HDPE and LDPE have many similarities. They’re made of the same polymerised ethylene molecules, and both have a very high tensile strength and impact resistance, with a low material weight. They are both resistant to chemical attack and moisture – and they’re equally recyclable.
But they each have very different structures, which gives them different properties and use cases. One is highly flexible, while the other is rigid. One can be crystal clear, while the other is more opaque by nature.
And even though they’re made of the same molecules, they can’t be recycled together, because of the different ways they are manufactured.
Polythene manufacturing: how HDPE and LDPE are made
LDPE is produced by chemical synthesis from ethylene, a hydrocarbon gas.
Single molecules of ethylene are linked together into chains under high temperatures and pressures. Catalysts added to the process promote a chemical reaction that allows single molecules to bond together and form long, branching chains. The transparent raw material can then be melted down into pellets of LDPE.
Read more: How Is Polythene Made?
HDPE is created in a very different way, in a process called cracking. Intense heat is applied to petroleum, which creates ethylene gas. The intensity of the heat causes the free-floating gas molecules to link together, forming polymers with a more crystalline structure, as opposed to branches.
Cracking gives the resulting polythene a sludgy appearance, but after further processing, it forms cloudy granules ready to be turned into any number of products that we all use day to day.
Physical properties and uses of LDPE
LDPE is soft and flexible. It can be made with excellent transparency, and extruded to extremely narrow widths without losing tensile strength. This makes it an excellent material for producing rolls of polythene film, sheets, and polythene bags. While LDPE can be made thicker for a variety of purposes, it is more likely to crack under mechanical stress than HDPE. This is actually beneficial in applications like car body parts; it’s lightweight, and offers cushioning in the event of an impact.
LDPE is used widely in commercial packaging, consumer packaging, logistics – as well as in crop farming, electronics and critical medical applications.
Read more: Every Use For Polythene
Physical properties and uses of HDPE
HDPE is rigid and durable, even at narrow gauges. It has a high degree of chemical resistance and a slightly higher melting point than LDPE, which can be helpful in some applications. HDPE’s more crystalline structure gives it greater strength, and also contributes to its natural opacity.
You’ve most likely directly experienced HDPE as packaging; it’s the material that supermarket milk bottles and laundry detergent bottles are made of. But it’s also an excellent building material, used for plumbing and pipework – as well as fuel tanks in car-making. It can be moulded into virtually any shape and is resistant to chemical attack from fuels and oils, meaning automotive manufacturers can make complex fuel tank shapes that save space and reduce weight.
Recycling HDPE and LDPE
LDPE has recycling number 4, and HDPE has recycling number 2. Both HDPE and LDPE are widely recycled, but despite their similarities, they must be separated and recycled separately.
This is because HDPE is denser than LDPE – and if the two materials were melted and combined, the resulting blend would be unstable, disintegrating with little effort applied.
Your commercial polythene packaging experts
The friendly, knowledgeable team at NPF Packaging are experts in fully recyclable commercial polythene packaging. We’ll help you find the right packaging solution for your business. Get a quote now, or call us on 01773 820415.