Investigation On The Application Of Lignin In PVC Composites

Lignin is the most abundant biopolymer second only to cellulose. The utilization of the vast quantities of lignin, available as by-products of the forest-using industries, becomes the most significant and challenging problem in lignin research field. Lignin and its derivatives have been used in the manufacture of plastic products and composites which offers potential for the provision of high-value composites. The study on lignocellulosic materials is of great importance to protect the environment and save the fossil resources. In this work, polyvinyl chloride/lignin composites (PVC-L composites) were prepared by melt compounding. The performances of PVC-L composites, as well as the correlation between the structural characteristics of lignin and the performances of PVC-L composites, were investigated. The modification methods of lignin that strengthen the bonding between lignin and PVC matrix were also studied. Furthermore, two kinds of interfacial modifiers were synthesized from lignin. The influences of these interfacial modifiers on the performances of PVC/wood composites were investigated. The interfacial modification mechanisms were discussed.The Vicat temperature of PVC-L composites increased with the increasing lignin loading, suggesting that the incorporation of lignin could extend the operating temperature range of composites. The investigation of rheological properties for PVC-L composites indicated that the addition of lignin exhibited positive influences on their thermal processing. However, the poor compatibility between lignin and PVC matrix limited the performances of PVC-L composites. The mechanical performances of PVC-L composites, including tensile strength, elongation at break and impact strength, decreased with the increasing lignin loading. Besides, the addition of lignin had negative influences on the thermal stability and flame-retardant properties of PVC-L composites. The influences of soda lignin species on the mechanical performances of PVC-L composites were not obvious. However, there were significant differences in mechanical performances of PVC-L composites prepared from lignin with varied separating methods. The tensile and impact strengths of PVC-L composites prepared from acetic acid lignin were better than those of composites prepared from alkaline lignin.The low molecular weight fraction of lignin accelerated the plasticization of composites and was dispersed in a rather uniformly form in PVC matrix than any other fractions. The PVC-L composites prepared from lignin fraction with low molecular weight also showed higher tensile and impact strengths in comparison with those of composites prepared by any other fractions. The addition of high-molecular-weight lignin fraction lowered the initial decomposition temperature but improved the maximum decomposition temperature of PVC-L composites. The heterogeneity of lignin, especially the wide molecular weight distribution, had negative impacts on the properties of PVC-L composites. The molecular weight of lignin should be controlled in a suitable range when lignin is used in polyblends. The improvement of specific surface area of lignin favored its dispersion in PVC matrix. The mechanical performances of PVC-L composites prepared from freeze-drying lignin were better than those of composites prepared from vacuum-drying lignin. Branched chains could be introduced into lignin molecules via hydroxypropylation reaction. The glass transition temperature (Tg) of lignin was lowered after reaction, reflecting that the hydroxypropyl lignin (HL) exhibited a higher degree of molecular mobility than un-reacted one. The mechanical performances of PVC-L composites prepared from HL were better than those prepared from un-reacted lignin. However, the performances of composites were always restricted by the structural characteristics of lignin, especially the molecular weight.Lignin was grafted with methyl methacrylate (MMA) by solution polymerization. The grafting products can be used as interfacial modifier for PVC/wood composites. As indicated by the data of contact angle, the hydrophilic wood surface was transformed into a hydrophobic one by the grafting product treatment. The grafting product exhibited a positive influence on improving the mechanical performances of PVC/wood composites. In comparison with untreated ones, the tensile and impact strengths of composites containing 30phr of wood flour treated with 2wt% grafting product (MMA/Lignin =2:1) were increased by 18.7% and 35.3%, respectively. Besides, the grafting products treatment improved the hydrophobicity of composites.Lignin amine was synthesized from lignin via Mannich reaction and was used in interfacial modification for PVC/wood composites. The experimental results indicated that the polymer-wood interfacial combination was strengthened. The lignin amine treatment provided almost equivalent improvement in mechanical performances of composites as aminosilane treatment does. The tensile and impact strengths of composites prepared from 30phr of wood flour treated with 2wt% lignin amine (nitrogen content 8.18%) were increased by 21.0% and 43.9%, respectively than those prepared from untreated wood flour. Furthermore, lignin amine treatment can also significantly reduce the water absorption of composites. The lignin based interfacial modifiers, including grafting product and lignin amine, resulted in the changes of the rheological properties for PVC/wood composites even if their addition level was limited.