Thermal, Mechanical and Chemical Analyses of Rapid and Self-Cured Prepolymerized Polyurethane Coatings

The influence of NCO/OH ratio of PU monomers on the thermal, mechanical and chemical properties of palm-based polyurethane (PU) was investigated. The prepolymerization of PU was conducted at NCO/OH ratio of palm kernel oil polyol (PKO-p) to 2,4-methylene diphenyl diisocyanate (MDI) at 200/100, 150/100, 100/100 and 75/100 with diethylene glycol (DEG) as chain extender at ambient temperature under nitrogen gas atmosphere. The palm-based PU prepolymer was casted onto a Teflon plate to a thickness of 50 µm. The presence of the urethane linkages (OCN-H) was identified from the FTIR spectrum. The disappearance of NCO peak in the FTIR spectrum at 2270-2250 cm-1 indicated that MDI had completely reacted with the PKO-p to form PU. The appearance of C=O peak at 1700 cm-1 showed that hydrogen bonds were formed between the soft segmented chain and hard segmented chain which were contributed by PKO-p and MDI respectively. Hence, urethane bond was the main polymeric chain in the PU. Besides, PU with the highest hard segment content (NCO/OH ratio: 200/100) exhibited great thermal stability (Tstable of 205°C). In addition, it showed the highest glass transition temperature (85°C) compared to other PU coating formulations. Endothermic peaks were detected in PU with low NCO/OH ratio of 150/100, 100/100 and 75/100 which was referred to the flash point of the polyols (PKO-p and diethylene glycol). However, PU with the highest content of hard segment possessed the lowest scratch resistance of 0.5 N. Steric effect due to benzene ring in MDI molecular chain caused restriction in the mobility of the PU chain.