Structure Formation And Variation Of Mass Fractions Of MUF And PMUF Co-condensed Resins

Melamine-urea-formaldehyde (MUF) and phenol-melamine-urea-formaldehyde (PMUF) thermo-setting resins are extensively used as exterior-grade adhesives in the wood industry. Although there are many problems to be confronted with in the field of resin formulations, with the development of science and technology and more and more application of modern analytical instruments, it is possible that scientific principles can be used to develop resins with improving performance on the basis of molecular structure.Today the research on MUF and PMUF resins around the world has just begun and few summary scientific literatures on their structure formation and mass fraction have been published. The main work in this paper included:1. Some work had been done on the structure formation of one PMUF resin made by a specific procedure. The samples taken during the preparation of the resin had been analyzed by ¹³C-NMR method. The effect of melamine added to UF resin on the structure formation of co-condensed resin had been discussed.2. Some work had been done on the variation of mass fractions in the different phases when the resins being prepared and stored in the way of GPC and TMA. The resins included sequential MUF resin (MUF1), non-sequential MUF resin (MUF2) and sequential resin PMUF.3. The thermal mechanic action of PMUF/pMDI exterior-grade adhesives with different relative weight proportion and the performance of particleboard made in laboratory had been researched.The variation of molecular mass distribution and the structure formation with the progress of the reaction and the change during the storage of the resins were studied in this paper. Two kinds of MUF resin were chosen, which are (i) sequential-type melamine-urea-formaldehyde MUF resin formulations where the sequence of addition of chemicals followed well-defined species reactivity principles. (ii) A non-sequential MUF formulation in which simultaneous melamine and urea competition for formaldehyde yielded a MF resin co-condensed with small amounts of urea, this resin drowned in reacted and unreacted monomeric urea species. Both the formulations used were industrial resins formulations in current use.For the two kinds of MUF resins, the study indicated that:1. After adding melamine to UF resins, the structure of the resin changed much. More fractions with higher molecular mass appeared. Thus the percentage of those increased.2. The results of the GPC analysis could give some proof on the co-condensation between urea and melamine species.3. The migration of hydroxymethyl groups when stored in the room temperature did effects on the structure and performance of the resin.4. To add melamine to UF resins would speed up the curing and increase the strength. Thus it was useful to increase the performance.5. In the non-sequential MUF resin, almost all of the urea was free because of the big difference of activity between melamine and urea. Even when some urea condensed with the MF species, the amount of the urea was small. So this type of PMUF resin could be seen as MUF resin drowned in urea. The change of the fractions during the preparation and storage of non-sequential MUF resin is not as...