Melamine‐Modified Waterborne Polyurethane Coatings: Experimental and DFT Study

Melamine (triazine)‐based crosslinkable waterborne polyurethane (WBPU) dispersions were prepared with defined dimethylolpropionic acid (DMPA, 21.10 mol%) and triazine (0.50 mol) contents. The hydrophobic properties (determined by water swelling (%) and water contact angle), mechanical strength, and adhesive strength of the respective coatings were evaluated. Compared with those of pristine WBPU, the hydrophobicity, adhesion strength, and mechanical strength of crosslinked WBPU increased. The interactions between the urethane‐urea groups and triazine and its derivatives 2,4‐diamino‐6‐dimethylamino‐1,3,5‐triazine (D1), 2,4‐diamino‐6‐diethylamino‐1,3,5‐triazine (D2), 2,4‐diamino‐6‐butylamino‐1,3,5‐triazine (D3), and 2,4,6‐tris[bis(methoxymethyl)amino]‐1,3,5‐triazine (D4 or HMMM) were investigated using density functional theory (DFT) optimization at the M06‐2X/6‐31+G(d,p) level. The DFT results revealed that D4 exhibited the strongest interaction energy and formed hydrogen bonds with WBPU. These theoretical findings strongly matched the experimental findings, because the coating with HMMM improved the protective properties among all the coatings. The addition of defined poly(ethylene oxide) glycol (PEG) also increased the shelf life of the dispersion without sacrificing material properties.