Mandla A. Tshabalala

Photostability and moisture uptake properties of wood veneers coated with a combination of thin sol-gel films and light stabilizers

Abstract In recent years, there has been increased interest in the use of inorganic UV blocking nanoparticles for photostabilization of wood surfaces. Photostability and moisture uptake properties of wood veneers coated with a combination of hybrid inorganic-organic thin sol-gel films and organic light stabilizers was investigated. The light stabilizers were applied by brushing, and the thin sol-gel films were deposi-ted on the wood surface by dipping in a sol prepared from a mixture of methyltrimethoxysilane, hexadecyltrimethoxysilane, and aluminum isopropoxide precursors. Although the sol-gel film improved the moisture resistance of the wood veneers, it showed mixed results on photostability. Under light and spray conditions in a Weather-Ometer™, specimens treated with a combination of sol-gel thin film and a UV absorber, tris-resorcinol triazine and an acrylic self-crosslink-ing binder, or treated with lignin stabilizer/tris-resorcinol triazine/acrylic self-crosslinking binder showed good overall weathering performance. Under permanent light conditions in the Weather-Ometer™, specimens treated with a combination of sol-gel thin film, a lignin stabilizer, and a water soluble UV absorber – namely: (2-hydroxyphenyl)-benzotriazole – revealed a good overall weathering performance. This study has demonstrated the feasibility of using a combination of organic light stabilizers and sol-gel deposits of hybrid inorganic-organic thin films to improve weathering resistance of softwood substrates.

Correlation of water vapor adsorption behavior of wood with surface thermodynamic properties

To improve the overall performance of wood-plastic composites, appropriate technologies are needed to control moisture sorption and to improve the interaction of wood fiber with selected hydrophobic matrices. The objective of this study was to determine the surface thermodynamic characteristics of a wood fiber and to correlate those characteristics with the fibers water vapor adsorption behavior. The surface thermodynamic properties, determined by inverse gas chromatography at infinite dilution or near zero surface coverage, were the dispersive component of the surface energy, surface acid-base free energy and enthalpy of desorption of acid-base probes, and surface acid-base acceptor and donor parameters (K A and K D ). Water vapor adsorption was expressed in terms of the percentage of weight gain (ΔW%) resulting from water vapor adsorption on the wood particles, calculated relative to their initial weight after preconditioning in a vacuum dessicator at room temperature. The results showed a strong correlation between ΔW% and K A , and between ΔW% and surface acid-base free energy of water desorption (ΔH AB water ), calculated from experimental K A and K D and values in the literature for acceptor and donor values of water. These results suggest that for substrates such as wood, whose surface Lewis acid-base properties are characterized by a relatively stronger tendency to accept electrons, the key to controlling water vapor adsorption is to manipulate the magnitude of ΔH AB water , primarily via K A , and to a lesser extent via K D .

Weathering Characteristics of Wood Plastic Composites Reinforced with Extracted or Delignified Wood Flour

This study investigated weathering performance of an HDPE wood plastic composite reinforced with extracted or delignified wood flour (WF). The wood flour was pre-extracted with three different solvents, toluene/ethanol (TE), acetone/water (AW), and hot water (HW), or sodium chlorite/acetic acid. The spectral properties of the composites before and after artificial weathering under accelerated conditions were characterized by Fourier transform infrared (FTIR) spectroscopy, the surface color parameters were analyzed using colorimetry, and the mechanical properties were determined by a flexural test. Weathering of WPC resulted in a surface lightening and a decrease in wood index (wood/HDPE) and flexural strength. WPCs that were reinforced with delignified wood flour showed higher ΔL* and ΔE* values, together with lower MOE and MOR retention ratios upon weathering when compared to those with non-extracted control and extracted WF.

Formulation and characterization of polysaccharide beads for controlled release of plant growth regulators

Owing to their chemical, physical, and functional characteristics, polysaccharides are considered to be the most versatile natural polymers. As a result, their properties have been exploited in various fields of research in the biomedical, pharmaceutical, cosmetic, food, and agricultural industries. A property of special interest is their ability to form systems or materials with unique physicochemical characteristics, such as hydrogels and micro- and nanoparticles for controlled release of active compounds. In the present study, polysaccharide beads formulated from alginate, cellulose powder, cellulose nanocrystals, starch, and xylan were reinforced with kaolin and surface-modified with polyethylenimine (PEI), a positively charged polyelectrolyte. Addition of kaolin improved the mechanical strength of the beads. Modification of the surface of the beads with PEI facilitated better control of the release rate of the plant growth regulator, phenylacetic acid (PAA). The physical properties of the beads were characterized by optical and scanning electron microscopy, and their mechanical strength was determined by an Instron 5565 Tensile Testing Machine. Cumulative release of PAA was measured by UV–Vis spectroscopy.

Spectroscopic analysis of the role of extractives on heat-induced discoloration of black locust (Robinia pseudoacacia)

Abstract To investigate the role of extractives on heat-induced discoloration of wood, samples of black locust (Robinia pseudoacacia) wood flour were extracted with various solvents prior to heat-treatment. Analysis of their color parameters and chromophoric structures showed that the chroma value of the unextracted sample decreased while that of the extracted sample increased after heat-treatment. Both samples showed broad diffuse reflectance UV-Vis (DRUV) absorption bands with maxima around 360–380 nm after heat-treatment due to the formation of conjugated double bonds, carbonyl functionalities, and quinoid structures. Compared with the unextracted sample, the dominant chromaticity of the extracted samples hypochromatically shifted and the peak became narrower. This result showed that extractives contribute mostly to the reduction in the light reflection on heat-treated wood. In addition to extractives, lignin and hemicellulose also contributed to the formation of color substances upon heat-treatment. The increase in C3/C2 ratio in X-ray photoelectron spectroscopy (XPS) spectra signified the oxidation reactions in the heating process. The increase in O1/O2 for extracted sample after heat-treatment and changes in DRUV and Fourier transform infrared spectroscopy (FTIR) spectra support the hypothesis that discoloration can also arise from the degradation of hemicellulose and the condensation reactions of lignin.

Properties of wood-plastic composites (WPCs) reinforced with extracted and delignified wood flour

Abstract The water sorption and mechanical properties of wood-plastic composites (WPCs) made of extracted and delignified wood flour (WF) has been investigated. WF was prepared by extraction with the solvent systems toluene/ethanol (TE), acetone/water (AW), and hot water (HW), and its delignification was conducted by means of sodium chlorite/acetic acid (AA) solution. A 24 full-factorial experimental design was employed to determine the effects of treatments and treatment combinations. WPCs were prepared with high-density polyethylene (HDPE) and treated WF was prepared by means of extrusion followed by injection molding, and the water absorption characteristics and mechanical properties of the products were evaluated. WPCs produced with extracted WF had lower water absorption rates and better mechanical properties than those made of untreated WF. WPCs containing delignified WF had higher water absorption rates and improved mechanical performance compared with those made of untreated WF.