The present invention relates to a novel process for continuous preparation of (co)polymers, block copolymers and graft copolymers of at least one olefinically unsaturated monomer by (co)polymerization, block copolymerization and graft copolymerization in bulk. The present invention also relates to a novel Taylor reactor for implementing this process.
In the text below, (co)polymerization, block copolymerization and graft copolymerization are referred to collectively as “polymerization”. Accordingly, the (co)polymers, block copolymers and graft copolymers are referred to collectively as “polymers”.
As is known, olefinically unsaturated monomers are polymerized continuously in bulk by a free-radical, anionic or cationic method in the presence or absence of small amounts—i.e., up to 25% by weight of the reaction mixture—of organic solvents. In the course of the polymerization reaction, the kinematic viscosity ν changes by a factor of at least 10, so that the handling of the polymers becomes difficult. It is therefore often necessary to take the polymerization only to a comparatively low conversion, for example, a maximum of 70 mol %, so that the reaction mixtures comprising polymers and monomers can still be mixed thoroughly and discharged from the reactors. It is then necessary to separate the unreacted monomers from the polymers, something which is complex and expensive from the standpoints of safety, energy, and process engineering. Monomers whose volatility is low can in many cases not be separated off at all, with the consequence that the monomer content of the polymers remains undesirably high.
In the German patent application DE 198 28 742 A1 it is proposed to conduct the polymerization of olefinically unsaturated monomers in bulk in a Taylor reactor under the conditions of Taylor vortex flow.
Taylor reactors, which serve to convert substances under the conditions of Taylor vortex flow, have been known for a long time. They consist essentially of two coaxial concentric cylinders of which the outer is fixed while the inner rotates. The reaction space is the volume formed by the gap between the cylinders. Increasing angular velocity ωi of the inner cylinder is accompanied by a series of different flow patterns which are characterized by a dimensionless parameter, known as the Taylor number Ta. As well as the angular velocity of the stirrer or rotor, the Taylor number is also a function of the kinematic viscosity ν of the fluid in the gap and of the geometric parameters, the external radius of the inner cylinder, ri, the internal radius of the outer cylinder, ro, and the gap width d, the difference between the two radii, in accordance with the following formula:Ta=ωirid ν−1(d/ri)1/2  (I)Where d=ro−ri.
At low angular velocity, the laminar Couette flow, a simple shear flow, develops. If the rotary speed of the inner cylinder is increased further, then, above a critical level, alternately contrarotating vortices (rotating in opposition) occur, with axes along the peripheral direction. These vortices, called Taylor vortices, are rotationally symmetric, possess the geometric form of a torus (Taylor vortex ring), and have a diameter which is approximately the same size as the gap width. Two adjacent vortices form a vortex pair or a vortex cell.
The basis of this behavior is the fact that, in the course of rotation of the inner cylinder with the outer cylinder at rest, the fluid particles that are near to the inner cylinder are subject to a greater centrifugal force than those at a greater distance from the inner cylinder. This difference in the acting centrifugal forces displaces the fluid particles from the inner to the outer cylinder. The centrifugal force acts counter to the viscosity force, since for the motion of the fluid particles it is necessary to overcome the friction. If there is an increase in the rotary speed, there is also an increase in the centrifugal force. The Taylor vortices are formed when the centrifugal force exceeds the stabilizing viscosity force.
If the Taylor reactor is provided with an inlet and an outlet and is operated continuously, the result is a Taylor vortex flow with a low axial flow. Each vortex pair passes through the gap, with only a low level of mass transfer between adjacent vortex pairs. Mixing within such vortex pairs is very high, whereas axial mixing beyond the pair boundaries is very low. A vortex pair may therefore be regarded as a stirred tank in which there is thorough mixing. Consequently, the flow system behaves like an ideal flow tube in that the vortex pairs pass through the gap with constant residence time, like ideal stirred tanks.
If, however, there is a sharp change in the viscosity ν of the fluid in the axial flow direction as conversion progresses, as is the case with polymerization in bulk, the Taylor vortices disappear or are not even formed. In that case, Couette flow, a concentric, laminar flow, is observed in the annular gap. There is an unwanted change in the mixing and flow conditions within the Taylor reactor. In this operating stage it exhibits flow characteristics which are comparable with those of the laminarly flow-traversed tube, which is a considerable disadvantage. For example, in the case of polymerization in bulk, there is an undesirably broad molecular mass distribution and chemical polydispersity of the polymers. Moreover, the poor reaction regime may result in considerable amounts of residual monomers, which then have to be discharged from the Taylor reactor. However, there may also be instances of coagulation and polymer deposition, which in some cases may even lead to blockage of the reactor or of the product outlet. Overall, it is no longer possible to obtain the desired products, such as polymers of comparatively narrow molecular mass distribution, but only those whose profile of properties does not meet the requirements.
In order to solve these problems, a Taylor reactor was provided having                a) an external reactor wall located within which there is a concentrically disposed rotor, a reactor floor and a reactor lid, which together define the annular reactor volume,        b) at least one means for metered addition of reactants, and        c) a means for the discharge of the product, where        d) during the polymerization there is a change in the viscosity ν of the reaction medium and        e) the reactor wall and/or the rotor are or is geometrically designed in such a way that the conditions for Taylor vortex flow are met over substantially the entire reactor length in the reactor volume.        
Condition e) is met by the annular reactor volume widening, in particular widening conically, in the direction of flow traversal. As a result, the known Taylor reactor is able substantially to solve the problem of maintaining the Taylor flow when there is a sharp increase in the kinematic viscosity ν in the reaction medium.
In the known Taylor reactor, the annular reactor volume is defined by the concentrically disposed rotor, the reactor floor, and the reactor lid. This means that the product outlet must be disposed at the side of the Taylor reactor or in the reactor lid and cannot be designed without edges. With this configuration, however, an undisrupted product discharge is difficult to realise, since edges and dead spaces result in deposition of polymers. Moreover, in this area, the Taylor flow which is still present in the highly viscous reaction medium may easily collapse, thereby impairing the mixing of monomers and polymers and increasing the likelihood of deposition at edges and in dead spaces.
In the known Taylor reactor, furthermore, the passage of the drive shaft for the rotor is rotated in the reactor lid. This means that the rotor is driven in the area in which the kinematic viscosity ν is at its highest. As a result, the seals and connections are subject to a particularly high mechanical load.
Owing to the disadvantageous combined effect of flow and geometric configuration, on the one hand the known Taylor reactor is unable to solve all of the safety and process engineering problems which occur in connection with polymerization in bulk and on the other hand it is still not possible to increase the monomer conversion to an extent where substantial freedom from monomers and a narrow molecular weight distribution and molecular weight polydispersity of the polymers are achieved.
It is true that the problem of insufficient mixing of the reactants may be solved to a certain degree by inserting a mixer upstream of the entry point for the reactants, as is described in the German patent application DE 199 60 389 A1. However, the problems set out above continue to occur in the outlet region in the case of polymerization in bulk.
The American patent U.S. Pat. No. 4,174,907 A discloses a Taylor reactor in which the rotor is mounted rotatably in the inlet region of the reactants. At its other end, the rotor is not mounted but instead ends substantially before the outlet region, which at its broadest point has the same diameter as the outer reactor wall. The outlet region narrows in the form of a funnel to an outlet pipe. The known Taylor reactor is used to mix liquids of different viscosity and electrical conductivity. Moreover, it may be used for the reaction of polyisocyanates with polyols. To what extent it can be used for the polymerization of olefinically unsaturated monomers in bulk, the American patent does not reveal.
In the known Taylor reactor, the drive shaft is passed through the reactor floor and is connected to the rotor in the inlet region of the reactants. However, in the inlet region of the reactants the rotor does not have the diameter which would be necessary in order to establish Taylor flow in this region. Moreover, the annular reaction volume does not widen in the direction of flow traversal. It is true that the American patent does specify, in column 10 lines 29 to 33, that the concentric parts may also have configurations other than the cylindrical—for example, substantially spherical or conical—configurations, but there is no teaching as to what configurations are of particular advantage for polymerization in bulk.
It is an object of the present invention to provide a novel Taylor reactor which no longer has the disadvantages of the prior art but which is instead especially suitable for the polymerization of olefinically unsaturated monomers in bulk, in the course of which the kinematic viscosity ν in the reaction medium multiplies by a factor of at least 10 in the course of the reaction. The novel Taylor reactor should readily permit the preparation of polymers with a conversion >70 mol % without being accompanied by the formation of gas bubbles and/or the deposition of polymers in the annular reaction volume and/or in the outlet region. Furthermore, the novel Taylor reactor should have a particularly long operational duration and service life.