Factors affecting the formulation of sunscreen products

Copyright John Woodruff, Creative Developments, UK

Abstract

In selecting the ingredients for any personal care product, the formulator must bear in mind a number of factors, such as ease of use, cost, and interactions between different ingredients. The formulation of a sunscreen product is influenced by these considerations, but also by a number of more specific ones relating to the efficacy of the actives used and the formulator has to study the solubility of the sunscreen, look for synergy between filters, evaluate the rheological and sensorial characteristics of the vehicle and be cognisant of the photostability of the active ingredients.

This paper will discuss some of these factors that are specific to sunscreens. The choice of emulsion type can influence the SPF and affect other properties such as water-resistance. Product rheology has been shown to dramatically affect SPF, and the use of rheological additives such as hydrocolloids in O/W emulsions or waxes in W/O systems to optimise these effects will be discussed. Further data show that choice of emollients in suncare is not just about optimising skin-feel; these ingredients can also affect the SPF.

Introduction

Many factors affect the formulation of sunscreen products. A product brief normally defines the SPF to be claimed, the degree of UVA protection and water-resistance required and whether it is to be a spray or a cream, a lotion or a gel. It should specify the materials to be used for the packaging components and for which markets the product is intended and give a cost ceiling for the ingredients.

These few parameters rapidly narrow down the choice for the formulating chemist. Debra Redbourn’s presentation will have shown the lack of harmonisation between the different regulatory bodies and the narrow choice of filters that this leaves. The physical form of the product will further narrow the choice. An inorganic oxide is obviously unsuitable for a clear gel product, a water-soluble one cannot be used in suntan oil. Some actives are incompatible with certain plastics; others may discolour badly on exposure to sunlight.

More subtle problems now arise; interactions between the sunscreen and other ingredients can seriously affect its efficacy. Photo-degradation can halve the SPF in a relatively short time and the products of photo-degradation may even cause irritation and allergies. Combinations of sunscreens may overcome the problem and then contravene an existing patent. Alternative materials may have restricted availability, be too expensive or just as the problems appear to be solved, the material may be the victim of an ill-informed media campaign.

Having overcome these problems the formulator will look for ways of providing the best possible product by maximising the efficacy of the actives in a stable composition that is a pleasure to apply.

q Ease of use

Material handling is an important aspect of factory production. Liquids and powders available as dispersions are more readily processed and are safer to handle than are powders.

q Solubility of the sunscreen

Sunscreen solubility presents many problems. There are few water-soluble materials although this is not normally a severe restriction. The only aqueous product likely to be encountered is a clear gel and the only suitable active is probably phenylbenzimidazole sulfonic acid that has to be neutralised in-situ. Failure to maintain a pH above 7 can create fascinating balls of crystals in the product.

The majority are oil-soluble but often not very! There are probably more failures because of this than any other single cause. Octyl triazone, benzophenone-3, butyl methylbenzylidene camphor (BMBC) and methoxydibenzoylmethane (MDBM) all crystallise if insufficiently solubilised. With an ever-increasing demand for higher SPF values the levels of actives are higher and cocktails of them compete for the solvent. Polar solvents may increase solubility but they also change the lmax of the active. They stabilise the ground state of polar compounds and decrease their lmax and stabilise the excited state of less polar compounds, increasing their lmax. Non-polar solvents decrease the lmax of non-polar sunscreens.

If the problems of shifting the lmax are remembered and checked then there is a wide selection of oils and esters that not only improve solubility but may also enhance the SPF. C12-15 alkyl benzoate combined with dipropylene glycol dibenzoate and PPG-15 stearyl ether benzoate can dissolve up to 30% benzophenone-3 and 16% butyl methoxydibutyl-methane (BMDBM = Avobenzone). Butyloctyl salicylate is a low viscosity oil that solubilises benzopheneone-3 and BMDBM and also improves its photostability. Hexadecyl benzoate and butyloctyl benzoate have similar solvent properties and diisopropyl sebacate and diisopropyl adipate are also good solvents that form non-occlusive films and are also soluble in ethanol.

The silicone ester, diisostearyl trimethylolpropane siloxy silicate is a good solubiliser for most organic sunscreens and is substantive to skin so that the active is held in a film that is also water-resistant. This material also works well with titanium dioxide and I have had increases of 25% in SPF in formulations to which this has been added. Octyldodecyl neopentanoate and dicaprylyl maleate are also good solubilisers that significantly enhance SPF values and dibutyl adipate and caprylyl carbonate are also good solvents as the chart shows.

q Interactions with ingredients

Despite being active chemicals with double bonds I can find little evidence for chemical reaction between the actives and the ingredients commonly found in cosmetic products. The main problems are those of pH, solubility and effect on SPF.

q Interactions between filters

Some combinations of filters are more prone to photo-degradation than others but I can find little evidence for chemical interactions. It has been reported that BMDBM crystallises out of solution in the presence of microfine titanium dioxide but this is most likely due to insufficient solubility of the BMDBM in the oil-phase and the titanium dioxide crystal then acts as a nucleus for crystal formation.

q Interactions with the packaging

Ethylhexyl methoxycinnamate (OMC or should it now be EHMC?) goes bright yellow in clear bottles and the solubility parameters of most liquid UV filters is similar to that for the polymers used in many packaging materials. This can result in the filter migrating into the plastic with a consequent degradation of the pack and loss of active in the formulation.

q Photostability of the actives

The diagram shows an absorbance curve for BMDBM, before and after irradiation. This is a problem that has only been properly recognised in the last few years but it can lead to significant loss of efficacy. In 1994 Gerd Dahms¹ compared photo-degradation of various sunscreens exposed to 5 MED in ethanol/water mixtures, in isopropyl myristate and in mineral oil and reported losses up to 52%. A paper by Craig Bonda² described the formulation of stable, high SPF, broad spectrum sunscreens that illustrated the use of butyloctyl salicylate, hexadecyl benzoate and butyloctyl benzoate to photo-stabilise BMDBM and mixtures of sunscreens that contain it. Work by Dr. Thomas Wünsch³ shows that while BMDBM is not photo-stable alone or in combination with OMC it can be stabilised by the addition of octocrylene or methylbenzylidene camphor (MBDC).

Diethylhexyl naphthalate is a new material currently approved for use in Australia, Japan and the USA and in the process of registration in Europe that is shown to significantly improve the photostability of BMDBM. It is a semi-viscous oily liquid with a high refractive index of 1.53 and a specific gravity of 1.02. It is insoluble in water, propylene glycol, and glycerin but freely soluble in most oils such as mineral oil, castor oil, and typical cosmetic esters. It is a good solvent for benzophenone-3, BMDBM, MBDC and ethylhexyl triazone.

q Cost effectiveness

The product must be affordable. Enhancing efficacy is one way of reducing costs.

q Legislation

Other speakers are covering this aspect in depth. We all know the frustration of non-harmonious agreements, not only over the materials that can be used but the lack of a universally approved SPF test method, of a way of measuring and declaring UVA protection and over the thorny question of water-resistance.

q Availability

The availability of some materials appears to be affected by two criteria. Lack of demand leads to loss of production as with ethyl dihydroxy PABA (not permitted in the EU or Japan) and benzophenone-5, which appears to be permitted but is not made. Other materials are made but sales are restricted e.g. drometrizole trisiloxane and terephthalylidene dicamphor sulfonic acid.

q Patent position

There have been 47 patents granted in the USA since 1996 that refer to Avobenzone. In that time there have been 203 patents referring to octocrylene and also 203 for benzophenone-3 but only 60 for compositions containing both. There have been more than 50 that refer to ethylhexyl methoxycinnamate, 27 references to phenylbenzimidazole sulfonic acid, 54 for 3-benzylidene camphor but strangely there were only 3 for drometrizole trisiloxane and I could not find any for bis-ethylhexyloxyphenol methoxyphenyl triazine or terephthalylidene dicamphor sulfonic acid. A large proportion of the patents discovered restrict the use of the materials indicated.

q Media comments

Cosmetics have always been a popular target for the media. Given the chance to introduce sex and it becomes irresistible. The public will remember the cancer-promoting and gender-bending possibilities of OMC and benzophenone-3 but not the scientific comment that put these stories into context.

q Sensorial characteristics of the product

This is closely related to rheological considerations. Applying product to large areas of the skin must be a pleasurable experience, more so if applied by a partner. Fortunately a product that takes time to apply smoothly and evenly is the most pleasurable and also is likely to maximise efficacy. Another important sensorial characteristic is perfume and it is essential that this is selected with due regard for potential photo-allergies.

q Rheological considerations

With the majority of skin care products the formulator looks for a product that is easily applied and that appears to be rapidly absorbed by the skin. This is not the case with sunscreen products that need to be applied as a thicker film, which must stay on the skin surface and that are needed as a continuous layer. The thixotropic rheology that helps the application of other products is to be avoided if the peaks and troughs of the skins topography are to be completely covered. Pseudoplasticity, whereby the composition has a low yield point but regains viscosity as soon as shear is removed, is the ideal.

In an o/w emulsion the rheology modifier affects these properties and we⁴ investigated the effect of different thickening aids on the SPF of an o/w emulsion containing 5% microfine titanium dioxide added as an aqueous dispersion to the water phase. In general SPF improves with increased viscosity and we found that the clay-type thickeners gave compositions with low yield points and pseudoplastic flow with higher SPF. The best results in the formulation studied were obtained using magnesium aluminium silicate (Veegum Regular), which is a good suspending aid and does not gel the product and with sodium magnesium silicate (Laponite XLG). Low levels of carbomers, an acrylates copolymer and a PVM/MA decadiene crosspolymer each gave improved SPF but lower results than with the clay-types. Xanthan gum as the sole thickener was disappointing but worked well in combination with Veegum. It should be said that SPF was only measured in-vitro and that some thickening aids react quite differently on human skin.

Additives	SPF& Ratio	Additives	SPF& Ratio
No additives	9.1 / 0.54	2.0% Veegum	13.3 / 0.52
2% Klucel 99-MF	4.5 / 0.49	1% Xanthan Gum	8.5 / 0.65
0.2% Carbopol Ultrez	10.1 / 0.56	2% Veegum / 0.5% Xanthan	11.3 / 0.55
0.20% Stabileze 06	11.4 / 0.53	2% Veegum / 1% Xanthan	14.5 / 0.54
1% Acrysol 25	12.1 / 0.56	2% Veegum / 1.5% Xanthan	16.7 / 0.52
2% Laponite XLG	15.1 / 0.51	2% Veegum / 0.2% Ultrez	11.2 / 0.53

Further work compared the effect of rheological additives with 5% microfine titanium dioxide added as a dispersion in C12-15 alkyl benzoate to the oil phase.

Additives	SPF& Ratio
No additives	11.9 / 0.45
1% Xanthan Gum	10.2 / 0.47
2.0% Veegum R	11.5 / 0.46
0.20% Stabileze 06	13.9 / 0.46

The differences were much less and it is thought that in an o/w emulsion with the titanium dioxide dispersed in the aqueous phase the polymer-type additives probably form a film incorporating the microfine oxide, which may roll up on application, giving a discontinuous film. This would not happen with the titanium dioxide dispersed in the oil phase.

The rheological characteristics of the samples were studied using a Brookfield DVII+ with a small sample adapter and software supplied by Brookfield Instruments. It was found that the more shear sensitive the composition the better the SPF results tended to be.

Similar work with w/o emulsions compared the effects of using different waxes to increase the viscosity of the emulsion. There were considerable differences in SPF with beeswax giving the best results and microcrystalline wax the worst. We tried various silicone waxes including C30-45 alkyl methicone, which gave average results in our standard formulation. However work published by Goldschmidt showed that adding 1% of cetyl dimethicone to a w/o cream containing 3% OMC increased the SPF from 11.5 to 13 and when added to a w/o lotion containing 5% titanium dioxide the SPF increased from 11.8 to nearly 14. A combination of titanium dioxide (3%) and OMC (3%) gave an SPF of 9.1 and when 0.25% cetyl dimethicone was added this was increased to 13.5. It is suggested that the emollient and film-forming properties of cetyl dimethicone contribute to this increase.

It is important that oil phase has the correct spreading characteristics and Cognis has further developed its cascade theory of emolliency and incorporated it into Synergistic Sun Systems. It describes the interactions between UV absorbers, pigments and cosmetic raw materials and the effects of emollients on sensorial characteristics and SPF values. Dahms also reviewed these aspects and I have examined the effects of the emollient on SPF values in a simple w/o emulsion containing titanium dioxide, as follows: -

Ingredient	%w/w
The variable oil	17.50
The variable wax	3.00
PEG-30 Dipolyhydroxystearate	2.60
TiO2; 50% dispersion in C12-15 Alkyl benzoate	10.00
Water, deionised	61.00
Magnesium sulfate	0.70
Propylene glycol	5.00
Preservative	0.20

The hot aqueous phase is added to the hot oil phase with stirring. The product is briefly mixed at high shear, cooled with stirring then briefly mixed at high shear again. The emulsifier is PEG-30 dipolyhydroxystearate available as Arlacel P135 from Uniqema. It provides soft glossy creams and was untroubled by all the different oils it was expected to emulsify although there were considerable differences in viscosity. We found that beeswax gave significantly higher SPF values than similar compositions made with microcrystalline wax; none of the latter exceeded SPF12.Twenty-three different oils and oil mixtures were substituted for the variable oil and the SPF measure in-vitro. The SPF varied from 13.6 to 20 and the UVB:UVA ratio was 0.50 – 0.60. The table that follows shows single oils only.

Oil	SPF	Oil	SPF
Ethylhexyl Stearate	20.10	Decyl Oleate	15.70
Cetearyl Octanoate	18.40	Mineral Oil	15.60
Octyldodecyl Neopentanoate	18.20	Ethylhexyl Palmitate	15.30
PG Dicaprylate	17.90	Ethylhexyl Dodecanol	15.30
Isopropyl Palmitate	17.20	C12-15 Alkyl Benzoate	15.20
Dioctyl Maleate	17.10	Capric/Caprylic Triglyceride	14.80
Cetearyl Isononanoate	16.70	Jojoba Oil	14.00
Isopropyl Isostearate	16.60	Triisocetyl Citrate	13.60

Much time was spent trying to find a relationship between the oil properties, the viscosity of the emulsion and the SPF results. In general there was an increase in viscosity with higher molecular weight oils and the thicker products tended to have higher SPF results. Mixtures of oils were tried and different water:oil phase ratios investigated. It was found that the system was stable up to about 70% aqueous phase although higher oil phase content emulsions had better spreading characteristics. At the IFSCC Congress in South Africa J. Hewitt described the results of extensive investigations into the factors affecting efficacy with microfine oxides⁵. He found that pre-dispersed systems have advantages over powder forms of TiO2, and that choice of emollients and emulsion rheology can have important effects on SPF.

q Synergy Between actives

Using combinations of sun filters is a necessity to obtain the higher SPF values required so finding synergy between them is a rewarding operation. Using combinations that improve photostability such as the addition of MBDC to BMDBM or OMC at least protects the expected SPF but it is also possible to improve on the sum of the parts. W. Johncock investigated mixtures of zinc oxide with octocrylene and with isoamyl p-methoxycinnamate and found significant increases in protection⁶. Hewitt and Housley studied the synergy that may be obtained when using inorganic oxides in conjunction with OMC and other organics⁷.

Thomas Wünsch⁸ found a mixture of 2.0% octyl triazone with 5.0% zinc oxide to be particularly effective and Ferrero⁹ found synergy between dioctyl butamido triazone and BMDBM. Both authors found that a small concentration of a UVA absorber could show a significant synergy with higher levels of UVB filters.

Enhancing SPF with non-active ingredients

Various materials have been proposed for increasing SPF including the use of botanical extracts such as Pongamol but many are more expensive than the recognized actives. However certain polymers act as physical blockers and this is an interesting area for investigation. Diglycol/CHDM/isophthalates/SIP copolymer is a water-dispersible polyester that is strongly substantive to skin as a water-resistant film. If incorporated in a sun protection product the organic actives are dissolved within this film, which also enhances SPF values by increasing the path length through scattering effects.

A styrene/acrylates copolymer has recently been introduced that has a similar action and this is illustrated in the diagram, supplied by courtesy of ISP. The copolymer is in the form of microscopic hollow spheres filled with water but as these dry in the cosmetic film on the skin water is lost by evaporation and is replaced by air. The microscopic air bubbles deflect radiation and the effective path length of the photons is therefore increased.

Maintaining the protective film throughout the claimed safe exposure time is of paramount importance. Diane Smith has described a series of trimethylpentanediol/adipic acid copolymers with good solvent powers for oil-soluble sunscreens that are substantive to skin as a water-resistant film. By altering the cross-linking of the polyester and the terminal group on the backbone the polymer properties are adjusted and may be used for slow release of sunscreen actives¹⁰.

Conclusions

There are many factors that affect the formulation of sun protection products but by careful selection of the active ingredients, the product form, the type of formulation used and the constituents of that formulation it is possible to deliver a safe, effective product at a reasonable cost, provided the final combination has not already been patented.

Index to Synonyms & Abbreviations

q Butyl Methylbenzylidene Camphor (BMBC)

q Methoxydibenzoylmethane (MDBM)

q Butyl Methoxydibenzoylmethane (BMDBM) = Avobenzone

q Ethylhexyl Methoxycinnamate (OMC)

q Methylbenzylidene Camphor (MBDC)

q Methylene Bis-Benzotriazolyl Tetramethylbutylphenol (MBBT)

q Disodium Phenyl Dibenzimidazole Tetrasulfonate (DPDT)

q Benzophenone-3 = Oxybenzone

q Benzophenone-4 = Sulisobenzone

References

1 Dahms, Gerd, Choosing Emollients & Emulsifiers For Sunscreen Products; Cosmetics & Toiletries Magazine, vol. 109, Nov 1994

2 Bonda C; Formulating Stable, High SPF, Broad Spectrum Sunscreens With Avobenzone, Annual Scientific Meeting of SCC, New York 1997.

3 Wünsch, Thomas; New Aspects in Sunscreens; European UV & Sunfilters Conference, Paris 1999.

4 Hewitt J., Woodruff J., Rheology Modifiers & Inorganic Sunscreens, In-Cosmetics Conference, 1998

5 Hewitt J., Woodruff J., Factors Influencing Efficacy of Oil-dispersed Physical Sunscreens, IFSCC Magazine, Vol.3 No.1, pp.18-23 (Jan/Mar 2000)

6 Johncock, W., Formulating Sunscreens with Microfine Zinc Oxide and Octocrylene, SCS Spring Conference, London, April 1998.

7 Hewitt J., Housley S., Cosmetics & Toiletries Manufacture Worldwide 2000, pp292-297.

8 Wünsch T., Synergistic Effects With High Performance UV-Filters XXI IFSCC International Congress 2000, Berlin

9 Ferrero L., Pissavini M., Perichaud C., Zastrow L., Experimental Design Application To Sunscreen Products. Demonstration Of A Synergistic Effect Between UVB And UVA Absorbers. Ibid

10 D Smith et al, Polymeric Ester Technology For Photoprotection Enhancement; European UV and Sunfilters Conference, Paris, 1999.