Soft Sells (June 01, 1999)

WHY TPE’S ?

Thermoplastic elastomers, or TPE’s, are turning up everywhere. From toothbrush handles to automotive gaskets, applications in TPE’s are exploding. Why?

There are many highly technical reasons, but as Daryl Davis, commercial manager for the Estane Thermoplastic Polyurethane Division of BFGoodrich Performance Materials declares simply: “TPE’s offer design flexibility versus rubber. They offer the performance of rubber type products with recyclability, and a lot more design options such as soft touch applications, and harder applications which retain good flexibility at both low and high temperatures. They bridge the gap between rubber and “hard” plastics.”

MOLDING TPE’S

Monica Gonzalez, marketing technical service manager, North America, for Advanced Elastomer Systems (In Canada, General Polymers, Toronto, 93) recommends that molders think of conventional thermoplastics as a candle: “It’s like wax. For most plastics, you only need to apply heat to make them flow. With TPE’s you need to apply a combination of heat and mechanical work.” Gonzalez discounts the need for special screw designs for good performance. “In practical terms, you have to mold the material very fast, and utilize high pressures as opposed to high temperatures. There are different ways to apply shear. One is to ram the material into the tool, and hand-in-hand with that, you need to have a machine which can provide the pressure. The third element is tool design. You need small gates and small runners, the opposite of typical plastics (practice).” Gonzalez emphasizes that the small gate and runner sizes are necessary to keep the material flowing freely.

A COMPOUND FOR EVERY NEED

TPE products are emerging with specific attributes designed to address product or process-specific issues such as productivity and part finish.

Estane 58284 TPU from BFGoodrich (Cleveland, OH 84) for example, is capable of producing surface finishes varying from matte to satin, depending on the processing temperatures. The product can also be run using both extrusion and injection processes, allowing dual-usage applications such as cable jacketing with overmolded connection assembly components.

Productivity is an issue which TPE molders know well. Roger Huarng, business manager for BASF’s Thermoplastic Polyurethane Elastomers Group (In Canada, BASF Canada, Inc., Rexdale, Ont.) describes a situation common to injection molders of elastomers, especially TPU’s:

“Typical TPU products, especially soft grades (75 Shore A to 95 Shore A), take much longer cycle times when compared to other plastics. They’re soft, and tend to stick in the mold, and tend to cool much slower, so the cycle time is fairly long. For example, a four by five inch, 80 to 100 thousandth (of an inch ) thick test plaque, with proper gate design and no sink marks, running an 85 Shore A resin can run easily 35 to 40 seconds.”

BASF’s Elastollan M-Series TPU’s (86) are formulated to address the productivity issue, by freezing faster than conventional TPU’s. An added side benefit of faster solidification is the virtual elimination of sink marks, reducing or eliminating the need for the traditional oversized mold gating. The M-series TPU’s can reduce cycle times by as much as 50 percent when compared to conventional TPE’s, especially in softer grades.

CONSUMER GOODS, AUTOMOTIVE APP’S DRIVE TPE SALES

One of the major drivers for TPE demand has been the popularity of sporting goods such as athletic shoes, and consumer goods with “soft touch” inserts on grips and handles.

An example is Elf Atochem Canada’s Pebax polyamide-polyether resins (Oakville, Ont., 88), which have found applications in the sporting goods market. Randy Shermet, Elf Atochem sales manager for the Polymers Division describes a typical product: “One of the first applications was in high-performance footwear where a lightweight resin with high elastic memory, high mechanical strength and abrasion resistance was desired.

Overmolding for grips and handles is another growing TPE application in consumer goods. GLS Corporation’s Dynaflex TPE (In Canada, CCC Plastics, Don Mills, Ont., 89) compounds are an example. Imperial Schrade overmolds Dynaflex on polypropylene to create a soft, slip-resistant grip on one of the firm’s lines of popular sporting knives. Dynaflex TPE’s are available in clear, colorable, and ultra-soft grades ranging from three to 90 Shore A durometer.

A unique application has been pioneered by Bayer’s Polymers Division, which has developed soft, half-moon-shaped magnifying lenses that adhere to sunglasses by molecular attraction, turning any pair of glasses into reading glasses. The product is molded from Bayer’s Texin aliphatic polyether-based TPU (Etobicoke, Ont.,90), specially compounded in an optical grade. Optical-grade resins are generally cleaner and more consistent than general industrial formulations.

Many of the same properties which make TPE’s ideal for sporting goods also benefit automotive OEM’s. Teknor Apex, for example, has developed a super-soft TPE compound for coinjection molding of a vibration seal onto a housing for sensitive electronics. Tekron 96-E0807A-03 resin (92) makes possible design innovation and parts consolidation because it can be overmolded or coextruded onto a rigid structure. CPL