Chemistry Reference
In-Depth Information
rubber can be used directly in the slab forms in which they are recovered from
solution polymerizations, because natural rubber is handled in that form and the
processing equipment in the rubber industry is designed to accommodate it.
12.4.2
Heterogeneous Systems
12.4.2.1
Heterogeneous Bulk Reactions
If the monomer and polymer are not mutually soluble, the bulk reaction mixture
will be heterogeneous. The high-pressure free-radical process for the manufacture
of low-density polyethylene is an example of such reactions. This polyethylene is
branched because of self-branching processes illustrated in reaction (8-89).
Branches longer than methyls cannot fit into the polyethylene crystal lattice, and
the solid polymer is therefore less crystalline and rigid than higher density
(0.935
0.96 g cm
2
3
) species that are made by coordination polymerization
(Section 11.5).
Ethylene is polymerized in high-pressure processes by free-radical reactions at
pressures of 1000
280
C. Ethylene is
3000 atm and temperatures of about 200
0.5 g cm
2
3
under these conditions.
Polyethylene remains dissolved in ethylene at high pressures and temperatures but
separates as an ethylene-swollen liquid in the lower ranges. The extent of long-
chain branching from chain transfer to polymer (Section 8.8.4) depends on the
local reaction temperature and concentrations of monomer and polymer. These
factors are determined in turn by the prevailing conversion of monomer to poly-
mer, the efficiency of mixing, and the local ethylene-polyethylene phases.
Various reactor designs can be employed to produce polyethylenes with about the
same average molecular weight and frequency of short branching. The molecular
weight distributions and long-chain branching will depend strongly on reactor
geometry and operation, however, and such polyethylenes are often clearly distin-
guishable in their processing properties and in some physical characteristics.
High-pressure ethylene polymerizations are continuous processes in which eth-
ylene and any comonomers, like vinyl acetate, are fed into tubular or stirred auto-
clave reactors. The reaction is ignited and sustained by periodic injections of
peroxide solutions while the polymerizing mixture travels through the reactor.
The ethylene and polyethylene leave the reactor and go into a primary separation
vessel which operates at a much lower pressure than the reactor. Most of the eth-
ylene is flashed off in this unit and recycled through compressors to the tube inlet.
Conversion per pass is of the order of 30% with ethylene flow rates about
40,000 kg/h. Since the polymerization is not isothermal, polymer properties
reflect the reaction history. Polyethylene made in the initial, cooler reactor
regions will have higher molecular weight and less branching than material made
in subsequent, hotter zones. The final product is a mixture of polyethylenes with
different molecular sizes as well as branch types and levels. Reactor operating
parameters can be varied to optimize manufacturing costs and polymer properties
a supercritical
fluid with density 0.4
