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ABSTRACT
Plastic recycling is the process of recovering scrap or waste plastic sand reprocessing the material into useful products, sometimes completely different in form from their original state. For instance, this could mean melting down soft drink bottles and then casting them as plastic chairs and tables. Typically a plastic is not recycled into the same type of plastic, and products made from recycled plastics are often not recyclable. When compared to other materials like glass and metal materials, plastic polymers require greater processing (Heat treating, Thermal depolymerization and monomer recycling) to be recycled. Plastics have a low entropy of mixing, which is due to the high molecular weight of their large polymer chains. A macromolecule interacts with its environment along its entire length, so its enthalpy of mixing is large compared to that of an organic molecule with a similar structure. Before recycling, most plastics are sorted according to their resin type. In the past, plastic reclaimers used the resin identification code (RIC), a method of categorization of polymer types, which was developed by the Society of the Plastics Industry in 1988. Polyethylene terephthalate, commonly referred to as PET, for instance, has a resin code of 1. Most plastic reclaimers do not rely on the RIC now; they use automatic sort systems to identify the resin, such as near infrared (NIR) technology. Some plastic products are also separated by color before they are recycled.
Types of Plastic
PET Polyethylene terephthalate - Fizzy drink bottles and oven-ready meal trays.
HDPE
High-density polyethylene - Bottles for milk and washing-up liquids.
PVC
Polyvinyl chloride - Food trays, cling film, bottles for squash, mineral water and shampoo.
LDPE
Low density polyethylene - Carrier bags and bin liners.
PP
Polypropylene - Margarine tubs, microwaveable meal trays.
PS
Polystyrene - Yoghurt pots, foam meat or fish trays, hamburger boxes and egg cartons, vending cups, plastic cutlery, protective packaging for electronic goods and toys.
OTHER
Any other plastics that do not fall into any of the above categories. - An example is melamine, which is often used in plastic plates and cups.
TECHNIQUES USED IN RECYCLING
Mechanical recycling
Hydrogenation
Gasification
Chemical de polymerization
Catalytic cracking and reforming
MECHANICAL RECYCLING
OPTIONAL STEPS:
1. Separation of plastics by resin type.
2. Washing to remove dirt and contaminants.
3. Grinding and crushing to reduce the plastics particle size.
4. Extrusion by heat and reprocessing into new plastic goods.
Ex: Polyvinyl chloride (PVC),Polyethylene terephthalate (PET)
HYDROGENATION
• Hydrogenation of plastics is a potential alternative for breaking down the polymer chain.
• Hydrogenation promotes the removal of hetero atoms, such as Chlorine (Cl), Nitrogen (N) and Sulphur (S) in the form of volatile compounds.
• Mainly due to the cost of hydrogen,
• The process need to operate under high pressure.
GASIFICATION
1. Gasification can be considered to be a partial oxidation process of carbonaceous material leading predominantly to a mixture of carbon monoxide (CO) and hydrogen (H2).
2. Gasification is an efficient treatment for polymeric waste
Further improvements were made to plastics at the turn of the 20th century. Another form of plastic called cellophane was created by Dr Jacques Brandenberger from Switzerland. This material was the first transparent fully-flexible and water-proof plastic wrap. In 1907, Leo Baekeland, a chemist from New York, invented a liquid resin called Bakelite, a thermoset plastic that was capable of retaining its shape under any condition. Bakelite was used in the manufacturing of military weapons and machines as well as electrical insulators.
There are four types of recycling processes that usually occur: primary, secondary, tertiary, and quaternary. The primary recycling process is recycling materials and products that contain similar features of the original product. This process is only feasible with semi-clean industrial scrap plastics, therefore this process is not widely used. Secondary recycling allows for a higher mixture of combination levels in plastics. When the secondary process of recycling is used it creates products such as fenceposts and any products that can be used in the substitution of wood, concrete, and metal. The low mechanical properties of these types of plastics are the reason why the above products are created. Tertiary recycling is occurring more and more today because of the need to adapt to the high levels of waste contamination. The actual process involves producing basic chemicals and fuels from plastic. The last form of recycling is the quarternary process. This quarternary process uses the energy from plastic by burning. This process is the most common and widely used in recycling. The reason this process is widely used is because of the high heat content of most plastics. Most incinerators used in the process can reach temperatures as high as 900 to 1000 degrees Celsius. For the sake of the environment the new techniques being used with the incinerators have decreased the amount of air pollutants being released.
By the 1920s, cellophane became a very popular material around the world. Later on, a young Harvard chemist called Wallace Hume Carothers succeeded in developing nylon, which was known as Fiber 66 at that time. By the 1940s, many other polymers were introduced to the world, and these included acrylic, PVC, neoprene, polyethylene, Teflon, SaranTM, and others. In the following decade, plastic began to be used in numerous products, ranging from packaging to new textiles, and it also paved the way for the invention of innovative products such as televisions and computers. In 2007, the total consumption of plastic had reached close to 100 million tones, and this has caused significant depletion of natural resources such as petroleum and natural gas.
ADVANTAGES:
It is not necessary to separate the different polymer types,
It is possible to mix plastic wastes with other
Non-plastic solid waste before gasification.
The profitability of a gasification process largely depends on the value and applications of the synthesis gas.
It has been further applied also to the processing of heavy petroleum fractions and natural gas.
CHEMICAL DE POLYMERIZATION
Chemical de polymerization is the most established method of plastic feedstock recycling, even though it is restricted to the recycling of condensation polymers and there are no applications of decomposition of other polymers.
Chemical recycling processes, plastic wastes can be remanufactured into valuable chemical feedstock by a large variety of thermal or catalytic processes.
The Ford hydro glycolysis process is a good example of these combined alternatives