A para-aramid solution, dissolving para-aramid material, recycling para-aramid material into para-aramid fibres, a para-aramid fibre and the use of a solvent

Abstract

A method of dissolving para-aramid comprising: arranging para-aramid material in a solvent, the solvent comprising dimethyl sulfoxide (DMSO), 1.5 wt / wt% to 10 wt / wt% potassium hydroxide (KOH) and 3.5 to 9.5 wt / wt% water; allowing the para-aramid to dissolve to produce a para-aramid solution. Also disclosed is use of a solvent comprising DMSO, 1.5 wt / wt% to 10 wt / wt% KOH and 3.5 to 9.5 wt / wt% water, for dissolution of para- aramid, and a para-aramid solution comprising para-aramid dissolved in a solvent, the solvent comprising DMSO, 1.5 wt / wt% to 10 wt / wt% KOH and 3.5 to 9.5 wt / wt% water. Further disclosed is a method of making para-aramid fibres from the solution.

Description

[0001] Method for dissolving para-aramid

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method of dissolving para-aramid material, and especially paraaramid fibres for the purpose of recycling, for example by extracting fibres from composite products. The invention also extends to use of a solvent for dissolution of para-aramid, to a para-aramid solution comprising para-aramid dissolved in the solvent, and to a method of making para-aramid fibres from the solution. The invention extends further to a method of recycling para-aramid fibres using the solvent.

[0004] TECHNICAL BACKGROUND

[0005] Para-aramid (poly para-phenyleneterephthalamide [PPTA]) materials such as fibres are commonly used in applications where strength, toughness and heat resistance are of critical importance: for example, they are commonly used as woven fabrics in protective clothing (such as bullet-proof clothing, stab-proof clothing or flame and heat resistant clothing), as well as finding use in applications such as fibre optics, aerospace, land mines and many other applications.

[0006] Para-aramid fibres are typically made by spinning fibres from a para-aramid solution. Concentrated sulfuric acid is used as the solvent in this process. Para-aramids have very poor solubility in most solvents - sulfuric acid is an effective solvent because it is a particularly strong acid and is therefore capable of dissolving large quantities of paraaramid, over 15% and typically up to 20 % by weight, vital for spinning strong fibres. It is also relatively inexpensive. However, sulfuric acid is not environmentally friendly for several reasons: it is extremely corrosive and toxic, particularly to marine life, and it cannot be recovered and re-used easily after it has been used for the process. Any spent acid must be neutralised to form salt products such as gypsum: for every Kg of para-aramid produced, 7kg of gypsum must be made to neutralise the acid.

[0007] The spun fibres are typically given a water-resistant sizing treatment. Fibres with a water- resistant treatment (i.e. a hydrophobic coating) are known henceforth as ‘dry fibres’. Dry fibres may be woven into a fabric, and are often coated with a resin, such as a phenolic resin. Producing such para-aramid components is a high-cost process: for example, a single unit of para-aramid body armour costs up to £3,000 to manufacture. Due to natural break-down of para-aramid fibres, such body armour typically has a shelf-life of five years. To date, there has been no viable method of recycling para-aramid fibre materials, and at end of life, articles made of para-aramid fibres are usually incinerated.

[0008] It is against this background that the invention has been devised.

[0009] SUMMARY OF THE INVENTION

[0010] Against this background, the invention resides in a method of dissolving para-aramid material. The method comprises arranging the para-aramid material in a solvent, the solvent comprising dimethyl sulfoxide (DMSO), potassium hydroxide (KOH) and water, and allowing the para-aramid to dissolve to produce a para-aramid solution.

[0011] The solvent is effective in dissolving para-aramid to produce a para-aramid solution, is commercially recyclable, non-corrosive, and environmentally friendly, thereby providing a clean and viable recycling route for para-aramid materials, such as para-aramid fibres. By virtue of this method, products containing para-aramid that have reached their end-of-life do not need to be incinerated but can instead be recycled to produce a para-aramid solution from which, for example, fibres can be spun. The recycling process is less energy intensive and more environmentally friendly than producing virgin para-aramid fibres due to avoidance of the polymer synthesis step. In particular, the above method avoids the need for sulfuric acid, which is used to produce virgin para-aramid fibres, and which is highly corrosive and cannot be easily disposed of.

[0012] The para-aramid material may dissolve partially in the solution, such that some solid or semi-solid material remains, or it may dissolve fully in the solution, that some no solid or semi-solid material remains.

[0013] The invention also extends to use of a solvent comprising DMSO, KOH and water, for dissolution of para-aramid.

[0014] The invention extends further to a para-aramid solution comprising para-aramid dissolved in a solvent, the solvent comprising DMSO, KOH and water. In any of the method, use or solvent described above, the solvent may comprise less than approximately 10 wt / wt% water, preferably between approximately 1.5 wt / wt% and approximately 10 wt / wt%, where wt / wt% represents the weight of water as a percentage of the weight of DMSO. The inventors have found that a small amount of water (i.e. in this range) produces an effective solvent.

[0015] In any of the method, use or solvent described above, the solvent may comprise between approximately 3.5 wt / wt% and approximately 9.5 wt / wt% water, preferably between approximately 4 wt / wt% and approximately 9 wt / wt%. This range has been found to produce particularly good dissolution of para-aramid.

[0016] In any of the method, use or solvent described above, the solvent may comprise between approximately 5.5 wt / wt% and approximately 7.5 wt / wt% water, preferably between approximately 6 wt / wt% and approximately 7 wt / wt% water. This range has been found to produce optimal dissolution of para-aramid.

[0017] The solvent may comprise between approximately 1.5 wt / wt% and approximately 10 wt / wt% KOH, preferably between approximately 2 wt / wt% and approximately 8 wt / wt%, more preferably between approximately 4 wt / wt% and approximately 6 wt / wt%, most preferably between approximately 4 wt / wt% and approximately 5 wt / wt%.

[0018] In any of the method, use or solvent described above, the balance of the solvent may be DMSO, such that the solvent may comprise between approximately 83.7% and approximately 95.2% DMSO by weight, preferably between approximately 85.5% and approximately 94.3% DMSO by weight, more preferably between approximately 88.1% and approximately 91.3% DMSO by weight, and most preferably between approximately 89.3% and approximately 90.9% DMSO by weight. In other embodiments, additional components may be added to the solvent mixture in addition to KOH, Water and DMSO.

[0019] In a particularly preferred solvent, the solvent comprises between approximately 5.5 wt / wt% and approximately 7.5 wt / wt% water (most preferably between approximately 6 wt / wt% and approximately 7 wt / wt% water), and between approximately 4 wt / wt% and approximately 6 wt / wt% KOH (most preferably between approximately 4 wt / wt% and approximately 5 wt / wt% KOH). In any of the method, use or solvent described above, the weight of para-aramid is preferably at least 7 % of the weight of DMSO, more preferably at least 10% of the weight of DMSO, and optionally is between 10% of the weight of DMSO and the maximum solubility of para-aramid in the solution (which may be for example 20% of the weight of DMSO). A para-aramid weight of at least 15% of the weight of DMSO may be particularly preferred for fibre spinning.

[0020] The invention also extends to a method of recycling para-aramid material into para-aramid fibres. The method comprises: dissolving para-aramid according to the method above, thereby forming a para-aramid solution, and forming regenerated fibres from the paraaramid solution. Forming the re-generated fibres may comprise spinning fibres from the solution, for example using an extrusion or wet-spinning method.

[0021] In any of the methods described above, the method of dissolving the para-aramid may comprise heating the solvent to a reaction temperature above room temperature. Heating the solvent in this way increases dissolution of the para-aramid.

[0022] The reaction temperature may be between approximately 60 °C and approximately 135 °C: this range provides a suitable balance between increasing dissolution and avoiding degradation of the para-aramid. Preferably the reaction temperature is between approximately 80 °C and approximately 120 °C, more preferably between approximately 95°C and approximately 120°C, and most preferably between approximately 95°C and approximately 105°C: these ranges further optimise the balance between increasing dissolution and avoiding degradation of the para-aramid.

[0023] The invention extends further to a method of making para-aramid fibres, the method comprising providing any of the solutions described above, and spinning para-aramid fibres from the solution.

[0024] The method may comprise spinning fibres into a coagulation bath. The coagulation bath may comprise any suitable coagulant, for example, de-ionised water; DMSO and water; acetone and water.

[0025] The step of providing the solution may comprise dissolving para-aramid according to any of the methods described above. The invention also extends to a para-aramid fibre made from the solvent described above, or using the method described above.

[0026] In any of the aspects above, the para-aramid material may comprise para-aramid fibres. For example the para-aramid material may comprise loose fibres, or it may comprise a woven or-non-woven fabric comprising para-aramid fibres.

[0027] It will be appreciated that preferred and / or optional features of any one aspect may be combined preferred and / or optional features of any other aspect: in particular the compositions of the solvent described above may be used with any method, use, solvent, or solution.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] So that it may be more readily understood, the invention will now be described with reference to the following drawings, in which:

[0030] Figure 1 shows the expected dissolution mechanism of para-aramid in the solvent of the invention;

[0031] Figure 2 is a graph indicating how viscosity of a para-aramid solution varies with paraaramid concentration, showing an Isotropic to Nematic transition;

[0032] Figures 3 and 4 are 1-D and 2-D NMR spectra respectively of a sample of para-aramid fibres dissolved in the solvent of the invention;

[0033] Figure 5 is a graph indicating the effect of water content of the solvent on para-aramid solubility;

[0034] Figure 6a and 6b are graphs indicating the effect of KOH content of the solvent on paraaramid solubility;

[0035] Figures 7a to 7f are a series of photographs of Samples 8A to 8F respectively, according to Example 8; Figure 8a to 8d are optical microscopy images of a solution according to Sample 8G of Example 8;

[0036] Figures 9a to 9h are photographs of Samples 9A to 9H of Example 9;

[0037] Figures 10a to 10g are photographs of Sample 10A to 10G respectively before and after dissolution;

[0038] Figure 11 is a schematic view is a graph indicating of apparatus for fibre spinning;

[0039] Figure 12 is a graph indicating tensile strength of Samples 11A to 11 E of Example 11 ;

[0040] Figures 13a to 13f are a series of optical microscopy images of the fibre samples of Figure 12, wherein the 25 micron scale shown in Figure 13a applies to all of Figures 13a to 13f;

[0041] Figures 14a to 14f are a series of scanning electron microscopy images of the fibre samples 11 A to 11 E; and

[0042] Figure 15 shows SEM images of a fibre spun according to Example 12.

[0043] DETAILED DESCRIPTION

[0044] The invention provides a recycling process for Poly para-phenyleneterephthalamide [PPTA] material, commonly known as para-aramid, through the use of an unconventional solvent. The inventors have surprisingly found that this unconventional solvent is successful in dissolving para-aramid to provide a solution of PPTA, without any obvious indication of degradation of the PPTA polymer chains.

[0045] While the invention can be used for dissolving any para-aramid material, having any form, it is of particular use in a fibre-to-fibre recycling system, in which para-aramid fibres are dissolved. These fibres may take the form of e.g. woven or non-woven fabrics, loose fibres, or other material that originates from fibres. For example, the fibres may be chopped into small pieces, or ground into powder or pulp to provide material that originates from fibres. The fabric or fibres may be coated or uncoated. Non fibre-forms may include, for example, powder, pulp, or any other solid or semi-solid form. In the forgoing description the para-aramid material will sometimes be referred to as para-aramid fibres, though it will be appreciated that the para-aramid material may take other forms, and need not have originated from fibres.

[0046] Where the method is a para-aramid recycling method, it includes a first stage of dissolving para-aramid material in the solvent to provide a recycled para-aramid solution, and a second stage of drawing recycled fibres from the recycled para-aramid solution.

[0047] In the first stage, para-aramid material (in this example virgin para-aramid fibres) is dissolved in a solvent mixture. The solvent mixture comprises dimethyl sulfoxide (DMSO), potassium hydroxide (KOH) and water. DMSO is the major constituent of the solvent, with KOH and water each making up a smaller proportion.

[0048] Throughout the forgoing description, unless otherwise indicated, the concentrations of the various components of the solvent and / or solution are indicated as a weight-by- weight percentage (wt / wt%) with respect to the weight of DMSO. For example, a solvent comprising 6 g of water and 5 g of KOH in 100 g of DMSO would be characterised using these wt / wt% units as a DMSO solvent containing 6 wt / wt% water and 5 wt / wt% KOH. If 10 g para-aramid were dissolved in the solvent, the para-aramid concentration would be 10 wt / wt%. In some of the examples presented below, a corresponding unit of wt% is also given. For avoidance of doubt, this wt% unit corresponds to the weight percentage of the component with respect to the total weight of the solvent. In the above example, using wt% units, the solvent would be characterised as containing 5.4 wt% water, 4.5 wt% KOH and 90.1 wt% DMSO.

[0049] The inventors have found that for this DMSO / KOH solvent system, a small amount of water (between approximately 1 wt / wt% and approximately 10 wt / wt%) is necessary to cause dissolution. This is surprising, because para-aramid is not soluble in water, and has no affinity to water: water content would therefore not be expected to impact its solubility. The inventors believe that the presence of a small amount of water increases the solubility of KOH in DMSO, thus a small amount of water increases the power of the solvent system. The inventors also found that the solvent system became less effective if water content was too high: the inventors believe that because water acts as an antisolvent for the para-aramid, causing precipitation, if the water content is too high, the effectiveness of the solvent system decreases. Thus, the inventors have found that the water content must be carefully balanced. A water content of approximately 3.5 to 9.5 wt / wt% provides good solubility levels, and 4 wt / wt% to 9 wt / wt% is preferable. A water content of 5.5 to 7.5 wt / wt%, and especially 6 to 7 wt / wt% is particularly preferred, as this shows especially high solubility. See Examples 4 and 5 for more detail.

[0050] A high KOH content will generally increase solubility of the aramid. In general the KOH content is limited at the uppermost end by the maximum solubility of KOH in DMSO. As noted above, the solubility of KOH in DMSO is affected by the water content. However, as KOH content increases, so does the risk of degradation of the aramid. Avoiding or at least minimising degradation is important for wet-spinning recycling. The KOH content is therefore a careful balance.

[0051] For the range of approximately 1% to approximately 10 wt / wt% water content, the KOH content is preferably between approximately 1.5 wt / wt% and approximately 10 wt / wt%. Preferably the KOH content is between approximately 2 wt / wt% and 8 wt / wt%, more preferably between approximately 4 wt / wt% and 6 wt / wt%, and most preferably between 4 wt / wt% and 5 wt / wt%

[0052] DMSO forms the major part of the solvent. Preferably the solvent comprises between approximately 83.7 % and approximately 95.2 % DMSO by total weight of the solvent, preferably between approximately 85.5% and approximately 94.3% DMSO by total weight of the solvent, more preferably between approximately 88.1% and approximately 91.3% DMSO by total weight of the solvent, and most preferably between approximately 89.3% and approximately 90.9% DMSO by total weight of the solvent.

[0053] The solvent may be made by a simple mixture of the DMSO, KOH and water in appropriate proportions. In one preferred example, the KOH and water may be mixed together first, with optional stirring or other agitation, until the mixture turns from cloudy to clear. DMSO may then be added, and mixed.

[0054] To dissolve para-aramid fibres in the solvent described above, a sample comprising para-aramid fibres is simply arranged in the solvent and allowed to dissolve. The mixture is preferably be stirred or otherwise agitated to aid dissolution. The sample of paraaramid fibre is added to the solvent, preferably as soon as the solvent has been mixed together. The sample may be added as a single batch, or it may be added in multiple batches, with each batch being allowed to dissolve before a subsequent batch is added.

[0055] Where the material to be dissolved comprises woven fibres such as a fabric or mat, the material may be cut or chopped into small pieces before dissolution. The material may also be processed in other ways, for example by chopping or grinding the material.

[0056] While dissolution is possible at room temperature, the solvent is preferably heated to a reaction temperature that is above room temperature to increase the solubility of the par- aramid fibres. The reaction temperature may for example be between approximately 60 °C and approximately 120 °C. 60 °C is sufficient to provide a noticeable increase in solubility. Over 120 °C risks degradation of the polymer. Preferably the reaction temperature is between 80 °C and 120 °C to provide a more notable increase in solubility while minimising risk of degradation, and most preferably 80 °C and 110 °C. A reaction temperature of approximately 100 °C to 120 °C is particularly desirable to maximise solubility, and a temperature of approximately 100 °C (i.e. between approximately 95 °C and 105 °C) has been found to provide a particularly good balance of these factors.

[0057] Where the solvent is heated, heating preferably takes place as soon as the solvent is mixed.

[0058] The above solvent has been found to form a solution comprising para-aramid polymer dissolved in the solvent, with high para-aramid loading. The inventors believe that this solvent exploits the ability of DMSO to co-ordinate with the potassium cation (K+) of KOH, increasing the basicity of OH anions, resulting in the removal of the N-H proton of the para-aramid polymer. Simply, this disrupts the intermolecular H-bonding between the para-aramid chains, thereby allowing solvent molecules between the polymer chains, resulting in dissolution of the para-aramid fibres. The H-bond disruption sites are shown schematically in Figure 1.

[0059] As noted above, the water content can be optimised to enable maximum solubility of the KOH (and hence maximise the OH anions that cause dissolution), without causing detrimental precipitation of the para-aramid. The constituents of the solvent (DMSO, KOH, and water) therefore act in synergy to dissolve the para-aramid fibres.

[0060] Surprisingly, given the presence of the alkali metal hydroxide KOH, which would be expected to degrade para-aramid fibres to at least some extent, there is no detectable degradation of the polymer chains using the above solvent system, even at temperatures of 120 °C (see Example 3 below). Dissolving para-aramid without any degradation of the polymer chains is critical to a successful recycling process (i.e. a process that can obtain high-grade near-virgin quality regenerated fibres), since any degradation of the chains will impact the quality of the regenerated fibres.

[0061] For the purposes of fibre spinning, it is particularly desirable to obtain a solution in which the para-aramid is in a nematic phase rather than the isotropic phase. The nematic phase is desirable for spinning, because in this phase the polymer chains are aligned, resulting in greater chain alignment in the fibres, and hence better fibre properties.

[0062] Figure 2 shows viscosity of para-aramid solutions in a sulfuric acid solvent, as a function of concentration. At concentrations of approximately 8 wt / wt% to approximately 10 wt / wt% para-aramid, the solution undergoes the phase transition from the isotropic phase to the nematic phase, and during the phase transition, the viscosity dramatically increases. This increase in viscosity can be an obstacle to achieving a solution with a concentration greater than approximately 8 wt / wt%. This is because as the fibres begin to dissolve in a solvent and reach loadings of approximately 8 wt / wt%, the solution enters the phase transition and viscosity greatly increases. At this stage, the very high viscosity prevents stirring or mixing of the mixture, which prevents the para-aramid fibres dissolving further.

[0063] Therefore, while concentration of para-aramid corresponding to the nematic phase is known to be desirable (typically over 8wt / wt% - though the concentration may vary to some extent in different solvent systems), in practice achieving fibre loadings in the nematic phase is particularly challenging. It is even more challenging in solvent systems that do not provide the same degree of solubility as sulfuric acid.

[0064] A particularly beneficial effect of the solvent described above is that para-aramid concentrations above 8 wt / wt% can be achieved. Example 8 below demonstrates that a para-aramid concentration of at least 10 wt / wt% (or 9% of the total weight of the solution) can be achieved, with low viscosity properties.

[0065] The solvent system described herein therefore achieves, for the first time, a high- concentration of para-aramid without the need for sulfuric acid. Recycled fibres can be spun from the resulting para-aramid solution. This may be achieved by any suitable spinning process, for example, by extrusion, or by standard wet-spinning techniques (see Examples11 and 12 for specific examples of fibre spinning). The resulting para-aramid solution may be used directly for the purposes of fibre spinning, or it may be adapted if appropriate.

[0066] The unconventional solvent system described above is effective in dissolving paraaramid fibres, enabling, for the first time, a feasible route for recycling of para-aramid fibres. Recycling para-aramid fibres in this way will reduce waste through incineration of para-aramid fibre products, and reduce the need for raw materials.

[0067] The process of recycling para-aramid fibres offered by this system is significantly more environmentally friendly than the process of creating new virgin para-aramid fibres, at least because the novel solvent system described is significantly more environmentally friendly than sulfuric acid, which is the solvent used in manufacturing virgin para-aramid fibres.

[0068] Unlike sulfuric acid, DMSO is also regarded as a green solvent because it is non-toxic, and is a natural product (part of the natural sulphur cycle), and is readily biodegradable.

[0069] Furthermore, unlike sulfuric acid, which cannot be re-used, and which must be disposed of carefully by conversion into gypsum due to its high toxicity to aquatic life, DMSO is reusable: its recycling is well documented and can be readily implemented into sustainable processes. Being bio-degradable, DMSO can also be disposed of safely if this is more desirable.

[0070] Recycling para-aramid fibres in this way also removes the need for incineration of end- of-life para-aramid fibres, which has further environmental benefits.

[0071] EXAMPLES

[0072] Example 1

[0073] Samples containing para-aramid fibres were prepared using a ‘resin coated paraaramid’. The resin was removed manually, and the fibres were cut down to approx. 1 inch. Solvent: 250 ml of DMSO was mixed with 750 mg of KOH and heated at 60 C for 3 h. The KOH was not ‘dry’ KOH but had been exposed to the atmosphere, and therefore contained small amounts of water.

[0074] The reaction was left to cool and para-aramid fibres were gradually added and allowed to dissolve, until full dissolution was observed. Dissolution was initially allowed to occur at room temperature. The mixture was subsequently heated to 80 C to encourage further dissolution.

[0075] Results

[0076] The solvent showed ability for dissolving para-aramid fibres at low concentration (<1 mg / ml) at low temperatures. Dissolution was confirmed by taking small aliquot of the solution and diluting it in water. If a precipitate was formed, it was concluded that at least partial dissolution had occurred. Upon heating at 80 ‘C, the solubility reached 7 mg / ml or 0.6 wt%.

[0077] Example 2

[0078] A sample was prepared in the same manner as Example 1 , but using dry KOH (i.e. with substantially no water content). There was no substantial observable dissolution of the para-aramid fibres.

[0079] Example 3

[0080] Nuclear Magnetic Resonance (NMR) measurements were used to check for degradation to the polymer chains. Samples were prepared using a solvent comprising 100 parts by weight (90.5 wt%) DMSO, 4.5 parts by weight (4.5 wt / wt%, or 4.07 wt%) KOH and 6 parts by weight (6 wt / wt% or 5.43 wt%) water.

[0081] To prepare the solvent, finely ground KOH powder was mixed with water in appropriate proportions. The solution was allowed to sit for a minute, agitating occasionally, until the solution changed from cloudy to clear. DMSO was then added to the solution with a stirrer bar. A sample of para-aramid fibre was added to the solution as soon as the solvent was mixed, with the sample amount corresponding to 1.5 wt / wt% para-aramid loading. The sample was fully immersed in the solvent.

[0082] Different samples were heated to reaction temperatures ranging from 60 to 120 °C, and were held at the reaction temperature for 3 hours.

[0083] The samples were analysed at 60 °C through NMR. 800 MHz NMR was required to obtain acceptable peak resolution for HSQC; 400 MHz was satisfactory for H-NMR.

[0084] No evidence of degradation was found in any of the samples.

[0085] Example spectra are displayed in Figures 3 and 4. The spectra were substantially identical in all samples. The two peaks at 7 and 8 ppm of the aromatic hydrogens were constant for all spectra across the temperature range, indicating depolymerisation was not occurring.

[0086] Example 4

[0087] Solvent mixtures were prepared with content according to Table 1 below:

[0088] Table 1: Solvent composition and solubility of para-aramid for Samples 4A to 4K. To prepare the solvent, finely ground KOH powder was mixed with water in appropriate proportions. The solution was allowed to sit for a minute, agitating occasionally, until the solution changed from cloudy to clear. DMSO was then added to the solution with a stirrer bar.

[0089] 3 grams of solvent mixture were placed in 5 mL vials and PA was loaded gradually to the system at 100 °C. The mixture was left until full dissolution was achieved through stirring, and a subsequent fraction was added until the viscosity becomes too high for further dissolution: at this point the para-aramid loading was deemed to be at its maximum.

[0090] The amount of PA dissolved was used to determine the resulting PA loading of the resulting solution.

[0091] The resulting para-aramid loadings are shown in Table 1 and in Figure 5. The results show that at least 1 wt / wt% solubility occurs across the entire range of samples.

[0092] Samples between 4 wt / wt% (3.7 wt%) and 9 wt / wt% (8.0 wt%) water content show an improved solubility, and samples at 6 wt / wt% (5.5 wt%) and 7 wt / wt% (6.4 wt%) show the highest solubility.

[0093] Example 5

[0094] To investigate optimum water content further, solvent mixtures were prepared with widely varying water content from 4.57% to 22.3%, according to Table 2 below.

[0095] Table 2: Solvent composition and qualitative solubility observations of various samples to test water content To prepare the solvent, finely ground KOH powder was mixed with water in appropriate proportions. The solution was allowed to sit for a minute, agitating occasionally, until the solution changed from cloudy to clear. DMSO was then added to the solution with a stirrer bar.

[0096] Para-aramid fibres corresponding to 6 wt / wt% loading were added to the solvent mixtures. The mixture was heated to 100 °C and stirred for 1 hr, after which the mixture was visually inspected to determine if the para-aramid had dissolved fully, mostly, partially or not at all.

[0097] The results demonstrated a reduction in solubility of the para-aramid fibres above

[0098] 12.5 wt / wt% water.

[0099] Example 6

[0100] To explore the effect of KOH content on para-aramid solubility, solvent mixtures were prepared with 100 parts by weight DMSO, 7 parts by weight water (i.e. 7 wt / wt%), and different KOH contents, up to the maximum solubility of KOH in the solvent mixture. For the water content of 7 wt / wt%, the maximum solubility of KOH was 5 parts by weight (i.e.

[0101] 5 wt / wt%, or 4.5 wt% KOH). Solvent mixtures were prepared with content according to Table 3 below:

[0102] Table 3: Solvent composition and solubility of para-aramid for Samples L to R.

[0103] To prepare the solvent, finely ground KOH powder was mixed with water in appropriate proportions. The solution was allowed to sit for a minute, agitating occasionally, until the solution changed from cloudy to clear. DMSO was then added to the solution with a stirrer bar.

[0104] 3 grams of solvent mixture were placed in 5 mL vials and PA was loaded gradually to the system at 100 °C. The mixture was left until full dissolution was achieved through stirring, and a subsequent fraction was added until the viscosity becomes too high for further dissolution: at this point the para-aramid loading was deemed to be at its maximum.

[0105] The amount of PA dissolved was used to determine the resulting PA loading of the resulting solution.

[0106] The resulting para-aramid loadings are shown in Table 3 and in Figure 6a. The results show that para-aramid dissolution occurred at all KOH concentrations, with maximum solubility of 7.5 wt / wt% observed at the maximum KOH content (5 wt / wt% or 4.5 wt%).

[0107] Example 7

[0108] To explore a wider range of KOH concentrations, solvent mixtures 1 to 6 were prepared with 100 parts by weight DMSO, 6 parts by weight water (i.e. 6 wt / wt%), and different KOH contents, according to Table 4 below. Three samples of each solvent mixture were added to glass vials, and used to dissolve aramid fibres, with each sample being used to dissolve a different loading of aramid (2 wt / wt%, 4 wt / wt% and 6 wt / wt%). The dissolution temperature was 80 °C, and the sample was stirred during dissolution. After 2 hours dissolution time, the degree of dissolution was assessed.

[0109] Table 4: Solvent compositions and qualitative solubility observations for various solvents The results, illustrated graphically in Table 6a, show a decrease in solubility of par- aramid above approximately 6 wt / wt%.

[0110] Example 8

[0111] Experiments were conducted to obtain a para-aramid concentration of over 8%.

[0112] Sample 8A to 8C

[0113] Samples 8A to 8C were comparative samples, not forming part of the present invention, in which K29 Kevlar was dissolved in > 96 % sulfuric acid in a proportion of 1.0 g K29 Kevlar to 10.0 g Sulfuric acid (i.e. 10 wt / wt% Kevlar). Samples 8A to 8C could therefore be used as a control against which other samples can be compared.

[0114] Samples 8D to 8F

[0115] For each of Samples 8D to 8F, DMSO, KOH, deionised water and K29 Kevlar were mixed in the proportions indicated in Table 5 below. The KOH was dissolved in the water in a vial before the DMSO was added. The K29 Kevlar was then immersed in the solution. The solution was stirred at 200rpm using a magnetic stirrer.

[0116] Table 5: Solvent compositions Samples 8D to 8F

[0117] Results

[0118] Samples 8A and 8D were initially heated to 80 °C and stirred for 9 hours, then inspected to determine the extent of dissolution.

[0119] Samples 8B and 8E were then heated to 90 °C and stirred for 9 hours, then inspected to determine the extent of dissolution. Finally, Samples 8C and 8F were heated to 100 °C and stirred for 9 hours, then inspected to determine the extent of dissolution.

[0120] The results are indicated in Table 6 below.

[0121] Table 6 qualitative observations for Samples 8A to 8F after dissolution and cooling to room temperature

[0122] Images of the solutions, in which the relative viscosities are clearly discernible, are shown in Figures 7a to f.

[0123] In Figures 7a and 7d, both Samples 8A and 8D are shown inverted, with the mixture clinging to the top of the vial. These samples show incomplete dissolution (i.e. a concentration less than 10 wt / wt%), and highly viscous behaviour.

[0124] In Figures 7b and 7e, sample 8B and 8E are both viscous and gel-like, and cling to the walls of the vial, showing a reduction in viscosity compared to 8C and 8F.

[0125] In Figures 7c and 7f, both samples 8C and 8F show complete dissolution (i.e. 10 wt / wt% concentration) and low viscosity that endures after 24 hours at room temperature.

[0126] Samples 8D to 8F, based on the solvent of the invention, therefore show parallel viscosity behaviour to Samples 8A to 8C, based on a conventional sulfuric acid solvent.

[0127] Sample 8G

[0128] A slightly larger scale was trialled to assess if the same behaviours would be observed. DMSO, KOH, deionised water and K29 Kevlar were mixed in the proportions indicated in Table 7 below. The KOH was dissolved in the water in a beaker before the DMSO was added. The K29 Kevlar was immersed in the solution.

[0129] Table 7 Solvent composition Sample 8G

[0130] Sample 8G was heated to 100 °C and stirred with a magnetic stirrer. Optical microscopy images were taken of the sample after stirring for 3, 6, and 9 hours respectively. Images are shown in Figures 8a to 8c respectively, showing a reduction in the number and size of fibres visible, until substantially no fibres are visible after 9 hours. At this time the solution was observed to be a viscous flowing solution, similar to Samples 8C and 8F after heating to 100 °C.

[0131] After 9 hours of magnetic stirring, two further hours of mixing were carried out using the mixer of a twin screw extruder. Optical microscopy was again carried out. The microscopy image is shown in Figure 8d, showing a homogenous solution with no fibres.

[0132] Example 9

[0133] Further experiments were conducted to produce still higher loadings of aramid.

[0134] Samples 9A to 9D

[0135] Samples 9A to 9D were comparative samples, not forming part of the present invention, in which K29 Kevlar was dissolved in > 96 % sulfuric acid. Samples 9A to 9D could therefore be used as a control against which other samples can be compared. The proportion of Kevlar, and the temperature at which the Kevlar was dissolved, are indicated in Table 8 below. The resulting solutions were assessed through visual observation.

[0136] Table 8: Aramid loading and dissolution temperature of samples 9A to 9D

[0137] Samples 9E to 9H

[0138] In all the below samples, a solvent was prepared by dissolving 0.45g KOH in 0.6 g deionised water in a beaker, and then adding 10g DMSO. In all samples, a quantity of K29 Kevlar according to Table 9 was cut with scissors into small pieces, and immersed in the solution. The solution was stirred at 200rpm using overhead stirring, for 9 hours, heating to the temperature shown in Table 9. The resulting solutions were assessed through visual observation.

[0139] Table 9: Aramid loading and dissolution temperature of samples 9E to 9G

[0140] Images of Samples 9A to 9H, reflecting the recorded visual observations, are shown in Figures 9a to 9g respectively.

[0141] The results indicate that loadings close to 12 or 15% are achievable at temperatures above 90 °C.

[0142] Example 10

[0143] Experiments were conducted on para-aramid fibres from different sources.

[0144] In all the below samples, a solvent was prepared by dissolving 2.48 g KOH in 3.30 g deionised water in a beaker, and then adding 55.07g DMSO (i.e. 4.5 wt / wt% KOH and 6.0 wt / wt% water). Fibre material according to Table 10 was immersed in the solution. Unless otherwise stated in Table 10, the quantity of fibre material was 3.30 g (i.e. 6 wt / wt%). Each mixture was heated to 80 °C and stirred every 30 minutes, for a total dissolution period of 6 hours. A visual assessment of each sample was then made, to assess the extent of dissolution.

[0145] Table 10: Fibre Sources for Samples 10A to 10G

[0146] Figures 10a to 10g are photographs of each sample before and after dissolution.

[0147] Example 11

[0148] Para-aramid fibres were dissolved in solvent mixtures to a concentration of 1 wt% paraaramid, to prepare solutions or ‘spinning dopes’ for the purposes of wet-spinning fibres.

[0149] 1 wt% was selected because this concentration produces a viscosity that is particularly suitable for spinning at room temperature. Traditionally, heating capability is required to spin at higher para-aramid loadings to overcome the increased viscosity of the solutions.

[0150] In a control sample, the para-aramid fibres were dissolved in a solvent of 96% H2SO4. In all other samples, the para-aramid fibres were dissolved in a solvent comprising 100 parts by weight (90.5 wt%) DMSO, 4.5 parts by weight (4.1 wt%) KOH and 6 parts by weight (5.4 wt%) water: the solvent was made according to the method described in Examples 3 to 5 above. The spinning dopes were transferred into syringes and degassed by placing the syringes in a vacuum.

[0151] Fibres were extruded from the spinning dopes by injecting from the syringes into a coagulation bath using the simple set-up shown in Figure 8. Solution was injected from the syringe 20 into a z-needle 22, with the exit 24 of the z-needle 22 located in the coagulation bath 26. The coagulation bath 26 was arranged on a rotating table 28, which was spun around an axis of R, thereby ‘spinning’ the ejected polymer into regenerated fibres 30.

[0152] Different coagulation baths were tested according to Table 11 below.

[0153] Table 11: Solvent composition, coagulation bath composition tensile strength, microscopy image and diameter of para-aramid fibre Samples Control and 11 A to 11E.

[0154] The fibres were collected and hung-dried, and high-resolution optical images and Scanning Electron Microscopy (SEM) images were taken of the spun fibres.

[0155] The tensile strength of the fibres was tested using the Instron 5543 mechanical tester. All samples were run using a paper frame at a loading rate of 1 mm / min. All fibres were cut to equal lengths of 40 mm for testing. A series of stress / strain curves were plotted displaying the relationship between tensile stress and tensile strain of the tested fibre. The tensile strength (commonly regarded as the best characterising metric for fibre quality) was calculated as the stress applied to the fibre at breakage.

[0156] The maximum tensile strength of the samples is provided in Table 11 above, and shown in Figure 12.

[0157] Comparing the control sample with sample 11A above (which each used the same coagulation bath of 30% acetone in distilled water), sample 11 A with the DMSO / KOH solvent significantly outperforms the control sample with the H2SO4 solvent, showing a tensile strength that is 2.4 times that of the control sample. When the coagulation bath is further optimised, for example in Sample 11 C, the tensile strength is even greater, being over three times that of the control sample.

[0158] Optical microscopy images of the spun fibres were obtained, and can be seen in Figures 13a to 13f. The 25 micron scale bar shown in Figure 12a applies to all figures in this set. The images show uniform fibres with few defects at this scale.

[0159] Comparing figure 13a (control) and Figure 13b (Sample 11 A), the fibre of Sample 11 A appears to have fewer defects, have a slightly smaller diameter, and look visually smoother. This observation translates into the mechanical properties of the fibres noted above. This relationship is also seen in, for example, Sample 11 C, which demonstrates the highest tensile strength, and also has the smallest diameter and smoothest appearance in the optical microscopy images (Figure 13d).

[0160] SEM images were obtained, and can be seen in Figures 14a to 14f. Images were obtained of the fibre surface as well as the fibre cross-section. The images show a particularly smooth surface for Sample 11 B, with few defects: this is expected to lead to good tensile strength.

[0161] Diameters of the fibres were measured from SEM images. The diameters are provided in Table 11 above: samples 11C and 11 E have particularly small diameters, which is expected to lead to good tensile strength. Example 12

[0162] Further spinning experiments were conducted at higher aramid loadings, using an Xplore MC15 twin-screw extruder. Tensile tests and microscopy studies were conducted on the resulting fibres.

[0163] Spinning

[0164] A spinning dope was prepared by dissolving 1 .35 g KOH in 1 .8 g deionised water in a beaker, and then adding 30g DMSO (i.e. 4.5 wt / wt% KOH and 6 wt / wt% water in DMSO). 1.8 g (i.e. 6 wt / wt%) of K49 para-aramid was immersed in the solution. The mixture was left to stir on a hotplate at 80 °C at 150 rpm, for a dissolution time of 3 hours.

[0165] The prepared dope was injected into the Xplore dope tank, which has a maximum capacity of 15 mL. The liquid was allowed to recycle inside the tank for 15 minutes before extrusion to ensure homogeneity.

[0166] The dope was spun into a coagulation bath with a coagulant of 20%DMSO-80% deionised water, using the following spinning parameters: Extrusion rate = 40 rpm;

[0167] Extruder temperature = 65 °C;

[0168] Collection rate = 70 rpm;

[0169] Die diameter= 0.15 mm.

[0170] Air gap = 1cm

[0171] The collected fibres were immersed in the coagulant for 10 minutes after spinning for further coagulation, before air drying.

[0172] Fibres were successfully obtained from the sample.

[0173] Tensile testing

[0174] Tensile testing was carried out at room temperature on an INSTRON 5543 at a 0.1 mm / min extension rate using a 100 N load cell. Fibre samples were fixed to a 10 mm gauge length paper frame with a cyanoacrylate binder, 5 replicates were taken for each sample. Diameter measurements were taken using an optical microscope, and an average of five measurements were taken over the length of the fibre image using the software package Image J. Results are shown in Table 12 below.

[0175] Table 12 Tensile test results and diameter assessment

[0176] Fibre Morphology

[0177] Scanning electron microscopy was used to assess surface morphology and cross- sectional shape of the fibres. An image of the spun fibres is shown in Figure 15. The overall fibre surface has a smooth morphology.

Claims

CLAIMS1 . A method of dissolving para-aramid material comprising: arranging the para-aramid material in a solvent, the solvent comprising: dimethyl sulfoxide (DMSO), potassium hydroxide (KOH) in a proportion between 1.5 wt / wt% and 10 wt / wt%; and water in a proportion between 3.5 wt / wt% and 9.5 wt / wt%; and allowing the para-aramid material to dissolve to produce a para-aramid solution.

2. The method of Claim 1 , wherein the solvent comprises between approximately 4 wt / wt% and approximately 9 wt / wt% water preferably between approximately 5.5 wt / wt% and approximately 7.5 wt / wt% water, more preferably between approximately 6 wt / wt% and approximately 7 wt / wt% water.

3. The method of any preceding claim, comprising heating the solvent to a reaction temperature above room temperature.

4. The method of Claim 5, wherein the reaction temperature is between approximately 60 °C and approximately 135 °C, preferably between approximately 80 °C and approximately 120 °C, more preferably between approximately 95 °C and approximately 120 °C, most preferably between approximately 95 °C and approximately 105 °C.

5. The method of any preceding claim wherein the solvent comprises between approximately 2 wt / wt% and approximately 8 wt / wt% KOH by weight, more preferably between approximately 4 wt / wt% and approximately 6 wt / wt% KOH by weight, most preferably between approximately 4 wt / wt% and approximately 5 wt / wt% KOH by weight.

6. The method of any preceding claim, wherein the solvent comprises between approximately 83.7% and approximately 95.2% DMSO by total weight of the solvent, preferably between approximately 85.5% and approximately 94.3% DMSO by total weight of the solvent, more preferably between approximately 88.1 % and approximately 91.3% DMSO by total weight of the solvent, and most preferably between approximately 89.3% and approximately 90.9% DMSO by total weight of the solvent.

7. The method of any preceding claim, wherein the weight of the para-aramid material is at least 7 % of the weight of DMSO, preferably at least 10% of the weight of DMSO, and optionally is between 10% and 20% of the weight of DMSO.

8. The method of any preceding claim, comprising causing shearing of the aramid sample and solvent during dissolution.

9. A method of recycling para-aramid material into para-aramid fibres, the method comprising: dissolving a para-aramid material according to the method of any preceding claim, thereby forming a para-aramid solution; and forming regenerated fibres from the para-aramid solution.

10. Use of a solvent comprising: dimethyl sulfoxide (DMSO), potassium hydroxide (KOH) in a proportion between 1 .5 wt / wt% and 10 w / wt% and water in a proportion between 3.5 wt / wt% and 9.5 wt / wt%; for dissolution of para-aramid.

11. The method or use of any preceding claim, wherein the para-aramid material comprises para-aramid fibres.

12. The method or use of Claim 11 , wherein the para-aramid material comprises a woven or-non-woven fabric comprising para-aramid fibres.

13. A para-aramid solution comprising para-aramid dissolved in a solvent, the solvent comprising: dimethyl sulfoxide (DMSO), potassium hydroxide (KOH) in a proportion between 1.5 wt / wt% and 10 w / wt% and water in a proportion between 3.5 wt / wt% and 9.5 wt / wt%.

14. The para-aramid solution of Claim 13, wherein the weight of para-aramid is at least15. A method of making para-aramid fibres comprising providing the solution of Claim 13 or Claim 14, and spinning para-aramid fibres from the solution.

16. The method of Claim 15, comprising spinning fibres into a coagulation bath, the coagulation bath comprising DMSO and water, and / or the coagulation bath comprising acetone and water.

17. The method of Claim 15 or Claim 16, wherein the step of providing the solution comprises dissolving para-aramid material according to the method of any of Claims 1 to 8, Claim 11 or Claim 12.

18. A para-aramid fibre spun from the solution of Claim 13 or Claim 14, or made using the method of any of Claims 15 to 17.

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