Method for stabilising PVC and method for forming carbon fibres from PVC TECHNICAL FIELD

[0001] The present invention relates to a method for forming carbon fibres from polyvinylchloride (PVC). The method may use virgin polyvinylchloride as a feed material. Alternatively, a mixed waste stream containing polyvinyl chloride and other materials may be used as a feed material. The present method also relates to a method for stabilising PVC fibres. The stabilised PVC fibres may be used to manufacture carbon fibres.

BACKGROUND ART

[0002] Polyvinyl Chloride (PVC) is one of the most commonly used polymers worldwide.

In 2018, PVC production is estimated to be 48.8 million tonnes with expectation to increase in the future. Waste PVC can be subjected to mechanical recycling to produce new PVC based materials. However, the task of recycling PVC materials does not come without significant challenges. Mechanical recycling requires a significant degree of homogeneity. Even small levels of impurity, including fillers and plasticizers, can represent significant property loss of the final product. Additionally, additives used in the polymer manufacture to provide specific properties hinder combined recycling, e.g. PVC-U and plasticized PVC. The low price of virgin polymer and the challenges required for its reprocessing contribute for the worldwide recycling rates of PVC being close to null.

[0003] Carbon fibres (CF) are very popular due to their physical, chemical and thermal properties. Polyacrylonitrile (PAN) dominates the current production of CF as the resin source. However, the high cost of PAN represents 50-65% of the high CF production cost. The replacement of PAN by PVC would enable a significant reduction of the carbon fibre price and contribute to a widespread use of this material into other less expensive applications.

[0004] The manufacturing of carbon fibres from PVC has been reported in literature. However, the reported method requires the prior conversion of the polymer into pitch, which is then spun and carbonized. Indeed, the relatively low softening point of PVC (~176°C) represents a challenge for the direct conversion of PVC fibres into carbon fibres. Typically, in CF production the polymer precursor undergoes stabilization at 200-300°C, prior to carbonization, to prevent fibre collapse. Consequently, for a successful conversion of PVC-fibres into CF this problem must be overcome. [0005] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

SUMMARY OF INVENTION

[0006] The present invention is directed to a method for stabilising PVC, such as PVC fibres, and to a method for producing carbon fibres, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

[0007] With the foregoing in view, the present invention, in a first aspect, resides broadly in a method for stabilising PVC comprising contacting the PVC with one or more chemicals or reactants that result in removal of chlorine from the PVC so that chlorine is removed from the PVC to thereby stabilise the PVC.

[0008] In one embodiment, the PVC comprises PVC fibres, and the removal of chlorine from the PVC thereby forms chemically stabilised fibres. In one embodiment, the method comprises a method for stabilising PVC fibres comprising contacting the PVC fibres with one or more chemicals or reactants that result in removal of chlorine from the PVC so that chlorine is removed from the PVC fibres to thereby stabilise the PVC fibres.

[0009] The PVC, such as PVC fibres, treated by the method of the first aspect of the present invention, show enhanced physical and shape stability (compared to untreated PVC) such that they retain their shape even if heated to a temperature above 176°C. The term “stabilising PVC” and its grammatical equivalents will be taken through the specification to have this meaning.

[0010] For convenience and brevity of description, the present invention will hereinafter be described with reference to stabilising PVC fibres. However, it will be appreciated that other forms of PVC, such as sheets, plates, rolls, etc, may also be stabilised by the method of the present invention.

[0011] In one embodiment, the PVC fibres are contacted with a solution including one or more chemicals or reactants that result in removal of chlorine from the PVC so that chlorine is removed from the PVC fibres to thereby stabilise the fibres. The solution may comprise an aqueous solution.

[0012] In one embodiment of the present invention, the PVC fibres are formed from a melt of PVC. In this embodiment, the PVC fibres may be formed by melt spinning or extrusion. In other embodiments of the present invention, the PVC fibres are formed from a solution containing dissolved PVC, for example, as described in detail below. In one embodiment, the method comprises producing the PVC fibres from PVC comprising the step of dissolving PVC into a solvent and mixing the solution of the solvent and dissolved PVC with an anti-solvent to form fibres of PVC. This may be as described in detail below.

[0013] The chemical stabilisation step of the present invention is desirably conducted at a temperature below the softening point of PVC, which is about 176°C. In some embodiments, the chemical stabilisation step takes place at a temperature of 175°C or less, or 170°C or less. Higher temperatures are believed to result in quicker reaction and hence less time to stabilise the PVC fibres. The stabilisation step should be conducted in a pressure vessel if the temperature being used is greater than a boiling point of any solutions or liquids used in that step. In some embodiments, the chemical stabilisation step is conducted at a temperature of from 100°C to 170°C, or at 110°C to 170°C, or at a temperature of between 130°C to 170°C, or at a temperature of about 150°C - 170°C. In some embodiments, the PVC is contacted with one or more chemicals or reactants at a temperature of from 100°C to 170°C, or at 110°C to 170°C, or at a temperature of between 130°C to 170°C, or at a temperature of about 150°C - 170°C. The chemical stabilisation step occurs more quickly at elevated temperatures. The desired temperature will be determined from the economics of the process, in which increased reaction rate must be balanced against increased heating expenses as a higher temperature is used. As mentioned above, the chemical stabilisation step is desirably conducted below the softening temperature of PVC so that the PVC fibres maintain their shape during the chemical stabilisation step. The present inventors believe that 150°C -170°C is a good temperature to conduct the chemical stabilisation step.

[0014] In one embodiment, the chemical stabilisation step comprises contacting the PVC fibres with an acid. In this embodiment, the chlorine that is removed from the PVC forms HC1.

In another embodiment, the chemical stabilisation step comprises contacting the PVC with an alkaline material, such as a base. In this embodiment, the chlorine that is removed from the PVC forms a chloride salt. In another embodiment, the chemical stabilisation step comprises contacting the PVC with a halogen under conditions such that the oxidation occurs to thereby remove some of the chlorine from the PVC.

[0015] In one embodiment, the PVC fibres are contacted with a strong acid. The chemicals or reactants may comprise a strong acid. In one embodiment, the strong acid is a concentrated strong acid. The strong acid may comprise or be selected from HC1, H2SO4 or HNO3. The strong acid may comprise HC1. The strong acid may be or comprise an acidic solution containing more than 10 wt% acid, or 15wt% or more acid, or 20 wt% or more acid, or 20 to 50 wt% acid, or 25 to 50 wt% acid, or about 30 wt% acid.

[0016] In one embodiment, the PVC fibres are contacted with a strong base or strong alkaline material. The chemicals or reactants may comprise a strong base. The strong base may comprise or be selected from NaOH or NH4OH. The strong base may be or comprise an alkaline or basic solution containing at least 5 wt base, or at least 10 wt% base, or 15wt% or more base, or 20 wt% or more base, or 20 to 50 wt% base, or 25 to 50 wt% base, or about 30 wt% base.

[0017] In one embodiment, the chemical stabilisation step comprises contacting the PVC with a halogen under conditions such that the oxidation occurs to thereby remove some of the chlorine from the PVC and the halogen comprises a chloride material. The chloride material may be selected from one or more of AICI3, NaCl, KC1.

[0018] The halogen may be in the form of a solution containing at least 5 wt% halogen compound, or at least 10 wt% halogen compound, or 15wt% or more halogen compound, or 20 wt% or more halogen compound, or 20 to 50 wt% halogen compound, or 25 to 50 wt% halogen compound, or about 30 wt% halogen compound.

[0019] The chemical/reactant(s) used in the chemical stabilisation step may comprise a solution containing the chemical/reactant, or an aqueous solution containing the chemical/reactant. The chemicals or reactants used in the chemical stabilisation step may comprise one or more of the chemicals or reagents discussed, for example, at paragraphs [0014- 0018]

[0020] The chemical stabilisation step may be conducted with a residence time of up to 100 hours, or from 1 hour to 80 hours, or from 6 hours to 72 hours, or from 12 hours to 60 hours, or from 24 hours to 60 hours. Experiments conducted by the present inventors to date have utilise a residence time of about 24 to 48 hours in this step.

[0021] Without wishing to be bound by theory, the present inventors believe that the chemical stabilisation step removes at least some Cl from the PVC molecules to thereby stabilise the PVC. A number of different mechanisms may be involved in this chemical stabilisation step, including one or more of cross-linking, aromatisation, formation of double bonds or addition of H to the PVC molecules. Different mechanisms may be associated with different reactants used in the stabilisation step. [0022] In embodiments where PVC fibres are stabilised, the chemically stabilised fibres may be converted to carbon fibres, as described below. In one embodiment, the method comprises converting the chemically stabilized fibres to carbon fibres. This may be as described in detail below.

[0023] In a second aspect, the present invention provides a method for producing carbon fibres from PVC comprising the steps of dissolving PVC into a solvent, mixing the solution of the solvent and dissolved PVC with an anti-solvent to form fibres of PVC, chemically stabilising the PVC fibres and converting the chemically stabilised PVC fibres to carbon fibres.

[0024] In one embodiment, the step of dissolving the PVC into a solvent comprises dissolving the PVC into a haloalkane, especially a C1-C12 haloalkane, or a Ci-C ₆ haloalkane (wherein halo may be one or more of fluoro, chloro, bromo or iodo). In one embodiment, the C1-C12 haloalkane is a Ci-Ci2chloroalkane, especially a C1-C6 chloroalkane. The C1-C12 haloalkane may have any number of halogen atoms. In one embodiment, the C1-C12 haloalkane is C _t X _y H _z , wherein X is fluoro, chloro, bromo, or iodo (especially chloro), t is an integer from 1- 12 (especially 1-6, more especially 2 or 3), y is an integer from 1 to 2t+2, and z is an integer from 0 (or 1) to 2t+2-y. In a preferred embodiment, the haloalkane is dichloro-ethane (C2H4CI2, also known as ethylene dichloride). However, other solvents for dissolving the PVC may also be suitable.

[0025] Accordingly, in one embodiment the solvent may be a haloalkane (as discussed above). In other embodiments, the solvent may comprise an aromatic compound or a substituted aromatic compound, especially a substituted aromatic compound having a single aromatic ring.

In another embodiment, the solvent may comprise a monocyclic aromatic that is optionally substituted by from 1-5 Ci- ₆ alkyl groups. Exemplary such monocyclic aromatic compounds include benzene, toluene and/or xylene. In a further embodiment, the solvent is or comprises a cycloalkanone, especially a C5-12 cycloalkanone, more especially cyclohexanone. In another embodiment, the solvent is or comprises an optionally substituted heterocyclic compound, especially a monocyclic heterocyclic compound or a saturated monocyclic heterocyclic compound, more especially a five membered heterocyclic compound or an oxygen-containing heterocyclic compound, most especially tetrahydrofuran. In a further embodiment, the solvent is a dialkylamidoalkane, especially a diCi-i ₂ alkylamidoCi-i ₂ alkane or a diCi- ₆ alkylamidoCi- ₆ alkane, more especially dimethylacetamide. In one embodiment, the solvent comprises at least one selected from the group consisting of: a haloalkane, a cycloalkanone, an optionally substituted heterocyclic compound, and a dialkylamidoalkane. In one embodiment, the solvent comprises at least one selected from the group consisting of: dichloro-ethane, cyclohexanone, tetrahydrofuran and dimethylacetamide.

[0026] In one embodiment, virgin PVC or essentially pure PVC or essentially pure PVC -U was dissolved into the solvent. In another embodiment, a mixed waste material comprising PVC and other plastics was mixed with the solvent and PVC dissolved into the solvent while the other plastics did not dissolve in the solvent. In one embodiment, producing the PVC fibres from PVC comprises producing the PVC fibres from a mixed material comprising PVC and other plastics; and wherein the step of dissolving PVC into a solvent comprises dissolving PVC into the solvent while at least some of the other plastics do not dissolve into the solvent; and especially wherein the method further comprises the step of separating (especially filtering) the solvent comprising dissolved PVC. In these embodiments, the method may comprise the further step of separating the other plastics material from the solution containing dissolved PVC. The other plastic material may include polyethylene terephthalate (PET), polyethylene, polypropylene, inorganic fillers and plasticisers. The other plastics may comprise one or more of polyethylene terephthalate (PET), polyethylene, polypropylene, an inorganic filler and a plasticiser. Any other non-PVC material that does not dissolve into the solvent may also be separated from the solution containing dissolved PVC.

[0027] In some embodiments, the step of dissolving the PVC into the solvent may be conducted at a temperature ranging from ambient temperature up to the boiling temperature of the mixture of PVC and solvent. In one embodiment, the step of dissolving the PVC into the solvent is conducted at a temperature of from 40°C to 90°C, or from 50°C to 80°C. A residence time of up to 1 hour may be used in this step, for example, 10 minutes to lhour. The present inventors have found that adequate dissolution of PVC can be obtained at a temperature of 60°C and a residence time of 15 minutes, although these values should not be considered to be limiting.

[0028] In the dissolution step, the ratio by weight of PVC to solvent may fall within the range of from 0.001 to 0.5, or from 0.01 to 0.2, or from 0.05 to 0.1. Again, these values should not be considered to be limiting.

[0029] The solution containing dissolved PVC was then mixed with an anti-solvent in order to form PVC fibres. The anti-solvent suitably comprises a liquid in which PVC is essentially insoluble and, desirably, in which the solvent is miscible. The anti-solvent may comprise or consist of an alcohol, especially an alkanol, more especially a C1-C12 alkanol or a C1-C6 alkanol, most especially ethanol. In another embodiment, the anti-solvent may comprise or consist of an alkane, especially a Ci-Ci2alkane, or a C3-C12 alkane or a Cs-Cg alkane, more especially hexane, most especially n-hexane. In another embodiment, the anti-solvent may comprise or consist of an alkanone, especially a Ci-Ci2alkanone, a C2-C6 alkanone or a C3-C6 alkanone, more especially acetone. In other embodiments, the anti-solvent may comprise an aromatic compound or a substituted aromatic compound, especially a substituted aromatic compound having a single aromatic ring. In another embodiment, the anti- solvent may comprise a monocyclic aromatic that is optionally substituted by from 1-5 Ci- ₆ alkyl groups. Exemplary such monocyclic aromatic compounds include benzene, toluene and/or xylene. In one embodiment, the anti-solvent comprises at least one selected from the group consisting of: a C1-C12 alkanol, a Ci-Ci2alkane, a Ci-Ci2alkanone and a monocyclic aromatic that is optionally substituted by from 1-5 Ci- ₆ alkyl groups. In one embodiment, the anti-solvent comprises at least one selected from the group consisting of: ethanol, hexane, acetone and toluene. However, other anti-solvents for forming PVC fibres may also be suitable. In one embodiment, the solvent is tetrahydrofuran and the anti solvent is ethanol.

[0030] In some embodiments, this step is conducted at ambient temperature. The solution containing dissolved PVC may be cooled prior to mixing it with the anti-solvent.

[0031] In some embodiments, the solution containing dissolved PVC is extruded into the anti-solvent to form PVC fibres. In other embodiments, the solution containing dissolved PVC is spun into the anti-solvent to form PVC fibres. In other embodiments, the solution containing dissolved PVC is injected into the anti-solvent to form PVC fibres.

[0032] The PVC fibres are typically removed from the anti-solvent and then converted to carbon fibres. In one embodiment, the PVC fibres are dried following removal from the anti solvent. The dried fibres may then be subject to further treatment.

[0033] In some embodiments of the present invention, the mixture of solvent and anti solvent from which the PVC fibres have been removed is subjected to a separation step to separate the solvent from the anti-solvent. The solvent can then be recycled to the dissolution step whilst the anti-solvent can also be recycled and reused. The separation step may suitably comprise a distillation step, although any other separation step known by the person skilled in the art to be suitable can also be used.

[0034] As PVC fibres cannot be directly stabilised by oxidative thermal treatment similar to those used in converting PAN to carbon fibres, an alternative route must be found in order to stabilise the fibres to enable the PVC fibres to retain their shape during the heat treatment steps that convert the PVC fibres to carbon fibres. The person skilled in the art will understand that producing carbon fibres from PAN involves an initial heat treatment to a temperature in the range of from 200°C to 300°C. This step is a thermal oxidative treatment which removes some of the nitrogen from the PAN polymer and causes a degree of cross-linking. Thus, this step is a thermal stabilisation step. As a result, the PAN fibres retain their shape during subsequent pyrolysis treatment to form the carbon fibres.

[0035] It is not possible to subject the PVC fibres to a purely thermal stabilisation step because the PVC fibres will soften at a temperature below the temperature at which thermal oxidative stabilisation will occur. As a result, the PVC fibres will lose their shape prior to thermal stabilisation occurring, thereby rendering it impossible to form carbon fibres therefrom. With this in mind, the present inventors have proposed that the PVC fibres be subject to a chemical stabilisation step in order to ensure that the PVC fibres maintain their shape at higher temperatures.

[0036] The chemical stabilisation step may be as described with reference to the first aspect of the present invention, as described herein. The chemical stabilisation step will typically comprise contacting the PVC fibres with one or more chemicals or reactants that will remove some of the chlorine from the PVC fibres to thereby stabilise the PVC fibres, with these reactions occurring at a temperature below the softening temperature of PVC. In some embodiments, the chemical stabilisation step is conducted at a temperature of from 100°C to 170°C, or at 110°C to 170°C, or at a temperature of between 130°C to 170°C, or at a temperature of about 150°C - 170°C. The chemical stabilisation step occurs more quickly at elevated temperatures. The desired temperature will be determined from the economics of the process, in which increased reaction rate must be balanced against increased heating expenses as a higher temperature is used. As mentioned above, the chemical stabilisation step is desirably conducted below the softening temperature of PVC so that the PVC fibres maintain their shape during the chemical stabilisation step. The present inventor believes that 150°C - 170°C is a good temperature to conduct the chemical stabilisation step.

[0037] The chemical stabilisation step may be conducted with a residence time of up to 100 hours, or from 1 hour to 80 hours, from 6 hours to 72 hours, or from 12 hours to 60 hours, or from 24 hours to 60 hours. Experiments conducted by the present inventors to date have utilise a residence time of about 24 to 48 hours in this step.

[0038] In one embodiment, the chemical stabilisation step comprises contacting the PVC fibres with an acid. In this embodiment, the chlorine that is removed from the PVC forms HC1. In one embodiment, the chemical stabilisation step comprises contacting the PVC fibres with HC1, either as hydrochloric acid or as gaseous HC1. In this step, the chlorine that is removed from the PVC forms further HC1.

[0039] In another embodiment, the chemical stabilisation step comprises contacting the PVC with an alkaline material, such as a base. In this embodiment, the chlorine that is removed from the PVC forms a chloride salt. In another embodiment, the chemical stabilisation step comprises contacting the PVC with a halogen under conditions such that the oxidation occurs to thereby remove some of the chlorine from the PVC.

[0040] In one embodiment, once the chemical stabilisation reaction has commenced, the fibres can be subjected to a thermal stabilisation step by heating to a temperature within the range of from 200°C to 300°C. The step of converting the chemically stabilized fibres to carbon fibres may comprise thermally stabilising the PVC fibres by heating the chemically stabilized fibres to a temperature of from 200 °C to 300 °C to form thermally stabilized fibres. Although this is above the softening temperature of PVC, the chemical stabilisation step has sufficiently stabilised the fibres by this time so that the fibres retain their shape and are able to be further stabilised by the thermally stabilisation. This thermal stabilisation step may be conducted with oxygen present in the atmosphere.

[0041] The thermally stabilised fibres may then be pyrolysed to form carbon fibres. The step of converting the chemically stabilized fibres to carbon fibres may comprise the step of pyrolising the thermally stabilized fibres to thereby form carbon fibres. The step of pyrolising the thermally stabilized fibres may comprise heating the thermally stabilized fibres to a temperature of up to 2800 °C or up to 1800 °C or up to 1300 °C or up to 800 °C (such as from 800 °C to 2800 °C; or from 800 °C to 1300 °C; or from 1300 °C to 1800 °C; or from 1800 °C to 2800 °C) to thereby form carbon fibres. The step of pyrolising the thermally stabilized fibres may comprise heating the thermally stabilized fibres to a temperature of at or greater than 800 °C to thereby form carbon fibres.

[0042] The pyrolization step typically involves heating the fibres to a temperature of about 800°C in an inert atmosphere. If it is desired to graphitise the carbon fibres, further heating to a temperature of about 1000°C or higher can be used. The step of pyrolising the thermally stabilized fibres may comprise heating the thermally stabilized fibres to a temperature of at least 1000 °C (such as from 1000 °C to 2800 °C; or from 1000 °C to 1800 °C; or from 1300 °C to 1800 °C) to thereby form graphetised carbon fibres. The present invention is intended to encompass all processes by which stabilised PVC fibres can be converted to carbon fibres and any such process known to the person skilled in the art is encompassed by the present invention.

[0043] The method of the present invention enables PVC to be used as a feedstock in the production of carbon fibres. The PVC may be virgin PVC or essentially pure PVC. The feedstock may comprise PVC contaminated with one or more of other plastics, plasticisers and inorganic fillers. This opens up the possibility of using recycled PVC as a feedstock for carbon fibre production. PVC is also a cheaper product than PAN, which is the material that is commonly used in current production of carbon fibres. As a result, the cost of the carbon fibres made from PVC should also be lower than current carbon fibres made from PAN.

[0044] In a third aspect, the present invention provides a method for producing PVC fibres from a mixed waste containing PVC and other materials or other plastics, the method comprising the steps of dissolving PVC into a solvent, mixing the solution of the solvent and dissolved PVC with an anti- solvent to form fibres of PVC.

[0045] In the third aspect of the invention, the solvent and the anti-solvent may be as described with reference to the first aspect of the invention. The method of the third aspect may further comprise a chemical stabilisation step, as described above. The method of the third aspect may further comprise a thermal stabilisation step as described above.

[0046] In a fourth aspect, the present invention relates to PVC stabilised by the first aspect. In a fifth aspect, the present invention relates to carbon fibres produced by the first or second aspects. In a sixth aspect, the present invention relates to PVC fibres produced by the method of the third aspect. Features of the fourth to sixth aspects of the present invention may be as described with reference to the first to third aspects of the invention.

[0047] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0048] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

BRIEF DESCRIPTION OF DRAWINGS

[0049] An embodiment of the invention will be described with reference to the following drawings, in which:

[0050] Figure 1 shows a flowsheet for carbon fibres from a mixed waste containing PVC; [0051] Figure 2 shows an SEM image of a cross section of a carbon fibre made from PVC in accordance with example 1 given below; and

[0052] Figure 3 shows an SEM image of carbon fibres made from PVC in accordance with example 6 given below.

DESCRIPTION OF EMBODIMENTS

[0053] The attached drawings have been provided for the purpose of illustrating a preferred embodiment of the present invention. Therefore, it will be understood that the invention should not be considered to be limited solely to the features as shown in the attached drawings.

[0054] Figure 1 shows a process flowsheet for a method for converting PVC mixed waste 10 into carbon fibres 34. In this process, the PVC mixed waste 10, which includes PVC, and other plastics such as PET, polyethylene, polypropylene, etc, is passed to a dissolution/filtration step 12. In step 12, the PVC mixed waste 10 is mixed with dichloroethane solvent. This dissolves the PVC but the other plastic materials are not dissolved. As a result, a mixture comprising a liquid phase of dissolved PVC in solvent and a solid phase containing other undissolved plastics is formed. This mixture is subject to a filtration step to remove the other undissolved plastics. The solution containing dissolved PVC plus solvent is removed from the dissolution/filtration step 12 via line 14. The contaminants, which comprise the undissolved other plastics, are removed at 16 and may be sent to disposal or for further recycling.

[0055] The solution of dissolved PVC plus solvent is fed via line 14 to solvent spinning step 18. In solvent spinning step 18, the solution of PVC dissolved in the solvent is spun into an anti solvent in which the solvent is miscible but PVC is not soluble. Ethanol is an example of a suitable anti-solvent. As the solution containing dissolved PVC in solvent is spun into the anti solvent, fibres of PVC are formed. The PVC fibres are removed at 26. The mixture of solvent and anti-solvent is removed at 20 and sent to distillation step 22. The solvent recovered from distillation step 22 is fed via line 23 back to dissolution/filtration step 12. The anti-solvent separated in the distillation step 22 is returned via line 24 to the solvent spinning step 18.

[0056] The PVC fibres 26 removed from the solvent spinning step 18 are dried to remove any residual solvent/anti-solvent therefrom. It is then subjected to chemical stabilisation step 28. In the chemical stabilisation step 28 of the process shown in figure 1, the PVC fibres are contacted with HC1, either as a strong acid or as gaseous HC1. Chemical stabilisation step 28 in the process of figure 1 is carried out at a temperature of 150°C and a residence time of 48 hours. Oxygen is present in the atmosphere in chemical stabilisation step 28. This results in some of the chlorine in the PVC coming out to form additional HC1 and cross-linking between polymer chains occurring. This stabilises the fibres.

[0057] The stabilised fibres are then fed to carbonisation step 30 in which the stabilised fibres are heated to a temperature of between 200 to 300°C to thermally stabilise the fibres and increase in cross-linking between the polymer chains, followed by heating in an inert atmosphere or in a vacuum to a temperature of up to 800°C (to form carbon fibres) or up to a temperature of 1000°C or greater if it is desired to graphitise the carbon fibres. HC1 that is formed in the thermal stabilisation step conducted in step 30 is returned via line 32 to the chemical stabilisation step 28. The final carbon fibres 34 are removed.

Example 1

[0058] Pure PVC powder, commercial unplasticized PVC (PVC-U), and PVC contaminated with PE, PP or PET (10 wt.% contamination) were used as feedstock for the preparation of PVC fibres. C2H4CI2 and ethanol were used as solvent and anti-solvent, respectively, for the PVC extraction and precipitation in fibre shape. For all cases, a PVC/ C2EI4CI2 ratio of 0.05 was used and dissolution took place at 60 ^° C for 15min. The contaminants were removed by filtration. The PVC + C2H4CI2 dye was manually extruded with the help of a syringe through a stainless steel needle (0.5mm ID) into an ethanol bath causing the precipitation of PVC into a continuous fibre. The precipitated PVC fibre was pulled out of the ethanol bath with the help of tweezers and continuously rolled. After, the PVC fibres were left to dry at room temperature overnight. No changes were observed when contaminants were added to PVC.

[0059] Once dried, the PVC fibres were chemically stabilized with concentrated HC1 (37% in water) at 150°C for 48 h. After stabilization, the fibres were carbonized, first under air atmosphere for 2h at 230 ^° C and secondly at 800 ^° C for lh. After chemical treatment with HC1 fibres retained 86.8% of their mass and after carbonization, the yield changed to 43.1% (relative to initial fibre weight). Typical PVC Cl content is around 56 wt.% and from literature is known that all Cl is removed, as HC1, at temperatures up to 350 ^° C. Therefore, the final carbon yield of the fibres should be very close to 100%. Figure 2 shows an SEM image of the cross-section of a carbon fibre made in accordance with this example.

Example 2

[0060] This example relates to chemical stabilisation of PVC fibres. In this example, several different chemical/reactants we used to chemically stabilise the PVC fibres. The stabilised PVC fibres were then converted to carbon fibres. In this example, the PVC fibres were stabilised by contacting with an aqueous solution of the chemical/reactants (in the table below, the chemical/reactants used are listed under the heading “Promoters”)· The chemical stabilisation step was carried out a temperature of 170°C, a period of 24 hours, and a concentration of promoter of 30 wt% in water. After the chemical treatment step, the samples were thermally treated at 230°C for 3 hours in nitrogen and carbonised/pyrolysed at 800°C for 1 hour in nitrogen. The results achieved are listed in table 1 below:

[0061] Table 1:

Example 3 :

[0062] In this example, the effect of the concentration of HC1 in the chemical stabilisation step on the conversion of PVC fibres into carbon fibres was investigated. Using the same reaction conditions as set out in Example 2, PVC fibres were stabilised with HC1 solutions containing 30% by weight HC1 and 10% by weight E1C1. The following results were obtained:

Table 2:

[0063] In Table 2 (and in Table 3 below), “yield (chemical”) is the yield following the chemical stabilisation step and “yield (final)” is the yield following the pyrolysis to form carbon fibres. Example 4

[0064] In this example, the effect of temperature during the chemical stabilisation step in the conversion of PVC fibres into carbon fibres was investigated. In this example, aqueous solutions of NaOH we used as the chemical reactant/stabiliser. The procedure as set an example 2 was followed, except for the variations in the chemical stabilisation step as noted in Table 3 below. The results were as follows:

Table 3

[0065] The results given above show that strong acids and strong bases are suitable chemical/reactants/promoters for use in the chemical stabilisation step. HC1 concentration should be greater than 10%. NaOH concentration of 10% does achieve acceptable results but a higher temperature in the chemical stabilisation step is desirable. Aluminium chloride being used as the chemical/reactant in the chemical stabilisation step also obtains suitable results.

Example 5

[0066] In this example, the effect of solvent and anti- solvent in the preparation of PVC fibres was investigated. In this example, 50/50 mixtures of PVC powder with polyethylene, PET and polypropylene, were dissolved in a range of solvents (solvents: cyclohexanone, tetrahydrofuran (THF), dimethylacetamide and dichloroethane) and the PVC solution was separated. Two different PVC / solvent ratios of 0.05 and 0.10 was used and dissolution took place at room temperature overnight and 60 °C for 15 min. A control solution of pure PVC was also prepared.

[0067] The resulting PVC solution was then formed into fibres by using a syringe to inject the solution into an antisolvent bath (anti- solvents: ethanol, hexane, acetone, toluene).

[0068] For the dissolution/purification/fibre forming experiments, the optimum conditions were PVC/THF at a ratio of 0.10 with dissolution taking place at 60°C for 15 min and an antisolvent of ethanol. High recoveries were obtained from contaminated PVC mixtures. Example 6

[0069] For the PVC fibre stabilisation / carbonisation, the following experiments were run:

1) Electrospun PVC fibres were used, prepared from the PVC solvent solutions, then dried at 105°C and weighed.

2) Stabilisation was done in a Teflon lined digestion bomb. Once dried, the PVC fibres were chemically stabilized with a promoter at 150°C for 48 h, then washed and again dried and weighed.

3) After stabilization, the fibres were carbonized, first under air atmosphere for 2h at 230°C and secondly at 800°C for lh. Final weights were recorded.

4) The range of conditions tested for stabilisation was: temperature 150 °C (for NaOH) and 170 °C; time: 1, 16, 24 h; stabilisation agent: NaOH (30% in ¾0), H2SO4 (30% in H2O), HC1 (10% and 30% in H ₂ 0) and H ₂ 0.

[0070] The optimal conditions for the chemical stabilisation step were found to be: temperature of 170°C, a period of 24 hours, and a concentration of stabilisation agent of 30 wt% in water. After the chemical treatment step, the samples were thermally treated at 230°C for 3 hours in nitrogen and carbonised/pyrolysed at 800°C for 1 hour in nitrogen. EDX (Energy Dispersive X-ray) analysis on the carbonised fibres showed that when using NaOH, most of the fibre is NaCl (about 70-80%). However, when HC1 is used, 97% of the fibre is carbon (22% overall yield). The fibres maintained shape and were extremely smooth as illustrated in Figure 3.

[0071] In the present specification and claims, the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0072] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0073] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.