UV Polishing - Materials and Process Indicators (I)

In recent years, UV (ultraviolet) coating technology has been gaining popularity in the printing industry. So what is UV? What is the use of UV for printing? What are the benefits of using UV technology for printing? and many more. Below we take these problems to get close to the UV.

UV is the abbreviation of Ultraviolet Ultraviolet in English. Ultraviolet light is a band in the electromagnetic spectrum. Waves with a wavelength of 100 nm to 400 nm are called ultraviolet rays. UV, which is generally referred to as printing, is actually an abbreviation of ultraviolet curing, that is, a process in which a printed product is dried under ultraviolet irradiation after printing.

First, the advantages and disadvantages of UV coating:

Advantage 1 There is no solvent in the formula, so there is no problem caused by solvent evaporation;
2 Fast curing at room temperature;
3 Reduce energy consumption. Curing speed, so that the required energy per unit area decreases;
4 can be coated with heat sensitive substrate;
5 The coating process is simple and easy;
6 Save space, much shorter than infrared heat drying oven;
7 Finished products can be stacked immediately;
8 Excellent coating performance.
Disadvantage 1 If you do not use harmful monomers, it is often difficult to formulate very low viscosity coatings;
2 expensive;
3 has skin irritation;
4 UV light source will produce ozone;
5 The coating has low adhesion and shrinkage.

Second, the chemical composition of UV coating.
The chemical composition of UV Varnish mainly includes radiation prepolymers, thinners and photoinitiators:
1. Radiation prepolymer. Prepolymers are chemical systems with residual unsaturation molecules. This molecule is capable of cross-linking with other unsaturated molecules when in certain conditions, from a liquid to a solid coating. It is required that these unsaturated molecules must be stable and not react with each other before crosslinking.
Prepolymer type:
1) Epoxy acrylate;
2) Acrylated oils;
3) Acrylic urethane;
4) unsaturated polyester;
5) Polyester acrylate;
6) Polyether acrylates, etc.
2. Thinner. Diluents are also chemical systems containing unsaturated molecules. Use it to adjust the viscosity, while it is a film-forming substance. Some situations can replace prepolymers. To make the viscosity reach the desired range, it is often necessary to add a small amount of volatile organic solvent to the mixture of prepolymer and diluent, but up to no more than 5%-10%.
Diluents are divided into active and inactive two. Inactive diluents include solvents and plasticizers. Solvents are mainly volatiles. Plasticizers impart certain flexibility to the cured coating, which not only reduces viscosity but also facilitates coating and leveling. The addition of a reactive diluent to the prepolymer can change the viscosity, tackiness, flexibility and hardness. Reactive diluents can be divided into monofunctional and polyfunctional groups.
Monofunctional reactive diluents, such as 2-ethylethyl acrylate, are flammable, volatile, and irritating to the skin. The polyfunctional reactive diluents have low volatility, high flash point, and low skin irritation.
The role of reactive diluents is as follows:
1Adjust the viscosity and leveling;
2 and photoinitiator system determine the curing speed;
3 with high molecular weight prepolymer molecules linked together to accelerate curing;
4 to improve and improve the performance of the cured coating.
3. Photoinitiator. Photoinitiators are defined as molecules that absorb radiant energy and undergo chemical changes to produce reactive intermediates that have the ability to initiate polymerization.
The photoinitiator does not participate in the curing, so the photoinitiator concentration is low, and the amount thereof requires that the initiator itself or its photochemical product must not adversely affect the chemical and physical properties of the polymer after curing. Benzophenones are usually used.
Photoinitiators In the process of initiating polymerization, two more substances are indispensable. Photosensitive type refers to a chemical reaction that transfers energy to a photoinitiator molecule to initiate polymerization after absorbing light energy, and the photosensitizer recovers to its initial inactive state. The other is called photoactive agent or photo-enhancer (usually amines). It mainly enhances the activation rate of photoinitiator. It does not absorb radiation by itself and does not initiate polymerization, but it can increase the curing rate.

Third, UV curing chemical process.
The basic composition of photo-curing materials are prepolymers, diluents, photoinitiators, auxiliaries, and pigments. The curing chemical process of light is excited by the absorption of photon energy by photoinitiators to form free radicals, which initiate polymerization of the prepolymer and the diluent (initiate molecular chain growth).

Fourth, in the print surface UV light oil process:
1. In addition to powder. In general, the printing machine has a powder removing device, and when the printing ink is large, dusting is commonly used to solve the dirt problem. However, the dusting glue sticks to the surface of the ink like sandpaper, and there is no gloss after applying UV varnish. Therefore need to remove powder. Removal method: First use a brush to sweep, the direction of the brush's rolling is opposite to the direction of the paper's movement; the powder is swept and then sucked into the outdoor through the suction duct. When the powder and the ink are mixed together into a granular form and cannot be cleaned, two light rollers can be used to press the surface of the paper to flatten the powder particles.
2. Corona treatment. Some printing materials, such as PP or PET, have poor affinity for UV varnish. Some inks contain waxes and are not compatible with UV oils. They can be used to improve the affinity of high voltage discharge and can be used with UV oil very evenly. Attached.
3. Priming oil. For oil-stained paper, first coat the white base oil, seal the pores of the paper, increase the whiteness, and then apply UV oil best. If the surface coating layer of thin white paper, if directly coated with UV varnish. The oil will quickly penetrate into the bottom layer and will not shine. Instead, it will reflect the bottom ash color and make the paper surface blue after glazing. If you first hit a layer of the bottom, then apply UV oil to solve this problem. Base oils are generally hot dry type.
4.UV polishing. Apply UV Varnish.
5. Cool. The UV-cured print was blown with a fan to reduce the surface temperature. One purpose is to avoid paper deformation and the other is to prevent post-curing from continuing.

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