Method for producing hollow fibrous membranes for...

Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Producing microporous article

Reexamination Certificate

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Details

C264S634000

Reexamination Certificate

active

06261510

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method for producing hollow fibrous membranes for microfiltration, ultrafiltration or gas separation, which fibrous membranes have an external diameter of 0.1-3 mm and a wall thickness of 10-500 &mgr;m, in which method a ceramic powder is mixed with a binder, the paste formed has sufficient viscosity to be extruded without heating, the paste formed is extruded to form hollow fibres by means of a spinneret and the powder particles are sintered to one another to obtain ceramic hollow fibres.
Such a method is described in EP-B-0693961.
BACKGROUND OF THE INVENTION
The use of membrane technology in industrial processes is increasing; continuous processing, relatively mild conditions and appreciable reduction of energy consumption are the greatest advantages compared with conventional separation processes.
At present, polymeric (plastic) membranes, in particular, are used. However, these have the disadvantage of a low chemical and thermal stability, a low erosion resistance and an unduly low rigidity, as a result of which they can be compressed and the properties may vary.
Ceramic membranes offer a solution to this. The present generation of commercially obtainable ceramic membranes is still too expensive and has a low ratio of the membrane surface per unit volume. The method described in EP-B-0693961 deals with this. Use is made of a thermoplastic binder. As a result of a relatively simple processing, the cost price of the microfiltration hollow fibrous membranes is a factor of 3 to 10 lower than that of the present generation of ceramic membranes, while the surface/volume ratio, at more than 1000 m
2
/m
3
, is up to 10 times higher. The abovemetioned method for making ceramic membrane fibres is eminently suitable for producing the fibres on an industrial scale.
OBJECT OF THE INVENTION
The object of the invention is to improve the method referred to in the introduction, as a result of which the product, ceramic fibrous membranes, can be produced more cheaply on an industrial scale.
SUMMARY OF THE INVENTION
According to the invention, the method mentioned in the introduction is characterized for this purpose in that a binder dissolved in water or another simple solvent or a thermosetting binder or an inorganic binder or a combination of said binders is used.
If a binder dissolved in water or another simple solvent is present, the paste formed will have to lose at least some of the water from the fibres as a result of evaporation caused by drying, at a temperature of at most 40° C., immediately after the extrusion process, but prior to the sintering, as a result of which the binder is gelated and the fibre rapidly acquires rigidity. Compared with EP-B-0693961, a lesser amount of raw materials is needed and the step in which the binder composed of a thermoplastic polymer has to be removed in a separate furnace at approximately 500° C. is unnecessary. The respective furnace is also no longer necessary. The relatively small amount of binder dissolved in water or another simple solvent can easily be removed during the sintering.
In general, only a few per cent of a binder composed, for example, of methylcellulose are needed to make the viscosity of the mixture of ceramic powder, water or a water-soluble binder great enough for said mixture to be capable of being extruded by means of a spinneret to form a hollow fibre.
As a result of evaporation of the water at room temperature or a somewhat higher temperature, the binder will be formed into a gel.
If a thermosetting binder is used, the binder will, however, be removed in a separate furnace at approximately 500° C. Thermosets become hard when heated as a consequence of crosslinking reactions In the cured state, they are no longer fusible, nor are they soluble, but at most swellable. In contrast to thermoplastic binders, the thermoset does not have to be cooled but can be cured, for example, by:
absorption of oxygen without the production of water or gases, such as epoxy polymers;
by increase in temperature, such as, for example, in the case of phenol formaldehyde;
by adding a small amount of catalyst, less than 1% by weight, usually a peroxide (such as, for example, methyl ethyl ketone peroxide), such as, for example, in the case of polyesters or allyls (diallylphthalate or diallylisophthalate).
If thermosets are used, approximately the same amount of raw materials is needed, but the strength of the green product will become greater during baking because the binder no longer becomes plastic. It is then also possible to use powders which have a different morphology.
If inorganic binders are used, use is mainly used of heteropolymers and hybrid heteropolymers. Heteropolymers are polymers having more than one type of atom in the main chain, such as, for example, silicates or siloxanes. Hybrid heteropolymers are polymers having inorganic atoms and having organic units, an example being ethene/silane copolymer. Inorganic binders can be cured by, for example:
carbon dioxide or an organic ester (for example, glycerol diacetate) in the case of inorganic silicates or
adding peroxide in the case of silanes or siloxanes.
Inorganic silicates have the advantage compared with the prior art (EP-B-0693961) that a smaller amount of raw materials is necessary and that the step in which binder has to be removed in a separate furnace at approximately 500° C. can be omitted. The furnace which is used for that purpose can also be eliminated. The inorganic residue which remains behind can be used to increase the rigidity of the membrane or to control the properties of the hollow fibrous membrane within a certain range. A lower sintering temperature can also be used and a relatively cheaper atmospheric furnace can be used for the nonoxidic membranes.
In order to prevent collapse of the fibres under all circumstances, they should be capable of being collected on a roller conveyor comprising a pair of rollers rotating in opposite directions.


REFERENCES:
patent: 4329157 (1982-05-01), Dobo et al.
patent: 4935199 (1990-06-01), Nishio et al.
patent: 29 19 510 (1979-11-01), None
patent: 197 01 751 (1997-07-01), None
patent: 0 047 640 (1982-03-01), None
patent: 2 736 843 (1997-01-01), None
patent: WO 91/05601 (1991-05-01), None

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