Acid-base interactions in the interpretation of aramid composite and fabric mechanics

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Abstract
This paper considers two facets of the role of surface properties in the performance and manufacture of aramid fabric-thermoset composites. The main factors considered are the intrinsic adhesion between the fibre and the matrix in the composite, and the fibre-fibre frictional intcractions during its manufacture. The former is elucidated by using essentially the Fowkes acid-base approach and the lattcr by examining the mutual interfacial shear characteristics of the aramid fibres. A number of aramid fibre surfaces have been prepared with a range of interfacial shear strengths and acid-base properties. The interfacial shear strength behaviour of these fibres was qualitatively determined by measuring the compressive properties of aramid fabrics, whilst the acid-base properties of the fibres were determined from wettability experiments. These fabrics were then incorporated into an epoxy resin to form laminates. Contrary to the predictions based on the interfacial energetics, the toughest aramid composites, as determined by mode 1 fracture mechanics, were the laminates whose fibres had the lowest potential acid-base interactions, and were thus the least likely to have the greatest interfacial adhesion. This increased toughness has been attributed to enhanced fibre bridging across the crack zone which resulted in additional volumetric energy dissipation during fracture. This phenomenon is apparently promoted by the low interfacial shear strength between the fibres in the fabric during manufacture. Thus, for these aramid fibre laminates, any improvements in the toughness due to increased interfacial interactions were effectively masked by the more significant decreases in the volumetric energy dissipated. These changes are suggested to be a result of changes in fibre/fabric structure in the laminatc which originate during manufacture.

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