Polyurethane Dispersion, or PUD, is understood to be a polyurethanepolymerresindispersed in water, rather than a solvent. Its manufacture involves the synthesis of polyurethanes having carboxylic acid functionality or nonionic hydrophiles like PEG incorporated into, or pendant from, the polymer backbone.
Background
There has been a general trend towards converting existing resin systems to waterborne resins, for ease of use and environmental considerations. Particularly, their development was driven by increased demand for solventless systems since the manufacture of coatings and adhesives entailed the increasing release of solvents into the atmosphere from numerous sources. The problem has always been that polyurethanes in water are not stable, reacting to produce a urea and carbon dioxide. Many papers have been published on the subject. For environmental reasons there is even a push to have PUD available both water-based and bio-based. PUDs are used because of the general desire to formulate coatings, adhesives, sealants and elastomers based on water rather than solvent, and because of the perceived or assumed benefits to the environment.
Synthesis
The techniques and manufacturing processes have changed over the years from those described in the first papers, journal articles and patents that were published. There are a number of techniques available depending on what type of species is required. An ion may be formed which can be an anion thus forming an anionic PUD or a cation may be formed forming a cationic PUD. Also, it is possible to synthesize a non-ionic PUD. This involves using materials that will produce an ethylene oxide backbone, or similar, or a chain pendant from the main polymer backbone. Anionic PUDs are by far the most common available commercially. To produce these, initially a polyurethane prepolymer is manufactured in the usual way but instead of just using isocyanate and polyol, a modifier is included in the polymer Backbone chain or pendant from the main backbone. This modifier is/was mainly dimethylol propionic acid. This molecule contains two Hydroxy group and a carboxylic acid group. The OH groups react with the isocyanate groups to produce an NCO terminated prepolymer but with a pendant COOH group. This is now dispersed under shear in water with a suitable neutralizing agent such as Triethylamine. This reacts with the carboxylic acid forming a salt which is water soluble. Usually diamine chain extender is then added to produce a polyurethane dispersed in water with no free NCO groups. Various papers and patents show that an amine chain extender with more than two functionalities such as a triamine may be used too. There is also a push to have a synthesis strategy that is non-isocyanate based.
Uses
Uses include industrial coatings, UV coating resins, floor coatings, hygiene coatings, wood coatings, adhesives, concrete coatings, automotive coatings, clear coatings and anticorrosive applications. They are also used in the design and manufacture of medical devices such as the polyurethane dressing, a liquid bandage based on polyurethane dispersion.
