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Film formation can take place in one of two ways: Binders of importance to the paint industry are: 3.2.1.1 Solvent-borne paints The binders are dissolved in solvents, both the natural bi

3 CLASSIFICATION AND PROPERTIES OF PAINT

3.1 Some basic chemistry

3.2 Classification

3.2.1 Physically drying

3.2.1.1 Solvent-borne paints 3.2.1.2 Waterborne paints 3.2.2 Chemically curing

3.2.2.1 Oxidative curing 3.2.2.2 Two-component curing 3.2.2.3 Other curing mechanisms 3.3 Classification Diagram

3.4 Properties of Individual Binder Types

3.4.2.1 Oxidative curing

• Alkyds

• Modified Alkyds 3.4.2.2 Two-component curing

• Epoxy

• Polyurethane

3.4.2.3 Other Film-forming Mechanisms

• Humidity curing

• Zinc Silicates (ethyl)

• Polyurethane (one-pack)

• Carbon Dioxide curing

• Zinc Silicates (alkali)

• Heat curing

• Silicone

3.1 Some basic chemistry

The properties of the binder in the paint mainly determine the film-forming properties of

a paint (ref 2.2.1)

A binder is always built up of small units (monomers) to a larger unit, the polymer If the polymer consists of more than one type of monomer, it is called a copolymer If the polymer is built up of the same monomer, it is called a homopolymer The number of

monomers in a polymer can vary from 5 to as much as 3500, and in connection withpaint the number is often quite high

In a dried and/or cured paint film the binder is either found as a bunch of long chains(the polymer chains) or as closely knitted networks (the polymer chains are connected

by cross-linking)

with cross-linkingThe size and shape of the binder polymer will influence the properties of the final paintfilm Longer chain molecules and denser network structures will increase toughness andresistance in the resulting binder, while smaller molecules will result in the bestpenetration into the substrate

3.2 Classification, based on type of binder

The manner in which paint films are formed is determining for many of the key

properties of a paint film This is the reason why paints are often classified according

to their film formation principles

Film formation can take place in one of two ways:

Binders of importance to the paint industry are:

3.2.1.1 Solvent-borne paints

The binders are dissolved in solvents, both the natural binders such as

tar and bitumen and the synthetic types like chlorinated rubber, acrylic and vinyl.

Film formation

Solvents evaporate Binder molecules are intertwined and attracted toeach other and to the substrate

3.2.1.2 Waterborne paints (dispersions)

Water-borne paints are dispersions of small binder particles in water.Very large binder molecules can be incorporated in the binder phase byutilising the dispersion technology

Used in a solvent-borne paint the same size of molecules would result

in too high viscosity, or if diluted to the same viscosity, in a very lowsolid content

Film formation

Water evaporates The binder particles deform and coalesce (or melttogether) assisted by coalescing agents (powerful solvent for the binderparticles) - until they form a continuous film, attached to the surface.Properties common to physically drying paints:

Ò Solvent borne

• Reversible, ie the coating, even months or years after application,

is soluble in its own or more powerful solvents Solventmolecules penetrate in between the binder molecules, push themapart and dissolve the binder

Solvent sensitive; consequent to being reversible, these paints arenot resistant to their own or stronger solvents

• Temperature independent for film formation, since no chemicalreactions are involved in the film formation

• Thermoplastic, ie physically drying coatings become soft atelevated temperatures Like other molecules, the polymersbecome more mobile at higher temperatures and, as they are not

cross-linked to each other, they have a certain freedom of ment, resulting in a softening of the coating.

move-• Excellent recoatability

Ò Waterborne

• Reversible to a certain degree, as the coalescing agent orsolvents of the same or stronger dissolution power will be able tore-dissolve the paint film; however, just adding water will notresult in a re-dispersion of the paint film Re-dispersion is neverpossible

• Solvent sensitive; consequent to the above, solvents similar orstronger in dissolution power to the coalescing agent in questionwill be able to attack the coating

• Temperature dependent for film formation, the softening pointwhere the binder particles are able to melt together, is normally5°C or slightly higher, minimum application temperature is 10°C

• Thermoplastic, as for solvent-borne paints

• Excellent recoatability

3.2.2 Chemically curing

In chemically curing paints the final binder molecules in the dry/cured paint filmare not present in the wet film The smaller "wet" molecules will during and afterapplication take part in a chemical reaction whereby new, larger bindermolecules are formed The film is formed by the cross-linking of the molecules(polymerization) and often by the evaporation of solvents also

Binders of importance to the paint industry are:

3.2.2.1 Oxidative curing

Binders which are most often dissolved in solvents and which form the

final binder with oxygen in the air, eg oleoresins, alkyds, modified

3.2.2.3 Other curing mechanisms

Binders can be formed by other chemical reactions or polymerisationprocesses, eg

• Reaction between binder molecules and water present as humidity inthe air like zinc ethyl silicate and one-pack polyurethane

• Reaction between binder molecules and carbon dioxide in the airlike sodium, potassium and lithium zinc silicates (alkali zincsilicates)

• Reaction triggered off by high temperature as in the case of silicone

by curing for a few hours at 200°C

Properties common to chemically curing paints:

• Irreversible, ie the cured coating is not dissolvable

• Resistant to solvents (consequence to being irreversible)

• Temperature dependent on film formation as the rate of the chemicalreaction taking place during curing is directly related to temperature,

ie there is a temperature limit below which film formation will nottake place

• Non-thermoplastic The binder molecules in the tight cross-linkingare not able to vibrate or move even at high temperature, ie the paintfilm does not become softer at higher temperatures

• Critical recoating interval Recoating must take place beforecomplete curing has taken place If curing is completed, the surfacemust be mechanically roughened before application of new paint

3.3 Classification Diagram

(most important binder types only)

PolyurethaneZinc Ethyl SilicatesZinc Alkali SilicatesSilicones

Others

EpoxiesPolyurethanePolyestersTwo-pack

OleoresinousAlkydsModified AlkydsOxidation

Chemicallycuring

AcrylicsVinyl acetate copolymerVinyl chloride copolymerStyrene-butadiene copolymerWaterborne

Chlorinated RubbersAcrylics

Vinyl

TarsBitumenSolvent borne

Physicallydrying

3.4 Properties of Individual Binder Types

The following is a survey of some of the more important binders used in coatings for theprotection of marine and industrial structures The properties listed are general and may,for the actual coating, vary somewhat according to formulation and possiblemodification

3.4.1 Physically drying

3.4.1.1 Solvent-borne paints

Tars and Bitumen Coal tars are obtained by distillation from coal, formerly from

gasworks, but today, mainly from steel works They consist of a mixture

of liquid tar oils and solid coal tar pitches

Bitumen is more often than not the heavy residues from oil distillations

though it may still be found in nature

As opposed to most other binders used for protective coatings, coaltars and bitumen are often used without pigmentation Their high content

of carbon make them virtually opaque and black

Tars and bitumen are low cost barrier coatings, often specified behindlinings and as maintenance coating in ballast tanks

General advantages and limitations of tars and bitumen are:

• Excellent water resistance

• Poor weathering, will crack when exposed to sunlight (due toevaporation of low boiling oils)

• Fair chemical resistance

• Poor solvent resistance

• Excellent penetration and adhesion

• Low cost

• Give rise to bleeding; ie small molecules (the low boiling tar oils)will migrate to the surface and cause discoloration of any non-blacktopcoat

• Black or very dark colours only

Due to the excellent water resistance of coal tar, it is often used incombination with other binders, epoxy, vinyl and polyurethane.

Chlorinated Rubbers

As the name indicates chlorinated rubber resin is formed by addingchlorine to rubber In the early days only isoprene from natural rubberwas utilised but, nowadays, most of the isoprene is extracted in the oilindustry

After application and the evaporation of its solvent, chlorinated rubberleaves a dense but very brittle film A softening agent, a plasticizer, istherefore always incorporated in a chlorinated rubber-based paint

Chlorinated rubber-based paints are used extensively as anticorrosivecoatings, coatings for concrete and other alkaline substrates

General advantages and limitations of chlorinated rubber coatings are:

• Good water resistance

• Relatively good weathering

• May yellow and chalk (chlorine content causes yellowing)

• Fair chemical resistance

• Good alkali resistance

• Poor resistance to animal and vegetable oils and fats

• Contain plasticizers (and many properties may depend upon the type

mono-Used on exterior steel work above the waterline

General advantages and limitations of acrylic coatings are:

• Relatively good water resistance

• Good weathering

• Good colour retention

• Low to fair resistance to animal and vegetable oils and fats

• Usually only fair wetting properties

Vinyl form fairly brittle films, so plasticizers might be incorporatedinto vinyl-based paints

Vinyl-based paints are used on exterior steel work, and when combinedwith tar extensively used as anticorrosive primer

General advantages and limitations of vinyl coatings are:

• Good to excellent water resistance

• Good weathering

• Good chemical resistance

• Resistant to weak solvents

• Fair resistance to animal and vegetable oils and fats

• Usually only fair wetting properties

• Relatively quick drying

• Relatively low solid content and, consequently contain high amount

3.4.1.2 Waterborne paints

General advantages and limitations of dispersions are:

• Contain only limited amount of high boiling solvents (the coalescingagent)

• No flash point

• Sensitive to high humidity during application

• Sensitive to low temperatures during storage and application (watermay freeze)

• Poor levellingAdditional properties can be obtained by combining various monomersinto different copolymers as indicated below

Vinyl acetate copolymer

Used in combination with various plasticizing monomers, vinyl acetatesare mainly used in house paints

Special properties:

• Relatively good water resistance

• Relatively good colour stability

• Low cost

Acrylic copolymer

The type and number of different acrylic monomers in an acrylic bindercan vary extensively, resulting in tailor-made properties for eachspecific binder

Acrylic-based paints are used as house paint (especially exterior) andwood varnish

As indicated, properties of acrylic binders can be very different, but ingeneral they have

• Good water resistance

• Good colour retention

• Good gloss retention

• Good adhesion to a variety of different substrates

The reaction product between isocyanate and the hydroxyl containingbinder, ie the polyurethane (ref 3.4.2.2), is present as complete binderparticles in the dispersion Paints based on a polyurethane dispersionare consequently one-component products

They are used as sole binder or in combination with acrylic as woodvarnish, floor varnish or paint, and paints for plastics

Special properties are:

• Good adhesion to plastics

• Good abrasion resistance

• Limited resistance to outdoor exposure

• High cost

Vinyl chloride copolymer

Used in combination with plasticizing monomers, eg vinylidenechloride and acrylic

Can be used as an anticorrosive primer on steel

Special properties are:

• Good water resistance

• Yellowing

Styrene butadiene copolymer

A ratio of approximately 60% styrene monomer and 40% butadienemonomer is used as binder in anticorrosive high-builds, on areas notexposed to sunlight

Special properties are:

• Good water resistance

• Gets brittle and yellow when exposed in UV-light (sunlight) andoxygen from the air

• Comparatively low cost

3.4.2 Chemically curing

3.4.2.1 Oxidative curing

The key component of paints forming film by an oxidation process, is adrying oil Drying oils are natural products, eg linseed oil, soya oil orfish oil Drying oil consists of fatty acids combined with glycerol Byreacting the fatty (drying oil) acid with other components, larger andmore complex molecules are formed, eg phenolic varnishes and alkydresins, having more durable properties

Alkyds

Alkyds are often referred to as being short, medium or long oil alkyds.Short oil alkyds contain less than 40% fatty acid and are mainly used incoatings for production-line application Medium oil alkyds containfrom 40-60% fatty acid, and long oil alkyds more than 60%

Alkyd paints are used on exterior steel work (above waterline) and ininterior dry rooms

General advantages and limitations of medium oil alkyds are:

• Poor water resistance after long periods of exposure

• Reasonable to good weathering

• Reasonable to good gloss retention

• Reasonable colour retention

• Poor alkali resistance

• Solvent is mineral spirits, a comparatively weak solvent

• Good penetration and adhesion

Modified Alkyds

Alkyds may be modified with a wide range of other binders to providenew binders with specific properties

Typical modifications are:

• Styrenated alkyd Copolymerising alkyd with styrene gives an

alkyd which is faster drying, more chemical and water resistant, butless tolerant of poor surface preparation

• Urethane alkyd Isocyanate modified alkyd resin improves the

drying somewhat, and the components are usually selected toprovide additional hardness Often used for primers required towithstand rough handling Due to their good through-drying proper-ties, they are favoured as binders in high-builds

• Silicone alkyd When the silicone content exceeds 30%, silicone

alkyd is durable and retains it's gloss Especially used for finishingcoats exposed to intense sunlight

• Epoxy ester Epoxy ester is sometimes, but wrongly, referred to as

a one-component epoxy Epoxy ester is a drying oil modified withepoxy This hard synthetic resin is faster drying and has better waterand alkali resistance than alkyds, but it chalks and is comparativelyexpensive

Epoxy esters exhibit good adhesion and good anticorrosive ties; often used for primers required to withstand rough handling orforced drying at elevated temperatures