What Is High-density polyethylene?
High Density Polyethylene is a polyethylene thermoplastic made from petroleum. HDPE is commonly recycled and made into composite wood or plastic lumber
HDPE is a Type 2 plastic commonly used in making containers for milk, motor oil, shampoos and conditioners, soap bottles, detergents, and bleaches.
High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called “alkathene” or “polythene” when used for HDPE pipes.
With a high strength-to-density ratio, HDPE is used in the production of plastic bottles, corrosion-resistant piping, geomembranes and plastic lumber. HDPE is commonly recycled, and has the number “2” as its resin identification code.
In 2008, the global HDPE market reached a volume of more than 30 million tons.
How is High Density Polyethylene (HDPE) Made?
HDPE is made under controlled conditions by applying intense heat to petroleum. This process, also known as “cracking,” helps create ethylene gas. During its production, the gas molecules will attach to form polymers, which then produce polyethylene.
HDPE is made in a 4-step process:
- HDPE is derived from petroleum and is a linear polymeric chain containing few branches. The process of HDPE manufacturing is different depending upon the manufacturer, but the first step usually involves taking petroleum-based hydrocarbons and applying high heat, causing the hydrocarbon chains to break down into more useful monomers such as ethylene molecules (in HDPE production, this ethylene is collected as a gas).
- After collecting the necessary monomers, metallic catalysts (titanium tetrachloride, diethylaluminum chloride, chromium oxide on silica, etc.) mix with ethylene gas and other compounds which catalyze into a slurry of polyethylene.
- This slurry is cleaned of remaining catalyst and the HDPE is recovered by extraction, filtration, and drying.
- At this point, the HDPE is processed and manufactured into stock or specific products.
This is a HIGHLY simplistic overview of the process, as polymerization organic chemistry is much more detailed (and organic chemists still do not fully understand the reaction mechanism!).
Regardless, note that HDPE is made from petroleum and is processed using catalysts to form long, linear, and dense polyethylene now known as high-density polyethylene.
Properties of High Density Polyethylene (HDPE)
HDPE is known for its high strength-to-density ratio. The density of HDPE ranges from 930 to 970 kg/m3.
Although the density of HDPE is only marginally higher than that of low-density polyethylene, HDPE has little branching, giving it stronger intermolecular forces and tensile strength (38 MPa versus 21 MPa) than LDPE.
The difference in strength exceeds the difference in density, giving HDPE a higher specific strength. It is also harder and more opaque and can withstand somewhat higher temperatures (120 °C/248 °F for short periods).
High-density polyethylene, unlike polypropylene, cannot withstand normally required autoclaving conditions. The lack of branching is ensured by an appropriate choice of catalyst (e.g., Ziegler–Natta catalysts) and reaction conditions.
HDPE is resistant to many different solvents, and is exceptionally challenging to glue; joints are typically made by welding.
The physical properties of HDPE can vary depending on the molding process that is used to manufacture a specific sample; to some degree, a determining factor is the international standardized testing methods employed to identify these properties for a specific process.
For example, in rotational molding, to identify the environmental stress crack resistance of a sample, the notched constant tensile load test (NCTL) is put to use.
Owing to these desirable properties, pipes constructed out of HDPE are ideally applicable for drinking water and waste water (storm and sewage)
Below is a table containing the material properties of HDPE. Note that each value is provided in a range, as various companies produce different blends of HDPE, each with its own set of material properties.
| Property | English | Metric |
| Density | 0.0337 – 0.0459 lb/in3 | 0.933 – 1.27 g/cm3 |
| Yield Strength (Tensile, @ -30.0 – 70.0 °C/ -22.0 – 158 °F ) | 1000 – 4350 psi | 6.89476 – 30 MPa |
| Modulus of Elasticity | 70 – 210 ksi | 0.483 – 1.45 GPa |
| Flexural Yield Strength | 2400 – 13200 psi | 16.5 – 91 MPa |
| Izod Impact, Notched | 0.375 – 14.1 ft-lb/in | 0.20 – 7.50 J/cm |
| Melting Point | 78.8 – 275 °F | 26.0 – 135 °C |
| Maximum Service Temperature (Air) | 176 – 248 °F | 80.0 – 120 °C |
| Linear Mold Shrinkage | 0.00500 – 0.0400 in/in | 0.00500 – 0.0400 cm/cm |
| Water Absorption | 0.0100 – 0.300% | 0.0100 – 0.300% |
| Coefficient of Friction | 0.200 – 0.280 | 0.200 – 0.280 |
Uses for HDPE Plastic
- Plastic bottles: Most people are informally introduced to the durable plastic through water bottles. HDPE is great for blow-molding applications, particularly food and beverage containers because it won’t leach into the bottles’ contents. In addition, HDPE is recyclable which makes it a great choice for these highly disposable products.
- Toys: HDPE is UV resistant which helps keep the toys resistant to the ray’s damaging and discoloring effects.
- Chemical containers: With its chemical resistant properties, HDPE is great for laundry, shampoo, conditioner, household cleaning products, motor oil, antifreeze and recycling bins. The strength of these bottles is increased when they’ve been colored or pigmented.
- Pipe systems: HDPE pipe grade sheet has a higher molecular weight compared to the standard HDPE used in the above examples. This strengthening with its UV-resistance makes it ideal for piping and outdoor applications. Pipe grade sheet has the ability to withstand -220 to 180 degrees Fahrenheit. It’s durable in most chemical interactions which make it useful in a lot of industrial applications.
AC Plastics has HDPE industrial sheets including pipe grade, smooth sheet, and matte sheet to suit a wide range of applications. No matter how large or small your order is, we can fulfill your order. Order today, or call us to speak to one of our specialists.
Below is a brief list of some of the many uses of HDPE plastic:
- Corrosion-resistant piping, HDPE sheet, and stock material
- Fuel tanks
- Food and beverage containers, plastic bottles, milk jugs, cups, etc.
- Shampoo/conditioner bottles, ointment tubes, personal care product containers, etc.
- Trash cans, recycle bins, plastic containers, etc.
- Bread bags, cereal box liners, food storage containers, etc.
- Laundry detergent bottles
- Recycled plastic lumber and composites
- Medical equipment
- 3D printing filament
- Boating components
- Coax cable insulators
- Sewage mains
- Pyrotechnic components
and many, many more applications.
Advantages of Using High Density Polyethylene (HDPE)
- Cost-effective
- Can withstand temperatures from -148 to 176 degrees Fahrenheit
- Non-leaching
- UV-resistant
- Dishwasher safe
- Resistant to most chemical solvents
- Stiff material
Disadvantages of Using High Density Polyethylene (HDPE)
- High thermal expansion
- Poor weathering resistance
- Subject to stress cracking
- Difficult to bond
- Flammable
- Poor temperature capability
- Sensitive to stress cracking
High Density Polyethylene (HDPE) vs Low Density Polyethylene (LDPE)
Both materials are highly durable, weldable, impact, chemical resistant in addition to malleable and manufacturable.
HDPE is a more rigid material than LDPE, but both are tough and wear-resistant. Finally, whereas LDPE is a transparent, branching variant of PE, HDPE has a linear structure and is opaque.
While both injection molding and additive manufacturing can process LDPE and HDPE, LDPE is best suited for injection molding. At the same time, HDPE can be machined utilizing CNC technology to assist in reaching precise tolerances.
High-density polyethylene and low-density polyethylene are two common polyethylenes with differing structures but similar properties. HDPE has a linear structure and is opaque, while LDPE is a transparent branched version of PE.
Both materials have excellent strength and weldability, boasting impact and chemical resistance alongside malleability and manufacturability.
While LDPE and HDPE can both be processed using injection molding and additive manufacturing, LDPE is best suited for injection molding whereas CNC machining HDPE can help achieve tight tolerances.
It’s up to product teams to do their research and determine which material is best suited for their upcoming project.