Water absorption and desorption kinetics

taken in moderation, cannot hurt anybody (Mark Twain). 

This is true for polyurethanes as long as it has been already reported that PUs may absorb moderate quantities of water which increase with the HS concentration [343]. In our study we have followed the increase in weight of already maturated 2 mm thick PU sheets previously anhydrided and then submitted to a controlled 

PUs sheets of different thickness, previously completely maturated, were anhy-drided at 60°C under vacuum of 0.1 mm Hg in the presence of anhydrous CaCl2 up to constant weights. On these films, the humidity absorption rate was measured by the increase of their weight under the same atmosphere conditions as those used in the maturation postcure process (e.g. 25°C and 50% RH). 

To determine water desorption, PU sheets of different thickness previously maturated, were immersed for 7 days in water, then the surface humidity was wiped off and maintained at the same controlled atmosphere of 50% RH and t = 25°C. The decrease of weight due to gradual drying was followed [17]. 

The gravimetric increase i.e., the variation of humidity (t/) with time for 2 mm thick PUs films is plotted as shown in Fig. 4.73, curve (b). As seen, water was absorbed very quickly. A saturation limit of U about 0.2% gravimetric was achieved in less than 2 days. This maximum degree of water saturation in PU sheets, corresponding to the atmospheric conditions of 50% RH and 25°C, was also confirmed by following the kinetics of desorption on similar PUs sheets which were previously hydrated by immersion in water and then kept under the same atmospheric conditions (Fig. 4.73, curve (a)). 

---