At present, there are four kinds of inks that can be used as heat shrinkable materials for printing: cationic UV inks, radical UV inks, solvent-based inks, and aqueous inks. Let’s take a look!
1, cationic UV ink
Cationic UV inks are less used in the field of heat shrinkable film printing, mainly because they are too expensive, and it is necessary to utilize a specific device to catalyze the polymerization reaction by using a photoinitiator to produce an acidic substance under irradiation of a UV lamp. The cationic agent induces a chain reaction of the epoxy group until the oxygen bond is broken or a glassy polymer is formed, and the chain reaction is terminated. The infrared light emitted by the UV lamp is the key to controlling the reaction rate and the complete reaction. To carry out the cationic polycondensation reaction, it is necessary to fully understand the properties of the film, to avoid cracking of the ink, and to ensure complete curing of the ink.
Cationic ink advantages
The reaction type is a typical ring-opening polymerization of an epoxy group, which increases the effective component content of the ink and is almost completely converted into an ink film.
Cathodic ink shortcomings
Longer reaction time
Since the cationic polycondensation reaction rate is slower than the radical polycondensation, it takes a long time for the ink film to crosslink and adhere to the substrate to reduce the pressure on the substrate, and the ink film shrinkage rate of the cationic cured product is generally less than 5%.
Since the reaction rate is slower than that of free radical polymerization, high temperature curing is often required.
High temperature curing combined with low ink film shrinkage tends to cause the edges of the ink film to be uneven and not smooth. In general, a UV light source of 80 W or more is selected to ensure rapid and complete curing of the cationic ink. Below this temperature, the durability and adhesion of the ink film are significantly reduced, and the odor is large.
Cationic polycondensation reaction will be terminated by atmospheric humidity
High humidity weather can cause constant changes in ink properties. Therefore, the temperature and humidity control system is a necessary device for cationic UV printing.
Ink composition selection is difficult
First, the price of the basic component of the cationic ink itself is higher than that of the free radical. Secondly, since the alkaline substance retards or even inhibits the progress of the reaction, the content of such substances in the ink cannot be excessively high. Acidic substances should not be used in excess, otherwise the viscosity of the ink will increase to become a gel. To this end, ink engineers must strictly control the type of pigment in the ink composition, surface treatment of the pigment to make its pH neutral. Therefore, many pigments cannot be used because they cannot meet the UV ink fluidity and viscosity requirements.
High requirements for dispersants
Cationic inks have strict regulations on the type of dispersant, which limits the amount of pigment added in the ink, resulting in poor color expression of the printed ink layer, and the color of the graphic is not bright.
The cationic initiator contains heavy metal elements, and it also produces a small amount of benzene substances, which pollutes the environment and endangers workers' health.
2, free radical UV ink
The free radical UV ink device also uses a UV lamp to illuminate the photoinitiator to induce a chain polycondensation reaction of the acrylate until the free radical reacts with the pigment, oxygen, additives or other free radicals, or vitrifies to terminate the chain reaction.
Under normal circumstances, there will be a small amount of secondary splitting after the ink is cured by UV light, but there are few real post-cure processes. Therefore, when the ink is cooled and attached to the substrate, adhesion, scratches, and the like may occur. The infrared light emitted by the UV lamp can accelerate the curing speed. This radical reaction is slightly worse than the cationic reaction, but there are other ways to obtain the desired curing effect.
â–½Free radical ink advantages
Fast response
Free radical inks are cured without too high temperature requirements. Under low-energy light sources, they can be cured quickly, which greatly reduces the heat of the substrate and avoids shrinkage of the heat-shrinkable film during printing. And its requirements for ink are not as harsh as cationic inks. Thus, a variety of pigment choices are sufficient to ensure good rheology of the ink on the YAG anilox roll. In addition, high pigment filling reduces the volatile components of the ink, reducing shrinkage and odor.
Wide printability
Since the free radical ink has no excessive restrictions on the composition selection, it can print a substrate with a strong alkalinity and print it in combination with an alkali-soluble, water-soluble flexographic printing ink.
Fast drying
The high adhesion allows the free radical ink to instantly adhere to the substrate and cure quickly under the illumination of a low energy source. In this process, the printing material is not heated, while the cationic ink is not required to be printed at high speed due to high curing energy requirements. This precisely solves the problem that UV flexographic printing productivity is lower than gravure printing. With a suitable curing device and pressure setting, free radical ink will break out of this bottleneck.
The new free radical UV technology can be produced without heavy metal elements and benzene-free substances, but the printing speed is slightly reduced.
â–½free radical ink shortcomings
Free radical inks react quickly and cure all the ingredients together quickly, which increases the pressure on the substrate.
Excessive pressure and a small amount of thermal energy of the UV lamp can cause shrinkage deformation of the film when printing the heat shrinkable film. The shrinkage also causes the free radical UV ink to be as flexible as other UV inks, and the affinity and adhesion are deteriorated.
Free radical inks are easily oxidized.
This can be done with high-energy sources, high-efficiency components, or with oxygen-absorbing initiators such as diamines and triamines. It should be noted that this is the experimental result obtained under the condition that the air condition is relatively stable and there is not much undulation. If it is used in actual production, further argumentation is needed.
Now some companies, such as XSYS's Flexocure XS UV radical polymerization type flexo ink, have been tested to achieve 70% shrinkage, and the product also has good adhesion, suitable for a wide range of materials, such as PVC, OPS, OPP, PETG, etc.
Even with a shrinkage of 70%, these new technologies of free-radically polymerized flexographic printing inks can exhibit good anti-friction properties without the need for primers and varnishes. These products have the advantages of high color strength, fast curing speed, excellent printability and print performance. It can realize both printing and printing processes, and its excellent curing performance and low viscosity are effective guarantees of production capacity.
3, solvent-based ink
Solvent-based inks are commonly used in gravure printing and consist of pigments, solid resins, volatile solvents, fillers, and additives. Most of the pigment components of gravure inks are organic pigments. Organic pigments contain metallic elements such as lead, chromium, mercury, and copper, and have certain toxicity. Solvent-based inks are the largest source of pollution in the printing industry. In 2004, the world's annual ink production was about 3.5 million tons, and solvent-based inks accounted for 30% to 40% on average. The annual amount of organic solvents brought by inks worldwide is as high as 1 million. More than tons. In addition, the ink also contains heavy metal elements that are harmful to the human body. The binder contains 40% to 50% of organic solvents, which can damage the skin of the human body through prolonged contact. High levels of organic solvents in the air cause passive damage to the respiratory system. In addition, the organic solvent used in the ink also reacts with some gases in the air to generate odorous chlorine and smoke, which seriously pollutes the air and has a great influence on the growth of various plants and destroys the ecological balance. Therefore, the conventional gravure solvent type inks tend to be eliminated. Instead, it is an alcohol-soluble composite plastic ink, a non-benzene solvent ink, and an aqueous ink.
Since aqueous gravure inks are not yet fully capable of replacing toluene inks, the ink industry has been striving to develop alcohol-soluble inks that are environmentally acceptable. The alcohol-soluble ink can effectively solve the problems of the damage caused by the toluene ink on the health and the solvent residue affecting the quality of the packaged food. In Europe and the United States, this development has become a trend. In Asia, such as Singapore, South Korea and other countries, toluene inks are being phased out and will be replaced by alcohol-soluble gravure inks. Alcohol-soluble inks have been widely used in the domestic gravure printing industry. It is expected that in the near future, alcohol-soluble inks will occupy a certain share of the shrink-label ink market in China.
4, water-based ink
Water-based inks are commonly used in flexographic printing. The fundamental difference between water-based inks and solvent-based inks is that water used in water-based inks is water rather than organic solvents, which significantly reduces VOC emissions, prevents air pollution, does not affect human health, is not easy to burn, is stable in ink, and is bright in color. Corrosive plate material, simple operation, low price, good adhesion after printing, strong water resistance and rapid drying, so it is especially suitable for packaging printed matter such as food, beverage, medicine, etc. It is recognized as environmentally friendly printing material in the world, and is also all printing inks at present. The only ink approved by the American Food and Drug Association. At present, 40% of the shrink film products in the United States use water-based inks, and other economically developed countries (such as Japan, Germany, France, etc.) use more and more water-based inks in film printing. According to relevant information, in the United States, the water-based ink that meets the VOC emission regulations must reach the ink composition in use, the proportion of volatile components is below 25%, and the moisture is above 75%; the ink with less moisture is not volatile under use. The ingredients are in the two standards of 60% or more.
At present, water-based inks in gravure have also made great progress. WE-type plastic film gravure aqueous ink produced by Beijing Yingkefan Chemical Products Co., Ltd. uses water as the main solvent, and uses a variety of high molecular polymers as film-forming substances, together with high-grade organic pigments, inorganic pigments and various additives. It meets environmental protection requirements and has good printing performance and adhesion fastness on BOPP, PE, PET and other plastic films with surface tension of (3.6 ~ 4.2) × 10-2N / m. After printing on the BOPP film at a speed of 70 m/min, the color printing and packaging factory showed that the printing effect and performance can reach the level of similar products with aromatic hydrocarbon as the main solvent.
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