INTRO: An extremely brief history of PET
Even if you know nothing about polymers or chemistry is just a forgotten nightmare from your high school days, you have probably heard about PET, and know that it’s the plastic that most bottles are made of.
It’s quite likely you have also heard that it’s an environmental problem, and that it end up polluting the oceans.
I’m going to skip most references to polymer chemistry. Suffice to say that PET is a polyester, which technically means that it contains several ester groups.
And yes, polyester fabric is also made from polyethylene terephthalate, or PET. PET is the most common of all the polyesters. Another- although lesser-known – polyester example is PLA, or polylactic acid, a bioplastic which already mentioned in this post.
PET synthesis
However, I said most, not all references to polymer chemistry, since some organic compounds will be mentioned lated when commenting on chemical recycling of PET.
In either reaction, an intermediate monomer bis(2-hydroxyethylterephthalate) (BHET) is formed. All these chemical substances will come up later, since chemical recycling is in a way reversing the polymerization reaction.
PET was developed by British chemists named Rex Whinfield and James Dickson in1941 while working at a small company called Calico Printer’s Association. After a series of agreements, DuPont that had already developed an improvement in its pipeline, acquired the manufacturing rights for the USA in 1946, In 1947 ICI (now part of AkzoNobel) did the sane for the rest of the world. PET was used to manufacture synthetic fibres such as Dacron or Terylene.
PET films were developed in the late 1950s (such as DuPont’s Mylar in 1952, or Melinex). Later, PET was modified for the manufacture of injection parts (generally reinforced with fiberglass).
In 1960, Amoco introduced the production of terephthalic acid (PTA) which facilitates the production of PET. In 1965 the Hans J. Zimmer ( from the Vickers-Zimmer company) was already producing three tons per day in continuous process, and in the early 70s the first pieces were manufactured oriented three-dimensional parts were manufactured.
The first to discover how to make PET bottles for carbonated soft drinks was Nathaniel Wyeth who patented the process in 1973.
PLASTIC WASTE MANAGEMENT
PET as most plastic present two huge environmental problems:
Plastic environmental issues are basically twofold:
- Their end-of-life problems
- Their non-renewable origin
For more info on this specially regarding the second point, you can go to this post.
Let’s have a look at how much plastic is recycled.
So, it seems that some Asian countries like India and South Korea lead the way. EU countries average is around 30% (more details later), and in the US only about 9% of the plastic is recycled.
More info about US plastic recycling rates:
The previous graphics show the recycling rates of all plastics. I have searched for more specific info about plastic packaging waste:
So we see that in Europe approximately 40% of the plastic produced is used in packaging. Let’s see the evolution of the plastic recycling rates in the EU:
Recycling facilities usually separate PET and high-density polyethylene (HDPE) containers like milk jugs. They are relatively clean and homogeneous materials, and recyclers handle enough of them to make extraction worthwhile.
If we look at the recycling rates of PET bottles by country:
Source: https://www.statista.com/statistics/1166550/plastic-bottle-recycling-rates-in-select-countries/
We find Norway is a spectacular exception in Europe, recycling 97% of its plastic bottles. 92 percent of the bottles recycled yield such high quality material, it can be used again in drink bottles. In some cases, the system has already reused the same material over50 times. More about this later.
PET RECYCLING OVERVIEW
Different types of PET Recycling.
- Primary recycling: Primary recycling, or re-extrusion, is just the ‘‘in-plant’’ recycling of the scrap materials that have similar features to the original products. It’s simple and cheap, but requires uncontaminated scrap,
- Secondary recycling: Secondary recycling, also known as mechanical recycling,
- Tertiary recycling: Tertiary recycling, more commonly known as chemical recycling,
- Quaternary recycling: It’s the recovery of energy content from the plastic waste by incineration. When the collection, sorting and separation of plastics waste are difficult or economically not viable, or the waste is toxic and hazardous to handle, the best waste management option is incineration to recover the chemical energy stored in plastics waste as thermal energy. However, it is thought to be ecologically unacceptable because of potential health risks from the air born toxic substances.
On this last point, I would like to comment on the use of pyrolysis for plastic waste. On this post I mentioned some initiatives to obtain oil from plastics.
Brightmark Energy has one of the largest such projects. It has started construction on a $260 million plant in the US to convert 100,000 tons per year of plastic waste into 68 million litres of diesel and naphtha.
Ashley Plastics Renewal from Brightmark on Vimeo.
MECHANICAL RECYCLING
And here’s a video that shows some of thesesteps (it omits the steps needed to create the pellets):
NAPCOR – PET Recovery from NAPCOR on Vimeo.
PET Mechanical Recycling Issues (and why not everybody can be like Norway)
The major issues for mechanical recycling are:
- PET waste is heterogeneous and highly contaminated, which makes mechanical recycling difficult. Also, small amounts of another in the PET matrix may significantly change the properties of PET, disturbing its use in conventional applications.
Back to Norway: The Norwegian system relies on two key incentives. First, the more companies recycle, the less tax they have to pay. If they reach a collective nationwide target of more than 95 per cent, then they don’t pay any tax at all. Second, customers must pay a deposit for each bottle, about 0.03 €. There are hundreds of thousands of ‘reverse vending machines’ where bottles can be returned you begin to understand Norway’s success on this front.
Today in the US, there are only 10 states that have Container Deposit Legislation (CDL), which have resulted in recycling rates of between 65% and 95% percent, according to Plastics News. Curbside systems are less efficient with recycling rates of just 20%.
- PET degrades each time it is recycled. Because of various factors such as temperature, ultraviolet radiation, oxygen, ozone, and mechanical stresses, leading to altered properties and reduced performance of recycled PET compared with virgin PET.
- Color: Recycled PET is an undesirable grey color, resulting from the presence of PET waste made from the same resin. Some PET bottle resins tend to yellow more than others when recycled, especially those containing multiple layers.
CHEMICAL RECYCLING
Chemical recycling involves the transformation of the PET polymer chain (either back to its monomers or a partial depolymerization to its oligomers and other industrial chemicals).
The main chemical-recycling processes for PET are: hydrolysis, glycolysis, methanolysis, and other processes. The resulting chemical substances for each are shown below, and have been mentioned in the introduction (DMT, EG… etc.).
Chemical recycling examples
- Thermoforming scraps and multi-layer trays (PET / PE / EVOH / PE);
- Scraps and films coupled with Aluminum (PET / PE / Alu / PE)
- Opaque PET bottles (containing fillers such as TiO2, CaCO3, Silica)
- Colored PET powders and fines
- Black PET trays
- PET / PP strapping
- Non-woven fabrics, TNT
- Polyester / Cotton blends
In 2019, Plastipak and Garbo announced an exclusive partnership for recycling PET on an industrial scale.
CHEMICAL OR MECHANICAL
Chemical recycling is considered to be the most sustainable option, since it leads to the formation of the raw materials (monomers) from which the polymer is originally made. In this way the environment is not surcharged and there is no need for extra resources for the production of PET.
In the past has not been cost effective to use this method because of the lower price of petrochemical feedstocks compared with the process prices for production of PET monomers or oligomers from PET waste.
Detractors list chemical recycling’s carbon and energy intensity compared to mechanical recycling, its economic viability, and low-yield as arguments against it.
Supporters for chemical recycling argue it has a lower environmental impact than virgin and works in tandem with mechanical recycling to treat waste volumes that would otherwise go to landfill or incineration.
Regarding the low yields and high cost, they attribute these to the current life cycle stage of the processes, and claim they will be solved thanks to the economies of scale and future technical breakthroughs.
PET RECYCLING CONCLUSIONS (my two cents…)
The ideal sitution would be for chemical and mechanical recycling to coexist.
Under my point of view the more important issues to be improved in order to facilitate plastics recycling:
- Cost-effective technologies recycling (specially for sorting).
- Design for recycling: fewer materials (ideally monomaterial packaging) and fewer parts, design for disassembly.
- Markets for recycled plastics: Assure quality, price and supply consistency.
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Improve recycling infrastructures to make them more effective
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Educate consumers on recycling and encourage recycling and purchasing goods containing recycled material.
April 2021, Bruno Rey – The Packaging Blog –
