Gluing is one of the finishing technologies of printed materials. By coating (or spraying or printing) a layer of colorless transparent coating on the surface of the printed material, the surface of the printed material can be thin and evenly transparent after being leveled, dried, and calendered. Bright layer.
Glossing can not only increase the surface smoothness of prints, make them more glossy, but also can protect printing artwork. It is widely used in packaging decoration, books and magazine covers, pictures, trademarks, advertisements, calendars, etc. Large-scale decoration, posters and other printed materials in the surface processing. Currently, glazing technology has been widely adopted as an extension of printing and an important means for print finishing. It has become an important means to achieve and improve the quality of printed materials.
The glazing process can be divided into oxidative polymerization glazing, solvent evaporation glazing, UV glazing and thermal curing glazing according to the glazing drying method.
UV varnish is UV curing, using UV varnish, after coating on the surface of the print, the varnish can be cured in a very short time by ultraviolet irradiation. Compared with other coating methods, UV coating has the following advantages:
(1) Varnish is 100% solids, no solvent evaporation, safe and environmentally friendly.
(2) Fast drying speed and high production efficiency. The general drying speed can reach 100-300 meters/minute, and the fastest is more than 500 meters/minute.
(3) Not limited by the type of substrate, not only on the surface of paper, aluminum, plastic materials, etc., but also can be applied to the surface of polymer boards, furniture, metal materials, wireless materials, etc.
(4) Low energy consumption and energy saving.
(5) Working at low temperature, can be used for the printing of heat-sensitive substrates (such as plastic).
(6) The quality is good. After the UV coating, the surface of the printed product is smooth and bright, without any problems such as curling, wrinkling, blistering, etc., and the surface hardness is high, and the abrasion resistance is not so easy. The chemical resistance is also good.
(7) After the polishing, the substrates can be processed immediately, no longer need to be hardened, and have higher adaptability in cutting, forming, folding, and the like.
Therefore, UV glazing, as an important glazing method, has become more and more widely used.
I. Composition of UV Varnish
UV varnish consists mainly of oligomers, reactive diluents, photoinitiators and other additives.
1. Oligomers
Oligomers, also known as prepolymers, are the most basic components in UV coatings. It is a film-forming substance whose properties play an important role in the curing process and the properties of the cured film. Structurally, the oligomers are all low-molecular resins containing C=C unsaturated double bonds, mostly acrylic resins.
At present, the oligomers commonly used in UV coatings include epoxy acrylic resins, polyurethane acrylic resins, and polyester acrylic resins. Epoxy acrylic resin has the characteristics of fast curing speed, low price, etc. The coated oil is coated on paper or paperboard to make it have good chemical resistance and high mechanical strength. Polyurethane acrylic resin has the characteristics of good flexibility, strong elasticity and high gloss, but it is more expensive and is often mixed with epoxy acrylic resin. Polyester acrylic resin has low viscosity, good flexibility and low price. Because of its good adhesion to non-absorbing substrates such as plastics, metals, etc., it is often used as a coating for such materials. It is generally used in combination with other resins, and sometimes it can be used as a thinner for polishing oils to adjust the ring. The viscosity of oxy acrylic and urethane acrylics.
2. Reactive diluent
Reactive diluents, also called cross-linking monomers, are a functional monomer. Its role in polishing oils is to adjust viscosity, cure speed, and cured film properties. Reactive diluents also contain C=C unsaturated double bonds on the structure, and most of the currently used acrylic monomers are used. Due to the different amounts of acrylic groups, they can be divided into three categories: mono-functionality, di-functionality and poly-functionality. Reactive diluents with different functionalities have different dilution effects and cure rates. In general, the higher the functionality, the faster the cure rate, but the worse the dilution effect. Commonly used difunctional glazing reactive diluents are: hexanediol diacrylate (HDDA), neopentyl glycol diacrylate (NPGDA), tripropylene glycol diacrylate (TPGDA), etc.; trifunctional There are: Trimethylolpropane triacrylate (TMPTA) and the like.
3. Photoinitiator
Photoinitiators are substances that absorb radiant energy and produce chemical intermediates that have the ability to initiate polymerization. They are the main components required for any UV curing system.
Photoinitiators can be divided into hydrogen abstraction and cracking types. Hydrogen abstraction photoinitiators need to be combined with a compound containing active hydrogen (generally known as co-initiators, usually amines) to form free radicals by hydrogen abstraction. It is a bimolecular photoinitiator, and benzophenone (BP) is commonly used in the coating oil. The photo-induced photo-initiator, after photoexcitation, decomposes free radicals in the molecule and is a monomolecular photoinitiator. Commonly used in the coating oil are 1173 (α-hydroxyisopropyl benzophenone), 184 (α-hydroxy cyclohexyl benzophenone) and the like.
4. Additives
Auxiliaries are mainly used to improve the performance of the ink. UV varnishes commonly used additives include stabilizers, leveling agents, defoamers and so on.
(1) Stabilizer
Stabilizers are used to reduce thermal polymerization during storage and improve the storage stability of the coating. The commonly used stabilizers include hydroquinone, p-methoxyphenol, p-benzoquinone, 2,6-di-tert-butylcresol and the like.
(2) Leveling agent
The leveling agent is used to improve the leveling of the surface of the coating film, prevent the occurrence of shrinkage, and also increase the gloss of the coating coating.
(3) Defoamer
Anti-foaming agents are used to prevent and eliminate the air bubbles produced during the manufacture and use of the coating.
Although ordinary UV coatings do not contain solvents, due to the generally high viscosity of the oligomers used, low molecular reactive diluents must be added for coating purposes. They generally have a certain degree of volatility or irritation. Still harmful to the environment. Therefore, waterborne UV coatings have been developed and developed. They do not require the use of reactive diluents to adjust the viscosity. Instead, they use water or a thickener to control the rheological properties of coating oils and solve the problems of volatility and irritation. An ideal environment-friendly product is the future development direction of Varnish.
Second, UV coating oil curing principle
Under ultraviolet irradiation, the photoinitiator in the UV varnish absorbs radiation energy to form free radicals or cations, initiates the polymerization and cross-linking reaction of the monomer and the prepolymer, and cures into a three-dimensional network in a short time. Structured polymers. At present, most of them are free radical polymerization systems.
Third, UV coating method
There are many UV glazing methods, and there are many classification methods. There are mainly the following classification methods:
1. According to the relationship between polishing machine and printing press
According to the relationship between polishing machine and printing machine, coating can be divided into two ways: offline coating and online coating.
Off-line glazing uses a special glazing machine to polish the prints, ie, printing and glazing are performed on their own special equipment. This kind of glazing method is more flexible and convenient, and the glazing equipment investment is small, which is more suitable for professional post-press processing manufacturers. However, this glazing method increases the transportation and transfer between the printing and glazing processes, and the production efficiency is low.
Online coating directly connects the glazing unit to the printing unit, ie, printing and glazing are performed on the same machine, which has high speed, high production efficiency, low processing cost, and reduces the handling of the printed matter and overcomes the problem caused by dusting. The various types of quality failures are the future direction of development. However, the on-line coating has high requirements for coating technology, coating oil, drying equipment, and coating equipment.
2. Classification by glazing method
Press the glazing method. Varnish can be divided into roller coating and printing glazing.
Roller coating is the most common method of glazing. The coating roll is used to spread the coating on the surface of the printed product.
Printing glazing coats lacquer on the prints through a glazing plate so that it can be applied locally. Currently used are gravure coating, flexo coating, offset printing, and screen polishing.
3. According to the product type
Press on no product type. The glazing can be divided into full-width light, partial glazing, extinction glazing, and artistic glazing.
The main role of full-frame light is to protect prints and improve the surface gloss of prints. Full-frame light is generally applied by roller coating.
Partial glossing is generally used to polish parts of graphics that need to be emphasized in printed matter. The use of a high-gloss picture of the glossed part is compared with a low-gloss picture that has no glossiness, resulting in wonderful artistic effects. The partial coating is performed by the method of printing glazing, in which the film obtained by screen printing is thicker, the effect is more obvious than other methods, and the cost is low. Currently, it is used more domestically, but its production efficiency is general. Lower.
Extinction coating uses UV matte oil, which has the opposite effect of ordinary coating. It reduces the glossiness of the printed surface and produces a special effect. Because the gloss is too high to stimulate the human eye to a certain extent, extinction coating is currently a popular glazing method.
The role of artistic glazing is to obtain special artistic effects on the surface of glazing products. If using UV pearlescent coating on the surface of the print, it will produce a pearl effect on the printed surface, making the printed material look grand, elegant and elegant.
Fourth, UV glazing should pay attention to the problem
In order to ensure the quality of UV coating, the following major issues should be noted when polishing:
1. Varnish selection
The glazing oil should first be selected according to the glazing method. Different glazing methods require different properties of the glazing oil. For this purpose, the varnishes with different properties need to be matched. That is, the application of Varnish for offline coating and in-line coating is different. The coating of the roller is different from that for printing coating. When the coating is applied, the varnish used for different printing methods is also different.
Second, the varnish should also match the ink on the printed surface. The surface tension of the coating varnish must be less than the surface tension of the ink, so that the varnish can be well wetted, adhered, and saturate on the graphic part of the printed surface, otherwise, the varnish layer after coating will produce a certain amount of Shrinkage, non-uniformity, and even trembling in some places.
If glazing products need to be post-processed such as bronzing or paste, the varnishes with relatively stable chemical properties should be used.
2. UV curing light source selection
Because the curing of the UV coating is based on the photoinitiator absorbing the radiant energy of the ultraviolet light to form free radicals, thereby initiating the polymerization and cross-linking reaction of the monomer and the prepolymer, therefore, the choice of the UV curing light source for the UV coating The drying speed of the oil has an important effect. The following factors should be considered when choosing a UV curing light source:
(1) The absorption spectrum of the photoinitiator is matched with the output spectrum of the light source as much as possible so as to maximize the use of the radiant energy of the light source and generate more free radicals, thereby increasing the drying speed of the UV coating oil.
(2) The efficiency of conversion of electrical energy to ultraviolet light should be high.
(3) Intensity should be appropriate, strong light, strong free radicals, high concentration, high reaction rate, adverse effects on cross-linking reaction, low intensity, too slow radical production, and very strong oxygen inhibition. Strong, these will make the UV coating dry too slowly.
(4) Long service life. The UV lamp will gradually age, and the intensity of the expired UV lamp will decrease a lot, making the UV coating dry too much.
(5) There should be a very good lampshade condenser, the same lamp, lampshade is good or bad, can make the efficiency difference of more than ten times.
(6) The shape is appropriate, the light can be evenly distributed on the printed matter, and it is easy to install, cheap, and safe
Glossing can not only increase the surface smoothness of prints, make them more glossy, but also can protect printing artwork. It is widely used in packaging decoration, books and magazine covers, pictures, trademarks, advertisements, calendars, etc. Large-scale decoration, posters and other printed materials in the surface processing. Currently, glazing technology has been widely adopted as an extension of printing and an important means for print finishing. It has become an important means to achieve and improve the quality of printed materials.
The glazing process can be divided into oxidative polymerization glazing, solvent evaporation glazing, UV glazing and thermal curing glazing according to the glazing drying method.
UV varnish is UV curing, using UV varnish, after coating on the surface of the print, the varnish can be cured in a very short time by ultraviolet irradiation. Compared with other coating methods, UV coating has the following advantages:
(1) Varnish is 100% solids, no solvent evaporation, safe and environmentally friendly.
(2) Fast drying speed and high production efficiency. The general drying speed can reach 100-300 meters/minute, and the fastest is more than 500 meters/minute.
(3) Not limited by the type of substrate, not only on the surface of paper, aluminum, plastic materials, etc., but also can be applied to the surface of polymer boards, furniture, metal materials, wireless materials, etc.
(4) Low energy consumption and energy saving.
(5) Working at low temperature, can be used for the printing of heat-sensitive substrates (such as plastic).
(6) The quality is good. After the UV coating, the surface of the printed product is smooth and bright, without any problems such as curling, wrinkling, blistering, etc., and the surface hardness is high, and the abrasion resistance is not so easy. The chemical resistance is also good.
(7) After the polishing, the substrates can be processed immediately, no longer need to be hardened, and have higher adaptability in cutting, forming, folding, and the like.
Therefore, UV glazing, as an important glazing method, has become more and more widely used.
I. Composition of UV Varnish
UV varnish consists mainly of oligomers, reactive diluents, photoinitiators and other additives.
1. Oligomers
Oligomers, also known as prepolymers, are the most basic components in UV coatings. It is a film-forming substance whose properties play an important role in the curing process and the properties of the cured film. Structurally, the oligomers are all low-molecular resins containing C=C unsaturated double bonds, mostly acrylic resins.
At present, the oligomers commonly used in UV coatings include epoxy acrylic resins, polyurethane acrylic resins, and polyester acrylic resins. Epoxy acrylic resin has the characteristics of fast curing speed, low price, etc. The coated oil is coated on paper or paperboard to make it have good chemical resistance and high mechanical strength. Polyurethane acrylic resin has the characteristics of good flexibility, strong elasticity and high gloss, but it is more expensive and is often mixed with epoxy acrylic resin. Polyester acrylic resin has low viscosity, good flexibility and low price. Because of its good adhesion to non-absorbing substrates such as plastics, metals, etc., it is often used as a coating for such materials. It is generally used in combination with other resins, and sometimes it can be used as a thinner for polishing oils to adjust the ring. The viscosity of oxy acrylic and urethane acrylics.
2. Reactive diluent
Reactive diluents, also called cross-linking monomers, are a functional monomer. Its role in polishing oils is to adjust viscosity, cure speed, and cured film properties. Reactive diluents also contain C=C unsaturated double bonds on the structure, and most of the currently used acrylic monomers are used. Due to the different amounts of acrylic groups, they can be divided into three categories: mono-functionality, di-functionality and poly-functionality. Reactive diluents with different functionalities have different dilution effects and cure rates. In general, the higher the functionality, the faster the cure rate, but the worse the dilution effect. Commonly used difunctional glazing reactive diluents are: hexanediol diacrylate (HDDA), neopentyl glycol diacrylate (NPGDA), tripropylene glycol diacrylate (TPGDA), etc.; trifunctional There are: Trimethylolpropane triacrylate (TMPTA) and the like.
3. Photoinitiator
Photoinitiators are substances that absorb radiant energy and produce chemical intermediates that have the ability to initiate polymerization. They are the main components required for any UV curing system.
Photoinitiators can be divided into hydrogen abstraction and cracking types. Hydrogen abstraction photoinitiators need to be combined with a compound containing active hydrogen (generally known as co-initiators, usually amines) to form free radicals by hydrogen abstraction. It is a bimolecular photoinitiator, and benzophenone (BP) is commonly used in the coating oil. The photo-induced photo-initiator, after photoexcitation, decomposes free radicals in the molecule and is a monomolecular photoinitiator. Commonly used in the coating oil are 1173 (α-hydroxyisopropyl benzophenone), 184 (α-hydroxy cyclohexyl benzophenone) and the like.
4. Additives
Auxiliaries are mainly used to improve the performance of the ink. UV varnishes commonly used additives include stabilizers, leveling agents, defoamers and so on.
(1) Stabilizer
Stabilizers are used to reduce thermal polymerization during storage and improve the storage stability of the coating. The commonly used stabilizers include hydroquinone, p-methoxyphenol, p-benzoquinone, 2,6-di-tert-butylcresol and the like.
(2) Leveling agent
The leveling agent is used to improve the leveling of the surface of the coating film, prevent the occurrence of shrinkage, and also increase the gloss of the coating coating.
(3) Defoamer
Anti-foaming agents are used to prevent and eliminate the air bubbles produced during the manufacture and use of the coating.
Although ordinary UV coatings do not contain solvents, due to the generally high viscosity of the oligomers used, low molecular reactive diluents must be added for coating purposes. They generally have a certain degree of volatility or irritation. Still harmful to the environment. Therefore, waterborne UV coatings have been developed and developed. They do not require the use of reactive diluents to adjust the viscosity. Instead, they use water or a thickener to control the rheological properties of coating oils and solve the problems of volatility and irritation. An ideal environment-friendly product is the future development direction of Varnish.
Second, UV coating oil curing principle
Under ultraviolet irradiation, the photoinitiator in the UV varnish absorbs radiation energy to form free radicals or cations, initiates the polymerization and cross-linking reaction of the monomer and the prepolymer, and cures into a three-dimensional network in a short time. Structured polymers. At present, most of them are free radical polymerization systems.
Third, UV coating method
There are many UV glazing methods, and there are many classification methods. There are mainly the following classification methods:
1. According to the relationship between polishing machine and printing press
According to the relationship between polishing machine and printing machine, coating can be divided into two ways: offline coating and online coating.
Off-line glazing uses a special glazing machine to polish the prints, ie, printing and glazing are performed on their own special equipment. This kind of glazing method is more flexible and convenient, and the glazing equipment investment is small, which is more suitable for professional post-press processing manufacturers. However, this glazing method increases the transportation and transfer between the printing and glazing processes, and the production efficiency is low.
Online coating directly connects the glazing unit to the printing unit, ie, printing and glazing are performed on the same machine, which has high speed, high production efficiency, low processing cost, and reduces the handling of the printed matter and overcomes the problem caused by dusting. The various types of quality failures are the future direction of development. However, the on-line coating has high requirements for coating technology, coating oil, drying equipment, and coating equipment.
2. Classification by glazing method
Press the glazing method. Varnish can be divided into roller coating and printing glazing.
Roller coating is the most common method of glazing. The coating roll is used to spread the coating on the surface of the printed product.
Printing glazing coats lacquer on the prints through a glazing plate so that it can be applied locally. Currently used are gravure coating, flexo coating, offset printing, and screen polishing.
3. According to the product type
Press on no product type. The glazing can be divided into full-width light, partial glazing, extinction glazing, and artistic glazing.
The main role of full-frame light is to protect prints and improve the surface gloss of prints. Full-frame light is generally applied by roller coating.
Partial glossing is generally used to polish parts of graphics that need to be emphasized in printed matter. The use of a high-gloss picture of the glossed part is compared with a low-gloss picture that has no glossiness, resulting in wonderful artistic effects. The partial coating is performed by the method of printing glazing, in which the film obtained by screen printing is thicker, the effect is more obvious than other methods, and the cost is low. Currently, it is used more domestically, but its production efficiency is general. Lower.
Extinction coating uses UV matte oil, which has the opposite effect of ordinary coating. It reduces the glossiness of the printed surface and produces a special effect. Because the gloss is too high to stimulate the human eye to a certain extent, extinction coating is currently a popular glazing method.
The role of artistic glazing is to obtain special artistic effects on the surface of glazing products. If using UV pearlescent coating on the surface of the print, it will produce a pearl effect on the printed surface, making the printed material look grand, elegant and elegant.
Fourth, UV glazing should pay attention to the problem
In order to ensure the quality of UV coating, the following major issues should be noted when polishing:
1. Varnish selection
The glazing oil should first be selected according to the glazing method. Different glazing methods require different properties of the glazing oil. For this purpose, the varnishes with different properties need to be matched. That is, the application of Varnish for offline coating and in-line coating is different. The coating of the roller is different from that for printing coating. When the coating is applied, the varnish used for different printing methods is also different.
Second, the varnish should also match the ink on the printed surface. The surface tension of the coating varnish must be less than the surface tension of the ink, so that the varnish can be well wetted, adhered, and saturate on the graphic part of the printed surface, otherwise, the varnish layer after coating will produce a certain amount of Shrinkage, non-uniformity, and even trembling in some places.
If glazing products need to be post-processed such as bronzing or paste, the varnishes with relatively stable chemical properties should be used.
2. UV curing light source selection
Because the curing of the UV coating is based on the photoinitiator absorbing the radiant energy of the ultraviolet light to form free radicals, thereby initiating the polymerization and cross-linking reaction of the monomer and the prepolymer, therefore, the choice of the UV curing light source for the UV coating The drying speed of the oil has an important effect. The following factors should be considered when choosing a UV curing light source:
(1) The absorption spectrum of the photoinitiator is matched with the output spectrum of the light source as much as possible so as to maximize the use of the radiant energy of the light source and generate more free radicals, thereby increasing the drying speed of the UV coating oil.
(2) The efficiency of conversion of electrical energy to ultraviolet light should be high.
(3) Intensity should be appropriate, strong light, strong free radicals, high concentration, high reaction rate, adverse effects on cross-linking reaction, low intensity, too slow radical production, and very strong oxygen inhibition. Strong, these will make the UV coating dry too slowly.
(4) Long service life. The UV lamp will gradually age, and the intensity of the expired UV lamp will decrease a lot, making the UV coating dry too much.
(5) There should be a very good lampshade condenser, the same lamp, lampshade is good or bad, can make the efficiency difference of more than ten times.
(6) The shape is appropriate, the light can be evenly distributed on the printed matter, and it is easy to install, cheap, and safe
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