Cross-linking heat-fugitive

For this reason, many attempts have been made over the years to produce a rubbery material which has a network structure over a useful temperature range but which, if heated further, loses this structure. In many cases this involves a form of cross-linking that is said to be heat fugitive. In Section 3.4 four types of heat-fugitive cross-link were identified, namely ... 

These materials have characteristics of both rubbers and thermoplastics. At room temperature they behave like cross-linked rubbers, but at elevated temperatures the cross-links effectively disappear (they are said to be heat fugitive) and the material may be processed as a thermoplastic. Unlike truly cross-linked (vulcanised) rubbers, these materials may be capable of disolution in solvents, although not necessarily at room temperature. 

Hexachloroplatinic acid and other platinum complexes are mainly used as soluble catalysts for the additive cure. The most active catalyst used recently for vulcanization of silicon rubber is the platinum-alkenylsiloxanes complex, mainly the platinum-vinylsiloxane complex (Karstedt s catalyst) (4). One important approach to the activated cure of silicone rubber makes use of various inhibitors or moderators added to the platinum catalyst to reduce, or temporarily inhibit, its catalytic activity in the presence of the alkenyl- and hydropolysiloxanes (see catalysis by Pt complexes). The catalyst is usually added to the reaction mixture in quantities related to the number of unsaturated (e.g., vinyl) substituents in the polysiloxane. Vinyl-terminated polydimethylsiloxane polymers (viscosity > 200 cSt) are typically cross-linked by methylhydrosiloxane-dimethylsiloxane copolymer with 15-50 mol% of polymethylhydrosiloxane. A typical catalyst is a platinum complex in alcohol, xylene, divinylsiloxanes, or cyclic vinylsiloxanes. The system is usually prepared in two parts (part A, vinylsiloxane -I- Pt (5-10 ppm) part B, hydrosiloxane -I- vinylsiloxane). Inhibitors stop the platinum catalyst they are volatile or react with silicone hydride cross-linker to become a part of the polymer network. Some of them are decomposed by heat or light (UV). A single-component system contains fugitive inhibitors of Pt. 

It is not out of place to give brief mention to the ionomers introduced by Du Pont in the mid-1960s. To produce these polymers an alkene, usually ethylene, is copolymerized with a few per cent of a second monomer such as an -carboxylic acid in order to introduce a few carboxylic acid groups into the chain. The copolymer is then blended with a metal salt which ionizes the acid group. Heat fugitive ionic cross-links form (between carboxylic groups via the metal... 

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