UV Resistance in Sealants

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Which sealants have the most UV resistance? Why is UV resistance important? What exactly is UV?

Let’s start with the basics. UV is short form of Ultraviolet. Light has UV rays, especially sunlight. UV resistance means the capability not to degrade or alter in the presence of UV.

This is important especially in outdoor applications, since most sealants are exposed and will face UV for most of its working life.

It may not be equally important indoors, but never understate the amount of UV an indoor sealant will face. It may be substantial.

The Damaging Effects of UV

UV has the ability to break down many things. Ever noticed a faded label or picture left out in the sun? Or even certain degradable plastic bags getting split apart? These are all the effects of UV.

Sealants are the same, and most have to be engineered for outdoor usage by resisting sunlight.

Despite this, some sealants will still be damaged and affected by UV after some time. This can range from weeks to years.

Sealant Types and UV Resistance

Here’s a brief breakdown of the sealant types and their resistance to UV.

High quality silicone sealants

Very good resistance. Almost no effect even after many years.

Poor quality silicone sealants

Normal resistance. Additives mixed into silicone polymers (to make it cheaper) will degrade in the presence of sunlight. Sealant will start getting chalky and rigid after a few months, and lose adhesion, making it very easy to peel off. Clear silicones will start turning more and more translucent and yellowish. White silicones may turn yellow.

Acrylic sealants

Good resistance. Can last years with minimal negative effects. However, poorer acrylic sealants with high water content or poor formulations (with added calcium carbonate to make it cheaper) may start cracking. This may not necessarily be an effect of UV – just heat ageing. They are somewhat correlated.

Hybrid polymer sealants (Soudal SMX hybrid polymer)

Very good resistance. Almost no change to integrity of sealant over many years. Minimal effect on adhesion.

PU sealants

Poor resistance, universally. Even high quality PU sealants will degrade in the presence of UV over time, as polyurethane as a chemical is susceptible to UV. Sealant will start to crack over a few months to years.

Conclusion

It may come as a surprise that PU sealants have poor resistance to UV, given their prevalence of use in exterior joints and expansion joints. Back then, it was more a necessity as PU sealants have better adhesion to exterior elements such as concrete, but in these days, hybrid polymers are widely available at cost-effective prices, which should replace PU sealants for such applications. However, traditional thinking has inertia, and change has been slow. Maybe more importantly is the price difference between PU and hybrid polymers – many companies and contractors are only interested in keeping a sealant in good shape for the first year, and so they always pick the cheaper choice.

Suitable substrates for Silicone sealants

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In our experience, many general purpose silicone sealants claim to be suitable for most common building materials. While this isn’t wrong, there are limitations to the adhesion of silicones on different materials.

The important note: silicone sealants can adhere to most materials, whether porous or non-porous. This includes glass, metal, wood, concrete, cement, stones, plastics etc. However the adhesive strength of silicone sealants can differ greatly among these materials.

There’s a general rule to this: silicone sealants have better adhesion to non-porous surfaces like glass and metal, and poorer adhesion to porous surfaces like wood and stone. This is not always 100% accurate (for example, plastics are non-porous but silicones do not adhere well to them), but it’s a good rule-of-thumb.

If there is no specific requirement for adhesive strength, it’s still possible to use silicones almost everywhere. However, if there are mechanical forces at work in the assembly, it may be better to use different sealant technologies if one or both of the substrates are porous.

The substrates where silicone sealants have the best adhesion to is glass and metal. This is why silicone sealants are so prevalent in window assemblies. It also has good adhesion to tiles and other bathroom fitting materials, which makes silicones a good choice in the bathroom.

For everything else, PU and hybrid polymer technologies would usually make a better choice. PU sealants have much greater adhesive strength to concrete, for example, but poorer adhesion to glass. In many ways, PU sealants and silicone sealants complement each other greatly, having good bonding strength to materials the other doesn’t.

However, at the top of the sealant food chain are still hybrid polymers, especially those based on Soudal’s SMX hybrid polymer technology. SMX polymers have greater adhesion than both silicones and PUs on a wider substrate spectrum. For most applications, you really can’t go wrong with hybrid polymers.

Still, for glass and metal, save your costs and stick to your tried-and-tested silicone sealants.