Cellulose is the principal structural component in green plants (Thomsen et al. 2005). Cellulose is a major quality component in agri-fibres and can be converted into mass produced products such as cellophane.
Lignin, which is indigestible by mammalian and animal enzymes, fills the spaces between the cells and is key for stalk strength. The fibre color changes from a dark to bright hue as the double bonded lignin structure is broken down by bacterial growth during the retting process or by the chemical or enzymatic preparations (Thomsen et al. 2005). The primary and secondary phloem layers amalgamate with the lignified primary xylem layer.
The primary xylem, otherwise referred to as hurd, is woody and thins to a pith which encircles the hollow hemp stalk (McPartland et al. 2000). The stalk consists of approximately 0.8% – 2.5% pectin, which is considered to be the main binder of the primary cell wall, middle lamella and of the cellulosic and non- cellulosic networks (Wang et al. 2003). Pectin helps to prevent the collapse of the structural and chemical constituent networks within green plants (Taiz and Zeiger 2002; Corriea and Roy 2005). Bast and hurd fibres contain both cellulosic and non-cellulosic properties (i.e. lignin) that differ in proportion. Thomsen et al. (2005) stated that while both bast and hurd fibres are cellulosic; a greater percentage of cellulose is located in the bast fibre while the hurd consist of a greater percentage of hemicellulose and lignin and less cellulose. Hemicellulose is made up of a group of tightly bonded heterogeneous polysaccharides located within the cell wall (Taiz and Zeiger 2002) that connects the cellulose and lignin. Glucose and xylose are the primary monosaccharides constitutes in hemp’s hemicellulose.
Meijer et al. (1995) stated that hemp fibres could be used as a wood substitute in pulp and paper production. The hemicellulose in hemp is similar to that of Canadian grown aspen tree fibres which are readily extracted with an alkali solution (Corriea and Roy 2005). Correia et al. (1998) determined that the physical properties of hemp bast fibre cells were similar to softwood tree species; which are currently used in Canadian pulp and paper production. Hemp bast fibres range from 0.5 – 10 cm in length; average lengths range from 1.5 – 5.5 cm with average thicknesses of 18-25 μ (micron) (Vignon et al. 1995; Bócsa and Karus 1998). The lengths of the hurd fibres typically do not exceed 0.5 mm (Bócsa and Karus 1998). Increasing fibre lengths have been associated with increasing paper strength (Bócsa and Karus 1998). The tensile strength of hemp bast fibres ranges from 593.72 to 1073.72 megapascal (MPa) (i.e. a unit of pressure or stress) depending upon the fibre separation method, i.e. untreated or alkalized treated (i.e sodium hydroxide (NaOH)) (Mwaikambo and Ansell 2006). Preliminary results from Manitoban grown hemp indicated that fibre from mature stalks produced stronger but stiffer and less elastic fibre from stalks cut early in the flowering stage (Moes and Empson 2000). Kamat et al. (2002) concluded that handsheet paper produced from unbleached pulp of hemp stalks harvested at 60- and 90-days after sowing produced insignificant differences in tensile strength (59 and 56 kilonewton/gram (kN/g) (i.e. an International System unit for force), respectively). Research conducted in Germanyby Müssig and Martens (2003) further concluded that plant age in hemp did not significantly affect fibre strength.
In 1997 and 1999,United Kingdom researchers, Murphy-Bokern and Bruce (2005) collected and processed matured hemp stems into composites. They determined that even though composite strength was not affected by delayed harvest, composite stiffness was reduced at an approximate rate of 10% per delayed harvest week. In addition, the authors concluded that harvest date did not affect the stiffness of the hand-extracted hemp fibre bundles (Murphy-Bokern and Bruce 2005).
European fibre hemp stems have been determined to consist of high-cellulose low-lignin bark (bast-long fibre) and low-cellulose high-lignin core (hurd-short fibre) (van der Werf et al. 1994b). Since the greatest percentage of cellulose is located in the bark (bast), van der Werf et al. (1994b) stated that stem value primarily depends upon the proportion of bast in the stem. Today, however, hurd value is on a rise due to its superior fibre qualities (i.e. absorbance, reduced dust, thermal and antibacterial properties) for animal bedding, building materials and bio-composites.
Complete references provided with written request.