Mechanism Of Synthesis Of Urea-formaldehyde Resin Under Initial Weak Acidity Condition

In this paper the synthesis mechanism of UF resin as well as the storage stability mechanism of MUF resin were studied by FTIR,13CP/MAsNMR, DSC and LC-ESI/MS. The UF resin and MUF resin were synthesized at initial weak acid condition. The technology of UF resin was optimized by response surface method. The curing dynamic of UF resin was affected by the times of urea addition and interval time at base addition phase, and the effect of different wood species on the curing of UF resin and MUF resin were also investigated. It was found that the carbocation of protonation formaldehyde increased under weak acid condition. This condition favored to the electrophilic substitution. The protonation formaldehyde was the main reaction body and three valence carbocation reacted with lone-pair electron N of urea of forming chemical bond. Meantime, H+ broke away and gave birth to methylolurea. A little of exceeded 1000 (molecular weight) long chain molecular was formed, which was linked by ether bond. Urea and formaldehyde processed nucleophilic substitution at base addition phase. Urea was the main reaction body at alkali condition, under which the reaction of negative ion urea and polarization formaldehyde formed methylolourea. Moreover, the amount of methylolurea began to reduce in the reaction solution and a little Uron generated. Besides, with the decreasing of amount of methylolurea, the condensation between methylolurea and methylolurea occurred. The 13C-NMR spectrum proved the condensation generated lined and branched ether. The resin was mainly composed of mixture of various long chain methylolurea and ether compound at addition phase. RCONHCH2OH was the main reaction body at acidic phase. When the integration of RCONHCH2OH and H+ formed RCONHCH2OH2+, The HMO calculation showed the structure of the positive ion CH2+ and OH2+ were not stable. The structure was prone to keep off H2O and finally formed RCONHCH2+. The electrophilic substitution of RCONHCH2+ with N or O of molecular chain brought to methylene and dimethylene linked macromolecule chain. The amount of molecular weight of macromolecule exceeded 1000 with the increasing of acid condensation, which was not the main component. The reaction mechanism of last addition urea and formaldehyde was the same to base addition phase. The reaction of urea and formaldehyde made to the equilibration molecular weight of resin, which formed to mixture of methylolurea. The UF resin was catalysed by acid and base. Moreover, acid and base participated in process of macromolecule synthesis reaction, which was synthesized under initial weakly acid.The synthetic technology of UF resin was optimized by response surface method. It was found that the solid content and curing time of UF resin could not be dramatically affected by the times of additions, the interval of urea, and the viscosity and hydromethyl content, free formaldehyde content of the UF resin and properties of plywood were prominently affected by the number of additions and the interval between additions of urea. The main chemical groups of UF resin were influenced by the number of additions and the interval of urea. When the times of addition was two and the interval of urea was 20 min, the performance of urea-formaldehyde resins and the properties of plywood were much better. The addition times and interval notably affected curing reaction. With the increase of the additional and interval, times, the apparent activation energy of curing system of resin was reduced. The curing reaction grade of UF resin was between 0 and 1, when the UF resin was synthesized under initial weak acidity condition. Different wood species affected the curing of UF resin and MUF resin. After different wood species powder mixed with UF resin and MUF resin, the curing activation energy of compound was lower than UF resin and MUF resin. Furthermore, collision factor of compound decreased. The curing degree of compound was also lower than UF resin and MUF resin, which was the UF resin and MUF resin mixed with wood powder. After the compound was heared to 180℃, the compound was investigated by FTIR. The result showed that wood powder did not react with UF resin and MUF resin. When wood was glued by resin, resin component mainly changed. The curing reaction grade of compound was between 0 and 1 non-elementary reaction.It was found that melamine content affected the water resistance of MUF resin. With the melamine content increased, the water resistance of MUF resin bonded product improved. Moreover, the curing crosslinking density of MUF resin was enhanced, owing to the increasing of triazine. Meantime, the reduction of acidic hydrolysis constant of resin improved the water resistance. The structure of MUF resin was special, which was synthesized under initial weak acidity. This impacted on storage stability of MUF resin. The amount of methylol decreased, which was resulted to the chemical reaction at MUF resin storage. The chemical reaction leaded to the change of linking molecular chain. The weak aggregates molecular chain was the main reason of forming molecular cluster, which was connected by hydrogen bond among the amine group in the molecular chain, or active group and water. The results showed the shape of colloidal particle was the same through microscope examined the MUF resin.