Lignin-Based Polyurethane Elastic Materials:Preparation,Structures,and Performances

Lignin,the second most abundant renewable resources in nature next to cellulose,is regarded as one of the most abundant green resources available in the 21st century.Polyurethane is a widely used synthetic polymer material.The huge production of polyurethane requires a large amount of polyols.Currently,the acquisition of polyols relies heavily on petrochemical resources.Pursuing new raw materials to replace petrochemical polyols is the key to the sustainable development of the polyurethane industry.Lignin has a large number of reactive functional groups such as hydroxyl groups,which makes it possible to be a natural ultraviolet light stabilizer.If the cheap and readily available lignin could be used to partially substitute petrochemical polyols to synthesize polyurethane material,its cost would be greatly reduced,and its overall performance might be improved.However,compared with common petrochemical polyols,lignin has complex molecular structure,large steric hindrance,and low reactivity of functional groups.Besides,lignin exhibits poor compatibility with polyurethane matrix is poor and thus could not be well dispersed in it.In this paper,lignin was pretreated by two methods in view of its molecular structure,including activation modification and catalytic depolymerization.On the one hand,activation modification was to modify lignin with a long chain of polyoxyethylene glycol.Converting its phenolic hydroxyl groups into aliphatic hydroxyl groups improved the reactivity with the isocyanate groups and compatibility with the polyurethane.On the other hand,the enzymatic hydrolysis lignin was partially depolymerized under alkaline conditions to reduce its molecular weight and increase its hydroxyl groups content.Then,the pretreated lignin was used to partially replace the petrochemical polyols to synthesize lignin-based polyurethane elastic foam and polyurethane elastomer,respectively.The effects of lignin on the comprehensive mechanical properties,thermal stability and reprocessability of polyurethane materials were investigated.The main research contents and conclusions are as follows:(1)The alkali lignin was graft-modified with PEG2000 to convert its phenolic hydroxyl groups to aliphatic hydroxyl groups,which increased its reactivity with isocyanates.Due to the introduction of the long chain of polyoxyethylene ether,the total hydroxyl content decreased while the aliphatic hydroxyl content increased.A highly resilient lignin-based polyurethane foam was then synthesized by partially replacing the petrochemical polyol with modified lignin.The long chain of grafted polyoxyethylene ether provided sufficient flexibility for the polyurethane foam and improved the compatibility between lignin and the polyurethane matrix.All lignin-based polyurethane foams exhibited good elasticity with elasticity recovery exceeded93%even though 50%of the polyol was replaced.The hysteresis tests showed that the introduction of lignin contributed to the energy dissipation during compression.(2)Enzymatic hydrolysis lignin was pretreated via base-catalyzed depolymerization.The results showed that the relative molecular mass of partially depolymerized enzymatic hydrolysis lignin significantly decreased,and the weight average molecular weight decreased from 4500 g/mol to 1800 g/mol,and the hydroxyl content also increased to some extent.The total hydroxyl content increased from 5.21 mmol/g to 5.62 mmol/g.Different proportions of partially depolymerized enzymatic hydrolysis lignin were used to partially substitute petrochemical polyols to synthesize polyurethane elastomer,which enhanced the strength and toughness of the elastomers.Specifically,the tensile strength and fracture absorption energy for5%DEL and 10%DEL respectively reached 60.7 MPa,57.8 MPa,and 263.6 MJ/m~3,244.5MJ/m~3.What's more,the elastic recovery also exceeded 95%,which were far superior to the polyurethane elastomer without lignin.The prepared lignin-based elastomer could still maintain high mechanical properties after reprocessing.In addition,lignin-based polyurethane elastomers also exhibited well shape memory properties.