GB2208611A

GB2208611A - Controllable microfilter

Info

Publication number: GB2208611A
Application number: GB8819096A
Authority: GB
Inventors: Scott Strachan
Original assignee: PENNWALT PIEZO FILM
Current assignee: PENNWALT PIEZO FILM
Priority date: 1987-08-11
Filing date: 1988-08-11
Publication date: 1989-04-12
Also published as: GB8719021D0; GB8819096D0

Abstract

The filter comprises two films (10, 12) of polyvinylidene fluoride (PVDF) in face-to-face relationship and formed in a central portion with aligned apertures (15) in the range 10 mu - 30 mu . Each film (10, 12) has electrodes (16, 18) on each face on either side of the central portion (14). Application of differential voltages to the electrodes (16, 18) produces relative lateral movement of the central portions of the films (10, 12) in a manner to vary the effective pore size of the filter by moving the apertures (15) out of and into register. Apertures may be cleaned by applying alternating voltage to vibrate the films combined with reverse flow. A silicone oil lubricant between the films facilitates relative movement. <IMAGE>

Description

Controllable microfilter This invention relates to a microfilter having a pore size of the order of microns or tens of microns, and in which the effective pore size is selectively controllable.

Microfilters having fixed, predetermined pore sizes are known, and can be produced for example by forming holes in a plastic film using a laser beam. One application of such filters is in separating bacteria or other microbiological products of processes such as fermentation.

Many fermentation processes produce a mixture of products which it would be desirable to separate by multiple filtration. With filters of fixed pore size, however, it is difficult to effect multiple filtration without physically changing filters, and the time involved in doing so can result in missing the time of maximum yield of the fermentation mass.

There is thus a need for a microfilter of controllably adjustable pore size, which need the invention (defined in claim 1 hereinafter) seeks to meet.

Another problem encountered with microfilters is clogging requiring cleaning. Preferred embodiments of the invention provide a means of dealing with this simply and rapidly.

An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 is an exploded perspective view illustrating the principle of sshe embodiment by reference to two films used therein; Fig. 2 is a sectional plan view of a filter in accordance with the invention; Fig. 3 is a perspective view of the filter of Fig. 2; and Figs 4A and 4B illustrate the mode of operation of the filter.

The filter utilises two films 10 and 12, which may suitably be identical. The films 10, 12 are of a polymeric piezoelectric material, suitably polyvinylidene fluoride (PVDF). A suitable material is KYNAR (trade mark) PVDF film by Pennwalt Corporation.

Each film 10, 12 has a central portion 14 provided with apertures indicated at 15, as will be described more fully below. On either side of the central portion 14, the films 10, 12 are provided with metallised electrodes 16a, 16b on the front face of the film and 18a, 18b on the rear face.

The electrodes are connected by leads 20 to voltage sources as will be described. Suitable methods of forming metallised electrode layers on PVDF film are well known in the art; for example, silk screen printing with silver ink.

As seen in Figs. 2 and 3, the films 10 and 12 are secured by a frame 22 around their edges in face-to-face relationship.

Conduits 24 and 26 are provided by means of which a liquid may be pumped through the apertures 16 to filter particles therein.

The apertures are so arranged that, in a rest condition, each aperture in the first film 10 is aligned in register with a corresponding aperture in the second film 12. This may be achieved by mounting the films 10, 12 in the frame 22 and then forming the apertures through both films simultaneously by a laser beam or particle bombardment.

The piezoelectric nature of the films 10 and 12 has the following result. If a positive voltage is applied across the right-hand electrodes 16a, 18a of the film 10, the film is lengthened in this area. Likewise, if a negative voltage is applied across the left-hand electrodes 16b, 18b on the same film, the length of the film in this area is reduced.

By impressing such voltages at the same time, the central portion 14 is shifted microscopically to the left. Applying similar but opposite voltages to the other film 12 causes its central portion to shift equally to the right. This is illustrated schematically in Fig. 4 where Fig. 4A shows the rest condition, Fig. 4B shows the effect of applying these voltages, the full line 15a denotes an aperture in the front film 10, and the dotted line 15b denotes an aperture in the rear film 12. P indicates the effective pore size. It is believed that aperture diameters in the range 0 - 30p can readily be achieved, and that a ratio of maximum to minimum effective pore size P of at least 20:1 is possible. The effective pore size within this range is of course selectable by varying the applied voltages.

In a modification (not illustrated) the front and rear films may each be a composite comprising PVDF bonded to a film of another plastics material such as polyethylene or PTFE. The two may be bonded together and then laser-drilled.

Alternatively, the non-piezoelectric film may be laserdrilled first and then bonded to PVDF film having relatively large apertures.

It is contemplated that, for microbiological applications, serial filters with aperture dimensions of 101l and 25p would give complete selectivity of filtration for products of interest.

Since the central portions of the films are in sliding contact, it is preferable to provide a lubricant between them. This may suitably be a film of silicone oil.

The electrodes may also be used to effect cleaning of the filter, by connecting them to an oscillator to produce vibration of the film. While doing so, the filter may be flushed with a suitable liquid in reverse flow.

Claims

1 A controllable pore size filter, comprising first and second films arranged face-to-face and having aligned apertures of a predetermined size, means securing the films relative to each other at their edges, and means for tensioning the films parallel to their planes in opposite directions to move said apertures out of register thereby reducing the effective pore size.

2 The filter of claim 1, in which said films are of a polymeric piezoelectric material and said means comprises electrodes arranged to distort each film when supplied with a voltage.

3 A controllable pore size filter, comprising: a first polymeric piezoelectric film having apertures formed in a central portion thereof, first electrodes formed on the front and rear of the first film at one side of the central portion, second electrodes formed on the front and rear of the first film at an opposite side of said central portion, a second polymeric piezoelectric film having apertures formed in a central portion thereof, third electrodes formed on the front and rear of the second film at one side of the central portion, fourth electrodes formed on the front and rear of the second film at an opposite side of said central portion, means controllably connecting said electrodes to selected DC voltages, frame means securing the first and second films adjacent their edges in face-to-face relationship, and conduit means communicating with the exposed faces of said central portions.

4 The filter of any preceding claim, including a film of lubricant between said films.

5 The filter of claim 4, in which said lubricant is silicone oil.

6 The filter of any of claims 2 to 5, including means selectively connecting said electrodes to an oscillator to produce vibration of the films for effecting cleaning of the filter.

7 The filter of any preceding claim, in which each said film is a composite of a piezoelectric polymeric film bonded to a non piezoelectric film.