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
1999-07-08
2001-05-29
Boykin, Terressa M. (Department: 1711)
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
active
06239225
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a process for the manufacture of impact resistant modified polymers by polymerization, especially by radical polymerization of vinyl-aromatic monomers and ethylene unsaturated nitrile monomers in the presence of soluble rubber.
Impact resistant modified polymers manufactured by radical polymerization of aromatic monomers and ethylene unsaturated nitrile monomers in the presence of rubber are known under the name “ABS polymers” or “ABS molding material” (acrylonitrile-butadiene-styrene). Another type of impact resistant modified polymers, known as “HI-PS” polymers (high impact polystyrene), are obtained by polymerization of vinyl-aromatic monomer in the presence of rubber.
Advantageous for ABS manufactured by solution or mass polymerization is the higher rubber efficiency and the avoidance of wastewater, as well as the smaller usage of pigments due to the lighter natural color compared to ABS manufactured by the emulsion process. Disadvantageous for ABS manufactured in mass or solution polymerization is the lack of surface luster (herein “gloss”) compared to ABS manufactured by emulsion polymerization. The lower gloss is the result of the relatively large, dispersed rubber particles. There was, therefore, no shortage of attempts to eliminate this disadvantage and manufacture lustrous/glossy ABS in mass or solution polymerization.
A continuous process for the manufacture of ABS polymerizates is described in DE 4 030 352 in which the phase inversion takes place in a Ringspalt reactor, in which the rubber phase passes over from the outer phase to the inner separated phase, and accordingly the polystyrene co-acrylonitrile phase from the inner phase to the outer connected phase. A disadvantage is that at least three reactors are needed for continuous polymerization, and that the shearing stress prevailing in the Ringspalt reactor is relatively small.
Continuous processes for the manufacture of presently relevant resin were described in JP 0408020 and in U.S. Pat. No. 5,210,132 (corresponding to EP-A376 232). In the process disclosed in the '020 document, the reaction solution is sheared by the use of a particle dispenser having wings or rotors rotating at high and low speeds alternatively. The dispersion of the rubber particles is said to be controlled by the speed of the rotation. The process disclosed in the '132 document refers to shear rates which are preferably equal to or greater than 300 s
−1
The application of shear is by a particle disperser having one shearing stirrer composed of rotatable blade or cylindrical rotor rotating at a high speed. The maximum shear rate demonstrated in the example (Example 33) is less than 3000 1/s and there is no indication at all of the criticality of shear rate to the reduction in particle size or the width of the particle size distribution. In fact, the document in column 12, lines 27-39, relates the distribution of the particles to the reaction conditions. Moreover, the working and comparative examples provide no suggestion respecting the present invention which resides, in part, in the finding of critical dependence of both the reduction in particle size and narrowing of the particle size distribution on the shear rate. Also presently relevant are U.S. Pat. Nos. 5,514,750 and 5,550,186 which disclosed the application of shear in relevant processes. Shear rates in the range of 2-2000 1/s were disclosed and higher shear rates were taught away from (see col. 10, line 43 in the '750 patent and col. 11, line 53 in the '186 patent). The disadvantage of the prior art processes is the energy inefficient operation of the rotor/stator/machinery, which leads first to heating of the reaction material, and only second to the breaking up of the rubber particles.
The inventive process relates to a continuous manufacture of impact resistant modified polymers having increased gloss and improved impact properties. The process comprises polymerization, preferably free-radical polymerization, of vinyl-aromatic monomers with or without ethylene unsaturated nitrile comonomers, in the presence of a soluble rubber and optionally in the presence of solvents. In the process, upon completion of the phase inversion, at least part of the reaction mixture is sheared at a rate of at least 30,000, preferably 35,000 to 20,000,0001/s using a device which entails no rotating parts. In view of the state of the art, it was surprising that such high shear rates do not lead to a breakdown of the phases, and that the process may be carried out in the presence of a solvent. It is also surprising that the process achieves higher gloss of the impact resistant modified polymers obtained. It is also surprising that a reaction mixture containing polymerizable mixture and potentially gel forming and easily crosslinkable rubbers can be subjected to high shear rates without formation of gel particles, hard spots or pluggage of the dispersing devices.
The process is preferably carried out in two or more reactors arranged in sequence. Stirred tank reactors with or without recycle loop, tower reactors or plug flow reactors, may be used and they may be filled or partially filled. Preferred are the homogeneous agitated reactors and plug flow reactors. In the case where two reactors are used, the monomer conversion in the first reactor is already sufficiently high that the first reactor is past the phase inversion, i.e., that rubber particles exist in a predispersed form. In cases where three or more reactors are used, it is possible to operate all three reactors after the phase inversion, or, the first reactor before, and the second and third reactors after the phase inversion. The inventive process is preferably carried out in two or three stirred tank reactors. In a specially preferred embodiment, the process is carried out in two such reactors.
The high shear rates may be generated by pumping the reaction mixture through static dispersing devices, containing no rotating parts, at high pressures. A static mixer may be used as a dispersing device. Common to all static mixers is that a liquid flow in a tube is constantly separated, relocated, combined, and redistributed by internal components. The pressure energy available is thereby dissipated in small volumes.
Also, a jet dispergator may be used as a dispersing device in which the pressure energy is dissipated in small volumes in a pressure relief nozzle. Other suitable static dispersion devices include microporous filters, microporous glass filters microfluidizers and Manton-Gaulin homogenizer nozzles. The jet dispergator is the preferred device.
A critical aspect of the invention therefore resides in that the weight average particle size of the rubber (herein “d
w
”) decreases, and the width of the size distribution of the particles (herein “d
w
/d
n
”, where d
n
denotes the number average particle size) narrows by shearing in accordance with the inventive process. In accordance with the inventive process, the application of shear at the inventive rate reduces dw and d
w
/d
n
by at least 20%, preferably by at least 30% relative to the values obtained by the process but for the application of shear stress at the prescribed rate. This application of shear stress is upon the completion of the phase inversion. A jet dispergator contains a pressure relief nozzle in which the available pressure energy is dissipated in the smallest possible volume in a dispersion zone, and with this a high volume specific dispersion power is achieved. Suitable design types are described in DE 195 10 651 (FIGS. 1, 2 and 6) and in EP 101 007 A2 (FIGS. 2, 3 and 4).
Suitable static mixers include the ones available from Sulzer company, Winterthur, Switzerland/Germany (Commercial identification SMX). Common to all static mixers is that liquid flow in a tube is constantly distributed, relocated, combined and re-distributed by internal components. The static mixers are thereby operated in a way similar to the jet dispergator, i.e., the available pressure energy is dissipated in the static
Burk Raymond D.
Kim Joong-In
Klinksiek Bernd
Michels Gisbert
Ostarek Ralph
Bayer Corporation
Boykin Terressa M.
Gil Joseph C.
Preis Aron
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