Klaus Jenni and Anna Howe examine how water-resistance, sand repellence and moisturisation properties make cationic emulsifiers a versatile alternative to conventional emulsifiers in sunscreens
Cationic emulsions have attracted increased interest in recent years due to their unique skin feel, which is drier and more powdery than that of anionic or nonionic emulsions. Following their introduction for skin, hand and baby care in the US, between 1999 and 2007 this market segment expanded into over 30 new products containing the cationic emulsifier distearyldimonium chloride (Figure 1). The application area first seen in the market was skin care for babies. Mennen marketed this line under the trade name Baby Magic using two cationic emulsifiers and S.C. Johnson & Son launched the second cationic emulsifier-based product, Curel, now sold under Andrew Jergens (Kao). Some of the latest product additions containing cationic emulsifiers are Unilever’s Dove Rich Nourishing Hand Cream and Johnson & Johnson’s Neutrogena Comforting Butter Body Cream. The success of these products can be attributed to the unique sensory profile that cationic moieties offer. Cationic emulsions give a smooth powdery after-feel that allows high active loadings. They are thus broadening the range of aesthetics available to the formulator.
SUITABILITY OF USE
Cationic moieties have typically been associated with hair care for conditioning. In this application, they impart lubricity, antistatic properties and smoothness to the hair fibres. Given these facts, cetrimonium chloride and behentrimonium chloride have been widely used in conditioning rinses, although they were regulated as preservatives up to the 6th Amendment of the EU cosmetics regulation. However, there was an exception for higher use levels in non-preservative applications which expired with the 7th Amendment.
The Scientific Committee for Cosmetic Products (SCCP) is now evaluating a dossier dealing with the safe use of these products as conditioners. Due to their eye irritation profile, it is expected that concentration limits in the range of a maximum of 0.5% for leave-on products and up to a maximum 3.0% for rinse-off products will be fixed for the mentioned monoalkyl quats.
However, as long chain dialkyl quaternaries are milder on the skin than long chain monoalkyl analogues and as alkylamidoamine quats are also known to be less irritating than their amide and spacer group-free analogues, this paper will concentrate on the evaluation of the former product types.
FORMULATING WITH CATIONIC EMULSIFIERS
Formulating with cationic emulsifiers is straightforward and similar to the techniques employed with the traditional non-ionic emulsifiers. They can either be added to the oil or the water phase. Similar to other hydrophilic o/w emulsifiers, additional stabilisation of the formulations is achieved by incorporating consistency enhancers such as stearyl alcohol, cetyl alcohol, glyceryl stearate or stearic acid. Cationic emulsifiers are incompatible with anionic moieties such as carbomers or anionic water soluble sunscreens such as phenylbenzimidazole sulfonic acid. These materials should be avoided or their use minimised.
Advantages offered by cationic emulsifiers for the preparation of sunscreens are high water resistance, which they impart to a formula without any addition of a polymeric film former, sand repellence and long-lasting moisturisation efficacy.
WATER RESISTANCE
Due to increased concern about sun induced skin damage, consumers are using sunscreens at the beach and throughout the year to protect their skin from premature ageing. While an oily feel may be acceptable for the beach, it is not acceptable for daily wear. The challenge for formulators is to develop high SPF formulas that protect the skin and wear well but do not feel oily. Cationic emulsifiers can offer a means to achieve the aesthetics along with the additional benefit of water resistance.
Water resistance is an important factor in sun care formulas, especially those intended to be used at the pool or on the beach. Data from an in-vitro water resistance study conducted according to a protocol published by a scientific collaboration between different industrial laboratories and testing institutes in Europe were gathered, with sunscreens containing the identical filter combinations as well as the identical emulsion base, varying only the emulsifier.
Tests methodology used the following:
l Rough PMMA Hélioplates as substrate
l Three plates per sample with five
measurement points on each plate
l Substrate amount of 1.0mg/cm2
l Temperature 36°C
l Volume: 5L water, conductivity 1-3 µS/cm
l Speed of stirrer: 175 rpm
l Time of water treatment: 40 min
l SPF test device: Optometrics SPF 290S
Analyser
l Accuracy: +/- 15% on the basis of an
SPF 18 formula
The study evaluated the water resistance of four emulsifiers: ceteareth-25, polyglyceryl-3 methylglucose distearate, distearyldimonium chloride (all three o/w) and polyglyceryl-4 diisostearate/ polyhydroxystearate/sebacate (w/o for comparison). The filter combination in all cases was a mixture of 5% ethylhexyl methoxycinnamate, 4% menthyl anthranilate and 4% ethylhexyl salicylate. Measurements and results are shown in table 2 and figure 2.
The result for the pure non-ionic emulsifier ceteareth-25 is somewhat higher than expected; in-vivo results for this emulsifier are usually below 50% water resistance. For an exact match with in-vivo results further adaptations to the method appear to be necessary.
The result for the lipid emulsifier polyglyceryl-3 methylglucose distearate is in line with expectations of previous in-vivo testing where the results are normally lower. Values between 50% and 65% have been reported.
The cationic emulsifier, however, leads to an emulsion that is 100% water-resistant without the usually incorporated film-forming polymers, and is comparable to most of its w/o counterparts. This allows a considerable cost reduction in the segment of water-resistant o/w sunscreens. The water-resistance can now be achieved without film formers at the same performance level as with w/o systems, especially those containing silicone emulsifiers.
For comparison reasons, some of the in-vitro results were repeated with an official in-vivo method. For that the Colipa guidelines for evaluating sun product water resistance from 2004 were chosen, the main parameters being:
l Drying time after application - 15-30 min Irradiation of the non-watered fields
l First stay in the Spa pool - 20 min
l Drying time in the air - 15 min
l Second stay in the Spa pool - 20 min
l Drying time in the air - 15 min
l Irradiation of the watered fields
l Water temperature - 29 +/- 2°C,
MgO and CaO content - 50 - 500 mg/L, no air bubbles
Two formulas containing the emulsifier distearyldimonium chloride, one formula with palmitamidopropyltrimonium chloride and two formulas with ester quats were chosen for this comparison. Formulas are shown in figure 3, with comparative in-vitro values summarised in table 3.
Comparing the resulting in-vivo water resistance values it is possible to say that, with the exclusion of one example, they tend to be lower than the corresponding in-vitro results. A safe prediction of whether a product is water-resistant or not is possible. All formulas that have been proven to be water-resistant in-vitro were also found to be water-resistant in-vivo because the criterion for that claim is that the value needs to be greater than 50%. However, differentiation between the claim water-resistant and very water-resistant cannot be made with this method. The Colipa recommendations for this claim are fixing an 80 min spa pool stay before the irradiation of the watered fields, which was not investigated with the described in-vitro method.
SAND REPELLENCE
Good sand repellency is a claim often made for sunscreens to be used on the beach. Sunbathers don’t really want sand particles which are transported by the wind to be deposited on their bodies, particularly just after having applied a new layer of sunscreen.
To check whether there is an advantage of cationic emulsions in this respect an in-vivo sand-repellence test was developed. An amount of 0.3g of the emulsion is applied to the well ventilated inner forearm (150cm2) of a panellist and rubbed in. The forearm must be dry and free of sweat. After one minute the forearm is sprinkled with 30ml of fine aquarium sand. After three sets of forceful clapping the residual attached sand is rinsed off with ethanol. The ethanolic suspension is dried to uniformity of weight at 40°C and weighed by an analytical balance and the value received is the amount of sand that was adhered to 150cm2 of skin.
With this test the difference between a strong and weak sand repellent formula is observable to the naked eye (see above).
A better differentiation can be found when the residual amounts of sand left on the forearm are shown (figure 4).
The weakest sand repellence in this diagram is found with 2.5g from a w/o market product containing polyglyceryl-2 dipolyhydroxystearate as an emulsifier. The carbomer-containing white label product does not have a good sand repellence, as expected. The value of the emulsion with methyl glucose sesquistearate is improved compared to the w/o type formulas and the carbomer-based emulsion.
The best overall values are achieved with the cationic sun screen emulsions.
Within this group, by far the best results are achieved
with distearyl-dimonium chloride (0.4g,
bar on the left
of the group)
and with distearoylethyl dimonium chloride (0.2g, bar on the right of the group).
These results demonstrate that cationic emulsifiers have a clear advantage over conventional emulsifiers for the property of sand-repellence.
MOISTURISATION
A comparison of the moisturising properties of a non-ionic versus a cationic emulsifier was conducted by an outside laboratory. Figure 5 outlines the results from a moisturisation study comparing glyceryl stearate SE against distearyldimonium chloride. The moisture measurements were made in May 2005 on 11 panellists over a 12-hour period using a Corneometer (Courage & Khazaka).
Corneometer readings were taken hourly. The results presented in figure 5 indicate that during the initial eight hours of the study the cationic emulsifier did not show an increased performance versus the glyceryl stearate SE regarding moisturisation. After eight hours the cationic film showed superior performance versus the glyceryl stearate SE. The slopes of the moisture loss curves were -0.438 and -1.155 respectively, thus the improvement of long-term moisture retention in the skin is 38% with the cationic emulsifier. This result is probably due to the ability of distearyldimonium chloride to form a substantive durable film on the skin in which the hydrophobic moieties of the emulsifier are directed to the outside.
EXAMPLE FORMULATIONS
With increasing concern about sun induced skin damage, consumers are using sunscreens not only at the beach but throughout the year to protect their skin from premature ageing. Although an oily feel may be acceptable for the beach, it is not acceptable for daily wear and the challenge to formulators is to develop sunscreen formulas that protect the skin and wear well but do not feel oily. Cationic emulsifiers can offer the formulator a means to achieve these aesthetics along with the additional benefit of water resistance.
Cationic emulsifiers offer the formulator a new tool in developing products that meet consumer demands. The unique properties of cationic emulsifiers enable the development of highly lipophilic systems that offer enhanced consumer benefits without sacrificing aesthetics. Cationic emulsions lend a smooth, powdery skin feel, which non-ionic and anionic emulsions do not. Applications that benefit from this technology include facial, hand and body, baby and foot care, sunscreens and other systems where a non-oily, highly lipophilic system is an advantage. For sun products, cationic emulsifiers offer the following benefits:
l Excellent emulsification
l A variety of possibilities for combining them with co-emulsifiers thus influencing skin feel and absorption behaviour
l Long-lasting skin moisturisation
l High water resistance properties with no need for film forming polymers
l Outstanding sand-repellence
l Excellent cost/performance ratio
References available at www.cosmeticsbusiness.com
Authors
Klaus Jenni & Anna Howe
Evonik Consumer Specialties, Evonik Goldschidt
tel +49 2011732018
e-mail: Klaus.Jenni@evonik.com