Low-Density Polyethylene (LDPE): Structure, Properties, and Characterization

What is LDPE?

Low-density polyethylene (LDPE) is a polyethylene made by high-pressure, free-radical polymerization of ethylene. The conditions of that process build a highly branched molecular chain, and that branching is what gives the material its low crystallinity and low density characteristics.

The “low density” in the name is therefore a direct consequence of branching. Branches interrupt the regular packing of chains, so fewer regions can crystallize, and a less crystalline polymer is also a less dense one.

It helps to place LDPE against its relatives. High-density polyethylene (HDPE) has little branching, packs tightly, and is stiffer and stronger. Linear low-density polyethylene (LLDPE) has a linear backbone with short branches but no significant long-chain branching, which gives it a different property balance again. LDPE sits apart from both because it carries both kinds of branching.

Molecular structure of LDPE

LDPE is characterized by a complex architecture and a wide variety of branching structures that result in a broad molecular weight distribution (MWD).

LDPE carries both long-chain branching (LCB) and short-chain branching (SCB), and having both is a distinctive feature. Both branch types come from the high-pressure, free-radical process. Short-chain branches form when the growing radical reacts back on its own chain a few carbons down, a “backbiting” step. Long-chain branches form when a radical transfers to an already-formed polymer chain and growth continues from that point.

This branched architecture is the root cause of nearly every LDPE property that follows: branching lowers crystallinity, lower crystallinity lowers density and melting range, and the same loss of close chain-packing raises flexibility. Long-chain branching in particular raises melt strength, which matters later when the product reaches processing. The degree of LCB and the breadth of the MWD also depend on how that high-pressure process is run.

How LDPE is made: autoclave vs tubular reactors

Commercial LDPE can be made by two different high-pressure reactor types: autoclave and tubular. The autoclave is a continuous stirred tank reactor (CSTR), where the polymer is back-mixed with the incoming ethylene feed. In a CSTR environment, some polymer chains may exit the reactor very quickly, while others remain inside for a longer period than the average residence time, resulting in a broad residence-time distribution. This broad residence-time distribution is the fundamental reason why autoclave-produced LDPE has a broader MWD and a higher level of LCB compared to tubular LDPE. Longer residence times for certain chains provide more opportunities for intermolecular chain transfer reactions, which generate LCB.

Autoclave reactor generates a higher level of LCB in LDPE

The tubular reactor consists of a folded high-strength steel tube that can span several kilometres. The reaction mixture flows at a linear velocity, resulting in all the polymer chains having a similarly short residence time in the reactor and a narrow residence-time distribution. As a consequence, the polymer chains have fewer opportunities for intermolecular chain transfer reactions, and the resulting product is a more uniform LDPE with fewer long-chain branches.

Tubular reactor generates fewer LCB in LDPE production

Autoclave LDPE (higher-LCB, broader-MWD) resins tend to suit extrusion coating, foam extrusion, and heavy-duty industrial packaging, while tubular LDPE (lower-LCB, narrower-MWD) resins tend to favour optical clarity.

Resolving these branching and MWD differences is what high-temperature GPC with multi-detection does, covered in the section Advanced Characterization of LDPE below.

LDPE Properties

Physical and thermal properties

A polymer’s degree of branching impacts its crystallinity. Both SCB and LCB lower the crystallinity of polyethylene, as they act as chain defects that interrupt the regular, 3-dimensional packing of ethylene units. This interruption reduces the ability of the chains to form organized structures or crystals. This weaker structural arrangement requires significantly less thermal energy to break apart and transition into a liquid.

LDPE’s crystallinity sits in the low range (45 to 60%), and its density is correspondingly low (0.915 to 0.935 g/cm³).

The low crystallinity of LDPE means that it melts at lower temperatures than their more crystalline relatives. The melting point of LDPE sits between 105 °C to 115 °C.

Rheological and processing properties

This is where LDPE’s long-chain branching pays off. The common processing descriptor is melt flow index (MFI, also reported as melt flow rate, MFR), which gives a quick read on how easily a grade flows when molten. MFI is an index inversely related to molecular weight, so a higher MFI points to a lower-molecular-weight, easier-flowing resin.

Long-chain branching gives LDPE high melt strength and pronounced shear-thinning. Those two traits are the reason why LDPE performs so well in blown film and extrusion coating, where the molten polymer has to stretch without tearing. The LDPE grades that excel in these applications are the autoclave-route resins.

Mechanical properties

Mechanically, LDPE is soft and flexible. It has a low flexural and tensile modulus, so it bends easily, with moderate tensile strength and high elongation at break. It also resists impact and tearing well, including at low temperature, and its surface hardness is low.

Compared with HDPE, LDPE trades stiffness and strength for flexibility and toughness, which makes it the better choice where a film or part needs to flex and stretch rather than hold a rigid shape.

Chemical and barrier properties

LDPE resists a broad range of chemicals, including acids, bases, and many solvents at ambient temperature, although some hydrocarbons and oils will cause it to swell. Its environmental stress-cracking resistance (ESCR) is a useful design property, particularly for containers and liners that see prolonged stress.

As a barrier material, LDPE has a split personality: it blocks moisture and water vapour well, while it lets gases such as oxygen through relatively easily. That combination shapes its packaging use, where it is often paired with other materials when oxygen sensitivity matters.

Electrical properties

LDPE is characterized by a low loss factor, high dielectric strength and high dielectric constant.

These dielectric properties allow for efficient transmission of electrical signals while minimizing energy loss, making LDPE an ideal choice for insulating electrical wires and cables.

Optical properties

LDPE offers moderate clarity with some haze and surface gloss. The optical behaviour is also a consequence of its molecular structure: the crystalline regions scatter light, so too much LCB reduces crystallinity and its optical properties.

Autoclave LDPE film is clear enough for most packaging while stopping short of glass-like transparency, while tubular LDPE grades are produced with low LCB to maximise clarity.

LLDPE vs LDPE

Linear low-density polyethylene (LLDPE) is easy to confuse with LDPE because they share similar names, but their properties differ significantly. The difference between them is structural, and it comes from how each is made.

LLDPE is a polyethylene made by low-pressure catalytic copolymerisation of ethylene with an α-olefin comonomer, which produces a linear backbone with controlled short-chain branching and no significant long-chain branching. This branching feature gives LLDPE different properties to LDPE.

The linear backbone in LLDPE gives it higher tensile and puncture strength, so it is favoured where toughness per micron counts. While LDPE’s long-chain branching gives it easier processing and higher melt strength, which is why it remains the workhorse for blown film and coating – although many film products blend the two to balance strength against processability.

LDPE has a higher level of long-chain branching as well as short-chain branching, while LLDPE has more short-chain branching

What is LDPE used for?

LDPE Plastic in Flexible Packaging and Films

Blown and cast film dominate LDPE consumption. The material goes into food packaging, bags, agricultural and construction sheeting, shrink and stretch films, and the liner layers in laminated packaging. Its clarity and seal performance are the main draws here. Squeeze bottles are an LDPE use where flexibility is wanted, though rigid bottles are more often made with HDPE or PET.

Industrial and Medical Grade LDPE Plastic Material

Beyond packaging, LDPE serves in wire and cable insulation, lids and closures, tubing, and single-use laboratory and medical disposables, where its chemical resistance and flexibility are useful.
Film and extrusion remain the primary conversion routes, while injection and blow molding account for a smaller share.

Advanced Characterization of LDPE

Density and melt flow index are in the standard specifications on any LDPE datasheet, and for good reason, they are quick to measure and they correlate with handling. However, both density and MFI are bulk values, meaning that each one collapses the whole material into a single number. That bulk number says nothing about how the polymer chains are distributed or how they are branched. Two LDPE grades can share the same density and the same MFI and still run differently on the same extruder or film line. The reason lays in their microstructure, and resolving it is what Gel Permeation Chromatography (GPC) – also called Size Exclusion Chromatography (SEC) – is built to do. For LDPE, high-temperature GPC with multi-detection is what separates an autoclave grade from a tubular one by its branching and molecular weight distribution, which is the practical reason the technique matters for this material.

Precise measurement of molecular weight distribution (MWD)

Melt flow index tracks an average related to molecular weight, but it cannot show the shape of the distribution behind that average. Two grades with the same MFI can have very different distributions: one narrow, one broad, one carrying a high-molecular-weight tail that the average conceals. Those differences change how a melt behaves even when the MFI numbers match.

GPC/SEC separates the chains by size and measures the full distribution, so its breadth and any tails become visible rather than hidden inside an average. For molecular weight distribution, averages and how dispersity (Đ) is defined, read our guide to molecular weight distribution in polymers.

Resolving branching and how it is distributed

Branching is the second piece of microstructure that bulk specifications miss. Density reflects crystallinity, which branching lowers, but it cannot tell you whether the branches are short or long, or how these branches are spread across molecular sizes. Long-chain branching is what gives LDPE its melt strength, so two grades with similar density can stretch and hold very differently in the melt depending on their branching. LCB is resolved by a GPC/SEC system with a concentration detector (Infrared or Refractive Index) paired with a viscometer or/and Light Scattering detector.

Why this analysis is performed at high temperatures

LDPE does not dissolve at room temperature, so this analysis runs by high-temperature GPC (HT-GPC), where the sample is dissolved in high-boiling-point solvents, commonly TCB or o-DCB at elevated temperatures of around 150–160 °C. For the full method, see our complete guide to high-temperature GPC.

LDPE related articles

• Poster: Improved long-chain branch detection in LDPE via extended Mark-Houwink calibration
• High temperature-size exclusion chromatography with triple detection to assess the molecular architecture of low-density polyethylene: Insight into branching and processability correlations
• Biodegradation of pre-treated low-density polyethylene (LDPE)