by Rudolf GORELIK, Cand. Sc. (Technology), Technology Director, R&D Co "Khimplast Engineering"
Discarded tyres and other rubber industry wastes do not decompose under natural conditions. This refuse stays on and builds up, gobbling up huge areas. A global problem now! Burning up or burying this dead matter is not a way out.
There is hard evidence-though not known to everybody, it is quite impressive: each year in industrial countries yet another old tyre is added per head of the population, man, woman and child. Discarded autotyres keep piling up. If we take the United States, the total number of thrown away tyres is 280 million pieces, weighing 3.3 mln tons; in Russia this stuff totals 600,000 tons-all that adds up every year, mind you! As much as 10 to 15 percent of the available tyres is thrown away annually.
It is high time to get down to utilization of worn autotyres and other polymers no one needs anymore. There is a good way out-have them ground or milled for subsequent repeat use within all kinds of mixtures in industrial production.
For one, rubber wastes can be sliced with milling cutters supplied with multiple cutting edges. Also, they can be crushed at cryogenic temperatures when rubber becomes brittle. Other innovative techniques have been developed lately-ozone and explosive fracturing as rub-
Change in the basic characteristics of silicone (a), fluorosilicone (b) and protector (c) rubber in bus tyres with the addition of active rubber powders. Dashed line, a value corresponding to service forms and records.
ber is brought to a stressed (deformed) state or cooled to ultralow temperatures (the latter option). The waste is thus reduced to crumb or powder (dust).
The crushed rubber particles are graded according to size and form; the finest ones are then distributed according to the value of their specific geometrical surface (relation of the surface area of particles to their mass), the presence of metal or textile cord * , their chemical composition and other factors. These characteristics are very important for the subsequent utilization of reprocessed products as additives to rubber melt mixes or to fresh rubber for the making of autotyres and other wear- resistant items.
Of particular significance is the chemical activity of the rubber powders thus obtained, namely, their ability to form stable molecular bonds with the stock material. Their reactivity is good enough if a compound ** containing 30 or 40 percent of additives shows little, if any, differences from specimens made of fresh raws (say, in elongation at rupture, tensile strength, etc.).
Meanwhile, longtime tests show that all conventional techniques of rubber processing, including cryogenic treatment (the technique most in use now) fall short of the desired effect.
The low chemical activity (reactivity) of rubber powders is usually explained by their small specific geometrical surface. This activity can be further increased by means of further crushing down to "motes" 100, 50 and even 20 mm in size. This procedure, however, is too energy expensive and costly.
In addition, extensive mechanical-and-chemical effects (crushing) make polymers succumb to attack by acid and other reactions whereby molecules are destroyed. So we get a material starkly different from the stock product. This is no proper procedure therefore.
We have developed a new way of attacking the problem of scrap rubber and polymer utilization. Our company's experts suggest a fine crushing technique at high volumetric stresses and at temperature 120 - 320 o C in the presence of modifiers *** . Extensive tests of this technology and appropriate equipment have proved the high efficiency of our method.
A compound containing up to 30 percent of powder has been found to be in no way different from 100 percent fresh rubber in physical and mechanical characteristics.
Depending on the designation of powders, they can be supplemented with a variety of modifiers-mineral or organic fillers, and liquids causing the vulcanized
* Cord-here, a twisted fiber added during the manufacture of automobile, aircraft and other tyres, and of other rubberized articles. - Ed .
** Compounds - here, composite materials obtained from polymers or monomers and designed for filling or impregnating current-carrying circuits and parts in electronics and radio electronics. - Ed.
*** Modifiers - substances improving a product's structure and quality. - Ed .
(cured) rubber to swell. If need be, several additions can be made simultaneously during reprocessing. That is to say, we have managed to combine both the crushing and the modification techniques.
Carrying out a wide range of research work and tests, we have seen that as much as 20 to 30 percent of scrap rubber could be utilized in the manufacture of tyres not at the expense of their quality. We also offer a number of materials and manufactured items that contain one and the same component. These include high-reactivity sorbents (granular and in sheet) of petroleum products. In addition to rubber powders, such sorbents also contain agglomerating (sintering) modifiers that "collect" oil product particles into larger congregations. Likewise present are polymer loosening agents and hydrophilic initiators (the latter activate water absorption).
Experts of the authority in charge of emergency, rescue and ecological operations (operating under the umbrella of the RF Ministry for Civil Defense, Emergency Situations and Eliminating the Aftermath of Natural Calamities) have tested our sorbents. Their verdict: the sheet absorbent is good for removing oil slicks from water surface; and the high-bulk density powder absorbent can well be used for cleaning the ground too. Low-bulk density powders can clear water areas having harbor booms (enclosures).
All these sorbents can be used both in static and in dynamic conditions. Thereupon they can be utilized - burned up in furnaces, they generate heat and electric power to boot.
A modified additive to asphaltic concrete can find uses in road building too. The point is that the durability of pavement depends a good deal on the bitumen component of road covering. Here stabilizers-substances protecting polymers against destruction-are most welcome as a remedy that will keep roadways from premature ageing. For this purpose we have developed a rubber-powder composite that catalyzes the formation of a three-dimensional structure in asphaltic concrete, increases its viscoelasticity and strength, and enhances the temperature range of its maintenance - from 80 o C to -45 o C. Our composite is added to asphaltic concrete directly in the process of its making. This composite is in two modifications - one for Russia's south, the other-for her north.
The range of materials we manufacture from scrap rubber is rather wide. These are polymer mixtures meant for the compacting and casting of various items, including those with cold endurance characteristics; these are also slabs and plates containing up to 80 percent of waste-rubber ingredients and used in the making of durable covers and for the insulation of residential houses and industrial premises (today heat losses through walls and roofs make up to 40 percent). Our list could be continued. Besides, we have developed several modifications of concrete: elastic concrete with improved vibration damping characteristics; high-durability, crack- and heat-resistant concretes. We can offer a range of other innovative building materials, such as roofing sheets, mastics and weather strips; granulated polyamide obtained from textile cord waste, and even artificial wood composed of sawdust, rubber crumb and plastic. These materials have been found to be in steady demand in the marketplace.
A comprehensive approach to the utilization of polymer wastes can provide a basis for a new industry. At any rate, essential conditions are on hand: reusable raws, demand for finished products, and available technology and equipment.
Illustrations supplied by the author
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