Dissolving pulp

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. 

Xanthation. The viscose process is based on the ready solubiUty of the xanthate derivative of ceUulose in dilute sodium hydroxide. The reaction between alkaU ceUulose and carbon disulfide must therefore be as uniform as possible to avoid problems with incompletely dissolved pulp fibers that wUl later have to be filtered out of the viscous solution. 

R. N. Armstrong and co-workers. Proceedings of the 5th International Dissolving Pulps Conference, TAPPI, Adanta, Ga., 1980. 

Both the sulfite and alkaline (kraft) methods can be modified to produce high purity chemical ceUulose. These pulps, usuaUy in the form of "dissolving pulps," are not only mosdy free of lignin and hemiceUulose, but the molecular weight of the ceUulose is degraded. This increases solubUity in alkah and provides desired viscosity levels in solution. These dissolving pulps are used to make derivatives such as sodium ceUulose xanthate [9051 -13-2] via alkah ceUulose, and various esters and ethers (see Cellulose esters Cellulose ethers). 

CeUulose is soluble only in unusual and complex solvent systems. The subject has been reviewed (84—87). Commercially, dissolving pulps, which have lower molecular weights, are used along with strong alkaU and derivatization. CeUulose subjected to high temperature and pressure during the steam explosion process can be dissolved in strong base (88). 

Recently, the alkah-soluble hemicelluloses of hardwood dissolving pulps have been investigated [28]. Their composition and molecular properties depended on the pulp origin and steeping conditions. The MGX of the j8-fraction from press lye had a low uronic acid content (ratio of MeGlcA to Xyl is about 1 20). The molecular weight of the hemicellulose fractions varied between 5000 and lOOOOg/mol. 

Attempts to remove hemicellulose for production of dissolving pulps with very low hemicellulose contents have shown that complete enzymatic hydrolysis of hemicellulose within the pulp is difficult to achieve. The xylan content in delignified mechanical aspen pulp was reduced from approximately 20 to 10%, whereas in bleached hardwood sulphite pulp the xylan content was decreased from 4 to only 3.5% even at very high enzyme dosages (50). The complete removal of residual hemicellulose seems thus unattainable, apparently due to modification of the substrate or to structural barriers. 

Itoh, T. Proc. Intern. Dissolv. Pulps Conf. 1987, 117-20. 

Shiraishi, N. Tappi Proc. 1987 Inter. Dissolving Pulps Con/., 95-102. 

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. 

In agreement with this hypothesis is the remarkable low rate constant with a never-dried beech sulphite dissolving pulp compared with the dried sample at identical conditions of aqueous acid hydrolysis (Table VI). 

Table VII. Influence of Disintegration and of Mechanical Strain on Acid and Enzymatic Hydrolysis of a Machine-Dried Spruce Sulphite Dissolving Pulp...

The standard procedure by Saeman et al. (I) involves manual stirring of the polysaccharide with 72% H2S04, standing at 30°C, and secondary hydrolysis at 100° or 120°C in a steam autoclave. While certain resistant polysaccharides are still incompletely depolymerized, decomposition of the more sensitive monosaccharides formed cannot be avoided. An alternative method by using trifluoroacetic acid was applied successfully for plant cell wall polysaccharides by Albersheim et al. (2) and for dissolving pulps and hemicelluloses by Fengel et al. (3). Highly crystalline cellulose was not well dissolved and not completely hydrolyzed by CFsCOOH. 

Fig. 6 MWD of different dissolving pulps (s2-s8), the vertical lines limit DP ranges of 100 and 2000, corresponding to the carbonyl contents in Fig. 7 below...

Table XVII is an example of the test result showing comparable efficiency of SAE, PAE and alkylphenol ethoxylates in resin removal from sulfite pulp. A Japanese patent specification(24) by Lion Fats and Oils describes 10-16 mole ethoxylates of secondary alcohols as extremely useful in deresination of pulp, especially in the manufacture of dissolved pulp for the rayon industry. According to the patent specification, to alkyls give the best results among 12 mole ethoxylates ethoxylates of from 10 to 13 moles of EO can give the best results among other ethoxylates of C secondary alcohols, and the optimum amount of SAE(12E0) is in the range between 0.2 and 0.6% by weight based on pulp.

Alpha-cellulose Alpha-cellulose, also known as chemical cellulose, is a highly refined, insoluble cellulose from which all sugars, pectin, lignin, and other soluble materials have been removed. It is commonly used in the production of nitrocellulose, carboxymethylcellu-lose, dissolving pulps, and other compounds. 

Dissolving pulp Dissolving pulps are also referred to as chemical cellulose. This pulp is taken into solution... 

THE TERM PULP is used to describe theraw material for the production of paper and allied products such as paper-board, fiberboard, and dissolving pulp for the subsequent manufacture of rayon, cellulose acetate, and other cellulose products. More specifically, pulp is wood or other biomass material that has undergone some degree of chemical or mechanical action to free the fibers either individually or as fiber bundles from an enbodying matrix. Paper,... 

While sulfite pulping is less popular than Kraft pulping, it is more prevalent in the production of dissolving pulps. Further, sulfite pulping permits recovery of ethanol from the spent pulping liquor before incineration, as is... 

Dissolving pulp, a bleached sulfite pulp from spruce, MbDoCell H3430 from Mb och DomsjS AB, Omskoldsvik, Sweden. 

Table V. Grafting of acrylonitrile (AN) onto MoDoCell dissolving pulp (MDC) and grease-proof pulp (GP) using Mb initiation.

Grafting of dissolving pulp (MDC) and grease-proof pulp (GP) (both are bleached sulfite pulps from spruce) and low-substituted ethyl cellulose (EC) and hydroxyethyl cellulose (HEC), all samples in never-dried state, with acrylonitrile (AN) using Mn +ions, complexed with pyrophosphate ip n inin amr >s... 

Table VII. Water retention of grafted HEC, dissolving pulp (MDC) and grease-proof pulp (GP) after alkaline hydrolysis, measured by three methods (Al, A2 and A3). Add-on (%) in grafting and yield in hydrolysis (YH in %) characterize the samples.

---