Investigation of polyurethane-based pressure-sensitive adhesives with castor oil as a polyol

Abstract

Renewable castor oil (CO) was investigated as a polyol for preparation of polyurethane-based pressure sensitive adhesives (PSAs). PSAs from the one-pot reaction of CO, poly(propylene glycol) (PPG), toluene diisocyanate (TDI) and DBTDL have all desirable properties for applications as surface protective films. NCO-terminated PPG-TDI adducts were prepared from reactions of PPG and TDI at room temperature (as designated as PPG-TDIR) and 100 °C (as designated as PPG-TDI100), respectively. The PSAs from reactions of CO and PPG-TDIR or PPG-TDI100 had higher peel strengths than those from reactions of CO and the mixture of PPG and TDI. PPG-TDIR resulted in a higher peel strength of the resulting PSAs than PPG-TDI100. A two-stage curing process of the partial curing of the mixture of CO and PPG-TDIR at 125 °C and the final curing in an ambient environment was better than one-stage complete curing at 125 °C in terms of enhancing the peel strength of the resulting PSAs. The NCO/OH molar ratio of 0.85 in the mixture of CO and PPG-TDIR resulted in the highest peel strength for the PSAs from CO and PPG-TDIR. The PSAs from CO and PPG-TDIR had good aging resistance and can be tailored for various applications such as labels and tapes. PPG was superior to poly(ethylene glycol) (PEG) in terms of enhancing the peel strength of the resulting PSAs. The curing mechanisms are discussed in detail.