Peroxide curing process materials

FluorosiHcones (FVMQ) have exceUent low temperature flexibUity properties coupled with good oil, fuel, and solvent resistance and exceUent aging properties. The materials are compounded and reinforced with fine particle fiUers, especiaUy sUica. The materials are mixed and processed on especiaUy clean equipment and are peroxide-cured. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

This material cures much faster than the peroxide curing offset, which results in much shorter curing times. This makes it suitable for moulding applications. It needs not to be processed in a PTFE treated mould, if the surface of the mould has the right electro-eroded structure. The reader may understand that this development is very recent and therefore no adhesion results can be provided in this review. 

From an analytical perspective, the erosslinking materials and process should be well understood. A reactive peroxide, for example, will generally not be evident in the analysis of a polymer crosslinked with this material. This will be evident when not fully decomposed during a cure process, for example, and the presence of non-reaeted peroxide may be a valuable indieation of an under-cure condition. Typically, the deeomposition products of dicumyl peroxide are what will be found analytically. These include aeetie aeid, eumyl aleohol, acetophenone, and others. Heating of most thermoplastics to a level that initiates deeomposition of the curing agent would also oxidize the base polymer. It is therefore neeessary to proteet the polymer with an antioxidant. GC/MS analysis of a typical peroxide-crosslinked polyethylene will reveal the peroxide decomposition products, an antioxidant, and no trace of the original peroxide. 

A new generation of LSRs has recently been commercialised which has been purpose designed for the manufacture of injection moulded and extruded parts by a fully automated process. These new materials overcome the earlier problems in using RTVs and result in finished parts with properties and temperature performance similar to conventional peroxide cured elastomers. 

The thickness of UV-cured samples is generally less than 100 /am. To cure sample thicknesses up to 1 cm or more, it is possible to use a dual initiation process. The heat of reaction developed during the UV cure of the material located close to the surface is used to decompose a peroxide that initiates the polymerization in the bulk. 

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