Cure shrinkage control of polymerization systems
Armeniades, C. D.
Doctor of Philosophy
Cure shrinkage is an inherent property of polymerizing systems due to the conversion of secondary bonds between monomer (or prepolymer) molecules to primary bonds, which have smaller interatomic distances. Cure shrinkage is highly undesirable: it impairs dimensional control and causes poor surface finish in molded polymers; it also generates setting stresses in highly filled systems. Previous methods for cure shrinkage control require special materials and conditions or entail the formation of voids. In this investigation, two processes were developed for producing polymer systems with zero shrinkage or slight expansion: (a) use of ammonia-modified montomorillonite as additive; and (b) microphase separation. The first method utilizes the dilatation of specially-modified montmorillonite (MMT) particles to counteract resin polymerization shrinkage. The MMT particles are first processed by replacing part of their hydration water with ammonia (which forms coordination bonds with SiO$\sb2$ in the mineral crystal) then dispersed into the resin. During cure at ambient temperatures the polymerization exotherm raises the temperature to 60-80$\sp\circ$C, breaking the SiO$\sb2$-NH$\sb3$ bonds; however the liberated gaseous ammonia cannot escape outside the resin-embedded MMT particles and forces them to dilate to more than twice their original size. By controlling the amount of ammonia-modified MMT added to the resin (in amounts of 6-10%) we obtain cured systems that show zero shrinkage and have no setting stresses. This can increase their strength by ca. 20-40%. The second method achieves cure shrinkage control by microphase separation. Certain multicomponent acrylic systems, when polymerized rapidly, separate into microdomains of different phases with a corresponding reduction in cure shrinkage. We attribute this phenomenon to lower efficiency in molecular chain packing at the interphase boundaries; the phase separation itself is attributed to local, diffusion-controlled composition changes during rapid cure. Accumulation of the local volume increase as the microphase boundaries gives rise to the observed reduction in cure shrinkage.