Preparation And Properties Of Enzymatic Hydrolyzed Lignin Modified Epoxy Resin Composites

Epoxy resin（EP）has been one of the thermosetting materials with the largest industrial demand by virtue of its high adhesive strength,superior insulation,excellent physical and mechanical properties,chemical corrosion resistance and dimensional stability.However,the inherent defects of epoxy resin curing materials,such as brittleness,flammability,and poor thermal conductivity,have dramatically limited their applications in the fields of building engineering,electronics and electrical appliances that require high comprehensive performance of materials.Nowadays,most epoxy resin modifiers rely on fossil resources to prepare.It is seriously contrary to the scientific concept of green and sustainable development.Therefore,scholars will pay more and more attention to the renewable biomass materials.Lignin is a sort of natural aromatic compound with the highest reserves in nature.The particularity of its skeleton structure endows this amorphous polymer with good heat resistance and strong ultraviolet absorption ability,etc.Moreover,lignin contains alcohol hydroxyl,phenolic hydroxyl,methoxy and other active functional-groups,which can replace chemical feedstocks for the synthesis and modification of polymers.In summary,the purpose of this paper is to modify the flame retardancy,thermal conductivity and mechanical properties of epoxy resin by using enzymatic hydrolysis lignin or its derivatives.It is generally known that lignin contains hydroxyl and benzene rings,so it can meet the structural requirements of macromolecular flame retardant charring agents.It also has the advantages of low cost,green and no pollution.Pyrophosphoric acid piperazine（PPAP）is a new environmentally friendly flame retardant with nitrogen-phosphorus synergy,which has excellent flame retardant properties.In this work,lignin was compounded with PPAP in the mass ratio of 1:1 as the flame retardant for EP.The combustion behavior,thermal stability and mechanical properties of epoxy resin composite materials,as well as the chemical structure,elemental content,microscopic morphology of the residual carbon layer after combustion were characterized.The introduction of lignin/PPAP compound flame retardant（L-P）effectively improved the flame retardancy and smoke suppression ability of epoxy resin materials.Compared with the neat EP,the limiting oxygen index of 20%L-P/EP increased from 22.2%to 27.5%.TGA results showed that the thermal stability of the EP composites was enhanced with the compound flame retardant.SEM images indicated that L-P obviously promoted the formation of carbon,which makes the residual carbon layer of epoxy flame retardant material denser after burning.However,the powder flame retardant was not conducive to the improvement of mechanical properties of epoxy resin.The impact strength of L-P/EP composites revealed a downward trend with the increase of L-P content.Currently,filling the matrix with carbon materials（expanded graphite,carbon nanotubes,graphene）is a typical approach to enhance the thermal conductivity of EP composites.Therefore,the enzymatic hydrolyzed lignin activated by K₂CO₃ was used as the precursor.It was carbonized under different pyrolysis temperatures in a tube furnace.Lignin-based carbon materials（KLPC）were prepared as the thermally conductive filler for epoxy resin.The results exhibited that the pore-forming ability of K₂CO₃ on lignin reinforced with increasing pyrolysis temperature.KLPC-10（pyrolysis temperature was 1000 ^oC）had the highest graphitization degree.Its specific surface area was up to 1588 m²·g^-1.Then,KLPC was filled into the matrix in different proportions to explore its influence on the thermal conductivity,mechanical properties and thermal stability of EP composites.The thermal conductivity of the 30%KLPC/EP increased by 105.26%compared to the unmodified EP.In addition,KLPC made the heat resistance and flexural capacity of the epoxy resin composites improve.In order to ameliorate the toughness of epoxy resin composite materials,the enzymatic hydrolysis lignin was subjected to an acid-alcohol condensation reaction with ethylene glycol,neopentyl glycol and sebacic acid.Liquid lignin-based polyester polyol（LLP）was synthesized to toughen epoxy resins.The effect of LLP on the mechanical properties and heat resistance of epoxy resin was comprehensively analyzed,and the optimal addition of LLP in the composite was determined.The bending strength and impact strength of 10%LLP/EP were up to 113.67MPa and 51.29 k J/m²,which were 5.25%and 27%higher than those of unmodified epoxy resin,respectively.The addition of LLP has greatly increased the storage modulus of epoxy resin composites.Meanwhile,LLP can promote the curing reaction to raise the degree of crosslinking of composites.It was found that the LLP flexibilizer could reduce the thermal decomposition rate of EP composites by TGA.Thus,this toughening strategy ensures the minimum sacrifice of the heat resistance of EP materials.A noval channel was put forward for the high-value utilization of lignin and the toughening modification of epoxy resin.