1 Introduction
Modern rotary screen ink jets, spray wax screens and traditional film screens all involve photographic processes. In the application of rotary screen photosensitive equipment and photosensitive adhesive products, although manufacturers will provide relevant exposure parameters, but in many cases, due to numerous changes in the factors that affect the accuracy of the exposure. These changes come from the light source's oldness, spectroscopy, and distance; the photosensitivity and amount of photosensitive materials; the thickness of the adhesive layer; the temperature and humidity of the environment and other factors. In actual operation, it is very important to properly master the sensitization process in order to obtain satisfactory circular nets for high-quality printing.
2 exposure time and light source relationship
2. 1 exposure time
The rotary screen exposure time involves the problem of exposure sensitization. In the process of plate making, overexposure will make the photosensitized film excessively sensitized, and the film will harden. The formed unexposured area will have poor expansibility and will not be easy to wash. It will cause the pattern to be difficult to appear. The lack of exposure will result in insufficient photosensitivity of the photopolymer. The film is tender and easy to wash out, resulting in the appearance of trachoma, zigzag patterns, in severe cases, the film will fall off. In short, mastering the correct exposure time is a very important process for the photographic process, which is associated with the exposure light source and the photosensitizer.
2. 2 exposure light source
Exposure light source refers to a light source that can cause chemical and physical changes in the photo-resist layer in a relatively short time during the exposure process. Choosing the right light source can reduce the exposure time, speed up plate making and improve plate quality. The cross-linking reaction of the rotary screen photoresist mainly takes place after absorbing ultraviolet light. The exposure light sources used are all ultraviolet light sources, and their spectral characteristics and intensity have a direct relationship with the plate making effect. Therefore, in the election
When using a light source, it must be considered that it matches the absorption wavelength of the photosensitive material used to obtain good results. The rotary screen photosensitive material is sensitive to ultraviolet light in the wavelength range of 300-400 nm, and the types of output light sources rich in this wavelength range include high-pressure mercury lamps and iodine-neon lamps. The wavelengths generated by these two types of light sources are more consistent with the absorption wavelength of the photosensitive material of the rotary screen, and can exert a higher light-sensing efficiency in the plate-making exposure, so that the photosensitive adhesive produces light-curing in a short time under light irradiation.
2. 2. 1 High pressure mercury lamp
The spectrum emitted by the high-pressure mercury lamp has the highest emission peak in the ultraviolet region, the highest peak at 365 nm, and close to the absorption wavelength of 355 nm in the photosensitive material of the rotary screen. The light source has high energy efficiency and low emissivity, and is a production circle. The ideal light source for web systems.
2. 2. 2 iodine lamps
The iodine deuterium lamp wavelength distribution in the 350 ~ 450 nm wavelength range, with the highest peak at 417 nm. There are two peaks at 400-350 nm, which is effective for most printing photosensitive materials. The light emitted by this light source is strong and has high luminous efficiency, but it has high radiant heat and a long start-up time.
Since the spectral characteristics and energy intensity of the above-mentioned light source meet the sensitization requirements of the rotary screen plate making, the two types of light sources are mainly used as the rotary screen photosensitive exposure light source.
2. 3 Light source attenuation and intensity effects
Exposure light sources generally have an average useful life of 500 to 1 000 h. In daily use, the amount of light will decay over time, leading to a decrease in the amount of light. Therefore, the exposure time is not constant. If the light source is used for a long time or exceeds the useful life, it is necessary to extend the exposure time accordingly or use a new light source.
In addition, the light source intensity also has an effect on the making of the net. The energy of the light source reaching the surface of the adhesive film is also the irradiation intensity obtained on the surface of the photosensitive material. This kind of irradiation intensity is not the bright intensity seen by the eye, because the ultraviolet light is invisible light. When the distance between the light source and the photoreceptor surface is fixed, the higher the light source irradiation intensity, the shorter the exposure time; the lower the light source irradiation intensity, The longer the exposure time.
3 photosensitive material properties, dosage and dark reaction
3.1 Characteristics of Photosensitive Materials
The photochromic agent used in the rotary screen system is a dichromate system, which becomes a photosensitive glue solution after being mixed with an organic colloid, and its photosensitivity directly affects the photosensitivity of the photosensitive colloid and the effect of making a mesh. Dichromate and its aqueous solutions do not possess photosensitivity when they are present alone. Only when they are mixed with organic colloidal substances that are oxidized, they produce a photosensitizing effect and are exposed to ultraviolet light, and they quickly interact with colloids in a short time. In conjunction with the reaction, the photo-resist film is cured to form a water-insoluble adhesive layer combined with a nickel mesh.
In fact, photosensitizers function to transfer energy. Wavelengths of different wavelengths have different energies, such as 300 nm UV energy of 399. 3 kJ, 360 nm UV energy of 333. 5 kJ, and 395 nm UV energy of 300. 8 kJ. These light waves first transmit energy to the photosensitizer, and initiate and supply the energy required for the photocrosslinking reaction of the photosensitizer and the organic colloid. For the dichromate photosensitizer, it cannot receive and transmit light of various wavelengths, but absorbs only the wavelength of the ultraviolet region, which is determined by the structure of the photosensitizer.
Dichromate is used as a rotary screen photosensitizer, absorbing mainly ultraviolet light in the 355 nm wavelength region (see Figure 3). Due to its highest peak at 210 nm, there is no suitable light source match. In addition, if the plate-making exposure is carried out under a glass plate, the highest peak wavelength of 210 nm cannot be utilized because the glass will strongly absorb and block the light transmission below 320 nm, which has no practical effect on the photosensitive layer. Therefore, you can choose to output richer and higher intensity light sources in the 360 ​​nm region.
3.2 The amount of photosensitive material
The photosensitive speed of the rotary screen photosensitive film is related to the amount of the dichromate photosensitizer. In the same weight of photosensitizer, the amount of dichromate used is smaller than the amount of photosensitization, and the degree of photocrosslinking is also large. However, when determining the amount of colloid, the amount of photosensitizer should be controlled according to the amount of formula. The appropriate amount of photosensitizer can shorten the photosensitivity, while the excess can have a bad effect. Excessive photosensitizer can accelerate the dark reaction of the photosensitive resin and deepen the color layer of the photosensitive layer, so that a part of the light that affects the photosensitive layer during exposure is absorbed by the photosensitizer on the surface of the photosensitive layer, so that the surface is over-cured and the interior of the adhesive layer is cured. Insufficient, causing the exposure to fail. In addition, if an excessive amount of dichromate photosensitizer is contained in the photosensitive resist, the photosensitizer crystals will precipitate on the surface of the adhesive layer after the photosensitive colloid is dried, which hinders the photosensitivity of the adhesive layer and makes the surface of the adhesive layer with dichromate. Crystals do not harden, affecting the photo-curing layer with sufficient photo-curing.
3. 3 dark reaction
In the dry state of the liquid net or the adhesive layer, the rotary screen photosensitive adhesive spontaneously undergoes a curing action in the absence of light, which is called a dark reaction. The dark reaction in the liquid state is manifested as the increase of the viscosity of the photosensitive liquid and the deepening of the color; when it is in the dry state, the dark reaction shows the difficulty of development and dissolution. The main reason for the dark reaction is that the dichromate oxidation reaction occurs under the effect of heat. Judging from the generalized reaction, the dark reaction is essentially the same as the photo-curing reaction, except that the dark reaction proceeds without the direct action of light energy, and the reaction speed is much slower than the photo-curing reaction. The speed of the dark reaction has a great relationship with temperature and humidity. The higher the temperature and humidity, the faster the reaction rate. The experiment shows that when the temperature rises from 25 °C to 42 °C, the dark reaction speed increases about 2 to 3 times for each 10 °C increase in temperature; when the temperature is 4 °C, the dark reaction becomes very slow.
In the case of rotary screen making, if the screen is coated with an already aged photosensitive adhesive, the amount of exposure during exposure is less than conventional, and the adhesive layer can be photocured. On the surface, the sensitivity seems to have improved, but it actually caused a decrease in solubility and development difficulties. If the just-prepared sensitized adhesive is applied to the substrate, the sensitized layer begins to react darkly. As time goes by, the dark reaction gradually strengthens, and the solubility of the gel layer becomes smaller and smaller. Over time, the adhesive layer will be completely cured. The result of the dark reaction can make the solubility of the visible and unspotted regions. The difference becomes smaller and smaller, and finally it is so small that no part of the light can be dissolved, and the image cannot be obtained.
To control dark reactions, the key is to control the temperature and humidity of the environment during drying and storage. As the temperature and humidity decrease, the dark reaction rate decreases. Therefore, the printing and dyeing factory rotary screen platemaking workshops are equipped with air-conditioning, so that the indoor temperature is maintained at about 20 ~ 25 °C, relative humidity is less than 50%, to minimize the dark reaction speed of the rubber screen layer.
4 Influence of Thickness and Relative Humidity on Photocuring
4.1 Effect of Adhesive Layer Thickness
The thickness of the adhesive layer depends on the viscosity of the adhesive. High viscosity, thick glue layer; low viscosity, thin glue layer. Because the photocuring starts from the surface of the adhesive layer and gradually penetrates into the adhesive layer, the thicker adhesive layer has a slow photocuring speed and requires a relatively large amount of exposure so that the adhesive layer can be cured from the surface to the interior without being soluble in water. The development of the thicker layer will also slow down after exposure. Of course, the adhesive layer should not be too thin. Although the adhesive layer is beneficial to the exposure time, light curing and development, the solid content in the adhesive layer is reduced when the network is manufactured, so that the film is poorly covered, the gloss is dull, the sand holes are large, and there is no sense of layer. The more important is that There will be problems with low print rates in production.
4.2 Influence of Relative Humidity
The photosensitive reaction of the dichromate photosensitizer in the rotary screen photoresist is an ionic reaction, so a certain amount of moisture is required in the photosensitive adhesive layer. In the production process, moisture in the gel layer gradually evaporates when the net is dried at a low temperature, and the adhesive layer is substantially dry during an effective pre-bake time. In fact, depending on the state of the atmosphere in the environment, the glue layer loses or gains moisture during drying and storage. That is, there is a small amount of moisture affected by the weather.
Water remains in the gel layer and its moisture content is related to the relative humidity of the operating environment. The relative humidity of the operating environment is large, and the water content of the adhesive layer is much; on the contrary, the moisture content of the adhesive layer is small. Moisture content has a great influence on the sensitivity of the adhesive layer and can accelerate or retard the light curing process. As the relative humidity rises, the water content in the adhesive layer increases, and the sensitivity increases, but the increased water content of the adhesive layer will affect the adhesive force of the adhesive layer and the metal mesh, the sharpness and resolution of the adhesive layer, and the development fastness of the adhesive layer. The fully dry or fully expanded adhesive layer has zero sensitivity.
Source: China Sensitive Materials Network