Among the various types of inorganic and organic fibers, carbon fibers (CFs) show the highest tensile strength and elastic modulus at relatively low density [1
]. This is why CFs, despite their high cost, are widely investigated and applied. The most promising CF application is as a reinforcement for polymer based composites [1
]. CF reinforced polymer composites have been widely used as engineering materials in the aeronautic and automotive industries. Specific applications of these composites include body structure for electric vehicles [4
], wet clutches to distribute torque in vehicle drive-trains of automatic transmissions and limited slip differentials [5
], deployable space structures [6
], heat exchangers [7
], electromagnetic shielding [8
], ballistic protection [2
], and some other engineering areas. For example, in [10
], a family of novel high-strength, lightweight structural epoxy/CF composites with self-healing function were proposed for potential use in aerospace and aeronautical structures, sports utilities, etc.
The properties of CF reinforced polymer composites strongly depend on the fiber shape and location in the matrix. Polymer composites filled with short CFs are widely used as materials with good mechanical and tribological performance [3
]. The technology of the formation of such composites is relatively cheap and easy [16
], moreover, they are suitable for production by additive manufacturing [18
]. Short CFs can either be chaotically distributed in a polymer matrix or oriented; in the latter, composites show anisotropy in mechanical [18
], thermal [18
], and tribological [19
] properties. However, short CF filled polymers do not achieve the mechanical characteristics required for high-performance structural application in the aeronautic and automotive industries. Realization of the excellent mechanical properties of CFs in the polymer matrix can be more effectively achieved by using continuous fibers as reinforcement.
Continuous CF reinforced unidirectional composites are popular objects for modeling and theoretical approaches [20
]. Unidirectional composites allow for the realization of CF mechanical characteristics better than 2D ones [1
]. Unidirectional composites based on thermoplastics can be easily produced by simply using impregnation with polymer melt [22
]. Application of unidirectional composites is restricted to very specific areas [6
], however, such composites are promising as a semi-product in advanced additive manufacturing [23
Most of the investigation and structural applications of CF reinforced polymer composites is based on 2D CF fabrics. Among the carbon fabrics, the most commonly used as reinforcement in polymer composites are woven fabrics, whereas knitted and braided fabrics are used much less often [3
]. The number of weave structures that can be produced is practically unlimited, but in the investigations and applications, based structures such as plain, twill and satin are most commonly used. The weave structure of reinforcing woven CF fabrics can significantly affect the mechanical behavior of polymer composites. For instance, a comparison of epoxy based composites reinforced with CF fabrics showed that composites with twill weave fabrics show a higher shear modulus, shear strength, and ultimate strength than composites reinforced with satin weave CFs [24
]. Similar results were observed in [25
], where tensile tests showed that epoxy based composites reinforced with twill weave CF fabrics possessed higher magnitudes of strength, modulus, and strain than composites containing satin weave CFs. Mechanical tests, carried out with epoxy based composites, showed that the tensile properties of composites reinforced with twill weave CFs were more stable with an increase in the strain rate than the properties of composites with plain and satin weave CF fabrics [26
]. The investigation of polyetherimide based composites showed that composites containing twill weave CF fabrics showed better tensile and flexural properties than composites filled with plain and satin weave CFs; however, composites with twill weave CFs had good wear resistance properties in adhesive mode only, whereas in abrasive wear mode, better properties were observed for composites with satin weave CFs and in erosive wear mode both satin and plain weave CF filled composites shows better behavior than the twill weave reinforced one [27
]. On the other hand, in [28
], it was reported than epoxy composites filled with twill weave CFs showed lower tensile characteristics than composites reinforced with plain and satin weave CFs, both in static and dynamic test modes. Furthermore, twill weave CFs are often used as the model object in theoretical investigations [29
] and can be a suitable material for the elaboration of new polymer based composites.
For critical structures operating under high loads and elevated temperatures, composite materials with matrices based on thermoplastic polymers are actively beginning to be applied. Thermoplastic based composites show a considerably higher static fracture toughness compared to the thermoset (epoxy) composites [3
]. Important advantages of thermoplastics are their unlimited shelf life, low curing duration, maintainability (i.e., the ability to correct defects and damage by reheating), the possibility of reforming defective products [31
], high environmental resistance, and high chemical resistance including to aviation fuels and oils [1
In recent years, a number of thermoplastic based CF reinforced composites have been elaborated and investigated [32
]. Among the thermoplastics, high performance polymers are of particular interest due to their thermal stability and high mechanical properties. Polysulfone (PSU) has one of the highest service temperatures of all melt-processable high performance polymers. The high temperature nature of the PSU allows them to be used in demanding applications that other polymeric materials cannot satisfy. PSU is highly resistant to acids, alkali, and electrolyte materials, oxidizing agents, surfactants, and hydrocarbon oils. Its resistance to high temperatures allows for the use of PSU as a flame retardant material [33
PSU is frequently used as a modifier for the epoxy matrix for CF reinforced composites [34
]. In the 1990s, CF reinforced PSU was widely considered to be promising in medicine for arthoplastic applications, where composites filled with short [37
] or unidirectional [38
] CFs were mainly used in these investigations. In recent years, only few papers related to PSU based CF reinforced composites have appeared. In [41
], the laminated gradient PSU composites with the formation of layers by parallel laying of continuous CFs as materials for artificial intervertebral discs were elaborated. The authors in [42
] report on the formation of thermal conductive short CF filled PSU composites by injection molding. No data on the PSU composites reinforced with CF fabrics were observed in the literature.
The aim of the present paper was to study the effect of CF surface modification by thermal oxidation on the mechanical and thermal properties of the polysulfone based composites reinforced with carbon fabrics. Whereas polymer melt impregnation, as was shown in [22
], is a suitable method for PSU based unidirectional CF composites, in the case of the less permeable structure of CF fabrics, this technique is not effective. Recently, [43
] suggested using a PSU solution impregnation method to obtain CF fabric reinforced PSU based composites. In the present study, we applied the PSU solution impregnation method to produce composites reinforced with CF twill weave fabrics.
2. Materials and Methods
2.1. Preparation of Carbon Fiber/Polysulfone Composites
Ultrason S2010 (BASF, Ludwigshafen, Germany) PSU and twill weave fabrics 3K-1200-200 (HC Composite, Moscow, Russia) based on high-modulus carbon fibers were used as the raw materials. Obtaining the composites was carried out in several stages including obtaining a polysulfone solution, impregnation of the CF with the solution; and drying, which resulted in prepregs (pre-impregnated fabrics). The prepregs were further cut into the desired shapes and molded by compression molding into the final composites. Figure 1
shows the scheme of the used process.
To obtain the PSU solutions, N-methyl-2-pyrrolidone (Eastchem, Jiangsu, China) was used as a solvent, and a 20 wt % PSU solution was formed using a magnetic stirrer. After impregnation, the carbon fabrics were dried at 100 °C. CF/PSU composites with various fiber to polymer ratios were produced: 50 wt % of the CF and 50 wt % of the PSU (denoted as 50/50); 60 wt % of the CF and 40 wt % of the PSU (60/40); and 70 wt % of the CF and 30 wt % of the PSU (70/30). These prepregs were stacked in the mold carefully to avoid misalignment. The stainless steel mold was previously coated with the mold release agent. During compression molding, the mold was heated to attain the temperature of 340 °C. The prepregs were compression molded at the above-mentioned temperature under the pressure of 10 MPa. Thereafter, samples were cooled under applied pressure. The specimens for investigation were cut from the composites in accordance with the standards for mechanical testing.
2.2. Carbon Fibers Surface Modification
To improve the fiber–matrix interfacial interaction, surface modification of the CF by thermal oxidation (TO) in an air atmosphere at 300, 400, and 500 °C was applied and the fibers denoted as TO 300, TO 400, and TO 500 °C, respectively. The muffle furnace was heated up to the desired temperature, then the fibers were loaded into the furnace and held for 30 min. The fiber to polymer ratio of 60/40 was used for all of the modified fiber reinforced composites.
2.3. Thermogravimetric Analysis
To study the kinetics of solvent removal, thermogravimetric analysis (TGA) was performed using the TA Instruments Q600 system (TA Instruments, New Castle, DE, USA). During the tests, the polymer solution was heated at a rate of 10 °C/min to various temperatures of isothermal exposure (80, 100, and 120 °C); after reaching the isotherm, air blowing was started. The total measurement time was 3 h. The weight of the samples of the studied materials was chosen as close as possible to each other and varied in the range of 20–25 mg.
2.4. Fourier-Transform Infrared Spectroscopy Analysis
The FTIR analysis was carried out using a Nicolet 380 spectrometer (Thermo Scientific, Waltham, MA, USA) with spectral range of 3750–650 cm−1 and resolution of 1 cm−1.
2.5. X-ray Photoelectron Spectroscopy
The x-ray photoelectron spectroscopy (XPS) studies of the CF were carried out using a ULVAC-PHI VersaProbe II spectrometer (ULVAC-PHI, Inc., Chigasaki, Kanagawa, Japan) with monochromatic Al Kα radiation (hν = 1486.6 eV), a power of 50 W, and a beam diameter of 200 μm. Atomic concentrations were determined from survey spectra using the method of relative elemental sensitivity factors. The binding energies (B.E.) of the photoelectron lines (C1s, O1s, N1s) were determined from high-resolution spectra taken at an analyzer transmittance energy of 23.5 eV and a data acquisition density of 0.2 eV/step. For decomposition of the photo peal, we used a Gaussian Lorentzian mix function and a linear background subtraction. XPS analysis of the modified fabrics was performed immediately after surface modification.
2.6. In-Plane Shear Strength Tests
In-plane shear strength of the composites was determined according to ASTM D 3846–02 using 80 × 10 × 4 mm samples at a crosshead speed of 1.3 mm/min. For the tests, a Zwick/Roell Z020 universal test machine (Zwick Roell Group, Ulm, Germany) was used. Compressive load was applied to a notched specimen of uniform width to measure the shear strength. The specimen was loaded edgewise in a supporting jig of the same description as that referenced in ASTM D 695 for testing thin specimens. A failure of the specimen occurs in shear between two centrally located notches machined halfway through its thickness and spaced a fixed distance apart on opposing faces. The distance between the notches was 6.5–8 mm.
2.7. Mechanical Tests
A Zwick/Roell Z020 universal test machine equipped with 1 and 20 kN sensors and a contact strain measurement system MultiXtens was used for mechanical tests. Tensile tests were performed in accordance with ISO 527:2009 for 110 × 10 × 2 mm samples and the flexural tests were performed in accordance with ISO 14125:1998 for 110 × 10 × 2 mm samples at a span length of 80 mm. Cross head speed during the tests was 10 mm/min. Five samples were tested for each type of composites in each test. Composite structure was investigated using a VEGA 3 TESCAN scanning electron microscope ((TESCAN ORSAY HOLDING, a.s., Brno–Kohoutovice, Czech Republic)) in a backscattered electron image mode. For the scanning electron microscopy (SEM) test, all specimens were sputter coated with a thin layer of carbon (10–15 nm) to provide the electrical conductivity of the samples.
2.8. Dynamic Mechanical Analysis
A Dynamic mechanical analyzer DMA Q800 (TA Instruments, New Castle, DE, USA) dynamic mechanical analyzer was used in these investigations. Specimens approximately 2 mm wide, 2 mm thick and 45 mm long were used for the DMA tests. The measurements were performed using a double cantilever clamp at a frequency of 1 Hz and deformation of 0.1%, in a temperature range from 30 to 220 °C and the heating rate was 2 °C/min.
The solution impregnation method was used to form PSU based composites reinforced with twill wave CF fabrics. The elaborated technique allowed us to achieve homogeneous impregnation of CFs with the polymer and to further remove the solvent from the composite almost completely. Low viscosity of the impregnation solution resulted in polymer penetration even between the individual CF filaments. To improve the adhesion between CF and PSU, the surface of the fabrics was modified by thermal oxidation in an air atmosphere. It was shown that the surface modification by heating at a temperature to 500 °C for 30 min significantly changes the functional composition of the CF surface. Hydroxyl –C–OH, ethereal –C–O–C–, carboxyl COOH–, carbonyl –C=O, and epoxy groups form on the CF surface. Shear strength tests and SEM analysis of the fracture surface confirmed that surface modification resulted in a significant increase in the adhesion between the CF and polymer matrix. The in-plane shear strength magnitude for composites reinforced with modified CF fabrics was found to be more than 1.5 times higher than for composites containing unmodified CFs. Fracture analysis showed that in the case of unmodified CF, destruction proceeded on the boundary between the CFs and PSU. In the case of modified CF, the destruction of composites occurred both through the polymer matrix and CF body.
Mechanical tests in the tensile and flexural modes were carried out for the obtained composites. Carbon fiber content affects the composites’ mechanical properties. At low CF content, the composite strength was not that high due to the significant content of the “weak” polymer phase. Increase in the fiber content provided an increase in the fiber packing density, while a decrease in the polymer interlayer between the individual fibers occurred. Decrease in the strength at a further increase in the CF content proceeded because the matrix content was too small to bind the CF and distribute the applied load to the CFs in this case. Surface modification of CF also led to an increase in the tensile and flexural properties of the composites. The maximum ultimate strength and Young’s modulus values were found in the case of the CF TO temperature value of 500 °C. In this case, the tensile strength and Young’s modulus of the composites increased from the initial 880 ± 44 MPa and 57.5 ± 2.2 GPa up to 1047 ± 28 MPa and 70.9 ± 2.6 GPa, respectively. The same behavior after the flexural tests was observed: from the initial 899 ± 27 MPa and 57.6 ± 2.8 GPa, the flexural strength and flexural modulus increased up to 1042 ± 32 MPa and 73.1 ± 3.5 GPa, respectively. Dynamic mechanical analysis showed that the investigated composites were stable up to the temperatures of 130–140 °C, and that an increase in the CF content was accompanied by an increase in the thermal stability of the composites. Surface modification of CFs also resulted in an increase in the composites’ thermal stability due to the decrease in the polymer chains mobility, which was caused by good interfacial interaction between the CFs and PSU.