FLEXIBLE PACKS 47 the dwell required for a 3-ply laminate is greater than that for a 2-ply laminate. The speed at which a machine can be run is limited by the speed at which heat can be supplied to the plastic film component of the material. It cannot be emphasised too strongly, that for a given supply of heat, and for a given material there must be a minimum dwell (and hence a maximum speed) beyond which heat cannot be supplied sufficiently fast to obtain a proper seal. Increase of pressure can apparently have the effect of producing a seal without increasing the heat supply, but this is nearly always at the expense of thinning the film, and therefore weakening the seal. The commonest cause of low strength sachets is excessive pressure on the platens indeed, this can often lead to physical damage, and actual fracture of the plastic film. It is recommended that the pressure used is the minimum which will serve to hold the faces of the material firmly together and permit a weld to be made. Reports on investigations into the problems of heat sealing have claimed that seal strength improved with increased pressure. This can be misleading in practice only a very short time can be allowed for platens to close, if reasonable production speeds are to be maintained. Consequently, there is a danger of delivering a heavy blow at the moment of sealing, which results in fracturing or weakening the film. The sealing mechanism should therefore be designed so that the final pressure is applied as gently as possible. It should be mentioned that an excessive supply of heat can have similar effects to excessive pressure. If the plastic film is allowed to flow too freely, it will flow away from the heat seal area and a weak seal will result. It is therefore important that a good degree of control of heat input, such as is achieved by a thermostat, should be used. Recent work • on heat sealing has established that maximum bond strength can be achieved by using radiused heater jaws. In practice, however, it is difficult to keep such jaws in perfect alignment. If this is not done, local weak spots can develop. An effective compromise is to use one radiused and one flat jaw. The radius can vary from I-" to 2" depending on the width of seal required. DEVELOPMENT OF MATERIALS As already stated, the first laminated shampoo pouch was made from cellulose acetate and Pliofilm. Since that time, Pliofilm has been used largely as the heat sealing medium in these pouches, although also laminated to combinations of aluminium foil and paper, as well as film. Its great advantage is that it heatseals over a fairly wide range of temperatures and thus sealing is not too critical. New materials have, however, become available, and are becoming available every day. These materials have advantages over materials previously used, as well as disadvantages.

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