UV-curable adhesives are single-part products that cure in seconds when exposed to UV light. They are normally optically clear for “invisible” bonding of glass and certain plastics and cure to form a high-strength, non-yellowing bond. Due to their rapid on-demand cure mechanism, they are convenient to use by hand or on fully automated high-speed production lines. There are a number of different factors which can affect UV adhesive cure. It is best to work with your adhesive and equipment suppliers to get a good match of adhesive, light system, and cure schedule.
There are a variety of different types of UV cure lamps. “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. Flood lamps and LED lights can range in intensity (the old adage “you get what you pay for”). UV-fluorescent tubes can be a cost-effective but slower way of curing UV-adhesive as the intensity of UV light is much lower.
Depending on how versatile your curing equipment is, different types of UV light bulbs are available. Their outputs give peak intensities at different wavelengths allowing users to match the bulb with the UV-adhesive optimum cure wavelength. Different photoinitiators inside the adhesive react with different light wavelengths. 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 appear the adhesive is taking longer to cure or is not curing. The problem is easily remedied by replacing the bulb.
Doubling the distance between the substrate and the lamp will reduce the light intensity by 75%, increasing curing time considerably. To reduce energy wastage and promote a faster cure, keep the light source as close to the adhesive as possible (this applies to spot lamps and LEDs). Take care not to overheat the 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. Exercise caution to ensure proper shielding of UV light to minimize exposure to skin and eyes. Flood lamp systems with parabolic reflectors require the parts to be placed as close to the lamp focal point as possible for a faster cure.
UV adhesive products have been designed with different cure speeds and wavelengths. Most plastic bonding UV adhesives have an aditional longer wavelength photoinitiator that reacts with 400-420nm wavelength and will react in normal daylight fairly well. Glass bonding products have a photoinitiator that activates with UV light of 365-400nm wavelength. It is important you have a UV lamp with a spectral output that peaks in the optimal range for the adhesive cure. High speed production items e.g., disposable medical devices such as tubes and connectors, use highly reactive UV adhesives. Slower curing UV adhesives are preferred for bonding decorative glass such as window bevels where users like to remove excess semi-cured adhesive “jelly” during the curing process for easy cleanup.
Some UV-cure adhesives offer dual cure mechanisms, which can influence cure speed. UV adhesives can have a secondary anaerobic cure. This works in the presence of metal and the absence of oxygen.) There are also products with a moisture cure, or a heat activated secondary cure. These adhesive products are suitable for applications where there are shadow areas.
Depending on the nature of the UV-curable adhesive, the depth of cure may be limited. If so, 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.
The type of UV light can also affect depth of cure. Lamp systems with high intensity peak below 365 nm will cure the surface extremely fast; quickly vitrifying the surface, blocking the UV light, and preventing the material below from curing. These systems are ideal for curing thin coatings or UV inks. Lamp systems with high intensity peaks around 385 nm or higher cure the material more uniformly and allow the UV light to penetrate and cure adhesives in thicker sections. The closest the peaks are to the visible range, the easier it will be to cure through larger gaps.
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 crystal clear and visible light will shine through them. The plastic can prevent the UV wavelength from passing through. To counter this problem, there are specialist plastic bonding UV adhesives. These have a longer wavelength photoinitiator that will react through UV-stabilized plastic.
Using a radiometer, it is possible to measure lamp output, dosage, and transmittance through substrate materials. These can offer digital readings of different UV band lengths (E.g. UVA, UVB, UVC.) Some will even generate a spectral output graph which helps determine at what wavelength peak output is achieved.
Permabond chemists are able to recommend or formulate UV adhesives to meet your specific requirements. Then assist in recommendations for curing equipment and production line integration. If you have an application you would like to discuss with a technical advisor or would like to receive further information, please contact help.europe@permabond.com.
