## 2 Understanding of Polyurethane with Water Expansion Properties
### Expansion Mechanism of Water-Expandable Polyurethane by Superabsorbent Polymers  
The polyurethane with water expansion properties contains the base resin (A-side) in liquid form and a superabsorbent polymer, and the curing agent (B-side) is thus stirred to form a membrane layer. The membrane layer is then cured through a urethane reaction to form a waterproofing layer, and the superabsorbent polymer distributed in the waterproofing layer expands as it absorbs water when the cured waterproofing layer comes into contact with moisture. When the superabsorbent polymer absorbs water, it traps it  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Title** & **Raw Materials** & **Type** & **Formula** & **Molecular Weight (g/mol)** & **Manufacturer** \\ \hline \multirow{8}{*}{**A-side**} & \multirow{2}{*}{isocyanate (TDI)} & Mixture & - & - & Covestro \\  & & (2,4-TDI;2,6-TDI = 8.2) & & & \\ \cline{2-5}  & EG (ethylene glycol) & Single Substance & C\({}_{2}\)H\({}_{6}\)O\({}_{2}\) & 62.06784 & Hanwha \\ \cline{2-5}  & bis (2-ethylhexyl) & \multirow{2}{*}{Single Substance} & C\({}_{24}\)H\({}_{38}\)O\({}_{4}\) & 390.6 & LG Chem \\ \cline{2-5}  & \multirow{2}{*}{antioxidant} & Tetrakis methane & & & \\  & Ocadecyl-3- & - & - & SONGWON \\  & propionate & & & & \\ \cline{2-5}  & mineral oil & - & - & - & Korea Petroleum \\  & & & & & Group \\ \hline \multirow{8}{*}{**B-side**} & \multirow{2}{*}{polyether polyol} & \multirow{2}{*}{-} & CH\({}_{3}\) & & \\  & & & 1 & & \\ \cline{1-1}  & & R-(O-CH\({}_{2}\)-CH) \({}_{n}\)-OH & & & \\ \cline{1-1} \cline{2-5}  & IPDA (isophorone & \multirow{2}{*}{Single Substance} & C\({}_{10}\)H\({}_{2}\)N\({}_{2}\) & 170.300 & Evonik \\ \cline{1-1}  & diamine) & & & & \\ \cline{1-1} \cline{2-5}  & bis (2-ethylhexyl) & \multirow{2}{*}{Single Substance} & C\({}_{24}\)H\({}_{38}\)O\({}_{4}\) & 390.6 & LG Chem \\ \cline{1-1}  & terophthalate & & & & \\ \cline{1-1} \cline{2-5}  & silica, Talc & - & SiO\({}_{2}\) & & \\ \cline{1-1} \cline{2-5}  & mineral oil & - & - & - & Korea Petroleum \\ \cline{1-1} \cline{2-5}  & bismuth & - & - & - & Jinyangchem \\ \cline{1-1} \cline{2-5}  & superabsorbent & - & - & - & Hanwha \\ \cline{1-1} \cline{2-5}  & polymer & - & - & - & \\ \cline{1-1} \cline{2-5}  & zeolite & - & - & - & \\ \hline \multirow{8}{*}{antioxidant} & Tetrakis methane & & & & \\ \cline{1-1}  & Ocadecyl-3- & - & - & SONGWON \\ \cline{1-1}  & propionate & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: The chemical composition of raw materials.  
Figure 3: The water absorption principle of superabsorbent polymers.  
and expands the volume of the polymer particles due to a repulsion between negative charges occurring when the volume of the polyurethane resin surrounding it also expands simultaneously due to the expansion pressure caused by the volume expansion of the superabsorbent polymer [15]. A conceptual diagram illustrating the expansion mechanism of the water-expandable polyurethane is shown in Figure 4.  
Application of Water-Expandable Polyurethane and the Waterproofing Mechanism in the Event of Damage to the Waterproofing Layer  
Figure 5 shows water-expandable polyurethane applied onto the cracked area of the sheet material to fill the crack and prevent (stop) water leakage, as the water-expandable polyurethane expands into a gel when it comes into contact with water.  
Figure 4: The expansion mechanism of polyurethane with water expansion properties.  
Figure 5: Cont.  
The development of polyurethane with water expansion properties is to improve the structural vulnerability of the joint between sheet materials that arises from the composite waterproofing method combining the membrane and sheet materials. Meanwhile, a general defect in sheet-based composite waterproofing occurs at the joint between sheets, and the use of water-expandable polyurethane as a reinforcing material for the sheet-based waterproofing joint makes it possible to stop water leakage due to the water expansion properties of the reinforcing material.