Physicochemical Characterization of Functional Lignin–Silica Hybrid Fillers for Potential Application in Abrasive Tools

Functional lignin–SiO2 hybrid fillers were prepared for potential application in binders for phenolic resins, and their chemical structure was characterized. The properties of these fillers and of composites obtained from them with phenolic resin were compared with those of systems with lignin or silica alone. The chemical structure of the materials was investigated by Fourier transform infrared spectroscopy (FT-IR) and carbon-13 nuclear magnetic resonance spectroscopy (13C CP MAS NMR). The thermal stability of the new functional fillers was examined by thermogravimetric analysis–mass spectrometry (TG-MS). Thermo-mechanical properties of the lignin–silica hybrids and resin systems were investigated by dynamic mechanical thermal analysis (DMTA). The DMTA results showed that abrasive composites with lignin–SiO2 fillers have better thermo-mechanical properties than systems with silica alone. Thus, fillers based on lignin might provide new, promising properties for the abrasive industry, combining the good properties of lignin as a plasticizer and of silica as a filler improving mechanical properties.

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abstract: Phenolic extract from the green tea leaves was used for the production of thermoset epoxy resins. The commercial green tea extract (GTE) was functionalized by the reaction with epichlorohydrin in the presence of phase transfer catalyst. The glycidyl ether derivative of the green tea extract (GEGTE) obtained with a good yield was cured in epoxy polymer with isophorone diamine (IPD) and the resulting network was compared to catechin-IPD and diglycidyl ether of bisphenol A (DGEBA-IPD) systems. The thermal and mechanical analyses of this bio-based epoxy polymer showed its high reactivity associated with a high crosslinking density ( T g : 140–190 °C), a high thermal resistance and interesting mechanical properties.

title: Thermokinetic behavior of powdered phenol-formaldehyde (PPF) resins
abstract: This study was conducted to investigate thermokinetic behavior of powdered phenol-formaldehyde (PPF) resins used for the production of wood-based panel products. Both fusion diameter and stroke cure time of the PPF resins were measured to characterize flow properties and reactivity of PPF resins. In addition, differential scanning calorimetry (DSC) was also used to obtain apparent glass transition temperature ( T g ) and activation energy ( E a ) of the PPF resins. The results showed that the stroke cure time of PPF resins was positively related with fusion diameter of the PPF resins. The T g of PPF resins depended on the heating rate with a linear relationship established between heating rate and the T g . The E a of PPF resins depended on types of PPF resin and spray-drying temperatures.

title: Relaxation behaviour of the amorphous components of wood
abstract: The viscoelastic properties of mod were investigated using dynamic mechanical thermal analysis and differential scanning calorimetry. Under a limited set of conditions, two separate glass transitions (Tg) could be identified with both techniques. These two transitions were identified as arising from the amorphous lignin and hernicellulose matrix in the wood cell wall. Moisture dramatically affected the temperature at which the two dispersions occurred and, consequently, the ability to resolve their independent responses. The relationship betweenTg and moisture for both components could be modelled with the Kwei equation, which accounts for the presence of secondary interactions. Annealing and specific interactions of a series of organic diluents were wed in an attempt to enhance the resolution of the two components values ofTg. Time-temperature superposition was shown to be applicable to wood plasticized with ethyl formamide, following Williams-Landel-Ferry behaviour over the temperature rangeTg toTg + 85° C. These observations allow certain conclusions to be drawn concerning the applicability of existing models of the wood cell wall's supermolecular morphology. Most notably, models of thein situ morphology of the three wood components can be limited to those which consider the amorphous matrix of lignin and hemicellulose to be immiscible.

title: Kinetic analysis of phenol–formaldehyde bonded wood joints with dynamical mechanical analysis
abstract: Cure development of phenol–formaldehyde (PF) resins has been extensively modeled based on chemical advancement. However, it is in situ mechanical development of wood–adhesive systems that is most relevant with process optimization such as hot-pressing of wood-based panels. The objective of this research is to examine the feasibility of applying common model-fitting kinetic analyses to describe cure development based on storage modulus development recorded with dynamic mechanical analysis (DMA). Dynamic three-point bending tests were conducted on a sandwich specimens composed of two wood adherends bonded with an adhesive layer. Two commercial PF resins of different molecular weights distributions (labeled as PF-high and PF-low, respectively) were used as adhesives. In addition, PF-high bonded wood joints were also wrapped by aluminum foil to investigate the effect of moisture loss. The specimen curing process was monitored using various isothermal and linear heating regimes. The results showed that the PF-low joints cured more slowly than the PF-high joints. The foil-wrapped PF-high joints displayed slower curing process than the unwrapped joints and rendered two peaks in the tan δ curves. These peaks were attributed to gelation and vitrification, respectively. Overall, model-fitting kinetic analyses were effective to describe the mechanical development of wood–adhesive systems.

title: Silica fiber–polybenzoxazine–syntactic foams; Processing and properties
abstract: Polybenzoxazine filled with chopped silica fibers and their syntactic foams of varying composition and densities were processed. The composition and density variations were achieved by regulating the relative concentrations of silica fiber and glass microballoons (MB). The variation of tensile, compressive, and flexural properties with change in composition was investigated. For the silica-fiber filled materials, the property attained a maximum at about 40% volume content of fiber, and thereafter, the properties showed a diminishing trend. The incorporation of microspheres significantly lowered the strength of silica filled materials. However, the decrease in the specific flexural strength was less pronounced and the strength was unaffected beyond a certain microsphere content. During processing and mechanical testing, a large quantity of fibers was fractured, which reduced the strength of silica filled systems. The diminution in material strength on embedding microspheres is attributed to the presence of stress concentrating loci as evidenced from SEM analysis. The various factors leading to the property variation with composition are discussed with microscopic analyses, like clustering of fibers, crack propagation, fiber pull out, and debonding of fibers from resin phase. Dynamic mechanical analysis revealed an improved damping property for the filled materials in contrast to the unfilled polymer. The Tg (deduced from tan δ maximum from DMTA) decreased in silica fiber containing materials and on incorporating the MB, the values reverted back to that of the neat polymer. Both silica and MB conferred better thermal and thermooxidative stabilities to the polybenzoxazine. However, the degradation mechanism is nonoxidative in nature.

title: Master curve and time-temperature-transformation cure diagram of lignin-phenolic and phenolic resol resins
abstract: The aim of this work is to generate both a master curve of resol resins based on the time–temperature superposition principle and their TTT cure diagrams. The samples used for this purpose were lignin–phenolic and phenol–formaldehyde resol resins. A TMA technique was employed to study the gelation of resol resins. In addition, a DSC technique was employed to determine the kinetic parameters through the Ozawa method, which allowed us to obtain isoconversional curves from the data fit to the Arrhenius expression. Establishing the relationship between the glass-transition temperature and curing degree allowed the determination of the vitrification lines of the resol resins. Thus, using the experimental data obtained by TMA and DSC, we generated a TTT cure diagram for each of resins studied.

title: Catalytic activity of graphite-based nanofillers on cure reaction of epoxy resins
abstract: The strong influence of graphite oxide (GO) nanofiller on the glass transition temperature ( T g ) of epoxy resins, generally attributed to restricted molecular mobility of the epoxy matrix by the nanofiller or to the crosslinking of GO layers via the epoxy chains, is investigated. The study confirms that large increases of the glass transition temperature of the nanocomposite can be observed in presence of GO. However, similar T g increases are observed, when the filler is a high-surface-area graphite (HSAG), lacking oxidized groups. Moreover, these T g differences tend to disappear as a consequence of aging or thermal annealing. These results suggest that the observed T g increases are mainly due to a catalytic activity of graphitic layers on the crosslinking reaction between the epoxy resin components (epoxide oligomer and di-amine), rather than to reaction of the epoxide groups with functional groups of GO. This hypothesis is supported by investigating the catalytic activity of graphite-based materials on reactions between analogous monofunctional epoxide and amine compounds.

title: X-ray photoelectron spectroscopy of reduced graphene oxide prepared by a novel green method
abstract: This work is focused on the surface characterization of reduced graphene oxide (rGO) obtained through reduction of graphite oxide. The reduction was performed using a green method (based on UV irradiation under N2) with low environmental impact, free of additional functionalization reagents, simple and low-cost. X-ray photoelectron spectroscopy (XPS) was applied for chemical characterization of surface C species showing a significant reduction of oxygen-containing functional groups from the surface of rGO.