LDPE and LLDPE in flexible packaging
LDPE is a polyethylene grade made by high pressure polymerisation. The process attaches long branches to the polymer chain, and the branches stop the chains packing tightly. The film that results seals at a low temperature and stays flexible when cold. Every property a packer notices on the line traces back to that one structural fact.
LLDPE is made a different way. Ethylene is copolymerised at low pressure with a short alpha olefin, which produces short branches of equal length instead of long random ones. Equal branches let the chains align, and alignment carries load. LLDPE therefore holds more tensile and puncture strength than LDPE at the same thickness. It is added to a sealing layer for exactly that reason.
This page covers polyethylene as a polymer. It sets out how the grades are made, what each attribute controls, and how the material behaves before it becomes a structure. How PE grades are combined into a recyclable laminate is covered on all-PE structures. It is not about polythene sheeting for construction, damp proof membrane or agricultural film.
Density is the attribute that classifies polyethylene
Polyethylene grades are named after their density, and density is set by branching. Branches occupy space between chains, space prevents crystallisation, and a less crystalline polymer weighs less per unit volume. Density is therefore not a property of polyethylene, it is a measurement of how the polymer was built.
| Grade | Published density | How it is made | Unique attribute |
| LDPE | 0.910 to 0.940 g/cm³ | High pressure free radical polymerisation | Long chain branching, which no other PE grade has |
| LLDPE | 0.915 to 0.925 g/cm³ | Low pressure copolymerisation with an alpha olefin | Short branches of controlled, equal length |
| HDPE | above 0.940 g/cm³ | Low pressure polymerisation, minimal comonomer | An almost linear chain, so the highest crystallinity |
Melting behaviour follows the same chain. Published melting for LDPE sits in the region of 105 to 115 °C, LLDPE melts slightly higher, and HDPE higher still. A crystal has to be broken before the polymer flows, so more crystal means more heat. More heat means a sealing jaw has to hold longer to close the same seal.
One consequence is worth stating plainly: a film cannot be both very soft and very stiff, because the same crystallinity governs both. Softness, seal initiation, stiffness and moisture barrier all move together along the density scale, in the same direction. No grade selection escapes that.
The comonomer decides how strong LLDPE becomes
LLDPE is not one material. Its strength depends on which alpha olefin was used as comonomer. The comonomer becomes the branch, and branch length decides how well neighbouring chains hold on to each other.
| Comonomer | Branch it creates | What it delivers |
| Butene | Two carbon branch, the shortest | The economical grade, adequate toughness, the usual default |
| Hexene | Four carbon branch | Clearly better tear and puncture resistance, the common compromise |
| Octene | Six carbon branch, the longest | The highest toughness and the widest downgauging margin, at the highest resin cost |
A longer branch reaches further into the neighbouring chain and forms what the literature calls a tie molecule, a chain that belongs to two crystals at once. Tie molecules are what stop a crack travelling. An octene grade will generally survive a drop that a butene grade of identical thickness and density does not. Two films can therefore share a specification sheet and behave differently in the same box.
Metallocene polyethylene and the single site catalyst
Metallocene polyethylene, written mPE, is LLDPE made with a catalyst that has one type of active site instead of many. One site produces chains of similar length and puts the comonomer in the same place on each of them. The molecular weight distribution narrows, and a narrow distribution changes three things that matter on a packaging line.
- Seal initiation drops, because there is no longer a population of high melting chains holding the seal open
- Hot tack rises, so the seal carries the product before it has cooled, which is what a vertical form fill seal machine needs
- Seal through contamination improves, so a seal closes over dust, powder or fat instead of failing at the point where the product crossed the seal area
The cost of a narrow distribution is processing. The same uniformity that improves the seal makes the melt less forgiving to extrude, so mPE is normally blended rather than used alone. It is specified where the seal is the failure point, not as a general upgrade.
Melt flow index, the number that decides how a resin runs
Melt flow index, MFI, measures how much polymer flows through a standard opening under a standard weight in ten minutes. It is an inverse reading of molecular weight. A high MFI means short chains and easy flow, a low MFI means long chains and stiff flow.
The trade is direct and it cannot be avoided. Long chains entangle, entanglement carries stress, and stress resistance is what stops a bag splitting. So the resin that runs easiest produces the weakest film, and the resin that produces the strongest film is the hardest to run. Every polyethylene film specification is a position taken on that line.
This also explains a failure that looks like a mystery on the line. Two batches with the same density and the same nominal grade can behave differently in the extruder and in the seal. Density and melt flow index are independent attributes, and only one of them usually appears in a purchase specification.
Additives change what the film does, not what it is
Polyethylene film is never pure polyethylene. Additives are compounded into the resin. Several of them work by migrating out of the polymer to the surface after the film is made. Their effect therefore changes with time and temperature.
| Additive | What it does | What it costs you elsewhere |
| Slip | Migrates to the surface and lowers the coefficient of friction, so film feeds instead of dragging | Migrates into the seal area and into print, so too much weakens seals and lifts ink |
| Antiblock | Puts microscopic particles at the surface so two layers do not stick together | Raises haze, so clarity falls as blocking resistance rises |
| Antifog | Lowers the surface tension of condensate so water spreads as a sheet instead of forming droplets | Is consumed over time and affects sealing, so it is specified against a shelf life |
| Processing aid | Coats the die and suppresses melt fracture at higher output | Can interfere with surface treatment and with print adhesion |
Coefficient of friction, COF, is the attribute most often specified without being understood. It is measured film against film and film against metal, and the two values are not interchangeable. A film that runs perfectly on one machine can jam on another because the sliding surface is different, not because the film changed.
Surface energy and why polyethylene must be treated
Polyethylene has a chemically inert, non polar surface with a low surface energy. Ink and adhesive need a surface with higher energy than their own in order to wet it and anchor. Untreated polyethylene rejects both. Corona treatment solves it by oxidising the surface in an electrical discharge. That creates polar groups and raises surface energy, measured in dynes per centimetre.
Treatment is not permanent. The oxidised groups reorient back into the polymer over weeks. The decay runs faster when the film is stored warm, or when slip additive is migrating to the same surface. This is why a film that printed cleanly on delivery can print badly from the same roll months later. A dyne level on a certificate describes the day of production, not the day of use.
Why polyethylene stops water and lets oxygen through
Permeation is solution followed by diffusion: a gas has to dissolve into the polymer before it can travel through it. Polyethylene is non polar, water is strongly polar, and a polar molecule will not dissolve into a non polar solid. Water vapour therefore barely enters the film, which is why polyethylene is a good moisture barrier without any additional layer.
Oxygen is a small, non polar molecule, and the amorphous regions between crystals give it plenty of room. It dissolves readily and moves easily, so polyethylene is a poor oxygen barrier at any thickness. Adding microns slows permeation in proportion, but no sealing-layer thickness a pack can realistically carry closes the gap to a barrier polymer. An oxygen sensitive product therefore needs a barrier layer of a different polymer, not a thicker sealing layer.
Crystallinity is the variable behind both. A gas cannot pass through a crystal, only around it, so a denser and more crystalline polyethylene is a better barrier to everything. That is the same crystallinity that raises the sealing temperature. It is why the sealing layer and the barrier layer are almost never the same layer.
Which polymer takes the oxygen barrier role, and what it costs in recyclability, is covered under barrier layer options compared.
Recycled polyethylene does not behave like virgin
Mechanical recycling puts polyethylene through heat and shear a second time. The polymer chain does not survive that unchanged. Chains break, and fragments recombine into branches that were never designed. The melt flow index of the recyclate drifts away from the grade it came from. A recycled resin is therefore a different material with the same name.
Four consequences show up in the film rather than in the certificate:
- Gels, small unmelted or cross linked points that appear as lenses in the film and become leak paths in a seal
- Colour drift, because the input stream was never one colour, which is why high recycled content and a clean white or transparent film pull against each other
- Odour, carried over from the previous contents and from degradation products, which matters most on low aroma products
- Batch to batch variation, since the input is a collected waste stream rather than a reactor output
This is the practical reason recycled polyethylene is usually placed in a layer that does not touch the product and does not carry the print, while the sealing and print layers stay virgin. The percentages that have to be reached, and the way they are calculated, are set out under recycled content requirements.
Frequently asked questions
Does polyethylene absorb flavour and aroma from the product?
Yes, and the effect has a name: scalping. Aroma compounds in food are mostly non polar, polyethylene is non polar, so the compounds dissolve into the sealing layer and stop reaching the person opening the pack. Fat soluble aromas are affected most, which is why citrus, mint, spice and some dairy products lose character behind a plain PE layer while the pack itself shows nothing wrong. Where aroma retention is part of the product, the contact layer is chosen for it rather than defaulted to.
Why do two rolls of the same polyethylene grade run differently?
Because a grade name fixes fewer attributes than people assume. Density and grade can match while melt flow index, comonomer type, additive package and corona treatment age all differ, and each of those four changes how the film feeds, seals and prints. A specification that names only the polymer and the thickness has left most of the behaviour unspecified.
Specifying more than a polymer and a thickness
Send the product, the shelf life you need and the machine it runs on. We come back with a structure and the documentation that goes with it.
Related resources
All-PE structures
What happens once these grades are combined into a laminate: structures, sealing behaviour, food contact and recyclability under the PPWR.
Multilayer structures
How layers are joined, what an adhesive contributes to bond strength, and where coextrusion is used instead of lamination.
Barrier layer options compared
Which layer takes the oxygen barrier once polyethylene has reached its limit, and what each option costs in recyclability.
Recycled content requirements
The Article 7 percentages, how the minimum is averaged per packaging type and format rather than per pack, and what mass balance allocation allows.