1.  Polyethylene production (PE)

Different grades of polyethylene such as low density polyethylene (LDPE), high density polyethylene (HDPE) or linear low density polyethylene (LLDPE) are produced commercially.  

Low density polyethylene (LDPE) and high density polyethylene (HDPE)

The LDPE or high pressure polyethylene is produced by radical polymerization. The HDPE or low pressure polyethylene is synthesized by co-ordination polymerization. Except LDPE, all other polymers of olefins are produced by co-ordination catalysts. LDPE obtained by radical polymerization differs structurally from HDPE produced by traditional Ziegler Natta co-ordination catalyst. The LDPE is more highly branched (both short and long branch) than HDPE and is therefore lower in crystallinity and density. The crystallinity of LDPE lies in the range of 40-60% and while that of HDPE in 70-90% . The density of LDPE and HDPE lie in the range 0.91 -0.93 g/cm³ and 0.94-0.96 g/cm³ respectively. Compared to LDPE, HDPE has increased tensile strength, stiffness, chemical resistance and upper used temperature. Most HDPE have number average molecular weights in the range of 50000 -250,000 and have wide range of applications such as bottles, housewares, toys, pails, film for grocery bags and food packing, pipe, tubing, cables etc.

Linear low density polyethylene (LLDPE)

Co-ordination copolymerization of ethylene in presence of small amount of α-olefins such as 1-butene, 1-hexene or 1-octene results in polyethylene that have structure, properties and applications equivalent to the branched LDPE produced by radical polymerization. This polyethylene is known as linear low density polyethylene (LLDPE) and has controlled amount of ethyl, n-butyl and n-hexyl branches respectively.

The polyethylene can be produced by following methods :

1. LDPE is produced by free radical high pressure bulk polymerization process
   
   
2. HDPE and LLDPE are produced by slurry-suspension process at moderately low pressure. The process is carried out over supported catalysts such as supported Cr or Cr organometallic or coordination catalysts such as metallocene or Ziegler –Natta catalysts. The reaction conditions are 80- 150°C and 20-35 atm pressure. Supported catalyst is typically suspended in alkene or cyclohexane solvent which also serve as the heat transfer medium. The Ziegler–Natta(TiCl₄ /Al - alkyl/MgCl₂) catalyst is more active than Cr based catalysts.
   
   
3. HDPE and LLDPE can also be produced by gas phase fluidized bed polymerization over supported CrCO₃ or Ti based catalysts.