Adhesion of Advanced TPEs for 'Soft-Touch' Over-Molding onto Rigid Substrates Exceeds Automotive Standards for Durability

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DETROIT, June 8, 1999…In tests measuring the bond strength of soft thermoplastic elastomers (TPEs) when over-molded onto rigid thermoplastics used in automotive components, Tekbond (trademark) TPEs from Teknor Apex exhibited adhesion far in excess of levels considered adequate in the industry, the company announced today at the Automotive & Transportation Interiors Expo.

"Using TPEs that form strong bonds to substrates, automotive manufacturers can eliminate costly assembly while incorporating valuable 'soft-touch' grips, cushions, or seals onto otherwise rigid components like knobs, handles, headlights, windows, housings, and ducts," said Charles E. Gates, Industry Manager. "What distinguishes Tekbond compounds from earlier TPEs for over-molding is their far greater versatility, making it possible to over-mold some plastics for the first time, reducing the number of TPE grades that must be inventoried, and often eliminating the need to modify mold designs or make process adjustments to accommodate different TPE compounds."

New data presented in a conference concurrent with the Automotive and Transportation Interiors Expo demonstrate that Tekbond TPEs maintain strong bonds even under hot, moist conditions and after immersion in automotive liquids like windshield washer fluid, according to Dexi Weng, Senior Polymer Development Chemist and one of the authors of the conference paper. The compounds also retained key physical properties after prolonged oven-aging.

Tekbond TPEs can be injection molded or coextruded onto a far greater range of rigid substrates than other 'over-molding' TPEs and are available in a broader selection of
hardnesses, according to Gates, who cited excellent adhesion to polycarbonate (PC) and PC blends, styrenics (ABS, SAN, and ASA) and their blends, polyesters, nylons (6 and 66), acrylics, cellulosics, vinyls, and other polymers. Shore A hardnesses range from 25 to 70.

Based on Teknor Apex's proprietary formulation and compounding technology, Tekbond TPEs are modified to broaden the range of substrates to which they can bond and to increase their bonding strength, according to Dexi Weng. His paper (co-authored with Charles Gates and titled An Advanced Family of TPEs for Over-Molding Soft Segments onto Rigid Automotive Components) discusses principles of bond formation and presents results of tests measuring the adhesion between Tekbond TPEs and diverse thermoplastic substrates.

Using a 180-degree peel test (a modified version of the ASTM D903 protocol for evaluating adhesives), researchers mounted partially over-molded test plaques in a tensometer so that the unbonded portion of the TPE layer was clamped in one jaw and the exposed portion of the substrate was clamped in the other; then they applied stretching force until the bond ruptured. The force at this point was recorded as the adhesion strength (in kilograms) per unit width (in decimeters) of the TPE strip.

The researchers tested as-molded samples of four Tekbond 6000 Series grades over-molded onto various substrates, as well as samples that had been exposed to air and water, respectively, at elevated temperatures ranging from 75 to 105 deg. C. "While a bond strength of 15 kg/dm is generally considered adequate for most applications, all of the as-molded samples exceeded this level, many exhibited values three times as great, and samples from one grade demonstrated bond strength five to six times as great," Weng said. "Upon exposure to hot and moist conditions, nearly all samples exceeded the 'adequate' level by significant margins."

The researchers also tested the adhesion strength of a typical pre-colored grade (Tekbond 6000-60 Tan 1851) and two developmental grades especially formulated for use with nylon substrates. In both cases bond-strength values far exceeded the "adequate" level, Weng said.

In explaining this excellent bonding performance, Weng reviewed the types of interaction that occur at the interface of TPE and substrate surfaces, pointing out that the strongest bonds occur when the interactions take the form of hydrogen bonding and covalent bonding. "Covalent bonding is a true chemical interaction, in which the molecules of the TPE and the substrate form an interphase region in which polymer-to-polymer linkages occur," Weng said. "We believe that hydrogen and covalent bonds are more prevalent with Tekbond compounds than with previous TPEs for overmolding."

As further evidence of this greater bond strength, Weng pointed out the difference between two types of bond rupture exhibited by sample plaques in the adhesion test. "In adhesive failure, the rupture occurs at the interface because no strong interphase has been formed. Cohesive failure, on the other hand, involves a rupture within the TPE layer itself, rather than at the interphase. Tekbond TPEs appear to exhibit cohesive failure more frequently than other TPEs for over-molding."