Natural fi bers, especially lignocellulosic fi bers extracted from plants, are gaining attention as polymer-matrix composite (PMC) reinforcements due to their comparative advantages over synthetic fi bers. Natural fi bers are relatively low cost, renewable, and biodegradable. Their production systems are associated with low equipment wear and are energy effi cient. In addition, the incorporation of lignocellulosic fi bers into PMCs may signifi cantly improve some mechanical properties. This article presents an overview of the advantages and drawbacks of applying natural fi bers, some of them relatively unknown, as reinforcements of PMCs. The mechanical behavior of composites incorporated with selected fi bers is discussed in terms of the effect of surface micromorphology and the fi ber/matrix interaction.

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By means of dimensional selection of natural lignocellulosic fibers, based on precise diameter measurements, it was recently possible to obtain fibers with relatively higher tensile strength. The present article overviews works on the statistical evaluation, through the Weibull analysis, of the ultimate tensile stress of eight lignocellulosic fibers: sisal, ramie, curaua, jute, bamboo, coir, piassava and buriti. It is shown that, for all of these fibers, the tensile strength holds an inverse relationship with the fiber diameter. Statistically this relationship conforms to a hyperbolic type of analytical equation, which discloses the possibility of unusually high strength fibers to be selected in association with very small diameters. A structural analysis using scanning electron microscopy offered an explanation to the strengthening mechanisms responsible for the superior performance of these dimensionally selected 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.

Recebido em 28/5/09; aceito em 25/11/09; publicado na web em 23/3/10We studied the adsorption of glyphosate (GPS) onto soil mineral particles, using FT-IR and Mössbauer spectroscopy. From IR measurements for samples collected under native vegetation of a forest reserve, bands at 1632 and 1407 cm -1 could be attributed to the interaction between the carboxylic group of GPS and structural Al 3+ and Fe 3+ on the surface of mineral particles; bands at 1075 and 1000 cm -1 were observed only for cultivated soil. Mössbauer spectra for these soils were definitely fitted using a broad central doublet in addition to the magnetic component. This multiple quadrupolar component may be attributed to all non-magnetic Fe 3+ contributions, including that of the GPS/Fe 3+ complex.