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Polyethylene Terephthalate (PET): A Comprehensive Review

Polyethylene Terephthalate (PET): A Comprehensive Review

PET is among those plastics which are an important part of your everyday life. It is an important commercial polymer having applications ranging from packaging, fabrics, films to molded parts for automotive, electronics and many more. You can find this famous clear plastic around you as water bottle or soda bottle container. Explore more about polyethylene terephthalate (PET) and find out what makes it a suitable choice in several applications. Learn about its key properties, how its blends are made with other thermoplastics and thermosets, processing conditions and ofcourse, benefits that make PET as No. 1 recyclable polymer worldwide.

Overview

What is PET plastic?

What is PET plastic?

Polyethylene terephthalate (PET or PETE) is a general-purpose thermoplastic polymer which belongs to the polyester family of polymers. Polyester resins are known for their excellent combination of properties such as mechanical, thermal, chemical resistance as well as dimensional stability.

PET is one of the most recycled thermoplastic and has the number "1" as its recycling symbol.

Molecular Structure of PET
Molecular Structure of Polyethylene Terephthalate
PET Chemical Formula: (C10H8O4)n

Recycled PET can be converted to fibers, fabrics, sheets for packaging and manufacturing automotive parts. Chemically, Polyethylene terephthalate is very much similar to Polybutylene Terephthalate.

PET is highly flexible, colorless and semi-crystalline resin in its natural state. Depending upon how it is processed, it can be semi-rigid to rigid. It shows good dimensional stability, resistance to impact, moisture, alcohols and solvents.

Commercially available PET grades include un-reinforced to glass reinforced, flame retardant and high flow materials for various engineering applications that typically require higher strength and or higher heat resistance. Addition of fillers like glass fibers, CNTs etc. help improve impact strength, surface finish, reduce warpage and several other benefits.

How PET is made?

How PET is made?

PET is an aliphatic polyester. It is obtained from polycondensation reaction of the monomers obtained either by:

  • Esterification reaction between terephthalic acid and ethylene glycol, OR
  • Trans-esterification reaction between ethylene glycol and dimethyl terephthalate

Dimethyl Trephthalate
Plus
Ethylene Glycol
Polymerization of PET
PET Structure

The reaction produces PET in the form of a molten, viscous mass which can be easily spun directly to fibers or extruded or molded into almost any shape.

PET Copolymers


Polyethylene terephthalate is available as a homopolymer and it can also be modified to produce copolymers (known as PETG or PET-G - polyethylene terephthalate glycol-modified) making it more desirable for a particular application.

The common modifiers which replace ethylene glycol or terephthalic acid to produce PETG are cyclohexane dimethanol (CHDM) and isophthalic acid respectively. There modifiers interfere with crystallization and lowers the polymer's melting temperature.

PET Bottles


Advantages & Key Properties of PET Resin

Advantages & Key Properties of PET Resin

The key advantages and features of polyethylene terephthalate are listed below.
    https://omnexus.specialchem.com/selection-guide/3d-printing-and-additive-manufacturing-polymers-and-processes
  • It has higher strength and stiffness than PBT
  • It is very strong and lightweight & hence easy and efficient to transport
  • It is known for its good gas (oxygen, carbon dioxide) and moisture barrier properties
  • It exhibits excellent electrical insulating properties
  • PET has broad range of use temperature, from -60 to 130°C
  • As compared to PBT, it also has higher heat distortion temperature (HDT)
  • It has low gas permeability, in particularly with carbon dioxide
  • PET is suitable for transparent applications, when quenching during processing
  • PET doesn’t not break or fracture. It is practically shatter-resistant and hence, a suitable glass-replacement in some applications
  • It is recyclable and transparent to microwave radiation
  • PET is approved as safe for contact with foods and beverages by the FDA, Health Canada, EFSA & other health agencies

PET Chemical Properties


  • Excellent resistance to alcohols, aliphatic hydrocarbons, oils, grease and diluted acids
  • Moderate resistance to diluted alkalis, aromatic & halogenated hydrocarbons

Limitations of Polyethylene Terephthalate


  • Lower impact strength than PBT 
  • Lower moldability than PBT, due its slow crystallization rate 
  • Affected by boiling water 
  • Attacked by alkalis and strong bases 
  • Attacked at high temperatures (>60°C) by ketones, aromatic and chlorinated hydrocarbons and diluted acids and bases 
  • Poor burning behavior

Polyethylene Terephthalate Blends with Thermoplastics and Thermosets

Polyethylene Terephthalate Blends with Thermoplastics and Thermosets

Blending of PET with other thermoplastics or thermosets is done to tailor new materials having improved performance with beneficial cost profiles to meet specific application demands. Blending also opens up new markets and applications potential without much investment and development.

The thermoplastic polymers that are used to produce blends with PET are polyethylene, polypropylene, polycarbonates, polystyrene, ethyl vinyl acetate and Acrylonitrile Butadiene Styrene. And epoxies, polyester resins, phenolic resins, elastomers such as nitrile butadiene rubber, styrene butadiene rubber etc. are among thermosets which are used to produce in PET blends.

  • PET modified with polyolefins are often glass fiber reinforced and used in injection molded automotive and industrial applications
  • PET/PC blends applications include those requiring a combination of excellent toughness, chemical and heat resistance long with high impact, tensile and flexural strength
  • Blending thermosets with PET significantly improves thermal, mechanical, impact resistance and flame retardant properties. These blends are mainly used for the production of automotive, aeronautic and electronic components

Polyethylene Terephthalate Blends with Thermoplastics and Thermosets


Processing Conditions for PET Resin

Processing Conditions for PET Resin

PET can be easily processed by injection molding, extrusion, blow molding and thermoforming. PET is generally extruded to produce films and sheets (can be thermoformed after) and blow molding is generally used to produce transparent bottles.

It is highly recommended to dry Polyethylene terephthalate for 2-4 hours at 120°C before processing. Up to 25% regrind can be used.

Blow Molding


  • Blow molding is generally used to produce transparent bottles
  • Mold temperature should lie between 10 and 50°C

PET Bottle Preform for Blow Molding
PET Bottle Preform for Blow Molding

Injection Molding


  • Melt temperature: 280-310°C
  • Mold temperature: 140-160°C to obtain a crystalline PET (for technical applications)
  • For transparent applications, mold temperature should lie between 10 and 50°C
  • Screw with an L/D ratio of 18-22 is recommended

Extrusion


  • PET is generally extruded to produce films and sheets (can be thermoformed after)
  • Extrusion temperature: 270-290°C

3D Printing


PET is an optimum polymer to produce 3D Printed objects having high flexibility and toughness. Certain modified PET compounds have been developed (such as PETG) for 3D Printing.

As discussed above, PETG is PET copolyester with glycol modification. PETG filament is more heat-resistant and tough than PLA, but easier to print than ABS. It offers higher strength, lower shrinkage, and a smoother finish.

  • Recommended hot end temperature: 240 and 260°C 
  • Bed temperature: 100°C
  • Retraction speed slow at 30mm/s or less

Toxicity and Recycling of PET

Toxicity and Recycling of PET

Polyethylene Terephthalate or PET products are 100% recyclable and is the most recycled plastic worldwide. PET can be easily identified by its recycling code #1.
PET Recycling Code

Low diffusion coefficient makes PET much more suitable than other plastic materials for use as a recovered, recycled material.

Post-consumer PET bottles are collected and processed through a series of special washing processes or by a chemical treatment to break down the PET into its raw materials or intermediates which are further used to produce recycled PET (rPET) flakes.

PET Recycling Facility
PET Recycling Facility


Then these recycled PET or rPET flakes are used in several applications some of which include:

  • Fiber for carpet, fleece jackets, comforter fill, and tote bags 
  • Containers for food, beverages(bottles), and non-food items 
  • Film and sheet 
  • Strapping

Further heat treatment of recycled PET flakes removes any volatiles making them safe and meet the requirements to be safe for direct food contact.

Get Inspired: Understand how you can improve the quality of recycled PET consistently and reliably to meet the increased demand in volume

According to ILIS study,
“PET itself is biologically inert if ingested, is dermally safe during handling and is not a hazard if inhaled. No evidence of toxicity has been detected in feeding studies using animals.

Negative results from Ames tests and studies into unscheduled DNA synthesis indicate that PET is not genotoxic. Similar studies conducted with monomers and typical PET intermediates also indicate that these materials are essentially non-toxic and pose no threats to human health.”


Watch Today!
An Interesting Video on Life Cycle of PET Bottle

(Source: Avery Dennison)

Also, PET bottles and containers that find their way to the landfill pose no risk of harm or leaching. Since the polymer is inert, it is resistant to attack by micro-organisms, and won't biologically degrade. PET bottles can also easily crushed flat and hence, takes up relatively little landfill space.


Find Suitable Polyethylene Terephthalate Grades


View a wide range of polyethylene terephthalate (PET) grades available today, analyze technical data of each product, get technical assistance or request samples.

Key Applications

Key Properties

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