A very important use of sulphur, if not of large amounts, is the vulcanization of rubber. Understanding where this fits in is an excuse for reviewing the nature of the material. Rubber was found in use by the South American natives when the Spanish arrived. They played with solid rubber balls, wore clothing made impermeable by rubber, and used rubber bottles. The Spanish could not make any use of this new material, and it remained a curiosity for over 200 years. In 1770, Joseph Priestley named the substance "rubber" from its ability to rub out pencil marks. If it was dissolved in turpentine, fabrics could be impregnated with it and made waterproof (the "MacIntosh"). It also made waterproof boots and similar objects, and by 1830 was widely used. Rubber is soluble in benzene, carbon disulphide, turpentine, and similar solvents. Rubber cement (originally rubber in benzene) was popular for art paste-ups, because any excess was easily removed by rubbing with a finger.
Rubber is elastic, water repellent and a good electrical insulator. However, it became brittle at low temperatures, sticky at high temperatures, had little mechanical durability and oxidized rapidly. It was found that sulphur improved the properties somewhat, but it was reserved for Charles Goodyear (1800-1860) to find in 1839 that if rubber was not only mixed with sulphur, but cooked at 120°-160°C for a sufficient time, it became practically a new substance. It was elastic over a wide range of temperature, becoming neither brittle nor tacky, mechanically more durable, and quite impermeable. If about 5% sulphur is used, the rubber is soft and flexible. When 32% sulphur is used, the product, called ebonite or hard rubber, is an excellent, durable thermosetting plastic, once popular as a structural material in all kinds of electrical apparatus. It has now been replaced by cheaper plastics.
Rubber was obtained from the latex of Hevea brasiliensis, a tree that grew wild in South America. The latex is a colloidal suspension of rubber that comes from just under the bark of the tree (it is not the sap). In this tree it is particularly pure and free of the resins that such liquids (which are fairly common--as in the dandelion and milkweed) usually contained. The suspension is coagulated with formic acid, and the liquid pressed out between rollers, forming sheets about 1/4" thick that are often smoked to sterilize them, and packed in bales for shipment. Seeds of H. brasiliensis were smuggled out of Brazil in 1876 by Sir Henry Wickham (1846-1928), and used to establish plantations in southeast Asia, the source of most of the natural rubber since that time.
Rubber is a polymer with the empirical formula (C5H8)x, with one C=C double bond to each unit. Sulphur cross-links the polymer chains, taking advantage of these double bonds. The density of rubber is about 0.915 g/cc, and it is a white elastic solid. It is essentially a (1,4) polymer of isoprene, the molecule shown in the diagram, but efforts to reproduce natural rubber exactly have been in vain. Natural latex contains about 35% rubber substance.
However, many very good rubber-like materials have been produced as artificial polymers. Natural rubber will retain its market only so long as it is cost-effective. The spur to artificial rubber production was the disruption of plantation rubber during World War II. Germany had produced artificial rubbers even during World War I.
About 80% of industrial rubber is used to make pneumatic tires and tubes. Much rubber is reclaimed from used rubber products, but not enough is done along these lines even now. The pneumatic tire was invented in 1877, and the cord tire, which replaced fabric with parallel cords and made the tire much more durable, in 1910. The mix for making a tire includes, besides rubber and sulphur, accelerators (CaO, MgO, organic compounds), softeners (stearic acid, paraffin, petroleum jelly), reinforcing agents (carbon black, zinc oxide), and antioxidants. The green tire is assembled with all its fabric, cord, and wire reinforcement, and put into the vulcanizing mold. Heat and pressure then make the finished tire.
Sulphur is an important fuel in pyrotechnic mixtures, because it is cheap and stable. It occurs in match heads, the most common pyrotechnical device, and was an ingredient of black powder. Black powder is a special mixture of 75% potassium nitrate, 15% charcoal, and 10% sulphur, more or less. The sulphur and charcoal are ground together dry so that the thixotropic sulphur thoroughly coats the active surface of the charcoal. Then the nitrate is added, and the mix is wet ground until homogenized.
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