Preparation and mechanical properties of functionalized graphene oxide and silane coupling agent modified carbon fiber/epoxy resin composites

ABSTRACT

To improve the interfacial bonding properties of carbon fiber-reinforced polymer (CFRP) composites, this study proposes an effective surface modification strategy for carbon fibers. A CF-KH560-PGO hybrid reinforcement was fabricated through chemical grafting, employing γ-glycidoxypropyltrimethoxysilane (KH560) as a coupling agent to bridge the carbon fibers (CF) with polyethyleneimine (PEI)-functionalized graphene oxide (GO). The presence of lamellar PGO has been observed on the surface of modified carbon fibers. The presence of the –NH₂ peak, in conjunction with the C–N peak, on the surface of the carbon fibers, served to verify the grafting of PGO. The synergistic effect of PGO and KH560 significantly enhanced the surface wettability of carbon fibers. This study systematically investigates the mechanical performance of composites fabricated through different CF modification approaches, along with the underlying interfacial reinforcement mechanisms. The modified carbon fiber composites exhibited remarkable tensile strength (669.7 MPa) and interlaminar shear strength (60.41 MPa), representing 49.89% and 20.36% improvements respectively over desized carbon fiber composites. Grafted PGO can provide abundant amino groups for chemical bonding with epoxy resins. That KH560 and PGO increased the thickness of the interfacial transition layer, thereby enhancing the load transfer at the composite interface.

Keywords:
Carber fiber; PEI; GO; Multi-scale reinforcement; Mechanical properties

1. INTRODUCTION

CFRP have a wide range of potential applications in a variety of fields, including aerospace, military, and wind power generation equipment, and high-end civilian products due to their high specific strength, strong designability, and high temperature resistance [¹, ², ³, ⁴]. CFRP composites consist of a polymer matrix, most commonly epoxy resin, reinforced with CFs. In these composites, the carbon fibers primarily bear tensile loads while the polymer matrix serves the dual purpose of transferring stresses between fibers and providing compressive strength to the overall structure [⁵]. Nevertheless, carbon fibers exhibit characteristically smooth surfaces and intrinsic chemical inertness. These inherent characteristics, coupled with their low surface energy, lead to suboptimal interfacial adhesion between the CF reinforcement and polymer matrix. Such interfacial incompatibility significantly limits the broader applications of CFRP composites [⁶, ⁷]. Therefore, surface treatment of carbon fibers to enhance the interfacial properties of composites has become a key research focus in academia. Methods such as oxidation, laser irradiation, mechanical abrasion, and plasma treatment can improve the bonding strength between fibers and resins by modifying the surface characteristics of carbon fibers. ŞENOL et al. [⁸] applied atmospheric pressure plasma activation (APA) treatment to carbon fiber surfaces and investigated the optimal treatment duration. This method demonstrated significant reinforcing effects while being environmentally friendly, leading to notable improvements in the tensile properties and fracture toughness of the composites. Subsequently, their research team utilized the acoustic emission (AE) method to systematically investigate the strength and damage mechanisms of composites fabricated from carbon fibers modified by three distinct surface treatment techniques: mechanical abrasion (MA), atmospheric pressure plasma activation (APA), and peel-ply (PP) [⁹], ultimately revealing characteristic damage mechanisms specifically associated with each surface treatment methodology.

Nanoscale reinforcement materials – particularly carbon nanotubes (CNTs), graphene (G), and GO have been extensively employed for CF surface modification owing to their favorable characteristics: low molecular weight, rapid reaction kinetics, excellent interfacial adhesion, and non-aggregation properties [¹⁰, ¹¹, ¹², ¹³, ¹⁴, ¹⁵]. Compared with other inorganic nanomaterials, GO demonstrates exceptional mechanical properties and superior dispersibility [¹⁶, ¹⁷, ¹⁸], making it an ideal candidate for surface modification. Studies have confirmed that constructing a GO-based transition layer at the fiber/matrix interface through surface deposition effectively enhances the interfacial performance of composites [¹⁹, ²⁰]. Many different methods have been used to prepare GO-reinforced CFRP composites, including chemical vapor deposition (CVD) [²¹, ²²], sizing agent coating containing GO [²³, ²⁴], and chemical grafting [²⁵, ²⁶]. The CVD modification process necessitates metal catalysts and elevated temperature conditions that may induce structural degradation in carbon fibers, consequently impairing the mechanical performance of resulting CFRP composites [²⁷]. Although the sizing agent coating method is more convenient to operate, the reinforcement effect is limited due to the mainly intermolecular interaction between the reinforcement layer and CF, with limited functional groups and bonding cooperation [²⁸]. Chemical grafting has gained widespread adoption as a surface modification technique owing to its tunable design parameters, precise reaction control, and environmentally benign characteristics. In a representative study, LI et al. [²⁹] employed 2-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) as a highly effective coupling agent. Through the molecular bridging effect of polyamidoamine (PAMAM) dendrimers, they achieved uniform GO grafting onto carbon fibers, resulting in significantly enhanced surface wettability. It is difficult to avoid the agglomeration phenomenon of GO after the grafting GO onto the CF surface by conventional chemical grafting methods. LI et al. [³⁰] utilized plasma treatment to graft NH₂ onto carbon fibers (CF), enabling the self-assembly of CF with graphene oxide (GO) through electrostatic interactions. The tensile strength of the resulting composite was 31.23% higher than that of pristine CF. XIONG et al. [³¹] employed hyperbranched poly(thioether-yne) (HPTY) as a bridging agent to chemically graft GO onto the CF surface. The modified composite exhibited a maximum tensile performance improvement of 38.2%. However, the agglomeration of GO on the CF surface was identified as a critical factor affecting the mechanical properties of the composite. Functionalized GO is a filler that can be used to improve the mechanical properties of CFRP, which contains reactive groups that promote chemical bonding with the matrix, and the presence of these reactive groups contributes to the dispersion of graphene oxide. However, there are still relatively few studies on functionalizing GO surfaces followed by grafting onto CF surfaces [³², ³³, ³⁴, ³⁵, ³⁶]. MA et al. [³⁷] used the trimer of hexamethylene diisocyanate (HDI) as a coupling agent to graft GO onto the surface of the carbon fiber in two ways: direct grafting of GO and HDI functionalization of GO followed by grafting, respectively. HDI-functionalized GO improved the agglomeration problem of GO compared to direct grafting, and the interfacial shear strength of the modified composites was increased by 39.91%. Functionalization of GO surfaces has gradually become a hot research topic.

Polyethyleneimine (PEI) demonstrates exceptional suitability for CF surface modification owing to its unique molecular architecture. The polymer’s abundant amine groups readily form amide linkages with functionalized carbon fiber surfaces, while its amphiphilic structure – combining hydrophobic vinyl segments with hydrophilic amine functionalities – ensures strong interfacial adhesion. These characteristics have established PEI as a widely adopted modifier in CF treatment protocols [³⁸, ³⁹, ⁴⁰, ⁴¹, ⁴²]. LIU et al. [⁴³] developed an innovative one-step dip-coating technique to construct a PEI/polydopamine (PDA) hybrid coating on CF surfaces. This approach significantly enhanced surface amine group density and surface energy, leading to remarkable improvements in composite mechanical performance. In a separate advancement, TIAN et al. [⁴¹] employed layer-by-layer electrostatic self-assembly to deposit PEI and nano-SiO₂ on CF substrates. This surface engineering strategy achieved an extraordinary 99.4% enhancement in interfacial shear strength compared to untreated fibers. WU et al. [⁴⁴] pioneered an alternative approach utilizing PEI as a molecular bridge to graft both conventional and porous GO onto CF surfaces. The porous GO-modified composites demonstrated superior performance with a 78.64% increase in shear properties, while simultaneously addressing the inherent agglomeration challenges associated with conventional GO. However, since the porous graphene oxide preparation process is complex and has a long lead time, PEI functionalization of GO is a simple and effective way to improve the interfacial characteristics of composites.

Surface activation of CF is typically performed prior to grafting amine-containing molecules onto CF. Currently, the most prevalent method involves oxidizing the CF surface followed by chlorination using thionyl chloride (SOCl₂). FENG et al. [⁴⁵] successfully grafted melamine onto carbon fibers through this chlorination approach, achieving a 20% improvement in the tensile properties of the resulting CFRP. While the chlorination process enables stable grafting of amine-containing molecules, its practical application remains limited due to the significant environmental hazards associated with this method. KH560 silane coupling agent is a commonly used bridging agent for CF surface modification [⁵, ⁴⁶, ⁴⁷], which has low cost and mild reaction conditions, can condense with the hydroxyl groups on the surface of CF itself, and contains epoxy groups, which are capable of forming a close chemical bond with amine-containing molecules. FENG et al. [⁵] subjected hydroxylated SiO₂ to KH560 grafting and self-assembled it with KH550-treated CF, and the tensile properties of the composites prepared from modified carbon fibers were enhanced by 40.6%. QIN et al. [⁴⁶] structurally modified KH560 by bonding its epoxy group with the amino group of phosphorotriamine and used it as a CF sizing agent for the surface treatment of CFRP, which successfully improved the interlaminar shear characteristics of the composites and enhanced the flame retardancy of the composites. XIONG et al. [⁴⁸] used KH560 as a coupling agent to graft CNT onto CF by one-step impregnation method, and utilized the spontaneous hydrolysis of the coupling agent to introduce epoxy functional groups on the surface of CF, and then grafted with carbon nanotubes afterward, and its interfacial shear strength was increased by 86.81%. This modification method is straightforward and demonstrates significant reinforcement effects. However, it suffers from limited chemical bonding between KH560 and carbon nanotubes, as well as a low density of amino functional groups on the carbon fiber surface. In contrast, PEI-functionalized GO exhibits a high concentration of amino groups, which can significantly enhance fiber-epoxy interfacial adhesion.

In this study, CF-KH560-PGO composite reinforcements were fabricated via a chemical grafting approach, utilizing KH560 silane coupling agent as a molecular bridge to connect CF with polyethyleneimine-functionalized graphene oxide (PEI-GO). The mechanical properties of the resulting CF-reinforced composites were systematically evaluated, and the interfacial reinforcement mechanism between CF and epoxy resin was thoroughly investigated. PGO effectively addresses the aggregation tendency of pristine GO while simultaneously enhancing the interfacial strength of CFRP. The KH560 silane coupling agent serves as an efficient bridge between CFs and PGO. During this process, the hydrolyzed product of KH560, Si(OH)₃, readily bonds with hydroxyl groups on the CF surface. while the self-polymerization of KH560 ensures high grafting efficiency on the CF substrate. Furthermore, the epoxy groups of KH560 form robust covalent bonds with the abundant amino groups on the PGO surface. The process is simple, inexpensive and the reaction conditions are mild. The presence of lamellar PGO can be observed on the surface of modified CF, analysis of their chemical bonding changes revealed the presence of –NH₂ peak as well as C–N peak, which verified the grafting of PGO, also the dual action of PGO and KH560 resulted in the improvement of CF surface wettability. The tensile strength and interlaminar shear strength of the composites prepared from modified carbon fibers were 669.7 MPa and 60.41 MPa, respectively, which were 49.89% and 20.36% higher compared to the composites prepared from desizing carbon fibers.

2. MATERIALS AND METHODS

2.1. Materials

The commercially available polyacrylonitrile-based CF (T300-3K) is provided by Jiangsu Wuxi Zhongfu Carbon Fiber Products Co. Epoxy resin (E51) and epoxy resin curing agent (T31) were purchased from Shandong Yousuo Chemical Technology Co. Graphene oxide (GO) was purchased from Nanjing Jicang Nanotechnology Co. 3-Glycidyl ether oxypropyltrimethoxysilane (KH560) (97%), polyethyleneimine (PEI), acetic acid (36 wt.%), nitric acid (68 wt.%) and sodium borohydride were provided by Shanghai Aladdin Biochemical Technology Co. Acetone, ethanol, acetic acid, diiodomethane and potassium hydroxide were analytical grade chemicals (minimum purity 99%) purchased from Tianjin Chemical Reagent Co. Deionized water was homemade in the laboratory.

2.2. Preparation of PEI functionalized GO (PGO)

The preparation process of PGO is shown in Figure 1, where reduction is carried out along with PEI functionalization of GO. PEI itself contains more amino groups, and the amino groups in the protonated state can react with the hydroxyl groups on the surface of GO to form hydrogen bonds, and react with the carboxyl groups to form ionic bonds. The reduced GO possesses superior mechanical properties compared to normal GO, while having more reactive functional groups such as hydroxyl and carboxyl groups than graphene (G) surfaces, which facilitates subsequent reactions [⁴⁹]. The PEI functionalization reaction adds –NH₂ group to the GO surface. Add 10 mL PEI to 50 mL anhydrous ethanol. Prepare 50 ml of KOH solution of 1 wt.%. GO was placed into the mixed solution of PEI and KOH and stirred at 78°C for 15 h, then 10 ml of 1 M NaBH₄ solution was prepared and kept in the reaction system for 4 h, at the end of the reaction, it was washed 5 times with deionized water, centrifuged to separate the PGO, and dried to obtain the desired sample.

Figure 1
Schematic of PEI functionalization on GO surface.

2.3. Carbon fiber (CF) pretreatment and modification process

The process of CF pre-treating and modifying is shown in Figure 2. The CF were soaked in acetone for 24 h for desizing and dried at 60°C for 3 h. Then, nitric acid was then heated for 3 h at 78°C in a water bath as the oxidizing solution. After washing with deionized water to pH neutral, the CF was dried in an oven to obtain the oxidized CF (CF-Oxidation). KH560 was dissolved in anhydrous ethanol to form a 5 wt.% solution of KH560, and the pH was adjusted to 5 with acetic acid, and ultrasonication was performed for 30 min to fully dissolve KH560. The oxidized CF was placed in KH560 solution and reacted at 50°C for 10 h to obtain the coupling agent-treated carbon fiber (CF-KH560). PGO was dissolved in deionized water to prepare 0.5 mg/ml, 1 mg/ml, and 2 mg/ml PGO solutions. The coupling agent-treated CF-KH560 was immersed into different concentrations of PGO solution, respectively, and heated at 60°C for 3 h with stirring. After drying, the modified CF (CF-KH560-PGO) was obtained.

Figure 2
Schematic diagram of CF surface pretreatment and modification process.

2.4. The preparation of Carbon Fiber composites (CFRP)

CFRP were prepared by hot pressing method. The bidirectional twill carbon fiber fabric was cut into square specimens measuring 20 cm × 20 cm. Epoxy resin (E51) and hardener (T31) are mixed in a 4:1 ratio for resin formulation, and the resin was degassed in an ultrasonic cleaner at 50°C for 30 minutes. Six layers of carbon fiber scrim were layered and resin was applied layer by layer and then placed into the mold, the fiber-to-resin mass ratio was 1:1. layers are laid in a manner as shown in Figure 3. After that, it was placed on the plate vulcanizing machine and molded at 80°C and 1 MPa for 30 min, taken out for demolding, and the corners were trimmed to obtain the carbon fiber composite board.

Figure 3
Schematic diagram of CF cloth and epoxy resin layup method.

2.5. Characterization

SEM (S-4800, Hitachi) was used to observe the surface morphology of PEI-modified GO and each stage of CF modification. The tensile fracture fracture morphology of CFRP was also observed. The functional groups present in the GO as well as CF modification stages were investigated by Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet iS10, USA) in the range of 400–4000 cm^-1. X-ray photoelectron spectroscopy (XPS, Thermo kalpha, USA) was utilized to test the CF surface chemistry. The Raman spectra of GO before and after modification were tested using a laser micro Raman spectrometer (inVia Reflex, Renishaw, Britain) with a laser wavelength of 532 nm.

The contact angles between CF and deionized water and diiodomethane were determined using a dynamic contact angle meter (DCAT 21, Data Physics Instruments, Germany), respectively, and its surface energy and components (polarity and dispersion) were calculated by the OWRK method (Equation 1).

(1) $${\gamma }_l^T(1+\cos \theta )=2{({\gamma }_l^d{\gamma }_s^d)}^{1/2}+2{({\gamma }_l^p{\gamma }_s^p)}^{1/2}$$

where ${\gamma }_l^T$ corresponds to the surface tension of the selected liquid, ${\gamma }_s^d$ and ${\gamma }_l^d$ correspond to the dispersive component of the surface energy of the solid and liquid, respectively, and the relative ${\gamma }_l^p$ and ${\gamma }_s^p$ correspond to the polar component of the surface energy of the solid and liquid.

CF surface modification effect on the tensile strength as well as the interlayer shear strength (ILSS) of CFRP was obtained by characterizing the interfacial bonding properties through tensile strength tests and short beam shear tests. The dumbbell-type tensile specimen with the size of 180 mm × 10 mm × 2 mm was prepared according to GB/T 1447-2005, and the shear specimen with the size of 12 mm × 4 mm × 2 mm was prepared according to ASTM D 2344. The specimens were divided into five groups, composites made from CF (CF-Desizing) and CF-KH560 after desizing and CF (CF-KH560-PGO_0.5, CF-KH560-PGO₁, CF-KH560-PGO₂) after treatment with different concentrations of PGO (0.5 mg/ml, 1 mg/ml, 2 mg/ml). Composites were made from each of the five groups of CF, at least five samples were tested per group. Tests were run on a universal testing machine at 3 mm/min for tensile and 1 mm/min for shear. Interlaminar shear strength was calculated using Equation 2.

(2) $$ILSS=(3P_{max})/4Wt$$

Where P_max is the ultimate load, W is the specimen width, and t is the specimen thickness.

3. RESULTS AND DISCUSSION

3.1. Surface functionalization of GO

As evidenced by the FTIR spectra in Figure 4(a), a distinct new peak emerges at approximately 1460 cm^-1 in the PEI-functionalized GO spectrum, corresponding to the characteristic C–N stretching vibration. This confirms the successful grafting of PEI onto the GO surface. Both GO and PEI-GO exhibit prominent absorption bands at 3438 cm^-1 (O–H stretching vibration) and 1626 cm^-1 (C=C skeletal vibration), which are attributed to the hydroxyl groups and carbon-carbon bonds, respectively. The peak at 1112 cm^-1 originates from the stretching vibration of the C–O group. The intensity of the peak (C=O) of PGO at 1725 cm^-1 was significantly reduced after the PEI-functionalized treatment. This is because the GO is reduced during the modification process, and the C=O on the GO surface is reduced. The distinct changes in the FTIR spectra of PEI-functionalized graphene oxide compared to pristine GO demonstrate the successful grafting of reactive functional groups onto the GO surface through PEI modification, along with the effective reduction of graphene oxide during this process. The Raman spectra of GO and PGO are shown in Figure 4(b), the primary spectral features of GO as well as PGO are characterized by the presence of D and G peaks at 1345 cm^-1 and 1580 cm^-1, respectively, where the peak at 1345 cm^-1 is due to the disorder in the graphite structure caused by surface functional groups [⁵⁰]. The intensity ratio (I_D/I_G) of the D and G bands increases with the increase in the amount of disorder in the graphitic material, with a significant improvement in the I_D:I_G value (1.211) for PGO compared to the ratio of GO I_D:I_G (0.991). This indicates that there is a significant increase in the disordered structure in the graphene oxide lamellae, the removal of oxygen-containing groups from the GO surface exposed more new defects. it is demonstrated that more sp3 carbon is involved in the surface modification of GO.

Figure 4
GO before and after PEI functionalization. (a) infrared spectrum, (b) Raman images.

SEM characterization reveals distinct morphological differences between GO and PEI-functionalized GO as shown in Figure 5. Pristine GO displays characteristic folded structures with well-defined creases. After PEI modification, three notable morphological transformations occur: first, the emergence of particulate features on the surface; second, partial filling of the original folds by PEI; third, a marked increase in surface roughness compared to unmodified GO.

Figure 5
GO SEM images before and after PEI functionalization. (a) GO, (b) PGO.

3.2. CF Surface modification

3.2.1. Surface morphology and properties

The carbon fiber modification process comprises two sequential steps following initial pretreatment: first, surface functionalization using KH560 silane coupling agent to introduce epoxy groups onto the CF surface, followed by covalent grafting of PGO through reaction with these activated epoxy sites. Figure 6 illustrates the morphological evolution of CF surfaces during different treatment stages. The desized CF (Figure 6(a)) exhibits numerous fine, longitudinal grooves that significantly enhance surface roughness. These microstructural features promote effective mechanical interlocking at the fiber-matrix interface. Following oxidation treatment, distinct dark features emerged on the carbon fiber surface (Figure 6(b)), resulting from oxidative etching of the fiber structure [⁵¹]. Subsequent KH560 silane coupling agent application partially infilled the surface grooves, leading to a reduction in surface roughness (Figure 6(c)). Figure 6(d-f) presents the surface morphology of KH560-treated CF after grafting with PGO solutions at varying concentrations. The modified CF surfaces exhibit irregular PGO lamellar structures, which enhance surface roughness. Notably, the density of grafted PGO demonstrates a concentration-dependent increase, with higher PGO concentrations yielding greater surface coverage. At 0.5 mg/mL (Figure 6(d)), sparse and small PGO flakes were observed. When the concentration increased to 1 mg/mL (Figure 6(e)), PGO displayed uniform surface distribution without detectable agglomeration. However, at 2 mg/mL (Figure 6(f)), large irregular PGO aggregates formed with significantly increased surface coverage. These results confirm that PEI functionalization effectively prevents GO aggregation while enabling controlled surface modification.

Figure 6
SEM images of carbon fibers at different stages of surface treatment. (a) CF-Desizing, (b) CF-Oxidation, (c) CF-KH560, (d) CF-KH560-PGO_0.5, (e) CF-KH560-PGO₁, (f) CF-KH560-PGO₂.

To investigate the effect of PGO grafting on carbon fiber surface roughness, atomic force microscopy (AFM) was employed to analyze the morphology of both desized carbon fibers and PGO-grafted carbon fibers. The results are shown in Figure 7, revealing significant differences between the two surfaces. Figure 7(a) shows the desized carbon fiber, where surface height variations primarily depend on inherent grooves, with a maximum height of 0.31 μm. After PGO grafting (Figure 7(b)), the maximum surface height increased to 0.96 μm. This change is attributed to PGO’s unique layered structure and the formation of particulate matter on GO surfaces after PEI treatment. The increased roughness of GO surfaces consequently led to a notable enhancement in the carbon fiber’s surface roughness.

Figure 7
AFM image of carbon fiber surface and height variation. (a) CF-Desizing, (b)CF-KH560-PGO₁.

3.2.2. Surface chemical composition and functional groups of modified carbon fibers

FTIR analysis was employed to characterize the surface chemical functionalities of CFs at various treatment stages (Figure 8). All samples including both desized CF and modified CF variants exhibited characteristic absorption bands at approximately 2950 cm^-1, corresponding to the asymmetric stretching vibration of C–H bonds. The characteristic absorption at 3252 cm^-1 corresponds to O–H stretching vibrations, while the 1074 cm^-1band arises from C-O stretching in primary alcohol moieties, collectively confirming hydroxyl group presence on the carbon fiber surfaces. Oxidation treatment significantly intensified the hydroxyl characteristic peak. Subsequent coupling agent modification introduced two new diagnostic bands at 1030 cm^-1 and 1180 cm^-1, corresponding to Si–O–Si and Si–O–C stretching vibrations respectively [⁵²], confirming both the silane coupling agent’s self-hydrolysis and its covalent attachment to the CF surface. Additionally, the emergence of a 1150 cm^-1 band, characteristic of epoxy ring stretching vibrations, verifies successful epoxy group introduction via KH560 grafting. The CF grafted by PGO showed a distinct peak at 1640 cm^-1, this is due to the stretching vibration of the C=C group on the surface of GO. The peak at 1737 cm^-1 becomes obvious, which is due to the stretching vibration of C=O groups on the surface of GO, and the number of C=O groups on the surface of CF is enhanced after grafting PGO, which indicates the successful grafting of GO. The appearance of one of the peaks at 1450 cm^-1 is caused by the vibration of the C–N group, this finding indicates that the grafting of PGO introduced –NH₂ to the CF surface was successful. This result also corresponds to the aforementioned GO infrared image (Figure 4(a)), it was shown that PGO could be stably grafted onto the coupling agent-treated CF surface.

Figure 8
FTIR profiles of CF at different surface treatment stages.

XPS analysis was conducted to characterize the surface elemental composition of carbon fibers at different modification stages (Figure 9). The wide-scan spectra revealed that desized CF surfaces predominantly contained carbon and oxygen, with carbon being the dominant element (O/C atomic ratio: 20.76%). Oxidation treatment significantly increased the oxygen content, elevating the O/C ratio to 23.42%. Subsequent KH560 silane coupling agent modification further enhanced surface oxygen concentration due to the introduced epoxy functional groups. At the same time, Si element was introduced on the CF surface, and the ratio of O to C content came to 32.38%. The element N appeared on the wide-scan spectrum of CF-KH560-PGO₁ surface, meanwhile, the higher elemental C content on the surface of PGO resulted in a decrease in the ratio of elemental O to elemental C on the surface of CF-KH560-PGO₁ compared to CF-KH560 (O/C of 26.40%). This proves the successful grafting of PGO and also indicates the introduction of new N-containing functional groups on the CF surface after modification.

Figure 9
XPS spectra of CF at different stages of surface treatment.

High-resolution C1s spectra deconvolution was performed to track functional group evolution across CF modification stages (Figure 10(a-d)). All spectra displayed characteristic peaks at 286.2 eV (C–O), 287.4 eV (C=O), and 288.4 eV (O–C=O) accompanying the dominant C–C peak (284.7 eV). Oxidation markedly intensified –OH and –COOH signatures while KH560 modification introduced a new C–Si bond (284.1 eV) and elevated surface oxygen content. The emergence of a C–N peak (285.7 eV) after PGO grafting confirmed both successful PEI-GO incorporation and amine group introduction, with spectral changes consistently corresponding to respective chemical modifications.

Figure 10
XPS high-resolution spectra of. (a) CF-Desizing C1s, (b) CF-Oxidation C1s, (c) CF-KH560 C1s, (d) CF-KH560-PGO₁ C1s, (e) CF-KH560 Si2p, (f) CF-KH560-PGO₁ N1s.

The surface chemical composition evolution was further investigated through high-resolution Si2p and N1s XPS analysis (Figure 10(e–f)). The Si2p spectrum of KH560-modified carbon fibers exhibited three distinct peak at 101.57 eV, 102.06 eV and 102.61 eV, corresponding to Si–O–C bonds confirming successful silane grafting, Si(OH)₃ groups from coupling agent hydrolysis, and Si–O–Si bonds formed through self-condensation respectively [⁵³]. Simultaneously, the appearance of a 401.7 eV N1s peak in CF-KH560-PGO₁ directly evidenced the presence of amine functional groups originating from PEI-modified graphene oxide, providing definitive confirmation of PGO covalent immobilization on the fiber surface.

3.2.3. Contact angle measurements

The contact angles between CF and deionized water as well as diiodomethane were tested using a contact angle meter and the surface energy was calculated by Equation (1), and the results are shown in Table 1 and Figure 11. This showed that the contact angle of KH560-treated CF with water came to 58.6° from 62.1° after desizing, and the wettability of CF was improved. This is because KH560 introduces epoxy functional groups to the CF surface. As the concentration of grafted PGO increased, the CF contact angle showed a gradual decrease. When the concentration of PGO was 0.5 mg/ml, the contact angle of CF with water was 47.4°, the lowest CF surface contact angle of 41.9° was observed at a PGO concentration of 2 mg/ml. As evidenced in Figure 11(b), PGO grafting substantially enhanced the surface energy of carbon fibers, with a measurable increase from 46.5 mN·m⁻¹ for desized CF to 59.2 mN·m⁻¹ for theCF-KH560-PGO₂. This is because the grafted PGO surface is rich in amino groups, this process leads to the presence of a substantial number of polar amino groups on the surface of the CF, resulting in a substantial increase in its polar strength. The amino group has the capacity to react with the hydroxyl group on the surface of the CF, thereby forming a hydrogen bond. In addition, the amino group can leading to the formation of an ionic bond through electrostatic interaction. The combined effect of chemical bonding and unreacted large amounts of amino groups effectively increases the polar component of the CF surface, which can greatly improve the activity of the CF surface and thus its surface energy.

Table 1
Contact angle and surface energy at different surface treatment stages of CF.

SAMPLE	θ (°)	σ (mN·m−1)
	WATER	DIIODOMETHANE
CF-Desizing	62.10	38.40	46.50
CF-KH560	58.60	36.70	48.70
CF-KH560-PGO0.5	47.40	33.90	55.30
CF-KH560-PGO1	45.40	32.50	56.70
CF-KH560-PGO2	41.90	29.00	59.20

Figure 11
CF different surface treatment stages. (a) Contact angle, (b) Surface energy.

3.3. Mechanical properties of the CFRP

The CF with different surface treatment stages were made into composites and tested in tensile test and the results are presented in Figure 12. The tensile strength of the composites made by CF-Desizing was 446.8 MPa, the tensile strength of the composites came to 512.5 MPa after treatment with KH560 coupling agent. This is because KH560’s epoxy groups react with the curing agent’s –NH₂ to promote matrix-resin bonding, and the coupling agent itself improves the wettability of CF. The tensile strength of CFRP composites exhibited a concentration-dependent trend, initially increasing before decreasing with higher PGO grafting concentrations. Optimal performance was achieved at 1 mg/mL PGO, yielding a maximum tensile strength of 669.4 MPa representing a 49.82% enhancement over desized CF composites (446.8 MPa). Figure 12(b) presents the tensile modulus of carbon fiber composites at different modification stages. Compared with composites made from desized carbon fibers, the tensile modulus actually decreased when lower PGO concentrations were used. This phenomenon occurred because PEI played a more dominant role in enhancing the composite’s toughness. The composite with a PGO concentration of 1 mg/mL exhibited the highest tensile modulus. This improvement resulted from the synergistic effect between KH560 and PGO, which established multiple reinforcement mechanisms at the fiber-resin interface. On one hand, this synergy optimized stress transfer pathways; on the other hand, it suppressed crack propagation through physical barriers and energy dissipation effects, thereby significantly enhancing interfacial tensile performance.

Figure 12
CFRP prepared by different modification processes CF. (a) Tensile stress-strain curve, (b) Tensile modulus.

This significant improvement stems from PGO’s lamellar structure augmenting CF surface roughness, thereby enhancing fiber-matrix mechanical interlocking. Concurrently, the abundant amino groups on the surface of PGO can establish a close chemical interaction with the resin. This combined physical and chemical effect can greatly increase the bonding ability between the CF and the resin. The composite properties are affected at lower or higher PGO concentrations, which may be due to the fact that some of the chemical bonds at the interface are exposed due to the absence or excess of their matching chemical bonds [⁵⁴]. At reduced PGO concentrations, diminished PGO deposition on carbon fibers results in inferior surface roughness development and consequently restricted mechanical interlocking effectiveness. Concurrently, the decreased amino group availability from sparse PGO coverage weakens fiber-resin chemical bonding. When the concentration of PGO is too high, at this time, although the surface roughness of the CF is high, but the excess of PGO makes the fiber itself morphology changed greatly. During the curing process, excessive PGO became dispersed around the resin matrix. This weakened the transition layer interface between the carbon fibers and matrix, reducing interfacial stress transfer capability. Such condition significantly affected the reinforced interfacial layer and compromised force transmission between fibers and resin. the mechanical properties are influenced by the microstructure. The appropriate amount of PGO plays a certain role in toughening the fibers, however, too high a quantity will affect the performance of the fibers, and at the same time this will cause impact on the reinforced interface layer, affecting the force transfer between the CF and the resin.

The interlayer shear strength of CFRP was tested using the short beam shear method; see Figure 13(b). Calculated from Equation 2, the ILSS of the composite prepared by CF after desizing was 50.08 MPa, and the ILSS of the composite after grafting PGO was 60.41 MPa. Compared to CF-Desizing, the CFRP ILSS treated with KH560 coupling agent and PGO increased by 20.63%. And from the load-displacement curves of CFRP (Figure 13(a)), the modified CFRP has increased modulus and load capacity. This indicates that the composites exhibit effective stress transfer at the interface layer and CF can improve the mechanical properties of CFRP after co-grafting treatment with KH560 and PGO.

Figure 13
CFRP prepared by different modification processes CF. (a) Bending load-displacement curve, (b) Interlaminar shear strength.

Figure 14 shows the morphology of the CFRP tensile fracture fracture described above. A large number of holes remaining due to fiber pull-out can be observed in the CFRP fracture made by CF-Desizing (Figure 14(a)). Large gaps between fibers and smooth fiber surfaces with low residual resin content (Figure 14(b)). At this point, there is insufficient infiltration between the fibers and the resin. The KH560 coupling agent treatment improved the wettability between fibers and resin, enhanced the fiber surface activity, and reduced inter-fiber porosity (Figure 14(c)). After PGO grafting, the carbon fiber surface roughness increased, enhancing the mechanical interlocking capability between fibers and resin. Meanwhile, the –NH₂ groups on PGO surfaces facilitated stronger chemical bonding interactions between carbon fibers and the resin matrix. Treatment with a coupling agent reduced the pores between the fibers and improved their wettability with the resin (Figure 14(c)). The holes in the CF-KH560-PGO_0.5 tensile fracture were further reduced, fiber-to-fiber porosity was further reduced (Figure 14(d)). At the optimal PGO concentration of 1 mg/mL, fiber pull-out induced voids were virtually eliminated in the fracture surface, with clearly observable resin residues adhering to exposed fiber surfaces. PGO grafting simultaneously enhanced CF surface roughness and introduced abundant amino functional groups (Figure 6(e)). This dual mechanism - combining chemical bonding and mechanical interlocking significantly improved fiber-matrix wettability, thereby strengthening interfacial adhesion (Figure 14(e-f)). When the PGO concentration increased to 2 mg/mL (Figure 14(g)), excessive grafting led to reappearance of voids in the fracture surface, accompanied by weakened fiber-resin interfacial bonding and consequent reduction in composite tensile strength.

Figure 14
The SEM images of CFRP Tensile Fracture. (a, b) CF-Desizing, (c) CF-KH560, (d) CF-KH560-PGO_0.5, (e, f) CF-KH560- PGO₁, (g) CF-KH560-PGO₂.

The shear fracture morphology of the composites was analyzed as shown in Figure 15. For composites prepared with desized carbon fibers (Figure 15(a)), traces left by fiber rebound were observed, indicating the primary shear failure mode was interfacial fracture between fibers and resin due to weak bonding. The desized carbon fiber surfaces appeared extremely smooth with minimal residual resin and large inter-fiber distances. In CF-KH560 composites (Figure 15(b)), no significant fiber-resin debonding was observed, and fibers showed obvious resin coating. This resulted from epoxy groups introduced by KH560 coupling agent treatment, which interacted with –NH₂ groups in the epoxy curing agent, enhancing fiber-matrix adhesion. PGO-treated composites (Figure 15(c))) demonstrated nearly complete resin encapsulation of fibers at fracture surfaces with tight fiber-resin integration. The fracture morphology revealed fish-scale-like resin patterns and broken fibers, indicating the failure mechanism shifted from initial interfacial failure to simultaneous fracture of both resin and fibers. Consequently, the multiscale reinforcement system constructed by KH560 and PGO effectively improved the composite’s shear strength.

Figure 15
CFRP shear fracture morphology. (a) CF-Desizing, (b) CF-KH560, (c) CF-KH560-PGO₁.

Figure 16 presents interfacial failure analysis of composites fabricated with differently treated CFs. The CFRP employing desized CF predominantly exhibited interfacial failure modes, attributable to insufficient fiber-matrix bonding. Matrix fracture initiates defect propagation toward the fiber-resin interface, ultimately inducing interfacial debonding and composite failure (Figure 16(a)). KH560 coupling agent treatment enables ring-opening reactions between its epoxy groups and the curing agent’s –NH₂, significantly enhancing fiber-resin interfacial bonding. This reinforcement alters the composite failure mode from interfacial debonding (caused by resin defect propagation) to cohesive failure involving simultaneous fracture of both resin matrix and carbon fibers (Figure 16(b)). The grafted PGO facilitates enhanced interfacial bonding through covalent interactions between its surface amino groups and the epoxy resin’s functional groups. At the same time, PGO’s lamellar structure enhances CF’s surface roughness, this mechanical locking and chemical bonding effect makes the composite material damage mode completely changed to the damage produced by the joint fracture of resin and fiber (Figure 16(c)), the mechanical properties of the composites were significantly improved at this time.

Figure 16
Interfacial damage analysis of composites made by CF at different treatment stages. (a) CF-Desizing, (b) CF-KH560, (c) CF-KH560-PGO.

4. CONCLUSIONS

In summary, a CF-KH560-PGO composite reinforcement was prepared using PEI-functionalized GO grafted onto a CF surface with KH660 as a bridge. And the CF-KH560-PGO composites were prepared by hot pressing method. which successfully improved the bonding ability between fibers and resin. The results demonstrated that the PGO exhibited uniform distribution across the surface of the CF. CF surface –NH₂ content has been significantly improved, the surface wettability is also improved, the surface energy increased from 46.5 mN·m^-1 after desizing to 59.2 mN·m^-1 after modification. Compared with the reinforced composites made of CF after desizing, the tensile property of CF-KH560-PGO/Epoxy composite material increased from 446.8 MPa to 669.7 MPa, which is a 49.88% improvement, and the interfacial shear property increased from 50.08 MPa to 60.41 MPa, which is a 20.63% improvement. CFRP’s enhanced mechanical properties result mainly from the presence of KH560 and PGO at the interface. KH560 acts as a bridge while introducing epoxy groups on the CF surface. The grafting of PGO not only improves the surface roughness of CF through its special lamellar structure, but also it also increases the amino content on the CF surface, resulting in a significant increase in CF surface activity, and the interfacial bonding between fiber and matrix is strengthened through the dual effects of mechanical locking and chemical bonding. The transition layer formed by KH560 and PGO is conducive to the uniform transfer of stress and can reduce the stress concentration, so that the damage mode of the composite material is changed from the damage caused by interfacial separation to the damage caused by the fracture of resin and fiber together.

5. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of the S&T Program of Hebei (grant number 22567635H) and the Natural Science Foundation of Hebei Province, China (grant number E2023202065).

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Publication Dates

• Publication in this collection
  31 Oct 2025
• Date of issue
  2025

History

• Received
  17 May 2025
• Accepted
  18 Aug 2025