Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-03-15
2002-10-08
Wilson, D. R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S329200, C525S330300, C525S330400, C525S330600, C525S344000, C525S345000, C525S370000, C525S371000, C525S387000, C526S919000
Reexamination Certificate
active
06462138
ABSTRACT:
Lessening residual monomers in liquid systems by adding a redox initiator system
The invention relates to a process for reducing the residual monomer content in a liquid polymer system by postpolymerizing with addition of a redox initiator system.
In the customary free-radically initiated polymerization of olefinically unsaturated monomers or monomer mixtures, the polymerization reaction normally proceeds only to a monomer conversion of from 90 to 99% by weight, irrespective of whether it is conducted in solution, in bulk, in suspension or in dispersion. The skilled worker is aware of the reasons for the incomplete reaction of the monomers (Trommsdorf-Norrish effect, reduction in diffusion rate, transfer and branching reactions, etc.). The resulting unreacted monomers which remain in the system (residual monomers) are undesirable for various reasons (reduction in degree of conversion, polymer contamination, odor, toxicity and/or flammability of the monomers, etc.). There has therefore been no lack of attempts to remove or reduce the amount of residual monomers remaining after the main polymerization. This step is referred to hereinbelow as postpolymerization.
Thus it is known, inter alia, to remove residual monomers by treatment with steam in a kind of steam distillation (cf. eg. EP-A 327 006, EP-A 650 977 or U.S. Pat. No. 4,529,753). The treatment, however, is complex and applies only to aqueous systems, and its success is dependent on the volatility of the target monomers. It is also customary following the main polymerization to add polymerization initiators again to the batch and to conduct an aftertreatment, known as postpolymerization, at an appropriate polymerization temperature so as to reduce the amount of residual monomers. In this context the addition of redox initiator systems has been widely described (cf. eg. U.S. Pat. No. 4,289,823, EP-A 9258, EP-A 241 127, EP-A 357 287, EP-A 417 960, EP-A 455 379, EP-A 474 415, EP-A 492 847, EP-A 522 791, EP-A 623 659). Frequently a twofold aftertreatment, in EP-A 9258 even a threefold aftertreatment, is recommended in order to polymerize fully the residual monomers. A repeat aftertreatment, however, is undesirable since it prevents the polymerization reactor being used again quickly for its principal purpose. The multiplicity of solutions recommended in the prior art is an indication that no satisfactory solution has been found. As indicated for example in EP-A 279 892, the success of the appropriate aftertreatment depends also on the size of the reactor used. For example, the temperature control and the homogenization behavior of large production reactors is very different from that of laboratory reactors, and simply transferring the residual monomer reduction experiences gained to large production reactors is in general not possible.
It is an object of the present invention to provide a process which can be carried out on a large scale and achieves a successful reduction in residual monomers by postpolymerization in a liquid system in a production reactor, by which is meant a reactor having a volume of more than 20 and preferably more than 100 liters.
We have found that this object is achieved and that the residual monomer content of a liquid solution, mixture, melt, suspension or dispersion of a polymer prepared by free-radical polymerization is reduced by postpolymerization with addition of a redox initiator system at a reaction temperature appropriate to it, which comprises metering—gradually, in portions or continuously—at least one of the redox initiator components required to initiate polymerization of the residual monomers into the reaction mixture in a production reactor, with a defined and extremely short mixing time of the liquid system in the production reactor, over a period (metering time) which is from about 10 to 250, in particular from 20 to 100, times the mixing time of the liquid system in said reactor.
An important technical parameter which plays a part in the process for controlled reduction of residual monomers in the liquid polymer system is the mixing time &thgr; of the liquid system in the production reactor used, by which is meant the time required to obtain a certain degree of homogenization by mixing and, in particular, by stirring. To determine the mixing time it is common to use the schlieren method and the chemical decolorization method. The latter involves adding a reagent to the liquid and coloring the liquid with an indicator. Then, at the beginning of mixing or stirring, the second reagent is added and a measurement is made of the time which elapses until the coloration disappears. The endpoint degree of homogenization depends on the excess of the added reaction component.
The mixing time is dependent, inter alia, on the Reynolds number, which depends in turn on the reactor form, on the type and speed of the stirrer and on the density and viscosity of the liquid system. Scaling up the mixing time from small to large reactors is difficult and always accompanied by errors, since there are no reliable literature data for calculating nonnewtonian liquids such as dispersions. For simple model calculations the literature (see eg. Ullmanns Encyklopädie der techn. Chemie, 4th Edition, Volume 2, pages 259 ff., especially 263-264 and FIG. 9) offers idealized relationships for liquids with no differences in density or viscosity, which allow an approximate calculation of a minimum mixing time. From the viscosity &eegr; of the liquid medium, its density &rgr;, the stirrer speed n and the stirrer diameter d and taking into account the boundary conditions it is possible to calculate the Reynolds number Re of the system and, from this number, the mixing time &thgr;:
Re=n d
2
&rgr;/&eegr;(1) n &thgr;=ƒ(Re)
In accordance with these equations, the mixing time for idealized stirred reactors (laboratory, pilot and production scale) was calculated for the case of an anchor stirrer and an aqueous polymer dispersion with a viscosity of 30 mPas and a density of 1 g/cm
3
, taking into account the geometric similarities; the results of these calculations are shown in the table below. The chosen stirrer speeds n are based on practical experience. As the diameter of the stirrer blade increases there is a rise in the stirring speed and hence in the shear to which the stirred material is exposed. In order to obtain a comparable and constant input of power into the variously dimensioned reactors and to avoid an excessive rise in the peripheral stirrer speed, it is common practice to reduce the stirrer speed as the size of the reactor goes up. The table below shows that the mixing time changes with the reactor size.
Stirrer
Peri-
Speed
diameter
pheral
Reynolds
Mixing
Reactor
n/s
(mm)
speed
number
time &thgr; (s)
Laboratory
2.5
110
860
1000
40
Pilot
1
1100
3400
40,000
60
plant
Production
0.666
2500
5200
140,000
120
Simply transferring the aftertreatment conditions from the laboratory scale, as is described in the prior art, to larger stirred vessels is hence not an option.
When the height to diameter ratio (H/D) of the reactors changes toward greater elongation, as is preferred on the basis of favorable heat dissipation, there is a sharp rise in the mixing time. For a cross-arm stirrer Ullmann (loc. cit.) gives: n&thgr;=16.5×(H/D)
2.6
. For a production reactor with a typical H/D of from 2 to 2.5, the mixing time rises by a factor of from 5 to 10, despite the fact that the cross-arm stirrer is a better mixer than the anchor stirrer.
In accordance with the process of the invention the liquid-system mixing time in the reactor should be extremely short. This requires not only slow metering of at least one of the components of the initiator system used but also its effective stirred incorporation into the liquid system. To achieve this it is possible firstly to choose a production reactor having suitable geometric parameters, a highly effective stirrer with appropriate speeds, or combinations thereof. Hence a reduction in mixing time can be achieved by using a close-clearance helical stirrer or c
Bächer Reinhard
Bauer Gerhard
Dobbelaar Johannes
Hübinger Wolfgang
Kirchner Werner
BASF - Aktiengesellschaft
Wilson D. R.
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