Comparative Physical–Mechanical Properties Assessment of Tailored Surface-Treated Carbon Fibres

materials

Article Comparative Physical–Mechanical Properties Assessment of Tailored Surface-Treated Carbon Fibres

Dionisis Semitekolos 1 , Aikaterini-Flora Trompeta 1 , Iryna Husarova 2, Tamara Man’ko 2, Aleksandr Potapov 2, Olga Romenskaya 2, Yana Liang 3, Xiaoying Li 3 , Mauro Giorcelli 4 , Hanshan Dong 3, Alberto Tagliaferro 4 and Costas A. Charitidis 1,* 1 Research Lab of Advanced, Composite, Nanomaterials and Nanotechnology (R-NanoLab), Materials Science and Engineering Department, School of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou str., GR15780 Zographou, Greece; [email protected] (D.S.); [email protected] (A.-F.T.) 2 Yuzhnoye State Design Oﬃce, Krivorozhskaya Street 3, 49008 Dnipro, Ukraine; [email protected] (I.H.); [email protected] (T.M.); [email protected] (A.P.); [email protected] (O.R.) 3 School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2SE, UK; [email protected] (Y.L.); [email protected] (X.L.); [email protected] (H.D.) 4 Department of Applied Science and Technology (DISAT), Politecnico di Torino, 10129 Torino, Italy; [email protected] (M.G.); [email protected] (A.T.) * Correspondence: [email protected]; Tel.: +30-210-772-4046

Received: 31 May 2020; Accepted: 7 July 2020; Published: 14 July 2020

Abstract: Carbon Fibres (CFs) are widely used in textile-reinforced composites for the construction of lightweight, durable structures. Since their inert surface does not allow effective bonding with the matrix material, the surface treatment of fibres is suggested to improve the adhesion between the two. In the present study, different surface modifications are compared in terms of the mechanical enhancement that they can offer to the fibres. Two main advanced technologies have been investigated; namely, plasma treatment and electrochemical treatment. Specifically, active screen plasma and low-pressure plasma were compared. Regarding the electrochemical modification, electrochemical oxidation and electropolymerisation of monomer solutions of acrylic and methacrylic acids, acrylonitrile and N-vinyl pyrrolidine were tested for HTA-40 CFs. In order to assess the effects of the surface treatments, the morphology, the physicochemical properties, as well as the mechanical integrity of the fibres were investigated. The CF surface and polymeric matrix interphase adhesion in composites were also analysed. The improvement of the carbon fibre’s physical–mechanical properties was evident for the case of the active screen plasma treatment and the electrochemical oxidation.

Keywords: carbon fibres; electrochemical treatment; fibre/matrix bond; mechanical properties; physical properties; plasma treatment; surface properties; tailored properties; textile-reinforced composites

1. Introduction Carbon fibres (CFs) are used as reinforcement for polymer composites, being among prospective materials in an aerospace domain [1]. An increased focus on carbon fibre-reinforced polymers (CFRPs) is specified by a unique set of their technically valuable properties, such as their high specific strength, low tensile strength at deformation, high thermal stability and conductivity. The main requirements to expand their applications are to enhance their mechanical properties while maintaining cost effectiveness of manufacturing process and final cost of the product. The physical and mechanical characteristics of composite materials directly depend on the adhesion strength of the CF surface and polymeric matrix interphase [2]. As a result of the significant

Materials 2020, 13, 3136; doi:10.3390/ma13143136 www.mdpi.com/journal/materials Materials 2020, 13, 3136 2 of 14 price of high-quality CFs, their application in various industries is currently not affordable [3]. CFs are usually manufactured from polyacrylonitrile, sinters or viscose precursor. The heat treatment of fibres obtained based on polyacrylonitrile under temperature higher than 1000 ◦C leads to almost full carbonization, as well as to more structured graphite microstructure and increased modulus of elongation [4]. The estimated cost of CF manufacturing with modulus of 270 GPa is twice higher than for fibres with modulus of 220 GPa. Improving the mechanical properties of CFs obtained from cost-effective raw materials will reduce the cost of the end product and expand the CFRPs market [5]. CFs have lots of surface defects, formed as a result of the production technology selected by industries, namely, the thermo-oxidizing method [6]. Selected treatments affect the roughness, surface morphology, and the porosity of the fibres [7]. During manufacturing, CFs accumulate defects, so the actual strength is significantly lower than the theoretical. To enhance the CF strength, carbon nanostructures, such as particles of nanographite, multi- and single-wall carbon nano-tubes, fullerenes and fullerene-like structures as modifying dopant for fibres have been investigated [8]. There are also techniques related to the doping of carbon nanotubes into the precursor, but these methods require complicated and expensive processes and equipment [9]. Another method with more feasibility is CF reinforcement using two-phase heat treatment [10]. However, with this method micro-stresses are removed, while micro-cracks remain in the material [11]. The adhesion strength can be increased by the formation of oxygen-containing active groups through surface modification that can create covalent bonds with the active groups of resin leading to a composite material with improved interfacial properties [12]. The main issue is that, during the modification of fibres, the sintering out of fibre pores, micro-cracks, and fibre destruction can occur [13]. In this regard, in order to select surface treatment methods and modes, which do not evoke fibre destruction, it is required to investigate their effect on single fibre strength. Surface treatments (continuous or batch type) can be categorized as: (1) oxidative and (2) non-oxidative treatments [14]. The oxidative treatments can be further subdivided into: (a) low-phase gas oxidations; (b) liquid phase oxidations carried out chemically or electrochemically; (c) catalytic oxidations. Non-oxidizing treatments that improve fibre–resin adhesion involve: (a) the deposition of more active carbon forms such as whiskers, pyrolytic carbon or (b) polymer grafting on the surface of carbon fibres [15]. The oxidative treatment of a carbon material results in [16]: Ion-exchange properties to the material; • Increases sorption capacity by developing porous structure and surface; • Increases mechanical strength; • Increases adhesion to polymers and inorganic binders. • The increase in carbon fibre strength by oxidation is based on the fact that oxidizing agent initially reacts with defective areas of the fibres, either by healing the imperfections by forming intermolecular bonds or by eroding the defects. Acid surface groups play an important role in the fibre/matrix interphase by: (1) forming chemical bonds with the matrix molecules, (2) improving the wettability of the fibre and (3) increasing the surface area of the fibre [17]. As result of the plasma treatment on carbon fibre surface, the reaction capacity between fibre and matrix surface grows due to the increase in the amount of COOH, –C–OH and =C=O groups onto the fibre surface [18]. According to the above, plasma treatment seems a novel and promising method of fibre surface modification of the chemical and physical structure of CF’s external layers. The main advantages and novelty of plasma treatment application onto CFs are: The possibility of process optimization, thanks to the wide range of controllable parameters; • The environmental safety, since it does not form chemical wastes; • The wide variety of gases, which can be used in such applications. • Lately, a new type of plasma treatment, active screen plasma (ASP), is being developed and discussed in this study, being of undisputable interest for modification of CFs surface. One of the Materials 2020, 13, 3136 3 of 14 main advantages of ASP process is floating potential of components, which allows nitrogenization and carbonization of insulating materials [19]. Moreover, ASP is compared to low-pressure plasma (LPP) in terms of the CFs properties tailoring. Specifically, the analysis of the effect on HTA-40 carbon fibre strength properties, due to their surface modification by electrochemical treatment, as well as by the two plasma treatments, is presented in this study, utilising advanced characterisation techniques such as single fibre testing.

2. Materials and Methods

2.1. Electrochemical Treatment Surface treatment of Tenax HTA-40 (Teijin, Tokio, Japan) carbon fibre was conducted in two stages, starting with an oxidative treatment (electrochemical oxidation) that is followed by a non-oxidative (electropolymerisation). The electrochemical oxidation was conducted via cyclic voltammetry to increase carbon fibre surface roughness and formation of active centres [16]. The fibres surface activation was followed by their electropolymerisation via chronoamperometry in solutions of different monomers: acrylic acid (polymer PAA), methacrylic acid (polymer PMAA), acrylonitrile (polymer PAN) and N-vinyl pyrrolidone (polymer PVP), as presented in [20]. All monomers were purchased from Acros Organics (Waltham, MA, USA) with a purity of 99%. Basic characteristics of HTA-40 CF can be found on Table1.

Table 1. HTA-40 CF speciﬁcations.

Number Nominal Tensile Tensile Filament Elongation Density Sizing CF of Linear Strength Modulus Diameter at Break (%) (g/cm3) Type Filaments Density (MPa) (GPa) (µm) HTA 40 Epoxy 6000 400 tex 3950 238 1.7 7 1.76 E13 1.3% w/w

2.1.1. Electrochemical Oxidation Electrochemical conditions were selected, considering the existing experience of PAN-based commercial carbon fibre treatment, which are focused on formation of oxygen-containing groups with high surface concentration [21]. Cyclic voltammetry was conducted in each specimen with multiple sweeps (5, 10, 15, 20) in the region of 3 V to 3 V (scan rate 100 mV/s), in a 5% solution of H SO − 2 4 purchased from Fisher Scientific (purity 95%). Comparing the tensile strength of the pristine and modified single CFs is one of the effective ways to assess the effect of the fibre treatment process on its mechanical properties [22]. To perform trade off analysis of different treatment methods effect on carbon fibre strength, the tensile stress at break of single fibres was determined. Regarding sample manufacturing, single CFs with length of 30 mm, selected from tows of the investigated materials, were pasted into mounting frame, providing its coaxial fixing in testing machine. Tensile strength tests were carried out at 20 samples from each batch.

2.1.2. Electropolymerisation Since under cyclic voltammetry, ﬁbre surface destruction is favoured with the increase in the number of scanning cycles, the following process was selected for ﬁbres’ treatment:

1. Electrochemical oxidation in 5% aqueous H2SO4 solution during 10 cycles of potential changes in a range of 3 V to +3 V with the velocity of 100 mV/s; − 2. Electropolymerisation in potentiostatic conditions.

Vinyl monomers, according to Gubanov ’s work [8], are well studied as carbon fibres coating. In this regard, four different monomers were selected, in particular, acrylic acid (polymer PAA), methacrylic acid (polymer PMAA), acrylonitrile (polymer PAN) and N-vinyl pyrrolidone (polymer Materials 2020, 13, x FOR PEER REVIEW 4 of 14 Materials 2020, 13, x 3136 FOR PEER REVIEW 4 of 14 methacrylicmethacrylic acid acid (polymer (polymer PMAA), PMAA), acrylonitrile acrylonitrile (polymer (polymer PAN) PAN) and and N-vinyl N-vinyl pyrrolidone pyrrolidone (polymer (polymer PVP),PVP), as as presentedpresented inin ourour previousprevious workwork in in [[22] 22[22]].. .Electropolymerisation Electropolymerisation was was conducted conducted under under room room temperaturetemperature in in a a 150-mL 150-mL single-chambersingle-chamber electrochemicalelectrocheelectrochemicalmical cellcell usingusing threethree electrodeelectrode system. system. After After the the finishingfinishingfinishing of of treatment, treatment, the the fibresfibres fibres werewere were washedwashed washed withwith with waterwater water andand and acetone,acetone, acetone, andand and drieddried dried inin in aa a furnace.furnace. furnace.

2.2.2.2. Plasma PlasmaPlasma Treatment TreatmentTreatment

2.2.1. Low-Pressure Plasma 2.2.1.2.2.1. Low-Pressure Low-Pressure Plasma Plasma The effect of LPP on modification of carbon fibres surface was studied in order to improve TheThe effecteffect ofof LPPLPP onon modificationmodification ofof carboncarbon fibresfibres surfacesurface waswas studiedstudied inin orderorder toto improveimprove interphase fibre adhesion to epoxy matrix in composites. The carbon fibre was rolled on a frame interphaseinterphase fibrefibre adhesionadhesion toto epoxyepoxy matrixmatrix inin cocomposites.mposites. TheThe carboncarbon fibrefibre waswas rolledrolled onon aa frameframe continuously to a length of 2 m, as shown in Figure1. Their surface was modified by LPP under the continuouslycontinuously to to a a length length of of 2 2 m, m, as as shown shown in in Figu Figurere 1. 1. Their Their surface surface was was modified modified by by LPP LPP under under the the pressure, range between 1–100 Pa. HTA-40 carbon fibre LPP treatment modes are presented in Table2. pressure,pressure, range range between between 1–100 1–100 Pa. Pa. HTA-40 HTA-40 carbon carbon fibre fibre LPP LPP treatment treatment modes modes are are presented presented in in Table Table 2.2.

((aa)) ((bb))

FigureFigure 1. 1. LPP LPPLPP ( (a(aa)) )control controlcontrol panel panelpanel and andand ( (b(b)) )chamber. chamber.chamber. Table 2. Carbon fibre LPP treatment modes. TableTable 2. 2. Carbon Carbon fibre fibre LPP LPP treatment treatment modes. modes. Treatment Gas ItemItem Number Reagent Reagent Treatment Time CapacityCapacity (W s) PressurePressure (Pa) Gas Item Reagent TreatmentTime (min)Time Capacity · Pressure ConcentrationGas NumberNumber (min)(min) (W·s)(W·s) (Pa)(Pa) ConcentrationConcentration 1 O2 5 100 20–40 75 2 1 О2 5 100 20–40 75 1 2О O2 5 5 100 200 20–40 20–40 75 75 2 22 ОО2 55 200 200 20–4020–40 7575 2.2.2. Active Screen Plasma 2.2.2.2.2.2. Active Active Screen Screen Plasma Plasma Under ASP treatment, a cathode is connected to a metallic screen, installed around the working Under ASP treatment, a cathode is connected to a metallic screen, installed around the working table,Under to which ASP floating treatment, potential a cathode is applied. is connected During to ASP a metallic treatment, screen, the installed plasma forms around on the the working metallic table, to which floating potential is applied. During ASP treatment, the plasma forms on the metallic table,screen to surface, which andfloating the carbonpotential fibres is applied. set on the During working ASP table treatment, are post the discharged plasma forms (Figure on2 the) with metallic ions, screen surface, and the carbon fibres set on the working table are post discharged (Figure 2) with screenelectrons surface, and radicals and the [18 carbon]. fibres set on the working table are post discharged (Figure 2) with ions,ions, electrons electrons and and radicals radicals [18]. [18].

FigureFigure 2. 2. Schematics SchematicsSchematics (left) (left)(left) and andand experimental experimentalexperimental cham chamberchamberber (right) (right) of of ASP ASP treatment treatment on on CFs. CFs.

ASPASP treatmentstreatments werewere carriedcarriedcarried outoutout in inin a aa Plasma PlasmaPlasma Metal MetalMetal 75 75kVA75kVA kVA + ++ 15 15KV15KV KV industrial industrialindustrial scale scalescale unit unit at at pressurespressures of ofof 20–110 20–11020–110 Pa. Pa.Pa. The TheThe voltage voltagevoltage controlled controlledcontrolled from fromfrom 250 250 to250 350 toto V 350350 was VV applied waswas appliedapplied between betweenbetween the active thethe screenactiveactive

Materials 2020, 13, 3136 5 of 14

(cathode)Materials and 2020 the, 13, wallx FOR ofPEER the REVIEW furnace (anode) during all processes. The CF tows were hung5 of on 14 a set of stainless-steel bars with a distance of 15 cm to the active screen (Figure2 right). The treatment currentscreen was (cathode) recorded and between the wall 60 of and the furnace 70 A, and (anode) the temperature during all processes. in the furnace The CF tows was recordedwere hung between on a set of stainless-steel bars with a distance of 15 cm to the active screen (Figure 2 right). The treatment 17 to 95 C. current◦ was recorded between 60 and 70 A, and the temperature in the furnace was recorded between ° 2.3. Characterisation17 to 95 C. Methods Scanning2.3. Characterisation electron microscopyMethods (SEM, JEOL 7000, Oxford Instruments, Oxfordshire, UK) were used to observeScanning the changes electron in the microscopy surface morphologies(SEM, JEOL 7000 of CFs., Oxford X-ray Instruments, photoelectronic Oxfordshire, spectroscopy UK) were (XPS),) analysesused were to observe performed the changes with XPSin the versa surface Probe morphologies 5000 (PHI of Electronics, CFs. X-ray photoelectronic Chigasaki, Kanagawa, spectroscopy Japan). The(XPS),) single analyses carbon were fibre performed tensile testswith XPS samples versawere Probe selected 5000 (PHI from Electronics, the tows Chigasaki, of treated Kanagawa, materials and cut intoJapan). 30 mm length. They were pasted into a mounting frame, providing its coaxial fixing in testing machine.The Tensile single strength carbon testsfibre tensile were carried tests samples out at were 20 samples selected for from each the batch. tows of The treated fibre’s materials tensile and strength was determinedcut into 30 mm at length. samples They of micro-plasticwere pasted into according a mounting to frame, ISO 10618 providing “Carbon its coaxial fibre—Determination fixing in testing of machine. Tensile strength tests were carried out at 20 samples for each batch. The fibre’s tensile tensile properties of resin-impregnated yarn” standard. Micro-plastic samples were manufactured strength was determined at samples of micro-plastic according to ISO 10618 “Carbon fibre— from carbon binder, impregnated with liquid Huntsman epoxy resin, at MAW 20 FB5/1 winding Determination of tensile properties of resin-impregnated yarn” standard. Micro-plastic samples were machinemanufactured (Mikrosam from AD, carbon Prilep, binder, North impregnated Macedonia). with After liquid winding, Huntsman the samples epoxy resin, were curedat MAW in furnace.20 BreakingFB5/1 load winding was machine monitored (Mikrosam using theAD, testing Prilep, machineNorth Macedonia). dynamometer After winding, (Universal the samples Testing were Machine, Instron,cured Norwood, in furnace. MA, Breaking USA). load was monitored using the testing machine dynamometer (Universal Testing Machine, Instron, Norwood, MA, USA). 3. Results and Discussion 3. Results and Discussion 3.1. Assessment of Electrochemical Treatment 3.1. Assessment of Electrochemical Treatment After the cyclic voltammetry, an enhancement of tensile strength was noticed (Figure3a). The maximumAfter the tensile cyclic strength voltammetry, value an of enhancement 4433 MPa, exceedingof tensile strength the strength was noticed of pristine (Figure fibres 3a). The by 20%, was achievedmaximum undertensile strength five cycles value of of treatment. 4433 MPa, Theexceeding ultimate the strength tensile stressof pristine value fibres decreases by 20%, withwas the achieved under five cycles of treatment. The ultimate tensile stress value decreases with the increase increase of treatment cycles (up to 20). Twenty cycles of treatment lead to strength reduction by 4%. of treatment cycles (up to 20). Twenty cycles of treatment lead to strength reduction by 4%. HTA-40 HTA-40 carbon fibre strength changing is not in conflict with the results of its surface microstructure carbon fibre strength changing is not in conflict with the results of its surface microstructure studies. studies.Based Based on microstructural on microstructural analysis, analysis, it was discovered it was discovered that foreign that inclusions foreign em inclusionserge at their emerge surface, at their surface,which which are not are present not present in the inpristine the pristine state material state material (Figure 3b). (Figure 3b).

(a)

(b) From left to right: pristine CF, 5C, 10C, 15C, 20C treatments.

Figure 3. Eﬀect of electro-chemical treatment on single ﬁbres strength (a) and microstructure (b).

Materials 2020, 13, x FOR PEER REVIEW 6 of 14 Materials 2020, 13, x FOR PEER REVIEW 6 of 14 Figure 3. Effect of electro-chemical treatment on single fibres strength (а) and microstructure (b). Materials 2020, 13, 3136 6 of 14 Figure 3. Effect of electro-chemical treatment on single fibres strength (а) and microstructure (b). After 5-15 cycles of electrochemical oxidation, a lot of new build-ups are formed at the fibre surface;AfterAfter however, 5-15 5–15 cycles cycles after of of 20 electrochemical electrochemical cycles their number oxidation, oxidation, sign ificantlya a lot lot of of newreduces. new build-ups build-ups Probably, are are thisformed formed can beat at theclassified the fibre fibre surface;surface;as external however, however, layer structure after 2020 cyclescycles changing theirtheir numberandnumber removal significantly sign ificantlyof new reduces.build-ups reduces. Probably, Probably,from the this surface,this can can be whichbe classified classified lead asto asexternalcarbon external fibre layer layer destruction structure structure changing [23].changing Using andand X-ray removalremoval photoelectronic of new new build-ups build-ups spectroscopy, from from the theit surface,was surface, discovered which which lead that— lead to carbontodespite carbon fibre the fibre formationdestruction destruction of [23]. active [23Us]. ingoxygen-containing Using X-ray X-rayphotoelectronic photoelectronic groups spectroscopy, at the spectroscopy,carbon it fibre was surface itdiscovered was discoveredunder that— 5–15 despitethat—despitecycles ofthe electrochemical formation the formation of active oxidation— of activeoxygen-containing oxygen-containingthe chemical groupscompounds groups at the between atcarbon the carbon thefibre fibres surface fibre oxidized surface under underby 5–15 the cycles5–15electrochemical cycles of electrochemical of electrochemical method andoxidation— oxidation—epoxy resin the thechemicalare chemicalnot key compounds for compounds defining between interphase between the the fibresstrength fibres oxidized oxidized in composites by by the the electrochemicalelectrochemical(Figure 4) [13]. methodThe method main and and reason epoxy epoxy for resinresin that areis are the not not lo key keyw concentration for for defining defining interphase of functional strength strength groups in in at composites the surface (Figure(Figure[24]. Therefore, 44))[ [13].13]. The cyclic mainmain reasonvoltammetryreason for for that that iswas is the the applied low low concentration concentration as a stage of functionalofof functional surface groups activationgroups at the at surfacethefor surfacefurther [24]. [24].Therefore,modification Therefore, cyclic via cyclic voltammetryelectropolymerisation. voltammetry was applied was applied as a stage asof a surfacestage activationof surface foractivation further modificationfor further modificationvia electropolymerisation. via electropolymerisation.

(a) (b) (a) (b)

(c) (d) (c) (d) Figure 4. High-Resolution XPS spectra of C1s for (a) Pristine CF, (b) after 5, (c) 10 and (d) 15 cycles of FigureFigureelectrochemical 4. 4. High-ResolutionHigh-Resolution treatment. XPSXPS spectra spectra of of C1s C1s for for ( (aa)) Pristine Pristine CF, CF, ( (bb)) after after 5, 5, ( (cc)) 10 10 and and ( (dd)) 15 15 cycles cycles of of electrochemicalelectrochemical treatment. treatment. To study the effect of electropolymerisation on HTA-40 carbon fibre properties, the tensile strengthToTo studystudy of single thethe e fffibreseffectect of andof electropolymerisation electropolymerisation their surface microstr on HTA-40onucture HTA-40 in carbon pristine carbon fibre state properties,fibre and properties, after the treatment tensile thestrength tensile at the strengthofelectronic single of fibres scanningsingle and fibres theirmicroscope and surface their were microstructure surface determined microstr in (Figuresucture pristine in 5 state pristineand and6). state after and treatment after treatment at the electronic at the electronicscanning microscopescanning microscope were determined were determined (Figures5 and(Figures6). 5 and 6).

(a) tensile strength (a) tensile strength Figure 5. Cont.

Materials 2020, 13, x FOR PEER REVIEW 7 of 14

Materials 2020, 13, 3136 7 of 14 Materials 2020, 13, x FOR PEER REVIEW 7 of 14 (b) Surface microstructure. From left to right: Pristine CF, PAA, PAN, PMAA, PVP treatments.

Figure 5. HTA-40 fibre tensile strength (a) and surface microstructure after electropolymerisation in solutions of different monomers (b).

Fibres after electropolymerisation in PMAA and PVP solutions have a tensile strength at the level of pristine fibre, 3600 MPa. A 20% strength reduction of fibres was noticed after treatment in PAN solution. During the investigation of the HTA-40 carbon fibre microstructure after electropolymerisation, it was noticed that in all monomer solutions formation of the corresponding polymeric(b) Surfacecoatings microstructure. at the fibre surface From occurred, left to ri e.ght:g., thePristine new CF,build-ups PAA, PAN,at the PMAA,fibre’s surface PVP were observed.treatments. Maximum amount of polymer was noticed under PAA treatment. The results of microstructural analysis are presented in Figure 6. Under electropolymerisation, polymer forms at Figure 5. HTA-40 ﬁbre tensile strength (a) and surface microstructure after electropolymerisation in the surfaceFigure 5.of HTA-40 HTA-40 fibre carbon tensile fibre, strength which (a )can and heal surface defects microstructure and enhance after its electropolymerisation strength. in solutions of didifferentﬀerent monomers ((bb).).

20μm 20μm

(a) (b)

20μm 20μm

(a) (b) 20μm 20μm (c) (d)

Figure 6.6. FibresFibres afterafter polymerisation: polymerisation: ( a()a HTA-40) HTA-40 with with PAA, PAA, (b) ( HTA-40b) HTA-40 with with PMAA, PMAA, (c) HTA-40 (c) HTA-40 with PAN,with PAN, (d) HTA-40 (d) HTA-40 with PVP.with PVP.

FibresIt should after be electropolymerisation noted that, according in PMAA to the and litera PVPture solutions [25] and have observation a tensile strength of its atsurface, the level the of pristinepolymerized fibre, 3600acrylonitrile MPa. A 20%formed strength into reductionpellets, exhi of fibresbiting wascrystalline noticed afterpolymer treatment properties in PAN associated solution. During the investigation of the HTA-40 carbon fibre microstructure after electropolymerisation, it was noticed that in all monomer solutions formation of the corresponding polymeric coatings at the fibre 20μm 20μm surface occurred, e.g., the new build-ups at the fibre’s surface were observed. Maximum amount of polymer was noticed under PAA(c) treatment. The results of microstructural(d) analysis are presented in Figure6. Under electropolymerisation, polymer forms at the surface of HTA-40 carbon fibre, which can heal defectsFigure 6. and Fibres enhance after polymerisation: its strength. (a) HTA-40 with PAA, (b) HTA-40 with PMAA, (c) HTA-40 withIt should PAN, (d be) HTA-40 noted with that, PVP. according to the literature [25] and observation of its surface, the polymerized acrylonitrile formed into pellets, exhibiting crystalline polymer properties associated It should be noted that, according to the literature [25] and observation of its surface, the polymerized acrylonitrile formed into pellets, exhibiting crystalline polymer properties associated

MaterialsMaterials2020 2020, 13, 13, 3136, x FOR PEER REVIEW 8 of8 of 14 14

with a unique two-dimensional order. Moreover, the observed crystalline structures could be with a unique two-dimensional order. Moreover, the observed crystalline structures could be attributed attributed to the residual electrolyte (ZnCl2) molecules that tend to form hexagonal crystal-like to the residual electrolyte (ZnCl ) molecules that tend to form hexagonal crystal-like deposits which deposits which demonstrate some2 properties of crystalline polymer, resulting from its planar order demonstrate some properties of crystalline polymer, resulting from its planar order [26]. After treatment [26]. After treatment in the solution of acrylic acid, similar build-ups are formed, with the polymer in the solution of acrylic acid, similar build-ups are formed, with the polymer covering the entire covering the entire surface of fibres. After treatment with PMAA and PVP, a smooth polymeric surface of fibres. After treatment with PMAA and PVP, a smooth polymeric coating is formed at the coating is formed at the fibre’s surface. fibre’s surface. To estimate the mechanical properties of composites with single fibre after To estimate the mechanical properties of composites with single fibre after electropolymerisation electropolymerisation in solutions of different monomers, the stress–deformation graphs of in solutions of different monomers, the stress–deformation graphs of specifically made samples were specifically made samples were obtained (Figure 7). obtained (Figure7). Based on the obtained data it was found that strength of composite with carbon fibre after Based on the obtained data it was found that strength of composite with carbon fibre after electropolymerisation in all analysed monomer solutions was in the range of 67–112 MPa, which electropolymerisation in all analysed monomer solutions was in the range of 67–112 MPa, which exceeds exceeds the strength of samples with pristine fibre of 50 MPa. The maximum strength of 112 MPa the strength of samples with pristine fibre of 50 MPa. The maximum strength of 112 MPa was achieved was achieved after PMAA, followed by 97 MPa of PAN, 81 MPa of PAA and finally 67 MPa of PVP. after PMAA, followed by 97 MPa of PAN, 81 MPa of PAA and finally 67 MPa of PVP.

(a) Test sample sketch (b) Strength–deformation graph

FigureFigure 7. 7.Test Test Sample Sample Schematic Schematic (a (),a), and and correlation correlation of of strength strength properties properties of of composites composites with with single single ﬁbrefibre after after electropolymerisation electropolymerisation with with di differentﬀerent monomers monomers (b ().b).

TheThe samples samples after after PAAPAA showshow thethe highest value of of elongation, elongation, which which is is 4%, 4%, probably probably due due to tothe theheavier heavier molecular molecular weight weight of polymer of polymer in comparison in comparison with withothers. others. The modulus The modulus of elasticity of elasticity of samples of sampleswith pristine with pristine fibre is fibre 3.7 GPa. is 3.7 After GPa. AfterРМАА PMAA treatment, treatment, it remained it remained practically practically at the at thesame same level level (3.8 (3.8GPa). GPa). All All other other types types of ofpolymerisation polymerisation led led to to a a redu reductionction of of the pristine fibrefibre modulusmodulus of of elasticity elasticity toto 2.5–3.5 2.5–3.5 GPa. GPa. According According to to the the literature literature [27 [27],], the the thickness thickness of of the the interlayer interlayer that that is is formed formed through through electropolymerisationelectropolymerisation between between the the carbon carbon fibre fibre and and the the matrix matrix is is crucial crucial to to the the tensile tensile and and flexural flexural propertiesproperties of of the the composite. composite. The The size size of of grafted grafted material material is is inversely inversely proportional proportional to to the the modulus modulus of of thethe material. material. The The results results of of the the conducted conducted experimental experimental studies studies showed showed that that the the most most prospective prospective treatmenttreatment is is PMAA, РМАА since, since it it leads leads to to maximum maximum properties properties improvement improvement of of composite composite with with treated treated carboncarbon fibre. fibre.

3.2.3.2. Assessment Assessment of of Low-Pressure Low-Pressure Plasma Plasma Treatments Treatments StudyingStudying of of single single fibres fibres strength strength after after treatment treatment with with oxygen oxygen plasma plasma with with the the capacity capacity of 100 of W100 andW and 200 W200 during W during 5 min 5 ledmin to led reduction to reduction of their of strengththeir strength by 10–15% by 10–15% in comparison in comparison with untreated with untreated fibre (Figurefibre 6(Figurea). However, 6a). However, during microstructural during microstructural analysis of HTA-40analysis fibreof НТА no visible-40 fibre defects, no visible inclusions defects, or structuralinclusions damages or structural of fibres damages were found of fibres (Figures were 8foundb and 9(Figures). 8b and 9).

Materials 2020,, 1313,, 3136x FOR PEER REVIEW 9 of 14 Materials 2020, 13, x FOR PEER REVIEW 9 of 14

(a()a )НТА НТА-40-40 fibres fibres strengthstrength (b (b) НТА) НТА-40-40 fibres fibres microstructure microstructure

FigureFigure 8. 8.8. Dependence DependenceDependence ofof ((a ofa)) НТА (a) HTA-40-40 fibresfibres ﬁbres tensiletensile tensilestrength strength strength and and (b ()b microstructure) andmicrostructure (b) microstructure on onLPP LPP treatment treatment on LPP modes. treatmentmodes. modes.

Variations in the surface properties after LPP treatment were, as evidenced by the intensity of Variations inin thethe surfacesurface propertiesproperties after LPP treatmenttreatment were, as evidencedevidenced by the intensityintensity of its electron reflection capability, observed in image tone density (Figure 9). Untreated fibres have a itsits electronelectron reflectionreflection capability,capability, observedobserved inin imageimage tonetone densitydensity (Figure(Figure9 9).). UntreatedUntreated fibresfibres havehave aa dark tint, while during LPP treatment with the capacity of 100W they become lighter and, finally, dark tint, while during LPP treatment with the ca capacitypacity of 100W they become lighter and, and, finally, finally, duringduring LPP LPP treatment treatment with with capacitycapacity of 200 W, W, they they acquire acquire the the lightest lightest tint. tint. Defects Defects at the at the surface surface are are duringnot observed. LPP treatment with capacity of 200 W, they acquire the lightest tint. Defects at the surface are not observed.

Figure 9. Microstructure of НТА-40 fibres before and after LPP treatment. Figure 9. Microstructure of HTA-40 fibres before and after LPP treatment. Figure 9. Microstructure of НТА-40 fibres before and after LPP treatment. Consequently, under the effect of LPP, a surface modification of carbon fibres is observed, while theirConsequently, strength reduces. under The the obtained effectof results LPP, correspo a surfacend modification to the data in of the carbon literature. fibres Plasma is observed, treatment while Consequently, under the effect of LPP, a surface modification of carbon fibres is observed, while theirof carbon strength fibres reduces. within The the obtainedtimeframe results of 2–30 correspond min leads to to a the reduction data in of the ultimate literature. tensile Plasma strength treatment by their strength reduces. The obtained results correspond to the data in the literature. Plasma treatment of8% carbon [28,29]. fibres within the timeframe of 2–30 min leads to a reduction of ultimate tensile strength by 8%of carbon [28,29]. fibres within the timeframe of 2–30 min leads to a reduction of ultimate tensile strength by 8%3.3. [28,29]. Accesment of Active Screen Plasma Treatments 3.3. Accesment of Active Screen Plasma Treatments 3.3. AccesmentA set of activeof Active screen Screen plasma Plasma (ASP) Treatments treatments under different treatment conditions in terms of gasA mixture, set of active pressure screen and plasmatime were (ASP) carried treatments out, and underthe details diff erentare listed treatment in Table conditions 3. A large innumber terms of gasof mixture,singleA set offibre pressureactive tensile screen and tests time plasma were were (ASP)carried carried treatments out out, an andd the theunder results details different are are listedshown treatment in in Table Table conditions3. A3. largeFigure numberin 10a,bterms ofof singlegascompared mixture, fibre tensilethe pressure treated tests and CFs were timewith carried werepristine outcarried ones and inout, the two resultsand groups: the are details (a) shown under are in liatmospherested Table in3. Table Figure of 25%N3. 10 Aa,b large2 + compared 75%H number2 theofwith single treated different fibre CFs treatmenttensile with pristine tests time; were ones(b) samecarried in twotreatment out groups: an timed the (a) of under results10 min atmosphere withare showndifferent ofin nitrogen 25%NTable2 3.content;+ Figure75%H and2 10a,bwith dicomparedc)fferent same treatmenttreatment the treated time;time CFs of (b) 5with samemin pristine and treatment pressure ones time inbut two of diffe 10 groups: minrent gas with (a) mixture. diunderfferent atmosphereIt nitrogencan be seen content; of that 25%N the and 2tensile +c) 75%H same 2 treatmentwithstrength different timeof ASP treatment of treated 5 min and time;carbon pressure (b) fibres same but are treatment dihigherfferent fo time gasr shorter mixture. of 10 time min It treated with can bedifferent CFs seen than that nitrogen longer the tensile time content; strengthones. and ofc)The same ASP tensile treated treatment strength carbon time of fibres of 5-min-treated 5 min are higherand pressure CFs for shorterare but incr diffe timeeasedrent treated 8% gas from CFsmixture. pristine than longerIt canones. be time However, seen ones. that Thethe tensile15- strengthminute-treated of 5-min-treatedASP CFstreated reduced carbon CFs the are fibresstre increasedngth are to higher 2809 8% MPa, from for shorter nearly pristine 10% time ones. lower treated However, than CFs the thanpristine the 15-minute-treated longer ones. time While ones. CFsThechanging reducedtensile the strength the ASP strength treatments of 5-min-treated to 2809 gas ratio MPa, ofCFs nearly H2 areand 10% incrN2 and lowereased keeping than8% from theother pristine pristine treatment ones. ones. conditions While However, changing the same the the15- ASPminute-treated treatments CFs gas reduced ratio of the H2 streandngth N2 and to 2809 keeping MPa, othernearly treatment 10% lower conditions than the pristine the same ones. with While the changing the ASP treatments gas ratio of H2 and N2 and keeping other treatment conditions the same

Materials 2020, 13, x FOR PEER REVIEW 10 of 14

Materialswith 2020the 10, 13 min, 3136 time, the tensile strength shows proportional relationship to the N2 contents, but10 the of 14 strength of lowest nitrogen content of 25%N2 treated CFs is similar to the pristine ones. When the treatment time duration was 5 min with the same treatment pressure, the tensile strengths of the 10 min time, the tensile strength shows proportional relationship to the N2 contents, but the strength of treated CFs are all higher than the pristine ones and the CFs treated with gas mixture of H2 and Ar lowestare shown nitrogen strength content increase of 25%N in big2 treated extent. CFs is similar to the pristine ones. When the treatment time duration was 5 min with the same treatment pressure, the tensile strengths of the treated CFs are all higher thanTable the3. Active pristine screen ones plasma and the(ASP) CFs trea treatedtments withconditions gasmixture for carbon of fibres H2 and and Ar their are single shown fibre strength increasetensile in big strengths. extent.

TableSample 3. Active Atmosphere screen plasma (ASP) Time treatments (min) Pressu conditionsre, Pa for (±20) carbon Tensile ﬁbres and strength, their single MPa ﬁbre tensilePristine strengths. - - - 3113

1 25%N2 + 75%H2 5 70 3349 Sample Atmosphere Time (min) Pressure, Pa ( 20) Tensile Strength, MPa ± Pristine2 25%N2 + - 75%H2 10 - 70 - 3099 3113 13 25%N 25%N22 ++ 75%H75%H22 15 5 70 70 2809 3349 2 25%N2 + 75%H2 10 70 3099 4 50%N2 + 50%H2 10 70 3304 3 25%N2 + 75%H2 15 70 2809 5 75%N2 + 25%H2 10 70 3399 4 50%N2 + 50%H2 10 70 3304 56 25%Ar 75%N2 ++ 75%H25%H22 5 10 70 70 3147 3399 6 25%Ar + 75%H2 5 70 3147 7 20%Ar + 80%H2 5 70 3422 7 20%Ar + 80%H2 5 70 3422 88 25%N 25%N22 ++ 75%H75%H22 10 10 50 50 3755 3755

FigureFigure 10. 10.Tensile Tensile strength strength ofof ASP-treatedASP-treated carboncarbon fibresfibres compared with with pristine pristine ones: ones: (a) ( aunder) under atmosphereatmosphere of of 25%N 25%N2 +2 +75%H 75%H22 withwith didifferentfferent treatment times; times; ( (bb) )same same treatment treatment time time of of 10 10 min min with with diffdifferenterent nitrogen nitrogen content; content; and and (c ()c) same same treatment treatment timetime of 5 5 min min with with different different conditions, conditions, asas indicated indicated in Tablein Table3. 3.

BasedBased on on the the outcomes outcomes of of the the single single fibrefibre tensiletensile strength and and the the other other characterisations characterisations [18], [18 ], twotwo newly newly designed designed treatment treatment conditions conditions (Table (Table4)—modified 4)—modified from from the the conditions conditions ofof samplesample number 1 and1 and 7, shown 7, shown in Tablein Table3—were 3—were set set up up for for ASP ASP treatments treatments on on the the carboncarbon fibres.fibres. Both Both ASP ASP treatments treatments keptkept a constanta constant pressure pressure (75 (75 Pa), Pa), time time (5(5 min)min) andand power (~23 KW) KW) but but with with different different gas gas mixture mixture compositions:compositions: ASP1 ASP1= =25% 25% N N2 2+ + 75%75% HH22 andand ASP2ASP2 = 5% Ar + 23.75%23.75% N N2 2++ 71.25%71.25% H H2. 2The. The surface surface morphologymorphology of of the the newlynewly treated treated and and pristine pristine CFs CFs were were observed observed by SEM. by As SEM. can Asbe seen can from be seen Figure from Figure11, the 11 surface, the surface of all fibres of all presen fibrested presented similar ridges similar and ridges striations and striationsparallel to parallelthe fibre toaxial the direction fibre axial directionowing to owing the PAN to the manufacturing PAN manufacturing process. process.No notice Noable noticeable surface damage surface can damage be observed can be after observed the afterplasma the plasma treatments, treatments, indicating indicating limited bombardment limited bombardment of the ions of to the the ions carbon to thefibre carbon surfaces fibre from surfaces the active screen plasma treatment comparing to the conventional DC plasma treatments. from the active screen plasma treatment comparing to the conventional DC plasma treatments.

Materials 2020, 13, 3136 11 of 14 Materials 2020, 13, x FOR PEER REVIEW 11 of 14

FigureFigure 11. 11.SEM SEM images images ofof HTA-40HTA-40 carboncarbon ﬁbres:fibres: ( (aa)) as as received; received; (b (b) )ASP1 ASP1 and and (c () cASP2) ASP2 treated treated (conditions(conditions shown shown in in Table Table3). 3).

CarbonCarbon fibre-reinforced fibre-reinforced polymer polymer composite composite samples samples were were made made with with ASP1- ASP1- and and ASP2-treated ASP2-treated CFs (conditionsCFs (conditions shown shown in Table in3 )Table and the3) and pristine the pristine ones for ones tensile for strength,tensile strength, interlaminar interlaminar shear and shear bending and strengthbending evaluation. strength Theevaluation. unidirectional The unidirectional carbon composite carbon samples composite were samples produced were by filament produced winding by methodfilament and winding the epoxy method used and is the the LY556 epoxy resin used system is the fromLY556 Huntsman. resin system Table from4 listed Huntsman. the strengths Table 4 of thelisted carbon the fibre-reinforced strengths of the polymer carbon (CFRP)fibre-reinforced composite polymer samples (CFRP) made composite with pristine samples and treated made with carbon fibrespristine under and modified treated carbon active fibres screen under plasma modified (ASP) treatmentactive screen conditions. plasma (ASP) The treatment tensile strength conditions. results testedThe tensile from the strength microplastic results tested composite from the samples microplastic show composite the ultimate samples tensile show strength the ultimate of pristine tensile and ASP1strength treated of CFs pristine possess and comparable ASP1 treated values, CFs 3420 possess and 3398 comparable MPa, respectively. values, 3420 Composites and 3398 made MPa, with therespectively. fibres treated Composites with the ASP2made condition with the showsfibres tr reducedeated with strength the ASP2 of 3117 condition MPa, which shows is reduced 9% lower thanstrength that of of the 3117 pristine MPa, which ones. is The 9% interfacelower than adhesion that of the strength pristine of ones. the The composites interfaceis adhesion reflected strength from the shearof the and composites bending testsis reflected results from shown the shear in Table and4. bending It can be tests seen results that theshown ASP in treatments Table 4. It can have be shownseen a moderatethat the ASP effect treatments on the shear have strength shown undera moderate current effect test on conditions, the shear instrength the order under ofa current 3% variation. test However,conditions, the bendingin the order strength of a 3% of ASP1 variation. treated Howe CFRPver, composite the bending were strength enhanced of ASP1 from 872treated and CFRP 65 MPa composite were enhanced from 872 and 65 MPa for pristine ones both along and across the fibre’s for pristine ones both along and across the fibre’s axis to 1015 and 76 MPa, respectively, a 16% increase axis to 1015 and 76 MPa, respectively, a 16% increase of the bending strength. With ASP2 treated of the bending strength. With ASP2 treated CFRP composite, the bending strength along fibres is CFRP composite, the bending strength along fibres is enhanced by 11%, but a decrease by 5% across enhanced by 11%, but a decrease by 5% across CF axis. CF axis. As a result of a strengths evaluation, ASP1-treated HTA-40 fibres revealed higher values of As a result of a strengths evaluation, ASP1-treated HTA-40 fibres revealed higher values of strengthstrength characteristics characteristics both both in in fibres fibres and and in compositein composite samples samples than than that that of theof the pristine pristine ones. ones. As As such, 9000-m-longsuch, 9000-m-long ASP1-treated ASP1-treated CFs have beenCFs have validated been by va makinglidated airspaceby making components airspace ofcomponents propellant tanksof (bypropellant one of the tanks co-authors, (by one Yuzhnoye).of the co-authors, The outcomes Yuzhnoye). of Th thee evaluationoutcomes of showed the evaluation a 12% improvementshowed a 12% in theimprovement strength of the in the tanks strength made of from the thetanks modified made fr CFsom the than modified the ones CFs made than from the ones the pristinemade from CFs. the pristine CFs. Table 4. Strengths of the CFRP composite samples made with pristine and treated carbon fibres under modifiedTable 4. active Strengths screen of the plasma CFRP (ASP) composite treatment samples conditions. made with pristine and treated carbon fibres under modified active screen plasma (ASP) treatment conditions. ASP1 Treatment ASP2 Treatment HTA-40 Carbon Fibre Treatment Pristine (75%H2 + 25%N2, (71%H2 + 5%Ar + ASP1 Treatment5 min, 75 Pa) 24%N , 5 min, 75 Pa) ASP2 Treatment2 (71%H2 + HTA-40Breaking Carbon tensile Fibre strength, Treatment MPa Pristine 3420.0 (75%H2 + 25%N 3398.02, 5 3117.0 5%Ar + 24%N2, 5 min, 75 Pa) Breaking strength under shearing, MPa 66.0min, 75 Pa) 68.3 63.9 Breaking strength under bending, 872.1/65.2 1015.01/76.4 972.8/61.6 Breaking(along tensile/cross strength, fibres), MPa MPa 3420.0 3398.0 3117.0

Breaking strength under shearing, 4. Conclusions 66.0 68.3 63.9 MPa Experimental studies of modifying the effect on HTA-40 carbon fibre surface and the Breaking strength under bending, physical–mechanical properties of pristine872.1/65.2 fibres were1015.01 performed / 76.4 for the first 972.8 time / 61.6 by means of cyclic(along/cross voltammetry, fibres), electropolymerization MPa in acrylic, methacrylic acids monomers, acrylonitrile, N-vinyl pyrrolidone media, as well as low-pressure plasma and active screen plasma treatments. 4. ConclusionsIt was determined that, under cyclic voltammetry, the maximum value of tensile strength, 4433 MPa, exceeding pristine fibres strength by 20%, was achieved in under five cycles of treatment. An increase in

Materials 2020, 13, 3136 12 of 14 the treatment cycles number results in decreasing the breaking stress value. The electropolymerization in PMAA and PVP solutions does not affect the tensile strength of pristine fibre. A decreasing of the fibre’s strength by 20% was noted after treatment in the PAN solution. Microstructural studies proved the formation of the corresponding polymeric coating at the fibre surface. The most prospective polymer coating is the PMAA, since it leads to maximum composite properties with treated carbon fibres (an increase of 124% in tensile strength), and does not cause a reduction of the mechanical properties of the fibre itself, which comes in accordance with our recent studies [30,31]. Under the modification of carbon fibres by low-pressure plasma treatment, CF’s tensile strength is reduced, while under modification by active screen plasma, all five-minute-treated CFs show increased tensile strength. Hence, in order to improve physical–mechanical characteristics of CFRPs, it is reasonable to modify CFs using cyclic voltammetry in a solution of 5% sulfuric acid under five cycles with further PMAA treatment and an active screen plasma treatment of ASP1. From this study, it was evidenced that the properties of CFs and subsequently the performance of CFRPs can be tailored after specific oxidised (electrochemical treatment and plasma) or non-oxidised (electropolymerisation) CF surface treatments.

Author Contributions: Conceptualization, D.S., A.-F.T. and I.H.; methodology, T.M. and O.R.; validation, Y.L.; investigation, D.S., Y.L., X.L., M.G.; resources, H.D., A.T., A.P. and C.A.C.; writing—original draft preparation, I.H., T.M., A.P. and O.R.; writing—review and editing, D.S., A.-F.T.; visualization, D.S., A.F.T.; supervision, I.H., H.D., A.T., C.A.C.; project administration, A.P., H.D., A.T., C.A.C.; funding acquisition, C.A.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by European Union’s Horizon 2020 Programme “Modified cost effective fibre based structures with improved multi-functionality and performance” (MODCOMP), under agreement no. 685844. Acknowledgments: The authors would like to acknowledge the researcher P. Kainourgios (R-NanoLab, NTUA, Greece) for the preparation of the electropolymerised carbon fibres. Conflicts of Interest: The authors declare no conflict of interest; The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Tiwari, S.; Bijwe, J. Surface Treatment of Carbon Fibres–A Review. Proc. Technol. 2014, 14, 505–512. [CrossRef] 2. Vautard, F.; Fioux, P.; Vidal, L.; Schultz, J.; Nardin, M.; Defoort, B. Influence of the carbon fiber surface properties on interfacial adhesion in carbon fiber–acrylate composites cured by electron beam. Comp. Part A Appl. Sci. Manuf. 2011, 42, 859–867. [CrossRef] 3. Oliveux, G.; Dandy, L.O.; Leeke, G.A. Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties. Prog. Mater. Sci. 2015, 72, 61–90. [CrossRef] 4. Xiao, H.; Lu, Y.; Zhao, W.; Qin, X. The effect of heat treatment temperature and time on the microstructure and mechanical properties of PAN-based carbon fibers. J. Mater. Sci. 2014, 49, 794–804. [CrossRef] 5. Koumoulos, E.P.; Trompeta, A.F.; Santos, R.M.; Martins, M.; Monterio dos Santos, C.; Iglesias, V.; Böhm, R.; Gong, G.; Chiminelli, A.; Verpoest, I.; et al. Research and Development in Carbon Fibres and Advanced High-Performance Composites Supply Chain in Europe: A Roadmap for Challenges and the Industrial Uptake. J. Comp. Sci. 2019, 3, 86. [CrossRef] 6. Dong-Kyu, K.; Kay-Hyeok, A.; Yun Hyuk, B.; Lee-Ku, K.; Sang-Yub, O.; Byung-Joo, K. Effects of electrochemical oxidation of carbon fibers on interfacial shear strength using a micro-bond method. Carbon Lett. 2016, 19, 32–39. [CrossRef] 7. Almutairim, M.K.; Felemban, R.A.; Pasha, S.E.; Abo khashaba, N.T.; Mubaraki, H.I.; Yankesa, R.A.; Algamdi, M.F.; Bakkar, A.M.; Aldouweghri, A.A.; Sannan, M.F.; et al. The Effect of Different Surface Treatments of Carbon Fibers and their Impact on Composites. Egypt. J. Hosp. Med. 2018, 70, 1275–1281. [CrossRef] 8. Gubanov, A. Development of the Process of Electrochemical Modification of the Surface of Carbon Fibre with the Aim of Increasing the Strength of Carbon Fibre. Dissertation Thesis, Dmitry Mendeleev University of Chemical Technology of Russia, Moscow, Russia, 2015. Materials 2020, 13, 3136 13 of 14

9. Termine, S.; Trompeta, A.F.; Dragatogiannis, D.; Charitidis, C.A. Novel CNTs grafting on carbon fibres through CVD: Investigation of epoxy matrix/fibre interface via nanoindentation. Matec Web Conf. 2019, 01014, 8. [CrossRef] 10. Titchenal, N.; Lam, H.; Ye, H.; Gogosti, Y.; Ko, F.; Liu, J.; Willis, P. SWNT and MWNT reinforced Carbon Nanocomposite Fibrils. In Proceedings of the 45th Structural Dynamics & Materials Conference, Palm Springs, CA, USA, 19–22 April 2004. [CrossRef] 11. Li, D.; Liu, H.; Chen, B.; Niu, D.; Lei, B.; Ye, G.; Jiang, W.; Shi, Y.; Yin, L.; Lai, G. Amorphous Carbon-Induced Surface Defect Repair for Reinforcing the Mechanical Properties of Carbon Fiber. Materials 2019, 12, 1244. [CrossRef] 12. Moosburger-Will, J.; Jäger, J.; Strauch, J.; Bauer, M.; Strobl, S.; Linscheid, F.F.; Horn, S. Interphase formation and fiber matrix adhesion in carbon fiber reinforced epoxy resin: Influence of carbon fiber surface chemistry. Compos. Interfaces 2017, 24, 691–710. [CrossRef] 13. Di Salvo, D.T.; Sackett, E.E.; Johnston, R.E.; Thompson, D.; Andrews, P.; Bache, M.R. Mechanical characterisation of a fibre reinforced oxide/oxide ceramic matrix composite. J. Eur. Ceram. Soc. 2015, 35, 4513–4520. [CrossRef] 14. Zhang, J. Different Surface Treatments of Carbon Fibers and Their Influence on the Interfacial Properties of Carbon Fiber/Epoxy Composites. Materials. Ph.D. Thesis, Ecole Centrale Paris, Hervé Biausser, France, 2012. 15. Procedure for Strengthening Carbon Fibres. Patent RU 2413799; MPK D01F 9/12, 10 March 2010. 16. Georgiou, P.; Walton, J.; Simitzis, J. Surface modification of pyrolyzed carbon fibres by cyclic voltammetry and their characterization with XPS and dye adsorption. Electrochim. Acta 2010, 55, 1207–1216. [CrossRef] 17. Kim, J.; Mauchauffé, R.; Kim, D.; Kim, J.; Moon, S.Y. Mechanism study of atmospheric-pressure plasma treatment of carbon fibre reinforced polymers for adhesion improvement. Surf. Coat. Technol. 2020, 393. [CrossRef] 18. Hansong, L.; Yan, Z.; Na, L.; Xiaoran, Z.; Xiao, H.; Shuang, L.; Wenkuo, L.; Kai, W.; Shanyi, D. Enhanced interfacial strength of carbon fiber/PEEK composites using a facile approach via PEI&ZIF-67 synergistic modification. J. Mater. Res. Technol. 2019, 8, 6289–6630. 19. Liang, Y.; Li, X.; Semitekolos, D.; Charitidis, C.; Dong, H. Enhanced properties of PAN-derived carbon fibres and resulting composites by active screen plasma surface functionalization. Plasma Process. Polym. 2020, 17, e1900252. [CrossRef] 20. Kainourgios, P.; Kartsonakis, I.A.; Dragatogiannis, D.A.; Koumoulos, E.P.; Goulis, P.; Charitidis, C.A. Electrochemical surface functionalization of carbon fibres for chemical affinity improvement with epoxy resins. Appl. Surf. Sci. 2017, 416, 593–604. [CrossRef] 21. Li, Z.; Wang, J.; Tong, Y.; Xu, L. Anodic Oxidation on Structural Evolution and Tensile Properties of Polyacrylonitrile Based Carbon Fibres with Different Surface Morphology. J. Mater. Sci. Technol. 2012, 28, 1123–1129. [CrossRef] 22. Bijwe, J.; Sharma, M. Carbon fabric-reinforced polymer composites and parameters controlling tribological performance. In Wear of Advanced Materials; Davim, J.P., Ed.; ISTE Wiley: Hoboken, NJ, USA, 2011; pp. 1–59. 23. Theodoridou, E.; Besenhard, J.O.; Fritz, H.P. Chemically modified carbon fibre electrodes: Part I. Bulk-functionalized carbon fibres. J. Electroanal. Chem. 1981, 122, 67–71. [CrossRef] 24. Jones, C. Effects of Electrochemical and Plasma Treatments on Carbon Fibres Surfaces. Surf. Interf. Anal. 1993, 20, 357–367. [CrossRef] 25. Subramanian, R.V.; Jakubowski, J.J. Electropolymerization on graphite fibers. Polym. Eng. Sci. 1978, 18, 590–600. [CrossRef] 26. Besenhard, J.O.; Jakob, J.; Möller, P.; Sauter, R.F. Gic-Formation in Neutral Aqueous Electrolytes: An Undesired Side-Reaction during Electrochemical Surface Oxidation of Highly Oriented Carbon Fibres. Synth. Met. 1989, 34, 719–724. [CrossRef] 27. Chang, J.; Bell, J.P.; Shkolnik, S. Electro-Copolymerisation of Acrylonitrile and Methyl Acrylate onto Graphite Fibres. J. Appl. Polym. Sci. 1987, 34, 21052124. [CrossRef] 28. Donnet, J.B.; Brendle, M.; Dhami, T.L.; Bahl, O.P. Plasma treatment effect on the surface energy of carbon and CFs. Carbon 1986, 24, 757–770. [CrossRef] 29. Sun, M.; Hu, B.; Wu, Y.; Tang, Y.; Huang, W.; Da, Y. Surface of CFs continuously treated by cold plasma. Comp. Sci. Tech. 1989, 34, 353–364. Materials 2020, 13, 3136 14 of 14

30. Semitekolos, D.; Kainourgios, P.; Jones, C.; Rana, A.; Koumoulos, E.P.; Charitidis, C.A. Advanced carbon fibre composites via poly methacrylic acid surface treatment; surface analysis and mechanical properties investigation. Comp. Part B Eng. 2018, 155, 237–243. [CrossRef] 31. Koumoulos, E.P.; Kainourgios, P.; Charitidis, C.A. Assessing the integrity of CFRPs through nanomechanical mapping: The effect of CF surface modification. Matec Web Conf. 2018, 188, 01006. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).