|
This is the favoured route although other raw materials such as alkoxysilanes can be used. Chlorosilanes and other silicone precursors are synthesised using the “Direct Process”, involving the reaction of elemental silicone with an alkyl halide thus,
Si + RX → RnSiX4-n (where n = 0-4)
Other Components in Silicones
Curing Additives
With the exception of RTV and liquid curing systems, silicone rubbers are usually cured using peroxides such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, t-butyl perbenzoate and dicumyl peroxide. Alkyl hydroperoxides and dialkyl peroxides have also been used successfully with vinyl containing silicones.
Hydrosilylation or hydrosilation is an alternative curing method for vinyl containing silicones and utilises hydrosilane materials and platinum containing compounds for catalysts. It is a 2-part process requiring mixing of 2 separate components, with the resulting material having a limited shelf life. Curing does not produce volatiles and heat cured conventional silicones with high tear strengths can be cured in this way.
Fillers
Reinforcing fillers are added to improve the otherwise poor tensile strength of silicones. Silica, in the form of silica fume with particle sizes in the range 10-40nm is the most preferred filler, although carbon black has been used. Fillers do interact with the vulcanisate, forming a pseudo-vulcanisation. This can occur either during mixing (creep hardening) or in storage (bin ageing).
Although milling can break down these structures, it is also common to add structure control additives or ant-structure additives to combat these reactions. Examples of these materials are siloxane-based materials such as diphenylsilane and pinacoxydimethylsilane.
Other Additives
Silicones have better fire resistant properties compared to natural rubbers. This property can be improved by the addition flame retardant additives such as platinum compounds, carbon black, aluminium trihydrate, zinc or ceric compounds. It should be noted that carbon black addition also increase electrical conductivity.
Ferric oxisde may also be added to improve heat stability, titanium dioxide and other organometallic compounds as pigments.
Manufacture
Silicones can be mixed/compounded using mixers of mills. However, due to the low viscosity close-fitting scrapers and cheek plates need to be used to ensure complete mixing. Forming can be carried out by conventional techniques such as injection moulding, extrusion and compression moulding. Care must be taken to take into account relatively large curing shrinkages and to avoid entrapped air.
Curing is generally rapid for most grades and folowed by a post cure treatment in an air oven at 200-250°C, for a period of 4-24 hours. This process serves to improve properties and remove residual peroxide products.
Liquid Silicone Rubbers
These are essentially two-part systems, supplied deaerated ready for use often in premetered equipment. Low injection pressures and low pressure forming techniques are sufficient. They cure after mixing the two separate portions, by processes such as hydrosilylation. Curing is often complete in as little as a few seconds at temperatures of about 200°C and post-curing is not usually required.
The low capital investment required for production mean that LSRs can compete with conventional silicones and organic rubbers.
Physical properties are comparable to general purpose grades and high strength peroxide cured elastomers. Furthermore, they exhibit self-extinguishing properties, with carbon black additions enabling them to satisfy UL-94 tests.
Room Temperature Vulcanising (RTV) Rubbers
These are available in one (RTV-1) and two-part (RTV-2) systems.
Single part systems consist of polydialkylsiloxane with terminal hydroxyl groups, which are reacted with organosilicon cross-linking agents. This operation is carried out in a moisture-free environment and results in the formation of a tetrafunctional structure. Curing takes place when materials are exposed to moisture. Atmospheric moisture is sufficient to trigger the reaction, and thickness should be limited if only one side is exposed to the moisture source. Curing is also relatively slow, reliant on moisture ingress into the polymer.
Two pack systems can be divided into two categories, condensation cross-linked materials and addition cross-linked polymers.
Condensation systems involve the reaction of silanol-terminated polydimethylsiloxanes with organosilicon cross-linking agents such as Si(RO)4. Storage life depends on the catalyst employed and ambient conditions.
Addition-cured materials must be processed under clean conditions as curing can be affected by contaminants such as solvents and catalysts used in condensation RTVs. These materials are suited to use with polyurethane casting materials.
|
