GB2217971A - Cigarette filters - Google Patents

Abstract

The invention is concerned with the problem of improving the efficiency of cigarette filters so that the total particular material per puff delivered to the smoker is as near as possible constant during the smoking of the cigarette. To overcome disadvantages of earlier proposals, the filter 14 is provided with a preferably conical chamber extending axially into the plug, the wall 20 of the chamber being impermeable to smoke and provided with flow-through apertures or slits 26 opening into the filter plug and extending longitudinally of the chamber whereby in use smoke drawn from the rod into the chamber passes into the filter material so that the flow-through apertures or slits become progressively blocked with solid material from the mouth end towards the rod end of the filter as the rod is smoked thereby increasing the path length of the smoke within the filter during the smoking of the rod. The density of the filter material increases along its length from the mouth end to the smoking material end to further flatten the natural yield profile of the cigarette. <IMAGE>

Description

IMPROVEMENTS RELATING TO ERS~EOR~RODS~OE~SMOKING~MA~ERIAL This invention is concerned with improvements relating to filters for rods of smoking material. In particular, the invention is concerned with the problem of improving the efficiency of cigarette filters so that the total particulate material per puff delivered to the smoker is as near as possible constant during the smoking of the cigarette. The yraph of total particulate material against puff as the cigarette is smoked will be referred to hereinafter as the "natural yield profile".

A technique is known in the cigarette filter art whereby tobacco smoke from a cigarette rod may be more efficiently filtered by increasing the flow path, for example, by passing it radically through a cigarette filter plug. Patents exemplifyinc tis technique include British patents 157111L (Cigarette Components Limited, 1319862 (British-American Tobacco Company Limited), 1360611 and 1358685 (Molins Limited) Although some of these patents teach an overall improvement in filter efficiency they do not teach increasing filter retention during the course of smoking so as to improve the performance of the cigarette by flattening the natural yield profile of the cigarette.

A patent that addresses the problem of flattening the natural yield profile of a cigarette is British Patent 1428018 (British American Tobacco Company Limited). This patent discloses a cigarette filter having a channel formed therei, the walls of the channel being impermeable to smoke, and the channel being enclosed except at the tobacco rod end where it communicates with the tobacco and at the other end which terminates in an orifice communicating with the filter material at an intermediate point in its length, the size of the orifice being such that it and/or the fitter material adjacent to it become substantially completely clogged with solid material when the tobacco is smoked in use.

The effect of this is said to be that, as clogging of the orifice continues, there is increased resistance to smoke passing through the portion of the filtering material surrounding the orifice so that the smoke generated by the burning tobacco passes directly through the main body of the filtering material. Due to the gradual change from a portion of the smoke flowing through the channel to all of the smoke flowing through the filter element, the device is said to accomplish a nearly uniform per-puff delivery of total particulate matter by reason of a steady increase in filter efficiency. Control of the per-puff delivery is further said to be achieved by providing a number of channels of unequal lengths, each with an orifice at its mouth end, so that the smoke flow shifts in turn from one channel to another as each orifice gets blocked, and finally to the main body of the filter.

A disadvantage of the proposal described in British patent 1428018 is that there is a severe restriction on the number of channels that could be formed in a cigarette filter: as shown in the embodiments illustrated and described in the specification to that patent no more than say six channels could reasonably be formed.

It is suggested in that specification that each channel should be provided with an orifice, and thus six orifices could be provided.

Such a limitation hinders the provision of a substantially flat natural yield profile and is likely to provide a 'step' change as each orifice becomes clogged.

A further disadvantage of the filter described in British patent 1428018 is that ventilation of the filter plug is not readily available since the channels themselves are adjacent the plug wrap or covering material of the filter plug and thus ventilation provided in such covering may provide access to the channel rather than to the filter plug.

There is also disclosed in co-pending European patent application 295835 a filter cigarette claimed to provide a constant delivery of dry particulate matter as the cigarette is smoked. The arrangement comprises a cigarette filter in which a perforate tube extends from the tobacco rod end and terminates short of the mouth end of the filter. The perforations in the tube become progressively blocked from the mouth end to the tobacco end thus causing the tobacco smoke to take even longer paths as the cigarette is smoked. Substantially constant dry particulate matter delivery results.

A disadvantage with the arrangement described in EP 295835 is that the tube is of relatively small diameter and could easily become blocked in use, thus causing all the smoke to pass through the filter plug without passing through the tube. Alternatively the apertures within the narrow tube may become clogged relatively quickly, thus reducing the effectiveness of the device. The small bore of the tube also substantially reduces the number of apertures available. These problems arise because a relatively small bore tube is utilised rather than a chamber having a relatively larye diameter compared with the diameter of the cigarette.

It is an object of the present invention to provide a filter cigarette with a substantially flat natural yield profile that overcomes disadvantages of the above proposals. It is a further object of the present invention to improve upoh the profiles obtainable by such proposals.

According to the present invention there is provided a filter for attachment to an end of a rod of smoking material, the filter comprising, a plug of cylindrical configuration made of porous filter material, a wrapper extending longitudinally of and circumscribing the plug so as to leave flow-through opposed ends of the plug, the plug being provided at that end of the plug adjacent said rod with a chamber in flow communication with the rod and extending axialLy into the plug towards and terminating short of the mouth end of the plug, the wall of the chamber being impermeable to smoke and provided with flow-through aperture means opening into the material of the plug whereby, in use with the rod of smoking material, smoke drawn from the rod into the chamber passes into the material of the plug through said flow-through aperture means so that the flow-through aperture means becomes blocked with solid material characterised in that the flow-through aperture means extends longitudinally of the chamber between the ends thereof whereby in use it becomes progressively blocked with solid material from the mouth end towards the rod end of the filter as the rod is smoked thereby progressively increasing the path length of the smoke within the filter during the smoking of the rod.

Compared with prior proposals the chamber provides a large surface in which to provide the access apertures and thus a far greater number of apertures can be provided. Slits or slots provide the equivalent of the greatest number of apertures in a given length of the chamber wall.

By increasing the availability of apertures compared with prior proposals a smoother transition is provided as each aperture becomes clogged and thus stepped changes are less noticeable. A smoother flatter curve is thus provided.

The use of a chamber also assists in channelling smoke into the filter plug. The use of a conical chamber or of a relatively large diameter chamber also avoids clogging of the chamber as opposed to the apertures leading to the filter plug.

The present invention also provides more efficient use of the filter plug. The greater number of apertures channel smoke to a greater number of paths through the filter, making better use of the material.

In a preferred embodiment of the invention, the density of the filter plug increases from the mouth end towards the tobacco end whereby the effectiveness of the filter is improved as the material becomes denser. Thus not only will the path length through the filter plug increase as the cigarette is smoked but the density and thus effectiveness of the plug material through which the smoke passes will also increase, thus further increasing the filtering effect and further flattening the natural yield profile of the product.

Alternatively or in addition, the use of a conical insert to form the chamber will vary the density of the filter material such that its effectiveness will vary over the length being denser at the large open end of the chamber and less dense at the mouth eno.

The variation in density need not be discontinuous with such an arrangement As the downstream (mouth) end becomes used the more effective upstream end will come into use.

The aperture means may extend from one end of the chamber to the other.

The flow-through aperture means may be provided by at least one longitudinally-extending slit or a longitudinally-extending plurality of holes extending between the ends of the chamber.

The arrangement of said at least one slit or the plurality of holes in the wall of the chamber may have a helical configuration.

The chamber preferably tapers towards the mouth end of the plug In a preferred embodiment the chamber may be blocked off at the mouth end so that smoke is prevented from entering the plug through the mouth end of the chamber.

The impermeable wall of the chamber is preferably provided by an insert of impermeable material provided with said flow-through aperture means.

The impermeable material may be cellulosic or plastics material.

In a preferred embodiment the cross-section of the rod end of the chamber is less than the overall cross-section of the plug thereby providing a second pathway for smoke into the pluy round the aperture.

The insert may be provided with a radial outwardly extending flange at the rod end so as to block passage of smoke from the rod into the plug via said second pathway.

The flange may be provided with at least one aperture so as to provide controlled passage of smoke from the rod into the plug via said second pathway.

The wrapper preferably includes a web of permeable plugwrap, and further preferably includes a tipping wrapper for attaching the plug to the rod.

The tipping wrapper may be provided with ventilation apertures.

The porous filter material is preferably cellulose acetate.

In a further embodiment the chamber may be wholly or partially filled with filter material having a pressure drop less than that of the said porous filter material.

The invention also includes a rod of smoking material, preferably tobacco, provided with such a filter.

The invention will now be described by way of example only with reference to the several embodiments shown in the accompanying non-scale diagrammatic drawings in which, Figure 1 is a longitudinal cross-section through a filter cigarette incorporating within the filter an insert according to the invention; Figure 2 is a side view of the insert of Figure 1; Figures 3 - 5 are side views of respective alternative filter inserts for the filter cigarette of Figure 1; Figure 6 is an oblique view of a flanged filter insert for the cigarette filter of Figure 1; Figure 7 is a side view of a further filter insert for the filter of Figure 1; Figures 8 and 9 are oblique cross-sectional views through two further filter inserts according to the present invention, and, Figure 10 is a graph illustrating the yield achieved using an insert of the form shown in Figure 4.

Referring to the embodiment of Figure 1 there is shown in longitudinal cross-section a filter cigarette 10 comprising a tobacco rod 12 abutting an acetate filter rod 14. The tobacco rod 12 is encased in cigarette paper 13, and the filter rod 14 is wrapped in air-permeable plug-wrap 15, as is standard practice.

The filter rod 14 is over-wrapped with tipping paper 16 which overlaps the tobacco rod 12 in a known fashion. The tipping paper 16 is provided with ventilating apertures 18 so as to allow the ingress of ventilating air into the filter.

Incorporated in the filter rod 14 is a hollow conical insert 20 made of an impermeable material such as plastics or paper with its longitudinal axis lying along the longitudinal axis of the filter rod, and having its open end or base 22 abutting the tobacco rod 12, and its apex 24, which is closed, adjacent the mouth end of the filter rod. The conical insert 20 is provided with a series of apertures 26, typically each 2 mm in diameter, arranged in a helical configuration from the base 22 of the insert to the apex 24 and providing communication between the interior of the insert and the acetate body of the filter. The diameter of the base 22 of the insert 20 is sufficiently smaller than the diameter of the filter rod 14 so that a quantity of smoke may enter the filter directly from the tobacco rod 12 rather than through the apertures in the insert.Figure 2 is a side view of the insert 20 of Figure 1 showing more clearly the helical or spiral arrangement of the apertures 26.

In forming the fitter rod 14, the insertion of insert 20 will cause the filter rod material to become compressed, the material being progressively compressed from the mouth end towards the tobacco end of the rod 14 from the closed end 24 of the insert 20 towards its open end 22 In operation, the apertures 26 in the conical insert 20 form in co-operation with the acetate filter material flow paths for the smoke of increasing resistance from the mouth end of the filter towards the tobacco rod end. Smoke drawn from the tobacco rod will initialLy take the path of least resistance via the aperture 26 nearest the apex 24 of the insert 20. After a number of puffs, 25 this particular aperture 26 nearest the apex 24 will become blocked by particulate material in the smoke. The next aperture 26 towards the base 22 of the conical insert 20 wilt then offer the flow path for the smoke of least resistance and will become the major flow path for the smoke. Finally, when all the apertures 26 become blocked, which will be towards the end of the smoking operation, the smoke will be constrained to take its path from the tobacco directly into the filter round the outside of the base of the insert. The path length of the smoke within the filter and the density of the filter material through which the smoke passes is thus progressively increased and brings about an increase in filter retention of particulate material during the course of smoking, thereby improving the performance of the cigarette by flattening the natural yield profile of the cigarette.

The density of the filter material may be varied using a multiple segment filter in which the segments, of differing densities, are assembled with the least dense towards the mouth end and the most dense towards the tobacco end of the filter plug. Such a filter may be used in place of or in addition to the variation in density created by inserting the insert 20 into the plug 14.

Figure 3 shows a conical insert 30 similar to the insert 20 of Figures 1 and 2 but provided with a continuous helical slit 36 15 from its base 32 towards its apex 34. The helical slit 36 acts in the same manner as the series of apertures 26 in the insert 20 of Figures 1 and 2 to change the path length of the smoke through the filter material and the density of the material through which it passes as the cigarette is smoked.

Figure 4 shows a conical insert 40 similar to insert 30 except that the slit 46 from the base 42 of insert 40 extends linearly towards the apex 44 of the insert. The operation of the insert in controlling smoke flow through the filter is similar to that of inserts 20 and 30.

Figure 5 shows as a further embodiment a conical insert 50 similar to the insert of Figure 2 except that the apex 54 of insert 50 is truncated and is open to the acetate filter at the mouth end of the filter, and the apertures 56 disposed in a series around the periphery of the insert 50 extend linearly from the base 52 to the apex of this filter insert. Again, the operation of this insert is similar to that of the inserts in Figures 1 - 4, except that more smoke may initially be drawn into the smoker's mouth through the relatively small thickness of acetate filter material that will overlie the open apex of the insert when it is in position within the filter.

Figure 6 shows a variation on the insert of Figures 1 and 2 in which the base of the insert 60 of Figure 6 is provided with a radially extending disc shaped flange 67 provided with apertures 68. As in Figures 1 and 2 the insert 60 is provided with apertures 66 corresponding to the apertures 26 of Figure 1 and fulfilling the same function. The size of the flange is such that it separates the filter material from the tobacco rod so that passage of smoke directly from the tobacco rod into the filter, as in the final stages of smoking described above with reference to Figures 1 and 2, is possible only through the apertures 68. The manufacturer is thus able, by controlling the size of the apertures 68 in conjunction with apertures 66, to further provide for particular circumstances of tobacco blend and ventilation.

The flange feature of Figure 6 may usefully be applied to the other embodiments of the invention Figure 7 shows an insert 70 that is not conical but cylindrical with one end 72 open to the tobacco rod and the other end 74 closed. A linear slit 76 is provided extending from the open towards the closed end. In a variation of this insert, an aperture (not shown) may be provided in the closed end 74. To achieve density variation in the smoke path, a multiple segment filter must be used in this embodiment since no density variation can be achieved by inserting the insert 70 into the filter plug material, the insert 70 being cylindrical rather than conical.

Figure 8 shows a filter cigarette 90 including a filter plug 81 incorporating an insert 80 made according to UK Patent No. 2077570 of Filtrona Ltd. The insert 80 comprises a polyethylene tube 82 open at one end to the tobacco rod 83 and closed at the downstream end by crimping forming fins 84, defining a conical chamber within the tube 82. Slits 85 are formed in the wall of the tube 82 to provide smoke access from the chamber within tube 82 into the material of the filter plug 81. The material of the filter plug is varied in density by insertion of the conical insert into the material of the plug. In addition the filter plug may be made of a number of segments of differing densities to provide a discontinuous variation in density from the mouth end to the tobacco end of the plug.

Operation of the device of Figure 8 is similar to that described above in relation to the embodiments of Figures 1 to 7.

Referring to Figure 9 there is shown a variation of the embodiment of Figure 8 in which the slits 85 are replaced by a series of holes 86 spaced around the surface of the tube 82.

With any of the embodiments of the invention described above the interior of the insert may be filled with a permeable material.

Example An insert of the form shown in Figure 4 was produced having a wall thickness of 0.7 mm, length 20 mm, diameter 4mm at the open end 42 and having slit 46 of width 0.8 mm. The conical insert of plastics material was inserted into a standard cigarette filter plug 22 mm in length, thereby compressing the filter plug material along its length, with minimum compression at the mouth end, adjacent the apex of the cone, and with maximum compression and thus density at the tobacco end adjacent the open end 42 of the cone.

Compared with the same filter without such an insert Figure 10 shows the dry particulate yield expressed as a percentage of total delivery of conventional cigarettes one provided with a standard filter and one with a standard filter modified by insertion of the insert LO.

It will be seen that there is an improvement in yiecd in the initial puffs and a suppression of yield in the final puffs with the modified filter thus providing a flattening of the overall yield profile.

With alt embodiments described above a chamber is provided in the filter plug and a plurality of apertures - or the equivalent in the form of a slit - is provided in the wall of the chamber to 15 provide access to the filter plug. Compared with prior proposals the chamber provides a larger surface in which to provide the access apertures and thus a far greater number of apertures can be provided. Slits or slots provide the equivalent of the greatest number of apertures in a given length of the chamber wall.

By increasing the availability of apertures compared with prior proposals a smoother transition is provided as each aperture becomes clogged and thus stepped' changes are less noticeable. A smoother flatter curve is thus provided.

The use of a chamber also assists in channelLing smoke into the filter plug. The use of a conical chamber or of a relatively large diameter chamber also avoids clogging of the chamber as opposed to the apertures leading to the filter plug.

The present invention also provides more efficient use of the filter plug. The greater number of apertures channel smoke to a greater number of paths through the filter, making better use of the material.

With any of the embodiments, an aperture may be provided in the end of the chamber closest to the mouth end of the filter whereby the initial puffs of tobacco smoke will pass to the smoker via this aperture, with the minimum of flow path through the filter material at minimum density. Thereafter all further puffs wilt be at increasing path lengths through increasing density thus providing a flattened natural yield profile.

The use of a filter material or filter segments of varying density and/or the use of a conical insert to form the chamber can vary the density of the filter material such that its effectiveness varies over the length being denser at the large open end of the chamber and less dense at the mouth end. As the downstream (mouth) end becomes used the more effective upstream end will come into use. This improves yet further the effectiveness of the design whereby the natural yield profile becomes further flattened.

The invention may be applied to smoking articles other than cigarettes, such as cigars, cigarillos, or cigarette holders, and may be usefully applied to articles using blends of tobacco, or reconstituted tobacco, or smokeable substitutes for tobacco.

Claims (20)

1. A filter for attachment to an end of a rod of smoking material, the filter comprising, a plug of cylindrical configuration made of porous filter material, a wrapper extending longitudinally of and circumscribing the plug so as to leave flowthrough opposed ends of the plug, the plug being provided at that end of the plug adjacent said rod with a chamber in flow communication with the rod and extending axially into the plug towards and terminating short of the mouth end of the plug, the wall of the chamber being impermeable to smoke and provided with flow-through aperture means openiny into the material of the plug whereby, in use with the rod of smoking material, smoke drawn from the rod into the chamber passes into the material of the plug through said flow-through aperture means so that the flow-through aperture means becomes blocked with solid material characterised in that the flow-through aperture means extends longitudinally of the chamber whereby in use it becomes progressively blocked with solid material from the mouth end towards the rod end of the filter as the rod is smoked thereby increasing the path length of the smoke within the filter during the smoking of the rod.

2. A filter as claimed in claim 1 wherein the density of the filter material varies over the length of the filter thus varying the smoke particulate retention effectiveness of the filter over its length, being greater at that end adapted to be attached to the rod of smoking material and decreasing towards the opposed end of the filter.

3. A filter as claimed in claim 2 wherein the filter is formed of a number of filter segments of differing densities.

4. A filter as cLaimed in claim 2 or 3 wherein the chamber tapers and is defined within an insert disposed within the filter, the presence of the insert varying the density of the filter material over its length, being denser around the larger end of the chamber and less dense around the smaller end of the chamber.

5. A filter as claimed in any preceding claim wherein the aperture means extends from one end of the chamber to the other.

6. A filter as claimed in any preceding claim wherein the flow through aperture means is provided by at least one longitudinally- extending slit or a longitudinally-extending plurality of holes extending between the ends of the chamber.

7. A filter as claimed in any preceding claim wherein the arrangement of said at least one slit or the plurality of holes in the wall of the chamber has a helical configuration.

8. A filter as claimed in any preceding claim wherein the chamber tapers towards the mouth end of the plug.

9. A filter as claimed in any preceding claim wherein the chamber is blocked off at the mouth end so that smoke is prevented from entering the plug through the mouth end of the chamber.

10. A filter as claimed in any preceding claim wherein the the impermeable wall of the chamber is provided by an insert of impermeable material provided with said flow-through aperture means.

11. A filter as claimed in any preceding claim wherein the impermeable material is cellulosic or plastics material.

12. A filter as claimed in any preceding claim wherein the crosssection of the rod end of the chamber is less than the overall cross-section of the plug thereby providing a second pathway for smoke into the plug round the aperture.

13. A filter as claimed in any preceding claim wherein the insert is provided with a radial outwardly extending flange at the rod end so as to block passage of smoke from the rod into the plug via said second pathway.

14. A filter as claimed in claim 13 wherein the flange is provided with at least one aperture so as to provide controlled passage of smoke from the rod into the plug via said second pathway.

15 A filter as claimed in any preceding claim wherein the wrapper includes a web of permeable plugwrap, and further includes a tipping wrapper for attaching the plug to the rod.

16. A filter as claimed in claim 15 wherein the tipping wrapper is provided with ventilation apertures.

17. A filter as claimed in any preceding claim wherein the porou filter material is cellulose acetate.

18. A filter as claimed in any preceding claim wherein the chamber is wholly or partially filled with filter material having a pressure drop less than that of the said porous filter material.

19. A filter for attachment to a rod of smoking material as hereinbefore described with reference to and as shown in any of the accompanying drawings.

20. A rod of smoking material having attached thereto a fitter according to any preceding claim.