GB2105566A - Tobacco smoke filtering device - Google Patents

Abstract

A tobacco smoke filter for cigarettes which presents initially relatively low pressure drop (54, 42) and relatively high pressure drop (40, 42) paths for the passage of smoke therethrough, the or each relatively low pressure drop having disposed thereacross a barrier (3) of material which is initially smoke-permeable but which is progressively blocked by the passage of smoke therethrough, whereby in use the initial preferential passage of smoke via the low pressure drop path or paths causes progressive blocking thereof so that the passage of smoke via the high pressure drop path or paths is correspondingly increased. <IMAGE>

Description

SPECIFICATION Tobacco smoke filtering device The present invention relates to tobacco smoke filters, and provides such a filter which presents initially relatively low pressure drop and relatively high pressure drop paths for the passage of smoke therethrough, the or each relatively low pressure drop path having disposed thereacross a barrier which is initially smoke-permeable but which is blocked by the passage of smoke therethrough, whereby in use the initial preferential passage of smoke via the low pressure drop path or paths causes blocking thereof so that the passage of smoke via the high pressure drop path or paths is increased.

Filters according to the invention help to even out the taste delivery over the smoking of the cigarette. The taste delivery of a conventional filter cigarette increases during smoking from a low, often unsatisfying, initial value to a high, often unacceptable, value; for a given overall delivery a filter according to the invention gives in comparison an increased initial and a reduced final value and hence a more even smoke.

In some preferred embodiments the said barrier has two plies of air-permeable sheet material (e.g.

paper), one of which is of high air-permeability (e.g. from 10 to 100 K, preferably about 20 K) and the other of which is of even higher airpermeability (e.g. 50 to 700 K, preferably about 500 K). The units of air permeability used herein are defined below. Preferably the barrier is disposed with the higher permeability ply on the downstream face of the lower permeability ply; it is believed that in use the drawn smoke jets through the lower permeability ply with the particulate phase tending to impact onto the higher permeability ply with progressive blocking of the pores of the former. This arrangement can give a smoother progressive increase of pressure drop (PD), and hence a smoother switch of smoke flow from low to high PD paths than has otherwise been obtainable.

In one type of embodiment according to the invention, the filter has an axially elongate core having at its periphery at least one passage which extends from one end of the core only partially towards the other end of the core and is in smokeflow communication with the core via means blockable by the passage of unfiltered tobacco smoke therethrough.The said means could for example be a barrier in the form of a core sleeve (e.g. wrap) which separates the core from the or each passage; this core sleeve might e.g. be of inherently low pressure drop, smoke-permeable material which is rapidly blocked by the passage of unfiltered tobacco smoke therethrough so that further such flow therethrough is reduced or stopped, or it might be of inherently smoke impermeable material having small perforations therethrough to give the same result; it could be a two-ply barrier of the type mentioned above. Any given said passage could comprise a groove in the core periphery and/or be defined between opposed core surface and profiled sleeve portions.

Both core and sleeve may be profiled to provide said passage(s).

In another type of embodiment the filter has at least one internal passage extending from one end of the filter only partially towards the other end thereof and in smoke-flow communication with a smoke filtering portion of the filter via means blockable by the passage of unfiltered tobacco smoke therethrough. The said means could for example be a barrier layer, surrounding the passage and separating it from the rest of the filter, this layer being initially of low pressure drop and permeable to smoke flow but becoming blocked by flow of unfiltered tobacco smoke therethrough so that further such flow is reduced or stopped; it could for example be a two-ply barrier as previously described.

Some preferred filters according to the invention have longitudinally adjacent core sections of relatively high and relatively low pressure drop (PD) surrounded by a profiled sleeve providing between itself and the core sections one or more passages extending from one end of the filter only partially towards the other, with a said blockable barrier disposed between the passage or passages and the low pressure drop core section.

The barrier could be of the two-ply variety mentioned above, or it could instead be a single ply of inherently permeable and/or perforate material. The initial low pressure drop path is thus provided by the low pressure drop core section, the passage(s) and said barrier therebetween, whilst the high pressure drop path is provided by the low and high pressure drop core sections. The high pressure drop core section, for example of a PD of 200 to 600 (e.g. 300) mm water gauge (Wg), may be a plug of staple fibres or tow /e.g. of cellulose acetate) or paper. The low pressure drop core section may be a lower pressure drop plug or a tubular member; such a lower pressure drop plug or tubular member may abut the higher pressure drop core section, or it may be longitudinally spaced therefrom to define a cavity forming part of the lower pressure drop core section.The said passage or passages may be in smoke flow communication (via said barrier) with the material of the lower pressure drop core section (when this material is smoke-permeable) and/or with a said cavity forming part of the lower pressure drop core section.

The higher pressure drop core section is suitably a plug of "Transorb" (Registered Trade Mark), a dense drawn down rod of bonded cellulose acetate filaments. The characteristics of "Transorb" rods, and methods for their production (which can remove the initial filament crimp) are described in U.K. Patent Specification No.

970,817. At least towards its downstream end, and in most cases over its whole longitudinal extent, the periphery of the high PD section is preferably substantially smoke impermeable e.g. due to provision of a fused (such as by heat or solvent) integral skin or of a sleeve (which may be a wrap or a coating, e.g. a coating of glue adhering the section to said barrier).

Herein, an air permeability of nK means an air permeability of n x 1000 mis/minute/10 square cms/1 00 mm Wg pressure -- as determined by measuring the pressure "p" in mm Wg generated by a flow of 1050 mls of air per minute through 10 square cms of the sample, and calculating from the equation: 100 air permeability = 1050 x P Filters according to the invention may be incorporated in filter cigarettes in conventional manner by means of a tipping overwrap. In the embodiments mentioned above where the filter has part of the low pressure drop path provided by a passage or passages from one end of the filter, it is this end of the filter which abuts the tobacco column of the filter cigarette.

Where filters according to the invention are initially formed, for supply to the filter cigarette production stage, with an outer wrap, they may be incorporated in filter cigarettes by either ring tipping or full tipping overwrap.

Some filters according to the invention require an outer wrapper to complete them, e.g. to complete formation of the low PD path(s). The invention also provides filter elements per se, for combination with an outer wrapper to yield filters according to the invention. Such elements, exemplified by the types described below, generally include the features of an integral or composite axially elongate core body having at least one external or internal passage extending from one end of the body only partially towards its other end, and means for permitting smoke-flow communication between passage(s) and body, the said means being blocked by the passage of unfiltered tobacco smoke therethrough. The elements will usually be produced initially in continuous rod form and immediately cut into finite lengths for further handling and filter cigarette production.A wrap can if desired be applied continuously around the element as it is formed and before cutting to finite lengths, but the resulting filters according to the invention then require further tipping (ring tipping or tipping overwrap) for incorporation in filter cigarettes; when the elements are produced and cut unwrapped, they are usually formed into filters according to the invention by the application of tipping overwrap during filter cigarette production.

For filter cigarette manufacture, a double length such element or filter is disposed with a wrapped tobacco rod abutting either end, tipping is applied to join the tobacco rods and intervening double element or filter together, and the resulting combination is cut in half to produce two filter cigarettes. The initially produced continuous filter or element is normally cut into even multiple (e.g.

sextuple) length units for supply to the filter cigarette manufacturer who then cuts these multiple lengths into double lengths for use in filter cigarette production as described above. It will be appreciated that, in the case of an individual filter or element length according to the invention which is asymmetric (i.e. whose two ends differ), the multiple and double lengths from which the individual lengths are eventually formed will usually be symmetrical and have identical ends. The multiple (including double) length elements and filters which are thus also provided by the invention comprise a plurality of unit elements or filters disposed integrally end-to-end with each said unit element or filter being disposed in mirror-image relationship to the or each integrally adjacent unit element or filter.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which like reference numerals indicate like parts and: Figure 1 is a perspective view of a filter element according to the invention; Figure 2 is a part sectional view of a filter and filter cigarette according to the invention employing the Fig. 1 filter element; Figure 3 is a longitudinal view through a filter cigarette employing a different filter and filter element according to the invention; Figure 4 is a longitudinal sectional view, on line IV IV of Fig. 5, through a filter cigarette employing a third filter element and filter according to the invention; Figure 5 is a section on lines V-V of Fig. 4;; Figures 6, 7 and 8 are schematic longitudinal sectional views of three different filters and filter cigarettes according to the invention, the Fig. 7 section being on lines VIl-VIl of Fig. 11; Figures 9 and 10 are cross-sectional views of the Fig. 6 embodiment through the region of its cavity and through its upstream end region respectively; Figure 11 is a cross-sectional view of the Fig. 7 embodiment through its upstream end region; and Figures 1 2 to 14 are schematic longitudinal sectional views of three different sextuple length filter elements according to the invention.

The filter element shown in Fig. 1 comprises a core 2 of bonded cellulose tow having a plugwrap 3 in surrounding engagement therewith. Core 2 and plugwrap 3 are partially crimped at one end 4 to provide a reduced section core 6 surrounded by four symmetrically disposed grooves or channels 8 separated by ribs or fins 10. The plugwrap 3 is of porous paper which offers little initial resistance to the passage of smoke therethrough but which is progressively blocked by the passage of unfiltered tobacco smoke therethrough. The element illustrated could if desired be provided with a plain cylindrical plugwrap.

The illustrated element can be made by continuously producing in conventional manner a continuous acetate tow rod wrapped in plugwrap 3, crimping this at intervals, and severing the resulting crimped rod transversely through adjacent crimped or uncrimped regions as previously explained this severing would be first into symmetrical multiple, then into symmetrical double, and finally into the asymmetrical single lengths.

The Fig. 1 filter element is incorporated in a filtered cigarette as shown in Fig. 2, where the element (shown in side elevation) has its end 4 abutted against a wrapped tobacco column 1 2 (shown in section) and is secured into the tobacco rod by a tipping overwrap 14 of air-impermeable material. In use, smoke from the tobacco rod is initially drawn preferentially along the low pressure drop route, i.e. along channels 8 and via plugwrap 3 into the uncrimped portion of core 2; the delivery of taste components to the smoker is thus relatively high.After the first few puffs (e.g. three orfour), however, plugwrap 3 becomes blocked by the passage of the unfiltered smoke therethrough, and smoke from the tobacco column is increasingly forced to take the higher pressure drop route into the reduced section 6 and along the whole length of core 2: the degree of filtering of the smoke thus increases, tending to counteract the increased tar etc. delivery that is normally encountered over the later puffs from a conventionally filtered cigarette. Thus a given overall tar delivery can be provided at a more even rate from puff to puff than with a conventional filter.

It will be appreciated that whilst an integral core 2 has been shown, numerous variations are possible; thus the core could be a composite of two abutting sections of different characteristics (e.g. different pressure drop); another possibility is for the buccal end of core 2 to be provided with an internal cavity.

The Fig. 3 embodiment of the filter according to the invention comprises a body 20 of smoke filtering material, e.g. bonded cellulose acetate tow, which has an air-impermeable plugwrap 32 and an internal cavity 22 open at its end 4. The cavity 22 is separated from surrounding body 20 by a tubular wall 26 (of the same material as that of plugwrap 3 of Figs. 1 and 2) which tapers within the filter to a closed end 28. This filter according to the invention is attached at end 4 to wrapped tobacco rod 12 by tipping overwrap 14.

The initial low pressure drop smoke path in this case is through cavity 22 and wall 26 into body 20, tar delivery thus being relatively high. After a few puffs, wall 26 becomes blocked by the passage of the unfiltered smoke therethrough, an increasing amount of smoke thus having to travel the full length of body 20 from its upstream end 4, so that filtering efficiency increases as before. End 28 is crimped in the general manner shown in Fig. 1 except that the crimping completely closes the tube of material 3, the ribs 10 extending inwardly to the closure region 28.

The Figs. 4 and 5 filter according to the invention has a core 2 made up of high pressure drop element 40 and a low pressure drop element 42 held together by a plugwrap 3. In surrounding engagement around plugwrap 3 is another wrapper 44 of smoke- and air-impermeable material which has longitudinally extending corrugations 46 extending from end 4 to terminate at circumferential corrugations 52 at end 50. Corrugations 46 define with core 2 longitudinal channels 54 which extend from end 4 and terminate at circumferential corrugations 52.

The plugwrap 3 is of the type described above in connection with Figs. 1 and 2. The filter includes wrapper 14 which is a tipping overwrap joining the filter at its end 4 to wrapped tobacco rod 12.

The initial low pressure drop path for the smoke in this case is along grooves 54, through the portion of plugwrap 3 at the ends of these grooves where it surrounds the low pressure drop core member 42, and then through the latter member to the buccal end 50; as with the other embodiments, initial tar delivery is thus fairly high. After the first few puffs, the plugwrap 3 through which the unfiltered smoke has passed into low pressure drop member 42 becomes blocked, and an increasing amount of smoke is obliged to pass instead into member 42 via the high pressure drop core member 40, with consequent increase in filtering effect. The walls of corrugations 46 could be perforate or have a certain degree of inherent permeability to allow passage of smoke (during the first few puffs) through all of the channels (54 and 53) formed between tipping overwrap 14, corrugations 46, and plugwrap 3.

Plugwrap 3 could be replaced by one having small perforations which initially pass smoke but then become blocked thereby, or by a two-ply wrap as previously described.

Likewise the tube 26 shown in Fig. 3 could be replaced by a tube of the above two-ply type of plugwrap (higher permeability ply innermost) crimped to close it at its downstream end.

The filters of Figs. 6 to 11 include a ventilating outer wrapper, the switching of.smoke flow from low to high PD paths being accompanied in these cases by an increase in the ventilating air flow through this wrapper and into the smoker's mouth. In each case the ventilating wrapper could be replaced by a non-ventilating one.

The Figs. 6, 9 and 10 filter has a low PD core section 42 formed by tubular member 61 and a cavity 63 by which the tubular member is longitudinally spaced from a higher pressure drop plug 40 of tobacco smoke filtering material.

Tubular member 61 may be of fibrous or nonfibrous smoke-permeable or -impermeable material. Plug 40 is suitably of gathered filamentary tow (e.g. of cellulose acetate) or paper, with a pressure drop of about 300 mm Wg.

Surrounding and joining the core portions is a sleeve 3 constituting the barrier which is initially permeable to smoke but becomes blocked by the passage of smoke therethrough. Sleeve 3 is suitably a wrapper, and is preferably adhered to members 61 and 40. Sleeve 3 is in this case a two-ply paper wrapper of which the innermost (upstream) ply has a permeability of 20 K and the outermost (downstream) ply a permeability of 500 K. Sleeve 3 could however be of any other material and structure performing the described function.

Immediately surrounding barrier sleeve 3 is a profiled (e.g. longitudinally corrugated) sleeve 44 (e.g. of paper) which may be permeable (but in this case is not) to air. The internal and external grooves 66, 68 provided by sleeve 44 are open at the end 50 of the filter around plug 40 and extend longitudinally therefrom only partially the length of the filter, terminating at a region 67 around the low pressure drop core section; in the region 67 the sleeve 44 may be simply left plain and unprofiled, a filling 80 (e.g. of thermoplastic material) making up the external diameter differential and closing off any passages which would otherwise be open at this end of the filter.

The internal grooves 66 may for example be from 0.5 to 1 mm deep and of about 1 mm pitch in which case in a conventional size filter they would be about 23 in number spaced uniformly around the core circumference. In use the filter can be attached to a wrapped tobacco rod 12 as shown by a ventilating tipping overwrap 14 which may be of inherently air-permeable material and/or provided with ventilating perforations (as at 16) opening into the external passages 68. In use the tobacco smoke flows, during a puff, from the tobacco rod 12 through the tubular element 61.

For initial puffs, at least the majority of the smoke flows through the barrier sleeve 3 into the internal passages 66 between sleeves 3 and 44 and proceeds down them to the smoker's mouth, this constituting the lowest pressure drop path.

The flow of smoke presented to the inner face of sleeve 3 jets through the 20 K porous paper and the particulate phase tends to impact onto the 500 K porous paper, thereby progressively blocking the pores. As a result, the switching of smoke flow from this initial path to that through high pressure drop element 40 takes place smoothly and progressively ensuring a smooth puff by puff increase in pressure drop.

As this progressive puff by puff increase in pressure drop occurs, a progressive increase in ventilating air flow through the tipping overwrap 14 occurs. This air flow passes into the outer passages 68 and proceeds down them to the smoker's mouth.

Figs. 7 and 11 depict another embodiment in which spaced segments 61 and 40 are similar to those utilised in Fig. 6 and are enwrapped as before with an air permeable two-ply barrier sleeve 3 (500 K ply innermost). A longitudinally corrugated sleeve 44 enwraps the assembly, a circumferential groove 77 closing passages 66 but not passages 68 which are blocked with thermoplastics material 80 at region 67.

In use smoke flow, in initial puffs, predominantly passes down the internal passages 66, the flow path being diverted at the interruptions 67 and passing through the initially low pressure drop wrap 3 and then out to the smoker's mouth.

As the wrap 3 becomes progressively blocked with smoke particulate matter, its pressure drop progressively rises and the smoke switches smoothly to flow through the high pressure drop segment 40.

As in the embodiment of Fig. 6 a smooth, progressive increases in filter pressure drop and filter ventilation results with each succeeding puff.

In modified embodiments there is no air dilution through overwrap 14, in which case both passages 66 and 68 are terminated at 77 (e.g. by an insert of thermoplastics material) and filling 80 may be omitted.

Fig. 8 shows yet another embodiment in which a core section 42 and a second core section 40 are axially butted together. Section 42 is a cylindrical body composed of porous material (e.g.

cellulose acetate fibres) and exhibiting a very low pressure drop -- preferably about 1 5 mm Wg.

Section 40 is a cylindrical body of porous material (e.g. cellulose acetate fibres) and exhibiting a high pressure drop, preferably in the region of 300 mm Wg. Sections 40 and 42 are enwrapped with barrier sleeve 3 of similar make-up to that of Fig. 6. Wrap 3 may be bonded to section 40, around its periphery. The assembly is enwrapped with longitudinally corrugated material 44, with passages 66 and 68 being terminated at upstream region 67 as in Fig. 6. A tobacco rod 12 is attached to the filter assembly by an overwrap of perforated (as at 16) or permeable tipping paper 14.

As shown by the arrow indication of smoke flow the operation of the filter during smoking is similar to that described for Fig. 6. The initial smoke flow is through the material of plug 42 and through barrier 3 into passages 66, but progressively switches to the path via plugs 40 and 42, with increase in ventilation via passages 68.

A further embodiment is provided by the reversal, end for end, of the design of Fig. 8, with the tobacco rod 1 2 abutting plug 40.

Various modifications of the illustrated embodiments are possible. For example in the embodiments of Figs. 6, 9, 10 and that of Fig. 8 the corrugated sleeve 44 may be permeable to smoke or air, inherently and/or by way of perforations; smoke and air may thus mix in the passages 66, 68 until the passage of smoke through barrier 3 into passages 66 ceases on blocking of barrier 3; in these modifications, throughout the smoking of a cigarette, both passages 66 and 68 are available for ventilation air flow, which may thus be increased compared to when only external passages can carry the air flow.

Furthermore, in all of Figs. 6 to 8, the outer wrapper (tipping overwrap) 1 4 may be nonventilating, the switch of smoke flow from low to high PD route still levelling out the taste delivery from puff to puff; in the absence of air dilution the Figs. 6 and 8 embodiments may otherwise remain substantially as already described, whether or not wrapper 44 is permeable or perforate, but as previously mentioned the Fig. 7 embodiment would have groove 77 plugged with filling 80 optionally absent - in which case wrapper 44 could be smoke-permeable or perforate. Also, in all illustrated and/or described embodiments, with or without air dilution, the orientation of the filter element relative to the tobacco rod could be reversed, with the present buccal end instead abutting the tobacco rod.In some instances of such reversal (e.g. that of Fig. 8), then if there is air dilution through the tipping overwrap the ventilating air, with blocking of the barrier 3, will be obliged to travel to the tobacco end and then through the core rather than via the initial more direct route to the buccal end.

Instead of using webs of inherently smokepermeable material which block in use to switch smoke from low to high pressure drop paths, one may employ plugwrap having small perforations which progressively block to like effect. Thus in the illustrated embodiments barrier 3 can be replaced by a plugwrap of smoke-impermeable material having such perforations at the downstream ends of the smoke by-pass passage(s).

In the embodiments illustrated in Figs. 4 to 11 the high PD core section 40 can have its circumferential surface rendered smokeimpermeable by a coating of adhesive by which it is secured to the barrier sleeve 3, but the necessary degree and physical extent of surface impermeability of the high PD section 40 can be provided in other ways as previously mentioned.

As previously explained, a filter element or filter according to the invention such as any of those illustrated will usually be produced initially in a continuous length from which even multiple length rods (i.e. each rod being an even multiple of the eventual individual element or filter) are cut, the multiple length rods subsequently being further subdivided into double length elements and then, during filter cigarette production, into the final single length elements. Examples of sextuple length rods according to the invention, which can be further subdivided into individual elements according to the invention, are illustrated schematically in Figs. 12 to 14.Fig. 12 shows a sextuple length element rod from which single elements and filters as in Figs. 1 and 2 can be produced by cutting firstly midway through the inboard longitudinally grooved sections at positions 82 to yield double length rods and then, during filter cigarette manufacture as described above, at positions 84 Fig. 1 3 shows a sextuple length rod which can likewise be cut firstly at 82 midway through the inboard tubular members 61, and then at 84 midway through the high PD members 40, as part of the production of individual elements, filters and filter cigarettes of the type shown in Fig. 6;; Fig. 14 shows a sextuple length rod which can likewise be cut at 82 through the inboard low PD members 42 and then at 84 through the high PD members 40 in the formation of individual elements, filters and filter cigarettes of the type shown in Fig. 8. In Figs. 1 3 and 14, the thermoplastic plugging material 80 of the final products is not shown; the material 80 could be applied locally to the relevant regions as part of the continuous rod formation; or to the finite multiple rod lengths; or to the subsequent double or single length products before, during or after incorporation into filter cigarettes.From consideration of Figs. 1 2 to 14, the form of multiple length rods suitable for forming other types of element, filter and filter cigarette according to the invention (e.g. those of Figs. 3 to 5, 7 and 11) will be readily apparent.

The multiple length rods can be formed with a surrounding plugwrap, which would normally be applied continuously to the initially produced element rod before cutting into multiple lengths; the material 80 would then need to be applied to the initial continuous element before wrapping.

The resulting filters could be attached to cigarettes by ring tipping or full tipping overwrap.

The end portions of the even multiple length rods according to the invention are the portions which abut the tobacco in the eventual filter cigarettes; thus for the embodiments of Figs. 6 to 8 the initially produced continuous rod is first cut into even multiple lengths through core sections 61 (Fig. 6), 40 (Fig. 7) and 42 (Fig. 8) respectively, these appearing at the ends of the multiple and double lengths and hence being the core sections of the eventual unit lengths which are against the tobacco in the completed filter cigarette.To obtain filter cigarettes with the elements in the reverse orientation, one simply cuts the initial continuous rods through positions displaced longitudinally from the above positions by a unit element length, i.e. through core sections 40 (Fig. 6), 61 (Fig. 7) and 40 (Fig. 8) respectively.

It will be appreciated that, for clarity of illustration, the drawings are, for the most part at least, not to scale; thus the radial depth of the corrugations 46 and 48 in the Figs. 4 and 5 is much exaggerated and their number and the diameter of the core are correspondingly reduced.

In practice, in all of the embodiments employing a corrugated barrier 3, substantially the whole of the diameter (about 8 mm) of the element or filter according to the invention will usually be taken up by the core, with the corrugations having a radial depth of for example about 0.25 to 1 mm, e.g.

0.5 mm, much as shown in Fig. 11. Where only one ventilating perforation 1 6 is shown for each external passage (56 or 68), the perforations may be more closely spaced with two or more communicating with each such passage.

Whilst Figs. 4 to 11 use a corrugated impermeable wrapper, other forms of profiled impermeable wrapper are possible. For example, the impermeable wrapper could be of a heavy duty paper (e.g. of 100 gí m2 or more) such as cartridge paper with channels 54, 56 (or 66, 68) constituted by longitudinal grooves formed therein, suitably by means of profiled impressing rollers; the surfaces of the paper could be left plain where the passages provided by the wrapper are to terminate.

Where the filtering core of a filter or filter element according to the invention is a composite of two or more core members, one or some need not have any filtering effect but simply impart pressure drop - e.g. in Figs. 4 to 11 the plug 40 might be replaced by a non-filtering plug having peripheral and/or internal channels for the passage of smoke substantially unfiltered therepast and/or therethrough but generating relatively high pressure drop. In such cases the switching of smoke from low to high pressure drop paths may be accompanied by little or no increase in filtration.

As indicated above a filter or element according to the invention may have a recess or cavity at the buccal end; e.g. in Fig. 4 the core 2 may stop short of end 50. Indeed the initial low pressure drop path may communicate directly with such a recess or cavity - e.g. in Fig. 4 plug 42 might be omitted.

It is indicated above that multiple length element and filter rods according to the invention can comprise a plurality of unit elements or filters according to the invention disposed integrally endto-end with each said unit element or filter being disposed in mirror-image relationship to the or each integrally adjacent unit element or filter. In the case of the multiple length embodiments shown in Figs. 13 and 14 (as compared to the unit lengths of Figs. 6 and 8) for example, it will however be seen that these do not have the filling 80 which is a necessary part of the unit lengths at least when incorporated in filter cigarettes.The invention accordingly provides also the multiple (and unit) length elements and filters in the absence of such filling 80 or the like which may be necessary in the filter cigarette to ensure the required increase in air dilution with switch of smoke flow from low to high PD paths.

The filling 80 when it is to be present as part of the multiple length rods could for example be applied locally to the barrier 3 by printing before the latter is wrapped around the core during initial continuous rod production; a wrap could if necessary be applied around the resulting composite element of core and surrounding barrier (preferably prior to cutting of the continuously produced rod into the multiple lengths) to protect the filling 80 on the latter during subsequent handling before and during filter cigarette production.

In embodiments of the type having a profiled sleeve 44 providing longitudinal internal and external passages which are to terminate at the same place, the passages can be so terminated by circumferential corrugation of the sleeve. For example in Figs. 6 and 8 the plain portion of wrapper 44 and filling 80 could be replaced by circumferential corrugation(s) in wrapper 44, as at 52 in Fig. 4; in Figs. 13 and 14 the plain portions around tubular members 61 (Fig. 13) and plugs 42 (Fig. 14) would then be replaced by circumferentially corrugated portions.

Whilst reference is occasionally made herein to the barrier being one which is blockable by the passage of unfiltered smoke, it is to be understood that the invention is not limited to the cases where smoke in practice reaches the barrier totally unfiltered. Thus the tobacco itself always acts as a filter; in Fig. 8 there is necessarily some filtration by plug 42; and whilst in other embodiments the smoke passes directly from tobacco to barrier there will frequently be some condensation of smoke constituents prior to the barrier and indeed filtering material could be positioned in this part of the path. e.g. in passages 66. Any filtering between tobacco and barrier is however preferably kept to a low value, since otherwise the barrier blocking effect may be reduced too far to give appreciable advantage.

Claims (31)

1. A tobacco smoke filter which presents initially relatively low pressure drop and relatively high pressure drop paths for the passage of smoke therethrough, the or each relatively low pressure drop path having disposed thereacross a barrier which is initially smoke-permeable but which is blocked by the passage of smoke therethrough, whereby in use the initial preferential passage of smoke via the low pressure drop path or paths causes blocking thereof so that the passage of smoke via the high pressure drop path or paths is increased.

2. A filter according to claim 1 wherein the barrier is of two-piy air-permeable sheet material one ply of which has an air permeability of from 10 to 100 K as herein defined and the other of which is of higher air permeability than the first.

3. A filter according to claim 2 wherein the higher permeability ply is on the downstream face of the lower permeability ply.

4. A filter according to any preceding claim which has an axially elongate core having at its periphery at least one passage which extends from one end of the core only partially towards the other and is in smoke-flow communication with the core via plugwrap blockable by the passage of tobacco smoke therethrough.

5. A filter according to any of claims 1 to 3 having at least one internal passage extending from one end of the filter only partially towards the other end in smoke-flow communication with a smoke filtering portion of the filter via a barrier layer which surrounds the passage and separates it from the rest of the filter and is blockable by the passage of tobacco smoke therethrough.

6. A filter according to any of claims 1 to 4 having longitudinally adjacent core sections of relatively high and relatively low pressure drop surrounded by a profiled sleeve providing between itself and the core sections one or more passages extending from one end of the filter only partially towards the other. there being disposed between the passage or passages and the low pressure drop core section a barrier which is initially smoke-permeable but becomes blocked by the passage of tobacco smoke therethrough.

7. A filter element comprising a filter according to any preceding claim without an outer wrapper.

8. A filter cigarette having a filter or element according to any preceding claim incorporated therein by a tipping overwrap.

9. A multiple length tobacco smoke filter or filter element comprising a plurality of unit filters or elements according to any of claims 1 to 7 disposed integrally end-to-end with each said unit element or filter being disposed in mirror-image relationship to the or each integrally adjacent unit element or filter.

10. A smoke filter substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

11. A filter element substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 2. A filter cigarette substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

13. A smoke filter substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

14. A filter element substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

1 5. A filter cigarette substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

1 6. A filter element substantially as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.

1 7. A smoke filter substantially as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.

1 8. A filter cigarette substantially as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.

1 9. A smoke filter substantially as hereinbefore described with reference to Figs. 6, 9 and 10 of the accompanying drawings.

20. A filter element substantially as hereinbefore described with reference to Figs. 6, 9 and 10 of the accompanying drawings.

21. A filter cigarette substantially as hereinbefore described with reference to Figs. 6, 9 and 10 of the accompanying drawings.

22. A smoke filter substantially as hereinbefore described with reference to Figs. 7 and 11 of the accompanying drawings.

23. A filter element substantially as hereinbefore described with reference to Figs. 7 and 11 of the accompanying drawings.

24. A filter cigarette substantially as hereinbefore described with reference to Figs. 7 and 11 of the accompanying drawings.

25. A smoke filter substantially as hereinbefore described with reference to Fig. 8 of the accompanying drawings.

26. A filter element substantially as hereinbefore described with reference to Fig. 8 of the accompanying drawings.

27. A filter cigarette substantially as hereinbefore described with reference to Fig. 8 of the accompanying drawings.

28. A multiple length filter element substantially as hereinbefore described with reference to Fig. 12 of the accompanying drawings.

29. A multiple length filter element substantially as hereinbefore described with reference to Fig. 13 of the accompanying drawings.

30. A multiple length filter element substantially as hereinbefore described with reference to Fig. 14 of the accompanying drawings.

31. A wrapped multiple length filter element according to claim 28, 29 or 30 which can be cut into unit filters according to claim 1.