Conversion melt rheology

The processing of polymeric materials such as plastics is characterized by a wide variety of distinct methods or techniques. Each technique has a different set of melt rheology requirements that are dictated by the processing mechanism and the equipment design. A qualitative assessment of the effect of major melt rheology properties on selected conversion processes is shown in Tab. 14-2. 

Tab. 14-2 Qualitative assessment of major melt rheological properties versus selected conversion processes.

Using a Rheometrics mechanical spectrometer and powdered polymer samples, the authors compared the rheological behaviour of two polymers with similar chemical compositions but different structures. The rheological profiles of polymers 21 and 22 were determined between 140 and 400°C by increasing the temperature at 10°C min from 140 to 190°C and from 300 to 400°C. In the predominant region of isoimide-imide conversion (190-300°C), the temperatme was raised by 2 or 5°C increments, the dynamic viscosity rj being measured at each temperature step. At 190°C, the viscosity of poly(isoimide) 21 was approximately 5 X 10 Pas and decreased to a minimum value of 10 Pas at 243°C as the polymer softened and melted. Thermal conversion to polyimide 22 concurrently... 

An important means of assessing the rheological behavior of polymer melts and concentrated solutions is the use of dynamic mechanical analysis. This consists in testing the mechanical response, or strain, of a polymeric material to a time-dependent stress, or, conversely, measuring the stresses generated by time-dependent strains. For simplicity, a... 

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