Abstract

Titanium dioxide pigments play an important role in protecting white and light colored paints against the influences of weathering. Careful pigment design enables the best protection and guarantees maximum coating lifetime. During the development of pigments and paints it is important to study and measure weathering influence in the most representative way. In this paper, a comparison is made between accelerated methods for studying weathering and outdoor exposure of several industrial paints. The impact of titanium dioxide on weather resistance, as well as the correlation between the testing methods, is evaluated and discussed.

Introduction

Titanium dioxide is a well-known white pigment; but beyond its exceptional whitening power, it can also increase the durability of a paint. TiO2 has a positive effect on the weather resistance of a coating due to its UV absorbing capacity. TiO2 absorbs UV light, which provides protection from UV degradation to the underlying resin molecules. After UV absorption, however, the energy of the UV photon must be transformed to another form of energy. In the clear majority of UV light absorption events, the energy absorbed by the TiO2 is changed into heat, without damaging the paint film. However, the UV light energy is sometimes changed into chemical energy in the form of chemical radicals. These radicals form on the TiO2 surface, but are mobile enough to travel to resin molecules, where they initiate a series of degradation reactions that ultimately lead to film failure. This process is called photocatalysis. Proper treatment of TiO2 pigments can reduce photocatalysis to a great extent, thereby turning the pigment into an efficient UV protecting agent without the negative photocatalysis effects. Such TiO2 grades are referred to as “durable” grades. The efficiency of this treatment can vary between 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 TiO2. Secondly, one can increase the amount of energy per photon by using more energetic light, such as UV-B light. A third method is increasing the number of photons, or light intensity. This is done using weatherometers with Xenon lamps (WOM).

Trying to accelerate the degradation is risky, since the different degradation reactions do have varying dependence on light intensity. The photocatalysis reaction changes with the square root of the light intensity, whereas the direct degradation of the resin is directly proportional to the light intensity. This implies that usage of high intensity UV light will increase the direct resin degradation more than the photocatalysis. Based on this, we can already assume that accelerated methods do not necessarily correlate well with true weather exposure.

The best way to accelerate the weather exposure is natural accelerated exposure (EMMAQUA).  In a typical exterior exposure, panels are attached to racks with the painted portion facing the sun.  In the natural accelerated test, panels are mounted facing away from the sun and towards a bank of mirrors.  The mirrors reflect the sunlight onto the panels.  This concentrates the sunlight, increasing its intensity by a factor of 10 or more.  The advantage of this type of exposure is that the balance between the different types of UV light is maintained at the same level as it is found in sunlight, and so we need not be concerned with unnatural reaction pathways that are initiated by UV-B or UV-C light. This accelerated test is however not yet well established in international standards and will not be further included in this study.

Now that we understand the different influences at play when trying to accelerate weathering, let us look at some examples of weatherability testing, and how TiO2 plays a role.

Experimental Methods

Having a standard method for measuring/assessing durability is one of the challenges in the coatings industry. Not every paint application has weathering standards. This study uses two internationally recognized standards for the construction industry. The first one is the GSB standard (GSB AL 631), using QUV-B as accelerated method (DIN EN ISO 11507). The second one is the 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 crosslinked 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.