GB1561706A - Tobacco-smoke filters - Google Patents

Description

(54) IMPROVEMENTS IN TOBACCO-SMOKE FILTERS (71) We, MINNESOTA MINING AND MANUFACTURING COMPANY, a corporation organised and existing under the laws of the State of Delaware, United States of America, of 3M center, Saint Paul, Minnesota 55133, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us. and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to filters for tobacco smoke and in particular to filter tips for cigarettes and filters for cigarette holders and pipes.

It is generally accepted that in tobacco smoke there are many dangerous compounds amongst which are, for example, acrolein and hydros gen cyanide.

Many different types of filter tips for cigarettes and filters for incorporation into pipes, cigarette and cigar holders have been proposed and amongst them is activated carbon.

The objective of these filter tips has been to absorb what have been thought to be the dangerous constituents of the tobacco smoke.

However, the various types of absorbent proposed tend preferentially to absorb components other than polar compounds such as acrolein and hydrogen cyanide.

It is therefore an object of the present invention to provide a filter for use in cigarette smoking which will absorb the polar components of tobacco smoke.

According to the invention there is provided a method of ueating tobacco smoke in which the smoke is passed in contact with a filter comprising a black sorbent thermoset foam which has been prepared by the pyrolysis of a liquid composition comprising at least one aromatic compound containing a nitrogen atom as part of a substituent of the aromatic suucture or containing nitrogen atoms both as part of the aromatic structure and as part of a substituent on the aromatic structure. This thermoset foam can be incorporated as small discrete particles in the filter tips of cigarettes or alternatively can be incorporated in cigarette holders or in chambers in pipes so that the tobacco smoke has to pass in contact with the foam. It is particularly advantageous for the smoke filters according to the invention to contain both the thermoset foam and activated charcoal.

These thermoset foams are found to have excellent sorption for polar molecules such as acrolein, hydrogen cyanide and the organic acids present in tobacco smoke and they tend not to have high physical adsorption and so do not absorb other components of tobacco smile as strongly as carbon with the result that the flavour to the smoker is not impaired to any substantial extent.

Another important advantage of using thermoset foams as tobacco smoke filters in accordance with the invention is that polar molecules tend to be irreversibly sorbed by the thermoset foam, and not released as the smoke gets hotter and hotter towards the end of the cigarette.

A further advantage of the invention may be that the foams used according to the invention preferentially remove acids from the tobacco smoke. This may be important in view of the theory that acidic tobacco smoke such as that produced from tobacco containing a high proportion of sugars appears to form nico- tine salts and therefore may cause the smoker to inhale more deeply in order to obtain comparable nicotine satisfaction with that of alkaline smoke with a consequent increase in risk of lung cancer.

Thermoset foams which are useful in forming tobacco smoke filters in accordance with invention are, for example described in detail in our United Kingdom Patent Specification Nos. 1,475,879 and 1,475,880 and in our United Kingdom Patent Specification No.

1,531,218. For a full description of these foams reference is directed to these specifications and that application.

As described in Specification No. 1,475,879 the black sorbent thermoset foams are prepared by the pyrolysis of a liquid composition comprising at least one aromatic nitrogen-containing compound having the general formula: X-Ar-Y in which Ar represents an aromatic nucleus on which X and Y are substituents and which optionally carries additional substituents, X represents a substituent having a negative Hammett sigma constant and attached to the aromatic nucleus by a nitrogen, oxygen or sulphur atom and Y represents a substituent having a positive Hammett sigma constant and attached to the aromatic nucleus by a nitrogen atom, the substituents X and/or Y optionally forming part of a ring fused onto the aromatic nucleus, the thermoset foam having a specific surface area of at least 50 m2/g.

Also as described in that specification the black thermoset foam may be one which is prepared by pyrolysis of a liquid composition comprising at least one aromatic nitrogen-containing compound having the general formula: X-Ar-Y in which Ar represents an aromatic nucleus on which X and Y are substituents and which optionally carries additional substituents, X represents a substituent having a negative Hammett sigma constant and attached to the aromatic nucleus by a nitrogen, oxygen or sulphur atom, and Y represents a substituent having a positive Hammett sigma constant and attached to the aromatic nucleus by a nitrogen atom, the substituents X and/or Y option- ally forming part of a ring fused onto the aromatic nucleus, the thermoset foam being neither carcinogenic nor otherwise toxic to humans and substantially free from low molecular weight fusible toxic components and so capable of being safely handled by man.

As described in Specification No. 1,475,880 the black sorbent foam can have a very high specific surface area and can be prepared by pyrolysing a liquid mixture of at least one aromatic nitrogen-containing compound having the general formula: X-Ar-Y in which Ar represents an aromatic nucleus on which X and Y are substituents and which optionally carries additional substituents, X represents a substituent having a negative Hammett sigma constant and attached to the aromatic nucleus by a nitrogen, oxygen or sulphur atom and Y represents a substituent having a positive Hammett sigma constant and attached to the aromatic nucleus by a nitrogen atom, the substituents X and/or Y optionally forming part of a ring fused onto the aromatic nucleus, and which is liquid under the pyrolysis conditions, and at least one Lewis acid metallic salt, and removing the Lewis acid metallic salt from the material resulting from the pyrolysis.

As described in Patent Specification No. 1,531,218 the foam can be prepared by a process in which a liquid mixture of at least one aromatic amine and sufficient concentrated nitric acid are allowed to react at elevated temperatures to give initially the evolution of nitrogen oxides followed by a pyrolysis reaction to give an expanded solid which is a black sorbent cross-linked thermoset foam insoluble in organic solvents.

These black sorbent thermoset foams are believed to have a poly-quinoxaline structure and so it appears that they have the general repeating structure:

with cross-linking between the chains of these repeating fused aromatic rings, the positions of the cross-links being from the carbon atoms to which the hydrogen atoms are attached after removal of the latter. This suucture makes the thermoset foam very stable both to heat and oxidation. Thus a piece of the foam can be held in a flame and, while it will glow and gradually burn away, it will stop burning immediately it is removed from the flame and is not apparently decomposed by this treatment.

This structure is consistent with the black colour of the foam and with the elementary analysis of the foam which gives a nitrogen content in the range of about 12 to 20%.

In this foam structure it appears that the nitrogen atoms impart the chemisorption properties. In addition to this, the foam may, depending upon its manner of preparation, contain some functional groups.

The aromatic nitrogen-containing compound is pyrolysed by heating it in a relatively low temperature, e.g. around 2000 C, but once pyrolysis starts, an exothermic reaction occurs and it appears that the temperature of the overall mass may increase to a temperature of the order of 300" G. Within this mass there may be however, localised regions where the temperature is much higher. Once the exothermic reaction starts no further external heating is required, the heating to pyrolysis temperature being merely to initiate the reaction.

When reaction occurs there is a quite sudden and large expansion to give a sponge of the thermoset foam which may have an apparent volume many hundreds of times the initial volume. Therefore, unless careful steps are taken to ensure that the pyrolysis temperature is maintained or the thermoset foams is purified, the foam will contain unreacted material and low molecular weight condensed fusible species which are carcinogenic or otherwise toxic to humans if the foam is handled.

In order to ensure that the thermoset foam be non-carcinogenic or otherwise non-toxic, i.e. it contains no more than traces of starting material or low molecular weight fusible species, e.g. no more than 0.001% by weight of either, and even residual traces such as these are usually included within the body of the foam and therefore precluded from contact when the foam is handled, it can be prepared in the pure state if substantially complete pyrolysis is ensured by enclosing the material during reaction so that in spite of the sudden and large increase in apparent volume the reacting material is maintained at the pyrolysis temperature after formation, or by exhaustively purifying it as prepared with dilute acids, water, dilute bases, water and organic solvents.

If required one can both exhaustively purify and reheat the theromset foam to at least its pyrolysis temperature.

The black thermoset foams are prepared by pyrolysis of a liquid composition to give the solid foam and not by charring of a carbonaceous solid to give a solid char. Thus, the pore structure of the foams become formed during the pyrolysis reaction and is not already present in the material being pyrolysed. In effect a condensation reaction appears to occur during the pyrolysis and so the aromatic nitrogen-containing compound should have a structure which readily condenses or the liquid mixture which is pyrolysed should contain in addition to the aromatic nitrogen-containing compound one or more compounds which promote the condensation of the aromatic nitrogencontaining compound, e.g. because they are dehydrating agents for and/or co-condense with, the aromatic nitrogen-containing compound.

There are a large number of ways in which liquid composition containing the aromatic nitrogencontaining compound can be pyrolysed. Thus an aromatic nitrogen-containing compound which is liquid at the pyrolysis temperature, can be pyrolysed on its own or in admixture with other components to give a liquid mixture at the pyrolysis temperature i.e. a mixture which is composed of liquid components or a solution, of one or more compounds in the other or others. These other components are, as noted above, chosen to promote the condensation of the aromatic nitrogen-containing compound.As noted above one group of compounds which promote this condensation are believed to be dehydrating agents for the aromatic nitrogen-containing compound and examples are strong mineral or organic acids and strong bases while another group of compounds are those which will co-condense with the aromatic nitrogen-containing compound and assist in forming cross-links in the foam structure since we believe this tends to define the desired pore structure in the thermoset foams of the invention at an early stage in the pyrolysis of the aromatic nitrogencontaining compound.

Examples of suitable strong acids are mineral acids such as sulphuric acid and phosphoric acid or organic acids such as methane sulphonic acid or fluorinated methane sulphonic acid, while examples of strong bases are the mineral bases such as sodium and potassium hydroxide. One can of course employ latent acids, i.e. compounds which readily decompose on heating to give strong acids and examples are amine salts of strong acids such as (CH,),NSO, and CH3CBr3.

The aromatic nitrogen-containing compound can also be pyrolysed in admixture with an organic acid such as oxalic acid or adipic acid which appear to co-condense with the aromatic nitrogen-containing compound and may in some instances give foams of increased specific surface area. When they are used however, it appears to be desirable additionally to include a strong acid in the liquid mixture which is pyrolysed.

The aromatic nitrogen-containing compound can further be pyrolysed in admixture with a soluble salt, preferably in the additional presence of a strong acid or base and examples of suitable soluble salts are sodium sulphate, sodium chloride, sodium bisulphate and primary sodium phosphate. It is believed that these soluble salts act rather like a filler forming layers or pores between condensed reacted molecules during the pyrolysis to assist in defining the structures required in a molecular scale. Thereafter, removal of these salts, e.g.

by leaching leaves the required pore structure.

The aromatic nitrogen-containing compound can be pyrolysed with a Lewis acid metallic salt as described in Specification No. 1,475,880 when foam of specific surface area of over 100 m2/g and often as high as 9002/g or even higher can be prepared.

We believe that the Lewis acid metallic salts should be capable of coordinating with amine groups and examples include aluminium chloride and ferric chloride, and the more expensive boron trifluoride, silicon tetrachloride, titanium tetrachioride, tin tetrachloride, and germanium tetrachloride. The preferred Lewis acid metallic salt is, however, zinc chloride which currently appears to give the highest specific surface areas.

Although we are not certain we believe that the Lewis acid metallic salt may also act rather like a filler forming layers between condensed reacted molecules during the pyrolysis to assist in defining the foam structure required on a molecular scale. Thereafter removal of the salt leaves the required pores giving the thermoset foams of the invention their high specific surface areas. In addition the Lewis acid metallic salts may have a corrosive action upon the thermoset foams so forming additional voids or pores.

When the mixture of aromatic nitrogencontaining compound and Lewis acid metallic salt is heated to pyrolysis temperature, a sudden and vigorous reaction occurs with the formation of a voluminous black sponge of the foam.

These sponges can then - be crushed to give a powder which destroys the large coids in the sponges and the Lewis acid metallic salt leached or otherwise removed from the powder or sponge to leave a black thermoset foam according to the invention having a microporous structure. The removal of the Lewis acid metallic salts can be readily achieved by washing the foam with dilute acid followed by water. In addition the foam can thereafter be exhaustively purified by washing with a dilute base, water, and an organic solvent.

The pyrolysis can be effected by heating the reaction mixture to a relatively low temperature, e.g. around 2000 C, but once pyrolysis starts, an exothermic reaction occurs as described above.

..e specific surface area of the foam used according to the invention depends upon the aromatic nitrogen-containing compound or mixture of that compound and other components but in cases other than those described in British Patent Specification No. 1,475,880 until after the Lewis acid metallic salt catalyst has been removed from the foam, the specific surface area can be increased by further pyrolysis e.g. at temperatures of 300 to 800" C of the thermoset foam under a controlled atmosphere, e.g. a nitrogen atmosphere which may be saturated with steam. During this further pyrolysis weight loss occurs but specific surface area increases.

The larger the surface area the larger the quantity of material which can be sorbed.

Thus, the pure non-carcinogenic and non-toxic thermoset foam preferably has a specific surface area of at least 50m2/g and the specific surface area of the foams can be at least 200m2/g and as high as 900m2/g.

The large and sudden increase in volume which occurs upon pyrolysis to give the thermoset foam occurs as a result of the release of steam and other volatile materials during the reaction. The result is a sponge having large voids. These do not contribute significantly to the specific surface area, which is a measure of the microporous structure of the foams.

This specific surface area can be measured by standard BET methods as described by Brunauer, Emmett and Teller in the Journal of the American Chemical Society, 60 page 309 (1938). We believe that it is in the pores of this microporous thermoset foam that polar molecules are chemically sorbed and so the higher specific surface area the larger the sorptive capacity of the foam.

The aromatic nitrogen-containing material can be any aromatic compound containing a nitrogen atom as part of a substituent of the aromatic structure or containing nitrogen atoms both as part of the aromatic structure and as part of a substituent on the aromatic structure. Generally the aromatic nitrogen-containing compound contains at least one nitro-group together with an amino and/or hydroxy group. Such compounds include heterocyclic aromatic compounds containing additional nitrogen atoms within the aromatic rings and multicyclic aromatic compounds. It is preferred that the amino or hydroxy group be in the ortho or para positions relative to the nitro group.

Preferred aromatic nitrogen-containing starting compounds include, for example, pnitroacetanilide, 2 - nitroaniline, 4 - nitroaniline, amino- nitrotoluenes, aminoitroxylene, 2,6 dibromo or chloro - 4 - nitroaniline, 4 nitrophenylhydrazine, bis - (2 - nitrophenyl urea), and nitronaphthyl amines having vacant positions ortho to the nitro and amino groups or mixtures of these.

Although the aromatic nitrogen-containing compound can be pyrolysed on its own, it is preferred that a mixture of it and an acid such as sulphuric acid or phosphoric acid or a mixture of an organic acid such as oxalic acid with a mineral acid, or a mixture of a strong acid and an excess of a salt such as sodium sulphate, or a strong base such as sodium hydroxide or potassium hydroxide or a mixture of it with a Lewis acid metallic salt, be pyrolysed. Such mixtures are usually pasty liquids at room temperature and, as they are slowly brought up to the pyrolysis temperature, a homogeneous solution forms. Then suddenly the vigorous reaction occurs with copious evolution of gases, believed to be mainly steam, and a voluminous black sponge of the thermoset foam is formed.

This manner of preparation often tends to give thermoset foams which have, as prepared, relatively low specific surface areas, e.g. when pyrolysis is effected in the presence of sulphuric acid, but which can be given increased specific surface areas by, for example, further pyrolysis as noted above. When the pyrolysis is effected in the presence of phosphoric acid, the foams often have quite high specific surface.

Once the voluminous black sponge has beer formed it can be crushed to give a powder of the thermoset foam. This destroys the large voids in the sponge but retains the specific surface area which is constituted by the microporous structure of the foam. Where a salt such as a Lewis acid metallic salt is present this might be removed before the microporous structure of the foam is given. The foam powder can then be used or alternatively treated to purify it from carcinogenic materials or other toxic components or simply to increase its specific surface area.

To purify the thermoset foam, it can be powdered and the powder washed successively with an acid, e.g. dilute HCl, or base, e.g.

dilute NaOH, and an organic solvent such as acetone. Between each washing the powder can be recovered by filtration and washed with water. In this way all traces of starting materi als and low molecular weight fusible species can be removed. Alternatively or in addition the thermoset foam powder can be further pyrolysed under higher temperature, e.g.

400" C, and an inert atmosphere, e.g. a nitrogen atmosphere. This also tends to increase the specific surface area of the foam.

The thermoset foams can be incorporated into cigarette filter tips in, for example, the ways shown in United Kingdom Patent Specifications Nos. 795,666 and 908,663 by replacing preferably a part of the activated carbon particles shown in those two Specifications with particles of the thermoset foam, or alternatively the filter can be as shown in United Kingdom Specification No. 795,420 by replacing preferably a part of the activated carbon and silica gel with the thermoset foam. Reference is therefore directed to these specifications for full details.

As prepared the thermoset foams used according to the invention are usually fine powders. If used in this form in filters they may cause a large pressure drop which may be undesirable. Preferably therefore the foams are converted to a granular form with a binder an example of which is starch.

The efficiency of thermoset foams for use in cigarette filter tips was investigated by studying the acidity of the smoke from: (1) untipped cigarettes, (2) cellulose acetate/paper tipped cigarettes, and (3) cellulose acetate/paper tipped cigarettes with various amounts of powdered thermoset foam added to the tip.

Care was taken to prepare tips with closely comparable pressure drop since it is known that the efficiency of filter tips is dependent on the pressure drop across the filter. An aqueous absorption system as described in the Journal of the National Cancer Institute, Volume 48, No. 6 June 1972 pages 1885 to 1890, was used and the pH of the extract solution and the volume of 0.1N NaOH solution required to return the solution to pH 7 was found for each type. The results are shown in the following Table.

TABLE 1

I Type of Tip pH of extract amount of 0.1N NaOH to Cigarette (if any) solution give pH 7 (ml) Cigarette A None 4.85 1.20 Acetate 5.00 0.38 Acetate + lmg foam 5.25 0.22 Acetate + 5mg foam 5.65 0.10 Acetate + lOmg foam 5.35 0.14 Cigarette B None 4.70 0.75 Acetate 5.20 0.25 Acetate + 5mg foam 5.35 0.15 Acetate + 7mg foam 5.25 0.19 Cigarette C None 4.65 1.35 Acetate 4.80 0.34 Acetate + 5mg foam 5.10 0.21 The thermoset foam can thus be seen to have removed the acidic components of the cigarette smoke selectively including probably fatty acids, phenols and hydrogen cyanide. One advantage of this sorbent foam, is that it is much less likely to suffer deactivation by neutral compounds such as plasticizers.

It can be seen that, although the normal tip removed much of the acidity of the smoke an appreciable improvement could be made by the addition of as little as 5 mg of the sorbent foam.

Samples of thermoset foams prepared according to Specification No. 1,475,880 using ZnCl2 (Sample A) and according to Specification No. 1,475,879 using H2PO, (Sample B) were evaluated as cigarette filter material.

The samples were compared with the active carbon normally used in "Lark" and Tempo" brand cigarettes. The analysis of the smoke given below in Table 2 are expressed as a percentage of material found in the smoke stream as compared with the active carbon control.

TABLE 2

Component Sample A Sample B CO + 5 % - 8.5% HCN -- 47.5% -18.7% NOX -- 7.8% -- 4.2% Phenol +56.7% --9.0% Acrolein -- 30.0% - O % Acetaldehyde +l20 % + 1.8% Isoprene +149 % As can be seen from these results the use of a foam according to the invention preferentially absorbs polar molecules such as HCN and acrolein from the smoke.Therefore if the foam and active carbon are combined in a tobacco smoke filter many dangerous compounds can be removed.

As pointed out above, the specific surface area of the thermoset foams can be varied within a range by varying its manufacture and method of treatment. It may well be important to choose the particular specific surface area of the thermoset foams for use in absorbing tobacco smoke in accordance with the invention since if the specific surface area is too large the foam may remove some of the desirable flavour imparting constituents of the smoke.

It is therefore believed that a foam of a somewhat lower surface area e.g., from 50 to 600 m2/g is to be preferred.

The words "Lark" and "Tempo" are registered Trade Marks.

WHAT WE CLAIM IS: 1. A method of treating tobacco smoke in which the smoke is passed in contact with a filter comprising a black sorbent thermoset foam which has been prepared by the pyrolysis of a liquid composition comprising at least one aromatic compound containing a nitrogen atom as part of a substituents of the aromatic structure or containnig nitrogen atoms both as part of the aromatic structure and as part of a substituent on the aromatic structure.

2. A method as claimed in Claim 1 in which the filter additionally comprises activated charcoal.

3. A method as claimed in Claim 1 or Claim 2 in which the foam is as claimed in any of claims 1 to 5 and 24 of Patent No. 1,475,879.

4. A method as claimed in Claim 1 or Claim 2 in which the foam is as claimed in Claim 8 or Claim 9 of Patent No.

1,475,880.

5. A tobacco smoke filter comprising a black sorbent thermoset foam which has been prepared by the pyrolysis of a liquid composition comprising at least one aromatic nitrogen-containing compound.

6. A filter as claimed in Claim 5 which additionally comprises activated charcoal.

7. A filter as claimed in Claim 5 or Claim 6 in which the foam is as claimed in any of claims 1 to 5 and 24 of Patent No. 1,475,879.

8. A filter as claimed in Claim 5 or Claim 6 in which the foam is as claimed in Claim 8 or Claim 9 of Patent No. 1,475,880.

9. A cigarette having a filter tip which comprises a filter as claimed in any of claims 5 to 8.

10. A tobacco pipe which has a filter for the tobacco smoke which is as claimed in

Claims (1)

1. claims 5 to 8.

   11. A cigarette holder for use in smoking a cigarette which includes a filter for the tobacco smoke which is as claimed in any of claims 5 to 8.