Moisture in cellulose

Contact with limited amoimts of water, either as liquid or that present as moisture in cellulose fibres, may cause ignition after a delay. 

In the 1980s, more attention has been increasingly paid to improving the physical properties, such as mechanical strength, stiffness, resistance to thermal deformation, and so forth, of the fiber-plastic composite products. It was recognized that moisture in cellulosic fiber leads to steam formation in the compounder and the extruder, increases porosity of the final product and decreases its density, accelerates the oxidation of the hot melt during processing, and makes the final product weaker and less durable. 

On increasing the moisture of cellulose from 0.5 to 16% the principal signals of cellulose shift a few ppm to higer fields. A similar, but much smaller shift is observed in cellulose acetate. The relaxation times T1 for Cl, C2, C3 and C4 diminish with increased moisture content. However, in the case of C6 there is no significant change. In the case of cellulose acetate, a similar general behaviour is observed. 

It is obvious that acid hydrolysis methods leave a number of unsolved problems and many minor disagreements to be ironed out. In general, however, the available results suggest that the natural celluloses consist chiefly of crystalline material which is only slowly eroded by acids. The non-crystalline fraction appears to be relatively more susceptible to hydrolysis than the crystalline fraction and to have a greater capacity to absorb moisture. In other words, the non-crystalline fraction is probably more reactive than the crystalline material, as Mark14 has suggested. In this connection the fact should not be overlooked that the surface layer of the crystallites is probably amorphous and hence relatively more reactive than the underlying layers. 

Rowell, R.M. and Rowell, J.S. (1989). Moisture sorption properties of acetylated lignocellulosic fibers. In Cellulose and Wood Chemistry and Technology, Proceedings of the 10th Cellulose Conference, Schuerch, C. (Ed.). John Wiley Sons, Inc., New York, pp. 343-355. 

Acetylation may be controlling the moisture sensitivity due to the lignin and hemlcellulose polymers in the cell wall but not reducing the sorption of moisture in the cellulose polymer because... 

An essentially physical change in cellulose, which can effectively reduce its moisture content, can indeed account for the observed increase in fold endurance loss under cyclic RH conditions,... 

Other examples are known from the work of Barlow and Hadaway (6) on insecticides in porous wall materials, who note examples of reactivation by moisture for cellulosic materials. The direct observation is recovery of insecticidal activity, but undoubtedly when the insecticide becomes available to the insect, it also becomes available for diffusion in the vapor phase. 

Cotton is seed hairs that protect the seeds in their pods from desiccation in the dry season and from moisture in the rainy season. Under the microscope, cotton appears as a collapsed, tubelike fiber twisted at irregular intervals. In its natural state it repels water because of a waxy substance covering the fiber. Once the wax has been removed, cotton becomes the most absorbent fiber known to man. Cotton is cellulose it withstands moderate alkaline conditions but is easily affected by acids. Because of twist in the fiber, cotton lacks glossiness. Mercerization, a chemical process, gives luster to cotton fiber by straightening the twist. 

In reviewing the literature on the effect of deacidification on paper, three important factors emerge (a) it is desirable to work with papers that are reproducible, well characterized, and similar in properties, both chemical and physical, to old papers (b) much more work needs to be done on the interaction of metals with the carboxyls in cellulose and (c) a standard procedure for accelerated aging that includes some moisture in the aging atmosphere is desperately needed. 

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... 

To study the structural features of preparations mixed polysaccharides, we investigated the process of sorptiewi of water vapor on a vacuum sorption setup of the McBain type (Fig. 2). The data show that preparations of mixed polysaccharides sorb a larger amount of moisture than cellulose, even in the case of mixed polysacdiaride (IV) where the number of hydroxyl groups decreases as a result (rf the forma-... 

It is natural that with the growing interest in cellulose acetate, chemists have investigated other organic cellulose esters. Cellulose formate, the ester of the lowest member of the fatty acid series, has been studied. sufficiently to show that it is unlikely to be of commercial im-portance. It is difficult to produce a high degree of esterification of cellulose with this acid, and the ester which is obtained is very limited in solubility, and is highly unstable toward moisture and elevated temperatures. 

Films for X-ray and portrait purposes require good rigidity for handling and high resistance to moisture in order to prevent unevenness at varying humidities. Cellulose acetate with comparatively large amounts of plasticizer has been widely used for this purpose. Cellulose... 

Pretreatment/activation involves swelling mostly chemical pulp fibers with glacial acetic acid and 4-7% moisture (on cellulose basis). Activation ensures uniform acetylation. Esterification involves the treatment of preactivated cellulose with a mixture of acetic acid and acetic anhydride at a consistency of about 10%. The acetic anhydride content of the mixture is approximately 25%, and this amounts to a 10-40% excess over stoichiometric requirement. The reaction proceeds at 50 °C. Hydrolysis refers to a process step in which the DS of the esterification product is reduced from its level of >2.9 to a level between 2.3 to 2.7 by the addition of 5 to 10% water. The temperature during hydrolysis is maintained at 40 to 80 °C. The consequence of hydrolysis is a uniform release of acetyl groups from the cellulose backbone so as to assure uniform solubility. Precipitation involves either the addition of the acetylation mixture into water or dilute acetic acid to produce a flake product, or the addition of water... 

Mobility of water in cellulose has been studied by solid-state and high-resolution NMR as a function of moisture content within the unfreezable moisture range (0-19% dry basis).Measurements of relative mobilities were based on relative intensities, transverse and longitudinal relaxation times and line shape analysis. At 2-16% moisture content (dry basis), water molecules reoriented anisotropically, suggesting an interaction with cellulose fibers. At moisture content below the monolayer value (2.8%, dry basis), 90% of the protons were immobile and no liquid deuterium signal was detected. A sharp increase in liquid or mobile intensity (accompanied by a decreased LW) and increases in NMR Ti and T2 relaxation times were observed as moisture increased above 9% (dry basis). 

Pioneering work in the development of an isocyanate-bound particleboard in the U.S. has been conducted by the Ellingson Lumber Company. This work has resulted in a proprietary process for bonding cellulosic materials with polymeric MDI to produce a multiple-ply structure panel (7.,. 8, 9.) Advantages cited for this process include a tolerance for up to 22% moisture in the wood raw material without predrying and the ability to also include significant quantities of bark and needles. 

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