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Advances in Cationic Polyurethane Dispersions and Blocked Isocyanates for Waterborne Coatings
Baxenden Chemicals Ltd, a Chemtura Company
Cationic polyurethanes can be prepared by incorporating dihydroxy-functional reagents such as N-methyldiethanolamine into their molecular backbone, and quaternizing the tertiary nitrogen atoms with alkylating agents or strong acids. An alternative method is to use a pre-quaternized monomer, for example N,N-bis(2-hydroxyethyl)-N,N-dimethyl quaternary ammonium methanesulphonate. This paper outlines some latest developments in cationic polyurethane dispersions (PUDs), and of associated blocked isocyanate crosslinkers/adhesion promoters. These latter materials will feature 3,5-dimethylpyrazole (DMP) as a patented, low temperature-unblocking agent. Our R&D emphasis is on optimizing the safety, health and environmental aspects of products for the formulator and end-user. When formulating cationic PUDs, the general guidelines applied to the more common anionic types are reversed. Additives must be fully compatible with positively-charged binders, and 1-K and 2-K coating systems supplied and applied at lower pHs. This widens the scope for using quaternary fluorochemical, silicone and fatty acid auxiliaries, e.g. as water repellants, softeners and anti-stats, as well as pigments and fillers with unusual surface charge. Applications for cationic PUDs and crosslinkers specifically include: textile & leather finishing, paper & vinyl coatings (especially for lock-down of anionic dyes used in ink-jet printing), stain blocking (e.g. primers over water-soluble or leachable tannin, nicotine and graffiti colored components), cathodic electrodeposition and fiber sizes for composites. Naturally, this covers coatings and finishes for a full range of substrates - metals, wood, glass, polymer films & plastics, concrete & masonry, etc. - that are applied in the factory (OEM) or in the field. Some appropriate examples and guide formulations will be provided, together with indicative results.