Zt8 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Polyethylene This may be used as a film, laminated to other materials or it can be extrusion-coated on to paper, film, or aluminium foil. Alone, polyethylene is a reasonable moisture vapour barrier but unfortunately it is a bad oxygen, grease, and odour barrier. It is therefore necessary to combine polyethylene with other materials, which supplement the properties stated above. As a heatsealing medium polyethylene is very good, giving a complete weld. The heatsealing, however, is more critical than with Pliofilm as the polyethylene melts, and will flow freely, if too much heat and pressure are applied. There are, of course, many grades of polyethylene, and it has been noticed that certain grades are affected by detergents and the like. They become brittle and tend to crack the correct grade for a particular product is therefore most important. Polypropylene This is a much more recent material and again can be used as a film, or extruded on to other materials. It has much better protective qualities than polyethylene, but unfortunately has a higher melting point which introduces complications in laminations where excess heat can cause delamin- ation. A lot of experimental work has still to be carried out on this material. Melinex This polyester film is not a heatsealing film in its simplest state, but it does display very good strength characteristics, together with excellent clarity, brilliance and barrier properties. Unfortunately, at the present time, compared with regenerated cellulose and cellulose acetate, it is rather expensive and although it has been used in laminated materials this has only been to a limited extent so far. Polyvinylidene chloride (Saran) Saran can be used as a film, or as a coating, applied from water dispersion or solution. It is a very protective resin, being an excellent barrier to moisture vapour, grease, odours and oxygen. In film forin, however, it is very difficult to use as it suffers greatly from static, and also requires impulse sealing. These difficulties make Saran a little-used film in laminations. When applied from solution, it is very difficult to get a continuous coating of Saran resin, and the advantage of using a highly protective resin is cancelled out by pinholes. Dispersion coating of Saran is designed to put on several coatings of Saran, if necessary, to give a stiltably continuous film. The coated material is usually extremely protective, depending upon the weight

FLEXIBLE PACKS 49 of coating. Saran-coated paper has been used on the Continent for shampoos but for thin liquids it was found to be not enough robust. It is, however, satisfactory for thick creams. Polyvinyl chloride Although not a new material in itself, several new grades of plasticised P.V.C. have recently come on to the market. These have lower melting points than the unplasticised grade, and should make handling far easier than it has been in the past on conventional heat sealing equipment. Polyamide Usually known as nylon, this material can be used as a film or can be extrusion coated on to paper or foil. This material has very good barrier properties for moisture vapour, grease, odour, and oxygen, although once again its melting point is rather high. This high melting point can be a virtue in that, like polypropylene, nylon can be used for steam sterilisation. When used alone it has to be impulse sealed, although it can be heat sealed by conventional means when it is the inside ply of a lamination. It is not possible to evaluate comprehensively all the materials which are available for packaging purposes, either as individual films or combined in two or three plies as laminations. When considering the packaging of any product, the following questions must be asked: 1. What material will give the required degree of protection against (a) gain or loss of moisture, (b) grease penetration, (c) oxygen penetration, (d) loss through absorption or penetration of essential ingredients scents, etc. (e) Will any ingredient in the product react with the material, due to plasticiser content, etc ? 2. Is the material capable of being printed economically to form an attractive container ? 3. Can the material be handled at commercial speeds on available machinery ? 4. Is the cost of the material such as to make the proposed combination commercially acceptable ? During recent years, a great number of combinations of materials have been tried, but only a few have been found to meet all the requirements outlined above. So far the following has been established: