Understanding the interfacial compatibility and adhesion of natural coir fibre thermoplastic composites

Abstract

An integrated physical–chemical–micromechanical approach is implemented to investigate the fibre–matrix interfacial compatibility and adhesion of natural coir fibre composites. Wetting measurements of the fibres and the matrices are carried out to obtain their static equilibrium contact angles in various liquids, and these are used to estimate the surface energies comprising of different components. The work of adhesion is calculated for each composite system, accordingly. Also, fibre surface chemistry is examined by X-ray photoelectron spectroscopy (XPS) to have more information about functional groups at the fibre surface, which assists in a deeper understanding of the interactions at the composite interfaces. Single fibre pull-out tests and transverse three point bending tests are performed on UD composites to measure interfacial shear strength and interfacial tensile strength respectively. The results suggest that the higher interfacial adhesion of coir fibres with polyvinylidene fluoride compared with polypropylene can be attributed to higher fibre–matrix physico-chemical interaction corresponding with the work of adhesion. Whilst the improvement of interfacial adhesion for coir fibres with maleic anhydride grafted polypropylene compared with polypropylene can probably be attributed to a chemical adhesion mechanism. In agreement with the interface evaluations, the flexural strength in longitudinal direction of the composites is largely correlated with their interfacial adhesion.

Introduction

Natural fibres extracted from different parts of plants typically have different surface chemistry properties. The surfaces of natural fibres have differences in the degree of hydrophilicity, are relatively rough and physico-chemically heterogeneous. For instance, the surface of flax fibres was found to be quite hydrophobic, containing fatty substances and waxes [1], [2], and similar for coir fibre surface a relatively high proportion of waxes was also identified [3], [4], while lignin was largely found on bamboo fibre surface [5]. The fibre surface properties strongly influence the fibre–matrix interactions in the composite.

Generally, the adhesion at the interface can be described by the following main interactions: physical adhesion related to surface energies of the fibre and the matrix, chemical bonding and thirdly mechanical interlocking created on rough fibre surfaces. Good interfacial adhesion initially requires a good wetting between the fibre and the matrix, to achieve an extensive and proper interfacial contact; for this the surface energies of the two materials are important parameters. The surface energy of a fibre generally should be higher than that of the liquid resin for a good wetting to take place during composite processing. Moreover, the surface energies will play an important role for keeping a stable contact after consolidation of the composite.

The interactions at fibre–matrix interface can be considered at different levels [6]. At the molecular level, the fibre–matrix interaction is determined by chemical groups presented on the surface of both phases. At this level, the interfacial adhesion depends on the physico-chemical interaction (e.g. van der Waals forces, acid–base interactions, hydrogen bond) and chemical bonds (covalent bonds). The physico-chemical interaction is quantitatively characterised by the work of adhesion. To understand the interactions at the molecular level, wetting parameters including contact angle, surface energy and work of adhesion have to be assessed. At the higher levels, the quality of the composite interface can be characterised by mechanical tests which are performed on either single fibre micro-composites or bulk laminate composites. In the former, a single fibre is embedded in a matrix block of different shapes and sizes. Then, interfacial shear strength (IFSS) is determined in various ways which comprise of pull-out, fragmentation and micro-indentation tests. Regarding bulk laminates, several testing techniques have been developed for unidirectional fibre composites such as transverse tensile and bending tests, short beam shear tests and the Iosipescu shear test. In these tests, the interface quality is characterised by either the transverse interfacial tensile strength (mode I) or interlaminar shear strength (ILSS).

Every abovementioned test has shown its advantages and limitations, which are mostly concerned with test sample preparation and properties of fibre and matrix, especially of the fibre surface [7]. In natural fibre composites, the dependency of the measured fibre–matrix interface properties on the particular test method can be aggravated due to their irregular geometry and surface condition. Therefore, a combination of test methods will provide a better understanding of the composite interface.

In this study, a combined physical–chemical–micromechanical interface study approach is implemented to investigate the interfacial adhesion between untreated and alkali treated coir fibre composites and various thermoplastics. Wetting measurements are carried out to determine the contact angles of fibres and matrices in various test liquids, which are used to estimate their surface energies and surface energy components. From these data, the fibre–matrix work of adhesion and interfacial energy are calculated to predict the physical adhesion and compatibility of the composites. The results are further interpreted in the light of the surface chemical composition of the fibres as determined by X-ray photoelectron spectroscopy (XPS). Flexural transverse three-point bending tests on unidirectional (UD) composites and single fibre pull-out tests are performed for determining interfacial tensile strength and interfacial shear strength (IFSS), to examine the interface quality and to obtain a deeper understanding of the interfacial interactions.