The role of thermal stabilization on the structure and mechanical properties of polyacrylonitrile precursor fibers

The thermal stabilization stage of polyacrylonitrile (PAN) fibers is characterized by a steady and continuous reduction in fiber diameter and linear density values together with color changes from reddish brown to shiny black with increasing stabilization time. Thermally stabilized PAN fibers acquire infusible and nonburning characteristics prior to the carbonization stage. Structural characterization of thermally stabilized polyacrylonitrile fibers was carried out using an indepth analysis of equatorial X-ray diffraction traces. Curve fitting of X-ray diffraction traces provided accurate peak parameters which were subsequently used for the evaluation of apparent crystallinity, apparent crystallite size and X-ray stabilization index. The results showed the loss of crystallinity due to the amorphization processes together with a steady and continuous decrease in lateral crystallite size with increasing stabilization time. With the progress of thermal stabilization, a new amorphous phase with a crosslinked and aromatized structure is formed which is expected to withstand high carbonization temperatures. Mechanical properties of the thermally stabilized PAN precursor fibers were found to be adversely affected with the progress of stabilization time. Due to the influence of thermal degradation mechanisms heavily involving chain scission along the fiber axis direction, tensile strength and tensile modulus values were found to decrease by significant proportions with the prolonged stabilization times.     

Modification of polyacrylonitrile precursor fiber with hydrogen peroxide

Polyacrylonitrile (PAN) precursor fibers were modified for different periods of time using hydrogen peroxide aqueous solution. A variety of tests were employed to characterize the fibers. The modification could induce cyclization and oxidation in the precursor fibers, as reflected by the changes in length and diameter of the fibers, and the results of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Compared with the unmodified fiber, the modified fibers released less heat during a heating process similar to stabilization of PAN precursor fiber. Also, the modified fibers showed lower characteristic temperatures on differential scanning calorimetry (DSC) thermograms, and lower onset temperature of weight loss on thermal gravimetry (TG) curves. The modified fibers had more surface defects and hence exhibited lower tenacity and tensile modulus. Compared with the unmodified fibers, however, the modified fibers had smoother surface and fewer defects after stabilization. The strain decreased with increasing temperature under a constant tension for all the fibers. At the temperatures above 200 °C, the shrinkage of the fibers decreased with the increase of modification time, because a certain degree of cyclization and oxidation occurred in modified fibers, making them shrink less in the temperature range equivalent to stabilization.     

Investigation of the effect of cupric chloride on thermal stabilization of polyamide 6 as carbon fiber precursor

An investigation on the role of cupric (Cu²⁺) ion incorporation during the thermal stabilization of polyamide 6 fibers was carried out using a combination of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) measurements. Cupric chloride pretreated and thermally stabilized polyamide 6 (PA6) fibers was characterized by a reduction in fiber diameter and linear density values together with color changes from light brown to black with increasing stabilization time. PA6 fibers were properly stabilized after 8 h of stabilization time prior to carbonization. The results obtained from DSC and TGA measurements indicated that there was an improvement in the thermal stability when cupric (Cu²⁺) ions were incorporated into the polymer structure. TGA thermograms showed the relative improvement in thermal stability as indicated by increasing char yield with progressing time. Char yield reached a maximum value of 33.6 % at 1000 °C for the cupric chloride pretreated PA6 fibers stabilized for 12 h at 180 °C. Experimental results obtained from DSC and X-ray diffraction methods suggested the loss of crystallinity as a result of perturbation of hydrogen bonds with progressing time. The formation of cupric ion-amide coordination bonds improved the thermal stabilization by encouraging the development of ladder-like structures. The investigation resulted in a new method of evaluation of X-ray stabilization index specifically intended for the thermally stabilized PA6 fiber.     

An in depth study of crystallinity, crystallite size and orientation measurements of a selection of poly(ethylene terephthalate)fibers

A selection of commercially available poly(ethylene terephthalate) fibers with different degrees of molecular alignment and crystallinity have been investigated utilizing a wide range of techniques including optical microscopy, infrared spectroscopy together with thermal and wide-angle X-ray diffraction techniques. Annealing experiments showed increased molecular alignment and crystallinity as shown by the increased values of birefringence and melting enthalpies. Crystallinity values determined from thermal analysis, density, unpolarized infrared spectroscopy and X-ray diffraction are compared and discussed in terms of the inherent capabilities and limitations of each measurement technique. The birefringence and refractive index values obtained from optical microscopy are found to decrease with increasing wavelength of light used in the experiments. The wide-angle X-ray diffraction analysis shows that the samples with relatively low orientation possess oriented non-crystalline array of chains whereas those with high molecular orientation possess well defined and oriented crystalline array of chains along the fiber axis direction. X-ray analysis showed increasing crystallite size trend with increasing molecular orientation. SEM images showed micro-cracks on low oriented fiber surfaces becoming smooth on highly oriented fiber surfaces. Excellent bending characteristics were observed with knotted fibers implying relatively easy fabric formation.     

X-ray diffraction studies of poly(aryl ether ether ketone) fibers with different degrees of crystallinity and orientation

A selection of commercially available poly(ethylene terephthalate) fibers with different degrees of molecular alignment and crystallinity have been investigated utilizing a wide range of techniques including optical microscopy, infrared spectroscopy together with thermal and wide-angle X-ray diffraction techniques. Annealing experiments showed increased molecular alignment and crystallinity as shown by the increased values of birefringence and melting enthalpies. Crystallinity values determined from thermal analysis, density, unpolarized infrared spectroscopy and X-ray diffraction are compared and discussed in terms of the inherent capabilities and limitations of each measurement technique. The birefringence and refractive index values obtained from optical microscopy are found to decrease with increasing wavelength of light used in the experiments. The wide-angle X-ray diffraction analysis shows that the samples with relatively low orientation possess oriented non-crystalline array of chains whereas those with high molecular orientation possess well defined and oriented crystalline array of chains along the fiber axis direction. X-ray analysis showed increasing crystallite size trend with increasing molecular orientation. SEM images showed micro-cracks on low oriented fiber surfaces becoming smooth on highly oriented fiber surfaces. Excellent bending characteristics were observed with knotted fibers implying relatively easy fabric formation.     

The characterization of polyacrylonitrile fibers stabilized by electron beam irradiation

This study investigates polyacrylonitile(PAN) fibers stabilized with various doses of electron beam irradiation (EBI) ability to produce carbon fibers. Feasibility was verified by FT-IR, the percent of gel fraction, density, DSC, XRD, and mechanical measurements. FT-IR spectra showed that the intensities of the stretching C??N bonds decreased at 2,244 cm^?1 with increasing EBI dose. This de crease was related to cyclization of nitrile groups during EBI-stabilization. The degree of cyclization was determined from the gel fraction and density tests. The gel content and density of PAN fibers stabilized by EBI increased with an increase in the EBI dose. Thermal properties were characterized by differential scanning calorimetry (DSC) and thermally activated reactions. DSC curves showed that EBI treatment influenced the quantity of released heat and the exothermic position at low temperature over a wide temperature range. The strongest diffraction peak from the PAN precursor fiber arose from the (100) plane; its stabilization index (SI) was evaluated by X-ray diffraction. The X-ray results showed that the peak intensity decreases gradually with increasing EBI dose. In addition, tensile strength decreased the EBI stabilization level.     

Production of carbon fibers modified with ceramic powders for medical applications

Carbon fibers and precursor polyacrylonitrile (PAN) fibres that contain either silica or hydroxyapatite particles, imbedded during the spinning process, were studied in this paper. The modified PAN fibers were thermally stabilized using a multi-stage process in the temperature range between 150 to 280 °C in an oxidative environment. Subsequent carbonization leading to obtain carbon fibers was carried on at 1000 °C in an argon atmosphere. The changes of properties of composite precursor fibers taking place during stabilization and carbonization processes were investigated by the combination of Differential Scanning Calorimetry, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy equipped with energy dispersive X-ray spectrometer and ultrasonic methods. Mechanical properties, such as tensile strength, static Young’s modulus, elongation at fracture were analyzed at each stage of thermal stabilization process. Additionally some traditional measurements like fiber diameter and mass were studied. Ceramic powders added to the spinning solution were present also in composites fibers after stabilization and carbonization process. Such modification allows to avoid the post-treatment operations, for example by coating or covering with films, which were usually necessary in order to obtain bioactive character of implants. Modification of carbon fibers using calcium phosphate or silica can lead to the development of a new materials for the manufacturing of implants which can establish direct chemical bonds with bone tissue after implantation.     

Effect of chemically modified date palm leaf fiber on mechanical,thermal and rheological properties of polyvinylpyrrolidone

Polyvinylpyrrolidone/date palm leaf fiber (PVP/DPL) biocomposites were prepared by melt mixing fabrication technique with different weight percentage of fibers. DPL fibers were chemically modified by acrylic acid in order to have better dispersion and compatibility with PVP matrix. The interaction of DPL fibers with PVP matrix was studied by Fourier transforms infrared spectroscopy (FTIR). Field emission scanning electron microscope (FESEM) was used for the study the morphology of chemically modified DPL fibers and PVP/DPL biocomposites. Mechanical properties were improved with fiber loading due to strong interfacial adhesion between PVP and DPL fibers. The storage modulus, loss modulus and tan delta values of PVA/DPL biocomposites were measured by DMTA. The rheological properties were investigated to study the shearing storage and loss modulii along with complex viscosity of biocomposites. The thermal and conducting properties of biocomposites were measured and compared with that of virgin PVP.     

Surface modifications of natural Kanchipuram silk (pattu) fibers using glow discharge air Plasma

Experimental investigations have been carried out to modify the surface properties of natural Kanchipuram silk (pattu) fibers using a low temperature DC glow discharge air Plasma. Silk is an externally spun fibrous protein secretion formed into fibers. Plasma treatment is an eco-friendly, dry, and clean process over wet chemical method and does not suffer from any environmental and health concerns. Experiments have been performed considering three parameters such as discharge current, treatment time, and working pressure. The structural, thermal, morphological, optical, and mechanical studies of raw and plasma treated silk fibers have been obtained out using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) spectroscopy, X-ray diffraction (XRD), thermo gravimetric analyzer (TGA), scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), diffuse absorbance spectroscopy, and tensile test. A comparative study has been done for the untreated and different treated fibers. Various characterization analyses reveal that surface roughness of the plasma treated silk fiber is increased and also crystallite size of treated samples is enhanced, plasma treated silk fibers maintain the whiteness effect and it is observed that UV transmittance region (A & B) is more for the treated fiber which signifies enhanced UV protection.     

Effect of surface treatment of jute fibers on the interfacial adhesion in poly(lactic acid)/jute fiber biocomposites

Jute fibers have immense potential to be used as natural fillers in polymeric matrices to prepare biocomposites. In the present study jute fibers were surface treated using two methods: i) alkali (NaOH) and ii) alkali followed by silane (NaOH+Silane) separately. Effects of surface treatments on jute fibers surface were characterized using fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) analyses. Further, the effects of surface treatments on jute fibers properties such as crystallinity index, thermal stability, and tensile properties were analyzed by X-ray diffraction method (XRD), thermo gravimetric analysis (TGA), and single fiber tensile test respectively. The effects of surface treatment of jute fibers on interphase adhesion between of poly(lactic acid) (PLA) and jute fibers were analyzed by performing single fiber pull-out test and was examined in terms of interfacial shear strength (IFSS) and critical fiber length.     

Synthesis and characterization of acrylic fibers-<Emphasis Type="Italic">g</Emphasis>-polyacrylamide

Graft copolymerization of acrylamide onto commercial acrylic fibers was carried out using benzoyl peroxide as a free-radical initiator in aqueous medium within the 75–95 °C temperature range. In this study, the effects of initiator and monomer concentration, the amount of fiber, polymerization time, and temperature on the graft yield were investigated. The optimum concentration for initiator was found to be 2.0×10⁻³ mol/l and the optimum temperature of 85 °C. The activation energy of the reaction was calculated to as 35.81 kJ/mol at the temperature interval of 75–95 °C. The structures and morphologies characterization of grafted fibers was investigated by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The thermogravimetric analysis data showed that the thermal stability of the acrylic fibers increased with graft yield. The scanning electron photographs showed that the homogeneous appearance of the fiber surface changed and a shell-like heterogeneous structure occurred at the surface with an increasing degree of grafting. The moisture content, water absorption, dyeability, and antimicrobial activity of grafted acrylic fibers were also reported. The results showed that grafting of polyAAm improved the moisture contain, water absorption, dyeability, and antimicrobial activity of fiber.     

Effect of surface treatment on the structure and properties of para-aramid fibers by phosphoric acid

The surface of para-aramid fiber was modified by phosphoric acid solutions (H3PO4) based on an orthogonal experimental design and analysis method. Statistical results indicate that treatment temperature is the most significant variable in the modification processing, while treatment time was the least important factor. The structure and morphology of the modified fiber were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction instrument (XRD), and scanning electron microscope (SEM). The results showed that some polar groups were introduced into the molecular structure of aramid fibers and the physical structure of the treated fibers was not etched obviously. The interfacial properties of aramid fiber/epoxy composites were investigated by the single fiber pull-out test (SFP), and the mechanical properties of aramid fibers were investigated by the tensile strength test. The results showed that the interfacial shear strength (IFSS) of aramid/epoxy composites was remarkably improved and the breaking strength of aramid fibers was not affected appreciably after surface modification.     

Depositon of ZnO on polyacrylonitrile fiber by thermal solvent coating

In this study, the surface functionalization of polyacrylonitrile (PAN) fibers was achieved by depositing ZnO nanoparticles using thermal solvent coating. surface morphology, crystalline structure, surface chemistry, thermal stability and washing stability of the ZnO coated PAN fibers were investigated by scanning electron microscope (SEM), X-ray diffractometer (XRD), Fourier transform Infra red spectroscopy (FT-IR), Thermo-gravimetric analyses (TGA) and washing stability test, respectively. In addition, the weight changes after coating and washing were studied at different coating and washing conditions. The SEM images revealed that the ZnO was well coated on the surface of the PAN fibers and the coating was obviously affected by the experimental temperature. The FT-IR spectra indicated the chemical features of the deposited ZnO nanostructures. The XRD patterns showed that there was a typical crystalline structure of ZnO nanogains formed on the PAN fibers after coating. The TGA results revealed that the thermal stability of the PAN fibers was improved by the ZnO coating. The experimental results of washing stability revealed the effect of temperature on the washing stability. Weight measurements indicated that the amount of ZnO deposited on PAN fibers increased with the increasing of coating temperature from 60 to 70 °C. Weight measurements also revealed that the weight of the ZnO coating on fibers decreased with the increase in washing temperature and washing time.     

Effects of surface treatment of ramie fibers in a ramie/poly(lactic acid) composite

The chemical and morphological properties of ramie fibers treated by chemical surface modification were examined with Fourier transform infrared (FT-IR) spectroscopy. The mechanical and thermal decomposition properties were evaluated with respect to tensile strength, tensile modulus and thermogravimetric analysis (TGA). Surface morphological changes were investigated with scanning electron microscopy (SEM). Finally, the capabilities of composites reinforced with various chemically treated fibers were analyzed by investigating tensile and impact strengths. Additionally, the thermal mechanical properties of the composites were investigated with thermal mechanical analysis (TMA). Based on the results of these analyses, we concluded that pectin, lignin and hemicellulose were removed and thermal stability was increased with chemical treatments. The composites reinforced with ramie fiber showed better properties compared with pure PLA matrix with respect to tensile and impact strengths. The peroxide-treated fiber composite had the smallest thermal expansion.     

Preparation,structure and properties of boron modified high-ortho phenolic fibers

Boron modified high-ortho phenolic fibers (o-BPFs) were prepared by melt-spinning from boron modified highortho phenolic resins (o-BPRs) with the weight-average molecular weight of 4973 g/mol, followed by being cured in a solution of formaldehyde and hydrochloric, and then heat-treated under high temperature. Gel permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy (NMR) were used to measure the average molecular weight and ortho/para (o/p) ratio of o-BPRs. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to characterize the chemical and morphological structures of o-BPRs and o-BPFs. Thermogravimetric analysis (TGA) was employed to examine the thermal stability properties of different resins and fibers and the tensile strength of fibers was measured by a tensile tester. It was found that under proper curing and heat-treatment conditions, the tensile strength of o-BPFs reached 213.6 MPa and the char yield in N₂ atmosphere at 800 °C attained 75.4 %. Compared with phenolic fibers (PFs), the decomposition temperatures at 5 % weight loss of o-BPFs in N₂ and air atmospheres were increased by 156.8 °C and 219.0 °C, respectively.     

Antimicrobial fibers based on chitosan and polyvinyl-alcohol

A series of antimicrobial fibers with different weight ratio of chitosan (CS) and polyvinyl alcohol (PVA) were fabricated via a primarily industrialized trail of wet-spinning method, and the morphology and structure of the resulting fibers were studied with the aid of scanning electron micrography (SEM), infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The CP60 blend fiber (60 % chitosan content) was confirmed as the best optimal sample among the blend fibers owing to strong intermolecular hydrogen bonds between PVA and chitosan and showed the maximum mechanical, antistatic, moisture absorption/desorption properties. The CP60 also exhibited good antimicrobial effects against Escherichia coli and Staphylococcus aureus as the chitosan fiber and could be recommended as the alternative material for the wound dressing and the food packing.     

Surface modification of polyester fibers by thermal reduction with silver carbamate complexes

In this study, the surface of polyester fiber was modified by means of thermal treatment with a silver carbamate complex. We used scanning electron microscopy (SEM), an X-ray diffraction technique (XRD), and X-ray photoelectron spectroscopy (XPS) to allow a detailed characterization of the silver-coated polyethylene terephthalate (PET) fibers. The results revealed remarkable changes in the surface morphology and microstructure of the silver film after thermal reduction. On SEM, the silver nanoparticles (AgNPs) were seen to be uniformly and densely deposited on the fiber surface. The XRD pattern of the silver-coated fiber indicated that the film has a crystalline structure. A continuous layer of AgNPs, between 30 and 100 nm in size, was assembled on the PET fibers. The PET/Ag composite was found to impart high conductivity to the fibers, with an electrical resistivity as low as 0.12 kΩ·cm.     

Effects of carbon nanotubes on morphological structure, thermal and flammability properties of electrospun composite fibers consisting of lauric acid and polyamide 6 as thermal energy storage materials

The ultrafine composite fibers consisting of lauric acid (LA) and polyamide 6 (PA6) as form-stable phase change materials (PCMs), were prepared successfully by electrospinning. The effect of carbon nanotubes (CNTs) on the structural morphology, phase change behaviors, thermal stability, flammability and thermal conductivity properties of electrospun LA/PA6 composite fibers was investigated by field-emission scanning electron microscopy (FE-SEM), differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), microscale combustion calorimeter (MCC) and melting/freezing times measurements, respectively. SEM observations indicated that the LA/PA6 and LA/PA6/CNTs composite fibers possessed flat and ribbon-shaped morphologies, but the neat PA6 fibers had cylindrical shape with smooth surface; and the average fiber diameters for LA/PA6 composite fibers decreased generally with the addition of CNTs. DSC measurements indicated that the heat enthalpies of the composite fibers were lower that that of neat LA powders, while the amounts of CNTs had no appreciable effect on the phase change temperatures and heat enthalpies of the composite fibers. TGA results showed that the addition of CNTs increased the onset thermal degradation temperature, maximum weight loss temperature and charred residue at 700 °C of the composite fibers, attributed to the improved thermal stability properties. It could be found from MCC tests that there were two-step combustion processes for composite fibers, and corresponded respectively to combustion of LA and polymer chains (PA6) in composite fibers. The addition of CNTs reduced the peak of heat release rate (PHRR) of electrospun composite fibers, contributing to the decreased flammability properties. The improved thermal conductivity performances of LA/PA6/CNTs composite fibers was also confirmed by comparing the melting/freezing times of LA/PA6 composite fibers with that of neat LA powders. The results from the SEM observation showed that the composite fibers had no appreciable variations in shape and diameter after heating/cooling processes.     

Flame retardancy,Thermal and mechanical properties of Kenaf fiber reinforced Unsaturated polyester/Phenolic composite

Unsaturated polyester (UP) resin has been blended with phenolic resin (PF) resole type at various ratios to obtain a homogeneous blend with improved flame resistance compared to its parent polymers. The polymer blend was reinforced with 20 wt% kenaf using hand lay out technique. Fourier transform infrared spectroscopy (FT-IR) was used to characterize changes in the chemical structure of the synthesized composites. The thermal properties of the composites were investigated using thermogravimetric analysis (TGA). The thermal stability of UP/PF kenaf composites co-varies with the PF content, as shown by the degradation temperature at 50 % weight loss. The char yield of the composites increases linearly with PF content as shown by the TGA results. The flammability properties of the composites were determined using the limiting oxygen index (LOI) and UL-94 fire tests. The LOI increased with the PF content while the composites exhibit improved flame retardancy as demonstrated by UL-94 test. The mechanical and morphological properties of the composites were determined by tensile test and scanning electron microscopy (SEM), respectively. The tensile strength and the Young’s modulus of the blend/composites slightly decreased with increasing PF content albeit higher than PF/kenaf fiber composites.     

Thermal conductivity and thermal expansion behavior of pseudo-unidirectional and 2-directional quasi-carbon fiber/phenolic composites

In the present paper, a variety of fiber reinforcements, for instance, stabilized OXI-PAN fibers, quasi-carbon fibers, commercial carbon fibers, and their woven fabric forms, have been utilized to fabricate pseudo-unidirectional (pseudo-UD) and 2-directional (2D) phenolic matrix composites using a compression molding method. Prior to fabricating quasi-carbon fiber/phenolic (QC/P) composites, stabilized OXI-PAN fibers and fabrics were heat-treated under low temperature carbonization processes to prepare quasi-carbon fibers and fabrics. The thermal conductivity and thermal expansion/contraction behavior of QC/P composites have been investigated and compared with those of carbon fiber/phenolic (C/P) and stabilized fiber/phenolic composites. Also, the chemical compositions of the fibers used have been characterized. The results suggest that use of proper quasi-carbonization process may control effectively not only the chemical compositions of resulting quasi-carbon fibers but also the thermal conductivity and thermal expansion behavior of quasi-carbon fibers/phenolic composites in the intermediate range between stabilized PAN fiber- and carbon fiber-reinforced phenolic composites.