The role of sizing resins, coupling agents and their blends on the formation of the interphase in glass fibre composites

Abstract

On fabrication, glass fibres are immediately coated with a sizing resin via an aqueous emulsion, to protect the fibres from damage and provide compatibility with the matrix. The choice of sizing system is not clear and is generally proprietary so that very few studies have examined individually the role of the components in the sizing emulsion. The single filament fragmentation test has been used to examine the stress transferred between an epoxy resin matrix and fibres which have been coated with: (1), an epoxy resin size of varying molecular weight; (2) silanes at different coating thickness; and (3) blends of the silane and resin size deposited from emulsion. The solubility of the dried sizing films in two matrix epoxy resins, and the changes in matrix properties have been examined. By utilizing two matrices of differing modulus, glass transition temperature and yield strength, the differences in the apparent or Kelly–Tyson interfacial shear strength (τa) or reciprocal normalized fragment length can be fully interpreted. The values of τa are all lower than for the control unsized fibres. The low molecular weight size was readily soluble and found to modify the properties of the two matrices in an opposing manner, causing the stress transferred to the fibres to converge. The medium molecular weight size was found to be insoluble. A swollen distinct interphase was considered to account for observed fragmentation data in the two resins. The high molecular weight size was found to modify both resins similarly, so that a difference in the apparent shear strength was observed. Addition of γ-aminopropylsilane to the sizing emulsion reduced the apparent interfacial shear strength of the high molecular weight sized fibres. It is postulated that this arises from a greater probability that a distinct interphase will form as a result of a chain extension reaction which reduces its solubility.