There are several different factors, which can affect UV cure on other materials such as adhesive. That is why users should consider the application of the right equipment on the curing process. This article addresses, in particular, the factors that influence adhesive UV curing.
Intensity of UV-Light
Different types of UV lamps can be used to cure adhesive. “Spot” lamps have a light guide with a concentrated beam of intense UV light. These are extremely powerful and can cure UV adhesive in one or two seconds. UV-fluorescent tubes can be cost-effective, but a slower way of curing UV-adhesive as the intensity of UV light is much lower.
Type & Age of Bulb
Depending on how versatile curing equipment is, there are different types of UV light bulb available. Their outputs give peak intensities at different wavelengths allowing users to match the bulb with the UV-adhesive optimum cure wavelength. Another important consideration is the age of the bulb. Although the lamp still appears to illuminate, UV-light bulbs have a limited lifespan as they lose their intensity in the UV spectra – it can seem the adhesive is taking longer to cure or is not curing at all.
Distance between Lamp and Substrate
Doubling the distance between substrate and lamp will reduce the light intensity by 75% – this increases curing time considerably. To reduce energy wastage and promote a faster cure, it is a good idea to keep the UV curing light sources as close to the adhesive as possible (this applies to spot lamps and LEDs). Take care not to overheat substrates. Some lamps give off a lot of IR radiation, which can damage certain plastics or cause shrinkage or discoloration in the adhesive from it getting too hot.
Different UV adhesive products have different cure speeds and cure wavelengths. Most plastic bonding UV adhesives have a longer wavelength photoinitiator, which reacts with 400-420nm wavelength and reacts in average daylight fairly well. Glass bonding products also have this feature, which is activated when exposed to UV light of 365-400nm wavelength. It is essential to have a UV lamp with a spectral output that peaks in the optimal range for the adhesive cure.
Thickness Of Adhesive
Depending on the nature of the UV-curable adhesive, depth of cure may be limited. If problems are experienced, it is possible to cure in layers, adding more adhesive then curing again. Alternatively, consider a different UV-cure product with an improved cure-through depth. Type of UV curing light source can also affect the depth of cure. Lamp systems with a high-intensity peak below 365 nm will cure the surface extremely fast, quickly vitrifying the body, blocking the UV light curing source and preventing the material below from curing.
Nature & Thickness Of Substrate
Some substrates reflect or absorb UV light making it difficult for light to reach the adhesive underneath. Most plastics contain a UV stabilizer, which prevents them from discoloring or going brittle in sunlight. Although many plastics appear to be clear and light will shine through them, the UV wavelength for the adhesive cure can be blocked.