The world of plastics has seen a growing demand of Thermo-Plastic Elastomers, under the general designation of TPE or synthetic rubbers. These materials are designed to put together the unique properties of rubber-like materials and the standard processing technology of thermoplastic commodities. Hence the great interest of the industry: processes for high productivity like injection molding can be successfully applied. Complex shapes, multi-material components and coated parts can all be obtained at fast production rates, giving life to colorful, soft, water- and chemical-resistant products.
We have tested on a CEAST Smart Rheo capillary rheometer several synthetic rubbers rated for injection molding, to verify the applicability of ISO 11443 and ASTM D3835 methods to these materials and provide indications for the processing parameters. We collected different samples covering a wide range of applications and final properties: SEBS (Styrene Ethylene Butylene Styrene), TPU (Thermoplastic Poly-Urethane), EPDM (Ethylene Propylene Diene Monomer), NBR (Nitrile Butadiene Rubber). The CEAST SR50 capillary rheometer used for the tests was equipped with a standard 15-mm diameter barrel, pressure transducers (full range from 10 to 200 MPa), 1-mm diameter capillary dies. Synthetic rubbers can be filled with mineral powders and reach considerably high viscosity, therefore the typical rheometer for these tests has a high force capability (here 50 kN or 11,250 lbf) and high-range pressure transducers (here up to 200 MPa or 29,000 psi, lower ranges were used for low viscosity grades). Conical inlet capillary dies may be a better choice for highly-filled materials.
The preferred shape of samples for capillary rheometer tests is pellets (granules); two samples came in the shape of blocks and we cut them down to small pieces to fill evenly in the test barrel. During this cutting operation, it's important to generate a minimum amount of heating and mechanical stresses, in order not to influence the properties by triggering some form of degradation. For the selection of test temperature we took as a reference the typical processing temperatures as advised by the same companies supplying the samples. SEBS was tested at 190 °C, TPU at 205 °C, EPDM at 60 to 120 °C, and NBR at 60 to 120 °C. For the last two samples we decided to investigate the whole processing window to study the viscosity dependence on temperature, an additional analysis available to determine the best processing conditions.
All the samples could be tested with repeatable results over at least two decades of shear rate, starting at 50 1/s and going up with a series of steps as per the standard testing procedure. The low-viscosity grade of EPDM in our hands proved to be particularly stable at high shear, reaching as high as 35,000 1/s at 90 °C without showing significant signs of flow instability. The viscosity was found to be decreasing by approximately 10% when increasing the temperature by 10 °C.