Enhanced Surface Properties of Carbon Fiber Reinforced Plastic by Epoxy Modiﬁed Primer with Plasma for Automotive Applications

polymers

Article Enhanced Surface Properties of Carbon Fiber Reinforced Plastic by Epoxy Modiﬁed Primer with Plasma for Automotive Applications

Kyeng-Bo Sim 1, Dooyoung Baek 1,2 , Jae-Ho Shin 1 , Gyu-Seong Shim 1, Seong-Wook Jang 1, Hyun-Joong Kim 1,2,* , Jong-Won Hwang 3 and Jeong U. Roh 4 1 Laboratory of Adhesion and Bio-Composites, Program in Environmental Materials Science, Seoul National University, Seoul 08826, Korea; [email protected] (K.-B.S.); [email protected] (D.B.); [email protected] (J.-H.S.); [email protected] (G.-S.S.); [email protected] (S.-W.J.) 2 Research Institute of Agriculture and Life Sciences, College of Agriculture and Life Science, Seoul National University, Seoul 08826, Korea 3 Kukdo Chemical Co., Ltd, Gasandigital 2-ro, Seoul 08588, Korea; [email protected] 4 Gumi Electronics & Information Technology Research Institute, Gumi 39171, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-880-4784; Fax: +82-2-873-2318

Received: 15 January 2020; Accepted: 25 February 2020; Published: 3 March 2020

Abstract: Carbon fiber reinforced plastic (CFRP) is currently used as a lightweight material in various parts of automobiles. However, fiber reinforced plastic (FRP) material may be damaged at the time of joining via mechanical bonding; therefore, adhesion is important. When bonding is conducted without surface CFRP treatment, interfacial destruction occurs during which the adhesive falls off along with the CFRP. Mechanical strength and fracture shape were investigated depending on the surface treatment (pristine, plasma treatment times, and plasma treatment times plus epoxy modified primer coating). The plasma treatment effect was verified using the contact angle and X-ray photoelectron spectroscopy. The wettability of the epoxy modified primer (EMP) coating was confirmed through surface morphology analysis, followed by observation of mechanical properties and fracture shape. Based on test data collected from 10 instances of plasma treatment, the EMP coating showed 115% higher strength than that of pristine CFRP. The adhesive failure shape also changed from interfacial failure to mixed-mode failure. Thus, applying an EMP coating during the automotive parts stage enhances the effect of CFRP surface treatment.

Keywords: epoxy; CFRP; lightweight; automobile; surface treatment

1. Introduction The automotive industry is currently focusing on the use of lightweight materials (such as composite materials, aluminum, magnesium, and plastics) to increase energy efficiency and reduce CO2 emissions. In general, a weight reduction of 100 kg corresponds to a CO2 emission reduction of 7.5–12.5 g/km. Therefore, weight reduction is becoming increasingly important in the automotive industry. Carbon fiber reinforced plastics (CFRPs) are used in various fields, including the aviation and automotive industries. The greatest advantages of CFRP materials are their light weight, high strength, corrosion resistance, and high rigidity [1,2]. Due to high cost, production processing issues, and restricted functionality, CFRP should be used with steel or other similar materials. Existing physical joining methods, such as riveting, bolting, and punching, are unsuitable for fiber-reinforced plastics (FRPs). Because FRP comprises multiple layers, it is prone to cracking around such physical joints, which causes significant damage to the substrate owing to propagation of these cracks [3,4]. Stress concentration is another major issue during

Polymers 2020, 12, 556; doi:10.3390/polym12030556 www.mdpi.com/journal/polymers Polymers 2020, 12, 556 2 of 14 physical joining as stress concentrates around bolts and rivets, leading to failure in these areas. Stress concentration is more severe for FRPs than for metals [5–7]. A solution for stress concentration is adhesive bonding. When using adhesives, stress is distributed evenly between the substrate and the surface of the substrate, thus avoiding the problem of stress concentration [8,9]. CFRPs are manufactured by various methods such as prepreg compression molding (PCM), CFRP, and autoclave. In this case, silicone release agents are used to facilitate demolding. However, these release agents remain on the PCM CFRP surfaces, interfering with chemical bonding and adhesion [10–12]. This induces surface irregularities via resin delamination during demolding. Proper pretreatment of a surface is necessary to improve adhesion performance. Pretreatment includes methods of removing surface contaminants, removing oxidized layers, improving wettability, chemical modification, and changing the surface roughness. Typically used surface treatment methods include abrasion and blasting for directly treating the surface. This allows for a low cost and simple surface treatment. The disadvantage is that it not only damages the surface resin but also the fiber, which can cause additional reliability problems and requires additional cleaning [13]. Another surface treatment method is the use of an excimer laser. This method selectively removes surface contaminants with little damage to the fibers. Depending on the wavelength and radiation, the surface cutting can be adjusted [11]. Although laser processing can be delicate, there are many limitations to its applications in automotive processes due to equipment cost. Among these surface treatments, plasma treatment induces chemical changes such as forming oxygen-containing groups on the surface; the surface roughness also shows changes such as smoothing [14,15]. The oxygen-containing groups on the surface can improve the surface wettability and adhesion performance, also increasing surface energy [16]. The plasma surface treatment process is also simple and relatively easy to introduce. In this study, an experiment was conducted in which a coating was formed on the activated CFRP surface after plasma treatment to create a single coating layer with strong chemical bonding. We investigated the chemical bonding strength depending on the number of plasma treatments. The effect was verified by evaluating the surface morphology and mechanical properties. The adhesion strength between the PCM CFRP and the coating layer was confirmed using a surface and interfacial cutting analysis system (SAICAS). After measuring the mechanical strength, the effect of the coating was confirmed by observing the fracture shape of the CFRP.

2. Materials and Methods

2.1. Materials The specimens used were PCM CFRP and steel plate. The PCM CFRP was composed of an epoxy resin matrix and plain weave prepreg (WSN-3KT, SK Chemicals, Seoul, Republic of Korea). The SK Chemicals prepreg was of a snap curing type. The PCM CFRP laminated 13 prepreg layers and laid up to 0◦ and 90◦. The laminates were cured in a compression molder at 150 ◦C for 5 min under a pressure of 2.0 MPa. The steel plate type used was cold rolled steel (CR340, Posco, Pohang, Republic of Korea); CR340 is widely used in pillar parts in the automotive industry.

2.1.1. Epoxy Modiﬁed Primer The epoxy modiﬁed primer (EMP) used YD-128, Dyhard 100 s, and Dyhard UR500. YD-128 (Kukdo Chemical, Seoul, Republic of Korea) is a bisphenol-A epoxy resin. The curing agent (Dyhard 100 s, Alzchem, Trostberg, Germany) and latent curing agent (Dyhard UR500, Alzchem, Trostberg, Germany) were blended with YD-128 and ethyl acetate according to an equivalent ratio of 1:0.7:0.6 of base epoxy, curing agent, and latent curing agent, respectively. Ethyl acetate was used to adjust the viscosity at the blending weight ratio of 18%. Detailed information on the materials used in this work is listed in Table1. PolymersPolymers 20202020,,12 12,, 556 x FOR PEER REVIEW 33 of 1414

Table 1. Epoxy modified primer composition. Table 1. Epoxy modiﬁed primer composition. Epoxy Modified Primer Epoxy Modiﬁed Primer Material Composition Equivalent Weight (g/eq) Form YDMaterial‐128 (Kukdo Chem.) Bisphenol Composition‐A epoxy Equivalent Weight187 (g/eq)Liquid Form YD-128Dyhard (Kukdo 100 Chem.) s (Alzchem.) Bisphenol-ADicyandiamide epoxy 18721 Powder Liquid DyhardDyhard 100 sUR500 (Alzchem.) (Alzchem.) DicyandiamideSubstituted urea 213 Powder Powder Dyhard UR500 (Alzchem.) Substituted urea 3 Powder Ethyl Acetate Solvent Liquid Ethyl Acetate Solvent Liquid

2.1.2. Plasma Treatment 2.1.2. Plasma Treatment An atmospheric pressure plasma unit (PLAMI‐α, APP, Hwaseong , Republic of Korea) was used An atmospheric pressure plasma unit (PLAMI-α, APP, Hwaseong, Republic of Korea) was used for CFRP surface activation with Ar and oxygen as the active gases at a flow rate of 10 mL/min. for CFRP surface activation with Ar and oxygen as the active gases at a flow rate of 10 mL/min. Plasma Plasma treatment was performed at a height of 5 mm and output power of 100 W. The exposure time treatment was performed at a height of 5 mm and output power of 100 W. The exposure time for one for one cycle of plasma was 10 s. The surfaces were treated with 0, 1, 5, and 10 plasma treatment cycle of plasma was 10 s. The surfaces were treated with 0, 1, 5, and 10 plasma treatment cycles. This cycles. This permitted increases in the ratio of oxygen‐containing groups. permitted increases in the ratio of oxygen-containing groups. 2.2. Epoxy‐Modified Primer Coating 2.2. Epoxy-Modified Primer Coating Figure 1 shows Epoxy‐Modified Primer Coating process. EMP coating was carried out in the Figure1 shows Epoxy-Modified Primer Coating process. EMP coating was carried out in the following manner: The CFRP surface was ultrasonically cleaned in ethanol for 10 min to remove the following manner: The CFRP surface was ultrasonically cleaned in ethanol for 10 min to remove release agent used during thermos‐compressing. Plasma treatment was conducted for surface the release agent used during thermos-compressing. Plasma treatment was conducted for surface activation. After 0, 1, 5, or 10 plasma treatment cycles on the CFRP surface, the CFRP was maintained activation. After 0, 1, 5, or 10 plasma treatment cycles on the CFRP surface, the CFRP was maintained at room temperature (23 °C, 55% RH) for 10 min. EMP coating was performed on the activated CFRP at room temperature (23 C, 55% RH) for 10 min. EMP coating was performed on the activated CFRP surface. Using an air compressor◦ spray gun at a pressure of 12 psi (82 kPa), 5 cm from the gun nozzle, surface. Using an air compressor spray gun at a pressure of 12 psi (82 kPa), 5 cm from the gun nozzle, the coating was applied from left to right. After EMP coating, the drying and curing steps were the coating was applied from left to right. After EMP coating, the drying and curing steps were conducted in an oven at 65 °C for 20 min and 80 °C for 10 min. After the ethyl acetate dried, curing conducted in an oven at 65 C for 20 min and 80 C for 10 min. After the ethyl acetate dried, curing was performed at 180 °C for◦ 30 min. It was then cooled◦ to room temperature (23 °C, 55% RH). Figure was performed at 180 C for 30 min. It was then cooled to room temperature (23 C, 55% RH). Figure1 1 shows the process of◦ EMP coating. ◦ shows the process of EMP coating.

Figure 1. Schematic of epoxy modiﬁed primer (EMP) coating process. Figure 1. Schematic of epoxy modified primer (EMP) coating process. 2.3. Contact Angle 2.3. Contact Angle The wettability according to the number of plasma treatments was conﬁrmed through contact angleThe measurements wettability according (Phoenix to 150, the SEO, number Suwon, of plasma Republic treatments of Korea) was using confirmed 2 µL deionized through contact water droplets.angle measurements The temperature (Phoenix was maintained150, SEO, Suwon, at 25 ◦C Republic while the of relative Korea) humidity using 2 μ wasL deionized 55%. water droplets. The temperature was maintained at 25 °C while the relative humidity was 55%.

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2.4. X‐Ray Photoelectron Spectroscopy 2.4. X-Ray Photoelectron Spectroscopy X‐ray photoelectron spectroscopy (XPS; AXIS SUPRA, Kratos, Manchester, UK) was used to X-ray photoelectron spectroscopy (XPS; AXIS SUPRA, Kratos, Manchester, UK) was used to verify verify the state of change of activated groups after plasma treatment. The XPS system used an Al Kα the state of change of activated groups after plasma treatment. The XPS system used an Al Kα radiation radiation source for surface chemical analysis. source for surface chemical analysis.

2.5.2.5. Surface Surface Analysis Analysis AA ﬁeld-emission field‐emission scanning scanning electron electron microscope microscope (FESEM, (FESEM, SIGMA, SIGMA, Gillingham, Gillingham, U.K.) and noncontactU.K.) and 3Dnoncontact surface proﬁler 3D surface (Nano profiler View-E1000, (Nano Nano View System,‐E1000, Daejeon, Nano System, Republic Daejeon, of Korea) Republic were used of Korea) to examine were theused surface to examine morphology the surface changes morphology of the EMP changes coating of after the EMP plasma coating treatment. after plasma treatment.

2.6.2.6. Surface Surface and and Interfacial Interfacial Cutting Cutting Analysis Analysis System System AA SAICAS SAICAS (SAICAS (SAICAS EN-EX, EN‐EX, Daipla Daipla Wintes, Wintes, Saitama, Saitama, Japan) Japan) was was used used to to directly directly evaluate evaluate the the degreedegree of of bonding bonding between the the CFRP CFRP surface surface and and the the coating coating layer layer using using a 0.2 a mm 0.2 mmwide wide diamond diamond blade. blade.Figure Figure 2 shows2 shows that thatthe thecoating coating was was first ﬁrst cut cut in in the the vertical vertical direction direction and thenthen inin the the horizontal horizontal direction.direction. Therefore, Therefore, the the peeling peeling force force exerted exerted by by the the coating coating on on the the CFRP CFRP surface surface was was conﬁrmed. confirmed.

Figure 2. Schematic of (a) surface and interfacial cutting analysis system (SAICAS) operation, (b) cutting blade. Figure 2. Schematic of (a) surface and interfacial cutting analysis system (SAICAS) operation, (b) 2.7. Lapcutting Shear blade. Test The lap shear test was performed using a universal testing machine (Allround Line Z010, Zwick, 2.7. Lap Shear Test Ulm, Germany) to conﬁrm the EMP coating eﬀect. Figure3 shows that the CFRP specimen was 25 100The mm lap 2shearand wastest was 2.5 mm performed thick, whereas using a theuniversal CR340 testing steel plate machine specimen (Allround was of Line the Z010, same Zwick, width × andUlm, length Germany) but was to 1.5confirm mm thick. the EMP The coating adhesives effect. used Figure were KSR-177 3 shows (Kukdothat the Chem, CFRP specimen Seoul, Republic was 25 of × Korea)100 mm and2 and G5022 was (Kukdo 2.5 mm Chem, thick, Seoul,whereas Republic the CR340 of Korea) steel plate with anspecimen equivalence was ratioof the of same 1:1.1. width Detailed and informationlength but was regarding 1.5 mm the thick. materials The usedadhesives in this used work were is listed KSR in‐177 Table (Kukdo2. The thicknessChem, Seoul, of the Republic adhesive of wasKorea) controlled and G5022 using (Kukdo a 200 µm Chem, shim stockSeoul, and Republic applied of over Korea) an area with of 25an equivalence12.5 mm2. ratio of 1:1.1. Detailed information regarding the materials used in this work is listed× in Table 2. The thickness of the adhesive was controlled using aTable 200 μ 2.mAdhesive shim stock composition. and applied over an area of 25 × 12.5 mm2.

Adhesive Materials Composition Equivalent Weight (g/eq) Form KSR-177 (Kukdo Chem.) Bisphenol-A Type 205 Liquid G-5022 (Kukdo Chem.) Polyamide 175 Liquid

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Figure 3. Figure 3.Schematic Schematic of testing lap lap shear shear strength strength of dissimilar of dissimilar materials. materials. 3. Results and Discussion Table 2. Adhesive composition.

When manufacturing PCM CFRP, a siliconeAdhesive release agent was applied to the mold for easy demolding. This releaseMaterials agent remainedComposition as a contaminant Equivalent on the PCM Weight CFRP (g/eq) surface. Form Such contaminants reduce adhesiveKSR‐177 performance (Kukdo Chem.) [17]. Therefore,Bisphenol‐A surface Type treatment is205 often required.Liquid G‐5022 (Kukdo Chem.) Polyamide 175 Liquid 3.1. Surface Modiﬁcation through Plasma Treatment 3.Plasma Results treatment and Discussion was conducted to activate the surface. The generation of oxygen functionalities can improveWhen the manufacturing wettability ofPCM PCM CFRP, CFRP. a silicone Figure 4release shows agent that thewas nature applied of to the the CFRP mold changed for easy from hydrophobicdemolding. to This hydrophilic release agent depending remained on the as numbera contaminant of plasma on treatmentthe PCM CFRP cycles. surface. The contact Such angle contaminants reduce adhesive performance [17]. Therefore, surface treatment is often required. with no plasma treatment was 86◦. Wettability increased sharply with increasing numbers of applied plasma treatments (0, 1, 5, 10 times); however, Figure4 shows that the wettability remained very 3.1. Surface Modification through Plasma Treatment low after four cycles were applied. The contact angle gradually decreased and stabilized after four treatmentPolymersPlasma cycles. 2020 , 12treatment The, x FOR experimental PEER was REVIEW conducted numbers to ofactivate plasma the treatments surface. wereThe 0,generation 1, 5, and 10.of oxygen6 of 14 functionalities can improve the wettability of PCM CFRP. Figure 4 shows that the nature of the CFRP changed from hydrophobic to hydrophilic depending on the number of plasma treatment cycles. The contact angle with no plasma treatment was 86°. Wettability increased sharply with increasing numbers of applied plasma treatments (0, 1, 5, 10 times); however, Figure 4 shows that the wettability remained very low after four cycles were applied. The contact angle gradually decreased and stabilized after four treatment cycles. The experimental numbers of plasma treatments were 0, 1, 5, and 10. The activated surfaces after 0, 1, 5, or 10 plasma treatment cycles were examined by XPS. The surface composition was observed to shift following plasma treatment. High‐resolution spectra of C1s showed that the peak intensities of the oxygen functional groups were dramatically changed for three components (284.6, 286.2, and 289.2 eV). The peak at 284.6 eV represented hydrocarbons (C–H), the peak at 286.2 eV represented alkoxy groups (C–O), and the peak at 289.2 eV represented carboxyl groups (O–C=O) [18]. The amount of oxygen increased with increasing numbers of plasma treatments; as a result, the peak intensities of the oxygen functional groups also increased [19]. Figure 5 shows that as the number of plasma treatments increased, the hydrocarbon peak intensity decreased while those of the alkoxy group and carboxyl group increased. The increased carboxyl group intensity indicated crosslinking with the epoxy [20].

FigureFigure 4. Contact 4. Contact angle angle according according to to the the number of of plasma plasma treatment treatment cycles. cycles.

Figure 5. C1s X‐ray photoelectron spectroscopy data of coatings with different numbers of plasma treatment cycles.

3.2. EMP Coatings Plasma‐treated materials are difficult to store for long periods because they show inferior endurance and, over time, they return to their original energy states [21]. To solve this problem of unsustainability of plasma treatment, we used EMP coatings. EMP coatings were prepared by blending an epoxy, curing agent, latent curing agent, and solvent. However, the EMP coating is not simply a single‐layer coating. The most important element in EMP coating is the chemical bonding of epoxides and amines with activated groups generated after plasma treatment. Figure 6 depicts possible chemical reactions between carboxyl groups and epoxy (Figure 6a), amines and epoxy (Figure 6b), and carboxyl groups and amines (Figure 6c). With a nonactivated surface, only epoxy and amine react with each other. However, the plasma treatment generates oxygen‐based polar functionalities such as ether (–O–), hydroxyl groups (–OH), carboxylic

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The activated surfaces after 0, 1, 5, or 10 plasma treatment cycles were examined by XPS. The surface composition was observed to shift following plasma treatment. High-resolution spectra of C1s showed that the peak intensities of the oxygen functional groups were dramatically changed for three components (284.6, 286.2, and 289.2 eV). The peak at 284.6 eV represented hydrocarbons (C–H), the peak at 286.2 eV represented alkoxy groups (C–O), and the peak at 289.2 eV represented carboxyl groups (O–C=O) [18]. The amount of oxygen increased with increasing numbers of plasma treatments; as a result, the peak intensities of the oxygen functional groups also increased [19]. Figure5 shows that as the number of plasma treatments increased, the hydrocarbon peak intensity decreased while those of the alkoxy group and carboxyl group increased. The increased carboxyl group intensity indicated crosslinking with theFigure epoxy 4. Contact [20]. angle according to the number of plasma treatment cycles.

FigureFigure 5. C1s5. C1s X-ray X‐ray photoelectron photoelectron spectroscopy spectroscopy data data of coatingsof coatings with with diﬀ differenterent numbers numbers of plasmaof plasma treatmenttreatment cycles. cycles. 3.2. EMP Coatings 3.2. EMP Coatings Plasma-treated materials are diﬃcult to store for long periods because they show inferior endurance Plasma‐treated materials are difficult to store for long periods because they show inferior and, over time, they return to their original energy states [21]. To solve this problem of unsustainability endurance and, over time, they return to their original energy states [21]. To solve this problem of of plasma treatment, we used EMP coatings. unsustainability of plasma treatment, we used EMP coatings. EMP coatings were prepared by blending an epoxy, curing agent, latent curing agent, and solvent. EMP coatings were prepared by blending an epoxy, curing agent, latent curing agent, and However, the EMP coating is not simply a single-layer coating. The most important element in EMP solvent. However, the EMP coating is not simply a single‐layer coating. The most important element coating is the chemical bonding of epoxides and amines with activated groups generated after plasma in EMP coating is the chemical bonding of epoxides and amines with activated groups generated treatment. Figure6 depicts possible chemical reactions between carboxyl groups and epoxy (Figure6a), after plasma treatment. Figure 6 depicts possible chemical reactions between carboxyl groups and amines and epoxy (Figure6b), and carboxyl groups and amines (Figure6c). With a nonactivated surface, epoxy (Figure 6a), amines and epoxy (Figure 6b), and carboxyl groups and amines (Figure 6c). With only epoxy and amine react with each other. However, the plasma treatment generates oxygen-based a nonactivated surface, only epoxy and amine react with each other. However, the plasma treatment polar functionalities such as ether (–O–), hydroxyl groups (–OH), carboxylic acids (–COOH), and esters generates oxygen‐based polar functionalities such as ether (–O–), hydroxyl groups (–OH), carboxylic (–COOR) [22]. Unreacted epoxides and functional groups react further to form strong chemical bonds with the coating layer at the CFRP interface.

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Polymersacids (–COOH),2020, 12, 556 and esters (–COOR) [22]. Unreacted epoxides and functional groups react further7 of 14 to form strong chemical bonds with the coating layer at the CFRP interface.

Figure 6. Chemical reaction between (a) carboxyl group and epoxy, (b) amine and epoxy, and (c) Figure 6. Chemical reaction between (a) carboxyl group and epoxy, (b) amine and epoxy, and (c) carboxyl group and amine. carboxyl group and amine. 3.2.1. Surface Morphology 3.2.1. Surface Morphology The surface morphology of the EMP coating was examined by scanning electron microscopy (SEM)The and surface noncontact morphology 3D microsurface of the EMP profiling. coating Figure was4 examinedshows that by without scanning any electron surface treatment,microscopy CFRP(SEM) showed and noncontact hydrophobic 3D microsurface characteristics. profiling. Figure Figure7 shows 4 shows that the that wettability without any of the surface EMP treatment, coating changedCFRP showed with increasing hydrophobic number characteristics. of plasma treatments, Figure 7 whileshows Figure that 7thea shows wettability that without of the anyEMP plasma coating treatment,changed thewith pristine increasing CFRP number is hydrophobic of plasma and treatments, P0 + EMP didwhile not Figure form a 7a uniform shows coating that without layer. The any SEMplasma image treatment, shows that the thepristine EMP CFRP coating is didhydrophobic not spread and and P0 formed + EMP numerous did not form shapes a uniform on the surface. coating Thelayer. noncontact The SEM 3D image surface shows profiling that the also EMP showed coating that did the not coating spread was and partially formed confined numerous in the shapes case of on P0the+ surface.EMP. The The plasma noncontact treatment 3D surface became profiling more hydrophilic also showed and that the wettability the coating improved was partially as shown confined in Figuresin the4 caseand7 .of The P0 observation + EMP. The of EMPplasma spread treatment gradually became improved more through hydrophilic SEM images and the and wettability surface profilingimproved images. as shown The in nonuniform Figures 4 and surface 7. The improved observation gradually. of EMP In spread the case gradually of the last improved P10 + EMP, through the noncontactSEM images image and showssurface the profiling formation images. of a uniform The nonuniform coating layer. surface improved gradually. In the case of the last P10 + EMP, the noncontact image shows the formation of a uniform coating layer.

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FigureFigure 7. 7. Scanning electronelectron microscopy microscopy (SEM) (SEM) and and noncontact noncontact 3D microsurface3D microsurface proﬁler profiler image image of epoxy of epoxymodiﬁed modified primer primer (EMP) (EMP) coating: coating: (a) P0 (+a)EMP, P0 + EMP, (b) P1 (+b)EMP, P1 + EMP, (c) P5 (+c)EMP, P5 + EMP, and (d and) P10 (d+) P10EMP. + EMP.

3.2.2. EMP Coating Adhesion Properties

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Polymers 2020, 12, x FOR PEER REVIEW 9 of 14 3.2.2. EMP Coating Adhesion Properties SAICAS equipment was used to confirmconfirm the binding forces between the CFRP and EMP coating layers. First, a diamond blade was used to vertically cut the coating layer at an angle of 60°. The layers. First, a diamond blade was used to vertically cut the coating layer at an angle of 60◦. The diamond blade cut down vertically and then, after reaching the CFRP interface, cut horizontally. The experiment waswas conducted conducted to to check check the the interface interface peel peel force force between between the CFRP the surfaceCFRP surface and EMP and coating. EMP Thiscoating. vertical This cut vertical was mainly cut was used mainly for depth used profiling for depth to profiling measure theto measure internal forcesthe internal of the coatingforces of layer. the Thecoating vertical layer. load The also vertical indicated load thealso thickness indicated of the the thickness coating layer. of the As coating the vertical layer. load As the approached vertical load 0 N, theapproached diamond 0 blade N, the approached diamond blade the CFRP approached surface. Thethe CFRP EMP coating surface. depth The EMP showed coating that P1depth+ EMP showed of 11 µthatm, P5P1 ++ EMP ofof 10.911 μµm,m, P5 P0 ++ EMPEMP of of 10.9 9.6 µ μm,m, and P0 + P10 EMP+ EMP of 9.6 of μ 8.1m,µ andm were P10 measured.+ EMP of 8.1 P10 μm+ EMPwere showedmeasured. the P10 best + EMP wettability showed and the the best thinnest wettability coating and the layer thinnest [23]. coating layer [23]. The horizontal force was measured during the peeling of the EMP coating from the CFRP surface. Figure 88 shows shows that that the the horizontal horizontal forces forces at at P0 P0 ++ EMP and P1 + EMP were 0.4 N. As the number of plasma treatmentstreatments increased,increased, additional additional chemical chemical bonds bonds formed, formed, as as indicated indicated by by P5 P5+ +EMP EMP and and P10 P10+ EMP,+ EMP, which which showed showed a horizontal a horizontal force force of 0.6 of N. 0.6 With N. zero With or zero a single or a plasma single treatment, plasma treatment, the thickness the thickness of the coating layer was not constant, and the graph of horizontal force fluctuated of the coating layer was not constant, and the graph of horizontal force fluctuated significantly. As significantly. As the number of plasma treatments increased, SAICAS showed that stronger chemical the number of plasma treatments increased, SAICAS showed that stronger chemical bonds and more bonds and more consistent coating layers formed. consistent coating layers formed.

Figure 8. Cont.

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Figure 8. (a) Schematic of surface and interfacial cutting analysis system (SAICAS) operation, (b) Figure 8.8. ((aa)) Schematic Schematic of of surface surface and and interfacial interfacial cutting cutting analysis analysis system system (SAICAS) (SAICAS) operation, operation, (b) vertical (b) vertical load of epoxy modified primer (EMP) coating, and (c) horizontal load of EMP coating. loadvertical of epoxy load of modified epoxy modified primer (EMP) primer coating, (EMP) coating, and (c) horizontal and (c) horizontal load of EMP load coating.of EMP coating. 3.3.3.3. MechanicalMechanical Test Test and and Fracture Fracture Analysis Analysis 3.3. Mechanical Test and Fracture Analysis TheThe lap lap shear shear test test was was used used to check to check the EMP the coatingEMP coating effect via effect measuring via measuring the mechanical the mechanical strength. The lap shear test was used to check the EMP coating effect via measuring the mechanical Givenstrength. the Given use of the CFRPs use of with CFRPs dissimilar with dissimilar materials materials in automobiles, in automobiles, the adhesion the adhesion of CFRP of to CFRP steel isto strength. Given the use of CFRPs with dissimilar materials in automobiles, the adhesion of CFRP to important.steel is important. Figure9 aFigure shows 9a the shows e ffect the of plasmaeffect of treatment plasma treatment changes on changes adhesive on bonding.adhesive Inbonding. addition, In steel is important. Figure 9a shows the effect of plasma treatment changes on adhesive bonding. In increasedaddition, mechanicalincreased mechanical properties properties were observed were observed with the with plasma the+ plasmaEMP coating. + EMP coating. With P0 With+ EMP, P0 + addition, increased mechanical properties were observed with the plasma + EMP coating. With P0 + FigureEMP, 9Figureb shows 9b noshows significant no significant di fference difference because because no chemical no chemical bonding bonding occurred; occurred; the epoxy the wasepoxy a EMP, Figure 9b shows no significant difference because no chemical bonding occurred; the epoxy simplewas a simple coating coating layer with layer no with chemical no chemical bonding bonding to the CFRP to the surface. CFRP surface. was a simple coating layer with no chemical bonding to the CFRP surface. TheThe lap lap shear shear results results showed showed no no significant significant di differencesfferences from from the the pristine pristine state. state. Figure Figure9 b9b shows shows The lap shear results showed no significant differences from the pristine state. Figure 9b shows thatthat asas the the number number of of plasma plasma treatments treatments increased,increased, thethe numbernumber ofof reactivereactive speciesspecies alsoalso increased,increased, that as the number of plasma treatments increased, the number of reactive species also increased, thusthus forming forming more more chemical chemical bonds. bonds. TheThe bondingbonding betweenbetween CFRPCFRP andand thethe coatingcoating layerlayer strengthened,strengthened, thus forming more chemical bonds. The bonding between CFRP and the coating layer strengthened, increasingincreasing the the lap lap shear shear strength; strength; this this is is unlike unlike P0 P0+ + EMP,EMP, which which had had no no chemical chemical bonds. bonds. The The interface interface increasing the lap shear strength; this is unlike P0 + EMP, which had no chemical bonds. The interface ofof the the coating coating layer layer was was weaker weaker than than that that of of the the adhesive. adhesive. When When the the interface interface of of the the coating coating layer layer is is of the coating layer was weaker than that of the adhesive. When the interface of the coating layer is strongerstronger than than the the adhesive, adhesive, thethe CFRPCFRP isis destroyeddestroyed oror thethe middlemiddle ofof thethe adhesiveadhesive layerlayer isis destroyed.destroyed. stronger than the adhesive, the CFRP is destroyed or the middle of the adhesive layer is destroyed. TheThe result result of of the the lap lap shear shear strength strength measurement measurement is important, is important, but observingbut observing the fracture the fracture shape shape is even is The result of the lap shear strength measurement is important, but observing the fracture shape is moreeven significantmore significant after measurement. after measurement. The adhesion The adhesion performance performance with the interface with the can interface be determined can be even more significant after measurement. The adhesion performance with the interface can be accordingdetermined to theaccording fracture to shape. the fracture In P0, shape. the adhesive In P0, strengththe adhesive was 13strength1.3 MPa, was 13 but that 1.3 of MPa, P10 but+ EMP that determined according to the fracture shape. In P0, the adhesive strength± was 13 1.3 MPa, but that increasedof P10 + EMP to 28 increased1.6 MPa. to 28 1.6 MPa. of P10 + EMP increased± to 28 1.6 MPa.

Figure 9. Cont.

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FigureFigure 9. 9. ((aa)E) Effectffect of of the the number number of of plasma plasma treatments treatments and and epoxy epoxy modified modified primer primer (EMP) (EMP) coatings coatings on theon lapthe shearlap shear strength strength and and (b) schematic(b) schematic of interactions of interactions during during lap shearlap shear strength strength evaluation. evaluation. The adhesive fracture surfaces were analyzed after lap shear testing using ImageJ image processing The adhesive fracture surfaces were analyzed after lap shear testing using ImageJ image software. A lot of information can be obtained from the adhesive fracture shape. The failure shape processing software. A lot of information can be obtained from the adhesive fracture shape. The and lap shear test were correlated and analyzed. In the case of P1 + EMP and P5 + EMP, the lap shear failure shape and lap shear test were correlated and analyzed. In the case of P1 + EMP and P5 + EMP, data were similar: 25.1 and 25 MPa, respectively. When we observed the shape of the fracture, a big the lap shear data were similar: 25.1 and 25 MPa, respectively. When we observed the shape of the difference was seen. The CFRP substrate fracture rate changed from 6% to 10%. The higher substrate fracture, a big difference was seen. The CFRP substrate fracture rate changed from 6% to 10%. The fracture rate indicated that the bonding force between the substrate and the adhesive was stronger. higher substrate fracture rate indicated that the bonding force between the substrate and the adhesive The EMP coating and simple plasma treatment also showed significant differences in failure mode, as was stronger. The EMP coating and simple plasma treatment also showed significant differences in depicted in Figure 10a. For P0 to P10, sample failure generally occurred at the interface, except for a failure mode, as depicted in Figure 10a. For P0 to P10, sample failure generally occurred at the few samples. This means that simple plasma treatment shows imperfect adhesion performance when interface, except for a few samples. This means that simple plasma treatment shows imperfect bonding steel to CFRP. Most of the samples of P1 + EMP to P10 + EMP showed mixed-mode failure adhesion performance when bonding steel to CFRP. Most of the samples of P1 + EMP to P10 + EMP and cohesive failure, except for P0 + EMP, which showed the need for surface treatment [24]. showed mixed‐mode failure and cohesive failure, except for P0 + EMP, which showed the need for Figure 10b shows the fracture proportion. In the case of simple plasma treatment (P0, P1, P5, and surface treatment [24]. P10), the rate of adhesive falling off from the CFRP interface was high. This indicated that the adhesion Figure 10b shows the fracture proportion. In the case of simple plasma treatment (P0, P1, P5, and between the CFRP interface and the adhesive force was not strong enough. Except for P0 + EMP, the P10), the rate of adhesive falling off from the CFRP interface was high. This indicated that the P1 + EMP, P5 + EMP, and P10 + EMP samples showed significantly lower interfacial failure rates in adhesion between the CFRP interface and the adhesive force was not strong enough. Except for P0 + CFRP. The CFRP substrate breaking rate also increased by 6%, 10%, and 13%, respectively. An increase EMP, the P1 + EMP, P5 + EMP, and P10 + EMP samples showed significantly lower interfacial failure in the substrate breaking rate indicated that the interfacial adhesion between CFRP and the adhesive rates in CFRP. The CFRP substrate breaking rate also increased by 6%, 10%, and 13%, respectively. was extremely strong, i.e., the interface and adhesive bonding was very good, leading to the breakage An increase in the substrate breaking rate indicated that the interfacial adhesion between CFRP and of the CFRP. the adhesive was extremely strong, i.e., the interface and adhesive bonding was very good, leading to the breakage of the CFRP.

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FigureFigure 10. 10.Observation Observation after after lap shear lap strengthshear strength measurement: measurement: (a) Fracture (a shape) Fracture and ( bshape) fracture and proportion. (b) fracture proportion.

4. Conclusions

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4. Conclusions Experiments were performed to improve the adhesion between dissimilar materials (steel and CFRP) in order to reduce the weight of automobiles. CFRP can show poor adhesion in a pristine state; however, the adhesive performance of diﬀerent materials can be improved through surface treatment. Plasma treatment was performed to generate activators (oxygen-containing functional groups) and improve the wettability of the surface; increases in oxygen-containing functional groups (C–O, O–C=O) were successfully observed (P10 > P5 = P1 > P0) to increase the carboxyl group. This increased carboxyl group cross-linked with epoxy. SAICAS was used to check the degree of chemical bonding between CFRP and the coating layer. Lap shear test results also showed that adhesive strength increased with increasing numbers of plasma treatment cycles. P10 + EMP showed the strongest chemical bonding and highest horizontal force. In the lap shear test, P10 + EMP surface treatment showed an increase of 115% compared to pristine CFRP. In terms of the fracture shape, the CFRP interfacial failure changed to adhesive failure and mixed-mode failure. This plasma + EMP coating process can solve the issue of the endurance of the plasma treatment. In addition, automotive parts fabricated by plasma + EMP coating process will be used frequently because it can be applied directly without surface treatment in the automotive body process.

Author Contributions: Conceptualization, writing-original draft, K.-B.S.; formal analysis, D.B., J.-H.S.; visualization, G.-S.S.; supervision, S.-W.J. and H.-J.K., methodology, J.U.R.; resource, J.-W.H. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Ministry of Trade (Korea) (S10076899, Development of adhesive bonding technique for joining CFRP and other materials used for lightweight transportation equipment). Acknowledgments: In this section you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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