Synthesis And Properties Of Divalent Metal Oxides Catalyzed High-ortho Phenol-Urea-Formaldehyde Resins

The water and weather resistance properties of Phenol-urea-formaldehyde (PUF) resin issimilar to that of phenol-formaldehyde (PF) resin, PUF resin could replace PF resin inmanufacturing outdoor using products. But there were still some drawbacks such as relativelyhigh curing temperature and low curing rate to restrict the widely application of PUF resin.Divalent metal oxides could be used as catalysts for the synthesis of high-ortho fast curing PFresin for its orientation on benzene ring ortho point. In this paper, divalent metal oxides wereused as catalysts for the synthesis of high-ortho fast curing PUF resin for its directing effect,the synthesis mechanism and curing properties of divalent metal oxides catalyzed high-orthofast curing PUF resin were also studied in this paper. The main results are summarized below:(1) Fast curing PUF resins could be prepared when divalent metal oxides were used ascatalysts. The optimized U:P molar ratio of PUF resin was0.5, then the influence of divalentmetal oxide type, content and adding stage, and parameters of reaction system to divalent metaloxides catalyzed PUF resin were discussed, the orthogonal experiment combined with wetshear strength, gel time and shelf life were used for the optimizing of PUF resin synthesistechnology. The optimized results were: ZnO (0.50wt%of phenol) added at the first synthesisstage, F:(U+P) molar ratio was2.0, NaOH: P molar ratio was0.6, the plywood wet shearstrength of this optimized PUF resin surpassed the minimum wet shear strength value oftypeⅠplywood required by GB/T17657-1999Standard, and the hot press time was significantshortened, the gel time was13%off and the shelf life was leveled off compared with thecontrol PUF resin.(2) High-ortho PUF resins could be prepared when divalent metal oxides were used ascatalysts.13C nuclear magnetic resonance (13C-NMR) was introduced for the qualitative andquantitative analysis about the structure changes of optimized PUF resin during its synthesisprocess, and the synthesis mechanism of different divalent metal oxides catalyzed and divalentmetal oxide added at different stage catalyzed PUF resin were also discussed. Addition reaction between phenol and formaldehyde was mainly occurred at the first synthesis stage combinedwith self-condensation reaction of formaldehyde units, the amount of methylol group (o-Ph—CH2OH) was much larger than that of methylol group (p-Ph—CH2OH) and total methylolgroups amount between phenols of the ZnO catalyzed optimized PUF resin was a little higherthan that of the control PUF resin at the first synthesis stage. Addition, self-condensation andco-condensation reaction were existed in the reaction system at the second synthesis stage,ZnO favored the addition reaction between formaldehyde and phenol than that betweenformaldehyde and urea, but ZnO could not effectively accelerate the self-condensation reactionof methylol groups between phenols. At the final synthesis stage, self-condensation andco-condensation reaction dominated the synthesis system, ZnO was conducive to improve theamount of methylol group (p-Ph—CH2OH), the methylene bridge (o,p-Ph—CH2—Ph) amountof ZnO catalyzed PUF resin was more than that of the control PUF resin, ZnO could make theurea units well incorporated into the co-condensed PUF resin. The influence of divalent metaloxide added at different stage to PUF resin lied in: ZnO added at the first synthesis stage couldincrease the amount of methylol group (p-Ph—CH2OH), ZnO added at the second synthesisstage catalyzed PUF resin had the minimum amount of unreacted urea, while ZnO added at thesecond synthesis stage was bad for the addition reaction between formaldehyde and phenol,ZnO added at the final synthesis stage was conducive to the co-condensation reaction, andself-condensation reaction of methylol groups between phenols. The influence of differentdivalent metal oxides to PUF resin lied in: ZnO catalyzed PUF resin had the maximum amountof methylol group (p-Ph—CH2OH) with the most obvious orientation effect of additionreaction between phenol and formaldehyde, MgO was the best catalyst for co-condensationreaction, and self-condensation reaction of methylol groups between phenols, CaO catalyzedPUF resin had the maximum amount of unreacted urea, the existence of CaO was bad for theco-condensation reaction of PUF resin. High-ortho PUF resins were prepared by addingdivalent metal oxides as catalysts.(3) Optimized PUF resin could achieve the goal of fast curing at lower temperature.Gelation test, DMA and DSC analysis were used to evaluate the curing properties of PUF resins, the results showed that, the gel time and apparent activation energy of optimized PUFresin were significant lower than those of control PUF resin, the rigidity and crosslinkingcuring degree of optimized PUF resin were significant higher than those of control PUF resin.For divalent metal oxide added at different stage catalyzed PUF resins, the gel time andapparent activation energy were decreased mostly for ZnO added at the first synthesis stagecatalyzed PUF resin, which achieved the goal of fast curing at lower temperature. The increaseof rigidity and crosslinking curing degree of ZnO added at the final synthesis stage catalyzedPUF resin were more than those of ZnO added at the first and second synthesis stage catalyzedPUF resins. For different divalent metal oxides catalyzed PUF resins, ZnO, MgO and BaO ascatalysts could significant decrease the gel time and apparent activation energy of PUF resins,these three divalent metal oxides facilitated the fast curing of PUF resins at lower temperature.CaO could not effectively decrease the gel time and apparent activation energy of PUF resin,CaO slowed the curing reaction of PUF resin. All the four divalent metal oxides in our researchcould effectively increase the crosslinking curing degree of PUF resins. There was littledifference between the reaction orders of divalent metal oxides catalyzed PUF resins and thecontrol PUF resin, and the reaction orders were fractions, it could be concluded that simpleproduct relationship was not existed between the curing reaction rate equation of PUF resinsand the initial concentration of reactants, a variety of chemical reactions were existedsimultaneously during the curing process of PUF resins, which belongs to complex reaction.(4) The orthogonal experiment was used to comprehensive analyze the influence of hotpress parameters and double-sided glue quantity to the plywood wet shear strength ofoptimized PUF resin, the optimized hot press technics was:140℃hot press temperature,4min hot press time,0.8MPa pressure,260g/m2double-sided glue quantity, the plywood wetshear strength of optimized PUF resin under this optimized hot press technics was far higherthan the minimum wet shear strength value of typeⅠplywood required by GB/T17657-1999Standard.