Patent Publication Number: US-2011048436-A1

Title: Smoking Article

Description:
FIELD OF THE INVENTION 
     The present invention relates to smoking articles such as cigarettes, and in particular to smoking articles that include a filter. 
     BACKGROUND OF THE INVENTION 
     When tobacco in a cigarette is burnt as the cigarette is smoked, tar is produced. In recent years, there has been a shift in consumer preference towards lower tar cigarettes. Accordingly, it is desirable to produce a cigarette that delivers less tar to the consumer. 
     Existing cigarettes have for many years included a filter that can help to reduce tar output. As the cigarette is smoked, the point of combustion moves closer to the filter. When the cigarette is first lit, the rod of unburnt tobacco itself provides a certain amount of filtration action, and can therefore help to reduce tar delivery. However, this action is lost as the tobacco rod is burnt to increasingly shorter lengths, resulting in increased smoke delivery for equivalent puff volumes during the latter puffs. It is also desirable to produce a cigarette that delivers less tar in the latter puffs to the consumer. 
     It is known to provide a hollow tube along a central axis of a portion of a filter. For example, GB 2177890 discloses a cigarette in which a filter comprises first and second elements, where the first element is adjacent the tobacco rod, and the second element is adjacent the mouth. The first element incorporates a thin central tube of heat-shrink material surrounded by cellulose acetate. When the cigarette is initially smoked, the tube provides a low resistance path to smoke through the first element (in preference to the smoke having to pass through the surrounding cellulose acetate). This low resistance path through the first element helps to compensate for the higher initial resistance of the long tobacco rod. However, as the cigarette warms up, the central tube collapses, and the vacated space is then occupied by expansion of the surrounding cellulose acetate material. The tube now no longer provides a low resistance path for smoke through the first element, with the smoke having to pass instead through the comparatively high resistance of the cellulose acetate of the first element. 
     A somewhat similar configuration is disclosed in EP 481596. In this document however, the tube does not collapse with heat. Rather, the tube is gradually blocked by smoke components that condense onto filter material at the end of the tube, whereupon the smoke must flow through the more resistant material around the filter. The tube may be provided with holes along its length to produce a more gradual transition. 
     The filtration properties of filters in existing cigarettes are largely independent of the amount of smoke presented to the filter, although there is a well-known relationship between flow and filtration efficiency. As a result, the performance of such filters, for example with respect to tar delivery, tends to vary with differing puff volumes and differing puff profiles. 
     SUMMARY OF THE INVENTION 
     The present invention provides a smoking article such as a cigarette having a filter located at one end. The filter comprises at least first, second and third sections disposed in succession along a cylindrical axis of the smoking article. Each of the first and third sections includes at least one tube running the length of the respective section. The second section comprises filter material of substantially higher filtration efficiency than the tubes of the first and third sections. 
     Such a filter can help to provide a more consistent level of tar, with a reduced dependency on the smoking regime or how much of the smoking article has already been consumed. For example, in one embodiment, the exit of the tube in the first section is at least partly blocked after a first amount of smoking Consequently more smoke now goes through the remainder of the first filter section (other than the tube), comprising some form of filter material. After a further amount of smoking, the entrance into the tube in the third section is at least partly blocked. More smoke is now routed through the remainder of the third filter section (other than the tube), again comprising some form of filter material. Accordingly, this embodiment provides a graduated and controlled rise in the filtration efficiency of the filter as the smoking article is consumed. Note that the rise in filtration efficiency occurs more rapidly for higher puff volumes and/or puff rates (since in this case the tubes block more quickly). In addition, the rise in filtration efficiency of the filter helps to compensate for reduced filtration by the unburnt tobacco rod itself as the smoking article is consumed. 
     In one embodiment, the tube(s) of the first section are coaxial with the tube(s) of the third section. The tubes of the first and/or third section may be located centrally within the smoking article (i.e. coaxial with the smoking article), or may be provided as grove(s) in the outer circumference of the filter. Another possibility is that a tube is located intermediate the centre and outside of the filter. The number and positioning of the tubes can be varied as desired to control the overall filter properties, and can vary from one filter section to another. 
     In one embodiment, the tube(s) of the first section and/or the third section have a diameter in the range 0.1-3 mm, or more particularly in the range 0.5-1.5 mm, and a length in the range 4-15 mm. The tube dimensions help to determine the overall properties of the filter. For example, it takes longer for a tube, and in particular the ends of the tube, with a greater diameter to become (at least partly) blocked during smoking. Accordingly, the tube dimensions can be adjusted depending on the desired properties of the filter and smoking article. Note that different tubes may have different dimensions (both in respect of tubes in different filter sections, and also in respect of multiple tubes in a single filter section). 
     In one embodiment, the tubes are hollow, and the first, second and third filter sections all comprise filter material, such as cellulose acetate, paper, and so on (different filter sections may comprise different filter material). For the first and third filter sections the filter material surrounds or embeds the tube(s) of these sections. The relative filtration efficiency of the filter material in the different sections can be used to control the overall properties of the filter. For example, in one embodiment, the filter material of the first filter section has a lower filtration efficiency than the filter material of the second and third filter sections. Consequently, the rise in filtration efficiency when a tube through the first filter section is at least partly blocked is smaller than the rise in filtration efficiency when a tube through the third filter section is at least partly blocked. 
     Ventilation may be provided around the circumference of the filter in one or more zones. The ventilation zone is preferably located downstream of a potential blockage point. In one embodiment, ventilation holes are provided on the outside of the second and/or third filter section. If the tubes through the first and third sections are centrally located (i.e. coaxial with the smoking article), then initially most of the airflow is along the central axis of the smoking article. However, as the ends of the tubes become at least partly blocked, and the flow of smoke is forced towards the outer portions of the smoking article, this increases the amount of air drawn in through the ventilation holes to dilute the smoke. In addition, the pressure differential, or pressure drop (PD), of the smoking article will increase. The positioning of the ventilation immediately downstream of the blockage points result in increased ventilation levels as the PD of the filter increases. 
     In one embodiment, the filter comprises additional pairs of sections in succession along the cylindrical axis of the smoking article after the third section. Each additional pair of sections comprises a first and second additional section. The first additional section includes at least one tube running the length of the first additional section, substantially parallel to the cylindrical axis of the smoking article. The second additional section comprises filter material of substantially higher filtration efficiency than the tube of the first additional section. Increasing the number of filter sections in this manner allows further control over the filter behaviour. For example, if a fourth and fifth filter section are provided analogous to the second and third filter sections, then the tube in fifth filter section will at least partly block some time after the tube in the third filter section becomes at least partly blocked. Such a filter therefore has more gradations in the rise of filter efficiency as respective tubes become blocked. 
     The invention also provides a filter for use in any smoking article such as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention will now be described in detail by way of example only with reference to the following drawings: 
         FIG. 1  illustrates a cigarette filter in accordance with one embodiment of the invention. 
         FIG. 2  illustrates the behaviour of the cigarette filter of  FIG. 1  for initial puffs in accordance with one embodiment of the invention. 
         FIG. 3  illustrates the behaviour of the cigarette filter of  FIG. 1  for second stage puffs in accordance with one embodiment of the invention. 
         FIG. 4  illustrates the behaviour of the cigarette filter of  FIG. 1  for third stage puffs in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is the schematic illustration of a cigarette  10  in accordance with one embodiment of the invention. Cigarette  10  has a generally cylindrical shape, and comprises a tobacco rod  15  (only shown partly in  FIG. 1 ) joined to a filter  20  with an outer wrap  21 . The filter is located at the mouth end  100  of the cigarette. The cigarette  10  is lit at the end of the tobacco rod  15  opposite to the mouth end. 
     Filter  20  comprises three sections  31 ,  41 ,  51  arranged along the cylindrical axis of the cigarette, where each section is cylindrical in shape. Section  31  is adjacent the tobacco rod  15 , section  51  is adjacent the mouth end  100 , and section  41  is between section  31  and section  51 . The configuration of cigarette  10  is therefore determined by a succession of three planes, all perpendicular to the cylindrical axis of the cigarette. The first plane defines the boundary between tobacco rod  15  and the first filter element  31 , the second plane defines the boundary between the first filter element  31  and the second filter element  41 , and the third plane defines the boundary between the second filter element  41  and the third filter element  51 . In one embodiment, the separation between the different planes (and hence the length of each filter section) is in the region 5-10 mm. (In other embodiments, the separation may be in the region 4-15 mm). 
     The first filter section  31  includes a tube  35  and the third filter section  51  also includes a tube  55 . Tubes  35  and  55  are both coaxial with the cylindrical axis of the cigarette and each tube  35 ,  55  extends the length of its respective section. The radius of tubes  35 ,  55  is significantly less than the radius of the cigarette. For example, the diameter for the tubes may only be approximately 0.1-3 mm. In one embodiment, each of tubes  35 ,  55  has a diameter of approximately 0.5-1 mm. Note that if the radius of a tube is 20% of the radius of the cigarette as a whole, the cross-sectional area of the tube is only 4% of the cross-sectional area of the cigarette. 
     The tubes  35 ,  55  may be made of paper, cardboard, plastic or any other suitable material and may have a porous or non-porous surface. In the embodiment shown, the tubes  35 ,  55  are hollow, but in other embodiments may have a filling that offers very little resistance to airflow. In the embodiments shown, tubes  35  and  55  are substantially the same as one another, but in other embodiments may vary as appropriate in term of dimensions, shape and/or other properties. 
     In the first section  31 , the tube  35  is surrounded by filter material  32  such as cellulose acetate, paper, polypropylene, or any other suitable material. This material is formed with a relatively low density, and hence a relatively low filtration efficiency (but significantly higher than the hollow tube). The middle section  41 , which does not contain a central tube, also comprises a filter material  42  such as cellulose acetate, paper, or polypropylene, etc. This material is formed with a relatively high density, and so has a high filtration efficiency (compared to that of material  32 ). In the third section  51 , the tube  55  is also surrounded by filter material  52  such as cellulose acetate, paper, polypropylene, etc. Again, this material is formed with a relatively high density, and so has a high filtration efficiency (compared to that of material  32 ). 
     The outer wrap  21  is provided with two circumferential rings of ventilation holes. The first ring of ventilation holes  43  is provided around the second section of the filter  41 , while the second ring of ventilation holes  53  is provided near the start (i.e. away from the mouth end  100 ) of the third section of the filter  51 . These ventilation holes allow air to be drawn into the cigarette filter and hence dilute smoke passing through. 
       FIG. 2  illustrates the main smoke flow through the cigarette  10  of  FIG. 1 , when the cigarette is first smoked, in accordance with one embodiment of the invention. Smoke from the burning tobacco travels down the tobacco rod  15  to filter  20 . The smoke now passes through the tube  35  of the first element  31  in preference to passage through filter material  32 , since tube  35  offers a lower resistance to the smoke flow. The smoke then passes through the high filtration efficiency material of the second section  41  before exiting at the mouth end  100  of the filter via the third section  51 . In the third filter section  51 , the smoke passes through the tube  55  in preference to the surrounding filter material  52 , due to the lower flow resistance of tube  55 . 
     At this stage, it is the second section  41  of filter  20  that provides the majority of the filtration activity of filter  20 . This filtration activity is supplemented by the passage of the smoke through the relatively long portion of unburnt tobacco rod  15  prior to reaching the filter  20 . Relatively little ventilating air is drawn in through ventilation holes  43 ,  53 , since the main smoke flow through the cigarette is close to the central axis of the cigarette  10 —i.e. away from the outside surface where the ventilation holes are located, and there is a low filter pressure drop upstream of the ventilation holes. 
     In the configuration of  FIG. 2 , smoke travels at a relatively high velocity through tube  35 , due to the low flow resistance, and then decelerates on entry into the second section  41  where the filter material  42  offers a much higher flow resistance. This deceleration results in a concentration of smoke at impaction point  36 —i.e. where the dense filter material  42  of the second element  41  is adjacent to the exit from tube  35 . The raised concentration at impaction point  36  leads to a deposition of tar on filter material  42 , which acts to increasingly block the exit from tube  35 . 
     This leads to the situation shown in  FIG. 3 , in which tube  35  is blocked by the deposition of material at impaction point  36 . (As used herein, “blocked” implies a partial or complete obstruction). Consequently, more smoke passing through the first filter section  31  now travels outside the tube  35  through the main body of the filter material  32 , which occupies the outer annulus of the first filter section  31 . This provides increasing filtration for the smoke travelling through the first filter section  31  (compared to when the smoke passed through tube  35  instead of filter material  32 ). 
     After having passed through filter material  32  of the first filter element  31 , the smoke is drawn through the second element  41 , and converges on the entrance  56  to tube  55 . The smoke flow then passes through tube  55  as previously described in order to exit the filter  20  at mouth end  100 . 
     The smoke flow through the second filter element  41  now passes somewhat closer to the ventilation holes  43  on the outside of the filter  20  (compared with the configuration of  FIG. 2 ). This is because with the central tube  35  blocked (either completely or partly), more smoke exits the first filter section  31  nearer to the outside of the cigarette. The restriction of the end of tube  35  creates an increase in pressure drop in the filter section upstream of the ventilation holes, thereby increasing the amount of ventilating air entering the filter. In addition, as more smoke passes through the radially outer portions of the filter, adjacent to the incoming ventilating air, more smoke is deflected radially inwards by the ventilating air towards the central portion of the filter and across the filter material, thereby providing an increase in filtration efficiency. 
     The convergence of smoke towards the region  56  of the second filter adjacent to the entrance to tube  55  leads to an increased density of smoke at this impaction point. This rise in smoke concentration again causes the deposition of smoke materials such as tar into the filter material. Accordingly, as the cigarette is smoked further, there is a build-up of tar at location  56 , which gradually blocks the entrance of tube  55  so that the exit of smoke from filter  20  via tube  55  is restricted. 
     This leads to the situation shown in  FIG. 4 . In the third filter section  51 , with smoke flow through the central tube  55  now restricted, more smoke passes through the filter material  52  surrounding the central tube—i.e. through the outer annulus of the third filter section  51 . The smoke is therefore subject to the high filtration efficiency of filter material  52 . This helps to compensate for the reduced filtering within tobacco rod  15  (which is now substantially shorter in length than when the cigarette was first lit). 
     In the configuration of  FIG. 4 , the flow through the second filter section  41  is largely parallel to the cylindrical axis of the cigarette and is generally located in the (radially) outer portions of the filter, nearer to the ventilation holes  43  and  53 . This increased flow adjacent to filtration holes  43  and  53 , combined with the overall increase in pressure drop of the filter due to the (partial) blockage of the entrance of the tube  55 , draws in increased ventilating air, resulting in greater dilution of the smoke reaching the mouth end  100 . 
     The harder a cigarette is puffed, the more tar is generated, the faster the smoke exits tube  35 , and the higher the impact filtration in  FIG. 2 . In addition, for each progressive stage, as smoke is gradually drawn towards the outer part of filter  20  and hence closer to ventilation holes  43 ,  53 , the ventilation through filter  20  increases. Cigarette  10  therefore demonstrates an increase in filtration efficiency, pressure drop, and also ventilation with increased tar generation. Furthermore, cigarette  10  has a relatively flat puff by puff tar profile (based on the combined filtering of the filter  20  and remaining tobacco rod  15 ). 
     Although  FIG. 1  illustrates one embodiment of the invention, the skilled person will be aware of many potential modifications of cigarette  10 . For example, filter sections  31  and/or  51  may be provided with multiple tubes. In addition, the tube or tubes may not necessarily be located along the central axis of cigarette  10 . For example, one possibility would be to have one or more grooves along the outer rim of the filter material  32  and/or  52  either to replace, or in addition to tubes  35 / 55  respectively. 
     Furthermore, although filter  20  comprises three sections, in other embodiments there may be additional sections. For example, a filter might be provided with a fourth section analogous to the second section, and a fifth section analogous to the third section. This would then provide a further gradation in the rise of filter efficiency. 
     It will be appreciated that the number, size, shape, material and structure of the tubes or grooves, the type and density of the filter material, and the number and structure of the filter sections can be adjusted to provide the desired overall filter characteristics—e.g. depending upon properties of particular tobacco, etc. In addition, although the present approach has been described generally in the context of cigarettes, it can be applied to a wider range of smoking articles, e.g. cigars. Accordingly, the scope of the present invention is defined by the appended claims and their equivalents.