The ubiquitous plastic shopping bag was first designed as a reusable carrier. Its inventor, Sten Gustaf Thulin, will have been appalled to see his creation turn into a single-use object. But this disposable tendency is common to all plastic materials, from screw taps to milk carton lining. With plastic recycling equipment, however, plastic waste can be reprocessed and given a second lease of life.
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Treat polyolefins with heat to depolymerize the plastic material
Plastics are synthetic organic polymers than be broken into the basic monomers and reconstructed into new material. The most widespread type of plastics, polyolefins, can be chemically depolymerized at high temperatures.
Heat the polymers to 380-480°C in a fluid bed heater until the chains start cracking and turn into gas. The monomer vapor is collected in a condenser and returned to liquid. Use catalysts like kaolin, quartz, or zeolite to improve heat transfer and decrease the temperature required for cracking.
Mix additives in the recycling system to produce plastic pellets
The clean monomers are ready to be refabricated into new plastic material. Melt the material through an extruder to produce plastic pellets.
Depending on the intended use of the end product, the raw material feed can be compounded with other additives such as fire-retardants, impact-absorbers, or colorings. Pellets from recycled plastic are a suitable base to manufacture masterbatches for the industry.
Use a twin-screw extruder for tighter control over the pelletizing process
Polymer modelling of plastics follows a similar sequence whether it is manufactured through a single-screw or twin-screw extruder. However, a co-rotating twin screw extruder allows greater control over the plasticating process.
As the feed material travels along the screw, friction forces and heat transfer from the barrel begin melting it. This process accumulates in a melt pool nearer to the inner barrel, where the polymer is subjected to increasing pressure and flows out through the die.
Separate the feed with specialized plastic waste processing equipment
Plastic types, for example HDPE, PVC and PET, all behave in different ways when subjected to mechanical and chemical processing. Separate the primary material into the right streams to prevent contamination down the plastic recycling production line.
A sink-float separator classifies the pieces by density. High-density plastic polymers sink when conveyed to a water-filled tank, whereas low-density pieces float. An alternative method is an optic scanner that sorts plastic pieces according to their capacity to absorb light.
Grind the plastic to improve the recycling process
Resize the feed material into uniform dimensions to prepare it for processing. Grind the plastic into flakes or filaments by passing the pieces through a shredder.
Besides making the stream more workable, reducing the size of the stream makes it easier to pick out impurities during the washing line stage. Contaminants such as metals or adhesive chemicals could compromise the quality of the final product. But resizing the feed material facilitates the segregation of valuable plastic from undesired particles.