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Progress in Organic Coatings 75 (2012) 33– 37

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings

jou rn al h om epage: www.elsev ier .com/ locate /porgcoat

odium montmorillonite clay loaded novel organic–inorganic hybrid composites:ynthesis and characterization

ishore K. Jenaa, K.V.S.N. Rajua, Ramanuj Narayana,b, T.K. Routa,b,∗

Organic Coatings and Polymers Division, Indian Institute of Chemical Technology, Hyderabad-500607, IndiaResearch, Development and Technology, Tata-Corus Group, Ijmuiden-1760, The Netherlands

r t i c l e i n f o

rticle history:eceived 28 October 2011eceived in revised form 2 March 2012ccepted 9 March 2012vailable online 10 April 2012

a b s t r a c t

Polyimide based organic matrix loaded with 1, 3 and 5 wt.% of sodium montmorillonite (Na+MMT)clay particles, i.e. (polyimide–clay-1, polyimide–clay-3 and polyimide–clay-5), were developed by ther-mal imidization method. Differential scanning calorimetry, thermogravimetric analysis and dynamicmechanical thermal analysis were used to understand thermal properties. Universal testing machine wasused to study mechanical strength properties. The results from thermal analysis indicated that the glass

◦

eywords:oatinghermomechanical propertiesandwich structuresynamic mechanical thermal analysis

DMTA)

transition temperature of the hybrid composite was increased by about 10 C and its thermal stability wasobviously improved, in comparison with pure polyimide. The investigation on the mechanical propertiesshowed that the tensile strength could be obviously increased by adding 1 wt.% (by weight) (Na+MMT)clay particles to the matrix. The gel content, adhesive strength, Impact strength, water absorption resis-tance and abrasion resistance were increased with increasing the clay concentration in polyimide matrix.

eat treatment

. Introduction

Polyimides have been used in a wide variety of applicationsuch as coatings, microelectronic packaging, high temperaturedhesives, and composites owing to their good thermal stability,xcellent mechanical strength, dielectric insulation, and chemicalesistance [1,2]. Combination of organic polymers and inorganicaterials is an exciting subject that has been receiving increas-

ng research attention during recent decades. Organic–inorganicybrid composites show novel properties such as mechanical [3],hermal [4], optical [5,6], electrical [7], and magnetical [8]. Theylso show wide potential applications in various areas such as inoatings [9], catalysis [10] and biotechnology [11].

Recently, clay particles are continued to be interesting filleraterials into polymers in developing cost-effective high perfor-ance coatings. Particularly, extensive research has been devoted

o develop polymer-layered nanocomposites, wherein filler inor-anic particles in a polymer matrix could exhibit improved physicalnd mechanical properties of the base polymers. The easily tailored

ore size and compositional variability available with montmoril-

onite clay particles when embedded into polymer network willrovide the versatile range of applications such as in coatings,

∗ Corresponding author at: Research, Development and Technology, Tata-Corusroup, Ijmuiden-1760, The Netherlands. Tel.: +31 611 623435; fax: +31 251 448706.

E-mail address: tkrout5@gmail.com (T.K. Rout).

300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2012.03.005

© 2012 Elsevier B.V. All rights reserved.

drug delivery and membrane separations [12,13]. One of the mostwidely used fillers in such applications is montmorillonite (MMT).Montmorillonite is one of the clay minerals, whose structure is likesandwich type with one octahedral Al2O3 sheet between two tetra-hedral SiO2 sheets. To make an organophilic clay intercalated withswelling agent, it is necessary to use clay that swells into water. Inthis aspect, synthetic mica, montmorillonite, saponite, and hectriteare appropriate for hybrid synthesis.

In this article, we report synthesis and properties ofpolyimide–clay hybrid films with various concentration of clayto investigate the properties of the hybrids. It was found thatthe higher concentration of the clay was much more effective toimprove the properties of polyimide. In order to investigate thecoating properties, the hybrid formulations were applied on totin foil and then imidized by a thermal procedure. The thermal,mechanical and coating properties with different clay content werecharacterized.

2. Materials and experimental

2.1. Materials

Untreated sodium montmorillonite (Na+MMT) clay particles,

3,3′,4,4′-biphenyltetracarboxylic dianhydride (BTDA), benzene-1,3-diamine (BDA) and 3,3′-oxydianiline (ODA) were obtained fromAldrich and used as received. 1-Methyl-2-pyrrolidone (NMP) andcetyl trimethyl ammonium bromide (CTAB) were obtained from

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hybrid coatings during imidization. For a pure polyimide sample,there are two remarkable weight loss temperatures. The first onestarts at about 270 ◦C and corresponds to 5 wt.% weight loss. Thisis probably due to the degradation of the polymer. The second one

4 K.K. Jena et al. / Progress in O

luka Chemicals (Osaka, Japan). The solvents were freed from mois-ure using 4 A molecular sieves before use.

.2. Preparation of organo-montmorillonite

The organifier was dissolved in deionized water at 60 ◦C andoncentrated HCl was added drop by drop into the organifier solu-ion to quaternize the amine group. The swelling agent CTAB wassed to increase the compatibility and inter layered spacing of thelay. Appropriate amount of Na-mont was preliminarily dispersedn deionized water at 60 ◦C by using ultrasound probe attached to

alvern Mastersizer 2000 (UK) particle size analyzer (20 W/cm2).he organifier solution was poured in to the suspension contain-ng Na-mont and the mixture was vigorously sonicated for 2 h.his technique is commonly employed when both high speednd high shear are required to create colloidal dispersion of finearticles. In order to avoid the excess heat generated during son-

cation, the sonication was repeatedly carried out in an alternateonication and cooling cycle of around 30 s. After sonication, theation-exchanged clay particles were collected by centrifuge andubsequently washed repeatedly with deionized water. In ordero ensure the complete removal of chloride ions, the filtrate wasitrated with 0.1 N AgNO3 until no further AgCl precipitated. Theroduct was then placed in a vacuum oven at 80 ◦C for 12 h. Theried product was ground to get the organo-montmorillonite.

.3. Synthesis of polyimide–clay hybrid

A 150 mL one-neck flask equipped with a nitrogen inlet waseated for 10 min with a heat gun under nitrogen flow toemove water and oxygen from the flask. The flask was chargedith (0.5 mol) (benzene-1,3-diamine (98%) and a 3,3′-oxydianiline

0.5 mol) and 100 g of NMP. (1 mol) of BTDA was added to theolution and the reaction mixture stirred under N2 conditionor 16 h to form a polyamic acid intermediate (PAA). PAA wassed as a precursor for the subsequent composite synthesis usingontmorillonite-clay and directly casted on the tin foil and cured

n oven at 250 ◦C for 5 min to form a solid film. Different syntheticteps for the preparation of polyimide–clay hybrid composites arehown in Scheme 1.

.4. Organic–inorganic hybrid film preparation

A thin film of formulation was cast onto tin foil using a manualiving applicator to make 10 �m solid films for testing the appro-riate properties. The tin foil were washed with acetone and driedefore casting a formulation. The wet thin films were cured usingir-circulated oven at 250 ◦C for 5 min.

.5. Instrumental characterization

The thermal properties of the films were measured using ther-ogravimetric analysis (TGA) Q500 (TA Instruments, Inc.) with a

eating rate of 10 ◦C/min under a N2 atmosphere. The weight of theamples was ranged from 5–10 mg. The modulus and glass tran-ition temperature (Tg) of hybrid samples were measured usingMTA IV instrument (Rheometric Scientific, USA) in tensile modet a frequency of 1 Hz with a heating rate of 3 ◦C/min by scanning thelms from 25 to 250 ◦C. Storage modulus (E′) and tan ı as a functionf temperature at a constant frequency were observed. DSC analysisas recorded on a Mettler Toledo DSC 821e, Switzerland. Sam-les were heated from 25 to 200 ◦C at a heating rate of 20 ◦C/min

nder nitrogen atmosphere at a flow rate of 30 mL/min. The impacttrength was determined using Izod Impact tester (PSI, India) forn-notched specimens conforming to ASTM D 256 specification. It

s a measure of brittleness of coating. Un-notched sample is held

c Coatings 75 (2012) 33– 37

as a vertical cantilevered beam and is impacted by a pendulum.The pendulum that swings and breaks the samples, the distancecovered by the pendulum after breaking the sample is measured.The machine is calibrated, so that if there is no sample, the dis-tance the pendulum swings will read on the indicator. This is thereason why Izod Impact is expressed in Torque. The torque is thendivided by the sample thickness and is expressed as J/m (joules permeter). For the adhesion test the metal discs were pretreated withacid cleaning (chemical pickling), washed with distilled water andacetone before application of coatings with spin coater. The adhe-sion strength of polymers on the metal surface was determined byPull-off test (Microtech Tensiometer, UK). A known amount of thehybrid films was placed in a Soxhlet apparatus and extracted withtoluene for 6 h. The extracted film was then dried for 80 ◦C to con-stant weight. The final weight of the extracted dry film, expressedas percentage of the initial weight, was assumed as the gel con-tent (the degree of polymerization). Water resistance of the filmswas measured by calculating % swelling by weight. To do this, pre-weighed dry films were immersed in deionized water for 50 h tostudy the water resistance at room temperature. After removingthe samples from the immersion bath, these were blotted with softtissue paper and weighed to calculate swelling ratio using

Swelling ratio(%) =[

(Ws − Wd)Wd

]× 100 (1)

where Wd is the weight of dry sample and Ws is the weight ofthe swollen sample. The abrasion resistance test was carried outaccording to ASTM D 4060-01. This test method was used to deter-mine the resistance of coatings to abrasion produced by the Taberabrasor, i.e. wheel CS 10 on coatings applied to a metal panel. Theweight of coated panel before rub was recorded. Then film wasrubbed for about 250 and 500 cycles and again the weights wererecorded.

3. Results and discussion

3.1. TGA analysis

TGA curves of the polyimide, polyimide–clay (1%),polyimide–clay (3%) and polyimide–clay (5%) are shown inFig. 1. From Fig. 1, one can see that no appreciable weight lossoccurs below 200 ◦C for all the samples. This indicates that solvents,such as NMP, and moisture are almost perfectly removed from the

Fig. 1. TGA thermograms of polyimide–clay hybrid coatings.

K.K. Jena et al. / Progress in Organic Coatings 75 (2012) 33– 37 35

prepar

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Scheme 1. Different synthetic steps for the

tarts at about 350 ◦C which is associated with the degradationf the protected polymer. The introduction of clay to Polyimideauses a slight weight loss above 358 ◦C and the thermal stabilityncreases with increasing the clay content. This might be observedue to the thermal insulator and mass transport barrier to theolatile products generated during decomposition behavior of the

lay. As shown in Fig. 1 and Table 1, the maximum decompositionemperature of the samples increases with the increasing clayontent of the hybrids [14,15]. From the weight loss curves, it islso noted that the degradation of Polyimide is largely reduced

able 1hermal-analysis data for the polyimide–clay hybrid coatings.

Sample code Ton (◦C) Tend (◦C) % Wt.

Polyimide 271.6 420.9 83.16Polyimide–clay (1%) 273.2 425.7 85.24Polyimide–clay (3%) 286.1 449.6 86.89Polyimide–clay (5%) 302.6 469.8 88.34

ation of polyimide–clay hybrid composites.

with incorporation of clay layer. Furthermore, the char yield valuesof the hybrid materials increase with increasing clay content.

3.2. Dynamic mechanical thermal analysis (DMTA)

The glass transition temperature and storage modulus

data of polyimide, polyimide–clay-1%, polyimide–clay-3% andpolyimide–clay-5% hybrid coatings are reported in Table 2. Poly-imide has a Tg of118.4 ◦C, which was shifted to 123.2, 130.6and 169.9 ◦C after incorporation of clay particles into polyimide,

remaining at Ton % Wt. residue remaining at

450 ◦C 550 ◦C 600 ◦C

19.43 9.32 9.11 23.16 18.31 18.16 27.91 21.33 20.31 32.18 24.19 23.71

36 K.K. Jena et al. / Progress in Organic Coatings 75 (2012) 33– 37

Table 2Glass transition temperature and storage modulus data for the polyimide–clay hybrid coatings.

Sample code Tg (◦C) DSC Tg (◦C) tan ımax E′ at 50 ◦C [dyn/cm2] E′ at (Tg + 5 ◦C) [dyn/cm2]

Polyimide 106.8 118.4 0.69 4.66 × 108 0.52 × 108

Polyimide–clay-1% 117.1 123.2 0.51 8 8

Polyimide–clay-3% 121.8 130.6 0.49Polyimide–clays-5% 156.4 169.9 0.44

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are reported in Table 4. The clay modified hybrid films showedhigher gel contents, suggesting their excellent solvent resistance.The increase of clay content in the hybrid films would induce crosslinking due to self condensation as well as hydrogen bonding with

Fig. 2. DSC thermograms of polyimide–clay hybrid coatings.

uggesting a restricted segmental motion of polyimide chains dueo intercalation [14].

.3. Differential scanning calorimetry (DSC)

The glass transition temperature of polyimide, polyimide–clay-%, polyimide–clay-3% and polyimide–clays-5% hybrid coatingsrom DSC study is reported in Table 2 and shown in Fig. 2. Thelass transition temperature has shifted to higher temperatures,iz., 117.1, 121.8 and 156.4 ◦C, respectively, for 1, 3 and 5 wt.%a+MMT-loaded polyimide. However, increase in Tg of polyimide

s due to increasing amount of clay particles that are inorganic inature.

.4. Tensile strength

The tensile properties of polyimide–clay hybrid films with thelay contents of 1, 3 and 5 wt.% are reported in Table 3 and shown inig. 3. It was found that the incorporation of clay particles into poly-mide increases the tensile strength. Clay (5 wt.%) containing hybridamples have a maximum value of tensile strength of 69.8 N/mm2.his might be attributed due to good dispersion of clay particles

n polyimide matrix leading to an efficient stress transfer. How-ver, when the clay content exceeds 5 wt.%, the tensile strengthbruptly decreases (data not reported). Both the clay content and

able 3ensile properties and impact strength of different polyimide–clay hybrid coatings.

Sample code Max. stress(N/mm2)

Elongation (%) Impactstrength (J/m)

Polyimide 58.2 63.1 121Polyimide–clay-1% 61.4 58.5 124Polyimide–clay-3% 64.1 49.9 138Polyimide–clays-5% 69.8 42.3 159

4.97 × 10 0.72 × 10 1.71 × 109 1.99 × 108

4.79 × 109 5.91 × 108

the clay size affect the tensile strength and elongation because clayaggregations would lead to stress concentrations on the interfaces[16].

3.5. Impact strength

The Izod impact strength of the polyimide, polyimide–clay (1%),polyimide–clay (3%) and polyimide–clay (5%) hybrid coatings arereported in Table 3.Table shows that the impact strength of thehybrid coatings increases with increasing clay concentration. Thisbehavior might be observed due to presence of inorganic networksin the system. Theses network structures stop the crack propaga-tion and increase the Impact strength [16].

3.6. Adhesive strength

The adhesion results of the polyimide, polyimide–clay (1%),polyimide–clay (3%) and polyimide–clay (5%) coatings are reportedin Table 4. Adhesion mainly depends on chemical interaction andmechanical interlocking at the interface. The adhesive strength ofPolyimide and polyimide–clay (1%) on GI, Al and Cu were 254.8,191.2, 123.7 and 272.1, 244.7, 183.4 kg/cm2, respectively. The adhe-sion strength of polyimide–clay (1%) on GI, Cu and Aluminumis much higher than the Polyimide. This is due to the inorganiccross-linked structure. The adhesive strength of hybrid coatingsis increased with increasing the clay concentration. This behaviormight be attributed due to the presence of more linkages in theinterface [17,18].

3.7. Gel content

The gel content (wt.%) results of the polyimide, polyimide–clay(1%), polyimide–clay (3%) and polyimide–clay (5%) hybrid coatings

Fig. 3. Tensile strength and elongation at break graph of polyimide–clay hybridcoatings.

K.K. Jena et al. / Progress in Organic Coatings 75 (2012) 33– 37 37

Table 4Adhesive strength, gel content, water absorption and abrasion resistance data of different polyimide–clay hybrid coatings.

Sample code Adhesive strength (kg/cm2) Gel content (%) Water absorption (%) Abrasion resistance ((mass loss (mg))

GI Al Cu After 250 cycle After 500 cycle

Polyimide 254.8 191.2 123.7 86 0.775 3.4 5.4

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Polyimide–clay-1% 272.1 244.7 183.4 88

Polyimide–clay-3% 299.9 269.4 214.8 93Polyimide–clays-5% 318.1 279.4 232.5 96

olymer; this will induce higher gel content of the film to improvets solvent resistivity [16].

.8. Water absorption property

The amount of moisture absorbed indicated by the weighthange in all the composites studied are given in Table 4. Fromhe table it is clear that, the water absorption of polyimide isound to be higher when compared to those of polyimide–clay (1%),olyimide–clay (3%) and polyimide–clay (5%) hybrid composites.his might be due to the fact that the addition of silica results innterlocking of both the polymer and the nanoparticles becomesougher due to a restriction in chain mobility, resulting in lesseroid content in hybrid leading to lesser water absorption [19].

.9. Abrasion test

The Taber abrasion test (ASTM D1044-99) performed on galva-ized iron (GI) panels were obtained to measure the mass decreaseaused by mechanical degradation of the protective layers. Thesehow a mass loss after 250 and 500 cycles at 1000 g load andith CS10 grinders. The mass losses of polyimide and hybrids are

eported in Table 4. The weight loss decreases with increasing theercentage of clay which means increasing of inorganic materi-ls. This is indeed a distinct improvement in abrasion resistance,hich is due to the interaction of clay particles with polymeratrix [20].

. Conclusions

Effects of Na+ mont-clay on the thermal stability, mechanical

trength and coating resistance of hybrid composites were inves-igated and discussed in this paper. The properties of the hybridoatings were characterized by TGA, DMTA, DSC and UTM. As theoncentration of clay increases from 0% to 5%, the gel content,

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impact strength, tensile properties, adhesion strength and abrasionresistance of hybrid films increased. TGA and DMTA results showthat the thermal stability and mechanical strength of the hybridcoatings is greatly improved by the introduction of layered clayparticles.

Acknowledgment

Kishore K Jena would like to acknowledge Council of Scientificand Industrial Research (CSIR, New Delhi, India) for the researchfellowship as well as permission for the publication.

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