Method Of Making Tobacco Smoke Filters

Abstract

A tobacco smoke filter, particularly for use with cigarettes, has a high filtration efficiency and novel construction, at least one end of the filter having an appearance similar to conventional filters comprising cylinders packed with crimped filaments or creped paper. The filter comprises a tube within which is a rod having an outer wall of filtering material and an inner part of supporting material. At one end the rod has the same cross sectional shape as the tube and is in engagement with the tube around its periphery, at the other end or between the ends, its cross sectional shape changes so that opposed surfaces of the filtering material engage each other, so that tobacco smoke drawn through the filter is constrained to pass through the wall of filtering material. The filter has a high efficiency at an acceptable pressure drop.

Parent Case Text

This is a division of application Ser. No. 101,295, filed Dec. 24, 1970 now U.S. Pat. No. 3,752,166.

Description

This invention is concerned with a tobacco smoke filter, for example, a cigarette filter, and with a method for its manufacture. Tobacco smoke filters in common use are made of cylinders filled with crepe paper, bonded crimped fibres, or bonded crimped filaments. They generally have a paper wrapper around the circumference. Recently, new types of tobacco smoke filters have been described in which smoke is caused to flow through the wall of a hollow permeable inner tube or lamina, which is arranged within an outer impermeable tube. Such a filter may have a high retention of tobacco smoke solids yet have a moderate pressure drop. A filter of this kind was invented by Richard M. Berger and Elwin W. Brooks and is described in U.S. Pat. No. 3,533,416. The appearance of the end of these filters is however quite different from that of the conventional filter. Further the methods proposed for the manufacture of the new types of filter appear in general to be more difficult to carry out than the conventional methods.

A primary object of the present invention is to provide a tobacco smoke filter in which smoke is constrained to pass through a wall of filtering material, which is circumferentially enclosed about an air permeable supporting part, which provides a passageway for the access of smoke to a relatively large surface of filtering material, and supports the filtering material during the formation of the filter and during its use.

Another object of the present invention is to provide a tobacco smoke filter in which the smoke will pass through the face of a laminar material but will have at least one end face similar in appearance to that of the conventional filled cylindrical filters.

Another object of the invention is to provide a method for the manufacture of the novel filters which uses techniques, many of which are known, in a novel combination, which may readily be employed on an commercial scale. Further objects become apparent upon onsideration of the following detailed description and drawings.

This invention provides a tobacco smoke filter comprising an axially extending tubular outer member, within said member an axially extending air permeable rod having a circumferential wall of filtering material which encloses an air permeable supporting part, at a first longitudinal position the outer circumferential surface of the filtering material being in engagement with the inner surface of the outer member, so as substantially to preclude passage of smoke in the axial direction between the rod and the outer member, and at a second position, longitudinally spaced from the first, opposed parts of the inner surface of the wall being in engagement so as substantially to preclude passage of smoke in the axial direction between them, so that tobacco smoke drawn through the filter passes through the wall of filtering material.

In the drawings

FIG. 1 shows, partly in section, a filter cigarette which includes a filter of the invention.

FIGS. 2, 3 and 4 show alternative forms which the rod in the filter may have.

FIG. 5 shows one form of the apparatus for performing the method and making the filter.

The embodiment shown in FIG. 1 consists of a filter 1, comprising a rod 2, enclosed by a tubular outer member 3 of stiff paper. The filter is attached to a paper wrapped tobacco rod 4 by means of an enveloping strip of cork tipping paper 5, which is adhered to the whole of the outer surface of member 3 and to part of the tobacco rod 4. The rod 2 has a circumferential wall 6 of air permeable filtering paper. Enclosed by the wall there is an air permeable supporting part 7 which consists of crimped filaments of cellulose acetate which are bonded to each other at their points of contact. The outer circumferential surface of the rod 2 has an end portion 10 in the form of a right circular cylinder with an outside diameter equal to the inside diameter of the outer member 3. Thus the end 10 is in engagement with the outer member 3 around its whole periphery so that no significant amount of tobacco smoke can pass in the axial direction, shown by the arrow, between the outer circumferential surface of the rod at the end 10 and the inner surface of the outer member 3. The other end 11 of the rod has a cruciform cross section with a smaller cross sectional area than that of the end 10. Four ribs 12 form the cross and in each rib the inner surfaces of the filter paper 6 are pressed together, a small quantity of fused cellulose acetate, produced by the fusion of a part of the supporting part during the manufacturing process, serving to adhere them to each other. Between the ends 11 and 10 lies a tapering portion 13 and a substantially cylindrical portion 14, which has grooves 15, in its peripheral surface. These increase the area through which tobacco smoke can pass. Smoke passing in the direction shown by the arrow enters the filter at the end 10, passes into the air permeable supporting part 7, and, since it cannot get out of the end 11 of the rod 2, the inner surfaces of the wall of filtering material being sealed effectively to each other, it travels through the wall 6 of the filtering material and is filtered.

This construction provides a large area of filtering material through which the tobacco smoke can pass so that the packing density of the material can be high without the pressure drop of the filter being high. The paper of which the tubular outer member 3 is made should be sufficiently thick and stiff to bend evenly over the ribs 12 without departing from a circular form. A paper weighing at least 45 g/m.sup.2 is usually needed.

In FIG. 2 an alternative form of rod 20, has two ends 21 and 21' which have the form of right circular cylinders. Contiguous with these are two tapering portions 23 and 23' which meet in a central portion 22. This comprises three ribs 24, of which two are to be seen, which are arranged around the axial portion 25 so that portion 22 has a Y shaped cross section. The air permeable supporting part 26, which fills the ends and tapering portions of the rod, is surrounded by a wall of filtering material 27, which forms the whole of the outer surface of the rod 20. The circumference of the ends 21 and 21' and the peripheral length of the portion 22 are approximately the same, so that it is possible to provide good support for the tubular outer member 3, by means of the tips of the ribs 24, when making the rod by deforming a cylindrical rod, without significantly altering the length of the periphery at the position of deformation. The tips of the ribs 24 lie thus on a circle coaxial with and of the same diameter as the ends 21 and 21'. The permeability of the paper which forms the wall 27 must be higher than that of the paper used to form the wall 6 of FIG. 1 since the smoke has to pass through two layers of paper. An air permeability of greater than 30,000 cc per minute is desirable.

In FIG. 3 a rod 30 is shown. This comprises ribs 31, a tapering portion 32 and a cylindrical end portion 33. It has the shape of one half of the rod shown in FIG. 2. In this embodiment however the air permeability of the paper should preferably be about 30,000 cc per minute, or less, since it is a single pass filter. A further rod is shown in FIG. 4. This comprises a central cylindrical portion 40 having crimped cruciform end portions 41 and 42. The wall and the supporting part which is within it are made of similar materials to those used in the embodiment of FIG. 2.

In FIG. 5 an embodiment of the apparatus for performing the method and making the filters of the invention is shown diagrammatically. A known device 50 supplies an opened filamentary tow 51 of cellulose acetate, having upon it droplets of triacetin, to the entry part 52 of a known filter rod forming machine 53. The latter comprises a garniture 54 through which a rotating drum 56 draws an endless tape 55. Rollers 57 tension the tape. A bobbin 60 supplies an elongate strip 61 of air permeable filtering paper, and a printing device 62 applies transverse stripes of adhesive to the strip 61. Within the garniture 54 the strip 61 is enveloped around the filaments 51, its edges are overlapped and adhered in place by an adhesive which comes from applicator 63. This enters a deforming device 66 which is provided with four heated rotating wheels, having radially directed projections on their circumferences which meet each other as they contact the rod 65. The rod 67 coming from device 66 comprises alternating portions in which the cross sectional area changes. For example, circular portions may alternate with cruciform portions, tapering or grooved portions lying between them generally as shown in FIGS. 1-4. The rod 67 enters the garniture 80, through which a drum 81 draws an endless tape 82, rollers 83 tensioning the tape. A bobbin 84 supplies an elongate strip 85 of substantially impermeable paper to the garniture 80 where it is enveloped around the rod 67. The strip 85 is held in place by a seam in which overlapping edges are adhered by an adhesive, which comes from the applicator 86, and is dried by heater 87. This forms a continuous rod 88. The tube formed from the strip 85 is in engagement with the undeformed portions of the rod 67 and enclose the deformed portions of reduced cross sectional area. A cut off 90 divides the continuous rod 88 into lengths 91, each of which comprises an even number of filters. The cut is made by the cut off 90 at the positions of deformation, or midway between the position of deformation. This may be seen by reference to FIGS. 1-4.

The triacetin, after an hour or two, causes the filaments of cellulose acetate to bond to each other. The adhesive stripes on the strip 61, are deposited in register with the positions of deformation to ensure the sealing of the inner surfaces of the filtering material to each other. A printing device similar to the printing device 62 may be used to apply stripes, in register with the undeformed portions of the rod 67, to the strip 85 to seal the inner surface of the tubular outer member to the outer surface of rod between the points of deformation.

If desired the parts of the apparatus between deforming device 66 and the cut off 90 may be dispensed with so that the latter divides the continuous rod 67 directly. The cut pieces may be fed, for example, to a suitable filter cigarette machine wherein they are assembled directly to tobacco rods and a strip of cork tipping paper, such as that indicated by reference numeral 5 of FIG. 1, may serve as a tubular outer member and as a means of attachment to the tobacco rod. They may also be assembled with other types of filter to form multiple filters in a known manner. If the strip 61 is made of paper having sufficient wet strength, the deformation of the rod 65 by the deforming device 66 may be facilitated by the provision of a steam heating jet arranged between heater 64 and device 66. This applies steam directly upon the rod 65. Other means of heating may also be employed to preheat the rod 65. If the filtering material or the supporting part are of thermoplastic material, its temperature during preheating or deformation should be raised to its softening point.

Suitable materials for forming the air permeable supporting part include, other thermoplastic materials such as fibres or filaments of polypropylene, polyethylene and lightweight longitudinally air permeable extruded sections which permit longitudinal passage of the smoke and access to the wall of filtering material. Other air permeable materials may be used for the supporting part, for example, a web of creped, porous paper. When using this material stripes such as those applied by device 62 of FIG. 5 should be used so as to adhere the inner surfaces of the wall of filtering material to each other and to the creped paper. The material to be used to form the wall which encloses the supporting part is chosen to suit the shape and size of the rod which is to be made. For example a rod such as that shown in FIG. 3 and having an area exposed to tobacco smoke of about 2.5 cm.sup.2 a length of 20 mm and a diameter of 8 mm may be made with filtering wall of paper weighing about 75 g/m.sup.2, an air permeability, of about 30,000 cc per minute and a thickness of about 0.25 mm. The air permeabilities referred to in this specification are the volume of air which passes per minute through a piece of the material, 10 cm.sup.2 in area, when the pressure difference between the faces of the material is 10 cm Water Gauge. In filters of the dimensions which are at present practical, the area of the outer circumferential wall exposed to the tobacco smoke lies between 2.0 and 7.0 cm.sup.2. For these the wall should preferably be formed of a laminar material with an air permeability between 2,000 and 40,000 cc per minute, the thickness between 0.05 mm and 0.4 mm and the weight between 40 and 100 g/m.sup.2. In general very thin paper is unsuitable, even when used in the embodiment shown in FIG. 2, as the pores of the paper may clog very easily if the paper is dense enough to be strong. If the pores are larger it may be too weak. For such a rod the weight of the paper is preferably not less than 45 g/m.sup.2 in order to provide the desired thickness. Laminar materials other than paper, such as bonded fibrous or filamentary webs and porous plastics, may be used.

When using as the supporting part bonded filaments of cellulose acetate or other thermoplastic fibres or filaments of a suitably small diameter, the wall of filtering material may be formed from the outer layer of these fibres or filaments by applying heated tools to compact the said layer and hold the fibres or filaments in place by partial fusion. For this purpose a rod substantially larger in diameter than the intended diameter of the rod in the completed filter is made. This is compressed into the appropriate shape, for example, that shown in FIGS. 1-4 by the heated tools.

The following examples illustrate the invention. Filters 20 mm long were made with the inner supporting part of 5,000 cellulose acetate filaments with a Y shaped cross section and a titer of 8 denier. A paper having a weight per unit area of 75 g/m.sup.2 and an air porosity of 30,000 cc per minute was wrapped around the rods by the method shown in FIG. 5 to form the filtering wall. The inner members were deformed to give cruciform or Y cross section portions at one end only of the filter. The results are --------------------------------------------------------------------------- given below:

Filter Outer Crimp Pressure Tar Nicotine Circum- Type Drop mm Retention Retention ference Water mm Gauge __________________________________________________________________________ 1. 24.95 + 68 68.0% 65.0% 2. 24.80 + 54 64.4% 62.9% 3. 24.80 Y 67 65.4% 61.6% __________________________________________________________________________

the pressure drop was measured by drawing air at a flow rate of 17.5 cc per second through the filters.

Such results cannot be obtained with prior art filters of packed paper or filaments.

The fibres of which paper is formed usually have a suitable diameter, however the synthetic fibres or filaments or non fibrous porous synthetic materials for use in this invention should have a small fibre or filament diameter or the dimensions of the material between the pores, in the case of non fibrous porous material, should be small. It is preferable, in order to obtain a satisfactory retention, that these dimensions should not exceed 14 microns.

In a further embodiment the rod was formed of a crimped continuous filamentary cellulose acetate tow, 8 denier per filament 20,000 total denier which formed the supporting part and a tow with smaller filaments, 1.6 denier per filament, total denier 61,000 which formed the filtering material. Each tow was opened, banded and sprayed with plasticiser in a known manner. The first tow was brough through an inner tubular guide, and the second was brought through an outer tubular guide, coaxial with and surrounding the first, so that at the exit of the guides the first tow had a circular cross section and the second an annular cross section surrounding the first. Immediately adjacent the outlet of the guides was a steam treating enclosure connected to a supply of saturated steam and having an internal diameter appropriate to that desired in the rod. The steam passed into the combined tows, causing the filaments to adhere firmly to each other. At the exit of th guides was an air cooling device of a known form. After the air cooling device a deforming device, such as device 66, was mounted. This deformed the continuous rod into an alternating succession of pieces having the form shown in FIG. 5. Upon leaving the deforming device the tube entered a garniture, such as garniture 80, in which it was enveloped in a continuous paper strip, which was kept tightly in place around the tube, by a lapped and stuck seam. This formed the tubular outer member. The rod was then cut into pieces by a cut off.

The space between the rod and the tube in the filter embodiment shown in FIG. 2 may be filled with particulate filtering materials such as distintegrated plastic foam or activated carbon granules. These may be incorporated using known apparatus mounted between the deforming device 66 and the garniture 80.

Claims

We claim:

1. A method of making tobacco smoke filters comprising the steps of forming an axially extending air permeable rod comprising an air permeable supporting part of bonded filaments or fibers enwrapped by an elongate air permeable paper strip which is retained around said supporting part by a lapped and stuck seam and forms a circumferential wall around said supporting part, deforming the rod at a position along its length to bring the inner surfaces of said circumferential wall into engagement, so as substantially to preclude passage of smoke in the axial direction between them, enclosing the rod by a tubular outer member formed by enwrapping the rod by an elongate substantially impermeable paper strip and bringing the inner surface of the outer member into engagement with the outer surface of the rod at a position longitudinally spaced from the position of deformation, so as substantially to preclude passage of smoke in the axial direction between them, and transversely cutting the enwrapped rod at the positions of deformation or midway between the positions of deformation to form lengths each of which comprises an even number of tobacco smoke filters.

2. A method according to claim 1 wherein at at least one of the said longitudinal positions the engaged surfaces are provided with an adhesive.

3. A method according to claim 1 wherein transverse stripes of adhesive in register with the positions of deformation, are deposited upon air permeable paper strip before the latter is enveloped around the supporting part.

4. A method according to claim 1 wherein the supporting part comprises thermoplastic material which is heated to its softening temperature during or before deformation.

5. A method according to claim 1 wherein the supporting part is formed by bonding filaments or fibres to each other whilst they are enclosed by said circumferential wall.

6. A method according to claim 1 wherein the supporting part comprises fibres or filaments of cellulose acetate, which are bonded to each other at their points of contact by means of a liquid plasticizer, which is applied to them.

7. A method according to claim 1 wherein the rod is deformed into a plurality of ribs.

8. A method according to claim 7 wherein the rod is deformed without significantly altering the length of the periphery at the position of deformation.