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我的联系方式: liuxingw3@163.com INTRODUCTION Superabsorbent polymer (SAP) as a special polymeric material can absorb a large amount of water and the water is hardly removed even under pressure. Because of its excellent characteristics, the superabsorbent is widely used in many applications such as disposable diapers, feminine napkins, soil for agriculture and horticulture, gel actuators, water-blocking tapes, drug delivery systems, and absorbent pads.1–4 Polyacrylate superabsorbents, which were more widely used in recent years, generally exhibit a very high absorbency in deionized water; they, however, have the problem of poor resistance to salts as evinced by their notable low absorbency exhibited to electrolytic solutions such as an aqueous common salt solution. The circumstances in which the superabsorbents are used always contain salts, such as disposable pads, sheets, and towels for surgery, adult incontinence, and feminine hygiene products. So, it is important to improve these superabsorbents’ salt resistance. Inorganic gel generally has excellent salt resistance, so if we prepare a hydrogel compound of polymer gel and inorganic gel, an excellent salt-resistant superabsorbent would be obtained. We have conducted a survey of the literature concerning salt-resistant superabsorbent polymers but found little information on this subject. On the basis of our previous studies, we prepared the salt-resistant superabsorbent as follows. At first, we prepared crosslinked sodium polyacrylate; then, we used ethylene glycol diglycidyl ether (EGDE) to crosslink the molecular chains existing at least in the vicinity of the surfaces of the crosslinked sodium polyacrylate; last, the surface-crosslinked superabsorbent was mixed with inorganic salt powders. The superabsorbent we obtained has excellent salt-resistance, hydrogel strength, and absorption rate. Moreover, it also has high water absorbency under pressure as to aqueous salt solution. The superabsorbent can be used in hygiene products such as feminine hygiene products, disposable diapers, and so on. EXPERIMENTAL Sample preparation Acrylic acid (AA, Beijing Dongfang Chemical Factory, Beijing, China) was distilled at reduced pressure (boiling point = 20-21°C at 0.5 mmHg). Sodium hydroxide (NaOH, Tianjin Chemical Regent Co., Tianjin, China), N,N-methylene-bis-acrylamide (bisAM, Fluka, Germany), ethylene glycol diglycidyl ether (EGDE, Fluka, Germany), hydrogen peroxide (H2O2, Jiangsu Sanmu Chemical Factory, Jiangsu, China), L-ascorbic acid (Vc, Xi’an Chemical Regent Co., Xi’an, China), sodium aluminate (NaAlO2, Shanghai Chemical Regent Co., Shanghai, China), and potassium dihydrogen hyphosphate (KH2PO4, Chengdu Chemical Regent Co., Chengdu, China) were of analytical grade. Ten milliliters of AA was added in a beaker and then 13.6 mL aqueous NaOH solution (25% in mass) was carefully added to partially neutralize AA. BisAM (35 mg) was added and the mixture was stirred at ambient temperature for 30 min. SALT-RESISTANT SUPERABSORBENT POLYMER Figure 1 (a) Curves of WAP0 and WAP versus reaction temperature. (b) Curve of hydrogel modulus versus reaction temperature Then, 0.5 mL H2O2 (0.1% in mass) and 0.2 mL Vc (0.1% in mass) were introduced into the reactor. The polymerization was carried out at 50°C under nitrogen for 8 h. The resulting polymer was dried at 150°C to a constant weight. After the polymer mentioned above was reswollen in excess water to remove the soluble materials, we acquired the gel. Then, the gel was redried, reweighed, ground, and milled through 26- to 90-mesh screen (SAP1). The sample was mixed with a solution compound of 1.2 wt%EGDE, 78.8 wt%methanol, and 20 wt % H2O at a ratio of 7g of the solution to 100 g of SAP1 particles. Then, the mixture above was heated for about 30 min at 150°C. The dried product was ground and milled again (SAP2). With the addition of 10 parts KH2PO4 and 4 parts NaAlO2 by weight to 100 parts dried product mentioned above (SAP2), we obtained the final product (SAP). The scheme of the polymerization, the surface-crosslinking, and the modification was Determination of water absorbency in physiological saline solution at atmosphere pressure (WAP0 ) The accurately weighted SAP (about 0.2 g) was immersed into a certain amount of physiological saline solution (0.9 wt % NaCl) and allowed to soak at ambient temperature for 30 min. The swollen polymer was filtrated through an 80-mesh sieve to remove nonabsorbed water and weighted to find the amount of the solution absorbed thereby. The water absorbency was calculated by using the equation where M and M0 denote the weight of the water swollen hydrogel and the weight of the SAP, respectively. Determination of water absorbency in physiological saline solution at applied pressure (WAP) The accurately weighted SAP (about 0.9 g) was uniformly placed at the surface of 200-mesh nylon fabric sealing the bottom of a plastic hollow cylinder 2.8 cm in inner radius and 6.0 cm in height. A cylinder 123 g in weight and 2.8 cm in radius was put into the plastic hollow cylinder (P _ 2 _ 103 Pa), weighted, and put Figure 3 (a) Curves of WAP0 and WAP versus neutralization degree of acrylic acid. (b) Curve of hydrogel modulus versus neutralization degree of acrylic acid. on a porous glass slice, which was placed in a evaporating dish filled with physiological saline solution (0.9 wt % NaCl) and allowed to soak at ambient temperature for 60 min, then removed from the solution and left draining, and weighted again to find the amount of the solution absorbed thereby.9 The water absorbency was calculated by using the equation whereM1 andM2 denote the weight of the total plastic cylinder with the superabsorbent and cylinder were placed in when the hydrogel was dry and swollen, and M0 denoted the weight of SAP. Determination of hydrogel strength (G) A sample in the form of swelled hydrogel was tested for the hydrogel strength by using an apparatus composed of micrometer gauge, weight support, and sample pond.10 The swelled hydrogel was obtained by allowing a salt-resistant superabsorbent polymer to swell in physiological saline solution (0.9 wt % NaCl) for 30 min. The height of the sample under pressure was read from the gauge. The pressure applied to the sample was calculated from the weights and the contract area of the sample pond. The hydrogel strength was the slope of the plot of the pressure applied to the sample versus the height of the sample. Figure 4 (a) Curves of WAP0 and WAP versus amount of initiator. (b) Curve of hydrogel modulus versus amount of initiator. RESULTS AND DISCUSSION Effect of reaction temperature on WAP0, WAP and G Figure 1 showed that WAP0, WAP, and G were increased as the reaction temperature increased up to 43°C, while it decreased when reaction temperature was higher than 56°C. There was an optimal reaction temperature range of 43–56°C, at which WAP0, WAP, and G all reached their own preferable values. It was known that polymerization velocity increased as the reaction temperature increased; that is to say, the monomer conversion increased, which would lead to the increase of insoluble parts, so that WAP0, WAP, and G increased before the optimal range. After 56°C, any further increasing temperature would result in the crosslinking and chain termination dominant, in accompaniment with WAP0, WAP, and G decreasing Figure 5 (a) Curves of WAP0 and WAP versus amount of crosslinking agent. (b) Curve of hydrogel modulus versus amount of crosslinking agent. Effect of reaction time on WAP0, WAP, and G Figure 2(a, b) showed that WAP0 increased monotonously as reaction time increased because the monomer conversion increased as reaction time increased in radical reaction but WAP and G could reach maximums, respectively, as reaction time increased. It is given that G relates to the network crosslink density by the equation where p is the polymer density, M c is the molecular weight of the network chains, 2 is the volume fraction of crosslinked polymer in equilibrium swollen gel [i.e., 2 1/(WAP0 )], 2is the volume fraction of crosslinked polymer in zerogel, and R and T are their usual meanings. Because 2 0 1 for the zerogel, G becomes According to eq. (4), we concluded that G increased as WAP0 increased, so that WAP and G decreased as reaction time increased in the range of 8–12 h. Effect of neutralization degree of acrylic acid on WAP0, WAP, and G Figure 3(a, b) showed that WAP0 and WAP increased as neutralization degree (ND) increased until it reached an optimal range of 70–72%. G decreased monotonously as ND increased. It was known that the electrostatic repulsion between attached carboxylate anions increased as the concentration of anions increased, so that the stretching extent of hydrogel network increased. Moreover, the increase in the osmotic pressure difference between hydrogel and external solution would lead to excellent WAP0 and WAP. A further increase in ND made part of the polymer hydrogel network become soluble, thus both WAP0 and WAP decreased in the range of 72–85%. The effect of the electrostatic interaction of charged groups on elastic free energy decreased the elastic modulus, so that G decreased monotonously as ND increased.11 Figure 6 (a) Curves of WAP0 and WAP versus amount of surface-crosslinking agent (EGDE). (b) Curve of hydrogel modulus versus amount of surface-crosslinking agent (EGDE). Effect of the amount of initiator on WAP0, WAP, and G Figure 4(a, b) showed that WAP0, WAP, and G reachedmaximums at nH2O2/nAA _ 5.2 _ 10_5, 8.0 _ 10_5, and 6 _10_5. It was known that monomer conversion increased as the concentration of initiator increased, so that WAP0, WAP, and G increased while nH2O2/nAA _5.2 _ 10_5, but with an increase in amount of initiator, the collision between monomer-free radicals also increased, which led to the increase of the soluble parts; thereby WAP0, WAP, and G increased, while nH2O2/nAA_ 7.5 _ 10_5. Effect of the amount of crosslinking agent on WAP0, WAP, and G Figure 5(a, b) showed that WAP0, WAP, and G reached maximums at mbisAM/mAA _ 1.9 _ 10_3, 2.7_ 10_3, and 2.4 _ 10_3, respectively. As we knew, the more the amount of bisAM, the higher crosslinking density of the hydrogel, and the higher the elastic chain force, the lower soluble parts of the polymer.12 The elastic chain force of the hydrogel network was the only negative effect on water absorbency13; however, the decrease of the soluble part of the polymer had a positive effect on water absorbency. This was why there existed maximums in Figure 5(a). According to eq. (3), the greater the crosslinking density, the higher the hydrogel modulus, but the result did not obey the equation, and G gradually decreased, when WbisAM/WAA _ 2.4 _ 10_3. It was mainly due to the high crosslinking density of the polymer hydrogel network, which decreased the degree of crystallization and reduced hydrogel modulus. Effect of the amount of surface-crosslinking agent on WAP0, WAP, and G The superabsorbent polymer particles (SAP1) were surface-crosslinked by EGDE to crosslink molecular chains existing at least in the vicinity of the surfaces of the superabsorbent polymer particles. The effect of the amount of surface-crosslinking agent (EGDE) on WAP0, WAP, and G should accord with the effect of amount of crosslinking agent. The result of Figure 6(a, b) proved in accordance. Figure 7 (a) Curves of WAP0 and WAP versus amount of inorganic salt (IS). (b) Curve of hydrogel modulus versus amountof inorganic salt (IS). Effect of the mass ratio of inorganic salt to initial superabsorbent on WAP0, WAP, and G Figure 7(a) showed that WAP0 and WAP increased as the amount of inorganic salt that consisted of 29 wt % NaAlO2 and 71 wt % KH2PO4 increased,while mIS/mSAP _ 0.05, but decreased gradually while mIS/mSAP _ 0.15. It was known that inorganic gel had excellent salt resistance so that the salt resistance increased with increasing the amount of inorganic salt. A further increase in the amount of inorganic salt decreased the osmotic pressure difference between the gel and the external solution, and WAP0 and WAP decreased. Figure 7(b) showed that G decreased monotonously as the amount of inorganic salt increased. This explained that the elasticity modulus of the hydrogel was higher than that of the inorganic hydrogel. Figure 8 (a) Curves of WAP0 and WAP versus molar ratio of sodium aluminate (NaAlO2) to potassium dihydrogen hyphosphate (KH2PO4). (b) Curve of hydrogel modulus versus molar ratio of sodium aluminate (NaAlO2) to potassiumdihydrogen hyphosphate (KH2PO4). Effect of the molar ratio of sodium aluminate to potassium dihydrogen on WAP0, WAP, and G Figure 8(a) showed that WAP0 and WAP increased as the amount of NaAlO2 increased, while nH2O2/nKH2PO4 _ 0.67. When the salt-resistant superabsorbent swelled in physiological saline solution, AlO2_ interacted with H2PO4_ and produced Al(OH)3 according to the following two reactions (a, b). From the reactions, we could see that the smaller the amount of NaAlO2 was, the less Al(OH)3 was produced, and the salt resistance of the superabsorbent decreased, while nNaAlO2/nKH2PO4 _ 0.67. The excessive NaAlO2 interacted with H2PO4 _ yielded to the ion pair, which formed an electric double layer, and the electric double layer kept the colloidal particles relatively stable when nNaAlO2/nKH2PO4 _ 0.67. As the molar ratio increased, more antiparticles entered the electric double layer and the stability of the colloidal particles was destroyed so that there existed an optimal range (0.67_ nNaAlO2/nKH2PO4_ 1.1), in which WAP0 and WAP had a higher value. Figure 8(b) showed that G decreased monotonously as the amount of NaAlO2 increased, this mainly due to the increase of Al(OH) CONCLUSION Crosslinked sodium polyacrylate after being surfacecrosslinked by EGDE and being modified by inorganic salt had excellent salt resistance and hydrogel modulus .The water absorbency first increased with increasing reaction temperature, neutralization degree of acrylic acid, amount of initiator, crosslinking agent, and surface-crosslinking agent, mass ratio of inorganic salt to initial superabsorbent, and molar ratio of sodium aluminate (NaAlO2) to potassium dihydrogen hyphosphate (KH2PO4) and then decreased continuously.The results indicated two opposite effects of the above-cited reaction conditions. The hydrogel modulus had the same tendency with water absorbency when it depended on reaction temperature, reaction time, amount of initiator, crosslinking agent, and surface-crosslinking agent, but decreased monotonously with increasing neutralizationdegree of acrylic acid, mass ratio of inorganic salt toinitial superabsorbent, and molar ratio of NaAlO2 to KH2PO4. [ Last edited by 渔火江枫 on 2008-4-19 at 08:12 ] |
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引言 超级吸收性聚合物作为一种特殊的聚合材料具有吸收大量水分的特性，水在即使在压力下也很难除掉。SAP由于其优异的特性而有很多的应用，如免洗尿布，女用纸巾，农业园艺处理污物用品，凝胶激励，堵水带，药物发送系统，吸水垫片。1－4聚丙烯酸吸收剂几年来被广泛使用，对去离子水有很高的吸收性。然而它们抗盐性能不佳，如它们对普通含水盐类这些电解质溶液吸收率极低就证实了这一点。这些吸收剂通常在诸如手术用免洗垫，片、毛巾，成人失禁，女性卫生用品等含盐环境中使用。因此。提高它们的抗盐性及其重要。无机凝胶通常具有很好的抗盐性。假如我们能制备一种以聚合物凝胶和无机凝胶为基础的水凝胶化合物，其必将具有优异的抗盐性。我们调研了很多有关抗盐性超级吸收性聚合物，发现相关信息极少。在已有的基础上我们按照下面所提方法制备了超级吸收剂———————— |
2楼2008-04-17 18:54:07
