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When last we visited this topic in 2015 through UL Prospector Knowledge Center[i], titanium dioxide was addressed as a pigment very well characterized and notably, the most important hiding pigment for paint. It was also an introductory article, and it was to educate anyone who wanted a general overview of the pigment. It explained that with nano TiO₂, due to its ability for self-cleaning, TIO₂  was finding use in applications in high-rise buildings for windows. In the last few years, it has been the target of regulation and formulation. Before we address the new topics, some review of the pigment.

Titanium dioxide, is also known as titanium (IV) and is the naturally occurring oxide of titanium, chemical formula TiO₂; CI 77891. It has many uses outside of paint, which include food, paper coatings, plastics, cosmetics, printing inks  and others. It is also used in cosmetics, food and feedstuff, textiles, rubber and pharmaceuticals. It is primarily obtained from ilmenite, rutile and anatase, but also occurs as brookite

There are two main processes for TiO₂: In 1916, the Titanium Pigment Corporation of Niagara Falls, New York and the Titan A/S, of Norway simultaneously began commercial TiO2 production with Sulfate Process.  In 1951 DuPont introduced the Chloride Process.

European Union Regulations – February 2020

The European Union has published the regulation to classify titanium dioxide as a suspected carcinogen by inhalation. The European Chemicals Agency’s Committee for Risk Assessment (RAC) announced June 9, 2017, which TiO2, a substance commonly used in paints, construction materials and other industrial and consumer goods, meets the criteria to be classified as a suspected cause of cancer via inhalation, under Category 2. It will be enforced after more than a year of a transition period.

This followed another European country that recommended cancer labeling for TiO₂. At the time, French regulators argued that the substance is likely a Category 1B carcinogen, meaning it is known to cause cancer in humans. The French regulatory body ANSES sought “harmonized classification” for the substance across the entire EU.

Many in the coatings industry, including the American Coatings Association, oppose the measure. The ACA issued a statement following the RAC’s 2017 announcement asserting that the recommendation is unfounded.  “There is considerable industry concern that the basis for the opinion is flawed and does not inform on risk to humans,” the statement read. “It is important to consider that the risks profiled are not attributable formulated products, like paint, where TiO₂ dust is embedded in the mixture.”

The ACA has repeatedly expressed concerns that all paint products containing the substance could be labeled as carcinogens under the EU’s classification system. The Titanium Dioxide Manufacturers Association, which represents the TiO₂ industry, has also previously issued a statement expressing its disappointment in the recommendation. As previously noted, TiO₂ is used in the food industry, along with powdered sugar in doughnuts and a multitude of other applications. Although Dunkin’ Donuts decided to eliminate the use of powdered TiO₂, was it really harmful?

In the past 20 years, nanomaterials have found greater utilization in many applications. This has been true also for TiO2, where the very small particles are very useful in cosmetics as well as catalytic self-cleaning. In cosmetics, the move away from organic ultraviolet absorbers has led to the use of nano-sized zinc oxide (ZnO) and TiO2. Both are highly effective in absorbing UV-sunlight.

“The scientific evidence is clear: There are no grounds for classifying TiO2 as carcinogenic for humans by inhalation,” said Robert Bird, chairman of the TDMA. “Also, classification would do nothing to increase the level of protection of human health and the environment, which is the whole point of the labelling and classification system.”

The REACH Committee of the European Commission met several times to discuss the matter last year, without coming to a majority decision, which put the assessment in the hands of the Commission now in the fall. The EU executive’s 18 July comments to the WTO come in response to US concerns that the proposed requirements “may be unnecessarily disruptive to billions of dollars in US-EU trade”.

The Commission said while substances classified as category 1 carcinogens have the “most significant consequences” such as bans, those listed as category 2 face “no such significant direct consequences”. It added that there would be “no or minor consequences” with regard to legislation on plant protection products, biocidal products, food additives, contaminants, water and pharmaceuticals.

Regarding other legislation, the use of titanium dioxide could continue under certain conditions, such as: an authorization being granted;  an exemption; or demonstration of safe use. This, the Commission said, would be the case for plastic food contact materials, toys, cosmetics and the EU Ecolabel.

And it confirmed that only the respirable particles of titanium dioxide – with a diameter smaller than 10µm – will be classified as a suspected carcinogen. It also proposed to introduce a note to limit the classification of mixtures to those placed on the market in powder form with a diameter smaller than 10µm if they contain 1% or more of such particles. Liquid mixtures (for example, paints) as well as solid mixtures will not have to be classified, it said.

For such non-classified mixtures, the Commission is proposing the addition of the following sentence on the label: ‘Warning! Hazardous respirable droplets/dust may be formed when sprayed/used. Do not breathe spay or mist/dust.’

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Applications

As stated previously, there are many applications of TiO₂, but paint is by far the largest (Figure 1).

Of all white hiding pigments, titanium dioxide, and specifically rutile, affords the best opacity/hiding (Figure 2).

The refractive index (RI) of air is 1.00. The greater the difference between a pigment and RI of air, the greater is the opacity and therefore hiding of the pigment. There are other paint opacifying technologies that leverage this optical effect as well.

CAS 13463-67-7	Rutile	Anatase	Brookite
Refractive Index	2.73	2.55	2.58
Density, g/cm3	4.23	3.78	4.11

 

TiO₂ does possess the highest opacity of paint pigments, but it is a relatively expensive pigment. “Spacers” are used in paint, so that the TiO₂ pigment particles are less agglomerated and are more readily amenable to optimum hiding.

As mentioned previously, nano-TiO₂ has also found utilization in the remediation of pollutants and deposited materials, on skyscraper windows and other structures not easily accessed. The market for photocatalyst is depicted in Figure 3, while the schematic  in Figure 4 represents what occurs.

It will be curious to see what regulations eventually occur that may affect how paint is made. Eliminating titanium dioxide as an ingredient could have devastating effects.

[i] https://knowledge.ulprospector.com/5187/pc-titanium-dioxide-introduction/
[ii] ricochetscience.com
[iii] Grandviewresearch.com
[iv] Blog.agchemigroup.eu

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