What Is Polyethylene? LDPE, MDPE and HDPE Explained
Polyethylene (PE) is the raw material behind products as different as a carrier bag and a water pipe. They all start from the same molecule; what separates them is how the polymer chains are arranged and how dense the resulting material is.
For a packaging buyer, the grade matters more than the family name. This article covers how the grades differ, what the resin identification code does and does not tell you, and how food-contact suitability is actually demonstrated in the EU. It closes with what applies at DEKA Plastik.
What is polyethylene?
Polyethylene is a polymer built from long chains of ethylene (C₂H₄) units, and it is the most commonly produced plastic in the world. As of 2017, more than 100 million tonnes of polyethylene resin were produced each year, around 34% of the total plastics market (source).
Being a thermoplastic, it softens when heated and sets again on cooling, which is why the same material can be blown into film, moulded into bottles or extruded into pipe.
What are the main types of polyethylene?
Polyethylene grades are classified mainly by density, which in turn follows from how branched the molecular chains are. More branching keeps the chains from packing tightly, so the material becomes lighter, softer and more flexible.
| Grade | Full name | Density (g/cm³) | Chain structure |
|---|---|---|---|
| LDPE | Low-density polyethylene | 0.910–0.940 | Many short and long branches |
| LLDPE | Linear low-density polyethylene | 0.915–0.925 | Substantially linear, many short branches |
| MDPE | Medium-density polyethylene | 0.926–0.940 | Intermediate |
| HDPE | High-density polyethylene | 0.941 and above | Few branches, linear |
Ranges and structural descriptions are taken from the Wikipedia article on polyethylene. The overlap between ranges is real: the boundaries are not sharp, and two resins sold under the same abbreviation can behave differently.
What is LDPE?
LDPE is low-density polyethylene, the first polyethylene grade ever made. It is produced at high pressure by free-radical polymerisation, and its single most common use is plastic bags (source).
Its heavily branched chains make it soft and pliable, with a typical melting point of 105–115 °C for commercial grades (source).
What are MDPE and LLDPE?
MDPE is medium-density polyethylene, with a density of 0.926–0.940 g/cm³ and a structure between LDPE and HDPE. Its melting behaviour sits with HDPE, typically 120–130 °C (source).
LLDPE, linear low-density polyethylene, has a density close to LDPE (0.915–0.925 g/cm³) but is made differently: by coordination polymerisation, for example with Ziegler–Natta catalysts, rather than by free radicals. The result is a substantially linear chain carrying many short branches (source).
What is HDPE?
HDPE is high-density polyethylene. With few branches, its chains pack closely, giving it a high strength-to-density ratio: tensile strength is around 38 MPa against 21 MPa for LDPE (source).
HDPE is familiar from bottles, jerrycans and pipes, but it is also a film resin: blown-film carrier bags with handles are also made from HDPE (source). It typically melts at 120–130 °C (source).
How do LDPE and HDPE compare?
The core difference between LDPE and HDPE is chain architecture: branched LDPE is soft and flexible, linear HDPE is stiffer and stronger. Most of the other differences follow from that.
| LDPE | HDPE | |
|---|---|---|
| Chain structure | Highly branched | Few branches, linear |
| Density | 0.910–0.940 g/cm³ | 0.941 g/cm³ and above |
| Typical melting point | 105–115 °C | 120–130 °C |
| Tensile strength | ~21 MPa | ~38 MPa |
| Resin code | 4 | 2 |
| Typical uses | Bags, film | Bottles, pipes, containers, carrier bags |
Sources: polyethylene, LDPE, HDPE.
Does resin code 4 mean a bag is recyclable?
No. The resin identification code identifies the polymer, nothing more: 4 for LDPE, 2 for HDPE. Since 2008 the codes have been administered by ASTM under standard D7611, and a code on a product does not indicate that it will be recycled where it is discarded (source).
The symbol itself was changed for that reason. Later revisions replaced the chasing arrows with a solid triangle to address consumer confusion about what the code means (source). For EU buyers, what the Packaging and Packaging Waste Regulation requires is a separate question, covered in our PPWR explainer.
Is polyethylene safe for food contact?
Polyethylene is widely used in food packaging, but suitability is shown by compliance and testing, not by the name of the material. In the EU, Regulation (EC) No 1935/2004 is the framework for all food contact materials, and Regulation (EU) No 10/2011 is the most comprehensive specific measure for plastics (source).
That regulation caps the combined transfer of substances into food with an overall migration limit of 10 mg/dm² of contact surface, or 60 mg/kg of food (source). In practice, a buyer asks the converter for a declaration of compliance and the supporting migration results for the specific product.
How is polyethylene film made?
Most packaging film is produced by blown (tubular) extrusion. Molten polyethylene is forced through a ring-shaped die and air is blown into the extruded tube to expand it, stretching and thinning the film (source).
Low-, medium-, high-density and linear low-density polyethylene can all be processed this way (source). The film then typically goes on to printing and cut-and-seal conversion; printing is covered in our article on flexographic printing.
What applies at DEKA Plastik
DEKA Plastik extrudes LDPE, MDPE and HDPE polyethylene film in its own production facilities, and carries out flexographic printing and cut-and-seal conversion on that film in-house.
Which grade goes into which product depends on the product. For some products the grade is fixed: our ziplock bags and zipper bags are made from MDPE and LDPE, and our perforated rolls from LDPE. For plastic bags and our other polyethylene bags, grade and gauge are set per order.
For several of our PE bag lines, material composition is documented in a numbered declaration prepared under the PPWR. Its scope is “garment and jacket bags, mailing bags, retail carrier bags and general purpose PE bags”. For those products, the declaration states that the film is blown in our facilities as a single layer and that the product is 100% polyethylene — no lamination, no co-extruded barrier layer, no metallised layer, and no paper, aluminium or polyamide. Printing ink is a surface coating, so printed and unprinted bags have the same composition. On request, 20% to 100% recycled polyethylene can be used and the product remains mono-material. For garment bags, the declaration covers the PE variant only; the OPP/CPP variant is outside its scope. Ziplock and zipper bags and our roll products are not named in the declaration’s scope either. Details are on our PPWR compliance page.
For food contact, products made in our facilities and intended to touch food fall under our food-contact declaration and are made from virgin polymer only. According to our PE bag technical data, overall migration is below 10 mg/dm² in 10-day tests at 40 °C across four food simulants (TS EN 1186-1/-5), with compliance declared against Regulation (EU) No 10/2011, Regulation (EC) No 1935/2004 and Regulation (EC) No 2023/2006. Production runs under an ISO 9001:2015 and ISO 22000:2018 certified system.
If you need help choosing a polyethylene grade for your application, get in touch.