Dry fibre

Cross-linking agents have been proposed for the improvement of chitin fibres in the wet state. Epichlorohydrin is a convenient base-catalysed crosslinker to be used in 0.067 M NaOH (pH 10) at 40 °C. The wet strength of the fibres was considerably improved, whereas cross-hnking had neghgible effect on the dry fibre properties. Of course, the more extended the chemical modification, the more unpredictable the biochemical characteristics and effects in vivo. Every modified chitin or modified chitosan fibre should be studied in terms of biocompatibiUty, biodegradabiUty and overall effects on the wounded tissues. 

During the delamination of the cell wall which takes place during refining (Figure 5.9), there is an increase in the water accommodated within non-crystalline zones, and the cell wall water content typically rises from around 1 to 3 g per g of dry fibre (Figure 5.10). 

AKDs are waxy, water-insoluble solids with melting points around 50 °C, and ASAs are viscous water-insoluble liquids at room temperature. It is necessary to prepare them as stabilised emulsions by dispersion in a cationic polymer (normally cationic starch). Small amounts of retention aid and surfactants may also be present. Particle size distributions are around 1 fim, and addition levels around 0.1% (of pure AKD or ASA) by weight of dry fibre. This is an order of magnitude lower than the amount of rosin used in rosin-alum sizing (1-2%). Emulsions of AKD are more hydrolytically stable than ASA, and the latter must be emulsified on-site and used within a few hours. 

It was preferable to use fibre having a low moisture content (up to 5 %), since the presence of high levels of moisture moisture would lead to the production of large quantities of acetic acid by-product. However, if oven-dry fibre was used, this led to clumping of the fibre. 

BP Chemicals developed a batch process capable of producing 100 kg of acetylated fibre per day, and a larger batch process, capable of producing 1 tonne per day, was to be constructed by Depac Engineering Ltd. Sheen (1992) also reported on an analysis of the costs for batch and continuous plants producing 10 000 tonnes of acetylated fibre per year at a WPG of 20 %. An essential part of the process to ensure commercial viability was the recovery of by-product acetic acid. The calculated costs in pounds sterling, based on fully dried fibre and in 1992 prices, are shown in Table 8.3. 

This reagent, applied to natural, dry fibres, colours the cellulose of cotton, flax, hemp, ramie, etc., violet, and that of wood blue, while lignified fibres are dyed yellow. 

A = percentage of dressing and colour, together fu ft. f - percentages of pure dry fibres... 

Figure 3.4. Water adsorbed in the cell wall of Picea mariana (Stone and Scallan, 1968). The amount of adsorbed water that is accessible to a polymer molecule increases as the size of the polymer molecule decreases. During pulping the cell wall is opened up with both pore volume and pore size distribution increasing with the degree of delignification. The yield is the ratio of the weight of oven-dry fibre remaining after pulping to the initial weight of oven-dry wood.

While dry fibre blending capacity was retained in some plants, the benefits of blowline blending meant that this approach has been adopted for an estimated 98% of MDF produced. Clearly the benefits of the blowline blending approach outweighed the cost of the extra resin needed where this approach was adopted. 

A) illustrates the effect of building cross-links (X) in a dry fibre where the molecular chains can also be bound together by hydrogen bonds (dotted... 

Figure 4 XRD pattern showing amorphous phase of dried fibre (bottom) which transformed into anatase only after firing to 500°C (top).

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