What is PE, and What is it Used For?
Polyethylene is a thermoplastic polymer with a variable crystalline structure and a vast range of applications depending on the particular type. It is one of the most widely produced plastics in the world, with tens of millions of tons produced worldwide each year. The commercial process (the Ziegler-Natta catalysts) that made PE such a success was developed in the 1950s by two scientists, Karl Ziegler of Germany and Giulio Natta of Italy.
There are several types of polyethylene, and each one is best suited for a different set of applications. Generally speaking, High-Density Polyethylene (HDPE) is much more crystalline, and is often used in entirely different circumstances than Low-Density Polyethylene (LDPE). For example, LDPE is widely used in plastic packaging, such as for grocery bags or plastic wrap. HDPE, by contrast, has common applications in construction (for example, in its use in the fabrication of drain pipes). Ultrahigh Molecular Weight Polyethylene (UHMW) has high-performance applications in things such as medical devices and bulletproof vests.
What Are the Different Types of Polyethylene?
Polyethylene is commonly categorized into one of several major compounds, the most common of which include LDPE, LLDPE, HDPE, and Ultrahigh Molecular Weight Polypropylene. Other variants include Medium Density Polyethylene (MDPE), Ultra-low-molecular-weight polyethylene (ULMWPE or PE-WAX), High-molecular-weight polyethylene (HMWPE), High-density cross-linked polyethylene (HDXLPE), Cross-linked polyethylene (PEX or XLPE), Very-low-density polyethylene (VLDPE), and Chlorinated polyethylene (CPE).
- Low-Density Polyethylene (LDPE) is a very flexible material with unique flow properties that makes it particularly suitable for shopping bags and other plastic film applications. LDPE has high ductility but low tensile strength, which is evident in the real world by its propensity to stretch when strained.
- Linear Low-Density Polyethylene (LLDPE) is very similar to LDPE, but offers added advantages. Specifically, the properties of LLDPE can be altered by adjusting the formula constituents, and the overall production process for LLDPE is typically less energy-intensive than LDPE.
- High-Density Polyethylene (HDPE) is a robust, moderately stiff plastic with a highly crystalline structure. It is frequently used in plastic for milk cartons, laundry detergent, garbage bins, and cutting boards.
- Ultrahigh Molecular Weight Polyethylene (UHMW) is an extremely dense version of polyethylene, with molecular weights typically an order of magnitude greater than HDPE. It can be spun into threads with tensile strengths many times greater than steel and is frequently incorporated into bulletproof vests and other high-performance equipment.
What are the Characteristics of Polyethylene?
Now that we know what it is used for, let’s examine some of the key properties of polyethylene. PE is classified as a “thermoplastic” (as opposed to “thermoset”), based on the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (110-130 degrees Celsius in the case of LDPE and HDPE respectively). A useful attribute of thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polyethylene liquefy, which allows them to be easily injection molded and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy), resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time, it would burn. This characteristic makes thermoset materials poor candidates for recycling.
Different types of polyethylene exhibit wide variability in their crystalline structures. The less crystalline (or amorphous) a plastic is, the more it demonstrates a tendency to soften gradually; that is, the plastic will have a wider range between their glass transition temperature and their melting point. Crystalline plastics, by contrast, exhibit a rather sharp transition from solid to liquid.
Polyethylene is a homopolymer, as it is composed of a single monomer constituent (in this case, ethylene: CH2=CH2).
Why is Polyethylene Used So Often?
Polyethylene is an incredibly useful commodity plastic, especially among product design companies. Because of the diversity of PE variants, it is incorporated into a wide range of applications. Unless it is required for a specific application, we don’t typically use polyethylene as part of the design process at Creative Mechanisms. For some projects, a part that will eventually be mass-produced in PE can be prototyped with other, more prototype-friendly materials like ABS.
PE is not available as a 3D printable material. It can be CNC machined or vacuum formed.
How is PE Made?
Polyethylene, like other plastics, starts with the distillation of hydrocarbon fuels (ethane in this case) into lighter groups called “fractions,” some of which are combined with other catalysts to produce plastics (typically via polymerization or polycondensation). You can read about the process in greater depth here.
PE for Prototype Development on CNC Machines and 3D Printers
PE is available in sheet stock, rods, and even specialty shapes in a multitude of variants (LDPE, HDPE, etc.), making it a good candidate for subtractive machining processes on a mill or lathe. Colors are usually limited to white and black.
PE is not currently available for FDM or any other 3D printing process (at least not from the two major suppliers: Stratasys and 3D Systems). PE is similar to PP in that it can be challenging to prototype with. You are pretty much stuck with CNC machining or Vacuum forming if you need to use it in your prototype development process.
Is PE Toxic?
In solid form, no. Polyethylene is often used in food handling. It could be toxic if inhaled and/or absorbed into the skin or eyes as a vapor or liquid (i.e., during manufacturing processes). Be careful and follow handling instructions for molten polymer in particular.
What are the Disadvantages of Polyethylene?
Polyethylene is generally more expensive than polypropylene (which can be used in similar part applications). PE is the second only to PP as the best choice for living hinges,.
If your company requires the use of PE to power your product, turn to a product design firm that knows the pros and cons of polyethylene and will be able to find a way to implement it or find a better substitute. To set up an appointment with the Creative Mechanisms team, contact us today.