Polyurethane dispersion
Polyurethane Dispersion, or PUD, is understood to be a polyurethane polymer resin dispersed in water, rather than a solvent. Its manufacture involves the synthesis of polyurethanes having carboxylic acid functionality or nonionic hydrophiles like PEG (polyethylene glycol) 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. Using  VOC exempt solvents is not a panacea as they have their own weaknesses. The problem has always been that polyurethanes in water are not stable, reacting to produce a urea and carbon dioxide. Many papers and patents have been published on the subject. For environmental reasons there is even a push to have PUD available both water-based and bio-based or made from renewable raw materials. 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 water-soluble 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 (DMPA). 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 but with polyurethane segments. 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. When blocked isocyanates are used there is no isocyanate (NCO) functionality and hence the water reaction producing carbon dioxide so dispersion is easier. Uses. They find  use in Coatings, Adhesives, Sealants and Elastomers. Specific 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. To improve their functionality in flame retardant applications, products are being developed which have this feature built into the polymer molecule. Weaknesses and disadvantages. Although they have excellent environmental credentials, waterborne polyurethane dispersions tend to suffer from lower mechanical strength than other resins. The wear  and corrosion resistance is also not as good and hence they are often hybridized. Other strategies used to overcome some of the weaknesses include molecular design and mixing/compounding with inorganic rather than polymeric materials. The use of an anionic or cationic center or indeed a hydrophilic non-ionic manufacturing technique tends to result in a permanent inbuilt water resistance weakness. Research is being conducted and techniques developed to combat this weakness. Simple blending has also been employed. This has the advantage in that if no new molecule has been formed but merely blending with existing registered raw materials, then  that is a way around the work required to get registration of the material under various country regimes such as REACH in Europe and TSCA in the USA. Because of the surface tension of water being so high, pinholes and other problems of air-entrainment tend to be more common and need special additives to combat. They also tend not to be manufactured with biobased polyols because vegetable based polyols don't have performance enhancing functional groups. Modification is possible to achieve this and enable even greener versions.