Flexibility is the name of the game when it comes to elastomers, both literally and figuratively, given their wide range of uses and ability to be stretched repeatedly and returned to near original shape. A decade or so ago, elastomer use was limited to some gasket and hose applications, or used for appearance, feel, and color. For the most part, they couldn’t be loaded with low-cost fillers; they had poor chemical and heat resistance, high-compression sets, and low-thermal stability.

Today’s elastomers come in composites that are more utilitarian, and their usage has exploded. Thermoplastic elastomers are considered the fastest growing segment in the polymer industry, says Paul Killian, technical manager for RTP, Winona, Minn., a supplier of elastomers. Applications include gaskets, seals, knobs, grips, hoses, tubing, keypads, electrical components, and much more. They can be infused with a variety of properties such as thermal and electrical conductivity, flame retardance, chemical, scratch, and mar resistance. They can be transparent or colored, and come in a wide range of softness levels. This list of capabilities continues to grow as material suppliers improve the polymers and develop new additives that allow use in applications that were not thought possible a few years back.

In addition, elastomers are becoming more environmentally friendly and addressing issues involving human health and safety, leading to the growing trend of removing plasticizers, halogens, and other chemicals that have been linked to negative environmental and health effects. Processing, too, has been made easier with elastomers that can be injection molded, extruded, blow molded, or fabricated by other methods. For instance, ExxonMobil Chemical, Houston, recently released a Santoprene thermoplastic vulcanizate (TPV) that can be cold insert-molded onto nylon materials.

Seals made from Simrit elastomers resist a wide range of chemicals.

Historically, achieving softer elastomers has been the main goal of elastomer developers targeting tactile applications, as a comfortable feel can enhance the customer’s perception of the product. Their work is paying off, as elastomers have gotten softer across the entire elastomer product range. Some reach 5 Shore A, which is basically a gel-like material, and can be injection molded or extruded. Other examples are TPVs that reach down into the 20 Shore A range, and TPE’s into the 30 Shore A.

As softness has improved, so too has bondability to a wider range of substrates. Overmolded part designs have become less dependent on mechanical interlocks to hold the material in place. Chemical bonds between an elastomer and a rigid material offer improved part integrity and longevity while eliminating the need for adhesives or surface pre-treatments. Killian says that one of his company’s newest TPE products, the RTP 6042 Series, has been formulated to bond to PC, ABS, PC/ABS, PC/PBT, PC/PMMA, and RTPU resins.

Elastomers are not just overmolded onto plastic resins; they can also be overmolded onto metal in an attempt to benefit from the softness and design flexibility of plastic and the strength and rigidity of metal. While research is still ongoing to develop an elastomer that adheres directly to a metal, as well as new ways to process them, a number of suppliers’ products can be used on metal. Inserts of steel, aluminum, brass, zinc, and other metals can be inserted into Texin and Desmopan elastomers from Bayer MaterialScience, Pittsburgh. In many cases, bond strength can be obtained by degreasing the inserts, applying an adhesive such as a polyurethane-based adhesive to the inserts and heating to between 104 DegC to 121 DegC before placing them into the mold. In other cases, mechanical interlocks might be required. An effective method for overmolding onto metal is full encapsulation, says David Barkus, president of Elastocon, Rochester, Ill., a company that offers several products that can be used with metal. In such cases, neither adhesives nor interlocks are required.

Elastocon elastomer was used to overmold the grip on the power tool.

No matter the substrate, the desire to combine the softness of elastomers with the rigidity of engineering thermoplastics is increasing. However, in the past, softness was sometimes achieved with chemicals such as plasticizers, which are being phased out in some markets and applications, says Joseph Kutka, technology launch manager for GLS Thermoplastic Elastomers, a PolyOne Co., McHenry, Ill. Phthalates are esters of phthalic acid and are mainly used as plasticizers to soften plastic material including PVC. Because they have been known to leach, they are being phased out of many products in the U.S. and European Union over health concerns.

GLS recently released the Versaflex CLE series, which is made without phthalates or plasticizers and currently has versions that are 95 Shore A, 90 Shore A, and 85 Shore A. Kutka says they plan to launch a 65 Shore A version, and have been able to reach this level without requiring any phthalates.

Bayer MaterialScience also offers a new plasticizer-free product. The company recently expanded its family of medical grade TPUs, including the Desmopan DP 2590 – 90A ester and the 9360A – 70SA ether. Both products meet the requirements of the FDA-modified ISO 10993 Part 1 “Biological Evaluations of Medical Devices” tests for 30-day indirect contact to blood applications. The company says it is the softest medical grade TPU in the company’s portfolio, with 90 Shore A and 70 Shore A hardness, respectively, which was achieved without the use of phthalates. The Desmopan material also features hydrolytic stability and microbe resistance.

Another chemical that is in question because of health concerns is bisphenol-A (BPA). PolyOne, Avon Lake, Ohio, introduced the BPA-free Edgetek compound made with Tritan copolyester from Eastman Chemical, Kingsport, Tenn. PolyOne is the exclusive compounder and marketer of filled Tritan copolyester products in North America. Edgetek compounds offer inherent chemical resistance and hydrolytic stability, which makes them useful in applications where heat and chemicals are prevalent, including small appliances, says Craig Nikrant, vice president and general manager, specialty engineered materials, PolyOne.

Environmental concerns have led some companies into producing bio-based products. Arkema, Philadelphia, with global headquarters in Paris, has developed the PebaxRnew, a bio-based TPE made from castor oil that is derived from castor beans. The beans grow in arid, inhospitable places, are not edible, and therefore do not compete with food production, says Basker Lalgudi, North American Market Manager - Polyamides.

The PebaxRnew, which has found increasing interest in consumer electronics, sports, and soft grips in consumer industries, is available with hardness ranging from 25 Shore D to 72 Shore D. The hardness level depends on the amount of amino 11 derived from castor oil that is incorporated into the polymer. The higher the percentage of amino 11, the harder the durometer. With a 72D Rnew material, the polymer has more than 90 percent renewable carbon atoms, while the softest, the 25D Rnew material, has a lower percentage of renewable carbon atoms. PebaxRnew is estimated to reduce global warming potential (GWP) by roughly 30 percent and usage of fossil energy by 25 percent. The biosourced PebaxRnew does not compromise any of its dynamic properties over a wide range of temperature (-50 DegC to 80 DegC).

Lalgudi says that the PebaxRnew is one of few polymeric materials that can take up to 70 percent of its weight in fillers, metal, or inorganic and still retain its flexibility. The flexural fatigue resistance, maximum energy return or rebound (compression set), and performance at subzero temperatures makes PebaxRnew an appropriate material for applications that need critical functionalities that cannot be met by thermoplastics and rubber materials, he says. It can be customized for applications that need permanent ESD, chemical resistance to kerosene and diesel, and other applications that need softening or impact properties.

Resistance to microbes, chemicals, water, and other fluids are improving across the elastomer spectrum, says Lynell Maenza, industry segment manager, Specialty Elastomers, ExxonMobil Chemical. A new entry in ExxonMobil’s Santoprene elastomer family is the W255 TPE series that comes in 45 Shore A, 55 Shore A, and 64 Shore A. It has been engineered for use as a seal in appliances such as dishwashers. Testing has shown that it is able to withstand not only the water it encountered, which by itself is an aggressive environment, but also detergent, rinsing aids, and dirt. Maenza adds that as machines work with less water to conserve the resource, the concentration of the water and other fluids will only get higher and the demands on plastic components even greater.

Another high-durability product is the SMR 1600 from Elastocon. The TPO material, which is the company’s first, is available in 64 Shore D, has a gravity density of .89, tensile strength of 20.2 MPa, and elongation before breaking of 542 percent. The material maintains its ductility at -30 DegC, and can keep a gloss level above 80 (ASTM D2457). It also has an innate ability to snap back after bending, which would make it a good material for flex applications, such as a living hinge, says Barkus. The product can be doubled over and, unlike most TPO products that get permanent stress whitening, the SMR 1600 will return to flat and there will be no permanent whitening.

Handheld computer utilizing elastomers from RTP.

Electrical conductivity is another capability appearing in many of today’s elastomers. Such materials can be useful in mitigating electrostatic discharge problems in electronic products. RTP, for instance, recently introduced a new series of electrically conductive TPV-based thermoplastic elastomer compounds - ESD C 2800 B Series. Historically, Killian says that conductive fillers raised the modulus and viscosity of materials, and negatively affected flexibility and processability. These new compounds achieve conductivity without sacrificing elastomeric feel, high tear strength, and easy moldability, he says. The series features a volume resistivity of 100 ohm/cm and consistent wide processing window across a Shore A hardness range of 55 to 85.

An elastomer’s ability to work in a wide range of temperatures is also a desired characteristic, says Joe Walker, corporate director of material development, Simrit, Plymouth, Mich. The company offers FluoroXprene, which Walker says is the world’s first fully fluorinated thermoplastic vulcanizate. It remains flexible down to -55 DegC and up to 177 DegC, although most grades that are attracting interest operate in the -40 DegC to 150 DegC range. Walker says that this outstrips conventional fluorocarbon materials that stop at –40 DegC. Because the FluoroXprene is fully fluorinated, the material has a very broad chemical resistance profile that would normally take multiple types of material to achieve that same end, says Walker.

Kraiburg TPE, Waldkraiburg, Germany, with offices in Duluth, Minn., offers three different TPE resins from its Thermolast K product group. They are suitable for gamma sterilization and permit autoclaving with superheated steam at temperatures of up to 134 DegC. It can bond to HDPE, PP, ABS, and PB/ABS resins. Its transparency properties make it possible to dye these compounds in any color.

Treknor Apex's new OBC material has a high chemical resistance and ease of processability.

Additives are helping to improve heat resistance and other properties. Zeon Chemicals, Louisville, Ky., offers its Hydrin blends of epichlorohydrin elastomers (ECO) that compounders add to a thermoplastic to develop customized TPEs. For instance, the blends can be used to fine-tune the damping characteristics of a compound, while retaining its flexibility, strength, dynamic properties, and oil resistance. It resists compression decay (creep), which means that it can be a replacement for natural rubber in some damping applications. Traditionally, natural rubber has been used for suspension-damping systems due to its low creep under a constant load, but high heat, oils and other fluids, can damage natural rubber.

Another popular additive is from Dow Chemical, Midland, Mich. The company offers its Infuse olefin block copolymer (OBC) additive that is being used by several companies, including Teknor Apex, Pawtucket, R.I., and GLS. Jordan Bast, Dow Chemical’s marketing lead for the Consumer Solutions Market, says it has heat resistance up to 120 DegC, which far exceeds Dow’s other elastomer products by as much as 60 DegC. A second selling point is that it has a 40 percent compression set at 70 DegC, which can be comparable to TPE-type performance at that temperature, says Bast.

Teknor Apex uses the OBC for its first vulcanizate product called Telcar OBC thermoplastic olefin elastomers. Vulcanizates are generally higher performance than other olefinic and styrenic TPVs because of their crosslinked rubber content, but in conventional TPVs that also yields a rubber-like surface. These new TPVs can be made to yield smooth, silky surfaces. The material features low isotropic shrinkage and flow characteristics that enable its use in complex molds with long flow paths. This can increase extrusion line speeds by 20 to 30 percent, says Andy Claytor, Teknor Apex’s sales director.

GLS uses the OBC for its Dynalloy OBC elastomers and also reports exceptional softness, unique feel, and a wide range of flow characteristics in injection molding and a new blow-moldable grade range. The five standard injection-molding grades include opaque and translucent materials and provide hardness levels ranging from 60 Shore A down to a 5 Shore A.

The ability to injection mold with so many grades is an example of the growing ease of processability. Another example is in the curing of elastomers. Improved curing times is an important attribute for a new product from Wacker Chemical, Adrian, Mich. The company has developed a UV-active silicone elastomer, the Semicosil UV, which cures at room temperature within a few minutes in the presence of UV light. Because the elastomer cures so quickly, manufacturers can encapsulate electronic components and PCBs faster and more economically right on the production line.

Transparency is another property that many elastomer suppliers offer. In the past, achieving transparency required a tradeoff in terms of chemical resistance and practical toughness, but that appears to be changing. The GLS Versaflex CL E95 TPE offers a balance of high clarity (haze <5 ASTM D 1003) and ruggedness, says Kutka. It has tensile strength at break of 17.2 MPa, elongation at break of 575 percent, and tear strength of 700 pounds per linear inch (pli).

Kraiburg also offers transparent adhesion compounds that provide the tactile appeal of a soft-touch finish with a high degree of transparency that allows the base product to be clearly seen through the finish. Color or special effects additive can be added to create high-clarity design elements.

These are just some benefits that elastomers can offer and some of the many reasons that this segment of the industry is growing so quickly. Designers looking at new elastomers can expect a broader range of softness levels, improved resistance to chemicals, greater impact and scratch resistance, improved thermal and electrical properties, and a whole host of color choices. On top of it all, the flexible elastomer materials can be customized to get just the right mix of these and other properties for a wide range of applications.

For more information, visit:
Arkema: basker.lalgudi@arkema.com
Bayer MaterialScience: scott.hunsberger@bayerbms.com
Dow Chemical: nelamb@dow.com
Elastocon: dbarkus@elastocon.net
ExxonMobil Chemical: tpe.answerperson@exxonmobil.com
GLS: info@glscorp.com
Kraiburg TPE: info-america@kraiburg-tpe.com
PolyOne: www.polyone.com/info
RTP: pkillian@rtpcompany.com
Simrit: information@simrit.us
Teknor Apex: tpe@teknorapex.com
Wacker Chemical: info.usa@wacker.com
Zeon Chemicals: askzeon@zeonchemicals.com

A Conformable Interface

When touched, the Elexium material from Willat Design Works conforms to the finger or hand that presses upon it.

A silicone elastomer and a silicone gel have have been combined into a new product that conforms to the contours of whatever touches it through a phenomenon called displacement. The goal was to create a more ergonomically comfortable gripping surface.

The technology, called Elexium, was developed by Boyd Willat, Willat Design Works, Los Angeles, a silicone-based designs and solutions provider. Momentive Performance Materials, Wilton, Conn., is providing the proprietary material formulation.

Willat, who previously designed the award-winning Sensa pen, says that the multi-material concept employs a molded outer skin made of a silicone elastomer that he calls a pod, and a gelatinous silicone material inside. When pressed, the material inside the pod acts more like water than it does a traditional elastomer, says Willat. When pushing on water, the water parts from the pressure and this material works the same way, he says. The Elexium material moves over and around whatever is pushing on it.

Because there is no push back as one would find with a compressible material, there is very little stress placed on the muscle. The same can be said for products that vibrate such as a lawnmower or power tool. These may enable product designers to engineer solutions that reduce hand fatigue and repetitive stress syndrome, Willat says.

When the finger or hand is moved away, the Elexium will return to its original shape in three to 10 seconds. It is hypoallergenic and is offered in a standard transparent color, but can be colored per customer specifications. It is available in durometers ranging from 10 Shore A to 80 Shore A. It has a typical tear strength of 150 pli, an elongation of approximately 65 percent, and can withstand temperatures from –76 DegF to 400 DegF. Additionally, bacterial and UV protection can be introduced. It cannot be used as a conductive material, but testing to achieve other mechanical properties is ongoing, Willat says.

For more information, email: Barbara@willatdesignworks.com