AFM measurements show that formulations 10 and 15 are
very tacky. We categorized the results into high adhesion (> 6
mN/m) and low adhesion (< 6 mN/m). That way, we can divided
the samples 20-65 into two groups:
(A) high hydrophobic adhesion 40, 40w, 50w, 55, 55w, 65
(B) low hydrophobic adhesion 20, 20w, 25, 25w, 30, 30w,
35, 35w, 45, 45w, 50, 65w
For most of the paints the ranking between high dirt pickup
and hydrophobic adhesion is fairly good. Only formulation 40
does not follow the rule. A simplistic explanation could be that
that the dirt pickup seen by eye is mainly linked to carbon black
giving the black appearance changing the lightness value L before
and after contamination. Carbon black is hydrophobic in nature
and that would then agree with the adhesion strength of a hy-
drophobic colloidal probe.
Anti-soiling effect due to harder surface induced
by nanoparticles
To evaluate the possibility of decreasing dirt pickup of coatings
with the help of surface hardening nanoparticles, alkyd binders,
acrylic binders and mixtures thereof we enriched with hydrophilic (Bindzil 40/220, average diameter 12 nm) or am-phiphilic (CC40, epoxy silane modified, average diameter 10 nm)
silica nanoparticles. The addition of the nanoparticles had a
strong influence on the microstructure of the coatings, as shown
in Figure 4.
Furthermore intendation tests using a microintender show
that coating films containing silica particles are harder than films
without particles. Another feature is that keeping the intender at
a fixed load results in a bigger creep in the latex coating compared to the coating containing silica nanoparticles. The impact
of 25 wt% nanoparticles on the surface properties of a latex
coating is shown in Table 1.
Microstructure of Plain Coating (top) and with added Silica
Nanoparticles (bottom).
Table 1: Hardness of a coating with and without silica using microindentation technique.
The above data gave the impetus of the idea that addition of
silica nanoparticles in a commercial paint might reduce dirt
pickup of the paint. Paints for dirt pick-up evaluations were formulated with 3 wt%, 6 wt%, and 12. 5 wt% silica nanoparticles
according to Table 2.
Table 2: Composition of Paint Formulations with Silica Nanoparticles.
We investigated commercial paints based on alkyd binders,
acrylic binders and mixtures thereof. We made two types of mod-
ification of the commercial paints:
(A) addition of 10 nm silica particles (40/220)
(B) addition of silane-modified 10 nm silica particles (CC40).
The latter type of modification makes the particles more am-phiphilic in character compared to the hydrophilic particles.
The previously described laboratory dirt pick-up test was
modified to include a heating step at 50 °C for 1 h after each
contamination cycle. In this way inclusion of dirt into the paint
due to binder movement is included in the result. A representa-
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