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1. What is the reaction principle of UV curing? UV curing means that under the irradiation of ultraviolet light, the photoinitiator absorbs the radiant energy of ultraviolet light and splits into free radicals, which initiates polymerization, cross-linking and grafting of the prepolymer, and solidifies into a network in a short time. High molecular polymer. Thereby, the UV coating, the ink, the binder, and the like are converted from a liquid state to a solid state in a few seconds to form a cured film.
The UV curing process can generally be divided into the following five stages.
(1) The photoinitiator absorbs ultraviolet light and becomes an excited photoinitiator.
(2) The photoinitiator in the excited state is unstable and easily decomposes to form free radicals.
(3) The interaction of the radical with the unsaturated group in the prepolymer initiates addition, crosslinking or polymerization to form a free radical intermediate.
(4) The free radical intermediate is subjected to a chain extension reaction to obtain a long-chain or network-like polymer radical intermediate.
(5) The long-chain or reticulated polymer radical intermediate produces a chain termination reaction, and the original liquid component is converted into a solid polymer.
2. What are the influencing factors of UV curing?
The main factors affecting the UV curing reaction are: the emission spectrum and radiation intensity of the UV lamp in the UV curing device, and the absorption spectrum of the photoinitiator in the UV ink, coating or adhesive and its formulation.
In the UV curing reaction, it is most important to determine whether the emission spectrum of the UV lamp in the UV curing device matches the absorption spectrum of the photoinitiator in the UV ink, coating or adhesive. If the two do not match, the photoinitiator can not be excited, and the UV curing reaction can not be completed smoothly; if the two do not match completely, it will easily lead to incomplete UV curing reaction; if the two match, the curing effect and UV The radiation intensity of the source is directly related. In addition to the spectral matching problem, the choice of UV curing equipment, UV inks, coatings and adhesives in the curing reaction also takes into account the effects of surface curing and deep curing to meet different application requirements.
Typically, UV curing uses a high pressure mercury lamp as the curing source (note that the quality and service life of the UV lamp directly affects the extent of the UV curing reaction). Of course, depending on the needs, it is also possible to add lamps (such as iron lamps, gallium lamps, etc.) with different additives to change the output spectrum of the UV lamps to meet specific curing requirements. In general, high UV source radiant intensity enhances the surface cure and deep cure of UV inks, coatings and adhesives.
3. How to reduce the occurrence of special odors during UV curing?
The odor emitted during UV curing is a decomposition product of a photoinitiator such as benzaldehyde or mercaptan, which has a special odor. We know that in the UV curing reaction, nitrogen is often used as a shielding gas to reduce the effect of oxygen and water vapor on the UV curing reaction (oxygen and water vapor will participate in the UV curing reaction, depleting the energy of the UV source, and generating ozone. ). In addition, the UV curing reaction was carried out under nitrogen protection conditions, and the photoinitiator was used in an amount of only 1/5 of that under conventional curing conditions. As the amount of photoinitiator is reduced, the amount of benzaldehyde or mercaptan decomposed therefrom is also reduced, so that the odor in the UV curing reaction is relatively small.
It can be seen that the use of nitrogen protection can reduce the generation of special odors to some extent, and a more effective solution to the special odor generation during UV curing is to select a polymerizable photoinitiator or a large molecular weight photoinitiator to Reduce the volatility of its decomposition product benzaldehyde or mercaptan.
The above content is selected from the 11th issue of “ Printing Technology·Packaging and Decoration †of Keyin Media in 2013. For more journal content, please pay attention to the journal channel, and click on the latest journal content to enter the electronic journal channel of Keyin.com.
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