FINE PARTICLEs IN THE COSMETIC INDUSTRY By H. W. HIBBOTT, M.Sc., PH.D., A.R.I.C. A lecture delivered before the' Society on Friday, January 4th, 1957 IN APPROACHING this subject the first thing of note is the immense amount of work that has been done particularly in the last thirty years. Apart from academic research, much work has been carried out by many industrial organisations, so that literature on the subject is very widely dispersed through scientific journals in this country, America and Europe. The main difficulty in preparing this paper has been rather what to leave out than what to put in. The information on particulate matter desired by one industry will differ appreciably from that of another, depending on the use for which the material is required. For instance, in cement the setting properties depend on the total surface of the particles, and this feature is important also for powdered fuels and explosives. On the other hand, many industries are interested in particle size range. In glass polishing the interest is in the very small fraction at the coarse end of the range of particle size, for in this case the incidence of one comparatively large particle in ten million can cause great trouble by deeper scoring of the glass surface. In the pharma- ceutical industry there is interest in the particle size of some medicaments, for where such products are ingested only slowly the particle size will control the rate of assimilation and therefore dictate the time of effectiveness. In the cosmetic industry the more important needs are performance and efficiency of grinding processes powder separation, transmission and collection control of particle size in finished products. In general, for cosmetic work particle sizes should be less than about twenty-five microns because solid particles above this size may be recogrdsed by sight and touch as single particles. Moreover, for dry applications to the skin the finer the particle size of a given material the more adherent it is to the skin surface. In considering the raw materials for this industry, naturally occurring minerals must be ground in some way manufactured materials may be produced chemically in a fine enough state of sub-division or may require to be ground after manufacture. The fineness of some matehals may be dictated by the needs of other large industrial users. Other materials may be produced specifically for the cosmetic industry. The range of particle size of materials used in this industry are of the following order: 186

FINE PARTICLES IN THE COSMETIC INDUSTRY 187 Talc ............ 150 5 microns Kaolin ............ 35 -- 1 micron or less Pptd. chalk .......... 10 -- below 1 micron Magnesium carbonate ... 5 -- below 1 micron Metal soaps ......... 50 -- 1 micron Zinc oxide ......... 5 -- 0.25 microns Titanium oxide ......... 0.5 -- 0.1 micron Natural iron oxide colours ... 50 -- 5 microns Synthetic iron oxide colours ... 25 -- 2 microns Lakes and pigments ...... 10 -- below 1 micron Processing of such materials involves mixing and grinding by wet or dry methods. For wet grinding methods, the triple roller mill and high- speed cone mill are most used. In both cases the grinding face clearances of the machines are controllable down to about 30 microns although usually grinding operations are carried on with gaps much wider than this. Perhaps of more interest for the purposes oi this paper are the dry grinding processes. The types of mill most favoured are the high-speed beater mills and to a small extent the vapour-jet pulveriser which are continuous feed processes, and ball mills which are batch processes. For a given material each type of mill has its own optimum efficiency in terms of increase of surface for a given energy input. Again each type of machine will have a best working range from a certain size of fed material to a certain size of ground product. These variations are connected with the friability of the material and design of machine. In high-speed beater mills the variables are feed rate, speed and and shape of hammers and design features such as incorporated sieves and air-classifying systems. In ball mills the performance will depend on the shape, size and load of balls, the load of material and the number and rate of revolutions of the machine. Wherever cushioning of coarse particles in a bed of fine particles can occur in a grinding process there will be loss of grinding effect and energy will be wasted in heating up the product. For the continuous operating machines close attention should be paid to keeping the feed rate as constant as possible. As will be appreciated from the general size range of cosmetic raw materials much of the capacity of grinding machines will be devoted to breaking up agglomerates of individually fine particles rather than primary comminution of oversize particles. In other words, such machines are operating largely as mixers but the energy input per given weight of material is appreciably greater than in conven- tional mixing machinery. In considering methods of particle size measurement it will become obvious, and needs to be kept in mind constantly, that adequate sampling of the material is of prime importance. Theory and practice of sampling is a large subject in itself and has a very extensive bibliography. One should be reasonably. sure that the sample to be examined is representative of the bulk.