Polyethylene Terephthalate (PET) - Uses, Properties & Structure

What is polyethylene terephthalate (PET)?

Polyethylene terephthalate (PET or PETE) is a general-purpose linear semicrystalline thermoplastic polymer. It belongs to the polyester family of polymers. These resins are known for their excellent combination of properties. These properties include mechanical, thermal, and chemical resistance as well as dimensional stability. Its chemical formula is C₁₀H₈O₄)_n.

Molecular Structure of Polyethylene Terephthalate

How polyethylene terephthalate (PET) is made?

Polyethylene terephthalate is an aliphatic polyester. It is obtained from the 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

The reaction produces PET in the form of a molten and viscous mass. This can be directly spun into fibers or extruded or molded into almost any shape. Chemically, Polyethylene terephthalate is very much similar to Polybutylene Terephthalate.

What properties assist in PET selection?

Polyethylene terephthalate 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.

Key features include:

Higher strength, heat distortion temperature (HDT) and stiffness than PBT
Very strong and lightweight & hence easy and efficient to transport
Good gas (oxygen, carbon dioxide) and moisture barrier properties
Excellent electrical insulating properties
Broad range of use temperature, from -60 to 130°C
Low gas permeability, particularly with carbon dioxide
Suitable for transparent applications, when quenching during processing
It doesn’t 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.
It is approved as safe for contact with foods and beverages by the FDA, Health Canada, EFSA & other health agencies. Food contact approved PET grades »
Chemical properties

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

The glass transition temperature of PET varies depending on the degree of crystallinity. It has a Tg of 65-80°C. It has a melting temperature of 240-270°C. Amorphous PET has a Tg of 65°C. The Tg increases with an increasing degree of crystallinity.

Crystallization occurs with a maximum crystallization rate of 178°C at:

the temperature range of 10°C above its Tg, and
up to 10°C below its melting temperature.

It normally reaches a crystallinity of 40-50%. It can also be polymerized to a co-polymer that cannot crystallize.

Access several optimized PET grades to find the best suited option for your application:

The addition of fillers improves impact strength, surface finish, and several other benefits. It also reduces warpage. Some filler examples include glass fibers, CNTs, etc.

What are the limitations of polyethylene terephthalate (PET)?

PET has immensely added to our daily life, but there still exist some drawbacks with the polymer.

The crystallized form of PET has:

lower impact strength
lower moldability

These properties are lower in comparison to PBT, due its slow crystallization rate.

Furthermore, PET in amorphous form is easily affected by boiling water, alkalis and strong bases. At high temperatures (>60°C), it can easily be attacked by ketones, aromatic and chlorinated hydrocarbons and diluted acids and bases.

A lot of plastic parts are molded in crystalline PET polyester to make PET capable of handling:

aggressive chemical environments as well as
elevated temperatures

Additionally, crystalline PET sometimes requires additives such as nucleating agents and solid particles of fillers and reinforcements. This makes it capable of applications beyond packaging.

What is PETG?

PET-G stands for Glycol modified polyethylene terephthalate. It is the copolymer form of polyethylene terephthalate homopolymer.

Features of glycol modification of PET through copolymerization include:

It improves processability (faster elongation rates and higher elongations).
It lowers the glass transition and melting temperature of PET.
It decreases the crystallization temperature and rate.
It is a polyester with good toughness and chemical resistance.
It also differentiates the properties of polyethylene terephthalate (PET).

As a technical material, PETG provides good mechanical properties. It also improves chemical and thermal behaviors with similar ease of use. All these comparisons are made with respect to PLA.

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

PETG-bottles-transparent

How is polyethylene terephthalate (PET) processed?

Polyethylene terephthalate can be easily processed by injection molding and extrusion. It is generally extruded to produce films and sheets and can be later thermoformed. Blow molding is generally used to produce transparent bottles.

Recommended drying time is 2-4 hours at 120°C before processing. Up to 25% regrind can be used.

Blow Molding

The blow molding process works by reheating a pre-molded PET preform. It is then automatically positioned into a mound.

Extrusion- and stretch blow molding can be used to fabricate polypropylene bottles. They are used for packaging products such as:

Extrusion Blow Molding	Stretch Blow Molding
Ketchup and sauces Cleaning products Mineral water Food and beverages	Water Pharmaceuticals Dried foods and spices Household cleaners Isotonic and sports drinks Baby formulas Housewares Liquid soaps and detergents

PET Bottle Preform for Blow Molding

Injection Molding

PET injection molding is one of the very important technologies for plastic processing. Since it is a hygroscopic material, it must be dried to a moisture content of 0.05% or less. This allows it to create a non-crystalline transparent preform.

If the moisture content is too high, the PET molecular chains can thermally decompose. This reduces the physical and mechanical properties as well as the crystallization rate. This thereby impacts the quality of the product.

This polyester is a heat-sensitive material with a narrow forming temperature. During the process, if the temperature is:

Too low – It is not good to make plastic parts. Depression and lack of material defects can occur.
Too high – It can cause a spill, nozzle salivation, and deep color change. It can also decrease mechanical strength and cause degradation.

Barrel temperature (unreinforced grades): 240~280°C
Barrel temperature of glass fiber reinforced PET: 250~290°C
Nozzle temperature: Should not exceed 300°CC
Melt temperature: 280-310°C
Mold temperature: 140-160°C to obtain a crystalline PET (for technical applications)
Screw with an L/D ratio of 18-22 is recommended
For transparent applications, mold temperature should lie between 10 and 50°C

Extrusion

Polyethylene terephthalate is generally extruded to produce films and sheets. After extrusion, they can be thermoformed for food packaging applications. Its extrusion temperature ranges between 270-290°C.

3D Printing

PET and PETG filaments are known to produce 3D Printed objects. These filaments have high flexibility and toughness. PETG filament is easier to print than ABS. It offers:

higher strength,
lower shrinkage, and
smoother finish.

The material also benefits from great thermal characteristics. This allows the plastic to cool efficiently with almost negligible warpage.

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

PET packaging accounts for more than 50 percent of total plastic waste. This extends its lifetime by reusing it as feedstock. This in turn offers a broadly available alternative to virgin raw materials. More recycled PET filaments are developed to produce unique designs and new products.

Explore some PET grades apt for 3D Printing/Additive Manufacturing in our database.

What happens when PET blends with other polymers?

Polyethylene terephthalate blends with thermoplastics or thermosets to tailor new materials. Blending materials have improved performance with beneficial cost profiles. It opens up the potential for new markets without much investment and development.

The following thermoplastics, thermosets and rubbers are used to produce blends with PET.

Thermoplastics	Thermosets	Rubbers
Polyethylene Polypropylene Polycarbonate Acrylonitrile Butadiene Styrene Polystyrene Ethyl vinyl acetate Polyester	Epoxy resins Phenolic resins	Styrene butadiene rubber Nitrile butadiene rubber

PET modified with polyolefins are often glass fiber reinforced. They are used in injection-molded automotive and industrial applications.

Applications with PET/PC blends require a combination of properties such as:

excellent toughness, chemical and heat resistance
along with high impact, tensile and flexural strength

Blending PET with PBT offers a great influence on mechanical properties like impact strength. These blends combine the fast crystallization rate of PBT with the low cost of PET.

Blending thermosets with PET improves thermal, mechanical, impact resistance, and flame-retardant properties. Used for the production of automotive, aeronautic, and electronic components.

The main objective of developing these blends is to improve cost-effectiveness, mechanical strength, flame retardancy, toughness, processability, etc.

Is polyethylene terephthalate (PET) recyclable?

Polyethylene Terephthalate is 100% recyclable. It is the most recycled plastic worldwide. It can be easily identified by its recycling code #1.

How rPET is processed?

It offers significant advantages (weight, durability, versatility) compared to alternative materials. This makes it a widely used polymer in the packaging and textile sectors. This class of polyesters has a short lifespan. Hence, almost all products marketed are transformed into waste. This makes it a priority target for recycling. A low diffusion coefficient makes it more suitable as a recovered, recycled material.

Post-consumer PET bottles are collected and processed:

through a series of special washing processes or
by chemical treatment

This breaks down PET into its raw materials or intermediates. They are further used to produce recycled PET (rPET) flakes. The growth of bottle recycling facilitates the development of processing technologies. This increases product purity and reduces operational costs.

PET Recycling Facility

What are the methods to recycle PET?

The most adopted methods for PET recycling are:

Chemical recycling by hydrolysis: This method involves the production of terephthalic acid (TPA) and ethylene glycol (EG). Reutilizing them for making other synthetic chemicals.
Mechanical recycling: Cost-effectiveness and environmental friendliness make mechanical recycling the most common PET recovery method.

Recycling and reprocessing cause a decrease in melt viscosity and average molecular weight. This can negatively affect the final product’s tensile and impact strength.

How to upgrade recycled PET bottles?

Bottle-grade rPET is upgraded into engineering-grade PET for injection molding. rPET is used as a matrix for composites based on glass fiber-reinforced virgin PET. This is a cost-effective and environmentally friendly way.

Applications of rPET

As with virgin PET, recycled PET or rPET flakes are used in several applications such as:

Fiber for carpet, fleece jackets, comforter fill, and tote bags
Containers for food, beverages (bottles), and non-food items
Automotive parts (carpets, sound insulation, boot linings, seat covers)
Film and sheet
Strapping, and
Industrial end-use items (geotextiles and roof insulation)

rPEt can also be blended in a ratio of virgin to recycled, depending on the application required.

Explore various PET grades with Post Consumer Recycled Content »

Is polyethylene terephthalate (PET) toxic?

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

According to the ILIS study:

PET itself is biologically inert if ingested.
It is dermally safe during handling.
It 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 non-toxic. They pose no threats to human health.

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 microbial attack and won't biologically degrade. PET bottles can also easily be crushed flat and hence, takes up relatively little landfill space.

Which class of polyester to choose?

Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) both belong to the polyester family. The use of alcohol resulting in an ester group makes these polymers distinctive.

In chemical composition, PBT differs only slightly from PET. PBT has a higher crystallization rate and lower melting point. PET can be either semi-crystalline state or amorphous. It is impossible to produce amorphous PBT parts under normal processing conditions. PBT crystallizes faster than PET and remains crystalline. In the case of PET, the time used to cool down the polymer decides its amorphous and crystalline behavior.

Compared to PBT, PET has the following features:

higher strength and rigidity,
less flexible tougher, and
lower chemical resistance.

So, if you need to make a plastic component that has better stiffness, toughness, and transparency at room or slightly elevated temperatures (~50°C), then PET is your material of choice over PBT.

Do you still feel PBT is the material of your choice? Know more about PBT here »

Which is a suitable packaging material - HDPE or PET?

Polyethylene terephthalate (PET) and High Density Polyethylene (HDPE) have some notable differences. Both these plastics are excellent packaging materials.

Appearance: PET is a clear plastic while HDPE is opaque.

Stress cracking: Polyethylene terephthalate (PET) bottles and containers have better resistance. HDPE is highly prone to environmental stress cracking due to its semi-crystalline nature. With higher molecular weight, crystallinity decreases. Thus, the plastic becomes more resistant to environmental stress cracking. Additionally, bottle-grade polyethylene terephthalate (PET) is amorphous and is engineered to prevent crystallization. If it crystallizes, it will lose its transparency as well as show poor impact resistance.

Durable and temperature resistant: HDPE containers are slightly more durable and temperature resistant. They have a high operating temperature of 160°F compared to 145°F for polyethylene terephthalate (PET) bottles.

Clarity: Due to this feature polyethylene terephthalate (PET) has natural CO₂ barrier properties to maintain food and beverage quality.

While HDPE is a cost-effective polymer, polyethylene terephthalate (PET) has a better strength-to-weight ratio. Thus, more products can be delivered with less packaging – less waste. It reduces overall transportation costs as well as carbon footprint.

Sustainability and recyclability are becoming key aspects when it comes to material selection. Both HDPE (code – 2) and PET (code – 1) are recyclable. The low diffusion coefficient makes polyethylene terephthalate (PET) much more suitable as a recovered, recycled material. Hence, it continues to remain a sustainable choice for the food and beverage packaging industry.

Do you still feel HDPE is the best fit for your packaging application? Know more about HDPE here »

Which is your material of choice - PET vs PVC?

Between Polyethylene terephthalate and Polyvinyl chloride, the selection depends on how you need the finished part to function and perform.

Among the two polymers, polyethylene terephthalate has the edge when it comes to recycling. PVC recycling is challenging due to the high Cl content of raw PVC (56% by weight). Of all plastics, PVC uses the highest proportion of additives. As a result, PVC requires separation from other plastics before mechanical recycling.

PVC is a rigid plastic that is moderately durable. It can become hard and break down with exposure to sunlight. This is not an ideal material for use in packaging material. Polyethylene terephthalate is a much stronger and more durable product. It can withstand UV rays and other natural elements. It is resistant to microbial attacks, lightweight, easy to transport, and also shatterproof.

When it comes to cost, PVC and PET are closely priced. However, polyethylene terephthalate contains more oil-based raw materials than PVC. Hence its price changes with fluctuations in oil prices. Also, PET scrap has a higher demand, leading to a higher scrap value. Overall, using polyethylene terephthalate is a cheaper alternative in the long run.

Do you still feel PVC is the material of your choice? Know more about PVC here »

Which is the best polymer among clear plastics?

When it comes to polymer clarity, Polycarbonate and Acrylic polymers are equally popular. They compete with polyethylene terephthalate for certain applications.

PC has the characteristics of good impact resistance. They are used to manufacture bottles, cans, and containers for packaging beverages. But at the same time, it has a very poor stress-cracking performance. Polyethylene terephthalate has better mechanical strength and its film strength is three times better than PC.

Choosing PET over PC might be meaningful if your application:

Has the potential where the polymer can fail under stress.
Involves exposure to certain oils and chemicals.

Polyethylene terephthalate is chemically resistant to household cleaners, graffiti, alcohol, and acid. It has a greater resistance to a broader range of chemicals than polycarbonate. The chemical resistance of PC is limited, and it is less desirable for harsh environment use.

At the same time, acrylic does not have the impact strength of either polyethylene terephthalate or polycarbonate. While PET products are usually food grade but cannot be used if exposed to UV rays. Acrylic is UV resistant with as little as 3% degradation outdoors over a 10-year period.

If you still feel Polycarbonate is your material of choice? Know more about PC here »

Or if Acrylics is your material or choice? Know more about Acrylics here »

Which film to choose from PET vs BOPP?

Usually, polyethylene terephthalate is a very stable film with good barrier properties. It is well suited for applications requiring a strong film with features like:

high tensile properties,
scuff resistance, and
durability.

It is stiffer and clearer than BOPP. BOPP films are more prone to absorption of oil and acidic products and become distorted by those substances.

What are the commercially available PET 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 Properties

	Property	POLYETHYLENE TEREPHTHALATE
Chemical Resistance
Acetone @ 100%, 20°C	Limited
Ammonium hydroxide @ 30%, 20°C	Satisfying
Ammonium hydroxide @ diluted, 60°C	Non Satisfactory
Ammonium hydroxide @ diluted, 20°C	Non Satisfactory
Aromatic hydrocarbons @ 20°C	Limited
Benzene @ 100%, 20°C	Limited
Butylacetate @ 100%, 60°C	Limited
Butylacetate @ 100%, 20°C	Limited
Chlorinated solvents @ 20°C	Limited
Chloroform @ 20°C	Limited
Dioctylphtalate @ 100%, 100°C	Limited
Ethanol @ 96%, 20°C	Satisfying
Ethyleneglycol (Ethane diol) @ 100%, 20°C	Satisfying
Ethyleneglycol (Ethane diol) @ 100%, 50°C	Limited
Glycerol @ 100%, 20°C	Satisfying
Grease @ 20°C	Satisfying
Kerosene @ 20°C	Satisfying
Methanol @ 100%, 20°C	Limited
Methylethyl ketone @ 100%, 20°C	Limited
Mineral oil @ 20°C	Satisfying
Phenol @ 20°C	Non Satisfactory
Silicone oil @ 20°C	Satisfying
Soap @ 60°C	Limited
Soap @ 20°C	Limited
Sodium hydroxide @ 10%, 20°C	Satisfying
Sodium hydroxide @ 10%, 60°C	Non Satisfactory
Sodium hydroxide @ 10%, 90°C	Non Satisfactory
Sodium hypochlorite @ 20%, 20°C	Satisfying
Strong acids @ concentrated, 20°C	Non Satisfactory
Toluene @ 20°C	Limited
Toluene @ 60°C	Non Satisfactory
Xylene @ 20°C	Limited
Electrical
Arc Resistance, sec	75 - 125
Dielectric Constant	3 - 4
Dielectric Strength, kV/mm	60
Dissipation Factor x 10^-4	20 - 200
Volume Resistivity x 10¹⁵, Ohm.cm	16
Mechanical
Elongation at Break, %	30 - 70
Elongation at Yield, %	3.8
Flexural Modulus, Gpa	2.8 - 3.5
Hardness Rockwell M	50 - 100
Hardness Shore D	85 - 95
Strength at Break (Tensile), MPa	45 - 70
Strength at Yield (Tensile), MPa	50 - 57
Toughness, J/m	140
Young's Modulus, GPa	2.8 - 3.5
Optical
Gloss, %	105 - 200
Transparency, %	70 - 90
Physical
Density, g/cm³	1.3 - 1.4
Gamma Radiation Resistance	Good
Glass Transition Temperature, °C	73 - 78
Shrinkage, %	0.2 - 3
Sterilization Resistance (Repeated)	Poor
UV Light Resistance	Fair
Water Absorption 24 hours, %	0.1 - 0.2
Service Temperature
Ductile / Brittle Transition Temperature, °C	-40
HDT @0.46 Mpa (67 psi), °C	75 - 115
HDT @1.8 Mpa (264 psi), °C	65 - 80
Max Continuous Service Temperature, °C	80 - 140
Min Continuous Service Temperature, °C	-40
Thermal
Coefficient of Linear Thermal Expansion x 10^-5, /°C	6 - 8
Fire Resistance (LOI), %	23 - 25
Flammability, UL94	HB
Thermal Insulation, W/m.K	0.29

Key Applications

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Comprehensive Guide on Polyethylene Terephthalate (PET)