The Examination of the Structure Property Relationships of Some Water-Dispersed Polyurethane Elastomers

Abstract

Water-dispersed or latex polyurethanes (PUs) have been known for many years. However, in the last decade there has been an increased level of R&D interest into these types of polymers, with potential use as coating and adhesive materials. In part, this research has been driven by the introduction of laws and regulations intended to reduce and eliminate the use of organic solvents, and so to minimise environmental pollution. Latex or emulsion forms of PUs have been investigated because of their specific end-use properties, in particular excellent adhesion and long term barrier properties.

This paper reports a study of some experimental formulations of water-dispersed polyurethanes based on tetramethylxylene diisocyanate (TMXDI), some polyesterols or polyetherols polyols and an aliphatic diamine chain extender. These have been synthesised as water dispersions, with dimethylol prioponic acid and triethylamine used as providing anionic centres and neutralising agent, respectively in the aqueous phase.

Some properties of the various formulations as dispersions were determined, including particle size, surface free energy, viscosity and dispersion stability. Stable dispersions were subsequently converted into coherent, dry, thin sheet materials, which were characterised using thermal analysis and some mechanical test methods.

The research has shown it is possible to produce stable water-dispersed urethane-urea copolymers, based on different molar masses of polyesterol or polyetherol precursors and TMXDI, and also where hard segment content was varied (i.e. by different ratios of combined dialcohol species to diamine). It was found that stable dispersions can be made with a broad range of internal emulsifier content in the polymer chain, that have low viscosity and low surface free energy. Particle size of the polymer dispersions was affect by emulsifier content: higher levels producing small particles. At low levels of emulsifier, the stability of the dispersions was increasingly poor, while at higher levels, viscosity increased due to small particle size and increasing coulombic effects. Stable dispersions cast into thin dry films of coherent elastomers, have good mechanical properties, similar to those of urethane-urea copolymers synthesised by water-free methods. Thermal analysis suggests that the degree of phase mixing of hard and soft segment in these polymers, parallels that reported for to urethane-urea copolymers synthesised by water-free methods.