Polydimethylsiloxane containing waterborne hydrophobic polyurethane coatings with good adhesion to metals: Synthesis and characterization

Waterborne polyurethanes (WPU) are of great importance in the industrial field as adhesives and coatings due to the reduced use of organic solvents in their formulations. However, one of the main drawbacks of these formulations is their high hydrophilic character, so in this work siloxane groups were introduced into the soft segment in order to solve this problem. Thus, the synthesis of polyurethane dispersions with different proportions of the polyols (poly-ε-caprolactone (PCL) and polydimethylsiloxane (PDMS)) was carried out following the acetone process in three stages: polyurethane synthesis, water dispersion of the polymer and acetone removal. The reactions were monitored by Infrared Spectroscopy (FTIR) and the particle size was determined by Dynamic Light Scattering (DLS). Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Size Exclusion Chromatography (SEC), and Proton Nuclear Magnetic Resonance (1H NMR) were used to characterize the polyurethanes. Finally, after coating the dispersions onto metal substrates, adhesion strength, contact angle and phase morphology were studied. The introduction of siloxane as soft segment increased the surface roughness and hydrophobicity but did not change the adhesion values.