Recent development in Waterborne Polyurethanes for coating applications

VERÓNICA L. MUCCI; MIRTA I. ARANGUREN; JAVIER I. AMALVY; MARIA E. V. HORMAIZTEGUI

Eco-Friendly Waterborne Polyurethanes: Synthesis, Properties, and Applications

CRC Press-Taylor & Francis Group

Lugar: Boca Ratón; Año: 2022; p. 253 - 262

The use of waterborne polymers for coating formulations has increased in the lastfew years due to restrictive regulations for the use of solvent-based systems [1]. Asan alternative to the latter, waterborne dispersions are more friendly due to theirlow or zero emission of volatile organic compounds (VOCs). However, becauseof their usually lower performance, new approaches and strategies are focused onimproving their performance [2]. From a coating point of view, the films shouldperform as barriers for substrate protection, and they should present good adhesionto the substrate, hardness (relevant for service life), and gloss (important for thefinal appearance in architectural and decorative coatings). In particular, waterbornepolyurethanes (WPUs) are one of the most popular waterborne coatings, with increasingparticipation in the market during the last years [3]. With a share in theprinting, ink, adhesive, and coating industries [4], WPU dispersions have advantagesover other waterborne polymers because of their versatile structuralproperties and the excellent elasticity of the resulting films. The wide number ofdiol components (including natural oils), different isocyanates, and chain extendersavailable allows the development of an enormous diversity of products with finalproperties tailored to the type of substrate. The careful selection of raw materials,relative proportions of chemicals involved in the formulation, as well as the processof synthesis, and adjustment of the variables involved, are responsible for thecharacteristics of the obtained suspensions and films [5], a subject treated in otherchapters of this book.Briefly, WPUs can be considered segmented polymers, constituted by soft segments(essentially the polyol) and hard segments (formed by the isocyanate, the chainextender, and frequently the internal emulsifier). The incompatibility of these segmentsleads to phase separation, with a large effect on properties [6]. Usually, WPUscontains ionic centers (internal emulsifier) that allow their dispersion in water in thepresence of a counterion that stabilizes the particles [7]. Anionic groups in waterbornepolyurethanes contribute to adhesion to the substrate, but other specific applicationsrequire the development of cationic and bio-based WPUs [8–10]. Additionally,nonionic internal emulsifiers have also been proposed, although they usually result indispersions less stable than those of ionic WPUs [9].Nanotechnology is an emerging field that may have a great impact on the performanceof coatings, inks, and adhesives, among other industrial products [11].The incorporation of nanoparticles into polymeric formulations is the most widelyused strategy to improve the performance of coatings. For instance, nano-silica andnano-alumina were used to obtain increased surface hardness and extended servicelife in coatings for industrial floors, while nano-silver was used in the formulationof hygienic paints. According to a recent market report (2021) about 90% of protectivecoatings are solvent-borne, 7% are water-based, and about 3% are powdersystems, with a high growth expected only by 2026 [12]. Despite the relativelylow share of WPUs in the market, Figure 15.1 shows the important increase inthe number of publications and patents for WPUs used for coatings that occurredduring the last decades [13]. The growth has been continuous, with a large increaseobserved during the last 15 years, evidencing the great industrial interest.