A slurry feed composed of water, PVA and fine particles are sprayed in droplet from into a spray dryer, with porous and spherical spray-dried products being recovered. However, the PV A binder is so hygroscopic that the strength of the products changes as the relative humidity changes. For the purpose of clarifying this phenomenon quantitatively, the specific surface area, hygroscopic moisture content and strength of the spray-dried products are examined experimentally and theoretically. The

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... clusions obtained are summarized as follows: 1. The hygroscopic moisture content of the products is estimated by the total of the moisture adsorbed on the surface of constituent fine particles covered somewhat with PV A and the moisture adsorbed by PVA itself 2. The strength of the products is measured under various conditions of relative humidity. The measured values are compared with the results calculated from the previously derived equation. As a result, it was found that the strength of the products could be estimated very well by taking account the effect of the variation in humidity on binder strength in the derived equation.