Beware not to create unnecessary post Covid19 materials disaster!

The problem:

We are currently experiencing an increasing focus on using UV-light to decontaminate surfaces on equipment, devices and even whole rooms. This focus has been dramatically accelerated by the Covid19 pandemy, since UV light has been shown to inactivate for SARS-CoV-2. However, it seems that it is generally not considered that many of the materials that are decontaminated with this method are not designed for exposure towards UV-light.

From our experience with UV resistance of materials, we’d like to raise a flag and put some focus on the possible issues with the use of UV light on materials that are intended for indoor use.

The technical background:

If you have ever taken a look at kids toys in a sandbox, you might have observed that the color of many plastics rapidly gets faded and the material gets brittle after lying outside for a given period. This is due to photo-degradation, caused by the UV light from the sun. Generally paint, polymers, elastomers and many textiles are prone to this. Metals and ceramics are generally not affected by UV light (unless they are coated, off cause).  In order to give a very simplified explanation of what happens, you can imagine plastic materials as an inter-tangled structure of long chains, sort of like a bowl of spaghetti. The polymer chains can having different lengths and they can bond to each (termed “cross links”), which all influences the overall properties of the material. The UV light can interact with these chains and break bonds, so that the length of the chains get reduced or break the cross links. Each time this happens, the properties of the material is altered a little bit. The amount of such reactions, depends on a lot of parameters, including intensity and spectral distribution of the UV light, exposure time and also the presence of water has an influence.

Polymers are not just polymers when it comes to UV-light, some have very poor resistance to UV light (ex. ABS), while other have far better resistance (ex. polycarbonate). In order to mitigate poor UV-resistance of a polymer, UV-stabilizers are used, which increase the properties for outdoor use. These are typically antioxidants, which block or reduce photo-oxidation reactions. So you might encounter ABS products for outdoor use that are perfectly fit the intended use, but they are so, only due to design considerations of the manufacturer.

Figure: Example of a polymer surface that has been exposed to accelerated UV light exposure simulating indoor use (just inside a window). The left side was shielded from the UV light, and the right side was exposed. Microcracks due to photo-degradation  can be seen (marked by the black arrows).

Also the fillers and additives that are used in a material are chosen with regards to the use for indoor or outdoor exposures.  As an example, white pigment for paint and plastic is very often titanium dioxide, TiO2, as it has excellent hiding power and low cost. Chances are that the vast majority of white wall paint you see around you has it’s white color due to the TiO2 pigment, and also for most plastic housings, coatings etc.

The TiO2 pigment comes in different crystal structures, mainly rutile and anatase. When synthesizing TiO2 pigment, the anatase structure is formed at a lower process temperature, and is hence a little bit cheaper than rutile, which is only obtained if the temperature is increased. Although both anatase and rutile are TiO2, and as a pigment posses almost identical properties for providing the white color, their properties are dramatically different when exposed to UV light. The anatase form of TiO2 is highly photocatalytic, and is used widely for its self cleaning properties when exposed to UV-light. In a simplified way, it can be said that it burn all organic material in its vicinity when exposed to UV light, and it does so extremely efficiently.

A paint manufacturer would always use a stabilized (not photocatalytic) pigment for outdoor use, as they know that the non-stabilized pigment will burn the surrounding binder and cause the paint to chalk if used outdoors. Stabilized TiO2 pigment has a very thin coating of an inert material that blocks the photocatalytic activity, or, for some cases, it can be rutile that has significantly less photocatalytic activity. For indoor use, this does not apply and since UV stabilization adds to the cost, it is often not considered.

The implications of decontaminating by UV light:

Let us start by stressing that using UV light can be very good and cost-efficient way of decontaminating various items, and for avoiding transfer of infections between individuals.

But the concept of using UV light repeatedly to decontaminate items needs a risk assessment. Especially the rising concepts of robots and autonomous vehicles with UV light, that are used for regular decontamination of hospitals, production facilities etc. might cause devastating material issues over time. The consequences will typically be chalking of the surface, increased surface roughness, fading colors, cracks and embrittlement and/or of the material.

So, the use of UV light for decontamination should be performed with caution and taking the risk imposed to the materials in consideration. If you are using UV light on metallic surfaces, you are most likely on the safe side, expect few cases such as colored anodized aluminum, where some color dyes will degrade in UV light. Apart from metals, ceramic surfaces are also generally safe as well, unless both material groups have a functional surface coating that is organic.

Materials where you need to be cautions, and where a risk assessment should be performed, include (non-exhaustive):

• Plastics
• Elastomers
• Surfaces coated by organic coatings/paints
• Textiles

We hope that this input will assist you in avoiding unforeseen consequences of decontamination by UV-light. Stay safe!