Nucleation of PET crystallization by metal hydroxides

PET formulations containing small amounts of nonalkali metal hydroxides were prepared by extrusion from the melt. The efficiency of the hydroxides as nucleating agents was estimated from the elevation of T_cc, the crystallization temperature upon cooling from the melt, normalized for the purpose of comparison to a single convenient M_w (or the corresponding intrinsic viscosity). The results show that metal hydroxides capable of releasing water within the processing temperature interval of PET are effective nucleating agents for the crystallization of the polymer. Hydroxides releasing water out of said interval do not nucleate PET. There are two proposed nucleation mechanisms: (a) a localized severe hydrolysis of PET by the water released in the immediate neighborhood of the hydroxide particles and (b) a localized supercooling of the PET in the vicinity of the particles by the released water. The hydrolyzed PET of sharply reduced molecular weight has a T_cc significantly higher than the T_cc of the unhydrolyzed majority of the polymer. Upon cooling from the melt, and with the possible localized supercooling, the PET in the immediate vicinity of the hydroxide particles crystallizes first, nucleating the rest of the polymer. At present we can not verify whether both mechanisms operate concomitantly and which one of them is dominant.