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in Wiley InterScience (www.interscience.wiley.com). The kinetics of the forward polycondensation reaction of polybutylene terephthalate (PBT) has been investigated using thermogravimetric analysis (TGA). PBT -prepolymer with an initial degree of polymerization of 5.5 was used as starting material. The PBT prepolymer was prepared from dimethyl terephthalate (DMT), and 1,4 -butanediol with 470 ppm wt (with respect to DMT) of tetrabutoxytitanate as catalyst. The process conditions in the TGA were such that the weight loss curves obtained from the TGA could be correlated uniquely and directly to the forward polycondensation reaction kinetics due to the absence of (1) mass transfer limitations in the removal of 1,4 butanediol from the melt, and (2) degradation reactions. It is shown that in the temperature range of 230 -270°C, the forward polycondensation reaction can be expressed using second-order kinetics in terms of the hydroxyl end group concentration of the PBT polymer. © 2005American Institute of Chemical Engineers AIChE J, 51: 622-630, 2005 Keywords: kinetics, polybutylene terephthalate, polycondensation, thermogravimetry IntroductionPolybutylene terephthalate (PBT) is a thermoplastic elastomer used extensively as an engineering plastic. PBT has excellent mechanical and processing properties and is, therefore, used in the electronics and automotive industry, and also in consumer goods and films. Industrial production of polyesters like PBT proceeds via a condensation reaction and is carried out industrially in two steps: (1) prepolymerization, and (2) polycondensation.During the prepolymerization step, monomers, such as dimethyl terephthalate (DMT), or terephthalic acid (PTA) undergo esterification reactions with diols like 1,4 butanediol (BDO) (in case of PBT) to form oligomers. Prepolymerization is carried out in a series of stirred reactors giving a prepolymer with a degree of polymerization ranging from 4 -10. The prepolymer reacts further to obtain high degrees of polymerization (Ϸ 100) during the actual polycondensation reaction. In this step the oligomers (with hydroxyl end groups, E OH ) formed during prepolymerization react giving diester groups (Z) with liberation of BDO (Figure 1).The polycondensation reaction is highly reversible and, hence, removal via devolatization of the volatile products, in this case the diol from the reaction mixture is an important factor in industrial reactors. Industrially, the polycondensation reaction is carried out between 230 -240°C using rotating disc contactors (RDC). RDCs provide a relatively large interfacial area, and are operated under vacuum of Ϸ1 mbar to enable rapid devolatization in order to speed up the polycondensation process. 1 For the RDCs, calculations show that the forward polycondensation reaction can enhance the devolatization mass-transfer rate with enhancement factors ranging from 1.5 -5. Details of the mass-transfer calculations for computing enhancement factors will be presented in a forthcoming publica- tion. In case of novel polycondensati...

The PBT polycondensation equilibrium constant at 255°C was determined using a batch reactor. Starting from a Polybutylene terephthalate (PBT) prepolymer having a degree of polymerisation of 12.7, equilibrium experiments were performed in the range of 1 to 50 mbar. The equilibrium degree of polymerisation (i) was determined indirectly using dilute solution viscometry of a solution of 1 weight % PBT in m ‐ cresol. The degree of polymerisation of PBT obtained at equilibrium in the range of 1 to 50 mbar at 255°C as a function of the BDO partial pressure (mbar) could be expressed by: i = 111.47 − 86.18 exp(−1.14 $P^{-1.44}_{bdo}$ ). The equilibrium solubility of 1,4 butanediol (BDO) in the PBT melt was derived form the BDO partial pressure using the Flory ‐ Huggins theory. The PBT polycondensation reaction equilibrium constant was related to the degree of polymerisation by the equation: $K_e = {0.187 \over 1 -19243 e^{-0.3932}} $ in the range i = 26 − 100. The PBT polycondensation equilibrium constant at high degrees of polymerisation is in line with literature data and thermodynamics.