Applying stretching in the modal spinning process

Abstract

The present invention relates to applying stretching in a Modal spinning process. In particular, the present invention relates to a process for producing Modal fibers, wherein stretching is applied to the filaments leaving the spinning bath in at least two stretching steps.

Description

Technical Field

[0001] The present invention relates to the Modal process.

[0002] According to the BISFA definition, Modal fibers are characterized by minimum requirements for tenacity and wet modulus. In addition to this, the technology used to manufacture Modal fibers with their unique combination of physical and textile properties, softness and fabric stability should comply with the requirements of the European Ecolabel and the IPPC guidance on Best Available Techniques (BAT) for process emissions resulting from proprietary manufacturing methods. Background Art

[0003] The key elements of the process are a balanced combination of the following factors compared to conventional viscose processes: low hemicellulose concentration in the impregnation lye, short aging time and higher DP of the cellulose, higher inputs of carbon disulfide (CS2) and sodium hydroxide (NaOH), use of special modifiers, viscose fibers with low cellulose concentration, high alkali ratio and high viscosity, lower concentrations of sulfuric acid and sodium sulfate and high zinc concentration in the spinning bath, higher number of spinneret holes, high drawing and low spinning speed and high specific formation and yield of sodium sulfate. In particular, the process can consist of the following process steps:

[0004] Slurry

[0005] All dissolving grade pulps of sufficient purity, reactivity, whiteness and appropriate viscosity can be used for the Modal process. Such pulps can be prepared from softwood or hardwood or even non-wood materials by sulfite or sulfate technology and used in the form of sheets, coils or flakes. The integrated preparation of pulp and fiber allows the use of wet pulp, which can be fed directly into the pulper without drying. The subsequent process steps in the viscose preparation require adjustments due to the physical and chemical properties of the pulp. The pulp may also contain impregnation aids, which are surfactants used to enable better penetration of the impregnation lye.

[0006] Pulping

[0007] Wood-based cellulose, such as baled dry pulp or wet pulp (48-50% dry matter), is mixed with a temperature-controlled sodium hydroxide liquor (impregnation liquor) containing 220-240 g / l total alkali in several pulpers equipped with special turbine stirrers. Alkali cellulose (AC) is formed by the reaction of cellulose with sodium hydroxide. Impurities like so-called "hemicelluloses" and their degradation products are dissolved in the liquor.

[0008] The pulp is stored in a pulp tank from which it is continuously pumped to an AC press which separates the alkali cellulose from the press alkali liquor.

[0009] Alkali Cellulose (AC)

[0010] After pressing the pulp in a drum press or a belt press, a wet cake of alkali cellulose containing 32-34% cellulose and 15-17% caustic soda is obtained. The AC is shredded to obtain a high specific surface area before the aging process.

[0011] AC aging (pre-aging)

[0012] The aging process is necessary to adjust the degree of polymerization of cellulose by depolymerization in the presence of air or oxygen in order to obtain the desired viscosity of the spinning solution. The crushed alkali cellulose is aged in a belt aging, aging drum, tower or cabinet device. Depending on the requirements, the aging time can be adjusted by changing the speed and temperature of the conveyor or drum. If necessary, the pre-aging process can be accelerated by adding a catalyst (such as cobalt chloride or manganese sulfate) to obtain an AC (SCAN) with an intrinsic viscosity of 400-450 ml / g.

[0013] After aging, the alkali cellulose is cooled in an alkali cellulose cooler, which operates as a fluidized bed reactor. A rotating cooling drum or a continuous flow conveyor can also be used.

[0014] xanthation

[0015] Alkali cellulose is reacted with CS2 to obtain sodium cellulose xanthate, which is soluble in dilute sodium hydroxide (dissolving lye). For this purpose, AC is fed into the xanthator via a conveyor belt and a weigh bin. The exothermic reaction of AC with carbon disulfide is controlled by cooling the AC.

[0016] The reactor used can be a wet churn equipped with an intensive kneader, a continuous twin screw kneader or a belt xanthogenizer. AC is reacted with liquid CS2 under vacuum and kneaded like dough until the reaction is complete. The CS2 input can be 35-38% based on cellulose.

[0017] As a byproduct of xanthate formation, CS2 also reacts with NaOH to form Na2CS3 and Na2S.

[0018] Sufficient degree of substitution is required to dissolve the xanthate in dilute NaOH and is expressed as the ɣ value (moles of CS2 / 100 moles of glucose units). The concentration of sodium hydroxide determines the alkali content of the viscose. By varying the degree of substitution, the alkali- / cellulose ratio and the average length of the chains, different viscose qualities can be achieved.

[0019] The xanthation apparatus may be of the Simplex or Suma type. The temperature at the start of xanthation may be 15-17°C and at the end of xanthation the temperature may be 30-35°C.

[0020] Dissolve

[0021] By adding cooled dissolved lye to the xanthate oxidizer, the mixture of lye and xanthate is discharged through a homogenizer into the dissolver.

[0022] The homogenization of viscose fibers is achieved in a fine homogenizer at low temperatures. Modifiers are added to the viscose fiber stock solution. The function of the modifier is to control coagulation and regeneration during the formation of filaments during the spinning process. Many different compounds or their blends can be used as modifiers: amines and polyamines, quaternary ammonium compounds, amides, heterocyclic nitrogen compounds, polyglycols, ethoxylated amino derivatives, ethoxylated fatty acid amines and amides, ethoxylated glycerol and ethoxylated fatty acids, esters or alcohols. Many examples of these compounds (which can be used alone or as a blend) are described in K. Götze, "Chemiefasern nachdem Viskoseverfahren", 3rd edition, Springer, 1967, pp. 635-641, the disclosure of which is incorporated herein by reference.

[0023] The modifier is used in an amount of 1.0-4.0% based on cellulose.

[0024] The modifier can be added during the dissolution process, preferably directly in the dissolver, after degassing and before the final filtration of the viscose fibers or directly before spinning.

[0025] This can be done by means of commonly known devices for introducing substances into the stream of a liquid medium. Preferably, after the addition point there is a device for providing a uniform distribution of the modifier in the viscose fibers, such as a static or dynamic mixer. Such mixers are well known and commercially available, for example Sulzer mixers. In a continuous process, the viscose fibers from the dissolver are further treated in two refiners to improve the dissolved state.

[0026] Unfiltered viscose fibers may have a cellulose content of 6.0-7.0%, an alkali content of 6.0-7.0% and a sulphur content of 1.7-2.0%.

[0027] Aging, Filtration and Degassing

[0028] After xanthate homogenization, the viscose fibers must undergo a successive ripening, degassing and filtering process.

[0029] The purpose of the aging in the tank is to achieve a better distribution of the xanthate groups on the cellulose chains and to obtain the desired aging index or gamma value for the subsequent spinning process.

[0030] Before spinning, the viscose fibers must be filtered to remove fibers, gel particles or other impurities, as these would block the fine spinneret holes and cause spinning defects. Instead of a conventional filter press, a continuous waste filter can be used, which is equipped with fine metal fleece fibers of different pore sizes. To obtain the best filtrate quality, the filtration process is carried out in two or three steps.

[0031] Before the viscose fibers are pumped to the spinning pot, continuous rapid degassing is performed to remove air bubbles.

[0032] The spun viscose fiber may have a ball-falling viscosity of 80-140 s, a gamma value of 57-64, a cellulose content of 6.0-7.0%, an alkali content of 6.0-7.0%, a sulfur content of 1.6-1.9% and an alkali / cellulose ratio of 0.95-1.1.

[0033] Spinning

[0034] Modal staple fibers are manufactured on a spinning machine consisting of a spinning trough containing a spinning bath, rotating godet rolls with variable drives and ceramic guides for taking up the filaments, and a final draw roll for collecting the individual tows after drawing in a hot secondary bath. Such a draw roll can be, for example, a two-set triple of squeeze rolls, which squeeze the tows and reduce chemical carryover to the acid wash, or a series of six or more non-squeezing rolls.

[0035] The viscose fibers are pumped by a gear pump, passed through a candle filter, and extruded through the fine holes of a spinneret immersed in a spinning tank.

[0036] Different spinning geometries can be applied for Modal fibers: diagonal or vertical spinning with two or three filament bundles per godet.

[0037] For ventilation purposes, the machine is fully encapsulated to recover the sulphide gases and achieve safe working conditions in the spinning area.

[0038] A new spinning bath enters the spinning tank from the bottom and its overflow is returned to the spinning bath system where it is degassed, filtered, excess water evaporated, excess sulfate crystallized, and make-up chemicals added to adjust the composition before being recycled back to the spinning machine.

[0039] After the viscose fibers are extruded through the spinneret holes, coagulation and regeneration occur in parallel. The viscose fibers coagulate and the high viscosity gel turns into a sol. During regeneration, the xanthate is decomposed by the acid of the spinning bath to become regenerated cellulose and the caustic soda of the viscose fibers is neutralized by the acid to form sodium sulfate. Both processes are affected by factors such as the composition and temperature of the viscose fibers and the spinning bath, the type and concentration of the modifier (which delays the regeneration process and its interaction with the spinning bath components), the spinning speed, the spinning geometry and the immersion length of the filaments, the spinning bath / viscose fiber flow rate and the diameter and design of the spinneret holes, and therefore require a careful balance to obtain fibers with the desired physical parameters without spinning defects.

[0040] The tow from two or three spinnerets is collected, guided by ceramic guides and wound three times around godet rolls. Stretched by a take-up roll at the end of the spinning machine, the tow is introduced into a hot secondary bath (stretching bath), where regeneration is done and high stretching is applied so that the filaments gain strength by fixing and orienting the cellulose molecules.

[0041] Due to delayed regeneration, the spinning speed in the Modal fiber process is much lower than the conventional viscose process and can be 25 to 40 m / min, and higher draws up to 125% can be applied.

[0042] Spinneret

[0043] Spinneret design and spinning geometry are key factors in obtaining high quality modal fibers at high line efficiency. Modal fibers need to be spun at much lower spinning speeds than viscose fibers, which can be compensated by spinnerets with significantly more holes. This clustered spinneret consists of a cup body arranged on a plate, which can be a full disk or annular plate. The diameter of the cup body can be 12 to 16 mm, and the hole diameter depends on the fiber drop. The total number of holes per spinneret can be as high as 100,000 or even higher. In order to achieve sufficient immersion length, the filaments are spun vertically or diagonally at a certain angle. The immersion length can be 40 to 80 cm.

[0044] Spinning bath

[0045] The spinning bath is an aqueous solution of sulfuric acid, zinc sulfate and sodium sulfate, which is circulated through the spinning machine. The viscose fibers are coagulated by the salts present in the bath and decomposed by the acid to obtain regenerated cellulose filaments, Na2SO4, CS2 and H2S. In the spinning bath circulation system, the excess Na2SO4 is removed by crystallization, the sulfide gas is collected and recovered or converted into sulfuric acid in the form of CS2, sulfur, and supplementary chemicals sulfuric acid and zinc sulfate are added to adjust the spinning bath composition.

[0046] In the spinning bath, the H2SO4 concentration may be 70-90 g / l, the Na2SO4 concentration 90-130 g / l, the ZnSO4 concentration 48-62 g / l, and the temperature 37-43° C. The spinning speed, spinneret size, modifier type, and spinning bath components (including temperature and immersion length) are not independent of each other but need to be well balanced to achieve the desired fiber properties.

[0047] Stretching Bath

[0048] The draw bath is an encapsulated hot acid bath containing the spinning bath components in diluted form at a temperature of 90 to 98° C. Its function is to apply maximum tow draw between the godet rolls and the draw unit while the regeneration process is still in progress, to wash the tow, thereby reducing carryover of spinning bath chemicals to post-processing, and to collect sulfide gases from the system for recovery.

[0049] Cutting

[0050] The tows from all godet rolls are collected on a drawing unit to obtain a thick cable which is drawn into the cable injector of a cutter with acidic water and cut into the desired staple lengths with a rotating self-sharpening cutter knife.

[0051] The cut staple fibers are then washed with acidic water into the CS2 tank of the washing track for post-processing.

[0052] As an alternative to the injection wet cutter, a dry cutter is used, which is a horizontal disk with a fixed knife. The cable is wound around the disk and cut by pressing the second disk towards the knife.

[0053] Post-processing

[0054] The different stages of the process are fluff formation, treatment with acidic water, desulfurization, bleaching, washing and finishing. The process is carried out on an encapsulated washing track, which can be a wire belt or an eccentric conveyor, which transports the fluff fibers through post-treatment during which they are rinsed with different liquids. Pressing rollers between the washing zones reduce the carryover of chemicals.

[0055] The short fibers from the cutter are washed into the CS2 tank and treated with hot acidic water. When the residual CS2 is released and subsequently recovered from the exhaust gas, the short fibers open up and form fluff, which is picked up by the washing track. The uniformity of the fluff is essential for uniform fiber quality.

[0056] In the first zone, residual sulphide compounds are removed by treatment with hot acidic water. After squeezing, the fluff is rinsed with soft water in countercurrent. Fresh water is added before finishing and reused for washing after bleaching, after desulphurization and after acid treatment.

[0057] Desulfurization is carried out by treatment with a dilute mixture of caustic soda and sodium sulfide to dissolve and remove sulfur residues.

[0058] Sodium hypochlorite is used as a bleaching chemical.

[0059] Prior to the final finishing step, the fluff is washed with soft water and dewatered with high pressure rollers to reduce liquid carryover into the finishing area and achieve maximum finish absorption.

[0060] Finishing is an important process to prepare the fiber surface for further processing in yarn spinning or nonwoven processing. The different components are mixed to get the best balance between bonding, slip and antistatic properties.

[0061] After finishing, the fluff is dewatered with strong pressing rollers to minimize the water content before drying. The compacted wet fiber mat requires a wet opening process, which is carried out in two or three stages, using spiked rollers of varying strengths to prevent fiber damage. The moist, opened fluff fibers are now ready for drying.

[0062] Dry, open, pack

[0063] Drying can be done in a belt dryer or drum dryer with countercurrent hot air. Screen drum dryers consist of a series of rotating drums, each with a wire sleeve and a ventilator inside. Hot air is sucked inside the drum through the fluff, which dries and presses the fluff against the wire sleeve. By blocking one half of the drum cylinder, the fluff is transferred to the next drum and turned over. After passing through the first zone of the dryer, the fluff is opened again to get an even moisture distribution and rearranged for the final dryer zone. At the end of the drying process, the fibers are re-wetted to adjust the fiber moisture to the desired level. After drying has been completed, a final opening process is carried out in one or two stages to produce a uniform and bulk product ready for packaging.

[0064] Spinning bath degassing and filtration

[0065] During the spinning process, CS2 and H2S are dissolved in the spinning bath and need to be removed by degassing. By spraying the spinning bath in a degasser under vacuum, the gas is released and used as enrichment gas to produce sulfuric acid.

[0066] After filtering the degassed spinning bath through sand or candle filters, the composition of the spinning bath is adjusted by dosing sulfuric acid and zinc sulfate and returned to the bottom tank of the spinning bath circulation.

[0067] evaporation

[0068] During spinning, the spinning bath is diluted by water taken up from the viscose fibers and by water formed by the neutralization of NaOH with sulfuric acid. In order to maintain the required spinning bath concentration, excess water must be removed by a thermal multi-stage evaporation device. High-efficiency flash evaporators operate with low specific steam consumption.

[0069] crystallization

[0070] In the Modal process with a high alkali ratio, the amount of sodium sulfate formed by the neutralization reaction is significant. This process by-product must be removed and recovered from the spinning bath.

[0071] This is done through a crystallizer that uses caustic soda or sulfuric acid as a vapor absorption method to cool the temperature of the spinning bath. Crystals of Glauber's Salt (Na2SO4*10H2O) are grown in large drums and separated by pusher centrifuges.

[0072] Calcination

[0073] In order to make transportable and usable sodium sulfate, it is necessary to remove the crystallization water of the thenardite. For this purpose, pressure calcination is used. After melting the thenardite, the melt is led through a calcination evaporator. The crystals are thickened and then separated by a second set of centrifuges. The anhydrous sodium sulfate is dried, stored and packaged.

[0074] CS2 recovery by condensation

[0075] A mixture of water vapor, CS2 and H2S is sucked out from the fluff formation unit. Most of the vapor is condensed and returned to the CS2 tank. The remaining gas passes through a condenser and is sucked out by a water jet, where the CS2 is condensed by cold water. A separator after the water jet separates the process water and the condensed CS2 from the uncondensed gases. The uncondensed gases are sucked out to the waste gas system for further processing into sulfuric acid. The liquid CS2 is separated in a settler and recycled to the viscose process without any additional cleaning.

[0076] CS2 Recovery by Adsorption (CAP)

[0077] CS2 offgas with a low concentration of H2S is recovered by the CAP process (Carbon Adsorption Plant). It is an intermittent process with a cycle of adsorption and desorption of CS2 by activated carbon. Before applying the adsorption process, H2S must be removed from the feed gas with NaOH in a gas scrubber. The mixture of NaOH and Na2S obtained can be used in the desulfurization process in the fiber post-treatment. The feed gas concentration should be high, but must be outside the explosion limits. The liquid CS2 from the desorption can be directly recycled into the process.

[0078] WSA (Wet Sulfuric Acid Plant)

[0079] Of all the technologies used to treat sulfide off-gases, the WSA process is the most versatile and effective in removing sulfide odors by converting CS2 and H2S into sulfuric acid, which can be used in the spinning process. By combining WSA with a condensation process, a reasonable amount of CS2 input can be recycled while achieving maximum purification efficiency. However, the economic disadvantage is that a valuable specialty chemical (CS2) is converted into a cheap commodity (sulfuric acid). Depending on the off-gas composition, additional sulfur and fuel (natural gas, oil) are used as additional inputs and steam is produced as a by-product.

[0080] Lean gas from spinning, rich gas from spinning bath degassing and sulfur are used as separate feeds for WSA. Sulfide compounds are efficiently converted into sulfuric acid by means of a catalyst.

[0081] Zinc Recovery

[0082] In the post-treatment, zinc is precipitated in the form of zinc sulfide from the overflow wastewater of the pickling system. The zinc sulfide is separated from the slurry and dissolved with sulfuric acid to give zinc sulfate again, which is metered into the spinning bath for the spinning process. Summary of the invention

[0083] Drawing of the viscose filaments is necessary to obtain a high orientation of the cellulose molecules and excellent tensile properties of the fibers. Usually, the filaments are guided by glass or ceramic guides and pulled out of the spinning bath via godet rollers. Due to the delayed regeneration in the Modal process, the filaments are still very weak when they leave the spinning bath. If drawing is applied at a too early stage of regeneration, the filaments break, resulting in spinning defects or long fibers. The filament bundle is wound several times around the godet rollers and the ceramic guides, so that drawing can be applied in one step between the godet rollers and the drawing rollers in air or in a hot secondary bath. If there are two sets of drawing rollers, drawing can also be applied in two steps.

[0084] The problem to be solved by the present invention is therefore to find a method for applying maximum stretching to the filaments in the Modal process in order to obtain a high orientation of the cellulose molecules and excellent stretching properties without breaking the filaments.

[0085] Surprisingly it has been found that maximum stretching and orientation can be achieved when the tow is collected from the individual godets and pre-stretched between the godets and the first set of stretching rollers. The tow is widely spread to obtain a flat ribbon which is introduced into the steam chamber and stretched between the two sets of stretching rollers. By providing a high and uniform temperature distribution, the hot steam can easily penetrate and heat the tow. The tow is plasticized, which can apply a high stretch of up to 130% in total, which provides a higher fiber orientation and better fiber stretching properties. Since all filaments are uniformly stretched at the same high temperature, filament breakage and long fibers can be avoided. The steam chamber can recover a high concentration of CS2 and separate this CS2 from the steam condensate. DETAILED DESCRIPTION

[0086] Therefore, one specific embodiment of the present invention is a method for producing modal fibers, which comprises at least the following method steps:

[0087] a. Preparation of alkali cellulose from cellulose raw materials,

[0088] b. Aging of alkali cellulose,

[0089] c. Xanthation,

[0090] d. dissolve,

[0091] e. Aging, filtering, degassing,

[0092] f. Spinning,

[0093] g. Stretch

[0094] h. Post-processing (including cutting, drying, opening, packaging),

[0095] i. Recycling of chemicals,

[0096] Wherein in step g, stretching is applied in at least two stretching steps, characterized in that the filament bundle is collected from each filament roller, the filament bundle is pre-stretched between the filament roller and a first set of stretching rollers by applying a first stretching, and the filament bundle is plasticized and stretched between the first set and a second set of stretching rollers by applying a second stretching.

[0097] In a preferred embodiment of the method according to the invention, the first stretching applied may be 30 to 70%. In another preferred embodiment, the second stretching applied may also be 30 to 70%. However, the sum of the first and second stretchings should not be higher than 130% to avoid damage to the fibers. According to the invention, a third or even a fourth stretching step may also be used. However, preferably, the total stretching should not be higher than 130% to avoid damage to the fibers.

[0098] In a preferred embodiment of the method according to the invention, the first stretching can be applied in air using a suitable ventilation system or in an enclosed hot secondary bath.

[0099] Preferably, the second stretching may be applied in a hot steam chamber located between the first and second sets of stretching rollers.

[0100] Most preferably, the tow entering the steam chamber is spread out to form a flat ribbon.

[0101] In a preferred embodiment of the method according to the invention, the temperature in the steam chamber is 105-120° C. and the residence time of the tow in the steam chamber is 15-20 seconds. Saturated steam may be applied. Due to the internal overpressure, the steam chamber must be properly sealed. Preferably, the steam chamber is encapsulated and the waste gases produced are sucked out of the chamber and recovered.

[0102] In a preferred embodiment of the method according to the present invention, the pre-stretching is carried out in a secondary bath having a bath temperature of 90-98° C. and an acid concentration of 15-30 g / l sulfuric acid.

Claims

1. A method for producing modal fibers, comprising at least the following steps: a. preparing alkali cellulose from cellulose raw materials, b. Aging of alkali cellulose, c.xanthation, d. dissolve, e. Ripening, filtering, degassing, f. Spinning, g. stretching, h. Post-processing: including cutting, drying, opening, packaging, i. Recycling of chemicals, wherein in step g, the stretching is applied in at least two stretching steps, characterized in that collecting the tow from each godet, pre-stretching the tow between the godet and a first set of stretching rollers by applying a first stretch, and plasticizing and stretching the tow between the first and second sets of stretching rollers by applying a second stretch, The second stretching is applied in a hot steam chamber located between the first and second sets of stretching rollers, the temperature in the steam chamber is 105-120° C., and the residence time of the filament bundle in the steam chamber is 15-20 seconds.

2. The method according to claim 1, wherein the first stretching applied is 30 to 70%.

3. The method according to claim 1, wherein the second stretching applied is 30 to 70%.

4. The method of claim 1, wherein the first stretch is applied in air using a suitable ventilation system or in an enclosed hot secondary bath.

5. The method of claim 1, wherein the tow entering the steam chamber is spread out to form a flat ribbon.

6. The method of claim 1 wherein the steam chamber is enclosed and the exhaust gases produced are sucked out of the chamber and recovered.

7. The method according to claim 1, wherein the pre-stretching is performed in a secondary bath having a bath temperature of 90-98°C and an acid concentration of 15-30 g / l sulfuric acid.