Properties of Polythene

When it was first synthesised by accident in 1898, nobody could have known what polythene would become.

Nobody could have known that the properties of polythene would eventually enable food production and logistics on such a scale, that the human population would experience food availability like never before. That it would facilitate an economic transformation, and empower the lives of billions.

And nobody could have known that it would become a threat to the environment.

But for all of this power, the properties of polythene do have limitations. And these limitations are important to know, because they dictate what polythene can and can’t be used for. So, let’s go over the physical and chemical properties of polythene – including its limits, and how we can work around them.

What is polythene?

Polythene is a type of plastic known as a polymer. It is made from a gas called ethylene. Ethylene gas is put under intense heat and pressure, which makes long, repeating chains of the gas molecules that bind together into a solid plastic.

Learn more – what is polythene?

Physical properties of polythene

Polythene is mighty stuff. Ridiculously strong for its weight, flexible, waterproof, and almost completely inert. This is why it has found a purpose in every industry you can think of. 

Let’s look at the physical properties of polythene in terms of heat, conductivity, tensile strength, and chemical resistance.

Quick facts

Operating temperature of polythene films

Low-density polythene (LDPE) typically has an operating temperature range of -40ºC to 90ºC.

LDPE retains its shape all the way down to -40ºC. But there’s a catch. Polythene becomes brittle at these temperatures, and cannot operate in sub-freezing environments for long. Polythene additives must be used to prevent cracking in poly products intended for freezer storage.

At the upper limit, polythene films can withstand up to 90ºC before they lose their shape – but again, there’s a catch. 90ºC can only be tolerated for short periods of time. Consumer-grade LDPE is typically completely molten at an average temperature of 110ºC. It begins to lose integrity long before that point.

Given this, the maximum safe operating temperature of LDPE for prolonged exposure is 65ºC. MDPE has better thermal capabilities, and a higher continuous service temperature, due to its high melting point. This means that MDPE can be used for things like hot foods and sous vide bags.

Polythene is harmful in its liquid and vapour forms, so it must never be pushed beyond its thermal limit.

Effects of UV radiation

Direct sunlight pounding down on dark polythene can cause temperatures to exceed the material’s safe limit. But in addition to this, prolonged exposure to UV light causes the polymer to break down and flake apart. UV inhibitors can counteract this effect – but high temperatures will always affect the material, because it’s a thermoplastic (meaning that heat melts it).

Tensile strength of polythene

As a thermoplastic, the mechanical performance of polythene depends on the operating temperature, which is why we covered that first. With that in mind, the following tensile properties are for room temperature polythene.

• Tensile strength at room temperature – 1,400 psi
• Tensile modulus – 57,000
• Tensile elongation at break – 100%
• Flexural modulus – 29,000 psi
• Shore hardness (D) – D45

If you have some context of these limits, you’ll see that the impact and tearing resistance of LDPE (compared to its weight) is astonishing. This is the primary reason it has become such a ubiquitous material for packaging.

Does polythene conduct electricity?

In simple terms, no – polythene does not conduct electricity.

But despite what’s taught in primary and secondary school, every material in the world conducts electricity to some degree. With enough voltage, electrical charge will pass through anything – like lightning travelling through air. So instead of a blanket yes or no, every material in the world is defined in terms of how well it conducts electricity.

The family of materials that do this very well are called conductors. Metals tend to be very good conductors, some more than others – like copper and gold.

The family of materials that conduct electricity very poorly are called insulators. Polythene is in this family, and does not conduct electricity readily. This makes it useful for electrical insulation over wiring or circuit boards, and as a packaging material for electronics (particularly anti static polythene).

Research into conductive and light emitting polymers has been ongoing since the 1990s, and under very specific circumstances, polymers can in fact become good conductors. The discovery that polyacetylene made with an overabundance of catalyst is a conductor was made decades ago, and today, there are several highly stable conductive polymers that have comparable electricity conductivity levels to copper.

That said, polythene does not fall into this group. However, as an insulator, it can be used as a flexible substrate for circuit boards or conductive ribbon connectors.

Chemical resistance of polythene

Polythene has incredible chemical resistance, and there’s only a handful of chemical compounds that it can’t be used with. It is known to be totally water resistant, able to package oils, and even has uses in fuel tanks.

It can withstand common organic solvents, degreasing agents, alcohol, and even some strong acids and bases, for prolonged periods. These properties are affected by temperature and chemical concentration – but for the most part, polythene is chemically stable in most practical applications.

It does have a major chemical weakness, in strong oxidisers. These are all the chemical compounds known to aggressively attack polythene in high concentrations:

This list is adapted from a study by SpillTech.

Using the properties of polythene beyond packaging

We have already discussed the uses of polythene extensively in our previous post, What is Polythene Used For? – where we cover everything from space exploration to salmon conservation. But the properties of polythene give it further uses, with the following specific examples from construction and agriculture.

Polythene as a vapour control barrier

Polythene is an excellent moisture barrier, due to the tight knit of the polymer chains. It’s not necessarily a gas barrier, though – some gas molecules are small enough to permeate polythene – but for general applications, it’s hard to beat.

In construction, polythene sheets are used to control the transmission of water vapour through walls, floors and roofing. By preventing moisture ingress, polythene helps maintain the integrity of building structures, preventing damp and mould growth. This can also enhance energy efficiency, by reducing heat loss and improving thermal insulation.

Polythene as electrical insulation

As discussed, polythene’s dielectric properties make it a valuable material for electrical insulation applications. Polythene effectively insulates electrical conductors, cables and wires, preventing short circuits, electric shocks and equipment damage. Polythene is found in consumer electronics, telecommunications and power distribution networks – wherever reliable insulation is important for efficient and secure operation.

Polythene’s greenhouse effect

The term “greenhouse effect” in the context of polythene refers to its role in agriculture and horticulture. Polythene films are employed as greenhouse covers, commonly as the covering for polytunnels. Transparent polythene lets solar radiation and heat inside the structure – but not all of it can escape, creating a localised climate that benefits plants. Farmers and gardeners can use polythene enclosures to extend their growing seasons, cultivate crops in non-optimal climates – and to protect plants from unexpected weather events.

Polythene also helps to regulate humidity levels, and provides a barrier against pests – giving higher crop yields and better quality. Ventilation and temperature can be controlled within to prevent overheating and to maintain optimal growing conditions.

How do the properties of polythene impact recycling?

The same piece of plastic can only be recycled about two or three times before its quality decreases to the point where it can no longer be used.

Virgin material is commonly added to up the quality, but eventually, the plastic reaches end of life. Recycling is important – and we need to keep at it. But reuse is a far better solution. So is upcycling spent plastic into bricks, or fuel.

One day, it’s all going to have to go back to the Earth. But the longer we can keep it useful, the longer it stays out of the environment. Even though polythene recycling has its limits, most material has a long way to go before it’s useless; so instead of disposal, reuse and upcycling could be the solutions to turn to.

Read more – Long-Term Solutions To Plastic Pollution

Polythene packaging, made to perform

NPF Packaging creates high-performance polythene products. We work well within the safe limits of polythene, in all our bags and sheets – and can even go beyond with additives, for bespoke requirements.

Get a quote now, or call us on 01773 820415 to start designing your polythene packaging.