A Comprehensive Understanding of 'TiO2 Pigment Durability'
While the benefits that TiO2 pigments bring to coatings are significant, there are some situations where TiO2 can be harmful to important properties of the final paint film. Perhaps the most familiar example of this to many paint formulators - particularly those who work in the industrial and/or OEM finishes - is the detrimental effect of TiO2 on paint durability. This effect, commonly referred to as "TiO2 durability" (which is actually a misnomer, since the TiO2 itself is completely durable - and hence the quotation marks in the title of this paper), is well recognized in the coatings industry. What is less well understood is that TiO2 affects paint durability in a number of ways, both good and bad. In this paper we review the many ways that TiO2 can affect paint durability, and how the interplay ultimately determines the longevity of a paint film.
Photocatalytic DegradationTitanium dioxide is an excellent absorber of ultraviolet light, and a single, quarter-micron particle of TiO2 will absorb in excess of 99% of all solar UV radiation that strikes it. The UV light energy is temporarily transformed into electronic energy in the form of an electronically excited TiO2 particle, and then the vast majority of this energy gets quickly converted into heat energy. The heat energy then dissipates from the paint film.
However, for every million light absorption events, anywhere from 1 to 10 will proceed through a different pathway. The energy contained in the excited TiO2 particle is transported to the pigment surface, where it can encounter absorbed water and oxygen. The resulting reaction transforms the energy yet again, this time into reactive chemical energy in the form of hydroxyl and superoxide radicals (Figure 1). These radicals are free to migrate from the pigment surface and react with polymeric resin molecules, ultimately leading to the destruction of the organic component of the paint film.
Photocatalytic degradation of organic binder can be prevented by taking advantage of the fact that it proceeds through a series of chemical reactions - one followed by the next - and that by preventing only one of these reactions, the entire sequence will be stopped. Coatings manufacturers can use additives that prevent the adsorption of UV light by TiO2 (e. g., UV light absorbers) or that prevent the reaction of the photogenerated hydroxyl and superoxide radicals with the binder (e.g., hindered amine light stabilizers - HALS).
While effective, these remedies add cost to the coatings, and the preferred route to reduced photocatalytic activity is for the pigment manufacturers to alter the pigment before it gets into the coating. They do this by completely covering the pigment surface with a layer of inert oxide (silica or alumina), which physically separates the absorbed water and oxygen from the TiO2 crystal (Figure 2).
The Beneficial Effects of TiO2 on DurabilityMost formulators are familiar with the detrimental photocatalytic effect of TiO2 on resin weatherability. However, TiO2 can have a very beneficial effect as well. This is because there is another set of degradation reactions that involve UV light, and TiO2 can affect the rates of these reactions.
This set of reactions, termed direct degradation, involves the direct absorption of UV light by the resin polymer. UV light photons are so energetic that this absorption event often causes weak chemical bonds within the resin to break (e. g., C-H bonds). The yield per photon for these reactions is typically many orders of magnitude greater than the yield per photon for photocatalytic degradations - that is, a UV light photon directly absorbed by the resin polymer is much more likely to cause damage than one that is absorbed by a TiO2 particle.
TiO2 pigment does not participate directly in this second set of reactions, but it does have a dramatic effect on their rates. This is because, as mentioned above, TiO2 is an excellent UV light absorber. By removing the UV component of sunlight, the TiO2 particles shield the binder molecules beneath them from direct degradation. This effectively limits direct degradation to the topmost layer of binder molecules.
A Balancing ActTiO2 has both positive and negative influences on paint film durability. A reasonable question to ask is: which is stronger? That is, does adding TiO2 to a paint improve its durability (due to TiO2 protection of the binder), or hurt its durability (due to photocatalytic degradation)? The simple answer is that for all but the most durable binders (i.e., the ones inherently resistant to direct degradation), the positive influence of TiO2 on durability more than outweighs its negative effects.
A dramatic example of this beneficial effect can be found in the plastics industry. The vinyl used in vinyl siding is typically very susceptible to degradation via direct absorption of UV light, and vinyl siding without TiO2 would last for no more than one or two summers. However, the incorporation of only a few percent TiO2 into the vinyl allows for this product to be warranted for 10, 20 and, in some cases, even up to 30 years. While resin molecules used in paints are not nearly this susceptible to direct degradation, it is true that in most cases the TiO2 pigment has a net beneficial effect, especially when a durable or superdurable grade such as Ti-Pure® R-960 or Ti-Select® TS-6200 is used.
Other ConsiderationsThe effect of TiO2 on paint durability is more complicated than just its effect on resin degradation rates. Paint consumers typically interpret durability in terms of the retention of appearance, and not, necessarily, as a resistance to chemical reaction. While the latter usually influences the former, the degree of influence can vary depending on the grade of TiO2. Said another way, a realistic durability goal for the paint manufacturer is to allow the film to "age gracefully," rather than to completely prevent degradation (which is impossible). By age gracefully, we mean that for a given amount of degradation (as measured by, say, weight loss), the degree to which film appearance changes should be minor. This relationship between amount of degradation and degree of appearance change is found to vary from one TiO2 grade to another.
This is shown schematically in Figure 3 for two paints that differ only in the grade of TiO2 used. In a typical durability study we measure gloss loss as a function of exposure time, but in the idealized graph in Figure 3 we instead follow it as a function of weight loss per unit area. At a given weight loss, Paint A suffers less gloss loss than Paint B. Based on this, a consumer would say that Paint A was the more durable, even though both paints have experienced the same amount of degradation.
What causes this variance, and how can a paint formulator take advantage of it? In the case of gloss, the variance is due to the effect of increasing pigment concentration near the film surface as the resin degrades. As mentioned above, most resin degradation, whether photocatalytic or direct, occurs near the film surface. As the resin degrades it erodes from the paint, primarily by being washed out during cleaning or exposure to rain. The TiO2 pigment is inert and remains behind in the film. The net effect is an increase in pigment volume concentration (PVC) in the immediate vicinity of the film surface (Figure 4).
Paint gloss typically decreases with increasing PVC, and this decrease is especially severe as PVC passes above critical (CPVC). Therefore, any property of the TiO2 or of the paint film that affects the point at which CPVC is reached will also affect the relationship between amount of degradation and degree of appearance change. Paint properties that affect the amount of degradation needed before CPVC is breached include initial PVC (paints formulated near CPVC will show faster gloss loss than paints formulated at lower PVC) and the amount of non-TiO2 pigment and/or filler present. The TiO2 pigment property that most affects CPVC is degree of dispersion. In general, a more poorly dispersed TiO2 (or a more difficult to disperse grade) will have a lower CPVC, and thus show more appearance change with time, than a better/easier dispersing grade. Therefore, the coatings manufacturer should choose, all else being equal, a more easily dispersed pigment for durable paint applications.
Good pigment dispersion also minimizes the rate of gloss loss in another way, in what can be thought of as a "tip of the iceberg" effect. Gloss is essentially a measure of film smoothness on the optical scale - roughly 1/2 micron. TiO2 pigment agglomerates exceed this size, and their presence near the topmost film surface hurts gloss. In the freshly applied paint, this negative effect is minimized by leveling of the resin above the agglomerates. At most, the agglomerates will cause a small bump or ripple on the actual paint surface. However, once the resin begins to degrade, what was once a small bump can quickly become a large surface defect. Initially only the "tip of the iceberg" was affecting surface optics - now the whole iceberg is exposed! If the pigment is initially well dispersed, though, this effect will be minimal since there are no large objects to expose (Figure 5), even if the overall amount of resin loss is the same. So once again we find that all other things being equal, a more easily dispersed pigment will appear more durable (suffer less change in appearance) than one that does not disperse well.