Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites

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Abstract

The objective of this study was to investigate the effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of hemp fibre/epoxy composites. Pectin removal by EDTA and endo-polygalacturonase (EPG) removed epidermal and parenchyma cells from hemp fibres and improved fibre separation. Hemicellulose removal by NaOH further improved fibre surface cleanliness. Removal of epidermal and parenchyma cells combined with improved fibre separation decreased composite porosity factor. As a result, pectin removal increased composite stiffness and ultimate tensile strength (UTS). Hemicellulose removal increased composite stiffness, but decreased composite UTS due to removal of xyloglucans. In comparison of all fibre treatments, composites with 0.5% EDTA + 0.2% EPG treated fibres had the highest tensile strength of 327 MPa at fibre volume content of 50%. Composites with 0.5% EDTA + 0.2% EPG  10% NaOH treated fibres had the highest stiffness of 43 GPa and the lowest porosity factor of 0.04.

Introduction

Cellulosic fibres are natural resources with important properties such as low density, high specific tensile strength and stiffness, and a high aspect ratio (average length over diameter of the fibres) [1]. Due to these properties and concerns about the environment, the application of natural cellulosic fibre reinforced polymer composites has received considerable attention in recent years [2], [3].

Hemp (Cannabis sativa) is a fast growing crop, which produces strong fibres that primarily lie beneath the epidermis in the cortex and form a ring in the phloem parenchyma [4]. Like wood fibres, hemp fibre cell walls are natural composites composed mainly of three classes of polysaccharides: cellulose, hemicellulose and pectins. Cellulose consists of β-1,4-linked glucan chains and is organized into microfibrils interlocked by xyloglucan (XG) [5]. The cellulose microfibrils and the cross-linked XG chains are generally considered as the two main components which provide cell wall strength. Pectins fill the spaces between cellulose and XG [5]. The pectins function as glue packing the microfibrils into final fibres that are approx. 20 mm in length and 10–40 μm in diameter [6]. The fibres make up the fibre bundles with varied sizes which are in turn organized into a fibre layer inside the cortex. Pectins and lignin in the middle lamellae (ML) join the fibres together [4], [7].

The most abundant pectic polysaccharide in plant cell walls is homogalacturonan (HG), which is a linear homopolymer of α-1,4-linked galacturonic acid that comprises approx. 70% of pectin. Besides HG, the pectic polysaccharides are mainly comprised of rhamnogalacturonan I (RG I) and rhamnogalacturonan II (RG II). RG I represents 20–35% of pectic substrates and consists of a backbone of alternating α-1,4-d-galacturonic acid and α-1,2-l-rhamnose units; the latter are usually decorated with homopolymeric side chains of β-d-galactose and α-l-arabinose [8]. RG II makes up 5–10% of the pectin, and consists of a HG backbone of 1,4-linked α-d-galacturonic acid residues decorated with different types of side branches [9].

The physicochemical properties of pectin are largely dependent on the degree of methyl and acetyl esterification. Low-methoxyl (LM) pectin (i.e. HG) has sufficient carboxyl groups for the formation of calcium-mediated interactions between two neighbouring pectin chains, as described by the “egg-box” model [10]. However, high-methoxyl (HM) pectin does not contain sufficient polygalacturonic acid residues un-methylated at the C-6 position to form a stable structure through calcium-mediated interactions. Instead, hydrogen bonding and hydrophobic interactions have been suggested important forces in maintaining a stable structure for HM-pectin [11], [12].

The principle behind fibre processing for the application of natural fibres in composites is to remove the non-cellulosic components (e.g. pectin, hemicellulose and lignin) to obtain well separated and cellulose rich fibres before use as reinforcement in composites. Traditional fibre processing methods such as field retting and water retting, however, are largely dependent on weather conditions (especially rainfall and temperature) and may damage the fibres if fibres are over retted [4], [13]. Enzymatic treatment, involving mainly pectinolytic enzymes, may offer an alternative method to degrade pectin from hemp fibre strips and provide a solution to the limitations of traditional fibre retting methods.

In treatment of fibre with enzymes, pectic polymers are released from ML and fibre cell walls by using pectinases (e.g. endo-polygalacturonase) that randomly hydrolyze the glycosidic bonds of the HG backbone to liberate monomeric, dimeric or oligomeric fragments [14]. Addition of chemical chelators (e.g. ethylenediaminetetraacetic acid (EDTA)) has been shown to promote enzyme catalyzed degradation of HG from cellulosic fibres during enzymatic treatments [15], [16]. The enhanced enzymatic degradation of HG results from the capacity of chemical chelators to form complexes particularly with calcium in pectin [17]. Furthermore, alkaline extraction with 10% NaOH is widely used for the isolation of hemicellulose from lignocellulosic biomass to obtain cellulose of high purity [18].

The objective of this study was to investigate the effect of sequential removal of pectin (e.g. homogalacturonan) and hemicellulose (e.g. xyloglucan) on the mechanical properties of fibres and their subsequent use in unidirectional hemp fibre/epoxy composites. Pectin removal from hemp fibres was carried out using EDTA alone and in combination with monoactive pectinase enzyme. In some experiments hemicellulose was also sequentially removed using 10% NaOH.

Section snippets

Plant material

Hemp (Cannabis sativa L.), variety USO-31, was grown in France (N48.8526°, E3.0190°(WGS84)) as described in detail by Liu et al. [4]. Hemp stem pieces with a length of 150 ± 10 mm were randomly collected from the stems. Before treatment, hemp bast fibre strips were manually peeled from the stem pieces, gently rinsed with warm water (40 °C) to remove dirt and then dried at 50 °C for 12 h. Field retting was carried out on whole stems for 20 days after harvest [4] as a comparison to the treatments

Pectin and hemicellulose removal from hemp fibres

Galacturonan content (GalA) of untreated hemp fibres of 8% was significantly decreased to 6% for 0.5% EDTA treated fibres, to 5% for 0.75% EDTA treated fibres, and finally to a little below 5% for 2% EDTA and 3% EDTA treated fibres (Table 1). In addition, there was a slight decrease in arabinan content, irrespective of the concentration of EDTA used in the treatments. No significant changes occurred to other components (Table 1).

The results are presumably due to the gradual increase in the

Conclusion

The impacts of pectin and hemicellulose removal from hemp fibres on morphology and mechanical properties of hemp fibres, and on the mechanical properties of fibre/epoxy composites were studied. Pectin and hemicellulose were removed from fibres by EDTA and enzyme treatments, and 10% NaOH treatment, respectively. The removal of pectin removed epidermal and parenchyma cells, produced more void spaces between fibres and resulted in improved fibre impregnation with epoxy matrix. These changes

Acknowledgements

The authors are grateful to the Danish Council for Independent Research supporting the CelFiMat project (No. 0602-02409B: “High quality cellulosic fibres for strong biocomposite materials”). The financial support of China Scholarship Council, China (CSC, no. 201304910245) for Ming Liu’s Ph.D. project is acknowledged. The support for short-term scientific mission from Nordforsk Researcher Network (Norway) to Ming Liu in June 2015 is acknowledged. Bo Madsen from Technical University of Denmark is

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