Composition, structure and thermal degradation of hemp cellulose after chemical treatments

https://doi.org/10.1016/j.polymdegradstab.2005.01.016Get rights and content

Abstract

The thermal degradation behaviour of hemp (Cannabis sativa L.) fibres under a nitrogen atmosphere was investigated by using thermogravimetry (TGA). The kinetic activation energy of treated fibres was calculated from TGA data by using a varied heating rate from 2.5 to 30 °C/min. The greater activation energy of treated hemp fibre compared with untreated fibre represented an increase of purity and improvement in structural order. A hydrophobic solvent affected the degree of non-cellulosic removal. Mercerisation and enzyme scouring removed non-cellulosic components from the fibre; however, structural disruption was observed after higher alkaline concentration, 20 %wt/v and longer scouring time, respectively. Structural disruption was observed by X-ray measurement. The FTIR results indicated an elimination of the non-cellulosic components by the mercerisation treatment and a specific removal of low methoxy pectin by use of pectate lyase enzyme (EC 4.2.2.2). An increase of temperature at the maximum rate of degradation and the rate of weight loss was characteristic of the purity and structure of treated hemp fibre.

Introduction

Composites derived from natural and sustainable resources, especially cellulose are increasing in importance due to their numerous applications and advantages. The composites require a strong fibre with good adhesion between matrix and fibre to enhance their final properties. The bast fibres from hemp (Cannabis sativa L.) were selected for pre-treatment. As a natural product, the complex fibre composite was created via biosynthesis. The bast fibres in hemp are bound by a central lamella and arranged in bundles, separated by the cortex parenchyma cell with pectic- and hemicellulosic-rich cell wall [1], [2]. The particular species, time of cultivation and weather produce differences in non-cellulosic composition [3], [4]. The hemp bundle bast fibres were found to contain a large amount of pectins (18%), hemicelluloses (16%) and a small amount of lignin (4%) [1], [2]. These chemicals are not thermally stable and tend to degrade at an early stage of heating. Further processing of a composite requires thermal stability information for materials selection and process operation. Removal of non-cellulosics from fibre surfaces was suggested to achieve this purpose. Various degrees of purity are required for different applications. Therefore, several methods have been applied to hemp cellulose.

Firstly, solvent extraction is an important method conducted to remove the extractable fraction from cellulosic fibres [3], [4]. This procedure may cause slight damage to the fibre structure and results in a more exposed cellulosic surface. Secondly, the chemical process of mercerisation is widely used to modify many types of cellulosic fibres. It is a well-known treatment for fibres using alkaline solution, prior to composite formation. Most of the non-cellulosic components and part of the amorphous cellulose can be removed by mercerisation. The treatment not only changes the chemical composition of fibres but can rearrange or transform the crystalline structure of cellulose I to cellulose II, especially when a high concentration of alkali has been applied. Thirdly, a recent method is a biological treatment process. The substrate of interest can be removed by a specific enzyme. Pectate lyase enzyme (EC 4.2.2.2) is recommended to remove low methoxy pectin. This process is an environmentally friendly process. Different treatments cause a variation in the degree of impurities removed as well as the degree of structural disruption. The effect of the difference in non-cellulosic composition and degree of structural disruption on the thermal stability is an important issue to be investigated. Several characterisation techniques are suitable; however, the focus here is on thermogravimetry [5].

Thermogravimetry is one of the most widely used techniques to monitor the composition [5] and structural [6] dependence on the thermal degradation of natural cellulose fibre. This is because the different compositions and supramolecular structures of cellulose behave differently when undergoing thermal degradation. The aim was to investigate the dependence of thermal degradation on the applied purification methods; solvent extraction, mercerisation and enzyme scouring. Thermogravimetry was used to calculate the kinetic activation energy of cellulose degradation based on mass loss of cellulose. The crystalline structure was observed by X-ray scattering and the crystallinity index was calculated. In combination with X-ray scattering analysis, an understanding of the structural and thermal degradation relationship of cellulose fibres with treatment methods was an objective. Other measurements such as FTIR and SEM were employed to assist with the interpretation of results.

Section snippets

Materials

Hemp (Cannabis sativa L.) was obtained from Australian Hemp Resource and Manufacture (AHRM). The fibre obtained was a green-dried stalk after decortication. Dry matter yield was 90% of field-dried yields. Scourzyme L, pectate lyase (EC 4.2.2.2), was kindly provided by Novozyme Australia Pty, Ltd.

Solvent extraction

The fibres were subjected to Soxhlet extraction with various solvents (acetone, benzene, ethanol and hexane) for 3 h to remove any waxes present and then air-dried.

Mercerisation

Dried fibres (2.5 g) were treated with

FTIR results

The aim of using FTIR is to measure the change of surface composition of fibres after treatment. Infrared spectra of hemp fibres after acetone extraction, mercerisation and enzyme scouring are shown in Fig. 1. In general, the spectrum of the solvent treated hemp fibre is similar to that of the untreated hemp. However, the vibration peak at 1733 cm−1 attributed to the Cdouble bondO stretching of methyl ester and carboxylic acid in pectin disappeared from mercerised fibres. This indicated the removal of

Conclusions

The treatment of hemp fibre by solvent extraction, enzyme scouring and mercerisation was conducted in this research. The FTIR results indicated a change of non-cellulosic components in the treated fibres. The X-ray crystallinity index depended on the method applied and the treatment conditions. Thermogravimetry revealed that thermal degradation of hemp depended mainly on the cellulose structure and the content of non-cellulosic components that were present in the fibre. The enzyme scoured

Acknowledgements

The authors gratefully thank King Mongkut's Institute of Technology North Bangkok (KMITNB), Thailand for a PhD scholarship.

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