The viscosity of the waterborne PUs increased when the
hydrophobic PTMEG diol was used for the preparation of the PUs,
even though the viscosity usually decreases as hydrophobic segments
increase [20]. The increased viscosities of the waterborne
PUs are mainly due to the decreased particle size in the dispersions.
It was reported that coating materials with high viscosity
(greater than 3000 mPa s) are desirable for textile coatings, since
coating materials with low viscosity can penetrate into the textiles
and leak out from the backside of the coated fabrics [9]. According
to our experimental results, the viscosities of all the waterborne
PUs are high enough to be used as coating materials without any
thickener.