Chemoviscosity cure effects

The cure effects on the chemoviscosity are two-fold the viscosity will initially decrease due to the increase in thermal effects but will eventually increase due to formation of the crosslinked network via the curing reaction. This is shown schematically for the chemoviscosity of a polyester resin during injection moulding in Figure 4.8. 

Extensive work in the literature has focussed on the determination of the cure effects on the chemoviscosity of thermosetting resin, and a summary of the effects of cure on various chemorheological models is given in Table 4.1. 

Various viscosity models have implicitly included the effects of gelation on the chemoviscosity, and these were reviewed in Table 4.2 incorporating gelation-conversion and glass-transition-temperature effects implicitly in the cure effects on chemoviscosity. Explicit models for the expression of gel time versus temperature and time are sparse, with empirical measurements mainly being used. 

Recent work has included tests that examine the effects of shear rate and cure simultaneously (which will also be discussed below). The models derived from these tests are recombined to provide an overall chemoviscosity model to be used in processing applications. The effect of pressure on chemoviscosity has not been studied extensively however, system pressure may be relevant to high-pressure injection-moulding and transfer-moulding processes. 

The effects of temperature and time on the chemoviscosity can also be described explicitly in terms of the extent of cure (a) from knowledge of the kinetics of the cure (i.e. (T, t)) and temperature by the following equation ... 

Thus, with a knowledge of the effects of gelation and an accurate chemoviscosity model, the optimum flow and cure conditions can be established. The measurement techniques for chemoviscosity and gelation effects will be reviewed next. 

The work of several authors (Peters et al, 1993, Halley et al, 1994) has demonstrated the use of non-isothermal dynamic sweep tests to examine the combined effects of shear rate and curing on the chemoviscosity of a highly filled epoxy resin simultaneously. These tests use a selected temperature ramp with repeated dynamic rate sweeps to investigate the effects on the chemoviscosity. The advantage of these tests is that the effects of shear rate and cure are not separated, which is similar to processing conditions. 

Malkin and Kulichikin (1991) initially reviewed the rheokinetics of cured polymers and highlighted the first empirical chemorheological models. They showed that for a simple homogeneous reaction with no diffusion limitations or gel effects for reacting epoxy-resin systems the chemoviscosity could be described by... 

Kim and Char (2000) examine the rheology of a PES-DGEBA-DDM system during isothermal cure. They found a fluctuation in viscosity at phase separation that could be simulated by a two-phase suspension model that incorporated chemoviscosity effects. 

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