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Title:
PROCESS FOR THE MANUFACTURE OF A FIBER COMPRISING META-ARAMID
Document Type and Number:
WIPO Patent Application WO/2021/070042
Kind Code:
A1
Abstract:
Process for manufacturing a fiber comprising meta-aramid having a breaking tenacity of at least 300 mN/tex comprising the steps of preparing a spin dope comprising meta-aramid and sulfuric acid having a concentration of at least 80 wt% and passing the spin dope through a spinneret into a coagulation bath, wherein the spin dope has a meta-aramid concentration of at least 10 wt%. The invention also pertains to a meta-aramid fiber, a multifilament yarn, a textile sheet and protective clothing.

Inventors:
BOERSTOEL HANNEKE (NL)
NIJENHUIS WIDO (NL)
GÜNES RASID (NL)
BOUDEBAH SAID (NL)
YAMAGUCHI YORIHISA (JP)
SATOH YUSUKE (JP)
Application Number:
PCT/IB2020/059360
Publication Date:
April 15, 2021
Filing Date:
October 06, 2020
Export Citation:
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Assignee:
TEIJIN LTD (JP)
International Classes:
D01D5/06; D01F6/60; D02G3/44; D04H1/4342
Foreign References:
JP2010095831A2010-04-30
US5432255A1995-07-11
KR20140075197A2014-06-19
KR20100001782A2010-01-06
KR20100001782A2010-01-06
US3094511A1963-06-18
EP0226137A21987-06-24
KR20140075197A2014-06-19
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Claims:
Claims:

1. Process for manufacturing a fiber comprising meta-aramid having a breaking tenacity of at least 300 mN/tex comprising the steps of preparing a spin dope comprising meta-aramid and sulfuric acid having a concentration of at least 80 wt% and passing the spin dope through a spinneret into a coagulation bath, wherein the spin dope has a meta-aramid concentration of at least 10 wt%.

2. Process according to claim 1 , wherein the fiber comprising meta-aramid has a breaking tenacity of at least 350 mN/tex, preferably at least 400 mN/tex, even more preferably of at least 450 mN/tex.

3. Process according to claim 1 or 2, wherein the spin dope has a meta-aramid concentration in the range of 10 - 30 wt%, preferably 12 - 20 wt%, more preferably 14-16 wt%.

4. Process according to any of the preceding claims, wherein the meta-aramid is co-poly(m-phenylene isophthalamide) comprising at most 5 molar% of aromatic moieties other than m-phenylene.

5. Process according to any of the preceding claims, wherein the spin dope is prepared by mixing meta-aramid and sulfuric acid.

6. Process according to any of the preceding claims, wherein the spin dope is prepared at a temperature in the range of 30 to 90°C.

7. Process according to any of the preceding claims, wherein the spin dope is passed through a gaseous medium after exiting the spinneret and before entering the coagulation bath.

8. Process according to claim 7, wherein the spin dope passes through a gaseous medium of a length in the range of 2 to 20 mm, preferably 3 to 15 mm, more preferably 5 to 10 mm.

9. Process according to any of the preceding claims, wherein the sulfuric acid has a concentration of at least 85 wt%, preferably at least 90 wt%, more preferably at least 95 wt%.

10. Process according to any of the preceding claims, wherein the fibers are heated in at least one heating step to a temperature in the range of 250 to 400°C, preferably in the range of 280 to 350°C, more preferably in the range of 300 to 320°C, optionally followed by a further heating step at a temperature in the range of 250 to 400°C, preferably in the range of 280 to 350°C, more preferably in the range of 300 to 320°C.

11. Meta-aramid fiber having a breaking tenacity of at least 300 mN/tex, obtainable by the process according to any one of claims 1 to 10.

12. Meta-aramid fiber having breaking tenacity of at least 300 mN/tex and a sulfonic acid group content of at least 0.001 wt%.

13. The meta-aramid fiber according to claim 11 or 12 having a breaking tenacity of at least 350 mN/tex, preferably at least 400 mN/tex, even more preferably at least 450 mN/tex.

14. The meta-aramid fiber according to any one of claims 11 to 13 having an organic solvent content below 250 ppm, preferably below 100 ppm, more preferably below 50 ppm.

15. Meta-aramid multifilament yarn comprising the meta-aramid fiber of any one of claims 11 to 14.

16. The meta-aramid multifilament yarn according to claim 15 wherein at least 50% of individual filaments have a round cross section such that the average ratio of [diameter of the minimum circumscribed circle] to [diameter of the maximum inscribed circle] is at most 1.3, preferably at most 1.1.

17. Textile sheet comprising the meta-aramid fiber of any one of claims 11 to 14 and/or the meta-aramid multifilament yarn of claim 15 or 16, preferably a knitted, woven or nonwoven textile sheet.

18. Protective clothing comprising the textile sheet of claim 17.

Description:
Process for the manufacture of a fiber comprising meta-aramid

Description:

The present invention relates to a process for manufacturing a meta-aramid fiber by preparing a spin dope comprising meta-aramid and sulfuric acid and passing said spin dope through a spinneret, and to the meta-aramid fiber obtained by said process. Furthermore, the invention also relates to a meta-aramid fiber having a sulfonic acid group content of at least 1 meq/kg, to multifilament yarn comprising said fiber and to a textile sheet and protective clothing.

Spinning processes for aramid fibers are known.

KR20100001782 describes aramid fibers and a manufacturing processes therefore. Various para-aramid polymers (e.g. paraphenylene terephthalamide, paraphenylene - 4,4’-biphenylene dicarboxylic acid amide or paraphenylene - 2,6- naphthalene dicarboxylic acid amide) may be used for the spinning process, wherein the para-aramid is dissolved in sulfuric acid, the spin dope is passed through a spinneret into a coagulation bath, the filaments are washed, dried and wound up. The time in the coagulation bath may be adjusted to improve the circularity of the filament cross section. KR20100001782 does not describe that meta-aramid is dissolved in sulfuric acid.

A spinning process for a meta-aramid fiber is disclosed in US3094511. This document discloses a dry spinning process wherein meta-aramid is dissolved in an organic solvent and passed through a spinneret into a heated air column. The latter step causes evaporation of the solvent. Also known are meta-aramid wet spinning processes where the spin dope is directly passed from the spinneret into the coagulation bath, as e.g. described in EP0226137. Also for the wet spinning processes organic solvents are used.

The use of organic solvents has several disadvantages. Due to legislation, the use of organic solvents might be banned or only be allowed under more stringent conditions in the future for environmental reasons.

Also, fibers spun with the use of organic solvents have to be extensively washed to remove the solvents, which is not economic.

Despite of extensive washing, meta-aramid fibers spun from organic solvents still contain organic solvents.

Therefore, there is the need for a spinning process for meta-aramid without the use of organic solvents.

KR20140075197 pertains to a meta-aramid composition providing discoloration prevention by adding antioxidants into the spin dope. KR20140075197 describes that a meta-aramid fiber is produced from a meta-aramid composition, after dissolving the meta-aramid composition in sulfuric acid with a concentration of 99% to a meta-aramid concentration of 20%. After passing the spin dope through a spinneret, the spin dope is passed directly and without any air gap into a coagulation bath, the filament is washed, dried and wound up. KR20140075197 does not disclose a meta-aramid fiber having a breaking tenacity of at least 300 mN/tex.

An object of present invention is to provide a meta-aramid fiber which is substantially free of organic solvents and has good mechanical properties, in particular a high breaking tenacity.

It is also an object of present invention to provide an improved meta-aramid fiber, in particular with improved fire retardancy and improved even cross section. In order to achieve the above objects, the present invention provides a process for manufacturing a fiber comprising meta-aramid having a breaking tenacity of at least 300 mN/tex comprising the steps of preparing a spin dope comprising meta- aramid and sulfuric acid having a concentration of at least 80 wt% and passing the spin dope through a spinneret into a coagulation bath, wherein the spin dope has a meta-aramid concentration of at least 10 wt% (based on the weight of the spin dope).

Preferably, the fiber comprising meta-aramid has a breaking tenacity of at least 350 mN/tex, more preferably at least 400 mN/tex and even more preferably at least 450 mN/tex. The instant process may also result in fibers comprising meta- aramid having a tenacity of at least 500 mN/tex.

The breaking tenacity of the fiber comprising meta-aramid is determined according to ASTM D7269-17 for multifilament yarns and according to ASTM D3822 for single filaments.

The fiber comprising meta-aramid may also be referred to as meta-aramid fiber.

For the purpose of this invention, the term meta-aramid refers to a class of wholly aromatic polyamide polymers and copolymers having at least 70%, preferably at least 80% and more preferably at least 90% of meta-oriented bonds between the aromatic moieties. In one embodiment, at least 95% or all (i.e. 100%) of the bonds are meta-oriented bonds. The amide bonds between the aromatic moieties are thus located substantially in the meta-oriented or nearly meta-oriented positions of the aromatic rings (as e.g. in a 1 ,3-phenylene group or 1,3-naphthalene group).

The meta-aramid of the present invention may be of recurring units of formula I and II:

-[-NH-Ar1 -NH-CO-Ar2-CO-]- (formula I)

-[-NH-AM-CO-]- (formula II), where Ar1 and Ar2 are aromatic, divalent, meta-oriented radicals which may be the same or different and at least one of Ar1 and Ar2 is a m-phenylene radical.

Meta-aramids may be produced by polymerization of meta-type aromatic amine and meta-dicarboxylic acid halide.

Suitable aromatic meta-diamines are meta-phenylenediamine, 3,4'- diaminodiphenylether, and 3,4'-diaminodiphenylsulfone; and derivatives thereof having substituents such as halogen atoms and/or alkyl groups having 1 to 3 carbon atoms, attached to the aromatic cyclic structures thereof, for example, 2,4- toluylenediamine, 2,6-toluylenediamine, 2,4-diaminochlorobenzene, and 2,6- diaminochlorobenzene, may be employed. Preferably, meta-phenylene diamine or mixed diamines containing meta-phenylene diamine in a content of 85 molar % or more, more preferably 90 molar % or more, still more preferably 95 molar % or more are employed.

Suitable aromatic meta-dicarboxylic acid dihalides are isophthalic acid halides, for example, isophthalic acid chloride and isophthalic acid bromide; and derivatives thereof having substituents, for example halogen atoms and/or alkoxy groups having 1 to 3 carbon atoms, for example 3-chloroisophthalic acid chloride and 3- methoxyisophthalic acid chloride may be employed. Preferably, isophthalic acid chloride and mixed carboxylic acid halides containing isophthalic acid chloride in a content of 85 molar % or more, more preferably 90 molar % or more, still more preferably 95 molar % or more, are employed.

The meta-aramid of instant invention may comprise further monomers besides meta-type aromatic diamines and meta-type dicarboxylic acid halides. Suitable copolymerization components usable in combination with the diamines and the carboxylic acid halides include benzene derivatives such as para-phenylene diamine, 2,5-diaminochlorobenzene, 2,5-diaminobromobenzene and aminoanisidines; and 1 ,5-naphthylenediamine, 4,4'-diaminodiphenylether, 4,4,- diaminodiphenylketone, 4,4'-diaminodiphenylamine and 4,4’- diaminodiphenylmethane. Suitable comonomeric aromatic dicarboxylic acid dihalides include terephthalic acid dichloride, 1 ,4-naphthalenedicarboxylic acid dichloride, 2,6-naphthalenedicarboxylic acid dichloride, 4,4'-biphenyldicarboxylic acid dichloride and 4,4'-biphenyletherdicarboxylic acid dichloride. In one embodiment the meta-aramid polymer of present invention may include such copolymerization components in a content of at most 15 molar%, preferably at most 10 molar%, more preferably at most 5 molar%.

In one embodiment, the meta-aramid used in present invention is co-poly(m- phenylene isophthalamide) comprising at most 5 molar% of aromatic moieties other than m-phenylene. In another embodiment, the meta-aramid is poly(m- phenylene isophthalamide).

In one embodiment the spin dope is free of para-aramid polymer, i.e. comprises less than 5 wt%, preferably less than 1 wt%, more preferably less than 0.5 wt%, and most preferably less than 0.1 wt% of para-aramid polymer (based on the weight of the spin dope).

In one embodiment, the spin dope comprises sulfuric acid having a concentration of at least 80 wt% and polymer, which polymer consists of meta-aramid as defined above. The spin dope may consist of polymer, preferably consisting of meta- aramid as defined above, and sulfuric acid. The spin dope (and the resulting fiber) may be free of additives, in particular antioxidants.

The spin dope preferably has a meta-aramid concentration in the range of 10 - 30 wt%, more preferably 12 - 20 wt%, even more preferably 14-16 wt%, based on the weight of the spin dope. The spin dope may be prepared by mixing meta-aramid and sulfuric acid. Mixing of the sulfuric acid and the polymer may be carried out using a (double screw) extruder or a (twin shaft) kneader, preferably including degassing.

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

Preferably, the spinning process of present invention is a dry-jet wet spinning process. This means, that the spin dope is passed through a gaseous medium after exiting the spinneret and before entering the coagulation bath.

The spin dope comprising meta-aramid and sulfuric acid is processed into fibers by passing the spin dope through a spinneret into a coagulation bath.

The spinning mass is deaerated and heated to spinning temperature.

The spinning temperature, i.e. the temperature at which the spin dope is lead to and passes the spinneret, is preferably at most 110°C, more preferably in the range of 25 to 80°C, or 45 to 60°C.

In the dry-jet wet spinning process, the liquid spin dope first passes through a non coagulating gaseous atmosphere, such as air, and immediately afterwards is led into a coagulation bath. In the gaseous zone (also referred to as air gap) through which the spinning mass passes, the meta-aramid is drawn.

After coagulation, the filaments formed are removed from the coagulation bath, washed, dried and taken up on a bobbin.

The spinnerets that are used in the process according to the invention may be of a type known in itself in the dry jet-wet spinning of fully para-aromatic polyamides. The gaseous non-coagulating medium preferably consists of air.

The gaseous 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 of 5 to 10 mm.

In the process according to the invention the spinning mass leaving the spinneret orifices is drawn in the non-coagulating gaseous medium. The degree of drawing, that is the ratio between the length of the filaments upon leaving the coagulation bath and the average length of the spinning mass upon leaving the spinning orifices of 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 entirely or partly consist of water or other substances, such as bases, acids, salts and organic solvents. The coagulation bath preferably consists of dilute aqueous sulfuric acid having a concentration of 0-40% by weight, preferably 2-20% by weight. In the embodiment where the coagulation bath comprises dilute aqueous sulfuric acid, the pH of the coagulation bath may have a pH of below 7, preferably below 2.

According to another embodiment, the coagulation bath may consist of a diluted caustic aqueous solution e.g. an aqueous NaOH solution with a concentration in the range of 0-10 % by weight, preferably 0.05 to 5% by weight and in particular 0.1 to 1% by weight. The coagulation bath may consist of water, in particular softened or demineralized water.

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

As small amounts of residual acid may have a detrimental influence on the fiber properties, the sulfuric acid used should completely be removed from the spun fibers, in particular by washing or neutralization and washing. The neutralization may be done by subjecting the fiber obtained by coagulation to a treatment at room temperature or at elevated temperature with solutions of alkaline substances, for instance caustic solutions of NaOH, NaHC03 or Na2C03. In one embodiment, the fibers are treated after coagulation with solutions having a NaOH concentration in the range of 0.1 to 2 wt%, preferably 0.3 to 1 wt%. Preferably, the neutralization solution has a pH of at least 9, more preferably at least 11. In one embodiment, the fibers are only treated with water (e.g. demineralized water or softened water) after coagulation, in particular once, twice, three or more than three times (only washing, without neutralization).

In a preferred embodiment, the fibers are washed, neutralized and again washed. After they have been washed, the fibers are dried. This may be done in any convenient way, either on-line or off-line. It is preferred that the drying should be carried out immediately after (neutralization and) washing, e.g. by passing the fibers over heated rollers having a temperature in the range of 50 to 220°C, preferably in the range of 75 to 200°C, more preferably in the range of 100 to 175°C or 125 to 150°C.

To increase the tenacity of the fibers, optionally, the fibers obtained in the process according to the invention may be subjected to a wet -drawing step. During the wet-drawing step, the coagulated, wet fibers are subjected to a tension resulting in a drawing ratio in the range of >1 to 2, preferably 1.1 to 1.5. At this stage of the process, the drawing ratio may be defined as [length of the fiber after wet-drawing step] / [length of the fiber before wet-drawing step]. For a continuous online process the drawing ratio may also be determined based on the speed of the godets guiding the yarn before and after the wet-drawing, thus [speed of godet after wet-drawing step] / [speed of godet before wet-drawing step].

Preferably, the wet-drawing step is carried out between the coagulation and the washing, during washing or after washing at a temperature above room temperature.

The fibers may be subjected to a heat treatment, in which the fibers are heated under tension in an inert or non-inert gas. The heat treatment may be carried out after the drying step (online) or otherwise after the winding of the fiber (offline).

The heat treatment may comprise one or multiple steps of heating under tension. In one embodiment, the process according to the invention comprises heating the fiber in at least one heating step to a temperature in the range of 250 to 400°C, preferably in the range of 280 to 350°C, preferably in the range of 300 to 320°C. The heating treatment of the fiber may comprise at least two steps. In one process according to the invention, the fiber obtained in the first heating step as described above is heated in a second heating step to a temperature in the range of 250 to 400°C, preferably in the range of 280 to 350°C, preferably in the range of 300 to 320°C.

In a preferred embodiment, during at least one of the heating steps a tension is applied which results in a drawing ratio in the range of 1.5 to 10, preferably 1.5 to 2.5 and during the other heating step no tension (relaxation) is applied or a tension with is sufficient to allow transport of the fiber over processing equipment (e.g. guiding rolls) or a tension which results in a drawing ratio of at most 1.5. Higher tension may be applied either in the first or the second heating step. Preferably, a higher drawing ratio is applied in the first heating step.

In another embodiment, during each of the heating steps a tension is applied, which results in a total drawing ratio in the range of 1.5 to 10, preferably 1.5 to 2.5. At this stage of the process, the drawing ratio may be defined as [length of the fiber after heating step] / [length of the fiber before heating step]. The drawing ratio refers to one heat treatment step, the total drawing ratio to the drawing achieved in all applied heat treatment steps (accumulated). For a continuous online process the drawing ratio may also be determined based on the speed of the godets guiding the yarn before and after the heat treatment, thus [speed of godet after at least one heating step] / [speed of godet before at least one heating step].

The process may include a wet-drawing step and a heat treatment.

The process according to the invention is a solvent-based process.

As solvent for the meta-aramid polymer, concentrated sulfuric acid is used. Preferably, the sulfuric acid has a concentration of at least 85 wt%, more preferably at least 90 wt% and even more preferably at least 95 wt%. Sulfuric acid having a concentration of at least 98 wt% or at least 99 wt% is particularly preferred.

The process results in a meta-aramid fiber, preferably being a multifilament yarn. The present invention also pertains to a meta-aramid fiber obtainable by the process of the invention, as disclosed in any of the embodiments above.

Within the scope of the invention fibers are to be understood as flexible units of matter having a high ratio of length to width (the width is defined across the cross- sectional area, perpendicular to the length of the fiber). The term fiber includes all usual types of fiber, such as filaments of particularly unlimited length, filament yarns comprising one or more twisted, co-mingled or non-twisted filaments (monofilaments and multifilament yarns), tow made up of a collection of a large number of filaments which are bundled practically without any twist being imparted to them, and the like. Filaments of practically unlimited length formed during spinning may, if desired, be cut into staple fibers, which may in their turn be processed into spun yarns. Filament yarns may also be cut into even smaller lengths called floe.

Furthermore, filament yarns may also be processed info pulp.

These fiber variants are encompassed by the term Tiber”.

Potentially, multifilament yarn may comprise fibers according to the invention and fibers of other materials. The cross section of the fiber or filament of the invention can be of any shape, but is typically circular (round).

The invention also pertains to a meta-aramid fiber having a breaking tenacity of at least 300 mN/tex and a sulfonic acid group content of at least 0.001 wt% (mass / mass fiber).

Generally, the sulfonic acid group content is at most 1 wt%, preferably at most 0.5 wt%, more preferably at most 0.3 wt%. Alternatively, this may also be expressed in parts per million resulting in a sulfonic acid group content of >1 ppm, preferably 5- 300 ppm, more preferably 10-100 ppm. The sulfonic acid groups are formed as a consequence of using sulfuric acid as solvent for the meta-aramid. To some extent, on the aromatic groups an aromatic substitution reaction takes place, such that hydrogen moieties on aromatic groups are replaced by sulfonic acid groups. Preferably, these groups are neutralized.

The sulfonic acid group content is determined by nuclear magnetic resonance analysis (NMR), in particular 1 H-NMR on as-spun, not-heat-treated samples. The method for determination of the sulfonic acid group content is provided in the example section.

Alternatively, the sulfur content may be determined to determine the degree of sulfonation of the fiber. In one embodiment, the sulfur content of the fiber according to the invention is at least 0.025 wt%, preferably at least 0.05 wt% (based on the weight of the fiber). The sulfur content may be determined on the obtained meta-aramid fibers, including on heat-treated fiber.

The sulfur content may be determined by inductively coupled plasma optical emission spectrometry (ICP-OES) as detailed in the example section.

Sulfonation of the meta-aramid fibers seems to be advantageous for the flame retardancy and flame resistance of the meta-aramid fibers. In particular, the LOI (Limiting Oxygen Index) of fibers of the instant invention is higher than that of meta-aramid fibers spun from organic solvents. Preferably, the LOI of the meta- aramid fibers according to the invention is increased by at least 10% compared to fibers spun from organic solvent.

The LOI is determined in accordance with ASTM D2863 as detailed in the example section.

Preferably, the meta-aramid fiber has a breaking tenacity of at least 350 mN/tex, more preferably at least 400 mN/tex and even more preferably at least 450 mN/tex. The meta-aramid fiber may have a tenacity of at least 500 mN/tex. The breaking tenacity of the meta-aramid fiber is determined according to ASTM D7269-17 for multifilament yarns and according to ASTM D3822 for single filaments.

One of the advantages of the meta-aramid fiber of current invention is the low content of organic solvents.

In one embodiment, the meta-aramid fibers have an organic solvent content below 250 ppm, preferably below 100 ppm, more preferably below 50 ppm, corresponding to an organic solvent content of below 0.025 wt% (based on the weight of the yarn), preferably below 0.01 wt%, more preferably below 0.005 wt%. This means that the combined content of organic solvents, in particular NMP (N- methyl pyrrolidone), THF (tetrahydrofuran) and DMAc (dimethyl acetamide) is below 250 ppm, preferably below 100 ppm and more preferably below 50 ppm. Meta-aramid fibers having an organic solvent content below 100 ppm may be referred to as “substantially free of organic solvents”. A very low residual organic solvent content may be the result of the solvents used during polymerization of the meta-aramid.

The organic solvent content may be determined by different methods, depending on the specific organic solvent. Generally, gas chromatography (GC), NMR (nuclear magnetic resonance) and MS (mass spectrometry) are suitable to determine the organic solvent content, e.g. the NMP or DMAc content, of the fibers. In the context of the current invention, the organic solvent content is determined by gas chromatography, as detailed in the example section.

The invention also pertains to a meta-aramid multifilament yarn comprising the meta-aramid fiber. The meta-aramid multifilament yarn may consist of meta- aramid fibers. Preferably, the meta-aramid multifilament yarn has a breaking tenacity of at least 350 mN/tex, more preferably of at least 400 mN/tex and even more preferably of at least 450 mN/tex.

Preferably, the meta-aramid multifilament yarn has an elongation at break of at least 15%, preferably at least 20%, more preferably at least 25%.

In one embodiment, the meta-aramid multifilament yarn has a breaking tenacity of at least 350 mN/tex and an elongation at break of at least 25%.

The mechanical properties of the meta-aramid yarn are determined in accordance with ASTM D7269-17.

In one embodiment, the invention pertains to a multifilament yarn wherein at least 50% of individual filaments have a round cross section such that the average ratio of [diameter of the minimum circumscribed circle] to [diameter of the maximum inscribed circle] is at most 1.3, preferably at most 1.1.

The term “minimum circumscribed circle” means the smallest or minimum circle which circumscribes the contour of the cross-section of one filament of the multifilament yarn at, at least, two points and contains the entire area of the cross- section of the one filament. The term “maximum inscribed circle" means the largest or maximum circle which inscribes the contour of said cross section at, at least, two points and is contained within said contour of the cross-section.

The average ratio of [diameter of the minimum circumscribed circle] to [diameter of the maximum inscribed circle] is determined by preparing a cross section of a multifilament yarn and determining the diameter of the minimum circumscribed circle, the diameter of the maximum inscribed circle and the corresponding ratio for 50 separate filaments and calculating the average of the ratio.

For a fully round circle, the minimum circumscribed circle and the maximum inscribed circle will overlap and therefore have the same diameter, resulting in a ratio of 1.0. The round cross section is another advantage of the meta-aramid fibers of present invention over the prior art meta-aramid fibers.

The meta-aramid fibers of present invention have a lower porosity than prior art meta-aramid fibers. Due to the lower porosity, the fibers of instant invention may have better mechanical properties.

Especially advantageous is the combination of properties of the instant meta- aramid fibers. The fibers have an even, round cross section, improved flame retardancy and a lower porosity.

Instant application also pertains to a textile sheet comprising the meta-aramid fibers and/or the meta-aramid multifilament yarn of present invention. The textile sheet may have the form of a woven, knitted or plaited textile sheet or fabric or a nonwoven textile sheet.

The meta-aramid fibers of present invention may be used for textile applications, e.g. in textile sheets, including knitted and woven fabrics, or cords used as reinforcement of hoses, in protective apparel, especially for fire-resistant application.

The meta-aramid fibers of present invention are especially suited for textile applications where the meta-aramid fibers or fabrics comprising the fibers are in direct skin contact.

Current invention also pertains to protective clothing comprising the textile sheet of instant invention. Protective clothing may e.g. be a glove, jacket, trouser or shirt.

The present invention is described in more detail with reference to the following examples, which should not be construed to limit the scope of present invention. Examples

Methods

1. Mechanical properties

The breaking tenacity, elongation at break and toughness at rupture of the meta- aramid multifilament yarns are determined according to ASTM D 7269-17.

2. Sulfonic acid group content

The sulfonic acid group content is determined by 1 H-NMR. 20 mg of as-spun non- heat-treated sample are dissolved in 1 ml_ DMSO-d6 and 550 mI_ thereof is transferred to a 5 mm NMR sample tube. 1 H NMR spectra are recorded at 300 K on a Bruker Avance III 400 MHz NMR spectrometer equipped with a BBFO-plus 5mm broadband probe. Spectra are recorded by co-adding 64 scans with a 30° excitation pulse using a pre-scan delay of 6 s and an acquisition time of 4 s. The 1 H NMR spectra obtained were referenced by setting the DMSO-d6 residual solvent signal to 2.5 ppm.

The sulfonic acid group content is calculated by the formula below and expressed in meq/kg (mmol/kg):

ASMPD = integral (singlet @ 8.97 ppm) [mol]

Ameta aramid = (integral 8.86 - 7.08 ppm - (6 * AsMPD))/8 [mol] sMPD = (ASMPD * 186.1844) [mg] meta aramid = (Ameta 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 determined by inductively coupled plasma optical emission spectrometry (ICP-OES). To 100 mg of fiber, 9 ml of concentrated nitric acid (70 wt%) is added. This mixture is exposed to microwave digestion in an Ultrawave (Milestone) until a clear liquid is obtained. The volume is adjusted to 25 ml by addition of MilliQ water. Precipitates are removed from this solution by filtration. The clear filtrate is analyzed by ICP-OES in a Perkin Elmer Optima 8300 DV apparatus. For the determination of the sulfur content emission lines at 181,972 nm and 180,669 nm wavelength are used.

4. Organic solvent content

The organic solvent content is determined by gas chromatography. About lOmg of fibers was collected and heated over 500°C in an electric furnace. Gas chromatography (Shimadzu Corporation, Ltd., Model: GC-2010) was used to measure the amide solvent amount vaporized from the fiber. Subsequently, residual solvent concentration in the fiber was calculated by using the calibration curve prepared by using an amide-based solvent as a standard sample.

5. Relative viscosity

A polymer sample is dried at 50°C in a vacuum oven for two hours for removal of water. The dried sample is then dissolved in sulfuric acid at room temperature overnight. The flow time of the 0.25 %(wA/) sample solution in 96 %(w/w) sulfuric acid was then measured at 25°C in an Ubbelohde viscometer (e.g. Schott AVS370). Under identical conditions the flow time of the solvent was measured as well. The relative viscosity was then calculated as the ratio between the two observed flow times.

6. Microscopy

The yarn is embedded in melted paraffin, which is left for about five minutes and solidified. Thereafter, the embedded sample is cut, vertical to the fiber axis, by a microtome to obtain a cut piece of 5 to 7 pm thick. Then, the cut piece is placed on a slide glass which is heated to melt the paraffin. Thereafter, the melt paraffin was removed by xylene and ethanol. Next, a cross-section of the fiber is observed and photographed by using an optical microscope (manufactured by NIKON CORPORATION; trade name “ECLIPSE” LV100N) to obtain a cross-sectional photograph. The magnification is selected within a range from 100 to 1000 as necessary.

7. LOI

The limiting oxygen index (LOI) iss determined in accordance with ASTM D2863.

A yarn specimen of 168000 dtex is prepared from each sample by combining the required number of yarns. The yarns are wound on a precision reel, with a yarn tension during winding of 5 ± 3mN/tex based on the nominal linear density of the yarn. This specimen is encircled with thin copper wire. Each specimen has a length of approximately 150 mm, a width of approximately 10 ± 0.5 mm and a thickness of approximately 3 ± 0.25 mm. The specimen is marked at approx. 50 mm from the end which is to be ignited. Immediately before testing, the specimen is conditioned for at least 88 hours at 23 ± 2°C and 50 ± 5% relative humidity. The specimen were tested according to option A of ASTM D2863 (top surface ignition).

Example 1

Fibers were spun from a spin dope comprising a m-aramid polymer (poly(m-phenyl isophthalamide) having a relative viscosity of 1.55.

The m-aramid polymer was mixed in a Theysohn 20 mm twin screw extruder at a temperature of 85°C and a speed of 300 rpm with 99.8 wt% sulfuric acid to a polymer concentration of 18 w/w% to obtain a spin dope.

The spin dope was processed into filaments by passing it at 55°C through filters and at 85°C through a spinneret, through an air gap and into a static coagulation bath (under the conditions indicated in Table 1). The coagulation bath had a temperature of 3°C.

Table 1: Settings in spinning process

The multifilament yarns obtained after coagulation were washed and neutralized by subsequently passing them through baths of water, 0.4% NaOFI and again water. The yarns were dried at 150°C and wound on the bobbin.

The properties of the yarns obtained after drying (also indicated as “as-spun”) were determined.

Table 2: Mechanical properties of the as-spun yarns For sample 1-1 the sulfonic acid group content was determined to be 116 meq/kg. Sample 1-1 was heat treated in a two-step process. Yarns were reeled off from the bobbin and lead through a first oven in which a draw ratio of 1.6 was applied, and through a second oven that was kept at a temperature of 333 °C and in which the draw ratio was kept at 1 , the draw ratio being 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 s. Example 2

Fibers were spun from a spin dope comprising a m-aramid polymer having a relative viscosity of 1.55. The m-aramid polymer was mixed in a Theysohn 20 mm twin screw extruder at a temperature of 55°C (2-1 and 2-3) or 60°C (2-2 and 2-4) and a speed of 450 rpm with 99.8 wt% sulfuric acid to a polymer concentration of 16 wt/wt% or 17.5 wt/wt% to obtain a spin dope.

The spin dope was processed into filaments by passing it through filters and through a spinneret, through a 5 mm air gap, in which a draw ratio of 3.41 was applied, and into a static coagulation bath (under the conditions indicated in Table 4). The coagulation bath had a temperature of 5°C.

Table 4: Spinning settings example 2

The multifilament yarns obtained after coagulation were washed and neutralized by subsequently passing them through baths of water, 0.25% NaOH and again water. The yarns were dried at 150°C and wound on the bobbin.

The properties of the yarns obtained after drying (also indicated as “as-spun”) were determined.

Table 5: Mechanical properties of the as-spun yarns Table 6: Relative viscosity and sulfonic acid group content of the fiber

Sample 2-3 was heat treated in a two-step process. Yarns were reeled off from the bobbin and lead through a first oven in which at a temperature of 305°C various draw ratios were applied, and through a second oven that was kept at a temperature of 333°C and in which the draw ratio was kept at 1 , the draw ratio being defined as the speed after the oven divided by the speed before the oven. The residence time in both ovens was calculated based on the speed between both ovens. The sulfur content of sample 2-3 was determined to be 0.18 wt%. Table 7: Heat treatment and mechanical properties of meta-aramid yarns treated in a two-step heating process

Example 3

Fibers were spun from a spin dope comprising a m-aramid polymer having a relative viscosity of 1.58.

The m-aramid polymer was mixed in a Clextral 53 mm twin screw extruder at a temperature of 45°C and a speed of 250 rpm with 99.8 wt% sulfuric acid to a polymer concentration of 12 wt/wt% to obtain a spin dope. The spin dope was processed into filaments by passing it at 50°C through filters and at 50°C through a spinneret containing 1000 capillaries of 65 pm diameter, through an air gap, in which the filaments were drawn by a factor of 2.9, and into a falling jet coagulation bath (under the conditions indicated in Table 1). The coagulation bath had a temperature of 5°C.

The multifilament yarns obtained after coagulation were washed and neutralized by subsequently passing them through baths of water, 0.35% NaOH and again water. The yarns were dried on a godet at 160°C. Optionally the yarns were hot drawn on heated godets and wound on the bobbin.

The properties of the yarns obtained were determined and are shown in table 9. Table 9: Mechanical properties yarns Example 4

Fibers were spun from a spin dope comprising a meta-aramid polymer having a relative viscosity of 1.55. The m-aramid polymer was mixed in a Theysohn 20 mm twin screw extruder at a temperature of 60°C and a speed of 450 rpm with 99.8 wt% sulfuric acid to a polymer concentration of 16 wt/wt% to obtain a spin dope. The spin dope was processed into filaments by passing it through filters and through a spinneret, through a 5 mm air gap, in which a draw ratio of 2.08 was applied, and into a dynamic coagulation bath (under the conditions indicated in Table 10). The coagulation bath had a temperature of 5°C. After the coagulation bath the yarns were wet drawn between 2 roller sets.

Table 10: Settings in spinninc process

The multifilament yarns obtained after coagulation and wet drawing were washed and neutralized by subsequently passing them through baths of water, 0.25% NaOH and again water. The yarns were dried at 150°C and wound on the bobbin.

The properties of the yarns obtained after drying (also indicated as “as-spun”) were determined. Table 11 : Mechanical properties of the as-spun yarns

Sample 4-2 was heat treated in a two-step process. Yarns were reeled off from the bobbin and lead through a first oven in which at a temperature of 315°C a draw ratio of 1 was applied, and through a second oven that was kept at a temperature of 315 °C and in which the draw ratio was varied, the draw ratio being defined as the speed after the oven divided by the speed before the oven.

Table 12: Heat treatment and mechanical properties of meta-aramid yarns treated

Example 5

Fibers were spun from a spin dope comprising a meta-aramid polymer having a relative viscosity of 1.55. The m-aramid polymer was mixed in a Theysohn 20 mm twin screw extruder at a temperature of 50°C and a speed of 300 rpm with 99.8 wt% sulfuric acid to a polymer concentration of 16 wt/wt% to obtain a spin dope. The spin dope was processed into filaments by passing it through filters and through a spinneret containing 106 capillaries with a diameter of 65 pm, maintained at temperature of around 75°C, through a 10 mm air gap, in which a draw ratio of 1.95 was applied, and into a dynamic coagulation bath. The speed after the coagulation bath was 40 m/min. After the coagulation bath the yarns were wet drawn 1.4 times between 2 roller sets. The yarns obtained after coagulation and wet drawing were washed and neutralized by subsequently passing them through baths of water, 0.25% NaOH and again water. The yarns were dried on a roller set at 220°C. The yarns moved to 2 successive roller sets, of which the settings are indicated in table 13. Table 13: Settings in spinning and hot drawing process

The yarns were wound on a bobbin. The mechanical properties of the yarn are shown in table 14. Table 14: Mechanical properties of the yarns

Example 6 - Organic solvent content

The NMP and DMAc content of commercially available TeijinConex® B, Nomex® and Yantai Tayho Advanced Materials’ meta-aramid yarn was determined by gas chromatography. The results are shown in table 13. Yarn according to the invention does not comprise organic solvent.

Table 15 - organic solvent content n.d.= not determined

Example 7 - Cross section of filaments

Micrographs of embedded bundles of comparative samples 1 to 3 (as used in example 6) and a meta-aramid yarn according to the invention were prepared. The micrographs depicting the cross section of the filaments are shown in Fig. 1. Panel a) shows a cross section of filaments of comparative sample 1 , panel b) of comparative sample 2, panel c) of comparative sample 3 and panel d) of the meta- aramid yarn according to the invention. As can be seen from the comparison, the filaments of the current invention have an even, round cross section, while the comparative filaments have an oblong cross section or a cross sections of varying diameter.

Example 8 - Limiting Oxygen Index

The LOI of meta-aramid yarns according to the invention and commercially available meta-aramid yarn was determined. The results are shown in table 14.

Table 16: LOI of meta-aramid yarns according to the invention and commercially available meta-aramid yarns.

The data show that the samples according to the invention have a higher LOI value than the comparative sample according to the prior art.