A plant-based adhesive can repair itself if damaged and could be more green than conventional gluesSmart glues can now be made from soybean oil and pine pulp. The adhesive polymer resins are called vitrimers and have versatile properties including shape-memory and self-healing abilities. The advance in environmentally-friendly ‘green chemistry’ is reported by researchers at the Chinese Academy of Forestry and Nanjing Forestry University in the journal Materials & Design.
The advantage of vitrimers is that they can maintain their rigid structure at a designed ‘service temperature’, but then be re-bonded, reshaped or reprocessed for other applications when heated above a ‘transition temperature’. The temperature changes trigger rearrangements in the chemical bonding within the vitrimer, sometimes assisted by the presence of a catalyst.
Plant-based vitrimers have been prepared previously, but it has proved difficult to make them with sufficiently high transition temperatures and strengths. The transition temperature must be significantly higher than any temperature the material might experience in its intended use.
The new and fully bio-based vitrimer has a transition temperature of 65 °C, well above that found in room temperature applications, and it also displays impressively high tensile strength.
“This achievement was challenging, due to the flexible molecular chains of vegetable oils,” explains He Liu of the research group.
The researchers considered a derivative of soybean oil called epoxidized soybean oil (ESO) as a promising monomer material to be linked into a polymer. ESO is available at low cost and its chemical epoxy groups readily react in ‘transesterification’ reactions with a variety of other chemical groups.
In the procedure developed by the researchers, other suitable reacting groups were found in a waste material from pine pulp processing and in the exudation of pines and conifers called rosin. The rosin was reacted with fumaric acid to make the second monomer, fumaropimaric acid FPA. A zinc-based catalyst then stimulated the monomer units to combine to form chains of the ESO-FPA vitrimer. Crucially, the FPA contributes a rigid chemical ring structure that brings desired strength into the final product.
When the researchers scratched samples of the vitrimer, the scratches could repair themselves after heating to 180 °C. This self-healing ability is attributed to the chemical bonds in the adhesive becoming reconfigured to match the original structure. They also tested the possibility of recovering from more drastic damage by using the vitrimer to bond metal sheets together then ripping them apart. This simulation of a failed joint was successfully re-bonded by heating and rejoining in the presence of the catalyst.
Deformations in shape could also be removed and the original shape recovered by a process of heating then cooling.
Easy recycling and reuse is an additional advantage of the ability of the chemical structure to be readily dismantled and then reformed into new shapes.
“This all opens up many new opportunities for using these materials,” says Liu.
The team still hope to address some chemical challenges in order to improve the process and move toward eventual commercialisation. “We would like to develop a catalyst-free process,” says Liu, while also targeting increased strength.
Article details:
Liu, He. et al.: “A fully bio-based epoxy vitrimer: Self-healing, triple-shape memory and reprocessing triggered by dynamic covalent bond exchange,” Materials & Design (2020).