Study On The Low Toxic Urea-formaldehyde Resin Adhesive Under The Low Curing Temperature

With the high quality natural wood resources sharply reduced in the worldwide and natural forest reserves protection project carried in china,the structures of wood resources have changed fundamentally.Gluing wood and wood composites,which are made of small-diameter lumbers and thinned timber become instead of the natural high quality wood,and will be the main trend of the wood-based panels that based on the Glue and the gulling technology.Urea-formaldehyde resin adhesive is the most widely used adhesives,but it has many shortcomings,such as lower water resistance,crisp adhesive layer,especially the highly free formaldehyde content and formaldehyde emission contaminate environment serious.Therefore,it's necessary to study low toxic UF which has low free formaldehyde content and formaldehyde emission,but its development is blocked by the longer curing time and the higher curing temperature.UF resin with different F/U molar ratio was synthesized and its properties were tested to choose an excellent UF resin with low molar ratio in this paper.Then different modifiers(such as PVAc,PAPI,resorcinol)and curing agent(such as ammonium chloride,p-toluenesulfonyl Chloride,ethylenediamine hydrochloride,oxalic acid,hydrochloric acid,phosphoric acid,etc.)are chosen to modify UF,and their influence of curing temperature and bonding strength for UF on the single and compound condition were discussed.In addition,the FTIR and DSC were applied to discuss about the internal active groups changed and curing reaction mechanism of UF by analysising on the samples of each stage of modified UF.The performance test of UF showed that,the viscosity,solid content of UF resin decreased with the F/U molar ratio increased.At the same time,the curing time significantly shortens and bonding strength showed a rising trend.In addition,free formaldehyde content of UF resin reduced with F/U molar ratio decreased,and there were no more changes when the molar ratio was less than 1.3.The 1.3 mole ratio of UF was used as research object in this paper.Single-factor experiments are used to research one-component curing agent and a single modifier which effect on the pH and water resistance bonding strength of UF resin.The results showed that,using ethylenediamine hydrochloride or toluene sulfonyl chloride as one-component curing agent,the pH dropped in a sharper range than using ammonium chloride,and the bonding strength appeared to be peak when ammonium chloride had 0.6%amount in UF,0.2%for ethylenediamine hydrochloride and 0.4%for toluene sulfonyl chloride.On the other condition,it was few effects on the pH of glue that just add PVAc or PAPI.But PAPI was better than PVAc to be as modifier for UF,both had no particular effect on pH of glue,but same impact for the bonding strength which arrived the top(1.43MPa,1.02MPa)when count up to 12%.The modified results of using compound modifiers indicated that,PVAc and PAPI worked together to be as modifier for UF,which was better than used them separately,the bonding strength was the best,1.61MPa,when there had 9%of PVAc combined with 6%of PAPI in UF.By the way,resorcinol just had a little function in it.With 5%flour,9%PVAc,3%PAPI,the results for modified UF using with 0.4% ammonium chloride as the main additive of two component curing agent showed that, the pH of glue sharply decline with usage enhancing of hydrochloric acid,phosphoric acid and oxalic acid as the curing additives.But the pH of glue changed little that the usage of ammonium sulfate,zinc sulfate,diethyl oxalate and trimethyl phosphate were increased as the curing additives.At the same time,the bonding strength of modified UF could meet the waterproof requirement ofâ…¡kind glue in the national standards when the hot pressing temperature was 60℃.The higher bonding strength was 1.49 MPa or 1.44 MPa while the curing additive was ammonium chloride and ammonium persulfate or ammonium chloride and zinc sulphate.The results for multi-component curing agent showed that,The pH of glue upward with the auxiliary Urotropine and triethanolamine.The bonding strength of UF resin was influenced by Different additives,for instant,it could be up to 1.55MPa if phosphoric acid and ammonium persulfate mixed together as auxiliary,but the triethanolamine as auxiliary could make it be weakened.Effects of curing agent(such as toluene sulfonyl chloride,ethylene diamine hydrochloride),PVAc,PAPI and resorcinol were analyzed by orthogonal experiments that according to L₉(3⁴)orthogonal design.The results showed that the curing agent toluene sulfonyl chloride and ethylene diamine hydrochloride affected significantly.Pressing board orthogonal experiment L₉(3⁴)and L₁₆(4⁵)took curing agent, hot-press temperature and specific pressure,hot-press time as investigation factors according to the different formulas of UF resin adhesive.The results showed that pressure,temperature had a greater effect on bonding strength.The best technology of pressing board was obtained with hot-press temperature 55℃,specific pressure 1.5MPa,hot-pressing time 19 min,two kinds of curing agent which were mixture of ammonium chloride,oxalate and Urotropine or mixture of ammonium chloride and phosphoric acid.The bonding strength of modified UF was 1.06 MPa with two component curing agent which was composed of ammonium chloride and phosphoric acid that reaches the request of typeâ…¡plywood in China National Standard when the hot pressing temperature was 40℃.The infrared spectrum showed that the intensity of characteristic IR absorption peak changed while PVAc was added to UF,so it could be indicated that some parts of hydrogen bonds broken in UF resin,and then its hydrophobicity enhanced.The characteristic absorption peak of isocyanate- group with asymmetric stretching vibration was increased when PAPI was added to UF and it reduced while acidic material was added,it could show that there were addition polymerization occurred between isocyanate-group and the groups with active hydrogen in UF,moreover crosslinking reaction between hydroxymethyl occurred.The DSC curve showed that,there was no significantly exothermic peak appeared on the curve of pure UF and UF with ammonium persulfate.It both appeared while UF with PVAc at 35℃and UF with PAPI at 25℃.The curing temperature of UF decreased under the condition of curing agent according to the exothermic peak dropped from 115℃to 90℃on the DSC curve when ammonium chloride was added to UF.On the test of compound curing agent,oxalic acid,hydrochloric acid, phosphoric acid as the additive of UF could reduced the curing temperature which rised when urotropine was added.Modifiers and curing agent were used to modify UF resin with low mole ratio in this paper.The low curing temperature of UF with low mole ratio could provide theoretical basis for enlarging the application of UF resin on the different wood-based panel.