Additives cure reaction products

The chemistry of silicones is summarized by following the steps necessary to produce a two-part, platinum-cured silicone containing vinyl-stopped polydimethylsiloxane, Si-H-on-chain siloxane, platinum catalyst and catalyst inhibitor. The process begins with silicon dioxide and follows the steps of conversion to sand to elemental silicon. Silicon is reacted with MeCl to make methylchlorosilanes in the methylchlorosilane reaction (MCS). The products from the MCS reaction are separated by distillation and then hydrolyzed and condensed to make the various siloxane polymers. Polymers with methyl, vinyl or Si-H functionality are made as required for the platinum addition-cured silicone product. 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

N-Nitrosamines, formed principally from the reaction of naturally occurring secondary amines with nitrites that may be added to foods or produced by bacterial reduction of nitrates, have been identified in many food systems including cured meat products, nonfat dried milk, dried malt and beer. In addition, the presence of less volatile and non-volatile N-nitroso compounds or their precursors in foods have been suggested from a number of model system studies. 

The formulation ofmoisture-ujriiig silicones includes a silicone polymer, filler, a moisture-reactive cross-linker, and sometimes a catalyst. A newer class of silicone sealants are known as the silicone latex sealants. These sealants are silicone-in-water emulsions that cure by evaporation of the emulsifying water. Addition-curing silicones in general are two-part systems that cure by the platinum-catalyzed reaction of a silicone hydride with typically a vinyl group attached to silicon. Because no by-products are generated by the cure, there are few volatiles and no shrink in thick sections. 

An ingredient added to an adhesive to reduce the concentration of base resin or binder is called a diluent. Diluents are principally used to lower the viscosity and modify the processing conditions of some adhesives. The degree of viscosity reduction caused by various diluent additions to a conventional epoxy adhesive is shown in Fig. 1.5. Diluents do not evaporate as does a solvent, but they become part of the final adhesive. Reactive diluents react with the resin base during cure, so that the final adhesive characteristics are determined by the reaction product of the binder and diluent. Nonreactive diluents do not react with the resin or curing agent and, therefore, more seriously weaken the final properties. Coal and pine tar are common nonreactive diluents. 

Cure system species, accelerators and their reaction products This class of additive can present problems as they are often thermally labile, reactive and, in some cases, have a degree of ionic character (e.g. zinc dithiocarbamate salts). In these cases LC-MS is a more appropriate technique than GC-MS. It is also easier to use LC-MS with a number of the approved food simulants as they can be injected directly into the instrument, being compatible with the mobile phase. In some cases the reaction products (e.g. aniline from diphenyl guanidine, and benzothiazole from thiazole and sulphonamide accelerators) are stable and so GC and GC-MS can be used. Peroxides are popular curatives for food use rubbers and the stable, breakdown products of these can be easily detected by GC-MS. 

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