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ADDITIVES | REINFORCEMENT


Figure 4: Tensile failure strength and elongation at break of mixtures of NBR-33 with different lignins at 50/50 ratio, including organosolv wheat straw (OWS), organosolv hard- wood (OHW), sodium hydroxide extracted hardwood lignin (SHW), Kraft softwood (KSW), and methanol fractionated low-molecular-weight Kraft softwood lignin (KSW_L). Source: ORNL


introducing nanoscale-dispersed biomass-derived lignin into nitrile rubber. The authors say the material is “a compositional analogue of ABS, where the styrene fraction is completely replaced with lignin.” The researchers say the material offers significantly higher toughness than ABS (Figure 4). “Temperature-induced controlled miscibility between the lignin and the rubber during high shear melt-phase synthesis allows tuning the material’s morphology and performance,” the researchers explain in a paper published in the journal Advanced Functional Materials. They say the product has “unprecedented” yield stress (15–45 MPa). Stiff thermoplastic elastomers would find


immediate use if they were based on low-cost renewables, the researchers argue. “The common bio-based thermoplastic elastomers have low softening points and most of these elastomers require expensive polyesters or polyamides that have poor hydrolytic stability, which limits their end uses. Hydrothermally stable, higher performance thermoplastic materials made of renewable polymer hard segments bridged with soft seg- ments will offer a great solution for renewable thermoplastic elastomers.”


Energy efficient The team’s energy-efficient method for synthesis- ing and extruding high-performance thermoplastic elastomers based on lignin involves fractionating a rigid, thermally malleable lignin that can then be melt-mixed with appropriate soft commodity rubbers to form high-performance polymers with precisely controlled morphology. Previous reports on incorporation of lignin into rubbers as a potential candidate for the replace- ment of the conventional reinforcement – carbon black – showed little effect on rubber reinforcing due to its large particle size and lack of interfacial


84 COMPOUNDING WORLD | October 2018


interactions, they say. “Our method…eliminates the need for costly and energy-intensive chemical func- tionalisation of lignin and obviates the need for a separate unit operation of polymer synthesis in a reactor with solvents,” the researcher team claims.


Morphology challenge “Common styrenic thermoplastic elastomers typically contain homogeneous distribution of chemically bonded domains of polystyrene (50–200 nm in length) in a soft matrix. Our research strategy was to replace polystyrene segments with lignin in similar soft matrices. The ‘grand challenge’ in this approach is that of controlling the morphol- ogy of lignin domains in the soft rubbery matrix and retaining these domains during processing, testing, and end use.” The ORNL group used nitrile rubber with 41% and 51% acrylonitrile contents (NBR-41 and NBR-51, respectively) with methanol-extracted low-molecular-weight fractions of Kraft softwood lignin. In their conclusion, the researchers report: “Temperature-induced controlled miscibility between the lignin and the nitrile rubber during high shear melt-phase synthesis allowed us to tune the material’s morphology and performance. Lignin–rubber interaction was further improved by using fractionated melt-stable lignin in the compo- sition with nitrile rubber of optimal acrylonitrile content. The products from equal-mass mixtures of lignin and rubber exhibit unprecedented yield stress and strain harden at large deformation.”


CLICK ON THE LINKS FOR MORE INFORMATION: � www.neg.co.jp/en/www.owenscorning.com � www.jm.com � www.maficbasalt.com � www.arcticminerals.com � www.ornl.gov


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