High performance wood composites from highly filled polybenzoxazine

Highly filled systems prepared by compression molding of Hevea brasiliensis woodflour filled polybenzoxazine composites with high mechanical properties and reduced water uptake has been developed. The effects of percent filler content and particle size of woodflour on the obtained composite's properties were examined. The low melt viscosity of BA‐a type polybenzoxazine allows substantial amount of woodflour to be easily incorporated into the composites. The results showed that mechanical properties from dynamic mechanical analysis and flexural test at filler content below the optimum filler packing show approximately linear relationship with filler loading. The outstanding compatibility between the woodflour and the polybenzoxazine matrix is evidently seen from the large improvement in the composite's T_g and char yield. Scanning electron micrographs of the composite also reveals substantially strong interface between the woodflour filler and the polybenzoxazine matrix. Water absorption of the composites is greatly reduced with increasing the amount of polybenzoxazine due to the inherent low water absorption of the matrix. The polybenzoxazine is; therefore, a highly attractive candidate as high performance lignocellulosic binder or adhesive and other related applications. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1240–1253, 2006     

Surface segregation of siloxane containing component in polysiloxane‐block‐polyimide and s‐BPDA/ODA polyimide blends

Surface segregation in polymer blend systems between 3,3′,4,4′‐biphenyltetracarboxylic dianhydride/4,4′‐diaminodiphenyl ether (s‐BPDA/ODA) polyimide and block copolymer based on polysiloxane‐block‐polyimide (SPI) has been investigated. These polyimide blends, having various compositions of the SPI, were processed by a solution casting method. The glass substrate used in the film‐casting process shows significant effect on the migration of surface segregated species to enrich the air‐exposed surface, whereas the more polar s‐BPDA/ODA tends to remain close to the polar glass substrate. X‐ray photoelectron spectroscopy reveals that even at low SPI concentration, the siloxane moieties in the block copolymer tend to segregate into the air side surface. Contact angle measurement evidently indicates an enrichment of the hydrophobic siloxane fraction on the blend film surface. The average water contact angle of glass side surface is 77°C whereas that of the air side is about 102°C in every blend ratio. This behavior confirms the surface segregation phase separation in these polymer blends. Finally, the surface morphology observed by atomic force microscopy also suggests segregation type of phase separation in these blend systems. POLYM. ENG. SCI., 47:489–498, 2007. © 2007 Society of Plastics Engineers.     

Toughening of polybenzoxazine by alloying with urethane prepolymer and flexible epoxy: A comparative study

The toughness of polybenzoxazine can effectively be improved by alloying with isophorone diisocyanate (IPDI)‐based urethane prepolymers (PU) or with flexible epoxy (EPO732). The experimental results, i.e., flexural testing and dynamic mechanical analysis, reveal that the toughness of the alloys of the rigid polybenzoxazine and the PU or the EPO732 systematically increases with the amount of either toughener, due to the addition of more flexible molecular segments in the polymer hybrids. The curing temperature of the benzoxazine resin (bisphenol A‐aniline type [BA‐a]) at about 225°C shifts to a higher value when the fraction of BA‐a in either alloy decreases. Interestingly, the enhancement in the glass transition temperature (T_g) of BA‐a/PU alloys is clearly observed, i.e., T_g of the BA‐a/PU alloys are significantly higher (T_g beyond 200°C) than those of the parent resins, i.e., 165°C for BA‐a and ?70°C for PU. However, this characteristic is not observed in the BA‐a/EPO732 alloy systems. The enhanced T_g of the BA‐a/PU alloy at a 70/30 mass ratio is found to be 220°C, while that of BA‐a/EPO732 at the same mass ratio is observed to be only 95°C, which further decreases as the content of epoxy fraction increases. Furthermore, the degradation temperature based on 5% weight loss in the thermogravimetric analyzer (TGA) thermograms of the BA‐a/PU alloys is found to improve with the presence of the PU, though the opposite trend is observed in the BA‐a/EPO732 systems. The char yield of both alloy systems is steadily enhanced with the increased benzoxazine content because the char yield of the polybenzoxazine is inherently higher than that of the two tougheners. Therefore, the polybenzoxazine alloys with the IPDI‐based urethane prepolymer, e.g., the 70/30 BA‐a/PU, are a promising system for a tough, high thermal stability polymeric network, suitable for both bulk and composite matrix applications. POLYM. ENG. SCI. 45:288–296, 2005. © 2005 Society of Plastics Engineers.     

Organic Heat Stabilizers for Polyvinyl Chloride (PVC): A Synergistic Behavior of Eugenol and Uracil Derivative

Recycling ability, mechanical, and thermal properties of PVC stabilized with organic heat stabilizers, i.e., uracil (DAU) and eugenol were investigated to substitute PVCs stabilized with commercial lead, Ca/Zn, and organic-based stabilizer for PVC pipe production. PVC stabilized with the DAU and the eugenol can be processable at 30 °C lower than that of the PVC stabilized with commercial heat stabilizers. The most remarkable short-term thermal stability belonged to the PVC stabilized with the DAU, and its original color can be maintained at least up to 3 processing cycles. Synergistic behavior in thermal stability of the PVC mixed with DAU and eugenol at mass ratios of 1.5:1.5 was observed. Mechanical properties of DAU- and eugenol-stabilized PVC were higher than the samples with other heat stabilizers. Glass transition temperature of the PVC stabilized with all heat stabilizers was determined to be 99 °C with the exception of the value of 89 °C for eugenol-stabilized PVC. Therefore, the DAU and the eugenol showed high potential to be used as an organic heat stabilizer for PVC because of their non-toxic and good heat resistance properties.     

Characterization of heat treated eastern redcedar (Juniperus virginiana L.)

The objective of the study is to investigate the influence of heat treatment on shrinkage, density, surface roughness, water absorption, diffusion coefficient, swelling and shear strength of eastern redcedar (Juniperus virginiana L.) samples. The anatomical structures of samples are also observed by scanning electron microscope (SEM). Specimens are exposed to temperature levels of 120 °C, 160 °C and 190 °C for time spans of 2 and 8 h. Based on the results of this study, dimensional stability in the form of shrinkage of the samples is improved by 2.68%, 1.40% and 1.49% for tangential, longitudinal and radial grain orientations as function of heat treatment, respectively. Heat treatment also enhances surface quality of the samples based on numerical values determined from stylus type of equipment. Water absorption, swelling values and diffusion coefficient of the samples are also reduced with heat treatment. Samples exposed to heat treatment have lower shear strength values, ranging from 25.12% to 52.67%, than those of control samples. It appears that all properties evaluated in this work are affected more pronouncedly as temperature and exposure time is increased.     

Performance Analysis of Flat-Plate Solar Collector Having Silver Nanofluid as a Working Fluid

A study on water solar collector performance having silver nanofluid as working fluid was carried out. In this study, 20-nm silver particles mixed with water at the concentrations of 1,000 and 10,000 ppm were undertaken in 3 small identical closed-loop flat-plate solar collectors, each with an area of 0.15 m × 1.0 m. The mass flux of the working fluid varied between 0.8 and 1.2 L/min-m² and the inlet temperatures were controlled in the range of 35–65°C. The tests were performed outdoor under a steady-state condition. The experimental results showed that at the same Reynolds number, the convective heat transfer coefficient of the nanofluid inside the solar absorber tube at 1,000 ppm was slightly higher than that of water, and at 10,000 ppm, the heat transfer coefficient was about 2 times that of water. This meant that the overall heat loss coefficient of the solar collector with nanofluid could be reduced and more solar heat gain could be obtained, especially with a high inlet temperature of the working fluid. In our experiments, for 10,000 ppm concentration of silver nanoparticles, the optical characteristic and the thermal loss characteristic of the solar collector, under steady-state condition with a mass flux of 1.2 kg/min-m², were 0.691 and 4.869 W/m²-K, compared with 0.684 and 7.178 W/m²-K, respectively for 1,000 ppm concentration and 0.702 and 8.318 W/m²-K for water. When the flow rate was different from the standard value, the solar thermal characteristics were also improved with the nanofluid.     

Effects of polyol molecular weight on properties of benzoxazine–urethane polymer alloys

Recently, a new thermoset resin namely benzoxazine (BA) resin has been developed. The polymer possesses several outstanding properties, such as, ease of synthesis, low viscosity, near‐zero shrinkage, lack of by‐product upon curing, high thermal stability, and high mechanical property. Moreover, the benzoxazine resins can be alloyed with various types of resins because of the various function groups in their structure. In this work, urethane elastomer (PU) is used to enhance toughness of the polybenzoxazine. The effects of polyol molecular weights on the properties of BA: PU polymer alloys are investigated. The experiment reveals that the similar curing peaks of the matrices at various polyol molecular weights, with the same urethane mass fraction, in the resin mixtures are obtained. The glass transition temperature increases from 160°C of polybenzoxazine to 240–245°C in the 70:30 BA:PU system. In addition, the char yield increases when the higher molecular weight of polyol is added. The flexural modulus of polybenzoxazine decreases from 6.2 GPa to be in the range of 2.2–2.8 GPa when 30 wt% of PU is presented in the alloys. Furthermore, the synergism with ultimate flexural strength is observed in the 90:10 BA:PU alloy for all molecular weights of the polyol used. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Surface-enhanced Raman spectroscopy to probe reversibly photoswitchable azobenzene in controlled nanoscale environments

We apply in situ surface-enhanced Raman spectroscopy (SERS) to probe the reversible photoswitching of azobenzene-functionalized molecules inserted in self-assembled monolayers that serve as controlled nanoscale environments. Nanohole arrays are fabricated in Au thin films to enable SERS measurements associated with excitation of surface plasmons. A series of SERS spectra are recorded for azobenzene upon cycling exposure to UV (365 nm) and blue (450 nm) light. Experimental spectra match theoretical calculations. On the basis of both the simulations and the experimental data analysis, SERS provides quantitative information on the reversible photoswitching of azobenzene in controlled nanoscale environments.     

Preparation of nanocrystalline SrTiO3 powder by sol–gel combustion method

In this research a sol–gel combustion route has been presented to synthesize strontium titanate (SrTiO₃:ST) nanocrystalline, using citric acid as fuel. The synthesis procedure was optimized by systematically varying the molar ratios of total metal nitrate to citric acid (MN:CA) from 1:1 to 1:3. The effect was investigated through XRD, SEM and TEM analysis. Analysis of XRD spectrum shows the complete of SrTiO₃ nanocrystalline, however, a minor phase of SrCO₃ impurity was found. Hence, an acid treatment process, with 1 mol/l HNO₃ solution and deionized water, was applied to remove the impurity. The results show that the appropriate condition to prepare the single phase nanocrystalline SrTiO₃ powders is MN:CA molar ratio of 1:3, coupled with an acid treatment process and at the lower calcination temperature of 500 °C. The particle size of powders was in nanometer ranges. The average crystallite size calculated from FWHM was about 23 nm. Morphology of powders was identified by SEM analysis. However, TEM estimated the average particle size about 7.5 nm after applying an acid treatment technique at 600 °C.