390 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 60 ø C., then the time for complete loss at 20 ø C. will probably be about two to three years. There are, of coarse, exceptions to this and the checking of perfume loss at room temperature can be a lengthy business. The permeability of non-volatile oils through P.V.C. sheet is very low indeed and can normally be ignored. Welding Properties Polyvinyl chloride may be welded either by heat or radio frequency methods. The melting-point of "rigid" or unplasticised P.V.C. is about 180 ø C., but plasticised P.V.C. has a lower melting-point and, if required, the figure must be determined for the particular formulation, as it varies consider- ably with variation in the plasticiser. In the case of heat sealing the tempera- ture of the jaws (which must, of course, be higher than the melting-point), the time of contact and the pressure needed must all be determined experi- mentally for a given formulation of P.V.C. sheet. A great difficulty in heat- sealing P.V.C. is the tendency of the plastic to stick to the heated jaws, but this can be partly overcome by a coating of polytetrafluorethylene on the welding jaws. Because of this difficulty, P.V.C. is usually welded by radio- frequency heating methods. As with heat sealing, the power required, the time of application and the pressure must all be determined experiment- ally, and will be found to vary if there is variation in the sheet, e.g., thickness, plasticiser content, dielectric content or inorganic pigments. Care should be taken to ensure that the inner surfaces are completely fused together, but not overheated, otherwise tearing may occur at the side of the welds. If correctly welded, the seals are as strong as the sheet itself. Printing Properties Different P.V.C. sheets have a different receptivity to printing inks, and it is necessary to find a sheeting which will print satisfactorily. The usual methods employed for printing P.V.C. are either the photogravure method or transfer printing, the latter being the most common. Simple practical tests on the printing machinery will show if a given P.V.C. sheet is receptive to the printing method in use. The adhesion of the printing ink may be tested simply by rubbing with the fingers. A more severe test is to press a piece of transparent adhesive tape on to the printing, remove the tape and examine the printing. Only a first-class print will remain completely adhered to the P.V.C. under this test. THE STORAGE OF COSMETIC PREPARATIONS IN CONTAINERS MADE FROM POLYVINYL CHLORIDE SHEETS All the above considerations of the constitution and properties of P.V.C. sheets have been made with respect to the P.V.C. alone. When the packed articles are stored, the product may have an effect on the P.V.C. or the

POLYVINYL CHLORIDE AS A PACKAGING MATERIAL 391 P.V.C. may have an effect on the product. In either case, these effects can only be found after long storage tests. They are, however, most essential as either may easily give rise to a complete loss of customer appeal of the packed product. The major changes are usually due to migration of the plastieiser. Certain preparations may leach out the plasticiser and this will be accom- panied by many changes in the properties of the P.V.C. A few examples are increased permeability to perfume, change of clarity of the plastic sheet, decrease in flexibility and increase in brittleness. In addition, any migration of the plasticiser into the product may give rise to an unpleasant odour. Some plasticisers themselves have distinct odours, others may be hydrolysed by the product, especially if it is alkaline, e.g., a shampoo or an emulsion having an alkaline aqueous phase. Plasticisers which are affected in this way include di-octyl phthalate, dibutyl phthalate and tetrahydrofurfuryl oleate. A measure of the migration of the plasticiser can be obtained by deter- mining the total plasticiser content of the original P.V.C. and samples of the P.V.C. which have been in contact with the product for varying times at different temperatures. The plasticiser is extracted from the P.V.C. by treatment with ether or ethanol, the solvent evaporated and the weight of non-volatile plasticiser obtained expressed as a percentage of the original P.V.C. sheet. It is, of course, advisable to choose a P.V.C. which loses very little of its plasticiser when stored in contact with the product. In this way changes in the properties of both the plastic sheet and the product on storage can be miniraised. In some cases the P.V.C. may be permeable to a certain constituent of the product which has a solubilising action on the dyestuff used for printing. In this case the printing is rapidly damaged and any other articles in contact with the pack may be contaminated. The permeability of P.V.C. to gases is also an important factor. There are many types of cosmetic preparations which have a long storage life when packed in full glass containers, but which may rapidly deteriorate in a plastic container when oxygen passes into the product. If any constituent of the preparation is susceptible to atmospheric oxidation, then the product should be tested for long periods in P.V.C. containers before finally accepting any particular formulation of P.V.C. as being suitable. •UMMARY When the cosmetic chemist is considering packing a given preparation in a P.V.C. container, it is not sufficient merely to accept any P.V.C. sheet as the raw material. Probably the ideal to aim at is a P.V.C. which: (a) does not discolour on storage