TPEs for overmolding wide range of resins

Have you noticed the proliferation of soft-touch grips and parts for personal care, power tool, appliance, and automotive applications? These products are hard to miss, which is exactly why their numbers are growing. The trend hasn?t escaped TPE material suppliers either, who have been diligently working to match the chemical compatibility of their products with a greater number of rigid substrate resins for better adhesion.

For example, at Plastics USA, Teknor Apex introduced a new family of TPEs for overmolding called Tekbond. Based on SEBS and available in natural, Tekbond 6000 and 6500 offer strong, permanent chemical bonds to dissimilar neat and glass-filled plastics across a broad spectrum. The 6000 grade covers PC, ABS, PC/ABS, PMMA (acrylic), cellulose propionate, and PCT. Series 6500 adheres to nylon 6/6, PC, ABS, PC/ABS, and PMMA (acrylic). Hardnesses for 6000 products range from 25 to 60 Shore A while the 6500 series ranges from 50 to 70.

One of the challenges with overmolding is getting good chemical adhesion between the TPE and the thermoplastic. TPEs are olefins and bond easily with other olefinic compounds such as PP. However, with dissimilar materials?polar substrates such as PC and nylon?TPEs are incompatible. Tekbond materials overcome that hurdle by creating uniform chemical bonds through compounding.

In addition to offering adhesion for multiple materials, the 6000 series adds a lower hardness level than previously available, according to Phil Morin, marketing manager. ?Human skin has a Shore A hardness of 55 to 60, so anything less than that is perceived as soft,? he says. Bond strength is also a plus with test results showing most bonds are above the minimum 6-lb/linear inch peel test value (see table). In testing the peel strength of Tekbond and ABS using a 180° peel test, the company reports values of 18 lb/linear inch after heat aging and 12.5 lb/linear inch after water immersion.

Processors who formerly had to inventory multiple TPEs for overmolding onto a variety of different thermoplastic substrates will benefit, Morin says. ?They can now stock natural-grade Tekbond compounds in different hardnesses and use color concentrates to overmold a range of parts.? Also, a low specific gravity for the compounds (.96) means more parts can be molded from a given lb of material. Both TPEs require no predrying and can be molded on standard presses with general purpose screws, Morin adds.

Current pricing for Tekbond products is $3.50/lb in truckload quantities, but the price varies according to grade. In the future, Teknor hopes to come out with a TPE product for adhesion to HIPS, PBT, TPOs, and other materials. For now, however, these two grade series span a fairly broad range of rigid thermoplastics.?Michelle Maniscalco

Teknor Apex
Pawtucket, RI
Phone: (401) 725-8000
Fax: (401) 725-8095
http://www.teknorapex.com