Since the first reported cases of COVID-19, we’ve been actively formulating and testing additives and materials in a bid to understand the factors at work, and identify routes to potential efficacy against coronavirus SARS-CoV-2, which causes COVID-19. In the initial absence of specific testing against this novel virus, we commissioned independent evaluation of a broad variety of active substances and materials against similar viruses, including influenza A, feline calicivirus, feline coronavirus and human coronavirus. The results of these tests have enabled us to develop a clearer picture of the efficacy of different active substances.
It’s important to note that the active substances used in any particular application are typically determined by suitability, but are also restricted by regulations. For example, the pool of ‘approved’ silver-based active substances in Europe or the US is limited to just a handful, so most companies in our sector will be using the same active substances as we use at Radical, and as we have tested. Whilst novel biocides are of interest, unless they have regulatory support they cannot be used in Europe or the US. For that reason, we have focused our attention on the active substances most commonly used in polymers and coatings: silver phosphate glass, silver zinc zeolite, zinc pyrithione and silver nitrate.
In order to evaluate these active substances, we first needed to determine the level of efficacy that would constitute a ‘meaningful’ result i.e. one which could be considered sufficient to translate into a tangible real-world effect. Based on discussions with leading laboratories, including IMSL and Virology Research Services, we concluded that the ISO21702 requirement of 90% reduction would only be useful if it was achieved within 2 hours, rather than the 24 hours specified by ISO21702. Indeed, the value of a 24 hour antiviral test, both in terms of consumer perception and actual benefit, is questionable.
For this reason, our criteria for the ISO21702 testing are…
Minimum: 90% reduction within 2 hours
Desirable: 99% reduction within 2 hours
Optimum: >99.9% reduction within 2 hours
The independent tests we have commissioned to date have demonstrated that it is extremely difficult to achieve meaningful antiviral performance in moulded or extruded polymers at an economically viable level of active substance. For example, even when used at 5% by weight in polypropylene, the most commonly used active substances (silver phosphate glass, silver zinc zeolite or zinc pyrithione) were unable to achieve a reduction of more than 80% of Human coronavirus within 2 hours, which we believe to be inadequate. It is worth noting that these levels would increase the cost of the moulded material by a factor of 5-10 times, dependent on the active.
In a thin coating – perhaps 5-10 microns thick – the entire active additive content is at or near the surface, ensuring it is all available to contribute to the antimicrobial effect. In a moulded piece, a significant proportion of the active substance cannot be close enough to the surface to contribute. For good antibacterial performance, the level of active substance required is relatively low, making it less expensive to use an additive in the polymer than undertake a secondary coating operation. However, given the very high levels at which the same additive must be used to provide anti-viral efficacy, the cost of a coating begins to look far more appealing.
To date, we have achieved a reduction of 99.9999% against Bacteriophage Phi6 in 3 hours and 93% of SARS-CoV-2 in 2 hours.
It is important to bear in mind that the additive which delivered this level of efficacy in these tests may well perform differently in another coating. This is always an important consideration with any antimicrobial additives, and one on which we are very happy to advise.