Study On The Mechanism And Performance Of Urea Formaldehyde Resins Modified With Lignin Derivatives

Urea formaldehyde?UF?resins are thermoset resins with the highest yield all over the world,which has a tremendous commercial value.However,there are several problems,including short storage period,poor initial viscosity,low water resistance,high formaldehyde emission and competition with food resources as fillers in the plywood production,which show negative effects on its usage in the practical application.On the other hand,lignin is a water-resistant and intertwined biopolymer that constitutes with hydrophobic phenylpropane units which are linked together by radical coupling.It has been widely utilized to modify UF resins in wood-processing industry.Unfortunately,the issues,such as low reactivity of lignin in UF resins and its complex reaction behaviors still remain unexplored.Therefore,in this study,model compounds were employed to interpret the reaction behaviors of lignin and factors during the synthesis of UF resins.After the addition of amphiphilic lignosulfonate,the storage stability of UF resins was significantly enhanced.In addition,the mechanism of lignosulfonate in UF resins was primarily explorted by analyzing the colloidal morphology,crystallization and structural changes of UF resins.In addition,lignin-based polyacid catalysts were synthesized and used to replace traditional inorganic Lewis acids as curing regents,which improved the water resistance of UF resins.The effects of lignin on curing behavior,crystalline behavior and thermal stability were systematically studied.Lignin-based chain transfer reagent was prepared by the reaction between polyol groups in alkali lignin and carboxylic acid groups in RAFT agent.Then,it was used to initiate the controlled polymerization of lauryl methacrylate?LMA?and methacrylate vanillin?VMA?to synthesize a series of lignin-based polyaldehydes,which improved the pre-press performance of UF resins.Chemical modification of cellulose with small aspect ratio and acid-insoluble lignin in corncob hydrolysis residue was utilized to replace food-based fillers in UF resins.The contents and conclusions are shown as follows:?1?Firstly,using 4-ethylphenol?LMH?,3-methoxy-4-hydroxyphenylpropane?LMG?,ether derivatives of LMH and LMG as model compounds,the reaction behaviors and factors during the hydroxyl methylation,condensation and the curing reaction of urea formaldehyde resins were studied by high performance liquid chromatography?HPLC?,liquid chromatography-mass spectrometry?LC-MS?,Fourier transform infrared spectroscopy?FTIR?,and nuclear magnetic resonance?NMR?.The results showed that for the hydroxyl methylation,the equilibrium conversion of LMG was 79.6%after240 min,compared to 99%yield in 45 min of 2HLMH.This resultindicated that the steric effect could influence the reactivity of lignin.Secondly,hydroxyl phenyl?H?units and the guaiacyl?G?units of lignin formed Ph-CH₂OCH₂-Ph and 5-5,5-2 were self-polymerized respectively when catalyzed by base and acid,and these results explained the reaction behaviors of lignin in UF resins.Additionally,methylated derivatives failed to react during the formation of UF resins,revealing that the phenolic hydroxyl groups have significant effects on the reactivity of lignin.The resultsin this section not only explained the poor reactivity of industrial lignin,but also offered theoretical backgrounds for using derivatives of lignin to modify UF resins in other sections.?2?In order to demonstrate the hypothesis that amphiphilic lignosulfonate could coat on the surface of UF resins colloidal particles to enhance the storage stability of UF resins.The potential interaction between lignosulfonate and UF resins was analyzed via Cryo-SEM,FT-IR,¹³C CPMAS NMR,zeta potential and POM.No chemical reaction between UF resins and lignosulfonate was observed.Adding lignosulfonate at different stages was also a key parameter to influence the storage stability,and the storage period of resins was significantly improved from 30 d to 200 d when adding lignosulfonate in the initial phase of hydroxyl methylation.Moreover,spherulite size of resins curing in room temperature decreased from 2-8?m to 0.5-3?m.Furthermore,lignosulfonate increased the surface charge of colloidal particles from-6.9 m V to-16.2 m V.After the elucidation of the mechanism,the effect of lignosulfonate on the curing process,crystallization and adhesive performance of UF resins was systematically evaluated by temperature variable rheology,XRD,TGA and mechanical analysis,respectively.Lignosulfonate slightly reduced the reactivity but increased curing temperature of UF resins,leaing the decreaed crystallinity from 60.04%to 52.80%.Furthermore,the shear strength of LUF1 resins was 0.9 MPa and formaldehyde release was 0.12 mg/L,which satisfied the demand of E₀grade class II plywood.Due to the excellent performance of lignosulfonate,it might be industrially used as a stabilizer in the UF resins production.?3?Lignin-based polyacid catalysts?MA-HL?were synthesized via hydroxymethylation and esterification to cure UF resins,which resolved the problem that inorganic curing agent usually absorbed water to accelarate reversible hydrolysis of resins.And there was a upward trend about the shear strength of resins,which rose from 1.0 MPa to 1.2 MPa.The effects on curing behavior and crystalline behavior were studied.Experimental analysis of melt rheology and curing time showed that when the addition of lignin-based polyacid was 5%,it was comparable to the effect of commercial additive.In addition,due to the rigid benzene molecules in MA-HL,the elasticity modulus of resins was6.07×10⁶ Pa,indicating an increase as compared to 3.68×10⁶ Pa catalysed by NH₄Cl.The results of XRD showed that lignin-based polyacid could improve the ordering of UF resins and the crystallinity to38.64%.?4?An esterification between 4-Cyano-4-?phenylcarbonothioylthio?pentanoic acid and alkali lignin was used to prepare lignin-based RAFT chain transfer reagent?L-CTA?.Then,the polymerization of lauryl methacrylate?LMA?and methacrylate vanillin?VMA?was initiated by L-CTA to synthesize a series of crosslinkable lignin-based polyaldehydes at room temperature.The glass transition temperature?T_g?could be adjusted from-60 ^oC to 48 ^oC.Lignin-based polyaldehydes improved the pre-press performance of UF resins and the production efficiency of plywood.The shear strength of lignin-based polyaldehydes could reach up to 3.5 MPa by partially cross-linking with polyprimary amine to form Schiff base structure.In addition,due to the dynamic features of the Schiff base structure,it showedself-healing properties.When 40%of aldehydes were cross-linked,the shear strength after first time self-healing could reach 2.9 MPa,which was 83.1%of the pristine adhesion.These results demonstrated that lignin-based polyaldehydes have the potential to be reused.?5?The direct carboxymethylation of corncob residues?CR?was used to convert the cellulose I structure?crystallinity,40.19%?to amorphous structures,which increased the compatibility of CR with UF resins.Through FTIR,¹³C CPMAS NMR,SEM,XRD and TGA analysis,the structural variations,micro-morphology and crystal behavior of corncob hydrolysis residues before and after modification were investigated.Although lignin in CR could impede the access to hydroxyl when modified by carboxymethylation,the degree of substitution of carboxymethyl could reach 0.42.The maximum thermal degradation temperature of carboxymethyl-CR was increased to 344.4 ^oC.Rheology analysis showed that carboxymethylation corncob residues?CMCR?could impart better viscosity adjustment and rheological control.SEM observation showed that CMCR could accelerate UF resins to form spherulites with the size of 1-3?m in the curing process.In addition,the analysis of plywood performance showed that the shear strength of plywood with CMCR as fillers could reach 1.04 MPa,which was equivalent to flour?0.99 MPa?.More importantly,the amount of formaldehyde emission was as low as 0.23 mg/L.These results indicated that carboxymethylated corncob residues could completely replace food resources fillers for the UF resin,which presents the promising practical values.