Incorporating antimicrobial properties into products may seem like a magic bullet destined to appeal to the germaphobe in everyone—after all their use is increasing in products we see all around us, from antibacterial soaps to kitchen countertop protection.

A variety of antimicrobial additives are commercially available that can be used to treat a wide spectrum of articles, providing them with additional protection against microbes such as bacteria, fungi, and algae. Treatment methods can range from temporary topical application to permanent incorporation of agents in host substrates.

Even with regular cleaning, it doesn’t take much for microbes to survive or grow on the unprotected surface of articles. Before deciding that antimicrobial treatment is the ideal solution for an application, it’s best to get an understanding of how incorporating them in thermoplastics resins and compounds affects and can possibly benefit components of an appliance.

Capabilities

Enlarge this picture Fig. 1. Chart shows result of a “slipcover” antimicrobial test (ISO 22196), using samples of polyurethane film following a 24-hour incubation period. Results, starting from left, are for untreated control sample, a sample treated with silver zeolite at 1 percent loading, a sample treated with silane-based Biosafe at 0.25 percent loading, and a sample treated with Biosafe at 0.5 percent loading.

With fears of pandemics in the news daily, anything that potentially makes products “safer” seems like a no brainer. However, antimicrobials are intended to keep articles themselves “fresher” or “cleaner.” Making implied health claims or other benefit claims for users of treated products runs afoul of government regulations controlling antimicrobials—which may subject an appliance to additional scrutiny to prove any claims.

Antimicrobials are a class of pesticides that are registered with and regulated by the U.S. Environmental Protection Agency (EPA) under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Treatment of articles with antimicrobial protection can be incorporated into many substrates, including plastic, as a means of combating the degradation effects of microbes such as bacteria, fungi (mold, mildew, and yeasts), and algae. Depending on environmental exposure conditions, untreated plastics can be susceptible to attack by microbes leading to unsightly discoloration, unpleasant orders, and a reduction in the materials physical properties.

Additives offering biostatic properties have been widely incorporated into products throughout Europe and Asia for years, and have recently become increasingly popular in the U.S. as product differentiations made by one company are often quickly followed by competitors to prevent market advantages. Many OEMs have discovered that incorporating antimicrobial properties into their products is a value-add for which consumers are willing to pay more. Antimicrobial properties are currently being added to a wide variety of applications, including household and consumer goods, medical equipment, business machines, and electronics equipment, along with many applications used outdoors.

Proper selection

Enlarge this picture The photos above illustrate the “slipcover” method of antimicrobial testing (ISO 22196). Samples of polyurethane film are shown following a 24-hour incubation period. The control image (A) is an untreated film with undesirable baterial growth. Image (B) shows a sample treated with a silver-based agent at called Alphasan at 0.75 percent loading/concentration, which is less than ideal for this particular additive. It shows some microbe spread and demonstrates the necessity to use antimicrobial additives in the proper amount to obtained desired efficacy. Image (C) is a film sample treated with 0.75 percent of the Biosafe silane additive and shows a complete kill of the microbes. The 0.75 percent loading of the Biosafe additive is optimal in this situation and shows the desired results of the plastic film not providing a host medium for microbe growth when properly treated with a compatible agent.

The first step of incorporating antimicrobial protection into products is learning about what antimicrobial treatments are available. In the case of plastic articles, there are numerous technologies currently on the market, each touting its own set of features and benefits.

OEM product designers and plastic processors often discover that the quickest, most efficient way to wade through the sometimes murky topic of new technologies is to seek out plastic suppliers of additive masterbatches or custom compounds who are experienced in working with the multiple variants commercially available.

Often the first points of discussion will center around what polymer the plastic article is to be made from and what types of environmental exposure it will receive for which antimicrobial protection is being sought. All of these factors greatly affect the functionality and protection that different antimicrobial additives can provide.

Another important consideration is cost. Antimicrobial treatments can range in price from inexpensive to cost prohibitive for some disposable or price sensitive consumer applications.

Even with a qualified partner, companies still need to do their own independent evaluation of suitability and effectiveness in a desired application. Some antimicrobials on the market, which have been in use for years, have environmental or toxicity concerns under certain usage conditions.

The final selection of an antimicrobial technology will depend on a combination of polymer selection, usage environment, desired efficacy, and, of course, price.

Available technologies

Rollers used in food processing application are made from a custom polyacetal (POM) compound developed by RTP Co. and contains a silver-based antimicrobial additive.

Three main classes of antimicrobials are in general use with plastic goods:

Silver-based:
The antimicrobial properties of silver have been known since the Middle Ages. In the presence of moisture, silver releases ions that disrupt the critical life processes of microorganisms, providing an effective means of combating their growth and spread. Silver can be easily incorporated into plastics where it provides inert biostatic properties. Silver is not toxic (to humans in concentrations used to provide antimicrobial treatment), flammable, or corrosive, and it provides long-term protection.

An advantage silver-based antimicrobials have over other technologies is its inorganic nature that provides high thermal stability, up to 500 DegC (932 DegF), allowing it to be used in the widest spectrum of polymers. Some silver-based antimicrobials are FDA compliant for use in various food contact applications and have passed biocompatibility testing for use in medical devices. One limitation of silver is that some commercial additives may limit colorability of products, but this is not the case for all additives of this type.

There are numerous companies that supply silver-based antimicrobials, the main differentiation between products is the amount of active silver in the additive and the method used to deliver the microbe-fighting silver ions to the article surface. They tend to be a more costly solution due to the precious metal characteristics of its primary component and because concentrations in finished goods generally need to be in the 1 percent to 3 percent range to maintain efficacy.

Organic-based:
There are many organic antimicrobials on the market, with triclosan being the most widely used. These types are routinely incorporated into consumer goods and can be used in plastics such as polyolefins, styrenics, and polyamides. The kill method inhibits the formation of fatty acids necessary for microorganisms to develop and reproduce.

As with any organic antimicrobial, triclosan’s main limitation in regard to plastics is its low heat stability of 260 DegC (500 DegF), which limits its use to resins that have processing temperatures below that point. It’s one of the most cost effective antimicrobial solutions, especially in comparison to silver-based options, making it a good candidate for disposable and commodity products.

There is debate within the scientific community concerning adaptation and whether the widespread use of triclosan allows resistant strains of microbes to develop as it’s flushed into or leaches into the environment. Triclosan does not provide any coloring limitations when incorporated into plastics.

While there are many suppliers of triclosan-based antimicrobial agents, the most widely known derivative for plastics is marketed under the trade name Microban.

Silane-based:
The newest entrant is a silane-based antimicrobial additive marketed using the Biosafe trade name. It provides permanent protection against the effects of microbial growth on plastic surfaces by rupturing the cell walls of microorganisms. Silane-based antimicrobials in liquid form for topical application have been widely used for decades.

The Biosafe additive is non-depleting and non-migratory, making it the antimicrobial of choice for applications made from nylon 6, acrylic, and polyurethane. It has passed ISO 10993-5, -10 and -11 biocompatibility testing and has been used in medical devices that have been FDA approved through the 510(k) process.

It is reported to be one of the safer antimicrobials from a material handling standpoint, and any handling effects are mitigated when incorporated in plastics, where it has an extremely low toxicity profile. Effective loadings in plastics are generally 0.5 percent to 1.0 percent, allowing it to be cost efficient. It has no coloring limitations.

Antimicrobial efficacy

Often antimicrobials alone can’t be relied upon to protect plastic articles, but when combined with routine cleaning they can provide a residual defense against the damaging effects of microbes.

After an appropriate antimicrobial technology has been selected, it is strongly advised that efficacy testing be done on an article in its finished form to ensure it provides the desired protection for the product based on its predicted environmental exposure conditions.

Third party microbiological testing labs provide such services, usually with an associated fee. However, some antimicrobial suppliers conduct initial efficacy testing free of charge to get their additive specified into new applications.

Product designers should make sure established ASTM and ISO procedures for testing antimicrobial efficacy in plastic articles are used. Each standard tests the effect antimicrobial agents have on specific types of microbes detailed in the procedure, so multiple tests may be required depending an applications potential usage criteria.

Government regulation

The inclusion of antimicrobial treatment in a plastic article does not allow these products to be marketed as having health benefits for people using them. Since antimicrobials are regulated as pesticides by the EPA, they have issued guidelines as to what marketing claims can be made regarding antimicrobial treated goods.

Manufacturers of products treated with antimicrobials should review Pesticide Release Notice 2000-1 available from the EPA Web site before offering their treated items for sale.

The EPA requires registration of all antimicrobials as pesticides under FIFRA. Antimicrobial pesticides are defined as substances or mixtures of substances used to destroy or limit the growth of microorganisms like bacteria, fungi, or algae on inanimate objects and surfaces.

There’s an exemption to this registration requirement by the EPA if the incorporated antimicrobial is intended to protect the article itself and no additional health claims or other benefits for users of the article are made.

For more information, visit: www.rtpcompany.com