In recent years, the screen printing industry has developed rapidly. Some people say that it has become an industry that is not easily satisfied by market demand. Its changes are changing with each passing day, constantly expanding into new areas of application while quickly introducing new products. Although there have been many changes in screen printing, the method of screen printing has not changed. The basis of the plate making and the chemical principle cannot be separated from the photosensitive adhesive. Therefore, in recent years, the photosensitive adhesive has also been continuously developed. Photoresist general properties:
Photoresist is composed of three basic ingredients: sensitizer, water and solids content. The sensitizer works on ultraviolet light. The combination of polyvinyl alcohol (PVA) solids with a photosensitizer produces a solid material that is insoluble in water.
Photosensitive adhesive structure:
There are three major families of photosensitive adhesives today: diazo photosensitive adhesives, double-sensitized photosensitive adhesives, and pure photopolymer sensitive adhesives.
(1) Diazo Photoschicht
The diazo sensitizer is a polyvinyl alcohol (PVA) and polyvinyl acetate (PVAC) emulsion that uses only a diazo sensitizer (photosensitive resin).
The diazonium salt is mixed with the basic emulsion before use.
In the cross-linking of PVA chains, cross-linking with diazo increases the molecular weight of the PVA material. The exposed part is changed, has water resistance, and remains as a stencil overlay. The unexposed portion maintains its water-solubility, and after being washed with water, it can pass through ink or color paste to form an image.
Diazo cross-linked emulsions (photosensitive adhesives) are primarily used for general screen printing applications using solvent-based inks.
(2) Dual Cure Photosensitive Adhesive (Dual Cure)
Double photosensitizer is a type of photosensitizer (photosensitizer), usually diazo and photopolymer. The dual sensitizers are mixed with a diazonium salt but have to be additionally linked to the photopolymer. The main component of the photopolymer in the photoresist contains acrylate chemistry and a photoinitiator. The component acts as a polyvinyl alcohol and a diazo resin at various sensitizations to various ultraviolet light source wavelengths. Crosslinking of the photopolymer increases the molecular weight of the material and thereby produces a durable and correct method of reproduction required by the original. Because the chemical composition is a double-sensitizer product, very accurate reproductions can be made.
Double photosensitizers have a wide range of applications for screen printing: screen printing plates made with dual photosensitizers are suitable for solvent screen printing inks, water-based screen printing inks, solvent/water-based screen printing inks and other etchings. Inks and colorants, and can ensure excellent reproduction quality and high stability of batch printing.
(3) Pure Photopolymer Photopolymer
This is the third type of photopolymer, which consists of 100% of photopolymer. Pure photopolymer photosensitive adhesives are currently marketed under the trademark Chroma/Tech PL by Chromaline. The photoresist does not contain diazo as a sensitizer appendage. It is pre-sensitized and does not require mixing. The photopolymer is accompanied by an intellectual enthalpy and sodium thiosulfate.
The above photoresist is fully sensitized. If you put it under ultraviolet light, you can complete its changes. This change is what has been mentioned earlier - light chain. The cross-linking or photopolymerization in the photo-sensitive adhesive causes the photo-resist to harden and solidify. In order to obtain the durability of the screen printing plate, it is necessary to specify the thickness of the photoresist, the number of screen meshes, the exposure time, and the solvent resistance.
The highest absorption point for diazo absorption ultraviolet light in the plate-making photosensitive adhesive occurs at a wavelength of about 371 nm. The clear photopolymer for use in plate-making photopolymers has desirable UV absorbance and has a wavelength of about 340 nm.
Ultraviolet lamps emit light waves at various wavelengths along the ultraviolet spectrum. The most important thing for screen printing personnel is to understand the different stages of sensitivity and reaction time of each photosensitizer. It is also important to know the aging of the UV lamp. The light radiation becomes weak and the exposure time should be adjusted accordingly. The plate printer equipped with a light meter can correct the fluctuation of ultraviolet light by automatically adjusting the exposure time. This can also lead to a balanced energy consumption during subsequent exposures.
Photosensitive adhesive performance of the three major families:
Finally, the characteristics of the three major families of Diazo, Dual Cure, and Pure Photopolymer were introduced. Although manufacturers produce different photosensitive adhesives, a comparison of the characteristics of the three types of photosensitive adhesives can be obtained.
(1) Diazo photosensitive adhesives are heat-sensitive, must be mixed, and have a short storage time. The solid content of the photosensitive adhesive is higher than that of the diazo photosensitive adhesive, which is more expensive. However, this type of photoresist can use standard exposure time, which is conducive to the correct reproduction of the image and is resistant to all screen printing inks.
(2) The pure photopolymer photosensitive adhesive usually has a relatively high solid content according to its composition, and has a short exposure time and a long storage time. Its price is more expensive than that of both nitrogen sensitive and dual photosensitizers. However, the photosensitive adhesive is different from the two photosensitive adhesives described above. It is non-thermosensitive and does not need to be mixed.
As mentioned earlier, today's highly developed screen printing technology has not changed its foundation, and although the development of the chemical industry has produced many kinds of photoresist, the basic principle of photoresist is still the same.
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