Fracture and interlaminar properties of clay-modified epoxies and their glass reinforced laminates

Abstract

The present work illustrates the experimental results of a project aiming to assess the benefits deriving from the matrix nanomodification of composite laminates made by vacuum infusion of woven glass fabrics. The following properties have been investigated: mode I fracture toughness and crack propagation resistance for neat and clay-modified epoxy, interlaminar shear strength, mode I delamination resistance for base and clay-modified epoxy laminates.

Available results indicate a significant improvement in the fracture toughness and crack propagation threshold of clay-modified epoxy. However, due to the nanofiller morphology, the behaviour of clay-modified laminates is still almost comparable to that of the base laminates.

Introduction

The chance to get substantial improvements of mechanical properties at low nanofiller volume fraction has arisen significant interest in the use of nanomodified epoxy resins.

It is well acknowledged that to achieve these results the nanofiller must be sufficiently dispersed and compatible with the epoxy resin. This requirement leads to a number of processing challenges, which depend on the adopted nanofiller.

As far as layered silicates are concerned, the dimensions of the clay platelets are of the order of microns in area, around 1 nm thick and arranged in stacks (tactoids). Complete exfoliation requires the separation of the tactoids from the primary particle, followed by the destruction of the order of the clay platelets within the tactoids.

In principle, a full exfoliation of the clay platelets will maximise the strength, modulus and toughness improvement [1], [2]. However a balance between an exfoliated and intercalated structure might be preferable to maximise enhancements in the mentioned properties [3], intercalated tactoids promoting some toughening mechanisms such as crack deflection or crack pinning [4].

The weak out-of-plane inter-laminar properties of laminates are definitely those with the greatest potential and need to be improved. Indeed, for ternary laminates, matrix toughness improvement itself is the most interesting and promising result, the interlaminar fracture behaviour of traditional composites being a weak matrix dominated property.

Unluckily, the research performed to date, aimed at translating resin properties to the fibre reinforced composite, has met with changing fortunes. Rice et al. [5] reported a 12% improvement in modulus for aerospace composite materials at 2 wt.% of organosilicate, without improvements in other mechanical properties. Timmerman et al. [6], reported negligible improvements in mechanical properties of nanoclay composites compared to traditional composites. However, they reported a significant reduction in transversal microcracking during cryogenic cycling thus indicating the need for careful selection of nanoclay concentration and surface modification. Becker et al. [7] have shown that improvements in crack opening fracture toughness can be achieved at low levels of clay addition. Quaresimin and Varley [8] reported “selective” improvements in toughness properties of carbon/clay-modified epoxy laminates due to the clay distributions: mode I toughness was slightly decreased while mode II slightly increased with respect to the values for neat epoxy laminates. The same behaviour was seen also for vapour grown carbon fibres (VGCF) modified laminates.

This paper presents the results of the ongoing studies carried out by the authors on the effect of nanomodification and its industrial potential by discussing the experimental results obtained on neat and nanomodified epoxy, as well as on neat and nanomodified epoxy laminates (ternary laminates).

After a brief description of the adopted materials and the manufacturing process, chosen for their industrial potential, we will present and discuss the experimental results obtained on nanomodified resins and laminates as well as investigations on the material morphology.