Polyethylene (PE) Explained
- Gavin Leo
Around since the 1950s, a German called Karl Ziegler and an Italian, Giulio Natta are credited with inventing the process that created polyethylene. Now, millions of tons of this commodity plastic product are produced every year.
However, the origins of this vital plastic actually date back a couple of decades earlier to the 1930s when Eric Fawcett and Reginald Gibson started experimenting with ethylene and benzaldehyde. The chemical reaction between these two carbon atoms created what is now known as polyethylene.
In 1936, Imperial Chemical Industries filed a patent for polyethylene. Polyethylene, or PE, was already a popular plastic to work with, and more and more uses for it were becoming apparent.
It was at this point that a chemist called Karl Ziegler discovered how to create high density material or high-density polyethylene (HDPE). The process was named after him and another chemist colleague, Natta, and is called the Ziegler-Natta polymerization process.
During World War II, polyethylene was used to insulate cables on radar equipment. Commercial production began in earnest after the war ended. Polyethylene was used to make products ranging from kitchen film to milk jugs, and garbage containers to toys.
What Is Polyethylene Plastic?
Polyethylene (PE) is a versatile thermoplastic, a homopolymer made of ethylene molecules with varying crystalline structures. It is widely used in everyday items, especially food packaging. Types of PE include:
- HDPE (High-Density Polyethylene): More crystalline, used for drain pipes.
- LDPE (Low-Density Polyethylene): Flexible, used in food packaging, plastic bottles, and bags.
- LLDPE (Linear Low-Density Polyethylene): Highly flexible, used for packaging films.
- VLDPE (Very Low-Density Polyethylene): Used in grocery bags and six-pack rings.
- UHMWPE (Ultra-High Molecular Weight Polyethylene): Used in medical devices and bulletproof vests.
- Cross-Linked PE: Enhanced with cross-links for applications like water piping and heating systems.
Polyethylene is a thermoplastic, meaning it melts and can be reformed without burning, making it ideal for injection molding. It differs from thermoset polymers, which cannot be reheated once set. PE variants like PET (Polyethylene Terephthalate) are used in applications like 3D printing.
How is PE made?
Polyethylene is made from ethylene molecules. An ethylene molecule is a stable molecule which is composed of two methylene units (CH2) linked together by a double bond between each carbon atom.
Ethylene gas is a colorless, flammable hydrocarbon found in natural gas and also created during crude oil refining. It requires a catalyst, like the famous Ziegler-Natta catalyst, to transform it into polyethylene; this process is referred to as polymerization. When the catalyst is introduced, the monomers react and link together to create polymer chains, which in turn form plastic. There are other types of polymerization.
Free radical polymerization uses a radical to break the double bond between the two carbon atoms in the ethylene molecule, leaving one side of the molecule open and available for bonding, allowing another molecule to link up and create the polymer chain. After these processes take place, you can shape the polymer into long threads or filaments and then convert them into pellets, which are used to manufacture familiar objects like plastic films, food containers, plastic bags, cable insulation, and fuel tanks.
Common Applications Of Polyethylene
There are so many different ways to use polyethylene. You’ll find PE in a range of items from products used every day, like food bags, carrier bags, plastic bottles, and flexible films, to more specialist items like medical implants, fishing nets, fuel tanks, and wear resistance liners for chutes.
Common Types Of PE and Their Properties
PE Grades
Uses
Ultra-high Molecular Weight (UHMWPE)
UHMWPE is also known as High Molecular Weight Polyethylene and is a tough, high density material that withstands wear and tear with longer chain structures and is used for gears, bearings, rollers and bushings.
Low-density Polyethylene (LDPE)
Low-density polyethylene LDPE is transparent and softer and more flexible because of its lower crystallinity. It is used for food containers and bottles.
Medium-density polyethylene (MDPE)
Less dense than HDPE, MDPE is used for gas pipes, packaging film, carrier bags, and screw closures.
Linear Low-density Polyethylene (LLDPE)
The softest and most bendable type of polyethylene, yet with remarkable tensile strength, is popular for packaging films used in different industries, including farming, to cover agricultural mulch to control weeds and insects.
High-density Polyethylene (HDPE)
The most popular PE format is used for detergent bottles and piping, products which need to be rigid in structure but then crushed and recycled after use.
Cross linked Polyethylene
Used for pipework systems in buildings including domestic water piping and radiant heating and cooling systems.
Common Types Of PE and Their Properties
Polyethylene has numerous uses in different industries and is always popular because of its strength, chemical resistance, and ability to withstand wear and tear and high temperatures. With a varied density range, PE is also economical to create, and the process is straightforward. Here are the key benefits of polyethylene.
Flexibility
Polyethylene is exceptionally flexible, making it the ideal choice for scenarios where there is a requirement for a material that can stretch and bend without stress cracking or breaking. It has high impact resistance, ideal for products that must operate in situations where there is the potential for damage or accidents.
Lightweight
Polyethylene is lightweight with low molecular weight, perfect for use in situations like car components or packaging, where weight is an essential factor.
Chemical Durability
Polyethylene has excellent chemical resistance, so it is unaffected by the use of solvents, strong acids, and other chemicals. Polyethylene’s chemical composition means it is the ‘go-to’ product for industries such as pharmaceuticals and food packaging.
Strength and Preservation
Whether you choose linear low-density polyethylene or high-density polyethylene (HDPE), the inherent molecular structure and mechanical properties offer low permeability.
Polyethylene is a great defender against the ingress of moisture and gases. It offers superior strength with a molecular structure that supports a variety of uses in the manufacturing and construction industries.
Strength and Preservation
Whether you choose linear low-density polyethylene or high-density polyethylene (HDPE), the inherent molecular structure and mechanical properties offer low permeability.
Polyethylene is a great defender against the ingress of moisture and gases. It offers superior strength with a molecular structure that supports a variety of uses in the manufacturing and construction industries.
Electrical Insulation
The mechanical properties of polyethylene provide superior electrical insulation so it’s a popular product in the electronic and electrical industries. It’s the perfect insulating material to control and inhibit the flow of electrical current.
Thermal Properties
Polyethylene resists elevated temperatures with an average melting point of 230 °F. Polyethylene’s mechanical properties can withstand sunlight, moisture, and low temperatures, which lends the material to use outdoors for the construction and agricultural industries.
Polyethylene is also corrosion-resistant and suitable for chemical storage.
Compatibility
Polyethylene is a friend to many other chemicals and materials, which is why it is seen in such a wide range of different industries, including construction, chemical processing, medical, and food packaging.
Odorless and Antimicrobial
Polyethylene is odor-free and doesn’t leave a residual smell, unlike other plastic materials. Its chemical composition creates a density structure that inhibits the growth of bacteria, so you could say the structure is antimicrobial. Again, its chemical properties support a multitude of uses in manufacturing and processing where hygiene and cleanliness are top priorities, such as medical equipment and food packaging.
Low-Cost Production
Polyethylene manufacturing is economical and efficient. Compared with other synthetic polymers in the total plastics market, it offers an attractive price point for products as varied as packaging film and fishing nets.
Chlorinated polyethylene (PE-C or CPE) is an even more inexpensive variation of the different types of polyethylene. This is where chlorine is substituted for some of the hydrogen atoms with typical specifications of around 34%-44% and not to be confused with polyvinyl chloride (PVC), which is one of a number of synthetic polymers that compete with polyethylene but is a totally different product.
Cost-Effectiveness
Polyethylene’s cost-effectiveness is another significant advantage. Its economic manufacturing processes and recyclability ensure lower production costs, made even more attractive if you factor in its durability and long life.
Recyclable
An essential element of the profile of any modern product is what happens to it at the end of its life. There has been a lot of controversy over single-use plastics like plastic bags. Happily, polyethylene can be transformed into new materials via different recycling processes.
It can be constantly melted, cooled and restructured into new shapes over and over again.
Why Choose Polyethylene for CNC Machining?
High density polyethylene is a versatile material with an exceptional strength-to-density ratio. It’s available in sheets, rods and special shapes, making it a great CNC machining material to cut on a mill or lathe. HDPE is easily machinable into parts of different shapes and sizes.
It is chemically resilient and has low friction. When you combine this with its strength, it’s easy to see why it has such appeal in CNC machining. It’s important to understand the chemical properties of polyethylene to achieve optimal machining results.
Why Choose Polyethylene for Injection Molding?
Polyethylene is a popular choice for injection molding because it has a wide range of physical and chemical properties. It is lightweight, low-cost, and resistant to chemicals. PE is also moisture-resistant and maintains its structural integrity at different temperatures.
However, PE’s strength depends on its mechanical properties and density, so it’s important to choose PE with the right specifications.
The Difference Between Polyethylene and Polystyrene
Polyethylene and polystyrene are two of the most popular polymers in use today. They are both impact resistant, lightweight and produced in different forms so are hugely versatile and have a lot of similarities. However, there are some clear differences that users need to be aware of. These include:
The HDPE and UHMW forms of Polyethylene are much stronger and more durable than polystyrene making it a favored choice for construction packaging.
Polyethylene is not as easy to shape and form as polystyrene; whilst PE is flexible to a degree, it still has a higher rigidity than polystyrene, even if you opt for low-density polyethylene. Polystyrene is ideal for the engineering sector because it is so easy to shape.
Both polyethylene and polystyrene are available in film and sheets; however, only polystyrene is available in foam forms, unlike PE, which is supplied in fibrous forms.
Polystyrene can withstand temperatures in excess of 200 Celcius.
Conclusion
Polyethylene’s track record across numerous industries and over many decades demonstrates its diverse advantages and versatility.
Polyethylene is flexible, lightweight, easy to produce, weather-resistant, and recyclable. It also has mechanical strength, electrical insulation properties, chemical durability, and cost-effectiveness, which is why it’s so popular in different sectors.
With sustainable production processes and recyclability, it looks like the story of polyethylene is far from over even in the 21st century.