Article for the Selective Removal of Hydrogen Cyanide From Tobacco Smoke

This invention relates to improvements in filters for tobacco smoke.

In recent years, there has been substantial interest in filters for tobacco smoke, particularly for cigarette filters, and of increasing interest is the improvement of additives for such filters which are capable of selectively removing individual smoke constituents. One such smoke constituent with respect to which selective removal is generally considered desirable is hydrogen cyanide. A number of materials have been suggested for the selective adsorption of HCN from tobacco smoke.

In U.S. Pat. No. 3,403,689 to Sublett et al., the use of sodium phosphite, potassium phosphite, lithium phosphite, sodium carbonate, potassium carbonate and lithium carbonate for this purpose is described, and in U.S. Pat. No. 3,403,690 to Horsewell et al., substances such as zinc acetate, zinc acetylacetonate, zinc isobutyrate and zinc trimethylacetate are mentioned.

In a later patent to Horsewell, U.S. Pat. No. 3,340,879, the use of poly (alkyleneimines) having a molecular weight in the order of 500 or higher is claimed to reduce volatile acidic components in tobacco smoke, presumably because of the availability of reactive imine sites. In U.S. Pat. No. 3,550,600, Horsewell et al. further described the combination of such polyethyleneimines, as well as other basic substances such as triethanolamine, sodium acetate, sodium carbonate and borax, capable of keeping the filter material substantially alkaline, in combination with zinc acetate, previously described by them for hydrogen cyanide adsorption for the removal of HCN, as well as steam-volatile phenols.

Still additional disclosures of filter additives for the selective removal of phenols appears in U.S. Pat. No. 3,605,759 to Owens et al. The Owens patent surveys the then-known state of the art as to HCN adsorption and points out that water-soluble inorganic salts, such as the alkaline metal carbonates are known to remove HCN, and that mild bases such as sodium bicarbonate will remove a portion of the acidic components of smoke. Owens claims that filters treated with a combination of an alkaline metal bicarbonate with a polyoxyethylene material show a much greater affinity for HCN from tobacco smoke than when the bicarbonate additive is used alone. In U.S. Pat. No. 3,618,619, Keifer describes a filter tow made from fiber spun from a solution containing HCN absorbants such as ZnO, Fe.sub.2 O.sub.3 and Cu.sub.2 O. The tow is then made active to absorb HCN by application of an agent such as triethylene glycol, triacetin, polyethylene glycol, diethyl citrate, etc. A later patent to Hammersmith, U.S. Pat. No. 3,802,441 further describes HCN adsorption with a mixture of zinc oxide and a (sodium or potassium) carbonate which is intimately dispersed in a plasticizer for cellulose acetate such as triethylene glycol diacetate and polyethylene glycol, and thereafter applied to the filter tow.

The present invention concerns an improved tobacco smoke filter for selectively removing hydrogen cyanide which contains a salt of a metal selected from the group consisting of zinc, copper, nickel and iron (II), chelated with a polydentate amine of relatively low molecular weight containing not more than about 10 --NH-- or --NH.sub.2 -- groupings which are capable of forming complex bonds with the metal ions. In the amine complexes of the present invention, the amine or imine nitrogen constitutes the principal bonding atom present in the molecular structure of the complex although hydroxy, carbonyl or ether oxygen groupings may also be present.

Within this class of new amine complexes found to be useful for cyanide adsorption from cigarrette smoke are complexes formed from the lower alkylenes diamines, triamines, tetramines and pentamines such as ethylenediamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine or other such amines as described by the general formula NH.sub.2 --(R--NH).sub.x --H, wherein R is ethylene and x is from 1 to 10. The ethylene group may bear a lower alkyl side chain, i.e., alkyl of 1-5 carbon atoms. In addition, as noted above, other amino compounds may be useful. Of particular significance in this respect are amino compounds such as the simple amino acids, for example, .beta.-aminopropionic acid and glycine.

As the metal compound suitable for use in the present invention, the water-soluble salts of zinc, copper, nickel and iron (II) may be used, such as zinc chloride, zinc acetate, zinc nitrate, zinc fluoride, zinc bromide, zinc oxalate, zinc phosphate, copper sulfate, nickel (II) chloride, copper (II) sulfate, and other similar compounds. In general, the inorganic compounds formed of these metals with physiologically innoculous anions are suitable.

Particularly preferred complexes in accordance with the present invention are those formed from ethylenediamine and diethylenetriamine and inorganic zinc and nickel compounds. The preferred inorganic complexes are known to have a different structural configuration as shown by x-ray analysis from zinc complexes such as zinc acetylacetonate described in Horsewell U.S. Pat. No. 3,403,690 for selective HCN adsorption. Where the zinc acetylacetonate complex is described in the literature as being a trigonal bi-pyramidal structure, the preferred complexes of the present invention have an octahedral structure. Copper in aqueous solution is 4-coordinated, forming square-planar complexes. The reason for this is connected with the Jahn-Teller effect.

It should also be noted that while it is believed that the iron (II) amine complex would be effective, such a complex tends to be unstable in the presence of oxygen and water, being oxidized to ferric oxide, a substance which does not complex with the amines. Accordingly, when the iron (II) complex is used, it should be under conditions which maintain its stability.

A number of amine complexes suitable for use in the present invention have been tested for HCN adsorption efficiency using simple screening techniques. The following is a partial list of the results of such screening test:

There appears to be a correlation between the stability of the complex and its ability to remove hydrogen cyanide. A possible explanation for the above correlation is the solubility of cellulose acetate in a wide range of organic compounds including these polydentate amines. In a strong complex, the amines will remain bonded to the metal and there should be very little tendency for the amine to migrate into the filter fiber where it would be less effective in removing hydrogen cyanide. Table II is a list of equilibrium constants for various complexes.

The stability constant of the zinc acetylacetone complex is over 1,000 times less than the corresponding amine complexes. It is estimated from the foregoing that the preferred compounds of the present invention should have a stability constant of at least about 10.sup.11.

A modification of the present invention concerns the further discovery that the complexes here found to be suitable for selective HCN adsorption interreact synergistically with polyalkylene glycols to provide even more effective adsorption for HCN, notwithstanding the fact that the polyoxyalkylene compounds used as synergists are substantially ineffective by themselves as selective adsorbants. This permits a substantially reduced level of metal complex required to produce the desired result. The polyoxyalkylene compounds suitable for use as synergists in accordance with the present invention are of the general formula:

wherein R is a saturated hydrocarbon radical containing from 2 to 6 carbon atoms, X is a polymeric chain having the formula --[OR'].sub.z --, R' being an alkylene radical having from 2 to 3 carbon atoms, and a is 2 or 3, and b is a small whole number from 1 to 2. Evaluation of the polyalkylene compounds as synergists has most generally involved the widely-available polyalkylene oxides of the formula HO(C.sub.2 H.sub.4 O).sub.x H known as Carbowaxes. These fall within the above generic formula R(XOH) wherein R is an ethylene grouping and X is --O--C.sub.2 H.sub.4 --.

The foregoing compounds should contain from 10 to 37% ether oxygen, based on the weight of the compound, the preferred materials containing from 16 to 37% ether oxygen. By "ether oxygen" is meant the group --O-- which occurs between successive alkylene groups in the "X" chain.

In selecting a suitable molecular weight, the vapor pressure of the synergist compounds should be sufficiently low that it will remain in the filter during storage and will not volatilize during the smoking of the cigarette. The vapor pressure of the synergist compounds should be below 40 mm. Hg at 25.degree. C., however, the preferred range is below 1 mm Hg at 25.degree. C. A further limitation on the molecular weight of the polymeric compound is that the compound should be soluble in the plasticizer for the filter fiber base. Best results should be obtained for polyalkylene oxides having a molecular weight between about 300 and 1,000, especially those which are liquid at room temperature.

Particularly preferred compounds are those derived from ethylene oxide.

Several typical compounds are illustrated by, but not limited to the materials set forth in Table III.

It is believed that these same polyalkylene compounds suitable as synergists for use with the amino complexes of the present invention also exhibit synergistic effects with respect to the organic zinc complexes described in the Horsewell U.S. Pat. No. 3,403,690, such as zinc acetylacetonate.

To prepare synergistic combinations of a zinc complex with a polyalkylene substance such as described above, the polyether is applied in conjunction with the inorganic complex onto the filter substrate. Appropriate steps are taken to assure thorough dispersion and co-mingling of the complex with the polyether synergist. Where those complexes are adequately soluble in the polyether or in a mutual solvent therefor such as water, preparation of an appropriate simple solution is convenient. However, if the complex or the polyether is insoluble or only partially soluble, it will be obvious to those skilled in the art that the suspension which results should be thoroughly homogenized before application to the filter substrate.

Thus, in preparing filters in accordance with the present invention, the inorganic complex is applied in a straightforward fashion, such as by dissolving it in water and applying it to the fibrous filter material by spraying a coating. Alternatively, other impregnating techniques may be used such as immersion, roll coating, syringe injection or other methods adaptable to commercial filter rod makers used to apply a liquid plasticizer to a filter tow.

In the absence of synergists, the filter is treated with a sufficient amount of the metal complex to provide effective HCN removal, usually from 1 to 8 mgs of metal ion per filter section for a typical cigarette. However, it has been found that one of the principal advantages of a polyether synergist according to the present invention is that the amount of metal complex required for effective HCN adsorption is reduced by from 50 to 75%. Thus, when used in combination with a polyether, it has been found most suitable to provide from 2 to 10 parts of the polyether for each part of metal ion and to apply from 0.2 to 4 mg of metal ion concentration per filter section for a typical cigarette. The most favorable concentration in combination with the synergist appears to be between 0.5 and 2 mg of metal ion.

Alternatively, filters can be prepared from the dry complex applied to the tow by any of the many available techniques developed for the application of solvents to cellulose filters. In another technique, the inorganic complex can be added at levels between about 0.5 to 10% by weight of an inert support such as pumice, Fuller's earth, powdered cellulose and the like, and 50 to 150 mg of this activated material pressed in a multisection cavity-type filter or applied as a solid to fibrous filter materials.

This invention will now be described in further detail with reference to specific embodiments thereof. Hydrogen cyanide analyses were run on a Technicon Auto Analyzer using the procedures of Collins et al., Tobacco Sci., 14, 12 (1970). The HCN values are the average of four determinations. Vapor phase analyses were performed by standard gas chromatography techniques.

Tris (ethylenediamine) zinc (II) chloride is prepared by adding 788 ml of ethylenediamine (3.0 molar equivalents) to an aqueous solution containing 533 gms of zinc chloride in 1800 ml of water. The plasticizing solution is Estrabond E (a mixture of 55 percent triacetin and 45 percent ethylene glycol 400). Filter rods (120 mm) were made at "standard" pressure drop and weight. The plasticizer solution and zinc complex solutions were applied separately to the tow with wick applicators. For a 20 mm filter section, an overall weight increase of 11.4 percent was observed with a zinc ion and polyethylene glycol concentration of 0.73 mg and 2.66 mg respectively for a 20 mm filter section. 85 mm cigarettes were then prepared containing a 20 mm filter section. Analyses for hydrogen cyanide and various other constituents are given in Table IV.

Filter rods were prepared using the same procedure as in Example 1 except that the delivery rate of the plasticizer and zinc complex solution was increased to afford a weight increase of 19.3 percent. Zinc analyses performed on a portion of the rods indicated 1.47 mg per 20 mm filter section. The polyethylene glycol level was estimated to be 5.3 mg. Analyses for hydrogen cyanide and various other constituents are given in Table IV.

Filter rods were prepared in accordance to the procedure outlined in Example 1, except that water was applied to the filter in place of the zinc complex. A 11.5 percent increase in rod weight was observed with the application of 2.75 mg of polyethylene glycol. Analyses for hydrogen cyanide and various other constituents are given in Table IV.

There is little or no adverse effect upon the taste of the smoke filtered with filters prepared by the procedures described in the above examples. Vapor phase results shown in Table IV indicate that HCN is selectively removed relative to other organic compounds.

Additional examples of the synergistic effects of a polyalkylene glycol are indicated in the following further tests:

The following series of experiments illustrate the difference in reactivity between copper, zinc and nickel complexes:

Ten grams of polyethylene 750 is added to a 30.1 ml solution containing tris (ethylenediamine) nickel (II) chloride prepared from 1 gm of NiCl.sub.2.6H.sub.2 O and 1.2 ml of ethylenediamine. 50 .mu.l of this solution was injected into a 20 mm filter section of a commercial 85 mm cigarette to give a concentration of 0.309 mg of Ni.sup.2+ ion and 12.5 mg of polyethylene glycol-750. Hydrogen cyanide analysis indicated 68 percent removal.

Ten grams of polyethylene glycol-750 is added to a 30.0 ml solution containing tris (ethylenediamine) zinc (II) chloride prepared from 1 gm ZnCl.sub.2 and 1.35 ml of ethylenediamine. 50 ul of the above solution was injected into a 20 mm filter section of a commercial 85 mm cigarette to give 0.59 mg of zinc and 12.5 mg of polyethylene glycol-750. Hydrogen cyanide analysis indicated 79 percent removal.

Ten grams of polyethylene glycol-750 is added to a 30.0 ml solution of bis (ethylenediamine) copper (II) sulphate containing 1.0 gm of CuSO.sub.4.5H.sub.2 O complexed with 1.00 ml of ethylenediamine. 50 .mu.l of the solution was injected into a 20 mm filter section of a commercial 85 mm cigarette to give filters containing 0.331 mg of Cu.sup.2+ ion and 12.5 mg of polyethylene glycol-750. Hydrogen cyanide analysis indicated 55 percent removal.

It will be understood that this disclosure is intended to cover any of the various complexes which exist in equilibrium when solutions of metal ions (M) are permitted to react with an organic ligand (am). There will be n such equilibria where n represents the maximum coordination number of the metal ion (M) for the ligand (am). For example, Zn.sup.2+ forms ethylenediamine zinc (II), bix (ethylenediamine) zinc (II) and tris (ethylenediamine) zinc (II) ions depending on the concentration of ethylenediamine and its equilibrium constants. For a general inorganic complex, these stepwise equilibria can be represented by the following equations and equilibrium expressions: ##EQU1##

The overall equilibrium Bn for these n independent equilibria is Bn = K.sub.1 K.sub.2 K.sub.3 . . . Kn where K.sub.1, K.sub.2, K.sub.3, are the individual equilibrium constants.