Study On The Mechanisms Of The Reactions In Synthesis Of Amino Resins Used As Wood Adhesives

The general theory of the amino resin synthesis has been establised since the middle of the last century. However, with the intensive stuies on the synthesis-structure-performance relationships, it has been realized that the resin chemistry is extremely complex and the original theory appears to be old and can not provide further supports for improvement and inovation of resin synthesis. Numerous studies in the past decades indicate that the resin structure and performance are closely related to the reaction conditions, but their relationships have not been well rationalized due to the limited undstanding on the reaction mechanisms. To overcome the difficulties in investigating the reaction mechanism by using experimental methods, the theoretical methods based on quantum chemistry were introduced in this work for the first time and the reaction mechanims of the UF, MF and MUF syntheis were theoreticlly and systematically studied. With the obtained reaction pathways, kinetics and therodynamics of the reactions, the main results in the experimental resin synthesises were explained and the competitive relationships of the different reactions, as well as the effects of the reaction conditions on them were elucidated. Based on the theoretical results some experiments were carried out and these experiments proived more evidences for elucidating the resin structure formation mechanism. The results of this work futher improved the theories of the resin chemistry and provided more theoretical supports for improving the synthetic procedure and optimizing the resin structures. The successful applications of quantum chemistry methods in this work imply that a new tool has been introduced to the area of resin research.The main findings and conclusions were drawn as follows:1. The Reaction System of UF Resin Synthesis1) The Neutral Hydroxylmethylation of UreaThe theoretical calculations show that the solvent water molecules can catalyze the addition between urea and formaldehyde and plays the role of mediating proton transfer. As a result, the potetial energy barrier was significantly lowered. The IRC calculations indicate that the zwitterionic-like intermediate does not exist on the potential energy surface and the reaction undergoes via one transition state. Although the reaction proceeds with a concerted mechanism, it exhibits asynchronous feature because the nucleophilic attack precedes the proton transfer.2) The Base-catalyzed UF ReactionsThe theoretical calculations suggest that the catalytic effect of the base (OH-) lies in its reaction with urea methylolurea to form amino anions which act as active nucleophilic intermediates in the hydroxymethylation and the condensation reactions.The condensations under alkaline condition proceed via SN2 mechanism. Both theoretical calculations and 13C NMR analysis on the reaction products produced from conditions of different molar ratio and temperatures indicate that the competitive formations of methylene ether and methylene bridges are affected by energy barrier and steric hindrance. The effects of the two factors on the relative contents of the two structures were rationalized.The energy barrier of the uron formation is higher than those of the formations of chain methylene ether bridges. This is in agreement with the experimental results that the content of uron is lower than those of the ether bridges under genneral conditions, particularly when the F/U molar ratio is around 2/1.3) The Acid-catalyzed UF ReactionsTheoretical calculations show that both protonated formadehyde and methanediol participate in the hydroxymethylation, but the former is energtically less favorable due to the formation of a stable hydrogen bonding complex between protonated formaldehyde and urea.SN1 mechanism was identified for the condensations of the methylolureas. The rate-determining step is the production of the carbocation through the elimination of water from protonated methylolurea. The formations of methylene bridges are both kinetically and thermodynamically more favorable than the formations of methylene ether bridges. However, the formations of branched methylene bridges through the reactions between di-or tri-methylolureas are slower due to the steric hindrance casued between the reactions of the secondary amino group (-NH-) with hydroxymethyl group. Differently, the reaction between free amino group and methylol group is not affected by streric hindrance and therefore the formation of linear methylene bridge becomes faster.Under acidic condition, the intra-molecular reaction resulting in uron structure is also energetically less favarable than the inter-molecular condensations. Relatively higher content of this species is dependent on higher molar ratio.The structures and components of the resin are determined by the thermodynamic nature of the reaction system. With the same molar ratio, the stronger or weaker acidic condition (or higher or lower concentration of the catalyst) only tetatively changes the competitive relationships of the reactions and theoretically, therefore the same resin would be obtained as long as the reaction time is long enough. However, the resin we generally obtain is indeed the intermediate product due to the limited viscosity. This is the right reason that differnt pHs lead to different resins4) Effects of The Molar Ratio at The Initial Alkaline Stage on The Final Strcture of the ResinThe classical theory states that the condensations under the alkaline conditions are of no industrial importance, but the experimental results in this work clearly show that more methylene ether bridges (including urons) at the alkaline stage lead to higher content of these structures in the final resin. Lowering the molar ratio at the initial alkaline stage will lower the content of the methylene ether bridges in the final resin.2. Base-Catalyzed MF ReactionsSimilarly, the MF condesations occurs with SN2 mechanism and the energy barriers are lower than those of UF condensations by 30-40kJ/mol, indicating the MF condensations would be faster. However, by comparing the experimental results, it was found that the condensation degree of the MF resin is actually lower than that of UF resin. The larger molecular weight, branched structures of M and some physical factors were employed to explain the different resinification conditions between UF and MF.The previous studies suggest that the relative content of the methylene ether and methylene bridges depends strongly on pH. However, our study indicates that the molar ratio is the key factor instead.3. The MUF Reaction System Under Alkaline ConditionThe theoretical calculations show that the energy barriers of co-condensation reactions are higher than those of MF self-condensations but lower than those of UF self-condensations.The co-condensation prodcuts produced through the reaction between N, N’-dimethylolurea and melamine under alkaline condition were detected by ESI-MS. The MS spectrum shows these products have higher relative abundance. However, considerable methylolmelamines were produced due to the transfer of formaldehyde from N, N’-dimethylolurea to melamine. Therefore, MF self-condensation products will form in the later period of the reaction.Because of the lower energy barriers of MF self-condensation reactions than those of co-condensation reactions, one-time addition of large quantity of melamine would lead to higher content of MF sel-condensation products and as a result, the degree of co-condenstions would be lowered. Therefore, keeping low concentration of melamine during the synthesis may be an effective way to enhance the co-condenation reactions.