Evaluation of Kraft Lignin as Natural Compatibilizer in Wood Flour/Polypropylene Composites Shupin Luo, Jinzhen Cao, Wenjing Sun MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Qinghua East Road 35, Haidian, Beijing 100083, China This study investigated the effect of Kraft lignin as nat- ural compatibilizer on the water absorption, thermal, mechanical, and interfacial properties of poplar wood flour (WF)/polypropylene (PP) composites. Varying con- tents (0.5, 1, 2, 4, and 8 wt%) of lignin were added to WF and PP by direct mixing, then the composites were prepared by two-screw extrusion and compression moulding. Results showed that lignin incorporation reduced the water absorption and postponed the ther- mal decomposition of the composites. Composites with lower lignin contents (0.5% and 1%) could get the optimal properties, and the excess lignin contents (4% and 8%) resulted in slight decrease in the mechanical properties. DMA and calculation of adhesion factor showed that the composites with 0.5% lignin had the best interaction between WF and PP. The morpholo- gies of fractured surface also indicated improved inter- facial adhesion between WF and PP from the addition of 0.5% and 1% lignin. POLYM. COMPOS., 00:000–000, 2015. V C 2015 Society of Plastics Engineers INTRODUCTION Natural fiber reinforced polymer composites, as an impor- tant branch in the field of composite materials, have been intensively investigated in recent decades. Compared with conventional synthetic fibers like glass and carbon, natural fibers have several advantages. They are low cost, highly available and renewable, low density, high specific strength and biodegradable [1, 2]. However, one of the main problems with using natural fibers as reinforcement is the poor compati- bility between the inherently hydrophilic fibers and hydropho- bic polymer matrices. The poor compatibility caused ineffective stress transfer throughout the interface, resulting in low performance of composites. This means that the reinforce- ment potential of natural fibers cannot be exploited to the full extent, especially for short fibers and particles [3]. Numerous strategies to improve the interaction at the fiber–matrix inter- face have been developed. Of these methods, modification of fibers [4] or adding coupling agents such as maleic anhydride grafted polymers [5] and silanes [6] have been most com- monly used. However, with rising environmental concerns and depletion of petro-chemical resources currently, it is desir- able to reduce the chemical input, energy cost, and wastes dur- ing the production. An alternative to these methods are the use of natural materials as additives in the composites. Lignin is the second most abundant biopolymer on our planet after cellulose, and it is one of the three main com- ponents found in the cell wall of natural lignocellulosic materials [7]. Large quantities of lignin are yearly avail- able from numerous pulping processes such as paper and bio-refinery industries. Most of them are burnt to produce energy although they can offer many other value-added uses. Only approximately 2% of the lignin from pulp and paper industry has been commercialized [8, 9]. The chem- ical structure of lignin is complex, largely depending on the origin, separation, and fragmentation processes [10]. Generally, it consists of phenylpropane units having vari- ous substituent functional groups. The main precursors are three aromatic alcohols, namely p-coumaryl, coni- feryl, and sinapyl alcohols, which undergo cross-linking and form the complex three-dimensional structure of lig- nin [11, 12]. Because of its various functional groups, lig- nin has been investigated as a stabilizer (antioxidant) for plastics and rubbers [13, 14] as well as in the formulation of dispersants [15], adhesives [16], and surfactants [17]. The effective use of blending lignin with various syn- thetic polymers such as polypropylene [18], polystyrene [19], polybutylene succinate [20], and polylactic acid [21] has been reported in literature. Furthermore, lignin was expected to be applied as a bio-compatibilizer between natural fibers and polymer matrix, since it contains polar hydroxyl groups and non-polar hydrocarbon and benzene rings. Previous studies investigated the effect of lignin extracted from coconut [22] and coir fiber [23] in PP/ fiber composites. But the extraction of lignin by sulfuric acid or organic solvent in the lab is relatively complicated and high-cost, which is not practical for extensive appli- cation in composites production. Correspondence to: J. Cao; e-mail: caoj@bjfu.edu.cn Contract grant sponsor: Fundamental Research Funds for the Central Universities, China; contract grant number: TD2011-14. DOI 10.1002/pc.23821 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2015 Society of Plastics Engineers POLYMER COMPOSITES—2015