A recent study was published in which via a thermomechanical "thiol-epoxy" click process, biocomposite and biodegradable epoxy vitrimer adhesives (ESOBV) with active borate ester linkages were produced from epoxidized soybean oil (ESO) and a dithiol borate ester curing agent (BDB). This research was published in European Polymer Journal.
Study: Strong and recyclable soybean oil-based epoxy adhesives based on dynamic borate. Image Credit: 271 EAK MOTO/Shutterstock.com
Importance of Polymer Adhesive
Polymer adhesives, which are used to join two or more homogeneous or heterogeneous items, are widely employed in a variety of sectors, such as automotive, architecture, electronics, and aviation. They are available in a variety of shapes and viscosities, or flow rates, and may be used to connect polymers, fiberglass, hardwood, granite, and metals.
Because of its exceptional adhesiveness, physical qualities, and damage tolerance, epoxy glue is one of the most extensively used sealants. Most epoxy resins, on the other hand, are made from petroleum compounds like bisphenol A glycidyl ether. Their basic ingredients are non-renewable, and the resulting goods are unable to decompose and recycle in even the mildest of conditions, resulting in significant waste contamination.
Epoxy adhesive substances are less impacted by water and steam than other matrix materials and have excellent corrosion resistance. Their primary drawbacks are the relatively expensive cost, extended cure time, and challenging handling.
Introduction to Vitrimer Synthesis
Vitrimer, a newly developed polymer, offers a novel approach to recycling existing epoxy thermosetting polymers. Natural feedstocks such as rosin acid, glycerol, cellulose, and fructose are used to make biobased epoxy trimers.
Because of their abundant availability, intrinsic degradability, and low cost, vegetable oils are one of the most widely used biomass resources in the manufacturing of biobased polymers. Several epoxy vitrimers have been made using epoxidized vegetable oils. Esterification of epoxidized soybean oil (ESO) with citric acid or fumaropimaric acid resulted in fully biobased vitrimers. By treating ESO with a vanillin-derived hardener, a vitrimer based on imine bonds was created.
In the latest research, a dithiol monomer (BDB) containing borate ester linkages was produced via a condensation procedure to give the ESO-based adhesives recyclability and reinforcement phase capability.
As all of the ESOBV began to cure at 117°C and showed a peak at 153°C on the DSC curves, a post-curing temperature of 155°C was chosen to finish the resin curing. It was also discovered that the molar ratio of sulfhydryl to epoxy had no discernible influence on the peak temperature.
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Due to the obvious molecular combination of flexible fatty acid chains from ESO and stiff benzene rings from BDB, all ESOBV adhesives had outstanding mechanical properties and breakage dilatation. The sealants in this study had much better tensile properties (18.74–25.14 MPa) than ESO thermosets treated using standard hardeners such as maleic anhydride. The fracture duration of the adhesives dropped dramatically as the BDB concentration rose, whereas the structural rigidity increased.
All the adhesives showed excellent thermal stability when the temperature was below 230°C. Due to structural reconfiguration, the stress relaxation time of ESOBV-1 reduced as the temperature was increased from 80 to 200 degrees Celsius.
The lap-shear strength (LS) of adhesives was also considerably increased. Due to the extreme frangibility of glass, the LS of a specimen cannot be evaluated. With the exception of polypropylene plastic, both liquid and recycled adhesives demonstrated good adhesion properties. Bamboo had a substantially higher LS of liquid adhesive (7.20 MPa) than wood (4.12 MPa), steel (5.67 MPa), or plastic (5.67 MPa) (0.49 MPa).
Future of Epoxy Adhesives
By 2028, the worldwide epoxy adhesive industry is estimated to be worth $10.5 billion. Because of the increased focus on the use of lighter materials in the automobile and aerospace industries, the epoxy adhesive sector is expected to grow significantly in the future. Adhesives allow for a decrease in overall product weight, which has been more important in the automobile and aerospace sectors in recent years.
In short, ESO was used to create a biobased epoxy vitrimer glue that had dynamical borate ester linkages. The effective manufacturing of BDB and merging between ESO and BDB was confirmed by FTIR and NMR studies. By varying the amount of BDB in the vitrimers, the architecture and characteristics of the vitrimers were altered.
Li, C. et al., 2022. Strong and recyclable soybean oil-based epoxy adhesives based on dynamic borate. European Polymer Journal, 162(5). 110923. Available at: https://www.sciencedirect.com/science/article/pii/S0014305721006571?via%3Dihub