Natural plant fibres have recently received a great attention as potential alternative replacement for synthetic fibres, as reinforcement of different polymer matrices for various engineering applications. This is due to their desirable properties such as low density and high specific mechanical properties, at the same time they are sustainable, renewable, and eco-friendly. However, one of the main issues that has limited the applications of plant fibres in polymer composites is their incompatibility with polymer matrices. This have resulted in poor interfacial bonding between the fibres and matrix, and consequently reduction in the mechanical properties of the composites. Therefore, surface modification of plant fibres is necessary to modify the fibre surface characteristics and to improve their surface adhesion properties. Chemical treatments are commonly used in modifying the surface characteristics of plant fibres, however, these treatments have some disadvantages such as consuming substantial volumes of liquids and chemicals as well as the treatment time could be hours and sometimes days. As an alternative, plasma treatment has attracted more attention in recent years, as it is simple, dry (solvent-free), environmentally friendly, and time-saving (reduces energy consumption). However, the efficiency of plasma surface treatments depend on several factors such as the nature of the substrate and the plasma operating conditions, that could affect the fibre main properties.
Therefore, in this study, plasma treatment as a prospective tool for simple and efficient fibre surface modification was used with the goal of improving the surface adhesion properties of ramie plant fibres without adversely affecting the fibre mechanical properties. The results reveal nanostructures at the fibre surface including microfibrils and elementary fibrils after plasma treatment, and their importance for fibre surface wettability, fibre-thermoset polymer interlocking, and fibre mechanical properties. All these peak at short treatment times. In addition, the problem of lack of adhesion between natural plant fibres and polymer matrices can be minimised if phenolic thermoset resin is used as a matrix for plant fibres. This is because some degree of interaction is expected to exist between the hydrophilic natural fibres and polar hydroxyl groups present in the structure of phenolic resins. However, the presence of microvoids in the structure of cured phenolic resins adversely affect the mechanical properties. The conventional approach to overcome this challenge and to produce void free structure is via utilising long heating cure cycles (3-4 days), which is not favourable for most industries due to time and energy consumption issues. Therefore, in this study, the tailoring of void size and distribution in the phenolic resins was introduced as an alternative approach to achieve a better balance between processing time and mechanical properties. For this purpose, two different catalysts (slow and fast action acid catalysts) and a short cure cycle (4 hrs) were used to alter the void size and distributions in the structure of phenolic resins. Long heat cure cycle (4 days) was also used to obtain void free phenolic structure without the addition of any catalyst and used as a reference sample in terms of flexural properties. The flexural fractured surfaces of all materials were investigated using low voltage scanning electron microscopy (LV-SEM), Fourier-transform infrared spectroscopy (FTIR). Results are compared to finite element modelling that confirmed the effects of different void size and distributions on the mechanical properties.
Finally, the effects of plasma treatment and cure cycle conditions on the properties of random short ramie fibres reinforced phenolic composites were investigated. A new method for making mats of short fibres was developed and used for the fabrication of composites containing plasma treated fibres. The results indicated clear effects for the plasma treatment and cure cycle conditions on the fibre-matrix interfacial bonding, and consequently the flexural properties of the composites.
