Riegel’s Handbook of Industrial Chemistry. Part 2

615 Pages·2019·194.65 MB·English

by  Kent J.A. (Ed.)

674 RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY
chain transfer agent, tints, suspending agent,
stabilizers, plasticizers, and other additives
usually are charged at this time although
delayed addition of one or more of these
additives is not uncommon. The reaction is Cycle
Ga.
almost always run in a nitrogen atmosphere.
Cooler
The batch then is heated through a predeter
mined temperature cycle, with the nitrogen
pressure maintained to prevent uncontrolled
boiling.
When the elevated portion of the tempera
ture cycle is complete, unreacted monomer
Ethylene or
may be removed from the batch by distillation. Propylene
The batch then is cooled below the glass Comonomet,
transition temperature of the beads and
Fig. 19.16. Unipol process for polyolefin manufac
discharged to a hold tank, from which it passes ture. (Courtesy Union Carbide.)
to a continuous centrifuge that removes most
of the suspension liquor and washes the beads agent, is fed continuously into the bottom of
with water, primarily to remove suspending a reactor containing polymer granules main
agent residues. The beads then pass to a dryer, tained in a fluidized state by the gas stream.
typically a cocurrent, hot air, rotary type. A transition metal catalyst is added separately.
Suspension processes have certain disad Reactor conditions are relatively mild: 50 to
e
vantages. Except for special applications such 1000 and 100 to 600 psi. Because of the large
as expandable polystyrene and ion exchange ratio of surface to volume of the granules
resins, the product beads are not favored for where the polymerization takes place, as well
critical fabrication steps because their rolling as the turbulent conditions in the fluidized
behavior often causes feeding difficulties with bed, there is very efficient transfer of the heat
extruders and injection-molding machines. To of reaction to the gas stream. As overheating
minimize these problems, the beads often are of the granules can be so effectively avoided,
blended with other ingredients or extruded there is very little tendency for them to
into pellets before being fed to these machines. agglomerate, and product quality control is
In addition, there are low levels of suspending greatly enhanced.
agent residues normally found in the product, After passing through the fluidized bed, the
which often affect optical clarity. More gas enters an expanded section of the reactor
importantly, costs are continually escalating where fine particles are disengaged. On exiting
for treating the large volumes of contaminated the reactor, the gas is pumped through a
water discharged from these processes, as well cooling heat exchanger and then is recycled
as for the control of organic vapor emissions. to the reactor. The granular product is
The trend, then, is to replace them wherever removed continuously from the reactor
possible with bulk processes that can avoid through a gas lock chamber. The small
these disadvantages. amount of residual monomer in the granules
at this point is purged off for recycling. The
Gas Phase Fluid Bed Processes. These devolatilized product granules, averaging 0.02
processes are of particular commercial impor to 0.04 inch, then are conveyed from the
tance with polyolefin production. A prominent system. With polyethylene, monomer yields
example of this technology is the Unipol are in the 97 to 99 percent range. Because
process, first commercialized in 1975.47 A flow the catalysts have very high productivity
sheet is shown in Fig. 19.16. A gaseous (105-106 lb polymer/lb transition metal),
monomer, such as ethylene or propylene, catalyst residue levels in the product are so
along with comonomers and chain transfer low that it is unnecessary to remove them.
SYNTHETIC RESINS AND PLASTICS 675
With polypropylene, catalyst selectivity for is reached. The second plateau, however, has
the isotactic isomer is so high that removal no practical significance because of the
of the small quantity of atactic polymer extremely high shear rates needed to reach it
formed is not needed. and the virtually unavoidable thermal effects
This process has a high level of environ that tend to mask it. The nonnewtonian
mental acceptability in view of its very low portion of the curve, on the other hand, is so
emissions of hydrocarbons and other waste frequently encountered in operations with
streams. polymers and concentrated polymer solutions
that it cannot be overlooked by designers and
operators.
POLYMER RHEOLOGY Because the nonnewtonian region in Fig.
19.17 usually can be reasonably approximated
Polymer melts and concentrated polymer
by a straight line on a log-log plot, the "power
solutions have many flow properties that
law" equation is a particularly straightforward
distinguish them from monomeric liquids and
and useful tool for modeling this behavior. It
solutions, and these properties can figure very
can be expressed by:
importantly in the design and operation of
process and fabrication equipment. (19.1)
A principal difference involves the effect of
where '1 = apparent viscosity, y = shear rate,
shear on viscosity. Monomeric liquids and
solutions exhibit newtonian behavior; that is, n = the flow behavior index, and subscript
zero denotes reference values, often taken at
viscosity at constant temperature is unaffected
the onset of nonnewtonian behavior. It can
by shear rate. Most polymer melts and
be seen that where n = 1, there is no
concentrated polymer solutions, however, are
shear rate effect, and behavior therefore is
nonnewtonian; their viscosity at a given
newtonian. For most polymers and concen
temperature will change as the shear rate is
trated polymer solutions, n is in the range of
increased. In general, it will drop as illustrated
0.25 to 0.5. Where n > 1, the viscosity
in the viscosity versus shear rate plot in Fig.
increases with shear rate, but such shear
19.17. At very low shear rates there is a
thickening materials are not encountered in
newtonian plateau where viscosity does not
practice with polymer solutions and melts.
change. Depending on molecular weight and
Many fluids also exhibit time-dependent
structure, a shear rate is reached where
viscous effects. With some pastes, drilling
viscosity starts to drop off. This dropoff
muds, and latex paints, for example, there is
continues until a second newtonian plateau
a drop in viscosity with time when they are
sheared at a constant shear rate. This effect,
known as thixotropy,48,49 is illustrated in Fig.
r
19.18. There is an opposite effect, rheopexy,
where there is an increase in viscosity with
Newtonian Plateau
time under shear. Fluids with this property
~ ~o ---- are relatively rare, however.
~ Viscosity generally decreases markedly with
UoI ~Non-newtonian
l;l shear-thinning region temperature, An Arrhenius-type equation
:;:
commonly is used to relate viscosity with
temperature in ranges well above 1'g:
( 19.2)
where A and B are constants characteristic of
Shear rate, y the material, and T is the absolute tempera
ture. Estimation of the constants requires
Fig. 19.17. Typical log viscosity vs.log shearrate plot
for shear-thinning fluids. viscosity data at two temperatures. For
676 RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY
higher average, Mz• Viscosity versus tempera
ture data are available in the literature.52,54,55
Melt viscosity also is a function of pressure, 52
but the change is negligible until one operates
in the range of 5000 to 25000 psi, where
viscosity increases by factors of lO to as much
as 500 can occur. 53 Because static pressures
in this range are encountered with injection
molding and some extrusion operations, the
effect cannot always be ignored.
Polymer melts and solids can exhibit
- Shearing stopped combinations of two types of behavior when
I
I deformed: (1) elastic, where the deformation
I
I energy is stored as potential energy and is,
ideally, fully and instantly recoverable; and
Time, t (2) viscous, where the deformation energy is
not stored but is dissipated over time as heat.
Fig. 19.18. Viscosity vs. time relationship for a thixo
These materials are called viscoelastic, and,
tropic fluid under constant shear until t = tt.
in analyzing their behavior, both types of
deformation must be taken into account.
temperatures in the range
The degree to which a viscoelastic material
I'g < T < (T + lOO°C), behaves as an elastic solid or a viscous liquid
g
depends largely on the time scale and pattern
the Williams-Landel-Ferry equation50 is more
of the applied stress and the response time
reliable:
required by the system. For example, a ball
10g('1/'1T,) = -17.4(T - I'g)/[51.6 + (T - I'g)] of Silly Putty dropped to the floor will bounce
elastically as if it were made of rubber. Here
(19.3 )
there is a very brief applied compressive stress
The viscosity of polymer melts at zero shear as the ball hits the floor. If that same ball is
is a function of the weight average molecular allowed to rest on the floor for a sufficient
weight, M where the molecular weight length of time, it will slowly start to flow and
W'
distribution (MWD) is narrow: end up as a puddle, like a viscous liquid.
Depending on composition, structure, and
'10 = KMj (19.4)
temperature, a viscoelastic material has a
For concentrated polymer solutions, the characteristic relaxation time, A, defined as the
following related equation is used: time required for a stress to decay to 1/ e of
its elastic response to stopping any change in
(19.4a)
strain. 56 The response time can be expressed
where '10 = zero-shear viscosity, K and K' are as:
proportionality constants, and c = polymer
A = '1/G ( 19.5)
solution concentration. The exponent j is
unity for low M and 3.4 where M exceeds where G is the modulus of the material.
W' w
a critical molecular weight, Me> at which there Figures 19.10 through 19.13 show generalized
is a shift to a more restricted mode of chain curves of modulus versus temperature for
movement. Me is a function of the solvent polymeric materials. There is a high-modulus
used (if any) and the composition and glassy state that drops off to a rubbery plateau
structure of the polymer; it usually is in the for uncured or lightly cross-linked polymers.
range of 5000 to 20,000.51 As the MWD Tg occurs at the inflection point of the dropoff.
broadens, the molecular weight in equation More heavily cross-linked polymers have a
(19.4) gradually shifts from Mw to the next higher plateau region above their I'g's, whereas
SYNTHETIC RESINS AND PLASTICS 677
there is a further dropoff of G for uncured, creep period. This effect is commonly observed
linear polymers beyond the rubbery plateau. with ordinary rubber bands, which, after
The ratio A/te, where te is the time scale weeks or months of remaining stretched, will
of the experiment, is called the Deborah no longer fully return to their original length
number.57 There is elastic response where the when released. The molecular chains slowly
Deborah number is high, as with the bounced and irreversibly began sliding by one another
Silly Putty (t is the very brief ground contact during the extended stretch period instead of
e
time of the bounce.) Where the Deborah behaving as end-anchored, parallel coiled
number is low, the behavior is viscous (as with springs. Light cross-linking (i.e., vulcanization
the protracted experiment, with Silly Putty or curing) of the rubber inhibits this chain
flowing on the floor). sliding effect. As cross-linking is increased,
Figure 19.19 shows schematically the pattern however, chain mobility will drop to a point
of strain response to an applied tensile stress where the desired rubbery behavior is lost.
with time t for a viscoelastic material. Many other viscoelastic effects are encoun
Following application of the stress at t 1, there tered in the manufacture, fabrication, and use
is an initial, rapid elastic deformation, produc of plastics. Extrudate swell as a melt emerges
ing a strain D. There ensues a nonlinear, strain from a die is a form of elastic recovery from
versus time period, following which the the extensional stresses applied as the melt
relationship becomes virtually linear. This passes through the die orifice. This must be
latter pattern, known as creep, can, in taken into account with many extrusion
principle, be observed with all solids if the operations. Shrink films are made by biaxial
time scale is long enough. At t2 the stress is stretching at temperatures slightly above I'g,
released, and the material initially contracts followed by quenching. The resulting built-in
elastically in a manner comparable to the film strain is relieved at an exceedingly slow
initial strain D. Following this, the strain rate at ordinary temperatures because of
versus time relationship approaches an the very long mean relaxation time. With
asymptotic value of nonrecoverable strain, sufficient heating, however, the relaxation
essentially equal to that developed during the time will drop by orders of magnitude. The
built-in strain then rapidly diminishes, and
the film shrinks.
A final example is the Weissenberg effect,58
where a viscoelastic fluid will flow at right
angles to the direction of a steady-state shear
stress. Thus, materials such as polymer
solutions or flour dough will climb up an
immersed rotating agitator shaft.
FABRICATION OF PLASTICS
The variety of means and the relative ease by
D c
which plastics can be formed into useful
objects are important advantages in comparing
Extension ---l-Recovery them with traditional materials. We will
survey here some of the principal fabrica
tion techniques employed today. Although
Time, t
our fundamental understanding of many of
Fig. 19.19. Idealized viscoelastic material response
these operations has broadened considerably,
(Burger's model) to constant, protracted tensile stress
especially since about 1950, a great deal of
starting at t1, followed by sudden release of the stress
art remains. In addition, the field is dynamic,
at t2'
678 RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY
with important design and control innovations stripping and house siding), or be coated
continually being introduced. directly over wire, cable, and cord, or onto
paper. In addition, extrusion is incorporated
into other types of fabrication procedures
Extrusion
such as injection molding and blow molding,
Extrusion is one of the basic methods of which will be described later.
fabrication, particularly for thermoplastics. It The workhorse design for plastics extruders
consists of shaping a material by forcing it is based on a single screw rotating in a
through a die, originally via a hydraulic ram. horizontal, cylindrical barrel, usually equipped
By the 1930s screw extruders, adapted from with a hardened liner. A schematic diagram
rubber technology, were replacing older is shown in Fig. 19.20. The extruder barrel
designs because they could be run con generally is equipped with temperature
tinuously, were adaptable to a much broader controlled sections along its length. Heating
range of materials than the older equip can be via electricity or heat transfer fluids,
ment, and were capable of vastly superior with cooling via forced air, water, or other
performance. coolant fluids. The screw often is cored for
In addition to their material-shaping coolant circulation along a portion of its
function, screw extruders can be designed for length. The feed material most often flows via
such varied tasks as mixing and blending, gravity from a funnel-type hopper through a
addition or removal of volatiles, expelling feed throat in the barrel and into the channel
insoluble liquids, and carrying out controlled of the screw. The screw is driven by a motor
chemical reactions. The extrudate might be in through a gear reducer with a thrust bearing
the form of pellets of a desired size and shape, positioned to absorb the rearward thrust of
usually intended for a subsequent fabrication the screw. The feed melts as it is conveyed
step. It also might be formed into pipe, tubing, along the screw channel, and at the down
sheeting, film, and profiles (such as weather- stream end of the barrel the melt is forced
Hopper Cooling Breoker PI ole
JacKel
Adapler
--I-+-+---- Molor Odv,
01,
Fig. 19.20. Elements of a single-screw extruder. (Bernhardt, E. C., Processing of Thermoplastic Materials,
copyright by the Society of Plastics Engineers, Inc., 1959. Van Nostrand Reinhold, New York.)
SYNTHETIC RESINS AND PLASTICS 679
TRAILING LEADING
EDGE EDGE
REAR RADIUS
SEC T I ON - ---t--- TRANSI T ION - --+_-ME TElliNG SEC TlON-----i
t---------- f. L. = fLiGH T LE NG TH - ------------1
1--_ _ _________ O.A.L.: OVERALL LENGTH --------------~
Fig. 19.21. Single-stage extruder screw. (Courtesy Spirex Corp.)
through a breaker plate, which frequently section, and, finally, a shallow-flighted meter
supports a screen pack to filter out con ing section. The length and flight depth of
taminants. From there the melt exits through each are determined largely by the frictional
a shaping die. Single-screw extruders usually and melting behavior of the feed, the rheo
are sized on the basis of the inside barrel logical properties of the melt, the pressure
diameter, which can vary from as small as 0.5 drop of the screen pack and die, and the
inch for special laboratory or industrial desired output rate. Melt thermal stability and
applications with capacities down to a few special mixing or blending needs also play
pounds per hour, to as large as 24 inches important roles in this screw design. The
(600 mm) and capacities of over 60,000 clearance between the screw flights and barrel
lb/hour. Most industrial units are in the 1.5 must be tight enough for efficient melting and
to 8-inch range. pumping and good heat transfer, but loose
The heart of the extruder is the screw, and enough to avoid binding. The radial clearance
the success of an extrusion operation is largely commonly employed is about 0.1 percent of
dependent on its design. Since the early the barrel diameter.
1950s, a large volume of literature has been With a solid feed to the extruder, such as
published on the theoretical and practical pellets, powder, beads, flakes, and regrind,
aspects of extruder screw design and opera alone or in combination, most of the energy
tion. 58,64,92 - 95 A typical single-stage extruder for melting comes from friction and viscous
screw is shown in Fig. 19.21. The screw helix dissipation rather than heat transfer through
is very commonly "square pitch"; that is, it the barrel wall. The melting of feed in the
advances one turn per unit length equal to screw channel, a complex process, is illustrated
the screw diameter. This is equivalent to a schematically in Fig. 19.22. With these designs,
helix angle of 17.7° at the outside of the screw. there is generally little heating or melting in
The principal screw variable with this design the feed zone of the screw (1). However, as
is the change in root diameter with length. the feed enters the transition zone, it starts to
The screw, as illustrated, usually has three be compacted because of the decreasing
sections: a relatively deep-flighted feed channel depth and is forced against the barrel,
section, followed by a tapered transition causing a sliding friction that creates a film
Fig. 19.22. Schematic of feed melting in a conventional single extruder screw channel. (Courtesy Spirex Corp.)
680 RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY
of molten polymer there (2). This molten where a is dependent only on screw dimen
material is collected as a melt pool ahead of sions.
the advancing flight (3). The molten polymer Pressure flow is caused by pressure in the
circulates in a helical path. As the material head of the extruder. If we assume that the
continues along the transition zone, this screw is stationary, but that there is melt under
melting continues. The melt pool width pressure at the die, the screw channel will act
increases, and the solid bed shrinks (4). At like a long rectangular orifice. The melt, forced
some point the diminished solids bed often by the pressure in the die head, will travel
will break up, leaving the remaining solid backward down the helical screw channel. In
particles dispersed in the melt pool (5 and 6). actual operation, pressure flow is a reaction
Because the high shear stresses previously to drag flow, caused by the die restriction and
developed against the barrel no longer are the pressure developed in the die head. It is
possible, the remaining melting is primarily influenced by screw channel dimensions,
via thermal conduction from the melt, which barrel diameter, melt viscosity, and back
is far less efficient than previous melting. Any pressure. If viscosity is assumed constant,
remaining solids generally will be melted in then:
the metering section where shear rates are
Qp = (h3S sin ¢ cos ¢)(dp/dl)/12rJ (19.7)
higher.
Although there are many complexities in where 1 = axial channel length, p = melt
this process that are beyond the scope of pressure, and rJ = melt viscosity. Here:
this chapter, the output characteristics of a
Qp = (f3/rJ)dp/dl
properly designed square-pitch screw can be
considered to be largely functions of the where f3 depends only on screw dimensions.
diameter, length, and flight depth of the Leakage flow is a backflow in the narrow
metering section. In addition to output, the clearance between the screw flight and the
design of this section is of major importance barrel caused by the pressure developed in
in setting melt temperature and flow uni the die head. It is usually negligible with
formity. The shallow-flighted metering section single-screw extruders except where there has
has been most tractable to theory because been significant screw wear.
geometric factors there are relatively simple, The net volumetric output of a metering
and melt properties can be most readily section as described above can be expressed
defined. In its most basic form, the flow from as:
the metering section, Q, is made up of three
Q = Qd - Qp = aN - (f3/rJ)dp/dl (19.8 )
components: drag flow, Qd, pressure flow, Qp,
and leakage flow, Ql' Figure 19.23 illustrates how output varies
Drag flow is simply the forward conveying with head pressure according to equation
action developed by the relative motion
between the screw and the barrel in the
absence of any downstream flow restrictions
such as a screen pack or die. With shallow
channels and a single-flighted screw of narrow
flight width: s
Qd = (nDhS cos2 ¢)N/2 .; 1--~r~~~:::::,.~~~\\:cl,e~O\le:0 \(\'&
(19.6) 0 s<'"
where D = screw diameter, h = flight depth,
S = axial channel width, N = screw revolu
tions per unit time, and ¢ = screw helix
angle = arctan(S/nD). Equation (19.6) also
can be expressed as: Head pressure
Fig. 19.23. l.haracteristic screw and die operating
Qd = aN curves. (Courtesy Monsanto.)
SYNTHETIC RESINS AND PLASTICS 681
(19.8), with different metering sections oper relatively low. Here there is an abrupt
ating under conditions of equivalent screw decrease in root diameter following the first
speed and melt viscosity, assuming newtonian metering section just upstream of the barrel
behavior and isothermal conditions. Three vent. This increase in flight depth is intended
metering section designs are compared: (1) a to result in partial fillage of the melt channel
short, deep metering section; (2) a long, deep in that zone, thereby preventing any melt
metering section; and (3) a shallow metering discharge from the vent. The vent thus may
section. The output value at zero head be open to atmospheric pressure or connected
pressure is the drag flow, and the figure shows to a vacuum system, depending on the
how this is increased by deeper screw flights. volatiles in question and the degree of removal
Comparison of the slopes of these lines shows desired. The vent zone length with its partial
that the output of a short, deep-flighted fillage provides residence time for release of
section is more sensitive to head pressure than volatiles. Following the vent zone, the melt
a longer section of the same flight depth. The passes in turn through a second transition
least sensitive configuration illustrated is that zone and a metering zone before emerging
where the flight depth is shallow. Because it from the die. The second screw stage must be
is generally desirable for output to be designed for a higher output rate than the
relatively unaffected by head pressure, de first stage to avoid melt flow from the vent,
signers will try to provide a long, shallow but not high enough to cause surging or other
flighted metering section. The metering section flow instabilities.
length may be limited by length requirements The Lj D ratio of screws will vary from about
of the other screw sections and the overall 20: 1 to 36: 1 or greater, depending primarily
length of the barrel. If the metering flight on melting, metering, and venting require
depth is too shallow, it may not be possible ments.
to obtain the necessary output without The single-screw extruder screws described
excessive shear; this could result in mechanical above have relatively limited capability for
degradation or overheating of the melt. Also mixing and blending; so numerous devices
shown in Fig. 19.23 are the output versus head have been developed in attempts to improve
pressure characteristics of two dies: one with on them-generally short sections with LjD
a large opening and one with a small opening. ratios of about 2: 1 to 5: 1, and often attached
Both lines pass through the origin. The at the discharge end of the screw. Two types
intersections of the screw and die operating are shown in Fig. 19.25. The Dulmage section
lines represent the operating points of those has shallow, multiple flights with semicircular
particular screw-die combinations. melt channels usually separated by three or
It frequently is necessary to remove volatiles more short, undercut cylindrical sections that
such as air, moisture, or solvents from the interrupt the laminar flow, dividing and
melt before it emerges from the die. For this recombining the melt many times. It often is
purpose a two-stage screw with a vented used with foam extruders. The Union Carbide
barrel, as illustrated in Fig. 19.24, commonly (Maddock) section features a series of semi
is used as long as the level of volatiles is circular grooves along the screw axis, of which
Fig. 19.24. Two-stage screw to be used with a vented barrel. (Courtesy Spirex Corp.)
682 RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY
OlA.MllGE SCWI
MIXING LANOS
OUTLET fLUTE
MELT FLUTEJ
MIXING LANO
UNI~~ CARBID( MIXER
Fig. 19.25. Single-screw mixing/blending sections. (Courtesy Spirex Corp.)
alternate grooves are open to upstream entry, generally functioning as a transition section,
while the others are open to downstream most of which feature a secondary or barrier
discharge. The mixing lands shown are flight, which creates two distinct channels, one
slightly undercut to provide a greater clearance for the melt and one primarily for the solids.
with the barrel, compared with the slightly The principle is illustrated schematically in
narrower wiping lands. The polymer is Fig. 19.26. The barrier flight has greater
pumped to the inlet groove and is subjected clearance with the barrel than the primary
to a brief, high shear as it passes over the flight. This allows melted plastic to flow over
undercut mixing land. Then it is pumped out it while blocking the passage of feed
of the discharge groove because it cannot solids. Melting in these barrier screw sections
escape over the tight-clearance wiping land. proceeds in a manner similar to that described
This arrangement screens out unmelted earlier, but the channel for melt gradually
materials and thus can be designed with enlarges in the downstream direction while
deeper channels than other designs have, for the solids channel gradually becomes narrower
greater output with equivalent or better and / or shallower. This decrease in size of the
mixing. It is used most often with polyolefins, solids channel prevents breakup of the solids
especially low density polyethylene. bed, which frequently occurs with conven
Barrier and other duel channel screws have tional screws as shown in Fig. 19.22. Melting
been developed in recent years, primarily therefore can continue by the efficient
to improve melting efficiency, mixing, and mechanism of frictional heating against the
output. There are numerous designs,59,60 barrel. The Double Wave screw,65 shown in
C0T
~MELT CHANNEL
I
rBARRIER fLIGHT / SOLIOS CHANNEL
'1 __ '.f!iSl.iiii~' I!! 'Mijl (' ti' l (( , ; ; 1 /
( !!!
® ® @ ®
Fig. 19.26. Schematic of feed melting in the channels of a barrier type screw. (Courtesy Spirex Corp.)
SYNTHETIC RESINS AND PLASTICS 683
CHANNEL A
DEEP
L BARRIER
CLEARANCE
Fig. 19.27. Double Wave screw. (Courtesy Spirex Corp.)
Fig. 19.27, is a variant generally employed as requirements are more critical, where melt
an extended metering section. It has two thermal stability is limited, or where chemical
channels of equal width separated by an reactions also are being carried out. There are
undercut barrier flight. The roots of each several design concepts, depending on whether
channel go up and down in wave fashion, such the screws are corotating or counterrotating,
that the depth of one channel is shallow while and the degree to which they intermesh. These
the channel across the barrier is deep. The systems and variants are illustrated in Fig.
melt is thereby forced back and forth across 19.28.
the barrier, and is subjected to alternately high With counterrotating extruders, the screws
and low shear. Any solids particles are wedged at any common point have the same helix
into each wave crest, where they are briefly angle but are oppositely pitched. Some
exposed to high shear. These features promote intermeshing types feature conical rather than
melting and mixing, and high output can be parallel, constant-diameter screws. The large
achieved at lower melt temperatures with diameter of such screws at the feed end
improved pressure stability. promotes heat transfer and allows greater
A feature increasingly incorporated into
some single-screw extruders is the grooved
barre1.61 With this design there are longi CO- ROTATING COUNTER-ROTATING
tudinal grooves in the barrel extending 3 to
5 diameters from the feed port, with their
depth decreasing to zero in linear fashion.
These grooves greatly increase the frictional
heat generation at the barrel surface, so the
solids conveying zone can develop very
NON-INTERMESHING
effective conveying characteristics with re
sultant high pressures. The extruder output
thus becomes controlled by the feed zone ~~--e
rather than by the plasticating and melt
~~-6
conveying zones. The results are reported to
be a greater and more stable output and
PARTIALLY INTERMESHING
reduced sensitivity of the output to pressure.
These advantages are at least partially offset
~c~c
by lower efficiency (i.e., kg/(kWh)), greater
-'ff51NPl)-G
wear, and increased design complexity. ~C
Twin-screw extruders are more expensive
than single-screw types, but they have been FULLY INTERMESHING
widely employed with hard-to-feed materials, Fig. 19.28. Twin-screw systems. (Courtesy Werner
where compounding and devolatilization & Pfleiderer Corp.)