Effect of Molecular Architecture on Polyurethane Coatings

Abstract

The physical, tensile and dynamic mechanical properties of segmented thermoplastic Polyurethane (PU) coatings, especially those with amorphous region, fundamentally depend on the properties of the Chain Extension Diols (CXD's). This research aimed to analyse and identify the best possible CXD's to react with Isophorone Diisocyanate (IPDI) to achieve optimum properties for a specific application. Ten common CXD's used by industry are Diethyleneglycol (DEG), Triethyleneglycol (TEG), 1,5-Pentanediol (PTD), 1,3-Propanediol (PPD), 1,4-Butanediol (BTD), 1,6-Hexanediol (HD), Butylethylpropanediol (BEPD), 2,2,4- Trimethyl-1,3-Pentanediol (TMPD), Propylene glycol (PG), and Neopentylglycol (NPG). These are used to create the hard segments of prepolymers and Polypropylene Glycol (PPG) of molecular weight 1000 or 480 is used to control the soft segment content. In this study soft segment content (SSC) of 45% and 55% by weight, average molecular weight between consecutive crosslinks (XLD) of 1500 and 2500, and NCO/OH ratios (NOR) of 1.6 & 1.4 were adopted for each pre-polymer. All possible permutations of these parameters were taken into consideration for every CXD. Free films of a three-layer coating of each PU were prepared in accordance with ASTM D823. Specimens were cut from cured films using a dog-bone cutter mounted on a mechanical press and analysed for thermal properties by Dynamic Mechanical Thermal Analysis (DMTA). Glass Transition Temperature (Tg) of polymers varied from 400C to 810C. Strength properties were measured using a Tensile Tester (Instron 5567). CXD is the foremost contributor to variation in properties. While SSC has significant influence over the modulus, the viscosity of prepolymers and cured film Tg were largely dependent on NOR, while XLD was the key factor in determining the tensile strength at break.