PLASTICS FOR PACKAGING 375 Plastics are man-made materials which, in the space of the last 25-30 years, have evolved from their laboratory cradles to a relatively prominent position in world economy. Not only can they compete with natural products as primary materials of construction, but often they can outstrip them in versatility and outstanding performance in one direction or another at no greater cost. They lend themselves to methods and rates of multiple produc- tion not to be achieved by the use of other materials. For hundreds of years containers have been formed from stone, glass, composite mineral products and metals. They have served mankind well because by alloying or varying the process of working they have yielded products that have satisfied the demands imposed by changes in industry and marketing. In plastics a deliberate exercise of chemical knowledge is yielding a range of products tailor-made for the functions they have to perform in service and to suit the working-up methods employed in forming them. Plastic products are often modified, as in the case of metals, by alloy- ing, but additionally they may be modified by adding plasticisers and fillers the variety of compounds so obtainable being almost unlimited. The plastics industry needs not only chemists and physicists, but engineers, toolmakers, designers, craftsmen and financiers, all of whom must understand the nature of the products. Plastics are relatively new entrants into the packaging field, having intrinsic strength, durability in form and colour, and freedom from corrosion among the features which distinguish and commend them and make their use worthwhile in the effort to apply a new material to the age-old problem of keeping one material apart from many others. In addition to ease of mass production, plastics incorporate other proper- ties inherently valuable for packaging purposes. Among them may be found materials, for exan•ple, possessing excellent resistance to water and chemical attack. Some are of low density, with a strength-to-weight ratio that is higher than steel. Many have good dimensional stability in difficult condi- tions and can withstand either high or low temperatures. The majority are non-toxic and therefore suitable for use in connection with foodstuffs and pharmaceuticals. Some are available in transparent grades, others in trans- lucent or opaque grades only, while most provide a complete colour range. Not all of the materials possess all of these characteristicq, but usually one with the combination required for a specific application can be found. To this width of choice is allied a facility of mass production which when necessary permits packages of extremely attractive design to be economically produced. There are some sixteen to twenty distinct chemical types of plastics of commercial importance. Their performance is determined by both the chemical properties, that is molecular structure, and by modifiers such as fillers, reinforcing fibres and/or plasticisers.

376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS DEFINITIONS In the plastics world one constantly hears of thermoplastic and thermo- setting materials. These are two broad classifications of products, the members of which have the common property of softening and becoming plastic at some stage of their manufacture. Tf•ermo•last{cs, like metals, are hard or soft according to temperature. They can be softened by heating and hardened by cooling, and this process of crossing the softening point on the temperature scale can be repeated as often as desired without appreci- able effect on the properties in either state. In a word, the process is fully reversible and heat produces a physical change only. Polystyrene, po]ythene and P.V.C. are typical thermop]astics. Tf•ermos•t•{ng compounds, on the other hand, are first softened by heat, but the continued application of heat sets up a chemical process within the material causing it to harden to a final condition which does not permit re-softening because of the chemical cross-linking which has taken place. It will readily be appreciated that these two classes offer very different possibilities as regards methods of shaping into finished articles and as to the service temperatures at which the finished articles will function satis- factorily. By and large, thermoplastic articles can only be used at tempera- tures below 100 ø C., whilst thermosetting materials are often serviceable at temperatures two or two and a half times this level. Thermosetting moulding compounds are usually made from resinous binders filled with either woodflour, fibrous substances, mineral powders or papers. The fillers impart special properties such as increased impact or tensile strength, heat or water resistance to the compound according to the type of filler selected. Plasticisers are used almost exclusively in the thermoplastics, although they do not improve all of them. The main function of the plasticiser is that of a softening agent to bring the working temperature of the thermoplastic --for example, P.V.C.--down to a suitable level and away from the natural working temperature which may be dangerously near or even above the decomposition point of the basic resin. Polystyrene and polythene are examples of thermoplastics whose natural softening point is controlled within its working limits without the use of added plasticisers. On the other hand, P.V.C. generally requires plasticisers. Common plasticisers are the high boiling esters like di-octyl phthalate and tri-tolyl-phosphate used in P.V.C. Di-methyl-phalate is one of the main plasticisers used in cellulose acetate and camphor is the principal agent employed to produce celluloid from cellulose nitrate. Some indications of the processes by which plastics are moulded and fabricated and some of their uses in packaging follow.