As the adhesive for composites, the resin matrix directly impacts the molding process and product performance of thermoplastic composites (TPCs). Carbon fiber-reinforced polyetherimide (CF/PEI) and polyaryletherketone (CF/PAEK) composites were prepared by a compression molding process. The interface, interlaminar, and low-speed impact properties of TPCs were studied. The results show that the interfacial shear strength of the CF/PEI composite is ~116 MPa, while the CF/PAEK composite is ~78 MPa. However, the interlaminar and low-speed impact performance of CF/PAEK is better than CF/PEI. Type I fracture toughness () and type II fracture toughness () of CF/PEI are ~1051 MPa and ~1060 MPa. But those of CF/PAEK are ~1786 MPa and ~2584 MPa, respectively. The compressive strength after the impact of CF/PAEK (~321 MPa) is 40% higher than CF/PEI (~230 MPa).

1. Introduction

Thermoplastic resins can be divided into general-purpose plastics, engineering plastics, and specialist plastics according to their properties and uses. Meanwhile, they can be divided into amorphous and crystalline polymers depending on whether they are crystallized. At present, specialist plastics are used as matrixes for thermoplastic composites (TPCs), mainly including amorphous polymers, polyethersulfone (PES), polyetherimide, semicrystalline polymer polyphenylene sulfide (PPS), and polyetheretherketone (PEEK). TPCs with good impact resistance and damage tolerance have attracted increasing attention in aerospace. In addition, compared to traditional thermosetting composites, TPCs also have the characteristics of recyclability, fast processing, and infinite shelf life of prepreg materials [13]. Therefore, TPCs have a broad market application space in the automobile industry, rail transit, transportation, and aerospace [47].

PEI is an amorphous thermoplastic polymer with high-temperature resistance, excellent mechanical properties, electrical properties, and dimensional stability. In addition, low melt viscosity endows it with good molding performance. Therefore, PEI received great attention during the development and application of early TPCs. As a member of the most common and widely applied semicrystalline thermoplastic resin, PEEK has been used as a matrix in aeronautical structural composites with good impact resistance, high-temperature resistance, and mechanical properties [8]. However, the harsh molding temperature of close to 400°C makes the process cost higher. Therefore, a low melting temperature has been developed and applied, which changes the regular structure of the main chain compared to that of PEEK. While maintaining PEEK’s high-temperature resistance and mechanical properties, the melting temperature of PAEK has been reduced by 30-40°C, which improves its process performance significantly. The molecular structure analysis shows that PEI and PAEK contain rigid functional groups and flexible ether bond structures, such as benzene rings or aromatic imides. The rigid functional groups endow them with high-temperature resistance and excellent mechanical properties, while the ether bond makes them good rheological properties required in the molding process of TPCs. The difference in the content of ether bonds in the molecular structure shows that PAEK has better toughness than PEI.

Several studies have shown that matrix properties influence the interfacial properties [911] and the mechanical properties [1219] of composites. During the composite failure process, microcracks initially occur in the matrix [20]. When matrix cracks propagate to the interface, two competing fracture failure mechanisms must be considered. On the one hand, if the interface bonding strength is strong enough, the matrix cracks can directly propagate through the fiber, which triggers a brittle failure of composites. On the other hand, if the bonding strength is weak, the crack of the matrix can be deflected along the fiber/matrix interface and continue to propagate along it. The propagation requires energy, which delays structural damage [21]. Therefore, the interfacial properties and the crack propagation resistance of the matrix are important factors affecting the mechanical properties of composites. Gu et al. [14] studied the effect of different viscosity PAEK resins on the interfacial properties of composite materials. The study showed that the interfacial shear strength (IFSS) between low-viscosity resin and fiber was about 23% higher than that of high-viscosity PAEK. The 90° tensile strength of the composite is increased by about 38% by improving the interfacial strength. The research group previously studied the effect of resin crystallinity on the interface properties. The results showed that the lower the crystallinity and smaller the crystals, the better the interface and mechanical properties of the composite materials [15]. Furthermore, the toughness of the resin matrix has an essential influence on the toughness of the fracture and the low-speed impact performance of the composite. [22] found that by improving the toughness of the matrix, the toughness of interlaminar fracture of composite materials can be improved, which is beneficial in preventing crack propagation, delaying the occurrence of delamination, and reducing delamination damage. Kim and Ye [23, 24] studied the effect of resin toughness on the interlaminar fracture toughness of CF/PEI composites. The results show that the fracture toughness for composites with strong interfacial properties is mainly affected by the properties of the matrix. With the improvement of the toughness of PEI resin, the interlaminar fracture toughness of CF/PEI composites has been effectively improved. As far as the author knows, there is relatively little literature on the influence of resin matrix on the properties of thermoplastic composites, and there is even a lack of systematic comparative studies on the properties of different thermoplastic composites.

In this paper, the effect of the crystallization behavior of PEI and PAEK on the interfacial properties of composites was discussed by microdebonding experiments. CF/PEI and CF/PAEK were prepared by compression molding. Meanwhile, the effect of the resin matrix on the interlaminar and low-speed impact properties of TPCs was studied. The study helps to understand the influence mechanism of matrix properties on the interface properties and mechanical properties of TPCs. At the same time, it can provide technical support and references for the design, manufacture, and application selection of TPCs in aviation and aerospace.

2. Materials and Methods

2.1. Raw Materials

The amorphous polymer PEI is provided by SABIC Company (ULTEM™ 1000F3SP, Saudi Arabia) and the semicrystalline polymer PAEK by Tangyuanxian Hairuite Engineering Plastics Co., Ltd. (PAEK-L, China). The mechanical properties of the two kinds of TPs are shown in Table 1. Figure 1 shows the molecular structure formulas of PEI and PAEK. The PEI molecular chain with aromatic imine ring structure is more rigid than PAEK with a benzene ring and low ether bond content (the mass fraction of oxygen in PEI ether bonds is about 5.41%, and the mass fraction of ether bond oxygen in PAEK is about 8.74-11.07%). The difference in structure makes PAEK resin have higher toughness and extensibility. The T700 grade CF is provided by Weihai Expand Fiber Co., Ltd. (TZ700S-12K, China).

CF/PEI and CF/PAEK thermoplastic prepregs were provided by Heilongjiang Yingchuang New Materials Co., Ltd. (TZ700S/PEI and TZ700S/PAEK-L, China), the areal density of the fiber is 149 g/m2, and the matrix content is 37 wt%. The nominal layer thickness of prepreg is 0.13 mm.

2.2. Preparation of Composites

The preparation of TPCs is shown in Figure 2. First, the prepreg was cut and stacked with a size of . Second, the preform was produced by ultrasonic spot welding (KH-2870Z, Kehai, China). Third, the welded preform was placed in a die, and the welded preform was placed in a mold and heated by a hot press (HS100/1-400/400, Hengsheng, China). The die was heated according to the molding process. The molding process of CF/PEI composite is , and that of CF/PAEK is , as shown in Figure 3. Finally, the pressure would be maintained until demolding below 100°C at a cooling rate of 1°C/min. The stacking sequence of test specimens is different. The 90° tensile is [0]16, the short beam shear is [0]40, the and are [0]24, and the low-speed impact is [(45/0/-45/90)]4S. The thickness of test specimens is 2 mm, 5 mm, 3 mm, and 4 mm, respectively. The internal quality of the composites was tested by metallographic microscopy and ultrasonic C-scanning, as shown in Figure 4. The typical scan parameters (transmission mode; ultrasonic frequency, 30 MHz; and signal gain, 28 dB) were carried out.

2.3. Crystallization Behavior Tests

A differential scanning calorimeter (DSC 250, TA, USA) conducted the thermal behavior tests. Under nitrogen protection, the sample weighing 3-10 mg was used for testing with a flow rate of 50 ml/min. First, the sample was heated to 400°C at a heating rate of 20°C/min and then kept for 5 minutes to erase thermal history. Second, they were cooled to 50°C at 1°C/min cooling rates. Finally, the sample was reheated to 400°C.

The crystallization behaviors of the polymers were observed by a polarizing microscope (DM4 P, Leica, Germany) equipped with a polarizer. First, a single CF was fixed on a hot stage, and the resin powder was evenly spread on the surface of the CF. Second, the polymer was heated to 360°C and kept for 5 minutes to ensure complete melt. Finally, the sample was cooled to 30°C at a rate of 1°C/min, and the crystallization morphology of the polymer on the fiber surface could be observed in real time.

2.4. Interface Property Tests

IFSS between fiber and resin was tested by a composite interface performance evaluation device (HM410, Tohei Sangyo, Japan). First, the monofilament separated from the fiber bundle was fixed on a concave metal fixture. Second, the metal fixture was installed in the furnace, where the temperature was increased to 380°C (PEI) and 350°C (PAEK) to melt the thermoplastics. The thermoplastics formed several elliptical microspheres on the surface of the carbon fiber due to surface tension. Finally, the sample was cooled to room temperature at 1°C/min. Precise force transducers record the tensile load during the test. IFSS is calculated by [14] where is the IFSS, is the maximum tensile load, is the fiber diameter, and is the embedded fiber length.

2.5. Mechanical Property Tests

All laminates of TPCs were prepared by a water-jet machine (OMAX, ProtoMAX, USA) and tested by a universal testing machine (5982-100kN, Instron, USA).

The 90° tensile properties were tested using ASTM D 3039. The specimen size is (), and sandpaper is used at both ends of the clamping.

The short beam shear properties were tested according to ASTM D 2344. The specimen size is , and the span-to-measured thickness ratio is 4.0.

The interlaminar fracture toughness tests referred to ASTM D 5528 and ASTM D 7905. The specimen size is . The prefabricated crack length is 50 mm for , and that of is 40 mm. The was measured using the double cantilever beam (DCB) test and the compliance calibration (CC) method. The numerical calculation is shown in where is the CC coefficient, which is the slope of the fit line versus plot using the visually observed delamination onset values and all the propagation values, is the maximum force, is the load point displacement, is the delamination length, and is the specimen width. The support span for the testing is 100 mm, and that of can be calculated using where is the CC coefficient, is the maximum force, is the crack length used in the fracture test, and is the specimen width.

The low-speed impact properties of composites were tested using a floor-standing impact system (CEAST 9350, Instron, USA) based on ASTM D 7136. The drop hammer weighed 10.28 kg with a 16 mm diameter impacter, and the impact energy was set at 6.67 J/mm. The nominal size of the specimen is . Compression strength after impact (CAI) performance tests was completed by a 600 kN universal mechanical testing machine (E45.605, MTS, USA) referring to ASTM D 713. The torque of the screws is 7 N·m, and the speed of the compressive testing is 1.25 mm/min.

2.6. Microscopic Morphology Analysis

The scanning electron microscope (Regulus SU8230, Hitachi, Japan) was used to observe the microscopic morphology of failed specimens and analyze the difference between CF/PEI and CF/PAEK. Before that, the exciting specimens were dried at 120°C for 4 h in a hot air circulation oven and then pretreated with gold spray for 60 s. The accelerating voltage is set at 10 kV.

3. Results and Discussion

3.1. Internal Quality of Thermoplastic Composites

Whether the matrix resin can impregnate the reinforcing fibers well will directly affect the final product’s performance for fiber-reinforced composites, so ensuring the internal quality of the composites is a prerequisite. Ultrasonic C-scanning is a relatively common test method for detecting the internal quality of composites. It can be seen from Figure 4 that the CF/PEI and CF/PAEK appear blue overall, and there is no obvious difference in distribution, indicating that the internal quality is similar and good. At the same time, obvious void defects were not found by observing the morphology of the composites, as shown in Figure 5.

3.2. Crystallization Behavior of Resins

The thermal properties and crystallization behaviors of the polymers are shown in Figure 6. A comparison of curves indicates an obvious difference in thermal properties. In Figure 6(a), PEI exhibits the typical thermal behavior of amorphous polymers, while in Figure 6(b), PAEK exhibits the typical thermal behavior of semicrystalline polymers. PAEK has two characteristic peaks during the first heating process. The first characteristic peak is the cold crystallization peak, and the second characteristic peak is the melting peak. The sharp crystallization peak represents the phase-transition process of PAEK in the amorphous region. There is no cold crystallization distinct peak in PAEK during the second heating process. Therefore, PAEK can be nearly fully crystallized at a 1°C/min cooling rate, and the crystallinity is approximately 33.8%. The crystallinity () of the resin can be calculated by where is the melting enthalpy of the DSC test and is the theoretic melting enthalpy of completely crystallized PAEK taken as 130 J/g [25].

It can be seen from Figure 7(a) that there is no crystalline structure inside the PEI, but as shown in Figure 7(b), a large amount of spherulites is generated inside the PAEK.

3.3. The Effect of Resin on the Interfacial Properties of Composites

Plots of IFSS stress versus displacement are shown in Figure 8. IFSS of CF/PEI is , and that of CF/PAEK is . Compared to the IFSS data of CF/PEI and CF/PAEK, the interfacial bonding strength of CF/PEI is higher than that of CF/PAEK under the same cooling conditions. The surface morphology after microdebonding experiments can help explain the phenomenon, as shown in Figure 9. It can be seen in Figure 9(a) that the surface of PEI resin is smooth with no apparent shrinkage phenomenon. However, the PAEK is rough and covered with different sizes of pits, as in Figure 9(b). PAEK with a specific dense spherulite structure is almost completely crystallized at a cooling rate of 1°C/min. This will generate internal stress at the interface between the matrix and CF due to solidification shrinkage. This mechanism can be better understood in Figure 10. However, some experimental studies have shown that, for some fiber systems, the transcrystalline structures on the fiber’s surface are conducive to improving the interface bonding strength. For example, Chen and Hsiao [26] studied the transcrystalline structure of various single-filament systems. The results showed that the IFSS had greater than 40% increases with transcrystalline designs compared to the system without transcrystalline facilities. It should be noted that the transcrystalline structures of PAEK are not found around the CF in this experiment, as shown in Figure 7(b).

Plots of 90° tensile strength versus strain are shown in Figure 11. 90° tensile strength and modulus of CF/PEI are and , while those of CF/PAEK are and . 90° tensile strength of CF/PEI is higher but has a lower modulus, which is consistent with the properties of the resin matrix. The failure morphology of the composites after testing is shown in Figure 12. The surface of the fibers is completely covered with resin, indicating that the failure of composites occurs in the resin layer. Therefore, both composites have good interfacial bonding strength, even stronger than the matrix. Furthermore, it can be found that the section morphology of the composites is quite different. The PEI has a relatively smooth surface, while the surface of PAEK is covered with dense hackle structures. This phenomenon is related to the property of the plastic deformation capacity of the resin.

3.4. Effect of the Matrix on Interlaminar Properties of Composites

The interlaminar shear strength (ILSS) of CF/PAEK () is higher than that of CF/PEI (). Figure 13 shows the typical plots of ILSS stress versus displacement, which shows that the CF/PEI has no apparent yield behavior, and the failure occurs instantaneously when the loading displacement reaches about 0.9 mm. In contrast, the stress of the CF/PAEK drops slowly after reaching its peak and then up and down multiple cycles. This difference in failure mode has a great relationship with the plastic deformation capacity of the matrix under a stronger interface strength. It can be seen from Table 1 that PAEK resin has better-notched impact toughness than PEI. The PEI is more brittle than PAEK, and cracks are easier to gather and produce larger through cracks. However, the PAEK with better plastic deformation capacity can reduce stress concentration and form many microcracks, which can absorb more energy with increased strain. Figure 14 can further explain this phenomenon. However, Gu et al. [14] believe that the interlaminar shear properties of thermoplastic composites are affected by the interfacial properties, and composites with stronger interfaces have higher ILSS. Still, it cannot explain our experimental results. Therefore, the ILSS of composites can be affected by both resin matrix and interfacial properties.

The of CF/PAEK () is higher than that of CF/PEI (). At the same time, the of CF/PAEK () is higher than that of CF/PEI (). Therefore, CF/PAEK has better interlaminar fracture toughness, mainly due to the greater plastic deformation capacity of PAEK resin. Figures 15(a) and 15(b) show the failure morphologies of the samples. After the failure of the test, the resin layer of the CF/PEI type I specimen is relatively smooth. On the other hand, the CF/PAEK composite cross-section has a rough resin appearance, which is conducive to the resin’s absorption of external energy during the test [15]. The fracture morphology of the type II sample is shown in Figures 15(c) and 15(d). The failure morphology of the resin matrix of the two composite materials is tear-like. It has the same orientation behavior as the stress mode (shear force) of the composites. The difference is that the tear size of the PAEK resin is relatively large. It can be found that a dense thermoplastic resin layer is attached to the fracture surfaces of the two composites, and the fibers are completely wrapped by the resin, which indicates that the interface is not damaged during the interlayer slip of the composites. Gao and Kim [27] considered the interlaminar fracture toughness of composites to be a complex interaction of two basic properties, the plastic deformation capacity of the matrix and the bond strength of the fiber-matrix interface. Therefore, for composites with strong interfacial strength, the plastic deformation capacity of the resin matrix determines the interlaminar fracture toughness. The resin’s force pattern during the test can be better understood in Figure 16.

3.5. Effect of Resin on the Low-Speed Impact Properties of Composites

Typical impact load-displacement and energy-time curves of CF/PEI and CF/PAEK are shown in Figure 17. The energy losses of CF/PEI and CF/PAEK are and , respectively, indicating that the absorption of impact energy by the composites is almost the same. More dense and volatile zigzags exist on the load-displacement curves of CF/PEI. These zigzags correspond to cracks and delamination.

The impact damage mechanisms in laminates constitute a complex process resulting from a combined interaction of matrix cracking, delamination, fiber shear, and fiber breakage [28, 29]. The indentation depth of CF/PEI () is about 20% lower than that of CF/PAEK (). The specific impact damage projection area of the composites for ultrasonic C-scan testing is shown in Figure 18. The damaged area of the CF/PEI composite is , and that of the CF/PAEK composite is . The damaged area is about 540% higher than the latter, indicating a difference in the damage mechanism between the CF/PEI and CF/PAEK. The differences between damage patterns can be identified with cross-sectional views (see Figure 19). Obvious delamination and fiber breakage occurred after CF/PEI impact failure. In contrast, the fibers on the back of CF/PAEK were severely broken, and there was no obvious delamination between layers. Zhang et al. [15] used XRM to reconstruct 3D samples of TPCs after impact, and the results showed that the composite with higher mainly absorbed energy through the plastic deformation of the resin matrix. Instead, they absorb energy primarily through layering. The interlaminar fracture toughness data in the previous section show that CF/PAEK has higher than CF/PEI composite, and PAEK resin has better plastic deformation ability. Therefore, the CF/PAEK composite mainly absorbs energy through the plastic deformation ability of the resin to reduce interlayer damage. However, the CF/PEI composite mainly absorbs impact energy through fiber fracture and layer crack propagation.

Plots of CAI stress versus displacement are shown in Figure 20. The CAI strength of CF/PEI () is lower than that of CF/PAEK (), indicating that the latter maintains high mechanical properties after impact damage. The residual strength is strongly related to the interfacial bond properties and but largely depends on the size of the damaged area after impact [30, 31]. The CF/PAEK has a lower and damaged area than the CF/PEI. Therefore, CF/PAEK composites exhibit higher CAI strength.

4. Conclusions

In this study, CF/PEI and CF/PAEK thermoplastic composites with good quality were successfully prepared by the hot pressing process. At the same time, their interfacial, interlayer, and low-speed impact properties were comparatively studied. CF/PEI has better interfacial properties than CF/PAEK, which may be related to the crystallization difference of the resin during the cooling process because the volume shrinkage of the PAEK resin during the crystallization process produces stress. When the stress is transmitted to the interface, the bond strength between the fiber and the resin is weakened. On the contrary, the interlayer performance of CF/PAEK composites is significantly better than that of CF/PEI. The and of the CF/PAEK are about 70% and 144% higher than those of the CF/PEI. This is mainly affected by the toughness of the resin matrix. The impact of PAEK’s better plastic deformation ability can absorb more energy to resist the damage of the resin matrix. Also affected by the deformation ability of the resin, the low-speed impact failure mechanisms of the two composites are different. CF/PEI composites mainly absorb energy from fiber failure and ply crack propagation. CF/PAEK composites mainly absorb energy through the plastic deformation ability of resin to reduce impact damage and exhibit higher compressive strength after impact. The CAI strengths of CF/PEI and CF/PAEK composites are 230 MPa and 321 MPa, respectively.

Data Availability

All data are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.


Thanks are due to Dr. Lei Liu and Dr. Mengyao Zhang for their help with the measurement and characterization. The authors are grateful for the financial support from the Fundamental Research Funds for the Central Universities (no. 2232022A-12).