UV ink composition and UV light source
Ultraviolet ink, that is, UV ink, uses ultraviolet radiation of a certain wavelength to polymerize and cure through an instantaneous photochemical reaction. UV curable inks were first applied to thick board printing in the United States in 1969. In 1971, Japan began to apply metal printing and self-adhesive printing. It is a type of environmentally friendly ink that has developed rapidly in recent years. It quickly occupies the packaging and printing market with its characteristics of instant curing, no volatile solvents, and simple and convenient application.
At present, in the production workshop of the xx business headquarters, the use of UV ink has occupied a large proportion. In order to make full use of the utility of UV ink, the following will introduce the basic composition of UV ink and UV light source in detail.
1. Composition of UV ink
The main components of UV ink are polymerizable prepolymers, photosensitive monomers, and photoinitiators, and the auxiliary components are coloring pigments, fillers, additives (leveling agents, defoamers, polymerization inhibitors), etc.
1. Polymeric prepolymer Polymeric prepolymer is an important component that determines the performance of UV varnish coating, and is the most basic component in UV ink. It is a film-forming substance, and its properties play an important role in the curing process and the properties of the ink film after curing. Generally classified according to the skeleton structure. Skeleton structure affects coating hardness, friction resistance, adhesion, light resistance, chemical resistance and water resistance, etc. Prepolymers, from a structural point of view, oligomers are all low-molecular resins containing "C=C" unsaturated double bonds, such as acryloyl, methacryloyl, vinyl, allyl, etc. There are mainly epoxy acrylate resins, polyurethane acrylate resins, polyester acrylate resins, polyether acrylate resins, polyacrylate resins, and unsaturated polyester resins. Under the same conditions, the photocuring speed of acrylic is the fastest, so most of the oligomers are acrylic resins.
2. Photosensitive monomer (reactive diluent)
UV ink and UV varnish need to adapt to the viscosity of the coating machine when coating. Generally, the viscosity of the prepolymer is reduced by adding 20% to 80% of the monomer, and the monomer itself polymerizes to form a cured film. part.
Reactive diluent, also called cross-linking monomer, is a functional monomer. Its function in ink is to adjust the viscosity, curing speed and cured film performance of ink. Reactive diluents also have "C=C" unsaturated double bonds in their structure, which can be acryloyl, methacryloyl, vinyl and allyl. In view of the faster photocuring speed of acryloyl, the reactive diluent currently used Most of the agents are acrylate monomers. Due to the different number of acryloyl groups, it can be divided into three types: monofunctional group, bifunctional group and multifunctional group. Functional group reactive diluents have different dilution effects and curing speeds. Generally speaking, the more functionality, the faster the curing speed, but the worse the dilution effect.
Traditional reactive diluents, such as styrene, first-generation acrylate monomers, etc., have strong toxic roots, and some acrylate monomers have a strong irritating effect on the skin. In order to reduce the irritation of reactive diluents to the skin, there are usually two methods: one is to increase the molecular weight of the monomer by ring-opening polymerization of ethylene oxide, propylene oxide and hexyl ester; the other is to change the structure of the monomer ester group; It is to change the previous method of esterification of alcohol, and the addition of alcohol to acryloyl group greatly reduces the skin irritation of multifunctional monomers. For example, when neopentyl glycol diacrylate is synthesized by esterification, the pH value (skin irritation Sex index) is 4.96, and when adopting addition synthesis, the pH value drops to 0.3.
Recently, some monomers with good performance have been developed, such as: alkoxy acrylate, carbonic acid monoacrylate, imidazolyl monoacrylate, cyclocarbonate monoacrylate, epoxy silicone monomer, silicone-based acrylic Esters, and vinyl ether monomers, etc. Some of them have been commercialized, such as: alkoxylated acrylates of Sartomer Company of the United States, UCB Company of Belgium, Henkel Company of Germany, and many vinyl ether monomers of ISP Company of the United States. When choosing a monomer, the following principles should be followed:
a. Low viscosity, good dilution effect;
b. Fast curing;
c. Good adhesion on the material;
d. Little skin irritation and toxicity;
e. Leaves no odor in the coating. 3. Photoinitiator A photoinitiator is a substance that can absorb radiation energy and produce an active intermediate with the ability to initiate polymerization through chemical changes. It is the main component required for any UV curing system. Photoinitiators can be divided into hydrogen abstraction type and cracking type: hydrogen abstraction type needs to cooperate with a compound containing active hydrogen (commonly known as co-initiator), and form free radicals through hydrogen abstraction reaction, which is a bimolecular photoinitiator The cleavage type is a single-molecule photoinitiator that decomposes into free radicals in the molecule after the photoinitiator is excited by light.
(1) Hydrogen abstraction type
Take benzophenone (BP) as an example: when benzophenone is used alone, vinyl monomers cannot be photopolymerized, and the requirements for it to become a photoinitiator are different. Their reaction mechanisms are different alkyl or aryl groups, and when hydrogen atoms are extracted from alcohols or ethers, oxygen can easily quench excited state benzophenones. When extracting hydrogen atoms from amines, because the ketone forms an excited state and immediately forms an excited state complex with the amine, avoiding the energy transfer to the oxygen molecule, so the amine system is not easy to be quenched by oxygen. Compared with the alcohol ether system, The possibility of energy transfer to the monomer is also reduced. Therefore, in practical applications, amine systems are generally used. In addition to benzophenone, such photoinitiators also include anthraquinones and thioxanthones, such as 2-isopropylthioxanthone (ITX), which is commonly used in UV inks.
(2) Cleavage type
Take benzoin ethers as an example: benzoin ether was once the most widely used photoinitiator, and its characteristic is that its excited state can be directly decomposed into two kinds of free radicals. The generated free radicals can initiate monomer polymerization. The excited state of benzoin ether has a short lifetime and is not easily quenched by oxygen or styrene, so it can be used in the polymerization of styrene. However, benzoin ether has different degrees of thermal decomposition even if it is not exposed to light, and the storage stability is not good. Generally, a stabilizer or a polymerization inhibitor is added. Currently, benzodimethylether (651) is commonly used. Such photoinitiators are commonly used in UV inks such as 1173, 184, 369, 907 and so on. Recently, some new photoinitiators have been developed, such as the acyl phosphorus oxide 819 and BAPO developed by Ciba, which have good absorption at 400urn and high photoinitiation efficiency; "Bleaching" effect; it is beneficial to dry and deep light assimilation; the final product is colorless, so it does not turn yellow, and is very suitable for screen printing UV inks with thicker ink films. Due to the high price of acyl phosphorus oxide photoinitiators, it is often used in combination with other photoinitiators, such as Ciba's 1700 (75% 1173 + 25% BAPO) and 1800 (75% 184 + 25% BAPO).
The selection of photoinitiators should follow the following principles:
a. High light absorption efficiency in the UV range;
b. Relatively stable;
d. Good compatibility with prepolymers and monomers;
e. Small smell;
f. Low cost.
4. Other additives
Auxiliaries are mainly used to improve the performance of inks. Commonly used auxiliaries in UV inks include stabilizers, leveling agents, defoamers, dispersants, waxes, etc.
(1) Stabilizer
Stabilizers are used to reduce thermal polymerization during storage and improve ink storage stability. Commonly used hydroquinone, p-methoxyphenol, p-benzoquinone, 2,6-di-tert-butylcresol, etc.
(2) leveling agent
The leveling agent is used to improve the leveling of the ink layer, prevent the generation of shrinkage cavity, make the surface of the ink film smooth, and also increase the gloss of the ink printing.
(3) Defoamer
The defoamer is used to prevent and eliminate the bubbles generated during the manufacture and use of the ink.
(4) Dispersant
The dispersant can make the pigment and the linking material in the ink wet well, so that the pigment has good dispersion in the ink, shorten the grinding time during ink manufacturing; reduce the oil absorption of the pigment to produce high-concentration ink; Prevent the aggregation and precipitation of pigment particles in the ink. Dispersants are generally surfactants.
(5) wax
The main function of the wax is to change the rheology of the ink, improve the water resistance and printing performance (such as adjusting the viscosity), reduce the disadvantages of smudging, paper hair removal, etc., and can form a smooth wax film on the surface of the dried ink film to improve the printing quality. friction resistance etc. In UV ink, wax also acts as an air barrier, reduces oxygen inhibition, and is beneficial to surface curing. However, adding too much wax to the ink or choosing the wrong type of wax will reduce the gloss of the ink, damage the ink transfer performance, and prolong the drying time.
2. UV light source
The performance of UV lamps that affect curing can be completely and accurately related to four characteristics: UV spectral distribution, irradiance, radiance, and infrared radiation.
1. spectral distribution
It describes the wavelength distribution of the phase radiant energy or radiant energy reaching the surface as a function of the wavelength emitted by the lamp. It is often expressed in a relative standardized term. In order to display the distribution of UV energy, the spectral energy can be combined into 10nm spectral bands to form a distribution table. This allows comparison between different UV lamps and easier calculation of spectral energy and power. The wavelength of ultraviolet light - the effective wavelength for ultraviolet curing is 200-400 (nm)
General on-line detection uses a multi-band ray Detector to characterize the spectral radiance or amount of radiation. They obtain useful relative information on the spectral distribution by sampling the radiant energy in a relatively narrow (20–60 nm) frequency band. Due to the different configurations of radiation Detectors from different manufacturers, it is possible but difficult to compare them with each other. There is no such standard to make comparisons between models and manufacturers.
Spectral Distribution Data for UV Lamps - Metal Halide and Mercury Lamps:
The high-pressure mercury lamp has a main wavelength of 365nm, and the range around 254nm, 303nm, and 313nm can effectively emit ultraviolet wavelengths, which are mainly used in the curing of UV varnish and ink; metal halide lamps mainly emit ultraviolet rays in the range of 200-245nm Wavelength, compared with high-pressure mercury lamps, long-wavelength ultraviolet rays emit more, and are mainly used in the curing of UV inks.
2．UV辐射度（Irradiance）：
辐射度是到达表面单位面积内的辐射功率。辐射度，以每平方厘米瓦特或豪瓦来表示。它随灯管的输出功率、效率、反射系统的聚焦以及到表面的距离不同而不同。（它是灯管及几何形状的特性，故与速度无关。）直接置于UV灯下的高强度、峰值聚焦功率参考为“峰值辐射度”。辐射度包括了所有有关电源功率，效率，辐射输出，反射率，聚焦灯泡尺寸及几何形状的因素。
由于UV可固化材料的吸收特性，到达表层以下的光能量要比表层的要少。在这些区域的固化条件可能有显著不同。光学厚度厚的材料（或者高吸收性，或者物理结构厚，或者两者有之）可能会减少光效率，从而导致材料深层的固化不充分。在油墨或涂层里，表面较高的辐射度会提供相对觉高的光能量。固化的深度更多地是被辐射度影响而不是较长的曝光时间（辐射量）。辐射度的影响对于高吸收性（高不透明度）的薄膜更重要。
高辐射度允许使用较少的光触发剂。光子密度的增加增多了光子—光触发剂的碰撞，从而补偿了光触发剂浓度的减少。这对于较厚的涂层会有效，因为表层的光触发剂吸收和阻碍了同一波长到达深层的光触发剂分子。
3．UV辐射量
到达表面单位面积的辐射能量。辐射量表示到达表面的光子总量（而辐射度则是到达的速率）。在任一给定光源下，辐射量与速度成反比而与曝光的数量成正比。辐射量是辐射度的时间累积，以每平方厘米Joules或转miliJoules表示，（遗憾的是，没有有关辐射度或光谱内容换为以辐射量测量的信息，它仅仅是被曝光表面能量的累积。）它的意义在于它是一台包括了速度参数和曝光时间参数的特性显现。
4．红外辐射密度：
红外辐射主要是由UV源的石英泡发射出来的红外能量。红外能量和UV能量一起被收集并聚焦在工作表层。这决定于IR的反射率和反射器的效率。IR能量可以被转换为辐射量或辐射度单位。但通常，它所产生的表面温度才是被注意的重要之处。它所产生的热量可能有害也可能有益。
There are many techniques for addressing the relationship between temperature and IR in conjunction with UV lamps. Can be divided into reducing emission, transmitting and controlling heat movement. The reduction in emission is achieved by using a small diameter bulb, since it is the surface area of the hot quartz that emits almost all the IR. Transmission reduction can be achieved by using a dichroic reflector (cold mirror) behind the lamp; or by using a dichroic window (hot mirror) between the lamp and the target. Heat movement reduces the temperature of the target—but only after the IR has caused the temperature to rise—cooling airflow or heat sinks can be used to control heat movement. The absorption of IR energy is determined by the material itself—ink, coating or substrate. Velocity has a significant effect on the temperature caused by incident IR energy and energy absorbed by the work surface. The faster the process, the less IR energy is absorbed, causing the temperature to rise. The production process can be accelerated by improving efficiency.
