Glass Transition Temperature Profile of the Curing Reaction of Unsaturated Polyester Resin

Abstract

ABSTRACT:The curing reaction of unsaturated polyester/styrene system was investigated by differential scanning calorimetry (DSC). Between gelation and vitrification (where T 8 of the curing system equals to the curing temperature), the propagation of the curing reaction is kinetically controlled whilst the termination reaction is diffusion controlled. After vitrification, both the propagation and termination reactions are diffusion controlled. The variation of T 8 during the curing reaction was calculated from the curing profile obtained from DSC measurements by using an empirical equation developed in a previous work. The experimental data indicated that there exist a critical point such that the time-temperature shifts of T 8 vs. ln(cure time) data at different cure temperatures formed a master curve between critical point and vitrification at an arbitrary reference temperature and yielded a single Arrhenius activation energy. These results indicated that polymerization between critical point and vitrification was T, dependent only.KEY WORDS Unsaturated Polyester / Glass Transition Temperature / Differential Scanning Calorimetry / Unsaturated polyester resin is one of the most important thermosetting resins for preparing molding compounds (SMC and BMC), resin transfer molding (RTM), hand lay up fiber reinforced plastics, and filament winding fiber reinforced plastics, etc. 1 • 2 Recently, the processing of thermosetting unsaturated polyester resins has received increasing attention from industry, especially from the automotives and aerospace industries.Curing of unsaturated polyester resins generally involves the transformation of low molecular weight amorphous solids by means of chemical reactions. The curing process is of particular importance in the making of structural composites, coatings, adhesives, and electronic encapsulants. An useful framework for understanding and conceptualizing the changes that occur during cure of a thermostting system is the isothermal time-temperature-transformation curves. 3 The curing of styrene-unsaturated polyester resins is a free radical polymerization. The reaction of styrene with unsaturated polyester includes not only kinetic reaction but also gelation and network formation. Several researchers 4 • 5 reported that before gelation "microgel" structure formed through intra-molecular cross-linking among the pendant C = C bonds of the polyester molecule. As the polymerization proceeds, the microgel particles growth both in particle sizes and numbers and the system gelled at a critical conversion where the microgel particles started to overlap. After gelation the termination between the free radical polymer chains becomes diffusion controlled, the rate coefficient for termination begins to decrease markedly and the reaction rate increases rapidly. During curing the glass transition temperature (T 8 ) of the system increases. As T 8 becomes higher than the cure temperature, the polymer network goes into the glassy state and the diffusion of monomer molecules in the polymer d...

Effect of Post-Cure on the Static and Viscoelastic Properties of a Polyester Resin

“…In the literature, at least for the last three decades, DSC studies are used to more accurately calculate the percentage of the crosslinking degree [ 33 , 34 , 35 , 36 ]. However, the reported studies usually refer to the system with a complete crosslinking reaction, which, as stated before, will not occur for the used resin nor for the tested temperatures.…”