Polyethylene (PE): What It Is, Types and Applications

Polyethylene (PE) is the highest-volume plastic produced worldwide, used in a wide range of applications, including packaging, pipelines, cable coatings and agricultural irrigation systems. Due to its widespread presence across the industry, it is essential to understand its composition, the different types available according to density, its main applications and the treatment it undergoes at the end of its useful life.

What is polyethylene and where does it come from?

Polyethylene (PE) is a thermoplastic polymer produced through the polymerization of ethylene, a gas derived from petroleum or natural gas. Of all the plastics included in the 7 standard types of plastic, it has the simplest chemical structure and is also one of the few identified by two different recycling codes: number 2 for its high-density version and number 4 for its low-density version.

As a polymer and thermoplastic, polyethylene can be melted and remoulded repeatedly without losing its essential properties. This characteristic underpins both its wide range of industrial applications and its recyclability.

Types of polyethylene according to density:

Density is the variable that determines the mechanical properties of polyethylene: the more ordered and linear its molecular chains, the higher the density and rigidity of the material; the more branched they are, the lower the density and the greater the flexibility. This property determines whether the final product is a rigid pipe or a flexible sheet.

High-Density Polyethylene (HDPE):

High-Density Polyethylene (HDPE) has few branches in its molecular structure, allowing its chains to pack more closely and achieve greater crystallinity. The result is a rigid, mechanically resistant material with good performance against heat and chemical agents. This combination of properties makes it the reference choice for water and gas pipes, rigid packaging and industrial components that must withstand pressure without deforming.

Medium-Density Polyethylene (MDPE):

Medium-Density Polyethylene (MDPE) lies between HDPE and LDPE, and its main advantage is resistance to stress cracking, a property that pure HDPE does not always guarantee. For this reason, it has become the reference material for gas distribution networks and irrigation systems, where pipes must absorb small movements without cracking.

Low-Density Polyethylene (LDPE):

Low-Density Polyethylene (LDPE) has a molecular structure opposite to that of HDPE: its high degree of branching reduces crystallinity and considerably increases flexibility. It is more translucent, has greater elongation capacity and better optical clarity, although this comes at the expense of reduced mechanical and chemical resistance. It is the predominant material used in films, bags and packaging wraps.

Linear Low-Density Polyethylene (LLDPE):

LLDPE modifies the structure of LDPE through a linear configuration with short, controlled branches, giving it greater resistance to tearing and puncture than conventional LDPE. It is commonly used in geomembranes and cable coatings, applications in which the material must withstand mechanical stress without breaking.

Properties that explain its industrial success:

Among the range of plastic properties, polyethylene stands out for several characteristics that explain its high production volume:

• Lightweight, with a low density even in its highest-density variant.
• Almost total impermeability, making it suitable for packaging and pipes.
• Good chemical resistance to acids, bases and common solvents.
• Effective electrical insulation, which is critical for cable coatings.
• Low production cost, a factor that facilitates its large-scale adoption.

Its main limitation compared with other polyolefins, such as polypropylene, is thermal resistance, as PE softens at lower temperatures.

Applications and Industries Using Polyethylene

The versatility of its four types allows polyethylene to be used across a wide range of industrial sectors. In packaging, it is the predominant material for bags, stretch film and flexible packaging. In construction, HDPE and MDPE are the standard materials for water and gas networks. In the agricultural sector, LDPE is used in greenhouses and irrigation systems, while in the electrical sector, LLDPE and LDPE protect cables and conduits.

It also has a significant presence in the domestic sphere, in products such as kitchenware, detergent containers and toys, among many other everyday applications made from this material.

The Polyethylene Recycling Cycle

Being the world’s highest-volume plastic also means being one of the materials with the highest recycling volumes. Identification codes 2 and 4, assigned to HDPE and LDPE respectively, allow each density to be processed on its own recycling line, preventing the mixing of materials with different behaviours.

From Waste to Recycled Pellets

The process follows the same stages of plastic recycling as any other polymer: collection, separation by density, washing, shredding and extrusion to obtain recycled pellets ready to be reincorporated into a new production process.

What Is Made from Recycled Polyethylene?

The recycled material retains a significant proportion of the original material’s mechanical properties, which explains the numerous applications of recycled polyethylene available today, ranging from bags and film to technical components, without this resulting in any significant loss of quality for the manufacturer incorporating it into its production process.

Frequently Asked Questions

Why Does Polyethylene Have Two Different Recycling Codes?

Because HDPE and LDPE, although they originate from the same monomer, have sufficiently different densities and behaviours to require separate processing. Codes 2 and 4 allow each type to follow its own recycling line.

Can HDPE and LDPE Be Recycled Together?

It is not recommended. Mixing them reduces the quality of the resulting material, which is why recycling plants separate them before processing, each through its corresponding line.

Which Type of Polyethylene Is More Impact-Resistant, HDPE or LLDPE?

It depends on the type of stress applied: HDPE performs better under pressure and structural loads, while LLDPE offers greater resistance to tearing and puncture in flexible materials.

Does Polyethylene Withstand Exposure to Sunlight and Weather Conditions Well?

Naturally, prolonged exposure to UV radiation can degrade it over time. For this reason, many outdoor applications, such as agricultural films, incorporate specific additives to extend their service life against solar radiation.

How Does Polyethylene Differ from Polypropylene?

Polyethylene offers greater flexibility and lower thermal resistance, while polypropylene is more rigid and withstands higher temperatures better. Both are polyolefins and are recycled through similar mechanical processes, although each has its own production line.

A Real Commitment to Recycled Polyethylene

HDPE, MDPE, LDPE and LLDPE exhibit different behaviours: some are rigid, others flexible; some are designed for pipes and others for lightweight films. However, they all share the same chemical origin and the same ability to be melted and reincorporated into a new production cycle, a property that not all plastics retain intact after several cycles of use.

At SINTAC Recycling, we are a leading reference in recycled polyethylene, with more than 35 years of specialisation and technical expertise. This experience positions us as a trusted partner for industry, providing solutions with the quality, traceability and consistency demanded by the market.