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FLEXO Magazine : May 2011
Technologies & Techniques An alternative to coating on press is to purchase a film already incor- porating oxygen barrier properties, manufactured using alternative coating methods. Exxon Mobil 2.1 mil oriented polyproylene (heat-sealable) is an example of such a film. Using Exxon Mobil’s characteristics chart as a target, the measurements of oxygen transmis- sion rate were compared to the oxygen barrier properties of the Exxon Mobil 2.1 polypropylene, indicating which plate method provides the most efficient way of applying the oxygen barrier coatings through flexographic printing. As the film was run through the press, I recorded the weight of a 2-inch-square piece for each treatment to check the barrier thickness being applied to the film. Sun Chemical’s product spec sheet suggests that every 2 square inches of material be applied with 7.74 mg to 15.48 mg of wet coating weight (3 g/M2) and a dry film weight of 0.46 mg to 0.93 mg (0.18-0.36 g/M2) in order to maximize the protective property of the oxygen barrier coating and not waste excessive material. The weight of the film with the corresponding coating weight can be found in Table 1. The coating weights were measured imme- diately after printing and therefore were on the high side of the dry film weights. It was presumed that additional weight would be lost due to additional evapora- tion of some solvent. OXYGEN TRANSMISSION RATE The film was first separated into four rolls: film with no treatment, film with control treatment (conventional plate), film with Treatment 1 (plate cell pat- terned plate) and film with treatment 2 (capped plate). Each film was then cut into two, 2 -inch squares (4 inches2) to be placed in the Oxygen Transmission Rate (OTR) Analyzer Model 8001 (Illinois Instruments). The OTR was measured for each 2-inch square, per film, until it reached a final result and automatically stopped at 1% convergence with a by- pass time of 30 minutes, and a sampling rate of 30 minutes. The temperature was 23° C with RH at 0.0%. The results of the permeability may be found in Table 2. Table 3 compares the oxygen transmis- sion rate of the best-achieved results with the film already treated with an oxygen barrier coating. CONCLUSION The results of this experiment provided beneficial information. In comparing the data in Table 2, it becomes obvious that, in general, coating the film with the barrier substantially lowered the oxygen transmission rate with a decrease from 917 OTR for the control plate to 166.5 OTR for the conventional method of printing. Interestingly, the weight of the coating on the conventional plate was 0.7 mg, while the weight of the coating for the plate cell pattern and capped plate were both 1.0 mg. The lack of increased barrier property for Treatment 1 may be a result of how uniform the plate cell treatment coating was applied. Further investigation is needed to determine why no substantial benefit occurred from the patterned plate, even though its average www.flexography.org may 2011 FLeXO 145 TAKE CONTROL Control your roll quality, speed, workflow and bottom line. Rotoflex Genesis - the newest, most advanced control system in the industry - gives you more control. Exclusive to Rotoflex inspection/rewind machines, Genesis guarantees fast, accurate fault recognition and placement, includes 100% matrix and missing label detection, and simplifies set-up and calibration. Best of all it lets you take control of your profits. www.rotoflex.com
Sustainable Spring 2011