Composites: Part B 51 (2013) 246–253

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Composites: Part B

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Poly(acrylamide-co-acrylate)/rice husk ash hydrogel composites.II. Temperature effect on rice husk ash obtention

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.03.027

⇑ Corresponding author at: Coordenação de Química, Universidade Estadual Valedo Acaraú, Av. da Universidade, 850, Campus da Betânia, 62040-370 Sobral-CE,Brazil. Tel./fax: +55 85 3611 6342.

E-mail address: almeida_quimica@yahoo.com.br (F.H.A. Rodrigues).

Jean de S. Cândido a, Antonio G.B. Pereira b, André R. Fajardo b, Nágila M.P.S. Ricardo c, Judith P.A. Feitosa c,Edvani C. Muniz b, Francisco H.A. Rodrigues a,b,⇑a Coordenação de Química, Universidade Estadual Vale do Acaraú, Av. da Universidade, 850, Campus da Betânia, 62040-370 Sobral-CE, Brazilb Departamento de Química, Universidade Estadual de Maringá, Av. Colombo, 5790, 87020-900 Maringá-PR, Brazilc Departamento de Química Orgânica e Inorgânica, Campus do Pici, Universidade Federal do Ceará, 60455-760 Fortaleza-CE, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 October 2012Received in revised form 15 February 2013Accepted 10 March 2013Available online 22 March 2013

Keywords:A. Polymer–matrix composites (PMCs)A. Smart materialsD. Thermal analysis

Superabsorbent hydrogel composites based on poly(acrylamide-co-acrylate) and rice husk ash (RHA)were prepared by free-radical copolymerization in aqueous media, using N,N-methylenebisacrylamide(MBA), as crosslinker and potassium persulfate (K2S2O8), as initiator. The effect of calcination tempera-tures (400–900 �C) for obtaining RHA was evaluated. FTIR, WAXS, SEM–EDS and TGA were applied tocharacterize a series of hydrogels filled with RHA. The hydrogels composites were formed with constantamounts of RHA (10 wt.%) and crosslinking agent (0.1 mol-%) in relation to the relative to the total massof acrylamide (AAm) and potassium acrylate (KAc) monomers (50–50 mol-%). A blank sample ofpoly(acrylamide-co-acrylate) hydrogel without RHA was used as control. A superabsorbent hydrogelcomposite, with a maximum absorption Weq > 1000 gwater/gabsorbent, was obtained with the RHA calcinedat 900 �C. In addition, the hydrogel composite showed to be sensitive to the pH variation and to the pres-ence of salts in the swelling media. From the results, it is possible to infer that the poly(acrylamide-co-acrylate)/RHA hydrogel composites presented good characteristics to be applied as soil conditioner forusing in agriculture.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Searching for technologies considered ecologically correct hasbecome of major concern in recent years. This new paradigm of eco-nomic development is directed to provide an improvement in wel-fare of further generations, incorporating in their conception theneeding of new production methodologies able to reduce the pollu-tion and the environmental impact. The large amounts of materialsthat present low degradation rate at ambient conditions havecaused severe environmental, economic, social and juridical issues.

The rice husk ash (RHA) is an agro-industrial residue resultingfrom the thermochemical conversion of rice husk, and as a waste,the majority of RHA generated is discarded, causing pollution. Thephysical and chemical properties of RHA are determined accordingto the methodology applied to its production (pyrolysis, gasifica-tion and/or combustion, for instance) and by some variables, suchas, type of equipment employed, burning temperature and time.Regardless the process of burning, the resultant ash has silica con-

tent around 74–97% [1–3]. On the other hand, temperatureachieved during calcination is the determining factor for theappearance of silica in amorphous or crystalline states [4,5].

Several researchers have utilized RHA to prepare zeolites andmesoporous materials [6,7], as an adsorbent for metal ions suchas Cd2+, Zn2+ and Ni2+ [8,9], and heavy metal such as lead and mer-cury from aqueous solution [10], as substitute for cement [11], asalternative source for active silica production [12,13], as fillersfor natural and synthetic rubbers [14–16] and for acrylamide–acry-late copolymer hydrogels [17]. Some organic matters, such as Con-go red and vacuum pump oil [18], palmytic acid [19], IndigoCarmine dye [20], Brilliant Green dye [21], Methylene Blue dye[22], can also be absorbed by RHA.

Superabsorbent hydrogels are insoluble in water and can absorband retain large amounts of aqueous fluids even under pressure.Therefore, superabsorbents have great advantages over traditionalwater-absorbing materials. Due to their excellent properties,superabsorbent hydrogels have raised considerable interest andthey have been widely used in several fields, such as, hygienicproducts, horticulture, gelactuators, drug delivery systems, as wellas water blocking tapes and coal dewatering [23–29].

Based on above description, this paper is a second of a series ofpapers in which superabsorbent hydrogels composites based on

JdS. Cândido et al. / Composites: Part B 51 (2013) 246–253 247

poly(acrylamide-co-acrylate) filled with RHA were formed. Thesuperabsorbent hydrogels composites were characterized by Fou-rier transform infrared spectroscopy (FTIR), X-ray diffraction(XRD), Scanning electron microscopy coupled to energy dispersivespectroscopy X-ray (SEM–EDS) and Thermogravimetry (TGA). Inthis paper the focus is on the temperature effect of RHA obtentionas well as on the swelling–drying capabilities of hydrogels.

2. Experimental

2.1. Materials

Acrylamide (AAm), acrylic acid (AAc), N,N,N0,N0-tetramethyl-ethylenediamine (TEMED), as catalyst, and potassium persulfate(K2S2O8), as initiator, were purchased from Sigma Aldrich (USA).N,N0-methylenobisacrylamide (MBA), as crosslinker, was obtainedfrom Pharmacia Biotech (USA). The acrylate salt (KAc) was ob-tained by the neutralization of acrylic acid with potassiumhydroxide.

The used rice husk originates from Mucambo/CE, Brazil. Asheswere produced through calcination in a muffle furnace at temper-atures ranging from 400 to 900 �C. The ashes, labeled as RHAT

(where T is the temperature applied during calcination), were pre-viously ground, and sieved through a 325 mesh (644 lm) sieveprior using in hydrogel formation. All reactants were of analyticalgrade and were used without further purification.

2.2. Poly(acrylamide-co-acrylate) hydrogel formation

2.1 g of AAm and 3.25 g of KAc were added to 30 mL of distilledwater, bubbled with nitrogen gas (to reduce the inhibiting effect ofthe oxygen in the radical polymerization). After 10 min, 16.2 mg ofK2S2O8 were added. Thus, MBA (0.05, 0.1 and 0.2 mol-% related tothe amount of monomers) and 100 L of TEMED solution 0.57 g L�1

were also added. The system was maintained under stirring andflowing nitrogen until the hydrogel formation (up to 30 min), afterit was left to rest for further 15 h at room temperature. The as-ob-tained material was cut in small pieces and washed in distilledwater to remove non-reacted monomers. The hydrogels wereoven-dried at 70 �C. The size distribution of particles spreads from9 to 24 mesh (2–0.71 mm). The poly(acrylamide-co-acrylate)hydrogel was labeled as PAMACRYL.

2.3. Poly(acrylamide-co-acrylate)/RHA hydrogel composite formation

The poly(acrylamide-co-acrylate)/RHA hydrogel compositeswere synthesized as described to the same methodology appliedto form the PAMACRYL hydrogel. However, the monomers weredissolved in a dispersion of RHA obtained at different calcinationstemperature (10 wt.% related to the total amount of monomers).The hydrogel composites were labeled as RHAG400, RHAG500,RHAG600, RHAG700, RHAG800 and RHAG900 (where the number sub-script is referent to the temperature of rice husk calcination).

2.4. Characterization

2.4.1. Fourier transformed infrared spectroscopy (FTIR)The FTIR spectra were obtained using Shimadzu FTIR-8300

equipment. The dried material was blended with KBr powderand pressed into tablets before spectrum acquisition.

2.4.2. Wide angle X-ray scattering (WAXS)The WAXS profiles of RHA and hydrogels were obtained through

a powder diffractometer Shimadzu model XRD 6000; with Cu Karadiation source at 30 kV and 20 mA.

2.4.3. Scanning electron microscopy coupled to energy dispersivespectroscopy X-ray (SEM–EDS)

The morphology of the hydrogels (with and without RHA) wasanalyzed through scanning electron microscopy coupled to EnergyDispersive X-ray Spectroscopy (SEM–EDS/Shimadzu, model SS550) operating at 10 keV. The hydrogels were immersed in distilledwater at room temperature until the equilibrium swelling has beenreached (approximately 24 h). Next, the samples were removedand immediately frozen by immersion in liquid nitrogen. Thereaf-ter, the frozen hydrogels were fractured and freeze-dried (Christ,Alpha 1–2 LD Plus) at �55 �C for 24 h. Then, the hydrogels weregold-coated by sputtering before observation by SEM–EDS.

2.4.4. Thermogravimetry (TGA)TGA was performed in a Simultaneous Thermal Analysis Sys-

tem, Netzsch (Model STA 409 PG/4/G Luxx) with a scanning rateof 10 �C min�1 under flowing N2(g) at 20 mL min�1 in a temperaturerange from 22 to 1000 �C.

2.5. Study of physical properties

2.5.1. Swelling experimentsInitially, the swelling tests were employed to determine water

uptake capacity of the materials. In this way, 15 mg of dried gelswere placed in 30 mL filter crucibles (porosity no. 0) pre-moist-ened and with a dried outer wall. This set was inserted in waterin such a way that the gel was completely submerged.

The crucible/hydrogel composite sample sets were removed atvarious time intervals, with the external wall of the set dried andthe system weighed. For each sample, 3 assays were performed(n = 3). The swelling capacity of the hydrogel composites wasdetermined by Eq. (1), where Weq is the gained water mass (ingrams) per gram of composite hydrogel (absorbent), m is the massof the swollen absorbent and m0 is the mass of the dry material[17,30]. The kinetics of swelling in each studied medium was eval-uated. The size distribution of the hydrogel composites remainedin the 9–24 mesh range.

Weq ¼ ½m=m0� � 1 ð1Þ

2.5.2. The effect of the ionic strength and type of metal ions on the saltThe hydrogels were immersed in distinct salt aqueous solutions

at different concentrations (0.001, 0.01, 0.05 and 0.10 M) and theswelling capabilities of gels were determined through the previ-ously described procedures [17,30]. NaCl and NaHCO3 solutionswere used for studying the anion effect while NaCl and CaCl2 solu-tions were used for evaluating the cation effect on swellingproperties.

2.5.3. Evaluation of pH effect on swelling capability of gelsThe effect of the pH on the swelling was also verified in buffer

solutions (pHs 2–12). The procedures were the same as describedabove. The ionic strength of the buffer solutions was kept constant(I = 0.1 M).

3. Results and discussion

3.1. FTIR and WAXS techniques

The FTIR spectra of RHA obtained at different temperatures areshown in Fig. 1a. The bands assigned to the main vibrational modesof SiAOASi bonds at 1100 cm�1 and 800 cm�1 and 471 cm�1 ap-pear in the spectra independent on temperature used for calcina-tion. These bands are attributed to the asymmetric andsymmetric stretching and angular deformation, respectively [31–

248 JdS. Cândido et al. / Composites: Part B 51 (2013) 246–253

33]. The broad band at 3548 cm�1 is assigned to the vicinal silanolwith hydrogen-bonded water. The intensity of band at 621 cm�1

can be observed on spectra obtained for RHA calcined atT P 700 �C and intensifies as the temperature used for calcinationsis increased. This band is characteristic of crystalline cristobalita[34,35]. The FTIR spectra of hydrogels formed using the ash ob-tained at different temperatures are shown in Fig. 1b. These spectrashow the appearance of the bands at 1100, 800 and 471 cm�1, as-signed to SiO2 and SiAOASi bonds [31–33]. Furthermore, a weakband at 621 cm�1 can be observed on FTIR spectra of hydrogel ob-tained using ashes burnt at T P 700 �C. Thus, the presence of crys-talline cristobalita [6,34] in hydrogel can be assigned. In addition,the bands at 1670 cm�1 and at 1564 cm�1, related to the C@Ostretching and to the NAO stretching, respectively, due to theformation of copolymers. These results indicate that copolymersand RHA component coexisted in superabsorbent hydrogelscomposites.

The WAXS patterns of RHA calcined at different temperaturesfor 2 h and cooled to room temperature (within the muffle for24 h) are shown in Fig. 1c. Such profiles provide information rela-tive to the main mineral constituint and structural parameters ofRHA. It can be noticed from WAXS profiles that RHA calcined at dif-ferent temperatures presented different structures corroboratingthe FTIR data. The ashes obtained at T < 700 �C showed no defineddiffraction peaks, but rather an amorphous halo that appears at ca.2h = 22�, which indicates the absence of crystalline domains, being

Fig. 1. (a) FTIR spectra of the RHA calcined in different temperatures; (b) FTIR spectratemperatures and (d) WAXS patterns of the hydrogels composite.

predominant the non-crystalline form of silica [13,35,36]. On theother hand, RHA obtained at 900 �C showed peaks of cristobalite,indicating that the material is predominantly crystalline[6,34,37]. Therefore, is possible to infer that when the rice huskis calcined at high temperatures the resulting RHA presents crys-talline structure.

The characteristics of amorphous and crystalline silica fromRHA calcined in different temperature are maintained in thehydrogel composites (Fig. 1d) but the diffraction peak assigned tothe crystalline portion is enlarged due to the presence of copoly-mer. The WAXS patterns and FTIR data evidence the formation ofthe hydrogel composite and suggest the presence of RHA into thepolymer matrix, as sketched in Fig. 2.

3.2. SEM coupled to energy dispersive X-ray scattering (SEM–EDS)

Fig. 3 shows the SEM micrographs of freeze-dried hydrogel filledwith rice husk ashes (RHA400, RHA600 and RHA900) after beingswelled at equilibrium. All samples exhibit porous structure charac-teristic of hydrogels, however changes in structure of material dueto different types of ash (crystalline or not) can be evidenced. For in-stance, the micrograph of RHAG400 hydrogel shows homogeneousporous distribution, but average size smaller than those of RHAG900,

while RHAG600 is in an intermediate condition. This phenomenoncould be responsible for the difference in the superabsorbent prop-erties among RHAG900, RHAG600 and RHAG400 composites. EDS tech-

of the hydrogels composites; (c) WAXS patterns of the RHA calcined in different

Fig. 2. Proposed model for the structure of hydrogel composite network (particle size 644 lm).

JdS. Cândido et al. / Composites: Part B 51 (2013) 246–253 249

nique was utilized to evaluate the filler dispersion into polymermatrixand the possible existence of silicate particles diffusion out-ward the hydrogel. Signal relative of silicon (from ashes) were evi-denced on images generated by EDS mapping technique,indicating the homogeneous dispersion of Si in whole matrix.

3.3. Thermal stability

TGA and DTG curves of PAMACRYL and hydrogels composites(RHAG400, RHAG600 and RHAG900) filled with 10 wt.% of RHA cal-cined at different temperatures are shown and compared inFig. 4. The decomposition curve of PAMACRYL and hydrogel com-posites could be divided into three steps. The first stage is in therange of 50–200 �C due to a loss of moisture present in the sam-ples. Following, the weight losses within the temperature of300–450 �C, which are attributed to the thermal decompositionof the carboxylate and amide side-groups of the copolymers, andalso MBA moieties in the network, leading to the evolution ofammonia and other gases [38,39]. During this period, the onsetsof the PAMACRYL and hydrogels composites filled with RHA400,RHA600 and RHA900 are similar and stability increases with increas-ing temperature in which the ashes were obtained, 391 �C (33.5%),386 �C (31.4%), 395 �C (33.4%) and 405 �C (33.3%), respectively. Thethird stage was attributed to the breakage of copolymer chains, inwhich it was observed a displacement for higher temperatures. Itcan be concluded from TGA data that the temperature in whichthe RHA was calcined has an influence on thermal stability of cor-responding superabsorbent composites. The hydrogel RHA900

could enhance the thermal stability to the highest degree amongthe hydrogel samples investigated. The properties of RHA in thesuperabsorbent composite polymeric network may be the mainreasons for the difference in TGA result of this system.

3.4. Kinetics of swelling

The swelling process of a hydrogel is controlled by chemical andphysical forces as well as by the consequent elastic response of the

constituent chains of the matrix. Swelling is primarily due to thepenetration of water into the hydrophilic polymer matrix by capil-larity and diffusion. On the other hand, the swelling rate of a super-absorbent is significantly influenced by swelling capacity, sizedistribution of powder particles, specific surface area, and apparentdensity of polymer.

Fig. 5a shows the kinetics of swelling in distilled water ofRHAG400, RHAG600 and RHAG900 composites and PAMACRYL. Theobserved trends in the swelling kinetics of four different samplesare very similar. The degree of swelling increased quickly duringthe first 20 min of immersion for all samples and ca. 90% of theswelling equilibrium value was reached in this time range. Then,a slower swelling process took place up to the equilibrium (Weq),around 30 min. The Weq values and the speed for reach the equilib-rium depend on the gel formulation (see the insertion on Fig. 5a).The presence of RHA greatly improved the water absorbency atequilibrium. Besides, RHAG900 composites took longest time toreach equilibrium. The PAMACRYL hydrogel presented waterabsorption capacity at equilibrium (Weq) of 645 gwater/gabsorbent

while RHAG400, RHAG600 and RHAG900 composites presented high-er water absorption capabilities of 781, 802 and 1077 gwater/gabsro-

bent, respectively.For evaluating the mechanism of the swelling process and the

temperature effect of RHA obtention on swelling kinetics of thesuperabsorbent hydrogels composites, the second order swellingkinetics model [40,41] was adopted to test the experimental data.The second order swelling kinetic model can be expressed asfollows:

t=W ¼ Aþ Bt ð2Þ

where

A ¼ 1ksw2

tð3Þ

and

B ¼ 1wt

ð4Þ

Fig. 3. SEM and EDS diffractograms of (a) RHAG400, (b) RHAG600, and (c) RHAG900 hydrogels composites.

250 JdS. Cândido et al. / Composites: Part B 51 (2013) 246–253

The A parameter corresponds to an initial swelling rate[(dW/dt)0] of the hydrogel, ks is the constant rate for swelling,Wt is a theoretical swelling value at equilibrium. Wt and ks

were calculated by fitting experimental data shown inFig. 5b to Eqs. (2)–(4) and the results are given in Table 1.For all the straight lines, the correlation coefficients (R2) werehigher than 0.999, indicating that the swelling processes of thesuperabsorbents obey the model utilized for predict theswelling.

The swelling rate constant (ks) for RHAG900 – was smaller thanthat of PAMACRYL. Although the introduction of RHA900 into PAM-

ACRYL network greatly improved the equilibrium water absor-bency, the necessary time to reach the equilibrium was higherleading to a slower swelling rate. The relationship between wateruptake capability and the temperature applied for obtaining theRHA is shown in Fig. 6.

According to the swelling assays it is verified the RHAG900 pre-sented highest water uptake because RHA900 has virtually no resi-due. RHA900 showed high crystallinity degree and the preferableintra-interactions among its silanol groups set free new sites onthe hydrogel matrix that could interact to water increasing theirwater uptake capacity.

Fig. 4. (a) TGA and (b) DTG curves of PAMACRYL and hydrogels composites (RHAG 400, RHAG600 and RHAG900) filled with 10 wt.% of RHA.

Fig. 5. (a) Degree of swelling and (b) plot of t/W versus t as a function of immersion time.

Table 1Effect of temperature of obtaining the RHA swelling kinetics of hydrogels compositesformed.

Hydrogel Weqa Wt

b teqc ks

d

PAMACRYL 645 ± 21 649 24 ± 3 6.54 � 10�4

RHAG400 781 ± 27 791 12 ± 2 1.15 � 10�3

RHAG500 773 ± 25 789 13 ± 4 1.91 � 10�3

RHAG600 802 ± 19 800 10 ± 1 1.42 � 10�3

RHAG700 939 ± 24 952 24 ± 4 2.49 � 10�4

RHAG800 1031 ± 26 1056 29 ± 5 1.19 � 10�4

RHAG900 1077 ± 32 1124 35 ± 4 7.40 � 10�5

a Experimental equilibrium swelling (gwater/gabsorbent).b Theoretical equilibrium swelling (gwater/gabsorbent).c Equilibrium time (min).d Swelling rate constant [(gabsorbent/gwater)/min].

Fig. 6. Effect of temperature for obtaining the RHA in the maximum swelling value(Weq) of hydrogels composites.

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3.4.1. Effect of salt solution on water absorbencyIt has been confirmed, through theoretical and experimental

considerations, that the presence of ions has great effect on thehydrogels swelling behavior [42–44]. In this work, the influenceof ions in the swelling capability of hydrogels was tested by theaddition of NaCl (0.001–0.1 M) or NaHCO3 (0.1 M) or CaCl2

(0.1 M) in the hydrogel-surrounding solution.

Table 2 presents the data collected in swelling measurements

for the hydrogels in NaCl solution. The sensitivity of hydrogel to

Table 2Weq (gwater/gabsorbent) as function of the ionic force for the hydrogels.

Hydrogel H2O NaCl (0.001 M) NaCl (0.01 M) NaCl (0.05 M) NaCl (0.1 M)

Weq Weq f Weq f Weq f Weq f

RHAG400 781 ± 27 445 ± 6 0.43 249 ± 3 0.68 118 ± 2 0.85 84 ± 1 0.89RHAG600 802 ± 19 459 ± 4 0.43 253 ± 2 0.68 123 ± 2 0.85 87 ± 1 0.89RHAG900 1077 ± 32 638 ± 7 0.41 345 ± 4 0.68 169 ± 3 0.84 114 ± 2 0.89PAMACRYL 645 ± 21 361 ± 4 0.44 195 ± 2 0.70 85 ± 1 0.87 63 ± 1 0.91

Fig. 7. pH effect on the degree of swelling.

252 JdS. Cândido et al. / Composites: Part B 51 (2013) 246–253

presence of salts can be related by the dimensionless factor f de-fined as [45]:

f ¼ 1� Wsaline

Wwater

� �ð5Þ

in which Wsaline and Wwater are, respectively, the swelling capacityin saline solution and in deionized water. According to Eq. (5) thef values range between 1 and zero. As close the value is of unit(1.0) higher is the sensitivity of hydrogel to the presence of salt.The opposite is valid: for f equal to zero the hydrogel would not pos-sess any sensitivity to presence of salt. The f values (Table 2) indi-cate that the hydrogel composites undego slightly less influenceto the presence of salt than PAMACRYL.

The increase in the ionic strength reduces the difference in theconcentration of movable ions between the polymer matrix andthe external solution (osmotic swelling pressure) and leads to animmediate contraction of hydrogel network.

The presence of divalent and trivalent cations in swelling solu-tion drastically reduces the swelling capacity of the hydrogels. Thisis due to the complexation ability of carboxymide or carboxylategroups and to the formation of inter and intramolecular complexes[46,47]. The hydrogel composites presented greater Weq values insaline solution (Table 3) as compared to PAMACRYL one.

Moreover, based on Weq values the hydrogel composites werenot sensible to the type (and size) of anion, because the values ofWeq were similar for Cl� and HCO�3 salt counter ions. An analogousobservation was recently reported for poly(acrylamide-co-acry-late) and cellulose nanowhiskers superabsorbent composites [48].

3.4.2. Equilibrium swelling in buffer solutions at various pHThe swelling behavior of PAMACRYL and composites (RHAG400,

RHAG600 and RHAG900) at several pH conditions was observed withthe use of buffer solutions at pHs 2–12 at constant ionic strength(Fig. 7). An increase in Weq value was observed as the pH of theexternal solution is increased [49]. In acidic medium, the carboxyl-ate anions are protonated and the anion–anion repulsive forcesvanishes; this leads to a minimum swelling of the hydrogel. Asswelled in buffer with higher pH values, the carboxylate groupsof hydrogels composites become ionized and the electrostaticrepulsion between ACOO� groups causes expansion of matrixand, consequently, increases Weq [50].

At pH > 4 the hydrogel swells much more due to the repulsionof ACOO� groups, while at pH 2 the hydrogel network fast col-lapses due to the shielding effect from excess of cations. The hydro-gel nanocomposites based on starch-g-poly(sodium acrylate)matrix filled with cellulose nanowhiskers studied by Spagnol

Table 3Weq (gwater/gabsorbent) as function of the type of anion and cation.

Hydrogel H2O NaCl NaHCO3 CaCl2

Weq Weq f Weq f Weq f

RHAG400 781 ± 27 85 ± 1 0.89 83 ± 2 0.89 26 ± 1 0.97RHAG600 802 ± 19 87 ± 2 0.89 79 ± 2 0.90 24 ± 1 0.97RHAG900 1077 ± 32 114 ± 3 0.89 117 ± 2 0.89 38 ± 2 0.96PAMACRYL 645 ± 21 63 ± 1 0.91 69 ± 1 0.89 17 ± 1 0.97

et al. [51] presented similar responsive behavior in relation topH. This behavior makes the hydrogels composites strong candi-dates to be utilized in controlled release systems. Hydrogels withresponses to variations of pH and temperature have been exten-sively studied within the referred class of ‘‘smart materials’’ whichhave been applied in wastewater and industrial effluents, con-trolled drug release, separation membranes among others [52–54].

4. Conclusions

Poly(acrylamide-co-acrylate)/RHA hidrogel composites present-ing relevant properties were successfully synthesized, as observedby FTIR and WAXS techniques. RHA proceeding from rice husk wascalcined in different temperatures from 400 to 900 �C. WAXS pat-terns showed that that RHA calcined at different temperatures pre-sented different structures confirming the FTIR data. The ashesobtained at T 6 700 �C showed no defined diffraction peaks, i.e.,presents silica with certain crystallinity. This study also evaluatedthe effect of crystalline or amorphous RHA on the water uptakecapability of the hydrogel. The presence of RHA900 on acrylam-ide–acrylate polymeric matrix improved the water-absorptionproperties of material, providing an increase of 67% in the Weq val-ues as compared to hydrogel without RHA particles. The RHA incrystalline form induces higher water uptake capacity (Weq) ofcomposites hydrogels due to the intra-interactions among silanolgroups on RHA make available new sites in the polymer matrix,which could interact to water. The hydrogels composite poly(acryl-amide-co-acrylate) filled with RHA proved to be adequate for theuse as soil conditioner. These preliminary results indicate thatthe hydrogels composites have great potential for their utilizationin the agronomic area.

Acknowledgements

The authors would like to thank the financial support by FUN-CAP (BPI 0280-106/08 and PIL – 139.01.00/09), by CNPq (Proc.507308/2010-7) and to COMCAP-UEM.

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