Patent Application: US-201715635159-A

Abstract:
a simple spray coating process can be utilized to create epoxy / hnt nanocomposites with vertically aligned nanotubes . important mechanical properties such as modulus and hardness values can be optimized and enhanced by controlling the level of nanotube dispersion during processing and the final orientation of the nanotubes . thus , a technologically relevant processing scheme can be used to fabricate hnt nanocomposites with a high level of control over nanotube alignment and the resulting mechanical properties .

Description:
one method to reinforce a polymer matrix is to include stiff and strong nanosized elements , such as carbon black particles , carbon nanotubes and graphene , inorganic particles of clay and metal oxides and bio - fillers of cellulose and wood . see , qian , h . ; greenhalgh , e . s . ; shaffer , m . s . p . ; bismarck , a ., carbon nanotube - based hierarchical composites : a review . journal of material chemistry 2010 , 20 , 4751 - 4762 , cai , d . y . ; song , m ., recent advance in functionalized graphene / polymer nanocomposites . journal of materials chemistry 2010 , 20 , 7906 - 7915 , azeez , a . a . ; rhee , k . y . ; park , s . j . ; hui , d ., epoxy clay nanocomposites &# 39 ; processing , properties and applications : a review . composites part b : engineering 2012 , and khalil , h . a . ; bhat , a . ; yusra , a . i ., green composites from sustainable cellulose nanofibrils : a review . carbohydr . polym . 2012 , 87 , 963 - 979 , each of which is incorporated by reference in its entirety . these fillers can be categorized as one - dimensional ( 1d tubes ), two - dimensional ( 2d sheets ) and three - dimensional ( 3d particles ) materials based on their geometric features . see , dresselhaus , m . s ., fifty years in studying carbon - based materials . physica scripta 2012 , 2012 , 014002 , which is incorporated by reference in its entirety . among them , 1d tubular nanoparticles have been attractive , in part owing to their anisotropic properties . see , sajanlal , p . r . ; sreeprasad , t . s . ; samal , a . k . ; pradeep , t ., anisotropic nanomaterials : structure , growth , assembly , and functions . nano reviews 2011 , 2 , 4 , which is incorporated by reference in its entirety . an example would be the dependence of young &# 39 ; s modulus and tension / compression strength on the direction of mechanical load in 1d nanoparticle filled composites . to fully exploit useful anisotropic properties , control of preferential particle alignment is essential and a widely - studied issue in composite reinforcement , with focus on fabricating both in - plane and out - of - plane oriented assemblies . dispersions of nanotubes in appropriate fluid media and use of either ( i ) exterior field forces ( electric field or magnetic field ), or ( ii ) shear forces induced by extrusion / injection flow have been explored with a variety of systems . see , lan , y . ; wang , y . ; ren , z . f ., physics and applications of aligned carbon nanotubes . advances in physics 2011 , 60 , 553 - 678 , martin , c . ; sandler , j . ; windle , a . ; schwarz , m . - k . ; bauhofer , w . ; schulte , k . ; shaffer , m ., electric field - induced aligned multi - wall carbon nanotube networks in epoxy composites . polymer 2005 , 46 , 877 - 886 , kaida , s . ; matsui , j . ; sagae , t . ; hoshikawa , y . ; kyotani , t . ; miyashita , t ., the production of large scale ultrathin aligned cnt films by combining ac electric field with liquid flow . carbon 2013 , 59 , 503 - 511 , camponeschi , e . ; vance , r . ; al - haik , m . ; garmestani , h . ; tannenbaum , r ., properties of carbon nanotube - polymer composites aligned in a magnetic field . carbon 2007 , 45 , 2037 - 2046 , kimura , t . ; ago , h . ; tobita , m . ; ohshima , s . ; kyotani , m . ; yumura , m ., polymer composites of carbon nanotubes aligned by a magnetic field . advanced materials 2002 , 14 , 1380 - 1383 , sulong , a . b . ; park , j ., alignment of multi - walled carbon nanotubes in a polyethylene matrix by extrusion shear flow : mechanical properties enhancement . j . compos mater . 2011 , 45 , 931 - 941 , ahadian , s . ; rámon - azcon , j . ; estili , m . ; liang , x . ; ostrovidov , s . ; shiku , h . ; ramalingam , m . ; nakajima , k . ; sakka , y . ; bae , h . ; matsue , t . ; khademhosseini , a ., hybrid hydrogels containing vertically aligned carbon nanotubes with anisotropic electrical conductivity for muscle myofiber fabrication . scientific reports 2014 , 4 , 4271 , ramón - azcón , j . ; ahadian , s . ; estili , m . ; liang , x . ; ostrovidov , s . ; kaji , h . ; shiku , h . ; ramalingam , m . ; nakajima , k . ; sakka , y ., dielectrophoretically aligned carbon nanotubes to control electrical and mechanical properties of hydrogels to fabricate contractile muscle myofibers . advanced materials 2013 , 25 , 4028 - 4034 , erb , r . m . ; libanori , r . ; rothfuchs , n . ; studart , a . r ., composites reinforced in three dimensions by using low magnetic fields . science 2012 , 335 , 199 - 204 , martin , j . j . ; fiore , b . e . ; erb , r . m ., designing bioinspired composite reinforcement architectures via 3d magnetic printing . nature communications 2015 , 6 , jalili , r . ; razal , j . m . ; wallace , g . g ., wet - spinning of pedot : pss / functionalized - swnts composite : a facile route toward production of strong and highly conducting multifunctional fibers . scientific reports 2013 , 3 , 3438 , and veedu , v . p . ; cao , a . ; li , x . ; ma , k . ; soldano , c . ; kar , s . ; ajayan , p . m . ; ghasemi - nejhad , m . n ., multifunctional composites using reinforced laminae with carbon - nanotube forests . nat mater 2006 , 5 , 457 - 462 , each of which is incorporated by reference in its entirety . other research has focused on composite fabrication directly from an aligned nanotube forest or an array synthesized by the chemical vapor deposition method ( cvd ) or electrochemical deposition . see , zhang , m . ; fang , s . ; zakhidov , a . a . ; lee , s . b . ; aliev , a . e . ; williams , c . d . ; atkinson , k . r . ; baughman , r . h ., strong , transparent , multifunctional , carbon nanotube sheets . science 2005 , 309 , 1215 - 1219 , and chen , t . ; cai , z . ; qiu , l . ; li , h . ; ren , j . ; lin , h . ; yang , z . ; sun , x . ; peng , h ., synthesis of aligned carbon nanotube composite fibers with high performances by electrochemical deposition . journal of materials chemistry a 2013 , 1 , 2211 - 2216 , each of which is incorporated by reference in its entirety . carbon nanotubes are popular in these methods due to their flexibility , and drawability from a growth substrate . see , jiang , k . ; li , q . ; fan , s ., nanotechnology : spinning continuous carbon nanotube yarns . nature 2002 , 419 , 801 - 801 , which is incorporated by reference in its entirety . naturally generated mineral clays such as montmorillonite , mica , talc , kaolinite and halloysite , however , are mechanically brittle and hard to synthesize using cvd . in addition , most of these mineral particles are much cheaper as compared to syntheses of carbon - based analogs . see , sivamohan , r ., the problem of recovering very fine particles in mineral processing — a review . international journal of mineral processing 1990 , 28 , 247 - 288 , which is incorporated by reference in its entirety . halloysite nanotubes ( hnts ), a naturally occurring clay mineral with a one - dimensional hollow cylindrical structure , are exceptionally stiff and hard for their ceramic chemical composition . see , guimaraes , l . ; enyashin , a . n . ; seifert , g . ; duarte , h . a ., structural , electronic , and mechanical properties of single - walled halloysite nanotube models . the journal of physical chemistry c 2010 , 114 , 11358 - 11363 , which is incorporated by reference in its entirety . the presence of the hollow lumen in hnts has also been extensively studied regarding their drug carrier / release properties and nanoreactor potential . see , lvov , y . m . ; shchukin , d . g . ; mohwald , h . ; price , r . r ., halloysite clay nanotubes for controlled release of protective agents . acs nano 2008 , 2 , 814 - 820 , levis , s . ; deasy , p ., characterisation of halloysite for use as a microtubular drug delivery system . international journal of pharmaceutics 2002 , 243 , 125 - 134 , and shchukin , d . g . ; sukhorukov , g . b . ; price , r . r . ; lvov , y . m ., halloysite nanotubes as biomimetic nanoreactors . small 2005 , 1 , 510 - 513 , each of which is incorporated by reference in its entirety . hnts have low surface charge and can be well dispersed in solvents and polymers of medium to high polarity . significant mechanical and thermal improvements have been demonstrated in starch , chitosan , gelatin , cellulose , pectin , and polyvinyl alcohol based composites . see , gaaz , t . s . ; sulong , a . b . ; akhtar , m . n . ; kadhum , a . a . h . ; mohamad , a . b . ; al - amiery , a . a ., properties and applications of polyvinyl alcohol , halloysite nanotubes and their nanocomposites . molecules 2015 , 20 , 22833 - 22847 , and rawtani , d . ; agrawal , y ., multifarious applications of halloysite nanotubes : a review . rev . adv . mater . sci 2012 , 30 , 282 - 295 , each of which is incorporated by reference in its entirety . to achieve their maximum potential as reinforcing agents in many applications , it is essential to control the orientation of the nanotubes and eliminate random distributions of tube orientations . the misalignment of particles will cause inefficiency in stress transfer , and cause the properties of hnt filled nanocomposites to be far below theoretical predictions . see , song , k . ; zhang , y . ; meng , j . ; green , e . c . ; tajaddod , n . ; li , h . ; minus , m . l ., structural polymer - based carbon nanotube composite fibers : understanding the processing - structure - performance relationship . materials 2013 , 6 , 2543 - 2577 , and xie , x . l . ; mai , y . w . ; zhou , x . p ., dispersion and alignment of carbon nanotubes in polymer matrix : a review . materials science & amp ; engineering r - reports 2005 , 49 , 89 - 112 , each of which is incorporated by reference in its entirety . of particular interest is the development of a facile process for producing nanocomposites in which the nanotubes are vertically aligned . fabrication methods that produce well - controlled , out - of - plane orientations provide an opportunity to fabricate 3d reinforced nanocomposites with highly directional properties . such nanotube arrangements , for example , can be exploited to generate well - defined nanotemplates or patterns for desirable magnetic , electrical and barrier properties in a cost - efficient way . see , wang , z . l ., zinc oxide nanostructures : growth , properties and applications . journal of physics : condensed matter 2004 , 16 , r829 , which is incorporated by reference in its entirety . a number of approaches have been used to control the orientation of nanotubes during processing . hydrodynamic flow has been demonstrated to produce unidirectional cellulose , mammalian motile cilia , and carbon nanotube based materials . see , håkansson , k . m . ; fall , a . b . ; lundell , f . ; yu , s . ; krywka , c . ; roth , s . v . ; santoro , g . ; kvick , m . ; wittberg , l . p . ; wågberg , l ., hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments . nature communications 2014 , 5 , guirao , b . ; meunier , a . ; mortaud , s . ; aguilar , a . ; corsi , j . - m . ; strehl , l . ; hirota , y . ; desoeuvre , a . ; boutin , c . ; han , y . - g ., coupling between hydrodynamic forces and planar cell polarity orients mammalian motile cilia . nature cell biology 2010 , 12 , 341 - 350 , and majumder , m . ; chopra , n . ; andrews , r . ; hinds , b . j ., nanoscale hydrodynamics : enhanced flow in carbon nanotubes . nature 2005 , 438 , 44 - 44 , each of which is incorporated by reference in its entirety . aerodynamic flow has been used in fabricating one - dimensional anisotropic materials , especially considering its industrial application in air - jet fiber spinning . see , angelov a , r ., air - jet spinning . advances in yarn spinning technology 2010 , 315 , which is incorporated by reference in its entirety . particle alignment mechanisms in both air and aqueous flow have also been thoroughly studied in theories and simulations . see , papthanasiou , t . ; guell , d . c ., flow - induced alignment in composite materials . elsevier : 1997 , which is incorporated by reference in its entirety . to date , there have been no studies demonstrating the controlled alignment of nanotubes via hydrodynamic flow in a spray coating process . disclosed herein is a method of making a coating comprising preparing a surface and spraying a mixture of a polymer with a plurality of nanotubes through a nozzle onto the surface . also disclosed is a coating including a polymer composite including a polymer and a pluraility of nanotubes aligned vertically to a surface . the polymer composite can be reinforced by controlling the orientation of nanotubes . in certain embodiments , the fiber in polymer matrix can be oriented parallel to loading direction ( i . e . vertical to the surface ) by a spray coating process . a spray coating process was used to control hydrodynamic flow to align the nanotubes . at the same time the elevated levels of viscosity in nanotubes suspensions preserved the nanotube orientations upon impacting the substrate surface . in certain embodiments , halloysite nanotube - filled epoxy composites can be fabricated using spray - coating methods . the halloysite nanotubes ( hnts ) can be aligned by the hydrodynamic flow conditions at the spray nozzle , and the polymer viscosity can help to preserve this preferential orientation in the final coatings on the target substrates . electron microscopy demonstrated a consistent trend of higher orientation degree in the nanocomposite coatings as viscosity increased . the nanoindentation mechanical performances of these coatings were studied using a hysitron tribindenter device . composites showed improvements up to ˜ 50 % in modulus and ˜ 100 % in hardness as compared to pure epoxy , and the largest improvements in mechanical performance correlated with higher alignment of hnts along the plane normal direction . this study has revealed favorable levels of anisotropic mechanical properties , mainly induced by particle orientation . achieving this nanotube alignment using a simple spray - coating method suggests potential for large - scale production of multifunctional anisotropic nanocomposite coatings on a variety of rigid and deformable substrates . the as - obtained halloysite nanotubes were characterized using a variety of methods to determine their physical and chemical properties . fig1 a shows the schematic crystal structure associated with the aluminosilicate composition of halloysite nanotubes , the outer layer is silica and the inner layer is alumina . the halloysite nanotubes used in this work have inner diameters of ˜ 15 nm , outer diameters of ˜ 40 nm , length ˜ 2 microns , and an average aspect ratio of around 50 , as shown in fig1 b . the rolled - up structure is clearly shown in fig1 c . the inter - layer spacing of ˜ 0 . 7 nm was confirmed by waxd measurements ( fig1 d ). see , joussein , e . ; petit , s . ; churchman , j . ; theng , b . ; righi , d . ; delvaux , b ., halloysite clay minerals — a review . clay minerals 2005 , 40 , 383 - 426 , which is incorporated by reference in its entirety . based on this number and the inner / outer diameter , the average number of layers in these hnts was calculated to be around 17 . a moisture content in ambient conditions of about 1 . 5 wt % ( insert in fig1 e ) was determined using thermogravimetric analysis ( tga ). the hnts showed stepwise thermal transitions centered around 150 and 500 ° c . the first transition is associated with the loss of freestanding water ( surface and interlayer ), and the second is assigned to dehydroxylation towards metakaolin , al 2 si 2 o 7 . a pristine hnt is al 2 si 2 o 5 ( oh ) 4 . n ( h 2 o ), where n = 0 for the dehydrated form and n = 2 for the hydrated form . this phase transformation as well as thermal stability can be seen from tga experiments . fig1 e shows the thermal profile of the hnts and the pure epoxy used in this study as a comparison from room temperature up to 900 ° c . the final degradation residue for the hnts is 84 . 5 wt % and for epoxy is 2 . 8 wt % in fig1 e . hnt filled epoxy composites were prepared using a simple spray coating technique , as shown in fig2 . batch suspensions of mixed epoxy / acetone / hnt are sprayed out of a spraying gun , and compressed air facilitates hnt alignment in the hydrodynamic flow . the viscosity of epoxy / acetone constrains the movement of hnt and preserves their preferential orientations . the particles upon exiting the spraying nozzle were accelerated in a gas stream and aligned along the flow . the particle movement was constrained within the viscous polymer fluid as the liquid suspension impacted the surface of the substrate . two parameters , namely the viscosity of the polymer suspension and particle concentration , were studied to determine their influence on the particle alignment within the polymer matrix . the sem images of fig3 show that the alignment of hnts along the plane - normal direction in the sprayed composites increases with increasing viscosity of the liquid suspension . the addition of acetone in spray coating decreases the viscosity , and can improve the ease of processing through high spraying speeds and more continuous ejection of liquid suspensions under the same pressure . however , in this study the decreased viscosity weakens the constraining of hnts in as - sprayed films and the flow - induced orientation that develops at the nozzle exit is not maintained in the composites processed from low viscosity suspensions . as shown in fig3 , all hnts in e 77 a 23 ( 23 vol % acetone and viscosity of 0 . 03 pa · s ) based composites are horizontally oriented within the coating plane direction ; in contrast , when samples were sprayed from pure epoxy - based e 100 a 0 suspensions ( 0 vol % acetone and viscosity of 1 pa · s ), the alignment of hnts changes from in - plane orientations towards the plane - normal direction ( z - axis as labeled in fig3 , section ( c ). as noted above , viscosity plays a significant role in the degree of orientation of the halloysite nanotubes in these spray - processed coatings . as shown in fig4 , the addition of acetone has a very strong effect in the reduction of the solution viscosity . in contrast , the addition of hnts led to relatively small changes in the viscosity of the suspensions . this is consistent with the observation as shown in fig3 . on one hand , an increase in viscosity from 0 . 03 to 1 pa · s produced composites displaying the most significant change in orientation , namely , from in - plane to out - of - plane . on the other hand , the addition of hnts only weakly modified the degree of particle alignment . as mentioned above , the 1d hnt nanoparticles tend to align themselves as they exit the nozzle . however , particles initially orientated out - of - plane in as - formed composites tend to relax to in - plane distributions , favoring a higher entropic state . the viscosity of the fluids used in the spraying process was the main factor that influences the timescale of this relaxation / disorientation phenomenon . under those specific rheological scenarios ( tested with shear rates of 631 s − 1 at 23 ° c .) as shown in fig4 , the time scale for halloysite to finally reach relaxed states was analyzed based on simple fluid mechanical principles . the addition of acetone to epoxy greatly lowered the viscosity ; as a comparison , the increase of hnt loadings did not change the viscosity much in suspensions containing up to 10 . 2 vol % acetone . the analysis considers single halloysite nanotubes ( diameter of 40 nm and length of 2 μm ) in a newtonian fluid ( viscosity values taken from fig4 ). external forces of gravity , buoyancy , and stoke &# 39 ; s drag were used to solve the equation of motion . the obtained reorientation time is plotted in fig1 . fig1 shows relaxation time from vertical to horizontal alignment for a single particle ( i . e ., diameter of 40 nm and length of 2 μm and viscosity values at shear rate of 631 s − 1 were taken from fig4 ) in various viscous fluids . in low viscosity processing solutions containing significant amounts of acetone ( i . e ., e 40 a 60 ) the particle relaxation time scale is on the order of 0 . 1 s . this short time for particle reorientation eliminates the possibility of transferring the as - sprayed coatings to a curing oven to preserve hnt orientation . on the other hand , for high viscosity processing solutions , rich in the epoxy component , it takes around 20 mins for the halloysite nanotubes to settle down , which provides a flexible time window for further processing ( oven curing in the present work ). it is worth mentioning that , although a single particle model was proposed in this model , the argument still holds for strongly interactive filler bundles . improved particle alignment can be achieved by ‘ crowding effects ’ ( see , xu , m . ; futaba , d . n . ; yumura , m . ; hata , k ., alignment control of carbon nanotube forest from random to nearly perfectly aligned by utilizing the crowding effect . acs nano 2012 , 6 , 5837 - 5844 , which is incorporated by reference in its entirety ), that is , increasing confinement from neighboring particles will enhance the degree of particle orientation . on the other hand , as shown below , the appearance of aggregates can have a deleterious effect on composite properties even though excellent nanotube orientation is preserved . the orientation of particles greatly influences the composite properties . see , derek hull ; clyne , t . w ., introduction to composite materials . 2nd ed . ; cambridge university press : cambridge , 1996 , which is incorporated by reference in its entirety . the young &# 39 ; s modulus ( e ) derived from equation 3 based on the measured reduced modulus ( e r ) values are plotted as a function of nanotube loadings in fig5 a . the measured hardness values are plotted in fig5 b . fig5 b displays consistent increasing trends with higher hnt content up to 1 vol % for formulation of specific epoxy and acetone contents . composite properties also displayed improvements with higher epoxy content ( i . e ., in more viscous sprayed suspensions with less acetone content ). these two mechanical properties of the sprayed and cured films composites showed significant increases from the pure epoxy control up to 1 vol %. beyond this concentration , both modulus and hardness values displayed a plateau region , suggesting that 1 vol % represents the percolation threshold for these hnt formulations . in the overall set of data there are modulus values that exceed the epoxy control by more than 50 %. comparable levels of enhancement in tensile modulus have been achieved for epoxy using 7 . 2 % 3d nanosilica particles . see , brunner , a . j . ; necola , a . ; rees , m . ; gasser , p . ; kornmann , x . ; thomann , r . ; barbezat , m ., the influence of silicate - based nano - filler on the fracture toughness of epoxy resin . engineering fracture mechanics 2006 , 73 , 2336 - 2345 , which is incorporated by reference in its entirety . carbon nanotubes , graphene and nanoclay have been added to epoxy at both low and high concentrations , and the reinforcement increases in modulus and hardness are generally between 10 % and 30 %. an ashby plot is provided in the supporting information ( fig1 ) to compare systematically the current results to other studies . because the viscosity of the spray processing fluid was shown to affect particle alignment ( fig3 ), normalized modulus and hardness values are plotted against viscosity in fig6 . non - monotonic behavior is observed for both modulus and hardness . both parameters increase with viscosity ( and orientation ) up to about 0 . 2 pa · s ( where the concentration of epoxy is about 93 % of the fluid phase of the processing fluid ) but a noticeable decrease in modulus and in hardness is observed when the fluid of the spray process is pure epoxy ( viscosity = 1 pa · s ). this drop off of properties are attributed to the appearance of significant amounts of nanotube aggregates in the highest - viscosity - processed films , an issue discussed in more detail below . for polymer composites filled with well - dispersed tubular particles , four parameters , ( i ) volume fraction , ( ii ) particle dimension and ( iii ) orientation and ( iv ) polymer / filler interactions determine the final bulk mechanical properties . see , halpin , j . c . ; tsai , s . w ., environmental factors in composite materials design . u . s . air force tech . rep . afml tr 1967 , 67 - 423 , which is incorporated by reference in its entirety . a balance of these parameters is needed to achieve stiffer and stronger properties in composites . in the present work the influence of item ( iv ) is ignored . polymer - filler interactions should be unchanged in the entire set of cured films since the surface chemistry of the as - received halloysite filler was not modified . on the other hand , the presence of some acetone in the processing fluid is apparently needed to ensure good hnt dispersion ; as mentioned above , processing from pure epoxy fluid resulted in significant nanotube aggregation . to examine the role of orientation , concentration and particle dimension on modulus and hardness for the set of nanocomposites , the cox - krenchel model ( see , cox , h ., the elasticity and strength of paper and other fibrous materials . british journal of applied physics 1952 , 3 , 72 , which is incorporated by reference in its entirety ) was employed , which is modified from the rule - of - mixture ( see derek hull ; clyne , t . w ., introduction to composite materials . 2nd ed . ; cambridge university press : cambridge , 1996 , which is incorporated by reference in its entirety ), considering volume effects , length efficiency and orientation factor in reinforcement . e c = e m v m + η l η o e f v f ( equation 1 ) in equation 1 e and v represent the modulus and volume fraction for epoxy matrix ( i . e ., e m and v m ) and hnt fillers ( i . e ., e f and v f ). here length efficiency factor , η l , and orientation efficiency factor , η o , were defined based on shear lag theory and krenchel &# 39 ; s method . see , mccrum , n . g . ; buckley , c . p . ; bucknall , c . b ., principles of polymer engineering . 2rd ed . ; oxford university press , usa : 1997 ; p 276 - 278 , and cox , h ., the elasticity and strength of paper and other fibrous materials . british journal of applied physics 1952 , 3 , 72 , each of which is incorporated by reference in its entirety . the length efficiency for particles with specific aspect ratio depends only on volume fraction and these length efficiency values were very similar in all of the composites ( i . e ., 81 % to 86 % as shown in fig1 a - 16c ). therefore , the analysis of orientation factor in the following section will expose the most significant influence on the mechanical properties . as shown in fig5 a and 5b , composites prepared from different viscous formulations showed increases in both modulus and hardness values up to 1 vol %. however , at any given hnt concentration in the plateau region above 1 vol %, there is dependence on the details of the formulation composition and on the method of processing , spray coating vs . spin coating ( fig5 a and 5b ). these property differences are attributed to hnt orientations in the final composite coatings ( as shown in fig1 and table 4 ). fig1 shows composite moduli at various volumes ( i . e ., 0 . 5 vol % and 1 . 0 vol %) showed the orientation factor trend distinctively . composites based on e 40 a 60 ( o ), e 77 a 23 (□), e 87 a 13 (⋆), e 93 a 7 ( δ ), and e 100 a 0 (⋄) are marked . a useful formalism for the analysis of orientation in composite mechanics is krenchel orientation factor , η o , defined in the following equation 2 , as defined above , η o can be obtained if i ( φ ) is known . it is often assumed that the distribution of rods , i ( φ ) as a function of φ , can be described by a gaussian or lorentzian distribution . η o is 0 for particles oriented perpendicular to the loading axis ( two - dimensional randomness in plane ), 1 for perfect orientation along the loading direction and 0 . 325 for three - dimensional randomly distributed particles . in previous studies i ( φ ) has been measured using polarized light microscopy , raman spectroscopy , x - ray diffraction , x - ray scattering , and raman scattering techniques . see , derek hull ; clyne , t . w ., introduction to composite materials . 2nd ed . ; cambridge university press : cambridge , 1996 , young , k . ; blighe , f . m . ; vilatela , j . j . ; windle , a . h . ; kinloch , i . a . ; deng , l . ; young , r . j . ; coleman , j . n ., strong dependence of mechanical properties on fiber diameter for polymer - nanotube composite fibers : differentiating defect from orientation effects . acs nano 2010 , 4 , 6989 - 6997 , blighe , f . m . ; young , k . ; vilatela , j . j . ; windle , a . h . ; kinloch , i . a . ; deng , l . ; young , r . j . ; coleman , j . n ., the effect of nanotube content and orientation on the mechanical properties of polymer - nanotube composite fibers : separating intrinsic reinforcement from orientational effects . adv . funct . mater . 2011 , 21 , 364 - 371 , song , k . ; zhang , y . ; meng , j . ; minus , m . l ., spectral analysis of lamellae evolution and constraining effects aided by nano - carbons : a coupled experimental and simulation study . polymer 2015 , 75 , 187 - 198 , pichot , v ; badaire , s . ; albouy , p . ; zakri , c . ; poulin , p . ; launois , p ., structural and mechanical properties of single - wall carbon nanotube fibers . physical review b 2006 , 74 , 245416 - 8 , chen , m . ; guthy , c . ; vavro , j . ; fischer , j . e . ; badaire , s . ; zakri , c . ; poulin , p . ; pichot , v ; launois , p . in characterization of single - walled carbon nanotube fibers and correlation with stretch alignment , mrs proceedings , cambridge univ press : 2004 ; p hh4 . 11 , and zhou , w . ; vavro , j . ; guthy , c . ; winey , k . i . ; fischer , j . e . ; ericson , l . m . ; ramesh , s . ; saini , r . ; davis , v . a . ; kittrell , c ., single wall carbon nanotube fibers extruded from super - acid suspensions : preferred orientation , electrical , and thermal transport . journal of applied physics 2004 , 95 , 649 - 655 , each of which is incorporated by reference in its entirety . the misalignment histogram ( fig7 ) was used to compute the krenchel orientation factors . 500 nanotube orientations were taken on around 15 fracture surfaces for each sample and processed using imagej software . gaussian fitting using origin 8 . 5 was conducted based on the fact that the fitted peak center is 0 ° for out - of - plane aligned particles and 90 ° for in - plane aligned particles . to generate a more quantitative understanding of the hnts &# 39 ; out - of - plane misalignments in the spray - processed composite films , statistical analyses of the sem images were conducted using origin software ( table 4 ). the statistical information was plotted and fitted in fig7 . assuming gaussian distribution of hnts orientations and no bias in out - of - plane orientation , the mean angle for in - plane orientation should be 90 ° and out - of - plane orientation should be 0 °. the fitted values of full - with at half - maximum ( fwhm ) suggest the deviations from average alignment . for e 77 a 23 , e 93 a 7 and e 100 a 0 based hnt composites , it is possible to fit values to a gaussian distribution function . the fact that e 87 a 13 samples cannot be fitted is due to the purely random particle distributions . the fitted normal function center of e 77 a 23 samples was located at ± 90 ° which indicates that the retardation of alignment loss provided by viscosity is weaker than gravitational effects and brownian motion , as described above in fig3 . consequently , hnts are essentially distributed parallel to in - plane x - y axes in this low - viscosity sample . comparatively , the hnts concentration also influenced particle alignment , as higher concentration associated with narrower fitted peaks . this observation demonstrates again an effective route for controllable orientation , from perfect parallel to vertical , by varying the viscosity . now that all the length efficiency and orientation factor parameters have been obtained , the particle intrinsic mechanical properties are accessible based on equation 1 . the effective reinforcements in modulus and hardness have been fitted and plotted in fig8 c and 8d and values also listed in table 1 . linear fitting of composites modulus ( fig8 c ) and hardness values ( fig8 d ) showed coherent trends . the closer the fitted curve to the experimentally measured data points , the closer r - square is to the value of 1 . the properties of the hnt particles derived from the three separate data sets ( based on spraying of the three different formulations listed in table 2 ) are reasonably consistent . compared to the literature reports , the effective reinforcement ( η · e eff ) achieved in this work , up to 182 gpa , is between the reinforcement of carbon nanotubes ( up to 417 gpa ) and alumina platelets ( 100 gpa ). see , jalili , r . ; razal , j . m . ; wallace , g . g ., wet - spinning of pedot : pss / functionalized - swnts composite : a facile route toward production of strong and highly conducting multifunctional fibers . scientific reports 2013 , 3 , 3438 , and erb , r . m . ; libanori , r . ; rothfuchs , n . ; studart , a . r ., composites reinforced in three dimensions by using low magnetic fields . science 2012 , 335 , 199 - 204 , each of which is incorporated by reference in its entirety . it is worth mentioning that the alumina particles are aligned while the carbon nanotubes are not . if length efficiency and orientation factor were taken into consideration , the reinforcement factor for carbon nanotubes would be higher . the hnt particles in the spray coatings have reached rather high reinforcement potential . this favorable situation is largely a result of the favorable normal alignment that exists in the coatings as well as the fact that hnts with relatively small diameters were used here ( around 40 nm ). as indicated previously in both experimental measurements and theoretical calculations , hnt mechanics are greatly influenced by size effects . see , lecouvet , b . ; horton , j . ; d &# 39 ; haese , c . ; bailly , c . ; nysten , b ., elastic modulus of halloysite nanotubes . nanotechnology 2013 , 24 , 105704 - 105711 , and guimaraes , l . ; enyashin , a . n . ; seifert , g . ; duarte , h . a ., structural , electronic , and mechanical properties of single - walled halloysite nanotube models . the journal of physical chemistry c 2010 , 114 , 11358 - 11363 , each of which is incorporated by reference in its entirety . for instance , hnts with diameter of 50 nm have a high modulus of about 600 gpa while a 160 nm hnt can only reach 20 gpa . in fiber - filled polymeric composites , percolation threshold effects lead to a regime of behavior in which mechanical properties do not show a continual increase as fiber concentration increases . see , kumar , a . ; chouhan , d . k . ; alegaonkar , p . s . ; patro , t . u ., graphene - like nanocarbon : an effective nanofiller for improving the mechanical and thermal properties of polymer at low weight fractions . composites science and technology 2016 , 127 , 79 - 87 , which is incorporated by reference in its entirety . this phenomenon has been reported in various filler loaded polymer composites , with percolation threshold values ranging from 0 . 5 % to 5 % 20 . in this study , significant improvement in both modulus and hardness values occurred up to 1 vol % ( fig5 a and 5b ). the average values beyond this percolation threshold were calculated and plotted as trend - lines in fig9 a and 9b . similar to the phenomenon observed in lower hnt loaded composites ( less than 1 . 0 vol %), higher orientation corresponds to more improved mechanical properties in higher hnts concentrated composites ( between 1 . 0 and 10 . 2 vol %). in addition , e 100 a 0 failed out of this trend mainly due to poorer dispersion quality . this plateau region independent of hnt concentration is now examined in more detail . three regimes based on the degree of particle interaction or the average distances between particles are defined in particle suspensions : dilute , semidilute , and concentrated . see , doi , m . ; edwards , s . d ., dynamics of concentrated polymer systems . part 1 .— brownian motion in the equilibrium state . journal of the chemical society , faraday transactions 2 : molecular and chemical physics 1978 , 74 , 1789 - 1801 , and doi , m . ; edwards , s ., dynamics of concentrated polymer systems . part 2 .— molecular motion under flow . journal of the chemical society , faraday transactions 2 : molecular and chemical physics 1978 , 74 , 1802 - 1817 , each of which is incorporated by reference in its entirety . the semidilute regime for hnts in this research was between 0 . 01 vol % and 2 vol % ( fig9 c ). in fig9 c , blue stars indicate the concentrations in this study and boundary between semidilute and dilute regime is 2 . 0 vol %. when the hnts concentrations reach the concentrated regime above 2 . 0 vol %, hnts interact with other particles in the form of a network . these interactions among particles lead to significant amounts of agglomerates in the final composites . fig1 a - 10d show the increased particle aggregates size and amounts . dispersion quality was always poorer at lower acetone concentrations ( fig1 e - 10h ; all square markers are 10 μm by 10 μm ). with the increase in hnt concentration , more bundles are formed . the aggregate size also increases . very distinguished are the 10 . 2 vol % composites which have the most aggregates and with bundle sizes even larger than 10 μm . as shown in fig9 d , the increase in hnt bundle size greatly decreases the efficiency of stress transfer . this will lead to the plateau region in young &# 39 ; s modulus and hardness in fig9 a and 9b despite the fact that concentration increases at each of the selected liquid viscosity ( along x axis of volume concentration ). for fixed hnt bundle size , fig9 d also displayed the reductions in modulus with higher misalignment angle . this is in line with the trends of plateau increases ( along y axis of modulus / hardness values in fig9 a and 9b ). moreover , e 100 a 0 showed lower mechanical properties than that in e 93 a 7 composites . this is attributed to the dispersion quality . as a comparison , the shear modulus of hnts with diameters ranging from 200 nm to 220 nm are 1 . 5 ± 0 . 26 gpa , while carbon nanotubes are 6 gpa for 4 . 5 nm bundles , 2 . 3 gpa for 9 nm bundles and 0 . 7 gpa for 20 nm bundles . see , salvetat , j . - p . ; briggs , g . a . d . ; bonard , j . - m . ; bacsa , r . r . ; kulik , a . j . ; stockli , t . ; burnham , n . a . ; forró , l ., elastic and shear moduli of single - walled carbon nanotube ropes . physical review letters 1999 , 82 , 944 - 7 , satcurada , i . ; ito , t . ; nakamae , k ., elastic moduli of the crystal lattices of polymers . journal of polymer science part c : polymer symposia 1967 , 15 , 75 - 91 , and popov , v . ; van doren , v ; balkanski , m ., elastic properties of crystals of single - walled carbon nanotubes . solid state communications 2000 , 114 , 395 - 399 , each of which is incorporated by reference in its entirety . considering the average particle diameter of 40 nm , shear moduli were assumed to be 2 , 5 , 10 and 20 gpa . as expected in fig1 e and 10h , the final composites showed larger - size aggregates with lower acetone content in suspension preparations . in summary , hollow tubular halloysite hnts were used as reinforcement fillers in transparent epoxy composites . a spray coating process was used to control hydrodynamic flow to align the particles ; at the same time the elevated levels of viscosity in hnts suspensions preserved the hnt orientations upon impacting the substrate surface . sem images showed an improvement of alignment with increasing viscosity . indentation results showed a consistent increase in modulus and hardness values with higher hnt orientation except for those composites processed from acetone - free epoxy fluids . fig1 shows with higher hnt alignment , composites showed more scratch resistance . fig1 shows the in - situ imaging of scratches during scratching tests . the drop in mechanical properties in e 100 a 0 composites was attributed to the poor dispersion quality in the absence of acetone . in summary , this study has revealed favorable levels of anisotropic mechanical properties , mainly induced by particle orientation . the work provides a new perspective for optimal coating design as well as a favorable , scalable processing methodology . materials : dragonite ™ hnt clay was obtained from applied minerals ( density 2 . 54 ± 0 . 03 g · cm − 3 , inner diameter 10 - 20 nm , outer diameter 30 - 60 nm , and aspect ratio between 20 and 200 . bet pore volume 20 %, surface area up to 100 m 2 · g − 1 , refractive index 1 . 534 ). epoxy 142 - 112 ( purchased from epoxy technology , inc ., density 1 . 18 g · cm − 3 ) and acetone ( vwr , density 0 . 79 g · cm − 3 ) were used as obtained . processing : the thin - film coatings were fabricated using a spray coating method , shown in fig2 . the nozzle ( manufactured by paasche ®) size is 1 / 50 ″, distance between spray gun and substrate ( usually a glass microscope slide ) is 1 ″, and compressed air pressure is 25 psi . the fluid phase of each formulation was made starting from 20 parts epoxy by weight and diluting with specific amount of acetone ( i . e ., epoxy : acetone weight ratios were 20 : 0 , 20 : 1 , 20 : 2 , 20 : 4 , and 20 : 20 , and these corresponded to epoxy volume percentages of 100 , 93 , 87 , 77 and 50 vol %). hnt was subsequently mixed with epoxy / acetone solutions . for each formulation , hnt concentrations with respect to epoxy in these various suspensions were 0 , 1 , 2 , 5 , 10 , 15 , 20 wt % ( i . e ., corresponding volume percentages of hnts are approximately 0 , 0 . 5 , 1 . 0 , 2 . 3 , 4 . 8 , 7 . 4 , 10 . 2 vol % based on known densities of constituents ). the mixtures were magnetically stirred for 5 mins , mechanically mixed for 1 min , and sonicated for around 2 hours to eliminate bubbles . the prepared batch solutions were then spray coated onto glass slides , cured using a dymax 5000 - ec flood lamp ( working distance 2 - 6 ″), with a total uv energy output of 225 mw / cm 2 at 1 . 0 inch in direct emission and wavelength mainly from 350 to 450 nm . the samples were abbreviated as e x a y h z , with x as volume percentage of epoxy in the liquid phase of acetone and epoxy , x being 100 , 93 , 87 , 77 , 40 for the five solutions mentioned above , y is the acetone volume percentage , and z is hnt volume percentage in the final cured samples ( i . e ., 0 , 0 . 5 , 1 . 0 , 2 . 3 , 4 . 8 , 7 . 4 , 10 . 2 vol %), as shown in table 2 . to study the hnt alignment effects on mechanical properties , composites with random in - plane hnt orientations were prepared using spin coating method . 1 ml mixed e 40 a 60 h z with z ranging from 0 to 10 . 2 vol % as mentioned were spin - coated onto glass slides at a rate of 5000 rpm , and cured in a similar fashion as the spray coating samples . nanoindentation measurements were conducted using a tribolndentator ( hysitron inc . ), equipped with a berkovich diamond tip ( semi conical tip with diameter of 1 μm ). indentation tests were operated in a displacement - control mode ; the displacement excitation is applied to the sample according to a programmed loading function while the force response is continuously monitored with a resolution of 1 nn . the loading function in this work consisted of a 5 s linear loading , a 5 s unloading segments with a los force dwelling at the peak load to reduce the influence of creeping effects . see , hu , h . ; onyebueke , l . ; abatan , a ., characterizing and modeling mechanical properties of nanocomposites - review and evaluation . journal of minerals and materials characterization and engineering 2010 , 9 , 275 - 280 , and bhushan , b . ; li , x ., nanomechanical characterisation of solid surfaces and thin films . international materials reviews 2003 , 48 , 125 - 164 , each of which is incorporated by reference in its entirety . the maximum displacement used was less than one tenth of the coating thickness ( about 200 nm ) to exclude the influence from the substrate . a total of 36 indents with lateral spacing of 2 μm were taken to obtain average reduced modulus and hardness values on both control and composite samples coated on glass slides . nanoindentation hardness is defined as the indentation load divided by the projected contact area of the indentation . it is the mean pressure that a material will support under load . from the load - displacement curve , hardness can be obtained at the peak load ( p max ) as , h = p max / a , where a is the projected contact area . for an indenter with a known geometry such as the berkovich tip used in this study , the projected contact area is a function of contact depth , which is measured by the nanoindenter in situ during indentation . the elastic modulus was calculated using the oliver - pharr data analysis procedure 64 beginning by fitting the unloading curve to a power - law relation , where s is the slope of the tangent to the loading curve at maximal load and a p is the contact area of the indenter . e r is related to the constituent properties by the instruments : where e and ν are the elastic modulus and poisson &# 39 ; s ratio for the sample , and e i and ν i are the same quantities for the diamond indenter . for diamond , e i = 1141 gpa and ν i = 0 . 07 62 , 63 , and for epoxy used here , ν = 0 . 3 . the bulk rheological response of epoxy / acetone solutions was measured at 25 ° c . using a cone - and - plate ( cp ) geometry ( 2 ° cone , diameter 60 mm , and truncation 58 μm , part # 513606905 ) on the ar - g2 rheometer ( ta © manufactured rheometer ). the steady shear viscosity of the solutions was measured at shear rates between 10 and 1000 s − 1 . differential scanning calorimetry ( dsc ) and thermo gravimetric analyzer ( tga ): dsc was performed using ta instrument q20 under n 2 . the samples were first heated to 200 ° c . to remove the thermal history , cooled to 0 ° c . at 3 ° c ./ min and then heated from 0 to 250 ° c . at heating rate of 5 ° c ./ min . tga was performed using ta instrument tga 2950 under n 2 from 30 to 900 ° c . at heating rate of 10 ° c ./ min . tem was used for the morphological investigation of the composites and the halloysite nanotubes using a jeol 2010 advanced high performance tem . in order to determine the structure of the halloysite nanotubes , the as - received powder of neat halloysite was suspended in ethanol , and a droplet of the suspension was deposited and dried on a carbon grid for tem studies . a field - emission high - resolution scanning electron microscope ( zeiss supra 25 , accelerating voltage 5 kv ) was used to image the composite film structures . all samples were sputter coated with a thin ( i . e ., 10 nm ) gold / palladium layer using a gatan high - resolution ion beam coater . samples were fractured in liquid nitrogen ( around − 200 ° c .) to expose the cross sectional structure of the thin film composite specimens . fig1 shows an increasing viscosity trend with higher epoxy content . the concentration of hnts in each figure remains constant . the particle concentration only slightly affects the viscosity compared to the significant change caused by the percentage of epoxy added . the shear rates in the rheometer measurements are from 10 to 1000 s − 1 and the temperature is set at 23 ° c . shear - aligned halloysite nanotubes were sprayed on a glass slide . the alignment of nanotubes is constrained by the epoxy and given enough time the nanotubes would lose their alignment and reach a state of relaxation . the tilted angle and rate depend on particle features ( i . e ., length , density , shape ), and liquid characteristics ( i . e ., viscosity , temperature , pressure ) 1 . the fluid consisting of either pure epoxy or different epoxy / acetone mixtures all displayed newtonian behavior , as shown in fig1 a - 12b . the orientation of the body would eventually be determined by inertia 1 . bodies with force and front - end symmetry were torque free when settling in stokes flow , so that the torques due to inertia were unopposed . this resulted in an eventual out - of - plane orientation for all particles 1 . see , liu , y . j . ; joseph , d . d ., sedimentation of particles in polymer solutions . journal of fluid mechanics 1993 , 255 , 565 - 595 , which is incorporated by reference in its entirety . fig1 a - 12b shows linear trends between shear force and shear rate are observed , which exhibits the characteristic of newtonian fluids . e 87 a 13 , e 77 a 23 and e 40 a 60 display the same newtonian liquid behavior due to the increase in acetone concentration . fig1 shows that relative flow motion past a falling particle in a fluid ( i . e ., a halloysite nanotube falling through the epoxy solution or melt ) generates drag force , f d , force due to gravity , f g , as well as buoyancy , f b . the sedimentation of the tubes until reaching steady state is equivalent to the steady flow past a stationary long body of halloysite . to simplify the problem , the micromechanics analysis model for a single hnt particle ( i . e ., diameter of 40 nm and length of 2 μm ) falling in a viscous fluid ( i . e ., viscosity taken from fig4 ) was plotted in fig1 . the time for the tube to reach steady in - plane state was calculated based on this micromechanics model ( fig1 ). according to stoke &# 39 ; s law , the force of viscosity on a small particle moving through a viscous fluid is given by , where f d is the friction force , known as stoke &# 39 ; s drag , acting on the interface between the fluid and particle . μ is the dynamic viscosity . see , munson , b . r . ; young , d . f . ; okiishi , t . h ., fundamentals of fluid mechanics . new york 1990 , 3 , 4 , which is incorporated by reference in its entirety . the liquid states studied here are all newtonian fluids . viscosity values were taken as a constant from experimental measurements . r is the quasi - radius of the object . ν is the flow velocity relative to the object . the single particle sedimentation procedure was analyzed by the equation of motion , where ρ particle and ρ fluid are the density values of the particle and the fluid , respectively , and g is the gravitational acceleration . to calculate the stability time , t , parameters of ν 0 and ν are needed . the initial injection velocity , ν 0 , can be obtained , ν 0 t spray πr gun 2 = v spray ( equation s8 ) where t spray is the time consumed for spraying a specific fluidic volume v spray , and r gun is radius of the spraying gun nozzle . at the equilibrium state , the excess forces of gravity and buoyancy will balance the stoke &# 39 ; s drag force , the resulting equilibrium velocity , ν , can be calculated via combining equations 51 and s5 , taking all the equations above , the calculated particle settling time was plotted in fig1 . in fig1 , carbon nanotubes , graphene , montmorillonitrile clay , silica , and current work data has been plotted . for prior art data , see , gojny , f . h . ; wichmann , m . h . g . ; fiedler , b . ; schulte , k ., influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites — a comparative study . composites science and technology 2005 , 65 , 2300 - 2313 , sun , l . ; warren , g . l . ; o &# 39 ; reilly , j . y . ; everett , w . n . ; lee , s . m . ; davis , d . ; lagoudas , d . ; sue , h . j ., mechanical properties of surface - functionalized swcnt / epoxy composites . carbon 2008 , 46 , 320 - 328 , gojny , f . ; wichmann , m . ; köpke , u . ; fiedler , b . ; schulte , k ., carbon nanotube - reinforced epoxy - composites : enhanced stiffness and fracture toughness at low nanotube content . composites science and technology 2004 , 64 , 2363 - 2371 , ayatollahi , m . r . ; 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wang , m . ; liu , m . ; motta , n . ; waclawik , e . ; yan , c ., recent advances in fabrication and characterization of graphene - polymer nanocomposites . 2012 , qi , b . ; zhang , q . x . ; bannister , m . ; mai , y . w ., investigation of the mechanical properties of dgeba - based epoxy resin with nanoclay additives . composite structures 2006 , 75 , 514 - 519 , wang , l . ; wang , k . ; chen , l . ; zhang , y . ; he , c ., preparation , morphology and thermal / mechanical properties of epoxy / nanoclay composite . composites part a : applied science and manufacturing 2006 , 37 , 1890 - 1896 , wang , k . ; chen , l . ; wu , j . ; toh , m . l . ; he , c . ; yee , a . f ., epoxy nanocomposites with highly exfoliated clay : mechanical properties and fracture mechanisms . macromolecules 2005 , 38 , 788 - 800 , zappalorto , m . ; salviato , m . ; quaresimin , m ., mixed mode ( i + ii ) fracture toughness of polymer nanoclay nanocomposites . engineering fracture mechanics 2013 , 111 , 50 - 64 , guevara - morales , a . ; 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dittanet , p . ; guild , f . j . ; kinloch , a . j . ; masania , k . ; pearson , r . a . ; taylor , a . c ., the modelling of the toughening of epoxy polymers via silica nanoparticles : the effects of volume fraction and particle size . polymer 2013 , 54 , 7022 - 7032 , and ma , j . ; mo , m .- s . ; du , x . - s . ; rosso , p . ; friedrich , k . ; kuan , h . - c ., effect of inorganic nanoparticles on mechanical property , fracture toughness and toughening mechanism of two epoxy systems . polymer 2008 , 49 , 3510 - 3523 , each of which is incorporated by reference in its entirety . tilted lines stand for specific modulus increase in percentage , and the slopes indicate reinforcement in modulus per unit particle concentration . it can be seen that the current work showed intermediate reinforcement efficiency between graphene and carbon nanotubes ; however , hnts cost is $ 2 / kg , while carbon nanotubes and graphene price range from $ 50 / g to $ 500 / g . the current work also achieves modulus increases beyond that from frequently used particles of montmorillonite and silica . the secret is in the particle alignment along loading direction . compared with cnt , graphene and clay particle , excellent dispersion quality allows for higher particle loadings and reinforcement ( fig1 ). tga experiments were used to confirm the concentrations of hnt in the final processed composites . data of this type is compared in fig6 a with the nominal hnt compositions , based on formulation compounding . it can be seen that the final composite concentrations are very consistent with the designed loadings ( fig6 b ). fig6 b shows the steady increase of mechanical properties with higher viscosity values in spray processing fluids . a drop at highest viscosity is attributed to the appearance of a significant number of aggregates in the samples . this also validates a stable distribution of hnts in the various viscous spray - processing formulations , where epoxy / acetone ratios vary considerably . macroscopic sedimentation was not observed even in 10 . 2 vol % hnt loadings . where g m is the shear modulus of polymer matrix , 2r is the distance from the fiber to its nearest neighbor fiber , l and d mean the length and diameter of the particle . at fixed fiber concentration below percolation ( i . e ., less than 1 vol % in the hnt nanocomposites ) and under uniform dispersion , length efficiency is only dependent on aspect ratio and concentration as shown in equations s7 to s9 . fig1 c shows how the length efficiency factor changes with lumped parameter na , and the insert demonstrates how na changes with volume concentrations . based on this curve from equation s7 , as well as the parameters including { circle around ( 1 )} shear modulus of 1 . 7 gpa for epoxy as calculated from the experimentally determined young &# 39 ; s modulus ( 4 . 5 gpa ), { circle around ( 2 )} poisson &# 39 ; s ratio ( 0 . 3 ), { circle around ( 3 )} tensile modulus for hnts of 300 gpa ( see , guimaraes , l . ; enyashin , a . n . ; seifert , g . ; duarte , h . a ., structural , electronic , and mechanical properties of single - walled halloysite nanotube models . the journal of physical chemistry c 2010 , 114 , 11358 - 11363 , and lecouvet , b . ; horion , j . ; d ′ haese , c . ; bailly , c . ; nysten , b ., elastic modulus of halloysite nanotubes . nanotechnology 2013 , 24 , 105704 , each of which is incorporated by reference in its entirety ), and { circle around ( 4 )} hnts concentration of 0 . 5 vol % generate a value of 5 for na . this corresponds to a length efficiency of 81 % in fig1 c . in this study , the hnt volume fractions , v f , varied from ˜ 0 . 5 vol % to ˜ 10 . 2 vol %, indicating the matrix volume fractions , v m , from ˜ 99 . 5 vol % to ˜ 89 . 8 vol %. this change in concentration , however , does not change the length efficiency factor significantly , with η i ranges from 81 % to 86 % ( insert in fig1 c ). therefore , the analysis of orientation efficiency factor will reveal their main influencing effectiveness on mechanical properties as discussed in the manuscript . the orientation factor can also be calculated based on composite mechanics . the spin - coating method produces a film with randomly orientated particles with an orientation factor η o of 0 . 2 . see , song , k . ; zhang , y . ; meng , j . ; green , e . c . ; tajaddod , n . ; li , h . ; minus , m . l ., structural polymer - based carbon nanotube composite fibers : understanding the processing - structure - performance relationship . materials 2013 , 6 , 2543 - 2577 , which is incorporated by reference in its entirety . a linear fitting of the experimental modulus values between 0 and 1 vol % in spin - coated films gives effective modulus of hnt of ˜ 312 gpa ( i . e ., moduli of 5 . 20 , 5 . 74 , and 5 . 81 gpa at hnt concentrations of 0 , 0 . 5 , and 1 . 0 vol %). at a rough estimation , e m = 4 . 5 gpa and e f 312 gpa , the composite modulus relative to orientation factor ( i . e ., 0 to 1 ) and fiber volume fraction ( i . e ., 0 to 1 vol %) is plotted in fig1 . the spray - coated samples were shown as white symbols on the contour so that their orientation factors at specific volumes of 0 . 5 vol % and 1 . 0 vol % become straightforward . the orientation factors showed consistent increase with viscosity values . fig1 shows that with a specific volume fraction , better consistency in particle orientation results in high modulus values . in spite of this relationship between orientation factor and modulus values , composite mechanics is not a straightforward method for particle orientation calculations . therefore statistical quantifications of hnt orientations in these composites are important and have been given in the manuscript discussion sections . where y c = y 0 + a /( fwhm * sqrt ( π / 4ln2 )), fwhm is the full width at half maximum and a is the area integrated . y 0 is base , x c stands for the fitted peak center which is 0 ° for out - of - plane aligned particles and 90 ° for in - plane aligned particles . the script access of the function is nlf_gaussian ( x , y0 , xc , a , w ). * see , guimaraes , l . ; enyashin , a . n . ; seifert , g . ; duarte , h . a ., structural , electronic , and mechanical properties of single - walled halloysite nanotube models . the journal of physical chemistry c 2010 , 114 , 11358 - 11363 , lecouvet , b . ; horion , j . ; d &# 39 ; haese , c . ; bailly , c . ; nysten , b ., elastic modulus of halloysite nanotubes . nanotechnology 2013 , 24 , 105704 , sato , h . ; ono , k . ; johnston , c . t . ; yamagishi , a ., first - principles studies on the elastic constants of a 1 : 1 layered kaolinite mineral . american mineralogist 2005 , 90 , 1824 - 1826 , and lu , d . ; chen , h . ; wu , j . ; chan , c . m ., direct measurements of the young &# 39 ; s modulus of a single halloysite nanotube using a transmission electron microscope with a bending stage . journal of nanoscience and nanotechnology 2011 , 11 , 7789 - 7793 , each of which is incorporated by reference in its entirety . hnt particles have been known for their high modulus , up to around 600 gpa at outer diameters of less than 50 nm . however , as tubes started aggregating , the accumulating defects and the lack of inter - tubular registry resulting from diameter differences and helicity variations will lead to decrease of effective modulus , especially shear modulus . for example , carbon nanotubes have been shown to have shear modulus of 6 gpa for 4 . 5 nm bundles , 2 . 3 gpa for 9 nm bundles and 0 . 7 gpa for 20 nm bundles . see , salvetat , j . - p . ; briggs , g . a . d . ; bonard , j . - m . ; bacsa , r . r . ; kulik , a . j . ; stöckli , t . ; burnham , n . a . ; forró , l ., elastic and shear moduli of single - walled carbon nanotube ropes . physical review letters 1999 , 82 , 944 , satcurada , i . ; ito , t . ; nakamae , k ., elastic moduli of the crystal lattices of polymers . journal of polymer science part c : polymer symposia 1967 , 15 , 75 - 91 , and popov , v . ; van doren , v . ; balkanski , m ., elastic properties of crystals of single - walled carbon nanotubes . solid state communications 2000 , 114 , 395 - 399 , each of which is incorporated by reference in its entirety . therefore a theoretical estimation of the average modulus dependent on bundle size will be necessary to understand the plateau region in fig9 a - 9d . the average effective modulus & lt ; e x & gt ; has been calculated using continuum mechanics ( equation s10 ). see , song , k . ; zhang , y . ; meng , j . ; green , e . c . ; tajaddod , n . ; li , h . ; minus , m . l ., structural polymer - based carbon nanotube composite fibers : understanding the processing - structure - performance relationship . materials 2013 , 6 , 2543 - 2577 , which is incorporated by reference in its entirety . modulus along longitudinal direction ( e 1 ), transverse direction ( e 2 ), and poisson &# 39 ; s ratio ( ν ) were from table 5 and also listed in fig9 d . shear modulus ( g 12 ) dependent on bundles size is not available experimentally ; here in this research were taken as 2 , 5 , 10 , and 20 gpa to show the average modulus change with bundle size . see , liu , t . ; kumar , s ., effect of orientation on the modulus of swnt films and fibers . nano lett 2003 , 3 , 647 - 650 , which is incorporated by reference in its entirety . the existence of plateau region can be attributed to two aspects . ( i ) from fig9 d , it can be seen that with the bundle size increase , the decrease in effective average modulus can be one order of magnitude lower . in addition , for the same bundle size , the average modulus was also found to improve with higher alignment . this is also consistent with the phenomenon as indicated from the trend line in fig8 a and 8b . ( ii ) the bundled structure not only influences the intrinsic particle modulus and hardness values but also affect the interaction between polymers and particles . fully dispersed and exfoliated nanotubes will have more contact area than aggregates , and reinforcement efficiency has also been found to be linearly proportional to interfacial area 54 . this could be another reason for the formation of plateau .