|E. David Santoleri |
Advanced Polymer Alloys
Melt-Processible Rubber (MPR) is poised to become the front-runner in the pack of elastic materials vying to replace rubber. Incentives to cut costs, improve performance, and satisfy the discerning consumer have led designers to look hard at this rubber alternative.
Globally, the technology has been so well received that its use is growing 7% annually. MPR is a partially crosslinked chlorinated olefin alloy and is fast becoming an industry-wide alternative to vulcanized rubber. Engineers and designers have found this new material can effectively replace rubber and other competing thermoplastic elastomers (TPEs) and thermoplastic vulcanates (TPVs). The challengers to date offer many rubber traits but have come up short when compared head-to-head with vulcanized rubber. Alcryn MPR is the only melt-processible rubber on the market. Manufactured by Advanced Polymer Alloys (a division of Ferro Corp.), Wilmington Del., it competes well when consumer expectations dictate specific rubbery properties previously offered only by the original.
Two manufacturers currently use MPR for a high-end flashlight and a new sander. During the design phase, MPR and a common TPE were compared. The results favored MPR because the material needs lower injection pressure, gives better knit line features and yields a more uniform part finish. In addition, MPR can be processed in heavy production applications using family molds, a multiple-part molding process that TPEs commonly can’t accommodate. This cost-controlling manufacturing process, coupled with the total reusability of MPR scrap, makes the economics of the new material hard to beat.
Even with these advantages, getting the public to buy this rubber replacement remains a critical area for manufacturers. Most consumers don’t have hard definitions for what rubber looks like or how it behaves. That leaves designers with no clear way of comparing MPR to vulcanized rubber or the competing thermoplastics. Designers have used TPEs and TPVs as rubber replacements, though to some degree consumers still don’t accept them as “rubber.” The problem: Many consumers think both materials fall short in appearance and touch. In contrast, MPR closely matches rubber’s look and soft feel.
Durability is another area on which designers must focus when specifying rubber components. Often thought of as a way to describe abrasion resistance, durability also indicates a material’s ability to withstand other harsh environments. In regard to abrasion, rubber is tough to beat. The reason is that rubber’s abrasion resistance is made possible by the addition of carbon. The tenacity of the carbon-fill gives rubber advantages over unfilled materials in abrasive situations. The carbon-fill also provides rubber with some protection from environmental deterioration, but it has its limitations. MPR is not as abrasion resistant as rubber, but it is a tough alternative in climates that are harsh either environmentally or industrially.
Look and feel
Look and feel are the first qualities that discerning consumers evaluate. Many people equate a glossy, shiny surface and a slippery cold feel with “cheap plastics.” What they find more desirable is a product with a satin surface and a soft, warm, high-friction feel. In addition, the average person may judge quality and product durability on looks alone. For example, consider a toy tractor: If the tires don’t look like those of the real thing, even a young child will brand it a cheap imitation. Anything that is supposed to be rubber must look and feel like the real thing. The bottom line is, consumers may not be able to scientifically describe rubber, but they know it when they see it.
Visually, most vulcanized rubbers change color or fade when exposed to heat and weather. Many familiar rubber parts (tires, hoses, and so forth) are pigmented with carbon black to help minimize the deterioration from UV light. MPR, however, stands up well to heat and UV. And it can be processed in various colors to provide several different looks, among them subtle pastels and striking neons. MPR technology lets designers create durable, highly colored products not possible with traditional rubber materials.
Tactually, one of rubber’s strongest attributes is its high-friction, nonslip surface. This is important for safety as well as for mechanical friction to engage or restrict motion. Coefficient of friction values give designers a way to gauge a material’s functional friction qualities in various applications.
To offer mechanical properties like those of rubber, a material must be soft and flexible (low stiffness) and recover elastically from deformation. And though a durometer reading indicates softness, it is not by itself a reliable measure of rubberlike properties. “Soft” is not the same as “rubbery.” Chewing gum and plasticized vinyl are “soft,” but they are not elastic like rubber.
Stress-strain properties are useful indications of flexibility and elasticity for comparing materials. The slope of the curve is a measure of flexibility or stiffness. The curve is a straight line for a 70A-hardness and flexible vulcanized nitrile rubber. This indicates the material will elastically recover as high as 100% elongation and meets the ASTM definition for rubber. Vulcanized rubber’s tensile stress at 100% strain is around 800 psi.
It is useful to compare the stress-strain curves of the 70A-hardness nitrile rubber and a 70A-hardness TPV. TPV has a different stress-strain curve than the nitrile TSE. The slope is much higher. At 25% strain, the TPV has a tensile stress about three times higher than the TSE. To the consumer, this makes TPV feel more rigid.
Besides being stiffer, TPVs are not as elastic as rubber. Typical TPVs display a “plasticlike” yield at around 35% strain. The data predicts that switching a 70A-rubber part to a 70A TPV would bring many users to look for “something softer and more elastic.”
In contrast, Alcryn MPR has the same flexibility as standard rubbers. The reason becomes apparent from stress-strain data for 70A-hardness Alcryn, 70A NBR, and neoprene vulcanized rubber. The slope of all three curves is the same. The stress-strain behavior of Alcryn is linear with no evidence of a plastic-yield point found with TPEs, and shows that Alcryn recovers elastically from 100% strain.
Another mechanical test to help describe rubber qualities involves a material’s ability to seal. The compression set test shows material compression performance. But this is a short-term test that may not accurately predict material creep under long-term loading. Vulcanized rubber compounds can be designed to maximize resistance to compression set. These will always outperform any TPE especially at high temperatures. MPR material scores well on the compression set comparison. Manufacturers are using MPR as a functional replacement for rubber in many sealing applications.
A better predictor of sealing performance is, however, a stress decay or creep-in-tension test. A comparison of the room temperature tensile creep for nitrile and neoprene rubber compounds with that of Alcryn and a TPV show that Alcryn resists creep in tension even after 1,000 hr. The MPR compares well to nitrile rubber and is clearly superior to TPV. For seal applications, designers should test material resistance to tensile creep and compression set under simulated service conditions.
All TPEs are by definition thermoplastic, which means that they will soften and lose mechanical properties at elevated temperatures. Material suppliers estimate service temperature limits in terms of mechanical property retention and resistance to heat-aging embrittlement. These qualities show that TPEs retain mechanical properties matching their much higher melting points, but they are not drop-in replacements for rubber. Alcryn MPR offers better performance than TPEs and is more durable than vulcanized rubber in applications up to 100°C.
It is well known that vulcanized rubber deteriorates in many environments. Synthetic elastomers have been specially formulated to combat the ravages of an industrial environment on seals, gaskets, hoses, drive-belts and so forth. But in many cases these elastomers fall short when rated for heat and oils under ASTM D2000 standards. In comparison, Alcryn MPR works with silicone greases, insecticides, agricultural spray, fuels or hydrocarbons and has a service temperature limit of 125°C. Other weathering environments that include rain, UV radiation, and ozone exposure, can also make rubber look bad and lose its properties. Ditto for synthetic TSEs and TPEs during outdoor use. Alcryn, however, offers excellent resistance to weathering. Even properly protected lightly colored grades work well in outdoor applications such as window and door seals for vehicles and buildings, and various sporting, farm and marine equipment. No longer is carbon black pigment the only additive protection available for outdoor use. Many shades of Alcryn will do the job just as well.
TPEs, TPVs, and Alcryn MPR are injection moldable, but MPR provides the fastest injection-molding time of the group. It also can be coextruded with rigid PVC providing dual-hardness profiles with integral sealing. Integral sealing eliminates the need for adhesives or secondary attachment operations.
Another useful manufacturing feature of Alcryn is a viscosity that depends primarily on shear and not temperature. Alcryn maintains its shape under low shear, an important feature for designers using profile extrusion and fast injection molding cycle times. Because of this stability, Alcryn can be calendered and thermoformed, processes impracticable for many TPEs. Alternatively, Alcryn flows well at high shear and relatively low temperatures (350°F). This is a necessary requirement for molding thick sections. The good flow quality provides relatively short cycle times during the production of thick parts such as rolls, bumpers and heavy seals.