multifarious applications of halloysite … · in this approach corrosion coat- ... fective...

282 D. Rawtani and Y.K. Agrawal

© 2012 Adva]ced Study Ce]ter Co. Ltd.

Rev. Adv. Mater. Sci. 30 (2012) 282-295

Corresponding author: Deepak Rawtani, e-mail: [email protected]

MULTIFARIOUS APPLICATIONS OF HALLOYSITENANOTUBES: A REVIEW

Deepak Rawtani and Y.K. Agrawal

Institute of Research & Development, Gujarat Forensic Sciences University, Sector 18-A, Near Police Bhavan,Gandhinagar 382007, Gujarat, India

Received: April 11, 2012

Abstract. Natural tubules Halloysite are unique and versatile material formed by surface weath-ering of aluminosilicate minerals and comprises of different proportion of aluminum, silicon,hydrogen, and oxygen. It has chemical formula of Al4Si4O10(OH)8

.4H2O. Nanotubular geometry ofhalloysites exhibit nanoscale dimensions. Basically, this tubular arrangement varies with differentregions. HNTs have high mechanical strength and modulus and these features make it an idealmaterial for prepari]g differe]t polymer based composites. Halloysites HNT’s) are bei]g usedfor so many varieties of biological and non-biological applications; remediation of environmentalcontaminants, act as a cargo for the delivery of drugs and various macro molecules, storage ofmolecular hydrogen and for catalytic conversion and processing of hydrocarbons. They are usuallyapplied in the fabrication of high quality ceramic white-ware, nanotemplates and nano scalereaction vessels. In conjunction with different epoxy (EP) composites halloysite used to improvethe mecha]ical properties of polymer. Various features of HNT’s like rigidity, higher aspect ratio,and easy dispersability in polymer matrix and more importantly its abundant availability andbiocompatibility make it a subject of fascination. In this review, we tried to summarize the variousfacet of halloysite nano tubes for the pertinence in the various research fields.

1. INTRODUCTION

The Halloysite nanotubes (HNTs) is a kind of alumi-nosilicate clay (Al2Si2O5(OH)4

.H2O with 1:1 layer)with hollow micro and nanotubular structure, andare mined from natural deposits in countries likeChina, New Zealand, America, Brazil, and France.HNTs are chemically similar to kaolinite and theyused in the manufacture of high quality ceramicwhite-ware. Many researchers have done much workon the characterization of the Halloysite clay. Re-cently it has reported that HNTs have typical dimen-sio]s of, 10–50 ]m i] outer diameter, 5–20 ]m i]i]]er diameter with 2–40 mm i] le]gth [1,2], seeFig. 1.Halloysite’s layer silicate a]d the crystal struc-

ture are formed by two building blocks: (i) sheets ofcorner sharing [SiO4] tetrahedra and (ii) sheets of

edge sharing [AlO6] octahedral. In the [SiO4] sheetsthe tetrahedra share three corners and the un-bounded tetrahedral apices are all pointing alongthe same direction[3]. Within the octahedral sheet,only 2/3 of the existing octahedra sites are filled byaluminium. It is referred to be a dioctahedral layer(term normally used in mineralogy). The layersbuilding up the final structure are composed by oneof each of these sheets so that the crystal structureof halloysite is described as 1:1 dioctahedral layersilicate. The water molecules are sitting betweentwo consecutive layers create slight differences inthe relative orientation between neighbouring layersmight give rise to the differences in the symmetry ofthe final structure. This structure of Halloysite elu-cidates the close similarity with structure of kaolin-ite and experimentally it is difficult to distinguishbetween them. As a matter of fact, the structure of

283Multifarious applications of halloysite nanotubes: a review

Halloysite- 7A °) is completely a]alogous to a dis-ordered kaolinite structure. Tubular or cylindricalparticles of different dimentions are normally foundwith halloysite even though some other morphologyhas also been reported (fibres, platy, etc.). Thus,the powder X-ray diffraction patterns are rather com-plicated with extensive overlapping of reflections ac-companied by a high background. As a result, X-ray diffraction powder patterns are virtually identicalfor both structures. As stated in the most notice-able difference is probably the particle shape. Thus,we adopt the convention that the presence of cylin-drical or tubular particles is indicative of halloysitewhile samples with platy particles might be namedas kaolinite [4-6].

Various patents exist for the use of halloysite asnanofillers and in controlled release technology fora range of active agents. This mineral is extensivelyused in industry for ceramics, cements andfertilisers. Besides these it has enormous potentialapplications because of their peculiar tubularlyshaped particles normally found in this mineral.Halloysite has also been used as a petroleum crack-ing catalyst in the past [6].

Such a peculiarity of Hallosite opens up the in-terest to wse it as a template in nanotechnology,i.e. In this context, it is physically impossible toreview the whole literature concerning this mineraland we just refer to a recent publication, which con-stitutes a compact and extensive review on thismineral.

2. HALLOYSITE NANOTUBE FORCORROSION PREVENTION.

Corrosion of metals is a serious technological prob-lem, and a variety of methods such as cathodic pro-

(a) (b)

Fig. 1. TEM images of Halloysite nanotubes (Courtesy of Dr. Bing Zhang).

tection, insulating coatings, and corrosion inhibitionhas been developed to overcome it. An inhibitor-en-hanced coating is one of the most effective meth-ods. Different type of inorganic corrosion inhibitorsincludes chromates, phosphates, molybdates, andnitrites avaliable. Drawbacks associated with theseinorganic inhibitors is their toxicity. Therefore, anintroduction of environmentally friendly corrosion in-hibitors for protective coatings is important.

Recent advancement has offered an opportunityfor fabrication of coatings with anticorrosion proper-ties by integration of nanoscale containers (carri-ers) loaded with the inhibitor or other active com-pou]d i]to existi]g “passive” films, thus desig]i]g]ew coati]g systems based o] the “passive” ma-trix-“active” co]tai]er structure. The co]cept behi]dthis is to develop nanocontainers, which can besensitive to the external (e.g., mechanical damage)or internal (e.g., pH changes) corrosion trigger

2.1. 2-mercaptobenzothiazole foranticorrosion

Halloysite nanotubes used for active release of cor-rosion inhibitors. In this approach corrosion coat-ings composed of hybrid sol-gel films were dopedwithin Halloysite nanotubes so they were able torelease entrapped corrosion inhibitors in a control-lable way. A silica-zirconia-based hybrid film can beused as an anticorrosion coating deposited on alu-minium alloy. Halloysite nanotubes with inner voidsloaded by corrosion inhibitors (2-mercaptobenzothiazole) and outer surfaces coveredlayer-by-layer with polyelectrolyte multilayers wereintroduced into the hybrid films. These sol-gel filmswith the nanocontainers provide long-term corrosion


protection in comparison with the undoped sol-gelfilm. In this approach corrosion trigger the releaseof corrosion inhibitor in controllable manner. Exer-tion of the inner Halloysite nanotube lumen used asa storage vacuoles for the corrosion inhibitor offersa novel way of fabricating composite.

Leakage of the loaded inhibitor from the interiorof halloysite can be prevented by modification ofouter surface of 2-mercaptobenzothiazole-loadedhalloysite nanotubes with deposition of alternatingpolyelectrolyte multilayer (poly(allylamine hydrochlo-ride)/ poly(styrene sulfonate). Function of the poly-electrolyte shell is to provide the release of the en-capsulated inhibitor in controllable manner with thechange in pH of surrounding environment around theHalloysite ]a]otube, a]d that’s why i]hibitor directlyreleases in the corrosion pit [7].

2.2. Benzotriazole for anticorrosion

Benzotriazole and its derivatives are the most ef-fective inhibitors used for protection of copper andtransition metals. Although benzotriazole is anefficient corrosion inhibitor for these metals, but inchloride-containing environment (e.g., seawater) en-vironment, its corrosion-inhibitive performance is notsufficient so the combination of corrosion inhibitorwith passive protection (such as paint coating) isrequired. A direct addition of benzotriazole into thepaint is not effective, because it is water-solubleand leaves empty voids in the coating layer, whichdecreases the barrier properties of the coating. Byintroduction of benzotriazole into paint within nanoor microscale encapsulating systems drasticallyimproves anticorrosion performance.

Halloysite nanotubes with dimension of 50 nmdiameter and ca. 1 m length used as containersfor the loading, storage, and sustained release ofbenzotriazole. Extended controlled release can beachieved by formation of the stoppers at tubeopenings. Benzotriazole release time in water wasin range from 10 to 100 h depending on the stopperformation at the cylinder ends. Entrapment ofbenzotriazole into clay within nano or microscaleencapsulating systems drastically improvesanticorrosion performance [7,8].

3. HALLOYSITE NANOTUBES FORTHERMAL RESISTANCE

Thermal stability and fire retardancy of thenanocomposites remarkably heightened by theincorporation or addition of HNTs. Normally HNTsused in the manufacture of high quality ceramic and

white-ware. In recent time, HNTs are being at-tempted to utilize as nanofiller in conjunction withvarious polymers like natural rubber, nitrile rubberand polypropylene. Halloysite nanotubes influencedthe fire performance of the composites, by develop-ing thermal insulation barrier at their surface duringburning. This created barrier either retard the burn-ing without stop or more often double the total burn-ing time.

3.1. Halloysite nanotubes and poly(propylene)

Incorporation of HNTs in Poly Propylene causessignificantly increased thermal stability and flameretardant effect of the PP/HNTs nanocomposites.Nanocomposites based on polypropylene and HNTsare prepared by melt blending. HNTs were dispersedin PP matrix evenly at nanoscale after facile modifi-cation. Cone calorimetric data also show the de-crease in flammability of the nanocomposites. En-trapment mechanism of the decomposition productsin HNTs also explained the enhancement of ther-mal stability of the nanocomposites. This enhancedthermal stability and decrease flammability were be-cause of barriers in heat and mass transport as wellas the presence of iron in HNTs [9].

2.2. Halloysite nanotubes and LLDPE(linear low density polyethylene)

Linear low density polyethylene (LLDPE) is an im-portant thermoplastic for the applications such aselectric wire, cable, film, pipe, and container. How-ever, its applications are limited due to its lowstrength, low softening point, and flammability andso on. Simultaneously, it is necessary to modifyLLDPE to get improved mechanical properties, flameretardancy as well thermal stability.

Halloysite nanotubes (HNTs) used with linear lowdensity polyethylene (LLDPE) to prepare compos-ites with enhance mechanical and prominant flameretardant properties. Poly Ethylene graft was usedas interfacial modifier in the LLDPE/HNTs compos-ites. HNTs have proved to be a promising reinforc-ing and flame retardant nano-filler for LLDPE. Thismechanical properties, flame retardancy, as well asthermal stability of the composites can be furtherenhanced by the addition of the graft copolymer.Addition of graft copolymer in LLDPE/HNTs com-posites not only facilitate the dispersion of HNTs inLLDPE matrix but also enhance the interfacial bond-ing [10].


2.3. Halloysite nanotubes and Nylon 6

Nylon 6 (PA6) is a semi-crystalline polyamide ther-moplastic with excellent properties such as chemi-cal resistance, ease in process ability and goodmechanical characteristics. So it used for wide rangeof fibre, film and engineering applications. Advan-tage associated with PA6 is its inherent degree offlame retardancy which attributed due to the pres-ence of nitrogen beside its favours thermal degra-dation of Nylon 6 via hydrolysis of the amide bond,followed by homolytic cleavage of covalent bonds.However, as a fire retardant PA6 do not exhibit sat-isfactory behaviour since upon ignition, intensiveflammable dripping causes increase chances of firehazard. Tubular nature with crystal structure andlow available hydroxyl groups at the surface HNTsmakes HNTs easily dispersable in polymer matri-ces compare to other nanoclays.

Recently HNTs are being used to improve ther-mal properties of polymers, by using HNTs as anadditive with polypropylene and poly (vinyl alcohol).They have also been co-cured with epoxy/cyanu-rate ester resins to form organic/inorganic hybrids.HNTs i] various co]ce]tratio]s 5–30 wt.%) are

used for making of composite materials with PA6by simple melt extrusion process. It has found thatrelatively high concentrations of additive (15 wt.%)are required to achieve the adequate levels of fireretardant property normally associated with nanoclay(or layered silicate) additives [11].

4. HALLOYSITE NANOTUBES ASFILLER FOR VARIOUSNANOCOMPOSITES

Nanocomposite has wide application in optical, elec-tronic, magnetic, and thermosensitive devices.These nanocomposites are prepared by templateassisted process by using various structures anddiversified samples from dissimilar biological spe-cies. However, these approaches which are beingemployed are very costly. So more optimised ap-proaches with cost effective modules are urgentlyneeded for the successful and more efficient fabri-cation of nanocomposites.

Halloysite nanoparticles are used an additive forenhancing the mechanical performance of polymers,specifically for strengthening and toughening of ep-oxies. There are many benefits associated withHNTs to used as filler in polymer based compos-ites.i) HNTs have low surface charges so there will beno intercalation and exfoliation needed compare to

other two dimensional nano clays fillers such asMontmorillonites (MMTs). So they provide ease inprocessing when mixed with other polymers to givehomogeneous particle dispersion.ii) Modifications at the surfaces of HNTs provide anopportunity to expand the basal spacing of HNTsby the intercalation of inorganic and organic com-pounds in their inter layers, which further enhancepossibility to produce a homogeneous mixture ofHNTs with polymers during blending.iii) Surface modification of HNTs enhances their wetability and bond formation with different polymers.iv) Expansion of base layers provides HNTsexfoliation.v) HNTs are comprised of siloxane and hydroxylgroups, which gives HNTs potential for the forma-tion of hydrogen bonds and hence improve disper-sion.vi) Larger luminal diameter of Halloysite nanotubescan accommodate different polymer molecules whichfurther offer polymeric composites.

4.1. 5FPPSyVNWe–TSPymeUnanocomposites

Halloysite nanotube an economically availablenanosized raw material, has gained growing interestto modify by polymers to form polymer-halloysitecomposite nanotubes. It is important to modify thehalloysite by keeping their tubular structure intactin resulting polymer-halloysite nanocomposites.Polymer chains can grow from both interior andexterior surfaces of the Halloysite nanotubes by atomtransfer radial polymerization (ATRP). Atom transferradial Polymerization (ATRP) used to give diversityto polymer brushes onto either planar substrates orindividual spheres surface. Nanocomposite could beeasily tuned by alteration of the monomer structure.Eventually, the composite nanotubes were evolvedto a core-shell coaxial structure after the interiorcavity was fully covered by the polymer. By thedissolution of the Halloysite template, polymericnanotubes and nanowires were derived. This canbe used to cast Halloysite based fabric and theseresulting nonwoven composites has interestingfeature of wettability [12,13].

4.2. Halloysite and ethylenepropylene dienemonomernanocomposites

Functionalization of clay minerals with organosilanesignificantly influences the properties of polymer-clay and polymer-nanotubes by raising their disper-


sion within the polymer matrix and thus enhance-ment of the mechanical properties ofnanocomposites. Modification of clay minerals canbe achieved by alkyl ammonium ions. EPDM chainswere intercalated into MMT after modification withtrimethyl octadecylamine and dimethylbenzyloctadecylamine, and exfoliated by using methyl bis(2-hydroxyethyl) coco alkylamine was used.

HNT gives uniform distribution in EPDM poly-mer with high loading capacity so it significantlyimproves the tensile, strength, thermal resistanceand morphological properties of composite. Surfacemodification increased the temperature at 5% massloss from 414 °C to 444 °C for ]a]ocomposites filledwith 10 phr HNT loading. Surface modification ofHNTs can be achieved by by -methacryloxypropyltrimethoxysilane and resultant effects of surfacemodification on the thermal and morphology prop-erties of HNT reinforced PP nanocomposites. Thetensile strength and tensile modulus at 100% elon-gation (M100) of the nanocomposites were higherthan those of EPDM/unmodified HNTs (EPDM/HNT)while the elongation at break decreased a little aftermodification of the HNTs [14-17].

4.3. Halloysite and styrene-butadienerubber nanocomposites

Rubber cannot be used for nanocomposites forma-tion alone because of their lower modulus andstrength so fillers must incorporated to provide thempractical applicability. The reinforcement of rubbersby particulate fillers such as carbon black and silicahas been extensively studied. Basically rubber/layered clay nanocomposites exhibit outstandingmechanical properties as well as loweredpermeability.

HNTs have been demonstrated to be an idealcomponent for fabricating high performance polymernanocomposites. Compare to carbon nanotubes(CNTs), the naturally occurring HNTs are muchcheaper and easily available. In addition, mosthydroxyls of HNTs are located in the inner side andthe combination of siloxane surface and tubulargeometry gives HNT’s a much better dispersio]property compared with other layered clay. Ultimateperformance of the polymer composites decide bytwo factors that is dispersion state of the filler andthe interfacial properties.

Carboxyl methyl cellulose sodium salt, poly(acrylic acid) sodium salt or poly (2-methoxyethylacrylate) (PMEA) or poly (vinyl acetate) can improvedispersion of clay minerals eg. montmorillonite andthe unsaturated acid, such as methacrylic acid

(MAA), was used as interfacial modifier for polymerin organics composites. This affinity of unsaturatedacid to the clay is due to the formation of hydrogenbonds between clay and the acid. Some carboxylicacid such as MAA is reactive to zinc oxide (ZnO) ormagnesium oxide (MgO) to yield unsaturated metalmethacrylates which show unusual reinforcingeffects for rubber compounds [18].

MAA is corrosive in nature so it utilized mini-mally in nanocomposites. Sorbic acid is an unsat-urated carboxylic acid which contains two functionalgroups, a carboxyl and a two conjugated carbon-carbon double bonds in each molecule. Sorbic acidused primarily in range of food and feed related prod-ucts and lesser extent in certain cosmetics andpharmaceuticals. HNT’s rei]forced i]terfacial prop-erties of styrene-butadiene rubber (SBR). Sorbic acid(SA) is used to improve the performance of styrene-butadiene rubber (SBR)/halloysite nanotubes (HNTs)nanocomposites. The strong interfacial bondingbetween HNTs and rubber matrix is resulted throughSorbic acid intermediated linkages.

It has demonstrated that by introduction of un-saturated acids such as (meth) acrylic acid, cro-tonic acid, and SA, into calcium carbonates(CaCO3) filled SBR compounds improve tensilestrength and the modulus compared with thevulcanizates without these acids. For obtaining highmodulus and strength of filler/rubber compositesincorporation of unsaturated acids like SA is nec-essary. SA improves interfacial interactions betweenof SBR/HNTs nanocomposites. Sorbic acid bondsSBR and HNTs through grafting copolymerization/hydrogen bonding mechanism. Significantly im-proved dispersion of HNTs in virtue of the interac-tions between HNTs and SA was achieved.

4.4. 5FPPSyVNWe–eTSxynanocomposites

Blending of epoxies in different proportion of HNTscan increase their fracture toughness, strength andmodulus, without losing their thermal properties.HNTs have large surface area because of their tinyparticle size, resultant HNTs tend to agglomerateunder the influence of the van der Waals force, so itis difficult to get homogeneous dispersion of HNTsin epoxies. This agglomeration is very difficult toeliminate by applying only moderate shear stressesprovided by the conventional mechanical blendinginstuments like ultrasonic vibration or using a stir-rer or a magnetic bar. Thus, sometimes use of se-vere shear stresses, such as the use of ball mill,may break-up the agglomerates and achieve a ho-


mogeneous dispersion of HNTs in the polymer ma-trix [19,20].For the preparatio] of Halloysite–epoxy

nanocomposites with improved homogeneity of HNTsin the epoxy matrix as well as enhanced mechani-cal performance, either mechanical mixing or ballmill homogenisation were used and with simulta-neous chemical treatment of surface. In potassiumacetate based surface treatment of halloysite, afterreducing the size of halloysite particle form clus-ters in the epoxy matrix with ball is very effectiveapproach.

4.5. 5FPPSyVNWe–0FUGSxyPFWeIGXWFINeRe–VWyUeRe UXGGeUnanocomposites

Nanocomposites incorporated with inorganic mate-rials gives better performance compared with theother polymer composites. Final acceptability andperformance of polymer nanocomposites incorpo-rated with inorganic material govern by Interfacialinteractions. The interfacial interactions betweenpolymer matrix and inorganics mainly include vander Waals force, hydrogen bonds, covalent bonds,and ionic bonds. There are so many approacheshave been developed to improve the interfacial inter-actions of the nanocomposites including the modi-fication of inorganics or matrix. Recently, Halloysitenanotubes (HNTs), has acquire attention to reinforcepolymers with unique reinforcing effects to differentpolymers such as epoxy resin, polypropylene, polya-mide, natural rubber, etc. have been demonstrated[21].xSBR carboxylated butadie]e–styre]e rubber)

is a copolymer of styrene, butadiene, and acrylicacid has the many carboxyl groups in chain whichprovide assistance in formation of hydrogen bonds.HNTs and xSBR by co-coagulation process are usedto prepare tailor made nanocomposites with improveinterfacial interactions via hydrogen bonding. ATR-FTIR (Attenuated total reflection Fourier transforminfrared spectroscopy) and X-ray photoelectronspectroscopy (XPS) studies on xSBR /HNTnanocomposites indicate the formation of hydrogenbonding between xSBR and HNTs. It also showedhigher content of HNT promotes higher vulcanizationand consequently Lower content of HNTs tends todelay the vulcanization of xSBR/HNT compound.The mechanical properties, like the modulus andhardness, are significantly increased by theinclusion of HNTs. The significant reinforcing effectsof HNTs are correlated to the co-coagulation processand strong interfacial interactions via hydrogen bond-ing [22].

4.6. 5FPPSyVNWe– GFVeI ‘LUeeR’nanocomposites

Halloysite nanotubes (HNTs) with natural rubber NR) are used to preapre ‘gree]’ composite.To e]-hance the mechanical and thermal properties of thecomposites, HNTs are being used as a filler for poly-mers. High aspect ratio of HNT is the responsiblefactor which makes it possible to use this clay inplace of silica filler. Clays containing silica surfacescan be chemically modify by organosilanes like bis(triethoxysilylpropyl)-tetrasulphide (TESPT), and thismodification of HNT was carried out by amino si-lane (3-aminopropyl) triethoxysilane (APTES).Sometimes, A silane coupling agent, bis(triethoxysilylpropyl)-tetrasulphide (TESPT), used toenhance the dispersion and physical properties ofthese composites in natural rubber. Reinforcing ac-tivity of HNTs was superior to commercial silicacoupled with the same amount of silane couplingagent. This has further been confirmed by Trans-mission electron microscopic imaging studies ofdispersion of the HNTs in the rubber matrix, whereasX-ray diffraction studies showed a little change ininterlayer spacing between the two silicate layersof HNTs [23].

5. HALLOYSITE NANOTUBES FORCELLULAR RESPONSE

Recently nanotubes are being used for tissue engi-neering as well as macro molecular delivery sys-tems. But as far as tissue engineering is concerncompatibility of nanotubes with biological matricesis a limiting factor. HNTs has similar geometry likeCNTs but have a much lower price and most impor-tantly they are available abundantly, besides all theyare biocompatible with cellular organelles and pro-vide high mechanical strength. Alumina and silicagroups are located on the surfaces of HNTs espe-cially on their crystal edges ease the formation ofhydrogen bonding interactions between HNTs andbiological components so it may make HNTs as anideal candidate for bionanomaterial and formationof bionanocomposite film.

5.1. Halloysite as a cargo forbiomaterials

Biocompatibility of Halloysite is essential for its po-tential application in biopolymer composites, boneimplants, controlled delivery of bio molecules, andfor in vivo protective coatings.

Quantitatively trypan blue and MTT (Methyl tet-razolium salt) measurements performed with


different origins cell line (cervical adenocarcinoma,HeLa, or breast cancer cells, MCF-7 cell line) sug-gest a model systems to measure the functionswith respect to nanotubes concentration andincubation time. These studies also indicate thathalloysite exhibits a high level of biocompatibilityand very low cytotoxicity, which renders it to be to agood candidate for household materials and medi-cine. HNTs interaction with cells and their intracel-lular cell uptake by different originating cell lineshas been observed. These functionalized halloysiteby amino propyl triethosilane (APTES) orfluorescently labeled polyelectrolyte layers, allowhalloysite uptake by the cells and were observedwith confocal laser scanning microscopy (CLSM)[24].

5.2. Halloysite for osteoblasts andfibroblasts response

Poly vinyl alcohol (PVA) is a biodegradable andbiocompatible synthetic polymer. Due to presenceof hydroxyl groups in polymer side chain of PVA, ithas high water solubility as well as high cross link-ing ability (film or hydrogel forming).Thou PVA pos-ses so many desirable properties for biomedicalapplications such as dialysis membrane, macromolecular delivery systems, wound dressing, artificialcartilage, and tissue engineering scaffold as wellas mechanical properties in the dry state but itshigh hydrophilicity limits its applications to use inliving systems. To improve the biological applicationas well as for enhancement of mechanical propertiesof PVA, PVA based bionanocomposites hasdesigned. Studies on PVA and PVA/HNTs basedbionanocomposite films has shown its potentialapplications in bone tissue engineering and drugdelivery systems. There are so many nanoclayssuch as mont morillonite, kaolinite, attapulgite, andother nanoparticles such as carbon nanotubes(CNTs) and nano-scaled metal powder are shownto have a significant effect on the performances ofPVA based bionanocomposites which are attributedto the strong interactions between the nanoparticlephase and polymer matrix. HNTs comprises of alu-mina and silica group located at the crystal edges,which ease the formation of hydrogen bond betweenHNTs and PVA so it may make HNTs as an perfectcandidate for PVA bionanocomposite film. Henceinternalization of HNTs in the PVA is necessary fortheir high performance in biomedical applications.

Cross linked PVA/HNTs bionanocomposite filmsprepared via solution mixing method by using glut-araldehyde as the crosslinking agent. The morphol-

ogy and physical properties of bionanocompositefilm were investigated thoroughly. The surface to-pography and chemistry of the films was character-ized by atomic force microscopy (AFM) and attenu-ated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, respectively. Blending withHNTs induced changes in nano topography andsurface chemistry of PVA films this will further en-hance the mechanical properties of PVA by the in-corporation of HNTs.

PVA and HNTs based bionanocomposites char-acterized by cell cultured studies. Osteoblasts(MC3T3-E1) and fibroblasts (NIH 3T3) cultured onneat PVA and PVA/HNTs bionanocomposite filmsand characterized by cell viability assay. Cellmorphological studies shown that modified filmincrease the cell adhesion and in vitro growth. Thesestudies also indicated that MC3T3-E1 cell behaviourstrongly responded to surface nanotopography. Onnanotube dominant surface, cells exhibited a sig-nificantly higher level of adhesion than on neat PVAfilm, whereas neat PVA showed higher degree ofosteoblast proliferation compared with PVA/HNTs.In compendious, these results provided insights intounderstanding of PVA and PVA/HNTsbionanocomposite films for their potential applica-tions in bone tissue engineering as well as macromolecular delivery systems [25].

6. HALLOYSITE NANOTUBE FORPOLYMERIZATION REACTIONS

Clay minerals and other inorganic materials usedas a host for the polymerization of several conductivepolymers. Various nanomaterial based approachesare available for polymerization but halloysite assupport matrix for polymerization either byintercalation or by immobilization is morereproducible.

Halloysite nanotubes used as catalytic supportin different polymerization as well as variousbiological processes because of following reasons:i) Halloysite nanotube improves the catalytic activ-ity of different molecular species.ii) Halloysite not only enhances the separation ofcatalyst but also it improves the recovery of cata-lyst from reaction media.iii) Halloysite due to their ionic interaction, facilitatethe recycling as well as improve the disposal ofcatalyst.iv) Halloysite has pre-defined tubular diameter whichallows the entrance of molecules with specific sizes.This caters the shape and size selectivity to thecatalyst.


v) Compare to other clay minerals, Halloysite showhigher reactivity and higher cationic exchange ca-p a c i t y .

These characteristics of Halloysite which makethem an attractive candidate to be used for supportmatrix for immobilization of catalyst inpolymerization reactions.

6.1. Halloysite nanotubes in in-situchemical polymerization

Polyaniline coated halloysite nanotubes (PANI/HNTs) prepared by the in-situ soap less emulsionpolymerization of the anilinium chloride adsorbedhalloysite nanotubes. Polyaniline (PANI) has beenknown as one of the most technologically importantconducting polymers because of its high electricalconductivity, easy producibility and environmentalstability.

Polyaniline usually combined with differentinorganic components to form nanocomposites withimprove physical, mechanical, and electricalproperties like enhanced solubility, conductivity,magnetic and optoelectronic properties, etc.

There are number of methods has been designedfor generating the polyaniline coated inorganicnanomaterials such as nanoparticles , nanotubes ,nanobelts, and silicate clays by in situ oxidativepolymerization, gamma radiation-induced chemicalpolymerization or electro polymerization. Amongthese nanocomposites methods synthesis by en-capsulation of inorganic nanomaterials inside theshell of polyaniline are more popular.

First report of the intercalation and polymeriza-tion of aniline within Halloysite nanotube was byexposure of the Cu (II)-doped Halloysite film to anilinevapor. In this method polyaniline was coated on thesurfaces of the Halloysite nanotubes via the in-situsoap less emulsion polymerization after the aniliniumchloride was adsorbed onto the halloysitenanotubes. HNT was dispersed in aqueous acidicsolution of aniline with continuous stirring and byultrasonic irradiation with using ammoniumpersulfate (APS) as oxidant. Morphological studieson PANI/HNTs hybrids, it influenced by acidity ofpolymerizing media and adsorption of anilinium chlo-ride on halloysite nanotubes [26].

6.2. Halloysite for biomimeticpolymerization of aniline

Nanostructured material is being used for supportmaterial since long time. In recent years interesthas been shifted to use of these nanostructured

material as a support for hematin in the aniline po-lymerization. Polyaniline (PANI) is a conductivepolymer has some advantages, like easy synthe-sis, environmental stability as well as wide elec-tronic and optical properties. Various chemical aswell as enzymatic polymerization methods are avail-able for polyaniline synthesis but among availablemethods for polymerization enzyme based polymer-ization has become very attractive because of itsenvironmental friendly approach in which hydrogenperoxide used as oxidizer and it generates wateras by-product.The only drawback associated withthis method is its high cost of the enzyme.

Biomimetic catalysts such as porphyrins (iron(III) tetrasulfonated tetraphenyl porphyrin),metallophthalocyanine and of tris (pyrazolyl)boratocopper are very effective for the polymeriza-tion of aniline. But these all are synthetic in origin.Hydroxyl ferriprotoporphyrin IX, commonly known ashematin is a biomimetic catalyst from a natural ori-gin. Hematin is insoluble in acidic media so it can-not be used directly in the synthesis of PANI, how-ever, it can be used for PANI synthesis after itsfunctionalization with a water soluble polymer likepolyethylene glycol. Chemical functionalization ofhematin broadens its pH range of catalytic activity.An environmental friendly approach based on chemi-cal functionalization , hematin was immobilized onto halloysite nanotubes, which was evaluated asbiomimetic catalyst for polymerization of aniline inaqueous acidic media. This method not only en-hances catalytic activity in acidic environment butalso involves minimum purification steps [27,28].

6.3. Halloysite for the atom transferradical polymerization

Atom Transfer Radical Polymerization (ATRP) is atransition metal based catalysis. One of the limita-tion associated with this method is, it require highconcentration of catalyst to obtain acceptable ratesfor the polymerization and because of this high con-centration of catalyst present in polymerization re-action causes the polymeric solution becomecoloured, thus process requires addition of one moreadditional separation step and that may further raisethe duration as well as cost of the process. Thiscan be minimize or overcome by Immobilization ofthe catalysts on inert support materials so that cata-lyst can be easily recovered and potentially recycledafter the termination of reaction.

There are several methods used for the immobi-lization of ATRP catalysts on different supports butuse of Halloysite clays as inert matrix for immobili-


zation purpose is a subject of more interest. Firstcatalyst supported ATRP was reported by Haddleton.In this supported catalyst matrix CuBr complexeswith Pyridyl methanimine ligand and that was ab-sorbed on support either physically or covalently. Inthe beginning of polymerization, supported ligandchelate with CuBr and form supported catalyst. Thephysically adsorbed catalysts produced polymerswith narrower poly dispersities than those producedby the covalently bonded catalyst. Main limitationof this method is loss of catalytic activity and thatmay be because of loss of supported catalyst dur-ing transfer, change in Cu(I) to Cu(II) and may bedegradation of catalyst during polymerization.

Immobilization of homogeneous catalysts on thesurface of a solid support, usually results in asignificant loss of the catalytic activity of catalyst,halloysite because of their nanometer size and regu-lar shape may reduce these drawbacks of immobi-lization.

Surface chemistry of Halloysite nanotubes al-lows efficiently heterogeneous catalyst for the poly-merization of methylmethacrylate (MMA) via an atomtransfer polymerization process. Silane is used asligand for this ATRP catalyst, as silane may en-hance the physical adsorption of the catalyst onthe support [29,30].

7. HALLOYSITE NANOTUBES AS APOTENTIAL DRUG DELIVERYVEHICLE

Efficient delivery of drug requires delivering the drugat intended site with predefined rate. These days,Halloysite has recognized as potential carrier forloading of cationic agents either by chemisorptionon its polyanionic faces or by entrapment into itshollow lumen or core. Many alternatives like Lipidmicrotubules or carbon nanotubules have similarmorphology sa well as similar drug loading capasityto Halloysite nanotubes but Halloysite because oftheir abundance and various surface properties area good candidate for the drug loading.i) Bi layered alig]me]t gives Halloysite’s ]a]otubesa cylindrical shape.ii) The inner and outer faces of tubular walls ofhalloysite nanotubes carries a net negative chargeso it function as polyvalent anion.iii) Halloysite nanotubes with amphoteric edges givenegative charge at high pH and positive charge atlow pH.iv) Unusual shape and charge distribution ofHalloysite’s favours face-to-edge attachme]t i]

aqueous suspension at slightly alkaline pH and fa-cilitates binding particularly of cations to unreactedfaces.

All these properties of Halloysite nanotubes makethe Halloysite a possible candidate for drug loadingeither with entrapping agents within the lumen oftubules using retardant polymers or by cationiccoating and other approaches for moderate releaserate, or by swapping intercalated water if presentfor low molecular weight agents.

Recently, Halloysite is also being tried for sus-tained delivery of drugs and one of these studiesdone by using diltiazem HCl as a model drugbecause of cationic nature of diltiazem HCl, itfacilitate binding with Halloysite faces and high solu-bility help in loading of drug into lumen of halloysite.But higher diffusion rate of diltiazem make it difficultto achieve adequate sustain release action.

A range of cationic polymers like polyvinylpyrolidone, chitosan cross linked with glutaradehydealso bind to Halloysite and used to achieve signifi-cant delayed drug release.

Beside these a range of non aqueous solventlike alkyl-2-cyanoacrylate and poly-iso-butylcyanoacrylate are also effective to reduce bust effectreported with aqueous coating systems [30,31].

Different nanomaterials used for entrapping ofdrugs but clay minerals provide dispersions atsubmicron level in aqueous media. Entrapping ofdrug molecules in micro- and nanoparticulatesystems is a helpful strategy for protecting drugsagainst the chemical as well as enzymaticdegradation, enhancement of aqueous solubility, toreduce dissolution rate and target drug release.

Among various clays minerals, Halloysite withhollow microtubule has been recently proposed asa natural vehicle for microencapsulation andcontrolled release of both hydrophilic andlipophilic drugs. 5-Aminosalicylic acid [5-ASA] isan anti-inflammatory drug. In the treatment ofulcerative colitis, it is require delivering 5-ASA atlarge intestine.

But problem associated with 5-ASA is, it rapidlyabsorbed at small intestine, and there is littlelocalization of the drug in the colon. So to get therequire effect of 5-ASA and to deliver the drugcontent at colon, 5-amino salicylic acid [5-ASA]adsorbed on Halloysite. Kinetic and equilibrium stud-ies of halloysite absorbed 5-ASA are comparable atdifferent temperatures and drug concentrations[32,33].


8. HALLOYSITE NANO TUBES AS ANIMMOBILIZATION MATRIX

Halloysite nanotubes used to mitigate the degrada-tion of enzymes and metal ions through their immo-bilization or encapsulation on to supports like assilica, zeolites, cationic exchanger montmorilloniteclay, silanized kaolinite, intercalated/delaminatedkaolinite, chrysotile, and other matrices. These tu-bular nanotubes can easily intercalate water mono-layers as well as other organic and inorganic com-pounds such as formamide. In among other avail-able clay minerals, Halloysites has their own ben-efits to use for immobilization.i) Metal complexes can be immobilized on Halloysitenanotubes both interior as well as extererior sur-face of tubular structure.ii) Halloysite associated immobilization are barredfrom undesirable effects like molecular aggregationand bimolecular self-destruction.iii) Halloysite nanotubes based immobilization fa-cilitates the recovery of catalyst so cost of materialpreparation get reduced.iv) Halloysite nanotubes based immobilization isstable chemically as well as thermally beside theyare having good mechanical properties. They arebased on non-toxic elements (Al and Si), so theyalso non toxic.v) Halloysite because of its pre-defined diameter,allowing the entrance of molecules with specificsizes that causes it gives shape and selectivity tothe catalyst.vi) Halloysite’s give regioselectivity to catalyst whe]only part of molecule allow penetrating the tube andcoming in contact with active site which are notselective in homogeneous media.

Halloysite offers higher reactivity and highercationic exchange capacity as compared to otherclays so it makes the Halloysite a potentially usefulsupport for the immobilization of catalytically activemolecules.

8.1. Halloysite for immobilization ofsilver nanoparticle

Silver (Ag) nanoparticles extensively used becauseof their catalytic potential. However, limitation as-sociated with silver nanoparticles guided catalysisis their recovery from the reacting mixtures aftercompletion of reaction. In order to reuse the cata-lysts or enhance their catalytic activities, silvernanoparticles can be immobilized onto the varioussupports such as a clay minerals, carbon nanotubesand polymeric materials. Halloysite nanotubes used

as support to immobilized silver nanoparticles basedcatalyst. The silver nanoparticles of about 10 nmdiameter were immobilized onto the halloysitenanotubes (HNTs) via the in situ reduction of AgNO3

by the polyol process. This immobilized matrix ofsilver nanoparticles used for the reduction of aro-matic nitro compound, 4-nitrophenol (4-NP) in pres-ence of NaBH4 in alkaline aqueous solutions [33].

8.2. Halloysite for immobilization ofmetalloporphyrins

Metalloporphyrins are macrocyclic complexes withability to perform hydrocarbon oxidation undermodest conditions. Iron (III) and manganese (III)based porphyrins have been the most flourishingmetalloporphyrins used for this purpose. Theproblem associated with these metalloporphyrinsystems used for hydrocarbon hydroxylation andepoxidation are degradation of oxidative catalyst inthe presence of inert substrates. There are manyapproaches used to mitigate the degradation ofmetalloporphyrin either through their immobilizationor encapsulation on to supports like silica, zeolites,montmorillonite clay, silanized kaolinite, interca-lated/delaminated kaolinite, chrysotile, and othermatrices.

There are many benefits associated withImmobilization of metalloporphyrin.a) Site specific isolation of the metal complex reducethe self-destruction of catalyst and dimerization ofresultant metalloporphyrin complex.b) Facilitated and efficient isolation of theheterogeneous catalyst from the reaction media.c) Catalyst can work as flow reactor cells.d) Catalyst can be recycle, which minimizes envi-ronmental impact.

Selectivity of oxidation reactions depend on themanner of metalloporphyrin attached to supportmatrix and this selectivity difference govern bydifferent binding modes. This matalloporphyrincontrol the formation structure of pore as well asthree dimensional network of matrix. Many types oflayered material, fibres and nanotubes or nanoscrollsare potential material for immobilization and encap-sulation of metallocomplexes. By immobilization,the homogeneous active species is transformed intoa heterogeneous catalyst, but their selectivity andactivity retained.

Halloysite, because of its crystal shape andgeometry offers vast opportunities for immobiliza-tion of metalloporphyrin. It takes place not only atthe surface but also inside the tubes thus leading


to highly active and selective oxidation catalysts.Immobilization of metalloporphyrin on Halloysite canbe performed by either under pressure or under stir-ring /reflux [34].

9. HALLOYSITE NANOTUBES FORREMEDIATION

Clays are being considered as promising adsorbentsdue to their accessibility, least cost constraint, largesurface area and high adsorption capacity. Numer-ous clays, such as perlite, dolomite, nontronite,montmorillonite, bentonite, zeolite, and sepiolitehave been tested for their adsorbtion ability for dyes.However, their local availability and nano-size in con-trast with other materials HNTs show greater prom-ise.

9.1. Halloysite and dyes

Dyes are widely used in industries such as tex-tiles, pulp mills, leather, synthesis of dyes, print-ing, food, and plastics, etc. Since many organicdyestuffs are harmful to human being and their dis-charge into water causes environmental problems,toxicity to aquatic as well as non aquatic microor-ganisms, and carcinogenicity. So the removal ofdyestuffs from wastewater has required with greaturgency. The dyes has broad spectrum of activitydue to their chemical structures which are primarilybased on the substituted aromatic groups. This com-plex chemical structure of dyes are responsible fac-tor because of which they are resistant to break-down by chemical, physical and biological means.Furthermore, any degradation by physical, chemi-cal or biological methods may produce small amountof toxic and carcinogenic products. Adsorption isknown to be a promising technique, which has greatimportance due to ease of operation and compa-rable low cost of involve in the discoloration pro-cess.

Adsorption has been developed to remove colourfrom dye-containing effluents and it is relatively low-cost technique for the treatment of dye contami-nated waste water. Halloysite nanotubes (HNTs), alow-cost available clay mineral, were tested for theability to remove cationic dyes such as Neutral Red(NR), Methylene blue, Malachite green and methylviolet from aqueous solution. Natural HNTs used asadsorbent and their adsorption potential increasedwith increase in dose of adsorbent, initial pH, tem-perature and initial concentration [35-37].

9.2. Halloysite for intercalation

The capability of minerals of the kaolin group to in-tercalate organic and inorganic compounds hasbeen evidenced long ago. Among the various mem-bers of the kaolin group, halloysite differs by thepresence of interlayer water and the morphology ofthe crystal, which are curved or rolled structure.Halloysite has been intercalated by a variety of guestmolecules such as amides, dimethyl sulfoxide, po-tassium acetate, aniline, hydrazine andformamide.

Numerous studies performed to differentiatesHalloysite from kaolinite as well as give understand-ing of intermolecular interactions arising in theinterlayer space between the intercalated moleculeand the host matrix. Clay minerals have a greatpotential to adsorb pollutants, such as heavy metalions and organic compounds. The AlgerianHalloysite, which is also a type of Halloysite werealso characterized for study the intercalation of Na+,NH4+, and Pb2+ acetate and apply these solids forremoving copper (II) ions from aqueous solutions.

Algerian Halloysite can be modified with Na+,NH4+, and Pb2+ acetate and by XRD and FTIRstudies also confirmed that intercalation exceed withtime. The adsorption of copper ions was explainedby electrostatic interaction between the copper (II)ions and negatively charged binding sites onhalloysite surface and ion-exchange of the cationsassociated with acetate within the interlayer space[38].

9.3. Halloysite nanotubes basedhydrogel as an adsorbent

Halloysite (HT) comprises of two-layered aluminiumsilicate and presence of water between the layerscreate a packing engendering causing them to curvebecause of this Halloysite can be used as an inor-ganic component to prepare polymeric hydrogelbased composite. These polymeric hydrogels usedas adsorbents for the removal of hazardous metalions or ionic dyes. Basically, these hydrogels arehydrophilic in nature but they do not dissolved inwater, making them easily separated from the aque-ous solution. Furthermore, due to presence of a largenumber of multi functional ionic groups within thesehydrogels give them surprising affinity for pollutantsin the surrounding water environment.

Earlier, there was no single evidence availablefor using this type of hydrogel adsorbent to removeammonium (NH+4) which is the inorganic ion form of


nitrogen pollution and is one of the responsible fac-tors for causing eutrophication. Nanocompositebased on, chitosan was used as the backbone tograft poly (acrylic acid) to form granular hydrogelcomposite with halloysite (HT) particles being em-bedded within the polymeric networks. And this alsoshow by introducing HT particles into the polymericnetworks, the resulting composite shows compa-rable adsorption capacity to that of pure polymerhydrogel for NH+4 removal [39].

10. HALLOYSITE NANO TUBE BASEDFABRICATION

HNTs have immense biological and non-biologicalapplications. They are being used for storingmolecular hydrogen, catalytic conversions,hydrocarbon processing, improvement of dispersion,differentiation studies and as an enzymaticnanoreactor. Mechanistic theories of composite for-mation explain that high content of clay incorpora-tion is necessary for prominent and heightened me-chanical strength of composites. However, incorpo-ration of large amount of clay with polymer in fabri-cation of nano composites result in poor dispersionand that leads to poor mechanical as well as opti-cal properties. Theses problem can be minimise byincorporation of Halloysite in fabrication of compos-ites. Naturally occurring, nanotubular Halloysite areused to reinforce the strength of the composite. Stud-ies also proves incorporation of up to 65%nanotubular clay exhibit high mechanical as wellas heat resistance [40].

Tubular halloysite also used as a template tofabricate Metallic Ni film or Nanoparticles throughthe process of electroless plating. Resultantproperties of material strongly depend on its micro-and nanostructure. Metallic nanoparticles or wiresare having useful electronic, magnetic, and catalyticproperties because of their nanoscopic size andshape. These all structures are utilized in templatedirected synthesis. For the arrangement ofnanocluster or wire artificial template like carbonnanotubes, ordered arrays of alumina, and porouspolymer membranes are used. Biological templates,like lipids, DNA, protein S-layers and microtubulesare used to conduct the nucleation, deposition andassembly of inorganic nanoparticles. These studiesfocused on the artificial as well as biomoleculartemplates, but naturally available tubular halloysiteis being widely used as template. Halloysitenanotube based templates are requiring activationbefore electroless plating to perform deposition ofNi nanoparticles on the outer surface and wires on

inner side cavity of the halloysite. This process wasi]itiated by prese]ce of colloidal S]–Pd acid solu-tion. Pd ions get reduced by methanol at halloysitesurface and that causes Ni particle deposited [41].

10.1. Non covalent functionalizationSK 5:T’V

Dispersion via non-covalent functionalization of car-bon nanotubes and biomolecules like protein, nucleicacid, amylase, starch and gum Arabic have beenmuch explored. This approach is based on directcontact between nanotubes and a biomolecules.Halloysite nanotubes have aluminols inside the tu-bular structure and siloxane groups outside thenanotubes beside these few silanols and aluminolsare exposed on the edges of the rolled sheets ofthe tubes. Combination of tubular geometry andsurface characteristics make the Halloysitenanotubes a good candidate for better dispersivitycompared to other clays.

Biocompatibility of Halloysite nanotubes can beimproved by non-covalent functionalization of HNTswith biomolecules. In one kind of green approachby application of mechanical force a supramolecu-lar complex of DNA and HNT and amylose and HNTin solid state were prepared. Thus formed DNA oramylose wrapped halloysite nanotubes show bet-ter dispersion in the DMSO/H2O solution [42,43].

10.2. Layer-by-layer coatings ofHalloysites on wood microfibers

Layer-by-layer (LbL) assembly approach used todeposit multilayer of nanoparticles coating onsoftwood fibres. Basic utility of this method is in thearea of pulping, paper processing and paper coating.This approach creates thin film of nano meter di-mension on large surfaces as well as on microfibersand cores. Lignocellulosic fibers because of theirsurface properties are necessary component of pa-per design as well as their quality enhancement.These fibers are negatively charged fibres but hy-drogen bonding between cellulosic fibres providesstrength to paper during paper formation and dry-ing. Recently Halloysite nanotubes were coatedthrough controlled layer-by-layer approach on woodfibres by alternating them with oppositely chargedpolyelectrolytes. By coating on fiber surface withcontinuous layers of nanoparticles changes their sur-face properties, and abolish the possibility of hy-drogen bond formation at the contact region betweenneighbouring fibers [44].


In other approach, LbL coating of lignocellulosefibers applied to substitute hydrogen bonding amonglignocellulose fibers by electrostatic attractions withsimultaneous application of positive and negativepolyelectrolyte fiber coatings. Resultant electrostaticbonding between fibers drastically enhances thestrength of paper sheets. This LbL basednanoassembly employed in great variety ofsubstances including linear or branched polyelec-trolytes, nanoparticles, and proteins.

10.3. Halloysite for differentiationstudies

Halloysite are ubiquitous clay which occurs in avariety of particle shapes and hydration states.Halloysite usually found with other clay minerals sothey are difficult to differentiate.

Halloysite can intercalate a monolayer of watermolecules but this interlayer water is weakly helda]d givi]g a basal spaci]g ]ear 10 Å to resulta]t]a]otubes. Halloysite with - 10 Å) form ca] readilydehydrate to give the Halloysite - 7 Å) form a]d thisform is very much similar to kaolinite. In compari-son to other clay minerals, Halloysite either hydratedform or dehydrated form has a greater propensityfor intercalating organic molecules [45].

Numerous intercalation methods have been de-veloped to ide]tify a]d qua]tify Halloysite- 7 Å) fromother clays as well as kaolinite mixtures. Basis ofthese methods are differences in the rate and ex-tent of formamide intercalation between Halloysiteand kaolinite.

Intercalation of formamide within the Halloysiteis conclusive for naturally dehydrated Halloysitesbut it is inconclusive for Halloysites with prior undergo]e ove] dried at 110 °C.

This also applies to some halloysites that havebee] dried at 40 °C because some water alwaysremains present in the interlayer space [46].

11. CONCLUSIONS

Halloysite nanotubes are not very common in na-ture being found in many subtropical and tropicalsoils, within individual origin and descent, a signifi-cant divergence arises in Halloysite morphology, de-pending on the exact forces of nature involved intheir formation. Minerals show silently different prop-erties in terms of size, surface area and porosityand indicating the possibility of potentially differentgrades of Halloysite available, some of which maybe suitable for biological applications and some maybe for non biological applications. It is very difficult

to summarize all the applications of Halloysites butfew of them are become very important componentfor fabrication, corrosion prevention, polymerization,immobilization, thermostability, macro moleculardelivery etc.

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multifarious applications of halloysite … · in this approach corrosion coat- ... fective...

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