nanotechno coatings presentation-vf

7/31/2019 Nanotechno Coatings Presentation-Vf

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Use and Benefits ofNanotechnology in Coatings

Characteristic of nanotechnology is theeffect that due to the size of the materialalone, new functionalities arise that lead to

new or improved product properties.

Thank you for attending.

Presented by Mark L. Drukenbrod, Ph.D.

Model of C60 Carbon fullerene


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Background in the industry

Transition from micro particles to nanoparticles

leads to changes in physical properties of matter

Increase in the ratio of surface area to

material volume

Shift of electromagnetic properties from massto quantum level


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Surface to volume ratio change is gradual as theparticle gets smaller, leads to dominance of the

behavior of atoms on the surface of the particle overthose at the center.

Affects both the isolated particle and itsinteraction with other materials, likesolvents

Leads to special properties, likechemical or abrasion resistance.


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Transition from classical to quantum mechanicsis not as gradual as the surface/volume transition.

As some particles get smaller, free electrons inthem start to behave like electrons bound byatoms in that they can occupy certain permittedenergy states.

These are known as quantum dots,

and can be conductive or semi-conductive. Are used in paint-onsensor technology


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History of nanoparticles in the coatings industry

Used since the time of the Roman empire

in porcelain glazes.

Most popular potential nanomaterial iscarbon black.

Most carbon black is not used in itsnano-state

Next most popular are metal oxide ceramics andsilicates, followed by pure metals, thennanostructures like fullerenes and nanotubes.


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A nanocrystal is a crystalline material with dimensionsmeasured in nanometers; a nanoparticle with a structure

that is mostly crystalline. These materials are of hugetechnological interest since many of their electrical andthermodynamic properties show strong size dependence,

and can therefore be controlled through carefulmanufacturing processes. Nanocrystals are also ofinterest since they often provide single domain crystalline

systems that can be studied to provide information thatcan help explain the behavior of macroscopic samples ofsimilar materials, without the complicating presence ofgrain boundaries and other defects.

Nanomaterial types: Nanocrystal


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Typical Nanocrystal Materials

Titanium Dioxide

Silicon


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Titanium Dioxide

Already intensively used in coatings, TiO2 in itsnanocrystalline form has several specific effects:

1. Size can be tuned to absorb various bandwidthsof UV radiation.

2. Material is very active chemically, and coatingsmade with it can remove pollutants from airthrough catalysis

3. In its nano-form, the material is essentially clear,rather than white, making it useful for opticalcoatings.


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Silicon

Nanocrystalline silicon is an allotropic form ofsilicon, similar to amorphous silicon (a-Si), in that

it has an amorphous phase. Where they differ,however, is that nc-Si has small grains ofcrystalline silicon within the amorphous phase.Most materials with grains in the micrometerrange are actually fine-grained polysilicon, sonanocrystalline silicon is a better term. The termnanocrystalline silicon refers to a range of

materials around the transition region fromamorphous to microcrystalline phase in thesilicon thin film.


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Silicon

nc-Si has useful advantages over a-Si, one beingthat if grown properly it can have a higher

mobility, due to the presence of the siliconcrystallites.

It also shows increased absorption in the red and

infrared wavelengths, which make it an importantmaterial for use in a-Si solar cells

One of the most important advantages of

nanocrystalline silicon is that it has increasedstability over a-Si because of its lower hydrogenconcentration.


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Manufacturing routes to Typical Nanocrystal Materials

Chemical reaction/growth:

1. Plasma Enhanced Chemical Vapor Deposition (PECVD)is a process mainly to deposit thin films from a gas state

(vapor) to a solid state on some substrate. The plasma isgenerally created by RF frequency or DC dischargebetween two electrodes where the gap is filled with the

reacting gases.

2. Chemical vapor deposition (CVD) is a chemical processused to produce high-purity, high-performance solid

materials. In a typical CVD process, the substrate isexposed to one or more volatile precursors, which reactand/or decompose on the substrate surface to produce

the desired deposit.


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Manufacturing routes to Typical Nanocrystal Materials

1. Ball or media milling

a. most materials are abrasive and destroy attritionmachinery, although ceramic inserts can be

purchased for media mills.

b. metallic and other contaminates in the nano-material is a limitation of this process.

2. Homogenizers and microfluidizers

a. equipment fails quickly due to extreme wear.

Attrition of mass material


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Nanomaterial types: Dendrimers and aerogels

Dendrimers are fully synthetic macromolecules

comprised of perfectly branched nano-scalerepeat units in layers emanating radially from apoint-like core.

Typical dendrimer structure


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The properties of dendrimers are dominated by thefunctional groups on the molecular surface. Dendriticencapsulation of functional molecules allows for theisolation of the active site, a structure that mimics thestructure of active sites in biomaterials because

dendritic scaffolds separate internal and externalfunctions. For example, a dendrimer can be water-soluble when its end-group is a hydrophilic group, like

a carboxyl group. It is theoretically possible to designa water-soluble dendrimer with internalhydrophobicity.


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Aerogels are a class of open-celled mesoporous solidmaterials possessing no less than 50% porosity byvolume. Typically, aerogels are composed of 90-99.8%air, with densities ranging from 1.9 to around 150mg/cm. At the nanoscale, an aerogel structurally

resembles a sponge and is composed of a network ofinterconnected nanoparticles. The term aerogel doesnot refer to a particular substance itself but rather to a

geometry a substance can take onin fact, aerogelscan be composed of a variety of materials includingsilica (SiO2), alumina (Al2O3), transition metal oxides,

carbon and others.


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Here we see a 5 pound brickbeing supported by a piece of

silica aerogel weighing slightlymore than 2 grams.

The world's lowest-density solid is a silica aerogel(the latest and lightest versions of this substancehave a density 1 mg/cm, or 1/1000 the density of

water.


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Nanomaterial types: Fullerenes

The fullerenes, discovered in 1985 by researchers at

Rice University, are a family of carbon allotropesnamed after Richard Buckminster Fuller and aresometimes called buckyballs. They are molecules

composed entirely of carbon, in the form of a hollowsphere, ellipsoid, or tube.


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Fullerenes continued

Fullerenes are stable, but not totally unreactive. Thesp2-hybridized carbon atoms, which are at their energyminimum in planar graphite, must be bent to form theclosed sphere or tube, which produces angle strain.The characteristic reaction of fullerenes is electrophilic

addition at 6,6-double bonds, which reduces anglestrain by changing sp2-hybridized carbons into sp3-hybridized ones. This causes a decrease in bond

angles and allows for the bonds to bend less whenclosing the sphere or tube, and thus, the moleculebecomes more stable.


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Other atoms can be trapped inside fullerenes to form

inclusion compounds known as endohedral fullerenes.

Fullerenes are sparingly soluble in many solvents.

Common solvents for the fullerenes include tolueneand carbon disulfide. Solutions of pureBuckminsterfullerene have a deep purple color.

Solutions of C70 are a reddish brown. Fullerenes arethe only known allotrope of carbon that can bedissolved in common solvents at room temperature.


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Solvents that are able to dissolve a fullerene extractmixture (C60 / C70) are listed below in order from

highest solubility. The value in parentheses is theapproximate saturated concentration.

1,2,4-trichlorobenzene (20 mg/ml) cyclohexane (0.054 mg/ml)

n-hexane (0.046 mg/ml) tetrahydrofuran (0.037 mg/ml)

acetonitrile (0.02 mg/ml) methanol (0.0009 mg/ml)

carbon disulfide (12 mg/ml) toluene (3.2 mg/ml)

benzene (1.8 mg/ml) chloroform (0.5 mg/ml)

carbon tetrachloride (0.4 mg/ml)



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The coatings industry uses fullerenes for their:

1. extreme hardness

2. ease of solubility

3. potential conductivity

4. ability to contain other atoms

5. potential pigment value

6. heat resistance



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Possible formulating and processing problems withfullerenes include:

1. Some fullerene structures are not soluble becausethey have a small bandgap between the groundand excited states.

2. Strong surface forces necessitate an aggressivedispersant regimen, some standard dispersionaids do not work.

3. The material is very abrasive, requiring specialprocessing equipment.


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Carbon Nanotubes

Carbon nanotubes are also allotropes of carbon. A

carbon nanotube is a one-atom thick sheet ofgraphite (called graphene) rolled up into a seamlesscylinder with diameter of the order of a nanometer.

This results in a nanostructure wherethe length-to-diameter ratio exceeds10,000. Such cylindrical carbon

molecules have novel properties thatmake them potentially useful in a widevariety of applications in coatings,

electronics, and optics.


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Carbon Nanotubes continued

The nature of the bonding of a nanotube is described by

applied quantum chemistry, specifically, orbitalhybridization. The chemical bonding of nanotubes arecomposed entirely of sp2 bonds, similar to those ofgraphite. This bonding structure, which is stronger than thesp3 bonds found in diamond, provides the molecules withtheir unique strength.

(0,10) nanotube diagram


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Nanotubes naturally align themselves into "ropes"held together by Van der Waals forces. Under high

pressure, nanotubes can merge together, tradingsome sp2 bonds for sp3 bonds, giving great possibilityfor producing strong, unlimited-length wires through

high-pressure nanotube linking.

A rope of single wall nanotubes, showing trifilar packing



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All nanotubes are very good thermal conductors alongthe tube, exhibiting a property known as "ballistic

conduction," but good insulators laterally to the tubeaxis. It is predicted that carbon nanotubes will be ableto transmit up to 6000 watts per meter per kelvin at

room temperature; compare this to copper, a metalknown for its good thermal conductivity, which onlytransmits 385 W/m/K. The temperature stability ofcarbon nanotubes is estimated to be up to 2800degrees Celsius in vacuum and about 750 degreesCelsius in air.



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Because of the symmetry and unique electronic structure

of graphene, the structure of a nanotube strongly affectsits electrical properties. For a given (n,m) nanotube, if n-mis a multiple of 3, then the nanotube is metallic,otherwise the nanotube is a semiconductor. Thus allarmchair (n=m) nanotubes are metallic, and nanotubes(5,0), (6,4), (9,1) are semiconducting. In theory, metallicnanotubes can have an electrical current density more

than 1,000 times greater than metals such as silver andcopper.



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The coatings industry uses nanotubes for their:

1. extreme hardness2. surface effects

3. potential conductivity

4. high linear and torsional strength

5. semiconductor effects

6. heat resistance/conductance



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Possible formulating and processing problems withfullerenes include:

1. The nanotubes display roping, and are difficult todisperse if not inhibited in some way.

2. Strong surface forces necessitate an aggressivedispersant regimen, some standarddispersion aids do not work.

3. The material is not strong in compression, andsome processing equipment can plasticallydeform it, changing its electrical and heat

characteristics.



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Quantum Dots

Quantum dots are semiconductor nanostructures thatconfine the motion of conduction band electrons,

valence band holes, or excitons (bound pairs ofconduction band electrons and valence band holes) inall three spatial directions.

The confinement can be due to electrostatic potentials(generated by external electrodes, doping, strain,impurities), the presence of an interface betweendifferent semiconductor materials (e.g. in core-shell

nanocrystal systems), the presence of thesemiconductor surface (e.g. semiconductornanocrystal), or a combination of effects.


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Various sized CdSe quantum dots excited by UV

Quantum Dots continued

One of the optical featuresof small excitonic quantumdots immediately noticeableto the unaided eye iscoloration. While thematerial which makes up a

quantum dot defines itsintrinsic energy signature,more significant in terms of

coloration is the size.


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The larger the dot, the redder (the more towards the redend of the spectrum) the fluorescence. The smaller the

dot, the bluer (the more towards the blue end) it is. Thecoloration is directly related to the energy levels of thequantum dot. The bandgap energy that determines the

energy (hence color) of the fluoresced light is inverselyproportional to the square of the size of the quantumdot. Larger quantum dots have more energy levels

which are more closely spaced. This allows thequantum dot to absorb photons containing less energy,i.e. those closer to the red end of the spectrum.


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The ability to tune the size of quantum dots isadvantageous for many applications. For instance, largerquantum dots have spectra shifted towards the redcompared to smaller dots, and exhibit less pronouncedquantum properties. Conversely the smaller particles allow

one to take advantage of quantum properties.

This makes quantum dots useful in optical coatings, smartor color changing coatings and in physical sensors over awide range of industries. These structures are used incombination with intrinsically conductive polymers in mostknown smart coatings.


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Methods of synthesis include:

1. Colloidal Synthesis, in which they are actually

grown

2. Electrochemical Synthesis, which allows thegrowth of large numbers of dots can befabricated in specific arrays

3. Pyrolytic Synthesis, which produces large

numbers of quantum dots that self-assembleinto preferential crystal sizes [(MBE) and(MOVPE)]


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Current non-research practice

Because making nanomaterials is still a difficult andexpensive process by coatings industry standards, andstabilizing the materials in a formulation is difficult, vendors

are offering premanufactured dispersions in variousconcentrations, much like pigment dispersions.

Available materials include:

1. n-SiO2

2. n-Al2O3

3. n-ZrO2 and ZrO

4. n-FexOy

5. TiO2


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n-SiO2

Nanoparticular silicon dioxide is used for the followingfunctions in coatings:

1. Transparent improvement of scratch resistance

2. Controlled modification of transparence and

translucence

3. Transparent functionalization of surfaces

4. Control of rheological properties


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n-Al2O3

NanoAlumina is used for the following functions in

coatings:1. Transparent improvement of scratch resistance

2. Controlled modification of transparence and

translucence



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n-ZrO2

NanoZirconia is used for the following functions in

coatings:1. Transparent improvement of scratch resistance



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ZnONanoZinc is used used for the following functions in

coatings:

1. UV absorption



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n-FexO

y

NanoFerrics are is used for the following functions incoatings:

1. UV absorption

2. Improving colorfastness


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TiO2

NanoTitania is used for the following functions in

coatings:

1. UV absorption

2. Controlled modification of transparence andtranslucence


4. Matching of refractive index in optical coatings


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Dispersants and formulation concerns in nanodispersions

All of the usual concerns apparent in pigment dispersionsare also critical in dispersing nanomaterials

pH, in general, is more critical

Traditional dispersants can be used

Biodispersants are a reasonable alternative if sizereduction is not to be accomplished in the process

To illustrate, lets look at an aqueous dispersion ofNanoAlumina in water with an average particle size of 350nm


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We will use 2 different dispersants:1. Anionic surfactant (ammonium salt of

polymethacrylic acid in 25% solution)

2. Bio-surfactant (aqueous solution ofrhamnolipids at 15% concentration

Graph 1 shows settling rate without dispersant added at various pH


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Settling rate as a function of pH without dispersant

The pre-established solid content was dispersed in 100mL distilled water. Suspension was thoroughly stirredfor 10 min, and transferred to 100 mL measuring jar,

where it was allowed to stand undisturbed for 20 to 24hours. Drop in the solid-liquid interface height wasnoted at regular intervals of time.

A i i f l


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Anionic surfactant results:

Settling rate of alumina nanoparticles with anionic dispersant in different concentrations

Same data as a function of pH


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Bio-surfactant result:

Settling rate of alumina nanoparticles with biosurfactant in different concentrations

Same data as function of pH


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Alumina nanoparticles dispersed in water in theabsence of any dispersant in highly acidic andalkaline media are stable. The IEP of alumina

nanopowder used was found to be at pH 9.2. Asynthetic polyelectrolyte (anionic surfactant)induces higher stabilizing action on aqueous

alumina nanodispersion. This dispersion was stablein a wide range of pH. The natural bio-surfactantalso acted as an effective dispersant. Alkaline pHfacilitates stabilizing action in presence of the

biosurfactant while in acidic pH range it is noteffective.


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Health concerns of working with nanoparticles

In the interest of full disclosure, there is much beingconjectured about the health effects of

nanotechnology They break down into the followingareas:

1. Pulmonary effects

2. Integumentary effects

3. Effects at the blood/brain barrier

4. Intestinal effects

5. Oxidative stress effects (liver)

B t il bl t thi ki


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Particles in the nano-size range can certainly enter thehuman body via the lungs and the intestines;penetration via the skin is less evident. It is possible that

some particles can penetrate deep into the dermis. Thechances of penetration depend on the size and surfaceproperties of the particles and also on the point of

contact in the lung, intestines or skin. After thepenetration, the distribution of the particles in the body isa strong function of the surface characteristics of the

particles. A critical size might exist beyond which themovement of the nanoparticles in parts of the body isrestricted.

Best available current thinking:


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The pharmaco-kinetic behavior of different types ofnanoparticles requires detailed investigation and adatabase of health risks associated with differentnanoparticles (e.g. target organs, tissue or cells) shouldbe created. The presence of the contaminates, such asmetal catalysts present in nanotubes, and their role inthe observed health effects should be considered along

with the health effect of the nanomaterials. Theincreased risk of cardiopulmonary diseases requiresspecific measures to be taken for every newly produced

nanoparticle. There is no universal "nanoparticle" to fitall the cases, each nanomaterial should be treatedindividually when health risks are expected.


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Thank you for your kind attention!

Time for questions and answers.

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