WO2023126391A1

WO2023126391A1 - Reinforced composite filament for an additive manufacturing application and method for manufacturing thereof

Info

Publication number: WO2023126391A1
Application number: PCT/EP2022/087857
Authority: WO
Inventors: Vincent BERTHE; Henri Perrin; Régis VAUDEMONT
Original assignee: Luxembourg Institute Of Science And Technology (List)
Priority date: 2021-12-29
Filing date: 2022-12-27
Publication date: 2023-07-06
Also published as: LU501120B1

Abstract

The invention relates to a filament (36) for an additive manufacturing application or a winding application, the filament comprising fibers (38) embedded in a reactive thermoplastic resin (40) comprising of (meth)acrylic polymer, (meth)acrylic monomer and organic peroxides, said filament further comprising a tie layer (44) coating the reactive thermoplastic resin, the tie layer being constituted by a reactive blend of: at least a polyamide and a reactive copolymer, said reactive copolymer comprising a reactive (meth)acrylic polymer containing acids and/or anhydrides. The invention also relates to a use of the filament for manufacturing an end product by an additive manufacturing process or by a winding technique. The invention also relates to a method for manufacturing such a filament. The invention enables a high-speed production (10 meters per minute) of a filament of high quality.

Description

REINFORCED COMPOSITE FILAMENT FOR AN ADDITIVE MANUFACTURING APPLICATION AND METHOD FOR MANUFACTURING THEREOF

Technical field

[0001 ] The invention relates to the field of composite materials that can be used in additive manufacturing or in winding applications.

[0002] In particular, the invention relates to composite materials used for the manufacturing of parts and structures, such as lightweight structures used in automotive, aircraft or space industry.

Background art

[0003] Composite filaments that are made from a thread or a roving of fibers, especially carbon fibers, are known from prior art under the name of continuous fiber reinforced filaments or prepregs.

[0004] In fact, composite filaments are commonly used to form 3D-printed parts by additive manufacturing processes wherein the part is formed from successive layers created by melting the filament. On the other hand, composite filaments can also have the form of a tape especially when used in a winding process, also called weaving process. In a winding process, a composite structure is formed by winding the tape using a robotized machine.

[0005] It is known from the state of the art that the roving is impregnated using a binder. For instance, a thermosetting binder based on a thermoset resin is commonly used to impregnate the thread of carbon fibers.

[0006] However, the thermoset resin presents considerable material formability limitations, especially when heated, inducing cracks during the printing operation (or insufficient flexibility).

[0007] In fact, the impregnating binder of the composite filament needs to be fully cured before the filament being usable in an additive manufacturing process. For that purpose, it is known from the art to use an oven configured to heat the thermoset resin to temperatures rising up to 400°C. However, in order to fully cure the thermoset binder, it usually needs around 10 min of total curing time, and knowing that the process time is dependent on the curing time, the slow curing process results in a slow filament production rate, typically having a speed ranging from 0,3 m/min to 1 m/min. Added to that, technical analysis has been carried out which supports that this method of curing a thermoset resin induces high number of voids and a random filament section.

[0008] Another disadvantage of using a thermoset resin-based binder for impregnating the composite filament, is the limited physicochemical compatibility between the filament and other thermoplastic materials used in a later stage of the forming process, i.e. 3D printing or overmolding. This insufficient compatibility results in a poor filament morphological quality during the additive manufacturing process and the 3D-printed part using that filament usually demonstrates mechanical properties which may not be seen as sufficient for every application.

[0009] In fact, a weak adhesion between the thermoset based continuous fiber filament and the thermoplastic polymer is observed during the 3D printing process. A relative motion (sliding) of the filament with respect to the resin can even be observed. This may impact the mechanical resistance of the final product and/or its longevity.

[0010] Publication document WO 2017/188861 A1 discloses an example of such known techniques. A prepreg which is considered as a commodity polymer is produced with the impregnation of fibers using a full bath of liquid thermoset resin coupled with curing using an oven. The production rate of the composite filament depends on curing time, which constitutes an intrinsic technological limit of productivity.

[0011 ] The filament disclosed in the document WO 2017/188861 A1 also presents limits of material formability and a limited production rate due to the curing time of the epoxy matrix. In fact, the filament is generated at a speed of around 1 m/min which leaves some margin for improvement. In addition, the question of multi-material assembly during the additive manufacturing process is not solved. [0012] Publication documents KR 2021 0096441 A, KR 2018 0034043 A, US 2021/299946 A1 , WO 2019/170463 A1 , EP 2 976 205 A2 disclose examples of composite filaments which also have room for imporvement.

Summary of invention

Technical problem

[0013] The present invention addresses the above-mentioned deficiencies and aims at providing a composite filament with a superior hot formability and a better compatibility with a large range of technical polymers during additive manufacturing for facilitating the 3D-printing or winding processes.

[0014] The invention further aims at providing a manufacturing method for the filament with a higher production speed.

[0015] Overall, the present invention enables to manufacture products of higher quality in a shorter amount of time.

Solution

[0016] The above-stated problem is solved by a filament for an additive manufacturing application or a winding application, the filament comprising fibers embedded in a reactive thermoplastic resin comprising (meth)acrylic polymer, (meth)acrylic monomer and organic peroxides, said filament further comprising a tie layer coating the reactive thermoplastic resin, the tie layer being constituted by a reactive blend of: at least a polyamide and a reactive copolymer, said reactive copolymer comprising a reactive (meth)acrylic polymer containing acids and/or anhydrides.

[0017] Both the compositions of the liquid reactive thermoplastic resin and the reactive blend used in the tie layer are compatible, the compatibility is particularly achieved due to the presence of the (meth)acrylic in both compositions. Advantageously, the compatibility results in an overall better formability and an enhanced homogeneity of the filament.

[0018] According to a preferred embodiment, the polyamide is polyamide 6 or polyamide 12 or a mixture thereof, said polyamide constituting 20 to 80 wt % of the tie layer. [0019] According to a preferred embodiment, the reactive (meth)acrylic polymer comprises poly(methyl methacrylate-co-methacrylic acid).

[0020] According to a preferred embodiment, the reactive (meth)acrylic polymer comprises poly(methyl methacrylate-co-methacrylic acid-co-di-methyl- glutaric anhydride).

[0021 ] According to a preferred embodiment, the weight ratio of polyamide particles to (meth)acrylic polymer particles is 0.25:1 to 4:1 and preferably 1.5:1.

[0022] According to a preferred embodiment, the (meth)acrylic polymer particles have an average size diameter (D50) of 0.1 pm to 10 pm, preferably from 2 pm to 8 pm.

[0023] The invention also relates to a use of the filament of any of the above- mentioned embodiments for manufacturing an end product by an additive manufacturing process or by a winding technique, and preferably a 3D- printing process.

[0024] According to a preferred embodiment, prior to a deposition of the filament to form the end product, the filament is wrapped in a shell material made of polyamide.

[0025] According to a preferred embodiment, the polyamide of the shell material is polyamide 6 or polyamide 12 or a mixture thereof, said polyamide forming at least 90 wt % of the shell material.

[0026] The invention also relates to a product obtained by the use of the filament in accordance with any of the preceding paragraphs regarding the use of the filament. As explained further in details below, a product obtained with the filament of the invention is structurally distinct from a product obtained otherwise. In this regard, pull-off tests can further support the distinction.

[0027] Preferably, the polyamide contained in the composition of the shell material wrapping the filament of the invention during a 3D-printing process is similar to the polyamide contained in the tie layer. Advantageously, this enables a good physicochemical adhesion between said tie layer and the shell material. Consequently, the formability and the homogeneity of the end product is enhanced, making it possible for the end product to be used for physically more demanding industrial utilizations such as lightweight structures used in automotive, aircraft or space industry.

[0028] The invention also relates to a method for manufacturing a filament aimed to an additive manufacturing application or a winding application, the method comprising: providing a thread of fibers; impregnating the thread with a liquid reactive thermoplastic resin mainly composed of (meth)acrylic polymer, (meth)acrylic monomer and organic peroxides; and co-extruding a tie layer of thermoplastic compound around the impregnated thread; wherein the compound is constituted by a reactive blend of: at least a polyamide and a reactive copolymer, said reactive copolymer comprising a reactive (meth)acrylic polymer containing acids and/or anhydrides.

[0029] The co-extrusion of the tie layer enables to produce a filament without having to wait for the impregnation resin to cure, thereby waiving the time constraint of known methods. Also, the tie layer improves the homogeneity and cross-section profile of the filament.

[0030] Preferably, the co-extrusion of the tie layer is done on the liquid reactive thermoplastic resin while it is not yet totally cured, and this advantageously enables a combination of chemical and mechanical interlocking, ensuring higher adhesion properties for the filament.

[0031 ] According to a preferred embodiment, the thermoplastic compound comprises a compatibilizer in an amount of 1 to 20 wt % in the tie layer, said compatibilizer is made of a reactive (meth)acrylic polymer or poly(styrene- co-glycidyl methacrylate) or poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate).

[0032] Preferably, the method further comprises a curing step of the liquid reactive thermoplastic resin right after co-extrusion of the tie layer. Said curing step can be achieved using an oven which ensures curing at a temperature that is lower than the melting temperature of the tie layer, the former being preferably around 100°C while the latter is preferably around 200°C. “around” is intended to mean plus or minus 10% of the given values. This ensures that the tie layer is not altered in shape or structure during curing of the resin.

[0033] Advantageously, the chemical composition of the thermoplastic compound constituting the tie layer enables solidification of said tie layer at least partially before curing the liquid reactive thermoplastic resin. The polymerization of the tie layer can optionally overlap the polymerization of the resin, meaning that the tie layer can still be solidifying while the filament enters the oven. The polymerization of the resin is controlled and a chemical diffusion of the liquid reactive thermoplastic resin into the tie layer occurs, thereby creating a chemical and mechanical bond.

[0034] According to a preferred embodiment, the thermoplastic compound is coextruded at a rate of about 10 meters per minute, “about” is intended to mean plus or minus 20% of the given value. Advantageously, the composition of the liquid reactive thermoplastic resin of the invention allows the impregnated thread to be sufficiently hard and only with a quick curing time in the oven, i.e. 3 minutes, said composition can fully cure subsequently outside the oven and at room temperature, thus enabling fast production rate.

Brief description of the drawings

[0035] Figure 1 is a schematic representation of a method for composite filament manufacturing;

[0036] Figure 2 shows a schematic representation of a reactive copolymer according to a first embodiment of the invention;

[0037] Figure 3 shows a schematic representation of the reactive copolymer according to a second embodiment of the invention;

[0038] Figure 4 shows a schematic representation of a first alternative for obtaining an anhydride;

[0039] Figure 5 shows a schematic representation of a second alternative for obtaining the anhydride;

[0040] Figure 6 illustrates a twin-screw extruder design achieving a conversion of poly(methyl methacrylate-co-methacrylic acid) PMMA-co-MAA into poly(methyl methacrylate-co-methacrylic acid-co-di-methyl-glutaric anhydride) PMMA-co-MAA-co-DMGAH;

[0041 ] Figure 7 illustrates a twin-screw extruder design achieving a reactive blend of components comprised in a tie layer of a filament of the invention;

[0042] Figure 8 is a SEM micrograph of a PMMA I PA12 blend at 40 I 60 weight ratio;

[0043] Figure 9 is a representation of two cross sections of the composite filament of the invention;

[0044] Figure 10 is an illustration of a 3D printing application of the composite filament;

[0045] Figure 11 is a representation of two cross sections of the filament used in the 3D printing process of figure 10.

Detailed description of the drawings

[0046] Figure 1 is a schematic representation of a method 100 for manufacturing a composite filament aimed to an additive manufacturing application or a winding application, the method 100 comprises a plurality of steps S102, S104 and S106. Said steps will be described within the established order.

[0047] A first step S102 of providing a roving or a thread of fibers for instance by unrolling a bobbin containing the thread. The thread of fibers can comprise any of the following list: carbon fibers, glass fibers, aramid fibers (kevlar), silicon carbide fibers, vegetable fibers (flax, hemp, etc.), polyester fibers (such as textilene), basalt fibers, any metallic continuous fiber, or any combination thereof. Preferably, the thread is made of dry carbon fibers, more preferably, 1 K and 3K carbon fiber available from Tejin.

[0048] A step S104 which consists of impregnating the thread with a liquid reactive thermoplastic resin. In fact, the impregnation is made with the thermoplastic resin being in a liquid state.

[0049] The liquid reactive thermoplastic resin is mainly composed of (meth)acrylic polymer and (meth)acrylic monomer which together form a (meth)acrylic monomer-polymer syrup. The liquid reactive thermoplastic resin further comprises a mix of initiators, like organic peroxides which advantageously enable fast curing of the resin, e.g. a curing duration of 3 minutes at 110°C.

[0050] Preferably, the liquid reactive thermoplastic resin is Elium® C585 provided by the company Arkema.

[0051 ] Preferably, the liquid resin has a dynamic viscosity being under 1 Pa.s (Poiseuille). Furthermore, the impregnation step S104 is made at room temperature, preferably ranging from 20°C to 26°C. Hence, the liquid resin is at low temperature.

[0052] The method 100 further comprises a step S106 of co-extruding a tie layer of thermoplastic compound completely and directly around the impregnated thread in an uncured state. The co-extrusion can be carried out by an extruder like a single screw extruder for example.

[0053] The thermoplastic compound of the tie layer is constituted by a reactive blend of materials according to two different embodiments. In this regard, a first embodiment consists of achieving a reactive blend of polyamide with a reactive copolymer comprising a reactive (meth)acrylic polymer containing acids. Whereas a second embodiment consists of using the same reactive blend as of the first embodiment and further adding anhydrides to it. The two embodiments will be further described later on in this description.

[0054] Preferably, the co-extrusion step S106 is performed at a high temperature, e.g. about 200°C or 220°C, which is above the melting temperature of the thermoplastic compound of the tie layer, i.e. 200°C or typically around 180°C.

[0055] Furthermore, and thanks to the fast curing capacity of the liquid reactive thermoplastic resin, the tie layer is co-extruded at a rate of about 10 meters per minute.

[0056] Right after the co-extrusion step S106, solidification of the tie layer can at least partially occur at room temperature. Advantageously, the solidification of the tie layer helps containing the thermoplastic resin which is impregnating the fibers and in a liquid state before curing the filament, and said solidification also enables maintaining a circular and homogeneous shape of the filament during and following the curing. [0057] The preparation of the material for the tie layer consists of blending polyamide and a reactive (meth)acrylic polymer. The polyamide is polyamide 6 or polyamide 12 or a mixture thereof.

[0058] Preferably, the thermoplastic blend of the tie layer uses polyamide 12 for the two embodiments of the invention, polyamide 12 will be designated by PA12 in this description, the PA12 is a Rilsamid® polyamide 12 available from Arkema.

[0059] In the first embodiment, the reactive blend is obtained by using the reactive (meth)acrylic polymer containing acids.

[0060] Figure 2 shows a schematic representation of such a reactive copolymer comprising the reactive (meth)acrylic polymer containing acids, wherein said (meth)acrylic polymer comprises poly(methyl methacrylate) “PMMA” which can be Altuglass 520L or Altuglass BS440, both can be provided by Arkema.

[0061 ] In fact, the reactive (meth)acrylic polymer containing acids comprises poly(methyl methacrylate-co-methacrylic acid) PMMA-co-MAA which will be further designated in this description as “functional PMMA”.

[0062] Preferably, the reactive (meth)acrylic polymer of the first embodiment fully corresponds to the functional PMMA.

[0063] In the second embodiment, the reactive blend is obtained by using the reactive copolymer comprising the reactive (meth)acrylic polymer containing acids, i.e. the functional PMMA of the first embodiment, along with the addition of anhydrides, thus obtaining poly(methyl methacrylate-co- methacrylic acid-co-di-methyl-glutaric anhydride) PMMA-co-MAA-co- DMGAH, which will be designated in this description as “modified functional PMMA”, and which has the chemical structure as depicted in figure 3.

[0064] Preferably, the reactive (meth)acrylic polymer of the second embodiment fully corresponds to the modified functional PMMA.

[0065] The added anhydride acid (di-methyl-glutaric anhydride, DMGAH) allowing the formation of the modified functional PMMA is preferably obtained according to a specific synthesis process which is derived in two different alternatives, both alternatives will be further developed in this description.

[0066] Figure 4 shows a schematic representation of a first alternative for obtaining the anhydride DMGAH. In this regard, the first alternative consists of joining two acids (methacrylic acids) MAA, from which a H₂O molecule is extracted, resulting in the formation of the (di-methyl-glutaric anhydride) DMGAH having the chemical structure depicted in figure 4.

[0067] Figure 5 shows a schematic representation of a second alternative for obtaining the anhydride DMGAH. To this effect, the second alternative consists of joining one methacrylic acid MAA and one ester, the latter corresponds to the PMMA in its regular structure, and the joint of the latter and the former result in a poly(methyl methacrylate-co-methacrylic acid) PMMA-co-MAA, from which a CH₃OH molecule is further taken away, thus chemically obtaining the (di-methyl-glutaric anhydride) DMGAH.

[0068] The first and second alternatives enable to obtain an identical DMGAH with the same chemical structure, as depicted in the right-hand side of both figures 4 and 5.

[0069] The addition of DMGAH to functional PMMA allowing the obtention of the modified functional PMMA according to the second alternative of the second embodiment, is preferably achieved by using a twin-screw extruder having a similar design to the one depicted in figure 6.

[0070] In reference to figure 6, the twin-screw extruder 2 is illustrated from a lateral side view. Preferably, the twin-screw extruder 2 comprises a screw diameter of about 18 mm and has a length to diameter ratio of about 60 L/D.

[0071 ] The twin-screw extruder 2 further comprises a functional PMMA inlet 4 from which dried pellets of poly(methyl methacrylate-co-methacrylic acid) PMMA-co-MAA are supplied.

[0072] Nitrogen gas is also supplied through the inlet 14. In fact, in order to prevent deterioration of the raw material (dried pellets of functional PMMA) due to oxidation, an inert gas is supplied to the interior of the extruder. Nitrogen gas is often used as this inert gas. [0073] The raw material is conveyed to the downstream end 6 of the twin-screw extruder 2 by rotation of the screws 8, the dried pellets of functional PMMA are subject to shear stress in a mixing section 10, this section enhances the mixing of the dried pallets.

[0074] The dried pallets of functional PMMA are extruded at a temperature comprised in a range of 250 to 280°C, and at a low flow rate in order to allow longer retention times. Around the downstream end 6, a degassing outlet 12 is arranged to enable extracting gas that can be retained in the mixture.

[0075] Based on poly(methyl methacrylate-co-methacrylic acid) a conversion is obtained in the twin-screw extruder 2 to form highly reactive di-methyl- glutaric anhydride units along the chain. In this regard, the conversion of the functional PMMA into the modified functional PMMA is achieved nearby the downstream end 6, wherein a modified functional PMMA outlet 14 is arranged and configured to allow the output of poly(methyl methacrylate-co- methacrylic acid-co-di-methyl-glutaric anhydride) PMMA-co-MAA-co- DMGAH.

[0076] In order to control the amount of DMGAH during the extrusion, some parameters are simultaneously controlled, such as temperature, residence time which is a function of both output speed of PMMA-co-MAA-co-DMGAH and screw speed, and shear stress which is dependent on screw speed and the extruder’s design.

[0077] The amount of anhydride acid is then further characterized by a FTIR (Fourier Transform Infrared Spectroscopy) characterization technique in order to obtain an infrared spectrum of absorption, emission, and photoconductivity information about the anhydride.

[0078] Figure 7 illustrates a twin-screw extruder design achieving a blend of the different components comprised in the tie layer of the filament of the invention.

[0079] In reference to figure 7, the twin-screw extruder 16 achieves a blending of polyamide PA12 with the (meth)acrylic polymer. For that matter, PA12 is present in a content of 20 to 80 wt % of the entire thermoplastic compound composition of the tie layer. [0080] In fact, the (meth)acrylic polymer according to any of the two abovedescribed embodiments, i.e. functional PMMA or modified functional PMMA, will be identified as PMMA further on in the present description.

[0081 ] The twin-screw extruder comprises a PA12 inlet 18, configured for inserting PA12 into the extruder, nitrogen gas can also be inserted, followed downstream by a first mixing section 20 and a PMMA inlet 22 along with nitrogen gas.

[0082] Upstream from the first mixing section 20, a compatibilizer inlet 19 can be arranged, and configured to supply PA12 with a compatibilizer. The compatibilizer can be made of (1 ) the reactive (meth)acrylic polymer, i.e. functional PMMA (PMMA-co-MAA); or made of (2) poly(styrene-co-glycidyl methacrylate) which is commercially available under the name of Joncryl®; or (3) made of poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate), the latter corresponds to Lotader® which is available from Arkema.

[0083] Advantageously, the compatibilizer can improve dispersion of polymer domains, thus obtaining an optimum dispersion of PMMA particles in PA12, which improves mechanical properties of the end product, such as a 3D- printed part for instance.

[0084] Furthermore, ageing resistance of tie layers based on reactively compatibilized blends should give better ageing resistance, i.e. an enhanced resistance to temperature, humidity and UVs.

[0085] The compatibilizer can be present in an amount of 1 to 20 wt % in the thermoplastic compound composition of the tie layer, and preferably in a content of 1 to 10 wt %, and more preferably at a content of 5 wt %.

[0086] Downstream of the PMMA inlet 22, a second mixing section 24 is arranged enabling blending of both PA12 and PMMA and the compatibilizer if it used during extrusion. Nearby a downstream area 26, a degassing outlet 28 is arranged to enable extracting retained gas in the mixture.

[0087] From an outlet gate 30, the blend is extruded, the latter corresponds to the thermoplastic compound and can be readily co-extruded with the resin- embedded fibers, so as to form the tie layer of the composite filament of the invention. [0088] The weight ratio of polyamide particles, i.e. PA12, to (meth)acrylic polymer particles, i.e. PMMA particles, is 0.1 :1 to 10:1 , and is preferably 0.25:1 to 4:1 and more preferably 1.5:1. Advantageously, the preferred weight ratio enables achieving a good repartition of PMMA particles in polyamide, which will enhance the diffusion of the (meth)acrylic monomers comprised in the impregnating thermoplastic resin of the filament into the PMMA particles comprised in the tie layer.

[0089] The blend is controlled during the extrusion, by monitoring on the one hand some parameters such as screw profile, temperature, melting temperature and screw speed. On the other hand, and in this extrusion configuration, the main characterization of the blend is performed after termination of the extrusion, i.e. outside the outlet gate 30, said characterization is done by performing evaluation mechanical properties of the blend, notably (tensile and adhesion), glass transition of the blend (DSC), and domain size measurement (SEM).

[0090] However, the characterization of the blend can still be performed simultaneously during the extrusion using for instance a specific online spectrometer.

[0091 ] Figure 8 is a SEM micrograph of the PMMA I PA12 blend at 40/60 weight ratio.

[0092] The (meth)acrylic polymer particles of the blend, i.e. any of PMMA-co-MAA particles or PMMA-co-MAA-co-DMGAH particles, have an average size diameter (D50) of 0.1 pm to 10 pm, preferably from 2 pm to 8 pm, and more preferably of about 4 pm.

[0093] In reference to a right side of figure 8, a magnified partial section of the micrograph blend shows the PMMA particles, 32, being homogenously distributed in the polyamide PA12, 34, that is advantageously achieved by the performed extrusion by the twin-screw extruder, 16 of figure 7, along with the well equilibrated weigh ratio between PMMA and PA12 particles.

[0094] Another factor helping with the homogeneous repartition of PMMA particles 32 in the polyamide matrix 34, is the average size diameter (D50) of said PMMA particles 34 that is comprised between 0.1 pm to 10 pm. [0095] Preferably, the dimensions of the twin-screw extruder 16 of figure 7 can be adapted to reduce the size of dispersed polymer nodules.

[0096] Figure 9 is a representation of two cross sections of the composite filament of the invention.

[0097] In reference to the left-hand side of figure 9, a (not to scale) cross-section of the filament 36 is illustrated according to the invention, notably, the filament 36 can be obtained according to the above-described method 100, and according to any one of the two described embodiments (figures 2-3) and any of the two alternatives (figures 4-5).

[0098] The illustrated cross section displays several fibers 38, that are preferably dry carbon fibers 38, forming a (continuous) thread containing a number of fibers ranging preferably from 1000 to 3000. However, the number of carbon fibers comprised in the filament 36 can be less than 1000 fibers or it can extend beyond 3000 fibers.

[0099] The fibers 38 are impregnated by the liquid reactive thermoplastic resin 40, thus forming the impregnated thread 42. The filament 36 further comprises the co-extruded tie layer 44 wrapping the impregnated thread 42.

[00100] The cross section of figure 9 further displays an interphase 46 formed by a portion of both the thermoplastic resin 40 and the tie layer 44, the portions have been blended together forming the interpenetrated area 44 around the fibers 38.

[00101 ] The filament 36 of the invention differs from a known filament of prior art, notably, through the presence of the tie layer 44 and the specific materials employed in the composition of each of the thermoplastic resin 40 and in the tie layer 44.

[00102] This interphase 46 results from the fact that the resin 40 and the tie layer 44 are in contact with each other when being both in a (semi)liquid phase. Chemical and mechanical bond ensue.

[00103] Said bond can be well observed in the longitudinal cross section A-A of the filament 36 arranged at the right-hand side of figure 9. [00104] In reference to the right-hand side of figure 9, the interphase 46 of the filament 36 shows a mechanical bond linking the impregnated thread 42 with the tie layer 44. In fact, the mechanical bond is particularly initiated by a prior chemical bond, which is the diffusion of the (meth)acrylic monomers comprised in the thermoplastic resin 40 into the PMMA particles 32 comprised in tie layer 44.

[00105] The chemical diffusion mainly occurs right after co-extrusion of the tie layer 44 and before curing of the filament 36. The mechanical bond however, is obtained during curing, wherein the diffused (meth)acrylic monomers through the interphase 46 polymerize and a physical combination of PMMA particles 32 along with the thermoplastic resin 40 occurs, thus forming a (meth)acrylic polymer.

[00106] Advantageously, polymerization enables a great interlocking between the tie layer 44 and the impregnated thread 42, thus forming an overall improved morphology and a better adhesion quality between the former and the latter.

[00107] The blend of PMMA and PA12 of the invention, particularly the one described along with figure 8, provides enhanced mechanical properties to the composite filament. To highlight this improvement, the results of pull-off tests are presented in table 1 below. The pull-off tests have been realized with filament samples comprising different tie layer compositions.

[00108] Table 1 :

[00109] The realized pull-off tests consist of mechanically pulling the filament of the invention until its break point. The right column of table 1 thus lists the tensile strength of the various filaments. The results of said tests are mainly around 9 MPa when the filament comprises a blend of PMMA and PA12 in its tie layer composition, and around 7 MPa when using a compatibilizer. Thus, demonstrating the superior joining properties of the filament.

[00110] It should be noted that, a filament sample which does not comprises a tie layer, has a severely penalized pull-off test. In fact, no adhesion for said filament is possible during the test, which explains the weak result of 0 MPa.

[00111 ] Figure 10 is an illustration of a 3D printing process with the use of a 3D printing machine 48, commonly known as 3D printer 48, using the composite filament 36.

[00112] In reference to figure 10, the 3D printer 48 is configured to unwind a bobbin containing the filament 36 of the invention. An additional bobbin provides a polymer filament 50, which can be polyamide 6 or polyamide 12 or a mixture thereof, and preferably polyamide 12.

[00113] The 3D printer 48 can co-extrude the polymer filament 50 onto the filament 36 of the invention using a co-extrusion die 52. Thus, allowing a joint between the filament 36 of the invention and the polymer filament 50, resulting in the output of a melted filament 54 through the heated 3D printer nozzle 56, the latter is configured to operate displacements in the three dimensions with respect to a horizontal table 58.

[00114] As a result, a 3D-printed part 60 is formed from successive layers of the melted filament 54. Advantageously, the 3D-printed part 60 demonstrates an improved material formability and an overall improved material health, i.e. micro voids and air canals are avoided.

[00115] Advantageously, the end product 60 obtained by the use of the filament 36 provide enhanced mechanical properties, enabling utilization of said product 60 in a very physically demanding configuration, such as a structural application, or in an automotive application, transportation, or in industry in general.

[00116] In addition to the 3D printing process, a winding or weaving technique operated by a winding machine and using a tape made from the composite filament 36 can also be an application to the filament of the invention resulting in the creation of a winded structure. Advantageously, the winded structure is a lightweight composite structure.

[00117] Figure 11 is a representation of two cross-sections of the filament used in the 3D printing process of figure 10.

[00118] In reference to the left-hand side of figure 11 , a (not to scale) cross-section of the melted filament 54 is illustrated according to the invention, and wherein a shell material 62 can be seen wrapping the filament 36, said shell material 62 is obtained from the co-extrusion of the polymer filament 50 during 3D printing. Thus, the shell material 62 is also mainly made of polyamide, and preferably from PA12.

[00119] Preferably, the polyamide content of the shell material 62 is of at least 50 wt % of the entire shell material 62, and more preferably in an amount of at least 90 wt %, and even more preferably, the shell material 62 is fully made of polyamide. The amount of polyamide in the shell material 62 is identical to the amount of polyamide in the polymer filament 50 of figure 10.

[00120] The cross section of figure 11 further displays a connection 64 formed at the interface of the tie layer 44 and the shell material 62, where the tie layer 44 and the shell material 62 have been linked together.

[00121 ] This connection 64 results from the fact that the polyamide 34 of the tie layer 44 partially melts due to heating means in the nozzle, 56 of figure 10, when the latter gets in contact with the shell material that also contains polyamide. Thus, a chemical compatibility occurs and from which a mechanical bond ensues.

[00122] The connection 64 can be well observed in the longitudinal cross section B- B of the melted filament 54 arranged at the right-hand side of figure 11 .

[00123] In reference to the right-hand side of figure 11 , the connection 64 shows a mechanical bond linking the latter with the tie layer 44. In fact, the cooling of the melted filament 54 at room temperature right after the additive manufacturing process enables the shell material 64 along with the tie layer 44 to solidify. Thus, forming a hard and resistant connection 64. [00124] Advantageously, the connection 64 enables achieving an overall improved morphology and a better adhesion quality between the filament 36 and the shell material 64, which is directly beneficial to the end product.

[00125] These illustrations show that the tie layer functions as a tying element with the thermoplastic resin 40 on the one hand and with the polymer material 50 added during manufacturing on the other hand.

Claims

1 . Filament (36) for an additive manufacturing application or a winding application, the filament (36) comprising fibers (38) embedded in a reactive thermoplastic resin (40) comprising (meth)acrylic polymer, (meth)acrylic monomer and organic peroxides, said filament (36) further comprising a tie layer (44) coating the reactive thermoplastic resin (40), the tie layer (44) being constituted by a reactive blend of: at least a polyamide and a reactive copolymer, said reactive copolymer comprising a reactive (meth)acrylic polymer containing acids and/or anhydrides.

2. Filament (36) according to claim 1 , characterized in that the polyamide is polyamide 6 or polyamide 12 or a mixture thereof, said polyamide constituting 20 to 80 wt % of the tie layer (44).

3. Filament (36) according to any of claims 1 or 2, characterized in that the reactive (meth)acrylic polymer comprises poly(methyl methacrylate-co-methacrylic acid).

4. Filament (36) according to any of claims 1 -3, characterized in that the reactive (meth)acrylic polymer comprises poly(methyl methacrylate-co-methacrylic acid- co-di-methyl-glutaric anhydride).

5. Filament (36) according to any of claims 1 -4, characterized in that the weight ratio of polyamide particles to (meth)acrylic polymer particles is 0.25: 1 to 4: 1 and preferably 1.5:1.

6. Filament (36) according to any of claims 1 -5, characterized in that the (meth)acrylic polymer particles have an average size diameter (D50) of 0.1 pm to 10 pm, preferably from 2 pm to 8 pm.

7. Use of the filament (36) of any of claims 1 to 6 for manufacturing an end product (60) by an additive manufacturing process or by a winding technique.

8. Use according to claim 7, characterized in that prior to a deposition of the filament (36) to form the end product (60), the filament (36) is wrapped in a shell material (62) made of polyamide. Use according to claim 8, characterized in that the polyamide of the shell material (62) is polyamide 6 or polyamide 12 or a mixture thereof, said polyamide forming at least 90 wt % of the shell material (62). Product (60) obtained by the use of the filament (36) in accordance with any of claims 7 to 9. Method (100) for manufacturing a filament (36) aimed to an additive manufacturing application or a winding application, the method comprising:

(S102) providing a thread of fibers (38);

(S104) impregnating the thread with a liquid reactive thermoplastic resin (40) mainly composed of (meth)acrylic polymer, (meth)acrylic monomer and organic peroxides; and

(S106) co-extruding a tie layer (44) of thermoplastic compound around the impregnated thread (42); wherein the compound is constituted by a reactive blend of: at least a polyamide and a reactive copolymer, said reactive copolymer comprising a reactive (meth)acrylic polymer containing acids and/or anhydrides. Method (100) according to claim 11 , characterized in that the thermoplastic compound comprises a compatibilizer in an amount of 1 to 20 wt % in the tie layer (44), said compatibilizer is made of a reactive (meth)acrylic polymer or poly(styrene-co-glycidyl methacrylate) or poly(ethylene-co-methyl acrylate-co- glycidyl methacrylate). Method (100) according to any of claims 11 or 12, characterized in that the thermoplastic compound is co-extruded at a rate of about 10 meters per minute.