JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 5 x 10 '• in. and the polyvinyl chloride 15 x 10 '* in. in thickness. Because of this variation, it is not possible to-make too close a com- parison between the different plastics, but the significance of the results lies in the fact that it is only the polyethylene sacs which show any increase in volume, i.e., diffusion of air into the sac. TABLE I Changes in Weight and Volume of Plastic Sacs containing Water-in-oil Emulsion when stored at various Temperatures • Weight after sto'rage [ Volume after storage Type and • (weeks) (weeks) thickness of '• 0 2 4 6 i 8 0 I 2 4 6 8 film øC gm. gm. gm. gm. gm. c.c. c.c. c.c. c.c. c.c. P.V.C. 20 94-2 93.8 93.0 92.4 91-4 100 100 100 100 99 15 x 10 '* in. 87.3 87-0 86-1 85.4 84.4 94 94 94 94 . .91 ß 37 100.6 97:0 94.5 92.7 90.9 I09 110 110 105 105 . 95'0 92-1 90.2 87.6 85-9 101 103 102 99 100 _. __. Pliofilm 20 110-8 112.1 111-2 110.5 109.2 133 133 133 130 130 2'5 X 10" in. 121'5 120'9 119-0 119'0 117'9 137 137 137 137 137•i • 116'1 115'7 114'4 113'7 112'8 157 157 157 157 151 : 37 125'2 122'9 120'5 117'9 114-2 153 153 150 150 134'9 130'5 128'0 124'8 120'7 153 153 150 150 14•i!!: -- --. Polyethylene 20 75.0 75.2 74-3 73-7 74.0 82 80 88 5 X 10-' in. .,:. 69.1 68-9 67.4 67-4 67-0 75 74 76 78 63.6 63.4 62.0 61-6 62-8 72 70 72 74 37 60.1 58.5 57.5 57.0 56-3 72 '80 90 100 •:: 78-6 78-0 76.5 76-0 74-2 85 94 105 110 ! 74 3 73-9 72.4 71.3 70.3 80 85 96 100 166

". X 10 -a in. Thickness 2'0 3.0 In the case of the sac 2 X 10 -s in. thick week and the value for the rate of loss in POLYETHYLENE AS A PACKAGING MATERIAL TABLE 2 The Effect of Thickness or' the I•olyethylene Film on the Rate of' Loss in Weight of Sacs when Stored Rate of Loss in weight mg./m.S/d. 18-25 ø C 3,600 1,000 430 37 ø C 10,600 6,800 stored at 37 ø C., bursting occurred after weight could not be determined. • •ii!i:iJ':: Experiments have been carried out i?!,:.iih which polyethylene of two •i ?•different thicknesses obtained from ii!11•i?the U.S.A. have been compared with :•?::ihe English variety 5 x 10 '• in. ?.thick. In each case, a sac of precisely !i:?!•the same dimensions was filled ?•'•(•:.:a reverse emulsion and the figures •[?::::Sho• in Table 2 obtained. :•:• •?{:•::. The rate of swelling of the different • ::• sacs was determined by storing them •?•:{•:in a d• atmosphere, i.e., over ::) ¾Calcim chloride, at S0 ø C., the change +•.:.::•m volme being measured by dis- {:-{•':::• placement. The thinnest -sac b•st •:'::(::•thin 48 ho•s • The sac, 3 x •?.¾::in. thick, showed an increase of :?volme of 50 c.c., whereas the ß : :? • :•(•thickest film, 5 x 10 '• in., showed ??•.' volme increase of 30 c.c. in 48 hours. {•{?/: It is quite clear from these experi- '•:•. .:: menis that if a polyethylene pack be •{:•:• envisaged, the thickess of the film '•:• ::: should not be less than 5 x 10'• in. •?:: and even then precautions should be ?•.•.: taken with regard to storage. •:: would appear from the results given that the loss in weight and the swelling are not straight-line lunc- h:.:':' tions of the thickness, aRhough the experiments are not by any means complete enough for the true rela- tionship to be determined. The evidence, such as it is, suggests that the permeability of the film varies inversely as about the square of the thickness. One of the difficulties encountered in the experiments, and which prevents accurate values, lies in the fact that polyethylene film, especially when made into "lay-flat tubing" is liable to have points of weakness which can develop into tiny pin- holes and with an increase of internal pressure, some of the con- tents are expressed through the tiny holes. Under the polarising micro- scope, the incidence of striations can readily be seen and incipient points of weakness can easily be detected. Pin-holing is considerably less with the thicker films. •DIscussION It would appear from the above results that polyethylene is an excellent membrane for demonstrat- ing the principles of diffusion. The 167