Author: Heng Petroleum and Chemical Technology Yang Aijun, Sunpan Feng, Zhang Zhenlin
The glazing of printed matter is a very important part in the post-printing process. The quality of the varnish directly affects the aesthetics and performance of the printed matter. After experiencing benzene-soluble varnish, alcohol-soluble varnish, water-based varnish, and UV varnish, can EB (ElectronBeamCuring) varnish replace UV varnish and become the mainstream product in post-print varnish? What are the advantages of EB varnish compared to UV varnish? What advantages can EB varnish bring to the printing industry?
This article shares your views on the above issues.
Table 1 Main properties of varnish after printing
Main performance
testing method
Performance discrimination
odor
Varnish residue, olfactory discrimination or chromatography
1 worst → 5 best
Drying speed
Different varnish methods are different (surface dry / hard work)
1 slowest → 5 fastest
luster
GB / T9754-88; gloss meter, 60 degree angle
0 is the most dumb → 100 is the brightest
Adhesion
GB / T1720-1989; cross-cut method
1 worst → 5 best
hardness
GB / T6739-1996; pencil method
5B → HB → 5H
Abrasion resistance
GB / T1768—1979 (1989); friction meter
1 worst → 5 best
Scratch resistance
ISO12137 (GB / T 9279); scribe method
1 worst → 5 best
Explosion-proof linear
GB / T1731-1993; bending method
1 worst → 5 best
Yellowing resistance
ASTMG23, IOS-MAT-0050; QUV
1 worst → 5 best
Anti-stick flower
Face to face (back), 4kg / cm2 pressure, 50 ℃ * 48h
1 worst → 5 best
Constructability
Construction method and drying method are not exactly the same
1 the hardest → 5 the easiest
Economy
Construction cost of the same varnish thickness (after drying)
1 lowest → 5 highest
Discussion on the performance of common varnishes
At present, the varnishes used in post-press processing mainly include solvent-based varnishes, water-based varnishes and UV varnishes, each of which has its own advantages in performance.
The main disadvantages of solvent-based varnish are: large odor, slow drying speed, not wear-resistant, easy to rub flowers, not environmentally friendly, and it is a product that has been eliminated.
Water and oil do not contain solvents, are environmentally friendly, and have no obvious shortcomings in performance. They are a product with excellent overall performance. There are a few shortcomings: the wear resistance and scratch resistance are not satisfactory. The lack is also very nerve-wracking. The product with high gloss has poor anti-sticking performance, which is not suitable for double-sided printing on coated paper. The product with good anti-sticking performance is not ideal for gloss.
UV oil is widely used due to its high gloss, good wear resistance, and scratch resistance, but its odor problems and yellowing problems cannot always be solved well. In recent years, in order to save costs, printing plants generally use paper with high filler and high brittleness. Therefore, the explosion-proof linearity of UV oil has attracted more and more attention. In addition, the contradiction between the flexibility, adhesion and anti-sticking of silk screen UV varnish is more prominent.
Water and oil are cheap, good in quality and moderate in overall performance. It is a product with strong vitality. Based on the limitation of its drying principle, it is impossible to think of a great improvement in performance. The advantages and disadvantages of UV oil are clear, how to avoid weaknesses or improve shortcomings, it is worth further discussion.
UV varnish performance analysis
UV oils are mainly composed of initiators, active monomers, oligomers and auxiliaries. The oligomers are usually used in combination of two or more types. They are the main components that determine the final physical properties of UV oils. Active monomers mainly provide oligomerization. It has the functions of crosslinking and reducing the viscosity of varnish. The additives provide the leveling, defoaming and surface smoothness properties of varnish, similar to the salt and monosodium glutamate in meals. The initiator receives ultraviolet light and produces free The base, initiating the oligomer and monomer reaction, and curing to form a film, the role is very critical.
Since the initiator is sensitive to ultraviolet rays, the varnish film will continue to absorb ultraviolet rays and turn yellow in the future storage process after construction. At the same time, the initiator reaction is incomplete, and the remaining part will make the UV oil smell larger. Furthermore, due to the limited efficiency of initiation, the monomer and oligomer reaction is also incomplete. The remaining part will cause sticky flowers and make the human allergic. This results in a slight lack of varnish resistance and anti-explosion linearity. In response to these inherent shortcomings of UV varnish, EB varnish came into being. EB varnish has an initiator removed, which has greatly improved the performance of varnish. The following will analyze the difference between the two. [next]
Performance difference between EB varnish and UV varnish
1. The difference between drying (curing) principle and equipment
UV varnish relies on the initiator in the varnish to absorb ultraviolet light, generate free radicals, and initiate monomer and oligomer reaction and curing. A domestic laboratory curing equipment can be 20,000 RMB, and domestic production equipment is about 200,000 to 300,000 RMB.
EB varnish is under the strong bombardment of the electron beam, the double bonds on the monomer and oligomer are forced to open, generate free radicals and complete the polymerization. An imported laboratory EB curing equipment costs about US $ 100,000-200,000, while the production equipment is between US $ 500,000-800,000.
At present, the core technology of EB curing equipment is in the hands of a few foreign companies. Because the mass production of such equipment is small, the price is relatively high. I believe that if this technology is generally 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, I believe the price will be lower.
2. The difference of 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 residual odor of UV oil, one is the residual initiator and active monomer, and the other is other small molecular substances generated after the reaction of the initiator. Improve the initiator of UV oil, and increase the degree of reaction, the smell will be improved, but it can not be completely removed.
There is no initiator in EB varnish, and the reaction degree exceeds 98%, so it will not have the problem of residual odor like UV oil.
3. Difference in yellowing resistance
We know that the paper that has been coated with UV oil will gradually turn yellow after being left for a long time. The main reason is that the residual initiator and the initiator product absorb ultraviolet light. Use expensive initiators or add ultraviolet absorbers to the UV oil It will help, but it can't be completely non-yellowing. EB varnish does not use an initiator, so this problem will not exist.
4. The problem of ink discoloration
While the UV lamp generates ultraviolet rays, it also generates infrared rays, which makes the surface temperature of the varnished paper higher. Since the production efficiency must be guaranteed, the paper is stacked together before the temperature drops. The inner layer of the paper will be 30- Keep it at 60 ℃ for a period of time, then some inks with poor heat resistance will change color. In addition to the influence of temperature, the initiator in the UV oil will still maintain a certain activity at a relatively high temperature, and it may have a chemical reaction with some active substances in the ink. One. By improving the UV lamp, reflector, and cooling system, the problem of ink discoloration will be solved to a certain extent, but it is also a solution to the symptoms.
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 real cold energy source and will be more suitable for some heat-sensitive substrates.
5. Comparison of curing degree
In order to understand the degree of curing of UV oil and UV oil, the concept of gel rate is introduced here. The measuring method is: make varnish into a coating film with a thickness of 100 μm, record the quality W0 of the coating film after curing, use acetone Extract for 48 hours, vacuum dry at 50 ° C for 8 hours, write down the extracted mass Wg. Then calculate according to the following formula:
Gel rate = Wg / W0 × 100%
The higher the gel rate, the higher the degree of curing. No matter how the UV oil increases the energy, the curing degree is only about 75%. The normal construction energy of the UV oil is 50-100mJ / cm2, which means that the UV oil we currently use will have 25-30% of small molecules on the surface of the printed matter. Substance residues, these small molecular substances, on the one hand, reduce the physical properties of the product, on the one hand, there are serious hidden dangers in safety, if it may cause human skin allergies, etc.
The curing energy of EB varnish reaches 40kGy (the unit of electron beam energy), the degree of curing can reach more than 98%, and the physical properties and safety have been well solved. [next]
6. The difference between scratch resistance, wear resistance and explosion-proof wire
Scratch resistance: Because the curing degree of EB varnish is much higher than that of UV varnish, plus the reason of oxygen inhibition during UV curing, the curing degree of the surface of UV varnish is lower than that of the interior. Under the condition of hardness, the scratch resistance of UV varnish is far inferior to that of EB varnish. Of course, increasing the hardness of the UV varnish, adding wear-resistant fillers and additives, etc. can improve the scratch resistance of the UV oil, but this will also bring the sacrifice of the linearity of the UV oil explosion-proof.
Wear resistance: We know that wear resistance is related to the flexibility of the coating film. The greater the toughness, the more wear-resistant the coating film, such as automobile tires (SBS rubber material) and sports soles (PU polyurethane material), etc., due to UV Insufficient curing of varnish can not obtain a coating film with good flexibility. Therefore, the wear 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 to EB varnish. It was found that the anti-explosion linearity of EB varnish is not as good as that of UV oil. Because the degree of curing of EB varnish is nearly 30% higher than that of UV oil, the higher the degree of curing, the greater the hardness of the product, the easier it is to be broken, and the anti-explosion linearity will become worse.
To truly compare the explosion-proof linearity of the two, you must adjust the formula of the two so that their hardness or scratch resistance is the same. At this time, you will find that the explosion-proof linearity of EB varnish will be better.
7. The difference between anti-sticky flowers
In the impression of ordinary people, there is no sticky flower in UV oil, but in some special cases, such as, UV oil is required to have a good explosion-proof linearity, then the cross-linking density of UV oil will be reduced as much as possible to make UV oil It has better flexibility and will not break when the paper is bent. However, due to the incomplete curing of UV oil, it may bring the risk of UV oil sticking to flowers.
In addition, in the silk screen UV oil, in order to obtain good flexibility and adhesion to the film, the varnish will be filled with thermoplastic resins, such as polyisobutyl acrylate, etc. These resins not only do not participate in curing themselves, but will Blocking the curing of other monomers and resins, this type of UV oil is more insufficiently cured, and the possibility of sticking flowers is greater.
In EB varnish, due to the violent bombardment of the electron beam, the monomers and resins that can react will fully react, and the possibility of sticking flowers is very small.
8. Difference in solvent resistance
In some cases, varnish will have solvent resistance requirements. For example, if you put PVC or PET film on the skylight above the color box, because the color box glue contains a small amount of solvent, if the UV oil has poor solvent resistance, it may cause UV oil. Stick to the film. In addition, the surface coating of film furniture also requires solvent resistance.
The measurement method of solvent resistance is as follows: a cotton cloth dipped in MEK (methyl ethyl ketone), one round trip, until the coating film is scratched. Figure 3 below is the difference between the solvent resistance of EB varnish and UV varnish, the coating thickness is 4 microns.
Under the same thickness, the solvent resistance of EB varnish is more than 4 times that of UV varnish. As the coating becomes thicker, the solvent resistance of EB varnish will become more obvious.
9. Does EB varnish need a primer like UV oil?
First, we need to understand why UV oil needs to be primed. In UV oil, the molecular weight of the active monomer is very small (about 300), the molecular weight of the oligomer is also very limited (about 500-1000), and the solvent type Compared with resin in varnish (20,000) and resin in water and oil (5000-100,000), the molecular weight is very different. When UV oil is applied to cardboard, monomers and oligomers will penetrate into the paper fibers In the loss of light on the surface. In order to ensure that the UV oil does not penetrate into the paper and all stay on the surface of the paper, high molecular weight products such as water oil are required for primer. At the same time, in order to ensure that the primer and UV oil have good adhesion, the primer also needs special design and matching .
EB varnish and UV varnish are very similar in formulation and selection, so EB varnish also needs a primer, but due to the sufficient curing of EB varnish, higher molecular weight resins can be used in EB varnish, which is not suitable for design. The EB varnish that needs to be primed provides greater possibilities and flexibility.
10. The difference in cost
I believe everyone will ask this question. UV oils used on packaging materials that are not sensitive to odor requirements do not use expensive ultraviolet light initiators. Compared with EB oils, these types of UV oils are comparable in price. However, for some of the more demanding UV oils used in gift packaging, medicine, tobacco, food and other outer packaging, the price of UV varnish will be higher than that of EB varnish. The overall cost may be the same. Since there is no industrial production data, an accurate answer cannot be given yet.
Summary of the advantages of EB varnish
Efficient: The curing speed of the single-curtain EB curing equipment can reach 900 meters per minute (calculated value), which is higher than the UV curing equipment with 3 UV lamps connected in series (about 200 meters per minute), and also higher than the current The fastest printing speed (726 m / min, calculated value, German Manroland 75 printing machine, 75,000 sheets), so EB varnish is the fastest drying varnish at present, and will not be used to improve printing efficiency Obstacles; enhance the added value of the product: the curing degree of EB varnish is much higher than that of UV varnish, which can make the product have better wear resistance, scratch resistance, solvent resistance, yellowing resistance and good smell, these Performance is unattainable by UV oil, water oil and other varnishes.
Safety and environmental protection: The surface of the printed matter after construction with EB varnish has no residual small molecular chemicals. Its safety and environmental protection are also unattainable by UV oil, and can be safely used in food packaging, tobacco and alcohol packaging , Packaging of medicines or articles in contact with the human body.
in conclusion
EB varnish has obvious advantages in performance compared to UV coatings. EB varnish is a better choice when processing high-end products or some products with special requirements, such as food packaging, tobacco and alcohol packaging, and packaging for items that contact human bodies. .
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