Wood coatings of PU-NC semi-interpenetrating polymer networks (semi-IPNs), made from blown castor oil (BCO), aromatic polyisocyanate (PTDI), and nitrocellulose (NC), wore prepared by sequential polymerization method. To investigate the effects of NC on the properties of PU-NC semi-IPNs wood coatings, three different viscosities of NC (1/2 sec, 1/4 sec, and 1/16 sec), and a PU catalyst [dibutyltin dilaurate (DBTDL)] Were synthesized in this study. The results revealed that the PU-NC semi-IPNs with 1/2 sec NC-containing coating had the shortest drying time, and their films had the best tensile strength, elongation at break, lightfastness, and solvent resistance. Dynamic mechanical analysis showed that PU-NC semi-IPNs with 1/2 sec NC or 1/16 sec NC had good interpenetration between PU and NC, whereas PU-NC semi-IPNs with 1/4 sec NC had a slight separation phase between PU and NC. In both PU-NC semi-IPNs with 1/4 sec NC and those with 1/16 sec NC, the addition of 0.2% DBTDL could effectively reduce the drying time of the coating and improve the film properties, including tensile strength, elongation at break, and lightfastness. In addition, the miscibility of PU and NC was examined in PU-NC semi-IPNs with 1/4 sec NC. (C) 2003 Wiley Periodicals, Inc.