The latest drying technology for glazing
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First, the continuous development of glazing drying technology
Applying (or spraying, printing) a layer of colorless and transparent paint 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 surface of the carrier. The glazing process for protecting the finishing and processing functions of printed graphics has been used as an important means of post-press finishing, and has achieved great results in the packaging processing of foreign trade export products. In the technological innovation of pre-press digital network and multi-color printing, post-press processing can achieve the overall revolution of printing technology only by using high-tech to achieve exquisite automation. The glazing process for cleaning products using clean energy and clean raw materials will be used as an important finishing method for post-press in the 21st century to meet the international standards of ISO14000 environmental management, and will be widely promoted in the printing and packaging industries.
With the continuous popularization of glazing processing, the development and development of glazing coatings and machinery are increasingly valued by related industries such as printing, packaging, chemical engineering and machinery. Glazing coatings have evolved from oxidative polymerization to solvent evaporation to thermal curing and photocuring. Relying on the polymerization of oxygen in the air to reduce the liquid glazing coating into a membrane, the equipment investment is less; relying on the solvent in the coating to evaporate and dry the film into a good leveling property, processing performance and a wide range of adaptation, but need to be glazed, Two steps of calendering; relying on the reactive functional groups contained in the polymer structure of the film-forming resin in the coating resin and the catalyst of the coating, the crosslinking process is heated to form a film, and the automatic glazing device has high production efficiency; after the coating absorbs the radiant light energy The ultraviolet glazing mechanism for drying the conjunctiva by polymerization of the internal structure of the molecule can be carried out synchronously with printing, and is used in one machine. The UV technology combines the printing and glazing process operations on a dedicated machine into the printing unit at one time.
The drying technology, which is important for the glazing process, is also constantly improving and developing. The drying treatment of the glazing coating film is progressed from an oxidative polymerization, a solvent-evaporated hot air drying type, to an electrothermal and ultraviolet drying treatment type. With the development of high-tech and the use of clean energy and the pursuit of the maximum use of clean energy, the electron beam drying method developed by various advanced scientific and technological means has been applied in developed countries in Europe and America. The rapid development of the latest drying system for UV/EB and thermal/electron beam hybrids will provide the printing and packaging industry with the latest high-energy curing principles and technologies at the turn of the century and the 21st century. Continuous use of technological innovation will make the glazing drying process more scientific, and further promote the quality and economic development of the printing and packaging industry.
Second, the comparison of several drying methods of glazing process
Solvent evaporation drying method
Solvent-based glazing coatings have different types and ratios of solvents used in different coating formulations, and the volatilization speeds during coating and drying are also different. The evaporation rate of the solvent is too fast, the leveling property of the glazing coating is not good, and the surface of the surface after drying and filming has streaks, blisters and the like which affect the quality of the surface smoothness. In the solvent evaporation process, moisture is condensed by absorbing external heat, causing cracking or blushing of the dried coating. The solvent evaporates too slowly, which causes problems such as poor drying of the conjunct film and poor anti-adhesiveness.
UV curing drying method
UV curing uses UV (Uitra)
Violet (UV) is a glazing process that illuminates the free radical polymerizable acrylate coating to form a film. The photoinitiator in the composition of such a coating absorbs the ultraviolet light energy and generates a radical upon activation to cause polymerization.
[PI]——-[PI]——–[free radical]
Photoinitiator
[free radical] + [monomer] + [oligomer] --- [polymer]
Cured coating
UV curing has the characteristics of fast curing and low temperature curing, which helps to improve product quality, maintain consistent color, shorten printing time, prevent air pollution, reduce fire hazard and improve working environment. It solves the adhesion caused by temperature and humidity changes of solvent-based glazing coatings.
The application of UV curing technology has developed rapidly in China's printing and packaging industry, and is often used in printing and post-press finishing. In 1994, UV-curable coatings for paper processing increased by 100% compared to 1993, and the annual consumption has reached 50 tons. It will increase year by year. In addition to the UV-excited phosphor developed by Peking University as the main raw material for the mainland to produce itself, it needs to be purchased from Hong Kong, Taiwan, Japan and other places to meet the demand every year.
Ultraviolet-cured illumination sources generally use high-pressure mercury lamps or metal halide lamps. The output power must be controlled at 80-120W/cm to ensure the curing speed of UV coatings is <0.5s. At the same time, there is a problem of the depth, degree of curing, and lag time of post-cure.
Electron beam (EB) heating drying method
The electron beam drying treatment method is a new type of energy conversion processing method used in the production of multi-color high-efficiency printing technology by the foreign developed countries in recent years to apply the original electron beam processing technology of perforation, groove cutting and cutting. An electron beam (EB) produces a concentrated, dense, directional electron flow through an electrical vacuum device. With 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 concentrated into a dense, small-loaded electron beam moving in the same direction. When an electron beam strikes a workpiece, the kinetic energy becomes heat, producing an extremely high temperature that allows any material to melt or vaporize instantaneously. It is considered by the scientific community to be the drying technology with the best consistency in the near future. The ink and glazing coating can be crosslinked and polymerized to the greatest extent without post-cure and solvent residue. At the same time, the heat energy rate is the highest.
Most high-speed production line printing operations require inks, adhesives, and varnishes to dry out in the unit. The use of electron beam (EB) alone as a means of drying is not economical in terms of price and space utilization.
Third, the trend of the United States and Europe
Paper, which is the carrier of printing and packaging, still dominates the main raw materials in the world market. However, plastics that are aesthetically pleasing, lightweight, and processable have been rapidly used in the food and beverage, daily necessities, make-up 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 rate of 3.9%. It will increase to $36.4 billion by the year 2000. Soft plastic packaging is growing faster than paper packaging, growing at an average annual rate of 7.2%, reaching $512 by the year 2000. Manufacturers are actively adopting new heat-sensitive plastic materials and corresponding innovative processing methods that do not cause shrinkage of the carrier substrate, replacing the traditional processing methods of post-printing finishing of existing paper products.
In the past, most printing and finishing machines (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 the chemical on the carrier, its use is strictly limited and modified within a time limit due to environmental regulations. External requirements have led to the development of processes such as UV curing and heat drying. UV drying treatment has a good surface layer curing effect, but does not penetrate or completely cure thicker coatings. At the same time, only 20% of the energy consumed is converted into ultraviolet light to cause curing. In addition, 60% of the electrical energy is released in the form of heat, and the remaining 20% is converted into visible light, and the energy utilization is particularly low.
The thermal dryer has 4 times more footprint 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 of the plastic packaging materials, causing the substrate to crease and shrink. After this drying treatment, the paper shrinkage is 1.25%, and the PE and BOPP films will shrink by 2% or more. This is not compatible with the use of new heat sensitive plastic materials.
Scientific and technical personnel in the United States and Europe introduce electron beams into existing UV or thermal drying processes. And the electron beam is mixed with any one of the drying methods, the advantages can be complemented, and the defects are weakened as much as possible. New products can be manufactured to increase production efficiency and profitability and enhance market competitiveness.
Fourth, electron beam hybrid drying
Any combination of UV curing or heat drying treatment combined with electron beam treatment allows the coating to be fully cured regardless of the thickness of the coating. The production speed is improved, and the carrier substrate is free from thermal damage and deformation, which can improve the application performance of the glazing coating, enhance the adhesion, and improve the finishing effect of the post-press finishing.
Low-energy, low-heat UV light cures the surface of the coating during UV/EB mixing. The depth of the coating is simultaneously dried by the electron beam. The power of the illumination source only needs 2-40W/cm, which is 60% more energy-efficient than the ordinary ultraviolet light system, and generates low heat, which avoids shrinkage deformation of the substrate.
In the thermal drying / electron beam mixing process, the energy consumption and heat generated are reduced by about 40%, the water-based glazing coating can be fully cured, the scratch resistance is good, and the least or no chemical transfer is produced. Flexible packaging materials, semi-rigid packaging materials and fast curing lamination without post-cure.
To ensure adequate curing, it is often necessary to remove oxygen from the material being treated with nitrogen during electron beam processing. In the hybrid process, nitrogen can be omitted and production costs are reduced.
Hybrid processing for printing and finishing, for multiple coatings in one channel, for multi-color printing and glazing for optimum curing on one machine for high gloss and scratch resistance The best processed products. Safe operation is similar to microwaves. It can improve product quality, save resources, and have many value-added advantages. It is a post-press finishing for the 21st century – the improvement direction of glazing will create a new development opportunity for the industry.