Extend the storage period of domestic solid photosensitive resin plates

The use of computer typesetting in combination with a solid photosensitive resin plate not only allows the original printing press to print high quality products, but also shortens the printing cycle considerably. Due to the simplicity of the plate-making equipment of the solid photosensitive resin plate, the process is easy to grasp and has no pollution, which is a good substitute material for eliminating the type printing.

Compared with imported products, domestic solid photosensitive resin plates have the advantages of strong applicability and low price (usually 1/3 to 1/4 of imported plates), but due to the problems in the storage period, it affects its popularization and application. For example, domestic industry standards require a storage period of 6 months. However, some manufacturers' products only have 4 months, sometimes only 2 months. Since it normally takes 15 to 20 days from the delivery to the user's hands, the time that the product can be used in the user's hands is very short, which limits the promotion and application of the solid photosensitive resin plate.

The problem of short shelf life of domestic solid photosensitive resin plates has the following three common conditions:

1. The newly produced plate is very easy to use, and development (rinsing) is difficult after one month, especially when the root is hardly dissolved near the base portion. Rinsing is more difficult after 2 months so that it cannot be developed.

2. After a period of storage, red spots began to appear inside. This part was hard to dissolve, and it was further expanded to the entire layout and could not be developed.

3. After the plate is stored for a period of time, snow-like or tetragonal crystals are precipitated on the surface of the plate, commonly known as "precipitation," and as time goes by, "precipitation" becomes more and more serious. Although platemaking can be developed, printing occurs. Flower spots cause waste products. This "precipitation" is the most serious in winter.

The reason for the first situation is that the manufacturers are concerned with the selection and use of the photosensitizer in the photosensitive resin and the binder of the photosensitive layer and the substrate. In the absence of light, it is the result of a dark reaction inside the photosensitive layer. Selecting the products of high quality manufacturers of these two materials can improve this situation.

The second case is due to too much impurities in the raw materials used to formulate the photosensitive resin. Some impurities contain active substances, and in the absence of light, radicals can be generated, and dark reactions form locally. To overcome this situation, a variety of high-purity reagents are used to enhance filtration and add substances that prevent dark reactions against impurities.

The third edition of "precipitation" is the most common cause for the majority of domestic manufacturers, and it is also the most important reason that affects the storage period of plates.

Through observation, it was found that when the water content of the plate is large, “precipitation” occurs easily; when the cover film is attached to the photosensitive layer, the “precipitation” is not likely to occur, and vice versa; when the temperature is low, “precipitation” easily occurs. When the temperature is relatively high, "precipitation" does not occur easily. Some manufacturers in order to prevent the "precipitation" phenomenon, in the capping of polyester film, first on the cover film roll some sensitized plastic, after pressure can make the cover film and the photosensitive layer tightly combined. Although a short time to prevent precipitation, but because of increased Moisture content still produces 'precipitation' after a month or two.

To completely solve the "precipitation," we must find out what the "precipitation" is. How does the "precipitation" phenomenon occur?

After our analysis and analysis, "precipitation" of the substance is the acrylamide monomer and sodium salt in the crosslinker. The acrylamide monomers and sodium salts are particularly water soluble and have a very high solubility in water. The normal water-containing string of the plate is about 12%, and a saturated solution containing the above substances will permeate outward along the molecular aperture of the photosensitive layer, and the initial stage is not serious due to the blocking of the cover film. Because the flatness of the layout is not enough or the film is entrained with air, capillary penetration is very easy to occur in this area. The solution is bound to the air molecules in this area, resulting in oversaturation and the formation of crystals. Especially when the temperature is low, crystallization is more likely to occur. This crystallization loosens the cover film and the photosensitive layer, and more air is introduced, which aggravates the occurrence of crystals on the layout. This results in a large area of ​​“precipitation” and makes the plate obsolete. "Precipitation" occurs more quickly and more severely than the "precipitation" formed by the crosslinking agent. In the whole process of preparing photosensitive adhesives, there is no substance containing sodium ions. Where does the sodium salt come from? Domestic solid photosensitive resin plates are of the Polyvinyl Alcohol (PVA) type. Polyvinyl alcohol is usually produced by the alcoholysis of polyvinyl acetate plus sodium hydroxide (also called “saponification” reaction) in the production process:

From the reaction, it can be seen that the sodium acetate content in the product is unavoidable in the production of polyvinyl alcohol, which is the root cause of the “precipitation” phenomenon. However, before 1992, the PVA produced by the Beijing Organic Chemicals Plant (the only manufacturer of PVA in the country) was crystallized in paraffin. In order to remove the paraffin in the product, it was rinsed many times. Sodium acetate is rinsed off together. The plate material made of polyvinyl alcohol with a low content of sodium acetate has almost no "precipitation" phenomenon. After 1994, the plant changed its process. At this time, there was no rinsing process, resulting in an excessively high content of sodium acetate, and the prepared plate was liable to “precipitate”. If polyvinyl alcohol with a low sodium acetate content is used, the "precipitation" phenomenon can be overcome.

When we formulated the sensitized adhesive, an additive was added, which can make the photosensitive layer have pore sealing effect after drying, making the solution with acrylamide and sodium salt no hole to drill, thus eradicating the "precipitation" The phenomenon greatly extended the storage period of the plate. At present, the storage period of our company's products has reached about one year, and the surface of the plate processed by the sealing hole is even and smooth.

In addition to the manufacturer's reasons, the user of the plate material also has a direct influence on the shelf life of the plate material. Storage conditions that are too cold or too hot can be detrimental to the storage period of the plate, and humid conditions can also shorten the life of the plate.

Source: Asia-Pacific Printing Network