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Recycling & degradation of polymers.

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Recycling & degradation of polymers.

  1. 1. Recycling is the process of making or manufacturing new products from a product that has originally served its purpose.
  2. 2.  Recycling of plastics should be carried in such a manner to minimize the pollution level during the process and as a result to enhance the efficiency of the process and conserve the energy.  Plastics recycling technologies have been historically divided into two general types: i. Primary, ii. Secondary,
  3. 3. i. Primary recycling involves processing of a waste/scrap into a product with characteristics similar to those of original product. ii. Secondary recycling involves processing of waste/scrap plastics into materials that have characteristics different from those of original plastics product.
  4. 4. 1) Inspection Workers inspect the plastic trash for contaminants like rock and glass, and for plastics that the plant cannot recycle. 2) Chopping and Washing The plastic is washed and chopped into flakes. 3) Flotation Tank If mixed plastics are being recycled, they are sorted in a flotation tank, where some types of plastic sink and others float. 4) Drying The plastic flakes are dried in a tumble dryer.
  5. 5. 5. Melting The dried flakes are fed into an extruder, where heat and pressure melt the plastic. Different types of plastics melt at different temperatures. 6. Filtering The molten plastic is forced through a fine screen to remove any contaminants that slipped through the washing process. The molten plastic is then formed into strands. 7. Pelletizing The strands are cooled in water, then chopped into uniform pellets. Manufacturing companies buy the plastic pellets from recyclers to make new products.
  6. 6. Advantages i. Lower labor costs. ii. Lower transportation costs, hand or ox carts often being used. iii. Innovative use of scrap machinery often leads to low entry costs for processing or manufacture. iv. Existing culture of reuse and recycling, & collection of these materials. Dis-Advantages i. Cost: “May not be economical to recycle small quantities of plastics. ii. Training of waste generators will be required, so they can separate the wastes properly. .
  7. 7. It is a change in the properties—tensile strength, color, shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids, alkalis and some salts.
  8. 8. Types of Polymer degradation • Thermal • Photo • Hydrolytic • Chemical • Biological • Degradation by irradiation • Pyrolysis
  9. 9.  Photo-induced degradation: Most polymers can be degraded by photolysis to give lower molecular weight molecules. Electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, X- rays and gamma rays are usually involved in such reactions.  Thermal degradation: Chain-growth polymers like poly(methyl methacrylate) can be degraded by thermolysis at high temperatures to give monomers, oils, gases and water.  Chemical degradation: Solvolysis: Step-growth polymers like polyesters, polyamides and polycarbonates can be degraded by solvolysis and mainly hydrolysis to give lower molecular weight molecules. The hydrolysis takes place in the presence of water containing an acid or a base as catalyst.
  10. 10. Ozonolysis:  Cracks can be formed in many different elastomers by ozone attack.  Tiny traces of the gas in the air will attack double bonds in rubber chains, with Natural rubber, polybutadiene, Styrene- butadiene rubber being most sensitive to degradation.  The problem of ozone cracking can be prevented by adding anti-ozonants to the rubber before vulcanization.  Ozone cracks were commonly seen in automobile tire sidewalls. Oxidation:  Polymers are susceptible to attack by atmospheric oxygen, especially at elevated temperatures encountered during processing to shape.  Many process methods such as extrusion and injection molding involve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken.
  11. 11. Galvanic action:  Polymer degradation by galvanic action was first described in the technical literature in 1990.  This was the discovery that "plastics can corrode", i.e. polymer degradation may occur through galvanic action similar to that of metals under certain conditions.  Biological degradation:  Biodegradable plastics can be biologically degraded by microorganisms to give lower molecular weight molecules.  To degrade properly biodegradable polymers need to be treated like compost and not just left in a landfill site where degradation is very difficult due to the lack of oxygen and moisture.

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