Synthesis,Property And Reaction Mechanism Of Glyoxal-Urea-Formaldehyde Cocondensed Resin

In order to decrease fundamentally the harm of formaldehyde to environment and human health, in this PhD thesis, the low volatile and nontoxic aldehyde, glyoxyal (G) was chosen to substitute for formaldehyde (F) to react with urea (U) to prepare glyoxal-urea (GU) resin and glyoxal-urea-formaldehyde (GUF) cocondensed resin. Firstly, the synthesis reaction and structure formation mechanism were studied by quantum chemical calculations. On the basis of the results of quantum chemical calculations, the optimal synthesis conditions will be determined after investigating the effects of various kinds of synthesis conditions on the resins’structure formation process and properties, including the molar ratios of material, reaction pH, reaction time, reaction temperatures and catalysts. Secondly, the structure and molecular weight distribution of the resins were investigated mainly by nuclear resonance carbon-13spectrum (13CNMR), matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS), Fourier transform infrared spectroscopy (FTIR) and ultraviolet and visible spectroscopy (UV-vis) metods. Then, the dynamic mechanical analysis (DMA) test was used to investigate the curing process of GUF resins by simulating the hot pressing process in plywood manufacture. The main results obtained from the above study are summarized as follows:(1) The results of chemical calculation indicates that under weak acid condition, the addition reaction of urea (U), monomethylol urea (MMU) and dimethylol urea (DMU) with glyoxal (G) and2,2-dihydroxyacetaldehyde (G1) is difficulty to take place. However, the addition reaction of urea (U), monomethylol urea (MMU) and dimethylol urea (DMU) with protonated glyoxal (p-G) and protonated2,2-dihydroxyacetaldehyde (p-G1) is easily to occur. And in the these reactions, the important carbocation reactive intermediates C-p-UG and C-p-UG1, C-p-G-MMU and C-p-G1-MMU, C-p-G-DMU and C-p-G1-DMU have been formed, respectively. In the condensation stage, the resins have been produced through the formation of two main chemical bond of C-N-C and C-O-C.(2) Based on the quantum chemical calculation results, GU and GUF resin have been synthesized under slightly acid condition. The dry shear strength of the plywoods bonded with the GU or GUF resins could meet the type III grade plywood requirement of GB/T9846.3-2004, and could be used in dry conditions, but their wet dry shear strength has yet to be improved.(3) In the UV-vis spectrum of GU and GUF resins, the adsorption peak of the Ï€â†'Ï€*transition appears, which suggests that the conjugated structure exists in the structure of GU and GUF resin. FTIR shows that there are mainly such functional groups as-OH,-NH-, C-N, C=O, etc. and compared with the reactive materials, many new adsorption peaks appear in the13CNMR spectrum of GU and GUF resin, corresponding to the signals of the sp3hybridized carbon and carbonyl (C=O) with different substitution degree in the resins, which indicates that the structure of the resins is probably very complicated.(4) The MALDI-TOF-MS spectrum show that the materials molar ratio has important effects on the condensation degree and the molecular weight’s distribution. The main peaks have appeared at the same adsorption position, but with different adsorption strength, which indicates that there are same repeating units in the resins with different molar ratio of MMU/G or DMU/G. Moreover, the peaks due to the dehydration condensation reaction of MMU or DMU also appear in the spectra and their strength increases with the MMU/G or DMU/G molar ratio, which indicates that at low MMU/G or DMU/G molar ratio much more glyoxal reacted with MMU or DMU and at high MMU/G or DMU/G molar ratio much more MMU or DMU occurs the dehydration condensation reaction, so in the resin’s synthesis, the appropriate MMU/G or DMU/G molar ratio is very important.(5) The results of chemical calculation suggested that the nucleophilic reaction activity of formaldrhyde (F) is higher than that of glyoxal (G), so in the system existing G, U and F, U reacts firstly with F, then G reacts with the unreacted U and intermediate product of MMU, the most important intermediate products in the nucleophilic reaction of U and F, and even the corresponding condensation reaction. The proper synthesis conditions of GUF resin are as follows: the reaction pH in the second stage is weak acid; the reaction time in the second stage is2h; the molar ratio of G/U/F is0.7:1:0.7. The peak temperature, peak E’and stable temperature range of E’of GUF samples with different molar ratio are basically the same, which suggests that the curing rate, the heat stability and the performance of the resins are basically the same.