Interfaces in Natural Fibre Composites: Effect of Surface Energy and Physical Adhesion

An integrated physical-chemical-mechanical approach was used to study the effect of adhesion on the mechanical strength of composites reinforced with bamboo and coir fibres, and three thermoplastic matrices (Polypropylene (PP), maleic anhydride grafted polypropylene (MAPP), and polyvinylidene fluoride (PVDF). Surface chemical analysis was conducted by X-ray photoelectron spectroscopy, XPS. Surface physical-chemistry was studied by determining surface energy components of fibres and matrices, by performing contact angle measurements with various known test fluids and employing a 3-component model for surface energy. Work of physical adhesion was calculated. Transverse 3-point bending tests on uni-directional composites were conducted to study interface strength and composite strength was determined by longitudinal flexural tests. Results show that increase in physical adhesion can explain the improved interface and longitudinal strength in bamboo-PVDF and coir-PVDF composites, as compared to the PP composites. For MAPP, no increase in physical adhesion is predicted, but particularly for coir a strong increase in mechanical interface strength and consequently an increase in longitudinal strength was observed, which may be attributed to a chemical adhesion mechanism.status: Published onlin

Mechanical behaviour and practical adhesion at a bamboo composite interface: physical adhesion and mechanical interlocking

Physical adhesion was experimentally determined by measuring contact angles with different liquids on bamboo and glass fibers, using the Wilhelmy technique, and by applying the acid-base theory for calculating the surface energy components and the theoretical work of adhesion. The mechanical strength of the interfaces was assessed by single fibre pull-out tests. In order to consider the real mechanisms of interfacial failure of natural fiber composites, the fibre matrix interfacial bond strength was characterized by the critical local value of interfacial shear stress, τ_d, and the radial normal stress at the interface, σult, at the moment of crack initiation. Both interfacial parameters are used for correlating thermodynamic work of adhesion and practical adhesion. Pull-out tests (taking into account friction), XPS, and profilometry techniques were used to study the influence of rough natural fibre surfaces on the interface between the fibre and a thermoplastic matrix, by comparing the mechanical behaviour at the interface of a smooth optical glass fibre with that of rough natural fibres. The results suggest that the physical and chemical compatibility between the bamboo fibre and the matrix does not improve substantially the composite performance if compared with glass composites. The relatively low off-axis strength of the bamboo fibres is suggested as the main reason for the low stress transfer capability at the fibre-matrix interphase. Furthermore, the pull-out process may be friction-dominated in bamboo fibre systems.status: publishe

The influences of fibre-matrix interfacial adhesion on composite properties in natural fibre composites

In this research, the interface between natural fibres and thermoplastic matrices is studied, in which fibre-matrix wetting analysis and interfacial adhesion are investigated to obtain a systematic understanding of the interface. In wetting analysis, the surface energies of the fibres and the matrices are estimated using their contact angles in test liquids. Then, work of adhesion is calculated for each composite system. For the interface tests, transverse three point bending (3PBT) tests on UD composites are performed to measure interfacial strength. XPS characterisation on the fibres is also carried out to obtain more information about the surface chemistry of the fibres. Unidirectional (UD) composites are examined to explore the correlation between the fibre-matrix interface and the final properties of the composites. The results suggest that the higher interfacial adhesion of the treated fibre composites compared to untreated fibre composites can be attributed to higher fibre–matrix physico-chemical interaction corresponding with the work of adhesion. In agreement with the interface evaluation, the flexural properties of the composites are significantly influenced by their interfacial adhesion.status: publishe

Equilibrium contact angle measurements of natural fibers by an acoustic vibration technique

Sorption of fluids during typical wetting experiments on natural fibers produces a zero receding contact angle situation, leading to an incomplete analysis of their wetting behavior. An acoustic vibration method was used to measure “equilibrium” contact angles on natural bamboo fibers. The correctness of the technique is verified by performing the experiment with polyethylene terephthalate (PET) fibers and films, and comparing these results with the average of the cosine functions of the advancing and receding angles for a given system. Surface energies and components of the surface energies of bamboo and PET fibers were estimated using the “equilibrium” contact angle data of various test liquids by using the acid–base approach. The results are in general agreement with X-ray photoelectron spectroscopy (XPS) analysis of the fiber's surface. The findings contribute to a better understanding of the complex phenomena occurring during wetting of natural fibers and suggest that the contact angle obtained by forcing relaxation through acoustic vibration is a reliable method for study the wetting behavior of natural fibers.publisher: Elsevier articletitle: Equilibrium contact angle measurements of natural fibers by an acoustic vibration technique journaltitle: Colloids and Surfaces A: Physicochemical and Engineering Aspects articlelink: http://dx.doi.org/10.1016/j.colsurfa.2014.04.054 content_type: article copyright: Copyright © 2014 Elsevier B.V. All rights reserved.status: publishe