Humins Blending in Thermoreversible Diels-Alder Networks for Stiffness Tuning and Enhanced Healing Performance for Soft Robotics

Abstract

Humins waste valorization is considered to be an essential pathway to improve the economic viability of many biorefinery processes and further promote their circularity by avoiding waste formation. In this research, the incorporation of humins in a Diels-Alder (DA) polymer network based on furan-maleimide thermoreversible crosslinks was studied. A considerable enhancement of the healing efficiency was observed by just healing for 1 h at 60 °C at the expense of a reduction of the material mechanical properties, while the unfilled material showed no healing under the same conditions. Nevertheless, the thermal healing step favored the irreversible humins polycondensation, thus strengthening the material while keeping the enhanced healing performance. Our hypothesis states a synergistic healing mechanism based on humins flowing throughout the damage, followed by thermal humins crosslinking during the healing trigger, together with DA thermoreversible bonds recombination. A multi-material soft robotic gripper was manufactured out of the proposed material, showing not only improved recovery of the functional performance upon healing but also stiffness-tunable features by means of humins thermal crosslinking. For the first time, both damage healing and zone reinforcement for further damage prevention are achieved in a single intrinsic self-healing system.

Keywords: Diels–Alder; biomass valorization; green composites; humins; immiscible blends; mild healing; self-healing; soft robotics; stimuli-responsive; thermoreversible.

Figure 1

Chemical structures of the reactives employed: (a) Jeffamine D400, (b) Furfuryl Glycidyl Ether, (c) BMI689, (d) humins (adapted from [5]), (e) FT-IR spectra of Humins-DA and raw humins, and (f) humins complex viscosity modulus variation as a function of temperature.

Figure 2

(a) TGA results of humins (black), BMI689-F400 (dark yellow) and Humins-DA blend (blue) at 5 K·min⁻¹, (b) DSC analysis of humins (black), BMI689-F400 (dark yellow) and Humins-DA (blue). Samples were cooled to −80 °C at 5 K·min⁻¹ and re-heated at 5 K·min⁻¹, (c) SEM pictures of the Humins-DA cross-section.

Figure 3

Dynamic rheology measurements at a heating ramp of 1 °C·min⁻¹ to determine the degelation of (a) BMI689-F400 and (b) Humins-DA. Frequencies (Hz): 0.3; 0.6; 1; 1.8; 3.1 (one curve for each frequency). A Tgel value of 123 °C is found for BMI689-F400 whereas no clear T_gel was found for Humins-DA despite the gel transition being shifted to lower temperatures.

Figure 4

Isothermal small amplitude oscillatory time sweeps at 60 °C for (a) Humins-DA and (b) humins. Both tests were performed at a frequency of 1 Hz and an amplitude of 1%.

Figure 5

(a) Fracture tests for BMI689-F400 (yellow dotted line), Humins-DA as synthesized (black solid line), Humins-DA treated isothermally at 90 °C for two hours (blue dashed line) and three hours (red dash-dotted line). Fracture tests for Humins-DA (black line), Humins-DA healed at 60 °C for 1 h (blue dotted line), and Humins-DA healed at 80 °C for 1 h (red dashed line) when treated isothermally at 90 °C for (b) one hour, (c) two hours, and (d) three hours.

Figure 6

SEM images of the extruded Humins-DA cross-section.

Figure 7

(a) Multi-material connectivity between BMI689-F400 (yellow) and Humins-DA (black) by healing two pieces together at 60 °C for 1 h. (b) Schematic representation of the multi-material robotic finger assembly. (c) Robotic multi-material finger built showing high flexibility. (d) Bending tendon-driven motion test of the prepared robotic finger. The blue curve corresponds to the finger position at rest, whereas the black curve corresponds to the final bent position. The rest of the curves, that is, orange, grey, dark yellow, purple and green, describe the finger transition from the position at rest to the bent state.

Figure 8

(a) Multi-material humins-based soft gripper grabbing a soft object. (b) Fractured multi-material finger showing great connectivity of the Humins-DA component under a healing treatment of 60 °C for 1 h. (c) Focalized healing-by-welding of Humins-DA upon damage at 100 °C for a few seconds.