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.
[7][8] For environmental reasons there is even a push to have PUD available both water-based and bio-based or made from renewable raw materials.
[9][10][11] 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.
The techniques and manufacturing processes have changed over the years from those described in the first papers, journal articles and patents that were published.
[26] Research has been done and published that shows it is not the dispersion speed, mechanical agitation or high shear mixing that has the biggest effect on properties, but rather the chemical makeup.
[40] To improve their functionality in flame retardant applications, products are being developed which have this feature built into the polymer molecule.
[43] On a similar theme, recent (post 2020) innovations have included producing a waterborne polyurethane that has embedded silver particles to combat COVID.
[45] Although they are perceived to have good environmental credentials[46][47][48][49][50][51][52][53][54][55][56][57][58] waterborne polyurethane dispersions tend to suffer from lower mechanical strength than other resins.
[62] 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.
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.
[65] Drying, curing and cross-linking is also not usually as good and hence research is proceeding in the area of post crosslinking to improve these features.
[29][82][83][84][85][86] Waterborne PUD- Acrylates based on epoxidized soybean oil that is also UV curable have been produced and are feasible.
[89] As ionic centers are introduced with waterborne PUDs, the water resistance and uptake in the final film has been studied extensively.
The nature of the polyol and the level of COOH groups and hydrophobic modification with other moieties can improve this property.
[98] Another method of hybridization is to make a PUD that is both anionic but with a very substantial nonionic modification utilizing a polyether polyol based on ethylene oxide.
[102] Since PUDs are usually considered green and environmentally friendly, techniques being researched also include capturing carbon dioxide from the atmosphere to make the raw materials and then further synthesis.