In studying the dynamics of drying of PBT and PET granulate, the parameters of the process that ensure sufficient process stability of their melts were determined. It was found that the rate of self-ordering of structural elements in the polymer substrate in drying is higher in PBT than in PET due to the greater flexibility of the macromolecules.At the end of the 1970s, polybutylene terephthalate (PBT) fibres appeared on the textile raw materials market. The good wet shape stability, high light and pilling resistance, and good dyeability of new textile materials predetermined the encouraging prospects for development of PBT fibre production [1, 2]. Pilot-industrial production of this polyester was organized in 2000 at Mogilevkhimvolokno OJSC.Drying is an important stage in preparation of PBT for processing. In studying the stability of the rheological properties of polyethylene terephthalate (PET) and PBT melts, it was found that high-temperature hydrolysis at 277°C is more intensive in the second case than in the first. Stable processing of PBT, even in molded articles, is only possible at a maximum moisture content of granulate going in for melting (W t ) of 0.04 wt. % [4]. For this reason, the kinetics and thermodynamics of sorption of water vapors and changes in the granulometric composition and structure of PET during preparation of the granulate for spinning (in storage, drying, etc.) were systematically investigated [5].An equation for a probable model of sorption that can be used to assess the equilibrium moisture content W e of amorphouscrystalline polymers as a function of the relative water vapor pressure and temperature in the range from 0 to 100°C was proposed previously in [6]:where W 0 is the limiting water vapor sorption in amorphous regions of the polymer substrate at T 0 = 293 K and relative humidity of ϕ = 1.0; α c is the degree of crystallinity; -μ 1 = RT ln(p/p 0 ) is the chemical potential of water, here (p/p 0 ) is the relative water vapor pressure; a 0 = δ ln W/(δT)Δμ 1 is the thermal sorption coefficient; E and n are parameters of Eq. (1). The equilibrium moisture content of the granulate at a given temperature will evidently essentially be a function of a c and ϕ.It was found that the structure of the granules formed by underwater granulation can be characterized as isotropic, and water is not only a hydrolyzing agent but also a structural plasticizer [7].We experimentally investigated drying of PBT granulate in comparison to the same process for PET granulate. The fraction of the amorphous phase of a polymer substrate to a significant degree predetermines the kinetics of mass transfer of water in the granules. The reciprocal ordering of segments of the macromolecules not related to crystallization begins in the amorphous state [8], resulting in the appearance of domain structures and formation of grain-like formations under 10 nm in size in the isotropic polymer substrate [9]. These features of crosslinking of hydrophobic semirigid polymers *Mogilev Man-

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