When it was first synthesised by accident in 1898, nobody could have known that polythene would become a wonder material. That it would enable food production and logistics on such a scale, that the human population would experience food availability that we never had before. That it would facilitate an economic transformation, and empower the lives of billions.
Or that it would become a threat to the environment.
But for all of its power, polythene packaging does have its limits – even when we want to recycle it. These limitations are important to know, because they dictate what polythene can’t be used for.
Learn about polythene trade tolerances
Thankfully, the list is small, considering that polythene has a use in practically everything we do as humans. So, let’s go over the physical (and chemical, for that matter) limits of polythene – including the limitations of recycling, and what we can do about it.
How far can polythene be pushed?
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. But some things are too much, even for this beast of a material – and they come down to heat, tensile strength, and chemical resistance.
Operating temperatures of polythene films
Polythene has a maximum and minimum temperature range that it can safely work in.
At the lower end of the scale, low-density polythene (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 far before that point.
Direct sunlight pounding down on dark polythene can cause temperatures to exceed this limit. What’s more, 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).
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.
Tensile strength of polythene
As a thermoplastic, temperature has an effect on the mechanical performance of polythene – which is why we covered that first. At room temperature, polythene has the following tensile properties:
- 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 can read these numbers and 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.
Chemical resistance of polythene
Polythene has incredible chemical resistance, and can’t be used with only a handful of chemical compounds. 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:
|Chemicals known to attack polythene|
|Acetic Acid||Ethylene Dichloride|
|Amyl Chloride||Ethylene Oxide|
|Aqua Regia||Furfuryl Alcohol|
|Bromine Liquid||Isopropyl Ether|
|Bromine Water||Methyl Bromide|
|Butyl Phenol||Oleum Concentrate|
|Carbon Tetrachloride||Sulfuric Acid Fuming|
|Chlorosulfonic Acid 100%||Tetrachloroethylene|
|Diphenyl Oxide||Trioctyl Phosphate|
|Ethyl Chloride||Wax Alcohol|
This list is adapted from a study by SpillTech.
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.