The basic principle of UV----The photoinitiator of UV ink/overprint varnish component absorbs light energy after exposure to ultraviolet light. The free group is generated by excitation, which initiates and leads to the photopolymerization crosslinking of unsaturated acrylate oligomers and active monomers to form cured ink layer/coating.
Ultraviolet (UV) is essentially a wave of electromagnetic radiation with a specific frequency (wavelength). Ultraviolet light is a wavelength of 40 ~ 400 nm of light. The light owning large energy and can trigger photochemical reactions are shown in the below table:
|
Category |
Wave length/nm |
Energy/ev |
|
Ultraviolet light |
40-400 |
31-3.1 |
|
Visible light |
400-800 |
3.1-1.55 |
|
Infrared light |
>800 |
>0.78 |
Ultraviolet light can be divided into vacuum ultraviolet light (<200 nm), medium ultraviolet light (200-300 nm) and near-ultraviolet light (300-400 nm). It can only propagate in vacuum because that vacuum ultraviolet light (also known as far ultraviolet light) less than 200 nm is easily absorbed and consumed in air, which has no practical significance in the application of light curing. Thus, the curing of UV ink is mainly to choose 200-400 nm ultraviolet light especially 300-400 nm near-ultraviolet light. The most effective wavelength range for curing UV inks is 350-420 nm because that the pigment of UV inks has the greatest transmittance at 360 NM wavelength. The photocuring process is completed when the fluid ink is printed on the printed matter and then irradiated by ultraviolet light.
UV ink/overprint varnish generally adopt unsaturated acrylate system materials. Though the curing and drying of the system belongs to the photopolymerization crosslinking reaction triggered by free groups and the photocuring process is done in an instant, in terms of mechanism, it undergoes the reaction process of formation of free radical, initiation and growth of chain, and termination of chain.
It is in an unstable energy excited state (R·) and rapidly decomposed into a large active free radical (R.) when the photoinitiator (R) absorbs UV energy. The free radical immediately causes the unsaturated compound (M) in the system to undergo polymeric cross-linking reaction and generate new free radical (RM.), which continues to react similarly with the surrounding unsaturated compound molecules, and the molecular chain continues to grow to form an increasing free group (RMm· or RMn·). The molecular weight increases rapidly (RMm+1· or RMn+1·) with the deepening of the reaction. The two newly generated free groups (RMm· and RMn·) combine with each other to form a chain during the photochemical reaction to form electrically neutral molecules (Pm, Pn or Pm+n), they loses their respective activity and the growth of the chain is terminated when. UV ink/ overprint varnish is also converted from colloidal/liquid to solid state and become an insoluble and refractory network hard coating with a variety of excellent properties.
Compare with ordinary offset printing ink, the advantages of UV ink are instant curing, 100% curing system, machine non-drying, very low VOCs. The shortcoming of the UV have poor fluidity, the narrow water width, the low printing gloss, the rubber roller has a choice, the roller easier dry and the special auxiliary materials need to be matched. LED-UV compared to traditional mercury lamps, the advantages are low power consumption, no ozone, long life and low maintenance cost. The disadvantage is the low band chip less than 315nm has not been mass produced and short life, high cost. Relatively mature 365nm, 385nm, 395nm and other bands need to match suitable LED-UV ink and overprint varnish. Supporting initiators cost is high and yellow. Among, LED-UV coating varnish is the most difficult to solve and accepted by customers. The problem of four-color ink is mainly caused by yellowing, the B-value of blue ink and black ink is high, and the cost of LED-UV ink is high.
It is necessary to ensure the adaptability of UV printing before the formal printing. For the new roller, the dual-purpose machine from the ordinary offset printing to the UV process, dry roll phenomenon may occur after a long time of printing resulting in poor ink transfer. The solution to this problem is: wash cleans UV vehicle-- UV ink oil wetting - inking - printing (select plates with more graphics) - observe- successive states of a print. In this process, it is necessary to stop the machine to observe the ink state on the roller once it is found that the printing concentration decreases, the field is empty, the paper wool is pulled, etc. The treatment process needs to be repeated if it is judged to be a dry roll, and the wetting time of the ink oil may be as long as 3-10 hours. We do not recommend dual-use equipment because that the first few prints will certainly have poor printing results from the process of ordinary offset printing to UV.
After the LED-UV system modification, it is recommended to re-confirm and adjust each pressure to a reasonable state after 1 week of official printing as the pressure of each plate roll changes during the printing process. After 2 weeks of printing, the printing curve can be made if the overall printing state is stable. Otherwise it is necessary to determine the cause of the problem and treat the right medicine.
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