José R.M. d’Almeida

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.

Avaliação do efeito de tratamentos superficiais sobre a força de adesão de braquetes em provisórios de resina acrílica

OBJECTIVE: To evaluate the influence of the surface treatment of acrylic resins on the shear bond strength of brackets bonded with composite resin. METHODS: Were fabricated 140 discs with autopolymerizing acrylic resin (Duralay™) and divided into 14 groups (n = 10). In each group, the specimens received a different type of surface treatment. Group 1= untreated surface (control), group 2= silane, group 3= aluminum oxide blasting (AOB), group 4= AOB + silane, group 5= diamond bur, group 6= diamond bur + silane, group 7= hydrofluoric acid, group 8= hydrofluoric acid + silane, group 9= phosphoric acid, group 10= phosphoric acid + silane, group 11= methylmethacrylate monomer (MMA), group 12= MMA + silane, group 13= plastic conditioner (Reliance®); group 14= plastic conditioner (Reliance™) + silane. After surface treatment the specimens were analyzed using a surface roughness tester. Subsequently, standard edgewise central incisor brackets (Morelli™) were bonded using Transbond XT™ light-cure adhesive system, according to the manufacturers instructions. RESULTS: The silane-based wetting agent had no statistically significant effect on bond strength values. Treatments with AOB and bur generated the highest topographical changes on the surface of acrylic resin as well as the highest roughness values. A nonlinear correlation was found between bond strength and surface roughness. Monomer + AOB treatment yielded the highest bond strength values. CONCLUSIONS: Silane failed to increase the bond strength between brackets and acrylic resin. We encourage further studies on this subject since the bond strength achieved in our study was extremely low.

Propriedades mecânicas de blendas de PS/resíduo de borracha: influência da concentração, granulometria e método de moldagem

Polystyrene was toughened by blending with rubber waste obtained from shoe and tire industries. The blends were twin-screw extruded and test specimens were compression and injection moulded. The effects of concentration (0-15% w/w) and particle size (1200 to 180 µm) of the waste rubbers on the blends mechanical properties were evaluated and their fracture surfaces examined. It was shown that tensile strength tends to lower and impact resistance to increase with increasing rubber content of decreasing particle sizes. Morphological analysis confirms these results. Injection moulded specimens showed greater sample uniformity and consequently, better mechanical properties than compression moulded samples.

Carbon nanotube hybrid polymer composites : recent advances in mechanical characterization

Abstract This chapter focuses on hybrid composites obtained with carbon nanotubes (CNT) and common reinforcement fibers, e.g., carbon fibers, into a polymeric matrix. Mechanical tests for their characterization are presented and discussed, with emphasis on more relevant methods. The impact of different types of CNT/fiber arrangements on the mechanical properties of polymer composites will also be discussed. The presented techniques include classical tensile, compressive, flexural and impact tests, along with interfacial tests for fiber/matrix adhesion evaluation (e.g., fiber pull-out, single-fiber fragmentation), fatigue testing, damage and delamination evaluation, and also dynamical–mechanical analysis. Hybrids based on thermoset matrices are presented and the text briefly covers CNT incorporation methods, that range from direct mixing with the matrix to direct incorporation into the reinforcement fiber, usually by chemical vapor deposition, type of matrix (e.g., epoxy and polyester) and type of fiber (e.g., glass, carbon, and aramid). The mechanical properties of the hybrid composites are sometimes compared with those from conventional, nonhybrid, fiber thermoset composites.