A Current Review of Water-Based Polyurethane Dispersions

Polyurethane-based materials are highly attractive compared to many thermoplastic and thermoset materials due to their benefits and effectiveness. The coatings industry is one of the important sectors within polyurethane-based applications. The American Society for Testing and Materials (ASTM) classifies polyurethane-based coatings according to their characteristic properties, curing methods, and the application areas where they are most effective¹. They are classified as single-component (1K) or two-component (2K) based on their curing mechanisms with reactive or non-reactive aspects. They can be powders and/or solids, dissolved in solvent, or water-based. As a technology, they are dissolved in water or solvent and can also be cured by radiation. Polyurethane-based coatings have numerous application areas, and their many excellent properties such as abrasion resistance, film flexibility, glossy and transparent finish coat, and chemical resistance are characteristics that distinguish polyurethane coatings from other polymeric systems. Among polyurethane coatings, water-based applications are highly attractive because they have low volatile organic compounds (VOC) or contain no volatile organic compounds. In global polyurethane-based coating consumption, solvent-based applications remain dominant². An increase in displacement by water-based systems is expected. Legal regulations in emission controls of VOC-containing products are projected to be a driving force in the development of high-performance water-based polymers. Figure 1 shows the increase in Water-Based Polyurethane Dispersion (PUD) and related publications since the early 2000s and appears to have reached its highest level to date. Additionally, this chart confirms market research suggesting that the PUD industry continues to grow in Asian countries⁴. [caption id="attachment_136170" align="aligncenter"] Figure 1. European Patent Office Statistics - Water-based polyurethane dispersion, February 2022[/caption] Water-based polyurethane dispersions on the market were first introduced in the 1950s. The industry had to wait until the early 2000s for completely solvent-free products⁵. The unique reactivity of isocyanate with active hydrogen atoms results in tremendous polymer innovation. The fundamental characteristic of water-based polyurethane dispersions is that multiple hydrogen bonds neighboring polymer chains consist of urethane and urea chemical groups. In the dry film, hard segments containing urethane and urea groups form strong hydrogen bonds through flexible soft segments, assembling themselves to create this structure. Hydrogen bond formation provides excellent properties to linear polymers even at low molecular weight. Water-based polyurethane dispersions have an important advantage; unlike other water-based polymer alternatives or solvent-based ones, their viscosity is not dependent on molecular weight. [caption id="attachment_136171" align="aligncenter"] Figure 2. Monodentate urethane structure and bidentate urea hydrogen bonding[/caption] Soft and hard segments are generally thermodynamically incompatible, which causes phase separation, and therefore this domain created in the chemical structure provides the material with a wide range of properties and application areas⁶. Application areas for water-based polyurethane dispersions extend from rimless glasses to hair styling products, from parquet coatings to denim trouser coatings, from sports clothing to safety workwear, from dispersion agents for paints and coatings formulations to binders and self-healing binders⁷. PUDs can be used as adhesives, coating materials, auxiliary binders, sizing agents, and UV-curable binders⁸. In typical PUD synthesis, initially a macromer is synthesized through step polymerization steps together with the hydrophilic portion providing stabilization in water, followed by isocyanate (NCO)-terminated prepolymer synthesis in acetone or methyl ethyl ketone solvents with an NCO/OH (hydroxyl) ratio greater than 1, with excess isocyanate. This is followed by a dispersion phase where the prepolymer spontaneously forms micelles in water. The dispersed prepolymer is then completed through chain extension polymerization to provide higher molecular weight, and the solvent is removed. The self-organized dispersed product forming micelles in water has been modified to be internally emulsified. For this reason, this renders the dispersed product sensitive to temperature changes. [caption id="attachment_136172" align="aligncenter"] Figure 3. Schematic representation of main components of PUD (water-based polyurethane-urea dispersions)[/caption] The unique properties of water-based polyurethane dispersions include an expanding list such as UV resistance, abrasion resistance, scratch resistance, moisture management, hydrophobicity-hydrophilicity, gloss control, strength, peelable films, wash resistance, impact resistance, and adhesion strength. Due to these properties, they occupy considerable space in the literature. Demand for disruptive innovation work with polyurethane-based solutions for fluorine-free waterproof materials remains current⁹. Not new but physical and chemical modifications using elements such as phosphorus, silicon, and sulfur to enhance the thermal and mechanical properties of PUDs remains one of the hot topics¹⁰. Based on recent research, polyurethane/acrylic hybrid systems form an important class for polymeric dispersion work conducted at industrial scale. This helps reduce the use of film-forming agents for floor coverings and wood systems¹¹. Low-isocyanate or isocyanate-free applications have been one of the industry's most important concerns for approximately twenty years. Synthesis of low-isocyanate or isocyanate-free polyurethane dispersions through new methods will become increasingly important in the future. Considering this industry focus, there are many projects supported by European Union funds that respond to these efforts with 'hybrid' technology solutions. Polyhydroxyurethanes (PHU) are among the most promising polymers that could help reduce isocyanate use in polymer structure. However, more effort will be required for their water-based dispersions. V. Besse and group¹² explain why and how PHUs have lower molecular weight. This explains why polyurethanes cannot be completely polyhydroxyurethane-based with currently known technology. Furthermore, companies pioneering the polyurethane industry are incorporating small initiatives that enable the use of renewable inputs for the polyurethane industry. For work to be conducted with the EU, wood coatings, technical textiles, and binders for recycled composite materials are rising trend topics for Turkish national economy¹¹. By 2050, the chemical industry in Europe will increasingly use products derived from CO₂ (carbon dioxide). The technology developing the conversion of CO₂ and CO (carbon monoxide) into polyols is a project funded by the European Union¹³. In the literature, there are studies describing that water-based polyurethane dispersions synthesized from advanced recycled polyols derived from polyurethane foam applications exhibit parallel properties to commercial polyurethane dispersions synthesized from newly synthesized polyols¹⁴. In conclusion, the versatility in the composition, properties, processability, and applicability of water-based polyurethane dispersions makes them promising materials for new fields requiring new and special requirements.

References 1. Alrashed M., 2013. "Polyurethane / Polysiloxane Ceramer Nanocomposite Coating for. Aircraft Applications" Thesis Advisor: Dr. Sadhan Jana. Rostyslav Dolog, Ph.D. 2. Mark F. Sonnenschein, 2020. Polyurethanes: Science, Technology, Markets, and Trends, 2^nd Edition, | Hoboken : Wiley, 2021. | Series: Wiley series on polymer engineering and technology 3. Ground Level Ozone Pollution, Volatile Organic Compound, United States Environmental Protection Agency (EPA) website 4. Global Overview of PUDs, Coating Industry-EMEA, IAL Consultants Market Report-2021 5. Bayer Materials Science, Coating 2013 Press Release 6. Erol Yildirim, Mine Yurtsever, Emel Yilgör, Iskender Yilgör,Garth L. Wilkes. Temperature-dependent changes in the hydrogen bonded hard segment network and microphase morphology in a model polyurethane: Experimental and simulation studies. Journal of Polymer Science Part B: Polymer Physics 2018, 56 (2) , 182-192. 7. Berezkin, Y., P.D. Schmitt, and S. Unal, Polyurethane dispersions for use in personal care products, 2008, Bayer Materialscience LLC, USA . p. 13pp 8. US9862824B2 ,Carbon fiber sizing agent, aqueous dispersion thereof, carbon fiber bundle applied with sizing agent, sheet-like article comprising carbon fiber bundle, and carbon fiber reinforced composite material 9. CN103628324A, Fluorine-free water-proofing agent and preparation method thereof as well as textile 10. Swapnil M. Vaidya, et al., Recent developments in waterborne polyurethane dispersions (WPUDs): a mini‑review on thermal and mechanical properties improvement, 2021,Springer Nature 11. Samane Mehravar, Nicholas Ballard, Radmila Tomovska, and José M. Asua, 2019. Polyurethane(PU) / acrylic hybrid waterborne dispersions: Synthesis, properties and applications Ind. Eng. Chem. Res., 12. V. Besse et al, How to explain low molar masses in PolyHydroxyUrethanes (PHUs), European Polymer Journal 71 (2015) 1-11 13. Carbon4PUR Press Release 14. Iyer D., Srivastava S., 'Upcycled Polyurethane Products Based-Waterborne Polyurethane Dispersions',Mattress Recycling Council Dispersion Report,2020

Derya Şara Department of Chemistry / Graduate Education Institute Istanbul Technical University Assoc. Prof. Serkan Ünal Integrated Manufacturing Technologies Research and Application Centre Sabancı University