A structure-property study of water-dispersible polyurethane anionomers

A number of novel water-dispersible and solution polyurethanes were prepared via the prepolymer mixing process. The principal diisocyanate, polyols and emulsifier used in these studies were isophorone diisocyanate (IPDI), polytetrahydrofuran diol (PTHF), cyclohexane dimethanol (CHDM) and 2,2-bis(hydroxymethyl) propionic acid (DMPA), respectively. The carboxylic acid groups of the emulsifier were neutralised with either an organic base (triethylamine) or an inorganic base (sodium or potassium hydroxide). The dispersion molecular weight was built up through chain extension by use of aliphatic diamine chain extenders (hydrazine monohydrate, carbodihydrazide or adipic dihydrazide). The dispersions studied remained stable throughout the period of the research. Structure-property studies were carried out on polyurethanes of similar composition prepared in solution (THF) and in water. The effect of the degree of chain extension on the molecular weight of the dispersions, in addition to the physical properties and morphology, were investigated for hydrazine monohydrate, carbodihydrazide and adipic dihydrazide derived systems. Variations in the polyol, polyol molecular weight, ionic component and the diisocyanate used and their effect on the polyurethane properties were also investigated. The effects of the neutralising counter-ion and the degree of neutralisation were investigated in addition to a study of the effect of the hard segment content for samples prepared with a PTHF soft segment of molecular weight 2000. Each dispersion was thoroughly examined and characterised according to their appearance, pH, viscosity and particle size. The DMPA content is well documented as being a primary factor with respect to the dispersion stability, but also significantly affects the pH, viscosity and particle size of the dispersion. Hence, for purposes of the studies described here the DMPA component remained constant so that this could be neglected from the discussion of the results obtained. In a subsequent examination of the polyurethane films, differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), Fourier transform infra-red spectroscopy (FT-IR), tensile testing and solvent resistance were employed for the study of the solid state morphology and physical properties of the cast films. The results obtained suggest that the advantages of water-based polyurethane dispersions over more conventional solvent-borne systems lies in the molecular weight (chain extension) achievable in water. Hence, this was found to affect significantly the solid state properties of comparable water-borne and solvent-borne systems. Variation of the polyol and its molecular weight, and the diisocyanate types resulted in polyurethanes of considerably different properties and morphologies. It was discovered that synergistic properties could be achieved from the use of isocyanate blends. The degree of neutralisation and the counter-ion employed contributed to a range of colloidal properties. Hard segment content was also found to be contributory to the differing properties and morphologies encountered for PTIlF2000 based polyurethanes.