Abstract
Recently, the preservation of natural resources and the strict environmental regulations have forced the composite industry to find alternative fiber reinforcements and resin systems that are ecofriendly to produce compatible composites. Therefore, the need for natural fiber-reinforced composites has never been as prevalent as it currently is. Natural fibers offer renewability, biodegradability, abundance, cost savings, and low specific gravity when compared to synthetic fibers such as glass and carbon. Though the strength of natural fibers is not as great as glass or carbon, the specific properties are comparable. Currently natural fiber composites have two main issues that need to be addressed: resin compatibility and water absorption. Thus, natural fiber composites are the main subject in this special issue. The authors have focused on the following topics.
In the paper entitled “Injection moulded biocomposites from oat hull and polypropylene/polylactide blend: fabrication and performance evaluation” by J. P. Reddy et al., oat hull fibre reinforced polypropylene (PP)/polylactide (PLA) based biocomposites were fabricated; their process engineering and performances were evaluated. The effect of ethylene propylene-g-maleic anhydride (EP-g-Ma) compatibilizer on mechanical properties of 30 wt% oat hull reinforced PP/PLA (90/10) blend composites was investigated. Thermal degradation parameters of the oat hull fibre were determined using thermogravimetric analysis. The effect of fibre reinforcement on crystallinity of oat hull fibre reinforced PP/PLA composites was studied by differential scanning calorimetry (DSC). Thermomechanical properties of the composites were analyzed by dynamic mechanical analyzer (DMA). The interfacial bonding between the fibre and the matrix was examined using scanning electron microscope (SEM). Significant improvement in tensile strength (40%) and flexural strength (46%) was observed with the addition of EP-g-Ma compatibilizer. DSC analysis of oat hull fibre reinforced composites showed an increase in the crystallization temperature (Tc) due to the nucleation effect of oat hull fibre. DMA results revealed that the storage modulus of PP/PLA/oat hull fibre composites was higher compared to PP/PLA blend throughout the investigated range of temperature.
In the paper entitled “Studies of moisture absorption and release behaviour of akund fiber” by X. Yang et al., an effort has been made to study the moisture absorption and release behaviour of akund fiber and its mechanical performance at relative air humidity ranging from 0% to 100%. The gain and loss in moisture content in akund fiber due to water absorption and release were measured as a function of exposure time under the environment, in which temperature is 20°C and humidity is 65%. The regression equations of the absorption and release process were established. From the study, it was found that the water absorption curve of akund is similar with those of cotton and kapok, and its water absorption ability is better than those of cotton and kapok. According to the regression analysis and comparison with cotton and kapok, the initial water absorption rate of akund fiber is a little lower, its balance moisture regain is bigger, and moisture releasing rate is faster. This feature shows that the akund fiber has less absorption moisture which is absorbed by hydroxyl directly and contains more indirectly absorbed moisture. This phenomenon may be decided by its chemical content, which has less cellulose content, and its big hollow structure.
In the paper entitled “The processing design of jute spun yarn/PLA braided composite by pultrusion molding” by A. Memon and A. Nakai, the fabrication of tubular jute spun yarn/PLA braided composite by pultrusion molding was presented. The intermediate materials were prepared by comingled technique. The braiding technique manufactured preform which had jute fiber diagonally oriented at certain angles with the glass fiber inserted into the braiding yarns along the longitudinal direction. The braided preformances were pulled through a heated die where the consolidation flow took place due to reduced matrix viscosity and pressure. The pultrusion experiments were done with jute/PLA comingled yarns and combined with glass fiber yarns to fabricate the tubular composite. Impregnation quality was evaluated by microscope observation of the pultruded cross-sections. The flexural mechanical properties of the pultruded were measured by four-point bending test.
The paper entitled “Developing simple production of continuous ramie single yarn reinforced composite strands” by H.-B. Kim et al. deals with long fiber-reinforced composite strands using continuous ramie yarns, in which polypropylene including Maleic anhydride grafted polypropylene (MA-PP) was used as a matrix material. The composite strands were fabricated by a new method called multipin-assisted resin impregnation (M-PaRI) process, for which the equipment was newly applied after conventional extrusion process. The composite strands were then pelletized and injection molded. Tensile strength and Young's modulus of the resultant short ramie/PP reinforced composites were investigated. Results show that this new process improved the mechanical properties of injection-molded specimens.
In the paper entitled “Continuous natural fiber reinforced thermoplastic composites by fiber surface modification” by P. Wongsriraksa et al., an intermediate material, which allows highly impregnation during molding, has been investigated for fabricating continuous fiber-reinforced thermoplastic composite by aligning resin fiber alongside reinforcing fiber with braiding technique. This intermediate material has been called “microbraid yarn (MBY).” Moreover, it is well known that the interfacial properties between natural fiber and resin are low; therefore, surface treatment on continuous natural fiber was performed by using polyurethane (PU) and flexible epoxy (FLEX) to improve the interfacial properties. The effect of surface treatment on the mechanical properties of continuous natural fiber-reinforced thermoplastic composites was examined. From these results, it was suggested that surface treatment by PU with low content could produce composites with better mechanical properties.
In the paper entitled “Effect of red mud and copper slag particles on physical and mechanical properties of bamboo fiber reinforced epoxy composites” by S. Biswas et al., series of bamboo-fiber-reinforced epoxy composites are fabricated by using red mud and copper slag particles as filler materials. Filler plays an important role in determining the properties and behavior of particulate composites. The effects of these two fillers on the mechanical properties of bamboo-epoxy composites are investigated. Comparative analysis shows that with the incorporation of these fillers, the tensile strength of the composites increases significantly, whereas the flexural strength and impact strength decrease with increase in filler content (red mud and copper slag fillers) in the epoxy-bamboo fiber composites. The density and hardness are also affected by the type and content of filler particles. It is found that the addition of copper slag filler improves the hardness of the bamboo-epoxy composites, whereas the addition of red mud filler reduces the hardness value of bamboo-epoxy composites. The study reveals that the addition of copper slag filler in bamboo-epoxy composites shows the better physical and mechanical properties as compared to the red mud filled composites.
In the paper entitled “Mechanical, thermal degradation, and flammability studies on surface modified sisal fiber reinforced recycled polypropylene composites” by A. K. Gupta et al., the effect of surface treatment of fiber on the mechanical, thermal, flammability, and morphological properties of sisal fiber (SF) reinforced recycled polypropylene (RPP) composites was investigated. The surface of sisal fiber was modified with different chemical processes such as silane, glycidyl methacrylate (GMA), and O-hydroxybenzene diazonium chloride (OBDC) to improve the compatibility with the matrix polymer. The experimental results revealed an improvement in the tensile strength to 11%, 20%, and 31.36% and impact strength to 78.72%, 77%, and 81% for silane, GMA, and OBDC treated sisal fiber-reinforced recycled polypropylene (RPP/SF) composites, respectively, as compared to RPP. The thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), and heat deflection temperature (HDT) results revealed improved thermal stability as compared with RPP. The flammability behaviour of silane, GMA, and OBDC treated SF/RPP composites was studied by horizontal burning rate by UL-94. The morphological analysis through scanning electron micrograph (SEM) supports and improves surface interaction between fiber surface and polymer matrix.