Vulcanisation as Applied to Tire Repairs.
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By Clayton Beadle and Henry P. Stevens.
ll'e hare ,.,ceiced front Mr. Clayton 1.1?..11a.Soc., and Dr. Henry P. eee. eeel., Ph.D., 14'.1.C. (analytical the following artick on the sub fee,' • rulcanisation as Applied to Tire Repairs." As the subject is one of great interest to if ors of commercial vehicles, and particularly to owners of light vans and cabs..we.nproduce the article in fa//.—En.
In recent years the repairs of tires by vulcanising has become very general and is now applied aboost universally to inner tubes in the place of the older method of merely " solutioning " on a patch. Although the latter method answers its purpose for the repair of Punctures on inner tubes of bicycle tires, it has been found to lw much less suited for the repair of motor tires, partly on account of the higher temperature developed, but largely upon the increased pressure to which they are subjected. 11/hen pneumatic tires were first used on motorcars, it was considered sufficient to solution-on a patch, and one of the leading manufacturers of motor tires still recommends this course and supplies materials for the purpose. The practice has, however, almost entirely given place to the method of repairing by means of vulcanisation, and in nine out of ten places where repairs are executed the method of " sticking on " a patch is condemned as useless. With the aid of the improved solution that is now obtainable, by carefully cleaning the surfaces before it is applied, and by allowing sufficient time to elapse before the patch is applied to the inner tube, a satisfactory repair can be effected, but such a mode of procedure is not always followed by private or trade users of motor vehicles.
The process now largely in vogue for filling in the cracks in outer covers is by means of a mass of rubber or rubber compound that is rendered plastic by the addition of some rubber solvent, such as carbon bisulphide. This compound is placed in the cut or gap which must first of all be dried, cleaned, and wiped out, and then moistened with a rubber solvent, such as coal tar naphtha. The crevices are then filled in with masses of the unvulcanised rubber compound, which hardens on the evaporation of the solvent ; this " plugging " up of the cracks protects the canvas from the rotting action of moisture, and gives the cover a better ap
pearance. Many engineers, however, do not think the results are worth all the trouble, and furthermore the plugged pieces of rubber are liable to come out in wet weather at the time when they are most needed. The plugging of these gaps certainly also keeps out small stones which are liable to become embedded in existing cuts, and, by their constant working, to cut the canvas and thus lead to the bursting of the outer cover. This filling in of crevices, which must be regarded as of secondary importance as compared with the mending of the inner tube, is now frequently
done by plugging the cuts with rubber compound, which is subsequently vulcanised by the application of heat.
The inner tube, which is, generally, about :-inch thick, requires a definite tune for the vulcanisation of a patch if done with a definite compound and at a uniform temperature; but a slit in an outer cover may vary in depth considerably and be of such a shape that very little vulcanising surface is exposed to the direct action of the vulcaniser. Hence it is that outer cover repairs nearly always require a longer time for the process of vulcanisation than do those to the inner tube.
Before reaching the work we wish to record, wc will refer to vulcanising patches by means of the cold vulcanising process. We have tested this method carefully, and we hope to give the results of our tests in a separate communication. We have succeeded in producing very satisfactory patches by this process, but can see reasons against its general adoption by repairers. Its employment entails a somewhat delicate chemical operation, and, therefore, the operation needs great care to ensure a good result, and furthermore the sulphur chloride— which is the active agent in coming in contact with the body of the tube— is liable to produce surface cracks ; hence the reason, we think, that the cold cure process has not been more universally adopted by repairers on the road. The only satisfactory and permanent repair appears to be that produced by hot vulcanisation, and the principle has been further extended to the complete retreading of worn down tires. We recently had occasion in the course of our work to investigate a number of the compounds which are sold for the purpose of effecting repairs and to subject them to a series of tests; as the results are of considerable interest both to motorists and to the rubber technologist we propose giving some account of these compounds and of the tests we have made.
In devising a recipe for a rubber mixing suitable for tire repairs there are a number of considerations which must be taken into account. In the first place the unvulcanised mixture must be supplied with such physical properties that the motor mechanic can easily adapt it to his requirements; it should, when slightly warmed, be soft, pliable, and adherent so that the pieces can be easily squeezed or pressed into the shape required to fill up the cut or hole. In the second place the chauffeur or mechanic ,at the garage, who, generally, wishes to effect the repair as quickly as possible (many commercial rubber mixings as used in the manufacture of rubber articles take an hour or more for vulcanisation), wants the repairing compound to be prepared in such a manner that it vulcanises quickly— say in 15 minutes at a practicable temperature or pressure. As a general rule, of course, the higher the temperature the shorter the time of vulcanisation, but in practice a temperature of 153deg. C (corresponding to about 6o11). steam pressure) is about the highest limit. A higher temperature than this would mean a higher pressure and a more strongly constructed vulcanising appliance to stand the increased pressure, and this would lead to a greater cost for the appliance; it would also take a longer time to heat up to the required temperature. There is, however, another point of great importance which is frequently overlooked : when a patch is being vulcanised on to a tire it is impossible to avoid heating the portions of tire adjacent to the patch; this has a distinctly objectionable effect on the surroundingrubber as it is liable to overvulcanise this portion of the tire, The longer the patch is heated and the higher the temperature (or pressure) employed the greater the damage done to the surrounding portions of the original rubber in the tire. When the tire
was originally manufactured the time and temperature of vulcanisation were most carefully adjusted by the manufacturer to bring out the best qualities of the mixture, so that any further heating is liable to produce the effect of over-vulcanisation and so impair the quality of the rubber. This effect is still more marked if the further heating be carried out some time after the rubber goods are made, as a certain amount of natural deterioration will already have set in, from which it follows that the damage resulting from heating during the course of vulcanising on a patch is greater with old than with new tires. And as it generally happens that , the repairs to punctures, etc., are needed on tires and tubes that have already undergone considerable deterioration as the result of the actions of light and heat, as well as of wear, this point should not be lost sight of. This will be apparent from the tests given later, which show the loss in strength with an old and a new tire respectively when treated in this manner. It is,
therefore, of the greatest importance that the vulcanising compound should "cure " easily,i.e., in a short time and at a low temperature (or pressure). The patch must, however, be sufficiently vulcanised, that is to say it must not be too much under-vulcanised, as this would mean a soft product which would be lacking in strength and wearing qualities. The compound should be of such a composition that when vulcanised under the above conditions it is as strong as the original on to which it is grafted. To summarise, then, a composition for tire repairs (t) should be : soft, pliable, and easily moulded when slightly warmed before vulcanisation ; (2) should vulcanise easily, that is at as tow a temperature or pressure), and in as short a time as possible; and (3) should be strong and of good wearing qualities after vulcanisation.
In the tests about to be recorded we examined some ecmmercial samples ot vulcanising compositions, as also a compound prepared by ourselves, which we found on careful testing to be an improvement upon the compounds in common use. The various compositions were vulcanised mostly for the same length of time, sometimes 15 and sometimes 17 minutes, and over a range of temperatures and pressures. For purposes of illustration we give in Table I the results obtained with a high-class commercial compound (A), a Para rubber mixing, and (B) one of our own compositions. The pieces after vulcanisation were subjected to physical tests in particular determinations of tensile strength, and the results in Table I (each figure of which is the mean of ten determinations) demonstrate the superiority of our own composition.
We may state here that the commercial compound (A) taken for comparison is the best of the several we have examined.
In the second series of experiments (Table II) we give the results of tests on a patch of rubber vulcanised on to the inner tube of a motor tire. Parallel tests were made using in one case vul
canising compound "A," and in the other compound " B." The rubber ot the inner tube itself was tested at points (adjacent to the patch) which were heated at the same time as the patch during the process of vulcanisa Lion. The inner tube was also tested in places where it had not been heated. A second series of experiments (Table II) was made on an old inner tube. This was a tube that had been in wear for a considerable time and, as the tests indicate, had considerably deteriorated, but was still in a useable condition. It will be seen that this old tube deteriorates much more rapidly than the new one when heated, except in cure No. where the loss is abnormally high for a pressure of 201b., the figure being practically the same as in cure No. 9 where the pressure was 251b. Taking the figures as a whole it will be seen that at a medium pressure, say 351b., the new tube loses 25 per cent, of its strength in 17 minutes, whereas the old tube loses 50 per cent. of its strength in the same time.
To bring out the relative advantages of repairing compounds when used on a good inner tube, the short summary (Table. III) giving only the breaking strain and vulcanising pressure is instructive (the time of vulcanisation is 17 minutes in all cases). A glance at Table III shows clearly : (a) how the original tube loses regularly in strength in direct proportion to the pressure (Or
temperature) to which it is submitted; (b) how the material of which the patch is composed (whether " A " or " B ") up to a certain point, gains in strength in direct proportion to the pressure (or temperature) employed ; and (c) that the material " B " vulcanises much more quickly than the material " A," that is to say the heat gets through the material of the patch much more quickly with "B" than "A," so that "B" is sufficiently vulcanised at a pressure (or temperature) much lower, and, therefore, less harmful to the original tire than "A." Thus a pressure of 2o1b. is sufficient to vulcanise "B" so that its strength is practically the same as the adjacent portions of the tire and only a very little below that of the original unheated tire. In the case of the material "A" a much higher temperature
or pressure is necessary. It further shows that all through "B" is stronger than "A," and that a pressure of 351b. produces a tougher product with the former than olb. with the latter.
As already stated time and temperature are in some measure interchangeable factors. Thus, assuming that in the case of compound "B," 2o1b. pressure at 17 minutes is sufficient, a somewhat equivalent result is obtained by reducing the time and increasing the pressure. The equivalents for all practical purposes of puncture repairs may be taken as follows : Pressure in lb. Time in minutes.
121b. 30
2 olb. 17
3o1b. 114 4o1b. 8 to to It must not be forgotten that the thickness of the material is a factor to be reckoned with and that in ordinary inner tube repairs the heat is transmitlid from one surface only, generally with a thickness of paper or cloth placed so as to prevent the rubber from
sticking to the metal surface. Furthermore it does not necessarily follow that the patches have reached, even at the end of the operation, a temperature equivalent to the steam pressure on the other side of the metal plate of vulcaniser.