Composition hardener/resin, effect

Effect of Hardener/Resin Composition on Degree of Cure. 

The effective area modd predicts that the stress concentration factor 7 should be independent of composition in conqmsites containing well-bonded rigid filler paitides. This prediction is supported by the compressive yield data for silica-loaded epoxy resins presented in Fig. 8 yield stress is linear with log (strain rate) for eadi material, and the dopes are identical in each case. The increase of yield stress with silica content must therefore be interpreted as a decrease in the pre-exponential factor rather than in 7. Young and Beaumont observed a similar large increase in the yidd stress of silica-loaded epoxy resins, and suggested an analc with precipi-taticm-hardening in metals ... 

The purpose of the study was a) to determine the effect of relative resin/hardener composition and ambient and elevated temperature curing schedules on the glass transition temperature, Tg, and on the degree of cure of the epoxy system, and b) to evaluate the agreement between the calculated degree of cure using the nth-order kinetic equation (1.1) (1, 2, 3) and the measured degree of cure. 

Table 12.1 gives typical values of the diffusion constant for a series of thermosetting matrices where the effect of polarity of the resin on the equilibrium on moisture concentration is illustrated. Epoxy resins are the network product of the reaction of a multifunctional epoxide monomer with a hardener. One of the important aspects of composite materials which needs to be recognised is that the material is synthesised at the same time as the component is manufactured. The precise chemistry of the final network can be uncertain but is a function of the chemical stmctures of the epoxide and hardener and/or catalyst, which determine the mechanism of cure. In this way, the cured resins can have differing polarities. 

Starting from the cure reaction mechanism, a proper cure rate law, describing the evolution of the system from initial to final state, can be proposed. In the case of a mechanistic approach, in which the reaction model consists of a set of chemical reaction steps, a set of (stiff) coupled differential equations has to be solved to describe the evolution of the important reacting species of the system. In this case, effects of the composition of the fresh reaction mixture (such as a stoichiometric unbalance of resin and hardener, the concentration of accelerator, initiator or inhibitor) and the influence of additives (such as moisture and fibres in composites) can be studied. Because this set of equations may be rather complex and/or even partly unknown, various simplifications have to be made. 

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