Abstract:
A cigarette is equipped with a filter incorporating nanofibers. The nanofibers can be used either alone or in conjunction with other filtering media such as larger fibers, paper, activated charcoal, etc. With a nanofiber filter, the filter need not be as dense as the molecules of the smoke become attracted to the nanofiber due to quantum mechanics instead of being sieved out. The efficacy of the filter is better than filters with larger diameter fibers.

Description:
SPECIFICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 10/896,762 filed Jul. 21, 2004, which claims the benefit of U.S. provisional patent application Ser. No. 60,481,111, filed Jul. 21, 2003. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to filters, and in particular to cigarette filters. 
       BACKGROUND OF THE INVENTION 
       [0003]    Cigarettes are frequently provided with filters to remove some of the contaminants, or hazardous components, of cigarette smoke before the smoke is inhaled. Such hazardous components include tar, carbon monoxide and various carcinogens. Various cigarette filters are known in the prior art which decrease the toxic effect of tobacco smoke. These consist of a base made of acetate, cellulose or acetate-cellulose fibers with substances applied to the base which have adsorption properties or are impregnated with these substances. Activated charcoal, inorganic and organic slats of various acids are used as the adsorbing agents. 
         [0004]    In one known form of construction, the filter body consists of a tow of continuous filaments, commonly cellulose acetate (acetate) filaments, arranged parallel to the longitudinal axis of the cigarette. In another known form of construction, the filter body consists of pleated or fluted paper compressed into a cylinder. The paper is subjected to a grooving process to allow it to be so pleated or fluted. Such forms of construction contain a single filter element and may be called “mono” filters. Another known form of construction is the so-called “dual” filter which contains two filter elements, namely a paper filter towards the interior and a tow filter towards the exterior of the cigarette. A further known form of construction is the so-called “triple” filter, which contains three elements, namely a paper filter and a tow filter, as in the “dual” construction, separated by an air gap or by an activated carbon filter. Paper filters are known to be generally more efficient at removing tar from tobacco smoke than are tow filters. High tar removable efficiency is particularly desirable in view of the trend towards low-tar cigarettes. 
         [0005]    A drawback of these filters is that the range of substances absorbed by them is restricted. The filters themselves can degrade on heating, which is accompanied by the formation of toxic compounds. 
         [0006]    Also, in the prior art, there exists filtered cigarettes that fall into the category of Less Harmful Cigarettes (LHCs) or Reduced Risk Cigarettes (RRCs). But these are chemical based and require extensive testing to prove efficacy. In fact, there continues to be debate about the efficacy of LHCs and RRCs. 
         [0007]    It would be desirable to provide a cigarette filter element which is capable of removing certain gas phase components of mainstream cigarette smoke, while not adversely affecting the flavor. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to provide a cigarette filter that removes contaminants or hazardous byproducts of cigarette smoke, such as tar and carbon monoxide, from cigarette smoke. 
         [0009]    It is another object of the present invention to provide a cigarette filter that filters out hazardous components of cigarette smoke by mechanical means. 
         [0010]    The present invention provides a cigarette that comprises a tobacco rod and a filter coupled to the tobacco rod. The filter comprises a tow made of nanofibers. 
         [0011]    In accordance with one aspect of the present invention, the pressure drop across the filter is between 50-150 millimeters wg. 
         [0012]    In accordance with another aspect of the present invention, the weight of the filter is between 50-400 g/1000 rods. 
         [0013]    In accordance with another aspect of the present invention, the filter tow is comprised of cellulose acetate. 
         [0014]    In accordance with another aspect of the present invention, the filter comprises activated charcoal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a longitudinal cross-sectional view of a portion of cigarette of the present invention, in accordance with a preferred embodiment. 
           [0016]      FIG. 2  is a schematic view of a prior art filter fiber illustrating gas flow around the fiber. 
           [0017]      FIG. 3  is a schematic view of a nanofiber used in the filter of the present invention, with an illustration of gas flow around the nanofiber. 
           [0018]      FIG. 4  is a longitudinal cross-sectional view of a portion of a cigarette in accordance with another embodiment. 
           [0019]      FIG. 5  is longitudinal cross-sectional view of a portion of a cigarette in accordance with another embodiment. 
           [0020]      FIG. 6  is a longitudinal cross-sectional view of a portion of a cigarette in accordance with another embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The present invention provides a filter so as to make a filtered cigarette  11 . The cigarette  11 , shown in  FIG. 1 , has smoking material  13  and a filter  15  located at one end. The filter  15  utilizes nanofibers  17  to remove harmful components of the smoke produced when the smoking material is burned. The filter does not adversely affect the taste of the smoke and produces less drag, or resistance, to the flow of smoke. The nanofiber filter can be substituted for any type of filter that uses fibrous filter elements, such as cellulose acetate filament filters (known as “tow”). 
         [0022]    The smoking material  13  is typically tobacco or a blend of tobaccos. The smoking material is in the form of a charge or roll and is contained in single or multiple layers to form a so-called “tobacco rod”. The tobacco rod is encompassed by a wrapping material  21  that is generally a paper-based product. The wrapping material can be one or more layers. 
         [0023]    The filter  15  contains fiber material  17  that is formed into a tow. During manufacturing, the tow is cut to length and is typically circumscribed with a wrapping material  21  that is generally a paper-based product. The filter is joined to the tobacco rod by wrapping material. 
         [0024]    The filter comprises nanofibers. “Nanofibers” are fibers with diameters of 1000 nanometers (nm) or less. In the preferred embodiments, the nanofibers are 500 nanometers or less, in order to provide slip-flow (which will be described below). The nanofibers can be 100-250 nm. 
         [0025]    The nanofibers can be made of a variety of materials. In the preferred embodiment, the nanofibers are made of cellulose acetate. The advantage of using cellulose acetate is that it is already used in conventional cigarette filters and thus does not change the taste of cigarette smoke when used in the nanofiber filter. Other materials suitable for use as nanofibers are organic polymers (such as PLA). Another material that can be used is PBI (poly benzimidazole). However, PBI may alter the taste. Also, carbon nanofibers can be used. After the carbon nanofibers are spun into a filament, the carbon can be activated, in accordance with conventional techniques. 
         [0026]    The nanofibers are made in accordance with conventional processes such as spunbonding, melt blown and electrospinning. Spunbonding is a non-woven manufacturing process involving direct conversion of a polymer into continuous filaments and the filaments are laid randomly into a non-woven fabric by thermal bonding. In melt blown, fibrous webs are produced directly from the polymers or resins using high velocity air to attenuate the filaments. In electrospinning, one step combines the charging of the polymer and the spinning of nano-scale fibers. Repulsive electrostatic forces are used to spin the fibers from a polymer solution or melt. 
         [0027]    In addition, carbon nanotubes can be used either alone, or in combination with nanofibers. 
         [0028]    The nanofibers are spun into filaments, in accordance with conventional techniques. The filaments are then made into the tow, also in accordance with conventional techniques. The tow has a denier of 1.0 or less per filament. The filament cross-section can be “Y” shaped. The tow weight is between 50-400 g/1000 rods and has a pressure drop of 50-150 mm wg. This is compared to a conventional tow, with larger diameter fibers, having a tow weight of between 540-700 g/1000 rods and a pressure drop of 250-435 mm wg. 
         [0029]    The nanofiber tow can be entirely cellulose acetate nanofibers or a combination of cellulose acetate nanofibers and activated charcoal nanofibers. 
         [0030]      FIGS. 2 and 3  show respectively a conventional filter fiber and the nanofilter fiber of the present invention, and associated gas flow velocities across the fibers. The conventional filter fiber  31  of  FIG. 2  is relatively large, about 2-3 microns in diameter. Filters containing conventional, large fibers work by sieving, wherein particles are blocked by the filter filaments and the fibers therein. As the gas passes around a fiber  31 , the velocity of the gas adjacent to the fiber drops to zero. Gas flowing a distance away from the fiber has a non-zero velocity; the further away from the fiber, the higher the velocity, until the velocity becomes constant. Classical filtration mechanics dictate that one assumes continuous flow around the individual fiber, exhibiting a no-slip behavior. This classical theory has proven its acceptability with large fibers on a relatively larger sized, macro scale, where the flow path was generally considered axially and the fiber size was larger than the molecules being filtered. 
         [0031]    The nanofiber  17  of  FIG. 3  exhibits slip-flow behavior. On a submicron scale, such as with nanofibers, one must consider that most particles are larger than the filter fiber media through which it is passing. As a result of the density of the filter media, the flow path becomes less and less linear (axial) as the molecules are forced to circumnavigate the fibers because of their relatively larger size. The generally accepted threshold for when slip-flow becomes relevant is when the fiber diameter is approximately 500 nanometers or less. In true slip-flow, the velocity of the gas is assumed to be non-zero. As can be seen in  FIG. 3 , the velocity of the gas that is adjacent to the nanofiber is non-zero. The velocity increases the further away the gas flow is from the nanofiber. As such, the drag force created by the molecules is less than in the case of non-slip-flow and results in a lower pressure drop across the filter. Consequently, this action results in more molecules being subject to the filter media and yielding higher rates of diffusion, retention and overall efficiency for a greater number of molecule types and sizes. Nanofibers offer the advantages which capitalize on the ability to create a more apparently dense filter media without the often corresponding pressure drop across the filter normally associated with a dense filter. In most cases, nanofiber air filtration devices exhibit longer life and a greater ability to entrap a larger number of contaminants. Quantum mechanics comes in to play when the molecules are coupled to the nanofibers. In particular, London van der Waals forces come into play between the molecules in the gas flow and the nanofibers. 
         [0032]    The contaminants typically found within the byproducts of tobacco smoke have a nominal size of approximately 0.2 microns or larger. The filter of the present invention effectively reduces the contaminants that exit the filter and are subsequently inhaled by the smoker by as little as 38% and as much as 72%. 
         [0033]    The pressure drop caused by the filter  17  is lower than conventional filters. This means that a smoker does not have to draw in as hard and consequently need not draw the smoke in as deeply into the lungs. 
         [0034]      FIGS. 4-6  show some other exemplary embodiments of the invention. The nanofiber filter  15  can be used in conjunction with other types of filters. For example in  FIG. 4 , there is a section  15  of the filter that is nanofibers and another section  41  of the filter that is microfibers, that is fibers that are 1 micron or larger in size. The different filter sections  15 ,  41  can be placed end to end as shown in  FIG. 4  or they can be concentrically arranged. 
         [0035]      FIG. 5  shows a nanofiber filter  15  with a rigid plug  43  at the mouthpiece end of the filter. The plug prevents the end of the filter from collapsing when placed in the mouth of a smoker. The plug can be used with a nanofilter fiber either by itself or a nanofilter fiber in conjunction with some other kind of filter. 
         [0036]      FIG. 6  shows a nanofiber filter  15  used with a cartridge  45  that encompasses the circumference of the filter. The cartridge can contain nanofibers, microfibers, activated charcoal, paper, etc. 
         [0037]    In addition, nanofibers can be used to make other types of filtration media. For example, nanofibers spun into filament can make air filters for use in heating, ventilation and air conditioning (HVAC) systems. The filter has a frame or cartridge that holds the nanofibers and the support structure for the nanofibers. A nanofiber has a lower pressure drop, thus increasing the overall efficiency of the HVAC system. 
         [0038]    The foregoing disclosure and showings made in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.