Dissolution behaviour and solubility of cellulose in NaOH complex solution
Introduction
Cellulose is the most important skeletal component in plants and the most abundant renewable material in nature (Dogan and Hilmioglu, 2009, Klemm et al., 2005). The non-thermal plastic nature and insolubility in most common solvents remain to be the challenges in processing cellulose for effective utilization. In fact, dissolution of cellulose without chemical modification or derivatization is difficult to achieve because of the rigid long-chain and strongly inter-molecular and intra-molecular hydrogen-bonded structure in cellulose (Fink et al., 2001, Zhang et al., 1999). More common than not, cellulose needs to be “activated” or made “accessible” to be dissolved, even though these notions are not clearly defined. Traditional production of regenerated cellulose fibers and films has been largely based on the viscose or the cuprammonium technologies, that generate hazardous environmental pollution (Kamide & Saito, 1986). Thus, identifying new solvent systems for cellulose processing would help to reduce these environmental problems.
Some solvents for cellulose, such as cuoxam, cuen, and cadoxen as well as lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) contain metal complexes (McCormick, Callais, & Hutchinson Jr, 1985). Others including N2O4/N,N-dimethylformamide (DMF) (Philipp, Nehls, Wagenknecht, & Schnabelrauch, 1987), NH3/NH4SCN (Cuculo, Smith, Sangwatanaroj, Stejskal, & Sankar, 1994), N-methylmorpholine-N-oxide monohydrate (NMMO) (Loubinoux & Chaunis, 1987) and ionic liquid (Swatloski, Spear, Holbrey, & Rogers, 2002) are limited to laboratory scale applications due to volatility, toxicity and high cost. Among the solvents developed, the NMMO/H2O system is the most powerful in attaining exceedingly high concentration solutions and has been commercialized to produce Tencel or Lyocell fibers. However, the NMMO/H2O system also has disadvantages of requiring high temperature for dissolution and antioxidants to avoid side reactions of solvents, causing degradation of cellulose, and high costs. Thus, it is not suitable for complete replacement of the viscose technology.
Kamide, Okajima, Matsui, and Kowsaka (1984) and Isogai and Atalla (1998) have systematically studied the solubility of microcrystalline cellulose and steam-exploded cellulose in aqueous NaOH systems in which native cellulose pulps have very limited solubility. Recently, Zhang's group (Cai and Zhang, 2006, Weng et al., 2004, Zhang et al., 2002, Zhang et al., 2001, Zhou et al., 2004) successfully developed aqueous NaOH solution systems with either urea or thiourea for cotton linter dissolution. The optimal solubility was found with either 7/12/81 NaOH/urea/H2O or 9.5/4.5/86 NaOH/thiourea/H2O in compositions. Both solvent systems were inexpensive and less toxic, but the precise structure of the solvent complex involved is still not clearly understood.
Recently, we developed a new complex aqueous solvent that consisted of NaOH, urea and thiourea and was capable of dissolving cellulose quickly when pre-cooled to temperatures between −8 and −12 °C (Jin et al., 2007, Zhang et al., 2009a, Zhang et al., 2009b). With the aim to gain basic understanding of the rapid dissolution of cellulose in this solvent, the present work was to further clarify the dissolution behaviour and solubility of cellulose based on this novel aqueous NaOH/urea/thiourea platform. The behaviour of cellulose solutions among the three aqueous NaOH/urea/thiourea, NaOH/thiourea and NaOH/urea solvent systems was also compared.
Section snippets
Materials
Cellulose used in this study was cotton linter, with a degree of polymerization of 520 and denoted as C520, supplied by Xinxiang Bailu Chemical Fibers Co. Ltd. (Henan, China). All cellulose samples were shredded into powder to pass 40 mesh and dried under vacuum at 70 °C for 24 h. All chemicals were of analytical grade and used as received. All concentrations of cellulose and solvent mixtures were expressed in weight percent unless specified. For dual or tri-component solutions, the concentration
Effect of NaOH concentration on solubility
The effects of NaOH concentration on cellulose dissolution kinetics was evaluated from 1 to 10% NaOH in 8/6.5 urea/thiourea aqueous solutions. The solubility of 7% cellulose improved significantly with increasing NaOH concentrations and peaked at 8% NaOH with 91% solubility (Fig. 1).
This is probably due to the concentration-dependent size of NaOH-water hydrates (Yamashiki et al., 1988). Crystalline cellulose chains are very densely packed, with an inter-sheet distance of about 10 Å and a
Conclusions
The rapid dissolution behaviour and solubility of cellulose in aqueous NaOH/urea/thiourea solution at a 8/8/6.5 mass ratio composition and pre-cooled to −10 °C was aided by vigorous stirring for 3 min and further dissolution at −2 to 0 °C for 7–10 min evaluated by 13C NMR, optical microscopy, solubility as well as viscometry. The low temperature, thought to be driven by entropy hydrate formation kinetics, played an important role in creating the stable hydrogen-bonded networks between cellulose,
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