Preparation Of Lignin-containing Polyurea Material And Its Applied Research In Coating And Adhesive Fields

Polyurea,synthesized from isocyanates and amino compounds,has been widely used in coating industry due to its fast reaction,solvent-free,excellent comprehensive performance and unique construction method.However,polyurea materials were of poor ultraviolet and yellowing resistance,and besides,polyurea was relatively expensive and derived from fossil fuel resources.It was of great significance to search low-cost and easily available biomass in the synthesis of polyurea materials for the sustainable development of polyurea industry.Lignin,the second largest natural polymer only second to cellulose,was of low utilization despite of its advantages such as highly availability and superior ultroviolet resistance.If lignin could be used as substitute materials in the synthesis of polyurea,not only lower cost but also improved comprehensive performance could be achieved.Until now,nearly no lignin-based polyurea material was reported.There were two main challenges for the development of lignin-based polyurea materials:(1)The structure of lignin was complex and the steric hinderance led to the low reactivity,and there is no amino groups on the structure of lignin.(2)Lignin was hard to disperse well and easy to reunite in the polymer matrix,showing poor compatibility with the polymer matrix.Based on the problems above,the main research content and conclusions of this work were as follows:(1)In order to enhance the reactivity of lignin and improve its compatibility with polyurea matrix,enzymatic hydrolysis lignin was grafted with polyetheramine through Mannich reaction,which could introduce high reactive amino group and flexible long polyetheramine chains in lignin structure,and thus could weaken the rigidity of lignin and reduce the agglomeration.FTIR sepctra,GPC,element analysis,ultraviolet spectra,potentiometric titration as well as thermogravimetry were used to characterize pristine lignin and D2000-grafted lignin.By controlling the reaction ratio between lignin and D2000,D2000-grafted lignins(LA,LB,LC)were prepared,whose grafting ratios of tests were respectively 49.2 wt%,34.4 wt% and 36.4 wt%,closing to the theoretical values of 50 wt%,33.3 wt% and 40 wt%,indicating the high conversion rate of grafting lignin with polyetheramine chains through Mannich reaction.(2)D2000-grafted lignin was used to partially substitute polyetheramine in the preparation of lignin-containing polyurea coatings.The effect of the microstructure and the substitute amount of lignin on the microstructure,chemical resistance,corrosion resistance,ultraviolet resistance and thermal stability of lignin-containing polyurea coatings were systemically studied.The results showed that modifying lignin with polyetheramine could significantly improve its compatibility with polyurea matrix and enhance the overall performance of polyurea coatings.Introducing aminated lignin could improve the acid resistance,ultraviolet resistance and thermal stability of polyurea coatings.When substituting polyetheramine with 30 wt% D2000-grafted lignin(LB),the tensile strength and elongation at break of the lignin-containing polyurea coatings(LB-30)double after ultraviolet aging for 3 days.(3)Lignin-containing polyurea adhesives were prepared by partially replacing polyetheramine D2000 with D2000-grafted lignin as well as introducing a chain extender containing dynamic disulfide bonds.In this way,the prepared lignin-containing polyurea adhesvies exhibited strong and repeated adhesivity,fast self-healing ability at room temperature,easily removability,excellent low-temperature resistance and superior thermal stability.The introduction of lignin was proved to endow materials with strong adhesion,while dynamic disulfide bonds play a key role for the reusable performance and rapid self-healing ability at room temperature.When substituting polyetheramine with 10,20,30 wt% aminate lignin,the average lap shear strength on beech boards during the reuse test increased from 1.79 MPa of the control blank sample to 5.51 MPa,7.92 MPa and 4.78 MPa,which were superior to two kinds of commercial EVA glues(4.5 MPa,2.18 MPa).The elongation at break of the lignin-containing polyurea adhesive with 10 wt% modified lignin substituting polyetheraimine(LC-10)could completely recover after self-healing at room temperature for 30 minutes.