Interactions of HEUR associative thickeners with waterborne polyurethanes
Polyurethane aqueous dispersions are examined with a variety of water-soluble polymers, and are compared with waterborne acrylic latices. Two particle size polyurethane aqueous dispersions are studied, the smaller size gives higher viscosities, in parallel with acrylic latices. Hydroxyethyl cellulose and hydrophobically-modified hydroxyethyl cellulose thicken surface-acid-stabilized acrylic latices. In contrast, polyurethane aqueous dispersions are not thickened effectively with cellulose ethers. The latter cause phase separation of the polyurethane dispersions, which increases with increasing particle size, in agreement with the concept of depletion or volume restricted flocculation. Polyurethane aqueous dispersions are more effectively thickened with hydrophobe-modified, ethoxylated urethanes (HEURs) than corresponding particle size acrylic latices. HEURs that are very inelastic in acrylic latex dispersions become very elastic in polyurethane dispersions. In a series of designed step-growth polymerization HEURs, synthesized in our laboratories, the viscosity increase is noted to depend on the 'effective hydrophobe size' of the terminal positions in HEUR thickeners. The effective terminal hydrophobe size is determined by the alkyl chain of the hydrophobic amine and the alkyl group of the diisocyanate with which it reacts. When the size of the hydrophobic amine is kept constant, but the alkyl units of the diisocyanate is increased in hydrophobicity, larger increases in viscosity are realized.(Author abstract)
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