Solid-state polycondensation reaction

M. Epple, A Detailed Characterization of Polyglycolide Prepared by Solid-State Polycondensation Reaction, European Cells and Materials, Vol. 5, p. 1-16,2003. 

M. Epple, A detailed characterization of polyglycolide prepared by solid-state polycondensation reaction, Macromol. Chem. Phys. 200 (1999) 2221-2229. 

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 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... 

Figure 5.3 Effect of nitrogen gas flow rate on the solid-state polycondensation process for PET reaction conditions, 259°C for 7h initial Mn, 16500, with a particle size of 0.18-0.25 mm data obtained by gas chromatographic analysis, employing a column of dimensions 8ft x 0.7.5 in o.d. [5]. Reproduced from Hsu, L.-C., J. Macromol. Sci., Phys., B1, 801 (1967), with permission from Marcel Dekker...

SOLID-STATE POLYCONDENSATION OF POLYESTER RESINS 4.3.7 Reaction Time... 

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. 

As discussed earlier, solid-state polymerization reactions are used to increase the degree of polymerization in the production of nylon-6 and nylon-6,6. The solid-state polymerization process has been studied by process simulation. Mallon and Ray [208] developed a comprehensive model to handle the reactions in polymers undergoing polycondensation reactions in the solid state. The polymer crystalline fraction is modeled as containing only repeat units,... 

Figure 1.5 Changes in M of sb-PLA as a function of the reaction time through the whole process involving the solid-state polycondensation conducted at 170 °C (Refs. 21,26, with permission).

Moreover, the molecular weight remained around 100 000 Da, being much lower than that of the PLLA obtained by the ring-opening polymerization of Z-lactide. Therefore, they examined the melt/solid polycondensation of lactic acid in which the melt polycondensation of Z,-lactic acid was subjected to solid-state polycondensation below Tm of PLLA [8]. In solid state, the polymerization reaction can be favored over the depolymerization or other side reactions. Particularly, in the process of crystallization of the resultant polymer, both monomer and catalyst can be segregated and concentrated in the noncrystalline part to allow the polymer formation to reach 100% [9]. Figure 3.2 shows the whole process of this melt/solid polycondensation of Z-lactic acid. In this process, a polycondensation with a molecular weight of 20 000 Da is first prepared by... 

The term solid state polycondensation (SSP) simply says that chain growth by condensation steps occur under conditions, where at least one reaction partner exists in the solid state. As discussed below, SSP encompasses a broad variety of condensation reactions and substrates. Seemingly, the first example of a SSP (but not recognized as such) was the synthesis of poly(4-hydroxybenzoic acid), poly(4-oxybenzoate) by dry distillation of 4-hydroxybenzoic acid (see Chap. 2, Refs. [64—66]). A more detailed discussion of syntheses of poly(4-hydroxybenzoic acid) will we presented in Sect. 14.3. The first section is dedicated to SSPs of peptide esters which were also described for the first time before World War 1. 

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. 

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