Pre-curing Characters And Regulating Mechanism Of Ureaformaldehyde Resins

The main materials in the field of furniture manufacturing and indoor decoration in China is medium density fiberboard(MDF), and Urea-Formaldehyde(UF) resin is the main adhesive in medium density fiberboard manufacture, which has taken up more than 90% of the amount of wood adhesives. In order to apply the adhesives to fibers uniformly and quickly, the blowline blending method has been adopted to dry the resined fibers. In blowline, the resined fibers were exposed to the environment of acidic, high temperature and humidity, which resulted in the polycondensation and pre-cured of resin. Once the resins which had lost the activity, the resined fibers would not be bonded together, leading to the physico-chemical properties of panels would decrease significantly. In order to keep the good performance of panels, the producers were nothing to do but to increasing the loading level of adhesives. Therefore, resolving the pre-curing problem before hot-pressing and improving the performance of resins was one of the important methods to cut down the production cost and to enhance the properties of panels.In order to provide theoretical basis for decreasing the pre-curing degree and loading level of UF resin, the pre-curing mechanism of UF resin was studied in this paper, and the pre-curing actions was discussed under the effects of three impact factors including temperature, solid content and pH value. By comparing the contribution rate of three factors to the main properties factors, the significant evaluation for three factors was determined. Meanwhile, the curing action and controlling mechanism of UF resin by adding latent curing agents was also discussed. The main research results were summarized as below:(1) Taking the UF resin added with NH4 Cl as the research objects, the curing time testing results indicated that the curing time was decreased with increasing pre-curing degree, and a linear relationship between the curing time and pre-curing time. The pre-curing course was analyzed by rheometer, differential scanning calorimeter(DSC), gel filtration chromatography(GPC) and matrix assisted laser desorption ionization time of flight mass spectrometry(MALDI-TOF MS), the results showed that the basic characteristics was that viscosity and molecular weight were increased, the hydroxymethyl content decreased while methylene content increased. According the change rule of activation energy, the pre-curing course were divided into two stages: in the first stage, the viscosity was increased with water evaporation, which resulted in the collision chances increased of micromolecules lead to the activity of resin was enhanced, just as it was shown as the active energy decreased. However, the content of hydroxymethyl and methylenes had changed a little, it means that the cross-linking degree was low, which led to the storage modulus was not changed obviously, but both dry shear strength and wet shear strength of plywood rose in this stage. In the second stage, with the cross-linking degree increased due to the content of hydroxymethyl was decreased a lot, the breakage of ethers appeared, and methylenes increased noticeable, which led to the storage modulus increased obviously, but both dry and wet shear strength of plywood declined resulted from the part of resin remained on the surface of veneer was cured.Taking the wet shear strength as the boundary, all performance indicators owed their critical values: the viscosity was 3.5×106mPa.s, the number-average molecular weight(Mn) and weight-average molecular weight(Mw) were the 3.85 times and 2.5 times of the initial value, respectively, while the content of methylene and hydroxymethyl were increased by 23.5% and decreased by 17% based on the initials value, respectively. When the values of the performance indicators were lower than the critical values, the wet shear strength was increased, whereas the wet shear strength were decreased.(2) The changes of the viscosity, thermal behavior, molecular weight distribution(MWD) and main functional groups of different pre-curing degree UF resin under the effect of three factors were analyzed, that is temperature, solid content and pH value. By comparing the contribution rate of the three factors, the results indicated that the temperature and pH value showed obvious effects on the changes of performance indicators including the curing conversion, the content of hydroxymethyl and methylene, and enthalpy. The order of these three factors affecting the pre-curing phenomenon of UF resin was pH value > temperature >solid content.(3) By adding two latent curing agents hexamine+ammonium persulfate+ p-methylbenzene sulfonic acid+diethanol amine(LGDE for short), ammonium persulfate+ diammonium hydrogen phosphate(GP for short), the curing properties and control mechanism of UF resin were discussed. The results indicated that, the onset temperatures of resins added LGDE and GP were 110.77°C and 108.02°C, respectively, both values were higher than 87.46°C for NH4 Cl. When the curing temperature was remained at 100 °C, the pH values of UF resins added with two latent curing agents were remained above 7.0, which resulted in the curing times of both two resins was longer than 10 minutes and the curing peaks were not turned up at isothermal temperature of 100°C. When the curing temperature was set higher than onset temperature of both latent agents, the descend rates of pH value of UF resins added with two kind of latent curing agents were slowly first and then became faster, and the curing time became shorter. Therefore, by using these two kinds of latent curing agents, the pre-curing phenomenon could be inhibited at lower temperature while delayed at higher temperature.