Note: Descriptions are shown in the official language in which they were submitted.
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SMOKING ARTICLE
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
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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
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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 unbumt 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 0.1-3mm, or more particularly in the range 0.5-
1.5mm,
and a length in the range 4-15mm. 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
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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.
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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:
Figure 1 illustrates a cigarette filter in accordance with one embodiment of
the
invention.
Figure 2 illustrates the behaviour of the cigarette filter of Figure 1 for
initial
puffs in accordance with one embodiment of the invention.
Figure 3 illustrates the behaviour of the cigarette filter of Figure 1 for
second
stage puffs in accordance with one embodiment of the invention.
Figure 4 illustrates the behaviour of the cigarette filter of Figure 1 for
third
stage puffs in accordance with one embodiment of the invention.
Detailed Description
Figure 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 Figure 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 5-10mm. (In other embodiments, the separation
maybe
in the region 4-15mm).
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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 0.1-3mm. In one embodiment,
each of tubes 35, 55 has a diameter of approximately 0.5-lmm. 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.
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Figure 2 illustrates the main smoke flow through the cigarette 10 of Figure 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 Figure 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 Figure 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
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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 Figure 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 Figure 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).
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In the configuration of Figure 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 Figure 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 Figure 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
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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.
