A wide variety of fibrous materials have been employed in tobacco smoke filter elements. However, the choice of materials for use in production of such filters has been limited because of the need to balance various commercial requirements. A very important property of a tobacco smoke filter is obviously its filtration efficiency, i.e., its ability to remove selected constituents from the tobacco smoke. However, the range of filtration efficiency has had to be compromised in order to satisfy other commercially important factors such as resistance to draw, hardness, impact on taste, and manufacturing costs.
Cellulose acetate has long been considered the material of choice in the production of tobacco smoke filters, primarily because of its ability to provide commercially acceptable filtration efficiency, on the order of about 50%, without significantly detracting from the tobacco taste, low resistance to draw, and filter hardness desired by the majority of smokers.
A significant component of the commercially desirable "taste" is provided by the standard plasticizers utilized in the production of filter elements from cellulose acetate fibers, usually triethylene glycol acetate or glycerol triacetate ("triacetin"). In conventional cigarette filter manufacturing, the plasticizer is commonly applied to the cellulose acetate fiber by spraying or wicking using art-recognized techniques. The tendency of the plasticizer to migrate toward the center of conventional cellulose acetate fibers reduces the level of plasticizer at the fiber surface, minimizing its taste-enhancing capability and limiting the shelf life of plasticized tow fibers before being processed into filter rods. The plasticizer is therefore usually added to the tow during the manufacture of the filter rods.
Cellulose acetate fiber plasticized in this manner and wrapped with paper into rod-like forms become bondable at the fiber contact points, enabling the formation of relative self-sustaining, elongated filter rods in two to four hours. This process can be accelerated by the application of gases at elevated temperatures simultaneously with the formation of the filter rod. Filter rods produced in this manner provide a tortuous path for the passage of tobacco smoke when discrete lengths of such material are utilized as tobacco smoke filter elements.
Filtration efficiency can be increased significantly through the use of small fibers which provide increased fiber surface area at the same weight of fiber. Solvent spun cellulose acetate fiber is commercially available only in fiber sizes down to 13 microns in diameter. To obtain finer cellulose acetate fiber, e.g., 10 microns or less, melt spinning of plasticized cellulose acetate resin would be required; however, the level of plasticizer necessary to directly spin such fine cellulose acetate fibers would render the resultant fibers very weak and commercially useless. Melt spun cellulose acetate of a larger diameter, which would require less plasticizer, would have to be drawn and crimped to produce such fine fibers for use in tobacco smoke filters. Unfortunately, melt spun cellulose acetate fibers can only be commercially drawn at relatively low draw ratios before the fibers break during processing. The inability to form and process very fine fibers of cellulose acetate places practical limits on the filtration efficiency capabilities of this material in the production of tobacco smoke filters.
Further, and very important commercially, by comparison with other polymeric materials such as the polyolefins, cellulose acetate is relatively expensive, costing, for example, on the order of more than three times as much as commercially available polypropylene in resin form. While attempts have been made to utilize other less expensive and more easily processed polymeric materials such as polypropylene in lieu of cellulose acetate in the manufacture of tobacco smoke filters, such efforts have been almost universally abandoned on a commercial level, primarily because of the undesirable impact of such materials on the taste properties of tobacco smoke. Also, such use is generally limited by the inability to easily bond the fibers in order to obtain the desired filter hardness at required resistance to draw.
Another problem with commercially available tobacco smoke filters, particularly cigarette filters, currently on the market is the difficulty in disposing of such materials after use. By bonding highly crimped cellulose acetate fibers at their contact points, conventional cigarette filters are designed to provide a significant volume of interstitial space for the passage of smoke. The bonded contact points of such filter elements degrade very slowly under normal environmental conditions resulting in high volume, long life, environmentally undesirable litter.