A Bugs Life

While Interest in antimicrobial additive is increasing, converters are still working on how they can develop real applications from the technology.

Interest in antimicrobial additives continues to grow. But as delegates at European Plastics News's Antimicrobials 2003 conference heard recently while the basic antibacterial performance of many additive is clear, there is still work to be done in understanding how that performance can be exploited in real applications.

One key area for future development is that of standards and testing methodologies for polymer applications.

Peter Askew, managing director of consultancy Industrial Microbiological Services, told delegates: "Definitions, standard test protocols and a fundamental understanding of antimicrobial activity in plastic materials is needed if credible solutions for genuine applications are to be realised."

The most commonly used method for determining antimicrobial efficiency is the Agar diffusion test, or variants of it. This qualitative test places a sample of the material to be tested on the surface of a bacterial loaded agar plate. Antibacterial performance is assessed by the size of the bacteria free ‘halo' that develops around the sample after a period of incubation.

Easy Test

Agar Diffusion is a simple and easy test and can show dramatic results. Critics say that these may not relate well to real world applications. Key areas of concern include the fact that the sample is in direct and aqueous contact with the bacterial agar. But the test also takes little account of the speed at which the antimicrobial additive functions and provides no measure of the timeframe over which the activity can be maintained.

Hong An Duong, manager of microbiological food safety and quality at TetraPak Research in Germany, says there is opportunity for manufacturers of antimicrobial additives and packaging to select tests that show their product in the best light.

He said that the first step towards more realistic testing must be to move to a quantitative methodology, such as the AATCC Viable Count Test Method, which is based on the film adherence principle. This involves applying a bacterial sample to the surface of the test material, covering it with a glass slide and then counting the surviving bacteria after incubation.

However, proving effectiveness does not necessarily prove suitability for a real application. For a TetraPak type application the antibacterial additive would have to maintain an antibacterial capability for the entire shelf-life of the product, which could be many months.

But other applications may have very different requirements. Trevor Lawson, research and director of hygiene services group Rentokil Initial, explained that in his company's area of business applications may call for bacterial elimination within a few minutes.

Lawson said that, as a potential end user, he was looking not only for standardized test methods that modeled real environmental exposure conditions, but also a degree of objectivity on the part of suppliers. In particular, he said there is a need to substantiate claims and highlight limitations.

Bob Hartog, senior microbiologist at TNO's nutrition and food research unit in the Netherlands, also highlighted the importance of adopting a sensible testing strategy on the part of the user. He recommended adopting a three-tiered approach: first test the basic antimicrobial performance of the additive system; then look at specific performance in simulated end use scenarios finally move on to durability and field trials.

Much of the conference focused on food packaging applications particularly active packaging solutions, in which the additives in the packaging are designed to interact with the packaged product to extend its lifetime. This revealed a big difference of opinion between some of the antimicrobial suppliers with some of those marketing or working with migratory additives hoping for changes in EU regulations that will open the way for such use.

But conference delegate Paul Morris, managing director of silver based antimicrobial masterbatch supplier Addmaster considered this hope misplaced. "It seems clear to me that the way to get food approval without [the antimicrobial additive] being classed as a preservative is to show no migration." he said. Many suppliers of additives that have high migration still hang on to the belief that their product will be okay without listening to facts. Even silver, including our ultra low migration product, will have its work cut out to prove the fact."

New Technology

While many of today's antimicrobial products are intended for incorporation into the bulk polymer there are still many attractions to using the products in coating form.

Avecia is just one company exploring the use of antimicrobial coating systems for plastics, claiming a much faster antibacterial performance that possible with compounded technology.

According to the company's biocides research and development manager Robert Sloss, incorporation of its Vanquish S antimicrobial into PVC or PP polymers can result in plastics that provide substantial reduction in bacterial numbers over a period of 24 hours. However, using the same product in a coating can give similar reductions within five minutes he said.

Sloss said that the company is developing coating formulations that use a variety of polymer systems to bind antimicrobial additives such as PHMB (polyhexamethylene biguanide) to the surface of the plastics part. These systems can be formulated to control the rate of antimicrobial release and to provide resistance to specific environmental factors.

Avecia is not yet ready to offer a commercial coating product. Sloss said that systems will - at least initially - need to be developed to meet specific end user requirements.

Addmaster believes the medical device sector will provide rapid growth opportunities for its range of silver based antimicrobial products over the next two years.

Managing director Paul Morris said that the Addmaster materials are already being used within the UK's National Health Service to help control cross infection risk. Feedback from companies supplying products using the Addmaster products into medical applications in other European countries is also positive.

However, Morris says that the ongoing success of antimicrobials in the medical sector rests, to at least some part, on avoiding the confusion over regulations that has held back antimicrobial development in the food packaging sector.

Selecting an antimicrobial additive for a particular application can be complex and time consuming. At Antimicrobials 2003, nano-porous silver additive manufacturer Bio-Gate detailed an automated screening system, which it believes can considerably shorten the process.

According to Bio-Gate managing director Thorsten Bechert, its screening process uses high speed automated optical measurement technology to determine quantitatively the adhesion and proliferation of microbes onto the surface of an antimicrobial containing plastic.

The Bio-Gate system is based around Elisa (enzyme-linked immunosorbent assay) technology. The company claims the system is suitable for product optimization and QA purposes.

Bechert says that the new screening system will play a key part in the development of its Nano-Silver BG business, which he says uses nano scale dispersions of elemental silver to provide antimicrobial performance in a range of medical and food packaging plastics applications.

Source - Europen Plastics News, January 2004

Biomaster is a registered trademark of Addmaster, based in Stafford UK. Addmaster is Europe's leading supplier of performance additives for a wide range of applications and industries. By working in partnership with its customers and focussing on research and development, highly innovative additives have been created and successfully introduced into many manufacturing environments. Addmaster's advanced products are being used by many of the world's industry leaders in the plastics, paper, paint, textile and coating industries.

Addmaster, founded in the year 2000, sees its major objective as capitalising on the lack of innovation which exists in these industry sectors, and has won many business awards for innovations in product development and marketing.