Interphase formation and environmental degradation in glass fiber/vinyl ester composites

Interphase formation mechanisms are identified, modeled, and correlated with degradation in glass fiber/thermoset polymer composites. This work focused on the effects of glass-fiber-surface coatings (sizings) in free-radical polymerizing vinyl ester/styrene resin. Prior research focused on silane coupling agents, which comprise only 10% of typical commercial sizing formulations. Well-defined models of commercial multi-component glass fiber sizings were formulated and applied to E-glass fibers in a manner consistent with commercial production.; Sized fiber were chemically analyzed before and after extraction with acetone, a solubility model for the resin. Solution FTIR, HPLC-MALLS and fiber XPS determined quantity and quality of physically- and chemically-adsorbed layers. In these systems, thermodynamic equilibrium dominates interphase formation and structure, not a diffusion-gradient mechanism.; Interphase environmental degradation (durability) was characterized by Interlaminar Shear Strength (ILSS) strength retention after hygrothermal exposure at 80°C, in unidirectional multi-fiber composites. Durability decreased with increasing amount of sizing inert relative to the matrix. Some sizing effects became less distinguished as the matrix degraded. Single fiber-microdroplet testing measuring interfacial shear strength showed similar results. Nonreactive species in the interphase would reduce the final interphase network crosslink density and reduce the durability of the interphase.; Species partitioning between the bulk and interphase are described using a multi-component model for polymer chemical potential with a complex non-linear solution. A critical aspect considered is the elastic response of the swollen bound phase. It was found that predicted interphase concentrations were very sensitive to the crosslink density of the bound sizing layer, Mc, which describes the sizing structure. Other physical properties are calculated from the interphase species concentration.; The thermodynamic analysis was validated by measuring the concentration of species in a model interphase, using FTIR-ATR, prepared on a germanium crystal. Bound sizing Mc was estimated for these systems. Interphase double bond concentration correlated linearly with the interphase dependent composite durability.; This analysis provides a basis for understanding of interphase behavior in glass fiber/thermosetting polymer composites, particularly a model for the chemical composition of the sizing induced interphase, taking into account sizing structure and composition. Further experimental work into composite interphase performance will benefit from the knowledge and material base developed.