EP0273806B1 - Electrochemical process for the treatment of carbon fibres, and fibres treated by this process - Google Patents

Description

The invention relates to an electrochemical process for the surface treatment of carbonaceous materials. It applies in particular to the surface treatment of carbon fibers in order to improve the adhesion of the fibers to the resin in a composite material formed of carbon fibers embedded in a matrix of synthetic resin.

The mechanical properties of a carbon-resin composite material are all the better as the interlaminar decohesion occurs for a higher shear stress and, consequently, that the carbon fibers adhere better to the resin. However, very high adhesion leads to a certain brittleness of the material, that is to say to a lack of resilience.

It has already been proposed to improve the adhesion of the fibers to the resin by treating the surface of the raw carbon fibers of manufacture either chemically or electrochemically. Chemical groups are thus generated on the surface of the fibers which promote the adhesion of the fibers to the resins largely by the creation of chemical fiber-matrix bonds, but also to a certain extent by an increase in Van der Waals interactions or interactions. dipoles between the two constituents fibers and resin, if necessary.

Electrochemical treatments of this type are for example described in the Damende of French patent No. 2,477,593. They essentially consist in immersing the fibers in an electrolyte solution by polarizing them positively with respect to a cathode. In particular, good adhesion is obtained by using ammonium and sodium sulfates and bisulfates as electrolytes, which are strong saline electrolytes.

These electrolytes comprising oxygenated anions and lead to the grafting of oxygenated groups on the carbon fibers. These oxygenated groups improve the adhesion of fibers to synthetic resins, but the treatment process can in certain cases give rise to a degradation of the mechanical properties of carbon fibers.

The earlier application also mentions tests carried out using strong acids and bases (sulfuric acid, phosphoric acid, sodium hydroxide) as electrolytes. There is then either an inhibition of the hardening of the impregnation resin, or poor resistance to thermal oxidation of the treated fibers.

Examination of the operating conditions of conventional electrochemical treatments shows that: in general, they involve acid, basic or saline solutions in an aqueous medium;

the potential applied between the anode made of carbon fibers and the cathode is sufficient to cause the decomposition of water and the appearance of a gaseous release of oxygen, phenomena well known in electrochemistry.

The electrolyte then includes reactive species which attack the carbon of the fibers to form oxygenated surface groups which promote fiber-matrix adhesion. The potential V _o of decomposition of water with release of oxygen is close to +1.7 volts relative to a reference electrode with saturated calomel, but can be lowered by certain electrolytes. In any event, anodic treatments carried out above V _o lead, whatever the electrolytes used, to the decomposition of water and to the formation of oxygenated groups (type C = O, COH, COOH. .) or even degradation of the surface of the fibers if the working potential V, is much greater than V _o . Only the relative proportions and the surface concentrations of these oxygenated groups vary from one process to another, and one can hardly expect to improve the resilience of the composite materials, the fiber-matrix interface generated by the oxygenated groups having essentially the same nature from one treatment to another.

French Patent No. 2,564,489 in the name of the Applicant describes a process for treating carbon fibers allowing the grafting of nitrogenous functions. In this process, the fibers are immersed in an aqueous solution of an amino compound which dissociates little water, so as to avoid an excessive reduction in V _o .

The object of the invention is to provide an electrochemical process leading to the grafting of nitrogenous groups to the surface of carbon fibers, avoiding the limitations linked to the use of an aqueous solution, in particular with regard to the speed of the electrochemical reaction. .

Another object of the invention is to graft nitrogen groups onto carbon in a form other than that of fibers, in particular in a divided form, with a view to catalytic use for example.

The subject of the invention is an electrochemical carbon treatment process in which the carbon is brought into contact with a solution of an amino compound in a dipolar solvent by polarizing it positively with respect to a cathode, characterized in that the solvent is an organic compound with high anodic oxidation potential, the solution being practically free of water.

Advantageously, the solvent is an aprotic dipolar solvent.

Three conditions must be met before nitrogenous substances can be grafted onto the carbon surface by electrochemical means.

The first condition is that the surface reactivity of the carbon is sufficient, which is the case of microporous carbons, graphitable carbons of low temperature and carbons with activated surface.

Carbons fall into two main categories: non-graphitable carbons and graphitizable carbons.

• un La et un Le faibles en diffraction de rayons X;
• une organisation de la microtexture en micropores par observation en microscopie électronique en transmission à haute résolution;
• une texture isotrope en microscopie optique.

Microporous carbons are carbons with a non-graphitizable turbostratic structure which are characterized by:

• a La and a Le weak in X-ray diffraction;
• an organization of the microtexture into micropores by observation in high resolution transmission electron microscopy;
• an isotropic texture in optical microscopy.

La and Le denote the dimensions of the basic textual unit, respectively parallel and perpendicular to the aromatic layers.

The size of the micropores is of the order of a few tens of nanometers; The rest remains low whatever the heat treatment temperature, the torsion of the layers being non-reducible.

Carbon fibers called "high resistance" and "intermediates", carbon blacks, certain pyrocarbons, belong to the category of microporous carbons.

The carbon of the "high resistance" fibers has a microtexture constituted by an assembly of basic textural units (UTB) formed by a turbostratic stack of two or three aromatic layers of small dimension [approximately 1 nm (10 A)]. The UTB are connected to each other by sp ³ type chemical bonds forming a joint with bending and twisting disorientations. A "high resistance" fiber is made up of UTB aggregates whose average orientation is that of the axis of the fiber. The surface of these fibers has a high density of sp ³ type bonds capable of being attacked electrochemically.

The "intermediate fibers" have UTBs slightly larger in size than those of the "high resistance"fibers; the density of surface bonds type _S p ³ remains high, although less.

The carbon of "high modulus" fibers is analogous to a non-graphitable pyrocarbon with large La, but it remains microporous. This type of carbon is not suitable for the treatment according to the invention, unless it is previously activated.

The "high modulus" fibers comprise UTBs of very different sizes, because they have undergone a "graphitation" stage between 2000 and 3000 ° C. The UTB are turbostratic stacks of several tens of aromatic layers providing reaching or exceeding, in particular on the surface, a size of 100 nm (1000 Å). Consequently, the density of joints between UTB is much lower than for "high strength" fibers, which gives "high modulus" fibers a surface reactivity very attenuated by the bonds between carbon atoms engaged in aromatic rings are very stable. The action of a nitrogen plasma on the surface of these fibers increases their reactivity by the ejection of carbon atoms from the aromatic surface layers and consequently allows the treatment according to the invention.

Graphitable carbons are characterized by a Le higher than La below about 1500 ° C, but their La increases above 1500 ° C and especially 2000 ° C (observation by high resolution electron microscopy in network fringes) in developing a three-dimensional periodic structure (graphitation).

When the treatment temperature is between 600 and 1000 ° C, La and Le are comparable (= 2.5 nm). Then, Le grows up to approximately 1500 ° C. From this temperature, La increases and becomes greater than Le.

Low temperature graphitable carbons (θ ≼ 1500 ° C) therefore have a microtexture which makes them sensitive to the action of an electrochemical treatment. High temperature graphitable carbons only become graphitable if their surface has been previously activated.

The second condition allowing grafting is that the working potential V _t is less than the decomposition potential V _SOL of the solvent or of the solvent + support electrolyte pair.

The organic solvent used in the treatment can be, in particular, acetonitrile, dimethylformamide, dimethyl sulfoxide. It is advantageous to add to the solution a support electrolyte also having a high anodic oxidation potential V _es , which depends on the nature of the organic solvent, such as lithium perchlorate, tetraethylammonium perchlorate or, for example, tetrafluoroborates, alkaline tetrafluorophosphates or quaternary ammoniums.

In general, the working potential V _t is limited by the oxidation potential of the support electrolyte, which varies with the solvent used, which leads to setting a potential V _SOL for a couple considered. The table below lists the V _SOL values observed for various solvents and supporting electrolytes.

Finally, third condition, the working potential V _t must be greater than the redox potential V _E of the amino compound or, if the amino compound has several amine functions, V _t must be greater than the redox potential the weaker. In order for the electrochemical reactions to occur quickly, we must make sure that the difference V _t ―V _E is high with V _t <V _SOL . It is also desirable that V _t is not too close to V _SOL , since we could then see parasitic electrochemical phenomena such as the passivation of the anode resulting from an accumulation of amine oxidation products forming a film on the electrode which may possibly remain on the surface.

The use of a non-aqueous electrolytic solution makes it easier to reconcile these latter two conditions and therefore to carry out treatments faster than those using an aqueous solution.

The amino compound used in the treatment is advantageously ethylenediamine whose redox potential V _E on vitreous carbon is approximately +1.2 Volt relative to a reference electrode with saturated calomel, in a mixture of acetonitrile - perchlorate of 0.1 M tetraethylammmonium (ie V _e = +0.9 Volt relative to an Ag / Ag electrode ⁺ 0.01 M).

As amino compound, it is also possible to use the amino 6 methyl 2 pyridine, tetramethylbenzidine, or any other compound having at least one redox potential lower than V _SOL .

The treatment is carried out under insufficient polarization to cause the decomposition of the solvent and of the support electrolyte. Good results are obtained by polarizing the fibers at a working potential V _t = 1.3 volts approximately with respect to a reference electrode Ag / Ag ⁺ 0.01 M, value significantly less than V _SOL for the acetonitrile + perchlorate pair. of lithium, or about + 2.3 volts with this electrode. V _t = 1.3 volts is located at the start of the ohmic regime of the polarization curve.

The invention also relates to a carbon, in particular in the form of fibers, treated by the process which has just been defined, as well as a composite material. The carbons treated according to the invention can also be powdered or divided carbons, provided that they also belong to the categories of microporous carbons and graphitable carbons of low temperature or that their surface has been activated.

• la figure 1 représente schématiquement un dispositif de laboratoire pour la mise en oeuvre du procède;
• la figure 2 est une courbe caractéristique de l'évolution du courant en fonction du potentiel imposé aux fibres;
• la figure 3 est un schéma d'une installation industrielle pour la mise en oeuvre du procédé dans le cas de fibres de carbone;
• la figure 4 est un schéma d'une installation industrielle pour la mise en oeuvre du procédé dans le cas de carbones divisés;
• la figure 5 est un schéma d'une installation de laboratoire pour le traitement de fibres de carbone par un plasma d'azote;
• la figure 6 regroupe un ensemble de spectres obtenus en spectroscopie de photoélectrons (ESCA) et en spectrométrie de masse d'ions secondaire (SIMS) relatifs à des surfaces de fibres de carbone COURTAULDS Grafil HT non traitées d'origine et ayant subi par la suite des traitements à l'hexaméthylène tétramine en milieu aqueux;
• la figure 7 représente un détail des pics de photoélectrons (ESCA) obtenu pour ces mêmes fibres traitées à l'hexaméthylène tétramine;
• la figure 8 regroupe un ensemble de spectres obtenus en ESCA et en SIMS pour des fibres COURTAULDS Grafil HT ayant subi un traitement à l'amino 6 méthyl 2 pyridine en milieu aqueux;
• la figure 9 regroupe un ensemble de spectres obtenus en ESCA et en SIMS pour des fibres COURTAULDS Grafil HT ayant subi un traitement à l'urée en milieu aqueux;
• la figure 10 regroupe un ensemble de spectres obtenus en ESCA et en SIMS pour des fibres COURTAULDS Grafil HT ayant subi un traitement à l'éthylènediamine dans l'acétonitrile additionné de perchlorate de lithium;
• la figure 11 regroupe un ensemble de spectres obtenus en ESCA et en SIMS pour les mêmes fibres ayant subi un triatement à l'amino 6 méthyl 2 pyridine dans l'acétonitrile sans électrolyte support;
• la figure 12 regroupe un ensemble de spectres obtenus en ESCA et en SIMS pour les fibres COURTAULDS Grafil HT ayant subi un traitement à l'éthylènediamine dans le diméthylformamide additionné de perchlorate de lithium;
• la figure 13 montre l'évolution de la contrainte de décohésion τ_d fibre-matrice pour les trois types de traitements en milieu aqueux mentionnés ci-dessus en fonction de leur durée. La résine employée est l'Araldite LY 556 + durcisseur HT 972 CIBA GEIGY:
• la figure 14 montre l'évolution de la contrainte de décohésion τ_d fibre-matrice pour les traitements à l'éthylènediamine dans l'acétonitrile additionné de perchlorate de lithium. On utilise deux résines époxyde, la résine CIBA GEIGY Araldite LY 556 et la résine NARMCO 5208;
• la figure 15 regroupe des spectres en ESCA pour montrer que de l'épichlorhydrine se fixe sur des fibres traitées à l'hexaméthylène tétramine et ne se fixe pas sur ces mêmes fibres non traitées.

The invention will be better understood thanks to the detailed description given below, by way of illustration and not limitation, of a few embodiments, and in the appended drawings, in which:

• Figure 1 schematically shows a laboratory device for the implementation of the process;
• Figure 2 is a characteristic curve of the evolution of the current as a function of the potential imposed on the fibers;
• FIG. 3 is a diagram of an industrial installation for implementing the method in the case of carbon fibers;
• FIG. 4 is a diagram of an industrial installation for implementing the method in the case of divided carbons;
• FIG. 5 is a diagram of a laboratory installation for the treatment of carbon fibers with a nitrogen plasma;
• FIG. 6 brings together a set of spectra obtained in photoelectron spectroscopy (ESCA) and in secondary ion mass spectrometry (SIMS) relating to surfaces of carbon fibers COURTAULDS Grafil HT untreated in origin and having subsequently undergone treatments with hexamethylene tetramine in an aqueous medium;
• FIG. 7 represents a detail of the photoelectron peaks (ESCA) obtained for these same fibers treated with hexamethylene tetramine;
• FIG. 8 groups together a set of spectra obtained in ESCA and in SIMS for COURTAULDS Grafil HT fibers having undergone a treatment with amino 6 methyl 2 pyridine in aqueous medium;
• FIG. 9 groups together a set of spectra obtained in ESCA and in SIMS for COURTAULDS Grafil HT fibers having undergone treatment with urea in an aqueous medium;
• FIG. 10 groups together a set of spectra obtained in ESCA and in SIMS for COURTAULDS Grafil HT fibers having undergone a treatment with ethylenediamine in acetonitrile added with lithium perchlorate;
• FIG. 11 groups together a set of spectra obtained in ESCA and in SIMS for the same fibers having undergone triatination with amino 6 methyl 2 pyridine in acetonitrile without support electrolyte;
• FIG. 12 groups together a set of spectra obtained in ESCA and in SIMS for the COURTAULDS Grafil HT fibers having undergone a treatment with ethylenediamine in dimethylformamide added with lithium perchlorate;
• FIG. 13 shows the evolution of the decohesion stress τ _d fiber-matrix for the three types of treatment in an aqueous medium mentioned above as a function of their duration. The resin used is Araldite LY 556 + hardener HT 972 CIBA GEIGY:
• FIG. 14 shows the evolution of the decohesion constraint τ _d fiber-matrix for the treatments with ethylenediamine in acetonitrile supplemented with lithium perchlorate. Two epoxy resins are used, the CIBA GEIGY Araldite LY 556 resin and the NARMCO 5208 resin;
• FIG. 15 groups ESCA spectra to show that epichlorohydrin binds to fibers treated with hexamethylene tetramine and does not bind to these same untreated fibers.

In the experimental device shown schematically in Figure 1, a tank 1 contains an electrolyte solution 2 in which is immersed a bundle of monofilaments or carbon fibers 3 forming an anode and coated in an insulating support 4. The anode, thus that a platinum cathode 5 and a reference electrode 6 -at saturated calomel for treatments in an aqueous medium; a 0.01 M Ag / Ag ^{+ system} in acetonitrile for treatment in a non-aqueous medium - which also immerse in solution 2, are connected to a potentiostat 7 which maintains a potential between the anode and the reference electrode of predetermined value, chosen so as to avoid the release of oxygen by electrolysis in an aqueous medium or the decomposition of the mixture of solvent and of the support electrolyte (LiCl0 ₄ ) in a non-aqueous medium.

Argon is bubbled into the bath, supplied by a tube 8 which opens below the fibers 3. This avoids the presence of dissolved oxygen in the bath.

The electrolytic bath 2 is either an aqueous solution of an amino compound, or a solution of an amino compound and a support electrolyte in a dipolar organic solvent. electrochemical reactions occurring at the interface of the solution and the fibers result in the grafting by nitrogen of nitrogen groups or molecules of the amino compound on the surface of the fibers.

The curve of FIG. 2 shows the evolution of the current 1 passing through the anode as a function of the potential V of the latter relative to the reference electrode. When the potential is sufficiently low, the current takes a value l _o virtually independent of the potential. For higher values, the current increases rapidly in a curvilinear part to reach a linear part characterizing the ohmic regime. The working potential V, is chosen at a maximum value but less than a value V _o for which the release of oxygen begins to manifest in an aqueous medium (examples 1, 2 and 3) or below the ohmic regime in the medium non-aqueous (Example 4). In examples 1 to 3 below, V _o is generally of the order of +1.7 volts (relative to the saturated calomel electrode) if the compound does not dissociate the water well and a potential of work V _t close to +1.5 volts. The working potential V _t is of the order of +1.3 volts (relative to the reference electrode Ag / Ag ⁺ ) in a non-aqueous medium (example 4), this value being close to the start of the ohmic regime. It is not advantageous to choose a lower value for V _t , which would slow down the electrochemical process.

The organic solvent (for example acetonitrile) must be free of water and dehydrated beforehand if it contains it. Another characteristic is that it must have a dipolar character in order to dissolve the support electrolyte, the nature of which does not matter, insofar as it does not intervene in the electrochemical processes (decomposition potential clearly greater than the potential V _t ). In addition, if the solvent is aprotic, it promotes the deprotonation of a cation radical. The choice of the dipolar solvent is based solely on the consideration that its decomposition potential is also clearly greater than V _t .

The curve of FIG. 2 does not generally show an oxidation-reduction peak of the amino compound, because the geometry of the electric field lines is complex in the vicinity of a multifilament electrode. The potential V _E is at present a quantity which has only been determined for a small number of amino compounds, although this depends on the solvent and the support electrolyte. It has been established that V _E ≃ +0.9 volts for ethylenediamine in acetonitrile + tetraethylammonium perchlorate + Ag / Ag ⁺ electrode, ie a value lower than the working potential (V _t = +1.3 volts) of example 4. Thus, the conditions V _E <V _I and V _I <V _SOL are fully satisfied.

An installation for the continuous treatment of fibers is shown diagrammatically in FIG. 3. A continuous wick 10, formed of a multitude of carbon fibers, unwinding from a reel not shown, passes on a roller 11 located at above an electrolyte bath 12 contained in a tank 13, then successively on two rollers 14 immersed in the bath 12 and finally on a roller 15 located above the bath before being wound up on a take-up reel not shown . The roller 15 (and possibly other rollers) is rotated by means not shown so as to advance the wire 10 continuously. Rollers 11 and 15 are connected to a positive output terminal of a potentiostat 16, a negative terminal of which is connected to a cathode 17 made of stainless steel immersed in the solution 12, so as to positively polarize the wire 10 relative to the cathode. A reference electrode 18 to the calomel is connected to a control terminal 19 of the potentiostat 16, which makes it possible to set the potential of the anode relative to the reference electrode to a desired value. This installation makes it possible to carry out the same type of treatment as the device in FIG. 1, but continuously.

An installation for the treatment of divided carbon is shown diagrammatically in FIG. 4. A bed of divided carbon 20 is retained by a fine platinum grid 21 acting as an anode, itself resting on a porous disc 22 obstructing a cylindrical column vertical 23 in glass. A second platinum grid 24 disposed above the carbon bed 20 constitutes the cathode. A reference electrode 25 plunges into the carbon bed 20. The enclosure 23 is filled with electrolyte 26 whose circulation in the cathode-anode direction is ensured by a pump 27 (the elements of the pump in contact with the electrolyte are chemically inert). The anode 21, the cathode 24 and the reference electrode 25 are connected to a potentiostatic device 28. This installation makes it possible to carry out the same type of treatment as the device of FIG. 1 for divided carbons.

We will now describe the method used to determine the adhesion of carbon fibers to a resin.

One end of a fragment of an insulated fiber is introduced into the movable jaw of a traction machine, shrinking it with a drop of tin solder, and the other end is coated in the resin over a length low enough that the force required to tear the fiber from the resin is less than the breaking force of the fiber.

où a est un coefficient tenant compte de la géométrie de l'encastrement du filament et des modules d'Young de la fibre et de cisaillement de la résine. L'expérience donne accès à la contrainte de décohésion moyenne _T donnée par la formule:

Ainsi :

The breakout force F _d is measured by means of the traction machine; the perimeter p of the section of the filament and the length implanted in the resin 1 are determined by means of a scanning electron microscope calibrated in magnification. Greszozuk established a theory of the pull-out test. It shows that the shear stress _T fiber-matrix is maximum at the embedding of the filament and that it decreases as one moves away from the embedding. At the time of decohesion, _T reaches τ _d , fiber-matrix decohesion constraint. τ _d is given by the formula:

where a is a coefficient taking into account the geometry of the embedding of the filament and the Young's moduli of the fiber and shear of the resin. The experiment gives access to the average decohesion constraint _T given by the formula:

So :

It is therefore advisable to correct the experimental values of the effect of embedding length. By varying the length embedded from one filament to another, we obtain a curve T = f (1) which is adjusted by least squares on the previous formula. We thus determine τ _d which is the interfacial decohesion stress and which characterizes the adhesion of the fiber to the resin and the ability of the fiber-resin interface to resist shearing. The points reported in Figures 13 and 14 each result from a set of measurements of T = f (1), from which we deduce _Td and an estimate of the error on τ _d for a confidence interval of 68%.

Examples 1 to 3 below are taken from the aforementioned French Patent No. 2,564,489, but the values of τ _d given there have been corrected for the effect of embedding length of the filament as indicated above, whereas the results indicated in the Patent did not take this correction into account. The effect of this is to slightly increase the values of _Td which are thus closer to reality, allowing a more precise comparison between the corresponding results and those provided by the present invention, reported in Example 4. All the values of τ _d given below are corrected and the error on T _d is estimated with a confidence interval of 68%.

Example 1

We start from HT type carbon fibers produced by COURTAULDS LIMITED, untreated from origin, and that is triated using the device of FIG. 1, the electrolytic bath being an aqueous solution of hexamethylene tetramine (tertiary amine) at 50 g per liter, pH = 8.62, the potential of the fibers being +1, 45 volts with respect to the reference calomel saturated electrode. The processing temperature is 20 ° C.

For the preparation of test specimens for measuring the interfacial decohesion stress T _of the CIBA GEIGY Araldite LY 556 resin (bisphenol A diglycidylether) with the hardener HT 972 (4-4 'diaminodiphenylmethane) cured for 16 hours at 60 ° C. then 2 hours at 140 ° C.

La figure 6 regroupe des spectres en ESCA et en SIMS effectués sur des fibres COURTAULDS HT non traitées (a, b, c) puis sur des fibres traitées 10 minutes (d, e, f) et 60 minutes (g, h, i). Un détail des pics Cls et Nls (ESCA) est donnée en figure 7.FIG. 13a relating to Table I shows the evolution of τ _d as a function of the processing time. Note that the treatment increases considerably τ _d and that it is practically stable from t = 10 minutes.

Figure 6 combines ESCA and SIMS spectra performed on untreated COURTAULDS HT fibers (a, b, c) then on treated fibers 10 minutes (d, e, f) and 60 minutes (g, h, i) . A detail of the peaks Cls and Nls (ESCA) is given in Figure 7.

An ESCA analysis (FIGS. 6a, d, g) makes it possible on the one hand to determine the nature of the elements present in the outer layer of the fibers, a layer whose thickness is approximately 5 nm (50 Å), on the other hand d '' obtain information on the chemical bonding status of these elements.

A spectrum in SIMS shows peaks corresponding to the various species of ions torn from the surface by the primary argon ion beam, in the composition of which enter the elements present on the surface of the fibers over a thickness of about 0.5 nm (5 Å). In negative SIMS (FIGS. 6b, e, h), the peak at mass 24 (secondary ions CC-) is characteristic of the carbonaceous substrate and serves as a reference. The peaks at masses 25 and 26 correspond to CCH ^- and CCH ₂ - ions for the untreated fiber which contains very little nitrogen. For the treated fibers, the peak at mass 26 contains CCH ₂ ions - and CN ions ^- originating from the nitrogenous surface groups. A convenient way of monitoring the enrichment of the surface of the fibers with nitrogen is to use the ratio R (N) defined as follows:

For oxygen, R (O) is defined analogously from the peaks at masses 16 and 17 (0- and OH ^- ). Table II groups together the analyzes carried out.

Although the treatments provide at least as much oxygen as nitrogen, examination of these data shows that nitrogen is localized on the very surface of the fibers, while oxygen is distributed below the surface.

In FIGS. 6c, f, i the spectra of positive secondary ions (SIMS positive) have been reported. We observe in Figure 6f (t = 10 minutes) peaks spaced apart with a mass periodicity of 14 units. They are absent from the spectra of the untreated fibers and treated for 60 minutes (FIGS. 6c and 6i). These massifs are characteristic of the fragmentation by the primary beam of a surface molecule comprising CH2 groups. This means that after 10 minutes of treatment, the molecule of hexamethylene tetramine or most of it is present on the surface and that after 60 minutes of treatment, only the groups -NH ₂ and = NH remain. fiber surface; the molecule of hexamethylene tetramine is gradually degraded by electrochemical reactions, without however that τ _d decreases. The -NH ₂ and = NH groups being less bulky than the hexamethylene tetramine molecule, it is normal for R (O) to be higher for t = 60 min than for t = 10 min.

FIG. 7 shows the photoelectron peaks Cls in Nls corresponding. For t = 10 and 60 minutes, the peaks Cls (Figures 7c and 7e respectively) have a shoulder relative to the peak Cls of the untreated fibers (Figure 7a), which shows that the carbon is partly bound with more electronegative elements than him, especially nitrogen; the Nls peaks (FIGS. 7b, d and f respectively for 0.10 and 60 minutes) are asymmetrical: their shape and their position in binding energy attest to the presence of -NH ₂ and = NH groups linked to the carbon substrate in a covalent manner.

Example 2

Treatments were carried out using the amino 6 methyl 2 pyridine (primary amine) as the electrolyte. The bath is an aqueous solution at 25 g per liter of amino 6 methyl 2 pyridine, pH = 10.06, the potential of the COURTAULDS HT fibers being +1.5 volts relative to the reference electrode with saturated calomel. The processing temperature is 20 ° C. For the measurements of τ _d , the procedure of Example 1 is reproduced.

FIG. 13b corresponding to Table III represents the evolution of τ _d as a function of the processing time. This curve has a maximum around 10 minutes of treatment and the value of τ _d obtained for this time is very comparable to that of Example 1 for the same duration.

• C: 72%
• N: 16,7% (épaisseur analysée: 5 nm)
• 0: 10,5%

In FIG. 8 are grouped together a set of ESCA and SIMS spectra for the one hour treatment. The ESCA analysis provides (Figure 8a):

• C: 72%
• N: 16.7% (thickness analyzed: 5 nm)
• 0: 10.5%

• R(N) = 4 (cf figure 8b relative à la SIMS négative)
• R(O) = 0,13

In SIMS, the surface enrichment ratios are:

• R (N) = 4 (see Figure 8b relating to negative SIMS)
• R (O) = 0.13

The peaks Cls and Nls (FIGS. 8d and 8e) show in a manner analogous to Example 1 that the nitrogen is bonded covalently to the carbon. It is located on the surface of the fibers: R (N) »R (O). The shift of the Nls peak towards the low binding energies with respect to the peak noted HI (FIG. 8e) relating to the treatment with hexamethylene tetramine (t = 60 minutes) comes from the fact that the analysis in ESCA detects the nitrogen engaged in the pyridine cycle of the amino molecule 6 methyl 2 pyridine. Its presence on the surface is attested by the detection of mass peaks spaced 14 units of mass in positive SIMS (Figure 8c). There is therefore grafting of the entire molecule of amino 6 methyl 2 pyridine even after 60 minutes of treatment. This grafting is done through the nitrogen of the amine function. Indeed, the same treatment carried out with methyl 2 pyridine, a molecule not comprising the NH ₂ group of the amino 6 methyl 2 pyridine, shows after analysis that only 1.6% nitrogen is fixed (ESCA) , that R (O) is small, ie 1.4 and that the peak masses observed with the amino 6 methyl 2 pyridine are here very attenuated (FIG. 8f). Nitrogen from the pyridine cycle is practically not involved in the electrochemical reaction.

The maximum of the curve τ _d = f (t) (FIG. 13b) can be linked to a partial deprotonation of the nitrogen by which the amino molecule 6 methyl 2 pyridine is grafted on the carbon. A small proportion of them is probably deactivated with respect to fiber-resin adhesion.

Example 3

Treatments were carried out using urea as the electrolyte. This body is an aminoamide comprising two amine groups.

We start with COURTAULDS HT carbon fibers which are treated using the device of FIG. 1, the electrolytic bath being an aqueous urea solution at 50 g per liter, pH = 7.42, the potential of the fibers being +1 , 5 volts with respect to the reference electrode at saturated calomel. The processing temperature is 20 ° C. The procedure of Example 1 is reproduced for the measurements of _Td .

FIG. 13c corresponding to Table IV shows the evolution of _Td as a function of the duration of the treatment.

• L'ESCA fournit pour le traitement d'une heure (cf fig. 9a):
• C: 76,3%
• N: 5,9%
• O: 17,8%
  et en SIMS négative (figure 9b) on trouve:
• R(N) = 4,3
• R(O) = 1,8

The increase in τ _d is also very significant in this example:

• The ESCA provides for the one hour treatment (see fig. 9a):
• C: 76.3%
• N: 5.9%
• O: 17.8%
  and in negative SIMS (figure 9b) we find:
• R (N) = 4.3
• R (O) = 1.8

Although the oxygen concentration is much higher than that of nitrogen, the enrichment of the surface is still very favorable for nitrogen. The peak Cls presents (FIG. 9d) a shoulder similar to those recorded in Examples 1 and 2. The peak Nls (FIG. 9e) is shifted towards the low binding energies relative to the peak Nls of hexamethylene tetramine (Example 1). We note in negative SIMS (FIG. 9b) a peak at mass 42 corresponding to CNO- ions. In positive SIMS (Figure 9c), peaks at masses 56 and 57 can correspond to the CON2 and CON ₂ H ⁺ ions. Thus, there is probably grafting of the urea molecule.

The treatments of Examples 4, 5, 6 below are carried out in a non-aqueous medium.

Example 4

The electrolyte is ethylenediamine (primary amine comprising two amine functions) in solution in dehydrated acetonitrile at the rate of 12 g per liter. A supporting electrolyte is added, lithium perchlorate at a rate of 21 g per liter of solution. The potential of the COURTAULDS HT fibers is +1.3 volts compared to a 0.01 M Ag / Ag ⁺ reference electrode containing acetonitrile. The temperature is 20 ° C., the experimental treatment device is that of FIG. 1. The procedure of Example 1 is also reproduced for the measurements of _Td .

FIG. 14a shows the evolution of _Td as a function of the duration of the treatment for a fiber coated in the Araldite LY 556 + HT 972 resin.

FIG. 14b shows the result obtained using the NARMCO 5208 resin, a resin sold by the company NARMCO and composed mainly of tetraglycidylmethylenedianiline and diaminodiphenylsulfone acting as hardener.

These data are reported in Table V.

en SIMS:

The surface analyzes corresponding to the 5 nm treatment provide: in ESCA:

in SIMS:

The peak Cls (FIG. 10d) has a very large shoulder attesting that a large part of the surface carbon is covalently linked to atoms more electronegative than it, in particular to nitrogen since R (N) and the concentration d are very high. Here again, nitrogen is located on the surface: R (N) »R (O). The oxygen - which can come from traces of water in the solution - is located below the surface. The asymmetry of the Nls peak (FIG. 10e) indicates the presence of -NH ₂ and = NH functions at the surface. In SIMS positive (Figure 10c), despite the importance of the Na ⁺ peak, we observe the presence of two small peak masses for masses close to 28 and 42. There is very probably grafting of the ethylenediamine molecule.

• ce traitement est très favorable au greffage de fonctions azotées, d'une manière beaucoup plus rapide et beaucoup plus dense que pour les traitements en milieux aqueux des exemples 1, 2 et 3;
• la contrainte de décohésion τ_d n'est cependant pas sensiblement supérieure à ce que l'on obtient en milieux aqueux avec la résine CIBA GEIGY LY 556.

These results call for some comments:

• this treatment is very favorable to the grafting of nitrogenous functions, in a much faster and much denser manner than for the treatments in aqueous media of Examples 1, 2 and 3;
• the decohesion constraint τ _d is however not significantly greater than that obtained in aqueous media with the CIBA GEIGY LY 556 resin.

We are then led to consider that the interface formed with this resin cannot withstand shear stresses greater than about 70 MPa. On the other hand, with the NARMCO 5208 resin, 105.5 MPa is reached (see Table V), the treatment reaching its maximum effectiveness around 2.5 minutes (see FIG. 14b).

It should be noted that τ _d for the NARMCO 5208 resin is worth 60.9 MPa for the untreated fibers against 28.1 MPa with the CIBA GEIGY LY 556 resin. This can be explained by considering that the NARMCO 5208 resin is more reactive that the CIBA GEIGY LY 556 resin vis-à-vis the bare surface of the untreated fibers and that direct fiber bonding can be established without surface grouping. Likewise, NARMCO 5208 resin reacts more easily with grafted surface groups, since practically maximum adhesion is reached around 2.5 minutes.

Measurements of τ _d made from a commercial TORAY T300 90A fiber ("high resistance" fiber) gave τ _d = 61 ± 4MPa with the CIBA GEIGY LY 556 resin, a value lower than that obtained in media. aqueous (examples 1, 2 and 3) and in a non-aqueous medium (example 4), which shows the effectiveness of the treatments using amino electrolytes.

• τ_d = 67,3 ± 2,9 MPa
• C: 17,7%
• N: 17,6%
• 0:9,4%
• R(N) = 15,2
• R(O) = 1,7

The effect of the working voltage in a non-aqueous medium is illustrated by the following results. All other things being equal, with V, = +1 volt with respect to the Ag / Ag ⁺ reference electrode and t = 5 minutes:

• τ _d = 67.3 ± 2.9 MPa
• C: 17.7%
• N: 17.6%
• 0: 9.4%
• R (N) = 15.2
• R (O) = 1.7

We note that the decohesion constraint is little affected by the lowering of V _t . On the other hand, the amount of surface nitrogen decreases by about a quarter.

Example 5

The electrolyte is the amino 6 methyl 2 pyridine in solution in dehydrated acetonitrile at a rate of 45 g per liter, without supporting electrolyte. The potential of COURTAULDS HT fibers is +1 Volt compared to a reference electrode Ag / Ag ⁺ 0.01 M in acetonitrile. The temperature is 20 ° C., the treatment device is that of FIG. 1. The duration of the treatment is three minutes.

FIG. 11a shows the peak Cls of the fibers thus treated, FIG. 11b the peak Nls, FIG. 11 c the spectrum in negative SIMS and FIG. 11d the spectrum in SIMS positive.

We find:

The shoulder of the Cls peak shows that the carbon is linked to more electronegative atoms than it. The energy position and the shape of the Nls peak indicate that the nitrogen is in the form of -NH ₂ or = NH groups, this being corroborated by the absence of peaks in positive SIMS peaks.

Although the potential V _t is low and although a support electrolyte is not used, there is a non-negligible grafting of nitrogen well located at the surface of the fibers (R (N) = 3.8).

Example 6

The electrolyte is ethylene diamine in solution in dehydrated dimethylformamide at the rate of 12 g / liter. A supporting electrolyte is added, lithium perchlorate at a rate of 21 g per liter of solution. The potential of the COURTAULDS HT fibers is, that is +1.45 Volt compared to a saturated calomel reference electrode (ECS) (equivalent to +1.15 Volt compared to an Ag / Ag reference electrode ⁺ 0.01 M in acetonitrile), ie +1.6 Volt with respect to this same electrode (equivalent to +1.3 Volt with respect to Ag / Ag ⁺ ). The processing temperature is 20 ° C, the processing time 5 minutes; the experimental treatment device is that of FIG. 1.

The corresponding surface spectroscopic analyzes (Table VI) were carried out with a device different from that used in Examples 1 to 5, 7 and 8. This second device was the subject of a calibration so that we can compare the results obtained in the present example with those cited in the other examples. The photoelectron energy scale (ESCA) is taken here in relation to the Mg Ka radiation and represents their kinetic energy while in the other examples the abscissa represents the EB binding energy of the photoelectrons with their emitting atom.

Although less effective than the treatments mentioned in Example 4 (solvent: acetonitrile), the treatments carried out in dimethylformamide, in particular with V _t = +1.45 Volt / ECS, bring significant quantities of nitrogen located on the surface even fibers. The result for V _t = +1.45 Sees / DHW is very comparable to those mentioned in Example 1 (t = 10 and 60 minutes) from the grafting point of view but for a significantly shorter treatment time (5 minutes ).

FIG. 12a represents the peak Cls after the treatment for V _t = +1.45 Volt / ECS. A broad shoulder towards low kinetic energies (strong bond energies in the atom) attests that carbon is chemically linked to atoms more electronegative than it. The Nls peak (Figure 12b) is centered at EB = 339 eV (854 eV in kinetic energy), the same value as that found for the Nls peak of the treatment for 5 minutes in acetonitrile (Example 4, see Figure 10e). Nitrogen is therefore covalently linked to carbon. Figures 12c and 12d are the negative and positive SIMS spectra.

FIG. 12e represents the peak Cls of the treatment for V, = +1.6 Volt / ECS. Its width at mid-height is greater than for V, = +1.45 Volt / DHW. Figure 12f shows the corresponding Nls peak, centered on EB = 399.8 eV, i.e. an offset of +0.7 eV with respect to V _t = +1.45 Volt. For oxygen, we find EB = 534.1 eV against EB = 532 eV for V, = +1.45 Volt. This indicates that nitrogen, oxygen and carbon are not in the same bonding state for these two treatments in dimethylformamide. Figures 12g and 12h show the negative and positive SIMS spectra.

l'oxygène ne pouvant participer notablement à l'adhésion puisque R(O) ne vaut que 0,21.We observe in SIMS negative (Figures 12c and 12g) the presence of chlorine (masses 35 and 37) and in SIMS positive (Figures 12d and 12h) a very significant peak due to lithium (masses 6 and 7). Consequently, lithium perchlorate is involved in the electrochemical reaction and it is not advantageous to approach too much V _soL (ie +2 volts for dimethylformamide + LiClO ₄ ) since the nitrogen grafting is less for V _t = +1.6 Volt / DHW (R (N) = 2.9 while R (N) = 6 for V _t = +1.45 Volt / DHW and we set 11% nitrogen against 13 % respectively). However, the grafting is effective, the location of the nitrogen being on the surface in the form of -NH ₂ or = NH groups. Since R (O) remains moderate, oxygen remains below the surface. The decohesion stress measured according to the procedure of Example 1 with the NARMCO 5208 resin is:

since oxygen cannot participate significantly in adhesion since R (O) is only worth 0.21.

Example 7

Using the conditions of Example 1, an original COURTAULDS HMU "high modulus" non-triat fiber was subjected to an hour treatment with hexamethylene tetramine. Originally, τ _d = 15.2 ± 1.7 MPa (with CIBA GEIGY LY 556 resin); after one hour of treatment τ _d = 15.2 ± 4.9 MPa. This means that the surface of these fibers is inert with respect to the electrochemical reactions taking place in Example 1.

These COURTAULDS HMU fibers were exposed to the action of a nitrogen plasma generated by an electromagnetic wave of frequency 12.57 MHz. The experimental laboratory device is shown in Figure 5.

A segment 5 cm in length 31 is disposed on a graphite support 32 placed inside a cylindrical envelope 33 cooled by circulation of water. Two external annular electrodes 34 are connected to a high frequency generator 35. A pump 36 maintains a nitrogen pressure close to 15 Pa in the enclosure thanks to a controlled nitrogen micro-leak 37. The power dissipated in the nitrogen plasma is approximately 100 watts.

Plasma treatment can be carried out continuously on a carbon wire by means of a suitable installation, not shown.

An exposure of the COURTAULDS HMU fibers for 30 seconds to the action of the plasma is sufficient for _{Td to} go from 15.2 ± 1.7 MPa to 64.7 ± 7.4 MPa under the conditions of example 1. Ion bombardment ejects carbon atoms from large aromatic structures carried by the surface of the fibers. Chemically active sites are thus created; they increase the reactivity of the surface of the COURTAULDS HMU fibers because, in ESCA and SIMS, no nitrogen is observed and a low oxygen ratio is entirely insufficient to justify the increase in _Td observed. Thus, the surface of these fibers becomes sufficiently reactive for the CIBA GEIGY LY 556 resin to adhere directly without the intermediary of surface functions. These plasma-treated fibers have a surface structure with many defects. The reorganization of the electronic processions around the vacant carbon atoms leaves unsatisfied chemical bonds: direct bridging between fibers and resin is possible. A fortiori, the surface of the fibers is sufficiently activated to be sensitive to the action of electrochemical treatments as described above.

Example 8

Original untreated COURTAULDS HT fibers and COURTAULDS HT fibers treated under the conditions of Example 1 for one hour were placed in the presence of epichlorohydrin in a sealed ampoule. The duration of the immersion was 22 hours at 120 ° C. Epichlorohydrin carries an epoxy group. The fibers are then cleaned three times with acetone to wash their surface of epichlorohydrin.

FIG. 15a represents the peak Cls (ESCA) of the non-triaturated COURTAULDS HT fibers. FIG. 15b represents the Cls peak after the treatment with hexamethylene tretramine in an aqueous medium after one hour. FIG. 15c represents the peak Cls for the fibers not treated and subjected to the action of epichlorohydrin. Finally, FIG. 15d shows the peak Cls of the fibers having undergone the surface treatment with hexamethylene tetramine and the action of epichlorohydrin.

Figures 15a and 15c show that the untreated fibers do not bind epichlorohydrin, unlike the treated fibers (Figures 15b and 15d). The very large shoulder of the peak Cls in FIG. 15d indicates that the epichlorohydrin is covalently attached to the nitrogen carried by the surface of the fibers triated with hexamethylene tetramine since, after one hour of treatment (cf. example 1) the surface contains only -NH ₂ or = NH groups grafted. These surface groups ensure the cohesion of the interface by means of carbon-nitrogen-resin covalent bonds.

All the results mentioned above lead us to think that the effects observed with regard to the grafting of nitrogenous groups or molecules proceed from a general mechanism.

• radical cation cation

The anodic oxidation of an amine (whether primary, secondary or tertiary), when studied in electrochemistry on platinum anode, generally involves the formation of a cation radical and then a cation. For example, for a primary amine where R is a group insensitive to redox under the conditions of the experiment:

• radical cation cation

The existence of cationic radicals was observed by infrared RAMAN spectroscopy in an acetonitrile solution containing tetramethylbenzidine and lithium perchlorate; using carbon fibers as the anode. The cation radicals appear when the first redox potential of this amine is exceeded, and the dications beyond the second potential.

• radial cation
  

As the cation cannot exist in water, the attack on carbon would take place as soon as the cation radical is formed according to the following mechanism, in an aqueous medium, valid for primary and secondary amines:

• radial cation
  

The carbon atoms involved in these reactions are surface atoms of a fiber.

The radical C 'can be combined with a cation radical. Reactions are possible with nucleophilic species present in the electrolytic solution, such as OH- ions:

These last two reactions justify the presence of oxygen groups which, moreover, remain in the minority on the very surface of the fibers (in an aqueous medium).

l'étape de la déprotonation est remplacée par l'élimination de l'un des trois groupements R', R" ou R"':

FibreFor tertiary amines

the deprotonation stage is replaced by the elimination of one of the three groups R ', R "or R"':

Fiber

These mechanisms, valid in an aqueous medium, remain so in a non-aqueous medium: the cation can have a certain stability in the organic solvent and react with the carbon of the fibers. Reactions involving C 'radicals and oxygen will remain in the minority as soon as the solvent has been suitably dehydrated.

• le potentiel de travail V_t est supérieur au potentiel d'oxydo-réduction V_E du composé aminé;
• le potentiel de travail V_t est inférieur au potentiel de décomposition V_d de l'eau ou V_SOL du solvant non aqueux additionné d'un électrolyte support.

These reactions take place if:

• the working potential V _t is greater than the redox potential V _E of the amino compound;
• the working potential V _t is less than the decomposition potential V _d of water or V _SOL of the non-aqueous solvent added with a support electrolyte.

• le potentiel de décomposition est repoussé vers les hauts potentiels dans la mesure où l'électrolyte support éventuellement utilisé est à fort potentiel d'oxydation électrochimique dans le solvant choisi;

By operating in a non-aqueous medium:

• the decomposition potential is pushed back towards the high potentials insofar as the support electrolyte possibly used has a high electrochemical oxidation potential in the chosen solvent;

• les groupements azotés tels -NH₂ et =NH ou des molécules entières du composé aminé servant d'électrolyte sont les espèces greffées majoritairement par l'intermédiaire d'une liaison covalente;
• la quantité de groupements de surface azotés greffés est augmentée par rapport à un traitement en milieu aqueux, et ce en un temps réduit, notamment avec l'acétonitrile;
• les adhésions obtenues sont très élevées;
• le respect des conditions de potentiel évoquées ci-dessus permet le greffage de la plus large variété de molécules aminées sur la surface de carbone des catégories appropriées ou sur la surface activité de carbones traités par un procédé convenable, l'utilisation d'un plasma par exemple.

V _SOL is not lowered by low pKb compounds as is the case in water;

• the nitrogen groups such as -NH ₂ and = NH or whole molecules of the amino compound serving as electrolyte are the species grafted mainly via a covalent bond;
• the amount of grafted nitrogenous surface groups is increased compared to a treatment in an aqueous medium, and this in a reduced time, in particular with acetonitrile;
• the adhesions obtained are very high;
• compliance with the potential conditions mentioned above allows the grafting of the widest variety of amino molecules on the carbon surface of the appropriate categories or on the activity surface of carbons treated by a suitable process, the use of a plasma by example.

Claims (17)

1. An electrochemical process for the surface treatment of carbon, in which the carbon is placed in contact with a solution of an aminated compound in a dipolar solvent by polarising it positively with respect to a cathode, characterised in that the solvent is an organic compound with a high potential of anodic oxidation, the solution being practically free of water.

2. A process according to Claim 1, characterized in that the aminated compound is selected from ethylene diamine, amino 6 methyl 2 pyridine and tetra methyl benzidine.

3. A process according to one of Claims 1 and 2, characterized in that the solvent is aprotic.

4. A process according to Claim 3, characterized in that the organic solvent is selected from acetonitrile, dimethylformamide and dimethylsulphoxide.

5. A process according to one of the preceding claims, characterized in that an electrolyte support, likewise having a high potential of anodic oxidation, is added to the solution.

6. A process according to Claim 5, characterized in that the electrolyte support is selected from lithium perchlorate, tetraethyl ammonium perchlorate, the tetrafluoroborates and the alkaline tetrafluoro- phosphates or quaternary ammoniums.

7. A process according to one of the preceding claims, characterized in that the polarisation of the tarbon is selected to be sufficiently low so as not to cause the anodic oxidation of the dipolar organic solvent and, if applicable of the electrolyte support, but to be sufficiently high for the aminated compound to undergo an anodic oxidation.

8. A process according to Claim 7, characterized in that the solution is constituted by ethylene diamine, acetonitrile and lithium perchlorate and in that the potential of the carbon with respect to a reference electrode AgIAg * 0.01 M is approximately +1.3 volt.

9. A process according to Claim 7, characterized in that the solution is constituted by ethylene diamine, dimethylformamide and lithium perchlorate and in that the potential of the carbon with respect to a saturated calomel reference electrode is approximately +1.45 volt.

10. Carbon treated by the process according to one of the preceding Claims.

11. Carbon treated according to Claim 10, characterized in that it comprises on its surface carbon- nitrogen bonds..

12. Carbon treated according to Claim 11, characterized in that it comprises on its surface -NH₂ and =NH groupings or all or part of the molecule of the aminated compound.

13. Carbon treated according to one of Claims 10 to 12, characterized in that it is obtained by the treatment of a carbon selected from the microporous carbons, the low temperature graphitable carbons and the activated surface carbons.

14. Carbon treated according to Claim 13, characterized in that its surface has been activated by the action of a nitrogen plasma.

15. Carbon treated according to one of Claims 10 to 13, characterized in that it is in the form of fibres.

16. A composite material formed of a matrix reinforced by carbon fibres according to Claim 15.

17. A composite material according to Claim 16, characterized in that the matrix is an organic resin reticulated by an aminated hardener.

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