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  • 1. Influence of Surfactant Sorbitan Monolaurate to Suspension Polymerization of Polyacrylic Acid Superabsorbent (Dewi Sondari) Akreditasi LIPI Nomor : 536/D/2007 Tanggal 26 Juni 2007 INFLUENCE OF SURFACTANT SORBITAN MONOLAURATE TO SUSPENSION POLYMERIZATION OF POLYACRYLIC ACID SUPERABSORBENT Dewi Sondari, Nuri Astrini, Lik Anah and Agus Haryono Research Center for Chemistry (P2K) - LIPI Kawasan Puspiptek Serpong 15314, Tangerang e-mail : sondaridewi@yahoo.com ABSTRACT INFLUENCE OF SURFACTANT SORBITAN MONOLAURATE TO SUSPENSION POLYMERIZATION OF POLYACRYLIC ACID SUPERABSORBENT. A superabsorbent polymer has been prepared by suspension polymerization reaction using acrylic acid, sodium hydroxide, cyclohexane, potassium persulfate as initiator and sorbitan monolaurate as surfactant. The effect of different weight ratio of acrylic acid/sorbitan monolaurate 20/1, 20/2 and 20/3 were investigated based on its swelling properties, chemical structure by using Fourier Transform-Infra Red (FTIR) Spectroscopy and the surface morphology by Scanning Electron Microscopy (SEM). The molecular weight of polyacrylic acid superabsorbent was determined by intrinsic viscosity measurements in 2 M aqueous sodium hydroxide at 25 oC. The study showed sample with weight ratio of acrylic acid/sorbitan monolaurate = 20/2 have best swelling properties. Key words : Superabsorbent polymer, Suspension polimerization, SEM, FT-IR, Surfactant ABSTRAK PENGARUH SURFAKTAN SORBITAN MONOLAURAT TERHADAP POLIMERISASI SUSPENSI SUPERABSORBEN ASAM POLIAKRILAT. Polimer superabsorben telah dibuat secara reaksi polimerisasi suspensi menggunakan asam akrilat, sodium hidroksida, sikloheksan, potassium persulfat sebagai inisiator, dan sorbitan monolaurat sebagai surfaktan. Pengaruh dari berbagai rasio berat asam akrilat/sorbitan monolaurate = 20/1, 20/2 and 20/3 telah dipelajari berdasarkan sifat mengembang (swelling properties), struktur kimia yang dianalisis menggunakan spektroskopi Fourier Transform-Infra Red (FT-IR) dan morfologi permukaan yang diamati menggunakan Scanning Electron Microscope (SEM). Berat molekul superabsorben poliasam akrilat dihitung dengan cara mengukur kekentalan intrinsik dalam larutan natrium hidroksida 2M pada suhu 25 oC. Hasil penelitian menunjukkan sampel dengan perbandingan berat asam akrilat/sorbitan monolaurat = 20/2 mempunyai nilai sifat mengembang yang terbesar. Kata kunci : Polimer superabsorben, Polimerisasi suspensi, SEM, FT-IR, SurfaktanINTRODUCTION Superabsorbent polymers can be described as For superabsorbent polymers used as medicalpolymer having a network structure and a moderate and sanitation materials, they must have high absorptiondegree of cross-linking. They can absorb a large amount rates and low residual levels of toxic chemicals. Usually,of water, considerably more than their dry mass. The the porous superabsorbent polymers are prepared byamount of the water absorbend can be in a range of adding chemical reagents to produce bubbles in thehundreds to thousands time to that of the polymer. These polymer [1].polymers have been extensively used as absorbents in Recent developments were also suggest thepersonal care products, such as infant diapers, feminine production of super-absorbent polymers from acrylichygiene products, and incontinence product. They have acid and a cross-linker by solution or suspensionalso received considerable attention for a variety of more polymerization. The type and quantity of cross-linkerspecialized applications, including matrices for enzyme control both the swelling capacity and gel modulus. Theimmobilization, bioabsorbents in preparative synthesis and the use of cross-linker polyacrylate super-chromatography, materials for agricultural mulches, and absorbents have been a popular topic in the polymermatrices for controlled release devices [1]. technology research and development [2]. 21
  • 2. Jurnal Sains Materi Indonesia Edisi Khusus Desember 2008, hal : 21 - 24Indonesian Journal of Materials Science ISSN : 1411-1098 In general, the functions of the surfactant used samples have been averaged over 3 independentin the polymerization, are that the polymerization measurements.mixture forms a stable emulsion, the surfactant does The water uptake is expressed in terms ofnot retard the polymerization reaction, and uniform swelling ratio (Q, g/g) was calculated by:polymer particles are formed without coagulation norseparation [3]. Q = (Wt - Wo - Wn) / Wo In this study, we investigated influence ofsurfactant sorbitan monolaurate to polyacrylic acid Where :superabsorbent properties into suspension W n = Weight of nylon clothpolymerization. Wo = Weight of dry hydrogel W t = Weight of swollen hydrogelEXPERIMENTAL METHODS FT-IR Spectrum and SEM AnalysisMaterials Infrared absorption spectra of polyacrylic Acrylic acid purity 99,8% was purchased from superabsorbent was studied by FT-IR using IRAldrich (Singapore), sodium hydroxide, cyclohexane Prestige-21 Shimadzu (Shimadzu Co., Japan). Thewere reagent grade from E-Merck, Potassium persulfate polymers were mixed with KBr and pressed to a plate forpure grade was purchased from E-Merck, and Sorbitan measurement. All of the spectra were measured 16 scansMonolaurate was purchased from Aldrich. at a resolution of 4 cm-1. A background spectrum containing no sample was substracted from all spectra. The surface morphology was observed using scanningMethods electron microscopy (SEM).Synthesis of Superabsorbent Polymers PolyacrylicAcid Measurement of Molecular Weight by Intrinsic Viscosity In 200 mL flask, acrylic acid having a purity of99.8% by weight was placed. While cooling it, 76.5 g of For the determination of viscosity-average22.6% by weight aqueous sodium hydroxide solution molecular weight (Dalton), the polyacrylic acid waswas dropped thereinto with stirring to neutralize dissolved in 2 M aqueous sodium hydroxide. Fivethe acrylic acid. Then, potassium persulfate was added different dilute solutions were used to do theand stirring was continued at room temperature to experiments on measurement of intrinsic viscosity (η).dissolve it. The Mark-Houwink equation relating to intrinsic 213 g of cyclohexane and sorbitan monolaurate viscosity with empirical viscometer constantwere charged into a 500 mL flask equipped with a reflux K = 0.0422 (mL/g) and a = 0.64 for polyacrylic acid wascondenser, and the surfactant was dissolved at room used to calculate the molecular weight using thistemperature with stirring. The aqueous solution equation [4].of partially neutralized acrylic acid salt was dropped tobe suspended therein. Polymerization reaction was RESULTS AND DISCUSSIONcarried out for 3 hours while keeping the bath temperatureat 70 oC [3]. Swelling Studies With the aim of investigating the effect ofSwelling Analysis surfactant on the material’s sorption properties an equilibrium swelling measurements in distilled water were The water absorption capability of hydro-gel performed on hydrogel samples, obtained from threewere measured by tea bag method using nylon cloth [5].The hydrogel (approximately 500 mg ) is weighed 250and poured into the bag. The open side of the bag 225 200is closed. The bag was then immersed into the 175 Swelling ratio 20/1 ratiodistilled water to swell. After a suitable period of time, 150 20/2 ratio 125the bag was taken out from the water. The bag was 100 20/3 ratio 20/2>20/3=20/1then hanged for 15 minutes to let the excess water to 75 50dripp off and then weighed to determine the amount of 25distilled water absorbed by the hydro-gel. Equilibrium 0swelling measurements in distilled water were carried 0 20 40 60 80 100 120 Tim e (m inute)out for all samples using an electronic microbalance(AdamAAA250L) with an accuracy of ±10-4 g. Reported Figure 1. Swelling ratio in distilled water vs time ofvalues of equilibrium water uptakes for each different various weight ratio of acrylic acid/sorbitan monolaurate22
  • 3. Influence of Surfactant Sorbitan Monolaurate to Suspension Polymerization of Polyacrylic Acid Superabsorbent (Dewi Sondari)different weight ratio of acrylic acid/sorbitan (20/1, 20/2and 20/3). The resulted swelling ratios are shown inFigure 1. Figure 1 shows that the water uptake capabilityof hydrogel was influenced by sorbitan monolauratecontent. Higher swelling capacity will be produced whenweight ratio of acrylic acid/sorbitan monolaurate = 20/2was put into reaction. The influence of sorbitanmonolaurate as the surfactant forms a stable suspension.A stable polymer will be obtained when the amount ofsurfactant, which is added, is optimum, in which the Figure 3. Scanning Electron Micrograph of the hydrogelpolymer product will be less harder and water swelling surface with weight ratio of acrylic acid/sorbitan monolaurate 20/1capacity will be higher. In the other hand, the increase ofsurfactant amount, the formed polymer during thepolymerization becomes a block and has such a lowwater absorption. Optimizing the amount of added surfactant topolymerization mixture, result in a more stablesuspension. This formed polymer particle sizes are smalland uniform, so that, increases the water absorptioncapability. When the particle sizes were smaller, thesurface area is better, and then the better surface areawill be have better water absorption. Figure 4. Scanning Electron Micrograph of the hydrogel surface with weight ratio of acrylic acid/sorbitanFT-IR and SEM Analysis monolaurate 20/2 The infrared spectroscopy was used to determinethe type of bonds present in the hydrogel. The infraredspectra in Figure 2 indicate that the hydrogel is containingcarboxyl group (-COOH). Spectra peak at 1734 cm-1 and800 cm-1 arised from symmetric stretching vibration ofC=O, and –C-C respectively. Another new peak is at 2852 cm-1 and 2924 cm-1arised from stretching vibration of –C-H. Thepolymerization between acrlylic acid using potassiumpersulfate as initiator was showed by disappearanceof spectrum peak at 3100-3000 cm-1 from double bond Figure 5. Scanning Electron Micrograph of the hydrogelof -HC=CH-. surface with weight ratio of acrylic acid/sorbitan monolaurate 20/3 sample 4 sample 5 SML 2x %T Samples with different surfactant amounts were collected, and their surface morphologies were investigated by SEM. A series of SEM micrographs of polyacrylic acid hydrogel with various surfactant are shown in Figure 3, 4, and 5. A From SEM analysis, it can be shown that morphology of the hydrogel surface B micrograph (weight ratio acrylic acid/sorbitan monolaurate 20/1 and 20/2) were less porous and compact compared with SEM image of 20/3 sample C in Figure 5. It is obviously illustrated in Figure 5 that 20/3 sample has more porous and compact morphology. 4000 SML 2x 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 1/cm The surfactant of sorbitan monolaurate Wave number (cm-1 ) influenced to the formed polymer particle size. An Figure 2. Infrared spectra of the polyacrylic acid optimum surfactant amount will provide a more porous superabsorbent with various weight ratio acrylic acid/ sorbitan monolaurate. (A) 20/1; (B) 20/2 and (C) 20/3 and compact of polymer structure. 23
  • 4. Jurnal Sains Materi Indonesia Edisi Khusus Desember 2008, hal : 21 - 24Indonesian Journal of Materials Science ISSN : 1411-1098Molecular Weight Determination by Intrinsic [5]. Y. ZHAO, H. SU, L. FANG and TAN, Polymer, 46Viscosity (2005) 5368-5376 The molecular weight of polyacrylic acidsuperabsorbent polymer were reported in Table 1.Table 1. Molecular weight of the polyacrylic acid superabsorbentpolymer Weight ratio acrylic acid/ Molecular No sorbitan monolaurate weight 1 20/1 135,397 2 20/2 191,594 3 20/3 217,274 Molecular weight depends on polymer chain,longer polymer chain will be produced when less initiatorwas put into reaction and depend on the amount ofmonomer. The functions of the sorbitan monolaurate assurfactant used in the polymerization, are that thepolymerization mixture forms a stable suspension, thesurfactant does not retard the polymerization reaction,and uniform polymer particles are formed withoutcoagulation nor separation.CONCLUSIONS Superabsorbent polymer has been prepared bysuspension polymerization reaction using acrylic acidand sorbitan monolaurate as surfactan. Sample withweight ratio of acrylic acid/sorbitan monolaurate = 20/2have best swelling properties. The morphology of polymer made of acrylic acid/sorbitan monolaurate with weight ratio of 20/3 has porousand compact structure more than others.ACKNOWLEDGEMENTS The authors are thankful for the financialsupported by DIPA thematic from Indonesian Instituteof Sciences (LIPI)REFERENCES[1]. CHEN, Y., LIU, Y.F., and TAN, H.M., Preparation of macroporous cellulose-based superabsorbent polymer through the precipitation method, BioResources, 3 (1) (2008) 247-254[2]. Absorbent Polymers: AMCOL International, http:/ /www.amcol.com/fctbook/absor.htm, (1996)[3]. United States Patent. 4,340,706. July. 20, 1982. Alkali metal acrylate or ammonium acrylate polymer axcellent in salt solution-absorbency and process for producing same, (1982)[4]. BRANDRUP, J., IMMERGUT, E.H., and GRULKE, E.A., Polymer Handbook, fourth edition, A Wiley- Interscience Publication, Canada.24