JP7584506B2 - Method for producing fibers containing meta-aramid - Google Patents

Description

The present invention relates to a method for producing meta-aramid fibers by preparing a spinning dope containing meta-aramid and sulfuric acid and passing the spinning dope through a spinneret, and to meta-aramid fibers obtained by the method. Furthermore, the present invention also relates to meta-aramid fibers having a sulfonic acid group content of 1 meq/kg or more, multifilament yarns containing the fibers, and fabrics and protective clothing.

The method of spinning aramid fibers is well known.

KR20100001782 describes aramid fibers and a method for producing the same. In the spinning method, various para-aramid polymers (paraphenylene terephthalamide, paraphenylene-4,4'-diphenylenedicarboxylic acid amide, paraphenylene-2,6-naphthalenedicarboxylic acid amide, etc.) can be used, in which para-aramid is dissolved in sulfuric acid, the spinning solution is introduced into a coagulation bath through a spinneret, and the filaments are washed, dried, and wound up. The time in the coagulation bath may be adjusted to improve the circularity of the cross section of the filaments. KR20100001782 does not describe dissolving meta-aramid in sulfuric acid.

A method for spinning meta-aramid fibers is disclosed in US 3,094,511. This document discloses a dry spinning process in which meta-aramid is dissolved in an organic solvent and passed through a spinneret with a heated air column. In the latter process, evaporation of the solvent occurs.

As described in EP 0226137, etc., a meta-aramid wet spinning method is also known in which the spinning solution is passed directly from a spinneret into a coagulation bath. Organic solvents are also used in the wet spinning method.

The use of organic solvents has several disadvantages. Legislation may in the future prohibit the use of organic solvents or allow their use only under more stringent conditions for environmental reasons. Also, fibers spun using organic solvents require extensive washing to remove the solvent, which is not economical. Meta-aramid fibers spun from organic solvents contain organic solvents, despite extensive washing.

Therefore, there is a demand for a method of spinning meta-aramid that does not use organic solvents.

KR20140075197 relates to a meta-aramid composition in which an antioxidant is added to the spinning dope to prevent discoloration. KR20140075197 describes that meta-aramid fibers are produced after dissolving the meta-aramid composition at a concentration of 20% in sulfuric acid at a concentration of 99%. After passing through a spinneret, the spinning dope is directly put into a coagulation bath without passing through an air gap, and the filaments are washed, dried, and wound. KR20140075197 does not disclose meta-aramid fibers having a breaking strength of 300 mN/tex or more.

The object of the present invention is to provide a meta-aramid fiber that is substantially free of organic solvents and has good mechanical properties, particularly high breaking strength. It is also an object of the present invention to provide an improved meta-aramid fiber, particularly a meta-aramid fiber with improved flame retardancy and cross-sectional uniformity.

In order to achieve the above object, the present invention provides a method for producing fibers containing meta-aramid having a breaking strength of 300 mN/tex or more, the method comprising the steps of preparing a spinning dope containing meta-aramid and sulfuric acid having a concentration of 80 wt% or more, and feeding the spinning dope through a spinneret into a coagulation bath, wherein the spinning dope has a meta-aramid concentration of 10 wt% or more (based on the weight of the spinning dope).

Preferably, the fiber containing meta-aramid has a breaking strength of 350 mN/tex or more, more preferably 400 mN/tex or more, and even more preferably 450 mN/tex or more. In addition, the method can produce a fiber containing meta-aramid having a breaking strength of 500 mN/tex or more.

The breaking strength of fibers containing meta-aramid is measured in accordance with ASTM D7269-17 for multifilament yarns and ASTM D3822 for single filaments.

Fibers containing meta-aramid are also sometimes called meta-aramid fibers.

For purposes of this invention, the term meta-aramid refers to a class of fully aromatic polyamide polymers and copolymers having 70% or more, preferably 80% or more, and more preferably 90% or more meta-oriented bonds between aromatic moieties. In one embodiment, 95% or more or all (i.e., 100%) of the bonds are meta-oriented bonds. Thus, the amide bonds between the aromatic moieties are substantially arranged in meta or near meta-oriented positions on the aromatic rings (e.g., as in 1,3-phenylene or 1,3-naphthalene groups).

The meta-aramids of the present invention may have repeat units of formulas I and II:
-[-NH-Ar1-NH-CO-Ar2-CO-]- (Formula I)
-[-NH-Ar1-CO-]- (Formula II),
Here, Ar1 and Ar2 are aromatic divalent meta-directed radicals which may be the same or different, and at least one of Ar1 and Ar2 is an m-phenylene radical.

Meta-aramids can be produced by the polymerization of meta-type aromatic amines and (meth)dicarboxylic acid halides.

Suitable aromatic (meth)diamines are metaphenylenediamine, 3,4'-diaminodiphenyl ether, and 3,4'-diaminodiphenyl sulfone; their derivatives having substituents such as halogen atoms and/or alkyl groups having 1 to 3 carbon atoms bonded to the aromatic ring structure, such as 2,4-toluylenediamine, 2,6-toluylenediamine, 2,4-diaminochlorobenzene, and 2,6-diaminochlorobenzene, may also be used. Preferably, metaphenylenediamine or a mixed diamine containing metaphenylenediamine in an amount of 85 mol % or more, more preferably 90 mol % or more, and even more preferably 95 mol % or more is used.

Suitable aromatic (meta)dicarboxylic acid dihalides are halides of isophthalic acid, such as isophthalic acid chloride and isophthalic acid bromide; their derivatives having substituents, such as halogen atoms and/or alkoxy groups having 1 to 3 carbon atoms, such as 3-chloroisophthalic acid chloride and 3-methoxyisophthalic acid chloride, can be used. Preferably, isophthalic acid chloride and mixed carboxylic acid halides containing 85 mol % or more, more preferably 90 mol % or more, and even more preferably 95 mol % or more of isophthalic acid chloride can be used.

The meta-aramid of the present invention may further contain a monomer other than the meta-type aromatic diamine and meta-type dicarboxylic acid halide. Suitable copolymerization components that can be used in combination with the diamine and carboxylic acid halide include para-phenylenediamine, 2,5-diaminochlorobenzene, 2,5-diaminobromobenzene, and benzene derivatives such as aminoanisidine; and 1,5-naphthylenediamine, 4,4'-diaminodiphenylether, 4,4,-diaminodiphenylketone, 4,4'-diaminodiphenylamine, and 4,4'-diaminodiphenylmethane. Suitable comonomer aromatic dicarboxylic acid dihalides include terephthalic acid dichloride, 1,4-naphthalenedicarboxylic acid dichloride, 2,6-naphthalenedicarboxylic acid dichloride, 4,4'-diphenyldicarboxylic acid dichloride, and 4,4'-diphenyletherdicarboxylic acid dichloride. In one embodiment, the meta-aramid polymer of the present invention may contain such a copolymerization component in a content of 15 mol % or less, preferably 10 mol % or less, and more preferably 5 mol % or less.

In one embodiment, the meta-aramid used in the present invention is a copoly(m-phenylene isophthalamide) containing 5 mole percent or less of aromatic moieties other than m-phenylene. In another embodiment, the meta-aramid is a poly(m-phenylene isophthalamide).

In one embodiment, the spinning dope does not contain para-aramid polymer, i.e., it contains less than 5 wt. %, preferably less than 1 wt. %, more preferably less than 0.5 wt. %, and most preferably less than 0.1 wt. % para-aramid polymer (by weight of the spinning dope).

In one embodiment, the spinning dope comprises sulfuric acid at a concentration of 80 wt % or more and a polymer, the polymer being comprised of a meta-aramid as defined above. The spinning dope may be comprised of a polymer, preferably a polymer comprising a meta-aramid as defined above, and sulfuric acid. The spinning dope (and the resulting fiber) may be free of additives, in particular antioxidants.

The spinning solution preferably has a meta-aramid concentration in the range of 10 to 30 wt%, more preferably 12 to 20 wt%, and even more preferably 14 to 16 wt%, based on the weight of the spinning solution.

The spinning solution may be prepared by mixing meta-aramid with sulfuric acid. The sulfuric acid and polymer may be mixed using a (twin-screw) extruder or (twin-screw) kneader, preferably with degassing.

Preferably, the spinning solution is prepared at a temperature in the range of 30 to 90°C.

Preferably, the spinning process of the present invention is a dry jet wet spinning process. This means that the spinning dope passes through a gaseous medium after leaving the spinneret and before entering the coagulation bath. The spinning dope, which contains meta-aramid and sulfuric acid, passes through the spinneret into the coagulation bath and is processed into fibers. The spinning mass is degassed and heated to the spinning temperature. The spinning temperature, i.e. the temperature at which the spinning dope is led through the spinneret, is preferably below 110°C, more preferably in the range of 25-80°C, or 45-60°C.

In the dry-jet wet spinning process, the liquid spinning dope is first passed through a non-coagulating gaseous atmosphere such as air and is immediately introduced into a coagulating bath. In the gaseous zone (also called air gap) through which the spinning mass passes, the meta-aramid is drawn. After coagulation, the formed filaments are removed from the coagulating bath, washed, dried and taken up on bobbins. The spinnerets used in the process of the invention may be of a type known per se for dry-jet wet spinning of fully para-aromatic polyamides. The gaseous non-coagulating medium preferably consists of air.

The gas medium (or air gap) preferably has a length in the range of 2 to 20 mm, more preferably 3 to 15 mm, and even more preferably 5 to 10 mm.

In the method of the present invention, the spinning mass leaving the nozzle of the spinneret is stretched in a non-coagulating gaseous medium. The degree of stretching, which is the ratio of the length of the filament when it leaves the coagulating bath to the average length of the spinning mass when it leaves the spinneret, may be in the range of 1.5 to 15, preferably 2 to 6.

The composition of the coagulation bath may vary. It may be composed in whole or in part of water and other substances such as bases, acids, salts, organic solvents, etc. The coagulation bath preferably consists of dilute aqueous sulfuric acid with a concentration of 0-40 wt%, preferably 2-20 wt%. In embodiments in which the coagulation bath comprises dilute aqueous sulfuric acid, the pH of the coagulation bath may have a pH of less than 7, preferably less than 2. In another embodiment, the coagulation bath may consist of a dilute aqueous caustic solution, e.g., an aqueous NaOH solution, with a concentration in the range of 0-10 wt%, preferably 0.05-5 wt%, especially 0.1-1 wt%. The coagulation bath may consist of water, especially softened or demineralized water.

The temperature of the coagulation bath can be any desired value. Depending on other spinning conditions, the temperature of the coagulation bath is typically in the range of -10°C to 50°C, preferably 0°C to 25°C, and more preferably 2°C to 10°C.

The sulfuric acid used must be completely removed from the spun fibers, in particular by washing or neutralization and washing, since small amounts of residual acid can adversely affect the properties of the _fibers . Neutralization may be carried out by treatment of the fibers obtained by coagulation with a solution of an alkaline substance, for example a caustic solution of NaOH, _NaHCO3 or _Na2CO3 , at room temperature or at elevated temperature. In one embodiment, the fibers are treated after coagulation with a solution having a NaOH concentration in the range of 0.1-2 wt%, preferably 0.3-1 wt%. Preferably, the neutralization solution has a pH of 9 or higher, more preferably a pH of 11 or higher.

In one embodiment, the fibers are only treated with water (e.g. demineralized or softened water) after coagulation, in particular once, twice, three times or more than three times (washing only, without neutralization). In a preferred embodiment, the fibers are washed, neutralized and washed again. After washing, the fibers are dried. This may be done in any convenient way, either on-line or off-line. Drying is preferably carried out immediately after (neutralization and) washing, for example by passing the fibers over heated rollers having a temperature in the range of 50-220°C, preferably 75-200°C, more preferably 100-175°C or 125-150°C.

Optionally, the fibers obtained by the method according to the invention can be subjected to a wet drawing step in order to improve the breaking strength of the fibers. In the wet drawing step, tension is applied to the coagulated wet fibers to a drawing ratio of 1-2 times, preferably 1.1-1.5 times. At this stage of the method, the drawing ratio can be calculated as the length of the fiber after the wet drawing step divided by the length of the fiber before the wet drawing step. In a continuous online method, the drawing ratio can be determined based on the speed of the godet (guide roller) that guides the yarn before and after the wet drawing step, i.e., the speed of the godet (guide roller) after the wet drawing step divided by the speed of the godet (guide roller) before the wet drawing step. Preferably, the wet drawing step is carried out between, during or after the coagulation and washing at a temperature above room temperature.

The fibre may be subjected to a heat treatment by heating under tension in an inert or non-inert gas. The heat treatment may be carried out after a drying step (online) or else after winding of the fibre (offline). The heat treatment may comprise one or more heating steps under tension. In one embodiment, the method according to the invention comprises heating the fibre in at least one heating step to a temperature in the range of 250-400°C, preferably 280-350°C, preferably 300-320°C.

The heat treatment of the fibers may consist of at least two stages. In one method according to the invention, the fibers obtained in the first heating step as described above are heated in a second heating step to a temperature in the range of 250-400°C, preferably 280-350°C, preferably 300-320°C.

In a preferred embodiment, at least one heating step is tensioned to a stretch ratio in the range of 1.5 to 10, preferably 1.5 to 2.5, and the other heating steps are either no tension (relaxed), or tensioned to allow the fiber to be transported over the processing equipment (guide rolls, etc.), or tensioned to a stretch ratio of 1.5 or less. Higher tension may be applied in either the first or second heating step. It is preferred to apply a higher stretch ratio in the first heating step. In another embodiment, tension is applied during each heating step, resulting in a total stretch ratio in the range of 1.5 to 10, preferably 1.5 to 2.5. At this stage of the method, the stretch ratio may be [length of fiber after heating step] divided by [length of fiber before heating step]. The stretch ratio refers to one heat treatment step, and the total stretch ratio refers to the stretch (cumulative) achieved in all heat treatment steps processed. In a continuous on-line method, the draw ratio may be determined based on the speed of the godet (guide roller) that guides the yarn before and after the heat treatment, i.e., [speed of the godet (guide roller) after at least one heating step] divided by [speed of the godet (guide roller) before at least one heating step].

The method may include a wet stretching step and a heat treatment.

The process according to the present invention is a solvent-based process. Concentrated sulfuric acid is used as a solvent for the meta-aramid polymer. Preferably, the sulfuric acid has a concentration of 85 wt% or more, more preferably 90 wt% or more, and even more preferably 95 wt% or more. Sulfuric acid with a concentration of 98 wt% or more or 99 wt% or more is particularly preferred.

The method results in a meta-aramid fiber, preferably a multifilament yarn. The present invention also relates to a meta-aramid fiber obtained by the method of the present invention, as disclosed in any of the above embodiments.

In the scope of the present invention, a fiber is understood as a flexible unit of material having a high ratio of length to width (width being defined across the cross-sectional area perpendicular to the length of the fiber). Fibers include all common fiber types, in particular filaments of unlimited length, filament yarns (monofilament and multifilament yarns) containing one or more twisted or untwisted filaments, and tows consisting of a collection of many filaments bound together with substantially no twist. The virtually unlimited length filaments formed during spinning may be cut into staple fibers as required, which in turn may be processed into spun yarns. Filament yarns may be cut into smaller lengths, called flocks. Furthermore, filament yarns may be processed into pulp. These fiber variations are encompassed by the term "fiber". Multifilament yarns may include fibers according to the invention and fibers of other materials. The cross section of the fibers or filaments of the present invention may be of any shape, but is typically circular (round).

The present invention also relates to meta-aramid fibers having a breaking strength of 300 mN/tex or more and a sulfonic acid group content of 0.001 wt% or more (mass/mass of fibers). In general, the sulfonic acid group content is 1 wt% or less, preferably 0.5 wt% or less, more preferably 0.3 wt% or less. It may also be expressed in ppm, so that the sulfonic acid group content is >1 ppm, preferably 5-300 ppm, more preferably 10-100 ppm. The sulfonic acid groups are formed as a result of using sulfuric acid as a solvent for the meta-aramid. In aromatics, aromatic substitution reactions occur to some extent in which the hydrogen moieties of the aromatic groups are replaced by sulfonic acid groups. It is preferred that these groups are neutralized.

The sulfonic acid group content is measured by nuclear magnetic resonance analysis (NMR), in particular ¹ H-NMR, on untreated yarn samples that have not been heat treated. The method for measuring the sulfonic acid group content is described in the Examples section.

The sulfur content can also be measured to obtain the degree of sulfonation of the fiber. In one embodiment, the sulfur content of the fiber according to the invention is 0.025 wt% or more, preferably 0.05 wt% or more (based on the weight of the fiber). The sulfur content may be measured on the resulting meta-aramid fiber (including the heat treated fiber). The sulfur content may be measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) as detailed in the Examples section.

Sulfonation of meta-aramid fibers appears to be advantageous in improving the flame retardancy and flame resistance of meta-aramid fibers. In particular, the LOI (Limiting Oxygen Index) of the fibers of the present invention is higher than meta-aramid fibers spun from organic solvents. Preferably, the LOI of meta-aramid fibers of the present invention is increased by 10% or more compared to fibers spun from organic solvents. LOI is measured according to ASTM D2863, as detailed in the Examples section.

Preferably, the meta-aramid fiber has a breaking strength of 350 mN/tex or more, more preferably 400 mN/tex or more, and even more preferably 450 mN/tex or more. The meta-aramid fiber may have a breaking strength of 500 mN/tex or more.

The breaking strength of meta-aramid fibers is measured in accordance with ASTM D7269-17 for multifilament yarns and ASTM D3822 for single filaments.

One advantage of the meta-aramid fiber of the present invention is that it has a low organic solvent content. In one embodiment, the meta-aramid fiber has an organic solvent content of less than 250 ppm, preferably less than 100 ppm, more preferably less than 50 ppm, which corresponds to an organic solvent content of less than 0.025 wt% (based on the weight of the yarn), preferably less than 0.01 wt%, more preferably less than 0.005 wt%. This means that the total content of organic solvents, especially NMP (N-methylpyrrolidone), THF (tetrahydrofuran), and DMAc (dimethylacetamide), is less than 250 ppm, preferably less than 100 ppm, more preferably less than 50 ppm. Note that meta-aramid fibers with an organic solvent content of less than 100 ppm are sometimes referred to as "substantially organic solvent-free." The very low content of residual organic solvents may be due to the solvent used during the polymerization of the meta-aramid.

The organic solvent content may be measured by various methods depending on the specific organic solvent. Gas chromatography (GC), NMR (nuclear magnetic resonance), and MS (mass spectrometry) are generally suitable for measuring the organic solvent content in the fibers, such as the NMP and DMAc content. In the context of the present invention, the organic solvent content is measured by gas chromatography, as detailed in the Examples section.

The present invention also relates to a meta-aramid multifilament yarn containing the meta-aramid fiber. The meta-aramid multifilament yarn may be made of meta-aramid fiber.

Preferably, the meta-aramid multifilament yarn has a breaking strength of 350 mN/tex or more, more preferably 400 mN/tex or more, and even more preferably 450 mN/tex or more. Preferably, the meta-aramid multifilament yarn has a breaking elongation of 15% or more, preferably 20% or more, and more preferably 25% or more. In one embodiment, the meta-aramid multifilament yarn has a breaking strength of 350 mN/tex or more and a breaking elongation of 25% or more. The mechanical properties of the meta-aramid yarn are measured in accordance with ASTM D7269-17.

In one embodiment, the present invention relates to a multifilament yarn in which at least 50% of the individual filaments have a round cross section, with the average ratio of the diameter of the smallest circumscribing circle to the diameter of the largest inscribing circle being 1.3 or less, preferably 1.1 or less.

"Minimum circumscribing circle" means the smallest or smallest circle that circumscribes the contour of the cross section of one filament of a multifilament yarn at at least two points and includes the entire area of the cross section of one filament. "Maximum inscribing circle" means the largest or largest circle that inscribes the contour of the cross section at at least two points and fits within the contour of the cross section. The average value of the ratio of the [diameter of the minimum circumscribing circle] to the [diameter of the maximum inscribing circle] is found by preparing a cross section of a multifilament yarn, determining the diameter of the minimum circumscribing circle, the diameter of the maximum inscribing circle, and the corresponding ratios for 50 separate filaments, and calculating the average value. In the case of a perfect circle, the minimum circumscribing circle and the maximum inscribing circle overlap and have the same diameter, so the ratio is 1.0.

This round cross-section is another advantage of the meta-aramid fibers of the present invention over the meta-aramid fibers of the prior art.

The meta-aramid fibers of the present invention have a lower porosity than meta-aramid fibers of the prior art. Because of the lower porosity, the fibers of the present invention may have better mechanical properties. Particularly advantageous is the combination of properties of the present meta-aramid fibers: the fibers have a uniform, round cross-section, improved flame retardancy, and lower porosity.

The application also relates to fabrics comprising the meta-aramid fibers and/or meta-aramid multifilament yarns of the invention. The fabrics may have the form of woven, knitted, or braided fabrics, or woven or nonwoven fabrics.

The meta-aramid fibers of the present invention may be used in textile applications, for example in fabrics, including knits and wovens, or in strings used to reinforce hoses, or in protective clothing, especially in fire-resistant applications. The meta-aramid fibers of the present invention are particularly suitable for textile applications in which the meta-aramid fibers or fabrics containing the fibers come into direct contact with the skin.

The present invention also relates to protective clothing comprising the fabric of the present invention. The protective clothing may be, for example, gloves, jackets, trousers, shirts, etc.

A micrograph of a cross section of a filament.

The present invention will now be described in more detail with reference to the following examples, which should not be construed as limiting the scope of the invention.

Measurement Methods 1. Mechanical Properties The breaking strength, breaking elongation, and breaking toughness of meta-aramid multifilament yarn are measured in accordance with ASTM D 7269-17.

2. Sulfonic acid group content The sulfonic acid group content is measured by ¹ H-NMR. 20 mg of a non-heat-treated sample of untreated yarn is dissolved in 1 mL of DMSO-d6, and 550 μL is transferred to a 5 mm NMR sample tube. ¹ H NMR spectra are recorded at 300 K using a Bruker Avance III 400 MHz NMR spectrometer equipped with a BBFO-plus 5 mm broadband probe. Spectra are recorded with 64 cumulative scans at an excitation pulse of 30°, using a prescan delay of 6 seconds and an acquisition time of 4 seconds. The resulting ¹ H NMR spectrum was referenced to the residual solvent signal of DMSO-d6 set at 2.5 ppm. The sulfonic acid group content is calculated using the following formula and expressed in meq/kg (mmol/kg):
A _sMPD = integral (singlet@8.97ppm) [mol]
A _{meta aramid} = (integral8.86-7.08ppm-(6*AsMPD))/8 [mol]
sMPD=(A _sMPD *186.1844) [mg]
meta aramid=(A _{meta aramid} *238.2414) [mg]
S=(sMPD/(sMPD+meta aramid))*(32.065/186.1844)*100%
Sulfonic acid group content = 106 * (S / 100) / 32.065 [meq / kg]

3. Sulfur Content The sulfur content may be measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). 100 mg of fiber is added with 9 ml of concentrated nitric acid (70 wt%). The mixture is decomposed in microwaves in an Ultrawave (Milestone) until a clear liquid is obtained. The volume is adjusted to 25 ml with MilliQ water. The solution is filtered to remove precipitate. The clear filtrate is analyzed by ICP-OES in a Perkin Elmer Optima 8300 DV instrument. Emission lines at wavelengths of 181,972 nm and 180,669 nm are used to measure the sulfur content.

4. Organic solvent content The organic solvent content is measured by gas chromatography. Approximately 1.0 mg of fiber was collected and heated to 500°C or higher in an electric furnace. The amount of amide solvent evaporated from the fiber was measured using gas chromatography (Shimadzu Corporation, model: GC-2010). The residual solvent concentration in the fiber was then calculated using a calibration curve created using an amide solvent as the standard sample.

5. Relative Viscosity Polymer samples are dried in a vacuum oven at 50° C. for 2 hours to remove moisture. The dried samples are then dissolved in sulfuric acid at room temperature overnight. The efflux time of a 0.25% (w/V) sample solution in 96% (w/w) sulfuric acid is then measured in an Ubbelohde viscometer (such as Schott AVS370) at 25° C. The efflux time of the solvent is also measured under the same conditions. The relative viscosity is calculated as the ratio of the two observed efflux times.

6. Microscopic Observation The thread is embedded in melted paraffin and left to harden for about 5 minutes. The embedded sample is then cut perpendicular to the fiber axis with a microtome to obtain a section of 5 to 7 μm thick. The section is then placed on a slide glass and heated to melt the paraffin. The dissolved paraffin is then removed with xylene and ethanol. Next, the cross section of the fiber is observed and photographed using an optical microscope (Nikon Corporation, product name "Eclipse" LV100N) to obtain a cross-sectional photograph. The magnification is selected as necessary within the range of 100 to 1000.

7. Letter of Inquiry
The limiting oxygen index (LOI) is measured according to ASTM D2863. From each sample, the required number of yarns are combined to produce a yarn sample of 168,000 dtex. The yarn is wound on a precision reel with a winding tension of 5±3 mN/tex based on the nominal linear density of the yarn. The sample is surrounded by a thin copper wire. Each specimen is approximately 150 mm long, 10±0.5 mm wide, and 3±0.25 mm thick. The specimen is marked approximately 50 mm from the end to be ignited. Immediately prior to testing, the specimen is conditioned at 23±2°C and 50±5% relative humidity for at least 88 hours. The specimens were tested according to ASTM D2863, Option A (top ignition).

Example 1
Fibers were spun from a spinning dope containing m-aramid polymer (poly(m-phenylisophthalamide)) with a relative viscosity of 1.55. The m-aramid polymer was mixed with 99.8 wt % sulfuric acid at a polymer concentration of 18 w/w % in a Theysohn 20 mm twin screw extruder at 85° C. and 300 rpm to obtain a spinning dope. The spinning dope was passed through a filter at 55° C., a spinneret at 85° C., an air gap, and then into a static coagulation bath to process into filaments (under the conditions shown in Table 1). The temperature of the coagulation bath was 3° C.

The multifilament yarn obtained after coagulation was subsequently washed and neutralized by passing it through baths of water, 0.4% NaOH, and again water. The yarn was dried at 150°C and wound onto a bobbin.

The properties of the resulting yarn after drying (also referred to as "untreated yarn") were measured.

The sulfonic acid content of sample 1-1 was measured to be 116 meq/kg. Sample 1-1 was heat treated in a two-stage process. The yarn was pulled from the bobbin and passed through a first oven where a draw ratio of 1.6 was applied and a second oven where a draw ratio of 1 was applied and the temperature was maintained at 333°C. The draw ratio is defined as the speed after the oven divided by the speed before the oven. The residence time in both ovens (based on the speed between the ovens) was 18.8 seconds.

Example 2
Fibers were spun from a spinning dope containing m-aramid polymer with a relative viscosity of 1.55. The m-aramid polymer was mixed with 99.8 wt % sulfuric acid at a polymer concentration of 16 wt/wt % or 17.5 wt/wt % in a Theysohn 20 mm twin screw extruder at 55° C. (2-1, 2-3) or 60° C. (2-2, 2-4) at 450 rpm to obtain a spinning dope. The spinning dope was passed through a filter, a spinneret, a 5 mm air gap with a draw ratio of 3.41 into a static coagulation bath and processed into filaments (under the conditions shown in Table 4). The temperature of the coagulation bath was 5° C.

The multifilament yarn obtained after coagulation was subsequently washed and neutralized by passing it through baths of water, 0.25% NaOH, and again water. The yarn was dried at 150°C and wound onto a bobbin. The properties of the yarn obtained after drying (also referred to as "untreated yarn") were measured.

Sample 2-3 was heat treated in a two-stage process. The yarn was pulled from a bobbin and passed through a first oven at 305°C with various stretch ratios and a second oven with a stretch ratio of 1 and a temperature of 333°C. The stretch ratio is defined as the speed after the oven divided by the speed before the oven. The residence time in both ovens was calculated from the speed between the two ovens. The sulfur content of sample 2-3 was measured to be 0.18 wt%.

Example 3
Fibers were spun from a spinning dope containing m-aramid polymer with a relative viscosity of 1.58. The m-aramid polymer was mixed with 99.8 wt % sulfuric acid in a Clextral 53 mm twin screw extruder at a temperature of 45° C. at a rotation speed of 250 rpm to obtain a spinning dope.

The spinning dope was passed through a filter at 50°C, a spinneret with 1000 capillaries with a diameter of 65 μm at 50°C, and an air gap, where the filaments were stretched 2.9 times and processed into filaments in a falling jet coagulation bath (under the conditions shown in Table 1). The temperature of the coagulation bath was 5°C. The multifilament yarn obtained after coagulation was subsequently washed and neutralized by passing it through a bath of water, 0.35% NaOH, and again water. The yarn was dried on a godet (guide roller) at 160°C. The yarn could also be hot-stretched on a heated godet (guide roller) and wound onto a bobbin.

The properties of the resulting yarn were measured and are shown in Table 9.

Example 4
Fibers were spun from a spinning dope containing a meta-aramid polymer with a relative viscosity of 1.55. The m-aramid polymer was mixed with 99.8 wt % sulfuric acid at a polymer concentration of 16 wt/wt % in a Theysohn 20 mm twin screw extruder at 60° C. and 450 rpm to obtain a spinning dope. The spinning dope was passed through a filter, a spinneret, a 5 mm air gap with a draw ratio of 2.08, and into a dynamic coagulation bath where it was processed into filaments (under the conditions shown in Table 10). The temperature of the coagulation bath was 5° C. After the coagulation bath, the yarn was wet drawn between two sets of rollers.

The multifilament yarn obtained after coagulation and wet drawing was subsequently washed and neutralized by passing it through baths of water, 0.25% NaOH, and again water. The yarn was dried at 150°C and wound onto a bobbin.

The properties of the yarn obtained after drying (also referred to as "untreated yarn") were measured.

Sample 4-2 was heat treated in a two-stage process. The yarn was pulled from the bobbin and passed through a first oven at 315°C with a stretch ratio of 1, and a second oven at 315°C with varying stretch ratios. The stretch ratio is defined as the speed after the oven divided by the speed before the oven.

Example 5
Fibers were spun from a spinning dope containing a meta-aramid polymer with a relative viscosity of 1.55. The m-aramid polymer was mixed with 99.8 wt % sulfuric acid at a polymer concentration of 16 wt/wt % in a Theysohn 20 mm twin screw extruder at 50° C. and 300 rpm to obtain a spinning dope. The spinning dope was processed into filaments through a filter, a spinneret with 106 capillaries with a diameter of 65 μm kept at a temperature of about 75° C., a 10 mm air gap with a draw ratio of 1.95, and a dynamic coagulation bath. The speed after the coagulation bath was 40 m/min. After the coagulation bath, the yarn was wet-drawn 1.4 times between two sets of rollers. The yarn obtained after coagulation and wet drawing was subsequently washed and neutralized by passing through baths of water, 0.25% NaOH, and again water. The yarn was dried on rollers set at 220° C. The yarn was transferred to two successive roller sets, the settings of which are shown in Table 13.

The yarn was wound onto a bobbin. The mechanical properties of this yarn are shown in Table 14.

Example 6 - Organic Solvent Content The NMP and DMAc contents of commercially ^{available meta-aramid yarns from TeijinConex®} B, ^Nomex® and Yantai Tayho Advanced Materials were measured by gas chromatography, and the results are shown in Table 13. The yarn according to the invention is free of organic solvents.

Example 7 - Cross-sections of filaments Photomicrographs were made of bundles embedded with comparative samples 1-3 (used in Example 6) and meta-aramid yarns according to the invention. Photomicrographs of cross-sections of filaments are shown in Figure 1. Panel a) shows cross-sections of filaments from Comparative Example 1, panel b) filaments from Comparative Example 2, panel c) filaments from Comparative Example 3, and panel d) filaments of a meta-aramid yarn according to the invention.

As can be seen from this comparison, the filaments of the present invention have a uniform circular cross section, while the filaments of the comparative examples have oval cross sections and cross sections with different diameters.

Example 8 - Limiting Oxygen Index The LOI of meta-aramid yarns according to the invention and a commercially available meta-aramid yarn was measured and the results are shown in Table 14.

This data shows that the samples according to the present invention have higher LOI values than the comparative samples according to the prior art.

Claims (9)

1. A method for producing a fiber comprising meta-aramid having a breaking strength of 300 mN/tex or more, comprising the steps of preparing a spinning dope comprising meta-aramid and sulfuric acid having a concentration of 80 wt % or more, and passing the spinning dope through a spinneret to a coagulation bath, the spinning dope having a meta-aramid concentration of 10 wt % or more, the fiber being heated to a temperature in the range of 250-400°C in at least one heating step, and optionally subsequently heated to a temperature in the range of 250-400°C in a further heating step . The method according to claim 1, wherein the breaking strength of the fiber containing meta-aramid is 350 mN/tex or more. The method according to claim 1 or 2, wherein the spinning dope has a meta-aramid concentration in the range of 10 to 30 wt %. The method according to any one of claims 1 to 3, wherein the meta-aramid is a copoly(m-phenylene isophthalamide) containing 5 mol % or less of an aromatic moiety other than m-phenylene. The method according to any one of claims 1 to 4, wherein the spinning dope is prepared by mixing meta-aramid and sulfuric acid. The method according to any one of claims 1 to 5, wherein the spinning dope is prepared at a temperature in the range of 30 to 90°C. The method according to any one of claims 1 to 6, wherein the spinning dope passes through a gaseous medium after leaving the spinneret and before entering the coagulation bath. The method according to claim 7, wherein the spinning solution is passed through a gas medium having a length in the range of 2 to 20 mm. The method according to any one of claims 1 to 8, wherein the sulfuric acid has a concentration of 85 wt% or more.

Images