Comprehensive Guide on Polyethylene Terephthalate (PET)
Polyethylene terephthalate (PET) is an aliphatic polyester. It has a semi-crystalline form when stable. It is used in everyday items and easily recyclable. It finds use in packaging, textiles, films to molded parts for automotive, electronics. It shows resistance to impact, moisture, alcohols and solvents.
Polyethylene terephthalate or PET belongs to the family of polyesters. It is widely used in everyday items and easily recyclable. It has a semi-crystalline form when stable. Most common application include rigid and flexible packaging as it is very light weight.
PET is among those plastics which are an important part of your everyday life. You find the polymer used in packaging, fabrics, films to molded parts for automotive, electronics and many more. For all the applications demanding lightweight and impact-resistant material, PET is of your choice. Also do not forget, PET is one of the most recycled thermoplastics as well.
Additionally, there have been continuous efforts to tailor PET properties for improved performance with beneficial cost profiles to meet high-end application demands.
At the same time, polypropylene, polyethylene and polyvinyl chloride are equally popular for these benefits, so how do you find which one is suited for you? In this guide, you will find out the key knowledge to find the suitable grade among thousands of options available.
PET and PBT - Class of Polyesters
Let’s begin by comparing apples with apples. Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) both belong to the polyester family. The key feature that makes two polymers distinctive is the use of alcohol resulting in ester group.
PBT differs only slightly from PET in chemical composition but more substantially in its properties such as its higher crystallization rate and lower melting point. Also, while PET can be either in the semi-crystalline state or in the amorphous state, it is not possible under normal processing conditions to produce PBT parts that are amorphous. PBT crystallizes faster than PET and remains crystalline. In case of PET, the time used to cool down polymer decides its amorphous and crystalline behavior.
Compared to PBT, PET has somewhat higher strength and rigidity, but it is less flexible and tougher and has lower chemical resistance. So, if you need to make a plastic component that has better stiffness, toughness and transparency at room as well as slightly elevated temperatures (~50C), then PET is your material your choice over PBT.
HDPE vs PET – Suitable Packaging Material
While PET and HDPE plastics are both excellent packaging materials, there are some notable differences which set these materials apart. First and foremost, it’s the appearance – PET is a clear plastic while HDPE is opaque.
In terms of stress cracking, PET bottles and containers have better resistance vs HDPE. HDPE is highly prone to environmental stress cracking due to its semi-crystalline nature. With higher molecular weight, crystallinity decreases thus plastic becomes more resistant to environmental stress cracking. Additionally, bottle-grade PET is amorphous and is engineered to prevent crystallization. If it crystallizes, it would lose its transparency as well as shows poor impact resistance.
HDPE containers are slightly more durable and temperature resistant, with a high operating temperature of 160° F compared to 145° F for PET bottles. As clarity is the most preferred feature of PET, it also has natural CO2 barrier properties to maintain food and beverage quality.
While HDPE is a cost-effective polymer, PET has better strength-to-weight ratio. Thus, more products can be delivered with less packaging – less waste. It reduces overall transportation cost as well as carbon footprint.
Today, sustainability and recyclability are becoming key aspects when it comes to material selection. Although both HDPE (code – 2) and PET (code – 1) are recyclable, the low diffusion coefficient makes PET much more suitable than other plastic materials for use as a recovered, recycled material. PET as well as rPET containers continue to remain a sustainable choice for food and beverage packaging industry.
PET vs PVC
Between PET and PVC, selection solely depends on the function of your part and how you need the finished part to perform.
Among the two polymers, PET has the edge when it comes to recycling. PVC recycling is challenging due to 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 which is moderately durable. It can become hard and eventually break down with exposure to sunlight. This is not an ideal material for using in packaging material. PET is much stronger and more durable product and can withstand UV rays and other natural elements. It is resistant to attacks by micro-organisms, is lightweight, easy to transport, and also shatterproof.
When it comes to cost, PVC and PET are closely priced. However, PET contains more oil-based raw materials than PVC and hence its price changes with fluctuations in oil prices. Also, PET scrap has a higher demand, leading to a higher scrap value. Overall, using PET is a cheaper alternative in the long-run.
PET vs Clear Plastics (Acrylic, Polycarbonate)
When it comes to polymer clarity, Polycarbonate and Acrylic polymers are equally popular and compete with PET for certain applications. While PC has the characteristics of good impact resistance and used to manufacture bottles, cans and various forms of containers for packaging beverages but at the same time it has a very poor stress cracking performance. Therefore, if your application has potential where polymer can fail under stress then your decision to choose PET might be meaningful. PET has better mechanical strength and its film strength is three times better than PC.
Also, if your application involves exposure to certain oils and chemicals, then you would prefer to choose PET over PC. PET is chemical resistant to household cleaners, graffiti-resistant, resistant to alcohol and acid, and overall has a greater resistance to a broader range of chemicals than polycarbonate. PC does have chemical resistance, but it is limited, and overall is less desirable for harsh environment use.
At the same time, acrylic does not have the impact strength of either PET or polycarbonate. While PET products are usually food grade but cannot be used if exposed to UV rays. Acrylic is inherently UV resistant with as little as 3% degradation outdoors over a 10-year period.
Toxicity and Recycling of PET
Usually, PET is a very stable film. It is well suited for applications requiring a strong film with high tensile properties, scuff resistance, and durability. It is stiffer and clearer than BOPP. While PET films have good barrier properties, BOPP films are more prone to absorption of oil and acidic products and become distorted by those substances.
What is PET made of?
Polyethylene terephthalate (PET or PETE) is a general-purpose linear semicrystalline 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 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
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 is one of the most recycled thermoplastics and 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.
PETG - Copolymer Form of PET
Glycol modified polyethylene terephthalate (PET-G) is a polyester with good toughness and chemical resistance. Glycol modification of PET through copolymerization improves the processability (faster elongation rates and higher elongations) and to differentiate the properties of polyethylene terephthalate (PET). It lowers the glass transition and melting temperature of PET and decreases the crystallization temperature and rate.
As a technical material, PET-G provides good mechanical properties and improved chemical and thermal behaviors than PLA but with similar ease of use. Check out available PETG grades with good chemical resistance.
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.
What Properties Assist in PET Selection?
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.
The key advantages and features of polyethylene terephthalate are listed below.
- 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
Explore various food contact approved PET grades as per your needs.
PET has a glass transition temperature of 65-80 °C, depending on degree of crystallinity, and a melting temperature of 240-270 °C. A fully amorphous PET has a Tg of 65 °C and Tg increases with increasing degree of crystallinity. PET crystallizes in the temperature range of 10 °C above its glass transition temperature up to 10 °C below its melting temperature, with maximum crystallization rate at 178 °C. PET normally reaches a crystallinity of 40-50%, but can also be polymerized to a co-polymer that cannot crystallize.
Limitations of Polyethylene Terephthalate
Although PET has immensely added to our daily life, there still exist some drawback with the polymer.
The crystallized form of PET has lower impact strength than PBT as well as it shows lower moldability than PBT, due its slow crystallization rate.
Furthermore, PET in amorphous form is easily affected by boiling water, alkalis and strong bases. Also, it can easily be attacked at high temperatures (>60°C) by ketones, aromatic and chlorinated hydrocarbons and diluted acids and bases.
In order to make PET capable of handling aggressive chemical environments as well as withstand elevated temperature, lot of plastic parts molded in crystalline PET polyester. Additionally, crystalline form of PET sometimes requires additives such as nucleating agents, as well as the solid particles of fillers and reinforcements, to make this polyester capable of applications beyond packaging.
Access several optimized PET grades to find the best suited option for your application:
- Glass fiber-reinforced PETs
- Heat stabilized PETs
- PETs with high impact resistance
- High strength PET
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 PET with PBT offer blends with greater influence on their mechanical properties, especially on impact strength. These blends are potentially attractive because can combine the fast crystallization rate of PBT with the low cost of the PET grade bottle.
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
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
The PET bottle blow molding process works by reheating a pre-molded PET preform which is then automatically positioned into a mound. Extrusion blow molding can be used to fabricate polypropylene bottles for packaging such products as ketchup and sauces, cleaning products, mineral water, and food and beverages. Stretch blow molding is commonly employed to produce PP bottles for water, pharmaceuticals, dried foods and spices, household cleaners, isotonic and sports drinks, baby formulas, housewares, and liquid soaps and detergents.
Blow molding is generally used to produce transparent bottles. Mold temperature should lie between 10 and 50°C
Injection Molding
PET plastic injection molding process is one of the very important technologies for plastic processing. Since PET is a hygroscopic material, it must be dried to a moisture content of 0.05% or less, so that it can create a good non-crystalline transparent preform. If the moisture content is too high, the PET molecular chains can thermally decompose and thus, reduce physical and mechanical properties as well as the crystallization rate, impacting the quality of the product.
PET is heat sensitive material and its forming temperature range is narrow. During the process, if the temperature is too low, it is not good to make plastic parts, depression, lack of material defects; on the contrary, the temperature is too high, will cause the spill, nozzle salivation, deep color change, the decline in mechanical strength, even cause degradation.
Barrel temperature (unreinforced grades): 240~280℃
Barrel temperature of glass fiber reinforced PET: 250~290℃
Temperature of nozzles: Should not exceed 300C
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
PET is generally extruded to produce films and sheets (can be thermoformed after) for food packaging applications. Its extrusion temperature: 270-290°C.
3D Printing
PET and PETG filaments are known to produce 3D Printed objects having high flexibility and toughness. 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. The material also benefits from great thermal characteristics, allowing 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
With PET packaging accounting for more than 50 percent of total plastic waste, extending its lifetime by re-using it as feedstock offers a broadly available alternative to virgin raw materials. More and more recycled PET filaments are developed that can be used to produce unique designs and new products.
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 offers significant advantages (weight, durability, versatility) compared to alternative materials, making it a widely used polymer in the packaging and textile sectors. As they have a short lifespan, almost all PET products marketed are transformed into waste, making PET a priority target for recycling. 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.
The growth of bottle recycling has been facilitated by the development of processing technologies that increase product purities and reduce operational costs.
As with virgin PET, 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
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.
The most adopted methods for PET recycling are chemical recycling by hydrolysis and mechanical blending. Chemical recycling involves the production of terephthalic acid (TPA) and ethylene glycol (EG), to reutilize them for making other synthetic chemicals. However, cost-effectiveness and environmental friendliness make mechanical recycling the most common PET recovery method.
Overall, recycling and reprocessing cause a decrease in melt viscosity and average molecular weight, which can negatively affect the final product’s mechanical properties in terms of tensile and impact strength. The use of recycled polyethylene terephthalate (PET) as a matrix for composite materials based on glass fiber reinforced virgin PET could be a cost-effective and environmentally friendly way to upgrade the bottle-grade recycled PET into engineering-grade PET for injection molding.
Furthermore, heat treatment of recycled PET flakes removes any volatiles making them safe and meet the requirements to be safe for direct food contact.
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.”
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.
What are the main applications of PET?
Polyethylene terephthalate (PET) is a chemically stable polyester, and its use has risen dramatically in the last few decades with a multitude of applications, ranging from food and drink containers to the manufacture of electronic & automotive components and as fibers in clothes.
Amorphous PET is used for the production of bottles and packaging due to its high transparency, which is very similar to that of glass. In other cases, a semi-crystalline state of polymer is used, which has a milky white color and is opaque.
Packaging Industry’s Favorite Food Safe PET
High resistance to various environmental factors along with an absence of harmful low molecular weight substances make it widely used in applications where it comes into contact with food (food industry, food packaging, household appliances). Also, it has good oxygen barrier qualities and can be used with oxygen absorbers to store bulk dry foods.
Polyethylene Terephthalate is used in several packaging applications as mentioned below:
Because Polyethylene Terephthalate is an excellent water and moisture barrier material, plastic bottles made from PET are widely used for mineral water and carbonated soft drinks
Its high mechanical strength, makes Polyethylene Terephthalate films ideal for use in tape applications
Non-oriented PET sheet can be thermoformed to make packaging trays and blisters
Its chemical inertness, together with other physical properties, has made it particularly suitable for food packaging applications
Other packaging applications include rigid cosmetic jars, microwavable containers, transparent films, etc.
PET monofilament is mainly used for making mesh fabrics for screen-printing, filter for oil and sand filtration, bracing wires for agricultural applications (greenhouses etc.), woven/knitting belt, filter cloth, and other such industrial applications.
High-end Engineering Applications Used Improved PET Grades
Thanks to good electrical insulating properties, high structural and dimensional stability, polyethylene terephthalate is widely used in electrical and electronics industry. It is an effective polymer to replace die casts metals and thermosets in applications like: electrical encapsulation, solenoids, smart meters, photovoltaic parts, solar junction boxes, etc. Polymer’s outstanding flow characteristics enable design freedom and miniaturization to produce high-performance parts.
PET is successfully being used in many applications in the automotive industry. It is currently being employed in the following applications: wiper arm and gear housings, headlamp retainer, engine cover, connector housings.
In the automotive applications, reinforced PET can be more suitable than reinforced PP. This is because the standard requirement for usage temperature for an automotive component is -40°C to +80°C, which is difficult to fulfil with reinforced PP for semi-structural components like body panels.
Polyester Fibers in Textile Industry
PET has wide applications in textile industry. PET polyester fabrics are manufactured using melt-spinning. In this technique, molten polymer is extruded through narrow channels to form fibres.
PET are strong, flexible, and offer additional benefit of less wrinkles and shrinkage over cotton. Polyester fabrics are light-weight, reduced-wind, drag-resistant and more resistant to tears.
Polyethylene terephthalate (PET) is an aliphatic polyester. It has a semi-crystalline form when stable. It is used in everyday items and easily recyclable. It finds use in packaging, textiles, films to molded parts for automotive, electronics. It shows resistance to impact, moisture, alcohols and solvents.
