Polyurethane aqueous dispersion

Figure 224 Scheme for an aqueous, anionically stabilised polyurethane dispersion. Source Klein H.P. and Schwab, M., Aqueous polyurethane dispersions - an environment friendly alternative for high quality paint systems, Resin News, number 29 (July) published by Hoechst AG, Frankfurt, 1993... 

J. Kozakiewicz, Aqueous Polyurethane Dispersions as Coating Binders, presented at Ekolak Symposium, Ustron, Poland, 1993. 

The general formula of an emulsified polyurethane or aqueous dispersion used for adhesive purposes is shown in Fig. 8.11. Aqueous polyurethane dispersions usually have 40% solids of a thermoplastic type (for aqueous inertness), aliphatic diisocyanate polymer (for light stability) and an anionic surfactant is used. However, if a polyurethane ionomer is prepared this does not contain added emulsifier, the polyurethane itself having its own emulsifier function. The incorporation of this function in the polyurethane chains as (a) sodium sulphonate or (b) sodium carboxylate groups is seen in Fig. 8.11. 

Polyesterpolyols based on dimer acids or dimer diol have been largely used in sol-vent-borne and water-based coating applications including alkyd resins and aqueous polyurethane dispersions. ... 

Water-reducible polyunsaturated acid substituted aqueous polyurethane dispersions are also being used (40). They can be made by reacting a diisocyanate with a polyol, monoglyceride of a drying oil, and dimethylolpropionic acid. The carboxylic acid groups are neutralized with a tertiary amine and dispersed in water. If aliphatic isocyanates are used, good color retention can be obtained. They are much more resistant to hydrolysis than conventional alkyd resins. Films also have excellent abrasion resistance. Cost can be reduced by blending in 10-20% of acrylic latex. 

Polycarbodiimides react with carboxylic acid and slowly enough with water so that they can be used in waterborne systems. The product of the reaction with a carboxylic acid is an AT-acylurea. Polycarbodiimides cross-link carboxylic acid-functional resins, including aqueous polyurethane dispersions and latexes (196, 197). Cross-linking occurs within several days at ambient temperature and faster with heat. 

Solutions (5-25% solids) of fully reacted polyurethanes based on MDI or IPDI are also used in coating applications. These coatings are used mostly as finish coats on fiexible substrates. Because of concern over airborne solvents aqueous polyurethane dispersions are used today. These polymers are prepared by incorporating ionic groups into their backbone, and both cationic and anionic groups are utilized in the preparation of the ionomers. The solvent and water resistance of films formed from aqueous dispersions can be improved by cross-linking. Ionomers are also used in the formulation of speciality adhesives. 

S.A. Madbouly, J.U. Otaigbe, A.K. Nanda, D.A. Wicks, Rheological behavior of aqueous polyurethane dispersions effects of solid content, degree of neutralization, chain extension, and temperature. Macromolecules 38 (May 3, 2005) 4014—4023. 

One-part, sohrent-bome polyurethane One-part, aqueous polyurethane dispersion ... 

Aqueous polyurethane dispersions are not new they have been available since the early 1970s. However, environmental concerns and impending legislation are in favour of non-solvent containing materials and reduction of VOCs making these systems commercially available nowadays. Performance of new aqueous dispersions has reached the point that they exceed the performance of some solvent-based systems [16]. These urethane dispersions can also often be processed with the same or similar equipment as solvent systems. They may be used as the sole resin or blended with other resin systems, such as acrylics. 

Another approach is to first prepare an isocyanate-terminated polyurethane in acetone. This is then chain extended with a sulfonate substituted diamine. The acetone solution is diluted with water, and then the acetone is removed by distillation leaving an aqueous polyurethane dispersion stabilized by charge repulsion of sulfonate groups. 

For soft finishes, solventbased and waterbased coatings are available both in middle and northern Europe, with the waterbased widely dominating the market. The chemical basis of waterbased finishes is based on a 2K polyol-isocyanate. Suitable polyols are special aqueous polyurethane dispersions. The isocyanate is provided in liquid form and usually contains a small amount of suitable solvents. Mixing to get the paint ready to spray can be done in small lots as most of these 2K materials exhibit a pot life of about one hour at ambient temperature. However, it is best to apply soft coatings through 2K automated mix equipment with moditied statical mixers. 

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