A Study On The Preparation Of Nanoscale Lignin And Its Reinforcing Effect On Nonpolar Rubber

As the most abound natural polymer next to cellulose, its application in rubber industryisbeneficial to reduce the environmental pollution of the pulping and paper making industry andimprove the added value of lignin. In polar rubber such as acrylonitrile-butadiene rubber, thecommercial products based on lignin have been realized. However, the poor compatibilitybetween lignin and nonpolar rubber, bad dispersion quality and low production efficiencyextremely restrict the application of lignin in nonpolar rubber. To solve this issue, this paperpresents several ways to prepare nanoscale lignin and aims to study the reinforcing effect andmechanism of the nano-lignin towards nonpolar rubber.In this work, colloidallignin-Poly (diallyldimethylammonium chloride)(PDDA)complexes (LPCs)were fabricated via self-assembly technology. Subsequently, natural rubber(NR)/LPCs nanocomposites were also prepared via latex co-coagulation method. The resultsshowed that PDDA intensively interacted with lignin via cation-π, π-π interactions, resultingin the stable LPCs in acidic pH and uniform dispersion of LPCs in NR matrix at nanoscale.The curing behavior indicated that lignin hindered the vulcanization and lowered thecrosslinking density, while the LPCs accelerated the vulcanization and increased thecrosslinking density. Dynamic mechanical analysis (DMA) indicated that LPCs had a goodcompatibility with NR and could restrict the mobility of NR segments, resulting in increasedglass transition temperature (Tg) of the composites. The homogeneous dispersion of LPCs inNR matrix could significantly improve the mechanical properties and thermal stability ofNR/LPCs composites.In aqueous alkali, lignin was viewed as a spherical microgel. While spread out in amonolayer or adsorbed on a surface, lignin was made up of flexible, disk-like molecules withapproximately the same thickness of2nm.According to this principle, we employed thelamina of montmorillonite (MMT) as a plane template to anchor cationic lignin (CL) on itstwo sides, resulting in the formation of CL-MMT hybrid materials (CLM). The isothermadsorption behavior and structure characteristicsof CLM were studied. The results showedthat CLM was individually dispersed nanosheets with tunable surface charge status and a thickness of about5nm when the mass ratio of CL to MMT is more than2:1and prepared atacidic or neutral pH. Compared to the co-coagulation of lignin and styrene-butadienerubber(SBR), CLM obviously accelerated the coagulation rate, due to the reduction of surfaceactivity of CL restricted by MMT. The nanoscale dispersion of CLM in SBR matrixsignificantly improved the tensile strength of CLM/SBR nanocomposites to14.1MPa byadding only10phr CLM and cardanol glycidyl ether(CGE) as compatibilizer. Dynamicmechanical analysis (DMA) showed that the glass transition temperature of SBR/CLMnanocomposites decreased with increasing CLM loading. Correspondingly, a specialinterfacial structure was proposed.Lignin as a thermoplastic polymer can melt and deform under heating. Therefore, thelignin and epoxidized natural rubber (ENR) was mixing under high temperature and shear,resulting in the thining of lignin and an obervious reinforcing effect on ENR. The effect oflignin content, temperature, water content, heat treatment time, epoxidization level andimidazole on the ring opening reaction, lignin size and the properties of composites werestudied. The results showed that the reactivity and the degree of ring opening reactiondepended on temperature, lignin content as well as epoxidization level. Specially, water couldsubstantially reduce the Tg of lignin and accelerate the reaction rate, resulting a finer lignindistribution. The reaction between lignin and ENR could improve the compatibility betweenthem, but extremely restrict the mobility of ENR molecules and decrease the strain-inducedcrystallization of ENR. However, imidazole in lignin could change the reaction mechanisimand control the reaction degree between lignin and ENR, thus resulting in controllablemechanical properties.Based on the ring opening between lignin and ENR, the lignin was employed ascrosslinker and reinforcing filler for ENR to prepare novel self-crosslinkable lignin/ENRcomposites (SLE). The curing behavior of the SLE indicated that the reaction order betweenlignin and ENR deviated from1. DMA suggested that the Tg of the SLE increased withincreasing lignin content, and there was apercolationthreshold between10phr and20phrlignin forsaltant elevation of Tg. Meanwhile, a physical model based on the viscoelastic behavior of polymer was employed to verify the strong interactions betweenlignin and ENR.Stress-strain curves and X-ray diffraction (XRD)suggestedthat thereinforcement effect on the SLE was mainly originated from lignin itself rather thanstain-induced crystallization.At last, the effect of themixing method and thechemical modification of ligninon SBRand the reinforcing mechanism were studied. The results showed that in lignin/SBR/ENRcomposites via masterbatch metod, lignin was uniformly dispersed at a submicron size andhightly compatible with ENR matrix, obviously improving the mechanical properites ofcomposites. Transmission electro microscopy (TEM) verified that the unmodified lignin (UL)showed amorphous shape and serious aggregation; All of the lignin modified withformaldehyde (LF), modified with glyoxal (LG) and the lignin modified with glutaraldehyde(LGD) were the condensed type of lignin with a spherical profile, and showed a decreasingdispersion ability according to above the order. Hydroxypropyl lignin (HL) was abranch-separating type of lignin and could inhibit the aggregation of lignin. By latexco-coagulation method, UL could be well dispersed in SBR matrix at molecular scale,accompanying intensive π-π interactions between them; HL could accelerate theco-coagulation rate and be dispersed at a size of about100nm in the presence of ENR. LFcould be dispersed at nanoscale and bonded to SBR molecules via chemical bonding,resulting in an enhanced compatibility with SBR matrix; LG and LGD were both dispersed ata larger scale than others. The order for the reinforcing effect of those lignins on SBR was asbelow: UL> HL> LF> LG, LGD. However, in the lignin/SBR composites via directlymixing method, only LF could be dispersed at a submicron scale and exhited a considerablereinforcing effect on SBR. And the mechanical properties of LF/SBR composites via directlymixing method were still inferior to that via latex co-coagulation method.