Pulped Phormium tenax leaf fibres as reinforcement for epoxy composites

Abstract

Leaf fibres from Phormium tenax (harakeke, New Zealand flax) were pulped at 170 °C with NaOH and anthraquinone. The pulp was wet laid to form mats, which were used to reinforce epoxy composites. The flexural modulus was almost as high as that measured for epoxy reinforced with glass chopped strand mat at the same weight fraction. The flexural strength was two-thirds that of the glass-reinforced composite. Failure was abrupt. SEM images showed torn fragments of fibre cell walls protruding from the fracture surface, indicating strong interfacial bonding. Good mechanical performance was attributed to the rarity of kink bands in the individual fibre cells, along with wrinkled cell-wall surfaces that enhanced the area of the fibre–matrix interface.

Introduction

Leaf fibres are often used to reinforce composite materials, primarily because of their low cost relative to bast fibres such as flax and hemp [1], [2]. Untreated leaf fibres provide little or no improvement in mechanical properties relative to unreinforced resin [3], [4], [5], [6], [7], [8]. Numerous chemical treatments have been studied [2]. Adding a binder, e.g., polyvinyl acetate, can enhance the reinforcing properties [9]. We assessed pulped leaf fibres as an alternative form of reinforcement.

Fig. 1 summarises flexural strengths reported for unsaturated polyester resins reinforced with untreated fibres from the leaves of three different species of plants [3], [4], [5], [6], [7], [8]. Unidirectional composites were excluded from this summary. Most of the data points for composites reinforced with abaca and/or sisal fibres showed no detectable strengthening. One study of sisal and two studies of pineapple leaf fibre (PALF) showed small improvements in flexural strength. Switching to an epoxy resin raised the flexural modulus for sisal-reinforced composites, without a significant improvement in strength [10].

The poor performance of leaf fibres might be associated with the fact that they are bundles of many fibre cells, often displaying 100 or more cells in a cross-sectional cut [1]. Only a small proportion of those cells contribute to the fibre–matrix interface. PALF bundle diameters, typically in the range 20–80 μm, are smaller than those for sisal bundle diameters, typically 50–200 μm [2]. The smaller diameters expose a higher proportion of fibre cells to the matrix, helping to account for the superior mechanical properties that are usually observed for PALF (Fig. 1). Further improvements in mechanical strength might be achieved by pulping leaf fibres to liberate the individual fibre cells, so that all of the cells contribute to the fibre–matrix interface. Duchemin et al. [11] tested that idea by pulping harakeke (Phormium tenax) leaf fibres at 170 °C, mixing the fibres with poly(lactic acid) and compression moulding. The composites were stronger than those reinforced with raw leaf fibres at the same fibre fractions. We extended that line of research by embedding harakeke leaf fibres in a thermosetting matrix.