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Superabsorbent composite. XIII. Effects of [Al.sup.3+]-attapulgite on hydrogel strength and swelling behaviors of poly(acrylic acid)/[Al.sup.3+]-attapulgite superabsorbent composites.

INTRODUCTION

Superabsorbents are crosslinked three-dimensional network of hydrophilic polymer chains that can absorb and retain a lot of aqueous fluids and the absorbed water is hardly removable even under some pressure. Because of excellent properties to traditional water absorbing materials (such as sponge, cotton, and pulp, etc.), superabsorbents have been widely used in many fields, such as hygienic products, horticulture, drug-delivery systems, gel actuators, and coal dewatering [1-4] since the first superabsorbent material was reported by the U.S. Department of Agriculture in 1961. Considerable interests have been drawn and many efforts have been made by lots of groups to modify properties including equilibrium water absorbency, swelling rate, and hydrogel strength of superabsorbents [5-8].

Recently, clay has become the focus for the preparation of inorganic-organic superabsorbent composite to improve swelling properties, enhance hydrogel strength, and reduces production cost of corresponding superabsorbents. Various clays, including montmorillonite [9, 10], attapulgite (APT) [11, 12], kaolin [13], mica [14, 15], bentonite, and sericite [16], have been introduced into neat polymeric network to obtain clay-based superabsorbent composites with improved properties. Most of these literatures indicate that the introduction of clays could improve properties of corresponding composites and reactive--OH group on the surface of clay is the focus. Quaternary ammonium salts were also often used to modify clay surface to change the interaction between clay and polymeric network, and then improve swelling behaviors of resulting superabsorbent composites [9, 15, 17, 18]. Main components of clay may also influence properties of corresponding superabsorbent composite; however, no information about this has been reported until now. In fact, the cations on the surface and in channels of clay may enhance osmotic pressure difference between polymeric network and external solution, and then increase equilibrium water absorbency. Furthermore, the cations could form intramolecular and intermolecular complex with hydrophilic groups on the polymer chains, and then improve hydrogel strength of corresponding superabsorbent composites [19]. So, it is really worthy to do some efforts on this point for the superabsorbent field.

APT is a kind of hydrated octahedral layered magnesium aluminum silicate with reactive--OH groups on its surface. A series of APT based superabsorbent composites have been synthesized by our group [11, 12, 17, 18, 20, 21], and comprehensive properties of these superabsorbents are improved by introducing APT. Chen et al. has reported that [Al.sub.2]([SO.sub.4])[.sub.3] could enhance crosslinking density of the poly (acrylamide-co-acrylic acid) superabsorbent, and then improves swelling behaviors [22]. Cations, such as [Na.sup.+] and [Ca.sup.2+], on the surface and in the channel of APT could exchange with other cations in the medium. On this basis, a series of [Al.sup.3+]-exchanged APT with Al[Cl.sub.3] solution of various concentrations were incorporated into the poly(acrylic acid) (PAA) polymeric network to investigate the effects of [Al.sup.3+]-exchange of APT on surface morphology and properties of the PAA/[Al.sup.3+]-APT superabsorbent composites, which may be helpful for us to understand the role of [Al.sup.3+] (a main component of APT and other clays) in clay-based superabsorbent composite polymeric network.

EXPERIMENTAL

Materials

Acrylic acid (AA) was purchased from Shanghai Chemical Reagent Corp. and distilled under reduced pressure before use. Ammonium persulfate (APS) was supplied by Xi'an Chemical Reagent Factory (Xi'an, China) and recrystallized from water before use. N, N'-methylenebisacrylamide (MBA) was purchased from Shanghai Chemical Reagent Corp. APT (supplied by Linze Colloidal, Gansu, China) was milled through a 320-mesh screen and treated with 37% hydrochloric acid for 72 h, followed by washing with distilled water until pH = 6 was achieved, and then dried at 105[degrees]C for 8 h. Other agents used were all analytical grade and all solutions were prepared with distilled water.

Preparation of [Al.sup.3+]-APT

APT powder (10.0 g) was suspended in 100 ml of Al[Cl.sub.3] aqueous solution of various concentrations (0, 0.01, 0.05, 0.10, 0.50, 1.00, and 1.86 M). The suspension was stirred mechanically at 1250 r/min at 20[degrees]C for 2 h, and then [Al.sup.3+]-APT was formed. The separated [Al.sup.3+]-APT were washed with large volume of distilled water to remove excess Al[Cl.sub.3] (until no [Cl.sup.-] can be detected by 0.1 M AgN[O.sub.3] aqueous solution in the filtrate), and then dried in an oven at 70[degrees]C for 6 h until the weight was constant. Main components of [Al.sup.3+]-APT was determined by X-ray Fluorescence Spectrometer (PAN alytical, Magix PW 2403 X).

Preparation of PAA/[Al.sup.3+]-APT Superabsorbent Composites

AA (7.20 g) was dissolved in 13.0 ml of distilled water and then neutralized at 5[degrees]C with 12.0 ml of sodium hydroxide solution (5 M) in a four-neck flask equipped with a stirrer, a condenser, a thermometer, and a nitrogen line. Certain amount (0, 0.38, 0.81, 1.83, 3.13, and 4.88 g, respectively) of APT or [Al.sup.3+]-APT powder was dispersed into the partially neutralized solution. Under nitrogen atmosphere, 5.3 mg of MBA was added to the above mixture. The mixture was stirred on a water bath at room temperature for 30 min, and then heated slowly to 70[degrees]C with vigorous stirring after 108.4 mg of initiator, APS, was charged into the mixed solution. The reaction system was kept at 70[degrees]C for 3 h under nitrogen atmosphere, and the resulting product was washed several times with distilled water and then dried in an oven at 70[degrees]C to a constant weight. Thus, the superabsorbent composites were prepared after the products were milled and screened. All samples used for test had a particle size in the range of 40-80 mesh.

Measurement of Equilibrium Water Absorbency and Swelling Rate

0.05 g of the sample was immersed in excess distilled water (500 ml) at room temperature for 4 h to reach swelling equilibrium. The swollen samples were then separated from unabsorbed water by filtering through a 100-mesh screen. The equilibrium water absorbency in distilled water of the superabsorbent composite, [Q.sub.eq], was calculated using the following equation:

[Q.sub.eq] = ([m.sub.2] - [m.sub.1])/[m.sub.1] (1)

where [m.sub.1] and [m.sub.2] are the weights of the dry sample and the swollen sample, respectively. [Q.sub.eq] is calculated as grams of water per gram of sample. Equilibrium water absorbency of the samples in 0.9 wt% NaCl aqueous solution, [Q'.sub.eq], was tested according to the same procedure.

The swelling rate of the samples was measured according to the following process. 0.05 g sample was poured into 500 ml distilled water. At certain time intervals, the water absorbency ([Q.sub.t]) of the sample was measured according to Eq. 1. The measurement condition is the same as that for equilibrium water absorbency.

Measurement of Hydrogel Strength

The hydrogel strength of the samples was measured according to a previously reported method [22]. After determining equilibrium water absorbency in 0.9 wt% NaCl aqueous solution, the swollen hydrogel was compactly placed into a glass tube (inner diameter is 1.66 cm) with one end sealed with a 100-mesh screen, and then different pressure was loaded on. According to the Flory equation [23], the relationship between compressive stress [sigma] and the deformation ratio [alpha] ([alpha] = deformed length/initial length) is as follows:

[sigma] = [G.sub.0] ([alpha] - [[alpha].sup.-2]). (2)

The elastic modulus ([G.sub.0]), which can be used to stand for the hydrogel strength, was determined from the slope of the above linear dependence.

Reswelling Capability

The reswelling capability of the superabsorbent composite was evaluated by measuring equilibrium water absorbency of it after several times of reswelling. 0.05 g of the sample ([m.sub.1] according to Eq. 1) was immersed in certain amount of distilled water in five 500 ml beakers to ensure the swelling equilibrium was achieved. The beakers containing swollen hydrogel were placed in an oven at 100[degrees]C until the hydrogel was dried thoroughly. The first beaker was charged with 500 ml distilled water and remained for 4 h at room temperature to achieve swelling equilibrium, and then the hydrogel was weighted ([m.sub.2] according to Eq. 1). Consequently, water absorbency of the sample after one time of reswelling was obtained according to Eq. 1. For the other four beakers, the same drying-swelling cycle was repeated for various times, and then equilibrium water absorbency of the sample after several times of reswelling was obtained.

[FIGURE 1 OMITTED]

Characterization

The micrographs of samples were taken using SEM (JSM-5600LV, JEOL). Before SEM observation, all samples were fixed on aluminum stubs and coated with gold.

RESULTS AND DISCUSSION

Effect of [Al.sup.3+]-Exchange of APT on Surface Morphology

SEM micrographs of PAA/APT and PAA/[Al.sup.3+]-APT are shown in Fig. 1. As can be seen, the PAA/APT shows a porous surface, however, the introduction of [Al.sup.3+]-APT forms a relatively planar surface. Obviously, this surface morphology change is attributed to the [Al.sup.3+]-exchange of APT, and then may has some influence on swelling behaviors of the corresponding superabsorbent composites.

Effect of Al[Cl.sub.3] Solution Concentration on Hydrogel Strength

The hydrogel strength curve for the PAA/[Al.sup.3+]-APT superabsorbent composite plotted against Al[Cl.sub.3] solution concentration is displayed in Fig. 2. The hydrogel strength increases with increasing Al[Cl.sub.3] solution concentration in the range of concentration investigated. According to the Flory theory, [G.sub.0] increases with increasing the crosslink density of polymeric network [23]. The increased [G.sub.0] as shown in Fig. 2 indicates the increase of crosslink density in the superabsorbent composite polymeric network. This relationship reveals that [Al.sup.3+]-APT acts as assistant cross-linker in the polymeric network. This may be attributed to the fact that [Al.sup.3+] on the surface or in the channel of APT really could form intermolecular and intramolecular complex with hydrophilic groups on the polymer chains, and then increase crosslinking density. The role of [Al.sup.3+]-APT is illustrated in Scheme 1. This result is in accordance with the effect of [Al.sub.2]([SO.sub.4])[.sub.3] in the poly(acrylamide-co-acrylic acid) system as reported by Chen et al. [22], and the effect of divalent and trivalent cations on swelling behaviors of the poly[sodium acrylate-co-3-dimethyl (methacryloyloxyethyl) ammonium propane sulfonate] system [24].

[FIGURE 2 OMITTED]

Effect of Al[Cl.sub.3] Solution Concentration on Reswelling Capability

The reswelling capability is an important property of superabsorbents besides equilibrium water absorbency, hydrogel strength, and swelling rate. The effect of Al[Cl.sub.3] solution concentration while preparing [Al.sup.3+]-APT on reswelling capability of the PAA/[Al.sup.3+]-APT superabsorbent composites is discussed in this part. Figure 3 shows variation of equilibrium water absorbency with reswelling times for the PAA/[Al.sup.3+]-APT superabsorbent composites incorporated with [Al.sup.3+]-APT by using Al[Cl.sub.3] solution of various concentrations. As can be seen, equilibrium water absorbency for the composites doped with [Al.sup.3+]-APT treated with Al[Cl.sub.3] solution lower than 1 M decreases rapidly with increasing reswelling times. However, when the concentration is higher than 1 M, the equilibrium water absorbency firstly increases a little, and then decreases with further increasing reswelling times. A relatively higher drying temperature could lead to irreversible collapses of hydrogels as had been reported previously [25, 26]. This collapses may be attributed to the broken of crosslinking points of the polymeric network during the drying process. As the concentration of Al[Cl.sub.3] solution is lower than 1 M, the crosslinking density of corresponding superabsorbent composites is relatively lower. These crosslinking points are partly destroyed and the crosslinking density further decreases during the drying process, and then the polymeric network had less space to hold water, which leads to the decrease of equilibrium water absorbency. As the concentration of Al[Cl.sub.3] solution is high enough, the corresponding superabsorbent composites were crosslinked intensively. With increasing reswelling times, a part of crosslinking points were destroyed and the resulting superabsorbent composites had a proper crosslinking density to hold a relatively higher amount of water. However, the crosslinking density decreases sequentially with further increasing reswelling times and induces the decrease of equilibrium water absorbency ultimately. From the above information, it can be deduced that [Al.sup.3+]-exchange could enhance reswelling capability of the superabsorbent composites. It also can be seen that the resulting dry samples after several times of reswelling still retain a higher degree of water absorbing capability. These superabsorbent composites may be proved useful as recyclable superabsorbent materials.

[ILLUSTRATION OMITTED]

[FIGURE 3 OMITTED]

Effect of Al[Cl.sub.3] Solution Concentration on Equilibrium Water Absorbency

The effect of [Al.sup.3+]-APT on equilibrium water absorbency of the PAA/[Al.sup.3+]-APT superabsorbent composites is investigated in this section. Figure 4 shows the variation of equilibrium water absorbency in distilled water and in 0.9 wt% NaCl solution for the PAA/[Al.sup.3+]-APT superabsorbent composites incorporated with [Al.sup.3+]-APT treated with Al[Cl.sub.3] solution of various concentrations.

As can be seen from Fig. 4, the equilibrium water absorbency in distilled water and in 0.9 wt% NaCl solution decrease evidently with increasing Al[Cl.sub.3] solution concentration. With increasing Al[Cl.sub.3] solution concentration, more [Al.sup.3+] could exchange with the other cations, such as [Na.sup.+] and [Ca.sup.2+], on the surface and in the channels of APT, and then may increase [Al.sup.3+] content. The XRF results in Table 1 indicate that the weight ratio of Al to Si for [Al.sup.3+]-APT really increases from 0.326 to 0.376 with increasing Al[Cl.sub.3] solution concentration from 0 to 1.86 M. While the amount of [Na.sup.+] and [Ca.sup.2+] decrease at the same time. The increased [Al.sup.3+] could form more intramolecular and intermolecular complex with hydrophilic groups of the PAA polymer chains as discussed earlier, and then increase the effective crosslinking density of corresponding superabsorbent composites, which is responsible for the decrease of equilibrium water absorbency. This phenomenon is similar to the effect of increasing crosslinker content as many literatures had reported [27-29]. It also can be seen from Fig. 4 that equilibrium water absorbency in 0.9 wt% NaCl solution is obviously lower than that of in distilled water. The shrinkage of the superabsorbent composites is mainly caused by the following facts. The external NaCl aqueous solution could decrease the osmotic pressure difference between the polymeric network and external solution. Moreover, the screening effect of penetrated counterions ([Na.sup.+]) on anionic hydrophilic groups could also restrict the expanding of polymeric network.

[FIGURE 4 OMITTED]

Effect of Al[Cl.sub.3] Solution Concentration on Swelling Rate

Since [Al.sup.3+]-APT has great influence on equilibrium water absorbency of the PAA/[Al.sup.3+]-APT superabsorbent composite, the effect of [Al.sup.3+]-exchange on swelling kinetics of the composite is investigated in this part. Figure 5 shows the swelling rate in distilled water of the PAA/[Al.sup.3+]-APT superabsorbent composites incorporated with [Al.sup.3+]-APT treated with different concentration of Al[Cl.sub.3] aqueous solution. It is found that the superabsorbent composites swell faster at the first 10 min, and then the swelling rate decreases. When [Q.sub.t]/[Q.sub.eq] < 0.60, the osmotic pressure and chain relaxation are responsible for the higher swelling rate, however, the swelling of the superabsorbent composites are only dominated by relaxation of the polymer chains when [Q.sub.t]/[Q.sub.eq] > 0.60, and then the swelling rate decreases. As indicated with dot line, [Q.sub.t]/[Q.sub.eq] reached 0.6 almost at the same time for all the samples except for that of incorporated with [Al.sup.3+]-APT treated with 1.86 M Al[Cl.sub.3] solution. This result is in accord with Bajpai's study in the poly (acrylamide-co-sodium acrylate) [30]. It also can be seen that the PAA/[Al.sup.3+]-APT superabsorbent composites incorporated with [Al.sup.3+]-APT treated with 0, 0.01, and 0.05 M Al[Cl.sub.3] solution need about 60 min to reach equilibrium water absorbency. However, for that treated with 0.1, 0.5, 1, and 1.86 M Al[Cl.sub.3] solution, about 120, 120, 180, and 180 min are needed, respectively. According to Buchholz and Graham [1], the swelling rate of superabsorbent could be significantly influenced by factors including swelling capability, particle size, specific surface area, and polymer density. The higher swelling capability of the PAA/[Al.sup.3+]-APT superabsorbent composites incorporated with [Al.sup.3+]-APT treated with 0, 0.01, and 0.05 M Al[Cl.sub.3] solution may be responsible for their higher swelling rate. Moreover, with increasing Al[Cl.sub.3] solution concentration, the crosslinking density of corresponding superabsorbent composites increases, which also could decrease the penetration of water molecules into polymeric network.

Effect of [Al.sup.3+]-APT Content on Equilibrium Water Absorbency

The effects of [Al.sup.3+]-APT content on equilibrium water absorbency of the PAA/[Al.sup.3+]-APT superabsorbent composites in distilled water and in 0.9 wt% NaCl solution are shown in Fig. 6, respectively. As can be seen, the equilibrium water absorbency in distilled water firstly increases from 475 to 1158g [g.sup.-1] with increasing [Al.sup.3+]-APT content from 0 to 20 wt%, and then decreases to 695g [g.sup.-1] with further increasing [Al.sup.3+]-APT content to 40 wt%. Adding proper amount of clay could increase hydrophilicity of corresponding superabsorbent composite as reported by Lee and Yang in the poly(sodium acrylate)/montmorillonite system [9], and then make the composites swell more. According to our previous study [11, 20],--OH on the surface of APT could react with acrylic acid and acrylamide, which could improve the polymeric network, and then enhance the water absorbency. The increase of equilibrium water absorbency with increasing [Al.sup.3+]-APT content may be attributed to similar reasons. The decreasing tendency of equilibrium water absorbency with further increasing [Al.sup.3+]-APT content may be attributed to the fact that additional [Al.sup.3+]-APT results in the generation of more crosslink points in the polymeric network, which increases the crosslinking density of the corresponding superabsorbent composites, and then elasticity of the polymer chains decreases. Additionally, the content of hydrophilic groups is lower at a higher [Al.sup.3+]-APT content, and then the osmotic pressure difference decreases, which also contributes to shrinkage of the superabsorbent composites.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

CONCLUSIONS

The effects of [Al.sup.3+]-APT on hydrogel strength and swelling behaviors of the PAA/[Al.sup.3+]-APT superabsorbent composites have been investigated. It was shown that [Al.sup.3+]-APT have great influences on hydrogel strength, equilibrium water absorbency, swelling rate, and reswelling capability of the superabsorbent composites. [Al.sup.3+]-APT could increase hydrogel strength and reswelling capability of the corresponding superabsorbent composites, but decrease equilibrium water absorbency and swelling rate. [Al.sup.3+]-APT not only acts as a substrate, but also serves as an assistant cross-linker in the polymeric network.

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Junping Zhang, (1,2) Yaogang Zhao, (1) Aiqin Wang (1)

(1) Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China

(2) Graduate School of the Chinese Academy of Sciences, Beijing 100049, People's Republic of China

Correspondence to: Aiqin Wang; e-mail: aqwang@lzb.ac.cn

Contract grant sponsor: Ministry of Science and Technology (863 major project), People's Republic of China; contract grant number: 2005AA2Z4030; contract grant sponsor: The science and Technology Major Project (Gansu Province); contract grant number: 2GS052-A52-002-07.
TABLE 1. Variation of main chemical components of [Al.sup.3+] exchanged-
APT with Al[Cl.sub.3] solution of various concentrations.

Chemical
components (%) 0 M 0.01 M 0.05 M 0.10 M 0.50 M 1.00 M

[Na.sub.2]O 1.878 1.802 1.748 1.652 1.520 1.501
MgO 3.022 3.025 3.075 3.132 3.178 3.203
[Al.sub.2][O.sub.3] 19.022 19.050 19.434 19.782 20.106 20.342
Si[O.sub.2] 65.997 65.417 65.026 64.655 64.333 62.764
[K.sub.2]O 3.990 4.000 4.061 4.121 4.130 4.142
CaO 0.138 0.131 0.120 0.106 0.097 0.084
[Fe.sub.2][O.sub.3] 5.513 5.524 5.543 5.572 5.589 5.595

Chemical
components (%) 1.86 M

[Na.sub.2]O 1.487
MgO 3.226
[Al.sub.2][O.sub.3] 20.463
Si[O.sub.2] 61.620
[K.sub.2]O 4.152
CaO 0.075
[Fe.sub.2][O.sub.3] 5.601
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Author:Zhang, Junping; Zhao, Yaogang; Wang, Aiqin
Publication:Polymer Engineering and Science
Geographic Code:9CHIN
Date:May 1, 2007
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