Continuous polymerization method

A continuous polymerization method to obtain polymer production in the form of fine particles from a liquid polymerization medium; the reaction being conducted in a polymerization reactor wherein mixing is effected by the action of a plurality of elliptical paddles mounted on each of dual rotating shafts, characterised in that the dual shafts are rotated in reverse directions to each other, and the paddles being enclosed by walls of the reactor with the inside surface of the walls closely defining the surface generated by the rotation of the ends of both sets of paddles; and with the ends of major axes of the elliptical paddles on one rotating shaft periodically approaching the ends of minor axes of the corresponding elliptical paddles on the other rotating shaft to effect a mixing action as well as a longitudinal shearing action across a notional interface between the two shafts. The method is particularly useful for the polymerization of trioxane.

This invention relates to a method of a continuous polymerization of a 
liquid polymerization medium to obtain fine particles of polymer product, 
the reaction being continuously effected in a polymerization reactor 
wherein mixing is effected by the action of a plurality of paddles mounted 
on each of dual rotating shafts. 
The homo- or co-polymerization of molten trioxan is widely practised. Thus 
the production of poly-oxymethylene (co-) polymer, is industrially very 
important in the production of polyacetal resin. 
The present invention 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 required. 
When molten trioxan (if desired containing material comonomer for example 
one or more of the monomers ethylene oxide, dioxolan, butanediol, formal 
and diethylene glycol formal) is polymerized in the presence of a strong 
acid e.g. phosophorous pentafluoride or perchloric acid or tin chloride or 
boron trifluoride, to give for example poly-oxymethylene, the very rapid 
reaction rate changes the liquid phase of the polymerization medium into a 
solid phase through a short intermediate slurry phase. 
If the reaction is effected without a comminuting step large blocks of 
stiff product will be obtained resulting in difficult handling, a 
deterioration in quality due to accumulated polymerization heat, and 
lowered polymerization yield. Reaction under a high shearing action is a 
particularly preferred technique for the prevention of large blocks of 
product and for providing effective removal of polymerization heat of 
which various detailed methods have been proposed. A reactor which is a 
mixer extruder having dual shafts supporting paddles is a useful apparatus 
because it imparts a high shearing action to the contents. For example 
published Japanese Patent specification No. 84890/76 discloses a dual 
shaft mixer comprising a combination of elliptical paddles. Such features 
have a disadvantage 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-cleaning 
action, the ability to fully granulate the contents of a polymerizing 
apparatus, and paddles free from polymer adhering thereto. However 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 solution of this problem published Japanese Patent 
specification No. 86794/78 discloses a method which restricts the degree 
of high shearing action 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 specified range, and if it increases too much 
the load on the final vessel providing high shearing becomes too high, and 
if the conversion is too low the degree of filling of the second reactor 
increases so as to cause agglomeration of solid particles leading to a 
deterioration of quality. Thus the method according to the said Japanese 
Patent specification 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 rotating in the same direction to vary the 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 changes according to the progress 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. However it has now been found that in such a mixer the 
shearing force automatically 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, the 
reaction being continuously effected in a polymerization reactor wherein 
mixing is effected by the action of a plurality of paddles mounted on each 
of dual rotating shafts, characterised in that the said dual shafts rotate 
in reverse directions to each other, and the said paddles are enclosed by 
walls of the said reactor the inside surface of the said walls closely 
defining the surface generated by the rotation of the ends of both sets of 
paddles; the ends of major axes of the said paddles on one rotating shaft 
periodically approaching the ends of minor axes of the corresponding 
paddles on the other rotating shaft to effect a mixing action as well as a 
longitudinal shearing action across a notional interface between said two 
shafts. 
The 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.

The mixer 1 includes a closed long narrow space 2 having a cross-section as 
shown in FIG. 2. The space 2 accommodates two shafts 3 and 4. On the first 
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.degree. or 60.degree., 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 inside surfaces in close contact with the paddles. The 
mixer 1 has an inlet port for charging the liquid polymerization medium 
and an outlet port 11 for discharging the solid product. The liquid medium 
e.g. troixan is 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 can be introduced from any direction. The catalyst can be 
charged also together with the starting material e.g. trioxan. As shown in 
FIG. 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 
FIG. 4 and FIG. 5. FIG. 4 shows the movement of the contents when the 
shafts rotate in the same direction, and FIG. 5, when the shafts rotate in 
reverse directions, the contents shown in hatched outline. In FIG. 4 the 
paddles rotate by 90.degree. in the stages (a).fwdarw.(b).fwdarw.(c). With 
respect to the space (B) enclosed by the paddles 5', 6' and walls 9, the 
space volume, while undergoing some change, is merely moved from 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 FIG. 5, which illustrates 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 between the paddles 5 
and 6, and longitudinal mixing and adequate shearing are effected. There 
is thus a significant difference between polymerization 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 reaction has not yet occurred or the reaction is less than 20% 
completed, the contents still being in liquid state. The requirements for 
the reactor mixer in 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 proceeds in the range from 20 to 60% completion. The 
required 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 full shearing force has been applied in the 
preceding stage), the liquid not remaining as a continuous phase. 
Requirements for the reactor in this stage are slow agitation which is 
enough to prevent adhesion between solid particles, heat removal, and a 
retention time to allow for completion of the polymerization. Shearing 
effects are not required. 
A feature of the dual shaft reactor described in the said Japanese Patent 
specification No. 84890/76 with elliptical paddles rotating in the same 
direction, which had been considered best before the advant of the present 
invention, is such that two corresponding paddles rotate always in contact 
with each other (with self-cleaning effect) and rotate the space defined 
by the paddles and the walls of the mixer, while changing its 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 with each other, is 
amall because of the low viscosity of the contents at this stage. These 
features are favourable also in the second stage where a strong shearing 
force is required; a reason why the same directional rotation system has 
been considered desirable. In the third stage, the contents are in 
substantially the form of solid particles, the volume of which and the 
interstitial spaces are difficult to change. If such contents are forced 
to change volume and form, they show a strong resistance and impose a very 
high load, and therefore the apparatus should be operated at a lower 
degree of filling. However a low filling degree leads to sinking of solid 
particles and uneven force exerted on the shafts resulting in bent shafts 
and increased load. Thus the operative range is extremely limited. 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 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 with 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 the apparatus of this invention has a similar function to 
the same direction rotation apparatus. 
In the second stage of reaction high shearing force is required, and the 
reverse directional rotation apparatus, 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 adhesion, the clearance between paddles is 
of significance in that it allows 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 always 
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 direction as the reaction stage proceeds, namely 
according to the phase change of the contents. 
For the above reasons, the same directional rotation reactor and reverse 
directional rotation reactor cannot be operated under the same conditions. 
In the conditions that attain enough filling and keep sufficient retention 
time for the reverse directional rotation reactor, 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 reactor is half that for the reverse 
directional reactor. Even in this range however the shafts of the same 
directional rotation reactor can be bent 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 permits reaction at a rate from zero to nearly 
100% and can be used also as the primary or secondary reactor in a 
two-stage reaction method. 
This invention will be further described with reference to the examples. 
EXAMPLE 1 
One hundred parts by weight of trioxan, 2.5 parts by weight of ethylene 
oxide, and 100 ppm boron trifluoride were charged into a reactor shown in 
FIG. 1. Water at 25.degree. 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%. 
EXAMPLE 2 
Materials of the same composition as Example 1 were reacted in the 
apparatus shown in FIG. 1, with a residence time of 2 min. The conversion 
at the discharge port was 60%. 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%. 
COMATIVE EXPERIMENT 
Similar 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 much that the paddles contacted the barrel and stopped 
the motor. Thus the experiment could not be continued.