Preparation And Research On The Curing Mechanism Of Compounded Adhesive Comprising Modified Isocyanate And Urea-formaldehyde Resin

Urea formaldehyde adhesive was widely used with the advantages such as low cost, high viscosity, high bonding strength and light color. However, the utilization of urea formaldehyde adhesive is always constrained due to low water resistance, evident frangibility, poor aging property and high deviated formaldehyde content. Isocyanate adhesive included in non-formaldehyde class is of well bonding property, water resistance and aging-resistant performance. Meanwhile, isocyanate possesses the advantages such as great reactivity and fine manufacturability. Used as wood adhesive, blocked isocyanate was required to be of low deblocking temperature (lower than100℃), high blocking efficiency and well stability under ambient situation.In this paper, the utilization and reaction mechanism of isocyanate was introduced and the necessity for blocking and emulsion of isocyanate was explained according to the drawbacks in practical application. Then the modified isocyanate was blended with urea-formaldehyde resin, and the curing mechanism of this mixture was studied systematically. The research was shown in detail as follows.(1) The effect of stir speed on blocked isocyanate was studied according to the determinations of blocking ratio, pH, viscosity and deblocking performance. The LPS, DSC, TGA as well as FTIR were employed to conduct the analysis of the blocking resultant. The FTIR results showed that blocked isocyanate could be deblocked within the temperature ranging from60to100℃and the deblocking reaction got faster from70℃on. DSC results indicated that the phenomenon of heat absorption was aroused within the temperature from50to100℃, which could testified that the deblocking reaction definitely occurred. It was also concluded that the deblocking reaction got faster with the increase of temperature and the time spent on deblocking reaction was around10min. The TGA results displayed that the blocked isocyanate prepared by different methods with different consumptions of accelerant and stabilizer obtained evidently different thermal properties. At last, it was found that the appropriate NaHSO3/NCO molecular ratio was1.2. While the accelerator consumption was0.2mol and the percent of stabilizer was0.5%, the blocked isocyanate emulsion possessed little size particles and well dispersity. Thereafter, the blocked isocyanate emulsion was washed by different kinds of solvents and then frozen dried. The DSC, TGA, FTIR, XPS and titration analysis methods were employed to characterize the resultant. The experimental results demonstrated that isocyanate could be blocked by sodium bisulfite completely and the deblocking reaction of blocked isocyanate could be accelerated by DMF. The sodium sulfite-blocked isocyanate could be debloced well at the temperature ranging from50to90℃.(2) The emulsified isocyanate was prepared with various methods under different situations. Then the effects of molecular ratio, emulsifier consumption and other factors on stability of emulsion were studied according to the experimental results about emulsifying phenomenon, viscosity and the content of isocyanate group. The experimental results indicated that the emulsion was most stable while the ratio of NCO/OH was50:1and the consumptions of stabilizer and emulsifier were0.9%and2%, respectively. In addition, the particle size distribution of isocyanate emulsion was analyzed. The experimental results showed that viscosity of oligomer, molecular ratio and the content of NCO played important roles on the appearance and distribution of particle size. The smaller the particle size, the more stable the emulsified isocyanate was. The results of particle size were corresponded with viscosity and pot life.(3) The following conclusions were obtained by the study on thermal stability and curing structure of blocked isocyanate modified UF adhesive:Under the optimized conditions, blocked isocyanate (BPI) prepared by synthesis played a catalytic role on the curing rate of UF resin adhesives. BPI has stronger catalysis than ammonium chloride on the UF adhesive curing process. The way that NaHSO3and PAPI were put into UF resin adhesive in the composite has longer curing time, but weak effect in UF curing process. Results of the DSC test show that the curing rate and the curing temperature effect of BPI was better than that NaHSC>3and PAPI were put into UF in the composite. The TGA results show that the blocked isocyanate was added to improve the thermal stability of UF adhesive at proper ratios of2:8. By FTIR analysis, the results show that the BPI compound with UF and UF mixed with NaHSO3and PAPI produce a new chemical bond:carbamate-amino bond, carbamate-methylene bond. Amino and substituted ureas have different absorption strength with different ratio of blocked isocyanate. The XPS analysis show that curing product contains-N-C=O-C and-N-C=O-both in UF adhesives prepared with BPI composite and NaHSO3and p-MDI mixed compound prepared by adding UF adhesive. The-NC=ON functional groups in BPI and UF composite adhesive has more content than that of-NC=OC, which indicating the isocyanate in the BPI has dominant reaction with amino in the UF. The curing product which NaHSO3and p-MDI mixed compound prepared by adding UF adhesive contains more-N-C=O-C than-N-C=O-N, which indicating that the isocyanate in the composite adhesive system has the dominant reaction with hydroxyl group in UF.(4) Meanwhile, a study was taken to test the law of the thermal reaction of emulsified isocyanate modified urea-formaldehyde resin adhesive. By comparing the experiments results, we can get the ratio of emulsified isocyanate compound with the urea-formaldehyde resin and the curing reaction rate and thermal stability of adhesive were affected by acidity of crosslinker. The results showed that emulsified isocyanate added in urea-formaldehyde resin can promote the curing reaction rate and improve the thermal stability of the composite adhesive, but with the ratio of isocyanate emulsion increasing, thermal stability has a downward trend. Acidity of curing agent has a certain impact on isocyanate modified urea-formaldehyde resin. The stronger acid curing agent is, the shorter period of application mixed micelles have. When the catalytic effect of curing agent is weak, the production cannot meet the actual needs, thus ammonium chloride is mixed with formic acid as curing agent (pH=1). FTIR and XPS results show that, EPU was mixed with UF urethane to produce carbamate-amino and carbamate-substituted urea bond; XPS analysis can be used to obtain better characterization of the structure of curing urea-formaldehyde resin modified by isocyanate, and the chemical structure can be quantitatively derived from cured products made by isocyanate and urea-formaldehyde resin.