Influence of ammonium lignosulfonate on the mechanical and dimensional properties of wood fiber biocomposites reinforced with polylactic acid

In order to fabricate green composites without the emission of toxic volatile compounds, wood fiber (WF) biocomposites reinforced with polylactic acid (PLA) were prepared utilizing the traditional hot-press manufacturing process. The effect of adding ammonium lignosulfonate (AL) on the mechanical and dimensional properties of the biocomposites was investigated. When the mass ratio of WF to PLA is 7:3 and the AL mass percentage is in the range from 1 to 20 wt%, the modulus of rupture of the WF/PLA/AL composites ranged from 21.95 to 36.96 MPa, the modulus of elasticity ranged from 3078.6 to 4157.8 MPa and the internal bonding strength ranged from 0.69 to 1.71 MPa, which showed a 4.0–75.1% increase in modulus of rupture, a 3.5–39.8% increase in modulus of elasticity and a 21.5–201.2% increase in internal bonding strength compared to the corresponding WF/PLA composites due to the reinforcing effect of AL. However, AL negatively affected the dimensional properties of the biocomposites. The thickness swell of the WF/PLA/AL composites ranged from 18.7 to 43.5% and the water absorption ranged from 47.1 to 78.9%. The thickness swell, water absorption and contact angle of the WF/PLA/AL composites increased with the AL mass percentage due to its high hydrophilicity, resulting in a 1.1–132.6% and a 3.4–67.4% increase in thickness swell and water absorption, respectively. AL had a positive effect on the crystallinity of the biocomposites due to its good surface activity as long as the AL mass percentage was below 20 wt%. The fractured surface of the WF/PL/AL composites clearly showed that a strong bond (mechanical interlocking and/or chemical bonding) exists at the interface resulting in an effective stress transfer.