Electromagnetic radiation can act both as a series of waves and as a stream of particles that are called photons. Shorter wavelength photons have greater energy than longer wavelength ones.
UV photons have shorter wavelengths so have more energy than visible light photons and are able to power a range of photochemical reactions. The most common photochemical reaction is called photosynthesis. In this reaction, visible light photons and UV photons are able to provide the energy needed for plants to convert carbon dioxide and water into carbohydrates and starches.
UV light can be used to harden particular glues, resins and inks by causing a photochemical reaction inside those substances. This process of hardening is called ‘curing’.
Apart from UV curing, there are a number of ways to harden (or set) substances such as glues, resins and inks, including:
- dissolving the substance in a solvent and then letting the solvent slowly evaporate away – many glues work this way
- heating the substance so that it hardens
- mixing in another substance that triggers the hardening process – Araldite is a well-known glue that uses this process.
UV curing has a number of advantages over these methods:
- It is faster – UV curing takes from seconds to minutes to harden a UV sensitive substance.
- It will cure the substance over a wide range of temperatures – the substance does not have to be heated or cooled to a particular temperature.
- It cures only the part of the substance that is exposed to the UV radiation – this means that large areas of the substance can be progressively cured as the UV radiation is moved over the substance.
- There are no problems with ventilation to remove evaporating solvent fumes.
Currently, there is much research into developing more types of resins and glues that are sensitive to UV and can be hardened in this way. One drawback with UV curing is that the substance needs to be relatively transparent so that the UV radiation can penetrate into the substance and cure it completely on the inside. UVC has shorter wavelengths than UVA so is less able to penetrate into a substance. UVC is used to harden surfaces such as vinyl flooring to make it more hard-wearing. UVA is used to harden thicker resins or resins that have white or coloured pigments mixed in them.
For a number of years, dentists have used UVA to quickly set small fillings made from a UV-sensitive resin. More recent developments in the type of resin used has enabled those fillings to be set using blue light with no UV content. This reduces the possible harmful effects of UV radiation on gums and other tissues in the mouth.