Les élastomères silicones

Abstract

The first silicone rubber was made in 1945. Now there are two types of silicone rubbers : heat vulcanizing silicone rubbers and room temperature vulcanizing silicone rubbers. They have in common many properties but they are quite different in their nature and uses. Heat vulcanizing silicone rubbers The formulation of these silicone rubbers (stocks or pastes) is as follows : Silicone gum 100 parts Reinforcing fillers 15 to 55 parts Semi-reinforcing fillers 0 to 100 parts Vulcanizing agents 0,5 to 5 parts Pigments 0 to 3 parts Additives 0 to 10 parts Silicone gums are linear high polymers, the general formula of which being as follows : (R1)3Si–[–O–Si(R1)2–]n–[–O–SiR1R2–]m–[–O–SiR1R3–]p–O–Si(R1)3 where : R1 = –CH3 R2 = –C6H5 R3 = –CH=CH2 or another organic radical. The methyl radical is the most important, it can be used alone and is present in every gum. The phenyl radical decreases the brittle point of the rubber. The vinyl radical is present in a very little proportion (1 for 500 to 1000 methyl groups). The gums containing vinyl radicals give silicone rubbers with many improvements: ease of vulcanization and very good “compression set” without any other additives. The fluoro-propyl or cyano groups bring oil and solvent resistance to the silicone rubbers. The reinforcing fillers are quite exclusively very fine silica. They give to the rubbers their mechanical properties. There is probably a reaction between the filler and the silicone gum. On the contrary, the semi-reinforcing fillers are numerous. They are used to decrease the cost and to increase the hardness of the rubbers. During the vulcanization, cross linkages are formed between the linear chain of the gum through the side groups attached to the silicon atom. The mechanism is not very well known and it might be different with the methyl and with the vinyl groups. The main vulcanizing agents are: benzoyl peroxide, 2-4 dichlorobenzoyl peroxide, tertiary butyl perbenzoate, ditertiary butyl peroxide and dicumyl peroxide. There are few words to say on pigments and additives. Some additives are used to improve heat stability. The interesting properties of heat vulcanizing silicone rubbers are: heat stability, weather and aging resistance, dielectric properties, chemical resistance. The mechanical properties are fair and the solvent resistance is poor except for the rubbers containing fluoro or cyano groups. Room temperature vulcanizing silicone rubbers Two types of RTV silicone rubbers are now available. The first type is a two component silicone rubber. It is composed of a paste containing the silicone gum (generally a methyl gum with some hydroxyl groups), fillers and eventually pigments and a cross linking agent, and of a separated catalyst (generally an organometallic salt). The vulcanization occurs at room temperature when the catalyst is added to the paste. In the second type called one component silicone rubber, the cross linking agent is the methyltriacetoxysilane and their is no catalyst. The vulcanization takes place at room temperature when the paste in exposed in a thin coat to the humidity of air. The RTV silicone rubbers have properties similar to that of heat vulcanizing ones but they are a little less heat stable. Silicone rubbers are young products and many improvements can be expected in the following years.