An approach towards tailoring interfacial structures and properties of multiphase renewable thermoplastics from lignin–nitrile rubber
Lignin-derived thermoplastics and elastomers with both versatile performance and commercialization potential have been an elusive pursuit for the past several decades. Lignin content has been limited to about 30 wt%, often requiring chemical modification, solvent fractionation of lignin, or prohibitively expensive additives. Each of these factors is a deterrent to industrial adoption of lignin-based polymers, limiting the potential of this renewable resource. Herein we describe high-performance multiphase thermoplastics made with a blend of 41 wt% unmodified industrial lignin and low-cost additives in a matrix of general-purpose acrylonitrile-butadiene rubber (NBR). Hardwood soda lignin (HSL) and softwood Kraft lignin (SKL) were blended under high shear conditions with NBR, carbon black (CB), polyethylene oxide (PEO), boric acid (BA), and dicumyl peroxide (DCP). This combination with SKL lignin in the proper proportions resulted in a thermoplastic with a tensile strength and failure strain of 25.2 MPa and 9%, respectively; it exhibited an unexpected tensile yield, similar to that of ABS, a commodity thermoplastic. The analogous HSL lignin compositions are tough materials with tensile strengths of 7.3–16.7 MPa and failure strain of 80–140%. The contrasting ductility and yield stress behavior were analyzed based on the compositions’ morphology and interfacial structure arising from the chemical nature of each lignin studied. The roles of CB as a reinforcement in the rubbery phase, DCP and BA as cross-linkers to create multiphase networks, and PEO to promote the adhesion and compatibility of lignin in commercial-grade NBR are also discussed in detail.