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Polyethylene Terephthalate - PET

Polyethylene Terephthalate (PET) is also abbreviated as PETE. This is the short form for polyethylene terephthalate. PET is synthesized by poly-esterification of ethylene glycol and terephthalic acid.

Polyethylene Terephthalate (PET) structure
Figure 01: Polyethylene Terephthalate (PET) structure

PET is commercially manufactured by batch process of trans-esterification reaction. There are 2 stages involved in this process. Those are esterification and polycondensation.

Esterification is a catalytical process. Catalyst is Zinc acetate, Mg, Ca, Co, Pb acetates. End products of the esterification process are Methanol and di (βhydroxy ethyl terephthalate).

The second step is the Polycondensation of di (βhydroxy ethyl terephthalate) under vacuum and heating. In such process result PET.

Polyethylene Terephthalate PET Manufacturing process
Figure 02: Polyethylene Terephthalate (PET) Manufacturing process

Properties of Polyethylene Terephthalate (PET)

PET is different from other engineering plastics. Typically, the properties of PET in which they differentiate themselves from other engineering plastics are their low water absorption in comparison to Nylon. Due to the low water absorption PET has high dimensional stability. PET has excellent electrical properties.

PET is hard solid of light cream color or transparent. It can be crystalline or amorphous. Molecular weight 20000-40000 g/mol.

Physical properties

PET has very low creep for self-reinforced (crystalline) morphology even at elevated temperatures. PET is an excellent weathering-resistant material and it has very good color stability. And also, PET is a very good heat and heat aging resistance material.

Tensile Strength (crystalline)2.5 N/mm²
Notched Impact Strength1.5 -3.5 KJ/m²
Maximum working temperature80 ℃
Density1.37 g/cm3

PET shows excellent resistance to chemical attacks and high environmental stress crack resistance. It shows very good resistance to dilute acids, aliphatic hydrocarbons, oils, grease, and alcohols. And PET shows moderate resistance to dilute alkalis, aromatic hydrocarbons, and halogenated hydrocarbons.

Disadvantages of PET

The main degradations way that Polyethylene Terephthalate can occur is hydrolytic and probably most important than thermal oxidation. When PET degrades, several things happen. Such as discoloration, chain sessions that result in reduced molecular weight, formation of acetaldehyde, and cross-links ("gel" or "fish-eye" formation), etc. Discoloration of the material is happening because of the formation of several chromophoric systems following prolonged thermal treatment at higher temperatures. Another problem arises when the optical requirements of the polymer are much higher. Such as applications like in packaging applications, This becomes a problem.

When PET is subjected to hydrolysis it forms Carboxylic acid and alcohol. 0.06% of water content is sufficient to reduce the molecular weight of PET by two times. Therefore, it is necessary to remove the moisture before processing. PET is dried at 70℃ for 4 hours prior to extrusion and injection molding.

PET processing and applications

Stretch blow molding and extrusion (fiber extrusion and sheet extrusion) are the most common PET processing techniques. Fibers, PET sheets, PET bottles, and tapes are the main applications of PET.

main applications of PET
Figure 03: main applications of PET

Polyethylene Terephthalate (PET) shows good barrier properties against O2 and CO2. Therefore, it is utilized in bottles for highly carbonated drinks and mineral water.

PET fibers are manufactured by the Fiber extrusion process. The main raw material for fiber production is granules of PET. A single screw extruder is used for PET processing. One extruder may have 20-100 of extrusion dies. The number of holes in the extrusion dies for spinning is distributed according to the fineness of the threads for 100-2000. The hole diameter of extrusion dies IS 0.2-0.6 mm.

Polyethylene Terephthalate PET fiber Manufacturing process
Figure 04: Polyethylene Terephthalate (PET) fiber Manufacturing process

Polyester is a versatile textile fiber, as it comes in a range of colors and can be woven into patterns. It is a cheap alternative to natural fibers such as wool.

PET is used for take-out food containers and prepared food trays. These trays can be warmed in the oven or microwaved. The basic chemical formula of these "dual oven-able" trays and containers are the same as PET bottles and jars. But they have special additives that crystallize and toughen the PET. So, these "dual oven-able" trays and containers have the ability to withstand the much higher temperatures of oven and microwave oven heating.

PET take out food containers
Figure 05: PET take-out food containers

Heat resistance, adhesion PET tapes are used for powder coating masking, High-temperature paintings, and galvanization. Other applications of PET tapes include audio/videotapes as well as mechanical components.

applications of PET
Figure 06: applications of PET

Polyethylene Terephthalate waste

Plastic bottles create environmental problems. More than 7.5 million tons of PET waste are dumped into the environment annually.

Ideonella sakaiensis is a bacterium from the genus Ideonella and family Comamonadaceae which is used to breaking down PET plastic. It was observed that this bacterium uses two different enzymes that reacted with water to break down PET plastics into terephthalic acid and ethylene glycol. These substances which on their own pose no threat to the environment.

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Cover Image was designed by using an image by Hebi B. from Pixabay

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