Santoprene has never been much of a joiner. The same polyolefin matrix that gives this family of thermoplastic vulcanizates (TPV) its chemical resistance and mechanical performance also imparts surface energies so low that Santoprene didn't stick to dissimilar substrates without the aid of an adhesive or primer. Advanced Elastomer Systems (AES) has now found a way around this reluctance to bond.
The company last month rolled out three new "enhanced bonding" grades that adhere to metals, polymers, and engineered textiles without the need for adhesives or primers. Available in 65, 80, and 85 Shore A hardnesses, these new elastomers feature modifications that allow them to form chemical bonds with other materials during melt processing. "Think of them as 'hot-melt TPVs," says AES technology manager Marvin Hill. "They process and function like a TPV but bond like a reactive hot-melt adhesive."
A brawny bond. Applied to their substrates during overmolding or extrusion operations, the new materials produce nothing if not a robust bond. AES tests show adhesive peel strengths ranging from 15 pli to more than 30 pli, depending on the substrate and processing methods. "It's really more of a structural bond than a tack bond," Hill notes.
The enhanced bonding grades represent just the latest effort in AES' strategy to make Santoprene adhere to a wide range of substrates without relying on pricey solvent-based adhesives. Starting in 1997, AES brought out grades that adhere to nylons, EPDM, and styrenic compounds like ABS.
The new enhanced bonding grades add etched fluoropolymers to the list of compatible plastics. More significantly, the technology extends Santoprene's reach into new classes of materials. According to Hill, the new bondable grades stick to aluminum, brass, and copper as well as to galvanized, cold-rolled, and stainless steels. They also adhere to polyester and other engineered textiles, including ones with epoxy coatings or carbon-fiber reinforcements. What's more, the enhanced bonding materials can also serve as tie layers in laminate structures made from otherwise incompatible materials. According to Hill, this tie-layer capability promises to allow the creation of new functional laminates-such as flame retardant or UV resistant materials-without worrying about the compatibility of individual layers.
Properties preserved. From a physical and mechanical property standpoint, the enhanced bonding grades stack up nicely to standard Santoprenes of equal hardness. With a specific gravity of 0.90, the new grades have one of the lowest gravities in AES' portfolio, opening up possibilities for weight-reduction uses.
The materials do have a slight temperature tradeoff: They cover a service temperature range from -50 to 121C, about 10 degrees lower than standard Santoprene. "Tear strength and elongation are comparable with other Santoprene grades," reports AES applications engineer Mike Goncy. The 85 Shore A grade, for example, has a tear strength of 1600 psi, an elongation at break of 750%. Likewise, the new bondable grades have abrasion resistance and flex fatigue life similar to standard grades.
Other than the slight loss of heat performance, the only other drawback to the new materials appears in injection molding applications, where it could be more susceptible to release problems. Because the bondable Santoprene sticks so well to metal, mold cavities need to be made from chrome steel or coated with Teflon, Goncy reports. And the material slightly limits part design freedom too. "Lots of undercuts would be tough," Goncy adds.
Many uses. Minor molding considerations aside, the bondable materials are poised to go into a host of diverse applications. Hill says the materials could serve in reinforced sheet goods for office furniture, appliances, containment systems, and conveyor belting.
Saint-Gobain Performance Plastics Specialty Hose Div. (Mantua, OH) has already used an enhanced bonding grade in its new Synflex carpet cleaning hose. It withstands temperatures up to 250F and handles pressure up to 1,250 psi, reports Product Manager Rachel Jones. "These hoses also weigh 2/3 less than traditional wire-braided rubber hoses," she adds.
And AES has been working with an undisclosed customer to develop integrated washers and gaskets, which would eliminate the solvent bonding of EPDM to metal washers. As an overmolding material the new bondable grades offer colorability and soft touch of previous elastomers-but with a better adhesion to the substrate than a mechanical bond alone would provide. Other uses under investigation include sound damping laminates. "You could use the new grades whenever you want to marry a flexible, colorable elastomer with rigid structural components," Hill concludes.
|Sparing the adhesive doesn't spoil the bond|
|Bond strength, pli|
|Substrate||Santoprene 80TB||Santoprene 85TL|
|Etched Teflon (Acton Type II)||22||19|
|Borosilicate glass (primed)||12||15|
|Carbon fiber fabric||29||32|
|(Source: Advanced Elastomer Systems)