Short- and Long-Term Water Absorption

Draining of water from a bulk material occurs very slowly compared to that from surface and near-surface layers. It takes place by a slow diffusion, not as a result of air movement, as at the surface area. 

To compare a short- and long-term water absorption, an experiment was conducted, in which several deck boards were submerged in water for a period from 24 h to 110 days. The experiment was performed in 2001, and two old (later discontinued) GeoDeck boards were chosen, one containing only rice hulls (65%) and HDPE (35% minus minor additives), another containing rice hulls and Biodac in approximately equal amounts (by weight). The second board had a very low density (specific gravity), of 1.09 g/cm compared with the current (2006) boards of 1.19-1.24 g/cm.  

TABLE 12.8 Water absorption by some deck boards 

Time period of submerging in water (days) Water absorption (%)  

Another similar experiment, such as shown in Table 12.8, was conducted with a different set of boards (GeoDeck, having only rice hnlls as a filler, was later discontinued), but following a different time schedule (Table 12.9). 

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Water absorption depends very significantly on the shape of composite profiles, and its dependence on brushed or unbrushed profiles is under study. Boards described in Tables 12.8 and 12.9 were brushed. For a comparison, unbrushed GeoDeck pickets (2" X 2" nominal cross-sectional size, for a composite railing system) showed the following figures for a short- and long-terms water absorption ... 

Earlier it was shown (Tables 12.4-12.6) that the lower the density, the higher the swelling of composite deck boards being submerged underwater. Table 12.12 shows short- and long-term water absorption for the same boards. 

The procedures for water absorption for immersed specimens are subdivided into short-term (2 h and 24 h) immersion and long-term immersion. All tests should be done in distilled water at ambient temperature (22-24°C or 72-75°F). Two-hour immersion is recommended for samples having a relatively high rate of water absorption and for thin specimens. At the end of the immersion time, the specimens should be removed from the water one at a time, all surface water wiped off, and weighed. For long-term immersion, the specimens should be removed, weighed, and placed back in water at-24 h time point, 7 days, and every 2 weeks, until the test is terminated or the specimens reach the water saturation level (see above). 

A long-term treatment of PS in a weak magnetic field also contributes to a short-wave shift of the OH stretching band (Fig. 2). In our recent work [5], it was found that the position of the valence absorption bands of water in PS silicon varies between 2900 and 3550 cm", which was concerned with variation of the energy of hydrogen bonds and water states in PS. This shift of the OH absorption... 

Water repellant treatments involve the deposition of a thin layer of a hydrophobic substance onto external surfaces and, to some extent, internal cell lumen surfaces. Generally the equilibrium moisture content is not significantly altered by these water repellent treatments, although the absorption rate is reduced, so that it may take months or even years to reach equilibrium. For short to medium term protection, polymer coatings can be applied to reduce moisture absorption. For example, short jute fiber/PLA composites, coated with 100 pm thick polypropylene plastic adhesive tape, have been shown to be moisture resistant in humid environments, although the effects were not long lasting [34]. 

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