Study Of Mechanism For Enhancing Interfacial Compatibility Between Lignin And PHBV

Lignin is abundantly produced as a by-product from papermaking and biorefinery industries that is widely avalible,renewable,biodegradable,thermally stable and inexpensive.It has been utilized as the filler or reinforcement fiber for blending with biodegradable plastics(e.g.PHBV)to manufacture composites.In the purpose of improving the poor compatibility between lignin and PHBV,in this study,lignin esterification using different acid anhydrides before blending with PHBV,and in-situ radical grafting between two phases initiated by DCP have been applied to prepare composites.The water absorption,tensile properties,spherulite morphology,thermal stability,and dynamic rheological behavior of the products were investigated.To enhance reactivity towards polymerization,lignin esterification with crotonic anhydride has also been explored,and the reaction sites were investigated by FTIR and NMR using the lignin model compound.The main conclusions are summarized as follows:(1)Lignin esterification was quite complete.The Tg,hydrophilicity,suface free energy,and polarity of lignin esters were lower than that of original lignins.The Mw of lignin esters gradually increased as the number of carbon atoms in the ester substituent increases from C2 to C6,the Tg generally decreased linearly,and the hydrophilicity and polarity reduced as well.(2)Esterification led to the more uniform dispersion of lignin in the PHBV matrix,and their surface energies are closer,therefore the interfacial bonding was enhanced.Water absorption of PHBV based blends decreased by about 50%after lignin esterification.Compared to blends with unmodified lignin,a progressive increase of tensile strength and elongation at break and reduction of Young's modulus could be observed with increasing ester carbon chain length in PHBV based blends.Among all the lignin esters,lignin propionate showed better performance than the others.Non-isothermal crystallization of PHBV from melt was depressed by lignin esterification,and the average radius of PHBV spherulites decreased,but the crystallinity was not significantly affected.Lignin hexanoate blends showed a Tg depression and smaller dynamic elastic and viscous moduli than the other lignin esters,suggesting that it acted as a plasticizer.(3)Free radical grafting during melt extrusion was initiated using DCP to improve interfacial adhesion via forming new chemical bonds between PHBV and lignin.At an optimal total loading of 2 wt%DCP,grafting efficiency(20.2%),tensile strength,Young's modulus,and storage modulus of the composite showed a maximum.Meanwhile,the adhesion factor calculated from DMA data was lowest for 2%DCP,indicating stronger interfacial adhesion.Tg,thermal stability,and viscosity of the composites were all increased as a result of grafting.On one hand,the crystallization process was suppressed and the crystallinity degree of PHBV was reduced by grafting.On the other hand,DCP induced grafting significantly reduced the spherulite size and increased nucleation density of PHBV,with the radius of spherulite in composites using 2%DCP reduced to around 160 ?m.(4)The in-situ polymerization happened in lignin crotonate during thermal treatment,the Tg,storage modulus and melt viscousity all increased.Introducing double bonds could enhance the reactivity of lignin towards polymerization,and the presence of DCP could further enhance the reactivity.Based on FTIR and NMR analysis of the lignin model compound,it can be concluded that during thermal treatment in the presence of DCP,C=C,C=O,and-CH2-groups are likely subjected to polymerization or crosslinking reaction.