text: 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 groups 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, a diamine chain extender is then added to produce a polyurethane dispersed in water with no free NCO groups but with polyurethane and polyurea segments. Dytek A is commonly used as the chain extender. Various papers and patents show that an amine chain extender with more than two functionalities such as a triamine may be used too. Chain extender studies have been carried out.
  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. Modifiers other than DMPA have been researched.
  It is also possible to introduce hydrophilicity into the polymeric molecule by using a modified chain extender rather than doing so in the polymer backbone or a pendant chain. Lower viscosity materials are often the result, as well as higher solids. A variation on this technique is to incorporate sulfonate groups. PUD/polyacrylate blends can be prepared this way also utilizing internal emulsifiers.
  Cationic PUD also introduce hydrophilic components when synthesized. This includes phosphonium entities. Techniques have and are being researched to improve the performance and water resistance properties by various techniques. This includes introducing star-branched polydimethylsiloxane.
  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. However, particle size distribution can be controlled by this to some extent.