Environmental Engineering Reference
In-Depth Information
3.5.1 Polyethylenes
In the US, ethylene monomer derived from natural gas is primarily used to
produce PE though other fossil fuels may also be used for the purpose. The
annual world production of PE is approximately 100 MMT.
Polymerizing ethylene in high-pressure (~100,000-350,000 kPa) reactors
yields an extensively branched variety of PE, the LDPE; branching is
responsible for its characteristic lower density. Essentially, the same
polymerization reaction can also be carried out at a considerably lower
pressure (~2000 kPa) when Ziegler-Natta catalysts or the more recently
developed metallocene catalysts are used. The former catalysts have
multiple sites that restrict linking of monomer molecules to a specific
regular orientation during polymerization, leading to broad molecular
weight distributions, while the latter catalysts yield PE with a relatively
narrow distribution. The low-pressure processes yield HDPE with very low
levels of chain branching and therefore higher crystallinity and relatively
high density. Often, a commoner hexane is used to control branching and
therefore the crystallinity of the product.
A third variety, LLDPE, is manufactured using specialized catalyst systems
and have a similar molecular structure but with short-chain branching (as
opposed to long-chain branching in LDPE). This variety is particularly
strong and is often used in the manufacture of thin-walled plastic bags.
Newer metallocene family of catalysts yields a grade of LLDPE with more
homogeneous molecular structure compared to the regular variety; this
mLLDPE can be used to down gauge packaging films as they can be tailored
as a very strong and transluscent plastic.
About 40% of PE produced is blow-molded into bottles. Only approximately
15% is used as plastic film. The rest is injection molded or extruded (into
pipe products). Both LDPE and LLDPE grades are used predominantly
in film applications (~75%), in extrusion coating of paperboard products
(<10%), and as cable coating with only a small fraction being injection
molded into container. HDPE absorbs very little water and has better
specific strength compared to LDPE. Therefore, it is popularly used in
molding milk jugs and other containers for packaging liquids including
chemicals.
Table 3.4
compares the properties of the different grades of PE
with PP.
