Differences in physical-chemical properties of the nonprinting areas for conventional and CtP process

Transcription

1 Differences in physical-chemical properties of the nonprinting areas for conventional and CtP process P. Y. Zitinski Elias 1,*, T. Tomasegovic 1, T. Cigula 1 1 Faculty of Graphic Arts, University of Zagreb, Getaldiceva 2, Zagreb, Croatia * Corresponding author: paulay6@yahoo.es, Phone: , Fax: Abstract The difference between printing and nonprinting areas in lithography is achieved through opposite physical-chemical properties. The printing areas attract hydrophobic substances printing inks. The nonprinting areas, made of aluminium-oxide, attract hydrophilic substances the fountain solution. In the printing process, the nonprinting areas are first covered with fountain solution, which makes them unable to adsorb printing ink. The amount and stability of the fountain solution adsorption on those areas has direct influence on the quality level of the reproduction. The aim of this paper was to determine how developing time in conventional and CtP plate making processes influences the physical-chemical properties of the nonprinting areas. For the research samples of conventional and CtP printing plate were made. All samples were exposed by light in optimal time and then immersed in developing solution in various times. The contact angle between fountain solution and nonprinting areas was measured. Results have shown differences in physical-chemical properties between those two types of plates. Keywords Plate making process, conventional process, CtP process, developing time, contact angle 1. Introduction One of the most commonly used printing techniques today is lithography. In this printing technique, the difference between printing and nonprinting areas is achieved by them having opposite physical-chemical properties. [1] The printing areas are made of photoactive layer that attracts oil and chemical substances with oil solvent printing inks. The nonprinting areas are

2 made of aluminium-oxide, which attracts water based substances the fountain solution. In the printing process, the nonprinting areas are first covered with fountain solution, which disables them to adsorb printing ink. The printing plates are made from aluminium foils. The aluminium surface must be processed mechanically and chemically to create rough aluminium surface covered with thin and porous layer of aluminium-oxide, which as said before, has the ability to adsorb water based solutions. [2] The processed aluminium foil is at the end covered with photoactive layer that enables photomechanical copying process of motives on the printing plate and because of its chemical properties, in the printing process attracts oily based printing ink. The plate making process consists mainly from two processes, exposure of the photoactive layer with determined light emission and the developing process, where a part of the layer is removed from the printing plate. [3] Both conventional and CtP technology are often used in lithography. While CtP, as a relative new way of plates making process, turns out to be a major breakthrough, due to the expenses of the shifting to this new technology and the expenses of maintenance, there are still printing houses, especially smaller ones, that rather continue to use the conventional plate making process then invest in new technology and change their well established workflow. 2. Technology and methods 2.1. Conventional and CtP plate making process The conventional plate making process sustains mainly of two processes, exposure and developing. In difference to the CtP process, exposure is made with help of a thin plastic foil. The places on the foil which are printed on prevent the light from passing through in the exposure process. Vice versa, the spots where the foil isn t printed on allow the light to pass, which causes the diazo layer to dissolve in the developer. [4] Since this process requires the preparation and use of the foil, the number of possible errors encrease [5], making the CtP process the number one choise for many printing houses. CtP stands for Computer to Plate, indicating a technology that relies almost entirely on a computer. Once the preflight check has been made by authorized personnel, all is done automatically. The computer first exposes the printing plate in the platesetter unit. This is usually done by means of a laser beam, the most expensive and delicate part of the CtP system. As a result, this type of technology is more expensive and requires more maintenance.

3 After the exposition process, the plate is often automatically carried onto the developing unit, which takes the plate through the developing process, afterwards laying the plate down to dry. The result is a high quality printing plate, and since the whole process is automatic, the process of making four printing plates can take no longer than half an hour [1], giving this technology a great advantage over the conventional process Measuring units To determine physical-chemical properties of the nonprinting areas on samples a measurement of the contact angle between prepared samples and commercial fountain solution. [6] Measurement was performed by Dataphysics OCA 30 goniometer. This type of modern measuring units enables great precision of the measurement. In figure 1 one can see the measuring unit and in figure 2 final step of the measurement. Figure 2 shows the end point of measurement, calculation of contact angle in defined time after first fluid-solid contact. The calculation of contact angle is made by defining the paths of solid and fluid and then software automatically calculates contact angle. Figure 1: Goniometer Dataphysics OCA 30 Figure 2: Measurement of the contact angle 2.3. Sample preparation Plate samples used for this research were conventional offset printing plate coated with diazo positive photoactive layer. Exposure of all samples was made Plural EXPO74 exposure unit, equipped with metal-halide lamp. First step was to define optimal exposure and developing time, which was made with Agfa-Gevaert control wedge. After exposure, all samples were developed in alkaline solution, sodium base of molar concentration 0.2 moldm -3. CtP plate samples were type of positive thermal plate, suitable for exposure with laser beam, which radiates wavelength of 830 nm. The developing process was conducted manually in strictly defined conditions. All samples

4 were developed in equal developer temperature of 22 ºC. Two sets of samples were developer in a developing solution with molar concentration of NaOH of 0.2 moldm -3 and one set of CtP samples was developed in commercial developing solution. Seven different times were used for developing process; optimal and six steps, +/- 1/15, 1/5, 1/3 from optimal developing time. 3. Results and discussion It is clearly visible in figure 3 that developing has significant influence on physical-chemical properties of nonprinting areas of the diazo conventional plate. One can see that with the increase of the developing time, the contact angle on the nonprinting areas decreases its value and reaches low point, but by further increase of the developing time the contact angle increases its value again. Figure 3: Influence of developing time on the contact angle on nonprinting surfaces of the conventional diazo plate Figure 4: Influence of developing time on the contact angle on nonprinting surfaces of the CtP plate with the CtP and with the NaOH developer

5 On the other hand, the measurements made with CtP plates shown in figure 4 indicate that both the CtP and NaOH developer react with the plate, dissolving the exposed areas. Both developers show similar behaviour at lower developing time, dissolving of the entire exposed layer is not completed, but with the increase of the developing time, exposed photoactive layer is completely removed. Both of the developing solutions cause similar behaviour of the wetting properties of the nonprinting areas in higher developing times. After reaching low point of contact angle (highest wetting properties), they cause increase of the contact angle. This fact means that, as seen before at conventional plates, developing solutions cause degradation of rough aluminium-oxide layer. One can see that commercial developing solution cause smaller degradation than NaOH solution. Figure 5: Comparison of wetting properties between all prepared samples Figure 5 shows the behaviour of wetting properties on all samples in dependence on developing time. It can be seen that developing time is very important factor on the conventional printing plate where increase of developing time cause very significant increase of contact angle. The CtP plates are not so sensitive on increase of developing time, the properties of nonprinting areas are changed but the change is not high. 4. Conclusions Performed investigations were based on the fact that the oscillations in chemical developing in the printing plate making process, either conventional or CtP could cause differences in the physical-chemical properties of the nonprinting areas.

6 The results obtained by this research proved the presumption that developing time has great influence on wetting properties of nonprinting areas. Comparing the CtP and conventional plates one can conclude that both can be developed in a solution of NaOH, but when using this kind of solution one must determine precisely the optimal developing time as to short or to long developing time cause significant degradation of wetting properties. In addition, on both plates wetting properties have similar behaviour; they increase its value until reaching maximum and then decrease again. Observing the results of contact angle measurement on CtP thermal plates one can see that both developer cause similar behaviour of the nonprinting areas. By all samples, results indicate that when dissolution of photoactive layer is finished, alkaline solution (developer) causes dissolution of aluminium-oxide peeks, decreasing roughness and reducing surface tension. Nevertheless, observing the value of contact angle and determining where its minimum is could be method for defining optimal developing time. 5. Literature [1] Mahovic Poljacek, S.; Agic, D. & Gojo, M.: Influence of the Chemical Processing on the CtP Printing Forms, Annals of DAAAM for & Proceedings of 17th International DAAAM Symposium, November 2006, Vienna [2] Shriver, D. F. & Atkins, P. W.: Inorganic Chemistry, 3rd Edition, W. H. Freeman and Company, 1999, New York [3] Urano T., Kohori K. & Okamoto H., Photosensitive Lithographic Printing Plate and Method for making a Printing Plate, Patent No.: US 6,689,537 B2, [4] Mahović Poljaček, Sanja; Cigula, Tomislav; Gojo, Miroslav: Formation and Defining the Different Aluminium Oxide Microstructures in Alkaline Solutions // International Journal of Material Forming, Springer Paris, 2008, Lyon [5] P. W. Atkins: Physical Chemistry, 6 th Ed., Oxford University Press, (1998) [6] MacPhee J.: Fundamentals of Lithographic Printing, Volume I, Mechanics of Printing, GATFPress, Pittsburg, 1998