Sustainable Green Hybrids of Polyolefins and Lignin Yield Major Improvements in Mechanical Properties When Prepared via Solid-State Shear Pulverization

Lignin, a byproduct of paper and pulp industries, is a sustainable, inexpensive biomaterial with potential as composite filler. Past research on polyolefin/lignin composites made by melt processing has led to modest increases in Young's modulus and drastic reductions in tensile strength and elongation at break relative to neat polymer. Here, green hybrids of low density polyethylene (LDPE) and polypropylene (PP) with 5-30 wt % lignin are made by solid-state shear pulverization (SSSP). Microscopy shows that SSSP leads to superior lignin dispersion and suppressed degradation when compared to melt-mixed composites reported in the literature. Composites made by SSSP exhibit major improvements in Young's modulus (81% and 62% increases for 30 wt % lignin in LDPE and PP, respectively, relative to neat polymer), tensile strength equal to or better than that of neat LDPE and near that of neat PP, and much better strain at break than reported in the literature for polyolefin/lignin composites. The SSSP-produced hybrids exhibit major increases in hardness, with 70/30 wt % PP/lignin hybrids reaching values near that of polycarbonate. Well-dispersed lignin improves LDPE and PP thermo-oxidative stability as shown by thermogravimetric analysis (∼35 °C increase in 20% mass loss temperature in air with 20 wt % lignin addition) and isothermal shear flow rheology. Lastly, SSSP-processed composites exhibit slightly improved crystallizability and melt viscosities at moderate to high shear rates that differ relatively little from those of neat LDPE and PP.