Effect of fiber surface treatments on the fiber–matrix interaction in banana fiber reinforced polyester composites

Cellulosic fibers have been used as cost-cutting fillers in plastic industry. Among the various factors, the final performance of the composite materials depends to a large extent on the adhesion between the polymer matrix and the reinforcement and therefore on the quality of the interface. To achieve optimum performance of the end product, sufficient interaction between the matrix resin and the cellulosic material is desired. This is often achieved by surface modification of the resin or the filler. Banana fiber, the cellulosic fibers obtained from the pseudo-stem of banana plant (Musa sepientum) is a bast fiber with relatively good mechanical properties. The fiber surface was modified chemically to bring about improved interfacial interaction between the fiber and the polyester matrix. Various silanes and alkali were used to modify the fiber surface. Modified surfaces were characterized by SEM and FTIR. The polarity parameters of the chemically modified fibers were investigated using the solvatochromic technique. The results were further confirmed by electrokinetic measurements. Chemical modification was found to have a profound effect on the fiber–matrix interactions. The improved fiber–matrix interaction is evident from the enhanced tensile and flexural properties. The lower impact properties of the treated composites compared to the untreated composites further point to the improved fiber–matrix adhesion. In order to know more about the fiber–matrix adhesion, fractured surfaces of the failed composites where further investigated by SEM. Of the various chemical treatments, simple alkali treatment with NaOH of 1% concentration was found to be the most effective. The fiber–matrix interactions were found to be dependent on the polarity of the modified fiber surface.     

Processability of flax plant stalks into functional bast fibers

Flax (Linum usitatissimum L.) is an agricultural crop that is being considered as cost effective alternative to glass in composites. Flax is nature's composite with strong bast fibers held together in bundles and located in the outer regions of the plant stem between the outermost cuticle-epidermis layer and the innermost, woody tissues. Agricultural production of this crop is environmentally beneficial because it is produced through photosynthesis and considered a naturally renewable and sustainable material. Its use in bio-based composites could help lessen our dependence on fossil fuels today and into the future. Despite the longevity of flax in world markets, particular problems exist to provide flax fiber economically for markets in the US and North America. Barriers to such provisions include consistent, high-quality retting methods, efficient processing of whole stems into cottonized fibers, and development of standards to judge fiber quality. Processability of plant stalks into usable fiber requires retting which selectively removes pectinaceous and matrix substances thus separating cellulose fibers from non-fibrous substances and easing mechanical cleaning. Enzymes have been tested for efficiency of retting, and recommendations are in place for particular fiber properties. Before this work began, no processing facilities existed in North America for producing high value fibers. To facilitate research on bast fiber retting and subsequent processing, the Flax Fiber Pilot Plant (Flax-PP), was designed and modified after the 'Unified Line'; it is the only research facility of this type in the US and effectively processes and extracts bast fibers from plant stalks. A separate cottonizing unit has been installed to complete processing of fibers for various applications. Each processing step creates a fiber product and byproduct both with potential uses in composites. The development of standards through ASTM International is a continuing project. Currently, emphasis is on new equipment to quantify properties of particular usefulness such as cleanliness, fineness, and color. The Flax-PP becomes a valuable tool for improving fiber processing and quality, and thus integrates all activities for a flax fiber industry in North America.     

Ultrasound-Assisted Surface-Modification of Wood Particulates for Improved Wood/Plastic Composites

The combined effects of alkali and ultrasound treatment of wood flour on the mechanical properties of polypropylene-based wood/plastic composites (WPCs) were examined. FT-IR measurements confirmed that the alkali treatment removed both hemicellulose and lignin from the wood, and there was an increase in the number of hydroxyl groups on the cellulose surface. This process was promoted by ultrasound treatment. Mechanical testing of injection-molded WPC samples revealed that alkali treatment improved both composite strength and modulus when polypropylene grafted with maleic acid was used as a coupling agent. The strength increase is due to improved adhesion between the fiber and matrix, while improved modulus is due to the removal of lignin and hemicellulose that are not as stiff as cellulose. Polarized optical microscopy showed the presence of well-defined polymer crystals on the surface of the modified wood, and this is also responsible for the improved mechanical properties. It is conclusively demonstrated that the combination of chemical treatment of wood and ultrasound assistance is more effective in improving the mechanical properties of the composites than the use of chemical treatment alone.     

Short sisal fiber reinforced polypropylene composites: the role of interface modification on ultimate properties

Sisal fibers have been used for the reinforcement of polypropylene matrix. The compatibilization between the hydrophilic cellulose fiber and hydrophobic PP has been achieved through treatment of cellulose fibers with sodium hydroxide, isocyanates, maleic anhydride modified polypropylene (MAPP), benzyl chloride and by using permanganate. Various fiber treatments enhanced the tensile properties of the composites considerably, but to varying degrees. The SEM photomicrographs of fracture surfaces of the treated composites clearly indicated the extent of fiber–matrix interface adhesion, fiber pullout and fiber surface topography. Surface fibrillation is found to occur during alkali treatment which improves interfacial adhesion between the fiber and PP matrix. The grafting of the fibers by MAPP enhances the tensile strength of the resulting composite. It has been found that the urethane derivative of polypropylene glycol and cardanol treatments reduced the hydrophilic nature of sisal fiber and thereby enhanced the tensile properties of the sisal–PP composites, as evident from the SEM photomicrographs of the fracture surface. The IR spectrum of the urethane derivative of polypropylene glycol gave evidence for the existence of a urethane linkage. Benzoylation of the fiber improves the adhesion of the fiber to the PP matrix. The benzoylated fiber was analyzed by IR spectroscopy. Experimental results indicated a better compatibility between benzoylated fiber and PP. The observed enhancement in tensile properties of permanganate-treated composites at a low concentration is due to the permanganate-induced grafting of PP on to sisal fibers. Among the various treatments, MAPP treatment gave superior mechanical properties. Finally, experimental results of the mechanical properties of the composite have been compared with theoretical predictions.     

Micromechanical Behavior of Carbon Nanotube-Covered SiC Fibers in Polymer Matrix Composites

The incorporation of nanotube-covered fibers in continuous fiber/epoxy composites has been shown to influence the mechanical, electrical, and thermal properties of the composite. Increased interlaminar shear stress, flexural strength and modulus have been reported in such composites over composites containing bare fibers. In this study, the microstructure and interfacial shear strength (ISS) of continuous silicon carbide fiber/epoxy composites with and without nanotubes grown from the SiC fiber surface were investigated with micro-Raman spectroscopy (MRS) and microscopy. The fibers with nanotubes grown from the surface were found to have a reduced ISS compared with the bare fibers. Electron microscopy showed good wetting of epoxy in the nanotube forests, but poor attachment of the nanotube forests to the fibers. These results suggest that the mechanism leading to improvements in bulk composite properties is not due to an improvement in the fiber/matrix ISS.     

Unsaturated polyester resin (UPR) reinforced with flax fibers,untreated and cold He plasma-treated: thermal,mechanical and DMA studies

Hemp, jute, flax, bagasse, coconut and bamboo fibers are some of the natural fibers that have attracted attention for the preparation of composite materials because of their low cost compared with synthetics fibers (glass, carbon). The performance of a natural fiber as reinforcement in composite materials is linked to its ability in term of adhesion with the synthetic matrix. This depends mainly on the quality of the fiber surface. In order to improve this adhesion, a thin reactive coating is generally used. In this study, cold He plasma treatments have been carried out on reinforcing flax fiber. Composites with unsaturated polyester resin (UPR) have been used with untreated flax fibers and plasma-treated fibers. The data characterizing the thermal, mechanical (dynamic and static) will be presented in order to analyze the efficiency of the He plasma treatment on the composite performances.     

Effect of extractives on the flexural properties of wood/plastic composites

This study investigated the effect of extractives in wood flour on the mechanical properties of wood-polypropylene (PP) composites. Three different solvents, acetone/water, dioxane/water and benzene/ethanol, were used to remove extractives in both pine and Douglas fir wood flour. X-ray photoelectron spectroscopy (XPS) confirmed that extraction resulted in a change in the surface composition of the wood flour. Differential scanning calorimetry showed no changes in the percent crystallinity of the PP matrix in the wood-PP composites and optical microscopy showed no detectable changes in PP spherulite size or shape between filled PP containing extracted and unextracted wood flour. A large increase in the strength of pine flour-PP composites was observed upon removal of extractives from pine flour. The Douglas fir flour-PP composites showed a smaller, but statistically significant, increase in strength upon removal of extractives, with the exception of the dioxane/water extracted Douglas fir. Significant differences were also observed in stiffness between extracted wood-PP and unextracted wood-PP composites with the exception of the dioxane/water extracted Douglas fir, which was not significantly different from the control.     

Effect of compatibilizers on the crystallization kinetics of cellulose-filled high density polyethylene

High density polyethylene (HDPE) is a ubiquitous material with versatile properties. It is produced and used in greater volume than any other thermoplastic. HDPE is often filled with a variety of materials for various applications. Glass fiber and wood flour are two common fillers for HDPE. This study investigated microcrystalline cellulose (MCC) as a filler in HDPE. The use of compatibilizers, or coupling agents, was investigated as a means of improving the dispersion of the cellulose filler in the HDPE matrix and the mechanical properties of the resulting composites. One compatibilizer was shown to improve the strength of the resulting composite. The stiffness was unaffected, as expected. Thermal properties were measured by means of differential scanning calorimetry. Analysis of the crystallization kinetics indicated that the Avrami coefficient was altered by the filler and was also modified by the presence of the compatibilizer. The presence of cellulose and/or compatibilizer increased the matrix degree of crystallinity. The two compatibilizers studied did not behave similarly and may have different mechanisms of compatibilization.     

Highly filled composites with thermoplastic elastomers matrix

The mechanical and Theological properties of blends based on commercial types of thermoplastic elastomers (TPEs) Finaprene with polystyrene and ethylene-vinylacetate copolymer, used as suitable materials to produce highly filled composites with magnetically hard fillers, have been investigated. The influence of a 90% to 93% filler content and the morphology of the fillers on mechanical, Theological, and magnetic properties of composites is discussed. The influence of changes of other technological parameters, such as type of treatment of the fillers and type of mixer, on properties of composites has been studied.     

Jute fiber-reinforced chemically functionalized high density polyethylene (JF/CF-HDPE) composites with in situ fiber/matrix interfacial adhesion by Palsule Process

Jute fiber-reinforced chemically functionalized polyethylene high density (JF/CF-HDPE) composites have been processed, by Palsule process without using any compatibilizer and without any fiber modification, by using chemically functionalized maleic anhydride grafted polyethylene (MAPE) as matrix, in place of polyethylene. Fiber/matrix interfacial adhesion generated in situ, due to interactions between jute fiber and the maleic anhydride of the CF-HDPE matrix, has been established by Fourier transform infrared spectroscopy and scanning electron microscope micrographs. Mechanical properties of the JF/CF-HDPE composites developed with in situ fiber/matrix interfacial adhesion in this study have been found to be higher than those of the CF-HDPE matrix and increase with increasing amounts of jute fibers in the JF/CF-HDPE composites, and are better than properties of literature reported and laboratory processed jute fiber/polyethylene composites with and without MAPE compatibilizer. Measured tensile modulus of JF/CF-HDPE composites compares well with values predicted by rule of mixtures, inverse rule of mixture, Hrisch Model, Halpin-Tsai equations, Nielsen equations, and with Palsule equation. The feasibility of developing natural fiber/maleic anhydride grafted polyolefin composites by Palsule process without using any compatibilizer and without any fiber treatment is demonstrated.     

Thermal behavior of chemically treated and untreated sisal fiber reinforced composites fabricated by resin transfer molding

Using thermogravimetric analysis (TGA), the thermal behavior of sisal fibers and sisal/polyester composites, fabricated by resin transfer molding (RTM), has been followed. Chemical treatments have been found to increase the thermal stability, which has been attributed to the resultant physical and chemical changes. Scanning electron microscopy (SEM) and infrared (FT-IR) studies were also performed to study the structural changes and morphology in the sisal fiber during the treatment. The kinetic studies of thermal degradation of untreated and treated sisal fibers have been performed using Broido method. In the composites, as the fiber content increases, the thermal stability of the matrix decreases. The treated fiber reinforced composites have been found to be thermally more stable than the untreated derivatives. The increased thermal stability and reduced moisture behavior of treated composites have been correlated with fiber/matrix adhesion.     

Performance of glass woven fabric composites with admicellar-coated thin elastomeric interphase

Adequate stress transfer between the inorganic reinforcement and surrounding polymeric matrix is essential for achieving enhanced structural integrity and extended lifetime performance of fiber-reinforced composites. The insertion of an elastomeric interlayer helps increase the stress-transfer capabilities across the fiber/matrix interface and considerably reduces crack initiation phenomena at the fiber ends. In this study, admicellar polymerization is used to modify the fiber/matrix interface in glass woven fabric composites by forming thickness-controlled poly(styrene-co-isoprene) coatings. These admicellar interphases have distinct characteristics (e.g. topology and surface coverage) depending on the surfactant/monomer ratios used during the polymerization reaction. Overall, the admicellar coatings have a positive effect on the mechanical response of resin transfer molded, E-glass/epoxy parts. For instance, ultimate tensile strength of composites with admicellar sizings improved 50–55% over the control-desized samples. Interlaminar shear strength also showed increases ranging from 18 to 38% over the same control group. Interestingly, the flexural properties of these composites proved sensitive to the type of interphase formed for various admicellar polymerization conditions. Higher surface coverage and film connectedness in admicellar polymeric sizings are observed to enhance stress transfer at the interfacial region.     

Visibly transparent & radiopaque inorganic organic composites from flame-made mixed-oxide fillers

Radiopaque composites have been produced from flame-made ytterbium/silica mixed oxide within a crosslinked methacrylate resin matrix. The refractive index of the filler powder increased with ytterbium oxide loading. A high transparency was achieved for a matching refractive index of the filler powder and the polymer in comparison to commercial materials with 52 wt% ceramic filling. It was demonstrated that powder homogeneity with regard to particle morphology and distribution of the individual metal atoms is essential to obtain a highly transparent composite. In contrast, segregation of crystalline single-oxide phases drastically decreased the composite transparency despite similar specific surface areas, refractive indices and overall composition. The superior physical strength, transparency and radiopacity compared to composites made from conventional silica based-fillers makes the flame-made mixed-oxide fillers especially attractive for dental restoration materials.     

Banana fiber/chemically functionalized polypropylene composites with in-situ fiber/matrix interfacial adhesion by Palsule process

Chemically functionalized maleic anhydride (MAH)-grafted polypropylene matrix has been used (in place of polypropylene as matrix with compatibilizer) to process banana fiber/chemically functionalized polypropylene (BF/CFPP) composites, without using any compatibilizer and without any fiber modification by Palsule process. Fiber/matrix interfacial adhesion generated, in-situ, due to interactions between BF and the MAH of the CFPP matrix has been established by Fourier transform infrared spectroscopy and scanning electron microscopy. Mechanical properties of the BF/CFPP composites developed by Palsule process with in-situ fiber/matrix interfacial adhesion in this study have been found to be higher than those of the matrix and it increases with increasing amounts of fibers in composites, and are better than properties of literature reported BF/polypropylene composites processed with compatibilizers. Measured modulus of BF/CFPP composites compares well with values predicted by rule of mixtures, Hrisch model, Halpin-Tsai equations and its modified Nielsen version, and with Palsule equation. The feasibility of developing natural fiber/MAH grafted polyolefin composites by Palsule process without using any compatibilizer and without any fiber treatment is demonstrated.     

Melt Flow Instabilities of Wood Polymer Composites

Melt flow instabilities during extrusion of wood polymer composites (WPC) containing 30–60 wt% wood flour (WF) have been investigated. The research emphasized elucidation of the extrudate surface tearing mechanism and its relation to wall slip. This interesting phenomenon has been known in the WPC industry for years; however, it has not received much research interest. It was observed that increasing the wood flour loading up to 50 wt% aggravated the surface tearing; however, addition of 60 wt% wood flour completely eliminated the surface defect due to strong wall slip and plug flow. It was also found that addition of lubricants and increasing the shear rate significantly improved the surface appearance of the filled compounds. Molecular weight and molecular weight distribution of the polymer matrix influence the melt flow properties of the composites. The significance of the entrance pressure measurement and its usefulness for quantitative assessment of filler–matrix interactions in composite materials is also demonstrated in this paper.     

A study of advances in characterization of interfaces and fiber surfaces in lignocellulosic fiber-reinforced composites

This article deals with the aspects of interfacial and surface characterization of natural fibers and their composites. Vegetable fibers and their composites have attracted the attention of scientists worldwide because of their favorable properties. The different chemical modifications of natural fibers and characterization aspects have been discussed. The adhesion between fiber and matrix is a major factor in determining the response of the interface and its integrity under stress. Therefore characterization of the interface is of utmost importance. Both fiber surface and polymer matrix surface can be modified to obtain a strong interface. Various treatments being used for the lignocellulosic surfaces and the characterization techniques have been illustrated. The four main techniques of interfacial characterization that are enumerated in this article are the micromechanical techniques, spectroscopic, microscopic and swelling techniques. The micromechanical techniques like fiber pull-out and fragmentation have been dealt with giving emphasis to experimental aspects. Recent studies dealing with interfacial study of different lignocellulosic fiber reinforced composites have also been cited.     

Effect of chemical treatment on dynamic mechanical properties of sisal fiber-reinforced polyester composites fabricated by resin transfer molding

The dynamic mechanical properties of treated sisal fiber-reinforced polyester composites fabricated by resin transfer molding (RTM) have been studied with reference to fiber surface modifications, frequency and temperature. The sisal fibers have been subjected to various chemical and physical treatments like mercerization, heating at 100°C, permanganate, benzoylation and vinyl tris(2-ethoxymethoxy) silane to improve the interfacial bonding with isophthalic polyester resin. Results indicated that treatment changed the storage modulus (E′), loss modulus (E″) and damping factor (tan δ) drastically at a wide range of temperature. The E′ value increased for every treatment, and is maximum for the composites fabricated by benzoylated-treated fibers. The T _g value obtained from the E″value showed an increase as compared to untreated fiber-reinforced composites. The alkali-treated fiber-reinforced composites showed lower tan δ value. Using Arrhenius' equation the activation energy was calculated and found maximum for the composites fabricated by alkali-treated fiber, which shows good fiber/matrix interactions.     

Lignocellulosic composites with grafted polystyrene interfaces

The effects of surface grafting of a polymer onto lignocellulosic fiber surface and processing methods on both the interfacial interactions and the resulting composite properties of the fiber-reinforced thermoplastic composites were investigated. Chemithermomechanical pulp (CTMP) wood fiber was used as a reinforcement, which has been chemically modified by radical polymer grafting of styrene onto the fiber surfaces. The chemically modified CTMP fiber was then compounded with polystyrene (PS). Two different processing methods, both compression and injection moldings, were performed to prepare the wood-fiber-reinforced composites. Experimental results showed that surface modification of wood fiber leads to an obvious increase in mechanical properties of the fiber-reinforced composites as compared to the untreated fiber composites. The enhancement of mechanical properties is much greater through injection molding compared with compression molding owing to occurrence of orientation, and better mixing and interaction between the fiber and the matrix by injection molding. An improvement in fiber wetting properties and adhesion by the matrix was observed through scanning electron microscopy for the surface grafted fiber reinforced composites. Untreated wood fiber exhibited a smooth surface without adhered polymer, indicating poor adhesion, while polymer attached to the surface was seen on treated cellulose fiber due to the higher fiber-matrix interactions.     

Effect of Carbon Nanotubes on the Mechanical Properties of Polypropylene/Wood Flour Composites: Reinforcement Mechanism

Maleic anhydride grafted polypropylene (PP-g-MA) was employed as the compatibilizer and carbon nanotubes (CNTs) or hydroxylated CNTs as reinforcements for polypropylene/wood flour composites. The results showed that when the PP-g-MA loading level was 10 wt%, the bending strength, tensile strength, Izod notched impact strength, and elongation at break of PP-wood composites were enhanced by 85% (66.3 MPa), 93% (33.7 MPa), 5.8% (2.01 kJ/m²), and 64% (23%), respectively, relative to the uncompatibilized composites. The introduction of pristine CNTs only improved slightly the overall mechanical properties of the compatibilized composites due to poor interfacial compatibility. Unlike CNTs, incorporating hydroxylated CNTs (CNT-OH) could significantly improve all of the mechanical properties; for instance, at 0.5 wt% CNT-OH loading, the flexural strength and tensile strength reached 68.5 MPa, and 40.4 MPa about 6.6% higher than that for the composites with the same CNT loading. Furthermore, CNT-OH also remarkably enhanced the storage modulus. Contact angle and morphology observations indicated that the increases in mechanical properties could be attributed to the improvements of interfacial interactions and adhesions of CNTs with the matrix and fillers.     

Sodium Borohydride as a Protective Agent for the Alkaline Treatment of Sisal Fibers for Polymer Composites

The vegetable fibers used for polymer matrix composites are usually treated to improve their adhesion with the matrix. The chemical treatment with sodium hydroxide (NaOH), although widely used, may damage the fiber surface structure, reducing its strength. The possibility of protecting vegetable fibers against alkaline chemical aggression by using hydride ions (H^–) was investigated in this work. Sisal fibers were modified by immersion in a NaOH aqueous solution (2, 5 and 10% wt/vol), with or without the addition of sodium borohydride (NaBH₄) (1% wt/vol), under variable conditions (immersion time and temperature). The effect of using NaBH₄ was investigated using fiber tensile and pull-out tests, critical length calculation, along with a Weibull statistical analysis. This agent was found to minimize sisal degradation under highly concentrated alkaline conditions in comparison with sisal treated with the pure NaOH solution. The results suggest the 5% wt/vol treatment for 60 min under room temperature in the presence of the hydride ions as the most suitable for sisal. This result may be extended to other vegetable fibers of similar composition and may promote their use in polymer composites.