GB2254016A

GB2254016A - Respirator filters

Info

Publication number: GB2254016A
Application number: GB9106461A
Authority: GB
Inventors: Michael John Rhymer; Caleb Roy Chatfield; Nigel Robert Bonnett
Original assignee: Sabre Safety Ltd
Current assignee: Sabre Safety Ltd
Priority date: 1991-03-26
Filing date: 1991-03-26
Publication date: 1992-09-30
Also published as: GB9106461D0

Abstract

A method of making a respirator filter, which includes charcoal granules 12 confined between grid assemblies 10 and 11 within a filter canister housing 8, includes the step of making at least one of the grid assemblies, which comprises an apertured metal grille 2, a support medium (3, Fig. 1) for preventing material passing through the grille 2, and a dust filter (4, Fig. 1), by forming a lip 5 of deformable resilient material extending outwardly beyond the circumference of the assembly so that the lip 5 holds the grille 2, the support medium and the filter as a single assembly. When used in a lower grid assembly, the lip 5 prevents charcoal dust escaping into gas breathed through the canister, and, when used on both upper and lower grid assemblies the lip 5 ensures that no breathing gas passing through the filter canister can bypass the bed of charcoal granules. Bead 14 of adhesive holds lip 5 against the wall of housing 8. <IMAGE>

Description

RESPIRATOR FILTERS This invention relates to respirator filters and more particularly to such filters which include a layer or bed of activated charcoal granules, or other absorbent granular or particulate material, for the removal of toxic gases.

In the manufacture of a respirator filter having a layer or bed of absorbent granular or particulate material, such as activated charcoal granules, the bed of charcoal granules is located in a canister between a pair of grid assemblies each of which usually comprises a circular apertured metal grille to which there are attached a support medium such as an open weave glass fibre mesh or scrim for preventing the charcoal granules from passing through the apertures of the metal grille, and a non-woven textile which may be a light weight viscose fabric for preventing charcoal dust from being carried in the breathing gas as this leaves the charcoal bed. The support medium and the dust filter are secured to the metal grille by bending over an edge of the metal grille to hold the other two components against the metal grille in a single grid assembly.The term "grid assembly" is used herein to denote an assembly of an apertured grille, a support medium such as an openweave glass fibre mesh and a dust filter such as a non-woven textile. The apertured grille, the support medium and the dust filter are "the members of the grid assembly".

In order to ensure that the granules are held firmly in place and the resistance to air flow through the bed of charcoal granules is controlled, the charcoal granules are compacted under a predetermined known load and held under that load between two grid assemblies. In the manufacture of respirator filters having a metal canister the desired compaction of the bed of charcoal granules has been achieved by placing the absorbent granular material in the canister on top of a first grid assembly, placing a second grid assembly on top of the absorbent granular material, applying the load to the second grid assembly to compact the absorbent granular material to the desired degree, and then swaging the metal canister to form a ring which holds the second grid assembly in the position where the desired compaction of the bed of absorbent granular material is maintained, and which also seals the grid assembly against the metal wall of the canister so that gas cannot pass between the grid assembly and the wall of the canister.

This method cannot however be used when the respirator filter canister is made of plastics material.

The upper grid assembly may be located permanently in a plastics canister by ultrasonic welding in a method corresponding to that used on a metal canister, the ultrasonic welding being in place of the metal swaging. However the use of ultrasonic welding in this context is not an attractive proposition.

A method suitable for use in a respirator filter comprising a plastics canister and a bed of charcoal granules has been to utilize a closure element for the canister, which closure element has a cylindrical flange located within the canister housing and serving during assembly of the closure element onto the canister housing to cause the upper or second grid assembly to compress the charcoal granules to the desired extent and retain them in such compressed condition. However this method is dependent on the closure element being securely fixed to the canister and also on very accurate measurement of the quantity of charcoal granules and a very precise selection of the granule size, if the desired compaction of the charcoal granules is to be obtained.

Whatever method of maintaining the desired compaction of the charcoal granules is employed, it is important that there be a good seal between the outer circumference of each grid assembly and the inner wall of the canister so as to ensure that gases are not able to by-pass the absorbent granular material and also that filtered gases carrying dust of the absorbent granular material, i.e. charcoal dust when charcoal granules are used as the absorbent granular material, are not able to escape past the grid assembly.

When a relatively long canister is used there are frequently variations in the diameter of the canister at different positions along the bore and such variations in diameter are liable to result in an imperfect seal between the grid assembly and the wall of the canister.

In accordance with the present invention a grid assembly designed for use in a filter canister containing a bed of charcoal granules is formed with a circumferential lip of a deformable resilient material such that the grid assembly, including the lip, is of greater diameter than the diameter of the canister at the position where the grid assembly is to be situated in the canister, and the grid assembly is inserted into the canister at the said position in which the lip is deformed by the wall of the canister so that the deformed resilient lip forms a continuous seal against the wall of the canister.

In a preferred method in accordance with the present invention the lip of deformable resilient material is also used to hold together the three integers of the grid assembly.

In accordance with this aspect of the present invention therefore there is provided a method of making a grid assembly for use in a filter canister having a bed of absorbent granular material which comprises the steps of assembling an apertured grille, a support medium in contact with one side of the grille for preventing granular material from passing through apertures in the grille, and a dust filter overlying the support medium, and forming an outwardly projecting lip of a deformable resilient material on the assembly such that the deformable resilient material overlies a circumferential portion of both the apertured grille and the dust filter and holds the apertured grille, the support medium and the dust filter as a single assembly.

Preferably the apertured grille, the support medium and the overlying dust filter are located in an injection moulding tool and the outwardly projecting lip of deformable resilient material is formed by an injection moulding process.

The deformable resilient material must have the necessary flexibility to enable the lip to deform when the grid assembly is inserted into a filter canister of gradually decreasing diameter. It should have a low compression set so that the resilience of the material is maintained over a period of years, and it must be chemically resistant to any toxic gases which will be present in gases to be filtered by the bed of activated charcoal granules or other absorbent granular material. The deformable resilient material should also be unaffected by changes in temperature within a range of -40 C to +800C, and it must be an injection mouldable material if it is to be formed in an injection moulding process.

A suitable material for the lip of a grid assembly in accordance with the present invention is a material comprising rubber particles dispersed in a plastics matrix, preferably a matrix of a polyolefinic material.

Such a material has elastic properties similar to rubber, and a particular example of such a material is Santoprene grade 101-64 which is a thermoplastic rubber material obtainable from Monsanto.

Further in accordance with the present invention there is provided a respirator filter which comprises a filter canister, a first grid assembly located in the canister, a bed of absorbent granular material supported on the first grid assembly, and a second grid assembly in contact with an upper surface of the bed of absorbent granular material, the second grid assembly including a circumferential lip of resilient material which is deformed by the wall of the canister to provide a seal between the second grid assembly. and the canister wall.

Advantageously, the first grid assembly of a respirator filter according to the present invention also includes a circumferential lip of resilient material which is deformed by the wall of the canister to provide a seal between the first grid assembly and the canister wall.

The present invention will be further understood from the following detailed description of preferred embodiments thereof which is made, by way of example, with reference to the accompanying drawings in which Figure 1 is a cross-sectional view of a portion of a grid assembly according to one embodiment of the present invention, the grid assembly having an outwardly projecting lip of deformable resilient material encapsulating the edges of the three components of the grid assembly; Figure 2 is a side cross-sectional view of part of a filter canister showing diagrammatically the introduction of an upper grid assembly in accordance with the present invention for applying and maintaining a predetermined load on a bed of activated charcoal granules placed on top of a lower grid assembly;; Figures 3 and 4 are enlarged detailed cross-sectional views showing the deformation of the outwardly projecting lip of the lower and upper grid assemblies respectively in positions which these grid assemblies adopt in the finished filter canister; Figure 5 shows in cross-section a portion of an alternative embodiment of a grid assembly according to the present invention including an outwardly projecting lip of deformable plastics material and Figure 6 shows the grid assembly of Figure 5 with the outwardly projecting lip deformed when the grid assembly is in position in the filter canister.

In the drawings the same or similar parts are designated by like reference numerals.

Referring to Figure 1 of the accompanying drawings there is shown in cross-section a portion of a grid assembly 1 which comprises an apertured metal grille 2, an open weave glass fibre mesh 3 and a lightweight non-woven viscose fabric 4. The circumferential edges of the metal grille 2, the glass fibre mesh 3 and the viscose fabric 4 are encapsulated within an injection moulded outwardly extending lip 5 of deformable resilient material preferably a material consisting of rubber particles dispersed in a polyolefinic matrix. In the embodiment of Figure 1 the lip 5 is formed with a thinner outwardly projecting tip 6 which is readily deformable by bending when the grid assembly is introduced into a filter canister of decreasing internal diameter.

Figure 2 of the accompanying drawings shows diagrammatically half of a filter canister housing 8 of plastics material, for example a polyamide material such as Nylon. The filter canister housing 8 constitutes a moulded substantially cylindrical body, the cylindrical wall of which has a 10 taper so that the internal diameter of the filter canister housing 8 gradually decreases as the distance of the diameter from the base 9 of the filter canister housing 8 decreases.

Grid assemblies similar to the grid assembly 1 of which a portion is shown in Figure 1 are used as lower and upper grid assemblies 10 and 11 within the filter canister 8 on either side of a bed 12 of charcoal granules.

The lower grid assembly 10 is inserted into the upper open end of the filter canister housing 8 where the diameter of the housing is widest and moved firmly into a position near the base 9 of the housing 8 as shown in Figure 2. The lower grid assembly 10 is so positioned near the base 9 of the filter canister housing 8 with the metal grille 2 lowermost and the outwardly projecting tip 6 of the lip 5 bent over from its natural position as shown in Figure 3, the non-deformed position of the tip 6 being shown in dashed lines in Figure 3 within the thickness of the wall of the canister housing 8. In this position the resilience of the deformed material makes a tight seal around the lower grid assembly 10, ensuring that all gas passing through the filter housing to outlet 13 must pass through the grid assembly 10.

In the manufacture of the gas filter the charcoal granules are introduced to form a bed 12 resting on the lower grid assembly 10. The upper grid assembly 11 is then moved into the filter canister housing 8 above the bed 12 of charcoal granules until it is in contact with the upper surface of the bed and a known load is applied through the upper grid assembly 11 to compact the bed 12 of charcoal granules to a predetermined extent. When the upper grid assembly 11 has reached the position in which the known load is applied to the bed 12, the deformation of the tip 6 of the lip 5 on the upper grid assembly 11 is as illustrated in Figure 4.Again, the undeformed position of the tip 6 is indicated in dashed lines in Figure 4 and the pressure exerted by the deformed resilient material of the tip 6 of the lip 5 on the wall of the canister . housing 8 maintains a gas seal around the upper grid assembly 11.

A bead of hot melt adhesive comprising a plastics material which is compatible with the plastics material of the canister housing 8, for example a polyamide based adhesive, is then injected via a nozzle of diameter of the order of 1 mm into contact with the lip 5 and the wall of the filter canister housing 8 along the whole edge of the lip 5. When the adhesive bead 14 has cured, the load may be removed and the upper grid assembly 11 is held in position by the cured bead 14 which holds the lip 5 securely against the wall of the filter canister housing 8.

In Figure 2 there is shown at 15 that when a grid assembly, in the case illustrated an upper grid assembly 11, is being introduced into the top of the canister housing 8 there is no deformation of the tip 6 of the lip 5 by the inner wall of the filter canister housing 8. However, by the time that the upper grid assembly 11 has been moved to the position where it exerts the desired compacting pressure on the bed of charcoal granules the interior diameter of the filter canister housing 8 is so reduced that the lip 5 is deformed as shown in Figure 4 in accordance with the method of the present invention.

Referring now to Figure 5, there is shown an alternative embodiment of a grid assembly in accordance with the present invention, the difference from the embodiment already described being in the shape of the outwardly extending lip 5 of the deformable resilient material. In Figure 6 the grid assembly of Figure 5 is shown in use as a lower grid assembly in position near the base 9 of the filter canister housing 8 where the outwardly extending lip 5 is placed in compression rather than being bent over as in the previous embodiment described.

The cross-section in Figures 5 and 6 is taken through a part of the metal grille 2 between adjacent apertures.

In a filter canister according to the present invention the presence of the deformable lip of deformable resilient material on the grid assembly ensures a continuous and effective gas seal around the grid assembly even if there should be a discontinuity or a break in the cured adhesive bead which holds the grid assembly in position in the filter canister. This good seal is maintained irrespective of variations in the diameter of the canister.

In our co-pending Patent Application No. of even date herewith there is described and claimed a method of making a respirator filter which comprises a filter canister having a substantially cylindrical wall of plastics material and a bed of absorbent granular material retained between lower and upper grid assemblies within the canister, the method including the steps of applying a desired compacting load to the bed of absorbent granular material through the upper grid assembly and, while the desired compacting load is applied, securing the upper grid assembly to the wall of the filter canister by injecting a hot melt adhesive into contact with both a circumferential rim of the upper grid assembly and the wall of the filter canister and allowing the hot melt adhesive to solidify, whereby the upper grid assembly maintains the desired compaction of the bed of absorbent granular material when the applied load is removed.

Claims

CLAIMS:

1. A method of making a respirator filter which includes a bed of absorbent granular material, the steps of the method including introducing into a filter canister a grid assembly having a lip of a deformable resilient material extending outwardly beyond the circumference of the grid assembly, moving the grid assembly to a desired position within the filter canister, and simultaneously causing the wall of the canister to deform the lip of the grid assembly whereby a continuous gas seal is formed between the lip of the grid assembly and the wall of the canister.

2. A method according to Claim 1 wherein the grid assembly is a lower grid assembly which is introduced into an empty filter canister and positioned near the base of the canister.

3. A method according to Claim 1 or Claim 2 wherein the grid assembly is an upper grid assembly which is introduced into the filter canister above a bed of absorbent granular material already present in the canister, and the upper grid assembly is moved into position in contact with the bed of absorbent granular material under a desired compacting load.

4. A method according to Claim 3 wherein a hot melt adhesive is injected into contact with the deformed lip of the upper grid assembly and the wall of the filter canister and is allowed to cure while the desired compacting load is applied to the upper grid assembly, whereby the cured adhesive holds the upper grid assembly in position to maintain the desired compaction of the absorbent granular material when the applied load is removed.

5. A method according to any one of the preceding Claims wherein the deformable resilient material constituting the lip holds together the members of the grid assembly.

6. A method according to any one of the preceding Claims wherein the lip of deformable resilient material includes an outwardly projecting tip of the deformable resilient material and wherein the outwardly projecting tip is bent over by contact with the wall of the canister when the grid assembly is positioned in the filter canister.

7. A method according to any one of Claims 1 to 5 wherein the outwardly extending lip of deformable resilient material is under compression as a result of contact with the wall of the canister when the grid assembly is positioned in the filter canister.

8. A method of making a grid assembly for use in a filter canister having a bed of absorbent granular material which comprises the steps of assembling an apertured grille, a support medium in contact with one side of the grille for preventing granular material from passing through apertures in the grille, and a dust filter overlying the support medium, and forming an outwardly projecting lip of a deformable resilient material on the assembly such that the deformable resilient material overlies a circumferential portion of both the apertured grille and the dust filter and holds the apertured grille, the support medium and the dust filter as a single assembly.

9. A method according to Claim 8 wherein the apertured grille, the contacting support medium and the overlying dust filter are located in an injection moulding tool and the outwardly projecting lip of deformable resilient material is formed by an injection moulding process.

10. A method according to Claim 8 or Claim 9 wherein the outwardly projecting lip of deformable resilient material is formed with an outwardly extending tip of thinner cross-section than the deformable resilient material immediately adjacent the outer circumferential surface of the apertured grille whereby the outwardly extending tip may be readily deformed by bending as a result of contact with a wall of the filter canister when the grid assembly is positioned in the canister in contact with the bed of absorbent granular material.

11. A respirator filter which comprises a filter canister, a first grid assembly located in the canister, a bed of absorbent granular material supported on the first grid assembly, and a second grid assembly in contact with an upper surface of the bed of absorbent granular material, the second grid assembly including a circumferential lip of resilient material which is deformed by the wall of the canister to provide a seal between the second grid assembly and the canister wall.

12. A respirator filter according to Claim 11 wherein the first grid assembly also includes a circumferential lip of resilient material which is deformed by the wall of the canister to provide a seal between the first grid assembly and the wall of the canister.

13. A respirator filter according to Claim 11 or Claim 12 wherein the circumferential lip or lips are formed of a thermoplastic rubber material comprising rubber particles dispersed in a plastics matrix.

14. A respirator filter according to Claim 13 wherein the rubber particles are dispersed in a polyolefinic matrix.

15. A respirator filter according to any one of Claims 11 to 14 wherein the lip or lips also act to hold the components of the or each grid assembly together.

16. A respirator filter according to any one of Claims 11 to 15 wherein the lip or lips are formed by an injection-moulding process.

17. A respirator filter according to any one of Claims 11 to 16 wherein the filter canister is made of a plastics material.

18. A method of making a respirator filter substantially as hereinbefore described with reference to the accompanying drawings.

19. A respirator filter substantially as hereinbefore described with reference to the accompanying drawings.