During processing and bubble growth processes, the melt viscosity changes with temperature, pressure, and blowing agent concentration. Therefore, measurement and prediction methods for viscosity characterization in terms of temperature, pressure, and blowing agent dependency are needed. This study demonstrates the applicability of in-line viscosity measurements during the foam injection molding process and a model for viscosity superposition and prediction. In the present study, polystyrene and a modified polylactide for foaming applications with nitrogen as blowing agent are investigated. By changing the injection speed, temperature, and blowing agent concentration, the process conditions are varied, and thus the resulting pressure drops within the in-line measurement die. The calculated shear rates and viscosities are shifted to a master curve by the application of superposition principles. The viscosity dependency on temperature is described by the Arrhenius equation, the pressure by the Barus equation, and for the blowing agent concentration, a novel Barus-like equation was derived and applied. The prediction of the master curve viscosity function was achieved by the power-law model in combination with the superposition principles and showed good agreement with the shifted in-line data. Finally, the in-line measurements and viscosity predictions are validated by comparing them to rotational and capillary rheometer measurements.

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Knowledge on diffusion coefficients in mixtures consisting of a polymer melt and a blowing agent is necessary for understanding and characterizing the bubble growth processes during foaming of thermoplastic polymers. Due to the lack of reliable literature data in process-relevant conditions, the diffusion coefficients are often assumed to be constant to solve the equations for bubble growth in polymer melts. This study demonstrates, for the first time, the applicability of dynamic light scattering for the characterization of molecular mass and thermal transport in binary mixtures consisting of a macromolecular polymer and a dissolved gas by the determination of mutual and thermal diffusivities in macroscopic thermodynamic equilibrium. In the present study, a mixture of polystyrene and nitrogen was selected as the model system, which has technical relevance. After evidencing that the obtained experimental signals are associated with hydrodynamic modes, the influence of pressure and temperature on mutual and thermal diffusivities is discussed and the results are compared with the literature. For the mixtures studied in this work, expanded experimental uncertainties (k = 2) of the obtained diffusivities have a minimum value close to 3% and are on average 13% for the thermal diffusivity and 7.4% for the mutual diffusivity.

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Hygroscopic polymers absorb and bind water. If not dried properly, the residual moisture can cause major problems for converting and affect the product quality significantly. Therefore, an effective drying down to an acceptable moisture level is essential for a successful production. Though data sheets give recommendations for drying parameters, these do not consider the actual current moisture up-take of the plastic pellets or the current ambient conditions. This paper investigates the interdependencies of polymer characteristics, like thermal properties and molecular structure, and the drying kinetics of the respective polymers. Tests are carried out with five different polymers on a State-of-the-Art desiccant dryer. The results show that distinctive drying behaviors can be attributed to the molecular structure of the respective plastic. This is reflected by the activation energy according to Arrhenius, the diffusion coefficient and the Flory-Huggins-Parameter, all showing a positive correlation with drying speed and therefore can be used as indicators to estimate drying times. Also, an industrial scale prototype of a microwave enhanced drying system was used to investigate the effect of microwave application on the drying kinetics. Experimental results show potential for reducing the drying times needed, especially for lower temperatures of the drying air and highly hydrophilic plastics. For higher temperatures, however, the prototype could not compete with the state-of-the-art desiccant dryer, due to heat losses and inefficient tubing of the prototype. Considering this, the benefits of microwave application could be shown representatively for polyamide 6 also at higher temperatures.