UV curing happens under the light of ultraviolet. Photoinitiator absorbs radiation energy of ultraviolet to split into free radicals,which causes prepolymers polymerize,crosslink and graft,then form the reticular polymers in a short time. UV curing will make UV coating,ink and adhesive cure in several seconds and form the images.
UV curing involves 5 phases as below:
(1)Photoinitiator absorbs ultraviolet to become excited state photoinitiator.
(2)Excited state photoinitiator is not stable, then it will split into free radicals.
(3)Unsaturated groups of free radicals and prepolymers are interacted with each other,which cause addition, crosslinking and polymerization to form free radical intermediates.
(4)free radicals intermediates makes chain propagation reaction to get chain- or net-high-molecular polymer free radical intermediates.
(5)Chain- or net-high-molecular polymer free radical intermediates chain termination to cause liquid groups change into curing polymers.
Influence factors of curing process for UV printers are UV curing equipment such as UV lamps’ emission and radiation intensity,and UV ink, coating and adhesive photoinitiator’s absorption spectrum and composition.During the UV curing process, it’s the most important thing to make sure UV lamps’ emission and radiation intensity matches UV ink, coating and adhesive photoinitiator’s absorption spectrum. If not match, photoinitiator cannot be excited, so the UV curing cannot be conducted smoothly. If they cannot match totally, the UV curing will be incomplete.If they match, the curing effect is directly related to radiation intensity of lights. Besides the match issue,the choice of UV curing equipment,UV ink, coating and adhesive should be accompanied with surface curing and deep curing effects to meet different application.
Usually UV curing use high pressure mercury lamp as curing illuminator.What should be paid attention to is that quality and service time of UV lamps are closely related to UV curing degree and effect. Of course, according to different requirements, different lamps of additive,such as iron lamp and gallium lamp, can be used to change the output spectrum of lamps to meet the special curing demands.Generally, high UV lamp radiation intensity can enhance surface curing and deep curing effects of UV ink, coating and adhesive.
During UV curing process, it will emit a smell which is caused by photoinitiator’s resolvent, such as benzaldehyde, mercaptan and so on.These substance is accompanied with special smell.it is known that during the UV curing process,nitrogen is often taken as protecting gas to decrease the influence of oxygen and vapor, because oxygen and vapor will participate in the curing process to consume the energy of UV lamps and generate ozone. Besides,with protection of nitrogen, the quantity of photoinitiator during the UV curing process is only one fifth of that under normal conditions. With the decrease of photoinitiator, benzaldehyde and mercaptan will decrease,too. Thus, the smell emitted during the curing process will be less.
Therefore, nitrogen protection will reduce the emission of smell, and the better way to reduce the emission of smell is to choose polymerizable photoinitiator or high molecular photoinitiator to decrease the volatility of benzaldehyde and mercaptan.