Safe antimicrobial technologies meet great industry need

The world around us is teeming with microscopic organisms such as bacteria, fungi and yeasts. In fact, scientists say that life as we know it would be impossible without microbes. 

Some microorganisms are highly beneficial to humans, helping us digest our food, decomposing our waste, and playing a key role in production of fermented foods like cheese, yogurt, beer, wine, vinegar and seitin. 

However, other microbes can make us ill, cause ferrous metals to rust and food to spoil, or lead to stains and odors that can damage products and property.

As a result, the demand for antimicrobial (AM) products is growing rapidly, with more than $1 billion now spent annually on biocides for items as diverse as food-processing conveyor belts, liquid soap, health and beauty aids, medical wound-care devices, catheters, and rubber mats.

By virtue of their function and end-use environment, certain products and applications are more prone to damage and deterioration from microbial growth than others. Food and beverage handling/processing equipment – whether a cutting board in a restaurant, appliance in the kitchen, or conveyor belting and rollers in a factory – are prime candidates. The same can be said about many packaging, medical device, and drug-production systems. Products used in wet or damp environments are also at risk. These could include anti-fatigue/anti-slip mats in medical and care settings and restaurants; caulk and weather-stripping for windows and baths; and, gaskets and seals in filtration systems. Furthermore, components located in hard-to-service areas of equipment provide similar challenges. Users must often disassemble machinery to clean these parts, or replace them entirely due to microbial growth.

Because of their chemical composition, certain materials are also more likely to support the growth of bacteria, yeast and fungi. Under certain conditions, rubber compounds are particularly vulnerable to attack due to their chemical constituents, which provide an ideal source of nutrients to support growth and reproduction of microbes. Additionally, rubber parts are often used in warm, moist environments rich in microorganisms that provide ideal conditions for the proliferation of these life forms. This, in turn, can lead to cleaning challenges requiring significant equipment downtime and disassembly and premature deterioration of molded rubber parts.

In the battle to control microorganisms, the best approach has traditionally been to create multiple hurdles to discourage growth and reproduction of these life forms. Hence, in applications most prone toward and sensitive to microbial buildup, not only are chemical disinfectants used regularly to keep surfaces clean, but manufacturers also – for an extra measure of protection – often incorporate antimicrobial agents into the product’s material matrix to further inhibit growth within and on the surface of the part.

Originally developed for use as preservatives in liquid systems such as latex paint, deodorants, shampoos, liquid soaps and other surface disinfectants, some of these chemicals have subsequently been adapted for use in solid materials such as rubber. However, use of integral organic biocides has been characterized by a number of shortcomings, virtually all of them linked to a tendency of these biocides to migrate out of the polymer matrix.

Since these particular chemicals have a tendency to bloom to the surface and subsequently leach out of the material in which they are incorporated, not all areas of a product are protected, leading to zones of inhibition and non-inhibition. Furthermore, the microbe-suppressing properties tend to diminish over time as the additive migrates away, with some parts losing efficacy after only a few cleaning cycles or uses. Additionally, as these chemicals move out of the material, they can impart a disagreeable taste and/or smell to a molded part.

Of greater concern, although they are often highly effective in controlling or suppressing microbial growth, many health experts question the safety of exposing humans and the environment to these chemicals. Consequently, some organic biocides have been deemed unsafe for use in medical and food-contact applications due to their inherent toxicity and the increased risk of exposure when the biocide leaches from the treated article. For example, a number of environmental and health organizations are now calling for legislation to limit the use of organic biocides, in particular encouraging consumers not to purchase certain products containing triclosan. Still other organizations have mandated a global ban on anti-fouling paints containing organo-tin by 2008 for marine applications.

Additionally, the majority of these organic biocides degrade at elastomeric processing temperatures exceeding 250C. In the case of triclosan and its chlorinated derivatives, heating can cause the chemicals to volatilize and corrode rubber-molding equipment. In certain situations, they may break down into dioxins (or dioxin-like molecules), a well-known class of highly toxic compounds.

Owing to both chemical composition and typical end-use operating conditions, there is a pressing need for safe and effective antimicrobial control of rubber parts used in numerous markets, including food-processing, packaging, medical and industrial applications. Recognizing this need, chemists at the Milliken Chemical Speciality Elastomers (MCSE) Business have developed a unique and patented family of heat-cured antimicrobial rubber compounds called Elastoguard. These materials contain a very special antimicrobial agent that avoids the problems traditionally associated with organic biocides, yet provides much more effective protection of rubber parts than untreated compounds. 

In rigorous testing, these new elastomers have demonstrated excellent incremental and long-term prevention of bacteria, yeast, and even fungi on both the surface and inside the treated material. This performance has been maintained even after repeated surface abrasion and washing. In fact, testing has shown that performance is actually enhanced over time by usage and by other actions that occasionally scuff the surface of the part, exposing more of the antimicrobial agent. 

The new and patented biocide used in Elastoguard compounds – Antimicrobial Alphasan – is a silver-sodium-zirconium-phosphate (silver-NZP) ion-exchange resin. This product permeates the entire rubber part, so it provides protection from skin to core without the zones of inhibition and non-inhibition common with organic biocides. Furthermore, the new silver-based agent is non-migratory, so parts achieve much more thorough and lasting protection. It is also extremely heat stable at temperatures exceeding 800C.

The new biocide works by slowly releasing silver ions, which are replaced in the NZP matrix by other common, positively charged ions via the process of osmosis. Once released, the silver ions are absorbed by the microbes (along with other essential ions), where they begin to interfere with the formation of vital enzymes used in energy production. This rapidly causes microorganisms to lose their ability to grow and reproduce. 

While highly effective as a biocide, silver-NZP is considered safe, non-irritating, non-toxic, does not affect color stability in light-colored compounds and requires no special handling during processing or subsequent use of those parts. In fact, the silver-based biocide has attained numerous regulatory registrations around the world for contact with food and potable water and for use in medical devices. Silver itself has a long history of use as a biocide, as well as millennia of use in utensils, cups, jewelry, and other decorative and functional devices. Today it is used in numerous medical treatments. Moreover, unlike antibiotics and certain organic biocides, silver’s mechanism of action against microbes is very non-specific, making it extremely difficult and rare for microorganisms to develop resistance. 

Elastoguard antimicrobial heat-cured rubber compounds are now commercially available from Milliken Chemical in various chemistries ranging from natural rubber to fluoroelastomers, and Shore A hardness values from 40 to 90. Within various product families, the compounds are offered in many presentations (forms), sizes, thicknesses, and formulated for molding, extrusion, or coating processes for customer convenience. All product lines feature sulfur-free peroxide cure systems, since sulfur tends to complex with silver, rendering it ineffective as a biocide

Milliken expects that potential uses for Elastoguard antimicrobial rubber could include coatings, seals, gaskets, O-rings, wheels, casters, diaphragms, bladders, packings, liners, tubing and belts for applications such as:
• Medical, pharmaceutical, food, dairy, and beverage processing and packaging equipment
• Large and small appliances
• Storage tanks and containers
• Filtration and fluid handling equipment
• Sporting goods, clothing and footwear
• Sanitary fittings for shower/sauna/bathrooms
• Conveyor belts and rollers
• Anti-fatigue matting
• Building materials/roofing membranes
• Pool/pond liners
• HVAC ducting in automobiles

Milliken & Company is a diversified international textile manufacturer based in Spartanburg, S.C. The company’s 14,000 associates work from 65 manufacturing facilities worldwide to produce more than 38,000 different textiles and chemical products for a broad range of industries. For more information on Elastoguard antimicrobial heat-cured rubber compounds, visit www.milliken.com.

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