Solid-state polycondensation drying

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. 

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. 

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 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. 

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