UV coating and waterborne coating quality control

Currently used in the market are oily glazing, waterborne glazing, and UV glazing. The glazing has decorative protection prints to improve the gloss of the prints, with moisture-proof waterproof, folding-resistant and wear-resistant. Judging from the role of environmental protection, water-based glazing and UV glazing are in line with the development trend in the future. Water-based glazing and UV glazing can be directly recycled without waste of resources and environmental pollution. UV glazing is used Packaging for food, medicine, tobacco, and cosmetics. With the promotion of glazing technology and people's understanding of glazing, UV glazing and water-based glazing are developing at an unparalleled speed. Countries like Europe and the United States have high requirements for environmental protection, and they have severe restrictions in some packaging fields. Standards stipulate that there is no room for survival for the film and oily coating that does not meet the requirements of environmental protection.

Due to the unprecedented fierce competition in the market, many manufacturers in order to save costs and generate profits, as much as possible the use of lower prices of varnish, will often result in printed products appear sticky, uneven coating, orange peel, appear needle Holes, pits, poor adhesion, hair growth, poor gloss, insufficient brightness, etc. The quality of printed products depends largely on the varnish, varnish drying, and production process. Only the actual conditions are flexible and good for these three points. Together with good glazing equipment, the quality control Great help.

UV Varnish

UV varnish consists mainly of oligomers, reactive diluents, photoinitiators and other additives.

First, 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.

Second, reactive diluent

Also called cross-linking monomer, it is a functional monomer. Its role in polishing oils is to adjust viscosity, cure speed, and cured film properties.

Third, 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.

Fourth, additives

Used to improve the performance of the ink. UV varnishes commonly used additives:

Stabilizer: used to reduce the thermal polymerization during storage and improve the storage stability of the coating.

Leveling agent: used to improve the leveling of the surface of the coating film to prevent shrinkage, but also increase the gloss of the coating coating.

Defoamer: used to prevent and eliminate the bubbles produced during the manufacture and use of the coating.

Waterborne coating

Waterborne varnish is composed of synthetic resin and water 45% synthetic resin and 55% water.

1. The film-forming substance of the main agent, film-forming resin, and polishing agent is usually a synthetic resin that affects the quality of glazing such as drying, adhesion, and gloss.

二、Adjuvants

1. Curing agent, to improve the film forming property of the aqueous main agent, and to enhance the poly-strength of the film forming degree;

2. Surfactant, reduce surface tension, improve leveling;

3. Defoamer, control foaming, needle eye, orange peel, pitting;

4. Desiccant, increase the drying speed of glazing, improve the adaptability of printed products;

5. Adhesion promoter, improve the adhesion of varnish printing products;

6. Wetting and dispersing agent to prevent sticking and improve wear resistance;

7. Plasticizers to improve folding resistance.

3. Solvents: Disperse and dissolve synthetic resins and auxiliaries. The main component is water (add certain ethanol to improve drying performance).

Varnish viscosity:

UV Varnish is generally adjusted according to the speed of the polishing machine.

60-200 meters / minute control in not more than 28 seconds;

20-50 meters/minute control in no more than 80 seconds;

Water-based coating oil with an offset press to 30-60 seconds;

20-50 seconds when polishing machine.

dry

UV Drying In general, the main supporting UV drying, infrared drying, cooling exhaust. Waterborne Varnish Drying mainly uses infrared rays and hot air drying.

UV drying affects the performance of the cured UV lamp. There are several factors: UV spectral distribution, radiant, radiation, and infrared radiation. Most UV rays contain two UV wavelengths, both of which work simultaneously. Short waves work on the surface, and longer waves act on the deep layers of the ink. This is because short-wave energy is absorbed by the surface and cannot enter deep layers. Short-wavelength underexposure can cause the surface to become sticky, while insufficient long-wave energy can cause adhesion difficulties. The efficiency of a UV lamp depends on how easy it is to launch photons into the curable material to initiate light triggering molecules. UV curing is determined by photon-molecule collisions. Spectral absorption energy is the absorption of a substance into a wavelength within a gradually increasing thickness. The more energy absorbed near the surface, the less energy is obtained in the deep layer. However, this situation varies with the wavelength. The total spectral absorbance includes all effects from light triggers, single molecules, oligomers, and additives including pigments.

The following should also be noted in actual production:

UV light oil viscosity is too small, the coating is too thin; ethanol and other non-reactive solvent dilution excess; UV oil coating is not uniform; paper absorption is too strong; will cause poor gloss, brightness is not enough according to the different conditions and according to the paper The speed of the polishing equipment is appropriately increased to increase the viscosity and coating amount of the UV varnish. For the paper with strong permeability and absorbency, a primer layer can be applied first. Some short-wave tubes can also be added as appropriate to increase leveling. Ultraviolet lamp aging, weakened; UV light oil storage time is too long; non-reactive thinner add too much; machine speed is too fast. It will cause poor drying, incomplete photocuring and sticky surface. The curing speed requirement is less than O. In the case of 5S, it must be ensured that the power of the high-pressure mercury lamp is generally not less than 12OW/cm, the lamp should be updated in time, and if necessary, a certain amount of UV varnish accelerator can be added to accelerate drying. UV light oil is small and the coating is too thin; the ink contains too much ink or dry oil; the surface of the ink is crystallized; the ink surface anti-adhesive material (silicone oil, dusting powder) will cause the surface of the printed matter to be coated with UV light. No, the flower coating is too thin; non-reactive diluents (such as ethanol) add too much; the surface of the printed matter is more dusty. It will cause white spots and pinholes in the UV coating. Care should be taken to ensure that the production environment and the printed surface should be kept clean, and that the thickness of the coating be increased. A small amount of smooth additives can also be added. The dilution is preferably a reactive diluent that participates in the reaction. UV Varnish is too high; pressure is not uniform; coating is too large; UV Varnish is poorly leveled. It may cause uneven surface coating, streaks and orange peel. In addition, in actual production, we must also pay attention to the maintenance of the temperature of the coating production at about 20°C, the characteristics of the paper, the heat resistance of the printing ink, and the resistance to alcohol, alcohol resistance, ester solvents, and good acid and alkali resistance, all of which have a great influence. Polishing quality.