10 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS though the corrosion products of aluminum are generally white in color and not as unsightly as the brown iron rust which develops on black iron or sometimes on tinplate. Protective coatings for metal plate are used both on the inside and outside of the caps. For aque- ous products, an oleoresinous type is generally used, though synthetic coatings are available which have good alcohol, acid and alkaline re- sistance. Similar coatings may be applied on the outside of the metal, or the metal base may be litho- graphed and, if lithographed, is pro- tected by a clear coating over the lithography. Very attractive colors varying from black through the solid colors and pastel shades to white can be used. In the manufacture of metal caps, the protective coatings are i•pplied to the plate in the fiat, and from the decorated plate blanks, generally of the shape of a stiff straw hat, are first produced. These blanks are then put into thread roll- ing machines which turn the bead, form the thread, and form the knurl if knurled caps are being produced. Knurling increases the finger hold and aids in the removal of closures. The knurling tools tend to rupture the coating and form focal points which can develop corrosion, though on the inside of the cap the exposed portions are generally protected by the liner and under normal storage the outside does not develop rust. It is interesting to note that the protective coatings and decorations are applied to the fiat sheet. The sidewall of the blank in which the threads are formed is subject to some stretch during the punching opera- tion. This stretches the' coating, and the coating is then further stretched when the threads are formed in the closure. It is really remarkable that, under this drastic treatment, protective coatings have been formulated which will take the stretch, remain properly adhered to the metal, and afford additional protection against rust. Metal caps are produced in sizes from 15 to 120 mm. The smaller sizes are more difficult to make and control to accurate dimensions, though closures from 18 mm. on up are quite satisfactory. Molded closures are relatively newcomers in the closure field, being about twenty years old. They are produced from compounded syn- thetic resins, and are of two types, namely, the thermoserring or those made from thermoplastic materials. Molded closures are produced to accurate dimensions, and these' di- mensions depend upon the accuracy to which the molds themselves are made, for the molding materials faithfully reproduce the various contours of both the cavities and force plugs which form the outside and the inside portion of caps. All molding materials shrink to some degree on cooling however, allow- ances for this are made in the manu- facture of the tools, so that accurate caps are produced. The'most generally used thermo- serring molding materill is of the phenol formaldehyde resin type.

THREE PRIME FACTORS IN SUCCESSFUL PACKAGING 11 The resin is filled with wood flour as an extender, with pigments and lubricants added. The standard type of wood flour filled material is available in the solid colors, though pastel shades cannot be produced since the resin is inclined to. be dark in color. This type of compound has good resistance to atmospheric moisture and will withstand steam autoclaving, though with some loss of gloss and a tendency toward roughness in the finished piece. Mineral-filled compounds are avail- able at a considerable increase in cost, which will withstand steam sterilization. The urea formaldehyde resins lend themselves to the molding of caps in the white or pastel shades. Alpha cellulose is generally used as the filler with the urea formaldehyde resins. Such caps have adequate moisture resistance for general use, but tend to chalk or change color when immersed in water, and do not resist steam sterilization. Origi- nally the urea formaldehyde caps were inclined to be slightly trans- lucent, and when a liner with back- ing which was opaque or dark in color was applied in the cap, the top portion would appear to be darker in shade than the sidewall. This con- dition can be corrected by using a more nearly opaque molding ma- terial or, in some cases, a lighter colored backing material, if avail- able. Melamine resins are currently being placed on the market. These can be produced in pastel shades and have excellent water resistance--in fact, will stand steam sterilization. Currently, they are high in cost. The thermoplastic molding mate- rials are just what their name im- plies, that is they soften with heat, even after molding. The thermo- plastic materials which have been used are cellulose acetate, cellulose aceto-butyrate, methyl methacryl- ate, ¾inylite, and polystyrene. Polystyrene has been used for the production of closures due to its ex- cellent acid and alkali resistance. It is also quite resistant to moisture attack. Transparent closures may be produced, though, generally speaking, they are unsightly on the package, due to the fact that the bottle finish threads and liner are readily apparent through the trans- parent materials, which does not add attractiveness to the package. These materials are available for the production of opaque closures. The thermoplastic materials are all sol- uble in organic solvents conse- quently, they do not lend themselves too well to use with perfume or toilet waters containing essential oils. While the closure itself may not, during the shelf life, be exposed to the contents of the container, during first use some of the product will remain on the thread and may then soften the inner portion of the cap. In use with perframes, some of the product may remain on the finger, and when the cap is again applied to the container, the essential oil may remain on the outside, being less volatile than the solvents contained in the product, and cause a softening of the molded part.