(10) 1 part ozokerit in 2 parts castor oil, and 1 part lamp-black added, makes an excellent application, as the boots will take a thin polish after. (11) Salad oil 1 pint, mutton suet 4 oz., white wax and spermaceti of each 1 oz., melted together, and applied to the boots warmed before the fire. (12) Much used by fishermen: Melt 3 oz. spermaceti in a ladle, and add 3/4oz. rubber, cut into thin shavings. When dissolved, add 1/2 lb, tallow, 2 oz. pure lard, and 4 oz. amber varnish. Mix well, and while still warm apply with a brush, giving two or three coats. It leaves a good polish, and is preservative as well as being waterproof.
Under ordinary conditions rubber for vulcanising is usually mixed with sulphur and heated to a high temperature, when chemical combination takes place between the sulphur and the rubber, producing a much more valuable compound for ordinary purposes than unvulcanised rubber; the former remaining soft at very low temperatures and firm at high temperatures, whilst the latter becomes hard and quite plastic respectively at those temperatures.
In making cloth for waterproof garments another method is employed for vulcanising the rubber, viz., by wetting its surface with a mixture of somewhere about 5-10 parts chloride of sulphur dissolved in 100 parts bisulphide of carbon, and then heating the fabric gently to evaporate away the excess of these substances; the rubber-covered cloth cannot be heated to a high temperature like the rubber alone, because the heat would be liable to injure the cotton, silk, or wool of the fabric, or destroy or injure the colours.
The bisulphide of carbon softens and penetrates the fine layer -of rubber, carrying with it the chloride of sulphur dissolved in it, and it is generally supposed that the chloride of tulphur breaks up, the sulphur combining with the rubber producing vulcanisation, and the chlorine, combining with the hydrogen producing hydrochloric acid which is liberated. This reaction is clearly not the correct one, and it is probable that the reverse is more in accordance with the facts, viz., that the chlorine of. the sulphur chloride combines with the rubber producing vulcanisation, leaving the sulphur in the free state, or only partially in combination with the rubber, because in rubber vulcanised by the cold process I have found free sulphur to be present.
From a piece of rubber-covered cloth I separated the rubber, and submitted it to analysis, by mixing it thoroughly in small pieces with pure sodium carbonate and igniting, then dissolving the whole in water and adding to it peroxide of hydrogen previously treated with excess barium chloride (to separate sulphuric of acid or sulphates).T he peroxide ensures the conversion of the lower oxides of sulphur into sulphuric acid, whilst the excess of barium chlorides precipitates the sulphuric acid in the solution, which is then weighed as barium sulphate.
Another portion of the made-up solution was neutralised, and the chlorine present titrated. The rubber previous to ignition, as above described, had been well boiled in water and dried to separate any hydrochloric acid which might be present, but only a faint trace of chlorine compound could be thus separated from the rubber.
The total sulphur present in the robber amounted to 2.60, and the total chlorine to 6.81 per cent.
The yellow-coloured sulphur proto-chloride is best adapted for vulcanising, because it does not act too strongly upon the rubber, whilst the dark coloured chloride of sulphur, containing as it does a large quantity of the higher chlorides of sulphur, is liable to render the rubber quite hard by vulcanising it too much. The theory generally adopted to explain this is, that these higher chlorides break up easily, liberating their sulphur which thus combines in greater quantity with the rubber; but my experiments and analyses prove that it is chiefly the chlorine and not the sulphur of the chloride of sulphur which produces the vulcanisation.
A rubber substitute, much used at present, is produced by acting on vegetable oils, such as rape, linseed, etc., with a mixture of chloride of sulphur and bisulphide of carbon; the oil becomes converted into a solid substance resembling rubber to some extent, but being much more brittle. This body is now used in large quantity for mixing with rubber for the purpose of cheapening its production. On analysis of some samples of this material I have invariably found that it contained a much greater proportion of chlorine than of sulphur, and this process therefore is a vulcanisation by chlorine rather than by sulphur.
Recently I analysed three samples of rubber substitute, the one termed "special" another "spongy" rubber substitute, the third being similar to the first in appearance. The first contained of sulphur 3.4 and of chlorine 7.6 per cent.; the second contained of sulphur 4.56 and of chlorine 8.22, and the third 2.67 of sulphur and 7.90 of chlorine per cent.
These rubber substitutes contain considerable quantities of oily matters soluble in ether, which I have also found to be chlorine and sulphur compounds of the oils. The first yielded 20.0 per cent., the second 14.3, and the third 11.5 per cent, of these thick oily matters soluble in ether. This oily substance from the first sample contained 2.6 per cent, of sulphur and 6.1 per cent, of chlorine, whilst that from the second contained 2.97 and 6.87 per cent, of sulphur and chlorine respectively.
Some rubber manufacturers regard this oily matter as injurious to the rubber, and reject any substitute which contains any considerable proportion of it. I have found, however, by experiment that this oily compound instead of acting injuriously on rubber, actually acts as a preservative of it; some rubber threads were smeared with this oily extract, some with ordinary (un-vulcanised) rape oil, and some left untreated; these were put into an incubator at 150° F. for a few days when it was found that the oil-treated rubber was quite soft and rotten, whilst the other two had remained sound; after a few days more, the original rubber threads had become quite rotten, whilst the threads smeared with the oily part of the vulcanised oil remained quite sound.
The first and second samples of rubber substitute were examined for soluble chlorides or hydrochloric acid, by boiling in water; the first gave 0.18 per cent, of chlorine soluble in water, and the second 0. 05 per cent.
It has been known for some time that copper salts exert a most injurious influence on rubber. Copper salts are sometimes used in dyeing cloth, which are afterwards employed for waterproofing with rubber, and it seems quite astonishing what a small quantity of copper is required to harden and destroy the rubber, and the destructive effect of copper is further enhanced if the cloth contains oily matter in which the copper has dissolved.
As an example, a piece of cloth, alleged to have damaged the thin coating of rubber on it, was found to contain copper, and with a view of demonstrating this point, I took one piece in its original condition. To the end of this I pasted a similar piece of cloth from which the oily and greasy matter* had been removed by ether, and to the end of this again, I pasted another piece of the same cloth, from which I had removed both oily and greasy matters and copper; these three pieces joined end to end into one, were then coated in the usual way with rubber, and then hung in an incubator at 150° F. In the course of a few days, ihe rubber on the original cloth had become soft, and it then hardened and became rotten and useless; the second piece, from which the greasy matters had been removed, then became quite hard and rotten, whilst the part from which both greasy mat ters and copper had been removed has remained in a perfectly elastic and good condition.
Prof. Dewar observed accidentally, that metallic copper when heated to the temperature of boiling water in contact with the rubber exerted a destructive effect upon it. With a view of finding whether this was due to the copper per se, or to its power of conducting heat more rapidly to the rubber, I laid a sheet of rubber on a plate of glass, and on it placed four clean discs, one of copper, one of platinum, one of zinc, and one of silver. After a few days in an incubator at 150° F. the rubber under the copper had become quite hard, that under the platinum had become slightly affected and hardened at different parts, whilst the rubber under the silver and under the zinc remained quite sound and elastic. This would infer that the pure metallic copper had exerted a great oxidising effect on jthe rubber, the platinum had exerted a slight - effect, whilst the zinc and silver, respectively, had had no injurious influence on it. A still more curious result was this, that the rubber thus hardened by the copper contained no appreciable trace of copper, the copper, therefore, presumably sets up the oxidising action in the rubber without its permeating it. (W. Thomson.)