Environmental stimulant-responsive hydrogels based on polyethylene glycol-derived polymer for underwater bonding and extraction of fragile wooden relics: Synthesis, characterization, and preliminary application

It has become widely acknowledged that certain underwater cultural relics, in a state of severe decay, should be salvaged for further protection. Underwater temporary reinforcement materials provide the possibility for such protection. These materials can achieve both the reinforcement of cultural relics underwater, as well as their reversible removal after extraction, without causing damage to the cultural relics in either process. In this context, two kinds of hydrogels based on polyethylene glycol-derived polymers were developed in this study. They are denser than water and can undergo gelatinization by UV-induced or oxidant-induced reactions in the underwater environment. Additionally, they possess enough mechanical strength to provide the necessary support in the bonding process. Both hydrogels have been successfully used to extract fragile underwater archaeological wooden artifacts in situ under laboratory conditions. The experimental results revealed that, in the case of UV-induced hydrogels, the compression strength was 115 kPa with a tensile strength of 20 kPa. For oxidantinduced hydrogels, the compression strength was 382 kPa, while the tensile strength was 8.2 kPa. During the actual extraction processes, the UV-induced hydrogel exhibited a lower water absorption ratio, maintained a more stable mechanical property underwater, and was easier to remove after extraction. On the other hand, the oxidant-induced hydrogel could complete the gelation process quickly through a rather convenient operation; however, it tended to absorb surrounding water and swell, thereby reducing its mechanical property. These two methods would be adapted to different underwater archaeological sites as convenient, environmentally friendly, and cost-effective novel methods for extracting fragile underwater wooden relics.