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Manufacture of poly(ethene) (polyethylene)

Manufacture of poly(ethene) (polyethylene)
Poly(ethene) is produced in three main forms:
low density (LDPE) (< 0.930 g cm-3)
linear low density ( LLDPE) (ca 0.915-0.940 g cm-3)
and high density (HDPE) (ca 0.940-0.965 g cm-3).
Low density poly(ethene) (LDPE)
The process is operated under very high pressure (1000-3000 atm) at moderate temperatures (420-570 K)
This is  a radical polymerization process and an initiator, such as a small amount of oxygen, and/or an organic peroxide is used.
Ethene (purity in excess of 99.9%) is compressed and passed into a reactor together with the initiator.      The molten poly(ethene) is removed, extruded and cut into granules.     Unreacted ethene is recycled.      The average polymer molecule contains 4000-40 000 carbon atoms, with many short branches.
High density poly(ethene) (HDPE)
HDPE is produced by three types of process.     All operate at relatively low pressures (10-80 atm) in the presence of a Ziegler-Natta or inorganic catalyst.      Typical temperatures range between 350-420 K.     In all three processes hydrogen is mixed with the ethene to control the chain length of the polymer.
(i) Slurry process (using either CSTR (continuous stirred tank reactor) or a loop)
The Ziegler-Natta catalyst, as granules, is mixed with a liquid hydrocarbon (for example, 2-methylpropane (isobutane) or hexane), which simply acts as a diluent.      A mixture of hydrogen and ethene is passed under pressure into the slurry and ethene is polymerized to HDPE.     The reaction takes place in a large loop reactor with the mixture constantly stirred (Figure 4).  On opening a valve, the product is released and the solvent is evaporated to leave the polymer, still containing the catalyst.      Water vapour, on flowing with nitrogen through the polymer, reacts with the catalytic sites, destroying their activity.     The residue of the catalyst, titanium(IV) and aluminium oxides, remains mixed, in minute amounts, in the polymer.
(ii) Solution process
The second method involves passing ethene and hydrogen under pressure into a solution of the Ziegler-Natta catalyst in a hydrocarbon (a C10 or C12 alkane).      The polymer is obtained in a similar way to the slurry method.
(iii) Gas phase process
Ethene polymerizes to form grains of HDPE, suspended in the flowing gas, which pass out of the reactor when the valve is released.
Modern plants sometimes use two or more of the individual reactors in series (for example two or more slurry reactors or two gas phase reactors) each of which are under slightly different conditions, so that the properties of different products from the reactors are present in the resulting polymer mixture, leading to a broad or bimodal molecular mass distribution.      This provides improved mechanical properties such as stiffness and toughness.
Linear low density poly(ethene) (LLDPE)
Low density poly(ethene) has many uses but the high pressure method of manufacture by which it is produced has high capital costs.     However, an elegant technique has been developed, based on both Ziegler-Natta and inorganic catalysts to produce linear low density poly(ethene) LLDPE, which has even improved properties over LDPE.     Any of the three processes, slurry, solution and gas phase, can be used when a Ziegler-Natta catalyst is chosen.      The gas phase process is used when the inorganic catalyst is employed.
The structure is essentially linear but because of the short chain branching it has a low density.      The structure gives the material much better resilience, tear strength and flexibility without the use of plasticisers.      This makes linear low density poly(ethene) an ideal material for the manufacture of film products, such as those used in wrappings.
The properties of the polymer, and hence its uses, can be varied by varying the proportion of ethene and co-monomer and by using different co-monomers.      All this can be done without shutting down the plant, an enormous advantage.
Uses of poly(ethene) (polyethylene)
The LDPE or LLDPE form is preferred for film packaging and for electrical insulation.
HDPE is blow-moulded to make containers for household chemicals such as washing-up liquids and drums for industrial packaging.     It is also extruded as piping.
Many plants can produce both forms of poly(ethene) and alter the amount that they produce of each type at short notice.    Both use a Ziegler (or Phillips) catalyst.       If pure ethene is used,  HDPE is formed.      LLDPE is produced when a small amount of another alkene, for example but-1-ene, is added to the ethene.     Another form, mLLDPE, is, at present, produced in much smaller quantities.
For LLDPE, HDPE process, whether slurry or gas phase applied, it needs pentane or hexane for the catalyst carriage.     Dongying Junyuan's pentane and hexane , have been exported to SABIC, FORMASA PLASTICS, SINOPEC etc for the polymerization since 2009, through its branch Dongy


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