Achieving high performance with titanium dioxide

There are many grades of titanium dioxide to select from which vary in characteristics such as particle size, coating type, crystal structure and purity. These differences in quality may dramatically influence the performance of the final formulation. In the following, an overview of the most important criteria to consider will be given in order to better differentiate the grades of titanium dioxide available for sun care product developments.

Influence of quality

The quality of titanium dioxide and its coating has a big influence on the stability and the performance of the final sun care formulation. Therefore, several aspects of quality can be assessed and compared by different test methods, as described below.

Crystal structure of the titanium dioxide core
Titanium dioxide exists in nature in three different crystal modifications: rutile, anatase and brookite. In personal care, both the anatase and rutile forms can be found; and in some cases, due to the manufacturing process, the resulting products are a mixture of crystal types. However, particularly in sun care applications, photostability is an issue and the crystal structure should be selected with care. As the rutile crystal structure is the most compact, it is more photo stable than the others and is therefore the best choice for sunscreen applications.

Influence of impurities
Another important quality aspect is the purity of the titanium dioxide. Coming from natural sources like Illmenite ore, titanium dioxide can retain certain impurities, such as iron, if it is not processed properly. Even low levels of transition metal ions such as iron can negatively influence the colour stability of a final formulation or create incompatibilities with other ingredients, like Butyl Methoxydibenzoylmethane (BMDBM). BMDBM is known to form complexes with transition metals, and with iron ions. BMDBM forms a complex which discolours the formulation red. As it remains the most effective UVA filter available, BMDBM is found in most sun care formulations and therefore a suitable titanium dioxide with a high purity should be used.

To easily identify qualities which are unsuitable, the following test procedure can be used: 10% of the commercial available titanium dioxide is dispersed in 85% caprylic/capric triglyceride. Then 5% BMDBM is added and thoroughly mixed. Twelve hours later (ambient temperature storage) a change in colour can be seen if the quality of titanium dioxide is not adequate. Figure 1 shows the obtained colours with different grades of titanium dioxide according to the described test procedure. The silica and dimethicone coated titanium dioxide (sample 5) showed very good compatibility with BMDBM.

Right choice of coating material
Titanium dioxide is usually micronised in order to improve UV absorbing performance and reduce whitening on the skin. However, uncoated, micronised titanium dioxide can be reactive and has the ability to act as a photo-catalyst. Such reactivity either in the formulation or on the skin once applied is obviously not desirable. Therefore, titanium dioxide should be well coated in order to avoid any direct contact of the titanium dioxide surface with the surrounding environment. The most often used coating material is alumina or aluminium hydroxide. Unfortunately, the aluminium coating can interact with other cosmetic ingredients ultimately leading to formulation instability. For example, aluminium will form complexes with BMDBM. These Al-BMDBM complexes, while colourless, are fairly insoluble in the oil phase and can lead to crystal formation. The crystals can become large enough to be felt on rub-in and will ruin the aesthetics of a formulation. More importantly, the crystallisation of the Al-BMDBM complex leads to a significant loss in UVA performance.

Figure 2 shows the kinetics of the formation of the Al-BMDBM complex at different temperatures. Whereas pure BMDBM crystals appear due to oversaturation (especially at low temperature) and often have the shape of needles, the Al-BMDBM crystals have a different shape and develop more at elevated temperature. Figure 3 shows the typical shapes of either Al-BMDBM crystals or pure BMDBM crystals in over-saturated formulations visualised under polarised light. A better alternative to an alumina or aluminium hydroxide coating is silica together with a second organic coating material such as dimethicone. The double layer ensures the completeness of the coating and minimises any incompatibilities in the formulation. Such a coating will not react with BMDBM and is fully compatible with all UV filters. The hydrophobic outer coating also allows for easy dispersibility in the oil phase.

A further aspect is the performance in UVA protection of silica/dimethicone coated titanium dioxide compared to alumina/aluminium hydroxide coated TiO2 in formulations containing BMDBM. Due to the above described complex formation and subsequent formation of Al-BMDBM crystals in the formulation, the protection in the UVA-range can be dramatically reduced. Figure 4 shows the absorption curves of both titanium dioxide grades after 12 weeks at 43°C in a formulation containing 4% BMDBM and 10% TiO2. A clear reduction in UVA absorbance can be noticed for the aluminium hydroxide coated sample.

Comprehensive coating
As mentioned above, a coating is necessary to avoid any undesired catalytic reaction in the formulation or on the skin. It is quite challenging to obtain a coating which fully covers the surface of such micro particles. Therefore big differences in the quality and integrity of coatings can be observed among commercially available titanium dioxides. One quick and simple test that can be performed to demonstrate the comprehensiveness of the coating is to expose the titanium dioxide to ascorbyl palmitate in an oily dispersion. When ascorbyl palmitate is brought together with a catalytic surface in an environment with dissolved oxygen, the ascorbyl group is quickly oxidised and forms brown reaction products. This colour development can be easily followed by visual assessment.

Figure 5 shows a typical series of commercially available coated titanium dioxide at 10% dispersed in 88% caprylic/capric triglycerides together with 2% ascorbyl palmitate. The photo was taken two hours after mixing all ingredients and clear differences in colours can be observed. The titanium dioxide grade with silica and dimethicone as coating (sample 5) shows almost no discoloration. This test is a short-term test – the colour development has to be assessed in the first hours. Over time, a browning in colour will occur in any sample, coming from the inherent oxidation of ascorbyl palmitate itself.

While the test above works in the dark and demonstrates catalytic activity, the titanium dioxide surface can also act as a photo-catalyst. This is especially undesirable for use in sun care products due to possible photo-catalytic reactions on the skin. Therefore, it is very important that photo-catalytic activity of the chosen grade is as low as possible. A possible way to measure photo-catalytic activity of coated titanium dioxide is shown in Figure 6. This method utilises the principle that an uncoated titanium dioxide surface photocatalyses the oxidation of 2-propanol into acetone and further into carbon dioxide. Therefore, a sample of 20 g of titanium dioxide is placed onto a Petri dish in a photo reactor. Then 2-propanol is injected into the gas phase of the reactor and the sample is irradiated with UV light. The degradation processes of 2-propanol and the formation of acetone and carbon dioxide is monitored by on-line FTIR measurements.

Figure 7 shows that the titanium dioxide coated with silica and dimethicone (sample 4) exerts almost no photo-catalytic reactions.

Performance

In addition to all the quality aspects of the titanium dioxide which are crucial for the stability of the final formulation, the performance as a UV-absorber is equally important. Several characteristics can influence this performance, such as particle size, amount of titanium dioxide in relation to coating material, dispersibility in the final formulation, etc. Furthermore, aesthetic aspects such as the transparency on the skin should be optimised while maintaining the quality and performance aspects. It is possible to test the efficiency of such a UV-filter in a sun care formulation by measuring the in vivo SPF of formulations with different grades of titanium dioxide. The next section of the article shows some concept formulations with different UV-filter combinations containing titanium dioxide.

Titanium dioxide particles not only absorb UV-light but also reflect it due to the particle nature. The larger the size of the particle, the more pronounced the appearance of titanium dioxide is on the skin, often described as a whitening effect. Such an effect is in most cases unwanted. In addition to the particle size, the ability to disperse the particles efficiently in the oil phase can impact the transparency of the final formulation. High shear mixers as well as an appropriate second coating of the titanium dioxide are key factors to achieve a good dispersion in the sun care product.

Sun care formulation concepts with titanium dioxide

In this section of the article, some concepts are given for different applications in sun care. They are designed to provide ideas as to how such concepts could look and can be used as a starting point for developing new sun care products.

Efficient sun care formula SPF 30
Most of the SPF 30 products available in the market contain titanium dioxide in order to achieve such a high protection factor. Table 1 shows two formulations which have a measured in vivo SPF well above 40 with both a ratio of 1/3 UVA protection. Formulation A is a cetyl phosphate based o/w emulsion while Formulation B is a w/o emulsion with polyglyceryl-2 dipolyhydroxystearate as emulsifier.

Both formulations demonstrate excellent performance and are highly efficient as can be realised by the high SPF which is achieved with only 15% and 17.6% UV-filter content respectively. These formulations are very attractive both from a cost perspective and from the sensorial aspect due to relatively low loading of filters.

Ultra high protection SPF 50/50+
There is a constant demand for ultra high protection formulations with a measured in vivo SPF of 50 or 60 (labelled as 50+). Such high SPFs need the addition of several UV filters as well as the use of titanium dioxide. Based on a synergistic combination of BMDBM, polysilicone-15 (Parsol SLX) and phenylbenzimidazole sulfonic acid (PBSA) together with titanium dioxide and one or two other UVB filter, we were able to achieve an in vivo SPF of >50 and >60 respectively. The UVA protection factor of this patented combination was in both cases above the necessary 1/3 of the labelled SPF and therefore these formulations fulfil the new European standard.

All-mineral sun care products
There is a constant trend to more natural and less chemical ingredients for cosmetic products. When it comes to sun protection formulations there is quite a big gap between what is allowed for a “bio/green” certificate and what is normally used to absorb the harmful UV light. Only mineral filters such as titanium dioxide and zinc oxide are presently categorised as “natural UV filters”. By combining these two filters, it is possible to develop natural, all-mineral sunscreens with both UVB and UVA coverage. As it is difficult to achieve sufficient UVA protection levels for SPFs over 20, it is recommended that the main application of such “all-mineral” concepts would be in day care or facial care protection products.

Conclusion

There are many different grades of titanium dioxide available on the market. Although all of them will have the name “titanium dioxide” in the ingredient listing of the final product, there are huge differences regarding coating, performance, incompatibilities, dispersibility, and many other relevant factors in sun care products.

This article lists the most important criteria to identify a good and easy-to-use grade of titanium dioxide which delivers a high level of in vivo performance as well as no short-term and long-term incompatibilities. Through the several tests performed, the importance of a complete and non-interactive coating is evident to prevent discoloration of formulations and other undesired reactions. As well, the negative influence of the alumina/ aluminium hydroxide coating due to the complex formation with BDMDM was demonstrated. One product, the titanium dioxide grade with a silica and dimethicone coating, has shown very good results in all of the described tests and in sun care concepts for high protection SPF products. This product provides the formulator a solution for use in modern formulations.

ABSTRACT
For a formulator who needs to choose an appropriate grade of titanium dioxide for a sun care formulation, it is important to know the key selection criteria for the products available on the market. In order to support such formulators, this article gives an overview of different test methods and shows comparative results between several titanium dioxide grades. The focus of the evaluation was on the overall quality of the products, the completeness of the coatings and the performance with respect to the Sun Protection Factor (SPF) and a proper UVA-balance.