Inkjet printing of polyurethane colloidal suspensions

found to be 45u. The dye used to make the colour gradients
was Disperse Red (Aldrich).
Results and discussion
Fig. 1 (a) A photograph of the Autodrop TM printer system, which
was used in these experiments; it is equipped with (b) a heatable printhead
(Microdrop Technologies GmbH).
It is equipped with a print-head that can move in the X, Y
and Z directions, and a platen, onto which substrates were
placed. All printing was performed at room temperature. The
Autodrop TM system has a working space of 200 6 200 6
80 mm, a positioning accuracy of 3 mm and has a stroboscopic
video camera, which is used to determine the correct
conditions for droplet formation. The print-head is an MD-
H-712-PH dispenser, which has a nozzle diameter of 100 mm; a
typical example is illustrated in Fig. 1b.
The Autodrop TM printer allows the user to determine the
number of droplets to be deposited in both the X and Y
directions, and to ascribe a value to the spacing between the
centres of each droplet; the so-called dot-spacing. Continuous
features are formed by making the dot-spacing between the
droplet’s centres small enough so that they merge on the
surface. 11 The structure’s surface and the height of the inkjet
printed areas were characterized using a white-light interferometer
(Nanotech Zoomsurf 3D, Fogale, France), which
has a vertical resolution of 7 nm and a horizontal resolution of
150 nm. A 56 objective was used for all measurements.
Finally, UV-VIS spectra were obtained using a UV-VIS
spectrophotometer (Flashscan 530, Analytik Jena, Germany.)
Materials
An aqueous polyurethane dispersion (LR 9005, BASF-AG,
Ludwigshafen, Germany) with an approximate solid content
of 40 wt% was used. The polyurethane had an average particle
size of 100–200 nm, a molecular weight of approximately
1600 g mol 21 , and its dispersion had a viscosity of 20 mPa s
at 23 uC. Microscopic slides, 3 6 1 inch (Marienfeld, Lauda-
Königshofen, Germany) were used as the substrates. These
were ultra-sonicated for 10 minutes in acetone. They were then
rinsed with acetone before being cleaned with sodium dodecyl
sulphate solution and washed with de-ionized water to remove
the soap. They were then treated with isopropanol vapour in a
reflux setup to remove the water and dried under a flow of air.
Finally, to remove any organic contamination, the substrates
were treated in a UV–ozone photo-reactor (UVP PR-100,
Upland, CA) for 20 minutes. The contact angle that the
aqueous polyurethane dispersion made with the cleaned glass
slide was measured (OCA 30, Dataphysics, Germany) and
The first part of our investigation was concerned with
obtaining the optimal jetting conditions for the 40 wt%
polyurethane dispersion, and determining if the polyurethane
dispersion needed diluting. In order to print structures with
appreciable heights, one can either use an ink with a high
solids content, 9 since this requires a smaller number of passes,
or one can perform a larger number of passes using a more
dilute ink; 12 this choice is dependent upon the type of inkjet
printer that is being used. Here, we adopted the approach of
using the high-solids ink, since the as-received dispersion was
40 wt% and the Autodrop TM system is a piezoelectric inkjet
printer, which is capable of dealing with inks with viscosities in
the range of 0.5–20 mPa s. 13 The viscosity of the as-received
dispersion was 20 mPa s, which is at the top of the printability
range using piezoelectric inkjet printing. 13 This value was
expected since viscosity is known to increase with increased
solids loading. 9 A 20 wt% dispersion was also prepared, by
simply diluting the as-received polyurethane suspension, in
case the 40 wt% dispersion could not be printed. A further
consideration was particle size since the ink was composed of
particles suspended in an aqueous carrier, i.e. it was not a
solution. However, the average particle size of the dispersion
was 100–200 nm, several orders of magnitude smaller than the
100 mm nozzle, and the dispersion itself was not seen to
agglomerate. The anticipated problem of nozzle clogging did
not occur.
Both dispersions were printed at room temperature using the
same pulse width (28 ms), frequency (200 Hz), and dot-spacing
(50 mm in X and 100 mm in Y). The only parameter that
needed to be changed was the voltage, which was increased
from 90 V (for the 20 wt% ink) to 96 V (for 40 wt%). For both
concentrations, no difference in drop formation was observed;
that is good, well-formed droplets were seen to be produced.
The inset of Fig. 2 shows a typical droplet produced using the
40 wt% dispersion. The increase in voltage, which was needed
to print the ink with the higher solids loading, was also found
to be necessary by Seerden et al., 9 who used 70 V for a 30wt%
solution compared to 80 V for a 40 wt% ink. It is known that
increased building rates can be achieved by operating the
print-head at higher frequencies, since frequency directly
influences the rate of droplet production. 9 Future research
using this material for rapid prototyping could investigate the
effect of an increased build rate; however, the scope of this
research was concerned with investigating the printability of
the polyurethane dispersion, and therefore frequency was kept
constant.
Fig. 2 shows the profiles obtained from printed, singlelayered
films of the 20 wt% and 40 wt% dispersions. It can
clearly be seen that the film obtained from the 40 wt%
dispersion produced a smooth profile, whereas the film
obtained from the 20 wt% dispersion produced an irregular
profile that showed a greater variation in height. It is suggested
that the profile observed for the 20 wt% solution is due to it
being more dilute. Furthermore, the dimple seen in the middle
This journal is ß The Royal Society of Chemistry 2007 Soft Matter, 2007, 3, 238–243 | 239