TiO2 Impact on Paint Weather Resistance
September 2017 www.coatingsworld.com Coatings World | 143
different TiO2 pigment types and can lead to important differences in a coating’s weather durability.
Super durable grades have a layer of silica, alone or in combination with other materials, on their surface that prevents these
radicals from forming. This silica layer is applied by the TiO2
manufacturer during pigment production. The fact that different grades of TiO2 have different radical formation rates is reflected by the labeling of TiO2 grades as being “non-durable”,
“durable” or “super durable”. Note that these designations do
not apply to the pigment itself—TiO2 is titanium metal rust and
as such is thermodynamically stable—but rather to the effect
that the TiO2 grade has on film durability.
The formulator has a choice of ingredients when developing
a new super durable paint or modifying an existing one. The
first choice is the correct resin, and super durable paints must
use highly durable resins. Since these resins tend to be quite costly compared to their low-durability counterparts, it is essential
that the formulator select the other ingredients in a way that
maximizes the durability performance and value of the resin.
Using the right super durable TiO2 grade is a critical aspect of
this. To develop and select the most durable paint, weathering
tests are essential.
Accelerating Paint Weather-Resistance
Measurements
Studying the weather resistance of a coating can be a complex matter. The best and most reliable method for studying
the weather resistance of a coating system is outdoor exposure
over several years. However, during the development of a paint
system, it is often necessary to assess the weather resistance
in a much shorter time frame. Therefore, different accelerated
weathering techniques have been developed. Before we compare some of these, it
is important to understand the complexity
of the weathering process. There are different ways a coating can degrade during
exposure to weather. In this study, we limit
the discussion to white paints.
1. One pathway is direct degradation
of the resin which is related to the effect of direct UV light. This degradation will mainly occur at the surface
in a pigmented system.
2. A second degradation mechanism is
the photocatalytic degradation related to the photocatalytic activity
of TiO2. Reactions with free radicals
produced at the surface of the TiO2
will occur in the vicinity of the TiO2,
because of its photocatalytic activity. Degradation will mainly occur
at, or close to, the surface here also,
because UV light is absorbed and
cannot reach beyond the surface of
the coating.
3. A last degradation pathway is temperature. Change in
temperature can cause different kinds of damage depending on the paint system. These include color changes, adhesion failures, or cracks. Increased temperature will also
accelerate many chemical reactions. We will not discuss
thermal degradation because its rate is independent of the
grade of TiO2.
Since photocatalysis is one of the mechanisms which can cause
degradation of a paint system, one can assume that reduced photocatalysis will, at least partially, slow down the weathering degradation. So, measuring the photocatalytic activity of TiO2 will
be a measure of the weather resistance of a coating system containing such pigments. This can be done by measuring the effect
of TiO2 on the light degradation of a simple organic molecule
like isopropanol. This is an over-simplified method, since in a true
paint many more complex reactions are taking place.
An elegant way of predicting the photocatalytic activity of
TiO2 is the measurement of the encapsulation efficiency of the
pigment. For silica-encapsulated TiO2, this can be done by measuring acid solubility.
However, this test only measures one aspect of degradation, and
will not always be an accurate prediction for final paint degradation.
The only way to get a realistic idea about the true weather
stability of paint is to do a lengthy outdoor weathering study of
the pigmented paint. Throughout the years, different methods to
study degradation in an accelerated way have been developed.
Since the degradation is caused by UV light (energy), there are
different ways to accelerate it by increasing the rate of energy
addition. There are three methods to do so. The first: increase the
temperature. However, as previously stated, this does not affect
TABLE 1: Weathering requirements per GSB.
Quality Standard Master Premium
Accelerated weathering UV-B (313 nm) UV-B (313 nm) UVB (313 nm)
Test duration 300 h 600 h 1000 h
Residual gloss Min 50% Min 50% Min 50%
Florida weathering
Months 12 36 60
UV energy (MJ/m²) Max 300 Max 840 Max 1400
Residual gloss Min 50% Min 50% Min 50%
TABLE 2: Weathering requirements per Qualicoat.
Quality Class 1 Class 1.5 Class 2 Class 3
Gloss( 60) Gloss( 60) Gloss( 60) Gloss( 60)
1000 h WOM (ISO 16474-2) Min 50% Min 75% Min 90%
1 yr. Florida Min 50% Min 65% Min 75%
2 yr. Florida Min 50% Min 65%
3 yr. Florida Min 50% Min 80%
7 yr. Florida Min 55%
10 yr. Florida Min 50%
