TiO2 Impact on Paint Weather Resistance
September 2017 www.coatingsworld.com Coatings World | 145
Qualicoat standard, using Xenon light exposure (ISO 16474-2).
Requirements for both standards are shown in Tables 1 and 2.
Results and Discussion
During the last decade, Chemours has evaluated 10 different industrial coating systems, including over 20 different TiO2 types.
There were seven polyester coil coat systems, one primid cross-
linked polyester powder coat system, one refinish polyurethane
coating and one high-bake melamine polyester automotive
topcoat system. All these systems were evaluated with QUV-B,
Xenon exposure and Florida exposure. This is the ideal basis for
a comparative study per both standards.
In Figure 1 the correspondence between QUV-B exposure
and Florida exposure is shown. According to GSB, 300 hours
should be an equivalent measure for one year of Florida exposure (standard), and 600 hours for three years of Florida
exposure. The former gives a reasonably good correlation, but
there are already some TiO2 pigment responses that do not correspond (Figure 1a). Longer exposure leads to much less correlation (Figure 1b). Figure 2 shows the correspondence between
WOM and Florida, per Qualicoat. The correlation gets worse
with longer exposure here as well (Figure 2).
This general overview suggests that one must be careful with accelerated weathering. Based on QUV-B, 70 percent of the coatings
would qualify for GSB standard class, the same based on Florida.
QUV-B would only qualify 50 percent for Master and 23 percent
based on Florida exposure. For Class 1 Qualicoat, almost all systems would qualify based on WOM and Florida. Only 40 percent
would qualify for Class 2 based on WOM, but much less ( 27 percent) would qualify for Class 2 based on Florida. From looking at
the graphs, one can see not only is the number of qualifications
less after Florida exposure, but that the types of paints that qualify under Florida can vary from those qualified under accelerated
methods. Certain paints would qualify under WOM and not under
Florida, and the reverse is also true. One can certainly draw the
wrong conclusions when only using accelerated weathering, ruling
out systems which might qualify in real-life weather exposure.
The influence of TiO2 is demonstrated in Figures 3a, b and
c. Here a primid crosslinked polyester powdercoat with 33 percent TiO2 load was evaluated with different TiO2 types. Twelve
different TiO2 types (chloride and sulphate) were evaluated.
All coatings were exposed in duplicate under QUV-B, Xenon
and Florida and evaluated according to GSB and Qualicoat.
Although the same resin was used in all coatings, a different
performance could be seen after Florida exposure. The most durable coats were obtained with so called “super durable” TiO2
grades (C2, C3 and C10). The least durable coats were obtained
with less durable TiO2 grades (C0, S2, S4 and S5). This difference is obvious after 3 years Florida but not, or hardly, visible
after WOM or QUV-B exposure.
Conclusions
It is clear that common accelerated artificial weathering methods
can lead to the wrong conclusion, especially when going to the
most durable paint systems. Paint producers must be cautious
when basing customer performance warranties on the results of
accelerated testing alone. It is also clear that TiO2, as an ingredient, plays a role in enhancing durability of white paints. However,
testing this in an accelerated way with artificial light is difficult
and can be inaccurate, especially for long-lasting systems. Super
durable TiO2 grades are designed to give optimal protection
against UV light. This is confirmed in paint studies using Florida
exposure, the most realistic and reliable test method. Conclusions
using accelerated studies must be treated with sufficient care. CW
Figure 3a. 1000 h QUV-B – superdurable PE powdercoat.
Figure 3a. 1000 h QUV-B – superdurable PE powdercoat.
Figure 3c. Superdurable powdercoat – Florida exposure.
