How-to, Tips and Articles: The Recent Advances in Methods of Surface Treatment of Carbon Fibers and Preparation of Carbon Fiber/Thermoplastic Composites

Abstract

Carbon ﬁbers (CFs) have high speciﬁc tensile strength, high modulus, and outstanding wear resistance, and are widely used for the reinforcement of advanced composite materials. CF-reinforced thermoplastic composites have received much attention because of their easy processability and recycling convenience compared with thermosetting composites. Surface treatment of CFs is generally employed to increase the surface functional groups and interfacial adhesion between the CFs and the surrounding polymer matrix. In this review, we explore recent advances in the surface treatment of CFs and preparation of CF/thermoplastic composites. The thermal, mechanical, and electrical properties of the composites are also discussed.

1. Introduction

Carbon ﬁbers (CFs) have been widely used to reinforce advanced composite materials because of their exceptional properties, such as high speciﬁc tensile strength, high modulus, and outstanding wear resistance. CFs can be classiﬁed as continuous CFs, long CFs, and short CFs (SCFs) based on the length of the ﬁber. CF-reinforced polymers exhibit outstanding mechanical properties and low density and are widely used in the ﬁelds of aerospace, transportation, and sporting goods [1–10].

In CF-reinforced composites, the polymer usually acts as the continuous phase (the matrix), and the CF serves as the discontinuous phase. The polymer matrices can be classiﬁed into thermosetting and thermoplastic resins. Thermoplastics can be further divided into general plastics, such as polyethylene (PE), polypropylene (PP), and acrylonitrile butadiene styrene (ABS) resins, and engineering plastics, such as polyamide (PA), polycarbonate (PC), polyetheretherketone (PEEK), polyetherimide (PEI), polyethersulfone (PES), and polyphenylene sulﬁde (PPS). Thermoplastics have received signiﬁcant attention as polymer matrices because of their lack of requirement for a curing stage and their less hazardous chemical compositions and improved recycling convenience and mass production capability compared with conventional thermosetting resins. Thermoplastic resins are often fabricated using conventional molding methods, such as injection−molding, rotational−molding, extrusion, vacuum forming, and compression−molding [11–19].

However, the surface of pristine CFs is non-polar, whereas, the polymer matrix generally exhibits polar character. The strength of the interfacial bonds between the CFs and matrix is consequently poor, thereby precluding achievement of the ideal mechanical properties of the composites. Thus, numerous CF surface treatment methods, such as wet chemical or electrochemical oxidation, ozone treatment, polymer or metal coating, and plasma treatment, have been proposed to increase the surface functional groups and interfacial adhesion between the CFs and surrounding polymer matrix [20–28].

In this report, various methods for surface treatment of CFs and the preparation of CF-reinforced thermoplastic composites are reviewed in detail. In addition, the thermal, mechanical, and electrical properties of the composites are discussed.

2. Surface treatment of CFs

Zhang et al. [29] treated CFs with a mixture of concentrated sulfuric/nitric acid in a reaction ﬂask. The ﬂask was placed in an ultrasonic water bath to maintain the treatment temperature. The treated CFs were washed several times with distilled water. Wu et al. [30] also treated CFs with nitric acid at 115 °C for 20–90 min. The CFs were then reﬂuxed with 1.0 N NaOH solution, extracted with distilled water, and dried in ambient air. Their results indicated that oxidation by nitric acid eﬀectively generated signiﬁcant amounts of acidic functional groups on the surfaces of CFs. Ryu et al. [31] treated CFs with aqueous ammonium carbonate solution using an original continuous treatment process. The length of the electrolytic treatment bath was 0.23 m, and the total oxidized ﬁber surface area was 0.032 m2. X-ray photoelectron spectroscopic (XPS) analysis indicated that the O1s/C1s and N1s/C1s ratios of the treated CFs increased with increasing current intensity.

Fig. 1. Schematic diagram of ﬂuorination reactor [27]. 1: F2 gas cylinder, 2: N2 gas cylinder, 3: O2 gas cylinder, 4: Buﬀer tank, 5: HF absorber, 6: reactor, 7: pressure gauge, 8: F2 absorber, 9: glass cock, 10: liquid nitrogen, 11: rotary vacuum pump.

Park et al. [27,32–35] investigated the eﬀect of oxyﬂuorination on the surface characteristics of CFs. The oxyﬂuorination reaction was performed in a batch reactor, as shown in Fig. 1. The ﬂuorine and oxygen mixtures were introduced into the reactor at room temperature and the reactor was then heated to the treatment temperature. After the reaction, the specimens were cooled and the reactor was purged with nitrogen. Fourier-transform infrared (FT-IR) analysis indicated that the oxyﬂuorinated CFs contained carboxyl/ester and hydroxyl groups and the –O–H peak was more intense than that of the ﬂuorinated counterparts. Park et al. [36] also studied the eﬀect of anodic oxidation of CFs on the fracture toughness of CF-reinforced epoxy composites. Electrochemical treatment of the CFs was conducted using a laboratory pilot- scale apparatus, as shown in Fig. 2. The electrolyte was a 10 wt% phosphoric acid solution. After anodic oxidation, the CFs were washed with distilled water and then rinsed by immersion in acetone for 2 h. Contact angle measurements indicated that anodic oxidation led to an increase in the surface free energy, mainly by increasing the polar component. Park et al. [37] further proposed an interpretation based on more precise linear free energy relationships for describing the acid−base interaction at a solid surface. The CFs were subjected to anodic treatment at an oxidation voltage of 1.6 V on a pilot scale. Electrochemical treatment of the CFs signiﬁcantly improved the acid−base character.

Osbeck et al. [38] investigated the eﬀect of ultraviolet-generated ozone treatment on the surface functionality and structure of CFs. The CFs were electrochemically treated in a base oxidation bath prior to further photosensitized oxidation in a UVO cleaner/oxidizer. XPS analysis indicated that functional groups such as hydroxyl species, alkoxides, carbonyl, and carboxyl groups were introduced in the ozone- treated CFs. Park et al. [39] also studied the gas−phase ozone treatment of CFs for introducing acidic oxygen functional groups onto the surfaces of CFs. Ozone treatment of the CFs was performed at ozone concentrations in the range of 10–40 mg/l at room temperature. FT-IR and XPS analyses indicated that oxygen functional groups, such as −OH, O−C=O, C=O, and C−O, were attached to the surfaces of the CFs after the ozone treatment.

Zhang et al. [40] treated CFs using an oxidation–reduction method followed by coating with vinyl silsesquioxane (VMS–SSO) to improve the interfacial properties of CF/polyarylacetylene (PAA) composites. The oxygen plasma treatment was performed in a plasma processor. The CFs were reduced with LiAlH4/THF saturated solution and then coated with VMS–SSO. The XPS results indicated that the number of polar functional groups increased after the redox reaction. The VMS–SSO coating treatment introduced vinyl groups, which could react with the PAA resin during the PAA curing process. Iwashita et al. [41] studied the treatment of CFs using either silane or titanate coupling agents. A CF bundle was immersed in a solution containing 1 wt% coupling agent and then dried in air at 120 °C for 30 min. The tensile strength of the carbonized composite was improved by both coupling treatments.

Park et al. [42] prepared a Ni–P coating on the surfaces of CFs to improve the impact resistance of CF–reinforced epoxy composites. The CFs were activated in nitric acid for 30 min and then sequentially activated in tin chloride and palladium chloride solutions. Nickel-plated CFs were obtained by dipping the CFs in a nickel bath, washing with distilled water, and drying in a vacuum oven at 120 °C for 12 h. X-ray diﬀraction (XRD) and XPS analyses indicated that Ni–P coating of the CFs led to an increase in the microcrystalline and amorphous phases. Park et al. [21,43–45] also performed electrolytic plating of metallic nickel on the surface of CFs to improve the interfacial adhesion and mechanical properties of CF/phenolic matrix composites. The CFs were activated in nitric acid, tin chloride, and palladium chloride solutions. Nickel-plated CFs were obtained by dipping the CFs in a nickel bath. The oxygen functional groups and metallic nickel on the CF surfaces were found to greatly aﬀect the mechanical interfacial behavior of the composites.

Fig. 2. Schematic representation of continuous electrolytic surface anodization process [36].

3. Sample preparation

3.1. ABS-based composites

3.1.1. Preparation method I

ABS resin was mixed with chloroform to form a paste; the desired amount of CFs was dispersed in the paste by mechanical stirring. The CF-ﬁlled paste was dried at room temperature and then molded by hot pressing at 200–210 °C for 5 min [46,47].

3.1.2. Preparation method II

The desired amount of CFs was chopped into 5−mm pieces and mixed with ABS resin in a Brabender mixer operating at 210 °C for 3 min. The mixture was then hot compression−molded at 200–210 °C for 5 min [46,48].

3.1.3. Preparation method III

A twin-screw extruder was preheated for 2 h and ABS was then used to clean the extruder for 10–15 min. The CF/ABS mixture was loaded into the machine at 260 °C using a rolling speed of 110 rpm. The CF/ ABS mixture was hot compression−molded at 240 °C and 150 kg/cm2 for 4 min [49–51].

3.2. PA-based composites

3.2.1. Preparation method I

PA was mechanically mixed with CFs and then extruded using a twin-screw extruder in the temperature range of 190–230 °C at a rotational speed of 20 rpm. The extrudate was pelletized, dried, and injection−molded into standard test samples [52,53].

3.2.2. Preparation method II

SCF-reinforced PA composites were prepared by melt mixing at 240 °C for 3 min using a laboratory-scale co-rotating twin-screw mini extruder. The compounds were subsequently injection−molded using a laboratory-scale injection−molding machine using a barrel temperature of 240 °C, mold temperature of 30 °C, and injection pressure of 8 bar [54–57].

3.2.3. Preparation method III

The CFs and solvent (1:5 wt/wt) were mixed and the mixture was treated by ultrasonic oscillation at 30 °C for 2 h. The PA pellets, solvent, CF suspension, and lubricant were combined in a reactor, stirred at 145 °C for 2–3 h, and then cooled to 105 °C. After distillation of the solvent, the precipitate was vacuum-dried and ball-milled. The CF/PA composite powders were thus obtained [58].

3.2.4. Preparation method IV

Unidirectional laminates with CFs and PAs were manufactured using a steel mold. The laminates with 40 vol% CF were obtained by sequentially winding CF cables over the PA sheets by alternatively stacking ﬁve PA sheets and ﬁve unidirectional CF layers. The two winding unidirectional prepregs were consolidated using hot compression−molding [59].

3.2.5. Preparation method V

Dried PA pellets were melted and forced to pass through a heated impregnation die using a single screw extruder at 230 °C. CF tows in the spool were pulled through the same die, where the CF tows were coated with a layer of PA, forming core/shell structured composite wire rods. The composite wire rods were cooled and chopped into core/shell- structured pellets [60].

3.2.6. Preparation method VI

Continuous CF-reinforced PA composites were prepared using a 3D printing system. The PA matrix was ﬁrstly printed and the continuous ﬁber reinforcement was then printed. The PA and ﬁber layers were printed onto a print bed using a hot end temperature of 263 °C [61].

3.2.7. Preparation method VII

Continuous−CF-reinforced PA 6 composites were prepared using a melt impregnation method by passing ﬁber bundles through a cross- head impregnation die. The volume fraction of the ﬁber was controlled in the range of 50.0 ± 2.0% and the thickness and width were 0.25 ± 0.03 mm and 7.00 ± 0.15 mm, respectively [62].

3.3. PC-based composites

3.3.1. Preparation method I

PC pellets were completely dissolved in tetrahydrofuran (THF). SCFs were dispersed in THF using a stirring and soniﬁcation process. The PC solution was added to the CF mixture. The mixture was sonicated and stirred for 15 min before being cast into a mold, and placed in a furnace at 50 °C for 7 min. Finally, the sheets were compacted at 180 °C and 25 kN for 10 min using a hot compress machine [63].

3.3.2. Preparation method II

SCF/PC composites were prepared at 300 °C by compounding with a twin-screw extruder for 1 h. The solid was compacted by a hot compressor at 180 °C and 25 kN for 20 min. The material was placed in a furnace at a temperature above the glass transition temperature (Tg) of PC [64,65].

3.3.3. Preparation method III

SCF-reinforced PC composites were prepared by treatment of the components in a micro-compounder at 295 °C for 3 min, followed by molding using an injection molding machine with a barrel temperature of 295 °C, mold temperature of 100 °C, and injection pressure of 10 bar [66].

3.4. PEEK-based composites

3.4.1. Preparation method I

Unidirectional composite specimens with ﬁber orientations of 0° and 90° were prepared by hot pressing CF/PEEK prepregs into uni-directional laminates and then cutting the laminates into specimens [67,68].

3.4.2. Preparation method II

The compounding of PEEK with CFs was achieved using a twin- screw extruder at 360–390 °C; a screw speed of 360 rpm and feed rate of 18 rpm were employed. The extrudate was continuously cooled in water and pelletized. Standard test bars were injection−molded at 180 °C [69].

3.4.3. Preparation method III

In this method, 30 wt% SCF-containing PEEK composites were prepared at processing temperatures of 360–390 °C using an injection−molding machine [70].

3.4.4. Preparation method IV

CFs and PEEK powder were dispersed in alcohol using a ball grinder. The mixture was dried at 90 °C for 24 h. The CF/PEEK composites were produced at 380 °C using an injection-molding machine. The resulting composite was injected into molds [71].

3.4.5. Preparation method V

Continuous CF-reinforced PEEK composite cylinders comprising a PEEK matrix, continuous CF, and internal lubricants, were produced by a ﬁlament winding process [72].

3.4.6. Preparation method VI

Unidirectional laminated long CF/PEEK composite plates were prepared by compression−molding. The micro-braided yarn was uni- directionally wound around a stainless-steel frame with adequate tension. The frame was then placed in a mold that was put on the platform of a hot press machine and subjected to programmed heating and pressure [73].

3.5. PEI-based composites

3.5.1. Preparation method I

Unidirectional CF tapes were clamped on a frame, followed by application of layers of a viscous solution of PEI in dichloromethane using a hand layup technique. Two unidirectional strokes of coats were applied and then dried under ambient environment. These dried prepregs were then compression−molded at 390 °C and 32 kN/m2 for 20 min [74,75].

3.5.2. Preparation method II

SCF-reinforced PEI composites were prepared using a twin-screw extruder. The screw temperature and rotation speed were 385 °C and 10 rpm, respectively. The extrudate was molded into a rectangular plate by compression−molding [76].

3.5.3. Preparation method III

PEI was dissolved in a mixture solvent, and continuous CF bundles were impregnated into the PEI solution. The solution-processed prepreg was allowed to dry on the drum and then vacuum-oven treated at 100–200 °C for 10 h. The prepreg was laid up and compressed into unidirectional composite panels using a hot-press molding method [77].

3.6. PES-based composites

3.6.1. Preparation method I

PES ﬁlms were pre-cut to the cavity dimensions and placed in the mold. A single CF was placed on the stack of PES ﬁlms. Both ends of the CF were ﬁxed to the mold with polyimide adhesive tape. Additional PES ﬁlms were placed on top of the CF. The mold was heated in a press- clave at 300 °C for 30 min and then slowly cooled to room temperature [78].

3.6.2. Preparation method II

PES was dissolved in a mixture solvent, and continuous CF bundles were impregnated into the PES solution. The solution-processed pre- preg was dried and treated at 100−200 °C for 10 h. The prepreg was laid up and compressed into unidirectional composite panels using a hot-press molding method [77,79,80].

3.6.3. Preparation method III

SCF/PES composites were prepared using a co-rotating twin-screw extruder. The temperature of the barrel was in the range of 360–375 °C. The extrudates were continuously cooled and pelletized. Standard test bars were obtained by injection−molding at 180 °C using a plastic-injection molding machine [81].

3.6.4. Preparation method IV

Unidirectional composites were fabricated with poly (phthalazinone ether sulfone ketone) (PPESK)/PES blends as the matrix and continuous CF as the reinforcement. Continuous CF bundles were impregnated into the polymer solution. The individual bundles were wound on−to a stainless−steel plate. The solution-processed prepreg was allowed to dry on the drum and then treated at 100–200 °C for 10 h [77].

3.7. PE-based composites

3.7.1. Preparation method I

The PE and CFs were mixed at 190 °C for 10 min using a two-roll mill. The mixture was melted by treatment at 190 °C for 10 min and compressed under a pressure of 18 MPa for 15 min, followed by quenching in water to obtain the ﬁlms [82–85].

3.7.2. Preparation method II

PE, CFs, and compatibilizers were mixed using a co-rotating twin screw extruder at 50–210 °C and 100 rpm. The extrudate was pelletized and molded using a laboratory-scale injection-molding machine at a barrel temperature of 200 °C and mold temperature of 30 °C [86].

3.7.3. Preparation method III

Several layers of a crossplied-woven roving CF mat were positioned individually on aluminum foil, and powdered high-density polyethylene (HDPE) was spread on these layers. The layers were placed on one another and enfolded in aluminum foil. The stacked laminate was placed between the platens of a high-temperature hydraulic press at 140 °C and compressed at 2500 psi for 2 h [87].

3.8. PPS-based composites

3.8.1. Preparation method I

PPS resin was melted and pulled into CFs and then knotted loosely around a CF monoﬁlament. The sample was placed onto a microscope melting point apparatus at 300 °C and kept for 1 min; the specimen was then removed from the hot plate and cooled to ambient temperature. The prepared samples were annealed at 120 °C for 12 h [88].

3.8.2. Preparation method II

PPS and CFs were compounded using a mini-twin-screw extruder at 230 °C for 10 min. The melted composites were continuously injected into the mold at an injection temperature of 250 °C [89–91].

3.9. PP-based composites

3.9.1.Preparation method I

CFs and PP particles were mixed together in a mortar. The mixture was subjected to strong mechanical stirring in ethanol for 1 h and ultrasonication for another 1 h. After complete evaporation of the ethanol, the mixture was compression−molded into sheets at 190 °C under a pressure of 15 MPa [92,93].

3.9.2. Preparation method II

PP and SCF were compounded using a co-rotating twin-screw mini-extruder at 230 °C. The compounds were subsequently injection−molded using an injection−molding machine employing a barrel temperature of 230 °C, mold temperature of 25 °C, and injection pressure of 8 bar [94,95].

3.9.3. Preparation method III

CF/PP composites were prepared by melt-blending PP with CFs. The mixed samples were compressed under a pressure of 7.5 MPa at 200 °C for 20 min using a hot press. The mold was cooled to room temperature using a cooling system [96,97].

3.9.4. Preparation method IV

PP resins and coupling agents were mixed in a single screw extruder. Long roving type CFs were impregnated into the PP die. The composite of PP impregnated with long CFs was water-cooled, pelletized with 10 mm pellets, and dried [98].

3.9.5. Preparation method V

Unidirectional and quasi-isotropic preconsolidated laminates were heated to the melting temperature of PP and then stamp−formed in a cold−matched metal tool. A continuous CF-reinforced PP prepreg with approximately 20% ﬁber was used. The prepreg sheet had a thickness of 0.4–0.5 mm [99].

4. Thermal, mechanical, and electrical properties

4.1. ABS-based composites

Li et al. [51,100] studied the tensile properties of HNO3-treated SCF/PA6/ABS composites. Their results indicated that the tensile strength and tensile modulus of the composites improved when the SCF content was increased from 10 wt% to 30 wt%. The tensile properties of the composites were also enhanced by increasing the PA6 content due to improved interfacial adhesion.

Lu et al. [46] investigated the electrical conductivity and shielding eﬀectiveness of CF/ABS composites. The resistivity of the nickel-coated CF/ABS composites decreased with an increase in the CF content. For the same CF content, the conductivity of the composites with nickel- coated CFs was much greater than that of the non-coated composites. The shielding eﬀectiveness of ABS resin ﬁlled with 10 vol% of nickel- coated CFs was 50 dB. Liang et al. [47] also investigated the resistivity of SCF/ABS resin composites. The composites displayed good conductivity when the SCF content was greater than 2 vol%. The resistivity of the composites decreased with increasing SCF length and surface treatment of the SCFs.

Tzeng et al. [48] studied the electromagnetic interference (EMI) shielding eﬀectiveness of copper- and nickel-coated CF-reinforced ABS composites. Their results indicated that the electroless-nickel-coated CF/ABS composites exhibited the best EMI shielding capability because of the longer ﬁber length distribution and excellent bonding between the nickel coating and ﬁbers. Huang et al. [50] also investigated the EMI shielding eﬀectiveness of CF/PC/ABS composites. Nickel-coated CFs produced by electroless nickel plating were the most eﬀective conductive ﬁllers for EMI shielding. Huang et al. [49,101] also studied the EMI shielding eﬀectiveness of metal-coated CF-reinforced PC/ABS composites. The results indicated that the resistivity of the nickel- coated CF/ABS composites increased signiﬁcantly after composite fabrication. The highest EMI shielding eﬀectiveness obtained with the composites was 65 dBm.

4.2. PA-based composites

Wu et al. [52] studied the mechanical, thermal, and morphological properties of glass-ﬁber- and CF-reinforced PA6 and PA6/clay nano- composites. Their results indicated that the mechanical and thermal properties of the PA6/clay nanocomposites were superior to those of the PA6 composite and there was no sacriﬁce of the impact strength. The heat distortion temperature of the CF-reinforced PA6/clay composites was almost 20 °C higher than that of the CF-reinforced PA6 composites. Yan et al. [58] studied the preparation and characterization of CF/PA12 composites for selective laser sintering. The CF/PA 12 composites exhibited higher thermal stability than pure PA12. The sintered CF/PA composites exhibited greatly enhanced ﬂexural strength and ﬂexural moduli compared with pure sintered PA12. Surface fracture analysis revealed that the CFs were encapsulated and bonded well with the PA12 matrix. Karsli et al. [54] investigated the tensile and thermo-mechanical properties of SCF-reinforced PA6 composites. Evaluation of the mechanical parameters indicated that the tensile strength, modulus, and hardness increased and the strain−at−break of the composites decreased with increasing CF content. Dynamic mechanical analysis (DMA) revealed that the storage modulus and loss modulus of the composites increased with increasing SCF content. Ma et al. [102] studied the failure behavior and mechanical properties of CF-reinforced PA6 laminates. Tensile strength analysis indicated that the CF/PA6 laminates exhibited step-like fracture in the interfacial fracture mode with weak interfaces.

Kurokawa et al. [53] investigated the gear performance of CF-reinforced PA12. Their results indicated that the composites exhibits excellent load-bearing characteristics when grease was present in the engagement region. The load-bearing capacity was enhanced by increasing the molecular weight of PA12. Botelho et al. [59] compared the mechanical behavior of CF-reinforced PA6 and PA66 composites. The elastic modulus, tensile strength, and compressive strength of the composite increased slightly with increasing CF content. Scanning electron microscopy (SEM) analysis revealed that damage to the composite occurred largely via shear failure at the ﬁber/matrix interface, where failure was initiated at relatively low stress. Hassan et al. [55] studied the structure–property relationship of injection-molded SCF- reinforced PA66 composites. Their results indicated that the tensile strength and tensile modulus of the composites increased with in- creasing CF content. Li [103] investigated the interfacial properties of O3-treated CF-reinforced PA6 composites. A 60% augmentation of the interfacial shear strength (IFSS) of the composites was achieved with O3-treated CF compared with the untreated counterparts. XPS analysis revealed that O3 treatment increased the amount of carboxyl groups on the CF surface, thereby enhancing the interfacial adhesion between the CFs and PA6 matrix. Do et al. [18] studied the eﬀect of PP on the mechanical properties and water absorption of CF-reinforced PA6/PP composites. The results indicated that the ultimate tensile strength, elastic modulus, elongation, density for weight reduction, and dimensional stability of the composites with PP exhibited were superior to those of the composite without PP.

Dickson et al. [61] investigated the inﬂuence of the ﬁber orientation, ﬁber type, and volume fraction on the mechanical properties of continuous CF-reinforced PA composites. The results indicated an up to 6.3-fold enhancement of the tensile strength of the composites compared with that of non-reinforced PA. Li et al. [62] studied the eﬀects of the thermal histories on the interfacial properties of continuous CF/PA 6 composites. Their results revealed that the IFSS of the composites decreased at slower cooling rates and higher annealing temperatures.

4.3. PC-based composites

Carneiro et al. [104] prepared PC composites reinforced with vapor- grown CFs. The tensile properties of the injection-molded specimens were marginally better than those of the unreinforced PC; however, the impact resistance was severely diminished by the addition of CFs. Park[105] studied the interfacial properties of CF-reinforced PC composites using two types of coupling agents. It was found that both coupling agents caused an increase in the IFSS due to chemical and hydrogen bonding at the interface between the functional groups in the CFs and polyacrylamide in the coupling agents. Montes−Morán et al. [106] investigated the eﬀects of plasma oxidation on the surface and inter- facial properties of CF-reinforced PC composites. Plasma treatment signiﬁcantly increased the interfacial shear strength of the CF/PC composites by increasing the number of surface functional groups. Hornbostel et al. [64] investigated the mechanical properties of triple composites of PC, single-walled carbon nanotubes (SWCNTs), and CFs. Small amounts of CNTs randomly distributed in the PC matrix did not necessarily enhance the mechanical stability, whereas greater mechanical improvements were attained by adding CFs to the composites. Choi et al. [63] studied the electrical and mechanical properties of PC composite sheets reinforced with vapor-grown CFs. Microscopic analysis revealed a homogeneous dispersion of CFs in the PC matrix. The electrical resistivity of the CF/PC composites decreased with increasing CF loading because of the presence of a good ﬁber network. The mechanical properties of the composite improved greatly because of the enhanced ﬁber orientation. Ozkan et al. [66] investigated the eﬀects of sizing materials on the mechanical, electrical, and morphological properties of SCF-reinforced PC composites. Their results indicated that the tensile strength, modulus, and electrical conductivity of sized CF/PC composites were higher than those of unsized CF/PC composites. SEM analysis revealed better interactions between the phenoxy-sized CF and PC matrix.

4.4. PEEK-based composites

Hanchi et al. [70] investigated the eﬀects of the operating temperature on the dry sliding friction and wear performance of a SCF- reinforced PEEK composite. The wear performance of the composite decreased when the temperatures was increased from below to above the Tg. In the sliding regime above the Tg, the friction and wear performance of the composite was signiﬁcantly better than that of neat PEEK. Xie et al. [69] investigated the tribological behavior of a PEEK composite reinforced with CFs and potassium titanate whiskers (PTW) using a pin-on-disk conﬁguration. Their results indicated that the CF/ PTW/PEEK composite exhibited excellent tribological performance under aqueous conditions. The results also revealed that the two ﬁllers worked synergistically to enhance the wear resistance of the composite.

Tewari et al. [107] characterized the solid particle erosion behavior of unidirectional CF-reinforced PEEK composites. The CF/PEEK composites exhibited semi-ductile erosion behavior. SEM observations indicated that the composites underwent erosion damage via matrix removal and exposure of the ﬁbers, ﬁber cracking, and removal of broken ﬁbers.

Almajid et al. [72] investigated the surface damage characteristics of continuous CF-reinforced PEEK composites under sliding and rolling contact. The composite with a normal ﬁber orientation exhibited the lowest speciﬁc wear rate under rolling wear conditions, whereas parallel ﬁber orientation resulted in the lowest speciﬁc wear rate in the case of sliding wear. Fujihara et al. [73] studied the eﬀect of the processing conditions on the bend-ability of continuous CF-reinforced PEEK composites. It was found that the bending performance of the composites was signiﬁcantly aﬀected at 440 °C. A lower fabrication temperature and a shorter holding time were the most suitable processing conditions for preparation of the CF/PEEK composites.

4.5. PEI-based composites

Xian et al. [108] studied the tribological behavior of SCF/PEI composites using both a block-on-ring and pin-on-disc rig under dry sliding conditions. The coeﬃcient of friction decreased and the wear resistance increased signiﬁcantly with the addition of 5–20 vol% CFs. Sharma et al. [75] studied the eﬀect of the orientation of long CFs on the mechanical and tribological properties of CF/PEI composites. It was found that the tribological properties of the composites deteriorated when the angle of orientation of the ﬁbers in the composites increased from 0° to 90°. Sharma et al. [74] also examined the eﬀect of the ﬁber orientation on the abrasive wear of CF/PEI composites. The Young's modulus, Poisson's ratio, toughness, and strain decreased when the angle of orientation of ﬁber with respect to the loading direction was higher. Unidirectional CF reinforcement signiﬁcantly enhanced all the strength parameters of PEI.

Arjula et al. [109] investigated the erosive wear behavior of uni- directional CF-reinforced PEI composites. The investigated CF/PEI composites exhibited semi-ductile behavior in low-speed erosive studies. Higher particle speeds resulted in a rougher surface because of severe ﬁber breakage and matrix erosion. Arjula et al. [110] also studied the eﬀect of the CF orientation on the erosion rate of CF/PEI composites, demonstrating that the rate of erosion of the CF/PEI composites was signiﬁcantly higher than that of neat PEI at higher impact angles. The rate of erosion of the composite was higher for 90° orientation than for 0° orientation of the ﬁbers.

4.6. PES-based composites

Yumitori et al. [78] studied the role of sizing resins in CF-reinforced PES. The sized CFs exhibited a higher interfacial shear strength than the unsized counterparts due to the strong interaction between the sizing resin in the CFs and the PES matrix. Wu et al. [79] investigated the processing and properties of solution-impregnated CF-reinforced PES composites, where the longitudinal ﬂexural modulus and ﬂexural strength were found to be 137 GPa and 1400 MPa, respectively. Their results also suggested that the transverse properties and interlaminar fracture toughness improved when the molecular weight of the polymer matrix was higher. Zheng et al. [77] blended PEI and PES to improve the rheological properties of a PPESK matrix and provide suﬃcient impregnation and consolidation during the hot-press molding treatment of CF/PPESK composites. The mechanical properties of the CF/PPESK/ PEI and CF/PPESK/PES composites were markedly improved, which was attributed to the good interfacial adhesion and low porosity resulting from the addition of PEI and PES to the PPESK matrix. Li et al. [111] investigated the cryogenic mechanical properties of SCF/PES composites using a graphene oxide (GO) coating. The GO-coated SCF/ PES composites displayed greatly enhanced cryogenic mechanical properties compared with the SCF/PES composites.

4.7. PE-based composites

Savas et al. [86] prepared CF-reinforced HDPE composites using PE copolymers as compatibilizers. Their results indicated that enhanced mechanical properties of the composites were achieved with all of the compatibilizers relative to the composites without compatibilizers. Chukov et al. [112] investigated the structural, mechanical, and tri- bological properties of SCF-reinforced ultra-high molecular weight PE (UHMWPE) composites. The results indicated that thermal oxidation of the CFs by ambient oxygen at 500 °C signiﬁcantly enhanced the interfacial interaction between the CFs and polymer matrix. Their results also revealed that the yield stress of the composites was almost twice as higher as that of pure UHMWPE. Erkendirci et al. [113] studied the quasi−static penetration resistance behavior of CF-reinforced HDPE composites. These composites exhibited a large eﬀective displacement for complete energy dissipation, where more energy was dissipated in the case of thinner laminates and less energy was dissipated for thicker composites.

Zhang et al. [82] studied the selective location and double−percolation of SCF-ﬁlled HDPE/isotactic PP (iPP) blends. The percolation threshold of the SCF-ﬁlled HDPE/iPP blends was found to be lower than those of the individual polymers. SEM analysis veriﬁed that the improvement in the electrical conductivity could be attributed to the selective location of the SCFs in the HDPE phase. Thongruang et al. [83] studied the correlation of the electrical conductivity and mechanical properties of HDPE ﬁlled with graphite and CFs. Their results indicated that the addition of CFs to the HDPE/graphite composites increased the conductivity relative to that of HDPE/graphite composites. Optical and electron micrographs revealed that the CFs exhibited preferential alignment depending on their length relative to the thickness of the composite ﬁlm. Xi et al. [114] studied the dielectric eﬀects on the positive temperature coeﬃcient (PTC) of SCF/PE composites. An excellent PTC eﬀect was achieved by adding SCFs to the PE matrix. The conductive CFs aggregated in series, where an equivalent circuit of a resistor and capacitor were arranged in parallel, representing the blends at these contact regions. Shen et al. [115] investigated the combined eﬀects of carbon black (CB) and CFs on the electrical properties of composites based on PE or a PE/PP blend. The volume resistivities of the HDPE/CB/CF and HDPE/PP/CB/CF composites were lower than those of the HDPE/CB and HDPE/PP/CB composites, respectively. The intensity of the PTC and the temperature coeﬃcient of resistivity of the HDPE/CB/CF and HDPE/PP/CB/CF composites increased appreciably with increasing CF loading.

4.8. PPS-based composites

Liu et al. [88] studied the interfacial micromechanical performance of CF-reinforced PPS composites using microbond analysis. The IFSS of the composites increased with the length of the embedded CFs at speeds below 0.02 mm/s and above 0.04 mm/s; however, the IFSS remained constant at speeds between 0.02 and 0.04 mm/s. Zhou et al. [89] investigated the eﬀect of CFs on the structural, mechanical, and tribological properties of PA6/PPS composites. Introduction of the CFs resulted in an increase in the strength, modulus, and hardness and a slight decrease in the breaking elongation rate and impact strength of the PA6/PPS blends. SEM observation revealed that the main wear mechanism of the composites was adhesive wear. Stoeﬄer et al. [91] investigated the mechanical and physical properties of PPS composites reinforced with recycled CF. Their results indicated that the mechanical properties of recycled CF-reinforced PPS composites were equivalent to those of compounds produced using industrial grades of virgin CF. Thermogravimetric analysis (TGA) revealed that the introduction of recycled CFs was not detrimental to the inherent thermal stability of PPS. Zhang et al. [116] studied the eﬀect of aminated PPS (PPS-NH2) on the mechanical properties of SCF-reinforced PPS composites. Amination improved the compatibility between the CFs and PPS matrix. The tensile strength, ﬂexural strength, and ﬂexural modulus of the CF/PPS composites with compatibilizers were better than those of the composites without the compatibilizer because of the enhanced adhesion at the interface of the CFs and PPS-NH2.

Xu et al. [117] investigated the tribological behavior of CF-reinforced PPS composite coatings under dry sliding and water lubricated conditions. The composite coatings exhibited lower friction coeﬃcients and higher wear rates than the pure PPS coatings under dry sliding. The composite coatings exhibited better wear resistance with water-lubrication than under dry sliding conditions. Jiang et al. [90] studied the friction and wear behavior of PPS composites reinforced with SCFs and sub-micro TiO2 particles. The lowest speciﬁc wear rate was obtained with 15 vol% CF and 5 vol% TiO2. SEM analysis revealed a positive rolling eﬀect of the particles between the two sliding surfaces, which protected the SCFs from being pulled out of the PPS matrix.

4.9. PP-based composites

Rezaei et al. [118] studied the eﬀect of the length of CFs on the thermo-mechanical properties of SCF-reinforced PP composites. Their results suggested that longer CFs imparted better thermo-mechanical properties to the CF/PP composites than shorter CFs. TGA also indicated that the thermal stability of the SCF/PP composites increased with increasing CF length. Wang et al. [119] studied the mechanical and thermal properties of SCF-reinforced PP composites coated with GO. The tensile, ﬂexural, and impact strengths of the GO-coated SCF/PP composites were greatly improved by the chemical reaction and mechanical interlocking between GO on the surface of the SCFs and the PP matrix. Diﬀerential scanning calorimetry (DSC) indicated that the GO- SCF/PP composites exhibited good thermal stability.

Fu et al. [120] investigated the fracture resistance of short-glass- ﬁber-reinforced and SCF-reinforced PP under Charpy impact load. The notched Charpy impact energy of the composites was found to increase with increasing glass ﬁber content and decreasing CF content. Han et al.[121] evaluated the surface treatment of CFs on the interfacial behavior of CF-reinforced PP composites. A 29.7% increase in the IFSS of the composites subjected to 3 min plasma treatment was achieved compared with that of the untreated specimens. The IFSS of the composites increased by 48.7% when treated with a silane coupling agent and subjected to 1 min plasma treatment compared with that of the untreated specimen. Karsli et al. [94] investigated the eﬀects of an irradiated PP compatibilizer on the properties of SCF-reinforced PP composites. Their results indicated an increase in the breaking strength values to 30%. SEM observation indicated that enhanced interfacial adhesion between the CF and PP matrix was achieved in blends comprising the PP matrix and irradiated PP. Arao et al. [95] studied the mechanical properties of CF/PP composites hybridized with nanoﬁllers. The IFSS and mechanical properties of the CF/PP composites improved with the addition of alumina, silica, and CNTs. The alumina particles were most eﬀective for enhancing the mechanical properties of the composites.

Akonda et al. [122] studied recycled-CF-reinforced PP composites. The tensile and ﬂexural strengths of the composites containing 27.7 vol% recycled CFs were 160 and 154 MPa, respectively. Wong et al. [123] investigated the eﬀect of coupling agents on the potential of recycled CF as a reinforcement for PP composites. The interfacial adhesion was improved by the addition of maleic anhydride grafted polypropylene (MAPP). The impact strength of the composites improved signiﬁcantly when MAPP was added due to greater compatibility between the CFs and matrix.

Ameli et al. [124] investigated the electrical properties and EMI shielding eﬀectiveness of CF/PP composite foams. The density of the composites decreased by 25%, the through-plane conductivity was enhanced to a maximum of six orders of magnitude, and the dielectric permittivity increased, resulting in an increase in the speciﬁc EMI shielding eﬀectiveness by up to 65% with the introduction of foaming. Zhao et al. [92] studied the eﬀect of CFs on the conductive properties of a segregated CB/PP composite. The percolation threshold of the composite decreased signiﬁcantly with the addition of 0.155 vol% CFs. Based on SEM analysis, the reduction of the percolation threshold was attributed to the construction of a shish-calabash-like conductive net- work. Hong et al. [96] investigated the EMI shielding eﬀectiveness of CF-reinforced PP composites in the presence of CNTs as a conductive ﬁller. The EMI shielding eﬀectiveness of the CF/PP composites increased with increasing mixing speed due to the enhanced dispersion of the CFs in the matrix, which resulted in improvement of the electrical networks, and thus the EMI shielding eﬀectiveness, in the CF/PP composites. Xu et al. [125] studied the liquid sensing behavior of conductive PP composites containing CFs and CB. The CF/CB/PP composite exhibited superior liquid sensing behavior compared with the CB/PP composite.

Cho et al. [98] achieved highly enhanced mechanical properties of long CF-reinforced PP composites by a combined method involving a coupling agent and surface modiﬁcation of CFs. The surface−modiﬁed CF-reinforced PP composite with 5 wt% bi-functional group-grafted PP as a coupling agent showed the highest tensile strength and ﬂexural strength, with respective increases of 1.5- and 1.7-fold respectively, compared with the PP/CF composite employing conventional maleic anhydride-grafted PP. Hou et al. [99] determined the processing conditions, such as stamping temperature, stamping time, stamping velocity, and pressure required for stamp forming of continuous CF-reinforced PP composites. An experimental two-dimensional matched die was successfully designed and employed for stamp-forming of continuous CF-reinforced PP composites.

4.10. Advantages and disadvantages of CFs in thermoplastic composites

The advantages of using CFs in thermoplastic composites are as follows [18,19,75,86,126,127]:

As advanced reinforcing ﬁber materials in thermoplastic composites, CFs have excellent mechanical, thermal, and electrical properties, good thermal conductivity, excellent corrosion resistance, a low linear thermal expansion coeﬃcient, and low density.
Compared to short glass ﬁbers, SCFs have low density, high speciﬁc strength and stiﬀness, excellent thermal and electrical conductivity, high wear resistance, and a low coeﬃcient of friction, making them attractive for many applications, especially in the automotive industry.
Continuous CF-reinforced thermoplastic composites have been extensively used in the aerospace, weaponry, automotive, and chemical industries because of their potential for construction of light- weight materials, high strength and stiﬀness, recyclability, repairability, and corrosion resistance.
CF-reinforced thermoplastics can be recycled by melting the material and reforming it into a new object.

The disadvantages of the use of CFs in thermoplastic composites are as follows [26,112,128,129]:

CFs are expensive compared with glass ﬁbers.
CFs have poor wettability and adhesion to the thermoplastic matrix.
Unidirectional CF-reinforced thermoplastic composites exhibit low strain under uniaxial tension.

4.11. Problems with CF-reinforced thermoplastic composites and possible solutions

4.11.1. High cost of CFs compared with glass ﬁbers

To obtain thermoplastics reinforced with SCFs, composites can be prepared by using recycled CFs. These ﬁbers are much cheaper than the initial CFs and can be used to reinforce thermoplastic polymers in order to improve their physical, mechanical, tribological, thermal, and elec- trical properties [112].

4.11.2. Poor wettability and adhesion of CFs to the thermoplastic matrix

The surfaces of CFs can treated using chemical methods, plasma methods, electrochemical methods, and sizing or coating methods to introduce chemical functional groups onto the ﬁber surface and im- prove the interfacial adhesion between the CFs and the thermoplastic matrix [128].

4.11.3. Low strain of unidirectional CF-reinforced thermoplastic composites under uniaxial tension

In order to increase the strain to tensile failure, diﬀerent techniques such as thin-ply hybridization, thin-ply CF-reinforced plastic angle ply lamination, wavy-ply sandwich structures, and interleaved lamination can be employed [129].

4.11.4. Negative inﬂuence of residual solvent in the solution-processed system on the performance of CF-reinforced thermoplastic composites

In solution-processed systems, CF-reinforced thermoplastic composite prepregs can be fabricated by impregnating CFs with a suitable polymer solution. This process can lower the viscosity of the resulting polymer solution and thus reduce the processing cost. However, the residual solvent may inﬂuence the performance of the composites. The residual solvent in solution-processed products should be removed by employing a higher molding temperature and/or longer molding time [75].

5. Conclusion

This review covered the recent advances in methods of surface treatment of CFs and preparation of CF/thermoplastic composites. Suitable surface treatments and preparation methods are important for achieving a uniform dispersion of CFs in polymer matrices and for increasing the interfacial adhesion between the CFs and matrix. Furthermore, the thermal, mechanical, and electrical properties of the composites were discussed in detail. Recent studies have focused on the preparation and characterization of nanoparticle-ﬁlled CF/thermoplastic composites as a means of achieving signiﬁcantly improved mechanical and electrical properties.

Source: Shan-Shan Yaoa, Fan-Long Jina, Kyong Yop Rheeb,∗, David Huic, Soo-Jin Parkd,∗∗

a Department of Polymer Materials, Jilin Institute of Chemical Technology

b Department of Mechanical Engineering, College of Engineering, Kyung Hee University

c Department of Mechanical Engineering, University of New Orleans

d Department of Chemistry, Inha University, South Korea

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The Recent Advances in Methods of Surface Treatment of Carbon Fibers and Preparation of Carbon Fiber/Thermoplastic Composites