Application of Radiation Curing Coating Innovation Technology (1)

1 Overview

Inmont, USA, first obtained the first patent for unsaturated polyester/styrene ultraviolet (UV) curable ink in 1946; Germany first UV-cured particle board coating into commercialization in the 1960s; the United States Ford first The application of electron beam (EB) radiation curing automotive parts and instrumentation surface coating. At the same time, the wood-coated EB curing line came out on the European market. The application of radiation-cured coating technology has experienced a period of rapid development in developed countries and regions until the end of the 1980s, maintaining an average annual growth rate of more than 15%. Since entering the 1990s, it has continued to grow at a rate of 12% per year. It is expected that it will not slow down by the end of this century, and there will be growth in some areas. The fundamental reason for the rapid development of radiation-curable coating technology lies in its fast curing speed and particularly excellent product performance. However, it has been further discovered in the development and application of radiation curing technology that the control of air pollution is a more important factor. Traditional paints use more volatile organic compounds (VOCs) as solvents and are exposed to high concentrations of VOCs for a long period of time and will cause great damage to the human body, especially the nervous system. Moreover, VOCs also react photochemically with nitrogen oxides in the air to produce O3 and fumes. The use of radiation-cured coating technology can greatly reduce VOCs and even eliminate the use of solvents at all.

This article focuses on the latest developments of radiation devices, initiators, monomers, and oligomers that are involved in the field of radiation curing in the world today, as well as the application status and development trends of radiation-cured coating technologies.
2 radiation curing device

2.1 Ultraviolet light irradiation device

The widely used ultraviolet light sources are filled with mercury vapor in quartz glass tubes and emit ultraviolet light under the excitation of electrodes or microwaves, and can be classified into high pressure (>106 Pa), medium pressure (~105 Pa) and low pressure according to the charge gas pressure. (10 ~ 100Pa) mercury lamp. The medium pressure mercury lamp is one of the most widely used ultraviolet light sources. The main components of the UV light source are: light source, power supply, cooling system, exhaust system, and light shielding system. The main focuses of the research on irradiation equipment are: (1) to reduce heat deposition on substrates and printing or transmission equipment, so as to open up new application fields, such as wide-web soft printing of flexible packaging materials; (2) improve arrival Curing surface irradiation power and maximum illumination to increase production speed and curing depth, and reduce the amount of photoinitiator.

2.1.1 Light Sources (Including Lamps and Reflectors)

Generally, the power of the UV lamp is 80-240 W/cm, the light intensity is 30-2000 W/cm2, and the lamp tube can be divided into two types: electrode type and electrodeless type. The electrode type lamp generates ultraviolet light in the arc discharge between the two electrodes, and the diameter is generally 20 to 25 mm. The electrodeless lamp excites light using microwaves generated by electromagnetic radiation and has a diameter of about 9-13 mm. The diameter of the lamp tube has a great influence on the infrared emission and reflection focusing. In recent years, a more detailed UV lamp has been developed. The mirror is usually a parabolic or elliptical highly polished aluminum plate that reflects not only ultraviolet light but also infrared light. Recent designs have focused on reducing the reflection of infrared light by the reflector in order to reduce the deposition of heat on the substrate. For this purpose, the reflector can transmit or absorb infrared light as much as possible. Another new development in mirror design is the use of more efficient focusing to increase maximum illumination. It is also designed to put a lens under the focus so that the non-reflected light is also focused on the focus.

2.1.2 Power Supply

The power supply consists of two parts: a high-voltage transformer and a capacitor. It is usually bulky and heavy, but in recent years it has developed a small, lightweight power supply that can be continuously adjustable or adjustable at small intervals and can provide up to 6 kW or more of power.

2.1.3 Cooling System

Since the UV lamp generates heat during operation, it is necessary to remove the heat in time for the system to function properly. Usually air or air and water dual cooling systems are used. Water is a more effective medium for heat dissipation, but water may cause leakage. Therefore, air cooling is more popular.

2.1.4 Exhaust system

Due to ultraviolet radiation, ozone that is harmful to the human body is generated and must be discharged outside the workplace. The exhaust system is usually also part of the cooling system.

2.1.5 Light shielding system

Ultraviolet light is harmful to human skin and eyes, so UV lamps must be equipped with a shield system, which is also part of the enclosure and exhaust cooling system. Since the emission spectrum of conventional UV lamps includes both UV, visible, and infrared light, only a small portion is absorbed by the photoinitiator to cure the coating, so the efficiency is not high. The main radiation band of the UV exciton is three narrow UV bands. They are: 172 nm xenon lamp, 222 nm plutonium chloride, 308 nm plutonium chloride. If the absorption band of the initiator coincides with the UV emission spectrum, the utilization efficiency can be greatly improved. UV flash can produce high flash intensity, up to several kW/cm2, mainly used for screen printing, which can greatly improve the degree of curing and depth. The UV flash lamp has great potential in the application of low-speed continuous irradiation treatment of three-dimensional object coating. In addition, the UV laser has a high energy and is a monochromatic light source that can be used for stereolithography.

2.2 Electron Accelerator

According to different ways of emitting high-energy electrons, it can be divided into scanning type and electronic curtain type. Modern electron accelerators have a grid to improve the uniformity of the electron beam, and are equipped with a touch screen and a micro-processing automatic control unit. Due to the polymerization inhibition of O2, the irradiation zone must be protected with N2. Although several improvements have been made to reduce the N2 consumption, this one is still the largest operating cost. Therefore, formulations with low sensitivity to O2 are now being developed. Electron radiation has many advantages such as high energy efficiency, strong penetrating power, even curable thick and colored coatings, and no need for initiators, but its application is limited due to high investment and high N2 cost. The current equipment improvement focuses on the scanning window design to reduce the energy loss when the electrons pass through and make it easy to repair to increase the working time of the equipment.

(to be continued)