When the ink and varnish are exposed to UV radiation, free radical photoinitiators interact with the adhesive. The type of photoinitiator determines the type of free radicals that are released, which in turn cause a difference in curing. One of the main tasks of configuring and using UV inks and varnishes is to optimize key characteristics whose effects may be contradictory: to have good runnability and distribution, quick response and flexibility, but also good Adhesion and minimal degeneration of color and sensation.
Interaction with radiant energy
Free radical photoinitiators only absorb the radiant energy generated by UV lamps in a specific part of the UV spectrum. The intensity of the radiation depends on the pigments in the UV ink, because these pigments absorb some UV radiation. Therefore, they have a considerable influence on the efficiency of generating free radicals, that is, the reaction to UV ink and thus the efficiency of the curing process. If the UV rays meet the pigment particles, you may see three different forms of reaction: a small portion of the radiation will be reflected back (exempted) on the surface of the pigment and can be used by the photoinitiator to generate the initiator. Depending on the absorption characteristics of the pigments involved, usually a larger portion of the radiation will be absorbed by the pigments and therefore will not be used by the photoinitiator to generate the initiating group. The part of the radiation (transmission) that is neither reflected nor absorbed can be absorbed by the photoinitiator and converted into chemical energy in the form of an initiator (photosplitting action).
From the position of the absorption curves α (λ) of the photoinitiators ITX, Irgacure 369 and TPO related to the UV lamp Hg (UV) radiation curve I (λ), it can be seen which photoinitiators are used in the peak of the UV lamp Inspire to react
The chemical, physical and spectral characteristics of the pigments contained in UV-curable inks are different from those of oxidatively-cured inks. If the absorption curve α (λ) of the photoinitiator is placed above the transmission curve τ (λ) of the pigment in the UV ink, then the reactivity of the relevant pigment dyeing to different photoinitiators in the UV range (250-400 nm) The effect can be transparent. The absorption curve (not shown) of the color pigment is similar to the transmission curve; only the black (K) is significantly higher
In order to cure UV varnishes, photoinitiators that mainly absorb radiation in the short-wave UV-C spectrum (200-280 nm) are usually sufficient, because the penetration of energy-rich UV-C radiation is reduced when no pigments are present, Will not weaken curing. If, for example, in online preprinting, the varnish competes with the pigment to absorb radiation, then a photoinitiator capable of absorbing UV-B, and most importantly UV-A radiation, must be used because radiation in this wavelength range Has a lower energy level, but has a greater penetration capacity.
The aging of UV lamps is mainly manifested by its own significant decrease in UV-C radiation, which can weaken the surface hardening and thus cause the surface to become sticky.
Task sharing
Normally, short-wave radiation that penetrates only the surface triggers curing and (surface) film formation, while longer-wave radiation that penetrates further cures the deeper layer and attaches the varnish to the substrate. The formulation of printing inks usually includes a photoinitiator for one of the following two functions-namely, alpha hydroxyketone (AHK) for surface hardening and alpha aminoketone (AAK) for deep curing or sunlight-sensitive bisaroyl phosphor Oxygen compounds (BAPO). Other modern photoinitiators used in printing inks are Ciba's Irgacure 369 and 379, isopropylthioxanthone (ITX), trimethylbenzoyl-diphenylphosphine oxide (TPO) and monoacyl-phosphinoxide (MAPO) . The yellowish benzophenone ether was initially replaced by benzyl derivatives such as benzyl dimethyl ketal (BDK). These were subsequently replaced by acetophenone (such as alpha hydroxy ketone), the latter and benzophenone (BP) are now commonly used to cure transparent varnishes.
When exposed to UV radiation, photoinitiators generate radicals through two major reactions: fragmentation and hydrogen abstraction. Most initiators immediately fragment ("decompose") into reactive free radicals with free electrons. Other initiators, including BP and ITX, require the presence of amines or amine-reconstructed acrylates, the so-called synergists. When the hydrogen atoms split, they can generate free radicals that react. In addition, ITX can act as a sensitizer for other photoinitiators (such as certain AAKs; usually combined with Ciba's Irgacure 907), thereby speeding up the curing process.
Notes on food packaging
Each packaging product has different requirements for printing inks, depending on the product that must be packaged with it. For packaging of aggressive products like household cleaners, if ink and varnish may come into contact with the packaged item, they are mainly required to have chemically resistant ingredients. For food and semi-luxury products, it is important that the ink and varnish must be neutral in terms of feel, that is, have no odor and will not spoil. This applies not only to cured adhesive films, but also to unconverted photoinitiators and their fission products. If there is migration of ink and varnish, that is, their particles migrate to the packaged items, even if they do not spoil, they are odorless, and they do not meet the requirements. This requirement also applies to the binding of children ’s books and textbooks. Today these products are often produced at low cost in critical countries, where the inspection of hazardous substances does not yet meet the EU standards.
Photoinitiators in fragmentation: benzyl ketal (1: decomposed into two free radicals each with a free electron), benzalkonone ether (2) and alpha hydroxyl?
Photoinitiators that are taking hydrogen: benzophenone (1: it combines with a hydrogen atom from a synergist to form diphenylmethanol, during which the synergist becomes a free radical) and ITX (2)
Migration barriers, such as aluminum foil in laminates, do not always prevent contamination by the packaging. For example, Italy confirmed in November 2005 that a very small amount of ITX was found in the packaging of children's milk drinks. The contact between the ink and the packaged object is a consequence of stickiness. If the inner surface of the packaging material that is not printed is in contact with the already printed outer surface in printing production, stickiness will occur. Although ITX may be originally harmless, it often leaves residues in packages containing fat or jam, and the long-term health effects of these residues have not been determined. After the scandal, at least in Italy, it was prohibited to use ITX-containing inks and varnishes for food packaging until further notice was received, and packaging manufacturers in other countries voluntarily gave up their use.
When benzophenone is used in food packaging, specific restrictions are imposed on migration. When developing UV-free varnishes, Vegra Group focused on eliminating photoinitiators such as benzophenone and texanol diisobutyrate (TXIB) (2,2,4- Methyl 1,3-pentanediol diisobutyrate).
Although, like BP, TXIB is not listed on the LD50 scale (which is used to indicate the lethal dose per unit body weight, half of the test animals that perform this measurement will die), but BP's material The safety data sheet does warn it that it is irritating to eyes, respiratory organs and skin, and harmful to aquatic organisms.
Interaction with adhesive
The choice of photoinitiator still depends on its working condition in varnish or ink. Liquid photoinitiators (such as a mixture of BP and special AHK) have advantages because they are easily dissolved or dispersed in the adhesive. Powdered photoinitiators (such as TPO) must be dissolved and then mixed.
In free radical curing varnishes, acrylic resins are not suitable for all applications. Although epoxy acrylates are extremely reactive and resistant to chemical reactions, at the same time they are very sticky and the varnish will be correspondingly hard. Polyester acrylates have relatively low viscosity levels and produce flexible varnishes, and their low price and good adhesion are a compensation for their general reactivity. Polyurethane acrylate can well resist chemical reactions, can be firmly attached to the substrate and generate a flexible glazing oil film, but usually the price is more expensive. Polyether acrylates are characterized by very low viscosity and relatively high reactivity. Silicone acrylate is only used for special purposes or as an additive. Their reactivity is relatively low, and usually expensive. The properties of acrylates, such as reactivity, viscosity or its potential irritation, in many cases depend on their chemical structure. For example, certain polyester acrylates have all the properties mentioned.
Odor production
When exposed to UV radiation, the photoinitiators mentioned above produce fission products, which may emit more or less detectable odors; some are actually odorless. In general, unnecessary excessive photoinitiators will increase the concentration of fission products, which may make the ink and varnish emit a strong smell, so that people feel their presence. It should be emphasized here that acrylate adhesives that do not form a network may have a unique odor that will decrease after polymerization. For several years it has been possible to provide improved acrylates which have been specially developed to reduce odor and skin irritation.
Undesirable odors may also be caused by paper coatings. In the absence of UV inks or varnishes, when exposed to UV radiation, paper coatings may produce a noticeable odor. This can be limited by irradiating unprinted paper. The generation of odor often indicates that the radiation level is too high, a more obvious sign is that the paper coating becomes brittle. This issue was discussed in detail in the third phase of KBA Process, "Compound Production Improves Quality". From all variables, we can deduce that in UV and composite offset printing, it is crucial to achieve the best balance between radiation level, photoinitiator and substrate sensitivity. Of course, UV varnishes must be compatible with UV inks or composite inks.
Ask your supplier
It is up to the manufacturer to decide which ingredients to use in UV inks and varnishes. Hazardous ingredients are listed on the material safety data sheet. The ink manufacturers of the European Printing Ink Association promise to discard toxic and environmentally hazardous materials that are included in an exclusion list. As a principle, if inks, varnishes and substrates come into contact with food, they should be tested for toxicity. Varnishes and inks can only be applied on the outer surface of food packaging and must comply with EU directive 82/711 / EEC. Typical raw materials used in the manufacture of synthetic materials must not exceed the specified migration limit. However, there is no official SML value for common raw materials used in inks and varnishes. To avoid unpleasant incidents, we recommend that you obtain detailed information from UV ink and varnish suppliers.
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