Water retention value

Important characteristics of chitosan are its MW, viscosity, DD (Bodek, 1994 Ferreira et al., 1994a,b), crystallinity index, number of monomeric units, water retention value, pKa, and energy of hydration (Kas, 1997). Chitosan has a high charge density, adheres to negatively charged surfaces, and chelates metal ions. 

Table VI. Water retention values for three AN-grafted EC-samples (DS = 0.40), hydrolyzed with aqueous NaOH-solution, measured using the three methods Al, A2 and A3. When two retention values (g/g) are given, the first (1) refers to a never-dried sample and the second (2) to a once-dried sample. The yield of hydrolysis (YH) is given as hydrolyzed product in % of grafted sample.

The water retention is time-dependent to various degrees for the samples studied. This is shown for A2-values vs. time in Figure. The EC and HEC samples are of DS=0.60 and MS=0.60, respectively, grafted with equal weight of AN and hydrolyzed in 1% aqueous alkali for 1.5 hrs at 100 C. The GP and MDC samples, containing native fiber structure, reach equilibrium much faster than the EC and HEC samples. All these measurements refer to once-dried samples. At 15-20 minutes exposure, the water retention values are of the same magnitude for the four samples. 

The water retention values (1) refer to never-dried samples after hydrolysis and (2) to once-dried samples. 

The cotton fiber is hydrophilic and porous. Upon immersion in liquid water, the cotton fiber swells and its internal pores fill with water. Pure cotton holds a substantial percentage of its dry weight in water under conditions of centrifugation. The amount of liquid water held depends upon the severity of the centrifugation used in testing. This is approximately 30% for the water of imbibition [281] or 50% for the water retention value [282]. Centrifugation conditions are less severe in the latter case. 

The fiber saturation point (FSP) of cotton is the total amount of water present within the cell wall expressed as a ratio of water to solid content. It is equivalent to the water of imbibition of the fiber, also called its water retention value. The FSP has been measured using solute exclusion, centrifugation, porous plate, and hydrostatic tension techniques. It occurs at RVP greater than 0.997 and from the review of the papers, it has been concluded that the studies have yielded a value for FSP in the range of 0.43 to 0.52 g/g [303]. 

Inspite of the fact that wilting point is a good indicator of lower limit of available water, there is enough evidence to indicate that wilting point is not a true intrinsic soil property As such there does not exist an rmique soil water retentivity value at which the water uptake by plants suddenly ceases, rather plant usually wilts at a point controlled by rate factor (both supply and demand). However, the 15 bar percentage has been found to be closely correlated with the permanent wilting point (Richard and Weaver 1943). 

WRV (Water Retention Value). The water retained when fibers are subjected to external force Is known as the Water Retention Value. Water retained by surface tension forces In addition to adsorbed water may be Involved In this determination. The methods used In determining this value are centrifuging (water retained under standard centrifuging) (11,12), hydrostatic tension (water retained under standard tension) (13), and suction (water retained under suction or vacuum) (14). All these methods can be employed at any temperature and the experimental techniques are quite simple. 

Effect of Beating on Hon-freezluR Water. It is well known that beating pulp results In an Increased water retention value. 

ScaUan, A. M., and J. E. Carles (1972). The correlation of the water retention value with the fibre samration point, Svensk Papperstid., 75, 699-703. 

Total amount of water in fibrous materials can be easily measured by use of gravimetric methods. There are also several methods that allow the measurement of the amount of total bound water, such as water retention value (WRV], fiber saturation point (FSC], and differential scanning calorimetry (DSC]. 

BNC demonstrates some unique properties such as high degree of crystallinity followed hy high water retention value and porous structure enabling the infusion of solids and liquids. BNC is safe in terms of the health risks caused by distributed and mobile nanoparticles, because the nanofibers of bacterial cellulose are immobilized in a stable network. The in situ production of BNC also enables obtaining products of designed, final shape (Fig. 21.13) [23,24,28]. 

Bacterial cellulose synthesized by Acetobacter xylinun has attracted much attention as a new biodegradable material, because it has unique properties different from those of plant cellulose [23]. For example, the water retention value of the never-dried bacterial cellulose is approximately 1000%, which is superior to the 60% observed in typical plant cellulose like cotton linters [24]. 

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