Testing oils for TPE
Successfully marrying the properties of plastics and rubbers, thermoplastic elastomers (TPE) are used in everything from car dashboards to toothbrushes and the soles of our shoes. In a recent study, Nynas researchers were able to confirm the high performance of the company’s highly refined oils in TPE materials.
Dr Kamyar Alavi
Senior Technical Advisor
"Thermoplastic elastomers have the desirable properties of elastomeric materials, such as the tactile feel, damping, and low temperature properties. But unlike common rubber material – which once vulcanised into a certain shape cannot be reshaped – TPE materials keep the melt processing characteristics of plastics, enabling them to be remoulded. This offers higher flexibility for both processing and recycling,” explains Dr Kamyar Alavi, Senior Specialist, Nynas.
Found in numerous applications and everyday goods, TPE materials are formulated compounds containing a TPE polymer and other components such as a polyolefin, filler, antioxidative additives and a process oil. The oil plays a vital role, as it is not uncommon for the oil loading to reach as high a level as that of the main polymer, the TPE.
“Many TPE materials are used in high-end and light-colour applications, so the ageing and colour stability of the products are of special importance. This, in combination with the high amount of oil, means that the colour stability of the oil is a key parameter for the overall performance,” says Kamyar Alavi.
To evaluate the performance of highly refined oils in TPE materials, Kamyar Alavi and his colleagues Tommie Ibert and Patrik Salomonsson recently conducted a comparative study. The performance of Nynas process oils NYFLEX® 223 and NYPAR® 330, as well as performance- optimised (additivated) versions of these oils, DP 223 and DP 330, were assessed alongside commonly used paraffinic TPE process oils.
“Since many of the final products are exposed to light, one of the most important aims of the project was to study the impact of the oil on the UV-ageing of the compound. To this end, we tracked the changes of key compound properties as a function of UV exposure time,” explains Alavi.
The results, shown in Figure 1, indicate that most ageing occurs in the early periods of exposure reaching almost maximum level after 168 h. Compounds with NYFLEX 223 perform at least at the same level as the paraffinic Gr I oils, SN500 and SN600. The enhanced naphthenic oil DP 223 shows significant improvement in colour getting closer to that seen in NYPAR 330 compounds, while DP 330 performs at an almost identical level as the paraffinic medical white oil (P-MWO).
Structurally, TPEs comprise thermoplastic hard blocks, interconnected by elastomeric mid-blocks. One of the most important TPE materials is the so called Styrenic Block Copolymers (SBC or TPS). In these, the thermoplastic block is made of polystyrene interconnected by the elastomeric mid-blocks such as butadiene or isoprene rubber. The role of the oil is to offer optimum plastification of the rubbery phase without inadequate interference with the hard (styrenic) phase.