Performance comparison of EB varnish and UV varnish in postpress processing
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Print glazing is a very important part of post-press processing. The quality of varnish directly affects the aesthetics and performance of printed matter. After experiencing benzene-soluble varnish, alcohol-soluble varnish, water-based varnish, and UV varnish, can EB (Electron Beam Curing) varnish replace UV varnish as the mainstream product in post-press varnish? What are the advantages of EB varnish and UV varnish? What advantages does EB varnish bring to the printing industry?
Discussion on the performance of commonly used varnish
At present, the varnish used in the post-press processing mainly includes solvent-type varnish, water-based varnish and UV varnish, and they all have different performances.
that the main disadvantages of the solvent-type varnish are: large odor, slow drying speed, no abrasion resistance, easy wiping, and no environmental protection, and are products that have been eliminated.
Water oil does not contain solvents, environmental protection, and has no obvious disadvantages in performance. It is a product with excellent comprehensive performance. There are some shortcomings that are not satisfactory for abrasion resistance and scratch resistance, and at the same time, gloss and anti-sticking properties. Lack of it is also very nerve-wracking. The product with high gloss has poor anti-sticking properties, and it is not suitable for double-sided printing on coated paper. The gloss of anti-sticking products is not ideal.
UV oil is widely used because of its high gloss, good wear resistance and anti-scratch property, but its odor problem and yellowing problem can not be solved well. In recent years, in order to save costs, printing companies generally use paper with high filling and brittleness, so the explosion-proof linearity of UV oil has been paid more and more attention. The contradiction between the flexibility, adhesion and anti-sticking properties of the other silk screen UV varnish is also prominent.
Water oil is cheap and good in quality, and its comprehensive performance is moderate. It is a very strong product. Based on the limitations of its drying principle, it is impossible to improve the performance. The advantages and disadvantages of UV oil are clear. How to avoid weaknesses or improve defects is worth further exploration.
UV varnish performance analysis
The UV oil is mainly composed of an initiator, a reactive monomer, an oligomer and an auxiliary agent. The oligomer is usually used in combination of two or more kinds, and is a main component that determines the final physical properties of the UV oil. The active monomer mainly provides oligomerization. It cross-links and reduces the viscosity of varnish. The additive provides the leveling, defoaming and surface slip properties of varnish. It is similar to salt and monosodium glutamate in food. The initiator receives ultraviolet light and produces freedom. The base, which initiates the reaction of the oligomer and the monomer, and solidifies into a film, is very critical.
Since the initiator is sensitive to ultraviolet rays, the varnish film will continue to absorb ultraviolet rays and yellow in color during the future storage process. At the same time, the initiator reaction is incomplete, the residual part will make the UV oil smell too large, and because the initiation efficiency is limited, the reaction between the monomer and the oligomer is not complete, and the residual part will cause sticky flowers and allergies to the human body. The rust resistance and anti-explosion linearity of varnish are slightly deficient. In view of these congenital insufficiency of UV varnish, EB varnish came into being, and the initiator of EB varnish was removed, which greatly improved the performance of varnish. The following is a brief analysis of the difference between the two.
Performance difference between EB varnish and UV varnish
‧ The difference between drying (curing) principle and equipment
UV varnish relies on an initiator in varnish to absorb ultraviolet light, generate free radicals, and initiate the reaction and solidification of monomers and oligomers. A domestic laboratory curing equipment can be 20,000 RMB, and domestic production equipment is about 200,000-300,000 RMB.
EB varnish is under the violent bombardment of electron beams, the double bonds on the monomers and oligomers are forcibly opened, generating free radicals and completing the polymerization. An imported laboratory EB curing equipment costs about 100,000-200,000 US dollars, while the production equipment is between 500,000-800,000 US dollars.
At present, the core technology of EB curing equipment is in the hands of a few foreign companies. Due to the small batch production of such equipment, the price is high. It is believed that if the technology is widely used, the reasonable price should be between 300,000 and 500,000 US dollars. If the manufacturing technology of the equipment is mastered by Taiwanese or domestic counterparts, the price will be lower.
‧ The difference in smell
Both UV oil and EB oil do not contain solvents, so the odor problem during construction does not exist. The so-called odor mainly refers to the odor remaining on the surface of the printed matter. There are two sources of odor in the UV oil residue, one is the residual initiator and the reactive monomer, and the other is the other small molecular substances produced by the initiator reaction. The UV oil initiator is improved and the degree of reaction is increased, and the odor is improved, but it cannot be completely removed.
There is no initiator in EB varnish, and the degree of reaction is over 98%, so there is no problem of residual odor like UV oil.
‧ The difference between yellowing resistance
We know that when the paper with UV oil is left for a long time, it will gradually turn yellow. The main reason is that the residual initiator and initiator products absorb UV light, use expensive initiator or add UV absorber to UV oil. It will help, but it can't be done completely. EB varnish does not use an initiator, so there is no such problem.
‧ Ink discoloration problem
When the UV lamp generates ultraviolet light, it also generates infrared rays, so that the surface temperature of the paper with the varnish is high. Because the production efficiency is guaranteed, the paper is piled up when it is too late to cool down, and the paper in the inner layer will be in the 30- At 60 ° C for a while, some inks with poor heat resistance will change color. In addition to the influence of temperature, the initiator in UV oil will still maintain a certain activity at a relatively high temperature, and may chemically react with some active substances in the ink, which is also the cause of discoloration of the ink caused by UV oil. one. Improvements to the UV lamp, reflector, and cooling system will solve the problem of ink discoloration to a certain extent, but it is also a solution to the problem.
In the curing process of EB varnish, the curing equipment does not generate infrared rays during the process of generating electron beams, and the temperature problem is well solved. Therefore, electron beam curing is a true cold energy source, and it is more suitable for some heat sensitive substrates.
‧ Comparison of curing degree
In order to understand the degree of curing of UV oil and UV oil, the concept of gel fraction is introduced here by measuring varnish to a coating film having a thickness of 100 μm, recording the film quality W0 after curing, using acetone. The mixture was extracted for 48 hours, dried under vacuum at 50 ° C for 8 hours, and the mass Wg after extraction was recorded. Then calculate as follows:
Gel rate = Wg / W0 × 100 %
The higher the gel fraction, the higher the degree of cure. The graph below compares the degree of cure of typical EB varnish and UV varnish.
As can be seen from the above figure, UV oil can only improve energy in any case, and the degree of curing is only about 75%. The normal construction energy of UV oil is 50-100mJ/cm2, which means that the UV oil we use now has 25- on the surface of the printed matter. 30% of small molecular substances, these small molecular substances, on the one hand, make the physical properties of the product decline, on the one hand, there are serious hidden dangers in safety, such as may cause skin allergies.
The curing energy of EB varnish reaches 40kGy (the unit of electron beam energy), the degree of curing can reach 98% or more, and the physical properties and safety are well solved.
‧ The difference between anti-scratch, abrasion resistance and explosion-proof line
Anti-scratch property: Since the curing degree of EB varnish is much higher than that of UV varnish, plus the cause of oxygen inhibition during UV curing, the surface of UV oil is cured to a lower degree than the inside, so the same coating film Under the condition of hardness, the anti-scratch property of UV varnish is far less than that of EB varnish. Of course, improving the hardness of UV varnish, adding wear-resistant fillers and additives, etc., can improve the anti-scratch property of UV oil, but this will bring the sacrifice of UV oil explosion-proof linearity.
Wear resistance: We know that the wear resistance is related to the flexibility of the film. The greater the toughness, the more wear-resistant the film, such as automobile tires (SBS rubber) and sports soles (PU polyurethane), due to UV The insufficiency of varnish curing cannot obtain a coating film with good flexibility. Therefore, the abrasion resistance of UV varnish cannot be compared with EB varnish.
Explosion-proof linearity: Once tested, the initiator in the UV oil formula was removed and changed into EB varnish. It was found that the blast resistance of EB varnish is not as good as that of UV oil. If this is the conclusion, it will be a big mistake. Because the curing degree of EB varnish is higher than that of UV oil by nearly 30%, the higher the degree of curing, the greater the hardness of the product, the easier it is to be broken, and the antiknock linearity will be worse.
In order to truly compare the explosion-proof linearity of the two, it is necessary to adjust the formula of the two to make their hardness or anti-scratch property the same. At this time, it will be found that the explosion-proof linearity of EB varnish is better.
‧ The difference between anti-sticking
In the general impression, UV oil does not have sticky flowers, but in some special cases, such as UV oil, it is required to have good explosion-proof linearity. At this time, the cross-link density of UV oil will be minimized to make UV oil. It has better softness and will not break when the paper is bent. However, due to the incompleteness of UV oil curing, there is a risk of UV oil sticking.
In addition, in the screen UV oil, in order to obtain good flexibility and adhesion to the film, a thermoplastic resin such as polybutyl acrylate may be added to the varnish, and these resins not only do not participate in curing themselves, but instead Blocking the curing of other monomers and resins, this type of UV oil cures less and is more likely to stick.
In EB varnish, due to the violent bombardment of the electron beam, the monomer and resin that can react are sufficiently reacted, and the possibility of sticking is small.
‧ The difference between solvent resistance
In some cases, varnish may have solvent resistance requirements. For example, PVC or PET film is attached to the skylight above the color box. Since the color box contains a small amount of solvent, if the solvent resistance of the UV oil is poor, UV oil may be caused. Stick to the film. In addition, the film surface coating has a solvent resistance.
The solvent resistance was measured by rubbing MEK (methyl ethyl ketone) in a cotton cloth and rubbing the film once. Figure 3 below shows the difference in solvent resistance of EB varnish and UV varnish, with a coating thickness of 4 microns.
Under the same thickness conditions, the solvent resistance of EB varnish is more than 4 times that of UV varnish. With the thickening of the coating, the solvent resistance of EB varnish will be more obvious.
‧ Does EB varnish need the same bottoming as UV oil?
First, we need to understand why UV oil needs to be primed. In UV oil, the molecular weight of the reactive monomer is very small (about 300), the molecular weight of the oligomer is very limited (about 500-1000), and the solvent type. Compared with the resin in varnish (20,000) and the resin in water oil (5000-100,000), the molecular weight differs greatly. When UV oil is applied to the paper, the monomer and oligomer will penetrate into the paper fiber. In the loss of the surface light. In order to ensure that the UV oil does not penetrate into the paper, all of it stays on the surface of the paper, and it needs to be grounded with high molecular weight products, such as water and oil. At the same time, in order to ensure good adhesion between the base oil and the UV oil, the base oil also needs special design and matching. .
EB varnish and UV varnish are very similar in formula selection, so EB varnish also needs base oil, but due to the sufficiency of EB varnish curing, higher molecular weight resin can be used in EB varnish. EB varnishes that require primers offer greater possibilities and flexibility.
‧ the difference in cost
I believe everyone will ask this question. For UV oils used on packaging materials that are not sensitive to odor requirements, such UV oils are comparable in price to EB oils because they do not use expensive UV photoinitiators. However, for some UV oils used in gift packaging, medicine, tobacco, alcohol and other outer packagings, the price of UV varnish will be higher than that of EB varnish, but due to the need for nitrogen or carbon dioxide gas protection during EB varnish construction, The overall cost may be comparable, and since there is no industrial production data, accurate answers are not currently available.
Summary of the advantages of EB varnish
Efficient: The curing speed of a single curtain EB curing device can reach 900 m / min (calculated value), higher than 3 UV lamps in series with UV curing equipment (about 200 m / min), but also higher than the current The fastest printing speed (726 m / min, calculated value, Germany Manroland 75 printing press, 75,000 pairs), so EB varnish is the fastest drying varnish at present, will not become the printing efficiency Barriers; added value of the product: EB varnish cures much higher than UV varnish, which can make the product have better wear resistance, scratch resistance, solvent resistance, yellowing resistance and good odor. Performance is not achievable with varnish such as UV oil or water oil.
Safety and environmental protection: The surface of printed matter after EB varnish construction has no residual small molecule chemicals. Its safety and environmental protection are also unachievable by UV oil. It can be safely used in food packaging, tobacco and alcohol packaging. , drug packaging or packaging of items in contact with the human body.
in conclusion
EB varnish has obvious advantages over UV coatings. EB varnish is a good choice when processing high-grade products or certain special requirements, such as food packaging/tobacco/wine packaging/human contact packaging.