One of the most common plastics, low-density polyethylene (LDPE), is known for its unique characteristics compared to other polyethylenes. A few of these characteristics include its softness, flexibility, and transparency. It is known by its recycling code number 4 and is a safe, moisture-resistant material with good chemical resistance.
In this blog post, we’ll explore what LDPE is, its fundamental characteristics, how it’s made, its major applications, and why it’s a standard material in many products.
What is LDPE?
Chemical Structure and Properties
A thermoplastic polymer, LDPE comes from the monomer ethylene (C2H4). What sets LDPE apart from other polyethylenes is its highly branched molecular structure. The polymer chain’s branching prevents the molecules from cramming too closely together, resulting in a lower-density material (0.910–0.940 g/cm³). Low-density polyethylene has lower crystallinity and a softer, more flexible surface than high-density polyethylene and linear low-density polyethylene (LLDPE).
Key Properties of LDPE:
- Density: 0.910–0.940 g/cm³
- Melting Point: 105–115°C (221–239°F)
- Flexibility: Very high, even at low temperatures
- Transparency: Naturally transparent or translucent
- Chemical Resistance: Excellent, especially to acids, bases, and alcohols
- Impact Resistance: High, especially at low temperatures
- Electrical Insulation: Good, making it suitable for cable coverings
- Water Absorption: Very low, providing good moisture resistance
The following table is information from AZO materials.
| Property | Value |
|---|---|
|
Density (g/cm³) |
0.92 |
|
Surface Hardness |
SD48 |
|
Tensile Strength (MPa) |
10 |
|
Flexural Modulus (GPa) |
0.25 |
|
Notched Izod (kJ/m) |
1.06+ |
|
Linear Expansion (/°C x 10-5) |
20 |
|
Elongation @ Break (%) |
400 |
|
Strain @ Yield (%) |
19 |
|
Maximum Operating Temperature (°C) |
50 |
|
Water Absorption (%) |
0.01 |
|
Oxygen Index (%) |
17 |
|
Flammability UL94 |
HB |
|
Volume Resistivity (log ohm.cm) |
16 |
|
Dielectric Strength (MV/m) |
27 |
|
HDT @ 0.45 MPa (°C) |
50 |
|
HDT @ 1.80 MPa (°C) |
35 |
|
Melting Temperature Range (°C) |
120 – 260 |
|
Mould Shrinkage (%) |
3 |
|
Mould Temperature Range (°C) |
20 – 40 |
These properties stem directly from LDPE’s unique, highly branched structure, which also makes it softer and more ductile than other polyethylenes.
What is the history of LDPE, and how is it made?
Accidentally discovered by Imperial Chemical Industries scientists John C. Swallow and M.W. Perrin in 1933, the process for creating LDPE was later developed into a high-pressure method to reproduce it. In 1939, large-scale production commenced. It became necessary for several military applications during World War II, including its use in radar cable insulation. When the war ended, its use in the civilian sector took off, revolutionizing industries such as food packaging due to its low cost and flexibility.
A process known as high-pressure free-radical polymerization became the standard method for creating LPDE. Here’s an overview of the process:
- Feed Preparation: Ethylene gas is purified and compressed to extremely high pressures (1000–3000 bar).
- Polymerization: Introducing the compressed ethylene to a reactor with free-radical initiators at high temperatures (up to 300°C), causing the ethylene molecules to form long, branched polymer chains.
- Cooling and Separation: The reaction mixture is rapidly cooled, and unreacted ethylene is separated and recycled.
- Pelletizing and Additives: The polymer is extruded, cut into pellets, and dried. At this step, additives are added to impart the desired characteristics and properties.
- Packaging: The finished LDPE pellets are packaged and shipped to manufacturers for further processing.
This high-pressure process is unique to LDPE and is a key reason for its distinctive properties.
What Industries Benefit from LDPE?
Low-density polyethylene (LDPE) maintains a notable combination of chemical resistance, flexibility, low weight, and ease of processing. The following are a couple of applications where LDPE really shines:
Packaging Industry
- Plastic Bags: Grocery bags, produce bags, and trash bags are examples of LDPE, a standard material used in everyday life.
- Fims and Wraps: Films like cling wraps are used in commercial and industrial kitchens to preserve freshness and protect food products from moisture and contamination.
- Flexible Packaging: LDPE creates pouches, baggies, and other flexible food packaging solutions.
Medical and Healthcare Industries
- Medical Supplies: It is a standard material in the medical field for sterile tubing, laboratory wash bottles, and medication dispensing systems.
Consumer Products Industries
- Disposable Products: From disposable cutlery and drinkware to food packaging, LDPE is a standard material. It is notably recyclable.
- Toys: A star material for creating soft, flexible, and safe toys for children, it is beneficial due to its non-toxic and sterilization properties.
Industrial and Construction Industries
- Trays and Containers: It is corrosion-resistant, making it an excellent option for general-purpose buckets, containers, and trays.
- Computer Components: Parts made with LDPE can provide protection and insulation for sensitive electronics.
- Pipes and Tubing: Suitable for lightweight manufacturing pipes and tubes for low-pressure applications.
- Corrosion-Resistant Surfaces: Work surfaces that are constantly exposed to harsh chemicals benefit from this material.
Electrical and Electronics Industries
- Wire and Cable Insulation: With excellent electrical insulation, it is an ideal choice for insulating wires and cables.
Agriculture
- Agricultural Films: LDPE is a standard material in greenhouse production, helping to regulate humidity and temperature, while mulch films suppress the growth of weeds.
