Binder depletion during the coating process and its influence on coating failure • a detailed analysis of the IGT dry pick test, 2012 Advanced Coating Fundamentals Symposium

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The process of applying a coating colour onto a porous paper substrate results in some loss of the aqueous phase into the substrate. Along with the water, some binder, latex or starch, may be lost, depleting the coating layer and reducing strength. This might be at least partly offset by enrichment of binder at the coating – base interface. In this study we have attempted to relate the effects of binder depletion on the type and location of failure observed in the dry pick test.

In earlier work we present evidence that there is a depletion of latex binder from the coating surface during the coating application stage, and a subsequent enrichment of the binder at the coating – basepaper interface. This effect is greatest with an absorbent basepaper and a coating pigment which has poor water retention characteristics, such as a narrow psd PCC.

The coating strength was measured using the IGT dry pick test, which causes failure to occur progressively as a result of the penetration of a viscous test oil into the coating layer, coupled with the increase in shear stress under acceleration. Since the oil is a single phase liquid, there is no separation of oil components during printing. This means that the results are not complicated by variations in tack force which would be the case using offset ink.