Synthesis and Characterization of Redispersible Latexes.

Abstract

Redispersible (RD) polymer powders is a special application of waterborne polymeric dispersions. Polymeric dispersions (latexes) are mostly known in liquid form, and their water content is around 35%-60% based on the total weight of the latex products. Converting emulsion polymers to powder can proceed to various benefits such as lower costs and most importantly, it enables dry applications. The dry final product need to be redispersed in water to form an emulsion having similar properties to the original latex. Consequently, the redispersed latex can be used in the different applications known for emulsion polymers. RD powders have been mainly developed as binders to be employed along with inorganic materials like cement, gypsum, hydrated lime, calcium carbonate, clay, etc. which have several applications such as tiling, flooring, insulation systems, sealing slurries and dry paints.The main drying process used for RD powders production from a latex is spray drying. This technique is based on water evaporation at elevated temperatures. Although spray drying has been used for decades as an industrial process, it is still a costly and energy-consuming process. It would be useful to develop a less energy-intensive technology that would not require such high investments and running costs. A possibility that is explored in this project is to develop redispersible latexes able to flocculate upon application of a specific stimulus, which enables water separation from the system in a more straightforward way.The aim of the project is to design and produce a copolymer latex system based on vinyl acetate and neodecanoic acid vinyl ester possessing redispersibility. The term "redispersible latex" in this study stands for a pH-responsive latex system, which can flocculate as a result of changing the pH anddeactivation of switchable stabilization system. This leads to the separation of the flocculated particles and water, and the remained aggregates of less water content can be dried under room conditions. In presence of water and appropriate pH in which the stabilizing moieties switch on, the particles can restore their stability and be redispersed in water by applying energy. It would be crucial to provide the soft polymer particles with a hard surface protection to avoid particles deformation and interpenetration during coagulation and drying processes.With this aim, pH-responsive moieties were successfully incorporated into the particle surfaces through two different strategies including: 1) in-situ through copolymerization with methacrylic acid as functional monomer, and 2) using alkali soluble resins containing methacrylic acid as switchable protective colloids. Both strategies led to production of film-forming latexes, which were efficiently coagulated by decreasing the pH, leading to production of a fine powder upon drying and grinding. The obtained powders were redispersible in water at basic pH, forming a new dispersion. In addition, the formulated ASR-based powder (with Kaolin), thanks to the hard ASR shell and reinforcing filler, represented a good temperature and blocking resistance making them a potential candidate for RD powders application.