research article effect of chitosan loading on the...

Research ArticleEffect of Chitosan Loading on the Morphological,Thermal, and Mechanical Properties of Diglycidyl Ether ofBisphenol A/Hexamethylenediamine Epoxy System

B. Satheesh,1 K. Y. Tshai,2 and N. A. Warrior1

1 University of Nottingham, University Park, Nottingham NG7 2RD, UK2University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia

Correspondence should be addressed to K. Y. Tshai; [email protected]

Received 19 June 2014; Revised 10 September 2014; Accepted 24 September 2014; Published 30 October 2014

Academic Editor: Suying Wei

Copyright © 2014 B. Satheesh et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The effect of chitosan filled diglycidyl ether of bisphenol A (DGEBA) epoxy systemwere investigated using the thermal, mechanical,and morphological properties. The mixing ratio of resin/hardener was kept constant while the chitosan of 1.0, 2.5, 5.0, 7.5, and 10weight percentage (wt%) was incorporated into the system.The thermal stability and the transition behaviour of the chitosan filledepoxy system were analysed through a differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fouriertransform infrared spectroscopy (FTIR) while atomic forcemicroscope (AFM) and scanning electronmicroscopy (SEM)were usedto investigate the morphology. It was observed that the additive tends to agglomerate, with the formation of clear phase separation,when the chitosan content increases above 5wt%. At lower chitosan loading (2.5 wt% and below), relatively uniform dispersionof the additive can be achieved. The thermal stability of the system increases with chitosan loading while the mechanical tensilestrength is compromised.

1. Introduction

Epoxy resins are molecules containing one or more 𝛼- or 1,2-epoxide groups, which can be crosslinked to form three-dimensional (3D) thermoset network structure. The degreeof crosslinking and hence the material properties can becontrolled by varying the amount of the resin hardenersystem, controlled curing over a wide range of temperaturesand introduction of additives or fillers. Epoxy has beenextensively used in a wide range of industrial applicationsowing to its excellent adhesiveness, resistance to heat andcommon chemical, mechanical strength, and electrical insu-lating properties. However, the material remains excessivelybrittle once cured and suffers from low impact strength,poor fracture toughness, and resistance to crack propagation.Dusek et al. [1] pointed out that alternating mechanismof network formation in DGEBA amine hardener systemdoes not offer the potential of partial segregation for theformation of inhomogeneous crosslinking. On the otherhand, more pronounced inhomogeneity may be promoted

through thermodynamic incompatibility or nonalternatingcuring mechanisms in more complicated thermosetting sys-tems. Over the past decade, various attempts to enhance thetoughness of epoxy system have been carried out by incorpo-rating fillers into epoxidesto produce a variant form of epoxyresin coupled with different chemical compositions. Reviewof the literature shows that majority of the previous workemployed thermoplastic, rubber, and elastomer compoundsas toughening agents in epoxy systems [2–13].

Zhang and Yu [5] observed that addition of semicrys-talline isotactic poly(phenyl glycidyl ether) (i-PPGE) mod-ifier resulted in change of particle size distribution, criticalintensity factor, and crystallinity. Ochi et al. [6] recordeda measureable level of improvement in toughness by intro-ducing a reactive elastomer carboxyl terminated liquidbutadiene-acrylonitrile rubber (CTBN). Nigam et al. [7] useda carboxyl terminated polybutadiene (CTPB) on novolacepoxy resin which demonstrated enhancement in the mate-rial toughness in tensility, flexure, and impact. Pereira Soareset al. [8] used hydroxyl-terminated polybutadiene (HTPB)

Hindawi Publishing CorporationJournal of CompositesVolume 2014, Article ID 250290, 8 pageshttp://dx.doi.org/10.1155/2014/250290

2 Journal of Composites

Table 1: Properties of the Hexamethylenediamine (HMDA) hardener.

Molecular Weight[g/mol]

Melting Point.[∘C]

Boiling Point.[∘C]

Flash Point[∘C]

Vapour Pressure[kPa]

Water Solubility[g/L] Additives Impurities

116.24 41 250 80 0.05 [0.4mmHg]at 25∘CCAL 800 at 15.6∘C None None

HOH2C

HOH2C

OO

O O O

HO

HONHCOCH3

NHCOCH3

Figure 1: Chitin molecular structure [14].

elastomer as toughening agent which showed that it ispossible to obtain a 100% increase in impact resistance withHTPB concentration up to 5 phr (parts per hundred rubber).Liu et al. [9] studied the effects of curing agents, that is, 4,4-diaminodiphenyl methane (DDM), dicyandiamide (DICY),and diethylphosphite (DEP), on the 4-(N-maleimidophenyl)glycidyl ether (MPGE) which revealed that the crosslinkedmaterial possesses high glass transition, good thermal sta-bility, and attractive flame retardant properties. Zhang [10]explained the advantages and disadvantages of using reactiveliquid rubbers, functionally terminated engineering thermo-plastics, reactive ductile diluents, and inorganic and hybridparticles as modifiers to improve fracture toughness. Cai etal. [11] conducted experiments on DGEBA epoxy resin withisocyanate terminated polyethers (ITPE) modifier showingthat the thermal stability decreases with the introductionof ITPE while the impact and flexural strength of thecured modified epoxy increase with ITPE loading. Brookeret al. [12] incorporated thermoplastic toughener poly(ethersulfone) copolymer into epoxy system reporting that theultimate tensile strength and fracture toughness increasedwith increasing thermoplastic content. Pearson and Yee [13]modified DGEBA with thermoplastic poly(phenylene oxide)(PPO) finding that the fracture toughness improved withPPO content in a linear fashion.

One of the most abundant and easily accessible nat-urally occurring polysaccharides commonly found in theexoskeleton of shrimps and crabs, chitin (2-Acetamido-2-deoxy–D-glucose, Figure 1), is a linear highmolecular weightcrystalline polysaccharide and, like cellulose, is made of 𝛽-(1→ 4) linked D-glucose. Chitosan (a biomedical cationicamino polysaccharide filler) is the N-deacetylated derivativeof chitin [14] which, instead of the acetyl group in chitin, hasthe –NH

2chain (Figure 2). The presence of high nitrogen

content (6.89%) in chitosan makes it feasible for use withepoxies while its antimicrobial properties provide addedadvantages. In the present study, chitosan is chosen as amodifier to DGEBA epoxy resin with hexamethylenediamine(HMDA) as a hardener to investigate the effects of chitosanloadings on the morphological, thermal, and mechanicalproperties of the resulting cured system.

HOH2C

HOH2C

OO

O O O

HO

HONH2

NH2

Figure 2: Deacetylated chitosan molecular structure [14].

2. Materials

2.1. Epoxy Resin. A room temperature curable DGEBA resinwith low viscosity, EPOFINE 564 (viscosity ranging between400 and 600MPa⋅s at 25∘C), suitable for the manufacture ofcomposite tools and lamination was supplied by Fine FinishOrganics Pvt. Ltd., India.

2.2. Coreactants/Hardener. Hexamethylenediamine (HMDA)was obtained from Planatec Lab Solutions, Hyderabad, India.The material was produced by hydrogenation of adiponitrileand the properties of HMDA are given in Table 1.

2.3. Chitosan. The chitosan used in this work was suppliedby India Sea Foods, Kochi, India. The process of extractingchitosan was carried out industrially by the supplier. Typicalproduction steps involved are demineralisation, deproteini-sation, decolouration, and deacetylation. The deacetylationpercentage depends on chitosan extraction method andrequires careful chemical treatment.The chitosan used in thisresearch was extracted from shrimps and the average chitincontent in shrimp shells was 17%. The shells extracted fromshrimps were first washed and then ground to powder witha size particle of 0.3–0.5mm. The powdered form of shrimpshells contains high percentages of chitin, calcium carbonate(CaCO

3), and proteins. The first stage of chemical process

starts with demineralisation where CaCO3is removed using

7% hydrochloric acid (HCl), in the ratio of 1 g of powderedshell to 10mL of HCl, and the mixture is left for 24 hrs inroom temperature. Upon demineralisation, deproteinisationtakes place where the demineralised powder is subjected to10% sodium hydroxide (NaOH) treatment, in the ratio of 1 gof demineralised powder to 10mL of NaOH, and the mixtureis treated at 60∘C for 24 hrs to yield chitin. Deacetylation ofchitin is carried out to remove the acetyl group, where theextracted chitin is treated with 50% NaOH at 60∘C for 8 hrs.Thedeacetylation achieved around 73%of chitosan in powderform.The chitosan powder is sun-dried and the final productcan now be used as filler in the epoxy-hardener system.

Journal of Composites 3

Table 2: Weight percentage (wt%) of chitosan.

Sample Group A B C D E FChitosan [wt%] 0.0 1.0 2.5 5.0 7.5 10.0

3. Methodology

The preparation of mixture containing epoxy-hardener sys-tem with chitosan was performed at ambient temperature.Curing is an exothermic process and the cure time isdependent on three parameters: (a) the weight of the epoxyresin used, (b) the weight of the hardener used, and (c) thecure temperatures and the surrounding environment.

Epoxy and hardener weights are kept constant at 31 g and9 g, respectively, whereas the wt% of chitosan was varied inthe fabrication, as shown in Table 2.

The specified quantity of epoxy resin is first mixed withchitosan powder using a magnetic stirrer maintained at300 rpm for 15mins, after which the predetermined amountof HMDA is added to the mixture and further stirred foranother 10mins. The resulting mixture was subsequentlyused for DSC analysis and to produce cured specimens formechanical test, TGA, SEM, FTIR, and AFM in accordancewith the procedures described below.

For DSC analysis, the resulting mixture in the range of7–10mg was held within a dedicated sample pan. The samplewas scanned between room temperature and up to 350∘C, at aheating rate of 10∘C/min, and the heat flux versus temperaturedata was recorded.

For the fabrication of cured specimens, the resultingmixture was poured onto glass mould of dimensions 15 cm ×15 cm × 3 cm where the mould surface was precoated withsilicon oil. Curing was allowed to take place within the glassmould to yield cured specimens of varying chitosan loadings.

TGA was carried out on a 10.0 ± 5.0mg specimento investigate the thermal degradation behaviour of thesamples and to observe the maximummass loss. The heatingtemperature was set within the range 25–850∘C, ramped at arate of 20∘C/min and under nitrogen flow rate at 50mL/min.

Specimens for SEM micrographs were prepared by cryo-genically fracturing the composite in liquid nitrogen. Thesamples were then sputter-coated with gold and micrographimages were taken under low vacuum mode, 10 kV voltagesand 220x magnification power within a Hitachi S-2400 SEMdevice.

For the AFM imaging, a flat fragment of the specimenwas securely held onto the centre of the steel sample puckusing a 2-sided adhesive tape and subsequently mounted onthe scanner tube. Contact mode AFM topographical scansusing silicon nitride cantilevers probe of 500 nm in radiuswere conducted on the composite specimen.

FTIR analysis was performed using a Thermo NicoletAvatar370 spectrometer to identify the resulting chemicalbonds of chitosan filled epoxy system. The cured specimenwas ground into powder using mortar and pestle. The finelygrinded specimen powder was subsequently mixed withpotassium bromide (KBr) powder in the ratio of 1 : 50 and

0

2

4

0 50 100 150 200 250 300 350

Hea

t flow

(m/W

)

Heat flow for neat epoxy

1st exothermic peak

2nd exothermic peak

−2

−4

−6

−8

−10

Temperature (∘C)

Heat flow for 1wt% chitosanHeat flow for 2.5wt% chitosan

Heat flow for 5wt% chitosanHeat flow for 7.5wt% chitosanHeat flow for 10wt% chitosan

Figure 3: DSC results of epoxy/hardener sample with varyingchitosan loadings.

pressed to a thin film pellet using a dye and bolt press assem-bly. Each active group of the component in the pellet hasits own characteristic absorbance or transmittance peak to aspecific infrared (IR) spectrum and the concentration of eachcomponent can be calculated from the area under its peak.KBr pellets with varying chitosan loadings were subjected toFTIR scan at IR spectrum from 800 to 4000 cm−1.

The tensile properties were determined using a ShimadzuAG-1 universal testing machine equipped with a 50 kN loadcell. Dumbbell specimens corresponding to BSI BS 903-A2were prepared using a standard cutting die. The measure-ments were carried out at a crosshead speed of 50mm/minaccording to ASTM D638 standard.

4. Results and Discussion

4.1. DSC Analysis. A plot of the DSC heat flow versustemperature results is shown in Figure 3. It can be observedthat the heat enthalpy curves with chitosan loading lowerthan 2.5 wt% revealed distinctive single exothermic peakat temperature around 60∘C and the peak area increaseswith chitosan loading. For composites with chitosan loadinggreater than 2.5 wt%, the exothermic peak area around 60∘Cis much reduced and an additional second exothermic peakappears at around 125∘C. This reaction is predominantlyinduced by the presence of excessive primary amines groupsalong the chitosan macromolecules that react with the epoxygroups in excess untilmaximumconversion.The secondpeakat higher chitosan loading is identified as the chitosan phaseinduced chemical cure of the sample.

As chitosan is not soluble in epoxy system, the incor-poration of chitosan filler tends to induce phase separationin epoxy. However, at low concentration the agglomerationof chitosan is relatively less noticeable. It is only at higherloadings that the agglomeration phenomenon is exposedwith the presence of a second peak. Similar observationswere reported by numerous researchers [4, 15, 16], where


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1 2 3 4 5 6 7 8 9 10Chitosan content in epoxy (%)

Neat epoxy

Glass temperature profile

Gla

ss tr

ansit

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tem

pera

ture

(∘C)

Figure 4: Variation of glass transition temperature with varyingchitosan content. SEM images demonstrate that the extent ofagglomeration tends to increase with increased chitosan content.

chemical reaction involved a more dominant additive inepoxy systems causing the existence of a second phase.The utilisation of HMDA hardener and the introduction ofexcessive free amine groups through chitosan chains mayhave improved the adhesion between constituent compo-nents. However agglomeration begins to form in sampleswith higher concentrations of chitosan.

The existence of a second phase, that is, chitosan phase,is more evident in the plot of variation in glass transitiontemperature, 𝑇

𝑔, in Figure 4. It is demonstrated that the

changes in 𝑇𝑔for specimens with 1.0, 2.5, and 5.0 wt% of

chitosan are negligible. However, a steep increase in𝑇𝑔can be

seen in specimens with chitosan loading higher than 5.0 wt%,recording 119.2∘C and 124.4∘C for 7.5 wt% and 10wt% ofchitosan, respectively. The increase in 𝑇

𝑔confirmed that the

epoxy composite reaches to the maximum conversion withexcessive presence of chitosan.

SEM images taken at each of the respective chitosanloadings demonstrate that nonhomogeneous mixture wasobtained when low concentration of chitosan filler is mixedwith the epoxy system. At higher loadings, it is evident thatthe chitosan phase is more dominant with rougher surfacedue to increased agglomeration.

The recorded improvement on thermal stability may berelated to the collective effect of reduced intermolecular spaceand bulky filler as a result of the agglomeration at higherchitosan loading. In the work of Shokralla and Al-Muaikel[17], the authors conducted an investigation on the effectsof fillers in varying thermal properties of epoxy/phenolicblends. It was observed that free volume is much reducedat higher filler loadings, incurring additional restriction tomolecular movement and rotation, which in turn causesthe loss of homogenous mixture and agglomeration takesplace. This phenomenon of space restriction was foundcapable of improving thermal stability through enhancedintermolecular vibrations. Similar observation was recorded

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0 100 200 300 400 500 600 700 800 900

Sam

ple w

eigh

t (%

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Shift intemperature

point at higher

chitosan loadings

−20 Temperature (∘C)

Neat epoxy

Onset of degradation

point

2.5wt% chitosan5wt% chitosan

7.5wt% chitosan10wt% chitosan

Figure 5: TGA curve of specimen with varying wt% of chitosan inDGEBA/hardener system.

by Park et al. [18] where the authors demonstrated that largerfillers were capable of improving thermal stability by restrict-ing molecular movement and rotation. It is hence evidentthat the chitosan phase is more dominant in specimens withhigher filler content, causing the agglomeration of bulky par-ticles. This phenomenon highlighted a great space reductionwithin DGEBA/HMDA and thus improved thermal stability.From the SEM images, it can be observed that the spacewithin the chitosan filled epoxy system is clearly reducedthereby restricting molecular movement.

4.2. Thermogravimetric Analysis (TGA). It can be seen fromFigure 5 that the thermal stability of chitosan filled epoxycomposite (found from the onset of degradation point)increaseswith chitosan loading. An increase of approximately10∘C is observed in the specimen with 5.0 and 7.5 wt% ofchitosan. The increase in thermal stability may be associatedwith the increase in crosslinking of the epoxy with chitosan.As chitosan is a cycloaliphatic amine; the amine groupspresent in chitosan could participate in the crosslinkingreactions with epoxy, hence imparting a higher thermalstability. However, in the case of 10 wt% chitosan loading,the agglomeration of the bulky chitosan particles reducesits probability to participate in the crosslinking reaction.Thereby, a reduced efficiency in enhancing thermal stabilityof the composite is seen as the onset of degradation pointwhich is slightly below that of the 7.5 wt% chitosan.The resultsshow that an increase in the chitosan content does improvethermal stability but at excessive loadings, that is, at 10 wt%and above, the shift in temperature shows that agglomerationleads to chitosan phase dominating the reaction, causingpoor crosslinking and a drop in thermal stability. At higherloadings, weight of the molecules increases and this wasdue to the agglomeration as exhibited by the SEM images.Increase in molecular weight could also have played a role inimproving thermal stability.


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80

90

100

800 1300 1800 2300 2800 3300 3800

Tran

smitt

ance

(%)

Neat epoxy2.5 wt% CS5 wt% CS

7.5 wt% CS10 wt% CS

1536

1004

2360.441262

1723

15901460

2915

2847

Wave number (cm−1)

Figure 6: FTIR spectra showing material interaction at differentwt% of chitosan loading.

4.3. Fourier Transform Infrared Spectroscopy (FTIR). TheFTIR absorption spectra are displayed in Figure 6. Significantabsorbance was observed at 2915, 2847, 2360, 1723, 1590, 1536,1460, 1395, 1262, and 1004 cm−1. The peaks at 1004 cm−1 wereassigned to the vibrational modes associated with saccharideunits in chitosan while the peak at 1262 cm−1 was due to thechitosan hydroxyl (–OH) group, which clearly indicates thatthe interaction of –OH groups has taken place [19, 20]. Bandsat 1723, 1590, and 1536 cm−1 show the amino group reactionsfrom chitosan. Although chitosan contains a large portion ofsugar molecules due to its polysaccharide nature, there arealso amine groups present whichmake chitosan very reactive.The peaks at 1262–1004 cm−1 were a result of the C–O/C–Nstretching and the peak at 2360 cm−1 was assigned to nitrilegroups stretching in the chitosan molecules [19, 20]. It isapparent from the recorded spectra that the absorption peaksincrease with chitosan loading. The higher FTIR absorptionrate observed at higher chitosan loadings can be related tothe excessive presence of amines, which makes the sampleshighly reactive because of the stretching due to C–O/C–Nbonds occurs during chemical curing. Therefore, the nitrileand amine bonds could be related to the presence of secondpeak in the DSC curve and the agglomeration at higherchitosan loadings. Overall, the FTIR investigation showedevidences of intermolecular bonding between epoxy andchitosan particles. Amine reaction from chitosan and –OHgroup reactions were the most important reactions revealedfrom the current FTIR readings.

4.4. Atomic Force Microscopy (AFM). Figure 7 shows theAFM images of the compositewith varying chitosan loadings.

It can be seen that relatively uniform filler distribution wasachieved with lower wt% of chitosan, where the surfacetopography was found to be more homogenized. Smoothsurfaces were observed for neat epoxy up to 2.5 wt% ofchitosan loading, after which agglomeration is clearly visible.At 5 wt% of chitosan, the particles begin to agglomerate andrevealed clear phase separation. This may be attributed tothe hydrophobic-hydrophilic interaction between the epoxyand the chitosan filler at large concentrations. AFM resultsindicated that the insolubility and agglomeration of chitosanin epoxy system is increasingly obvious at higher loadings.Radhakumary et al. [21] and Rutnakornpituk et al. [22]attempted to address the reaction mechanism of chitosanmolecule through a discussion on the high hydrophilicitynature of chitosan. The authors’ work revealed that –NH

2

molecules present in chitosan are capable of acting as reac-tion site for epoxy, which at low filler loadings providesthe homogenisation effect, and such reaction was foundto be similar to epoxy/hardener reaction in conventionalcrosslinked system. However, at higher filler loadings thehydrophobic-hydrophilic interactions between the epoxy andfiller exceeded the interaction of the –NH

2functionality of

chitosan.

4.5. Young’s Modulus and Ultimate Tensile Strength (UTS).The Young’s modulus was computed as the steepest slope inthe stress-strain response of the composite and the result isdepicted in Figure 8. It can be seen that the measured elasticmodulus increases with chitosan loading but the magnitudedropped slightly in specimen loaded with 10wt% of chitosan.With increasing chitosan loadings, the reaction becomesmore amine rich and dominates the curing process, leading toamore rigid and brittle composite with highermodulus. Neatepoxy without chitosan on the other hand shows a reactionwhere the ring opening by the –OH groups took place withepoxy monomers. In the case with excessive chitosan, thatis, 10 wt%, the agglomerated bulky chitosan particle of sizeapprox. 40 𝜇m reduced the probability of its participationin crosslinking, indicating a potential threshold of chitosanloading for modulus enhancement.

Figure 9 shows the ultimate tensile stress response atvarying wt% of chitosan. The results demonstrated thatincreasing the chitosan content had an effect on reducing thefracture strength of the epoxy thermoset. Excessive chitosanresulted in an amine rich formulation and is capable ofenhancing crosslinking and therefore increases the UTS.However, in the scenariowhere agglomeration took place, thebulky chitosan molecules begin to dominate and the reactionprobability of excess epoxy with free amine in chitosandropped, leading to poor crosslinking, and subsequently theUTS dropped as a result. Notably adding chitosan alone toDGEBA without hardener induces material brittleness andfracture and this was found to occur prior to yielding. Onthe other hand, the introduction of amine hardener in thepresence of chitosan leads to the formation of 3D networks.This tends to induce more ductile behaviour; however theUTS remained low due to chitosan agglomeration in thesample.


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tic m

odul

us (M

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Figure 9: Ultimate tensile strength of epoxy specimen at varyingwt% of chitosan.


5. Conclusions

Chitosan obtained from the exoskeletons shells of shrimpsis used as a filler to epoxy/hardener thermosetting poly-mer. The results successfully demonstrated the variation inthermal, morphological, and mechanical properties of thethermoset composite system with varying chitosan loadingsranging from 0 to 10wt%.The polycationic nature of chitosanwith –NH

2and –OH groups was found capable of acting

as the reaction site in producing a relatively homogenousmixture and reveals an improvement in thermal stability.However, the UTS are being compromised due to agglom-eration at higher filler loadings and miscibility of chitosanmolecules decreases. DSC scan indicates the presence ofphase separation at higher chitosan loadings, manifesting asthe second exothermic peak in the thermogram. The firstexothermic peak denotes the homogenous phase whereasthe second exothermic peak revealed the chitosan phaseinduced chemical cure of the epoxy system. SEM confirmsthat, by adding small quantity of chitosan to epoxy system,homogeneity of the mixture is disturbed while at higherchitosan loadings, clear phase separation is evident due toincrease in agglomeration. AFM images further support theexistence of this chitosan phase separation and agglomerationat higher loadings. The increase in 𝑇

𝑔at chitosan loadings

greater than 5.0 wt% can be attributed to its hydrophilicnature and immiscibility with epoxy, whereby the bulkyagglomerated molecular size of chitosan greatly restrictedmobility during curing reaction and the enhanced inter-molecular vibrations capable of improving thermal stability.The FTIR spectroscopic investigation showed evidences ofintermolecular bonding between epoxy and chitosan parti-cles.The absorption peak was found to increase with chitosanloading.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

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