Effects of Partial Dehydration and Freezing Temperature on the

Jul 26, 2010 - Superabsorbent polymers (SAPs) were prepared from carboxymethyl chitosan (CMCS) cross-linked to a gel, concentrated by partial ...
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Ind. Eng. Chem. Res. 2010, 49, 8094–8099

Effects of Partial Dehydration and Freezing Temperature on the Morphology and Water Binding Capacity of Carboxymethyl Chitosan-Based Superabsorbents Akram Zamani*,†,‡ and Mohammad J. Taherzadeh† School of Engineering, UniVersity of Borås, 50190 Borås, Sweden 50190 and Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, 41296 Go¨teborg, Sweden SE-412 96

Superabsorbent polymers (SAPs) were prepared from carboxymethyl chitosan (CMCS) cross-linked to a gel, concentrated by partial dehydration in a rotary evaporator (at 70, 85, and 100 °C), frozen at -5, -20, and -196 °C, and then freeze dried. A 0.9% aqueous solution of CMCS was gelled by addition of glutaraldehyde and partially dehydrated to 1.3-16.8% dry matter (DM) before freeze drying. The water binding capacity (WBC) of the products was up to 171 g/g of superabsorbent. The best results were obtained when 32-81% of the water in the gel was removed in the evaporator at 85-100 °C, and the concentrated gel (1.3-4.7% DM) was frozen in liquid nitrogen at -196 °C before freeze drying. On average, these SAPs, according to SEM micrographs, had a porous sponge-like structure and absorbed 35 and 32 g/g of saline and urine solutions after 10 min exposure, respectively. The corresponding WBC of two commercial polyacrylate-based SAPs was 34-57 g/g for saline and 30-37 g/g for urine solutions. Introduction Superabsorbent polymers (SAPs) are loosely cross-linked hydrophilic polymers with a very high capacity to absorb and retain aqueous solutions. SAPs have many applications in medical and sanitary products, with generally short life-time requirements. Therefore, there is an obvious need for developing biobased SAPs that undergo quick and safe decomposition after usage.1–3 Chitosan, which is a cationic polysaccharide, is produced by deacetylation of chitin, the most abundant biopolymer in the world after cellulose. Chitin, chitosan, and their derivatives can be used as a possible renewable source for production of SAPs.4,5 Carboxymethyl chitosan (CMCS) was recently applied for production of SAPs with a high water binding capacity (WBC) in water and saline solution. Preparation of SAPs from CMCS can be performed by cross linking of CMCS in aqueous solutions to a gel followed by a drying process.5 Drying is a critical step during the preparation of SAPs from CMCS and affects the WBC of the final product. Air drying of the cross-linked CMCS results in a product with poor WBC, while a foaming technique with, e.g., n-pentane as the blowing agent, during the air drying of the gel, results in formation of a porous structure of CMCS with high WBC.5 Alternatively, SAPs can be prepared via freeze drying of CMCS gel.5 This drying method results in good properties of the SAPs. However, freeze drying of dilute gels is an expensive process. Additionally, increasing the concentration of CMCS solution is restricted because of a very high viscosity of concentrated polymer solution and the limited solubility of CMCS in water. It would be thus interesting to investigate a predrying method to increase the concentration of the gel after the cross linking and before the freeze drying without decreasing the WBC of the final product. Among different drying methods, predrying in a vacuum rotary evaporator showed promising results in our preliminary experiments and became the basis for the current work. * To whom correspondence should be addressed. Tel.: +46-334354360. Fax: +46-33-4354008. E-mail: [email protected]. † University of Borås. ‡ Chalmers University of Technology.

The objective of this study was partial dehydration of CMCS gel in an evaporation step in order to decrease the amount of water which should be eliminated via freeze drying. A secondary goal in this work was to investigate the effect of freezing temperature on the properties of the final freeze-dried SAPs prepared from CMCS. Experimental Section Materials. A commercial medium Mw chitosan (degree of deacetylation 85%, viscosity 120 mPaS, and ash content less than 1%, supplied by BioLog, Germany) was used in this work for production of CMCS. Two commercial polyacrylate-based superabsorbents, Sumitomo SEIKA (5A70) and Aqualic ca. (L520), were used in this work as reference SAPs, and the WBCs of the products were compared to their WBCs. Production and Characterization of Carboxymethyl Chitosan (CMCS). CMCS was prepared according to Pang et al.6 with minor modifications. In brief, 675 g of NaOH was dissolved in 1 L of water, cooled in an ice bath, added to 500 g of chitosan, and mixed. Then, 4 L of 2-propanol was added to this mixture and stirred manually while cooling in an ice bath. A solution of 750 g of monochloroacetic acid in 1 L of isopropanol was subsequently added in 10 equal portions and mixed. It was important to cool down the mixture and avoid raising the temperature during addition of monochloroacetic acid. Then, the mixture was left for 4 h at room temperature without further mixing to obtain CMCS. The carboxymethylation reaction was stopped by adding 70% ethanol (6 L) to the mixture, and CMCS was separated by filtration. It was then washed five times with 70% ethanol and dried at 50 °C. The product was the sodium salt of CMCS, which was used for production of superabsorbents. CMCS was characterized by FTIR7,8 and conductometric methods.5 The acid form of CMCS was prepared via the reaction of the sodium salt of CMCS with hydrochloric acid as previously described.5 This acid form was used for FTIR analysis (DuraSample IR, SensIR Technologies, Danbury, CT). The degree of substitution (Ds) of CMCS was measured with the conductometric method according to a previous report.5 Preparation of CMCS Gel and Its Partial Dehydration. The dried CMCS was dissolved in water and centrifuged to remove any insoluble particles. The final concentration of CMCS

10.1021/ie100257s  2010 American Chemical Society Published on Web 07/26/2010

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was 0.9%. Then, glutaraldehyde (0.02 g per g of CMCS) was added and mixed for 2 min at room temperature to form a gel. This cross-linked CMCS gel was partially dehydrated in a 20-1 vacuum rotary evaporator (Laborota 20, Heidolph, Schwabach, Germany) equipped with a vacuum pump (PC 3004 VARIO, Vacuubrand, Wertheim, Germany). For the evaporations, 1.0 kg of the CMCS gel was heated in a 10-l evaporation flask at 70-100 °C (the temperature of the heating medium) for different periods of time. The flask rotation speed was adjusted to 80 rpm, and the vacuum level was controlled automatically on 130-200 mbar during evaporation. After the desired evaporating time, the partially dried gel was collected from the evaporation flask. Final Drying of Partially Dehydrated CMCS Gels. The concentrated CMCS gels were rubbed with a metal screen to make them homogeneous. Then, three equal portions of the concentrated gels were frozen at (a) -5 °C in a freezer, (b) -20 °C in a freezer, and (c) -196 °C by immersing in liquid nitrogen. The frozen gels were then freeze dried and stored at room temperature for further analyses. In order to prepare a reference CMCS-SAP, a fresh gel (0.9%) without a dehydration step was also frozen in the above conditions and freeze dried. Measurement of the Water Binding Capacity (WBC). The freeze-dried samples were milled in a kitchen mill to obtain a powder with less than 1.0 mm in particle size. Then, 0.025-0.05 g of the powder was placed in nonwoven bags and immersed in water, saline (0.9% NaCl), or synthetic urine solution (19.4 g of urea, 8.0 g of sodium chloride, 0.6 g of calcium carbonate, and 2.05 g of magnesium sulfate heptahydrate in 970 g of water).9 The exposure times were 1, 10, and 100 min for pure water and 10 min for salt and urine solutions. The swollen gels were then collected from the bags and weighed, and the WBCs of the samples were calculated according to the following equation WBC )

W2 - W1 W1

(1)

where W1 is the weight of the SAP before immersion and W2 is the weight of the swollen gel. The water binding capacity (WBC) is calculated as grams of absorbed liquid per gram of the SAP. The experiments were performed at least in duplicate, and the standard deviation of the results was less than 10%. Scanning Electron Microscopy. The morphologies of different dried CMCS gels were investigated by scanning electron microscopy (SEM). The dried samples were subjected directly to a high-performance scanning electron microscope (Quanta 200 ESEM FEG, FEI, ORA), and the images were taken using an Everhart Thornley Secondary Electron Detector (ETD) in high-vacuum mode at 5 kV. Results and Discussion Carboxymethyl chitosan has previously shown a high potential for preparation of SAP in a process that involves cross linking of CMCS in aqueous solution followed by drying the provided gel.5 The method of drying of CMCS gel has significant effects on the WBC of the product. A porous structure with high WBC can be obtained by freeze drying the CMCS gel. However, freeze drying of dilute gels may not be economically feasible.5 Therefore, in this work, a partial dehydration step prior to freeze drying was developed in order to increase the concentration of hydrogel. Preparation and Characterization of CMCS. CMCS was successfully produced in 500 g batches by carboxymethylation

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Figure 1. FTIR spectrum of CMCS used in this study.

of chitosan. The FTIR spectrum of the acid form of the product is shown in Figure 1. The presence of CdO and C-O stretching vibration peaks (at 1728 and 1238 cm-1, respectively) confirms the occurrence of the carboxymethylation reaction. The peaks at 1510 and 1619 cm-1 represent the NH3+ group, pointing to the carboxymethylation reaction occurring mainly at OH the positions, and the product is O-carboxymethyl chitosan.7,8,10 Conductometric analysis indicates the degree of substitution of the prepared CMCS as 0.9. Production of SAP from CMCS. A CMCS gel (0.9% dry matter) was prepared by cross linking the CMCS with glutaraldehyde. Then, the obtained gel was either freeze dried directly or partially dehydrated in a vacuum rotary evaporator prior to freeze drying. The fresh and dehydrated gels were frozen at -5, -20, and -196 °C and then freeze dried. The WBCs of the dried gels were measured at different conditions, and the results are summarized in Figures 2 and 3. When the fresh CMCS hydrogel was frozen at -20 °C and subjected directly to freeze drying (without partial dehydration), the final product was able to absorb 60-142 g/g of water within 1-100 min (Figure 2). The WBCs of this SAP in salt and urine solutions were, respectively, 27 and 25 g/g after 10 min exposure (Figure 3). Effect of Partial Dehydration of Gel on WBC of the Freeze-Dried Products. Dehydration of the 0.9% dry weight gel was performed at 70, 85, and 100 °C for 15-300 min, and the final concentration of the gel is given in Table 1. At 70 °C, 32-85% of the initial water was removed from the gel within 2-5 h (Table 1). The concentrated gels (1.3-6.2%) were frozen at -20 °C and freeze dried, and their WBCs were measured in pure water and in urine and salt solutions (Figures 2a, 3a, and 3d). On average, the WBC of the product in pure water was 25-82 g/g after 1-100 min exposure, which is significantly lower than the WBC of the fresh freeze-dried gel (Figure 2a). The same phenomena were observed in salt and urine solutions, in which the WBC was decreased by 40% and 49%, respectively, due to dehydration. However, at this dehydration temperature, increasing the concentration of the gel from 1.3 to 6.2 did not have any significant effect on the WBC of the final product. Dehydration of CMCS gel at 85 and 100 °C generally resulted in faster water removal and higher WBC of the final freezedried products in comparison with the products obtained at 70 °C (Table 1 and Figure 2a-c). At 85 °C, a 2.5% gel was obtained after 45 min and the SAP prepared from this gel (by freezing at -20 °C and freeze drying) showed the highest WBC. It absorbed 32-148 g/g of pure water after 1-100 min and 20 g/g of salt and 18 g/g of urine solutions after 10 min. At this evaporation temperature, further increasing the concentration of the gel resulted in decreasing the water binding capacity of the final products (Figure 2b).

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Figure 2. WBC of freeze-dried CMCS-SAPs prepared by partial dehydration at 70, 85, and 100 °C and freezing at -20, -196, and -5 °C after 1 (b), 10 (9), and 100 min (2) exposure in pure water. X-axes show the concentration of gels after partial dehydration.

When the evaporation was performed at 100 °C, 32% of the water was removed within just 15 min and a 1.3% gel was obtained. It was then frozen at -20 °C and freeze dried. The WBC of this product was 27-109 g/g after 1-100 min exposure in water and 21 and 23 g/g after 10 min exposure in salt and urine solutions. Further increasing the concentration of the gel to 3.5% at this temperature did not change the WBC of the final SAP significantly. Interestingly, an improvement in WBC was observed by increasing the concentration of the gel to 4.7% at 100 °C. The WBC of the SAP prepared from this gel in pure water was 28-129 g/g after 1-100 min exposure, while it was 28 and 24 g/g after 10 min exposure in salt and urine solution, respectively. Further increasing the concentration of gel to 16.8% at 100 °C resulted in decreasing the WBC (Figures 2c, 3c, and 3f). Effect of Freezing Temperature on WBC of the FreezeDried Products. For all of the evaporation conditions discussed above, the WBC of SAPs prepared from concentrated gels was lower than the one prepared from fresh gel when the samples were frozen at -20 °C. Therefore, the effect of freezing temperature was studied by performing the freezing step at -5, -20, and -196 °C, and results are summarized in Figures 2 and 3. When the fresh CMCS gel was subjected directly to the freeze drying, the freezing temperature had a significant effect on the WBC of the freeze-dried product. In general, freezing the gel at -20 °C resulted in a SAP with the highest WBC. The WBC of the fresh freeze-dried gel after 1-100 min exposure in pure

water was 32-148, 47-127, and 27-160 g/g while frozen at -20, -196, and -5 °C, respectively, before freeze drying (Figure 2). By freezing the fresh gel at -196 and -5 °C, the WBC was, respectively, decreased by 4.5% and 22.7% in saline and 16% and 45% in urine solution in comparison with the WBC of fresh gel frozen at -20 °C (Figure 3). Despite the fact that freezing the fresh gel at -20 °C resulted in a SAP with the highest WBC (Figures 2 and 3), we got completely different effects of freezing temperature for the partially dehydrated samples. The best results generally were obtained when the concentrated gels were frozen quickly in liquid nitrogen prior to freeze drying. When the dehydration was performed at 70 °C, the WBC of the concentrated gels frozen at -20 and -5 °C in pure water was significantly lower than the WBC of the fresh gel frozen at the same condition (Figures 2a, 2g, 3a, and 3d). In contrast, when the freezing of the concentrated gels was performed at -196 °C, an improvement in the WBC was obtained and the products had an enhanced WBC compared to the ones formed by freezing at higher temperatures (Figures 2d, 3a, and 3d). The best result was obtained when 66% of the initial water was removed from the gel and a 2.7% gel was obtained. For this gel, by freezing in liquid nitrogen, the WBC in pure water was 50-136 g/g after 1-100 min exposure. As shown in Figure 2a and 2d, these values are quite comparable with the WBC of the best fresh gel (frozen at -20 °C). Similarly, the WBC of liquid nitrogen frozen concentrated gels in salt and urine solutions was higher than the WBC of the ones frozen at -5

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Figure 3. WBC of freeze-dried CMCS SAPs prepared by partial dehydration at 70, 85, and 100 °C and freezing at -196 (2), -20 (9), and -5 °C (b) after 10 min exposure in salt and urine solutions. X-axes show the concentration of gels after partial dehydration. Table 1. Water Removal from CMCS Gel in a Rotary Evaporator at 70, 85, and 100°C and Different Evaporation Times to Final Concentration of the Gels evaporation temperature (°C)

evaporation time (min)

final concentration of gel (%)

water removal (%)

70 70 70 85 85 85 85 100 100 100 100

0 120 180 300 45 60 120 135 15 30 45 60

0.9 1.3 2.7 6.2 2.5 5.6 6.3 16.7 1.3 3.5 4.7 16.8

0 32 66 85 64 84 86 95 32 74 81 95

and -20 °C. Additionally, the 2.7% gel frozen in liquid nitrogen showed enhanced WBC in salt solution compared to the best SAP prepared from fresh gel (Figure 3a and 3d). For the evaporations performed at 85 and 100 °C, a similar phenomenon was observed and SAPs with the highest WBC were obtained from concentrated gels by applying the liquid nitrogen freezing technique (Figures 2 and 3). By using liquid nitrogen and performing the evaporation at 85 °C, the WBC of the freeze-dried concentrated gels was not decreased significantly compared to the fresh gel unless the concentration of the gel was increased to 16.7% in the evaporator (Figure 2e). At this evaporation temperature, the highest WBC was obtained for 2.5% gel frozen in liquid nitrogen that was obtained during 45 min evaporation. For this product, except for very brief (i.e., 1 min) exposure in pure water, the WBC in all other conditions was higher than the WBC of the best 0.9% freeze-dried gel

Figure 4. SEM micrographs of the fresh gel (0.9%) dried by freeze-drying (a-f) and air-drying (g and h) methods. Prior to freeze drying, the fresh gel was frozen at -20 (a and b), -5 (c and d), and -196 °C (e and f). The bars in left and right micrographs are 1000 and 200 µm, respectively.

(prepared by freezing at -20 °C) (Figure 2b and 2e). Noticeably, although 64% of the initial water was removed via heating at 85 °C, the WBC in salt and urine solutions was increased by 37 and 9% respectively (Figure 3b and 3e). As mentioned in the previous section, when the evaporation temperature was 100 °C, the best results were obtained for the 4.7% gel. Freezing this gel in liquid nitrogen led to a significant improvement of the WBC. This product was one of the best SAPs prepared in this study, which absorbed 40.1 g/g of saline and 32 g/g of urine solutions in 10 min, which are 61% and 17% respective improvements in WBC compared to the best freeze-dried fresh gel (Figure 3c and 3f). Additionally, there was no significant difference in pure-water-WBC between this SAP and the best 0.9% gel (Figure 2f). Effect of Partial Dehydration and Freezing Temperature on Morphologies of the SAPs. Morphologies of the dried CMCS gels were examined using SEM micrographs. The SEM images of freeze-dried fresh gel (Figure 4a-f) indicate that freeze drying of the CMCS gel resulted in formation of porous structures. In contrast, air drying of the fresh gels at 30 °C ended with formation of a totally nonporous product (Figure 4g and 4h). The enhanced WBC of freeze-dried CMCS gels compared

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Figure 5. SEM micrographs of products obtained from partially concentrated gel (4.7% prepared at 100 °C) by freezing at -20 (a and b), -5 (c and d), and -196 °C (e and f) and freeze drying. The bars in left and right micrographs are 1000 and 200 µm, respectively.

to oven-dried products5 is probably the direct effect of the higher porosity generated during freeze drying. In the freeze-drying technique, water acts as a porosity generator such that sublimation of ice crystals leaves empty pores in the structure of dried product.11 The SEM micrographs of fresh freeze-dried gel indicate that the freezing temperature had a significant effect on the morphology of the products (Figure 4a-f). The fresh gel frozen at -20 and -5 °C in a freezer, before freeze drying, exhibited a porous sheet-like structure, whereas the gels frozen at -196 °C had a porous sponge-like structure. The dependence of the structure of freeze-dried materials on the freezing temperature has been previously reported in several studies.12–14 Doillon et al.12 investigated the effect of freezing temperature on the structure of collagen-based wound dressings and reported similar sheet-like and sponge-like structures, respectively, at -20 and -196 °C. These authors suggest that rapid cooling in liquid nitrogen resulted in a regular linear growth of ice crystals and formation of channels connecting the pores on the surface and inside the frozen matrix. The channel structure consequently may lead to a regular sublimation of ice between the surface and the interior of the collagen and end with a sponge-like structure. In contrast, slow freezing, at higher temperatures, led to formation of large ice crystals which were not connected to form channels. The irregular growth of ice crystals may result in irregular sublimation and partial collapse of pores during the freeze drying, which can lead to generation of sheet-like structures.12 Alteration of the morphology of CMCS-SAPs by changing the freezing temperature, in the current study, may follow similar mechanisms. Similar sheet-like and sponge-like structures were observed by respective fast and slow freezing of 81% concentrated gel (obtained after 45 min dehydration at

Figure 6. SEM micrographs of the products obtained from partially dehydrated gels: 2.5% (a and b), 2.7% (c and d), and 16.8% (e and f) respectively prepared at 85, 70, and 100 °C by freezing at -196 °C and freeze drying. The bars in left and right micrographs are 1000 and 200 µm, respectively.

100 °C) (Figure 5a-f). The sheet-like structures of this dehydrated gel, however, were to some extent more collapsed than the fresh gel frozen and freeze dried at identical conditions. The higher porosity of the fresh gel may be due to the presence of a higher amount of porosity generator, i.e., water, in this gel. Reduction of WBC due to dehydration (after freezing at -5 or -20 °C) is probably the result of the lower porosity of the final products (Figure 2a-c and 2g-i and Figure 3a-f). On the contrary, fast freezing of concentrated gel in liquid nitrogen ended with a porous sponge-like structure similar to the one obtained from fresh gel (Figures 4e, 4f, 5e, and 5f). It has been previously reported14 that a higher cooling rate during the freezing process increases the rate of nucleation of ice crystals and as a result led to formation of smaller ice crystals and smaller pores in the final freeze-dried product. In this study, the increased number of ice crystals in the dehydrated gel frozen at -196 °C ended with formation of a SAP with higher porosity and consequently higher WBC compared with products obtained by freezing at -5 or -20 °C (Figures 2, 3, and 5). The morphologies of the 46-81% dehydrated gels were not significantly affected by dehydration temperatures, and 2.5%, 2.7%, and 4.7% gels (prepared at 85, 70, and 100 °C, respectively) had similar SEM micrographs after fast freezing and freeze drying (Figures 5e, 5f, and 6a-d). At higher dehydration level (95% dehydration at 100 °C) however, fast freezing and freeze drying of the concentrated gel did not result in a porous spongelike structure (Figure 6e and 6f). The lower porosity which ends with lower WBC (Figures 2f, 3c, and 3f) might be due to the absence of enough water to form ice crystals and generates a porous structure.

Ind. Eng. Chem. Res., Vol. 49, No. 17, 2010 Table 2. WBC of Reference Polyacrylates (commercial SAPs) in Pure Water, Saline, and Urine Solution WBC (g/g)

5A70 (Sumitomo SEIKA)

L520 (Aqualic ca)

water, 1 min water, 10 min water, 100 min salt, 10 min urine, 10 min

151 463 498 57 37

43 213 273 34 30

WBC of the Reference Synthetic Superabsorbents. Two commercially available synthetic superabsorbents were used as reference SAPs, and their WBCs were examined at different conditions (Table 2). In spite of the high WBC of these materials in pure water (up to 500 g/g), their WBCs were decreased significantly in salt (34-57 g/g) and urine solutions (30-37 g/g) after 10 min. At identical conditions, the respective average WBC of the best CMCS-SAPs, obtained by partial dehydration of fresh gel to 1.3-4.7% at 85-100 °C, freezing in liquid nitrogen, and freeze drying, was 35 and 32 g/g, respectively. In summary, by applying the partial dehydration at 85-100 °C and liquid nitrogen freezing prior to freeze drying, not only 32-81% of water was removed from the CMCS gel but also the final freeze-dried products were superabsorbents with a commercially acceptable WBC in salt and urine solutions. Conclusion Partial dehydration of CMCS gels followed by fast freezing and freeze drying generated porous sponge-like structures with a commercially acceptable WBC in salt and urine solutions. Nomenclature CMCS ) carboxymethyl chitosan Ds ) degree of substitution DM ) dry matter SEM ) scanning electron microscopy SAP ) superabsorbent polymer WBC ) water binding capacity

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ReceiVed for reView February 2, 2010 ReVised manuscript receiVed July 13, 2010 Accepted July 13, 2010 IE100257S