Development of hemp fabric reinforced composites

Development of hemp fabric reinforced composites

One of the activities producing a destructive impact on the environment is uncontrolled or illegal timber harvesting. It is a response to high market demand due to the modernisation of lifestyles. The awareness of world’s population of the global environmental problems related to this issue has increased the popularity of natural fibre reinforced composites. Its availability and the data available on its properties tend to provide assurance for its application in many fields. The main objective of this project is to develop material using woven hemp fabric (WHF) and vinyl ester resin as a potential alternative to the utilisation of woods or engineered wood products. Apart from the composite development, fire retardant, water absorption and fatigue properties of the WHF reinforced composite are investigated.

Some characterisations of the fabric properties were undertaken to verify the data given by the supplier as well as to collect more data about the fabric since the data given by the supplier is usually more general and inadequate for technical purposes. Investigation was done on the properties of two batches of a similar nominal quality which were obtained within a three month time interval. The weight of fabric was found to be lower than what was provided by the supplier. Other properties measured were fabric densities, yarn sizes, yarn crimp percentage, cloth cover factors, chemical composition, thermal properties as well as fabric strength. It was concluded that the properties of the two batches are most likely not only identical from a textile point of view, but are also similar from an engineering point of view. Characterisation on the composite made of WHF with different layering orientations and vinyl ester (HVE) were found to have consistent density and fibre volume fraction due to properties of WHF. In terms of tensile, flexural and impact properties, depending on the layer orientation, their properties were slightly different. Inferential statistical analysis confirmed layering orientation affected the mechanical properties of the fabricated composite. Nevertheless, the differences among them less than 10% which suggests that any layering of this fabric can be used for composite fabrication. The properties of fabricated composites were comparable to some wood and engineered wood product. However, their densities were found to be higher than those of woods.

When the WHFs were treated with sodium hydroxide (NaOH), fire retardant chemical (FR) and combination of both (NaOH+FR), their densities and shrinkage properties increased due to the swollen fibres. The fire retardant properties of treated fabric also increased and this were proven by the burning, themogravimetry analysis and limiting oxygen index tests. However, all the treatments decreased the mechanical strength of WHFs. All these treated fabric were then utilised to reinforce vinyl ester using [0, 90]10 layering orientation. The changes in the WHFs’s physical properties increased the density (ranging from 1.1 to 1.21g/cm3) and fibre volume fraction (ranging from 33 to 37.12%) of HVEs. Although the mechanical properties of HVEs made of treated WHFs were found to be decreased due to poor adhesion between hemp fibres and vinyl ester, they are still comparable to woods and engineered wood products. The improvements in fire retardant properties for treated fabrics were also proven to enhance their HVEs’ fire retardant properties. HVE made of WHF treated with FR was found to be the best against the fire and its mechanical properties were still suitable for use as an alternative to woods and engineered wood products. This kind of HVE was then used to analyse it degradation properties (mechanical and fire retardant properties) when subjected to water absorption.

In terms of HVEs’ water absorption properties, the maximum water uptake and time to reach saturation point were 3.27% and 552 hr respectively. The diffusion coefficient calculated using Fick’s law, which is 4.71E-06 mm2/s and this was found lower than wood products. In terms of mechanical properties, the presence of water reduced the tensile strength and modulus up to 24% and 39% respectively due to the penetration of water which weakened the adhesion between the fibres and resin. The flexural properties was increased after 2688 hr of water immersion due to the swollen fibres which is attributed to high amount of water infiltration thus fill the gaps between the fibre and the matrix. The presence of water also degraded its fire retardant performance. It was found that the durability or fastness of fire retardant chemical treatment on this HVE composite was between 168 hr and 840 hr of water immersion. In terms of fatigue properties, the fatigue strength coefficient, b, was found to be 0.12 and these HVEs were tested under low load cycle with higher stress level ranging from 50% up to 80%. As suggested by other worker, the safety limits for sample HVE-UT (untreated WHF) and HVE-FR (FR treated WHF) are defined to be 30 and 24MPa respectively.

At the end of this study, a ‘material selection guide’ was established for the use by relevant stake holders when producing WHF reinforced vinyl ester. This selection guide is important which can be related in many ways such as in product life cycle, determination of cost/energy, identification of suitable application etc.