Defining of time-dependent contact angle of liquids on the printing plate surfaces.

Cigula, Tomislav ; Poljacek, Sanja Mahovic ; Gojo, Miroslav 等

1. INTRODUCTION

Among various surface analytical methods available, determination of the contact angle and wettability properties remain as a standard methods for characterisation of the different surfaces (Lander et al., 1993). By determination of contact angle between the defined liquids and certain surface it is possible to get the wetting properties of the solid surface, information about the homogeneity and roughness characteristics of the surface, information about the interaction between the liquid and the solid. These kind of measurement fall into the tensiometry area, where the contact angle is defined through the interfacial energy of the solid-liquid, liquid-vapor and solid-vapor interfaces (Hamraoui, et al., 2000). In most situations equilibrium state is hardly to reach, which leads to the fact that precise description of the wetting characteristics becomes quite complex. On the other hand, the information of the contact angle depends on the speed and direction of movement of the liquid droplet on the surface. These facts point out that characterization of wetting properties of surfaces is highly complex and that absolute results are hard to achieve.

Previously, the contact angle was measured and results of these measuring were published under the assumption that the droplet of the liquid rests on a solid surface (static contact angle). Information about contact angle was useful for the liquid droplets whose shape stabilises immediately after attaching to a solid surface.

The aim of this paper was to determine which changes occur in the liquid droplets on printing plate surfaces when the contact angle is changed over time (dynamic contact angle) (de Ruijter, 1998). The speed of spreading depends on a combination of several factors, and can be understood from variations in the contact angle over time (Fig. 1).

[FIGURE 1 OMITTED]

Results obtained in this paper will be usefull for two reasons. The first one will be usefull in offset reproduction where the functionality of the printing plate depends on the fountain solution and printing ink adsorption. The second goal is directed to easier determination of contact angle and possible standardization of the measurements based on goniometry principles.

2. INFORMATION

Aluminium surface suitable for use as an offset printing plate consists of two different areas: ink-receptive image areas which carry a photosensitive coating and fountain solution-retaining non-image areas. In order to improve the fountain solution adhesion on the aluminium oxide film and to enhance the adhesion of the photosensitive coating during the printing process the foil is roughened by electrochemical graining and anodic oxidation (Dimogerontakis et al., 2006; Limbach et al., 2003). During the printing process, printing plates are first covered with fountain solution which has to be adsobed on nonprinting areas (aluminium-oxide), and afterwards is covered with printing ink which then adsorbes on the printing areas (photosensitive coating).

3. EXPERIMENTAL

Videobased, optical contact angle measurement was performed by DataPhysics OCA30 device. It ensures the static and the dynamic characterization of liquid/solid interfaces by contact angle measurement procedure, the requirement for the calculation of surface free energy. In this paper contact angle was measured by using the sessile drop method and surface free energy were calculated by using Owens-Wendt-Rabel and Kaelble (OWRK) analysis method (1) (Data Physics, 2006).

[[gamma].sub.l] = [[gamma].sup.d]l + [[gamma].sup.P]l (1)

[[gamma].sub.s] = [[gamma].sup.d]s + [[gamma].sup.p]s (2)

where [[gamma].sub.1] and [[gamma].sub.s] are the surface free energy of liquid and solid respectively, [gamma] [sup.d] is the dispersive and [gamma] [sup.p] the polar components of the surface free energy (surface tension). Wetting properties of non-printing and printing areas of printing plates were calculated by measuring the contact angle of three liquids of known surface free energy and viscosity (Tab. 1) (van Oss et al., 1993). Contact angles of liquids were defined from average values of seven liquid droplets placed on different areas of the same printing plate sample. Contact angles of liquids were calculated after 0.2s, 0.4s, 0.6s, 1.0s and 2.0s of droplet relaxation.

Printing plate samples were prepared to the standardized processing procedure (ISO 12218:1997). The samples were exposed for 75s and chemically processed in NaOH solution at the temperature of 24[degrees]C (pH=12.68; [chi]=8.35 [mScm.sup.-1]).

4. RESULTS AND DISCUSSION

In Fig. 2 results of the relative contact angle measured on the printing areas are presented. One can see that there is a small difference in contact angle values of glycerol during the time period. The values of contact angles measured with water and diiodomethane on mainly dispersive solid surface do not change in time. This could be the consequence of their surface free energies while these two liquids are mainly dispersive (diiodomethane) or polar (water).

Higher difference can be seen on mainly polar surface (nonprinting areas). Results are shown in Fig. 3. Significant changes of contact angle values depending on measured time can be seen. On mainly dispersive solid surface (printing areas) only glycerol has shown lowering of contact angle values while on polar surfaces all samples have shown decreasing of values during the time. The highest decrease is occurred by diiodomethane, it has the smallest surface free energy (Tab. 1). On the other hand, the smallest decrease is measured by glycerol which is probably the consequence of its higher viscosity (Tab. 1). Results of the surface free energy calculation can be seen in Fig. 4. It can be seen that these results are in correlation with results shown in Figs. 2 and 3. The value of surface free energy of printing areas is not significantly changed during the time, as only one factor in its calculation has been changed. On the other hand, on non-printing areas (polar) the value of surface free energy has changed notably, increasing its value for nearly 30%.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

5. CONCLUSION

In this paper, the wetting characteristics of printing and non-printing areas of the offset printing plates were defined by measuring the spreading contact angle of standardized liquids. Results have shown that contact angle values and values of surface free energie of non-printing areas (polar characteristics) have been significantly changed during the time period of measurement (slightly less than 30%). On the other hand surface free energy of printing areas has not significantly changed during the time.

One can conclude that dynamic contact angle can give more complex information about the printing plate surface than static one. On the other hand, the limitation of this research can be expressed through usage of only one method for defining of surface wetting properties. The usage of, for instance, profilometric methods for surface characterization in correlation with dynamic contact angle could bring more accurate results. In future, further research has to be directed to the studies related to polar (non-printing) surfaces on the printing plates, which are obviously, because of their porous and rough characteristics, highly sensitive in liquid-solid interfaces.

It is highly important in graphic reproduction process where functional properties of printing plates, and consequently quality level of final graphic product, depend on the wetting properties of surface structures.

6. REFERENCES

Dimogerontakis, Th.; Van Gils, S.; Ottevaere, H.; Thienpont H. & Terryn, H. Quantitative topography characterisation of surfaces with asymetric roughness induced by AC-graining on aluminium, Surf. Coat. Technol. 201, 918-926 (2006)

Hamraoui, A., Thuresson, K.; Nylander, T. & Yaminsky, V. Can a Dynamic Contact Angle Be Understood in Terms of a Friction Coefficient? Journal of Colloid and Interface Science 226, 199-204 (2000)

ISO 12218:1997. Graphic technology--Process control--Offset platemaking

Lander, L. M.; Siewierski, L. M.; Brittain, W. J. & Vogler, E.A. A Systematic Comparison of Contact Angle Methods. Langmuir, 9, 2237-2239 (1993)

van Oss, C. J.; Giese, R. F.; Li, Z.; Murphy, K.; Norris, J.; Chaudhury, M. K. & Good, R. J. (1993) Contact Angle, Wettability and Adhesion, K. L. Mittal (Ed.), VSP, Utrecht, The Netherlands

de Ruijter, M.; Kolsch, P.; Vouea, M.; de Coninck, J. & Rabe J.P. Effect of temperature on the dynamic contact angle Colloids and Surfaces A: Physicochemical and Engineering Aspects 144 (1998) 235-243

***Data Physics Instr. GmbH, Operating manual OCA, 2006

Tab. 1. Surface free energy ([[gamma].sub.lv]) and their dispersive
([gamma].sup.d.sub.lv]) and polar ([[gamma].sup.p.sub.lv]) components
and viscosity of liquids

 Surface free energy [gamma]
 ([mNm.sup.-1])

Liquid [[gamma].sub.lv] [gamma].sup.d.sub.lv]

Diiodomethane (Strom) 50.8 50.8
Glycerol (van Oss) 64.0 34.0
Water (Strom) 72.8 21.8

 Surface free energy
 [gamma]
 ([mNm.sup.-1])
 Viscosity
Liquid [gamma].sup.p.sub.lv] (mPas)

Diiodomethane (Strom) 0.0 2.78
Glycerol (van Oss) 30.0 1412
Water (Strom) 51.0 1.002