The process of making high density polyethylene starts with a procedure known as “cracking”, whereby petroleum is heated in a “cracking column” that imbues the petroleum with enough energy to brake carbon-carbon and carbon-hydrogen bonds into smaller hydrocarbons (namely methane) that can enter their gas phase and rise up and out of the column to enter the next step of the process. Now the methane gas is oxidised to form ethane and water, and then oxidised further to form ethylene (ethene). (Galadima & Muraza, 2016)
HDPE itself can be produced through three different methods. The three methods involve relatively low pressures, ranging from 10 to 80 atmospheres, and temperatures, ranging from 350 to 420 Kelvin. (The Essential Chemical Industry online, n.d.)
The first method is known as the slurry method, which works as follows; first a diluent consisting of a liquid hydrocarbon mixture that is relatively unreactive is mixed with the Ziegler-Natta catalyst. Next a mixture of hydrogen and ethylene is fed under pressure into the slurry, polymerising ethylene to high density polyethylene. (The Essential Chemical Industry online, n.d.)
The second method is the solution process. This method is similar to the slurry method except for the hydrocarbon diluent, which is alkane in this process.
The third method is the gas phase process. In this process our reaction takes place with a Philips catalyst in a fixed bed reactor, where the hydrogen and ethene merely pass over the catalyst. Grains of HDPE form as the ethene is polymerised and are then suspended in the gaseous mixture until the valve of the reactor is open and the grains can be extracted. (The Essential Chemical Industry online, n.d.)
The grains produced through any of the above-mentioned processes are refined into HDPE granules or pellets (figure 3-1). (The Essential Chemical Industry online, n.d.)
Figure 3-1 (The Essential Chemical Industry online, n.d.)
High-Density Polyethylene 