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FLEXO Magazine : June2010
Technologies & Techniques 32 FLeXO june 2010 www.flexography.org Figure 2. MeK resistance of 100 mj/cm2 uV samples before and after subsequent eB curing in air (0 = complete ink removal, 5 = no visible effect on ink layer) Figure 1. Tape adhesion of 100 mj/cm2 uV samples before and after subsequent eB curing in air. ethyl ketone (MEK) solvent was determined using four layers of cloth wrapped on the round end of a 24oz ball-peen hammer. The cloth was saturated with MEK and rubbed back and forth 10 times over the cured ink using only the force from the weight of the hammer head. The relative damage to the ink layer was recorded on a scale of 0 to 5 (0 = complete ink removal, 5 = no visible ink removal). Table 2. Flexographic inks for Dual uV-eB Curing Cyan Magenta Yellow Black Component (Weight %) Acid Modified Epoxy Acrylate 20 20 20 20 PONPGDAa 10 10 10 11 EOTMPTAb 8 8 8 7 Yellow 14 Flexo Dispersionc -- -- 50 -- Red 57.1 Flexo Dispersionc -- 50 -- -- Blue 15.4 Flexo Dispersionc 50 -- -- -- Black 7 Flexo Dispersionc -- -- -- 50 PL-LOW-Nd 12 12 12 12 a. Propoxylated neopentyl glycol diacrylate b. Ethoxylated trimethylol propane tricrylate c. Sun Chemical d. Photoinitatior system (Palermo Lundahl – Low Migration, Nestle Compliant) resulTs and discussion Inks were applied at two different weights and cured at three different applied UV energy levels. The optical density of the ink films along with cured ink properties are shown in Table 3. As expected ink adhesion and MEK resistance improved with increasing UV exposure. In many cases inks applied at the lower weights showed better adhesion com- pared to the heavier weights. Tape adhesion is dependant on the curing at ink/substrate interface; therefore, the decrease in tape adhesion was likely caused by reduced through-cure as a result of decreased light penetration through the more dense ink layer. The relatively low MEK rub resistance con- firmed that in most cases the inks were not completely cured by UV alone. Higher apparent MEK resistance was observed for the heavier ink layers due to the fact that more ink must removed to reach the underlying substrate. Table 3. uV Cured ink properties Tape Adhesion (%) MEK Resistance 75 mj/cm2 100 ft/min 100 mj/cm2 75 ft/min 150 mj/cm2 50 ft/min 75 mj/cm2 100 ft/min 100 mj/cm2 75 ft/min 150 mj/cm2 50 ft/min Cyan 2.65 OD 0 0 0 0 0 0 Cyan 2.73 OD 0 0 0 0 0 0 Magenta 2.61 OD 0 10 20 0 0 2 Magenta 2.69 OD 0 0 10 0 1 4 Yellow 1.39 OD 0 0 0 0 0 0 Yellow 1.44 OD 0 0 0 0 0 1 Black 1.95 OD 70 80 90 1 3 4 Black 2.07 OD 50 50 60 2 4 4 The partially UV cured ink samples from above were sub- jected to subsequent EB curing. EB irradiation (30kGy) was conducted both in air and with nitrogen inerting (<200ppm) of the reaction chamber. The results for curing in air are summarized in Table 4. The results clearly show that EB is effective for completing the curing of the inks. Improved ink adhesion was observed for all samples after EB exposure. This was expected since the EB can easily penetrate the ink layer and provide curing at the ink/substrate interface. MEK rub resistance was also improved for all samples after EB cur- ing. The improvement in tape adhesion and MEK resistance after EB appeared to be independent of the initial UV energy exposure. This indicates that EB effectively completes the cur- ing to about the same degree independent of the initial level of UV cure. EB induced changes in ink film properties for the 100 mj/cm2 UV samples are shown in Figures 1 and 2. A photograph of representative samples is shown in Figure 3. In this case the magenta ink shows increasing MEK resis- tance with increasing UV exposure; however, tape adhesion remains poor in all cases. The poor tape adhesion is likely due to limited UV energy reaching the ink/substrate interface with this high density (2.69) ink layer. After EB exposure, MEK resistance is greatly improved and appears to be about the same for all three samples. Adhesion is also greatly improved for all samples after EB exposure. This shows that the beam is effectively completing the curing at the ink/film interface. FLX_June2010_mech.indd 32 6/10/10 9:39 AM