The Amination Of Lignin And The Applications As Hardeners Of Epoxy Resins

Second to cellulose, lignin is one of the most abundant renewable resources produced by plants. The application of this environmentally friendly biodegradable lignin resource of being as substitutes of dwindling petroleum oil based materials is an important aspect of sustainable development. Numerous reactive hydroxyl groups including both phenolic and aliphatic hydroxyl groups and carboxyl groups exist in lignin molecular. These structural features make lignin a potential source of preparing network materials. Synthesis of various functional derivatives from lignin has attracted increased attentions from researchers in recent years. In this paper, aminated derivatives of lignin containing a great quantity of primary amine and secondary amine groups were synthesized by a new two-step method after degradation and purification of lignin and feasibility study of amination reaction with a lignin model compound.With the existence of amino groups in the modified lignin, it was served as a curing agent of epoxy resins instead of a filler of the polymer materials. The main contents and conclusions of this paper are listed as follows:1Degradation, purification and characterization of alkaline lignin:Solid base catalysts MgO/CaO were used to catalyze degradation of lignin. Molecular Weight (MW) of the degradation products were reduced to below1000. An acid-precipitation method was used to remove the residual cellulose, inorganic salt, sugar and other impurities in original lignin material. After degradation and purification of the lignin, the purity of the lignin improved from83.98%to97.04%, total hydroxyl, alcoholic hydroxyl groups and phenolic hydroxyl increased from6.88%,3.061%and3.819%to9.62%,3.524%,6.096%respectively. Thermogravimetric analysis (TGA) result showed good thermal stability of the purified lignin.2Amination modification of a lignin model compound:guaiacol (GGA) was served as the model of alkali lignin. Aminated derivative of GGA was prepared from guaiacol, epichlorohydrin and diamine by a two-step epoxidation-amination method. The chemical structure of guaiacol and its derivatives were characterized by FTIR, GC-MS, and NMR. The results indicated that the target product was synthesized successfully. The investigation of amination reaction kinetics of the epoxidation compound with amine compound confirmed that introducing amine group into lignin molecules by this two-step method is feasible.3Synthesis of aminated lignin derivative:the aminated lignin possessing primary amine and secondary amine groups was synthesized through a two-step process, and the effects of reaction conditions on the target product have been discussed. At the first stage, epoxidation reaction of lignin was significantly affected by the alkaline condition and the dropping speed of epichlorohydrin. The proper synthesis conditions of epoxy lignin were achieved at50℃,8h, and with a ratio of hydroxy group to epichlorohydrin at1:1.1and NaOH concentration at12%. At the amination step, the proper reaction conditions were:80℃,4h, and with a ratio of epoxy group to amine group at1:8. In the qualitative amine color test, aminated lignin samples with different content of amine groups showed blue and blue-purple colors, visually proved the introduction of primary amine groups. The results of FTIR, XPS and the element analysis also confirmed that amine groups have been introduced into the structure of lignin and the amount of the incorporated amino groups was significant. The TGA revealed good thermal-mechanical performance of the products.4Curing the bisphenol A epoxy resin with aminated lignin:the aminated lignin was used as a curing agent of bisphenol A epoxy resin with different viscosity, the crosslinking reaction kinetics of the aminated lignin reacted with epoxy resin were investigated by using a non-isothermal method. The thermal stability, mechanical properties and water absorption of the epoxy resins cured by the aminated lignin were researched. The FTIR results proved the reactivity of the aminated lignin with the epoxy resin. The result of crosslinking reaction kinetics showed that the curing reaction was a complex curing reaction. The proper curing conditions were:100℃,180min. Both appearance features and scanning electron microscopy (SEM) images indicated that the aminated lignin had good compatibility with epoxy resins. In addition, the glass transition temperature (Tg), thermal deformation temperature (Td) and the thermo gravimetric analysis (TGA) results revealed that the epoxy resin had better thermal stability compared with ones cured by a common hardener. The mechanical properties of the epoxy resin cured by the aminated lignin were improved with the increase of content of the lignin due to the introduction of rigid molecule. The water absorption of epoxy resin E51contained the aminated lignin was2.740%(GCC135:2.584%) comparing with1.561%(GCC135:1.973%) without lignin, indicating that the addition of lignin slightly reduced the water resistance of the epoxy resin..5Curing the bisphenol F epoxy resin with aminated lignin:the possibility of using the aminated lignin prepared in this laboratory as a crosslinker of epoxy resin was explored. FTIR spectra confirmed that the synthesized aminated lignin could react with epoxy groups and serve as a hardener. Transparent and homogeneous epoxy resin films could be formed with less than60% of the aminated lignin in the hardener mixture after the resins were cured at following procedures:90℃,90min. Tg and Td of the epoxy resin cured by the aminated lignn individually increased26℃,20℃compared with the one without lignin. The Tg value of the epoxy resin was improved with the increase of post-cured temperature, time and the addition of the aminated lignin. The aminated lignin had a positive effect at the initial degradation stage of the epoxy resin. The mass loss of the epoxy resin cured by the aminated lignin before300℃was small around only3.57%while that of the one without the aminated lignin in the hardener system was7.79%. The results revealed that the thermal behavior of the epoxy resins was improved because of the introduction of the aminated lignin. Similar to bisphenol A epoxy resin, the mechanical properties of the bisphenol F epoxy resin cured by the aminated lignin were also improved with the increase of content of the lignin. The water absorption of bisphenol F epoxy resin cured by the aminated lignin was1.319%comparing with0.996%without lignin.The results showed that the aminated lignin containing a great quantity of primary amine and secondary amine groups could serve as a curing agent of epoxy resins.