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Industry focus: overview of special inks (III)

2. UV curing inks

due to the high cost and time-consuming baking process in the iron printing process, fashion can cause ink discoloration. Therefore, UV curing process has been widely used in iron printing process

when using UV drying ink, just install a few UV quartz lamps on the printing machine, but the lamp power, installation method and other factors will affect its curing rate

the curing of UV ink requires appropriate energy to activate the components (photoinitiators) in the ink. Ultraviolet lamp provides this kind of energy. This kind of lamp is generally a high-pressure quartz mercury lamp (that is, the arc tube made of quartz glass is sealed with high-purity mercury vapor, argon and other inert gases), and its emission wavelength is about 200 nanometers (the wavelength of light used for light curing is generally between 200 and 450 nanometers. Although the energy below 200 nanometers is large, it is easy to be absorbed and the penetration ability is weak). The power is generally 200 W/h. The tube wall temperature of this lamp can reach 400 ~ 800 ℃. The service life is generally 1000 1500 hours

in fact, only about 25% of the rays of this lamp are in the ultraviolet region, and only 8% of the wavelengths can be used to activate the photoinitiator, and the other 17% of the ultraviolet light has no effect on the initiator. Other rays are visible and infrared, which are useless for the curing of ink

because ultraviolet rays can damage people's eyes and skin, and the ozone emitted by this lamp is also toxic, exhaust devices, devices to cool the lamp tube and protective covers should be installed when using this lamp. At present, there are two kinds of protective covers widely used: elliptical and parabolic. The internal surface of the cover is generally coated with a layer of aluminum film, which has two functions: reflecting light and playing a protective role

as for the installation position of the lamp, it depends on the structure of the printing machine. 3. Detection and control are various. In multi-color printing machines, there are some methods of installing one lamp in one color, and there are also methods of installing multiple lamps behind four colors, and so on. Only when the light wave emitted by the lamp is matched with the absorption spectrum of the photosensitizer, the best effect can be achieved

we know that some dry oils and resins can also be partially polymerized (cured) after adding some appropriate photoinitiators and UV irradiation, and then slowly oxidized and dried later. Admittedly, this kind of printing cannot be applied (or re constructed) immediately, because its later drying time is too long. Therefore, people have further developed inks that can be cured immediately after UV irradiation

modern UV drying inks generally have three main components, namely monomer (as binder), pigment and photoinitiator (photosensitizer). Monomers can be combined with free radicals to polymerize, and with the transfer of activity to new units, high molecular weight polymers are constantly produced. As this happens almost immediately, the drying of the ink takes only about 1/150 seconds

generally speaking, substances containing polymerizable unsaturated double bonds can almost be used as UV curing materials. The light solid rate of the double bond is arranged in the following order: acrylic acid, methacrylic acid, vinyl. Unsaturated polyester, acrylic polyether, polyester and alkyd, acrylic epoxy, acrylic polyurethane, polybutadiene and other substances can be used as UV curing materials

ideally, pigments should not absorb any ultraviolet light. In fact, it is certainly impossible. All kinds of colored bodies can absorb varying degrees of ultraviolet light, and black can absorb more ultraviolet light than other colors, which is well known. It can be understood that the higher the transmittance of the pigment, the better, because the UV curing material must absorb ultraviolet rays to cure. If the transmittance of the pigment is low, it is impossible for the ultraviolet rays to irradiate the interior of the film, which will delay the curing or make the curing incomplete

from the UV transmittance of several colors of four-color printing ink, it can be clearly seen that their curing rate order is magenta, yellow, cyan and black. This is because the transmittance of black pigment is the worst and that of carbon black is zero. Later, a carbon black grafted with ethylene monomer was developed. When using this carbon black to make light curing ink, the curing speed was much faster

the principle of light curing is that the double bond in the resin (binder) is initiated by the free radical of the photoinitiator, so that the chain polymerization reaction occurs to crosslink and cure the ink. The reaction mechanism is the same as that of the general free radical type reaction, which is divided into chain initiation, chain growth, chain transfer and chain termination (termination)

photoinitiator is stable under normal conditions, but it will form free radicals after proper light (energy) irradiation. This highly active unit will be transferred and combined with monomer without dust and sundries

the curing principle of UV ink polymerized by free radicals initiated by light can be roughly expressed as follows:

(1) initiation. High energy short wavelength light can directly initiate polymerization. For example, benzophenone, a photochemical initiator, can decompose into free radicals after UV irradiation

(2) growth. The chain group formed in the initiation (phase) forms chain growth due to the addition of monomer

(3) termination (termination). The growth reaction can continue until the monomer is exhausted. There are two kinds of expression forms of terminal reaction, coupling and disproportionation

photosensitizers can generally be classified into six categories: (a) carbonyl compounds, (cost reduction will become the biggest pressure faced by power battery enterprises; (b) peroxides, (c) polysulfide compounds, (d) azo, diazo, azide compounds, (E) elemental organic compounds, and (f) halogen compounds. Among them, (E) it mostly has polymerization inhibition effect on photopolymerization reaction, (f) the free radical formed as photosensitizer is extremely unstable, (c) it is as difficult to control as peroxide in the process of photolysis, and it is easy to yellowing, (d) nitrogen will escape during photolysis, making the film form bubbles. Relatively speaking, (a) is ideal

the photoinitiators for UV ink are generally benzophenones, and then some other initiators are appropriately mixed. It depends on the type of pigment and the requirements of printing products

the curing rate can be increased by using chain transfer agents (such as thioglycol dodecyl ester) in this kind of ink

this kind of ink is characterized by fast drying (curing), which can achieve the so-called instant drying. However, the drying condition may vary with the function of the ink, light conditions, the thickness of the imprint, and the characteristics of the substrate (such as paper, iron sheet, aluminum foil, plastic film). Generally, it is about 0.2 1 second

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