Edited by David S. Hotter
David S. Hotter
You don’t have to look far to find “soft-touch” thermoplastic elastomers (TPEs) in action. In fact, you probably used them before you got to work today, as you brushed your teeth, shaved with a razor, or drove to the office. TPEs are making their way into a myriad of consumer applications including personal-care products, sporting goods, power tools, kitchen utensils, automobiles, and even pens and pencils. These soft, flexible polymers add comfort as well as functionality to many of the products engineers design today.
Market data show that 50 to 70% of all handles will have some form of a soft grip by the year 2010. As a result, material suppliers are developing new elastomers to meet demanding applications. Some of the latest advances include super-soft materials, easier processing, the ability to bond to engineered plastic substrates, and resins that can be pigmented to almost any color.
A soft touch
Thermoplastic elastomers are a class of polymers with a soft and flexible nature like rubber materials, yet are processed with the same equipment as thermoplastics. TPEs were first developed in the 1950s as melt-processible materials that could be formed by injection molding, extrusion, blow molding, rotomolding, and thermoforming.
TPEs have been traditionally categorized into two classes: Block copolymers, which include styrenics, copolyesters, polyurethanes, and polyamides; and thermoplastic/elastomer blends and alloys, comprised of thermoplastic polyolefins and thermoplastic vulcanizates.
Conventional TPEs are considered two-phase materials comprised of a hard thermoplastic phase that is mechanically or chemically mixed with a soft elastomer phase. The resulting material shares the characteristics of both.
TPEs give engineers several advantages over thermoset rubbers, including lower fabrication costs, faster processing times, little or no compounding, recyclable scrap, and processing by conventional thermoplastic equipment. In addition, the processing equipment consumes less energy and maintains tighter tolerances than that of rubber processing.
There are several drawbacks, however. TPEs have relatively low melting temperatures, making them unusable in high-temperature applications. They usually require drying before molding since they are hygroscopic, or moisture absorbing. Manufacturers must use TPEs for high-volume applications for it to be economical. And molders accustomed to working with rubber have little experience using TPE materials and equipment.
As for material properties, TPEs come in durometers, or hardnesses, ranging from 3 Shore A to 70 Shore D, which roughly translates into going from very soft and flexible to semirigid. Although they have relatively low melt temperatures, TPEs resist continuous exposure to temperatures up to 275°F, with spikes up to 300°F. On the other end of the temperature spectrum, they remain flexible at temperatures down to -81°F. The polymers have outstanding dynamic fatigue resistance, good tear strength, and resist acids and alkalis, ultraviolet light, fuels and oils, and ozone, and maintain their grip in wet and dry conditions.
Although TPEs got their beginnings by replacing rubbers at a cost savings, engineers now use them to add flexibility and comfort to designs. “Our materials used to replace thermoset rubbers by reducing cycle times and consolidating parts,” explains Peter Burnett, consumer-market manager at Advanced Elastomer Systems (AES) L. P., Akron, Ohio. “Now we help engineers make money rather than save it, by differentiating their products and making them more attractive to consumers.”
Material suppliers control flexibility and strength, along with other material characteristics, by varying the amount of hard and soft components used during compounding. When developing a TPE blend, most of the focus is on what type of environment the end product will see.
“Each resin family we produce has different physical and mechanical properties that target specific markets and applications,” explains Phil Morin, industry manager at the Plastics Division of Teknor Apex Co., Pawtucket, R.I., “We can make super soft materials as well as fluid or oil-resistant materials.” Engineers develop formulations by giving a TPE as much flexibility and softness as possible without sacrificing any of its critical qualities.
Coefficient of friction is another quality that depends on the product’s end use. “We add slip agents or tacky components to the chemistry to control the level of ‘stick’ depending on whether a manufacturer wants a smooth, tactile feel or a sticky grip,” says John Marshall, marketing manager at GLS Corp., Cary, Ill. “For example, a golf grip needs to be sticky but the grip on a pen should feel soft and smooth. These characterisitcs are controlled by the material’s frictional properties.”
Besides making components more comfortable, elastomer grips absorb energy, noise, and vibrations, which makes them ideal for power tools. “We are trying to quantify the damping qualities of our TPEs and how long they extend the time a consumer can comfortably use a tool such as a drill, for example,” says Scott Conway, business development manager for consumer products at AlliedSignal Plastics Inc., Morristown, N.J. “We are also exploring ways to reduce the noise of products using an elastomeric layer on devices such as leaf blowers by combining TPEs and rigid plastics.” Still, few consumer-product manufacturers take advantage of TPEs.
Once engineers choose the best TPE for an application, they must select a method for attaching the grip to the device. In the past, they relied on adhesives and mechanical features designed into products, such as undercuts and holes that helped lock thermoset rubber or olefinic-based TPE components in place. Unfortunately, those aren’t the most effective bonding methods and they put too many constraints and limitations on substrate designs.
The molecular structure of TPEs makes it a challenge to chemically bond them to other resins. Most TPEs are olefin based, which makes it easy to bond them to high-density polyethylene or polypropylene. It’s more difficult to overmold TPEs on engineering thermoplastics such as the styrenics because the two materials are inherently incompatible, chemically speaking.
The latest advancements in TPE formulations are resin blends that adhere directly to engineering thermoplastics without any special features and can be applied by insert molding (overmolding) and two-shot injection molding. The new formulations let designers mold grips onto a wider range of thermoplastics, particularly high-strength materials such as high-impact polystyrene, glass-filled nylon, and polycarbonate, which are commonly used in power tools, sporting goods, and electronics.
AlliedSignal and Advanced Elastomer Systems joined forces to develop applications and markets for nylon-bondable “soft-touch” products. Under a nonexclusive agreement, the companies collaborate on application development and technical support, but market products separately.
“We used to apply conventional adhesives to secure grips, but they couldn’t provide us with the confidence needed for long-term adhesion and peel strength, plus there were always environmental issues and concerns,” says AlliedSignal’s Conway. “Two-shot and insert molding chemically bond the grip to the substrate, eliminating peel-strength concerns, reducing labor, and increasing control over tolerances.”
Engineers at AES found that the differential melt temperature between substrate and elastomer for its new nylon-bondable resin promotes using a two-shot method. “With two-shot molding, the nylon doesn’t cool much from its 500°F melting temperature and therefore doesn’t act as a heat sink when injecting the elastomer,” explains Kevin Gase, senior design engineer for AES. “Insert molding can also be used but it requires preheating nylon inserts up to 250°F, which increases labor costs.”
New materials are also easier to mold in thin-wall sections, which is crucial for consumer electronics where weight is a concern. Consumer-electronics manufacturers can use TPEs for grip strips, for example, because new resins flow easily across long, thin channels in part molds.
Cleveland-based BFGoodrich Specialty Chemicals takes another approach to developing elastomers that bond to plastic substrates. “We found that engineers prefer to mold ‘soft-touch’ elastomers onto rigid substrates instead of using adhesives and mechanical locks; yet most elastomers don’t provide the necessary adhesion, compatibility, and durability,” says Elliott Pritikin, automotive commercial manager at BFGoodrich. “To answer these needs, we developed a material system consisting of a rigid and a flexible TPU formulation. The two materials — Estaloc thermoplastic and Estane elastomer — have similar chemical makeup. This chemical compatibility helps the two materials form a strong bond without adhesives, giving automotive manufacturers, for example, a ‘one-stop shop’ for interior applications such as ignition bezels.”
Molding parameters and tooling costs become a concern when molding elastomers onto rigid plastics. “There are several considerations when molding low-durometer materials onto rigid plastics,” explains John Conlin, program manager at Phillips Plastics Corp., Prescott, Wis. “For instance, the durometer of the elastomer affects the gate design used to deliver resin to mold cavities. Higher-durometer materials, greater than 70 Shore A, are easier to process because they behave more like rigid materials. Low-durometer materials, from 40 to 60 Shore A, are more challenging.”
Where the gate attaches to the part is crucial and adds to the complexity of tooling. “Aesthetics are critical on parts such as knobs and grips, since end users will feel any flaws,” says Conlin. “Therefore, we conceal gate marks underneath parts or by using subgates attached to a post, which must be removed by a secondary process.”
While the trend is toward using two and three-shot molding to form elastomers over rigid plastics, tooling costs and manufacturing volumes remain important considerations. Multishot molding creates stronger chemical bonds compared to overmolding because the substrate material stays in the same tool and doesn’t cool. Mechanical locks are another option, but they drive up tooling costs by requiring complex slides, lifters, and through-holes in the mold.
The amount of automation and parts handling during molding adds to manufacturing costs. “The cost of complex tooling used for two-shot molding may seem prohibitively high initially,” adds Conlin, “but can save money for high-volume applications by slashing manufacturing costs, especially when compared to labor-intensive processes such as overmolding, which requires more handling during molding.”
In living color
Colorability is another characteristic that sets TPE grips apart from those made with thermoset rubber-based elastomers. Thermoset rubbers are naturally black because of their ethylene-propylene-diene-terpolymer (EPDM) component. In contrast, natural TPE resins range from clear to opaque white, making them easy to pigment.
Thermoplastic vulcanizates (TPVs), a type of TPE, are more challenging to color because they consist of a blend of polypropylene and very fine EPDM rubber particles. The EPDM particles makes it impossible to develop a clear resin. However, recent developments offer whiter and cleaner TPVs that can produce bright colors.
“In the past we only offered black resins. Our newest elastomer, Santoprene 8000, produces vibrant colors and metallics when pigmented,” says AES’s Burnett. “It is easier to color because it’s a much whiter resin, using up to 50% less pigments compared to former blends.”
Recent trends show that pigmented resins are moving into industrial applications, where customers want colorability. “It’s difficult to formulate a resin that is easy to color and stands up to harsh chemicals, such as for power tools, but we are working on new formulations,” explains GLS Corp.’s Marshall.
The appliance industry is also getting a boost from the colorability of TPE resins. “We provide plastic/elastomer systems that match the demands for appliance white,” says AlliedSignal’s Conway. “Our joint venture with Advanced Elastomer Systems to develop nylon-bondable TPVs lets us compete for applications now using steel stampings covered with painted urethane foam. Thermoplastic elastomers outperform urethane foams because foams don’t have the strength to resist tears if they get nicked. Glass-filled nylon handles covered with Santoprene last 10 to 15 years, which is often the life of the appliance.”
Though most “soft-touch” elastomers are being used for consumer products, material suppliers aren’t concerned with the sometimes fickle attitudes and transient nature of marketing trends. “‘Soft-touch’ materials are much more than a fad,” says AES’s Barnett. “A lot of the attempts companies make to differentiate themselves in the market are just passing trends, but we don’t see that as the case for TPEs. Elastomers are answering consumers’ needs and providing a level of performance they have come to expect.”