Analysis of Expansion Characteristics According to Deterioration Conditions for Superabsorbent Polymer Content of a Polyurethane Waterproof Material with Water Expansion Properties

Abstract

Polyurethanes with water expansion properties are manufactured by incorporating polyurethane resins and superabsorbent polymers. This study aimed to present an optimal mixing ratio of a superabsorbent polymer that can express the effective waterproofing performance of a polyurethane material with water expansion properties. The evaluation results confirmed that the waterproofing performance was exerted at a superabsorbent polymer mixing ratio of more than 10% and an expansion ratio of 150% or more, which were then set as the minimum standard value for ensuring waterproofing performance. In addition, chemical erosion (by hydrochloric acid, sulfuric acid, nitric acid, alkali, and sodium chloride), freeze-thaw, and wet-dry cycles were set as deterioration conditions to determine the optimal mixing ratio for the superabsorbent polymer under conditions of use, and the maximum expansion ratio was evaluated after pretreatment under the deterioration conditions. It was found that, based on the minimum standard value for the waterproofing performance set above, the superabsorbent polymer mixing ratio that can ensure an expansion ratio of 150% or more under all deterioration conditions was 14%.

1. Introduction

In general, waterproofing materials with water expansion properties used in building construction include bentonite waterproofing membranes and acrylate waterproofing materials. These materials are processed and manufactured separately to be applied as waterproofing materials using the water expansion properties of the materials themselves. This has the advantage of utilizing the water expansion properties of the materials, but poses a limitation for ensuring the mechanical properties and chemical resistance performance of the waterproofing materials required under deterioration conditions.

Figure 1 shows the problems of bentonite and acrylate waterproofing materials. The bentonite waterproofing material exhibits problems with drying shrinkage, insufficient expansion in salt water or alkaline environments, and release after pre-condensation due to water absorption during the construction process [1]. The acrylate waterproofing material also exhibits problems with drying shrinkage, gelation due to a reaction with water, and poor resistance to external force [2].

Recently, in order to address these problems, a technology that combines polyurethane resins with superabsorbent polymers with water expansion properties (hereafter referred to as water-expandable polyurethane) has been applied as an alternative technology. Water-expandable polyurethane is used to seal the damaged section, since it changes into a gel (swelling) when water comes into contact with the damaged area [3,4]. Therefore, water-expandable polyurethane is used as a liquid membrane waterproofing material and applied as a reinforcing material for joints or areas vulnerable to leaks [5].

The core of water-expandable polyurethane is the “swelling rate” that can exert its performance so that the waterproofing effect appears when damage occurs. The swelling rate is determined by the content of the superabsorbent polymer in the polyurethane resin. The superabsorbent polymer has been widely applied as a general material used in daily necessities such as diapers, sanitary napkins, and ice packs [6]. Accordingly, the quality of the products and the method for using superabsorbent polymers suitable for the purpose of the product (mixing ratio, etc.) have been stabilized in industries where superabsorbent polymers are already in use. New materials are being developed using this material in the waterproofing industry as related to construction [7].

The water-expandable polyurethane is basically based on a polyurethane resin (A-side) and includes a superabsorbent polymer in the curing agent (B-side). It is a material designed to ensure that the superabsorbent polymer is densely distributed throughout the polyurethane membrane when it is stirred on the A-side. Therefore, the waterproofing layer has the mechanical and chemical properties of the polyurethane and can additionally secure water expansion properties when it comes into contact with water. This characteristic allows for the same water expansion property and waterproofing function, while solving the problem of reduced waterproofing performance due to durability degradation, such as drying shrinkage, freeze-thaw, and chemical erosion, in the existing water-expandable materials such as bentonite or acrylate [8,9].

Based on these advantages, water-expandable polyurethane is increasingly used as a waterproofing material; however, the leakage problem has not yet been adequately solved using the existing method [10]. This is due to a lack of research on the responsiveness to various deterioration conditions in the actual field, as well as a lack of related evaluations and analyses data regarding the expansion ratio, as water-expandable polyurethane was the only application of a superabsorbent polymer available in the waterproofing field. This suggests that the waterproofing effect of the current water-expandable polyurethane is not suitable for various deterioration conditions. Thus, there is a need to present an optimal mixing ratio that can exhibit the waterproofing effect under actual deterioration conditions.

This study analyzed how the swelling rate according to the mixing ratio of the superabsorbent polymer used in water-expandable polyurethane changes depending on the deterioration conditions, and attempted to propose the optimal mixing ratio of superabsorbent polymers enabling the water-expandable polyurethane waterproofing material to exhibit and sustain its waterproofing performance in various deterioration environments.

2. Understanding of Polyurethane with Water Expansion Properties

2.1. Fabrication of Superabsorbent Polymers and Water Absorption Principle

The superabsorbent polymer incorporated into the polyurethane with water expansion properties is a powdered polymer with an average particle size of 10–220 µm, a pH of 5.5–6.5, and a specific gravity of 0.6–0.8. Unlike general water-soluble polymers, it does not dissolve in water, it has the ability to absorb water in amounts up to hundreds of times its own weight, and it can retain water even under external pressure [11]. The superabsorbent polymer is fabricated by the polymerization of acrylic acid and partially neutralized acrylic acid through an initiator. The properties of the superabsorbent polymer are summarized in

Table 1. The properties of superabsorbent polymers.

Item	Unit	Property
Particle size	µm	10–220
pH	-	5.5–6.5 (at 5.0 g/L 0.9% NaCl solution)
Specific gravity	g/cm2	0.6–0.8 (apparent density)
Decomposition temperature	°C	200 °C or higher

, and the molecular structure is shown in Figure 2 [12].

As shown in Figure 3, the superabsorbent polymer is a material with a network structure. It has a large number of hydrophilic groups, such as carboxyl (-COOH), at the ends and absorbs water molecules through ion-water interactions via ionization. The principle is that due to the network structure of the carbon chain, the absorbed water molecules are trapped in it, and the polymer expands further due to the repulsion of negative (-) charges to absorb more water molecules [13,14].

2.2. Chemical Composition of Raw Materials

The polyurethane with water expansion properties takes the form of a liquid and is a two-component coating material divided into a base resin and a curing agent. The raw material composition of the water-expandable polyurethane is summarized in

Table 2. The chemical composition of raw materials.

Title	Raw Materials	Type	Formula	Molecular Weight (g/mol)	Manufacturer
A-side	isocyanate (TDI)	Mixture (2,4-TDI:2,6-TDI = 8:2)	-	-	Covestro
	EG (ethylene glycol)	Single Substance	C2H6O2	62.06784	Hanwha
	bis (2-ethylhexyl) terephthalate	Single Substance	C24H38O4	390.6	LG Chem
	antioxidant	Tetrakis methane Octadecyl-3- propionate	-	-	SONGWON
	mineral oil	-	-	-	Korea Petroleum Group
B-side	polyether polyol	-		-	MSNS
	IPDA (isophorone diamine)	Single Substance	C10H22N2	170.300	Evonik
	bis (2-ethylhexyl) terephthalate	Single Substance	C24H38O4	390.6	LG Chem
	silica, Talc	-	SiO2		TOSOH
	mineral oil	-	-	-	Korea Petroleum Group
	bismuth	-	-	-	Jinyangchem
	superabsorbent polymer	-	-	-	Hanwha
	zeolite	-	-	-	Baltimore Innovations
	antioxidant	Tetrakis methane Octadecyl-3-propionate	-	-	SONGWON

.

2.3. 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 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.

2.4. 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.

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.

3. Test Methods and Result for Analysis of Swelling Property of Water-Expandable Polyurethane

The evaluation of the swelling rate and waterproofing performance according to the superabsorbent polymer content (0–20%) was carried out to determine the optimal mixing ratio of the superabsorbent polymer for the waterproofing performance of polyurethane with water expansion properties. The superabsorbent polymer mixing ratio and the volume expansion ratio when the superabsorbent polymer exerts its waterproofing performance were confirmed based on the evaluation results. Next, the volume expansion ratio of the water-expandable polyurethane, depending on the superabsorbent polymer content, was evaluated according to the application of deterioration conditions. Based on the volume expansion ratio for each deterioration condition, the volume expansion ratio at the point in time of the above waterproofing performance expression was compared and analyzed to propose the optimal mixing ratio of the superabsorbent polymer.

3.1. Swelling and Leakage Test Methods and Results

3.1.1. Swelling Rate Test Methods and Result

(1) Swelling Rate Test Methods

① Specimen Preparation

To identify the volume expansion ratio of the polyurethane with water expansion properties according to the superabsorbent polymer content, polyurethanes were prepared by adjusting the content of the superabsorbent polymer to be constant. In order to clearly confirm the change of water expansion properties according to the content, the adjustment of the superabsorbent polymer content was planned to increase by 1% of the weight of the curing agent (B-side), as shown in

Table 3. The superabsorbent polymer mixing conditions.

Division	Superabsorbent Polymer Content
Sample number	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	#11	#12	#13	#14	#15	#16	#17	#18	#19	#20
SAP Content (%)	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20

. A total of 20 test specimens were fabricated by increasing the superabsorbent polymer content by up to 20%.

The test specimen was fabricated by stirring the base resin (A-side) and the curing agent (B-side) for 5 min with the use of a mechanical stirrer, applying the coating material with a thickness of 2 mm, and curing it in a constant temperature chamber at 20 ± 2 °C for 168 h.

Table 4. The specimen fabrication conditions.

Division	Condition
Mixing ratio (A-side:B-side)	1:4.25
Mixing method	Mechanical stirrer, 5 min stirring
Coating thickness	2 mm
Curing	(20 ± 2) °C, 168 h

and Figure 6 show the specimen fabrication conditions and status.

② Calculation of Swelling Rate

The swelling rate test on the prepared specimen was conducted in accordance with the test method specified in a KS M 3736 Waterproofing sheet with expandable bentonite. The test was done by taking about 2 g of the sample, putting it in a graduated cylinder (100 mL), and filling 50 mL of distilled water up to the 50 mL scale mark of the graduated cylinder containing the sample by means of a burette. Then, the volume of the water remaining in the burette is set as the initial volume (mL). After that, enough distilled water is added to the graduated cylinder to expand it for 120 h, and the distilled water is then removed to measure the volume after expansion in the same way as the initial volume measurement. The swelling rate is calculated using Equation (1), and the status is shown in Figure 7 [16].

$${\mathrm{V}}_e=\frac{V_1-V_2}{V_1}\times 100$$

(1)

where: V_e—swelling rate (%); V₁—initial volume (mL); V₂—volume after expansion (mL).

(2) Swelling Rate Test Result

Table 5. The swelling rate evaluation results according to the superabsorbent polymer content.

Division	Superabsorbent Polymer Content
Sample number	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	#11	#12	#13	#14	#15	#16	#17	#18	#19	#20
Swelling Rate (%)	15	23	52	61	74	97	106	125	141	150	171	186	196	210	229	245	262	275	280	298

shows the swelling rate evaluation results of the water-expandable polyurethane according to the superabsorbent polymer content.

The evaluation results showed that the swelling rate of the water-expandable polyurethane increased with an increase in the superabsorbent polymer content.

3.1.2. Leakage Test Methods and Result

(1) Leakage Test Methods

① Specimen Preparation

In accordance with the method shown in Section 3.1.1, Specimen Preparation, the water-expandable polyurethane is applied for each superabsorbent polymer mixing ratio, and the cured water-expandable polyurethane specimens are prepared by dividing them into two 200 × 70 × 2 mm sheets according to the superabsorbent polymer mixing ratio.

② Leakage Test Method

The prepared specimen is placed between two glass plates of the same size as the specimen and fixed to make a total of two specimens. The two specimens are placed 1 mm apart (to simulate a crack width of 1mm), butted together, and fixed to create a crack environment. A plastic cylinder of ∅100 mm and 100 mm in height is installed on the top, creating one specimen. The specimen fabrication process is shown in Figure 8.

The prepared specimen is immersed in distilled water for 24 h, only the water from the surface of the specimen is then removed, and the cylinder of ∅100 mm × 100 mm is filled with distilled water to a height of 80 mm. After that, the presence of leakage is checked through cracks in the lower part of the specimen. It is determined that if leakage occurs within 60 min from the time of filling with distilled water, there is no waterproofing performance, and if leakage dose not occur after 60 min, waterproofing performance exists. The test is terminated when the waterproofing performance exerts as the superabsorbent polymer content increases, and the test status is shown Figure 9.

(2) Leakage Test Result

Figure 10 shows the waterproofing performance evaluation results of water-expandable polyurethane according to the superabsorbent polymer mixing ratio.

The evaluation results confirmed that the waterproofing performance was not exerted in the samples incorporating 1–9% superabsorbent polymer, but the waterproofing performance was exerted in the samples with a superabsorbent polymer mixing ratio of 10% or higher. This suggests that the expansion ratio of the material increases to the simulated crack area in the samples with a superabsorbent polymer mixing ratio of more than 10%, and the simulated crack area is thus water-tightened/waterproofed with sufficient expansion pressure. On the other hand, as the expansion ratio is low in the samples with a superabsorbent polymer mixing ratio of less than 10%, the water-expandable urethane does not sufficiently expand to the simulated crack area, and thus leakage occurs through the crack.

3.1.3. Analysis of Evaluation Results

The above leakage test results showed that the water-expandable polyurethane incorporating more than 10% superabsorbent polymer exhibited waterproofing performance. In addition, the swelling rate evaluation results confirmed that the expansion ratio of a superabsorbent polymer mixing ratio of 10%, which exhibits waterproofing performance, was 150%. That, is, it can be confirmed that the waterproofing performance is exerted only when the minimum expansion ratio of the water-expandable polyurethane is 150% or higher. Figure 11 shows it.

3.2. Analysis of Expansion Characteristics of Water-Expandable Polyurethane under Deterioration Conditions

The above evaluation results were obtained under no treatment condition and without deterioration conditions. However, it is expected that if the deterioration condition is applied, the expansion ratio of the water-expandable polyurethane will decrease due to material deterioration. Therefore, in order to derive the optimal mixing ratio of the superabsorbent polymer that meets the condition of a minimum expansion ratio of 150% for waterproofing performance derived from the above evaluation, the following swelling rate evaluation was carried out based on the deterioration conditions.

3.2.1. Swelling Rate Test Method and Results under Deterioration Conditions

The swelling rate test under deterioration conditions was conducted in the same manner as the Section 3.1.1 Swelling Rate Test Method. The deterioration of specimens complies with the following conditions.

(1) Specimen Preparation

In order to set the deterioration conditions of water-expandable polyurethane, a test environment was created in compliance with the chemical treatment conditions of specimens, or the hydrochloric acid, nitric acid, sulfuric acid, sodium chloride, and alkali treatment conditions specified for the KS F 4935 Sealer of Injection Type for Water Leakage Maintenance of Adhesive Flexible Rubber Asphalt Series (hereafter KS F 4935), obtained from Korean Industrial Standards (KS). The chemical treatment conditions specified in KS F 4935 are conditions that reflect the chemical environmental factors affecting waterproofing materials in the underground environment. If the water-expandable polyurethane is applied to the underground structures, it is possible to investigate the characteristics of change in the expansion ratio due to chemical influences [17]. In addition, the test environment was set up in accordance with the freeze-thaw test method specified in the KS F 2604 Standard Test Method for Frost Resistance of Exterior Wall Materials for Buildings (Freezing and Thawing Method), given that a freeze-thaw environment is created due to seasonal changes when the water-expandable polyurethane is applied to the above-ground structures such as rooftops [18]. Lastly, repeated wet-dry treatments were carried out to confirm the characteristics of change in the expansion ratio when subjected to environmental conditions that create a wet/dry environment due to repeated wet-dry cycles. Because there is no test method specified in KS for repeated wet-dry treatments, the test environment was created in accordance with the repeated wet-dry test method specified in the Detailed evaluation and Technical Review Operation Standards for the Selection of Waterproofing Method inside Waterworks Facilities in the Seoul Waterworks Authority [19]. The deterioration conditions of specimens by item are summarized in

Table 6. The deterioration conditions.

Pretreatment Item	Temperature and Heated Welding Speed		Standard
Hydrochloric acid	2% hydrochloric acid, immersed for 168 h		KS F 4935
Nitric acid	2% nitric acid solution, immersed for 168 h
Sulfuric acid	2% sulfuric acid solution, immersed for 168 h
Alkali	solution of first-grade calcium hydroxide saturated in 0.1% sodium hydroxide solution, immersed for 168 h
Sodium chloride	10% sodium chloride solution, immersed for 168 h
Free-thaw		Freezing in air and thawing in water, 50 cycles	KS F 2604
	1 Cycle	Freezing at −20 ± 2 °C for 80 min, thawing (spraying) at 10 °C–30 °C for 20 min
Repeated wet-dry cycle		Repeated wet-dry, 50 cycle	Detailed Evaluation and Technical Review Operation Standards for the Selection of Waterproofing Method inside waterworks Facilities in the Seoul Waterworks Authority
	1 Cycle	Maintained for 1 h at 20 °C and 98% humidity; 1 h required to raise the temperature and humidity from 20 °C and 98% to 60 °C and 50% Maintained for 1 h at 60 °C and 50% humidity; 1 h required to lower the temperature and humidity from 60 °C and 50% to 20 °C and 98% Maintained for 1 h at 20 °C and 98% humidity; 1 h required to lower the temperature from 20 °C at 98% humidity to −10 °C Maintained for 1 h at a temperature of −10 °C; 1 h required to raise the temperature from −10 °C to 20 °C at 98% humidity

, and the pretreatment status is shown in Figure 12.

(2) Swelling Rate Test Method under Deterioration Conditions

The results of the swelling rate test conducted to analyze the expansion ratio of polyurethane according to the superabsorbent polymer content are summarized in

Table 7. The swelling rate test results under deterioration conditions according to the superabsorbent polymer content.

Sample Number	Mixing Ratio (%)	Swelling Rate according to Deterioration Conditions (%)
		Non-treatment	Hydrochloric Acid	Nitric Acid	Sulfuric Acid	Alkali	Sodium Chloride	Freeze-Thaw	Repeated Wet-Dry Treatment
#1	1	15	12	12	13	14	13	14	14
#2	2	23	18	19	18	20	20	21	21
#3	3	52	40	42	41	44	43	47	47
#4	4	61	46	49	47	51	50	54	54
#5	5	74	55	59	56	61	61	65	66
#6	6	97	72	76	73	81	80	84	85
#7	7	106	77	82	81	87	85	92	93
#8	8	125	91	97	95	102	100	108	108
#9	9	141	102	110	106	116	113	123	120
#10	10	150	108	116	111	122	120	128	127
#11	11	171	122	131	127	140	135	148	146
#12	12	186	133	142	138	149	149	159	158
#13	13	196	142	148	144	156	157	165	164
#14	14	210	152	158	155	169	168	180	175
#15	15	229	166	172	170	185	179	192	191
#16	16	245	175	187	178	196	195	205	208
#17	17	262	189	200	190	207	208	224	217
#18	18	275	198	209	203	219	218	230	230
#19	19	280	197	208	203	222	221	236	236
#20	20	298	213	221	215	236	235	251	250

and Figure 13.

The evaluation results confirmed that the swelling rate increased as the superabsorbent polymer content increased in all the specimens; however, the degree of expansion varied depending on the superabsorbent polymer mixing ratio. In the case of the untreated specimen, the swelling rate increased from 15% at a mixing ratio of 1% to 298% at a mixing ratio of 20%. Moreover, the swelling rate was 12% to 213% for the hydrochloric acid treatment, 12% to 221% for the nitric acid treatment, 13% to 215% for the sulfuric acid treatment, 14% to 236% for the alkali treatment, 13% to 235% for the sodium chloride treatment, 14% to 251% for the freeze-thaw treatment, and 14% to 250% for the repeated wet-dry treatment. The above results suggest that when the superabsorbent polymer is exposed to deterioration conditions, the absorption recovery ratio decreases to a certain extent, and the swelling rate does not recover to 100%. In addition, when analyzed based on the minimum swelling rate of 150% for water-expandable polyurethane to exert its waterproofing performance, the test specimens subjected to deterioration may not exert the waterproofing performance at a superabsorbent polymer mixing ratio of less than 13% under hydrochloric acid, nitric acid, and sulfuric acid treatment conditions. However, it can be confirmed that all the specimens subjected to deterioration can exert waterproofing performance at a superabsorbent polymer mixing ratio of 14% or higher. In addition, it was found that the degree of decrease in the swelling rate according to each deterioration treatment varies. Therefore, each evaluation was conducted regarding the degree of decrease in the swelling rate according to deterioration conditions when compared to the case of non-treatment. The evaluation results are summarized in

Table 8. The analysis of the degree of decrease in the swelling rate according to the deterioration conditions for each superabsorbent polymer content.

Mixing Ratio (%)	Swelling Rate (%)
	Non-Treatment	Hydrochloric Acid	Nitric Acid	Sulfuric Acid	Alkali	Sodium Chloride	Freeze-Thaw	Repeated Wet-Dry Treatment
2	100.0	80.0	83.2	83.8	90.0	87.1	94.9	96.0
4	100.0	76.9	82.5	80.2	87.9	86.5	92.5	91.8
6	100.0	76.1	80.9	79.2	85.0	83.0	89.8	90.4
8	100.0	75.1	80.0	77.4	83.8	82.8	87.9	89.1
10	100.0	74.4	79.3	75.9	82.9	82.8	88.0	89.0
12	100.0	73.9	78.8	75.4	83.1	82.3	86.6	87.4
14	100.0	72.8	77.6	76.5	81.8	80.2	87.0	87.7
16	100.0	73.2	77.2	76.1	81.3	79.8	86.6	86.3
18	100.0	72.1	77.7	75.0	82.6	80.3	87.1	85.0
20	100.0	71.9	77.5	74.0	81.4	80.0	85.1	84.7
22	100.0	71.6	76.5	74.5	81.9	78.9	86.6	85.2
24	100.0	71.4	76.2	74.3	80.0	80.3	85.5	84.9
26	100.0	72.7	75.3	73.3	79.8	80.1	84.4	83.8
28	100.0	72.5	75.1	73.9	80.4	79.9	85.9	83.5
30	100.0	72.3	75.0	74.4	81.0	78.1	84.0	83.3
32	100.0	71.4	76.3	72.8	80.0	79.5	83.9	84.8
34	100.0	72.0	76.2	72.7	79.0	79.4	85.4	82.9
36	100.0	71.9	76.0	74.0	79.6	79.2	83.6	83.5
38	100.0	70.3	74.4	72.4	79.5	79.1	84.3	84.2
40	100.0	71.6	74.3	72.3	79.3	78.9	84.1	84.0

and Figure 14.

3.2.2. Analysis of Evaluation Results

The analysis found that there is a difference in the degree of the decrease of the swelling rate according to the deterioration conditions when compared to the case of non-treatment. In the case of the hydrochloric acid treatment, the swelling rate was 80 to 70.3% compared to the case of non-treatment. Additionally, it was 83.2 to 74.3% for the nitric acid treatment, 83.8 to 72.3% for the sulfuric acid treatment, 90.0 to 79.3% for the alkali treatment, 87.1 to 78.9% for the sodium chloride treatment, 84.9 to 83.6% for the freeze-thaw treatment, and 96.0 to 82.9% for the repeated wet-dry treatment when compared to the case of non-treatment. Based on these results, it was confirmed that among the deterioration conditions, the hydrochloric acid treatment has the greatest influence on the swelling rate, followed by the sulfuric acid treatment, the nitric acid treatment, the sodium chloride treatment, the alkali treatment, the freeze-thaw treatment, and the repeated wet-dry treatment. This suggests that as the content of the superabsorbent polymer increases, the swelling force to expand in the same area increases, thereby offsetting the decrease of the swelling rate due to deterioration conditions.

4. Suggestion and Verification of Optimal Mixing Ratio for Waterproofing Performance

4.1. Suggestion for Optimal Superabsorbent Polymer Mixing Ratio

The waterproofing performance of water-expandable polyurethane varies depending on the swelling force; that is, the degree of expansion when the coating material comes into contact with water. As previously identified, the minimum swelling rate for water-expandable polyurethane to exert its waterproofing performance was 150% for water-expandable polyurethane, which is commonly used in practice.

Therefore, based on the swelling rate evaluation results according to the deterioration conditions in Table 7, the optimal mixing ratio of the superabsorbent polymer to ensure a swelling rate of 150% or higher is presented in

Table 9. The optimal superabsorbent polymer mixing ratio for water-expandable polyurethane to secure waterproofing performance according to deterioration conditions.

Division	Non-Treatment	Hydrochloric Acid	Nitric Acid	Sulfuric Acid	Alkali	Sodium Chloride	Freeze-Thaw	Repeated Wet-Dry Treatment
Mixing ratio (%)	More than 20	More than 14	More than 14	More than 14	More than 13	More than 13	More than 12	More than 12

. It is adjudged that the water-expandable polyurethane can stably secure waterproofing performance at a mixing ratio of 14% or higher for the hydrochloric acid treatment, the nitric acid treatment, and the sulfuric acid treatment, 13% or higher for the alkali treatment and the sodium chloride treatment, and 12% or higher for the freeze-thaw treatment and the repeated wet-dry treatment.

4.2. Verification of Waterproofing Performance

In order to verify the effectiveness of the optimal superabsorbent polymer mixing ratio to ensure waterproofing performance under deterioration conditions, the waterproofing performance evaluation was carried out again in accordance with the deterioration conditions presented in the Section 3.2.1 Swelling Rate Test Method and Results and the test methods presented in Section 3.1.2 Leakage Test Methods and Results. However, for the evaluation, the specimens were fabricated using a mixing ratio of 14%, the highest superabsorbent polymer mixing ratio, so that they could respond to all deterioration conditions. The evaluation results are shown in Figure 15.

As shown in the above evaluation results, the specimen fabricated by setting the superabsorbent polymer mixing ratio to 14% exhibited waterproofing performance under all deterioration conditions. Based on this result, the present study proposes a mixing ratio of 14% as the optimal superabsorbent polymer mixing ratio for water-expandable polyurethane to exhibit the appropriate swelling ratio and waterproofing performance.

5. Conclusions

The waterproofing performance of the currently used water-expandable polyurethane for a certain crack area was evaluated according to the deterioration conditions. The evaluation found that the waterproofing performance decreases under the deterioration conditions, but there was no research data on the quantitative swelling rate change characteristics. Accordingly, the evaluation of the swelling rate change characteristics of water-expandable polyurethane according to the content of the superabsorbent polymer was conducted as basic research. The evaluation results confirmed that the swelling rate of the material increased proportionally as the content of the superabsorbent polymer increased, and the swelling rate did not recover to 100%, as the absorption recovery ratio was partially reduced under deterioration conditions. Therefore, since the material may suffer the degradation of expansion characteristics when exposed to various deterioration conditions in the long term, proper care should be taken not to expose the material to the deterioration conditions during on-site construction.

In addition, it was confirmed that the decrease of the swelling rate compared to the case of non-treatment varies depending on the deterioration conditions, and that the hydrochloric acid treatment has the greatest effect on the swelling rate, followed by the sulfuric acid treatment, the nitric acid treatment, the sodium chloride treatment, the alkali treatment, the freeze-thaw treatment, and the repeated wet-dry treatment. Based on the decreased swelling rate characteristics according to deterioration conditions for each superabsorbent polymer content presented above, the optimal mixing ratio of the water-expandable polyurethane under each of the deterioration conditions was proposed in this study.

Therefore, it is expected that the findings of this study can be used as basic data to determine the optimal mixing ratio, considering the expansion characteristics, in the development of polyurethane-based waterproofing materials using superabsorbent polymers.

However, this study has its limitations in that the material used for evaluation was limited to the superabsorbent polymer, and combined deterioration conditions were excluded from the evaluation. Therefore, for the applicability of waterproofing materials using superabsorbent polymers in various fields, the additional evaluation of materials and items need to be included in the scope of further research so that changes in waterproofing performance can be examined from a multilateral perspective.