polyacrylate super water absorbents and their biodegradabilities

Journal Pre-proof Radiation-synthesized polysaccharides/polyacrylate super water absorbents and their biodegradabilities Lorna S. Relleve, Charito T. Aranilla, Bin Jeremiah D. Barba, Alvin Kier R. Gallardo, Veriza Rita C. Cruz, Carlene Rome M. Ledesma, Naotsugu Nagasawa, Lucille V. Abad PII:

S0969-806X(19)30310-X

DOI:

https://doi.org/10.1016/j.radphyschem.2019.108618

Reference:

RPC 108618

To appear in:

Radiation Physics and Chemistry

Received Date: 29 April 2019 Revised Date:

22 November 2019

Accepted Date: 24 November 2019

Please cite this article as: Relleve, L.S., Aranilla, C.T., Barba, B.J.D., Gallardo, A.K.R., Cruz, V.R.C., Ledesma, C.R.M., Nagasawa, N., Abad, L.V., Radiation-synthesized polysaccharides/polyacrylate super water absorbents and their biodegradabilities, Radiation Physics and Chemistry (2019), doi: https:// doi.org/10.1016/j.radphyschem.2019.108618. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Radiation-synthesized polysaccharides/polyacrylate super water absorbents and their biodegradabilities Lorna S. Rellevea,*, Charito T. Aranillaa, Bin Jeremiah D. Barbaa, Alvin Kier R. Gallardoa, Veriza Rita C. Cruza, Carlene Rome M. Ledesmaa, Naotsugu Nagasawab, Lucille V. Abada a

b

Philippine Nuclear Research Institute-Department of Science and Technology Commonwealth Ave., Diliman, Quezon City, 1101, Philippines

National Institutes for Quantum and Radiological Science and Technology (QST) 1233 Watanuki, Takasaki, Gunma, 370-1252, Japan *Corresponding author: [email protected]

Abstract Different super water absorbents (SWA) based on polysaccharides/poly(acrylic acid) (PAAc) were prepared by gamma irradiation to be used as soil water retainers. Polysaccharides, kappa-carrageenan (seaweed and semi-refined forms) and cassava starch were used in the preparation of SWA to improve biodegradability. Gel fraction of the different SWAs ranged from 31% to 97% and degree of swelling reached up to about 5890 g H2O/g dry gel. The physico-chemical and mechanical properties of the SWA were influenced by the degree of neutralization, macromolecular properties of the polysaccharide and irradiation dose. FTIR and TGA analyses showed successful incorporation of polysaccharides in the network structure through formation of covalent bond. Biodegradability test by microbial oxidative degradation analyzer (MODA) showed that cassava starch/PAAc SWA biodegraded at a rate of 42% in 85 days compared to 11% of pure PAAc. The cassava starch/PAAc SWAs retained water up to more than 20 days in sandy soil and still absorbed water after 62 days with wet/dry cycles in pot experiment.

Keywords: polysaccharides; polyacrylate; gamma irradiation; super water absorbent; biodegradability;

1. Introduction Super water absorbents (SWA) can absorb a very large amount of water (many hundred times their dry weight) and retain it even under pressure. They are synthesized by crosslinking highly hydrophilic macromolecules. Super water absorbents are used in a wide variety of applications such as disposable absorbents, in agriculture, and other fields where water absorption is needed. As soil water retainers or soil conditioners, super water absorbents are developed to improve the physical properties of soil in view of: (a) increasing their waterholding capacity, (b) increasing water use efficiency, and (c) increasing plant performance (Jhury, 1997). Desired features of super water absorbents are high gel content, swelling capacity, fast swelling and good mechanical strength of the swollen gel. Most of the superabsorbents available on the market are 100% polyacrylate-based products, therefore not biodegradable. Biodegradability of super water absorbent is a very important property for protecting the environment. It has been found that the polyacrylate main chain degraded in the soils at rates of 0.12-0.24% per 6 months (Wilske et al., 2014). It is the aim of this study to synthesize biodegradable super water absorbents through the incorporation of polysaccharides into the polyacrylate structure by radiation methods. Polysaccharides like starch, carrageenan, cellulose derivatives etc. are good candidate raw materials for various applications because of their renewability, availability at low cost, biocompatibility and biodegradability. Kappa-carrageenan is sold commercially as dried seaweed, semi-refined and refined. The main source of raw material for the carrageenan industry is Kappaphycus alvarezii, a red alga, widely cultivated in the Philippines. Semirefined kappa-carrageenan (SRKC) is used as an alternative for the refined carrageenan which still contains cellulose as in the original seaweed. It is considerably a cheaper form of carrageenan. Starch is one of the most abundant substance in nature with unlimited supplies. Starch is produced from grain or root crops. Cassava is one of the main agricultural crops in the Philippines. Cassava production in the Philippines has reached 2.806 million metric tons in 2017 (Philippine Statistic Authority, 2018). About 20% of cassava production is utilized for starch processing (Bacusmo, 2000). Radiation technology has emerged as an environment-friendly, commercially viable technology with broad applications in health, environment agriculture and industry (IAEATECDOC, 2004). This technology is based on the use of ionizing radiation to modify the structures and properties of materials in different applications. Gamma and electron beam irradiation are found to be very effective methods for constructing three-dimensional polymeric networks of super water absorbent. Both offer advantages over conventional physical or chemical methods of network formation: elimination of need for toxic initiators and crosslinking agents, mild reaction conditions, negligible by-product formation, fast gelation and room temperature process. It is easy to control the properties of the materials by the adjustment of the radiation dose. In this report, super water absorbents from polysaccharides/polyacrylate were synthesized and characterized. The gel properties, IR spectra, TG profile, swelling kinetics, biodegradability and soil water retention and efficiency of the different radiation-synthesized polysaccharide/polyacrylate-based SWAs are presented. The influence of different polysaccharides on the gel properties of SWA is discussed.

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2. Materials and methods 2.1 Materials Dried seaweed, Kappaphycus alvarezii was obtained from Mioka Biosystem Corporation Philippines. Semi-refined kappa-carrageenan and refined iota-carrageenan were produced by Shemberg Corporation Philippines. Cassava starch and carboxymethyl cellulose (degree of substitution 0.8) were produced by Matling Industrial and Commercial Corporation Philippines, and CPKelco, Philippines, respectively. Acrylic acid monomer was purchased from Nippon Shokubai Indonesia. Commercial superabsorbent based on a potassium polyacrylate formulation, was used for comparison.

2.2 Preparation of SWA 2.2.1 Pure PAAc Pure PAAc gels were prepared by using an aqueous solution of acrylic acid (AAc) monomer with a final concentration of 20% w/w. Partial neutralization of acrylic acid was done using sodium hydroxide until it reached 0, 50 and 75% degree of neutralization (DN). DN refers to the amount of acrylic acid neutralized in the solution calculated based on mole percentages. 2.2.2 Seaweed/PAAc Dried seaweed was mixed with deionized water in a reaction vessel and stirred vigorously at 450 rpm for 30 min at 80 °C to form a viscous solution. This is referred to as heat gelatinization. Temperature was lowered to 60 oC before adding partially neutralized acrylic acid (25, 50 and 75% DN using NaOH). The slurry was mixed for an additional 15 min. The final concentration of dried seaweed and AAc in the mixture was 3% and 15-20% w/w respectively. 2.2.3 SRKC/PAAc Semi-refined kappa carrageenan (SRKC) based gels were prepared similarly to Seaweed/PAAc using heat gelatinization, with final concentrations of SRKC and AAc at 3% and 20% w/w respectively. AAc was partially neutralized with NaOH until DN was 0, 50 and 75%. 2.2.4 Starch/PAAc The formulation and preparation of Starch/PAAc was adapted from the “Guideline on development of hydrogel and oligosaccharides by radiation” (FNCA, 2017). Cassava starch (10% w/w) was gelatinized by heating in water or in alkali solution at room temperature. In heat gelatinization, AAc (20% w/w) was partially neutralized with potassium hydroxide (38% DN) before adding to the starch. In alkali gelatinization, starch was first dispersed in KOH solution (equivalent to 38% DN) at room temperature until it became viscous. AAc (20% w/w) was then added the mixture and stirred for 30 min. Additional gels were prepared using heat gelatinization with 20% AAc (50% DN using NaOH) and 3% starch, and with 15% AAc (75% DN using NaOH) and 0-15% starch. 3

2.2.5 CMC/PAAc and IC/PAAc These gels were prepared using heat gelatinization with final concentration of 3% polysaccharide with 20% AAc partially neutralized using NaOH until 50% DN. 2.2.6 Irradiation and processing SWA formulations were poured into plastic pouches, sealed and gamma-irradiated at the PNRI Irradiation Facility with doses of 15-45 kGy at a dose rate of 0.5 kGy/h. Dosimetry was monitored using ethanol-chlorobenzene (ECB) dosimetry system following ISO/ASTM51538-17 standards. The resulting gels were cut into cubes, air-dried and ovendried at 50 oC for 24 h. Samples were further ground to granules (1-3 mm) for evaluation of swelling kinetics, biodegradation and soil-water retention.

2.3 Characterization of gel properties of SWA 2.3.1 Gel fraction (GF) and degree of swelling (DS) The degree of swelling of the SWA was determined by gravimetric method, adapted from (Al-Assaf et al., 2016) with some modification. SWA samples (about 0.2 g) dried to a constant weight were immersed in 1 L of deionized water at room temperature for 7 days to ensure maximum swelling, i.e. swelling values plateaus and no longer changes. Water was changed daily. The swollen gel samples were periodically weighed after removing the surface water with a tissue paper. After 7 days, the swollen samples were removed from the water and dried to constant weight to get the gel fraction. The gel fraction and degree of swelling were calculated as follows: Gel fraction: (%): = Degree of swelling = where Wd is the weight of dried insoluble part after immersion for 7 days, Wi is initial weight of dried sample after irradiation and Ws is the maximum weight of swollen gel. All measurements were done in triplicates. 2.3.2. Gel strength (GS) of swollen SWA Dry gels were immersed in water for 24 h or until it absorbed 100 g of water per gram of dried gel. Compression test using the Zwick Roell Universal Testing Machine was carried out on the swollen gel using a 10 mm-diameter stainless steel probe at a crosshead speed of 3 mm/min. 2.3.3. Swelling kinetics Selected SWA formulations were ground to 1-3 mm granules and immersed in deionized water. The gels were weighed by filtering through a stainless-steel sieve (200 µm). The sample was immersed again in water and the procedure was repeated for different swelling times. The DS at different swelling time was calculated using equation in Section 2.3.1. 4

2.4 FTIR analysis SWA samples were washed extensively by immersing them in deionized water for 7 days, with frequent change in water, in order to extract sol fraction. The gels were then dried and subjected to FTIR analysis. The infrared spectra (600-4000 cm-1) were examined by a Perkin Elmer Spectrum One FTIR spectrophotometer in attenuated total reflectance (ATR) mode with a resolution of 4 cm-1. 2.5 Thermogravimetric analysis As with FTIR analysis, SWA samples were washed extensively and dried before subjecting it to thermogravimetric analysis. Thermal decomposition properties of the SWA samples (~10 mg) were investigated using a Netzsch STA449 F3 Jupiter over the temperature range of 251000 °C with a heating rate of 10 °C/min under nitrogen atmosphere. 2.6 Biodegradation test The biodegradation test was done in the laboratory of National Institutes for Quantum and Radiological Science and Technology, Takasaki, Japan. The microbial biodegradability of the super water absorbents was evaluated by measuring released carbon dioxide (CO2) using the Microbial Oxidative Degradation Analyzer (MODA) (Saida Ironworks Co., Ltd). The scheme of MODA is shown in Fig. 1. The test was performed as previously described (Nagasawa et al, 2004). Briefly, 5 g of the sample (dry granules) was mixed well with 320 g of rinsed sea sand, 60 g of compost and 95 g of water. This was then placed in the heated reaction column. The temperature inside the reaction column was maintained at 35°C. Moisturized air (CO2 free) was flowed through the sample inside the reactor and the flow rate was kept at 30 ml/min. The air carrying CO2 (formed due to polymer decay) that flowed out of the reactor was passed through a series of columns. Ammonia, which could be formed from the decomposing sample, was trapped in sulfuric acid solution and water vapor was absorbed into the first two columns (silica gel and calcium chloride). The CO2 was collected quantitatively by soda lime while water generated during the reaction was trapped in the last calcium chloride column. The amount of produced CO2 was calculated as a difference in the weight of two last columns (containing soda lime and calcium chloride) at the beginning and during the testing period. Pure compost mixed sea sand was used as a blank sample and cellulose powder as a reference sample. Biodegradation was calculated as indicated below: % Biodegradation = 2.7 Soil water retention and efficiency Four hundred grams of dry sandy soil (1:1 garden soil:sand) were placed in plastic pots. Soil was taken from the PNRI vicinity. SWA samples in the concentration range of 0.25-1% (based on the weight of soil) were applied as either dry granules mixed with the soil or as pre-swollen gels arranged in between layers of the soil. Set-ups without SWA and with commercial product were used as controls and reference, respectively. Each pot was initially wetted with 600 mL (dry granules) or 300 mL (pre-swollen gels) of water. Pots were rehydrated with 400 mL of water roughly every two weeks and weighed after allowing the set-ups to drip for one day. Pots were weighed daily for a period of 53 days. Water content was calculated as: 5

Water content (%) = 2.8 Statistical analysis Data was evaluated using GraphPad Prism 8. Numerical data are presented as mean values ± standard deviation when applicable. Means of different treatment groups/parameters/samples were analyzed using ANOVA including post-tests such as Bonferroni test. Values were considered significant at p<0.05.

3. Results and Discussion 3.1. Radiation crosslinking of poly(acrylic acid) in aqueous solution Formation of poly(acrylic acid) super water absorbent gel in this study using radiation involved the polymerization of acrylic acid monomer followed by crosslinking interactions. Free radical polymerization of acrylic acid monomers can be initiated to form homopolymers. Ionizing radiation leads to the formation of vinyl radicals that propagates with addition of the vinyl monomers (Reaction 1). A key advantage of radiation-induced polymerization is that it has lower activation energy which translates to high conversion rate. The covalent bonds of water and PAAc dissociates causing the formation of radicals. Radiolysis of water molecules produces hydrated electrons (eaq-), hydroxyl radicals (HO·) and hydrogen radical (H·) (Reaction 2). Likewise, the C-H bond of ionized PAAc breaks, then a polymer radical and a hydrogen radical are formed in Reaction 3. Radical transfer reactions between PAAc and hydroxyl and hydrogen radicals dominate to form polymer radicals. These polymer radicals recombine causing gel formation. As monomers are converted to polymers, polymer buildup occurs. With continuous radiation exposure, these polymers can be reactivated which leads to the formation of branched polymer molecules, such that remaining monomers can be “homografted” to its own polymer. When conversion reaches very high values, crosslinked gels can be formed (Adolphe Chapiro, 1979). The most important reactions of radiation crosslinking of PAAc in aqueous solution as described by Jabbari and Nozari (2000) are shown from Reaction 4 to 7. AAc → PAAc

(1)

H2O → H·, HO·, eaq-, H2, H2O2, H3O+

(2)

PAAcH → PAAc· + H·

(3)

PAAcH + (H· or HO·) → PAAc· + HOH or H2

(4)

PAAc·m + PAAc·n → PAAcm‒PAAcn

(5)

PAAc·m+n → PAAcm + PAAc·n

(6)

PAAc·m+n + O2 → PAAcm+n‒O2· → PAAcm‒C(O)H + PAAcn‒O·

(7)

6

Reaction (4) involves the reaction of the HO· and H· radicals with poly(acrylic acid) by Habstraction at the α and ß-positions to the carboxylic acid. Most ß-macroradicals of PAAc are converted α-macroradicals by rearrangement (Ulanski et al., 1996). In reaction (5), it is the pathway for increasing the gel formation. In reaction (6), polymer degrades and in reaction (7), molecular oxygen present reacts with polymer radical to give a decomposition product, aldehyde.

3.2. Radiation synthesis of polysaccharide/PAAc SWA An important feature of super water absorbent, especially when used in agricultural application, is its biodegradability. This ensures that the product remains eco-friendly and reduces pollution contribution. While the market is dominated by polyacrylate-based SWA, increasing environmental awareness has shifted the demand to biodegradable SWA, usually achieved by a blend of natural polymers such as polysaccharides. This reduces reliance on petroleum-based, nonrenewable resources while producing sustainable, environment-friendly and nontoxic products. The process of preparing polysaccharide-polyacrylate blend super water absorbents involves first the gelatinization of the polysaccharide in order to fully extend their chains and improve dissolution and mixing. This is achieved by either heat or alkali treatment under shear stress. The heat or base added breaks down intermolecular association of polysaccharide molecules allowing hydrogen bonding with water. High shear forces, on the other hand, allows for faster diffusion of water into the polysaccharide network by physical tearing apart its granules and reducing its crystallinity (Burros, Young, & Carroad, 1987; Wen, Rodis, & Wasserman, 1990). Another component in the process involves neutralization of the carboxylic groups of the acrylic acid component. This replaces H+ with Na+ or K+ with the addition of the corresponding hydroxide base. Upon contact with water, these ions are hydrated which reduces their attraction to the carboxylate ions and allows the Na+ or K+ ions to move more freely inside the network. This contributes to the osmotic driving force for the diffusion of the water into the gel (Witono et al., 2014). Neutralization is done before or after crosslinking for optimum swelling or absorbency (Francis et al., 2004, Sutradhar et al., 2015, Wang et al., 2010, El‐Mohdy et al., 2007, Bhuyan et al., 2016). In this study, neutralization was done before crosslinking for easier processing. As in the radiation synthesis of PAAc gels, polysaccharide/PAAc SWA was subjected to gamma irradiation simultaneously. The bulk of the reactions occurring in the mixtures of the polysaccharide and acrylic acid monomer are mediated by species formed from the radiolysis of water molecules. Reactive species (hydrated electrons, hydroxyl radicals and hydrogen atoms) can give rise to several possible mechanisms within the mixture as summarized in Fig. 2. Abstraction of hydrogen on the polysaccharide backbone can form macroradicals on its surface, which can lead to chain scission (a), especially at oxygenated solutions; various transformations of the sugar units (b) (G. Ershov, 1998); or these radicals can serve as graft points for the polymerization of acrylic acid (c) (Mittal, Ray, & Okamoto, 2016). Polymerization and crosslinking mechanisms of acrylic acid discussed in the previous section may also occur in the mixture (d) as well as recombination of macroradicals between either the polysaccharide, chain scission products or polyacrylic acid chains (e) (Al-Assaf et al., 2016; Makuuchi, Yoshii, Aranilla, & Zhai, 2000). While it is difficult to fully elucidate the nature of the reactions within the mixture and which mechanisms predominate, the properties exhibited by the synthesized hydrogels suggest the formation of a three-dimensional network 7

resulting from significant crosslinking. Graft copolymerization, while a possible mechanism, is much more difficult to elucidate, though several studies use this term liberally (El-mohdy, Hegazy, & El-rehim, 2006; Kiatkamjornwong, Chvajarernpun, & Nakason, 1993). Another mechanism that has also been suggested is noncovalent, physical entanglement of gelling type polysaccharides to the crosslinking polymer (PAAc) to form a interpenetrating network (Abad, Relleve, Aranilla, & Dela Rosa, 2003; Jing, Yanqun, Jiuqiang, & Hongfei, 2001). The properties of radiation-synthesized pure PAAc and polysaccharide/PAAc SWAs are presented in succeeding discussions.

3.2.1 Pure PAAc Table 1 presents the crosslinking of PAAc with 20% AAc concentration at different DN. PAAc gels with 0% DN had almost 100% gel fraction. Partial neutralization of acrylic acid showed some decrease in gel fraction especially at 75% DN. Several studies noted that there is less recombination of macroradicals at higher pH due to electrostatic repulsion of polymer chains (Ulanski and Rosiak, 1994, Zhu et al., 1998). Poly(acrylic acid) acts like a polyelectrolyte with a pKa value of ~6.2 (Oosawa, 1971). Below pKa, carboxylic acid groups are predominantly undissociated. Above pKa, PAAc chains are ionized and can adapt a stiff and extended conformation due to repulsion of the negative charges of the carboxylate species. For radical recombination of neighboring chains to occur, these repulsive forces must first be overcome. Prior to irradiation, pH values of the acrylic acid solution were 2.2, 5.01 and 5.85 at 0, 50 and 75% DN, respectively. These values correlate well with the observed trend in gel fraction. At pH 5.85, it approaches the pKa of PAAc wherein more carboxylic groups are ionized that caused the gel fraction to decrease at 75% DN. Swelling capacity, on the other hand, increased with increasing DN. Expectedly, higher swelling values were obtained from gels with lower gel fractions due to less restriction of the hydrogel network. Low osmotic diffusion may also be observed when carboxylic acid groups are less ionized, as with low DN (Bhuyan et al., 2016, Fekete et al., 2016). While 50% DN gels may offer a good balance of sufficient crosslinking and swelling capacity, poor biodegradability of acrylate-based gels remains a significant constraint in its utilization as superwater absorbents in agricultural applications. Incorporation of natural polymers like polysaccharides were explored as an alternative.

3.2.2. Seaweed/PAAc SWA Acrylic acid solutions with 15% and 20% concentration were investigated for the preparation of seaweed/PAAc super water absorbent, as previous study reported this concentration range produced SWA with good properties (Sanju et al., 2004; FNCA Guideline, 2017). The results are summarized in Table 2. At 25-50% DN, the gel fraction ranged from 31 to 85%. The amount of gel fraction measured gives stability to the SWA since it will be able to retain or hold water at a higher degree without simply leaching out in the soil when used as water retainer. Gel fraction was found to increase with decreasing DN (p<0.05), similar to pure PAAc gels. Notably, samples with 15% AAc at 75% DN did not form gels even at absorbed dose of 45 kGy. The pH of this solution before irradiation was 5.2, which approached the pKa of PAAc of ~6.2. This is also above the pKa of the main component of the seaweedkappa-carrageenan (~2). Both groups, -COOH and -OSO3H of kappa-carrageenan are mostly ionized at this pH and therefore they have extended conformation. Greater repulsive force between these ionized groups prevents polymer radicals from recombining and competitive 8

reactions, such as chain scissions, proceed with higher probability as charge density increases (Sakurada & Ikada, 1963). The 15% AAc alone at this DN also did not form gels as will be shown in the later section. This was also observed in the study by Lappan & Uhlmann (2010) on PAAc crosslinking in dilute solutions, wherein no gelation was observed when initial pH was greater than 5. Meanwhile, seaweed/PAAc using 20% acrylic acid formed gels at 75% DN. Here, the increased concentration limits the spacing between the mixture components and forces the polymers to come together, enough to overcome charge repulsion and lead to network formation with gel fraction of 31-33%. This remains significantly lower than samples at lower DN. There is less electrostatic inhibition of crosslinking at lower DN as discussed earlier. All gel fractions recorded for this formulation are evidently lower than that of pure PAAc, owing mostly to the filler effect of the added polysaccharide which competes with PAAc for the free radicals. On the other hand, the addition of the hydrophillic polysaccharide improved the swelling capacity of the gels and loosens the 3D network for better water absorption and stress transfer. The swelling capacity of seaweed/PAAc SWA with 25-75% DN (15-45 kGy), ranged from 347 to 5890 g H2O/g dry gel and gel strength of 1-10 kPa. Among the samples in this formulation, 25% DN was found to be suitable for the preparation of seaweed/polyacrylate SWA as it had higher gel fraction accompanied by acceptable degree of swelling especially at lower doses.

3.2.3 Semi-refined kappa-carrageenan/PAAc SWA The characteristics of SWA made from SRKC/PAAc are presented in Table 3. The SRKC/PAAc SWA had gel fractions in the range of 62-78% at 50-75% DN and absorbed doses of 15-30 kGy. There was an appreciable crosslinking as indicated by an increased gel fraction even at 75% DN and 15 kGy as compared to seaweed-based-SWA. While the major component of Kappaphycus alvarezii is KC, there may be some residual amount of proteins, fats and minerals in the unprocessed seaweed, along with phenolic bioactive components that could reduce the efficiency of the radiation process (Prabha, Prakash, & Sudha, 2013). SRKC, on the other hand, has gone through extraction processes that remove these impurities such that it is mostly comprised of only KC and a small amount of cellulose (McHugh, 2003). The gel fraction still increased with decreasing degree of neutralization like with seaweedbased SWA and pure PAAc. The SRKC/PAAc with 0% DN exhibited high gel fractions (9597%) compared to the partially neutralized SWA. In addition, at 20% AAc and 50-75% DN, both SRKC/PAAc and seaweed/PAAc had lower gel fraction compared to the pure PAAc while at 0% DN, SRKC/PAAc and pure PAAc had almost the same gel fraction. Thus, higher percentage of SRKC was incorporated when AAc was unneutralized. This could be attributed to the acid effect. Prior to irradiation, pH level of 0% DN sample was found to be 2.9. In acidic solutions, there is less electrostatic inhibition of crosslinking and the presence of H+ can increase G(H•) and encourages abstraction of hydrogen preferentially from saturated organic molecules (A Chapiro & Dulieu, 1977; Suda Kiatkamjornwong & Meechai, 1997). Recombination of these radicals can form T type molecules when KC chain attaches to PAAc or H type molecules from recombination of PAAc chains (Makuuchi et al., 2000). Furthermore, if the polymerization/crosslinking of acrylic acid and the degradation of the polysaccharide occurred separately, the maximum gel fraction for the 0% DN should be 87%. However, gel fraction was recorded at 95-97% even after extensive washing of sol fraction. 9

SRKC in this condition was effectively incorporated into the gel either by crosslinking or grafting mechanisms (Abad et al., 2003). The degree of swelling ranged from 556 to 1693 g H2O/g dry gel for SRKC/PAAc SWA gels at 50% and 75% DN and with doses of 15-30 kGy. The swelling of SRKC/PAAc SWA with 0% DN were rather very low with values of 29-71 g H2O/g dry gel. Unneutralized gels of CMC/PAAc and potato starch/PAAc were reported also to have low degrees of swelling due to low osmotic diffusion when carboxylic acid groups are less ionized (Bhuyan et al., 2016, Fekete et al., 2016). This type of SWA may not be useful in agricultural or hygiene products due to low water absorption.

3.2.4. Cassava starch/PAAc SWA The formulation of cassava starch/PAAc SWA, as adapted from the FNCA guidelines, was 20% acrylic acid, 10% starch and 38% degree of neutralization (solution pH measured at 4.78). In the synthesis process, the gelatinization of starch can be achieved by heating with water or mixing with alkali solution at room temperature. Table 4 gives the gel properties of the cassava starch-based SWA. Gel fraction, degree of swelling and gel strength were all significantly higher in alkali gelatinization, especially at doses 20-25 kGy (p<0.05). Thermal gelatinization involves the thermal disordering of crystalline structures in native starch granules. Once heated past its gelatinization temperature, typically between 57-65 °C, hydrogen bonding of starch molecules is disrupted, and water molecules become attached to their hydroxyl groups which leads to greater swelling and dissolution (Liu, Xie, Yu, Chen, & Li, 2009). On the other hand, in alkali treatment, the base ions disrupts the hydrogen bonding by deprotonation accompanied by some physico-chemical changes on the starch structure (Ragheb, Abdel-Thalouth, & Tawfik, 1995). Because of ionic interactions and transformations, alkali treatment seemed to be more effective in the gelatinization process and yielded better results than simple heat gelatinization. Pure PAAc synthesized using the same conditions (20% AAc, 38% DN, 15 kGy) had the swelling capacity and gel strength of 1076 g H2O/g dry gel DS and 39 kPa, respectively. The swelling of pure PAAc was much greater than that of SWA containing heat or alkali-treated starch but had lesser gel strength. These characteristics of SWA suggested incorporation of starch in the gel network. In addition, if only PAAc formation in the gel is accounted for, gel fraction should be at around 70% if full conversion was achieved. However, starch-based SWA had higher gel fractions ranging from 80-94% and so starch molecules must be effectively incorporated in the SWA such that they were retained despite sol extraction. The behavior of cassava starch/PAAc fixed at only 15% AAc and 75% DN with varying starch concentration and doses of 15-45 kGy was also studied and presented in Table 5. No gels were obtained at 0-5% starch and these results are similar to seaweed/PAAc. At higher starch concentrations of 10-15%, substantial gel fractions in the range of 42-61% were obtained. This highlights the importance of sufficient starch concentration in the SWA formation. Possibly, increase in the total substrate concentration pushed the reactants in close proximity enough to form covalent bonds despite charge repulsion between polymer chains at relatively higher pH.

10

3.3. Influence of different polysaccharides on the gel properties of SWA at fixed conditions Fig. 3 shows the effect of different polysaccharides, CMC, SRKC, IC and cassava starch, on the gel properties of PAAc SWA with fixed polysaccharide and AAc concentrations, and DN, i.e. 3% polysaccharide, 20% AAc and 50% DN. As seen in the figure, both carrageenanbased SWAs synthesized at 15 kGy absorbed large amounts of water compared to other polysaccharide-based SWAs. The presence of sulfate groups in carrageenan may have led to the increased degree of swelling. Presence of charges within the SWA causes an osmotic pressure difference between the gel and water and creates a driving force that diffuses the solvent into the gel. This effect can be tapered by increasing crosslinking density as a function of dose, which limits diffusion and expansion. The difference in the gel fraction of KC and IC-based SWAs, though similar in sugar structure, is probably due to differences in charge density and gelation mechanism. IC is more sulfated than KC, and has a greater charge density of 1.49 per disaccharide while KC is at 0.96 (Hugerth, Caram-Lelham, & Sundelöf, 1997). While helpful in aiding in swelling, the negative charges impede radical recombination during crosslinking. Moreover, their gelation mechanism generally follows the transition from random coils to helix formation. In KC gels, the double helices further associate together and this mechanism aids in the formation of tightly bound interpenetrating network between KC chains and PAAc. IC, on the other hand, only form double helices without association, such that any non-covalently bonded chain may get extracted during washing (Thomas, 1997). CMC/PAAc had the highest recorded gel fraction, owing possibly to the additional crosslinking capability of its carboxymethyl groups (Kume, Nagasawa, & Yoshii, 2002). This may also explain its comparatively low swelling and high gel strength despite also having a relatively high charge density (~1.11). Starch-based gel had lower swelling than the other polysaccharides especially considering it has the lowest gel fraction. Unlike the carrageenans and CMC, starch is a neutral molecule with high pKa at 12.6 (Bertolini, 2009). At fixed DN for all polysaccharides with initial mixture pH at ~4.9, only starch-based gels are non-ionized and lack the extra charges that facilitate osmotic water absorption. The gel strengths of the different followed the order of CMC > starch > carrageenan. CMC leads because of its relatively higher crosslinking density (gel fraction) while extensive hydrogen bonding and rigidity of starch granules translate to better gel strength despite lower gel fraction (Lin, Liang, & Chang, 2016). Carrageenans, on the other hand, form relatively weak gels (Thomas, 1997).

3.4. FTIR and TG analyses Characterizations were done on the representative samples from the different polysaccharide/polyacrylate-based SWA to check for its composition after sol extraction. Conditions were fixed at 3% polysaccharide, 20% AAc, 50% DN and 15 kGy dose. FTIR spectra are shown in Fig. 4. Pure polysaccharides show distinctive peaks at 3400, 2920 and 2850, and 1100-950 cm-1 corresponding to O-H, C-H and C-O stretching of sugar units. Pure PAAc, on the other hand, show unique peaks at 1700 and 1560 cm-1 due to carboxylic and carboxylate groups of the polymer. As expected, all gel fractions exhibited peaks from acrylic acid indicating presence of polyacrylic acid in the gel. PAAc also exhibits absorption peaks which coincide with polysaccharide peaks, which made it difficult to distinguish. However, PAAc only has weak absorption at 1100-950 cm-1 but some of the gels, particularly SRKC/PAAc, starch/PAAc and CMC/PAAc, showed more intense peaks at this region indicating successful incorporation of the polysaccharide. 11

The thermograms of the SWA gel fractions and their components are shown in Fig. 5. The thermal degradation profile of pure PAAc occurred in three steps, (i) dehydration at less than 200°C, which is also the general first step for all the other samples; (ii) decarboxylation and loss of small molecules (e.g. H2O, CH4, etc.) at 200-330°C; and (iii) chain scission and depolymerization at greater than 350°C. Broad DTG peaks at the second and third decomposition stage may have been due to the different chain lengths formed during PAAc homopolymerization (Calahorra, Cortázar, Eguiazábal, & Guzmán, 1989; Nishizaki & Yoshida, 1981). The Tmax indicated by the DTG peak was at 453°C. This is slightly higher than the Tmax of PAAc in literature, usually at 380-410°C (Bin-Dahman, Jose, & Al-Harthi, 2015; Dubinski, Grader, Shter, & Silverstein, 2004; Jose, Shehzad, & Al-Harthi, 2014), suggesting additional thermal stability which may have been caused by crosslinking. The main decomposition of seaweed was at 150-207°C. The seaweed-PAAc generally follows the thermogram of pure PAAc, indicating that the gel was made primarily of only PAAc. The same goes for IC which decomposes mainly at Tmax 232°C but whose gel generally follows that behavior of pure PAAc. SRKC, on the other hand, was found to start its main decomposition at 188°C with Tmax at 212°C. SRKC/PAAc also exhibited a small decomposition Tmax at 178°C suggesting possible grafting of SRKC. Grafted polysaccharides typically exhibit lower thermal stability than pure polysaccharides due to free volume effects (Athawale & Lele, 1998; Tanodekaew et al., 2004) . The same trend could be seen for starchAAc and CMC-PAAc whose Tmax shifted from 310°C to 305°C and 285°C to 256°C respectively. The results from both FTIR and TG indicate possible grafting of AAc onto SRKC, starch and CMC but not on seaweed and IC. Again, other residues found in seaweed and the higher charge density of IC may obstruct their participation in the grafting or crosslinking of the SWA hydrogel.

3.5. Biodegradability Biodegradability, which is a desired characteristic of super water absorbent, is the ability of a material to be converted into CO2 through the action of microorganisms such as bacteria, fungi, and algae. Despite the impressive characteristics of polyacrylate SWA, its low biodegradability, typically less than 1% rate of degradation in soil per six months, has been a significant setback in its utilization (Wilske et al., 2014). This has been the driving force in the research for amended SWA formulations that can integrate the usefulness of polyacrylate SWA with acceptable biodegradability. Incorporation of water absorbing natural polymers like polysaccharides has been the attractive option. Biodegradation was studied using MODA, which has been demonstrated to be an effective test method for determining the ultimate biodegradability of plastic materials (Hoshino et al., 2007). Weight loss method cannot accurately determine a degree of biodegradation over 70%, because the remnants of materials that have already degraded to 70% cannot be totally recovered (Kunioka et al., 2009). Furthermore, the advantage of using MODA over conventional soil burial test is that degradation is measured as the byproduct of microbial processing without ambiguity. Soil burial test can often mistake run-off of low molecular weight fragments as full degradation.

12

To evaluate the improvement of biodegradability of the polyacrylate SWA with the incorporation of polysaccharides, biodegradation tests by MODA were carried out. Formulations tested were the following: (a) 10%starch/20%AAc at 38% DN and 15 kGy prepared by heat method (S/PAAc); 3%SRKC/20%AAc at 50% DN and 15 kGy (SRKC/PAAc); and (c) 20%AAc at 50% DN and 15 kGy (Pure PAAc). The samples were tested without extraction of sol fraction in order to simulate real-world application which eliminates the extraction step for convenient and economical processing. Results of biodegradation tests after 85 days in the controlled compost are shown in Fig. 6. The control used was cellulose which showed around 10% biodegradation within 3 weeks and a steady incremental increase henceforth. This slow degradation could be attributed to the crystalline structure of cellulose. Polysaccharide-based SWA showed an initial biodegradation curve at 3 weeks which could be attributed to polysaccharides trapped within the crosslinked network (as in an interpenetrating gel model). Afterwards, a steady and fast increase was observed with S/PAAc. The SRKC/PAAc, on the other hand, had low biodegradation rate (12.9% in 85 days). This may have been caused by the acidification of the compost resulting from the sulfate that was released after biodegradation of unbound SRKC in the first few days of incubation and could cause inhibition of the microbial activity. Similarly, the commercial product had negative values of CO2 which suggest possible toxic effect of the sample and/or its degradation products on the microbes in the compost. Among the SWA tested, the cassava starch-based SWA had the highest biodegradability with a rate of 42% in 85 days compared to pure PAAc with a rate of 11%. The biodegradation of pure PAAc may be due to the low molecular-weight fragments from the soluble fraction which are more susceptible to microbial degradation. The incorporation of starch induced by radiation processes resulted in higher biodegradation rate of the SWA which is useful for environment friendly SWA products.

3.6. Swelling rate The swelling rate is one of the important characteristics of a super water absorbent for hygiene products, agricultural and other applications. For example, the SWA should quickly absorb water upon irrigation of soil. For swelling of hydrogels, it may be expressed by the following equation: t/DS = A + Bt where DS is the degree of swelling at time t, B =1/DSeq is the reverse of the equilibrium swelling or A = 1/(ksSeq2) is the reciprocal of the initial swelling rate of the gel and ks is the swelling rate constant. The initial rate of swelling, theoretical equilibrium swelling, and swelling rate constants are presented in Table 6. The values of theoretical equilibrium swelling were relatively in good agreement with the values obtained experimentally. The initial swelling rate (ri) is the velocity before the networks are strained and begin to retard swelling. The initial rate of swelling of commercial SWA was comparably higher than the radiation-processed SWAs. The swelling rate of the radiation processed super water absorbents needs to be improved. Swelling rate of the super water absorbents was found to be related to the porosity of the network. A rapid swelling superporous hydrogel (SPH) has been synthesized through the gas blowing technique by acid decomposition of bicarbonates (Sanju et al., 2009, Chavda et al., 2012). It has been shown that the presence of interconnected microscopic pores makes SPHs absorb water very rapidly and swells to equilibrium in a short period. This process could be integrated into the current method of preparation of SWA. 13

3.7. Soil water retention and efficiency of cassava starch/polyacrylate SWA Based on the data obtained, cassava starch-based SWA appears to be an ideal SWA material for agriculture due to its biodegradability, gel properties, and low cost. The soil water retention properties of S/PAAc were characterized. Fig. 7 shows the soil water retention of S/PAAc at different application rate (applied as dried granules) in sandy soil. As expected, the water holding capacity of soil was found to increase with the concentration of SWA. The starch-based SWA can increase the water content to 92%, 76%, 61% and 48% with the addition of 1%, 0.75%, 0.50% and 0.25% SWA, respectively. The water content of the control (without SWA) was 38%. In the soil with 1.0 % SWA, the amount of water was still 50% even after 16 days while soil with 0.75% and 0.5% SWA had only 25% and 18% retention, respectively. For the soil without SWA and with 0.25% SWA, almost no water was present after 16 days. Thus, an application rate in the range 0.5-1.0% could be effective. Efficiency of water retention of starch-based SWA was evaluated by extending to longer incubation period with wet/dry cycles. Fig. 8 shows the water retention pattern for 62 days of S/PAAc and commercial product in sandy soil. In the first cycle or first rehydration (day 21), water absorption of the S/PAAc-amended soil decreased from 80% to 53% and decreased further to 39% after third cycle. The commercial SWA was relatively stable during the 34day incubation period as demonstrated by the consistent value of water content at each cycle. These results showed that the SWA crosslinked at 15 kGy was not efficient as it loses its water retention properties on day 34 (i.e. its water content almost similar to the control) while the S/PAAc-20 kGy SWA had still the capacity to absorb 53% water content after 62 days. These differences in results could be due to different crosslinking density or due to difference in cation exchange from the soil. Studies on further optimization of properties of cassava starch-based SWA and the cause of loss of efficiency will be reported on our next publication.

4. Conclusions Different super water absorbents were prepared from different polysaccharides and acrylic acid by simultaneous gamma irradiation. SRKC/PAAc, starch/PAAc and CMC-PAAc were successfully synthesized as proven by gel fraction, FTIR and TGA analyses. Important considerations in the synthesis of SWA with good properties include gelatinization of polysaccharide, with alkali method proving to be more effective, and partial neutralization of acrylic acid which encourages radical recombination and creates osmotic force for swelling. Biodegradation test supports the improvement of biodegradability of acrylate-based-SWA with the incorporation of starch. Cassava starch-based SWA appears to be a promising SWA material for agriculture due to its biodegradability, gel properties, and low cost.

Acknowledgements The authors thank Dr. Jordan F. Madrid and Mr. Patrick Jay E. Cabalar of Philippine Nuclear Research Institute for helpful discussions and Mr. John Andrew A. Luna for his experimental assistance. They are also grateful to National Institutes for Quantum and Radiological Science and Technology for allowing biodegradation studies to be conducted in their laboratory. 14

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17

Table 1. Gel properties of pure PAAc super water absorbent with different degree of neutralization (20% AAc). GF (%)

DS (g H2O/g dry gel)

Dose (kGy)

0% DN

50% DN

15

100 ± 2

30

97 ± 1

GS (kPa)

75%DN

0% DN

50% DN

75% DN

0% DN

50% DN

75% DN

94 ± 3

87 ± 4

30 ± 2

845 ± 63

2094 ± 343

1061 ± 3

15 ± 6

4.2 ± 3.6

98 ± 2

90 ± 3

26 ± 4

454 ± 9

1175 ± 76

474 ± 78

9±1

3.7 ± 0.8

The pH of solutions with 0%, 50%, and 75% DN were 2.20, 5.01, and 5.85, respectively. Gel strength at 24 h swelling

Table 2. Gel properties of seaweed/PAAc super water absorbent with different concentration of AAc and degree of neutralization (3% seaweed, 15-20% AAc) AAc (%)

15

20

Dose (kGy)

GF (%) 25% DN

50% DN 31 ± 12

DS (g H2O/g dry gel) 75%DN

25% DN

--

GS (kPa)

50% DN

75% DN

25% DN

50% DN

75% DN

840 ± 88

5890 ± 1405

--

3.9 ± 0.7

nd

--

15

80 ± 4

30

85 ± 2

61 ± 13

--

531 ± 75

2144 ± 449

--

4.3 ± 1.5

1.1 ± 0.2

--

45

81 ± 2

53 ± 1

--

347 ± 24

1893 ±

--

9.5 ± 2.4

2.0 ± 0.4

--

15

70 ± 2

56 ± 3

33 ± 3

1426 ± 151

1527 ± 385

4522 ± 622

5.7 ± 1.1

3.2 ± 0.2

nd

30

70 ± 2

60 ± 2

31 ± 15

1035 ± 160

1055 ±

4177 ± 2776

7.8 ± 0.7

5.1 ± 0.5

nd

51

26

The pH of solutions from 15% AAc, with 25%, 50%, and 75% DN were 4.19, 4.70, and 5.20, respectively. The pH of solutions from 20% AAc, with 25%, 50%, and 75% DN were 4.47, 5.08, and 5.91, respectively. Gel strength at 24 h swelling “--” no gel formation; “nd” no data (below detection limit); means between varying DN are significantly different (p<0.05)

Table 3. Gel properties of SRKC/PAAc super water absorbent with different degree of neutralization (3% SRKC, 20% AAc) GF (%)

DS (g H2O/g dry gel) 50% DN 75% DN

Dose (kGy)

0% DN

50% DN

75%DN

0% DN

15

97 ± 2

72 ±7

62 ± 2

71 ± 17

1357 ± 449

30

95 ± 2

78 ±2

74 ± 2

29 ± 5

556 ± 34

GS (kPa) 0% DN

50% DN

75% DN

1693 ± 103

186 ± 3

1.0 ± 0.3

nd

1174 ± 83

313 ± 5

6.0 ± 1.8

nd

The pH of solution with 0%, 50%, and 75% DN were 2.88, 4.87, and 5.38, respectively. Gel strength at 24 h swelling “--” no gel formation; “nd” no data (below detection limit; means between varying DN are significantly different (p<0.05)

Table 4. Gel properties of cassava starch/PAAc super water absorbent (10% starch, 20% AAc, 38% DN) Dose (kGy)

GF (%)

Heat Gelatinization Method GS DS (kPa) (g H2O/g dry gel)

GF (%)

Alkali Gelatinization Method DS GS (kPa) (g H2O/g dry gel)

15

80 ± 14

182 ± 37

100 ± 20

93 ± 2

194 ± 3

110 ± 22

20

80 ± 1

95 ± 5

43 ± 17

92 ± 1

183 ± 11

94 ± 37

25

94 ± 4

87 ± 6

74 ± 14

93 ± 1

165 ± 5

133 ± 13

The pH of the solution was 4.78. Gel strength at 24 h swelling Means between heat and alkali gelatinization group are significantly different (p<0.05) at 20 and 25 kGy.

Table 5. Crosslinking behavior of cassava starch/PAAc super water absorbent at 15% AAc and 75% DN Starch (%) 0 3 5 10 15

15 kGy ---51 ± 1 61 ± 3

GF (%) 30 kGy ---42 ± 3 60 ± 1

45 kGy ---44 ± 2 55 ± 2

DS (g H2O/g dry gel) 15 kGy 30 kGy 45 kGy ---------114 ± 20 305 ± 73 377 ± 64 102 ± 13 144 ± 8 114 ± 20

The pH of solutions with 3%, 5%, 10% and 15% starch concentration were 5.5, 5.75, 5.46 and 6.1, respectively. “--” no gel formation; means between varying starch (%) are significantly different (p<0.05)

Table 6. Swelling kinetics parameters of different super water absorbents Samples SRKC/PAAc S/PAAc Pure PAAc Commercial

ri (g H2O/g dry gel)/min 6.35 6.43 6.63 9.79

ks x 10-6 (g dry gel/ g H2O)/min 0.057 4.880 0.074 0.640

Theoretical DSeq (g H2O/g dry gel) 1667 179 1423 400

Experimental DSeq (g H2O/g dry gel) 2353 ± 170 182 ± 137 1374 ± 110 431 ± 34

R2 0.9765 0.9992 0.9933 0.9988

19

Fig 1. Scheme of Microbial Oxidative Degradation Analyzer (MODA)

20

Fig. 2. Summary of covalent reaction schemes of radiation processing of polysaccharide and acrylic acid monomer mixture

Fig. 3. Influence of different polysaccharides on the gel properties of SWA (3% polysaccharide, 20% AAc, 50% DN). Gel strengths were measured at swelling of 100 g/g for CMC and 70 g/g for IC, Starch and KC. The pH of the solution with CMC, IC, Starch and KC were 4.89, 4.92, 4.84, and 4.87, respectively.

21

Fig. 4. FTIR spectra of SWA formulations, pure polysaccharides and PAAc.

22

Fig. 5. TGA profiles of SWA formulations and their respective DTG (inset).

23

Fig. 6 Biodegradation of the radiation-synthesized super water absorbents and commercial product

24

Fig. 7. Water retention of cassava starch/PAAc at different application rate (dry granule application)

Fig. 8. Efficiency of water retention of cassava starch/PAAc and commercial SWAs (preswollen) in soil with wet/dry cycles (S/PAAc-15 kGy-conducted in cold weather condition while S/PAAc-20 kGy in summer) 25

Highlights Super water absorbents (SWA) based on polysaccharides/polyacrylate were synthesized by gamma irradiation and characterized. Gel fraction and degree of swelling reached up to 97% and 5890 g H2O/g dry gel, respectively. Cassava starch-based SWA biodegraded at a rate of 42% in 85 days.

CONFLICTS OF INTEREST STATEMENT

Manuscript title: Radiation-synthesized polysaccharides/polyacrylate super water absorbents and their biodegradabilities

The authors declared no conflicts of interest.

Authors names:

Lorna S. Relleve Charito T. Aranilla Bin Jeremiah D. Barba Alvin Kier R. Gallardo Veriza Rita C. Cruz Carlene Rome M. Ledesma Naotsugu Nagasawa Lucille V. Abad

Author Statement

Manuscript title: Radiation-synthesized polysaccharide/polyacrylate super water absorbents and their biodegradabilities

Lorna S. Relleve: Conceptualization, Methodology, Validation, Writing-Original draft preparation, Writing-Reviewing and Editing, Visualization, Supervision, Project Administration Charito T. Aranilla: Conceptualization, Methodology, Visualization, Supervision Bin Jeremiah D. Barba: Investigation, Validation, Formal Analysis, Visualization, WritingReviewing and Editing, Alvin Kier R. Gallardo: Investigation, Validation, Formal Analysis, Visualization, Veriza Rita C. Cruz: Investigation Carlene Rome M. Ledesma: Investigation Naotsugu Nagasawa: Methodology, Investigation, Validation, Visualization, Supervision Lucille V. Abad: Conceptualization, Methodology, Visualization, Supervision

Listed above are the specific contributions made by each author for this manuscript.

Lorna S. Relleve Corresponding Author Philippine Nuclear Research Institute – Department of Science and Technology Commonwealth Avenue, Diliman, Quezon City, 1101, Philippines Tel: (632) 8929-6010 to 19; Fax: (632) 8920-16-46 E-mail: [email protected]