Preparation and investigation of unsupported multilayer coating composites concerning their in-plane stress-strain and bending characteristics

In many converting processes, the material behaviour of coated paper and especially of its coating layers is responsible for paper quality performance. Quality imperfections of printed products addressed in this paper are coating cracking and fold cracking which can lead to expensive claims. This study presents the results of a new methodology for preparation and testing of unsupported multilayer coating films concerning in-plane stress and bending behaviour. By applying the new methodology, multilayer coating composites were prepared and tested concerning material properties like e.g. E-modulus, in-plane stress-strain characteristics and bending characteristics. The influence of the synthetic binder (latex)-starch ratio and pigment type on the strength properties of unsupported single coating layers and multilayer coating composites was investigated. The in-plane stress-strain and bending stress measurements showed the embrittling or "hardening" effect of starch application on stress-strain curves (yielding behaviour) and bending behaviour of single and multilayer composites. Variation in pigment type at the equal latex-starch ratio had a comparably lower influence on stress-strain behaviour than the starch application; finer pigments with a steep particle size distribution led to a more ductile "softer" material behaviour at a lower level of maximum strength. The combination of fine pigments, low starch and high synthetic binder (latex) amount resulted in "soft" in-plane stress-strain behaviour at sufficient strength level and good bending stress development. The behaviour of multilayer coating composites is determined by the behaviour of the more rigid single coating layer leading to a coating composite with high E-modulus and low yielding behaviour. Low E-modulus and high yielding behaviour of multilayer coating composites lead to a lower bending stresses, which should lead to a less critical fold cracking behaviour (CaF).