Chemistry Reference
In-Depth Information
transfer and prevent inclusion of gelled polymer (“fish eyes”) in the product.
Cleanliness of the reactor walls is very important because the productivity of the
equipment is enhanced by longer intervals between shutdowns for cleaning. To
this end, some phenolic coatings have been designed that inhibit polymer buildup
by terminating free-radical reactions on the walls (cf. Section 8.9).
Modern reactors are made of stainless steel and have capacities up to as much
as 180 m
3
(50,000 U.S. gal). Designs vary, and include top and bottom entry stir-
rers and multiple impellers. Agitation of the reaction mixture must be given seri-
ous attention since many monomers are less dense than water while their
polymers are more dense. The mixing systems must then pull monomer down
from the surface of the charge and lift polymer off the floor of the reaction
vessel.
The major process for poly(vinyl chloride) production is the suspension sys-
tem. Typical reaction temperatures are 50
65
C. As the reaction proceeds, a
conversion (
76%) is reached at which the only monomer left in the system is
that absorbed in the polymer particles. This occurs when the monomer concentra-
tion is about 30 wt% in the particles. The occurrence of this phenomenon is sig-
naled by a drop in the reactor pressure. Normal pressures in the autoclaves are
initially about 150 psig (pounds per square inch, gauge), and it is usual to carry
out polymerizations until the pressure drops to about 20
70 psig, depending on
the reaction temperature. Water may be injected into the reaction vessel as the
polymerization proceeds, to compensate for the volumetric contraction between
monomer and polymer. This also helps prevent the reaction mixture from becom-
ing too viscous. The water addition also enhances the cooling capacity of the
reactor because it increases the heat transfer area on the walls.
Porosity is a desirable characteristic of the particles in many applications of
poly(vinyl chloride). If the product is to be used as a “dry-blend” resin, it is
required to soak up substantial quantities of liquid plasticizers and still remain
free-flowing. The structure and porosity of PVC granules is affected strongly by
the choice of organic suspending agents, which are different types of partially
hydrolyzed poly(vinyl alcohols). The required porosity is enhanced also by rap-
idly removing the unreacted monomer which is occluded in the particles. For this
reason, such suspension polymerizations of poly(vinyl chloride) are not driven to
conversions much greater than about 80%. Polymer intended for extrusion into
pipe and other nonplasticized applications is taken to higher conversions to
enhance the reactor productivity and the bulk density of the PVC granules.
Higher bulk densities result in greater production rates during subsequent extru-
sion, because the polymer powder is fed more efficiently from the extruder hop-
per into the conveying screws. Higher polymerization temperatures also result in
higher final bulk densities, but these conditions produce lower PVC molecular
weights.
Sudden increases in reaction rate, called “heat kicks,” are sometimes encoun-
tered toward the end of the PVC reaction. This probably results from the deterio-
rating heat conductivity of the polymer particles as the monomer concentration in
B
