Waterborne polyurethane adhesives grafted with carbon dots for enhancing dispersion, cohesion, and adhesion

Abstract

Carbon dots (CDs) possess abundant functional groups on their surfaces and can be easily modified, allowing for covalent bonding with waterborne polyurethane (WPU) to create eco-friendly adhesives that exhibit both high cohesion and strong adhesion. However, as a type of nanofiller, CDs often aggregate, hindering effective dispersion. To address this dispersion challenge and enhance the properties of WPU adhesives, CDs were chemically grafted onto the macromolecular chains of WPUs using a silane coupling agent (KH550) as a linking agent. This method improved the adhesion strength of the resulting WPU nanocomposite adhesives, demonstrating the scientific validity of the approach. The successful grafting was confirmed through Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy. Optimizing the material ratios between the silane coupling agent and CDs significantly reduced CD aggregation in the WPU adhesives, as shown by scanning electron microscopy images that indicated efficient dispersion of the CDs. Malvern particle size analysis and thermogravimetric analysis revealed that when the CDs-to-KH550 mass ratio was 1:10, the water dispersion particle size of silane-modified CDs (KCD) decreased from 447.7 nm to 160.0 nm, and the total weight loss reduced from 53.07 % to 35.55 %. Furthermore, the adhesion properties of WPU adhesives modified with KCDs (KCD-WPU) were notably enhanced. The U-peel strength of the KCD-WPU adhesive on the steel test sheet increased by 31.8 %–510 N m⁻¹ when the KCD content was 0.1 wt.%. The tensile strength of the KCD-WPU adhesive was 62.6 % greater than that of the original WPU adhesive. We proposed a mechanism for the improvement of properties in WPU adhesives grafted with a small amount of CDs. This research presents an effective solution to the agglomeration of CDs in WPU adhesives, thereby enhancing both cohesive and adhesive strength. This study lays the groundwork for designing high-performance WPU adhesives.