Advances in mechanical and physicochemical performances of recycled carbon fiber reinforced PEKK composites

Abstract

In recent years, waste storage and recycling have been important issues. All efforts shown in these fields aim to help resolve concerns related to climate change. In this study, recycled CF/PEKK thermoplastic composite materials were reviewed to reduce the environmental impact of materials during the production phase as well as economic costs and ensure product continuity. Comparisons between unprocessed or virgin material and recycled materials were performed by bending and interlaminar shear strength (ILSS) analyses. In addition to mechanical tests, physicochemical tests such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and fiber void ratio (FVC)-porosity tests were conducted. Images of the samples were observed by scanning electron microscopy (SEM) to interpret changes in the structure before and after the recycling. According to the experimental results, it was found that the virgin thermoplastic composite showed 68.05% higher flexural strength and 7.85% higher flexural modulus compared to the recycled thermoplastic composites. The average ILSSs were measured as 81.8 MPa and 64.7 MPa for the virgin and the recycled thermoplastic composites, respectively. Hence, it is obvious that the recycled composites could be used for applications that require less strength and durability. Finally, it was concluded that recyclable materials can be reused in the aero structures.

Introduction

Special materials with special features are in demand for fast-developing technologies. Researchers can satisfy some of the strict material requirements using composite materials. Nearly every day, new products are introduced to the market. The necessity to produce various parts with the necessary high strength is prevalent in the sports equipment industry as well as in industries like aerospace, defense, and automotive. Particularly, the aerospace has a high interest in high-performance structures, even simple structures, which need lightweight components with ultra-mechanical qualities through the use of modern composite materials. Advanced composites have significantly displaced conventional structural materials in key load-carrying airplane structures in recent years (for instance the Boeing 787 consists of approximately 50% advanced composites by weight) [1].

In recent years, composite demands in the aerospace industry have been increased significantly. The use of thermoplastics with recyclable properties has increased with the rising amount of waste generated via the increase in demands, and decrease in raw materials that is gaining importance every day considering that environmental damages can be prevented through recycling. Thermoplastic matrix composites offer many advantages compared to thermoset matrix composites. They can be recycled and reshaped, even though they cannot completely retain their mechanical, chemical, and thermal properties. Basically, thermoplastic composites may be disassembled into little parts, melted, and then remolded using molten material [2, 3]. The recycling of thermoplastic composites can be achieved through mechanical, thermal, and chemical methods. In the year of 2021, Brujin and van Hattum [4] conducted research on a carbon fiber-reinforced polyphenylene sulfide (PPS) access panel that was manufactured using post-industrial waste. Blazsó [5] performed studies to reduce environmental concerns for pyrolysis. Regarding the pyrolysis of halogenated flame retardants in polymer composites, certain techniques are mentioned for reducing or removing dangerous and damaging substances from pyrolysis products of halogenated flame retardants. Eriksson et al. [6] compared the properties of uncured as well as cured materials and mechanically ruptured glass fiber-reinforced thermoplastic composite products. After mechanical fragmentation and injection molding with resin reinforcement, the impact of fiber breaks on the product's mechanical qualities, such as tensile strength, was studied. Beg et al. [7] examined the mechanical properties of the reworked product through disintegration and injection molding. Their evaluation led to the conclusion that there were decreases in the mechanical properties. It was also concluded that the failure strain gradually increased compared to each previously recovered product. The decrease in mechanical properties revealed that the bond between matrices attached to the fiber gradually decreased. According to Poulakis et al. [8], glass fiber/polypropylene obtained using solvents such as methanol, xylene, and toluene has a better tensile strength than the initial product. Celik et al. [9] studied thermal and mechanical properties of recycled and virgin polyethylene terephthalate (PET). They focused on melting temperature, glass transition, and crystallinity to investigate mechanical properties. In addition, they examined effects of processing steps on molecular weight and crystallinity distribution for virgin and recycled PETs. In this study, the changes in the thermal and mechanical behavior of the structure were examined. An experimental study has been conducted by LeBlanc [10] to determine effects of filament size, pressure, material position, and temperature in the die cavity for T-shaped pieces’ quality. Basic results show that machining a complex property at filling pressure is sufficient to achieve rated mechanical properties. It was concluded that the mechanical performance for specified tests was not adversely affected by moderate porosity. According to the technical readiness level evaluation made by Rybicka et al. [11], the mechanical approach for carbon fiber reinforced composites and the pyrolysis method for glass fiber-reinforced composites should be chosen. It was stipulated that the fiber lost its continuity properties because of mechanical fragmentation. Studies have revealed that recycling by mechanical processes is only possible to a certain extent, due to the loss of mechanical properties [12,13,14]. Howarth et al. [15] settled that recycled fibers can be used in industries that demand less mechanical power than blank/plate energy, which provides advantages for the environment. In the study conducted by Bernatas et al. [16], it was decided that at a minimum temperature close to 400–450 °C, carbon fibers that retain 95–99% of their initial mechanical capacity will be obtained by the pyrolysis method. They stated that for short fiber-reinforced thermoplastics, where fiber breakage during reprocessing has a lower effect on reinforcing qualities than long fiber-reinforced polymers, the simplicity and relatively low cost of mechanical recycling give it the greatest potential [17]. In addition, reprocessing has been connected to increased fiber degradation, as has been reported for thermoplastics reinforced with glass fiber [7, 18].

In the literature, although most studies have brought the recycling of thermoplastic composites to a certain maturity, they are not sufficient. The importance of this study is to compare changes in the mechanical and physicochemical structure of monolithic composite plates during recycling. In light of all mechanical and physicochemical tests, the strength and hardness of the recycled aero structures are measured and compared to similar studies in the literature. It is known that the change in angle orientation at this point affects the strength of the solid composite sheet. Moreover, after recycling of carbon fiber material/poly ether ketone ketone, SEM imaging of the recycled and virgin plaque was performed to examine changes in the structure.

Experimental studies

The composite material TC1320 produced by TORAY Cetex was used as PEKK reinforced prepregs to be used in the samples. Toray Cetex® TC1320 is available as a unidirectional (UD) tape, a fabric prepreg, and as reinforced thermoplastic laminates (RTLs) of varying thicknesses [19]. In experimental studies, carbon fiber/PEKK prepregs with symmetry and balance properties were preferred. Characteristic features of carbon fiber/PEKK prepregs are summarized in Table 1.

Table 1 Characteristics of commercial CF/PEKK prepregs used in experimental studies

In order for thermoplastic composites to become formable, first of all, a consolidated plate production is provided. Consolidated plate production was carried out in an autoclave. Following this, the waste structure was shredded and reshaped with the hot press method. As shown in Fig. 1, hand lay-up was the initial manufacturing process for fiber-reinforced composite structural elements. Note that the quality of the parts depends on the operator's expertise. Layup of thermoplastic composite panels and parts is conceptually alike to lay-up with thermosetting composites such that plies are cut to shape and applied ply by ply. Before the lay-up stacking $({[0/0/45/90/-45/0/90]}_{s})$ sequence of the prepregs was determined, mechanical requirements need to be considered. Afterwards, consolidation of 14 layers of CF/PEKK were carried out in an autoclave.

Fig. 1

Preparation of the hand lay-up process

As seen in Fig. 1, the bagging process was carried out by using a Kapton separator film, vacuum bag and tape, which can withstand high temperatures. The tool-laminate assembly was initially put inside an autoclave, which is a large tank with temperature and pressure controls, and the bag was linked to the vacuum system. In order to cure the part, pressure and temperature were applied to the laminate in a predetermined cure cycle. In addition, pressure was used to compact the laminate at the required fiber volume content, conform it to the tool surface, and collapse any gaps that might arise. Corresponding curing parameters are presented in Table 2.

Table 2 Autoclave curing parameters

After the production of the consolidated plate, the press forming process was carried out to obtain the final part. Waste parts of the composite were used within the scope of this work. Obtained waste pieces were separated according to their size. For the shredding process, equipment working in principle analogue to a small-sized shredder was used. Thanks to the shredder, fractures were observed in the fibers and matrix, which were joined by strong bonds. Also, the primary rationale for employing mechanical recycling is based on the observation that it results in relatively minimal reduction in mechanical properties compared to other methods used in thermoplastic recycling, such as mechanical degradation, as reported in the existing literature. Images of the waste composites are presented in Fig. 2.

Fig. 2

Production waste

Shredding was carried out for waste materials with the same ply sequence. Herein, it was expected that studs in the geometrical arrangement within the recycled plate were minimized. Shredding is the process of breaking down raw materials to be reintroduced into production of small pieces. As can be seen in Fig. 3, the waste plate was broken into pieces of approximately 1 in (25.4 mm).

Fig. 3

Shredded thermoplastic composite plate

First of all, the lay-up tool for the consolidation process was determined. A tool made by invar materials was preferred because it is commonly exposed to high temperatures. At high temperatures, geometrical changes in invar materials stay within tolerances [19]. A release film, which can withstand high temperatures, is located on the set in Fig. 4. Subsequently, it was fixed with a flash breaker with the same features to keep it stable. Production wastes with about 25 mm dimensions, which were in shredded form, were placed on the tool.

Fig. 4

Layout of the shredded thermoplastic composite plate

In addition, thermocouples were placed on the tool surface to monitor the controlled heating cycle (Fig. 5). The thermocouples were placed directly on the tool and divided into zones. Shredded composite plates were placed in these zones. Hence, during and after the stamping, it was checked whether the temperature of the consolidated thermoplastic first plate was between the corresponding initial temperature values.

Fig. 5

Preparation on the tool for recycling

The first consolidated plate had a thickness of 2 mm. Data were fed into the press—in order to manufacture the recycled plate with the same dimensions and 2 mm space between press and tool. Moreover, pressure was used to compact the laminate at the required fiber volume content, conform it to the tool surface, and collapse any gaps that might arise. The press can rise to a maximum temperature of 380 °C increasing by 10 °C/min. When it reached 380 °C, the dwell time was about 30 min. It was put through a presser with a force of 100 kN. The hydraulic press arm traveled a distance of 250 mm until the distance between it and the tool was 5 mm. When the distance between the press and the tool was 2 mm, a 30 min cool down was performed. During this process, the thermoplastic resin in the structure of the part changed from solid to molten. In this way, the resin was homogeneously dispersed around fibers which lost their continuous feature. Remarkably, if the molten structure had been press-formed on a paving set with a shaped geometry, it would have taken the corresponding form directly. Due to the fragmentation that occurs, the main aircraft structure tends to lose some of its properties, as stated by the initial design requirements. Hence, when it comes to characteristics, recycled composite sheets are typically preferred for non-essential parts. Consequently, these sheets find applications in aircraft structures that are not directly subjected to loads. The recycling process by pressing can be seen in Fig. 6.

Fig. 6

Recycling with press forming

It was continued until the part temperature reached 60 °C with a cool-down rate of 10 °C/min. Subsequently, the demolding process was performed at 60 °C. Figure 7 shows the recycled plate. Although production of the thermoplastic composite was carried out under high vacuum, porosity occurs quite frequently since in addition to the rupture of the fibers during shredding, the resin ratio in the structure is reduced compared to the consolidated plate due to fractures in the matrix. However, as a result of the first visual inspection after the forming, no defects such as porosity were observed on the surfaces of the part.

Fig. 7

Recycled thermoplastic composite plate

Results and discussion

In this study, EN 2562 ILSS and ASTM D 2344 bending tests were applied using INSTRON instruments to examine mechanical properties of recycled and unused composite sheets. While examining these properties, one set of tests was applied. Results were shared on 6 samples for each set. Obtained results are given in detail below.

The ILSS values are known as an indicator of the interlaminar adhesion strength of composites. The ILSS represents the maximum shear stress at half the thickness of the sample at the time of the first fracture. Test samples of 20 × 10 × 2 mm³ dimensions were obtained from the plate with fiber orientation in the 0°-90° direction. It was tested according to the ASTM D2344 standard to find the shear strength between layers. For this test, 1 set of 6 test samples was applied. The ILSS is calculated by using Eq. 1

$${F}^{sbs}= (\text{0,75}x P)/ (b x h)$$

(1)

where the parameters are described as following: F^sbs is the ILSS, in MPa. P is the maximum applied force, in N. b is the specimen width, in mm. h is the specimen thickness, in mm.

In this study, the ILSS of CF/PEKK and recycled CF/PEKK composites under different loads was investigated according to the ASTM D 2344 standard. As seen in Tables 3 and 4, according to the test results, the average ILSS value was determined as 81.8 MPa in virgin composites produced from carbon fiber reinforced PEKK prepregs. In the recycled composites, the average ILSS value was determined as 64.7 MPa.

Table 3 The ILSS testing results of virgin CF/PEKK

Table 4 ILSS testing results of recycled CF/PEKK

Results of the shear strength test between layers are directly related to mechanical and chemical bonds that prepreg layers established with each other. Results obtained from the virgin composite plate are consistent with values in the literature and the certificate values transmitted by the prepreg manufacturer [20]. The comparison of the values obtained from the test results is mentioned in the following sections. However, there is a 20.9% decrease in the values obtained from the recycled composite plate. The decrease in the ILSS might be the result of three factors; firstly, mechanical properties of the thermoplastic are somewhat lost by recycling, secondly, the degree of adhesion between carbon fiber and resin might have decreased, and lastly, with the fragmentation of the structure, fiber-matrix adhesion decreased in the rupture regions. Moreover, CFs lost their continuity possibly yielding lowered mechanical properties. Virgin composite plate samples with an average thickness of 2.07 mm showed an average ILSS value of 81.8 MPa. The average thickness of the recycled composite plate samples was 1.96 mm, and the average ILSS was recorded as 64.7 MPa.

The coefficient of variation for virgin composite plate samples is 3.95%. The coefficient of variation for recycled composite board samples lies over 5%. The main reason for this is that the fiber produced during recycling is not the same in all parts of the plate. At some points, the resin ratio is higher. This situation also affects the results obtained.

The flexural strength is known as the maximum stress in the outermost fiber on the compressive and tensile side of the sample. In this study, bending behavior and fracture mechanisms of CF/PEKK and recycled CF/PEKK composites under different loads were investigated according to the EN 2562 standard. Results were obtained on five samples for each case. The flexural strength is calculated by using Eq. 2

$${\sigma }_{b}=\frac{3 {P}_{R }{l}_{v}}{2 b {h}^{2}}$$

(2)

where the parameters are characterized as follows:

${\sigma }_{b}$ is the flexural strength, in MPa. ${P}_{R}$ is the load at failure, in N. b is the width, in mm. h is thickness, in mm. ${l}_{v}$ is the distance between supports, in mm.

A composite plate was produced from 14 layers of CF/PEKK with a thickness of 2 mm. Thereupon, the produced plate was shredded and reshaped by the mechanical recycling method. Subsequently, two sets of test coupons, 10 mm wide and 100 mm long, were cut from the 2 produced plates. The part thickness was not homogenous at every point. The main reason for this is the random positioning of waste after mechanical fragmentation during recycling. Then, the heating process started, and the thickness was determined with the stamp forming method. The results obtained are given in Tables 5 and 6.

Table 5 Flexural testing results of the virgin CF/PEKK

Table 6 Flexural testing results of recycled CF/PEKK

Force–displacement curves were obtained as shown in Figs. 8 and 9. As a result of the bending test performed with the test coupons prepared from the virgin composite plate, the average flexural modulus was found to be 49.15 GPa. When the same test was repeated for the plate produced using mechanical recycling methods, the flexural modulus was found to be 45.57 GPa. One of the main reasons for the 7.28% decrease can be named as the loss of continuity of fibers during mechanical recycling. In addition, the thickness and fiber ratio was not constant at every point of the recycled plate. This causes a decrease in strength.

Fig. 8

Force–displacement diagrams of the virgin CF/PEKK

Fig. 9

Force–displacement diagrams of the recycled CF/PEKK

Physicochemical tests were performed to characterize both physical and chemical properties of the polymer. Within the scope of this study, the DMA and DSC tests were applied to the composite coupon with TA instruments. The results obtained in this context are summarized in detail in the following sections. In the DSC, on the other hand, measures the amount of energy absorbed or released when the sample is heated, cooled or kept at a constant temperature. Thermogravimetric Analysis (TGA) measures the mass change of the sample as a function of temperature or time. Bhuyan et al. [21] evaluated the results of the TGA analysis along with the DSC analysis. They concluded that the TGA graph which were obtained reflects the stability at normal temperature and pressure. In an another study, thermal stability was analyzed with the TGA and the DSC thermograms and it was found that the hydrogels were thermally stable up to 290 °C [22]

When a load is applied to the polymer material, it deforms. If it is an ideally elastic material, the deformation disappears when the load is removed. Elastic deformation occurs in proportion to the applied load. However, in polymers, deformation does not occur linearly. It exhibits a viscous behavior. The deformation rate is directly proportional to the load. Some materials, such as polymers, exhibit both elastic and viscous behavior, and such materials are called viscoelastic materials [23]. Viscoelastic properties of materials are measured by using a load motor that increases and decreases at a frequency determined by the materials with the DMA test. Knowledge acquired through the strategic use of three parameters of force, temperature, and time provides the basis for predicting the polymer performance in real-world applications. A graph of the storage modulus and loss modulus of recycled and virgin material temperature given in Figs. 10 and 11 provides a schematic graphical representation of the elasticity.

Fig. 10

Storage modulus-temperature of the recycled CF/PEKK

Fig. 11

Storage modulus-temperature of the virgin CF/PEKK

The test specimen was placed in the thermal chamber between movable and fixed fixtures. Frequency, amplitude, and temperature range suitable for the tested material were entered. The analyzer was moved slowly over the specified temperature range and voltage oscillation was applied to the test sample. As a result, T_g loss and T_g onset values were determined. Table 7 provides results for the recycled and virgin materials.

Table 7 Result of the dynamic mechanical analysis

The glass transition temperature is one of the main distinguishing features of polymeric materials. It is defined as the temperature limit where the substance loses its glassy property and gains viscosity. When energy (heat) is given to polymer materials, they expand, and the attraction force decreases as the temperature increases. The temperature reaches such a point that the entire chain structure in the structure is activated. The temperature at which this chain structure is activated is the glass transition temperature. The molecular structure of the material is determined by how the unit structures are connected to each other. These are some isomeric structures in addition to linear, branched, cross-linked, and network structures. Most linear polymers and branched polymers with flexible chain structures are thermoplastics. Such polymers are generally produced by methods in which heat and pressure are applied simultaneously. Therefore, branching occurs with the effect of heat and pressure applied [24].

In line with the results obtained, the T_g value of the recycled thermoplastic composites decreased by 2.83% compared to the T_g value of the virgin thermoplastic composites. The reductions in T_g values are relatively minor. These variations can be influenced by the testing environment and the individual conducting the test. This reduction is visible even within test samples obtained on a test plate.

The DSC test provides information on the degree of crystallinity of the recycled and the virgin CF/PEKK composite plates, which is also correlated with the brittleness, toughness, hardness or modulus, optical clarity, creep or cold flow, barrier resistance, and long-term stability. The sample was heated twice in the DSC in order to destroy the thermal history of the polymer. However, only the second heating was considered for data analysis. The test procedure was applied as follows. First, the test sample was heated to 50 °C at a rate of 10 °C/min, and the sample was stabilized at 50 °C for 5 min. The sample was then reheated to 400 °C at a rate of 10 °C/min. Finally, it was cooled down to 50 °C at a rate of 10 °C/min. These processes were applied during the first thermal cycle. It was then subjected to the second thermal cycle. Immediately after the first thermal cycle without turning on the system, it was restarted under a dry inert gas atmosphere at 10 °C/min to 400 °C. The test sample, whose entire thermal cycle was completed, was cooled to room temperature (RT) at a rate of 10 °C/min. Since the first cycle material may have internal stresses or inclusions which are undesired effects that determine the existence of crystalline phases in the polymer on the DSC. In this study, peak crystallization temperatures (T_c) during the cooling values, peak melting temperatures (T_m) during heating values, as well as the associated enthalpies were represented in the graphs belonging to the DSC schematically as shown in Figs. 12 and 13.

Fig. 12

The DSC graph of the recycled CF/PEKK

Fig. 13

The DSC graphs of the virgin CF/PEKK

According to these graphs, the concept of enthalpy and melting temperature were obtained as an endothermic peak on the DSC trace. In the curve, the melting temperature for the recycled CF was found to be 337.91 °C while 337 °C is stated in the manufacturer's technical data sheet. The degree of crystallinity values was calculated based on the theoretical melt for 100% crystalline polymers considering the fiber content by weight using Eq. 3.

$${X}_{\text{c}}=\frac{\Delta { H}_{\text{m}}}{\Delta { H}_{\text{f }}(1-{\alpha })}\text{x }100{\%}$$

(3)

where the parameters are described as follows: ${X}_{c}$ is the degree of crystallinity. $\Delta {H}_{m}$ is enthalpy of fusion measured at the melting point, ${T}_{m}$. ${H}_{f}$ is enthalpy of fusion of completely crystalline polymer. $\alpha$ is the weight content of fiber in the composites.

In this equation, heats of melting and cold crystallization are in J/g. The term ∆H_m° is a reference value and represents the melting point of the polymer if it is 100% crystalline. According to Chang et al. [25], the melting temperature of PEKK, which is considered as a 100% crystal, was calculated as approximately 130 J/g. Using this datum, the degree of crystallinity was calculated. The results obtained are 16.9, 17.1, 17.1, 20.1, 15.97, 15.76%, respectively, for the degree of crystallinity of the recycled CF/PEKK material. The mean value was calculated as 17.15%. The results obtained are 19.5, 19.52, 20.58, 24.79, 23.84, and 21.06%, respectively, for the virgin CF/PEKK material. The mean value was calculated as 21.54%. Hence, the degree of crystallinity decreases for the recycled composite since the polymer chains might be aligned more regularly in the virgin composite leading to higher long-range order. Note that this results are in good agreement with the decreased glass transition temperature.

The above cases are an alternative way of obtaining melting temperature and crystallinity with the DSC test, which provides relatively rapid results on polymeric composite materials for the aerospace industry. The porosity or void ratio is defined as a measure of voids in a material's structure. It expresses the mass difference of samples before and after the extraction of the resin using sulfuric acid. In such manner, the difference between weights can be defined as void ratio. Determining whether a composite part contains voids can be determined by various ultrasonic methods. Inspection reference panel (IRP) production ensures that the void ratio in the structure is produced within acceptable limits. For thermoset composites, this value drops to 2–5% in primary air structures. For non-destructive testing methods (such as ultrasonic scanning or manual pulse echo), scanning was done with reference to the IRP panel. Results were compared accordingly, and volume as well as porosity ratio in the structure were determined. Correspondingly, it was confirmed if the properties of the composite are within acceptable limits.

The EN 2564 standard was taken as a reference in order to determine the void ratio in the structure. Volume voids can be found if the volume ratio of fibers and matrix is known. The density of the resin in the structure is 1.3 g/cm³, while the density of the fiber is 1.85 g/cm³. While determining the porosity or void ratio, the resin content by volume is calculated according to Eqs. 4 and 5

$${V}_{r}=\left(100-{W}_{r}\right)x\frac{{\rho }_{c}}{{\rho }_{r}}$$

(4)

where ${V}_{r}$ is the resin content as a percentage of the initial volume. ${W}_{r}$ is the fibre content as a percentage of the initial mass. ${\rho }_{c}$ is the specimen density expressed. ${\rho }_{r}$ is the density of the cured resin.

The void content by volume is calculated according to Eq. 5.

$${V}_{o}=100-[ {W}_{r}\frac{{\rho }_{c}}{{\rho }_{f}}+\left(100-{W}_{r}\right)\frac{{\rho }_{c}}{{\rho }_{r}}]$$

(5)

${V}_{o}$ is the void content as a percentage of the initial volume. ${W}_{r}$ is the fiber content as a percentage of the initial mass. ${\rho }_{f}$ is the fiber density. ${\rho }_{c}$ is the specimen density. ${\rho }_{r}$ is the density of the cured resin.

According to the given formula, values in Table 8 were obtained. For the test sample, six test coupons of 20 mm length, 10 mm width, and 2 mm thickness were cut by a water jet machine. Three test coupons were cut from the recycled thermoplastic composite plate and three from the virgin thermoplastic composite plate. For this test, test coupons were obtained from different points on the plate. When the virgin thermoplastic composite plates in the structure were compared among themselves, the average fiber ratio was 60.17%, while the resin ratio was 35.51%. When a similar situation was compared for test coupons obtained from the recycled thermoplastic composite plate, the fiber ratio is 55.92% and the resin ratio is 40.40%. When the recycled and the virgin thermoplastic composite plates were compared, it can be seen that a 7.04% decrease in fiber for the virgin thermoplastic composite plate and 11.77% increase in resin in comparison with the recycled thermoplastic composite plate occurs.

Table 8 Result of the fiber volume content

The decrease in fiber content in the structure of the thermoplastic CF/PEKK composite occurred during the grinding and screening process of mechanical recycling. During the recycling process, fibers less than 25 mm passed through the sieve. In this state the fiber content in the structure decreased. As a result, void ratios in the structure were calculated as 3.68% in the recycled thermoplastic composite plates and 4.33% in the virgin thermoplastic composite plates. This difference between virgin and recycled thermoplastic composite plate may be related to their manufacturing processes since the consolidation method of the virgin thermoplastic composite plate was carried out in an autoclave. In this method, the pressure inside the structure was 7.1 bar. However, the consolidation process of the recycled thermoplastic composite plate was carried out with a hot press. The pressure applied in this method was about 15 bar. Therefore, it can be expected that the void ratio in the structure is decreased with the increase in the applied pressure.

With a SEM, high-energy electrons interact with the sample to generate electron and photon signals which reveals the morphology of materials. The pores in the structure, fiber orientation, and fiber matrix adhesion were investigated with SEM images. Electrons are scattered in different areas and gathered by the z detector, and images are obtained as a result of processing the collected signals with the microscope software. In order to evaluate and observe changes in the structure, images were taken from fracture surfaces of virgin and recycled thermoplastic composite plates with the SEM by using HITACHI instruments. Hence, the structure of fibers and the resin after the transformation process was examined.

Firstly, this process was applied to test coupons obtained from the virgin thermoplastic composite plate. The orientations of the fibers in the virgin thermoplastic composite plate were suitable for the desired orientation in the design as seen in Figs. 14 and 15. Angular deviations were within tolerances. Since the fibers were not subject to mechanical disruption, fibers had continuity properties.

Fig. 14

The SEM image taken at the fracture region for the virgin CF/PEKK-1

Fig. 15

The SEM image taken at the fracture region for the virgin CF/PEKK-2

For a different test coupon, this procedure was repeated. The findings depicted in this image indicate a consistent fiber orientation. In simpler terms, it demonstrates that there were no alterations in the angles of the fibers within the structure. In addition, there were resins of similar thickness around the fibers. The main reason for this, is the use of resin-impregnated plies. Thus, almost the same amount of resin occurs at every point. It ensures that the strength in the structure is similar at every point. The average thickness of the fibers was measured as 7.35 nm. Figure 16 shows results obtained by SEM.

Fig. 16

The SEM image taken at the fracture region for the virgin CF/PEKK-3

The edge of the different test coupon was also imaged using the SEM. According to these results, porosity formed on the surface was negligible. During the consolidation, air bubbles formed on the coupon surface were completely evacuated by applying vacuum. In the sample taken from a different point, the orientation and continuity of the fibers were preserved.

The whole process was repeated for the recycled CF/PEKK material. Dimensional sieve analysis was applied for waste composites that had undergone mechanical degradation. Therefore, fibers with a size of about 1 in² are distinguished. Obtained images are given in Figs. 17 and 18. The figures reveal that the orientations of the fibers differ due to the irregular placement of the fibers. This situation causes decrease in the mechanical properties compared to the virgin CF/PEKK material.

Fig. 17

The SEM image taken at the fracture region for the virgin CF/PEKK-4

Fig. 18

The SEM image taken at the fracture region for the recycled CF/PEKK-1

The SEM images obtained from the recycled thermoplastic composite plate are given in Fig. 19. First, they were taken from the surface of the part and pin holes were seen on the surface. It was observed that pinholes were formed because the air bubbles on the surface were not removed from the structure.

Fig. 19

Pin holes on the surface

Figure 20 indicates that there are some fractures on the surface. When evaluated in detail, the orientation of the fibers was completely lost during recycling. During the hot press application after the mechanical transformation, the distribution of the resin was not evenly in each region. This situation leads to the fact that there cannot be the same resin and fiber ratio at every region.

Fig. 20

Region of the fracture

According to the results obtained from the recycled CF/PEKK plate, angular deviations were observed in the fibers in the structure. Angular deviations have affected the mechanical properties of the part. The reductions in mechanical properties have occurred due to angular deviations. Because fibers provide the transmission of the load, fibers that have lost their angular orientation and continuity characteristics cannot fully fulfill this task. Corresponding angular deviations are shown in Fig. 21. Since the fragmented structure of certain dimensions was used by sieving waste materials during mechanical fragmentation, there were parts that maintain its continuity properties in 25 mm length.

Fig. 21

Angular deviation

According to images taken from the SEM, the CF/PEKK plate, which was structurally produced to simulate the properties of a primary structural part, differed from the recycled CF/PEKK. The reason for the situation was applied to the mechanical recycling process. Plates suitable for continuity properties and angular tolerance could be obtained using chemical and thermal recycling methods.

Composite materials with polymer matrix, which stand out with their low-density feature, are the most preferred composite material class. It is possible to obtain composite materials with superior mechanical properties by using thermoplastics. PEKK shows mechanical and physicochemical properties suitable for use in the aerospace industry. Ongoing studies show that PEKK can be used in the production of primary structural parts. Mechanical and physicochemical properties of CF/PEKK thermoplastic composite plates, which are currently being reviewed for use in the aviation industry, have been examined. Within this regard, the recyclable CF/PEKK composites were compared with the virgin CF/PEKK material.

During the flexural test, the material was exposed to both compression and tensile forces while the test specimen bends under the influence of the force applied from above. Since the continuity of the fibers is lost after mechanical disintegration, test results obtained from the virgin thermoplastic composite plate have resulted in higher strength results in comparison with the recycled thermoplastic composite plates that have completely lost their continuity properties. In the ILSS test, virgin thermoplastic composites were found to give higher values than the recycled thermoplastic composites. The average ILSS was measured as 81.8 MPa. In the recycled thermoplastic composites, the ILSS was recorded as 64.7 MPa. The ILSS test includes the interfacial bond strength of fiber and matrix, the interlaminar bond strength, and mechanical properties of the matrix. Although the same reinforcement material is chosen in both composite structures, the continuity properties of the fibers are different as a result of mechanical fragmentation. During its mechanical breakdown, some of the reinforcement material in the structure became waste. Therefore, the proportion of reinforcement material in the structure also changed. Tests also revealed that there is more resin in the virgin thermoplastic material in contrast to the recycled material. During consolidation, the adhesion strength between the layers during the autoclave process and hot press application also differed. However, the expected result was that there would be a positive interaction, but this result could not be obtained due to the decrease in the amount of resin in the structure. Thermal changes in the structure of the material were obtained by the DMA and the DSC tests. The change in the T_g value of the recycled and the virgin CF/PEKK material may have been effective because the sample was produced using different production methods, and tested including test coupons with different masses. However, deviations in the results were mainly negligible. In addition, crystallization degrees were determined with results obtained from untouched and recycled plates. This value averages at 17.15% for the recycled CF/PEKK and 21.54% for the virgin CF/PEKK. In addition, the porosity/void ratio in the building was also examined. While it was 3.68% in recycled thermoplastic composite plates, it was calculated as 4.33% in untreated thermoplastic composite plates.

According to the images obtained by the SEM, the continuity properties and orientations of the virgin CF/PEKK material were preserved. However, the same was not the case for the recycled CF/PEKK material. Because the continuity properties of the fibers were lost during mechanical disintegration and chopped fibers were obtained, orientations were differentiated during hot press forming and recycling. Therefore, mechanical test results from the virgin plate gave enhanced results in contrast to the recycled plate. However, recycled thermoplastic composites may still be used in secondary and cosmetic aero structures.

Conclusions

In this study, recycling of carbon fiber reinforced PEKK thermoplastic composite materials was evaluated. Comparisons between unprocessed or virgin material and recycled materials were made by bending and interlaminar shear strength (ILSS) tests.

According to the flexural test results of the virgin and recycled thermoplastic composite plates, the virgin thermoplastic composite showed 68.05% higher flexural strength and 7.85% higher flexural modulus compared to the recycled thermoplastic composites. While the tests of virgin thermoplastic composites were carried out on six samples out of one set, the average strength value in the sets was 879.92 MPa, and the average modulus value was 49.15 GPa. In six recycled thermoplastic composite samples out of one set, the average flexural strength value was recorded as 523.6 MPa, and the average flexural modulus value was 45.57 GPa.