Blowing agent compositions and compositions curable to give elastomeric silicone foams

Blowing agent compositions (a) comprising aqueous emulsions containing organopolysiloxanes (1), emulsifiers (2), water (3) and thickeners (4) which may be used in the preparation of elastomeric silicone foams. The blowing agent compositions are used in curable compositions to form elastomeric silicone foams which comprise the blowing agent compositions (a), diorganopolysiloxanes (b), crosslinking agents (c) and, if appropriate, crosslinking catalysts (d) and optionally fillers (e). Elastomeric silicone foams are prepared by mixing the blowing agent compositions (a), with curable organopolysiloxane compositions containing diorganopolysiloxanes (b), crosslinking agents (c) and, if appropriate, crosslinking catalysts (d) and optionally fillers (e) and also optionally additional substances and then the resultant mixture is cured at temperatures in the range of from 100.degree. to 250.degree. C. with simultaneous foaming to form the elastomeric silicone foams.

The invention relates to blowing agent compositions for the preparation of 
elastomeric silicone foams, processes for their preparation, compositions 
which are curable to give elastomeric silicone foams and processes for the 
preparation of the elastomeric silicone foams. 
BACKGROUND OF THE INVENTION 
Compositions which are curable to give elastomeric silicone foams are known 
in the art. In order to prepare foams of this type, curable compositions 
are treated with blowing agents which decompose when heated with the 
evolution of gas and thus effect foaming of the compositions during the 
curing reaction. In this regard, reference may be made to, for example, 
U.S. Pat. No. 2,857,343, U.S. Pat. No. 5,019,295 and GB-A 1,130,674. 
In the curing reaction based on diorganopolysiloxanes containing Si-bonded 
vinyl groups and diorganopolysiloxanes containing Si-bonded hydrogen 
atoms, which reaction is catalyzed by platinum or platinum compounds, 
hydrogen is evolved as the blowing agent gas, which facilitates foaming, 
by the addition of water, organic alcohols or diorganopolysiloxanes 
containing Si-bonded hydroxyl groups. In this context, reference may be 
made to U.S. Pat. No. 4,189,545, U.S. Pat. No. 4,613,630, U.S. Pat. No. 
4,871,781 and EP-B 227 233. 
According to EP-B 97 915 and U.S. Pat. No. 4,391,765 an elastomeric foam is 
obtained by mechanical production of a foam from an aqueous, reactive 
silicone emulsion, adding surfactants and thickeners, in order to render 
the foam stable until the water is removed, and subsequent drying of the 
foam in an oven or in air, or subsequent exposure of the foam to an amount 
of microwave energy sufficient to remove water. 
U.S. Pat. No. 4,584,324 discloses a pressurized composition comprising an 
aqueous emulsion which contains a crosslinking silicone polymer, 
emulsifier, water and optionally filler, crosslinking agent and thickener 
and which on drying at room temperature cures to give an elastomeric film 
and sufficient aerosol blowing agent, such as nitrogen, nitrogen oxide, 
isobutane, propane, dichlorodifluoromethane or trichlorofluoromethane, to 
convert the composition into a foam if it is reduced to atmospheric 
pressure at 25.degree. C. The resultant foam yields an elastomeric foam 
after removal of water. The object of the invention was to provide 
water-based blowing agent compositions suitable for the preparation of 
elastomeric silicone foams. A further object was to provide compositions 
which are curable to give elastomeric silicone foams and which do not 
contain any toxic blowing agents or blowing agents liberating toxic 
decomposition products. 
SUMMARY OF THE INVENTION 
The foregoing objects and others which will become apparent from the 
following description are accomplished in accordance with this invention, 
generally speaking, by providing a blowing agent composition comprising 
(a) aqueous emulsions containing organopolysiloxanes (1), emulsifiers (2), 
water (3) and thickener (4) which may be used in preparing elastomeric 
silicone foams. 
The invention also relates to a process for preparing blowing agent 
compositions (a), in which the aqueous emulsions containing 
organopolysiloxane (1), emulsifiers (2) and water (3) are mixed with 
thickeners (4). 
Also the invention relates to compositions which are curable to elastomeric 
silicone comprising the blowing agent composition (a), 
diorganopolysiloxanes (b), crosslinking agents (c), if appropriate, 
crosslinking catalyst (d) and, if appropriate, fillers (e). 
The invention also relates to a process for preparing elastomeric silicone 
foams, which comprises mixing the blowing agent composition (a), 
diorganopolysiloxanes (b), crosslinking agents (c), if appropriate, 
crosslinking catalyst (d) and, if appropriate, fillers (e) and also 
optionally additional substances together and thereafter curing the 
mixtures at a temperature of from 100.degree. to 250.degree. C. with 
simultaneous foaming to form the elastomeric silicone foams. 
DESCRIPTION OF THE INVENTION 
The blowing agent in the blowing agent compositions of this invention is 
water. Water evaporates under the action of heat or microwave energy 
during the curing reaction, expands and effects foaming of the 
compositions during the curing reaction. The gas evolved, water vapor, has 
the advantage of being nontoxic, nonflammable and odorless. In addition, 
an extremely fine dispersion of water is achieved by means of the blowing 
agent composition of this invention. 
The blowing agent compositions (a) of this invention preferably have a 
viscosity of from 1.times.10.sup.5 mPa.s to 1.times.10.sup.8 mPa.s at 
25.degree. C., and more preferably from 5.times.10.sup.5 mPa.s to 
5.times.10.sup.7 mPa.s at 25.degree. C. The aqueous emulsions used in the 
preparation of the blowing agent compositions (a) of this invention can be 
any desired silicone emulsion. 
The organopolysiloxanes (1) preferably used are those composed of units of 
the formula 
##EQU1## 
in which R represents the same or different monovalent hydrocarbon radical 
having from 1 to 18 carbon atoms per molecule or a monovalent substituted 
hydrocarbon radical having 1 to 18 carbon atoms per radical, the R.sup.1 
represents the same or different and represents hydrogen or a monovalent 
hydrocarbon radical having from 1 to 8 carbon atoms per radical or a 
monovalent substituted hydrocarbon radical having 1 to 8 carbon atoms per 
radical, x is 0, 1, 2 or 3, y is 0, 1, 2 or 3 and the sum x+y is 0, 1, 2 
or 3. 
Organopolysiloxanes (1) which are liquid or solid at room temperature can 
be used. 
The organopolysiloxanes (1) are preferably straight-chain 
organopolysiloxanes of the general formula 
EQU (R.sup.1 O).sub.t R.sub.3-t SiO(SiR.sub.2 O).sub.r SiR.sub.3-t 
(OR.sup.1).sub.t (II), 
in which R and R.sup.1 are the same as above, r is an integer having a 
value of from 50 to 2000, and t is 0 or 1, or cyclic organopolysiloxanes 
of the general formula 
EQU (R.sub.2 SiO).sub.s (III), 
in which R is the same as above and s is an integer having a value of from 
3 to 100, or organopolysiloxane resins of the general formula 
##EQU2## 
in which R and R.sup.1 are the same as above, a is 0, 1, 2 or 3, with an 
average of from 0.9 to 1.8, and b is 0, 1, 2 or 3, with an average of from 
0.0 to 0.5. 
Although not shown in formula (II), up to 10 mol % of the 
diorganopolysiloxane units can be replaced by other siloxane units, such 
as R.sub.3 SiO.sub.1/2, RSiO.sub.3/2 and SiO.sub.4/2 units, in which R is 
the same as above; but usually only by siloxane units present with 
impurities which are more or less difficult to avoid. 
The organopolysiloxane of formula (IV) is preferably an organopolysiloxane 
composed of RSiO.sub.3/2 units or an organopolysiloxane composed of 
RSiO.sub.3/2 and R.sub.2 SiO units or an organopolysiloxane composed of 
R.sub.3 SiO.sub.1/2 and SiO.sub.4/2 units as well as, optionally, R.sub.2 
SiO units, where R is the same as above. 
Examples of radicals represented by R are alkyl radicals, such as the 
methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, 
n-pentyl, iso-pentyl, neo-pentyl and tert-pentyl radicals; hexyl radicals, 
such as the n-hexyl radical; heptyl radicals, such as the n-heptyl 
radical; octyl radicals, such as the n-octyl radical and isooctyl 
radicals, such as the 2,2,4-tri-methylpentyl radical; nonyl radicals, such 
as the n-nonyl radical; decyl radicals, such as the n-decyl radical; 
dodecyl radicals, such as the n-dodecyl radical; octadecyl radicals, such 
as the n-octadecyl radical; alkenyl radicals, such as the vinyl and the 
allyl radicals; cycloalkyl radicals, such as cyclopentyl, cyclohexyl and 
cycloheptyl radicals and methylcyclohexyl radicals; aryl radicals, such as 
the phenyl, naphthyl and anthryl and phenanthryl radicals; alkaryl 
radicals, such as o-, m- and p-tolyl radicals, xylyl radicals and 
ethylphenyl radicals; and aralkyl radicals, such as the benzyl radical and 
the .alpha.- and the .beta.-phenylethyl radicals. 
Examples of substituted radicals represented by R are cyanoalkyl radicals, 
such as the .beta.-cyanoethyl radical; and halogenated hydrocarbon 
radicals, for example halogenoalkyl radicals, such as the 
3-chloro-n-propyl radical, the chloromethyl radical, the 
3,3,3-trifluoro-n-propyl radical, the 2,2,2,2', 2', 2'-hexafluoroisopropyl 
radical, the heptafluoroisopropyl radical and the perfluorohexylethyl 
radical, halogenoaryl radicals, such as the o-, m- and p-chlorophenyl 
radicals, and the tetrafluoroethyloxypropyl radical. 
Examples of radicals represented by R.sup.1 are alkyl radicals, such as the 
methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl and tert-butyl 
radicals. 
Examples of substituted radicals represented by R.sup.1 are the 
methoxyethyl radical and the ethoxyethyl radical. 
Particularly preferred organopolysiloxanes (1) are straight-chain 
diorganopolysiloxanes of formula (II), and more preferably 
dimethylpolysiloxanes which have a viscosity of from 100 to 100,000 mPa.s 
at 25.degree. C. 
It is possible to use one type of organopolysiloxane (1) or a mixture of at 
least two different types of organopolysiloxanes (1). 
In order to prepare the aqueous emulsions used in the preparation of the 
blowing agent compositions (a) of this invention, organopolysiloxanes (1) 
are emulsified with water and the addition of emulsifiers by the methods 
generally employed in silicone chemistry. Depending on its chemical 
nature, the emulsifier can be initially introduced either into the water 
phase or into the oil phase. The emulsifying step can be carried out in 
conventional mixing equipment suitable for the preparation of emulsions, 
such as high-speed stator-rotor stirrers according to Prof. P. Willems, 
such as are known under the registered trade name "Ultra-Turrax". 
If solid organopolysiloxanes, and in particular those of formula (IV), are 
used for the preparation of the aqueous emulsions at room temperature, the 
emulsions are preferably prepared by the procedure described in U.S. Pat. 
No. 5,039,724. According to this procedure, organopolysiloxanes which are 
solid at room temperature are dissolved in low molecular weight 
organopolysiloxanes which are liquid at room temperature and these 
solutions are emulsified with water with the addition of emulsifiers and 
optionally additional substances. The dissolution of the 
organopolysiloxanes which are solid at room temperature in the 
organopolysiloxanes which are liquid at room temperature and the 
emulsifying step can be carried out in conventional mixing equipment 
suitable for the preparation of emulsions. 
The emulsifiers (2) used to prepare the aqueous emulsions can be all the 
ionic and nonionic emulsifiers described heretofore, both individually and 
in the form of mixtures of different emulsifiers, with which it has been 
possible heretofore to prepare stable aqueous emulsions of 
organopolysiloxanes. Those emulsifiers which are described in U.S. Pat. 
No. 4,757,106 can also be used. Nonionic or anionic emulsifiers or 
mixtures of nonionic and anionic emulsifiers are preferably used. Nonionic 
emulsifiers preferably used are fatty alcohol polyglycol ethers or 
partially saponified polyvinyl alcohols. Fatty alcohol polyglycol ethers 
are available, for example, under the trade names "Arlypon SA4" or 
"Arlypon IT16" from Grunau and partially saponified polyvinyl alcohols are 
available, for example, under the trade name "Polyviol W25/140" from 
Wacker. The anionic emulsifiers used are alkyl sulfates, 
alkylbenzenesulfonates or alkylsulfonates. 
The thickeners (4) used are preferably silicon dioxide which has a BET 
surface area of at least 50 m.sup.2 /g, preferably 50 to 400 m.sup.2 /g 
and more preferably from 100 to 200 m.sup.2 /g, such as silicas prepared 
pyrogenically or precipitated silicas, which optionally are rendered 
completely or partially hydrophobic. Further examples of thickeners (4) 
are highly branched polyacrylic acids having a viscosity of from 5000 to 
50,000 mPa.s at 25.degree. C., for example those available under the trade 
name "Carbopol" from B.F. Goodrich Chemical, cellulose ethers having a 
viscosity of from 5000 to 50,000 mPa.s at 25.degree. C., such as 
hydroxyethylcellulose and carboxymethylcellulose, for example those 
available under the trade name "Tylose" from Hoechst, and xanthans having 
a viscosity of from 5000 to 50,000 mPa.s at 25.degree. C., for example 
those available under the trade name "Kelzan" from G. M. Langer. 
The thickeners are preferably uniformly dispersed in the form of powder or 
granules by continuously stirring into the aqueous silicone emulsions 
comprising organopolysiloxanes (1), emulsifiers (2) and water (3). Mixing 
is generally carried out at a temperature of preferably from 5.degree. to 
40.degree. C., and more preferably from 15.degree. to 30.degree. C., and 
under a pressure of preferably from 800 to 1200 hPa, and more preferably 
from 1000 to 1020 hPa. Mixing can be carried out in any desired vessels 
and in any desired sequence, preferably in mixing equipment without a high 
shear effect. 
The blowing agent composition (a) preferably contains from 10 to 50% by 
weight, and more preferably from 15 to 40% by weight, of 
organopolysiloxanes (1), from 1 to 7% by weight, and more preferably from 
2 to 6% by weight, of emulsifiers (2), from 20 to 80% by weight, and more 
preferably from 30 to 70% by weight, of water (3) and from 5 to 40% by 
weight, and more preferably from 10 to 20% by weight, of thickener (4). 
The diorganopolysiloxanes (b) used in the compositions of this invention 
can be all the diorganopolysiloxanes which have been or could have been 
cured by means of free radical formation or by adding Si-bonded hydrogen 
onto Si-bonded alkenyl groups to form elastomeric foams. 
The diorganopolysiloxanes (b) used in the compositions of this invention 
are preferably those of the general formula 
EQU Z.sub.n R.sup.2.sub.3-n SiO(R.sup.2.sub.2 SiO).sub.m SiR.sup.2.sub.3-n 
Z.sub.n (V), 
in which R.sup.2 represents the same or different monovalent hydrocarbon 
radical having from 1 to 18 carbon atoms per radical or a monovalent 
substituted hydrocarbon radical having 1 to 18 carbon atoms per radical, Z 
is a hydroxyl group, n is 0 or 1, and m is an integer which has a value 
such that the average viscosity of the diorganopolysiloxanes is from 
1.times.10.sup.6 to 1.times.10.sup.9 mPa.s at 25.degree. C., and more 
preferably from 5.times.10.sup.6 to 1.times.10.sup.8 mPa.s at 25.degree. 
C. 
In addition to the diorganosiloxane units (SiR.sup.2.sub.2 O), other 
siloxane units can be present within or along the siloxane chains of 
formula (V) shown above; although these are generally not shown in such 
formulas. Examples of such other siloxane units, which usually are present 
only as impurities, are those of the formulas R.sup.2 SiO.sub.3/2, 
R.sup.2.sub.3 SiO.sub.1/2 and SiO.sub.4/2, in which R.sup.2 is the same as 
above. However, the amount of such other siloxane units other than the 
diorganosiloxane units is preferably at most 10 mol %, and in particular 
at most 1 mol %, based on the weight of diorganopolysiloxane (1). 
Examples of radicals represented by R.sup.2 are alkyl radicals, such as the 
methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, 
n-pentyl, iso-pentyl, neo-pentyl and tert-pentyl radicals; hexyl radicals, 
such as the n-hexyl radical; heptyl radicals, such as the n-heptyl 
radical; octyl radicals, such as the n-octyl radical and isooctyl 
radicals, such as the 2,2,4-trimethylpentyl radical; nonyl radicals, such 
as the n-nonyl radical; decyl radicals, such as the n-decyl radical; 
dodecyl radicals, such as the n-dodecyl radical; octadecyl radicals, such 
as the n-octadecyl radical; alkenyl radicals, such as the vinyl and the 
allyl radicals; cycloalkyl radicals, such as cyclopentyl, cyclohexyl and 
cycloheptyl radicals and methylcyclohexyl radicals; aryl radicals, such as 
the phenyl, naphthyl and anthryl and phenanthryl radicals; alkaryl 
radicals, such as o-, m- and p-tolyl radicals, xylyl radicals and 
ethylphenyl radicals; and aralkyl radicals, such as the benzyl radical and 
the .alpha.- and the .beta.-phenylethyl radicals. 
Examples of substituted radicals represented by R.sup.2 are cyanoalkyl 
radicals, such as the .beta.-cyanoethyl radical; and halogenated 
hydrocarbon radicals, for example halogenoalkyl radicals, such as the 
3-chloro-n-propyl radical, the chloromethyl radical, the 
3,3,3-trifluoro-n-propyl radical, the 2,2,2,2',2',2'-hexafluoroisopropyl 
radical, the heptafluoroisopropyl radical and the perfluorohexylethyl 
radical, halogenoaryl radicals, such as the o-, m- and p-chlorophenyl 
radicals. 
It is preferred, because of their availability, that the majority of the 
radicals R.sup.2 be methyl radicals. The other radicals which may 
optionally be present are preferably vinyl radicals. If the compositions 
are curable by free radical formation to form elastomers, the 
diorganopolysiloxanes (b) then contain preferably from 0 to 3% by weight, 
and more preferably from 0 to 0.5% by weight, of Si-bonded vinyl groups. 
The vinyl groups optionally present can be located at the end of the chain 
in the two terminal units R.sup.2.sub.3 SiO.sub.1/2 and/or within the 
chain in the R.sup.2.sub.2 SiO units of formula (V) above. 
If n has a value of 0 and the compositions are curable by adding Si-bonded 
hydrogen onto Si-bonded alkenyl groups to give elastomers, at least two of 
the radicals R.sup.2 per molecule in the diorganopolysiloxanes (b) of 
formula (V) above must be alkenyl radicals, preferably vinyl radicals. The 
alkenyl radicals, preferably vinyl radicals, are preferably located in the 
two terminal units R.sup.2.sub.3 SiO.sub.1/2 and/or within the chain in 
the R.sup.2.sub.2 SiO units of formula (V) above. If the alkenyl radicals 
are vinyl radicals, the diorganopolysiloxanes (b) preferably contain from 
0.01 to 10% by weight, and more preferably from 0.03 to 5% by weight, of 
vinyl groups. 
It is possible to use one type of diorganopolysiloxane (b) or it is 
possible to use a mixture of at least two different types of 
diorganopolysiloxanes (b). 
Blowing agent compositions (a) are preferably used in the compositions of 
this invention in amounts of from 0.5 to 8% by weight, and more preferably 
from 1 to 5% by weight, based on the total weight of diorganopolysiloxanes 
(b) used. 
If crosslinking of the compositions of this invention is effected by means 
of free radicals, the crosslinking agents (c) used are organic peroxides, 
which serve as the source of free radicals. Examples of organic peroxides 
are acyl peroxides, such as dibenzoyl peroxide, bis(4-chlorobenzoyl) 
peroxide, bis-(2,4-dichlorobenzoyl) peroxide and bis-(4-methylbenzoyl) 
peroxide; alkyl peroxides and aryl peroxides, such as di-tert-butyl 
peroxide, 2,5-bis-(tert-butylperoxy)-2,5-dimethylhexane, dicumyl peroxide 
and 1,3-bis-(tert-butylperoxy-isopropyl)-benzene; perketals, such as 
1,1-bis-(tert-butyl-peroxy)-3,3,5-trimethylcyclohexane; and peresters, 
such as diacetyl peroxydicarbonate, tert-butyl perbenzoate, tert-butyl 
peroxyisopropyl carbonate, tert-butyl peroxyisononanoate, dicyclohexyl 
peroxydicarbonate and 2,5-dimethylhexane 2,5-diperbenzoate. 
It is possible to use one type of organic peroxide (c) or it is possible to 
use a mixture of at least two different types of organic peroxides (c). 
Peroxides are preferably used in the compositions of this invention in 
amounts of from 1 to 5% by weight, and more preferably from 2 to 3.5% by 
weight, based on the total weight of the diorganopolysiloxanes (b) used. 
If crosslinking of the compositions of this invention is effected by adding 
Si-bonded hydrogen onto Si-bonded alkenyl groups in the presence of 
catalysts which promote this addition, the crosslinking agents (c) used 
are organopolysiloxanes containing at least two Si-bonded hydrogen atoms 
per molecule. Straight-chain, cyclic or branched organopolysiloxanes 
containing units of the general formula 
##EQU3## 
in which R.sup.2 is the same as above, k is 0, 1, 2 or 3, 1 is 0 or 1 and 
the sum of k+1 is 0, 1, 2 or 3, with the proviso that each molecule 
contains at least 2, and more preferably at least 3, Si-bonded hydrogen 
atoms per molecule are used. Organopolysiloxanes of the general formula 
EQU H.sub.g R.sup.2.sub.3-g SiO(R.sup.2 SiO).sub.o (R.sup.2 HSiO).sub.p 
SiR.sup.2.sub.3-g H.sub.g (VII), 
in which R.sup.2 is the same as above, g is 0 or 1, o is an integer and p 
is 0 or an integer, the sum of o+p being an integer which has a value such 
that the average viscosity of the organopolysiloxanes is preferably from 5 
to 1000 mPa.s at 25.degree. C., and more preferably from 10 to 500 mPa.s 
at 25.degree. C., with the proviso that each molecule contains at least 2, 
and more preferably at least 3, Si-bonded hydrogen atoms per molecule are 
used. The organopolysiloxanes having at least 2 Si-bonded hydrogen atoms 
preferably contain from 0.1 to 1.7% by weight, and more preferably from 
0.4 to 1.2% by weight, of Si-bonded hydrogen. 
Organopolysiloxanes containing at least 2 Si-bonded hydrogen atoms per 
molecule are preferably used in amounts of from 0.5 to 10, and more 
preferably from 1 to 5, gram atom of Si-bonded hydrogen per mol of 
Si-bonded vinyl groups in the diorganopolysiloxanes (b). 
The catalysts (d), which promote the addition of Si-bonded hydrogen onto 
Si-bonded alkenyl groups, which are used are preferably a metal from the 
group comprising the platinum metals or a compound or a complex thereof 
from the group comprising the platinum metals. Examples of such catalysts 
are platinum metal and finely divided platinum, which can be present on 
supports such as silicon dioxide, aluminum oxide or active charcoal, 
platinum compounds or complexes, such as platinum halides, for example 
PtCl.sub.4, H.sub.2 PtCl.sub.6.6H.sub.2 O and Na.sub.2 PtCl.sub.4.4H.sub.2 
O, platinum-olefin complexes, platinum-alcohol complexes, 
platinum-alcoholate complexes, platinum-ether complexes, platinum-aldehyde 
complexes, platinum-ketone complexes, including reaction products of 
H.sub.2 PtCl.sub.6.6H.sub.2 O and cyclohexanone, platinum-vinylsiloxane 
complexes, such as platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane 
complexes, which may be free from or contain detectable inorganically 
bonded halogen, bis(gamma-picoline)-platinum dichloride, 
trimethylenedipyridine-platinum dichloride, dicyclopentadiene-platinum 
dichloride, dimethyl sulfoxide ethyleneplatinum(II)dichloride, 
cyclooctadiene-platinum dichloride, norbornadiene-platinum dichloride, 
gamma-picoline-platinum dichloride, cyclopentadiene-platinum dichloride 
and also reaction products of platinum tetrachloride with olefin and 
primary amine or secondary amine or primary and secondary amines according 
to U.S. Pat. No. 4,292,434, such as the reaction product of platinum 
tetrachloride dissolved in 1-octene with sec-butylamine, or 
ammonium-platinum complexes according to EP-B 110 370. 
The catalyst (d) is preferably used in amounts of from 0.5 to 200 ppm by 
weight (parts per million), preferably in amounts of from 2 to 50 ppm by 
weight, calculated as elemental platinum and based on the total weight of 
organopolysiloxanes used. 
When the compositions are crosslinked by adding Si-bonded hydrogen onto 
Si-bonded alkenyl groups in the presence of catalysts which promote this 
addition, the compositions can contain inhibitors. Examples of inhibitors 
are 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, benzotriazole, 
dialkylformamides, alkylthioureas, methyl ethyl ketoxime, organic or 
organosilicon compounds which have a boiling point of at least 25.degree. 
C. at 1012 mbar (abs.) and at least one aliphatic triple bond according to 
U.S. Pat. No. 3,445,420, such as 1-ethynylcyclohexan-1-ol, 
2-methyl-3-butyn-2-ol, 3-methyl-1-pentyn-3-ol, 
2,5-dimethyl-3-hexyn-2,5-diol and 3,5-dimethyl-1-hexyn-3-ol, inhibitors 
according to U.S. Pat. No. 2,476,166, such as a mixture of diallyl maleate 
and vinyl acetate, and inhibitors according to U.S. Pat. No. 4,504,645, 
such as maleic acid monoesters. 
The compositions of this invention can contain both reinforcing and 
non-reinforcing fillers (e). 
Examples of reinforcing fillers (e), that is fillers having a BET surface 
area of at least 50 m.sup.2 /g are pyrogenically produced silicon 
dioxides, precipitated silicon dioxides, carbon black, such as furnace 
black and acetylene black, and silicon/aluminum mixed oxides having a 
large BET surface area; and fibrous fillers, such as asbestos, graphite 
fibers and synthetic fibers. 
Examples of non-reinforcing fillers, that is fillers which have a BET 
surface area of up to 50 m.sup.2 /g are quartz, diatomaceous earth, 
calcium silicate, zirconium silicate, zeolites, montmorillonites, such as 
bentonites, metal oxide powders, such as aluminum, magnesium, titanium, 
iron, zinc, manganese or cerium oxides and mixed oxides thereof, barium 
sulfate, calcium carbonate, gypsum, silicon nitride, silicon carbide, 
boron nitride, glass powders and plastic powders and mixtures of Teflon, 
graphite and carbon black. 
The fillers can be rendered hydrophobic, for example by treatment with 
organosilicon compounds, such as hexamethyldisilazane, organosilanes or 
organosiloxanes, or by etherification of hydroxyl groups to alkoxy groups. 
It is possible to use only one type of filler (e) or it is also possible 
to use a mixture of at least two types of fillers (e). 
Fillers (e) are preferably used in the compositions of this invention in 
amounts of from 10 to 120% by weight, and more preferably from 25 to 80% 
by weight, based on the total weight of the diorganopolysiloxanes. 
The compositions of this invention can also contain other additives which 
are generally used in preparing compositions which are crosslinkable to 
form elastomeric foams, such as plasticizers, for example 
diorganopolysiloxanes of the general formula 
EQU (R.sup.1 O).sub.u R.sup.2.sub.3-u SiO(SiR.sup.2.sub.2 O).sub.v 
SiR.sup.2.sub.3-u (OR.sup.1).sub.u (VIII), 
in which R.sup.1 and R.sup.2 are the same as above, u is 0 or 1 and v is an 
integer which has a value such that the average viscosity of the 
diorganopolysiloxanes is from 10 to 500 mPa.s at 25.degree. C., inorganic 
or organic pigments, antioxidants, heat stabilizers, agents for improving 
the electrical properties, antihydrolysis additives, so-called reversion 
stabilizers and flame-retardant additives. 
The individual constituents of the compositions of this invention can be 
mixed with one another in any desired manner, for example in stirrers, 
mixers, kneaders or roll mills. 
The blowing agent compositions (a) are stored separately from the other 
constituents. 
Moldings from the compositions of this invention can be prepared under 
atmospheric pressure by extrusion or calendering, for example with 
subsequent vulcanization in the form of a milled sheet, or under pressure 
by calendering, for example with subsequent vulcanization in an rotocure 
machine (continuous vulcanization under pressure between a steel drum and 
a steel belt), foam molding, such as transfer pressing or HTV injection 
molding, or production of a milled sheet with subsequent foam molding. 
If crosslinking of the compositions of this invention is effected by 
organic peroxides, curing is carried out with simultaneous foaming, 
preferably at temperatures of from 120.degree. to 250.degree. C. 
If crosslinking of the compositions of this invention is effected by 
organic peroxides and processing of the compositions of this invention is 
carried out at atmospheric pressure, the organic peroxides used are 
preferably dibenzoyl peroxide, bis-(4-chlorobenzoyl) peroxide, 
bis-(2,4-dichlorobenzoyl) peroxide or bis-(4-methylbenzoyl) peroxide and 
curing is carried out with simultaneous foaming, preferably at 
temperatures in the range of from 160.degree. to 230.degree. C. 
If crosslinking of the compositions of this invention is effected by 
organic peroxides and processing of the compositions of this invention is 
carried out under pressure, the organic peroxides preferably used are 
tert-butyl perbenzoate, dicumyl peroxide, tert-butyl peroxyisopropyl 
carbonate or 2,5-bis-(tert-butylperoxy)-2,5-dimethylhexane and curing is 
carried out with simultaneous foaming, preferably at temperatures in the 
range of from 130.degree. to 200.degree. C. 
If crosslinking of the compositions of this invention is effected by adding 
Si-bonded hydrogen onto Si-bonded alkenyl groups in the presence of 
catalysts which promote this addition, curing is carried out with 
simultaneous foaming at temperatures of preferably from 100.degree. to 
250.degree. C. When processing is carried out under atmospheric pressure, 
curing is preferably carried out at from 100.degree. to 230.degree. C. and 
when processing is carried out under pressure curing is carried out at 
from 130.degree. to 250.degree. C. 
The compositions of this invention can be used in the production of foamed 
round cords, foamed sealing profiles, foamed insulating tubes, foamed mats 
and cushions, foamed damping elements, foamed stoppers, foamed seals and 
foamed roller coatings. 
In the following examples all parts and percentages are by weight, unless 
otherwise specified. 
Preparation of blowing agent compositions A to M 
(A) About 16 parts of a hydrophilic, pyrogenic silica having a BET surface 
area of 150 m.sup.2 /g (commercially available under the trade name "HDK 
V15" from Wacker-Chemie) were stirred slowly into 100 parts of an 
oil-in-water emulsion which contained 35% by weight of a 
dimethylpolysiloxane end-blocked by trimethylsiloxy groups and having a 
viscosity of 350 mPa.s at 25.degree. C., 60% by weight of water and 5% by 
weight of a fatty alcohol polyglycol ether (commercially available under 
the trade name "Arlypon IT 10" from Grunau) until a homogeneous paste was 
formed. A blowing agent composition (A) was obtained. 
(B) About 18 parts of a hydrophilic, pyrogenically produced silica having a 
BET surface area of 150 m.sup.2 /g (commercially available under the trade 
name "HDK V15" from Wacker-Chemie) were stirred slowly into 100 parts of 
an oil-in-water emulsion, which contained 35% by weight of a 
dimethylpolysiloxane end-blocked by trimethylsiloxy groups and having a 
viscosity of 100,000 mPa.s at 25.degree. C., 59% by weight of water, 3% by 
weight of a fatty alcohol polyglycol ether (commercially available under 
the trade name "Arlypon IT 10" from Grunau) and 3% by weight of sodium 
dodecylbenzenesulfonate, until a homogeneous paste was formed. A blowing 
agent composition (B) was obtained. 
(C) About 16 parts of a hydrophilic, pyrogenically produced silica having a 
BET surface area of 150 m.sup.2 /g (commercially available under the trade 
name "HDK V15" from Wacker-Chemie) were stirred slowly into 100 parts of 
an oil-in-water emulsion, which contained 35% by weight of a 
dimethylpolysiloxane end-blocked by trimethylsiloxy groups and having a 
viscosity of 12,500 mPa.s at 25.degree. C., 59% by weight of water, 3% by 
weight of alkyl sulfate (commercially available under the trade name 
"Genapol CRT 40" from Hoechst AG) and 3% by weight of sodium 
alkylsulfonate (commercially available under the trade name "Emulgator 
K30" from Interorgana), until a homogeneous paste was formed. A blowing 
agent composition (C) was obtained. 
(D) About 12 parts of a hydrophilic, pyrogenically produced silica having a 
BET surface area of 150 m.sup.2 /g (commercially available under the trade 
name "HDK V15" from Wacker-Chemie) were stirred slowly into 100 parts of 
an oil-in-water emulsion, which contained 30% by weight of an 
organopolysiloxane composed of 20 mol % of methyl-(phenylethyl)-siloxy 
units and 80 mol % of methyl-dodecyl-siloxy units and having a viscosity 
of 1000 mPa.s at 25.degree. C., 65% by weight of water and 5% by weight of 
fatty alcohol polyglycol ether (commercially available under the trade 
name "Arlypon IT10" from Grunau), until a homogenous paste was formed. A 
blowing agent composition (D) was obtained. 
(E) About 20 parts of a pyrogenically produced silica having a BET surface 
area of 200 m.sup.2 /g which has been rendered hydrophobic (commercially 
available under the trade name "HDK H20" from Wacker-Chemie) were stirred 
slowly into 100 parts of an oil-in-water emulsion described above under 
(A) until a homogeneous paste was formed. A blowing agent composition (E) 
was obtained. 
(F) About 18 parts of a hydrophilic, pyrogenically produced silica having a 
BET surface area of 300 m.sup.2 /g (commercially available undr the trade 
name "HDK T30" from Wacker-Chemie) were stirred slowly into 100 parts of 
an oil-in-water emulsion described above under (A) until a homogeneous 
paste was formed. A blowing agent composition (F) was obtained. 
(G) About 16 parts of a hydrophilic, pyrogenically produced silica having a 
BET surface area of 150 m.sup.2 /g (commercially available under the trade 
name "HDK V15" from Wacker-Chemie) were stirred slowly into 100 parts of 
an oil-in-water emulsion, which contained 35% by weight of a 
polymethylsilsesquioxane containing a total of 10 mol % of hydroxyl groups 
and ethoxy groups, 60% by weight of water and 5% by weight of a partially 
saponified polyvinyl alcohol (commercially available under the trade name 
"Polyviol W25/140" from Wacker-Chemie), until a homogeneous paste was 
formed. A blowing agent composition (G) was obtained. 
(H) About 14 parts of a hydrophilic, pyrogenically produced silica having a 
BET surface area of 150 m.sup.2 /g (commercially available under the trade 
name "HDK V15" from Wacker-water emulsion, which contained 50% by weight 
of a silicone resin containing 25 mol % of polydimethylsiloxane units and 
75 mol % of polymethylsilsesquioxane units containing a total of 10 mol % 
of hydroxyl groups and ethoxy groups, 45% by weight of water and 5% by 
weight of fatty alcohol polyglycol ether (commercially available under the 
trade name "Arlypon IT16" from Grunau), until a homogeneous paste was 
formed. A blowing agent composition (H) was obtained. 
(K) About 18 parts of a hydrophilic, pyrogenic silica having a BET surface 
area of 150 m.sup.2 /g (commercially available under the trade name "HDK 
V15" from Wacker-Chemie) were stirred slowly into 100 parts of an 
oil-in-water emulsion, which contained 30% by weight of a 
dimethylpolysiloxane end-blocked by trimethylsiloxy groups and having a 
viscosity of 12,500 mPa.s at 25.degree. C., 2% by weight of a silicone 
resin of the average composition [(Me.sub.3 SiO.sub.1/2).sub.0.7 
(SiO.sub.4/2).sub.1.0 (OEt).sub.0.04 (OH).sub.0.01 ].sub.30, 65% by weight 
of water and 3% by weight of a fatty alcohol polyglycol ether 
(commercially available under the trade name "Arlypon IT16" from Grunau), 
until a homogeneous paste was formed. A blowing agent composition (K) was 
obtained. 
(L) About 35 parts of a hydrophilic, precipitated silica having a BET 
surface area of 170 m.sup.2 /g (commercially available under the trade 
name "Ultrasil VN3" from Degussa) were stirred slowly into 100 parts of an 
oil-in-water emulsion described above under (A) until a homogeneous paste 
was formed. A blowing agent composition (L) was obtained. 
(M) About 30 parts of a precipitated silica having a BET surface area of 
100 m.sup.2 /g which has been rendered hydrophobic (commercially available 
under the trade name "Sipernat D17" from Degussa) were stirred slowly into 
100 parts of an oil-in-water emulsion described above under (A) until a 
homogeneous paste was formed. A blowing agent composition (M) was obtained 
.

EXAMPLE 1 
About 1.5 parts of blowing agent composition (A) and 0.5 part of a paste 
composed of equal parts of bis-(2,4-dichlorobenzoyl) peroxide and a 
dimethylpolysiloxane end-blocked by trimethylsiloxy groups and having a 
viscosity of 350 mPa.s at 25.degree. C. (commercially available under the 
trade name "Vernetzer E" from Wacker-Chemie) and 1.7 parts of a paste 
composed of equal parts of dibenzoyl peroxide and a dimethylpolysiloxane 
end-blocked by trimethylsiloxy groups and having a viscosity of 350 mPa.s 
at 25.degree. C. (commercially available under the trade name "Vernetzer 
B" from Wacker-Chemie) were mixed, on a roller, into 100 parts of an HTV 
silicone rubber mixture which contained 67 parts of a dimethylpolysiloxane 
end-blocked by trimethylsiloxy groups and having a viscosity of 10.sup.7 
mPa.s at 25.degree. C., 10 parts of a dimethylpolysiloxane end-blocked by 
trimethylsiloxy groups and having a viscosity of 35 mPa.s at 25.degree. C. 
and 23 parts of a hydrophilic, pyrogenically produced silica with a BET 
surface area of 130 m.sup.2 /g (commercially available under the trade 
name "HDK S13" from Wacker-Chemie). The resulting mixture was then 
extruded to give foam profiles or foam tubes and vulcanized in a hot air 
duct. Using an extruder die size of 8 mm in diameter (round cord), a 
vulcanization time of 5 minutes at 200.degree. C. and an output rate of 
about 1.5 m/min, a homogeneously foamed round cord was obtained having a 
diameter of 13 mm and having circular pores, with a homogeneous pore size 
distribution and a foam density of 0.6. 
EXAMPLE 2 
About 2 parts of blowing agent composition (B), 0.5 part of the "Vernetzer 
E" described in Example 1 and 1.7 parts of the "Vernetzer B" described in 
Example 1 were mixed, on a roller, into 100 parts of an HTV silicone 
rubber mixture which contains 75 parts of a dimethylpolysiloxane 
end-blocked by trimethylsiloxy groups and having a viscosity of 10.sup.7 
mPa.s at 25.degree. C. and 25 parts of hydrophilic, pyrogenically produced 
silica with a BET surface area of 150 m.sup.2 /g. The resulting mixture 
was then extruded to give foam profiles or foam tubes and vulcanized in a 
hot air duct. Using an extruder die size of 10 mm in diameter (round 
cord), a vulcanization time of 5 minutes at 180.degree. C. and an output 
rate of about 1.5 m/min, a homogeneously foamed round cord having a 
diameter of 19 mm and a foam density of 0.4 was obtained. 
EXAMPLE 3 
About 2 parts of blowing agent composition (H), 0.5 part of the "Vernetzer 
E" described in Example 1 and 1.7 parts of the "Vernetzer B" described in 
Example 1 were mixed, on a roller, into a mixture containing 50 parts of 
the HTV silicone rubber mixture described in Example 1 and 50 parts of the 
HTV silicone rubber mixture described in Example 2. The resulting mixture 
was then extruded to give foam profiles or foam tubes and vulcanized in a 
hot air duct. Using an extruder die size of 15 mm in diameter (round 
cord), a vulcanization time of 5 minutes at 200.degree. C. and an output 
rate of about 1.5 m/min, a homogeneously foamed round cord having a 
diameter of 24 mm and a foam density of 0.5 was obtained. 
EXAMPLE 4 
About 1.5 parts of blowing agent composition (L), and 2 parts of the 
"Vernetzer B" described in Example 1 were mixed, on a roller, into 100 
parts of the HTV silicone rubber mixture described in Example 1. The 
resulting mixture was then extruded to give foam profiles or foam tubes 
and vulcanized in a hot air duct. Using an extruder die size of 10 mm in 
diameter (round cord), a vulcanization time of 5 minutes at 220.degree. C. 
and an output rate of about 1.5 m/min, a homogeneously foamed round cord 
having a diameter of 13 mm and a foam density of 0.7 was obtained. 
EXAMPLE 5 
About 1.5 parts of blowing agent composition (C) and 2 parts of 
bis-(4-methylbenzoyl) peroxide (commercially available under the trade 
name "Interox PMBP" from Peroxid-Chemie) were mixed, on a roller, into 100 
parts of the HTV silicone rubber mixture described in Example 1. The 
resulting mixture was then extruded to give foam profiles or foam tubes 
and vulcanized in a hot air duct. Using an extruder die size of 8 mm in 
diameter (round cord), a vulcanization time of 5 minutes at 200.degree. C. 
and an output rate of about 1.5 m/min, a homogeneously foamed round cord 
having a diameter of 14 mm and a foam density of 0.6 was obtained. 
EXAMPLE 6 
About 2 parts of blowing agent composition (D) were mixed, on a roller, 
into 100 parts of an HTV silicone rubber mixture which contained 49 parts 
of a dimethylpolysiloxane end-blocked by trimethylsiloxy groups, composed 
of dimethylsiloxane units and methylvinylsiloxane units, having a 
viscosity of 2.times.10.sup.7 mPa.s at 25.degree. C. and containing 0.04% 
by weight of Si-bonded vinyl groups, 19 parts of a dimethylpolysiloxane 
end-blocked by dimethylvinylsiloxy groups, composed of dimethylsiloxane 
units and methylvinylsiloxane units, having a viscosity of 
2.times.10.sup.7 mPa.s at 25.degree. C. and containing 0.25% by weight of 
Si-bonded vinyl groups, 1.4 parts of a dimethylpolysiloxane end-blocked by 
dimethylvinylsiloxy groups, composed of dimethylsiloxane units and 
methylvinylsiloxane units, having a viscosity of 8.times.10.sup.6 mPa.s at 
25.degree. C. and containing 4.5% by weight of Si-bonded vinyl groups, 3 
parts of a copolymer composed of trimethylsiloxane units, dimethylsiloxane 
units and methylhydrogenosiloxane units, having a viscosity of 50 mPa.s at 
25.degree. C. and containing 0.5% by weight of Si-bonded hydrogen atoms, 
27.5 parts of hydrophilic, pyrogenically produced silica with a BET 
surface area of 300 m.sup.2 /g and 0.06 part of 
platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex having a 
platinum content of 1% by weight, based on elemental platinum, and 0.04 
part of ethynylcyclohexanol. The resulting mixture was then extruded to 
give foam profiles or foam tubes and vulcanized in a hot air duct. Using 
an extruder die size of 15 mm in diameter (round cord), a vulcanization 
time of 5 minutes at 200.degree. C. and an output rate of about 1.5 m/min, 
a homogeneously foamed round cord having a diameter of 20 mm and a foam 
density of 0.7 was obtained. 
EXAMPLE 7 
About 2 parts of blowing agent composition (A), 0.5 part of the "Vernetzer 
E" described in Example 1 and 1.7 parts of the "Vernetzer B" described in 
Example 1 were mixed, on a roller, into 100 parts of the HTV silicone 
rubber mixture described in Example 2. The resulting mixture was then 
processed at room temperature on a two-roll rolling mill to give a milled 
sheet 7 mm thick and sheets were punched out of the milled sheet. After 
the sheet surfaces had been fixed with a fabric layer, the sheets were 
vulcanized at atmospheric pressure under hot air at a temperature of 
200.degree. C. for 5 minutes. Homogeneously foamed sheets having a 
thickness of 20 mm, oval pores and a foam density of 0.45 were obtained. 
EXAMPLE 8 
About 1.5 parts of blowing agent composition (A) and 1 part of tert-butyl 
perbenzoate (commercially available under the trade name "Trigonox C" from 
Peroxid-Chemie) were mixed, on a roller, into 100 parts of the HTV 
silicone rubber mixture described in Example 1. The resulting mixture was 
then processed at room temperature on a two-roll rolling mill to give a 
milled sheet 14 mm thick. A sheet section (15 cm.times.15 cm.times.1.4 cm) 
was placed in a square spring mold (15 cm.times.15 cm.times.3 cm). The 
spring mold was then placed in a hot press and the rubber was vulcanized 
under pressure for 5 minutes at a temperature of 140.degree. C. A 
homogeneously foamed square having dimensions of 15 cm.times.15 cm 
.times.3 cm, oval pores and a foam density of 0.55 was obtained. 
EXAMPLE 9 
About 1.5 parts of blowing agent composition (A) and 1.7 parts of the 
"Vernetzer B" described in Example 1 were mixed, on a roller, into 100 
parts of the HTV silicone rubber mixture described in Example 1. The 
resulting mixture was then processed at room temperature on a two-roll 
rolling mill to give a milled sheet 16 mm thick. A sheet section (15 
cm.times.15 cm.times.1.6 cm) was placed in a square spring mold (15 
cm.times.15 cm.times.3 cm). The spring mold was then placed in a hot press 
and the rubber was vulcanized under pressure for 5 minutes at a 
temperature of 140.degree. C. A homogeneously foamed square having 
dimensions of 15 cm.times.15 cm.times.3 cm, oval pores and a foam density 
of 0.65 was obtained. 
EXAMPLE 10 
About 10 parts of an iron(III) oxide (commercially available under the 
trade name "Bayferrox 130B rot" from Bayer AG), 1.5 parts of blowing agent 
composition (A), 0.5 part of the "Vernetzer E" described in Example 1 and 
1.7 parts of the "Vernetzer B" described in Example 1 were mixed, on a 
roller, into 100 parts of an HTV silicone rubber mixture which contained 
72 parts of a dimethylpolysiloxane end-blocked by dimethylvinylsiloxy 
groups, composed of dimethylsiloxane units and methylvinylsiloxane units, 
having a viscosity of 2.times.10.sup.7 mPa.s at 25.degree. C. and 
containing 0.05% by weight of Si-bonded vinyl groups and 2 parts of a 
dimethylpolysiloxane end-blocked by hydroxyl groups and having a viscosity 
of 35 mPa.s at 25.degree. C. and 26 parts of a hydrophilic, pyrogenically 
produced silica having a BET surface area of 150 m.sup.2 /g. The resulting 
mixture was then extruded to give foam profiles or foam tubes and 
vulcanized in a hot air duct. Using an extruder die size of 8 mm in 
diameter (round cord), a vulcanization time of 5 minutes at 200.degree. C. 
and an output rate of about 1.5 m/min, a homogeneously foamed round cord 
having a diameter of 14 mm and having circular pores, a homogeneous pore 
size distribution and a foam density of 0.6 was obtained. 
EXAMPLE 11 
About 10 parts of a silanized ground quartz (commercially available under 
the trade name "Silbond 600 TST" from Quarzwerke Frechen), 1.5 parts of 
blowing agent composition (A), 0.5 part of the "Vernetzer E" described in 
Example 1 and 1.7 parts of the "Vernetzer B" described in Example 1 were 
mixed, on a roller, into 100 parts of the HTV silicone rubber mixture 
described in Example 10. The resulting mixture was then extruded to give 
foam profiles or foam tubes and vulcanized in a hot air duct. Using an 
extruder die size of 8 mm in diameter (round cord), a vulcanization time 
of 5 minutes at 200.degree. C. and an output rate of about 1.5 m/min, a 
homogeneously foamed round cord having a diameter of 12 mm and having 
circular pores, a homogeneous pore size distribution and a foam density of 
0.5 was obtained. 
EXAMPLE 12 
About 8 parts of the iron(III) oxide described in Example 10, 1.5 parts of 
blowing agent composition (A), 0.5 part of the "Vernetzer E" described in 
Example 1 and 1.7 parts of the "Vernetzer B" described in Example 1 were 
mixed, on a roller, into 100 parts of an HTV silicone rubber mixture which 
contained 55 parts of a dimethylpolysiloxane end-blocked by 
dimethylvinylsiloxy groups, composed of dimethylsiloxane units and 
methylvinylsiloxane units, having a viscosity of of 2.times.10.sup.7 mPa.s 
at 25.degree. C. and containing 0.15% by weight of Si-bonded vinyl groups 
and 14 parts of a dimethylpolysiloxane end-blocked by hydroxyl groups, 
composed of dimethylsiloxane units and methylvinylsiloxane units, having a 
viscosity of 35 mPa.s at 25.degree. C. and containing 0.8% by weight of 
Si-bonded vinyl groups and 31 parts of a hydrophilic, pyrogenically 
produced silica having a BET surface area of 150 m.sup.2 /g. The resulting 
mixture was then extruded to form foam profiles or foam tubes and 
vulcanized in a hot air duct. Using an extruder die size of 8 mm in 
diameter (round cord), a vulcanization time of 5 minutes at 200.degree. C. 
and an output rate of about 1.5 m/min, a homogeneously foamed round cord 
having a diameter of 10 mm and having circular cores, a homogeneous pore 
size distribution and a foam density of 0.85 was obtained. 
EXAMPLE 13 
About 10 parts of an aluminum oxide (commercially available under the trade 
name "Alcoa T60" from Alcoa), 1.5 parts of blowing agent composition (A), 
0.5 part of the "Vernetzer E" described in Example 1 and 1.7 parts of the 
"Vernetzer B" described in Example 1 were mixed, on a roller, into 100 
parts of the HTV silicone rubber mixture described in Example 10. The 
resulting mixture was then extruded to form foam profiles or foam tubes 
and vulcanized in a hot air duct. Using an extruder die size of 8 mm in 
diameter (round cord), a vulcanization time of 5 minutes at 200.degree. C. 
and an output rate of about 1.5 m/min, a homogeneously foamed round cord 
having a diameter of 16 mm and having circular pores, a homogeneous pore 
size distribution and a foam density of 0.55 was obtained. 
EXAMPLE 14 
About 8 parts of a titanium oxide (commercially available under the trade 
name "Titanoxid P25" from Degussa), 1.5 parts of blowing agent composition 
(A), 0.5 part of the "Vernetzer E" described in Example 1 and 1.7 parts of 
the "Vernetzer B" described in Example 1 were mixed, on a roller, into 100 
parts of the HTV silicone rubber mixture described in Example 10. The 
resulting mixture was then extruded to form foam profiles or foam tubes 
and vulcanized in four UHF ducts each 1.5 m long, through which hot air at 
a temperature of 130.degree. C. additionally flowed. Using an extruder die 
size of 8 mm in diameter (round cord), a vulcanization time of 6 minutes 
at a microwave energy of 4.times.3 kW and a frequency of 2700 MHz and an 
output rate of about 1.0 m/min, a homogeneously foamed round cord having a 
diameter of 11 mm and having circular pores, a homogeneous pore size 
distribution and a foam density of 0.75 was obtained.