Imaging Mechanism of Thermal CTP Plate
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The data show that the CTP plate in 2003 accounted for about 30% of the total global plate, and is expected to reach more than 60% by 2007. Among them, thermal CTP technology is currently the fastest growing. The global thermal CTP installed capacity accounts for about 60% of the total, and it accounts for more than 55% in China. The reason why thermal technology has developed so rapidly is that it has the advantages of high printing durability, high resolution and bright room operation, and is recognized as the future development direction of CTP technology.
Many types of thermal CTP plates have been developed by world-renowned plate suppliers. The thermal CTP version is divided into two types: the negative image type and the positive image type. The CTP version of the negative image type is partially exposed when the infrared scanning is performed, and the positive image type is the non-graphic partial exposure. In addition, the CTP plate that does not require development after exposure is called a process-free plate, and it is currently a hot spot for various plate suppliers. The imaging mechanism of the thermal CTP version is varied, and the CTP version can be classified according to its difference. The following describes the imaging mechanism and application of various thermal CTP plates.
1, negative-type thermal CTP plate material
1.1 Heat-crosslinked negative image CTP version pre-heated before development
This is the earliest developed product, the technology is very mature, commercialization, practicality and application. Currently, all plate suppliers have production.
Such plates are typically formed by uniformly applying a heat sensitive layer to a granulated and anodized or coated polyester substrate. The heat sensitive layer typically includes a film forming resin, a crosslinking agent, an infrared absorbing dye, and a photothermal acid generator. The imaging mechanism is: when infrared light illuminates the plate, the infrared dye absorbs light energy into heat energy, and the acid generator generates acid. Under the catalysis of acid, the resin in the exposed area is crosslinked to form a latent image. After the pre-heat treatment, the resin in the exposed region is sufficiently cross-linked, and the non-exposed region does not react. The non-exposed areas were removed by lye development.
1.2 base ink-injected negative CTP plate
Such plates are generally waterless offset plates, the aluminum-based surface of which is a layer of hydrophilic metal oxide, which is in fact also ink-receptive. The undercoat of the plate material is of a photodecomposable type and serves as a photosensitive imaging layer and a binder resin for the bonding plate and the top coat. The top coat is a cross-linked, cured silicone rubber that is oleophobic and does not absorb infrared radiation.
Its imaging mechanism is: when infrared light illuminates the plate, the undercoat layer undergoes photodecomposition reaction, and the adhesion between the binder resin and the silicone rubber in the exposed area is reduced, and the silicone rubber in the exposed area is removed during development and the decomposition is performed. The coating resin exposes the surface of the substrate to form a portion of the ink-repellent image.
Post-exposure development of such media typically utilizes mechanical stripping or solvent cleaning to remove the silica gel. Therefore, the silicone layer may be scratched or rubbed during development to damage the layout. In addition, KPG has developed a process-free base-based ink-based CTP version, which will be described later.
2, positive-type thermal CTP plate material
2.1 Solubility change type positive image CTP plate material
The structure of such a plate is generally coated with a heat sensitive layer on a hydrophilic plate, and the heat sensitive layer is insoluble in an alkaline developing solution and is ink-receptive. The imaging mechanism is: when the infrared laser is scanned, the exposed area is physically or chemically changed to become alkaline water-soluble, and the exposed portion is removed by an alkali developing solution.
The patented technology is that the heat sensitive layer is composed of a film-forming resin, an infrared absorbing agent, a resisting solvent and other auxiliaries. Prior to exposure, the resistive solvent forms a hydrogen bond association with the film-forming resin, causing a barrier effect that makes it difficult to dissolve in the lye. In the case of infrared exposure, the infrared absorbing agent generates heat, so that the sensible composition is decomposed and combined, so that the solubility in the lye is increased to achieve development imaging. This process is generally considered to be a process of physical change. Such imaging layers include alkali soluble polymers and infrared sensitive skin layers that are not penetrated by the developer.
2.2 Thermal cross-linking CTP plate with preheating
Its imaging composition includes an alkali soluble resin, a latent protic acid, a crosslinking agent, and an infrared absorber (carbon black). The imaging mechanism is: when the infrared laser is irradiated, the unknown effect of the exposed area can still retain good alkali solubility; the unexposed part does not change. During the pre-heat treatment process before development, the unexposed area undergoes thermal crosslinking curing based on an acid-catalyzed reaction (the presence of a latent protonic acid), which reduces the alkali solubility; while the exposed area remains alkali-soluble. The alkaline developing solution removes the exposed portion during development.
This process of plate imaging is exactly the opposite of the conventional heat-crosslinked negative CTP version, in which the cross-linked and non-cross-linked parts are just adjusted. This is because the infrared absorbing agent used is different from the negative pattern, in which carbon black and the protonic acid generated by infrared exposure have an unknown effect, so that the exposed area can still maintain alkali solubility.
3, free of processing CTP plate material
According to the data, all companies have taken the treatment-free thermal CTP plate as the key research direction, and proposed a large number of design plans, and new plans are constantly being proposed. This is because such a plate can save development processing costs and improve efficiency.
3.1 polarity change type processing-free CTP plate
The imaging mechanism of this type of plate is that the entire layout is usually hydrophilic before exposure, and the polarity of the exposed area changes during infrared scanning. This change can be physical or chemical. This portion is made hydrophilic to oleophilic. After the printing plate is exposed, it is not necessary to carry out the development process, that is, the printing requirements are satisfied. This process is shown in Figure 5. Of course, there are also some designs that first design the layout to be lipophilic, and the polarity is enhanced to become hydrophilic during exposure, so that there is a hydrophilic and oleophilic zone on the printing plate, and it does not need to be developed.
There are many design schemes and methods for changing the polarity of the printing plate. In summary, it can be divided into physical mode and chemical mode. The physical polarity change plate usually has some hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder resin. The thermoplastic particles have a critical agglomeration temperature, which softens or melts under heat to cause the particles to aggregate, in the hydrophilic layer. Hydrophobic aggregates are formed in the middle. Chemically Polarized Platen The thermosensitive resin is typically a homopolymer or copolymer containing pendant polar functional groups that react under thermal action with other functional groups to form a hydrophobic structure.
3.2 Infrared ablation type free processing CTP plate material
The infrared ablation type CTP plate is a positive type thermal plate which uses metal as a molten layer. The molten metal layer is coated on a hydrophilic aluminum plate which is lipophilic. The imaging mechanism is that the metal layer in the exposed region is melted into droplets under high-intensity laser radiation, and the contact angle with the aluminum plate base becomes large, and can be removed without developing, thereby exposing the hydrophilic plate base. The process is shown in Figure 6.
The other is to apply an ink-repellent layer on the substrate with metal particles as a hydrophilic layer of the photothermal conversion agent. The metal particles can be embrittled during scanning exposure and can be easily removed. Alternatively, the hydrophilic layer of the metal oxide or metal hydroxide and the acid-containing hydroxy resin may be coated on the ink-receiving layer, sometimes with a hydrophilic surface layer.
3.3 base ink-free type processing-free CTP plate material
This is a waterless offset printing plate. The plate is first coated with an ink-repellent intermediate layer on the substrate and then coated with a special silicone copolymer on the ink-receiving layer. This ink-repellent silicone copolymer acts as a heat sensitive layer and is thermally decomposed. The imaging mechanism is that the heat-sensitive layer of the exposed region undergoes a thermal decomposition reaction, and these decomposed silicone copolymers are easily removed during the exposure process or the usual procedure after exposure to expose the intermediate ink-repellent layer.