Influence of plate making process and developing solutions on the nonprinting areas of offset printing plates.

Baracic, Marina ; Cigula, Tomislav ; Tomasegovic, Tamara 等

1. INTRODUCTION

The conventional plate making process sustains mainly of two processes, exposure and developing (MacPhee, 1998). To obtain best possible quality of made printing plate one must optimize all processes involved.

The commonly used method for determining the optimal time of exposure and developing is by using the Agfa-Gevaert control wedge with 20 grey steps. This method is an optical one, without using measuring unit, it is observer dependent which makes it hard to optimize. Furthermore this method takes into account only the remaining part of the photoactive layer on the surface of the printing plate not the physical-chemical properties of the nonprinting areas.

The aim of this research is to determine how the exposure and developing process influence on nonprinting areas of the printing plate.

2. SURFACE PHENOMENA

2.1 Adsorption

Adsorption is the process of binding atoms or molecules on a surface of a material, and it is a consequence of surface energy (Atkins, 1998). In a bulk material, all the bonding requirements of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates.

2.2 Wetting

Wetting is the ability of a liquid to maintain contact with a solid surface. If the molecules of the liquid have a stronger attraction to the molecules of the solid surface than to each other, wetting of the surface occurs. One way to quantify a liquid's surface wetting characteristics is to measure the contact angle of a drop of liquid placed on the surface of an object. The contact angle is the angle formed by the solid/liquid interface and the liquid interface measured from the side of the liquid.

The contact angle between fountain solution and printing plate should be as small as possible to obtain a better level of adsorption and a higher print quality.

3. MATERIALS AND METHODS USED

For this research, conventional printing plates with positive diazo layer were used. The samples were made altering the exposition and developing times, using the Agfa-Gevaert control wedge to define the optimal ones. Variations were [+ or -] 1/15, [+ or -] 1/5 and [+ or -] 1/3 of optimal time. After exposure the printing plate is immersed in alkaline solution to remove soluble part of the photoactive layer (Urano et al., 2004). The samples were immersed in KOH and NaOH solutions of molar concentration 0,2 mold[m.sup.-3].

All samples were developed individually in controlled conditions, the same developer volume and temperature; freshly prepared to avoid influence of solution saturation (Mahovic et al., 2008). KOH and NaOH developer were compared before developing process by their pH and electrical conductivity values. The pH of the KOH developer was 13.05, electrical conductivity 8.43 [mScm.sup.-1], and the NaOH developer had a pH of 12.68, and electrical conductivity of 8.35 [mScm.sup.-1]. The temperature of both developers in process was 22[degrees]C.

For determining changes of physical-chemical properties of nonprinting areas, contact angle measurement between prepared samples and a commercial fountain solution was performed (Gojo et al., 1998). The fountain solution was made of dematerialized water in which an additive to increase the electrical conductivity in volume concentration of 2%, and a puffer solution in volume concentration of 2.5% to enable fountain solution to maintain constant pH value and to decrease solution's surface tension, 2-propanol was used in volume concentration of 10%. The solution's pH value was 4.48, electrical conductivity 1.31 [mScm.sup.-1] and surface tension of 0.4275 [mNcm.sup.-1].

The contact angle measurement was performed on the Dataphysics OCA 30 goniometer, where on the prepared sample of the printing plate a drop of fountain solution with defined volume (2.5 [micro][m.sup.3]) was dropped. Dosing of liquid and the sample positioning is software controlled.

[FIGURE 1 OMITTED]

A high quality lens system and video system with high-speed CCD camera ensures high-quality recording process damping. The measurement was performed five times on each prepared sample by shifting the sample on a measuring table to avoid contact of fluid drops on sample. The measuring process was filmed, which enabled that measurement of contact angle is performed in the defined time from contact of fluid with solid surface to the measuring position. The contact angle was measured by the Sessile drop method, where the angle is measured between the base line (the line of touch of the surface) and the tangent on the drop curve in the point where solid, liquid and air phase meat.

4. RESULTS AND DISCUSSION

Results indicate that the increase of exposition and developing time changes the surface of the printing plate. In Figures 2 and 3 one can see the behavior of the contact angle value depending on exposure and developing time. When exposure is to short the lipophilic photoactive layer is not completely removed and the wetting of those areas with water based fountain solution is lowered. By increasing this parameter the value of contact angle is decreased to a minimum. Further increase of the exposition time causes increase of the value of the contact angle. Since the value of contact angle indicates the adsorbing point of the fountain solution on the nonprinting surfaces, the results suggest that the best quality of the nonprinting areas is not achieved on sample made by using defined optimal values (exposure 75 s). When developing with NaOH minimum value is achieved with 1/15 lower exposure, but when developing with KOH the difference is significant, 1/5 lower than optimal time. In Figure 2 one can also notice the difference between the effect of used developing solution (KOH and NaOH) on the plate surface. KOH acts more aggressively than NaOH, it causes larger impact on surface properties and decrease of wetting with fountain solution.

Similar behavior can be seen in Figure 3. The differences of the contact angle are significant. Contact angle decreases to a lowest value and then increases its value again. After reaching the minimal contact angle, the KOH developer starts to affect the nonprinting areas faster and in a more aggressive way than the NaOH developer, which was already calculated when varying the exposure time, with developing time set as a constant value.

In Figures 2 and 3 can be noticed that minimum value of contact angle is achieved when developing samples with NaOH developer in both cases, when varying exposure or developing time. While comparing Figures 2 and 3 one can see that the measured steps in exposure time cause larger difference in contact angle value than by varying the developing time in both developers.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

5. CONCLUSION

Observing results of contact angle measurement one can see significant changes in surface properties of nonprinting areas but the changes are larger when exposure time is varied. Increasing time of both processes causes decrease of contact angle to the minimum value, after that point, value increases.

When exposure time is to short the photochemical reaction is not finished and the photoactive layer is resistant to alkaline solution. Extending the energy input, the photochemical reaction is finished and entire layer is removed by developer. The question remains why the contact angle value rises with increase of the exposure after reaching its minimum value. There are two possibilities, it can become more dissoluble and then alkaline solution starts dissolving aluminum-oxide sooner or becomes less dissoluble so it is harder to remove and gives those areas reduced hydrophilic properties, but this needs to be further investigated with other measuring methods.

Further, the results indicate that when dissolution of photoactive layer is finished, alkaline solution (developer) causes dissolution of aluminum-oxide peeks, decreasing roughness causing decrease of surface free energy. It is visible that the developer KOH causes faster increasing of the contact angle after the point of its minimal value than the solution of NaOH developer does. Lower value of the contact angle is reached by using NaOH as a developer. One can conclude that NaOH is better to use to achieve higher printing plate quality.

Observing the value of contact angle and determining process parameters where its minimum value is reached could be method for defining optimal exposure and developing time. Therefore, the further research will be made to determine the behavior of the printing areas when contact angle measurement is used to determine optimal times.

6. REFERENCES

Atkins, P.W. (1998). Physical Chemistry, 6th Ed., Oxford University Press, ISBN 0-19-850101-3, Oxford

Gojo, M.; Lovrecek M. (1998). Characterization of Surfaces on the Offset Printing Plate, Proceedings of 1st International Symposium on Novelties in Graphic, Ljubljana

MacPhee J. (1998). Fundamentals of Lithographic Printing, Volume I, Mechanics of Printing, GATFPress, ISBN 0-88362-214-9, Pittsburg

Mahovic Poljacek, S.; Cigula, T.; Gojo, M. (2008). Formation and Defining the Different Aluminum Oxide Microstructures in Alkaline Solutions, International Journal of Material Forming, Vol 1, Suplement 1, (January 2008.), 463-466, ISSN 1960-6214

Urano T., Kohori K. & Okamoto H., Photosensitive Lithographic Printing Plate and Method for making a Printing Plate, Patent No.: US 6,689,537 B2, 2004