| Elizabeth
Connock BSc (Hons), Chimica Oggi (January/February 1998), pp. 38-40.
Arguably no single class of materials has had such an impact on
modern cosmetic formulation as the silicones. Unlike the majority
of herbal extracts whose main appeal is their name and picture on
the label the silicone compounds are added purely for their functionality
and, one may say, despite their name in the ingredient listing.
The first silicone compound to make an impact was dimethyl siloxane,
commonly called silicone oil and now known as dimethicone according
to the International Nomenclature of Cosmetic Ingredients. Initially
its principal appeal was its waterproofing properties and its main
use was in barrier creams. However it was soon realised that 2 -
3% of dimethicone greatly improved the skin-feel of cosmetic compositions
and reduced the soaping effect of the triethanolamine-stearate emulsions
which were popular at that time.
The viscosity of dimethicones can be precisely controlled and they
exhibit true Newtonian behaviour. Standard grades are available
ranging from 1cs to 60,000cs and by careful selection of the correct
viscosity material a wide range of properties may be conferred on
the finished product. Thus the lower viscosity products are miscible
with mineral oil and isopropyl myristate and are useful for aerosol
applications, reducing the whitening effect of aluminium salts in
antiperspirant sprays. The 100cs - 500cs are the traditional silicone
"oils" used as additives in skin care formulations and
the higher viscosities are used in barrier products or may be dissolved
in cyclomethicone for use in hair serums.
The next breakthrough in silicone based ingredients was the volatile
silicones and, in particular, the cyclomethicones. Two grades are
in common use, the tetramer and the pentamer: the tetramer has a
viscosity of 2.5cs and a boiling point of 176°C whereas the
pentamer is 4.2cs and 210°C, respectively. Both materials are
volatile at skin temperature and on evaporation they impart a soft,
dry, velvety feel to the skin. They find application in liquid makeup
as pigment dispersing aids; the resulting product leaves a smooth,
matt finish on the skin. Their main field of application is in deodorants
and antiperspirants. In aerosol products they act as solvents, replacing
the alcohol or isopropyl myristate content; they reduce stickiness
and the tendency for nozzles to clog and are not so cooling on evaporation
as ethanol. In stick products they give a non-sticky product which
applies without drag. In sun products they are useful as solvents
for the active material and as dispersing aids for micronized oxides.
In emulsions they give a soft, smooth product which is pleasant
to apply and has an unique skin feel.
Silicone compounds with their soft, silky feel would seem to be
natural additives for hair care products but, long before 2-in-1
shampoos and hair serums appeared, a suspension of amodimethicone
with a quaternary ammonium salt was used in hair conditioners and
in conditioning hair sprays. Initially a tallowtrimonium salt was
used but now the better products use cetrimonium chloride as the
cationic conditioning aid. Another recent introduction for hair
conditioning is trimethylsilylamodimethicone. This branched chain
silicone fluid gives excellent wet and dry combing and is available
in emulsion form, greatly simplifying its incorporation into hair
conditioning products. The hair serums are generally solutions of
dimethicone or dimethiconol in cyclomethicone, the latter reducing
product viscosity and improving spreadability while the dimethicone
or dimethiconol is deposited on the hair shaft as a thin film, giving
an unique feel of slip and smoothness and conferring a high gloss.
Incorporating phenyl trimethicone into the product enhances the
gloss still further and hexamethyldisiloxane may be added to increase
volatility. This material is also used as a replacement for ethanol
in antiperspirants and aerosols. Phenyl dimethicone was introduced
as an alternative to dimethicone; although considerably more expensive
it gives an unrivalled sheen to hair. Its low viscosity and complete
solubility in isopropanol or ethanol make it an obvious additive
in hair sprays. Similarly, diphenyl dimethicone is soluble in 99%
ethanol and organic solvents and has a very low surface tension
so is also used to add gloss and enrich hair colour when incorporated
in hair sprays and blow-dry styling aids.
Hair product patents in the nineties have been dominated by those
for 2-in-1 shampoos incorporating silicone compounds. One (REF 1)
filed by Helene Curtis includes an excellent description of the
difficult handling of recently shampooed hair, both when wet and
dry, and describes the benefits of incorporating suitable conditioning
materials in the shampoo. Cationic materials have good conditioning
properties but are generally incompatible with the anionic cleansing
and lathering agents. To overcome these problems various patents
disclose the use of non-ionic, amphoteric and zwitterionic compounds
used in conjunction with cationic materials. Another difficulty
in formulating a conditioning shampoo is an instability problem
that results from incorporating a non-volatile silicone in a formulation
and keeping the non-soluble silicone suspended in stable form without
detracting from the performance of the shampoo.
The Helene Curtis patent used a fatty acid to improve conditioning
and product stability, other patents cite the use of xanthan gum
or a carbomer as suspending agents and a quaternary imidazolinium
compound used in combination with a non-volatile polyalkyl siloxane
or a polyether siloxane copolymer was also the subject of a patent
(REF 2). Yet another patented 2-in-1 shampoo formulation cited a
silicone-polyether copolymer in conjunction with quaternium-27 and
claimed the conditioning effect could be further improved by the
addition of ethylene glycol monostearate although it was more probably
used to hide the impossibility of dissolving the silicone (REF 3).
According to a Procter & Gamble patent, for best results the
silicone should be insoluble in the shampoo and a polyalkyl siloxane,
a triphenyl dimethicone or a polyether siloxane copolymer is preferred
with long chain acyl derivatives used as suspending agents (REF
4). Dimethicone is also available as a water-based emulsion with
up to 60% solids content depending on the source. The emulsifiers
are laureth-4 and laureth-23, the non-ionic nature of which does
not interfere with either anionic cleansing agents or cationic conditioning
aids.
Whereas the majority of 2-in-1 shampoos are based on insoluble
silicone compounds suspended in an opacified shampoo, trimethylsilylamodimethicone
may be used to prepare clear shampoo. It is available as a microemulsion
with octoxynol-40, isolaureth-6 and glycerine and is readily dispersed
in aqueous systems including shampoos and pump-spray conditioning
mists. Many blends of silicone compounds are available; sometimes
this hides the true nature of the material, other times it is useful
if the blend needs sophisticated mechanical or chemical process
plant to ensure its homogeneity. One useful blend seems to be dimethicone
with trimethylsiloxysilicate; it is soluble in isopropanol and isopropyl
myristate and greatly enhances the durability of makeup. Other blends
include dimethicone and/or dimethiconol with cyclomethicone and
cyclomethicone and/or dimethicone with dimethicone copolyol.
Dimethicone copolyols were probably the first water-soluble silicone
compounds to find cosmetic application. They are surface active
and particularly useful as emulsifiers when preparing water-in-(volatile)silicone
emulsions. This has led to spectacular improvements in such diverse
products as antiperspirants, liquid makeup and inorganic sunscreens.
More recently dimethicone copolyols have been used in the preparation
of triple phase emulsions such as water-in-silicone-in-water. The
key feature of the siloxane emulsifier is its ability to form an
efficient, high molecular weight viscoelastic film at the water
and oil interface. The siloxane emulsifier remains intact as the
water-in-oil emulsion is then dispersed further in the new water
phase to form the multiple phase emulsion (REF 5). This system allows
water soluble active ingredients to be encapsulated within the dispersed
aqueous phase and protected from atmospheric oxidation or from antagonistic
material in the outer, aqueous, continuous phase.
A further development described in the same paper is for a non-aqueous
emulsion whereby propylene glycol is the dispersed phase and which
can act as a carrier for water-sensitive active materials. The continuous
phase is a volatile cyclomethicone which, on evaporation, deposits
the active on the skin in a form similar to that of liposomes. Suggested
applications are the delivery of enzymes, vitamins and antioxidants
from preservative-free products that are aesthetically pleasant
to use. There are many other applications for dimethicone copolyols
including their action as plasticizers in hair styling products,
as emulsion stabilizers and as emollients in aqueous-alcoholic lotions.
Such diversity does not come from just one material; the name is
generic for a class of compounds which vary from a low viscosity
liquid to a pasty solid and from water soluble to water insoluble
so careful selection is required for specific applications.
Silicone esters were introduced in the early 1990s; diisostearoyl
trimethylolpropane siloxy silicate proving to be particularly useful
in liquid makeup and sunscreen products. It combines good lubricity
and spreadability with excellent skin substantivity. It is an efficient
solvent for organic sunscreens and enhances the water resistance
and SPF values of both organic and inorganic based preparations.
Work by Woodruff (REF 6) demonstrated the importance of good dispersion
of the inorganic oxide in a suitable vehicle and a paper published
by Connock (REF 7) showed a significant increase in SPF values when
silicone esters formed part of the non-aqueous phase. The growth
in sun protection products based on micronized inorganic oxides
has been nearly as spectacular as that of silicone compounds and
the latter materials have contributed substantially to the success
of the former. Cetearyl methicone is a waxy solid which was initially
recommended for protective hand products but its excellent lubricity
means it can contribute substantially to enhancing SPF values by
ensuring the deposit of an even film of product. Cetearyl methicone
does not significantly hinder transepidermal water loss yet imparts
a high degree of water resistance to products.
The safety of silicone compounds is well established and the CTFA
CIR (REF 8) report states that silicone compounds do not readily
cross membrane barriers and are not absorbed through the skin. They
are not metabolized by the body or by micro-organisms and are relatively
innocuous when administered orally or parentally. Their cosmetic
uses are widespread, appearing in approximately 40% of all personal
care products currently on sale in the USA and their main function
is as co-solvents, dispersing aids and film formers and as replacements
for ethanol and isopropyl myristate in particular applications.
Their chemical and physical properties may be carefully controlled
in manufacture which enhances their appeal as functions can also
be precisely controlled. They have been responsible for many major
advances in cosmetic preparations, improving both their efficacy
and their aesthetic properties and have lead to many novel applications.
Finally, as is the case with so many cosmetic ingredients, many
so called specialities tend to hide behind trade names but now,
with legislation enforcing ingredient labelling, it is apparent
that many are no more than useful commodities. Purchasing them as
such can save money while not diminishing their properties.
REFERENCES
REF 1: Helene Curtis, Patent 0 555 691 A1; Patent for a stable,
conditioning shampoo; filed 27/01/1993.
REF 2: General Electric Co; Patent 0 581 442 A2; Novel shampoo
composition; filed 28/06/1993.
REF 3: General Electric Co; Patent 0 598 531 A2; 2-in-1 shampoo
and conditioner with silicone-polyether copolymer; filed 08/11/1993.
REF 4: Procter & Gamble Co; Patent 0 615 742 A1; A shampoo
composition comprising an insoluble, non-volatile silicone; filed
30/05/1994.
REF 5: A Zombeck and G Dahms; Novel formulations based on non-aqueous
emulsions of polyols in silicones; proceedings of the 19th IFSCC
Congress, Sydney, Australia, 1996.
REF 6: J Woodruff; Formulating sun care products with micronized
oxides; Cosmetics and Toiletries Manufacture Worldwide 1993.
REF 7: A & E Connock (Perfumery & Cosmetics) Ltd; The use
of silicone esters in enhancing SPF values of inorganic sunscreens.
REF 8: Silicone and silicone compounds etc; The Cosmetic Ingredient
Review; published by the CTFA.
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