NO773391L - FILTER MATERIAL. - Google Patents

Abstract

Filter material of non-woven cellulose ester, in particular as a tobacco smoke filter

Description

The present invention relates to a filter material with a sheet-like structure, particularly suitable for filtering tobacco smoke. The filter will effectively remove the harmful components in tobacco smoke, especially tar and nicotine, without forcing stronger puffs or suction on the cigarette. The invention also relates to methods for producing said sheet structures as filters.

The most important area of use for the material of the invention is as a filter for the removal of inhalable particles of all types and as a liquid filter. Used as a filter for removing inhalable particles, the filter according to the invention can be used together with a cigarette or another smoking item such as a pipe, cigar or cigarette or cigar holder. However, it will be clear from the following description that the filter material according to the invention can also be advantageously used in surgical masks, for air filtration and in face masks for protection against smoke, dust or "smog".

Many types of filter materials have been proposed, particularly to reduce the amount of harmful constituents in tobacco smoke that reach the user's respiratory system. In addition to being able to remove a relatively high proportion of the harmful components in the smoke, however, a satisfactory filter must also work without significantly obstructing the air flow or smoke flow through the filter so that too strong suction or draft is forced. For filtering tobacco smoke, the filter material must not change the taste of the tobacco smoke by introducing a flavor of its own. Another factor in the manufacture of a satisfactory two tray filter is that it must be able to be manufactured cheaply so as not to make the final smoking article too expensive.

Although a large number of fiber materials have been used as filter material in this connection, only fibers of wood pulp and cellulose acetate have gained significant market penetration.

Wood or paper pulp fibers are generally used in the form of a paper which is corrugated and/or compressed into a rod form for connection with a cigarette, while cellulose acetate is generally used in the form of a rope of substantially continuous longitudinal filaments which are preferably crimped so that short sections of the individual strands run randomly and non-parallel towards and apart from each other in relation to the rope's longitudinal direction.

Ordinary paper filters are most often characterized by a higher filtering capacity measured on the basis of the smoke removal effect, but will also adversely affect the smell and taste of the filtered smoke stream. Furthermore, the phenol selectivity of the paper filters will be significantly lower than when using ordinary cellulose acetate rope filters. Paper filters will also tend to collapse during smoking mainly because they absorb moisture from the tobacco smoke and the smoker's mouth. The compressibility of paper filters at a given pressure drop is also usually greater than that of ordinary rope filters of similar weight.

Compared to paper filters, ordinary cellulose acetate rope filters fulfill to an admirable extent the requirements set for good draft passage and cheap manufacture while avoiding all the aforementioned disadvantages of paper filters. Therefore, most of the filter cigarettes now on the market are equipped with this type of filter material, despite the fact that cellulose acetate rope filters have relatively less smoke removal effect than paper filters for a given penetration (suction strength, drag strength).

Many filter types are proposed for to avoid this disadvantage while retaining the advantages of the cellulose acetate and thereby arrive at a filter which has the advantages of both filter types in one filter in terms of high smoke removal effect and usable suction passage. A common method of achieving this is to combine cellulose acetate rope filters and paper filters into a single filter. This is done by placing a short stick of paper filter and a short stick of rope filter next to each other in a so-called "double filter". Such a filter will be relatively expensive compared to a regular "filter tip" since it is necessary to manufacture two separate rods and join them together to form a single filter. It has therefore alternatively been proposed, as in the U.S. patent 3,396,061, to join a web of cellulose acetate rope and paper tap immediately prior to filter forming. Combined paper/acetate filters can be produced using this method only slightly more expensive than normal filters. However, these filters are not considered completely satisfactory since there is a tendency for the two filter materials to be unevenly distributed over the filter's cross-section, which is an unfavorable system since two-bay smoke sucked through part of the filter will only come into contact with one filter material, while smoke that sucked through another part of the filter only comes into contact with the other filter material. Thereby, no part of the smoke will be filtered through both substances. The distinctly undesirable taste given off by a paper filter is also still present to a degree that corresponds to the amount of filter paper used in the filter.

Another method for utilizing cellulose acetate staple fibers in filters made only of cellulose acetate has consisted in orienting the fiber direction in the fiber structure perpendicular to the smoke flow. However, whether in the form of non-woven webs, felted mats or cylindrical rods, these fiber structures lack dimensional stability and require a binder to keep the strands in the desired order. These structures also do not have the effect of a normal paper filter because the surface area of the staple fibers is smaller than that of the paper fibers. The use of binders presents other problems as they reduce the available filtering surface area, add undesirable flavor to the filtered smoke and are a limiting factor in the manufacture of filters due to curing or setting time.

It is therefore an aim of the present invention to provide a sheet-like structure with staple fiber orientation, with a high surface area and for use as a filter material. Another purpose of the invention is to provide a method for producing materials suitable for processing into a sheet-like structure with a high surface area and without a binder. Another aspect of the invention is to provide a filter rod with a high surface area, which can be divided into

garett filters.

These and other features of the invention will appear more clearly from the following detailed description.

According to the invention, a sheet-like structure with a high surface area consisting of interwoven non-woven cellulose ester fibers is provided which maintains the fiber orientation without a binder and which exhibits a high filtration effect with acceptable suction. The filter material consists of a web of cellulose ester staple fibers and from approx. 5 to 35% based on the weight of the staple fibres, fibrillar cellulose ester material which gives rise to large voids, has a high surface area and is termed "fibrous".

The term "fibrets" in the present sense indicates a fibrillar cellulose ester material with a high surface area of over 5.0 m 2 per g, lengths of less than 1000 and diameters of

about. 0.0 to 50^u.

The term "binder" in the present sense indicates solvent-free substances that have the ability to glue the fiber material together by forming a contact mass of foreign origin on the fibers. This definition of "binder" especially excludes substances which are partial solvents for the fibers and which are more correctly termed plasticizers, such as e.g. triacetin, triethylene glycol diacetate and mixtures containing such and other plasticizing additives such as e.g. mixtures of polyethylene glycol and triacetin.

The term "high surface area" is used here for materials with a surface area of more than 1 m 2 /g. Comparisons between surface areas can be put into perspective by noting that paper suitable for filtration purposes has a surface area of 1.2 to 3.2 m 2 /g while cellulose acetate staple fibers of 6 mm length and with a filament denier of 1.8 , 3.0 and 8.0 have a surface area of 0.35, 0.26 and 0.20 m 2 /g respectively.

The staple fibers used for the production of the web are preferably cellulose ester staple fibers of the usual type with a fiber length of approx. 3 to 15 mm and one denier per filament on

about. 1.0 to 8.0. Preferably, the staple fibers have a length of

6 to 9 mm and a filament denier of approx. 1.0 to 3.0. The sheet-like material is wet-laid from an aqueous slurry of cellulose ester staple fiber and cellulose ester fiberlets (fibrets) using conventional wet-laying equipment for non-woven material. The sheet-like material preferably has a basis weight of 20 to 40 g/m 2 , a surface area of over 1 m 2 /g and an abrasion resistance of 200 to 1000 g per 5 cm. The sheet material is preferably corrugated before it is formed into cigarette stick material. When the rod is cut into lengths suitable as a tobacco smoke filter, it has a suction resistance measured as the air pressure drop across the length of the filter in the range of 30 to 200 mm water column at a flow of 17.5 ml/sec. for

20 mm filter length.

The invention will be more easily understood in connection with the attached figures where: fig. 1 shows a schematic drawing of a system for the production of fiber plates suitable for use according to the invention,

fig. 2 is a block diagram of a method for producing sheet material according to the invention,

fig. 3 is a vertical view of an arrangement for shrinking sheet material according to the invention and forming the shrunken material into a cylindrical filter rod,

fig. 4 is a photomicrograph of a cellulose acetate fiber according to the invention,

fig. 5 is a photomicrograph of a sheet material according to the invention.

Reference is made to fig. 1 where a schematic drawing illustrates the production of a highly fibrillated acetate fiber. As shown in the drawing, a supply of cellulose acetate dissolved in acetone, or possibly in acetic acid, is pumped through a capillary needle. The end of the needle is located in the constriction of a venturi tube carrying a coagulation fluid, usually hot or cold water. The high velocity of the water flow in the venturi throat portion serves to dilute the acetate solution stream and extract the solvent during the formation of a fiber. By changing the concentration of the solution, the water flow, the temperature of the water, or by adding other solvents to the water flow, the size, degree of fibrillation and length of the produced fiber can be regulated. Special apparatus and conditions for the production of fiber dishes according to the invention are shown in fig. 2. The apparatus in fig. 2 is also of the venturi type, however the acetate solution is extruded above the constriction in the venturi tube. Examples of fibers produced according to example 2 are illustrated in fig. 4 showing a photomicrograph at 600 times magnification. Fibers produced by this method or in previously known ways can then be converted into sheet material as shown in fig. 2.

One looks at fig. 2 is a block diagram illustrating a typical method using wet laying machinery for the production of non-woven sheet material according to the invention. As can be seen from the flow diagram, a slurry of fibers is fed from a proportioning vessel through a stirrer or mill to a storage tank where a slurry of staple fibers is introduced from a supply of staple fibers. In the mixing tank, the mass is stirred to a uniform dispersion and the amount of liquid is adjusted. The mixer supplies slurry to the inlet box above a fourdrinier machine where the wet web is laid down, runs through the dryer and is finally wound up on the roller. In fig. 5 you see an example of

a sheet-like product according to the method, in the form of a photomicrograph magnified 100 times. One looks at fig. 5 that the fibers are entangled in each other and in the staple fibers while forming a physical bond. This bonding effect is illustrated by the fact that non-woven sheets made of staple fibers and produced in the same way have practically no tear strength.

Sheet material from the final roll is then processed as shown in fig. 3. A sheet-like web 2 cut to a suitable width is fed from the feed roller 1 into the nip between a pair of corrugating rollers 3 with drift, where these corrugating rollers 3 form folds and partial breaks in the longitudinal direction of the web's direction of movement. Eliminators 4 for static electricity are placed below the corrugating rollers 3 to prevent the web 2 from being sucked up on the corrugating rollers 3. The corrugated sheet material 2 is fed over a crown roller 5 and to the inlet funnel

6 on a rod machine together with a wrapping strip 7 which is supplied by the roller 8. The sheet-like material which is harvested in the transverse direction and folded in the longitudinal direction and wrapped cylindrically exits the hopper 6 and the end 9 is supplied with adhesive material through the applicator 10, the the glued rod is passed through a heated forming tool 11 so that the finished filter rod 12 is formed. The entire apparatus is driven with the advantage of a conveyor belt 13 from drive rollers 14. The rod material 12 can then be divided into suitable lengths which can then be cut into cigarette filter plugs. As previously mentioned, the sheet material according to the invention consists of a continuous non-woven web of cellulose ester staple fibers and from approx. 5 to 35% cellulose ester fibers based on the weight of the finished sheet. It is preferred that the filter material contains from 10 to 20% cellulose ester fibres. It is also an advantage if both the staple fiber and the fiber plates are cellulose acetate. The filter material according to the invention is further characterized by a surface area of between 1 and 5 m 2 /g, a porosity AP through a circular sheet section with a 25 mm diameter of approx. 1 to 70 mm at a flow rate of 200 ml/min. Preferably, however, the filter material has a surface area between 2 and 5 m<2>/g.

As mentioned, the fibers used in filter material according to the invention can be produced by any of the known methods such as, for example described in the U.S. patent 3,342,991 or 3,441,473, which consists in air spraying a dilute solution of the cellulose ester into the air and then in a cooling bath of water or as described in U.S. patent 2,988,469 where a cellulose acetate solution is extruded in a fast-moving gas stream to produce fibers without hail formation. Production of fiber nets for use as filter material according to the invention preferably takes place as described in fig. 1 on the drawings or according to the description in the U.S. patent application serial no. 638,242 (1975) which uses a rotary spinning process as follows: 1) a cellulose ester solution is formed which preferably contains from 5 to 15 weight percent cellulose ester in a solvent containing from 2 to 20 weight percent non-solvent which is miscible with the solvent for the cellulose ester and from 80-98% by weight solvent for the cellulose ester which is miscible with said non-solvent

2) the cellulose ester solution is fed in a suitable manner, e.g.

by pumping through a rotary flange to the extrusion die

3) a substantially cylindrical disc revolving about its axis is placed in a heated precipitation bath containing non-solvent for the cellulose ester and up to 10% by weight of an organic solvent which can be mixed with said non-solvent 4) the precipitation bath is maintained at a temperature from approx. 60° C and up to below the boiling point of the selected non-solvent, preferably approx. 95° C 5) pumping in more liquid which has the same composition as the precipitation bath liquid into the precipitation bath and past the circumference of the rotating disc in a flow rate of at least 0.5 litres/min per extrusion opening, preferably at least 1.0 l/min per extrusion opening, 6) extruding the acetate solution through one or more openings or capillary needles which are placed along the circumference of the rotating disc or optionally placing a wall or plate around the circumference of the rotating disc and optionally extruding the solution through nozzles in the wall or along the circumference of the disc into in the precipitation bath, which extrusion nozzles or openings have a diameter of

0.1 to 1.0 mm 7) precipitating the cellulose ester solution in the precipitation bath and removing the precipitate from the bath, 8) boiling the fibrous precipitate preferably in water for at least 5 min, preferably 10 to 30 min, to remove residues of organic solvent and to expand and harden the porous fibril structure, 9)

homogenize the fiber plates and reduce the particle size of the cellulose ester fiber plates and 10) the cellulose ester fiber plates may optionally undergo a drying operation for complete or partial drying of the fiber plates prior to the production of the filter material according to the invention.

A cellulose ester such as cellulose acetate, cellulose triacetate, cellulose acetate butyrate, benzyl cellulose or mixtures of these is used. Preferred fibers are formed from cellulose acetate dissolved in a solvent containing two miscible components: an organic solvent such as acetone, methyl ethyl ketone, acetaldehyde or ethyl acetate and a liquid non-solvent for the cellulose ester, such as water, methanol or ethanol. The liquid non-solvent comprises from approx. 2 to 20 percent by weight of the mixture. The preferred organic solvent is acetone which is miscible with the preferred non-solvent which is water. The concentration of cellulose ester in the solvent mixture should be from 5 to 15 percent by weight and preferably 5 to 8 percent. Below 5 percent cellulose ester content, the process will be economically unprofitable. Because the organic solvent evaporates during the process and can give rise to various problems in recycling and the presence of flammable volatile solvents, it is beneficial to keep the organic solvent content as low as possible. However, by keeping the content of cellulose ester relatively low, the mixture has a relatively low viscosity and can be handled and extruded more easily without substantial resealing of the extrusion openings. Mineral additives such as TiC^/BaSO^ and A^O^ can be added to the solution if desired. These can be added in quantities of up to 50% by weight of the acetate as part of the solids content of the mixture, and are ground in a ball mill to a fine particle size.

Cellulose esters according to the invention are preferably produced from acetylated wood pulp with over 90% by weight hemicellulose. However, wood pulp of a lower quality can also be used, i.e. wood pulp with a hemicellulose content

from 5 to 10 percent by weight. It is therefore preferred to use pure cellulose esters. The purity is linked to the filtration capacity, which denotes the number of kilograms of dissolved ester that can be filtered through a typical first filtration medium in a factory plant before the medium clogs to an undesirable degree. Values of from 146 to 293 kg/m 2 filter surface are typical for marketed textile qualities of esters. Cellulose esters with sealing values of less than 146 kg/m 2 can, however, also be suitable for the production of fiberboard for the present purpose. This means that fibers according to the invention can be produced from cellulose esters with a lower degree of purity than is considered suitable for filament purposes.

When fibers for use in filter material according to the invention are produced by spray spinning as described in connection with fig. 1 in the drawings, high-pressure water is preferably used as the spray medium. By using water, a uniform spraying process is achieved without sealing problems and the dimensions of the precipitated product appear to be finer than with the product manufactured according to U.S. patent application 638,242 (1975). By changing the water flow and temperature, the length of the fibers can be changed. Cold water and/or high flow rates will reduce the fiber length. The solution preferably contains 10% solid dissolved in 90% acetone, 10% water. Solids concentrations of between 5 and 12% can be used, where the highest concentrations give a coarser and stringy material and low concentrations are economically unfavourable. The mixture of solvents can vary between 100% acetone to 60% acetone 40% water, with little effect on the product's properties. However, it will be understood that all cellulose ester solutions previously described are suitable in connection with the process in the U.S. patent application serial no. 638,242 (1975) will also be suitable for use in spray spinning as described in connection with fig. 1 on the present drawings. Mineral additives such as Ti02/BaSO^ and A^O^ can also be added if desired.

The preferred fibers according to the invention have an extremely large surface area per weight unit. While ordinary cellulose acetate filaments have a surface area of approx. 0.25

m 2 /g cellulose ester fibers used for the production of filter material according to the invention have a surface area of between 12 and 25 m 2 /g in most cases, but always have a surface area of more than 5.0 m 2 /g. As previously mentioned, the cellulose ester staple fibers used will preferably be of the usual type with a fiber length of approx. 3 to 15 mm and one denier per filament of approx. 1.0 to 8.0. It is an advantage if the staple fiber has a length of 6.4 to 9.5 mm and a filament denier of 1.4 to 3.0. The fiber cross-section may be a regular circular shape when extruded through a round die or other cross-sectional shapes formed by extrusion through non-circular die openings, Y-shape, X-shape or meat bone cross-section and the like. The cellulose ester staple agent can be selected from cellulose acetate, cellulose propionate, cellulose butyrate, cellulose benzoate, cellulose acetate formate, cellulose acetate propionate, cellulose acetate butyrate and the like. The esters can be matured and acetone-soluble like ordinary cellulose acetate or can be substantially fully esterified, i.e. contain fewer than 0.29 free hydroxyl groups per anhydroglycose unit, such as cellulose triacetate. The preferred cellulose ester staple fiber is cellulose acetate.

As mentioned in connection with fig. 2, sheet material according to the invention can be produced from a slurry of cellulose ester staple fibers and cellulose ester fibers in water. Between 5 and 35% and preferably 10 and 20% of fibers should be contained in the mixture based on the weight of the fiber material. The solid should constitute between 0.005 and 0.1% and preferably between 0.01 and 0.03% of the weight of the slurry.

The staple fibers and fiber dishes should be mixed thoroughly and distributed evenly in the slurry. This can be achieved by stirring or mixing either manually or with conventional mixing equipment. The staple fibers and fiber dishes can be added separately to the water and mixed. However, when the fiberboards are prepared with water as the cooking medium for expanding and hardening the fibrillar structure, the fiberboard/water cake, with the addition of extra water if necessary, can be used as a basis for the slurry of staple fiber/fiberboard/water, which in this case will be produced by adding the appropriate amount of staple fibers to the fiberboard/water mixture.

As previously mentioned, the slurry is deposited on a four-drinier wire in ordinary papermaking equipment to form a sheet material which has application as filter material, e.g.

in sheet form for use in face masks and respirators or in corrugated and compressed form for use as a cigarette filter. Cigarette straight filter rods which are produced from corrugated filter material according to the invention provide as good or higher filtering effect at a given pressure drop than filter rods made from a similar corrugated paper web, with a significantly improved taste.

The invention is further illustrated by the following examples where all percentages and quantities are on a weight basis and all temperatures are in °C where nothing else is stated.

Example 1. For the production of spin-spun fibers for sheet-shaped filter material according to the invention, a 7.5% solution containing cellulose acetate of fiber quality with an acetyl number of approx. 55, according to the following recipe:

by first mixing acetone and water and then adding the acetate flakes. The mixture is gently tumbled until the cellulose acetate is

completely dissolved. 20% by weight TiC^ trace material is added on the basis of the solution's dry weight (solids weight). Using apparatus as described in parallel U.S. application serial no. 608,416, the solution is placed in a storage tank and pumped through a line with a gear pump to a hollow shaft through a rotas ionic coupling into the inner part of a 113 cm hollow disk with a rotational speed of 2,900 rpm (peripheral speed = 1390 m/min). The disc is immersed in a precipitation bath for the solution, containing substantially water heated to 75 to 85° C. The hollow rotating disc has three 0.36 mm diameter openings along the circumference of the disc. The disc rotates with an annular wall or ring at a distance of 4.8 mm from the circumference. Water that maintains a temperature between 75 and 85° C is pumped into the precipitation tank and is passed through the annular space between the disc's circumference and the wall at a flow rate of 6.7 l/min per opening. Short, voluminous fibers with a high degree of fibrillation are created by the rapid precipitation of the cellulose acetate and the shear and tensile forces in the solution stream from the extrusion openings.

The precipitated material is swept to the surface of the precipitation bath and, in case of overflow, a collecting cloth where part of the water and acetone is separated from the fiber material. The collected material is boiled for approx. 20 min at a pressure of 1.5 kg/cm 2 to remove residual solvent and harden the hybrid structure. The fiber material is then dispersed again in water and introduced into a homogenizer of the type Gaulin 15M (Gaulin Corporation, Everett, Massachusetts, U.S.A.) where the fiber length is reduced at a pressure of 210 kg/cm 2 to approx. 500^u or less. The fiber material is suction-filtered and forms a cake containing approx. 12% by weight cellulose acetate fiber and 88% water. When the fibers are taken up from the precipitation bath, they have a relatively soft structure and a relatively short fiber length. After homogenization and heat treatment, the fibers are no longer soft and flowing but hardened and have a somewhat expanded and voluminous structure. The fibers generally have an uneven shape and vary in length from approx. 1 to 500 yU and in diameter 1 to

50 . The fiber boards and 6 mm staple fibers with denier per filament

= 1.8, 3.0 and 8.0 (Y cross section) were formed into sheets in a laboratory sheet former. The fibrets were then dispersed to 0.5% concentration in a Waring mixer and the slurry dispersed

further in 1000 liters of water containing 227 g of staple fibres. The slurry was pumped onto a 60 x 40 mesh brass screen moving at a speed of 1.65 m/min. A vacuum was applied under the sieve (20 to 25 cm Hg) to remove the water, and the resulting web was transferred to a felt web and finally to two dryers containing steam at pressures of 1.4 and 2.1 atmospheres. The 30 cm wide webs produced were 0.094 to 0.096 mm thick and had properties as shown in the following Table 1:

Strips were cut from these sheets or webs in widths as shown below and these were passed through heated corrugating rollers containing 10 teeth/cm, with a surface temperature of approx. 120° C. The corrugated strips were scraped together and wrapped with paper into filter rods with a length of 90 mm and a circumference of 25 mm. The rods were cut into 20 mm filter tips which were attached to 65 mm tobacco columns to measure the filtration efficiency (% removed by the filter) of total particulate matter (Solid Remover Efficiency = SRE), nicotine alkaloids (Nicotine Removal Efficiency = NRE) and " tar" as defined by the U.S. Federal Trade Commission (Tar Removal Efficiency = TRE). The results appear in the following table II:

The sheets or webs that contained fibers with a low number of deniers per filament seems to be better in several ways. The retention of fibers in the sheet machine is improved and you get a weaker and more easily corrugable sheet. The formed tip weights and pressure drop are reduced and for 1.8 denier/filament you get a significantly increased smoke removal effect.

Example 2. For the production of spray binding fibers for the filter material according to the invention, the solution according to example 1 is again used. A nozzle and spray apparatus from "Spraying Systems Company" no. 22B, U.S.A. is used, the solution is filled in the storage tank and pumped through a centrally located 0.10 mm extrusion die a quantity of 420 g/min. Felling

and dilution water with a temperature of 60 to 65° C is pumped through the three openings surrounding the extrusion nozzle at a rate of 9 to 10 l/min at a pressure of 12.6 kg/cm 2. The solution-water mixture exits through a nozzle with diameter 2.8 mm placed 3.56 mm from the dissolution nozzle into a tube filled with water where the fibers are precipitated. The fibrettes are collected and cleaned and formed into paper as described in example 1, except that the material is cooked at atmospheric pressure and homogenization is omitted.

Example 3. Example 2 was repeated except that the spray-spun fibrett material was exposed to atmospheric boiling and passed through a Gaulin homogenizer.

Example 4. Example 2 was repeated except that the syringe spinning material was boiled under a pressure of 120°C and homogenized through a Gaulin homogenizer. The samples from Example 2,

3 and 4 were formed into sheet-like webs and underwent tests, the results of which are shown in the following table:

* interpolated or extrapolated from data at higher and lower pressure drops *-\the high wear resistance is rinsed drying at a higher temperature with steam tank pressure of 2.4 5 and 3.15 kg/cm 2.

As you will see from the figures in Table III, boiling under pressure and homogenization will produce a finer, smaller-sized material that produces a weaker and easier to corrugate sheet which in turn improves the smoke removal effect at a given pressure drop.

In order to find the effect of the fiberboard content in the web material produced according to the invention, samples of the rotary bottom fiberboards according to example 1 and the injection-spun fiberboards from example 4 were combined with staple fibers and wood pulp fibers into webs containing between 5 and 100% fiberboards. The most important data appears in table IV below:

As can be seen from Table IV, the figures indicate that the fiberboard content is related to the sheet strength but does not have a greater effect on the smoke removal effect at a content lower than 30%. When the fibers are added in amounts of more than 30%, the sheet strength becomes so high that the corrugation process does not open up the structure sufficiently to make a good filter. Accordingly, an increasing reduction of the smoke removal effect takes place as the fibrate content increases above 30%. It is also found that the tip weight increases with increasing . fiber content, which would make large fiber content undesirable from an economic point of view. At a low fiber content of 5%, the smoke removal effect is high, partly due to the staple fiber used with a low denier per filament and partly due to the transparency of the corrugated sheet. The sheet strength is relatively low and the material is difficult to handle in sheet and rod form for the same reason.

Example 5. Staple fibers 3 denier/filament 6 mm length of cellulose acetate were used together with cellulose acetate fibers according to example 4 for the production of sheet material. A 30 cm wide laboratory fourdrinier machine with a 90 x 100 mesh wire cloth was used and the drying conditions were such that unpressed sheets were obtained (0.7 kg/cm 2 steam pressure on the drying rollers). The sheet material formed was then converted into cigarette filters according to the method of fig. 3 on the drawings. The figures from this example are shown in Table V:

Example 6. Cellulose acetate staple fibers 1.8 denier/filament, 6 mm, were used together with cellulose acetate fibers according to Example 4 to make sheet material. The staple fiber and fiberboard content in the output slurry was reduced to 113 and 32 g per 945 1 water, respectively. A 30 cm wide laboratory paper machine with 90 x 100 mesh wire cloth was used and the drying conditions were such that unslipped sheets were obtained (0.7 kg/cm<2> steam pressure on the drying rollers). The example is illustrated by the following table VI:

The figures from tables V and VI indicate that there is an optimal sheet weight for filters produced from these sheet materials in the range of 26 to 36 g/m 2. It will naturally be understood that the optimal sheet weight will vary depending on other physical dimensions that apply to the sheet.

In order to find the smoke filter effect of the cigarette filter rod made of sheet material according to the invention in comparison with filter rods previously known, a sheet material was made according to the following example 7: Example 7. Example 6 was repeated with a sufficient amount of fibers to form a sheet containing 17, 3% fibers and a sheet weight of 26.8 g/m 2. The sheet was longitudinally corrugated and hand rolled into a cigarette filter. The filter was judged in relation to marketed known filters. The most important data can be found in the following table VII:

As will be seen from the figures from Table VII, it is clear that the corrugated acetate sheet has superior smoke removal ability compared to the other types and is therefore preferred. When, however, for various reasons such as e.g. what smokers prefer, if it is desirable to change the manufacture of filters or reduce the filter effect, the sheet material can be needle-punched or torn rather than corrugated before it is rolled into a filter. Alternatively, wood pulp can be mixed into the cellulose fiber and cellulose staple mixture used to produce the sheet material. To demonstrate the aforementioned reduction of the filter effect, the sheet material from example 7 was needle-punched before hand-rolling, torn up before hand-rolling and wood pulp was added, after which the relevant data appears in Table VIII:

The most preferred method for adjusting the smoke removal effect for sheet material according to the invention is layered laying of corrugated sheet material and cellulose acetate rope before forming filter rods. The manufacture of such layered filters is described in detail in U.S. Pat. patent 3,396,061. However, it is preferred to corrugate the layered structure at the same time. When e.g. the corrugated sheet material from example 7 is layered and simultaneously corrugated, with 3.3 denier per filament cellulose acetate rope on a 50/50 weight basis, a filter weighing 0.154 g and pressure drop 78 mmf^O will give a smoke removal efficiency of 65.9%

To test the effect of fibrett-staple fiber blends

in sheet filters, sample sheets were made according to example 1, except that a hand former was used (Nobel and Woods sheet former). Acetate and polyester staple fibers with varying fiber content were used to produce the sheets. These were cut into 1 3/4 diameter slices and mounted in Cambridge filter holders to determine the smoke removal effect against tobacco smoke,

which in this case was simulated by an aerosol in the particle range 0.1 to 1.0. Two types of samples were made, one consisting of normal thin sheets (0.1 to 0.2 mm thick) and the other thick sheets (1.4 to 1.6 mm), which roughly corresponded to the pressure drop in normal face mask filters. For the thin sheets, pressure drop comparisons were made by stacking the sheets in the Cambridge holder. The known marketed material which was used for comparison consisted of discs cut out of face mask materials. One comparison material was a BM 2166 dust respirator sold by Mine Safety Appliances Co., Pittsburg, Pennsylvania, U.S.A. and the other consisted of glued

fibers used as molded face mask of brand TC-21C-132

(No. 8710) manufactured by Minnesota Mining and Manufacturing Co., Minneapolis, Minnesota, U.S.A. The results appear in the table

IX:

It is clear that non-woven staple fibers are effective for the removal of tobacco smoke particles in both thick and multilayered thin systems. Non-woven cushions according to the invention prove to have as good or greater smoke removal effects than previous materials in the area of high pressure drop.

Claims (26)

1. Sheet-shaped filter material of non-woven cellulose ester fibers characterized by a content of from 5 to 35% cellulose ester fibers. 2. Filter material as specified in claim 1, characterized in that the cellulose ester fibers are made of a material in the group cellulose acetate, cellulose triacetate, cellulose acetate butyrate, cellulose benzoate or mixtures thereof. 3. Material as set forth in claim 1, characterized in that the fibers consist of cellulose acetate. 4. Material as stated in claim 1, characterized in that it contains from 10 to 25% cellulose ester fibres. 5. Material as stated in claim 4, characterized in that the cellulose ester fibers are selected from the group of cellulose acetate, cellulose triacetate, cellulose acetate butyrate, benzyl cellulose or mixtures thereof. 6. Material as set forth in claim 4, characterized in that the fibers consist of cellulose acetate. 7. Tobacco smoke filter in the form of sheet-shaped non-woven cellulose ester filter material as stated in claim 1. 8. Non-woven filter material of fibrous cellulose ester containing from 65 to 95% cellulose ester staple fibers and from 5 to 35% cellulose ester fibrils. 9. Sheet-shaped filter material as stated in claim 8, characterized in that the fiber plates consist of cellulose acetate. 10. Filter material as stated in claim 8 containing from 10 to 25% cellulose ester fibres. 11. Sheet-shaped filter material as stated in claim 8, characterized in that the cellulose ester staple fiber is selected from cellulose acetate, cellulose propionate, cellulose butyrate, cellulose benzoate, cellulose acetate formate, cellulose acetate propionate, cellulose acetate butyrate and mixtures thereof. 12. Sheet-shaped filter material as stated in claim 8, characterized in that the cellulose ester staple fiber consists of cellulose acetate. 13. Sheet-shaped filter material as stated in claim 12, characterized in that the staple fiber has a denier per filament of 1.0 to 8.0 and a fiber length of approx. 3 to 16 mm. 14. Filter material as stated in claim 12, characterized in that the staple fiber has a denier per filament from approx. 1 to 8 and a fiber length of approx. 3 to 16 mm. 15. Tobacco smoke filter in the form of non-woven filter material of fibrous cellulose ester according to claim 8. 16. Method for producing a continuous and self-supporting sheet-shaped filter material of cellulose ester staple fibers characterized by a content of from 5 to 35% cellulose ester fibers. 17. Method for the production of a self-supporting, non-woven sheet-shaped filter material of cellulose ester fibers, characterized in that a slurry of cellulose ester staple fibers and cellulose ester fibers is prepared in water, the slurry is mixed into a homogeneous mixture of staple fibers and fiberboards, deposits the slurry on a porous surface that allows water to run off from the deposited fiber mat, dries the fiber mat and picks up the formed web or sheet from said porous surface. 18. Method as stated in claim 17, characterized in that the aqueous slurry contains fiber material consisting of from 65 to 95% cellulose ester staple fibers and from 5 to 35% cellulose ester fibers. 19. Method as stated in claim 17, characterized in that the cellulose ester staple fiber has a denier per filament of approx. 1.0 to 8.0 and a length of approx. 3 to 16 mm. 20. Method for producing a cigarette filter rod, characterized by producing a continuous, self-supporting, sheet-shaped filter material of cellulose ester staple fiber with. 5 to 35% cellulose ester fiberlets, corrugates the sheet, folds the corrugated sheet into a rod-like shape, and maintains the folded corrugated sheet in a rod shape by means of a wrap strip. 21. Method as stated in claim 20, characterized in that the corrugated sheet is layered with a web of cellulose acetate cigarette rope before folding into a rod shape. 22. Method for producing an opaque, voluminous fibrillar structure characterized by preparing a solution of cellulose ester in a solvent consisting of a non-solvent which is miscible with the solvent for the cellulose ester and a miscible organic solvent agent for the cellulose ester, leads the cellulose ester solution to an extrusion nozzle located in or above the constriction in a venturi tube through which a coagulating liquid is passed, which coagulating liquid is a non-solvent for the cellulose ester, whereby a precipitate is obtained in form of cellulose ester fibres. 23. Process as stated in claim 21, characterized in that the cellulose ester is cellulose acetate. 24. Method as stated in claim 22, characterized in that the coagulation liquid is water. 25. Method as set forth in claim 22, characterized in that the precipitate is homogenized to reduce the particle size of the precipitated cellulose ester. 26. Method as stated in claim 25, characterized in that the homogenized precipitate is boiled for at least 5 minutes.