370 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS urea formaldehyde wood filled urea formaldehyde paper filled and Melamine formaldehyde is also used, but to a less extent than others mentioned. Thermoplastic materials used are polythene and polystyrene. Caps from various of these materials can be made in practically any shape and colour, also they can have trade marks or letters embossed upon them. In the manufacture of caps from thermosetting materials either heated flat-bed presses or automatic rotary presses are used. With either type of production an excess charge of powder, sometimes in the form of a pellet, is delivered to each mould, which after the moulding has taken place, causes a flash ring to be produced on the caps. This is either removed by a separate operation or automatically in the case of rotary machines, immediately after the caps have been moulded. A point worth noting here in regard to the cost of caps is that the heat and time cycle needed to manufacture them is broadly in accordance to weight and thickest section. A conventional well- designed cap will mould without difficulty, but if a customer, for some decorative reason, requires a cap with a thick section, difficulties will be experienced. The essential characteristics of moulding powders are as follows: 1. The strongest is wood-filled phenol. This can only be supplied in black, brown and some red colours it is not so easy to use as wood- or paper-filled urea. 2. Most deeply coloured caps are made from wood-filled urea and the pastel type paper-filled urea. 3. Melamine formaldehyde moulding powder is appreciably more expen- sive than phenol or urea and is generally only used for specialised purposes. Caps made of thermoplastic materials are mottided by conventional injection moulding methods, polythene being used for the manufacture of snap-on caps and polystyrene used for the manufacture of caps used for the bottling of certain acids. An important feature of screw caps is the liner material used. With moulded caps the majority of liners are glued in place, and it is found in practice that for specific bottling purposes the use of definite types of liner materials is preferable. These liner materials are usually composed of one of the following alternatives: 1. A backing material of white wood pulp or composition cork with a facing material. 2. Wax impregnated white wood pulp or composition cork. 3. A homogenous rubber or plastic material. Reasons for and against the use of either backing material are as follows: (a) Composition cork has the greatest resiliency and gives a better sealing effect.

BOTTLE CLOSURES 371 (b) White wood pulp has the best appearance. (c) It is alleged that wood pulp is less prone to mould growth than com- position cork however, no definite proof of this has been advanced, and given the right circumstances either would be a vehicle for mould growth and subsequently may promote mould growth. It is worth mentioning here that conditions that may cause deterioration of cap liners are damp or dirty storage and broken containers. A list oi commonly used liner facing materials with a brief description is as A co-polymer of vinyl chloride and vinyl acetate on paper. A film of polythene of thickness 0.002 in. to 0.005 in. on facing paper or direct on backing material. follows: Vinylite Vinylseal Polythene Telecothene Alkathene Ceresine Oil varnish base, one type prepared from tung and linseed oil, on paper. Blackol British type cashew nut polymerised resin pigmented black on varnishable Kraft paper. Resistol Melamine formaldehyde impregnated paper with alkyd type resin. Crystal Cap Similar to Resistol. Tin Foil Pure metal of thickness 0.025 mm. to 0.040 mm. lamin- ated to paper or direct to backing material. Aluminium Foil Pure metal of thickness 0'025 mm. to 0'063 mm. laminated to paper or direct to backing material. Pliofilm Rubber hydrochloride on paper. Permaceal Similar to Pliofilm. Saran Polyvinyldene chloride on paper. Homogenous rubber or plastic liner material such as polythene or poly- vinyl chloride is used for some bottling purposes. The chief disadvantages in the usual use of these are ß 1. If liners made from them can be glued in, only hand assembly is practical, this adversely affects overall cost of cap. 2. Rubber liners invariably have a characteristic odour. 3. With plastic liners there is a lack of resiliency a,nd a tendency to flow when in use, but no evidence can be offered to prove this affects sealing efficiency. In the mineral water bottling trade returnable moulded caps with rubber ring liners have been used for some years now and appear to be gaining favour.