tive chart for the impact of silica amount is shown in Figure 5 for
an alkyd coating. Generally the dirt pickup is reduced with
higher amounts of silica present in the formulation. Note that
all formulations are painted the same day as they are produced.
This is important to be aware of since hydrophilic silica would
flocculate the paint upon storage.
Figure 6 shows a summary of the results after three contamination cycles for different binder systems without silica particles and with 12. 5 wt% CC40 or 40/220. For almost all paints
the dirt pickup is lower for silica-modified paints compared to
the commercial paints. This indicates that the change in microstructure and harder surface reduce the dirt pickup.
Figure 5: ΔL Values for an Alkyd Coating after Three Cycles of Heated Contamination with Standard Dirt with CC40 (◊) and 40/220 (●).
panels have been distributed at random over the rack. The rack
is placed 45° facing south to increase paint degradation by sun
light. The results of this weathering experiment are shown in Figure 8. From these data it is clear that the silica nanoparticles formulated into the paints help to reduce dirt pickup in three of the
four tested paints. This finding can provide the possibility for reduced dirt pickup by reformulation of the commercial paints
available today.
Figure 8: ΔL results of Outdoor 9 Months Weathering Experiment of 4 Different Paint Formulations Without Silica Particles and With 12. 5 wt% CC40 or 40/220.
Figure 6: ΔL Values after one cycle (light grey), two cycles (dark grey) and three cycles (black) of heated contamination with standard dirt on different binder systems and 12. 5 wt% silica additve.
Anti bio-fouling
Another important issue for coatings is so called bio-fouling –
fungi and other micro- and macro- organisms grow on or into
the coating – degrading and uglifying the surface, as shown in
Figure 9.
74 | Coatings World
Figure 7
Sample Rack at the Test Field of “SP
Trätek” in Bogesund.
To further investigate the correlation
between silica particles and dirt pick-up
we placed 9 replicas of
each paint and silica-modified paints outdoors for 9 months in
Bogesund, Sweden, as
shown in Figure 7.
The painted wood
Classical solutions to avoid growth of fouling species on land
under water cannot longer be used due to legal restrictions. For
example tributyl tin (TBT) was banned in 2003 for marine ap-
plications. TBT is poisonous to several marine species but very
effective for antifouling purposes. In Europe, there are several
new regulations coming up and one of the toughest is the biocide
directive BPD. The consequence of the biocide directive is that
the most efficient substances against mold and algae are being
phased out. The substances that are allowed are efficient but leak
out to quickly from today’s coatings leaving it susceptible for bi-
ological growth including mold growth. Several strategies to
avoid or minimize fouling can be thought of. Here we will limit
Figure 9: One sample with (to the left) and one sample without novel biocide (to the right) left for three months under water at the Swedish west coast. Courtesy: Hans Elwing, Gothenburg University.
www.coatingsworld.com
April 2012
