*3.5. Effects of Reaction Conditions on SAPC Performance* 3.5.1. Effect of SA Content

The effect of SA content on water absorption of SAPC is shown in Figure 5a. Water absorption increased from 736 to 933 g/g, with the SA content changing from 10 wt. % to 15 wt. %, and then decreased to 763 g/g, with SA content further increasing to 20 wt. %. When the content of SA, which act as the basic skeleton, was too low, AA self-polymerized, resulting in the inability to form an effective 3D network to absorb water. With much higher SA content, sharply increased viscosity resulted in the system, restricting the monomer movement and reducing the conversion rate. Thus, a less developed 3D polymer network and suppressed water absorption capacity could be expected.

**Figure 5.** Effects of (**a**) SA content (ST = 15 wt %, KL = 5 wt %, neutralization degree (ND) of acrylic acid (AA) = 80%, potassium persulfate (KPS) = 0.5 wt %, N, N -methylenebisacrylamide (MBA) = 0.15 wt %), (**b**) KL content (ST = 15 wt %, SA = 15 wt %, ND of AA = 80%, KPS = 0.5 wt %, MBA = 0.15 wt %), (**c**) ND of AA (ST = 15 wt %, SA = 15 wt %, KL = 4 wt %, KPS = 0.5 wt %, MBA = 0.15 wt %), (**d**) KPS content (ST = 15 wt %, SA = 15 wt %, KL = 4 wt %, ND of AA = 80%, MBA = 0.15 wt %) and (**e**) MBA content (ST = 15 wt %, SA = 15 wt %, KL = 4 wt %, ND of AA = 80%, KPS = 0.9 wt %) on the water absorption of SAPC. These contents are based on the weight percentage of raw materials and AA (20 mL). Qeq represents the water absorption of swelling for 4 h.

#### 3.5.2. Effect of KL Content

Figure 5b depicts the effect of KL content on water absorption. Water absorption of SAPC first increased and then decreased with the increase of KL content from 3 wt. % to 7 wt. %. KL with a large number of hydrophilic groups could be chemically cross-linked with a polymer chain, forming a cross-linked polymer network with KL particles as the additional cross-linking points. The introduction of appropriate KL particles weakens the hydrogen-bonding interaction between carboxyl groups in the polymer and reduces the physical entanglement of the grafted polymer network chain [34,35]. Nevertheless, with excessive KL (>4 wt. %), the excess KL particles increased the degree of cross-linking, leading to a decrease of the penetration space of water molecules. In addition, the extra kaolin particles were also filled into the polymer network space in the form of physical filling. Yet the water absorption rate of KL itself was low, as a result, the water absorption of SAPC inevitably decreased. Therefore, appropriate KL loading is critical to obtaining SAPC with high water absorption capacity.

#### 3.5.3. Effect of Neutralization Degree of AA

The water absorptance capacity of SAPC was closely related to the neutralization degree (ND) of AA (Figure 5c). AA has higher reaction activity and polymerization rate than sodium acrylate [36]. Water absorption of SAPC increased as ND increased from 65% to 80%. When ND was lower, the polymerization reaction completed rapidly within a short time, resulting in a highly cross-linked network structure with low swelling capacity. With the increase of ND, the polymerization rate decreased correspondingly. In addition, the increase of -COO<sup>−</sup> groups and Na+ content in the polymer network increased both the repulsive force between the anions on the polymer chains and the osmotic pressure difference between the inside and outside of the polymer network, which was conducive to the entry of water or other small molecules [37–39]. However, when ND was higher than 80%, water absorption dropped significantly, attributed to the reaction of -COO− groups with excess Na+, resulting in weakening of the repulsive force.

#### 3.5.4. Effect of KPS Content

Water absorption was significantly affected by the initiator concentration and the average kinetic chain length. With low KPS content (0.3–0.9 wt. %), an integral polymer network could not be formed due to the few grafting points and a large number of unreacted monomers in the reaction system. With too much KPS (0.9–1.1 wt. %), the excessive free radicals terminated the propagating chains earlier, shortening the average kinetic chain length. In both scenarios, low water absorption capacity resulted (Figure 5d).

#### 3.5.5. Effect of MBA Content

The effect of the MBA content on water absorption of SAPC is shown in Figure 5e. Water absorption of SAPC increased first and then decreased with the increase of MBA content from 0.1 to 0.2 wt. %, and a maximum value of 1156 g/g was achieved with 0.15 wt. % MBA loading. Lower or higher MBA content resulted in reduced water absorption. The water absorption capacity of resin was closely related to its spatial network structure. When the content of MBA was not sufficient, cross-linking density was low and a complete spatial network structure could be established, leading to low water absorption and poor mechanical properties of SAPC. However, a higher concentration of MBA produced a denser cross-linked structure, which made it difficult for liquid molecules to enter.

The optimal reaction conditions of SAPC were when the masses of ST, SA, KL, KPS and MBA were 15 wt. %, 15 wt. %, 4 wt. %, 0.9 wt. % and 0.15 wt. % of AA, respectively. The neutralization degree of AA was 80%. In this work, SAP and SAPC were synthesized under the optimal conditions for further tests.

### *3.6. Performance Tests of SAPC*

3.6.1. Swelling Kinetics of SAPC in Distilled water

The pseudo-second order swelling kinetics model (Equation (3)) and the Ritger-Peppas model (Equation (4)) were used to analyze the experimental swelling data to evaluate the swelling behavior of SAPC, with results shown in Figure 6a,b [40–42]:

$$\frac{t}{q\_t} = \frac{1}{(k\_2 q\_\varepsilon^2)} + \frac{t}{q\_\varepsilon} \tag{3}$$

$$F = \frac{q\_t}{q\_\varepsilon} = \text{kt}^n\tag{4}$$

**Figure 6.** (**a**,**b**) Swelling kinetics of SAPC (4 wt. % KL), (**c**) Swelling capacity in different salt solutions, (**d**) Water retention at different temperatures, (**e**) The swelling capacity of SAPC (4 wt. % KL) in different pH solutions and (**f**) the swelling mechanism at various pH values.

To calculate *n* and *k*, take the natural logarithm of Equation (4):

$$
\ln F = \ln \left(\frac{q\_t}{q\_\varepsilon}\right) = \ln k + \ln n \, t \tag{5}
$$

where *t* is absorption time and *k2* is the rate constant of the pseudo-second order model. *qt* and *qe* correspond to water absorption of SAPC at time *t* and at equilibrium, respectively. *F* denotes fractional uptake at time *t*. *k* and *n* are the characteristic constants of the polymer and the diffusion index, respectively.

From Figure 6a, water absorption of SAPC increased rapidly in the first 15 min, reached swelling equilibrium at 15 min, and then stabilized after 15 min. The maximum water absorption was 1200 g/g. It was also found that the relationship curve between *t* and *t/qt* was linear with R2 value (0.99935) close to 1, indicating that the swelling behavior of SAPC could be fitted by the pseudo-second order kinetics model. Since this model is based on chemical adsorption assumptions, it suggests that the chemisorption was the main way of the water absorption process.

Based on the polymer chain relaxation rate and the relative diffusion rate of water into the polymer network, the water diffusion mechanism can be classified into five types. That is, pseudo-Fickian diffusion (*n* < 0.5), Fickian diffusion (*n* = 0.5), non-Fickian diffusion

(0.5 < *n* < 1.0), Case II transport diffusion (*n* = 1) and relaxed diffusion (*n* > 1) [43–45]. These equations were applied to the initial stages of swelling. According to Figure 6b, within 0–15 min, 0.5 < *n* < 1, indicating that the water diffusion mechanism was consistent with the non-Fickian diffusion mechanism, the diffusion and relaxation were considered to be isochronally effective. After 15 min, *n* < 0.5, the main reason of swelling was the diffusion of water molecules in the polymer network.

#### 3.6.2. Swelling Behavior in Salt Solutions

The influences of cation and saline type on water absorption were studied using FeCl3 and NaCl, with results shown in Figure 6c. For the same saline solution, as cation concentration increased, water absorption of SAPC decreased. This result was due to the charge screening effect of cations, which reduced the osmotic pressure difference between the polymer network and the external saline solution and decreased the repulsive force between the -COO− groups of the polymer chain. Comparing the two absorption curves, it was found that water absorption of SAPC was lower in the multivalent cation solution. This is attributed to the complexation between carboxylate anions of chains and multivalent cations, resulting in an increase of the network cross-linking density and a drastic decrease in water absorption [46,47].

#### 3.6.3. Water Retention Properties

The water retention of fully swollen SAPC at different temperatures, i.e., 25, 45 and 60 ◦C, was studied, and the results are shown in Figure 6d. As can be seen, water retention of SAPC decreased with time, and the water retention curve at 25 ◦C was gentler than that at 45 and 60 ◦C. At 25 ◦C, the water retention rate was about 80% after 12 h. However, the water retention rate of SAPC at 45 ◦C and 60 ◦C still reached about 37% and 6% after 12 h, respectively. The above results showed that SAPC had excellent water retention ability and thus broad application prospects.

#### 3.6.4. Swelling Behavior in Buffer Solutions

As a new superabsorbent composite, the swelling capacity in different pH solutions has great influence on its application in various fields. Hence, the swelling behavior of SAPC in different buffer solutions was studied, with results shown in Figure 6e and the mechanism illustrated in Figure 6f. At low pH values (pH ≤ 4), due to the higher concentration of H+, most of the -COO<sup>−</sup> groups were transformed into -COOH groups, which weakened the electrostatic repulsion between -COO− groups and strengthened the hydrogen-bonding interaction between the -COOH groups of polymer chains [48]. Therefore, water absorption was low. With an increase of pH value (4 ≤ pH ≤ 7), water absorption of SAPC increased, owing to gradual ionization of -COOH groups, which resulted in an increase of electrostatic repulsion between -COO− groups. At a high pH value (pH ≥ 7), as Na+ concentration increased in the solution, water absorption decreased correspondingly. The decrease was due to the charge shielding effect of excessive Na+ on the -COO− groups, which weakened the electrostatic repulsion force and reduced the osmotic pressure difference between the inside and outside of the polymer network. Generally, this type of SAPC had excellent pH tolerance between 4 and 10.

#### **4. Conclusions**

To summarize, a series of SAPC was successfully prepared by free radical polymerization of SA, ST, AA and KL in aqueous solution. The optimal reaction conditions of SAPC were achieved when the mass of ST, SA, KL, KPS and MBA were 15 wt. %, 15 wt. %, 4 wt. %, 0.9 wt. % and 0.15 wt. % of AA, respectively, with a neutralization degree of AA 80%. With optimized reactants concentrations, a maximum water absorption of 1200 g/g was obtained. FT-IR spectra confirmed the success of the polymerization reaction. SEM images revealed the rough and porous surface of SAPC which was conducive to the liquid entering into the polymer network. XRD patterns proved that KL was uniformly dispersed in the

polymer matrix. TGA spectra indicated that the addition of KL improved the thermal stability of SAPC. The swelling kinetic of SAPC was studied, showing that the swelling behavior of SAPC followed the pseudo-second order kinetic model and the non-Fickian diffusion model. Along with excellent water absorption capacity, the fabricated SAPC also had excellent water retention property, good salt tolerance in monovalent salt solution and pH tolerance between 4 and 10, making it promising in many applications.

**Author Contributions:** Conceptualization, M.C. and Z.W.; methodology, M.C. and Z.W.; validation, M.C. and Z.W.; formal analysis, M.C. and Z.W.; investigation, M.C., C.Z., B.C.; resources, M.C., Z.L., Z.W.; data curation, M.C., C.Z., B.C.; writing—original draft preparation, M.C. and Z.W.; writing review and editing, M.C., X.C., Z.W.; visualization, M.C. and Z.W.; supervision, D.Z. and Z.W.; project administration, Z.W.; funding acquisition, M.C. and Z.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by grants from the National Natural Science Foundation of China (contract grant number 31270608) and the Heilongjiang Educational Committee (contract grant number 1511385).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data availability upon request.

**Conflicts of Interest:** The authors declare no conflict of interests.

#### **References**

