by Pei-Zhen Jian, Prakash Sanjeevaiah and Krishna Venkataswamy, Star Thermoplastic Alloys and Rubber
Bonding thermoplastic elastomers to polar and non-polar substrates has been widely studied in the past few decades to create products that enhance ergonomic comfort and functionality. Overmolding a TPE compared to two-shot molding is a litmus test for bonding the material to the substrate. The TPE must wet the cold surface and must melt a few nanometers of the surface of the substrate that is being overmolded on to facilitate bonding.
The surface energy of the TPE and the substrate must match closely to obtain good bonding. Surface energy of 34 mN/m and above is considered polar. TPEs with lower surface energy than 34 mN/m are considered non-polar. Olefin copolymers and thermoplastic vulcanizates (TPVs) are very non-polar, with a surface energy around 28 or below. Styrene block copolymers have a dual behavior of being both polar and non-polar due to their chemistry. The styrene end blocks are polar, and the mid blocks of ethylene and butylene are non-polar.