FLEXIBLE PACKS 45 far none have been produced which show an all-round improvement. As far as the inner ply is concerned, this has to provide a reasonable barrier to moisture, and to the penetration of the ingredients incorporated in the product. From the available plastic material, Pliofilm has proved to combine the necessary characteristics, coupled with its suitability for heat sealing on sachet forming and filling machines. This characteristic is referred to more fully when dealing with heat sealing problems. The use of cellulose acetate as the outer ply was determined mainly on account of its dimensional stability, coupled with clarity and suitability for printing. Various grades of regenerated cellulose film have been tested from time to time as an alternative to cellulose acetate but the dimensional variation has been such that the tendency to delamination is greatly in- creased, and on the whole it has not been considered a satisfactory alternative. DEVELOPMENT OF Iq'ON-TRANSPARENT PACKS As mentioned earlier, transparency is not always desirable consequently a demand developed for an opaque pack. The first material to be used was a lamination of paper and Phofilm, which had the advantage of being opaque, and was rather cheaper than the acetate Pliofilm lamination. This make-up was found to have severe limitations. The moisture loss through the lamination was considerable, and the strength of the sachet, even when sealed under the best conditions, was marginal. There was also an increased tendency of moisture loss. To overcome this, a ply of aluminium foil was introduced between the paper and the Pliofilm, producing a triple lamination--paper/foil/Pliofilm. This materiM was an extremely good barrier to moisture, and also made up into a much stronger sachet than paper Pliofilm. There were, however, still some difficulties notably a tendency for the aluminium foil to delaminate from the Pliofilm when the sachets were stored for any length of time, particularly at elevated temperatures. This was due to ingredients of the shampoo getting through the Pliofilm and weakening the bond. This problem is far more acute when an impermeable barrier such as aluminium foil is next to the Pliofilm, as the latter becomes saturated, causing the film to swell and become detached from the foil. Paper/foil/ Pliofilm is still used for certain products, mainly rather viscous creams e.g., face packs. The next development was foil/paper/Pliofilm, that is, with paper sandwiched between the film and the foil. When made up this produced a sachet with good strength characteristics and also allowed the brilliant

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.