Utilization of secondary sludge as filler in composites: surface energy and final mechanical properties

Abstract

Secondary sludge from pulp and paper mills can be considered as a potential filler for composite industry. The surface thermodynamics of the secondary sludge from two different pulp and paper mills, processing capability, and material characteristics of biocomposites filled by secondary sludge were studied in this study. Inverse gas chromatography (IGC) has been employed to study the surface characteristics of the secondary sludge. Also, the cellular biopolymers were extracted and their surface energy determined by IGC. Based on the surface thermodynamics and the chemical structure of the secondary sludge, Nylon 11 was selected as the polymeric matrix. The dispersive component of surface energy for the secondary sludge samples was obtained in the range 60—42 mJ/m2 measured at 313—373 K, which is high enough to allow the biosolid to be coupled with conventional polymeric resins. The manufactured nylon/sludge composites showed acceptable, yet not improved, mechanical strength. Also, 10% of dried sludge as filler proved to be an effective amount, which is sufficient to fill but not deteriorate the tensile and flexural strengths of the composite. Sludge-filled composites compounded by a twin-screw extruder exhibit considerably better tensile properties than those compounded by the K-mixer. Maleated polyolefins used as coupling agents also improved the composite’s mechanical properties significantly.