384 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS POLYVINYL CHLORIDE AS A PACKAGING MATERIAL K. G. JOHNSON, B.Sc., Ph.D., A.R.I.C.* Following a briet review of the various sheet plastic materials used for packaging, the author describes the composition and physical properties ot polyvinyl chloride sheets in detail. Particular attention is given to the effect oi the product on the sheet and oi interaction between them. THE PRACTICE of packing cosmetic preparations in containers fabricated from plastic sheet is becoming increasingly popular. The container generally consists of either two sheets of plastic welded together all round or a piece of plastic tubing welded at each end. The plastic .sachet so formed may be a variety of shapes---square, triangular or imitation bottle shape are all commonly used. The quantity of the preparation packed is sufficient for one application, so that after use the packing material is discarded. There are several types of cosmetic preparations which are suitable for packing in this form, including powders, liquids and creams. The choice of polymers is also quite wide, but many of the plastics available in sheet form have only a limited usage in the cosmetic field. Regenerated cellulose sheets are affected considerably by moisture or humidity changes and tend to become dimensionally unstable. They are normally only used in laminated sheets. Cellulose acetate sheets are dimensionally more stable than regenerated cellulose, but the moisture resistance is poor and sealing presents problems. Rubber hydrochloride is quite resistant to moisture and gives welds of great strength, but it ages rapidly, especially in sunlight, becoming brittle and losing hydrochloric acid. Polystyrene sheets are too brittle for general use. Polyvinylidene chloride possesses extremely high resistance to water and gases and is very resistant to attack by most chemicals and solvents. The sheets are quite resistant to humidity changes. They are, however, somewhat rigid and present difficulties in welding and printing operations. Straight polyvinylidene chloride is not produced commercially on a large scale, usually being in the form of a co-polymer with polyvinyl chloride. 'This co-polymer is more flexible than the straight polyvinylidene chloride and has a water resistance between those of polyvinylidene chloride and polyvinyl chloride. The main disadvantages are poor printing properties-- usually a soft co-polymer coating is necessary to facilitate the adhesion of printing inks--and high cost due to the high specific gravity (1.85) and the * County Laboratories, Ltd., Stanmore, Middlesex.

POLYVINYL CHLORIDE AS A PACKAGING MATERIAL 385 need to keep the polymer free from heavy metals which cause decomposition if present in quantities as low as a few parts per million. • Polythene sheeting is used to a certain extent for packaging cosmetics. Polythene is a general term for polymerised ethylene, the molecular weight of which can be varied according to the pressure in the polymerisation vessel. It is a typical hydrocarbon with a low specific gravity (0.92) and a melting- point of 115 ø C. No plasticiser is required with polythene, so that migration problems do not occur. The sheet is quite strong, with an exceptionally high tear-strength, but it is not possible to weld polythene by high frequency heating, and this is a disadvantage. The selective permeability of polythene is also a great disadvantage, the resistance to water and water vapour being high, but the permeability to gases and oils is unfortunately also high. Consequently any cosmetic preparation containing oil and water will preferentially lose this oil, changing its own composition and causing the external surfaces of the pack to become soaked in oil. The high permeability to gases allows them to pass into a pa. ck containing an aqueous liquid, especially at elevated temperatures, resulting in an increase in volume and the development of pressure inside the pack. 2 There is also a variety of laminated materials available consisting of two or more very thin sheets. Probably the most useful and widely used plastic sheet for the packaging of cosmetic materials is polyvinyl chloride. There are many factors, however, which affect the suitability of the P.V.C. sheet for any particular preparation and it is some aspects of the suitability of P.V.C. sheet that are discussed. CONSTITUTION OF POLYVINYL CHLORIDE Straight polyvinyl chloride is a hard, horny material with a melting-point of about 180 ø C. It is fairly rigid and has to be plasticised with ester-type plasticisers, which have the effect of giving pliability to the material and reducing the melting-point. After plasticising, it can readily be callendered in the form of sheets which are translucent and weaker than sheets made from unplasticised or rigid P.V.C. The sheet material is very sensitive to temperature effects, being very soft and difficult to handle when warmed and very hard and brittle if cooled below room temperature. There are several plasticisers in general use and these include dibutyl phthalate, di-2-ethylhexyl phthalate, di-2-ethylhexyl adipate, some sebacic acid esters, tetrahydrofurfuryl oleate, which gives pliability at low temperatures, tri- cresyl phosphate, which imparts flame resistance, and certain chlorinated hydrocarbons. The choice of plasticiser is very essential as they affect several properties of the P.V.C., including strength, pliability, brittleness at low temperatures, ease of welding, resistance to the passage of gases, water vapour, perfume, etc. Certain plasticisers can affect the odour of products