Preparation and Application of Lignin-based Adhesives with Ultra-low Formaldehyde Emission
In order to address the problems of water resistance,poor weather resistance and potential formaldehyde release of urea-formaldehyde resins,lignin-based adhesives(LPUs)with ultra-low formaldehyde emission meeting the requirements of ENF grade Ⅰ plywood were prepared by co-condensation with phenol(P),urea(U)and industrial alkali lignin(L).The effects of L,P and U mass ratio on the gel time,free formaldehyde content,storage stability,curing reaction activity and physicochemical properties of LPUs were investigated.The results showed that the maximum ratio of mL/(mP+mU)was up to 1∶1.With the decreasing mass ratio of P and U,the gel time of LPUs resin was gradually shortened by 40％,the free formaldehyde content increased from 0.05％to 0.18％,and the storage period was reduced from 127 d to 14 d,showing the physical and chemical properties of urea-formaldehyde resins.The enthalpy change behavior of the curing process monitored by DSC showed that the reaction activation energy of LPUs resin only reached 2％-7.8％of that of commercial phenolic and urea-formaldehyde resins.The boiling water resistance and formaldehyde emission of LPU1-LPU4 resin plywood prepared with mL/(mP+mU)=1 and with the amount of phenol not less than that of urea,met the requirements of ENF grade Ⅰ plywood.The glue spread of LPU2 resin could be reduced to 66.7％compared to that of the industrial production process.At this time,the boiling water resistance and formaldehyde emission of plywood could still be ensured to meet the requirements of ENF grade Ⅰ,which could effectively reduce the production cost of plywood.
ligninco-condensationplywoodboiling water resistant shear strengthformaldehyde emission
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