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Factors Affecting UV-Cure

UV cure

UV-curable adhesives are single-part  products which 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.  They are convenient to use by hand or on fully automated high-speed production lines  due to their rapid on-demand cure mechanism. 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.

INTENSITY OF UV-LIGHT:
Different types of UV lamp 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. Flood lamps and LED lights can range in intensities (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.

TYPE & AGE OF BULB
Depending on how versatile your 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 (different photoinitiators inside the adhesive are designed to 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 at all. The problem is easily remedied by replacing the bulb.

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 light source 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 discolouration in the adhesive from it getting too hot.  Caution must be exercised to ensure proper shielding of UV light to minimise 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 faster cure.

ADHESIVE PHOTOINITIATOR
Different UV adhesive products have been designed with different cure speeds and cure wavelengths. Most plastic bonding UV adhesives have a longer wavelength photoinitiator which reacts with 400-420nm wavelength and will react in normal daylight fairly well.  Glass bonding products have a photoinitiator which is activated when exposed to UV light of 365-400nm wavelength. It is important you have a UV lamp with a spectral output which peaks in the optimal range for the adhesive cure.  Highly reactive UV adhesives are often used on high speed production items e.g. disposable medical devices such as tubes and connectors. 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 an easy clean up.

Some UV-cure adhesives offer dual cure mechanisms which can have an influence on cure speed. UV adhesives can have a secondary anaerobic cure (this works in the presence of metal and absence of oxygen), moisture cure or a heat activated secondary cure. These adhesive products are suitable for applications where there are shadow areas that may not receive enough UV light to cure the adhesive.

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 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 often used when 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.

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 stabiliser which prevents them discolouring or going brittle in sunlight. Although many plastics appear to be crystal clear and light will shine through them, the UV wavelength for the adhesive cure can be blocked. To counter this problem, there are specialist plastic bonding UV adhesives with a longer wavelength photoinitiator which will react through UV stabilised plastic.

It is possible to measure lamp output, dosage and transmittance through substrate materials by using a radiometer. These can offer digital readings of different UV band lengths (E.g. UVA, UVB, UVC) and 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 and 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.