MECHANICAL RECYCLING

Mechanical recycling uses only mechanical methods to reprocess plastic waste and this typically involves grinding, washing, separating, drying, regranulating, and compounding . The recyclates from these processes can be used to replace virgin polymers in the production of new plastic articles. For thermoplastics, after remelting the recyclate can be processed by injection or rotational moulding, extrusion, and heat pressing . These techniques are not applicable to thermoset plastics, which will not remelt.

Mechanical recycling of plastic waste is a mature, well-established industry that operates well below its capacity due to a range of technical, financial, and policy challenges. In the EU, in 2016, 8.4 million tons of plastic were recycled, but 11.3 million tons were burned in incinerators, while around 7.4 million tons were landfilled .

Among these challenges, the following have proven difficult for the industry to overcome. At a technical level, many plastics have become more complex, multi-layered and contain many additives (including toxic additives) that preclude or inhibit the ability of mechanical recyclers to process or sell them. Competition from cheap, virgin-production plastics using petrochemicals is so great that the volume and scale required to collect, clean, and process waste plastics limits the market for plastic recyclate. Waste incineration in some regions and countries (EU, Japan, Scandinavia, and the US) competes for supplies of recyclable plastic.

Policy makers in most countries have not yet moved to mandatory recycled plastic content in products or to government procurement policies in order to drive demand for recycled plastic, support a circular economy, and provide a significant boost to investment in plastic recycling. The over-reliance on a waste export model by many developed countries has left their domestic recycling infrastructure undeveloped, with insufficient investment and little domestic demand. Australia is an example of a country that relied heavily on plastic waste exports to China, while neglecting its domestic recycling infrastructure only to find itself in a crisis when China’s National Sword policy was implemented.49 Many countries have relied on export of low value plastic waste from high-income to low- and middle-income countries that recycle some of the waste while the rest is burned or dumped in the environment contaminating the food chain. Such exports diminish the need for the establishment of domestic mechanical recycling industries for plastic waste in many high-income countries.

KEY TECHNICAL PROCESSES

Collection

For an efficient and profitable mechanical plastic recycling system, source separation and collection systems from the public, commercial, and industrial sources is very important. These should be supplemented with targeted collection schemes from agricultural, automotive, and Waste Electrical and Electronic Equipment (WEEE) sources.50 The main collection methods used in many developed countries are curbside collection, drop-off locations, buy-back, and deposit-refund programs. These focus on plastic packaging and consumer products. Industrial waste plastics (cut offs and scraps) sourced directly from factories can be particularly valuable due to lack of contamination from comingling with organic wastes. The cleaner the incoming plastic waste, the less resource-intensive is the cleaning phase. In low-income countries, the collection, sorting, and cleaning processes are less sophisticated and have high risks for workers, but can also be very efficient .

Sorting and cleaning

Mixed plastics mostly arrive at the mechanical recycling facility contaminated with organic matter and other materials, and must be sorted, separated, and cleaned to facilitate mechanical recycling. The first step is the removal of non-plastic materials such as metal, wood, and paper. Shredding may occur during the nest steps to facilitate separation. The separation of rigid plastics from non-rigid plastics (such as chip packets and foils) is followed by division into colored and clear plastics. Finally, polymer types have to be separated into different fractions. Metals can be removed by magnets and eddy currents. Non-rigid plastics can be separated by blowers and wind sifters. Color separation is conducted by optical color recognition sensors .

Polymer Sorting

Mixed polymers decrease the value of recyclate and contribute to degradation of strength and other properties of the final products or cause difficulties in processing due to different melting temperatures. Polymer sorting is conducted by direct and indirect methods. Direct methods include density separation , but for mono separation, more advanced techniques may be necessary due to density overlap such as flotation or froth flotation , use of centrifuges, or hydrocyclones. Indirect methods involve the use of optical scanners to detect and separate polymer types. The most commonly used technology is the FT-NIR (Fourier Transform Near-Infrared) sensor, but it can falsely detect black plastics and contaminants. The sensor field is developing rapidly, and black plastic sensors and PVC-sensing equipment is now in use . The introduction of bioplastics has also recently raised concern about contamination of the recycling chain and potential incompatibility with fossil fuel-based polymers .

Remelting and extrusion

The sorted, cleaned, and shredded polymers may then be subject to remelting (except thermoset plastics) and extrusion, where they are formed into bulk pellets (nurdles) to be sold to plastic product manufacturers. With some polymers such as PET, solid-state polycondensation (SSP) within a vacuum is applied at specific temperatures (180–240 °C), causing post-consumer contaminants to rise to the surface of the PET and to be removed by the vacuum force .