Lignocellulosic fibres hemicellulose

If water-insoluble polymers, such as cellulose or wood fibres are present in the solution, precipitating hemicelluloses attach easily onto the surfaces. This also occurs in current processing of lignocellulosic materials such as in alkaline pulping, during which acetyl groups in xylan and mannan are removed, with the result that hemicelluloses are associated more tightly with cellulose in pulp than in wood fibres. Hemicelluloses are indeed important for the paper technical properties of pulp fibres (38). 

DMSO/TBAF is highly efficient as a reaction medium for the homogeneous esterification of cellulose by transesterification and after the in situ activation (see below) of complex carboxylic acids. The acylation using acid chlorides and anhydrides is limited because the solution contains a certain amount of water caused by the use of the commercially available TBAF trihydrate and the residual moisture in the air-dried polysaccharides. Nevertheless, this system has shown a remarkable capacity for the esterification of lignocellulosic mataials, for example, sisal fibres, which contain about 14 per cent hemicellulose [28]. The DS values of cellulose acetate prepared from these fibres with acetic anhydride in mixtures of DMSO/TBAF were found to decrease with increasing TBAF concentration from 6 to 11 per cent (Table 16.4), due to the increased rate of hydrolysis both of the anhydride and the ester moieties. 

The twin-screw extruder has proved to be a versatile tool for continuous treatment of vegetable matter, either for food or non-food applications. Trials for the direct alkaline extraction of xylans from wheat bran in a twin-screw extruder were unsuccessful. Bran impregnation with sodium hydroxide in the twin-screw extruder was very efficient, but it was necessaiy to make the separation between the hemicellulosic gel and the lignocellulosic matrix in another apparatus and remained difficult without a dilution to a L/S ratio of 50. Bran and straw co-extrusion was therefore investigated to be able to reduce the L/S ratio. Straw fibres form a dynamic plug in the restrictive elements of the screw profile just after the filtration zone. The pressure induced in the extruder sheath by the cellulosic fibres enabled the liquid/solid separation. 

---