Changlei Xia

Property enhancement of kenaf fiber composites by means of vacuum-assisted resin transfer molding (VARTM)

Abstract This work was aimed at applying vacuum-assisted resin transfer molding (VARTM) technique to reinforced polymer molding products made of vegetable fibers. Kenaf (Hibiscus cannabinus L. Malvaceae) bast fibers were preformed into mat by means of a cold press. The unsaturated polyester resin was infused into the preforms at a vacuum pressure of 1.3–1.6 kPa. The examination of the mechanical properties and microstructure of the prepared composites indicated that the modulus of elasticity (MOE), modulus of rapture (MOR), and tensile strength (TS) of the VARTM composites were increased by 65.5%, 30.7%, and 41.7%, respectively, compared to the traditional hot-pressing composites. The dynamic mechanical analysis (DMA) revealed that the VARTM composite moduli in the temperature range of -50°C–200°C were doubled. The observations by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and mercury porosimetry confirmed that the interfacial compatibility between the kenaf fibers and the polyester resin was substantially improved.

Property enhancement of soy protein isolate-based films by introducing POSS.

To enhance the mechanical and water-resistant properties of soy protein isolate (SPI) based films, hydrophobic TriSilanolPhenyl polyhedral oligomeric silsesquioxanes (POSS) was incorporated to modify the SPI films. POSS has three SiOH groups in a molecular, which is employed to cross-link SPI with the help of 3-glycidoxypropyltrimethoxysilane (GPTMS). POSS is a structure of eight phenol groups, playing a critical role in improving the physical and mechanical properties. The X-ray diffraction (XRD) and attenuated total reflectance (ATR) Fourier transform infrared spectroscopy (FT-IR) were used to characterize the films. The degree of reaction of SiOH groups in the POSS was estimated to be 53.0% according with the absorbance of ATR FT-IR spectra. Although the elongation at break was reduced by 52.6%, the tensile modulus, tensile strength and 10% offset yield strength were significantly increased by 86.6%, 34.0% and 56.8%, respectively, due to the cross-linking reactions among SPI, GPTMS and POSS. The results of water-resistant tests showed that the 24-hour water absorption was dramatically reduced by 54.7%.

Self-activation for activated carbon from biomass: theory and parameters

Self-activation is a process that takes advantage of the gases emitted from the pyrolysis process of biomass to activate the converted carbon, which saves the cost of activating agents and decreases the environmental impact, compared with conventional activation processes. An activation model was developed to describe the mechanism of the activation process, and it was examined by the self-activation experiments using the kenaf core as a raw material. The relationships among the parameters, yields, specific surface areas, and specific pore volumes were quantified. The results showed that the ideal temperatures for the self-activation process of the kenaf core were found between 970–1090 °C. The yield of 9.0% for the activated carbon from the kenaf core provided the highest surface area per gram of biomass, while the yields of 5.5–13.8% could achieve 90% of the highest. The developed activation model can be used to explain the relationship between the yields, specific surface areas, and specific pore volumes, effectively.

Soy protein isolate-based films cross-linked by epoxidized soybean oil

Epoxidized soybean oil (ESO) is an environmentally friendly cross-linking agent derived from soybean, having multiple epoxy groups in its molecules. It can effectively improve tensile strength and water resistance of soy protein isolate (SPI)-based films. The properties of the SPI-based films were characterized by X-ray diffraction and attenuated total reflectance Fourier transform infrared spectroscopy. The best performance of the SPI-based films was achieved when the ESO addition was 2.5%, for which tensile modulus, tensile strength and 10% offset yield strength were increased to 265.0 MPa, 9.8 MPa and 6.8 MPa, respectively. Compared to untreated SPI-based films, these were increases of 695.6%, 139.8%, and 246.6%, respectively. However, the elongation at break was decreased by 67.6% due to the cross-linking between SPI and ESO. The SPI-based film modified by 5% ESO had the best water-resistance property and reduced the 24 hour water absorption from 209.1% to 45.9%, which was a significant decrease of 78.1%.

Hybrid boron nitride-natural fiber composites for enhanced thermal conductivity

Thermal conductivity was dramatically increased after adding natural fiber into hexagonal boron nitride (hBN)/epoxy composites. Although natural fiber does not show high-thermal conductivity itself, this study found that the synergy of natural fiber with hBN could significantly improve thermal conductivity, compared with that solely using hBN. A design of mixtures approach using constant fibers with increasing volume fractions of hBN was examined and compared. The thermal conductivity of the composite containing 43.6% hBN, 26.3% kenaf fiber and 30.1% epoxy reached 6.418 W m−1 K−1, which was 72.3% higher than that (3.600 W m−1 K−1) of the 69.0% hBN and 31.0% epoxy composite. Using the scanning electron microscope (SEM) and micro computed tomography (micro-CT), it was observed that the hBN powders were well distributed and ordered on the fiber surfaces enhancing the ceramic filler’s interconnection, which may be the reason for the increase in thermal conductivity. Additionally, the results from mechanical and dynamic mechanical tests showed that performances dramatically improved after adding kenaf fibers into the hBN/epoxy composite, potentially benefiting the composite’s use as an engineered material.

Scalable Fabrication of Natural-Fiber Reinforced Composites with Electromagnetic Interference Shielding Properties by Incorporating Powdered Activated Carbon

Kenaf fiber—polyester composites incorporated with powdered activated carbon (PAC) were prepared using the vacuum-assisted resin transfer molding (VARTM) process. The product demonstrates the electromagnetic interference (EMI) shielding function. The kenaf fibers were retted in a pressured reactor to remove the lignin and extractives in the fiber. The PAC was loaded into the freshly retted fibers in water. The PAC loading effectiveness was determined using the Brunauer-Emmett-Teller (BET) specific surface area analysis. A higher BET value was obtained with a higher PAC loading. The transmission energies of the composites were measured by exposing the samples to the irradiation of electromagnetic waves with a variable frequency from 8 GHz to 12 GHz. As the PAC content increased from 0% to 10.0%, 20.5% and 28.9%, the EMI shielding effectiveness increased from 41.4% to 76.0%, 87.9% and 93.0%, respectively. Additionally, the EMI absorption increased from 21.2% to 31.7%, 44.7% and 64.0%, respectively. The ratio of EMI absorption/shielding of the composite at 28.9% of PAC loading was increased significantly by 37.1% as compared with the control sample. It was indicated that the incorporation of PAC into the composites was very effective for absorbing electromagnetic waves, which resulted in a decrease in secondary electromagnetic pollution.

Enhancement of mechanical and thermal properties of Poplar through the treatment of glyoxal-urea/nano-SiO2

Glyoxal is a cross-linking agent that could effectively improve dimensional stability and water resistance with compromising the mechanical properties of wood materials. This study explores relevant disadvantages of glyoxal-treated wood, and in an effort to overcome the drawbacks, an environmental-friendly glyoxal-urea (GU) resin is synthesized from urea and glyoxal, and combined with nano-SiO2 to treat Poplar wood. Results showed that the mechanical properties of the GU resin-treated wood were significantly increased compared to those of wood treated with glyoxal alone, and that incorporation of nano-SiO2 in the GU resin further improved performance. The fracture morphology of GU/nano-SiO2-treated wood was also characterized, indicating increased elasticity. Scanning electron microscopy and energy dispersive X-ray (SEM-EDX) results showed that GU and nano-SiO2 existed not only in the wood cell lumens, but also in the cell walls. Fourier transform infrared spectroscopy (FT-IR) test results showed the formation of GU resin and the incorporation of GU resin into the wood samples. Thermal analysis results demonstrated that thermal properties were improved after the incorporation of GU and nano-SiO2 compared to samples solely glyoxal-treated. Improvement can most likely be attributed to increased cross-linkage length among the celluloses, and/or the filling effect of GU/SiO2 in the voids in wood cell walls.

Controlling pore size of activated carbon through self-activation process for removing contaminants of different molecular sizes

Self-activation was employed for the manufacturing of activated carbon (AC) using kenaf core fibers, which is more environmentally friendly and cost-effective than the conventional physical/chemical activations. It makes the use of the gases emitted from the thermal treatment to activate the converted carbon itself. The mechanism was illustrated by the Fourier transform infrared spectroscopy and mass spectrometry analysis of the emitted gases, showing that CO2 served as an activating agent. The AC from self-activation presented high performance, for instance, the Brunauer-Emmett-Teller surface area was up to 2296 m2 g-1, Using the Density Functional Theory (DFT), the pore volume (PV) was determined to be 1.876 cm3 g-1. Linear relations of PVDFT-micropore/iodine number, and PVDFT-mesopore/tannin value were established, indicating a strong relationship between the pore structure of AC and its adsorbing preference. Adsorption results for copper (II) and rhodamine 6G also indicated that the pore size of AC should be designed based on the molecular size of the contaminants.