In this study, different strategies to improve the mechanical properties of physically foamed high-density polylactic acid sheets were examined to produce polylactic acid foam sheets with tailor-made mechanical properties. The first part was the determination of the effect of different blowing agents (CO 2 and N 2 ) on the foam morphology. The second part of the study was the modification of the formulation. For this purpose, both a linear and a branching chain extender and a thermoplastic elastomer were used to improve the elongational properties (tensile modulus and strain at break) of the polylactic acid foam sheets. Additionally, the effect of the addition of cellulose fibers on the foam morphology and the mechanical properties was investigated. All experiments were carried out on a laboratory flat-film line. This extrusion line consists of a 30-mm single-screw extruder attached with a 250-mm flat sheet die. The results show a strong influence of the material formulation on the mechanical properties of the high-density foam sheets. Both the mechanical properties and foam morphology could be improved by the right material formulation. The addition of the thermoplastic elastomer leads to a better foam morphology and also to a reduced brittleness of the foam sheets. Furthermore, it could be demonstrated that cellulose fiber can be used as a nucleating agent for polylactic acid but causes a further decrease in the strain at break.
The bulk temperature is a very important parameter for the processing of rubber compounds. Currently, standard injection molding simulation is taking into account just shear heating, while elongational heating is neglected by most available software tools. A new viscous model for the prediction of temperature changes in rubber compounds flowing through conical dies and runner segments was deduced where elongational heating is considered as well. Shear viscosity was directly measured on a Capillary Rheometer and elongational viscosity was calculated from the inlet-pressure drops using a new approach. To verify the theoretical calculations experiments on a rubber injection molding machine were carried out by means of a specially designed mold. Varying injection speed and the die angle bulk temperature measurements in the purged material were performed. The measured bulk temperature rise showed a dependence on the flow rate and the cone angle with the flow rate being dominant. The results of the temperature measurements showed good correlation with the model results with an average error being <5%. POLYM.ENG. SCI., 55:701-709, 2015.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.