jcm.sagepub/Journal of Composite Materials
jcm.sagepub/content/early/2012/04/18/0021998312443398The online version of this article can be found at:
DOI: 10.1177/0021998312443398
published online 3 May 2012
Journal of Composite Materials Yi Liu, Hong Chen, Junping Zhang and Aiqin Wang
nanocomposites
Effect of number of grindings of attapulgite on enhanced swelling properties of the superabsorbent
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- May 3, 2012
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JOURNAL OF
C O M P O S I T E
M AT E R I A L S Article
Effect of number of grindings
of attapulgite on enhanced swelling
properties of the superabsorbent
nanocomposites
Yi Liu1,2,Hong Chen1,2,Junping Zhang1,3and Aiqin Wang1,3
Abstract
The incorporation of clay has become a preferred method for superabsorbent composite preparation because of its great influence on the swelling properties.The mechanical chemistry shows that grinding treatment could change the physical and chemical properties of clay.Therefore,in this article,the grinding treated attapulgite were used for prep-aration of sodium carboxymethyl cellulose-g-poly(acrylic acid)/attapulgite superabsorbent nanocomposites by the solu-tion polymerization using ammonium persulfate as an initiator and N,N0-methylenebisacrylamide as a crosslinker.The effects of number of grindings on the specific surface area and other characteristics of attapulgite,and the swelling ability, swelling kinetics,saline and pH sensitivity of the superabsorbent nanocomposites were systematically investigated.The results indicated that the increase in number of grindings could decrease the length of single crystals and dissociate part of crystal aggregates of attapulgite,which affected the dispersion of attapulgite in the polymeric matrix,thus leading to change in swelling behavio
rs of the superabsorbent nanocomposites.The swelling tests showed that the superabsorbent nanocomposites with attapulgite grinded for6times were the best in the reswelling abilities,initial swelling rate,saline and pH sensitivity.
Keywords
Superabsorbent nanocomposite,attapulgite,grinding treatment,sodium carboxymethyl cellulose,swelling behavior
Introduction
Superabsorbents are hydrophilic polymers with three-dimensional network,which are capable of absorbing lots of aqueous solution and have found extensive applications in variousfields such as agriculture,1,2 coal dewatering,3drug delivering,4,5environmental decontamination,6–8etc.Recently,organic–inorganic superabsorbent composites based on natural polysac-charides and inorganic clays have received considerable attentions in both academic research and industrial application due to their excellent hybrid properties superior to each individual component as well as sur-prising environmentally friendly characteristics.9–13 Among various polysaccharides,cellulose is the most abundant natural polymer with good biodegrad-ability and biocompatibility,but poor solubility i
n water and many organic solvents and poor reactivity of cellulose limit its wide application.Sodium carbox-ymethyl cellulose(CMC)is a representative cellulose derivative with carboxymethyl groups(ÀCH2COONa) bonded to some hydroxyl groups on cellulose back-bone.CMC is soluble,chemically reactive and strongly hydrophilic,rendering its application in superabsorbent fields attractive and promising.
Attapulgite(APT)is a kind of hydrated octahedral layered magnesium aluminum silicate with reactive
1Center of Xuyi Attapulgite Applied T echnology,Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences,PR China
2Graduate University of the Chinese Academy of Sciences,PR China
3Key Laboratory for Attapulgite Science and Applied T echnology of Jiangsu Province,Huaiyin Institute of T echnology,PR China
Corresponding author:
Aiqin Wang,Center of Xuyi Attapulgite Applied T echnology,Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences,Lanzhou 730000,PR China;Key Laboratory for Attapulgite Scie
nce and Applied T echnology of Jiangsu
Province,Huaiyin Institute of T echnology,Huaian 223003,PR China
Email:aqwang@licp.cas
Journal of Composite Materials
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DOI:10.1177/0021998312443398
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ÀOH groups on the surface and natural crystals.14 The incorporation of APT for superabsorbent nano-composites preparation can greatly enhance the swel-ling properties.14–17Various chemical treatment methods,such as surfactant,18acid activation19,20or heating treatment,21–23have been used to treat clays and improve mechanical or other properties and to expand corresponding applicationfields.However, little information regarding the introduction of APT with physical treatment methods into the superabsor-bent nanocomposites has been reported.
The physical treatment method of clays was proved to be an efficient method.Several literatures concerning grinding clays have been reported.24–27The mechanical chemistry shows that grinding treatment could change the physical and chemical properties of clays.
Here,much interest is directed toward APT with grinding treatment(gr-APT)and it is expected that the swelling properties of corresponding superabsor-bent nanocomposites could be improved.Thus in this article,sodium carboxymethyl cellulose-g-poly(acrylic acid)/attapulgite(SPA)superabsorbent nanocompo-sites were prepared using CMC,acrylic acid(AA)and gr-APT.The effects of the number of grindings on the specific surface area and other characteristics of APT, and the swelling ability,swelling kinetics,saline and pH sensitivity of the SPA superabsorbent nanocomposites were systematically investigated.
Experimental
Materials and equipment
Sodium carboxymethyl cellulose(CMC,chemical pure, 300–800mPa s(25g/L,25 C))was obtained from Sinopharm Chemical Reagent Co.,Ltd.,Shanghai, China.Acrylic acid(AA)was obtained from Shanghai Chemical Reagent Corp.(Shanghai,China),distilled under reduced pressure before use.Ammonium persul-fate(APS)was purchased from Xi’an Chemical Reagent Factory(Xi’an,China).N,N’-methylenebisacrylamide (MBA)was supplied by Shanghai Chemical Reagent Corp(Shanghai,China).Attapulgite(APT,supplied by Mingguang Colloidal Co.Ltd.,Anhui,China)was milled through a200-mesh screen,and dried to constant weight at105 C before use.Other reagents used were all analytical grade and distilled water was used for prepar-ing solution.
Preparation of gr-APT
The APT powder was grinded for0to10times under buhrstone mill with the diameter of22cm and screened through a200-mesh sieve.The samples were denoted as gr-APT0to gr-APT10according to the number of grindings.
reaction to a book or an article
Preparation of SPA superabsorbent nanocomposites CMC(1.030g)was dissolved in30mL of distilled water in a four-neckflask equipped with a stirrer,a conden-ser,a thermometer and a nitrogen line.Under nitrogen atmosphere,CMC solution was stirred on an oil bath at 60 C for30min,and then5mL of APS solution con-taining72.0mg of APS was dropped into the reaction flask for10min to generate radicals.MBA(0.0216g) and gr-APT(0.94g)powder were dispersed into the neutralized AA(7.20g)solution at0 C with7.6mL of sodium hydroxide solution(8mol/L).The resulting suspension was added dropwise to the reactionflask at 50 C.Then the reaction system was heated slowly to 70 C and kept for180min under nitrogen atmosphere. The resulting gel products were obtained from theflask with a hook,washed with distilled water for several times,expressed as SPA0to SPA10according to the gr-APT used and were dried in an oven at70 C to a constant weight.The superabsorbent nanocomposites were milled and passed through40–80mesh screens. Measurement of equilibrium water absorbency and swelling rate
About0.05g of the sample was immersed in excess dis-tilled water(300mL),at room temperature for240min to reach swelling equilibrium.The swollen samples were rid of unabsorbed water byfiltering through a 100-mesh screen for10min.The equilibrium water absorbency in distilled water of the superabsorbent nanocomposites,Q eq,was calculated using the follow-ing equation
Q eq¼ðm2Àm1Þ=m1ð1Þwhere m1and m2are the weights of the dry and swollen samples,respectively.Q eq is the equilibrium water absorption defined as grams of water per gram of sample.
The swelling rate of the samples in distilled water was measured according to the following process.
0.05g sample was poured into300mL aqueous solu-tion.At predetermined intervals,the water absorbency (Q t)of the sample was measured according to the simi-lar procedure as that for the measurement of equilib-rium water absorbency.The water absorbency in saline solutions and solutions with different external pH values was measured in the same way as that in dis-tilled water.The pH of aqueous solution was adjusted using1mol/L of hydrochloric acid or sodium
2Journal of Composite Materials0(0)
hydroxide solutions.The pH values were measured with a pH meter(DELTA–320).
Reswelling ability
The sample(0.05g)was immersed in certain amount of distilled water for240min to ensure the swelling equi-librium was achieved.The swollen hydrogel was placed in an oven at100 C until the gel was dried
thoroughly. Equal amount of water was added to the dried gel and placed in the oven again.Similar procedure was repeated and then equilibrium water absorbency of the sample for each time was obtained. Characterization
The pore volume(PV),pore size(PZ),and specific sur-face area(SA)of gr-APT were measured using an Accelerated Surface Area and Porosimetry System (Micromeritics,ASAP2020)at77K.The surface morphologies of gr-APT and corresponding superab-sorbent nanocomposites were investigated using SEM (JSM-5600LV,JEOL).Before SEM observation,all samples werefixed on the aluminum stubs and coated with goldfilm using an acceleration voltage of20kV. FTIR spectra of samples were recorded using potas-sium bromide pellets on a Thermo Nicolet NEXUS TM spectrophotometer in the range of4000–400cmÀ1.TEM images were obtained with a JEM-2010high-resolution transmission electron microscope (JEOL,Tokyo,Japan)at an acceleration voltage of 200kV and100,000Âmagnifications,and the sample was ultrasonically dispersed in ethanol before observation.
Results and discussion
FTIR analysis
FTIR spectra of gr-APT with different number of grind-ings(0,2,6,8)and the corresponding superabsorb
ent nanocomposites are shown in Figure1.The absorption bands were all agreed to the characteristic absorption peaks of APT.28Compared to the FTIR spectrum of gr-APT0,the absorption bands of other APT samples ascribed to Si–O–Si stretching vibration(1022.14and 980.19cmÀ1)and Si–O–Si bending vibration(511.84 and469.90cmÀ1)were obviously enhanced with the increase in number of grindings,which suggested that the grinding treatment can alter the microstructure of APT.For the FTIR spectra of the superabsorbent nano-composites,new bands at1710.99cmÀ1(C¼O stretching ofÀCOOH)and1604.86cmÀ1(asymmetric stretching ofÀCOOÀgroups)and at1454.42and1408.37cmÀ1(symmetric stretching ofÀCOOÀgroups)appeared in the spectra of the superabsorbent nanocomposites.In addition,the(Al)O–H stretching vibration at 3608.54cmÀ1,the(Si)O–H stretching vibration of APT at3545.63cmÀ1and theÀOH bending vibration at 1651.26cmÀ1disappeared in the spectrum of the super-absorbent nanocomposites.The absorption band of APT at1022.14and980.19cmÀ1(Si–O–Si stretching vibration)appeared in the spectra of the superabsorbent nanocomposites,and shifted to1026.81and 974.18cmÀ1.The information indicated that APT parti-cipated in the graft copolymerization reaction through its activated silanol groups.14However,FTIR spectra of the superabsorbent nanocomposites with different gr-APT did not exhibit obvious difference.
FESEM and TEM analyses
For investigating the change of surface morphology, the microstructures of gr-APT with number of grind-ings of0,2,6and8and the corresponding superabsor-bent nanocomposites were observed and shown in Figure2.It can be seen that grinding treatment has great influence on microstructure of APT.The length of single crystals of APT without grinding treatment is about1m m,and the single crystals aggregate together due to the existence of hydrogen bond and Van der Waals forces.29For gr-APT2,it can be seen the nests collapsed and were partly dissociated and some of the single crystals become shorter.For gr-APT6,the length of single crystals is even shorter than that of gr-APT2, and a large part of aggregates was dissociated.The length of most single crystals is about0.1m m (gr-APT8),which is one tenth of that of APT without grinding treatment,and part of the single crystals reunites to be particles.Owing to low dispersion and existence of aggregates,APT aggregates as large par-ticle and few single crystals in the superabsorbent nano-composites from the FESEM micrograph of SPA0can be clearly observed(Figure2(e)),while single crystals of SPA2dispersed more uniformly than SPA0and several aggregates can be seen in the three-dimensional net-work of the superabsorbent nanocomposites(Figure 2(f)).The dispersion of APT in the SPA6was more uniform,and many single crystals in the superabsor-bent nanocomposites can be seen(Figure2(g)).Few single crystals can be observed and large aggregates of granules can be observed(Figure2(h)),as the single crystals of gr-APT8were very short and most of them were in the form of granules.
The TEM images of SPA0,SPA2,SPA6and SPA8 are shown in Figure3.The dispersion of APT sam-ples in the polymeric matrix can be clearly observed from Figure3,which was similar to that mentioned above.
Liu et al.3
Effect of the number of grindings on water absorbency
It is known that the dispersion of clay in polymeric matrix is important for an organic–inorganic nanocom-posite.30The effects of the number of grindings of APT on water absorbency of the superabsorbent nanocom-posites and the SA of gr-APT with different numbers of grindings were investigated,as shown in Figure 4(a).As can be seen,with increase in the number of grindings of APT,the water absorbency of the corresponding super-absorbent nanocomposites increased,and reached to the maximum as gr-APT 6was introduced,and then
decreased with further increase in the number of grind-ings on APT.In addition,the SA of gr-APT increased until to gr-APT 2and then hardly changed with the increase in number of grindings except for gr-APT 6.The SA of gr-APT 6was the minimum compared to other APT samples with grinding treatment,indicating appropriate SA value of APT facilitated the water absorbency of superabsorbent n
anocomposites.The phenomena may be attributed to that the prime grind-ing treatment dissociated the aggregates and shortened the single crystals.This will lead to better dispersion of APT in the matrix of superabsorbent nanocomposites,as validated by the FESEM micrographs in Figure
2
Figure 1.FTIR spectra of gr-APT (a)and corresponding superabsorbent nanocomposites with gr-APT (b).
4Journal of Composite Materials 0(0)

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