| By
Elizabeth Connock B.Sc
It is not only people that suffer from sunburn, it can also happen
to cosmetic products. More accurately solar radiation in the ultra
violet (UV) region can cause photo-degradation of cosmetics, toilet
waters and perfumes.
Solar energy reaching the earth’s surface is broadly divided
between infrared, visible light and ultra violet radiation. The
human eye is sensitive to the visible spectrum between 400 and 700nm,
which appears as white light but comprises the colours of the rainbow.
Infrared is light of wavelength above 700nm and is thought harmless
to humans. Ultra violet radiation has a wavelength between 280 and
400nm and it is this that can cause damage to both human skin and
cosmetic products.
The products most at risk are perfumes and toilet waters that can
change both in smell and colour, carbomer gels that lose viscosity
and shampoos, shower gels and bath foams that lose colour and may
change in odour, but any product that is packed into a clear bottle
may be affected by direct sunlight.
This article will discuss the problems of UV degradation of cosmetic
products and suggest methods of overcoming the problem without resorting
to opaque packaging.
Keywords
Photo-degradation, Perfumes, Carbomer Gels, Shampoos, Stability,
Ultra Violet (UV), Benzophenone-2, Benzophenone-6, Benzophenone-9,
K-Value, Sun Protection Factor (SPF).
Introduction
Over the last decade or so the effect that high-energy short wave
solar radiation has on human skin has received much publicity as
the major cause of the visible effects of ageing and of skin cancers.
To meet public demand the cosmetic industry has developed high SPF
sun protection products for leisure use and has introduced moderate
SPF protection into everyday applications such as moisturizers and
make up. However any cosmetic product that is packed in a transparent
container and exposed to direct sunlight is also liable to suffer
accelerated ageing effects and change in colour and smell. Many
may also lose viscosity; active ingredients can lose their activity
and chemical instability will be accelerated.
To overcome photo-instability the obvious answer may be to pack
the cosmetic composition into opaque containers or to use deeply
coloured glass. However such measures detract from the product’s
presentation. An alternative is to introduce sun protection into
the formulation in much the same way as sun protection for consumers
is now built into moisturizers and sun products. Whereas to protect
consumers an SPF of 10 or more is needed and at an active level
exceeding 5%, the products themselves only need very low levels
of UV absorber for long-term protection.
UV absorbers work by absorbing high-energy radiation and dissipating
it at a lower level and as heat. For the protection of human skin
there are a restricted number of effective materials that are on
a positive list of permitted absorbers in Europe and most of the
world. However these ingredients are not ideal for product protection.
Fortunately a small number of suitable materials are permitted where
their purpose is to protect the product and these are the benzophenones;
specifically benzophenone-2, benzophenone-6 and benzophenone-9.
Benzophenone, or more correctly tetrahydroxybenzophenone, or more
correctly still bis(n,ndihydroxyphenyl) methanone, consists of two
benzene rings joined by a carbonyl group and carrying four hydroxyl
groups. The position of the hydroxyl groups gives the benzophenone
its common designation. Thus benzophenone-2 is 2,2', 4,4'-tetrahydroxybenzophenone
with a molecular structure as follows.
Common Name: Benzophenone-2
EINECS No: 205-028-9
CAS Registry No: 131-55-5
Chem. Name: 2,2',4,4'-Tetrahydroxybenzophenone
K-Value: 58
The K value is a measure of the effectiveness of a sunscreen chemical
and is the ratio of maximum absorbance to its concentration in grams
per litre measured in a cell of 1 cm path length.
The absorption spectrum of benzophenone-2 is shown below and it
can be seen that it has two maxima, one at 285nm and another at
345nm. It gives some UVB protection but is particularly useful in
the UVA region, the region that is the principle cause of colour
loss and perfume degradation.
Benzophenone-2 is a yellow powder, soluble in ethanol and propylene
glycol but is insoluble in water and nonpolar oils. It is soluble
at user concentrations in ethanol/water mixtures, olive oil, peanut
oil and many fatty acid esters. When used to protect perfumes, toilet
waters, after shaves etc. the incorporation of benzophenone-2 is
straightforward; 0.2% to 0.5% is generally sufficient and at this
level it is readily soluble in the formulations typically used for
such products. Care must be taken to maintain the pH at 7 or less
as higher pH values may lead to discolouration through oxidation
of the tetrahydroxybenzophenone structure.
When used to protect other cosmetic products against colour loss
the constraints of solubility and pH must be acknowledged. In most
cases 0.1 – 0.2% is sufficient and if surfactants, glycols,
ethanol or oils are present its limited solubility in water should
not cause a problem. Soluble colours used in cosmetic products frequently
have absorption maxima in the UVA range and it is these colorants
that discolour or fade on exposure to sunlight. In an experiment1,
Blue No.1 (CI 42090), Blue No.2 (CI 73015), Yellow No.5 (CI 19140),
Yellow No.6 (CI 15985), Red No.2 (CI 16185) and Acid Red 14 (CI
14720) were each tested for light-fastness in a typical shampoo
base and in each case 0.1% Benzophenone-2 greatly improved the resistance
to fade.
Carbomer gels are frequently used as the basis for hair styling
products, skin balms and moisturizers. If exposed to sunlight the
carbomer is broken down by oxidation and the gel structure degrades
with a consequent loss of viscosity. Adding 0.1 – 0.2% Benzophenone-2
to the gel retards this effect and stability is restored (ref 2).
As many carbomer gels are also coloured the addition of benzophenone-2
confers a double benefit.
Benzophenone-6 is dihydroxy-dimethoxy benzophenone or more correctly,
bis(2-hydroxy-4-methoxy phenyl) methanone and the two hydroxyl groups
of benzophenone-2 are replaced by two methoxy groups in the 4 position.
It is insoluble in water and ethanol but is soluble in organic
solvents used in nail enamels and this is its main field of application.
It is also used in the paints industry for UV protection. Its absorption
spectrum shows broadband protection between approximately 270 and
350 nm and it has a K value of 49.
The third material used to protect products from sunburn is benzophenone-9.
More correctly termed 2,2'dihydroxy-4,4'-dimethoxybenzophenone-5,5'-disodium
sulfonate, it has an additional sodium sulphonate group in the 5
position on each of the benzene rings.
It shows maximum absorption at about 280 nm and a gradual decrease
in absorption towards visible light. It is only soluble at about
1% in ethanol and is not as effective in alcohol-based perfumery
products as benzophenone-2. However it is soluble at 5% in water
thus finding application in surfactant-based products to protect
product colour when packed in clear containers and in clear gel
products to protect the polymer that gives the gel its structure.
It is also less prone to oxidation at alkaline pH.
Summary
So sunlight can cause visible signs of ageing in products as well
as in people but this effect may be delayed by incorporating a suitable
UV absorber into the product. Benzophenone-2 is the material of
choice to protect perfumes and alcohol-based perfumery products;
benzophenone-6 is used to protect nail enamels and other solvent-based
materials and benzophenone-9 is used to protect shower gels, bath
foams and other products with a high water content.
References
Ref. 1 Sperling-Vietmeier, K; UV Absorbers for Product Protection,
1990
Ref. 2. USP 6,007,846, Scented body gel having particulate matter
in the form of glitter with predetermined shapes, December 1999.
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