Contact Angle

The R2P printing process was analyzed to deter- mine the optimum relationship between contact angle and substrate at each stage of the printing process。 Equation 1 was developed, in which selec- tion of printing substrates and inks is standardized according to contact angle。 To develop the optimum relationship in this experiment, the UV LED poly- mer ink was transferred from Stage 1 (the off pro- cess) to Stage 3 (the setting process) consecutively by use of the roller。 Throughout this process, the roller removed the ink in Stage 1 and made direct contact in Stage 2 to transfer the UV LED-curable ink on to the cliche´。 The embossed parts of the cliche´ removed the ink from the roller but left the remaining ink with its patterns on the substrate of the roller。 After this step, in Stage 3 the remaining UV LED polymer ink was transferred to the final substrate。 From this information, the contact angles for the cliche´ and the final substrate should be smaller than that for the roller for optimum patterning quality while printing。

h ðStage 1Þ > h ðRollÞ > h ðStage 2Þ; h ðStage 3Þ  (1)

Figure 5a shows results for contact angles between the pristine, 40。7 cP, and diluted solution, 25 cP, of the UV LED-curable ink and its different substrates。 From this figure, M4 had the largest contact angle with the UV LED-curable ink (h = 52。1°) whereas PET had the lowest contact angle (h = 20。5°)。 According to Eq。 1, M4 substrate was used in the coating process and is set on Stage 1 for the off pro-

cess。 M8, M1, and M3 substrates are chosen for use on the roller which is the mechanism of transfer of the reverse-offset roll-to-plate printing process。 The glass cliche´ is placed on Stage 2 where the patterning process occurs。 M2, kapton PV 9101, kapton PV 9102, and PET substrates are all used on Stage 3 in the setting process, because of their their small contact angles。 Similarly to the process used for choosing the optimal substrates for the pristine solution, the di- luted solution, Fig。 5b shows the substrates with the highest potential for being used as the roller, Stage 1 and Stage 3, while Stage 2 remains constant。 From this figure, the optimum substrates to be used for the roller are kapton PV 9102 and M8, according to Eq。 1。 The substrates, kapton PV 9101 is most suitable for Stage 1 and PET is most suitable for Stage 3。 Unlike Fig。 5a, the three substrates with the largest contact angles in Fig。 5b were not satisfactory for Stage 1 due to the solutions viscosity。

The plot in Fig。 6 shows the different contact angles for the three selected substrates used for the roller and the six other substrates used for Stage 1, Stage 2, and Stage 3, for the pristine UV LED-cur- able ink。 Analysis of the information shows that the larger the contact angle difference between the substrate used for the roller and the substrates used for Stage 3, the more accurate and precise the printing pattern will be in reverse-offset roll-to- plate printing。

Fig。 4。  Plot of viscosity as a function of shear rate for a dilatant non-Newtonian  fluid。

Fig。 5。 (a) Images and results for contact angles between pristine UV LED-curable ink and nine different substrates; (b) images and data for contact angle between diluted UV LED-curable ink and eight different substrates。

Fig。 6。 Graphical data for contact angle differences between the roller and Stage 1, Stage 2, and Stage 3 for the pristine  solution。

The plot in Fig。 7 shows the different contact angles for the two selected substrates used for the roller and the substrates used for Stage 1 (kapton PV 9101), Stage 2 (glass cliche´), and Stage 3  (PET)

for the dilute solution of the UV LED-curable ink。 Equation 1 indicates that the larger the contact angle difference for the roller and Stage 3, the more accurate and precise will be the pattern。 At the beginning of the experiment, the contact angles were measured for eight possible substrates; how- ever, as the ink was being spin coated at a speed