Plastic recycling is the reprocessing of plastic waste into new and useful products. When performed correctly, this can reduce dependence on landfills, conserve resources and protect the environment from plastic pollution and greenhouse gas emissions. Although recycling rates are increasing, they lag behind those of other recoverable materials, such as aluminium, glass and paper.
Pritish Kumar Halder takes a brief look at the Plastic recycling process in this blog.
The global recycling rate in 2015 was 19.5%, while 25.5% was incinerated and the remaining 55% was disposed of in landfills. From the beginning of plastic production in the 20th century, until 2015, the world has produced some 6.3 billion tonnes of plastic waste, only 9% of which has been recycled, and only ~1% has been recycled more than once.
Recycling is necessary because almost all plastic is non-biodegradable and thus builds up in the environment, where it can cause harm. For example, approximately 8 million tons of waste plastic enter the Earth’s oceans every year, causing damage to the aquatic ecosystem and forming large ocean garbage patches.
The total amount of plastic ever produced worldwide, until 2015, is estimated to be 8.3 billion tonnes. Approximately 6.3 billion tonnes of this has been discarded as waste, of which around 79% has accumulated in landfills or the natural environment, 12% was incinerated, and 9% has been recycled, although only ~1% of all plastic has ever been recycled more than once.
Collecting and sorting
Recycling begins with the collection and sorting of waste. Curbside collection operates in many counties, with the collections being sent to a materials recovery facility or MBT plant where the plastic is separated, cleaned and sorted for sale. Anything not deemed suitable for recycling will then be sent for landfill or incineration. These operations account for a large proportion of the financial and energy costs associated with recycling.
Sorting plastic is more complicated than any other recyclable material because it comes in a greater range of forms. Glass is separated into three streams (clear, green and amber) metals are usually either steel or aluminum and can be separated using magnets or eddy current separators, paper is usually sorted into a single stream. By comparison about six types of commodity polymer account for about 75% of plastics waste, with the remaining 25% consisting of a myriad of polymer types, including polyurethanes and synthetic fibers which can have a range of chemical structures.
Sorting through waste by hand is the oldest and simplest method of separating plastic. In developing countries this may be done by waste pickers, while in a recycling center workers pick items off a conveyor-belt. It requires low levels of technology and investment,
Plastics can be separated by exploiting differences in their densities. In this approach the plastic is first ground into flakes of a similar size, washed and subjected to gravity separation. This can be achieved using either an air classifier or hydro cyclone, or via wet float-sink method.
In electrostatic separators, the triboelectric effect is used to charge plastic particles electrically; with different polymers being charged to different extents. They are then blown through an applied electric field, which deflects them depending on their charge, directing them into appropriate collectors. As with density separation, the particles need to be dry, have a close size distribution and be uniform in shape. Electrostatic separation can be complementary to density separation, allowing full separation of polymers, however, these will still be of mixed colours.
Sensor based separation
This approach can be highly automated and involves various types of sensors linked to a computer, which analyses items and directs them into appropriate chutes or belts. Near-infrared spectroscopy can be used to distinguish between polymer types, although it can struggle with black or strongly coloured plastics, as well as composite materials like plastic-coated paper and multilayered packaging, which can give misleading readings.
Optical sorting such as colour sorters or hyperspectral imaging can then further organise the plastics by colour. Sensor based separation is more expensive to install but has the best recovery rates and produces more high-quality products.
Plastic waste can be broadly divided into two categories; industrial scrap and post-consumer waste. Scrap is generated during the production of plastic items and is usually handled completely differently to post-consumer waste. It can include flashings, trimmings, sprues and rejects. As it is collected at the point of manufacture it will be clean, and of a known type and grade of material, and is usually of high quality and value. As scrap is mostly traded company-to-company rather than via municipal facilities, it is often not included in official statistics.
Plastics are reprocessed at anywhere between 150–320 °C (300–610 °F), depending on the polymer type, and this is sufficient to cause unwanted chemical reactions which result in polymer degradation. This reduces the physical properties and overall quality of the plastic and can produce volatile, low-molecular weight compounds, which may impart undesirable taste or odor, as well as causing thermal discoloration. Additives present within the plastic can accelerate this degradation.
For instance, oxo-biodegradable additives, intended to improve the biodegradability of plastic, also increase the degree of thermal degradation. Similarly, flame retardants can have unwanted effects. The quality of the product also depends strongly on how well the plastic was sorted. Many polymers are immiscible with one another when molten and will phase separate (like oil and water) during reprocessing. Products made from such blends contain many boundaries between the different polymer types and cohesion across these boundaries is weak, leading to poor mechanical properties.
Although thermoset polymers do not melt, technologies have been developed for their mechanical recycling. This usually involves breaking the material down to a crumb, which can then be mixed with some sort of binding agent to form a new composite material. For instance, polyurethanes can be recycled as reconstituted crumb foam. Tire recycling similarly produces crumb rubber, which can be used as aggregate. Various devulcanization technologies have also been developed to allow the recycling of rubber wastes, though few of these are commercially important.