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In the past, inks based on mineral oil based binders were used primarily for printing on plastic films. In the past few years, as printers, processors, and end customers have been calling for better performance systems, people have come to the conclusion that UV technology is the best alternative to oil-based inks. This article describes the state and innovation of these two ink systems in detail from the perspective of the ink manufacturer Siegwerk.
Special requirements for printing on films and foils Synthetic substrates, such as plastic films and other non-absorbent printing substrates, are appearing more and more frequently in the printing industry, especially in offset printing. The challenges of finding suitable offset ink systems for these applications include:
* Good printability,
* Improved operating characteristics for use on faster and faster presses,
* Strong adhesion and scratch resistance on non-absorbent substrates.
The surface structure of a typical plastic film does not allow the ink to solidify compared to most paper and paperboard substrates. It is impossible to support the drying and adhesion of the ink by the method of penetrating the substrate. In addition, the presence of fountain solution in oil-based offset inks often impairs the drying process. Therefore, a good ink balance is a key factor affecting the drying process. Special oil-based inks have been developed for synthetic substrates to meet the specific technical requirements for the quality of printed products. However, when using oil-based inks, it is still extremely difficult to achieve a good compromise between fast drying, safe stacking, adhesion and scratch resistance.
The advantages of UV curing technology should be used here. This includes
* Immediate hardening of the ink layer,
* The impact of the amount of fountain solution is small, and
* Quickly prepare for subsequent processing.
The development of improved radiation-curing printing inks
When UV ink systems were first introduced into the printing industry, they were criticized for their printing habits and adhesion problems in offset presses. These problematic properties have been successfully overcome by new materials and innovative ink formulations.
Printability of UV inks
In non-absorbent substrates such as plastic films, the fountain solution cannot penetrate into the surface. First-generation UV inks tend to accumulate on ink rollers, plates, and/or blankets because they absorb excess fountain solution and thus cause loss of tack. Here, the optimized ink balance improves printability. Subsequent generations of inks optimize the absorption and release of the fountain solution, exhibiting much wider boundaries between excessive wetting and insufficient wetting. In the past few years, alcohol-free printing has been growing. However, especially in the field of printing films and foils, the use of isopropyl alcohol has proven to be a better choice. The reduction in the surface tension of the fountain solution facilitates optimal printing and good ink-water balance. Depending on the configuration of the press, printing plate, etc., an alcohol substitute can be used instead of IPA (isopropanol).
UV ink adhesion
In the past, UV inks showed limited adhesion to films and foils, but these problems have been successfully overcome by newly developed special ingredients and optimized ink formulations.
However, one basic condition is that a substrate such as PVC must not contain any plasticizers, static eliminators and other substances that may reduce adhesion.
The recommended surface tension for PVC is 35 millinewtons/meter. For substrates made of ABS, PP, PET, PE and PS, surface tensions exceeding 40 mN/m will be critical. These substrates should also be free of problematic additives, such as static eliminators, whose separation may prevent uniform ink adhesion. The correct surface tension of the synthetic substrate will depend on the formulation of the material and will therefore depend on the substrate manufacturer. Manufacturers also sometimes use the so-called corona discharge technology to treat the surface of the diaphragm electrically. When the substrate is stored for a long time, the surface tension may drop, so corona pretreatment must be performed directly on a web or sheet-fed press. If non-pretreated substrates are used for reasons of economy, it is recommended that this alternative method be used.
In addition, the molecular structure of the cured ink layer affects the adhesion properties, flexibility, and scratch resistance.
UV curing layer flexibility
UV inks and coatings are usually easy to shrink during curing. The thicker the ink layer, especially the UV coating layer, the greater the volume of shrinkage. One result is reduced adhesion.
Especially in cross-cut tests, the adhesive force of the tape can exceed the adhesion of the ink/paint and the print substrate, leaving the ink-coating layer away from the substrate. Adhesion quality also largely depends on the use of a highly flexible ink/coating/binder system that reduces shrinkage.
UV ink curing
Different curing properties can also affect the adhesion of the ink/paint layer. If the ink is not completely cured, adhesion may be impaired due to insufficient crosslinking of the ink/paint layer. In rare cases, overcured ink/coating layers can become brittle due to excessive shrinkage, which can lead to reduced flexibility and adhesion.
The versatility of UV inks
In the early days of UV technology, it was seldom possible to use specially formulated inks for film and foil printing at the same time for paper and board printing because the viscosity of such oligomers was adjusted too high to ensure adhesion. Today, the optimized quality of UV film printing inks allows them to be used on paper-based substrates in many cases.
UV system requirements in film printing
The film-forming quality of the UV inks can be significantly improved by using doping lamps and by putting them in a nitrogen atmosphere (inert UV). The so-called cold UV system reduces the amount of heat emitted and reduces the stack temperature, thus preventing the film and foil from changing in size; however, it also slows the polymerization of UV inks and coatings. If the cross-linking of the molecules takes place in nitrogen, the polymerization will be accelerated, allowing the printing speed to be increased. It is essential to test the adhesion of inks and coatings in all jobs. Unlike scratch resistance, there is usually no improvement in adhesion after 12 hours of printing.
The nature of sensory and molecular migration
Printed films are used for packaging, shrink films, cosmetic packaging, labels, and many other uses. Some of these applications have special technical requirements for UV inks and coatings, such as
* The smell should be small
* No effect on the taste of packaged foods,
* There is no migration to the packaged food.
In the context of increasing customer demand, legislation and better analysis, ink manufacturers must meet new requirements every day. The quality of the ink is formed by special materials that minimize the effects of organ sensations, such as stimulation of sensitive nerve endings such as olfactory and taste nerves in the mucosa, while reducing molecular migration. The optimised photoinitiator system, the use of high molecular weight ultrapure monomers and oligomers, and the modification of the formulations have resulted in a very low migration and thus help meet new requirements.
With all the high-purity materials and sophisticated manufacturing techniques, the user should still check and adjust the ink supplier's recommendations in the technical environment (printer, UV system, printing speed, etc.) of the pressroom that meets the legal requirements. quality. In addition to inks and coatings, data on organ perception and migration are influenced by many other parameters in addition to the ink manufacturer's influence. Special inspection of applicable cleaners and dampening additives. After UV curing, the printed substrate may also produce an inherent odor. Careful handling and storage of printed jobs is another important factor. If you have a problem or have completed a job in a particular job, especially in a sensitive area of ​​food packaging, it is recommended that you contact your local ink and coating manufacturer's representative for the best technical support and advice.
Summary
Printing on substrates with ever-increasing amounts of impervious surfaces—from plastic diaphragms to metallized substrates or even sheet metal—requires ink manufacturers to continually develop their products. The ever-increasing press speed and the resulting shorter drying time pose special challenges to people.
Because oil-based systems do not shrink, they continue to be used for special purposes. However, the continuous improvement of UV technology in ink formulation and manufacturing (especially in terms of organoleptic quality), control of pre-tone tone values, and physics of radiating equipment will continue to dominate the printing industry. The status of growth.
Special requirements for printing on films and foils Synthetic substrates, such as plastic films and other non-absorbent printing substrates, are appearing more and more frequently in the printing industry, especially in offset printing. The challenges of finding suitable offset ink systems for these applications include:
* Good printability,
* Improved operating characteristics for use on faster and faster presses,
* Strong adhesion and scratch resistance on non-absorbent substrates.
The surface structure of a typical plastic film does not allow the ink to solidify compared to most paper and paperboard substrates. It is impossible to support the drying and adhesion of the ink by the method of penetrating the substrate. In addition, the presence of fountain solution in oil-based offset inks often impairs the drying process. Therefore, a good ink balance is a key factor affecting the drying process. Special oil-based inks have been developed for synthetic substrates to meet the specific technical requirements for the quality of printed products. However, when using oil-based inks, it is still extremely difficult to achieve a good compromise between fast drying, safe stacking, adhesion and scratch resistance.
The advantages of UV curing technology should be used here. This includes
* Immediate hardening of the ink layer,
* The impact of the amount of fountain solution is small, and
* Quickly prepare for subsequent processing.
The development of improved radiation-curing printing inks
When UV ink systems were first introduced into the printing industry, they were criticized for their printing habits and adhesion problems in offset presses. These problematic properties have been successfully overcome by new materials and innovative ink formulations.
Printability of UV inks
In non-absorbent substrates such as plastic films, the fountain solution cannot penetrate into the surface. First-generation UV inks tend to accumulate on ink rollers, plates, and/or blankets because they absorb excess fountain solution and thus cause loss of tack. Here, the optimized ink balance improves printability. Subsequent generations of inks optimize the absorption and release of the fountain solution, exhibiting much wider boundaries between excessive wetting and insufficient wetting. In the past few years, alcohol-free printing has been growing. However, especially in the field of printing films and foils, the use of isopropyl alcohol has proven to be a better choice. The reduction in the surface tension of the fountain solution facilitates optimal printing and good ink-water balance. Depending on the configuration of the press, printing plate, etc., an alcohol substitute can be used instead of IPA (isopropanol).
UV ink adhesion
In the past, UV inks showed limited adhesion to films and foils, but these problems have been successfully overcome by newly developed special ingredients and optimized ink formulations.
However, one basic condition is that a substrate such as PVC must not contain any plasticizers, static eliminators and other substances that may reduce adhesion.
The recommended surface tension for PVC is 35 millinewtons/meter. For substrates made of ABS, PP, PET, PE and PS, surface tensions exceeding 40 mN/m will be critical. These substrates should also be free of problematic additives, such as static eliminators, whose separation may prevent uniform ink adhesion. The correct surface tension of the synthetic substrate will depend on the formulation of the material and will therefore depend on the substrate manufacturer. Manufacturers also sometimes use the so-called corona discharge technology to treat the surface of the diaphragm electrically. When the substrate is stored for a long time, the surface tension may drop, so corona pretreatment must be performed directly on a web or sheet-fed press. If non-pretreated substrates are used for reasons of economy, it is recommended that this alternative method be used.
In addition, the molecular structure of the cured ink layer affects the adhesion properties, flexibility, and scratch resistance.
UV curing layer flexibility
UV inks and coatings are usually easy to shrink during curing. The thicker the ink layer, especially the UV coating layer, the greater the volume of shrinkage. One result is reduced adhesion.
Especially in cross-cut tests, the adhesive force of the tape can exceed the adhesion of the ink/paint and the print substrate, leaving the ink-coating layer away from the substrate. Adhesion quality also largely depends on the use of a highly flexible ink/coating/binder system that reduces shrinkage.
UV ink curing
Different curing properties can also affect the adhesion of the ink/paint layer. If the ink is not completely cured, adhesion may be impaired due to insufficient crosslinking of the ink/paint layer. In rare cases, overcured ink/coating layers can become brittle due to excessive shrinkage, which can lead to reduced flexibility and adhesion.
The versatility of UV inks
In the early days of UV technology, it was seldom possible to use specially formulated inks for film and foil printing at the same time for paper and board printing because the viscosity of such oligomers was adjusted too high to ensure adhesion. Today, the optimized quality of UV film printing inks allows them to be used on paper-based substrates in many cases.
UV system requirements in film printing
The film-forming quality of the UV inks can be significantly improved by using doping lamps and by putting them in a nitrogen atmosphere (inert UV). The so-called cold UV system reduces the amount of heat emitted and reduces the stack temperature, thus preventing the film and foil from changing in size; however, it also slows the polymerization of UV inks and coatings. If the cross-linking of the molecules takes place in nitrogen, the polymerization will be accelerated, allowing the printing speed to be increased. It is essential to test the adhesion of inks and coatings in all jobs. Unlike scratch resistance, there is usually no improvement in adhesion after 12 hours of printing.
The nature of sensory and molecular migration
Printed films are used for packaging, shrink films, cosmetic packaging, labels, and many other uses. Some of these applications have special technical requirements for UV inks and coatings, such as
* The smell should be small
* No effect on the taste of packaged foods,
* There is no migration to the packaged food.
In the context of increasing customer demand, legislation and better analysis, ink manufacturers must meet new requirements every day. The quality of the ink is formed by special materials that minimize the effects of organ sensations, such as stimulation of sensitive nerve endings such as olfactory and taste nerves in the mucosa, while reducing molecular migration. The optimised photoinitiator system, the use of high molecular weight ultrapure monomers and oligomers, and the modification of the formulations have resulted in a very low migration and thus help meet new requirements.
With all the high-purity materials and sophisticated manufacturing techniques, the user should still check and adjust the ink supplier's recommendations in the technical environment (printer, UV system, printing speed, etc.) of the pressroom that meets the legal requirements. quality. In addition to inks and coatings, data on organ perception and migration are influenced by many other parameters in addition to the ink manufacturer's influence. Special inspection of applicable cleaners and dampening additives. After UV curing, the printed substrate may also produce an inherent odor. Careful handling and storage of printed jobs is another important factor. If you have a problem or have completed a job in a particular job, especially in a sensitive area of ​​food packaging, it is recommended that you contact your local ink and coating manufacturer's representative for the best technical support and advice.
Summary
Printing on substrates with ever-increasing amounts of impervious surfaces—from plastic diaphragms to metallized substrates or even sheet metal—requires ink manufacturers to continually develop their products. The ever-increasing press speed and the resulting shorter drying time pose special challenges to people.
Because oil-based systems do not shrink, they continue to be used for special purposes. However, the continuous improvement of UV technology in ink formulation and manufacturing (especially in terms of organoleptic quality), control of pre-tone tone values, and physics of radiating equipment will continue to dominate the printing industry. The status of growth.
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