The use of hemp fibers as reinforcement in composite materials has increased in recent years as a response to the increasing demand for developing biodegradable, sustainable, and recyclable materials. Hemp fibers are found in the stem of the plant which makes them strong and stiff, a primary requirement for the reinforcement of composite materials. Themechanical properties of hemp fibers are comparable to those of glass fibers. However their biggest disadvantage is the variability in their properties. Composites made of hemp fibers with thermoplastic, thermoset, and biodegradable matrices have exhibited good mechanical properties. A number of hemp fiber surface treatments, used to improve the fiber/matrix interfacial bonding, have resulted in considerable improvements in the composites’ mechanical properties.

This paper presents the results of the experiments undertaken to evaluate various physical and mechanical properties of hemp fibres. The study of these properties is vital for comparison with similar properties of synthetic fibres and for assessing hemp fibres’ suitability for use as reinforcement in composite materials. The properties of hemp fibres were found to be good enough to be used as reinforcement in composite materials. However, the issues of relatively high moisture content of fibres, variability in fibre properties, and relatively poor fibre/matrix interfacial strength were identified as factors that can reduce the efficiency with which these fibres can be utilised.

Natural fiber composites have experienced a renaissance in the last two decades as a response to societal demands for developing eco‐friendly, biodegradable and recyclable materials. They are now being extensively used in everyday products as well as in automotive, packaging, sports and construction industries. Hemp fiber is being used in most of these products because of its superior mechanical properties. Like other natural fibers, hemp fibers require modifications in order to improve their properties and interfacial bonding with polymer matrices, and to reduce their hydrophilic character. These modification methods can be grouped into three major categories: chemical, physical and biological. Chemical methods use chemical reagents to reduce fibers' hydrophilic tendency and thus improve compatibility with the matrix. They also expose more reactive groups on the fiber surface to facilitate efficient coupling with the matrix. Physical methods change structural and surface properties of the fiber and thereby influence the interfacial bonding with matrices, without extensively changing the chemical composition of the fibers. They are cleaner and simpler than the chemical methods. Biological methods use biological agents like fungi, enzymes and bacteria to modify the fiber surface properties. These methods are not toxic like chemical methods and are not energy‐intensive like physical methods. This paper presents an overview of recent developments in these methods. It is concluded that these methods almost invariably result in improvement in fiber/matrix interfacial bonding, resulting in increase in mechanical properties of the composites.

The effect of hybridization of hemp fibers with glass fibers on the impact and fatigue properties of the hybrid biocomposites made from them has been studied. The laminates were made by hand layup followed by compression molding, using unsaturated polyester resin as the matrix. Hemp fibers were in the form of short randomly oriented mat and glass fibers were in the form of chopped strand mat. Replacement of about 11% of hemp fibers with glass fibers in hybrid composites increased their impact damage tolerance considerably. Following impact at 4 J energy, hybrid composites lost only about 30% of their intrinsic strength and stiffness, compared to 70% loss for hemp fiber composites at same impact energy. However, at this concentration of glass fibers in hemp fibers, the improvement in impact damage tolerance was limited to 15 J impact at which level the hybrid composites lost almost 90% of their intrinsic strength and stiffness. Hybrid hemp-glass fiber composites showed improvement in fatigue strength but no improvement in fatigue sensitivity was observed compared to hemp fiber composites.

The effects of alkalization surface treatment on hemp fiber properties and the properties of hemp fiber–reinforced polyester composites have been studied. Hemp fibers were exposed to 1, 5, and 10% sodium hydroxide (NaOH) solutions. The tensile properties and interfacial shear strength of all alkalized fibers were found to lie within the range of nonalkalized fibers. Laminates were made of alkalized fibers with unsaturated polyester resin, using hand lay‐up and compression moulding. Alkalization of fibers at low concentrations of 1 and 5% resulted in improvements in tensile and fatigue properties of composites made from these fibers, but no such improvements were observed for 10% alkalized fiber composites. The improvements were attributed to improvement in fiber/matrix bonding after this treatment, which was also confirmed by scanning electron microscopy images. No improvement in impact damage tolerance was observed for any of these three alkalized fiber composites. Immersion in distilled water reduced water absorption compared with nonalkalized fiber composites; however, the tensile properties in water were similar to those for nonalkalized fiber composites. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers