UV curing of protective layer of optical disc

The UV curing process is used for the protective layer of the optical disk, the UV adhesive of the DVD bonding process, and the curing process of the UV ink in the printing process.

Analyze the UV spectrum in the frequency domain. The longest wavelength closest to visible light is UV V, which ranges from 445 to 340 nm and penetrates the opaque components in the aluminum layer and the UV ink. Moving to a high frequency band, UV A in the wavelength range of 350 to 320 nm and UV B in the wavelength range of 320 to 280 nm can increase the speed of UV processing and increase the degree. At the top of the UV spectrum, UV C has a shorter wavelength (280-200 nm) and usually only affects the surface of the material.

Generally, the UV lamps in the curing system emit UV light of long wavelengths and short wavelengths. The ratio of the two should be strictly set: if the long wavelength is underexposed, the adhesion between the protective adhesive and the substrate is weak; if short If the exposure of the wavelength is too short, the surface is not completely processed. The latter can have a positive meaning. Incomplete curing of the surface of the protective layer can maintain the partial adhesion of the surface and facilitate the adhesion of the printing ink.

The following discusses how UV lamps cure liquids. The UV treatment material contains a photoinitiator that is sensitive to UV light. Once activated by UV light, the polymer chain reaction begins. This process is called polymerization. When triggered by UV light, the photoinitiator causes the monomers of short molecular chains to recombine to form molecular chains. When the short-chain molecules form long chains (polymerization process), the material gradually hardens and gradually solidifies. When all short-chain molecules form long chains, the material becomes solid.

Photoinitiators are very sensitive to UV light and photoinitiators can be selected based on both ends of the UV spectrum (long wavelength and short wavelength). A variety of photoinitiators can be used in the formulation of a UV gel. The strength of the polymerization process depends entirely on how much photoinitiator is triggered by the UV light, that is, how much energy is transmitted by the UV light that passes through the protective glue, and exposure, UV wavelength and light intensity are the main factors affecting the latter. .

In addition to photoinitiators and monomers, UV treatment materials also contain oligomers. The oligomers act as a solvent and determine the physical and chemical properties of the material (such as hardness, toughness, chemical resistance). The monomer affects the speed of curing. For different applications, the ratio of monomers and oligomers varies greatly, some formulated monomers may account for 10%, and other formulated monomers may exceed 70%. Sometimes the formula also contains other ingredients to improve adhesion and stability.

A very important factor must be considered in the production of platters: speed. Since most UV light sources also radiate infrared light (heat) at the same time, system designers must weigh the pros and cons in the following contradictions: too much UV light, too much heat; too little UV light, too slow.

UV light source has three strict indicators: irradiance, radiation, infrared radiation. Irradiance represents the intensity of the light source, in milliwatts per square meter, and is related to the design of the curing system, including efficiency, optical design, and focusing. The amount of radiation represents the amount of radiant energy received in a given area, in millijoules per square centimeter. The irradiance test value is fixed (always) while the radiation dose test value is time-varying.

These specifications, along with the characteristics of the UV formulation, determine the distance of the light source from the substrate and the rotational speed of the substrate to ensure that the UV light penetrates the protective glue and cures at the end of the process.

The infrared filter can eliminate part of the infrared radiation before the UV light reaches the substrate. Absorption of infrared radiation can generate heat and the use of heat sinks, water-cooled jackets, or air-cooled systems can quickly dissipate heat. The use of medium-pressure mercury lamps can usually achieve lower infrared radiation: the use of quartz tube electrodes at one end. The electric arc is used to evaporate the mercury in the tube, providing a rich source of UV light. The addition of a metal halide additive can adjust the UV spectrum (eg, increase the brightness of the long wavelength band), which can be achieved using metal halide lamps.

The third type is an electrodeless mercury lamp, developed by the Fusion UV System. The use of microwaves to provide system energy, due to the absence of electrodes, allows the use of smaller size quartz tubes. One of the advantages of electrodeless mercury lamps is that they turn on and off very quickly. Conventional UV lamps require warm-up time, during which time the output is unstable and may last for several minutes. To eliminate this instability, UV lamps are usually in a low-power, normally open state and use a shutter to prevent UV light from reaching the disk.

The final design of the UV curing system is still based on practical applications. Curing the protective coating is the simplest, curing UV inks is a little more complicated.

The main problem with the curing UV ink process is that various solid pigments absorb UV light in a wide or narrow band. The green pigment absorbs UV light while the white pigment reflects UV light. In addition, infrared light is easily absorbed by a certain pigment (for example, black), and additional heat is generated in the curing process to finally deform the disk. In the UV ink curing process should be strictly selected light source, process cycle and pigment categories.

The selection of UV lamps and adhesives in the DVD bonding process corresponds to the long wavelength band of the UV spectrum. Because the PC substrate absorbs short-wavelength UV light, the adhesive formulation cannot contain only photoinitiators that have an activating effect on the high-frequency spectrum (short wavelength).

The layer thickness is also an important process parameter, the CD protection layer is usually 5-7 μm, and the DVD adhesion layer is about 50 μm. In the bonding process, in the adhesive sandwiched between two PC substrates, the heat generated by the infrared absorption or curing process (polymerization is an exothermic reaction) cannot be directly dissipated into the air. This means that the temperature of the substrate is critical at the beginning of the UV curing process.