The potential of paper mill sludge for wood–plastic composites

•Wood–plastic composites were made with paper mill sludge produced by three different pulping processes.•The effects of primary and secondary sludge on composites properties were assessed.•Higher cellulose content improved the composite properties.•Secondary sludge had a negative impact on the composite properties.•The ash content in the sludge showed a positive impact on the WPC properties.

Recent studies have demonstrated the potential of primary sludge (PS) as reinforcing fibers and secondary sludge (SS) as binder or co-binder in wood–plastic composites (WPC). A comparative study was conducted using paper mill sludge produced by three different pulping processes at two SS to PS ratios. The objectives were to determine the impact of PS and SS on the development of high density polyethylene (HDPE) WPC properties. Sludge produced by thermomechanical pulping (TMP), chemithermomechanical pulping (CTMP), and Kraft pulping were used at three different proportions (20%, 30%, and 40%) for composite manufacturing. The use of mixed sludge containing 30% SS resulted in lower tensile, flexural, and impact performance of the WPC compared to mixed sludge containing only 10% SS for the three pulping processes. Sludge type had a significant impact on the WPC physical and mechanical properties. Kraft sludge produced the best WPC properties, followed by CTMP and TMP sludge. Increasing the sludge proportion produced increasingly negative impacts on water absorption and thickness swelling, but improved the flexural and tensile properties.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide