The true nature of Polyvinyl chloride’s proclaimed antimicrobial properties
Polyvinyl chloride (PVC) is one of the most abundant polymers; with about 40 million tonnes1 produced annually, it ranks third in terms of global polymer use. PVC is frequently utilised in a range of applications – from vinyl records to construction materials like window frames, drainpipes, and flooring, as well as in food packaging and medical equipment like bags and face shields. It can be formed into a variety of intriguing shapes and easily pigmented. However, this very practical plastic is not popular only for its high versatility, but also for its proclaimed inherent antimicrobial properties. Our Technical Team took a closer look at why this property of PVC could be more complex than one might assume.
What is Polyvinyl chloride (PVC)?
PVC is a long chain of carbons, with one chloride attached to every other carbon. It is quite a simple structure, which can repeat tens of thousands of times in one single chain. PVC itself is a white, granular powder that can be melted and formed into shapes. These temperatures can be quite high, usually around 180 °C. It is thermoplastic, which means that once it is shaped, it can be reheated and remoulded. However, PVC will begin to decompose at around 135 °C and can immediately discolour – from pink to brown to black – and become brittle if exposed to these processing temperatures without a heat stabiliser. Heat stabilisers can work in a variety of ways to protect the polymer, but their essential function is to ensure that the PVC gets to the end mostly intact.
Hydrogen chloride: a by-product of PVC breakdown
An extremely potent acid called hydrogen chloride can be released as a by-product when PVC breaks down. Due to the high temperatures, it is expelled as a gas; a release which can also trigger further breakdown of PVC chains. This is referred to as a self-catalysing reaction – the by-product of a reaction can initiate the same process and create an accelerating cycle. Hydrogen chloride, however, can become trapped in between the PVC chains and eventually slowly release itself from the product over time. This makes hydrogen chloride a Volatile Organic Compound (VOC) as a result. VOCs are substances that slowly emit from a variety of materials and have been linked to illnesses including sick building syndrome2. Currently, a variety of regulations are in place to reduce human exposure to VOCs.
Understanding how Hydrochloric acid makes PVC antimicrobial
Because of the powerful biocidal nature of hydrochloric acid (HCl), PVC is recognised for having what are described as “inherent” antibacterial qualities. This implies that PVC is naturally antimicrobial and cannot support the growth of bacteria or fungi on its surface. However, this is not entirely correct. The Volatile Organic Compound that is hydrochloric acid (HCl) is antimicrobial; PVC is not. Microbes cannot survive on a surface of a product which is actively releasing HCl. This release, however, is not a permanent feature of PVC. Due to restrictions on the amount of VOCs allowed, the amount of released HCl gradually decreases to a level where it is no longer efficient in killing bacteria. As a result, there is nothing stopping microbes from flourishing on the product’s surface.
Microbes are able to extract nutrients out of polymers such as PVC in order to not only survive but grow, with some microbes having the ability to double in count in as little as 20 minutes. This poses a potential issue for the PVC product since microbes are now causing it to deteriorate. There is also a chance that the microbial contamination may cause unpleasant odours and stains. At this point, a long-term solution would have to be included.
Integrating scientifically proven and long-lasting BioCote® antimicrobial technology
BioCote® is aware of many materials, such as PVC, off-gassing and producing biocidal VOCs that can distort the results of many antimicrobial tests, such as ISO 22196, ISO 20743 and ASTM G21. These VOCs can hide any issues contained within the formulations and may hide that our antimicrobial silver is being deactivated by something. Tin, as a popular heat stabiliser, is a common problem in PVC for silver, as one tin atom can reduce 2 atoms of silver to metallic silver. Metallic silver cannot kill microbes, and this behaviour can therefore be hidden by the “inherent” properties seen. This would mean your antimicrobial is not working, and the product continues to be exposed to microbial contamination further down the line, rather than being protected long term.
Because of this, BioCote® requests that materials such as PVC are “aged”. This procedure heats the PVC just enough to allow the release of VOCs like HCl without causing further degradation that would produce more HCl. This is done alongside a control sample, which is tested side by side. If the control sample passes the antimicrobial tests, and is showing antimicrobial efficacy, the samples are aged further. Once the sample is able to grow bacteria/fungi on its surface, then the results of the test are considered valid. Typically, PVC would need to be aged for a minimum of 1,000 hours to release all the VOC content. For us at BioCote®, this procedure has previously revealed formulation problems in products that proved to be antimicrobial, but only for a short-term period. We have worked with these manufacturers to address such issues in order to guarantee successful long-term antimicrobial protection that our global client base has come to rely on.
The difference between BioCote® and PVC’s inherent antimicrobial properties
While VOCs cannot be avoided through polymer processing, it is important that they are not ignored. If antimicrobials are being incorporated, it is important to make sure the technology is working efficiently and continuously in the product. Otherwise, why is it there? You cannot make antimicrobial claims on products that are “inherently” antimicrobial, while there is a genuine possibility of microbial damage to the product that can start any minute once it has left factory doors. At BioCote®, we ensure the best quality of our products and yours, and can take all the extra steps needed to provide you with data that can independently demonstrate that the antimicrobial technology is effective not only on the day it was manufactured into your product, but even up to 25 years.