Aqueous Coating Technology

In the 1980s, the processing technology of paper products was raised in China's printing industry. The main production process is the traditional oil film. The process of the production process is as follows: the plastic film is coated with a binder, and after being pressed by a blanket cylinder and a heating roller through a drying tunnel, the sheet is glued together with the paper to form a paper-plastic product. The water-based film is a new type of film coating technology that was introduced in 1996. Because the process uses water-based green glue, it does not need heating and drying equipment, high-speed energy-saving and high gloss, and it is welcomed by many manufacturers. However, due to the short time for the water-borne coating, the lack of relevant information, many manufacturers encountered many problems in the actual production can not be solved. The process flow of water-based film coating is: paper feeding in the paper feeding section, paper feeding in the paper feeding table, film coating adhesive, printed film pressing, winding, drying, and slitting. This article will begin with the basic process description of the water-borne coating, in order to play a role in attracting jade.

plastic film

The plastic film used for water-based film coating is basically the same as the conventional oil-based film, and is mainly a polypropylene film, and a polyvinyl chloride film, a polyethylene film, a polyester film, and the like can also be used. Film thickness is generally 0.01 ~ 0.02mm, requires high transparency, light transmittance of more than 90%, and must be light resistance, chemical resistance, geometric dimensions, a certain degree of mechanical strength.

Water-based film and oil film have different requirements for the quality of plastic film due to their different processes and adhesives.

1. The corona treatment requirement of the film on the water film is lower, and the film storage time is longer, while the oil film has higher corona treatment requirements on the film.

2. The water-based film coating requires high rewinding quality and surface flatness of the film. Because of the water-based film, the water-based adhesive is poor in initial adhesion, and the tension and pressure on the film are very small. Therefore, the wrinkles on the film and the different rewinding and tightness will affect the water-based film; Because of the good initial adhesiveness of the adhesive and the tensile and hot pressing of a certain strength, the film can make up for the defects of the above film quality.

Adhesive

Water-based film adhesives are also called water-based adhesives, and the main raw material ratios are: vinyl acetate-ethylene polymerization emulsion (VAE) 80%, rosin 5%, emulsifier 4%, water 8%.

Waterborne rubber production process is relatively simple, but the processing process is demanding. The production steps are as follows: The rosin is first put into a stainless steel reactor and melted, and then the emulsifier and water are added, and the melting temperature is controlled at 80°C. Looking through the sight glass, when it is in a molten state, the VAE emulsion is added and stirred for 1 hour. And then through the colloid grinding, 200 mesh filter, can be packed out of the library.
The quality standard of water-based glue is: the storage period of the glue liquid can not exceed 1 year, and the glue liquid can not be stratified. The viscosity of the glue liquid can not be lower than 200 seconds (coating 4# cup, 20°C).

Water film coating equipment

The aqueous laminating machine includes: a paper feeding portion, a paper conveying positioning portion, a gluing portion, a pressure combining portion, a winding portion, and a slitting portion.

Water-based film does not need to be heated and dried, and it will be wound directly, so the machine speed is faster. The maximum speed can reach 80 meters / minute (about 7,000 sheets / hour), comparable to the speed of ordinary offset press.
The paper feeding section and the paper conveying positioning section are the same as a general paper offset printing press. The gluing section and the pressurizing section are similar to the oily coating machine. The difference is that the water-based laminator has no heating drying tunnel, the distance between the gluing section and the pressure-compressing section is relatively short, the gluing section is relatively simple, the pressure-compressing section is at normal temperature, there is no pressure roller, and the pressure is also small. The take-up section is the same as the take-up section of a general manual oil film machine.

Factors Affecting the Quality of Waterborne Coatings

Apart from the quality and quality of raw materials, the quality of waterborne coatings is also affected by the following factors:

1. The influence of the state of the ink on the printed matter The state of the ink on the printed matter mainly refers to the size of the image area on the paper, the thickness of the ink layer, and the performance of the ink. Their influence on the film is mainly the effect on the adhesion strength of the printed matter and the film. When the printed matter is large in the ground, the ink layer is thick, and the area of ​​the graphic is large, the ink changes the surface characteristics of the paper with many voids, hinders the penetration and diffusion of the adhesive, makes the print and the plastic difficult to bond, and is prone to delamination, not easy to dry, etc. problem.

2. The effect of diluting agent If white ink is used as diluting agent in the ink, the obvious silty particles in the white ink make it not tightly combined with the linking material, which has an obstructive effect on the adhesion and may cause the adhesion to be weak.

3. Effect of water-based glue Water-based glue has a certain ability to “powder” on the printed matter on the print, and can improve the gloss of the film. However, if you spray too much, there will be snow-white spots. Increasing the amount of glue can improve this situation.
In addition, if gold or silver ink is used for printing, if the amount of glue is too large, the water will soak the gold and silver inks, causing the gold and silver ink blots to become black.

4.Influence of ink drying conditions The ink is not completely dry and can cause blistering and delamination of the product after lamination.

5. The impact of ordinary water laminating machine Because the ordinary water laminating machine does not have a drying tunnel, the glue liquid mainly penetrates and dries, so it is not suitable for coating gold, silver cardboard and hot stamping products. The special water-based laminating machine is equipped with a drying tunnel, similar to the oil-based laminating machine, which can cover the gold and silver cardboard, but the speed is low.

6. The effect of paper The first is the effect of paper deformation. Due to the poor initial viscosity of the water-based glue, the viscosity of the water-based glue is very low at the beginning of the drying. If the paper is severely deformed, after the film is coated, the paper and the film will be separated from each other due to the deformation of the paper, resulting in air bubbles after drying.
Followed by the paper thickness and stiffness. When the paper is thick or high in stiffness, the water-borne film dragging film is slightly longer, which will affect the subsequent process of aligning the paper.

Author: Cui Xiaoming, etc.

Reprinted from: Printing Technology