Chemical Interactions in Diboride-Reinforced Oxide-Matrix Composites

Thermochemical analyses of interfacial reactions in titanium, zirconium, and hafnium diboride reinforced oxidematrix composites have been carried out to evaluate the chemical compatibility. The chemical reactivity of these diborides with oxygen and the high volatility of B₂O₃( l ) at reduced oxygen pressures are concerns during processing and operating conditions. The thermochemical stability and the vaporization behavior of B₂O₃( l ) are discussed in terms of partial pressures of dominant gaseous species of the boron-oxygen system at 1700 and 2300 K. The TiB₂/ZrO₂ and TiB₂/HfO₂ systems are thermodynamically stable in a limited oxygen pressure range. The TiB₂/Al₂O₃ system is stable, but the reactions in this system may apparently be accompanied by formation of gaseous products (B₂O₃, AlO, Al₂O, and lower boron oxides) in the presence of elemental oxygen. These thermochemical considerations are very useful in evaluating the effectiveness of oxides as diffusion barrier coatings on diboride reinforcements.     

Asbestos replacement aspects: Modification of the fibre-matrix interphase in lonomer based composites

Asbestos fibres, of the chrysotile variety, and chopped carbon fibres were pretreated by an in-situ polycondensation technique eventually resulting in a polyamide coating on the fibre surface. Ionomer based composites containing either carbon or asbestos fibres in random in plane fibre orientation were prepared, and the influence of this coating process on the tensile properties was investigated. It was found that for the asbestos-filled composites the presence of the nylon 6,6 interlayer improves the tensile performance, especially at moderate polyamide depositions. This is not the case with the pretreated carbon-filled composites for which carbon fibres with higher polyamide contents are preferred. Combinations of the treated asbestos fibres with carbon and/or aramid fibres may be used to reduce the asbestos content in asbestos-only based engineering plastics.     

Electromagnetic properties of ferroelectric-ferrite ceramic composites

The NiFe₂O₄ and Pb(Zr_0.52Ti_0.48)O₃ ceramic composites are prepared by conventional solid state reaction method. The microstructural and morphological properties of the composites are studied by X-ray diffraction analysis and scanning electron microscopy. Results indicate that the ferrite and piezoelectric phases can co-exist in the composites. The relationship between the dielectric constant and frequency (40 Hz–40 MHz) as well as temperature is also investigated. It is noted that the transition temperature of the composites is about 380 °C. A double electric hysteresis loop is observed in our composites. The magnetic properties are relatively weak according to the magnetic hysteresis loop. With increase in the content of the ferrite phase, the permeability increases. The electromagnetic properties of the as-prepared composites are tunable according to the content of the NiFe₂O₄ phase.     

Electrooptic studies on polymer‐dispersed liquid‐crystal composite films. III. Poly(methyl methacrylate‐co‐butyl acrylate)/E7 and poly(methyl methacrylate‐co‐butyl acrylate)/E8 composites

Composite films composed of poly(methyl methacrylate‐co‐butyl acrylate) (PMMABA) and nematic‐type liquid crystals E7 and E8 (commercial products from E. Merck, Darmstadt, Germany) were prepared through solvent casting in chloroform. The morphology and electrooptic responses were studied. Scanning electron microscopy observations showed that the liquid‐crystal phase (E7 or E8), as larger, elongated, interconnected cavities, was continuously embedded in a spongelike PMMABA matrix. At a specific level of the liquid‐crystal (E7 or E8) loading (30/70 wt %), the effects of the voltage, temperature, and frequency of an applied alternating‐current electric field on the transmittance of the composite films were measured with a He–Ne laser (wavelength = 632.8 nm). The results were interpreted in terms of the aggregation structure, interfacial interaction, and solubility of the liquid crystal in the matrix polymer. The results indicated that, under these experimental conditions, the output could be controlled to a desired level by the selection of suitable liquid crystals to prepare polymer‐dispersed liquid‐crystal, electrooptic, active composite films with a response time of the order of only milliseconds or less. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Swelling properties of chemically modified oil palm empty fruit bunch based polyurethane composites

In this study, the diffusion of various types of solvent in oil palm empty fruit bunch/polyurethane composites, produced from chemically modified empty fruit bunches, was investigated. The solubility parameters and polymer–solvent interaction parameters of the produced composites were determined. The void contents of the composites were also determined before swelling tests to eliminate the free solvent present in the system. From the results obtained, we found that the diffusion of the solvents was dependent on the compatible group available and the voids present in the system. The solubility parameters of the empty fruit bunch/polyurethane composites with different degrees of chemical modification were 11.6 and 11.7 (cal/cm^?3)^1/2. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A comparative study of short nylon fiber—Natural rubber composites prepared from dry rubber and latex masterbatch

A novel method for the preparation of short nylon fiber–natural rubber composites was developed in which short fibers chopped to approximately 6 mm were incorporated in the latex stage and processed into sheet form. By this method, mixing cycle time was reduced without compromising the fiber dispersion. Fiber breakage during mixing was reduced. The new composites when compounded with a dry bonding system based on hexamethylenetetramine, resorcinol and hydrated silica (HRH) showed improved modulus, tensile strength and abrasion resistance compared to conventional composites. Tear strength, resilience, and compression set were similar to the conventional composites. SEM analysis indicated better interaction between matrix and fibers in the case of latex master batch. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal and mechanical properties of epoxy composites reinforced by a natural hydrophobic sand

If a low weight percentage of crude fine fillers can improve properties of polymer materials directly without complicated chemical treatment process involved, it will be significant for many industrial applications. Our previous study indicated that a kind of Cancun natural sand could be an effective filler material for polymer composites. In this current work, the epoxy composites reinforced by this kind of natural sand particles were prepared and thermal and mechanical properties of the composites containing up to 5 wt % of the sand particles were characterized. Results showed that the highest flexural strength appears in the epoxy composite containing 1 wt % sand particles. A damage model was used to interpret the flexural properties, which showed an acceptable agreement with the experimental results. The glass transition temperature, high temperature storage modulus, and dimensional stability of the sand/epoxy composites monotonically increased with the addition of the sand particles. The sand particle/epoxy composites also displayed a noticeable enhancement in thermal conductivity. Theoretical analysis showed that in addition to conduction, other heat transport mechanisms played roles in the improved heat transmission through the composites. As a natural porous micron-scale material, Cancun sand has the potential for applications in cost-effective composites with enhanced mechanical and thermal properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Green and self-lubricating polyoxymethylene composites filled with low-density polyethylene and rice husk flour

Polyoxymethylene (POM) composites filled with low-density polyethylene (LDPE) and rice husk flour (RHF) were prepared by injection molding. The POM/5 wt % LDPE/7.5 wt % RHF composite exhibited the lowest wear rate, whereas the coefficient of friction remained low, and the POM/5 wt % LDPE/5 wt % RHF composite had the best mechanical properties. X-ray diffraction analysis was carried out, and the worn surfaces were examined with scanning electron microscopy. The results showed that the addition of the filler reduced the crystallinity degree of the POM composites. The main wear mechanism for unfilled POM was adhesion, whereas for the POM composites, wear seemed to occur mainly by fatigue and abrasion. It was experimentally confirmed that the POM composite filled with LDPE and RHF, which is well-performing, low-cost, and environmentally friendly, could be a potential material for tribological applications. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of temperature on the optical and electro-optical properties of poly(methyl methacrylate)/E7 polymer-dispersed liquid crystal composites

Experiments on the effect of temperature on the optical and electro-optical behaviors of polymer-dispersed liquid crystals (PDLCs) are considered. Composite films composed of poly(methyl methacrylate) (PMMA) and the nematic-type liquid crystal (LC) E7 were prepared by solvent casting in chloroform. The PDLC film contained droplets of E7 from 10 to 80 wt % in a PMMA matrix. Morphological studies illustrated the formation of isolated droplets of LC due to phase separation, and their homogeneous distribution increased with increasing E7 content. Thermo-optical studies showed an increase in the nematic–isotropic transition temperature of composites, which indicated preferential solvation during the phase-separation process. The electro-optical characteristics were studied under the conditions of an externally applied square wave electric field with a He–Ne laser (λ = 632.8 nm) as a light source. The responses improved as the E7 content in PMMA increased. Semipermanent memory effects were noticed in composites at higher temperatures. Changes in the transmittance due to thermal variations provided the possibility of using such a device as a temperature sensor. The results obtained indicate that under these experimental conditions, the output can be controlled to the desired level by the selection of a suitable loading of LC to prepare PDLC electro-optically active composite films with a response time on the order of only a few milliseconds. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Coincidence Doppler broadening spectroscopy in polyvinyl chloride after doping with Al2O3

Coincidence Doppler broadening of annihilation radiation (CDBAR) and Vickers hardness techniques were performed to study pure Al₂O₃, pure polyvinyl chloride (PVC), and doped PVC with different concentrations of Al₂O₃ (10–50%). The CDBAR ratio curves with respect to pure PVC were presented and reflect the momentum distribution of all the samples. The peak around 14.5 ×10^?3 m_oC in the CDBAR ratio curves suggests a large contribution of positron annihilation with the Al₂O₃. There is a linear correlation between the height of this peak and the Al₂O₃ concentration. The S‐ and W‐parameters were extracted from the CDBAR spectra and increase with increasing the Al₂O₃ concentration showing discontinuity at 30% of Al₂O₃ concentration on PVC. The present data confirmed that there is no positronium formation in pure Al₂O₃ as a result of smaller S‐parameter. The Vickers hardness increases with increasing the Al₂O₃ concentration in PVC showing a linear dependence with two different slopes depend on the Al₂O₃ concentration range. A correlation between the Vickers hardness (macroscopic data) and the W‐parameter (microscopic data) was observed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008