HiTone Resins for Enhanced Pigment Affinity of Powder Coatings
by Eric Blokhuizen
Jurjen Bolks
Stefan Brendeke
Paul Vercoulen
November 1, 2008
Powder coating manufacturers are increasingly
searching for tools to improve their cost performance balance. Solutions that
can lead to cost reductions are, for example, the use of higher filling levels,
or applying thinner layers. However, such solutions could result in a decrease
in properties like flow, gloss and hiding power. Using higher-level epoxies,
flow additives and pigment additives can overcome these kinds of problems. On
the other hand, it does cause adverse side effects and can lead to cost
increase, yellowing and a decrease in powder stability.
At higher pigment levels the appearance of
powder coatings becomes worse. High pigment levels are required to reach a
desired hiding power, especially in thin-layer applications. The difference in
appearance between a standard and a highly filled system can be seen in Figure
1.
Resins are developed based on a special concept, the
HiTone concept, to overcome flow and appearance problems at higher pigment
levels. The purpose of this study is to achieve a better pigment affinity of
resins without compromising on important properties like flow, gloss and hiding
power. As already mentioned, the pigment affinity can be improved by using the
HiTone concept. This paper discusses the effect of the solution found on the properties
of pigmented powder coatings. Besides the appearance, it is expected that the
extrusion and milling behaviour of powder coatings with a higher pigment level
will be affected. The effect of high-filled systems on milling behaviour is
also discussed.
Experimental
A standard 70/30 hybrid resin and two 70/30
hybrid resins based on the HiTone concept (Uralac® P 770 and Uralac
P 772) were synthesized. Uralac P 772 is a faster-curing version of Uralac P
770. Furthermore, a standard 60/40 hybrid resin and a 60/40 hybrid resin based
on the HiTone concept (Uralac P 760) were synthesized.
The properties of the 70/30 hybrid resins are
given in Table 1, and the properties of the 60/40 resins are given in Table 2.
The resins were tested in formulations with 33 and 50 wt% pigment. For
these tests, TiO2 pigment (Kronos® 2310) was used.
Typical 70/30 and 60/40 hybrid formulations of the 33 and 50 wt% pigment loaded
systems are given in Table 3.
Powder coatings were prepared by extrusion of the formulations using a
Prism TSE 16 PC twin extruder with 16
mm diameter screws and L/D 14. The extrusion temperature was set at 120 °C, the
extrusion speed at 200 rpm, and the torque was kept between 75 - 90%. After
extrusion, the extruded material was ground by a Retch ZM 100 centrifugal mill
and sieved through a Retch Vibro sieve with a mesh of 90 µm. Finally, the
powder coatings were sprayed electrostatically onto steel panels (Q-panel S 46)
with a GEMA PG1 spraying gun and placed vertically in a UT 6120 hot-air oven
from Heraeus. Coatings with a layer thickness of 60 µm were used for
evaluation.
Evaluation
Flow was visually evaluated on coated panels and compared to PCI
standards. In addition, the flow was evaluated with a white light
interferometer from Veeco. This device is able to rapidly measure a 3D scan of a large surface area
(e.g., 2x2 mm2 with a single scan) with a height resolution of a few
nanometers and a lateral resolution of 600 nanometers. By using an automatic XY
table and appropriate “stitching” software, the interferometer was able to
measure even much larger samples (e.g., 10x2 mm2).
Coating flexibility was tested via reverse impact, according to test
method ASTM D 2794. Coated steel panels were subjected to an impact of 160
inch-pound on the back of the panel. Also ALMg3 panels were coated and cured
for 10 minutes in a BYK-Gardner gradient oven between 150 - 250 °C. These
panels were subjected to an impact of 160 inch-pound. Gloss and haze were
measured at 20° and 60° with the BYK Gardner reflector meter, according to the
standard ASTM D 523.
Heat resistance was determined by placing a cured panel in the oven at 240 °C
for 10 minutes and 220 °C for one hour. Colour differences between an untreated
panel and the treated panel were evaluated.
Salt spray tests were conducted on a cured panel with a vertical scratch of 1
mm, according to ASTM D 2803. The cured panel was placed in salt spray
equipment for 500 hrs. After 500 hrs, the average creepage from scratch was
measured. The powder and coating Tgs were determined by differential
scanning calorimetry (DSC) at a heating rate of 5 °C/min.
Results
Haze
Haze values are an indication of the appearance. In general, if the haze
value is low the appearance is good; if the haze value is high the appearance
is bad. Haze values of the final coatings were measured by the BYK Gardner
reflector meter. Haze measurement results are shown in Figure 2 for different
pigment concentrations. A comparison between standard resins and the new,
developed resins, Uralac P 770, P 772 and P 760, was made.
The standard resins showed low haze values up to a concentration of 40-45 wt%.
Above a concentration of 45 wt% pigment, an increase in haze was shown.
Conversely, Uralac P 770, P 772 and P 760 showed much lower haze values up to a
concentration of 55 wt% pigment versus the references.
Summarizing the above-mentioned experimental
results, it can be stated that compared to standard resins, HiTone resins can
be much higher filled with titanium dioxide without compromising haze
properties.
Gloss
Gloss is also an indication of the appearance. In general, a high gloss
value indicates a good appearance. Gloss values were measured by the BYK
Gardner reflector meter. Figure 3 shows a comparison of the gloss values
between standard resins and the HiTone resins (P 770, P 772 and P 760) at
different pigment concentrations. In this figure, gloss 20° is plotted as a
function of the pigment concentration.
As can be seen, coating gloss, based on standard resins, starts to decrease at
a pigment concentration of 40 wt%. On the other hand, gloss decrease at higher
pigment loadings was much less for systems based on the HiTone resins. The
results on gloss and haze indicate that the pigment affinity of HiTone resins
is much better than the pigment affinity of standard
resins.
Flow
Flow depends on many factors. In general, flow will improve if the
reactivity of the system is lowered. Also other parameters such as viscosity
and surface tension can influence flow. It is expected that flow will improve
at higher pigment levels if the pigment affinity of the resin is improved. Flow
results at different pigment concentrations are shown in Figure 4. A comparison
was made between standard resins and HiTone resins. The flow was measured by
comparing standardized PCI panels with coated panels made with HiTone resins or
standard resins.
The flow at lower pigment concentrations was comparable, but at higher pigment
concentrations differences in flow were observed. HiTone resins showed better
flow than standard resins at higher pigment concentrations. This indicates that
the improved pigment affinity of HiTone resins improves the flow properties,
especially at higher pigment loadings.
In addition to the flow measurement with PCI panels, the leveling of the
powder coating surface was evaluated with the white light interferometer. White
light interferometer pictures were taken of panels with a pigment concentration
of 50 wt%. Pictures of the white light interferometer are shown in Figure
5.
As can be seen from Figure 5 the Uralac P 770 levels better than the standard resin
at a pigment concentration of 50 wt%. According to the flow results, HiTone
polyesters showed a better flow at higher pigment levels than standard
polyesters. It is clearly demonstrated that the use of the HiTone concept
improves the flow ability of highly filled systems.
Milling Behaviour
To determine the influence of high-filled systems on the specific power
consumption during grinding, grinding experiments were performed on an
industrial scale. The specific power consumption is expressed by the required
electrical power per kg of product to mill the product to a defined particle
size. For the grinding experiments Uralac P 770 was tested over the full range
of pigment concentrations. Also Uralac P 760 with 50 wt% pigment was tested.
Because smaller particle sizes are needed for thin-layer applications, the
difference in specific power consumption between a particle size distribution
(PSD) of an average size D50 of 30 µm and 50 µm was determined. The experiments
were performed on an air classifier mill ACM10 of Hosokawa. Limiting conditions
during grinding were the maximum power consumption of the grinding motor and
the classifier motor and the maximum pressure drop over the system. The effects
of these conditions were especially observed during grinding of the highest
pigment loaded systems to the smallest particle size. To overcome this, the
speed of the grinding disk and the classifier were adjusted. The most relevant
test results obtained are given in Tables 4 and 5.
As can be seen in Tables 4 and 5, the speed of the grinding disk and the
classifier were adjusted for the grinding experiments with 50 wt% pigmented
systems to a D50 of 30 µm. This adjustment was done to prevent
clogging.
The above-mentioned results are graphically represented in Figure 6. The
diagram clearly shows the effect that with increasing pigment content the
specific power increases.
From
Figure 6 the general conclusion may be drawn that with higher pigment loading
rates a tougher milling behaviour is observed, especially in combination with a
small PSD. Nevertheless, the tougher milling behaviour is independent of the
polyester type used and is directly related to the increased pigment content.
It can be concluded that the milling behaviour of HiTone resins is not
different compared to standard resins.
Other Coating Properties
As already mentioned, the purpose of this study
is to improve the pigment affinity of polyester resins without compromising on
important properties. Haze, gloss, flow and milling behaviour have been
discussed. The results of tests done to determine other properties are
presented in Table 6. A comparison was made between a standard 70/30 hybrid,
Uralac P 770 and Uralac P 772, and a comparison was made between a standard
60/40 hybrid and Uralac P 760. Only the results with 33 wt% and 50 wt% TiO2
are indicated.
The
impact resistance was measured on a gradient
panel, which was cured in a gradient of 150-250 °C for 10 minutes. The temperature
at which the coating starts to have a good impact is shown in Table 6.
From the results given in Table 6 it is clear
that properties like chemical resistance, heat stability, powder stability and
salt spray resistance are not affected by using the HiTone concept. Also powder
Tg and coating Tg are comparable. The standard curing
HiTone resins (Uralac P 770, Uralac P 760) are a little bit slower than the
standard hybrid resins but for fast-curing 70/30 hybrid systems Uralac P 772
can be used.
Conclusions and Discussion
In this paper, the use of resins with
improved pigment affinity has been described. Excellent results are achieved by
using the HiTone concept. Properties of coatings made with HiTone resins Uralac
P 770, P 772 and P 760 have been discussed. These resins have been filled up to
50 wt% pigment while maintaining properties like flow, haze and gloss.
Performance on other important coating properties such as powder stability,
flexibility, heat and chemical resistance, powder and coating Tg has
not been affected.
Furthermore,
the milling behaviour of HiTone resins has been discussed. Hitone resins with a
low and high pigment amount have been milled to a D50 of 30 and 50 µm. A
comparison has been made with standard polyesters. HiTone resins showed no
difference in milling behaviour compared to standard resins.
To conclude, with HiTone resins a much
higher pigment loading rate can be reached than conventionally possible, whilst
retaining high aesthetic and mechanical properties. The good results enable a
broader formulation and application window. Possible application areas are for
example, thin layers, domestic appliances and white goods, high-filled primers,
ceiling tiles, heat ventilation and airco, metal housing, metal furniture and
many others. n
This paper was presented at The Waterborne Symposium sponsored by
The University of Southern Mississippi School of Polymers and High Performance
Materials and The Southern Society for Coatings Technology, 2008, New Orleans,
LA.
For more information, visit www.dsmpowdercoatingresins.com.
|
