GB2271709A - Cigarette filter - Google Patents

Abstract

To regulate the perceived characteristics of smoke received by a smoker, a filter has within an uncrimped and uniform tube zones of higher 1 and lower 2 flow resistance, and means for permitting smoke flow from filter material surrounding the tube into the tube. <IMAGE>

Description

CIGARETTE FILTER The invention relates to cigarette filter rod elements and cigarettes incorporating such elements.

It is concerned with such filter rod elements which have the purpose of evening out the response of such an element in terms of particulate matter transmitted to the smoker, as the cigarette is progressively smoked.

We have in EP-A-0310257 and 0453299 made proposals for such a filter rod element in which in essence we have shown the filter rod element having an inner barrier tube one end of which is closed off. In EP-A-0310257 the barrier tube is impermeable and smoke communication between its inside and smoke filter material lying outside it is assured by holes. In EP-A-0453299 the barrier tube is made up of a microfibrous material permeable to smoke but having a comparatively high flow resistance.

In both cases closure is by crimping the tube.

Experience has shown that under certain circumstances, particularly under the influence of moisture, the crimp may come undone so that a virtually unimpeded through path becomes available through the filter and the desired effect is lost.

Remedial measures can be taken but the present invention solves the difficulty and at the same time provides a core filter element which is as simple to manufacture by taking an entirely different approach, while achieving results as good as or superior to those in these two earlier proposals.

In Figure 1 of our EP-A-364253 (the other Figures show crimped-end tubes) we have shown another form of progressive filter where smoke is forced to flow at least once through the peripheral wall of the barrier tube by placing a non-porous disc centrally at one end of the rod element and a non-porous annulus at the other end.

It has now been found however that it is unnecessary to have a complex and difficult-to-manufacture arrangement of that type if the present construction is adopted.

In the present invention the barrier tube is a continuous open and uniform tube from end to end of the filter rod element. It is surrounded by filter medium which is in turn surrounded by and in continuous contact with a conventional plug wrap of a filter rod element.

There is means for communication of smoke between the inside of the barrier tube and the filter material outside the tube, either by virtue of the tube being formed of micro fibrous material or by virtue of the tube being of impermeable material and having apertures in it. Within the barrier tube are provided zones of respectively high and low flow resistance.

The high flow resistance zone may be entirely impermeable and the low flow resistance zone may be devoid of material; but preferably both zones are occupied by permeable but differently flow-resistant material. Any suitable materials may be used, such as tow, granular material, foam material, fibrous material and paper or paper-like material.

In a preferred embodiment of the invention the two zones are both occupied by permeable solid material but with the pressure drop across the whole axial length of the high flow resistance zone being at least double and preferably at least ten times the pressure drop across the whole axial length of the low flow resistance zone. However, it is preferred that the length in the axial direction of the element of the high flow resistance zone should be not greater than the length of the low flow resistance zone and preferably not less than one tenth of the length of the low resistance zone.

When the barrier tube is of impermeable material there should be an aperture or apertures in it adjacent to the high flow resistance zone (that is to say preferably within a range of 1 to lOmm from the end of that zone); when the barrier tube is of a microfibre permeable material such apertures are not essential but may be provided if it is desired to produce a particular flow or filtration effect.

The high pressure area may be at one end of the filter rod element, and this end may be assembled either next to the tobacco in a cigarette or at the free end of a cigarette.

In any case, the construction defined is extremely easy and efficient to assemble with existing filter-building technology and machinery.

Particular embodiments of the invention will now be described with reference to the accompanying drawings wherein Figure 1 is a diametrical cross-section through a filter rod element which is a first embodiment of the invention; Figure 2 is a section on the plane AA of Figure 1; Figure 3 is a section on the plane BB of Figure 1; Figure 4 is a diametrical section through a second embodiment; Figure 5 is a section on the plane CC of Figure 4; Figure 6 is a diametrical section through a third embodiment; Figure 7 is a cross-section on the plane DD of Figure 6; Figure 8 is a diametrical cross-section of a fourth embodiment; Figure 9 is a cross-section on the plane EE of Figure 8; and Figure 10 is a diametrical cross-section through a cigarette embodying the invention.

In the first embodiment we see a cigarette filter element 6 with core zones 1 and 2 within a uniform openended barrier tube 3. This is in turn surrounded by conventional filter material 4 and a conventional plug wrap 5. Such plug wraps are usually permeable.

The core zone 1 has a higher flow resistance than core element 2. For this purpose they are occupied by different or differently-prepared materials. These abut each other and are tightly surrounded by the tube 3. In particular it is important to prevent leakage past the high flow resistance zone 1 between its edges and the tube 3 and for this purpose it may be desirable for there to be an adhesive or bonding layer applied between the barrier tube and the material making up the zone 1.

The enclosed pressure drop of the high flow resistance zone should be greater than 50 mm WG per 10 mm 3 length at 17.5 cm3/second air flow rate. For this purpose a suitable material can be 2.5Y/52,000 cellulose acetate tow.

When this has a diameter of 5.5mm (the core tube 3 will preferably have a diameter between 3 and 6mm) and is 6mm in axial length the enclosed pressure drop is approximately 3 180mm WG at 17.5cm /sec air flow, over its axial length.

Those skilled in the art will recognise however that there are many other materials which could be suitable for occupying this core zone such as porous solids, granules, foams, fibrous material, paper or paperlike materials and conventional cellulose acetate tow materials of denier per filament ranging from 1.5 to 25 (with appropriate total denier). In the extreme case the material may be impermeable.

The core zone 2 on the other hand is of material with low flow resistance and preferably the enclosed pressure drop of the material used in this part should be below lOOmmWG per lOmm length at the same air flow rate as above. For example, this zone can be made using an 8Y/14000 (8Y denier per filament, 14000 total denier) acetate tow.

The enclosed pressure drop across such a material is 40mmWG under the conditions stated. Again any suitable material which will provide an adequately low flow resistance is acceptable, for example foam or fibrous material, paper or paperlike material and conventional cellulose acetate tow materials.

The filtration efficiency of the zone 2 should be as low as possible. For this purpose as will be seen in one of the later embodiments this zone may be devoid of material. However it is preferable for material to be there for simplicity in the manufacturing process and for the support that it gives to the barrier tube and filter material outside it. In any case, a filtration efficiency of this zone of less than 50% is desirable.

To achieve the desired flow resistance characteristics in the zones 1 and 2 the enclosed pressure drop per unit length of the material used in zone 1 should be higher than that per unit length of the zone 2; overall, the enclosed pressure drop across zone 1 in the axial direction should be at least double and preferably ten times the enclosed pressure drop in the axial direction over the zone 2.

The lengths of the respective zones, e and f in Figure 1 can be in any relationship but it is preferred that the high flow resistance zone is not longer than the low flow resistance zone and a preferred range is 1:10 to 1:1.

The barrier tube is made of at least one layer of microfine fibres, usually such as those produced by meltblow process and usually of a thermoplastic material such as polypropylene. The diameter of the microfine fibres ranges between O.lpm and lOum and the layer has a thickness in the range between 0.05mm and 2mm. Its density will usually range between 0.04 and 0.3 g/cm3.

For ease of handling this layer of micro fine fibres is usually supplied with one or two layers of protective non-woven fabric made up of coarser fibres and these may readily be incorporated in the filter rod element since the protective fabrics are porous.

Suitable materials are known, for example, a commercially available laminate fabric known as EHP (also known as Evolution or Kimguard) made by Kimberley Clark of the USA and similar laminates which are suitable are SMS, Blue Drake, LX 43, LX 293, LX 274, LX 275 and LX 276 all made by Kimberley Clark.

One grade of EHP laminate which has been tested and proved to be effective has a total thickness of 0.5mm, a 2 total fabric weight of 60gm/m2 with the three layers (coarse/fine/coarse) being approximately of the same thickness. Each layer has about equal fabric weight of 2 20gm/m2 .

The barrier tube therefore offers a comparatively high resistance, highly effective filter path between the inside and outside of the tube, in either radial direction.

The filter material 4 is conventional and is a material such as cellulose acetate tow 8Y/39,000. Other known materials for providing an appropriate overall filter flow resistance can be used here. Usually the flow resistance of the material 4 will be higher than that in the low flow resistance zone 2 of the core but lower than that of the high flow resistance zone.

A second embodiment is seen in Figures 4 and 5.

This embodiment 7 varies from the first in that the barrier tube 3 is penetrated by apertures 8 adjacent the high resistance zone. The apertures may be single or multiple but should offer a total opening area in the range 0.1mm2 to 10mm2 and it or they should open into the low flowresistance zone between 1 and lOmm from an end of the high flow resistance zone 1. When such apertures are present the barrier tube 3 may be of an entirely impermeable material such as is seen in barrier tube 10 of the third embodiment of filter rod element 9 seen in Figures 6 and 7.

In either case, the aperture(s) 8 act to even-out the response of the filter in the manner described for the crimped-end prior art in EP-A-310257.

Figures 8 and 9 show in an embodiment 11 how the low flow resistance zone 12 may be devoid of material.

However, as indicated above, this may be less preferable in manufacture and/or use because of the absence of internal support for the barrier tube.

Figure 10 shows an embodiment of a cigarette 13 having tobacco 14 wrapped in conventional cigarette paper 15 and having a filter element according to any of the embodiments described united to the tobacco by overwrapper 16, with the high flow resistance zone 1 of the core adjacent to the tobacco 14 of the cigarette. This situation is preferred since if the filter rod element were assembled the other way round onto the cigarette the material 1 would be at the free end of the filter and there is a slight risk that it could become dislodged in transport or during smoking. Functionally, however, the action of the filter would not be affected substantially by its orientation in relation to the tobacco of the cigarette.

All of the embodiments the filter described are readily made on modified filter making machines and into a cigarette in conventional cigarette making machines.

Any of them may be ventilated by apertures transversely through them, in known manner, provided that such ventilation does not interfere with the proper functioning of the filter.

Broadly this function is to achieve an evening out of the perceived total particulates received by the smoker as the cigarette is progressively consumed. The effect of the high resistance zone is to divert all or most of the smoke flow through the filter material 4 whence it is pulled back into the low flow resistance zone 2 of the core either through aperture 8 or through the wall of the permeable barrier tube 3. As smoking progresses, the apertures 8 or the portion of a permeable barrier tube 3 adjacent to the high flow resistance zone 1 becomes progressively partially blocked by solids and/or condensates and as the burning area of the tobacco approaches the mouth of the user the smoke is progressively increasingly forced to pass through a greater length of the filter material 4, achieving a greater extraction of solids at a time when, without the filter, the solid particulates would be perceived by the smoker to be excessive.

Claims (7)

CLAIMS:

1. A smoking article filter having a tube which is continuously uniform from end to end of the filter, is open-ended and is surrounded by filter medium, with means of communication between the inside of the tube and the filter medium, and there being within the tube zones of respectively different flow resistances, one being higher than the other.

2. A filter according to claim 1 wherein the zone of higher resistance is completely impermeable.

3. A filter according to claim 1 or claim 2 wherein the zone of lower resistance is empty.

4. A filter according to claim 1 wherein the zones are both occupied by material and are permeable.

5. A filter according to claim 4 wherein the pressure drop across the zone of higher resistance is at least ten times the drop across the zone of lower resistance.

6. Filters substantially as herein described with reference to and as illustrated in the accompanying drawings.

7. A cigarette incorporating a filter according to any one of the preceding claims.