Note: Descriptions are shown in the official language in which they were submitted.
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CRYOTROPIC HYDROGELS AND THEIR USE AS FILTERS
Technical field
[0001] The present invention relates to the field of cryotropic hydrogels and
their use for filters. The present invention further relates to the design of
smoke
filters including a cigarette smoke filter using crosslinked porous materials.
The
invention further concerns a method for the filtration of smoke.
Background
[0002] Cigarettes have always been a topic of conflict between the health
organizations and the cigarette companies because of harmful and carcinogenic
compounds. In the 1950s, in the wake of rapidly growing scientific evidence
that cigarettes cause lung cancer, companies tried to make special filters
which
would arrest the tar and other components from being inhaled with the smoke.
Over the years several advancements have been made in regard to filter design.
In general, the filters have been designed using fibers made of thermoplastic
polymeric materials. For example US patent 4,059,121 describes a filter
element made of solid fibers oriented in randomly manner. The diameter of
these
fibers range up to 5 pm. While, this patent talks about a single filter
element, US
patent 4,149,550 describes a filter core made of randomly oriented fibers and
a crust with densely packed fibers. Thus along with the filtering efficiency,
the
strength of the porous structure of the filter too has been targeted to be
enhanced
by this procedure. While these filters are made of single polymeric material,
US
patent 4,579,130 proposes to use two different types of crystalline polymeric
materials which are mixed and extruded to form the fibers. The patent further
proposes to enhance the mechanical strength of the filter by enhancing the
adhesion between the fibers which are thermally and mechanically treated for
this purpose. US patent 4,869,275 describes the design of an entangled web of
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very low bulk density made of plastic fibers of micron size diameter.
Filtration
efficiency of as high as 80% is achieved in this filter. US patent 4,961,415
describes the use of non-woven thermoplastic polypropylene fibers, but here,
in
addition, the filter consists of regions made of fused columnar structure
which
does not increase the pressure drop but impart mechanical strength to the
filter.
Similarly US patent 5,538,019 envisages randomly oriented filaments formed
into loop springs using which a desired degree of firmness and pressure drop
is
achieved. Filter of a different kind is proposed in US patent 5,586,987 which
depicts the design of a thermoplastic elastomer-based fibrous nonwoven web
formed into a bag filter with an open, a close and inside surface. A bi-
component sheath-core filter is envisaged by US patents 5,633,082 and
6,026,819 which describe the use of thermoplastic material like polypropylene,
polybutylene terephthalate and ethylene-vinyl acetate copolymer. These
polymers
in different composition are used to form the core and the sheath and a
seamless
continuity over these two regions. While the above patents desires essentially
to
achieve high filtration efficiency while maintaining the mechanical integrity
of
the filter, these are several other patents which have been proposed to
improve
the quality of smoke being inhaled, by the addition into the matrix of the
filter of
different components and molecules which either diffuse slowly into the smoke
or
arrest something toxic. For example, US patent 5,1 15,823 proposes to impart
flavor to the smoke by a novel design which comprises of a filtering zone and
a
flavor enhancing zone containing the flavor. Similarly, US patents 5,746,231
and 6,164,288 propose dispersing of a humactant like sodium pyroglutamate,
chlorophyllin, vegetable oil etc. into the filtering material which captures
the
moisture in the smoke. This moisture then helps in wet filtering of the smoke.
US
patent 6,530,377 proposes to incorporate these molecules inside microcapsules
dispersed through the matrix of the filter. Beside these humactants, there are
other type of molecules that facilitate removal of toxic chemicals have been
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proposed. For example, US patent 6,792,953 describes the use of cellulose
fibers impregnated with metallic ions like copper and iron containing
porphyrins.
Similarly, US patent 7,104,265 proposes addition of copper and iron
phthalocyanine into the filter material. Beside the smoke quality, the
disposal of
the filter material has also been a targeted issue. For example, there are
patents
which propose to design biodegradable cigarette filters; US patent 5,817,159
proposes to design an interpenetrating network made of two water soluble
polymers. Importantly, the fibers for such filters are prepared without using
any
organic solvent. Similarly, US patent 5,91 1,224 uses polyvenylalcohol as the
fiber material which too is biodegradable. Since, polyvinylalcohol is
hygroscopic, the patent describes also the special treatment procedure for
carefully controlling the moisture of the fibers. While the above patents
describe
fibrous design of the filter, US patent 5,360,023 envisages filter made of web
of
paper incorporating a carbonaceous material. The papers arranged in a way to
provide longitudinally extending channels.
[0003] Details of preparation of cryogels have been dealt with in US patent
applications 10/492404 and 10/552034. Previously cryogels have been used
for variety of applications e.g. for chromatographic separation methods and as
biomaterials. Another application has been proposed by US patents 5,288,503
and 5,460,715 which have described respectively cryogel diffusion barrier for
release of therapeutic agents and as a filtering medium for separating blood
cells from plasma.
[0004] Whereas the existing filters have provided some hope for a safer
cigarette smoking, the particle count in the inhaled smoke in traditional
cigarettes (Capstan , ITC brand) is still very high: 109 particles per
centimeter
cube of the smoke or higher. Thus there is room for improvement regarding
cigarette filters.
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Summary
[0005] It is an object of the present invention to obviate at least some of
the
disadvantages of the prior art and provide an improved particle-filter as well
as
a method for the filtration of smoke.
[0006] It is described to use a polymeric material known as cryogel which
have been found to reduce the particle count significantly (10' particles per
centimeter cube) without increasing the pressure drop any significantly.
[0007] There is provided a particle-filter comprising at least one hydrogel
formed in aqueous solution at a temperature below 0 C.
[0008] There is further provided a cigarette comprising at least one particle-
filter as described above.
[0009] There is also provided a method for the filtration of smoke comprising
the steps:
providing at least one filter comprising a macroporous hydrogel formed in
aqueous solution at a temperature below 0 C, and
filtering the smoke through the filter.
[00010] Further aspects and embodiments are defined in the appended claims,
which are specifically incorporated herein by reference.
[00011] Advantages of the invention include that:
= The cigarette filter is prepared without using any organic solvent and the
crosslinked network is not soluble in water in contrast to many other types of
filter materials.
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= The filter material does not cause any harm to the environment or to the
human body and is biodegradable.
= The filter allows use in both dry condition or by incorporating an amount of
water in it. While water enhances the arresting effect of the particulates
that is
the tar materials and other components in the smoke, it increases the pressure
drop. Since the filter material is hydrophilic it contains the water in its
pores
without allowing the water to wet the surface of its container e.g. the paper
wall
of a conventional filter.
= It is possible to achieve filtration via physical adsorption onto the
surface of
the cryogel material or by specific chemical reaction if the crosslinked
network is
suitably functionalized.
= When the filtration occurs by physisorption, the filter can be recycled
after
simple washing with water.
= The crosslinked structure provides mechanical strength to the filter without
significantly increasing the pressure drop
= The mechanical strength i.e. the deformability of the material can be tuned
by
controlling the pore size distribution of the filter.
= The filter material is macroporous which helps in reducing the pressure
drop.
= The macroporous structure is tuned by varying the water to monomer ratio
and the monomer to crosslinker ratio in the pre-polymer solution.
= The filter can be used as a substitute of the conventional cigarette filters
or
can be used as an additional filter to the native cigarette filter.
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Definitions
[00012] Before the invention is disclosed and described in detail, it is to be
understood that this invention is not limited to particular compounds,
configurations, method steps, substrates, and materials disclosed herein as
such
compounds, configurations, method steps, substrates, and materials may vary
somewhat. It is also to be understood that the terminology employed herein is
used for the purpose of describing particular embodiments only and is not
intended to be limiting since the scope of the present invention is limited
only by
the appended claims and equivalents thereof.
[00013] It must be noted that, as used in this specification and the appended
claims, the singular forms "a", "an" and "the" include plural referents unless
the
context clearly dictates otherwise.
[00014] If nothing else is defined, any terms and scientific terminology used
herein are intended to have the meanings commonly understood by those of skill
in the art to which this invention pertains.
[00015] "Cryogel" is used herein to denote a hydrogel formed in aqueous
solution at subzero temperatures.
[00016] "Hydrogel" is used herein to denote a network of crosslinked water-
soluble polymer chains.
[00017] "A macropourous" material is a material containing pores with an
average diameter greater than 50 nm but not more than 50 pm, along with
interconnectivity between the pores.
[00018] "Smoke" is used herein to denote a collection of airborne solid and
liquid particulates and gases emitted when a material undergoes combustion or
pyrolysis. For instance smoke often comprises particles comprising tar.
Examples
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of substances in cigarette smoke include but are not limited to methane,
acetylene, ethane, propene, chloromethane, propane, ethanol, acetaldehyde,
butene, ethanol, acetonitrile, acrolein, acetone, acrylonitrile, isoprene,
pentadiene, 2-butanone, hexane, benzene, dimethylfuran, pyridine, toluene,
benzopyrine, napthalene, N-nitrosamine, chloro-biphenyls, phenols, 2-
pentanone, 3-buten-2-one, 3-pentanone, ethylbenzene, isobutyronitrile,
isoprene,
o-xylene, propionaldehyde, propionitrile, m-/p-xylene, styrene and toluene.
[00019] "A supermacroporous" material is a material comprising pores greater
than 1000nm (1 pm) along with interconnectivity between the pores.
Brief description of the drawings
[00020] Further objects and advantages of this invention will be more apparent
from the ensuing description when read in conjunction with the accompanying
drawings wherein:
[00021] Figure 1 shows scanning electron microscope image of a typical
cryogel sample. The image shows the presence of pores and the pore wall made
of the polymeric material. The pore size is a function of the polymer and the
crosslinker concentration.
[00022] Figure 2 shows schematic of the experimental set up for particle count
analysis in the smoke emitted by a burning cigarette. The cigarette 2 1 is
first
attached to one end of a cylindrical holder 22 which contains also a smoke
filter
23. This filter is either a conventional filter or one pre-designed of cryogel
material. The SMPS 25 (Scanning Mobility Particle Sizer Spectrophotometer)
sucks in the smoke 24 at constant flow rate which brings in the tar and other
content from the burning cigarette and analyze it to yield the particle size
distribution of the smoke 24.
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[00023] Figure 3 shows number density NP of particles per cc volume of smoke
is plotted against diameter DP (nm) of particles. There is represented the
particle
count in smoke released respectively by a single (D) and double (C)
conventional
cigarette filter, and the particle count in smoke released by John silver
cigarette
filter (E). There is further shown particle count in smoke emitted by
polyacrylamide cryogel filters prepared by using 6% (A) and 7% (B) by weight
of monomer respectively. The monomer to crosslinker ratio is maintained at 2:1
for both the gels.
[00024] Figure 4 shows number density NP of particles per cc volume of smoke
is plotted against diameter DP (nm) of particles. There is represented the
particle
count in smoke released respectively by a single (H) and double (G)
conventional cigarette filter, and the particle count in smoke released by
John
silver cigarette filter (I) as well as that emitted by agarose-alginate (2:1)
cryogel
filter (F).
[00025] Figure 5 shows number density NP of particles per cc volume in smoke
released by the acrylamide cryogel filters prepared with varying monomer to
cross-linker ratio plotted against the particle size DP in nm. Here the
monomer
concentration is kept constant at 6%. J denotes a 3:1 Cryogel, K denotes a 4:1
Cryogel.
[00026] Figure 6 shows pressure drop (Pa) across the filters plotted against
the
flow rate (cc/min) of gas through the filters. There is represented pressure
drop
across a conventional cigarette filters Capstan (L) and an auxiliary cigarette
filter-
John Silver (N). There is also represented pressure drop offered by cryogel
filters
made of Agarose-alginate(2:1) (M) and Acrylamide (6%,3:1) (0) respectively.
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[00027] Figure 7 shows Number density NP of particles per cc volume of smoke
is plotted against diameter DP (nm) of particles. Comparison of particle
reduction
by agarose-alginate cryogel filters in wet (Q) and dry (P) condition.
Detailed description
[00028] In a first aspect there is provided a particle-filter comprising at
least one
hydrogel formed in aqueous solution at a temperature below 0 C.
[00029] There is in one embodiment described a highly efficient particle-
filter
comprising a cross-linked inter-connected macroporous hydrogel structure to
reduce particulate components in mainstream cigarette smoke.
[00030] There in one embodiment provided a cigarette filter which has been
prepared by using crosslinked material e.g. "cryogel" synthesized using
acrylamide, or N-isopropylacrylamide monomers but not excluding others. There
are also described filters synthesized by cryogel matrices of synthetic
polymers
like agarose and/or alginate. These materials are prepared without using any
organic solvent and are biodegradable. The filtration process is achieved via
physical adsorption onto the surface of the cryogel material or by specific
chemical reaction if the crosslinked network is suitably functionalized. The
filter is
recyclable and biodegradable. One advantage of the crosslinked structure is
that it provides mechanical strength to the filter without significantly
increasing
the pressure drop. Furthermore, both these parameters can be tuned by
controlling the pore size distribution. This cigarette filter has been found
to
reduce the content of particles comprising tar in the cigarette smoke by
several
orders of magnitude from what is achieved by the conventional filters used in
cigarettes. The filter can be used in both wet and dry condition. The
macroporous nature and interconnectivity makes these cryogel filters as highly
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efficient substitutes of the conventional cigarette filters or can be used as
an
additional filter to the native cigarette filter.
[00031] The invention discloses the application of porous crosslinked
materials
like cryogel as the base material for designing cigarette smoke filters. The
cross
linking polymerization is in one embodiment carried out inside a hollow solid
cylinder in order to achieve a cylindrical structure (monolith) of the filter
of
specific dimensions to suit for application of use as filters.
[00032] In one embodiment the particle-filter is a cigarette filter.
[00033] In one embodiment the hydrogel of the particle-filter is insoluble in
water.
[00034] In one embodiment the hydrogel of the particle-filter comprises water.
This has the advantage that water arrests particles, for example particles
comprising tar in cigarette smoke.
[00035] In one embodiment the hydrogel of the particle-filter is hydrophilic.
This
has the advantage that the filter contains water in its pores without allowing
the
water to wet the surface of its container, e.g. the paper wall of a
conventional
filter.
[00036] In one embodiment the hydrogel of the particle-filter is
supermacroporous.
[00037] In one embodiment the hydrogel of the particle-filter comprises at
least
one material selected from the group consisting of poly-acrylamide, agarose,
and alginate.
[00038] In one embodiment the hydrogel is formed at a temperature below -
10 C. In one embodiment the hydrogel is formed at a temperature below the
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freezing point of the solution comprising the monomer. In an alternative
embodiment the hydrogel is formed at a temperature 10 degrees Celsius below
the freezing point of the solution comprising the monomer.
[00039] In one embodiment the hydrogel of the particle filter is formed
without
using any organic solvents. This has the advantage that it does not cause harm
to the environment and the human body.
[00040] In a second aspect there is provided a cigarette comprising at least
one
particle-filter as described above.
[00041] In a third aspect there is provided a method for the filtration of
smoke
comprising the steps:
providing at least one filter comprising a macroporous hydrogel formed in
aqueous solution at a temperature below 0 C, and
filtering the smoke through the filter.
[00042] In one embodiment the at least one filter is recycled.
[00043] In one embodiment the at least one filter is washed with water, when
it
is recycled.
[00044] In one embodiment particulate matter in the smoke is reduced. In one
embodiment the reduction of particulate matter is more than 95 wt%. In another
embodiment the reduction of particulate matter is more than 99 wt%. In yet
another embodiment the reduction of particulate matter is more than 99.9 wt%.
In an alternative embodiment the reduction of particulate matter removes
essentially all particles.
[00045] In one embodiment the amount of at least one substance in the smoke is
reduced, wherein the at least one substance is at least one substance selected
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from the group consisting of methane, acetylene, ethane, propene,
chloromethane, propane, ethanol, acetaldehyde, butene, ethanol, acetonitrile,
acrolein, acetone, acrylonitrile, isoprene, pentadiene, 2-butanone, hexane,
benzene, dimethylfuran, pyridine, toluene, benzopyrine, napthalene, N-
nitrosamine, chloro-biphenyls, phenols, 2-pentanone, 3-buten-2-one, 3-
pentanone, ethylbenzene, isobutyronitrile, isoprene, o-xylene,
propionaldehyde,
propionitrile, m-/p-xylene, styrene and toluene. In most cases the amount of
several or all of the above substances in smoke are reduced. In one embodiment
all of the above substances are reduced. The above substances are abundant in
smoke such as smoke from cigarettes.
[00046] In one embodiment the amount of the above at least one substance in
the smoke is reduced by more than 90 wt%. In a further embodiment the amount
of the at least one substance in the smoke is reduced by more than 95 wt%. In
another embodiment the amount of the at least one substance in the smoke is
reduced by more than 99 wt%. In another embodiment the amount of the at least
one substance in the smoke is reduced by more than 99.9 wt%.
[00047] The hydrogel is in an alternative embodiment made from at least one
type of monomers or polymeric precursors selected from the group consisting of
acrylamide, N-isopropylacrylamide, acrylonitrile, N-vinylcaprolactam,
chitosan,
gelatin, alginate, agarose and poly(vinylalcohol).
[00048] The cryogel is in one embodiment synthesized by using a
polymerization reaction in the water medium under freezing conditions which
yield a spongy, elastic and supermacroporous material. Normally the freezing
conditions are below 0 C.
[00049] In general the filters according to the present invention are useful
for
filtering gases comprising particulate matter and substances in gas phase, for
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example the smoke generated by a cigarette, dust laden smoke in many
industrial situations e.g. mines, blast furnaces, cement plants etc. Cryogel
filters
can be employed in disposable particulate respirators for filtration of
aerosols
and fine particulates suspended in air, which are associated with human
disorders like asthma, lung cancer etc.
[00050] Other features and uses of the invention and their associated
advantages will be evident to a person skilled in the art upon reading the
description and the examples.
[00051] It is to be understood that this invention is not limited to the
particular
embodiments shown here. The following examples are provided for illustrative
purposes and are not intended to limit the scope of the invention since the
scope
of the present invention is limited only by the appended claims and
equivalents
thereof.
Examples
[00052] The cryogels of poly(acryl amide) [poly(AAm)] were synthesized by
radical polymerization at -12 C during 12 hours using monomers of acryl amide
(AAm) with N, N-ethylene bisacrylamide (MBAAm) as cross-linking agent.
Similarly, agarose-alginate cryogels were fabricated by cross-linking agarose
and alginate with the help of glutaraldehyde at -12 C for 16 hours.
[00053] When the polymerization occurs at sub zero temperature the water
freezes and form nucleated crystals which grow and prevents the polymer
reaction from occurring inside the crystals of water. After the polymerization
reaction is complete the gel is brought back to the normal temperature at
which
the crystals melt resulting in the porous structure. The sizes of these pores
depend
on the polymer concentration, cross-linker concentration and the rate of
cooling.
The moisture content of the filter is controlled by drying it to a desired
extent.
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[00054] The supermacroporous nature of these hydrogels was defined visually
by scanning electron microcopy and ESEM (hydrogels in hydrated state). The
quantitative estimation was done by mercury porosimetry, uptake of water and
cyclohexane. The evaluated pore size is up to 200pm.
[00055] The approach is to exploit the filtration properties of polymeric
matrices
called as "cryogels" to reduce the particulate count of smoke by using
cryogels
as auxiliary or substitute to conventional filters.
[00056] Cryogel filters have been used as auxiliary filters to observe
reduction
in total particle matter (TPM) in main stream cigarette smoke. Cigarette smoke
coming out of the cigarette butt having a single conventional cigarette filter
has
about 109 particles per centimeter cube ("D" in Figure 3). These "cryogel
filters"
are macroporous materials having many advantages over the currently used
cigarette filters as they remove significantly higher number of particles
comprising tar compared to traditional auxiliary filters like John Silver or
Capstan's cigarette filter used as auxiliary filter. As can be seen in Figure
3,
acrylamide cryogel (7% monomer, 2:1) as auxiliary filter reduces total
particulate count by a factor of about 104 in comparison to TPM of cigarette
having its filter ("D" in Figure 3). Even if an additional cigarette filter is
used as
auxiliary filter with the cigarette ("C", Figure 3) the TPM reduction is less
in
comparison to the reduction offered by cryogel filters. Similarly 6%
acrylamide
cryogel also reduces particulate count by a factor of 103 to 102 when compared
to auxiliary Capstan cigarette filter and auxiliary John Silver filter
respectively
(Figure 3).
[00057] Agarose-alginate cryogel filter gives a particle reduction by a factor
of
104 in comparison to the particle reduction by traditional cigarette filter,
when
both of them were used as auxiliary filters (Figure 4).
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[00058] Moreover, the synthesis protocol of the cryogel filters can be varied
to
design cigarette filters with desired reduction in particulate count and
optimum
draw resistance. As the polymer concentration of the cryogel increases the
particle count decreases e.g. the particle reduction offered by acrylamide
cryogel filter with 7% (total monomer) concentration is higher than that
offered
by 6% acrylamide cryogel filter (Figure 3). Similarly on decreasing the
polymer
to cross linker ratio in the cryogel the particle count passing through
"cryogel
filter" also decreases (Figure 5). This change in porosity by varying the
polymer
concentration and the cross linker to polymer ratio gives the cryogel of
different
porosity which changes the total particulate count (TPM) in the smoke.
[00059] Along with particle count reduction, the draw resistance of the
filters is
also an important property. To characterize these cryogel filters for draw
resistance, we measured the pressure drop across these filters when nitrogen
gas
was flowed through these cryogels at different flow rates and compared them
with the pressure drop of capstan cigarette filter and John silver filter.
Figure 6
shows that both acrylamide and agarose-alginate cryogel filters have
comparable pressure drop as with capstan cigarette filter, thus making them
eligible candidates for use as cigarette filters.
