Study On Binder Migration Mechanism During The Consolidation Of The Structure Of Coatings

Binder is an important component of coating color. It has an important effect on the stability of coating process, coating structure and print quality. Binder migration and redistribution with respect to pigments during coating consolidation play a key role in determining the structure and performance of coated products.Large research efforts on binder migration have been made in the last few decades and a lot of interesting results such as capillary rise effects, marangoni effect, boundary wall effect and other particle migration mechanisms generated. However, there is still no agreement in the industry on whether particulate latex binders migrate to the coating surface, and if they do, when and how does the latex migration take place.A new mechanism that assumes latex migration to the coating surface mainly takes place by Brownian motion before capillary formation, after coating application has been developed. The new mechanism is based on the classic Einstein particle diffusion theory and is consistent with Lepoutre’s experimental findings that latex migration mainly occurs before FCC. It has also been shown that the new mechanism could coherently and systematically explain various latex migration behaviors observed under different conditions.In this work, surface elemental compositions of model coatings on impervious substrate dried under various conditions have been characterized using XPS technique. Surface content of carbon was converted to latex content using a newly established standard curve. Coating gloss, brightness, roughness, porosity, ink receptivity, print gloss and printing density under different coating process conditions (such as delay time, coating solid, binder content, drying temperature and coating weight) are measured to investigate the effect of binder migration on the structure and quality of model coatings.The results showed that delaying heating after coating application greatly reduced surface carbon content, providing solid evidence that drying conditions did affect latex accumulation at the surface. The results also showed that heating was more effective at inducing latex migration when applied at the initial drying stage, and that heating applied after a critical delay time corresponding to reaching the FCC caused little or no latex enrichment at the surface, proving that latex migration mainly takes place when the coating is dried in liquid phase. It was also shown that surface carbon content increased with decreasing initial solid content of the coating suspensions, implying that surface latex enrichment was not induced during coating application. These results contradict both the conventional capillary transport migration mechanism and the boundary wall effect concept. However, a recently proposed Brownian motion induced latex migration mechanism appears to provide a satisfactory explanation to those latex migration phenomena.In addition, the effects of binder migration on the structure and performance of coated product also have been investigated. Ink absorption is one of the major factors that affect printability of coated papers and boards. Conventional wisdom considers that ink absorbency is mainly affected by porosity or void fraction of the coating layer, which is in turn affected by binder content in the coating formulation. On the other hand, it has been found in this work that ink absorption depends not only on porosity, but also on drying conditions of the coatings. When dried under room conditions, ink absorption increased with increasing coating porosity, as observed in the literature. However, when dried at higher temperature with hot-air, ink absorbency increased with increasing solid content of the coating colors, or with decreasing coating porosity. This phenomenon is attributed to more pronounced latex migration under faster drying, leading to the formation of a more closed coating surface.The results are explained perfectly using a new mechanism proposed by Zang and others that assume latex particles mainly migrate to the surface by Brownian motion.