Plastic and nonmetallic article shaping or treating: processes – Vacuum treatment of work – To degas or prevent gas entrapment
Reexamination Certificate
1998-10-21
2002-03-05
Chin, Peter (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Vacuum treatment of work
To degas or prevent gas entrapment
C264S127000, C264S005000, C264S008000, C264S009000, C162S146000, C162S157500, C428S364000, C428S421000, C428S422000
Reexamination Certificate
active
06352660
ABSTRACT:
The present invention relates to a process for preparing composite fibers and diaphragms as used, for example, in chlor-alkali electrolysis.
There are basically two types of chlor-alkali electrolytic cells for the production of caustic soda and chlorine from sodium chloride: mercury and diaphragm. In the diaphragm process, a porous diaphragm separates the anode and cathode compartment. An aqueous sodium chloride solution flows from the anode compartment through the diaphragm into the cathode compartment, where hydrogen is produced at a steel cathode. The effluent cell liquor comprises sodium hydroxide as well as sodium chloride. The chlorine produced at the anode is obtained in gaseous form. Modern diaphragm cells feature adjustable, activated titanium anodes and increasingly diaphragms densified with synthetic polymer fibers instead of the traditional asbestos diaphragms.
Diaphragms are formed of a basic structure of organic polymer fibers which holds inorganic materials. Various processes for preparing such diaphragms or for preparing the composite materials used for preparing the diaphragms are known.
U.S. Pat. No. 4,680,101 describes a process for preparing diaphragms by mixing a dispersion of polytetrafluoroethylene (PTFE) fibrils, polypropylene fibers and a perfluorinated ion exchange material in water and applying the slurry to a perforated steel plate cathode covered with a cellulose filter paper. After removal of the volatiles, the diaphragm is dried at from 120° C. to 130° C. and, after cooling, impregnated with a solution of partially hydrolyzed silicon alkoxide and zirconium alkoxide. Then the diaphragm is dried again.
EP-B-0 196 317 describes a process for preparing fiber composite materials by using a ball mill to hot mix a PTFE dispersion with zirconium dioxide and sodium chloride, which initially causes the dispersion medium to escape. After the mixing, the product obtained is separated from the ball media used. It comprises irregularly shaped, partly branched fibers consisting of a composite of the PTFE used and the finely divided zirconium dioxide. The second inorganic material, sodium chloride, assists in the fiber formation process and can be dissolved out by the brine before or during the subsequent application. The fibers obtained can then be used to prepare a diaphragm. Prior art diaphragms do not always exhibit the desired high flow resistance, which prevents backmixing of the caustic obtained during the electrolysis. The diaphragms obtained are accordingly not of sufficient quality for all applications.
Not all the above-described process variants are suitable for preparing fibers for chlor-alkali electrolysis diaphragms. Not just any branched fiber can be used for preparing chlor-alkali electrolysis diaphragms. The diaphragms obtained from the fiber do not always have the required defined flow resistance.
The flow resistance of a diaphragm determines the rate of flow of the brine through the diaphragm. The flow rate also depends on the pressure forcing the brine through the diaphragm. In the field, the pressure is regulated by the difference in head between the brine feed and the catholyte effluent. Suitable values range, for example, from 20 to 70 cm of liquid column. This flow rate in turn has a direct bearing on the concentration of the caustic produced. In addition, the applied current density has no influence on the optimal flow rate. The concentration of caustic obtained should range from 100 to 150 g/L. In the field, this requires flow rates of 20-30 L/m
2
h and current densities from 2 to 2.5 kA/m
2
, for example.
The use of a ball mill for preparing the fibers leads to problems due to incomplete removal of the water in the dispersion. Said incomplete removal of water can cause rusting of the steel balls used, in which case PTFE will collect on the rust-roughened surfaces of the steel balls, preventing adequate fiber formation. To circumvent this problem, the starting materials have to be mixed and dried in another apparatus. This makes the process costly. In addition, at the end of the ball milling step, the balls used have to be separated off to isolate the fibers. This separation step is costly. It can take the form of sieving, for example.
It is an object of the present invention to provide a process for preparing such composite fibers as permit the preparation of diaphragms having a defined flow resistance to meet the technical requirements of a chlor-alkali electrolysis cell.
We have found that this object is achieved according to the invention by the process for preparing composite fibers by
(a) mixing a PTFE or PTFE copolymer dispersion or powder with a finely divided inorganic material and a fiber forming material,
(b) shear heating the resulting mixture to a temperature at which sheared PTFE or PTFE copolymer becomes flowable without showing signs of de-composition while removing the dispersion medium, if a PTFE or PTFE copolymer dispersion is used,
(c) cooling the mixture to below 70° C.,
(d) mix shearing the mixture at below 70° C. to form the composite fibers.
The invention proposes that shearing the mixture of PTFE or PTFE copolymer, finely divided inorganic material and fiber forming material especially at less than 70° C. provides fibers which permit the preparation of improved diaphragms having a defined flow resistance.
The heating in step (b) is preferably to more than 70° C., particularly preferably to more than 100° C., especially 130-180° C. Thereby coarse clumpy fiber hanks are formed. The cooling in step (c) and the shearing in step (d) are each preferably carried out at 20-60° C. A lower temperature in step (d) makes the mixing and shearing more difficult because of the increased stiffness of the material. In this step a chopping of the material and a separation into free flowing fibers is performed.
The invention further proposes that the shearing of the mixture in step (d) is advantageously carried out in mixers at a Froude number of more than 1. This requires the use in this step of mixers having a Froude number of more than 1. In this case the cooling in steps (c) and (d), respectively, is not necessary.
The Froude number is a measure of the intensity of mixing and is defined as Fr=r
2
/g where =2 .f, f=fre-quency, r=radius, g=gravitational constant. The frequency is determined from the speed of the mixing tool. The radius is the largest distance between the mixing tool and the shaft.
Examples of suitable mixers are Eirich mixers, ring tub mixers, ring layer mixers, DRAIS mixers. It is similarly possible to use a Löodige mixer fitted with additional choppers whereby Froude numbers of more than 1 can be achieved. A particularly preferred high intensity mixer is an Eirich mixer which is characterized in that it has a rotating mixing pan and a mixing tool rotor which selectively rotates or contrarotates. The mixing tool can reach a very high speed of more than 2000 rpm. The mixing tools are whisk- or stirrerlike tools which can have diverse geometric shapes and which ensure thorough mixing and an input of a high level of mixing energy. A wall scraper prevents material sticking to the walls. Eirich high intensity mixers are available from Maschinenfabrik Gustav Eirich, Hardheim, Germany.
The process can preferably be carried out in a vacuum mixer which can be heated. Vacuum mixers are provided by Eirich. These mixers perform the so-called EVACTHERM® process (of Eirich).
The heating of these mixers is performed by steam or hot steam which is led directly onto the mixture, and by the heating jacket of the mixer. The temperature of the jacket which is heated with steam as well may be adjusted by applying pressure or lower pressure. This specific advantage of these mixers is the possibility to rapidly cool the content. By injecting water and subsequently evacuating the mixer content may be cooled to the desired temperature (less than 70° C.). The invention relates also to the use of these types of mixers with a Froude number of more than 1 in the production of composite fibers.
Customary
Bröckel Ulrich
Friedrich Holger
Hecky Kurt
Hoppe Klaus-Dieter
Palm Peter
BASF - Aktiengesellschaft
Chin Peter
Halpern M.
Keil & Weinkauf
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