No abstract

In this doctorate dissertation, the influence of beech, fir and poplar wood species on curing kinetics of commercial urea-formaldehyde (UF) adhesive was investigated. With this objective, the following thermal analysis methods were utilized: differential scanning calorimetry (DSC), dielectric analysis (DEA) and dynamical mechanical analysis (DMA). DSC measurements were obtained in dynamical scanning regime with the heating rates (β) of 5; 10; 15 i 20 o C/min. Test samples consisted of pure UF adhesive, UF adhesive / wood flour mixes and UF adhesive / wood extractives mixes of selected wood species. Catalyst (NH 4 Cl) addition was 0.2% (per oven dry weight) for all test samples. Results of peak temperature (T p ), enthalpy of reaction (ΔH), and activation energy (E a ) were analyzed for each UF adhesive system. In addition, the isoconverzional models of Ozawa-Flynn-Wall (OFW) and Kissinger-Akahira-Sunose (KAS) were applied on the DSC test data.DEA tests proceeded in the isothermal regime in the real hot pressing conditions using veneer samples of the selected wood species as adherents. UF adhesive was applied with the NH 4 Cl addition of 0.2% (per oven dry weight). Press temperatures were 120; 140 i 160 o C. Dielectric parameters were recorded through IDEX sensor applied on the bond line of veneer samples and connected to LCR meter. Degree of reaction (α) was obtained on the basis of electric conductivity. DMA rheological tests were performed in the sinusoidal shear stress regime at the temperature of 80 o C. Test samples consisted of pure UF adhesive and the UF adhesive / wood extractives mixes. Two separate sample series were used, one with the NH 4 Cl addition of 0.2% (per oven dry weight) and the other without catalyst addition. Gel time was determined at the crossover of the storage modulus (G') and loss modulus (G"), resulting in the loss factor tanδ=1.Results of DSC and DMA tests on UF adhesive / wood extractive mixes, showed significant influence of pH value and acid buffer capacity of selected wood species on kinetics of curing reaction. Lower values of said chemical characteristics of a given extractive, resulted in decrease of peak temperature and activation energy, as well as in shorter gel time for a corresponding mixture. Both, DSC measurements of UF adhesive / wood flour mixtures and DEA results of UF adhesive cure in the hot pressed veneer samples, have showed significant influence of wood anatomy structure on curing behavior of UF adhesive.

The influence of Narrow Leaved Ash (Fraxinus angustifolia Vahl. ssp. Pannonica Soo & Simon) pre-treatments on the curing behavior of urea-formaldehyde (UF) adhesive was studied. Differential scanning calorimetry (DSC) was used to monitor the curing reaction of UF adhesive mixed with non-treated wood flour (control series) and wood flour of Narrow leaved Ash obtained after different pre-treatments of wood particles. Three different pre-treatments were applied, one with sodium carbonate solution (0.03 g/g of dry wood), one with acetic acid solution (0.06 g/g of dry wood) and one with distilled water. The curing reaction of different adhesive systems showed no apparent differences in peak temperature values, and there were no significant differences in the enthalpy of the reaction. However, it was assessed that the activation energy has lower values for the UF systems with alkaline and acid treated wood flour (76.64 and 76.92 kJ/mol) in regard to the UF systems with control (untreated) wood flour and water treated wood flour (79.28 and 79.14 kJ/mol). In addition, the application of iso-conversional models have shown different curing behavior of UF adhesive systems.

The goal of this paper is to analyze the chemical composition of untreated and treated wooden chips from the native narrow-leaved ash (Fraxinus angustifolia Vahl. ssp. Pannonica Soó & Simon). In order to determine the effect of pretreatment with liquid hot water (LHW) on changes in chemical composition, the content of moisture, cellulose, lignin, minerals (ash), extractives soluble in hot water, extractives soluble in organic solvents, for treated and for untreated wooden chips was determined. This was done in accordance with TAPPI and ASTM standard methods. The properties of wooden chips, treated for 30 min and 60 min at a temperature of 100 °C, are compared to untreated wooden chips and changes in the chemical composition that occurred are defined as a result of the applied treatments. The research was performed under controlled conditions in a laboratory, and the results of treatments were the subject of comparative analysis. Applied treatments had a statistically significant effect on decreasing the content of extractives. The content of cellulose and hemicellulose increased in the treated wooden chips compared to untreated wooden chips, while the lignin content did not significantly change.

The formaldehyde potential of particleboards imported on Serbian market was evaluated in this research. The sampling and testing of particleboards were conducted in two discrete periods: first was during 2011 and the second was in the period of 2015-2016. For both periods, the samples of raw particleboards originated from three different producers. The perforator method (EN 120) was used to test the formaldehyde content in the particleboard samples. In general, the results have showed that the majority of the samples belong to the E1 class of particleboards. In addition, 19 % of the particleboards sampled in 2011 and 31 % of the boards sampled in 2015-2016 were found to have formaldehyde content below 50 % of E1 class requirements.