Solid-state polycondensation temperature

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. 

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. 

Besides the pressure (vacuum) and the flow rate of the gas, temperature is the major experimental variable in SSP and is of the highest importance for the economy of the process. Temperature dependence data for the solid-state polycondensation process are shown in Figures 5.4-5.7. According to the results... 

For a long period of time, too litde attention has been paid to the content and the role of oligomers in the spinning process. Due to the equilibrium conditions in the reaction mixture, PET contains about 1-2% of oligomers. In certain conditions, this amount can be reduced to values below 1 % by solid-state polycondensation (SSP) processes. Figure 13.8 shows the variation of the oligomer content as a function of temperature and time during SSP processes. 

The occurrence of fluorescence is often related to inappropriate processing conditions in molten-state and solid-state polycondensation (SSP) (presence of oxygen, high temperature, long retention time, etc.), as well as the later drying of chips where prolonged residence times can occur. 

Solid-state polycondensation (SSP) is thus a technique applied to thermoplastic polyesters to raise their molecular weight or IV. During solid-state polycondensation, the polymer is heated above the glass transition temperature and below the melt temperature of the polymer either under an inert gas or under vacuum. Increasing the intrinsic viscosity requires a residence time of up to 12 h under vacuum or under inert gas, at temperatures from 180 to 240 °C. 

High-molecular-weight polyesters cannot be made by polymerization in the molten state alone - instead, post-polymerization (or polycondensation) is performed in the solid state as chips (usually under vacuum or inert gas) at temperatures somewhat less than the melting point. The solid-state polycondensation of polyesters is covered in detail in Chapters 4 and 5. 

Levites, E. I., A. V. VoLOKHiNA, and G. I. Kudrayavtsev Solid phase polycondensation. IV. Solid phase co-polycondensation of aminoacids and diamine salts of dicarboxylic acids. Vysokomolekul. Soedin. 5, 875 (1963). Bagramayants, B. A., A. K. Bonetskaya, N. S. Yenikolopyan, and S. M. Skuratov The reason for the ligh temperature coefficient in solid state polycondensation of cu-amino acids. Vysokomolekul. Soedin. 8, 1594 (1966). Cohen, S. M., and E. Lavin Polyspiroacetal resins. Part I. Initial preparation and characterization. J. Appl. Polymer Sci. 6, 503 (1962). 

The S5mthesis of aromatic high-molecular polyketones by the low-temperature solid-state polycondensation of 4,4 -bisphenoxybenzophenone and isophthaloylchloride in the presence of AICI3 in 1,2-dichlorethane is possible [337]. Obtained pol miers are thermoplasts with glass transition temperature 160 °C and melting point 382 °C. 

The final stage of a polycondensation in suspension represents a polycondensation in the solid state. Polycondensation in the solid state is particularly suitable for producing polyamides. Here, too, a continuous precondensation is first performed to molecular weights between 1000 and 4000. The products are then spray-dryed and are polycondensed further at temperatures of 200-220 "C under nitrogen. This further polycondensation occurs relatively quickly. In order to obtain molecular weights between 1000 and 15,000 with polymers from hexamethylene diamine and adipic acid, the time required at 216°C is 16 h. If the molecular weight of the precondensate is increased to 4000, however, 2 h is sufficient. Since the temperatures are lower than for polycondensation in the melt, better end products are also obtained (less discoloration, etc.). 

Transesterification is the main reaction of PET polycondensation in both the melt phase and the solid state. It is the dominant reaction in the second and subsequent stages of PET production, but also occurs to a significant extent during esterification. As mentioned above, polycondensation is an equilibrium reaction and the reverse reaction is glycolysis. The temperature-dependent equilibrium constant of transesterification has already been discussed in Section 2.1. The polycondensation process in the melt phase involves a gas phase and a homogeneous liquid phase, while the SSP process involves a gas phase and two solid phases. The respective phase equilibria, which have to be considered for process modelling, will be discussed below in Section 3.1. 

According to the principles of polycondensation, all of the above reactions will also take place during SSP. The conditions for the latter, however, are different as this process is carried out at lower temperatures in a non-homogeneous environment. In order to examine the kinetics of SSP, some assumptions have to be made to simplify the analysis. These are based on the idea that the reactive end groups and the catalyst are located in the amorphous regions. Polycondensations in the solid state are equilibrium reactions but are complicated by the two-phase character of the semicrystalline polymer. 

Figure 5.7 Degree of polycondensation as a function of retention time and temperature [12b]. From Weger, F., Solid-state postcondensation of polyesters and polyamides, presentation given at the FrankI and Thomas Polymer Seminar, 16 June, 1994, Greenville, SC, USA, and reproduced with permission of EMS Inventa-Fischer, GmbH Co. KG...

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