PRODUCTION AND PROPERTIES OF GLASS CONTAINERS 41 I)ISCUSSION MR. C. BLOOM: I think there is little doubt that the two major dis- advantages of glass are its brittleness and its weight. The weight factor has been very well examined in recent years and as we all know, great improvements have been made but glass will never, of course, compete with plastics in this direction. With regard to brittleness, however, I would have thought that it would pay to investigate this much more thoroughly than it has been, at least as far as I am aware. It is not fully realized that the cost of protecting glass containers during transit is so considerable that a fair differential in the basic price between glass and plastic containers can exist and still make the latter as economical in use. When this differential is reduced even further, the economics of packaging could be very appreciably weighted in fayour of the plastic bottle. The average glass bottle uses only about 1% of the strength of glass. Ordinary glass fibres have a tensile strength of 1 million p.s.i. but the glass is weakened by pits, scratches, chips and abrasions. I would have thought, therefore, that this presents a tremendous challenge to the glass industry. Is there any likelihood of serious inroads into this problem in the near future ? In discussing glass containers, one cannot divorce the closure from the bottle and I wonder therefore whether I may be allowed to make some observations. The selection of a suitable closure, particularly for moisture sensitive products, is extremely important. The key to this problem is the wad rather than the cap, and I have found that a wad of natural rubber is the most effective way of getting a good seal. Rubber seems to be better than cork faced with tinfoil (which provides a seal better than most wads) because it does not itself absorb moisture and because it binds to the glass under all conditions of temperature change. I should warn users that in order to obtain valid information on the effectiveness of a particular closure for a glass container, it is vital to alternate the conditions of storage, particularly with respect to temperature. I have found that whilst butyl rubber and solid polythene wads function well if the temperature does not change, they fail if the temperature of storage is allowed to fluctuate, in both instances due to a high permanent set. Rubber has the disadvantages that it is somewhat costlier than more conventional wads, and that it can impart an odour to the product unless the grade used is very carefully selected. I have found the recently developed polyethylene cone liner to be particularly good for liquid products. THE LECTURER: Glass is very fragile. This fragility can be reduced but you have to pay for it. Glass manufacturers are at the moment

•42 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS manufacturing aerosol bottles coated with plastic, and also bottles with plastic coatings tempered in the same way as glass tumblers. Glass bottles are made at high speeds and tempering would make an article very much more expensive. But is it worthwhile for the price involved ? MR. C. BLOOM: Can you give us any information on lightweight bottles ? THE LECTURER: Provided the bottle is well designed and the glass is well distributed, the container will be quite satisfactory with a uniform wall thickness of 0.020". The strongest type of bottle is the round one, which is approximately as wide as it is high square and oval bottles are next in regard of strength, and the rectangular bottle is the weakest. The bottle must be strong enough to withstand contact with other bottles in production and handling. A "multi-point" contact bottle is a much better proposition than a "one-point" contact bottle. The weakest part of a jar or bottle is the bottom half inch. "In-sweeping" of a bottle helps in production. Sharp shoulders should be avoided. Properly designed lightweight containers can be as good as thicker walled bottles. In order to dispense with the re-use of bottles, lightweight containers have been introduced in the United States. As a result, sterilization, etc., is no longer necessary. MR. A. MARSH: Using a semi-automatic process for small runs, is it reasonable to expect and rely on a fully automatic process to produce bottles of the same specification, or are the limits widened on a semi- automatic line ? THE LECTURER: A high speed machine has to be set up with six sets of moulds, and usually a few sample bottles are run off which should be the same as eventual production, but it is very difficult to relate semi- and fully automatic. It depends on the individual factory set-up. Manufac- turers who set up for short production runs would probably use the same machines for bulk production. FLEXIBLE PACKS F. W. NOBLE* Presented at the Symposium on "Packaging in the Cosmetic Industry", organised by the Society at Harrogate, Yorks., on 5th July 1961. The application of flexible films, such as polythene, Pliofilm, etc., to cosmetics, with particular reference to unit packaging of liquid shampoo, and some of the mechanical problems involved are discussed. TH• USE of plastics in packaging has extended very rapidly during the last few years. It is necessary, therefore, to define what we mean by "flexible * Robinson Waxed Paper Co. Ltd., Bristol.