Latest glazing drying technology

First, the continuous development of drying technology

A layer of colorless and transparent paint is applied (or sprayed or printed) on the surface of the printed matter to form a thin and uniform transparent bright layer on the surface of the print after being leveled, dried, calendered, and cured to enhance the smoothness of the surface of the support. The glazing process that protects the finishing and printing functions of printed graphics and texts has become an important means of finishing after printing, and has achieved great results in the packaging and processing of foreign trade export products. In the technological innovation of prepress digital network and high-efficiency multi-color printing, post-press processing can only accomplish the complete revolution of printing technology by using high technology to achieve exquisite automation. The glazing process using clean energy and clean raw materials for the production of cleaning products will be used as an important post-print finishing tool for the 21st century to meet the ISO14000 environmental management international standards. It will be fully promoted in the printing and packaging industry.

With the increasing popularity of glazing processing, the research and development of glazing coatings and machinery are increasingly valued by related industries such as printing, packaging, chemicals, and machinery. Glazing coatings have evolved from oxidative, solvent-vapor-soluble to heat-curable and photocurable types. Reliance on oxygen in the air to polymerize the liquid glazing coating to dry the film investment equipment less; rely on the solvent solvent evaporation drying film forming a good leveling, processing performance and a wide range of adaptation, but the need for glazing, Two steps of calendering; Reliance on the active functional groups contained in the film-forming resin in the coating material and the catalyst of the coating in case of thermal cross-linking reactions. The automatic film coating machine has a high production efficiency; After absorbing the radiation energy of the coating, , The molecular internal structure of the polymerization reaction drying the conjunctiva UV glazing mechanism, can be synchronized with the printing, a multi-purpose machine. UV technology allows the printing and coating processes to be performed on a dedicated machine to be integrated into the printing unit at one time.

As an important polishing technology, the drying technology has also been continuously improved and developed. The coating drying process of glazing coatings has evolved from hot-air drying of oxidative polymerization and solvent evaporation to electrothermal and UV drying. With the development of hi-tech and the use of clean energy and the pursuit of clean energy, the electron beam drying method integrated with various advanced scientific and technological means has been used in developed countries in Europe and America. The rapid development of the latest UV/electron beam and thermal/electron beam hybrid drying systems will provide the latest high energy curing fundamentals and technologies for the printing and packaging industries at the turn of the century and the 21st century. The continuous application of technological innovation will make the coating drying process more scientific and further promote the quality improvement and economic development of the printing and packaging industry.

二、Comparison of several drying methods for polishing process

Solvent evaporation drying method

Solvent-based glazing coatings have different types and proportions of solvents for different coating formulations, and the evaporation rate during coating and drying is also different. The evaporation speed of the solvent is too fast, the leveling property of the coating material is not good, and the quality problems such as streaks and tresses that affect the surface smoothness appear on the surface after the film is dried. And in the process of solvent evaporation, it absorbs external heat and induces condensation of moisture, causing cracking or whitening of the dried coating. Solvent evaporation is too slow and can cause problems such as insufficient curing of the conjunctiva, poor resistance to stickiness, and the like.

UV curing drying method

UV curing is the use of UV (Uitra Violet or ultraviolet) irradiation energy to cure the free-radical-polymerized acrylic coatings into film. Photoinitiators in the components of this type of coating absorb the light energy of the ultraviolet light and generate free radicals in the excited state, which leads to a polymerization reaction.

[PI]-------[PI]--------[Free radicals]

Photoinitiator excited photoinitiator

[radical]+[monomer]+[oligomer]--[polymer]

Cured coating

UV curing has the characteristics of rapid curing and low temperature curing, which helps improve product quality, maintain consistent color, shorten printing time, prevent air pollution, reduce fire risk and improve working environment. Solve the adhesion of solvent-based glazing coating due to temperature and humidity changes.

The use of UV curing technology in China's printing and packaging industry has developed rapidly, mostly used in printing and finishing after printing. In 1994, UV-curable coatings for paper processing increased by 100% compared with 1993. The annual consumption has reached 50 tons. It will increase year after year. In addition to ultraviolet radiation-excited fluorescent powder developed by Peking University as its main raw material, the mainland itself needs to be purchased from Hong Kong, Taiwan, Japan and other places to meet demand.

UV curing light sources generally use high-pressure mercury lamps or metal halide lamps. The output power must be controlled at 80-120W/cm to ensure that the UV coating cure speed is < 0.5s. At the same time there is the problem of the depth of cure, the degree of curing, and the post-curing lag time.

Electron beam (EB) heating drying method

Electron beam drying treatment method is a new type of energy conversion processing method used by foreign developed countries in recent years for the application of electron beam processing technology such as perforation, notching, cutting and other mechanical processing to a multi-color high-efficiency printing process. Electron beam (EB) is a polymerized, dense, directional electron flow generated by an electrical vacuum device. Using the high power density electron beam processing method, the electrons emitted from the hot cathode are controlled by the electrostatic field of the control electrode and the accelerating anode to be condensed into a dense, small-surface-mounted electron beam that moves in the same direction. When the electron beam hits the workpiece, the kinetic energy becomes thermal energy, producing an extremely high temperature that can instantaneously melt or vaporize any material. It is considered by the scientific community as a drying technology with the best processing consistency in the near future. Allows inks and coatings to be maximally crosslinked and polymerized without post cure and solvent residue. At the same time, the use of heat energy is highest.

Most high-speed production line printing operations require that the inks, adhesives, and coatings be dried in the unit. The use of electron beam (EB) alone as a dry treatment method is not economical in terms of price and space utilization.

III. Trends in the United States and Europe

As a carrier for printing and packaging, paper still occupies the dominant position in the world market. But beautiful, durable, lightweight, and highly processable plastics have rapidly increased in the United States and Europe in the markets for food, daily necessities, makeup and hygiene products and other product packaging. At the end of 1995, sales of soft and semi-rigid paper and soft plastics in Europe and the United States reached 66.4 billion U.S. dollars, an increase of 31% over 1990. Since then, paper packaging has grown at an average annual rate of 3.9%. It will increase to 36.4 billion U.S. dollars by the year 2000. The development of soft plastic packaging is faster than paper packaging, with an average annual growth rate of 7.2%. By the year 2000, it will reach US$512. Manufacturers are actively adopting new heat-sensitive plastic materials and their corresponding innovative processing methods that do not cause shrinkage of carrier substrates, replacing traditional processing methods of existing paper products after finishing.

In the past, most printing and finishing machinery (lines) were designed to handle solvent-based inks and coatings. Because the volatilization of the solvent causes environmental pollution and the residue and transfer of chemical substances on the carrier, its use is strictly limited by the environmental protection regulations and is limited to a period of modification. External requirements led to the development of UV curing and heat drying processes. Ultraviolet drying treatment has good effect on surface curing, but it cannot penetrate or completely cure thicker coating. At the same time, only 20% of the electric energy actually consumed is converted into ultraviolet light to generate a curing effect. The other 60% of the electric energy is released as heat, and the remaining 20% ​​is converted into visible light, and the energy utilization is particularly low.

The thermal dryer covers 4 times more space than UV curing equipment and 3 times more than electron beam curing equipment. The heat generated by the thermal drying system can easily cause many plastic packaging materials to overheat and cause the substrate to sag and shrink. After this drying process, the shrinkage of the paper is 1.25%, and the PE and BOPP films will shrink by 2% or more. This is incompatible with the use of new heat-sensitive plastic materials.

Scientific and technical personnel in the United States and Europe have introduced electron beams into existing UV or thermal drying processing lines. And with electron beam and any one of the drying methods mixed treatment, advantages can be complementary, defects as weak as possible. It can create new products, increase production efficiency and profits, and enhance market competitiveness.

Fourth, electron beam mixing drying

Combining either UV-curing or heat-drying treatment with electron beam treatment makes it possible to completely cure the coating regardless of the thickness of the coating. Increased production speed, no thermal damage to the carrier substrate, can improve the application performance of glazing coatings, enhance adhesion, and improve the finishing effect after finishing.

In the UV/electron beam mixing process, low energy, low heat UV light can cure the surface of the coating. The depth of the coating is simultaneously dried by the electron beam. The power of the light source is only 2-40W/cm, which is 60% energy saving than the ordinary UV light system. The low heat generation avoids the shrinkage deformation of the substrate.

During heat drying/electron beam mixing, energy consumption and heat generation are reduced by approximately 40%, allowing waterborne gloss coatings to be fully cured, with good scratch resistance, and minimal or no chemical transfer Soft packaging materials, semi-rigid packaging materials, and rapid curing lamination without post-curing.

In order to ensure sufficient curing, it is usually necessary to remove oxygen from the treated material by nitrogen during electron beam treatment. In the hybrid process, nitrogen can be saved, reducing production costs.

The hybrid process is used in printing and post-finishing finishing and can be applied in multiple passes in a single pass to achieve optimal multi-color printing and curing of glazing on one machine to produce high-gloss, scratch-resistant The best processed product. Safe operation is similar to microwave. Can improve product quality, save resources, have many value-added advantages. It is the finishing process for the post-press finishing of the 21st century – the direction of improvement of SHIFT will create a new development opportunity for the industry.