Studies On Uron Structure Reducing Free Formaldehyde Content And Emission Level Of UF Resin And Plywood

Urea formaldehyde resin (UF resin) accounts for 80% in all wood based panel adhesives, it is the main adhesive used in wood industry in China. Compared with other adhesives, UF resin has many advantages, such as available raw material, low cost, excellent agglutinative intention and so on. Because of these advantages, it has been used in the manufacturing of the plywood, flake board, fiber board, and the interior decoration, which can not be replaced easily by any other kind of adhesive. On the other hand, it has some disadvantages: poor water-proof quality, short storage-time, quickly ageing, especially for the high content of free formaldehyde which limit UF application. So it is important to study and empolder modifiers using in UF resin at the same time make sure other properties meeting the practical requirement.Based on the previous study work, this paper improved the synthesis technics of Uron resin and changed the mixing method of uron resins with normal UF resins. Through measuring the viscosity, curing time, and free formaldehyde content of UF resins, manufacturing plywood, testing formaldehyde emission values and bond strength, and examining with ¹³C-NMR analysis, this paper studied effects of the structure and amount of uron resin on reducing free formaldehyde content of mixed UF resins and formaldehyde emission level of plywood bonded by the corresponding UF resins, and also analyzed the working mechanisms. The conclusions are as follows:(1) There is a distinct effect of Uron resins on viscosity, curing time and free formaldehyde content of UF resins, the higher adding amount of uron resins, the lower viscosity values and free formaldehyde content and the longer curing time of mixed UF resin, the most reduction of free formaldehyde content is about 80% (from 0.35% to 0.05%). The effect varied as the Uron resin's type and the F/U molar ratio, generally speaking, the lower F/U molar ratio of Uron resin, the better reducing formaldehyde content effect.(2) Compared with the long time (24 hours) synthesized Uron resins, the shorter one (Uron-C) can improve the bond strength of UF resin, the most increase of this item is about 29.3%. All of the Uron resins can reduce the formaldehyde emission level of plywood, the most value is 84%.The experiment indicated that mixing Uron-D with UF-C can decrease the formaldehyde emission level and improve the bond strength as well, the mixing weight ratio of Uron to UF is 20:100. The time of manufacturing Uron resins can be shortened to 8 hours, it still has the effect of reducing free formaldehyde content and decreasing formaldehyde emission level of the plywood, considering about the requirement of practical manufacturing. The suitable weight ratio of Uron resin to UF resin is 20:100, then the mixed UF resin has a perfect performance.(3) Using Uron-C as a raw material to synthesize UF resin, the free formaldehyde content, curing time, viscosity of new UF resin, the bond strength, and the formaldehyde emission level of the plywood were studied, the author found that the synthesis craft of UF resin was changed as increasing of the adding amount of Uron-C. The higher dosage of Uron-C, the later coming of reaction ending time, which made the condensation reaction period longer, so the viscosity value of UF resin became higher and the curing time became longer.(4) Comparing the ¹³C-NMR spectrum of sample CC-2 with Uron-C, the relative peak areas proportion of the carbonyl peak of Di-substituted urea to Tri-substituted urea of sample CC—2 is higher than in Uron-C's. The author inferred that the mechanism of how uron resins effects the properties of UF resins was that the uron cycle structure opened and reacted with free formaldehyde and its oligomeric glycol forms.