Dissolving grade pulp

Heteropolysaccharides are, in general, highly desirable components of paper-grade pulps while they are not tolerated in dissolving-grade pulps used for the production of regenerated cellulose products and cellulose derivatives. A vast body of literature exists on the effects of hemicellu-loses on the papermaking process. 

CeUulose is the most abundant polymer, an estimated 10 t being produced aimuaUy by natural processes. SuppUes for the rayon industry can be obtained from many sources, but in practice, the wood-pulping processes used to supply the needs of the paper and board industries have been adapted to make the necessary speciaUy pure grade. Of the 3 x 10 t of wood used by the paper and board industry (13) in 1989, about 6 x 10 t were purified to provide the 2.5 x 10 t of dissolving pulp required by the viscose processes. 

Our experimental work started from the following original cellulose samples (a) acetate-grade, bleached cotton linters, DP 1800 (b) hot, refined, spruce, sulphite-dissolving pulp, machine dried, ca. 93% a-cellulose, DP — 750 (c) never-dried, normal, rayon-grade, beech sulphite pulp, ca. 90% a-cellulose, DP = 825 (d) commercial cellulose powders obtained by hydrolytic degradation and/or mechanical disintegration of cotton linters or spruce sulphite pulp. 

If this is required it is achieved by using either hydrogen peroxide or chlorine (or a hypochlorite) dissolved in water to remove the coloured residues that are in the cellulose fibres. It is usual to bleach only the chemical pulp made from the sulphite or soda processes, as these are grades probably used for white paper and the white plies of board. 

Wood pulp is the principal raw material of the lyocell process in terms of cost and volume. The grade used is similar to the dissolving pulp used for viscose rayon but has a slightly lower degree of polymerisation (DP) Tencel fibres have a DP of 500 to 550. The pulp is pulled from the reels into a shredder, which cuts the pulp into small pieces for mixing with the amine oxide solvent. The amount of pulp fed to the mixer has to be accurately measured so that the cellulose content in solution is closely controlled. 

---