Recyclable composite materials and recyclable curable polymer compositions

Abstract

Herein described is a recyclable composite material, the composite material comprising: a curable polymer composition comprising: i) a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; and ii) a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; and a fibre reinforcing material. Herein is also described methods of recycling said recyclable composite material and a kit of parts for preparing a recyclable composite material. The present invention is also directed towards a recyclable curable polymer composition and a method for forming a recyclable curable polymer composition.

Description

RECYCLABLE COMPOSITE MATERIALS AND RECYCLABLE CURABLE POLYMER COMPOSITIONSFIELD OF THE INVENTION

[0001] The present invention is directed towards a recyclable composite material and a method of recycling said recyclable composite material and a kit of parts for preparing a recyclable composite material. The present invention is also directed towards a recyclable curable polymer composition and a method for forming a recyclable polymer composition.BACKGROUND OF THE INVENTION

[0002] High performance thermoset plastics and polymers provide lightweight, durable, and mechanically robust materials for a wide range of industrial applications such as high durability coatings, structural composites, lightweight components for aerospace and automotive application, and structural adhesives.

[0003] Thermoset polymers, which make up ~20 % of all polymeric materials, are based on high density crosslinked polymer networks traditionally assembled using non-reversible covalent bonding. The covalent network structure of thermoset polymers renders them singleuse, and non-recyclable in their current form. Epoxy resins are one of the leading thermoset technologies for industrial applications, due to their excellent mechanical strength, versatility, and robustness. Over 323,000 tonnes of epoxy resin is produced in Europe per annum. A major limitation of epoxy resin technology is the limited opportunity for chemical or mechanical recycling. This issue is highlighted in applications such as wind turbine blades, where it is estimated that -250,000 tonnes of epoxy thermoset polymers reside that are destined for landfill or incineration once the wind turbines have reached the end of their useful life. Without lightweight, structural epoxy-thermosets, renewable wind-energy would not be possible, yet their inherent, almost indestructible, molecular structure renders them unsuitable for conventional recycling processes. Therefore, further, curable polymer compositions are required.

[0004] The present inventors found it was desirable to provide a cured / curable polymer composition that closely resembles a thermosettable / thermoset polymer composition, by having versatile material performance and advantageous mechanical properties, but that is able to be recycled, and incorporated into a recyclable composite material with ease.

[0005] As such, the present inventors aimed to overcome the problems of the prior art, and provide recyclable composite materials comprising curable polymer compositions, where therecyclable composite material may be recycled (i.e. molecule recovery and separation from mixed material) with ease, whilst still having versatile material performance and advantageous mechanical properties.SUMMARY OF THE INVENTION

[0006] At its most general, the present invention provides a recyclable composite material comprising a curable polymer composition comprising a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more and a primary amine, and a fibre reinforcing material. The present invention also provides a method of recycling said recyclable composite material, a recyclable curable polymer composition formed from an epoxy resin precursor having an epoxy functionality of 2 or more, methods for forming both the recyclable polymer composition and recyclable composite material, and a kit of parts for preparing a recyclable composite material.

[0007] The present inventors have realised that a recyclable composite material comprising a cured / curable polymer is desired, due to the significant environmental and scientific challenges currently surrounding the recycling of cured materials. In particular, the present inventors have realised that a recyclable composite material comprising a cured / curable polymer that has similar mechanical properties to a thermoset polymer is desired.

[0008] Previously, epoxy resins have been chemically modified to produce dynamic cured materials; however, such materials have not benefitted from the high glass transition temperature or other properties desirable for recyclable composite materials. Further to this, an efficient process for the chemical or mechanical recycling of such modified materials has not been demonstrated, the present invention aims to address these and the above-mentioned challenges.

[0009] Surprisingly, the present inventors have found that a recyclable composite material comprising a curable polymer composition comprising a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more and a primary amine, and a fibre reinforcing material, can be recycled to provide recycled fibre reinforcing material in a reusable form that is free from polymer. Even more surprisingly the present inventors have found that the composite materials described herein can be recycled to provide undamaged fibre reinforcing material that is comparable to the virgin reinforcing fibre. Current processes for recovering fibre, such a pyrolysis, are unable to provide fibres in a good and useable condition.

[0010] In a first aspect, the invention provides a recyclable composite material, the recyclable composite material comprising: a curable polymer composition comprising: i) a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; and ii) a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; and a fibre reinforcing material.

[0011] The present inventors surprisingly found that the combination of a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more and a primary amine, and a crosslinking agent comprising boronic acid and / or boronic ester groups resulted in a curable polymer composition that can be used to form a composite material that can be recycled in a simple and efficient process.

[0012] The present inventors have surprisingly found that the recyclable composite material was able to be recycled in a chemoselective process to separate the cured polymer composition and fibre reinforcing material to provide reusable fibre reinforcing material. Even more surprisingly, the present inventors found the recyclable composite material was able to be recycled to provide undamaged fibre reinforcing material that was free from polymer, and comparable to the virgin fibres. This was particularly surprising as current state-of-the-art solvolysis and pyrolysis techniques commonly used to attempt to recycle cured composites are unable to provide undamaged polymers and fibres. Surprisingly, the present inventors found that the crosslinking of the cured polymer composition was reversible and allowed for facile separation from the fibre reinforcing material, to provide closed-loop recycling. Furthermore, the recyclable composite material was found by the present inventors to display good chemical resistance, as it remained undamaged and insoluble upon exposure to chemically aggressive liquids.

[0013] The present inventors surprisingly found that recyclable composite materials according to the present invention, which have chemical recyclability designed into their molecular structure, offer an opportunity to replace existing single-use materials with more sustainable alternatives.

[0014] In a second aspect, the present invention provides a method of recycling the recyclable composite material according to the first aspect, the method comprising: providing a solution comprising a polyol, or monofunctional boronic ester, ormonofunctional boronic acid; putting the recyclable composite material into the solution; and removing the cured polymer composition from the fibre reinforcing material with the solution.

[0015] In embodiments, the polyol is a diol.

[0016] The present inventors found that such a method was chemoselective, and able to simply separate the cured polymer composition and fibre reinforcing material to provide reusable fibre reinforcing material. Even more surprisingly, the present inventors found the reusable undamaged fibre reinforcing material was free from polymer, and comparable to the virgin fibres.

[0017] Beneficially, such a method of recycling the recyclable composite material may be a scalable and versatile approach for tackling the issue of recyclability of cured / curable polymer compositions and composites comprising such cured / curable polymer compositions.

[0018] In a third aspect, the present invention provides a recyclable curable polymer composition, the composition comprising: i) a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; and ii) a primary amine; and a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; wherein the polyfunctional epoxy resin precursor comprises an aromatic epoxy resin precursor.

[0019] The present inventors surprisingly found that such curable polymer compositions may be used to produce recyclable polymer composites. Such recyclable polymer composites may be able to be recycled to provide re-usable fibre reinforcing materials. Advantageously, recyclable polymer composites comprising such curable polymer compositions may be recycled to provide undamaged fibre reinforcing materials free from polymer and comparable to virgin reinforcing fibres.

[0020] Furthermore, the present inventors found that the curable polymer compositions can be further improved by providing a composition comprising an aromatic polyfunctional epoxy resin precursor. This allows for the production of improved curable polymer compositionswhich have improved physical properties such as improved Tgand / or improved Young’s moduli.

[0021] The present inventors also found that control of the molecule architecture through the polymerisation process resulted in the development of recyclable curable polymer composition with varied physical properties. For example, some of the recyclable curable polymer compositions displayed good flowability, making them suitable for applications in pre-pregs and laminates, whilst other recyclable curable polymer compositions resisted flow for optimal shape stability even at elevated temperatures. Composite materials formed from both flowable and flow resistant polymer compositions according to the present invention were recyclable.

[0022] In embodiments, the ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine is from about 1 :2 to about 2:1. The present inventors found that the degree of polymerisation of the epoxyamine component in the curable polymer composition may be controlled by the presence of a polyfunctional epoxy resin precursor and the ratio of epoxide groups to NH groups.

[0023] Previously, the maximum degree of polymerisation and molecular weight of the epoxyamine has been controlled by using monofunctional epoxy resin ethers, such as phenyl glycidyl ether, for example, in ANDERSON Lynn, SANDERS, Edward W., UNTHANK, Matthew G, Recyclable thermosets based on modified epoxy-amine network polymers, Mater. Horiz., 2023, 10, pages 889 to 898, where the monofunctional epoxy resin ether is used to control polymerisation.

[0024] The present inventors have found that materials produced using a monofunctional epoxy resin ether to limit polymerisation have relatively low glass transition temperatures (Tg), for example below about 60 °C, and relatively low Young’s moduli, for example below about 150 MPa. Recyclable curable polymer compositions according to the invention were found by the present inventors to result in cured polymer compositions having increased Tgand Young’s moduli.

[0025] In a fourth aspect, the present invention provides a kit of parts for preparing a recyclable composite material, the kit comprising: a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; a primary amine; a crosslinking agent comprising boronic acid and / or boronic ester groups; and a fibre reinforcing material.

[0026] In a fifth aspect, the present invention provides a method for forming a recyclable composite material, the method comprising: a) providing a recyclable curable polymer composition according to the first or third aspects of the present invention; b) providing a fibre reinforcing material; c) contacting the recyclable curable polymer composition and fibre reinforcing material to form a recyclable composite material.

[0027] In a sixth aspect, the present invention provides a method for forming a recyclable polymer composition, the method comprising: a) providing a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; b) combining a crosslinking agent comprising boronic acid and / or boronic ester groups with the polyfunctional epoxy resin precursor and primary amine to form a curable polymer composition; wherein the polyfunctional epoxy resin precursor comprises an aromatic epoxy resin precursor.

[0028] The inventors found the method allows for the use of widely used and structurally diverse polyfunctional epoxy resins, polymerised with equally diverse amines. The present inventors surprisingly found that when the formed polymers are crosslinked with boronic acid and / or boronic ester groups (to form cured materials), a range of tailored material types are accessible, with tuneable physical properties.

[0029] Furthermore, recyclable curable polymer compositions prepared by this method were also found by the present inventors to result in cured polymer compositions having increased Tgand Young’s moduli.

[0030] In embodiments, the ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine is from about 0.5:1 to 2:1.

[0031] Surprisingly, the present inventors found the degree of polymerisation of the epoxyamine component in the curable polymer composition may be controlled by the ratio of epoxide groups in the polyfunctional epoxy resin precursor to primary amine (R-NH2).

[0032] Unlike ‘traditional’ epoxy-amine thermosets, the present inventors found this method allowed the polymer backbone to be based on epoxyamine polymerisation, generating p- amino diol groups, which are subsequently complexed to form dioxazaborocane crosslinks.

[0033] The present inventors also found the above method allows for the reduction or removal of monofunctional epoxy resins such as phenyl glycidyl ether from the polymer composition.

[0034] Certain embodiments of the present invention may provide one or more of the following advantages:• cured polymer compositions with an improved Tg;• cured polymer compositions with an improved Young’s modulus;• reduction, or in some cases negation of use of monofunctional epoxy resin ethers, such as phenyl glycidyl ether;• versatile cured / curable polymer compositions;• desirable and tuneable physical properties;• industrially applicable process;• chemoselective process;• desired ease of recycling;• desired ease of synthesis; and• good chemical resistance;• tack-free polymer-fibre composites having flow properties such that they may be moulded or re-moulded;• repairable or re-mouldable polymer-composites.

[0035] The details, examples and preferences provided in relation to any particular one or more of the stated aspects of the present invention apply equally to all aspects of the present invention. Any combination of the embodiments, examples and preferences described herein in all possible variations thereof is encompassed by the present invention unless otherwise indicated herein, or otherwise clearly contradicted by context.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The disclosure will further be illustrated by reference to the following figures:

[0037] Figure 1 shows variations in Young’s modulus displayed by cured polymers according to the present invention;

[0038] Figure 2 shows a gel permeation chromatogram of a recyclable composite material, according to the present invention, after being placed in THF for 48 hours;

[0039] Figure 3 shows a gel permeation chromatogram of a recyclable composite material, according to the present invention, after being placed in a solution of THF and excess pinacol for 48 hours;

[0040] Figure 4 shows the recyclable composite material of Figure 3, after removal from the THF and pinacol solution;

[0041] Figure 5 shows a recyclable composite material, according to the present invention, prepared by a pre-preg method.

[0042] Figure 6 shows the recyclable composite material of Figure 5, after being placed in a solution of THF and excess pinacol for 8 days.DETAILED DESCRIPTION

[0043] The present invention is based on the surprising finding that using a combination of a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more and a crosslinking agent comprising boronic acid and / or boronic ester groups surprising provides a curable polymer composition with chemical recyclability designed into its molecular structure.

[0044] The use of such a curable polymer composition surprisingly resulted in a recyclable composite material that may be recycled in a simple and efficient manner to provide reusable fibre reinforcing material. Surprisingly, the fibre reinforcing material was found to be undamaged, free from polymer, and comparable to the virgin reinforcing fibre material.

[0045] The present inventors also found that improved physical properties, including an improved Young’s modulus and improved Tgsuitable for recycling were able to be achieved. Additionally, the present inventors found that the degree of polymerisation of the epoxy-amine component may be able to be controlled by the ratio of epoxide in the polyfunctional epoxy resin precursor to primary amine.

[0046] As used herein, the term “recyclable” in e.g., recyclable composite material, refers to a composite material that may be put through a process such that parts or the whole of the composite material may be reused again to form a new product. For example, a recyclable composite material includes where fibres from a composite material may be recovered and reused.

[0047] As used herein, the term “epoxy resin precursor” in e.g., a polyfunctional epoxy resin precursor, refers to a compound that participates in a chemical reaction to produce an epoxy resin. For example, an epoxy resin precursor is a compound used for the production of epoxy resins. For example, Bisphenol A is an epoxy resin precursor.

[0048] As used herein, the term “thermoset” in e.g., a thermoset polymer composition, refers to a cured or partially cured polymer composition comprising a permanently crosslinked polymer.

[0049] As used herein, the term “curable” in e.g., a curable polymer composition, refers to a polymer composition comprising a polymer and crosslinking agent that is curable into a partially cured or cured polymer. The crosslinking, and hence curing is reversible. The reversibility is due to the incorporation of dynamic covalent chemistry.

[0050] As used herein, the term “cured” in e.g., a cured polymer composition, refers to a cured or partially cured polymer composition. Specifically, a polymer composition comprising reversible dioxazaborocane crosslinks between polymer chains. In absence of a stimulus, these cured polymer compositions behave as thermosets, showing high chemical resistance and dimensional stability, but when the stimulus is applied, the dynamic bonds become activated, enabling the polymer composition to rearrange its topology on a molecular level. The crosslinking in the cured polymer composition is reversible. In other words, the cured polymer composition is a covalent adaptable network.

[0051] As used herein, the term “tack free” in e.g., tack free polymer, refers to a polymer that no longer feels sticky to the touch. T ack may be measured by ASTM standards ASTM-D8336- 24 and ASTM D6195-22.

[0052] As used herein, the term "monofunctional" in e.g. monofunctional epoxy resin, means that the epoxy resin includes one reactive epoxy group per molecule.

[0053] As used herein, the term "difunctional" in e.g. difunctional epoxy resin, means that the epoxy resin includes two reactive epoxy groups per molecule.

[0054] As used herein, the term "polyfunctional" in e.g. polyfunctional epoxy resin means that the epoxy resin includes two or more reactive epoxy groups per molecule.

[0055] As used herein, the term "aliphatic" in e.g. aliphatic epoxy resin precursor means that the epoxy resin does not comprise an aromatic functional group. Aliphatic epoxy resin precursors may be linear, branched or cyclic compounds. For example, the aliphatic epoxy resin precursor may be butanediol diglycidyl ether.

[0056] As used herein, the term "aromatic" in e.g. aromatic epoxy resin precursor means an epoxy resin comprising an aromatic functional group, an aromatic group being an organic compound with a conjugated planar ring system and delocalized pi-electron clouds. For example, the aromatic epoxy resin precursor may be epoxidized bisphenol A resin.

[0057] As used herein, the term " crosslinking agent " means a molecule that comprises two or more reactive functional groups that links two polymer chains by covalent, dative covalent or ionic bonds. In particular, the crosslinking agents have boronic acid and / or boronic ester groups and form dioxazaborocane crosslinks between two polymer chains.

[0058] As used herein, the term "alkyl" means all variants possible for each number of carbon atoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1 ,1- dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, etc.

[0059] As used herein, the term "substituted", in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

[0060] As used herein, the term “glass transition temperature” and “Tg” refers to the range of temperatures at which a material transitions from a brittle or glassy state to a viscous or rubbery state. The glass transition temperatures referred to here represent glass transition temperature midpoint values unless otherwise stated.

[0061] As used herein, the term ““tensile testing” or “Young’s modulus” refers to a measurement of stiffness, which is the ratio (GPa or MPa) of tensile stress to strain when a force is applied to a material.

[0062] As used herein, the term “free from monofunctional epoxy resin precursor” refers to polymer compositions where monofunctional epoxy resin precursor has not been added to the composition to achieve a particular effect. For example, where the monofunctional epoxy resin precursor is present in the polymer composition in an amount of less than about 0.5 wt.%, or less than about 0.3 wt.%, or less than about 0.2 wt.%, or less than about 0.1 wt.%, or less than about 0.05 wt.%, or less than about 0.01 wt.%, or less than about 0.005 wt.% based on the total weight of the polymer composition.

[0063] As used herein, the term "about" when referring to a number or a numerical range means that the number or numerical range referred to is an approximation within experimental variability (or within statistical experimental error), and thus the number or numerical range may vary. Typical experimental variabilities may stem from, for example, changes andadjustments necessary during scale-up from laboratory experimental and manufacturing settings to large scale as is known to those familiar with the art of manufacturing. Such changes can vary between 1% and 10% of the stated number or numerical range.

[0064] As used herein, the term "comprising" (and related terms such as "comprise" or "comprises" or "having" or "including") has an open meaning and therefore a composition comprising described features may comprise additional components in addition to the described features.

[0065] When ranges are used herein, all combinations and sub-combinations of ranges and specific embodiments therein are intended to be included.

[0066] Abbreviations used herein have their conventional meaning within the chemical and biological arts, unless otherwise indicated.

[0067] The features described below may be included in any one of the first to sixth aspects of the present invention as appropriate. Any combination of the features described herein in, and all possible variations thereof are encompassed by the present invention unless otherwise indicated herein, or otherwise clearly contradicted by context.Recyclable Composite Material

[0068] The present invention provides a recyclable composite material, the composite material comprising: a curable polymer composition comprising: i) a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; and ii) a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; and a fibre reinforcing material. Curable Polymer Composition

[0069] The composite material comprises a curable polymer composition. In other words, the composite material comprises a polymer composition that is curable to form a cured or partially cured polymer composition. The curable polymer composition may be cured to form a covalent adaptable network.

[0070] The curable polymer composition comprises a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more and a primary amine, and a crosslinking agent comprising boronic acid and / or boronic ester groups.

[0071] The polymer is reversibly crosslinkable by the crosslinking agent. When cured, the polymer is reversibly crosslinked by the crosslinking agent to form a cured polymer composition. The polymer may be reversibly crosslinked by dioxazaborocane bonds.

[0072] In embodiments, the curable polymer composition may be described as comprising: a) a polymer formed from a primary compound and a secondary compound; and b) a tertiary compound; where the primary compound is a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and the secondary compound is a primary amine; and wherein the tertiary compound is a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent.

[0073] In embodiments, the primary, secondary and tertiary compounds are separate distinct compounds. The primary, secondary and tertiary compounds may be described as chemical distinct entities. In other words, the polyfunctional epoxy resin precursor, primary amine and crosslinking agent are separate distinct entities.

[0074] In embodiments, the primary amine nor the polyfunctional epoxy resin precursor contain boronic acid or boronic ester groups. In other words, the primary amine and polyfunctional epoxy resin precursor are free from boronic acid or boronic ester groups.

[0075] The primary, secondary and tertiary compounds may be any combination of the polyfunctional epoxy resin precursor, primary amine and crosslinking agent described herein.

[0076] In other words, the curable polymer composition comprises a) a polymer formed from i) a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and ii) a primary amine; and b) a crosslinking agent comprising boronic acid and / or boronic ester groups, where the polymer is reversibly crosslinkable by the crosslinking agent.

[0077] The curable polymer composition may be a viscous homogeneous mixture. In some embodiments, the curable polymer composition may be formed as a polymeric film. The polymeric film may be a liquid or solid film, for example, the curable polymer composition may comprise a viscous liquid film.Polyfunctional Epoxy Resin Precursor

[0078] The curable polymer composition comprises a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more. For example, the polyfunctional epoxy resin precursor may have di or tri epoxy functionality. In an embodiment, the polyfunctional epoxy resin precursor has a functionality of 2.

[0079] The polyfunctional epoxy resin precursor may be any suitable polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more.

[0080] In some embodiments, the polyfunctional epoxy resin precursor comprises an aromatic polyfunctional epoxy resin precursor. The use of an aromatic epoxy resin precursor may be beneficial as aromatic epoxy resins commonly have excellent high temperature resistance, good corrosion resistance and low viscosity. In alternative embodiments, the polyfunctional epoxy resin precursor is an aliphatic epoxy resin precursor.

[0081] In some embodiments, the polyfunctional epoxy resin precursor comprises a glycidyl ether moiety.

[0082] In embodiments, any suitable alcohol, diol, triol or tetraol or combinations thereof may be epoxidized to form a suitable polyfunctional epoxy resin precursor.

[0083] In some embodiments, the polyfunctional epoxy resin precursor comprises a hydrocarbyl moiety comprising up to 22 carbon atoms.

[0084] Examples of suitable polyfunctional epoxy resin precursors having an epoxy functionality of 2 or more include, but are not limited to, bisphenol-A type epoxy resin precursors, bisphenol-F type epoxy resin precursors, phenol or cresol novolac type epoxy resin precursors, cycloaliphatic epoxy precursors such as bis-(3,4-epoxycyclohexyl)-adipate, ether derivatives precursors including diol derivatives such as 1 ,4-butanediol diglycidylether, pentaerythritol, and neopentyl glycol diglycidylether, glycidyl ethers precursors such as poly glycol diglycidyl ether, epoxidized aliphatic diol precursors, epoxidised trimethylolpropane and epoxidised pentaerythritol, or combinations thereof . It will be understood that the above list is by way of example and that other suitable epoxy materials are available.

[0085] In some embodiments, the polyfunctional epoxy resin precursor may be selected from a group comprising: an epoxidized phenol-acetone resin precursor, a diglycidylether of bisphenol A epoxy resin precursor, a diglycidylether of bisphenol F epoxy resin precursor, an epoxidized phenol-formaldehyde resin precursor, an epoxidized novolac resin precursor, an epoxidized cresol-formaldehyde resin precursor, or a combination thereof.

[0086] In preferred embodiments, the polyfunctional epoxy resin precursor may comprise butanediol glycidyl ether (BGE) or epoxidized bisphenol A resin (DGEBA), or a combination thereof.

[0087] In some embodiments, the polyfunctional epoxy resin precursor may comprise more than one epoxy resin precursor having an epoxy functionality of 2 or more. For example, in some embodiments, the polyfunctional epoxy resin precursor may comprise at least a first anda second epoxy resin precursor. The first and second epoxy resin precursors may be any suitable polyfunctional epoxy resin precursor as detailed above.

[0088] In embodiments, the first and second epoxy resin precursors are different polyfunctional epoxy resin precursors to one another.

[0089] In an embodiment, the first epoxy resin precursor comprises an aromatic epoxy resin precursor and the second epoxy resin comprises an aliphatic epoxy resin precursor. For example, in an embodiment, the first epoxy resin precursor may comprise a bisphenol A epoxy resin precursor, such as Bisphenol A diglycidyl ether, and the second epoxy resin precursor may comprise a difunctional epoxy resin, such as 1,4- Butanediol diglycidyl ether.

[0090] In embodiments, where the polyfunctional epoxy resin precursor comprises at least a first and a second epoxy resin precursor, and the first epoxy resin precursor has a functionality of 2, and the second epoxy resin precursor has an epoxy functionality greater than 2, the molar ratio of first epoxy resin precursor to second epoxy resin precursor may be from about 1 :1 to about 40:1.

[0091] In some embodiments, the polymer is formed solely, or substantially solely, from the polyfunctional epoxy resin precursor. In embodiments, the polymer is not formed from a monofunctional epoxy resin precursor. In such embodiments, the polymer is free from a monofunctional epoxy resin.

[0092] The present inventors found that using a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and no, or little monofunctional epoxy resin was particularly beneficial, as it resulted in cured polymers with improved Tgand improved Young’s modulus.

[0093] Monofunctional epoxy resin precursors are commonly used to control the degree of polymerisation. The use of a majority of polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more allows for the amount of monofunctional epoxy resin precursors to be reduced, as the degree of polymerisation of the epoxy-amine component in the curable polymer composition is able to be controlled using the ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine. This may be advantageous in cases where the monofunctional epoxy resin precursor is toxic.Monofunctional Epoxy Resin Precursor

[0094] In some embodiments, the polymer is formed from a monofunctional epoxy resin precursor. In other words, in embodiments, the polymer is formed from a polyfunctional epoxy resin precursor and a monofunctional epoxy resin precursor.

[0095] In embodiments, the molar ratio of epoxy groups derived from a polyfunctional epoxy resin precursor to epoxy groups derived from a monofunctional epoxy resin precursor may be from about 1 :1 to 40:1 , for example the molar ratio of polyfunctional epoxy resin precursor to monofunctional epoxy resin precursor may be about 40:1 , 20:1 , 10: 1, 4:1 , 2:1 or 1 :1. For example, where the molar ratio of epoxy groups derived from a diepoxide to epoxy groups derived from a monoepoxide is 1 :2, 50 % of the epoxide groups may be from a monofunctional epoxy resin precursor and 50% of the epoxide groups may be from a difunctional epoxy resin precursor. The present inventors speculate higher amounts of monofunctional epoxy resin precursor may result in very low molecular weight polymers.

[0096] Use of a monofunctional epoxy resin precursor may be beneficial in some circumstances, as the monofunctional epoxy resin precursor may be able to control polymerisation of the epoxy-amine polymer, and therefore by definition the average functionality of p-amino diol groups per polymer chain. Therefore, the use of a monofunctional resin precursor, alongside the first epoxy resin precursor may allow for molecular-level material design, controlling factors critical to material performance including (i) polymer molecular weight (ii) average polymer functionality and (iii) cross-link density.Primary amine

[0097] The curable polymer composition comprises a polymer formed from a primary amine (R-NH2).

[0098] The R group on the amine may be aliphatic linear, aliphatic cyclic or aromatic.

[0099] In embodiments, the primary amine comprises a hydrocarbyl moiety comprising up to 22 carbon atoms. In some embodiments, the hydrocarbyl moiety is a linear, branched or cyclic C2-C22 alkyl moiety. In some embodiments, the hydrocarbyl moiety is a C5-C22 aryl moiety.

[0100] The aliphatic linear, aliphatic cyclic or aromatic R group may be substituted in a variety of normal ways. For example, the R group may be saturated or unsaturated to include allylamine and amines derived from unsaturated fatty acids or alcohols.

[0101] In embodiments, a hydrocarbyl group of the primary amine may be substituted with one or more substituents selected from a halide, such as F or Cl, a C1-C4 alkoxy moiety, or a C1-C4 haloalkoxy moiety.

[0102] In embodiments, the primary amine comprises a linear alkyl having between 6 and 12 carbons atoms, for example, in embodiments, the primary amine comprises a cyclohexyl, dodecyl or hexyl moiety, or a combination thereof.

[0103] In some embodiments, the primary amine is n-hexylamine or cyclohexylamine.Crosslinking Agent

[0104] The polymer is reversibly crosslinkable by the crosslinking agent. The crosslinking agent comprises at least two crosslinking groups. The crosslinking groups are selected from boronic acid and boronic ester groups, or a combination thereof.

[0105] Any crosslinking agent comprising at least two crosslinking groups selected from boronic acid and boronic ester groups or a combination thereof would be suitable.

[0106] In embodiments, the crosslinking agent is selected from a boronic acid or ester based on an aliphatic or aromatic molecule core containing 20 carbon atoms or less, excluding the carbon atoms contained with the ester groups of the boronic esters. For example, in embodiments, the crosslinking agent comprises a boronic acid or ester-based molecule containing between 2 and 6 boronic acid / ester groups based on an aliphatic or aromatic molecule core containing 20 carbon atoms or less, excluding the carbon atoms contained with the ester groups of the boronic esters.

[0107] In embodiments, the crosslinking agent is selected from a group comprising: 1 , 4- phenylenediboronic acid tetrabutyl ester (TBEBA), an ester of 1 ,4-phenylenediboronic acid or 1 ,4-phenylenediboronic acid or a combination thereof.

[0108] When the curable polymer composition is cured, the crosslinking agent forms crosslinks between the polymer to form a cured polymer composition.

[0109] In embodiments, the crosslinks formed between the polymer comprise dioxazaborocane crosslinks formed from complexation of p-amino diol groups of the polymer with the boronic acid or boronic ester groups of the crosslinking agent.

[0110] The present inventors found the crosslinks formed between the polymer are reversible, and therefore, the polymer may be recycled.Degree of Crosslinking

[0111] The degree of crosslinking of the polymer corresponds to the percentage of potential crosslinking sites of the polymer crosslinkable by the crosslinking agent.

[0112] The degree of crosslinking of the polymer by the crosslinking agent is determined by the ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine. For example, to provide for 50 % degree crosslinking, if 0.0334 moles of NH2 groups are present in a primary amine, 0.0167 molar equivalents of boronic ester groups would be used.

[0113] In embodiments, the degree of crosslinking of the polymer by the crosslinking agent is at least about 5%, at least about 10%, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least 90%.

[0114] The ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine is at least about 0.05:1 , at least about 0.1 :1 , at least about 0.2: 1, at least about 0.3:1 , at least about 0.4:1 , at least about 0. 5:1 , at least about 0.6:1 , at least about 0.7:1 , at least about 0.8:1 , at least about 0.9:1 , or at least 1 :1.

[0115] In embodiments, the degree of crosslinking of the polymer by the crosslinking agent is in a range from about 5% to about 200%, from about 10% to about 175%, from about 20% to about 150%, from about 30% to about 100%, or from about 40% to about 90%, or from about 50% to about 80%.

[0116] In embodiments, the ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine is from about 0.05:1 to 2:1 , for example, from about 0.1 :1 to 1.75:1, from about 0.2:1 to 1.5:1 , from about 0.3:1 to 1 :1 , from about 0.4:1 to 0.9:1 , or from about 0.5:1 to 0.8:1. In embodiments, the boron groups may be in excess.

[0117] In an embodiment, the degree of crosslinking of the polymer by the crosslinking agent is in a range from about 40% to about 140%. In embodiments, the ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine is from about 0.4:1 to 1.4: 1.Molar Ratio of Epoxy ResimPrimary Amine

[0118] The present inventors found that the degree of polymerisation may be controlled by the ratio of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine (R-NH2). For example, a diepoxide has 2 epoxy groups, and a primary amine such as n-hexylamine amine has 2 NH groups.

[0119] The ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine may be from about 1 :2 to 2: 1 , for example, from about 1 :1 to 1 .15: 1 , or from about 1 : 1 to 1.1 : 1.

[0120] In some embodiments, the molar ratio of diepoxide groups to molar equivalents of NH groups is from about 1 :2 to 2:1 , for example, about 1:1 to 1.15:1 , or about 1 : 1 to 1.1 : 1.

[0121] In an embodiment, the molar ratio of diepoxide groups to molar equivalents of NH groups is about 1 :1.

[0122] In embodiments where the polyfunctional epoxy resin precursor comprises more than one polyfunctional epoxy resin precursor, the molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor includes the total moles of epoxy groups from each of the polyfunctional epoxy resin precursors.Fibre Reinforcing Material

[0123] In embodiments, the fibre reinforcing material comprises organic, inorganic, metal, natural or aramid fibres or combinations thereof. For example, the organic fibre may be a carbon fibre. The inorganic fibre may be a glass fibre. The natural fibre may be a flax or hemp fibre.

[0124] In an embodiment, the fibre reinforcing material comprises carbon fibre, glass fibre, a metal fibre, or a combination thereof.

[0125] In a preferred embodiment, the fibre reinforcing material comprises carbon fibre.

[0126] In an embodiment, the fibre reinforcing material is woven.Method of Recycling a Composite Material

[0127] The present invention further provides a method of recycling the recyclable composite material, the method comprising: providing a solution comprising a polyol, or monofunctional boronic ester, or monofunctional boronic acid, or an aqueous solution comprising a phase transfer catalyst; putting the recyclable composite material into the solution; and removing the cured polymer composition from the fibre reinforcing material with the solution.

[0128] The present inventors surprisingly found that the cured polymer composition may be removed from the recyclable composite material in a chemo-selective disassembly reaction. The reversible crosslinking of the cured polymer composition was surprisingly found by the present inventors to allow for facile separation from the fibre reinforcing material, and to provide closed-loop recycling. Therefore, the method provided reusable fibre reinforcing material. The method was also surprisingly found to provide undamaged polymers and fibres, which is in stark contrast to state-of-the-art solvolysis and pyrolysis techniques commonly used for cured composites.

[0129] Any polyol comprising at least a 1, 2-, 1 , 3-, or 1 , 4 - diol group may be suitable. In embodiments, the polyol comprises at least a 1 , 2-, 1 , 3-, 1 , 4- diol group and comprises less than 20 carbon atoms. In embodiments, the diol may be selected from the group comprising: 1 , 2 -ethanediol, 2, 3-butanediol, 1 , 3-propanediol, 2, 4-pentanediol or combinations thereof.

[0130] In embodiments, the polyol is a diol. In embodiments, the diol comprises pinacol. The use of pinacol may be particularly advantageous as it is a simple, low cost and non-hazardous reagent.

[0131] In embodiments, the monofunctional boronic ester or monofunctional boronic acid comprises a mono-functional boronic acid or ester, based on an aliphatic or aromatic molecule core containing 20 carbon atoms or less, excluding the carbon atoms contained with the ester groups of the boronic esters. In embodiments, the monofunctional boronic ester or monofunctional boronic acid comprises monofunctional phenylboronic ester or monofunctional phenylboronic acid. Without wishing to be bound by theory, the present inventors believe any monofunctional boronic ester or monofunctional boronic acid would be suitable and act to displace the crosslinker.

[0132] In embodiments, the aqueous solution comprising a phase transfer catalyst further comprises a comprises a metal hydroxide. The aqueous metal hydroxide may be an alkali metal, such as sodium or potassium. In embodiments, the phase transfer catalyst is a quaternary ammonium salt. In embodiments, the quaternary ammonium salt is tetrabutylammonium bromide. In embodiments, the aqueous solution comprising a quaternary ammonium salt is an aqueous solution of tetrabutylammonium bromide and sodium hydroxide.

[0133] In some embodiments, the solution further comprises a solvent. Any solvent that can solubilise the polymer once the crosslinker has been removed may be suitable. For example, solvents such as THF, acetone, methanol, ethanol, propanol, butanol, toluene, xylene, DMF, DMSO, ethyl acetate, propyl acetate, butyl acetate, methylethyl ketone, 2-methyl THF, diethyl ether, 1,4 dioxane, tert-butyl methyl ether or combinations thereof may be suitable.

[0134] In embodiments, the solution further comprises an organic solvent. In an embodiment the solvent is THF.

[0135] In an embodiment, the solution comprises pinacol and THF.Kit of Parts

[0136] The present invention further provides a kit of parts for preparing a recyclable composite material, the kit comprising: a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; a primary amine; a crosslinking agent comprising boronic acid and / or boronic ester groups; and a fibre reinforcing material.

[0137] In some embodiments, the kit further comprises a monofunctional epoxy resin precursor.

[0138] In some embodiments, the polyfunctional epoxy resin precursor may comprise more than one epoxy resin precursor having an epoxy functionality of 2 or more. For example, in some embodiments, the polyfunctional epoxy resin precursor may comprise at least a first and a second epoxy resin precursor.

[0139] The present invention further provides a kit of parts for preparing recyclable curable polymer composition, the kit comprising: a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; a primary amine; and a crosslinking agent comprising boronic acid and / or boronic ester groups.Recyclable Curable Polymer Composition

[0140] The present invention further provides a recyclable curable polymer composition, the composition comprising: a polymer formed from i) a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; and ii) a primary amine; and a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; wherein the polyfunctional epoxy resin precursor comprises an aromatic epoxy resin precursor.

[0141] Such a curable polymer composition may have improved properties such as Tgand Young’s moduli.

[0142] The polyfunctional epoxy resin precursor may comprise any of the features detailed above, for example, in some embodiments, the polyfunctional epoxy resin precursor is selected from a group comprising: an epoxidized phenol-acetone resin precursor, a diglycidylether of bisphenol A epoxy resin precursor, a diglycidylether of bisphenol F epoxy resin precursor, an epoxidized phenol-formaldehyde resin precursor, an epoxidized novolac resin precursor, an epoxidized cresol-formaldehyde resin precursor, or a combination thereof.

[0143] In preferred embodiments, the polyfunctional epoxy resin precursor may comprise butanediol glycidyl ether (BGE) or epoxidized bisphenol A resin (DGEBA), or a combination thereof.

[0144] The crosslinking agent may comprise any of the features detailed above, for example, in preferred embodiments, the crosslinking agent comprises 1, 4- phenylenediboronic acid tetrabutyl ester (TBEBA).

[0145] In embodiments, the ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine is as described above, for example from about 0.05:1 to 2: 1. For example, the ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine is from about 0.1 :1 to 1.75:1 , from about 0.2:1 to 1.5:1 , from about 0.3:1 to 1 :1 , from about 0.4:1 to 0.9:1 , or from about 0.5:1 to 0.8:1 , wherein the degree of crosslinking of the polymer by the crosslinking agent is determined by ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine. In embodiments, the boron groups or the NH2 groups may be in excess.

[0146] The primary amine may comprise any of the features detailed above, for example, in embodiments, the primary amine comprises a linear C2-C22 alkyl moiety, for example n-hexylamine.

[0147] In embodiments, the molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine is from about 1 :2 to about 2: 1 , or from about 1 : 1 to about 1.15:1 , or from about 1 : 1 to about 1.1 :1.

[0148] In embodiments, the polyfunctional epoxy resin precursor comprises more than one polyfunctional epoxy resin precursor, for example, a first and a second polyfunctional epoxy resin precursor. In embodiments, the first and / or second polyfunctional epoxy resin precursor is aromatic, the aromatic epoxy resin precursor is at least about 50 wt.% of the polyfunctional epoxy resin precursor.

[0149] The curable polymer composition may be cured to form a cured polymer composition.

[0150] The curable polymer composition may be cured in a stepped cure, where the temperature is gradually raised over time.

[0151] In an alternative embodiment, the present invention provides a recyclable curable polymer composition, the curable polymer composition comprising: a polymer formed from: i) a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; and ii) a primary amine; and a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; wherein the ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine is from about 0.5:1 to 2:1.

[0152] In such an embodiment, the polyfunctional epoxy resin precursor may comprise an aromatic epoxy resin precursor.Method of Forming a Recyclable Polymer Composition

[0153] The present invention further provides a method for forming a recyclable polymer composition, the method comprising: a) providing a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; b) combining a crosslinking agent comprising boronic acid and / or boronic ester groups with the polyfunctional epoxy resin precursor and primary amine to form a curable polymer composition; wherein the polyfunctional epoxy resin precursor comprises an aromatic epoxy resin precursor.

[0154] The use of an aromatic epoxy resin precursor may help to provide improvements in Tgand Young’s moduli.

[0155] The polyfunctional epoxy resin precursor and primary amine may first be combined together before being subsequently combined with the crosslinking agent. In other words, the polyfunctional epoxy resin precursor and primary amine may react to form a polymer before the crosslinking agent is added. In embodiments, the method for forming a recyclable polymer composition comprises a) forming a polymer from a polyfunctional epoxy resin precursor and a primary amine, where the polyfunctional epoxy resin precursor has an epoxy functionality of 2 or more; and b) crosslinking the polymer by combining the polymer with a crosslinking agent comprising boronic acid and / or boronic ester groups. In embodiments, the polyfunctional epoxy resin precursor comprises an aromatic epoxy resin precursor.

[0156] Alternatively, the polyfunctional epoxy resin precursor, primary amine and crosslinking agent may all be combined at the same time. In other words, the polymerisation of the polyfunctional epoxy resin precursor and primary amine may occur in the presence of the crosslinking agent. In some embodiments, the method for forming a recyclable polymer composition comprises a) i) providing a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; a) ii) providing a crosslinking agent comprising boronic acid and / or boronic ester groups; and b) combining the crosslinking agent, polyfunctional epoxy resin precursor and primary amine together to form a curable polymer composition. The polyfunctional epoxy resin precursor may comprise an aromatic epoxy resin precursor.

[0157] In embodiments, combining the crosslinking agent with the polyfunctional epoxy resin precursor and primary amine may be carried out at a temperature of about 20 °C to 70 °C, about 30 °C to 65 °C or about 40 °C to 60 °C. In embodiments, combining the crosslinking agent with the polyfunctional epoxy resin precursor and primary amine may be carried out at ambient temperature, about 25 °C.

[0158] In embodiments, the method further comprises the step of: c) curing the curable composition. The curable polymer composition may be cured at a temperature between about 20 °C to about 250 °C, for example curable polymer composition may be cured at a temperature between about 50 °C to about 200 °C, or about 80 °C to about 170 °C.

[0159] In some embodiments, the method may further comprise curing the curable polymer composition at ambient temperature, for example, at about 25 °C. In some embodiments, the method may further comprise curing the curable polymer composition at ambient temperature for at least about 1 hour, at least about 2 hours, at least about 3 hours, at least about 5 hours, at least about 12 hours, at least about 18 hours, at least about 24 hours.In some embodiments, the method may further comprise curing the curable polymer composition at ambient temperature for up to about 6 hours, up to about 12 hours, up to about 18 hours, up to about 6 hours, up to about 24 hours.

[0160] In some embodiments the method may further comprise curing the mixture at an elevated temperature. In some embodiments the method may further comprise curing the mixture at an elevated temperature of at least about 30 °C, at least about 50 °C, at least about 75 °C, at least about 100 °C, at least about 125 °C, at least about 150 °C, at least about 175 °C, or at least about 200 °C. In some embodiments, the method may further comprise curing the mixture at an elevated temperature for up to about 30 minutes, up to about 45 minutes, up to about 1 hour, or up to about 2 hours.

[0161] The mixture may be cured in a stepped cure, where the temperature is gradually raised over time. The stepped cure may comprise two or more, or three or more cures at a temperature between about 20 to about 200 °C.

[0162] In some embodiments, the composition may be cured at ambient temperature for about 16 hours, and subsequently cured at an elevated temperature, for example, about 80 °C, for about 1 hour, cured again at an elevated temperature, for example, about 120 °C, for about 1 hour and cured again at an elevated temperature, for example, about 170 °C, for about 1 hour.

[0163] The curable polymer composition may be cured under an inert atmosphere, for example, the curable polymer composition may be cured under a blanket of nitrogen or argon. In an alternative embodiment, the curable polymer composition may be cured in the atmosphere.

[0164] In an alternative embodiment, the present invention provides a method for forming a recyclable curable polymer composition comprises: a) providing a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; b) combining a crosslinking agent comprising boronic acid and / or boronic ester groups with the polyfunctional epoxy resin precursor and primary amine to form a curable polymer composition; wherein the molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine is from about 0.5:1 to 2:1.Cured Recyclable Polymer Composition

[0165] The present invention also provides a cured recyclable polymer composition. The cured recyclable polymer composition is a polymer composition, formed by curing a curable polymer composition described herein.

[0166] The cured polymer composition may comprise a polymer having the following Formula (I):

[0167] In embodiments, the sum of n + m = 1. In embodiments, n is in the range of 0.2 to 1 . In some embodiments, m is in the range of 0 to 0.8. The ratio of m and n depends on the crosslink density, for example, for an 80% crosslinked system, n = 0.8 and m = 0.2.

[0168] In embodiments, Ri is an hydrocarbyl group, preferably aliphatic or aromatic.

[0169] The hydrocarbyl group may be substituted with one or more substituents each individually selected from the group consisting of a halide, such as F or Cl, a C1-C4 alkoxy and a C1-C4 haloalkoxy. In some embodiments, R1 is a linear, branched or cyclic alkyl, preferably a C2-C22 alkyl. In alternative embodiments, R1 is an aryl, preferably C5-C22 aryl.

[0170] In a preferred embodiment, R1 is a linear alkyl having between 6 and 12 carbon atoms, such as cyclohexyl, dodecyl or hexyl.

[0171] L is a linker group of the formula X1p-L1-X2q. In embodiments, L1is a C1-C20 aliphatic group or C5-C14 aromatic ring group or a C3-C8 cyclic hydrocarbon group; and each of X1and X2is a C1-C20 linear or branched saturated or unsaturated aliphatic hydrocarbon group; and wherein each of X1, X2and L1is optionally substituted with Y, wherein Y is a Ci-C20 linear or branched aliphatic hydrocarbon; halogen-substituted C1-C20 linear or branched aliphatic hydrocarbon; halogen; -OR4; -NR42; -NO2; -SO3H; -C(=O)R4; -C(=O)OR4; - OC(=O)NR42; -C=N; -SR4; -P(=O)R42; -OC(=O)OR5; -NC(=O)OR5;- SO2R5; -SOR5; in which R4is H, a C1-C20 branched or straight chain alkyl group or phenyl optionally substituted with Y; and R5is a C1-C20 branched or straight chain alkyl group and p and q are independently 0 or 1.

[0172] In an embodiment, each of X1and X2, where present, is independently a Ci-Cs linear or branched saturated or unsaturated aliphatic hydrocarbon group.

[0173] In an embodiment, L1is a Ci-Cs linear or branched hydrocarbon group or a C5- C14 aromatic ring group.

[0174] In an embodiment, L1is C1-C4 linear or branched hydrocarbon group, or is benzene, or naphthalene.

[0175] In an embodiment, L1is a C2 alkyl group, or is benzene, or naphthalene.

[0176] In some embodiments, L1is unsubstituted.

[0177] In embodiments, the cured polymer composition comprises a polyfunctional epoxy resin. In embodiments, the polyfunctional epoxy resin comprises any polyfunctional epoxy resin resultant from the polyfunctional epoxy resin precursors described herein. The polyfunctional epoxy resin may be selected from a group comprising: an epoxidized phenolacetone resin, a diglycidylether of bisphenol A epoxy resin, a diglycidylether of bisphenol F epoxy resin, an epoxidized phenol-formaldehyde resin, an epoxidized novolac resin, an epoxidized cresol-formaldehyde resin, or a combination thereof.

[0178] In embodiments, the cured polymer composition may be moulded or remoulded.

[0179] In embodiments, the cured polymer composition is tack-free.Method for Forming a Recyclable Composite Material

[0180] The present invention further provides a method for forming a recyclable composite material, the method comprising: a) providing a recyclable curable polymer composition; b) providing a fibre reinforcing material; c) contacting the recyclable curable polymer composition and fibre reinforcing material to form a recyclable composite material.

[0181] In embodiments, contacting the recyclable curable polymer composition and fibre reinforcing material may be by infusion. The infusion may be vacuum assisted.

[0182] In embodiments, the infusion may be vacuum assisted using a vacuum in the range of about 0.5 kPa to 20 kPa, for example, in the range of about 10 kPa to 100 kPa, or in the range of about 15 kPa to 50 kPa.

[0183] In embodiments, an air permeable membrane may be used during infusion.

[0184] In embodiments, contacting the recyclable curable polymer composition and fibre reinforcing material may be by forming a stack of the curable polymer composition and the fibre reinforcing material.

[0185] In embodiments, the curable polymer composition is formed as a layer, for example, as films or sheets.

[0186] In embodiments, the contacting of the recyclable curable polymer composition and fibre reinforcing material may involve heat.

[0187] The present inventors surprisingly found that use of a curable polymer composition as described herein allowed for a layer of a cured polymer to be stacked with a layer of fibre reinforcing material, and then for the cured polymer to flow across the fibre reinforcing material to combine with the polymer layer below when slightly heated, due to the flow characteristics of the curable polymer composition.

[0188] In embodiments, the recyclable composite material may be formed by forming a stack of two or more layers of recyclable curable polymer composition and / or fibre reinforcing material, and infusing the layers with uncured or partially cured curable polymer composition.

[0189] In embodiments, a stack comprises N fibre reinforcing material layers and N+1 polymer composition layers, wherein the stack has alternating layers of fibre reinforcing material and polymer composition. For example, in one embodiment, the stack may comprise 3 fibre reinforcing material layers and 4 polymer composition film or sheet layers. The polymer composition layers may comprise cured or curable polymer composition.

[0190] In embodiments, the stack is pressed to form the composite material.

[0191] In embodiments, the stack is hot pressed to form the composite material.

[0192] The hot pressing may comprise pressing the stack with a pressure of about 0.1MPa to about 10 MPa, for example, about 0.5 to about 2 MPa, or about 0.5 to about 1 MPa. In an embodiment, the hot pressing may comprise pressing the stack with a pressure of about 1 MPa or less.

[0193] In embodiments, the hot pressing may comprise heating the stack to a temperature of about 100 to 200 °C, about 120 to 180 °C, or about 150 to about 160 °C. In an embodiment, the hot pressing may comprise heating the stack to a temperature of about 155 °C.

[0194] In an alternative embodiment, the present invention further provides a method for forming a recyclable composite material, the method comprising: providing an uncured or partially cured recyclable polymer composition; impregnating at least one fibre reinforcing material layer with said recyclable polymer composition; forming a stack of the impregnated at least one fibre reinforcing material layer.EXAMPLES

[0195] The following non-limiting Examples are provided for further illustration of the present invention. Thus, these examples should not be considered to restrict the present disclosure, but are merely in place to teach how to carry out the processes and obtain the products of the present disclosure.Materials and Methods

[0196] Commercial reagents were purchased as detailed below, and were used without further purification unless otherwise specified.

[0197] The bisphenol A diglycidyl ether (DGEBA) used was D.E.R. 332 Epoxy resin supplied by Olin, U.S.

[0198] The breather fabric was breather cloth supplied by Easy Composites, U.K.

[0199] The carbon fibre was C200T-T300 carbon fibre.

[0200] The infusion mesh was FM100 resin infusion mesh supplied by Easy Composites, U.K.

[0201] The vacuum bagging film was VB160 vacuum bagging film supplied by Easy Composites, U.K.

[0202] The bag sealing tape was ST150 bag sealing tape supplied by Easy Composites, U.K.

[0203] The crosslinker benzene- 1 ,4-diboronic tetrabutyl ether (TBEBA) was prepared according to the following method. Benzene-1 ,4-diboronic acid (6.2817 g, 0.0379 mol) was charged to a round bottom flask, dissolved in excess butanol (150 ml), and stirred for 30 minutes to give a colourless solution. The resulting solution was concentrated under reduced pressure on a rotary evaporator to give the resulting product (TBEBA) as a viscous yellow liquid. 13.48 g, yield = 91.4%.Measurement TechniquesTensile Testing Method

[0204] Samples were heated in the oven at 170 °C for 45 minutes, 24 hours prior to tensile testing, to remove thermal history. Polymer films were then laser cut into dogbonesaccording to ASTM standard 638-14-type V and samples tested on either an Instron 5969 or Instron 3343 with a displacement ramp rate of 10 mm per minute at room temperature.Glass Transition Analysis:

[0205] Glass transition (Tg) analyses were conducted on all polymeric materials using a Perkin-Elmer Pyris DSC 8500 and analysed on Pyris software (version 11.1.1.0492). Samples had previously been thermally cured in an oven at 160 °C. Sample masses of 3-6 mg were weighed into standard aluminium DSC pans with vented lids and were heated from -50 °C to 160 °C at a constant rate of 200 °C per minute. Samples were then cooled to -50 °C and heat to 250 °C at a constant rate of 200 °C per minute for two consecutive cycles. The Tg was taken on the second of the consecutive cycles and reported as the midpoint of the endothermic step in the heat flow signal output (Tg onset and endpoint were also recorded).Gel Permeation Chromatography Procedure:

[0206] Samples were dissolved in THF (2 mg mL-1) and filtered through 0.2 mm nylon filters. Samples were analysed using an Agilent 1260 infinity II system equipped with a Rl and viscometry detector, fitted with PLgel MiniMIX-E and PLgel MiniMIX-D columns in sequence, using a THF mobile phase and a flow rate of 0.6 mL min-1. Analysis was performed against a calibration curve of polystyrene standards (EasiVial PS-M supplied by Agilent).Recyclable Curable Polymer Compositions and Composites

[0207] The present inventors found that a recycling process that allows selective disassembly and dissolution of cured polymer compositions and composites comprising such polymer compositions would be beneficial, as it may allow for separation and purification of the molecular building blocks of the cured polymer composition and polymer composites, ready for reuse. However, the curable polymer compositions, and polymer composites from the recyclable curable polymer compositions also need to be sufficiently chemically resistant and have desirable mechanical properties.

[0208] To test suitability of the recyclable curable polymer compositions according to the present invention for use in composites, five Examples were screened. The Examples were prepared by combining commercially available epoxy resins and aliphatic amines with a benzene-1 ,4-diboronic tetrabutyl ether crosslinker.

[0209] In Example 1 , difunctional epoxy resin 1,4- butanediol diglycidyl ether (BGE) was used as a the polyfunctional epoxy resin precursor alongside the monofunctional epoxy resin phenyl glycidyl ether (PGE). PGE was used to control the maximum degree of polymerisation and molecular weight that the epoxy-amine polymer could achieve. In Examples 2 to 4, the polyfunctional epoxy resin precursor comprised BGE and Bisphenol A diglycidyl ether (DGEBA). In Example 5, the polyfunctional epoxy resin precursor comprised solely DGEBA.Example 1

[0210] 1 ,4- Butanediol diglycidyl ether (BGE, 4.00 g, 0.020 moles of BGE, 0.040 moles of epoxy groups), phenyl glycidyl ether (PGE, 1.49 g, 0.010 moles of PGE, 0.010 moles of epoxy groups) and n-hexylamine (HA, 2.50 g, 0.025 moles of HA, 0.050 moles of NH groups) were combined in a round bottom flask under nitrogen with stirring. To the reaction mixture, TBEBA was added (3.86 g, 0.010 moles of TBEBA, 0.020 moles of boron atoms, sufficient to react with 80% of the theoretical maximum number of p-amino diol functional groups on the epoxy-amine polymer, the ratio of molar equivalents of boron groups to NH2 groups (or moles of HA used) groups being 0.8:1). The reaction mixture was stirred at 55 °C for 1 hour and 45 minutes under nitrogen whilst the viscosity slowly increased, before drawing down onto a nonstick PTFE sheet using a 400 pm drawdown bar to control film thickness. The resulting films were allowed to cure at room temperature for 16 hours under nitrogen, followed by a post cure schedule ramp at 80 (1 hour), 120 (1 hour) then 170 °C (1 hour, total 3 hours thermal cure). After cooling to room temperature, the resulting cured polymer composition films were removed from the PTFE sheet to provide Example 1. The resulting polymer composition was a tack free network polymer. As such, this Example details the preparation of a ‘tack free’ network polymer that can be pressed with fibre to form a tack free fibre composite.

[0211] Although described herein, for brevity, the amounts of each reagent used for the preparation of Example 1 is shown below in Table 1.Examples 2 to 4

[0212] Examples 2 to 4 were prepared, as above, but with a 1 :1 molar ratio of BGE and Bisphenol A diglycidyl ether (DGEBA), instead of the 2:1 molar ratio of BGE and PGE used in Example 1. The amounts of each reagent used for the preparation of Examples 2 to 4 is shown below in Table 1. Each of Examples 2 to 4 had a different degree of crosslinking,which was varied by changing the mole % (relative to moles of HA used) of TBEBA in the formulation.Example 5

[0213] Example 5 was prepared, using the methodology of Example 1. However, for Example 5 the only epoxy used was the difunctional DGEBA. There was no BGE or PGE used. The amounts of each reagent used for the preparation of Example 5 is shown below in Table 1.Table 1 - Curable Polymer Compositions of Examples 1 to 5

[0214] The present inventors found all of the Examples formed low viscosity homogeneous mixtures suitable for casting into sheets, and that each provided a solid, tack free, polymer film after the 3 hour thermal cure. The polymer films were covalent adaptable network polymers. The present inventors also found the low viscosity homogeneous mixtures would be suitable for infusion or the preparation of pre-pregs. Furthermore, the present inventors found that the polymer compositions may be shaped, moulded, re-shaped or remoulded with the application of heat.

[0215] Tganalysis of each of the polymer films was conducted. The Tgresults are shown below in Table 2.Table 2 - Tganalysis of Examples 1 to 5

[0216] The present inventors found that curable polymer compositions with suitable Tgvalues were successfully formed for all of the Examples. Thus, curable polymer compositions with suitable Tgvalues may be formed from a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; and ii) a crosslinking agent comprising boronic acid and / or boronic ester groups.

[0217] The present inventors surprisingly found that the polyfunctional epoxy resin precursor influenced the Tgof the polymer films. The present inventors also found the Tgof the polymer films was able to be tuned by the degree of crosslinking.

[0218] Example 1 , containing the monofunctional PGE and difunctional aliphatic BGE displayed a suitable Tg.

[0219] Examples 2 to 4, comprising the difunctional aliphatic BGE and the difunctional aromatic DGEBA displayed higher Tg, with lower or equal degrees of crosslinking than Example 1. The highest degree of crosslinking of 80% for Example 2 showed an almost three-fold increase in Tgcompared to Example 1. The Tgof Examples 2 to 4 demonstrated the effect of crosslinking, with the Tgincreasing with the degree of crosslinking. The present inventors speculate that the increase in Tgmay be a result of the reduced molecular mobility with increased crosslinking.

[0220] Example 5, containing the difunctional aromatic DGEBA showed the highest Tg. The degree of crosslinking was slightly higher than Example 1 , but the Tgwas surprisingly found by the present inventors to be more than three times as high.

[0221] Surprisingly, the present inventors found that polymer compositions free from the monofunctional PGE provided even further improved Tg’s, and were able to be synthesized using the method according to the present invention. Reducing, or removing PGE from the polymer may be beneficial in some cases.

[0222] The inventors also investigated the Young’s modulus (YM) of each Example polymer film. The present inventors found that the YM was improved significantly for Examples 2 to 5, in comparison to Example 1.

[0223] As shown in Figure 1 , as with the Tg, the YM can be controlled and tailored by changing the degree of crosslinking. Examples 2 to 4 were able to achieve YMs of 285-435 MPa using a BGE:DER polyfunctional epoxy resin precursor, which was seen to be an improvement vs the maximum YM data of 119 MPa of Example 1. Overall, the polyfunctional epoxy resin precursor of BGE and DGEBA was shown to give a desirable blend of low viscosity and flexibility, with improved Tgand improved YM.

[0224] As demonstrated by Example 5, the present inventors found that using DGEBA as the sole polyfunctional epoxy resin precursor, and increasing the degree of crosslinking to 100% resulted in the highest YM. Surprisingly, Example 5 gave a YM of 707.2 MPa, which is significantly higher than previously published YM results for similar materials. The present inventors surprisingly found using an aromatic type amine, DGEBA, was beneficial and led to a cured polymer composition with significantly improved Tgand YM.

[0225] Furthermore, the present inventors found the degree of polymerisation may be controlled by controlling the ratio of the ratio of primary amine (R-NH2) to diepoxide in the polymerisation formulation.

[0226] In summary, the present inventors found that the Young’s moduli of recyclable cured polymer compositions may be significantly enhanced through selection of optimised building blocks. For example, Example 5 displayed the highest YM, at 100% of the maximum theoretical degree of crosslinking. This compared favourably to Example 1 , which had a much lower Young’s modulus. Example 2 also displayed a much higher YM than Example 1 , even though the degree of crosslinking was the same (80%).

[0227] The present inventors prepared an additional Example following the method of Example 1 , using PGE and BGE, but increased the highest possible theoretical degree of crosslinking to 100%, however even this polymer composition only had a YM of 127.86 MPa, which was still significantly lower than any of Examples 2 to 5, despite all of Examples 2 to 5 having theoretically similar or lower degrees of crosslinking. Whilst not wishing to be bound by theory, the present inventors spectate this is possibly due to optimisation of the epoxy-amine polymerisation component, and the lack of monofunctional epoxy resin precursor, even at lower theoretical degrees of crosslinking.

[0228] The present inventors also prepared an additional Example, using BGE and PGE as in Example 1. 1 ,4- Butanediol diglycidyl ether (BGE, 4.00 g, 0.020 moles of BGE, 0.040 moles of epoxy groups), phenyl glycidyl ether (PGE, 1.49 g, 0.010 moles of PGE, 0.010 moles of epoxy groups), n-hexylamine (HA, 2.50 g, 0.025 moles of HA, 0.050 moles of NH groups) were combined in a round bottom flask under nitrogen with stirring. The resulting reaction mixture was allowed to polymerise for 1 hour at 60 °C, followed by 1 hour at 100 °C followed by 1 hour at 140 °C, to provide the epoxy-amine polymer. The epoxy-amine polymer was then dissolved in toluene (40 mL). Benzene-1 , 4-diboronic acid crosslinker (1.65 g, 0.010 moles of benzene-1 , 4-diboronic acid, 0.020 moles of boron atoms, sufficient to react with 80% of the theoretical maximum number of p-amino diol functional groups on the epoxy-amine polymer, the ratio of molar equivalents of boron groups to NH2 groups (or moles of HA used) groups being 0.8:1) that had been pre-dissolved in 25 mL ethanol was then added to the toluene epoxy-amine polymer solution with stirring. The resulting gel mixture was stirred at room temperature for 1 hour. The solvent was then removed under vacuum, and the resulting polymer composition was collected and ground to a powder. The polymer composition was then made into polymer sheets by hot pressing the powder in the appropriate dimensions at 180 °C for 20 minutes, under moderate manual force. The resulting polymer composition was a tack free network polymer. As such, this Example details an alternative preparation to the preparation described in Example 1, of a ‘tack free’ network polymer that can be pressed with fibre to form a tack free fibre compositePreparation of Recyclable Composite Materials from Example 1

[0229] A recyclable composite material was prepared from the tack-free, cured polymer film of Example 1. Aligned carbon fibre was used as the fibre reinforcing material.

[0230] Four layers of tack-free, cured polymer composition and three layers of fibre reinforcing material were stacked up in an alternating order, and placed between two layers of silicon sheets with air channels to allow air to escape. The areal weight of the fibre was 32 g / rm2and the areal weight of the cured polymer composition was 180 to 215 g / m-2. A metal block was added to the top of the stack and then this was placed in a vacuum bag and compressed between two heated Instron plates (180°C) and allowed to equilibrate for 10 minutes. The Instron machine was then ramped up from 25 Ns-1up to 3250 N (13 bar (1.3 MPa) ) and held for 15 mins.

[0231] The present inventors found, that when heated and pressed, the polymer flows across the fibre and into the adjacent polymer sheet. The interface between the polymersheets cannot be detected by cross-section and examination by microscopy. The present inventors found this behaviour contradictory to the nature of thermoset polymers.

[0232] The resulting recyclable composite material prepared from polymer Example 1 was cut into two roughly equal halves for the purpose of demonstrating both the chemical resistance (Example 1A) and the ability to chemically recycle and recover the carbon fibre (Example 1 B).Recyclability of Composite Material

[0233] The chemical resistance and recyclability of each recyclable composite material 1A and 1 B produced from the cured polymer composition Example 1 , was tested.

[0234] The recyclable composite materials 1A was placed in the solvent tetrahydrofuran (THF) for 48 hours. THF is a solvent commonly used to dissolve noncrosslinked (thermoplastic) polymers.

[0235] The THF solution was sampled after 48 hours, and the sample run on gel permeation chromatography (GPC) to see the molecular weight distribution of any solubilised polymer. The results, as shown in Figure 2, show that only low molecular weight polymers were extracted from the recyclable composite material, indicating that the cured structure of the recyclable composite material remained in-tact. Figure 2 also includes a photograph showing the recyclable composite material after being in THF, which show the composite is intact and that the polymer remains attached to the carbon fibre. Whilst recycling of the composite material is desired, it is essential that high-performance cured polymer compositions exhibit a broad spectrum of chemical resistance, remaining undamaged and insoluble upon exposure to chemically aggressive liquids and solutions. The present inventors pleasingly found the immersion of the recyclable composite material in THF resulted in the composite showing no material integrity loss comparable to the insoluble properties of a ‘traditional / epoxy-amine’ thermoset, illustrating the recyclable polymer compositions according to the present invention had good chemical resistance.

[0236] The recyclable composite materials 1 B was placed into a solution of THF and 15 times excess of pinacol (based on the theoretical concentration of crosslinks in the polymer) for 48 hours. After 48 hours, the cured polymer had completed separated from the carbon fibre and dissolved into the solution. The carbon fibre was then removed from the THF-pinacol solution, washed with fresh THF, and dried.

[0237] The molecular weight distribution of the THF-pinacol solution with the dissolved cured polymer was determined via GPC and the molecular structure analysed by NMR. As shown in Figure 3, the GPC showed a large peak initially due to the excess of pinacol in thesolution and the boronic pinacol ester produced when dioxazaborocane groups are removed, and a GPC trace containing higher molecular weight polymers (than the THF only experiment) indicating breakdown and dissolution of the polymer structure. This also demonstrates the chemoselectivity of the disassembly process, as THF alone did not result in dissolution.

[0238] Figure 4 shows photographs of the clean carbon fibre free from cured polymer. Figure 4 also shows SEM micrographs confirming the fibres were undamaged, free from polymer and comparable to the virgin fibres.

[0239] These tests demonstrate the present invention provides a viable mechanism for recovery of molecular building blocks from the recyclable composite materials according to the present invention. The recyclable composite materials have been shown by the present inventors to be able to be recycled in a chemoselective process to separate the cured polymer composition and fibre reinforcing material to provide reusable fibre reinforcing material, which was found by the present inventors to advantageously be undamaged fibre reinforcing material that was free from polymer, and comparable to the virgin fibres. This was particularly surprising given the current state-of-the-art solvolysis and pyrolysis techniques are unable to provide undamaged polymers and fibres. The present inventors found that the crosslinking of the cured polymer composition was reversible and allowed for facile separation from the fibre reinforcing material.

[0240] The chemical resistance and recyclability of the recyclable composite material produced from the cured polymer composition Example 1 was also tested in an alternative solution. The recyclable composite material (Example 1 B) containing ~10g of polymer, was immersed in 90 mL of toluene and 20 mL of 10% (w / v) aqueous solution of sodium hydroxide and 0.1g of tetrabutylammonium bromide (1% w / w). The mixture was stirred at room temperature for 24 hours. At room temperature, after this time, the polymer was dissolved, and the carbon fibre was removed from the solution. The present inventors also found that the recyclable composite material may be placed in the toluene, sodium hydroxide and tetrabutylammonium bromide solution and heated for 2 hours at 80 °C to dissolve the polymer and recover the carbon fibre. As such, the present inventors found that the polymer component of the recyclable composite material may be disassembled and dissolved via a variety of chemo-selective processes, including the use of solution comprising an organic solvent, a phase transfer catalyst and an aqueous solution of sodium hydroxide, or the use of a diol such as pinacol.Examples 6 to 7

[0241] Two further curable polymer compositions Examples were prepared. On the basis of the findings relating to Example 5, DGEBA was used as the polyfunctional epoxy resinprecursor, as it was found in Example 5 to give high Tgpolymeric films when sufficient boronic esters was introduced to react with 100% of the theoretical number of p-amino diol groups (degree of crosslinking = 100%). n-hexylamine was used as the primary amine (RNH2). The ratio of molar equivalents of epoxy groups of the DGEBA to molar equivalents of NH groups of the v was 1 :1. The mole % of crosslinking was varied by changing the mole % (relative to primary amine) of TBEBA in each formulation. The cross-linking was varied by including sufficient boronic esterto react with 100% (repeat of Example 5), 140% (Example 6) and 200% (Example 7) of the theoretical p-amino diol groups in the epoxy-amine polymerisation (where Example 6 and Example 7 have an excess of boronic ester groups relative to the theoretical number of p-amino diol groups). It was surprisingly found that the highest Tgexample was with 140% of boronic ester (relative to hexylamine), thus that there was a 40% excess of boronic esters relative to the theoretical number of p-amino diol groups.

[0242] The method of formulating the polymer films of Examples 6 to 7 was as follows.

[0243] DGEBA diepoxide ( 5.69 g, epoxy equivalent weight = 170, 0.0334 mols of epoxy groups) was heated to 55 °C with stirring, before adding n-hexylamine (1.69 g, amine hydrogen equivalent weight = 50.5, 0.0334 moles of NH groups, 1 :1 molar ratio) and stirring for a further 5 minutes at 55 °C. TBEBA (boron equivalent weight = 195.09) was then added with stirring to form a homogenous (clear) solution, which was poured onto a layer of PTFE and drawn down using a 150 mm wide, 400 pm depth draw down bar.

[0244] The amount of TBEBA added varied for Examples 6 to 7 (Example 6 = 4.56 g, 0.0234 mol, 140 mol% of boronic ester groups based on moles of RNH2, or Example 7 = 6.51 g, 0.0334 mol, 200 mol% of boronic ester groups based on moles of RNH2).

[0245] The film was allowed to harden and then layered with perforated PTFE-release film sheet, peel-ply fabric and a breather fabric and placed in a vacuum bag and vacuum applied. The resulting sample and sealed (static) vacuum bag were then placed in an oven at 70 °C and cured using the following method.

[0246] The vacuum bag and sample were held at 70 °C for 1 hour before increasing the oven temperature to 110 °C for a further 2.5 hours. After this time, the vacuum bag and sample were removed from the oven, and allowed to cool to room temperature. The sample was removed from the vacuum bag and the perforated PTFE-release film sheet, peel-ply fabric and breather fabric were manually removed from the polymer sample.

[0247] The polymer sample was then returned to an oven and heated from 80 °C to 150 °C where the sample was held for 30 minutes, before heating further to 160 °C for 10minutes. The polymer sample was then removed from the oven and allowed to cool to room temperature.

[0248] Each resultant polymer was subjected to glass transition Tganalysis. The results are shown below in Table 3.Table 3 - Tganalysis and Crosslinking of Examples 5 to 7

[0249] As above, all of the Examples demonstrated high Tg. The degree of crosslinking was found to influence the Tg. The polymer film with 140 mol% crosslinker was found to result in the highest Tg.Preparation of Composite Materials

[0250] Two different methods of manufacturing a recyclable composite material according to the present invention were investigated. The polymer composition of Example 6 was used for testing manufacturing methods, as it resulted in the highest Tg.Method 1 : Pre-impregnation (pre-preg) Method

[0251] A polymer composition was prepared according to the procedure described above for Example 6, using 140 mol% crosslinker. The resulting polymer composition was impregnated into carbon fibre (the fibre reinforcing material) at a polymer to fibre mass ratio of 43:57 using a 400 pm depth draw down bar.

[0252] After a 5 minute hold time, the mixed cured pre-impregnated fibre / polymer samples were cut into egual strips measuring 5 cm by 13 cm. The pre-preg was then assembled in a vacuum bag, three layers at time, degassing under in a vacuum bag for 30 minutes each time, until a total of 9 layers were prepared.

[0253] The final polymer composite was then layered top and bottom with perforated release film, peel-ply fabric and breather fabric and placed in a vacuum bag and vacuum applied. The resulting sample and sealed (static) vacuum bag were then placed in an oven at 70 °C and cured using the following method.

[0254] The vacuum bag and sample held were at 70 °C for 1 hour before increasing the oven temperature to 110 °C for a further 2.5 hours. After this time, the vacuum bag and sample were removed from the oven, allowed to cool to room temperature and then the sample was removed from the vacuum bag and the perforated release film sheet, peel-ply fabric and breather fabric manually removed from the polymer sample. The polymer- composite sample was then returned to an oven and heated from 80 °C to 150 °C where the sample was held for 30 minutes, before heating further to 160 °C for 10 minutes. The polymer- composite sample was then removed from the oven and allowed to cool to room temperature, to give the recyclable composite material sample shown in Figure 5.

[0255] The present inventors found that defect free polymer films were achievable by using a vacuum bag in the early stages of film cure, as the vacuum bag prevented gassing and bubble formation in the resulting polymer film.

[0256] The present inventors found the curable polymer composition is suitable for impregnation, and therefore found a recyclable composite material may be made by impregnating carbon fibre with curable polymer compositions according to the present invention.Method 2: Vacuum Assisted Infusion Method

[0257] Carbon fibre dry fabric was used as the fibre reinforcing material, and was cut into 9, 14cm-14cm, squares (60.42 g). PTFE tape was stuck to a heat proof mat (mould) as a non-stick surface, so the composite and the carbon fibre squares could be layered up. ST150 bag sealing tape was stuck around the exterior of the mat leaving room for additional layers. FM100 infusion mesh was cut slightly small area than the peel ply (but larger than the fibres), with a corner cut for the exit connector, and placed over the peel ply. Connectors were placed as far apart within the bag as possible (over the top of the peel ply but not the mesh). VB160 vacuum bagging film was placed over the top and sealed with the perimeter ST150 tape. The bag was pierced over the connectors and disposable PVC tubes were inserted. With the ST150 tape, the tube and connector were sealed to keep airtight. The exit tube was connected to a catch pot which connects to a vacuum pump to prevent polymer entering the vacuum pump. The vacuum was then turned on checking the tape around the edge of the bag, the connectors for any leaks. The entry and exit pipes were then clamped and the vacuum turned off and checked for leaks over a 10 minute window.

[0258] A polymer composition was then prepared as described for Example 6, scaled to ensure sufficient polymer composition to fully infuse the fibres according to the follow calculation:Total mass of polymer = (1.2 x the mass of carbon fibre) + mass of mesh and peel ply

[0259] The resin was then infused into the pre-prepared fibre by turning on the vacuum and unclamping the entry tube. Once the curable polymer composition reached the exit pipe and the curable polymer composition in the beaker ran dry, the tubing was clamped, and the vacuum was turned off. The bag was then transferred to the oven and heated at 70 °C for one hour, then heated to 110 °C for a further 1 hour to provide a recyclable composite material.

[0260] As with the pre-preg method, the present inventors found that defect free polymer films were achievable by using a vacuum bag in the early stages of film cure, as the vacuum bag prevented gassing and bubble formation in the resulting polymer film.

[0261] The present inventors found the curable polymer composition is suitable for infusion, and therefore found a recyclable composite material may be made by using a vacuum assisted infusion method. This further demonstrates the versatility of the curable polymer compositions.Recyclability of Composite Material

[0262] A sample of the composite manufactured by the pre-preg method (described above) was submerged in a 100 mL of THF (solvent) containing 10 wt.% pinacol at 35 °C for5 days. After 5 days, the polymer had largely depolymerised and separated from the fibres. The samples were placed in a fresh solution of 100 mL of THF (solvent) containing 10 wt.% pinacol at 35 °C for 3 days and then rinsed with a solution of pinacol (10 wt.%) in THF. Figure6 shows photographs of the recovered fibres. The recovered fibres were separated from the bulk polymer in good condition.

[0263] These tests show the present invention provides a recyclable composite material, a recyclable curable polymer composition, and a method of easily recycling the composite material to provide recovered fibre reinforcing material free from polymer, and comparable to the virgin fibres. The present inventors found that the combination of dioxazaborocane crosslinking and use of a diol, such as pinacol, allowed for easy recovery of useable fibre reinforcing material.

[0264] Although TBEBA was used in the Example as the crosslinking agent, the present inventors consider any boronic acid / ester containing molecule with two or more boronic functional groups could crosslink the reaction product of the epoxy-amine polymerisation, and therefore would be suitable and provide similar results. Without wishing to be bound by theory, the present inventors consider that any crosslinking agent comprising at least 2 boronic acid or ester groups capable of reacting with the beta-amino diol groupsformed from the epoxy-amine polymerisation would be suitable as a network polymer or “gel would be formed.

[0265] Furthermore, although pinacol was utilised in the Example, the present inventors speculate any diol containing molecule, for example, any 1 ,2-, 1 ,3-, or 1 ,4-diol containing molecules containing less than 20 carbon atoms would be suitable and produce similar results.

[0266] Whilst epoxy resins have previously been chemically modified previously to create dynamic or reversible cured materials, such materials rarely benefit from the high Tgdesirable properties for recyclable materials. Further to this, an efficient process for the chemical or mechanical recycling of these materials has not been demonstrated.

[0267] The present inventors found that in recyclable composite materials according to the present invention, the polymer and fibre reinforcing material can be easily separated to provide reusable fibre reinforcing material. Surprisingly, the fibre reinforcing material was found to be undamaged, free from polymer, and comparable to the virgin fibres. The process of recycling was also found by the present inventors to be efficient and highly applicable.

[0268] Pleasingly, the present inventors also found that Tgand YM of the resultant cured polymer compositions can also be improved by using the recyclable curable polymer compositions as described herein.

Claims

1. CLAIMS1 . A recyclable composite material, the recyclable composite material comprising: a curable polymer composition comprising: i) a polymer formed from a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; and ii) a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; and a fibre reinforcing material.

2. The recyclable composite material as claimed in claim 1 , wherein the ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine is from about 1 :2 to 2:1 , for example, from about 1 :1 to 1.15:1 , or from about 1 :1 to 1.1 :1.

3. The recyclable composite material as claimed in claim 1 or claim 2, wherein the polyfunctional epoxy resin precursor comprises an aromatic polyfunctional epoxy resin precursor.

4. The recyclable composite material as claimed in any one of the preceding claims, wherein the polyfunctional epoxy resin precursor comprises a glycidyl ether moiety.

5. The recyclable composite material as claimed in any one of the preceding claims, wherein the polyfunctional epoxy resin precursor comprises an epoxidized phenolacetone resin precursor, a diglycidylether of bisphenol A epoxy resin precursor, a diglycidylether of bisphenol F epoxy resin precursor, an epoxidized phenolformaldehyde resin precursor, an epoxidized novolac resin precursor, an epoxidized cresol-formaldehyde resin precursor, or a combination thereof.

6. The recyclable composite material as claimed in any one of the preceding claims, wherein the polyfunctional epoxy resin precursor comprises at least a first and a second epoxy resin precursor.

7. The recyclable composite material as claimed in claim 6, wherein the first epoxy resin precursor comprises an aromatic epoxy resin precursor and the second epoxy resin precursor comprises an aliphatic epoxy resin precursor.

8. The recyclable composite material as claimed in any one of the preceding claims, wherein the curable polymer composition further comprises a monofunctional epoxy resin precursor.

9. The recyclable composite material as claimed in any one of claims 8, wherein the molar ratio of epoxy groups derived from the polyfunctional epoxy resin to the molar ratio of epoxy groups derived from a monofunctional epoxy resin precursor is about 1 :1 to 40: 1.

10. The recyclable composite material as claimed in any one of claims 6 to 9, wherein the first epoxy resin precursor has an epoxy functionality of 2 or more and the second epoxy resin precursor has an epoxy functionality of 2 or more.

11. The recyclable composite material as claimed in any one of the preceding claims, wherein the crosslinking agent is selected from a group comprising: 1 , 4- phenylenediboronic acid tetrabutyl ester (TBEBA), an ester of 1 ,4-phenylenediboronic acid or 1 ,4-phenylenediboronic acid or a combination thereof.

12. The recyclable composite material as claimed in any one of the preceding claims, wherein the ratio of molar equivalents of boron groups of the crosslinking agent to NH2 groups of the primary amine is from about 0.05:1 to 2:1 , for example, 0.4:1 to 1.4:1 or 0.5:1 to 0.8:1.

13. A method of recycling the recyclable composite material of any one of the precedingclaims, the method comprising: providing a solution comprising a polyol, or monofunctional boronic ester, or monofunctional boronic acid, or an aqueous solution comprising a phase transfer catalyst; putting the recyclable composite material into the solution; and removing the cured polymer composition from the fibre reinforcing material with the solution.

14. The method of recycling the recyclable composite material according to claim 13, wherein the polyol is a diol, such as pinacol, and / or the monofunctional boronic ester is a phenylboronic ester and / or monofunctional phenylboronic acid and / or wherein the aqueous solution comprising a phase transfer catalyst, such as a quaternary ammonium salt, further comprises a metal hydroxide, such as sodium hydroxide.

15. The method of recycling the recyclable composite material according to claim 13 or claim 14, wherein the solution further comprises a solvent.

16. A recyclable curable polymer composition, the composition comprising: a polymer formed from: i) a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; and ii) a primary amine; and a crosslinking agent comprising boronic acid and / or boronic ester groups, wherein the polymer is reversibly crosslinkable by the crosslinking agent; wherein the polyfunctional epoxy resin precursor comprises an aromatic epoxy resin precursor.

17. The recyclable curable polymer composition as claimed in claim 16, wherein the polyfunctional epoxy resin precursor comprises an epoxidized phenol-acetone resin precursor, a diglycidylether of bisphenol A epoxy resin precursor, a diglycidylether ofbisphenol F epoxy resin precursor, an epoxidized phenol-formaldehyde resin precursor, an epoxidized novolac resin precursor, an epoxidized cresol-formaldehyde resin precursor, or a combination thereof.

18. The recyclable curable polymer composition as claimed in claim 16 or 17, wherein the ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor to molar equivalents of NH groups of the primary amine precursor is from about 1 :2 to 2:1 , or from about 1 :1 to 1.15: 1, or from about 1 :1 to about 1.1:1.

19. The recyclable curable polymer composition as claimed in any one of claims 16 to 18, wherein the aromatic epoxy resin precursor is at least about 50 wt.% of the polyfunctional epoxy resin precursor.

20. The recyclable curable polymer composition as claimed in any one of claims 16 to 19, wherein the ratio of molar equivalents of boron groups of the crosslinking agent to amine (N-atoms) groups of the primary amine is from about 0.2:1 to 1:1.

21. A kit of parts for preparing a recyclable composite material, the kit comprising: a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more; a primary amine; a crosslinking agent comprising boronic acid and / or boronic ester groups; and a fibre reinforcing material.

22. The kit of claim 21 , further comprising a monofunctional epoxy resin precursor.

23. A method for forming a recyclable composite material, the method comprising: a) providing a recyclable curable polymer composition of any one of claims 1 to 12 or 16 to 20; b) providing a fibre reinforcing material; c) contacting the recyclable curable polymer composition and fibre reinforcing material to form a recyclable composite material.

24. A method for forming a recyclable curable polymer composition, the method comprising: a) providing a polyfunctional epoxy resin precursor having an epoxy functionality of 2 or more, and a primary amine; b) combining a crosslinking agent comprising boronic acid and / or boronic ester groups with the polyfunctional epoxy resin precursor and primary amine to form a curable polymer composition; wherein the polyfunctional epoxy resin precursor comprises an aromatic epoxy resin precursor.

25. The method for forming a recyclable curable polymer composition according to claim 24, wherein the ratio of molar equivalents of epoxy groups of the polyfunctional epoxy resin precursor, to molar equivalents of NH groups of the primary amine precursor is from about 1 :2 to 2:1.

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