Modification of interactions in polypropylene/lignosulfonate blends

• Weak interactions develop between polypropylene and lignosulfonate resulting in complete immiscibility.
• Interfacial adhesion can be improved by a maleic anhydride grafted polypropylene coupling agent.
• Local deformation processes (debonding and the fracture of lignosulfonate particles) determine the strength of the blends.
• Deformability is small both with and without coupling, which represents the largest obstacle of practical application.

Polypropylene (PP)/lignin blends were prepared in a wide composition range (0–70 vol%) from a PP homopolymer and two lignosulfonates of different compositions by homogenization in an internal mixer and compression molding. Thermodynamic considerations and dynamic mechanical analysis (DMTA) were used for the estimation of interactions and miscibility, while mechanical properties were characterized by tensile testing. Local deformations were followed by acoustic emission measurements, and structure was analyzed by scanning electron microscopy (SEM). The results showed that the structure of the blends form during processing by the breakup of large lignin particles which soften at the high temperature of processing. Weak interactions develop between PP and lignin, since lignin molecules interact with each other strongly and prevent the mixing of the two components. Dispersed structure forms in the entire composition range studied, co-continuous structure cannot develop at all. Interfacial adhesion can be improved through the addition of a maleic anhydride grafted polypropylene (MAPP) coupling agent. Depending on the strength of interfacial adhesion the blends fail either by debonding or by the fracture of lignin particles. Although coupling improves the strength of the blends, deformability is very small especially at larger lignin contents hindering practical application.

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