Thermal imaging and application of violet laser CTP plate

First, thermal imaging CTP plate

The thermal imaging CTP plate has a single photosensitive layer structure, and particularly it uses thermal energy generated by the laser for thermal mode recording, unlike the light mode, which is affected by halation or scattering, and dot reproducibility is good. At the same time, since there is no photochemical reaction process, platemaking can be performed under bright white light conditions. Thermal imaging equipment suppliers have screens, Creo Sai Angel and more than a dozen companies, with good service, mature technology, and diverse applications. The thermal imaging CTP plate was supplied by Mitsubishi Chemical, Fujifilm, Kodak, Boli, and Agfa, and the plate was completely practical. Every year, remarkable progress has been made in plate material development at a multiple rate.

In 1995, the negative thermal imaging CTP plate was first introduced. It utilizes the principle of generating a thermal polycondensation reaction with a photoacid generator, so that after exposure it is heated and the image quality tends to fluctuate. In 1997, a new positive-acting thermal imaging CTP plate without heating was released and became a mainstream product in Japan. The key crosslinked polymeric group of the plate contains a near-infrared pigment, which absorbs near-infrared rays and generates heat conversion, and the polymer in the light-receiving part becomes an alkali-soluble substance, thereby forming a positive-working plate. Because there is no photochemical reaction at all, laser plate making can be performed under white light. When a high-power-output thermal laser is focused on a CTP plate, the temperature of the surface of the photosensitive layer at a thickness of about 2 μm will reach a temperature of nearly 1000° C., and the temperature of the photosensitive layer of the aluminum plate base near the heat transfer is good. At 500°C, there is a large temperature difference between the two layers, so it is difficult to increase the contrast of the development. Japan's Mitsubishi Chemical Corporation has developed a PNSC technology (PhotopolymerLayer Nano Structure Control) with a high-density polymerized structure near the surface layer of the photosensitive layer for the above-mentioned temperature distribution, which makes the plate development stability, surface strength, and printability Image quality has been improved. This is the first thermal imaging plate that was introduced in Japan in 1998, the LT-1 CTP plate.

Second, violet laser CTP plate

The development of semiconductor lasers is surprising. In 1999, Nichia Chemical Industry Co., Ltd. announced that it launched a new generation of DVD semiconductor lasers at 410 nm and became a big news. Seven European companies on Drupa 2000 announced the launch of their own violet laser CTP devices, all using 5mW low-power lasers. Supporting suppliers of plates are rare.

If the sensitivity of the photosensitive resin at 410 nm is increased to more than 10 μJ/cm2, the sensitivity of visible light corresponding to the longer wavelength of 450 nm or more can be reduced, so that it is possible to operate under a yellow safety lamp. The low-cost violet laser CTP will have practical value. The sensitivity of the printing plate required by the 5 mW internal drum violet laser device is estimated by the following exposure conditions of the laser device: 5 mW laser; the output of the laser on the layout is 0.5 mW; the scanning area of ​​the CTP device is 1 m2; The laser scanning density was 2 500 dpi; the rotation speed of the rotating mirror was 20 000 rpm; the sensitivity required for exposure time at 5 minutes was 7.5 μJ/cm 2 . However, this sensitivity has exceeded the limit sensitivity of photopolymerization theory.

In this aspect, the photopolymer has a small amount of thermal decomposition at room temperature and has a reactive group that escapes through the oxygen barrier layer above the photopolymerizable layer and is then trapped by oxygen. Due to the presence of this dark reaction, the polymerization of acrylic monomers cannot be practically achieved. On the other hand, shutting off the oxygen to allow the polymerization reaction to proceed must possess a certain amount of reactive groups, which requires a minimum amount of light energy. Based on the rate of penetration of oxygen molecules, the lowest light energy (limit sensitivity) required for the photopolymer to produce a polymerization reaction can be inferred. For example, when the radical generating agent (RR) has a thermal decomposition rate of 8.2×10 10 mol/cm 2 ·sec at 25°C, correspondingly, the traveling speed of oxygen molecules passing through the polyvinyl alcohol layer (for example, 2 μm thick) is 2.1 x 1013 mol/cm2.sec, so that the polymer is stable at room temperature. The limit light energy required for a photopolymer to produce a photopolymerization reaction can be expressed by the number of photons equivalent to the traveling speed of the oxygen molecule that penetrates into 2.1×10 13 mol/cm 2 .sec, so that the limit light energy can be found to be 13 μJ. /cm2.

To further increase this limit sensitivity, Mitsubishi Chemical has developed a photopolymerization catalyst using a new DAP (Double Amplified Photoinitiation) technology.

The catalyst can improve the absorption efficiency of the sensitizing dye on the laser light energy, transmit the light energy to the radical initiator, and accelerate the formation of the radical. This is the first sensitization mechanism, the light sensitization mechanism. At the same time, the following chain reactions are also generated: (1) an addition reaction between an active group and acrylic acid obtained by the above sensitization reaction to form an acrylic group; (2) an acrylic group and a sensitizing dye and a reactive group are generated The initiator generates a reaction to induce decomposition of these initiators to form a large number of active groups; (3) These reactive groups generate an addition reaction with acrylic monomers to generate acrylic groups again. The above (1)-(3) step chain reaction is formed by the second sensitization mechanism (chemical sensitization mechanism) that makes the quantum yield greatly exceed that of the radical initiator.

That is to say, DPA refers to a sensitization initiation mechanism that utilizes both of the above accelerated reaction mechanisms in a photoinitiator. Mitsubishi Chemical successfully developed the CTP plate that exceeded the limit sensitivity of 10 μJ/cm2 for the first time using DPA technology. This was preceded by a live demonstration of the photopolymer plate LV-1 on Drupa 2000 and the successful production of light using the 5 mW laser. Polymer plates have overturned common knowledge recognized by the world's CTP industry.

Drupa95 introduced us to the thermal imaging CTP device and plate. At that time, it attracted attention as a new technology that could perform platemaking work under white light. In the following five years, after many improvements to its devices and plates, it has been perfect as a system. In particular, this technology has developed into a DI printer capable of configuring a laser imaging head on a printing press, and is entering a new on-demand printing market. The thermal imaging CTP has indeed reached a practical stage and can be expected to be officially promoted.

The PNSC technology developed on the basis of Mitsubishi Chemical LT-1 plate has been further improved, making the development tolerance greater, "LV-2" as its product has recently been listed, and in Japan and other countries won praise. With respect to the violet laser CTP, there are still some unavoidable issues, such as the perfection of the laser imaging device (including the increase of the output power of the laser on the plate), the provision of photopolymer plates by more manufacturers, and the like. However, the system is considered to be able to constitute a cheap CTP system, especially through the efforts of European manufacturers to develop, it can be considered that it will be promoted in the European and American markets.