Chemorheology of Thermosets

Thermosets (e.g., unsaturated polyester, epoxy, urethane) are small molecules containing functional groups, which undergo chemical reactions (commonly referred to as “cure”) in the presence of an initiator(s) or a catalyst(s). In a broader sense, thermosets can be regarded as being parts of reactive polymer systems, which include pairs of polymers (e.g., blends of maleated polyolefin and nylon 6, as presented in Chapter 11) that undergo chemical reactions during compounding, and mixtures of an elastomer and a vulcanizing agent that undergo cross-link reactions (commonly referred to as vulcanization) at an elevated temperature. The subject of investigating the rheological behavior of reactive polymer systems is referred to as “chemorheology.” Since chemorheology is such a very broad field of investigation, one must specify the polymer system under consideration, classifying as chemorheology of thermosets, chemorheology of reactive polymer blends, chemorheology of elastomer vulcanization, and so on. In this chapter, for a number of reasons we restrict our presentation to the chemorheology of thermosets only. These reasons include (1) the limited space available here, meaning that it is not possible to present the chemorheology of every reactive polymer system, (2) thermosets play a very important role in polymer processing from an industrial point of view, and (3) the presentation of the chemorheology of thermosets in this chapter lays the foundation for the presentation of processing of thermosets in Chapters 11–13 of Volume 2. In the 1970s and 1980s, considerable amounts of effort were spent on investigating the chemorheology of thermosets. There are many experimental techniques that have been used to investigate the cure kinetics of thermosets: differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, dielectric measurements, and rheokinetic measurements. There are monographs (Kock 1977; May 1983; Turi 1981) and a comprehensive review article (Halley and Mackay 1996) on the subject. A better understanding of the chemorheology of thermosets requires an understanding of the kinetics of chemical reactions during cure. It can then easily be surmised that an understanding of the chemorheology of thermosets is much more complex than the rheology of thermoplastics presented in Chapter 6 through Chapter 12.