The Crosslinking of Polyesters:a Study of network formation by physical methods

Abstract

Unsaturated polyester resins copolymerize with styrene giving crosslinked products, The useful properties of the crosslinked material depend greatly on the extent of completion of the reaction, which is difficult to determine chemically. It is important to know how to assess the degree of crosslinking, and how physical, electrical and mechanical properties change during the curing reaction. A commercial polyester, of known constitution, was cured isothermally under standardized conditions. Dielectric constant, electrical dissipation factor, d.c. volume resistivity, refractive index, tensile strength, dynamic shear modulus, logarithmic decrement, Young's modulus at audio frequencies, volatile content, % extractable matter, and resistance to various aqueous solutions were all measured at intervals throughout the curing process. Because the conditions were standardized, the relationship between the various properties could be considered. In addition, an infrared spectroscopic method was devised by which the extent of chemical reaction could be assessed in an approximately quantitative way. This method possesses the advantage that no prior removal of unreacted material is necessary. The extent of reaction of styrene monomer, as determined spectroscopically, was in agreement with the determination by volatile content. Changes in resistivity were found to be related to the weight fraction of reacted unsaturation. The maximum slope of the semilogarithmic resistivity- time curve was proportional to the reaction rate. Apparent activation energies by resistivity, refractometry and gel time measurement were similar. Tensile strength did not become appreciable until over half the reaction had occurred It was then proportional to the percentage of reacted unsaturation. A small fraction of the polyester did not undergo reaction, but remained unattached, acting as a plasticizer. Electrical and mechanical damping terms were the quantities which changed most in the latter stages of cure, and seem useful for assessing the degree of crosslinking in this region.