HAIR LACQUER SYSTEMS AND THEIR EVALUATION 481 xII PVP .......... 2.0% Ethanol (absolute) ...... 41.6% Water .......... 34.4% isoButane ........ 22.0% CORROSION When a mixture of propellants 11 and 12 is used for a pressurized hair lacquer, the water content of the finished pack must be kept to a minimum to prevent the occurrence of serious corrosion. It was thought that the cause of the corrosion was due to the acid produced as a result of hydrolysis of propellant 11 until Sanders 25 showed that the major cause for the develop- ment of acidity was due to the reaction of ethyl alcohol with propellant 11. Following this discovery, further work carried out showed that nitromethane added at 0.3% to propellant 11 was an efficient stabiliser for this alcohol/ propellant system 26'". The use of propellant 12 alone, usually at about 35ø,/0, considerably reduces the risk of corrosion, and many pressurized hair lacquers using this system are now being marketed. It should be noted that aluminium containers used for any t•vpe of hair lacquer must be internally lacquered with a system giving 100% protection. When tinplate containers are to be used for water-based hair lacquers, the internal protection of the container becomes of paramount importance. Details of the various lacquer systems used for the body and side seams of tinplate containers are given by Johnsen •8'•9, and Garten aø. Prolonged shelf tests are necessary for water-based hair lacquers, and test containers should be filled on a production basis as well as in the laboratory, because there are some factors not yet fully understood which affect shelf life. The amount of air present in the containers is certainly an important factor. COST The chlorofluorohydrocarbon propellants are expensive by comparison with the hydrocarbon propellants and, usually, unless the concentrate in- cludes expensive perfumes or other ingredients, with the concentrate itself. Propellants 11 and 12 are normally used, as cost rules out the use of pro- pellant 114, despite its advantages in some respects. The tendency to use high concentrate/low propellant ratios has been encouraged by the pre- ponderance of pressure filling equipment available, and further economies result from the increased filling rates.

482 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The water-based products offer not only the advantage of a cheaper propellant system but also of a cheaper concentrate, as about half of the ethyl alcohol used in the conventional hair lacquer is replaced by de-ionised water. The extra cost of internally lacquered containers, special valves, and the increased capital cost incurred as a result of the need for additional safety equipment for hydrocarbon filling, do not materially offset the saving. It might be noted that the somewhat slower drying time of these lacquers is not always preferred. PROPELLANT/CONCENTRATE RATIO AND CHOICE OF VALVE TYPE The particle size distribution, and the spray pattern, are dependent upon several factors including the surface tension and viscosity of the product, the ratio of propellant to concentrate, the type of propellant, and the type of valve used. The higher the proportion of propeRant in the emerging product, the greater is the volume of vapour formed and thus the greater is the degree of atomization. The higher the vapour pressure of the propellant used, the more rapid is the vaporization, and again the degree of atomization is greater. A hair lacquer based on 70/30 or a 65/35 propellant/concentrate ratio, using a standard valve actuator, will produce a suitable spray, but as the ratio of propellant to concentrate decreases, the spray becomes progressively jet- like and a valve with a mechanical break-up actuator must be used to produce a suitable spray. These actuators vary in design and some are more efficient than others, but the general principle of operation is the same. The exit of the liquid stream is mechanically impeded so that some of the kinetic energy of forward movement is used to disrupt the liquid mass. The jet which would be produced by a standard actuator is replaced by a relatively coarse spray of lesser forward velocity. Figures 1--6 demonstrate the effect of the various combinations of pro- pellant type, propellant/concentrate ratio, and valve actuator. The spray pattern apparatus described by Gunn-Smith and Platt a• was used to record the spray patterns. The following basic concentrate, coloured with Gentian violet, was used throughout. DMHF resin ........ 6.25% Lanolin derivative ...... 0.45%o Propylene glycol ........ 0.45 ø/o Perfume .......... 0.75% Industhai Methylated Spirits 74 o.p. .. 92.10•o The spray pattern shown in Fig. I is suitable for all types of product, but is particularly useful for products where considerable holding power is required. The relatively narrow spray cone enables individual curls to be