Thermal stability of chemically crosslinked moisture-cured polyurethane coatings

Polyurethane prepolymers are widely used in reactive hot melt adhesives and moisture-cured coatings. The segmented moisture-cured formulations, based on polytetramethylene glycol (PTMG-1000)/trimethylol propane (TMP)/isophorone diisocyanate (IPDI) and PTMG/TMP/ toluene diisocyanate (TDI), were prepared with NCO/OH ratio of 1.6 : 1.0. The excess isocyanate groups of the prepolymers were chain extended in the ratio of 2 : 1 (NCO/CH) with different aliphatic diols and 4 : 1 with different aromatic diamines. The surplus isocyanate groups of the formulations were completely reacted with atmospheric moisture, and the thermal stability of the postcured materials obtained as cast films were evaluated by thermogravimetric (TG) analysis. It was observed that initial degradation temperatures were above 270degreesC, with two- or three-step degradation profiles. The degradation parameters were evaluated using the Broido and Coats-Redfern methods. The thermal resistance of moisture-cured formulations using diisocyanates with the cycloaliphatic structures (IPDI) and the aromatic TDI, at the same NICO/OH ratio (1.6), and TMP content were compared from the isothermal TG experiments at different temperatures and dynamic TG experiments at different heating rates in nitro-en and oxygen environments. The observation suggests that polyurethane-containing sulfone groups and straight-chain diol chain extenders were more stable. It was also observed that at lower temperature polyurethane, prepared from aliphatic diisocyanates (IPDI), was more stable than the aromatic diisocyanate (TDI) containing polyurethanes. At high temperature, the stability order follows the reverse trend. (C) 2005 Wiley Periodicals, Inc.