Historically, broad spectrum lamps have been used to cure UV adhesives, but with advances in LED technology, the use of lamps containing LEDs has become more popular over the past few years due to the lower cost of ownership.
The term "UV adhesive" used to refer to any adhesive that requires light curing is actually misleading as it refers to adhesives that cure in the ultraviolet part of the spectrum and adhesives that cure in the visible (VIS) part of the spectrum. Mixtures. The generally accepted definition of pure UV adhesives is that they cure between 100 and 400nm, while VIS adhesives cure between 400 and 780nm.
The UV spectrum can be further subdivided into vacuum ultraviolet (100 – 200nm), UV-C (200 – 280nm), UV-B (280 – 315nm) and UV-A (315 – 400nm).
Broad-spectrum UV lamps should contain a filter at the lamp output to remove all of these except UV-A, which works in a similar way to the ozone layer above Earth, filtering out these same UV wavelengths.
broad spectrum light
These typically emit light between 200nm and 550nm and are therefore suitable for curing almost all light curing adhesives on the market. Mercury bulbs or their derivatives are used in these lamps because of their relative simplicity of manufacture. As mentioned above, bulbs operate over a wide range of wavelengths, relying on filters in the glass housing to remove the harmful aspects of the UV rays.
When setting up the manufacturing process for broad spectrum lamps, factors to consider are: a) the time it takes for the bulb to reach operating intensity after it is switched on (approximately 5 minutes); will go through an aging process, which means that after about 50 hours, the intensity will drop to 20% of its normal working intensity; c) Since the lamp needs 15 - 20 minutes to cool down after turning off the lamp, the lamp cannot be switched on again during this period , because the mercury in the bulb needs to condense, so turning the bulb back on before then will cause the bulb to break; d) The lifespan of the bulb is around 1,000 hours (depending on the bulb),
Broad-spectrum lights emit approximately 30% UVA light, 10% visible light, and 40% infrared. The upshot of this is that it can be an added advantage for some adhesives, such as light-activated epoxies, as these cure exothermically, and the additional heat from the bulb can reduce cure time. But since these lights do generate heat, care needs to be taken to ensure the bulbs are properly ventilated. Also, some temperature sensitive substrates will not work with these lights - they may warp and may even emit smoke. Mercury bulbs also emit a wide arc of light from the exit area of the lamp, so some kind of shielding needs to be placed around the work area to protect the operator and any casual passers-by. Pigmented polycarbonate is often used for this purpose, as most grades of polycarbonate act as UV blockers.
led lights
Unlike broad spectrum lights, LED lights operate at very specific wavelengths, either 365nm, 400nm or 460nm. A 365nm LED lamp will cure adhesives containing only UV photoinitiators; a 460nm LED lamp will cure adhesives containing only VIS photoinitiators, while a 400nm LED lamp straddles the border between the two and will cure most adhesives containing either A photoinitiator adhesive. For this reason, 400nm LED lights are the most popular of the three wavelengths as they will cure most light curing adhesives on the market. Also, 365nm LEDs tend to be more expensive than other LEDs, making the lights more expensive, sometimes by considerable amounts. Given that most adhesives can be cured with 400nm lamps, it also makes it easier to switch between adhesives without investing in new lamps.
Another notable difference from broad-spectrum lamps is that these lamps are cold lamps due to the heat being removed from the LEDs through active or passive cooling. Therefore, there is no cool down period when the LED lights are off, which results in a system that cures on demand - the lights are only 'on' when light is needed. So when the broad spectrum lamp is on during production, say 8 hours a day, it's actually only curing for a fraction of the time. During the same 8-hour shift, LED lights will be "on" for significantly less time, resulting in significantly lower energy consumption. Combined with the longer lifetime of LED lights (>20,000 "on" hours), it results in significantly lower operating costs for LED lights and a faster return on investment.
The intensity of light produced by LED lights is typically one or two orders of magnitude greater than that produced by broad-spectrum lights. LED lamps are capable of producing up to 85,000 W/cm2 (measured directly at the adhesive with a DELOLUX 80 400nm LED lamp), while for broad spectrum lamps the light intensity will be in the range of hundreds of W/cm2, resulting in Faster adhesive cure, shorter cycle times and higher UPH when using LED lights.
process control
Whether mercury bulbs or LED lamps are used to cure uv adhesives, the curing process needs to be controlled as closely as possible to ensure consistent results. This means that the light intensity reaching the adhesive is the important measurement parameter, not the power or energy of the lamp. Luminous intensity is defined as the radiant performance per unit area, usually in watts per square centimeter (W/cm 2 ). This can be controlled by changing the intensity of light produced by the lamp, assuming the lamp has this capability, or by changing the distance of the lamp from the adhesive - moving the lamp closer to the adhesive will increase the intensity of light reaching the adhesive while moving it Moving further away reduces the light intensity. It is difficult to define this relationship due to many factors, but in general, doubling the distance from the lamp to the adhesive will reduce the light intensity by 30-40%, assuming no change in other variables.
Another factor to consider is the measurement frequency. For a new production line, the light intensity should be measured several times per shift, while for a more established process it can be measured daily or even weekly. The type of lamp used can affect this: Mercury bulbs, as the light intensity seen varies greatly over the lifetime of the bulb, should be measured more frequently than LED lamps, as they last significantly longer and have the least intensity The change of this life.
