New Family of 'Soft-Touch' TPEs for Overmolding that Adhere

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Innovative thermoplastic elastomers (TPEs), supplied in a wide range of hardnesses, can be overmolded directly onto most rigid thermoplastics used for rapidly proliferating "soft-touch" grips and multi-component parts in appliance, automotive, and other major markets, Teknor Apex announced today at a news conference during Plastics USA/98.

The new TPEs, called Tekbond (trademark of Teknor Apex), are derived from styrene-butylene-ethylene-styrene (SEBS) and are off-white in natural grades, allowing pigmenting to virtually any color. The new TPEs will bring greater flexibility to new-product development, cut costs by reducing manufacturers’ material inventory requirements, and streamline mold design and processing operations, according to company executives.

Tekbond 6000 Series grades, with Shore A Hardnesses from 25 to 60, adhere without modification to many neat and glass-filled polycarbonate (PC); ABS; PC/ABS alloys; acrylic; cellulose propionate; and polycyclohexanedimethanol terephthalate (PCT). Series 6500 TPEs, available with hardnesses from 50 to 70, can be overmolded onto neat and glass-filled nylon 6,6, as well as PC, ABS, PC/ABS, and acrylic.

"Tekbond TPEs represent the culmination of extensive R&D addressing an industry need for more versatile overmolding materials for fast-growing ‘soft-touch’ grip applications," said Bertram Lederer, Group Vice President of Teknor Apex. "We have yet to fully explore the potential of these unique TPEs, but are excited by their initial acceptance among designers and processors."

The adhesion achieved by Tekbond products to dissimilar plastics involves strong and
permanent chemical bonds, according to a presentation by Phil Morin, Marketing Manager, Thermoplastic Elastomers. "Most polymers are completely incompatible, but Tekbond TPEs will adhere to many dissimilar plastics, including very difficult polar substrates such as polycarbonate and nylon."

The new TPEs exhibit the mechanical properties required for overmolded grips and other parts, including high tear strength and elongation values over 400%. Prior to their market introduction, Tekbond compounds were overmolded onto various rigid thermoplastics and subjected to accelerated testing to quantify adhesion integrity. "The results show that the bonds achieved with Tekbond are very strong and permanent, nearly always above the 6 pounds per linear inch (p.l.i.) peel test values considered the minimum requirement for most overmolding applications," said Morin.

The technical challenges involved in developing Tekbond TPE products went beyond facilitating adhesion to multiple materials, he said. "A key hallmark of these new compounds is that they can adhere to dissimilar plastics at Shore A Hardness levels as low as 25. Custom-tailored TPEs for overmolding, on the other hand, typically adhere to only one or two rigid plastics and are generally restricted to the 50-70 Shore A Hardness range."

Morin stated that lower hardness levels are important for two reasons. Softer materials are required for applications like automotive components with molded-in gaskets or seals that absorb vibration or dampen noise, he pointed out. "The other factor is that human skin generally falls in the 50 to 55 Shore A Hardness range. With Tekbond TPEs, design engineers now have the capability to incorporate much softer grips into consumer products for a more pleasing or luxurious feel."

"We predict that the single greatest benefit afforded by Tekbond TPEs will be cost reduction brought about by eliminating the need to inventory multiple overmolding compounds for different thermoplastic substrates," said Morin. "Processors can now stock natural-grade Tekbond compounds in different hardnesses and use color concentrates to overmold a range of
parts." The low specific gravity of Tekbond products (0.91 to 1.02), he added, results in a lower cost/in3, "permitting more parts to be molded from a given pound of resin than with many other thermoplastics."

In product design, Morin said, the availability of a single TPE product for overmolding will allow companies to change rigid substrates without incurring the "penalties of a replacement TPE" such as tooling modifications and development delays. "Tekbond products also open up the opportunity to overmold the same TPE onto different thermoplastic substrates," said Morin, "which permits common tooling designs and the use of a single injection machine to overmold multiple parts." An equally important advantage, he added, is better color matching because the same base material can be used.

"The new TPEs are non-hygroscopic and therefore require no pre-drying before processing," Morin said. "They can be run on standard injection molding equipment without modification."