Processing during curing

The ion thus produced may itself react with another epoxy-group in a process which forms the first crosslink (Reaction 4.16). This reaction may occur at both ends of the molecule of the diglycidyl ether, so that a crosslinked stmcture can easily be built up from these substances. Reaction becomes complicated by the fact that the epoxy-group may also react with the hydroxy-groups that form as the epoxy ring opens up during cure. Thus the finished resin may contain a complicated array of structures within the three dimensional network. 

FIGURE 17.7 Results for the Bl through B8 and A2 and A3 sihca samples showing the Mooney viscosity at 100°C, and the d3mamic mechanical properties measured hy mhher process analyzer (RPA) during curing of the compounds. 

The final physical properties of thermoset polymers depend primarily on the network structure that is developed during cure. Development of improved thermosets has been hampered by the lack of quantitative relationships between polymer variables and final physical properties. The development of a mathematical relationship between formulation and final cure properties is a formidable task requiring detailed characterization of the polymer components, an understanding of the cure chemistry and a model of the cure kinetics, determination of cure process variables (air temperature, heat transfer etc.), a relationship between cure chemistry and network structure, and the existence of a network structure parameter that correlates with physical properties. The lack of availability of easy-to-use network structure models which are applicable to the complex crosslinking systems typical of "real-world" thermosets makes it difficult to develop such correlations. 

In Tobacco. At the time of harvesting, fresh tobacco leaves do not contain measurable amounts of nitrosamines (<5 ppb). However, these compounds are formed during curing, aging and fermentation. Their concentrations depend primarily on the content of proteins, alkaloids, agricultural chemicals and nitrate in the tobacco, as well as on the processing conditions which lead to the reduction of the nitrates. 

We thus assume, that the biochemical processes during tobacco curing may be different from the ui vitro N-nitrosation of secondary and tertiary amines (30). 

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