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Technical Briefs and Manuals: English
Rubber is produced from natural or synthetic sources. Natural rubber is obtained from the milky white fluid called latex, found in many plants; synthetic rubbers are produced from unsaturated hydrocarbons.
Long before Colombus arrived in the Americas, the native South Americans were using rubber to produce a number of water-resistant products. The Spaniards tried in vain to copy these products (shoes, coats and capes), and it was not until the 18th century that European scientists and manufacturers began to use rubber successfully on a commercial basis. The British inventor and chemist Charles Macintosh, in 1823, established a plant in Glasgow for the manufacture of waterproof cloth and the rainproof garments with which his name has become synonymous.
A major breakthrough came in the mid 19th century with the development of the process of vulcanisation. This process gives increased strength, elasticity, and resistance to changes in temperature. It also renders rubber impermeable to gases and resistant to heat, electricity, chemical action and abrasion. Vulcanised rubber also exhibits frictional properties highly desired for pneumatic tyre application.
Crude latex rubber has few uses. The major uses for vulcanised rubber are for vehicle tyres and conveyor belts, shock absorbers and anti-vibration mountings, pipes and hoses. It also serves some other specialist applications such as in pump housings and pipes for handling of abrasive sludges, power transmission belting, diving gear, water lubricated bearings, etc.
In this brief, we will be looking primarily at the reclamation and reuse of scrap tyres. This is simply due to the fact that this is the major source of waste rubber in developing countries.
Natural rubber is extracted from rubber producing plants, most notably the tree Hevea brasiliensis, which originates from South America. Nowadays, more than 90% of all natural rubber comes from these trees in the rubber plantations of Indonesia, the Malay Peninsula and Sri Lanka. The common name for this type of rubber is Para rubber.
The rubber is extracted from the trees in the form of latex. The tree is 'tapped'; that is, a diagonal incision is made in the bark of the tree and as the latex exudes from the cut it is collected in a small cup. The average annual yield is approximately 2 ½ kg per tree or 450kg per hectare, although special high-yield trees can yield as much as 3000kg per hectare each year.
The gathered latex is strained, diluted with water, and treated with acid to cause the suspended rubber particles within the latex to coagulate. After being pressed between rollers to form thin sheets, the rubber is air (or smoke) dried and is then ready for shipment.
There are several synthetic rubbers in production. These are produced in a similar way to plastics, by a chemical process known as polymerisation. They include neoprene, Buna rubbers, and butyl rubber. Synthetic rubbers have usually been developed with specific properties for specialist applications. The synthetic rubbers commonly used for tyre manufacture are styrene-butadiene rubber and butadiene rubber (both members of the Buna family). Butyl rubber, since it is gas-impermeable, is commonly used for inner tubes. Table 1 below shows typical applications of various types of rubber.
Table 1: Applications of different classes of rubber in the manufacture of vehicle tyres. Download the full PDF version to see this picture/table/diagrams.
The raw materials that make up tyres are natural and synthetic rubbers, carbon, nylon or polyester cord, sulphur, resins and oil. During the tyre making process, these are virtually vulcanised into one compound that is not easily broken down.
The modern process of rubber manufacture involves a sophisticated series of processes such as mastication, mixing, shaping, moulding and vulcanisation. Various additives are included during the mixing process to give desired characteristics to the finished product. They include:
Fillers are used to stiffen or strengthen rubber. Carbon black is an anti-abrasive and is commonly used in tyre production. Pigments include zinc oxide, lithopone, and a number of organic dyes. Softeners, which are necessary when the mix is too stiff for proper incorporation of the various ingredients, usually consist of petroleum products, such as oils or waxes; pine tar; or fatty acids. The moulding of the compound is carried out once the desired mix has been achieved and vulcanisation is often carried out on the moulded product.
To understand the process of vulcanisation it is worth discussing, briefly, the molecular structure of rubber. Crude latex is made up of a large number of very long, flexible, molecular chains. If these chains are linked together to prevent the molecules moving apart, then the rubber takes on its characteristic elastic quality. This linking process is carried out by heating the latex with sulphur (other vulcanising agents such as selenium and tellurium are occasionally used but sulphur is the most common). There are two common vulcanising processes.
The proportion of natural and synthetic rubber used for tyre manufacture depends on the application of the particular tyre.
Rubber recovery can be a difficult process. There are many reasons, however why rubber should be reclaimed or recovered;
There is an enormous potential for reclamation and reuse of rubber in developing countries. There is a large wastage of rubber tyres in many countries and the aim of this brief is to give some ideas for what can be done with this valuable resource. Whether rubber tyres are reused, reprocessed or hand crafted into new products, the end result is that there is less waste and less environmental degradation as a result.
In developing countries, there is a culture of reuse and recycling. Waste collectors roam residential areas in large towns and cities in search of reusable articles. Some of the products that result from the reprocessing of waste are particularly impressive and the levels of skill and ingenuity are high. Recycling artisans have integrated themselves into the Hydraulic oil traditional market place and have created a viable livelihood for themselves in this sector. The process of tyre collection and reuse is a task carried out primarily by the informal sector. Tyres are seen as being too valuable to enter the waste stream and are collected and put to use.
In Karachi, Pakistan, for example, tyres are collected and cut into parts to obtain secondary materials which can be put to good use. The beads of the tyres are removed and the rubber removed by burning to expose the steel. The tread and sidewalls are separated - the tread is cut into thin strips and used to cover the wheels of donkey carts, while the sidewalls are used for the production of items such as shoe soles, slippers or washers (WAREN Report).
There are many ways in which tyres and inner tubes can be reused or reclaimed. The waste management hierarchy dictates that re-use, recycling and energy recovery, in that order, are superior to disposal and waste management options. This hierarchy is outlined in Table 3 below.
Damaged tyres are, more often than not, repaired. Tubes can be patched and tyres can be repaired by one of a number of methods. Regrooving is a practice carried out in many developing countries where regulations are slacker and standards are lower (and speeds are lower) than in the West. It is often carried out by hand and is labour intensive.
The use of retread tyres saves valuable energy and resources. A new tyre requires 23L of crude oil equivalent for raw materials and 9L for process energy compared with 7L and 2L respectively for retreading. Tyres of passenger vehicles can generally be retreaded only once while truck and bus tyres can be retreaded up to six times. Retreading is a well established and acceptable (in safety terms) practice. The process involves the removal of the remaining tread (producing tyre crumb - see later) and the application and vulcanisation of a new tread (the 'camel back') onto the remaining carcass. In Nairobi about 10,000 tyres a week are received for retreading (Ahmed).
Secondary reuse of whole tyres is the next step in the waste management hierarchy. Tyres are often put to use because of their shape, weight, form or volume. Some examples of secondary use in industrialised countries include use for erosion control, as tree guards, in artificial reefs, fences or as garden decoration. In developing countries wells can be lined with old tyres, docks are often lined with old tyres which act as shock absorbers, and similarly crash barriers can be constructed from old tyres. Old inner tubes also have many uses; swimming aids and water containers being two simple examples.
The next step in our hierarchy involves the material being broken down and reused for the production of a new product. As mentioned earlier, in developing countries this hand reprocessing of rubber products to produce consumer goods is well established and the variety of products being made from reclaimed tyres and tubes is astonishing. The rubber used in tyres is a relatively easy material to reform by hand. It behaves in a similar manner to leather and has in fact replaced leather for a number of applications. The tools required for making products directly from tyre rubber are not expensive and are few in number. Shears, knives, tongs, hammers, etc., all common tools found in the recyclers' workshop, along with a wide range of improvised tools for specialised applications. Shoes, sandals, buckets, motor vehicle parts, doormats, water containers, pots, plant pots dustbins and bicycles pedals are among the products manufactured.
Another way in which physical reuse can be achieved is by reducing the tyre to a granular form and then reprocessing. This can be a costly process and there has to be a manufacturer willing to purchase the granules. Crumb rubber from the retreading process can be used in this way, as it is a good quality granulated rubber. The reprocessing techniques used are similar to those described in earlier chapters. Granulate tends to be used for low-grade products such as automobile floor mats, shoe soles, rubber wheels for carts and barrows, etc., and can be added to asphalt for road construction, where it improved the properties of this material.
This article was added to our catalogue on Tuesday 24 October, 2006.
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