Since municipal sewage systems often spill into rivers and oceans, debris and other debris is observed. PET has a density of about 1.3 and therefore decreases in freshwater and seawater. PET is known to break down when exposed to sunlight and oxygen.  Microplastics located at the bottom of the river or seabed can be absorbed by small marine life and thus enter the food chain. Since PET has a higher density than water, a significant proportion of microplastics can be precipitated into wastewater treatment plants. Very light PET fibers can enter the air, directly through dryers or wind. Fibres can travel long distances and migrate to fields where they are ingested by farm animals or delivered to the human food supply on by-products. To date, there is virtually no information on the lifespan of synthetic polymers in the environment.  Polyethylene terephthalate (PET) is a condensation polymer produced by esterification of ethylene glycol (EG) with terephthalic acid (TPA) or dimethyl terephthalate (DMT) and has the general formula (OOCC6H5COOCH2CH2CH2)n. Unlike the carbon-carbon compounds in the previously discussed polymers, polyesters are based on carbon-oxygen-carbon compounds, in which one of the carbons is part of a carbonyl group.
The outstanding properties of PET films as a food packaging material are their high tensile strength, excellent chemical resistance, low weight, elasticity and stability over a wide temperature range (−60° to 220°C). PET films are most often used in the biaxially heat-stabilized form. Due to the recyclability of PET and the relative abundance of post-consumer waste in the form of bottles, PET as a carpet fiber is rapidly gaining market share. Mohawk Industries launched everSTRAND in 1999, a PET fiber that is 100% recycled after the consumer. Since then, more than 17 billion bottles have been recycled into carpet fibers.  Pharr Yarns, a supplier to many carpet manufacturers, including Looptex, Dobbs Mills and Berkshire Flooring, produces BCF PET (Bulk Continuous Filament) carpet fibre containing at least 25% post-consumer recycled content. DuPont claims that PTT has similar performance to traditional PBT plastics (see Table 10.30), which offers mold properties, strength, stiffness, dimensional stability, and PBT-like finishing qualities. In addition to this external (post-consumer) recycling of polyester bottles, there are a number of internal (pre-consumer) recycling processes in which the wasted polymer material does not leave the production facility for the free market and is instead reused in the same production cycle.
In this way, fiber waste is reused directly for fiber production, preform waste is reused directly for preform production, and film waste is reused directly for film production. This process involves converting bottle waste into flakes, drying and crystallizing the flakes, plasticizing and filtering, and granulating the flakes. The product is an amorphous regranulat with an intrinsic viscosity between 0.55 and 0.7 dl/g, depending on how the PET flakes were completely presealed. About 60% crystallization is the upper limit for commercial products, with the exception of polyester fibers. Transparent products can be produced by rapid cooling of the molten polymer below the glass transition temperature Tg to form an amorphous solid.  Like glass, amorphous PET is formed when its molecules do not have enough time to organize themselves in an orderly and crystalline manner while the melting is cooled. At room temperature, the molecules are frozen on the spot, but when enough thermal energy is returned to them by heating on Tg, they start moving again so that the crystals can germinate and grow. This process is called solid-state crystallization. Virent Cy has successfully produced paraxylene called BioFormPX from 100% renewable vegetable sugar. In combination with existing PET technology with Bio-MEG, companies can offer their customers 100% natural, renewable and plant-based PET. BP partnered with Virent and Johnson Matthey (in April 2019) to commercialize this process. PET can be mixed with other thermoplastics and thermosets to improve its performance and use it for various purposes.
The purpose of the production of its mixtures is to improve the economy and its properties such as mechanical strength, flame retardant, transformability, etc. It can also be referred to by the brand names Terylene in the United Kingdom, Lavsan in Russia and the former Soviet Union, and Dacron in the United States. .