Improvements In and Relating to Polymeric Membranes

Abstract

Disclosed is a method and apparatus for manufacturing a continuous web of polymeric membrane and for continuous downstream processing of said web. The apparatus (10) comprises: a casting station (20) for casting the continuous web (M); a carrier (24) for carrying the web downstream; a membrane drier (30) downstream of the carrier, for drying the web; and a brushing station (40) downstream of the drier for brushing the web. Said drier is located immediately downstream of the carrier, and upstream of said brushing station. The apparatus (10) further includes an additional drying station (50) downstream of the brushing station (40). Brushing after drying retains more surfactant in the membrane which is useful for certain applications. In addition, initial drying eliminates virtually all solvents from the membrane, but leaves some non-solvent (e.g. water) within it, which in turn fixes the surfactant on the nitrocellulose fibers, which improves significantly the consistency and reproducibility of the membrane.

Claims

1. A method for manufacturing a membrane comprising the steps of: a) providing a membrane casting mixture containing at least a polymer, a polymer solvent, and a surfactant; b) casting said mix on a carrier; c) initially drying said mixture, in order to evaporate at least the majority of said solvent and to thereby form a porous polymeric membrane and thereby fixing said surfactant in the membrane; d) removing particles of dust on the surface of said dried membrane; and e) further drying the membrane.

2. A method as claimed in claim 1 wherein said removal of dust particles is by means of physical contact between the membrane and a dust removal member, for example by means of brushing, wiping or rubbing, or is by means of a flow of liquid for example by means of a jet of water.

3. A method according to claim 1 wherein said initial drying is performed by means of a heated drum, over which the membrane passes, said drum being heated to at least 50 degrees Celsius, and preferably between 50 and 100 degrees Celsius, and more preferably to 68 to 84 degrees Celsius.

4. A method according to claim 1 wherein said initial drying includes allowing at least 95% by weight and preferably 98% about by weight of the polymer solvent content of the mixture to evaporate.

5. A method according to claim 2 wherein said dust removal is by means of brushing and includes rotating a cylindrical brush in a partially filled bath of fluid, and guiding the membrane to make contact with, and pass over an upper part of the rotating brush.

6. A method according to claim 1 wherein said further drying includes drying by means a set of drum dryers.

7. A method according to claim 6, further including the step of rolling said membrane to form a roll.

8. Apparatus for manufacturing a continuous web of polymeric membrane and for continuous downstream processing of said web, the apparatus comprising: a casting station for casting the continuous web, a carrier for carrying the web downstream, a membrane drier downstream of the carrier, for drying the web, a dust removal station, for example a brushing station, downstream of the drier for brushing the web, wherein, said drier is located immediately downstream of the carrier, and upstream of said brushing station.

9. Apparatus as claimed in claim 8, further including an additional drying station downstream of the dust removal station or brushing station.

10. Apparatus as claimed in claim 8 wherein the membrane drier includes a heat drum which makes contact with the web in use and is heated to at least 50 degrees Celsius, and preferably between 50 and 100 degrees Celsius, and more preferably to between 68 and 84 degrees Celsius.

11. Apparatus as claimed in claim 8 wherein said dust removal station comprises a brushing station which includes a rotatable cylindrical brush in a bath partially filled with fluid, and guides for guiding the web to make contact with, and pass over an upper part of the rotatable brush.

12. Apparatus as claimed in claim 8 wherein said additional drying station includes a set of drum dryers.

13. A porous cast membrane manufactured by the apparatus of claim 8.

14. A membrane according to claim 13 wherein said membrane contains about 0.9 to about 1.2% of surfactant by weight, and preferably has little or no surface dust.

15. A membrane according to claim 13 wherein the amount of residual surfactant contained in the membrane is controlled by the drying temperature that the membrane is subjected to prior to surface dust removal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention, together with its objects and the advantages thereof, may be understood better by reference to the following description taken in conjunction with the accompanying drawings.

[0018] The invention can be put into effect in numerous ways, illustrative embodiments of which are described below with reference to the drawings, wherein:

[0019] FIG. 1 shows a schematic side view of membrane production apparatus; and

[0020] FIG. 2 shows a graph of experimental results.

[0021] Referring to FIG. 1, there is shown membrane forming and processing apparatus 10 which is capable of forming a continuous web of membrane material, and continuously processing the web. The apparatus comprises a membrane casting station 20, a membrane drier 30, a dust removal station 40, for example a brushing station, a final drier 50 and a rolling station 60. In this arrangement, the casting station 20 includes a vat 22 containing a liquid mixture of polymer dissolved in a solvent, together with water and a surfactant. In use the liquid mixture is wiped into a moving carrier 24 having a local environment which allows evaporation of the solvent and water, and thereby allowing a porous membrane web M to be formed on the carrier 24, consisting substantially of unevaporated parts of the mixture.

[0022] In use the membrane M is pulled off the carrier in a continuous web and is pulled in the direction of the arrows through the remainder of the apparatus. The membrane drier 30 includes a drum 32 heated to between 50 and 100 degrees Celsius, which evaporates any residual solvent or water, such that at least 95%, preferably about 98%, of the initial liquids (solvents and water) are removed from the membrane web M. Then the membrane web M travels on to the brushing station 40 at which the underside of the web Mu is bushed by a rotating cylindrical brush head 42, which in turn is located in a tray 44 partially filled with water 46 such that the brush head 42 is partially submerged in use. The underside of the web Mu corresponds to what is called the air side of the web. In other words, Mu is the side of the web which is exposed to the air when it is formed on the carrier 24. This is the useable side of the web. Brushing the underside only of the web means that water or other washing fluid can more readily drip off the web, back into the bath as the web moves out of the brushing station.

[0023] Thence, the web M is dried again by a set of drum dryers 50, this time to remove all remaining liquids, such that the membrane is dry. An extractor hood 52 is used to collect any vapours emitted by the web M as it is dried for a second time Then the web is formed into rolls 62 at the rolling station 60. It will be appreciated that various guide rollers are employed to manipulate the web M as shown, but their positon and size are merely illustrative of many possible configurations.

EXAMPLES

[0024] The following mixture was used to cast a membrane:

[0025] Cellulose nitrate polymer 7% by weight;

[0026] Solvent 85.5% by weight;

[0027] Water 7.4% by weight; and

[0028] Surfactant 0.1% by weight.

[0029] The cast membrane was subjected to initial drying on a heated drum of the type illustrated (30) at different temperatures, before being brushed in at a brushing station of the type show in FIG. 1 (40). One test run (V1) was made with a heating drum temperature of 68 degrees Celsius, another run (V2) was made at 76 degrees Celsius, and another run (V3) was made at 84 degrees Celsius. For each run V1,2 and 3, the membranes where brushed to remove dust, and finally dried to remove all liquids. For comparison, another test run (V4) was made without attempting the dry the cast membrane before brushing. For each run, an assay of the quantity of surfactant was performed to determine the residual fixed surfactant present after the respective processes were performed. The assay results are shown in FIG. 2, where it can be seen that the membranes from V1, V2 and V3 have significantly more residual surfactant than the V4 process which involved brushing before initial drying took place. Whilst it is marginal the lower temperature drying process of V1 outperformed V2 and V3 in terms of residual surfactant. The inventors have noted that, since drying temperature effects the residual surfactant levels, the drying temperature can thus be used to control the level of surfactant in the finished membrane.

[0030] Subsequent experiments have showed that the expected surfactant content for cellulose nitrate membranes whose dust is removed after initial drying, according to the invention is 0.89% minimum and have a typical range of 1.00 to 1.20%, i.e. an average of 1.1% for a drying drum temperature of around 68 degrees Celsius. However, observations of commercially available membranes produced according to the Beer et al process described above show surfactant levels of less than 0.5%. In other words, the process according the invention provides around twice as much residual surfactant in porous cast membranes. Where the surfactant has a beneficial effect in use, e.g. increased hydrophilicity, it can be seen that membrane manufactured according embodiments of the invention, having little or no filter dust, is significantly more useful compared to prior membranes made in a similar way. In addition, the initial drying of the membrane has been found to be important because this eliminates virtually all solvents from the membrane, but leaves some non-solvent (e.g. water) behind (about 32 to 5%), which allows the fixing of the surfactant on the nitrocellulose fibers, improving significantly the consistency and reproducibility of the membrane.

[0031] Although embodiments have been described and illustrated, it will be apparent to the skilled addressee that additions, omissions and modifications are possible to those embodiments without departing from the scope of the invention claimed. For example a membrane brushing technique is described in detail above, although removal of surface dust particles could be achieved in other ways such as mechanical wiping or rubbing of the membrane surface for example using a resilient flat blade for example a polymeric wiper blade similar to an automotive windscreen wiper blade, or even directing a jet of liquid at the membrane's surface.