The industrial production of aqueous polyurethane
dispersions (PUDs) is nowadays a well established
technology. There are two main synthetic routes to produce
PUDs: the acetone process (a former process developed by
Bayer AG) and the pre-polymer process (developed as an
alternative response to the patented acetone process).
Comparatively to the acetone process, the pre-polymer
process has one major advantage since it requires none or
only small amounts of acetone.
The pre-polymer process, at present, is being forced to
readapt due to ongoing developments, partly motivated by
process constraints, raw materials restrictions and the need
to obtain a true solvent-free product. Allied to this fact it is
worth mentioning the European REACH legislation, which
is having a considerable influence on the PUD industry.
Most of the industrially produced PUDs use dimethylol
propionic acid (DMPA) as the internal emulsifier. DMPA
is sparingly soluble in the reactive mixture and needs to be
previously dissolved in an organic solvent, usually Nmethyl-
2-pyrrolidone (NMP). NMP is difficult to remove
and will remain in the final product.
There are some alternatives to achieve the NMP-free
concept. Among them we can refer the direct NM P
replacement by an equivalent solvent, the DMPA
replacement by an equivalent hydrophilising diol but with
better solubility in the reactive mixture and the preneutralization
of DMPA prior to reaction with isocyanate
