THE COLLAPSIBLE TUBE pigment, which had a pH of 7 and yet it was through the wall of a pure tin tube within a month. Finally, there are the electrochemical investigations which can be quite revealing in view of the fact that such a high proportion of metallic corrosion is electrolytic in origin. Simple potential measurements between the metal and a calomel electrode in the product have little or no value. Extremely useful information can, however, be obtained by setting up a cell with the product in between a Noble metal electrode and the metal of the tube wall. A microammeter, preferably in the form of a recorder, placed across this cell, will reveal its current-producing characteristics, and these in turn are a measure of the product's corrosive powers in a rather highly magnified form. I have found it convenient to utilise a Cambridge Recording Thermo- couple instrument in conjunction with a shunt assembly which can be switched to give 0-60, 0-120, 0-300 microamp ranges whilst keeping the total circuit resistance constant. The recording is continued over a period of four to five days and it has been possible, by comparing the records of a large number of products of well-known behaviour, to interpret the results with considerable accuracy. There are at least four distinct types of curve, certain irregularities, and an overall current value to guide the assessment. Once again, we do not claim 100 per cent accuracy because there is the inevitable, somewhat ill-defined, dividing line between safe and unsafe. The method is particularly good for comparing the effectiveness of experimental corrosion inhibitors since a result is obtained so quickly. It is beyond the scope of this lecture to go into greater detail, but as tube suppliers it is our practice to submit to this test any product which our customers care to send along. PROTECTIVE COATINGS If, by one means or another, you have discovered that your product is corrosive and you have been unable to inhibit this corrosion, it is necessary to fall back upon protective coatings. In this event, one thinks almost exclusively in terms of the aluminium tube. To apply such a coating on a tin tube is really a case of "gilding the lily" and a waste of good money. In the case of the lead, or tin-plated lead tube, the most effective coatings cannot be used owing to their high-storing temperatures, and I have already commented upon the adhesion and flexibility difficulties common to a lead surface. A coating of microcrystalline wax is probably the best suggestion which can be offered in these cases, although one must remember that such a coating is subject to hydrolysis, or even actual solution, when in contact with some products. For the alurainium tube there are three distinct types of protective coating: (a) The "High Bake," fully polymerised and therefore insoluble resin- based lacquers, which include Araldite, Epikote and phenolic resins. 113

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (b) The "Low Bake" lacquers, which are resins applied in solution from which the solvents are then driven off at moderate temperatures to leave a protective film which retains its full solubility characteristics. These are re- presented by the vinyl, Vinylitc, ethyl cellulose and even nitrocellulose types. (c) The micro-crystalline waxes. Which do you select ? It is a rough approximation to say that the list is in order of cost and effectiveness. Given a new product, my approach is to try it against a wide range of coatings and leave the customer to make the final choice if two or more are equally suitable. Those in group (a) have the highest chemical resistance and very few products affect them. They fail in contact with such things as dimethyl phthalate, which softens the film, mixtures of menthol and oil of wintergreen which cause stripping, and in the presence of products which are naturally vicious pin-holing agents containing high proportions of wetting agent (e.g., shampoo/conditioning creams). The latter concentrate on the inevit- able weak spots which exist in a proportion of such coatings and cause penetration at isolated points. Of the three types mentioned, Araldite is outstandingly good for its combination of high chemical resistance with good flexibility and adhesion. Epikote-based lacquers, with a chemical constitution that has much in common with Araldite, also have high chemical resistance and are less subject to application troubles. There has been some difficulty in getting a modification which could provide the exact balance in physical. characteristics needed for the collapsible tube, and this has delayed their general introduc- tion into this field. Improvements during the last year have given us lacquers which are now on trial and are showing very good results. The phenolic types are rather weak in respect of flexibihty, and efforts to improve. this inevitably lead to a reduction in actual chemical resistance. Before" leaving this group, two other points are worth mentioning. There is a tendency with certain products for these coatings to have their flexibility :'. decreased although the coating may be otherwise unaffected. Generally, the'.:.': decrease is small and only shows up on severe bending or stretching of the.::.: tube wall and can be ignored. More serious in its effect is the power which these coatings retain of causing slight discoloration of certain types of product. Fortunately, the effect appears limited to a few transparent,? colourless gels, which take on a faint brown tint, and to some of those! modern oxidation dyestuff systems which seem to have their oxidation! ' process very slightly catalysed so that a partial--unfortunately often.:i variable--premature colour development results. Normally, these lacquers/ are brownish straw-coloured, but a type of Araldite in a pigmented off-white. shade is also available with a somewhat reduced chemical resistance. The Low-Bake lacquers, of which the white-pigmented Vinylitc is easilY::i i the best known, are useful where the degree of protection desired is 114