Improvement in the Water-Absorbing Properties ofSuperabsorbent Polymers (Acrylic Acid-co-Acrylamide)in Supercritical CO2

Li Ma,1 Le Zhang,2 Ji-Chu Yang,1 Xu-Ming Xie2

1Department of Chemical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China2Institute of Polymer Science & Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

Received 1 October 2001; accepted 21 March 2002Published online 18 September 2002 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/app.11210

ABSTRACT: Superabsorbent resins prepared by ultravio-let radiation-inducing polymerization techniques withacrylic acid/acrylamide were treated with supercritical car-bon dioxide (SC-CO2). The water-absorbing properties of thetreated resins were greatly improved. The water-absorbingproperties of resins treated with SC-CO2 in the pressurerange of 10–35 MPa and the temperature range of 40–60°Cwere studied. The effects of the treatment time and depres-surizing speed of CO2 after treatment were also examined.Obviously, different results were found for particles of dif-ferent sizes. Smaller particles were more efficient under thesame treatment conditions. Samples were tested with differ-ential scanning calorimetry. The results showed that the

plasticizing effect of CO2 reduced the glass-transition tem-perature of the polymer, and it was proposed that the plas-ticization effect might have led to polymer chain redistribu-tion and better flexibility. Minor changes in the surfacemorphology of the particles were observed with scanningelectron microscopy. The extraction of the unpolymerizedmonomers by SC-CO2 was also studied. © 2002 Wiley Period-icals, Inc. J Appl Polym Sci 86: 2272–2278, 2002

Key words: superabsorbent resin; supercritical carbon diox-ide; absorption; photopolymerization; poly(acrylic acid-co-acrylamide)

INTRODUCTION

Supercritical fluid has attracted the attention of chem-ical engineers for many years because of its adjustablesolvent properties. Supercritical carbon dioxide (SC-CO2) is a nonflammable, nontoxic, and relatively in-expensive environmentally benign fluid. It has mod-erate critical conditions (Tc � 31.1°C, Pc �7.38 MPa)that make it convenient for many applications. Inpolymer engineering, SC-CO2 can be used as a poly-merization solvent and as a foaming agent. It also canbe used to plasticize many polymers. There are manyinvestigations concerning the effects of CO2-treatedpolymers. Chen et al.1 reported that the gas-transportand sorption properties of polycarbonate membranescould be changed after CO2 treatment. Their studyshowed that the free volume of the membranes in-creased with an increase in the exposed CO2 density.Mizumoto et al.2 investigated how the crystallinestereocomplex of isotactic poly(methyl methacry-late) and syndiotactic poly(methyl methacrylate)could form, induced by high-pressure CO2. For the

CO2-treated polymer, the average cell size de-creased and the average cell density increased as theamount of the stereocomplex that was formed in-creased. Fourier transform infrared spectroscopywas used to investigate the interactions of CO2 withcarbonyl oxygen in polymers, as reported by Kazar-ian et al.3 It was proposed that this interactioncaused the CO2 plasticizing effect.

Since the U.S. Department of Agriculture4 reportedthe first superabsorbent polymer, there has been con-siderable interest in the preparation and properties ofsuperabsorbent polymers. In a series of investigations,a number of preparation methods have been explored,including solution polymerization and inverse sus-pension polymerization.5,6 Photopolymerization is anew method requiring less time and more moderateconditions. In all products, acrylic acid (AA)- andacrylamide (AM)-based superabsorbent polymershave received considerable attention in the last 4 de-cades.7–9

Because of the comprehension of the frame of thepolymer networks and the interactions of CO2 withpolymers, it can be presumed that the water-absorbingproperties are probably affected in SC-CO2. In thiswork, the effects of SC-CO2 on superabsorbent poly-mers were investigated under different treatment con-ditions. Differential scanning calorimetry (DSC) andscanning electron microscopy (SEM) were also used

Correspondence to: J.-C. Yang (yjc-dce@mail.tsinghua.edu.cn).

Contract grant sponsor: Open Laboratory Foundation ofTsinghua University.

Journal of Applied Polymer Science, Vol. 86, 2272–2278 (2002)© 2002 Wiley Periodicals, Inc.

for the determination of the mechanism of the phe-nomena.

EXPERIMENTAL

Sample preparation

The sensitizer benzophenone (BP; Shanghai ReagentCo., C.R.), AA (Beijing Yili Fine Reagent Co., A.R.),AM (Tianjin Reagent Research Laboratory, A.R.), andsodium hydroxide (Beijing Chemical Plant, A.R.) wereall used as received, except that AA was partly neu-tralized with sodium hydroxide, and the neutraliza-tion ratio was 65%.

The photopolymerization process includes threestages—excitation, initiation, and propagation—asshown in eqs. (1)–(3), respectively:

BPO¡h�

�BP�S 3 �BP�T (1)

(2)

(3)

where the superscript S represents the singlet state,T represents the triplet state, and R represents thefunctional groups OOH, OONa and ONH2.

Weighed quantities of monomers and BP were dis-solved in distilled water until saturation. The reactionmedium was deoxygenated with nitrogen before be-ing put into the reaction cell. A 4-mL solution wasdeposited onto the bottom of the cell. Then, a block ofquartz glass was put onto the cell, and deoxygenationcontinued. The assembled unit (Fig. 1) was irradiatedunder UV irradiation from the top side at room tem-

perature. The resulting product was cut into smallpieces and vacuum-dried at 90°C to a constant weight.

SC-CO2 treatments

As shown in Figure 2, a stainless steel pressure vesselwith a jacket was designed for the process. First, thevessel was maintained at the desired temperature fora long time. The particles were put into a small beakerwith a mesh cover. The beaker was then loaded intothe vessel. The air was removed from the apparatus byCO2 purging. CO2 was pumped into the apparatusuntil it reached the desired pressure range. This con-dition was maintained for a given period of time. TheCO2 depressurization rate was controlled with a valvesystem.

Sorption curve measurements

The dry gel prepared by photopolymerization wasmilled through 20–40-mesh screens. The dry gel(0.0900–0.1000 g) was immersed in water at roomtemperature. Swollen gel was then separated fromunabsorbed water with a 200-mesh screen andweighed every minute. The water absorption in thefirst 5 min was designed to show the absorbing rate ofthe gel. Two days later, the equilibrium water absorp-tion was measured. In this study, the water absorptionwas calculated with the swelling ratio (Q):10

Q � �Ws � Wd�/Wd (4)

where Ws is the weight of the swollen hydrogel andWd is the weight of the dry hydrogel.

Characterization

A 2910 modulated differential scanning calorimeterfrom TA Instruments Co. was used to test the polymer

Figure 1 UV irradiation setup for polymerization: (1) ob-turator, (2) bracket, (3) jacketed vessel, (4) shutter, (5) UVirradiator, and (6) exhaust fan.

Figure 2 High-pressure apparatus used for treating thepolymer samples: (1) CO2 cylinder, (2) filter, (3) cooler, (4)pump, (5) water bath, (6) treatment autoclave, (7) tempera-ture controller, and (8) pressure controller.

SUPERABSORBENT POLYMERS IN SUPERCRITICAL CO2 2273

glass-transition temperature (Tg). The temperaturewas brought to 0°C with liquid nitrogen and was keptthere for 5 min. The thermograms were recorded at aheating rate of 10°C/min. An SEM instrument (Hita-chi S-520) was used to observe the polymer morphol-ogies. The samples were prepared by gold sputtering.The acceleration voltage was 20 kV.

RESULTS AND DISCUSSION

Effect of the SC-CO2 density on the absorbingproperties of the polymer

The sample particles in the 300–450-�m range wereimmerged in CO2 at a series of pressures and temper-atures for 3 h. The pressures ranged from 6 to 35 MPa,and the temperatures ranged from 40 to 60°C. Thedepressurizing process, at a speed of 2–4 MPa/10min, was slow. The CO2 densities, which were calcu-lated by the Peng–Robinson equation,11 were corre-lated with the water absorption every minute. Asshown in Figure 3, the water-absorbing rate increasedwith the CO2 density increasing, up to 0.8 g/mL.

Beyond 0.8 g/mL, there was a decrease in the water-absorbing rate with the CO2 density increasing.

As shown in Table I, the DSC results indicated thatTg’s of the treated samples decreased. These resultsproved that the free volume of the treated samplesincreased. As a good plasticizing agent, SC-CO2 mo-bilized the polymer chains of the samples. The moreflexible chain framework improved the water-absorb-ing properties of the polymer. We correlated Tg withthe density of CO2, and the result is shown in Figure4. The equilibrium absorption results are shown inFigure 5. In comparison with Figure 3, Figure 4 showsa decreasing trend for Tg the corresponds to the in-creasing trend for the water-absorbing rate. The low-est Tg occurred at the CO2 density of 0.8 g/mL inFigure 4, and the water-absorbing rate of the polymerwas most rapid at the same treatment density. Thisresult reflected the relationship between the free vol-ume, which reflected the chain flexibility of the poly-mer, and its water-absorbing rate. That is, the moreflexible the polymer chains are, the more rapid thewater-absorbing rate is for the polymer.

TABLE ITg Results of the Samples Tested by DSC

SampleTemperature

(°C)Pressure

(MPa) Time (h) Tg (°C)

d 108.26d1 40.0 30.0 3.00 96.04d2 40.0 20.0 3.00 89.92d3 40.0 10.0 3.00 96.59d4 40.0 20.0 1.00 98.98d5 40.0 20.0 5.00 84.58d6 50.0 20.0 3.00 84.92d7 60.0 20.0 3.00 92.59

Figure 3 Effect of the CO2 density on the absorbing prop-erties of the samples treated for 3 h and depressurizedslowly.

Figure 4 Effect of the CO2 density on the Tg values of thesamples treated for 3 h and depressurized slowly.

Figure 5 Equilibrium absorption capacity of the samplestreated for 3 h and depressurized slowly.

2274 MA ET AL.

Effect of the depressurizing speed after treatmenton the absorbing properties

In the experiment, we controlled the depressurizingspeed with a valve system. There were some differ-ences in the polymer absorbing curves for differentdepressurizing processes. Particles that were in thedepressurizing speed range of 20–40 MPa/10 min andparticles that experienced the same treatment condi-tions and a different depressurizing speed range of2–4 MPa/10 min were compared. Figures 6 and 7show that the water-absorbing speed of the particlestreated by the higher speed depressurizing processwas degraded. A coherent phenomenon for the parti-cles could be observed in the high depressurizingspeed process. In this process, the sudden release ofCO2 from the polymer induced the abrupt shrinkageof the swelling particles. It was assumed and pro-posed that the high depressurizing speed process ofCO2 could induce a decrease in the flexibility of thepolymer chains like the thermal annealing process.A thermal annealing process can make a polymer

crisp, and so it would affect the water-absorbingproperties of the polymer. Therefore, a slow depres-surizing process was necessary for the whole exper-iment.

In Figure 3, the water-absorbing rate of the samplesis shown decrease with a CO2 density greater than 0.8g/mL. This can be attributed to a relatively rapiddepressurizing process after the treatment. It washarder to control the release speed of the higher den-sity CO2 for most valve systems during the initialdepressurizing process. Therefore, selecting a properCO2 density and a corresponding depressurizingspeed was an important step for better absorptionproperties.

Effect of the treatment time on the absorbingproperties of the polymer

The effects of the CO2 treatment time on the water-absorbing properties are shown in Figure 8. The de-pressurizing process was slow. The water absorptionof the samples obviously increased with the increasingtreatment time in the first 5 min. A higher degree ofswelling in CO2 could be attained with a longer swell-ing time. It would lead to polymer chain redistributionand better chain flexibility. With the treatment time ex-tended, the swelling degree would reach an upper limit.

Different effects on polymer particles of differentsizes

Figures 9 and 10 show the water absorption of thesamples with diameters of 300–450 �m and less than300 �m at 20 MPa and 40°C. The water-absorbingcurves of the smaller particles are very different fromthose of the bigger particles. The water-absorbing rateof the smaller particle was faster, and it reached ab-sorption equilibrium in a shorter period of time. Com-

Figure 6 Effect of the depressurizing speed on the absorb-ing curve for the samples treated at 60°C and 20 MPa for 3 h.

Figure 7 Effect of the depressurizing speed on the absorb-ing curve for the samples treated at 40°C and 10 MPa for 3 h.

Figure 8 Effect of the CO2 treatment time on the absorbingproperties of the samples treated at 40°C and 20 MPa anddepressurized slowly.

SUPERABSORBENT POLYMERS IN SUPERCRITICAL CO2 2275

paring the water-absorption curves before and aftertreatment, we find that the effect on the smaller par-ticles was more efficient than that on the bigger par-ticles in the same treatment progress. A possible ex-planation was the rule of solvent transport in poly-mers. Lin et al.12 studied the transport phenomenon ofsolvent–penetrant systems in spherical glassy poly-mers, and they found that the penetration time fordifferent radii could be predicted. This study indi-cated that the penetration of SC-CO2 in the polymerparticles was affected by the particle size. In the sametreatment time, it could penetrate smaller particlesmore completely and thereby improve the particleabsorbing properties more efficiently.

Morphologies of the polymers by SEM

The photomicrographs in Figure 11 show that theparticles were irregular in shape, just like the particlesfrom solution polymerization reported by Omidian et

al.5 This occurred because that the particles wereground in a mortar. The representative particles 300–450 �m in size were photographed, as shown in Fig-ures 12 and 13. It can be observed in Figure 12 thatthere were some eyelets on the surfaces of the treatedsamples. The swelling of SC-CO2 in the polymerformed these defects. These were similar to the defectson the polycarbonate membrane formed by the rapidremoval of absorbed CO2 from the membrane matrix,as reported by Chen et al.1 There were rougher sur-faces on the particle rupture region by abrasion, asshown in Figure 13. The swelling imprint can be ob-served in the SEM micrographs also. These changes inthe sample surface can lead to a slight increase inpermeability. However, these minute changes werenot enough to affect the absorbing properties of thepolymer so efficiently.

Effect of SC-CO2 extraction on the polymer

The extraction of the unpolymerized monomers bySC-CO2 was studied by a gravimetric method.

Figure 9 Effect of the particle size on the absorption prop-erties of the samples treated at 40°C and 20 MPa for 0.5 hand depressurized slowly.

Figure 10 Effect of the particle size on the absorption prop-erties of the samples treated at 40°C and 20 MPa for 3 h anddepressurized slowly.

Figure 11 Morphologies of the particles by SEM, amplified50 times: (a) untreated and (b) treated at 40°C and 20 MPafor 3 h and depressurized slowly.

2276 MA ET AL.

Weight-change data for samples were measured im-mediately after the treatment process in CO2. In TableII, we find that the weight changes were very slightunder all conditions. A similar experimental method

was introduced in McHardy and Sawan’s book.13 Thisdemonstrates that the extraction of the unpolymerizedmonomers by SC-CO2 was very slight, and it was notthe main factor affecting the water-absorbing speedunder the experimental conditions.

Figure 12 Morphologies of the polymer surfaces by SEM,amplified 1000 times: (a) untreated, (b) treated at 40°C and20 MPa for 3 h and depressurized rapidly, and (c) treated at40°C and 20 MPa for 3 h and depressurized slowly.

Figure 13 Morphologies of the polymer surfaces on theparticle rupture region by SEM, amplified 1000 times: (a)untreated, (b) treated at 40°C and 20 MPa for 3 h anddepressurized rapidly, and (c) treated at 40°C and 20 MPafor 3 h and depressurized slowly.

SUPERABSORBENT POLYMERS IN SUPERCRITICAL CO2 2277

CONCLUSIONS

Superabsorbent polymer samples prepared by ultra-violet radiation-inducing polymerization techniqueswere treated with SC-CO2.

The effects of CO2 treatment on the water-absorbingproperties of the polymer were assessed. It was shownthat a proper CO2 density range enhanced the absorb-ing properties of the samples. A longer treatment timeand a slower depressurizing process were also bene-ficial.

Comparing the water-absorbing properties beforeand after treatment, we found that the effects onsmaller particles were more efficient than those onbigger particles with the same treatment progress.This can be explained by the rules of solvent transpor-tation in polymers.

The DSC results showed that the plasticizing effectof CO2 reduced Tg of the polymer. It was proposedthat the plasticization effect could lead to polymer

chain redistribution and better flexibility. Minutechanges in the surface morphology of the particleswere observed with SEM. These changes were notenough to affect the absorbing properties of the poly-mer very much. The extraction of the unpolymerizedmonomers by SC-CO2 was very slight and could notaffect the water-absorbing speed under the experi-mental conditions.

References

1. Chen, S.-H.; Huang, S.-L.; Yua, K.-C.; Lai, J.-Y.; Liang, M.-T. JMembr Sci 2000, 172, 105.

2. Mizumoto, T.; Sugimura, N.; Moritani, M.; Sato, Y.; Masuoka, H.Macromolecules 2000, 33, 6757.

3. Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C. L.; Eckert,C. A. J Am Chem Soc 1996, 118, 1729.

4. Hamielec, A. E.; MacGregor, J. F. In Polymer Reaction Engineer-ing; Reichert, K. H.; Geiseler, W., Eds.; Hanser: New York, 1983.

5. Omidian, H.; Hashemi, S. A.; Sammes, P. G.; Meldrum, I. Poly-mer 1999, 40, 1753.

6. Melekaslana, D.; Okay, O. Polymer 2000, 41, 5737.7. Zhou, W.-J.; Yao, K.-J.; Kurth, M. J. J Appl Polym Sci 1997, 64,

1001.8. Okay, O.; Sariisik, S. B. Eur Polym J 2000, 36, 393.9. Kiatkamjornwong, S.; Chomsaksakul, W.; Sonsuk, M. Radiat

Phys Chem 2000, 59, 413.10. Chen, J.; Park, K. J Controlled Release 2000, 65, 73.11. Peng, D. Y.; Robinson, D. B. Ind Eng Chem Fundam 1976, 15, 59.12. Lin, J.-S.; Hwang, C.-C.; Lin, C.-M.; Lai, J.-Y. Chem Eng Sci 2000,

56, 151.13. McHardy, J.; Sawan, S. P. Supercritical Fluid Cleaning: Funda-

mentals, Technology and Applications; Noyes: Westwood, NJ,1998.

TABLE IIWeight Changes of Samples Treated

with SC-CO2 for 3 h

SampleTemperature

(°C)Pressure

(MPa)Weight Change

(%)

d1 40.0 30.0 0.10d2 40.0 20.0 �0.21d3 40.0 10.0 �0.08d6 50.0 20.0 0.04d7 60.0 20.0 �0.14

2278 MA ET AL.