Preparation Of Structural Strand Board Containing Preservatives And The Reaction Mechanisms Of Its Components

As one of the most important wood-based composites which can be used outdoors, the application of stuctural strand board was restricted by its decay reisitance. Thus, it is necessary to develop preservatives suitable for structural stand board and corresponding application methods. In this study, ammonium pentaborate (APB), the mixture of APB and poly enthylene (PEG), nano size copper oxide, and basic copper carbonate were respectively introduced into strand or flake board manufacturing process. The effect of the loading level and the application method of these additives on the physical and mechanical properties, fungi and termite resistance of strand board were investigated, futher more,13C CP/MAS NMR, differential scanning charametry (DSC), fourier transform infrared spectrometer (FT-IR) were used to study the effect of APB, wood and copper compounds on the curing kinetics of phenol formaldehyde (PF) resin and the molecular structure change of cured PF resin. In this study, the "second curing process" was first observed and proposed which is completely different from the previous ingestivations. In addition, the PF resin and its mixtures with additives were analysized by chemical reaction models and quantitative way as well.The main results can be concluded as follows:(1) After introducing APB into structural strand board, the termite resistance was significantly improved and the formaldehyde emission levels were greatly reduced. The three layer structure was proved as an effective way to compensate the reduction of physical and mechanical properties caused by APB.(2) The addition of PEG eliminated the adverse effect of APB on the internal bond strength of strand board. With the increasing adding amount of PEG, the internal bond strength of the board increased to a certain extent. According to the analysis of the physical and mechanical properties, the best formula was 2.80% APB (based on the weight of oven dried strand) with 40% PEG-4000 (based on the weight of PF resin).(3) Based on the analysis of reaction activation energy, reaction enthalpy and molecular change by FT-IR, the concept of "second curing process" was put forward while investigating the curing process of PF resin with APB. The characteristics of "second curing process" were as follows:first, more stable methylene bridges are formed, followed by the addition reaction between eliminated formaldehyde and-B-O- group to the active groups on PF resin, finally, the reaction between released ammonia and formaldehyde to produce hexamethylenetetramine (hexamine). (4) The reaction order of PF resin did not change with the addtion of APB and poplar powder. The addition reaction in the early stage of the curing process was accelerated, since the curing system transformed from consistent one to disperse one caused by poplar powder. In addtion, the reaction activation energy of the curing system decreased as the pH buffer effect of APB provided a friendly curing environment to PF/Wood/APB system. 125℃was a critical temperature to the reaction rate of curing systems. The comparison of the experimental and predicted cure conversion rates at low temperature range showed a good agreement between measured and predicted data. However, with the increasing of heat temperature, the experimental data are relatively higher than the predicted ones.(5) Nano size copper oxide improved the internal bond strength, modulas of rupture (MOR) and modulas of elasticity (MOE), but basic copper carbonate decreased the internal bond strength and increased the 24 h water absorption. The DSC result showed that both kinds of copper component improved the curing reactions of PF resin, futher more, the addition of nano copper oxide increased the reaction enthalpy of PF resin and made it cured much more completely. The crystal distance of cellulose kept stable with the addition of nano size copper oxide and basic copper carbonate, but the crystallinity was reduced slightly which maybe due to the improvement of the thermal conductivity of strand board mat caused by copper compounds. The curing of PF resin promoted the fixation of both nano copper oxide and basic copper carbonate in flake board significantly.(6) The result of 13C CP/MAS NMR indicated that the addition of APB resulted in the bonding between boric acid ions and unreacted hydroxymethyl groups, and made the chemical shift move to low magnetic field as well, which provided strong evidence to the concept of "second curing process". The spectrum area of PF, APB and poplar increased due to the overlapping of the chemical shifts of wood constituents and PF resin, however, the interaction between the main chemical groups in cured PF resin has not been changed. The chemical shift of carbon in hydroxymethyl in PF resin moved to high magnetic field with the addition of nano size copper oxide and basic copper carbonate, but the whole molecular structure of cured PF resin has not been changed significantly. Meanwhile, at the same curing condition, nano size size copper oxide improved the curing degree of PF resin slightly...