I. Overview
UV inks are ultraviolet curing inks. The ink can be rapidly cross-linked and cured under ultraviolet (UV) irradiation. It is well known that traditional inks are composed of a binder, a solvent, a colorant (a pigment or a dye), and a small amount of additives. The ink has a suitable privacy and body, and generally contains a considerable amount of solvent (mostly organic solvents). During printing and drying, these solvents gradually volatilize into human space. In other words, the solvent is only useful when it is inked. After printing, it is all ejected from the ink film and becomes useless "scrap." However, once it enters the atmosphere, it becomes a harmful pollutant.
Nowadays. People attach great importance to the prevention of public hazards and the protection of the environment. International environmental protection agencies and governments of all countries have formulated a series of policies and regulations to limit the amount of pollutants and their hazards, just as the standard for organic compounds in the atmosphere (VOC), hazardous substances in water. Content standards, etc. It is not difficult to predict that existing organic solvent inks are bound to be replaced by less (or less) solvent-based inks (UV inks) or water-based inks with less pollution and less harm. Solvent, printing and drying, there is no or little volatile gas overflow, it is a UV-curing dry _ dry type.It has good stability and bright color in application, it is a kind of green ink.Composition of UV ink and traditional ink, There are similarities and great differences.
The initial UV ink is the technology of the American company nmont, which is characterized by the use of unsaturated polyester and styrene monomer as the connecting material, and at the same time adding the photoinitiator, coloring agent and additives. The subject of the ink is (i) the volatilization of styrene monomer; (2) the curing ink film lacks the flexible color (s) ink curing and drying is greatly disturbed by oxygen. Various factors have hindered the promotion and application of the initial UV ink. Only in the sixties and seventies of the 20th century, Americans first cited compounds such as polyfunctional acrylic vinegar as a new ink binder and matched with appropriate photoinitiators, pigments, and additives to create epoch-making significance. UV curing inks, and in 1969, for the first time groundbreaking applications on substrates such as metals, opened a new era of UV inks. Then, Western European countries, Japan, Canada, and Australia also scrambled to start UV ink research, development and application. Since then, UV inks and related technologies have flourished.
Today, the application of UV inks has covered almost all types of printing such as offset printing, offset printing, gravure printing, screen printing, flexographic printing, and inkjet printing. Viewed from UV inks, some are clear inks (colored or colorless and transparent), some are color inks, and some are functional inks with special properties and applications.
In short, the current UV inks and related technologies have been developed to a fairly mature level. They can fully satisfy various printing methods and printing requirements for almost all substrates, and have a very wide application market.
In the middle and late 20th century, U NT inks and their applications were in a period of great development. They reached a considerable degree in terms of ink varieties, application fields, and market size. Table 3 shows the statistics from 1996 to 1997.
In recent years, UV inks have made great progress in serialization and functionalization. Color UV inks, transparent UV inks, paper UV inks, metal UV inks, plastic UV inks, and UV inks And their special thinners and cleaning agents have made their debut. Various functional UV inks are used in practical applications to fully meet the market's application requirements. Novel packing and packaging equipment, arts and crafts, advertisements for light boxes and new crystals for anti-counterfeiting have appeared one after another.
China's UV inks and related technologies started late and developed rapidly. Since the mid-to-late 1990s, UV inks have grown at an annual rate of over 25%. According to statistics from related parties, the market usage in 1998 was about 2 000 tons, of which 1 030 tons of domestic UV inks. At present, most of the UV inks that foreign manufacturers can produce have domestically produced and sold corresponding products and are now moving toward depth.
In recent years, microelectronics technology has advanced by leaps and bounds. Various credit cards, payroll cards, shopping cards, membership cards, conference cards, and IC cards, industrial P cards, and smart cards have been put on the market. New recording media such as CD-R, DVD, etc. Become an indispensable part of people's learning, life, and entertainment. Due to the incomparable characteristics of UV inks in traditional inks, applications in these areas have been rapidly promoted and popularized in recent years.
General UV inks and conventional inks are electrical insulators. However, since the appearance of the conductive ink (including the UV conductive ink), the printed pattern has a conductive function, which provides a new and rare new material for the electronic product to develop in the direction of “light, thin, short, and smallâ€.
In short, UV inks and related technologies are inconsistent with traditional inks regardless of the quality of existing products, the improvement of the printing environment, or the improvement of production efficiency. It is not difficult for people to see that UV inks and related technologies have been playing an increasingly important role in various areas of printing and are getting closer and closer to our daily lives.
Two. UV ink composition and classification
UV inks are a complex composition with photoactivity. Among them are reactive oligomers (prepolymers), reactive monomers (diluents), photoinitiators, colorants (pigments or dyes), and additives. Different printing methods, different application requirements and different types of substrates, UV ink ratios and the selection of related raw materials have corresponding changes, and the latitude allowed for this change is generally large. Table 4 shows the composition and ratio of ordinary UV inks.
(a) Reactive oligomers
Reactive oligomers form the main part of the ink film. Its molecular structure, what kind of functional group on the molecular chain, where the functional group is located on the main chain, how the molecular weight and the polarity of the molecule, will directly affect the photochemical reaction of the system and the physical and chemical properties of the cured ink film ( Hardness, softness, gloss, corrosion resistance, weatherability, adhesion, etc.). Therefore, we should make appropriate choices of reactive oligomers depending on the application and use. Commonly used reactive oligomers include epoxy bisacrylic acid vinegar, polyurethane bisacrylic acid vinegar, polyester bisacrylic acid A F3 and polyether bisacrylic vinegar. In the formulation, in order to improve certain properties of the ink film, it is also possible to add a certain amount of certain polymer resin (such as acrylic resin, unsaturated polyester, etc.) in the binder.
In addition, the introduction of silicone modified acrylic vinegar will significantly improve the solvent resistance, especially the heat resistance of the ink. The addition of a small amount of saturated chlorinated acetal can also increase the adhesion of the ink to certain substrates.
(b) Active monomers
Reactive monomer is also called reactive diluent. It generally refers to an organic compound containing one or more acrylic groups (CHZ=CH-C.) in the molecule. The function of the compound in UV ink is twofold: First, it is used as dilution. Agents to adjust ink anvility and rheology; second is to directly participate in the curing of the photochemical reaction, to increase the system cross-link density, thereby enhancing the ink film strength.
When selecting active monomers, low toxicity, low irritation, low volatility are generally required, and the anvil is smaller and the activity is higher. The active monomers with different chemical structures have a great influence on the dilution effect of the photocurable ink and the physical properties of the cured ink film, and must be carefully selected in the application. For reference, some active monomers and their main properties are listed in Table 6 below.
The most important issue to be aware of is that in order to increase the adhesion of the ink film to the substrate (generally plastic), it is advantageous to choose a reactive monomer that is moderately aggressive to the substrate. At the same time, many factors such as the toxicity and irritation of the active monomer and its effect on the volume shrinkage of the cured ink film cannot be ignored. In short, when choosing reactive diluents, their effects on the ink's overall performance cannot be ignored.
(c) Photoinitiators
Photoinitiators are extremely important for the curing and drying of UV inks. It can be said that without the direct participation of the photoinitiator, curing and drying of the UV ink is unthinkable. Its role in the photocuring system is to absorb ultraviolet light energy and form highly active radicals, thereby initiating the photopolymerization cross-linking reaction of active materials such as reactive oligomers and reactive monomers, and the molecular structure of photoinitiators. How it will determine the distribution and intensity of its UV absorption spectrum. In other words, the photo-induced effect of the initiator is determined. Experience has shown that when the UV absorption wavelength of the photoinitiator is in the range of 200 to 400 nm, especially in the range of 320 to 400 nm, the UV curable effect of the UV ink is ideal. In the application, in order to enhance the priming effect, two or more mixed photoinitiators are often used. In addition, references to photosensitizers and photoactivators have a certain role in the photopolymerization cross-linking reaction of the curing system.
Commonly used photoinitiators include photodecomposition type and hydrogen abstraction type, and photoinitiators containing sulfur and the like have both functions of photodecomposition and hydrogen abstraction. Its name, performance, features and practicality are now applied in Table 7.
When selecting photoinitiators, the following principles should be considered: (1) High UV absorption efficiency; (2) Good storage stability; (3) No odor or small odor; (4) No toxicity or low toxicity; Cured ink film is not yellowed; (6) Miscible with the resin, not easy to precipitate; (7) The price is appropriate.
What needs to be mentioned here is that the UV ink is light-cured and dried, usually at room temperature, and its cross-linked curing is mostly free-radical. However, the ion-curing system, though strict in requirements, is due to the small volume shrinkage of the cured ink film. Strong adhesion, polymerization and cross-linking are not interfered with by oxygen, and have recently emerged in the application of UV inks.
(d) Colorants
The exception of the UV ink colorant is no different from traditional inks. However, the photocuring rate of the ink may be affected by the type, nature and physical properties of the colorant. How the colorant absorbs or reflects ultraviolet light, and how the ultraviolet light penetrates the colorant particles will directly affect the speed of UV ink curing and drying. The black (strongly absorbing) ink film generally has a slow photocuring and drying speed; the white (strongly reflective) ink film has a fast curing and deep curing and slow drying; the yellow, blue, and red color ink films have a relatively fast light curing speed, and The above sequence is accelerated in sequence. In the color printing, in order to obtain a good printing effect, the printing of each color ink should be generally performed in the order of the slow first and then fast (photocuring drying speed), ie, black, yellow, blue, and red. In addition, the lightfastness of pigments or dyes has a great influence on the application of outdoor UV inks.
When selecting the colorant, the following points should be noted: (The power hiding power or coloring power should be strong; (2) The light resistance and weather resistance are better; (3) The wetting ability is better with the connecting material so that the particles can be dispersed; (4) ) Impermeability is good, can not afford to float when printing, the color does not migrate from the ink film after printing; (5) good storage stability; (C) cheaper prices.
In short, the choice of pigment or dye is a comprehensive problem, it should be considered together with the connection materials, photoinitiators and light sources to obtain satisfactory results.
(v) Additives
In order to improve certain properties of the ink, a small amount of leveling agents, plasticizers, polymerization inhibitors, wetting agents, defoamers, stabilizers, and anti-spattering inks are often added in the UV ink manufacturing process.
III. UV ink curing mechanism
Ultraviolet (UV) is essentially an electromagnetic wave. Its wavelength range covers 40 to 400 nm. It has more energy and can trigger photochemical reactions. Because the ultraviolet light within 200 nm (far ultraviolet) is easily absorbed by air, the UV ink is cured and dried, often using 200 to 400 nin, especially the ultraviolet light (near ultraviolet) of 320 to 400 nm wavelength as the light source. Figure 1 shows the electromagnetic wave diagram.
Different wavelengths of light, with different energies, can be calculated by the following formula: E = hV = h · C / where: E-energy (Erg / photons) h-Planck constant v-light frequency C A light enters a wavelength of light at a speed in vacuum
For UV-curable inks, the most commonly used resin system is unsaturated acrylates. The cure drying of this system is a free-radical initiated photopolymerization cross-linking process. See Table 8.
Free radical formation
When the photoinitiator (R) absorbs ultraviolet light energy, it is in an unstable energy excited state (R*), and rapidly decomposes to generate a highly active radical (R·), which immediately initiates the unsaturated in the system. Photopolymerization crosslinking reactions occur with compounds (M1-reactive oligomers, reactive monomers, and other unsaturated compounds or resins). However, it should be noted that not all free radicals will initiate the above reaction. The reason is that some of the radicals may be consumed by oxygen molecules dissolved or miscible in the system, and other radicals may also be bonded to each other to lose their activity. In fact, the radicals that can really participate in the photopolymerization cross-linking process are only the remaining part. Therefore, to ensure that the reaction speed of UV ink photopolymerization and cross-linking does not have a great influence, it is a crucial issue to suppress the degree of radical deactivation. Chain initiation and growth
As described above, the radicals that have just been decomposed generate an immediate photopolymerization crosslinking reaction of the unsaturated compound (M) in the system to generate a new radical (RM+). The free radicals continue to react similarly with the molecules of the surrounding unsaturated compounds. For this reason Han Fu carries on, and the chain is growing, which is the free radical in growth (RMm or RM-) o As the reaction continues to deepen, the molecular weight increases rapidly (RMm + 1 or RM.},) However, as in the previous phase of reaction, some of the free radicals also lose their activity due to various reasons. The free radicals (RMm or RM) in the transfer growth of the chain, if reacted with other active substances in the system (VIII), will generate another free radical (A·), which is what people often call chain transfer. Termination of Chains When two newly generated radicals (RMm and RM) combine to form a neutral molecule (Pm" Pn or Pln+n) during the photochemical reaction, their respective activities have been Loss, the growth of the chain will be terminated (the termination of bond formation.) In addition, when an active molecule A, after capturing an electron from the free radical, the radical will also lose its activity, and the growth of its chain will also be terminated (lost). Electronic termination).
According to the different radical formation mechanism, there are roughly two types of photoinitiators used in UV-curing inks: one is an intramolecular photodegradation type (such as benzoin ethers), and the other is an intermolecular hydrogen abstraction type. (such as thioxanthone/organic amine system). After the former absorbs ultraviolet light energy, the initiator decomposes inside the molecule and releases radicals; the latter under the action of light, the hydrogen proton acceptor (thioxanthone) forms a complex with the hydrogen proton donor (organic amine). The body and the acceptor take hydrogen atoms from the donor, generating free radicals. In photochemical reactions, the former type of bower {the hair sprayer is susceptible to oxygen interference in the air (impeded), and the latter type of initiator is not easily influenced by oxygen to affect the photocuring rate. However, the latter has the disadvantage that the cured ink film tends to yellow.
In short, there are many types of UV-curable inks. In the process of formulation, what kind of reactive oligomers, what kind of reactive monomer, what kind of photoinitiator, what kind of colorant and what kind of additive should be used according to the purpose and actual needs, Carefully choose. The mix and match between them is also ever-changing, and the implementation must be flexible and must not be overshadowed.
UV inks are ultraviolet curing inks. The ink can be rapidly cross-linked and cured under ultraviolet (UV) irradiation. It is well known that traditional inks are composed of a binder, a solvent, a colorant (a pigment or a dye), and a small amount of additives. The ink has a suitable privacy and body, and generally contains a considerable amount of solvent (mostly organic solvents). During printing and drying, these solvents gradually volatilize into human space. In other words, the solvent is only useful when it is inked. After printing, it is all ejected from the ink film and becomes useless "scrap." However, once it enters the atmosphere, it becomes a harmful pollutant.
Nowadays. People attach great importance to the prevention of public hazards and the protection of the environment. International environmental protection agencies and governments of all countries have formulated a series of policies and regulations to limit the amount of pollutants and their hazards, just as the standard for organic compounds in the atmosphere (VOC), hazardous substances in water. Content standards, etc. It is not difficult to predict that existing organic solvent inks are bound to be replaced by less (or less) solvent-based inks (UV inks) or water-based inks with less pollution and less harm. Solvent, printing and drying, there is no or little volatile gas overflow, it is a UV-curing dry _ dry type.It has good stability and bright color in application, it is a kind of green ink.Composition of UV ink and traditional ink, There are similarities and great differences.
The initial UV ink is the technology of the American company nmont, which is characterized by the use of unsaturated polyester and styrene monomer as the connecting material, and at the same time adding the photoinitiator, coloring agent and additives. The subject of the ink is (i) the volatilization of styrene monomer; (2) the curing ink film lacks the flexible color (s) ink curing and drying is greatly disturbed by oxygen. Various factors have hindered the promotion and application of the initial UV ink. Only in the sixties and seventies of the 20th century, Americans first cited compounds such as polyfunctional acrylic vinegar as a new ink binder and matched with appropriate photoinitiators, pigments, and additives to create epoch-making significance. UV curing inks, and in 1969, for the first time groundbreaking applications on substrates such as metals, opened a new era of UV inks. Then, Western European countries, Japan, Canada, and Australia also scrambled to start UV ink research, development and application. Since then, UV inks and related technologies have flourished.
Today, the application of UV inks has covered almost all types of printing such as offset printing, offset printing, gravure printing, screen printing, flexographic printing, and inkjet printing. Viewed from UV inks, some are clear inks (colored or colorless and transparent), some are color inks, and some are functional inks with special properties and applications.
In short, the current UV inks and related technologies have been developed to a fairly mature level. They can fully satisfy various printing methods and printing requirements for almost all substrates, and have a very wide application market.
In the middle and late 20th century, U NT inks and their applications were in a period of great development. They reached a considerable degree in terms of ink varieties, application fields, and market size. Table 3 shows the statistics from 1996 to 1997.
In recent years, UV inks have made great progress in serialization and functionalization. Color UV inks, transparent UV inks, paper UV inks, metal UV inks, plastic UV inks, and UV inks And their special thinners and cleaning agents have made their debut. Various functional UV inks are used in practical applications to fully meet the market's application requirements. Novel packing and packaging equipment, arts and crafts, advertisements for light boxes and new crystals for anti-counterfeiting have appeared one after another.
China's UV inks and related technologies started late and developed rapidly. Since the mid-to-late 1990s, UV inks have grown at an annual rate of over 25%. According to statistics from related parties, the market usage in 1998 was about 2 000 tons, of which 1 030 tons of domestic UV inks. At present, most of the UV inks that foreign manufacturers can produce have domestically produced and sold corresponding products and are now moving toward depth.
In recent years, microelectronics technology has advanced by leaps and bounds. Various credit cards, payroll cards, shopping cards, membership cards, conference cards, and IC cards, industrial P cards, and smart cards have been put on the market. New recording media such as CD-R, DVD, etc. Become an indispensable part of people's learning, life, and entertainment. Due to the incomparable characteristics of UV inks in traditional inks, applications in these areas have been rapidly promoted and popularized in recent years.
General UV inks and conventional inks are electrical insulators. However, since the appearance of the conductive ink (including the UV conductive ink), the printed pattern has a conductive function, which provides a new and rare new material for the electronic product to develop in the direction of “light, thin, short, and smallâ€.
In short, UV inks and related technologies are inconsistent with traditional inks regardless of the quality of existing products, the improvement of the printing environment, or the improvement of production efficiency. It is not difficult for people to see that UV inks and related technologies have been playing an increasingly important role in various areas of printing and are getting closer and closer to our daily lives.
Two. UV ink composition and classification
UV inks are a complex composition with photoactivity. Among them are reactive oligomers (prepolymers), reactive monomers (diluents), photoinitiators, colorants (pigments or dyes), and additives. Different printing methods, different application requirements and different types of substrates, UV ink ratios and the selection of related raw materials have corresponding changes, and the latitude allowed for this change is generally large. Table 4 shows the composition and ratio of ordinary UV inks.
(a) Reactive oligomers
Reactive oligomers form the main part of the ink film. Its molecular structure, what kind of functional group on the molecular chain, where the functional group is located on the main chain, how the molecular weight and the polarity of the molecule, will directly affect the photochemical reaction of the system and the physical and chemical properties of the cured ink film ( Hardness, softness, gloss, corrosion resistance, weatherability, adhesion, etc.). Therefore, we should make appropriate choices of reactive oligomers depending on the application and use. Commonly used reactive oligomers include epoxy bisacrylic acid vinegar, polyurethane bisacrylic acid vinegar, polyester bisacrylic acid A F3 and polyether bisacrylic vinegar. In the formulation, in order to improve certain properties of the ink film, it is also possible to add a certain amount of certain polymer resin (such as acrylic resin, unsaturated polyester, etc.) in the binder.
In addition, the introduction of silicone modified acrylic vinegar will significantly improve the solvent resistance, especially the heat resistance of the ink. The addition of a small amount of saturated chlorinated acetal can also increase the adhesion of the ink to certain substrates.
(b) Active monomers
Reactive monomer is also called reactive diluent. It generally refers to an organic compound containing one or more acrylic groups (CHZ=CH-C.) in the molecule. The function of the compound in UV ink is twofold: First, it is used as dilution. Agents to adjust ink anvility and rheology; second is to directly participate in the curing of the photochemical reaction, to increase the system cross-link density, thereby enhancing the ink film strength.
When selecting active monomers, low toxicity, low irritation, low volatility are generally required, and the anvil is smaller and the activity is higher. The active monomers with different chemical structures have a great influence on the dilution effect of the photocurable ink and the physical properties of the cured ink film, and must be carefully selected in the application. For reference, some active monomers and their main properties are listed in Table 6 below.
The most important issue to be aware of is that in order to increase the adhesion of the ink film to the substrate (generally plastic), it is advantageous to choose a reactive monomer that is moderately aggressive to the substrate. At the same time, many factors such as the toxicity and irritation of the active monomer and its effect on the volume shrinkage of the cured ink film cannot be ignored. In short, when choosing reactive diluents, their effects on the ink's overall performance cannot be ignored.
(c) Photoinitiators
Photoinitiators are extremely important for the curing and drying of UV inks. It can be said that without the direct participation of the photoinitiator, curing and drying of the UV ink is unthinkable. Its role in the photocuring system is to absorb ultraviolet light energy and form highly active radicals, thereby initiating the photopolymerization cross-linking reaction of active materials such as reactive oligomers and reactive monomers, and the molecular structure of photoinitiators. How it will determine the distribution and intensity of its UV absorption spectrum. In other words, the photo-induced effect of the initiator is determined. Experience has shown that when the UV absorption wavelength of the photoinitiator is in the range of 200 to 400 nm, especially in the range of 320 to 400 nm, the UV curable effect of the UV ink is ideal. In the application, in order to enhance the priming effect, two or more mixed photoinitiators are often used. In addition, references to photosensitizers and photoactivators have a certain role in the photopolymerization cross-linking reaction of the curing system.
Commonly used photoinitiators include photodecomposition type and hydrogen abstraction type, and photoinitiators containing sulfur and the like have both functions of photodecomposition and hydrogen abstraction. Its name, performance, features and practicality are now applied in Table 7.
When selecting photoinitiators, the following principles should be considered: (1) High UV absorption efficiency; (2) Good storage stability; (3) No odor or small odor; (4) No toxicity or low toxicity; Cured ink film is not yellowed; (6) Miscible with the resin, not easy to precipitate; (7) The price is appropriate.
What needs to be mentioned here is that the UV ink is light-cured and dried, usually at room temperature, and its cross-linked curing is mostly free-radical. However, the ion-curing system, though strict in requirements, is due to the small volume shrinkage of the cured ink film. Strong adhesion, polymerization and cross-linking are not interfered with by oxygen, and have recently emerged in the application of UV inks.
(d) Colorants
The exception of the UV ink colorant is no different from traditional inks. However, the photocuring rate of the ink may be affected by the type, nature and physical properties of the colorant. How the colorant absorbs or reflects ultraviolet light, and how the ultraviolet light penetrates the colorant particles will directly affect the speed of UV ink curing and drying. The black (strongly absorbing) ink film generally has a slow photocuring and drying speed; the white (strongly reflective) ink film has a fast curing and deep curing and slow drying; the yellow, blue, and red color ink films have a relatively fast light curing speed, and The above sequence is accelerated in sequence. In the color printing, in order to obtain a good printing effect, the printing of each color ink should be generally performed in the order of the slow first and then fast (photocuring drying speed), ie, black, yellow, blue, and red. In addition, the lightfastness of pigments or dyes has a great influence on the application of outdoor UV inks.
When selecting the colorant, the following points should be noted: (The power hiding power or coloring power should be strong; (2) The light resistance and weather resistance are better; (3) The wetting ability is better with the connecting material so that the particles can be dispersed; (4) ) Impermeability is good, can not afford to float when printing, the color does not migrate from the ink film after printing; (5) good storage stability; (C) cheaper prices.
In short, the choice of pigment or dye is a comprehensive problem, it should be considered together with the connection materials, photoinitiators and light sources to obtain satisfactory results.
(v) Additives
In order to improve certain properties of the ink, a small amount of leveling agents, plasticizers, polymerization inhibitors, wetting agents, defoamers, stabilizers, and anti-spattering inks are often added in the UV ink manufacturing process.
III. UV ink curing mechanism
Ultraviolet (UV) is essentially an electromagnetic wave. Its wavelength range covers 40 to 400 nm. It has more energy and can trigger photochemical reactions. Because the ultraviolet light within 200 nm (far ultraviolet) is easily absorbed by air, the UV ink is cured and dried, often using 200 to 400 nin, especially the ultraviolet light (near ultraviolet) of 320 to 400 nm wavelength as the light source. Figure 1 shows the electromagnetic wave diagram.
Different wavelengths of light, with different energies, can be calculated by the following formula: E = hV = h · C / where: E-energy (Erg / photons) h-Planck constant v-light frequency C A light enters a wavelength of light at a speed in vacuum
For UV-curable inks, the most commonly used resin system is unsaturated acrylates. The cure drying of this system is a free-radical initiated photopolymerization cross-linking process. See Table 8.
Free radical formation
When the photoinitiator (R) absorbs ultraviolet light energy, it is in an unstable energy excited state (R*), and rapidly decomposes to generate a highly active radical (R·), which immediately initiates the unsaturated in the system. Photopolymerization crosslinking reactions occur with compounds (M1-reactive oligomers, reactive monomers, and other unsaturated compounds or resins). However, it should be noted that not all free radicals will initiate the above reaction. The reason is that some of the radicals may be consumed by oxygen molecules dissolved or miscible in the system, and other radicals may also be bonded to each other to lose their activity. In fact, the radicals that can really participate in the photopolymerization cross-linking process are only the remaining part. Therefore, to ensure that the reaction speed of UV ink photopolymerization and cross-linking does not have a great influence, it is a crucial issue to suppress the degree of radical deactivation. Chain initiation and growth
As described above, the radicals that have just been decomposed generate an immediate photopolymerization crosslinking reaction of the unsaturated compound (M) in the system to generate a new radical (RM+). The free radicals continue to react similarly with the molecules of the surrounding unsaturated compounds. For this reason Han Fu carries on, and the chain is growing, which is the free radical in growth (RMm or RM-) o As the reaction continues to deepen, the molecular weight increases rapidly (RMm + 1 or RM.},) However, as in the previous phase of reaction, some of the free radicals also lose their activity due to various reasons. The free radicals (RMm or RM) in the transfer growth of the chain, if reacted with other active substances in the system (VIII), will generate another free radical (A·), which is what people often call chain transfer. Termination of Chains When two newly generated radicals (RMm and RM) combine to form a neutral molecule (Pm" Pn or Pln+n) during the photochemical reaction, their respective activities have been Loss, the growth of the chain will be terminated (the termination of bond formation.) In addition, when an active molecule A, after capturing an electron from the free radical, the radical will also lose its activity, and the growth of its chain will also be terminated (lost). Electronic termination).
According to the different radical formation mechanism, there are roughly two types of photoinitiators used in UV-curing inks: one is an intramolecular photodegradation type (such as benzoin ethers), and the other is an intermolecular hydrogen abstraction type. (such as thioxanthone/organic amine system). After the former absorbs ultraviolet light energy, the initiator decomposes inside the molecule and releases radicals; the latter under the action of light, the hydrogen proton acceptor (thioxanthone) forms a complex with the hydrogen proton donor (organic amine). The body and the acceptor take hydrogen atoms from the donor, generating free radicals. In photochemical reactions, the former type of bower {the hair sprayer is susceptible to oxygen interference in the air (impeded), and the latter type of initiator is not easily influenced by oxygen to affect the photocuring rate. However, the latter has the disadvantage that the cured ink film tends to yellow.
In short, there are many types of UV-curable inks. In the process of formulation, what kind of reactive oligomers, what kind of reactive monomer, what kind of photoinitiator, what kind of colorant and what kind of additive should be used according to the purpose and actual needs, Carefully choose. The mix and match between them is also ever-changing, and the implementation must be flexible and must not be overshadowed.
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