NEW INSIGHTS INTO THE FREEZE-THAW STABILITY OF POLYMER LATEXES

摘要

Freeze-Thaw (F/T) stability of polymer latexes and waterborne coating formulations is of considerable industrial importance. Because of environmental concerns and legislative measures, there has been in recent years a new driving force for the coatings industry to develop low- or zero-VOC (Volatile Organic Compounds) waterborne formulations. In a lower-VOC formulation ( < 50 g/L), the amount of antifreezes such as glycols that can be added has to be substantially reduced and is most probably no longer sufficient to impart F/T stability. It is a challenging goal that next-generation polymer latexes be improved in such a way that they are not only F/T-stable themselves but are also capable of providing F/T stability to the coatings formulations which are based on them. Partly because of the lack of adequate microscopic techniques, fundamental aspects of F/T stability have not been studied extensively and are not well-understood. The objective of this work is to try to clarify and better understand the effects of key factors affecting the F/T stability of polymer latexes. In addition to following viscosity changes during F/T cycles, we apply advanced cryogenic SEM technique coupled with a high-pressure freezing apparatus, which allows us to visualize micro-structural changes and to study the effects of the freezing and thawing processes separately. Relationships between F/T stability and polymer and colloidal characteristics are analyzed. Cryogenic electron micrographs also provide information for understanding the cause of viscosity rise during the F/T processes in a F/T-unstable latex.