Method of improving the fire retardancy of polyamide parts

Abstract

The present disclosure relates to a method of improving the fire retardancy of polyamide parts. In particular, it has been found that the application of a treatment solution comprising at least one phosphate ester can help to improve the fire retardancy of parts comprising such materials.

Description

[0001] METHOD OF IMPROVING THE FIRE RETARDANCY OF POLYAMIDE PARTS

[0002] FIELD

[0003] The present disclosure relates to a method of improving the fire retardancy of a polyamide part, to a fire-retardant polyamide part obtainable via said method, and to the use of a phosphate ester for improving the fire- retardancy of a polyamide part.

[0004] BACKGROUND

[0005] In many industries, the requirement for certain parts or materials to exhibit fire-proof or fire- retardant properties is becoming increasingly prevalent (for example due to government regulations). However, applying such treatments to parts comprising polyamide materials has proven to be extremely challenging.

[0006] It is therefore an aim of the present disclosure to provide a method of improving the fire retardancy of parts comprising such materials.

[0007] SUMMARY

[0008] According to a first aspect of the present disclosure, there is provided a method of improving the fire retardancy of a polyamide part, the method comprising the steps of: a) providing a part comprising a polyamide material; b) providing a treatment solution comprising at least one phosphate ester; c) a heating step, wherein the treatment solution is heated to a temperature of at least 150°c; and d) an application step, wherein the heated treatment solution is applied onto a surface of the polyamide part in order to improve the fire- retardancy of said part.

[0009] Advantageously, it has been found that by pre-heating the treatment solution to a temperature of at least 150°c, upon application onto a part comprising a polyamide material, the phosphate groups contained within the treatment solution are able to bind onto the polyamide part.

[0010] As such, when the polyamide material is subsequently exposed to a flame, the presence of the phosphate groups will cause the material to char thereby forming a protective layer which interrupts the combustion process, thereby preventing further combustion of the material and hence improving the fire retardancy of the part.

[0011] Furthermore, phosphate esters as described herein are non-halogenated. Halogenated compounds are not found in nature and are therefore very difficult to break down. This means that they tend to be extremely persistent in the environment which can lead to contamination of local ecosystems as well as wider environmental problems, such as the depletion of the ozone layer. Moreover, halogenated fire retardants can result in adverse health effects in both animals and humans. For example, they may possibly lead to endocrine and thyroid disruption, immunotoxicity, reproductive toxicity, and cancer.

[0012] As such, the use of a treatment solution containing one or more non-halogenated phosphate esters as a non-halogenated fire-retardant treatment solution provides a more environmentally friendly alternative when compared to other know fire- retard ants which may traditionally be used for this purpose.

[0013] It shall also be appreciated that the non-halogenated phosphate esters described herein are considered to be more environmentally friendly than their halogenated counterparts, as they are generally less toxic and are less persistent in the environment.

[0014] Optionally, the at least one phosphate ester has a general formula according to formula (I):

[0015] R1Ck ' / °

[0016] R22O 'URK3formula (I) wherein R1, R2, and R3are each independently selected from hydrogen; a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group; with the proviso that not all of R1, R2, and R3are hydrogen.

[0017] Optionally, R1, R2, and R3are each independently selected from hydrogen; a linear or branched Ci to C30 alkyl group, optionally wherein at least one C atom is replaced by O; a linear C2 to C12 alkenyl group substituted with an alkoxy group, wherein the alkyl group bonded to the oxygen atom of the alkoxy group is selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl; and an aryl group selected from phenyl, optionally substituted with an alkyl group as defined herein at the ortho, meta, and / or para position, or naphthyl; with the proviso that not all of R1, R2, and R3are hydrogen. Optionally, the at least one phosphate ester comprises a phosphate ester according to general formula (II), (III), or (IV): formula (II) formula (III) formula (IV) wherein R4, R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group.

[0018] Optionally, the phosphate ester may comprise a phosphate ester according to general formula (III) or (IV), and wherein R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group.

[0019] Optionally, R4, R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group optionally wherein at least one C atom is replaced by O; and an aryl group. Optionally, R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to Cw alkyl group optionally wherein at least one C atom is replaced by O, a phenyl group, and a phenyl group substituted with at least one Ci to Cs linear or branched alkyl group at the ortho, meta, and / or para position.

[0020] Optionally, the at least one phosphate ester is selected from trimethyl phosphate, dimethyl phosphate, triethyl phosphate, diethyl phosphate, tripropyl phosphate, triisopropyl phosphate, tributyl phosphate, triisobutyl phosphate, tripentyl phosphate, tris(2-ethylhexyl) phosphate, triphenyl phosphate, 2-ethylhexyl dipentyl phosphate, tri(2-isopropylphenyl) phosphate, isodecyl diphenyl phosphate, cresyl diphenyl phosphate, tri-o-cresyl phosphate, tri-p-cresyl phosphate, tricresyl phosphate, tris(2-butoxylethyl) phosphate, or combinations thereof.

[0021] Optionally, the at least one phosphate ester has a boiling point in the range of about 110°c to about 450°c.

[0022] Optionally, the at least one phosphate ester has a boiling point in the range of about 190°c to about 280°c.

[0023] Advantageously, it has been found that phosphate esters having a boiling point between 190°c and 280°c are beneficial since they can be effectively applied onto polyamide parts via both liquid and vapour immersion methods.

[0024] Optionally, the at least one phosphate ester is selected from trimethyl phosphate, triethyl phosphate, diethyl phosphate, triisopropyl phosphate, triisobutyl phosphate, tris(2-ethylhexyl) phosphate, tri(2-isopropylphenyl) phosphate, tris(2-butoxylethyl) phosphate, or combinations thereof.

[0025] Optionally, the treatment solution comprises triisobutyl phosphate. Advantageously, triisobutyl phosphate is extremely stable and hence the use of this particular phosphate ester helps to avoid decomposition of the treatment solution at high temperatures.

[0026] Optionally, the polyamide material comprises an aliphatic polyamide, an aromatic polyamide, or a semi-aromatic polyamide.

[0027] Optionally, the polyamide material comprises an aliphatic polyamide selected from Polyamide 11 , Polyamide 12, Polyamide 46, Polyamide 66, or a combination thereof.

[0028] Optionally, the treatment solution is provided as a liquid, and step d) comprises submerging the part into the treatment solution. Advantageously, it has been found that providing the treatment solution as a liquid and then submerging the part into the liquid treatment solution is a particularly effective method for causing the phosphate groups contained within the treatment solution to bind onto the polyamide part.

[0029] Optionally, the at least one phosphate ester has a boiling point in the range of about 200°c to about 450°c.

[0030] Advantageously, it has been found that phosphate esters having a boiling point between 200°c and 450°c are particularly effective for liquid treatment methods since they enable the treatment solution to be heated to higher temperatures without causing the treatment solution to evaporate.

[0031] Optionally, the at least one phosphate ester is selected from triethyl phosphate, diethyl phosphate, triisopropyl phosphate, tributyl phosphate, triisobutyl phosphate, tris(2-ethylhexyl) phosphate, triphenyl phosphate; 2-ethylhexyl dipentyl phosphate, tri(2-isopropylphenyl) phosphate, isodecyl diphenyl phosphate, cresyl diphenyl phosphate, tri-o-cresyl phosphate, tri- p-cresyl phosphate, tricresyl phosphate, tris(2-butoxylethyl) phosphate, or combinations thereof.

[0032] Optionally, step c) comprises heating the treatment solution comprising at least one phosphate ester to a temperature in the range of about 150°c to about 200°c, and further optionally to a temperature of about 180°c.

[0033] Advantageously, heating the treatment solution to a temperature in the range of about 150°c to about 200°c helps to improve the binding of the phosphate groups onto the polyamide part without causing thermal damage (such as warping or plastic deformation) to the part.

[0034] Optionally, the method further comprises, prior to step d), heating the part to a temperature below a melting temperature of the part, and further optionally to a temperature of up to 150°c

[0035] Advantageously, it has been found that pre-heating the part prior to the application of the treatment solution helps to further improve the binding of the phosphate groups onto the polyamide part.

[0036] Furthermore, keeping the temperature of the part below its melting temperature avoids causing thermal damage (such as warping or plastic deformation) to the part during the aforementioned heating step. Optionally, step c) comprises heating the treatment solution so as to cause the treatment solution to vaporise, and wherein step d) comprises condensing the vaporised treatment solution onto the surface of the polyamide part

[0037] Advantageously, it has been found that vapour processing methods (such as the one described above) provide a more controllable and more easily automated alternative to traditional liquidbased processing methods.

[0038] Optionally, the at least one phosphate ester has a boiling point in the range of about 110°c to about 300°c.

[0039] Advantageously, it has been found that phosphate esters having a boiling point between 110°c and 300°c are particularly effective for vapour treatment methods since they enable the treatment solution to evaporate more easily during step c) of the claimed method.

[0040] Optionally, the at least one phosphate ester is selected from trimethyl phosphate, dimethyl phosphate, triethyl phosphate, triisobutyl phosphate, tripropyl phosphate, tris(2-ethylhexyl) phosphate, tri(2-isopropylphenyl) phosphate, or combinations thereof.

[0041] Optionally, the method further comprises cooling the polyamide part prior to step d), further optionally to a temperature between 0 and - 30°c.

[0042] Advantageously, cooling the polyamide part helps to encourage the vaporised treatment solution to condense onto the surface of the part during step d).

[0043] Optionally, the method comprises placing the polyamide part into a processing chamber and, during step d), an interior of the processing chamber is maintained at a pressure in the range of about 0.1 kPa to about 50 kPa (1 mBar to 500 mBar), and further optionally in the range of about 5 kPa to about 30 kPa (50 mBar and 300 mBar).

[0044] Advantageously, maintaining the interior of the processing chamber at a pressure below 1 Bar allows the treatment solution to be vapourised at lower temperatures than would otherwise be possible when performing the method at atmospheric pressure. This helps to reduce the likelihood of causing thermal damage to the part during processing.

[0045] Optionally, the part is an additively manufactured part. The additively manufactured part may be a powder-based additively manufactured part or may be a filament-based additively manufactured part. According to a second aspect of the present disclosure, there is provided a fire-retardant polyamide part obtainable by the method according to the first aspect of the present disclosure.

[0046] According to a third aspect of the present disclosure, there is provided a use of a phosphate ester for improving the fire- retard a ncy of a polyamide part.

[0047] Optionally, the phosphate ester used for improving the fire- retard a ncy of the polyamide part may be tri-isobutyl phosphate.

[0048] Optionally, the polyamide part may comprise polyamide 12 or polyamide 11 .

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Embodiments of the disclosure will now be described with reference to the accompanying drawings, in which:

[0051] Figure 1 is a flow diagram depicting a method of improving the fire retardancy of a polyamide part according to an aspect of the present disclosure;

[0052] Figure 2 is a schematic diagram of an apparatus for applying a fire-retardant treatment solution onto the surface of a polyamide part;

[0053] Figure 3 is a schematic diagram of an alternative apparatus for applying a fire-retardant treatment solution onto the surface of a polyamide part; and

[0054] Figure 4 is a schematic diagram of a further alternative apparatus for applying a fire-retardant treatment solution onto the surface of a polyamide part.

[0055] DETAILED DESCRIPTION

[0056] Figure 1 shows a schematic illustration of a method of improving the fire retardancy of a polyamide part.

[0057] At a first step 101 of the method, a part comprising a polyamide material (i.e., a polyamide part) is provided.

[0058] As used herein, the term “polyamide” is intended to take its usual meaning in the art and refers to any polymer in which the repeating units in the molecular chain are linked together by amide groups (i.e. a functional group that consists of a carbonyl group and a nitrogen atom). The polyamide material may comprise an aliphatic polyamide, an aromatic polyamide, or a semi-aromatic polyamide.

[0059] As used herein, the term “aliphatic polyamide” takes its usual meaning in the art and refers to polymers often formed by polycondensation reactions between diamines and dicarboxylic acids or their derivatives.

[0060] The aliphatic polyamide may be selected from Polyamide 11 , (also referred to as PA 11 , or Nylon 11), Polyamide 12 (also referred to as PA 12, or Nylon 6, 12), Polyamide 46 (also referred to as PA 46, or Nylon 4,6), Polyamide 66 (also referred to as PA 66, or Nylon 6,6), Polyamide 69 (also referred to as PA 69, or Nylon 6,9), Polyamide 610 (also referred to as PA 610, or Nylon 6,10), Polyamide 612 (also referred to as PA 612, or Nylon 6,12), or a combination thereof.

[0061] The aliphatic polyamide may be selected from Polyamide 11 , (also referred to as PA 11 , or Nylon 11), Polyamide 12 (also referred to as PA 12, or Nylon 6, 12), Polyamide 46 (also referred to as PA 46, or Nylon 4,6), Polyamide 66, or a combination thereof.

[0062] As used herein, the term “aromatic polyamide” takes its usual meaning in the art and refers to polyamide polymers whereby at least 85% of the amide linkages are attached directly to two aromatic rings, generated from aromatic diamines and diacids.

[0063] The aromatic polyamide may be selected from a para-aramid or a meta-aramid. The aromatic polyamide may be selected from p-phenylene terephthalamide (PpPTA) (often denoted by the brand name Twaron®), poly-paraphenylene terephthalamide (often denoted by the brand name Kevlar®), or poly(meta-phenyleneisophthalamide) (often denoted by the brand name Nomex®).

[0064] As used herein, the term “semi-aromatic polyamide” takes its usual meaning in the art and refers to polyamide polymers including a combination of aliphatic and aromatic polyamides.

[0065] The semi-aromatic polyamide may be selected from poly(hexamethylene terephthalamide), or poly(hexamethylene isophthalamide).

[0066] For example, it may be the case that the part comprises a different grade of aliphatic polyamide including, but not limited to, Polyamide 66, Polyamide 46, Polyamide 69, Polyamide 610, Polyamide 612 and / or Polyamide 11 ; an aromatic polyamide including, but not limited to, p- phenylene terephthalamide, poly-paraphenylene terephthalamide, or poly(meta- phenyleneisophthalamide); or a semi-aromatic polyamide including, but not limited to poly(hexamethylene terephthalamide), or poly(hexamethylene isophthalamide). In other words, in some embodiments, the polyamide material may be an aliphatic polyamidearomatic polyamide co-polymer.

[0067] It shall be appreciated that the polyamide part may be manufactured using any type of suitable manufacturing process including, but not limited to, injection moulding, compression moulding, thermoforming, extrusion, and / or computer numerically controlled (CNC) machining.

[0068] It shall also be appreciated that in some examples, the polyamide part may be manufactured using additive manufacturing (i.e., 3D printing) and hence in some examples the polyamide part may be an additively manufactured part.

[0069] In examples in which the polyamide part is an additively manufactured part, the polyamide part may be manufactured using powder-based additive manufacturing methods, such as Selective Laser Sintering or Multi-Jet Fusion, and hence may be a power-based additively manufactured part or, alternatively, may be manufactured using filament-based additive manufacturing methods, such as Fused Filament Fabrication, and hence may be a filament-based additively manufactured part.

[0070] It shall also be appreciated that in other examples, the part may be manufactured using other suitable powder or filament-based additive manufacturing methods, and hence the present disclosure is not solely limited to polyamide parts manufactured using the additive manufacturing methods which are specified above.

[0071] During powder-based additive manufacturing processes, such as Selective Laser Sintering or Multi-Jet Fusion, a first layer of powder build material is laid down onto a build bed. The first layer of powder build material is then sintered to form a first sintered layer of the part. A second layer of powder build material is then laid onto the first sintered layer. The second layer of powder build material is then subsequently sintered to form a second sintered layer of the part. Subsequent layers are then applied and sintered in the same fashion thereafter, until a part having a desired shape has been built from multiple sintered layers.

[0072] Meanwhile, during Fused Filament Fabrication, a filament material is heated and extruded through a nozzle which selectively deposits the material on the build platform. Once a first layer of build material has been deposited, either the platform is lowered, or the nozzle is lifted so that one or more further layers can be extruded on top of the first layer until a part having the desired shape is constructed.

[0073] It has been found that material at the surface of an additively manufactured part may often exhibit a different, less favourable structure to that of the material which is beneath said surface. As such, the presence of less favourable structures at the surface of a given part can result in additively manufactured parts having a rough surface finish. The material at the surface of the part can exhibit a looser structure / morphology, due to the presence of un-sintered or semisintered build material at the surface of a given part, which is typically much weaker than the material beneath the surface of the additively manufactured part.

[0074] It has been found that the less favourable material at the surface of the part can inhibit effective binding of various solvents or treatment agents to the more favourable underlying material which makes up the additively manufactured part.

[0075] As such, in examples in which the polyamide part is manufactured via additive manufacturing processes (particularly powder-based additive manufacturing processes such as Selective Laser Sintering), the raw “as printed” polyamide part may undergo an initial treatment process prior to the fire-retardant treatment process in order to smooth the surface(s) of the polyamide part and / or to remove any unfavourable or un-sintered build material provided at the surface of the polyamide part before the fire-retardant treatment solution is applied.

[0076] For example, in some embodiments the polyamide part may be treated with another solvent or plasticizer to condition the part prior to the application of the fire-retardant treatment solution. For example, the polyamide part may be treated with another polar solvent such as Hexafluoroisopropanol (HFIP) or benzyl alcohol to partially or fully dissolve and smooth the outer layer of the part to create a more uniform surface.

[0077] Without being bound by theory, the presence of a polar solvent on the surface of the polyamide part may increase the attraction of the fire-retardant treatment solution to condense onto the surface of the polyamide part and bond with the polyamide part, thereby strengthening the fire- retardant properties of the part.

[0078] Referring now to step 102 of the method, once the polyamide part has been provided, and optionally once the polyamide part has undergone an initial treatment process, a fire-retardant treatment solution is selected for applying onto a surface of the polyamide part in order to improve the fire- retard a ncy of said part (as shall be described in detail below).

[0079] In the method according to the present disclosure, the treatment solution comprises at least one phosphate ester.

[0080] As used herein, the term “phosphate ester” takes its usual meaning in the art and relates to a class of organophosphorus compounds. Phosphate esters are derived from an alcohol and phosphoric acid and may also be referred to as organophosphates or OPEs. The term “fire-retardant” is defined herein to mean a substance which, when applied to a given substrate (e.g., a polyamide part) prevents or inhibits the burning of said substrate when exposed to a naked flame.

[0081] The at least one phosphate ester may comprise a phosphate ester according to general formula (I): formula (I) wherein R1, R2, and R3are each independently selected from hydrogen; a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group; with the proviso that not all of R1, R2, and R3are hydrogen.

[0082] As described herein, the term "alkyl" is intended to take its usual meaning and relates to a group of atoms that consist of an alkane that has had one hydrogen atom removed.

[0083] It may be the case that R1, R2, and R3are each independently selected from hydrogen; a linear or branched Ci to C30 alkyl group, optionally wherein at least one C atom is replaced by O; a linear C2 to C12 alkenyl group substituted with an alkoxy group, wherein the alkyl group bonded to the oxygen atom of the alkoxy group is selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl; and an aryl group selected from phenyl, optionally substituted with an alkyl group as defined herein at the ortho, meta, and / or para position, or naphthyl; with the proviso that not all of R1, R2, and R3are hydrogen.

[0084] The alkyl group may have a carbon length in the range Ci to C30. It may be the case that the alkyl group will be linear or branched. It may be the case that the alkyl group will be linear. It may be the case that the alkyl group will be branched. The exact length of the alkyl group may vary. It may be the case that the alkyl group has a carbon length in the range Ci to C20. It may be the case that the alkyl group has a carbon length in the range Ci to C12. It may be the case that the alkyl group has a carbon length in the range Ci to C10. It may be the case that the alkyl group has a carbon length in the range Ci to Cs. It may be the case that the alkyl group has a carbon length in the range Ci to C7.

[0085] It may be the case that the linear or branched alkyl group is a Ci to C30 alkyl group substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof. It may be the case that the alkyl group is a linear Ci to C30 alkyl group, and is substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof.

[0086] As used herein, the term “hydroxyl group” is intended to take its usual meaning and denotes a functional group with the chemical formula -OH. As used herein, the term “amino” is intended to take its usual meaning and denotes a functional group that consists of a single nitrogen atom bonded to two hydrogen atoms. As used herein, the term “carboxyl group” is intended to take its usual meaning and denotes an organic functional group consisting of a carbon atom bonded to an oxygen atom via a double bond and to a hydroxyl group by a single bond with the chemical formula -C(=O)OH or -COOH.

[0087] It may be the case that the alkyl group is a linear or branched alkyl group, wherein at least one C atom is replaced by a heteroatom selected from N, S, or O. As used herein, the term “heteroatom” takes its usual meaning in the art and indicates that non-carbon atoms have replaced a carbon atom in the backbone of a molecular structure. It may be the case that the alkyl group is a linear alkyl group, wherein at least one C atom is replaced by a heteroatom selected from N, S, or O. It may be the case that the alkyl group is a linear alkyl group, wherein at least one C atom is replaced by O (i.e. forming an ether). It may be the case that the alkyl group is a C4to C30 linear alkyl group wherein at least one C atom is replaced by O. It may be the case that the alkyl group is a C o C10 linear alkyl group wherein at least one C atom is replaced by O. It may be the case that the alkyl group is a C4to C10 linear alkyl group wherein at least one ofthe third, fourth, fifth, sixth, seventh, or eight C atom of the alkyl group is replaced by O (wherein the first C atom is the C atom bonded to the O atom of an O-P bond of formula (I)). It may be the case that the alkyl group is a C4to C10 linear alkyl group wherein at least one of the third, fourth, or fifth C atom is replaced by O.

[0088] Examples of alkyl groups include, but are not limited to, substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, te / Y-butyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isoheptyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, 2-butoxyethyl, and 2-ethylhexyl. The alkyl group may be selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, 2-butoxyethyl, and 2-ethylhexyl. The alkyl group may be selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, 2-butoxyethyl, and 2-ethylhexyl. As used herein, the term “alkenyl” is intended to take its usual meaning and denotes a hydrocarbon group formed when a hydrogen atom is removed from an alkene group and comprises at least one carbon-carbon double bond. The alkenyl group may have a carbon length in the range C2to C30. It may be the case that the alkenyl group will be linear or branched. It may be the case that the alkenyl group will be linear. The exact length of the alkenyl group may vary. It may be the case that the alkenyl group has a carbon length in the range C2to C20. It may be the case that the alkenyl group has a carbon length in the range C2to C12. It may be the case that the alkenyl group has a carbon length in the range C2 to C10. It may be the case that the alkenyl group has a carbon length in the range C2 to Cs. It may be the case that the alkenyl group has a carbon length in the range C2to Cs.

[0089] The at least one carbon-carbon double bond may be internal and / or in the terminal position. As used herein, the term “internal” refers to any carbon atom other than the terminal carbon atom. The at least one carbon-carbon double bond may be internal. The alkenyl group may comprise two or more double bonds. It may be the case that the at least one carbon-carbon double bond is substituted with an alkyl group as defined herein, an alkoxy group, an amino group, or a combination thereof. As used herein, the term “alkoxy” is intended to take its usual meaning and denotes the functional group containing an alkyl group bonded to an oxygen atom. The alkyl group bonded to the oxygen group may be any alkyl as defined above. It may be the case that the alkyl group bonded to the oxygen atom has a carbon length in the range Ci to C30. It may be the case that the alkyl group bonded to the oxygen atom will be linear or branched. It may be the case that the alkyl group bonded to the oxygen atom will be linear. The exact length of the alkyl group bonded to the oxygen atom may vary. It may be the case that the alkyl group bonded to the oxygen atom has a carbon length in the range Ci to C20. It may be the case that the alkyl group bonded to the oxygen atom has a carbon length in the range Ci to C12. It may be the case that the alkyl group bonded to the oxygen atom has a carbon length in the range Ci to Cw. It may be the case that the alkyl group bonded to the oxygen atom has a carbon length in the range Ci to Cs. It may be the case that the alkyl group bonded to the oxygen atom has a carbon length in the range Ci to C7. Examples of an alkyl group bonded to the oxygen atom may include, but is not limited to, substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, te / Y-butyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isoheptyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, and 2-ethylhexyl. The alkyl group bonded to the oxygen atom may be selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl. The alkyl group bonded to the oxygen atom may be selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2- ethylhexyl.

[0090] Examples of the alkenyl group may include, but are not limited to, substituted or unsubstituted ethene, propene, butene, pentene, hexene, octene, nonene, and decene. It may be the case that the alkenyl group is selected from ethene, propene, butene, pentene, hexene, octene, nonene, and decene. It may be the case that the at least one carbon-carbon double bond is substituted with an alkoxy group as defined herein (i.e. an alkenylalkoxy group). In instances where the at least one carbon-carbon double bond is substituted with an alkoxy group, examples of the alkyl group bonded to the oxygen atom of the alkoxy group may include, but is not limited to, substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert- butyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isoheptyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, and 2-ethylhexyl. The alkyl group bonded to the oxygen atom of the alkoxy group may be selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl. The alkoxy group of the alkenylalkoxy group may include, but is not limited to, propoxy, butoxy, hexoxy, heptoxy, or octoxy. The alkenylalkoxy group may be selected from 1 -methylethoxy or 2-butoxyethyl.

[0091] As used herein, the term “aryl” is intended to take its usual meaning and denotes any organic group derived from an aromatic hydrocarbon by the removal of a hydrogen atom. Examples of aryl groups include, but are not limited to phenyl, optionally substituted with an alkyl group as defined herein at the ortho, meta, and / or para position; or naphthyl. It may be the case that the aryl group is selected from phenyl, optionally substituted with an alkyl group as defined herein at the ortho, meta, and / or para position. Examples of alkyl groups include, but are not limited to, substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, te / Y-butyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isoheptyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, and 2-ethylhexyl. The alkyl group may be selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl. It may be the case that the phenyl group is substituted with at least one Ci to Cs linear or branched alkyl at the ortho, meta, and / or para position. It may be the case that the phenyl group is substituted with an alkyl group selected from methyl, ethyl, propyl, isopropyl, butyl, or isobutyl at the ortho, meta, and / or para position. It may be the case that the aryl group is selected from phenyl, o-tolyl, m-tolyl, p- tolyl, xylyl, isopropyl phenyl, and isobutyl phenyl.

[0092] It may be the case that R1, R2, and R3 are each independently selected from a linear or branched Ci to C10 alkyl group optionally wherein at least one C atom is replaced by O, a phenyl group, and a phenyl group substituted with at least one Ci to Cs linear or branched alkyl group at the ortho, meta, and / or para position. It may be the case that R1, R2, and R3 are each independently selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, isodecyl, 2-butoxyethyl, phenyl, o-tolyl, m-tolyl, p- tolyl, and isopropyl phenyl.

[0093] It may be the case that one of R1, R2, and R3is a linear or branched Ci to C10 alkyl optionally wherein at least one C atom is replaced by O; and two of R1, R2, and R3are selected from a phenyl group, or a phenyl group substituted with at least one Ci to Cs linear or branched alkyl group at the ortho, meta, and / or para position. It may be the case that one of R1, R2, and R3is methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, decyl, 2-butoxyethyl, or isodecyl; and two of R1, R2, and R3are selected from phenyl, o-tolyl, m-tolyl, p- tolyl, and isopropyl phenyl.

[0094] It may be the case that R1, R2, and R3are selected from a phenyl group, or a phenyl group substituted with at least one Ci to Cs linear or branched alkyl group at the ortho, meta, and / or para position. It may be the case that R1, R2, and R3are selected from phenyl, o-tolyl, m-tolyl, p- tolyl, xylyl, isopropyl phenyl, and isobutyl phenyl.

[0095] It may be the case that one of R1, R2, and R3is hydrogen; and two of R1, R2, and R3are selected from a linear or branched Ci to C10 alkyl optionally wherein at least one C atom is replaced by O. It may be the case that one of R1, R2, and R3is hydrogen, and two of R1, R2, and R3are selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, 2-butoxyethyl, and isodecyl. It may be the case that R1, R2, and R3are selected from a linear or branched Ci to Cw alkyl optionally wherein at least one C atom is replaced by O. It may be the case that R1, R2, and R3are selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, decyl, 2-butoxyethyl, and isodecyl.

[0096] The at least one phosphate ester may comprise a phosphate ester according to general formula (II), (III), or (IV): formula (II)

[0097] Rs<pP

[0098] R6OOHformula (III) formula (IV) wherein R4, R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group.

[0099] The Ci to C30 alkyl group may be branched. It may be the case that the Ci to C30 alkyl group will be linear. The exact length of the alkyl group may vary. It may be the case that the alkyl group has a carbon length in the range Ci to C20. It may be the case that the alkyl group has a carbon length in the range Ci to C12. It may be the case that the alkyl group has a carbon length in the range Ci to C10. It may be the case that the alkyl group has a carbon length in the range Ci to Cs. It may be the case that the alkyl group has a carbon length in the range Ci to C7. It may be the case that the linear or branched alkyl group is substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof. It may be the case that the alkyl group is linear and is substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof. It may be the case that the alkyl group is a linear or branched alkyl group, wherein at least one C atom of is replaced by a heteroatom selected from N, S, or O. As used herein, the term “heteroatom” takes its usual meaning in the art and indicates that non-carbon atoms have replaced a carbon atom in the backbone of a molecular structure. It may be the case that the alkyl group is a linear alkyl group, wherein at least one C atom is replaced by a heteroatom selected from N, S, or O. It may be the case that the alkyl group is a linear alkyl group, wherein at least one C atom is replaced by O (i.e. forming an ether). It may be the case that the alkyl group is a C4to C30 linear alkyl group wherein at least one C atom is replaced by O. It may be the case that the alkyl group is a C4 to C30 linear alkyl group. It may be the case that the third, fourth, fifth, sixth, seventh, or eight C atom of the alkyl group is replaced by O (wherein the first C atom is the C atom bonded to the O atom of an O-P bond of formulae (II), (III), and (IV)). It may be the case that the third, fourth, or fifth C atom is replaced by O. Examples of alkyl groups include, but are not limited to, substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, te / Y-butyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isoheptyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, 2-butoxyethyl, and 2-ethylhexyl. The alkyl group may be selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl. The alkyl group may be selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, 2-butoxyethyl, and 2-ethylhexyl.

[0100] The C2 to C30 alkenyl group may be linear or branched. It may be the case that the C2 to C30 alkenyl group will be linear. The exact length of the alkenyl group may vary. It may be the case that the alkenyl group has a carbon length in the range C2 to C20. It may be the case that the alkenyl group has a carbon length in the range C2 to C12. It may be the case that the alkenyl group has a carbon length in the range C2to C10. It may be the case that the alkenyl group has a carbon length in the range C2 to Cs. It may be the case that the alkenyl group has a carbon length in the range C2 to Ce. The alkenyl group may have at least one carbon-carbon double bond. The alkenyl group may have two or more carbon-carbon double bonds. The at least one carbon-carbon double bond may be internal and / or in the terminal position. The alkenyl group may comprise two or more double bonds. It may be the case that the at least one carbon-carbon double bond is substituted with an alkyl group as defined herein, an alkoxy group as defined herein, an amino group, or a combination thereof. Examples of the alkenyl group may include, but are not limited to, substituted or unsubstituted ethene, propene, butene, pentene, hexene, octene, nonene, and decene. The alkene may be selected from ethene, propene, butene, pentene, hexene, octene, nonene, or decene. It may be the case that the at least one carboncarbon double bond is substituted with an alkoxy group as defined herein (i.e. an alkenylalkoxy group). In instances where the at least one carbon-carbon double bond is substituted with an alkoxy group, examples of the alkyl group bonded to the oxygen atom of the alkoxy group may include, but is not limited to, substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, te / Y-butyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isoheptyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, and 2-ethylhexyl. The alkyl group bonded to the oxygen atom of the alkoxy group may be selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl. The alkoxy group of the alkenylalkoxy group may include, but is not limited to, propoxy, butoxy, hexoxy, heptoxy, or octoxy. The alkenylalkoxy group may be selected from 1 -methylethoxy or 2-butoxyethyl.

[0101] The aryl group may be selected from phenyl, optionally substituted with an alkyl group as defined herein at the ortho, meta, and / or para position; or naphthyl. It may be the case that the aryl group is selected from phenyl, optionally substituted with an alkyl group as defined herein at the ortho, meta, and / or para position. Examples of alkyl groups include, but are not limited to, substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, te / Y-butyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isoheptyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, and 2-ethylhexyl. The alkyl group may be selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl. It may be the case that the aryl group is selected from phenyl, o-tolyl, m-tolyl, p- tolyl, xylyl, isopropyl phenyl, and isobutyl phenyl.

[0102] The phosphate ester may comprise a phosphate ester according to general formula (II), wherein R4is selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched Ci to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; or an aryl group. It may be the case that R4is selected from a linear or branched Ci to C30 alkyl group optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched Ci to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; or an aryl group. It may be the case that R4is selected from a linear or branched Ci to C30 alkyl group optionally wherein at least one C atom is replaced by O; or an aryl group. It may be the case R4is selected from a linear or branched Ci to C10 alkyl group, a phenyl group, or a phenyl group substituted with at least one Ci to C5 linear or branched alkyl group at the ortho, meta, and / or para position. It may be the case that R4is selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, isodecyl, phenyl, o-tolyl, m-tolyl, p- tolyl, or isopropyl phenyl. It may be the case that R4is selected from methyl, ethyl, isopropyl, isobutyl, 2-ethylhexyl, 2-butoxylethyl, or isopropyl phenyl.

[0103] The phosphate ester may comprise a phosphate ester according to general formula (III) or (IV), wherein R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group. It may be the case that R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group. It may be the case that R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group optionally wherein at least one C atom is replaced by O; and an aryl group. It may be the case that R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C10 alkyl group optionally wherein at least one C atom is replaced by O, a phenyl group, and a phenyl group substituted with at least one Ci to C5 linear or branched alkyl group at the ortho, meta, and / or para position. It may be the case that R5, R6, R7, R8, and R9are each independently selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, isodecyl, phenyl, o-tolyl, m-tolyl, p- tolyl, and isopropyl phenyl. It may be the case that R5, R6, R7, R8, and R9are each independently selected from methyl, ethyl, isopropyl, isobutyl, 2-ethylhexyl, 2-butoxylethyl, and isopropyl phenyl. It may be the case that R5, R6, R7, R8, and R9are each independently selected from ethyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, 2-ethylhexyl, 2-butoxylethyl, phenyl, o-tolyl, p- tolyl, and isopropyl phenyl.

[0104] The phosphate ester may comprise a phosphate ester according to general formula (III), wherein R5and R6are selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group. It may be the case that R5and R6are selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O. It may be the case that R5and R6are selected from a linear or branched Ci to C10 alkyl. It may be the case that R5and R6are selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, isodecyl. It may be the case that R5and R6are selected from ethyl.

[0105] The phosphate ester may comprise a phosphate ester according to general formula (IV), wherein R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group. It may be the case that R7, R8, and R9are each independently selected from a linear or branched Ci to C10 alkyl group optionally wherein at least one C atom is replaced by O, a phenyl group, a phenyl group substituted with at least one Ci to C5 linear or branched alkyl group at the ortho, meta, and / or para position. It may be the case that R7, R8, and R9are each independently selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, 2-butoxylethyl, isodecyl, phenyl, o-tolyl, m-tolyl, p- tolyl, and isopropyl phenyl. It may be the case that R7, R8, and R9are each independently selected from methyl, ethyl, isopropyl, isobutyl, 2-ethylhexyl, 2-butoxylethyl, and isopropyl phenyl. It may be the case that R7, R8, and R9are selected from a linear or branched Ci to C10 alkyl optionally wherein at least one C atom is replaced by O. It may be the case that R7, R8, and R9are selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, decyl, 2-butoxyethyl, and isodecyl. It may be the case that R7, R8, and R9are selected from a phenyl group, or a phenyl group substituted with at least one Ci to C5 linear or branched alkyl group at the ortho, meta, and / or para position. It may be the case that R7, R8, and R9are selected from phenyl, o-tolyl, m-tolyl, p- tolyl, xylyl, isopropyl phenyl, and isobutyl phenyl. It may be the case that one of R7, R8, and R9is selected from a linear or branched Ci to C10 alkyl, and two of R7, R8, and R9are selected from a phenyl group, or a phenyl group substituted with at least one Ci to C5 linear or branched alkyl group at the ortho, metal, and / or para position. It may be the case that one of R7, R8, and R9is selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-ethylhexyl, isodecyl, and two of R7, R8, and R9are selected from phenyl, 0- tolyl, m-tolyl, p- tolyl, xylyl, isopropyl phenyl, and isobutyl phenyl. The phosphate ester may be selected from trimethyl phosphate, dimethyl phosphate, triethyl phosphate, diethyl phosphate, tripropyl phosphate, triisopropyl phosphate, tributyl phosphate, triisobutyl phosphate, tripentyl phosphate, tris(2-ethylhexyl) phosphate, triphenyl phosphate; 2- ethylhexyl dipentyl phosphate, tri(2-isopropylphenyl) phosphate, isodecyl diphenyl phosphate, cresyl diphenyl phosphate, tri-o-cresyl phosphate, tri-p-cresyl phosphate, tricresyl phosphate, tris(2-butoxylethyl) phosphate, or combinations thereof.

[0106] The boiling point of the phosphate ester may be in the range of about 110°c to about 450°c. The boiling point of the phosphate ester may be in the range 120°c to 400°c. The boiling point of the phosphate ester may be in the range 150°c to 400 °c. The boiling point of the phosphate ester may be in the range 200°c to 400 °c. The boiling point of the phosphate ester may be in the range 150°c to 200 °c.

[0107] Advantageously, phosphate esters as described herein are non-halogenated. Halogenated compounds are not found in nature and are therefore very difficult to break down. This means that they tend to be extremely persistent in the environment which can lead to contamination of local ecosystems as well as wider environmental problems, such as the depletion of the ozone layer. Moreover, halogenated fire retardants can result in adverse health effects in both animals and humans. For example, they may possibly lead to endocrine and thyroid disruption, immunotoxicity, reproductive toxicity, and cancer.

[0108] As such, the use of a treatment solution comprising one or more non-halogenated phosphate esters as a non-halogenated fire-retardant treatment solution provides a more environmentally friendly alternative when compared to other known fire- retard ants which may traditionally be used for this purpose.

[0109] It shall also be appreciated that the non-halogenated phosphate esters described herein are considered to be more environmentally friendly than their halogenated counterparts, as they are generally less toxic and are less persistent in the environment.

[0110] The phosphate ester may be selected from trimethyl phosphate, triethyl phosphate, diethyl phosphate, triisopropyl phosphate, triisobutyl phosphate, tris(2-ethylhexyl) phosphate, tri(2- isopropylphenyl) phosphate, tris(2-butoxylethyl) phosphate, or combinations thereof.

[0111] The boiling point of the phosphate ester may be in the range of about 190°c to about 280°c.

[0112] Advantageously, the aforementioned group of phosphate esters are particularly beneficial since they feature boiling points which are low enough be usable in vapour application techniques but also high enough to be usable in liquid application techniques without losing significant amounts of the treatment solution due to evaporation of the one or more phosphate esters (as shall be described in greater detail below).

[0113] Once a suitable fire-retardant treatment solution has been selected, the treatment solution undergoes a heating step 103 during which the treatment solution is heated to a temperature of at least 150°c before being applied onto a surface of the polyamide part during an application step 104 to improve the fire- retard a ncy of the part.

[0114] An apparatus 200 suitable for performing the aforementioned heating 103 and application 104 steps shall now be briefly described with reference to Figure 2.

[0115] The apparatus 200 is made up of a reservoir in the form of a bath 202 and a heating element 204.

[0116] The bath 202 is configured for containing a treatment solution 206 comprising at least one phosphate ester to be used during the fire-retardant treatment process and is of a size and configuration such that a polyamide part 210 can be entirely submerged within the treatment solution 206 during the application step 104.

[0117] The heating element 204 is located proximate to the bath 202 and is operatively coupled to a controller 240 which is configured for controlling the heating element 204 so as to heat the treatment solution 206 to a desired temperature during the heating step 103.

[0118] The precise temperature to which the treatment solution 206 is heated during the heating step 103 varies based on the material being processed and the flashpoint of the treatment solution 206 but is typically in the range of about 150°c to about 200°c.

[0119] Notably, it has been found that pre-heating the treatment solution 206 to temperatures within the aforementioned range helps to improve the binding of the phosphate groups (contained within the treatment solution 206) to the atoms at the surface of the polyamide part 210 when the polyamide part 210 is submerged into the treatment solution 206 during the application step 104.

[0120] Without being bound by theory, the phosphate ester may interact with the surface of the polyamide part 210 via London dispersion forces, dipole-dipole interactions, or hydrogen bonding through the polar functional groups of the phosphate ester, namely the phosphate (- PO4) and ester functionalities (-COO-) can interact with the amide linkages in the polyamide part 210 via hydrogen bonding. The interaction at the surface of the polyamide part 210 is optimised through implementation of methods according to the disclosure. Furthermore, keeping the temperature of the treatment solution 206 below 200°c helps to reduce the likelihood of the polyamide part 210 being subjected to thermal damage (such as warping or plastic deformation) during the application step 104.

[0121] It is generally preferred that the phosphate ester(s) used within the liquid treatment solution 206 have a boiling point which is greater than 180°c, and preferably greater than 200°c. This allows the treatment solution 206 to be heated up to the desired temperature during the heating step

[0122] 103 without causing some (or all) of the phosphate ester(s) to evaporate.

[0123] For example, the phosphate ester(s) used within the liquid treatment solution 206 may be selected from triethyl phosphate, diethyl phosphate, triisopropyl phosphate, tributyl phosphate, triisobutyl phosphate, tris(2-ethylhexyl) phosphate, triphenyl phosphate; 2-ethylhexyl dipentyl phosphate, tri(2-isopropylphenyl) phosphate, isodecyl diphenyl phosphate, cresyl diphenyl phosphate, tri-o-cresyl phosphate, tri-p-cresyl phosphate, tricresyl phosphate, tris(2- butoxylethyl) phosphate, or combinations thereof.

[0124] In such examples, the boiling point of the phosphate ester(s) may be in the range of about 200°c to about 450°c.

[0125] In addition to the heating step 103 described above, in some examples the polyamide part 210 may also optionally undergo a further heating step (prior to the application step 104) in which the polyamide part 210 is heated to a temperature below a melting temperature of the part (i.e., below its melting point). The precise temperature to which the polyamide part 210 is heated during this further heating step varies based on the material being processed but is typically up to temperatures of about 150°c.

[0126] Notably, it has been found that pre-heating the polyamide part 210 prior to the application step

[0127] 104 helps to further improve the binding of the phosphate groups (contained within the treatment solution) to the surface of the polyamide part during the application process.

[0128] Once a suitable treatment solution has been selected and the treatment solution 206, and optionally the polyamide part 210, have been heated to the desired temperature, the heated treatment solution 206 is then applied onto a surface of the polyamide part 210 during application step 104 so that the phosphate groups contained within the heated treatment solution 206 are able to bind onto the surface of the polyamide part 210.

[0129] As set out above, in the example illustrated in Figure 2, the heated treatment solution 206 is provided as a liquid and so, during the application step 104, the liquid treatment solution 206 is applied onto the surface of the polyamide part 210 via submerging the polyamide part 210 into the liquid treatment solution 206 provided within the bath 202. Once the polyamide part 210 has been immersed in the treatment solution 206, the part 210 is then left for a period of time to allow the phosphate groups contained within the treatment solution 206 to bind onto the surface of the part 210.

[0130] It has generally been found that immersion times in the range of about 5 seconds to about 1 hour are suitable for performing this treatment, although it is generally preferred to leave the surface of the polyamide part 210 exposed to the treatment solution 206 for an immersion time in the range of about 10 seconds to about 5 minutes to help reduce the likelihood of damaging the part 210 due to overexposure to the treatment solution 206.

[0131] Once the desired immersion time has expired, the part 210 is removed from the treatment solution 206 and any treatment solution 206 which remains on the surface of the part 210 is removed.

[0132] As alluded to above, it shall also be appreciated that in other examples, the treatment solution may be heated and applied onto the surface of the polyamide part via alternative means.

[0133] An alternative apparatus for heating and applying a fire-retardant treatment solution onto the surface of a polyamide part shall now be described with reference to Figure 3.

[0134] The apparatus 300 illustrated in Figure 3 is made up of a gas-tight processing chamber 302, a reservoir 304 and a vapour distribution system 306.

[0135] The gas-tight processing chamber 302 is sized to be able to receive a part 310 (e.g., a polyamide part) within its interior volume, which may be placed within the processing chamber 302 or suspended from a series of racks or hangers (not shown) provided within the interior volume of the processing chamber 302.

[0136] The gas-tight processing chamber 302 is fluidically-connected to the reservoir 304 via the vapour distribution system 306 to allow vapourised treatment solution 308 to be delivered into the processing chamber 302 (as shall be described in greater detail below).

[0137] In the illustrated example, the vapour distribution system 306 is provided in the form of a pipe having a first end located at an outlet of the reservoir 304 and having a second end located at an inlet of the processing chamber 302. The vapour distribution system 306 also includes a valve 305, which is in operable communication with a controller 340, to enable the introduction of the vapourised treatment solution 308 from the reservoir 304 into the processing chamber 302 to be controlled. The apparatus 300 further includes a vacuum pump 307 which is in fluid communication with the interior volume of the processing chamber 302. The vacuum pump 307 is also in operable communication with the or a controller 340 such that, in use, the vacuum pump 307 can be controlled to adjust the pressure applied to the interior of the processing chamber 302 during operation.

[0138] The apparatus 300 also includes a pair of heating elements 309a, 309b. The heating elements 309a, 309b are in operable communication with the or a controller 340 such that, in use, the heating elements 309a, 309b can be controlled to heat or vaporise the treatment solution as may be required during the heating 103 and application 104 steps.

[0139] In the embodiment illustrated in Figure 3, the first heating element 309a forms part of a wall of the processing chamber 302 and is configured to heat the vapourised treatment solution 308 which has already been introduced into the processing chamber 302.

[0140] Meanwhile, the second heating element 309b is associated with the reservoir 304 and is configured to vaporise the liquid treatment solution contained within the reservoir during the heating step 103 prior to its introduction into the processing chamber 302

[0141] However, in alternative embodiments such as the arrangement illustrated in Figure 4, the liquid treatment solution may be evaporated via a heating element 309c situated between the processing chamber 302 and the reservoir 304, thereby causing the liquid treatment solution to be evaporated before entering the chamber, whilst avoiding the need to heat all of the liquid treatment solution contained within the reservoir 304. It shall also be appreciated that the arrangement of Figure 4 also allows more precise dosing of the liquid treatment solution.

[0142] It shall also be appreciated that in some embodiments, the apparatus may comprise one or more flow rate sensors (not shown) located along the pipe together with a flow pump for more accurate dosing of the liquid treatment solution.

[0143] Referring now back to the method illustrated in Figure 1 , once a suitable treatment solution has been selected and loaded into the reservoir 304 of the apparatus 300, during heating step 103 the treatment solution is subsequently heated via the heating element 309b until at least some of the treatment solution 308 provided within the reservoir 304 becomes vaporised.

[0144] To reduce the amount of energy required to cause the treatment solution to vaporise, it is generally preferred that the phosphate ester(s) used within the treatment solution 308 have a boiling point which is less than 220°c. It has also been found that phosphate ester(s) having lower boiling points tend to produce hotter vapours which tends to result in more effective processing. For example, the phosphate ester(s) used within the vapour treatment solution 308 may be selected from trimethyl phosphate, dimethyl phosphate, triethyl phosphate, triisobutyl phosphate, tripropyl phosphate, tris(2-ethylhexyl) phosphate, tri(2-isopropylphenyl) phosphate, or combinations thereof.

[0145] In such examples, the boiling point of the phosphate ester(s) may be in the range of about 110°c to about 220°c.

[0146] Once at least some of the treatment solution contained within the reservoir 304 has been vaporised during the heating step 103, the vapourised treatment solution 308 is delivered from the reservoir 304 into the processing chamber 302 via opening the valve 305 of the vapour distribution system 306 and allowing the vapourised treatment solution 308 to flow into the processing chamber 302.

[0147] Once the vapourised treatment solution 308 has been introduced into the processing chamber 302, the treatment solution 308 may also be further heated (via the heating element 309a) if additional heating of the treatment solution 308 is required. The precise temperature to which the treatment solution 308 is heated varies based on the material being processed and the flashpoint of the treatment solution being used but is typically in the region of about 200°c.

[0148] It shall also be appreciated that the walls of processing chamber 302 may also be heated to maintain a suitably high temperature within the processing chamber 302 (e.g., up to approximately 200°c) following the introduction of the vaporised treatment solution 308 to aid the processing of the part 310 and to help prevent the vaporised treatment solution 308 from condensing onto the walls of the processing chamber 302.

[0149] In some examples, the vacuum pump 307 may also be operated prior to and / or during the introduction of the vapourised treatment solution 308 to maintain the interior of the processing chamber 302 at a negative pressure during the application step 104. Typically, the pressure within the processing chamber 302 is maintained below 100 kPa (1 bar).

[0150] By creating a negative pressure environment within the processing chamber 302 it has been found that the treatment solution 308 can be vaporised at lower temperatures than would otherwise be obtainable under atmospheric conditions. This helps to reduce the likelihood of the polyamide part 310 becoming thermally damaged during the application process and also enables the process to be performed with a wider range of the phosphate esters such as those which would not normally vaporise at temperatures in the range of about 200°c under ambient atmospheric conditions. Furthermore, the application of a negative pressure to the processing chamber 302 also aids in drawing the vapourised treatment solution 308 from the reservoir 304 and into the processing chamber 302 upon opening of the valve 305.

[0151] The precise pressures which are applied to the interior of the processing chamber 302 tend to vary based on the thermodynamic characteristics of the treatment solution which is being used for a given process and based on the temperature conditions within the processing chamber 302.

[0152] However, during the application step 104, the pressure within the processing chamber 302 will typically be maintained in the range of 0.1 kPa to 50 kPa (1 mBar to 500 mBar), with optimal results typically being achieved at pressures in the range of 5 kPa to 30 kPa (50 mBar to 300 mBar).

[0153] It will be understood that the temperature of the surface of the polyamide part 310 is initially lower than a condensation temperature of the vapourised treatment solution 308. As such, any vapourised treatment solution 308 which comes into contact with the surface of the polyamide part 310 will be subsequently cooled which will cause it to condense onto the surface of the polyamide part 310.

[0154] It shall also be appreciated that in some examples, the polyamide part 310 may also be precooled, typically to a temperature in the range of about 0°c to -30°c prior to placing the part 310 within the processing chamber 302 to help further encourage the vapourised treatment solution 308 to condense onto the surface(s) of the part 310.

[0155] As increasing amounts of vaporised treatment solution 308 are introduced into the processing chamber 302 via the vapour distribution system 306, the pressure and the temperature of the vapourised treatment solution 308 within the processing chamber 302 will also subsequently increase, thereby causing more and hotter treatment solution 308 to condense onto the surface of the part 310 (since at higher pressures, higher temperatures are required to vaporise the treatment solution 308).

[0156] After application of the treatment solution 308, the part 310 is then left for a period of time to allow the phosphate groups contained within the treatment solution 308 to bind onto the surface of the polyamide part 310.

[0157] It has generally been found that immersion times in the range of about 5 seconds to about 1 hour are suitable for performing this treatment, although it is generally preferred to leave the surface of the polyamide part 310 exposed to the treatment solution 308 for an immersion time in the range of about 10 seconds to about 5 minutes to help reduce the likelihood of damaging the part 310 due to overexposure to the treatment solution 308.

[0158] Once the desired immersion time has expired, the treatment solution 308 is removed from the surface of the part 310 via suction within the help of the vacuum pump 307, increasing the temperature to aid the evaporation of the treatment solution 308 from the part surface and / or via reducing the pressure within the processing chamber 302 so as to cause the treatment solution 308 present within the processing chamber 302 to re-vaporise.

[0159] Optionally, the application step 104 may be repeated after the removal of treatment solution from the surface of the part to further improve the fire-retardant properties of the additively manufactured part if further improvements are required after the first processing run.

[0160] The re-vaporised treatment solution can then be evacuated from the processing chamber 302 via a suitable outlet 303 associated with the processing chamber 302 so that the polyamide part 310 applied with the fire-retardant treatment can be safely retrieved by an operator.

[0161] Alternatively, the treatment solution may be evacuated from the processing chamber 302 by increasing the pressure within said processing chamber 302, opening the outlet 303 and collecting the condensed treatment solution at the bottom of the processing chamber 302.

[0162] It has been found that polyamide parts subjected to the aforementioned treatment processes exhibit improve fire- retard a ncy when compared to untreated polyamide parts as shall be evidenced in the experimental section below.

[0163] In particular, it is believed that the presence of the one or more phosphate groups (which bind onto the molecules of the polyamide part during the aforementioned the treatment processes) cause the polyamide material to char upon being exposed to a naked flame, thereby forming a protective layer which interrupts the combustion process and hence prevents further combustion of the material, thereby resulting in a more fire-retardant polyamide part being achieved.

[0164] Unless otherwise stated, each of the integers described may be used in combination with any other integer as would be understood by the person skilled in the art. Further, although all aspects of the disclosure preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art. Further, in the discussion of the disclosure, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

[0165] In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about". In order that the disclosure may be more readily understood, it will be described further with reference to the specific examples hereinafter.

[0166] EXPERIMENTAL EXAMPLES

[0167] Example 1

[0168] In a first experimental example, five 2mm thick samples of Polyamide 12 UL94 were printed using an MJF 5210 3D printer.

[0169] The samples were then subjected to a smoothing treatment using an a 1 ,1 , 1 ,3, 3, 3- Hexafluoroisopropanol (HFIP) solvent before undergoing treatment with a fire-retardant treatment solution. The samples were treated with HFIP vapours at 100°c under 300 mbar pressure for an exposure time of 2 minutes. During this process, the processing chamber was maintained at a temperature of 30°c.

[0170] Following the smoothing treatment, the samples were submerged (or “hot dipped”) into a triisobutyl phosphate liquid treatment solution, which had been heated to a temperature of 150°c, for an immersion time of approximately 1 minute.

[0171] The samples were then removed from the liquid treatment solution and dried in a drying oven maintained at a temperature of 60°C for 2 hours.

[0172] The five samples were then each in turn held to a naked flame for a period of 10 seconds before being removed. The time taken for each sample to “self-extinguish” was then recorded as a “burn time” which is displayed in Table 1 below.

[0173] N.B - Any samples which did not self-extinguish following removal from the naked flame were considered to constitute a failure (F). Example 2

[0174] In a second experimental example, five 2mm thick samples of UL94 Polyamide 12 were printed using an MJF 5210 3D printer.

[0175] The raw “as printed” samples were then submerged (or “hot dipped”) into a tri-isobutyl phosphate liquid treatment solution, which had been heated to a temperature of 180°c, for an immersion time of approximately 1 minute.

[0176] The samples were then removed from the liquid treatment solution and dried in a drying oven maintained at a temperature of 60°C for 2 hours.

[0177] The five samples were then each in turn held to a naked flame for a period of 10 seconds before being removed. The time taken for each sample to “self-extinguish” was then recorded as a “burn time” which is displayed in Table 2 below.

[0178] N.B - Any samples which did not self-extinguish following removal from the naked flame were considered to constitute a failure (F).

[0179] Example 3

[0180] In a third experimental example, five 2mm thick samples of UL94 Polyamide 12 were printed using an MJF 5210 3D printer.

[0181] The samples were then subjected to a smoothing treatment using an a 1 , 1 ,1 , 3,3,3- Hexafluoroisopropanol (HFIP) solvent before undergoing treatment with a fire-retardant treatment solution. The samples were treated with HFIP vapours at 100°c under 300 mbar pressure for an exposure time of 2 minutes. During this process, the processing chamber was maintained at a temperature of 30°c. Following the smoothing treatment, the samples were submerged (or “hot dipped”) into a triisobutyl phosphate liquid treatment solution, which had been heated to a temperature of 180°c, for an immersion time of approximately 1 minute.

[0182] The samples were then removed from the liquid treatment solution and dried in a drying oven maintained at a temperature of 60°C for 2 hours.

[0183] The five samples were then each in turn held to a naked flame for a period of 10 seconds before being removed. The time taken for each sample to “self-extinguish” was then recorded as a “burn time” which is displayed in Table 3 below.

[0184] N.B - Any samples which did not self-extinguish following removal from the naked flame were considered to constitute a failure (F).

[0185] Example 4

[0186] In a fourth experimental example, five 2mm thick samples of UL94 Polyamide 12 were printed using an MJF 5210 3D printer.

[0187] The raw “as printed” samples were then submerged (or “hot dipped”) into a tri-isobutyl phosphate liquid treatment solution, which had been heated to a temperature of 200°c, for an immersion time of approximately 30 seconds.

[0188] The samples were then removed from the liquid treatment solution and dried in a drying oven maintained at a temperature of 60°C for 2 hours.

[0189] The five samples were then each in turn held to a naked flame for a period of 10 seconds before being removed. The time taken for each sample to “self-extinguish” was then recorded as a “burn time” which is displayed in Table 4 below. N.B - Any samples which did not self-extinguish following removal from the naked flame were considered to constitute a failure (F).

[0190] Example 5

[0191] In a fifth experimental example, five 2mm thick samples of UL94 Polyamide 12 were printed using an MJF 5210 3D printer.

[0192] The samples were then subjected to a smoothing treatment using an a 1 ,1 , 1 ,3, 3, 3- Hexafluoroisopropanol (HFIP) solvent before undergoing treatment with a fire-retardant treatment solution. The samples were treated with HFIP vapours at 100°c under 300 mbar pressure for an exposure time of 2 minutes. During this process, the processing chamber was maintained at a temperature of 30°c.

[0193] Following the smoothing treatment, the samples were submerged (or “hot dipped”) into a triisobutyl phosphate liquid treatment solution, which had been heated to a temperature of 200°c, for an immersion time of approximately 30 seconds.

[0194] The samples were then removed from the liquid treatment solution and dried in a drying oven maintained at a temperature of 60°C for 2 hours.

[0195] The five samples were then each in turn held to a naked flame for a period of 10 seconds before being removed. The time taken for each sample to “self-extinguish” was then recorded as a “burn time” which is displayed in Table 5 below.

[0196] N.B - Any samples which did not self-extinguish following removal from the naked flame were considered to constitute a failure (F).

[0197] Example 6

[0198] In a sixth experimental example, a tri-isobutyl phosphate treatment solution was vaporised via heating the solution to a temperature of approximately 200°C under the atmospheric pressure of l OOOmbar.

[0199] Five 2mm thick samples of raw “as printed” UL94 Polyamide 12 (printed via a MJF 5210 3D printer) were then subsequently exposed to said vapours to allow the vapourised treatment solution to condense onto the samples. The samples were then left exposed to the condensed treatment solution for an immersion time of approximately 1 minute.

[0200] The five samples were then each in turn held to a naked flame for a period of 10 seconds before being removed. The time taken for each sample to “self-extinguish” was then recorded as a “burn time” which is displayed in Table 6 below.

[0201] N.B - Any samples which did not self-extinguish following removal from the naked flame were considered to constitute a failure (F). Example 7

[0202] In a seventh experimental example, three 2mm thick samples of UL94 Polyamide 12 (printed via a MJF 5210 3D printer) were subjected to a smoothing treatment using an a 1 ,1 , 1 ,3, 3, 3- Hexafluoroisopropanol (HFIP) solvent before undergoing treatment with a fire-retardant treatment solution. The samples were treated with HFIP vapours at 100°c under 300 mbar pressure for an exposure time of 2 minutes. During this process, the processing chamber was maintained at a temperature of 30°c.

[0203] Following the smoothing treatment, a tri-isobutyl phosphate treatment solution was vaporised via heating the solution to a temperature of approximately 200°C under the atmospheric pressure of WOOmbar, and the samples were subsequently exposed to said vapours to allow the vapourised treatment solution to condense onto the samples. The samples were then left exposed to the condensed treatment solution for an immersion time of approximately 1 minute.

[0204] The three samples were then each in turn held to a naked flame for a period of 10 seconds before being removed. The time taken for each sample to “self-extinguish” was then recorded as a “burn time” which is displayed in Table 7 below.

[0205] N.B - Any samples which did not self-extinguish following removal from the naked flame were considered to constitute a failure (F).

[0206] Although the disclosure has been described above with reference to one or more preferred examples, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1 . A method of improving the fire retardancy of a polyamide part, the method comprising the steps of: a) providing a part comprising a polyamide material; b) providing a treatment solution comprising at least one phosphate ester; c) a heating step, wherein the treatment solution is heated to a temperature of at least 150°c; and d) an application step, wherein the heated treatment solution is applied onto a surface of the polyamide part in order to improve the fire- retardancy of said part.

2. The method according to claim 1 , wherein the at least one phosphate ester has a general formula according to formula (I):formula (I) wherein R1, R2, and R3are each independently selected from hydrogen; a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group; with the proviso that not all of R1, R2, and R3are hydrogen.

3. The method according to claim 2, wherein R1, R2, and R3are each independently selected from hydrogen; a linear or branched Ci to C30 alkyl group, optionally wherein at least one C atom is replaced by O; a linear C2 to C12 alkenyl group substituted with an alkoxy group, wherein the alkyl group bonded to the oxygen atom of the alkoxy group is selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isodecyl, and 2-ethylhexyl; and an aryl group selected from phenyl, optionally substituted with an alkyl group as defined herein at the ortho, meta, and / or para position, or naphthyl; with the proviso that not all of R1, R2, and R3are hydrogen.

4. The method according to claim 2, wherein the at least one phosphate ester comprises a phosphate ester according to general formula (II), (III), or (IV):formula (II)formula (III)formula (IV) wherein R4, R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group.

5. The method according to claim 4, wherein the phosphate ester may comprise a phosphate ester according to general formula (III) or (IV), and wherein R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group, optionally substituted with a hydroxyl group, an amino group, a carboxyl group, or a combination thereof and / or optionally wherein at least one C atom of the linear or branched Ci to C30 alkyl group is replaced by a heteroatom selected from N, S, or O; a linear or branched C2 to C30 alkenyl group, optionally substituted with an alkyl group, an alkoxy group, an amino group, or a combination thereof; and an aryl group.

6. The method according to claim 4 or claim 5, wherein R4, R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C30 alkyl group optionally wherein at least one C atom is replaced by O; and an aryl group.

7. The method according to any of claims 4 to 6, wherein R5, R6, R7, R8, and R9are each independently selected from a linear or branched Ci to C10 alkyl group optionally wherein at least one C atom is replaced by O, a phenyl group, and a phenyl group substituted with at least one Ci to Cs linear or branched alkyl group at the ortho, meta, and / or para position.

8. The method according to any preceding claim, wherein the at least one phosphate ester is selected from trimethyl phosphate, dimethyl phosphate, triethyl phosphate, diethyl phosphate, tripropyl phosphate, triisopropyl phosphate, tributyl phosphate, triisobutyl phosphate, tripentyl phosphate, tris(2-ethylhexyl) phosphate, triphenyl phosphate, 2-ethylhexyl dipentyl phosphate, tri(2-isopropylphenyl) phosphate, isodecyl diphenyl phosphate, cresyl diphenyl phosphate, tri- o-cresyl phosphate, tri-p-cresyl phosphate, tricresyl phosphate, tris(2-butoxylethyl) phosphate, or combinations thereof.

9. The method according to any preceding claim, wherein the at least one phosphate ester has a boiling point in the range of about 110°c to about 450°c.

10. The method according to any preceding claim, wherein the at least one phosphate ester has a boiling point in the range of about 190°c to about 280°c.11 . The method according to any preceding claim, wherein the at least one phosphate ester is selected from trimethyl phosphate, triethyl phosphate, diethyl phosphate, triisopropyl phosphate, triisobutyl phosphate, tris(2-ethylhexyl) phosphate, tri(2-isopropylphenyl) phosphate, tris(2-butoxylethyl) phosphate, or combinations thereof.

12. The method according to any preceding claim, wherein the treatment solution comprises triisobutyl phosphate.

13. The method according to any preceding claim, wherein the polyamide material comprises an aliphatic polyamide, an aromatic polyamide, or a semi-aromatic polyamide.

14. The method according to any preceding claim, wherein the polyamide material comprises an aliphatic polyamide selected from Polyamide 11 , Polyamide 12, Polyamide 46, Polyamide 66, or a combination thereof.

15. The method according to any preceding claim, wherein the treatment solution is provided as a liquid, and wherein step d) comprises submerging the part into the treatment solution.

16. The method according to claim 15, wherein the at least one phosphate ester has a boiling point in the range of about 200°c to about 450°c.

17. The method according to claim 15 or 16, wherein the at least one phosphate ester is selected from triethyl phosphate, diethyl phosphate, triisopropyl phosphate, tributyl phosphate, triisobutyl phosphate, tris(2-ethylhexyl) phosphate, triphenyl phosphate; 2-ethylhexyl dipentyl phosphate, tri(2-isopropylphenyl) phosphate, isodecyl diphenyl phosphate, cresyl diphenyl phosphate, tri-o-cresyl phosphate, tri-p-cresyl phosphate, tricresyl phosphate, tris(2- butoxylethyl) phosphate, or combinations thereof.

18. The method according to any of claims 15 to 17, wherein step c) comprises heating the treatment solution comprising at least one phosphate ester to a temperature in the range of about 150°c to about 200°c, and optionally to a temperature of about 180°c.

19. The method according to any of claims 15 to 18, wherein the method further comprises, prior to step d), heating the part to a temperature below a melting temperature of the part, and optionally to a temperature of up to 150°c20. The method according to any of claims 1 to 14, wherein step c) comprises heating the treatment solution so as to cause the treatment solution to vaporise, and wherein step d) comprises condensing the vaporised treatment solution onto the surface of the polyamide part21 . The method according to claim 20, wherein the at least one phosphate ester has a boiling point in the range of about 110°c to about 300°c.

22. The method according to claim 20 or 21 , wherein the at least one phosphate ester is selected from trimethyl phosphate, dimethyl phosphate, triethyl phosphate, triisobutyl phosphate, tripropyl phosphate, tris(2-ethylhexyl) phosphate, tri(2-isopropylphenyl) phosphate, or combinations thereof.

23. The method according to any of claims 20 to 22 wherein the method further comprises cooling the polyamide part prior to step d), optionally to a temperature between 0 and - 30°c.

24. The method according to any of claims 20 to 23, wherein, the method comprises placing the polyamide part into a processing chamber and wherein, during step d), an interior of the processing chamber is maintained at a pressure in the range of about 0.1 kPa to about 50 kPa (1 mBar to 500 mBar), and optionally in the range of about 5 kPa to about 30 kPa (50 mBar and 300 mBar).

25. The method according to any preceding claim, wherein the part is an additively manufactured part.

26. The method according to claim 25, wherein the additively manufactured part is a powder-based additively manufactured part or wherein the additively manufactured part is a filament-based additively manufactured part.

27. A fire-retardant polyamide part obtainable by the method according to any preceding claim.

28. The use of a phosphate ester for improving the fire- retard a ncy of a polyamide part.

29. The use of a phosphate ester according to claim 28, wherein the phosphate ester is triisobutyl phosphate.

30. The use of a phosphate ester according to claim 28 or 29, wherein the polyamide part comprises polyamide 12 or polyamide 11 .

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