1. Continuous development of glaze drying technology
Apply (or spray, print) a colorless and transparent coating on the surface of the printed matter. After leveling, drying, calendering and curing, a thin and uniform transparent bright layer is formed on the surface of the printed matter to enhance the smoothness of the carrier surface. The glazing process, which protects the finishing and processing functions of printed graphics and texts, has become an important means of post-press finishing, and has achieved great results in the packaging and processing of foreign trade export products. In the technical innovation of pre-press digital networking and printing multi-color high efficiency, only after the use of high-tech to achieve exquisite automation can complete the overall revolution of printing technology. The glazing process that uses clean energy and clean raw materials for the production of clean products will be used as an important post-press finishing tool for the 21st century to meet the international standards of ISO14000 environmental management, and will be fully promoted in the printing and packaging industries.
With the continuous popularization of glazing processing, the research and development of glazing coatings and machinery have received more and more attention from printing, packaging, chemical, machinery and other related industries. Varnishes have evolved from oxidative polymerization type, solvent volatilization type to thermal curing type and photo curing type. The equipment that relies on the oxygen in the air to polymerize and dry the liquid glazing paint to form a film requires less investment; the solvent that evaporates from the paint to evaporate and dry to form a film has good leveling performance, wide processing performance and wide application range, but it needs to be glazed, Two processes of calendering; relying on the active functional groups contained in the polymer structure of the film-forming resin in the coating and the catalyst of the coating to produce a cross-linking reaction when heated, drying and forming the automatic coating equipment with high production efficiency; relying on the coating to absorb the radiant light energy The UV light glazing mechanism for the polymerization of the internal structure of the molecule to dry the conjunctiva can be synchronized with the printing, and it can be used in one machine. Ultraviolet technology makes the process operations of printing and glazing on special machinery merged into the printing unit at one time.
The drying technology, which is an important glazing process, has also been continuously improved and developed. The drying process of coating film formation of glazing coating has developed from hot air drying type of oxidative polymerization and solvent volatilization to electric heating and ultraviolet light drying type. With the development of high and new technology, the use of clean energy and the pursuit of maximum use of clean energy, the electron beam drying method developed by the integration of various advanced scientific and technological means has been used in developed countries in Europe and America. The rapid development of the latest ultraviolet / electron beam and thermal / electron beam hybrid drying treatment system will provide the latest high-energy curing basic principles and technologies for the printing and packaging industries at the turn of the century and the 21st century. The continuous use of technological innovation will make the glaze drying process more scientific, and further promote the quality improvement and economic development of the printing and packaging industries.
Second, the comparison of several drying methods of glazing process
Solvent evaporation drying method
Solvent-based glazing paints have different types and ratios of solvents due to different paint formulations, and their volatilization rates during coating and drying are also different. The solvent volatilization rate is too fast, the leveling property of the glazing coating is not good, and the surface of the film after drying and film formation has streaks, trachoma and other quality problems that affect the surface smoothness. In the process of solvent volatilization, moisture condensation is induced by the absorption of external heat, which makes the dried coating crack or white. If the solvent evaporates too slowly, it will cause insufficient drying, poor curing of the conjunctiva, and poor resistance to adhesion.
UV curing drying method
Ultraviolet curing is a glazing process that uses UV (Uitra Violet) energy to cure radical polymerized acrylic coatings into a film. The photoinitiator in the components of this type of coating absorbs the light energy of ultraviolet rays, generates free radicals after being excited, and causes polymerization reactions.
[PI] ------- [PI] -------- [free radical]
Photoinitiator excited state photoinitiator
[Free radical] + [Monomer] + [Oligomer] ------- [Polymer]
Cured coating
Ultraviolet curing has the characteristics of fast curing and low temperature curing, which helps to improve product quality, maintain consistent colors, shorten printing time, prevent atmospheric pollution, reduce fire risk and improve working environment. Solve the adhesion of solvent-based glazing paint due to temperature and humidity changes.
The application of ultraviolet curing technology has developed rapidly in China's printing and packaging industries, and is mostly used in printing and post-press finishing. In 1994, UV curing coatings for paper processing increased by 100% compared with 1993, and the annual consumption has reached 50 tons. It will increase year after year. In addition to the ultraviolet-excited phosphor developed by Peking University as the main raw material produced in mainland China, it needs to be purchased from Hong Kong, Taiwan, Japan and other places every year to meet demand.
High-pressure mercury lamps or metal halide lamps are generally used as the ultraviolet curing light source. The output power must be controlled at 80-120W / cm to ensure the curing speed of UV coating <0.5s. At the same time, there are also problems of depth, degree of curing, and lag time of post-curing.
Electron beam (EB) heating and drying method
The electron beam drying treatment method is a new type of energy conversion processing method in which the developed countries have used the original application of electron beam processing techniques such as perforation, grooving and cutting in the multi-color high-efficiency printing process. Electron beam (EB) is to generate a concentrated, dense electron flow with a certain direction through an electric vacuum device. Using high-power-density electron beam processing methods, the electrons emitted from the hot cathode are controlled by the electrostatic field of the control electrode and the accelerated anode to be concentrated into a dense, small electron beam moving in the same direction. When the electron beam hits the work piece, the kinetic energy becomes heat energy, generating extremely high temperature, which can instantaneously melt or vaporize any material. It is considered by the scientific community to be a drying technology with the best treatment consistency in the near future. It can make the ink and glazing coating crosslink and polymer to the greatest extent without post-curing and solvent residue. At the same time, the rate of thermal energy is the highest.
Most high-speed production line printing operations require inks, adhesives and glazing coatings to dry in the unit. The use of electron beam (EB) alone as a drying method is not economical in terms of price and space utilization.
3. Trends in the US and Europe
Paper used as a carrier for printing and packaging still occupies the dominant position of the main raw materials in the world market. However, plastics that are beautiful, durable, light in weight, and have good processability have rapidly increased in the food and daily necessities, cosmetics, and hygiene products and other product packaging markets in the United States and Europe. At the end of 1995, sales of soft and semi-rigid paper and soft plastics in Europe and the United States reached US $ 66.4 billion, an increase of 31% over 1990. Since then, paper packaging has grown at an average annual growth rate of 3.9%. By 2000 it will increase to 36.4 billion US dollars. The development of flexible plastic packaging is faster than that of paper packaging, with an average annual growth rate of 7.2%, reaching 512 US dollars by 2000. Manufacturers are actively adopting new heat-sensitive plastic materials and their corresponding innovative processing methods that do not cause shrinkage of the carrier substrate to replace the traditional processing methods of post-press finishing of existing paper products.
In the past, most printing and post-press production 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 restricted by the environmental protection regulations and strictly restricted and modified within a time limit. External requirements have led to the development of processes such as UV curing and heat drying. Ultraviolet drying treatment has good surface curing effect, but it cannot penetrate or completely cure thick coatings. At the same time, only 20% of the electrical energy consumed is converted into ultraviolet light for curing, another 60% of the electrical energy is released in the form of heat, and the remaining 20% ​​is converted into visible light. The energy utilization is particularly low.
The thermal dryer takes up 4 times more floor space than UV curing equipment and 3 times more than electron beam curing equipment. The heat generated by the thermal drying system tends to overheat many plastic packaging materials and cause the substrate to crease and shrink. After this drying process, the paper shrinkage rate is 1.25%, and PE and BOPP films will shrink by 2% or more. This is very incompatible with the use of new heat-sensitive plastic materials.
Scientific and technical personnel in the United States and Europe introduced electron beams into existing ultraviolet or thermal drying production lines. Combined with electron beam and any of the drying methods, the advantages can be complemented, and the defects can be weakened as much as possible. You can manufacture new products, improve production efficiency and profits, and enhance market competitiveness.
4. Electron beam hybrid drying treatment
Combining any of ultraviolet curing or heat drying treatment with electron beam treatment, no matter how thick the coating is, the mixed treatment can make it completely cured. The production speed is increased, and there is no thermal damage and deformation to the carrier substrate, which can improve the application performance of the glazing coating, enhance the adhesion, and improve the post-press finishing processing effect.
In the mixed process of ultraviolet light / electron beam, low energy and low heat ultraviolet light can cure the surface of the coating. The deep part of the coating is also dried by electron beam. The power of the light source is only 2-40W / cm, which is 60% energy-saving than the ordinary ultraviolet light system, low heat generation, and avoids the shrinkage and deformation of the substrate.
During the thermal drying / electron beam mixing process, both energy consumption and generated heat are reduced by about 40%, which can fully cure the water-based glazing coating, good scratch resistance, and produce minimal or no chemical transfer Flexible packaging materials, semi-rigid packaging materials and fast curing lamination without post-curing.
In order to ensure sufficient curing, nitrogen is usually required to remove the oxygen of the material to be processed during the electron beam treatment. In the mixed treatment, nitrogen can be omitted, which reduces the production cost.
Hybrid processing is used in printing and post-press finishing. It can perform multiple coatings in one channel. It can complete the optimal curing of multi-color printing and glazing on one machine, producing high gloss and scratch-resistant The best processed products. Safe operation is similar to microwave. Can improve product quality, save resources, and has many advantages of added value. It is the 21st century post-press finishing-the improvement direction of glazing, which will create a new development opportunity for the industry.
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