The softer side of plastics: New TPEs/TPVs tougher, more pleasing to the touch

The many different variants of thermoplastic elastomers (TPEs) are increasingly making inroads into the market, offering an alternative to such traditional materials as thermoset rubbers.

The makers of TPEs, which have been continuously pushing the envelope, engineering tougher, more aesthetically pleasing TPEs, have fueled this trend. As a result, this new generation of materials is being used in an increasing range of commercial and consumer applications, broadening the range of target markets for the plastics processor.

SOFTER YET STRONGER

Jack Avery, a 34-year veteran of the North American plastics industry and president of Avery Plastics Consulting in Salt Lake City, Utah, said research focusing on toughening TPEs and making them more aesthetically pleasing came about when the materials began replacing rubber on consumer products, particularly on power tools.

Before TPEs, it was common for a power tool manufacturer to add a rubber handle or grip to improve the tool’s feel and to improve ergonomics. Rubber dampens vibration, lowering the stress on a person’s hand and wrist that occurs with prolonged use of a power tool.

“Today, (TPEs) are used in everything from toothbrushes to hair brushes and handles,” Avery added. “For old fogies like me, that ‘soft-touch’ [added by] TPEs just feels better (than rubber).”

But when TPEs began to be used extensively in power tools, they had to become tougher, better able to withstand the heat produced by these tools and the often physically punishing environment they were used in. These requirements challenged manufacturers of TPEs to improve the toughness and heat-resistance of the resins, while improving the feel, Avery said.

H. Chul Lee, engineering polymers business director for the Mississauga, Ont.-based E.I. du Pont Canada Company, said his company’s Advantech product is an ethylene elastomer compound that has ‘soft-touch’ characteristics, which makes it attractive to plastics processors looking to add aesthetic value to parts or products.

“If you are talking about ‘soft-touch,’ there are two different kinds,” Lee said. “There are those that are used for everyday grip-type [applications] which you see a lot on cooking utensils or on tools. Then there are ‘soft-touch’ areas that are not as frequently touched, but [TPEs] gives a nice aesthetic appearance and when you do end up touching them it is soft.”

Lee said plastics processors that make interior components for automobiles would benefit from Advantech’s combination of toughness and softness.

As well, Advantech can be thermoformed with standard machinery and tweaked to meet the unique physical and process requirements of any plastics processor.

Plastics processors would also be interested in E. I. du Pont Canada’s Hytrel TPE, which combines the flexibility of rubber with the processability of thermoplastics, Lee added.

Hytrel can be processed by conventional thermoplastic processes like injection molding, blow molding, calendering, rotational molding and extrusion, as well as metal-casting. Hytrel can be used to manufacture parts varying from power tool handles to heel cushioning systems for athletic shoes.

Santoprene recently delivered a softer TPE that combines the properties of a vulcanized elastomer but with a softer feel, said Seth Barron, strategic marketing manager for Santoprene Specialty Products Group, with Advanced Elastomer Systems (AES), an affiliate of ExxonMobile Chemical in Akron, Ohio.

Santoprene’s TPE grade X82115-25 was designed for a wide-range of consumer and packaging applications that require a softer grip, high heat tolerance, and resistance to chemicals and other fluids.

SUSPENDING RUBBER

Along with the X82155-25, Santoprene has available a thermoplastic vulcanizate (TPV) that contains vulcanized cross-linked rubber in a continuous matrix of polypropylene.

The best way to describe this TPV is to liken it to a fruit Jell-O salad. The pieces of fruit suspended in the Jell-O are the vulcanized cross-linked rubber; the Jell-O is the polypropylene. Combined, the Santoprene TPV has a great deal of flexibility, a 25 Shore A softness rating and a surprising degree of chemical and heat resistance.

“Our material behaves very much like a cured rubber in that it has excellent sealing quality,” Barron said. “You can compress it and it will have an excellent seal and it will maintain that seal over a long period of time.”

For example, Knoxville, Tenn.-based Radio Systems Corporation used the Santoprene TPV in its SportDog SD-1200 SportHunter remote control transmitter to help dog owners guide behaviour from a distance.

The Santoprene TPV is insert-molded on the front and back of SportDog SD-1200’s yellow polycarbonate case to give the device a ‘soft-touch’ surface and prevent slippage in wet weather. At the same time, the Santoprene TPV provides sealability for the device’s battery door seal and for the integrated gaskets that are molded along the interior of the case.

FLOW ENABLES MULTI-MATERIAL PROCESSING

Avery said many of today’s improved TPEs and TPVs flow better, meaning the materials are more easily used in conventional plastics processing machines, including those for multi-material molding.

“From a materials point of view, (TPEs/TPVs) need to have improved flow because what you find in most of the plastics processing applications is that the (TPEs/TPVs) are not the primary material,” Avery explained. “What they provide is an added benefit, such as an improved seal or an improved sense of touch. The primary material in a product is usually a higher performance material, in some cases glass-reinforced with a greater modulus than the (TPEs/TPVs).”

Because of the improved flow, it is possible to use these TPEs/TPVs in multi-component, two-shot molding machines. In a typical molding of a multi-part product, a plastics processor would initially mold the main component, usually from a harder plastic material, like a glass-reinforced nylon. Then this component would be removed from the mold and inserted into another mold on another molding machine where the TPE/TPV would be overmolded onto the main plastic component. For example, when manufacturing a toothbrush with a TPE grip, the hard plastic of the brush would be molded first, and then the soft grip made from a TPE material would be overmolded onto it.

A two-shot machine would do this process in one step by first molding the harder component of the toothbrush and then overmolding the TPE onto the initial component to produce the finished part.

SOFTENING TPUS

Gerry DiBattista, channel manager for TPU injection with Bayer MaterialScience in Pittsburg, Penn., said his company’s most recent thermoplastic polyurethanes (TPUs) are made to have the improved flow needed for use in two-shot molding processes and to reduce cycle times.

The Desmopan 600 Series of TPU resins (DP-6064A, DP-6386A and DP6045DU) have ratings ranging from 65 Shore A to 45 Shore D, and solidify more quickly and flow more freely in injection molding systems.

“Part of the issue with TPUs is that you can make them softer, but there were always several [complications],” DiBattista said. “The softer TPUs became, the less friendly they became to processors. They tended to stick more, take a longer time to set up and they became temperamental, often breaking down with heat.”

Another problem was that TPUs tended to ‘cold harden,’ meaning the TPU starts out soft but hardens over time.

Bayer MaterialScience’s goal with this new TPU product was to avoid these pitfalls by improving the flow of the TPU, and by getting it to solidify more quickly, up to 50 per cent or more faster versus traditional polyether TPUs, according to the company.

As well, the Desmopan Series has high microbe- and hydrolysis-resistance and is recyclable, which Bayer MaterialScience said is ideal in the processing of injection molded parts.

“Another nice fe
ature with these (resins) is that they cycle very quickly,” DiBattista added. “Two-shot processing (takes a lot of time), because the second shot is done with softer material so it tends to take a lot longer to set up (the process) and that affects overall cycle time. This has often been a limiting factor in two-shot processes.”

DiBattista said tests run by Bayer MaterialScience has shown that its Desmopan Series only takes several minutes to harden, with the best results being about one minute in total cycle time and as short as 20 seconds.

TPES’ LASTING BONDS

Avery said a lot of current research with TPEs involves tweaking TPEs to form more lasting bonds with specific kinds of materials. For example, overmolding TPEs onto nylon is fairly difficult. But recently, TPE-makers have designed grades that have better bonding capabilities with nylons, olefins and polycarbonates. These ‘targeted’ TPEs could help reduce the time it takes to get a product to market.

For example, one tool manufacturer experienced delays getting a product onto shelves because it had difficulty tracking down the right material for a ‘soft-touch’ grip, Avery said. It took six months for the company to find a TPE that would properly bond with the material the tool was made from.

“In the past few years, the specificity of the TPEs has improved the compatibility with the base plastic and TPE materials to get better performance,” he added.