Imaging mechanism of thermal CTP plate

Since Drupa95, CTP technology has developed rapidly in the world, and the matching CTP plate material has also developed rapidly. The data shows that in 2003, CTP plates accounted for about 30% of the total plates in the world, and it is expected to reach more than 60% by 2007. Among them, the thermal CTP technology is currently the fastest growing. The global thermal CTP installed capacity accounts for about 60% of the total, and it also accounts for more than 55% in China. The reason why thermal technology develops so quickly is because of its advantages such as high print resistance, high resolution, and bright room operation. It is recognized as the future development direction of CTP technology.

World-famous plate material suppliers such as Agfa, KPG (Kodak Polylight) and Fuji (Fuji) have developed various types of thermal CTP plates. The thermal CTP version is divided into two types: the negative type and the positive type. The CTP version of the negative type is partially exposed during infrared scanning, and the positive type is non-graphic type. In addition, the CTP plate that does not require development after exposure is called a processing-free plate, which is currently the focus of research by various plate suppliers. The imaging mechanism of the thermal CTP version is diverse, and the CTP version can be classified according to its difference. The imaging mechanism and application of various thermal CTP plates are introduced below.

1. Negative heat-sensitive CTP plate

1.1 Thermal cross-linked negative picture CTP version preheated before development

This is the earliest product developed, the technology is very mature, the degree of commercialization and practicality is high, and it is widely used. Currently, various plate material suppliers have production.

This type of plate is usually made by uniformly coating the heat-sensitive layer on an aluminum plate that has been granulated and anodized or coated with polyester. The heat-sensitive layer usually includes a film-forming resin, a cross-linking agent, an infrared absorbing dye, and a photothermal acid generator. The imaging mechanism is: when infrared light irradiates the plate, the infrared dye absorbs the light energy and converts it into heat energy, and the acid generator generates acid. Under the catalysis of the acid, the resin in the exposed area crosslinks to a certain extent, forming a latent image. After pre-heat treatment, the resin in the exposed area is fully cross-linked, but the non-exposed area is not reacted. The non-exposed areas are removed by alkaline development. The imaging process is shown in Figure 1.

Fig.1 Thermal cross-linked negative image CTP plate imaging process

1.2 Ink-based negative-printing CTP plate

This type of plate material is generally an anhydrous offset printing plate, and Agfa, KPG, Fuji and other companies have related patented technologies. Taking Agfa's technology as an example, the aluminum surface of the plate is a layer of hydrophilic metal oxide, which is actually ink-friendly. The lower coating of the plate material is a photodecomposition type, which is used as the photosensitive imaging layer and the bonding resin that bonds the plate base and the upper coating layer. The top coat is cross-linked and cured silicone rubber, which is oleophobic and does not absorb infrared radiation.

Its imaging mechanism is: when the infrared light irradiates the plate, the lower coating layer will undergo a photolysis reaction, the adhesion between the adhesive resin and the silicone rubber in the exposed area is reduced, and the silicone rubber in the exposed area and the decomposed lower area are removed during development The coating resin is exposed on the surface of the substrate to form an ink-friendly graphic part. The imaging process is shown in Figure 2.

Fig. 2 CTP plate image formation process

Such plates are usually developed by mechanical stripping or solvent cleaning to remove silica gel after exposure. Therefore, during the development process, the silicone layer may be scratched or rubbed to damage the plate surface. In addition, KPG has also developed a treatment-free base-based ink-friendly CTP version, which will be introduced later.

2. Positive figure type thermal CTP plate

2.1 Solubility-changing positive figure CTP plate

The structure of this plate is generally coated with a heat sensitive layer on the hydrophilic substrate, the heat sensitive layer is insoluble in alkaline developer and ink-friendly. The imaging mechanism is: when the infrared laser scans, the exposed area is heated and undergoes physical or chemical changes to become alkali-water soluble, and the exposed portion is removed with an alkali developer. The development process is shown in Figure 3.

Figure 3 The development process of the positive-dissolving positive CTP plate

The patented technology of KPG and Fuji is that the heat-sensitive layer is composed of film-forming resin, infrared absorber, solvent-resisting agent and other additives. Before the exposure, the resist formed a hydrogen bond with the film-forming resin, resulting in a dissolving effect, making it difficult to dissolve in the alkaline solution. During infrared exposure, the infrared absorber generates heat, which causes the heat-sensitive composition to decompose and increase its solubility in the lye to achieve development imaging. This process is generally considered to be a process of physical change. Agfa's plate imaging layers of this type include alkali-soluble polymers and infrared-sensitive surface layers that are impervious to developer.

2.2 Thermal cross-linking with pre-heated positive CTP plate

This type of plate material belongs to Agfa's patented technology. Its imaging composition includes alkali-soluble resin, latent protonic acid, cross-linking agent and infrared absorber (carbon black). The imaging mechanism is: when 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 preheating process before development, the unexposed area undergoes thermal cross-linking and curing based on the acid-catalyzed reaction (the presence of latent protonic acid), which reduces the alkali solubility; while the exposed area remains alkali soluble. The alkaline developer removes the exposed part during development. See Figure 4 on the next page.

Figure 4 Thermal cross-linking with preheated positive image CTP plate development process

This process of plate imaging is exactly the opposite of the conventional thermal cross-linked negative CTP version, that is, the cross-linked and non-cross-linked parts are just adjusted. This is because the infrared absorber used is different from the negative pattern. In this process, carbon black and the proton acid produced by infrared exposure have an unknown effect, so that the exposed area can still maintain alkali solubility.

3. No processing CTP plate

According to the data, all companies take the treatment-free thermal CTP plate as the key research direction, and have put forward a large number of design schemes, and new schemes are constantly being proposed. This is because this type of plate material can save development processing costs and improve efficiency.

3.1 Polarity change-free processing CTP plate

Figure 5 Polarity change processing-free CTP plate development


The imaging mechanism of this type of plate is: before exposure, the entire plate is usually hydrophilic. During infrared scanning, the polarity of the exposed area changes. This change process can be physical or chemical. Make this part from hydrophilic to lipophilic. After the printing plate is exposed, it does not need to be developed again to meet the printing requirements. This process is shown in Figure 5. Of course, there are also some design schemes that first design the plate to be lipophilic, which makes the polarity enhancement become hydrophilic during exposure, so that there are hydrophilic and lipophilic areas 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, which can be divided into two types: physical and chemical. Physically polarized plates usually have some hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder resin. The thermoplastic particles have a critical aggregation temperature, which is softened or melted under heat to cause the particles to aggregate. In the hydrophilic layer In the formation of hydrophobic aggregates. The chemically polarized plate thermal resin is usually a homopolymer or copolymer containing pendant polar functional groups. These hydrophilic functional groups react with other functional groups under heat to form a hydrophobic structure.

3.2 Infrared ablation-free CTP plate

Figure 6 Infrared ablation-free CTP plate development process


Agfa's infrared ablation CTP version belongs to the positive thermal version, which uses metal as the molten layer. The molten metal layer is coated on a hydrophilic aluminum substrate, which is oleophilic. The imaging mechanism is: the metal layer in the exposed area is melted into droplets under high-intensity laser radiation, and the contact angle with the aluminum plate becomes larger, which can be removed without development, exposing the hydrophilic plate. The process is shown in Figure 6.

Fuji's thermal ablation plate is different from Agfa. It is coated with an ink-receptive layer on the plate base, with a hydrophilic layer of metal particles as a photothermal conversion agent. During scanning exposure, metal particles can make it brittle and easy to remove. Or a hydrophilic layer coated with metal oxide or metal hydroxide and acid hydroxyl-containing resin on the ink-receptive layer, and sometimes a hydrophilic surface layer.

3.3 Base-based ink-friendly CTP-free plate

Figure 7 The development process of substrate-free ink-friendly CTP plate

This is a patent of KPG and belongs to the waterless offset printing plate. This plate material is coated with an ink-receptive intermediate layer on the plate base, and then coated with a special silicone copolymer on the ink-receptive layer. This layer of ink-repellent silicone copolymer, as a heat-sensitive layer, is of thermal decomposition type. The imaging mechanism is that the thermally sensitive layer in the exposed area undergoes thermal decomposition reaction. These decomposed silicone copolymers are easily removed during the exposure process or the usual procedure after exposure, exposing the intermediate ink-receptive layer. The development process is shown in Figure 7.

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