RECYCLING OF TIRE RUBBER

 RECYCLING OF TIRE RUBBER

In 1840, Charles Goodyear invented the process "Vulcanization" to convert soft natural rubber into a hardened form by the formation of cross-links in between rubber molecules, bridged by Sulphur atoms. Simply Vulcanization can be defined as the "heating of rubber with sulfur" which is an irreversible process and is predominantly used to produce automobile tires.

The global consumption of natural rubber for the tire industry is 76% of all the natural rubber produced globally. According to the year 2019 statistics, over 3 billion tires are manufactured annually, worldwide. The tire is a composite product of natural rubber, polymer derived from non-renewable petroleum products, various chemicals, steel, and fabrics and is designed for use in a single generation. Once the tire is worn out, it becomes an industrial waste. There are serious concerns about the disposal of used tires in the environment such as possible fire hazards(tire fires), insect breeding in water retained in the tire, and so on.

Disposing of waste tires is a challenge. This is a type of waste that take800-1000 hundreds of years to decompose and is non-biodegradable. 

Traditional disposal techniques of used tires

• Used as an alternative fuel for the cement industry
• Landfills applications and the making of an artificial reef to prevent coastal erosion

Actually, the above-mentioned techniques are inefficient ways to dispose of used tires. There is a risk of the addition of toxic gases such as dioxins, dibenzofurans, NOx, and SO2, Moreover the heavy metal in ash, for example, Mn, Cr, and Pb cause soil and water pollution. If so what are the most economical and environment-friendly ways available to dispose of used tires?

1. Tire Retreading

Retreading is a process in which the removal of old worn-out tread area of the tire and replaced it with a new portion of rubber. Retreading is done by using two processes called hot cured processes or conventional process and cold cured or pre-cured process. Retreading decelerate the addition of used tires as a waste to the environment. Retreading provides the opportunity to extend the usable life of valuable resources and to delay the ultimate need the for disposal of used tires. 

Buffing dust is a valuable by-product that you obtain during the tire retreading process. When you remove the old worn-out tread rubber you receive the rubber particles in the form of crumbs. You can reduce the particle size of these crumbs by further grinding and can be used as a filler for compound mixing, not only for reducing the cost but also for maintaining the desired physical properties.     

    2. Reclaiming

Reclaiming is a process that utilization of rubber in waste tires by the application of heat and chemical agents. This process involves essentially one of the depolymerization reactions. Reclaimed rubber can be used to produce various rubber products, including tires, by replacing some of the new rubber. Powdered reclaimed rubber is used to produce, plastics tanks, artificial turfs, waterproof coating, and asphalt modifications. 

The special characteristic of reclaimed rubber is it improves the processibility of natural and synthetic rubbers but weakens the mechanical properties.

    3. Pyrolysis

The process of pyrolysis breaks down the waste tire rubber into its constituent components oil, gas, and carbon black in the absence or lack of Oxygen. 

In pyrolysis process involves the heating(450-500°C) of the whole tire or shredded tire in a reactor vessel in an Oxygen-free environment. Then the rubber becomes softened and degrades into smaller molecular mass: liquid pyrolysis oil(or bio oil}, Carbon black, and metal.  

• Pyrolysis oil or bio-oil is an alternative source of energy that can be used for boiler fuel or for electric generation
• Carbon black can be used for metallurgy, production of paint and building materials, the manufacture of fuel briquettes
• The gas generated during the pyrolysis process can be used for system operation and electricity generation
• Metal cords of breakers and bead wire bundles can de be used in the metallurgical industry

    4. Ambient Grinding

This involves several steps, using a series of machinery such as granulators and cracker mills interspersed with magnets to remove steel and air separators to remove fibers. Vulcanized rubber is first reduced to small chips. Then these chips are further broken by using granulators and cracker mills set at the close nip. The ambient grinding process produces a significant amount of heating and results in rubber degradation. 

    5. Cryogenic Grinding

Little or no heat is generated during this process. It produces fine grind, fairly smooth fracture surfaces. Due to no heat generation during processing, there is no degradation of rubber. In cryogenic grinding liquid Nitrogen is used to freeze the rubber which is then passed through the hammer mill. The most significant feature of this process is that almost all fiber or steel is liberated from the rubber resulting in a high yield of usable product and little loss of rubber. 

  6. Solution Grinding

In this method, first rubber particles are swelled in a solvent, typically used fatty acids for swelling and are then subjected to a conventional grinding process. Mesh sizes of 60-120 are commonly produced. These rubber dust are widely used in tire compounds because of their uniformity and cleanliness.

 7. De-vulcanization of rubber

I will discuss this in a future post