Chemistry Reference
In-Depth Information
Table 6.1
Typical conditions for some preparation of high-density polyethylene
Ziegler-Natta process
Cr
2
O
3
/support
MoO
3
/support
75
C
140
C
234
C
Approx. temperature
Approx. pressure
60 psi
420 psi
1,000 psi
Usual state of the polymer in reaction mixture
Suspension
Suspension
Suspension
They demonstrated that the catalyst can be immobilized on silica. The product yields ultrahigh
molecular weight polyethylene. Increased polymerization temperature resulted in higher activity, but
lower molecular weight of polyethylene.
6.1.3 Commercial High-Density Polyethylene, Properties, and Manufacture
High-density polyethylene (0.94-0.97 g/cm
3
) is produced commercially with two types of catalysts:
1. Ziegler-Natta type catalysts
2. Transition metal oxides on various supports
The two catalytic systems are used at different conditions. Both types have undergone evolution
from earlier development. The original practices are summarized in Table
6.1
.
The Ziegler process yields polyethylene as low as 0.94/cm
3
in density, but process modifications
can result in products with a density of 0.965 g/cm
3
. The transition metal oxide catalysts on support,
on the other hand, yield products in the density range of 0.960-0.970 g/cm
3
.
The original development by Ziegler led towhat appears to be an almost endless number of patents for
various coordination-type catalysts and processes. As described in Chap.
4
,
such catalysts have been
vastly improved. Progress was made toward enhanced efficiency and selectivity. The amount of polymer
produced per gram of the transition metal has been increased manyfold. In addition, new catalysts, based
on zirconium compounds complexed with methyl aluminoxane oligomers (sometimes called Kominsky
catalysts), were developed. They yield very high quantities of polyethylene per gram of the catalyst. For
instance, a catalyst, bis(cyclopentadienyl)-zirconium dichloride combined with methylaluminoxane, is
claimed to yield 5,000 kg of linear polyethylene per gram of zirconium per hour [
14
].
An important factor in the catalysts activity is the degree of oligomerization of the aluminoxane
moiety. The catalytic effect is enhanced by increase in the number of alternating aluminum and
oxygen atoms. These catalysts have long storage life and offer such high activity that they need not be
removed from the product, because the amount present is negligible [
14
,
15
]. This makes the work-up
of the product simple.
The continuous solution processes are usually carried out between 120 and 160
C at 400-500 lb/in.
2
pressure. The diluents may be cyclohexane or isooctane. In one zone reactors, the solid catalyst is evenly
dispersed throughout the reactor. In the two zone reactors (specially constructed), the polymerizations
are conducted with stirring in the lower zone where the catalysts are present in concentrations of
0.2-0.6% of the diluent. Purified ethylene is fed into the bottom portions of the reactors. The polymers
that form are carried with small portions of the catalyst to the top and removed. To compensate for the
loss, additional catalysts are added intermittently to the upper “quiescent” zones.
In suspension or slurry polymerizations, various suspending agents, like diesel oil, lower petro-
leum fractions, heptane, toluene, mineral oil, chlorobenzene, or others, are used. The polymerization
temperatures are kept between 50 and 75
C at only slightly elevated pressures, like 25 lb/in.
2
. If these
are batch reactions, they last between 1 and 4 h. The slurry reactor is illustrated in Fig.
6.2
.
Polymerizations catalyzed by transition metal oxides on support were described variously as
employing solid/liquid suspensions, fixed beds, and solid/gas-phase operations. It appears, however,
that the industrial practices are mainly confined to use of solid/liquid suspension processes. The
