Articles

Elizabeth Connock B.Sc (Hons)
A&E Connock (Perfumery & Cosmetics) Ltd

Abstract

Incorporating physical exfoliants into cosmetic cleansing preparations is increasingly popular. Early products relied on the abrasive effect of broken nut shells in standard cosmetic bases and were as appealing as sandpaper. Little was understood about the relationship between suspending power and yield values and the standard method of preventing the material either floating to the surface or sinking to the bottom of the product was to make it as viscous as possible.

Now there are many exfoliants available to the cosmetic chemist from both natural and synthetic sources. The particle size and abrasive qualities of each type can be strictly controlled enabling the desired level of exfoliation to be precisely formulated and a better understanding of rheological properties enables stable, elegant products to be made with the exfoliant distributed evenly throughout.

This article will review the many different natural and synthetic exfoliants available; it will discuss their properties and applications and suggest different methods by which they may be incorporated into a variety of formulations.

Introduction

Pumice stone has been used for centuries as a means of removing dead skin, particularly the hard skin from feet. It may be assumed that oatmeal was tried as a cleansing aid but a review of the classic books on cosmetic chemistry does not show the use of any physical additives to the cold creams and vanishing creams described therein. The first references to specific cleansing preparations other than soap do not appear until the ‘50s and the majority of these were still variations on the beeswaxborax cold cream type preparation although some formulations were based on non-ionic emulsifiers.

This was the time of progress in skin care preparations and of the development of new ingredients to satisfy demand. Soon triethanolamine stearate was being used as both emulsifier and cleansing aid and ethoxylated surfactants were suggested for ìwashableî skin cleansers instead of relying on the solubilising effect of w/o emulsions. Oatmeal appeared in preparations by Yardley and Holiday Magic in the 60’s but it would appear that it was not until the early 80’s that physical scrubbing aids or exfoliants were added to cleansing preparations with ground apricot shells appearing in Aapri by Gillette.

The success of this product stimulated a surge of interest and very soon every nut shell, kernel or husk that could be reduced to small particles was being added to cleansing creams, lotions and body washes. In the rush to join the bandwagon scant attention was paid to the physical shape of the scrubbing aids, to the microbiological problems they could introduce or to ensuring the long term stability of the product. Sharp edges felt like broken glass, a variety of new fungi appearing on the product surface added interest for the microbiologist but horrified the consumer and the particles floating to the surface or sinking to the bottom of the container did nothing to enhance the products visual appeal.

However the marketing department still insisted that there was a market niche; the material suppliers were as ready as always to satisfy a demand and the cosmetic formulator just needed time. First the processing was improved to eliminate sharp edges and a variety of softer materials were tried. Hydrogenated vegetable oils were moderately successful as the waxy particles broke up on application and rubbed out while visibly collecting grime. Corn (Zea mays) cob granules gave a noticeably abrasive feel but disintegrated readily without damaging sensitive skins and nut meals often proved more consumer acceptable than the harder shell granules. Irradiation of the natural materials solved the microbiological problems although care was still needed as preservative efficacy is affected by the presence of particulate matter. Perfectly spherical polymeric materials were introduced with precisely controlled particle size and it only needed the formulator to stabilise the end-product for a multitude of new products to reach the market.

Formulating Problems and Product Stability

If particulate matter is introduced into a lotion or surfactant system it will either float to the surface or sink to the bottom. The obvious solution to this problem would appear to be to increase the viscosity. However even if a surfactant product is turned into a ringing gel the particles may still migrate. The answer is to increase the suspending power of the product. Suspending power is measurable as yield value and this must be sufficiently high to overcome the effects of gravity or buoyancy on the particles.

Yield value is defined as that stress which must be applied before flow will start and, although related to viscosity, it is more dependent on the characteristics of the rheological additive used. Yield value has traditionally been determined by measuring the viscosity of the material at two speeds using a Brookfield Viscometer. As the speed of rotation of the spindle is increased materials that undergo shear thinning give a lower viscosity measurement. Applying the formula 2r*(V1-V2)/100 where r is the lower spindle speed, V1 the viscosity at this speed and V2 the viscosity at the higher speed, gives a yield value in dynes/cm2. A report published by Goodrich Performance Chemicals (1) gives the following formula which predicts the yield value needed to overcome the pull of gravity or effect of buoyancy on spherical particles of known radii and densities.

Required Brookfield Yield Value = [4/3 R(D-Do)g] dynes/cm2

where

R = particle radius (cm)
D = particle density (g/cm3) Do = medium density (g/cm3)
g = acceleration due to gravity = 980 cm/s2

Because the factors applied to the viscosity determinations are very high the measurement of the Brookfield Yield Value is prone to error.

Also different spindle and speed combinations are necessary to measure values across a wide viscosity range which affects results. If a Brookfield viscometer is fitted with the small sample adapter the yield value according to the Bingham equation

p = po + v1 * D where p = Shear Stress, D = Shear Rate v1 = Plastic Viscosity (cP), p o = Yield value (dynes/cm2)

may be readily determined using suitable software. Results are more reproducible, confidence levels are calculated automatically and a graph is plotted enabling the user to apply subjective judgement to validity of the results. Unfortunately the results are different to the Brookfield method.

The formula for determining the yield value necessary to suspend solid particles is not easy to apply in practice; it is dependent on the particle radius and density being known and is only truly valid for spherical particles. A practical method was devised which would determine the yield value necessary to maintain some commonly used physical exfoliants in stable suspension when centrifuged.

Determination of Yield Value required for suspending various exfoliants

Experimental Details

Three commonly used rheology modifiers were used to prepare gels of different strengths. The viscosity, Brookfield Yield Value and Bingham Yield Values were determined. Various physical exfoliants were then added to the differing gels and centrifuged to assess stability. The yield values were determined at which the distribution of the physical exfoliant remained unchanged when centrifuged at 2,500 rpm for two minutes and at 5,000 rpm for two minutes.

Comments

Various additives were used for initial experiments before the following choices were made.

Carbopol Ultrez 10 Polymer (Carbomer) was selected because of its ease of use.

Carbopol ETD 2020 (Acrylates/C10-30 alkyl acrylate crosspolymer) was selected because it is more stable in the presence of electrolytes than Carbopol Ultrez.

Stabileze QM (PVM/MA decadiene crosspolymer) was selected because if its high yield values.

The centrifuge testing proved very rigorous; many commercially available products with additives were tested and most were found to separate when centrifuged at 2,500 rpm. for two minutes. However emulsions are normally required to remain stable at 5,000 rpm so for each material tested the yield values were determined at which the distribution of the physical exfoliant remained unchanged when centrifuged at 2,500 rpm for two minutes and at 5,000 rpm for two minutes. It may be seen from Table 2 that the majority of materials could be suspended satisfactorily at 2,500 rpm with Brookfield yield values between 200 and 635 dynes/cm2, equating to approximately 50 to 130 dynes/cm2 on the Bingham scale. However to achieve stability at 5,000 rpm some materials required Brookfield yield values in excess of 2000 dynes/cm2 which was only achieved using Stabileze QM.

In order to equate the centrifuge tests with the yield values, A-C Polyethylene 9A (granules) were suspended in Carbopol ETD 2020 gels and the centrifuge speed at which they just showed signs of separation determined. Results are shown in Table 3.

Practical Considerations

The laboratory work predicts the yield value necessary to achieve a stable product. It enables products to be formulated that are opaque and for which determining stability would be very difficult. It is not necessary to formulate products with a solid consistency and shower gels with suspended beads, cleansing emulsions, exfoliating scrubs and face and body washes may all be made at a viscosity to suit the application and preferred packaging.

When formulating such a product the basic formula is first decided, the most suitable physical exfoliant selected and the yield value to ensure its suspension either calculated or found from the table. The yield value of the base formula is then measured and adjusted by the addition of a suitable rheology modifier.

Carbopol Ultrez 10 Polymer is easy to use, has a relatively high yield value for its viscosity but is adversely affected by electrolytes. Carbopol ETD 2020 has a similar viscosity profile, it imparts a similar yield value and is tolerant to low levels of electrolytes. Stabileze QM gives higher yield values than the Carbopols for a given concentration but it is necessary to heat it in water to 800C to ensure full hydration. Like many other modifiers all three additives require neutralisation to reach maximum viscosity.

The Acrylates/steareth-20 methacrylate copolymers available as Acrysols from the Rohm and Haas Company have a softer, creamier feel than many of the polymeric additives and may be preferred for cleansing emulsions. Veegum has long been used as a suspending agent and is particularly effective if used in conjunction with xanthan gum.

Most rheological additives are anionic; for a cationic system the cationic guars are effective.

The yield values of rheological additives are affected by other constituents of the formulation and proper consideration must be given to stability at higher temperatures. Fortunately most of the above modifiers are little affected but using traditional surfactant thickeners such as salt or PEG-150 distearate can result in considerable loss of yield value at higher temperatures. The exfoliant itself also has an effect depending on its particle size and the concentration added.

1. Goodrich Performance Chemicals; Measurement and understanding of yield value in personal care formulations; 1995.

2. Goodrich Performance Chemicals; Bulletin 12, Flow and suspension properties; 1997

Example Formulations

AEC 1934/3: Facial Cleanser with Jojoba Wax Beads (Ingredients in table below)

This Gentle Cleansing Face Wash uses sodium cocoyl isethionate in combination with sodium cocoamphopropionate as a mild surfactant system and jojoba wax beads as non-abrasive physical exfoliants. They are held in suspension by the Carbopol ETD 2020. Allantoin is included as a healing aid, cetyl lactate for its unique skin feel, extract of Awapuhi for moisturising and the Pamplemousse fragrance for its superb refreshing aroma. The product is applied to a wet face and worked into a gentle lather with a damp cosmetic sponge. It is rinsed off after application.

Mixing Instructions

Heat the water to 40C, add the Carbopol ETD 2020 and continue heating to 65C with mixing. Add the items of Stage B in order with mixing but avoid introducing air into the product. When addition is complete check that the materials are properly dissolved then start cooling with continuous slow mixing. Add Stage C items below 35C and finally add the Jojoba Beads with gently mixing.

The Brookfield Yield Value before the addition of the Jojoba Wax Beads was 610 dynes/cm2 and 420 dynes/cm2 after addition. This drop in yield value is not uncommon and occurred despite a near 50% increase in viscosity. The product was completely stable at 2,500 rpm but showed some separation at 5,000 rpm. The addition of 0.1% xanthan gum is sufficient to stabilise the product at 5,000 rpm.

AEC 1935/2: Hard Skin Remover (Ingredients in table below)

This cleansing emulsion has been designed to remove hard skin from feet. The oil phase softens the hard callus to facilitate its removal by the gentle abrasive action of the A-C Polyethylene (granules) while the mixture of tea tree oil, peppermint oil and menthol leave the feet feeling cool and refreshed.

Directions: Wash the feet, massage the Hard Skin Remover into the soles and backs of the feet, massage for 5 - 10 minutes then remove the excess product with a damp sponge or tissue.

Mixing Instructions

Heat the Oil Phase (Items 1 - 6) to 70C, adding the AEC Cyclopentasiloxane just prior to bringing the two phases together to minimise loss. Heat the Water to 40C, disperse the xanthan gum and bring temperature to 70C. Add the Oil Phase to the Aqueous Phase, preferably with high shear mixing. After 5 minutes mixing change to an anchor or contra-rotating type mixer and mix slowly while cooling and add the essential oils, menthol, preservatives and colour when below 35C and finally mix in the AEC Polyethylene Spheres.

The Brookfield Yield Value before the addition of the Polyethylene Spheres was 630 dynes/cm2 and 680 dynes/cm2 after addition. The product was completely stable at 2,500 rpm but showed slight separation at 5,000 rpm.

The exfoliants and majority of other ingredients are available through A & E Connock (Perfumery & Cosmetics) Ltd., Alderholt Mill House, Fordingbridge, Hampshire SP6 1PU, England. Tel +44 (0) 1425 653367, Fax +44 (0) 1425 656041, email: sales@connock.co.uk