Mechanical Activation-Assisted Solid Phase Esterification Of Lignin And Effect Of The Products On The Properties Of PVC

Lignin,one of the three principal components of lignocellulosic biomass,has the highest energy and is abundant in source.It has many characteristics,such as renewability and biodegradability,non-toxic,diverse rich functional groups,strong antioxidant properties and weather resistance,thermoplastic,glassy state transformation,and so on.The industry lignin was got as a byproduct of pulp in paper-making and biofuels industries,however,only approximate 2%of it was used commercially due to its brittle,low reactivity,poor compatibility with polymer,limited application scope,and so on.The remainder still mainly been burned as energy source or discard as waste,leading to the waste of resources and growing environmental problems.Therefore,chemical modification of lignin,which could improve its performances and expand its applications,would have important economic and social significance,ascribing to that it would not only make full use of waste but also reduce the cost of paper-making and biofuels industries.In this papar,for the purpose of improving the reactivity of lignin,embroading its application fields,and avioding the difficulties of separation and environmental pollution in traditional liquid phase method,lignin was treated by mechanical activation(MA)to improve its reactivity and lignin esters were prepared by MA-assisted solid phase synthesis(MASPS)with a customized stirring ball mill as MA solid phase reactor.Then,the lignin and its esters were applied in the modification of PVC.The main works and results were as follows:(1)In order to learn about the effect of MA on structure changes and reactivity in further chemical modification of lignin,lignin was treated by MA,and the structure and reactivity in further esterification were studied.The chemical structure and morphology of MA-treated lignin and the esterified products were analyzed by chemical analysis combined with Ultraviolet/Visible(UV/Vis)spectrometer,Fourier transform infrared spectroscopy(FTIR),Nuclear magnetic resonance(NMR),Scanning electron microscopy(SEM),and particle size analyzer.The results showed that MA led to the changes of molecular structure and morphological structure of lignin.The change of molecular structure mainly showed that the contents of aliphatic hydroxyl,phenolic hydroxyl,carbonyl and carboxyl groups increased while the content of methoxyl groups decreased after MA treatment.The change of morphological structure indicated that MA led to the decrease of particle size and the increase of specific surface area and roughness of surface in lignin.The reactivity of lignin was enhanced significantly for the increase of hydroxyl content and the improvement of mass transfer in chemical reaction caused by the changes of molecular structure and morphological structure.(2)In order to investigate and analyze the characteristics of acetylation of lignin by MASPS and the properties of the products,acetylated lignin was prepared by a MASPS technology,and the structure and properties were studied by UV/Vis,FTIR,NMR,SEM,differential scanning calorimetry(DSC),and thermal gravimetric analysis(TG).The product prepared by traditional liquid phase synthesis(LPS)and thermal solid phase synthesis(TSPS)were used as the comparisons.The results showed that the acetylation mainly took place at the aliphatic hydroxyl when acidic catalysts such as H2SO4 and ZnCl2 were used.While both phenolic hydroxyl and aliphatic hydroxyl participated in the reaction when basicity catalyst such as pyridine,sodium acetate and dimethyl aminopyridine(DMAP)were used.The optimum preparation condition of acetylated lignin by MASPS was reaction temperature 80 ?,reaction time 1.5 h,milling speed 300 rpm,anhydride dosage of 3 mol/C9,and catalyst dosage of 2%with DMAP as catalyst.The degree of esterification(DE)of the acetylated lignin produced by MASPS technique was 77.59%,while DEs of those produced by LPS and TSPS were only 42.29%and 27.54%,respectively.The reactivity of phenolic hydroxyl was higher than aliphatic hydroxyl groups,and acetylation of aliphatic hydroxyl mainly took place at the ? of ?-O-4.The chemical structure of the acetylated lignin prepared by MASPS was similar to that by LPS and TSPS,but the difference of DE and contact way led to different morphology and thermal properties.The condensed state of granular particles became a dispersed state and the particle size decreased significantly after the esterification due to the consumption of large amounts of hydroxyl groups during esterification,resulting in the destruction of hydrogen bonds between lignin molecules.The acetylated lignins prepared by MASPS and TSPS were irregular block with coarser surface and loose structure,while the one prepared by LPS was mostly regular balls with smooth surface and compact structure.Esterification improved the flow properties and thermal stability of lignin,which had some difference between the acetylated lignin prepared by different methods due to the different structural characteristics and DE.(3)In order to investigate and analyze the characteristics of acylation of lignin by MASPS with different anhydrides and the properties of the products,acylated lignins with the substituents of different length of carbon chain were prepared by MASPS with different anhydrides as acylating agents.Their structure and properties were analyzed by UV/Vis,FTIR,NMR,SEM,DSC,and TG.The results showed that both aliphatic hydroxyl and phenolic hydroxyl groups participated in the reaction during acylation of lignin with non-cyclic anhydride.The reactivity of acylating agent decreased with the chain length of acyl groups,and the different reactivity of acylating agents resulted in different relative content of phenolic and aliphatic substituents in the products.The reactivity of cyclic anhydrides was weaker than that of non-cyclic anhydrides,and only aliphatic hydroxyl of lignin participated in the acylation.The unsaturated bond in acylating agent also reduced its reactivity.The destruction of intermolecular hydrogen bonding by esterification and erosion of acylating agent on lignin led to the loose interior and rough surface of acylated lignins.All acylated lignins except maleated lignin had lower glass transition temperature(Tg)than original lignin,and the To decreased with chain length of acyl groups.The acylated lignins prepared with cyclic anhydrides had higher Tg and worse thermal stability than that prepared with non-cyclic anhydrides with same length of carbon chain.(4)In order to learn about the esterification mechanism of lignin,the esterification of lignin model compounds with the DMAP,and sodium acetate or H2SO4 as catalyst were studied.The structures of product were analyzed by UV/Vis,FTIR,and NMR.The results showed that the esters were formed through an ion pair of acetate and acetylpyridinium or a four-membered or six-membered-ring transition state when DMAP was used as a catalyst.Sodium acetate played a part of salt when it was used as a catalyst.Water was formed by dehydration and carbonization of lignin when sulfuric acid was used as a catalyst.Then,the proton which promoted the esterifcation,was got by the dissociation of sulfuric acid assisted by the water from reaction and absorption.The oxidation might happen when DMAP and sodium acetate were used as catalysts and the benzene ring may change when H2SO4 was used as a catalyst.The reactivity of three phenylpropane units in lignin has the following order:p-hydroxyphenyl(H)>guaiacyl(G)>syringyl(S).The reactivity of phenolic hydroxyl was greater than the aliphatic hydroxyl when DMAP was used as a catalyst while the reactivity of aliphatic hydroxyl was greater than the phenolic hydroxyl when sodium acetate or H2SO4 was used as a catalyst.The reactivity of H2SO4 was so low that the reaction of phenolic hydroxyl in lignin was almost invisible although the reaction of phenolic hydroxyl in lignin model compounds was visible(5)In order to learn about the application prospect of lignin and its esters in PVC,the lignin(lignin esters)/PVC composites were prepared and their structure and properties were analyzed by universal mechanical testing machine,congo red test,UV/Vis,FTIR,SEM,DSC,TG,and rheometer.The results showed that the proper preparation process was moulding time 10 min,moulding temperature 165 ?,moulding pressure 7 MPa,and lignin dosage 15%(w).The thermal stability and mechanical properties of lignin/PVC composite could be improved when the lignin were activated,esterifed or complex modified before moulding process.The non-cyclic anhydrides acylated lignin/PVC composites had good thermal stability and mechanical properties,and these properties were nearly not affected by the length of carbon chain.The succinylated lignin/PVC composite had poor dynamic thermal stability and mechanical properties.But the both properties were good for the maleated lignin/PVC composite,which had a poor property at static thermal stability.The static thermal stability of maleated lignin/PVC composite could be improved when the COOH in maleated lignin were converted to salt.At the same time,the lignin and its esters improved the aging resistance of PVC.The abilities of lignin and its esters such as capturing free radicals,crosslinking reaction with PVC during moulding process,reducing the decomposition activation energy of composites and good compatibility with PVC were the reasons that lignin affected the stability and mechanical properties of composites.The research results of this paper provided theoretical and technical support for the controllable synthesis of lignin esters.The studies of structure-property relations of lignin and its application in PVC provided a theoretical basis for efficient utilization of lignin.