Opening the SATRA Bulletin archives
Examining the working lives of our shoemaking predecessors – through the pages of the SATRA magazine.
Published continuously since 1935, SATRA Bulletin magazine has long provided valuable information to companies working in the footwear and leather industries. In this issue, we continue our occasional journey into the Technology Centre’s archives in order to re-visit an article published some 56 years ago that provides an insight into the technologies and construction methods available to previous generations of shoemakers.
From the Monthly Bulletin of the British Boot, Shoe & Allied Trades’ Research Association – January 1950
Alternatives to leather soles
The trend, in America, towards the greater use of rubber and synthetic soling materials in place of leather has been widely commented on. Towards the end of the war nearly half the civilian boots and shoes had soles not made of leather, but this was due to the enforced use of alternatives and by the year 1947 the proportion of leather soled shoes and slippers was back to 74%. In 1948 a really significant change began because, despite lower sole leather prices, leather lost ground substantially and was used for the outsoles of only 65% of shoes and slippers. Figures for 1949 are not yet available, but the same trend has apparently continued, and one well-informed estimate is that in 1950, 200 million pairs will have outsoles not made of leather; this would bring leather below 60% of the total. It seems certain that this reduced use of leather is due to a substantial shift in public demand and is not due to a change in the relative prices of leather and its competitors. There has been a vogue for crepe and rubber soles of bold patterns and bright colours, but it is the popularity of materials such as Neolite, Avonlite and Panolite which is of special significance.
Naturally enough, no detailed technical information is available about the composition of these soling materials, but it is known that Neolite is mainly, if not entirely, compounded from constituents which, in other proportions, are used to make a well-known synthetic rubber. So also, no doubt, are the other products which have similar characteristics. The American synthetic rubber industry, built up with remarkable speed during the war, now has an enormous output capacity. It is a strategic asset, and is protected by regulations which, broadly speaking, require that a specified percentage of all rubber consumed shall be synthetic. It is also a source of large potential profits for the big chemical, rubber and oil firms, and great efforts have been put into the development of commercial outlets for synthetic rubbers and allied products. The vast advertising campaigns which have established the new soling materials are an illustration of this. It has been said that advertising expenditure on leather soles of all makes is less than 1% of the amount spent on promoting its competitors. It is not surprising that the traditional centuries-old acceptance of leather as the best material for shoe soles has been shaken; but it is on their merits that the new synthetics must ultimately keep or lose their market. It is too early yet to do more than speculate, but it is significant that, though no longer novel, their popularity still grows.
Characteristics of American synthetic solings
The R.A. [the Research Association – now SATRA] has not yet completed all the work which it is planned to carry out on the examination of these materials, either from the point of view of their physical properties, their working characteristics, or their behaviour in wear. The comments which follow are therefore tentative and are based on a first examination of one of the products named. In colour it can be mistaken for sole leather. It is about 20% heavier than leather, but a little lighter than most natural rubber compositions. It works well in most shoe-making operations. Chanelling seems to present some difficulty as the lip tends to close, but we believe that despite this the material is often channelled; possibly a plough or other device is used on the stitching machines. Also it is doubtful if outsole moulding is a practical proposition, so that the use of the material for high-heeled shoes, especially of stuck-on construction, may be limited. Edges can be set and bottoms polished with wax to look quite attractive, and we believe they can also be coloured.
The material is harder than most rubber compositions. Heel top-pieces are exceptionally hard, and specimens of sheeting which we have examined suggest that thin soles are made harder than those of heavier substance. The hardness of this product contributes to its ability to insulate the foot when walking on rough surfaces or gravel and is particularly important on women’s shoes. Another characteristic is its lack of rubber-like resilience; when bent it feels stiffer than most compositions and recovers slowly. This, in some degree, is a property of most synthetic rubbers and plastics. In some applications it is an asset but in others it is a disadvantage; for instance, it may cause overheating of tyres of tanks and other heavy vehicles. Whether this property has any importance in shoe soles remains to be proved.
Measured by our standard test for permanent set, the resistance to spread of this material seems to be inferior to a good natural rubber composition. However, this is one of several respects in which existing physical tests may be misleading and we should be surprised to find that spreading is a serious fault; indeed it is claimed that this type of sole helps to keep the shoe in shape. Soles and heels of this material do not mark floors; nor are they slippery on wet or dry surfaces, but in this respect we cannot yet make a comparison with natural rubber solings. Finally, in length of life this material is claimed to last more than twice as long as leather and our tests indicate that this is a very conservative estimate, both for soles and heels. Nevertheless, it is probably not superior to many natural rubber compounds in this respect; for instance, as experimental mix to which we have given wear tests had considerably more than ten times the life of leather soles and heels, and a commercial product had over five times the life. For most purposes there is little point in having soles with longer life than is given by many existing compounds, but top-pieces which last longer would be a boon.
Summing-up, it is tempting to conclude that the only two really noteworthy properties of this synthetic soling are, firstly, its hardness, which gives a pleasing feeling of firmness like leather, when standing and walking, and secondly, its colour. This assessment may be very incomplete and may miss subtle and important points, but it provides a starting point for experiments in this country. Several British firms are already making attempts to reproduce the characteristics of these American solings without using synthetic rubbers. It is a crucial point whether synthetics, especially the butadiene-styrene type, are essential to the production of materials of this kind because their production has not yet been undertaken commercially in this country.
The production of synthetics
In America the petroleum refineries supply abundant cheap raw materials for the production of butadiene and other synthetic rubber constituents. Alcohol, made from agricultural grains, is of comparable importance. The possibility of using natural gases from oil well has also been considered and may, by now, be an actuality. These gases have hitherto gone to waste in incalculable quantities. There are other raw materials from which butadiene can be made; in Germany the starting point was carbide which requires coal, lime and cheap electric power for its production. In Russia, alcohols from the fermentation of potatoes and other agricultural products have been used. Synthetic alcohols have made considerable headway in this country for general industrial use, coal being the basic raw material for their production, but it is doubtful, in view of the complexity of the chemical processes and the high cost of coal, if butadiene made from synthetic alcohols could compete with that produced from other raw materials. In the production of styrene, which is used in synthetic rubbers and is also an important plastic, this country is not at such a great disadvantage. Except for small quantities derived from petroleum by-products, coal is a necessary raw material and production of styrene is likely to expand.
The purpose of this brief outline is to indicate some of the reasons why the development of home produced synthetic rubber has been comparatively slow. New technical developments may, of course, put a different complexion on the matter, but even if the present position is accepted, it does not mean that American developments in soling materials can be dismissed as of no importance to the British shoe industry. Firstly, it is necessary to give fresh study to properties which are desirable in sole leather for different types of shoes. Some of the directions in which present methods of assessment are incomplete have been indicated in this article, but there are still other considerations, including the importance of "breathing". Is it better for health and foot comfort to have a material which "breathes" or to have one which keeps out the wet, even when nearly worn out? The answer no doubt depends on the season of the year. So also may the heat conducting properties which are desirable. To find the answers to such questions is important work for the Research association. Secondly, manufacturers of soling materials have a great opportunity to develop new products based on natural rubber and other ingredients available in this country. The high process of sole leather makes the time particularly propitious. Thirdly, the possibility of further dollar savings needs consideration. The use of crepe and existing types of rubber composition is already substantial, but the limit of public demand for these materials may have been reached. If American type synthetic soles and heels would find an additional market here, the import of the necessary synthetics (assuming them to be essential ingredients) might well be justified by the saving in expenditure of hides at their present very high prices.
Reprinted from SATRA’s Monthly Bulletin, January 1950.
This article was originally published on page 38 of the February 2016 issue of SATRA Bulletin.