Melt-blown nonwoven fabric, process for producing same and the uses thereof

The disclosure relates to a melt-blown non-woven fabric based on cellulose esters, with fibers of mean diameter less than about 10 microns. The fabric contains 0-10 wt. % extractable softener, has a reflection factor determined according to DIN 53 145 Part I (1992) of more than 60% and the cellulose ester has a degree of substitution DS of about 1.5-3.0. The softener is preferably water-extractable. A melt-blown non-woven fabric is produced with the cellulose ester as follows: a cellulose ester, cellulose acetate, with a DS of about 1.5-3.0, in particular 1.7-2.7, is mixed with softener in a weight ratio of about 2:1 to 1:4 and simultaneously heated and melted. The mixture of softener and cellulose ester has a melting index MFI (210/2.16) according to DIN 53 735 of about 400 to 5 g/10 min., in particular 300 to 50 g/10 min. The melt is worked in a melt-blown spinning device into a melt-blown non-woven fabric and the softener is then extracted with a softener solvent to leave a proportion of 0-10 wt. %. The melt-blown non-woven fabric is especially suitable as a filter material.

FIELD OF THE INVENTION
 The invention relates to a melt-blown formed fabric based on cellulose
 esters, in particular on cellulose acetate, with fibers of an average
 fiber diameter of less than approximately 10 .mu.m, a process especially
 suited for its production as well as advantageous applications of the
 melt-blown formed fabric.
 BACKGROUND OF THE PRIOR ART
 Melt-blown formed fabrics meet the ISO definition for formed fabric
 materials (ISO 9092:1988). According to it, a material is referred to as a
 formed fabric material if a) the fiber fraction is more than 50% by weight
 (except chemically broken down plant fibers) and the fibers have a
 coefficient of fineness greater than 300 or b) the following conditions
 are met: 1) the fiber fraction is more than 30% by weight (except
 chemically broken down plant fibers) and the fibers have a coefficient of
 fineness greater than 300 and 2) the density is less than 0.40 g/cm.sup.3.
 This ISO regulation is also observed by the formed fabrics explained in
 further detail in the following, with these being produced according to
 the melt-blown process or a melt-blown technique. Without wishing to see
 this as a restriction, the melt-blown process can be described as follows:
 i.e. the melt-blown filaments, fibers and formed fabrics are generally
 produced as follows:
 The particular synthetic material is placed into an extruder in which it is
 melted. From the extruder the melt is moved into the spinning head which
 comprises the melt-blown spinneret, which is the central component of the
 process. Here the melt is brought to the required processing temperature.
 The nozzle itself comprises a number of capillary bores. On both sides of
 the nozzle bores are disposed openings for the primary process air which
 is under pressure. Below the nozzle is a stacking arrangement in the form
 of a driven traveling screen or a revolving screen through which the
 fibers are drawn in and stacked to form a formed fabric.
 As the melt exits from the nozzle bores, it comes into contact with
 relaxing hot primary process air at high speed. In the process the melt of
 each capillary bore is torn apart and drawn into a large number of fine
 fibers. In this process the filaments largely are torn to form fibers.
 This is in contrast to other spin formed fabric processes in which fiber
 breaks must be prevented. Through the primary process air stream cold
 ambient air, referred to as secondary air, is drawn in and conducted to
 the fibers and filaments being formed. The generated filaments and fibers
 are consequently cooled directly under the spinneret. The fibers are
 subsequently stacked on the above cited stacking arrangement to form a
 formed fabric and are wound. Melt bonding between the fibers, as a rule,
 does not take place. The fiber lengths are, as a rule, of the order of
 magnitude of 5 to 50 cm. The fiber diameter is very small and, for example
 in connection with the invention described in the following, is less than
 approximately 10 .mu.m.
 Further information about the melt-blown process can be found in U.S. Pat.
 No. 3,825,379 (Exxon Research and Engineering Co.) as well as U.S. Pat.
 No. 4,714,647 (Kimberly Clark Corp.).
 U.S. Pat. No. 4,869,275 also addresses the melt-blown process for the
 production of a formed fabric from various starting materials. As suitable
 starting materials are cited polyolefins (polypropylene, polyethylene and
 ethylene/propylene copolymers), polystyrene, polyester (polyethylene
 terephthalate), Nylon (6, 66 and 610), polymethylene methacrylates and
 generally also cellulose acetate. This patent does not specify the degree
 of substitution of this cellulose acetate when used in the described
 process. The unusual reference that even cellulose acetate is suitable
 ("even cellulose acetate" s. column 5, paragraph 1) indicates that it is
 only conditionally suitable. This is also in agreement with the technical
 findings that the narrow temperature interval between melting temperature
 and decomposition range largely excludes the conversion of the cellulose
 ester into processable melts, for example in the case of cellulose
 triacetate, and, in the case of lower melting cellulose acetopropionates,
 is still connected to incipient product damage (cf. Kunststoff-handbuch
 3/1 Hansa Verlag, 1992, p. 411). If, in fact, cellulose acetate were
 processed into a melt-blown formed fabric at a high "melt temperature"
 which must be assumed, an undesirable strong degradation would occur. The
 degradation products would have a strong disadvantageous effect in various
 applications, thus in particular also when used as filter materials in
 tobacco smoke filters. Precisely this application is emphasized in U.S.
 Pat. No. 4,869,275. However, in the description of the especially
 practical embodiments, cellulose acetate is not taken into consideration.
 Due to the decomposition of cellulose acetate, which must be anticipated
 according to the known process, the quality of the obtained melt-blown
 formed fabric would also be impaired because no satisfactory degree of
 whiteness develops. In view of the decomposition of cellulose acetates at
 relatively high temperature, it should be pointed out that, beginning at
 180.degree. C., a marked chemical decomposition occurs which can be
 detected inter alia through the formation of furfural.
 According to Example 5 of U.S. Pat. No. 3,509,009 a portion of the
 cellulose acetate and a portion of diethylphthalate (as softening agent)
 are melt-spun at a temperature of 170.degree. C., so that decomposition of
 the cellulose ester used is largely excluded, but the product properties
 are dominated in an undesirable way by the softening agent. Such high
 content of softening agent restricts the application properties to the
 effect that too low a melting point is set as well as softening agent
 migration or exudation and exhalation can occur.
 SUMMARY OF THE INVENTION
 On the basis of the above described prior art, the invention is based on
 the object of further developing a melt-blown formed fabric of the above
 cited type such that it is not thermoplastic up to a temperature of
 approximately 180.degree. C., has a desirably high reflection factor or
 degree of whiteness and, if desired, can be used for advantageous filter
 materials, in particular for filter materials of cigarettes and for the
 filtration of gases or fluids, in particular of blood. Moreover, the
 invention describes an especially advantageous process for the production
 of such melt-blown formed fabric.
 According to the invention this object is achieved when the fabric
 comprises approximately 0 to 10 percent by weight of an extractable
 softening agent, has a reflection factor (R.infin.), determined according
 to DIN 53 145 Part 1 (1992), of more than approximately 60% and the
 cellulose ester has a degree of substitution DS of approximately 1.5 to
 3.0.
 The invention thus provides access to melt-blown formed fabrics comprising
 cellulose ester, which comprise little or even no softening agent, which
 previously could not have been considered to be possible.
 The melt-blown formed fabric according to the invention comprises fibers of
 cellulose esters. These can be, for example, cellulose acetate, cellulose
 acetobutyrate, acetopropionate and propionate and the like. Preferred is
 cellulose acetate.
 The degree of substitution DS of the cellulose ester used according to the
 invention is between approximately 1.5 to 3.0, in particular between
 approximately 1.7 to 2.7, wherein the range from approximately 2.2 to 2.6
 is especially highly preferred. If the value falls to less than 1.5,
 damage of the polymer skeleton through dehydration must be anticipated.
 The targeted goals can also be attained with a degree of substitution of
 approximately 3.0, however, at this value undesirable crystallization and
 phase separation can occur. These undesirable drawbacks can be
 counteracted with a higher content of extractable softening agent up to
 approximately 10 wt %, however, if a lower softening agent content is
 targeted, it is advantageous to lower simultaneously the degree of
 substitution DS to at least approximately 2.7, in particular at least
 approximately 2.6.
 In spite of the unusually good degree of whiteness, which will be discussed
 further, the melt-blown formed fabric according to the invention contains
 only up to approximately 10 wt. %, in particular approximately 2 to 8 wt.
 %, of an extractable softening agent, in particular in the form of a
 water-extractable softening agents. Consequently, the invention takes into
 account the relevant application purposes in which the fraction of
 softening agent cannot be too high since the product otherwise would be
 dominated in an undesirable way by the softening agent. Rather, the
 product properties should largely derive from the cellulose ester. The
 precise adjustment of the softening agent content within the specified
 framework of approximately 0 to 10 wt. % depends on the particular
 application of the formed fabric. Accordingly, it is left to the
 discretion of the expert to optimize the softening agent content
 quantitatively in individual cases within the scope of the invention. It
 has been found in using the melt-blown formed fabric in filter cigarettes
 to be desirable to adjust a softening agent content of approximately 5 to
 10 wt. %, in particular when as the softening agent triacetin is used. It
 is known, for example, that triacetin affects positively the taste of the
 tobacco smoke and the specific retentions of cellulose acetate. A content
 of softening agent exceeding 10 wt. % would restrict the application to
 the effect that too low a melting point would occur as well as softening
 agent migration or exudation and exhalation and, in addition, undesirable
 adhesion. Furthermore, in the event of its use in filter sticks a high
 softening agent content would have a negative effect on the hardness of
 the filter sticks. In applications subject to food law regulations the
 softening agent content is kept as low as possible within the scope of the
 invention, in particular to nearly 0. The same applies for medical
 applications, such as for example in blood filters.
 The softening agent used within the scope of the invention not only needs
 to develop a plastification effect. But, the softening agent, at the end
 of the production process must be present in a content above 10 wt. %,
 must be extractable from the melt-blown formed fabric with a suitable
 solvent such that the object of the invention of approximately 0 to 10 wt.
 % is set. In terms of their chemical and physical structure the cellulose
 ester fibers are to be largely unchanged in the process. As softening
 agent have proven to be suitable triacetin, ethylene and propylene
 carbonate, triethyl citrate, triethylene glycol diacetate, Carbowax.RTM.
 (polyethylene glycols of a molecular weight of 200 to 14000, produced by
 UCC, USA) and/or sulfolane (tetrahydrothiophene-1,1-dioxide). Triacetin is
 used with particular advantage since it can be extracted rapidly and
 effectively with water.
 The degree of polymerization DP of the cellulose esters, in particular of
 the cellulose acetate, is not critical and can be within a relatively wide
 range. However, special advantageous results are attained if it is between
 150 to 400, in particular between approximately 180 to 350. If the degree
 of polymerization falls below approximately 150, a too high fraction of
 oligomers would is present such that during the extraction of the
 softening agent, a large portion of the cellulose ester would
 simultaneously be extracted. If the upper limit value of approximately 400
 is exceeded, the melt index in the melt-blow process described hereinbelow
 becomes too high which would have a disadvantageous effect on the process.
 In individual cases this problem could be reduced by raising the content
 of the softening agent, but this would mean additional expenditures in
 practicing the invention, in particular in connection with the removal or
 recovery of the softening agent.
 Within the scope of the invention in view of the various fields in which
 the melt-blown formed fabric according to the invention can be used, of
 critical importance is a minimum reflection factor, also called degree of
 whiteness, of the formed fabric. The reflection factor or the degree of
 whiteness is measured according to DIN 53 145 Part I (1992) corresponding
 to ISO 2469 (1977). Herein an Elrepho device by Zeifs is used. A formed
 fabric sample folded in 8 layers one on top of the other is therein
 diffusely illuminated with an Ulbricht globe and measured perpendicularly
 to the sample plane (measurement geometry d/0) at 457 nm (by means of
 spectral band filters). Reference is here to the barium sulfate whiteness
 standard. The reflection factor or whiteness within the scope of the
 invention is more than 60%, in particular more than 70% or even
 approximately 90%. The whiteness is in particular a measure of the purity
 of the product according to the invention. If this were brownish or
 yellowish, this would mean that during the production undesirable and
 non-controllable decomposition products had been formed. For this reason
 the consumer would reject such product in the event of usage in the
 cigarette manufacturing industry. The disadvantage of an unsatisfactory
 whiteness degree can surprisingly also not be remedied by working in white
 pigments, such as titanium dioxide, during the production process. It is
 consequently an especially clear indication of the chemical purity of the
 cellulose ester fibers. This view point plays a predominant role in
 various areas, for example, when using the formed fabric in the biomedical
 field, in particular in blood filtration.
 It can in individual cases be of advantage that the cellulose acetate is
 present in the form of a polymer blend, in particular with aliphatic
 polyesters and/or acetylated starches. In this case not only the desired
 properties can be optimized, such as for example the biological
 degradability in connection with aliphatic polyesters (cf. in this
 connection DE-C 39 14 022) but, beyond that, the feasibility of saving
 costs. This is evident in another application area from EP-A 0 622 407 to
 which reference will be made.
 In order to attain the effects desired with the invention, the fiber
 diameter, such as is obtained in general according to the melt-blown
 process, must be less than approximately 10 .mu.m, in particular between
 approximately 2 to 8 .mu.m. The standard diameter of a filament obtained
 according to the dry-spin process, in contrast, is between approximately
 15 and 40 .mu.m. Fibers having a smaller diameter have the advantage that
 they have a greater specific surface and thus yield also greater activity
 in the desired application fields, in particular in filtration. Within the
 scope of the invention fibers of an average fiber diameter of less than
 approximately 8 .mu.m can readily be adjusted. The especially advantageous
 practical range is between approximately 5 and 8 .mu.m. The fiber diameter
 is the mean diameter. Here a number of fibers are measured using a
 scanning electron microscope and subsequently the mean value is formed.
 In principle, if desired, to the melt obtained after the melt-blown process
 according to the invention to be described hereinbelow, active substances
 can be added, such as for example agriculturally active substances,
 pharmacologically active agents, selective and other filtration aids, for
 example for the selective retention, aroma substances, additives for
 biological degradability, etc. They are preferably melt-compatible.
 The melt-blown formed fabric according to the invention can advantageously
 be produced when a cellulose ester, in particular cellulose acetate, of a
 degree of substitution of approximately 1.5 to 3.0, in particular of
 approximately 1.7 to 2.7, is mixed with a softening agent at a ratio by
 weight of approximately 2:1 to 1:4 while the mixture is being heated and
 converted to a melt, and the mixture of softening agent and cellulose
 ester has a melt index MFI (210/2.16) according to DIN 53 735 of
 approximately 400 to 5 g/10 min, in particular 300 to 50 g/10 min, the
 melt is processed in a melt-blown spinning device to form a melt-blown
 formed fabric and subsequently the softening agent is extracted with a
 solvent in which the softening agent is soluble such that a fraction of
 approximately 0 to 10 wt. % remains. In order to convert the starting
 materials into a melt, they are preferably heated to a temperature of more
 than approximately 100.degree. C. The especially suitable melt temperature
 depends on the individual case and can be determined by an expert solely
 conventionally. However, a temperature of 240.degree. C. should not be
 exceeded since otherwise undesired decomposition phenomena would occur.
 The melt-blown formed fabric obtained according to the invention comprises,
 as shown, a low fraction of extractable softening agent of approximately 0
 to 10 wt. %. Due to the way in which the process is conducted the
 decomposition of the cellulose ester used is largely eliminated. It is not
 required that work be carried out in a protective atmosphere to avoid
 undesirable oxidative processes. It is of advantage if the melt is
 subjected to the melt-blown process immediately after its production,
 since otherwise undesired degradation reactions can occur. Thus, a special
 advantage of the process according to the invention lies that it can be
 carried out continuously. Thus, the mixing and the spinning advantageously
 take place in a single process step so that the mixture from the extruder
 is supplied immediately to the melt-blown spinneret. The process according
 to the invention consequently represents a marked simplification with
 respect to the carrying-out of the process.
 For carrying out the melt-blown process according to the invention it is
 advantageous if the ratio by weight of softening agent to cellulose ester
 is adjusted to approximately 3:2 to 2:3, consequently in the practical
 embodiment preferably to approximately 1:1, which also corresponds to the
 demands of U.S. Pat. No. 3,509,009. However, the present invention differs
 in the process from the teaching according to U.S. Pat. No. 3,509,009
 because it absolutely requires the use of a suitable solvent for the
 softening agent. Accordingly, a solvent for the extraction of the
 softening agent is used according to the invention, which however, does
 not impair the chemical and physical structure of the cellulose ester
 fibers.
 The type of mixing of softening agent and cellulose esters, optionally with
 further additives, is not subject to significant restrictions. It has been
 found that the mixing of cellulose ester and softening agent is carried
 out especially advantageously in a twin-screw extruder. The shear
 necessary for optimum mixing of the starting materials is attained which
 leads to an especially advantageous homogenation of the starting material.
 It is preferred to use a parallel twin-screw extruder.
 The process according to the invention is controlled especially
 advantageously in the melt-blown spinning device if at the spinneret and
 the spinning head of the spinning device a temperature of approximately
 180.degree. to 240.degree. C., in particular of approximately 200 to
 230.degree. C. is kept. If the temperature is lower than approximately
 180.degree. C., the result can be an insufficient fineness of the product
 of the process. If the upper limit of 240.degree. C. is exceeded,
 undesirable degradation occurs.
 The softening agents usable within the scope of the invention have already
 been discussed earlier, in particular the advantageous use of the
 water-extractable softening agents in the form of triacetin. In the case
 of a water-extractable softening agent, the obtained melt-blown formed
 fabric is simply conducted into a water bath for the extraction of the
 softening agent. The process according to the invention can here be
 carried out with the special advantage that a normal water bath
 (approximately ambient temperature), i.e. without heating, can be used for
 the extraction. In the presence of high softening agent content, the
 application of a hot extracting bath is even of disadvantage since the
 melt-blown formed fabric in this case has a melting range such that its
 structure is impaired or even destroyed.
 It is especially advantageous if the formed fabric leaving the melt-blown
 spinning device is transferred to a stacking arrangement, in particular in
 the form of a screen or traveling screen or revolving screen, pressed to
 adjust the desired thickness, and subsequently the softening agent is
 extracted. It is in principle also possible to carry out the extraction
 before the molding. If desired, the melt-blown formed fabric can also be
 structured during the molding. The structuring takes place in order to
 obtain the structure advantageous for the later use, for example in the
 case of its use in cigarette filters, longitudinal fluting, in connection
 with surface enlargement.
 Lastly, it can in individual cases be advantageous to incorporate in the
 formation of the melt-blown formed fabric simultaneously filaments, in
 particular cellulose acetate filaments. Two options described in detail in
 DE 35 21 221 exist in principle. In this respect reference is expressly
 made to them. In general the incorporation of filaments leads to an
 improvement of the mechanical properties, in particular of the tensile
 strength of the material.
 It is also of special advantage if the melt-blown formed fabric leaving the
 spinning device is deposited onto filter tow processed so as to be flat or
 on paper for the formation of a compound structure on a base, in
 particular in the form of a formed fabric comprising a cellulose acetate
 filter tow. In the event a base of formed fabric is used, the expert,
 depending on the intended end use, can determine the formed fabric
 suitable in each case without any problems. For example, in the case of
 the further use of the melt-blown formed fabric according to the invention
 in filter cigarettes, preferably a cellulose acetate formed fabric should
 be used. But possible are also any closed support, such as for example the
 paper already cited. The compound structures obtained in each case can be
 advantageously molded and/or structured for regulating its thickness.
 A special advantage of the process according to the invention lies in the
 fact that the targeted melt-blown formed fabric can be produced without
 requiring special additive substances, such as for example any auxiliary
 processing agents.
 Based on its properties, the melt-blown formed fabric according to the
 invention is suited especially advantageously as filter material. The
 formed fabric, for example in tobacco smoke filters, in particular in
 cigarette filters, and especially in double filters for ultralight
 cigarettes, is used for the filtration of gases and liquids, such as for
 example sterile filtration of beverages as well as especially
 advantageously for the filtration of blood.
 If the melt-blown formed fabric according to the invention is used in
 cigarette filters, these are readily disintegratable. Furthermore, a low
 degree of substitution DS of the cellulose ester, in particular of the
 cellulose acetate, leads to especially favorable biological degradability.
 The filter materials according to the invention not only show a better
 filter effect than the materials known so far, they also meet without
 restriction the taste requirements. This applies in particular to
 cellulose acetate in connection with a residual content of triacetin
 softening agent.
 In the following the invention will be explained in further detail in
 conjunction with examples.

EXAMPLE 1
 Cellulose acetate having a DP of 220 and a DS of 2.5 was placed by means of
 a gravimetric dosing device into the charging opening of the first zone of
 a parallel twin-screw laboratory extruder with a screw diameter of 25 mm,
 a length of 48 D and 15 zones. In the second zone triacetin was supplied
 as the softening agent at a ratio of 2:3 (1:1.5) by means of a
 reciprocating piston pump. The temperature in zones 1 and 2 were
 30.degree. C., in the third zone the temperature was 110.degree. C., in
 the fourth zone it was 150.degree. C. The temperature of zones 5 to 11 was
 150.degree. C. and of zones 12 to 15 it was 175.degree. C. At a screw
 speed of 150 RPM a homogeneous melt was obtained. The melt obtained was
 converted via a round section die continuously into a strand and the
 latter was cooled below the melting temperature and reduced with the aid
 of a strand granulator into cylindrical granulates of 2 mm diameter and 3
 mm length. The granulate obtained was supplied to a melt-blown laboratory
 spinning device comprising an extruder, intermediate block, melt tube,
 spinning head spinneret, hot-air device, stacker and winder. The
 temperature in the extruder of the melt-blown laboratory spinning device
 was increased from 100.degree. C. at the inlet to 170.degree. C. at the
 extruder outlet. The intermediate block and the melt tube were set to
 200.degree. C. The temperature in the spinning head was 230.degree. C. The
 air temperature was 265.degree. C. The quantity of air was adjusted to 70
 m.sup.3 /h. At these process parameters a melt pressure of 125 bars
 developed. The weight throughput was 7.7 kg/h. The fibers generated with
 the spinning device were deposited on a receiving belt and continuously
 drawn off under the spinning device such that a weight per unit area of
 132 g/m.sup.2 was obtained. By means of a wind-up device the formed fabric
 was wound to form a roll. The roll of formed fabric was subsequently
 supplied to a washing device filled with water comprising two successive
 vats and the softening agent comprised in the formed fabric was rinsed out
 to a remaining content of 0.3%. The formed fabric was subsequently dried
 with a drying unit at 160.degree. C. up to a residual moisture content of
 4.8%. The mean fiber diameter of the formed fabric obtained thus was 8.4
 .mu.m. The reflection factor (R.infin.), relative to the barium sulfate
 white standard, was 65%.
 EXAMPLE 2
 Cellulose acetate with a DP of 220 and a DS of 2.5 was placed by means of a
 gravimetric dosing device into the charging opening of the first zone of a
 parallel twin-screw laboratory extruder with a screw diameter of 25 mm, a
 length of 48 D and 15 zones. In the third zone triacetin was added as the
 softening agent at a ratio of 3:2 (1.5:1) by means of a reciprocating
 piston pump. The temperature in the first and second zone was 50.degree.
 C., in the third 100.degree. C. and in the fourth zone 120.degree. C. The
 temperature of zones 5 to 10 was 140.degree. C. and of zones 11 to 15 it
 was 150.degree. C. The weight throughput was 3.2 kg/h. At a screw speed of
 190 RPM, a homogeneous melt was obtained. The melt obtained was supplied
 directly to a laboratory belt-blown spinning device described under
 Example 1, which, however, in contrast to Example 1, no longer required an
 extruder since the material to be processed was already present in the
 form of a melt. In this case the melt-blown spinning unit followed
 immediately the parallel twin-screw laboratory extruder. The intermediate
 block and the melt tube were set to 170.degree. C. The temperature in the
 spinning head spinneret was 210.degree. C. Air temperature was 255.degree.
 C. The air quantity was adjusted to 60 m.sup.3 /h. At these process
 parameters a melt pressure of only 73 bars developed. The fibers generated
 with the spinning device were deposited on a receiving belt and drawn off
 continuously under the spinning device such that a weight per unit area of
 176 g/m.sup.2 resulted. The formed fabric obtained in this way was
 conducted directly into a washing device described as in Example 1, and
 the softening agent contained in the formed fabric was rinsed out to leave
 a residual content of 5.5%. The formed fabric was subsequently dried with
 a drying arrangement at 150.degree. C. to allow a residual moisture
 content of 6.3%. The mean fiber diameter of the formed fabric obtained was
 5.7 .mu.m. The reflection factor (R.infin.), relative to the barium
 sulfate white standards, was 74%.