Siloxane-based elastomer-forming compositions comprise a first a,w-dihydroxyl polydiorganosiloxane, a second a,w-dihydroxyl polydiorganosiloxane, a third a,w-dihydroxyl polydiorganosiloxane, an organosilicon compound having at least three silicon-bonded substituents selected from the group consisting of hydrogen and alkoxy groups, and a condensation catalyst. The number average molecular weight ratio of the first a,w-dihydroxyl polydiorganosiloxane to the second a,w-dihydroxyl polydiorganosiloxane is in the range of 20 to 80 and the number average molecular weight ratio of second a,w-dihydroxyl polydiorganosiloxane to the third a,w-dihydroxyl polydiorganosiloxane is in the range of 1 to 6. The molar ratio of the first a,w-dihydroxyl polydiorganosiloxane to the second a,w-dihydroxyl polydiorganosiloxane is in the range of 0.01 to 0.15 and the molar ratio of the second a,w-dihydroxyl polydiorganosiloxane to the third a,w-dihydroxyl polydiorganosiloxane is in the range of 0.2 to 2. These compositions are substantially solvent-free and result in elasomers having improved strength without fillers.

The present invention relates to elastomer-forming compositions, more 
specifically siloxane-based elastomer-forming compositions which are 
substantially free from organic solvents and which show improved strength 
without the use of reinforcing filler particles. 
In some applications it is important that siloxane elastomeric materials 
are provided, which have a good mechanical strength, but which do not have 
a high viscosity prior to curing to the elastomeric phase. Systems have 
been provided which have a relatively low viscosity before curing, and 
give a high strength when cured to elastomeric form, but they tend to use 
organic solvent materials to reduce the inherent viscosity of 
elastomer-forming siloxane compositions which have been formulated with 
reinforcing filler particles. There is a need to provide compositions 
which achieve a low viscosity prior to curing to an elastomer form, 
without the use of organic solvents, which are environmentally 
undesirable, but which retain good mechanical properties. 
Some systems have been suggested by Madkour and Mark in Polymer Bulletin 
31, 615-621, 1994 and in Macromolecular Reports A31, 153-160, 1994. 
Suggested systems include what are referred to a bimodal systems, in which 
a mixture of functionally-terminated polydimethyl siloxanes were used and 
end-linked. Trimodal systems were also prepared and tested (using three 
specified different molecular weight siloxane polymers), but the authors 
state that although "changing from a unimodal distribution to a bimodal 
distribution significantly improves mechanical properties, changing from a 
bimodal distribution to a trimodal distribution does not give a further 
improvement in properties and may actually be detrimental". 
We have now found that by carefully selecting the components of a trimodal 
distribution and their ratios, substantial improvements can be obtained. 
According to the invention there is provided a siloxane-based 
elastomer-forming composition comprising (A) a first 
.alpha.,.omega.-dihydroxyl polydiorganosiloxane, (B) a second 
.alpha.,.omega.-dihydroxyl polydiorganosiloxane, (C) a third 
.alpha.,.omega.-dihydroxyl polydiorganosiloxane, (D) an organosilicon 
compound having at least three silicon-bonded hydrogen or alkoxy groups 
and (E) a condensation catalyst, characterized in that the number average 
molecular weight ratio of A/B is in the range of 20 to 80, the number 
average molecular weight ratio of B/C is in the range of 1 to 6, the molar 
ratio of A/B is in the range of 0.01 to 0.15 and the molar ratio of B/C is 
in the range of 0.2 to 2. 
Each of the components of the compositions according to the invention is 
known and commercially available, and will be described in more detail 
below. 
Components (A), (B) and (C) are .alpha.,.omega.-dihydroxyl 
polydiorganosiloxanes, which are substantially linear materials having an 
average general formula (I) 
##STR1## 
wherein R denotes an organic group, preferably a hydrocarbon group, more 
preferably an alkyl group or an aryl group, e.g. methyl, ethyl, n-propyl, 
isobutyl, n-hexyl, n-octodecyl, and n is an integer. R may be saturated or 
unsaturated, aliphatic or aromatic, but is preferably a group having up to 
8 carbon atoms, most preferably no more than 3 carbon atoms. It is 
particularly preferred that at least 80% of all R groups are alkyl groups, 
preferably methyl groups. 
Most preferred are .alpha.,.omega.-dihydroxyl polydimethyl siloxanes. Such 
siloxane materials are well known and are commercially available with a 
wide range of chain lengths and molecular weights. The actual molecular 
weight of the materials used is not most important. The ratio of molecular 
weight of the different components (A), (B) and (C) is however very 
important. The ratio of number average molecular weight of A/B is from 20 
to 80, preferably 30 to 50, and of B/C is from 1 to 6, preferably 2 to 4. 
From the conditions indicated above, it is clear that component (A) will 
have the highest molecular weight, and component (C) the lowest, thus 
making (A) the longest polymer and (C) the shortest. It is, however, 
preferred that component (A) has a number average molecular weight of from 
20,000 to 500,000, preferably 50,000 to 200,000, and that component (C) 
has a number average molecular weight of from 200 to 3000, preferably 500 
to 2500. 
Apart from the importance of the ratios of number average molecular weight 
of the polymers used as components (A) to (C), it is also important that 
the components (A), (B) and (C) are used in the correct molar proportions. 
The number of moles used should reflect a ratio of A/B of from 0.01 to 
0.15, preferably 0.01 to 0.1, more preferably 0.02 to 0.06, and a ratio of 
B/C of from 0.2 to 2, preferably 0.5 to 1.5. Values outside the specified 
ratios tend to give elastomeric siloxane materials which have lower 
mechanical strength. 
Component (D) is an organosilicon compound which functions as a 
cross-linker for the components (A) to (C) during the curing process, 
causing a three-dimensional network to form upon curing of the 
compositions according to the invention. In order to be active as 
cross-linker, it is important that component (D) has at least three sites 
per molecule in which it can react with any one of components (A) to (C). 
The curing mechanism of elastomer-forming compositions comprising 
.alpha.,.omega.-dihydroxyl polydiorgano-siloxanes, is preferably a 
condensation reaction, in which the silanol group of components (A) to (C) 
react with silicon-bonded hydrogen atoms, releasing hydrogen gas, or with 
silicon-bonded alkoxy groups, releasing alcohols. Accordingly component 
(D) has either at least three silicon-bonded hydrogen atoms per molecule 
or three silicon-bonded alkoxy groups. Component (D) may be silanes or 
siloxanes and may be a linear, branched or resinous compound. 
Where component (D) has silicone-bonded hydrogen atoms, it is preferable 
that component (D) has the average formula (II) 
##STR2## 
wherein R' denotes a hydrocarbon group having up to 8 carbon atoms, R" 
denotes a hydrogen atom or a group R', provided at least three R" groups 
are hydrogen atoms, a has a value of 0 or an integer and b is an integer 
with a value of at least 1. It is more preferred that component (D) has at 
least three silicon-bonded alkoxy groups. The most preferred components 
(D) are either silanes or small resinous siloxanes. Preferred silanes have 
the general formula R'Si(OR').sub.3 or Si(OR').sub.4, wherein R' is as 
defined above, preferably alkyl having up to 4 carbon atoms. Examples of 
suitable silanes include methyltrimethoxy silane, phenyl trimethoxy 
silane, ethyl tripropoxy silane, hexyl trimethoxy silane, tetra ethoxy 
silane and tetra n-propoxy silane. Alternatively, but less preferred than 
silanes, component (D) may be a resinous siloxane, preferably consisting 
of monovalent siloxane units of the formula R".sub.3 SiO.sub.1/2 and 
tetravalent siloxane units of the formula SiO.sub.4/2, preferably in a 
ratio of from 0.6/1 to 1.5/1. 
The amount of component (D) used has to be sufficient to provide sufficient 
cross-linking to give the elastomer formed sufficient mechanical strength, 
while maintaining sufficient flexibility. It is preferred that the ratio 
of silicon-bonded hydrogen or alkoxy groups of component (D) to silanol 
groups in components (A), (B) and (C) combined is in the range of from 1/1 
to 10/1. For compositions wherein component (D) is a silicon-bonded 
hydrogen containing compound, the ratio is preferably from 1/1 to 5/1, 
most preferably 1/1 to 3/1. For compositions where component (D) has 
silicon-bonded alkoxy groups, the ratio is preferably from 2/1 to 10/1. 
Component (D) may also comprise more than one organosilicon compound 
useful as cross-linker for the compositions according to the invention. It 
has been found that some combinations of different compounds may give 
improvements in mechanical strength, particularly tear-strength. One such 
combination is the use of a tetra-alkoxy silane with a trialkoxy silane in 
a ratio of from 2:1 to 1:2, e.g. tetraethoxy silane and phenyl trimethoxy 
silane. 
Component (E) is a catalyst which encourages the condensation of components 
(A), (B) and (C) with (D). Any suitable condensation catalysts may be 
used, preferably metal salts of carboxylic acid, e.g. tin or lead salts. 
Particularly suitable condensation catalysts are tin or lead salts of 
octoates or acetates, for example dibutyltin dioctoate, dibutyltin 
dilaurate, dibutyltin di-ethyl-hexoate, dibutyltin diacetate, lead 
octoate, lead ethyl-hexoate and lead acetate. 
Catalysts may be provided as 100% solid materials or may be diluted in 
suitable solvents or dispersed in suitable dispersion media. 
It is preferred to use from 1 to 4% by weight of the catalyst, based on the 
weight of the total composition, preferably 1.2 to 2%, or from 0.001 to 
0.1 molar parts by weight of the actual metal per 100 g of the 
composition, preferably 0.002 to 0.05 molar parts. Levels as indicated 
will ensure a good cure, while still maintaining a reasonable pot life of 
the compositions according to the invention. Particularly preferred for 
improved pot life are mixtures of tin and lead salts, preferably in a 
50/50 weight ratio of the active catalyst species. 
Optionally additional ingredients may also be incorporated in compositions 
according to the invention. One such ingredient is (F) an organosilicon 
compound having at least three silicon-bonded hydrogen atoms. Although 
such ingredient is suitable as cross-linking component (D), it may be 
added in addition to component (D) at a level of 1 to 2% by weight based 
on the total weight of components (A) to (D). The organosilicon compound 
(F) will act as a curing accelerator, whichever cross-linking component 
(D) is used. Preferably the organosilicon compound (F) is added where 
component (D) is an organosilicon compound having at least three 
silicon-bonded alkoxy groups. 
Where component (D) has silicon-bonded alkoxy groups, it is also beneficial 
to add component (G), which is an organosilicon resin consisting only of 
monovalent siloxane units of the formula R.sup.a.sub.3 SiO.sub.1/2 and 
tetravalent siloxane units of the formula SiO.sub.4/2 in a ratio of from 
0.4/1 to 1.2/1, preferably 0.6/1 to 0.9/1 wherein R.sup.a is an alkyl 
group, preferably having up to 6 carbon atoms, most preferably methyl, or 
a hydroxyl group, provided no more than about 5% by weight hydroxyl groups 
are present on the resin molecule, preferably from 2 to 3% by weight. 
Component (G) is usually a solid material, and is preferably provided in a 
suitable solvent, e.g. xylene or toluene, for example as a 50 to 800 
solids solution. Where this component (G) is used at high levels, i.e. 
above 10% by weight based on the total weight of components (A) to (D), 
preferably above 15%, an improvement in tear strength is also observed. 
Other optional ingredients include adhesion promoters, e.g. 
amino-functional or epoxy-functional organosilicon compounds, colorants, 
dyes, preservatives, cure inhibitors, fillers, which may be strengthening 
or non-strengthening fillers or solvents. However, it is preferred to 
leave out solvents or strengthening fillers, as the invention is 
particularly concerned with compositions without such ingredients. 
Compositions according to the present invention may be prepared by any 
convenient method, e.g. by mixing of all the ingredients till a 
homogeneous mixture is obtained. In order to provide a storage stable 
composition, it is preferred that the ingredients are mixed in two parts, 
whereby the catalyst (E) is kept separated from the cross-linking 
component (D), preferably in sealed containers which do not allow ingress 
of moisture. One suitable way of packing compositions according to the 
invention is by incorporating part of the mixture of components (A), (B) 
and (C) with component (E) into a first part and the remainder of the 
mixture of (A), (B) and (C) with component (D) in a second part, so that 
mixing parts 1 and 2 in a 1/1 ratio will provide an elastomer forming 
composition according to the invention. Alternatively component (E) is 
kept separately and is added as a second part in the desired ratio to the 
rest of the composition in a first part. 
Compositions according to the invention may be cured to elastomers by 
exposing them to moisture e.g. to air at ambient temperatures. 
Cure of the compositions can be accelerated by heating the compositions 
when applied to appropriate substrates. 
Compositions according to the present invention are particularly useful as 
sealing compounds, e.g. conformal coatings for electronic application or 
intricate moulding materials. These uses are particularly of interest as 
the initial viscosity of the compositions can be kept relatively low, 
allowing easy flow of uncured materials. Viscosities could be as low as 
100 mPa.s at 25.degree. C., and are preferably kept below 50,000 mPa.s. 
Particularly useful viscosities are in the range of 500 to 10,000 mPa.s at 
25.degree. C. 
There now follow a number of examples and comparative examples which 
demonstrate the benefits of the compositions according to the invention. 
All parts and percentages are by weight, unless otherwise indicated. All 
viscosities are given at 25.degree. C. 
In the examples the following compounds were used as ingredients of the 
compositions. Ingredient (1) was a high viscosity 
.alpha.,.omega.-dihydroxy polydimethyl siloxane having a number average 
molecular weight of 100,000 and a viscosity of 50,000 mPa.s. Ingredient 
(2) was a high viscosity .alpha.,.omega.-dihydroxy polydimethyl siloxane 
having a number average molecular weight of 146,000 and a viscosity of 
360,000 mPa.s. Ingredient (3) was a high viscosity 
.alpha.,.omega.-dihydroxy polydimethyl siloxane having a number average 
molecular weight of 71,000 and a viscosity of 13,500 mPa.s. Ingredient (4) 
was a high viscosity .alpha.,.omega.-dihydroxy polydimethyl siloxane 
having a number average molecular weight of 62,000 and a viscosity of 
10,000 mPa.s. Ingredient (5) was an .alpha.,.omega.-dihydroxy polydimethyl 
siloxane having a number average molecular weight of 4,600 and a viscosity 
of 70 mPa.s. Ingredient (6) was an .alpha.,.omega.-dihydroxy polydimethyl 
siloxane having a number average molecular weight of 38,500 and a 
viscosity of 2000 mPa.s. 
Ingredient (7) was an .alpha.,.omega.-dihydroxy polydimethyl siloxane 
having a number average molecular weight of 2000 and a viscosity of 40 
mPa.s. Ingredient (8) was an .alpha.,.omega.-dihydroxy polydimethyl 
siloxane having a number average molecular weight of 400 and a viscosity 
of 20 mPa.s. Ingredient (9) was tetraethoxy silane. Ingredient (10) was 
phenyltrimethoxy silane. Ingredient (11) was a trimethylsiloxane 
end-blocked polymethylhydro siloxane having a viscosity of about 30 mPa.s. 
Ingredient (12) was a trimethylsiloxane end-blocked polymethylhydro 
siloxane polydimethyl siloxane copolymer having a viscosity of about 5 
mPa.s and 0.7% hydrogen. Ingredient (13) was a 50/50 mixture of dibutyltin 
di-2-ethylhexoate and lead-2-ethylhexoate with some organic solvent. 
Ingredient (14) is an organosilicon resin consisting only of monovalent 
siloxane units of the formula Me.sub.3 SiO.sub.1/2 and tetravalent 
siloxane units of the formula SiO.sub.4/2 in a ratio of from 0.6/1 to 
0.9/1 wherein Me denotes a methyl group.