Finishing of carbon fibres

Abstract

The process of carbon fibre finishing by various finishing agents is studied. Analysis of the obtained data show that the test finishing agents, vis-à-vis the standard one, improve the properties of the fibre and the fibreglass based on it. The strength of monofilaments containing the finishing agent is slightly lower than that of thread containing a standard finishing agent, but the degree of attainment of the strength in fibreglass and, consequently, the value of the attained strength is much higher when the test finishing agents are used. The test finishing agents of 2 and 3 type increase the abrasion resistance of carbon tows by 80%.High-strength, high-modulus carbon fibres are nowadays the most effective reinforcing fillers of composite materials. Most commonly, epoxy resins are used as a binder in such composites. The activity of nonpolar surface of carbon fibres submitted to heat treatment at about 1500°C in nonoxidizing medium for its manufacture leads to weak reaction with polar groups of epoxy resins.Two methods are employed to intensify their reaction. The surface of the carbon fibres are activated (oxidized), which increases the specific surface of the fibre and facilitates formation of surface oxygen-bearing groups, and a thin layer of oligomers, generally containing active epoxy or amino groups, is laid on the surface. Such a layer markedly increases adhesive interaction between the active groups of the carbon fibre and the functional groups of the binder.The function of the finishing agent, however, is not merely to promote adhesion between the filler and the binder in the composite. Since carbon fibres are highly brittle and wear out on abrasion easily, their protection against breaking in subsequent treatment processes (weaving, producing prepreg, winding, etc.) acquires special importance. The finishing agents for carbon fibres must therefore have affinity to the fibre and the binder as well as high lubricating properties. Further, the finishing agents must reduce stresses developed on the fibrematrix interface under dynamic loads due to the difference in the deformation properties of these components of the composite. Finishing agents consisting of epoxy resins have been used in Russia so far to produce carbon fibres. After drying, a layer of such resins, in spite of its small thickness, remains brittle and transmits this brittleness to the fibres without enhancing the ability of the fibre to undergo subsequent treatment.In this work, for finishing carbon fibres we used a mixed composition containing aqueous dispersions of epoxy resins and polyurethane and aqueous solution of trialkoxysilane. Polyurethane was used to stabilize the dispersion and enhance the elasticity of the finished fibre, and trialkoxysilane played the part of elastic cross-linking of the finishing agent molecules and for imparting the properties of the lubricant to it. As a result of preliminary studies of various versions of finishing agent preparations from a number of starting compounds, the basic compositions that ensure...