478 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS addition of plasticizers or softeners is necessary to control the flexibility, adhesion, gloss, clarity and moisture sensitivity of the film. Many ingredients used in hair lacquer formulations possess plasticizing or softening properties, although they may be included in the formulation to fulfil other functions. Lanolin derivatives, cetyl and oleyl alcohols, isopropyl myristate and other similar materials used as hair "conditioning" and spreading agents, may act as softeners or true plasticizers. Perfumes, which may be present in con- centrations as high as 20ø,/o in relation to the resin content, contain essential oils, esters, ketones, alcohols, etc., and their plasticizing or softening effect may be considerable. Silicone fluids, included to increase water repellency, lubricity and gloss, act similarly. Since the addition of true plasticizers is also necessary, the combined plasticizing effect of the various ingredients must be carefully checked to ensure that the final film is satisfactory in all respects. It is not only necessary to check that the final concentrate and propellant are compatible, but also to ensure that, during the drying of the film, there is no gelling or precipitation of any individual ingredient which will result in a lustreless film of poor adhesion. This applies particularly to the water- based product xvhere the solvent composition changes sharply as the film dries. Some ingredients function as softeners rather than as true plasticizers and may exude from the film, which may or may not be desirable because, although the exudate may act as a lubricant and impart anti-static pro- perties, it may also render the film dull. Examples of the types of plasticizers used with various film formers have already been given, and it is only necessary to state the general principles underlying the choice of a plasticizer, which are governed by the physical properties of the resin concerned, and by the properties required of the final film. With the more hygroscopic resins, water-miscib!e and humectant plasticizers should either be avoided or used in very low proportions. Glycols increase the tackiness of PVP films sharply. For the water-resistant resins, water-miscible plasticizers such as diethylene glycol oleate, triethyl citrate, and glycols may be used. Plasticizers, or modifying agents with surface active properties, will assist in the spreading of the hair lacquer over the hair. This property is more important witIt the water-based product because of the higher surface tension of the deposited droplets. An interesting class of "stiffening" agents for pressurized hair lacquers has been described •0. The incorporation of certain substituted benzophenone derivatives, particulxrly 2.2%.4•tetrahydroxybenzophenone, is reported to increase markedly the stiffness of hair lacquer films.

HAIR LACQUER SYSTEMS AND THEIR EVALUATION 479 PROPELLANT SYSTEM The pressurized hair lacquer is a surface coating product, and so the quantity and composition of the propellants used must produce a relatively coarse spray, in conjunction with the particular valve used. This aspect has now assumed greater importance because if inhaled spray particles do constitute a health hazard, then the danger increases as the particle size decreases. Several other major considerations, including flammability characteristics, corrosion problems and cost, also govern the choice of propellant system. The chlorofluorohydrocarbon propellant systems used for hair lacquers usually consist of a mixture of propellants 11 and 12, or propellant 12 alone. Methylene chloride and 1,1,1-trichloroethane are also used in conjunction with propellant 12, serving both as solvents and pressure depressants. A combination of propane and propellant 11, referred to below, has not, so far, been used in Great Britain, largely because the odour of the propane available has been poor, although it is now improving. The ratio of propellant 11 to propellant 12 may be varied, although the 50/50 mixture is most frequently used, partly because of economic con- siderations. By increasing the proportion of propellant 11 to propellant 12, the internal pressure of the pack is reduced and a wetter spray of larger average particle size results. The refillable "mother and daughter" packs normally contain a propellant mixture having a preponderance of propellant 11, as the necessary over-pressurizing of the "mother" pack with nitrogen may otherwise create pressures which are too high for safety, particularly as a larger can is used for the "mother" pack. It might be noted here that the refillable "daughter" pack must be thoroughly purged of air. The use of propellant 12 alone reduces some of the problems of compati- bility and perfume stability. It also has the dubious advantage of giving the purchaser the impression that the pack contains a greater quantity of product, as the volume occupied by a 65ø,/0 concentrate/35% propellant 12 fill is about 15--20% greater than that occupied by a product containing 35% concentrate and 65% of a propellant 11/12 mixture. FLAMMABILITY The flammability of hair lacquers is under investigation, but having regard to the vast number which have been produced and used, without mishap, it may well be that the danger is exaggerated. Nevertheless, the formulator of a pressurized hair lacquer must consider this aspect carefully, and choose the least flammable formulation consistent with efficiency. The methods employed for testing the flammability of pressurized pro- ducts, and the interpretation of the results obtained thereby are, themselves, the subject of controversy. However, the aerosol industry throughout the