NOVEL TECHNIQUES FOR INTERFACE/INTERPHASE TAILORING IN ARAMID - EPOXY COMPOSITES AND EFFECT ON MECHANICAL / THERMO-MECHANICAL PROPERTIES

Abstract

It is a well known fact that aramid fibres exhibit poor adhesion with most matrices, and many research efforts have been devoted to improve this adhesion. A novel chemical treatment has been developed in this thesis for the surface modification of aramid fibres that is based on the N-acylation of surface amide groups with an acid anhydride. Adhering to the defined objectives, we have shown that the treatment is mild, not degrading significantly the mechanical properties of the fibres. There was only a slight decrease in fibre strength up to 2.89%, the SEM of treated surfaces revealed only a marginal increase in surface roughness. The surfaceanhydride interaction was indicated by differential scanning calorimetry (DSC) and confirmed by FTIR and XPS techniques. The fibre surface treatment increased interfacial shear strength in model epoxy composites by about 21% and interlaminar shear strength in the unidirectional treated fibres-epoxy laminates up to 16% over the control samples. A significant increase in storage modulus in composites containing treated fibres has been recorded. In recent years, graphene /graphene oxide (GO) – a two dimensional nano material derived from graphite by oxidation and/or exfoliation of its layers – has shown great promise as nano reinforcement in polymer composites, and for tailoring interphases. In a novel approach, we have tailored the interphase in aramid-epoxy model composites by chemically grafting GO on functionalized surface of aramid fibres thus producing a multiscale reinforcement and studied the effect on interfacial shear property. In addition to improving fibre-matrix adhesion, the approach has exhibited the potential of simultaneously improving the fibre strength, a feature which may be beneficial for improving properties of the existing materials. The reactions were facilitated in both acidic and basic reaction conditions which resulted in different end-properties. The plasma co-polymerized functional primer coating has been characterized by contact angle goniometry, micro Raman and X-ray photoelectron spectroscopy. Raman and FTIR spectroscopy was used for the characterization of graphene oxide. Better strength and adhesion properties were observed for fibres treated in alkaline GO bath as compared to those treated in acidic condition, the mean tensile strength of the treated fibres improved by 7.9% over control sample. A significant improvement up to 44.68% was observed in interfacial shear strength (IFSS) in model composites of modified fibres with epoxy. vi Recent studies have demonstrated that addition of small amounts of functionalised CNTs in epoxy matrix improved the interlaminar properties of glass and carbon fibre composites. In a separate study in present thesis, Woven Kevlar® Fabric/ nanoreinforced epoxy laminates have been prepared by hand layup technique, wherein a small quantity (0.2 to 0.5 wt%) of acid-oxidized MWCNTs was dispersed in the epoxy matrix before laminate fabrication. Flexural strength and dynamic mechanical properties have been measured for epoxy-CNTs nanocomposites as well as for multiscale hybrid composites and the interlaminar shear strength (ILSS) for the later. A 20.26% improvement in average flexural strength and 16.37% improvement in flexural modulus were observed in epoxy-CNTs nanocomposites over the neat epoxy with only 0.2 wt% additions of MWCNTs. The enhanced mechanical properties of the nanomodified matrices reflected accordingly in the flexure tests of corresponding multiscale aramid-CNTs-epoxy composites, the best properties among the formulations were observed for the hybrid composite containing 0.2 wt% CNTs. An enhancement of 15.88% in flexural strength was observed over aramid fabricepoxy laminate without CNTs while elongation at break was also higher. Improved mechanical properties associated with the addition of nanophase were also exhibited in DMTA analysis. The short beam shear method for measuring ILSS proved unsuitable for the composites under study.