Rheological Study of Injection Molded LDPE Foam
Xue Chen, Marie-Claude Heuzey, Pierre J. Carreau
Ecole Polytechnique
Canada
Keywords: Cellular foamed polymer, rheological properties, injection molded
Low-density polyethylene (LDPE) was selected to make the polymeric foam with an injection-molding machine by dry mixing the LDPE with a chemical blowing agent. First, the decomposition behavior of the chemical blowing agent was studied by thermogravimetric analyzer (TGA). Then the LDPE containing the chemical blowing agent was processed in injection molding at high injection speed to make cellular foams. Most bubbles were found to have a closed structure in the final foam product. Typical cell diameters were of 41mm and 30 mm in volume and number average, respectively and a cell density of 6.88´106/mL (pure polymer) was obtained.
The linear viscoelastic properties of LDPE and injection molded LDPE foams have been investigated using a controlled stress rheometer with a concentric disk geometry. The measurements were carried out under nitrogen in the linear regime at a set temperature of 120oC. This selection of temperature was considered from thermal stability of the foams. The effect of different volume fraction of bubble in foams was also studied. For all bubble volume fractions, both the loss modulus and storage modulus of the foams were found to be significantly smaller than the moduli of the LDPE and decrease with the volume fraction. The Palierne model (1) was used to fit and predict the linear behavior of the polymeric foams. An excellent agreement between the experimental data of small amplitude oscillatory tests and the theoretical predictions is obtained for different bubble volume fractions.. The bubble volume fractions used in the calculations were obtained from the density measurements of the foams and LDPE. The effects of the cell morphology and of the interfacial tension between the bubbles were assumed to be negligible. In other words, the linear viscoelastic properties of LDPE foams depend only on the linear viscoelastic properties of LDPE and the volume fraction of bubbles.
1. Palierne, J.F., Rheol. Acta, 29, 204 (1990)