380 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS HEAT SEALING Heat sealing requires that the surfaces to be joined shall be melted when in contact with each other. SOLVENT WELDING Consists of wetting the two surfaces of a plastics film with a solvent which will soften the film and then by applying pressure to bring the two softened surfaces into co•tact. ADHESIVE BONDING Consists o[ cementing two surfaces together with a suitable bonding agent. A FEW REMARKS ON THE PROPERTIES OF PLASTIC I•IATERIALS Dimensional Stability Some special purpose grades of thermosetting materials have been designed to give satisfactory service at high temperatures, in some cases as high as 200 ø C., but it is unwise to use the general purpose materials at temperatures in excess of 100 ø C. By their nature thermoplastic materials will soften at elevated tempera- tures. A guide to the temperatures that these matehals will stand will shortly be published as part of Part Seven of the Packaging Code being prepared by British Standards Institution Table II gives similar information. Water and Water Vapour Permeability The water and water vapour permeability of a plastic is easy to measure and it is possible to classify materials according to the amount of water that will pass through a given membrane in a given time. Generally it is sufficient to establish from published data whether or not a particular plastics material offers an effective barrier to water or water vapour and to carry out an ad hoc test. Most plastics offer very good resistance to the passage of water, the notable exception being, of course, poly~vinyl alcohol, which dissolves in water and is used for the manufacture of water soluble sachets to contain shampoo, medicines, etc. Odour Permeability The transmission of a vapour through a material takes place by a process of solution of the molecules of vapour at one surface followed by diffusion in the body and subsequent evaporation at the other surface.

PLASTICS FOR PACKAGING 381 Most odour molecules are large and therefore the rate of diffusion is small. Solubility can often be used to provide a useful guide to the resistance of a film to a particular odour: if the odour can be typed chemically the behavi- our of the material as a barrier to the chemical will indicate its behaviour as a barrier to the odour. Gas Permeability Quite a lot is known about the gas permeability of plastics materials. I cannot give the information here because of the wide range of gases en- countered in industrial processes, but most plastics manufacturers will be able to supply the answer to any particular problem. Flammability Very few plastics materials burn readily. It can be assumed that with the exception of cellulose nitrate which, as you know, is highly inflammable, most plastics materials will continue to burn slowly when once ignited. An exception is P.V.C. which is self-extinguishing. Chemical R•sistanc• It is impossible to give an effective answer to all the questions that can be posed on the chemical resistance of plastics. However, Table III gives the resistance of certain plastics films to a number of chemical reagents. This table has been prepared by a committee of the British Federation and I am obliged to them for permission to show it. Shapes and Designs of Packs It will be agreed that plastics materials have afforded the pack designer with a most versatile material with which to work. Do not, however, expect that your designer can always satisfy both the problem of eye appeal and TABLE II Cellulose Acetate Cellulose Acetate Butyrate Ethyl Cellulose Polyethylene Terephthalate Polystyrene and Toughened Polystyrene Polythene Nylon Polyvinyl Chloride Copolymers of Polyvinyl Chloride Polyvinyl Alcohol Copolymers of Vinylidene Chloride Maximum Service Temperature øC. 55-70 60-75 70-80 160-180 74-85 70-80 130-150 50-60 50-60 90-110 55-70