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.

On the gelation of humins: from transient to covalent networks

“…However, the use of humins as a physical gel allows these materials to be reprocessed under mild thermal conditions that can minorly influence their mechanical properties if appropriately designed. 29,71 This can apply for a limited number of reprocessing cycles that will be influenced by the processing conditions, which in turn also reinforces the reprocessed materials due to the growth of stiff humin clusters.…”

“…Polymeric materials such as porous foams, 21–24 thermoset resins, 25–27 elastomers 28 or polymer blends 29 have been investigated for value-added applications including capacitors, 30 binders, 31 catalysis, 32,33 adsorbents, 34,35 reinforced composites 36,37 or self-healing robotics. 29…”

Highly Reactive Biomass Waste Humins Derived from Photocatalytic Polymerization of 5-Hydroxymethylfurfural for Self-Healing Polymers

“…According to Cerdan et al, the electron-withdrawing substituents on the furan groups in A-H prepared from the thermal-acid method lower the reactivity toward the DA reaction, resulting in failure in producing covalently crosslinked gels. 50 In this study, the electron-withdrawing groups such as aldehyde groups in L-H prepared by photocatalysis were significantly reduced, and the reactivity of L-H for DA reaction was improved. 1 H NMR and FT-IR spectra of DA-L-H are shown in Figures S7 and S8.…”

Laser sintering of self-healable and recyclable thermoset networks

“…The incorporation of dynamic bonding in polymers allows the creation of stimuli-responsive, high-performance materials with enhanced features [8,9]. Those include self-healing [10,11], shape memory [12,13], self-assembly [14], biodegradability [15] or recyclability [16], finding applications in fields such as drug delivery [17], tissue engineering [18], optics [19], sensors [20], coatings [21,22] or actuators [23][24][25][26].…”