Direct observation of freeze-thaw instability of latex coatings via high pressure freezing and cryogenic SEM

• Published in: JCT research Vol. 3; no. 2; pp. 109 - 115
• Main Authors: Zhao, Cheng-Le, Porzio, Shane, Smith, Alan, Ge, Haiyan, Davis, H. T., Scriven, L. E.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Heidelberg Springer 01.04.2006; American Coatings Association, Inc
• Subjects: Applied sciences; Coatings. Paints, varnishes and inks; Exact sciences and technology; Film formation and curing, properties, testing; Forms of application and semi-finished materials; Latex and emulsion; Latex paint; Polymer industry, paints, wood; Technology of polymers; Thermal properties; United States; physical properties; Graft copolymer; Water base paint; colloids; Property formulation relationship; cryogenic SEM; Cyclic ether copolymer; emulsions; waterborne; Butyl methacrylate copolymer; Coating material; acrylics; VOC; Ethylene oxide copolymer; Experimental study; Methyl methacrylate copolymer; flocculation; Latex; Freeze thaw cycle; freeze-thaw stability; Storage stability; Morphology; latexes; SEM; Methacrylate copolymer; architectural
• ISSN: 1547-0091; 1935-3804; 2168-8028
• DOI: 10.1007/s11998-006-0013-6

Despite its industrial importance, the subject offreeze-thaw (F/T) stability of latex coatings has not been studied extensively. There is also a lack of fundamental understanding about the process and the mechanisms through which a coating becomes destabilized. High pressure (2100 bar) freezing fixes the state of water-suspended particles of polymer binder and inorganic pigments without the growth of ice crystals during freezing that produce artifacts in direct imaging scanning electron microscopy (SEM) of fracture surfaces of frozen coatings. We show that by incorporating copolymerizable functional monomers, it is possible to achieve F/T stability in polymer latexes and in low-VOC paints, as judged by the microstructures revealed by the cryogenic SEM technique. Particle coalescence as well as pigment segregation in F/T unstable systems are visualized. In order to achieve F/T stability in paints, latex particles must not flocculate and should provide protection to inorganic pigment and extender particles. Because of the unique capabilities of the cryogenic SEM, we are able to separate the effects of freezing and thawing, and study the influence of the rate of freezing and thawing on F/T stability. Destabilization can be caused by either freezing or thawing. A slow freezing process is more detrimental to F/T stability than a fast freezing process; the latter actually preserves suspension stability during freezing.