10 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In addition, with aluminium collapsible tubes the annealing conditions should be such that there is maximum lubricant removal as well as the formation of the optimum oxide film for lacquer adhesion. This involves something of a compromise, as cleaning of the surface is more efficient at higher temperatures, whilst the type of oxide film formed at these tempera- tures is not the one that gives maximum adhesion. Dealing with our second conclusion, it is very obvious that one of the main drawbacks to present one-lacquer systems is the fact that the lacquer is applied at an early stage of the container's production, i.e., following annealing or degreasing, and that no matter how many coats are applied, or how good the film build, damage and possibly contamination must occur during subsequent operations. Secondly, there is a limit to the film thickness which can be applied without brittleness. Finally, high speed automatic spray- ing leaves something to be desired in the way of continuity. Recent systems overcome these defects by utilizing two coats of dissimilar matehals, the second coat being applied at the final stage of the container's production. This type of system gives very good protection, for apart from compensating for flaws in the first lacquer coat, the barrier provided by a laminate of dissimilar organic materials has very high resistance to penetration. Flush coated wax and spray coated vinyl-based lacquers are at present in production use as final coats for use with corrosive products, although wax itself, and spray coating in the case of the vinyls leave much to be desired. Development work in this connection is mainly directed at substituting thermoplastic polymers for wax, and flush coating plastisols or organosols based on high molecular weight vinyl resins, in place of spray coated vinyl lacquers. In this way, very thick films of inert solids can be built up as a final coat. TESTING The only effective way of determining the suitability or otherwise, of lacquered aluminium containers is by subjecting a sufficient number of filled specimens to shelf life tests at varying temperatures, and by extracting specimens at regular periods and examining the product and product/lacquer contact areas visually and by means of a surface microscope. Care should be taken to ensure that the specimens are representative of bulk production, and that the specimen containers have been through all the stages of pro- duction, otherwise, as has been mentioned earlier, a more favourable result may be obtained than with bulk production containers. ELECTRO-CHEMICAL TESTS Two standard types of test are in use for determining the standard of lacquer coverage on aluminium containers. The first consists basically of

PROTECTIVE LACQUER SYSTEMS FOR ALUMINIUM CONTAINERS 11 filling the container with an electrolyte, usually acidified copper sulphate, inserting an electrode and passing a small voltage D.C. current through the cell thus formed. The current or voltage is then noted as are any areas on the container on which metallic copper has been plated out. The second method is based on the same principle, i.e., the resistance of a lacquered surface is inversely proportional to its porosity, and consists of filling the container with a 2% solution of sodium chloride, inserting an electrode and measuring the ratio of the resistances encountered, by a current passing through the test container and an unlacquered container. A 2 volt 800 c.p.s. current is used to obviate polarization effects which may give fallacious results. Rapid information may be gained regarding the film penetrative effects of a product by using a galvanic cell of the Denison type, in which a cutting from a standard lacquered container is made the anode, and a noble metal the cathode, with the actual product the electrolyte. The cell is then coupled to a recording micro-ammeter and a current/time curve obtained which is characteristic of the product under examination in relation to the lacquered anode. When using any method of inspection based on electro-chemical measure- ments, it is essential to first establish correlation with shelf life trials, as the observations obtained by these methods are quite meaningless in themselves. In investigational work it is sometimes necessary to examine a detached lacquer film, and this can best be achieved by filling the container with molten wax or a hot melt vinyl compound, cooling and then dissolving the aluminium in hydrochloric acid or caustic soda. A replica of the container is thus obtained with the complete lacquer film exposed on the outside which may be examined in detail if care is taken in handling it. In conclusion, it is hoped that this paper, which has attempted only to state the present general position and draw attention to defects and some causes of failure, indicates that sufficient is known to make advances in the field of protective lacquering systems so that suitable containers will be available whenever required. (Received: 17th May 1961) GENERAL REFERENCES Bailey, K.C. The Elder Pliny's chapters on chemical subjects 61-101 (1932). (Edward Arnold, London.) Denison, I.A. Corrosion Handbook. 1038. E.P.A. Technical Digest No. 17. Luff, G. & Frederighi, T. Alurninio, No. 4. 391. (July 1954.) Mayne, J. E.O. Corrosion 5, (1952). (Butterworths Scientific Publications, London.) Uhlig, H.H. Corrosion Handbook (1948). (Chapman & Hall, London.)