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The processing of cellulose for production of regenerated cellulose and cellulose derivatives re-quires a cellulose of high purity, i.e. cotton linters or dissolving pulp. The latter can be produced either by prehydrolysis-kraft or acid sulfite pulping. In both processes, the conditions must be chosen such that the remaining amount of hemicellulose in the fibers is reduced to a minimum.
Furthermore, bleaching to high brightness is required in order to remove all lignin. The pulp yield in these processes is low and in the order of 35 % making it a rather expensive product.
An alternative and more energy-efficient way of separating the wood components from each other has been suggested and is usually referred to as the “wood explosion” process (Figure 8.3). Here, the wood material (or other biomass) is treated with steam at temperatures in the range of 190–240 °C for a few minutes followed by a rapid release of the pressure. This forces the material to “explode” with formation of individual fibers and fiber bundles whereas volatile extractives can be collected separately. Under the conditions of the steam treatment, wood acids are liberated and acid hydrolysis of the polysaccharides takes place together with simultaneous hydrolysis and condensation reactions of the lignin. Most of the hemicelluloses are degraded into low molecular weight sugars and oligosaccharides and can be removed by washing with water. A redistribution of lignin in the fiber walls also occurs and results in a rather facile elim-ination of a large portion of the lignin by extraction of the fibrous material with either aqueous alkali, sodium sulfite or an organic solvent. The remaining material consisting of cellulose with a low to medium degree of polymerization together with the remainder of the lignin and some degraded carbohydrates like furfural-derived polymers can be easily bleached giving a rather pure cellulose. Unfortunately, the process suffers from the difficulty in obtaining a homoge-neous reaction in the biomass material and this results in high amounts of shives. Therefore, the process has not yet reached the commercial scale.
Figure 8.3. Principal scheme for the “wood explosion” process and the types of products that can be obtained.
Typical reaction conditions are 190–240 °C, 1–5 min.
From cellulose, a large variety of derivatives, esters and ethers, can be manufactured. The largest volume is, however, regenerated cellulose which is produced as Rayon fibers, Cello-phane and Lyocell fibers (Figure 8.4). Several major difficulties are encountered in the produc-tion of cellulose-based products such as the heterogeneity of the starting material, the reproducibility of the experimental conditions, the heterogeneous phase of the reaction, the pu-rification difficulties, the effluent disposal and the product quality control. In addition, there has been no real driving force to further develop the technologies used because of the strong compe-tition from the petroleum-based industry.
Figure 8.4. Major uses of cellulose derived products in the world with production volumes in tonnes ×103. Data from 1990.
Thus, a successive decline in the production of cellulose-based fibers has been encountered during the last 20 years. At present (2002), the global production of manufactured fibers is in
Wood explosion
chemicals Ethanol Cellulose derivatives
Regenerated cellulose
Chemical (dissolving) pulp 3,920
Alkali cellulose Alkali cellulose
Xanthation Acetylation Nitration Etherification
Viscose solution Cellulose ester Cellulose nitrate Cellulose ethers
Viscose
the order of 36 million tons/y, an increase of 155 % from the production in 1982. Of this produc-tion, only some 6 % (~2.2 million tons) is based on cellulose, however (Figure 8.5).
Figure 8.5. Worldwide production of manufactured fibers shared by fiber types in 1982 and 2002.
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The traditional way of making regenerated cellulose fibers and films is by treating the cellulose with strong alkali (mercerisation) in order to adjust (decrease) the degree of polymerization (DP) to a suitable value followed by reaction with carbon disulfide. The solution that is formed is termed viscose and this is also the name of the process. Chemically, the mercerisation con-verts the cellulose I to cellulose II which subsequently is converted to cellulose xanthate by re-action with carbon disulfide. The xanthate is dissolved in aqueous sodium hydroxide and allowed to equilibrate in order to get the substitution as evenly distributed as possible. Finally, the xanthate is pressed through a spinnerette into a solution of sulfuric acid where the acid
re-cellulosic 21%
nylon 11% cellulosic 6%
acrylic 8%
polyester 58%
olefin 17%
1982 2002
Figure 8.6. Process steps and chemical reactions encountered in the manufacturing of rayon fibers.
filtering/deaeration/
generates the cellulose as fine filaments resulting in rayon fibers. The process and the chemical reactions are schematically shown in Figure 8.6. In a similar way, cellophane can be made by pressing the viscose solution through a fine slit.
A newly developed alternative to the viscose process is a direct dissolution of the cellulose in NMMO (N-methyl-morpholine-N-oxide, Figure 8.7) and subsequent precipitation of the cellu-lose filaments in an NMMO-water mixture. These fibers are termed Lyocell fibers and like the rayon fibers, their major use are in textiles.
Figure 8.7. N-methyl-morpholine-N-oxide, NMMO, a cellulose solvent used to manufacture Lyocell fibers
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Carboxymethylcellulose (CMC) is one of the most important cellulose derivatives. In Sweden, it is manufactured by Metsä-Serla Chemicals AB in Skoghall which has an annual production capacity of around 20,000 tonnes (1995). The process involves mercerisation of the starting cel-lulose, usually dissolving pulp, followed by reaction with sodium monochloroacetate to form an ether linkage. After neutralisation, washing and beating the product is dried as its sodium salt.
Normally, the degree of substitution (DS) is around 0.60–0.95. The process is outlined in Fig-ure 8.8.
CMC has a wide area of use. Highly purified CMC is used in the food, pharmaceutical and cosmetic industry when a taste- and smell-free non-toxic thickening agent, stabilizer or dispers-ing agent is needed. Typical examples are ice cream, tooth-paste, deodorants and schampoos. A somewhat less pure, highly viscous and water-soluble form of CMC is used in various industrial applications such as dispersing agent, flow property regulator and for the development of thin films in e.g. paper coating colors.
In Sweden, the non-ionic cellulose derivative ethylhydroxyethylcellulose (EHEC) is manu-factured by Akzo Nobel Surface Chemistry AB in Örnsköldsvik with an annual production of around 20 000 tons (1995). In the manufacturing of EHEC, the cellulose is first mercerized and subsequently reacted with ethyleneoxide to form hydroxy-polyethoxy ether groups along the cellulose chain. Typically, a DS ~1.2 is obtained. In a second reaction step, ethylchloride is used to introduce ethyl ether groups with a DS of around 0.8–1.0. The process is outlined in Figure 8.9. In water, EHEC forms colloidal solutions that are used for water retention in cement and other applications in the construction industry. Other important uses are as thickening and dis-persing agents and as stabilizer in water-based latex paints.
N O O CH3
N-methyl-morpholine-N-oxide
Figure 8.8. Process steps in the manufacturing of carboxymethylcellulose (CMC) together with a simplified chemical structure of CMC.
Figure 8.9. Process steps in the manufacturing of ethylhydroxyethylcellulose (EHEC) together with a simplified chemical structure of EHEC.
Several products based on cellulose acetate are produced commercially having different de-grees of substitution along the cellulose chain. Major uses are as lacquers, fibers, photographic films and fabrics (Table 8.1). Normally, the reaction is carried out in acetic acid with acetic an-hydride as the acetylation reagent and sulfuric acid as the catalyst. The reaction conditions de-termine the DS but also the solvent and the catalyst.
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A further cellulose ester of commercial interest is cellulose nitrate which also can be pro-duced with a variety of DS as shown in Table 8.2. The process involves treatment of the cellu-lose (dissolving pulp) with a mixture of nitric acid and sulfuric acid in which the treatment conditions determine the DS of the product. The crude product is washed with water and subse-quently treated with boiling sodium carbonate solution in order to adjust the degree of polymer-ization (“stabilpolymer-ization”) before beating and dewatering to form the final product. The process is outlined in Figure 8.10.
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Figure 8.10. Process steps in the manufacturing of cellulose nitrate together with a simplified chemical structure.
In Sweden, cellulose nitrate is manufactured by Bofors Explosives AB in Karlskoga which has an annual capacity of around 1000 tons (1995). The DS is around 2.2–2.5 with more than 50 % of the substitution in the positions 2 and 3. The products are gunpowder and explosives, made by mixing of low and high nitrated cellulose together with a solvent like ethanol.
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Na2CO3
A different way of substituting the cellulose is by graft polymerization. Although several modes of adding reactive compounds to cellulose have been tried, these attempts have so far been without much success. One major difficulty encountered in these experiments is the rela-tive ease by which homopolymers are formed together with the desirable copolymerization.
Therefore, comparatively large losses of the grafting compound can be obtained. One example is shown in Figure 8.11 where radical polymerization between cellulose and vinyl chloride has been tried with the major product being polyvinyl chloride.
Figure 8.11. Radical initiated polymerization of vinyl chloride on bleached softwood kraft pulp. Amount of copo-lymerization (C) and homopocopo-lymerization (H) respectively.