Journal of Composite Materials http://jcm.sagepub.com/ Hemp fibre and its composites − a review Asim Shahzad Journal of Composite Materials published online 15 August 2011 DOI: 10.1177/0021998311413623 The online version of this article can be found at: http://jcm.sagepub.com/content/early/2011/08/04/0021998311413623 Published by: http://www.sagepublications.com On behalf of: American Society for Composites Additional services and information for Journal of Composite Materials can be found at: Email Alerts: http://jcm.sagepub.com/cgi/alerts Subscriptions: http://jcm.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav Downloaded from jcm.sagepub.com by Asim Shahzad on September 6, 2011 JOURNAL OF COMPOSITE Article MATERIALS Journal of Composite Materials 0(0) 1–14 ! The Author(s) 2011 Hemp fiber and its composites – a review Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998311413623 jcm.sagepub.com Asim Shahzad Abstract 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. The mechanical 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. Keywords Hemp, natural fiber composites, mechanical properties, surface treatments, life cycle assessment Introduction oldest. The Columbia History of the World states The invention of polymers in early 20th century laid the that the oldest relics of human industry are bits of foundation of a revolution in materials. This revolution hemp fabric discovered in tombs dating back to had an inherent drawback with respect to the environ- approximately 8,000 BC. The flowering tops and, to a ment because of poor biodegradability of these mate- lesser extent, leaves of hemp produce resin secretions rials. This meant that disposal of this material put extra containing the narcotic 9-Á tetrahydrocannabinol burden on already dwindling landfill resources in most (THC) for which marijuana and hashish are famous. countries. The annual global disposal of millions of Since industrial hemp produces less than 0.2% THC, tons of plastics, especially from packaging, has raised it cannot be used as a narcotic. the demands for looking for new means of managing Hemp is an annual plant native to central Asia and this non-biodegradable waste. These environmental known to have been grown for more than 12,000 years. issues have resulted in considerable interest in the devel- It probably reached central Europe in the Iron Age and opment of new composite materials based on biode- there is evidence of its growth in the UK by the Anglo- gradable resources, such as natural fibers as low-cost Saxons (800-1000 AD). It is now grown mostly in the and environment-friendly alternative for synthetic EU, Central Asia, Philippines, and China. According to fibers. Hemp, sisal, and flax are some examples of the Food and Agriculture Organisation (FAO), almost half natural fibers being used in composite materials. of the world’s industrial hemp supply is grown in Synthetic polymers have been conventionally used as China, with most of the remainder being cultivated in matrices in composite materials which are not eco- Chile, France, the Democratic People’s Republic of friendly. Now new matrix materials are also being Korea, and Spain.1 developed based on natural and renewable resources, In hemp plant, fibers are contained within the tissues for the development of ‘green’ biocomposites. Polyactic of the stems which help to hold the plant erect. In acid, soy oil, and lignophenolic resins are some exam- doing so these fibers impart strength and stiffness to ples of such biodegradable matrix materials. Materials Research Centre, School of Engineering, Swansea University, Swansea, UK. Hemp Corresponding author: After sisal, hemp is the most widely used natural fiber Asim Shahzad, Materials Research Centre, School of Engineering, as reinforcement in composites. Hemp is naturally one Swansea University, Swansea SA2 8PP, UK of the most ecologically friendly fibers and also the Email: [email protected] Downloaded from jcm.sagepub.com by Asim Shahzad on September 6, 2011 2 Journal of Composite Materials 0(0) the tree. This high strength and stiffness of hemp Cellulose, hemi-cellulose, and lignin are basic compo- fibers makes them a useful material to be used as rein- nents that define physical properties of fibers. Cellulose forcement in composite materials. There has been is the stiffest and the strongest organic constituent an exponential increase in the use of hemp for various in the fibers. Perhaps the biggest drawback of hemp applications in recent years. According to FAO, world fibers is the variability in their composition as shown production of hemp fiber grew from 50,000 tons in in Table 1. This, invariably results in variability in their 2000 to almost 90,000 tons in 2005.1 Hemp currently physical and mechanical properties. Diameters and accounts for less than 0.5% of total world production properties of natural fibers vary significantly depending of natural fibers. upon factors such as source, age, retting and separating As Annie Kelly wrote in the Guardian in 2006,2 only techniques, geographic origin, rainfall during growth, fifteen years ago, any farmer trying to grow hemp in and constituents’ content.4 Britain could be arrested. But in 2006, grown under Nishino5 has identified the following factors that can license, more than 3,500 acres of hemp was harvested cause the variability in the physical and mechanical as an industrial crop, processed, and made into a pleth- properties of natural fibers: ora of natural products, including insulation, horse bed- ding, fabric, bio diesel, and paper. ‘Hemp is back and is Materials Microscopic: crystallinity; microfibril throwing off its hippy shackles to emerge as one of angle; crystal modifications the UK’s fastest growing sustainable industries’, she Macroscopic: fineness; porosity; wrote. The National Non-Food Crops Centre size and shape of lumen (www.nnfcc.co.uk) recognizes hemp as an established History: source; age; retting and minor crop. The ban on hemp cultivation, imposed in separating conditions; geographical 1971 under the Misuse of Drugs Act, was finally over- origin; rainfall during growth turned in 1993. Campaigners successfully argued that Measurement tensile speed; initial gauge length; although industrial hemp was a variety of the cannabis conditions moisture; temperature; different plant, it could be grown as a legitimate crop as it con- cross-section of fibers at different tained practically no THC. Still permission of the Home points Office is required to grow hemp. Only one company in Britain has the license to grow hemp. Hemcore, the UK’s first large-scale hemp company, has seen rapid Morvan et al.17 showed that the diameter of flax growth over the last few years. It now owns the only fiber bundles is greater in the middle part of the stem hemp processing plant in the UK and currently con- than in the bottom or top regions. Although the cross tracts 40 farmers to grow 3,500 acres of hemp a year, section of natural fiber bundles is polygonal in shape, which it converts into industrial materials. It currently the major dimension is referred to as the diameter in provides all BMW 5 Series cars with hemp door panels, literature and this convention has been followed in this as well as making high-quality horse bedding. review. The diameter was also found to vary depending The main constituents of hemp fibers are cellulose, on the development stage of the plant, the diameter hemi-cellulose, lignin, and pectin as shown in Table 1. being lower during flowering/capsulation stage than Table 1. Chemical composition of hemp fibers as reported by different authors Cellulose Hemicellulose Pectin Lignin Other (%) (%) (%) (%) (%) Reference 67.0 16.1 0.8 3.3 2.8 [6] 74.4 17.9 0.9 3.7 0.8 [7] 74.0 18.0 1.0 4.0 [8] 55.0 16.0 18.0 4.0 7.0 [9] 76.0 11.5 1.3 3.2 [10] 57–77 9–13 [11] 75.1 <2 8.0 [12] 70–74 17.9–22.4 0.9 3.7–5.7 0.8 [13] 75.6 10.7 6.6 [14] 78.3 2.9 [15] 76.1 12.3 1.6 5.7 3.3 [16] Downloaded from jcm.sagepub.com by Asim Shahzad on September 6, 2011 Shahzad 3 mature capsule or seed maturation stages. It is natural properties with considerable scope for variation. to expect that this variability in diameter will also Apart from their high tensile strength and stiffness, impart variability to the mechanical properties of the their high aspect ratio (length/diameter ratio) and fibers. lower density make hemp fibers a good material to be Catling and Grayson18 measured the diameters of used as reinforcements in composite materials. Their hemp fiber bundles and found the average to be disadvantages for use in composite materials, like 30 mm with a range of 11.68–31.96 mm. The average most other natural fibers, are their non-uniform and hemp fiber length was found to be 8.46 mm with a non-smooth surfaces, variability of properties, and range of 1–34 mm. Olsen and Plackett19 found the aver- low resistance to water absorption and decay.