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In the literature it is well known that foaming is a highly complex process which is influenced by e.g. the process parameters, the choice of polymer and its structure as well as by the blowing agent type and concentration. Due to the dependency of both, shear and elongational viscosity on such influencing factors, the melt viscosity is highly influenced during the foaming process. Thus, the viscosity is a crucial parameter for the foaming process. A collapse of single bubbles or even of the whole foam structure is often a result of a too low elongational viscosity and the associated melt strength. Therefore, polylactide (PLA) is difficult to foam. In this study the melt viscosity in shear and extensional flow, and the melt strength was increased by modifying PLA with dicumyl peroxide on a twin-screw extruder. Thus, the polymer chain structure was changed from linear to branched and strain hardening was observed during uniaxial viscosity measurements with a Sentmanat extension rheometer at different temperatures and Hencky strain rates. The uniaxial elongational viscosity of the modified PLA including the observed strain hardening was predicted at different temperatures and different Hencky strain rates with the molecular stress function model (MSF). Because of the good accuracy of the MSF model predictions in comparison to the uniaxial viscosity measurements, the equibiaxial viscosity was modeled as well since equibiaxial deformations mainly occur during the foaming process. Therefore, the elongational viscosity of modified polymers for foaming applications can be characterized and predicted. [ABSTRACT FROM AUTHOR]

Generally, Thermoforming is used for the production of simple shaped packaging or housing parts, which do not feature any additional functions. However, by the use of heat conductive plastic materials, a heat loss function can be implemented to the thermoformed parts. Increased filler contents are correlated with higher heat conductivity. Unfortunately, higher filler content also leads to a degraded thermoformability of plastic sheets. As a result, simulation becomes more and more important, when operating the thermoforming process at its limits. To obtain high predictive accuracy in thermoforming simulation, material models as well as data fitting methods are needed, which match with the real thermoforming material and process. In this investigation the use of the K-BKZ model for thermoforming simulation of heat conductive plastics is evaluated. Two different kinds of parameter fitting algorithms are examined. On the one hand, the K-BKZ model is fitted by a reverse-engineering procedure using the IKT Thermoforming-Material-Characterization-test (TMC-test) as reference test. On the other hand, classic rheological measurements by means of rotational rheometry and Rheotens-test are used for the data fitting of material parameters. As reference material a polystyrene is combined with two graphite fillers (spherical and platelet). The predictive accuracy of the material model and the two parameter fitting procedures are quantified by a comparison of the simulation with real thermoforming experiments. [ABSTRACT FROM AUTHOR]

Polyamide 6 can be produced by anionic ring-opening polymerization of ε-caprolactam using e. g. sodium-caprolactamate as a catalyst. One of the best known anionically polymerized polyamide 6 is so called cast polyamide 6, used for particularly stressed parts. The reason for its good mechanical properties is the comparably high molecular weight. When using cast polyamide 6 for extrusion or injection molding, its high molecular weight can be a disadvantage due to its very high viscosity. To enhance processability, the flow behavior of these materials has to be adjusted. Besides the use of a lubricant, shorter chains would be another way to achieve better processability. In this work, the latter is performed by compounding anionically polymerized cast polyamide 6 using a chain scissorer, e. g. terephthalic acid. Compounding of dry-blends made of cast polyamide 6 and said terephthalic acid is done on a twin-screw extruder type ZSK 26MC of Coperion, Stuttgart. Mechanical properties and flow behavior were investigated afterwards. Surprisingly, it could be shown, that adding very small amounts of terephthalic acid did not reduce but raise viscosity significantly. However, higher amounts of terephthalic acid reduce these viscosities, as expected. Additionally, a reduction of viscosity can be seen over time, because remaining ingredients seem to still be active. Mechanical properties such as Young’s modulus or tensile strength are not affected by adding terephthalic acid and are comparable to those of injection molding polyamide 6. Solely impact strength was slightly reduced, when a larger amount of terephthalic acid was added. [ABSTRACT FROM AUTHOR]

Cast polyamide 6 is a high-quality material due to its long polymer chains. Because of sprues and post-processing, a comparably large amount of the produced material, up to 30 %, becomes waste. Until today, this high-quality waste is burned or added into standard virgin polyamide 6 raising its molecular weight. Instead of using cast polyamide waste as an additive or combustion material, it would be beneficial to process high-molecular weight recyclates made of this cast polyamide 6. To enhance its processability, using a lubricant as a processing additive is a reasonable approach. Our previous findings within this field revealed an unusual behavior of a lubricant consisting of a polyester modified wax, which is already used for standard polyamide 6 without known problems. In this work, milled cast polyamide 6 chips obtained from machining are washed and dried and then used for producing high-molecular weight recyclates. In literature the deposition of such waste is recommended because of pollution and contaminations. It is hence possible to get clean material for recycling. The recycling step itself is performed on a twin-screw extruder type ZSK 26MC of Coperion, Stuttgart, by processing a dry-blend of cast polyamide and said lubricant. Rheological measurements show increasing complex viscosities of recyclates depending on the lubricant’s content. It is possible to achieve viscosities higher than those of the solely processed polyamide recyclate. Even the viscosity of unprocessed cast polyamide 6 can be reached. Compared to virgin extrusion polyamide 6, the processed recyclates achieve higher tensile strengths, while an increasing amount of lubricant does neither affect Young’s modulus nor tensile strength. It is also shown that the lubricant raises impact strength significantly. [ABSTRACT FROM AUTHOR]

Polylactide (PLA) is the most important bioplastic on the market due to its good mechanical properties and a permanent growth of the production capacity. But in terms of extrusion foaming the melt strength and extensibility of commercial polylactide is too low. Therefore such commercial grades need to be modified to induce crosslinking, chain extension or branching by means of reactive extrusion on a twin-screw extruder. In this work different modifiers (chemical as well as physical modifiers) were used. The most effective results were achieved with organic peroxide. With this modifier the molecular weight, the melt viscosity, the melt strength and the crystallization rate were improved leading to foams with a closed-cell structure and improved compression behavior. Furthermore, the modifier was found to be more effective than a commercial modifier. [ABSTRACT FROM AUTHOR]