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Chopped kenaf fibers, poly(lactic acid) (PLA), and ammonium polyphosphate (APP) were uniformly compounded by twin-screw extrusion process, and then kenaf/PLA biocomposites with various APP loadings were fabricated by compression molding. The effects of APP on the flame retardant, tensile, dynamic mechanical, and thermo-dimensional properties were investigated. The biocomposites exhibit highly increased flame resistance, showing the limiting oxygen index increased with increasing the APP loading and the UL-94 V-0 level with 20 wt.% and higher APP loadings. The thermal stability of the biocomposites was increased, whereas the thermal expansion behavior was decreased with increasing the APP loading. The incorporation of APP into the PLA matrix contributed to simultaneously enhancing not only the flame retardancy, thermal stability, and thermo-dimensional stability, but also to enhancing the tensile and storage moduli of kenaf fiber/PLA biocomposites.

The effect of multi-walled carbon nanotubes (MWCNT) coating in the presence of polyethyleneimine (PEI) of different molecular weights (MW) on the interfacial shear strength (IFSS) of carbon fiber/acrylonitrile–butadiene–styrene (ABS) and carbon fiber/epoxy composites was investigated. The IFSS between the carbon fiber and the polymer was evaluated by means of single fiber microbonding test. The results indicated that uses of the carbon fibers uncoated and coated with pristine, low MW PEI-treated, and high MW PEI-treated MWCNT significantly influenced the IFSS of both thermoplastic and thermosetting carbon fiber composites as well as the carbon fiber surface topography. The incorporation of low MW (about 1300) PEI into the carboxylated MWCNT was more effective not only to uniformly coat the carbon fiber with the MWCNT but also to improve the interfacial bonding strength between the carbon fiber and the polymer than that of high MW (about 25,000) PEI. In addition, carbon fiber/epoxy composite exhibited the IFSS much higher than carbon fiber/ABS composite due to the chemical interactions between the epoxy resin and amine groups existing in the PEI-treated MWCNT.

Biocompatible barrier membranes with both hydrophobic and hydrophilic surface properties provide critical backup for guided regeneration at localized bone defects without soft tissue invasion. As a surface modified functional barrier, saturated fluorocarbon (CF4)-immobilized nanofibrous silk fibroin (SF) membranes were fabricated by electrospinning for a fibrous non-woven mat, water vapor treatment for insolubilization, and followed by CF4 gas plasma treatment for top surface hydrophobization. Plasma-treated SF nanofiber membranes maintained a non-woven mat structure without shrinkage and deformation in a five-month biodegradation test. From in vivo rabbit cranium perforation model, nanofibrous SF membranes prevented soft tissue invasion and facilitated volumetric bone regeneration compared with the control groups. New bone ingrowth in bone defects at 4 and 8 weeks after surgery was visualized by trichrome staining. Medical application of fluorocarbon-immobilized nanofibrous SF barrier membranes could be one of the practical approaches for guided bone regeneration. Open image in new window

Silk Fibroin (SF) nanofibers containing various silver compounds, such as silver tetrafluoroborate, silver phosphate, silver sulfadiazine, and silver acetate, were prepared by electrospinning. Silver compounds were dissolved into SF solution in 98% formic acid. In this system, the formic acid was used both as a co-solvent of SF and silver compounds, and as a reducing agent for silver ions in solution. The average diameters of SF nanofibers varied depending on the type of silver compounds added. The Ag element in the SF nanofibers was confirmed to be mainly Ag nanoparticles by XPS results. The in vitro cytotoxicity of silver compounds in SF nanofibers on normal human keratinocytes and fibroblasts was examined. Overall, the silver compounds in SF nanofibers are cytotoxic to both normal human epidermal keratinocytes (NHEK) and normal human epidermal fibroblasts (NHEF) cells. Open image in new window

Saturated fluorocarbon (CF4) immobilized silk fibroin (SF) nanofibrous membranes were prepared and characterized for biomedical applications. Biocompatible barrier membranes that provide both hydrophobic and hydrophilic surface properties on each side are critical to prohibit soft tissue invasion into localized bone defect. As a barrier membrane, SF nanofibrous mat was fabricated by electrospinning method, and then subsequently modified with water vapor treatment for insolubilization in water and CF4 gas plasma treatment for surface hydrophobization. Morphology of SF nanofibrous mats were observed by scanning electron microscopy. Conformational change of insolubilized SF nanofibers was confirmed by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and 13C nuclear magnetic resonance (NMR) spectroscopy. Immobilized fluorine atoms on CF4 plasma treated SF nanofibrous membranes were detected using electron spectroscopy for chemical analysis (ESCA). Water contact angle of the SF nanofiber membrane surface was analyzed by varying plasma input power and time. Insolubilized SF nanofibrous membrane maintained nonwoven mat structure without deformation after water immersion. SF nanofibrous membranes showed significant increment of water contact angle from 99.7° to 141.2° by CF4 gas plasma treatment. Fibroblasts on plasma untreated SF nanofibrous membranes were well attached and spread than a control tissue culture polystyrene dish. Fibroblasts on the CF4 gas plasma treated SF nanofibrous membrane showed significantly lower proliferation behavior than plasma untreated SF nanofibrous membranes. Fluorocarbon immobilized SF nanofibrous barrier membrane will be useful for biomedical applications such as a guided bone regeneration. Open image in new window

In this study, various surfactants were added to control the gelation time of silk fibroin (SF) aqueous solution. The gelation behaviors of SF aqueous solution in the presence of surfactant were investigated with attenuated total reflectance infrared, SEM, and a viscometer. When surfactants other than chitooligosaccharide were added into an SF aqueous solution, the gelation time of the solution was decreased under the fixed conditions. Particularly, anionic surfactant was found to be more effective than non-ionic and cationic surfactants in accelerating the gelation of SF. In addition, the conformational changes of SF hydrogel with or without surfactant were investigated in a time-resolved manner using infrared spectroscopy. Conformational transitions of SF nanofibers from random coil to β-sheet forms were strongly dependent on the inherent properties of surfactant, and on the different interactions between surfactant and SF molecules in aqueous solution. This approach to controlling the gelation of SF aqueous solution by the surfactant, and to monitoring their conformational changes on a real-time scale, may be critical in the design and tailoring of SF hydrogels useful for biomedical applications.

Exfoliated graphite nanoplatelets (EGN) were successfully coated with a liquid rubber amine-terminated poly(butadiene-co-acrylonitrile) (ATBN) at various concentrations. The rubber-coated EGN was incorporated into epoxy resin at different contents. The result revealed that the impact toughness of EGN/epoxy composite was increased by about 18 % with increasing the ATBN coating concentration. The impact strength, the flexural and dynamic mechanical properties, and the thermo-dimensional stability of EGN/epoxy composites were simultaneously enhanced by the incorporation of 1 wt% EGN coated with 10 wt% ATBN rubber into epoxy, which turns out a very small amount of 0.1 wt% ATBN compared to the epoxy resin.

In this study, biodegradable poly(D,L-lactic-co-glycolic acid) (PLGA) fibers were prepared by a melt-electrospinning and treated with plasma in the presence of either oxygen or ammonia gas to modify the surface of the fibers. The effects of processing parameters on the melt-electrospinning of PLGA were examined in terms of fiber morphology and diameter. Among the processing parameters, the spinning temperature and mass flow rate had a significant effect on the average fiber diameter and its distribution. The water contact angle of melt-electrospun PLGA fibers decreased significantly from 123 ° to 55 ° (oxygen plasma treatment) or to 0 ° (ammonia plasma treatment) by plasma treatment for 180 sec, while their water content increased significantly from 2.4 % to 123 % (oxygen plasma treatment) or to 189 % (ammonia plasma treatment). Ammonia gas-plasma enhanced the surface hydrophilicity of PLGA fibers more effectively compared to oxygen gas-plasma. X-ray photoelectron spectroscopy analysis supported that the number of polar groups, such as hydroxyl and amino groups, on the surface of PLGA fibers increased after plasma treatment. Overall, the microfibrous PLGA scaffolds with appropriate surface hydrophilicity and fiber diameter could be fabricated by melt electrospinning and subsequent plasma treatment, without a significant deterioration of fiber structure and dimensional stability. This approach of controlling the surface properties and structures of fibers could be useful in the design and tailoring of novel scaffolds for tissue engineering.

Green composites that, in general, consist of cellulosebased natural fibers and polymer resins have many advantages like environmentally friendliness, light weight, costeffectiveness, biodegradability and acceptable mechanical properties over conventional glass fiber-reinforced polymer composites. However, natural fibers of hydrophilic character may cause weak interfacial adhesion with a polymeric resin of hydrophobic character, leading to reduction of the properties and performances of resulting green composites. Due to lack of interfacial adhesion between raw natural fibers and polymers, there have been many papers reporting the modification of fiber surfaces and the effect on the interfacial properties of green composites. In order to modify natural fiber surfaces and consequently to enhance the interfacial bonding to a polymer, chemical methods like alkalization and silane coupling and physical methods like electron beam irradiation and plasma treatment are frequently used. Electron beam technique has often been utilized for surface modification and property improvement of polymer materials like fibers, films, plastics and composites for the last decades. It may remove surface impurities and alter surface chemical characteristics at an appropriate irradiation condition. Electron beam processing is a dry, clean and cold method with advantages such as energy-saving, high throughput rate, uniform irradiation and environmental safety. Poly(lactic acid), referred to as PLA, is obtainable from natural resources, thermoplastic and completely biodegradable. As it combines with natural fibers like jute, kenaf, etc., the material would be not only highly reinforced but also ecobenign and biodegradable. The objective of the present work is to explore the effect of jute fiber surface modification on the interfacial shear strength of jute-PLA green composites. The paper deals with the change of single fiber tensile property, fiber surface topology, and chemical composition occurring in jute fibers irradiated at different dosages of electron beam, discussing on their effect on the interfacial adhesion between jute fibers and PLA.

In this study, polycardanol, which was synthesized by enzymatic oxidative polymerization of thermally treated cashew nut shell liquid (CNSL) using fungal peroxidase, was partially or fully cured using methyl ethyl ketone peroxide (MEKP) as initiator and cobalt naphthenate (Co-Naph) as accelerator. The curing behavior of polycardanol was extensively investigated in terms of curing temperature, curing time, concentration of initiator and accelerator, and the monomer-to-polymer conversion of polycardanol by means of differential scanning calorimetry (DSC). The curing behavior significantly depends on the thermal condition given and it was monitored with the change of the exotherms as a function of temperature. The optimal conditions for fully curing polycardanol are 1 wt% MEKP, 0.2 wt% Co-Naph, curing time 120 min, and curing temperature 200 °C. This study suggests that a polycardanol with high monomer-to-polymer conversion would be useful for processing a polycardanol matrix composite under the optimal conditions of curing.

The purpose of this study was to conduct a thermal analysis of the hydrolysis and degradation behavior of biodegradable polymers and bio-composites at 50°C and 90% relative humidity (RH). With increasing hydrolysis time, the thermal stability and degradation temperature of polybutylene succinate (PBS) slightly decreased. The glass transition temperature (Tg) and melting temperature (Tm) of PBS and the anti-hydrolysis agent treated PBS did not vary significantly with increasing hydrolysis time, whereas those of the trimethylolpropane triacrylate (TMPTA)-treated PBS slightly increased. With increasing hydrolysis time, the storage modulus (E’) values of the bio-composites decreased, whereas those of the TMPTA treated bio-composites slightly increased. Also, the tan values of the anti-hydrolysis agent and TMPTA treated PBS-BF bio-composites were slightly lower than those of the non-treated bio-composites, due to the reduction in their degree of hydrolysis. The tanδmax peak temperature (Tg) of the anti-hydrolysis agent treated bio-composites was not significantly changed, whereas that of the TMPTA treated bio-composites was increased.

In this study, the thermal properties of bio-flour-filled, polypropylene (PP) bio-composites with different pozzolan contents were investigated. With increasing pozzolan content, the thermal stability, 5% mass loss temperature and derivative thermogravimetric curve (DTGmax) temperatures of the bio-composites slightly increased. The coefficient of thermal expansion (CTE) and thermal expansion of the bio-composites decreased as the pozzolan content increased. The glass transition temperature (Tg), melting temperature (Tm) and percentage of crystallinity (Xc) of the bio-composites were not significantly changed. The thermal stability, thermal expansion and Xc of the maleic anhydride-grafted PP (MAPP)-treated bio-composites were much higher than those of non-treated bio-composites at 1% pozzolan content due to enhanced interfacial adhesion. X-ray diffraction (XRD) analysis confirmed the crystallinity of pozzolan-added bio-composites. From these results, we concluded that the addition of pozzolan in the bio-composites was an effective method for enhancing the thermal stability and thermal expansion.

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.

In the present study, the fluorescence behavior of a phenylethynyl-terminated imide (LaRC PETI-5) resin, a bismaleimide (BMI) resin, and various LaRC PETI-5/BMI blends with different blend compositions has been characterized as a function of heat-treatment temperature, using a steady-state fluorescence technique with a front-face illumination method for solid-state films. It is observed that there are distinguishable changes in the spectral shape, size, and position of fluorescence with varying heat-treatment temperature in the pure and blend samples. The result is qualitatively explained in terms of charge transfer complex formation as well as microenvironmental change with local mobility and viscosity occurring in the LaRC PETI-5, BMI, and their blends during the cure process. The result also implies that a steady-state fluorescence technique may be a useful tool to understand the processing conditions of polyimides and their blends in the film form on the basis of their thermo-photophysical responses.

The surface of polyacrylonitrile-based carbon fiber was appropriately coated with dilute phosphoric acid to protect or retard its thermal oxidation occurring during an exposure to high temperatures above 600°C in air. Microscopic behavior on surface damage and size change in the uncoated and coated fibers was compared using thermogravimetric and scanning electron microscopic techniques. The result showed that the state of the carbon fiber surface was found to be remarkably improved by the coated phosphorous compound, minimizing the surface pitting, and size reduction of carbon fiber by high temperature oxidation. The microscopic evidence for the effective protection of carbon fiber from thermal oxidation also indicated that use of this technology may be positively considerable to other related materials.

The thermal resistance and ablation characteristics of composite materials are properties which play a very important role in the field of aerospace technology. When composite materials with excellent characteristics are exposed to extremely high temperatures, above 1500 °C, not only their surface and structure but their intrinsic features can be protected from the decomposition which can occur at these temperatures. The ablation of a material depends on intrinsic conditions of the material, and also on extrinsic conditions such as thermal, chemical and mechanical factors according to the variables of theoretical or practical environments [1]. Carbon/ phenolic composites have been based on the continuous development of rayon-based polyacrylonitrile-based (PAN-based) and pitch-based carbon fibres since the 1950s; they are widely used because of their excellent ablative resistance [2]. Carbon/ carbon composites are effective materials for parts used in extreme environments which require very low erosion rates [3]. Cost, space and experimental-simplicity constraints in the study of the characteristics of these ablative materials lead to the frequent use of lab-scaled chemical burners instead of a rocket motor, which would be closer to the practical situation in its performance. There have been several reports [2, 4] on ablative materials, but the detailed results which are the reason for the excellent ablative resistance of carbon-fibre-reinforced composites and the microscopic behaviour of ablation have been little reported. Consequently, the aim of this work is to investigate, from a microscopic viewpoint, the ablative behaviour of PAN-based carbon-fibre-reinforced phenolic composites treated with acid, in an ablation test [5] with an oxyacetylene burner. The carbon fibre used in the present work is a high-strength PAN-based carbon fabric (ACELAN 12KM-450HSW) with 3000 and 12 000 filaments and a 8-H/S texture, manufactured by Tae Kwang Industries. A phenolic resin (KRD-HM2) of a resol type was used (manufactured by Kolon Chemical Company). The diameter of a single filament of the carbon fibre was 6.8/.~m. The fibre density was 1800 kg m -~. The solid content of the phenolic resin used was 59.8%. The char yield at 1000 °C under a N2 gas was measured to be 69.5% [6]. The prepreg was prepared from the carbon fabric and the resin