Preparation And Properties Of Functional Lignin/polyurea Composites

Lignin is the second largest biomass resource in plant resources after cellulose and has the advantages of abundant yield,diverse functional groups,high carbon content and good biocompatibility,which makes the use of lignin to develop new biomass composites a current research hotspot.Polyurea is a new type of polymer material developed to meet the needs of the environment and is widely used in many civil and military applications due to its low cost,impact,corrosion and abrasion resistance.However,as polyurea materials continue to develop,their own shortcomings limit their further application,such as the need for improved overall performance,functional products to be developed and flammability.In response to these problems,lignin/polyurea composites（L-PUA）were prepared in this paper using lignin as the reinforcing phase.The addition of lignin improves the overall properties of the matrix.Afterwards,the composite is gradually endowed with self-healing properties and flame retardant properties through the introduction of dynamic covalent bonds and flame retardants.This not only extends their functionalisation properties but also facilitates the high value use of lignin.The main research contents and results are as follows:（1）Polyurea（PUA）was prepared by one-step method,and then L-PUA was prepared by compounding refined alkali lignin with it,and the influence of lignin introduction on various properties of composite was tested.The results of the study show that the lignin can be well dispersed in the polyurea matrix without agglomeration within a certain range of addition amount,which contributes to the performance of the matrix.Thermogravimetric tests show that the addition of lignin improves the thermal stability of the composite.The maximum degradation rate of material was reduced from 14.07%/min to 8.11%/min,while the amount of carbon residue was also significantly improved.This is mainly due to the large number of aromatic structures in the lignin,which are very stable and resistant to decomposition at high temperatures,thus enhancing the high temperature thermal stability of the composite.The transmittance of L-PUA in the UV region was almost zero,which significantly improved the UV shielding performance of the composite.The mechanical properties of L-PUA increased and then decreased as the amount of lignin added increased,with the best mechanical properties were L₂-PUA.The tensile strength and modulus of elasticity of L₂-PUA increased by 225%and 666%respectively,indicating that the introduction of lignin was able to take up the stresses transferred from the matrix,thus improving the mechanical properties of the material.（2）After the amino-terminated product was synthesized by polyurea reaction,the lignin/self-healing polyurea composites（T-L-PUA）was prepared by introducing terephthalaldehyde（TA）and reacting with amino-terminated polyurea to undergo a Schiff base reaction to generate dynamic imine bonds.The research results show that the thermal stability and the UV shielding performance of T-L-PUA is significantly enhanced with the increase of lignin ratio.The best mechanical property appears at 20 wt%of lignin addition,and the corresponding tensile strength of T-L-PUA is 12.44 MPa,which is 937%higher than that of pure polyurea elastomer.T-L-PUA exhibits good self-healing properties.When the repair time reached 5 h,the cracks almost completely disappeared.The test results of tensile properties show that the self-healing efficiency of T-L-PUA is positively correlated with the repairing time,and the recovery efficiencies of the tensile strength and elongation at break of T-L₂-PUA after48 h of repairing at room temperature are above 90%.This is mainly due to the reversible dynamic imine bonds contained within T-L-PUA and the intramolecular hydrogen bonds,which can be reversibly broken and rebuilt at room temperature,thus achieving the result of repairing the material’s own properties.In addition,the T-L-PUA can also be recovered by the hot-pressing process and solvent dissolution process,and the mechanical property remain basically unchanged after remolding.（3）The flake flame-retardant（PM）was generated by the reaction of phytic acid（PA）and melamine（MEL）.Lignin/polyurea composites with flame retardant and self-healing properties（P-T-L₂PUA）were prepared by introducing a flame retardant PM into T-L₂-PUA,which had the best mechanical properties in the previous chapter,to improve the flammable weakness of composite.The thermogravimetric results show that the residual carbon content of PM is 37%at 700°C,which means that PM has better thermal stability and is conducive to the subsequent improvement of the flame retardancy of the composite.The introduction of PM into the self-healing lignin/polyurea composite resulted in the carbon residue of P-T-L₂PUA is increased by up to 54.04%compared with T-L₂-PUA.As the amount of PM added increased,The LOI values of the composites were improved from 20.5to 24.8 thanks to the addition of PM.The heat release rate and heat release amount had a significant reduction during the combustion process,with a maximum reduction of 19.3%in the total heat release.The charcoal layer obtained after combustion was more complete and denser,and the original structure of the material was better preserved,which improved the flame-retardant property of the composite.This is due to the fact that during combustion,PM not only decomposes to produce polyphosphate to promote the formation of char in the matrix,but also releases non-combustible gases to dilute volatile combustible substances,thus acting as a flame retardant.In summary,in this paper,lignin/polyurea composites were prepared by introducing alkali lignin into polyurea materials.Due to the existence of lignin,the thermal stability,mechanical properties and UV shielding properties of the polyurea were improved.Afterwards,lignin/self-healing polyurea composites were prepared by introducing dynamic imine bonds into the polyurea matrix,which further endowed L-PUA with good self-healing functions;on this basis,a synthetic flame retardant was introduced into the T-L-PUA system,which finally improved the flame retardancy of polyurea composites.