Abstract
Today’s industries are increasingly demanding high performance materials as substitutes for conventional materials. Polymer matrix composites (PMCs) can frequently be these substitutes due to their versatility and sublime performance. The combinations of these components result in unique properties which cannot be achieved with a single material. Carbon fiber (CF) reinforced PMCs are lightweight with high performance and have replaced traditional materials in many applications. The purpose of this project was to prepare a CF-PMC using recycled carbon fiber (RCF) recovered from CF-PMC components after their useful life time and to compare the properties to a virgin CF-PMC. Polyetheretherketone (PEEK) is an aerospace-grade polymer that is now commonly reinforced with carbon fiber. In this work, RCF-PEEK and CF-PEEK PMCs were prepared using an injection molding machine that imparts uniform, high shear to provide superb mixing, and the morphology, mechanical properties, thermal properties, and electrical conductivity were characterized. Mechanical property results reveal that RCF-PEEK PMCs have a higher flexural modulus but lower impact strength than CF-PEEK at similar concentration. Morphology images indicate uniform distribution and strong fiber-matrix interaction for both RCF and CF in PEEK. Thermal property results showed that CF-PEEK and RCF-PEEK PMCs are similar in their transition temperatures. Evaluation of electrical conductivity data reveal that CF-PEEK can be used as an insulating material and RCF-PEEK can be used as a semiconducting material. This comprehensive analysis of RCF-PEEK PMCs is beneficial in order to develop the full potential of PEEK and its composites for future applications in materials science. Overall, RCFs can be used in certain applications with similar effectiveness to virgin CFs. Optimization of carbon fiber recycling will lead to the RCF becoming more and more prevalent and practical for industrial applications.
1. INTRODUCTION
Materials science is an interdisciplinary field involving the design and discovery of new materials, along with determining unique properties which can open up materials for additional uses. The research of characterizing PMCs is significant because it could potentially indicate novel uses for the composites. Compared to traditional materials, PMCs have displayed enhanced properties such as high stiffness and transverse rupture strength, while still maintaining a low weight
[1].Due to their unique set of properties, PMCs have been studied for many possible applications, such as beverages, electronics, and automobiles [2] [3] [4]. Composites allow for two dissimilar materials to be combined while optimizing beneficial properties from each component. Creating CF-PMCs enables scientists to provide an alternative to traditional materials in industries such as healthcare, civil infrastructure, and renewable energy. Following the creation of structural carbon and glass fibers, there has been a continuous increase in the utilization of PMCs for structural purposes. In many cases, the specific mechanical properties of the composite can be used to yield better structural efficiency in comparison to common metallic structures [5].
2. BACKGROUND
PMCs have great friction and wear performance to counter-act the thinning, cracking, and debonding of fibers [6]. This has made them an ideal substance to be used in many industrial applications [7]. PMCs are divided into reinforced plastics and advanced composites, which usually differ in terms of strength and stiffness [8]. Also, they are more amenable for near-net shape manufacturing of intricately shaped products. Despite widespread employment and research, conventional PMCs still demand high concentrations of reinforcement phase in order to achieve the desired mechanical properties, principally stiffness [9].
CF are light-weight, strong, and stiff, making them an appealing candidate for polymer reinforcement [10]. CF have the highest specific modulus and specific strength of all reinforcing fibers [6]. The demand for CF has grown annually at a rate of 12.5% since 2012, and the global market is valued at over $1.7 billion. The biggest demand comes from the aerospace, wind energy, and automotive industries [11].
Polyetheretherketone (PEEK) is an aromatic engineering polymer which is used in very demanding application environments due to its excellent mechanical properties, thermal stability, and chemical resistance. PEEK has been studied for uses in power and energy generation and is increasingly being used in biomedical applications such as fracture fixation and artificial joints because of its biocompatibility, low density, and similarities to human bone [12] [13] [14]. CF-PEEK offers high compressive and tensile strength and high modulus [10]. Thus, it has a wide collection of industrial applications [6].
New technological advancements have allowed recycling of the CF component from CF-PMCs. Pyrolysis is used to burn off the polymer at elevated temperatures in an inert environment, leaving only carbon fiber, or RCF. The RCF may be reused for a limited amount of cycles [15]. Carbon fiber recycling is rather scarce: glass fiber recycling constitutes the overwhelming majority (approximately 98% in weight) of all recycled composites [16]. Reclaiming CF is beneficial because CF is expensive and very energy-intensive to produce. RCF is less expensive and requires less energy to produce than CF.
Recently, thermoplastic composites have been used to a greater extent to replace metals in many industrial, transport, and sporting applications [16]. Thermoplastics are polymers that can be remelted without changing their properties unlike thermosets which are cross-linked and unable to be remolded. Since carbon fiber reinforced composites create waste and exhaust finite resources, there is a growing interest in developing industrial scale solutions which are environmentally sustainable [17]. Furthermore, stricter environmental legislation, specifically within the European Union, has forced companies to take major strides in reducing their overall environmental footprint.
However, recycling CF reinforced composites has proven to be a difficult challenge. CF is regarded as higher quality than RCF, and there is a lack of certainty in the performance of RCF in new PMCs, which has caused a reluctance in market adoption [18]. In previous research, PMCs created with RCF exhibited weaker mechanical properties because RCF are usually a mixture of different grades that may be reprocessed collectively [16]. Further, when RCFs are randomly distributed throughout the PMC, the property improvements are reduced making them inappropriate in highly structural applications. Properties of RCFs are greatly influenced by fiber length. The probability of defects increases for longer fibers, making longer CFs more fragile and thus limiting their utility. RCFs offer many advantages but must be studied extensively prior to adaptability and market acceptance.
3. METHODS
3.1 Materials
RCF and CF (Toho Tenax) were mixed with PEEK (Solvay KT 820 NT) to prepare RCF-PEEK and CF-PEEK PMCs. Carbon fibers are composed of long carbon fiber chains (Fig. 1)and are obtained from either petroleum pitch or polyacry-lonitrile (PAN) [19]. Compared to other fibers such as glass, they are more resistant to stress corrosion and stress rupture failures [15]. CF offers a stiffness-to-weight ratio higher than that of steel by virtue of its rigid polymer backbone. When used to reinforce polymers, CFs improve chemical resistance, temperature tolerance, tribological performance, and mechanical properties of polymers [6].
Fig. 1. Carbon Fiber Structure [20]
PEEK is a rigid, aerospace-grade, semi-crystalline aromatic thermoplastic polymer [7]. The excellent mechanical properties, thermal stability, and chemical resistance of PEEK can be attributed to its relatively stiff aromatic polymer backbone (Fig. 2). Because of this, PEEK has been used in various engineering fields for many years. First commercialized in 1980, PEEK is currently the most widely used aromatic polyketone with an elastic modulus of 3.5 GPa and tensile strength of approximately 100 MPa [21].
Fig. 2. Polyetheretherketone Molecule [22]
3.2 Sample Preparation
RCF-PEEK and CF-PEEK PMCs were prepared at 0, 10, and 20 wt. % CF in PEEK and 0, 10, 20, and 30 wt % RCF in PEEK using a Negri Boss V55-200 injection molding machine (Fig. 3) with a novel design to impart uniform, high shear for good mixing. The sample information and labels are in Tables 1 and 2. Injection molding is a commonly used method to mold thermoplastics [23]. It involves a cyclic process of melting plastic, conveying molten plastic through the barrel, forcing the molten plastic through the nozzle under pressure, filling the mold, cooling, and ejecting the molded part. The processing temperatures and pressures varied for each concentration. After processing, specimens were stored at room temperature prior to mechanical and thermal testing.
TABLE I VIRGIN CF-PEEK SAMPLE COMPOSITIONS
TABLE II RECYCLED CF-PEEK SAMPLE COMPOSITIONS
Fig. 3. Negri Bossi V55/200 Injection Molder.
3.3 Flexural Testing
Mechanical properties are physical properties that materials exert when a force is applied on them. There has been an increasing interest in mechanical properties of PEEK over the past 20 years [24]. Establishing the mechanical performance of PEEK and fiber reinforced PEEK composites will be valuable in characterizing the usefulness of a material under different conditions, including those found in medical practices [25].
A three-point flexural test was used to determine the flexural strength and flexural modulus of the samples, according to ASTM D 790 using a Mechanical Testing System QTest/25 (Fig. 4) [26]. The flexural strength is the maximum stress attained during a flexural test. The flexural modulus indicates the indicates the material stiffness and is measured by the slope of the stress-strain curve at the beginning of the test in the linear region. Five specimens per sample were tested and data averaged. The width (b) and thickness (d) of the specimens were measured using a caliper. The support span-to-depth ratio must be controlled in the test in order to minimize the shear stress exerted on the specimen and must be 16:1, according to ASTM D790. The ASTM International standards is an organization that creates a standard for a wide range of applications from systems to services. ASTM D790 is used to test the flexural properties of plastics. The span length (L) and crosshead rate (R) were calculated using the following equations:
After the samples were prepared, the rectangular cross section of the specimen was put on the two supports of the flex machine with the midpoint in the center. The supports were separated to the calculated span length. A preload between 2 and 5 N was put on the sample and then the machine was rezeroed. The specimen was deflected until a rupture occurred or maximum strain of 5% was reached. The stress-strain curve, flexural modulus, and flexural strength for each sample were recorded.
Fig. 4. Three-Point Mechanical Testing System QTest/25 Flexural Test.
3.4 Impact Testing
Impact testing is a mechanical test method that measures the ability of a material to resist high-rate loading and absorb energy. It helps to assess doubts about the effects of collision damage on the structural stability of a material [5]. The impact energy measures how much energy is absorbed during a dynamic impact. The dimensions of the specimens tested were 3.2 mm x 12.7 mm x 63.5 mm. Specimens were first notched using a Dremel notching tool (Fig. 5). Notching is a method used to cut a V-shaped indentation in a specimen. The Izod impact strength was measured on notched specimens using the Instron Dynatup POE 2000 (Fig. 6), according to ASTM D256. After the impact from the pendulum, the net energy absorbed by the material was calculated by subtracting energy left over from the starting energy of the pendulum. Ten specimens per sample were tested and data was averaged. The Izod impact strength is reported from the data.
Fig. 5. Notching Machine.
Fig. 6. Instron Dynatup POE 2000 Impact Tester.
The DSC results produced data on the temperature and energy of thermal transitions, including glass transition, melting temperature, crystallization temperature, heat of fusion and heat of crystallization.
3.5 Differential Scanning Calorimetry
Thermal properties of RCF-PEEK and CF-PEEK were measured using a TA Instruments Q 1000 Differential Scanning Calorimeter (DSC) (Fig. 7). DSC measures temperatures and heat flows of a material in a controlled atmosphere. It provides both qualitative and quantitative information about the changes that the material undergoes during heat change.
In preparation for DSC, two specimens per each concentration of RCF-PEEK and CF-PEEK were first cut using a SKIL 3386 Band Saw. Each specimen was cut into thin rectangles of thickness 1 mm and width 3.5 mm. The samples were weighed using a Mettler AE 240 High Precision Scale and cut with a razor blade to be 10 mg with a 0.5 mg margin of error. The DSC specimens were placed in standard aluminum pans using tweezers and enclosed using the T-Zero Press. The conventional DSC heat/cool/reheat method was used starting at room temperature, heating to 400 C, cooling to 0 C, then heating to 400 C at a rate of 10 C/min for each step. The reheat step of DSC is crucial to the data because it counteracts the potential thermal history of the polymer when it is made.
Fig. 7. TA Instruments Q1000 Differential Scanning Calorimeter.
3.6 Scanning Electron Microscopy
Scanning Electron Microscopy (SEM) is a characterization method used to take high resolution pictures of specimens in order to determine the resulting morphology (i.e. fiber-matrix interaction, fiber distribution within the polymer, etc). SEM specimens were prepared by the cold fracture of molded specimens using liquid nitrogen. The samples were cut into small 1-2 cm samples which were doused in liquid nitrogen and broken parallel to flow. The flow direction is the direction the materials streamed into the mold during injection molding. The fractured surfaces were mounted on aluminum studs, gold coated to a thickness of 5 nm, and placed under a vacuum overnight prior to observation. A ZEISS SUPRA 40 Field Emission Scanning Electron Microscope (Fig. 8) was used with both in-lens and secondary electron detectors to observe dispersion and distribution of RCF or CF within PEEK and RCF or CF particle-matrix interactions. Pictures of the specimens were taken at various magnifications (500X, 1000X, 2000X). Accelerating voltages of 5 keV to 20 keV were used for virgin and recycled CF-PEEK observations, respectively.
Fig. 8. ZEISS SUPRA 40 Field Emission Scanning Electron Microscope.
3.7 Electrical Conductivity
Electrical conductivity is a characterization method in which the rate that an electrical current can pass through a composite is measured to calculate resistivity and conductivity. Resistivity and conductivity can be used to indirectly predict many properties of materials.
Electrical conductivity was measured in accordance with ASTM D257 for conductance of insulating materials. Testing occurred at ambient temperature using the Keithley 2450 SourceMeter connected to a computer with KickStart software. Using the IV-Characterizer, the sample current at each voltage was recorded when a linear voltage sweep from 0-100 volts was applied to the specimen. The samples were placed in the chamber and secured in place between two strip or guard electrodes using glass rectangles. The test chamber was locked to control outside factors (e.g. humidity) which could affect results. A rubber stopper was placed directly above the top guard electrode to prevent the sample from moving during testing. Preparation for electrical conductivity testing entailed both the top and bottom surface of each sample being covered with conductive silver paint. The silver paint is also porous enough for moisture to diffuse through them so that the specimen can be conditioned after the testing. This minimizes the effect of humidity. After the test was started, the data logger would stream readings directly to the PC for safe archival and exportation. The conductivity ( 𝞼v) was determined with the following equation involving area (A), thickness (t), and volume resistance (Rv) for a given specimen:
4. RESULTS
4.1 Flexural Mechanical Properties
Flexural stress-strain curves for PEEK and RCF-PEEK samples appear in Fig. 9 and show increasing flexural modulus and flexural strength with increasing RCF concentration, as is typical for CF-PEEK PMCs. Flexural stress-strain curves for PEEK, RCF-PEEK, and CF-PEEK appear in Fig. 10. RCF-PEEK samples at the same concentration as CF-PEEK samples display higher flexural modulus and flexural strength. The RCF-PEEK and CF-PEEK specimens fractured prior to 5 % strain, while PEEK specimens did not fracture. The flexural modulus of 10 and 20 wt. % CF-PEEK was 6.60 GPa and 9.19 GPa, whereas the modulus of 10 and 20 wt. % RCF-PEEK was 7.82 GPa and 11.59 GPa, respectively. The increase in flexural modulus and flexural strength between RCF-PEEK and CF-PEEK was more pronounced at higher concentrations of 20 wt. % CF. Thus, RCF-PEEK PMCs have higher modulus and strength than CF-PEEK PMCs, suggesting better load transfer from RCF to PEEK than CF to PEEK during flexural loading.
4.2 Izod Impact Strength
Impact data revealed that CF-PEEK samples have higher impact strength than RCF-PEEK samples (Fig. 11). CF-PEEK samples absorbed more energy upon impact, resulting in a hinge fracture type (H), while RCF-PEEK samples suffered complete fracture (C). A hinge fracture occurs when crack propagation loses energy prior to reaching the opposite side of the specimen, indicating superior toughness.
Fig. 9. Flexural Stress-Strain of RCF-PEEK.
Fig. 10. Flexural Stress-Strain of RCF-PEEK and CF-PEEK.
Fig. 11. Impact Strength of RCF-PEEK and CF-PEEK PMCs.
It is interesting to note that the CF-PEEK PMCs have higher impact strength than PEEK, and RCF-PEEK PMCs have equivalent or slightly higher impact strength than PEEK up until 30 wt. % RCF in PEEK. Since 30 wt. % CF-PEEK PMCs were not able to be processed for this work, a direct comparison between CF and RCF at 30 wt. % is not possible. The exemplary impact analysis results for CF-PEEK PMCs is attributed to the exceptional mixing during the novel injection molding process used in this work.
4.3 Thermal Properties
Thermal properties are associated with a composite’s reaction to a change in temperature. Temperature is an important characterization method to be studied due to its ability to reveal a material’s performance in different environmental factors. The melting temperature (Tm), crystallization temperature (Tc), and glass transition temperatures (Tg) are shown in the curves produced from DSC (Fig. 12).
A line is indicated on Fig. 12 to indicate the Tm, Tc, and Tg of PEEK. The peaks of the samples are generally in line with that of PEEK in Fig. 12a and 12b. This means that the melting temperatures in both the first heat and second heat of the recycled CF-PEEK samples are consistent with the melting temperatures of the virgin CF-PEEK samples for both fiber type and fiber concentration (Fig. 12a and b). The crystallization temperatures of the recycled CF-PEEK samples are similar to the crystallization temperatures of the virgin CF-PEEK samples in fiber type but vary slightly with the change in concentration of RCF and CF (Fig. 12c). However, this may not be a significant change because of only slight variations between the concentrations.
Fig. 12. Thermal Properties of RCF-PEEK and CF-PEEK (a) First Heat, (b) Second Heat, (c) Cooling.
4.4 Morphology
Morphology images of 20 wt. % CF-PEEK and 20 wt. % RCF-PEEK samples appear in Fig. 13 at various scales as the magnification increases from 500x, 1,000x, and 5,000x. Images at the 100 micron scale show that both CF and RCF are well distributed throughout the PEEK in a similar manner. There is strong fiber-matrix interaction in both RCF-PEEK and CF-PEEK samples, as PEEK wets the fibers well and adheres to the fiber surfaces. Fiber orientation in the flow direction is most notable in the RCF-PEEK sample. SEM images indicate that CF and RCF interact with PEEK in a similar manner.
Fig. 13. Recycled and Virgin CF-PEEK Samples with SEM.
4.5 Electrical Properties
Conductivity is used to predict the dissipation factor properties and dielectric breakdown (at low frequencies) of materials. Volume conductivity from 0 to 100 volts was calculated. The 30 RCF-PEEK L2 displayed slightly better conductivity with increasing voltage as shown in Fig. 14. At 100 Volts, the 30 RCF L1 demonstrated the highest conductivity at 0.22 S/m out of all samples. The 10 RCF-PEEK L2 and 20 CF-PEEK L1 showed slight increase in conductivity with increasing voltage. Conductivity of 20 RCF-PEEK increases exponentially. (Fig. 15). At 100 volts, 30 RCF-PEEK L1 had around eight times greater conductivity than CF-PEEK L1, 200,000 times greater conductivity than 20 RCF-PEEK L1, and 4,000,000 times greater than 20 CF-PEEK L1 (Fig. 16 and Fig. 17). Longer samples tended to have lower conductivity.
Due to the high resistivity of neat PEEK, only data from CF- Fig. 16. Conductivity at 100V. PEEK and RCF-PEEK samples was shown. Overall, short length and high weight percent RCFs in PEEK displayed superior conductive capabilities, as compared with CFs in PEEK. CF-PEEK and short-length samples are better suited for applications in electrical insulation.
Fig. 14. Voltage vs. Conductivity.
Fig. 15. Voltage vs. Conductivity.
Fig. 16. Conductivity at 100V.
Fig. 17. Conductivity at 100V.
5. CONCLUSIONS
RCF-PEEK and CF-PEEK were characterized through flexural testing, Izod impact testing, DSC, SEM, and electrical conductivity testing. The results successfully characterized the mechanical, thermal, structural, and electrical properties of pure PEEK, RCF-PEEK, and CF-PEEK.
The properties of RCF-PEEK and CF-PEEK are similar in terms of their high strength. The flexural test revealed that the flexural modulus of RCF-PEEK was greater than that of CF-PEEK. This means that RCF-PEEK is stiffer than CF-PEEK. According to the impact test, CF-PEEK is able to absorb more energy upon impact than RCF-PEEK. RCF-PEEK is more brittle than CF-PEEK because it was easier to break. In some applications, RCF-PEEK could be a feasible alternative. More concentrations of carbon fiber should be tested for further analysis of the relationship between virgin and recycled carbon fiber composites.
The DSC analysis showed that RCF-PEEK has similar thermal properties and behaviors compared to CF-PEEK. The transition temperatures of recycled CF-PEEK were, on average, consistent with virgin CF-PEEK. SEM was used to see how the carbon fibers interact with the polymer. The pictures revealed that fiber distribution and fiber-polymer interaction is similar in RCF-PEEK and CF-PEEK, indicating that the carbon fibers mixed similarly with PEEK. Electrical conductivity analysis revealed that CF-PEEK would be suitable for use as an electrical insulator. Based on the data, terse and high weight percent RCF-PEEK have potential applications in electronics as semiconductive elements.
Overall, RCF-PEEK can be used in similar applications to those of CF-PEEK. Carbon fiber can be effectively resourced in order to reinforce polymers. Properties of RCF-PEEK would be useful in many industries like healthcare, automotive, aerospace, and infrastructure. Using RCF would drastically decrease the cost of carbon fiber and increase its accessibility. It would also be environmentally sound because it reduces polymer waste and the use of non-renewable resources associated with polymer production.
Further research should be conducted to decrease the variability in the making of PMCs reinforced with RCF.
Direct comparisons between CF and RCF at 30 wt. % were not possible because 30 wt. % CF-PEEK PMCs were not able to be processed for this work. More concentrations of CF-PEEK and RCF-PEEK should be compared in future studies. For industries to increase usage of RCF-PMCs, more research must be done on their properties in certain circumstances. New innovations in the recycling of carbon fiber composites must try to preserve the enhanced characteristics of virgin carbon fiber. Additional research is needed to determine the extent of the disparities between CF and RCF PMCs.
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