.i?'• d•i•'OLYETHYLENE AS A PACKAGING MATERIAL l•?. 711" By ROBERT H. MARRIOTT, D.Sc., F.R.I.C.' ':': • INTRODUCTION PLASTIC made by polymeri'sing and known generally as Polyet•hylene is a very useful material :from the point of view of the cos- industry. Interest lies notably the fact that it requires no plasti- r.,and maintains its flexibility remarkably well over a wide range temperatures. The fact that no is required eliminates the that so often arise when )ther plastic materials are employed. often a plastic which has to be rapidly deteriorates owing the migration of the plasticiser, rubber chloride, which is vn under the trademark "Plio- can also deteriorate, i.e., i.become:" brittle, even though no Plasticiser is normally employed. Another advantage of polyethy- is that it is readily heat-sealable indeed, the temperature of need not be higher than 100 ø C. The melting point of polyethylene is of the order of 110 115 ø C. and it is quite easy to fuse polyethylene to produce a seal which can be far stronger than the ..!.film itself. Its relatively low fusion Technical Director, ii:-•'!?i:: County Perfumery Co., Ltd., London. point enables polyethylene to be employed for making bottles, which are blown in a way similar to that used in manufacturing glass con- tainers. Polyethylene bottles possess a characteristic flexibility-which enables them to be employed for packaging liquid cosmetic prepara- tions, the user merely squeezing the bottle to eject the required quantity for use. They have considerable attractiveness as well as a good life, since the flexibility of the plastic is well maintained, and unless certain materials, notably of a polar charac• ter, are in long contact, embrittle- ment does not occur. Other desirable. features of polyethylene are its relatively low specific gravity ('92) and the fact that it is perfectly resistant to moulds and other micro- organisms. Although polyethylene is imper- vious to water, it does allow water vapour to diffuse through it. Within the ordinary range of temperatures, it is not seriously acted on by oils, although even at ordinary tempera- tures it will absorb 3 to 5 per cent of its weight of mineral oil. It will also absorb, or allow to be dissolved within it, essential oils, and reduce the intensity of odour of the prepa- ration with which it is in contact, 163

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS but in this respect it is superior to polyvinyl chloride. On the other hand, it is sensitive to oils when the temperature is somewhat elevated and certainly at temperatures above 60 ø C. it is liable to disintegrate when in contact with the white oil nor- mally employed in cosmetic manu- facture. This has the effect that, with an oil-containing emulsion, especially if the emulsion be of the reverse type, i.e., water dispersed in the oil, the outer surface of .the polyethylene container- becomes greasy-and• indeed, if the tempera- tures be even warm, staining of an outer paper wrapping can occur. If the emulsion, however, is of the obverse type, i.e., oil in water, this danger is considerably lessened, especially if the outer phase contains surface-active materials which induce the surface of the polyethylene to be preferentially wetted by water. The important point to remember, however, in the case of polyethylene, is the fact that although the rate of diffusion of water vapour through the film is by no means high, it is permeable to gases such as nitrogen, oxygen, hydrogen and carbon di- oxide. ß In respect of its water vapour permeability, polyethylene is more porous than rubber hydrochloride although less permeable than poly- vinyl chloride. • EXVRmETAL Experiments have been carried o(•t to determine the suitability of poly- ethylene for holding reverse emulsions such as hair cream.' It was 164 found that when the product was held in a sealed tube of the material, swelling of the sac occurred. Investi- gation showed that this swelling was due to the inward diffusion of air which replaced the water vapour which diffused outwards. A sealed package containing a water-in-oil emulsion and freely exposed to the ordinary laboratory atmosphere commences immediately to swell although the rate is very slow. Afteoe,• the lapse of 3 or 4 weeks, evidence of swelling is quite easy to discern bY visual examination and handling th e package. The precise extent of swelling can easily be determined•! however, by measuring the displace• !• ment in water of the package afte.•Y: various intervals • of time. It wa• found by storing packages at variøfi:i temperatures, e.g., 37 ø , 50 ø and 60 C., the rate of,swelling increased wi't• increase of temperature, and'!• 60 ø C. the distension was so grea• after 4 weeks that bursting of pack was prone to occur. The taken to reach the point when burS{• ing was imminent varied inverselY::"• • the temperature of storage. amount of distension, as was not due in any way to the exP•'• sion of the air within the change of temperature, since determinations were carried immersing the sac in cold water.:? •:• analysis was carried out on the '?• content in the sads. •e values . were •uaHy identical with thø•:• of the ordina• atmosphere. presum•l swelling, therefore, was brought about by the diffusion: :•f "• . into the sealed sac.