THE ROLE OF SIZING RESINS, COUPLING AGENTS AND THEIR BLENDS ON THE FORMATION OF THE INTERPHASE IN GLASS-FIBER COMPOSITES

On fabrication, glass fibres are immediately coated with a sizing resi n via an aqueous emulsion, to protect the fibres from damage and provi de compatibility with the matrix. The choice of sizing system is not c lear and is generally proprietary so that very few studies have examin ed individually the role of the components in the sizing emulsion. The single filament fragmentation test has been used to examine the stres s 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 s izing films in two matrix epoxy resins, and the changes in matrix prop erties have been examined. By utilizing two matrices of differing modu lus, glass transition temperature and yield strength, the differences in the apparent or Kelly-Tyson interfacial shear strength (tau(a)) or reciprocal normalized fragment length can be fully interpreted. The va lues of tau(a) 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 stre ss transferred to the fibres to converge. The medium molecular weight size was found to be insoluble. A swollen distinct interphase was cons idered to account for observed fragmentation data in the two resins. T he high molecular weight size was found to modify both resins similarl y, so that a difference in the apparent shear strength was observed. A ddition of gamma-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. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.