Ultraviolet light (UV) curing uses the photosensitivity of photoinitiators (photosensitizers) to form photoinitiators under ultraviolet light to stimulate the decomposition of ecological molecules into free radicals or ions, so that unsaturated organic matter undergoes chemical reactions such as polymerization, grafting, and cross-linking to achieve the purpose of curing.
Treatment of infrared radiation from UV lamps:
Ultraviolet high-pressure mercury lamps convert 60% of their total power into infrared radiation, and the bulb surface temperature can rise up to 700-800℃. In order to avoid overheating of materials, high-power lamps and multi-lamp systems are used in UV curing devices, so various measures are generally taken in the device to cool the lamp tube, reflector lampshade and substrate.
The common used methods: First is air cooling, which is the most widely used method today and has a low cost; Second is water cooling, which involves installing a water shell outside the lamp. This method has good effects but is more expensive; Third, installing an optical sheet to isolate infrared radiation from the solidified material, which is suitable for products that are easily deformed.
When using photocuring coatings, it is necessary to try to adjust the temperature. While it is necessary to avoid material overheating, the coating temperature should be increased. Raising the temperature is conducive to the curing reaction.
The role of reflective lampshade
The installation of reflective lampshade can make full use of the intensity of UV lamp to the cured object surface. The reflective cover reflective effect directly affects the product quality. The reflective effect of the reflective cover is 30-40% higher than that of ordinary reflective sheets. It can reflect more than 90% of the UV light to product surface, which is greatly improving work efficiency.
Several factors affecting deep curing of UV radiation
After UV coatings are irradiated by ultraviolet light, the photoinitiator is triggered to generate free radicals or ions. These free radicals or ions react with the double bonds in the prepolymer or unsaturated monomer to form monomer genes. These monomer genes start a chain reaction to generate polymer solid molecules, and a complete curing process ends.
Several factors that affect its deep curing.
1) Influence of UV energy
The main condition for deep curing of UV radiation is that the molecules must absorb light quanta with sufficient energy to become excited molecules, decompose into free radicals or ions, and make unsaturated organic matter undergo chemical reactions such as polymerization, grafting, and cross-linking to achieve the purpose of curing. The UV energy of the photoinitiator in the UV coating supplied exceeds or is insufficient to the required energy, such as
① It is known that the UV energy required by the photoinitiator is normal if the energy supplied is scientifically and reasonably greater than the required energy to ensure thorough curing.
② If the UV energy required by the photoinitiator is unknown and excessive supply is blindly carried out, this practice will not only waste energy, but also cause negative effects of excessive curing, such as explosion, reverse curing reaction, etc.
③ When the UV energy is insufficient, the UV energy must be moderate, that is, it cannot be excessive or insufficient, so as to avoid incomplete curing.
2) Influence of UV coating thickness
The thickness of UV coating plays a key role in the effect of UV curing. If the coating thickness is too high, the drying time will be relatively long under the irradiation of the same power light source, which will affect the drying of UV coating. On the other hand, the substrate surface temperature will be too high, causing the substrate to deform; if the coating thickness is too thin, the surface gloss will be poor. In addition, the thickness of the UV coating needs to be properly adjusted under different conditions such as the hue, temperature, curing speed over different substrates.
3) Influence of curing distance
The distance between UV lamp and reflector and the irradiated object surface is 7-8cm for the best curing performance (the strongest UV energy), but depending on the different curing substrates, the general distance is about 10-15 cm. If the distance is too low, the UV lamp surface high temperature will destroy the substrate; if the distance is too high, the UV energy is small, thus the curing can not be completely processed. The curing distance must be adjusted appropriately according to the substrate, coating, lamp power, etc.
4) Influence of curing speed
According to the different substrates, coatings, curing distance, etc., the curing speed of the equipment should be adjusted appropriately. If the curing speed is too fast, the UV coating on the substrate surface will be sticky or the surface might be dry but the inside will not; if the running speed is slow, the surface of the substrate will age.
