What is Low-Density Polyethylene (LDPE)?

Low-density polyethylene is a type of thermoplastic in the polyethylene family. It’s formed of long chains of ethylene molecules called monomers.

Its chemical formula is (C₂H₄)_n— the same as high-density polyethylene. Their differences lie in density, as their names suggest. Once LDPE is created, it’s typically flexible and transparent in color.

It’s a common choice for manufacturing plastic goods like cling wrap and juice containers because it’s corrosion-resistant, flexible, durable, and low-cost.

On products, you’ll recognize it through the number four in a triangle (usually on the bottom or side of the item).

How LDPE Is Made?

LDPE is prepared from gaseous ethylene under very high pressures (up to about 350 megapascals, or 50,000 pounds per square inch) and high temperatures (up to about 350 °C [660 °F]) in the presence of oxide initiators.

These processes yield a polymer structure with both long and short branches. Because the branches prevent the polyethylene molecules from packing closely together in hard, stiff, crystalline arrangements, LDPE is a very flexible ma terial. Its melting point is approximately 110 °C (230 °F).

To make low-density polyethylene, you’ll need to call on the radical polymerization process. This involves extreme heat (up to 570 degrees Kelvin) and an immense amount of pressure (up to 3,000 atm).

First, the raw petroleum-based materials are broken down into small molecules, then the ethylene gas is separated and funneled into a reactor.

In the reactor, it’ll come into contact with an initiator like oxygen or organic peroxide. Then, the polymerization will begin.

Low-density polyethylene can shapeshift a little, becoming pellets or granules. These formats make it easy to turn the material into other products.

LDPE is a common material for injection molding and plastic extrusion processes. Other fabrication processes that can use this type of polyethylene include:

Blow molding
Thermoforming
Vacuum casting
Film making

Properties Low-Density Polyethylene (LDPE)

LDPE (low density polyethylene) is a soft, flexible, lightweight plastic material. LDPE is noted for its low temperature flexibility, toughness, and corrosion resistance.

It is not suited for applications where stiffness, high temperature resistance and structural strength are required.

It is often used for orthotics and prosthetics. LDPE has good chemical and impact resistance and is easy to fabricate and form.

Below is a table that shares the different properties of low-density polyethylene:

Property	English	Metric
Density	0.0331-0.0347 (lb/in3)	0.915-0.960 (g/cm3)
Tensile Strength, Yield	1100-2030 (psi)	7.60-14.0 (MPa)
Tensile Strength, Ultimate	1300-7830 (psi)	8.96-54.0 (MPa)
Modulus of Elasticity	22.0-42.1 (ksi)	0.152-0.290 (GPa)
Elongation at Break	226-650 (%)	226-650 (%)
Hardness, Shore D	42.0-56.0	42.0-56.0
Melting Point	216-235 (oF)	102-113 (oC)
Processing Temperature	293-644 (oF)	145-340 (oC)
Moisture Vapor Transmission (@37.8oC/100oF)	1.04 (cc-mil/in2-24hr-atm)	0.410 (cc-mm/m2-24hr-atm)
Water Vapor Transmission	0.0167-1.55 (g/100in2/day)	0.260-24.0 (g/m2/day)

Applications Low-Density Polyethylene (LDPE)

Polyolefins (LDPE, HDPE, PP) are a major type of thermoplastic. LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic parts for computer components, and various molded laboratory equipment.

Its most common use is in plastic bags. Other products made from it include:

Trays and general-purpose containers.
Corrosion-resistant work surfaces.
Parts that need to be weldable and machinable.
Parts that require flexibility, for which it serves very well.
Very soft and pliable parts such as snap-on lids.
Six-pack rings.
Juice and milk cartons are made of liquid packaging board, a laminate of paperboard and LDPE (as the waterproof inner and outer layer), and often with of a layer of aluminum foil (thus becoming aseptic packaging).
Packaging for computer hardware, such as hard disk drives, screen cards, and optical disc drives.
Playground slides.
Plastic wraps.
Plastic bags.
Plastic containers.
Pipes.
Housewares.
Battery cases.
Automotive parts.
Electrical components.

What is LDPE plastic used for?

LDPE is used for manufacturing various containers, dispensing and squeeze bottles, tubing, plastic parts of computer components, moulded laboratory equipment and many caps and closures. It is robust enough to be almost unbreakable.

It’s most common use is in plastic bags for food shopping and also in the dry cleaning plastic covers that protect your newly cleaned clothes.

Commercially, it’s mainly used as plastic film and packaging to protect a number of products, using it’s tough, flexible and transparent qualities.

LDPE Plastic Recycling Process

During the recycling process, used LDPE is chopped up into small pieces and then formed into pellets.

These pellets can then be turned into a variety of products and may have a small amount of virgin LDPE added to improve tensile strength and reliability.

Many recycled LDPE products will be black in colour as it’s easier to turn clear plastic black, hence the use in bin liners, agricultural film, irrigation pipes, floor tiles and furniture.

Processing LDPE is one of the easiest plastics to work with. It offers the potential for injection, film and blow moulding and options to make resulting plastic material that is anti-slip, anti-static, anti-bacterial and flame retardant.

The challenge for the recycling facility will be to clean and decontaminate the collected plastic packaging in an efficient way.

Advantages and Disadvantages

Low density polyethylene has good chemical resistance and electrical properties, but it has a high thermal expansion and tends to fail under mechanical and thermal stress.

Glass reinforcement improves its thermal expansion, strength, rigidity and temperature resistance.

There are good reasons to use low-density polyethylene, but depending on the type of product you’re manufacturing, there are also disadvantages to be wary of.

Advantages.

Flexible: LDPE bends, stretches, and has low crystallinity.
Moisture resistance: LDPE has no problem deflecting moisture and water.
Impact and chemical resistance: LDPE won’t break down if it’s exposed to diluted or concentrated chemicals.
Low price point: LDPE is fairly cheap to produce and can be recycled.

Disadvantages:

Cracking and lower strength: LDPE might crack or break when it has to hold heavier objects.
Has a Max Temperature: LDPE can handle pretty hot temperatures, but above 221 degrees Fahrenheit, it will start melting.
High gas permeability: Gasses like carbon dioxide can easily slide through LDPE.
Poor UV resistance: LDPE struggles to keep its shape, color, and integrity when it’s exposed to UV rays.
Recycling can be tricky: LDPE is recyclable, but not every product made of it can go in standard home or business recycling containers. It will need to be sorted properly or specially processed at particular facilities, especially for softer, more flexible items like cling film.

Low-Density vs High-Density Polyethylene

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. A trusted manufacturing partner can help you make these decisions confidently.