courtesy of DataPhysics Instruments

 Offset printing is a widely used technique in the production of printed products such as books, newspapers, and packaging. The process relies on the transfer of an image from an inked surface to the substrate. When used in combination with a lith­ographic process, it comprises the use of a dampening solution and an oil-based printing ink. The inked surface (plate) is di­vided into hydrophilic regions, which accept the dampening solution and repel the ink, and hydrophobic regions which repel the dampening solution and accept the ink.

 If the process is functioning optimally, it can be confirmed that the non-printing regions of the plate are devoid of ink. It is essential to ascertain the precise interfacial tension between the dampening solu­tion and the printing ink at the interface to avert the possibility of their mixing. The OCA system, developed by DataPhysics Instruments, is a highly robust and reliable apparatus for measuring the optical contact angle and performing contour analysis. The system employs the pendant drop method to accurately determine interfacial tension between liquid/liquid interfaces. The following note will illustrate the pendent drop technique with ink and three different dampening solutions commonly utilised in offset printing.

Experiment

In this study, the total surface ten­sion was determined for three differ­ent dampening solutions and one ink, along with the polar and dispersive parts, using an OCA with the pen­dent drop method. Images or movie sequences of the pendant drops were recorded using the integrated USB 3.0 camera, thereby facilitating subse­quent analysis of the drop contour. To ensure the accuracy and reproducibil­ity of the results, three measurements were taken for each sample.

In order to utilise the pendant drop method with the greatest accuracy, it is essential that the drop shape exhib­its a clear deviation from a spheri­cal shape. Typically, a “pear-shaped” configuration is attained just before the drop detaches with a secondary reflection in proximity to the needle. Furthermore, the diameter of the nee­dle is also a critical factor that affects the outcome. It is recommended by DataPhysics Instruments that a nee­dle diameter of 1 mm or above should be used for the analysis of pendant droplets.

Technique and Method

The surface tension of a liquid and the interfacial tension between two liq­uids can be easily determined with an optical contact angle measuring and contour analysis system of the OCA series from DataPhysics Instruments (Fig. 1) by analysing the shape of a pendant drop hanging or floating on a needle.[1] Depending on the density of the phases, the drop can be dosed downwards with a needle or upwards with a bent needle (Fig. 2b).

The polar part and the dispersive part of the surface tension of a liquid can be determined by measuring its total surface tension against air, and the interfacial tension against a non-polar liquid.

Conversely, the interfacial tension between two opaque liquids can be calculated using the polar and dis­persive parts of the surface tension of each liquid, as obtained. This is a crucial method in numerous technical applications, including the production and processing of paints and inks.

A preliminary test was conducted to ensure the purity of the liquids used as the surrounding medium, as well as the cleanliness of the syringe. It should be noted that the test liquids and surrounding phase employed in interfacial tension measurement must be immiscible. In this study, non-polar solvents, specifically n-dodecane and n-perfluorohexane, were employed. The surface tensions of these two liq­uids were therefore measured three times using the pendant drop method (Table 1).

Similarly, the total surface tensions of the dampening solutions and ink were also determined using the pendant drop method.

In order to determine the disper­sive and polar parts of the samples, the interfacial tension between the dampening solutions and n-dodecane was measured. Since oil-based inks are alkane soluble, n-perfluorohex­ane was used as the surrounding phase. However, as the density of the oil-based ink is lower than that of n-perfluorohexane, the ink drop was dispensed ‘upwards’ (Fig. 2a). A spe­cial dosing needle was used for cap­tive drop and bubble measurements (Fig. 2b).

With the polar and dispersive parts of the dampening solution and the ink surface tension, the interfacial ten­sion between different dampening solutions and inks can be calculated automatically by the software.

Results & Discussion

The total surface tension, along with its dispersive and polar parts, was determined for the dampening solu­tions and ink. Fig. 3 illustrates that the highest surface tension is exhibited by Dampening solution 1, with a value of 52.07 mN/m. In contrast, Dampening solutions 2 and 3 exhibit lower surface tensions, with values of 46.28 mN/m and 39.10 mN/m, respectively.

Furthermore, the ratio of polar to dispersive parts in Dampening solu­tions 1 and 3 is comparable, with a ratio of approximately 50:50, while Dampening solution 2 exhibits a ratio of approximately 40:60.

This indicates that it contains a greater proportion of the dispersive part than the other two solutions. In comparison with the dampening solutions, the ink exhibits a lower surface tension, accompanied by a higher proportion of the disper­sive part, at approximately 90:10.

Based on the total surface tension and the polar and dispersive parts, the interfacial tension of the ink in relation to the dampening solutions was evaluated.

Fig. 4 illustrates that the interfacial tension between the ink and Dampening solution 2 is the lowest, with a value of 6.51 mN/m. These findings are consistent with the previous results, which indicated that Dampening solution 2 has the high­est proportion of the dispersive part amongst all the dampening solutions and is therefore closer to the propor­tion of the dispersive part present in the ink. In contrast, the highest value of interfacial tension is observed between Dampening solution 1 and the ink, at 10.23 mN/m.

It is widely accepted that the interfacial tension between the dampening solution and the ink should be maximised to ensure the formation of a distinct interface. Consequently, this result suggests that the Dampening solution 1 has the highest potential as a candidate for further development.

This study demonstrates that meas­uring the total surface tension of liq­uids alone is insufficient for obtaining comprehensive results. Consequently, it may lead to the inappropriate selec­tion of liquid combinations for further application. The quantification of the polar and dispersive parts of the sur­face tension is of significant impor­tance for formulation designers, as it enables the prediction of the degree of interaction between two liquids when brought into contact.

Summary

The optical contact angle measuring and contour analysis system of the OCA series from DataPhysics Instru­ments are the reliable, straightfor­ward solution for determining surface tensions with their polar and disper­sive parts of dampening solutions and ink. Based on these measurements, the interfacial tension between liq­uids can be readily calculated. This study clearly shows that measuring the total surface tension of liquids alone is insufficient for obtaining comprehensive results. It is crucial for those engaged in the formulation process of dampening solutions and inks to gain a deep understanding of the polar and dispersive components of surface tension. This enables the appropriate selection of liquid combi­nations for further applications.

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