GB2032936A - Organopolysiloxane compositions which can be cured to give elastomers for the manufacture of moulds - Google Patents

Description

SPECIFICATION Organopolysiloxane compositions which can be cured to give elastomers for the manufacture of moulds The present invention relates to "two-component" organopolysiloxane compositions which harden, at or above ambient temperature, to give silicone elastomers which are particularly useful in moulding polyurethane foam articles. Two-component organopolysiloxane compositions, which can be used for producing moulds from silicone elastomers, have already been proposed on the industrial market and in the chemical literature (see, for example, French Patent 1,418,114 and Japanese O.P.I. Specification 50/066,553). These compositions can contain various additives which facilitate their processing or their conversion to elastomers, such as a, co-dihydroxydiorganopolysiloxane oils having a low viscosity (see French Specifications 2,272,145 and 2,300,1 14). The moulds manufactured therefrom have the characteristic that they reproduce the details of the patterns with sharpness and precision; they also make it possible, by virtue of the non-stick character of slicone rubber, to improve mould-release rates. However, during successive cycles in the manufacture of the mouldings, the inner walls of the moulds tend to be attacked quite rapidly by one or more constituents of the mixtures poured into the moulds. If these mixtures do not contain aggressive constituents; it is not necessary to have means of protecting the walls of the moulds; this is generally the case for organic mixtures used for polyester, polystyrene, polyethylene, and polyvinyl chloride mouldings. In fact, these means complicate the moulding/mould-release cycles and the expense created by their use can outweigh the advantage gained by increasing the number of mouldings possible. On the other hand, if one or more of the constituents is aggressive, as is the case with mixtures for polyurethane mouldings and, more precisely, mouldings made of polyurethane foams, it is then essential to use means which counteract this aggressivity; otherwise, the number of mouldings drops to a third or even a quarter of the number possible when using mixtures which do not contain aggressive constituents. Amongst the most effective and simplest means which can be employed, the prior application of film-forming organic compounds to the inner walls of the moulds should be mentioned; a fine layer of a protective coating is thus formed (see French Patents 2,040,714 and 2,043,388 and filed Japanese O.P.I. Specification 49/005,160). This application is preferably effected by spraying solutions of filmforming compounds in organic solvents; this is fairly easy to carry out with moulds having a relatively even internal surface which is accessible and of small dimensions. In the case of moulds of large dimensions, the walls of which posses a complicated relief, such as ..he moulds used for reproducing instrument panels and objects d'art, it is very difficult to obtain a uniform and continuous film-forming coating.Furthermore, a treatment of this type requires, on the one hand, the availability of large surfaces for drying the walls of the moulds, and, on the other hand, the use of a device for recovering the solvents. It is therefore desirable to find even simpler, and in particular less expensive, means which make it possible to achieve a large number of polyurethane mouldings for each mould made of silicone elastomer. For this purpose, according to the present invention there is provided, for producing moulds which are resistant to mixtures convertible to polyurethane, organopolysiloxane compositions which harden at or above ambient temperature and comprise (the parts being expressed by weight):

mPa.s at 25[deg]C, in which the organic radicals bonded to the silicon atoms are chosen from amongst methyl, ethyl, n-propyl, vinyl and phenyl radicals, at least 80% of these radicals being methyl radicals and at most 5% being vinyl radicals, B. 1.5 to 12 parts of crosslinking agents chosen from amongst:

which may or may not contain halogen and has from 1 to 8 carbon atoms, or a cyanoalkyl radical having from 3 or 4 carbon atoms; the symbols R', which are identical or different, represent alkyl radicals

represents a methyl or ethyl radical; and the symbol a represents zero or 1;and (ii) the products resulting from the partial hydrolysis of the silanes of the formula Si(OR')4, the symbols R' having the meaning given under (i), C. 10 to 150 parts of pulverulent fillers, D. alpha , dihydroxydiorganopolysiloxane oils having a viscosity of 10 to 350 mPa.s at 25[deg]C, in which the organic radicals bonded to the silicon atoms are chosen from amongst methyl, vinyl and phenyl radicals, at least 50% of these radicals being methyl radicals and at most 3% being vinyl radicals, and E. catalysts of the formula (R"COO)2SnQ2' in which the symbols R", which are identical or different, represent aliphatic hydrocarbon radicals having from 6 to 25 carbon atoms and the symbols Q, which are identical or different, represent alkyl radicals having from 1 to 6 carbon atoms, characterised in that: 1) the oils D are used at a rate of 7 to 23 parts, preferably 8 to 21 parts, per part of tin provided by the catalysts E, and 2) the catalysts E are introduced in a sufficient amount to provide from 0.22 to 1 part of tin, preferably 0.25 to 0.95 part, per 100 parts of the compositions.

mPa.s at 25[deg]C, preferably from 1,000 to 80,000 mPa.s. at 25[deg]C, are linear polymers which essentially consist of diorganosiloxyl units of the formula Z2SiO and are blocked at each end of their chain by a hydroxyl group; however, the presence of monoorganosiloxyl units of the formula ZSIO1.5 and/or siloxyl units of the formula SiO2 is not excluded in a proportion of at most, say, 2%, relative to the number of diorganosiloxyl units. The symbols Z, which are identical or different, represent the organic radicals described above, that is to say methyl, ethyl, n-propyl, vinyl or phenyl radicals; these radicals are preferably methyl radicals. By way of illustration of units represented by the formula Z2Si0, those corresponding to the

may be mentioned. The oils A are generally marketed by silicone manufacturers. Furthermore, the processes for their manufacture are well known and are included, for example, in French Patents 1,134,005, 1,198,749 and 1,226,745. One of the most common processes consists, in a first stage, in polymerising diorganocyclopolysiloxanes with the aid of catalytic amounts of alkaline or acid agents, and then in treating the polymers with calculated amounts of water. These amounts of water are larger, the lower is the viscosity of the oils to be prepared. Subsequently, in a 2nd stage, the oils are isolated by removing, at a temperature which is generally above 100[deg]C and under a pressure which is preferably below atmospheric pressure, the diorganocyclopolysiloxanes present when equilibrium is reached and also the relatively low molecular weight polymers formed during this reaction. Before distilling the volatile products, it is recommended to neutralise the alkaline or acid agents used as polymerisation catalysts. The oils A can be used by themselves or in the form of mixtures containing at least 2 oils of different viscosities. The crosslinking agents B are introduced into the compositions of the invention at a rate of 1.5 to 12 parts, preferably 1.7 to 10 parts, per 100 parts of the oils A. They are chosen from amongst the

of the formula Si(OR')4. By way of illustration of hydrocarbon radicals which may or may not contain halogen and which have from 1 to 8 carbon atoms, and which are represented by the symbol R, there may be mentioned: alkyl radicals which may or may not contain halogen and which have from 1 to 6 carbon atoms, such as methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, chloromethyl and 3,3,3-trifluoropropyl radicals, alkenyl radicals having 2 or 3 carbon atoms, such as vinyl and allyl radicals, cycloalkyl and halogenocycloalkyl radicals having from 3 to 7 carbon atoms, such as cyclopentyl, cyclohexyl, methylcyclohexyl, 2,2-dichlorocyclopropyl, 2,2-difluorocyclopropyl and chlorocyclohexyl radicals, and mononuclear aryl and halogenoaryl radicals having from 6 to 8 carbon atoms, such as phenyl, tolyl, xylyl, chlorophenyl and trichlorophenyl radicals. The cyanoalkyl radicals having 3 or 4 carbon atoms, which are represented by the symbol R, include, specifically, /}-cyanoethyl and y-cyanopropyl radicals. By way of illustration of alkyl radicals having from 1 to 4 carbon atoms, which are represented by the symbols R', there may be mentioned methyl, ethyl, n-propyl, isopropyl and n-butyl radicals. Concrete examples which may be mentioned of monomers of the formula RaSi(OR')4-a are methyltrimethoxysilane, methyltriethoxysilane, chloromethyltrimethoxysilane, ethyltrimethoxysilane, vinyltrimethoxysilane, cyclohexyltrimethoxysilane, 2,2-dichlorocyclopropyltriethoxysilane, phenyl-tris-

represents any number ranging from 0.3 to 3.7. These mixed silanes can be pure products or mixtures; they are prepared, for example, by heating, in one and the same reactor, two tetraalkoxysilanes, one of the silanes carrying ethoxy radicals and the other n-proxy radicals or, alternatively, one of the silanes carrying methoxy radicals and the other n-butoxy radicals. The polymers resulting from the partial hydrolysis of the silanes of the formula Si(OR')4, for example of tetraethoxysilane, tetra-(n-propoxy)-silane and mixed silanes described above, consist of a high proportion of units of the formula (R'0)2SiO, the other units present including those of the formulae

Characterisation of these polymers, which are customarily referred to as "polysilicates", can be based on the proportion of alkoxy groups and/or of silica therein. The processes for their preparation are well known; they are described, for example, on pages 648 to 653 of "Chemistry and Technology of Silicones" by W. Noll. However, in order to be compatible and/or reactive with the other ingredients employed for preparing the compositions of the invention (in particular with the oils A and the oils D), the polysilicates prepared in this way should possess the ability to dissolve in hydrocarbon solvents, such as toluene, xylene and methylcyclohexane, in a proportion of at least 50 parts of polysilicate per 100 parts of solvent. The fillers C are introduced at a rate of 10 to 150 parts, preferably 15 to 140 parts, per 100 parts of the oils A. They are preferably pulverulent materials having a mean particle diameter of more than 0.1 micron, such as a diatomaceous silica, ground quartz, aluminium silicates, mixed aluminium/magnesium silicates, mica powder, calcium carbonate, powdered glass, the oxides of aluminium, titanium, iron, zinc, chromium, zirconium and magnesium, boron nitride, lithopone, graphite, glass fibres and synthetic polymers in the form of fibres and powders. These fillers can optionally be modified on the surface by treatment with low molecular weight organosilicon compounds such as diorganocyclopolysilazanes, diorganocyclopolysiloxanes and hexamethyldisilazane. The fillers treated in this way generally contain from 3 to 30% of organosilicon compounds, relative to their initial weight. The use of materials having a particle diameter of less than 0.1 micron, such as pyrogenic silica and precipitation silica, is not excluded; however, it is not recommended, in particular in amounts of more than 30 parts per 100 parts of the oils A, because such materials cause a considerable increase in the viscocity of the compositions, which makes it difficult to process them by casting. The a,co-dihydroxydiorganopolysiloxane oils D are introduced at a rate of 7 to 23 parts, preferably 8 to 21 parts, per part of tin provided by the catalysts E. These oils are also linear polymers having a viscosity of 10 to 350 mPa.s at 25[deg]C, preferably 15 to 300 mPa.s at 25[deg]C, and consist essentially of diorganosiloxyl units of the formula Z'2SiO blocked at each end of their chain by a hydroxyl radical. The symbols Z', which are identical or different, represent methyl, vinyl or phenyl radicals, preferably methyl radicals. By way of illustration of units represented by the formula Z'2SiO, those corresponding to the

mentioned. Some of the oils D are marketed by silicone manufacturers; moreover, they can easily be manufactured by the simple hydrolysis of diorganodichlorosilanes or diorganodiacetoxysilanes in an aqueous medium containing a basic agent such as ammonia or sodium bicarbonate. Othe processes can be employed, for example the polymerisation or hexaorganocyclotrisiloxanes with the aid of clay activated by an acid, in a water/acetone medium. The catalysts E are introduced into the compositions of the invention in sufficient amounts to provide from 0.22 to 1 part of tin, preferably 0.25 to 0.95 part of tin, per 100 parts of the compositions of the invention. They correspond to the abovementioned formula (R"COO)2SnQ2. The aliphatic hydrocarbon radicals having from 6 to 25 carbon atoms, which are represented by the symbols R", include linear or branched, alkyl and alkenyl radicals. Concrete examples which may be mentioned of alkyl radicals of this type are 2-ethylhexyl, n-octyl, n-decyl, n-dodecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-octadecyl, n-eicosyl, n-docosyl and npentacosyl radicals. Concrete examples which may be mentioned of alkenyl radicals of this type are the radicals

Furthermore, concrete examples which may be mentioned of the alkyl radicals having from 1 to 6 carbon atoms, which are represented by the symbols Q, are methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl, n-pentyl and n-hexyl radicals. The catalysts E can be pure products or mixtures; the latter can be prepared, for example, by reacting mixtures of carboxylic acids (originating from natural glycerides such as copra oil and palm oil) with diorganotin oxides. By way of illustration of these catalysts E, those corresponding to the following formulae may be

Sn(n-C4H9)2. Apart from the constituents A, B, C, D and E, various adjuvants can be introduced into the compositions of the invention, such as inorganic or organic pigments and agents for improving flame resistance. These pigments and agents together should generally not represent more than 15% of the weight of the fillers C. Moreover, additives based on organopolysiloxane polymers or on organic compounds, which influence the physical characteristics of the compositions and the mechanical properties of the elastomers, can be used at a rate of at most, say, 50 parts, preferably 3 to 40 parts, per 100 parts of the oils A. Amongst these additives, there should be mentioned more especially:

mPa.s at 25[deg]C, which are essentially formed by the units of the abovementioned formula Z'2SiO and terminal units of the formula Z'3SiO0.5, in which formulae the symbols Z' have the meaning indicated above in relation to the units of the oils D. Generally at least 80% of the radicals represented by symbols Z' are methyl radicals and at most 3% are vinyl radicals. These oils are marketed by silicone manufacturers and can be prepared by proceeding in accordance with techniques which are now well developed and are described, for example, in French Patents 978,058, 1,025,150 and 1,108,764.

25.C are preferably used. 2) organic compounds which are inert towards the constituents A, B, C, D and E and are capable of readily dispersing in the compositions and of not exuding from the elastomers. By way of illustration of these organic compounds, there may be mentioned: aliphatic monocarboxylic acids having from 6 to 25 carbon atoms, such as 2-ethylhexanoic acid, lauric acid, palmitic acid, stearic acid, oleic acid, linoleic acid and linolenic acid, and ester prepared from the abovementioned aliphatic monocarboxylic acids and from aliphatic monohydric alcohols having from 2 to 20 carbon atoms, such as ethanol, propanol, isopropanol, 2-ethylhexanol, octanol, decanol. dodecanol, pentadecanol, octadecanol and eicosanol. Specific examples of these esters which may be mentioned are isopropyl palmitate, 2-ethylhexyl stearate and dodecyl oleate. In order to assist the curing of the composition of the invention, which are generally used in the form of layers having a thickness of, for example, more than 2 cm, it is recommended to introduce water at a rate of at most 1 part per 100 parts of the oils A. This addition of water is not necessary if the fillers C contain a sufficient amount thereof. In order to facilitate its incorporation, the water is preferably added in the form of a dispersion in a paste

the fillers C. The organopolysiloxane compositions of the invention can easily be manufactured by mixing the various constituents in suitable apparatuses such as malaxators and kneading machines. In order to obtain homogeneous compositions, it is recommended initially to mix the oils A with the fillers C; the whole can be heated for at least 30 minutes at a temperature above 80[deg]C so that the fillers are completely wetted by the oils. The other constituents, that is to say the crosslinking agents B, the low viscosity oils D, the catalysts E and, if appropriate, various additives and adjuvants and water, can be added to the resulting mixture, preferably heated to a temperature below 80[deg]C, for example of the order of ambient temperature. Compositions of this type are not stable on storage and they must therefore be employed rapidly even though the joint presence of the abovementioned amounts of the oils D and the catalyst E makes it possible to achieve working times of at least 70 minutes. For packaging and storage, these compositions cannot therefore contain all the constituents A, B, C, D and E. From the industrial point of view, they must be manufactured in the form of two components, both of which are stable on storage. A first component, which is stable on storage, can comprise the constituents A, B, C and D; it is preferably prepared by introducing the crosslinking agent B and the oil D into the abovementioned homogeneous mixture formed by malaxating the oil A and the filler C. The 2nd component then comprises the catalyst E. At the time of use, the catalyst E is simply introduced into the first component; it can be introduced either as such or, alternatively, in order to facilitate its dispersion (because it is used in small amounts), in the form of a homogeneous mixture comprising, for example, apart from the catalyst, the abovementioned additives based on organopolysiloxane polymers and/or on organic compounds, and part of the fillers C employed. For example, these homogeneous mixtures can comprise 100 parts of additives based on organopolysiloxane polymers and/or on organic compounds, 5 to 50 parts of inorganic filler C and 1 to 10 parts of catalyst E. Other ways of presenting the two-component compositions can be singled out, for example a first component containing the oil A, the filler C and the oil D and a second component containing the crosslinking agent B and the catalyst E. In all cases, both the components must be sufficiently fluid for compositions having a viscosity ranging, for example, from 10,000 to 200,000 mPa.s at 25[deg]c to be formed when these components are mixed. These compositions, which remain sufficiently fluid for at least 70 minutes, preferably for at least 90 minutes, after the two components have been mixed, can be used more especially for the manufacture of moulds from silicone elastomers; however, they can be used for other applications such as the encasing of electronic equipment and the coating of metal surfaces or of textile or cellulosic materials. The moulds manufactured are intended for the production of mouldings from cellular or non-cellular organic polymeric materials. Amongst these materials, there may be mentioned polyesters, polyethylene, polypropylene, polyamides, polyvinyl chloride and polystyrene. However, it is recommended to use these moulds for the production of mouldings from polyurethane because the moulds have a remarkable resistance to attack by the constituents of the mixtures, in particular polyisocyanates, which gives rise to the polyurethanes. Surprisingly, this resistance is achieved by means of the amounts of tin which are provided by the catalyst E; the specified amounts of the oils are provided with these amounts of tin in order to obtain compositions having the abovementioned working times. The moulds produced for the compositions of the invention make it possible to manufacture a number of polyurethane mouldings which is at least 50% greater than the number of mouldings previously manufactured with moulds made of silicone elastomers. The polyurethane mouldings thus obtained can form a wide variety of articles, for utilitarian or decorative purposes, such as instrument panels, arm rests, furniture and objets d'art. The following Examples further illustrate the invention. (The parts and the percentages are expressed by weight.) EXAMPLE 1 8 catalysed compositions C, to C8 are manufactured, operating in the following manner for each catalysed composition: 40 parts of a diatomaceous silica having a mean particle diameter of 3 microns and 60 parts of alumina having a mean particle diameter of 1 micron are gradual introduced into a malaxator

25.C. The contents of the malaxator are then heated for 2 hours at 110[deg]C. When the temperature of the resulting mixture has fallen to about 25[deg]C, 8 parts of n-propyl silicate and an amount, specified below,

into the malaxator. An amount, also specified below, of a tin derivative is then added to this mixture. The composition catalysed in this way is kept for 10 minutes under a pressure of 4,000 Pa in order to remove the bubbles of gas. For each catalysed composition, its initial viscosity is measured and its working time is established by noting the time after which the composition is no longer capable of spreading when it is placed on a horizontal surface. With each catalysed composition, moreover, a mould is manufactured from silicone elastomer by pouring the composition into a master mould consisting of 2 parts, namely a counter-mould, made of a polyester resin/glass fabric laminate, and a moulding made of epoxy resin (representing the pattern to be reproduced), which fits into the counter-mould to leave a sufficient space for the formation of the mould made of silicone elastomer. The whole is left to stand for 24 hours at ambient temperature. The resulting mould made of silicone elastomer is subsequently released and it is then left to stand for 8 days in the atmosphere. 168 g of a mixture intended for the manufacture of semi-rigid polyurethane foams is cast into the mould cavity, the approximate dimensions of which are 140 x 70 x 40 mm. This mixture is prepared by mixing: 100 parts of a polyether-triol having a molecular weight of about 5,000, containing 75% by number of primary hydroxyl groups, having a hydroxyl number of 35 and having a viscosity of 1,100 mm /s at 20[deg]C, 20 parts of 3,3'-dichloro-4,4'-diaminodiphenylmethane, 0.9 part of a 33% strength solution of triethylenediamine in dipropylene glycol, 0.5 part of N,N-dimethylethanolamine, 18 parts of fluorotrichloromethane and 30 parts of 4,4'-diisocyanatodiphenylmethane. The mouth of the mould is immediately closed and, after 6 minutes, a block of semi-rigid foam, which possesses all the details of the pattern on its surface, is released from the mould. This operation for the manufacture of blocks of foam is repeated and is not stopped until sticking starts to occur at the interface between the wall of the mould and the surface of the blocks of foam. The number of blocks manufactured is noted under these conditions.

having a viscosity of 50 mPa.s at 25[deg]C, which were used and also the amounts of the tin derivative used, the nature of the tin derivative, the percentage of tin present in each catalysed composition, the

is present in each catalysed composition, the viscosity of each catalysed composition in the freshly prepared state, the working time of each catalysed composition and the number of blocks of foam manufactured.

The tin derivative (XI) is the one used in Example 1 of U.S. Patent 3,919,136; this derivative contains 25.2% of tin; it is prepared by repeating Example 1 of U.S. Patent 2,597,920, which describes the heat treatment of an equimolar mixture of dibutyltin oxide and diethyl phthalate. Experiments C5, C6, C7 and C8 are given by way of comparison. Experiment C5 shows the value of a large amount of the tin derivative; amounts which are reduced by half (compared with C,) cause a considerable drop in the number of blocks manufactured. Experiment C6 shows that a deficiency of oil having a viscosity of 50 mPa.s at 25[deg]C does not make it possible to achieve an industrially viable working time, it being necessary for this time to be at least 70 minutes. Experiments C7 and C8 show that not all diorganotin carboxylic acid salts can be used. EXAMPLE 2 30 parts of a diatomaceous silica having a mean particle diameter of 3 microns are gradually

having a viscosity of 15,000 mPa.s at 25[deg]C and having 0.4% of methylvinylsiloxy units. The contents of the malaxator are then heated for 2 hours at 1 15[deg]C. When the temperature of the mixture has dropped to about 25[deg]C, 3 parts of a 50/50 by weight mixture consisting of ethyl silicate and methyltrimethoxysilane, 1 part of water and 7 parts of an a,co-dihydroxydimethylpolysiloxane oil having a viscosity of 80 mPa.s at 25[deg]C are introduced into the malaxator.

siloxane oil having a viscosity of 10,000 mPa.s at 25[deg]C 20 parts of diatomaceous silica having a mean particle diameter of 3 microns and 20 parts of a tin derivative having a melting point of 5[deg]C and the

average of 14.6 carbon atoms, are added to the resulting mixture. The composition catalysed in this way is kept for 10 minutes under a pressure of 4,000 Pa in order to

siloxane oil having a viscosity of 80 mPa.s at 25[deg]C. Its viscosity is 22,000 mPa.s at 25[deg]C and its working time is 1 hour 40 minutes. With this catalysed composition, a mould is manufactured from silicone elastomer by following the procedure indicated in Example 1. Using this mould, blocks of polyurethane foams are then manufactured by casting the foam mixture also described in Example 1. The operation is stopped when it is observed that sticking is starting to occur between the inner wall of the mould and the surface of the blocks of foam; 202 blocks of foam are manufactured under these conditions.

Claims (11)

1. A hardenable organopolysiloxane composition which comprises, by weight:

mPa.s at 25[deg]C, in which the organic radicals bonded to the silicon atoms are methyl, ethyl, n-propyl, vinyl and phenyl radicals, at least 80% of them being methyl radicals and at most 5% being vinyl radicals, B. 1.5 to 12 parts of a crosslinking agent which is; (i) a silane of the formula RaSi(OR')4_a, in which R represents a hydrocarbon radical of 1 to 8 carbon atoms optionally containing halogen, or a cyanoalkyl radical having from 3 or 4 carbon atoms; each R', which is identical or different, represents an alkyl radical having from 1 to 4 carbon atoms or a radical of the formula CH2CH2OG, in which G represents a methyl or ethyl radical; and a is zero or 1, or (ii) a product resulting from the partial hydrolysis of a silane of the formula Si(OR')4, in which R' is as defined above, C. 10 to 150 parts of pulverulent filter,

which the organic radicals bonded to the silicon atoms are methyl, vinyl and phenyl radicals, at least 50% of them being methyl radicals and at most 3% being vinyl radicals, and E. a catalyst of the formula (R"COO)2SnQ2' in which each R", which is identical or different, represents an aliphatic hydrocarbon radical having from 6 to 25 carbon atoms and each Q, which is identical or different, represents an alkyl radical having from 1 to 6 carbon atoms, the oil D being present in an amount from 7 to 23 parts per part of tin provided by the catalyst E, and the catalyst being present in an amount to provide from 0.22 to 1 part of tin per 100 parts of the composition.

2. A composition according to claim 1 in which the oil D is present in an amount from 8 to 21 parts per part of tin provided by the catalyst E, and the catalyst E is present in an amount to provide from 0.25 to 0.95 part of tin per 100 parts of the composition.

3. A composition according to claim 1 or 2 which comprises, by weight, per 100 parts of oil A, 1.7 to 10 parts of crosslinking agent B and 15 to 140 parts of filler C.

4. A composition according to any one of claims 1 to 3 in which the oil D has a viscosity of 15 to 300 mPa.s at 25[deg]C.

5. A composition according to any one of claims 1 to 4 in which the catalyst E is one specifically identified herein.

6. A composition according to claim 1 substantially as described in example 1 or 2 or part thereof.

7. A two component pack such that on mixing the two components a composition as claimed in any one of the preceding claims is obtained, the first component comprising constituents A, C and D and the second component comprising constituent E.

8. A pack according to claim 7 in which constituent C is in the first component.

9. Process for the preparation of a composition as claimed in any one of claims 1 to 6 which comprises a) preparing a mixture whch is stable on storage by mixing the oil A and the filler C and then introducing the crosslinking agent B and the oil D, and b) adding the catalyst E at the time of use.

10. A composition as defined in claim 1 whenever prepared by a process as claimed in claim 9.

11. A silicone elastomer mould obtained from a composition as claimed in any one of claims 1 to 6 and 10.