Microbes and rubber go together like kids and candy stores. This affinity springs in part from the chemical makeup of rubber compounds. "They often contain additives that act as nutrients for bacteria and fungi," notes Bhawan Patel, director of development for Milliken Chemical's Elastomers Business (www.millikenchemical.com). What's more, rubber parts often see use in the warm, moist environments that bacteria, fungi, and yeast crave. Now, the company's scientists have come up with a new way to stop these rubber-loving bugs dead in their tracks.
They've developed a family of patented antimicrobial rubber compounds that contain a silver-based additive shown to inhibit, or even halt, the growth of bacteria, fungi, and yeast. This Elastoguard family encompasses the full range of thermoset elastomers—from workaday rubbers like EPDM and NBR to high-performance fluoroelastomers. Shore A hardnesses for Elastoguard range from 40 to 90. Compounds can be formulated for a variety of rubber processes, including molding and continuous extrusion.
And though Elastoguard currently includes thermoset elastomers, Milliken hasn't ruled out the possibility of extending it into thermoplastic materials if there's enough demand. "The technology works in thermoplastic elastomers," Patei notes.
Elastoguard takes a very different approach. It relies on micron-sized crystals of silver-sodium-zirconium-phosphate (silver-NZP) encapsulated within the vulcanized rubber matrix. This additive, which Milliken has named Alphasan, works by slowly releasing silver ions that interfere with microbe growth. Patel says it has several key advantages over traditional organic biocides. Because the Alphasan remain embedded throughout the part, it won't leech or wear away. "Abrading the surface of the part can actually rejuvenate the antimicrobial properties," Patel says. And silver-NZP remains heat stable to more than 800C, allowing it to survive during processing. It also avoids the taste and smell associated with liquid traditional biocides.
And finally, it seems safe. Patel reports that Elastoguard has received FDA approval for direct food contact as well as for use in medical devices. It has also been approved for use with potable water. Elastoguard applications currently under development include food processing, packaging machinery, and medical equipment.
As for the mechanical properties of the Elastoguard compounds, silver-NZP has only an indirect effect. The additive itself is used at such low loadings—generally well under 5%—that it doesn't change the properties of the base rubber, according to Patel. Elastoguard rubbers do, however, require peroxide curing agents rather than sulphur-based agents—because the latter reacts with silver. Patel says peroxide-cured rubbers exhibit compression set improvements over comparable sulphur-cured materials. At the same time, the peroxide-cured rubbers also have a higher modulus, less elongation, and less tear strength. None of the property differences are so extreme that they can't be designed around, he adds.