Degradation and stabilization of cosmetic polyetherurethane in alcoholic solution

A polyetherurethane (PEU) was synthesized for potential cosmetic applications by coupling poly(tetramethylene oxide) (PTMO, Mn = 2000 g mol−1) and poly(ethylene oxide) (PEO, Mn = 2000 g mol−1) with 4,4′-diphenylmethane diisocyanate (MDI), without using a chain extender. The PEU polymer, synthesized with PEO and PTMO in the ratio of 1:3 by weight, was soluble in ethanol–water mixed solvents. The solution formed a thin film in situ on the skin by coating, which was flexible and elastomeric with appropriate skin adhesiveness. However, PEU was susceptible to degradation when it was exposed to an ethanol–water (80/20 v/v %) mixed solvent and stored in an air-filled and sealed bottle at 60°C for 4 weeks. These conditions resulted in deterioration of PEU molecular weight (Mn), viscosity, and mechanical properties. Peak analysis of the Fourier transform infrared spectrum of the aged PEU revealed that >97% of the urethane carbonyl bonds remained, whereas ether bonds were significantly reduced (∼82% of initial value). The degraded PEU contained ester bonds, which were confirmed by proton and carbon-13 nuclear magnetic resonance spectroscopy. This observation suggests that oxidative chain cleavage rather than hydrolysis was the dominant reaction in the degradation process. Two approaches were adopted to minimize oxidative degradation of PEU dissolved in an ethanol–water (80/20 v/v %) mixed solvent; they were, applying a nitrogen environment and adding an antioxidant (1.8 wt % dry PEU). The results indicate that reduction of oxidative degradation produced a synergistic effect. Vitamin E was a more effective antioxidant than butylated hydroxytoluene (BHT), which is a typical antioxidant for commercial polyurethanes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2270–2276, 2003