Optical disk protective glue coating process

Protective gel coating is a well-known process, but this process has evolved over the years, with producers seeking a balance between cost efficiency and process stability.

After the disc has been sputtered with a reflective layer, the next step is to spin-coat a UV protective glue or paint (sometimes involved). The reflective layer is very thin and can be damaged if it is not protected. The metal layer itself cannot be treated. Contact with the metal layer can result in a decrease in reflectivity. Even if there is no physical contact with the reflective layer, most of the materials will react with the air quickly and will inevitably reduce the reflectivity to below the lower limit of the reflectance by 70%. The protective glue prevents chemical reactions of the metal layer and also prevents physical damage of the metal layer.

The protective glue process consists of two main steps: spin-on protective glue and UV treatment. The early spin coaters were very large, using parallel conveyors to feed, and distinguished themselves from the current system in many important ways: the protective layer material was not UV-treated, but solvent-based, which meant that the spin-coated disks were not only Drying is required, and vaporized volatile solvents (generally toxic) need to be safely removed from the work area. This not only increases the complexity of the machinery, but also increases the cost. In particular, environmental protection laws in many places require the establishment of a solvent recovery system to strictly control the waste discharged into the air.

Nowadays, the protective rubber process is considered to be a mature technology. Compared with the earlier mechanical devices, today's systems show light and high-efficiency characteristics in terms of feeding. The main components of the UV spin coater include a blanking bowl slightly larger than the disk size, a blanking arm, a free blanking nozzle, a feed chute, a material pump, and a motor for rotating the disk.

In order to obtain a uniform spin coating effect, a pre-quantified protective glue is fed to the center of the disk. The disc speed is very low at the time of blanking to ensure that the protective tape dropped on the disc becomes evenly distributed. Obviously, in order to ensure uniform coating on the platter, it is necessary to strictly control the intrinsic viscosity of the material, the coating temperature, and the rpm of the platter. Once the protective glue is in place, the disc rotates at a high speed of 2000-6000 rpm, forcing the protective glue to coat the outer edge of the disc. The material thrown off the edge of the disc is returned to the feed tank to avoid waste.

At the outer edge of the disc, there is an unsputtered area (this area is intentionally hidden when the reflective layer is plated), so the protective adhesive will directly adhere to the PC substrate and close the reflective layer to protect the air from the reflective layer. erosion.

As with other optical disc production process stages, the protective glue process should also avoid defects. The main problem of the protective glue process is the splash spots and uneven thickness that occur when the coating is applied. Because the protective glue is applied to the back of the disc, it does not seem to be very important, and it is not. To meet the Red Book standards, the disc thickness must be 1.2mm, and the allowable tolerance range is very small. This thickness is the thickness of the entire disc, including PC substrate, protective adhesive and so on. The problem with CD-R becomes even more important: if the protective glue is too thin, it is not enough to protect the groove and the dye layer; if the protective glue is too thick, it affects the total number of times the disc is burned.

The physical properties of UV adhesives are also very strict. The center hole is slightly smaller or the cartridge sleeve is slightly larger, or the combination of the two may cause the disk to be difficult to remove from the cartridge. Users generally use a method of bending the disk to remove the disk, so the protective layer must have sufficient plasticity To resist bending. Protective adhesives also need to have excellent adhesion properties, especially for very smooth gold reflective layers. The protective adhesive layer is very thin (usually 5 ~ 7μm), should have very tough characteristics to protect the reflective layer and pits susceptible to erosion. The protective adhesive should also have low shrinkage and certain resistance to peeling to prevent the protective layer from falling off during the process of moving the disc.

Although the primary purpose of the protective adhesive is to protect the reflective layer, the printing properties of the protective adhesive surface (whether it is a white printed substrate and the final printed label) should also be considered when developing the protective adhesive formulation. Because the printing of the disk is performed on the protective layer, the printing medium and the protective layer should have better adhesion characteristics. Most UV protective adhesives appear to be transparent or yellowish, which can slightly affect the color of the substrate and ink.

The protective glue is applied to the surface of the reflective layer of the disc in the form of a liquid, and the UV curing treatment is required after the coating is completed.
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