Solid-state polycondensation

Because of the need to provide enough interfadal area to allow the removal of volatile by-products, the polymer has to be in a powdery form. One of the several optimization problems of these processes is to specify an economical starting particle size. 

A practical difficulty is the possible tendency of the particles to stick, which will make the process unfeasible. It is counteracted by starting with polymers with sufficiently high crystallinity, and by adding glass beads. Another problem may be the sublimation of oligomers, as in nylon-6, which may clog the bed. A precrystallization step for PET, to make it attain at least 40% crystallinity before SSP starts, is present in industrial processes since early 1970 s. 

The overall polydispersity of polymer will be greater than the equilibrium value (2 

In the same way as in heterogeneous catalysis, effective diffusion coefficients (for fluxes with respect to the total geometric area) are decreased relative to the values in the melt Dy because of the obstruction due to the crystalline phase at a volume fraction (j) and because of the tortuosity factor xo (which depends on the structure of the solid, values of 1.5 to 3 being common) the relationship is given in Eq. (47). 

Notice that volume and mass fractions Wa- of the crystalline phase are different because of the slight difference in density with respect to the amorphous phase. 

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Many studies on the modelling of esterification, melt polycondensation, or solid-state polycondensation refer to the reaction scheme and kinetic data published by Ravindranath and co-workers. Therefore, we will examine the data sources they have used over the years. The first paper concerned with reactor modelling of PET production was published by Ravindranath el al. in 1981 [88], The reaction scheme was taken from Ank and Mellichamps [89] and from Dijkman and Duvekot [90], The kinetics for DEG formation are based on data published by Hovenkamp and Munting [60], while the kinetics for esterification were deduced... 

In industrial PET synthesis, two or three phases are involved in every reaction step and mass transport within and between the phases plays a dominant role. The solubility of TPA in the complex mixture within the esterification reactor is critical. Esterification and melt-phase polycondensation take place in the liquid phase and volatile by-products have to be transferred to the gas phase. The effective removal of the volatile by-products from the reaction zone is essential to ensure high reaction rates and low concentrations of undesirable side products. This process includes diffusion of molecules through the bulk phase, as well as mass transfer through the liquid/gas interface. In solid-state polycondensation (SSP), the volatile by-products diffuse through the solid and traverse the solid/gas interface. The situation is further complicated by the co-existence of amorphous and crystalline phases within the solid particles. 

The phase equilibria of the most important compounds will be described in the following section. In the sections thereafter, we will treat mass transport in melt-phase polycondensation, as well as in solid-state polycondensation, and discuss the diffusion and mass transfer models that have been used for process simulation. 

Depending on the size and shape of the polymer particles, solid-state polycondensation (SSP) is performed at temperatures between 220 and 235 °C, which lie above the glass transition temperature ( 70-85 °C) and below the melting point (measured by DSC 245-255 °C) of PET. The temperature range for operation of SSP is rather small because on the one hand, the temperature should be as... 

The chemistry of the solid-state polycondensation process is the same as that of melt-phase poly condensation. Most important are the transesterification/glycolysis and esterification/hydrolysis reactions, particularly, if the polymer has a high water concentration. Due to the low content of hydroxyl end groups, only minor amounts of DEG are formed and the thermal degradation of polymer chains is insignificant at the low temperatures of the SSP process. 

The papers of Mallon and Ray [98, 123] can be regarded as the state of the art in understanding and modelling solid-state polycondensation. They assumed that chain ends, catalysts and by-products exist solely in the amorphous phase of the polymer. Because of the very low mobility of functional groups in the crystalline phase, the chemical reactions are modelled as occurring only in the amorphous phase. Additionally, the diffusion of by-products is hindered by the presence of crystallites. The diffusivity of small molecules was assumed to be proportional to the amorphous fraction. Figure 2.32 shows the diffusion coefficients for the diffusion of EG and water in solid PET. 

To increase the PET molecular weight beyond 20 000 g/mol (IV = 0.64 dL/g) for bottle applications, with minimum generation of acetaldehyde and yellowing, a further polycondensation is performed in the solid state at low reaction temperatures of between 220 and 235 °C. The chemistry of the solid-state polycondensation (SSP) process is the same as that for melt-phase polycondensation. Mass-transport limitation and a very low transesterification rate cause the necessary residence time to increase from 60-180 minutes in the melt phase to... 

For the solubility of TPA in prepolymer, no data are available and the polymer-solvent interaction parameter X of the Flory-Huggins relationship is not accurately known. No experimental data are available for the vapour pressures of dimer or trimer. The published values for the diffusion coefficient of EG in solid and molten PET vary by orders of magnitude. For the diffusion of water, acetaldehyde and DEG in polymer, no reliable data are available. It is not even agreed upon if the mutual diffusion coefficients depend on the polymer molecular weight or on the melt viscosity, and if they are linear or exponential functions of temperature. Molecular modelling, accompanied by the rapid growth of computer performance, will hopefully help to solve this problem in the near future. The mass-transfer mechanisms for by-products in solid PET are not established, and the dependency of the solid-state polycondensation rate on crystallinity is still a matter of assumptions. 

Ravindranath, K. and Mashelkar, R. A., Modeling of polyethylene terephthalate) reactors. IX. Solid state polycondensation process,./. Appl. Polym. Sci., 39, 1325-1345 (1990). 

Similar to virgin PET, solid-state polycondensation (SSP) is the method of choice to increase the molecular weight in mechanical recycling processes. 

Gey, W., Langhauser, W., Heinze, H. Rothe, H. J. and Freund, P., Process for the solid state polycondensation of linear polyesters, US Patent... 

Buxbaum, L. H., Solid-state polycondensation of poly(butylene terephthalate), J. Appl. Polym. Sci., 35, 59-66 (1979). 

Chang, T. M., Kinetics of thermally induced solid state polycondensation of poly(ethylene terephthalate), Polym. Eng. Sci. 10, 364-368 (1970). 

Chen, F. C Griskey, R. G. and Beyer, G. H., Thermally induced solid state polycondensation of nylon 66, nylon 6-10 and poly(ethylene terephthalate), AIChEJ., 15, 680-685 (1969). 

Devotta, I. and Mashelkar, R. A., Modelling of polyethylene terephthalate reactors - X. A comprehensive model for a solid-state polycondensation process, Chem. Eng. Sci., 48, 1859-1867 (1993). 

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