation. Sources for these radiations are known. The distance from the source can be varied according to the users preference or equipment. The duration of exposure will depend upon the specific composition and the energy of the radiation. However, for most commercial radiation sources the time can be short such as a fraction of a second to a few seconds.

The polydiorganosiloxanes of (A) are aliphatically unsaturated siloxane polymers which have at least three diorganosiloxane units per molecule and an average of at least two aliphatically unsaturated diorganosiloxane units per molecule. The polydiorganosiloxanes can be cyclic, linear, low viscosity fluids, gums and mixtures of these polymeric species. The diorganosiloxane units can be dimethylsiloxane units, methylvinylsiloxane units or units of the formula

These diorganosiloxane units are combined to provide at least two aliphatically unsaturated diorganosiloxane units per molecule, forming copolymers or they can be combined to provide polydiorganosiloxanes having all units with aliphatic unsaturation, such as polymethylvinylcyclosiloxanes.The linear polydiorganosiloxanes are terminated with siloxane units selected from trimethylsiloxane, methylphenylvinylsiloxane, dimethylvinylsiloxane and units of the formula

wherein R is a methyl, ethyl, propyl or phenyl radical. These polydiorganosiloxanes are known. The silacyclopentenylsiloxy units defined by the above formula includes two isomers as illustrated below, namely

The preparation of the silacyclopentenyl usually results in a mixture of isomers. Inasmuch as there is no problem in using the mixture, separation is not attempted. The silacyclopentenyl containing copolymers are disclosed in U.S.

Patent No. 3,509,191.

The mercaptoorganopolysiloxanes of (B) contain combinations of two or more of the following units: dimethylsiloxane units, trimethylsiloxane units, and units of the formulae

where R is an alkyl radical containing from 1 to 3 carbon atoms, including a methyl, ethyl and propyl radical, or a phenyl radical, R' is a methyl or ethyl radical and n is from 1 to 4 inclusive, preferably n is 3 and R and R' are both methyl radicals. The mercaptoorganopolysiloxanes for use in the present invention are those which have at least two mercapto groups per molecule and no more than 10 mol percentSH containing siloxane units based on the total number of siloxane units of the mercaptoorganopolysiloxane.Preferably, the mercaptoorganopolysiloxanes have a molecular weight greater than 5000 and no more than 3.5 percent by v eight --SH groups and the most preferred are those with no more than 2.2 percer - by weight -SH groups. The mercaptoorganopolysiloxanes are known in the art as evidenced by the prior art cited herein. The silacyclopentane mercatpo siloxanes can be prepared by the method defined in U.S. Patent No. 3,655,713. The mercaptoorganosiloxanes which contain terminal units of the formula

can be prepared by reacting a hydroxyl terminated polydimethylsiloxane and a mercaptoalkyl trialkoxysilane of the formula HSCnH2nSi(ORt)3 in the presence of solid potassium hydroxide or potassium silanolate catalysts.The potassium silanolate catalyst is preferred for the higher viscosity polydimethylsiloxanes. The mercaptoalkyltrialkoxysilane is preferably used in excess of 10 percent by weight over stoichiometric amounts. The resulting product is essentially a polydimethylsiloxane terminated with units of the formula

but there may be some small amounts of units wherein two SiOH groups have reacted with one mercaptoalkyltrialkoxysilane molecule, but these amounts are sufficiently small for the character of the terminated polydimethylsiloxane not to be noticeably altered.

Fillers are used in the compositions of this invention. The fillers can be both treated and untreated reinforcing fillers, such as fume silica and fume silica having triorganosiloxy groups, such as trimethylsiloxy groups on the surface, carbon black or precipitated silica, and extending fillers such as crushed or ground quartz, diatomaceous earth, and calcium carbonate. The amount of filler is enough to provide a change in the physical characteristics of the cured elastomer compared with a cured elastomer without a filler. Preferably, the amount of filler is at least 5 parts per 100 parts by weight of the combined weight of (A) and (B). Certain fillers, such as carbon black, can be present in smaller amounts. Carbon black is of particular interest, because it is a strong absorber of ultraviolet radiation, but it still will cure to an elastomer in accordance with this invention, although the thicknesses curable are not as great with carbon black as with other fillers, thicknesses such as 0.30 to 0.40 mm are considered thick sections.

The compositions used in this method are made by mixing (A) and (B) in a weight ratio range sufficient to provide a molar ratio of moles of-SH groups in (B) per mole of C=C bond in (A) of from 0.2:1 to 10:1, preferably from 1.5:1 to 5:1.

Also, the composition must contain at least one of either (A) or (B) which has at least 100 siloxane units per molecule, preferably at least 200 siloxane units per molecule.

Although not required, the compositions can contain other additives such as benzophenone which can make the curing step even faster although it is very fast already, gelation inhibitors can be used such as p-methoxyphenol, although most compositions are sufficiently stable without such stabilizers and the gelation inhibitors can lengthen the cure time. Certain free radical sources can be added, but are not really needed and can significantly hasten gelation at room temperature and should be avoided.

The method of this invention is useful for making thick section elastomers such as for films, webs, sheets and coating. The cure is fast and because the compositions contain fillers, they have improved strength and other useful properties such as durometer, modulus and tear strength.

The following examples illustrate the present invention.

Example 1 A. A composition was prepared by mixing 60.9 g. of a methylphenylvinylsiloxy terminated polydimethylsiloxane having a viscosity at 250 C. between 25 and 35 pascal-seconds (Pa.s) (hereinafter identified as Polymer D), 6.0 g. of a trimethylsiloxy terminated polydiorganosiloxane having 5 mol percent methyl(gamma-mercaptopropyl)siloxane units and 95 mol percent dimethylsiloxane units and having a viscosity at 25"C. of 0.0015 metre2/second (m2/s), (hereinafter identified as Polymer A), 22.3 g of a fume silica filler having the surface modified with trimethylsiloxy units and 1.0 g. of benzophenone.The composition was formed into a film 0.99 mm thick and then exposed to UV radiation, using an Ashdee continuous belt two lamp curing module having Hanovia (Registered Trade Mark) medium pressure lamps rated at 7874 watts per linear metre, for 13 seconds under an air atmosphere. The composition cured to an elastomer having a tensile strength of 1379 kilopascals (kPa) at break and an elongation at break of 480 percent and a Die "B" tear strength of 5954 newtons per metre (N/m). The tensile strength and elongation were determined by ASTM-D412 and the tear strength was determined by ASTM-D-624.

B. Another composition was prepared as described in A. above except that the benzophenone was left out. This composition was formed into a film 2.4 mm thick and then exposed to an electron beam from a 2 megavolt electron beam unit with a focused beam and a 0.3048 m sweep. The film was exposed under a nitrogen atmosphere to provide 5000 joules per kilogram (J/kg). The composition cured to an elastomer having a tensile strength at break of 3861 kPa, an elongation at break of 1200 percent and a Die "B" tear strength of 18,213 N/m.

Example 2 Compositions were prepared by mixing the ingredients as defined below, formed into films and exposed to UV radiation as defined in Example 1. The film thickness and exposure times were as shown in Table I. The physical properties were determined by the procedures described in Example 1 and were as shown in Table I.

A. 106.4 g. of a methylphenylvinylsiloxy terminated polydimethylsiloxane having a viscosity at 250C between 8 and 12 Pa.s (hereinafter identified as Polymer B), 15.0 g. of Polymer A, 39.4 g. of the fume silica filler as defined in Example 1, 1.8 g. benzophenone.

B. 71.2 g. of a methylphenylvinylsiloxy terminated polydimethylsiloxane having a viscosity at 250C between 1.8 and 2.4 Pa.s (hereinafter identified as Polymer C), 50.0 g. of Polymer A, 40.0 g. of the fume silica filler as defined in Example 1, 1.07 g. of benzophenone.

C. 0.43 g of polymethylvinylcyclosiloxane mixture having 3 to 8 siloxane units per molecule, 37.6 g. of a trimethylsiloxy terminated polydiorganosiloxane having 2 mol percent methyl(gammamercaptopropyl)-siloxane units and 98 mol percent dimethylsiloxane units and having a viscosity at 25"C of 0.0015 m2/s, 11.4 g. of a fume silica filler having the surface modified with dimethylvinylsiloxy units, 0.38 g. of benzophenone.

D. 2.7 g. of polymethylvinylcyclosiloxane mixture having 3 to 8 siloxane units per molecule, 97.3 g. of Polymer A, 30.0 g. of the fume silica filler as defined in Example 1.

E. 65.9 g. of a dimethoxy(gamma-mercaptopropyl)-siloxy terminated polydimethylsiloxane having a viscosity of 0.0015 m2/s, 9.57 g. of Polymer A, 25.0 g. of fume silica filler as described in Example 1, 1.1 g. of benzophenone.

Example 3 Compositions were prepared by mixing the ingredients as defined below, formed into films and exposed to electron beam radiation as defined in Example 1.

The film thickness and the dosages were as shown in Table 2. The physical properties were determined by the procedures described in Example 1 and were as shown in Table 2.

A. 53.2 g. of Polymer B, 16.4 g. of Polymer A, 25.0 g. of the fume silica filler as defined in Example 1.

B. 100.0 g. of Polymer C, 18.5 g. of Polymer A, 41.5 g. of the fume silica filler as defined in Example 1.

C. 100.0 g. -of Polymer D, 14.0 g. of Polymer A, 30.0 g. of the fume silica filler as defined in Example 2, C.

D. 100.0 g. of Polymer B, 19.3 g. of Polymer A, 32.7 g. of the fume silica filler as defined in Example 2, C, 500.0 g parts by weight para-methoxyphenol per million parts by weight composition.

E. 107.0 g. of Polymer B, 14;2 g. of Polymer A, 34.4 g. of the fume silica filler as defined in Example 1, 1.0 g. of benzoyl peroxide, 500.0 g. parts by weight para-methoxyphenol per million parts by weight composition.

F. 107.0 g. of Polymer B, 15.2 g. of Polymer A, 34.4 g. of the fume silica filler as defined in Example 1, 1.0 g. of N,N-azobisisobutyronitrile.

G. 106.0 g. of Polymer B, 14.8 g. of Polymer A, 34.8 g. of the fume silica filler as defined in Example 1, 1.0 g. of di-tertiary-butyl peroxide.

H. 53.2 g. of a dimethylvinylsiloxy terminated polydimethylsiloxane having a viscosity at 250C between 8 and 12 Pa.s, 16.4 g. of Polymer A, 25.0 g. of the fume silica filler as defined in Example 1.

1. 59.0 g. of a methylphenylvinylsiloxy terminated polydiorganosiloxane gum having 99.7 mol percent dimethylsiloxane units and 0.3 mol percent methylvinylsiloxane units and having a Williams plasticity of about 1.5 mm, 41.0 g. of a fume silica filler having the surface treated with trimethylsiloxy units, diemthylsiloxane units and methylvinylsiloxane units, 16.5 g. of Polymer A.

J. 44.7 g. of Polymer B, 4.9 g. of a dimethylvinylsiloxy terminated polydiorganosiloxane having 78 mol percent dimethylsiloxane units and 22 mol percent methylvinylsiloxane units (hereinafter identified as Polymer E), 53.3 g. of Polymer A, 29.9 g. of fume silica filler as defined in Example 2, C, 500.0 parts by weight para-methoxyphenol per million parts by weight composition.

K. 9.57 g. of Polymer E, 65.9 g. of a dimethoxy(gamma-mercaptopropyl)-siloxy terminated polydimethylsiloxane having a viscosity at 250C of 0.0015 m2/s, 25.0 g. of fume silica filler as described in Example 1.

L. 50.0 g. of a dimethyl(gamma-mercaptopropyl)-siloxy terminated polymethylsiloxane having a viscosity at 250C of 0.00424 m2/s, 0.48 g. of a trimethylsiloxy terminated polymethylvinylsiloxane having a viscosity at 250C between 0.000025 and 0.000035 m2s, 12.5 g. of fume silica filler as described in Example 1.

Example 4 Elastomers were obtained by curing the following compositions in the film thicknesses described in Table 3 and with the dosage of electron beam radiation as shown by Table 3.

A. 106.4 g. of Polymer B, 13.0 g. of Polymer A, 59.7 g. of calcium carbonate filler.

B. 106.4 g. of Polymer B, 13.0 g. of Polymer A, 59.7 g. of crushed 5 micron quartz filler.

TABLE 3 Film Dosage, thickness, Composition J/kg mm A. 7500 5.59 B. 5000 3.35 Example 5 A composition was prepared by mixing 3 g. of a polymethylvinylcyclosiloxane mixture having 3 to 8 siloxane units per molecule, 97 g. of Polymer A and 2.5 g. of carbon black. The composition was formed into a film 0.38 mm thick and exposed to UV radiation as described in Example 1 for 2.7 seconds and an elastomer was obtained.

TABLE 1 Film Exposure Tensile Tear Strength Thickness Time, Strength, Elongation, Die"B", Composition mm sec. kPa O/n N/m A. 1.32 5.3 2551 430 16,637 B. 1.22 8 1138 515 3,853 C. 1.27 5.3 827 175 D. 1.27 1.4 1103 60 E. 1.02 8 724 220 2,277 TABLE 2 Film Tensile Tear Strength Dosage, Dosage, Thickness, Strength, Elongation Die "B" Composition J/kg mm kPa % N/m A. 5000 2.13 5240 800 27,320 B. 5000 1.52 4964 550 C. 5000 1.52 5137 820 24,518 D. 5000 1.52 6895 540 23,817 E. 5000 1.47 2586 430 12,784 F. 5000 1.57 3551 580 5,954 G. 5000 1.60 4826 685 5,254 H. 15000 2.29 1724 540 11,383 I. 5000 1.52 5240 140 8,231 J. 5000 1.52 1896 80 1,226 K. 5000 2.29 3172 295 17,338 L. 5000 1.60 3827 455 11,033 WHAT WE CLAIM IS: 1.A method of forming a thick section elastomer which comprises (I) mixing to form a composition (A) an aliphatically unsaturated polydiorganosiloxane having at east three diorganosiloxane units per molecule wherein each unit of a combination forming the molecule is selected from dimethylsiloxane units, methylvinylsiloxane units and units of the formula

any terminal siloxane units present being trimethylsiloxane units, methylphenylvinylsiloxane units, dimethylvinylsiloxane units or units of the formula

wherein R is an alkyl radical containing from 1 to 3 carbon atoms or a phenyl radical, there being at least two aliphatically unsaturated siloxane units per molecule, (B) a mercaptoorganopolysiloxane comprising a combination of units selected from dimethylsiloxane units, trimethylsiloxane units, units of the formula

units of the formula

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

TABLE 2 Film Tensile Tear Strength Dosage, Dosage, Thickness, Strength, Elongation Die "B" Composition J/kg mm kPa % N/m A. 5000 2.13 5240 800 27,320 B. 5000 1.52 4964 550 C. 5000 1.52 5137 820 24,518 D. 5000 1.52 6895 540 23,817 E. 5000 1.47 2586 430 12,784 F. 5000 1.57 3551 580 5,954 G. 5000 1.60 4826 685 5,254 H. 15000 2.29 1724 540 11,383 I. 5000 1.52 5240 140 8,231 J. 5000 1.52 1896 80 1,226 K. 5000 2.29 3172 295 17,338 L. 5000 1.60 3827 455 11,033 WHAT WE CLAIM IS: 1.A method of forming a thick section elastomer which comprises (I) mixing to form a composition (A) an aliphatically unsaturated polydiorganosiloxane having at east three diorganosiloxane units per molecule wherein each unit of a combination forming the molecule is selected from dimethylsiloxane units, methylvinylsiloxane units and units of the formula

any terminal siloxane units present being trimethylsiloxane units, methylphenylvinylsiloxane units, dimethylvinylsiloxane units or units of the formula

wherein R is an alkyl radical containing from 1 to 3 carbon atoms or a phenyl radical, there being at least two aliphatically unsaturated siloxane units per molecule, (B) a mercaptoorganopolysiloxane comprising a combination of units selected from dimethylsiloxane units, trimethylsiloxane units, units of the formula

units of the formula

units of the formula

units of the formula

units of the formula

and units of the formula

wherein R is an alkyl radical containing from I to 3 carbon atoms inclusive or a phenyl radical, R' is a methyl or ethyl radical and n has a value of from 1 to 4 inclusive, there being in the mercaptoorganopolysiloxane an average of at least two mercapto containing siloxane units per molecule and no more than 10 mol percent -SH containing siloxane units based on the total number of siloxane units in the mercaptoorganopolysiloxane, (A) and (B) being combined in a weight ratio sufficient to provide a molar ratio of moles of --SH groups in (B) per mole of vinyl radical in (A) of from 0.2:1 to 10: :1, at least one of (A) and (B) having at least 100 siloxane units per molecule, and (C) a filler in an amount sufficient to alter the physical characteristics of the final cured elastomer compared with a cured elastomer without filler (II) forming the composition from (I) into a shape having a thickness of at least 0.25 mm, and thereafter (III) exposing the shaped composition of (II) to ultraviolet radiation or electron beam until the composition is cured to an elastomer.
2. A method as claimed in claim 1 substantially as described with reference to any one of the Examples.
3. An elastomer whenever prepared by a process as claimed in Claim 1 or Claim 2.