Correlation between the Microstructure and the Mechanical Properties of PAN-based Stabilized/Carbonized Fibers

8 Figure 2.3 Tensile strength and modulus of carbonized fiber as a result of different stabilization times. 15 Figure 2.4 Tensile strength and modulus of carbonized fiber as a result of changing carbonization time. 18 Figure 2.7 Tensile strength and modulus of carbonized fiber as a result of changing the elongation ratio in LT carbonization.

20 Figure 2.8 Tensile strength and modulus of carbonized fiber as a result of changing the elongation ratio in HT carbonization. 21 Figure 2.9 Tensile strength and modulus of carbonized fiber as a result of changing the HT carbonization temperature. 25 Figure 3.1 (a) FT-IR spectra and (b) Percentage of recated nitrated nitriles and dehydrogenation index of the stabilized fiber.

Introduction

Carbon fibers

The choice of precursor for carbon fiber
The manufacturing process of Polyacrylonitrile (PAN) based carbon fiber

Stabilization heat treatment

PAN precursor
Reaction in stabilization heat-treatment

Carbonization heat treatment
Radial heterogeneity in carbonized fiber
Object of Thesis

The production of PAN-based carbon fiber involves the synthesis and subsequent spinning of the polymer prior to stabilization and carbonization. However, this research is based on the relationship between results and parameter of the production of carbon fiber carbon fiber. Skin-core structure is known in the carbon fiber research field, but the exact reasons for the occurrence of this phenomenon are not well understood [32, 33].

That of the formation of the reasons of the skin core has been proposed the presence of oxygen heterogeneously in the cross section of the carbon fiber due to the different oxygen diffusion [34-38]. In this study, the correlation between the microstructure and mechanical properties of carbon fiber during the heat treatment process was investigated, and these results can be used to design the optimized manufacturing process. Parameter Optimization for Polyacrylonitrile Based Carbon Fiber with High Strength and High Modulus in Continuous Production Line.

Figure 1.1. Chemical reation schmes during (a) stabilization and (b) carbonization of PAN fiber [15]

Parameter Optimization for High Strength and High Modulus

Introduction
Experimental

Material
Continuous stabilization and carbonization line

Results and discussion

Effect of the stabilization time on the strength of carbon fiber
Effect of the carbonization time on the strength of carbon fiber
Effect of the low temperature carbonization elongation ratio on the strength of carbon fiber
Effect of the high temperature carbonization elongation ratio on the strength of carbon
Effect of the High carbonization temperature on the strength of carbon fiber
Parameter optimization in continuous carbonization process

Conclusion

The tensile strength of carbonized fibers increases and then decreases gradually, showing a maximum value at 135 minutes of stabilization time. The tensile modulus of carbonized fibers increases and then decreases, showing a maximum value at 120 minutes stabilization time. This accounts for the increase in the strength of carbonized fibers obtained by carbonization of stabilized PAN.

The tensile strength and modulus of charred fiber as a result of varying charring time is shown in Figure 2.4. The highest Weibull modulus of carbonized fiber was shown in the fiber treated for 2 minutes of carbonation time. The tensile strength and modulus of carbonized fiber as a result of varying elongation ratios in LT carbonization is shown in Figure 2.7.

The highest tensile strength of carbonized fibers appeared in 3% shrinkage ratio in the HT carbonization process. On the other hand, increasing temperature affects the defect structure of the charred fiber and decreases the tensile strength.

Figure 2.1 Schematic diagram of the pilot scale continuous stabilization line

Microstructure and Mechanical Properties of Poly-

Introduction

Experimental

Materials
Continuous stabilization condition and sample codes
Characterization

FT-IR
Differential scanning calorimetry
Expansion diameter in DMF
Raman Spectroscopy
Wide-angle X-ray diffraction
Density
Single fiber tensile test
Scanning electron microscope

The intensity of precursor is used as a reference intensity to obtain the degree of ring formation. The thermal behavior of the fiber was investigated by differential scanning calorimetry (DSC, Q200, TA Instruments Inc.). The first peak of the DSC curve is related to the cyclization and the cyclization index was obtained using the following equation [56].

Where 𝐻𝑃 is the integrated area of the PAN precursor from the DSC curve between the cyclization reaction area in J/g, and 𝐻𝐿 is the treated area of the stabilized fiber from the DSC curve in the same way. The PAN and the stabilized fiber were dissolved in 7 g of dimethylformamide (DMF) 1% by weight and heated in a hot plate at 70 oC for 24 hours. The expansion diameter ratio (EDR) was calculated with effective diameter obtained in tensile test section and expansion diameter.

Raman spectra of PAN and stabilized fiber were obtained using an Alpha 300s micro Raman spectrometer (WITec) equipped with a 532 nm laser. Where, L(hkl) is the plane crystallite size (hkl), FWHM is the full width at half width of the diffraction, 𝜃 is the Bragg angle and 𝜆 is the X-ray wavelength. Density of PAN and stabilized fiber He pycnometer (Accupyc II 1340, Micromeritics, Inc) was measured.

The pycnometer measured the fiber volume and calculated the density using the previously measured mass. A single filament test was used to determine the mechanical properties of PAN and stabilized fiber. For the preliminary step of the tensile test, the linear densities of the fibers were obtained using a vibroscope.

The tensile fracture morphologies of PAN and stabilized fiber were observed with a scanning electron microscope (SEM, Nanonova 230, FEI Co.) at an accelerating voltage of 10 kv.

Result and Discussion

Chemical Structure during Stabilization Process
Thermal Property Changes during Stabilization
Intermolecular Crosslinking analysis
Microstructure Changes during Stabilization
Crystal Structure Changes during Stabilization
Mechanical Property Changes during Stabilization
Fracture morphology

Shown in Figure 3.1 (a), FT-IR spectrum of each zone fiber was obtained to investigate the chemical structural change during stabilization. This development was related to the cyclization reaction during stabilization and was expressed numerically as a percentage of reacted nitriles in Figure 3.1 (b). The precursor fiber was fully dissolved in DMF solution and L1 fiber formed as a gel state shown in Figure 3.5 (a) and (b) respectively.

The radial expansion of fiber was obtained and the expansion diameter measured by OM shown in Figure 3.6. The stabilized fiber had an anisotropic property along the fiber axis and the WAXD pattern of the fiber was analyzed in equatorial, meridional and azimuthal scan shown in the Figure 3.9. The crystal size of PAN structure at 2θ = 17 o was similar from PF to L3 fiber shown in Table 3.4.

The PAN structure in the crystal phase lost orientation during the stabilization reaction, and the ladder structure of PAN fiber appeared with orientation. The density of stabilized fibers gradually increased above L3, while the linear density of stabilized fibers was similar in the stabilization phase. The linear density determined the weight in grams per unit length of a single filament and, taking into account the stabilized fiber heat-treated in the iso-length condition, the mass of the fiber remained comparable to that of the PAN precursor fiber .

In this experiment, the tensile strength decreases due to the decrease in the interaction of molecular structure in stabilized fiber to the chemical reaction of PAN structure. On the other hand, the tensile modulus of stabilized fiber remained the same up to the L3 fiber condition, after which the tensile modulus of stabilized fiber decreased. From the WAXD analysis, the crystal orientation of PAN structure was destroyed at L3 fiber and this affected the decrease in tensile modulus.

To show the strain hardening mode, the second point of increase in strength was obtained from the stress-staining curve shown in Figure 3.13. To study the fracture morphology of stabilized fiber, the SEM images were obtained and shown in Figure 3.14. The fractured morphology of PAN precursor fiber (Figure 3.14 (a)) was a hard fracture along the cross section.

Figure 3.2. Photographs of samples : (a)PF; (b) L1; (c) L2; (d) L3; (e) L4; (f) L5; (g) L6; (h) L7

Conclusion

From fibers L1 to L3, the inner region changed from tough to brittle, and on the other hand, the surface region remained a tough fracture. Therefore, the stabilization reaction first occurred in the core, where the amorphous phase can be dominantly formed in the fiber than on the surface [75].

Microstructure and Mechanical Properties of Poly-acrylonitrile-

Introduction

Experimental

Materials
LT and HT carbonization process
Characterization

Density
Single fiber tensile test
Thermogravimetric analysis
Elemental analysis
X-ray photoelectron spectroscopy
FT-IR
Wide-angle X-ray diffraction
Raman Spectroscopy

The densities of carbonized fibers were measured with a He-pycnometer (Accupyc II 1340, Micromeritics, Inc) at room temperature (23 oC). The densities of carbonized fibers were reported in the average value obtained through 10 times of testing, meeting conditions of 0.005 standard deviation or less [41]. The mechanical properties of carbonized fibers were performed using a single filament tensile testing machine (FAVIMAT+, Measured Solutions Inc.).

The linear densities of the carbonized fibers were measured with a vibroscope prior to tensile testing for a fiber thickness of 25.4 mm. The effective diameters of carbonized fibers were calculated as a correlation between the linear density and the density of carbonized fibers. The residual mass of carbonized fibers represented the remaining mass after the carbonization process compared to stabilized fibers and was calculated by following equation [76, 77].

Retained mass LDs− LDc . LDs Where LDs is linear density of stabilized fiber, LDc is linear density of carbonized fiber and TE is the total elongation ratio of carbonized fiber during carbonization process. The fracture morphologies of carbon fiber converted tensile test were investigated by the scanning electron microscope (SEM, NOVA NanoSEM 230, FEI, USA) at an accelerating voltage of 10 kV. Fourier Transform Infrared (FT-IR) spectroscopy was performed on the carbonized fibers with an infrared microscope (Cary 670, Varian, Co.) using Attenuated Total Reflectance (ATR) mode. The absorption spectra were obtained in the range of 600 cm-1 to 4000 cm-1 at spectral resolution of 2 cm-1 by collecting 256 scans.

The orientation factor, f(002), is a parameter to obtain the orientation of the crystal plane (002) in carbonized fibers. Raman spectra of samples were acquired using an Alpha 300s micro Ramana spectrometer (WITec) equipped with a 532 nm laser and a power of 0.5 mW. To obtain the Raman spectrum on the surface of carbonized fibers, the carbonized fiber was mounted on microscope slides.

Cross-sectional surfaces of fibers were obtained using an optical microscope (Leica, DM2500M) at 100x magnification.

Table 4.2 Carbonization condition during LT carbonization.

Result and Discussion

During the carbonization process, the reactions during carbonization were known as a combination of pyrolysis, condensation, and graft hitization in the carbonized fiber [53]. As shown in Figure 4.2 (a), the density of charred fibers increased up to stage 3 with maximum value and decreased in stage 4. Therefore, in stage 2, the intermolecular cross-linking by dehydrogenation affects to release the hydrogen and oxygen atom in charred fibers under heat treatment.

At stage 3, intermolecular crosslinking by denitrogenation occurred predominantly, which had an effect on the loss of nitrogen content. The results of X-ray diffraction in Figure 4.5(a), showed the unreacted PAN structure in SF fibers, which meant that the PAN fiber did not completely change into the ladder structure during the stabilization. Figure 4.7 shows the crystal size and Herman's orientation factor from the WAXD analysis and the results are described in Table 4.5.

The result of Figure 4.7 (a), the growth of crystal size showed the difference way in each stage. On the other hand, the orientation greatly increased at stage 3, even in the shrinkage reaction. In stage 1, the D band increased due to the increase in ladder structure by cyclization reaction.

In step 3, the amorphous area greatly increased in the core due to the shrinking process. In step 4, the D and G band increased because the graphitization occurred in the charred fiber. On the other hand, the gas could not escape in the core region and this affected smaller size of graphitic structure.

Above stage 3, the graphic structure became similar as shown in ID/IG. 49]Johnson, W.J.E.S.P.B.V., Handbook of Composites., Structure of PAN-based carbon fibers and relationship to physical properties. Naebe, A route to reduce energy consumption in the thermal stabilization process of carbon fiber production.