Solid-state polycondensation molecular weight

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. 

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

Figure 5.9 Effect of crystallinity on the solid-state polycondensation of PET, shown as the number-average molecular weight as a function of time. Conditions fluidized bed polymerization at 230°C particle size, 35-48 mesh superficial velocity of nitrogen, 43cm/s [6]. From Chang, T. M., Polym. Eng. Sci., 10, 364 (1970), and reproduced with permission of the Society of Plastics Engineers...

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. 

Chul-Yung Cha The molecular weight distribution of poly (ethylene tereph-thalate) from solid state polycondensation. Polymer Preprints, Am. Chem. Soc. Polymer Div. 6, 84 (1965). 

Waste of polyesters and polyamides may have suffered from the hydrolytic depolymerization. Thus, it may be necessary to increase the molecular weight by solid-state polycondensation under high vacuum, or by reactive coupling using such di-functional agents as di-glycidyls. Prior to melt processing, the resins should be dried to less than 0.01 wt% of moisture, re-stabilized and (if compounded with immiscible polymers) compati-bilized. 

For example, solid state polycondensation (SSP), a way to increase the molecular weight directly on pellets of solid PET, is the evidence that Sb(III) residual is still active in solid polymer. [25-28] On the contrary less information are available about the effect of Ti(IV) residual on SSP for PBT. It is a matter of fact that in all the studies about thermal and thermooxidation degradation, the effects of the eatalytic system are usually ignored because the catalyst is always present as residual in every PBT sample. It is simply a background effect. 

The trend of I. V. in PBT 1 and 4, since it cannot be related to the accuracy of the method (0.002 dL/g), is very interesting. If on one hand the slight decrease from 7 to 47 h can be attributed to a slight degradation, the increase after the first 7 h can be related to a small increase of molecular weight. This effect is very intriguing and can be related both to the possibility of a solid state polycondensation on PBT (SSP) and reticulation processes. 

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.). 

The concentration of the remaining monomer decreases to zero at 120 and 140 °C where the polymerization systems turn to solid state, whereas at 160 °C the monomer consumption reaches a plateau with the melt-state retained. The same technique is utilized to increase the molecular weight of PLA in the solid-state polycondensation of lactic acid vide infra). 

Solid-state polycondensation Can attain high molecular weights can be used for formed objects Requires inert gas sweeps... 

Stereoblock PLA was synthesized by solid-state polycon-densation of a 1 1 mixture of PLLA and PDLA [34]. In the first step, PLLA and PDLA having a medium molecular weight were melt polycondensated. The PLLA and PDLA were then melt blended in a 1 1 weight ratio to allow formation of their stereocomplex, and the blend was subjected to solid-state polycondensation. Some process optimization with regard to polymerization conditions was done and molecular weights exceeding 100.000 g/mol were... 

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... 

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