With the development of science and technology, metal printing technology has made considerable progress. The biggest breakthrough in recent years is the introduction and successful practice of UV curing technology that has been maturely applied in the field of paper printing. Compared with traditional heat-curable inks, UV printing has the advantages of low energy consumption, high efficiency, and high cost performance.
Unlike paper printing, metal printing generally requires the use of a lacquer-type primer. Since UV ink cannot penetrate into the primer and metal, it is difficult to form good adhesion like printing on paper, which has become a major problem in the industry.
Through the theoretical study of adhesion, we can find the root cause of the poor adhesion of the above-mentioned UV ink, and then find a solution accordingly. Below we analyze it from three aspects: primer, ink and process.
The primer is the carrier of UV ink, and the adhesion is the force between the two. The performance of the primer will inevitably affect the adhesion of the UV ink. At present, there are mainly two kinds of primers for metal printing: varnish and color paint. The performance indicators discussed here are different from conventional processing performance indicators, and more emphasis is placed on the performance of the surface after forming the coating, including surface tension, polarity, resistance, surface roughness. Degree, reactive group, electrostatic adsorption, etc., they are the key factors affecting the adhesion of UV ink.
Surface tension From the point of view of the formation mechanism of adhesion, the carrier can only be effectively wetted by the attachment, and the two can exert force at the interface, thereby forming an effective adhesion. The main parameter of this wettability is the surface tension of the carrier and the attachment. The wetting rule is that the attachment with low surface tension can be completely wetted on the carrier with high surface tension, thereby forming a strong attachment, and vice versa. Repulsive force will be generated and adhesion will be deteriorated. According to this theory, increasing the surface tension of the primer can have a positive effect on UV ink adhesion.
At present, the primer used for printing iron is mainly polyester amino baking paint system, and it is necessary to consider increasing the surface tension of the paint when selecting materials. For example, when selecting amino resins, it should be noted that the surface tension of methyl etherified amino resins is higher than that of butyl etherified amino resins, and silicone additives that reduce surface tension are often not used.
Polarity Regarding the adhesion between UV inks and organic coatings such as primers, the intermolecular forces account for a large proportion. These forces mainly include hydrogen bonds and van der Waals forces. The polarity of the molecules has a great impact on this. influence. In general, the more polar the molecule, the greater the force.
The material used in the printing iron primer is not very polar. The polarity is mainly provided by some reactive functional groups. During the baking curing process, as these functional groups disappear through chemical reaction, the polarity of the coating will decrease. This can explain that in the production process, the temperature and time of the oven will affect the curing of the coating, which in turn affects the adhesion of the UV ink. On the other hand, when selecting coating raw materials, more polar materials should be selected to improve UV ink adhesion.
Resistance In practical applications, when the ink layer is cured very well, that is, the resistance is excellent, UV inks tend to adhere poorly. This is mainly because before the UV ink is cured, it has a certain erosion and penetration effect on the carrier, which can greatly improve the adhesion of the UV ink.
Surface Roughness Surface roughness also has a significant effect on adhesion. A smooth surface creates a mirror effect that makes the adhesion between layers very poor. The most common example is that in the construction process of wall paint and wood paint, it is often necessary to use grinding to improve the adhesion of the paint. There are two main mechanisms of action. First, there are many holes, “tunnels”, cracks, etc. on the rough surface from the microscopic point of view. During the wetting process, the attachments will penetrate into these points and play an anchoring role through mechanical occlusion, thereby enhancing the adhesion. Second, the rough surface can greatly increase the surface area of the object, so that the interface between the carrier and the attachment can be significantly improved, thereby improving the adhesion. Adding an appropriate amount of filler to the coating can reduce the gloss and flatness of the coating, and the adsorption of the filler itself can often improve the adhesion of UV inks.
Reactive groups, hydrogen bonds and van der Waals forces are only secondary valence forces. If the carrier and the attachment can react chemically to form chemical covalent bonds, this force is the primary valence force, and the strength is much higher than the secondary valence force. Therefore, some auxiliaries rich in reactive reactive groups are added to the primer, and they are controlled to remain in the coating. During printing, they are chemically cross-linked with the reactive groups in the UV ink through UV light irradiation, which can greatly enhance the adhesion.
As another player in adhesion, UV inks are just as important in their performance as primers. Formulators need to improve ink adhesion in many ways. The focus is still on surface tension and material polarity, while curing rate and choice of additives are also influencing factors.
Surface tension and material polarity The main resins of UV inks are generally some prepolymer resins rich in unsaturated bonds, such as acrylates, polyurethane modified acrylates, polyester modified acrylates, epoxy modified Acrylics, etc. According to the aforementioned wetting law, compared with the primer, the UV ink should appropriately reduce the surface tension in order to better wetting and spreading on the carrier. At the same time, the material selected for UV ink should also have a strong polarity, in order to better generate intermolecular force with the carrier. These can be achieved by the properties of the selected prepolymer resin.
Cure Rate The adhesion of UV inks can be significantly affected by the cure rate. One of the main reasons for poor adhesion is that the curing is too fast. There are two specific manifestations. On the one hand, it is cured when it is not fully wetted and spread, and good interfacial force cannot be achieved. On the other hand, the chemical reaction will cause the material to shrink, and the resulting internal stress that is inconsistent with the direction of the adhesion will not be released during rapid curing, which will affect the adhesion between the attachment and the carrier.
The curing rate of UV ink mainly depends on the unsaturated bond content of the selected material, the type and amount of photoinitiator, and the power of the energy lamp.
Selection of additives In order to achieve the desired effect of UV ink, many additives are often needed, such as leveling agent, polymerization inhibitor, dispersing agent, etc. In addition, some adhesion promoters can also be selected to enhance the adhesion of the ink layer.
This refers to the control of UV ink printing, which mainly includes the following aspects.
The power of UV lamp UV lamp is the main factor affecting the curing of UV ink. The power is too low, the energy value is not enough, and the ink layer is not fully cured. If the power is too high, the energy value is too high, the ink cures too fast, the internal stress increases, and the adhesion becomes poor.
It should be noted that the UV lamp has an aging phenomenon, and the energy will gradually decrease after long-term use. Therefore, it is necessary to use an energy meter to test the UV curing equipment frequently to ensure that the energy value is within a reasonable range.
In recent years, the industry has begun to introduce UV LED light curing lamps. Compared with traditional mercury lamps, its wavelength is narrower, more targeted, and the curing speed of ink is faster. In addition, because it is a cold light source, there is no preheating effect, and it is difficult to combine the ink with the primer, resulting in poorer adhesion.
However, since the light intensity of the UV LED lamp can be adjusted, it is possible to find the UV curing strength for the best adhesion through practice. In addition, you can also consider adding an infrared preheating system. In a word, how to solve the adhesion of UV LED system is the main research direction of coating and ink suppliers.
Thickness of ink layer For some colors with special requirements, a thicker ink layer is required to achieve color fullness and brilliance, and the thicker the ink layer, the more difficult it is for UV light waves to penetrate, resulting in the phenomenon that the surface is dry and the inside is not dry, and then Affects ink adhesion. At the same time, the thicker ink layer will also produce greater shrinkage, resulting in stronger internal stress and reduced adhesion.
Ink balance In the UV printing process, the fountain solution will have an effect on the ink. Improper pH value can easily lead to the imbalance of ink and water, and the ink is emulsified, which in turn affects the curing of the ink, and finally has an adverse effect on the adhesion of the ink.
To sum up, there are many factors affecting the adhesion of UV ink. How to solve this problem requires the joint efforts of all participants in the production chain. Collaborating with all parties to promote technological progress in the industry is the only way to promote the development of the industry.