PET (plastic with the number 1 in the triangle) recycling - the science

 Triangle Number 1 (PET)

PET (PolyEthylene Terephthalate) is represented with the number 1 logo, and is considered perfectly safe to drink from. PET is a thermoplastic polymer resin and doesn’t actually contain Polyethylene.  It ha a high melting point of 245°c, which leads to it being formed and recycled in alternative ways.

Uses of PET Plastic

• Non-food bottles
• Fibres for clothing
• Sleeping bags
• Transparent carbonated drinks bottles

How is PET recycled? 

1: Granules

Having been sorted, reduced in a plastic grinder, washed and dried, the PET flakes are sent to two homogenisation silos, where they are continually agitated as material is added. At the end of the line is an aerodynamic separator that separates materials according to their size/weight. At the exit from the separator, the final refining stage, the flakes are sorted by infra-red cameras and undesirable items are removed by blowing. The selected material is then sent to four quality-control silos where a representative sample is taken of the average quality in the silo. Depending on the result, the flakes will be stored in big-bags or in an intermediate silo to be fed into the extruder.
 

 
But before they are actually extruded, plastics to be recycled are first preheated in a dedicated machine, then taken by a second mechanical feed screw to two hoppers for hot-air-drying. Then the plastic is deposited at the entrance to the extruder machine's screw, which, by turning against the walls of a duct heated by electrical elements, softens it considerably. The PET dough continues its path along the screw, where any impurities are released (all of which are converted into gases) by pump systems attached to the extruder.
 

 
Then come the phases where the material is extruded and cut into plastic rods (or monofilaments) and water-cooled before being filtered and granulated using pre-calibrated grids to produce the size of product required. Once conditioned, it will be stored in an intermediate silo for quality control, carried out by laboratories that, using special small extrusion machines, are able to simulate on a small scale plastics manufacturing methods similar to those that will be used by the end-user of the recycled plastic materials. In this way, the goods are checked to ensure conformity (colour, grade, density, tensile modulus, degree of pollution, impact-resistance, etc.).
 

 
After they have spent a certain time in the reactor, the granules are evacuated and again sent to intermediate silos. Once the silos are full, the material is stored in big-bags. 

2: Powder

The micronisation of plastics consists in pulverising the flakes until an extremely fine powder is obtained. Of a quality lower than that found in regenerated products (since impurities have not been removed and the plastics have simply reduced to a minimum size) but still useful and costing less to produce, it is still greatly prized by the plastics industry and often mixed with top-quality materials to manufacture multi-layer products by plastic rotary moulding (a combination of pure material for the upper layers and micronised binders for the intermediate parts).

 

PVC and PET that are most often treated in this way. Mixtures of the ground material to be micronised are first loaded into silos to be homogenised before being conveyed pneumatically to micronisers. These two-plate machines (one fixed and the other moving) blow the flakes onto the walls by centrifugal force before a rotor smashes the edges off the material. Then, during the final powder micronisation stage, it is sieved down to the desired particle size (often less than 300 microns in diameter) and stored in tanks.

 

I will try to find out which system is used here on Menorca.

My brain hurts now after all this tecnical stuff!!

The blog song for today is "Heartbreaker" by Led Zep, get out your air guitar!!

TTFN