Note: Descriptions are shown in the official language in which they were submitted.
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This invention re~a-tes to a method of continuous
polymerization of a liquid polymerization medium to obtain
fine particles of polymer product? the reaction being
continuously effected in a polymerization reactor wherein
rnl~ing is effected by the action of a plurality of paddles
mounted on each of dual rotating shafts~
The ho~o- or co-polymerization of molten trioxan
is wid~ly practised. Thus the production o~ poly-o~ymethy-
lene (co-) polymer, is industrially ~ery important in the
production of polyacetal resin.
The present in~ention is particularly suitable
to such continuous polymerization of trioxan, although it
can be used for other processes wherein a phase change
takes place and in which a desired granulating step is re-
quirsd.
When moltQn trio~an ~if desired containingmaterial comonomer for example one or more o~ the monomers
ethylene o~ide, dioxolan, butanediol, formal and diethylene
glycol formal) ls polymerized in the presence of a strong
acid e.g, phosphorous penta~luoride or pQrchloric acid or
tin chloride or boron trifluoridQ, to giVQ for example
poly-oxym~thylene, the vQry rapid reaction rate changes
the liquid phase of ~e poly~lerization medium into a solid
phase through a short intermediatQ slurry phase.
If the reaction is e~fected without a comminuting
step large blocks of stiff p~oduct will be obtained resul-
tlng in difficult handling, a deterioration in quality due to
accumulated polymerization heat, and lowered polymerization
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yield. Reaction under a high shearing action is a
particularly preferred technique for the prevention of
large blocks of product and for providing e~ective removal
of polymeri~ation heat of which various detailed methods
have bee~ proposed. A reactor which ls a mixer extruder
having dual shafts supporting paddles is a useful apparatus
I because it imparts a high shearing ac-tion to the contents.
¦ For example published Japane~e Paten-t specification No.
¦ 84890/76 discloses a dual shaft mlxer co!nprising a combina-
tion of elliptical paddles. Such features have a disad-
I vantage however when used for polymerization reactions
! in that the dual shafts all rotate in the same direction~
The features of this system are the strong shearing action
~ on the contents, a self clsaning action, the ability to fully
I 15 granulate the conten-ts of a polymeriæing apparatus, and
paddles free from polymer adhering thereto. Howe~er such
- advantages are offset by -the higher loads that are applied
to the rotating shafts, and for safe operation the contents
of the vessel must be restricted. For the solutlon of
this problem published Japanese Patent specification No.
86794/78 discloses a method which restrlcts the degree of
high shearlng actior. to a lower value and provides a
second reactor of lower shearing action. Such two-stage
reaction techniques however restrict the conversion obtained
to a specif`ied range, and if it increases too much the load
on the final vessel providing high shearing becomes too
high, and i~ the conversion is too low the degree of filling
of the 4econd reactor increases so as to cause agglomeration
of solid particles leading to a deterioration o~ quality.
Thus the method according to the said Japanese Patent
specifica-tion No. 86794/78 is limited in adaptability to
change of material quality and product grade. It is
therefore desirable to provide an optimum shearing action
in the same reactor in accordance with the progress of
reaction, While it is possible in dual shaft apparatuses
using shafts rotatlng in the same direction to vary the
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shearing force by changing the pitch of the screws or by changing the clearance
inside the apparatus, since the progress of the reaction depends upon slight
changes of the reaction conditions and material quality, such apparatus is not
readily adaptable. Thus there is a need for apparatus in which shearing action
ch~m ges according to the progr0ss of reaction.
Hitherto a paddle-type dual shaft mixer the shafts of which rotate in
reverse directions to each other has not been considered as a polymerization
apparatus because it effects only low shearing force and is not self-cleaning.
~lowever it has now been found that in such a mixer the shearing force automatical-
ly changes in the desirable direction corresponding to changes in phase
occurring in liquid phase polymerization reactions.
The invention provides a method of continuous polymerization of a
liquid polymerization medium to obtain fine particles of polymer product by
conducting the reaction in a polymerization reactor wherein ~.ixing is effected
by the action of a plurality of elliptical paddles mounted on each of dual
rotating shafts, characterised in that the shafts are rotated in reverse
directions to each other~ and the paddles are enclosed by walls of the reactor
with the inside surface of major axes of the elliptical paddles on one rotating
shaft periodically approaching the ends of the minor axes of the corresponding
elliptical paddles Oll the other rotating shaft to effect a mixing action as
well as a longitudinal shearing action across a notional interface between
said two shafts.
'l'he method according to this invention can expeditiously be used
for polymerization reactions in which a liquid-to-solid phase-change occurs,
particularly for the continuous polymerization of trioxan.
This invention is hereinafter described and
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illustrated in the accompanying drawings, of which
Figure 1 is a schematic elevation of the mixer
reactor 1 used in the method of the invention, the broken
portion showing the position of the shafts;
Figure 2 is a cross-sectional view on line A-A
in Figure 1, and
Figure 3 is a partial elevation of a shaft of
the mixer.
The mixer 1 includes a closed long narrow space
2 having a cross-section as shown in ~igure 2. The space
2 accommodates two shafts 3 and 40 On the flrst shaft 3
and second shaft 4 are mounted a plurality of paddles 5, 6,
7, 8, ,.. in an arrangement whereby corresponding paddles
on both the shafts engage with each other alternately.
Successive paddles on the same shaft are,displaced for
example by 90 or 60 , to vary the mixing characteristics~
Skewed feed paddles 7, 8 are also included in the paddles,
Around the periphery of the paddles an enclosing wall 9
' is provided with its lnside surfaces in close contact with
20 ' the paddles. The mixer 1 has an inlet port for charging
the liquid poly~leri~ation medium and an outlet port 11 ~or
discharging the solid product. The liquid medium e.g.
trioxan ls charged from the charging port 10 into one end
of the Mixer reactor 1, and the catalyst is introduced
through the 'catalyst inlet 12 and mixed with the liquid
medium, and,the solid product is discharged from the
discharging port 11 provided at the other end. The
position of the catalyst inlet 12 is not limited to the
upper portion of the mixer, and the catalyst ca,n be
introduced from any direction. The catalyst can be
charged also together with the starting material e.gO
trioxan. As shown in Figure 3 a feed screw 13 is
positioned near the charging port and pushes forward the
contents. The skewed feed paddles 7 arranged between the
adjacent non-skewed paddles help to push the contents
forward.
The relationship of the movements of paddles and
contents when the dual shafts rotate in the same direction
or in reverse directions is shown in Figure 4 and Figure
~5. ~igure 4 shows the movement of the contents when the
shafts rotate in the same direction, and Figure 5, when the
shafts rotate in reverse directions, the contents sho~n in
hatched outline. In ~igure 4 the paddles rotate by 90
in the stages (a~ (b) -~ (c). With respec-t to the space
(B) enclosed by the paddles 5~, 6~ and walls 9, the space
volume, while undergoing some change, is merely moved Prom
right to left. Thus, only a small mixing effect is obtained
in this process, while the large resistance increases the
load applied on the apparatus. In contrast to this in
~igure 5, which lllustratss the invention, the space (E)
in the stage (a) is decreased by compression when moving
from stage (b) to (c), the space (G) being gradually
expanded. Therefore the contents move in the arrowed
direction (F) through the clearance bet~een the paddles
5 and 6, and longitudinal mixing and adequate shearing
! 20 are effected. There is thus a significant difference
between po:Lymerization processes using co-directional
rotation of the shafts and by the reverse-directional
rotation, as hereafter further described.
As set forth in published Japanese Patent
specification No. 86794/78, the polymerization of trioxan
is divided into three stages:
In the first stage rapid ~eaction has not yet occurred
I or the reaction is 1Q~ than 20% completed~ the contents
still being in liquid state~ The requirements for the
reactor mixer i~ this stage is merely a good mixing ability,
In the second stage, reaction proceeds with a rapid phase
change from liquid to solid. The reaction proce~ds in the
range from 20 to 600/o completion. The re~uired properties
of the reactor mixer are strong shearing effects and good
removal of heat. The third stage results in the formation
of fine particles of solid (providing ~ull shearing force
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has been applied in the preceding stage), the liquid not
remaining as a continuous phase~ Requirements for th~
reactor in this stage are slow agitation which is enough
to pr~vent adhesion between solid particles, heat removal,
S and a retention time to allow for completion of the poly-
merization. Shearing effects are not required.
A feature of the dual shaft reactor described
in the said Japanese Paten-t specifica-tion No. 84890/76
with elliptical paddles rotating in the same direction,
which had been considered best before the advan-t of the
pres~nt in~ention, is such that two corresponding paddles
rotate always in contact with each othsr (with self-
cleaning effect) and rotate the space deflned by the
I paddles and the walls of the mixer, while changing its
1 15 volume and shape to effect substantial deformation of the
contents. This feature has favourable effect in the
first stage of reaction, but the effect arising from the -
fact that the paddles are always in contact ~ith each
other, is aalall because o~ the low ~iscosity of the
contents at -this stage. These features are favourable also
in ths second stage where a strong shearing force is re-
¦ quired; a reason why the same directional rotation systemhas been eo;nsidered desirable. In the third stage, thQ
contents are in substantially the form of solid particles,
the ~olume of which and the interstitial spaces aredifficult to change. If such contents are forced to
change ~olume and form, they show a strong resistance and
j impose a very high load, and therefore the apparatus should
be operated at a lower degree of filling. Howe~er a low
fllling degree leads to sinking of solid par-ticles and uneven
- force exerted on the shafts resulting in bent shafts and
increased load. Thus the operztive range is extremely
linlited~ To increase the retention time, in addition,
the length/diameter ratio must be increased, which will
further increase shipping of the rotating shafts.
In contrast, in the dual shaft reactor with
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paddles rotating in reverse directions used according
to this invention, though coupled elliptical paddles
contact at the end of the major axis of a paddle wlth the
end of the minor axis of the other paddle, other parts
of the paddles do not contact each other on rotation.
Thus the reactor is not self-cleaning in the usual sense.
In the first stage of reaction, the problem of mixing
low viscosity liquids has little correspondence with the
direction of rotation, and th~ apparatus of this invention
has a similar function to the same direction rotation
apparatus.
In the second ~tage of reaction high shearing
force is roquired9 and the reverse directional rotation
apparatus7 which is not a self-cleaning type, at first
sight appears to be disadvantageous with weak shearing
force~ In fact however the contents at this stage,
having a strong tendency to stick to each other, hardly
move from the clearance between the paddles, and good
shearing action is effected by the reverse directional
rotation apparatus, like the same directional rotation
apparatus. The clearance between paddles has little
significance. In the third stage wherein solid particles
have relatively weak adhesion9 the clearance between
paddles is of significance in that it allo~s the particles
to move into another space through it. Therefore the
resistance and load are kept lower even at higher filling
degrees. In addition, since the paddle surface is
alwayq rubbed by solid particles, undesirable sticking
of polymer hardly occurs in spite of the paddles not being
self cleaning. Thus the reverse directional rotation
apparatus has such characteristics that the exertion of
the shearing force, that is the load on the apparatus,
automatically changes in a desirable direc-tion as the
reaction stage proceeds9 namely according to the phase
change of the contents.
For the above reason~, the sa~e directional
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rotation reactor and re~erse directional rotation reactor
cannot be operated under the sqme conditions. In the
conditions that attain enough filling and keep sufficient
retention time for the reverse directional rotation reactor9
the same directional rotation reactor cannot operate because
of greatly raised resistance of the solid filling and the
maximum filling degree in the operative range for the same
directional rotation reaotor is half that for the reverse
directional reactor. Even in this range however the
shafts of the same directional rotation reactor can be
ben* during agitation Because of this whipping effect
the clearance between the paddles and the barrel must be
made large enough to prevent their contact. This results
in a thick layer on the barrel walls leading to poor heat
removal and lowered product quality. The operation at
lower filling degree extends retention -time and also causes
lowered quality.
In the reverse directional rotation apparatus
used in the method of this invention, the automatic change
of characteristics in the same reactor fully responds to
the change of reaction rate due to the change of reaction
conditions, material quality and grade Thus the reactor
used according to this invention per~its reaction at a rate
from zero to nearly 100% and can be used also as the prirnary
or secondary reactor in a two-stage reaction method.
This invention will be further described with-
reference to the examples.
Exam~le 1
One hundred par-ts by weigh-t o-f trio~an, 2.5 parts
by weight of ethylene oxide, and 100 pprn boron trifluoride
were charged into a reactor shown in ~igure 1. Water at
25 C was passed through the jacket. The shafts were
rotated in reverse directions at 45 rpm. After a residence
time of about 8 min. a finely powdered product was obtained
from the discharge port. Unreacted monomer content in the
product was about 2%.
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Exa~ple 2
Materials of the same composition as Example 1
were reacted in the apparatus shown in Figure 1, with a
residence time of 2 min. The conversion at the discharge
port was 600/o. This reactant was further fed into an
agitator having paddles inside a cylinder which was
water-cooled and agitated for 10 min. The unreacted
monomer content in the product taken out of the agitator
was 2%.
Comparative Experiment
Sirnilar polymerization was tried in the same
reactor as in Example 1, with the shafts rotated in the
same direction. Upon the start of polymerization, the
load on the apparatus increased substantially and the
shafts whipped so muc~ that tho paddles contacted the
¦ barrel and stopped the motor. Thus the e~periment could
¦ not be continued.
