Synthesis of superabsorbent resin with the properties of temperature tolerant, salt tolerant, and water absorbency deferred

2.1 Materials

Acrylamide (AM), potassium hydroxide (KOH), ammonium persulfate (APS), sodium hydrogen sulfite (SHS), acrylic acid (AA), N,N-dimethylene bisacrylamide, and polyvinyl alcohol (PVA) were all used as received.

2.2 Procedures

AA (7 ml) mixed with distilled water (3 g) in a beaker cooled by ice water before use and the solution was stirred for some time. Then, the AA solution was partially neutralized by adding KOH (5.9812 g) aqueous solution followed by the addition of AM (0.7196 g), PVA (1.75 g), distilled water (9.4 g), and crosslinking agent N,N-dimethylene bisacrylamide (3.75×10^-5 mol). The mass fraction of reactant materials was about 40% (wt%) in the aqueous solution; then, the beaker was set in a water bath with selected constant temperature. Finally, the initiator of APS/SHS [1.04×10^-4 mol APS/SHS=1 (mol/mol)] quantity weighed was added into the reactant system at 60°C under the conditions of fully stirring. After the radical polymerization in situ for 2 h at 60°C, the temperature was raised to 70°C and the reaction proceeded for 3 h. The SAR products were dried at 60°C in the oven for 12 h. The obtained products were milled and screened by a 40- and 50-mesh sieve, respectively.

2.3 Measurement of WA and 0.9% NaCl solution absorbency (SA)

A sample (0.1 g) of dried SAR was put into the nylon tea bag (100 mesh) and then immersed in water or 0.9% NaCl solution at 25°C, and the liquid in the nylon tea bag was stirred at the same time. The absorbency was determined by recording volumes of remaining liquid at different time intervals when no liquid drop out of the bag anymore. The absorbency of water/0.9% NaCl solution, expressed by Q, was calculated by Eq. (1) as follows:

In the equation, absorbency is expressed by milliliters of liquid retained per gram of dried SAR; V and M₀ denote the volume of absorbed liquid and the weight of dried SAR, respectively.

2.4 WA measurement at 90°C

A sample (0.1 g) of dried SAR was immersed in water (500 ml) for 24 h at 90°C. Absorbency was determined by weighing the swollen SAR (the swollen SAR was drained in a 100-mesh nylon tea bag for 5 min before weighing). Eq. (2) of WA at 90°C is as follows:

Absorbency is expressed by grams of water retained per gram of dried SAR; M and M₀ denote the weight of the water swollen SAR and dried SAR, respectively.

3.1 Effect of reaction temperature on absorbency

The effect of reaction temperature on absorbency of SAR was investigated in the temperature range from 40°C to 80°C. The result in Figure 1 showed that the WA increased from 110 to 210 ml/g when temperature increased from 40°C to 60°C but decreased from 60°C to 80°C.

Figure 1

Effect of reaction temperature on absorbency.

APS/SHS are combined thermal initiator in radical polymerization. As the half-life of its decomposition become shorter with increasing reaction temperature, the decomposition rate of APS/SHS increased, which resulted in the increase of the number of radical centers in unit time. Thus, the degree of polymerization is due to the increase of the termination rate as well as the temperature dependence of the chain transfer reaction. Therefore, the molecular weight decreases when polymerization temperature is increased. According to Eq. (3) [11], WA is decreased. In our experiment, the optimal reactive temperature is 60°C:

where Q_m stands for swelling ratio, i/V_u is the concentration of fixed charge referring to the unswollen networks, S is the ionic strength of the swollen solution, X₁ is the polymer-solvent thermodynamic interaction parameter, V₁ is the molar volume of water, M_c/ρ_p is the ratio of the average molecular weight of network chains and the density of polymer, and M_n is the average molecular weight of the polymer before crosslinking.

3.2 Effect of reactant ratio on absorbency

According to the data in Table 1, the absorbency of SAR decreased with the increase of C_m, but generally it is difficult to synthesize SAR when C_m is <30%, and the absorbency is also difficult to measure exactly if a large quantity of water soluble materials exists in the products. The average kinetic chain length (v) increases with increasing concentration of reactant according to Eq. (4) [12], and absorbency decreases according to Eq. (3). Our experiment is consistent with Flory’s network theory.

where k_p, k_i, and k_t are the rate constants for propagation, initiation, and termination, respectively; f is the efficiency of initiation of the initiator; and [I] and [M] are the initial concentration of initiator and monomer, respectively.

3.3 Effect of initiator content on WA

The effect of the initiators content [C_i, mass ratio of initiator and reactant, APS/SHS=1:1 (mol/mol)] on WA of SAR is plotted in Figure 2. The result shows that the WA of SAR increased greatly when C_i increased from 0.2 to 0.5. Because APS/SHS was a kind of thermal initiator in radical polymerization, and the degree of polymerization decreased with the increase of APS/SHS content, the polymerization rate was evidently enhanced when C_i increases, which leads to the increase of the polymerization rate. Therefore, the molecular weight decreases, and this results in the increase of the relative amount of the polymer chain ends in SAR networks. (1-2M_c/M_n)^-1 in Eq. (3) expressed the correction for the imperfection of the network resulted from chain ends. The more chain ends in the networks, the smaller M_n became, which resulted in higher imperfection of the networks. It can be proposed from Eq. (3) that network with more chain ends had higher Q_m value with the same M_c. In addition, our experimental results were in agreement with the Flory’s network theory.

Figure 2

Effect of initiator content on WA.

3.4 Effect of PVA content on temperature tolerant and absorbency deferred

The effects of PVA content on temperature (25°C and 90°C) on WA of SAR were investigated while unchanging the other reaction conditions. The results in Figure 3 indicate that WA of SAR gradually decreased both at 25°C and 90°C when the mass ratio of PVA and AA (R_PVA/AA) increased from 0.1 to 0.25. The reason for it was that the content of polycarboxylate with higher WA in SAR decreased gradually. In addition, the results also show that the trend of WA of SAR immersed in water for 24 h was similar both at 25°C and 90°C, but the WA at 90°C was about two times higher than that at 25°C when R_PVA/AA≥0.1. The WA of SAR networks without PVA was only 210 ml/g and sharply decreased to 53 ml/g when the temperature rose to 90°C. It can be concluded that the introduction of PVA into SAR networks indeed improved the property of SAR temperature tolerant and provided the SAR with WA deferred.

Figure 3

Effect of PVA content on temperature tolerant/WA deferred.

3.5 Effect of AM content on the property of SAR salt tolerance

It can be seen in Figure 4 that the content of SA in SAR increased from 5 to 60 ml/g and WA increased from 45 to 370 ml/g when of AM and AA (R_AM/AA) varied from 0.10 to 0.8. Hydrophilic amide, a kind of nonion group, was not easy to dissociate in aqueous solution, and metallic ions have little effect on its hydrophilic capacity. Thus, the salt-tolerant property of SAR was improved by introducing AM into SAR networks. In addition, the improvement of WA of SAR might result from the cooperative effect of manifold groups such as amide, carboxylate, and hydroxyl in SAR networks. However, both WA and SA decreased evidently when the R_AM/AA was more than 0.14. It might be responsible for the weaker hydrophilic ability of amide compared with that carboxylate.

Figure 4

Effect of AM content on the property of SAR salt tolerant.