46 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS aluminium surface to be used for display purposes. There was a slight loss in protection quality since the "sandwich" with paper in the middle allowed an edge loss of moisture, but by and large the matehal has proved adequate for liquid and cream shampoos, and is now the most widely used sachet material for these commodities. The use of the paper layer gives the pack a rigidity which allows it to withstand handling without the rather unpleasant creases, which are characteristic of film/foil/Pliofilm laminations, which was an alternative make-up and for which there has been a limited demand. A good deal of research has gone into finding improved adhesives between foil and paper, and between paper and Pliofilm to withstand the effects of heat sealing and the shampoo, with the result that with existing material, delamination has been eliminated to a large extent. PRINCIPLES OF HEAT SEALING It may be relevant to consider some of the points which arise when deciding the suitability of a material, with regard to machine performance. In forming a sachet to contain a liquid, it is essential that the inner component of the material should be capable of forming a complete weld• Only by doing this, can a liquid-tight seal be obtained. It has also been established, that there is a certain minimum gauge of material--dependent on the type of film---below which it is not possible to achieve a seal sufficiently strong to withstand the normal stresses to which a sachet may be subjected during transit and handling. In the case of Pliofilm, we would not normally recommend that a gauge below 140 is used, and with polythene a gauge of 200. The action of welding the films is normally carried out by heated platens which are brought together, and hold the two plies of material in position while the weld is made. The temperature at the face of these platens, the pressure exerted on the material by them, and the time they are actually in contact with the material (the dwell) are the three most important factors in determining the efficiency of the seal. Some machines effected a heat seal by driving the two plies of material between a pair of heated rotating wheels. This method is not considered a satisfactory one for liquid sachets, as the period of dwell is of necessity too short for reliable sealing. In making a heat seal, sufficient heat must be supplied through the material--which may be a 2- or 3-ply laminate, to at least partially melt the. plastic film, and sufficient pressure must be applied to fuse the two faces. into one. A certain minimum time will be required for the heat to pene-- trate the material and raise the film to its softening point. It follows that.

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.