Elastomeric silicone sponge

A method of producing silicone elastomeric sponge is disclosed. The method comprises freezing an aqueous silicone emulsion, then thawing and drying to produce a silicone sponge containing irregular closed cells. The aqueous silicone emulsion comprises water, an anionically stabilized hydroxyl endblocked polydiorganosiloxane, an organic tin compound, and a colloidal silica, the emulsion having a pH of from 9 to 11.5 inclusive. The sponge produced by the method is useful as insulation and gasketing at high and low temperatures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of producing silicone elastomeric sponge 
from an aqueous emulsion of polydiorganosiloxane, colloidal silica, and an 
organic tin compound. 
2. Description of the Prior Art 
A method of foam production from elastomeric emulsions is known as the 
Talalay process. As described in U.S. Pat. No. 2,432,353, a natural rubber 
latex is compounded with curing agents, then caused to expand by mixing 
with hydrogen peroxide. While expanding, the mixture is poured into an 
aluminum mold. After the expanding mixture fills the mold, the contents of 
the mold are frozen by immersing the mold in a brine at -30.degree. C. The 
mold is then connected to a vacuum and a supply of alkaline calcium 
chloride brine at -2.degree. C. is drawn through the expanded, frozen 
mixture to cause an irreversible coagulation to take place. The mold is 
then transferred to a live steam vulcanizer where it is heated for 25 
minutes at 125.degree. C. to vulcanize the natural rubber. The vulcanized 
foam article is then removed from the mold, washed, centrifugally 
extracted and dried. Talalay teaches the preliminary formation of the foam 
can be effected by any of the known procedures, such as mechanical 
whipping, chemical gas generation, or physical release of gas or vapor. 
In the book "Neoprene Latex", by J. C. Carl, published by E. I. Dupont 
DeNemours and Co. (Inc.) (1962), a process of converting specialized 
neoprene latex to foam is described. The latex fillers, curing agent, 
accelerator, and foam stabilizing surface active agents are mixed, then 
the mixture is stirred rapidly to whip in air and create a froth. The 
froth is stirred until it is refined to a smooth cream. A gelling agent is 
then added and the froth placed in a mold. The foam produced by the 
gelling of the froth is usually cured by exposured to steam. After curing, 
the foam is washed and dried. 
The processes as discussed above require the gelling of the froth to 
stabilize the foam by the use of either an internal gelling agent as 
discussed for a neoprene latex or an external gelling agent as discussed 
with a natural rubber latex. This gelling step must be carefully 
controlled in order to produce satisfactory foam. 
SUMMARY OF THE INVENTION 
A silicone elastomeric sponge is produced by freezing, then thawing and 
drying a silicone emulsion. The silicone emulsion comprises water, an 
anionically stabilized hydroxyl endblocked polydiorganosiloxane, colloidal 
silica, and an organic tin compound, the emulsion having a pH in the range 
of 9 to 11.5 inclusive. The emulsion can also contain an organic amine, a 
thickener, and a filler other than colloidal silica. 
The method of this invention is much simpler than previous methods of 
producing foam from emulsions. The process consists only of freezing the 
emulsion, then thawing and drying. There is no separate coagulation and 
vulcanization steps required as is necessary with previous methods of 
producing foam or sponge. The production of sponge by this method does not 
create any problems with toxic vapor or pollutants caused by gases or 
vapor given off by the process such as those found with many previous foam 
and sponge processes. 
DESCRIPTION OF THE INVENTION 
This invention relates to a method comprising (a) freezing a silicone 
emulsion which provides an elastomeric product upon removal of the water, 
having a pH in the range of 9 to 11.5 inclusive, comprising water, 100 
parts by weight of an anionically stabilized hydroxyl endblocked 
polydiorganosiloxane having an average molecular weight of greater than 
10,000, an organic tin compound, and at least one part by weight of 
colloidal silica, long enough to freeze the water and produce a solid 
frozen article; (b) thawing the solid frozen article yielding a wet 
elastomeric sponge-like article; and (c) drying the wet elastomeric 
sponge-like article until the water is removed and an elastomeric sponge 
is obtained. 
The composition of the emulsion that is used in the method of this 
invention to produce silicone elastomeric sponge allows the emulsion to be 
transformed into a sponge by freezing the emulsion, then thawing and 
drying without any further additions of materials such as coagulating 
agents as are commonly used in producing foam by previous methods. The 
emulsion used in this invention can be frozen to produce a solid frozen 
article at which time it undergoes a change. When the solid frozen article 
is thawed it is no longer a liquid. The frozen article when thawed is a 
wet, elastomeric sponge-like article which is capable of maintaning its 
shape when removed from the container in which it is frozen. The freezing 
and thawing steps transform the originally liquid emulsion into a firm, 
jelly-like mass. Drying the wet elastomeric sponge-like article by 
exposing to ambient air or by heating then yields a cured elastomeric 
sponge. The cured elastomeric sponge is composed of primarily closed, 
irregularly shaped cells. The nature of the cells can be varied by varying 
the ratio of water to solids in the original emulsion, the thickness of 
the emulsion layer being frozen, and the rate of freezing. If the solids 
content of the emulsion is low, the wet article must be supported 
throughout the drying step as the sponge may not have enough strength to 
withstand the weight of the entrapped water. Higher solids content results 
in a sponge with a higher density. 
The effect of the thickness of the emulsion layer being frozen on the final 
cured sponge is not clearly resolved. When thin films of emulsion, such as 
1 mm, are frozen, the cured layer is not a sponge, but a solid elastomeric 
film. When layers of greater than 10 mm are frozen, the cured layer is a 
sponge. If a sponge of less than 10 mm thickness is desired, it is 
necessary to cut it from a thicker layer or one can mechanically mix air 
into the emulsion before it is frozen so that the emulsion is a froth as 
it is being frozen. 
When it is desired to make a sponge of lower density than is readily 
obtained by simply freezing the liquid emulsion, the emulsion is 
mechanically stirred to mix air into the emulsion to produce a froth. This 
froth is then frozen, thawed, and dried to yield an elastomeric sponge of 
a lower density. 
The simplest embodiment of the method of this invention is freezing the 
emulsion until solid, thawing at ambient air temperature, and then 
allowing the wet sponge to dry by exposure to ambient air temperature 
until the sponge dries. Since the sponge is primarily of closed cells, 
such a procedure takes several days if the sponge is of a thickness such 
as 20 mm. The process of drying can be accelerated by drying in an oven at 
elevated temperatures. The wet sponge should not be exposed to 
temperatures approaching 100.degree. C. as the vapor pressure of rapidly 
evaporating water can rupture the wet sponge. After the wet sponge is 
partially dried so that the sponge has greater strength, the temperature 
can be raised to speed the drying process. The thawing step and drying 
step can be combined by heating the solid frozen article directly without 
a separate thawing step. 
The emulsion, before being frozen, can also contain additional ingredients 
such as an organic amine, a thickener, filler other than colloidal silica, 
and the common additives for silicone elastomers such as heat stability 
additives, compression set additives, and pigments. Additives are selected 
to maintain the emulsion at a pH in the range of 9 to 11.5 inclusive. 
Additives should also be evaluated for their effect on the storage 
stability and ultimate physical properties of the sponge. Such additives 
can be added as an aqueous emulsion or in a finely divided dry form. 
Fillers other than colloidal silica can be semi-reinforcing and extending 
fillers that are not acidic such as diatomaceous earth, finely ground 
quartz, alkaline clays, titanium dioxide, and non-acidic carbon black. 
The emulsion used in the method of this invention comprises water, an 
anionically stabilized hydroxyl endblocked polydiorganosiloxane, an 
organic tin compound, and colloidal silica, the emulsion having a pH in a 
range of 9 to 11.5 inclusive. Such an emulsion is described in U.S. Pat. 
No. 4,221,688, issued Sept. 9, 1980, of Johnson, Saam, and Schmidt, said 
patent being hereby incorporated by reference to describe the emulsion and 
how to manufacture it. 
The hydroxyl endblocked polydiorganosiloxanes are those which can be 
emulsified and which impart elastomeric properties to the product obtained 
after the removal of the water from the emulsion. Such hydroxyl endblocked 
polydiorganosiloxanes should have a weight average molecular weight (Mw) 
of at least 10,000. Hydroxyl endblocked polydiorganosiloxanes with a lower 
Mw range, such as 5000 to 10,000, do not provide strong elastomeric 
products. Tensile strengths and elongations at break improve with 
increasing molecular weight, with reasonable tensile strengths and 
elongations obtained above 30,000 Mw and the best tensile strengths and 
elongations obtained above 50,000 Mw. The maximum weight average molecular 
weight is one which can be emulsified and which will give elastomeric 
properties to the product obtained after the water is removed from the 
emulsion. Weight average molecular weights up to about 1,000,000 for the 
hydroxyl endblocked polydiorganosiloxane are expected to be practical. The 
preferred Mw for the hydroxylated polydiorganosiloxanes are in the range 
of 200,000 to 700,000. The viscosity of the polymer obtained upon removal 
of the water from the emulsion will vary from about 75 Pa.s at 25.degree. 
C. to about 4,000 Pa.s at 25.degree. C. with the preferred range from 
about 1,000 Pa.s at 25.degree. C. to 3,000 Pa.s at 25.degree. C. 
The organic radicals of the hydroxyl endblocked polydiorganosiloxane can be 
monovalent hydrocarbon radicals containing less than seven carbon atoms 
per radical and 2-(perfluoroalkyl)ethyl radicals containing less than 
seven carbon atoms per radical. Examples of monovalent hydrocarbon 
radicals include methyl, ethyl, propyl, butyl, isopropyl, pentyl, hexyl, 
vinyl, cyclohexyl and phenyl and examples of 2-(perfluoroalkyl)ethyl 
radicals include 3,3,3-trifluoropropyl and 2-(perfluorobutyl)ethyl. The 
hydroxyl endblocked polydiorganosiloxanes preferably contain organic 
radicals in which at least 50 percent are methyl. The hydroxyl endblocked 
polydiorganosiloxanes are essentially linear polymers containing two 
organic groups per silicon atom, that can include trace amounts of 
monoorganosiloxane or triorganosiloxane groups present as impurities of 
the manufacturing process. The preferred hydroxyl endblocked 
polydiorganosiloxanes are the hydroxyl endblocked polydimethylsiloxanes. 
The most preferred hydroxyl endblocked polydiorganosiloxanes are those 
prepared by the method of anionic emulsion polymerization described by 
Findlay et al. in U.S. Pat. No. 3,294,725, issued Dec. 27, 1966, which is 
hereby incorporated by reference to show the methods of polymerization and 
to show the hydroxyl endblocked polydiorganosiloxane in emulsion. Another 
method of preparing hydroxyl endblocked polydiorganosiloxane is described 
by Hyde et al. in U.S. Pat. No. 2,891,920, issued June 23, 1959, which is 
hereby incorporated by reference to show the hydroxyl endblocked 
polydiorganosiloxanes and their method of preparation. These methods and 
others are known in the art. The hydroxyl endblocked polydiorganosiloxanes 
used in the emulsion are those which are anionically stabilized. As used 
herein, "anionically stabilized" means the hydroxyl endblocked 
polydiorganosiloxane is stabilized in emulsion with an anionic surfactant. 
Anionic surfactants are preferably the salt of the surface active sulfonic 
acids used in the emulsion polymerization to form the hydroxyl endblocked 
polydiorganosiloxane as shown in U.S. Pat. No. 3,294,725 cited above 
which is hereby incorporated by reference to show the surface active 
sulfonic acids and salts thereof. The alkali metal salts of the sulfonic 
acids are preferred, particularly the sodium salts. The sulfonic acid can 
be illustrated by aliphatically substituted benzenesulfonic acids, 
aliphatically substituted naphthalene sulfonic acids, aliphatic sulfonic 
acids, silylalkylsulfonic acids and aliphatically substituted 
diphenylethersulfonic acids. 
One of the advantages of the emulsions described herein is the relatively 
small amount of surfactant or emulsifying agent needed to maintain a 
stable emulsion. The amount of anionic emulsifying agent can be less than 
2 weight percent of the emulsion, wherein this amount can result from the 
neutralized sulfonic acid wherein the sulfonic acid is used in the 
emulsion polymerization method for the preparation of the hydroxyl 
endblocked polydiorganosiloxane. Other anionic emulsifying agents can be 
used, for example, alkali metal sulfonicinoleates, sulfonated glyceryl 
esters of fatty acids, salts of sulfonated monovalent alcohol esters, 
amides of amino sulfonic acids such as the sodium salt of oleyl methyl 
tauride, sulfonated aromatic hydrocarbon alkali salts such as sodium 
alpha-naphthalene monosulfonate, condensation products of naphthalene 
sulfonic acids with formaldehyde, and sulfates such as ammonium lauryl 
sulfate, triethanol amine lauryl sulfate, and sodium lauryl ether sulfate. 
Although not specifically required, one can optionally include nonionic 
emulsifying agents in addition to the anionic emulsifying agents. Such 
nonionic emulsifying agents can be illustrated by saponins, condensation 
products of fatty acids with ethylene oxide such as dodecyl ether of 
tetraethylene oxide, condensation products of ethylene oxide and sorbitan 
trioleate, condensation products of phenolic compounds having side chains 
with ethylene oxide such as condensation products of ethylene oxide with 
isododecylphenol, and imine derivatives such as polymerized ethylene 
imine. 
Colloidal silica is a required ingredient of the emulsions. Any of the 
colloidal silicas can be used. These colloidal silicas are well known in 
the art and many are commercially available. Although any of the colloidal 
silicas can be used including fumed colloidal silicas and precipitated 
colloidal silicas, the preferred colloidal silicas are those which are 
available in an aqueous medium. Colloidal silicas in an aqueous medium are 
usually available in a stabilized form, such as those stabilized with 
sodium ion, ammonia, or an aluminum ion. Aqueous colloidal silicas which 
have been stabilized with sodium ion are preferred because the pH 
requirement can be met by using such a sodium ion stabilized colloidal 
silica without having to add additional ingredients to bring the pH within 
the range of 9 to 11.5. The term "colloidal silica" as used herein are 
those silicas which have particle diameters of from 0.0001 to 0.1 
micrometer. Preferably, the particle diameters of the colloidal silicas 
are from 0.001 to 0.05 micrometer. The relative amounts of hydroxyl 
endblocked polydiorganosiloxane and colloidal silica can vary over a wide 
range, such as from 1 part to 150 parts by weight of colloidal silica for 
each 100 parts by weight of hydroxyl endblocked polydiorganosiloxane. 
Amounts of colloidal silica from 10 to 50 parts by weight for each 100 
parts by weight of hydroxyl endblocked polydiorganosiloxane are preferred 
in the method of the instant invention. 
The silicone emulsion has a continuous water phase in which there is a 
dispersed phase which comprises an anionically stabilized hydroxylated 
polydiorganosiloxane and colloidal silica. For this silicone emulsion to 
maintain a storage stability and also be curable to an elastomer after the 
emulsion is stored, the pH of the silicone emulsion must be within the 
range of 9 to 11.5 inclusive. The silicone emulsions which have the best 
storage stability and still have the ability to form elastomers at ambient 
conditions at any point during the storage stable period are those which 
have a pH in the range of 10.5 to 11.2. 
A silicone emulsion of hydroxyl endblocked polydiorganosiloxane and 
colloidal silica does not provide a useful elastomeric product when the 
water is allowed to evaporate at ambient conditions immediately after the 
emulsion is prepared. An aging period is necessary before an elastomer can 
be formed from the emulsion, but such an aging period can take a long 
time, such as up to five months. The addition of an organic tin compound, 
preferably a dialkyltindicarboxylate, can be used to reduce the aging 
period to one to three days. After the aging period, an elastomeric 
product can be obtained by the removal of the water under ambient 
conditions. Dialkyltindicarboxylate can be used in amounts of from 0.1 to 
2 parts by weight for each 100 parts by weight of the hydroxyl endblocked 
polydiorganosiloxane, preferably about 0.25 to 1.5 parts by weight are 
used. Dialkyltindicarboxylates include dibutyltindiacetate, 
dibutyltindilaurate, and dioctyltindilaurate. The preferred 
dialkyltindicarboxylate is dioctyltindilaurate. 
The long-term storge stability of the emulsion has been found to be 
improved by the addition of an organic amine. The organic amine can be 
primary, secondary, or tertiary amines which contain carbon, hydrogen, and 
nitrogen, and can also contain oxygen, and which are water soluble in the 
amounts required. These organic amines include diethylamine, 
ethylenediamine, butylamine, hexylamine, morpholine, monoethanolamine, 
triethylamine, and triethanolamine. The preferred amine is diethylamine. 
The organic amines can be added neat or in aqueous emulsion provided that 
they do not cause the anionically stabilized hydroxyl endblocked 
polydiorganosiloxane emulsion to break during their addition. For this 
reason, it is preferable to add the amine as an aqueous solution. 
A thickening agent can be used to adjust the viscosity of the emulsion. 
Suitable thickeners are available commercially and would be selected for 
their stability and usability in thickening the emulsion at a pH in the 
range of 9 to 11.5 inclusive. Some of the useful thickeners include the 
classes of cellulose derivatives, alkali salts of polyacrylates and 
polymethacrylates, sodium and ammonium salts of carboxylate copolymers, 
and colloidal clays. 
The ingredients used in preparing the emulsion used in the method of this 
invention can be mixed together by any suitable means. In a simple batch 
operation, for instance, the anionically stabilized hydroxyl endblocked 
polydiorganosiloxane emulsion is placed in a mixing container, the 
colloidal silica in the form of a colloidal dispersion is added with 
mixing, the organic tin compound is also added in the form of an emulsion, 
sufficient amine is added to adjust the pH to the required range, and the 
mixture is stirred until uniform. A thickening agent or other incidental 
additives can also be stirred in. 
To obtain the preferred pH range of 10.5 to 11.2, it will usually require 
adjusting the pH after the siloxane polymer, colloidal silica, organic tin 
compound, and any additional ingredients have been mixed. The pH is 
adjusted with the amine compound or with an alkali metal hydroxide or a 
combination thereof. The preferred alkali metal hydroxide is sodium 
hydroxide. For the purposes of this invention, the term "pH" means the 
electrical potential measured on commercially available glass electrodes 
designed for this purpose when the glass electrode is immersed in the 
emulsion. The electrode is calibrated with a standard buffer solution 
which gives a pH of 10. 
The method of this invention is useful for producing silicone elastomeric 
sponge. The emulsion used in this method uses inexpensive raw materials, 
requires simple processing, is a one-part system, and has a long shelf 
life. The method of this invention requires simple processing, using 
equipment that is readily available. The sponge produced has primarily 
irregular shaped closed cells throughout the cured article. The cured 
sponge is useful for thermal insulation and gasketing, particularly where 
the sponge is subjected to extremely high or low temperatures which are 
within the known operating limits of silicone elastomers. The sponge, 
because of its closed cells and resiliency at both high and low 
temperatures, is particularly suitable as a gasket for sealing irregular 
surfaces as it easily conforms to the surface under low pressure.

The following examples are included for illustrative purposes and should 
not be construed as limiting the scope of the invention which is properly 
delineated in the claims. All parts are parts by weight. 
EXAMPLE 1 
A mixture was prepared using 200 parts of an anionically stabilized 
hydroxyl endblocked polydimethylsiloxane emulsion with a pH of about 2 and 
62 percent by weight polymer, 1 part of morpholine, 2 parts of a 50% by 
weight emulsion of dioctyltindilaurate hereinafter called Tin Emulsion A, 
and 100 parts of a sodium ion stabilized colloidal silica dispersion 
present as a 30 percent solids by weight dispersion in water. This 
colloidal silica dispersion had a pH of about 10. It is hereafter referred 
to as Colloidal Silica A. 
The mixture was allowed to age for 48 hours at room temperature. A sample 
to about 56 g was placed in a container and the container placed in a 
freezer at -18.degree. C. for 24 hours. The frozen sample was then allowed 
to thaw at room temperature for 6 hours. At this point, the sample was a 
cured, spongy elastomer saturated with water. The cured, spongy elastomer 
was removed from the container and dried in an oven at 70.degree. C. After 
drying, the sample was a very tough, cured silicone elastomer sponge with 
very small cells. 
EXAMPLE 2 
Additional experiments were run to evaluate the shelf life of the system. 
A. A mixture was prepared comprising 200 parts of the anionically 
polymerized polydimethylsiloxane emulsion of Example 1, 0.3 part of a 
sodium lauryl sulphate surfactant, 7.6 parts of a 2 percent sodium 
hydroxide solution, 100 parts of the Colloidal Silica A, 10 parts of 
finely divided titanium dioxide, 1 part of morpholine, 5 parts of an 
acrylic thickening agent, and 1 part of Tin Emulsion A. This emulsion had 
a viscosity of 6 Pa.s at 25.degree. C. when measured on a Brookfield 
viscometer using a No. 3 spindle at 2 rpm. 
B. A mixture was prepared identical to mixture A except that there was used 
1.5 parts of a derivative of a xanthan gum as the thickening agent in 
place of the acrylic thickening agent. This emulsion had a viscosity of 
22.3 Pa.s at 25.degree. C., measured as above described. 
About 90 days after the mixtures A and B were manufactured, they were 
tested by producing sponge following the procedure used in Example 1. Both 
emulsions produced very tough sponges.