Filters For Tobacco Smoke

Abstract

The invention relates to a tobacco-smoke filter comprising regions extending substantially longitudinally of the filter and separated by a diaphragm or diaphragms of vapour-porous material into at least one region providing a through path for smoke and at least one region which is filled with carbon and is closed against smoke flow through the filter. The regions may be formed by two co-axial tubes, an inner diaphragm tube of the porous material and an outer tube of substantially smoke-impervious material. Alternatively the carbon may be contained in one or more closed bags or envelopes of the porous material enclosed within an outer tube providing a through path for the smoke.

Description

Many forms of filters for tobacco smoke are known which remove the particulate phase of the smoke by mechanical means. These filters, composed for example of filamentary or fibrous materials, have, however, a low efficiency for the retention of volatile constituents of tobacco smoke. It is known that such constituents in vapour form can be removed from the smoke to some extent by adsorption on a suitable surface or by chemical reaction. An effective material for this purpose is, for example, carbon, possibly in the form of activated charcoal. The material may be provided as a granular bed between two filter plugs of fibrous or filamentary filtering material or may be dispersed in or on other filtering material. It is, however, known that carbon, thus used, may have an adverse effect on the taste of the tobacco smoke by producing a so-called "off flavour." It is an object of the invention to provide a filter containing carbon which removes the volatile or vapour-phase constituents to an acceptable degree, but which has little or no adverse effect on the taste of the smoke. More particularly, it is an object to provide a filter in which the carbon is protected against the deposition of smoke particles, while allowing the adsorption of vapours.

According to the invention, a tobacco-smoke filter comprises regions extending longitudinally of the filter and separated by a diaphragm or diaphragms of vapour-porous material, for example highly porous paper, into at least one region providing a through path for smoke and at least one region which is filled with carbon, possibly in the form of activated charcoal, and is closed against smoke flow through the filter. Conveniently the diaphragm may be tubular and the said regions located one within another.

Thus such a filter may comprise two concentric tubes, an inner tube of the vapour porous material and an outer tube of substantially smoke-impervious material, the annular space between the tubes being filled with carbon granules or powder and being blocked at each end, while the inner tube, without a carbon filling, is open at both ends to afford a smoke channel. Suitably the ratio of the diameter of the inner tube to that of the outer tube may be of the order of from 1:4 to 1:2.

Alternatively, the inner tube may be filled with carbon and blocked similarly at each end, the outer tube being spaced from the inner tube, possibly by longitudinal ribs, to leave an open-ended annular smoke channel. Suitably the tube-diameter ratio may be of the order from 5:10 to 9:10.

The packing of the carbon particles may be significant in that the greater the surface area available, the more favourable are conditions for the diffusion and thus the removal of the vapour constituents. Defined in terms of weight per unit length of filter a range of 10-150 mg/cm, preferably 40-100 mg/cm, is suitable. The particle size is preferably within a range of 12 to 150 British Standard Specification mesh.

The "porosity" of the porous material, preferably paper, should be at least 1000 cm.sup.3 /min/10cm.sup.2 per 10 cm water-gauge pressure and is preferably within the range of 32,000 to 600,000.

Such a filter is preferably used as a centre section between two end sections of fibrous material, such as cellulose acetate or paper, or in dual filter adjacent to the tobacco rod. The whole filter may be enclosed within a customary sleeve of tipping paper and attached to a cigarette in conventional manner.

On smoking a cigarette through such a filter, the smoke particles pass through the smoke path, the inner tube or annular space as the case may be, without flowing through the carbon. Analysis of the smoke shows that a reduction in the amount of volatile constituents, particularly aldehydes and hydrogen sulphide is obtained, although the taste of the smoke is unaffected. The reduction is achieved because volatile constituents diffuse through the inner tube into the carbon-filled region in which they are adsorbed. The particulate matter does not diffuse through this tube. Substantially, therefore, the carbon is prevented from being contaminated by smoke particles and carbon/particle interactions do not occur. A further advantageous effect observed is that the proportion of total volatile constituents remains more uniform, from puff to puff, than is the case if the smoke flows through a carbon bed.

Examples of ways of carrying out the invention and of results thereby achieved will now be more fully described with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a longitudinal section through a cigarette filter with an annular carbon-filled region,

FIG. 2 a similar view of a filter with a central carbon-filled region,

FIG. 3 a similar view of a filter with a bag-like carbon-filled region,

FIGS. 3a and 3b being opposite end views of the filter,

FIG. 4 a graphical representation of filter-efficiency results obtained,

FIG. 5 a perspective view of a carbon-filled bag,

FIG. 6 a perspective view of a filter section comprising three such bags, and

FIGS. 7 and 8 are a cross section and a longitudinal section, on the line VIII--VIII in FIG. 7, of a filter with a rolled, carbon filled, bag.

In all of the following examples, cigarettes with filters having an outer-tube diameter of 8 mm were smoked under standard conditions of 1 puff of 2 seconds duration every minute and 35 ml volume. The measured filtration efficiency for total volatile aldehydes was taken as an index of the effectiveness with which the carbon removed vapour-phase constituents from cigarette smoke.

Example 1

the triple filter illustrated in FIG. 1 comprises a section 1 in accordance with the present invention interposed between filter-plug sections 2 and 3 of cellulose acetate at the mouth end and adjoining the tobacco rod 4 respectively, all enclosed within a smoke-impervious wrapper tube 5. The section 1 comprises an inner tube 6 of highly vapour-porous paper which extends co-axially within the tube 5 and bounds a central smoke channel 7 the annular space 7' between the tubes being filled with carbon particles and being blocked at both ends by impervious annular closures 8. These closures may be produced from a plastics material, for example a polyvinyl acetate adhesive, used as a sealing compound or by discs of smoke-impervious material having a central hole in register with the tube 6.

In a specific instance, the overall length of the filter was 71 mm. The tube 5 was made of smoke-impervious paper and the tube 6 of a paper having a porosity of 34,000 cm.sup.3 /min/10cm.sup.2 per 10cm water-gauge pressure. The tube 6 was 58 mm in length and 4.5 mm in diameter. The space 7' between the tubes 5 and 6 contained 720 mg of a granular carbon known as grade 203B (available from Sutcliffe Speakman Ltd). The total pressure drop of the filter was 7.1 cm water gauge. The filter removed 45 percent of the total volatile aldehydes from the smoke. For comparison purposes, cigarettes were prepared with filters which contained a bed of 50 mg of the carbon granules positioned between two cellulose acetate sections and designed to remove the same proportion of volatile aldehydes from the smoke.

Cigarettes with the two kinds of filters were offered to a panel of smokers with particular experience of evaluating the flavour characteristics of carbon-filter cigarettes. The panel found that the comparison, granular-bed, filter produced a high level of the undesirable off-flavour characteristic of carbon-filter cigarettes, whilst the other filter, with the porous inner tube 6, produced none of this flavour.

Example 2

The filter construction of FIG. 2 differs from that of FIG. 1 in that the granular carbon is disposed in the space 9' within the tube 6. This space is blocked at both ends, at 10, whereas the annular smoke channel 9 is open at both ends. With one filter of this kind, 53 mm long, the outer tube 5 was made of a plastics material. The inner tube was made of paper having a porosity of 200,000 cm.sup.3 /min/10cm.sup.2 per 10 cm water gauge. It was 40 mm in length and 4.5 mm in diameter and was filled with 282 mg of the same granular carbon as in Example 1. The filter, whose pressure drop was 3.2 cm water gauge, removed 48 percent of the total volatile aldehydes from the smoke. A comparison filter was prepared in the same manner as in Example 1, but with a 55 mg carbon bed expected to have the same aldehyde-filtration efficiency. The panel of smokers again noted a high level of the characteristic off-flavour from the comparison cigarettes, but none from the cigarettes with filters containing carbon within the porous tube.

Example 3

In the filter of FIG. 3, the granular carbon is contained in a porous bag 11 disposed within the tube 5. The ends of the bag 11 are sealed along straight lines 12,13 at right angles to each other, as shown in FIGS. 3a and 3b, producing fin-like formations 14 which support the bag 11 within the tube 5 and assist in ensuring favourable smoke-flow around the bag.

With one filter of this kind, 53 mm long, the tube 5 was of impervious paper. The bag 11, of paper of 200,000 cm.sup.3 /min/10 cm.sup.2 per 10 cm water gauge porosity, was 40 mm in length and 4.5 mm diameter. It was filled with 235 mg of granular carbon, grade BPL available from Pittsburgh Activated Carbon Co. The pressure drop of the filter was 4.5 cm water gauge. On smoking under standard conditions, a reduction of 40 percent of the total volatile aldehydes was observed. When compared by the panel with cigarettes with a granular-bed filter having 70 mg of carbon giving the same filtration efficiency, cigarettes having filters as described with reference to FIG. 3 did not produce the off-flavour imparted by the carbon-bed filter.

Example 4

Further filters were prepared as described in Example 3, but with the bag 30 mm long and containing 150 mg of the same carbon, the overall pressure drop being 3.0 cm water gauge. The total volatile aldehyde filtration efficiency was determined on a puff by puff basis for these filters (A) and for a conventional filter (B), containing a 50 mg bed of carbon, which removed substantially the same average percentage (29 percent) of the volatile aldehydes. The puff by puff results, given in the following table, are shown in FIG. 4 as efficiency n plotted against the puff number PN.

______________________________________ Filtration Efficiency Puff Number for Total Volatile Aldehydes (%) A B ______________________________________ 1 27 70 2 36 60 3 31 49 4 27 41 5 28 33 6 30 37 7 26 22 8 29 12 9 25 0 10 28 0 11 21 0 Average (all puffs) 28 29 ______________________________________

Filter A removed a similar fraction of the total volatile aldehydes for each puff. In contrast, filter B removed a very high fraction of the aldehydes from the first puff, but rapidly declined in performance, removing none of the aldehydes from the final three puffs.

Example 5

Filters were prepared as described in Example 3, but the bag 11 was made of porous paper which had been so perforated that the overall porosity was raised to 600,000 cm.sup.3 /min/10 cm.sup.2 per 10 cm water gauge. The bag, 30 mm long, contained 161 mg of carbon of the same grade. The filter had a total pressure drop of 2.9 cm water gauge and removed 31 percent of the total volatile aldehydes. Compared with control filters containing 50 mg of carbon and having similar filtration efficiency, the former filters did not produce the off-flavour observed with the latter filters.

Example 6

Filters were prepared as described in Example 3, but the bag 11, 30 mm in length and 6 mm in diameter, was filled with 259 mg of carbon of the same grade. The pressure drop of the filter was 3.1 cm water gauge. These filters removed 32 percent of total volatile aldehydes without producing the off-flavour, as detected from comparison filters having granular beds of 54 mg of carbon.

Example 7

Filters were produced as in Example 3, but containing 3 bags, each of 2.75 mm diameter and 30 mm length and each filled with 51 mg of carbon of the same grade. These filters, which had a pressure drop of 3.3 cm water gauge removed 34 percent of the total volatile aldehydes. A panel could not detect the off-flavour found with comparison filters having granular beds of 59 mg of carbon.

Example 8

Further filters were prepared as described in Example 3. The annulus between the bag 11 and the tube 5 was filled with: (a) 1,700 mg of inert granules of unglazed porcelain, known as anti-bumping granules, or (b) open-cell polyethylene foam supplied by Monsanto Chemicals Ltd, or (c) bloomed cellulose-acetate tow. The bag 11 was 30 mm long and contained 155 mg of carbon of the same grade. The pressure-drops for the complete filters were 3.5, 5.7 and 4.8 cm water gauge and the total volatile aldehyde filtration efficiencies 46 percent, 32 percent and 43 percent respectively.

Granular-bed filters, of comparable efficiency, containing 82 mg, 51 mg and 74 mg of carbon respectively produced high levels of the characteristic carbon flavour, which was absent with the former filters.

Example 9

Filters were prepared as described in Example 3. The bags 11, 30 mm long and 4.5 mm in diameter, contained other grades of carbon, namely (a) 140 mg of MF3, supplied by Chemviron Ltd, and (b) 114 mg of AC45, supplied by The British C.E.C.A. Co. Ltd, the pressure drops being 2.8 and 3.0 cm water gauge and the total aldehyde-filtration efficiency 30 percent and 33 percent respectively. The off-flavour was absent from these filters. Comparable granular-bed filters containing 36 mg of MF3 and 40 mg of AC45 carbon respectively produced high levels of the off-flavour.

Example 10

Further filters were prepared as described in Example 3, but using polyethylene for the outer tube 5. The tube was ventilated near the cellulose-acetate section 2 at the mouth end by making holes in the tube 5 and then wrapping the whole filter in a porous tipping paper, supplied by Papeteries de Malaucene.

The ventilation reduced the cigarette pressure drop by 55 percent. The porous bag, 30 mm long and 4.5 mm in diameter, contained 149 mg of the same grade of carbon as in Example 3. The combination of ventilation and porous bag reduced the delivery of total volatile aldehydes by 73 percent. Comparable carbon-bed, non-ventilated, filters containing 183 mg of the same carbon produced high levels of the characteristic off-flavour, which was absent with the former filters.

Example 11

To prepare the filter illustrated by FIGS. 5 and 6, carbon powder (52-100 British Standard Specification mesh) of the grade AC45 referred to above was placed on a sheet of paper of porosity 200,000 cm.sup.3 /min/10 cm.sup.2 per 10 cm water gauge and the sheet was folded over to the form of a bag 15 and sealed down the longitudinal edge 16 and at both ends 16'. Three such bags, 30 mm long and approximately 7 mm wide each contained 40 mg of carbon. The three bags 15 were assembled in a triangular configuration (FIG. 6) between two cellulose acetate sections (not shown), within an impervious paper tube 5. The filter, which had a pressure drop of 2.7 cm water gauge, removed 55 percent of total volatile aldehydes from the smoke. In smoking-panel tests, a comparable cigarette with a granular-bed filter containing 79 mg of the same carbon was found to produce a high level of the characteristic off flavour, which was absent from cigarettes with filters prepared as described.

Example 12

For the filter illustrated in FIGS. 7 and 8, 115 mg of carbon powder (100-150 British Standard Speciification mesh) of the aforesaid grade AC45 was laid between two 100 mm .times. 25 mm sheets of very porous paper (porosity 200,000 cm.sup.3 /min/10 cm.sup.2 per 10 cm water gauge). All of the edges of the two sheets were glued together to form a closed bag 17 containing the carbon 18. The bag 17 was rolled from the flat form to the spiral form shown in FIG. 7 and as in previous examples, was placed between cellulose-acetate sections 2, 3 to form a 25 mm long filter, enclosed in an impervious paper tube 5, with a pressure drop of 3.5 cm water gauge. The filter removed 57 percent of total volatile aldehydes from the smoke. A comparable cigarette with a granular-bed filter containing 84 mg of the same grade of carbon produced high levels of the characteristic off flavour, which was absent from the filter of the Example.

Preferably, the bag 17 is rolled with a sheet of longitudinally corrugated paper in the spiral space 19 between the turns of the rolled bag. This will ensure that different parts of the bag do not come into contact with one another, thus increasing the effective area of bag surface through which smoke constituents can diffuse and also the number of smoke passage ways.

The same end can be achieved by making the bag 17 itself from corrugated porous paper. Contact between different parts of the paper is then limited to a number of points. The corrugations also increase the surface area for a given length of bag.

A bag or bags produced in one of the ways described in Example 12 may be disposed in the filter otherwise than in a single spiral form, as seen in cross section. For example, such a bag may be rolled in the form of a C with both free ends further rolled inwardly. Indeed, the bag may be incorporated in the filter with a random disposition in the cross section.

The bag or bags need not occupy the entire cross section within the tube 5. A bag may be rolled or folded into the filter together with a longitudinally extending filter material, such as a cellulose acetate tow or paper.

Indeed, in all cases, the smoke channel or channels, whether central and/or peripheral, may be packed with a filtering material (not carbon) for the removal of particulate matter, for example with fibrous or filamentary cellulose acetate, paper or the like.

Alternatively, the smoke channel may be packed with an inert granular material, as described in Example 8, or with glass spheres or chippings. Such a packing will provide more intimate contact between the smoke and the porous paper, thus assisting the diffusion of volatile smoke constituents into the carbon. The filtration efficiency of the filter for volatile constituents can thereby be enhanced above the value obtained for the filter without such a packing, again without adverse effect on the taste.

Preferably and as described above, the carbon is in the form of loose granules or powder. However it could alternatively be used in the form of a charcoal or other carbon rod or tube disposed as previously described. Carbon may be bonded by a resin such a polyethylene, an organic material such as paraffin wax or an inorganic material such as trisodium phosphate.

Claims

We claim:

1. A filter for a smokeable article intended to be disposed downstream from a smokeable charge, comprising a smoke treating chamber formed by a substantially cylindrical outer wall of the filter element in close proximity to the downstream end of a smokeable charge, a longitudinally oriented vapor porous membrane mounted within said cylinder, said membrane containing a charge of particulate carbon, the upstream and downstream ends of said carbon charged membrane being effectively sealed or blocked against the passage of smoke, the space not occupied by said membrane defining longitudinally extending continuous and uninterrupted passageway or passageways extending for a length at least equal to that of the contained carbon charge, such that the smoke emerging from the downstream end of a smokeable charge preferentially courses through the said continuous and uninterrupted passageways without being forced through said contained carbon charge.

2. A filter according to claim 1 wherein the porous membrane is tubular and the carbon charge is contained within.

3. A filter according to claim 1 wherein the porous membrane is tubular and the carbon charge is contained within the space between the tube and said outer wall.

4. A filter according to claim 1 wherein the carbon charge is contained within at least one closed envelope of porous membrane enclosed within said outer wall.

5. A filter according to claim 4 wherein a plurality of said envelopes are disposed in a triangular form within said outer wall.

6. A filter according to claim 1 wherein the carbon charge is contained in at least one closed envelope of porous membrane, said envelope being folded within and longitudinally of said outer wall.

7. A filter according to claim 1, disposed as a centre section between two end sections of fibrous material.

8. A filter according to claim 1, wherein the vapor porous membrane is made of highly porous paper with a porosity of at least 32,000 cm.sup.3 /min/10 cm.sup.2 per 10 cm water gauge pressure.