PHYSICOCHEMICAL INTERACTIONS BETWEEN MONTMORILLONITE AND POLYMERIZING SYSTEMS: EFFECT ON PARTICLE-REINFORCED COMPOSITES (CONCRETE, ZERO-SHRINKAGE, EXPANDABLE)
Doctor of Philosophy
Highly filled polymer composites, such as polymer concrete (PC), suffer from setting stresses generated during the cure of the resin binder, when polymerization shrinkage is hindered by the close packing of filler and aggregate particles. Setting stresses impair significantly the strength of the cured composite. Current zero-shrinkage and expanding polymer concrete formulations achieve these properties with a sacrifice in strength. In this investigation, a novel system was developed for producing zero-shrinkage and expanding polymer concrete composites with concomitant enhancement in strength. This was achieved by dispersing small amounts of the hydrated mineral montmorillonite (MMT) into the resin and was found effective with three different resin binders (polyester, epoxies, and acrylics). Most resins require less than 2% MMT to produce zero-shrinkage systems with flexural strengths and splitting tensile strengths 30% and 16% greater than conventional PC; higher MMT contents create PC systems which expand upon curing or generate positive hydrostatic pressure during constant-volume cure. We have examined here the complex physicochemical interactions between the polymerizing resin and the dehydrating mineral, which give rise to the observed cure-expansion and strength enhancement. This required extensive experimental work, including strength measurements, X-ray diffractometry, differential scanning calorimetry, thermomechanical analysis, scanning electron microscopy, X-ray spectroscopy, and gas chromatography/mass spectrometry data. Based on the results of these measurements we propose a mineral-resin interaction mechanism that involves the migration of organic species from the curing resin into the MMT crystal structure; these organic molecules replace some of the ordered hydration water, released by the mineral at the temperatures generated by the exothermic polymerization reaction. The MMT-resin bonding takes place through the molecules of silane coupling agents, thus contributing to the strength enhancement of the composite. The observed expansion at cures above 100(DEGREES)C is due to the release of highly disordered water, which remains distributed in the pores and internal structure of MMT, rather than forming a discrete gas phase.