40 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS giving rise to splitting. This effect is accentuated when extra mechanical strains are set upon the bottle by use of inserts in the neck or where tightly fitting screw caps are used. Moulded polythene bottles are excellent for unperfumed aqueous pro- ducts and have a good performance when packed with O/W emulsions, W/O emulsions, oils and greases all have a tendency to permeate the con- tainer and may give rise to environmental stress cracking. Another important item in considering containers for cosmetics is the. closure. With sachets, this is an integral part of the container and no. extra components are needed. With a moulded container, however, at separate component, such as a plug, screw cap or dispensing type of closure, e.g. a spray fitment, has to be used. Where polythene or polystyrene has been found to be suitable for the container it is reasonable to assume that the same material would serve for the closure, but one has to obtaJin the necessary resilience in one component in order to provide an efficient sealL With a rigid container, therefore, it is usual to employ a flexible plug or snap-on type ooe closure, and xvith a less rigid container, such as a polythene bottle, a harder material is used such as polystyrene or urea/formaldehyde resin. Two rigid components can be used together if a wad is used in the cap, the wad being of some resilient material with a facing which is resistant to the action of the product. It would not, for example, be practicable to employ a tinfoil wad with a product containing su]phur as this would lead to unsightly staining of the wad. The two most common facings used with cosmetics are Vinylite or Pol•hene, the former being a co-polymer of vinyl chloride and vinyl acetate. Both these wads comprise films of the plastic material laminated to a paper base which is then further laminated to a backing which may be felt, cork or woodpulp. A feature of the plastic bottle is its "squeeze" property and a number of manufacturers are changing to this type of container for packaging products such as talcum powder. This pack has great appeal and enables the contents to be dispensed easily and economically. Such a container, however, is not resistant to ingress of gases and moisture or to the egress. of perfume. The solution to this problem is to establish thickness and properties of materials with minimum expected permeability which will still give a bottle with adequate squeeze properties to dispense the powder. Once this has been decided upon, storage tests have to be carried out to ensure that the necessary shelf life can be obtained. Testing of Materials 1. EVALUATION OF BASIC MATERIALS Many of the tests in this section will be carried out by the suppliers of the packaging materials during its development and production and most

LABORATORY EVALUATION OF NEW PACKAGING MATERIALS users will not normally need to carry them out. It may be, however, of interest to outline them here. The results obtained from raw material evaluation can be used only as ß a general guide to the suitability of a material for any specific application. In our experience, routine materials assessment can seldom substitute for packaging suitability tests on the final pack. (a) Mechanical tests for flexible films These can be classified into two groups those requiring shock or impact loading and those which require a slow rate of straining. Our impact •testing of thermoplastic films is carried out on either the Double Tear /Elmendorf Machine or the Ball Dart Impact Tester. The Elmendorf Paper Tearing Tester has been adapted to measure the properties of materials with quite high impact strengths. This gives useful readings except that with certain polythene blown films there is a tendency for the tears to propagate, not in a straight line, but skewed at an angle, thereby producing a meaningless reading on the Elmendorf scale. To •vercome such behaviour we often use narrow Elmendorf Double Tear test pieces. Shock Strength and Fold Strength of blown layflat tubing are measured on the Elmendorf. Basically the procedure is to cut the test piece for shock strength with the long side at right angles across the machine direction on the film a similar test piece for fold strength is prepared by cutting the long edges centrally across the fold in the layflat material. After a small V-shaped incision is placed in the edge of test piece this is placed in the jaws of the tester and pulled apart. The scale reading is taken as a measure of the shock strength and fold strength of the material respectively. The Ball Drop Impact Tester is also used to measure the impact resistance of polythene film under defined conditions. The basis of the method is very simple. A round hemispherical dart (half a billiard ball) is fixed to a shaft of variable weight and this is dropped vertically from an electromagnet on to the test film from a height of 2(• inches. The principle of the method is to take four sets of ten test pieces from each test film and having pre- viously determined the range of dart weights within which the films break, count the percentage of film failure for each of the four sets of ten test pieces, dropping a different dart weight for each of the four sets. A graph of dart weight versus percentage failure will yield almost a straight line and then it is a simple matter to read off the impact strength of the material, which is defined as the dart weight required to cause 50 per cent failure. This method has been shown to be satisfactory for polythene films of mod- erate extensibility. For very extensible films and also low impact strength