Synthesis and Characterization of Cellulose Based Superabsorbent Polymer Composites Ahmad Zainal Abidin a, N. M. T. P. Sastra a, G. Susanto a, H.P.R.Graha a Abstract Superabsorbent polymer composite (SAPC) is a material that has an ability to absorb big amount of water in a short time when it is dry. in this study, sap composites have been synthesized by copolymerising monomers of acrylamide (AM) and acrylic acid (AA) and then reinforcing with celluloses, using variations of ammonium persulphate (APS) concentration as an initiator and n,n-methylene bisacrylamide (MBA) concentration as a crosslinker, i.e. 0.2 wt%, 0.6 wt%, 0.8 wt%, and 1 wt% for each of them and also variation of cellulose concentration, i.e. 5 wt%, 7.5 wt%, and 10 wt%. The sap materials produced were then characterized by determining their rate and capacity of water absorption, observing the FTIR spectra and also SEM photographs. Results of the experiment showed that the water absorption capacity of the sap was affected by concentrations of APS and MBA, and the maximum capacity was obtained 339 g water/g SAP at composition of 1 wt% APS and 0.2 wt% MBA. Addition of cellulose does not improve the absorption capacity, but it helps the polymer to absorp water faster. The higher the addition is the faster the rate of absorption. Besides that, addition of cellulose makes the sap stronger. FTIR spectra give the result that the compound of C-O from cellulose has a peak in range of 1040-1140 cm -1 when it is grafted with AA and AM. Observation morphology of sap using SEM with enlargement 5000x shows the structure of the pores of the polymer s surface uniquely for each variation. Keywords: acrylamide, acrylic acid, cellulose, superabsorbent polymer composite. a Department of Chemical Engineering, Faculty of Industrial Technology, Bandung Institute of Technology. Jalan Ganesha 10, Bandung, INDONESIA, 40132. Corresponding author e-mail address: zainal@che.itb.ac.id Introduction Superabsorbent Polymer (SAP) is known as a material absorbing lots of water in short time and it will keep the water bonded inside. This capability is due to functional groups in the molecular structure, OH, - NH 2, -COOH, -CONH and SO 3H. in over 50 years, SAP is already applied in health area, such as for diapers and women sanitary. Further researches make this material appropriate for application in other fields such as agriculture, pharmacy, and engineering. SAP here was synthesized from two monomers of acrylamide (Am) and acrylic acid (AA) using crosslinker of N,N-methylene-bisacrylamide (MBA) and initiator of ammonium persulphate (APS). Cellulose from paper waste was used as a reinforcement to strengthen the SAP and improve its absorbance performance, which is useful for diapers and water sanitary applications. Figure 1 shows the part of polymer tissue. Polymer chains in SAP are hydrophilic because they are made from CONH of acrylamide and COOH of acrylic acid. When water is absorbed into the SAP, there is an interaction between the polymer and water, which is called hydration and hydrogen bond creation. Swelling mechanism in SAP happens when the water is diffused by the polymer because of the hydrophilic group (carboxylic acid). After it is in the equilibrium condition, the water will make hydrogen bonds. At last, the water will be detained in SAP which will keep swelling (Li and Wang, 2005). 8 P a g e Green Chemistry Section 1: Material Chemistry, Ahmad Zaenal Abidin, et al.
ISBN 978-602-285-049-6 Figure 1. Hydrophilic bond and the crosslink in SAP (Elliot, 2007) Methodology The substances for making the SAP were obtained from manufacturer and used in the experiment directly. The substances are: a. Acrylamide (Am, pure, solid, Merck, Germany) b. Acrylic acid (AA, pure, liquid, Merck, Germany) c. N,N-methylethylene-bisacrylamide (MBA, pure solid, Merck, Germany) d. Ammonium persulphate (APS, pure solid, Merck, Germany) e. Cellulose (pulp from paper waste, solid, Badan Besar Pulp dan Kertas, Indonesia) f. Sodium oxide (NaOH, pure, solid, Chemical Engineering Department ITB, Indonesia) 10 ml acrylic acid, 10 gr acrylamide, 75 ml NaOH 5 M, and 70 ml demineralized water were stirred togetherin a round flask. The concentrations of MBA and APS were varied and mixed in the solution. Reaction time was about 4 hours at temperature of 70 C. to make the composite, cellulose was added into the round flask. particles, which were restrained in 60 mesh, were used as samples in the measurement. Further characterization was conducted using FTIR and SEM. FTIR is to determine chemical bonds and groups existing in SAP and also grafting between polymer and cellulose, while SEM is to observe the physical characteristic of the SAP. Results and Discussion Absorption Capacity This experiment produced two kinds of superabsorbent polymer, i.e. SAP from AA-Am and SAPc from AA-Am-Cellulose. The highest absorption capacity obtained from SAP is 339 g/g at composition of APS 1% and MBA 0.2% as shown in Figure 2. Absorption capacity of SAP is calculated with equation (1): %capacity = m t m o m o 100%% (1) Greater value of% capacity indicates the greater volume of absorbed water. The absorption rate of SAP is determined by measuring the volume or mass of water that can be absorbed in time unit. The measurement was done in predetermined time span such as5 seconds, 15 seconds, 30 seconds, etc. Before the measurement, SAP must be in dry condition in the form of small powder.therefore SAP was crushed with blender and then heated in oven of 70 C until it was fully dry. The dry SAP was sized usinga sieve of 40-60 mesh. The Figure 2. Absorption capacity of AA-Am with concentration of APS 1% The absorption capacity relates to tissue structure of polymer as shown in the following Flory Equation: (2) In this equation, crosslink density is an important element to control the swelling capacity of SAP. Higher concentration of crosslinker can produce smaller micro pores in the polymer. These micro pores are places in polymer to keep the water inside. Therefore, the more Green Chemistry Section 1: Material Chemistry, Ahmad Zaenal Abidin, et al. P a g e 9
crosslinker in the polymer makes fewer water can be absorbed. Somehow, lower concentration of crosslinker can make the polymer dissolved in water. The relation between absorption capacity and cross linker sconcentration is shown in Flory s equation (3). (3) Increasing initiator concentration will decrease the molecule mass of polymer and increase number of single polymer chains. These single chains will not have a contribution in improving absorption capacity. Therefore, the capacity of the polymer will decrease by increasing initiator. Lower initiator concentration, somehow, produces lower capacity of absorption. This is because of the less number of free radicals produced by the initiator and so the less number of crosslinks formed. The composite of AA-Am-Cellulose has been made by varying the cellulose s concentration. This composite uses the 1 wt% APS and 0.2 wt% MBA concentrations which produces the highest capacity of absorption (339 g/g). The result of cellulose addition in SAP is shown in Figure 3. The absorption capacity of SAPc decreases as cellulose concentration increases, while the physical form of SAPc looks harder and stronger. It seems that increasing cellulose in the SAPc reduces the number and micro pore size in the SAPc as confirmed by SEM observations. Figure 4. The initial rate of absorption of SAP (no cellulose) Figure 5. The Initial rate of absorption of SAPc with 5 wt% of cellulose Figure 6. The Initial rate of absorption of SAPc with 7,5 wt% of cellulose Figure 3. Absorption capacity of SAPc in various % cellulose Absorption Rate of SAP When SAP absorbs water, the most interesting moment is at first 30 seconds of absorption at which the SAP fastly swells and becomes bigger (see Figure 8). The rate of absorption is shown in Figure 4 to Figure 7 below. Figure 7. The initial rate of absorption of SAPc with 10 wt% of cellulose 10 P a g e Green Chemistry Section 1: Material Chemistry, Ahmad Zaenal Abidin, et al.
ISBN 978-602-285-049-6 Figure 8. (a) SAP particles before absorption (b) SAP after absorption From Figure 4 to 7, we can see that there is an advantage of cellulose addition to the SAP in term of absorption rate. Cellulose gives a significant effect in the first 30 second absorption. Based on the slope of the graphs, the SAPc graph has higher slope than the SAP graph. This means that SAPc has faster absorption rates than SAP. This fact gives an indicationthat SAPc is suitable for applications in diapers or women sanitary, that needquick absorption, while SAP may suit the application in agriculture that needs big capacity of absorption. Based on the data, cellulose addition in SAP reduces the capacity of absorption. This may be explained as follow. in SAP, the hydrogen bonds are the active points that can so react with water. The water can make a lot of new OH chains so that the polymer looks bigger and heavier because of water trapped inside the polymer. In Figure 9, cellulose inserts the interchain space in the polymer during the polymerisation process. The internal cellulose (red dots) cannot have an interaction with water (blue arrows) because they are blocked by either the polymer or the external cellulose (green dots). The external cellulose reacts to the water and creates the hydrogen bonds to increase the absorption rate. Unfortunately, all cellulose make the polymer becomes inflexible so it cannot swell in maximum size. SAP Characterization by using SEM and FTIR The main purpose of SAP characterization by using SEM is to discover the physical structure of the polymer, especially the number of micropores on the polymer. These micropores are places for permeating the water and for interacting the external stimulant and hydrophilic groups of polymer grafting (Gao, et al, 2008). The amount of micropores will affect the absorption capacity of SAP. There are many SEM microphotographs that show physical structure of SAP, such as waving, pores, aggregate structure, irregular shape, and else. Based on the previous research (Gao et al, 2001; Li and Wang, 2005), however, the micropores structure is the most important factor that affect absorption capacity because micropores can increase surface area of the polymer. The wider surface area can give higher possibility of the interaction between hydrophilic groups and water to take place and also for water permeation. When cellulose becomes composite materials, SAPc, cellulose will be trapped in the polymer. It does not always have an interaction with water because there is a barrier on the polymer s surface and closes the cellulose s surface in the SAP. This trapped cellulose can decrease the performance of SAP although the polymer still is swelling and absorbing the water. The amount of water absorbed is around 280 g/g, much lower if it is compared to the non-composite SAP, 339 g/g at maximum capacity. The cellulose trapped inside the polymer is assumed to fill the empty space interchains. Even the monomers and cellulose are hydrophilic, these bonds can limit the capacity of absorption. Therefore, the rate of absorption is fast but the capacity is smaller. The illustration of this trapped-cellulose can be seen in Figure 9. Figure 10. SEM of AA-Am with 5000x enlargement In Figure 10, we notice that SAP surface is furrow sand, meaning having a lots of pores and hence having higher ability to absorb water (Li and Wang, 2005; Liu and Rempel, 2007)). This absorption is also affected bybulk density and porosity of SAP (Gao et al, 2001; Li and Wang, 2005). The higher bulk density of SAP has the smaller capacity, while the bigger porosity has the bigger capacity. Figure 9. Illustration of the trapped cellulose inside SAP Green Chemistry Section 1: Material Chemistry, Ahmad Zaenal Abidin, et al. P a g e 11
Nomenclature m t = polymer mass after t time of absorption [gram] m o = initial polymer mass (at dry) [gram] Q = degree of swelling i/v u = Concentration of non-swelling network S * = concentration of ionic solution V 1 = Water molar concentration V e/v o = crosslink density v = kinetic chain length M = monomer concentration I = number of initiator Figure 11. SEM of SAPC with 2500 times enlargement As previous explained, SAPc has faster absorption rate than SAP, but the capacity is lower. The faster absorption may be due to contribution of the crack shown in Figure 11 while the lower capacity may be due to an impact from the cellulose addition in which it wrecks the furrows and closes the pores in SAPc. Conclusions Graft polymerisation was used to make composite of SAP from AA, Am, and cellulose in a solution. The biggest capacity of absorption from this research is 339 g/g for SAP and 280 g/g for SAPc. Cellulose addition to SAP can increase the rate of absorption and the strength of hydrogel. These properties are supported by morphological observation using SEM showing that the SAP surface has a lot of pores while the SAPc surface has many cracks and less pores.therefor SAP is suitable for agricultural application and SAPc is for medical application. Acknowledgments Researchers want to give an appreciation to those who help us for this research: 1. Dr. Aditianto from Department of Material Engineering ITB as his help in SEM analysis. 2. DIKTI as the sponsor for Student Creativity Program 2010. References Elliot, M. (1997). Superabsorbent Polymers. BASF Product Development Scientist. http://chimianet.zefat.ac.il/download/superabsorbant_polymers.pdf Gao, J., Wang, A., Li, Y., Fu, Y., Wu, J., Wang, Y., and Wang, Y. (2008). Synthesis and Characterization of Superabsorbent Composite by Using Glow Discharge Electrolysis Plasma. Reactive and Functional Polymers, 68(9): 1377 1383. DOI: 10.1016/j.reactfunctpolym. 2008.06.018 Gao, D., Heimann, R.B., Lerchner, J., Seidel, J., and Wolf, G. (2001). Development of A Novel Moisture Sensor Based on Superabsorbent Poly(Acrylamide)- Montmorillonite Composite Hydrogels. Journal of Material Science, 36 (16): 4567-4571. DOI: 10.1023/A:1017971811942. Li, A andwang, A. (2005). Synthesis and properties of clay-based Superabsorbent Composite. European Polymer Journal, 41 (7) 1630 1637. DOI: 10.1016/j.eurpolymj.2005.01.028 Liu, Z, S. and Rempel, G.L. (2007). Preparation of Superabsorbent Polymers by Crosslinking Acrylic Acid and Acrylamide Copolymers. Applied Polymer Science, 64(7): 1345 1353. doi: 10.1002/(SICI)1097-4628(19970516)64:7 12 P a g e Green Chemistry Section 1: Material Chemistry, Ahmad Zaenal Abidin, et al.