Felipe Perissé Duarte Lopes

High Strength Natural Fibers for Improved Polymer Matrix Composites

A statistical evaluation based on the Weibull method was performed to correlate the mechanical properties and the diameter of different lignocellulosic fibers. The sisal, rami and curaua fibers were found to have a hyperbolic correlation between their ultimate strength and diameter. This permitted to select thinner high strength fibers, with over 1000 MPa, as reinforcement for the strongest polymer composites ever fabricated with these fibers. A structural analysis was conducted by electron microscopy to identify the strengthening mechanism for both, the high performance fiber and their improved polymer composites.

Tenacidade ao entalhe por impacto Charpy de compósitos de poliéster reforçados com fibras de piaçava

Rigid piassava fibers of the specie Attalea funifera Mart have been recently investigated as effective reinforcements for polymeric composites. The works performed so far in these composites have shown a reasonable increase in the mechanical strength with the amount of fiber. Since some possible application for piassava composites would demand resistance to impact, the objective of the present work was to carry out a study on the notch toughness, measured by a pendulum hammer, of rigid and aligned piassava fiber reinforced polyester matrix composites. Standard specimens of composites with up to 40 wt.% of continuous piassava fibers were evaluated by means of Charpy tests and their fracture microstructure investigated by scanning electron microscopy. The results obtain showed that piassava composites present a notch toughness behavior that is comparatively superior to that of other natural fiber reinforced polymeric composites.

Comportamento Mecânico e Características Estruturais de Compósitos Poliméricos Reforçados com Fibras Contínuas e Alinhadas de Curauá

Polymeric matrix composites reinforced with natural lignocellulosic fibers are now being used in many fields of practical interest. This has motivated scientific and technological investigations on both, traditional fibers, such as sisal and jute, as well as those, like curaua, which present promising characteristics. Until today, however, the works performed on the behavior of curaua reinforced polymeric composites used short, discontinuous and randomly oriented fibers. As a consequence the attained mechanical strength was relatively small. In the current work the properties of polyester matrix composites reinforced with up to 30 wt. % of continuous and aligned curaua fibers was investigated. These composites were bend tested and the fracture surface was observed by scanning electron microscopy. The results showed strength values higher than those obtained by other researchers in curaua composites with short and non-oriented fibers. Microstructural observations revealed an effective adhesion between the fibers and the matrix, which contributed to the mechanical performance of the present composites.

Tensile behavior of lignocellulosic fiber reinforced polymer composites: Part II buriti petiole/polyester

The current interest for natural fibers as an environmentally correct composite reinforcement has motivated the investigation of new possibilities. For instance, the fibers extracted from the petiole of the buriti palm tree were recently found to have adequate mechanical properties to reinforce polymer composites. Therefore, the present work evaluates the tensile properties of polyester composites incorporated with thinner buriti petiole fibers for improved mechanical performance. Composites with up to 40% in volume of buriti petiole fibers embedded in orthophtalic polyester matrix were post-cured and then ruptured in tension. Fracture surfaces were analyzed by scanning electron microscopy. A marked increase in the tensile strength was found with the amount of buriti fibers. The fracture analysis revealed aspects of the bonding condition at the fiber/matrix interface, which could be associated with the composite performance.

Interfacial Shear Strength in Lignocellulosic Fibers Incorporated Polymeric Composites

Lignocellulosic fibers have been recognized as attractive fillers for different types of matrices in polymeric composites. Their advantages such as recyclability and renewability are unique characteristics for composites used as automobile components and building structural panels. In view of the hydrophobic behavior of most polymers and the hydrophilic nature of lignocellulosic fibers, poor adhesion is observed between lignocellulosic fibers and the polymeric matrix, which results in lower mechanical properties. Pullout tests have been successfully used to determine the interfacial shear stress in synthetic fiber-reinforced composites, but little has been reported in the case of lignocellulosic fiber–polymer composites. This chapter presents an overview on the determination of the interfacial strength of lignocellulosic fibers–polymer matrix composites including some obtained by the authors on Brazilian fibers such as curaua, ramie, and piassava, considered as reinforcement for composites. Concluding remarks and suggestions indicate some future works.

Tensile behavior of lignocellulosic fiber reinforced polymer composites: Part I piassava/epoxy

The fibers extracted from the piassava palm tree, scientifically known as Attalea funifera, are among the stiffest lignocellulosic fibers being considered for polymer composite reinforcement. Characterization of piassava composites have been carried out for different polymeric matrices and mechanical tests. In this work the tensile properties of DGEBA/TETA epoxy matrix composites reinforced with up to 30% in volume of continuous and aligned piassava fibers were evaluated. Tensile specimens post-cured at 60 o C for 4 hours were room temperature tested and the corresponding fracture analyzed by scanning electrons microscopy. The results showed a decrease in both the tensile strength and the elastic modulus of the composites up to 30% with an increase at 40% of piassava fibers to values above those of the pure epoxy. The fracture analysis revealed a weak fiber/matrix interface, which could account for the comparative low performance of these composite in tensile tests up to 30% of volume fraction. The relatively large amount of stronger piassava fibers accounts for the better performance of the composite with 40% in volume fraction.

Charpy impact resistance of alkali treated curaua reinforced polyester composites

Natural fibers obtained from cellulose-based plants are being used as reinforcement of polymer composite owing to both environmental and technical advantages. One important technical characteristic of most lignocellulosic fibers is the bend flexibility, which allows them to resist impact forces. As a consequence, there is an increasing application of these lignocellulosic fibers in automobile parts that, during a crash event, should absorb the impact energy without splitting into sharp pieces. The present work investigates the toughness behavior of polyester composites reinforced with up to 30% in volume of alkali treated continuous and aligned curaua fibers by means of Charpy impact tests. It was found that the incorporation of treated curaua fibers increased the composite absorbed impact energy but not as much as in composites reinforced with non-treated fibers. Macroscopic observation, and scanning electron microscopy analysis of fracture surface, revealed that the main mechanism for the increase in the Charpy notch toughness is the interfacial rupture between the curaua fiber and the polyester matrix.

Weibull analysis for the diameter dependence of the elastic modulus of curaua fibers

The curaua fiber is one of the strongest lignocellulosic fibers and is currently being considered as reinforcement of polymer composites for industrial applications such as automobile interior components and bicycle helmets. The tensile strength of the curaua fiber was found to display an inverse variation with its corresponding equivalent diameter. Since the stiffness of the fiber is also important for its use as composite reinforcement, the present work investigated the dependence of the elastic modulus of curaua fibers with the associated diameters. The results confirmed the existence of an inverse dependence between the elastic modulus and the fiber diameter. In principle, this could allow a selection of stiffer curaua fibers to be used as reinforcement in polymer composites with comparatively higher elastic modulus. A possible mechanism for this inverse dependence is discussed following structural differences between thicker and thinner fibers.

Dimensional Analysis and Surface Morphology as Selective Criteria of Lignocellulosic Fibers as Reinforcement in Polymeric Matrices

In recent years, there has been a resurgence of interest for the use of renewable materials such as plant fibers, also called “lignocellulosic fibers”, due to increasing environmental concerns along with the unique characteristics of these fibers. These include abundant availability, renewability, biodegradability, as well as low wear and tear of equipments, particularly when processing their composites. In comparison with today’s most used synthetic fibers such as glass fiber, lignocellulosic fibers offer the advantage of lesser health hazards and of course lower cost. In addition, they help in generating employment, particularly in rural sector, by leading to better living standards of the rural population. Thus, the utilization of these fibers has both short-term objectives, through the synthesis and characterization of composites, and long-term objectives, to use them as alternates for synthetic fibers and possible substitute for wood. This has driven the researchers to bring out data on the source and availability of all the useful lignocellulosic fibers, cataloging their available information on morphology and properties as well as current uses. A sound knowledge of the morphology of these fibers helps the understanding of their observed properties in terms of structural parameters, such as number, size, and shape of cells, chemical constituents, as well as the fracture mechanism in these fibers. Further, a careful examination of various properties of these fibers indicates that they are inconsistent probably due to the nonuniformity in dimensions and the defects in these fibers. The latter may be present either inherently or due to their processing. As a consequence, highly scattered properties are observed, which may be one of the drawbacks for their use as engineering materials. These limitations could, in principle, be overcome through an individual selection of fibers with approximately the same dimensions and properties by knowing the nature of correlation of fiber dimensions with a given property whereby the strongest fibers based on selection of their diameters could be separated. However, this may pose some problem with tedious work. Development of a scientific methodology does become essential to the selection of these fibers particularly for their application as reinforcements in various matrices to render them reliable, similar to synthetic fiber products.

Propriedades mecânicas e termomecânicas de compósitos com partículas de diamante dispersas em matriz epoxídica modificada na razão resina/endurecedor

The mechanical properties of composites with dispersed diamond particles in epoxy matrix cured with different proportions of hardener to monomer ratio, characterized by the resin/hardener ratio (phr) were studied. An investigation on the thermo-mechanical behavior of these composites was also carried out by dynamical mechanical analysis (DMA). In the present work a complete evaluation of the mechanical properties was carried in a wide interval of phr associated with possible technological applications. Composites with up to 30 wt. % of diamond particles dispersed in type DGEBA/TETA epoxy matrix were fabricated with phr ratios varying from 7 to 21. For all investigated conditions, the composite strength decreased with the amount of incorporated diamond. Matrices with phr above the stoichiometric 13 were associated with composites with better mechanical performance. The DMA results showed that the storage modulus increases with the amount of diamond particles incorporated in the composite. The values obtained for the delta tangent, allowed an evaluation of possible mechanisms that contribute to the thermal mechanical performance of these composites.