Preparation And Characterization Of Lignin-based Non-isocyanate Polyurethanes

Polyurethane is one of the most important polymers in the world,however,its raw material isocyanate is toxic and thermosetting polyurethane is unreprocessing.The preparation route of reprocessable non-isocyanate polyurethanes(NIPUs)has attracted much attention.Lignin is the most abundant aromatic compound in the word and has great potential to substitute petroleum for synthesizing polyurethane.Therefore,it is of great significance to study the synthetic technical route and related properties of lignin-based NIPUs.However,there exist some problems of lignin-based NIPUs,such as relatively harsh synthesis conditions,single technical route and weak mechanical properties.Lignin derivative vanillin and lignin was used as raw materials to synthesize reprocessable lignin and its derivative based NIPUs though chemical modification under relatively mild conditions in this paper.The main results in this study are listed below:Cyclic carbonates of vanillin(C-Van)was synthesized from lignin derivative vanillin by epoxidation and CO2 cycloaddition.Vanillin based NIPUs(Van-NIPUs)were prepared by different stoichiometric feed ratios of diethylenetriamine and tris(2-aminoethyl)amines.The effects of cross-linking density on the properties of Van-NIPUs were investigated.The results showed that the tensile strength of Van-NIPUs increased first and then decreased with the increase of crosslinking density,and the highest tensile strength was 46 MPa.The relaxation time and the time required for complete healing of surface scratches of Van-NIPUs decreased with the decrease of crosslinking density,and the lowest relaxation time and the time required for complete healing of surface scratches were 21 s and 15 min,respectively.When the crosslink density of Van-NIPUs was the highest,the tensile strength of the sample increased from original 36.93 MPa to 39.71 MPa,after one reprocessing treatment,and it could be fully degraded in a 0.1 mol/L hydrochloric acid solution.Levulinate enzymatic hydrolysis lignin(LEHL)was synthesized from enzymatic hydrolysis lignin(EHL)by chemical modification.Prepolymer was synthesized from bis(5-membered cyclic carbonate)and different diamines,and Schiff base reaction was carried out with LEHL to prepare lignin-based NIPUs based on 5-membered cyclic carbonate.The effects of flexibility of diamines on the properties of lignin-based NIPUs were investigated.The results indicated that the elongation at break of lignin-based NIPUs increased with the increase of the flexibility of the diamines,and the maximum elongation at break is 157.10%,while the tensile strength is 8.18 MPa.Under the effect of 4-(dimethylamino)pyridine(DMAP),its relaxation time was less than 30 min.Meanwhile,it maintained a tensile strength higher than 60%after two reprocessing treatment.The repair rate of its surface scratches was 94.44%after hot pressing at 130? for 10 min.LEHL was used as the raw material,the prepolymer synthesized by bis(6-membered cyclic carbonate)(BCC)and excess polyetheramine(D-400)was as used the reaction substrate,the Schiff base reaction was carried out to prepare lignin-based NIPUs based on 6-membered cyclic carbonate.The effects of different stoichiometric feed ratios of BCC and LEHL on the properties of lignin-based NIPUs were investigated.The results showed that the tensile strength of lignin-based NIPUs increased with the increase of LEHL content.When the LEHL content reached 45.53%,the highest tensile strength of 2.86 MPa was obtained.The relaxation time of lignin-based NIPUs decreased with the increase of LEHL content,and the lowest relaxation time was 9 s.When the LEHL content reached 31.13%and 45.53%,respectively,the samples maintained a tensile strength higher than 75%after two reprocessing treatments.In order to improve the mechanical properties of lignin-based NIPUs,epoxy lignin(EEHL)was synthesized from EHL by chemical modification,and the ring-opening reaction was carried out with the prepolymer synthesized by BCC and D-400 to prepare lignin-based hybrid non-isocyanate polyurethane(H-NIPUs).The results showed that the tensile strength of lignin-based H-NIPUs increased with the increase of EEHL content.When the EEHL content reached 40.63%,the highest tensile strength of 20.25 MPa was obtained.And it maintained a tensile strength higher than 10 MPa after two reprocessing treatments.