Study On The Oxidation And Esterification Of Lignin And Its Application In Polyurethane Materials

Lignin is an important renewable resource,which is the second most abundant natural polymer.The abundance of lignin is just less than cellulose.Since the structure of lignin is complex,the industrial applications of lignin are quite limited.Lignin is often discarded with agricultural by-products or papermaking waste water,which leads to the waste of resources and the pollution of the environment.After modification,lignin can be used as the raw material or the modification filler of some polymers.Therefore,the fossil resources,which are non-renewable,can be saved.Making full use of lignin is conducive to the sustainable development of the national economy and achieves good social benefits.In this paper,steam explosion lignin was used as the starting material for the preparation of two kinds of modified lignin.Firstly,steam explosion lignin was oxidized by a common oxidizer NaClO.The oxidative modification derivatives of lignin(called lignin-derived polycarboxylic acids,LPCAs)were prepared,and the recovery and reuse of lignin in papermaking waste water were explored.Secondly,steam explosion lignin was transformed into acid-precipitated lignin.Later,acidprecipitated lignin was modified by esterification with ethylene glycol and thus a new kind of lignin-based polyol,which is called lignin esterified by ethylene glycol(LEEG),was prepared.These two kinds of modified lignin were both introduced into waterborne polyurethane(WPU)elastomers and rigid polyurethane foams(RPUFs),and hence four kinds of polyurethane composites with modified lignin were prepared: LPCAs/WPU composite,LPCAs/RPUF composite,LEEG/WPU composite,LEEG/RPUF composite.In detail,the research content of this paper is presented as follows:1.Lignin was oxidized in control by varying the reaction conditions of the oxidation by NaClO and thus lignin-derived polycarboxylic acids(LPCAs)were prepared.After the oxidation by NaClO,the benzene rings of lignin were disrupted to a certain degree.The content of carboxyl of lignin was increased.The molecular weight of lignin was decreased obviously.The best reaction condition was proposed as follows: the mix ratio [n(NaOCl)/m(lignin)] was 12mmol/g,the reaction temperature was 30°C and the reaction time was 5 to 30 min.Under the best reaction condition,a great deal of LPCAs were prepared for the following experiments.2.LPCAs were introduced into WPU elastomers and the LPCAs/WPU composite was prepared.The water resistance test showed that the water solubility of WPU elastomers was increased by LPCAs.With the increase of the LPCAs loading,the tensile stress at 100% strain and other properties of the composite first increased and then decreased.When the LPCAs loading was 2.5wt%(based on PPG-1000),the tensile stress at 100% strain of the composite was the highest.With the thermogravimetric analysis,it is supposed that LPCAs tended to get into the soft segment regions of WPU in the composite.LPCAs had the interactions,like hydrogen bonds and van der Waals forces,with the soft segment of WPU,and thus LPCAs modified the mechanical properties of WPU elastomers.3.LPCAs were introduced into RPUFs and the LPCAs/RPUF composite was prepared.With the loading of LPCAs increasing,the expansion ratio of the composite first increased and then decreased,while the compressive strength and the compressive modulus of elasticity first decreased and then increased in general.When the loading of LPCAs was between 0.13wt% and 0.33wt%(based on polyether polyol),the compressive strength of the composite was nearly the same as the blank RPUF,but the expansion ratio increased.When the loading of LPCAs was between 0.53wt% and 0.67wt%,the expansion ratio of the composite was almost equal to the blank RPUF,but the compressive strength was increased evidently.Therefore,LPCAs can improve the mechanical properties of RPUFs to a certain degree.4.Using Steglich esterification,acid-precipitated lignin was modified by esterification with ethylene glycol.The modified lignin,i.e.lignin esterified by ethylene glycol(LEEG),had a lower content of carboxyl and a higher content of aliphatic hydroxyl.LEEG,which was used as a kind of lignin-based polyol,was introduced into WPU elastomers and then LEEG/WPU composite was prepared.LEEG didn't change the water resistance of WPU elastomers obviously.When the loading of LEEG was between 0.5wt% and 3wt%,the tensile stress at 100% strain of the composite was increased to a certain degree,but the tensile strength was decreased at the same time.It is supposed that LEEG mainly existed in the composite in a form of particles.The contact area between LEEG and the segments of WPU was relatively small,and then the interactions like hydrogen bonds and van der Waals forces were fewer.Therefore,LEEG had a relatively small influence on the mechanical properties of WPU elastomers.5.LEEG were introduced into RPUFs and the LEEG/RPUF composite was prepared.When the loading of modified lignin was the same,the expansion ratio of LEEG/RPUF composite was smaller than that of LPCAs/RPUF composite,indicating that the hindering effect of LEEG on the foaming process was more obvious than that of LPCAs.Besides,when the loading of LEEG was smaller than 1.00wt%,the compressive strength of the composite didn't vary obviously with the change of the loading of LEEG.It is supposed that the interactions between LEEG and the segments of polyurethane were relatively weak,so that LEEG didn't show evident reinforcing effect on RPUFs.