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Coatings for metal packaging must maintain adhesion and not crack while the metal is bent and
stretched during can formation. Food can coatings must withstand retort processing which occurs after filling (1) and most beer can coatings must withstand pasteurization. UV-curable cationic coatings containing cycloaliphatic epoxide resins are used in rigid packaging applications because their toughness and adhesion helps them withstand can end formation, retort and transit abrasion, and because UV is an efficient, low VOC coating and decorating process (2).
Previous studies have shown that flexibility and hardness of cationic coatings were directly and
inversely proportional, respectively, to cycloaliphatic epoxide and polyol molecular weights. Data analysis predicted increasing flexibility and hardness of the same coating were generally opposing goals (3,4).
In an attempt to increase flexibility and hardness simultaneously, epoxy novolac resin concentration and functionality and coating thickness were included as variables in this study. Epoxy novolac resins were expected to increase hardness but decrease, flexibility and it was anticipated changing coating thickness might help balance these seemingly opposing but desirable pmperties.
Designed sets of experiments and statistical analysis were used to determine the importance of the effects of the variables and their interactions on properties. The effects of the interactions were demonstrated using contour plots.