Table of Contents
Reviewed and updated by ICD - Week 43/2026
PE plastic is made by polymerizing ethylene gas in two ways: at very high pressure, 1,000 to 3,000 bar, for LDPE; and at low pressure, from a few bar to about 40 bar with a metal catalyst, for HDPE and LLDPE. The reaction conditions decide how much the chains branch, and the degree of branching decides density, flexibility and use. This guide covers making virgin plastic from ethylene; distinguishing the types and their properties is in what PE plastic is.
1. From ethylene to polyethylene
Ethylene is a gas obtained from cracking oil or natural gas and purified carefully before it enters the reactor. Polymerization opens the double bond of each ethylene molecule and links them into long chains; commercial PE has chains of several thousand to tens of thousands of units (NZIC). Two factors decide the product: how the reaction is initiated, by free radicals or by a metal catalyst, and the accompanying pressure. From these come three main product branches.
| LDPE | HDPE | LLDPE | |
|---|---|---|---|
| Polymerization route | Free radical, very high pressure | Metal catalyst, low pressure | Metal catalyst, low pressure, with comonomer |
| Pressure | 1,000 - 3,000 bar | about 5 - 40 bar | about 20 bar (gas phase) |
| Temperature | 80 - 300 °C | 70 - 110 °C | 70 - 110 °C |
| Branches per 1,000 carbons | 2 - 50, including long branches | 5 - 10, mostly short | short branches from the comonomer, 2.5 - 3.5% |
| Density | 915 - 940 kg/m³ | 940 - 960 kg/m³ | 915 - 925 kg/m³ |
The figures in the table are compiled from the sources listed at the end. The pressure ranges for HDPE and LLDPE are typical values for each technology (slurry 0.5 to 1 MPa, Phillips loop 3 to 4 MPa, gas phase about 2 MPa), not a single number. In LDPE the heavily branched chains pack together poorly, so the density is low and the material is flexible.
2. LDPE: very high pressure and free radicals
The LDPE reaction runs at 1,000 to 3,000 bar and 80 to 300 °C, initiated by oxygen or peroxide, in a stirred autoclave or a tubular reactor hundreds of meters long (Ullmann). An autoclave typically converts under about 20% of the ethylene per pass, a tubular reactor up to about 35%; unreacted ethylene is recovered and recycled. This explains why LDPE plants are expensive: compressing the gas to that pressure and keeping the temperature within a safe limit are both hard. The history starts there too: the ICI group discovered in 1935 that compressing ethylene at very high pressure produces a solid, and commercial LDPE production began in 1938 (ELCD document).
3. HDPE and LLDPE: low pressure, metal catalysts
Chemists found low-pressure polymerization routes from the 1950s: the Phillips chromium oxide catalyst in 1950 (3 to 4 MPa, 70 to 100 °C) and the Ziegler titanium catalyst in 1953 (atmospheric pressure to a few bar, 50 to 100 °C) (ELCD document). Thanks to the catalyst, chains line up straight with few branches, so HDPE has a high density. Later, metallocene catalysts widened the scope for tuning. All three commercial reactor types are in use.
| Technology | Method | Typical conditions |
|---|---|---|
| Slurry | The PE particles formed are suspended in a solvent such as hexane and separated by centrifuge | 0.5 - 1 MPa, 80 - 90 °C (Hoechst); Phillips loop 3 - 4 MPa, 100 °C |
| Gas phase | A fluidized bed of dry PE particles, no solvent needed | about 2 MPa, 70 - 110 °C |
| Solution | Both polymer and catalyst dissolve in a solvent | Used for LLDPE and for grade-switching HDPE/LLDPE |
LLDPE is made by adding a small amount of a short-chain olefin, usually 1-butene or 1-hexene, to the ethylene. The comonomer creates regular short branches, so LLDPE has flexibility close to LDPE because it has the same density range of 915 to 925 kg/m³. The gas phase uses no solvent, so no solvent recovery step is needed.
4. After the reactor: pelletizing and quality control
The polymer powder or solution is mixed with stabilizers, antioxidants and additives, then extruded through a die and cut into pellets. These are the virgin PE pellets sold to converters. Finished pellets are checked on three groups of properties.
Density tells whether it is LDPE, LLDPE or HDPE. Melt flow rate (MFR, measured at 190 °C under a 2.16 kg load) shows which technique the pellets suit: about 1 to 20 g/10 min for injection molding, 0.2 to 0.7 for blow molding and 0.02 to 0.07 for film (manufacturer Ipek Plastik). Mechanical strength and chemical resistance, measured to customer requirements, complete the set.
5. Recycled pellets: a different chain
Recycled PE pellets do not start from ethylene but from scrap: sorted, washed, ground, dried, extruded through a screen and cut into pellets. This process is simpler than virgin pellets, but the result depends heavily on how clean the scrap is, and mechanical properties fall with each reprocessing, so recycled pellets are usually used for products that do not touch food. Details are in the article can PE plastic be recycled.
Frequently asked questions
How is PE plastic made?
By polymerizing ethylene gas. LDPE uses free radicals at 1,000 to 3,000 bar and 80 to 300 °C; HDPE and LLDPE use metal catalysts at low pressure.
How do LDPE and HDPE differ in production?
LDPE polymerizes at very high pressure with free radicals, so its chains branch heavily; HDPE uses a catalyst at low pressure, so its chains are straight with few branches and the density is higher.
How is LLDPE made?
By polymerizing ethylene with a small amount of a short-chain olefin such as 1-butene or 1-hexene using a metal catalyst, usually in a gas-phase reactor.
What decides the quality of PE plastic?
The purity of the ethylene, the type of catalyst or initiator, pressure, temperature and comonomer type, then the additives at pelletizing.
What is melt flow rate (MFR) used for?
It shows which processing technique the pellets suit: high MFR suits injection molding, low suits blow molding and film.
Contact ICD Vietnam for advice
ICD Vietnam Industrial Manufacturing Co., Ltd.
North: 3rd Floor, Thang Long A1 Building, Bau Hamlet, Thien Loc Commune, Hanoi, 0983 797 186 / 090 345 9186 / 090 5859 186
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References
- NZ Institute of Chemistry, The Manufacture of Polyethylene (pressure 1000-3000 bar, 80-300 °C for LDPE; branched chains) - nzic.org.nz
- Whiteley K.S., Polyethylene, Ullmann’s Encyclopedia of Industrial Chemistry (autoclave, tubular, slurry and gas-phase reactors) - ugr.es
- ELCD document on polyolefin production (catalyst history, branches per 1000 carbons, gas-phase conditions) - inference.org.uk
- Ipek Plastik, PE Polyethylene (melt flow rate by purpose: injection molding 1-20, blow molding 0.2-0.7, film 0.02-0.07 at 190 °C/2.16 kg) - ipekplastik.com.tr
- Wikipedia, Polyethylene (classification by density, melting point, UHMWPE, PEX, 34% market share) - en.wikipedia.org
