47762547 Surface Coating1

8/3/2019 47762547 Surface Coating1

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PaintPaintPaintPaint cancan bebe defineddefined asas aa fluidfluidmaterialmaterial whichwhich whenwhen spreadspread overover aa

surfacesurface inin aa thinthin layer,layer, willwill formform aasolid,solid, cohesivecohesive andand adherentadherent filmfilm..

GenerallyGenerally usedused forfor decorativedecorative

purposespurposes sincesince longlong..


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Ingredients of PaintIngredients of Paint

y Liquid paint contains three major ingredientstogether

y Major ingredients arex Pigments (including Extenders)

x Binder (or film former)x Solvent or Thinner

Pigment is dispersed in the binder and constitutes the paintfilm. The properties depend largely on the nature of binderas well as pigment.

Solvent is used to render the pigment/binder mixturesufficiently fluid for application as a thin film after which it islost by evaporation and plays no part in the performances ofthe dry paint film.


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y Other additives used arex Driers

x Anti-skinning

x Anti settling agents

x Fungicides

x Surface active agents assist pigment dispersion


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PigmentsPigments

y Finely divided solids, insoluble in the media

y Average particle size vary from 0.2-10 m

y

May be organic or inorganicy Confers the following properties

Colour aesthetic appeal

Obliteration or hiding power

Protective properties

Corrosion repression


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Classification of PigmentsClassification of Pigments

Pigments

Inorganic

Natural

True Pigments Extenders

Manufactured

Extenders True Pigments

Organic(Manufactured)


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y Extenders are inorganic in nature

y They differ from true pigments in their

behaviour when dispersed in organic media

y True pigments exhibit opacity or hiding power

in varying degree, whereas extenders are

practically transparent.

y Extenders are used in certain types of paints

(notably undercoats, primers and some low-

gloss finishes) to modify or control physical

properties like gloss and flow properties.y They make no contribution to colour (unless

impure) or to opacity.


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y A pigmented film is more weather

resistant than an unpigmented film of thesame binder.

y Majority of natural pigments are oxides or

hydroxides of iron, may also contain clay

or siliceous matter.

y These colours are less bright than

corresponding manufactured oxides and

hydroxides.


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Inorganic and organic pigments

y Manufactured inorganic pigments containwhites and wide range of coloursincluding yellows, reds, oranges, greens

and blues.y Carbon black is usually included in

inorganic pigments.

y Organic pigments cover the entirespectrum range, but brilliance and opacityvary considerably.

y There are no white organic pigments


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Inorganic and organic pigments

y Organic pigments are generally brighter

than inorganic counterpart

y Chemical constitution of pigments and

dyes is set out in the colour index.


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Wetting agentWetting agent

y Wetting is the ability of a liquid to maintain contact witha solid surface, resulting from intermolecular interactions

when the two are brought together. The degree of wetting

(wettability) is determined by a force balance

between adhesive and cohesive forces.

y Wetting is important in the bonding or adherence of two

materials. Wetting and the surface forces that control

wetting are also responsible for other related effects,

including so-called capillary effects. Regardless of the

amount of wetting, the shape of a liquid drop on a rigidsurface is roughly a truncated sphere. Various degrees of

wetting are summarized in the table.


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Wetting of different fluids. A shows a fluid with very little

wetting, while Cshows a fluid with more wetting. A has a

large contact angle, and Chas a small contact angle.

Droplet of water on an ideal surface.


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AdhesiveAdhesive forces between aforces between a liquidliquid andand solidsolid cause a liquid drop to spread acrosscause a liquid drop to spread across

thethe surfacesurface.. CohesiveCohesive forces within the liquid cause the drop to ball up andforces within the liquid cause the drop to ball up and

avoid contact with the surface.avoid contact with the surface.

Contactangle

Degree ofwetting

Sol./Liq.interactions

Liq./Liq.interactions

= 0Perfect

wettingstrong weak

0 < < 90high

wettability

strong strong

weak weak

90 < 180low

wettabilityweak strong

= 180perfectly

non-wettingweak strong

The contact angle (), as seen in Figure 1, is the angle at which the liquid-

vapor interface meets the solid-liquid interface. The contact angle is

determined by the resultant between adhesive and cohesive forces. The

tendency of a drop to spread out over a flat, solid surface increases as the

contact angle decreases. Thus, the contact angle provides an inverse

measure of wettability.


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Wetting agentWetting agent

y Surfactants are compounds that lower

the surface tension of a liquid, allowing

easier spreading, and lowering of

the interfacial tension between twoliquids, or between a liquid and a solid.

Surfactants may act as detergents, wetting

agents, emulsifiers, foaming agents,and dispersants.


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Dispersing AgentDispersing Agent

y A dispersant or a dispersing agent or

a plasticizer or a superplasticizer is either a

non-surface active polymer or a surface-

active substance added to a suspension,usually a colloid, to improve the

separation of particles and to

prevent settling or clumping. Dispersantsconsist normally of one or

more surfactants, but may also be gases.


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y In order to provide optimal performance,

pigment particles must act independentlyof each other in the coating film and thus

must remain well dispersed throughout

manufacture, storage, application, and film

formation. Unfortunately, colloidal

dispersions such as the pigment

dispersions in liquid coatings are

inherently unstable, and they must bestabilized against the flocculation that

might occur.


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AntiAnti--sagging agentsagging agent

y

Rheological additive used toprevent sagging in wet coatings where it

increases the viscosity.

y Sagging

Coating defect: unsightly gravity-driven

flow of a paint film applied on vertical

surfaces. Caused by to too much flow,

often related to application technique orenvironment. Sagging can be prevented by

rendering the paint pseudo-plastic, but

this may deteriorate the leveling.


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AntiAnti--settling agentsettling agent

y Additive that impedes sedimentation of

pigments, extenders and fillers by

imparting increased low shear viscosity or

a small yield value to liquid paints.


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AntiAnti--skinning agentskinning agent

y Substance added to a material to prevent

or retard the processes of oxidation or

polymerization which result in the

formation of an insoluble skin on thesurface of the wet coating.


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AntiAnti--floating/flooding agentfloating/flooding agent

y Additive preventing floating/flooding defects

in coating applications.

y Floating/Flooding

Coating defect: concentration of pigments at

the surface of the paint (floating) or in stains

and patches inside the coating film (flooding).

This defect leads to a change in color.


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FungicidalAgentFungicidalAgent

y Substance capable of destroying or

preventing the growth of fungi. Fungicides

do not provide any residual protection

from future mould growth.

Ref:http://www.specialchem4coatings.com/resources/glossar

y/index.aspx?id=A


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Pigmentary PropertiesPigmentary Properties

y Particle Size: influences gloss, opacity, freedom

from settlement and consistency

y Size or avg diameter is usually expressed in m

(m=0.001mm)y Not uniform particle size. Varies over a range

y Modern pigments are generally free from both

coarse particles and ultrafines.

y In many natural pigments and extenders the

particle sizes are greater than those of prepared

pigments.


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y Determination of Particle Size: by sieve

analysis method.

y Sieving methods are of little use for

pigments at present time as these are

supplied with a particle size ranging from


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Particle Size DistributionParticle Size Distribution

y A pigment contains a range of particle

sizes. Hence it is usual to express this as a

particle size distribution, i.e. the

percentage (by no or wt) occurringbetween certain limits, e.g. below 0.1 m,

0.1 to 0.2 m, 0.2 to 0.3 m and so on.


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Particles % by wt or noParticles % by wt or no vsvs ParticleParticle

SizeSize


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y A very large no of methods have been suggested

for the determination of particles size distribution.

1. Sedimentation Methods based on Stokes

equation. Pigments are separated into a no of

fractions under gravity or by the use of centrifuge

v = [2r2 (1

2)g]

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v=velocity of fall of the particle

r=radius of particle 1=density of particle

2=density of liquid =coeff of viscosity of the liquid

g=gravitational constant


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y Also r = d/2 v=h/t

d=[1812)gt]


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Air Elutriation MethodAir Elutriation Method

y

The general principle underlying the methodsconsists of determination of the proportion of a

pigment which is removed by an upward flow of a

gas at a given velocity in a vertical column.

y The Stokes equation is used to calculate the largestsize of particle removed by a gas at the given

velocity.

y In the determination the gas velocity is first

adjusted so that the smallest particles are carriedoff and collected. The velocity is then increased to

remove the next fraction and the process repeated

until the entire range has been separated.


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y There are many other methods for

determining particle size distribution,like

Optical Microscope method, Electron

Microscope method, shadowing, surfacereplica etc.


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Surface Area of Pigment ParticlesSurface Area of Pigment Particles

y Many paint properties, including viscosity,

flow characteristics and dispersion

stability can be influenced by interaction

between pigment and medium.

y Adsorption of certain polymer fractions

and/or driers onto the pigment surface

takes place frequently, and the totalamount of adsorption will depend on the

nature and surface area of pigment

particle.


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y Surface area (m2/gram)=6/D

Where D=volume/surface mean diameter,i.e. diameter of a particle possessing

same volume-to-surface ratio as the

whole sample

=specific gravity of the pigment

6=factor applicable only to spherical

particles; with particles of irregular shape

the expression can give only approximatefigures.


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Surface area and oil absorptionSurface area and oil absorption

y The oil absorption value of a pigment

(represents the amount of oil required to coat

the surface and to fill the voids) increases with

decreasing particle size.y Smaller voids result form closer packing of

smaller particles are more than offset by the

increased surface area.

y Pigment particles may be smooth or irregularindividuals, or they may be aggregates of various

sizes.


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y These will all behave as individual particles

towards oil and other media.

y In case of aggregates the practical surface

area will be appreciably less than total

surface area of component particles.

y Difference depends on type of pigment anddegree of grinding.

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47762547 Surface Coating1