GB2133269A

GB2133269A - Tobacco smoke filter

Info

Publication number: GB2133269A
Application number: GB08235520A
Authority: GB
Inventors: Ernest Brian Hayes
Original assignee: Filtrona PLC
Current assignee: Essentra PLC
Priority date: 1982-12-13
Filing date: 1982-12-13
Publication date: 1984-07-25
Also published as: HK86589A; PT77550B; GB2133269B; NL191528B; BE898391A; ZA838698B; NL8304093A; PT77550A; NL191528C

Abstract

A cigarette filter comprises a filtering core (2), a smoke permeable first wrapper (4) around the core, and a second wrapper (6) around the first wrapper, the first wrapper being corrugate and thereby providing at least between itself and the second wrapper passages extending longitudinally of the filter from at least one end thereof, each said passage extending only part of the core length and the said passages contributing at least 4% of the overall filter cross sectional area. The wrapper corrugations may be formed by embossing the wrapper so that its permeability is increased. <IMAGE>

Description

SPECIFICATION Tobacco smoke filter Cigarette filters are known comprising a filtering core, a smoke permeable first wrapper around the core, and an air-permeable second wrapper around the first wrapper, the first wrapper being corrugate and thereby providing at least between itself and the second wrapper passages extending longitudinally of the filter from at least one end thereof.

When such a filter is drawn on in use, external air can pass through the second wrapper into the passages between the first and second wrappers.

Conventional such filters (of about 8 mm diameter) have about 22 such longitudinal passages between the first and second wrappers, the total cross sectional area of these passages amounting to about 3.5% of the overall cross sectional area of the filter.

The invention provides a filter of the mentioned type in which the longitudinal passages extend only part of the first wrapper length and contribute at least 4% or 5% of the overall filter cross sectional area.

According to the invention, careful control over embossing of the first wrapper can give improved filtering performance compared to the conventional filters; this may be achieved by increasing the depth of the corrugations, preferably with increase in surface area of the corrugations available to smoke, and optionally along with increase in the corrugation pitch-i.e. widening of the corrugations with reduction in their number.For example, compared to a said conventional filter having 22 of the said passages about 0.2 mm deep at a pitch of about 1.2 mm, one type of filter according to the invention has 1 5 such passages about 0.45 mm deep and at a pitch of about 1.5 mm; another filter according to the invention is characterised only by deeper (0.32 mm) such passages, the number of which is in fact slightly increased (23, at a pitch of 1.05 mm) compared to that in the mentioned conventional filter. The area of the corrugated wrap available for smoke filtration is generally a little less than the total corrugated surface area, since the crests of corrugations will generally be against the outer wrapper and hence not effective to filter.

In filters according to the invention, the air-permeability of the embossed (corrugated) inner wrapper is preferably from 7 K to 1 20 K Filtrona units.

In filter cigarettes according to the invention, the longitudinal passages may be open at the buccal end, or open to the tobacco. Some passages may extend to one end of the filter and others to the other, but each passage terminates short of at least one end of the filter. The passages will generally be parallel to the filter axis, but this is not essential.

In the production of the mentioned priorfilters and of filters according to the invention, the first wrapper is corrugated as required by embossing before being wrapped around the filtering core. In the mentioned conventional filters, the embossing has little or no effect on the permeability of the wrapper; we have unexpectedly found that with the deeper embossing employed according to the invention, especially in the case wher this embossing provides 20 or so of the passages between first and second wrappers in the finished filter, the wrapper porosity increases markedly with embossing depth whilst the filtering effect due to the embossed wrapper remains high provided that the embossing is not so deep as to cause appreciable splitting of the wrapper; the invention preferably employs embossing depth within this latter range (without appreciable splitting of the inner wrapper) where the enclosed pressure drop (PD) of the resultant filter is substantially independent of embossing depth whilst the enclosed total particulate retention of the resultant filter remains high. It is to be noted that, unless otherwise specified, all pressure drops, retentions etc. herein are "enclosed" values, i.e. values measured with the normally permeable outer wrapper sealed off from the external air.

The effect of embossing depth on permeability of the inner wrapper is illustrated by the measurements given in Table 1. The wrappers here were of a preferred form being plain and unembossed at one end (termed the bar) with embossed corrugations (termed the flutes) extending therefrom to the other end. The permeability measurements were made on embossed wrappers removed from completed filter rods, and each PD value and pair of permeability values quoted is the average of 6 readings.

Table 1 Rod PD Embossing Increase (enclosed) depth Mean permeability INutesl mm. Wg. mm. Bar Flutes bar) Prior art 660 0.2 6.6 K 8.3 K 1.26 Example 1 568 0.3 8.9 K 26.9 K 3.02 Example 2 545 0.35 6.9 K 87.5 K 12.68 Example 3 547 0.40 7.3 K 119.3 K 16.34 A graph of enclosed rod PD against flutes/bar permeability ratio (i.e. effectively against embossing depth) is given in Figure 1 of the accompanying drawings. The mean bar permeability value for these readings was 7.4 K Filtrona units.

Table 1 and Figure 1 show that whilst the deeper flutes employed according to the invention result in an advantageously reduced enclosed filter rod PD compared to that of the conventional filter with shallower grooves, the enclosed rod POD for Examples 1 to 3 according to the invention is substantially independent of flute depth. In Examples 1 to 3 and the comparative prior art sample, there are in each case the same number (22) of the said flutes or passages between the inner and outer wrappers.

Table 2 below compares the enclosed pressure drop and enclosed retention values for the rods of above Examples 1 to 3 according to the invention with those for the prior art sample of Table 1 and of a plain rod of cellulose acetate tow (without the embossed inner wrapper). The rods tested were in each case 20 mm long. In the Table NR stands for nicotine retention, and TR for tar retention. Accompanying Figures 2 and 3 are graphs, taken from the Table 2 values, illustrating the increase with embossing depth of improvement in TR over plain acetate (Fig. 2) and of enclosed rod PD over plain acetate (Fig.

3) the TR and enclosed rod PD advantages over plain acetate are plotted against embossing depth expressed as flutes/bar permeability ratio as taken from Table 1.

Table 2 Example Prior art 1 2 3 PDmm.Wg 109 96 94 92 TR % 60.5 60.0 61.0 59.5 NR % 53.3 53.5 54.0 51.5 Plain acetate TR % at same PD 57.5 55.2 54.8 54.5 % TR advantage over plain acetate 3.0 4.8 6.2 5.0 Plain acetate PD at same%TR 127 124 130 121 PD advantage over plain acetate 18 28 36 Accompanying Fig. 4 is a diagrammatic cross sectional view through the fluted portion of a conventional filter having a corrugated permeable air wrapper, and Figs. 5 and 6 are similar views of improved filters according to the invention. In each of Figs. 4 to 6, 2 is the bonded cellulose acetate tow core of the filter, 4 is the embossed portion of the permeable inner wrapper, and 6 is the permeable outer wrapper.In each case the cores 2 are essentially the same, and the inner wrapper is unembossed and of plain cylindrical form at one end of the filter. The relevant details of the filters of Fig. 4 to 6 are summarised, in comparison with a plain acetate filter (i.e. with no corrugated inner wrapper 4), in the following Table 3; in every case the tip length was 20 mm., the enclosed tip PD was 85 mm. Wg., and the core was of 2.1 d/f (denier per filament) RSK (RHodia Super Krimp) cellulose acetate tow. In the Fig. 6 embodiment, the surface area of the wrapper available for filtration is appreciably increased, and the embossing is within the mentioned preferred range where the enclosed filter PD is independent of embossing depth. For the tests reported in Tables 2 and 3 and 4 the retentions are measured with the corrugations (when present) extending from the tobacco end to short of the buccal end of the filter; and all tar retentions quoted are the water- and nicotine-free (WNF) values.

Table 3

Total cross sectional area Pitch Total cross of grooves as a Surface area of Filter Depth of of sectional percentage of grooves available Nicotine Figure No. of grooves grooves area of grooves cross section of for filtration Tar retention retention No. grooves m.m. m.m. mm2 filter mm2/mm length % % 4 22 0.20 1.20 1.8 3.5 26.4 57.5 52.7 6 23 0.32 1.05 3.2 6.4 32.2 62.0 56.0 5 15 0.45 1.50 5.1 10.2 27.0 61.0 54.0 Plain Nil - - - - - 54.0 acetate Table 4 below gives retention values at various enclosed rod PD's for plain acetate rods (without an embossed inner wrapper), for rods of the prior art type shown in Fig. 4, and for rods according to the invention as shown in Figs. 5 and 6.

Table 4 Tow type Tip length Tip circ. Tip P.D. d/f (mum) (mm) (mm) RSK % TR %NR 20 24.65 47 2.1 43.5 - 20 24.65 58 2.1 47 - Plain 20 24.65 70 2.1 50 acetate 20 24.65 85 2.1 54 20 24.65 110 2.1 58 - 20 24.63 64 2.1 49.7 45.9 20 24.63 85 2.1 57.5 52.7 Figure 4 20 24.63 97 2.1 62.2 55 20 24.63 107 2.1 -61.6 57.7 20 24.63 142 2.1 66.8 62.6 20 24.65 .46 3 44. 39 20 24.65 50 3 46.5 41 20 24.65 53 3 46 41.5 20 24.65 47 2.5 45 41.5 Figure 5 20 24.65 53 2.5 47.5 42.5 20 24.65 71 2.5 53.5 46.5 20 24.65 46 2.1 46.5 41 20 24.65 58 2.1 51 44.5 20 24.65 70 2.1 54 47 20 25.16 81 2.1 60 53 20 25.13 85 2.1 61 54 20 25.12 97 2.1 63 56 20 24.71 110 2.1 65 58 20 25.12 76 2.1 58.8 52 20 25.12 82 2.1 63 55 Figure6 20 25.13 85 2.1 62 56 20 25.12 88 -2.1 61 54 As indicated above, the completed filters according to the invention generally have a permeable wrapper (which may be of inherently permeable material and/or perforate) around the corrugated inner wrapper. This permeable outer wrapper may be the tipping overwrap by which the filter is incorporated in a filter cigarette, such overwrap being applied only at the cigarette manufacturing stage.In this case, the filter rod is initially produced with the corrugated permeable wrapper exposed, and the invention includes such rods as well as the completed filters with the outer wrapper attached. Such rods can also be used with advantage (with passages open at the buccal or the tobacco end) in conjunction with an impermeable outer wrap, e.g. tipping overwrap, and this also forms part of the invention.

Filter rods according to the invention can be made by embossing the smoke-permeable wrapper to form the part-length longitudinal corrugations therein, and then wrapping and securing the embossed wrapper around a core of "live" (i.e. not yet fully hardened) gathered and bonded fibres or filamentary tow so that the corrugations impress themselves into the core surface; in the finished product the core surface thus conforms substantially to the corrugate wrapper. The fibres or tow are preferably of cellulose acetate. The embossed wrapper and live core are preferably formed and combined in continuous lengths and the continuously produced rod then cut into finite lengths. These may be a multiple (e.g. sextuple) of the length of the eventual individual rods.

The structure of a filtering core with part length corrugations and an integral smoke-permeable wrapper conforming to the corrugated and plain core surface portions means that all smoke passing between core and corrugation must pass through the wrapper and be filtered thereby.

The above-mentioned procedure of securing the pre-corrugated permeable wrapper around a live core so that the wrapper corrugations become impressed in the core is applicable and of advantage in general, i.e. whatever the size and shape of the corrugations, and whatever their length.

Examples 1 to 3 and Figs. 5 and 6 described above relate to filters according to the invention in which the outer wrapper (six in Figs. 5 and 6) is a ventilating wrapper, though air dilution via this wrapper is prevented in the tests reported. Precisely the same embodiment as described in Examples 1 to 3 and Figs. 5 and 6 can advantageously be employed with the modification of an impermeable nonventilating outer wrapper in place of the ventilating wrapper (the same "enclosed" PD's and retentions of course applying" and such embodiments are included within this invention. In such non-ventilated embodiments according to the invention, the longitudinal passages may be closed at the buccal end, or the filters may be employed in the reverse orientation.

Claims

1. A cigarette filter comprising a filtering core, a smoke permeable first wrapper around the core, and a second wrapper around the first wrapper, the first wrapper being corrugate and thereby providing at least between itself and the second wrapper passages extending longitudinally of the filter from at least one end thereof, each said passage extending only part of the core length and the said passages contributing at least 4% of the overall filter cross sectional area.

2. A filter according to claim 1 wherein the first wrapper corrugations are formed by embossing, without tearing, to a degree such that the wrapper permeability is increased and to within the range wherein the enclosed filter pressure drop is substantially independent of embossing depth.

3. A filter according to claim 2 wherein the embossing of the first wrapper increases its surface area.

4. A filter according to any preceding claim wherein the first wrapper is integral and its corrugated and uncorrugated,regions conform to the core surface.

5. A filter according to any of claims 1 to 4 wherein the second wrapper is air-impermeable.

6. A filter according to any of claims 1 to 5 wherein the second wrapper is air-permeable.

7. A filter substantially as hereinbefore described in any of Examples 1 to 3 or with reference to Fig. or 6.

8. A filter according to any preceding claim without the second wrapper.

9. A method of making a filter rod which comprises forming a longitudinally corrugated airpermeable plugwrap and wrapping and securing it around a live permeable core of bonded fibres or filaments so that the plugwrap corrugations become and remain impressed in the core.