SPECIFICATIONCurable organosiloxanesThis invention is concerned with organosiloxane compositions curable by exposure to ultra violet radiation.
Polysiloxane compositions intended to be cured by exposure to ultra violet radiation have been proposed for treatments of various materials including cellulosic substrates for example paper to provide for example release coatings. Among compositions proposed for such treatments are compositions comprising organosiloxanes having organomercapto groups available for reaction and organosiloxanes having unsaturation, in combination with photoinitiator. Compositions have been proposed comprising olefinically unsaturated organosiloxanes in which the unsaturation is provided by siloxane units including vinyl, allyl, acryloxy, methacryloxy or cinnamoyloxy groups.
Also, in our European patent application 157 540 (85301939.6) there is described and claimed a curable organosiloxane composition comprising (A) an organosiloxane having in the molecule at least two siloxane units of the general formula
wherein R represents a divalent saturated aliphatic hydrocarbon group having from 3 to 8 inclusive carbon atoms, R' represents a monovalent hydrocarbon group having from 1 to 6 inclusive carbon atoms and free of aliphatic unsaturation, an alkoxy group having from 1 to 4 carbon atoms or an alkoxyalkoxy group having from 2 to 6 carbon atoms and a has a value of 0, 1 or 2, any remaining units having the general formula
wherein R" represents a monovalent hydrocarbon group having from 1 to 6 carbon atoms and free of aliphatic unsaturation and b has a value of 0, 1, 2 or 3 at least 50 percent of the totalR' and R" groups being methyl, (B) an organosiloxane having in the molecule at least two units of the general formula
wherein Q represents a monovalent hydrocarbon group having from 1 to 6 inclusive carbon atoms and free of aliphatic unsaturation, an alkoxy group having from 1 to 4 carbon atoms or an alkoxyalkoxy group having from 2 to 6 carbon atoms and c is 0, 1 or 2, any remaining units having the general formula
wherein 0' is as defined for R" and d has a value of 0, 1, 2 or 3, at least 50 percent of the total 0 and Q' groups being methyl, and (C) a photoinitiator.
The mercapto organosiloxane systems are highly desirable especially those which can be cured to a substantially odour free condition in a comparatively short time.
Compositions according to said European patent application have been found to provide cured coatings having acceptable properties for many uses. Thus, preferred compositions according to said specification comprising benzophenone as photoinitiator can be cured to provide smear resistant coatings by passage beneath a mercury vapour lamp at a rate of up to about .10 metres per second (20 feet per minute). For many purposes this is satisfactory but, nevertheless, it is desirable to provide a composition capable of curing more quickly. The extent to which the cure may be accelerated by increasing the amount of benzophenone used is limited by compatibility considerations, and by the rate at which photoinitiation is effected by the benzo phenone.
We have found, surprisingly, that the speed of cure of UV curable compositions based on polysiloxanes having mercapto groups and polysiloxanes having unsaturation and including benzophenone when subjected to UV radiation can be markedly increased by use of certain additional photoinitiation materials selected from photoinitiators that produce initiator radicals by intramolecular photocleavage.
The present invention provides in one of its various aspects a curable composition comprising (A) an organosiloxane having mercapto siloxane units, (B) an organosiloxane having unsaturation and (C) photoinitiator material comprising benzophenone and characterised in that the composition comprises a second initiator capable of producing initiator radicals by intramolecular photocleavage.
In a composition according to the invention, the organosiloxane (A) having mercapto groups is preferably an organosiloxane having in the molecule at least two siloxane units of the general formula
wherein R represents a divalent saturated aliphatic hydrocarbon group having from 3 to 8 inclusive carbon atoms, R' represents a monovalent hydrocarbon group having from 1 to 6 inclusive carbon atoms and free of aliphatic unsaturation, an alkoxy group having from 1 to 4 carbon atoms or an alkoxyalkoxy group having from 2 to 6 carbon atoms and a has a value of 0, 1 or 2.Any remaining units of this organosiloxane preferably have the general formula
wherein R" represents a monovalent hydrocarbon group having from 1 to 6 carbon atoms and being free of aliphatic unsaturation and b has a value of 0, 1, 2 or 3 at least 50 percent of the total R' and R" groups being methyl.
The organosiloxane (A) may be a homopolymer consisting only of units (i) for example as in the cyclic siloxane, or may be a copolymer containing both (i) and (ii). In the general formula (i) R may be any divalent saturated aliphatic group having from 3 to 8 carbon atoms, for example -(CH2)3-, -CH2CHCH3CH2-, -(CH2)4-, and -(CH2)6-. The substituent R', when present, may be for example methyl, ethyl, propyl, phenyl, methoxy, ethoxy or methoxyethoxy. In the units (ii) of copolymers, R" may be for example methyl, ethyl, propyl or phenyl. At least 50 percent, and preferably substantially all, of the total R' and R" substituents should be methyl. It is therefore preferred that each R' and each R" is methyl.
The mercaptoalkyl substituents of the organosiloxane (A) may be attached to any of the silicon atoms in the molecule, that is they may be present in chain terminating units HSR(R')2SiOo5, or in chain units HSR(R')SiO or HSRSiOl 5. The organosiloxane (A) may have a molecular size from the disiloxanes to high molecular weight polymers and may have the consistency of a freely-flowing liquid or a resinous solid. when the compositions of this invention are intended for the provision of release coatings on paper and other flexible substrates the preferred organosiloxanes are polydiorganosiloxanes having from about 50 to about 500 siloxane units and a viscosity of from 5 x 10-5 to 10-2 m2/s at 25"C, at least three of the said siloxane units, and preferably from 5 to 20 percent of the siloxane units more preferably 5 to 10 percent thereof having therein a HSRgroup attached to silicon.
Organosiloxanes having HSR- groups are in general known substances and methods for preparing such organosiloxanes will be evident to those skilled in the art. For example, according to one method a silane bearing silicon-bonded hydrolysable atoms- or groups and a mercaptoalkyl group HSR- is hydrolysed and condensed to prepare a mixture of cyclic and linear siloxanes which is then mixed with cyclic and/or linear siloxanes having units
and the mixture equilibrated employing a suitable catalyst. The mixture preferably contains a source of endblocking units e.g. hexamethyldisiloxane but such source may be omitted e.g.
where a high molecular weight polydiorganosiloxane is required. Another, less preferred method comprises reacting a hydroxyl-terminated polydiorganosiloxane with a silane HSRSiRa(OAlk)3~a wherein OAlk is an alkoxy group and a is 0, 1 or 2.
In a composition according to the invention the organosiloxane (B) having unsaturation is selected with a view to crosslinking the polymers via addition reaction between the unsaturation and the mercapto groups of the organosiloxane (A). The unsaturation is provided by groups present in siloxane units of the organosiloxane (B) which groups preferably are aliphatic or alicyclic groups having a sole double bond available for the addition reaction. These groups may be for example vinyl, allyl, acryloxy, methacryloxy or cinnamoyl groups but are preferably cyclohexenylethyl groups.Preferred organosiloxanes (B) have in the molecule at least two cyclohexenylethyl substituted siloxane units of the general formula
wherein 0 represents a monovalent hydrocarbon group having from 1 to 6 inclusive carbon atoms and being free of aliphatic unsaturation, an alkoxy group having from 1 to 4 carbon atoms or an alkoxyalkoxy group having from 2 to 6 carbon atoms and c is 0, 1 or 2, any remaining units having the general formula
wherein Q' is as defined for R" and d has a value of 0, 1, 2 or 3, at least 50 percent of the total 0 and Q' groups being methyl.
Examples of suitable 0 substituents are methyl, ethyl, propyl, phenyl, methoxy, ethoxy and methoxyethoxy. These organosiloxanes (B) may be homopolymers (e.g. cyclic siloxanes) consisting only of siloxane units having unsaturation (e.g. units (iii)) or they may be copolymers consisting of such units and units (iv). In the copolymeric units (iv) each 0' may be for example methyl, ethyl, propyl or phenyl. At least 50 percent, and preferably substantially all, of the totalQ and Q' groups are methyl groups.
The cyclohexenylethyl groups may be attached to any of the silicon atoms in the organosiloxane molecule. For example, when the organosiloxane has a linear molecule the cyclohexenylethyl groups may be attached to terminal silicon atoms or non-ternninal silicon atoms. As in the case of the organosiloxane (A) the organosiloxane (B) may vary in molecular size from a freely-flowing liquid to a gum-like or resinous solid. The preferred organosiloxanes (B) for use in coating compositions for flexible substrates are polydiorganosiloxanes having a viscosity of from about 2x10 5m2/stoabout5xl0 3 m2/s.
Examples of organosiloxanes (B) which may be used in the invention include
wherein X represents the cyclohexenylethyl group
and Ph represents the phenyl group. In those compositions intended for application to provide coatings adherent to paper and having release performance in relation to pressure sensitive adhesives we prefer to employ substantially linear organosiloxane copolymers according to the general formula
where m and n are values greater than 0 and where Q' and X are as referred to above, and Q' is preferably methyl.
Preferably the values m and n are such that the ratio m/n is as large as practicable bearing in mind the requirements for application viscosity, cure profile and properties of the cured compositions e.g. release characteristics of the cured coating. Preferred copolymers are such that n has a value up to about 20 for example 5; the value of m may conveniently be selected in accordance with desired characteristics of the composition and cured film and may be for example from about 140 to about 500.
The preferred organosiloxane (B) may be prepared by the addition of vinylcyclohexene to an organosiloxane having silicon-bonded hydrogen atoms, for example a trimethylsiloxy end-stopped copolymer of methylhydrogensiloxane and dimethylsiloxane. The addition reaction is best carried out in the presence of a platinum catalyst. According to another method of preparing organosiloxanes (B) vinylcyclohexene can be reacted with a polymethylhydrogen siloxane and the resulting polymer equilibrated with a source of dimethylsiloxane units such as a cyclic dimethylsiloxane. In another method vinylcyclohexene can be reacted with a trialkyloxysilane and the resulting cyclohexenylethyl trialkoxysilane then reacted with a hydroxylterminated polydiorganosiloxane.In another method, vinylcyclohexene can be reacted with methylhydrogen dichlorosilane in presence of a platinum catalyst and the resulting cyclohexenylethyl dichloromethylsilane hydrolysed and polymerised for example in presence of sources of dimethylsiloxane units and trimethylsiloxane units.
The photoinitiator benzophenone is capable of producing initiator radicals by intermolecular hydrogen abstraction in presence of a hydrogen donor compound. We believe that under the influence of ultra violet radiation the benzophenone is excited to the triplet ketone which is capable of abstracting a hydrogen atom from a donor -RSH group to yield initiating radicals.
This effect may be represented by the scheme
Initiators capable of producing initiator radicals by intramolecular photocleavage are subject to cleavage under influence of ultra violet radiation to yield initiating radicals. A variety of such photoinitiators is commercially available. However, in order to achieve inter alia effective improvement of the cure rate of compositions comprising organosiloxanes (A) and (B) and benzophenone, it is necessary to select the material employed as the second initiator in accordance with the nature of the other ingredients employed in the composition. It is also necessary to employ the initiators in selected amounts in order to achieve desirable improvements.Materials suitable for use as the second initiator include 2-hydroxy-2-methyl- 1 -phenyl-propan- 1-one,
hereinafter referred to as second initiator (a), 1-hydroxycyclohexylphenyl ketone
hereinafter referred to as second initiator (b) and benzildimethylketal (i.e. a,a-dimethoxy-a-phenyl- acetophenone)
hereinafter referred to as second initiator (c). Of these materials, second initiators (a) and (b) are preferred, with second initiator (a) being the most preferred. In general, second initiator (a) may be used in quantities of 50% or 200% by weight of the benzophenone in mercaptosiloxanecyclohexenylethylsiloxane systems and in mercaptosiloxane-vinylsiloxane systems.For best results in mercaptosiloxane-cylcohexenylethylsiloxane systems we prefer to use the benzophenone and second initiator (a) in a ratio of 1:2 by weight. With this second initiator (a) we also prefer to use the benzophenone in an amount of not less than about 1% by weight of the composition and not more than the maximum amount which is soluble in the system which is to say not more than about 1.5% by weight of the composition. Second initiator (b) should be used in quantities of 50% or 200% by weight of the benzophenone in mercaptosiloxane-vinylsiloxane systems so that the benzophenone and second initiator together provide a total of about 1.5% photoinitiator by weight of the composition.In mercaptosiloxane-cyclohexenylethylsiloxane systems the benzophenone and second initiator (b) should be used in a ratio of 1:2 by weight and the benzophenone should be used in an amount of not less than about 1% by weight of the composition. Second initiator (c) should be used in quantities of 50% by weight of the benzophenone in mercaptosiloxane-vinylsiloxane systems so that the benzophenone and second initiator together provide a total of about 1.5% photoinitiator by weight of the composition. In mercaptosiloxane-cyclohexenylethylsiloxane systems the benzophenone and second initiator (c) should be used in a ratio of 1:2 by weight and the benzophenone should be used in an amount of not less than about 1% by weight of the composition.
Under the influence of ultra violet radiation the preferred second initiator (a) undergoes intramolecular cleavage which may be represented by the scheme
The compositions of this invention may be prepared by simply mixing the organosiloxanes and initiators in any order. In the undiluted state the mercapto and unsaturated organosiloxanes react together in the presence of initiator and ultra violet radiation. They may, however, be stored in the mixed state in the absence of such radiation, for example in lightproof containers or storage areas. For maximum storage stability it is preferred to provide the compositions as a two package system, the organosiloxanes (A) and (B) being packaged separately and the benzophenone and second initiator being present in one or both packages.
For the production of crosslinked products the organosiloxane (A) preferably has at least two mercaptosiloxane units (i) per molecule and the organosiloxane (B) has at least two unsaturated groups per molecule and the sum of the HSR- groups in (A) and the unsaturated groups in (B) should be at least 5. It is normally preferred that the organosiloxane components of the compositions be employed in proportions such that there is an excess of mercaptoalkyl groups with respect to the unsaturated groups. In preferred compositions for paper coating we prefer to employ the organosiloxanes in quantities to provide a ratio of RSH groups to unsaturated groups of -C=C- of about 2:1. However, depending on the relative molar contents of the organosiloxanes and their molecular weights the relative weight proportions of the organosiloxane components may vary within wide limits.
Compositions according to this invention may also contain ingredients normally present in curable coating compositions. For example, the compositions may be diluted with qrganic solvents to facilitate application to some substrates. However, when a significant amount of solvent is present it may be necessary to subject the coated substrate to elevated temperatures prior to curing in order to effect solvent removal therefrom. Other ingredients which may be present include fillers, pigments and additives for modifying the release or other properties of the coating.
Compositions according to the invention find use in various fields, for example in the paints, surface coatings and adhesives arts. They are however particularly suitable when employed as thin coatings. They may be applied as coatings to a variety of substrates and cured thereon by exposure to actinic radiation, particularly in the form of ultra violet radiation. Although curing will take place slowly in the presence of normal daylight it is preferred to accelerate the cure rate by exposure to lamps which emit U.V. light, preferably with a wavelength in the range from 250 to 450 nm for example medium pressure mercury lamps. The compositions may be applied to substrates such as metals e.g. aluminium, iron, steel and copper, plastics e.g. polyamide, polyester, polyethylene and polypropylene, siliceous materials e.g. ceramics and cement, and textiles e.g. cotton and synthetics.They are particularly useful for the formation of release coatings on cellulosic materials such as paper, plastics coated paper and paper board. They may be applied to the substrate employing any suitable means such as dip coating, spraying, doctor blade or gravure roll.
The present invention also provides a method of producing a release coating on a cellulosic or other support surface comprising application thereto of a coating of not less than about 1 gm-2 of a composition according to the invention, and exposing the coating to U.V. radiation.
In order that the invention may become more clear, there now follows a description of example curable compositions and their use. It is to be clearly understood that the example compositions have been selected for description to illustrate the invention by way of example only and not by way of limitation thereof.
Example 1A base formulation coating composition having a viscosity of about 1000 cS (about 10~3 m2/s) was made up by mixing together 2 parts by weight of the polyorganosiloxane
and one part by weight of the polyorganosiloxane
there being a ratio of 1.92 mercapto groups per cyclohexenyl group present.
Various amounts of benzophenone and a second initiator namely second initiator (a) (2 hydroxy-2-methyl-1-phenyl-propan-1-one) were mixed with portions of the base formulation to provide curable compositions comprising photoinitiators in amounts by weight per 100 parts by weight of the composition as shown in Table 1. These compositions were coated onto SuperCalendered Kraft paper using a blade coater at a level of about 1g per square metre to provide a layer of substantially uniform thickness. The coated paper was exposed to light at a wavelength of about 250 to 450 nm from a medium pressure mercury vapour lamp rated at 80 w/crn held at a distance of 50mm and focussed by an elliptical reflector.The coated paper was fed beneath the lamp at various speeds and the smear, rub off and migration characteristics of the resulting coatings were observed. Results are shown in Table 1.
These characteristics were assessed immediately after passage of the coated paper beneath the lamps. Smear characteristics were observed by pressing a finger onto the surface with moderate pressure and drawing it across the surface. Ready spreading of the coating by the finger was awarded "Y", some slight spreading was awarded "S" and no spreading was awarded "N". Rub off (RO) characteristics were observed by rubbing the finger to and fro several times on the surface. Removal of the coating, slight removal of the coating and no removal of the coating in this test were awarded "Y", "S", "N" respectively. Migration (Mig) characteristics were observed by adhering a strip of Sellotape to the coating by finger pressure.
The Sellotape was removed from the coating and immediately looped and the adhesive surfaces pressed against each other. Good adhesion was recorded as no migration ("N"), slightly reduced adhesion was recorded as some migration ("S") and poor adhesion was recorded as migration ("Y")TABLE 1AMOUNT OFCOMPOSITION INITIATOR PAPER SPEED CHARACTERISTICSSecondBenzo- Initiaphenone tor (a) (m/sec) Smear/RO/Mig.
1 A 1.5 0 0.076 N N N0.102 NNN0.127 S N N1 B 0 1.5 0.076 NSN 0.102 N Y N0.203 YyY 1 C 1.0 0.5 0.102 N N N0.127 SNN0.152 SNS 1 D 0.5 1.0 0;102 NNN 0.152 N N N0.203 SYN1 E 1.0 2.0 0.203 N N N0.229 NNN0.254 N S SN = No Y = Yes S = Slight RO = Rub OffExample 2A base formulation coating composition having a viscosity of about 7x 10-4 m2/s was made up by mixing together 2 parts by weight of the polysiloxane
and one part by weight of the polysiloxane
there being a ratio of 1.92 mercapto groups per cyclohexenyl group present.
Various amounts of benzophenone and second initiator (a) were mixed with portions of the base formulation to provide curable compositions comprising percentage amounts (by weight in the composition) of photoinitiators as shown in Table 2. These compositions were coated ontoSuper Calendered Kraft paper using a blade coater at a level of about 1 g per square meter to provide a uniform thin layer. The coated paper was exposed to light at a wavelength of 250 to 450 nm from a medium pressure mercury vapour lamp rated at 80 w/cm held at a distance of 50mm and focussed by an elliptical reflector. The coated paper was fed beneath the lamp at various speeds, and the smear, rub off and migration characteristics of the resulting coatings were assessed as described in Example 1. Results are shown in Table 2.
TABLE 2AMOUNT OFCOMPOSITION INITIATOR PAPER SPEED CHARACTERISTICSSecondBenzo- Initiaphenone tor (a) -(m/sec) Smear/RO/Mig.
2 A 1.5 0 0.051 N N N0.076 SNS 0.152 YNY2 B 0 1.5 0.076 N N N0.102 N N N0.152 S N S2 C 1.0 0.5 0.102 NNN 0.127 N N N0.152 N N S0.203 SNY2 D 0.5 1.0 0.152 N N N0.178 N N N0.203 S N N2 E 1.0 2.0 0.203 NNN 0.229 N N N0.254 SNS N = No Y = Yes S = Slight RO = Rub OffAs may be appreciated from the Tables, the rate at which the photocurable paper coatings reached the desired state, i.e. "NNN" (No smear, No rub off, No migration) is considerably influenced by the photoinitiators present. Results using Compositions 1A, 1B, 2A and 2B show that the cure rate, for these particular combinations of polymers using either of the first and second initiators separately, permits a maximum paper speed of about 0.102 m/sec.By keeping constant the total weight of photoinitiator material present but using combinations of first and second initiator this rate can be increased to permit paper speeds of 0.15 m/sec or more. See for example Compositions 1D and 2D. Thus the combination of first and second initiators provides a more rapid curing than can be achieved with either separately, at the same level of addition i.e. a synergistic effect is demonstrated.
The benzophenone limit of solubility is less than 1.5% and that of the second initiator is about 2% in the organosiloxanes. Nevertheless, by practice of the invention, cure rates which permit paper speeds of about 0.23 m/sec can be achieved as demonstrated by compositions 1E and 2E. The improvement in cure rate by a factor in excess of 2 is seen as highly advantageous.
Example 3To a base formulation as described in Example 1 were added amounts of benzophenone and, as second initiator (b) 1-hydroxycyclohexylphenyl ketone or (c) benzildimethylketal. The amounts by weight of benzophenone and second initiator per 100 parts by weight of the composition used are shown in Table 3. The compositions were coated onto Super Calendered Kraft paper and irradiated with UV as described in Example 1. The fastest speeds of paper feed at whichthe smear, rub off and migration tests yielded an N/N/N test rating assessed as in Example 1was recorded and is shown in Table 3.
TABLE 3COMPOSITION INITIATOR FASTESTPAPER SPEEDBenzophenone (b) (c) (m/sec)3 A 1.5 0 0 0.1123 B 0 1.5 0 < 0.112 3C 0 0 1.5 < 0.1123 D 0.5 1.0 0 < 0.1123 E 1.0 2.0 0 0.1523 F 1.0 0 0.5 0.1273 G 0.5 0 1.0 < 0.1123 H 1.0 0 2.0 0.178From these results one may see that using these compositions best results are achieved withcompositions 3E and 3H.
Example 4A base formulation was prepared comprising 2 parts by weight of the polysiloxane (CH3ViSiO)nwhere Vi represents the vinyl group -CH=CH2 and n has a value of about 5 and 98 parts byweight of the polysiloxane
there being a ratio of about 2.7 mercapto groups per vinyl group present. Various amounts of benzophenone and the second photoinitiators (a), (b) and (c) employed in Examples 1, 2 and 3 were mixed with portions of the base formulation to provide curable compositions comprising by weight per 100 parts by weight of the composition amounts of photoinitiators as shown inTable 4. The compositions were coated onto Super Calendered Kraft paper and irradiated withUV as described in Example 1.The fastest speeds of paper feed at which the smear, rub off and migration tests yielded N/N/N test rating assessed as in Example 1 was recorded and is shown in Table 4.
TABLE 4FASTESTCOMPOSITION AMOUNT OF INITIATOR PAPER SPEEDBenzophenone (a) (b) (c) (m/sec) 4A 1.5 0 0 0 0.1274 B 0 1.5 0 0 < 0.112 4C 0 0 1.5 0 < 0.1124 D 0 0 0 1.5 < 0.1124 E 1.0 0.5 0 0 0.178 4F 0.5 1.0 0 0 0.1124 G 1.0 2.0 0 0 0.1784 H 1.0 0 0.5 0 0.2034 J 0.5 0 1.0 0 0.2034 K 1.0 0 2.0 0 0.1274 L 1.0 0 0 0.5 0.1524 M 0.5 0 0 1.0 0.127From these results one may see that using these compositions best results are achieved with compositions 4E, 4G, 4H, 4J and 4L.
As may be seen from the results shown in the tables, second initiator (a) may be used in quantities of 50% or 200% by weight of the benzophenone in compositions which use mercaptosiloxane-cyclohexenylethylsiloxane systems or mercaptosiloxane-vinylsiloxane systems. Best results in composition which use mercaptosiloxane-cyclohexenylethylsiloxane systems are shown with compositions in which benzophenone and the second initiator (a) are used in a ratio of 1:2 by weight and the benzophenone is used in an amount of 1% by weight of the composition.
Compositions using second initiator (b) show satisfactory results when this second initiator is used in quantities of 50% or 200% by weight of the benzophenone in compositions which use mercaptosiloxane-vinylsiloxane systems and the benzophenone and second initiator together provide a total of about 1.5% photoinitiator by weight of the composition. In compositions using mercaptosiloxane-cyclohexenylethylsiloxane systems the combination of benzophenone and second initiator (b) provides satisfactory results when the benzophenone and second initiator (b) are used in a ratio of 1:2 by weight and the benzophenone is used in an amount of not less than about 1% by weight of the composition. Compositions using second initiator (c) show satisfactory results when used in quantities of 50% by weight of the benzophenone in compositions using mercaptosiloxane-vinylsiloxane systems and the benzophenone and second initiator together provide a total of about 1.5% photoinitiator by weight of the composition. In compositions using mercaptosiloxanecyclohexenylethylsiloxane systems the benzophenone and second initiator (c) can be used in a ratio of 1:2 by weight and the benzophenone should be used in an amount of not less than about 1% by weight of the composition.