| In 2015,the Chinese wood-based panel production was close to 287 million cubi c meters,an increase of 4.8% over the previous year.The primary adhesive kind used in panel industry is still urea-formaldehyde(UF)resin,accounting for more than 70%of the total consumption of wood adhesives.Compared with other adhesives,UF resi n has many advantages,such as sufficient raw material supply and low production cos t.However,there existed problems of formaldehyde emission from UF-bonded wood c omposite panels and poor water resistance.In recent years,the state,local,industry,e nterprises have issued new environmental standards to further reducethe formaldehyde emission from wood composite panels.Reducing the formaldehyde emissionfrom panel s and improving the wet strengthwhile maintaining the cost advantage are main directi ons and aim of UF resin research.Based on the preliminary work,in this study,a low molar ratio of F/U=1.05 was used in resin synthesis.The synthesis method,structures,and performance of high-eth er content urea-formaldehyde(UF)and urea-modified UF(UMF)resins were investigat ed.The specific studies included the effects of synehtsis method(regular and high-ethe r)and melamine addition point on basic features of resins as well as the structure an d performance of the new high-ether UMF resins.Resin synthesis methods included re gular and high-ether.The regular procedure including acid-base-acid 3-steps.While the high-ether procedure used a high intital F/U molar ratio(2.7),and the pH value of c ondensation step was 3.0.The methylene-ether content could be increased under these conditions.The melamine addition amount was 5%,based on the amount of urea.The melamine addition points were: the beginning of addition reaction,the end of the fir est condensation reaction,and the end of the second condensation step.Seven kinds of regular/high-ether UF and UMF resins were therefore synthesized.13 C NMR and TG analysis were used to determine resin structures and to study the heat stability of res ins,respectively.Three-layer plywood was made and the mechanical property as well as formaldehyde emission of the plywood were measured.The study results are as fol lows:1.Compared with regular resins,high-ether resins generally have longer curing tim e,pot life,and storage time;lower formaldehyde content,and better water resistance.The wet shear strength of plywood bonded by high-ether UF resin of 1.05 molar ratio could reach 0.75 MPa,reaching national standerd of type II plywood.2.When melamine was added at beginning of resin’s synthesis,condensation produ ct’s linear methylene ether bond content is increased in resin,there is no free or low instead melamine.Due to a combination of melamine and formaldehyde,urea and for maldehyde hydroxyl methylation level decrease,the number of methylene reduce,so th e remaining hydroxymethyl content in resin increase.Synthesis’ s UMF resin’s bond str ength and water resistance is higher;while UMF resins synthesized by adding melami ne at the end of condensation step has lower formaldehyde emission from plywood.3.While adding melamine at the end of condensation step,there is no obvious ch ange of the gross of methylene ether bond and hydroxymethyl.But type I of hydroxy methyl contentin increase,type II of hydroxymethyl reduce.43%-43% of the melam ine in the form of free or low hydroxyl methyl exists in liquid resin,not involved in the main structure of the resin.4.High ether content UMF resin’s bond strength and water resistance gradually de creased with melamine added backwards,but have little influence on the formaldehyde emission from plywood.The highest wet shear strength is 1.06 MPa when melamine was added at the beginning of addition step and resin was synthesized using high-eth er method.Above all,in this article the synthesis’ s high ether content UF and UMF resin ha ve different structures and properties comparing with regular resin,in the man-made board production practice,we could make full use of the characteristics of this kind of resin,exert its advantages by changing the types of curing agent,optimize the hot-pr essing process,which having important practical significance and value to the producti on practice. |
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