When selecting antimicrobial additives for use in a polymer, there are six key considerations:
1. Processing parameters
Organic antimicrobial additives tend to have low boiling points relative to plastics, which can lead to an unknown proportion gassing-off during injection moulding or extrusion, leading to uncertainty over the level remaining in the finished product. In contrast, most inorganic antimicrobial additives are more thermally stable, with some even suitable for processing at temperatures over 400°C.
2. Material properties
Incorporating any additive or filler into a polymer will generally have an effect on the material’s properties, The effect may be imperceptible, or even beneficial, but is most often detrimental, resulting in anything from a small shift in colour to a catastrophic loss of impact strength or flame retardancy. This is such a critical consideration, we have written a separate article on the subject.
3. Regulatory compliance
In most territories there is strict regulation of antimicrobial materials, dictating the product types and concentrations at which the antimicrobial additives, or active substances, can be used. In recent years, most organic antimicrobials have seen these permitted concentrations reduced while others have had their usage largely prohibited. While some inorganic antimicrobial additives have been removed from the market, either as a result of lack of regulatory support or because of inherent risks, in general terms inorganic additives would be considered safer from both a consumer and environmental perspective.
While broad regulatory support is important, it is also crucial to confirm suitability for the specific application. For example, while there are numerous antimicrobial additives with EFSA approval for food contact applications in Europe, those same additives may not have the appropriate PT4 support on the European Biocidal Product Regulations or UK BPR permitting use in food & feed area applications.
3. Migration & leaching
Organic antimicrobial additives will typically migrate to the surface of the plastic, often over a relatively short period. This can lead to leaching into the surrounding environment and a reduction in the efficacy. Inorganic antimicrobial additives are typically less prone to migration and leaching, with the associated benefit of greater longevity.
5. Transparency
While this is really just another material property, it can be very difficult to achieve good transparency, so it’s worthy of specific consideration. Broadly speaking, when using antimicrobial additives in a polymer it is only possible to preserve the transparency by using one of a very small range of silver based antimicrobial additives, typically those based on phosphate glass. Closely matching the refractive index of the additive to that of the polymer is crucial, although it may also be necessary to mitigate the discolouration caused by some grades of the additive.
6. Antimicrobial efficacy
Obviously a key requirement, the efficacy should be evaluated using a suitable test method, such as ISO2196. It is important to consider the nature of the environment in which the end product will be used, in case this influences the test parameters. For example, it would be more relevant to test an antimicrobial refrigerator lining at 4°C, rather than the standard 35°C dictated by ISO22196. Similarly, while the standard test organisms of Staphylococcus aureus and Escherichia coli are useful indicators of performance against gram-positive and gram-negative bacteria respectively, in some use cases it is more appropriate to test against specific organisms e.g. Listeria monocytogenes & Salmonella enteritidis for products used in food processing or perhaps pseudomonas
Navigating the process of selecting an antimicrobial additive for your product can be daunting, but we’re here to help at every stage, whether it’s identifying the most appropriate additive, evaluating the material properties, ensuring regulatory compliance, testing the antimicrobial performance or tailoring the material to meet the specific demands of your application.
