Semipermeable membranes

Details Semipermeable membranes Semipermeable membranes

1999-09-10

2004-11-16

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S650000, C210S652000, C210S490000, C210S500270, C210S500250, C095S045000, C096S004000, C096S006000, C528S025000, C427S255120

Reexamination Certificate

active

06818133

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to semipermeable membranes based on organically modified silicic-acid polycondensates, to a process for preparing them and to their use in gas exchange and in separation techniques, especially in gas separation, dialysis, pervaporation, and micro-, ultra- and hyperfiltration. The membranes of the invention are flat or tubular membranes.
BACKGROUND OF THE INVENTION
For the separation of mixtures of substances a very wide variety of membrane materials are known, all of them being capable of improvement in terms of their serviceableness and their economics. For instance, known membrane materials, such as cellulose acetate, have poor temperature and pressure stability and swell severely in organic solvents. A consequence of the poor temperature stability, pressure stability and solvent resistance is that pore size changes continually under service conditions and consequently may result in nonreproducible results and in short membrane service lives.
Ultrafiltration, for example, is carried out predominantly in an aqueous system, so that the requirements of the membranes used for the ultrafiltrations, in terms of mechanical and thermal stability (sterilizability to 140° C.), resistance to acids and alkali's, and customizable hydrophilic/hydrophobic properties are particularly stringent. The polymers used to date to produce membranes are unable to meet these requirements at the same time and, for example, their relatively good thermal stability up to about 140° C. is accompanied by a lack of sufficient mechanical stability.
The surface modifications often necessary for various applications (adjustment of porosity, adsorption behavior, etc.) necessitate further, subsequent processes. Furthermore, materials modified in this way have the disadvantage of having only a modified monolayer at the surface, so making them extremely sensitive to mechanical and chemical exposure.
Commercially customary polymers such as polyethylenes, polypropylenes, polysulfones, polyimides, polymethacrylates, etc. have poor gas permeability (e.g., to O
2
, CO
2
, etc.). An increase in permeation on the basis of these polymer types is possible only by incorporating pores. For example, hollow polymer fibers provided with a defined, interconnecting porosity are obtainable directly only by highly complex spinning processes or by subsequent and thus additional process steps. Where polymers modified in this way are used, for example, for gas exchange in fluid systems, there exists the risk of passage of the fluid phase. For example, in the case of O
2
/CO
2
exchange in the blood in oxygenators during operations on the open thorax, the pores harbor a considerable hazard potential. In the case of relatively long operations in particular, the passage of blood through the pores is observed fairly frequently.
Very high gas permeation values without porosity are realizable only with highly specific polymers (silicones, substituted polysilyipropynes, etc.). The high gas permeability is achieved, however, only at the expense of extreme reductions in the mechanical properties. As permeability increases there is a reduction in strength and modulus of elasticity, i.e., the material becomes increasingly softer. Self-supporting thin films and stable hollow fibers of low wall thickness are therefore not possible. Films and hollow fibers having a degree of permeability which can be established over wide ranges are possible only on the basis of very different types of polymer in conjunction with different production techniques.
Membranes based on silicic-acid heteropolycondensates exhibit excellent resistance to acids and organic solvents and are also highly stable in the pH range up to about 10.
From DE 27 58 415 C2 it is known to process silicic-acid heteropolycondensates to porous membranes by mechanically cutting the polycondensates, which are obtained in compact blocks, into very thin slices which are then used—directly or after being ground beforehand—as membranes. Since, however, the silicic-acid heteropolycondensates are generally not sufficiently elastic, the membrane slices break on cutting, and also the necessary membrane surface area is usually not achieved by this method.
DE 29 25 969 C2 describes another process for producing porous membranes on the basis of silicic-acid heteropolycondensates at the interface between an organic and an aqueous phase. Since, however, the resulting membranes have a high water content owing to the contact with an aqueous phase, the risk exists of excessive shrinkage on drying, and of associated cracking. The hydrolytic polycondensation of the starting components to silicic-acid heteropolycondensates proceeds with substance egress, so that shrinkage of the polycondensates takes place unavoidably.
With the membranes described in DE 27 58 415 C2 and DE 29 25 969 C2, the shaping operation to form the membrane, and its curing, take place by means of an inorganic condensation, i.e., by the construction of an Si—O—Si network. These membranes have very poor mechanical properties; the mechanical stabilities only rarely satisfy the requirements made of them. Furthermore, these membranes are brittle and inflexible.
EP 0094060 B1 discloses a further process for producing membranes on the basis of silicic-acid heteropolycondensates. In that process polycondensation is carried out on the surface of a support which supports the membrane. Here again, the operation of shaping to form the membrane, and its curing, take place by inorganic condensation, i.e., by the construction of an inorganic network. The resultant membranes are supported and not self-supporting.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide semipermeable membranes for gas exchange and for separations, the exchange capacity of said membranes being variable over wide ranges and adapted to the requirements of the particular application. It is another object to provide membranes that may be supported but also that may be self-supporting and in a tubular or flat form. It is yet another object to provide membranes having the permeability and flexibility that can be varied over wide ranges and adapted to the requirements of a particular application. Yet another object is to provide membranes that combine high mechanical stability with high permeability, particularly with respect to gases, inter alia, to allow use for gas exchange and for separation without risking penetration of the fluid phase. Still further, even with high permeation values, such membranes may remain self-supporting. Another object is to provide membranes that are toxicologically acceptable and thus suitable for use in the medical sector.
It is a further object of the present invention to provide a process to manufacture semipermeable membranes having properties that can vary over wide ranges. In embodiments of the process, variation of the process steps allow control of the chemical and physical properties of the membrane, within wide ranges, to the requirements of particular applications. Moreover, embodiments of the process are simple, rapid and inexpensive to carry out. By means of the process it should be possible to manufacture membranes which meet the above-mentioned requirements. Furthermore, the process should also be suitable for the continuous production of hollow fibers and flat membranes. In addition, the surface modifications which are often necessary for various applications, for example, in order to avoid blood coagulation, in order to adjust the polarity, adsorption characteristics, etc., should be realizable both during the synthesis of the material, i.e., in situ, and also subsequently.
One embodiment of the invention is a process for producing a semipermeable membrane, comprising forming a semipermeable membrane from a low-viscosity to resinous liquid produced by hydrolytic polycondensation of a material comprising at least one compound selected from the group consisting of:
a compound of formula I
wherein
R=alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising betw

Affiliated with

Also associated with

Rating

Semipermeable membranes does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Semipermeable membranes, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semipermeable membranes will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3348023