GB2090543A

GB2090543A - A method for bonding a filter membrane to a plastics support

Info

Publication number: GB2090543A
Application number: GB8139099A
Authority: GB
Original assignee: Millipore Corp
Current assignee: EMD Millipore Corp
Priority date: 1979-11-05
Filing date: 1980-11-05
Publication date: 1982-07-14
Also published as: JPS56129016A; GB2063089A; GB2063089B; CA1148094A; SE447795B; FR2468397A1; SE8007745L; GB2090543B; DE3041729C2; JPS6243723B2; DE3041729A1; FR2468397B1

Abstract

The method comprises absorbing a predetermined amount of solvent into the pores of the membrane (68), the solvent being a solvent for the plastics support (88) but the membrane, contacting the dry support with the wetted membrane, maintaining contact until the solvent solvates the support and wicks a portion thereof into pores of the membrane, then removing the solvent. Excess solvent may be supplied to the membrane to fill the membrane pores, the excess being removed by spinning the membrane. <IMAGE>

Description

1 GB 2 090 543A 1

SPECIFICATION

A method for bonding a filter membrane to a plastics support This invention relates to filter units and concerns methods for bonding a filter membrane to a plastics support in the manufacture of such units.

According to the present invention there is provided a method for bonding a filter membrane to a plastics support comprising absorbing a predetermined amount of solvent into the pores of the membrane, said solvent being a solvent for the support but not the membrane, contacting the dry support with the wetted membrane, maintaining contact until said solvent solvates said support and wicks a portion thereof into pores of the membrane, then removing the solvent.

Such a method is reliable and reproducible, readily automated, and minimizes damage to the filter membrane and occlusion of its filter volume.

The solvent may be removed by drying.

Preferably, said pores are substantially filled with solvent, without excess. Conveniently, the filter can be saturated with excess solvent and spun to remove the excess.

Using the method as defined, only a minimum contact pressure between support and filter is required which avoids mechanical damage to the membrane.

The structure of the resulting filter and support is believed to be different from and superior to structures obtained with prior techniques whereby heat solvent or adhesive is applied to the support. By the present method, plastics material of the support is wicked or imbibed only into the membrane pores over the ribs or the like, without significant lateral spread which occludes filter area. Indeed, the imbibed plastics material need not, and normally does not extend through the membrane, thereby leaving portions over the ribs, remote therefrom, with oblique channels through the membrane. Experience to date, for example, indicate that for a given fluid, fluid pressure and support structure, the present procedure reduces flow rate by only about 15%. The ribs or other formations preferably have a curved surface to minimize contact.

The bonding procedure according to the present invention is applicable to the bonding 120 of a filter membrane to any support structure having fine support features, as shown for example in U.S. patents 3,854,907; 3,730,353; and 3,471,019. Heretofore, fine ribbed or dimpled supports have not been bonded to the filters for reasons of occlusion, control and the like. Bonding by the method of the present invention is practical and can provide significant back-flow protection against pressure surges and, in some cases, can obviate the need for check valves. The method can also be conveniently applied to laminate membranes to fibrous plastics supports such as supports of woven cloth, non- woven felts, and spun-bonded filaments for reinforcement.

A specific embodiment of the present invention will now be described by way of example, and not by way of limitation, with reference to the accompanying drawings in which:

Figure 1 is a partial plan view of one part of a two part filter support.; Figure 2 is a partial plan view of similar to Fig. 1 showing the second part of the two part support; Figure 3 is a section on the line 3-3 of Fig.

Figure 4 is a section on the line 4-4 of Fig.

Figure 5 is a section on the line 5-5 of Fig. 1; Figure 6 is a section on the line 6-6 of Fig. 2; Figure 7 is a section similar to Figs. 3 and 4 showing the two parts assembled, together with superimposed filter units shown in broken lines; Figure 8 is a section similar to Figs. 5 and 6 showing the two support parts assembled; Figure 9 is a partial plan view, to reduced scale, showing the reverse side of the support part shown in Fig.4; and Figure 10. is a partial plan to reduced scale, of the reverse side of the part shown in Fig. 2.

Referring to the accompanying drawings, a filter unit 38 comprises two interfitting parts 50 and 52. Part 50 comprises a flat disc portion 54 (Fig. 10) which carries concentric upstanding ribs 56 and is provided with radial cutaway channels 58 beneath the ribs 56.

Part 50 also has integral with the base 54 a solid rim 60 which has upper and lower flat lands 62 and 64. Part 50 is also provided with an inner circumferential land 66. Inner land 66 and outer land 64 together with the surfaces of the intervening ribs 56 define a common plane to which a membrane filter 68 is bonded as hereinafter described.

Both parts 50 and 52 have a central opening 70 and 72 respectively which are adapted for axial flow of liquid therethrough. Between the edge of opening 70 and the inner circumferential land 66, part 50 has upstanding circumferential coupling means 74 adapted for permanent bonding to the complementary coupling of a superposed filter unit 38 to lock the two units together and space their adjacent filters 68 apart so that liquid may flow between them. Coupling 74 may have a surface of any desired configuration, but conveniently as shown is provided with an upstanding tongue on its surface.

The part 52 fits within the part 50 as shown in Figs. 7 and 8 and is bonded perma- GB 2 090 543A 2 nently thereto to make a unitary support for filters 68 bonded to its upper and lower surface. Part 52 has a flat central portion 80 (Fig. 9) similar to the portion 54 of the part 50 and is also provided with upstanding ribs 82 and radial channels 84 which are bridged by the ribs 82. The part 52 has an outer rim 76 which fits within the interior of the rim 60 of the part 50 inside the land 62. The part 52 has an inner circumferential land 78 which, when assembled with the part 50 as shown in Figs. 7 and 8, defines a common plane with the land 62 and the surfaces of the ribs 82. Between the openings 72 and the land 78, the part 52 is provided with upstanding coupling means 86 which as shown has a groove complementary to the tongue of the coupling means 74 of part 50.

When the parts 50 and 52 are assembled and bonded together, preferably by welding with heat or solvent, a unitary support 88 for membrane filters 68 is provided. The membranes 68, one on each of the upper and lower surfaces of the support 88, are perma- nently bonded to the lands 66 and 64 and the tops of ribs 56, and the lands 62 and 78 and to the tops of ribs 82.

As illustrated in Fig. 7, the completed filter unit 88 has a hub about the opening 70, 72 comprising the coupling means 74 on one side and the coupling means 86 on the other side, one coupling means being complementary to the other. As shown in Fig. 7 in broken lines, like filter units 38 are superim- posed and bonded at their coupling means, preferably by solvent welding. The filter units 38, comprising a completed support 88 and the two filter membranes 68, may be bonded into stacks of any desired height and secured within a housing to form a filter cartridge. The size of the cartridge can be varied by varying the number of units 38 included, and by varying the height of spacer 26.

The channels 58 and 84 of parts 50 and 52, respectively, terminate in openings 90 and 92 at the central openings 70 and 72, respectively, of the parts 50 and 52. In assembling the part 52 within the part 50, the channels 58 and 84 may be aligned, which will provide alignment for the openings 90 and 92, but such alignment is not required. The spaces between the ribs 56 and 82 provide arcuate channels by which liquid passing through the filters 68 may flow to the radial channels 58 and 84 and then through the openings 90 and 92 into the central opening 70, 72, of the filter element 38. Such spaces between the ribs and radial channels provide internal passageways for liquid flowing through the membrane filters 68 to the central opening 70, 72 from which filtrate flows axially to the fitting 32 and to an outlet conduit.

Preferably the parts of the units, other than the membrane filters 68, are of a common plastics material, for example polycarbonate.

The membranes 68 are microporous cast films of polyvinylidene flouride or bonded or matted fibers, preferably having pores less than about 25 microns in size. Such membranes and methods for making them from a variety of polymeric materials are known. By being supported flat, without folds, and by being bonded to support ribs intermediate their peripheral margins, the membranes are well anchored against flow in either direction and are not subject to the stresses of folding or of flexing during use.

The membranes 68 are bonded to the sup- ports 88 by the following procedure. A predetermined amount of a solvent for the plastics material of support 88, which is a nonsolvent for the membranes, is absorbed into the membrane pores. Each membrane is lightly pressed on the lands and rib surfaces of the support. The solvent solvates the plastics material, preferably a thermoplastics material, of the support and wicks portions thereof into the membrane's pores lying above the surfaces of the lands and ribs, but not substantially into the flow passages of the pores between the ribs, thereby providing firm anchorage with a minimum of occulsion of filter volume. A predetermined amount of sol- vent can be premeasured and applied, and will spread throughout the membrane by capillarity. Preferably however, it has been found convenient to spin the membrane, to apply excess solvent to the membrane, and to remove the excess by the centrifugal force. This procedure substantially saturated the membrane pores, without excess, and is readily automated. After light contact sufficient to wick solvated plastics material into the membrane's pores, the membrane is dried. This has not been found to be a critical operation and normal air drying after a dwell time of 60 seconds or less has been found suitable. For the preferred materials, a dwell time of three seconds is sufficient. Any suitable solvent for the plastics comprising support 88 may be used.

Careful testing has established that the above bonding procedure effectively bonds the membranes liquid-tight against flow of liquid except through the membranes. It has also shown that the area of each membrane occluded from the flow of liquid is substantially less than is the case when other bonding techniques are used, for example by applying the solvent directly to the support or by the use of heat and pressure. The resulting bond is believed to be at least partly mechanical, cross-sections of such membranes having shown extensions of support material filling the pore portions adjacent to the support.

While the coupling means on the surfaces of the units 38 may conveniently extend each from the surface of the disc above the adja- cent membrane to provide spacing between 3 GB 2 090 543A 3 adjacent discs, one of them can provide all or a majority of the required height and spacing.

While the filter supports are herein described for convenience and clarity as being round and having inner and outer circumferential lands, the supports may have any other convenient shape, oval or polygonal.

Similarly, while the central flow opening is preferably round and axially located, the open- ing may be off centre and of other shapes if desired. Also, the term "ribs" as herein used is intended to include any fine support structure, continuous o discontinuous, useful for providing mechanical support to a membrane filter while providing flow paths for liquids passing through the filter. In addition to discontinuous or continuous ribs as shown, for example, aligned or randomly arranged upstanding dimples of any desired cross- section may be used.

The present invention is useful with fibrous or cast microporous and ultrafiltration membranes having a pore size less than 25 microns, and particularly less than 1 micron. A unit made in accordance with the present invention may be used as a prefilter or as a final filter for blood, bacteria or the like. It is especially useful as a sterilizing unit for liquids. When used as a final filter, prefilter materials can be included in a cartridge to protect the final membranes 68. Such coarse filters may be superposed over the membranes 68, or a cylinder of prefilter material can be bonded to the periphery of a stack of units 38. Cast microporous membrane filters are preferred.

Claims

1. A method for bonding a filter mem- brane to a plastics support comprising absobing a predetermined amount of solvent into the pores of the membrane, said solvent being a solvent for the support but not the membrane, contacting the dry support with the wetted membrane, maintaining contact until said solvent solvates said support and wicks a portion thereof into pores of the membrane, then removing the solvent.

2. A method according to claim 1 wherein said pores of the membrane are substantially fillet with solvent with excess.

3. A method according to claim 1 wherein excess solvent is applied to the membrane to fill the pores, and the excess is removed by spinning the membrane.

4. A method of bonding a filter membrane to a plastics support substantially as hereinbefore described with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.-1 982. Published at The Patent Office. 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.