Process Parameters Influencing SSP

The complex nature of SSP depends on many factors, all of which can affect the course of the reaction and the quality of the final product. An optimum combination of such variables thus determines the reaction process and needs to be discussed in detail. Therefore, some of the generally known aspects are reported in the following sections. 

Not only quality reasons, but also economic considerations, make it necessary to reduce the particle size. Generally, the development of an economic SSP process is characterized by the trend to a reduced particle size. The commonly used cubic chips size (2-4 mm) is reduced to that of the known small nylon chips ( 1.5 mm diameter, spherical shape). The size of the chips can be defined by the length of the cubes and is easily measured. Due to their irregular shape, however, determination of their average volume is difficult. Therefore, the definition weight (g) of 100 pieces has become popular. 

The grinding of crystallized chips has been well-known for about 50 years but yields problems with respect to handling and processing. Very small particle sizes can be achieved by grinding under cryogenic conditions. Lower-IV prepolymers ( 0.40 dl/g) are preferred for this process. The handling of smaller particles becomes more difficult due to the increased content of fines and their separation, particularly in continuous gas flow technology. Fines are defined as extremely 

The activity of Ti catalysts in SSP depends on the kind of stabilizer fed into the reactor. In the production of PET, phosphorous-containing chemicals are commonly added as stabilizers. These products improve the thermal stability, particularly in processing, which results in reduced degradation and discoloration and are therefore of importance with respect to quality. Such materials are added during the production of the prepolymer. These stabilizers are mainly based on phosphoric or phosphonic (phosphorous) acids or their esters. 

Small amounts of these acids or other phosphorus compounds containing acidic OH groups inhibit the SSP process. This inhibition of Ti-catalyzed polycondensation is related to the formation of stable adducts between the acidic phosphorus compound and the catalyst [53], Such a problem can be overcome by the use of certain compounds, for example, nonyl phosphite, in exact, equivalent amounts [54], 

Titanium- and tin-containing systems show high efficiency but tend to discolor the polymer. PET based on Ti catalysts also exhibits a lower ciystallization rate in comparison with antimony. This phenomenon may be caused by the lower catalyst concenti ation and dierefore die reduced nucleation. 

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