Effect of hydroxyl terminated polybutadiene number-averaged molecular weight on chemorheological behavior of polyurethane networking

The time available for proper casting of the polyurethane (PU) system after isocyanate addition (called pot life) affects the macrodiol microstructure. In this research, the effect of molecular weight ( Mn ) of hydroxyl terminated polybutadiene (HTPB) as an important macrodiol on the pot life of polyurethane system was rheologically investigated. The chemorheological behavior of polyurethane systems based on five different HTPBs with Mn ranging from ۲۸۱۰ to ۴۴۵۰ g/mol and two common diisocyantes (TDI and IPDI) were studied in situ. The results showed that by increasing Mn , the rotational viscosity as well as the constant reaction rate of urethane formation increases; whereas the pot life of the polyurethane system decreases. The psuedoplastic behavior of the samples showed the minimum rate constant/longest pot life at a critical molecular weight (Mc) of about ۳۱۰۰ g/mol, which may be due to physical chain entanglements. Rheomechanical spectrometry interestingly depicted that tan δ originated-gel point occurs before the intersection of stored modulus (G') and loss modulus (G") (tgel< tG'=G"). The kinetic model developed by Hsich showed excellent agreement with the cross-linking data of three polyurethane systems.

Keywords:

HTPB , number-averaged molecular weight , pot life , Gel Time , Dynamic Rheological , polyurethane system