EP3512900A1

EP3512900A1 - Cleavable epoxy compositions based on amine- and disulfide-containing additives

Info

Publication number: EP3512900A1
Application number: EP17780993.6A
Authority: EP
Inventors: Martin Lahn HENRIKSEN; Mogens HINGE; Rasmus Konge JOHANSEN; Jens Bomholdt Ravnsbæk
Original assignee: Danish Technological Institute; Aarhus Universitet; Vestas Wind Systems AS
Current assignee: Danish Technological Institute; Aarhus Universitet; Vestas Wind Systems AS
Priority date: 2016-09-14
Filing date: 2017-09-13
Publication date: 2019-07-24
Also published as: WO2018050189A1

Abstract

The present invention relates to epoxy compositions, which may be disassembled by immersion in an appropriate solvent. In particular, the present invention relates to the addition of amine-and disulfide-containing compounds, such as L- cystine, to commercially relevant epoxy-based compositions to prepare sustainable polymer composite materials that can be fractionated by non-harsh solvents.

Description

Cleavable epoxy compositions based on amine- and disulfide-containing additives

Technical field of the invention

The present invention relates to epoxy compositions, which may be disassembled by immersion in an appropriate solvent. In particular, the present invention relates to the addition of amine- and disulfide-containing compounds, such as L- cystine, to commercially relevant epoxy-based compositions to prepare

sustainable polymer composite materials that can be fractionated by non-harsh solvents.

Background of the invention

The ever-increasing energy consumption places high demands on the production of green energy. This in addition to the increasing strain on the world's resources requires reconsidering of the choice of materials used e.g. in the aviation, automotive, ship and wind industries. As an example, wind turbine blades must be highly durable structures in order to withstand the forces they experience during their lifetime. While there are several approaches to blade manufacturing, one of the most widely applied methods is based on infusing structural fibre-bundles with a resin to obtain the required shape. The result is highly durable blades, however, the challenge arises when the wind turbine reaches end-of-life as the material composing the blades is non-recyclable. Consequently, the blades are typically incinerated or dumped in a landfill; here the composite material making up the blades has lost all value.

Thus, one major goal concerning sustainability is to optimize the use of the world's resources by keeping materials as high value resources for as long as possible creating value far beyond end-of-life of the product. To this end, it is necessary to overcome the lack of recyclability of the elements of the composite. The disassembly ensures the re-entry of the material in a production cycle either as raw material for new products or in other composite structures. Current state-of-the-art composites typically used in e.g. wind turbine blades are based on layers of carbon-fibre/glass-fibre joined together with an epoxy matrix, as depicted in figure 1A. This conventional approach uses thermosets, i.e. a polymer that cures irreversible, as the polymeric matrix. This has several advantages like relatively easy mixing of monomers (often liquid) and fibres before polymerization. Moreover, the irreversible curing ensures a system that is providing the necessary strength and durability for high demanding utilisation. Unfortunately, this also results in a non-recyclable composite, which renders the technology less sustainable.

To address this key challenge of recyclability, one strategy is to utilize cleavable crosslinking for thermoset-based composites as depicted in figure IB.

It is worth noting that many productions use thermosets, due to its abilities as a material with great strength, chemical resistance and low heat sensitivity. Thus, epoxy-based composite materials are also utilized in the aviation, automotive and ship industries and any advances achieved within the development of new recyclable epoxy-based composite materials may therefore be extrapolated to a variety of industries.

At present, many fabricants are employing a two-component epoxy in

combination with either glass- or carbon-fibre reinforcement. Dismantling of carbon-fibre reinforced epoxy-based composites has been demonstrated using thermal treatment. However, the size of many products built from these composites makes it challenging to accomplish such thermal treatment.

Cleavable crosslinking systems for polymer matrices have been derived. One reported approach is the disulfide-based system, which can be reversible cleaved using external stimuli. For example, the disulfide moiety may be utilized by swelling (e.g. in CHC , THF, MeOH, DMF) and addition of a reductant, e.g. thiols, or electrochemical reduction. The disulfide-based system has found its application in other materials, such as gels and films, and has the benefit of being resistant to many environmental aspects, including heat, salt concentration, UV, shear and pH due to the nature of some disulfides. Successful incorporation of a disulfide in an epoxy-based composite material may therefore open up the possibility of a versatile and sustainable alternative to today's non-recyclable fibre reinforced epoxy composites. A disulfide-based epoxy composition has recently been proposed in what was described as a dynamic thermoset (de Luzuriaga et a/., Materials Horizons, 2016). It was demonstrated that disulfide bridges could be incorporated in an epoxy matrix and the dynamics of the thiol-disulfide exchange was utilized to dismantle the matrix. Further, it was shown that a grinded matrix could be reassembled by heating and high pressure. However, the disulfide compound, 4-aminophenyl disulfide (AFD), was in that instance used as a dynamic hardener, i.e. as a direct substitute of a classic hardener. Thus, the approach of de Luzuriaga et al. is not applicable to typical commercial epoxy-based composites that already contain a hardening agent. Furthermore, AFD was added in large amounts and dismantled by use of a solution of 2-mercaptoethanol in DMF.

Hence, the provision of an improved epoxy-based composite material that may be disassembled under mild conditions would be advantageous. In particular, the provision of an additive for epoxy compositions that may be directly utilized in conjunction with typical commercial epoxy-based composites and facilitate disassembly of otherwise non-recyclable material using green chemistry would be advantageous.

Summary of the invention

In the present invention, amine and disulfide-moeities are introduced into the epoxy matrix by addition of a disulfide-additive containing polyamine cross- linkers. This approach will incorporate a cleavable cross-link in the epoxy composition, facilitating fragmentation of epoxy-based polymer composites. To achieve the fragmentation, a swelling-assisted methodology is utilized. This process will leave the structural units (e.g. glass- or carbon-fibre) separated from the polymer matrix and they may subsequently be collected and recycled.

Thus, an object of the present invention relates to the provision of an improved epoxy-based polymer composite that may be disassembled under mild conditions while retaining or improving the advantageous mechanical properties of state of the art epoxy compositions and composites thereof.

In particular, it is an object of the present invention to provide an epoxy composition that solves the above mentioned problems of the prior art with non- sustainability of epoxy-based polymer composites by incorporation of an additive into the epoxy matrix, thereby enabling disassembly of epoxy-based composite materials using green chemistry. Thus, one aspect of the invention relates to an epoxy composition comprising the reaction product of a set of reactants comprising :

a) at least one epoxy resin,

b) at least one hardener, and

c) at least one additive represented by formula (I),

wherein

R¹, R², and R³ are independently selected from the group consisting of H, and an optionally substituted C1-C5 alkyl group,

R⁴ is selected from the group consisting of -H, -OH, -SH, -CN, -OCN, and -COOR, wherein

R is selected from the group consisting of H and an optionally substituted C1-C5 alkyl group, and

n is an integer from 1-10.

Another aspect of the present invention is a method of manufacturing an epoxy composition comprising the steps of:

a) swelling of an additive according to formula (I), wherein

R⁴ is selected from the group consisting of -H, -OH, -SH, -CN, -OCN, and COOR, wherein

n is an integer from 1-10,

b) pre-treatment of the at least one epoxy resin with the swollen additive to obtain a pre-treated solution, and

c) mixing said pre-treated solution with the at least one hardener,

thereby forming said epoxy composition.

Yet another aspect of the present invention is the use of a compound represented by formula (I)

wherein

R⁴ is selected from the group consisting of -H, -OH, -SH, -CN, -OCN, and COOR, wherein R is selected from the group consisting of H and an optionally substituted C1-C5 alkyl group, and

n is an integer from 1-10,

as an additive in an epoxy composition further comprising :

a) at least one epoxy resin, and

b) at least one hardener.

Brief description of the figures

Figure 1 shows the state-of-the-art method of producing polymer composites comprising epoxy matrix as well as the inclusion of cleavable cross-linkers proposed in the present invention. The incorporation of cleavable cross-linkers enables the introduction of a disassembly strategy in which the structural units/fibres of the polymer composite may be recycled. Figure 2 shows the typical formation of an epoxy matrix. The amine hardener reacts with the least hindered spot on the epoxy in order to open the ring and form an amine and an alcohol (1), which subsequently reacts with another epoxy unit (2). The created macromolecules can now be used to cross-link the rest of the matrix (3). This curing continues until a network is formed with low to none mobility. Also shown is the epoxide unit (4) which is the essential moiety of all epoxy resins.

Figure 3 shows (A) the mixture before curing, (B) the amine reacting with epoxies, (C) the curing at elevated temperature, and (D) the epoxy matrix from a macro perspective.

Figure 4 shows the standard epoxy (left) and an epoxy composition comprising 20 mol% L-cystine (right) in acetic acid at 70°C. It is shown as (A) initial, (B) 265 minutes, and (C) 530 minutes immersion in the solvent. While nothing noteworthy happens to the standard epoxy, the epoxy composition comprising 20 mol% L-cystine is gradually swollen and eventually dismantled.

Figure 5 shows a series of epoxy compositions comprising various mol% of L- cystine immersed in acetic acid. The samples were tested in duplicates with samples A and B corresponding to 5 mol% L-cystine, C and D corresponding to 10 mol% L-cystine, and E and F corresponding to 20 mol% L-cystine. The effect of acetic acid on the epoxy compositions comprising a varying quantity of L-cystine is shown before (A) and after (B-C) immersion in the solvent. In all cases, the epoxy composition is dismantled.

Figure 6 shows (A) ATR-FTIR spectrum of the standard cured epoxy matrix (1), applied hardener (2) and epoxy resin (3), and (B) ATR-FTIR spectrum of the standard, Cys20mol%, Hyd20mol% and Phy20mol% epoxy matrices from top to bottom, respectively. (C) Normalized spectra of Cys20mol% (top), Hyd20mol% (middle), and Phy20mol% (bottom) versus STD.

Figure 7 shows (A) liquid state NMR of epoxy resin (dot line), hardener (solid line) and solid state NMR of the cured standard epoxy matrix (broad peaks, solid line). The insert from 180-100 ppm shows the aromatic area. The septet at 39.52 ppm corresponds to the DMSO solvent peak. (B) Solid state NMR spectra of the standard, Cys20mol%, Hyd20mol% and Phy20mol% epoxy matrices from top to bottom, respectively. Figure 8 shows the volumetric swelling as a function of time for the STD

(triangle), Cys20mol% (circle) and Hyd20mol% (square) modified epoxy matrices immerged in demineralized water at 70 °C with (solid symbols) and without (open symbols) sonication. Figure 9 shows tensile testing of glass fibres infused with standard epoxy (A) or CYS5mol% modified epoxy (B).

Figure 10 shows a series of compositions comprising glass fibres infused with standard epoxy, Cys5mol% modified epoxy or Cysl0mol% modified epoxy immersed in acetic acid.

The present invention will now be described in more detail in the following. Detailed description of the invention

Prior to discussing the present invention in further details, the following terms and conventions will first be defined :

Definitions

Nomenclature of samples

The following notation for samples is used : xxxxyymol%, where xxxx may be either of cys (L-cystine), hyd (2-hydroxyethyl disulfide), or phen (4-aminophenyl disulfide), and yymol% may be either of 5mol%, 10mol%, 15mol% or 20mol% corresponding to "the percentage of active mole sites of the standard hardener, which have been exchanged with the new additive. "STD" is used for the standard epoxy made without any additives.

Mol%

In the present context, mol% is the mol% of substituted active sites. Hence, if the original hardener has four active sites, substituting 5% with an additive that has two active sites doubles the amount of additive. On the other hand, substituting with an additive containing eight active sites will result in an addition of half the amount of additive, thereby ensuring one to one active site substitution between hardener and additive.

Epoxy resin

In the present context, the term "epoxy resin" refers to a low molecular weight epoxy monomer, pre-polymer or higher molecular weight polymer, which normally contain at least two epoxide groups. Thus, the epoxy resin compound may be, but are not limited to, saturated or unsaturated aliphatic, cycloaliphatic, aromatic or heterocyclic compounds, which possesses at least one epoxy group. However, epoxy resins of the present invention may also include monoepoxides, diepoxides, polyepoxides or mixtures thereof. Figure 2, compound 4 illustrates a generic structure of an epoxide monomer.

Such compounds may be substituted with one or more non-interfering

substituents, such as halogens, hydroxyl groups, ethers, aryls and alkyls or combinations thereof. In the present invention, particularly suitable epoxy resins include, but are not limited to, bisphenol A epoxy resins, bisphenol F epoxy resins, novolac epoxy resins, aliphatic epoxy resins, glycidylamine epoxy resins and combinations thereof.

An example of epoxy resins suitable for the present invention may include, but are not limited to, the glycidyl polyethers of polyhydric phenols and polyhydric alcohols. Thus, epoxy resins of the present invention include the diglycidyl ethers of resorcinol, catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP (1,1- bis(4-hydroxylphenyl)-l-phenyl ethane), bisphenol F, bisphenol K,

tetrabromobisphenol A, phenol-formaldehyde novolac resins, alkyl substituted phenol-formaldehyde resins, phenol-hydroxybenzaldehyde resins, cresol- hydroxybenzaldehyde resins, dicyclopentadiene-phenol resins, dicyclopentadiene- substituted phenol resins tetramethylbiphenol, tetramethyl-tetrabromobiphenol, tetramethyltribromobiphenol, tetrachlorobisphenol A; and any combination thereof.

Hardener/curing agent

In the present context, the term "hardener" refers to compound that reacts with an uncured epoxy resin to produce a cross-linked epoxy matrix. In the present context, the terms "hardener" and "curing agent" are used interchangeably.

The reaction of a hardener with an epoxy resin forms a thermoset structure with improved mechanical, chemical and heat resistance properties.

Hardeners may be chosen from, but are not limited to, polyfunctional amines, acids (and acid anhydrides), phenols, alcohols, thiols, ketones, aldehydes, isocyanate, and cyanide. Amine hardeners may be primary, secondary, or tertiary amines of aliphatic, cycloaliphatic, or aromatic types, preferably primary amines.

The hardener may also be a compound comprising an amine group and an alcohol group such as, but not limited to, monoethanolamine; 2-amino-2-hydroxymethyl- 1,3-propanediol; 2-amino-2-methyl-l,3-propanediol; diethanolamine and mixtures thereof.

Additive

In the present context, the term "additive" refers to a compound that is partly substituting and/or added to an epoxy composition comprising an epoxy resin and a hardener, such as commercially available epoxy composition. It is important to note that the additive in the present invention is not used to entirely replace any components of standard epoxy compositions. The additive is substituting a fraction of hardener present in the epoxy composition, thereby creating a three- component epoxy composition. Consequently, the epoxy compositions of the present invention may be utilized in conjunction with already available epoxy compositions. Additives of the present invention are defined by the presence of an amine group and a disulfide bridge. The additive may also encompass other functionalities such as carboxylic acids, ethers, halogens or other functional moieties. It is preferred that the additive comprise carboxylic acids. Reaction product

In the present context, the term "reaction product" refers to the reaction product of the reaction between an epoxy resin, a hardener and an additive. This reaction product forms an epoxy matrix. The size of the reaction product may vary depending on the choice of epoxy resin, hardener and additive. Furthermore, the size of the reaction product will vary depending on the material ratio between epoxy resin, hardener and additive used in the formation of the reaction product. Structural fibre

In the present context, the term "structural fibre" refers to a component that is mixed with an epoxy composition to form a polymer composite material. The terms "structural fibre" and "structural unit" is used interchangeably herein. Examples of structural fibres including sizing may be chosen from, but are not limited to, glass, carbon, cellulose, ceramics, natural fibres, pre-pregs, continuous fibres, platelet materials and graphene. Swelling

In the present context, the term "swelling" or "swollen" refers to a state of an epoxy composition sample, in which it maintained its appearance, just scaled in three dimensions, upon immersion into a solvent. Thus, a swollen sample is not dismantled or mechanically broken, but still constitutes a single piece of material.

Mechanically broken

In the present context, the term "mechanically broken" refers to a state of an epoxy composition sample, in which it is physically separated in a few (e.g. 2-5) large otherwise non-affected pieces upon immersion into a solvent.

Mechanical breakage may happen because the test species gets stuck in the test tube and then broken apart due to the force from swelling. Thus, a mechanically broken sample is not dismantled, but constitutes several large pieces of material.

Dismantle

In the present context, the term "dismantle" refers to a state of an epoxy composition sample, in which it is separated into a plethora of very small bits of material upon immersion into a solvent. The terms "dismantle", "disassemble" and "fractionated" is used interchangeably herein. It is also known as chemical dismantling.

Thus, a chemically dismantled sample is not mechanically broken, but constitutes a plethora of very small bits of material.

Curing

In the present context, the term "curing" refers to the process, in which the epoxy resin and hardener reacts to form a thermoset structure. In the present curing may also be performed subsequent to addition of the additive to the epoxy composition.

In the present invention, it is preferred that curing is performed at elevated temperatures. It is particularly preferred that the curing temperature is performed at or above the glass transition temperature. The temperature may be increased step-wise to control the rate of curing and ensure that heat is not accumulated from the exothermic reaction. However, in some incarnations of the present invention, curing may also be performed at ambient temperature.

Glass transition temperature, T_g

In the present context, the glass transition temperature, T_g, is defined as a transition from a glassy more brittle state, to a rubbery or more flexible state. ASTM E1356 - 08(2014) is used as the standard test method for assignment of the T_g by differential scanning calorimetry (DSC).

Elastic modulus (Emoduius), ultimate tensile strength (UTS), elongation at break (%EL at break) and toughness

The Emoduius, UTS and %EL at break is calculated according to ISO 527 1- 5 : 2012. Toughness is given as the integral from start until rupture, and is described as

Where e is the strain, e_B is the strain at break and σ is the stress.

Epoxy matrix formation

The simplest type of reaction to form an epoxy matrix is a two-component mixture of an epoxy resin and a hardener. The hardener is typically, but not limited to, a primary amine due to the reactivity and the number of reactive sites. Alcohols, thiols or isocyanates can also react with epoxies. Schematic

representations of the formation an epoxy matrix are given in figures 2 and 3. The epoxide unit is the essential moiety of all epoxy resins. An epoxide is a cyclic ether with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. The present invention is not limited to any specific types of epoxy resins and the specific experiments shown herein is only an example of a commercially available epoxy composition. Thus in the present invention, the epoxide unit (see figure 2, item 4) may be associated/combined with any suitable group. Epoxy composition comprising an amine- and disulfide-containinq additive

In the present invention, additives are added to the epoxy matrix in order to obtain epoxy compositions with new features. Mainly, it is the ambition of the present invention to introduce cleavable cross-linkers into the epoxy matrix, thereby obtaining epoxy compositions that may be disassembled by green chemistry, i.e. non-harsh solvents. Such epoxy compositions can be used in the preparation of sustainable epoxy-based polymer composites that may be fragmented into recyclable parts due to the introduction of the cleavable cross- linkers. The inventors have found that additives with both amine- and disulfide-moieties were suitable for preparing additives enabling the disassembly of epoxy

compositions.

Thus, a first aspect of the present invention relates to an epoxy composition comprising the reaction product of a set of reactants comprising :

a) at least one epoxy resin,

b) at least one hardener, and

a) at least one additive represented by formula (I),

(I) wherein

n is an integer from 1-10. Another aspect of the present invention relates to an epoxy composition comprising :

a) at least one epoxy resin

b) at least one hardener, and

c) at least one additive represented by formula (I),

wherein

n is an integer from 1-10.

In some epoxy compositions, the epoxy resin and hardener may be encompassed in a single molecule. As an example, this may be the case if an aromatic amine is incorporated into an epoxy resin. It is to be understood within the present context that such "pseudo" one-component epoxy compositions are considered to be equivalent to the two-component epoxy compositions recited herein, as they comprise both an epoxy resin element and a hardener element. Therefore, addition of an additive to an epoxy composition according to the present invention is not limited by the number of other components in the epoxy, but is applicable to epoxy compositions comprising one, two or more components besides the additive.

The additive of the epoxy composition can take several forms and may thus be described by a selection of embodiments. An embodiment of the present invention relates to an epoxy composition as described herein, wherein the optionally substituted C1-C5 alkyl groups comprises one or more substituents selected from the group consisting of a C1-C5 alkyl, halogen, cycloalkyls, heterocycloalkyls, aromatic and heteroaromatic groups. Another embodiment of the present invention relates to an epoxy composition as described herein, wherein R⁴ is -COOH.

Yet another embodiment of the present invention relates to an epoxy composition as described herein, wherein n is 1.

A further embodiment of the present invention relates to an epoxy composition as described herein, wherein R¹ is H. An even further embodiment of the present invention relates to an epoxy composition as described herein, wherein R² is H. Still another embodiment of the present invention relates to an epoxy composition as described herein, wherein R³ is H. In a particularly preferred embodiment, R¹, R² and R³ are H, R⁴ is COOH and n = l

L-cystine is interesting for the investigation of a sustainable epoxy system as an additive, since it contains both a disulfide bridge together with primary amines and carboxylic acids. If the disulfide bridge of L-cystine is broken by e.g. a thiol- disulfide exchange it yields the thiol counterpart known as the amino acid cysteine.

The main issue, when working with L-cystine is its solubility. The powerful internal hydrogen bonding interactions makes the compound hard to dissolve. A way to break the bonds is to change the pH. The pKa of L-cystine is 1.50 and 8.80 for the carboxylic acid and amine, respectively. Methanol and concentrated hydrochloric acid can be used as media to lower the pH and break the hydrogen bonds, hereby causing the L-cystine to swell.

L-cystine is easily available from natural sources and the use of L-cystine as additive is therefore perfectly aligned with object of providing new sustainable materials. Thus, a preferred embodiment of the present invention is related to an epoxy composition as described herein, wherein the additive is cystine, preferably L- cystine.

The additive according to the present invention may in principal be added to any epoxy compositions. Thus, the use of the additive is not limited to addition to any particular epoxy composition.

Thus, an aspect of the present invention relates to the use of a compound represented by formula (I)

wherein

n is an integer from 1-10, as an additive in an epoxy composition further comprising :

a) at least one epoxy resin, and

b) at least one hardener. For instance, the present invention is readily applicable to already commercially available epoxy compositions and may as such immediately be used to introduce new features to known epoxy-based polymer composites. Many different types of epoxy compositions i.e. mixtures of epoxy resins and hardeners) is currently applied in industries relying on epoxy-based polymer composites, all of which may be mixed with the additive of the present invention.

Thus, an embodiment of the present invention relates to an epoxy composition as described herein, wherein the at least one epoxy resin is selected from the group consisting of bisphenol A epoxy resins, bisphenol F epoxy resins, novolac epoxy resins, aliphatic epoxy resins, glycidylamine epoxy resins and combinations thereof.

Another embodiment of the present invention relates to an epoxy composition as described herein, wherein the at least one epoxy resin is selected from the group consisting of bisphenol A diglycidyl ether, bisphenol F diglycidyl ether,

butandioldiglycidylether and combination thereof.

A further embodiment of the present invention relates to an epoxy composition as described herein, wherein the at least one hardener is selected from the group consisting of amine hardeners, anhydride hardeners, phenol hardeners, thiol hardeners and combinations thereof.

A still further embodiment of the present invention relates to an epoxy

composition as described herein, wherein the at least one hardener is selected from one or more of the group consisting of poly(oxypropylen)diamine, 3- aminomethyl-3,5,5-trimethylcyclohexylamine,

4,4'-Methylenebis(cyclohexylamine) and combinations thereof.

Epoxy compositions may comprise one or more epoxy resins and one or more hardeners. Many available epoxy compositions obtain their enhanced thermal and mechanical features from the interplay between several epoxy resins and several hardeners. The additive of the present invention is not limited to epoxy

compositions comprising a single epoxy resin and a single hardener, but may be applied to any epoxy composition comprising any number of epoxy resins and hardeners. An example of a commercially available epoxy resin comprising two individual epoxy resins is a resin A containing 2,2-bis[p-(2,3- epoxypropoxy)phenyl and l,4-Bis(2,3-epoxypropyloxy)butane. An example of a commercially available hardener comprising two individual hardeners is a hardener B containing poly(oxypropylene) diamine and 3-aminomethyl-3,5,5- trimethyl-cyclohexylamine. The mixture of resin A and hardener B is one example of a typical epoxy composition used in epoxy-based polymer composites.

Thus, an embodiment of the present invention relates to an epoxy composition as described herein, wherein the at least one epoxy resin comprises 2,2-bis[p-(2,3- epoxypropoxy)phenyl and l,4-Bis(2,3-epoxypropyloxy)butane, and the at least one hardener comprises poly(oxypropylene) diamine and 3-aminomethyl-3,5,5- trimethyl-cyclohexylamine.

The present invention relates to epoxy compositions that comprise at least three different components; epoxy resin, hardener and additive. In contrast, most currently available epoxy composition comprise only two components; epoxy resin and hardener. Introduction of a third component in an epoxy composition may potentially alter thermal or mechanical properties of the original epoxy

composition. Thus, it is important that the amount of hardener substituted by additive does not impair the thermal or mechanical properties of the resulting epoxy composition. The inventors have found that epoxy compositions comprising the additive according to the present invention maintain or display enhanced thermal and mechanical properties. Thus, an embodiment of the present invention relates to an epoxy composition as described herein, wherein the additive constitutes less than 25 mol%.

Another preferred embodiment of the present invention relates to an epoxy composition as described herein, wherein the additive constitutes 0.01-25 mol%, such as 0.05-20 mol%, such as 0.1-15 mol%, such as 0.5-10 mol%, such as 1-5 mol%.

Preparation of epoxy composition

The preparation of the epoxy composition may extend beyond simple mixing of each component. The incorporation of the additive in the epoxy matrix, and ultimately the thermal and mechanical properties of the resulting epoxy

composition, may be influenced by the method of preparation. Therefore, the method of manufacture of the epoxy composition may be optimized by introducing process steps to enhance the incorporation of additive in the epoxy matrix.

Consequently, a second aspect of the present invention relates to a method of manufacturing an epoxy composition comprising the steps of:

a) swelling of an additive according to formula (I),

wherein

n is an integer from 1-10,

c) mixing the pre-treated solution with the at least one hardener,

thereby forming the epoxy composition. The inventors have surprisingly found that such a procedure results in an epoxy composition, which maintains or display enhanced thermal and mechanical properties. Depending on the choice of epoxy resins and hardeners, the epoxy composition may be further improved by thermal treatment of the epoxy composition.

Thus, an embodiment of the present composition relates to a method as described herein, wherein the epoxy composition is cured subsequent to step c) by applying a predetermined specific temperature profile.

The method of manufacture may be further enhanced by selection of suitable solvent for swelling the additive prior to pre-treatment with the epoxy resin.

Therefore, a preferred embodiment of the present invention relates to a method as described herein, wherein the swelling of the additive is performed by mixing of the additive in an alcohol followed by titration with an acid.

Another embodiment of the present invention relates to the method as described herein, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol and combinations thereof.

A further embodiment of the present invention relates to the method as described herein, wherein the acid is selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, citric acid, lactic acid, malic acid and combinations thereof.

Depending on the additive, it may also be suitable to use a base as solvent for swelling the additive prior to pre-treatment with the epoxy resin. Thus, an embodiment of the present invention relates to the method as described herein, wherein the swelling of the additive is performed by mixing of the additive in an alcohol followed by titration with a base, such as NaOH.

Epoxy-based polymer composite materials

One preferred use of the additive or epoxy composition of the present invention is as a component in epoxy-based polymer composite materials. Such materials typically comprise structural unit, such as fibres, and an epoxy composition.

Together these components form highly durable fibre reinforced polymer composites that may be used within a wide range of industries such as the aviation, automotive, ship and wind industries. To date, one major drawback of such fibre reinforced polymer composites is that they can hardly be picked apart when reaching end-of-life. Introduction of the additive of the present invention enables the fractionation of these materials and potentially make possible the recyclability of the structural units. Thus, an aspect of the present invention relates to a polymer composite material comprising :

a) structural fibres, and

b) the epoxy composition as described herein. A preferred embodiment of the present invention relates to the composite material as described herein, wherein the structural fibres are of a material selected from the group consisting of glass, carbon, cellulose, ceramics, natural fibres, pre-pregs, continuous fibres, platelet materials and graphene. The three-component epoxy composition comprising structural fibres as described herein may be used for the preparation of epoxy-based polymer composites within a wide range of industries such as the aviation, automotive, ship and wind industries. Thus, an aspect of the present invention relates to a wind turbine blade

comprising structural fibres and a resin matrix, the resin matrix comprising the epoxy composition as described herein.

Another aspect of the present invention relates to a wind turbine blade comprising the polymer composite material as described herein.

Fractionation of epoxy-based polymer composite material

By inclusion of the amine- and disulfide-containing additive described herein, cleavable cross-linkers are efficiently introduced into an epoxy composition of choice. These cross-linkers may be cleaved by use of an appropriate solvent thereby resulting in the disassembly of the epoxy matrix. As the epoxy matrix is the component that binds the structural units of epoxy-based polymer composite materials together, such a disassembly of the epoxy matrix also results in the fractionation of the composite into structural units.

Thus, an embodiment of the present invention relates to the composite material as described herein, wherein the composite material can be fractionated by immersion into a dismantling solvent. The efficiency with which the dismantling solvent works is erratic and it is not readily predictable which groups of solvents will disassemble the epoxy matrix and not simply swell, mechanically brake or have no effect on the epoxy matrix. Thus, the effect of the dismantling solvent cannot be predicted by characteristics such as chemical structure, polarity or acidity of the solvent.

The inventors have surprisingly found that a few selected solvent are particularly suitable for the dismantling of epoxy matrices comprising an additive according to the present invention.

Thus, a preferred embodiment of the present invention relates to the composite material as described herein, wherein the dismantling solvent comprises a compound selected from the group consisting of acetic acid, chloroform and N- methyl-2-pyrrolidone or a mixture thereof.

In a preferred embodiment the dismantling solvent may comprise compounds with similar reactivity to the additive of the present invention, and may include compounds selected from the group consisting of amines such as aliphatic amines and/or aromatic amines and reducing agents, such as disulfides. Disulfides may include dithiothreitol. Dismantling solvents may comprise the compounds in concentrated form, i.e. as the sole component or in dilute form, including e.g. aqueous compositions.

A particularly preferred embodiment of the present invention relates to the composite material as described herein, wherein the dismantling solvent comprises acetic acid. Furthermore, an aspect of the present invention relates to a method of

fractionating a polymer composite material as described herein by contacting the polymer composite material with a dismantling solvent.

An embodiment of the present invention relates to the method as described herein, wherein the dismantling solvent comprises a compound selected from the group consisting of acetic acid, chloroform and N-methyl-2-pyrrolidone. To reach the object of providing sustainable epoxy-based polymer composite materials the present invention includes:

• Provision of a sustainable/non-toxic additive for epoxy compositions

• Utilization of the additive to provide an epoxy composition that allows

disassembly of the epoxy matrix

· Provision of a scheme/method for fractionation of epoxy-based polymer composite materials under mild conditions.

It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention.

All patent and non-patent references cited in the present application, are hereby incorporated by reference in their entirety. The invention will now be described in further details in the following non-limiting examples.

Examples

Example 1 : Materials and methods

Chemicals

Methanol≤99.9% HPLC grade (MeOH), hydrochloric acid 37% (HCI), L-cystine ≤98.5%, 2-hydroxyethyl disulfide technical grade, 4-Aminophenyl disulfide 98%. Commercial epoxy resin contains propane, 2,2-bis[p-(2,3-epoxypropoxy)phenyl]-, polymers (BADGE) and l,4-Bis(2,3-epoxypropyloxy)butane. Commerical hardener contains poly(oxypropylene) diamine and 3-aminomethyl-3,5,5-trimethyl- cyclohexylamine.

Preparation of epoxy matrices

Standard epoxy

100.00 g of commercial epoxy resin is mixed by hand with 30.00 g of commercial hardener in a glass beaker. The beaker is moved to a vacuum chamber, where it is degassed for 20 minutes or until no bubbles can be found leaving the mixture. The mixture is poured into a mould and cured by applying a predetermined specific temperature profile.

L-cystine epoxy composition

The overall procedure is the same as for the standard epoxy mixture, but the L- cystine is swollen before addition to the epoxy matrix.

10.00 g L-cystine is added to 100 mL MeOH. 12 mL HCI is added dropwise under stirring until L-cystine swells. The swollen mixture is poured into a petri dish and left in the oven at 80°C overnight. The commercial epoxy resin is pre-treated with the swollen L-cystine, before it is mixed with the commercial hardener. The pre-treatment is done in an ultrasound bath at 55°C for 30 hours. L-cystine is expected to have six active sites, neglecting any reaction kinetics, whereas the commercial hardener have four reaction sites. 11.45 g swollen L-cystine is mixed with 190.94 g commercial epoxy resin for the pre-treatment and is afterwards divided into smaller amounts to make various additive-containing epoxies, e.g. 106.00 g of the pre-treated solution is mixed with 24.00 g commercial hardener in order to make a 20 mol% additive-containing epoxy. 4-aminophenyl disulfide epoxy composition

The overall procedure is the same as for the standard epoxy mixture. The number of moles is corrected depending on the number of reactions available on the additive molecule. A variation can be made where the epoxy resin and 4- aminophenyl disulfide is pre-treated at 55°C for 30 hours before the commercial hardener is added and the mixture is cured. 2-hydroxyethyl disulfide epoxy composition

The overall procedure is the same as for the standard epoxy mixture. The number of moles is corrected depending on the number of reactions available on the molecule. The terminal alcohols can attack one epoxy each, where the

commercially available hardener uses primary amines, which can attack two epoxies each. This means that for each mole of subtracted commercial hardener two moles of 2-hydroxyethyl disulfide is added. Resulting in 5 - 10 - 20 mol% exchanged additive is 2.51 g and 28.50 g commercial hardener - 5.04 g and 27.00 g commercial hardener - 10.09 g and 24.00 commercial hardener, respectively.

Tested solvents for disassembly of epoxy compositions

Diethyl ether (1), chloroform (2), methanol (3), 1,1,1-tricholoroethane (4), ethyl acetate (5), hydrochloric acid (6), sulfuric acid (7), acetonitrile (8), demineralized water (9), 1,4-dioxane (10), acetic acid (11), 2-ethoxyethanol(12), xylene (13), cyclohexanone (14), and N-methyl-2-pyrrolidone (15) are all bought from Sigma Aldrich and used as received without further purification. Test element fabrication

Test epoxy matrix elements were prepared in pieces of roughly the dimension 10x20x4 mm³, with an approximate weight of 0.5-1.0 g (referred to as test coupons). Glass transition temperature

The glass transition temperature of the cured epoxy is measured with a

Differential Scanning Calorimeter (DSC). The thermogram of the epoxy is used to determine the glass transition temperature (T_g). The sample is heated from 0°C to 200°C with a scan rate of lOK/min. Analysis is performed on the second up-scan as first scan is thermal homogenization.

Tensile testing

The test coupons used for testing follows the ISO 527-2 standards type 1A

(directly-molded multipurpose test). The instrument used is a LR150k plus (LLOYDTM) with a click-on axial extensometer with a gauge length of 10 mm. The Emoduius, UTS and %EL at break is measured in accordance with ISO 527-2 and is based on six test specimens. Swelling experiments

The test specimens are put in a 20 mL glass vial with snap cap, where approx. 12- 15 mL solvent is added (in excess to cover the sample).

The mass is measured on a scale and the volume by Archimedes' principle. The swelling ratio is calculated by equation 1 :

Vol .Swelling?'* ' 100%,

Solution-state NMR spectroscopy

The experiments were conducted using an AVANCE-III HD (Bruker) 400 MHz spectrometer. The epoxy resin, hardener, 2-hydroxyethyl disulfide, and 4- aminophenyl disulfide was diluted in 1.5 mL deuterated DMSO (ref. 99.8% D, Euriso-Top). L-cystine was diluted in 1.5 mL D2O (ref. 99.98% D, Sigma-Aldrich) and 0.1 mL hydrochloric acid (37 wt%, Sigma-Aldrich). Post-analysis was done in MestReNova (v. 10.0.2-15465)

Solid-state NMR spectroscopy

An AVANCE-III (Bruker) wide-bore 700 MHz NMR spectrometer with a standard 4.0 mm HXY triple-resonance MAS probe (14 kHz rotation) was used. The matrices were cut with a 3 mm router bit, and the shavings were packed into 4 mm NMR rotors. ^- -^C CP MAS (cross-polarization magic-angle spinning) spectra were acquired at 15-20 °C with 2048 scans collected for all epoxy matrices. Post- analysis was done in MestReNova (v. 10.0.2-15465).

FTIR spectroscopy

Infrared spectra were measured with a Nicolet FTIR Spectrometer (iS5, Thermo Fisher Scientific, USA) controlled with the OMNIC software (Thermo Fisher

Scientific, USA). The background was subtracted from the recorded sample spectra. Solid samples of the matrices were placed on the ATR (attenuated total reflectance) crystal (iD7, Thermo Fisher Scientific, USA) and held firmly with a torque wrench. Epoxy resin and hardener liquid samples were applied dropwise to the crystal window. Data were corrected for wavelength-dependent penetration depth followed by baseline correction.

Example 2 : Incorporation of additives in the epoxy matrix and glass transition temperature of epoxy compositions

The formation of the epoxy compositions (e.g. curing and incorporation of additives) was investigated and compared to a standard epoxy with known characteristics.

The epoxy matrices were characterized by FTIR to investigate the curing process. FTIR spectra of the cured standard epoxy matrix can be found in Figure 6A together with spectra of the epoxy resin and hardener. The cured STD epoxy matrix (Figure 6A, curve 1) shows a broad O-H band at 3403 cm^-1, and there are no bands from either oxirane, or primary or secondary amines, indicating a fully cured material.

FTIR spectra showing the region 1750-750 cm ¹ of the modified and STD epoxy matrices are shown in Figure 6B. Normalized spectra of Cys20mol% (top),

Hyd20mol% (middle), and Phy20mol% (bottom) versus STD are given in figure 6C and show the differences between the modified matrices and the standard matrix. The peaks originating from the additives recur in the normalized spectra, thus supporting the indication that the additives have been incorporated in the matrices.

In addition to FTIR experiments, the epoxy compositions were also chararcterized by solution-state NMR and solid-state NMR. Solution-state NMR spectra of the epoxy resin and the hardener is provided in Figure 7A, together with solid-state NMR spectra of the cured epoxy matrix (STD). Comparing the cured STD epoxy matrix to the epoxy resin, it can be seen that the peak at 44 ppm is absent, whereas the oxirane peak at 51 ppm cannot be identified, indicating a fully cured epoxy matrix. Figure 7B shows the solid-state spectra of the standard and modified epoxy matrices. A shoulder at 44 ppm (β-carbon) and the better- resolved peak at 52 ppm (a-carbon) show the presence of L-cystine in the Cys matrix. Similarly, the spectrum of Hyd20mol% shows a clear signal at 61 ppm, indicating the presence of 2-hydroxyethyl disulfide (-CH2-OH) in the Hyd matrix. For Phy20mol%, the other aromatic peaks at 150 and 137 ppm indicate incorporation of 4-aminophenyl disulfide into the Phy matrix. In all cases, the peak at 18 ppm (-CH3) has reduced intensities owing to mass reduction of the commercial hardener, and no peaks from the oxirane or carbons neighbouring the primary amine groups (44-51 ppm) can be identified, indicating full curing.

The glass transition temperature of the standard epoxy and epoxy compositions comprising L-cystine, 2-hydroxyethyl disulfide and 4-aminophenyl disulfide, respectively were measured as described herein. All T_g values are given in Table 1.

Name Tg Note

STD 83.6

Cys5mol% 81.3 US48h+

Cys5mol% 78.7 OB30h

Cys5mol% 82.3 US30h

Cysl0mol% 72.2 US48h+

Cysl0mol% 74.8 OB30h

Cysl0mol% 82.4 US30h

Cysl0mol% 77.5 US30h+C

Cys20mol% 70.7 US48h+

Cys20mol% 72.1 OB30h

Cys20mol% 76.6 US30h

Hyd5mol% 72.2

Hyd5mol% 74.8

Hydl0mol% 63.3

Hydl0mol% 57.0

Hyd20mol% 43.6

Hyd20mol% 47.2

Phen20mol% 91.7 Mix

Phen20mol% 91.5 Pretreat Table 1 : T_g values are measured on the second run with a heating rate of

10°C/min on a DSC. US=Ultrasound bath, OB=Oil bath, C=Centrifuge. "Mix" corresponds to mixing the epoxy resin, hardener and additive as described for the STD sample. "Pretreat" corresponds to preparing the sample according to the method of the L-cystine epoxy composition.

The mixtures with same compositions have T_g values, which indicates the process is very reproducible. The trend is that the 2-hydroxyethyl disulphide lowers the T_g as larger amounts are introduced. The 4-Aminophenyl disulfide increases the T_g when high concentrations are added. Whereas the amount of L-cystine is similar at all ratios compared to the standard epoxy system.

Example 3: Tensile testing of epoxy compositions

The tensile properties of the standard epoxy and epoxy compositions comprising L-cystine, 2-hydroxyethyl disulfide and 4-aminophenyl disulfide, respectively were tested as described herein. The calculated Emoduius, UTS, %EL at break and toughness for all epoxy compositions tested can be found in Table 2 as the average value of the tested specimens. Uncertainty is given as standard deviation.

Hyd5mol% 2456±614 47.1±4.5 38.6± 12.1 13.4±3.2 n = 6

Hydl0mol% 3406±547 59.8± 1.2 46.2±24.6 18.5± 1.7 n = 6

Hyd20mol% 1137± 172 18.7± 1.6 367.9±69.9 58.2±7.8 n = 6

Phen20mol% N/A 54.5±5.0 N/A N/A Mix, n=4

Phen20mol% N/A 66.1±4.0 N/A N/A Pretreat, n = 5

Table 2: US=Ultrasound bath, OB=Oil bath, C=Centrifuge, n = number of replicates. The general trend is that 2-hydroxyethyl disulfide lowers the listed mechanical properties. On the other hand, 4-Aminophenyl disulfide and L-cystine seems to enhance the matrix properties with a higher Emoduius and UTS when introduced in lower concentrations. The matrix is becoming a bit more brittle as well, since %EL at break is lower than for the standard matrix. Overall, the toughness are roughly the same for 4-Aminophenyl disulfide, L-cystine and the standard epoxy. For both hydroxyethyl disulfide and L-cystine, it seems that the mechanical properties are reduced when too much is introduced and it becomes more flexible with less strength. The general enhancement of the listed mechanical properties in the matrix seems to be best, when the pre-treatment of L-cystine is done in an ultrasound bath for 30h compared to the other noted pre-treatments.

In table 2, values for "%EL at break" and "toughness" were extracted directly from the tensile test instrument. Values for "%EL at break" and "toughness" may also be calculated manually from the raw data of the same experiments. Utilizing the manual approach of calculation from the raw data gives values for "%EL at break" and "toughness", which are proportionally identical between samples, but have different absolute values (see table 3). Only the values of a subset of the samples is displayed in table 3. Table 3

Name Emodulus UTS [MPa] %EL at Toughnes Note

[MPa] break [%] s [MPa]

STD 2976±937 55.7±3.6 2.9±0.5 1.0±0.2 n=6

Cys5mol% 4035±780 60.9±4.1 2.2±0.6 0.9±0.2 US 30h, n=6 Cysl0mol% 4403± 1279 58.8±6.1 1.8±0.3 0.6±0.1 US 30h, n = 6

Cys20mol% 3632± 1435 40.8±6.6 1.3±0.2 0.3±0.1 US 30h, n = 6

Hyd5mol% 2456±614 47.1±4.5 2.6±0.6 0.7±0.1 n = 6

Hydl0mol% 3406±547 59.8± 1.2 2.7±0.8 1.0±0.3 n=6

Hyd20mol% 1137± 172 18.7± 1.6 19.3±3.5 3.0±0.4 n=6

Phen20mol% N/A 66.1±4.0 6.5±0.7 2.8±0.5 Pretreat, n = 5

US=Ultrasounc bath, n = num ber of replicates.

The data shows that the mechanical characteristics of the L-cystine modified epoxy are comparable to the standard epoxy without any additive.

Example 4: Swelling of epoxy compositions in different solvents

Epoxy compositions comprising no additive (STD), L-cystine, 2-hydroxyethyl disulfide or 4-aminophenyl disulfide was tested for swelling in a variety of solvents as described herein.

The solvents used were diethyl ether (1), chloroform (2), methanol (3), 1,1,1- tricholoroethane (4), ethyl acetate (5), hydrochloric acid cone. (6), sulfuric acid cone. (7), acetonitrile (8), demineralized water (9), 1,4-dioxane (10), acetic acid (11), 2-ethoxyethanol(12), xylene (13), cyclohexanone (14), and N-methyl-2- pyrrolidone (15).

The experiments showed that the standard epoxy composition and epoxy compositions comprising 2-hydroxyethyl disulfide and 4-aminophenyl disulfide depending on the solvent, mechanically brake or swell to different degrees.

It was found that the epoxy composition comprising L-cystine behaved somewhat different than the three other epoxy compositions. Whereas the epoxy

composition comprising L-cystine mechanically broke when immersed into e.g. 1,4-dioxane and cyclohexanone, it were instead surprisingly rapidly dismantled when immersed into acetic acid, chloroform and N-methyl-2-pyrrolidone. In contrast, acetic acid and N-methyl-2-pyrrolidone mechanical broke the standard epoxy. Furthermore, the epoxy composition comprising 2-hydroxyethyl disulfide was also mechanically broken by N-methyl-2-pyrrolidone. In table 4 is given the rates of the volumetric swelling after 10 days at 70 °C (unless otherwise stated) of the standard, Cys20mol%, and Hyd20mol% epoxy matrices in a variety of solvents.

Table 4

a] No sonication. [b] With sonication. [c] Temperature 30 °C.

D: Dissolved, B: Mechanically broken, F: Fractionated.

Table 4 shows that the modified epoxy matrices tend to swell, break, or fractionate more than the STD epoxy matrix. It is important to distinguish between the terms mechanically broken and fractionated. The fractionated matrices are reduced to very small pieces (see also example 6), whereas the mechanically broken matrices form larger pieces during swelling in the vials.

Additionally, the water retention was tested for each of the epoxy compositions. Water uptake is a central parameter for applications of epoxy materials, and was tested by immersion of the matrices in demineralized water at elevated temperatures. The results are given in Figure 8, showing that the modified epoxy matrices does not swell more than standard epoxy i.e. less than 5%). The results indicate no significant difference in water uptake, independent of both additive and sonication treatment. From the initial swelling data, the densities of the selected epoxy matrices were determined (n>40) as p(STD) = 1.173±0.008 g mL ¹, p(Cys20mol%) = 1.192±0.009 g mL ¹, and p(Hyd20mol%) = 1.185±0.004 g mL^"1. The similarity of densities compared to STD indicates that no significant changes in density occur upon modification.

In conclusion, the data shows that the modified epoxy matrices have the same resistance to water as the standard epoxy matrix, while displaying increased solvability in other solvents, such as N-methyl-2-pyrrolidone, acetic acid and chloroform. Furthermore, the densities of the modified epoxy matrices are comparable to the standard epoxy matrix, thereby permitting a strong, yet lightweight material.

Example 5: Disassembly of epoxy compositions in acetic acid

The behaviour of epoxy compositions comprising L-cystine, 20 mol% L-cystine was further tested in acetic acid (glacial) against the standard epoxy. The glass vials were left on a hotplate at 70°C and photographs of the swelling process were acquired continually. Three photos of this experiment is shown in figure 4.

The photographs illustrates that the 20 mol% L-cystine epoxy starts to swell and slowly dismantles over time, whereas little happens to the standard epoxy. The 20 mol% L-cystine epoxy behaved similar in chloroform and N-methyl-2- pyrrolidone.

Example 6: Disassembly of epoxy compositions comprising different mol% L- cystine in acetic acid, chloroform and N-methyl-2-pyrrolidone

The effect of the amount of additive was tested by immersion of epoxy

compositions comprising a varying quantity of L-cystine (5 mol%, 10 mol%, and 20 mol%) in acetic acid, chloroform and N-methyl-2-pyrrolidone. The glass vials were left in a water bath at 70°C. Photographs (figure 5A-C) document the influence of acetic acid on immersion of epoxy compositions comprising a varying quantity of L-cystine before (A) and after (B-C) immersion in the solvent.

The samples were tested in duplicates with samples A and B corresponding to 5 mol% L-cystine, C and D corresponding to 10 mol% L-cystine, and E and F corresponding to 20 mol% L-cystine.

The experiment showed no lower boundary of additive concentration for dismantling the epoxy composition comprising L-cystine in acetic acid, chloroform and N-methyl-2-pyrrolidone.

For efficient dismantling of the epoxy matrix, it is important that the matrix fractionates into small fragments instead of breaking into larger pieces. In the present experiment, fractionation in acetic acid yielded average grain diameters of 3.2± 1.3 mm, 2.0±0.8 mm, and 2.4± 1.1 mm for Cys5mol%, Cysl0mol%, and Cys20mol%, respectively. Thus, the method accomplish the aim of fractionating the epoxy matrix to small fragments and is a suitable method for recycling of composite material components. Example 7: Polymer composite material comprising glass fibres and epoxy composition with additive

Polymer composite materials comprising glass fibres mixed with two different modified epoxy compositions (Cys5mol% or Cysl0mol%) were tested for their tensile properties and benchmarked against two polymer composite materials based on the most utilized commercially available epoxy compositions

(762H/766H and STD). The 762H/766H epoxy compositions is a standard composition, which is known to cure fast. Representative tensile tests of the polymer composite materials comprising glass fibres mixed with standard (STD) epoxy composition and Cys5mol% are depicted in figures 9A and 9B, respectively. The quantified results of the tensile testing is given in table 5. Table 5

The data confirm that the polymer composite materials comprising glass fibres mixed with L-cystine modified epoxy compositions have comparable tensile properties as the commercially available epoxy compositions (762H/766H and STD).

Disassembly of the polymer composite materials comprising glass fibres mixed with the Cys5mol%, Cysl0mol% or standard epoxy composition were tested as described in examples 5 and 6 (see figure 10) utilizing acetic acid as solvent. In the composite materials comprising L-cystine (vials in middle and right, figure 10) it was possible to separate the glass fibres from the epoxy matrix (see pulp in bottom of vials, figure 10), whereas no separation was achieved for the composite material comprising the standard epoxy matrix (no pulp in bottom of vial, figure 10). Additional experiments support that acetic acid mixed with up to 50% water still achieves the same effect of separation of fibres from epoxy matrix (data not shown). Thus, the Cys-modified epoxy resins may be used for generation of composite materials possessing great mechanical properties, while providing the opportunity to separate fibres from the epoxy matrix if desired.

Overall conclusion of examples

To conclude, chloroform, acetic acid and N-methyl-2-pyrrolidone showed to be able to dismantle the L-cystine epoxies without further additives added . The standard epoxy and epoxy made with 2-hydroxyethyl or 4-aminophenyl disulfide did not show the same dismantling tendencies.

The 4-aminophenyl disulfide compound is an interesting component for enhanced mechanical properties and a higher Tg, but not as a recyclable epoxy as was the same case for the 2-hydroxyethyl disulfide compound. The L-cystine matrix was the only compound showing the promising dismantling properties.

It is worth noting that the solvents facilitating the dismantling of the epoxy composition comprising L-cystine are very different in chemical structure. They are not limited to only being depending on the polarity or acidity of the solvent, e.g. acetic acid worked whereas concentrated hydrochloric acid did not.

Furthermore, it was confirmed that the modified epoxy compositions comprising additives were fully cured with the additives intrinsically incorporated into the epoxy matrix, yielding epoxy compositions with comparable water resistance and mechanical properties as the standard epoxy composition.

References

de Luzuriaga A.R., Martin R., Markaide N., Rekondo A., Cabahero G., Rodriguez J., and Odriozola I., "Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber- reinforced thermoset composites" Materials Horizon (2016), 3, 241-247.

Claims

1. An epoxy composition comprising the reaction product of a set of reactants comprising :

a) at least one epoxy resin,

b) at least one hardener, and

c) at least one additive represented by formula

wherein

n is an integer from 1-10.

2. The epoxy composition according to claim 1, wherein the optionally substituted C1-C5 alkyl groups comprises one or more substituents selected from the group consisting of a C1-C5 alkyl, halogen, cycloalkyls, heterocycloalkyls, aromatic and heteroaromatic groups.

3. The epoxy composition according to any one of the preceding claims, wherein R⁴ is -COOH.

4. The epoxy composition according to any one of the preceding claims, wherein n is 1.

5. The epoxy composition according to any one of the preceding claims, wherein R¹ is H.

6. The epoxy composition according to any one of the preceding claims, wherein R² is H.

7. The epoxy composition according to any one of the preceding claims, wherein R³ is H.

8. The epoxy composition according to any one of the preceding claims, wherein said additive is cystine, preferably L-cystine.

9. The epoxy composition according to any one of the preceding claims, wherein the at least one epoxy resin is selected from the group consisting of bisphenol A epoxy resins, bisphenol F epoxy resins, novolac epoxy resins, aliphatic epoxy resins, glycidylamine epoxy resins and combinations thereof.

10. The epoxy composition according to any one of the preceding claims, wherein the at least one epoxy resin is selected from the group consisting of bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, butandioldiglycidylether and combination thereof.

11. The epoxy composition according to any one of the preceding claims, wherein the at least one hardener is selected from the group consisting of amine hardeners, anhydride hardeners, phenol hardeners, thiol hardeners and combinations thereof.

12. The epoxy composition according to any one of the preceding claims, wherein the at least one hardener is selected from one or more of the group consisting of poly(oxypropylen)diamine, 3-aminomethyl-3,5,5-trimethylcyclohexylamine, 4,4'-Methylenebis(cyclohexylamine) and combinations thereof.

13. The epoxy composition according to any one of the preceding claims, wherein the at least one epoxy resin comprises 2,2-bis[p-(2,3-epoxypropoxy)phenyl and l,4-Bis(2,3-epoxypropyloxy)butane, and the at least one hardener comprises poly(oxypropylene) diamine and 3-aminomethyl-3,5,5-trimethyl-cyclohexylamine.

14. The epoxy composition according to any one of the preceding claims, wherein the additive constitutes less than 25 mol%.

15. The epoxy composition according to any one of the preceding claims, wherein the additive constitutes 0.01-25 mol%, such as 0.05-20 mol%, such as 0.1-15 mol%, such as 0.5-10 mol%, such as 1-5 mol%.

16. A wind turbine blade comprising structural fibres and a resin matrix, the resin matrix comprising the epoxy composition of any one of claims 1-15.

17. A method of manufacturing an epoxy composition comprising the steps of: a) swelling of an additive according to formula (I),

wherein

n is an integer from 1-10,

c) mixing said pre-treated solution with the at least one hardener,

thereby forming said epoxy composition.

18. The method according to claim 17, wherein said epoxy composition is cured subsequent to step c) by applying a predetermined specific temperature profile.

19. The method according to any one of claims 17-18, wherein said swelling of 5 the additive is performed by mixing of the additive in an alcohol followed by titration with an acid.

20. The method according to claim 19, wherein said alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol and combinations

10 thereof.

21. The method according to any one of claims 19-20, wherein said acid is selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, citric acid, lactic acid, malic acid and combinations thereof.

15

22. A polymer composite material comprising :

a) structural fibres, and

b) the epoxy composition according to claims 1-15.

20 23. The composite material according to claim 22, wherein said structural fibres are of a material selected from the group consisting of glass, carbon, cellulose, ceramics, natural fibres, pre-pregs, continuous fibres, platelet materials and graphene.

25 24. The composite material according to any one of claims 22-23, wherein said composite material can be fractionated by immersion into a dismantling solvent.

25. The composite material according to claim 24, wherein said dismantling solvent comprises a compound selected from the group consisting of acetic acid,

30 chloroform and N-methyl-2-pyrrolidone or a mixture thereof.

26. A wind turbine blade comprising the polymer composite material according to any one of claims 22-23.

35 27. Use of a compound represented by formula (I) wherein

n is an integer from 1-10,

as an additive in an epoxy composition further comprising :

a) at least one epoxy resin, and

b) at least one hardener.

28. An epoxy composition comprising :

a) at least one epoxy resin

b) at least one hardener, and

c) at least one additive represented by formula

wherein

R¹, R², and R³ are independently selected from the group consisting of H, and an optionally substituted C1-C5 alkyl group, R⁴ is selected from the group consisting of -H, -OH, -SH, -CN, -OCN, and COOR, wherein

n is an integer from 1-10.

29. A method of fractionating a polymer composite material according to any one of claims 22-23 by contacting said polymer composite material with a dismantling solvent.

30. The method according to claim 29, wherein said dismantling solvent comprises a compound selected from the group consisting of acetic acid, chloroform and N-methyl-2-pyrrolidone.