Polyethylene resin is polyethylene in the form it is bought and sold: a synthetic thermoplastic made by polymerising ethylene and supplied as pellets for converters to extrude or mould into products. It comes in three commodity types, LDPE, LLDPE and HDPE, which differ in how branched the polymer chain is and so in density. Within a type, a grade is identified by its density and melt flow rate.
For synthetic polymers the words resin and plastic are not clearly separated, so polyethylene resin is a plastic.
Polyethylene is listed in the US FDA’s food-contact regulation for olefin polymers, 21 CFR 177.1520, on condition that the resin meets that section’s specifications when tested by its methods.
How Polyethylene Resin Is Made
Polyethylene resin is made by polymerising ethylene by one of two routes: at very high pressure with free-radical initiators, which gives LDPE, or at low pressure on a catalyst, which gives HDPE and LLDPE.
The monomer is ethylene, a gas produced by steam cracking ethane or naphtha.
Whichever route made it, the polymer is then extruded with any additives the grade carries and cut into pellets, the form in which polyethylene resin is sold.
Types of Polyethylene Resin
The three commodity types are LDPE, LLDPE and HDPE, and they are told apart by density, which follows from how branched the chain is.
The bands below are ASTM’s density types, originally set out in ASTM D1248.
| ASTM type | Density (g/cm³) | Resins in the band |
|---|---|---|
| I | 0.910–0.925 | LDPE and LLDPE |
| II | 0.926–0.940 | MDPE |
| III | 0.941–0.959 | HDPE with a little butene or hexene comonomer |
| IV | 0.960 and above | HDPE homopolymer |
Another classification puts the boundaries elsewhere. Judge a grade by its density figure, because the type label depends on whose table it follows.
Very-low-density PE (VLDPE) reaches below the low-density band. Ultra-high-molecular-weight PE (UHMWPE) and cross-linked PE (PEX) are defined by something other than density: UHMWPE by a melt flow rate below 0.1 g/10 min even at 190 °C under 21.6 kg, and PEX by crosslinking a PE base resin during or after extrusion.
Low-Density Polyethylene (LDPE)
LDPE is the polyethylene of the high-pressure route. Its long and short branches put it in the low-density band and make it soft and flexible.
LDPE goes into packaging film, such as bags, lamination and coextrusion film, and into flexible mouldings such as closures, household items and toys. It carries resin identification code 4.
Linear Low-Density Polyethylene (LLDPE)
LLDPE is a copolymer of ethylene with a small amount of 1-butene, 1-hexene or 1-octene, made on the low-pressure catalytic route. Its backbone is linear, and the comonomer adds short, uniform branches that keep the chains from packing closely.
LLDPE competes with LDPE for the same film markets — agricultural film and food, consumer and industrial packaging — and gives the tougher film of the two.
Metallocene LLDPE (mLLDPE) is the same copolymer made on a single-site metallocene catalyst, which spreads the comonomer more evenly across the chains than a Ziegler-Natta catalyst does.
High-Density Polyethylene (HDPE)
HDPE is the least-branched polyethylene: an ethylene homopolymer, or a copolymer with a small amount of butene or hexene to control branching, made on the low-pressure catalytic route. With so few branches, HDPE is the stiffest and hardest of the three types, with the highest melting point.
HDPE goes into blow-moulded drums, jerry cans and thin-walled food containers, and into injection-moulded caps and closures. It is also extruded into pressure pipe. It carries resin identification code 2.
Properties of Polyethylene Resin
Polyethylene is tough and chemically resistant, and density sets most of its other properties. It is a plastic, not a rubber: a semi-crystalline thermoplastic.
As density rises, stiffness, tensile yield strength and chemical resistance go up, and impact strength and stress-crack resistance go down. The table shows the two ends of the range, one injection-moulding grade of each type from two producers’ datasheets.
| Property | LDPE injection grade | HDPE injection grade | Test method |
|---|---|---|---|
| Density | 0.918 g/cm³ | 0.954 g/cm³ | ISO 1183 |
| Melting temperature | 106 °C | 130 °C | ISO 11357 |
| Tensile modulus | 160 MPa | 945 MPa | ISO 527-2 |
| Tensile stress at yield | 8 MPa | 23 MPa | ISO 527-2 |
| Shore D hardness | 51 | 61 | ISO 868 |
These are typical datasheet values and do not serve as specification limits.
Polyethylene absorbs negligible water and resists salt solutions, non-oxidising acids, alkalis, alcohols, fats and oils at room temperature. It is also an electrical insulator.
Each of polyethylene’s disadvantages either calls for a particular grade or rules it out of the application:
- Low heat limit: a producer of HDPE sheet rates it for continuous service from −50 to +70 °C, and up to +80 °C only where it carries no significant mechanical stress.
- UV degradation: unprotected polyethylene degrades in sunlight. For PE pipe, 2–3% finely divided carbon black is the most effective screen.
- Stress cracking: resistance differs widely between grades of one type. A loaded part in contact with detergents or surfactants needs a grade chosen on its ESCR figure.
- Strong oxidisers: nitric acid, chromic acid and halogens attack it, and aromatic and halogenated hydrocarbons make it swell.
- Flammability: it sustains burning in normal air, and applications with a fire rating need a flame-retardant grade.
Within a type, a grade is identified by two figures, density and melt flow rate, and each means something only with its test condition attached.
Melt flow rate for polyethylene is measured at 190 °C under a stated load. At 2.16 kg it is the melt index; low-flow grades are quoted at 21.6 kg, as the high-load melt index. One HDPE grade is listed at 0.80 g/10 min at 2.16 kg and 52 g/10 min at 21.6 kg on the same datasheet, so two figures taken at different loads never share a column.
Density depends on how the test specimen was moulded, and the methods differ: ASTM D792 is not equivalent to ISO 1183-1 Method A. Before setting a certificate of analysis beside a datasheet, check that both quote the same method and condition.