It is well known that tobacco smoke contains more than four thousand constituents in the form of liquid, solid, or vapor. Cigarette filters commonly used now are made of cellulose acetate, cotton, rayon, or paper. Among these filter materials, over 90% of the filter cigarettes made in the U.S. and a majority of filter cigarettes made in the world use a single-segment cellulose acetate filter. The performance of these filters in terms of pressure drop generation and smoke filtration efficiencies are somewhat limited because of certain requirements for cigarette filters.
The prior known fibrous filters are capable of removing varying percentages of tar and nicotine from cigarette smoke depending on the amount of fibrous material compacted into them, their length, their circumference, their resistance to draw, the surface characteristics of the fiber, the configuration of the fiber, and other factors. These filters, however, show substantially no independent control of the filtration of nicotine from cigarette smoke, without changing the filtration of tar.
U.S. Pat. No. 3,424,173 mentions organic acids such as citric acid as filter additives to remove a higher percentage of nicotine than tar from cigarette smoke. However, it was also mentioned that the addition of an acid to the filter can cause hydrolytic degradation of the fiber by prolonged contact with the applied acid, thus generating acetic acid which gives the filter an objectionable odor and taste. U.S. Pat. No. 3,424,172 discloses a filter containing citric acid that is partially esterified with an alcohol such as, ethanol, to leave at least one free carboxyl group. The application of additives was done by spraying or passing the fibers through a bath of the coating material. The fibers made by this method improve the filtration of nicotine significantly.
Known methods of applying additives in the filter are to dust the additive on the filter or to spray aqueous solution on the filter material. It was observed that the effectiveness of the additive in selective removal of nicotine depends on the total surface area of the additive in the filter. Usually, the total surface area of the additive in powder form applied by dusting is significantly less than that of the additive applied with an aqueous solution. It was observed that the filters dusted with citric acid powder showed a significantly lower performance in selective filtration of nicotine compared with the filters sprayed with citric acid solution. Even though spraying aqueous solution of citric acid on the tow makes the filters effective in selective filtration of nicotine, this application method presents operational problems such as accumulation of tacky deposits on the plugmaker processing rolls, delivery rolls and garniture and a need for a dryer for removing moisture once the solution is applied. It would be beneficial to enhance the nicotine filtration efficiency of tobacco smoke filters while avoiding the above problems.