Polyethylene Terephthalate: Uses, Advantages, and Disadvantages
Learn about this material, its characteristics, and its toxicity.
Polyethylene terephthalate (or poly(ethylene terephthalate), PET, or PETE) is the polyester family's most common thermoplastic polymer resin. PET has a glass transition temperature of 67–81 °C and a melting point of 260 °C.
This article explains what PET is and how it is made. It also looks at the different grades of Polyethylene Terephthalate, its characteristics, uses, and the advantages and disadvantages of using it in plastic injection molding.
What Is Polyethylene Terephthalate?
Polyethylene terephthalate is a clear, strong, and lightweight plastic widely used for packaging foods and beverages. The essential elements of PET are ethylene glycol and terephthalic acid, combined to form a polymer chain.
PET was first created in the mid-1940s by DuPont chemists attempting to develop new synthetic fibers. DuPont later labeled its PET fiber as "Dacron." Now, more than half of the world's synthetic fiber is made from PET. Polyethylene terephthalate is called polyester when used as a fiber or fabric. When used for packaging or other applications, it is called PET or PET resin.
How Is Polyethylene Terephthalate Made?
Polyethylene terephthalate is made from ethylene glycol. Ethylene glycol is usually called "MEG," standing for monoethylene glycol. PET is made from a combination of MEG and dimethyl terephthalate (DMT) (C6H4(CO2CH3)2) but primarily terephthalic acid, also called "PTA" for purified terephthalic acid.
As of 2022, ethylene glycol is made from natural gas, and terephthalic acid comes from p-xylene created from crude oil. An antimony or titanium compound is usually employed as a catalyst, a phosphite is added as a stabilizer, and a bluing agent such as cobalt salt is added to mask potential yellowing.
After PET is made in a chemical reaction, the resulting spaghetti-like strands are extruded to be processed and manufactured. The strands cool and are cut into small pellets. The resin pellets can then be heated to a molten liquid that can be easily extruded or plastic injection molded.
What Are the Grades of Polyethylene Terephthalate?
The following is a list of Polyethylene Terephthalate (PET) grades:
The commercial grades of PET are flame retardant, heat resistant, glass-reinforced, and various other engineering materials offering good strength. Fillers are added to improve the surface finish, reduce warpage, and provide other desired benefits.
What Are the Characteristics of Polyethylene Terephthalate?
Polyethylene terephthalate has high strength and stiffness. It exhibits excellent electrical insulating properties. It is very lightweight. It has good gas (oxygen, carbon dioxide) and moisture barrier properties. It has a broad range of use temperatures, from -60 to 130 °C. PET has low gas permeability, particularly with carbon dioxide.
Polyethylene terephthalate is suitable for transparent applications, when quenching during processing. It doesn’t break or fracture. It is shatterproof, recyclable, and transparent to microwave radiation.
What Are the Uses of Polyethylene Terephthalate?
Polyethylene terephthalate is used for textiles, packaging, and other applications. Almost all single-serving and two-liter bottles of carbonated soft drinks and water sold in the U.S. are made from PET. It is also commonly used for packaging salad dressings, peanut butter, cooking oils, mouthwash, shampoo, liquid hand soap, window cleaner, and tennis balls. Special grades of PET are used for carry-home food containers and prepared food trays that can be warmed in the oven or microwave.
What Are the Advantages of Polyethylene Terephthalate in Plastic Injection Molding?
The advantages of Polyethylene Terephthalate as a material are that it is shatterproof, is non-reactive when in contact with food or water, and provides a high strength-to-weight ratio. Being lightweight also provides for cost savings when transporting products using PET packaging.
The glass conversion temperature of PET is about 165℃, and the temperature range of the material is 120~220℃. PET has a strong hygroscopicity at high temperatures. For glass fiber reinforced PET materials, bending deformation is very easy to occur at high temperatures. The material liquefies at high temperatures (as opposed to burning) also makes it an ideal material for recycling. Hence it is considered to be a green plastic.
What Are the Disadvantages of Polyethylene Terephthalate in Plastic Injection Molding?
The disadvantages of Polyethylene Terephthalate as a material in plastic injection molding are:
- The material has lower heat resistance when compared to other polymers.
- Resins made from PET can oxidize, resulting in a degradation in the taste of food and beverages when the items have a long shelf life and are stored in PET packaging.
- PET is not a biodegradable plastic, which may be a disadvantage depending on the intended application.
Injection molding is mainly used to enhance the molding of PET. In general, PET can only be molded by a screw-type injection molding machine.
How Does Polyethylene Terephthalate Affect Human Health?
Polyethylene terephthalate does not affect human health. While many consumers have erroneous assumptions about PET, it does not contain BPA, phthalates, dioxins, cadmium, lead, or endocrine disruptors. Therefore, drinking water from a polyethylene terephthalate bottle that has been left in a hot car, frozen, or washed and used more than once, does not pose any health risk.
Is Polyethylene Terephthalate Toxic?
Polyethylene Terephthalate is not considered toxic. Heat treatment of recycled PET flakes removes any volatiles, making them safe and meeting the requirements for direct food contact.
According to the ILIS study, PET is biologically inert if ingested, is dermally safe during handling, and is not a hazard if inhaled. In addition, no evidence of toxicity has been detected in feeding studies using animals. The study states, “Similar studies conducted with monomers and typical PET intermediates also show that these materials are essentially non-toxic and pose no threats to human health.”
Additionally, PET bottles and containers that are discarded in landfills pose no risk of harm or leaching. Because it is inert, it is resistant to attack by micro-organisms and won't biologically degrade. PET bottles can also easily be crushed flat to take up relatively little landfill space.
What Is the Difference Between Polyethylene Terephthalate and High Density Polyethylene (HDPE)?
HDPE and PET are both very popular for manufacturing bottles. They are quite similar on the surface but do have some differences that set them apart.
High-density polyethylene (HDPE) is a ubiquitous plastic used in manufacturing. HDPE can withstand temperatures ranging from -110° to 165° F. Unlike PET, HDPE is not clear but can be used to make semi-opaque bottles.
Polyethylene terephthalate isn’t as temperature-resilient as HDPE. PET does protect against diluted acids, oils, and alcohols that attack HDPE. Like HDPE, PET can be matched to any color.
Is Polyethylene Terephthalate Plastic Good for 3D Printing?
Yes, polyethylene terephthalate is suitable for 3D printing. PET 3D filament is an easy material to use. Its temperature range for the PET extruder is quite wide at 160°C to 210°C. The advantages of using PET for 3D printing are that it is food-safe, waterproof, has a glass-like appearance, and does not produce odor or fumes. The disadvantages are that PET may take some tweaking to find the right temperature for printing. In addition, it has a wide range of printing temperatures and may be slow to print.
This article summarized the characteristics, uses, advantages, limitations, and toxicity of Polyethylene Terephthalate.
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