Effect of Cooling Rate on Freezing of Wool-Water Mixtures

Abstract

The effect of the cooling rate on freezing of a wool-water system is studied using Dsc at cooling rates (CR) that vary from 1.25 to 40 K/min. By means of centrifugation, an equilibrium quantity of interstitial water in the fibers ( Ws) is determined to be 0.48 g/g of wool. Two samples with different water contents are used: sample A is more than Ws, i.e., Wc = 0.54, and sample B is 0.43 g/g of wool. Characteristic shape changes in a broad exotherm ranging from temperatures of about 230 to 260 K are observed in DSC curves. A freezing mechanism of interstitial water is proposed on the basis of supercooling. The swollen wool is assumed to be a dispersion system consisting of domains defined as structurally favored sites, permitting water migration for ice crystal growth. The shape change with CR can be explained by differences in the freezing temperature of water of different volumes in the domains. Kissinger's equation is applicable to this crystallization process. Values of activation energy for both A and B samples are nearly the same, about -115 kJ/mol. Activation energy is considered to be the sum of the two terms for crystallization and migration of water, and the latter value is presumed to be about 49 kJ/mol. The important finding is that ice crystals in sample A cooled at 1.25 K/min contain an amount of water corresponding to about 38.2% of the so-called "nonfreezing water" Wu, which remains unfrozen at a CR above 5 K/min.