Cellulose water sorption

The sorption of water by excipients derived from cellulose and starch has been considered by numerous workers, with at least three thermodynamic states having been identified [82]. Water may be directly and tightly bound at a 1 1 stoichiometry per anhydroglucose unit, unrestricted water having properties almost equivalent to bulk water, or water having properties intermediate between these two extremes. The water sorption characteristics of potato starch and microcrystalline cellulose have been determined, and comparison of these is found in Fig. 11. While starch freely adsorbs water at essentially all relative humidity values, microcrystalline cellulose only does so at elevated humidity values. These trends have been interpreted in terms of the degree of available cellulosic hydroxy groups on the surfaces, and as a function of the amount of amorphous material present [83]. 

Interestingly, the ball-milling studies of microcrystalline cellulose by Nakai (Table 4 [53]) have shown that the Wm values obtained from water sorption... 

Another typical property of cellulose and its derivatives dependent on water sorption is the swelling of the fibre that occurs under the influence of certain solutions such as aqueous sodium hydroxide or an ammoniacal solution of cupric oxide, i.e. cuprammonium . The process of swelling does not start with sorption as in the instance of water. In the first stage of swelling the liquid penetrates the molecular chains of the cellulose, gradually coming in contact with all of them so that chemical combination takes place to form alkali celluloses, (C6H,0O5) NaOH and (C6H,0O5)2 NaOH. 

Sorption and swelling are accompanied by the evolution of heat (Katz [33]). Water sorption causes the evolution of some 3.5-6.0 kcal/mole. The heat of swelling in the presence of sodium hydroxide depends on its concentration. When a 5-18% sodium hydroxide solution is used, the heat of swelling amounts to 13-30 cal/g of cellulose. 

Maskavs M., Kalninsh M., Reihmane S., Laka M., Cbemyavskaya S. (1999) Effect of water sorption on some mechanical parameters of composite systems based on low-density polyethylene and microcrystalline cellulose. Mechanics of Composite Materials, 35, 1, 79-90. 

Buckton G, Yoremochi E, Yoon NL, Moffat AC. Water sorption and near IR spectroscopy to study the differences between microcrystalline cellulose and silicified miicrocrystalline cellulose after wet granulation. Int J Pharm 1999 181 41 7. 

Surprisingly, the association between TPS and cellulose fibres (polysaccharide-based composites) has been little analysed. In the literature, cellulose fillers used in association with a plasticized starch matrix are commercial paper fibres,10 potato pulp microfibrils,11 12 tunicin whiskers and paper bleached pulp.13-14 The literature reports that these cellulose fillers improve the tensile strength. Besides, the composite water sorption seems to be decreased. This behaviour is related to the well known lower water uptake of cellulose compared to starch.10-14... 

Hemicelluloses are very hydrophilic polymers and are largely responsible for the water sorption behavior exhibited by plant fibers (along with pectins when present). In many plant and wood species, the hemicelluloses tend to interface between the cellulose and the lignin (Shimizu, 1991) [32]. 

Moisture Regain. The advantage of this method is that water vc r normally does not penetrate well-defined crystallites of cellulose (26). Thus the amount of moisture adsorbed a sample after conditioning at a i >ecific relative ve r pressure and tenf)erature can be used to estimate its accessibility and degree of order. In one method using water sorption (27), the fraction of amorf ous material (F ) has been calculated from the sorption ratio (SR) of the sanple using the equation... 

Hailwood and Horrobin ( ) develqped an equation for water sorption of cellulose based on a solution theory. It permits the calculation of the fraction of the san )le inaccessible to water vapor. However, Hailwood and Horrobin assumed in the development of their equation that an ideal solid solution of polymer, hydrated polymer and water is formed. This assutrption has laeen questioned in the general discussion following the presentation of their paper and also by McLaren and Rcwen (29). 

Water vapor at room temperature will not penetrate well-defined crystallites but will be adsorbed in the amorphous regions. Consequently, moisture sorption measured gravimetrically at a given relative vapor pressure and temperature has been used to determine order in cellulosic materials. In the case of Valentine [252] and Jeffries [253], the fraction of ordered material was obtained by correlating moisture sorption with values obtained by the deuterium oxide method. Hailwood and Horrobin [254] developed an equation for water sorption of cellulose based on a solution theory that allowed the calculation of the fraction of the sample inaccessible to water. 

The working group in the COST 90 bis Project has developed a reference material (microcrystaUine cellulose, MCC) and a reference method for measuring water sorption isotherms, and conducted a collaborative study to determine the precision (repeatability and reproducibility) with which the sorption isotherm of the reference material may be determined by the reference method. A detailed procedure for the resulting standardized method was presented, and the factors influencing the results of the method were discussed [158-160]. 

The initial differential heat of sorption of the system is 67.5 cal/g acetone, a figure which is rather modest as compared to the 240 cal/g water in the system cellulose-water. The concavity of the initial part of the curves towards the pressure axis is, accordingly, smaller. 

Pizzi A (1987) The structure of cellulose by conformation antdysis. 5. The cellulose II water sorption isotherm. In Macromol J (ed) Sci-Chem A24(9) 1065-1084. 

Joly C, Gauthier R, Escoubes M (1996) Partial masking of cellulosic fiber hydrophilicity for composite applications. Water sorption by chemically modified fibers. Water sorption by chemically modified fibers. J Appl Polym Sci 61 57-69... 

Sapieha S, Caron M, Schreiber HP (1986) Kinetics and equilibria of water sorption in LLDPE-cellulose composites. J Appl Polym Sci 32 5661-5663... 

Figure 7.5 Water sorption behavior for different starch-cellulose composite films.

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