Synthesis and properties of spray-applied high solid content two component polyurethane coatings based on polycaprolactone polyols

A series of polycaprolactone (PCL) polyols was synthesized via the ring-opening reaction of ε-caprolactone initiated by small molecular polyols including ethylene glycol (EG), 1,4-butanediol (BDO), diethylene glycol (DEG), neopentyl glycol (NPG), 2,4-diethyl-1,5-pentanediol (PD-9), 1,4-cyclohexanedimethanol (CHDM), trimethylolpropane (TMP), di-trimethylolpropane (Di-TMP), di-pentaerythritol (Di-PE) and maltitol. IR and 1H NMR spectra of PCLs disclosed that ε-caprolactone reacted with the hydroxyl groups of small molecular polyols, then the target PCLs were successfully prepared and possessed similar molecular skeleton structures. The GPC data summarized the measured number-average molecular weights of PCLs approached to their theoretical values, and the polydispersity indexes (PDI) were ranged from 1.35 to 1.58. PCLs prepared with maltitol displayed the highest viscosity up to 42000 mPa s, while products obtained from diol substrates including EG, BDO, DEG, NPG and PD-9 exhibited the lower viscosities in the range of 160-300 mPa s. The cured films of PCLs prepared from polyisocyanate curing agents exhibited good impact resistance, adhesion, flexibility, higher hardness and displayed better thermal stability with 5% weight loss at 260 °C. PCLs obtained from TMP (PCL-7, TMP/3ε-CL) and Di-TMP (PCL-8, Di-TMP/4ε-CL) exhibited the higher films hardness and lower viscosities at 100% solid content. The viscosities of PCL-7 and PCL-8 were 1320 and 3900 mPa s, respectively, and the VOC contents of spray-applied 2K-PU coatings prepared from PCL-7 and PCL-8 were low to 180 g/L.