INCREASED VOLUME SYNTHETIC FIBRES, PROCEDURE FOR PRODUCING THEM AND THEIR USE, IN PARTICULAR FOR FILTERS

Abstract: Description is made of an increased volume synthetic fibre consisting of a porous central core and a large number of short porous lateral threads integral with the core and distributed along the whole length of the fibre so as to form a voluminous branching struc­ ture. There is also a description of the procedure used to produce the fibre, which consists of the mixing of a synthetic polymer with an inflating agent which expands when heated, the spinning by means of melting the mixture which results in the inflation and fring­ ing of the threads produced, caused by the inflating agent, followed by the drawing and fixing of the threads thus produced while they are still hot. Examples of the use of the fibres in question are given by filters and padding.

Description: The invention regards increased volume synthetic fibres, the procedure used to produce them, and the use of the fibres, with special reference to the production of filters. It is known that synthetic fibres produced by means of spinning have a'compact' structure. For certain uses requiring fibrous masses with a certain degree of porosity, for example in the manufacture of filters, the porosity required is in reality that which can be obtained with non-woven fabrics or rovings, even carded ones, i.e. a porosity that is, so to speak, 'inter-filamentary', created by the interstices formed between the individual fibres that make up the non-woven fabrics. Known from FR-A-1 490 987 is a process for manufacturing fibrous products wherein rods or strips of polymeric foams are converted in the stretching operation to an integral fibrous network. Known from US-A-3 939 849 is a cigarette filter formed from foamed polypropylene rods. Now a new type of porous structure fibre has been found which is able to replace the porous fibrous masses so far used in all kinds of applications, with considerable advantages in terms of the consumption of materials and of cost. Therefore the main purpose of this invention is to create a new type of porous synthetic fibre enabling savings to be made in terms of materials and costs in applications which require the use of porous fibrous masses. Another of the purposes of the invention is to create a procedure for the production of the abovementioned porous fibres able to be performed using the same type of machinery traditionally used to produce conventional 'compact' fibres. The final purpose of the invention is to create one specific practical application of the fibres that are the subject of the invention, or more specifically, to create a filter for cigarettes. As part of this final purpose, the invention has the aim of creating a filter for cigarettes which is highly selective with regard to the tar contained in cigarette smoke and also possesses a high condensation capacity with regard to the various distillates of the smoke itself. Another important aim of the invention is that of creating a cigarette filter which satisfies the requirements of the user in terms of rigidity and which at the same time is able to increase the absorption of the products of combustion of cigarette tobacco. Another purpose of the invention is to create a cigarette filter which does not alter the taste of the cigarette, in terms of the tobacco, and which is at the same time easy to breathe through and able to filter effectively the harmful substances generated by the combustion of the cigarette. A further purpose of the invention is to obtain a high degree of condensate absorption for each cigarette, together with the capacity of the filter to hold a high degree of moisture, nicotine and tar. Yet another aim of the invention is that of creating a cigarette filter, and a procedure for manufacturing it, which as well as considerably reducing the costs of producing the filter also makes it possible to considerably increase the quality of the absorption of the harmful substances produced during the inhalation of the smoke. All these and other purposes, which will become clearer in the paragraphs below, are achieved by a synthetic fibre consisting of a porous central core and a large number of porous lateral filaments integral with the core but shorter than it; these filaments are distributed along the whole length of the fibre so as to form a ramified fibre structure with increased voluminosity. According to another aspect of the invention, these purposes are achieved by a procedure for producing increased volume synthetic fibres each consisting of a porous central core and a number of porous lateral filaments integral with the core and shorter than it; these filaments are distributed along the whole length of the fibre so as to form a ramified structure. This procedure is characterized by the fact that it consists of: a) the cold mixing of a fibre-forming synthetic polymer with an inflating agent b) spinning by means of melting the mixture formed as at a) in order to obtain the said ramified structure through the inflation and fringing of the fibres caused by the said inflating agent c) the drawing of the ramified structure fibres obtained as at b), and d) the fixing of the fibres by means of heating in a furnace. Finally, according to a further aspect of the invention the purposes set are achieved by a procedure for the manufacture of a cigarette filter consisting of the following phases: a) cold mixing of polypropylene with an inflating agent and a porogenous agent: b) melting and spinning of the mixture formed in a): c) drawing of the tow of polypropylene obtained in step b): d) preferably impregnation of the tow in an aqueous solution of stiffening substances containing, if necessary, a suspended porogenous agent; e) crimping f) stabilization of the tow by means of heating in a furnace; g) treatment with plasticizer; h) making up into cylindrical shapes for cigarette filters. The new type of porous fibres according to the invention have a special ramified structure due to the presence of a porous central core and a large number of short lateral porous filaments, shorter than the core but integral with it, distributed uniformly along its whole length. This special fibre structure is achieved by means of the procedure which is one of the subjects of the invention. Thanks to the inclusion of the fibre-forming polymer, the inflating agent and the subsequent heating during the phase of melting the mixture for the spinning of the fibre, the inflating agent first forms gaseous compounds trapped in the fibre in the form of micro-bubbles which, following further expansion due to the heating, "explode" and cause the fibre to fringe, at least on the surface, with the consequent formation of the previously described porous ramified structure. The subsequent phase of drawing the fibre completes the "fringing" effect of any remaining micro-bubbles giving rise to the finished ramified structure of the fibre in question which is then fixed by means of heat treatment, as happens with traditional fibres. It has been discovered that the best results are achieved by preparing the fibres to which the invention from polypropylene or from copolymers of propylene with ethylene in various proportions, such as, for example, the commercially available copolymers which contain up to 50% of ethylene in the copolymer. Thanks to the intrinsic properties of these polymers and in particular to their visco-elastic properties and their consequent high resistance to elongation and high tensile strength, the process of "fringing" and ramification does not lead to the breaking of the central core of the fibre which in practice acts as the carrying frame for the structure obtained. The fibres according to the invention are therefore more voluminous than traditional fibres and also than mechanically carded fibres, for example, with the result that it is possible to achieve the same degree of porosity by using a smaller quantity of polymer to produce the fibre (for example. a quantity at least 10% smaller by weight) or to obtain a considerably greater amount of porous fibre with an equal amount of polymer. In the spinning process required to obtain the fibre as per the invention, a large number of fibres is obviously obtained, as with all spinning processes. Thanks to the special nature of the process which includes the inflating agent in the fibre forming polymer, and as a result of the fringing effect of the said inflating agent, the ramified structure of each fibre will interpenetrate that of the adjacent fibres, so that rovings are obtained which are directly suitable for many kinds of application involving the use of porous fibres. The inflating agent to be used according to the invention may be one of a series of compounds acting chiefly as expanding agents, in particular, azobicarbonamide, 4-4-hydroxybis((benzenesulphonyl)hydrazide, ammonium carbonates and bicarbonates and/or alkaline metals. Of this particular preference is given to azobicarbonamide since it gives rise to extended ramification of the fibre. The process as per the invention is preferably performed by mixing the polymer and the inflating agent in a weight ratio of from 0.05 to 1.0%. The spinning through melting is preferably performed by using special "X" or "Y" profile dies at a temperature which varies according to the specific polymer in question, but which for polypropylene and its copolymers is generally from 260 to 310°C. The drawing of the fibres thus obtained is generally carried out with a drawing ratio from 1:2 to 1:3, while fixing is performed in the traditional way (for example in a furnace at a temperature of approximately 105-130°C). The fibres or tow obtained by means of the procedure as per the invention can be used in all applications that until now have required the use of porous fibrous masses, in particular filters, padding, etc. In this type of application the fibrous mass can be added to by means of additives, adjuvants, auxiliaries, etc., selected according to the specific use required. Thus, one particular application of the fibres that are the subject of the invention is, for example, that of the preparation of filters for cigarettes, as is described in more detail in the paragraphs below. As mentioned previously, the procedure for the preparation of such a filter implies the addition during the mixing phase a) of a porogenous agent as well as the inflating agent. The porogenous agent, as per the invention, can be mixed to the polymer, for example to the polypropylene before spinning, during the cold mixing step, or it can be applied to the fibre at a subsequent step, after the forming of the tow of drawn spun fibres. Porogenous substances that are especially suited for the invention are: calcium carbonate, talc and amorphous silica. The particle size of the amorphous silica is preferably less than 1 micron. One of the porogenous agents that is particularly active in holding back the harmful substances contained in cigarette smoke is calcium carbonate. After the spinning phase, the porogenous agent is distributed statistically on the threads of both the core and the lateral filaments. This makes each thread highly absorbent and at the same time extremely rigid. Moreover, the filaments tend to join together, by means of the lateral threads, thus giving rise to a tow of polypropylene which acts as a support, inasmuch as it has a large number of interstices inside it, for the other absorbent and sizing substances used in the impregnation phase during the "foulard" bath. The mixtures used during the finishing phase contain porogenous substances, such as, for example, CaCO3 prepared in particular with anti-static and lubricating substances, such as stearic acid, bathed in antistatic oleating substance, such as polyethyleneglycol, and with the addition of absorbent sizing substances such as starch. These mixtures enable the product to undergo crimping to increase the voluminosity of the polypropylene tow, without problems of processability. Moreover, during the filter making phase the CaCO3 does not become powdery, thanks to the stearic acid coating the individual particles. Another advantage is that as well as bonding the filaments together these substances also absorb the products contained in the tobacco smoke and the product thus obtained effectively condenses the distillates of the smoke so that they can be cooled as a result of the large number of interstices that exist between the various threads that make up the filter itself. The polypropylene tow, preferably having a total count of from 3300 to 6100 tex (30000 to 55000 denier) and formed by fibres which each have a count of from 0.3 to 0.94 tex (3 to 8.5 denier), is then inserted into a filter-making machine which advantageously has the rollers in a closed position at 1×10⁵-1.8×10⁵Pa (1-1.8 atmospheres); in addition, polyvinyl pyrrolidone is used as a plasticizer during the filter-making phase in order to achieve greater cohesion. The procedure for the manufacture of the cigarette filter, which is the subject of the invention, consists of the following phases. Cold mixing of the various components in the form of flakes and highly stereospecific base polypropylene with MI = 12 in a slow mixer at a temperature of approximately 20°C for a period at about 30 revolutions/min. Then spinning by melting is performed at a temperature of between 260°C and 310°C with the use of the previously described "Y" or "X" section dies in order to create the lateral threads on the individual filaments which are charged with porogenous agents such as, for example, calcium carbonate. After spinning the filaments are treated with anti-static substances and then bathed in water to eliminate as much of the anti-static as possible in order to create a product that is non-toxic. After the water bath, the polypropylene tow is passed over the first roller (a slow roller) at a temperature of 60°C - 80°C and is then drawn in a steam furnace at 120°C with a drawing ratio of between 1:2 and 1:3, before being fed onto a second roller (a fast roller) at a temperature of approximately 120°C. After the tow has passed over the fast roller at a temperature of 120°C, it is subjected to a "foulard" bath in an aqueous solution of lubricants, for example polyethyleneglycol, which also consists of porogenous inorganic charges, CaCO3 prepared with lubricating, anti-static stearic acid, and sizing substances such as starch; the concentrations of the abovementioned substances varies according to the charge required to obtain filters of the compactness desired. The subsequent phases of the procedure consist chiefly of phases of the mechanical type, such as for example the wringing of the tow and the crimping of the same to increase its voluminosity, forming on it about 5-8 waves/cm. Finally the tow is stabilized in a furnace at a temperature of 105-130°C at a speed of approximately 2-5 meters per minute, followed by packing with presses and the unwinding of the polypropylene tow in order to be fed into an opener on the filter making machine which will have rollers closed at a pressure of 1×10⁵-1.8×10⁵Pa (1-1.8 atmospheres); cellulose paper is also used for the making of the filters. As an example, we give below an example of the manufacture of a filter: 1) Mixing for 15 minutes in a Battaggion type slow mixer at 30 rpm at a temperature of 20°C of the following components: 1a) 97.8% of polypropylene, fibre type, highly stereospecific, melt index = 12, containing: 0.2% of calcium stearate (anti-acid) 0.15% of heat stabilizer (anti-oxidant) 1b) 2% of white flake containing: 1% of TiO2 0.5% of CaCO3 0.5% of low density polyethylene, MI = 20 1c) 0.2 azobicarbonamide flake , containing: 0.08% of azobicarbonamide 0.12% of low density polyethylene MI = 20 2) Melting and spinning of the tow under the following conditions: 2a) Use of a temperature profile of: 260°C in the feed zone 290°C in the body of the extruder 300°C in the filter zone 290°C in the die zone 2b) Use of a filter before the die composed of three 10,000 mesh/sq.cm. mesh filters. 2c) Use of "Y" section dies Tow on output from die with 2.26 tex (20.4 denier) per hole. 3) Treatment with an anti-static product 4) Hot drawing in steam at 120°C with a temperature of 80°C before the furnace and a roller temperature after the furnace of 120°C. Drawing ratio 1:3. The characteristics of the tow thus obtained are as follows: - count of each filament: 0.75 tex (6.8 denier) - total count: 3929 tex (35,360 denier) 5) "Foulard" bath treatment of tow In an aqueous solution containing: 5a) Starch solution in water with traces of diluted acetic acid to assist hydrolysis into: maltose (C₁₂H₂₂O₁₁)and Dextrin (C₆H₁₀O₅) n where n = 50 - 60. 5b) 50% solution of polyethyleneglycol (steeping) and 50% CaCO3 with stearic acid on the outside of each particle (this is necessary for lubrication purposes during the subsequent crimping phase and to avoid the powdering of the CaCO3 when the tow is put into tubes of cellulose paper to make the filter). 5c) Solution for 'foulard' bath After mixing the two solutions, the solution for the 'foulard' bath must contain, in total: 8.3% starch 8.3% CaCO3 25% polyethyleneglycol, having 600 monomer units 58.4% water 6) Wringing 7) Crimping to create 8 waves/cm. of undulation on the filaments. 8) Passage through furnace at 105 - 110°C to stabilize the tow at a furnace belt speed of 3 meters/ min. 9) Packing of the tow in order to avoid folds and/or twists. 10) The total count of the tow , with the addition of starch and CaCO3, becomes: 4840 tex (43,600 denier). The table below gives comparative data between the analysis of a filter as per the invention as prepared in the way described above and a traditional cellulose acetate filter. FUNCTIONAL ANALYSIS OF A P.P. FILTER AS COMPARED WITH A CELLULOSE ACETATE FILTER P.P.FILTER CELL.AC.FILT. Length of cigarette (mm) 84 84 Weight of cigarette - filter (gr) 1080 1080 Weight of tobacco cylinder (gr) 0.925 0.9235 P inhalation of cig. (Pa) (mm/water) 980-1080 (100-110) 980-1080 (100-110) Relative humidity of environment of smoke test (%) 12.46 13.16 P inhalation only of 20 mm. filter (Pa) (mm/water) 400 (41) 480 (49) no. of inhalations by smoke machine needed to finish cigarette (no. of inhalations) 13.7 13.5 Residue of filter and cigarette after smoke test (mm) 28 28 Condensate + Humidity + Nicotine (mg/cigarette) 38.27 34.77 Humidity held in filter (mg/ cig.) 4.37 4.26 Nicotine held in filter (mg/ cig.) 1.39 1.26 Tar held in filter (mg/cig.) 32.5 29.25 Weight of one filter (mg) 177.5 156.5 Total count (tex) (denier) 4847 (43,620) 4000 (36,000) In practice it was shown how the cigarette filter and the procedure for producing it are particularly advantageous with regard to the reduction of the amount of tar contained in the tobacco smoke and to the high condensation of the distillates of the smoke to enable cooling in the large number of interstices between the various filaments that make up the thread itself. As the invention has ben conceived it can be modified in many ways and still remain within the sphere of the concept of the invention; moreover, all the details can be replaced by technically equivalent elements. In practice any materials can be used and any dimensions adopted according to the requirements of the state of the art.

Claims: Synthetic fibre characterized in that it comprises a porous central core and a large number of porous lateral filaments integral with said core and shorter than it, said lateral filaments being distributed uniformly along the whole length of said fibre to form an increased volume ramified structure. Fibre according to claim 1 consisting of a polymer to be chosen from either stereospecific polypropylene or copolymers of propylene-ethylene. Tow of synthetic fibre comprising a plurality of fibres according to claim 1 on 2 wherein the ramified structure of each fibre interpenetrates with the ramified structure of the surrounding fibres so as to form a porous mass of increased voluminosity. Cigarette filter comprising a tow of synthetic fibres according to claim 3 wherein the fibres are made of polypropylene. Cigarette filter according to claim 4 wherein said tow is impregnated with particles of calcium carbonate. Cigarette filter according to claim 5 wherein said particles of calcium carbonate are treated with stearic acid. Cigarette filter according to any one of claims 4 to 6 wherein said tow is impregnated with polyethyleneglycol. Cigarette filter according to any one of claims 4 to 7 wherein said tow is impregnated with starch. Cigarette filter according to any one of claims 4 to 8 wherein said tow has a total count of from 3300 to 6100 tex (30000 to 55000 denier) and is formed by fibres which each have a count of from 0.3 to 0.94 tex (3 to 8.5 denier). Process for making synthetic fibres of Claim 1 of an increased volume ramified structure, said process being characterized in that it comprises: a) the cold mixing of a fibre-forming synthetic polymer with an inflating agent; b) the melting and spinning of the mixture formed as at a) in order to obtain the said ramified structure through the swelling and fringing of the fibres brought about by the action of said inflating agent; c) the drawing of the fibres of said ramified structure obtained as at b); and d) the fixing of the fibres by means of heating them in a furnace. Process according to claim 10 wherein the spinning of the mixture occurs through X-section or Y-section orefices. Process according to claim 10 wherein said polymer is chosen from between polypropylene and copolymers of propylene-ethylene and said inflating agent is chosen from among azobicarbonamide, 4-4-hydroxybis (benzenesulphonil) hydrazide, ammonium carbonates and bicarbonates and/or alkaline metals. Process according to claim 12 wherein said polymer is polypropylene and said inflating agent is azobicarbonamide. Process according to claim 10 wherein in the mixing step a) the weight ratio between said polymer and said inflating agent is between 0.05% and 1%. Process according to claim 13 wherein the melting and spinning is carried out at 260-310°C, the drawing is carried out with a drawing ratio of between 1:2 and 1:3, and the furnace fixing is performed at a temperature of 105-130°C. Process for making a cigarette filter comprising the steps of the process of claim 10 for making synthetic fibres of an increased volume ramified structure wherein said synthetic polymer is polypropylene and said process comprises the following steps: a) cold mixing of polypropylene with an inflating agent and a porogenous agent; b) melting and spinning of the mixture formed in a) in order to obtain the said ramified structure through the swelling and fringing of the fibres brought about by the action of said inflating agent; c) drawing of the polypropylene tow obtained in step b); d) preferably impregnation of the tow in an aqueous solution of stiffening substances, containing where necessary, a suspension of porogenous agent; e) crimping; f) stabilization of the tow by means of heating in a furnace; g) treatment with plasticizer; h) making into small cylinders for use as cigarette filters. Process according to claim 16 wherein the spinning of the mixture occurs through X-section or Y-section orefices. Process according to claim 16 wherein said porogenous agent is chosen from among amorphous silica with a particle size less than 1 micron, calcium carbonate and talc. Process according to claim 18 wherein said porogenous agent consists of calcium carbonate. Process according to claim 16 wherein said inflating agent is chosen from among azobicarbonamide, 4-4-hydroxybis(benzenesulphonyl) hydrazide, ammonium carbonates and bicarbonates and/or alkaline metals. Process according to claim 20 wherein said inflating agent is azobicarbonamide. Process according to claim 16 wherein said aqueous solution of stiffening substances contains starch and preferably traces of diluted acetic acid to assist the hydrolysis of the starch into maltose and dextrin. Process according to claim 22 wherein said aqueous solution contains polyethyleneglycol. Process according to any one of claims 16 and 18 to 23 wherein said solution contains a suspended porogenous agent, preferably formed by particles of calcium carbonate.

Inventors: CUNDARI, FRANCESCO
Applicants: S.P.T. S.R.L.
Application year: 1986
Main classification: D01F1
IPC codes: D01F6, D01F1, D01D10, D01D5, A24D3