DN Staff

February 19, 1996

14 Min Read
Elastomers expand design options

For your next design project, what kind of material will best fit your needs? One that is soft? Hard? Rigid? Flexible? Forgiving? Chemical resistant? UV stable? Processes fast? Is lightweight? Doesn't cost much?

Kryptonite is out of this world when it comes to price and where to find it. What then? Try elastomers. They come in all types of compounds--everything from natural rubber to such exotic blends as elastomeric thermoplastics and thermoplastic polyolefins. And, if a standard grade doesn't do the job, these materials can be tailored to meet nearly any design goal or budget.

With so many benefits in their favor, it should come as little surprise that elastomers are experiencing expanded uses in many industries, especially the electronics, automotive, and medical arenas. Here's a look at what goes into the alphabet soup that makes up these materials, as well as some innovative applications for their use.

Computers contain extremely delicate circuitry and parts that must remain free of contaminants and foreign particles. That's why Seagate Technology, Scotts Valley, CA, turned to 3M's Specialty Fluoropolymers Department's Fluorel(TM) fluoroelastomer to process custom-made seals for its computer disk drives. The elastomers help seal the die-cast aluminum disk-drive boxes, which have mechanical clearances of less than 2 millionths of an inch.

The Fluorel elastomeric seals not only repel the intrusion of contaminants, but also control humidity and air flow within the drives. And the specially formulated compounds avoid the natural elastomer emissions, or "outgassing" that occurs inside the drives due to temperature changes.

The Fluorel fluoroelastomers come in more than 35 grades. Recently, 3M introduced proprietary "incorporated cure" food-grade elastomers. The 5000 Series Fluorel FG FDA-approved compounds "have superior characteristics over amine-type cure systems in the areas of compression set resistance, scorch properties, bin stability, processability, steam and water resistance, moldability, and shelf life," says William H. Donahue, market development manager.

Further expansion of the fluoropolymer lines seems certain now that 3M has entered into a joint venture with Hoechst AG, Frankfurt, Germany, a producer of PTFE and fluorocopolymers sold under the Hostaflon(R) tradename. Subject to government clearance, the company, to be headquartered in St. Paul, MN, is expected to begin operations by mid-1996. The high-performance fluoropolymer products would be tailored for such markets as automotive, chemical processing, and electrical.

Ultimate design goal for Navistar engine sealing system would reduce six parts down to three. Completed initial design phase replaces middle HNBR lathe-cut seal, potentially susceptible to extrusion and cavitation under pressure, with solid dual-elastomer seal backup to the Viton O-ring.

Parts reduction. In another seal application, a dual-elastomer should reduce parts considerably for Navistar International Transportation Corp., Chicago, for the fuel-injection system in its International T 444E, 7.3-liter V-8 engine. The medium-duty diesel engine is said to be the first to fully incorporate engine electronics and the technologically-advanced HEUI (Hydraulically actuated, Electronically controlled Unit Injection) fuel system.

"Our ultimate design goal is to replace a six-part injector sealing system--three sealing O-rings, two lathe-cut, backup seals, and a metal spacer--with just three parts," says John Serio, seal design specialist for Navistar's Engine and Foundry Div. "Another design goal is to increase throughput of our all-robotic seal installation and inspection equipment."

Previous designs had included seal materials and backup structure materials selected for their primary function in the seal packs. "Our direction is to employ dual-elastomer seals that satisfy sealing, strength, rigidity, and performance needs in a single sealing component," Serio explains. "Initial results are very promising, and we are confident of reaching our sealing-system goal."

For this project, Navistar called on Acadia Polymers, Roanoke, VA, the company that had originally presented the dual-elastomer static-seal concept for the injection system. "The first new dual-elastomer back up ring is in place and performing well," reports Jack Powell, president and general manager of Acadia's ETI Div., Burr Ridge, IL. "A second seal project, considerably more challenging, is in the prototype testing stage. Results so far are encouraging."

The original six-part sealing system employed three O-rings made from DuPont's Viton(R) copolymer fluoroelastomer as static seals. The injector tips are positioned at the center of the cylinder around the combustion bowl of the piston. The bottom O-ring seal prevents fuel from contacting high temperatures near the bottom of the cylinder head. The middle seal separates high-pressure engine oil from the diesel fuel.

For Acadia, designing and manufacturing a dual-elastomer seal as a middle seal component provided the initial challenge. Concerns existed over the potential for improper sealing due to both extrusion and cavitation of the HNBR (hydrogenated nitrile rubber) lathe-cut, back up seal. Acadia delivered a glass-filled PTFE ring on the OD, with fluorocarbon bonded to the ID. Subsequent testing by Navistar engineers verified that the dual-elastomer performs as a solid back-up to the O-ring.

Phase two, the redesign of the top seal, is currently in progress. It would eliminate the metal washer, the lathe-cut seal, and the O-ring. If successful, one PTFE and Viton dual-elastomer seal will replace all of the top injector-seal components.

A clutch rubber connection. Another interesting application involving HNBR comes from Germany. In this case, the rubber parts, made of Therban(R) HNBR from Bayer Corp., Pittsburgh, PA, replace the steel spring segments in clutch friction plates. Using the Therban technology, Textar KBI GmbH in Leverkusen considerably improved the friction clutches, according to Bayer's Huge Vernaleken, manager of the Research and Development Section in Bayer AG's Rubber Business Group.

The new clutch system has better abrasion characteristics, allowing thinner friction linings to be used. This, in turn, resulted in reduced material requirements, a lighter clutch system, and a decrease in the clutch plate's inertia. The clutch linings are vulcanized onto the backing plate so there is no need for rivets and central plates to mechanically hold the part in place. This enables the full strength of the lining to be employed.

Moreover, the Z-shaped connection elements molded from the oil- and fuel-resistant elastomer perform well at high temperatures. And the material's elastic behavior enables the clutch to be engaged smoothly, virtually simulating the non-linear, progressive spring characteristics of conventional steel spring segments.

Sorting out softness. If its a "soft" material you require for your design, there's a number of thermoplastic elastomer (TPE) resins that should fill the bill. The only question that remains is how soft do you want the material to be? And, when it comes to selecting the softest offering, there appears to be some friendly competition in the marketplace.

For instance, GLS Corp., Cary IL, lays claim to having the "world's softest TPE for injection molding" in its Dynaflex(R) G-6703 rubber compound. The material has a 3 Shore A hardness.

"Two years ago we introduced a 13 Shore A TPE, and, based on the success of that product, industry has demanded an even softer TPE," says John Marshall, GLS market development manager. "Our Dynaflex G-6703 pushes the barrier of how soft a TPE can be and still be injection moldable." Commercial applications include: toys, sporting goods, high-performance active-wear, and medical products. Industrial applications include: bumpers, pads, and shock-absorbing products.

Consider the simple act of opening a door. Taken for granted by most people, getting a grip to turn a slick, smooth doorknob can be an exasperating effort for the disabled, elderly, small children, or those stricken with arthritis. Two Colorado inventors have a solution based on GLS's Dynaflex TPE and the formation of the Knobbles Doorknob Twister Co.

"Early tests through initial production runs showed us TPEs were the materials of choice," says Knobbles Doorknob Twisters President Bill Youmans. "But we experienced countless difficulties, including extruding the materials and achieving the desired softness and tactile feel in the end product, as well as getting the appropriate grip on the doorknobs."

Use of a suitable material proved to be a major stumbling block--until Youmans and partner Patrick Going saw their hobby of mountain biking offer a solution. Both noted that the handlebar grips of their bikes were made from a material that had the properties they sought. A call to the bike company led them to GLS Corp.

Yet another problem arose. The Dynaflex TPEs were some of the softest on the market, but they had been limited to injection-molded, rather than extruded parts. GLS Sales Engineer Luke Kimble and Ranger Plastics Extrusions, Inc., Arlington, TX, took on the challenge. Several weeks of experimentation ensued. "We had several readily available dies to use that were similar to the end product everyone had in mind," explains Ranger President John Earnest. The result was a product with a 13 Shore A hardness that "extruded extremely well."

The Knobbles doorknob twisters were first introduced in only one size, since they could be easily stretched for most doorknobs. Recent new uses, however, include barbecue grill propane gas-tank handles and outdoor water faucets. Knobbles also conveniently fit over the caps of soft-drink bottles to facilitate their removal, and they are offered with UV stabilizers, glow-in-the-dark crystals, and a variety of colors. Installation requires users to simply soak the Knobbles cover in hot water for one minute, then stretch it over or around the knob.

Meanwhile, Advanced Elastomer Systems (AES), L.P., Akron, OH, recently announced the worldwide commercialization of "the industry's softest, fully vulcanized" TPE. The Santoprene(R) grade 111-35 has a 35 Shore A hardness "to meet the demand for a very soft, flexible TPE with rubber properties for use in such sealing applications as fluid delivery, mechanical rubber goods, and appliances," says Jay Griffith, AES worldwide product director.

Initially, the product is offered in black and is designed as a user-friendly, injection-molding grade. "As with all grades of Santoprene rubber, it is easy to process and has inherent recycling advantages over rubber," Griffith adds.

No vacuum for TPU use. Add thermoplastic polyurethane elastomer (TPU) compounds to the elastomeric alphabet soup that continues to find expanding uses in the industrial world. Typical of this forward march into new and innovative applications are several components for vacuum cleaners made by Miele & Cie, Bielefeld, Germany, and Vorwerk Elecktrowerke, Wuppertal, Germany.

The quality characteristics of Desmopan(R), a thermoplastic elastomer from Bayer AG, Leverkusen, Germany, proved to be the deciding factor for its use in the vacuum-cleaner parts. "The essential advantages of Desmopan are good abrasion resistance with good elasticity, flexibility over a wide temperature range, and good resistance to oils, greases, and many solvents," explains Bayer's Johann Thim. "Desmopan also contains no plasticisers," he adds.

Among the parts produced by the two German companies are a fender with integral dust-chamber seal, various castors, a brush holder, and crevice nozzle. And the parts have another thing going for them: they can be recycled without difficulty, either in combination with virgin material, or as 100% recyclate.

In addition to Desmopan, Bayer produces Texin(R) TPU resins. Texin 285 is recommended for applications where high flexibility is required, Desmopan 453, a harder, more rigid grade, for low-deflection, load-bearing applications, and Texin 390 for applications where hardness and modulus needs fall between the other two grades.

But the Desmopan story doesn't end here. Bayer recently announced a new line of Desmopan TPUs that "promises to eliminate the dilemma of using a polyester thermoplastic polyurethane for its cost-effectiveness, chemical stability, and mechanical properties, or a polyether TPU for its good hydrolysis resistance, microbe stability, and low temperature flexibility," says Jergen Haettig, technical marketing specialist at Bayer's Polymers Div., Pittsburgh.

Even more "exciting" grades are on the way, according to Jerry Robbins, technical and marketing manager for Texin at the Polymers Div. Among the the new developments to watch for are blow molding and rotomolding grades.

Dynamic duo. It's too early to predict what elastomeric materials might emerge, but a new company formed by DuPont and Dow Chemical, DuPont Dow Elastomers, has the potential to crank out a variety of new and innovative compounds. In fact, in announcing the joint venture, the companies said that they will focus on "the discovery, development, production, and sale of thermoset and thermoplastic elastomer products used primarily in the automotive, chemical process, general rubber, wire and cable, construction, molded goods, durable adhesives, and oil and polymer modifications markets."

"DuPont Dow Elastomers will link DuPont's strength in marketing synthetic rubber with Dow's Insite(TM) technology," says Don Duncan, the venture's president and CEO. He adds: "Combining the science of DuPont and Dow will greatly enhance our ability to meet rapidly evolving customer needs on a global basis by providing the broadest portfolio of elastomer products in the world--from ethylene elastomers to fluoroelastomers."

The single-site catalyst Insite process has resulted in elastomeric copolymers of ethylene and octene. They are sold under the trade name Engage(R) polyolefin elastomers (POEs). The Intellectual Property Owners Association and R&D Magazine have presented the process with their top awards.

"You can expect to see ethylene and styrene copolymers making the scene in the near future as result of this process," says Chris Pappas, the joint venture's global business vice president in charge of ethylene elastomers based on the proprietary process. Polypropylene and adhesive compounds are also on the way, according to Ed Gambrell, Dow's vice president of Insite Technology.

Cyber Contacts

You can reach the following companies mentioned in this feature using the services of the World Wide Web. Please tell them that you were referred by Design News.
3M http://www.mmm.com
DuPont http://www.dupont.com

Silicone solutions. Another member of the elastomer family, silicones, is also finding expanded uses, particularly in the medical area. Silastic(R) BioMedical grade materials from Dow Corning, Midland, MI, provides a good example. Not only has the material passed the standard USP Class V and USP Implant (5 day) tests for elastomers, but it has survived a battery of other tests, including hemolysis, skin sensitization, mutagenicity, cell culture, USP Pyrogen, and 90-day implant.

John Hilliard, manager of Dow Corning's medical materials business, reports that outside testing costs have already reached $1 million. "Other indirect costs will total another $1 million, making the total investment at least $2 million to have the ISO/Tripartite battery of tests completed on our materials," he adds.

Because of this impressive list of test results, George L. Anstadt, an Ohio veterinarian, turned to Silastic for the design of his Anstadt heart cup. The heart-shaped device, produced by Specialty Manufacturing, Saginaw, MI, fits over the heart in cardiac arrest to resuscitate the patient. By aspirating itself onto the heart, the cup actuates the heart to continue pumping. Arstadt attributes a large part of his successful design to the technical support provided by Dow Corning. The original heart cup contained silicone only in the diaphragm of the the device. Now, the entire device consists of silicone.

The cup attaches itself to both ventricles via a continuous vacuum at the apex of the cup. Once the cup is in position, positive and negative pneumatic forces operate a diaphragm within the cup to actuate the ventricles into their normal systolic and diastolic configurations. Appropriate delivery of uniform positive and negative forces to the ventricles rapidly returns the failing, asystolic, or fibrillating heart to its function as a blood pump.

Once the heart is actuated, the physician can focus attention on bodily wounds caused by a gunshot, stabbing, or accident that could cause cardiac arrest. "It can take the 'emergency' out of treatment," says Anstadt.

Also, the cup may stay in a patient for days to lengthen the amount of time needed to locate a transplant heart. Anstadt tells about a patient whose use of the heart cup even eliminated the need for a heart transplant. "Such patients have a life expectancy of only five years; this patient has more of a chance to live a full life with her own heart," says Anstadt. The heart cup has been through clinical trails at Duke University and is "on the verge" of being on the market.

So take your pick of the alphabet soup of elastomers. No doubt you will find the material that will best fit your application. If not, any of the companies mentioned can tailor that special material for you, many within a few days. But, if you still can't find the exact material you desire, wait a while. With all the activity going on in the elastomeric marketplace, that special material won't be long in the making.

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