Process for Producing a Fine Pored Silicone Foam

The invention relates to a process for producing a fine pored silicone foam by free foaming the two-component foam reaction mixture in which the reaction component A, a silicone composition, contains a silicone polymer having at least two double bonds per molecule, preferably vinyl groups, an organo-platinum catalyst, fumed silica, water, and if appropriate, further additives (alone or in combination) such as color pigments, foam regulators flame retardants, fillers and the like. The crosslinking component B is polydimethyl hydrogen siloxane. Compared to other known foams, such as polyurethane foam, PVC foam, or chloroprene foam, silicone foams exhibits a number of favorable properties which make a wide application range of these foams possible. These properties include above all: UV resistance, resistance to salt water, physiological harmlessness, odor free, high comfort of wear, when used as or in items of clothing chemical resistance, resistance to aging, tightness with respect to fluids, breathability or gas permeability, low combustibility and low foam density.

EP 0 416 229 A2 discloses a process for producing a fine pored, soft-elastic, sealing foam for lid, casing and cavity seals, in which the reaction components of the foam are processed in a low pressure, two-component meter mixing machine and made to foam in a free rise process without the addition of heat. One reaction component contains a silicone polymer having at least two double bonds in the molecules, preferably vinyl groups, and an organo-platinum catalyst, as well as further additives, if appropriate, The other component is formed from hydrogen siloxane as the crosslinking agent. A small amount of water being added to component A as an additional crosslinking agents and as a blowing agent. The addition of water causes the hydrogen siloxane to react almost completely during the foaming of the silicone composition so that a highly hydrogen blowing effect is obtained. A post reaction, having detrimental effects on the quality of the silicone foam is avoided.

The invention is based on silicone polymers in which water is used as a blowing agent and hydrogen siloxane is used as a crosslinking agent. It is the objective of the invention to produce a high quality fine pored, competitively priced foam, preferably in the form of foam sheets, finding a wide range of applications, with prolonged pot life and process time (modified LSR quality) which are easily controlled in a production environment.

According to the invention, this objective is accomplished by the following features of the process:

component A of the composition contains a complexed organo-platinum catalyst, for example with butanol, whereas cross-linking agent component B contains silicone polymers having at least two double bonds per molecule, preferably vinyl groups, as well as fumed silica;

the reaction mixture is pressurized in a storage vessel for dissolving a gas into the reaction mixture before it is withdrawn from the storage vessels; and

the mixture is heated to a temperature of 150.degree.-220.degree. C. to generate the foam.

In the process according to the invention components A and B are preferably used in the following compositions:

100 g of a silicone polymer having at least two double bonds per molecule, preferably vinyl groups,

0.2-5 g of a complex organo-platinum catalyst,

5-40 g of fumed silica,

0.05-5 g of water, and if appropriate,

further additives (alone or in combination) such as color pigments (e.g., carbon black, iron oxide red), foam regulators (e.g., fluorotensides), flame retardants, fillers (e.g., quartz powder), etc.

100 g of silicone polymer having at least two double bonds per molecule, preferably vinyl groups;

5-40 g of fumed silica, and

3-20 g of a Sill functional polyorganosiloxane crosslinking agent.

The above-mentioned components A and B can be processed at a ratio of 1:1 and, at this mixing ratio, have a pot life of at least three days at 20.degree. C. The viscosity of the two components is approximately 100,000 mPa.s.

For both components A and B, fluorosilicone polymers used as the silicone polymer may be advantageous.

In the process according to the invention, the high viscosity silicone composition is processed at a viscosity range of 30,000 to 500,000 mPa.s, whereby a high quality silicone foam is obtained which is capable of withstanding mechanical stresses

Use of the complexed organo-platinum catalyst prevents premature foaming of the reaction mixture, thus making it possible to store the reaction mixture or to pressurize the reaction mixture over a prolonged period of time, which would be at least two hours, but can generally be longer, depending on the pressures used. In general, it is advisable to pressurize the mixture of the two components A and B with a pressure of at least 70-145 psi.

The reaction mixture which has been pressurized over a period of several hours contains the gas finely dispersed in a dissolved form so that it can act to nucleate the foam in the subsequent foaming reaction. For reasons of cost alone, compressed air or compressed nitrogen is preferable as the pressurizing gas, although other gases, such as inert gases, can be used as well. If the dissolved gas was not introduced into the reaction mixture, no foaming reaction producing the desired fine pored foam would result

The reaction mixture into which the dissolved gas has been introduced is made to foam when applied over an area. In doing so, heating the reaction mixture as required for the foaming process can be accomplished by passing the reaction mixture through a continuous heating furnace or the like.

The two above-referenced components A and B are preferably produced such that they are free of air bubbles. Before being pressurized, it is preferable to mix so that no air bubbles are entrained in the mixture. This can be done by static mixing in the absence of air, e.g., when passing through a mixing pipe or mixing screw. If air bubbles are contained in the silicone composition, e.g., by simple stirring of the two components A and B in an open mixing vessel, then a coarse celled silicone foam is obtained which is unsuitable for the various application purposes. When dissolved air or any other gas is taken up in the reaction mixture in the absence of air bubbles, it is guaranteed that a high quality silicone foam is formed which has a high number of fine cells and is very homogenous.

If the reaction mixture made from the two components does contain entrained air bubbles, it is advisable to deaereate the mixture by means of a vacuum treatment prior to pressurizing.

The reaction mixture, which is now free of entrained air bubbles but mixed with the dissolved gas, is applied over an area with the desired coating thickness by a doctor blade or spread coating process. The mixture is heated, e.g., in a continuous heating oven, to a temperature of 150.degree.-220.degree. C. Depending on the thickness of the reaction mixture layer and the nature of the basic body supporting it, the foaming and curing reactions occur within 0.5 to 5 minutes. Since hydrogen is released by a chemical reaction taking place during the foaming process, it is advisable to use a continuous dryer or continuous furnace with regulated circulating of fresh air which can be controlled by hydrogen detectors. The reaction mixture can also be applied onto a base, such as a metal sheet, a metal foil (e.g., an aluminum foil), or a tissue band such as a PTFE tissue band, and made to foam. An adhesion promotor (silane adhesion promotor) can be used, if appropriate, in order to obtain strong bonding of the silicone foam to the base. The silicone foam can also be processed into a foam sheet, which can be suitably wound into a roll with a separating film being inserted in between. The common thickness of the silicone foam layers are 2-30 min. The raw density of the silicone foam is generally 0.2-0.8 g/cm.sup.3. From the silicone foam produced in the process according to the invention, wound into a roll or cut into blocks, high quality seals can be produced. Heat shields can be produced by lining with aluminum foil, or fittings can be produced for the interior items of clothing, in particular for surfing or diving suits, by lining with flexible tissue. Otherwise, the silicone foam produced in the process according to the invention can be used for many other purposes due to its high foam quality and its low cost production process. It is also possible to line both sides of the silicone foam or silicone foam sheets produced in the process according to the invention.