Green Preparation And Properties Comprehensive Of Lignin/Polyurethane Adhesives

At present,the commercial polyurethane adhesives are mainly solvent-based products,and the organic solvents used in these traditional polyurethane adhesives are toxic and volatile,which will cause pollution in the process of production and use.Therefore,high performance,pollution-free and environment-friendly waterborne polyurethane gradually becomes a new trend of research and development.However,waterborne polyurethane has some disadvantages,such as low mechanical strength,poor water resistance,and easy to be damaged in the process of use.Lignin is one kind of phenolic polymer and rich in hydroxyl and phenolic groups.These groups can partially replace polyols and react with isocyanates,improving the performance of waterborne polyurethane.In order to solve this kind of problem and implement the concept of green preparation,in this paper,dimethylglyoxime,one chemical that can be obtained in industrial batch,is used as the construction unit to prepare waterborne polyurethane via the principle of oxime carbamate thermal reversible bonding.In order to provide the theoretical guide and technical support for the utilization of waterborne polyurethane adhesives.The main research contents and results of this paper are as follows:First,the functional monomer with two methyl ethyl two oxime groups(DMG)was introduced for the design of waterborne polyurethane.Chain extender,catalyst and neutralizer were also used to synthesize the waterborne polyurethane emulsion with dynamic network structure.Based on the fixed content of DMG,the effect of hard segment content on the properties of waterborne polyurethane was investigated.The results showed that the particle size of the waterborne polyurethane emulsion was nano dispersed(particle size<100 nm)with a good dispersion stability.The hard segment content had great influence on the mechanical properties of the waterborne polyurethane adhesives.The reasonable hard segment content and structural design could give the adhesive the extremely high bonding strength(25.1 MPa)compared to the general commercial hot-melt adhesive(<6.0 MPa),and 12.3 MPa hydrolysis strength,The results show that the repairing efficiency is up to 96.5%(160oC,10 min),demonstrating super strength,stability and the possibility in scale-up.Secondly,lignin/waterborne polyurethane composite emulsion(LWPU1)was prepared via introducing lignin into the WPU1 system.The effect of the lignin content on the properties of the LWPU1 was investigated.Results showed that when the lignin was added,the particle size of emulsion was increased but still nano-dispersed.This prepared emulsion showed good stability and the resultant solid film demonstrated a good capacity in ultraviolet absorption.A tensile strength of 34.9 MPa,toughness 655.2 MJ/M~3 and elongation at break 389.7%can be obtained for the LWPU1 adhesive when the lignin content is 10 wt%.More important,this adhesive still maintained a tensile strength of28.7 MPa after water soaking for 24 h,demonstrating a good water resistance.The effect of catalyst content on the properties of the LWPU1 system was also investigated.The results showed that proper amount of catalyst can accelerate the reaction and consume the isocyanate groups of the composite membrane,improving the water resistance and mechanical strength.When the catalyst content is 4 wt%,a bonding strength of 44.8 MPa,compared with the non-catalyst(34.9 MPa),the tensile strength is increased by 28.4%,and repairing efficiency of 80.2%can be achieved,showing good self-repair ability.More important,this adhesive still maintained a tensile strength of 37.2MPa after water soaking for 24 h,Upon being heat treated,the mechanical properties and the repairing efficiency of the composite membrane are increased by 4.2%and 2.0%(150oC,4%),respectively,it means that heating can promote the further synthesis of dynamic oxime-carboxate structure and enhance the interaction between chain segments.