Recycling of pre-impregnated fiber materials

Abstract

A pre-impregnated material recycling process comprising the steps of: a) supplying at least one pre-impregnated material comprising a first partially crosslinked thermosetting polymer resin and a reinforcement comprising fibers, b) applying a vitrimerization reaction mixture to the at least one pre-impregnated material, c) placing the at least one pre-impregnated material coated with the vitrimerization reaction mixture onto a mold of a predetermined shape, d) applying thermocompression to the at least one pre-impregnated material at a temperature (T1) greater than or equal to the glass transition temperature (Tg) of the first resin, so as to activate the reaction between the vitrimerization reaction mixture and the thermosetting polymer leading to the formation of a vitrimer polymer and the transformation of the at least one pre-impregnated material into a vitrimer composite material of the predetermined shape. Figure 4

Description

Title of the invention: Recycling of fiber-reinforced pre-impregnated materials

[0001] The present invention relates to the field of pre-impregnated composite materials intended to form fiber-reinforced composite materials. In particular, the invention relates to a method for recycling unused pre-impregnated composite materials so as to enable them to be reshaped and / or repaired. According to a second aspect, the invention relates to an intermediate material comprising a pre-impregnated composite material coated with a vitrimerization reaction mixture, in particular obtained at the end of an intermediate step of the recycling method.

[0002] Fiber-reinforced composite materials are mainly used as a replacement for metals for lightening structures, and this in a wide range of applications: sport (cycles, board sports), transport (structure of cars, trains), aerospace (structure of airplanes, space shuttles). These systems consist of a thermosetting polymer resin, crosslinked during the shaping of the material, and a reinforcement, in particular a fiber armor, which can be of variable nature (mainly carbon fibers, glass fibers, polymer fibers, etc.) and organization (woven according to different models, non-woven).

[0003] One way of shaping these composites is to pre-impregnate the fiber armor with the non-crosslinked resin, so as to obtain a pre-impregnated composite material. This pre-impregnated composite is then covered with a protective film known as "backing", allowing the pre-impregnated to be rolled and transported in a reel to the place of use. Indeed, since the resin is not crosslinked at this stage, it has a sticky nature which prevents the pre-impregnated materials from being stacked and thus stored before use. At the time of use, the pre-impregnated rolls are cut with their backing to the correct size from the reels, then the pre-impregnated sheets without their backing are stacked so as to obtain the desired composite thickness and are hot-pressed to the desired shape.During heating, the resin crosslinks irreversibly, and the composite thus formed is then demolded.

[0004] While the use of pre-impregnated composites allows for great simplicity of implementation, such as the absence of resin handling by the user and the possibility of custom cutting, it nevertheless has its drawbacks. Firstly, the crosslinking reaction of the resins in pre-impregnated composites occurs at any temperature, although the lower the temperature, the slower the reaction progresses. This implies either storing the rolls of pre-impregnated materials under specific conditions, typically via refrigeration, or using them quickly once received. Thus, in practice, significant lengths and masses of materials Prepregs can be lost because, having exceeded their shelf life recommended by the supplier at a given storage temperature, the crosslinking reaction has advanced too far to allow them to be shaped. As a result, a large quantity of these prepregs becomes waste, causing a significant environmental issue. In addition, the cutting process for prepreg materials involves the existence of cutting scraps. These scraps are currently difficult to find applications, even if some are beginning to emerge at industrial levels, but which systematically lead to a significant devaluation of the material and are typically used for consumer goods.

[0005] One of the aims of the present invention is to overcome at least one of the aforementioned drawbacks. To this end, the present invention proposes a method for recycling pre-impregnated materials comprising the steps of:

[0006] a) providing at least one pre-impregnated material comprising a first thermosetting polymer resin, partially crosslinked, and an armor comprising fibers,

[0007] b) application of a vitrimerization reaction mixture to the at least one pre-impregnated material,

[0008] c) placing at least one pre-impregnated material coated with the vitrimerization reaction mixture on a mold of a predefined shape,

[0009] d) applying thermocompression to the at least one pre-impregnated material at a temperature Tl greater than or equal to the glass transition temperature Tg of the first resin, so as to activate the reaction between the vitrimerization reaction mixture and the thermosetting polymer leading to the formation of a vitrimer polymer and the transformation of the at least one pre-impregnated material into a vitrimer composite material of the predefined shape.

[0010] Thanks to the method of the invention, it is possible to recycle pre-impregnated materials, the first resin of which is partly crosslinked and which is out of use for reasons of aging (storage conditions in conditions which do not allow partial crosslinking of the first resin to be avoided and / or the storage duration given by the supplier is exceeded) or which comes from the offcuts of a pre-impregnated material. It is in fact possible to carry out hot reshaping or to locally repair the first resin using a vitrimerization reaction mixture which uses covalent dynamic chemistry leading to vitrimers whose viscoelastic properties are similar to those of glass.

[0011] Thus, the application of a mechanical force at a temperature Tl above or equal to the glass transition temperature Tg of the vitrimer polymer makes it possible to soften the prepregs in order to reshape or repair them.

[0012] According to one arrangement, the temperature T1 is between 180 and 280°C.

[0013] The terms 'recycling' or 'recycle' mean the reshaping and / or repair of crack(s) appearing in the first resin of the pre-impregnated material for a new use and the obtaining of a vitrimer composite material which can be reshaped or reshaped in turn by the presence of the vitrimer resin.

[0014] The terminology relating to a first thermosetting polymer resin, 'partially crosslinked' is well known to those skilled in the art of polymer and pre-impregnated materials as being a resin which retains accessible functions for crosslinking or for an ancillary reaction, here an exchange reaction for vitrimerization.

[0015] In particular, in partially crosslinked epoxy-amine resins, there are still available amine functions. These are the ones that allow the exchange reactions. There are also still available epoxy functions, but these can react directly with the added anhydrides. In addition, these partially crosslinked resins are still soluble in certain solvents, for example DMF, which is not the case when crosslinking is complete. It is in particular possible to dissolve between approximately 30% and 50% by mass in DMF in 24 hours at room temperature of the first resin of the prepreg from step a) of the process. This first partially crosslinked resin can effectively lead to vitrimerization.

[0016] The placement of the pre-impregnated material in the mold of a predefined shape, which may be a press plate, comprises the positioning of one or more pre-impregnated materials on the mold so as to orient the fibers in the desired manner and to superimpose several pre-impregnated materials until the desired thickness of composite material is reached.

[0017] According to one possibility, the recycling process comprises the steps of:

[0018] - e) arrangement of the vitrimer composite material shaped according to step d) on a second mold of a determined conformation, and

[0019] - f) applying thermocompression to the vitrimer composite material at said temperature Tl so as to reshape the vitrimer composite material.

[0020] It is thus possible to give a second life to the vitrimer composite material without devaluation or reduction of the mechanical properties of the initial material. By the expressions 'reshaping' or 'reshaping' is meant a remodeling, a reshaping in another conformation or repairing defects, such as cracks appearing in the vitrimer resin. This is achieved by softening the vitrimer resin and applying pressure to the composite.

[0021] According to one arrangement, the vitrimerization reaction mixture comprises precursor compounds of a second resin, such as an epoxy-amine, epoxy-anhydride, epoxy-phenol system or an epoxy-disulfide system, and a catalyst for transformation into vitrimer resin, chosen from an organometallic compound or organic, for example zinc acetate or a ternary amine.

[0022] This composition allows the thermoactivated exchange of functions in the first initial resin which leads to the transformation into vitrimer whose internal structure is a particular form of crosslinking, called "dynamic" or "reversible", where the covalent bonds between macromolecules can be undone and remade, with the possibility of interchanging certain molecules; their internal chemical network is permanent but its topology can be reorganized thanks to exchange reactions which maintain the number of bonds constant.

[0023] Concretely, the vitrimerization reaction mixture has a maximum viscosity of 2 Pas (2000 cP) so that the pre-impregnated material can be coated uniformly.

[0024] According to one possibility, the second resin comprises accessible hydroxyl groups, so as to allow function exchange reactions with the first thermosetting resin.

[0025] Concretely, the molar ratio of the epoxide functions to the anhydride functions is at least 1. Thus, the stoichiometry of the precursor compounds of the second resin is such that all of the epoxide groups can react, so that accessible hydroxyl groups remain on the network.

[0026] According to one characteristic, the proportion of transformation catalyst is between 2 and 15% by weight of the reaction mixture.

[0027] The vitrimer resin transformation catalyst can also act as a crosslinking accelerator for the thermosetting polymer of the prepreg. This results in an acceleration of vitrimerization.

[0028] Concretely, the application of the heat treatment of step d) is for example carried out in a temperature range Tl from 160°C to 280°C. This temperature range varies according to the glass transition temperature Tg which is linked to the nature of the thermosetting polymer but also to the fibers used and to the nature of the catalyst for transformation into vitrimer resin. The maximum temperature of the temperature range Tl is determined by the degradation temperature of the compounds (typically observed in ATG analysis). The volatility and stability of the components of the vitrimerization formulation can also limit this maximum temperature.

[0029] According to one characteristic, the pressure used in step d) is in a range from 2 to 20 MPa. The value of the optimal pressure depends on the shape, the surface quality, and the desired vitrimerization speed. High pressure improves vitrimerization (the covalent bonds are closer or “more in contact” and can exchange better).

[0030] According to one arrangement, the application of the reaction mixture is carried out by spraying rization on the at least one pre-impregnated material. For example, the reaction mixture and its spraying are carried out in an independent temperature range and between ambient and 80°C under an inert atmosphere or in air, and of course at a temperature lower than that of the crosslinking. The flow rate of the manual or automatic spraying can be between 0.01-0.1 mL / cm2 / s. According to this embodiment, the composition of the reaction mixture further comprises additives to adapt the mixture to the spray application method. The additives are in particular diluents or rheo-thickening additives. This application method advantageously makes it possible to control the quantity of mixture applied.

[0031] According to an alternative embodiment, the application of the vitrimerization reaction mixture is carried out by:

[0032] - i) soaking the at least one pre-impregnated material in a mixing bath vitrimerization action for coating said at least one pre-impregnated material,

[0033] - ii) filtration of the vitrimerization reaction mixture, and recovery of said at less a coated pre-impregnated material.

[0034] After filtration and drying, the coated prepregs can be stored without temperature conditions, used as is, or transported to their place of shaping.

[0035] According to a particular embodiment, the first resin of the at least one pre-impregnated material is made of epoxy-amine polymer, in particular DGEBA for the epoxy part and the amine is chosen from diethylenetetramine, a polyetheramine, an aromatic amine or a mixture of these compounds, and the vitrimerization reaction mixture is made up of DGEBA (acronym for Bisphenol A diglycidyl ether), phthalic anhydride, zinc acetate, and an acetone solvent. These reactive compounds of the reaction mixture are available from the supplier Sigma-Aldrich.

[0036] According to another example, the first resin of the at least one pre-impregnated material is made of phenolic resin, in particular a mixture of cardanol for phenol, citric acid, and formaldehyde, and the vitrimerization reaction mixture is made up of DGEBA (an English acronym for Bisphenol A diglycidyl ether), phthalic anhydride, zinc acetate, and an acetone solvent.

[0037] The process is also applicable to other resins, such as for example:

[0038] - polyester resins (epoxy-anhydride),

[0039] - BMI (bismaleimide) resins, and

[0040] - cyanate ester resins.

[0041] According to one arrangement, the method further comprises the steps of total recycling of: - g) placing the vitrimer composite material in a composition comprising at least one monofunctional alcohol, - h) apply a heat treatment at a temperature T2 greater than or equal to Tg so as to allow the dissolution of the vitrimer resin and the recovery of the fibers of the armor so as to recycle the compounds of the vitrimer composite material.

[0042] Thus treated, the vitrimer composite material is recycled and its recovered fibers can be reused. Preferably, the vitrimer composite material is completely recycled.

[0043] According to a second aspect, the invention further provides an intermediate material, which may in particular be obtained at the end of step b), or ii) of the recycling process, the intermediate material comprising a coated pre-impregnated material comprising a first partially crosslinked thermosetting polymer resin, a weave comprising fibers, the first resin being coated with a vitrimerization reaction mixture suitable for vitrimerization of the thermosetting polymer during the application of a heat treatment at a temperature Tl greater than or equal to the glass transition temperature Tg of the thermosetting polymer. This intermediate material has the advantage of being able to be reused as such in a reshaping step to lead to a vitrimer composite material, to be stored or to be transported to the shaping site without conditions of storage duration or temperature.The application of a temperature Tg is sufficient for the vitrimerization of the first thermosetting resin. It can be applied in conjunction with compression to obtain a very high quality composite material with a high fiber volume fraction.

[0044] According to other characteristics, the recycling method of the invention comprises one or more of the following optional characteristics considered alone or in combination: - Step d) of thermocompression is carried out over a period of 3 to 30 minutes. - Pre-impregnated material is a three-dimensional part, e.g. a film, a coating. - The armor is made of carbon fibers. - The armor is made of glass fibers. - The armor is made of natural fibers, including fibers of cellulose of various origins, chitosan. - The armor is made of synthetic fibers, particularly PA and PET. In this case, the heat treatments used in the process are limited by the melting temperature of the fibers. - The mold of a predefined shape is different or identical to the second mold of a specific conformation. - The epoxy-disulfide system used in the vitrimerization reaction mixture comprises a disulfide chosen from 2-aminophenyl disulfide, and compounds carrying silyl ether bonds such as 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyl disiloxane.

[0045] Other characteristics and advantages will appear on reading the detailed description below, of two non-limiting examples of implementation, made with reference to the appended figures in which:

[0046] [Fig. 1] represents an example of an ester function exchange reaction by transester fication in a dynamic covalent network according to one embodiment of the invention.

[0047] [Fig.2] represents an example of a disulfide function exchange reaction in a dynamic covalent network according to another embodiment of the invention.

[0048] [Fig.3] represents an example of a reaction of exchange of silyl ether functions in a dynamic covalent network according to yet another embodiment of the invention.

[0049] [Fig.4] represents the composition of a vitrimerization reaction mixture according to an embodiment of the invention.

[0050] [Fig.5] represents a first step of the anhydride-epoxy reaction from a alcohol.

[0051] [Fig.6] illustrates a second stage of the anhydride-epoxy reaction of the mixture re vitrimerization action.

[0052] [Fig.7] illustrates a function exchange reaction between an epoxy-anhydride system partially crosslinked and a free amine present in the epoxy-anhydride vitrimerization system according to one embodiment of the invention.

[0053] As will be seen in the detailed examples below, the invention consists in allowing the reuse of pre-impregnated materials that are no longer in use due to aging (storage outside ideal conditions, storage for too long), such as offcuts. To do this, the method consists in applying a reaction mixture of activator compounds to the pre-impregnated material which will integrate with the first pre-impregnated resin and will allow the final composite material to have the characteristics of a dynamic covalent network, capable of exchange reactions so as to allow it to be reshaped when hot, despite a significant state of crosslinking, by taking advantage of the use of dynamic covalent chemistry. Dynamic covalent networks, or vitrimer networks, are crosslinked polymer networks having the particularity of possessing thermosetting characteristics at low temperature and thermoplastic characteristics at high temperature.These characteristics arise from the presence of dynamic covalent chemical bonds, that is, . Exchange reactions can occur between chains and rearrange the network topology, while maintaining a constant crosslinking rate (refer to an example of an exchange reaction [Fig.l]). These exchanges are kinetically controlled and can only take place once the glass transition temperature of the polymer has been exceeded (conditions the mobility necessary for the reactions) and once the minimum temperature for the reaction to take place has been reached, linked to the activation energy of the exchange reaction.

[0054] The invention therefore consists in using the residual reactivity of the first incompletely crosslinked resin to integrate chemical motifs allowing the production of a dynamic covalent network. The added motifs are preferably similar to those of the first resin used in the pre-impregnated, such as epoxy-amine or epoxy-anhydride systems etc., or may be based on different exchange chemistries disulfide, silyl ethers - figures 1 to 3))

[0055] Typically, epoxy amines create amines, epoxy anhydrides create esters. In both cases, disulfide or silyl ether bonds are integrated into the existing epoxy-amine network. This can be done in particular by adding precursors carrying groups reactive with the epoxies or the existing amines in the first partially crosslinked resin (e.g.: alcohols or amines that can react with epoxy, anhydrides, epoxy that can react with amines)

[0056] Typically, if disulfide bonds are desired, a compound bearing a disulfide bridge and at least two amines or alcohols can be reacted to become incorporated into the existing resin and act as a dynamic crosslinker.

[0057] Example of implementation 1: application of a vitrimerization reaction mixture by impregnation via a solvent bath.

[0058] A vitrimerization reaction mixture (figures 4 to 6) is formed from:

[0059] - 500 ml of acetone,

[0060] - 25 g of DGEBA II epoxy (0.073 mol),

[0061] - 16 g of phthalic anhydride I (0.108 mol), and

[0062] - 10 g of zinc III acetate (0.045 mol)

[0063] Then 50 g of pre-impregnated materials reinforced with carbon fibers, based on a first epoxy-amine resin, stored at room temperature for more than 30 days, with dimensions of 1 cm by 2 cm, are immersed in the bath of the reaction mixture described above at room pressure and temperature.

[0064] After 24 hours of immersion with gentle stirring at room temperature, the pre-impregnated materials are filtered from the reaction mixture, then dried at room temperature for 4 hours.

[0065] Two pre-impregnated materials are then placed against each other, then between 2 steel plates covered with fibered Teflon. The whole thing is placed in a hot press, heated to 240°C, under 10 MPa of pressure, for a period of 5 minutes. The pressure is then removed, and then the two pre-impregnated materials are recovered. Adhesion is then observed between the two pre-impregnated materials, without apparent destruction of the fibrous structures. The first thermosetting resin having now crosslinked and being vitrimerized, a vitrimer composite material is obtained. According to another possibility, at least two pieces of pre-impregnated materials are juxtaposed two by two in the mold so as to enlarge the surface of the final vitrimer composite material. According to yet another possibility, at least two pieces of pre-impregnated materials are arranged on top of each other so as to give the desired shape and properties to the final vitrimer composite.

[0066] The vitrimer composite material thus formed is then reshaped under stress and temperature. The assembly of 2 prepregs is placed on a mold having an angle at 90°C, and heated to 240°C. A stress is gradually applied to the composite until it reaches 10 MPa. The vitrimer composite is capable of deforming without breaking until it matches the angle of the mold. It retains this shape after cooling.

[0067] For comparison, when 2 pieces of pre-impregnated materials not previously immersed in the vitrimerization reaction mixture are compressed under the same conditions, numerous cracks are visible on the surface due to the detachment of the crosslinked resin from the fibers.

[0068] Example of implementation 2: application of the vitrimerization reaction mixture by spraying

[0069] A vitrimerization reaction mixture (figures 4 to 6) is formed from:

[0070] - 50 ml of acetone,

[0071] - 25 g of DGEBA II (0.073 mol)

[0072] - 16 g of phthalic anhydride I (0.108 mol) and

[0073] - 10 g of zinc III acetate (0.045 mol)

[0074] This solution is then sprayed onto the surfaces of two pre-impregnated materials reinforced with carbon fiber, based on a first epoxy-amine resin, stored at room temperature for more than 30 days, with dimensions of 1 cm by 2 cm.

[0075] The two pre-impregnated materials are then hot-compressed as described in Example 1. A vitrimer composite material is obtained, without the presence of defects, cracks, or fiber breakage. It can be reshaped in a second mold of a predefined conformation for its future application.

[0076] According to other possibilities not described in the detailed examples, the vitrimerization reaction mixture can be made up of other systems allowing functional exchanges and dynamic covalent chemistry, such as epoxy / phenol, epoxy / amine ([Fig.7]), epoxy / disulfide systems.

[0077] Thus, the present invention has the advantage of allowing the reuse of pre-impregnated materials no longer in use or cutting waste and in particular the production of a vitrimer composite material which can be reshaped several times during its lifetime. The invention therefore allows, by a simple to implement and inexpensive process, the reuse of pre-impregnated waste which is not highly valued and the production, with this waste, of a fully recyclable composite material, unlike existing processes.

Claims

Claims

1. A method for recycling pre-impregnated materials comprising the steps of: - a) providing at least one pre-impregnated material comprising a first, partially crosslinked thermosetting polymer resin and a weave comprising fibers, - b) applying a vitrimerization reaction mixture to the at least one pre-impregnated material, - c) placing the at least one pre-impregnated material coated with the vitrimerization reaction mixture on a mold of a predefined shape, - d) applying thermocompression to the at least one pre-impregnated material at a temperature (Tl) greater than or equal to the glass transition temperature (Tg) of the first resin,so as to activate the reaction between the vitrimerization reaction mixture and the thermosetting polymer leading to the formation of a vitrimer polymer and the transformation of the at least one pre-impregnated material into a vitrimer composite material of the predefined shape.,

2. Recycling method according to claim 1 comprising the steps of: - e) placing the vitrimer composite material shaped according to step d) on a second mold of a determined conformation, - f) applying thermocompression to the vitrimer composite material at said temperature (Tl) so as to reshape the vitrimer composite material.

3. A recycling method according to one of the preceding claims, wherein the vitrimerization reaction mixture comprises precursor reagents so as to form a second resin comprising accessible hydroxyl groups so as to allow function exchange reactions with the first thermosetting polymer resin.

4. Recycling method according to one of the preceding claims, in which the vitrimerization reaction mixture comprises precursor reagents of a second resin, such as an epoxy-amine system, epoxy-anhydride, or an epoxy-disulfide system, and a catalyst (III) for transformation into vitrimer resin, chosen from an organometallic or organic compound, for example respectively zinc (III) acetate or a ternary amine.

5. Recycling process according to claim 4, wherein the proportion of catalyst (III) is between 2 and 15% by weight of the vitrimerization reaction mixture.

6. Recycling method according to one of the preceding claims, in which the application of the vitrimerization reaction mixture is carried out by spraying onto the at least one pre-impregnated material.

7. Recycling method according to one of claims 1 to 5, in which the application of the vitrimerization reaction mixture is carried out by: - ​​i) dipping the at least one pre-impregnated material in a bath of vitrimerization reaction mixture for coating said at least one pre-impregnated material, - ii) filtration of the vitrimerization reaction mixture, and recovery of said at least one coated pre-impregnated material.

8. Recycling method according to one of claims 1 to 7, in which the first resin of the at least one pre-impregnated material is made of epoxy-amine polymer and the vitrimerization reaction mixture consists of DGEBA (II), phthalic anhydride (I), zinc acetate (III), and an acetone solvent.

9. Recycling method according to one of the preceding claims, which comprises the total recycling steps of: - g) placing the vitrimer composite material in a composition comprising at least one monofunctional alcohol, - h) applying a heat treatment at a temperature (T2) greater than or equal to (Tg) so as to allow the dissolution of the vitrimer resin and the recovery of the fibers of the armor so as to recycle the compounds of the vitrimer composite material.

10. An intermediate material comprising a coated prepreg material comprising a first partially crosslinked thermosetting polymer resin, an armor comprising fibers, the first resin being coated with a vitrimerization reaction mixture suitable for vitrimerization of the thermosetting polymer during the application of a heat treatment at a temperature (Tl) greater than or equal to the glass transition temperature (Tg) of the thermosetting polymer.