The latest ANTIMICROBIALS for plastics

Microbes may be nothing more than tiny single-celled organisms, but they’re harder to kill than Rasputin. They’re resilient enough, in fact, that scientists have revived them from fossils more than 250 million years old. They also like to multiply, doubling in number every 20 minutes under the right conditions.

And while microbes have their benefits — we can thank their fermentation mechanism for beer and wine, for example — in a hospital environment they’re deadly. In Canada alone, up to 10 per cent of hospital patients acquire post-operative microbe/bacterial-related infections, resulting in thousands of deaths annually.

Sounds like a very good reason for the medical industry to increasingly turn to antimicrobial plastics for applications that can protect against pathogens while remaining cost-effective. Antimicrobials are added to plastics for two primary purposes: as an active biocide to kill germs, and as a biostabilizer/preservative for the plastic. The difference? Plastics with active biocides are often formulated for use in implantable medical devices and items with known high infection potential such as catheters. The basic requirements for antimicrobials used as either biostabilizers or active ingredients are low toxicity to humans, animals, and the environment; compatibility with processing aids and other additives; and — needless to say — no negative impact on the properties or appearance of the plastic article.

There’s no doubt that antimicrobials will continue to play an increasing role in protecting plastics from damage by microbes — but it’s worth asking which types of antimicrobials. The chemistries used for biocides compounded into plastics have been undergoing major changes since the 2013 phase-out in Europe of the traditional workhorse biocide, oxybisphenoxarsine (OBPA), due to concerns about toxicity. OBPA is still currently allowed in Canada and the U.S., but for how much longer is anybody’s guess.

HI-YO, SILVER

There’s a proven successor antimicrobial material to OBPA already in place: silver. Silver and most silver compounds are toxic for bacteria, algae and fungi by inhibiting the essential functions of the cell walls and interfering with the functions within the cells. That’s the good news. The bad news is that, since silver-based antimicrobials require higher concentrations to be effective against fungi and algae, they’re not particularly cost-effective alternatives to OBPA for traditional plastic protection.

But with the pressure on to replace OBPA — just in case — the use of silver-ion antimicrobials is nevertheless growing in medical applications. And material suppliers are stepping up with new silver-based formulations. BASF, which offers Irgaguard silver-ion antimicrobials for industrial applications, supplies fully compounded polymer systems containing medical-grade, silver-ion releasing HyGentic additives for medical applications. Grades include HyGentic SBC (styrene butadiene copolymer), HyGentic PA (glass-filled nylon) and HyGentic SA (acrylic modified PS); custom formulations can also be developed in a wide range of plastic materials. And the company is currently developing products that provide added functionality beyond antimicrobial performance — for example, a PU compound with both anti-thrombotic and antimicrobial performance for vascular access devices is in advanced development.

PolyOne Corporation’s WithStand antimicrobial additive solutions, available since 2010, are targeted for medical devices, medical packaging, and other healthcare applications. Last year, PolyOne launched Smartbatch HC additive concentrates, which combine WithStand antimicrobials and OnColor HC colorants for healthcare applications.

Also last year, Clariant added Sanitized MedX silver antimicrobials to its line of Mevopur masterbatches for medical applications. The materials, which can be combined with pigments, are targeted for such applications as catheters, surgical instruments, and preservative-free pharmaceutical packaging.

Plastics Color Corporation (PCC) recently introduced its silver-ion based MicroBlok antimicrobial compounds, targeted for medical devices and  medical packaging applications, as well as consumer markets. “MicroBlok is engineered into resins during our compounding process, eliminating the need for secondary steps and additional material modifications on the customer’s part,” said Tim Workman, PCC’s vice president of business development technology. “The product line is custom-blended in a wide variety of resins, including TPU, PC, ABS, PP and PE. MicroBlok is appropriate for virtually any molding or extrusion application, and can be custom-formulated for any special manufacturing process.”

In 2013, Sabic Innovative Plastics launched nine new antimicrobial compounds featuring silver technology, each of which has been tested for log reduction values according to ISO 22196-2007. Five of the grades provide a high antimicrobial effect with log reduction values above 4, representing more than a 99.99 per cent reduction in pathogens, the company said; the other four compounds have a lower antimicrobial effect with log reduction values below 4, representing a 99.0 to 99.99 per cent reduction in pathogens. The new grades are based on the four Sabic resins that are most commonly used in medical devices: Lexan EXL copolymer, Lexan PC, Xenoy PC/PBT, plus PP resins with and without glass fibre reinforcement. Target applications include fluid and drug delivery systems, surgical instruments, and monitoring and imaging devices.

But not quite every new antimicrobial is silver-based. Americhem Inc. recently developed the nShield line of antimicrobial masterbatches in partnership with additive supplier Life Material Technologies. Targeted for use in sanitary applications for kitchens, bathrooms and other functionalities, the product line complies with statutes from the Environmental Protection Association in the U.S. and the Biocidal Products Regulation in Europe. “The nShield masterbatches offer good compatibility with transparent polymers like PC, acrylic, SAN and ABS,” said Life Material’s chief executive Tom Ellefsen. “In addition to sanitary ware, our antimicrobial additive technologies are appropriate as antifungal agents in high-temperature and transparent polymers, as well as plastics used outdoors. Products are also available for nylon and polyester fibres, as well as products that allow for an ultra-low letdown rate.”

So is the battle against the omnipresent microbe being won? Not exactly, but when it comes to eliminating them as sources of hospital-acquired infections, we may finally be slugging our way towards a draw.