Process for the recycling of para-aramid waste

Abstract

A process for the recycling of para-aramid waste, comprising: - feeding para-aramid waste comprising sulfuric acid into a rotor-stator apparatus, - contacting the para-aramid waste with an aqueous coagulant in the rotor-stator apparatus, - applying shear to grind and coagulate the para-aramid waste to obtain para-aramid polymer particles. The invention is also directed to a para-aramid particle obtained by the process and to a process for the re-use of para-aramid waste and the continuous para-aramid fiber obtained thereby.

Description

[0001] CQN3299

[0002] Process for the recycling of para-aramid waste

[0003] Description:

[0004] The application relates to a process for the recycling of para-aramid waste to obtain para-aramid polymer particles, to para-aramid polymer particles obtainable by such process, to a process for the re-use of para-aramid waste and the continuous para-aramid fiber obtained thereby.

[0005] Aramid fibers, in particular para-aramid fiber are used for a large variety of applications, including textiles, nonwovens, ropes, fiber-reinforced composites, cables, penetration-resistant materials (e.g. anti-ballistic vests), tires and conveyor belts.

[0006] It is desirable to reduce the carbon foot print of para-aramid fibers and the waste disposal of aramid end-of-life products and of waste produced during manufacture by the recycling of para-aramid fibers.

[0007] A well-known way to recycle aramid fibers is by producing pulp, i.e. fibrillated short fibers from aramid short-cut. However, pulp is only suitable for a number of applications and does not have the mechanical properties of continuous paraaramid fiber. Furthermore, pulp is commonly used in small percentages in composite materials, making it difficult to recycle the pulp after incorporation in such materials. This limits the number of times the aramid can be recycled through the pulp process. This route may therefore be considered as down-cycling. Hence, recycling methods that allow the manufacture of para-aramid materials other than pulp are of interest, in particular methods that allow the manufacture of material that have comparable properties as materials based on virgin para-aramid polymer.

[0008] CN106564134A describes a method for degrading para-aramid spinning waste, wherein the spinning waste is crushed by a horizontal centrifugal crusher, the broken scraps are transferred to a digestion tank wherein the broken scraps are subjected for 50 to 200 hours to an acidic treatment at a temperature in the range of 100 to 300°C under stirring, introducing the treated broken scraps into a neutralization tank where hydroxide solution is added for neutralization and filtering the suspension after neutralization. This process is not efficient and economical and requires long chemical treatment steps. Further, due to the long acidic treatment at high temperature, it is likely that either monomer intermediates such as terephthalic acid, monomers (PPD, p-Phenylenediamine) or severely degraded polymer are obtained.

[0009] CN 102560748 describes a process to recycle continuous para-aramid yam or spin dope waste from the production. The process requires an elaborate pre-treatment of the continuous aramid yarn including unwinding of the yam, drying, soaking in saturated NaHCOs solution for 2 to 24 hours and washing the yam, before sulfuric acid is added to the continuous yam to dissolve the yam. The resulting spin dope is discharged into plastic bags where it is allowed to solidify. The para-aramid is hydrolyzed by the acid which lowers the viscosity of the polymer. Subsequently, the solidified slurry is melted in an oven and processed into fibers. The process results in a yam which is suitable for staple fiber production.

[0010] CN109514758 is directed to a recovery process for para-aramid spinning waste. The process involves the step of crushing para aramid spinning waste to obtain granular waste (of 4 to 6 mm size) and feeding this granular waste into a water washing tank for soaking and repeated washing and further soaking. The waste material is then subjected to pressure filtration and then dried in a vacuum dryer to give a solid powder. JP2009292984 describes a method for treating a waste spin dope comprising PPTA and concentrated sulfuric acid, resulting into a separation of the polymer and sulfuric acid by a simple method. The solidified spin dope is crushed to form a granular material having an average particle size of 1 mm or more. Subsequently, the granular material is separated into polymer and sulfuric acid by washing it with water, acid, or aqueous alkali. The method of JP2009292984 requires lengthy washing. To avoid polymer degradation due to the lengthy treatment process, the washing liquid should preferably be cold.

[0011] WO2021 / 123210 discloses a continuous recycling process for para-aramid fibrous waste. The fibrous material is dissolved in sulfuric acid. The resulting spin dope is further processed into continuous fibers by known spinning processes. WO2021 / 123210 is silent on how to obtain polymer particles.

[0012] There is a need for an efficient recycling process for para-aramid waste which can utilize a range of aramid materials in various forms as input and which results within short processing time in high quality para-aramid polymer particles that can be easily further processed.

[0013] To this end, present application provides a process for the recycling of para-aramid waste, comprising:

[0014] - feeding para-aramid waste comprising sulfuric acid into a rotor-stator apparatus,

[0015] - contacting the para-aramid waste with an aqueous coagulant in the rotor-stator apparatus,

[0016] - applying shear to grind and coagulate the para-aramid waste to obtain paraaramid polymer particles.

[0017] The para-aramid waste comprising sulfuric acid and an aqueous coagulant are contacted with each other in a rotor-stator apparatus. The para-aramid waste comprising sulfuric acid may be added in various forms, including in the form of a polymer slurry, solution or particulate material. In a rotor-stator apparatus very high shear forces are generated in the gap between a moving (rotor) and a stationary part.

[0018] As the rotor (or rotors) turn(s) at high speeds within the stationary stator, it creates a vortex circulating the material through the gap between the rotor and stator, mechanically shearing and cutting the para-aramid waste. Use of the rotor-stator apparatus has the advantage that the size reduction of the material to be recycled and the contacting with the coagulant (to separate para-aramid polymer and sulfuric acid) take place simultaneously. Due to this, the process of instant invention is very short which enables an efficient and a fast washing process. Preferably, the rotor-stator apparatus has an axial inlet for the material to be treated (the para-aramid waste comprising sulfuric acid and the aqueous coagulant) and a radial outlet for the treated material. Alternatively, the feed stream can also be injected radially into the rotor and the gap. Preferably both, the rotor and stator are provided with devices which produce shearing forces and cutting, such as tooth systems and tooth profiles, cutting edges or blades, preferably, the device comprises toothed rims with cutting edges. This is different from the prior art which employs stirring, crushing or soaking. In a conventional mixer or stirrer, the material is present in a vessel and moved by stirrer blades or the like. Such stirring will not be sufficient to efficiently coagulate and mill the paraaramid waste into polymer particles in a short time. With instant process, paraaramid polymer particles may be created which are suitable for direct use in a spin dope in a fast and efficient manner. Compared to instant process, the prior art processes which involve stirring have multiple disadvantages, in particular: significantly longer duration of treatment (which often correlates with polymer degradation), lower coagulation speed, due to longer residence time larger equipment needs to be employed, particle size control is more difficult and acid removal may be more difficult for the obtained polymer particles. In contrast, the use of a rotor-stator apparatus allows a much faster process with equipment of smaller scale and the particle size distribution of the obtained polymer particles is better controllable. Preferably, the gap between the rotor and the stator of the rotor-stator apparatus has a width in the range of 0.01 to 5 mm, more preferably in the range of 0.05 to 2 mm, even more preferably in the range of 0.1 to 1 mm. The gap width may be used to adjust the particle size of the resulting para-aramid polymer particles. If the gap width varies over the length of the gap, the indicated width refers to the smallest distance between the rotor and stator.

[0019] Preferably, instant process is a continuous process and the aqueous coagulant and the para-aramid waste are applied through the stator onto the rotor. In this event, the coagulant and the para-aramid waste comprising sulfuric acid are both fed coaxially (both streams forming one joint stream before contacting the rotor from the same direction) into the rotor-stator apparatus. The choice for the feed location on the rotor-stator apparatus for the coagulant and the para-aramid waste comprising sulfuric acid may be made dependent on the viscosity of the paraaramid waste comprising sulfuric acid. Alternatively, the coagulant and the paraaramid waste comprising sulfuric acid may be applied counter currently to the stator and rotor (e.g. one of them through the drive shaft of the rotor). It is preferred that the para-aramid waste comprising sulfuric acid and the coagulant are both fed coaxially into the rotor-stator apparatus.

[0020] Preferably, the aqueous coagulant is selected from water and an aqueous solution of sulfuric acid. In one embodiment, the aqueous coagulant comprises 0 to 50 wt%, preferably 2 to 30 wt% of sulfuric acid.

[0021] Optionally, the same mixture of para-aramid waste comprising sulfuric acid and aqueous coagulant is passed multiple times through the rotor-stator apparatus to further optimize the size of the para-aramid polymer particles.

[0022] The mass ratio between aqueous coagulant and the para-aramid waste comprising sulfuric acid may be in the range of 400:1 to 1 :1 , preferably 50:1 to 5:1. The mass ratio may be chosen in dependence on the concentration of the paraaramid waste comprising sulfuric acid (influencing the viscosity of the polymer slurry, solution or dispersion), such that a higher amount of coagulant is added for a slurry, solution or dispersion with a higher concentration of polymer. In the context of the present specification aramid refers to an aromatic polyamide comprising or consisting of aromatic fragments directly connected to one another via amide fragments. Methods to synthesize aramids are known to those skilled in the art and typically involve the polycondensation of aromatic diamines with aromatic diacid halides.

[0023] For the purpose of this application, the term para-aramid refers to a class of wholly aromatic polyamide polymers and copolymers having at least 60%, preferably at least 80% and more preferably at least 90% of para-oriented bonds between the aromatic moieties. In one embodiment, at least 95% or all (i.e. 100%) of the bonds are para-oriented bonds.

[0024] Typical para-aramids are poly(para-phenylene terephthalamide) (PPTA), poly(4,4'- benzanilide terephthalamide), poly(para-phenylene-4,4'-biphenylene dicarboxamide) and poly(para-phenylene-2,6-naphthalene dicarboxamide), 5,4'- diamino-2-phenylbenzimidazole or poly(para-phenylene-co-3, 4' -oxidiphenylene terephthalamide) or copolymers thereof.

[0025] Preferably, the para-aramid waste comprises poly(para-phenylene terephthalamide) (PPTA).

[0026] The starting material, i.e. the para-aramid waste comprising sulfuric acid is subjected to shearing and cutting forces. Generally, the para-aramid waste added to such rotor-stator apparatus is centrifugally accelerated in a process chamber. The shear rates at which the rotor-stator apparatus can be operated are usually high or very high. Such high shear rates may be realized with a rotation speed of 2500 to 6000 rpm, preferably 3500 to 5000 rpm.

[0027] An example of a rotor stator apparatus is a Condux LV15 15 / N3, now offered by Netzsch Trockenmahltechnik GmbH.

[0028] In instantly claimed process, the para-aramid waste is directly, when coming into contact with the coagulant, milled and cut into small polymer particles by the simultaneous action of the rotor and the stator. Only a short processing time in the rotor-stator apparatus is required (e.g. less than 1 minute per one time passing though the rotor-stator apparatus, preferably less than 30 seconds or less than 10 seconds per one passage through the rotor-stator apparatus) to effectively precipitate the para-aramid polymer. In some instances, the processing time may be as short as 1 second. Hence, the residence time, i.e. total time of para-aramid waste in the rotor-stator apparatus (accumulated in the event of multiple passages) is in one embodiment at most 2 minutes, preferably at most 1 minute, more preferably at most 30 seconds or even at most 15 seconds.

[0029] Preferably, the treatment of the para-aramid waste in the rotor-stator apparatus takes places at a temperature of at most 70°C, preferably at most 50°C, more preferably at most 40°C. Optionally, the rotor-stator apparatus and / or the coagulant may be cooled to realize the desired temperature.

[0030] The advantage of using a rotor-stator apparatus in this process is that the paraaramid polymer is very efficiently and in short time processed to polymer particles. Due to the very short processing times, polymer degradation does not occur or only to a very limited extent. Hence, instant recycling process results in polymer that basically has the same properties and can be used in the same way as ‘virgin’ para-aramid polymer (i.e. by polymerization newly produced polymer). Primarily indicative of the quality of the para-aramid polymer is the relative viscosity, qrel. Also indicative may be the width of the molecular weight distribution. The polymer degradation may hence be assessed by determining the difference between the relative viscosity of the polymer of the para-aramid waste (i.e. the starting material of the process) and of the para-aramid polymer particles (i.e. the product of the process). The relative viscosity qrel is defined as the viscosity ratio of a 0.25 wt% solution of the para-aramid polymer in 96% sulfuric acid compared to the pure solvent (96% sulfuric acid).

[0031] To determine the relative viscosity, a polymer sample (for example polymer powder or particle, fiber) 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 % (w / v) sample solution in 96 % (w / w) sulfuric acid is subsequently measured at 25°C in an Ubbelohde viscometer (e.g. Schott AVS370). Under identical conditions the flow time of the solvent is measured as well.

[0032] Preferably, the loss of viscosity qrel of obtained para-aramid polymer particles compared to the aramid waste material used as starting material is at most 20%, preferably at most 15%, more preferably at most 10% and even more preferably at most 5%.

[0033] After coagulation, the para-aramid polymer particles may be subjected to a neutralization step and optionally to a washing and drying step. Preferably, the para-aramid polymer particles are provided onto a belt filter to separate the paraaramid polymer particles from the coagulant and sulfuric acid by filtration. Alternatively, the particles may be separated from the coagulant by other well- known methods to separate solids from liquids, e.g. centrifugation, batch filtration, decantation or cyclone separation. The filtrate or the supernatant (comprising mainly the coagulant and sulfuric acid) may preferably be recycled (at least partially) and again be re-used as coagulant in the precipitation step in the rotorstator apparatus.

[0034] The neutralization may be carried out by contacting the polymer particles with an aqueous NaOH or KOH solution.

[0035] Washing may remove the remnants of the sulfuric acid, the coagulant and the neutralization agent. The para-aramid polymer particles are preferably supplied to a counter current washing unit. As washing solution water or alkaline aqueous solutions may be used. After the washing step, the para-aramid polymer particles may be dried as commonly known, preferably using hot air, e.g. in a fluid bed dryer, a flash dryer, contact dryers, infrared dryers, microwave heaters or a vacuum dryer or combinations thereof.

[0036] Preferably, the para-aramid polymer particles are dried to a moisture content of approximately 0.1 to 2 wt%, preferably less than 1 wt%, more preferably less than 0.5 wt%. The moisture content is determined by drying a known amount of para- aramid polymer particles at 150°C in an infrared dryer (e.g. Sartorius MA160). Drying stops when the loss of weight is less than 1 mg / 30 seconds (weight is constantly monitored). The relative loss of weight during this drying process is considered the sample’s moisture content.

[0037] Instant process may be carried out batch-wise or continuously, preferably continuously.

[0038] The para-aramid waste used as starting material may comprise para-aramid spin dope waste and / or para-aramid fibrous waste. Para-aramid spin dope waste and para-aramid fibrous waste may be used in different forms and may be pre-treated in different manners prior to feeding the para-aramid waste into the rotor-stator apparatus.

[0039] The para-aramid fibrous waste as one of the potential starting materials of instant process is usually obtained from or comprises to-be-recycled aramid fiber. The para-aramid fibrous waste may be derived from post-customer aramid waste material, i.e. materials which have been used in products and which have reached their end-of-lifetime, including but not limited to ropes, penetration-resistant articles (e.g. ballistic vests), fabrics (woven or knitted), nonwovens, papers, composites including aramid fibers, cables, reinforcement layers in tires, optical fiber cables, (umbilical) hoses, uni-directional shields. Hence, the para-aramid fibrous waste may comprise para-aramid short-cut fibers, pulp, pieces of woven or knitted fabric, pieces of non-woven or pieces of paper or any mixture thereof.

[0040] The para-aramid fibrous waste may be derived from pre-customer aramid waste material, i.e. waste materials created during the production process, e.g. aramid fiber waste created during production or converting processes or aramid yarn having low quality.

[0041] Depending on the form of the to-be-recycled waste material, the waste material may be subjected to a size-reducing step to obtain the para-aramid fibrous waste, e.g. by cutting, chopping, breaking, milling, shredding, hammering or a combination of such methods. Also, hazardous components (e.g. finishes) may be removed from the to-be-recycled waste material or the para-aramid fibrous waste prior to starting the process. Preferably, the pre-treatment and the process do not comprise an alkaline treatment of the material-to-be recycled or the para-aramid fibrous waste before being contacted with sulfuric acid.

[0042] The para-aramid fibrous waste usually comprises non-continuous para-aramid fiber in the form of short-cut fibers, pulp, pieces of woven or knitted fabric, pieces of non-woven or pieces of paper or any mixture thereof. In addition to non- continuous para-aramid fiber, the para-aramid fibrous waste may comprise other, non-para-aramid fibers and / or non-fibrous material, the latter e.g. derived from the to-be-recycled material. The para-aramid fiber and other optionally present fibers together form the fibrous component of the para-aramid fibrous waste.

[0043] The para-aramid fibrous waste may comprise at least 50 wt% of para-aramid fiber (in any of the forms mentioned above), preferably at least 70 wt% of para-aramid fiber, preferably at least 80 wt% or even at least 90 wt% of para-aramid fiber (based on the weight of the fibrous component of the para-aramid fibrous waste). The para-aramid fibrous waste may comprise at least one type of fiber other than para-aramid fibers (referred to as non-para-aramid fiber), for example at most 50 wt% of non-para-aramid fibers, preferably at most 30 wt% of non-para-aramid fibers, more preferably at most 20 wt%, more preferably at most 10 wt% or most preferably at most 5 wt% of non-para-aramid fibers (based on the weight of the fibrous component of the para-aramid fibrous waste). Preferably, the para-aramid fibrous waste consists of para-aramid fibers. The non-para-aramid fibers may be selected from meta-aramid fibers, thermoplastic fibers, natural fibers, regenerated fibers or fibers made from rigid rod polymers or any combination thereof.

[0044] Thermoplastic fibers are well known and include e.g. acrylic fibers (e.g. modacrylic fibers), polyamide fiber (such as e.g. Nylon fibers) and polyester fibers. Natural and regenerated fibers include e.g. cotton fibers, cellulose fibers, viscose fibers, (e.g. Rayon or Lyocell fibers). Rigid rod polymer fiber other than aramid fibers are e.g. polybenzimidazole (PBI) fibers and polybenzobisoxazole (PBO) fibers. For the purpose of this application, the term meta-aramid refers to a class of wholly aromatic polyamide polymers and copolymers having at least 60%, 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). In one embodiment, the meta-aramid is poly(m-phenylene isophthalamide).

[0045] For obtaining a good quality para-aramid polymer it is advantageous if the paraaramid fibrous waste substantially consists of para-aramid fibers, i.e. comprises at least 95 wt% of para-aramid fiber (based on the weight of the para-aramid fibrous waste). Preferably, when using para-aramid fibrous waste for the recycling process, the para-aramid waste consists of sulfuric acid and para-aramid fibers.

[0046] The para-aramid fibrous waste is combined with sulfuric acid to obtain para-aramid waste comprising sulfuric acid suitable for use in instant process. Preferably, the sulfuric acid has a concentration of at least 95%, more preferably at least 98% and even more preferably of at least 99%.

[0047] A solution of the para-aramid fibrous waste in sulfuric acid is produced by mixing the para-aramid fibrous waste with solid or liquid sulfuric acid. In the embodiment where liquid sulfuric acid is used, a liquid solution is obtained. In the embodiment where frozen sulfuric acid is used, a solid mix is obtained. Solid sulfuric acid is obtained by freezing sulfuric acid below its solidifying temperature.

[0048] Preferably, the mixing the of the para-aramid fibrous waste and the sulfuric acid is carried out continuously. Preferably, in the instant process, the para-aramid fibrous waste and the sulfuric acid are mixed in a mixing device with a continuous flow.

[0049] In the embodiment wherein solid sulfuric acid is used, the mixing may take place at an initial temperature in the range of -5 to 10°C, more preferably a temperature in the range of 0 to 8°C. Subsequently, the temperature is increased to a temperature in the range of 50-100°C, preferably 70-90°C. In the embodiment wherein liquid sulfuric acid is used, the mixing takes place at a temperature in the range of 20 to 120°C, more preferably at a temperature in the range of 50 to 90°C.

[0050] In this embodiment, the mixing device is preferably used with settings that create a high shear rate for an efficient mixing of the aramid fibrous material and the sulfuric acid and subsequent dissolution of the aramid fibrous material.

[0051] The mixing device may for example be a kneader or extruder, preferably a single shaft kneader, double shaft kneader, single screw extruder or twin screw extruder. The dispersion or solution may have a para-aramid concentration in the range of 10 to 25 wt%, preferably 15 to 21 wt%, more preferably 18 to 20 wt%

[0052] The para-aramid waste comprising sulfuric acid may also comprise or consist of para-aramid spin dope waste. Para-aramid spin dope waste may be created during the manufacturing of para-aramid fibers, e.g. when starting a spinning process, when stopping a spinning process and cleaning the equipment or when mistakes occur during the preparation of the spin dope. Para-aramid spin dope waste comprises para-aramid polymer and sulfuric acid, where the para-aramid polymer is usually present at a concentration of 18 to 20 wt%.

[0053] During the manufacturing of para-aramid fibers the para-aramid spin dope waste may initially be collected and stored. Preferably, the para-aramid spin dope waste is stored at a temperature of at most 60°C, more preferably at most 40°C or at room temperature. Preferably, the storage vessel is air-tight to avoid the ingress of moisture or water. Usually, the para-aramid spin dope waste will be solid.

[0054] The para-aramid spin dope may be pre-treated in different manners.

[0055] In one embodiment, for further processing, the para-aramid spin dope waste will be heated at a temperature in the range of 40 to 95°C, preferably in the range of 50 to 85°C to yield a solution. In another embodiment, the solid para-aramid spin dope is shredded to obtain pieces of about 0.5 to 2 cm (maximum dimension). In one embodiment, the pre-treatment does not comprise steps other than a heat- treatment or a size reduction. In some embodiments, the para-aramid spin dope waste does not require any pre-treatment, because it is already present in the form of pieces or large particles.

[0056] The para-aramid waste comprising sulfuric, if present in the form of a solution (e.g. obtained after melting the para-aramid spin dope waste), may be subjected to a filtration step prior to feeding into the rotor-stator apparatus. The filtration may remove any particles, e.g. solid contaminations formed when contaminants contact sulfuric acid, larger than 100 pm, preferably larger 50 pm by employing a sieve with a suitable opening size.

[0057] Usually, no further sulfuric acid is added to the para-aramid spin dope waste, since it comprises sulfuric acid. However, a small amount of sulfuric acid may be added to compensate for any moisture that has entered the para-aramid spin dope waste, e.g. during storage.

[0058] The para-aramid spin dope waste in the form of a solution or particulate pieces may be directly added to the rotor-stator apparatus.

[0059] In one embodiment, a metal detector may be used to detect any metal foreign parts for subsequent removal that may be present in the para-aramid spin dope waste prior to adding it to the rotor-stator apparatus.

[0060] Instant invention is also directed to a para-aramid polymer particle obtainable by the process of any one of the embodiments described above.

[0061] In contrast to virgin aramid polymer obtained by polymerization which has not been processed in sulfuric acid, the para-aramid polymer particle as described herein is derived from material which previously has been in contact with a spinning or processing solvent, usually sulfuric acid. Hence, instant invention is also directed to a para-aramid polymer particle having a sulfonic acid group content of at least 1 meq / kg of polymer, preferably of at least 5 meq / kg of polymer. The sulfonic acid group content of the polymer is determined after degradation of the polymer. This is done by dissolving the aramid in methane sulfonic acid with approximately 2% water and heating to 85 °C for at least 100 hours. Subsequently, high performance liquid chromatography (HPLC, column: Phenomenex Gemini C18; 150x4.6mm; dp=5 pm, Temperature: 40 °C, Flow: 1.0 ml / min, Eluent A: ammonium acetate buffer 0.01 M pH 8.3, Eluent B: methanol) is performed on the yielded monomers to determine the number of sulfonic acid groups. HPLC is performed using a gradient program from eluent A to eluent B as follows: At the start the flow contains 100% eluent A, this is maintained for 5 minutes. Subsequently, the eluent is switched to 20% A and 80% over a period of 1 min, and maintained at this composition for 5 more minutes. Next, the eluent is switched back to 100% A over a period of 1 minute, after which this is maintained for 5 more minutes before the experiment is ended.

[0062] The para-aramid polymer particle as referred to above may take the form of powder or crumb having a range of sizes and morphology, depending on the conditions (e.g. the gap width and the total residence time) chosen for the treatment in the rotor-stator apparatus. Preferably, the particles will have a size of less than 2 mm, preferably less than 1 .5 mm, more preferably less than 1 mm, determined by a vibration sieving machine. More preferably, the particles may have a size of less than 0.9 mm or even less than 0.8 mm. A known amount of sample is sieved over three sieves (sieves with openings of 250, 1000 and 1400 pm). The amount of material sieved on each sieve is weighed and expressed in percentages of the total amount of polymer taken for analysis.

[0063] Preferably, the para-aramid polymer particle has a bulk density of at most 400 kg / m3, preferably of at most 200 kg / m3, more preferably of at most 100 kg / m3The bulk density is determined by drying a sample of the para-aramid polymer particles to a moisture content of at most 0.5 wt%. The mass of a sample of the dried para- aramid polymer particles is determined by weighing the material. Next, the volume of this sample is determined by measuring in a graduated cylinder. Finally, the dry mass is divided by the volume to calculate the bulk density.

[0064] Due to the processing, it was found that the para-aramid polymer particles may have a high purity, i.e. a low amount of contaminants, such as e.g. organic contaminants. Preferably, the para-aramid polymer particles contain less than 0.5 wt%, more preferably less than 0.25 wt% of organic contaminants, even more preferably less than 0.15 wt% of organic contaminants, determined by solvent extraction. In fact, virgin polymer particles may comprise an higher amount of organic contaminants due to the polymerization taking place in organic solvents. Due to the processing of recycling material, the para-aramid polymer particle preferably has a low organic solvent content, in particular a low N- methylpyrrolidone (NMP) content.

[0065] It was found that the para-aramid polymer particles can be efficiently washed to remove the solvent / coagulant, are easily dissolved for further processing, have a limited bulk density (corresponding to a high bulk volume) and a high purity.

[0066] The invention also pertains to a process for the re-use of para-aramid waste, comprising:

[0067] - providing a para-aramid polymer particle according to the process of the invention

[0068] - dissolving the para-aramid polymer particle in sulfuric acid to obtain a spin dope,

[0069] - processing the spin dope into continuous para-aramid fibers by passing the spin dope through a spinneret.

[0070] Preferably, the sulfuric acid has a concentration of at least 95%, more preferably at least 98% and even more preferably of at least 99%.

[0071] In one embodiment the sulfuric acid has a concentration of at least 100%, or even at least 102%. Sulfuric acid having a concentration of more than 100% may be obtained by combining sulfuric acid and oleum. Liquid or solid (i.e. frozen) sulfuric acid may be used.

[0072] The spin dope may be prepared in a mixing device, such as a kneader or extruder, preferably a single shaft kneader, double shaft kneader, single screw extruder or twin screw extruder.

[0073] In the embodiment wherein solid sulfuric acid is used, the mixing takes place at an initial temperature in the range of -5 to 10°C, more preferably a temperature in the range of 0 to 8°C. Subsequently, the temperature of the aramid spin dope is increased to a temperature in the range of 50-100°C, preferably 70-90°C.

[0074] In the embodiment wherein liquid sulfuric acid is used, the mixing as well as the spinning of the mixed aramid spin dope takes place at a temperature in the range of 20 to 120°C, more preferably at a temperature in the range of 50 to 90°C.

[0075] In this embodiment, the mixing device is preferably used with settings that create a high shear rate for an efficient mixing of the aramid fibrous material and the sulfuric acid and subsequent dissolution of the aramid fibrous material.

[0076] In one embodiment of instant process, one spin dope is prepared based on liquid sulfuric acid and one spin dope is prepared based on solid sulfuric acid. Both spin dopes are subsequently combined to form a combined spin dope which is processed into continuous fibers.

[0077] Preferably, the spin dope has a para-aramid concentration of 15-21 wt%, even more preferably a para-aramid concentration of 18-20 wt%, based on the weight of the spin dope.

[0078] The spin dope is processed into a continuous aramid fiber by the well-known dryjet wet spinning process. This process is described in more detail for instance in US3414645 and US4016236. The dry-jet wet spinning process comprises extruding the liquid spin dope into a non-coagulating gaseous atmosphere, such as air, and immediately afterwards into a coagulation bath. In the air zone (also referred to as air gap) through which the spinning mass passes, the polymer is drawn.

[0079] After their coagulation the filaments formed are removed from the coagulation bath, washed, dried and taken up on a bobbin. In one embodiment, after a first washing step, the filaments are neutralized and washed again before being dried. The filaments may be subjected to a wet or hot drawing step before or after taking them up on a bobbin.

[0080] The air gap may have a length of 2-100 mm, preferably it has a length of 4-20 mm, more preferably of 6-15 mm.

[0081] 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 5-20 wt%.

[0082] 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.

[0083] As small amounts of residual acid may have a detrimental influence on the fiber properties, it is preferably that 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, NaHCOs or Na2CO3.

[0084] 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.

[0085] In one embodiment of the process, virgin para-aramid polymer is added to the spin dope or two spin dopes are combined, one spin dope based on virgin para-aramid polymer and one spin dope based on para-aramid waste. Virgin aramid polymer refers to aramid polymer which has not been (at least partly) processed into shaped or oriented articles by a solvent-based spinning process prior to instant process. Hence, the continuous para-aramid fibers obtained by this embodiment of the process will comprise recycled and virgin para-aramid polymer. The total amount of virgin polymer added into the spin dope or the combined spin dopes may be in the range of 5 to 95 wt% (based on the total amount of polymer in the spin dope), preferably in the range of 10 to 80 wt%, more preferably in the range of 20 to 70 wt%.

[0086] In another embodiment, no virgin para-aramid polymer is added.

[0087] The invention is also directed to the continuous para-aramid fiber obtained by the process of re-use and multifilament yam comprising the fibers.

[0088] Preferably, the continuous aramid fibers obtained by this process have the same or basically the same properties as continuous aramid fibers produced from 100% virgin para-aramid polymer while they comprise recycled para-aramid polymer, i.e. polymer that has been processed before being used as input for the process of instant invention (after polymerization). Thus, the resulting multifilament yam comprising recycled para-aramid polymer may have a breaking tenacity (tenacity at break) of at least 1800 mN / tex, a modulus of at least 50 GPa and an elongation at break at least 2%, more preferably a breaking tenacity of at least 2000 mN / tex, a modulus of at least 65 GPa and an elongation at break of at least 2.5%, even more preferably 2100 mN / tex, a modulus of at least 70 GPa and an elongation at break of at least 3%.

[0089] The mechanical properties are determined according to ASTM D7269. The invention is further illustrated by the following, non-limiting examples.

[0090] Examples Example 1

[0091] The recycling of spin dope waste, and the recovery of PPTA polymer powder from this recycling stream, was evaluated with this example.

[0092] Spin dope waste in the form of a solid spin dope crumble (particles of about 0.5 to 1.5 mm) comprising PPTA and sulfuric acid (19.8 wt% polymer concentration) was collected. Approximately 300 g of this solid spin dope crumble were fed into a Condux LV15 15 / N3, while simultaneously adding demineralized water (coagulant). The ratio between coagulant and spin dope is 25:1 , the rotor stator gap is 0.3 mm and the processing time in the rotor-stator is between 0.25 to 0.5 seconds (one passage only). The outflow is caught in a filtration setup that retains the solid particles while allowing liquid (primarily water and sulfuric acid) to pass through. The solid material on the filter is washed with an excess of demineralized water (approximately 10 L). Vacuum was applied over the filter to facilitate dewatering of the solid. After washing with water, the material was found to still be strongly acidic, as determined by pH paper (pH 0-1 ). To remove acid from the polymer, a neutralization step was applied by washing the obtained material in 200 mL of a 1% sodium hydroxide aqueous solution. After 2-5 minutes of washing in this solution, by means of agitation, the liquid was removed using vacuum filtration. To remove excess sodium hydroxide, the solid material that was retained on the filter was washed with an excess of demineralized water, by constantly adding and removing water on the filter, until the liquid adhering to the solid material was found to have a pH of around 7 (approximately 10 L of water was applied). After removal of the liquid using vacuum filtration a still wet solid material was obtained. This was dried completely by placing it in an oven at 80 °C under vacuum for 8 h. The obtained polymer crumb was visually evaluated and compared to virgin polymer powder. The recovered polymer was found to consist out of polymer particles.

[0093] Example 2

[0094] The recycling of para-aramid fibrous waste material to obtain para-aramid polymer powder was evaluated with this example.

[0095] Chopped para-aramid (PPTA) fibers were mixed with sulfuric acid in a Drais planetary mixer at room temperature for 8 hours to obtain a solid crumble with an aramid concentration of 19.8 wt%.

[0096] This crumble was processed in a Condux LV15 15 / N3 setup according to the procedure described in Example 1 using demineralized water as coagulant to obtain solid para-aramid polymer particles. The product was visually evaluated and was found to consist out of polymer particles.

[0097] The amount of organic contamination before and after the procedure was determined using solvent extraction showing an extractable content of 0.98% on the chopped fiber and 0.14% on the PPTA polymer product, indicating that the procedure also reduces the amount of contamination in the aramid material.

[0098] Example 3

[0099] The coagulation of liquid spin dope and the recovery of para-aramid polymer powder was evaluated with this example.

[0100] Liquid para-aramid spin dope comprising PPTA and sulfuric acid (polymer concentration of 19.8 wt%) was obtained directly from a spinning line and approximately 300 grams of spinning solution was fed to a Condux LV15 15 / N3 rotor stator apparatus in liquid form while simultaneously feeding demineralized water. Following the process described in Example 1 , the PPTA was coagulated from the spin dope to from a solid PPTA polymer crumb. The product was visually evaluated and was found to consist out of polymer particles. Example 4

[0101] Instant process was compared to the process of JP2009292984 by this example. Polymer particles were obtained by coagulation of PPTA spin dope (19.85% PPTA in sulfuric acid) in a rotor-stator coagulator comparable with Example 3.

[0102] After coagulation the polymer particles were washed to neutral (i.e. until the washing liquid has a pH of >5).

[0103] Subsequently, the washed particles were dried to a moisture content of < 0.5% (sample 4).

[0104] As comparative sample, the process of JP2009292984 was re-worked.

[0105] First, solid PPTA spin dope (19.2% PPTA concentration) was shredded into stripelike particles. Subsequently, the stripe-like particles were sieved in two steps to obtain particles having a size of 2-6 mm (as required by JP2009292984).

[0106] These particles were washed with excessive demi-water at room temperature in the following manner. 100 gram of the particles were added to 1.8 liter of demi- water and stirred for a few minutes after which the water was removed. This was repeated 5 times. After this the particles were again added to 1 .8 L demi-water and stirred for 1 hour after which the water was removed, which was repeated 2 times. After this the product was again added to 1 ,8L demi-water and stirred overnight (~18 hours). After this washing procedure, the pH of the washing water as well as determined on the product was >5.

[0107] The washed particles were dried to a moisture content of < 0.5% (comparative sample 1 ).

[0108] When observing the morphology of the particles of sample 4 and comparative sample 1 visually, immediate differences are visible.

[0109] While sample 4 comprises very fine and light yellow particles, comparative sample 1 has larger, roundish, more solid and darker particles.

[0110] Figure 1 shows 3 grams of sample 4 (left beaker) and 3 grams of comparative sample 1 (beaker) with a top view (left panel) and a side view (right panel) for both. It is immediate apparent that the volume of the sample according to the invention is much larger and also the morphology and colour of the particles are different.

[0111] Subsequently, the dissolution properties and the bulk density of sample 4 and comparative sample 1 were determined.

[0112] To determine the dissolution time, 3 gram of PPTA particles (sample 4 or comparative sample 1) were added with 97 gram of 99.8% sulfuric acid into a glass reactor containing a metal stirring device (in total 100 gram, 3% polymer concentration). Immediately after adding the sulfuric acid the stirrer was started and set to 200 rpm. The torque, supplied by the motor to the stirring device, expressed in Volt, was continuously determined as a relative measure of the dissolution. In addition, the temperature of the solution was also measured. After a certain time the torque will start to increase after which it remains on a constant level. Based on visual interpretation (glass reactor) it appears that the moment when torque is remaining more or less constant, the polymer is completely dissolved. The time needed for reaching constant torque is defined as the dissolution time.

[0113] Table 1 shows the results of the dissolution test.

[0114] Table 1

[0115] As can be seen, the particles obtained by the process according to the invention (Sample 4) dissolve much quicker than the particles obtained by the process described in JP2009292984. A shorter dissolution time is advantageous for any industrial process where the polymer is further processed. Subsequently, the bulk density of particles was determined by pouring the particles into a metered glass vessel. From the measured value, the bulk density was calculated.

[0116] Sample 4: ~60 kg / m3(3 g / 50 ml) Comparative sample 1 : ~500 kg / m3(3 g / 6 ml).

[0117] This comparison shows the large difference in bulk volume and bulk density between the samples.

[0118] This comparison shows that the process of instant invention where the grinding and coagulation take place simultaneously and in very short time results in polymer particles with attractive properties.

[0119] Example 5

[0120] In this example, yam was spun from the polymer particles.

[0121] Polymer particles were obtained by coagulation of PPTA spin dope (19.85% PPTA in sulfuric acid) in a rotor-stator coagulator comparable with Example 3. After coagulation the polymer particles were washed to neutral (i.e. until the washing liquid has a pH of >5). Subsequently, the washed particles were dried to a moisture content of < 0.5%.

[0122] Preparation of a spin dope having a total aramid concentration of ca. 19.3% was done by mixing 30% of these polymer particles and 70% virgin aramid polymer (p- phenylene terephthalamide) and sulfuric acid in a Nauta mixer (Hosokawa, 1000 L) for 20 hours. Mixing was started at a temperature of 8 °C and during the mixing the temperature increased to a final temperature of 20 °C (spin dope 1 ).

[0123] In the same way, an aramid spin dope was prepared by combining virgin aramid polymer (p-phenylene terephthalamide) and sulfuric acid (spin dope 2). Hence, spin dope 2 is based on 100% virgin PPTA polymer, while spin dope 1 comprises a combination of PPTA polymer based on the recycling process of this invention and virgin PPTA polymer.

[0124] From both spin dopes multifilament aramid yam was spun using a dry-jet wet spinning process. This was done by dosing the solid spin dope into a kneader where it is heated to 85 °C to obtain a liquid spin dope. In the kneader, the dope is transported to the outlet where it is fed to a series of pumps. After filtration, the liquid spin dope is extruded through a spinneret containing 1000 holes of 59 pm into air where it is drawn, and immediately afterwards into a coagulation bath. After their coagulation the filaments formed were removed from the coagulation bath, washed, neutralized, washed again, dried and taken up on a bobbin. The yam was spun with a linear density of ca. 1680 dtex.

[0125] The mechanical properties for the multifilament yams were determined according to ASTM D7269 after conditioning at 20°C and 65% relative humidity for 14 hours in accordance with ASTM D1776. In addition, the relative viscosity of both yams was determined as described above. The results are shown in table 2.

[0126] Table 2: Properties of multifilament yams

[0127] The mechanical data show that the multifilament yam prepared with polymer comprising the recycled polymer of this invention has basically the same properties as conventional multifilament yam prepared from 100% virgin polymer.

Claims

Process for the recycling of para-aramid wasteClaims:

1. A process for the recycling of para-aramid waste, comprising:- feeding para-aramid waste comprising sulfuric acid into a rotor-stator apparatus,- contacting the para-aramid waste with an aqueous coagulant in the rotor-stator apparatus,- applying shear to grind and coagulate the para-aramid waste to obtain paraaramid polymer particles.

2. The process according to claim 1 , wherein the para-aramid waste comprises para-aramid spin dope waste and / or para-aramid fibrous waste.

3. The process according to claim 1 or 2, wherein the para-aramid polymer particles are subjected to a neutralization step and optionally to a washing and drying step.

4. The process according to any of the preceding claims, wherein the para-aramid waste comprising sulfuric acid is subjected to a filtration step prior to feeding into the rotor-stator apparatus.

5. The process according to any of the preceding claims, wherein the aqueous coagulant comprises 0 to 50 wt%, preferably 2 to 30 wt% sulfuric acid.

6. The process according to any of the preceding claims, wherein the mass ratio between the para-aramid waste comprising sulfuric acid and the aqueous coagulant is in the range of 400:1 to 1 :1 , preferably 50:1 to 5:1 (wt / wt).

7. The process according to any of the proceeding claims, wherein the residence time of the para-aramid waste in the rotor-stator apparatus is at most 2 minutes, more preferably at most 1 minutes, even more preferably at most 30 seconds or at most 15 seconds.

8. The process according to any of the proceeding claims, wherein the treatment in the rotor-stator apparatus takes places without active heating or cooling, preferably at ambient temperature.

9. A para-aramid polymer particle obtainable by the process of any one of claims 1 to 8.

10. A para-aramid polymer particle having a sulfonic acid group content of at least 1 meq / kg of polymer preferably at least 5 meq / kg of polymer.11 . The para-aramid polymer particle of claim 9 or 10, having a particle size of at most 2 mm, preferably at most 1.5 mm, more preferably less than 1 mm.

12. The para-aramid polymer particle of any one of claims 9 to 11 , having a bulk density of at most 400 kg / m3, preferably of at most 200 kg / m3, more preferably of at most 100 kg / m3.

13. A process for the re-use of para-aramid waste, comprising:- providing a para-aramid polymer particle according to the process of any one of claims 1 to 8,- dissolving the para-aramid polymer particle in sulfuric acid to obtain a spin dope,- processing the spin dope into continuous para-aramid fibers by passing the spin dope through a spinneret.

14. The process according to claim 13, wherein virgin para-aramid polymer is added to the spin dope.

15. A continuous para-aramid fiber obtained by the process of claim 13 or 14.

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