Hairsprays are usually applied to dry hair and hold it in place by a combination of coating each hair with a thin deposit of stiff polymer, and ‘gluing’ hairs together at points where they cross. Such products evaporate rapidly and the effect is ‘instant’, i.e. the product is applied to the finished hairstyle to hold it in place. Hairsprays were revolutionized by the introduction of the aerosol with its concomitant huge increase in product performance. The pump-spray revolution has been altogether quieter, though no less effective, with concern over ozone depletion adding to already increasing sales. It is now possible to achieve a very fine spray, a choice of spray angles and different delivery quantities per stroke by careful selection of the most suitable pump. Further legislation to control the emission of volatile organic compounds (VOCs) may ultimately favor pump sprays at the expense of aerosols, although water-based aerosol sprays using lower levels of VOCs are being developed.
The degree of hold given by a hairspray may be varied quite easily and is dictated by fashion; more natural styles require only a light degree of hold, while complex styles may require a very stiff finish to hold them in place.
Various descriptive terms, such as light hold, medium hold, stronghold, ultra hold, and mega hold, have been used, the only apparent limitation being the degree of imagination of the marketing department. There is, of course, a limit to the degree of hold which may be obtained but, in practice, this limit is high enough to permit the formulation of a range of products covering normal usage requirements.
The main ingredients in a hairspray are:
- Polymer
- Solvent
- Plasticizer
- Neutralizer
- Perfume
- Other additives
Polymers
Early hairsprays used shellac in either its natural or its dewaxed form to provide the hold (hence the name lacquer). Shellac gives a brittle film that flakes off easily yet is difficult to remove completely. It has now been displaced by modern synthetics, most of which are derived from acetylene.
The first of these to make a big impact on the hair-care market was polyvinyl pyrrolidone (PVP). This is widely available in different degrees of polymerization, which are usually referred to by the K number, derived from the viscosity of a dilute solution of the PVP relative to that of water. K numbers between 30 and 90 are common.
PVP is very hygroscopic and absorbs sufficient water from humid atmospheres to appreciably soften the deposited film and render it unacceptably tacky. Copolymerizing the vinyl pyrrolidone with a harder, more hydrophobic material, such as vinyl acetate overcomes this, and copolymers in which the ratio of VP to VA can be varied at will.
More recently, resins have continued to improve on the balance of properties obtainable, but such optimization always gives rise to trade-offs of properties. Ideally, a hairspray polymer should be soluble in ethanol, and be hydrophilic enough to be easily removed from the hair by shampooing, but hydrophobic enough to be compatible with hydrocarbon propellants for aerosol use. It should give a powerful hold without brittleness, yet be easily removed by combing or brushing, and must have good adhesion, yet not be sticky. All these properties preferably should be combined with antistatic and conditioning properties, and a competitive price.
Some polymers have free acidic groupings in their molecules and can be fully or partly neutralized with a suitable base, modifying their properties considerably. The bases most commonly used are 2-amino-2-methylpropane-1-ol (AMP), 2-amino-2-methyl-1,3-propanediol (AMPD), triethanolamine (TEA), triisopropanolamine (TIPA) and tris (hydroxymethyl) amino methane (Tris).
For any given polymer system, an increase in solids will give a firmer hold. However, the relationship is not always linear and, where very firm holds are required, better results can sometimes be obtained from a mixture of polymers. The viscosity of the formulation should be kept as low as possible and a suitable pump spray chosen (the lower the viscosity, the easier it is to obtain a fine spray).
Hairspray polymers can be quite difficult to handle. Many are sold as concentrated, highly viscous solutions (usually in ethanol), which are hard to empty out of drums and have a remarkable propensity to stick to anything and everything. Those supplied in solid form are more amenable, although more efficient stirring may be required and batch processing times may be longer.
Solvents
For non-aerosol products, there is really only one solvent-ethanol either alone or diluted slightly with water. It has a suitably rapid evaporation rate, an inoffensive odor that is not too difficult to mask, excellent solvent properties for most resins, and is reasonably priced.
Isopropanol has been used in some countries with a high excise duty on ethanol but has a slightly slower evaporation rate and an objectionable smell. Ethanol B, denatured with Bitrex (CTF A name denatonium benzoate) and marked with t-butyl alcohol, is now standardized throughout the EC.
In some countries (e.g. Norway and Sweden) additional denatonium benzoate must be added to comply with requirements for alcohol denaturing.
The addition of water to the solvent system may be necessary:
a) to improve the solubility of an ingredient not fully soluble in ethanol;
b) to retard evaporation rate;
c) to reduce cost; and
d) to prevent precipitation from an acid-resin-based system that has been neutralized to a high level. A high percent neutralization will often increase water solubility and reduce alcohol solubility and, in some cases, a colloidal solution results. In the long term, precipitation can occur when the colloidal particles coalesce.
Plasticizers
Plasticizers are used to modify the properties of the polymer film, making it more flexible. Only small quantities are normally required; 5% of the dry weight of the polymer is a good rule of thumb. A huge number of different materials have been used as plasticizers, including a variety of esters (usually liquid), various silicones (phenylmethyl silicones have good alcohol solubility and are especially useful), silicone glycol copolymers, proteins, polyols, lanolin derivatives, etc. It should be remembered that other materials in the formulation, for example, perfume, may act as plasticizers, even if that is not the purpose of their inclusion. A poor choice of plasticizer may have adverse effects such as weakening and dulling of the film. This can occur when the plasticizer is not fully miscible with the resin, leading to a discontinuous film with high local concentrations of plasticizer. A useful check is to examine some evaporated product on a glass microscope slide to check whether the deposited film is dull or discontinuous. Although plasticizers are not always necessary, the stiffer the polymer film, the more plasticizer is likely to be needed.
Neutralizers
The amount of neutralizer required for a given percent neutralization can either be obtained from polymer manufacturers’ literature, or simply be calculated from the acid value of the resin. The polymer properties may be modified by the neutralizer used, and it is wise to follow the polymer manufacturers’ recommendations.
Perfume
Hairsprays are not difficult to perfume, in that there is not usually a solubility problem. Perfumes should be strong in odor and, to avoid interference with the plasticizing of the resin should be used at low levels. The raw odor of the ethanol may be quite difficult to mask and the residual odor on the hair may need careful control.
Other additives
Preservatives are unnecessary in ethanol-based products such as hairsprays. Many other additives, such as vitamins, proteins, amino acids and herbal extracts, are used. Most of these are present in very small amounts and cannot seriously be expected to dramatically affect product performance. Ultraviolet filters have become a fashionable addition and some work has demonstrated that they may be of value in leave-on styling products, although some of the studies have used unrealistically high levels of UV filters.
There seems to be little doubt that hair can be damaged appreciably by ultraviolet light (and other environmental influences).
References:
- Tazi, M. and Login, R.B. (1990) Progress in water-based hairspray. Presented at the SCS symposium on The Future of Hair Care Technology, Harrogate, November 1990.
- Petter, P.J. (1987) Trends in hair care products. Soap, Perfumery Cosmet. 60(95) 29.
- Petter, P.J. (1989) Acetylene derived polymers and their applications in hair and skin care. J. Cosmet. Sci. 11(1) 35.
- Williams, D.F. and Schmitt, W.H. (1992) Chemistry and Technology of the Cosmetics and Toiletries Industry
- Georgalas, A. (1993) Photoprotection of hair. Toilet. 108(3) 107.
- Dubrief, C. (1992) Experiments with hair photodegradation. Toilet. 107(10) 9
