Abstract
Biobased circular materials are alternatives to fossil-based engineering plastics, but simple and material-efficient synthetic routes are needed for industrial scalability. Here, a series of lignin-based vitrimers built on dynamic acetal covalent networks with a gel content exceeding 95% were successfully prepared in a one-pot, thermally activated, and catalyst-free “click” addition of softwood kraft lignin (SKL) to poly(ethylene glycol) divinyl ether (PDV). The variation of the content of lignin from 28 to 50 wt % was used to demonstrate that the mechanical properties of the vitrimers can be widely tuned in a facile way. The lowest lignin content (28 wt %) showed a tensile strength of 3.3 MPa with 35% elongation at break, while the corresponding values were 50.9 MPa and 1.0% for the vitrimer containing 50 wt % of lignin. These lignin-based vitrimers also exhibited excellent performance as recoverable adhesives for different substrates such as aluminum and wood, with a lap shear test strength of 6.0 and 2.6 MPa, respectively. In addition, recyclability of the vitrimer adhesives showed preservation of the adhesion performance exceeding 90%, indicating a promising potential for their use in sustainable circular materials.
Highlights
Cross-linked polymers, known as thermosets, are one of the most important classes of polymeric materials owing to their permanent covalent bond structure, which renders them with outstanding mechanical properties and resistance to solvents and heat, both of which are of broad importance in industrial applications.[1]
Our approach to lignin-based vitrimers starts with the preparation of the lignin-based acetal dynamic networks through a thermal, catalyst-free “click” addition of softwood kraft lignin (SKL) to poly(ethylene glycol) divinyl ether (PDV) (Figure 1)
The traditional way of acetal formation by this route involves the use of strong acids as a catalyst under mild reaction conditions.[57−59] we decided to avoid acid catalysis to simplify our approach and avoid possible acid traces remaining after the synthesis that could catalyze the degradation of acetals, severely affecting the stability of the covalent crosslinking network
Summary
Cross-linked polymers, known as thermosets, are one of the most important classes of polymeric materials owing to their permanent covalent bond structure, which renders them with outstanding mechanical properties and resistance to solvents and heat, both of which are of broad importance in industrial applications.[1]. Generate synthons able to react and form dynamic covalent networks, in addition to the use of catalysts to trigger the reversibility of dynamic covalent bonds.[13,37,40−42] This preprocessing or fractionation of natural polymers, together with the need of catalysts, hinders their scalability toward industrial processes and limits their potential applications This present scenario poses an important challenge to avoid the chemical or physical pre-modification and functionalization of renewable raw materials that are available in a sufficient scale and at a low cost for industrial end uses. We demonstrate the lignin-based vitrimers as recoverable adhesives for metal and wood substrates, with preservation of adhesive strength exceeding that of benchmark materials
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