L. H. de Carvalho

Analysis of the tensile strength of polyester/hybrid ramie–cotton fabric composites

Abstract Plain weave hybrid ramie–cotton fabrics were used as reinforcement in polyester matrix composites. The tensile strength of the composites was determined as a function of the volume fraction and orientation of the ramie fibers. Composites were tensile tested with ramie fibers oriented parallel, (0), to the tensile axis and with various stacking sequence configurations (0/90). The results obtained showed that the main parameter governing the tensile properties of the composites was the ramie volume fraction parallel to the direction of the tensile axis. The contribution of the cotton fibers was shown to be minimal. Indeed, the results obtained for the tensile strength of the (0) composites were shown to follow a common rule of mixtures law, disregarding the contribution of the cotton fibers. Values of tensile strength of up to 338% greater than that of the matrix were obtained which shows the potential of the ramie fiber as reinforcement in lignocellulosic fiber composites.

Influence of Fiber Orientation on the Mechanical Properties of Polyester/Jute Composites

Tensile and impact properties of compression molded unsaturated polyester/jute composites were investigated as a function of fiber content and orientation. Both unidirectional fiber composites with 0-50% weight fiber content and a randomly distributed short-fiber composite with 30% w/w were manufactured. The unidirectional composites were tested along and transversally to the fiber axis. Higher values for all mechanical properties were obtained when long-fiber oriented composites were tested along the fiber axis, even at low fiber content (10% w/w). The tensile behavior of the unidirectional composites qualitatively followed the theoretical isostrain and isostress behaviors, when the tests were conducted along and perpendicular to the fiber axis, respectively. The tensile properties of the composites tested perpendicular to the fiber were dominated by the strain at the fiber-matrix interface. Properties for randomly distributed short-fiber composites were found to be intermediate between those obtained with long-fiber oriented composites with the same fiber load tested along and across the fiber direction.

Influence of Water Absorption and Pre-drying Conditions on the Tensile Mechanical Properties of Hybrid Lignocellulosic Fiber/Polyester Composites

It is well known that humidity can deleteriously affect composite mechanical properties. In the present study the tensile properties of hybrid jute/cotton and sisal/cotton fabrics— polyester matrix composites were analyzed as a function of fiber content, immersion time in distilled water and of fabric drying just prior to composite manufacture. The results indicate that, as expected, tensile properties increase with fiber content, and that sisal reinforced composites were slightly more affected by water exposure than jute reinforced ones. This behavior was attributed to the higher affinity of sisal fibers toward water absorption. Fabric drying immediately before their incorporation onto the composites was shown to play an important role on composite mechanical properties. Fabric pre-drying led to composites with lower water absorption and higher overall mechanical properties. These composites also displayed a smaller strain at rupture, evincing better fiber to polymer matrix interaction. The data obtained suggests that water acts as a plasticizer for the systems investigated and that plasticization occurred in the early stages of composite immersion in water.

Immersion Temperature Effects on the Water Absorption Behavior of Hybrid Lignocellulosic Fiber Reinforced-Polyester Matrix Composites

Although economic, ecological, processing and property considerations suggest that it is very attractive to use lignocellulosic fibers as reinforcement in polymer matrix composites, moisture can strongly and deleteriously affect their properties. In this work the water absorption behavior of sisal/cotton, jute/cotton and ramie/cotton hybrid fabric reinforced composites is evaluated. The effect of the temperature of immersion, fiber volume fraction, and predrying of the fabrics before their incorporation onto the composites are evaluated. Sisal was shown to be the most hygroscopic of the fibers analyzed, and its presence leads to higher values of the maximum water content and of the diffusion coefficient of sisal/cotton reinforced composites. Under the range of temperatures analyzed (30–60°C) the volume fraction of the fibers, rather than the temperature itself, was shown to be the main parameter governing water absorption. Predrying usually lowers maximum water content, although for sisal/cotton reinforced composites a reverse trend was observed for the composites with higher volume fractions. This behavior was again attributed to the higher hydrophilic behavior of sisal fibers.

Comparison of the Mechanical Behavior of Plain Weave and Plain Weft Knit Jute Fabric-Polyester-Reinforced Composites

The tensile and impact behavior of jute fabrics—polyester composites–were evaluated as a function of the fabric style (knitted or weaved cloths), fiber weight fraction, and direction of the applied load. The tensile properties of plain-weave-fabric-reinforced composites (PWF) were higher than those of plain weft knit cloth composites (WKT) and were dependent on fiber content and test direction. The properties of the WKT, however, were independent of these variables. The results obtained indicate that the orthogonal fiber alignment of weaved cloths favors anisotropy, while the interconnected loops in knit fabrics favors isotropy. The results also indicate weak fiber-matrix interactions in both fabrics and a better fabric impregnation for the plain weave fabric if compared with that of the knit fabric. The impact strengths of both composites were higher than that of the matrix and were shown to increase with fiber content. WKT-reinforced composites showed better impact absorption capacity than PWF composites. This behavior is attributed to the influence of the weaving pattern of the fabrics and to the differences in fabric impregnation by the matrix.

Accelerated Aging of Hybrid Ramie-Cotton Resin Matrix Composites in Boiling Water

Abstract The effect of accelerated aging by immersion in boiling water on the tensile behavior of a hybrid ramie-cotton fabric polyester composite was evaluated. Under these conditions, fast water absorption by the lignocellulosic fibers took place, causing a sharp reduction in the tensile strength of the composite during the first 5 h of immersion. A decrease in Youngs modulus and an increase in the deformation at break were observed at longer immersion times. It was shown that resin embrittlement occurred under the aging conditions used in this work, which was attributed to the leaching out of low molecular weight substances. Therefore, the plasticizing effect observed with the composites was attributed to the presence of the lignocellulosic fibers. The water absorption kinetics were satisfactorily modeled using the Fickian model.

Mechanical, Morphological, and Structural Characteristics of Caroa (Neoglaziovia variegata) Fibres

The surface morphology, thermal behaviour and tensile mechanical properties of raw caroa (Neoglaziovia variegata) fibres have been characterised. The influence of fibre washing with flowing tap water on the surface morphology and thermal properties of these fibres has also been investigated. The results show that gummy tissues are attached to the surface of the raw fibres and that washing leads to the partial removal of these tissues and to the exposure of the inner fibrillar structure of the fibres. Thermogravimetric analysis indicates that fibre thermal degradation is characterised by three main degradation stages corresponding, respectively, to water release at low temperatures, followed by decomposition of hemicellulose and of α-cellulose. Fibre washing produces an increase in the fibre thermal degradation temperature, Tonset, and in the thermal decomposition rate. These characteristics are associated with the removal of gummy tissues and with a decrease in the compactness of the fibres respectively. The tensile properties obtained place caroa as a low-to-medium-strength fibre when compared with other lignocellulosic fibres, but with a strength level similar to that of coir fibres, which, nowadays, are used commercially on a fairly large scale.

Water sorption of a polyurethane/bentonite composite : Effects of bentonite content and drying conditions

In the present work, compression molded polyurethane/bentonite composites were manufactured and their water sorption characteristics were determined as a function of filler content and drying conditions. The matrix was a polyurethane derived from castor oil and the filler was a commercially available sodium bentonite. Filler contents varied from 0 to 10% by weight. The effects of three filler drying conditions, that is, 1 h at 200°C; 1 h at 110°C, and 24 h at 110°C, on the water sorption behavior of PU/bentonite composites with 5% w/w filler loading were also analyzed. The experimental results show that the water sorption behavior of the composites could be satisfactorily modeled by the Fickian model, and that the amount of absorbed water increases with the filler loading. It was also observed that, within the first 200 h of immersion, composite water sorption was lower than that of the polymeric matrix alone. The data evince that although only minor differences were noted, water sorption is reduced when the filler was dried at higher temperatures.

Water Sorption of Vegetable Fiber Reinforced Polymer Composites

Despite the ever-growing worldwide interest in the use of lignocellulosic fibers as reinforcement in either thermoset or thermoplastic matrices, the use of these fibers to replace synthetic ones, is limited. The reasons for these limitations are associated with the vegetable fiber’s heterogeneity, lower compatibility to most polymers, inferior durability, flammability, poorer mechanical properties and higher moisture absorption when compared with synthetic fibers. Nevertheless, despite these drawbacks, vegetable fiber reinforced polymer composites are lighter in weight, more sustainable and can be used for non-structural products. Strategies to minimize these drawbacks include fiber and or matrix modification, the use of compatibilizers, fiber drying and the concomitant use of vegetable and synthetic fibers, for the production of hybrid composites, the latter being an unquestionable way to increment overall mechanical and thermal properties of these hybrid systems. Here we present data on the water sorption of polymer composites having thermoset and thermoplastic matrices as a function of vegetable fiber identity, content and hybridization with glass fibers. Our data indicates that, regardless if the matrix is a thermoset of a thermoplastic, water absorption tends to be relatively independent of vegetable fiber identity and to be significantly dependent of its content. Fiber drying prior to composite manufacturing and hybridization with glass fibers leads to lower overall water absorption and higher mechanical properties.

Moisture Diffusion in Unsaturated Polyester Composites Reinforced with Macambira Natural Fiber: A Finite-Volume Approach

This paper presents a theoretical and experimental study about water absorption in unsaturated polyester polymer composites reinforced with vegetable fibers, with particular reference to macambira fiber. A mathematical modeling based on the liquid diffusion theory has been proposed and numerical procedures using the finite volume technique are presented and discussed. Results of the water absorption kinetic and moisture content distribution for the polymer composites are shown and analyzed. The knowledge of moisture distribution inside the composite is essential for determination of areas that may show delamination problems (moisture induced degradation) due to the weakness of the fiber-matrix interface and consequently reduction in the mechanical properties of the composites.