Optimising feel of inorganic sunscreens

Optimising transparency

It is well known that the optical properties of inorganic sunscreens such as titanium dioxide depend critically on particle size.1 Titanium dioxide pigment is highly opaque to visible light, due to its high refractive index. As particle size is reduced, the peak of the attenuation spectrum shifts to shorter wavelengths, giving improved UV attenuation while the visib le attenuation decreases. It has been shown previously that TiO2 can be made transparent to visible light if the particles are small enough.1 Unfortunately, as has been demonstrated,2 such a material is also largely transparent to UVA and UVB light, and so has low efficacy as a sunscreen, leading to something of a dilemma.

This dilemma arises partly from the fact that any sample of a particulate material contains a distribution of particle sizes. Theoretical calculations1 indicate that, to achieve good UVB and UVA protection the optimum mean particle size for TiO2 is 40-60 nm. However, a TiO2 sample which has such a mean particle size, and which has a typical size distribution for commercial grades of TiO2, also includes a significant number of larger particles which interact with visible light and hence cause whitening on skin.

Optimising TiO2 manufacturing process

What is required is a grade of TiO2 which has a mean particle size as close as possible to the optimum for UV protection, but without the larger “whitening” particles. In other words, a narrower distribution of particle sizes is needed. This distribution must then be maintained, as much as possible, through into the final formulation. The particle size distribution of the TiO2 is dependent on the thermodynamics of the process by which the particles are made, and with conventional production methods it is virtually impossible to control the conditions well enough to obtain the required narrow size distribution. However, by using a novel production process based on precipitation from a solution of titanium oxydichloride, the necessary control of particle size distribution can be achieved. Figure 1 shows electron micrographs of conventional fine particle TiO2 and the new material. While both samples contain the small particles required for good UV attenuation, it is apparent that the conventional material also contains a number of larger particles, whereas the particles in the new grade are of a much more uniform size.

Dispersion

In order for the TiO2 to be transparent on skin, however, the optimised particle size distribution must be maintained in the final formulation. To achieve this, the particles must be incorporated into an optimised pre-dispersion. In the dry powder state the particles tend to aggregate, and these aggregates can be difficult to break down in formulation. Figure 2 shows whitening data for W/O emulsions, containing the conventional and new grades of TiO2. The “whitening index” is measured by applying a thin film of emulsion onto a black card, and measuring the difference in whiteness (L-value on the L a b colour scale) of the card, before and after applying the product.

From this data it can be seen that the new grade does indeed give improved transparency (less whitening) compared to existing grades. More importantly, however, it is clear that in order to achieve optimum transparency, the TiO2 must be used in a pre-dispersed form. When it is incorporated into the formulation as a powder, much of the transparency improvement is lost.

Optimising skin feel

A key question here is what does “optimised” mean? Of course this is very subjective, and depends on the preferences of the individual consumer. In the case of the Asian consumer, a key requirement that is often quoted is that products must have a “light” feel. But what does “light” mean in this context? By combining the results of several sensory studies, we were able to arrive at a list of properties which characterise “light” skin feel:

_ Not oily.

_ Not greasy.

_ Not sticky or tacky.

_ No residue.

_ Very good spreading.

When considering a titanium dioxide dispersion, there are three key aspects which could conceivably influence the skin feel: particle size, particle coating (surface treatment), and carrier fluid.

Particle size

A previous paper3 described studies carried out to determine how these parameters influence the skin feel of formulations containing TiO2. Formulations were made which incorporated TiO2 grades of different particle size and size distribution, and these were assessed by a trained sensory panel. No significant differences in skin feel properties were observed between the different grades.

In the same study, the effects of hydrophilic and hydrophobic particle coatings were also considered. Inorganic sunscreen particles often give a “dry” or “draggy” skin feel, and this was most apparent when the particles were coated with other inorganic materials (alumina and silica) to impart a hydrophilic character. However, coating with a hydrophobic (lipophilic) surface treatment (alumina and aluminium stearate) resulted in a more lubricious skin feel, giving a perception of improved spreadability.

Carrier fluid

The emollients used in a formulation exert a very significant influence on the overall skin feel of the product. Therefore, in the case of a sun care formulation which contains a TiO2 dispersion, one would expect that the carrier fluid used in the dispersion to be important in determining the sensory properties of the formulation. When we consider the sensory attributes which contribute towards a “light” skin feel, as listed above, it is easy to understand why silicone fluids, in particular high spreading fluids such as cyclomethicone, have become very popular in Asian sun care formulations. One might expect, therefore, that the ideal dispersion would use a silicone fluid as the carrier, and indeed such dispersions are commercially available. However, a further point that we must also consider is the intended function of the product. In most cases, it is necessary to compromise between the ideal skin feel and product functionality or efficacy, and sun care products are a prime example of this. TiO2 dispersions based on a pure silicone carrier have two significant disadvantages in sun care:

_ Silicone fluids tend to be poor solvents for organic UV filters.

_ To achieve a stable dispersion in a pure silicone requires silicone-based dispersing agents, which are usually only effective in the pure silicone system. This limits the versatility of the dispersion, since incorporating other (non-silicone) oils in the formulation is likely to destabilise the dispersion. Nevertheless, the advantages of silicones, for example in terms of skin feel and water-resistance, make it highly desirable to include a silicone in the dispersion.

A suitable compromise, therefore, is a dispersion with a mixed carrier system, comprising cyclomethicone pentamer and an organic co-emollient, for example an ester. One advantage of this approach is that it facilitates the use of a dispersant (polyhydroxystearic acid, or PHSA) which is far more versatile than silicone-based dispersing agents. The usefulness of this dispersant is evidenced by the fact that inorganic sunscreen dispersions using PHSA are the subject of a number of patents.4

In deciding what to use as co-emollient, the following performance parameters were considered:

_ Solvency for organic UV filters.

_ Moisturisation.

_ Substantivity. _ Effect on skin elasticity.

Also, of course, the chosen emollient should not compromise the skin feel benefits of the silicone fluid.

Based on these requirements, two co-emollients were selected for development work: isopropyl isostearate and propylene glycol isostearate.

Figure 3 shows some of the key skin feel parameters for these emollients, compared to cyclomethicone pentamer, as assessed by a trained sensory assessment panel. Figure 4 shows the clinical parameters for IPIS and PGIS.

All three emollients show high spreadability and very low stickiness. IPIS shows very low greasiness; PGIS is slightly more greasy, but has an advantage over IPIS in terms of residue. IPIS is an excellent moisturiser, and also shows good substantivity, but gives relatively little benefit in terms of skin elasticity. PGIS is exceptional in promoting skin elasticity, and also gives good performance in moisturisation and substantivity.

Optimising dispersion

Having chosen the appropriate TiO2 grade, coating, and carrier systems, the final step is to optimise the dispersion recipe in order to achieve a stable, fluid dispersion with the maximum solids content possible. Dispersion stability is achieved by adsorption of PHSA to the particle surface, forming a stabilising layer around the particle of thickness ä. This steric repulsion prevents close approach of the particles to a separation of <2ä. (Fig. 5). Adsorption of PHSA is by association of its anchoring group (COO-) with the particle surface. In good solvent conditions (e.g. pure co-emollient) the tail group is highly solvated in the carrier fluid, extending the chains and providing good stability. In less favourable solvent conditions (e.g. by addition of a nonsolvent such as CM), the solubility of the tail group is reduced the chains become more coiled and bulky at the surface, restricting the adsorption of further polymer molecules. This reduces the thickness and density of the stabilising layer. If the proportion of non-solvent is too high, the dispersion is unstable. In this case it was found that a 50/50 blend of cyclomethicone and co-emollient was a good compromise, delivering the skin feel benefits of the silicone while ensuring a stable dispersion.

The level of dispersant must also be optimised. If this is too low, the protective barrier is incomplete and particles can flocculate into a three-dimensional structure as shown in Figure 6. Increasing the level of dispersant decreases viscosity as the steric repulsion prevents flocculation. However, if the dispersant level is too high, the size of the stabilisation layer around each particle is increased and the dispersant layers of neighbouring particles begin to overlap, forming a weak repulsive gel.5 This causes the viscosity to increase once again. Therefore, the optimum dispersant concentration can be identified by making dispersions at various concentrations and measuring the residual viscosity. This results in a “dispersant demand curve” which shows a minimum at the optimum dispersant concentration. This is illustrated in Figure 7 for the dispersion based on cyclomethicone and PGIS.

The end result of this work was a new TiO2 dispersion, containing 45% solids, and using a 50/50 blend of cyclomethicone and propylene glycol isostearate as the carrier. This provides optimised UVB protection and transparency, together with the type of skin feel desired by Asian consumers.

References

1 Robb J.L., Simpson L.A., Tunstall D.F. Drug & Cosmetic Industry 154(3), 32-39 (1994).

2 Woodruff J. Cosmetics & Toiletries Manufacture Worldwide 179-185 (1994).

3 Hewitt J.P., Hine H., Kessell L.M. 23rd IFSCC Congress, Orlando, Poster 218, (2004).

4 For example, US patents 5599529, 5366660, 7101427 B2.

5 Kessell L.M., Naden B.J., Tadros Th.F., 23rd IFSCC Congress, Orlando, Poster 213, (2004).

ABSTRACT
This article describes how a new titanium dioxide dispersion was developed specifically to meet the needs of Asian skin. By using a new manufacturing process for TiO2, together with an optimised dispersion process, exceptional transparency on skin was achieved. However, to meet the needs of Asian consumers, this had to be combined with excellent skin feel, while maintaining efficacy in the formulation. This was achieved by combining a silicone carrier fluid with an isostearate ester to deliver optimum sensory properties combined with solvency for organic sunscreens and flexibility in formulation. The use of such a mixed carrier system was found to facilitate use of a more versatile dispersing agent, thus giving a dispersion which is compatible with a wide range of other oil phase ingredients. Dispersant demand experiments were conducted to determine the optimum level of dispersing agent to disperse the particles effectively in the mixed carrier system.