The Synthesis And Curing Properties Research Of Tannin-NIPU Wood Adhesive

The research and development of biobased formaldehyde-free wood adhesive always the one of most attractive topics for the long-term development currently in wood industry.In this study,tannin non-isocyanate polyurethane(NIPU)wood adhesive was prepared with the main raw materials,biomass condensed tannin(mimosa tannin),and analyzed it’s bonding and curing mechanism.The wood-based panel samples,including particleboard and/or plywood,were utilized to further evaluate their performance which bonded with tannin-based wood adhesives.Additionally,some apparently drawbacks were pointed out during tannin based NIPU adhesive application,and the targeted improvement measures were proposed as well.The main results were obtained here by following:The abounded condensed tannin hydroxyl groups(R-OH)are the main reaction locations for urethane structure formation.In the preparation,tannin molecular was carbonated firstly on tannin hydroxyl groups,then,hexamethylenediamine(HMDA)was further linked with them to form the “urethane structure bridge”,then further connecting other carbonated tannin-derived products,resulting to adhesive layer with threedimensional network.The three-layer plywood bonded with tannin-based NIPU wood adhesive under two preparation conditions,i.e.,hot-press temperature 220℃ for 6 min and 200℃ for 10 min,all obtained the optimal bonding performance,which can satisfy the requitement of China National Standard GB/T 9846-2015 for type Ⅱ plywood sample.The results for wood particleboard,which obtained under 220℃ for 10 min with the panel density 0.7 g/cm3,shown that the inter bond of samples can reach 0.6 MPa,the MOR is12.8 MPa,and the MOE is 2385 MPa,respectively.From that we can see the mechanical properties(except the 24 h thickness swelling rate of water absorption)can satisfy the requitement of China National Standard GB/T 4897-2015 for type P2 particleboard sample.One of main shortage to tannin based NIPU wood adhesive is high curing temperature,normally over 200℃.Herein,partially bio-based diglycidyl glycerol ether(GDE)served as enhancer and modifier to decrease the curing temperature of tanninbased NIPU.The basic reaction mechanism of the modification process derived from the highly reactivity between epoxide and amino groups.The epoxide groups of GDE can link some tannin-derived products which containing free amino groups,as a bridge to further enhance the adhesive network.Moreover,the additional crosslinking reaction also do occur between tannin-OH group and GDE,which can further enhance the bonding performance of tannin-based NIPU wood adhesive.The results shown that the best bonding performance obtained under a proportion around 27% GDE addition,based on the solid content of tannin NIPU adhesive,the shear strength under the dry condition and the wet condition after 63℃ hot-water soaked can reach 1.41 MPa and 0.85 MPa,respectively.Those value can over the minimum requirement of China National Standard GB/T 9846-2015 for type Ⅱ plywood sample.The mixture of polyethylenimine(PEI,boiling point around 250℃)and tannin was utilized to modify tannin NIPU wood adhesive,for decreasing the usage and evaporation of HMDA.The amino groups of PEI can substitute HMDA for forming urethane structure in adhesive system.The stability of cured adhesive network was improved by high branched PEI structure.In addition,the reactions of Stiff and Michael addition between tannin and PEI will further enhance the adhesive three-dimensional network,leading to a more stable adhesive property than raw tannin NIPU adhesive.After that,the hexamethylenediamine ratio in modified tannin adhesives have reduced by 12%,and the biomass content ration,meanwhile increased by around 2-5%,comparing to raw tannin based NIPU adhesive.The results shown that the performance(except thickness expansion rate of water absorbing)of laboratory particleboards can meet other physical and mechanical properties requirements of type P2 particleboard in China National Standard GB/T 4897-2015,while the tannin /PEI mass ratio is 4:1.Therefore,the modification solution by appropriate proportion between PEI and tannin can reach the target,and the properties of particleboard samples can reach the same performance with unmodified adhesive sample.Additionally,a certain amount of GDE(6.67 g,based on the weight of tannin adhesive)was used to further enhance the performance of particleboard.The samples were prepared under 190℃ for 7 min,which can satisfy other physical and mechanical properties requirements of type P2 particleboard in China National Standard GB/T 4897-2015.But the water resistance of the modified adhesive still needs to improve.Non-furanic humins,a biomass material,was used as starting material to prepare NIPU wood adhesive by using the preparation method based on tannin NIPU wood adhesive.The hydroxyl groups from humins structure were carbonated firstly,similar to tannin materials,then obtained the urethane structure by using HMDA into the reaction mixture.The limited numbers and reactivity of hydroxyl groups from humins structure resulting to restricted urethane structures which cannot supporting to a good bonding performance.The property of particleboard bonded by humins NIPU wood adhesive,such as internal bonding strength(IB),only can reach 0.44 MPa,just meet the standard requirement(GB/T 4897-2015,≥0.4 MPa,type P2).And herein,tannin resource served as modifier,mixed with humins solution firstly,then to make tannin-furanic-NIPU wood adhesive.the particleboard was selected to evaluate the properties of new wood adhesives.The results shown that tannin can significantly improve the bonding performance of furanic NIPU adhesive,normally,the IB value can reach 0.65-0.7 MPa.