Fibers and Polymers 2008, Vol.9, No.5, 593-603
Effect of Processing Route on the Composition and Properties of Hemp Fibre Sandra Korte* and Mark P. Staiger Materials Engineering Group, Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch, 8020, New Zealand (Received May 23, 2007; Revised June 22, 2008; Accepted August 25, 2008) Abstract: There is great interest in the plant Cannabis sativa (hemp) as a source of technical fibres for the reinforcement of polymers in composite materials due to its high mechanical properties. In this work, the effect of enzymatic, hydrothermal and alkaline treatments on the composition and mechanical properties of hemp fibre are compared. The influence of enzyme concentration and treatment time was examined (2.5-80 % Pectinex® Ultra SP-L, 6-48 hrs). Additionally, hydrothermal (170 oC, 10 bars) and alkaline treatments (18 wt. % NaOH, 40 oC) were used as pre-treatments to observe their effect on subsequent enzymatic treatment. The composition of hemp fibre was analysed by wet chemistry and Fourier transform infrared spectroscopy, while microstructure and mechanical properties were examined by scanning electron microscopy and tensile testing, respectively. Enzymatic treatment resulted in extensive fibrillation and removal of non-cellulosic components, especially when combined with hydrothermal treatment. However, a lengthy enzymatic treatment or combinative enzymatic-alkaline treatment led to extensive fibre breakdown that was accompanied by a pronounced reduction in the mechanical properties. Enzymatic treatment decreased Young’s modulus and tensile strength by 77 and 73 % respectively, and alkaline treatment by 83 and 36 %. The hydrothermal treatment resulted in only minor changes in these properties. Keywords: Natural fibre, Hemp, Enzyme, Hydrothermal treatment, Alkaline treatment
Introduction
hemicellulose, lignin and proteins. Further in, the secondary cell wall, with the largest proportion of cellulose within the cell wall, consists of three layers of cellulose fibrils with varying axial orientation that are bound by a matrix of lignin and hemicellulose. The basis for fibre processing is the removal of lignified pectins from the middle lamella, thereby allowing the separation of the primary fibres from their bundles. For composite applications, fibre treatments aim to improve the mechanical properties of the reinforcing fibre and enhance the adhesion between fibre and matrix, thus improving the interfacial strength and overall composite performance [8,9]. In particular, fibre treatments are able to increase the surface area of the reinforcement available for chemical and/or mechanical bonding to a polymer matrix, thereby improving stress transfer within the final composite material [1,10-12]. Greater acceptance of biocomposites will hinge on the development of fibre treatment processes that are economically viable, environmentally sustainable and do not severely degrade mechanical properties [13]. The life-cycle analysis of biocomposites has shown that traditional fibre processing steps such as mercerization, water-retting and bleaching have negative environmental impacts due to the copious consumption of water and polluting by-products [14,13,6]. The recyclability and degradability of enzymes has stimulated research into their application to natural fibre treatment [15-23,11]. Enzymatic treatment has also been shown to be a more reproducible processing route compared with the highly variable bacterial processes that occur during field retting [15,14]. Treatments in caustic soda have been adapted from mercerization of cotton and applied to natural fibres with differing process parameters, typically sodium hydroxide
Mounting concerns for the environment have sparked renewed interest in the development of biodegradable, mechanically sound alternatives to synthetic fibres through the use of natural fibres found in the leaf or stalk (bast) of plants such as hemp, flax, ramie and sisal [1]. Natural fibres that are high in cellulose can be embedded in a polymer matrix to serve as an inexpensive, biodegradable and renewable alternative to glass fibres [2]. Firstly, natural fibres must be extracted from the leaf or stem and then broken down (fibrillated) further to produce suitable reinforcement for polymer composites. It has been reported that natural fibrereinforced polymer composites (also referred to as ecocomposites or biocomposites) have specific strengths and stiffnesses comparable with their glass fibre counterparts [1,3-6]. Biocomposites have so far been selected for a number of low load bearing industrial applications, especially in the automotive sector [7]. Natural fibres generally consist of bundles of individual cells (or primary fibres). The primary fibres transport water and nutrients through the plant, facilitated by a hollow central canal (or lumen). There are four distinct regions surrounding the lumen: the primary, secondary and tertiary cell walls, and the middle lamella, each varying in composition, morphology and purpose [4]. The middle lamella at the exterior of the cell wall is comprised predominantly of lignified pectins that act to cement primary fibres into bundles. Adjacent to the middle lamella is the primary cell wall that consists of a disorganised arrangement of cellulose fibrils embedded in a matrix of pectin, *Corresponding author:
[email protected] 593
594
Fibers and Polymers 2008, Vol.9, No.5
(NaOH) concentration, treatment duration and temperature and allowed fibre shrinkage [24,25,8,26-31]. Mwaikambo et al. [32] reported the strength and stiffness of hemp after alkaline treatment to almost double with low NaOH concentrations (0.16-0.24 %), while a decrease in these properties is typical with higher sodium hydroxide concentrations (2-20 wt.%). After treatment with 20 % NaOH, Ouajai et al. [29] and Gassan et al. [25] observed a decrease in stiffness for hemp and jute fibre to 13 and 16 %, respectively, while the initial strength of the fibres decreased by 30 and 70 %, respectively. The untreated fibre exhibited elongation before break of ~2-2.5 %, while treatment with increasing NaOH concentrations has reported increases in elongation of up to 7.5 % [33,34]. Degradation of the cell wall components and removal of amorphous non-cellulosics are proposed to be possible causes for the observed decrease in stiffness and strength [8,29]. This is in contrast to Gassan et al. where stiffness and strength of jute was observed to increase by ~250 and 300 %, respectively, after a 20 minutes isometric alkaline treatment at 25 wt.% [25]. It is proposed that the removal of amorphous components reduces the density of the interfibrillar volume, allowing alignment of the cellulose microfibrils in the direction of the applied tensile load (i.e. decreasing the microfibril angle). Heat, steam (or hydrothermal) and steam explosion treatments have also been found to be useful in fibrillating natural fibres. Rong et al. [35] and Yang et al. [34] observed the strength and elongation at break of sisal to increase by 10-30 % and 10-40 %, respectively, while the fibre stiffness remained constant after a heat treatment at 150 oC in air. This heat treatment proved to be superior with regard to strength and stiffness in comparison with a 2 % NaOH treatment. The application of steam explosion has also been shown to be highly effective [14,36-38]. For example, Kessler et al. [36] reported a 10 % increase in strength and elongation at break compared with the mechanical separation of flax fibre. Various parameters of enzymatic treatment have been investigated. The most influential parameters are known to be the enzyme activity, enzyme concentration, treatment duration and use of additives such as chelating agents (e.g. EDTA) [9,11,15-18,20,21,39,40-43]. Dreyer et al. [40] compared the mechanical properties of hemp after treatment with three different enzyme solutions with an alkaline treatment in 0.4 % sodium carbonate at 100 oC. The enzymatic treatments led to large variations in strength from
