Interfacial Adhesion in Natural and Synthetic Fibre Composites: a Physical-Chemical-Mechanical Approach (Grensvlakhechting in natuurlijke en synthetische vezel composieten: een fysisch-chemisch-mechanische methodiek)

Natural fibres are a realistic and ecological alternative to synthetic fibres as reinforcement for polymer composite materials due to their low cost, environmental friendliness, natural abundance, and mechanical properties. The latter allows the design of composite materials to meet specific mechanical properties. For instance, bamboo exhibits a combination of low density (~1.4 g/cm³) and high stiffness (~43 GPa) and strength (~800 MPa), while coir fibres are not very strong and stiff, but exhibit high strain to failure (approximately 40%).However, their potential as reinforcing agent is reduced when compatibility problems with polymer matrices arise at the interface. The generally hydrophilic nature of natural fibres produces low interfacial interactions with certain important hydrophobic thermoplastic matrices, such as polyethylene and polypropylene, leading to a poor interfacial strength.Mechanical properties of composite materials can be greatly affected by the bond strength at the fibre-matrix interface. When a composite structure is loaded, the load is transferred from the matrix to the fibres mainly through shear stresses at the interface. Load transfer increases with increasing interfacial (shear) stresses, thus improving the composite strength. The adhesion at the interface can be described as a combination of physical adhesion (related to wettability), chemical bonding, and mechanical interlocking. In this study, the wettability and compatibility of the fibre and the matrix is assessed by the analysis of their surface energies. This is done by measuring contact angles, which is a quantitative measure of solid-liquid molecular interactions and thus provides information on the surface energy of solids.This dissertation develops an interdisciplinary and integrated approach that deals with the physical, chemical, and mechanical aspects of natural bamboo fibre composite interfaces. The wetting and mechanical behaviour at the interface of smooth and isotropic synthetic fibres (glass, polyethylene terephthalate) are compared with that of a rough and anisotropic natural fibre (bamboo). Atomic force microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and optical profilometry techniques were used to study the fibres topography and chemistry. A new approach, based on an autoclave treatment, to reduce the noise in performing contact angle measurements on rough natural fibre surfaces is presented. The results indicate that the high concentration of lignin on the surface of bamboo fibres is responsible for their surface wetting properties.The wetting dynamics of various test liquids on bamboo fibres is analysed by applying the molecular-kinetic theory of wetting. The surface free energy components are calculated according to the acidbase theory. These values are then used to calculate the theoretical work of adhesion, spreading coefficient, wetting tension, and interfacial energy for analysing the fibre-matrix compatibility. Additionally, a novel way to measure equilibrium contact angles by using sound excitation is proposed. This way it is possible to better take into account both polar and dispersive surface energy components. The findings suggest that the contact angle obtained by forcing relaxation through acoustic vibration is a reliable method to study the wetting behaviour of natural fibres.Moreover, understanding of the stress state in the composite after processing and during mechanical testing is required for a correct analysis of adhesion properties. The mechanical strength of the interfaces was assessed by single fibre pull-out tests, and transverse 3-point bending tests. The fibre matrix interfacial bond strength was characterized by the critical local value of interfacial shear stress. Since during crack initiation in the pull-out test, the crack surfaces move directly apart, it is possible to correctly relate the theoretical work of adhesion with the normal stress at the debond point. This radial normal stress at the interface at the moment of crack initiation is also used in this study as a parameter for correlating thermodynamic work of adhesion and practical adhesion.

Publication year:2014

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