Colored aramid fibers

Abstract: Colored, high strength, high modulus p-aramid fibers are prepared by including an organic pigment which is soluble in but not degraded by concentrated sulfuric acid in a p-aramid spinning dope wherein the solvent is concentrated sulfuric acid and spinning the pigment containing dope through an air gap into a coagulation bath and washing and drying the resulting fibers. The dissolved pigment is precipitated by the coagulation bath as particles with a diameter of more than 0.01 to 0.50 microns.

Description: Field of Invention This invention relates to colored, high strength, high modulus p-aramid fibers and a process for preparing them. High strength, high modulus p-aramid fibers are known from U S-A-3 869 429 (Blades). These fibers are extremely difficult to dye. Some improvement in dyeability can be obtained by mechanically crimping these fibers while wet but dye penetration is limited to the crimp nodes of the individual filaments and the mechanical properties of the fibers are degraded. Colored p-aramid fibers of relatively low strength and modulus are known from U S-A-3 888 821 and British Patent 1,438,067. These patents disclose the wet spinning of poly( p -phenylene terephthalamide) from sulfuric acid solutions which also contain dissolved dyes. The dyes used are vat dyes or copper phthalocyanine pigment. EP-A-21 484 describes a process for spinning poly(p-phenylene terephthalamide) fibers by blending finely divided solidified sulfuric acid (purity ≧ 96% by weight) with finely divided polymer to a mixture in solid state containing 16 to 21% by weight polymer. Pigments may be added during blending. The blend is deaerated (preferably in the solid state), its temperature is raised to 70 to 100°C, and the molten mixture is spun. The filament is then passed through an air gap of 1 to 100 mm, where it is stretched at a stretch ratio of from 1.9 to 10. It is then passed through a coagulation bath (e.g. water) at a temperature of 0 to 25°C, and washed in aqueous solutions of alkaline substances. The fibers obtained preferably have a tenacity of at least 17 cN/tex and an initial modulus of at least 350 cN/tex. The aramid used preferably has an inherent viscosity of ≧ 3.5. The older but non-prepublished EP-A-295,672 discloses high strength, high modulus p-aramid fibers which are characterized in that they contain 0.01 to 6% by weight of a completely organic pigment distributed uniformly throughout the fibers and having no particles or agglomerates larger than 0.01 µm. Brief Description of the Invention This invention provides high strength, high modulus p-aramid fibers exhibiting a filament tenacity of at least 18 g/d and a filament initial modulus of at least 400 g/d, characterized in that they contain 0.01 to 6% by weight of a completely organic pigment selected from the group consisting of (1) monoazo and disazo pigments, (2) anthanthrone pigments, (3) indanthrone pigments, (4) pyranthrone pigments, (5) violanthrone pigments, (6) flavanthrone pigments, (7) quinacridone pigments, (8) dioxazine pigments, (9) indigoid and thioindigoid pigments and (10) isoindolinone pigments, distributed throughout the fibers and having particles or agglomerates with a diameter of more than 0.01 to 0.50 µm. Monoazo and disazo pigments have the structure wherein R₁, R₂ and R₃ are chloro, nitro, methyl, methoxy, or hydrogen, R₄ is hydroxy, and R₇ is wherein R₅ and R₆ are hydrogen, methyl, or chloro. Anthanthrone pigments have the structure wherein R₁, R₂ and R₃ are -H, -Cl, or -Br. Indanthrone pigments have the structure wherein R₁, R₂ and R₃ are -H, -OH, -Cl, -Br, -NH₂, or fused aromatic groups, R₄ and R₅ are -H, -CH₃, or -C₂H₅. Pyranthrone pigments have the structure wherein R₁, R₂ and R₃ are -H, -Cl, or -Br. Violanthrone pigments have the structure wherein R₁, R₂, and R₃ are -H, -Cl, -Br, -OCH₃, -OC₂H₅, or a fused aromatic group. Flavanthrone pigments having the structure wherein R₁, R₂ and R₃ are -H, -Cl, -Br, -OH, an aromatic group of a fused aromatic group. Quinacridone pigments have the structure Dioxazine pigments have the structure wherein R₁ and R₂ are -H or -Cl and R₃ and R₄ are -CH₃ or -C₂H₅. Indigoid pigments have the structure wherein R₁, R₂, R₃, R₄, R₅ and R₆ are -H, -Cl, -Br, -CH₃ or -NH₂ and thioindigoid pigments have the structure wherein R₁, R₂, R₃, R₄, R₅ and R₆ are -H, -Cl, -NH₂, -OC₂H₅, -SC₂H₅, -CH₃, -OCH₃, phenyl or fused aromatic groups. Isoindolinone pigments have the structure The preferred monoazo pigment is Colour Index Pigment Red 3. The preferred disazo pigment is Colour Index Pigment Red 242. The preferred anthanthrone pigment is Colour Index Pigment Red 168. The preferred indanthrone pigment is Colour Index Pigment Blue 60. The preferred pyranthrone pigment is Colour Index Pigment Orange 40. The preferred violanthrone pigment is Colour Index Pigment Blue 65. The preferred flavanthrone pigment is Colour Index Yellow 24. The preferred quinacridone pigment is Colour Index Pigment Red 122. The preferred dioxazine pigment is Colour Index Pigment Violet 23. The preferred indigoid and thioindigoid pigments are Colour Index Pigment Red 88 and Colour Index Pigment Red 86, respectively. The most preferred isoindolinone pigment is Colour Index Pigment Yellow 173. The organic pigments of the foregoing structures are those pigments named in the Colour Index published by the Society of Dyers and Colourists. The colored high strength, high modulus p-aramid fibers of this invention have visible colorant particles when viewed under an electron microscope. The particles or agglomerates are consistently smaller than 0.50 in diameter. Above 0.50 µm in diameter, particles cause a decrease in the tenacity attainable; and, as particles increase in size, tensile strength decreases further. The fibers have a yarn tenacity of at least 18 g/d (15.9 dN/tex) and an initial modulus of at least 400 g/d (354 dN/tex). Filament tenacity is often higher, by as much as 3 gpd (2.6 dN/tex). This invention also provides a process for the preparation of high strength, high modulus p-aramid fibers comprising the steps of: a) agitating a mixture of: i) sulfuric acid having a concentration of at least 98%; ii) p-aramid polymer having an inherent viscosity of at least 4 in an amount which is at least 18 weight percent of the mixture; and iii) pigment b) heating the mixture with continued agitation to a temperature of 80 to 105°C to form a uniform solution; c) extruding the solution through a spinneret; d) passing the extruded solution through a non-coagulating fluid layer 0.5 to 2.5 centimeters thick such that the spin stretch factor is 3 to 10; e) passing the stretched solution into and through an aqueous coagulating bath having a temperature of -5 to 25°C to form filaments; and f) washing the filaments with water and/or dilute aqueous alkali, characterized in that the pigment is a completely organic, sulfuric acid soluble, pigment in an amount which is from 0.01 to 6 weight percent based on the p-aramid polymer, and is present in the fiber in form of particles and agglomerates with a diameter of more than 0.01 to 0.5 µm. The spin stretch factor is the ratio of the velocity of the filaments as they leave the coagulating bath to the velocity of the extrudate as it leaves the spinneret. Detailed Description of the Invention The para-oriented aromatic polyamides (p-aramids) useful in the present invention are those described in U S-A-3 869 429 in which rigid radicals are linked into polymer chains by amide groups. The chain-extending bonds of the rigid radicals are either coaxial or parallel and oppositely directed. The rigid radicals may be single-ring radicals, multi-ring radicals in which the chain-extending bonds are para-oriented, fused ring radicals or heterocyclic radicals. Preferred rigid radicals are 1,4-phenylene, 2,6-naphthalene, 1,5-naphthalene, 4,4'-biphenylene, trans-1,4-cyclohexylene, trans-trans-4,4'-bicyclohexylene, 1,4-pyridylene and 1,4-phenylene groups linked by trans-vinylene, ethynylene, azo or azoxy groups. The polyamides may be substituted with simple groups such as chloro- and methyl groups. Both homopolymers and copolymers are suitable as long as the rigid radicals are as defined above. Up to 5 mol percent of non-conforming radicals may be included. The polyamides may be prepared by reaction of a suitable aromatic acid halide with a suitable aromatic diamine in a non-reactive amide solvent which may contain solubilizing salts such as LiCl or CaCl₂. The polyamide should have an inherent viscosity of at least 4. By high strength is meant a yarn or filament tenacity of at least 18 g/d (15.9 dN/tex). By high modulus is meant having a yarn or filament initial modulus of at least 400 g/d (354 dN/tex). The single fibers of the present invention usually have a denier of 0.5 to 15 but such is not critical. The purely organic pigments suitable for use in the present invention are soluble in sulfuric acid having a concentration of at least 98%, but are insoluble in water or organic solvents and do not degrade appreciably in 98% sulfuric acid at 95°C when held at that temperature for three hours. Indications of pigment degradation include change of color in the final fiber, bleeding of the pigment into the coagulation bath and precipitation of the pigment from the polymer solution. The amount of organic pigment will depend on the tint desired and the type of organic pigment used but in general 0.01 to 6% by weight pigment in the fibers provides useful results. Suitable organic pigments may show a change in color when dissolved in concentrated sulfuric acid but will return to the original color on coagulation and washing of the fibers. The chemical structures of preferred organic pigments have been defined above. Organic pigments with an inorganic component are generally unsatisfactory. It has been found that some vat dyes may, also, dissolve in sulfuric acid spinning solutions without severe degradation, and some such spinning solutions may be spun to yield fibers having extremely small particles of vat dyes therein -- on the order of less than 0.01 µm. In contrast to the purely organic pigments of the present invention, however, vat dyes have been found to interfere with the crystal structure of the fibers and to cause a severe decrease in fiber tenacity. In the process of this invention, sufficient p-aramid polymer having an inherent viscosity of at least 4.0 is mixed with cold sulfuric acid having a concentration of at least 98% and the desired amount of sulfuric acid soluble organic pigment to provide, when heated, a dope having a p-aramid concentration of at least 18% by weight. The dope is heated to 80-105°C with stirring and degassed. The hold-up time of the dope may be 1-3 hours in a commercial spinning process. The dope is extruded through a spinneret having orifices with a diameter of 0.025 to 0.125 mm through a layer of non-coagulating fluid, usually air, into an aqueous coagulating bath having a temperature of -5 to 25°C. The air gap may be from 0.5 to 2.5 cm but preferably is about 0.7 cm. The yarn is further washed with dilute alkali and/or water and wound up on bobbins. The fibers are of the same color as the original organic pigment added. No color is lost to the aqueous coagulation bath. Measurements and Tests Linear Density This is usually calculated as denier, that is, the weight in grams of a 9000-meter length of yarn. Multiplication of denier by 1.1111 yields linear density in dtex. Tensile Properties Tenacity is reported as breaking stress divided by linear density. Modulus is reported as the slope of the initial stress/strain curve converted to the same units as tenacity. Elongation is the percent increase in length at break. Both tenacity and modulus are first computed in g/denier units which, when multiplied by 0.8826, yield dN/tex units). Each reported measurement is the average of 10 breaks. Tensile properties for yarns are measured at 24°C and 55% relative humidity after conditioning under the test conditions for a minimum of 14 hours. Before testing, each yarn is twisted to a 1.1 twist multiplier (for example, nominal 1500 denier yarn is twisted about 0.8 turns/cm). Each twisted specimen has a test length of 25.4 cm and is elongated 50% per minute (based on the original unstretched length) using a typical recording stress/strain device. Tensile properties for filaments are measured at 21°C and 65% relative humidity after conditioning under test conditions for a minimum of 14 hours. A single filament is mounted to provide a test length of 2.54 cm using 3B Pneumatic Action Clamps with neoprene faces (available from Instron Corp.). Rate of elongation is 10% per min. Tensile properties of filaments are normally at least as large as the properties for yarns. Inherent Viscosity Inherent viscosity (η inh ) is measured at 30°C and computed from η inh = ln(t₁/t₂)/c where t₁ = solution flow time in the viscometer t₂ = solvent flow time in the viscometer c = polymer concentration of 0.5 g/dL, and the solvent is concentrated sulfuric acid (95-99 wgt %). Twist Multiplier The twist multiplier (TM) correlates twist per unit of length with linear density of a yarn being twisted. It is computed from TM = (Denier) 1/2 (tpi)/73 where tpi = turns/in TM = (dtex) 1/2 (tpc)/30.3 where tpc = turns/cm Particle Size The fibers of this invention have colorant particles or agglomerates with a diameter consistently smaller than 0.50. Example and Comparative Example Sulfuric acid having a concentration of 100.1% (24,235 g) was cooled in a reaction vessel to -5°C by a circulating -25°C glycol jacket. Poly(p-phenylene terephthalamide) having an inherent viscosity of 6.3 (5,889 g) and Sandorin Blue RL (Pigment Blue 60) powder (176.7 g) were added to the reaction vessel. The mixture was stirred while the temperature was gradually increased to 85°C. The mixture was stirred for two hours at 85°C under a reduced pressure of 33.25 mbar (25 mm (Hg)) to eliminate air bubbles. The resulting dope was extruded through a filter pack and then through a 267 hole spinneret having spinning capillaries 0.063 mm in diameter, and finally through an air gap of 0.7 cm length into an aqueous coagulating bath at 5°C. The extruded dope was stretched 6.3 X in the air gap. The resulting fibers were further washed with dilute aqueous alkali and water, dried on a roll at 180°C and wound up at 732 m/min. No color was lost to the coagulating bath. Pigment level was 4% based on weight of fiber. Yarn tenacity/elongation/modulus was 18.3 g/d/2.6%/674 g/d (16.1 dN/tex/2.6%/595 dN/tex). As a comparison, a spin was, also, conducted identical with the Example, above, except using 4% of a vat dye identified as C.I. Vat Violet 1. The fibers from that spin had tenacity/elongation/modulus of 15.5g/d/3.1%/516g/d (13.7 dN/tex/3.1%/456 dN/tex). To further determine differences between the pigmented fibers of the Example and the dyed fibers of the Comparative Example, it was determined that the Orientation Angle (OA) and the Apparent Crystallite Size (ACS) for those fibers and for a control fiber made according to Example 1 but with no color additives, exhibited the following qualities: Fiber OA(deg) ACS(Å) Control 11.9 53.5 Example 11.6 53.7 Comparative Ex. 19.7 47.1 Orientation Angle and Apparent Crystallite Size are determined as described in U S-A-3 869 429. Lower Orientation Angle values indicate higher degrees of polymer orientation and increased tensile strengths. To observe the differences between fibers having the pigment of this invention and fibers having dye, photomicrographs were made of the fiber product of this example and comparative example. Sample fibers were embedded in an epoxy resin, cut using an ultra microtome along a direction at 45 degrees to the fiber axis into a 200 nm (2000 Å) thick specimen, and examined on a cut surface using an electron microscope at 500 - 10000 X total magnification. Sections were, also, made in the longitudinal direction (along the fiber axis). Fig. 1 is a photomicrograph of a cross-section of the fiber of the example with Sandorin Blue pigment. The dark spots in the cross-section are particles of pigment which precipitated from its initial solution in the spinning dope on contact with the coagulation bath after spinning was complete. The particles, while apparently only relatively few in number, represent a part of the pigment concentration which serves to give the fibers a brilliant blue appearance. The pigment particles which are visible are a uniform 0.1 µm in diameter. Fig. 2 is a photomicrograph of a cross-section of the fiber of the comparative example with the C.I. Vat Violet 1 vat dye. There are no particles evident in the photograph. It is not understood what mechanism explains this; but, because significant loss in tenacity occurred, it is probable that the dye became bound to the polymer in such a way as to disrupt crystallization to some extent. Figs. 3 and 4 are photomicrographs of longitudinal sections of the fibers of the example and comparative example, respectively. The observations are the same as for Figs. 1 and 2.

Claims: High strength, high modulus p-aramid fibers exhibiting a filament tenacity of at least 18 g/d and a filament initial modulus of at least 400 g/d, characterized in that they contain 0.01 to 6% by weight of a completely organic pigment selected from the group consisting of (1) monoazo and disazo pigments, (2) anthanthrone pigments, (3) indanthrone pigments, (4) pyranthrone pigments, (5) violanthrone pigments, (6) flavanthrone pigments, (7) quinacridone pigments, (8) dioxazine pigments, (9) indigoid and thioindigoid pigments and (10) isoindolinone pigments, distributed throughout the fibers and having particles or agglomerates with a diameter of more than 0.01 to 0.50 µm. The fibers of Claim 1 characterized in that the p-aramid is poly( p -phenylene terephthalamide). The fibers of Claim 1 characterized in that the organic pigment is selected from the group consisting of Colour Index Pigment Red 3 and Colour Index Pigment Red 242. The fibers of Claim 1 characterized in that the organic pigment is Colour Index Pigment Blue 60, Colour Index Pigment Orange 40, Colour Index Pigment Blue 65, Colour Index Pigment Yellow 24, Colour Index Pigment Red 122, Colour Index Pigment Violet 23, Colour Index Pigment Red 88, Colour Index Pigment Red 86 or Colour Index Pigment Yellow 173. A process for preparation of high strength, high modulus p-aramid fibers comprising the steps of: a) agitating a mixture of: i) sulfuric acid having a concentration of at least 98%; ii) p-aramid polymer having an inherent viscosity of at least 4 in an amount which is at least 18 weight percent of the mixture; and iii) pigment b) heating the mixture with continued agitation to a temperature of 80 to 105°C to form a uniform solution; c) extruding the solution through a spinneret; d) passing the extruded solution through a non-coagulating fluid layer 0.5 to 2.5 centimeters thick such that the spin stretch factor is 3 to 10; e) passing the stretched solution into and through an aqueous coagulating bath having a temperature of -5 to 25°C to form filaments; and f) washing the filaments with water and/or dilute aqueous alkali, characterized in that the pigment is a completely organic, sulfuric acid soluble, pigment in an amount which is from 0.01 to 6 weight percent based on the p-aramid polymer, and is present in the fiber in form of particles and agglomerates with a diameter of more than 0.01 to 0.5 µm. Process of Claim 5 wherein the p-aramid is poly( p -phenylene terephthalamide). Process of Claim 5 characterized in that the organic pigment is selected from the group consisting of (1) monoazo or disazo pigments, (2) anthanthrone pigments, (3) indanthrone pigments, (4) pyranthrone pigments, (5) violanthrone pigments, (6) flavanthrone pigments, (7) quinacridone pigments, (8) dioxazine pigments, (9) indigoid and thioindigoid pigments and (10) isoindolinone pigments.

Inventors: LEE KIU-SEUNG, LEE, KIU-SEUNG
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1988
Main classification: D01F1
IPC codes: D01F1, D01F6