PET bottles are one of the most common things available to a consumer today. It is so ubiquitous that rarely does a single day pass by without a consumer having to see at least one product packaged in PET. Since it is so widely available, readily used and easily disposed of, there is a need to discuss what happens to the plastic before and after the consumers handle the product. The events up to the production of bottles from polyethylene terephthalate resin is called the upstream life cycle and the events after the consumption of the product, including disposal come under downstream life cycle. Polyethylene terephthalate is a highly recyclable material, a large amount of PET produced gets recycled into other products.

Get To Know PET

PET is one of the most popularly used plastic materials in the world. Over 80 million metric tonnes are produced every single year. This versatile material is an aliphatic polymer. PET is a thermoplastic polymer used extensively for bottle and fiber manufacture. According to the resin identification code, PET is number 1. It is the most recyclable plastic produced. It was first made in the U.S. in the 1940’s by chemists at DuPont, who were researching polymers textile production. It was called ‘Dacron’ initially.

PET or polyethylene terephthalate is produced by the reaction of terephthalic acid with ethylene glycol. The monomer unit of PET is ethylene terephthalate C₁₀H₈O₄. It is produced by a polycondensation reaction of ethylene terephthalate, which is produced by the esterification reaction of terephthalic acid and ethylene glycol.

What Makes PET A Great Resin?

PET is hard, stiff and resistant to most solvents. It is quite strong, and resins can exist either in semi crystalline or amorphous form. Polyethylene terephthalate is usually transparent in amorphous form, but has lower strength. Semi crystalline PET is harder and not transparent. However, PET used in the manufacture of packaging materials is amorphous.

The chemical properties of polyethylene terephthalate make it an ideal material for food packaging and bottle manufacture. It is resistant to most gasses and solvents. It is strong and has good impact resistance, as well as dimensional and temperature stability. It is not heavy, making it quite easy to carry around. PET is fairly resistant to reactions with food and microorganisms. Another great thing about polyethylene terephthalate, especially with regard to bottles, is the fact that PET is shatter resistant, unlike glass bottles.

The PET Bottle

PET’s high strength to weight ratio, transparency, shatterproof nature and high recyclability make it a great choice for packaging materials. One of the most important uses of polyethylene terephthalate resin is in the manufacture of bottles.

PET bottles are manufactured by the process of blow molding. It can be done in a single step or different step, depending upon the machines that are used for blow molding. One step molding involves the preform being transformed into the bottle in one step within the same machine. Two step molding, on the other hand, requires two machines. One step molding is comparatively more efficient compared to two step molding.

The PET bottle was invented by a man named Nathaniel Wyeth and was later patented by DuPont, a leading manufacturer of plastics. PET can be used to bottle a large variety of drinks, food products and other goods. Mostly, PET bottles contain water or other soft drinks or juices. Other commonly bottled liquids include milk, vinegar and oil. After bottling, the bottles are closed with an anti-tamper ring and sent for labeling.

The Life Of A PET Bottle

THE PRODUCTION

• Crude oil is the raw material from which PET is produced. Crude oil is distilled in refineries and broken down into different components. They can be grouped into three fractions: light, middle and heavy. PET is produced from naphtha, which is part of the light fraction of hydrocarbons.
• Naphtha is used to produce ethylene glycol and terephthalic acid (or dimethyl terephthalate), which are then chemically combined by polycondensation to produce the polymer polyethylene terephthalate in the form of pellets.
• These pellets are then used in the manufacturing process of bottles. They are first melted down in order to form the liquid resin. Then it is used to make a preform, which is a small tube which will be later processed in order to form a plastic bottle.
• The preform undergoes molding processes, either blow molding or injection molding in order to make a plastic bottle. Usually, the preform is heated up and air is blown into the tube to expand the preform in the shape of a bottle.
• Once the bottle is made, it is ejected from the machine and cooled down. It is then used to bottle various liquids including water.
  
  

THE CONSUMPTION

After production, PET bottles are filled with different liquids and sent out into the market. Plastic bottles, especially PET, are much preferred due to the ease of handling. It is strong, lightweight and easy to carry, and can be reused as and when needed. It is also very cheap, and is a good substitute for glass, considering its great physical and chemical properties. After consumption, PET bottles usually end up in the garbage. A large part of it is recycled, however some are eventually sent to landfills, where they essentially do not decompose at all.

THE RECYCLING

PET bottles do not decompose. It is not a biodegradable material. Hence, the next step in its life cycle is recycling. Polyethylene terephthalate is one of the most recyclable materials, which is why it has the resin identification code number 1. If it is not recycled, PET bottles often end up in landfills.

• PET bottles are collected from garbage dumps, curbs or from houses and sent for recycling.
• Materials are sorted and separated from one another. Once enough material has been collected for PET, they are sent for mechanical recycling.
• During mechanical recycling, plastic is shredded into small pieces and heated up to make resin. However, mechanical recycling is not very repeatable as long polymer chains get broken down into smaller chains.