All About Polyethylene (PE) Plastic
Polyethylene or “PE” is a thermoplastic that also happens to be the most common plastic. It accounts for roughly 34% of the total plastics market. It has desirable physical properties such as high ductility, high impact strength, and very great chemical resistance. There are different types of polyethylene that are used in different applications. High-density polyethylene is dense and relatively more crystalline and is used in rugged environments like construction. Low-density polyethylene is used in plastic bags and packaging.
In general, different types of polyethylene have variations in their crystalline structures. The less crystalline the structure, the more is its tendency to gradually soften. The more crystalline the structure, the faster is its transition from solid to liquid form. Being a thermoplastic material, Polyethylene has a melting point of 110oC to 130 oC. It can be heated to that temperature, cooled, and then reheated again without any significant degradation. The ability of polyethylene to liquefy at around 110oC makes it a good fit for injection molding.
History of Polyethylene
The earliest record of polyethylene creation dates back to 1898. It was at this time that a German chemist named Hans von Pechmann made polyethylene by accident while he was investigating diazomethane. Diazomethane was not a preferred material for industrial applications because it was known to be very unstable.
However, around the year 1933, a process for polyethylene synthesis that can be used in an industrial environment was discovered at the Imperial Chemical Industries (ICI) by Eric Fawcett and Reginald Gibson. Later on, in 1935, this process was further refined by Michael Perrin (a chemist at ICI), and by 1939, large-scale production of low-density polyethylene commenced.
In the US in the year 1944, a company named Bakelite Corporation (based out of Texas) began large-scale commercial production of polyethylene under license from Imperial Chemical Industries. A major landmark was crossed in the 1950’s when catalysts were discovered to help the polymerization process that is vital to polyethylene production. These catalysts resulted in the production of high-density polyethylene throughout the 1950’s, 60’s, and 70’s.
Today, over 100 million tonnes of polyethylene resin are produced annually.
Manufacturing of Polyethylene
Polyethylene is made via the polymerization process. It all starts with the distillation of hydrocarbon fuels into fractions or lighter groups. These groups or monomers are then brought in contact with a catalyst to drive the polymerization process. The polymerization reaction is an exothermic reaction.
Nowadays, coordination polymerization is the more commonly used process, which involves metal chlorides and metal oxides. The most commonly used catalysts are called Ziegler-Natta catalysts and Phillips catalysts. Polyethylene is also sometimes produced using the radical polymerization process. However, this method requires the use of high-pressure apparatus.
Once the polyethylene is made, there are sometimes instances where separate polyethylene parts need to be joined together to make a larger product. In such cases, the polyethylene parts are joined using the following methods:
- Laser Welding
- Ultrasonic Welding
- Hot Gas Welding
- Heat Fusion
- Heat Sealing
Different grades of Polyethylene
There are different types of polyethylene materials available. They are classified differently based on their density and branching. Those two factors significantly affect the mechanical properties. The various types of polyethylene are:
- High-Density Polyethylene
- Low-Density Polyethylene
- Linear Low-Density Polyethylene
- Very-Low-Density Polyethylene
- Medium Density Polyethylene
- Ultra-high-molecular-weight Polyethylene
- Cross-linked Polyethylene
Applications of Polyethylene
Packaging: High-Density Polyethylene (HDPE) is used to make crates, trays, and bottles for commonly available groceries. Bottle caps, cans, and drums are also made using HDPE. The high impact strength of HDPE makes it the material of choice for making such packaging items.
Low-Density Polyethylene (LDPE) is used to make films, plastic bags, trash bags, and other food packaging material. The good thing about LDPE is that it is low-cost and provides good physical flexibility.
Pipes and fittings: Polyethylene is used extensively to make a variety of pipes and fittings. HDPE is used in gas pipes, water pipes, sewer pipes, as well as coatings on steel pipes. HDPE provides excellent resistance to chemicals and hydrolysis which makes it the material of choice for those applications. LDPE, on the other hand, is used to make water pipes and hoses because of its low water absorption and plasticity.
Electrical: Polyethylene is a good insulator and is used in making insulation for coaxial cables and cable jacketing.
Medical: Certain specific types of polyethylene like ultra-high-molecular-weight polyethylene has high toughness and is resistant to cuts and wear. It is also chemical resistant. So, it is used to make artificial joints, knee replacements, and hip replacements. It is also used to make certain sections of implants.
Toys: One of the biggest applications of HDPE is toys. In fact, one-third of all toys are made using HDPE. High tensile strength is one of the useful properties of HDPE.
Consumer products: Garbage containers, kitchenware, housewares, ice boxes, bowls, buckets, ketchup bottles, etc. are all made using polyethylene.
Automotive: Fuel tanks in vehicles are made using polyethylene (HDPE).
Polyethylene Prototypes using CNC, Injection Molding and 3D printer machines
Polyethylene is a great material for use with CNC machines. It comes in the form of sheet stock, rod, and many other specialty shapes thanks to its variants such as HDPE and LDPE. Polyethylene works well with a machining process that involves a mill or lathe. It has desirable properties such as decent impact strength and toughness, which translate into great machinability. The polyethylene used with CNC machines is normally black or white.
Polyethylene is currently not available in FDM or used for any 3D printing process. It is considered to be a difficult material to make 3D printed prototypes. Hence, you have to use a CNC machine to make polyethylene prototypes.
Polyethylene, being a thermoplastic, can be used in an injection molding machine because it can be re-heated, melted, and cooled without major degradation. Normally, the polyethylene is fed into the injection molding machine and heated to its melting point. The liquid form is then injected into the mold to take the desired shape. The main advantages of using polyethylene over other polymers in an injection molding process is its low cost, ease of recycling, and strong impact strength, especially at low temperatures.
Disadvantages of Polyethylene
- Polyethylene is toxic when in the liquid form. It can cause problems if it is inhaled or absorbed into the skin. Eyes also get affected by polyethylene vapor. Hence, it is extremely important to follow all safety guidelines when handling liquid polyethylene.
- Polyethylene tends to turn brittle when it is exposed to sunlight for long periods of time.
- Polyethylene is not very biodegradable. Hence it takes a long time to decompose in a land-fill and tends to accumulate.
- Polyethylene is made from hydrocarbons and fossil fuels. Hence, it is not environmentally sustainable. Its production and combustion tends to give out carbon dioxide, which is a greenhouse gas.
Properties and Specs
|Scientific Name||Polyethylene (PE)|
|Resin Identification Code||2(HDPE), 4 (LDPE), 1 (PET)|
|Tensile Strength||1000 PSI (LDPE), 2900 PSI (HDPE)|
|Dielectric Constant||2.25 at 20 oC|
|Specific Gravity||0.92 (LDPE), 0.95 (HDPE)|
|Melting Temperature||110oC (LDPE), 130oC (HDPE)|
|Flexural Strength||800 PSI (LDPE), 3000 PSI (HDPE)|
|Maximum Continuous Use Temperature||65oC|
|Shrink Rate||0.02 – 0.05 in/in|
|Izod Impact Strength||1064 J/m (LDPE), 150 J/m (HDPE)|
|Elongation at break||150% (HDPE), 400% (LDPE)|
|Rockwell Hardness||M10 (LDPE), M65 (HDPE)|
|Poisson’s Ratio (v)||0.46 (HDPE)|
|Heat Deflection Temperature||85oC at 67 PSI|
|Injection Mold Temperature (Typical)||21oC to 66oC|
|Greases and Oils||Poor/Average|
The section on 3D printing polyethylene is wholly wrong; polyethyene is very commonly used in 3D printing. I use it myself. In fact, I use more of it than any other filament chemistry, and it prints exceedingly well.