Inkjet printing of polyurethane colloidal suspensions

PAPER
www.rsc.org/softmatter | Soft Matter
Inkjet printing of polyurethane colloidal suspensions
Antje M. J. van den Berg, ab Patrick J. Smith, ab Jolke Perelaer, ab Wolfgang Schrof, c Sebastian Koltzenburg c
and Ulrich S. Schubert* ab
Received 13th July 2006, Accepted 15th November 2006
First published as an Advance Article on the web 12th December 2006
DOI: 10.1039/b610017a
An aqueous 40 wt% dispersion of polyurethane has been successfully printed at room temperature
using a piezoelectric inkjet printer. Simple layered structures, as well as dots, were made and
subsequently analyzed using white-light interferometry. A single layer was found to have a
structure height of 10 mm; a value that suggests that this polyurethane dispersion may be suitable
for use in rapid prototyping, since tall structures can be rapidly produced using only a few
printing passes. Finally, by the simple addition of a water-soluble dye, colour gradients were
produced using this printing technique.
Introduction
In the last decade, inkjet printing has moved away from its
traditional applications in graphic arts and product marking to
become a versatile fabrication technique. It is now routinely used
to form polymer and organic LEDs for flexible display applications,
1 to fabricate electronic circuits, 2,3 and for the controlled
deposition of nanoparticles. 4 It has also been employed in
various analytical applications such as sample preparation for
MALDI-TOFMS measurements. 5 Inkjet printing has gained
increasing use with polymeric coatings and thin-film fabrication,
since different parameters such as application and processing can
be readily altered and quickly tested when one is developing a
new formulation. Libraries of coatings with different compositions,
homogeneities and film thickness can be easily prepared
using inkjet printing technology and subsequently characterized
using high-throughput analytical techniques. 6
The inkjet printer used in this study precisely placed droplets
by mechanically positioning the droplet-generating print-head
directly above the substrate. It employs the drop-on-demand
(DOD) method, in which droplet formation is achieved by
changing the voltage that is applied across the piezoelectric
ceramic ‘jacket’ which surrounds the ink chamber. This voltage
change results in a pressure pulse, which in turns causes a
column of ink to be ejected from the print-head’s orifice. This
column rapidly assumes the familiar droplet shape, due to
surface tension, well before it makes contact with the substrate.
A primary consideration for inkjet printing is that liquids must
fit the physical and rheological requirements of fluid flow in
the print-head, especially viscosity, in order to be successfully
ejected. If the ink is too viscous then a large pressure pulse is
needed to generate a droplet, whereas if the surface tension is
too low, the print-head will generate attendant satellites as well
as the desired droplet, which reduces the resolution of the final
as-printed feature. The two main parameters that were varied
a Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven
University of Technology, PO Box 513, 5600 MB, Eindhoven,
The Netherlands. E-mail: u.s.schubert@tue.nl; Fax: +31 40 247 4186
b Dutch Polymer Institute (DPI), PO Box 902, 5600 AX, Eindhoven,
The Netherlands
c BASF AG, Polymer Physics, 67056, Ludwigshafen, Germany
in this study were the voltage applied across the piezoelectric
actuator and the pulse width, the time taken for the voltage to
return back to its starting value.
Inkjet printing has found considerable use in the field of
rapid prototyping, in which a two-dimensional layer is printed
repeatedly to produce three-dimensional (3D) structures.
Rapid prototyping, or layered manufacture as it is sometimes
called, has become useful to industry since it allows swift
visualization of concept models. It was first employed by Sachs
et al. 7 who prepared ceramic components by printing a binder
phase into a bed of powder. After the requisite number of
layers was printed, the free powder was removed, leaving the
3D object. This approach, however, has limitations since the
use of a powder bed limits the range of composition variation.
Evans and co-workers 8 developed a direct inkjet deposition
method, whereby they used dilute particle suspensions and a
modified, commercially-available desktop printer to build up
ceramic structures ‘voxel by voxel’. The main disadvantage of
this approach was the very dilute nature of the ceramic inks,
which caused an increase in processing time since a greater
number of layers had to be deposited in order to obtain a
specified thickness. A quicker way to build structures makes
use of a ‘phase change’ ink, which either solidifies upon impact
with the substrate, such as molten wax, 9 or undergoes a gel
transition via a photo-induced cross-linking step. 10
In this paper, we present an initial investigation into printing
layered 3D structures using a photo-curable polyurethane
dispersion; since the research was preliminary we omitted the
final step of adding a photo-initiator. Simple structures were
printed such as five-layer pyramids and step structures; these
were subsequently examined using white-light interferometry
to determine height and resolution. Finally, a water-soluble
dye was added to the polyurethane dispersion, and step
structures were printed to obtain colour gradients.
Experimental
Instrumentation
For printing, an Autodrop TM system (Microdrop Technologies
GmbH, Norderstedt, Germany) was used; as shown in Fig. 1a.
238 | Soft Matter, 2007, 3, 238–243 This journal is ß The Royal Society of Chemistry 2007

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

Fig. 2 Two profiles obtained by white-light interferometry of singlelayered
films printed with 20 wt% and 40 wt% polyurethane
dispersions. The inset shows an image of a typical droplet of the
40 wt% dispersion produced by the inkjet printer.
of the 20 wt% solution may possibly be a consequence of the
preferential deposition of suspended material towards a drying
feature’s boundary, which was reported by Deegan et al. 14
Certainly, it is fair to assume that for the 20 wt% solution more
water has to evaporate in order for the feature to dry.
Therefore, there is a greater propensity for material to migrate
during drying, for the more dilute solution.
On account of its obtained profile and printability, it was
decided to use the 40 wt% dispersion for further experiments.
Subsequently, a series of dots of the 40 wt% polyurethane
dispersion was printed in order to observe individual
behaviour. A typical sequence is illustrated in Fig. 3. The
average height of each individual deposit was measured as
being approximately 3 mm, which suggests that it may be
suitable for rapid prototyping, where the ability to form thick
layers quickly is desirable. This value of height was attributed
to the high contact angle that the dispersion formed with the
cleaned substrates. Clearly a lower contact angle would
have led to a decrease in droplet height and a broader base.
The height of the individual droplets is lower than for the
corresponding films. This was expected as dot-spacing, which
is the distance between the centres of two deposited droplets,
influences structure height. 15 The more dot-spacing is
decreased, the greater the height of the printed structure.
The decrease in volume was measured for the deposited
droplets, using white-light interferometry, and was found to be
43%. The dimple apparent in the centre of each dried droplet is
attributed to drying.
In order to produce functional structures, the individual
polyurethane droplets were printed in such a way that the
descending droplet partially overlapped with the preceding
droplet to produce linear features. Well-defined lines with a
width of 224 ¡ 12 mm were obtained on the as-cleaned glass
slides using a dot-spacing of 100 mm. By printing a sequence of
lines whose inter-line spacing corresponded to the dot-spacing,
homogeneous films with an average height of 2.50 ¡ 0.2 mm
were obtained. Ten layers were printed on top of each other,
which resulted in a film thickness value of 59 ¡ 22.6 mm.
However, these ten-layered films were not homogeneous since
successive layers were deposited on top of earlier layers that
had not thoroughly dried.
Therefore, in order to prepare a well-defined thickness
gradient within one structure, a drying step of 5 minutes was
required before printing the next layer. It is worth commenting
Fig. 3 An image obtained by white-light interferometry of a sequence of single dots of 40 wt% polyurethane dispersion. The inset shows their
associated profiles, which demonstrates their height and suggests the suitability of this dispersion for rapid prototyping.
240 | Soft Matter, 2007, 3, 238–243 This journal is ß The Royal Society of Chemistry 2007

that all printing was performed at room temperature and the
time of this drying step could be reduced, or removed, by
heating the platen of the printer. The as-obtained thickness
gradient resembled a step structure, i.e. the base layer was the
largest in terms of area and each subsequent, and higher, step
had a reduced area. An initial 6 6 6 mm square base was
printed and allowed to dry, and then a second 4.5 6 6mm
rectangular layer was printed. Two more step-like layers of 3 6
6 mm and 1.5 6 6 mm, respectively, were printed to produce a
thickness gradient. (This process was done repeatedly and was
found to be highly reproducible.) Fig. 4 shows a typical
thickness gradient; each constituent step is composed of a
single layer, i.e. one printing pass produced the layer, and can
be clearly seen. The thickness of each step is as follows:
layer 1 = 12.5 ¡ 3.53 mm, layer 2 = 13.0 ¡ 4.94 mm, layer 3 =
8.50 ¡ 2.82 mm and layer 4 = 6.00 ¡ 8.48 mm. The decrease in
layer thickness can be explained by the fact that the previous
layer is partially dissolved by the layer printed on top of it. For
the first layer this effect does not count since this one is printed
directly on the glass substrate. It can also be seen that the
highest step displays the greatest variation in thickness and, as
can be seen from Fig. 4, it has a pronounced slope towards the
back of the step structure. This slope may be a consequence of
the combined ‘settling’ of the three underlying layers.
Using this step-building method, a square-based, pyramidlike
structure was subsequently obtained by printing five layers
on top of each other; again each layer was produced using a
single pass of the print-head and was allowed 5 minutes to dry.
The base was 10 6 10 mm and the area of each subsequent
step decreased by 2 mm per side; with the uppermost layer
being 2 6 2 mm. The height of the pyramid was determined by
white-light interferometry and found to be 87 mm, with each
step having the following thicknesses: layer 1 = 14.5 ¡
3.04 mm, layer 2 = 17.5 ¡ 1.62 mm, layer 3 = 9.55 ¡ 2.54 mm,
layer 4 = 13.7 ¡ 4.03 mm and layer 5 = 25.5 ¡ 7.99 mm. The
resultant structure, as shown in Fig. 5, has distinct, welldefined
steps; the plan view (Fig. 5a) shows each layer clearly.
For the 3D representation (Fig. 5b), the edge definition is
slightly ‘sharper’ at the front of the structure then at the back.
This is also visible in the section (Fig. 5c) whereby the edges at
the left of the picture are sharper and more distinct then those
on the right. This difference in edge profile is explained as
being due to the direction of print-head travel. The material at
the left hand side of each of the layers, shown in the section
(Fig. 5c), was printed last. Therefore, this material had more
time to dry, since the print-head travelled from right to left,
and subsequently began building a new layer on the right hand
side of the previously printed layer.
Polyurethane has been used in the preparation of microcontact
stamps and in other patterning applications. 16 It may
be possible that, with the use of specially prepared surfaces,
micro-contact stamps could be produced from polyurethane
using inkjet printing. Although there is a clear increase in
height for each layer that is printed, stamps produced using
inkjet printing and unprepared surfaces are unlikely to gain
acceptance on account of their dimensions being a couple of
orders of magnitude larger.
The final part of our investigation involved the fabrication
of colour gradients, which were obtained by adding 0.5 wt%
Disperse Red to the polyurethane dispersion. Thickness
gradients, similar to those shown in Fig. 4 but composed of
five layers, were printed. Fig. 6 illustrates two of the colour
gradients that were prepared. The most intense red colour
was seen in the area that was composed of five layers. It is
suggested that these colour gradients could be used as
colour filters 17,18 for creating two-dimensional gradients on a
substrate that has been coated with a photo-curable material.
19,20 The UV-VIS spectra of the films are shown in Fig. 7.
It can be seen that polyurethane and glass exhibit similar
absorbance behaviour in the visible spectrum to that of glass,
Fig. 4 An image and a profile, obtained by white-light interferometry, of a thickness gradient produced by printing a structure composed of welldefined
steps of 40 wt% polyurethane dispersion.
This journal is ß The Royal Society of Chemistry 2007 Soft Matter, 2007, 3, 238–243 | 241

Fig. 5 An image and a profile, obtained by white-light interferometry, of a pyramid-like structure of 40 wt% polyurethane dispersion obtained
from printing successively smaller layers.
whereas the trace for glass/polyurethane/Disperse Red displays
a clear absorbance in the blue–green part of the visible
spectrum.
Conclusions
An aqueous 40 wt% dispersion of polyurethane has been
successfully printed at room temperature using a piezoelectric
inkjet printer. Simple layered structures such as pyramids and
step-structures, as well as dots, were printed. These were
subsequently analyzed using white-light interferometry. The
height of the individual dots was found to be 3 mm, whereas
a single-layered film and the component layers of a step
structure were found to have heights of about 10 mm. This
value in height, for the film and layers, was attributed to
dot-spacing, which is the distance between the centres of
deposited droplets. This value suggests that this polyurethane
dispersion may be suitable for use in rapid prototyping, since
Fig. 6 A photograph of the colour gradients produced by the
addition of 0.5 wt% Disperse Red to the 40 wt% polyurethane
dispersion.
Fig. 7 UV-VIS spectra of the printed 40 wt% polyurethane films with
and without the addition of 0.5 wt% Disperse Red.
242 | Soft Matter, 2007, 3, 238–243 This journal is ß The Royal Society of Chemistry 2007

tall structures can be rapidly produced using only a few
printing passes. A possible drawback in using this dispersion
could be the 5 minute drying time that was used for each
printed step. However, this could be reduced or even removed
if the dispersion was printed at higher temperatures. Finally,
by the simple addition of a water-soluble dye, colour gradients
were produced using this printing technique. As expected, a
more intense hue was seen in those areas that contained more
layers. This suggests that this polyurethane dispersion could be
used in the preparation of colour filters.
Acknowledgements
The authors would like to thank BASF-AG for their
stimulating discussion and financial support.
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