The influence of synthetic amine and biobased imine-based sizing on the interface of carbon fibre-reinforced bio-epoxies

Composite materials are well known to be unsustainable, partly due to the use of fossil-based resins, while their performance strongly depends on the adhesion between the fibre reinforcement and the matrix. The quality of the adhesion is influenced by the surface properties of the fibres and by sizing agents which are applied to the fibres. This paper compares a bio-based imine sizing agent and a synthetic amine sizing agent, evaluating their influence on both fibre surface properties and overall composite performance when deployed in a bio-based epoxy resin (SR Infugreen 810). Multiple surface characterisation techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact angle measurement, were conducted to evaluate the surface morphology, roughness and wettability of sized carbon fibres. To establish their compatibility or lack thereof with Infugreen 810, interlaminar shear strength (ILSS) testing and dynamic mechanical thermal analysis (DMTA) were conducted. These results show that the bio-based sizing significantly reduced the roughness of the carbon fibre surface compared to the synthetic one. Additionally, no chemical interactions were observed between the bio-based imine sizing and the bio-epoxy resin due to the absence of nucleophilic groups in the sizing molecular structure. While for the synthetic sizing, the primary amine group is reacting with the epoxide group, which in turn impacts adhesion strength and mechanical properties. The bio-based sizing led to a 12.5 % reduction in interlaminar shear strength (ILSS) compared to the synthetic sizing, along with a 6.7 % reduction in activation energy. These findings provide valuable insights for optimizing fibre surface treatments to enhance the performance of sustainable composite materials.