Characterization of polyurethane networks structure and properties based on rapeseed oil derived polyol

Abstract

Polyurethane (PU) networks were synthesized from rapeseed oil (RO) and triethanolamine (TEA) polyols, as well as from modified RO/TEA polyol with ε-caprolactone (CPL) and triethylene glycol (TEG) additives. The study was carried out to determine the impact of molecular weight between cross-links (Mc), urethane group concentration and dangling chain concentration on structure of PU networks, physical, mechanical and thermal properties. The Mc of PU networks was varied using isocyanates (4,4′-methylene diphenyldiisocyanate (MDI), polymeric MDI Suprasec 2651, a polymeric MDI Lupranate M20) separately or in a mixture; the functionality of isocyanates was changed in range from 2.0 to 2.7. Using The Fourier transform infra-red spectral analysis (FTIR-ATR) the impact of Mc and urethane group concentration on PU networks to form hydrogen bonds was determined. Hydrogen bonding index (HBI) was calculated using ratio COb/COfree, parameter R was calculated using ratio NHb/NHfree. It was found out that with increasing Mc of PU networks, HBI and R also increases. Glass transition temperature (Tg) of PU networks was determined using differential scanning calorimetry (DSC). Using model compound bis(2-hydroxyethyl)methylamine (MDEA) for PU networks synthesis, it was discovered that long dangling chains from RO/TEA are screening polar groups within PU networks, thus H-bond formation is interfered and as a result Tg and tensile strength decreases significantly in PU networks obtained from RO/TEA polyols. The volume contraction (Δυ) during PU-forming reaction is determined by using experimental densities of PU and raw material additive densities. Higher Δυ was for PU networks with higher urethane group concentration and higher Mc. The cohesion energy density (CED) and Van der Waals volume (Vw) was calculated for groups in PU networks structure. There is a correlation between CED and tensile strength of the obtained PU. The thermal properties of PU networks were characterized using thermogravimetric analysis (TGA). The initial weight loss (5wt.%) occurs at lower temperature with increasing Mc and urethane group concentration.