Polyethylene terephthalate (PET) belongs to the family of polyesters. It has a semi-crystalline form when stable. It is recyclable and shows resistance to impact, moisture, alcohols, and solvents.
It is among those plastics which are an important part of your everyday life. The polymer finds use in packaging, fabrics, and the textile industry. It is also used in films to mold parts for automotive, electronics, and many more.
Additionally, continuous efforts are made to tailor PET properties. This improves performance with beneficial cost profiles to meet high-end application demands.
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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.
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:
- Glass fiber-reinforced
- Heat stabilized
- High impact resistance
- High strength
- Flame retardant
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.
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 saucesCleaning productsMineral waterFood and beverages | WaterPharmaceuticalsDried foods and spicesHousehold cleanersIsotonic and sports drinksBaby formulasHousewaresLiquid 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.