Journal of Composite Materials

In this study, nano-SiO₂ has been used as a high reactive pozzolan to develop the microstructure of the interfacial transition zone between the cement paste and the aggregate. Mechanical tests of blended cement-based concretes exposed that in addition of the pozzolanic reactivity of nano-SiO₂ (chemical aspect), its particle grading (physical aspect) also revealed considerable influences on the blending effectiveness. It was concluded that the relative permeability reduction (relative to the control concrete made with plain cement) is higher for coarser nano-SiO₂ after 90 days of moisture curing. However, finer nano-SiO₂ particles showed better effects in early ages. These phenomena can be due to the free spacing between mixture particles that was associated with the global permeability of the blended cement-based concretes. This article presents the results of the effects of particle size ranges involved in nano-SiO₂ blended Portland cement on the water permeability of concrete. It is revealed that the favorable results for coarser nano-SiO₂ reflect enhanced particle packing formation accompanied by a reduction in porosity and particularly in particle spacing after 90 days.

Thermoplastic vulcanizates (TPVs) are a specific group of the so called thermoplastic elastomers. The main characteristic is the existence of a crosslinked rubber phase obtained by dynamic vulcanization in the presence of the thermoplastic matrix. This article studies TPVs based on ground tyre rubber (GTR), high-density polyethylene, and ethylene propylene diene monomer rubber. Vulcanization is performed by a new peroxide developed to resist high temperatures and an standard one. The aim of this study is optimize the formulation in order to include GTR, while maintaining a good balance of properties in the final TPV material. The use of GTR would improve the possibilities of recovering tyre waste. A detailed study regarding the influence of each component in the final mechanical properties has been carried out. The swelling properties, ATR infrared spectroscopy, TGA, and DSC analysis indicated a high degree of crosslink and good adhesion between the matrix and the rubber phase. Morphology of the composites was assessed by scanning electron microscopy. A composite containing a combination of peroxides and 40/30/30 of HDPE, EPDM, and GTR was found to show a good balance of characteristics regarding mechanical properties, crosslinking, and adhesion between phases.

A series of blends of sago pith waste (SPW) and poly(vinyl alcohol) (PVA) were prepared. Mechanical and water absorption properties of the composites have been investigated. In this study, variable amounts of plasticized SPW (pSPW) and PVA (pPVA) were processed in the presence of glycerol as plasticizers. Composites were compression molded and evaluated. The addition of pSPW reduced the tensile properties of the composites, lowering the elongation and increasing Young’s modulus. The reduction in mechanical strength with the addition of pSPW was a general phenomenon due to the poor interfacial adhesion between the pPVA and Pspw, which can be proved by the scanning electron microscope observations. The percentage of water absorbed of the pPVA/pSPW biocomposites was higher than either the pPVA or pSPW alone while pSPW showed better water resistance compared to pPVA because of the restricted mobility exerted by the cellulose fibers. The incorporation of SPW into PVA decreased both the mechanical and water absorption properties.

The thermal, viscoelastic, mechanical behavior of polymers filled with dispersed zeolite and oil shale is studied as a function of temperature, grain size, and filler concentration. It was found that the thermal conductivity of epoxy—zeolite composite increases with different zeolite grain sizes and takes a higher value in case of the 63 μm grain size composite. The observed enhancement in the thermal conductivity of zeolite composites correlates well with that of the electrical conductivity. The thermodynamic results exhibit a slight increase in the glass transition temperature of the polystyrene/oil shale composites, and shift in the observed relaxation peaks with increasing the oil shale content. The plastic deformation of PS/oil shale composites shows that the elastic modulus increases and the compressive yield stress decreases with oil shale content. The Eyring theory of yielding could predict the dependence of the yield stress on the applied strain rate. The predicted activation volume and activation energy showed dependence on the oil shale grains sizes and content.

Aramid and aramid copolymer fibers are used in a wide variety of military and civilian applications; however, the long-term effects of environmental exposure on tensile properties are still not well understood. The current effort investigates the effect of hygrothermal conditioning on the tensile properties of Kevlar^® KM2 ^®, Twaron^®, and the newly available Russian copolymer, Armos^® high performance fibers. Moisture uptake studies show that at room temperature, water diffuses more slowly into the copolymer Armos ^® (D = 8.7 × 10^-13 cm²/s) compared to the Kevlar^® KM2^® and Twaron^® homopolymers (D = 2.16 × 10^-12 cm²/s and D = 1.8 × 10 ^-12 cm²/s, respectively). Tensile properties have been measured for these aramid fibers that have been conditioned in water at 40°C, 60°C, 80°C, and 100°C for periods of 17 and 34 days. For both aramid and aramid copolymer fibers, hygrothermal conditioning did not significantly change fiber tensile properties below 80°C. At the most extreme condition of 100°C, 34 days, aramid fibers showed significant loss of tensile strength (58% for KM2 and 34% for Twaron^®), while a reduction in tensile strength of 13% (Armos^®) was observed for aramid copolymer (Armos^®) fibers. Conditioned fibers exhibited no significant change in mass as a result of the conditioning procedure and FTIR spectroscopy results did not indicate signs of chemical or thermo-oxidative change due to hygrothermal conditioning. These results suggest that in aramid fibers, the primary mechanism of degradation at temperatures between 80°C and 100 °C is due to the ingress and egress of moisture in the highly ordered core structure of the fiber. The presence of water in the intercrystalline regions of the fiber core enable segmental chain motion that can relax tie molecules, alter crystal orientation, and change apparent crystallite size. Because of differences in molecular architecture and phase morphology, the aramid copolymer, Armos^®, is less susceptible to degradation of tensile properties under these conditions.

The mass manufacture of tires and the difficulty for their elimination or storage constitutes a serious environmental problem. At present, several methods for the recycling of tires are used, such as mechanical crushing, in which the steel vulcanized rubber and the fibers are separated; this rubber is being used in numerous applications like pavements, insulators, footwear, etc. This study proposes a second option for obsolete tires, demonstrating their utility as dielectrics. In order to do so, ground tire rubber (GTR) has been combined with polyvinyl chloride (PVC), to obtain a composite of a polymeric matrix reinforced with GTR. In order to determine the behavior of this composite material, the electrical and mechanical tests are presented as well as, more briefly, microstructure and thermal analyses, undertaken for the various mixtures of PVC with GTR (concentrations of 0%, 5%, 10%, 20%, 40%, 50%, and 70% GTR), and three GTR particle size categories (<200 μm, 200—500 μm, and >500 μm), in a range of temperatures that varied from 30° C to 130° C, and with frequencies from between 1 × 10^-2 Hz and 3 × 10⁶ Hz. The dielectric tests have allowed for an analysis of dielectric constant, dielectric loss factor, dielectric modulus, etc. On the other hand, the mechanical analysis has involved the Young’s modulus, tensile strength, elongation at break, and toughness. Mechanical and dielectric results point out that below 20% of GTR the material features for mechanical or electrical applications are not significantly altered.

Micromechanical analyses were conducted for the prediction of transverse thermal conductivity of laminated composites. We reproduced and reinvestigated both analytic and numerical models with regular and randomly distributed fibers in matrix material. A parametric study was conducted for wide ranges of fiber volume fractions and fiber-to-matrix thermal conductivity ratios. The numerical solutions using finite element (FE) analysis were compared with various analytic solutions from simple and enhanced rule or mixtures and an effective inclusion method (EIM). It was found that the EIM yields a reasonably agreeable solution with the FE solution using a hexagonal-array of regular fiber distribution for wide ranges of fiber volume fraction and fiber-to-matrix thermal conductivity ratios, which makes the EIM a useful method in predicting various multiphysical transverse properties of composites. Comparison of the results from the regular- and random-fiber models indicates that the transverse thermal conductivity of composites can significantly be affected by the random fiber distributions, especially at high fiber volume fractions. A similar conclusion was made for the foams with random pore distribution. It was shown that the predictions with the random fiber distribution agree well with the experimental data.