Steps of Recycling PET Bottle
1. Purification and decontamination
The success of any recycling concept is hidden in the efficiency of purification and decontamination at the right place during processing and to the necessary or desired extent.
In general, the following applies: The earlier in the process foreign substances are removed, and the more thoroughly this is done, the more efficient the process is.
The high plasticization temperature of PET in the range of 280 °C (536 °F) is the reason why almost all common organic impurities such as PVC, PLA, polyolefin, chemical wood-pulp and paper fibers, polyvinyl acetate, melt adhesive, coloring agents, sugar, and protein residues are transformed into colored degradation products that, in their turn, might release in addition reactive degradation products. Then, the number of defects in the polymer chain increases considerably. The particle size distribution of impurities is very wide, the big particles of 60–1000 µm—which are visible by naked eye and easy to filter—representing the lesser evil, since their total surface is relatively small and the degradation speed is therefore lower. The influence of the microscopic particles, which—because they are many—increase the frequency of defects in the polymer, is relatively greater.
The motto "What the eye does not see the heart cannot grieve over" is considered to be very important in many recycling processes. Therefore, besides efficient sorting, the removal of visible impurity particles by melt filtration processes plays a particular part in this case.
Workers sort an incoming stream of various plastics, mixed with some pieces of un-recyclable litter.
2. Bales of crushed blue PET bottles
Bales of crushed PET bottles sorted according to color: green, transparent, and blue.
In general, one can say that the processes to make PET bottle flakes from collected bottles are as versatile as the different waste streams are different in their composition and quality. In view of technology there isn't just one way to do it. Meanwhile, there are many engineering companies that are offering flake production plants and components, and it is difficult to decide for one or other plant design. Nevertheless, there are processes that are sharing most of these principles. Depending on composition and impurity level of input material, the general following process steps are applied.
Bale opening, briquette opening
Sorting and selection for different colors, foreign polymers especially PVC, foreign matter, removal of film, paper, glass, sand, soil, stones, and metals
Pre-washing without cutting
Coarse cutting dry or combined to pre-washing
Removal of stones, glass, and metal
Air sifting to remove film, paper, and labels
Grinding, dry and / or wet
Removal of low-density polymers (cups) by density differences
Caustic wash, and surface etching, maintaining intrinsic viscosity and decontamination
Clean water rinsing
Air-sifting of flakes
Automatic flake sorting
Water circuit and water treatment technology
Flake quality control
3. Impurities and material defects
The number of possible impurities and material defects that accumulate in the polymeric material is increasing permanently—when processing as well as when using polymers—taking into account a growing service lifetime, growing final applications and repeated recycling. As far as recycled PET bottles are concerned, the defects mentioned can be sorted in the following groups:
Reactive polyester OH- or COOH- end groups are transformed into dead or non-reactive end groups, e.g. formation of vinyl ester end groups through dehydration or decarboxylation of terephthalate acid, reaction of the OH- or COOH- end groups with mono-functional degradation products like mono-carbonic acids or alcohols. Results are decreased reactivity during re-polycondensation or re-SSP and broadening the molecular weight distribution.
The end group proportion shifts toward the direction of the COOH end groups built up through a thermal and oxidative degradation. The results are decrease in reactivity, and increase in the acid autocatalytic decomposition during thermal treatment in presence of humidity.
Number of polyfunctional macromolecules increases. Accumulation of gels and long-chain branching defects.
Number, concentration, and variety of nonpolymer-identical organic and inorganic foreign substances are increasing. With every new thermal stress, the organic foreign substances will react by decomposition. This is causing the liberation of further degradation-supporting substances and coloring substances.
Hydroxide and peroxide groups build up at the surface of the products made of polyester in presence of air (oxygen) and humidity. This process is accelerated by ultraviolet light. During an ulterior treatment process, hydro peroxides are a source of oxygen radicals, which are source of oxidative degradation. Destruction of hydro peroxides is to happen before the first thermal treatment or during plasticization and can be supported by suitable additives like antioxidants.
Taking into consideration the above-mentioned chemical defects and impurities, there is an ongoing modification of the following polymer characteristics during each recycling cycle, which are detectable by chemical and physical laboratory analysis.
Increase of COOH end-groups
Increase of color number b
Increase of haze (transparent products)
Increase of oligomer content
Reduction in filterability
Increase of by-products content such as acetaldehyde, formaldehyde
Increase of extractable foreign contaminants
Decrease in color L
Decrease of intrinsic viscosity or dynamic viscosity
Decrease of crystallization temperature and increase of crystallization speed
Decrease of the mechanical properties like tensile strength, elongation at break or elastic modulus
Broadening of molecular weight distribution
The recycling of PET bottles is meanwhile an industrial standard process that is offered by a wide variety of engineering companies.