Curing process nucleation

The relative effectiveness of nucleating agents in a polymer can be determined by measuring recrystallization exotherms of samples molded at different temperatures (105). The effect of catalyst concentration and filler content has been determined on unsaturated polyesters by using dynamic thermal techniques (124). Effects of formulation change on the heat of mbber vulcanization can be determined by dsc pressurized cells may be needed to reduce volatilization during the cure process (125). 

Voids can be formed by either entrapment of air mechanically or by one of two nucleation processes. Mechanical entrapment could include (1) entrained gas bubbles from the resinmixing operation, (2) bridging voids from large particles or particle clusters (quenched DDS curing agent, airborne particles, or paper release agent), (3) voids from wandering tows, fuzz... 

Halftime of crystallization ti/2 as a function of crystallization for cured ER/EO blend reveals that addition of cured ER resin to PEG crystallizable thermoplastic depresses the overall crystallization rate of PEO, and at a fixed the overall crystallization rate decreases significantly with increasing the concentration of the cured resin ER (Fig. 3.38). Application of the Avrami extrapolation resulted in n values comprised between 3.5 and 5, not changing as the content of ER in the blend is increased. That means the incorporation of cured ER does not affect significantly the nucleation, and growth process is under the conditions the authors selected for the crystallization of PEO. 

Reaction Injection Moulding.— By contrast with the epoxy resins, polyurethanes cure at much lower temperatures, 40 °C upwards. The reactants are usually polyether or polyester triols and diols (polyols) and isocyanates or isocyanate-tipped polyester piepolymers. The process problems are essentially those of mixing the reactants sufficiently rapidly to achieve composition uniformity and to control the temperature in the mould. Where foams are produced, control of bubble nucleation is an additional complicating factor e.g. Menges and Schwesig ). 

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