The properties of silicone rubber mean it is used in many applications in which antimicrobial properties are beneficial, such as medical devices, food moulds, kitchenware and construction materials. While it is relatively straightforward to achieve good antimicrobial efficacy in most grades of silicone, it is important to consider the potential interaction of the active substance with components of the silicone, particularly the catalyst or accelerator.

One of the most common problems we are asked to overcome is discolouration of antimicrobial silicone. In most instances, the silicone has turned yellow or brown, sometimes immediately after curing but more often after a few weeks or months.

Often, the discoloration has been masked, temporarily, with pigments or optical brighteners. We have even seen the discolouration rather disingenuously passed off as “confirmation that the antimicrobial is working”. Ultimately, the discolouration is a result of an adverse reaction that obviously affects the appearance of the silicone, but also in all likelihood will lead to compromised antimicrobial efficacy. It’s also, most cases, entirely avoidable.

Generally, the reaction is between the antimicrobial active substance and the catalyst/accelerator. Determining which active substances are likely to lead to discolouration is very straightforward, using a simple compatibility test.

While relatively uncommon, it is not unknown for antimicrobial additives to be adversely affected by some pigments, particularly those containing sulphur or chlorine. When working with silicone it is imporant to evaluate efficacy with each individual colour.