NL8103900A - PROCESS FOR CONTINUOUS POLYMERIZATION FROM A LIQUID POLYMERIZATION ENVIRONMENT TO A FINE-PARTICULAR POLYMER PRODUCT. - Google Patents

Description

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Process for the continuous polymerization of a liquid polymerization medium into a fine particulate polymer product.

The invention relates to a process for the continuous polymerization of a liquid polymerization medium to a fine particulate polymer product, wherein the reaction is continuously discharged into a polymerization reactor, in which mixing takes place by means of a number of vanes, mounted on each of two rotary shafts. The homo- or copolymerization of molten trioxane is widely used. For example, the preparation of polyoxymethylene (co-) polymer is technically very important in the preparation of polyacetal resin.

The invention is particularly suitable for such a continuous polymerization of trioxane, although it can also be used for other processes where a phase transition takes place and a desired granulation step is required.

When molten trioxane (which optionally contains a comonomer material, for example one or more than the monomers (epoxyethane, dioxolan, butanediol, formal and diethylene glycol formal)., The presence of a strong acid, for example, phosphorus pentafluoride or perchloric acid or tin chloride or boron trifluoride, is polymerized. for example, poolyoxymethylene, the very high reaction rate converts the liquid phase of the polymerization medium into a solid phase via a short intermediate phase in knitting form.

If the reaction is carried out without a crushing step, large blocks of stiff product will be obtained, resulting in difficult handling, deterioration in quality due to the accumulated heat of polymerization and a reduction in polymerization yield. A particularly preferred technique for preventing the S! 9 3 9 0 0 I ^ * I.

Formation of large blocks of product and to provide effective removal of the heat of polymerization is the high shear reaction, of which various detailed methods have been proposed. A reactor constructed as a mixing vane with two vane-bearing shafts is a useful device because it exposes the contents to a high shear. For example, published Japanese Patent No. 84890/76 discloses a two-axis mixer comprising a combination of elliptical blades. However, when used for polymerization reactions, this device has the disadvantage that the two shafts both rotate in the same direction. The important aspects of this system are the strong shearing action on the contents, a self-cleaning action, the ability to completely granulate the contents of a polymerization device, as well as keeping the blades free of adhering polymer. However, these advantages are offset by the higher loads applied to the rotary shafts, so that the contents of the vessel must be limited for safe operation.

As a solution to this problem, published Japanese Patent No. 86794/78 discloses a method in which the degree of the strong shearing action is reduced to a smaller value and a second reactor with a smaller shearing action is provided. Such 2-stage reaction techniques limit the degree of conversion achieved to a prescribed range and, if this degree of conversion increases too much, the! load on the last vessel, which causes the high shear, is too high, while if the conversion rate is too small, the degree of filling of the second reactor increases such that agglomeration of the solid particles is caused, leading to deterioration of the quality. The method according to said Japanese patent no.

Thus, in practicality, 35 86794/78 is limited to changing the quality of the material and product.

It is therefore desirable to provide optimum shearing action in one and the same reactor depending on the course of the reaction. Although in two-axis devices employing rotating shafts in the same direction, it is possible to change the shearing force by changing the pitch of the worms or by changing the clearance in the interior of the device, such an apparatus is not readily adaptable because the course of the reaction is dependent on minor changes in the reaction conditions and material quality. There is thus a need for a device in which the shearing action is subject to change depending on the course of the reaction.

A two-shaft vane type mixer, the shafts of which rotate in opposite directions relative to each other, has hitherto not been considered a polymerization device because it produces only a small shear force and is not self-cleaning. However, it has now been found that in such a mixer the shear force automatically changes in the desired direction according to the phase changes that occur in liquid phase polymerization reactions.

The invention now provides a process for the continuous polymerization of a liquid polymerization medium into a fine particulate polymer product, wherein the reaction is carried out continuously in a polymerization reactor. In which the mixing takes place by means of a number of blades, which are arranged on each of two rotating shafts, the method being characterized in that the two shafts rotate in opposite directions to each other and the blades are surrounded by walls of the reactor the internal surface of the walls accurately delimiting the surface of revolution determined by rotation of the ends of both groups of blades, and the ends of the major axes of the blades on the one rotating axis being the ends of the minor axes of the periodically approximate corresponding blades on the other rotary axis to produce a mixing action as well as a longitudinal shearing action over an imaginary interface between the two shafts.

The process according to the invention can be successfully used for polymerization reactions, in which a phase transition from bloom to solid occurs, in particular for the continuous polymerization of trioxane.

The invention is described below and will be elucidated with reference to the annexed drawings, in which Figure 1 shows a schematic view of the mixing reactor 1 used in the method of the invention, with the broken-away part the location of the shafts shows;

Figure 2 shows a cross-sectional view along line A-A in Figure 1; and

Figure 3 shows a partial view of a shaft of the mixer.

The mixing device 1 comprises a closed long narrow space 2 with a cross-section, as shown in figure 2. In the space 2, two shafts 3 and 4 are accommodated. A number of blades 5,6,7,8 ... are arranged on the first shaft 3 and the second shaft 4 in such an arrangement that corresponding blades on the two shafts interlock alternately. For example, successive blades on the same axis have been displaced through 90 ° or 60 ° to vary the mixing characteristic. Beveled blades 7,8 are also included under the blades. . A surrounding wall 9 is provided around the circumference of the blades, the inner surfaces of which come into precise contact with the blades. The mixer 1 has a supply port for supplying the liquid polymerization medium and a discharge port 11 for discharging the solid product. The liquid medium, for example trioxane, is fed through the feed port 10 at one end of the mixing reactor 1, while the catalyst is fed through the catalyst feed 12 and mixed with the liquid medium and the solid product through the discharge port 11, the other end is provided, .8103900 i ** * -5- is discharged. The location of the catalyst feed 12 is not limited to the top of the mixer, since the catalyst can be fed from any direction.

The catalyst can also be supplied together with the starting material, for example trioxane. As shown in figure 3, a conveying worm 13 is provided near the supply port, with which the contents are pushed away. The chamfered transport vanes 7, which are interposed between the adjacent non-chamfered vanes, 10 help to advance the contents.

The relationship between the movements of the blades and the contents when the two shafts rotate in the same direction or in opposite directions is shown in figure 4 and figure 5. Figure 4 shows the movement of the contents when the shafts rotate in the same direction and Figure 5, if the shafts rotate in opposite directions, the contents being indicated by the hatching. In figure 4, the blades in the steps (a1 - (bl - (c)) are each rotated through 90 °. As for the space (Bl, which is enclosed by blades 5 ', 6', and the walls 9, the room volume, while undergoing some modification, is simply transported from left to right.This process achieves only a small mixing effect, while the high resistance increases the load applied to the device 25. In Figure 5 illustrative of the invention instead, the space (E) at the stage (a) is reduced by compression as it progresses from stage (b) to (c), while the space (G) is gradually increased, therefore the contents move in the 30 the arrow indicated direction (F) through the clearance between the blades 5 and 6, while longitudinal mixing and satisfactory shearing are effected, so there is a significant difference between polymerization processes, where a rotation of the shafts in the same direction and 35 e and rotation of the shafts in the opposite direction is applied, as described in more detail below.

As set forth in published Japanese Patent No. 86794/78, the polymerization of 8103900 <# -6-trioxane is divided into three steps:

In the first stage, the rapid reaction has not yet occurred or the reaction is less than 20% complete and the contents still flake in the liquid state. The requirements to be met by the mixing reactor at this stage is simply good mixing power.

In the second stage, the reaction proceeds with a rapid phase transition from liquid to solid. The reaction proceeds within the range of 20-60% of the total conversion. The required properties, which the mixing reactor must meet, are then strong shearing effects and good heat removal.

The third step leads to the formation of fine solid particles (provided that total shear force was applied in the previous step), while the liquid no longer remains as a continuous phase. The requirements to be met by the reactor at this stage are slow agitation sufficient to prevent adhesion between the solid particles, dissipation of heat, and residence time allowing polymerization to complete. Shear Effects are not required.

A typical welding aspect of the two-axis reactor described in the aforementioned Japanese Patent No. 84890/76 with elliptical blades rotating in the same direction, which device was considered best prior to the invention, is that two associated blades always rotate in contact with each other (with the self-cleaning effect) and the space passing through the blades. the walls of the mixer are determined, rotated, while the volume and shape are changed to cause significant deformation of the contents. This aspect has a favorable effect in the first stage of the reaction, but the effect due to the fact that the blades are always in contact with each other is small because of the low viscosity of the content in this reaction. stairs. These aspects are also beneficial in the second stage, where a strong shear force is required? 8103900 --7- a reason why the same direction rotating system is considered desirable. In the third stage, the content is essentially in the form of solid particles, the volume and spacing of which are difficult to change.

If such a content is forced to change in volume and shape, it shows a strong resistance and gives rise to a very high load, which makes it necessary to operate the device at a smaller filling degree. However, a small degree of filling causes the solid particles to sink down and an uneven force to be exerted on the shafts, resulting in shaft deflection and increased load. The business process is therefore extremely limited. In addition, in order to extend the residence time, the ratio of length to diameter must be increased, which will further increase the displacement of the rotating shafts.

On the other hand, the two-axis reactor having counter-rotating blades, which is used according to the invention, although the coupled elliptical blades are at the end of the major axis; of a blade coming into contact with the end of the minor axis of the other blades, the other portions of the blades do not touch each other during rotation. The reactor is thus not self-cleaning in the usual sense. In the first stage of the reaction, the problem of mixing liquids of low viscosity has virtually no connection with the direction of rotation, and the device according to the invention therefore has the same function as a device using the same direction of rotation.

In the second stage of the reaction, a high shear is required and the device, in which rotation is applied in the opposite direction and which is not of a self-cleaning type, appears at first sight to be disadvantageous because of the small shear force. In fact, the contents at the staircase, which have a strong tendency to stick together, are barely apparent from the clearance 8103900 -8-

φ V.

to move between the blades and good shearing action is brought about by the reverse rotation device as in the same rotation device. The play between the blades 5 is of little importance. In the third stage, in which the solid particles exhibit a relatively weak adhesion, the clearance between the blades is important, because this makes it possible for the particles to move to another space via this clearance. This keeps the resistance and the load smaller even at higher fill levels. There the. solid particles always rubbing along the surface of the blades, undesired adherence of the polymer hardly occurs despite the fact that the blades are not self-cleaning. The device with the rotation in the opposite direction thus has such properties that the application of the shearing force, ie the load on the device, automatically changes in a desired direction as the reaction stage proceeds, namely depending on the phase. transition of the 20 contents.

For the above reasons, the reactor with the rotation in the same direction and the reactor with the rotation in the opposite direction cannot be operated under the same conditions. Under the conditions, 25 'in which a sufficient filling and a sufficient residence time are maintained for the reactor with the rotation in opposite directions, the reactor with the rotation in the same direction cannot be operated due to the greatly increased resistance of the solid filling and -30 the maximum fill degree within the operating range of the reactor with the same direction of rotation is half the fill level for the reactor with the opposite direction of rotation. Even within this range, the shafts of the reactor can bend with the same direction of rotation during stirring. Due to this pendulum effect, the clearance between the blades and the cylinder must be made large enough to prevent mutual contact. This gives rise to a thicker layer on the walls of the 8103900-9 cylinder, resulting in poor heat dissipation and deterioration in product quality. The residence time is extended by operating at a smaller filling degree and, moreover, a deterioration in quality is caused.

In the reverse rotation apparatus used in the process of the invention, the automatic change of properties in the same reactor responds entirely to the change in reaction rate due to the change in reaction conditions and material quality. Thus, the reactor used according to the invention makes it possible to carry out reactions at a conversion degree from 0 to almost 100% and it can also be used as a primary or secondary reactor in a 2-stage reaction process.

The invention is further described by the examples.

Example I

100 parts by weight of trioxane, 2.5 parts by weight of epoxyethane and 100 parts per million of boron trifluoride were fed to a reactor as shown in figure 1. Water was passed through the jacket at 25 ° C. The shafts were rotated in opposite directions at 45 rpm. After a residence time of about 8 minutes, a fine powdery product was obtained from the discharge port. The unconverted content. monomer in the product was about 2%.

Example II

In the apparatus, as shown in Figure 1, 30, materials of the same composition as in Example 1 were reacted at a residence time of 2 minutes. The conversion rate at the discharge port was 60%. This reaction product was further fed to a stirrer equipped with blades in a cylinder and cooled with water and stirred therein for 10 minutes. The unreacted monomer content in the product obtained from the stirrer was 2%.

8103900 * -10- c

Comparative test

An attempt was made to carry out a similar polymerization in the same reactor as used in Example 1, the shafts of which rotated in the same direction.

After the start of the polymerization, the load on the device increased significantly and the shafts swung so much that the blades came into contact with the cylinder and the engine stopped. The trial could therefore not be continued.

8103900

Claims (3)

1. A method for the continuous polymerization of a liquid polymerization medium into a fine particulate polymer product, wherein the reaction is carried out continuously in a polymerization reactor, in which the mixing takes place by means of a number of vanes mounted on each of two rotary shafts characterized in that the two shafts rotate in opposite directions to each other and the blades are surrounded by walls of the reactor, the internal surface of the walls precisely delimiting the orbit surface which, by rotation of the ends of Heather groups of blades are determined, and the ends of the major axes of the blades on one rotating axis periodically approach the ends of the minor axis of the corresponding blades on the other rotary axis to effect a mixing action as well as a shearing action in lengthwise across an imaginary interface between the two axes. 2. Process according to claim 1, for the continuous polymerization of trioxane, optionally in combination with comonomer. 3. Method as claimed in claim 1 or 2, substantially as described above and explained with reference to the accompanying drawings. 81 0 3 9 0 ü