Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
1999-09-23
2001-10-09
Wilson, Donald R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C526S060000, C526S061000, C528S490000
Reexamination Certificate
active
06300468
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing porous, spherical particles in the non-nanoscalar range of polymeric material.
Spherical particles are used, for example, on a large scale as catalysts, catalyst supports, adsorbents, drying agents or ion exchangers.
For most of the applications named, particles are required with a uniform spherical shape and a narrow spectrum of particle sizes, in order to make a more uniform packing and a high packing density, for example, in a solid bed reactor, possible. If the particles are used in a moving bed reactor, they are expected to have a higher abrasion resistance.
If the particles are to be used as catalysts or catalyst supports, they must have, in addition to a narrow particle size spectrum, a certain surface area and a specific pore volume.
The spherical particles composed of inorganic oxides are obtained, for example, by the generally known sol-gel method, in which, to begin with, a sol is produced in the form of a solution, suspension or dispersion. Subsequently, the sol is converted into droplet particles, which are thereupon caused to gel. During this process, the droplets assume a spherical conformation and solidify in a suitable gaseous or liquid medium. After a wet process, the product is dried and tempered. The finished product consists of spherical, porous, oxide particles.
On the other hand, polymer dispersions are tiny polymer globules, which are dispersed uniformly in a liquid medium. Approximately, 30% of all synthetic resins based on styrene, butadiene, acrylic acid derivatives or vinyl esters are produced and processed at the present time in the form of dispersions.
For emulsion polymerization, monomers, to begin with, are dissolved in water or emulsified in the form of very fine droplets with addition of surfactants. After the polymerization reaction is started, during which monomers join together to form long-chain molecules, the droplets grow at the expense of the dissolved monomer and finally cure with cross linking into small spherical, non-porous particles, 50 to 1,000 nm in diameter (depending on the surfactant content).
It is furthermore known that porous, non-nanoscalar polymer beads can be produced by suspension polymerization in the presence of inert components, which can later on be removed from the polymer.
The polymerization of the monomers is carried out here under such conditions, that the inert components are finely dispersed in the polymer particles formed during the polymerization.
In a subsequent step of the process, the inert components are removed by evaporation, extraction or some other way, which maintains the desired porosity in the polymer particles.
This method is suitable, however, only for polymers, the chains of which form a rigid, coherent lattice structure, such as cross-linked aromatic polymers (German patent 3,202,479).
Moreover, porous particles, composed of polyvinyl chloride, can be produced by atomizing polymer solutions in a heated stream of gas and, at the same time, evaporating the solvent. The choice of solvent is important here, since any molecular weight change is to be avoided and, in any case, no solvent residues may remain in the polymer.
SUMMARY OF THE INVENTION
It was an object of the invention to provide a modified process for the production of porous polymer beads ranging from 1 to 1,000 micrometers in diameter.
Pursuant to the invention, polymers that are not cross linked, that is, linear polymers, especially polyolefins or PVC, are dissolved in a solvent at an elevated temperature and subsequently cooled to room temperature. When selecting the solvent, it is important to choose a solvent, in which the polymers used have a high solubility at elevated temperatures and a low solubility at low temperatures, so that, as the temperature drops, the polymers precipitate once again. It is a further criterion for the selection of the solvent that the polymer forms true or colloidal solutions with the solvent.
It was found, for example, that polymers not only dissolve in halogenated aromatic hydrocarbons, alkyl esters or phenyl ethers, but also separate once again when the polymer solution cools down, so that, in an unexpected manner, porous polymer beads are formed.
DESCRIPTION OF PREFERRED EMBODIMENTS
In one embodiment of the invention, dichlorobenzene is used as halogenated hydrocarbon for dissolving the polymers. Ortho-, meta- as well as para-dichlorobenzenes or their mixtures are suitable. Preferably, 1,2-dichlorobenzene is used.
In a further embodiment, amyl acetate is used as alkyl ester or anisole is used as phenyl ether for dissolving the polymer.
The polymers are dissolved at temperatures close to the boiling points of the solvents used, such as 100 to 180° C. and preferably 140 to 180° C. The use of pressure is also possible.
As uncrosslinked, linear polymers, which may also be branched, polyolefins, preferably polypropylene, polyethylene or polyvinyl chloride are used.
In a preferred embodiment, polypropylene is used.
The polymer solution can be cooled quickly or slowly.
In one embodiment of the invention, the solution is cooled at the rate of 1 to 3° C. per minute.
It is also within the scope of the invention to age the polymer solution by lowering the temperature to a value between 60 and 90° C. over a period of 0 to 24 hours. Preferably the aging takes place over a period of 1 to 3 hours, during which the temperature is lowered to a value between 60 and 80° C. After the aging, the solution is cooled further to room temperature.
The size of the polymer beads is affected by the way in which the polymer solution is cooled.
The size of the polymer beads is also affected by varying the cooling time and the aging temperature, respectively.
If, for example, the aging time is 1 to 2 hours and the aging temperature is 70° C., polymer beads with an average diameter of 110 to 120 &mgr;m can be obtained. The aging temperature usually is between 60 and 100° C. It was noted that, as the aging temperature decreases, the diameter of the beads increases.
For example, microspheres with an average diameter of less than 100 &mgr;m are obtained if aging is omitted.
The total solids content of the polymer in the solution is a further factor that influences the polymer bead size. Accordingly, a total solids content of 20% has proven to be advantageous for optimum bead formation. A total solids content of less than 20% leads to the formation of distinctly smaller spheres.
If the hot polymer solution is sprayed, atomized or divided into droplets in a cooling medium in a known manner for cooling, mesobeads or macrobeads are obtained with an average diameter of 100 to 200 &mgr;m and 1,000 to 5,000 &mgr;m, respectively.
The resulting polymer beads are separated, washed in a solvent, in which the solvents, used to dissolve the starting polymer, are soluble, and subsequently dried. The solvent, used to wash the polymer beads, should not form an azeotropic mixture with the solvents used to dissolve the polymers.
Preferably, alcohol is used for the washing.
If 1,2-dichlorobenzene is used as solvent, preferably isopropanol is used for the washing, since isopropanol does not form an azeotropic mixture with 1,2-dichlorobenzene, so that separation by distillation is readily possible.
If amyl acetate is used as solvent for the polymer, the polymer beads can also be washed with isopropanol.
It is also within the scope of the invention to cool the hot polymer solution by introducing it in a known manner, such as spraying or dividing it into droplets, into a cooling medium, such as cold or liquid air, solid carbon dioxide or liquid nitrogen.
It has proven to be advantageous to wash the beads several times in order to make certain that the solvent is removed completely.
Pursuant to the invention, it is possible to produce not only porous polymer beads from the pure polymers. It is also possible to use physical or chemical mixtures of polypropylene, polyethylene or polyvinyl chloride as starting material.
In order to influence the physical and functional properti
Bretz Karl-Heinz
Derleth Helmut
Francois Philippe
Rasche Heinz-Helmer
Cheung William
Crowell & Moring LLP
Solvay Deutschland GmbH
Wilson Donald R.
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