Measurement of Thermoplastic Properties of Packaging Materials

A combination of elevated temperature, tension and dimensional changes are common in converting and packaging processes. Paperboard and polymer films, either alone or as a combined material, can undergo either desired or unwanted deformations. Here, two examples of testing the deformation behaviour are presented and two novel test methods for packaging materials are introduced. The development of new paper-based packaging requires improvement in the thermoformability and extensibility of paper. However, the extensibility of paper can be highly dependent on the forming conditions, which may be either an advantage or a hindrance for the forming performance. VTT's novel, laboratory scale, 2-dimensional thermoforming tester was improved in order to better simulate the process conditions of thermoforming. The results clearly show the softening effect of the forming temperature on strength and stiffness, which are temporarily reduced, and the improving effect on the extensibility of polymer covered paperboard. The influence of mechanical and novel chemical treatments of the bleached softwood kraft pulp on the improved extensibility of paperboard are presented. The quality of printing on polymer film can be adjusted by web tension, drying process conditions and by taking account of the thermoplastic properties of the material. A special electric heating device was mounted on to a high strain-rate tensile tester in order simulate the heat-induced changes in the tensile behaviour of the polymer film. The heating device formed a small non-contacting climate chamber, which was able to heat the test samples to a target temperature of 30-110 °C within a couple of seconds. Thermoplastic elongation of polymer film was measured by utilizing constant tension under cyclic temperature variation in order to simulate web tension and elongation during printing and drying. The studied polymer film showed extensive increases in strain due to heating and contraction during the subsequent cooling periods and permanent deformation after the heating and cooling cycles.