Starch-based reversible adhesive: Effect of off-stoichiometric ratios on transesterification vitrimer properties

Bio-based epoxy vitrimers are gaining attention as sustainable alternatives to traditional epoxies due to renewable starting materials and recyclability, enabling material circularity. Transesterification, a dynamic bond reaction mechanism in vitrimers, relies on ample hydroxy and esters groups in the polymer network to rearrange under moderate conditions. To reduce reliance on petroleum we investigated starch, a natural feedstock rich in hydroxy groups. Ester bond densities were adjusted with an ester containing crosslinker (pentaerythritol tetrakis(3-mercaptopropionate) (PETMP)) at various epoxy to crosslinker molar ratios (1:0.7, 1:1, 1:1.25 and 1:1.5) with TEMPO as the catalyst. The reformation efficiency of the vitrimer increased to 239 % from 31 % when the ratio changed from 1:1 to 1:1.5. Essential dynamic behavioral parameters (dynamic bond exchange activation energy (E_a(d)), Arrhenius prefactor (τ₀), and freezing transition temperature (T_v)) were analysed. Adding excess amount of crosslinker decreased the T_g, crosslink density, E_a(d) and T_v of the starch-based epoxy vitrimer. Adhesion tests on birch veneer found that a 1:1.5 ratio had the highest adhesion performance at 3.02 ±0.7 MPa with an adhesion recovery as high as 91 % averaging 61 +/- 23 %. These insights into density of dynamic bonds and active functional groups will contribute to developing new bio-based vitrimers with designed circularity.