Glue that can be used in water? Get a new underwater adhesion skill!

Produced by: Science Popularization China

Author: Qin Chenxi (Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences)

Producer: China Science Expo

In the past, during the Chinese New Year, families would use homemade paste to paste Spring Festival couplets, but the couplets pasted in this way are generally not firm and are very easy to fall off once soaked by rain. In addition to paste, there are many adhesive materials in daily life, such as tire repair glue and transparent tape. During use, the elders often tell us to wipe the water on the surface before sticking, otherwise it will not stick firmly. This shows that water on the surface will weaken the interfacial adhesion, which is actually a kind of bonding failure caused by the destruction of the adhesion interface by water molecules.

couplet

(Photo source: Veer Gallery)

Adhesion is indispensable to human life. Adhesion-related materials and technologies have been applied to various industries, such as aerospace, aviation, marine mechanical engineering (coating adhesion, adhesives), intelligent robots (reversible adhesion and sensing devices), wearable devices (electronic skin) and biomedicine (wound dressings, bone adhesion and rapid hemostasis). However, for conventional adhesives, it is difficult to ensure good adhesion once in a humid or water-containing environment. This is mainly because the water film between the adhesive and the substrate hinders the full contact between the two and the formation of intermolecular interactions. Therefore, removing this layer of interfacial water becomes the key to achieving super strong underwater adhesion.

How to remove it? To date, researchers have developed a variety of different water removal methods, such as through extrusion, hydrophobic repulsion and adsorption. Specifically, researchers remove interfacial water and promote interfacial contact based on the fabrication of surface micro-nano structures and the application of preload. The hydrophobic segments in the adhesive polymer have good hydrophobicity and can effectively remove interfacial water through hydrophobic repulsion. In addition, the use of water-absorbing fillers (including inorganic substances and hydrophilic polymers) can also effectively remove interfacial hydration films. However, a single physical dehydration method cannot completely remove interfacial water, especially the hydrated water between the adhesive and the substrate surface, so it is difficult to achieve high-strength underwater adhesion.

What method can effectively and thoroughly remove interfacial water and ensure good adhesion of the interface?

Adhesive Applications

(Photo source: Veer Gallery)

Recently, the team led by Researcher Zhou Feng from the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, proposed a physical and chemical coupled method for removing interfacial water film, which includes dehydration processes at multiple scales.

Firstly, based on the excellent wettability of the adhesive, physical replacement of interfacial water is achieved at the millimeter scale. Secondly, the air film formed by the chemical reaction between the isocyanate fragments in the adhesive and water can achieve physical shielding of interfacial water at the micrometer scale. Finally, the consumption of interfacial bound water is achieved based on chemical reactions at the molecular scale.

Different from the single physical dehydration method, this physical and chemical synergistic approach can effectively remove water from the macroscopic interface to the microscopic bound water, ensuring the excellent underwater adhesion performance of the adhesive. Based on this method, the researchers developed a wet adhesive, which is different from the glue used in daily life (once it encounters water, it immediately becomes a hard plastic without stickiness). This glue can be used directly underwater.

Once the wet adhesive is squeezed onto the surface of the object to be adhered, it will quickly infiltrate and spread; at the same time, a circle of bubbles will be generated around the adhesive, which will help to drain the water on the surface and provide conditions for full contact between the glue and the adhesive interface; as time goes by, the fluidity of the wet adhesive disappears, and its cohesive energy increases. Based on the fully formed interface contact state and interaction in the early stage, the adhesive will firmly bond the two target contact surfaces together. The adhesive has very excellent adhesion properties, with an adhesion strength of 1600kPa in water (fresh water), equivalent to an adhesion area the size of a palm, and can hang a weight of 1.6 tons (equivalent to the weight of an ordinary car).

Compared with fresh water, seawater has a more complex composition. The salt and high pH in seawater have a strong destructive effect on adhesives, which can easily lead to hydration, dissolution and failure of adhesives. However, in seawater, the adhesive still maintains high adhesion strength and stable adhesion performance due to its functional groups' inertness to water, stable cross-linking network and deep dehydration mechanism.

What is the use of this adhesive? In the context of building a strong maritime nation, my country is gradually moving from "light blue" to "deep blue".

Adhesives are widely used in various underwater projects, such as bonding and sealing of hulls, underwater pipelines, and underwater robot components. However, in harsh seawater environments, the bonding interface is easily corroded by seawater and fails, which will bring immeasurable losses. Therefore, how to detect and repair it in time is one of the most important links in dealing with emergencies.

This adhesive can be directly operated underwater, deeply dehydrated, has high-strength adhesion, and has a self-detection function. It has very important applications in the field of underwater engineering. The adhesive with added carbon nanotubes has sensing properties, and its safety status can be monitored based on the conductivity feedback of the adhesive itself. For example, if a ship is suddenly damaged during navigation, the conductivity of its adhesive material will suddenly change, and we can take immediate measures to repair the damaged area with water. In addition, with its good bonding performance and adaptive cross-linking characteristics without external energy input, this adhesive is also expected to be used for underwater sand fixation.

Schematic diagram of underwater adhesive dehydration process and adaptive adhesion

(Image source: Reference [1])

Dynamic process of air film removal of interfacial water

(Image source: Reference [1])

The adhesion research results have great application potential in underwater sealing and adhesion fault detection, and provide support for the development of simple, convenient and fast repair technology. It is expected to achieve rapid and effective solution to leakage problems during ship movement. We are currently jointly developing related products with user units.

In underwater adhesion, interfacial water is a very important influencing factor, so multi-scale dehydration is the prerequisite for achieving strong interfacial adhesion. At present, the research on multi-scale dehydration methods is still in its early stages, and more dehydration methods need to be further explored. We believe that the research results of this part will provide theoretical basis and technical support for the development of high-performance adhesive materials for different applicable environments.

References:

[1]Qin C, Ma Y, Zhang Z, et al. Water-assisted strong underwater adhesion via interfacial water removal and self-adaptive gelation [J]. Proceedings of the National Academy of Sciences, 2023, 120(31), e2301364120.