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Preparation and properties of porous poly(sodium acrylate-co-acrylamide) salt-resistant superabsorbent composite.

INTRODUCTION

Superabsorbent polymers, as crosslinked hydrophilic polymers, have the ability to absorb and retain large amounts of aqueous solution, and the absorbed solution is scarcely released even under certain load. Based on their fine properties, they have been widely used in many fields, such as personal care products, agriculture, food packaging, artificial snow, and biomedical applications (1-3), especially in hygiene area, where 90% of superabsorbents have been consumed according to report.

Superabsorbents used in hygiene area need to satisfy with the following features: (1) high water absorbency both at atmospheric pressure and under load; (2) rapid swelling rate; (3) excellent hydrogel strength, resilience, and dispersion (1), To be content with above demands, numerous works have been carried out (4-18). First, most of work mainly paid their attention to the improvement of water absorbency at atmospheric pressure (4-7). Besides, some reports showed that the strength of the swollen hydrogel and the swelling rate could be improved by surface crosslinking and use of crosslinking agent mixture (7-10). Surface crosslinking was a process of secondary crosslinking utilizing unreacted functional groups on the surface of materials to improve some surface performances, such as hardness, modulus. And the hydrogel strength, resilience, and dispersion could be improved by introducing clay into superabsorbents^ blending with inorganic hydrogel, or forming interpenetrating networks (11-14). But the most important progress is the successful preparation of fast swelling surperabsorbents (15-18). All above work make the preparation of more comfortable superabsorbent used in hygiene area become achievable. However, the influences of reaction conditions on different properties are different. While one property is improved, other properties would decrease usually, i.e., as the absorbency at atmospheric pressure increased immoderately, the absorbency under load, the swelling rate, the hydrogel strength, resilience, and dispersion would decrease; similarly, as the absorbency under load increased excessively, the absorbency at atmospheric pressure, the swelling rate, resilience, and dispersion would decrease correspondingly. So it is impossible to obtain superabsorbent with high levels for all properties, and a feasible way is to find a balance point among these properties to meet the realistic application.

Because the material used in hygiene area is saliniferous, the salt resistant study of products in this field become imperative, and the absorbency study of superab sorbent in physiological saline will obtain more objective results for further application. On the basis of our previous works about superabsorbent with high water absorbency both at atmospheric pressure and load, and excellent strength, resilience and dispersion of swollen hydrogel (10), (19), (20), especially on our work of copolymer of partially neutralized acrylic acid and acrylamide (10), we focused our interesting on preparing a product which possessed rapid swelling rate, excellent hydrogel strength, resilience, and dispersion after swelling and relative high water absorbency under certain load and atmosphere pressure in salt solution. First, the suitable foaming agents, interpenetrating polymer, and crosslinking agents were selected. Then, the factors influencing water absorbency and other properties were investigated. The water absorbency of superabsorbent composite prepared at optimal conditions in 0.9 wt% NaCl aqueous solution at atmospheric pressure and certain load (P [approximately equal to] 2 x [10.sup.3] Pa) were 61 g [g.sup.-1] and 16.7 g [g.sup.-1] respectively. Besides, the swelling rate was very fast, and it could reach 22.003 x 10 3 g [(g s).sup.-1]. Moreover, the swollen hydrogel possessed excellent hydrogel strength, resilience, and dispersion. All these properties were better or equivalent to that of sample used in hygiene area presently.

EXPERIMENTAL

Materials

Acrylic acid (AA, Beijing Eastern Chemical Works, China, CP) was used directly. Sodium hydroxide (NaOH), hydrogen peroxide (H2O2), L-ascorbic acid (Vc), phos-phoryl trichloride (POCl3), ethoxyethane (Et20), methanol (MeOH), glycerol (G), propanol (PA), butanol (BA), urea (Ur), ammonium hydrogen carbonate (NH4HCO3), ammonium nitrate (NH4NO3), sodium chloride (NaCl), sodium acrylate (AANa), triethylamine (TEA), and tetrahydrofu-ran (THF) were analytical grade and purchased from Xi'an Chemical Reagent Plant, China. Ethyleneglycol diglycidyl ether (EGDE, Fluka, Switzerland) was chemical purity and used as received. W-methylenebisacryla-mide (NNMBA, China Medicine (Group) Shanghai Chemical Reagent Corporation, China, CP) and trihydrox-ymethyl propane glycidol ether (6360, Wuxi Wells Synthetic Material, China) were used as received. Sodium bicarbonate (NaHCQ3) and sodium carbonate (Na2C03) were analytical grades and purchased from Tianjin No. 6 Chemical Reagent Factory, China. Aluminum sulfate (Al2(S04)3) was analytical grade and purchased from Tianjin No. 3 Chemical Reagent Factory, China. Polyethylene glycol (PEG, Fluka, Mn = 20000) were used directly. Octadecanol (OT, Tianjin No. 2 Chemical Reagent Factory, China, CP) were used as received. Allyl alcohol (AAO) was analytical grade and purchased from Shanghai Chemical Reagent Station Center Chemical Plant, China. Acrylamide (AM, Beijing Eastern Chemical Works, China, CP) was used directly. The 1,2-propandiol and 1-propanol were analytical grades and purchased from Gangzhou Chemical Reagent Factory, China. Ethanol (EtOH), isobutanol (IPA), and butanol (BA) were analytical grades and purchased from Tianjin No. 6 Chemical Reagent Factory, China.

Synthesis of Semi-IPNs Porous Salt Resistant Superabsorhent

Preparation of Triene Propanol Phosphate (TTP). The synthetic reaction was conducted under N2, in an oven-dried 200 mL four necked flask equipped with a magnetic stirrer, an N2 line (attached to a bubbler), an addition funnel with a pressure equalizing side arm and a thermometer. The flask was further dried with a heat gun under positive N2 flow, and kept in a salt ice bath for 15 min. Then, it was charged with AAO (42 mL), THF (30 mL), and TEA (84 mL). And the mixture of POC[l.sub.3] ([n.sub.AAO]/[n.sub.POCL3] = 1/3) and THF (30 mL) was added dropwise into the stirred colorless solution. The resulting bright yellow solution was allowed to stir at above salt ice bath for 2 h, then at ambient temperature, 35 and 55[degrees]C for 7 h, respectively. The mixture was filtered to remove the triethylamine hydrochloride. The resultant mixture was distilled to separate the solvent, and then quenched by addition of 50 mL of deionized water and Et20. The Et20 layer was washed successively with the following solvent: water (200 mL); 5 wt% HC1 aqueous solution (200 mL); saturated NaHC[0.sub.3] aqueous solution (200 mL); brine (200 mL). Then, the solution was dried using anhydrous N[a.sub.2]S[0.sub.4] and concentrated to obtain TTP. It was characterized by nuclear magnetic resonance hydrogen ((1) HNMR), and the content was determined by gas chromatography/mass spectrometry (GC/MS). A synthetic scheme was illustrated in Scheme 1.

Preparation of Sodium 1-Octadecanol Phosphate (SOP), The synthetic reaction was conducted in 200 mL three-necked flask equipped with a magnetic stirrer, an addition funnel with a pressure equalizing side arm and a condenser tube. The flask was kept in a water bath, and charged with E[t.sub.2]O (100 mL) and OT (24 g). And POC[l.sub.3] ([n.sub.AAO]/[n.sub.POCL3] = 1/1) was added quickly to the stirred solution. Then, it was allowed to stir for 6 h. The resultant mixture was distilled to separate the solvent. After that, NaOH aqueous solution (10 wt%) and HC1 aqueous solution (5 wt%) were added successively until a neutral product was obtained. The synthetic scheme was illustrated in Scheme 2.

Preparation of Semi-IPNs Porous Salt-Resistant Super-absorbent Composite. The polymerization was carried out under nitrogen atmosphere in a four necked flask equipped with a magnetic stirrer, thermometer, and gas inlet tube at a constant temperature. PEG and distilled water were added into the flask and stirred until PEG dissolved. Then, 0.12 mol of freshly distilled AA was neutralized carefully by NaOH aqueous solution. After that, agents were added sequentially as follows: AM ([n.sub.AM]/[n.sub.AA] = 30.3%), NNMBA ([m.sub.NNMBA]/[m.sub.AA] = 7.14 x [10.sup.-4]), 6360 ([V.sub.6360]/[V.sub.AA] = 0.188%), TTP ([V.sub.TTP]/[V.sub.AA] = 0.375%), MeOH ([V.sub.MeOH]/[n.sub.AA] = 0.408), PA ([n.sub.PA]/[n.sub.AA] = 0.225), and BA ([n.sub.BA]/[n.sub.AA] = 0.183). During this process, the stirrer was kept running till all above agents were mixed homogeneously. Then, the initiator system of [H.sub.2][O.sub.2] (0.10 mol [L.sup.-1]) and [V.sub.c] (0.028 mol [L.sup.-1]) in equal volume, with the ratio of [n.sub.H2O2]/[n.sub.AA] = 3.34 x [10.sup.-4], were introduced at 20[degrees]C. After stirrer stopped, the reactive system was set at 52[degrees]C for 3 h. Finally, the resulting polymer was dried at 150[degrees]C to a constant weight, then ground it into powder, and milled through 26-90 mesh screen, and the product between the two mesh screens was named SAPl. The preparing process of sample was shown in Scheme 3.

The modification of the superabsorbent copolymer was carried out as follows: 4.0 g SAPl was well mixed with an aqueous solution containing EGDE ([m.sub.EGDE]/[m.sub.SAPI] = 2.23 x [10.sup.-7]), distilled water ([m.sub.H2O]/[m.sub.SAPI] = 3.75%), G ([m.sub.G]/[m.sub.SAPI] = 1%) and PA ([m.sub.PA]/[m.sub.SAPI] = 1%). The mixture was heated at 100[degrees]C for about 30 min, and then the resultant was cooled to room temperature. Ground it again, thus SAP2 was obtained. Then mixing SAP2 with [A[l.sub.2](S[O.sub.4]).sub.3] and N[a.sub.2]C[O.sub.3], got final product, SAP3.

Characterization

The Structure Characterization of TTP. The structure of TTP was characterized by (1) HNMR and GC/MS. (1) HNMR was carried out on a Bruker NMR spectrometer (Model: Aspect 300 MHz). GC/MS (Model: Trace GC MS 2000) was used to confirm the content of TTP.

The Morphology Characterization of SAPl. After some SAPl were synthesized, they were prepared into cuboid samples and over dried or freeze-dried, respectively. Thereafter, the fracture morphology of xerogels was determined using a scanning electron microscope, JSM-5600LV SEM (Japan). During the determination, the accelerating voltage and the magnification were set at 20 KV and x 100, respectively.

Properties Measurement

The particle size distribution was determined according to the literature (21) and the distribution of the particle size was as follows: <26 mesh, 0 wt%; 26-45 mesh, 20 wt%; 45-65 mesh, 40 wt%; 65-80 mesh, 36 wt%; 80^90 mesh, 3.6wt %; >90 mesh, <0.5 wt%. Both the range of particle size and their distribution would well affect the characteristics of absorption, especially the kinetics and dispersion, so samples with particle size in the range of 26-90 mesh were used in tests.

Water Absorbency at Atmospheric Pressure ([W.sub.Po]). The accurately weighed 0.200 g sample was immersed into a beaker containing 100 mL 0.9 wt% NaCl aqueous solution and allowed to swell for 30 min at room temperature and atmospheric pressure. The swollen hydrogel was filtrated through a 280 mesh sieve to remove the non absorbed water and weighed. The water absorbency at atmospheric pressure was calculated by the following equation: where M is the weight of the swollen hydrogel and M0 stands for the weight of dried sample.

[W.sub.Po] = (M - [M.sub.0])/[M.sub.0] (1)

where M is the weight of the swollen hydrogel and [M.sub.0] stands for the weight of dried sample

Water Absorbency Under Load ([W.sub.P]). The accurately weighed 0.900 g sample was spread uniformly in a weighed glass tube (2.8 cm in diameter) with one endsealed with a 280 mesh fabric, and then a 123 g plastic cylinder was placed on the sample ([rho] [approximately equal to] 2 x [10.sup.3] Pa). Then put it into a beaker containing 0.9 wt% NaCl aqueous solutions, and swelled for 60 min at room temperature. After that, removed non absorbed solution and weighed it again. The water absorbency under load was calculated according to the following equation:

[M.sub.p] = ([M.sub.2] - [M.sub.1] - [M.sub.0])/[M.sub.0] (2)

where M0 denotes the weight of dry sample, M1 is the weight of glass tube and load, M2 denotes the weight of glass tube with the swollen hydrogel and load.

Swelling Rate (Wt). The accurately weighed 1.000 g sample was spread uniformly into a 100 mL beaker and started to stir. After that, 25 mL 0.9 wt% NaCI aqueous solution was added to the beaker. When 1/2 of the solution was added, time recorder was started till the magneton rotation stopped. The swelling rate was calculated by the following equation:

[W.sub.t] = 1/[t.sub.a] (3)

where [t.sub.a] denotes the time recorded.

Dispersion of the Hydrogel Particles. After determining [W.sub.P], whether the swollen hydrogel particles adhered to each other was observed. And the dispersion of the hydrogel particles was evaluated by the adhering degree. Less adhering degree meant better dispersion.

Strength of the Hydrogel. After determining [W.sub.p0], the swollen hydrogel particles were compactly placed into a glass tube with one end sealed with a 280 mesh fabric at a certain height, then a pressure of 2.2 x [10.sup.2] Pa was loaded on the glass tube and the height was labeled too. The strength of the hydrogel was evaluated by the height difference before and after pressure load. Less height difference meant better hydrogel strength.

[FIGURE 1 OMITTED]

Resilience of the Hydrogel. After the hydrogel strength experiment was finished, the loaded weight was discharged and the sample was set for 5 min, and the height of hydrogel was labeled again. The resilience of hydrogel was evaluated after and before unloads. Larger height difference meant stronger resilience.

Permeability of the Hydrogel. After superabsorbent composite swelled for 30 min at atmospheric pressure, put them onto a 280 mesh fabric to drain solution. At the same time, the time was recorded till solution did not drop in 10-min period, and the spending time in this process was used to evaluate permeability. The longer consumed time, the poorer permeability.

RESULTS AND DISCUSSION

Characterization

Figure 1 showed the structural formula and the 91) HNMR (CDC(l.sub.3]) spectroscopy of TTP. Each peak related to hydrogen types in the structural formula could be assigned as follows: (1) HNMR (CDC[l.sub.3]) [delta] (ppm): 5.844 (m, [H.sub.c]), 5.292 (dd, [H.sub.a]), 5.169 (dd, [H.sub.b]) and 4.447 (m, [H.sub.d]). These [delta] values evidenced that the product was TTP.

The results of GC/MS showed that the content of TTP was 93%.

Selection of Reaction Components

Selection of Foaming Agent. It was well known that pore structure of superabsorbent composite affected some absorption characteristics, especially Wt, so the effects of six different foaming agents on Wt were investigated and the results were listed in Table 1. It was apparent that the optimal foaming agent was PA. The vaporization point of PA was 97.1[degrees]C (boil point, b.p: 97.1 [degrees]C), which was near to the boiling point of water (b.p: 100[degrees]C). During drying, PA evaporated from the hydrogel before water, and endowed the sample with rigid pore structures. Moreover, a large amount of water had been evaporated as PA completely vaporized, which would decrease the probability of pore's swelling, and the pore structures could not be swelled again after water evaporated from the hydrogel completely. The pore structures were remained effectively after drying (15), thus, the high swelling rate was obtained. For MeOH (b.p: 69CC), EtOH (b.p: 78[degrees]C) and Ur (decomposition temperature, d.t: 50-60[degrees] C), they vaporized from the hydrogel at low temperature. Because a large amount of water existed in hydrogels for a relatively long period, the formed pores could be swelled again, and the pore structures shrinked and collapsed seriously in drying process, which would form lots of closed pores and decrease the swelling rate. For IPA (b.p: 108[degrees]C) and BA (b.p: 117.7CC), due to their higher boil points, their role as foaming agents could not be developed completely, so the pores formed were less than that of PA. As a result, Wt decreased.
TABLE 1. Influences of foaming agent on Wt of superabsorbenl composite.

Foaming agents  Quantities (mol)  Wt x [10.sup.-3] (g [s.sup.-1])

MeOH                       0.025                            3.530
MeOH                       0.049                            5.764
McOH                       0.074                            5.291
EtOH                       0.017                            3.759
EtOH                       0.034                            6.849
EtOH                       0.052                            6.329
PA                         0.013                            4.167
PA                         0.027                            8.475
PA                         0.040                            6.061
IPA                        0.011                            7.530
IPA                        0.022                            8.130
IPA                        0.032                            7.519
BA                         0.011                            6.452
BA                         0.022                            7.194
BA                         0.032                            5.556
Ur                         0.013                            7.634
Ur                         0.025                            8.403
Ur                         0.038                            8.000

Note: Values of foaming agents given in Table 1 and 2 denoted the gas
molecular numbers after they vaporized from reaction system completely.


To verify above analyses, the morphology of over-dried SAP 1, which were prepared with the same mole of MeOH, PA and BA as foaming agents and non-foaming agents respectively, were characterized, and the results were displayed in Fig. 2 (a: MeOH as foaming agent, b: PA as foaming agent, c: BA as foaming agent, d: non-foaming agents). It could be found after oven dried, SAP1 prepared with PA as foaming agent existed many pores in xerogels, and the pores number prepared with BA as foaming agent were obviously smaller than that of PA, and SAP1 prepared with MeOH as foaming agent and non-foaming agents hardly had any pore. This verified our analyses about the variation of Wt were reasonable.

In addition, it could be found that sample had a maximum with increasing amount of foaming agents no matter what foaming agent was used, which might be caused by the morphology variation of superabsorbent composite. When the higher amount of foaming agent was in reaction system, more pores were formed and the pore walls could be thinner. Thus, the pore structure could be destroyed easily by water in the hydrogel, which would further lead to shrink and collapse of pores and decrease the swelling rate.

Based on the experimental results in Table 1, it could be easily considered that a foaming agent complex, which could vaporize at the different temperature stage, could give a more inspiring result. To check this thought, the influence of foaming agent complex on Wt was further studied, and the results were shown in Table 2. The optimal combination of foaming agents was MeOH, PA and BA ([n.sub.MeOH]/[n.sun.PA]/[n.sub.BA] = 0.025/0.027/0.022), this could be attributed to the synergistic effect of foaming agent complex: In the polymerization, a lot of heat was released, and MeOH evaporated from the reaction system and formed rigid pore structures in reaction system. At the later drying stage, PA vaporized, which would form new pores or reinforce the pores formed by MeOH. At the later evaporation stage of water, BA vaporized, and the same procedure happened, more pores produced and earlier formed pores were reinforced further. Finally, the sample contained many open channels and the high swelling rate was obtained.
TABLE 2. Influences of foaming agent complex on Wt of superabsorbent
composite.

Foaming agent                   Quantities (mol)    Wt x [10.sup.-3]
complex                                             (g [s.sup.-1])

Propanol-methanol                     0.042, 0.025             5.519
Propanol-methanol                     0.027, 0.025             9.149
Propanol-methanol-                    0.027, 0.049             7.358
N[H.sub.4]HC[O.sub.3]          0.027, 0.025, 0.025             4.900
Propanol-methanol-             0.027, 0.025, 0.011             6.277
isobutanol
Methanol-propanol-      0.025, 0.027, 0.011, 0.038             8.026
butanol-urea
Methanol-propanol-      0.025, 0.027, 0.022, 0.025             9.083
butanol-urea
N[H.sub.4]HC[O.sub.3]-         0.013, 0.025, 0.027            10.246
methanol-propanol
Methanol-propanol-             0.025, 0.027, 0.022            14,650
butanol


[FIGURE 2 OMITTED]

When N[H.sub.3]HC[O.sub.3] or N[H.sub.4]N[O.sub.3]was used as foaming agent with MeOH and PA, the swelling rate of sample was lower than that prepared in the presence of MeOH, PA and BA. For the combination of NH4HCO3, MeOH and PA, the lower vaporized temperature of N[H.sub.3]HC[O.sub.3] (d.t: 50[degrees]C) and MeOH would produce many pores in the early stage, and the formed pores would shrink or collapse as bulk water vaporized, so the combination had no benefit to the swelling rate. For the combination of N[H.sub.3]N[O.sub.3] (d.t: 150[degrees]C), MeOH and PA, due to the higher vaporization point and no the function of rigid pore structures of N[H.sub.3]N[O.sub.3], it could not prevent the shrink of pores effectively.

[FIGURE 3 OMITTED]

Selection of Semi-IPNs Polymer. The use of foaming agents enhanced the swelling rate of superabsorbent, but weakened some properties, such as the water absorbency under load, hydrogel strength, dispersion, and resilience. To overcome these disadvantages, the technology of semi-IPN was used. In this work, PEG and PVA were used. The effects of them on properties were compared and the results were listed in Table 3. From it, it could be found that PEG was the better one, which was due to the better solubility of PEG in water at room temperature.
TABLE 3. Influences of semi-IPNs polymer on [W.sub.Po] and Wt of
superabsorbent composite.
                             [W.sub.Po]     Wt x [10.sup.-3]  Other
Sorts of semi IPNs polymer  (g [g.sup.-1])  (g [s.sup.-1])    preperties

Without semi IPNs polymer               89            14.620  Bad
PEG (0.5g)                              86            14.556  Good
PVA (0.5g)                              82            11.236  Bad


To investigate the relationship between the absorption properties and structure, the morphologies of three samples dried under freezer dryer, which were semi-IPNs SAPl prepared with MeOH, PA and BA as foaming agent, non-semi-IPNs prepared with MeOH, PA and BA as foaming agents, and non-semi-TPNs copolymer without foaming agent, were studied, and the morphology of them were shown in Fig. 3 (a: semi-IPNs SAPl with MeOH, PA and BA as foaming agents, b: non-semi-IPNs with MeOH, PA and BA as foaming agents). It could be found the fracture of semi-IPNs kept the pore structure more regular than that of non-semi-IPNs, while the fracture of SAP1 prepared without foaming agent had little pores. This verified the semi-IPNs structure could increase the hydrogel strength and eliminate the shrink and collapse, which further ensured high Wt.

Selection of Crosslinking Agent. After introducing foaming technique and semi-IPNs polymer into reaction system, the effects of 6360 on properties of superabsorb ent composites were investigated and the results were listed in Table 4. It was obvious that when the volume ratio of 6360 to AA was 0.188%, the sample had better properties. However, the strength of swollen hydrogel particles was still not good enough comparing with that of the product what had been used in hygeian area, the reason was that the crosslinking reaction of epoxy groups was step polymerization process, which needed higher temperature, and could not crosslink polymers effectively at the early stage. As a result, polymers formed at the earlier stage could not be crosslinked, which would lead to poor hydrogel strength. So, two other crosslinking agents, NNMBA and TTP, which contained vinyl groups and had different reaction activities, were used in polymerization to obtain a homogeneous network to improve the properties of superabsorbent composites, in which NNMBA and TTP were used to crosslink the polymer in the earlier and middle stage (7).
TABLE 4. Influences of 6360 on [W.sub.Po], Wi, and other swelling
properties of superabsorbent composite.

6360           [W.sub.Po]   Wt x        Hydrogel    Hydrogel  Hydrogel
(V.sub.6360]  (g           [10.sup.-3]  strength  dispersion  resilience
/[V.sub.AA]   [s.sup.-1])  (g
vol%)                      [s.sup.-1])

0.125                  65       12.610  Good      Bad         Bad
0.18                   60       17.452  Good      Bad         Good
0.25                   57       13.699  Good      Bad         Good
0.313                  47       10.000  Good      Bad         Bad


Selection of Surfactant. After above mentioned agents and technology were used, most properties of superab-sorbent, such as [W.sub.Po], [W.sub.P], Wt, dispersion, hydrogel strength, and resilience, were improved but the permeabilities. According to literature (15), the strong surface tension between hydrogel and water was the basic reason. To improve the permeability of hydrogel, surfactants, such as Span, Tween, detergent, sodium dodecyl benzene sulfonate, Soap, and SOP, were used, and the results showed that SOP ([m.sub.SOP][m.sub.SAPI] = 1/40) was the best. It might be attributed to the stronger hydrophilicity of phosphate group core and stronger hydrophobicity of longer aliphatic chain of OT.

Optimization of Reaction Conditions

Effect of Neutralization Degree(ND) of AA on [W.sub.Po], [W.sub.P], and Wt. The effect of ND of AA on [W.sub.Po], [W.sub.P], and Wt were shown in Fig. 4. As we could see, [W.sub.Po] and Wt increased with the increase of ND of AA till they reached maximums at 73.4%, while [W.sub.P] decreased monotonously. With increasing ND of AA, the electrostatic repulsion of--CO[O.sup.-] on the polymer chians increased (22), and the difference of osmotic pressure between hydrogel and external solution increased (23), So [W.sub.Po] increased. However, high ND also led to low [W.sub.Po]. Because NNMBA and TTP were more reactive than 6360 (7), most of A A, NNMBA, TTP, and AM reacted in the early stage of polymerization. When acrylate was polymerized, only 6360 could act as crosslinking agent. The polymerization rate of acrylate was low, and the temperature of system was low. Moreover, introduction of foaming agents also resulted in low temperature. Thus, the crosslinking efficiency was low, and the soluble polymer increased. So, [W.sub.Po] decreased when ND of AA was more than 73.4%.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The swelling rate of sample was mainly determined by the ability of water acquiring and transporting of hydro-philic groups on polymer chains (24). With the increase of ND, the difference of osmotic pressure between hydro-gel and external solution increased, and the ability of polymer chains acquiring and transporting water increased too. At a high ND of AA, the non-homogeneous network structure could not preserve the channels effectively. So, Wt was low.

With the increase of ND, the crosslinking density decreased (25). The network elasticity would decrease, and the ability of polymer network resisted the pressure from environment decreased. Therefore, [W.sub.P] decreased too.

Effect of AM on [W.sub.Po], [W.sub.P], and Wt. The influence of AM ([n.sub.AM][n.sub.AA]) on [W.sub.Po], [W.sub.P], and Wt were shown in Fig. 5. As AM was less than 30.3%, [W.sub.Po] and Wt increased with the increase of AM, and after that, they decreased. But [W.sub.P] increased monotonically. AM was a non-ionic monomer, so the external conditions had little effect on properties, such as, [W.sub.P], and Wt. Moreover, the collaborative absorbent effect among different hydro-philic groups resulted in higher absorbency (8), (26). As a result, [W.sub.P] increased. On the other hand, the hydrophily of--CON[H.sup.2] was less than--CO[O.sup.-], the hydrophilicity of the polymer network decreased with the increase of AM. Therefore, when AM was higher than 30.3%, [W.sub.Po] decreased.

As--CON[H.sub.2] existed in polymer network could lorm hydrogen bond with water and accelerate the diffusion of water. At the same time, [W.sub.Po] increased when AM was lower than 30.3%. So, Wt increased. However, after that, [W.sub.Po] decreased, and finally led to the decreasing of Wt.

Conirary to [W.sub.Po] and Wt, [W.sub.P] kept increasing with the increase of AM. AM was more reactive than AA (10), higher activity meant that polymerization conducted more vigorous, thus, a lot of heat was given off, which would lead to high crosslinking degree of 6360. As a result, the network elasticity increased, and [W.sub.P] kept increasing with the increase of AM.

Effect of Initiator on [W.sub.Po], [W.sub.P], and Wt Figure 6 revealed that [W.sub.Po], [W.sub.p], and Wt increased with the increase of initiator ([n.sub.H2O2][n.sub.AA]) and then started to decrease at 3.34 x [10.sup.-4], 2.92 x [10.sup.-4], and 3.34 x [10.sup.-4], respectively. At low initiator concentration, the number of free radicals, the crosslinking density and the polymerization rate was low. Thus, soluble parts would increase, and [W.sub.Po], Wt, and [W.sub.P] were low. With increasing initiator, crosslinking degree increased. The collapse of pore structures in drying could be prevented effectively. So [W.sub.Po], Wt, and [W.sub.P] increased. With further increasing initiator, polymerization rate was strongly enhanced. However, the part with low polymerization degree increased in sample, and the rigidity of polymer network decreased. So, [W.sub.Po], Wt, and [W.sub.P] decreased. The difference of initiator corresponding to the maximum [W.sub.Po], Wt, and [W.sub.P] was caused by the network elasticity.

[FIGURE 6 OMITTED]

Effect of NNMBA on [W.sub.Po], [W.sub.P], and Wt. The effect of NNMBA ([m.sub.NNMB]/[m.sub.AA]) on [W.sub.P], [W.sub.P], and Wt were shown in Fig. 7. With increasing NNMBA, [W.sub.Po], Wt, and [W.sub.P] had maximums at 7.14 x [10.sup.-4], 7.14 X [10.sup.-4] and 7.74 X 1(T4, respectively. The reactivity of NNMBA, AA, and AM were higher than that of AANa (7), At low NNMBA content, the polymer chains could not be crosslinked effectively. Linear polymer produced, the rigidity of polymer network was low, and pore structures were destroyed in drying process. Thus, [W.sub.Po], [W.sub.P], and Wt were low. With the increase of NNMBA, the crosslinking density increased. An appropriate polymer network could form. So, [W.sub.Po], [W.sub.P], and Wt increased. However, excessive NNMBA resulted in restriction of expansion of network. Thus, [W.sub.Po], [W.sub.P], and Wt decreased. The difference of NNMBA corresponding to the maximum [W.sub.Po], Wt, and [W.sub.P] was also caused by the network elasticity.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Effect of TTP Content on [W.sub.Po], [W.sub.P], and Wt. As shown in Fig. 8, [W.sub.Po] and Wt increased with increasing amount of TTP (VTTp/VAA), but they decreased when TTP was higher than 0.375%. For [W.sub.P], it increased all along. The function of TTP in the reaction system was similar to NNMBA, so the variation of [W.sub.Po], [W.sub.P], and Wt could be understood easily.

Effect of PEG Content on [W.sub.Po], [W.sub.P], and Wt. The influence of PEG content (mpEG/wAA) on [W.sub.Po], [W.sub.P], and Wt were shown in Fig. 9. [W.sub.Po], [W.sub.p], and Wt first increased, and then decreased. However, PEG contents corresponding to the maximums were different, which were 18.5, 19.0, and 19.0%o, respectively. The long chains of PEG could physically entangle on superabsorbent network, and form semi-IPNs structure to enhance the elasticity of network, which would retain the pore structures effectively. Thus, [W.sub.Po], [W.sub.P] and Wt increased. However, the hydrophilicity of PEG was weaker than P [AA-co-AM], increasing PEG excessively would decrease the water holding capacity, thus, [W.sub.Po] decreased. And the variation trends of Wt and [W.sub.p] could be attributed to the same reason.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Effect of Reaction Time on [W.sub.Po], [W.sub.P], and Wt Figure 10 showed the influence of reaction time on [W.sub.Po], Wt, and [W.sub.P]. [W.sub.Po], Wt, and [W.sub.P] reached maximum at 2.5, 3, and 2.5 h, respectively. The process of polymerization was a radical reaction, while crosslinking reaction of 6360 was step polymerization process, the rate of polymerization would be more quickly than crosslinking reaction of 6360. As a result, crosslinking density increased with the increase of reaction time. On the other hand, the monomer conversion increased with the increase of reaction time. So, [W.sub.Po], Wt, and [W.sub.P] increased. However, when reaction time prolonged, the increase of crosslinking degree led to the decreasing of space of hydrogel network. So, [W.sub.Po] and Wt decreased. For [W.sub.P], the increase of crosslinking degree could improve the network elasticity, so the maximum occurred at 3h.

CONCLUSION

A novel semi-IPNs porous salt-resistant superabsorbent composite was prepared by solution copolymerization of AA and AM using PEG as semi-IPNs polymer, NNMBA, TTP and 6360 as crosslinking agents, MeOH, PA and BA as foaming agents, Vc an H202 as initiators, surface cross-linking and blending with inorganic salt and SOP. The influences of reaction conditions on properties of superab-sorbent composite were investigated, and the optimal conditions were as follows: for water absorbency both at atmospheric pressure and load, the ratios of AM, H202, NNMBA, TTP, 6360, and PEG to AA were 30.3 mol%, 3.34 x [10.sup.-4] mol, 7.14 x [10.sup.4] wt, 0.375 vol%, 0.188 vol%, and 19 wt%, respectively. And ND of AA and reaction time were 73.4% and 2.5 h. The water absorbency of superabsorbent composite prepared at optimal conditions in 0.9 wt% NaCl aqueous solution were 61 g [g.sup.-1] and 16.7 g [g.sup.-1] respectively, and the swelling rate reached 22.003 x [10.sup.3] g [(g s).sup.-1]. The excellent hydrogel strength, resilience, permeabilities, and dispersion of swollen hydro-gel were also observed. Based on these properties, this kind of superabsorbent composite could be used in hygiene products, such as feminine hygiene products, and disposable diapers.

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Correspondence to: Zhenbin Chen; e-mail: zhenbinchen@163.com

Contract grant sponsor: Graduate Tuor Funds of Department of Educa-tion of Gansu Province; contract gram number: 1001ETC094; contract grant sponsor: Doctorial Program Funds of the Lanzhou University of Technology; contract grant number: SB01200806.

DOI 10.1002/pen.22034

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 201 I Society of Plastics Engineers

Zhenbin Chen, (1), (2), (3) Fang Dong, (1), (2) Mingzhu Liu, (3) Xiaohua Qi (3)

(1.) State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou, 730050 Gansu, China

(2.) School of Material Science and Engineering, Lanzhou University of Technology, Lanzhou, 730050 Gansu, China

(3.) School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000 Gansu, China
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Author:Chen, Zhenbin; Dong, Fang; Liu, Mingzhu; Qi, Xiaohua
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Date:Dec 1, 2011
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