PP vs PE: which is best for your component?

Anyone looking for differences between polypropylene (PP) and polyethylene (PE) may struggle to see them at first, but this guide outlines the differences in Polypropylene and polyethylene. These two both belong to the thermoplastic family but have distinct characteristics and in this article we cover:

What is polypropylene?
What are polypropylene’s properties?
Does polypropylene have fatigue resistance?
What is polyethylene?
What is polyethylene used for?
Can you recycle polypropylene and polyethylene?
Characteristics of polypropylene and polyethylene
When should you use PP vs PE?

Although PP and PE (polypropylene and polyethylene) both share similar material properties, there are pros and cons to each you need to know about when selecting the right injection molded component. 

What is polypropylene?

Polypropylene (PP) is a semi-crystalline thermoplastic made from the polymerisation of propylene monomers. Otherwise known as polypropene, its chemical formula was discovered in 1951 by Paul Hogan and Robert Banks before being refined into a resin for mass production by Professor Giulio Natta in 1954.

Polypropylene is widely used in its homopolymer form, which contains only propylene monomers. Adding ethylene into the polymerisation process creates PP copolymers. These have slightly different characteristics to the virgin material, including varying melting temperatures between 266°F and 338°F. This makes the PP better at higher temperatures.

PP random copolymers are made when ethylene or butene bonds are introduced in the polymer chain and up to 6% of the polymer's mass is ethene that's spread at different points on the molecular chain.

PP block copolymer is when 5% to 15% of the mass is ethene integrated in a regular pattern across the chain.

Learn more about polypropylene in our guide, Everything you need to know about PP plastic.

What are polypropylene’s properties?

The popularity of PP is due to its beneficial material characteristics. These properties include:

• Flexibility – the semi-crystalline structure of PP means it can be bent, moved and compressed without completely losing its shape.
• Toughness – PP’s ability to move and flex is combined with its ability to perform its function under stress and fatigue.
• High resistance – to water, erosion from certain chemicals, dilute acids and alkalis, melting or deforming from heat. It also insulates from electricity.
• Lightweight – PP has one of the lowest densities of all polymers, though this can vary depending on the specific molecular structure.
• Variable translucency – the chemical composition of the PP means it can range from opaque to transparent.

These properties and the ability to adapt PP’s chemical nature means its suitable for a range of applications, including packaging materials:

• Medical equipment such as syringes, files, and dishes.
• Automotive industry for details and trims such buttons and instruments.
• Food packaging, film, plates, and trays.
• Plastic bottles and tubs containing cosmetics and cleaning products.
• Fibers for bags, ropes, clothing, and carpets.

Does polypropylene have fatigue resistance?

Polypropylene is often used in applications where a high resistance to repeated loading is necessary. That is with and without reinforcements and fillers.

The thermoplastic nature of PP also means it’s a great option to use the injection molding process to make plastic components. Polypropylene components include feet, rivets, plugs, cable ties and knobs used in manufacturing sectors.

What is polyethylene?

Polyethylene (PE) is also a semi-crystalline thermoplastic. However, unlike PP, it’s made from the polymerisation of ethylene monomers. Classified as a polyolefin and known as polyethene, the manufacturing process for PE was also developed in the 1950s by German Karl Ziegler and Italian Giulio Natta. Indeed, one of the processes used to create PE is called the Ziegler-Natta catalyst.

The specific processes used to create PE with different densities. This is because the molecular structure of the PE changes according to the process, meaning they have different levels of amorphous and crystalline qualities. This variability in density is a quality that’s unique to PE and enables the creation of polymers with a variety of characteristics.

There are two main types of polyethene and each has its own qualities and best uses. These are: high density polyethylene and low density polyethylene. Specifically, if the molecular structure is a linear chain, it is a high density polyethylene HDPE, because of the closeness of the molecules. With its higher density, HDPE is more rigid. If the structure is branched, the links between molecules are greater, meaning it's low density polyethylene LDPE. Chains with shorter branches lead to the creation of linear low density polyethylene (LLDPE), which has a molecular structure that balances both amorphous and crystalline qualities.

What is polyethylene used for?

The different densities of PE have various molecular structures. This also means the physical properties of each type of PE (high density polythylene HDPE and low density polythylene LDPE) is slightly different. However, there are some common qualities and chemical properties, such as heat resistance, that all PE polymers share, including:

• Good resistance to water, electricity, heat, friction, and chemical resistant.
• High levels of tensile strength and toughness.
• Varying translucency, from opaque to transparent.
• Good flexibility, allowing it to be bent without losing its shape.

Note however, that despite having low toxicity it is low on the limiting oxygen index (LOI) and can drip during burning which can lead to flames spreading rapidly. Sometimes, however, it is sometimes possible to add a flame retardant during processing.

It has a heat resistance of up to 276°F but the thermal resistance/strength is low compared with some other plastics.

These beneficial qualities make PE suitable for many different uses and products, including for the packaging industry:

• Components in consumer appliances
• Medical protective equipment, patches, and containers.
• Hydraulic pipes, hoses and plumbing fittings for industrial applications.
• Packaging including films, trays, bottle caps and plastic bags.

Being a thermoplastic with a melting point of between 230°F and 266°F, PE can also be used to create injection molded components such as rivets, inserts, feet and caps.

Recycling process: can you recycle polypropylene, and can you recycle polyethylene?

Polypropylene can be recycled and polyethylene can be recycled. Indeed, HDPE, LDPE and PP are all classified under Resin Identification Code (RIC) recycling symbols two, four and five respectively.

Able to be cleaned, melted down and reformed into pellets or other products (such as automotive parts, food packaging and injection molded components) through heating processes such as injection molding, both PE and PP also have low levels of toxicity. This means potentially damaging gases aren’t released when the polymers are reformed.

However, as with other plastics, there are difficulties in recycling PP and PE when they are mixed with other polymers. This is because extra processing is required to separate each type of resin to then break it down into reusable plastics. 

By choosing Essentra Components for PP and PE components, businesses can benefit from manufacturer’s expertize in polymers and the injection molding process to get the best quality parts with minimal environmental impact. This includes a range of polypropylene security seals using a minimum of 45% recycled material. 

Characteristics of polypropylene and polyethylene

When should you use PP vs PE?

Though PP and PE share very similar characteristics and are similarly cost effective, there are some subtle differences between them. Businesses need to consider this when selecting their final injection molded components:

• They are made from different types of monomers, propylene and ethylene.
• Polypropylene vs polyethylene UV resistance: PP has little UV resistance compared to PE.
• PE has a lower melting point than PP.
• PE can take a transparent form while PP can only be made translucent.
• High chemical resistance: PP is slightly more rigid and chemically resistant than PE.

It should also be noted that the exact nature of each type of PP and density of PE means they have particular characteristics. So, when making the final decision as to which polymer might be best for your component, it’s important to get advice from an expert from the plastic industry and in injection molding manufacturing.

These experts will help you understand the huge variety of PE and PP derivatives available so you can find the best option for your needs. They will also be able to balance the type of plastic with the mold and injection molding process needed to make a specific component. This will ensure the final component is high quality and has all the characteristics you require.