Indian Journal of Fibre & Texti le Research Vol. 3 1 . March 2006, pp. 202-2 14

Polymer/clay nanocomposite based coatings for enhanced gas barrier property

Mangala Joshi', Kushan Banerjee & Prasanth R Department of Texti le Technology, Indian Institute of Technology, Hauz Khas, New Delhi 1 10 0 1 6, India

and Vikas Thakare

Aerial Delivery Research & Development Establishment, Defence R & D Organisation, Agra Cantt, Agra 282 00 1 , India

This paper aims at exploring the revolutionary field of nanotechnology and some of i ts promising aspects in the innovative field of polymeric nanocomposites because they show substantially improvec physical properties as compared to neat polymers. The polymer layered s i l icate nanocomposites are an i mportant class of hybrid organic- inorganic materials with improved mechanical, thermal and thermomechanical properties. They also show superior UV and chemical resistance and are widely being investigated for i mproving the gas barrier and flame retardant properties. The common synthesis techniques to produce polymeric layered s i l icate nanocomposites and their feasibi l ity as coatings for textiles to i mprove the property mix have been discussed along with the i mproved properties of these materials. Polymer nanocomposite based coatings for enhanced gas barrier properties have also been reviewed. A feasibi l i ty study on polyurethane /c1ay nanocomposite based coating for inflatable has been done and it is found that the polyurethane /c1ay nanocomposite based coated fabric shows an encouraging result on i mproving the gas barrier property for i nflatables.

Keywords: Gas barrier property, Layered s i l icate nanoccomposite, Nanocoatings, Polymer nanocomposite, Polyurethane/clay nanocomposite

IPC Code: Int. CI.8 88283/00, C0 1 833/00

1 Introduction Polymer nanocomposites (PNC) are a new class of

composites for which at least one dimension of the dispersed particles i s in the nano range - 1 0- 1 00 nm. They combine the concepts of both composites and nanometer size materials . S ince nanometer s ize grains, fibers and plates have dramatically increased surface area compared to their conventional s ize materials, the properties of these nanosized materials are altered compared to conventional materials. They often have properties that are superior to conventional microscale composites because of the strong interaction between components and can be synthesized using simple and inexpensive techniques. As compared to neat resins, these composites have a number of significantly improved properties i nc luding tensile strength, modulus, heat distortion temperature, gas barrier properties, flame retardant property etc. This aspect of nanotechnology, showing dramatically improved material properties, has the potential of finding applications as engineering plastics, polymer products, rubbers, adhesives and coatings. 1 .2

a To whom all the correspondence should be addressed. E-mai l : mangala@textile. i i td.ernet . in

The hybrid organic- i norganic nanocomposites have shown a great interest since they frequently exhibit unexpected hybrid properties synergistically derived from the two components. One of the most promising candidates is based on the organic polymers and inorganic clay minerals, consisting of si l icate layers.3A Polymer layered si l icate nanocomposites show superior mechanical properties (e.g. 40% increase in tensile strength at room temperature), heat resistance (e.g. 1 00% i ncrease in heat d istortion temperature), i mproved abrasion resistance, chemical resistance, and gas barrier ( 1 00-fold decrease in O2 & H20 permeabil ity) compared to neat polymers, just by the addition of 0. 1 - 1 0 vol. % of nanoclays. Coatings and fi lms based on these organic­inorganic hybrid materials are endowed with exceptional mechanical and thermal stability, allowing small thickness and making them very interesting for various applications. The organic component in the matrix offers the advantages of mechanical toughness and flexibil ity while the inorganic component provides hardness, thermal stabil i ty and gas barrier properties.

Coated fabrics are used extensively in defence. transportation, health care, architecture. space and

JOSHI el al.: POL YMERJCLA Y NANOCOMPOSITE BASED COATINGS 203

sports products, and to control the environmental pollution. Particularly, the impermeable coated fabrics find a very large application i n aerospace/aeronautics/defence as i nflatable, which include balloons for passive air defense and surveillance, floatation systems for recovery of missi les, space payload and emergency landing of helicopters on sea, etc . The use of polymer coatings i n these applications i s l imi ted because o f the low scratch resistance, poor weathering resistance, poor adhesion to substrate or h igh coating permeabi l i ty to air/gas. Further enhancement i n properties, l ike gas barrier, chemical and environmental resistance, scratch and abrasion resistance, adhesion, thermal resistance and flame retardancy, is desirable for advanced applications i n the fields of defense and aerospace. An i nnovation used to overcome these l imitations is the i ncorporation of nanosized particle/fil lers into polymer base. Polymer nanocomposites have recently attracted attention of coating industry because they show substantial improvement in physical properties as compared to neat polymers.s.7

This paper reviews the synthesis and properties of polymer layered s i l icate nanocomposites and their feasibil i ty as coatings in textiles for enhanced gas barrier and other physical properties as reported i n l iterature. A feasibil ity study o n polyurethane /c1ay nanocomposite based coating for inflatable has also been done and reported i n this paper.

2 Polymer Layered Silicate Nanocomposites In the late 1 980s, Toyota Central Research Labs of

Japan teamed up with Ube Industries Ltd. (Japanese resin supplier) to produce a new composite polymer consisting of nylon-6 interphased with layers of montmorillonite, layered s i l icate clay. The clay greatly improved the mechanical properties of the nylon with very small filler loading (less than 5% by wt). Toyota subsequently used this material for t iming belt cover and other under the load automotive applications, capital izing on the materials heat resistance and d imensional stabil i ty. The work was significant because clay platelets, which are just 1 0 A thick, are dispersed homogenously i n the polymer matrix at the nanometer level. The prospect of dramatic weight savings and improvement i n properties set off research activity for applying this technology to wide variety of polymers, both thermoplastics and thermosets. Over the last decade, polymer layered nanocomposites have been a hot

topic of research among researchers from both academics as well as i ndustry.s.9

2.1 Layered Silicates

Layered s i l icates (alternati vely referred to as 2 : 1 layered aluminosil icates, phyllosi l icates, clay minerals and smectites ) are the most commonly used i norgani c nanoelements in PNC research to date. I -)

The chemical structure of montmorillon i te i s : Mx (AI4_x Mgx) S is 020 (OH)4, where M is the

monovalent cation; and x , the degree of i somorphous substitution (between 0.5 and 1 .3) . Their crystal lattice cons ists of two-di mensional layers where a central octahedral sheet of alumina or magnesia i s fused to two external s i licate layers. Depending on the precise chemical composition of the clay, the sheets bear a charge on the surface and edges, this charge being balanced by counter ions, which reside in part in the in terlayer spacing of the clay . Stacking of the layers by weak dipolar or van der Waal forces lead to i nterlayer or galleries between the layers.

The galleries are normally occupied by inorganic cations (Na+, Ca++) balancing the charge of the oxide layers. These cations are readily ion-exchanged with .a wide variety of positively charged species .The number of exchangeable interlayer cations is also referred to as the cation exchange capacity (CEC). This is generally expressed as mill iequiv./100g and ranges between 60 and 1 20 for relevant smectites . Thus, i n layered s i l icates, (Fig. 1 ), the van der Waals interlayer or gallery containing charge compensating cations (M+) separates covalently bonded oxide layers (0.96 nm thick), formed by fusing two s ilica tetrahedral sheets with an edge shared octahedral sheet of either alumina or magnesia.

Pristine layered s i licates usual ly contain hydrated Na+ or K+ ions. One i mportant consequence of the charged nature of the clays is that they are generally highly hydrophi l ic species, and hence naturally i ncompatible w ith a wide range of polymer types. Therefore, the clay often must be chemically treated to make i t organophil ic . When the i norgani c cations are exchanged by the organi c cations, these are called organically modified layered s i l icates (OMLS). Generally, this can be done by ion exchange reactions with cationic surfactants i ncluding primary, secondary, tertiary and quaternary alkyl ammonium or alkyl phosphonium cations. These cations in the organosi l icate lower the surface energy of the i norgani c host and improve wetting characteristics of the polymer matrix or i n some cases i nit iate the

204 INDIAN 1. FIBRE TEXT. RES .. MARCH 2006

i .� -1 nm '" 0. If) � 1! .. p:j

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polymerization of monomers to improve the strength of the interface between the inorganic and polymer . 1 0 matnces.

2.2 StruCture

The complete dispersion of clay into the polymer reduces the micron size clay particles to nanosize and causes their dispersion throughout the polymeric resin . The dispersion of clay tactoids in a polymer matrix can result in the formation of three types of

. H 9 h . F' 2 nanocomposltes ' as s own ill ig . . The first type is the conventional microcomposite,

where polymer is unable to i ntercalate between the s i l icate sheets and a phase-separated composite is obtained (Fig. 2a) whose properties are same as that of traditional composites. The second type is the intercalated structure (Fig. 2b) in which a single (and sometimes more than one) extended polymer chain is intercal ated between the s i l icate layers, resulting in well ordered multi layer morphology bui l t up with alternating polymeric and inorganic layers. In case of intercalations, the organic component is i nserted bet ween l ayers of clay such that the interlayer spacing is ex panded but the layers sti l l bear a well- defined spatia l relat ionship to each other. The third type is formed when the s i l i cate l ayers are completely

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Fig. 2-Different types of composite aris ing from the interaction of layered s i licates and polymers: (a) phase separated microcomposite; (b) intercalated nanocomposite; and (e) ex fol iated nanocomposi te.

separated and the i ndividual layers are distributed throughout the res in matrix (Fig. 2c) and ensure complete random dispersion of layers with in the polymer matrix. Normal ly, a nanocomposite may be partially i ntercalated or exfoliated or a combination of two structures.

The d ispersion of clay in the polymer matrix to form i ntercalated or exfoliated structure is often investigated using X-ray diffraction (XRO) and transmission electron microscopy (TEM) techniques.

2.3 Synthesis

Complete dispersion of clay tactoids in a monomer or polymer matrix i nvolves three steps, such as ( i ) wetti ng the surface of clay tactoids by monomer or polymer molecules, ( i i ) intercalation of the monomer i nto the clay galleries, ( i i i ) exfoliation of clay layers. In polymer nanocomposi tes, fol lowing routes ensure i ncorporation of clay i nto the polymer matrix at nanolevel .

2.3. 1 Direcl Illlercalatioll

This method consists of spontaneous penetration of the polymer in the two-dimensional i nterlayer space by mixing the s i l icate and the polymerl • e. g. poly (ethy lene oxide) . Most polymers are incompatible with the s i l icate, making this method unsuitable.

2.3.2 In sitll Intercalalive Polymerization

This method consists of insertion of molecules or ions act ing as monomers. which cou ld be included to polymerize with in the intracrystal l ine region of the two-dimensional sol id . The presence of trans i tion

JOSHI et al. : POL YMER/CLA Y NANOCOMPOSITE BASED COATINGS 205

metal ions as exchangeable cations are included i n the structure of the layered solids to obtain the composites. This is the most successful approach to date, although i t probably l imits the ult imate applicabi l i ty of these systems l O, e .g. polyamide 6 -

clay nanocomposite. I n a typical synthesis, modified clay is dispersed in the monomer caprolactum , which is polymerized to form Polyamide 6 - clay hybrid as an exfoliated composite. Complete exfoliation may be preceded by intercalation of the monomer in the clay .

2.3.3 Ultrasound Irradiation

Ultrasound irradiation, as a new technology, has been widely used in chemical synthesis . When ultrasonic waves pass through a liquid medium, a large number of micro bubbles form, grow and collapse in a very short time (about a few microseconds) , which i s called ultrasonic cavitations. These ul trasonic cavitations can generate a local temperature as h igh as 5000 K and local pressure as high as 500 atm, a heat ing and cooling rate greater than 1 09 Kls-a very rigorous environment. Therefore, ultrasound has been extensively applied in the dispersing, emulsifying, crushing and act ivating of particles 10 , e.g. polyanil ine/nano-Si02

2.3.4 So/ulion Polymerizatioll

The layered si l icate i s exfoliated into si ngle layers using a solvent in which the polymer (or a prepolymer in case of insoluble polymers such as polyimide) i s soluble. It i s well known that such layered si l icates, owing to the weak forces that stack the layers together can be easily dispersed in an adequate solvent. The polymer then adsorbs onto the delaminated sheets and when the solvent is evaporated (or the m ixture precipitated), the sheets reassemble, sandwiching the polymer to form an ordered multilayer structure.8

Polar solvent can be used to synthesize i ntercalated polymer-clay nanocomposi tes . l o The organoclay at first is dispersed in a non-polar solvent such as toluene. Alkyl ammonium treated clays swell considerably in non-polar organic solvents, forming gel structures. The polymer, d issolved in the solvent, i s added to the solution and intercalates between the clay layers. The last step consists in removing the solvent by evaporation, e.g. polyurethane/clay nanocomposite l l and Gelatin/MMT.

2.3.5 Emu/sian Polymerization

It i s a new method to synthesize polymer nanocomposites. Synthesis is based on one step emulsion polymerisation. I t el iminates the environmental problems associated with the solution

polymerisation process, and involves addition of surfactants with unmodified s i l icate clay under stirring conditions. Monomer is fed wi th the init iator and the emulsion proceeds under vigorous agitation conditions. The reaction mixture is cooled to room temperature. The final product is obtained after filtration and washing several t imes with water. I t i s then dried under reduced pressureS. 1 2 , e .g . poly (methyl methacrylate/MMT).

2.3.6 Melt B1endillg (Compoundillg)

Intercalation with the aid of an extruder has been achieved by mixing the modified si l icate with polymers in the melt l ) , e.g. polypropylene nanocomposites by melt compounding organophil ic clays with maleic anhydride grafted pP. 1 4 The layered s i l icate is mixed with the polymer matrix in the molten state. Under these conditions and specially if the layer surfaces are sufficiently cQmpatible with the chosen polymer, the polymer can crawl .into the interlayer space and form either an intercalated or an exfoliated nanocomposite. In this technique, no solvent is required. Melt blending (compounding) depends on shear to help delaminate the clay and can be less effective than ill situ polymerization 111 producing an exfoliated nanocomposi te . 2.4 Properties

The polymer layered si l icate nanocomposites possess a unique combination of properties at very low volume fractions of the fil ler, not achievable by the conventional composi tes. The properties of interest for their application as coati ngs for various substrates, l ike textiles, plastics, metal and wood, are as given below.

In conventional polymer clay composites, fil lers form aggregates which remain undisturbed when mixed into the polymer. Thus, the i ncreased tensile and other properties may be achieved by using h igher conventional filler loading at the expense of increased weight and decreased gloss. On the other hand, in polymer nanocomposites, uniform dispersion of these nanosized filler particles produces an ultra large interfacial area /unit volume between the nanoparticle and the host polymer. These nanoelements are characterized by very h igh aspect ratio (often up to 2000) (refs 8, 9). Thus, their effective dispersion in polymer matrix in combination with adequate i nterfacial �dhesion between filler and polymer and nanoscopic dimensions between nanoelements can account for same effects at lower loadings than with conventional filler, thereby achieving weight reduction.

206 INDlAN J. FIBRE TEXT. RES., MARCH 2006

Polymer layered si l icates nanocomposites exhibit superior mechanical characteristics (e.g 40% increase in tensile strength at room temperature), heat resistance (e.g 100% increase i n heat distortion temperature), chemical resistance, gas barrier (e.g- 1 00-fold decrease in O2 & H20 permeabil ity) and improved abrasion resistance compared to the neat resin, just by the addi tion of 0. 1 - 1 0 vol. % of nanofiller l .2 , e.g. nylon nanocomposites, where addition of 4 wt% alumino si l icate creates a substantial increase i n strength and modulus due to i ncrease in interfacial i nteractions between the si l icate layer and the polymer chains.

Nanofil lers like layered s i l icates have the potential to reduce the permeabil i ty of polymers. It is believed that these nanoelements present a torturous path (due to large aspect ratio of the si l icate layers) for the gases/liquids, thereby slowing their lower rate of passage. Relatively low ti l ler concentrations are required (2%) (ref. 2). For example, as a result of nanoscale dispersion, water vapour permeabi l i ty is reduced by about five folds in polyurethane nanocomposites for biomedical applications. I I

These nanocomposites show superior physical properties. such as thermal and mechanical, wi thout adversely affecting the optical properties (translucency) of the sample. The reason is that the size of the dispersed particles is very small « 1 00 nm) and they do not scatter l ight, maintaining the optical clarity of the sample.

I mproved flame retardancy shown by these nanocomposites can be of advantage. Most flame safe coatings protect the substrate by forming an i nsulation layer during combustion . In the same manner, nanoelements l ike multi-layered si l icate appears to act as an excellent insulator and thus improves flame

I � retardancy . . -

3 Polymer Nanocomposite based Gas Barrier Coatings Properties of tradit ional materials change and the

behaviour of surfaces start to dominate the behaviour of bulk materials at nanoscale. Such effects i nclude ultraviolet (UV) blocking, antistatic and conduction capabil it ies, which have been taken as advantage of improving the properties of paints and coatings. It has been reported that by the i ncorporation of nanoparticles, the thin film coatings have stronger bonds and better flexibil i ty with l i ttle cost difference. These coatings are smoother, stronger and more durable. When used on products, the results range from scratch resistant and self-clean ing surfaces to

moisture absorbi ng clothing.5.7. 1 5 Many companies around the world are using the properties of nanoparticles and are i ncorporating them within their coatings. Organic-inorganic hybrid coati ngs are of i ncreasing interest in industry due to their w idespread applications.

The improved gas barrier property is of signifi cance for the appl ications where retention of gases or air i s very critical, e.g. packaging, inflatables (balls , tyres, etc) and special i ty applications, l ike hot air balloons. stratospheric balloons. floatation system and other aerial delivery systems used in defence. Although polymeric materials in the form of films or coated fabrics are attractive candidates for each of these and many other applications, the issue, which concerns the manufactures, i s minimization of gas permeabi lity.

The migration of gases through polymeric materials has become a crit ical factor for the abi l i ty of food packagers to i ncrease the shelf l i fe of products or retention of gases (H2 or He) in hot air balloons and air i n tennis bal ls . The migration of CO2 out of soda bottles can reduce shelf l i fe by allowing the soda to become flat; O2 migrating into beer bottles react with the beer to make it stale. The migration of oxygen through auto and truck t ires causes steel belt to rust. reducing tyre ' s l i fe. I n other words, maximizing the gas barrier properties of the polymers is highly desirable for such appl i cations .

The research on the in troduction of clay and clay l ike nano particles. both natural and synthetic into polymers i s centered on the maximization of these barrier properties. The four factors that are important to maximize the barrier properties ot' polymer/clay nanocomposites are:

Exfoliat i Oil The aspect ratio of the particles must be such that

they are very thin in cross-section, but show a large surface area in their flat d imensions, with the objective of maximizing gas barrier properties while minimizing the effect of the particle on material density, weight, colour, etc. The structures. where clay particles are completely exfoliated into i ndividual platelet, have a thickness of the order of one nanometer with lengths and widths of the order of 500nm.

Compatibilization The s i licate clay layers have polar hydroxyl groups

and are hydrophil ic , and therefore generally

JOSHI et al.: POL YMERICLA Y NANOCOMPOSITE BASED COATINGS 207

incompatible with non - polar organic polymers. The natural 'alumino s i l icate' MMT clays have thus to be functionalized to make it compatible with organic polymer matrix .

Orientation The dispersed particles must be oriented so that the

flat surface of clay is parallel to the surface of film, which helps in max imizing the barrier properties.

Reaggregation During final processing, the part icles have to be

prevented from reaggregating or clumping. The studies show interesting applications of

polymer nanocomposites as coatings with attractive combinations of properties not achievable by neat polymeric conventional coatings. Both scientis ts as well as industrial ists are actively engaged i n exploring the potential appl ications of polymer nanocomposites as advanced coating materials and some of the developments in this area are described in this paper.

Studies on a novel nanocomposite approach for reducing gas permeabi l i ty through biomedical polyurethane membranes have been reported. I I

Nanocomposites were prepared using commercially available poly (urethane urea) (PUU) and two organical ly modified layered s i l icates (OLS) . Wide angle X-ray diffraction experiments showed that the sil icate layer spacing in the nanocomposites i ncreased significantly as compared to that i n neat OLS, signify ing the formation of intercalated PUU/OLS structures. The nanocomposite materials exhibit increased modulus with increasing OLS content, while maintaining polymer strength and duct i l i ty . Water vapour permeabil i ty was reduced by about five folds at the highest OLS contents, as a result of PUUlinorganic composite formation. Clays are believed to increase the balTier properties by creating a maze or 'tortuous path' (Fig. 3) that retards the movement of gas molecules through the matrix resin .9

The water vapour permeabi l i ty for the PUU/OMLS nanocomposites is shown in Fig. 4 i n terms of Pc/Po [the ratio of the permeabi l i ty coefficient of the nanocomposite (Pc) to that of the neat PUU (Po)]. The nanocomposite formation results in a dramatic decrease in H20 vapour transmission through the PUU sheet. The lines i n Fig. 4 are based on the theoretical values calculated using the tortuosity model for the aspect ratios of 300 and 1 000 (ref. 1 6) . The comparison between the experimental values shown as points and the theoretical model suggests a

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gradual change in the effective aspect ratio of the fil ler.

Polyurethane/clay nanocomposites have been prepared by both in situ polymerization and solution intercalation. 1 6 Exfoliation of nanocomposites was i ndicated by XRD and TEM. The X-ray analysis showed that the exfoliation occurred for low organophi l ic montmorillonite (OMont) content, whereas for h igher contents the intercalated clay rearranged to a minor extent. The mechanical and dynamic-mechanical analyses showed an i mprovement i n elastic modulus and y ield stress but a decrease i n stress and strain at breaking on increasing the clay content.

The transport properties have been investigated l6 by using water vapour as . a hydrophil ic solvent and dichloromethane as a hydrophobic one. I n fact, in this way the barrier and the physical properties of a multiphase system composed of hydrophilic regions (OMont) and dispersed hydrophobic phase (PU) were studied. Figure 5 shows the logarithm of permeabil ity

208 INDIAN J . FIBRE TEXT. RES., MARCH 2006

to water vapour as a function of OMont content i n polyurel'hane nanocomposi tes (NPU). The permeabi l i ty decreases l inearly up to 20% of OMont, then a plateau is reached. Since zero conc. diffusion coefficient (Do) and sorption parameter (5) of the NPU has opposite trends, it is concluded that the permeabi l i ty behaviour, at low activi ties, is l argely dominated by the diffusion phenomenon.

For hydrophobic vapour, the logarithm of permeabi l ity of the nanocomposites to dichloromethane as a function of the OMont content in NPUs is shown in Fig. 6. As observed for the water vapour, a h igher content of OMont in the hybrids gives rise to a lower values of the permeabi l i ty, and the improvement i n barrier properties (significant u p to 20% of OMont).

Polyester nanocomposi tes for h igh barrier applications, e.g. the polyester having dispersed therein at least one layered clay material which i s cation exchanged wi th an organic cation salts for h igh barrier applications i n packaging materials, has also been reported. 17 The US patent l 8 claims an i nvention, which relates to a polymer-clay nanocomposite having an improved gas permeabi l i ty compris ing a melt-processible matrix polymer and i ncorporated therein, a layered clay material i ntercalated wi th a mixture of at least two organic cations. Dispersion of organophi l ic montmoril lonite i n organic solvents for solvent-based nanocomposite coati ngs has been studied . ' 9 Thi s study aims to determine the relevant parameters control l ing the organophi l i c montmori l lonite dispersion in various organic solvents which can be used as dispersion media for polymer coatings.

Recently, Wi lson Sport ing Goods have i ntroduced the Wi lson double core tenn is ball . The manufacturers claim that these balls retain their pressure and bounce for twice as long as conventional tennis bal l s . 2° This i s because of the inner core of the bal ls , coated wi th a nanocomposi te, that inh ibi ts the permeation of air by 200% from the inside of bal l . The c�ating was developed by InMat Inc . , ut i l iz ing aligned vermicul i te platelets to impede path of air from escaping. The platelets have an aspect ratio from 1 0000 to 1 and show significant i ncrease in the path/ travel distance for the air. The patented coati ng is known as Air D­fense® and is mixed at low v iscosity and hence low shear stress. The techonology i s being extended to t i re i ndustry , where these coat i ngs would inhibi t the migration of oxygen i nside the t ire, which affects the l ife of the steel t i re cord. This further offers the

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opportuni ty to i mprove fuel efficiency and pressure retention and to reduce recycl ing and incineration costs. I nMat Inc . plans to continue development of soccer bal ls , footballs, bicycle t ires, automobi le t ires and truck t i res based on thi s technology.

Another in terest ing European patent2 1 reports an invention called ' tubeless tire' in which air permeabi l ity i s significantly decreased. It relates to a tubeless t ire wherein an air chamber i s formed between the inner face of a t ire body and a res in . A gas barrier l ayer, compris ing an i nprganic layered compound having a part icle s ize of almost 5 /-1m and an aspect rat io of 50-5000 and a resin , is formed on the i nner face of the t ire body.

A series of polymer-clay nanocomposites was prepared by d ispers ing the i norgani c MMT clay into

JOSHI et al. : POL YMERICLA Y NANOCOMPOSITE BASED COATINGS 209

organic matrix polystyrene-acrylonitri l e (PSAN) by ill situ thermal polymerization22 and used as coatings on cold rol led steel . The PSAN-clay nanocomposi te coatings showed superior anticolTosion properties as compared to bulk PASN. This has been attributed to about 30% and 72% reduction i n H20 and O2 permeabi l i ty respecti vely by the addit ion of low clay loadings of 3 wt%. These nanocomposite coat ings also improved thermal stabi l i ty .

Another US patent23 described coat ings and films based on wax/clay nanocomposites having appl ications as protective packaging and industrial coat ings with improved chemical resistance and gas barrier properties

Nanocor24 have developed nanoclay based polymer nanocomposites for use in packaging l i ke beer bottles, which act as a barrier allowing for thinner material a subsequently l ighter weight and greater shelf l i fe. I t i s estimated that these containers w i l l gain a n extra 60 days of shelf l ife, reducing spoil age and decreasing overall costs.

Nanolok™ technolog/5 for coat ing was pioneered by I nMat Inc . , showing extremely efficient gas balTier to elastomeric substrates whi le retain ing a h igh degree of flexibi l i ty. These coat ings are aqueous suspensions of nanodi spersed s i l icates and elastomers and can be applied by spray, dip or rol l coat ing processes to a variety of substrates. These coat ings showed reduced weight, h igh gas barrier, flexibi l i ty and environment friendl iness.

Nanocomposite coat ings(, based on UV curing of clay or col loidal s i l ica fil led acryl ic or epoxy resins are also being studied. The coating showed improved impact resistance, tensi le strength and hydrophobicity. Clay also imparts surface roughness and makes it an efficient matting agent. Nylon- I I coat ings fil led wi th nanosized si l ica and carbon black have been reported to give an improvement of 35% in scratch and 67% in wear resistance, and 50% decrease in water vapour transmission rate through nano reinforced coati ngs compared to pure polymer coat ings.7

Research on s i l icon oxide th in fi lms, developed for gas barrier protection of polymer-based components has been reviewed.26 The relationships between ( i ) coating defects, cohes ive strength and in ternal stress state, and ( i i ) i n terfacial i n teractions and related adhesion to the substrate have been discussed.

High luster. flexible mult i layered fil m wi th a polyamide outer l ayer contain ing nano-dispersed fi l l ing material and its u t i l i zation for packaging

foodstuffs is also reported.27 Aromatic mono-, di- or tri-hydroxy compounds have been used alone or with peel strength-improving addit ives, i n association with a polyurethane adhesive for adhering a polyurethane­coated nylon fabric to another polyurethane-coated nylon fabric to form a polyurethane-coated nylon composite.28

The properties of polymer nanocomposite based coati ngs can be further enhanced by surface engineering of nanopart icles to ideally sui t the needs of speci fic appl ications. The chemistry of polymer nanocomposite formulations can be opt imized to achieve coatings with best performance. The synthesis of these nanoparticles i n larger quantities i s also being

�9 attempted.-

4 Polyurethane/clay Based Coatings for Infla­table-An Investigation Most commonly used materials for inflatables

include coated fabrics based on nylon, polyester or kevlar as base material and polyurethane or rubbers [neoprene, butyl rubber, hypalon, n itrile, EPDM (ethylene propylene terpolymer), etc] as polymeric coatings. F i lms of polyester (Mylar) and polyvinyl fluoride (Tedlar) have also been used.:1lU I The coating materials must meet the i mportant cri teria of h igh strength to weight ratio, max imum impermeabi l i ty to gases, high resistance to degradation due to UV rays. hydrolysis, abrasion and other environmental factors. and ease of fabrication ( i .e. RF, thermal and adhesive sealabi l i t ies) . Polyurethane qual i fies as an excel lent coating material for i nflatable owing to i ts i nherent properties l i ke RF sealab i l i ty, good adhesion property, abrasion resi stance, sui tabl i ty for frequent flex appl ication (packing-unpacking cyc l ic) , flexibi l i ty at low temperature, resistance to ozone, U V radiat ion, mold and mildew /fungus, and other environmental factors.32

However, i t has certain drawbacks, such as lower thermal stabi l i ty and poor barrier properties. To overcome the these drawbacks, the use of novel polyurethane/MMT (clay) based nanocomposites as coatings for intlatables has been explored in an ongoing reseaI'ch project at Department of Text i le Tech nology, I ndian Inst i tu te of Technology, Delh i . A brief report on the above i nvestigation is described here.

4.1 Materials and Methods 4. 1. 1 MateriaLs

Nylon-6,6 fabric [80 gsm, 40 endslinch. 40 picks/inch and 75 kgf break ing strength (warp &

2 1 0 INDIAN J. FIBRE TEXT. RES., MARCH 2006

weft)) , procured from M/s Kusumgar, Mumbai, was used for the study. The polymer used for coatings was thermoplastic polyurethane (Estane-T -54620), obtained from Noveon I nc. , USA. The cross-l inking agent used was Imprafi x-TH ( Bayer). The modi fied organophi l l ic MMT day (Closite® 30) was procured from Southern Clay Products Inc. , USA and solvents N-N-dimethyl formaldehyde ( DMF) and toluene (GR) were obtained from Merck.

A knife-over-rol ler type laboratory coating machine from Benz (UK), consisting of rollers, doctors blade, rubber b lanket and pin frame, was used for the coating.

4. 1.2 Characterization 4. 1 .2. 1 XRD Analysis

PU/c1ay nanocomposites were scanned for X-ray diffraction analys is on Phi l l ips X-Ray System [X' Pert Pro Console (PANanlytical)] using the experimental conditions (40 kY, 30 rnA, and 0.04°/s scanning rate) w ith n ickel fi l ter.

4. 1.2.2 TEM Analysis

TEM was carried on a Phi l i ps CM- 1 2 operati ng at 80 kY. The TEM grids were mounted on sample holder and the brightness of the electron beam was minimized. Precautions were taken to min imize motion and beam damage of the sample.

4. 1 .2.3 TGA Analysis

The thermogravi metric analysis (TGA) for the PU/clay nanocomposites was performed on Perkin­Elmer TGA 7 under N2 atmosphere. The samples were heated at 20 DC/min from room temperature to 850 °C.

4.1.3 Testing

The breaking force of coated fabrics was tested on l nstron 4202 using strain rate of 1 20 mm/mi n and a gauge length of 1 5 cm. The w idth of the samples was 5 cm. An average of five test readings of the sample specimens was taken. The tearing strength of coated fabric was tested on I nstron 4202 using strain rate of 1 00mm/min and a gauge length of 1 0 cm. The thickness of the fibres and coated fabric was tested on Essedial Thickness Tester. Ten readi ngs were taken and an average was calculated.

The hydrogen (H2) gas permeabil i ty of coated fabric sample was tested as per the ASTM-D- 1434 method. For this, a test set up located at ADRDE, Agra, I ndia, was used. The fabric sample was

clamped across a gas cell/chamber and the gas i s allowed to diffuse through the fabric for a fixed duration (24 and 48 h) . The volume of gas escaped during thi s period is measured and converted into standard units of permeabi lity.

4.1.4 Preparation of PUiClay Nanocomposites 4. 1 .4. 1 Preparation of Clay Dispersion

Clos i te® 30 was taken in DMF solvent i n a beaker and subjected to u ltrasonic v ibration in an ultrasonicator for about 25 h . This resulted i n breaki ng of the clay particles to very fi ne s ize and very uniform dispersion of the clay i n DMF. A 2.5 wt % dispersion of the clay i n DMF was prepared. This dispersion was used as stock solution for preparing different PU-c1ay nanocomposites of varying clay concentrations.

4. 1.4.2 Preparation of PU SO/Iltion

A uniform 1 5 wt % PU solution was prepared in DMF: Toluene (60:40) solvent mixture, usmg mechanical stirring for 4 h at 70 DC

4. 1 . 4.3 Preparation of PU/Clay NWIOCOlllposite

PU/c1ay nanocomposite was synthesized by solution intercalation/exfol iation route. The required quantity of PU resi n and clay dispersion in DMF: toluene (60:40) solvent mixture was taken and subjected to mechanical stirring at 80° C for about 5 h so as to obtain 1 5 wt % nanocomposite solution. The clay content was varied i n the range 0.5, 1 , 2, 3,4,5 ,7 and 1 0 wt %. The nanocomposite solutions were coated on glass plates and then dried under controlled conditions to get thin fi lms. These fil ms were used for characterization of different nanocomposites by X-ray, TEM and TGA analyses.

4. 1 . 4.4 Preparation of COaled Ny/o/l Fabrics

The neat PU and PU/clay nanocomposite solutions thus prepared were used to coat the nylon-6,6 fabric on the knife-over-rol ler type coating machine. The outer side was given two tie coats of 1 5 gsm each with ester grade PU solution and 5 wt % crossl inker and dried at temperature around 1 00 °C. The subsequent coats were given with PU-nanoclay composite solution. each of approx . 1 5 gsm, w ithout using crossl inker and finally cured at 1 30° C for 1 -2 min. The i nner s ide was coated s imilarly but with ether grade PU as i t has better gas barrier properties. The total add-on on the fabric was in the range of 1 50- 1 70 gsm and thickness of coated fabrics was i n the range of 0. 1 9-0.25 mm at 200 gf/cm2 pressure for

JOSH I et al.: POL YMERICLA Y NANOCOMPOSITE BASED COATINGS 2 1 1

differen t nanocomposite samples wi th varymg clay weight %.

4.2 Results and Discussion 4.2.1 Characterizatioll of Pu/Clay Nallocomposites

The X-ray diffractogram of MMT clay shows a diffraction peak at 28 = 6.2°, corresponding to a basal spac ing of 1 4 A between the c lay platelets (Fig. 7 ) . XRD analysis i s bei ng popularly used to ident ify intercalated and exfoliated structures. 8.9 The in tercalation of the polymer chains i ncreases the interlayer spac ing in comparison to the spacing of organoclay used, leading to shi ft i n diffraction peak towards lower angle values. As far as exfol iated structures are concerned, the diffraction peak due to clay is general ly dimin ished or no more v i sible i n XRD because of a too large spacing between layers or because the ordering of c lay structure is no more present.

The absence of any diffraction peak in the 28 range 1 .5°- 1 0° for all nanocomposite samples i nd icates a exfol iated structure although other factors such as clay di lut ion, preferred orientation and peak broadening need to be considered as wel l ."

4.2.2 TEM Analysis

TEM allows a qual i tati ve understanding of the internal structure, spatial distribution of the various constituents and defects through direct visual ization.8.9 The TEM micrographs of PU/clay nanocomposite are shown in Fig . 8. The s i l icate l ayers of the organoclay are seen as dark l i nes of about 1 nm in exfol iated layers and as thicker l i nes for in tercalated structures dispersed i n PU matrix . The clay appears to be d ispersed i n the matr ix both as intercalated and exfol iated structures. However, TEM qual i tative analysis is sometimes considered i nsuffic ient to describe c lay d ispersion and may be misleading for several reasons. Firstly, the small areas examined by TEM may not be representat ive of the overall microstructure; and secondly, i t can have multiple structures and a range of s izes.

4.2..1 TGA Allalysis

The TGA curves of PU and PU/clay nanocomposites are shown in F ig . 9. I t is c lear that PU/clay nanocomposi tes start degrading at higher temperature and the temperature for 5 wt % loss i s substantially higher than for prist ine polymer. The char y ield at near 8500 C i n N2 atmosphere also increases substantial ly in presence of the c lay

_ 1 000/0 Clay

�---_____ 1CP1oNC ------------- . 3%NC

------..----------------.. - 1%N.C

3 4 5 6 8 2 9 Scale

Fig. 7-X ray diffractograms for neat Clay (Cloisite 30B) and PUiclay nanocomposites with I . 3 and 1 0% clay

Fig. 8-TEM of polyurethane tilled with 2 wt % of nanoclay lea) l oonm, 80kY, direct magnification x56oo; and (b) 1 00nm, 80kY. direct magnification x 7 1 00]

(Table 1 ) . The i ncreased char y ield leads to reduced flammabi l i ty of samples. The flame retardant mechan ism i nvolves a h igher performance carbonaceous s i l icate char, which bui lds up on surface

2 1 2 INDIAN J . FIBRE TEXT. RES . . MARCH 2006

Q) :J

duri ng burning. This i nsulates the underlying material and slows the mass loss rate of decomposit ion products.14 The i mproved thermal stabi l i ty of PU/c1ay nanocomposites as coat ings give an added advantage of superior thermal and flame retardant properti es to the coated fabric .

4.2.4 Breakillg Force alld Tearing Strength o/ Coated Fabrics

The nanoclay reinforced PU coated fabrics show higher breaking force as compared to neat PU coated ones. The force increases with the increase in clay content and shows a maxima around 3-4 wt %, beyond which poor dispersion and agglomeration do not contribute to the breaking force (Fig. 1 0) .

The PU/ clay nanocomposite coating of around 5 wt % shows a maximum value of tearing stress at break, which i s 42% higher than pristine PU coated fabrics (Fig. I I ) . The resistance to growth of fracture measured in terms of stress at break i ndicates that clay tethering and uniform dispersion of c lay particles inhibit the stress concentration and delayed crack propagation as also reported by Pattanayak and Jana. 3S

4.2.5 Gas Per/lleability

The PU/ clay nanocomposi te coated fabrics with varying clay percentages ( 1 ,3 ,5,7 & 1 0 wt %) were ini tially tested for air permeabi l i ty and it was observed that about 3-4 wt % clay could reduce the ' air permeabi l i ty significantly . Hence, the coated fabric with 3 wt% nanocomposite was tested for H2 gas permeabi l i ty (Fig. 1 2) . It is observed that the permeabi l i ty to hydrogen reduces by about 36 % at clay content of 3 wt % as compared to neat PU

:2 75 en Q) cr:: � Ol � 50 ;!2. o

25

o 1 60 240 320 400 480 560 640 720 800 880 Temperature ee)

Fig. 9-TGA curves of pure PU and PUlclay nanocomposites

coating at the same thickness. The process needs to be optimized to further improve the gas barrier property of nanocomposite coated fabrics The permeabi l i ty of coated fabrics depends strongly on fabric th ickness and it decreases with increasing thickness. The clays are believed to increase the barrier properties by creating a tortuous path that retards the progress of gas molecules. i .e. gas diffusion through the matri x res in as reported by several authors . �·�J6

Table 1-5% wt loss temperature and char yield at 850"C in N2 atm

Clay. % 5% wt loss Char yield at temp . . DC 850D C. %

E u � CI oX

� G.I � o -CI c::

oX III G.I ... aI

o 0.5 1 .0 2.0 3.0 4.0 5.0 7.0 1 0.0 1 00

200

1 85

170

Ether based PU

338 34 1 345 347 352 363 358 348 336

•

1 55 1/ 1 40

0 2

•

Ester Ether Ester based PU based PU based PU

34 1 (l.0 345 0. 1 0 35 1 0. 1 1 356 0. 1 2 368 0.85 364 1 . 1 3 35 1 1 .79 348 2.34 342 2.95

4 6 8 C lay (w t %)

0.0 0. 1 1 0.34 0.37 0.66 1 .33 2.23 2.28 3.02

70. 1 96

1 0

• w a rp

• weft

1 2

Fig. I O-Variation in breaking force (warp and weft) with clay wt�k

� Ol 16

::=:- 14 .r. en 12 c ill � 1 0 Ol c 8 .�

..

� 6 +-----,-----,------.-----r----� o z 4 6

Clay (wt%) 8 1 0

Fig. I I-Variation of tearing force (warp) with clay wt%

JOSHI et al.: POLYMERICLA Y NANOCOMPOSITE BASED COATINGS 2 1 3

5 (a) 4

3 >: 2 '" � N

E 1 -.. � c- o >-. 0. 18 0 . 19 0.21 0.23 0 .25

1) 1 .5 (b) O wt%. l .38 '" Q) r--E Q;

Cl. 3 INt%. 0.88 ,...---

0.5

O +------L----L------r------L---�----_. 0.25 0.25

Thickness (mm)

Fig. 1 2-Plots of hydrogen permeabi lity at 10 cm water column pressure: (a) 3% nanocomposite coated fabric vs thickness (b) 0% and 3% nanocomposite coated fabric having same coating thickness

5 Conclusions Polymer nanocomposi te is a rapidly growing area

of nanoengineered materials, providing l igh ter weight alternatives to conventional fi l led polymers wi th value added properties. The possible app lications i n coated fabrics industry, i n particular, and coating industry, i n general , are very exciting. A prel iminary investigation on PU/clay nanocomposite based coated fabric shows an encouraging result on improving the gas barrier property for intlatables. The major challenges would be dispersion of nanoparticles at nanolevel, sophisticated characterization techniques to investigate the level of dispersion and the avai lab i l ity of nanoparticies in bulk. Moreover, substantial fundamental research is sti l l necessary to provide a basic understanding of these material� to enable i ts ful l exploi tation.

Acknowledgement The authors wish to acknowledge the financial

support from ADRDE (Minis try of Defence, Govt. of

I ndia) , Agra. They also wish to express their heartfelt grati tude to M r M L Sidana, Director, ADRDE, Agra, for his constant encouragement and guidance during the course of this work.

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