The name is absent

sirable. For example, proper orientation and crystallization conditions are needed to
secure useful fibers and films; on the other hand, segregation of highly viscous phases
may lead to catastrophic consequences like plugged lines or overheated reactors. Folie
and Radosz [1] presented a review on the relationship between polymer processing
and the phase equilibria of the system in the commercial high-pressure polyethy-
lene process (HPPE). HPPE is used to produce low density polyethylene (LDPE) by
free-radical polymerization [2, 3] and LLDPE by using single-site homogeneous met-
allocene catalysts [4, 5]. The process is carried out in supercritical ethylene, which
is both the reactant and the solvent for the polymer. The production of LDPE is
carried out in a single-phase region to facilitate the heat removal from the exothermic
polymerization reaction and ensure adequate reaction temperature control. Efficient
temperature control is required to avoid forming cross-linked materials. Again, seg-
regation of the viscous polymer rich phase increases the probability of forming hot
spots in the reactor and initiating the explosive runaway reactions [6, 7].

On the other hand, polymerization reaction to produce LLDPE is carried out in a
two-phase region. Phase separation in the autoclave reactor is achieved by lowering
the pressure or adding an anti-solvent, such as N₂ to the reaction mixture. LLDPE
produced this way exhibits superior film properties because of narrower molecular
weight distribution (MWD) and less long chain branches [8]. An undesirable phase
transition is the polymer precipitation due to the cooling of the reaction mixture.
This leads to the deposition of polymer films, which impairs heat transfer and re-

<<   15   16   17   18   19   20   21   >>

More intriguing information