Composition of Paints

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Composition of Paints
Paints are made of numerous components, depending on the method of application,
the desired properties, the substrate to be coated, and ecological and economic
constraints. Paint components can be classified as volatile or nonvolatile.
Volatile paint components include organic solvents, water, and coalescing agents.
Nonvolatile components include binders, resins, plasticizers, paint additives, dyes,
pigments, and extenders. In some types of binder, chemical hardening can lead to
condensation products such as water, alcohols, and aldehydes or their acetals, which
are released into the atmosphere, thus being regarded as volatile components.
All components fulfill special functions in the liquid paint and in the solid coating
film. Solvents, binders, and pigments account for most of the material, the proportion
of additives being small. Low concentrations of additives produce marked
effects such as improved flow behavior, better wetting of the substrate of pigment,
and catalytic acceleration of hardening.
Solvents and pigments need not always be present in a coating formulation.
Solvent-free paints and pigment-free varnishes are also available.
The most important component of a paint formulation is the binder. Binders
essentially determine the application method, drying and hardening behavior, adhesion
to the substrate, mechanical properties, chemical resistance, and resistance to
weathering.

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Binders and Resins
Binders are macromolecular products with a molecular mass between 500 and
ca. 30000. The higher molecular mass products include cellulose nitrate and polyacrylate
and vinyl chloride copolymers, which are suitable for physical film formation.
The low molecular mass products include alkyd resins, phenolic resins, polyisocyanates,
and epoxy resins. To produce acceptable films, these binders must be
chemically hardened after application to the substrate to produce high molecular
mass cross-linked macromolecules.
Increasing relative molecular mass of the binder in the polymer film improves
properties such as elasticity, hardness, and impact deformation, but also leads to
higher solution viscosity of the binder. While the usefulness of a coating is enhanced
by good mechanical film properties, low viscosity combined with low solvent content
are also desirable for ease of application and for environmental reasons. Therefore,
a compromise is necessary.
The low molecular mass binders have low solution viscosity and allow low-emission
paints with high solids contents or even solvent-free paints to be produced.
Here, the binder consists of a mixture of several reactive components, and film
formation takes place by chemical drying after application of the paint. If chemical
hardening occurs even at room temperature, the binder components must be mixed
together shortly before or even during application (two- and multicomponent systems).
Today, most binders are synthetic resins such as alkyd or epoxy resins.
The natural resin most commonly used as a binder today is rosin, which is often
tailored by chemical modification to suit specific applications. Also, many synthetic
hard resins mainly based on cyclohexanone, acetophenone, or aldehydes, are used
in the paints industry. Hard resin binders increase the solids content, accelerate
drying, and improve surface hardness, luster, and adhesion.
Most synthetic binders are softer and more flexible thant hard resins. Consequently,
they impart good elasticity, impact resistance, and improved adhesion, even to
critical undercoats, as well as offering adequate resistance to weathering and chemicals.
These binders are produced with a property profile tailored to suit particular
application methods and to comply with a range of technical requirements, including
environmental protection, low toxicity, and suitability for recycling and disposal.

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Plasticizers
Plasticizers are organic liquids of high viscosity and low volatility. The esters of
dicarboxylic acids ( e g , dioctyl phthalate) are well-known examples. Plasticizers
lower the softening and film-forming temperatures of the binders. They also improve
flow, flexibility, and adhesion properties. Chemically, plasticizers are largely inert
and do not react with the binder components. Most binders used today are inherently
flexible and can be regarded as "internally plasticized" resins. For this reason, use
of plasticizers has declined.

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3. Pigments and Extenders
Pigments and extenders in coatings are responsible for their color and covering
power, and in some cases give the coating film improved anticorrosion properties.
Pigments and extenders are finely ground crystalline solids that are dispersed in the
paint. They are divided into inorganic, organic, organometallic, and metallic pigments.
By far the most commonly used pigment is titanium dioxide. As a rule,
mixtures of pigments are used for technical and economic reasons. The hiding power
and tinting strength of a paint depend on the particle size of the pigment. The usual
size range aimed at is 0.1 -2.0 pm, which means that the pigment has a high surface
area that must be wetted as effectively as possible by the binder components to give
the coating film good stability, weathering resistance, and luster. This is achieved by
bringing the pigment and binder into intimate contact under the influence of high
shear forces. The high hiding power of some pigments enables them to be partially
replaced by the cheaper extenders such as barium sulfate, calcium carbonate, or
kaolin. Extenders have a particle size distribution similar to that of the pigments and
are incorporated into the coating in the same way. The concentration of pigment in
coating films is expressed by the pigment volume concentration (PVC). This is the
ratio of the volume of pigments and extenders to the total volume of the nonvolatile
components. Each coating system has a critical pigment volume concentration
(CPVC) at which the binder just fills the free space between the close-packed pigment
particles. At higher pigment concentrations, the pigment particles in the coating film
are no longer fully wetted by the binder, leading to a marked deterioration in coating
film properties such as luster, stability, strength, and anticorrison properties.

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4. Paint Additives
Paint additives are auxiliary products that are added to coatings, usually in small
amounts, to improve particular technical properties of the paints or coating films.
Paint additives are named in accordance with their mode of action.
Leveling agents promote formation of a smooth, uniform surface on drying of the
paint. Suitable materials include certain high-boiling solvents such as butyl ethers of
ethylene glycol, propylene glycol and diglycols, as well as cyclohexanone and alkylated
cyclohexanones, and in some cases aromatic and aliphatic hydrocarbons. Low
molecular mass resins (e.g., some polyacrylates and silicones) are also used. Solid
leveling agents, such as special low molecular mass resins, are also useful for improving
the surface properties of films produced from powder coatings. Flow agents act
by reducing the paint viscosity during drying. The effectiveness of a particular flow
agent depends on the type of binder and the drying or hardening temperature.
FilniTformation promoters, which are closely related to flow agents, reduce the
film-forming temperature for film formation from dispersions, leading to a surface
that is as pore-free and uniform as possible. Certain high-boiling glycol ethers and
glycol ether esters are used, often in combination with hydrocarbons.
Wetting Agents, Dispersants. and Antisefting Agents. Wetting agents from one of
the largest groups of coating additives. These are surfactants which aid wetting of the
pigments by the binders and prevent flocculation of the pigment particles. This leads
to the formation of a uniform, haze-free color and a uniformly high luster of the coating film. This group also includes the dispersants, which give good pigment
wetting and hence optimum dispersion of the pigments in the paint, thereby preventing
sedimentation particularly of high-density pigments. As well as good wetting
properties, some pseudoplasticity is also necessary. Antisetting agents have similar
characteristics to dispersants.
Ant$oaming agents are used to prevent foaming during paint manufacture and
application and to promote release of air from the coating film during drying.
Various products are used, including fatty acid esters, metallic soaps, mineral oils,
waxes, silicon oils, and siloxanes, sometimes combined with emulsifiers and hydrophobic
silicas.
Catalysts are added to paints to accelerate drying and hardening. They include
drying agents (driers, siccatives), which, in the case of the air-drying binders (including
some alkyd resins or unsaturated oils), accelerate decomposition of the peroxides
and hydroperoxides that form during the drying process, thereby enabling radical
polymerization of the binders to take place. The driers used are mainly metallic soaps
such as cobalt naphthenate; manganese, calcium, zinc, and barium salts; and zirconium
compounds.
Various products are used to catalyze the cross-linking of binder systems at room
temperature. For acid-catalyzed systems such as polyester - melamine resin systems,
free acids, their ammonium salts, or labile esters are suitable. while for base-catalyzed
systems such as polyester - isocyanate, tertiary amines or dibutyltin dilaurate
are used. The amount of catalyst used must be such that the pot life is not impaired.
Antifloating and antiflooding agents prevent horizontal and vertical segregation of
pigments with different densities and surface properties. This prevents differences in
the color and luster of the surface of the film, which can lead to a blotchy appearance.
Antiskinning agents are added to air-drying paints to prevent surface skin formation
caused by contact with atmospheric oxygen. In the film, they produce uniform
drying and prevent shrinkage (wrinkling). Chemically, these materials are antioxidants
such as oximes, which evaporate with the solvents during the drying process.
Matting agents are used to produce coatings with a matt, semi-matt, or silk finish.
They include natural mineral products such as talc or diatomites and synthetic
materials such as pyrogenic silicas or polyolefin waxes. Matting can also be obtained
by special formulations that exploit the incompatibility between binder components
and their cross-linked structures.
Neutralizing agents are used in waterborne paints to neutralize binders and stabilize
the product. Ammonia and various alkylated aminoalcohols are used, depending
on the type of binder and method of application. On hardening, the amines mainly
evaporate along with the water.
Thickening agents control the rheological properties of paints of various types.
They include inorganic (mainly silicates), organometallic (titanium and zirconium
chelates), naturally occurring organic (mainly cellulose ethers) and synthetic organic
products (polyacrylates, polyvinylpyrrolidone, polyurethanes).
Preservarives (biocides, fungicides) prevent the attack of paint systems, principally
water-based, by microorganisms.
Corrosion inhibitors are used to prevent the formation of corrosion products when
waterborne paints are applied to metallic substrates (flash rust). They include oxidizing salts such as chromates, metaborates, nitrites, and nitrates; organic amines or
sulfur-containing products; and organic salts (benzoates, naphthenates, octoates).

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5. Solvents
Solvents are compounds that are normally liquid at room temperature. Those
most commonly used in coatings technology are aromatic and aliphatic hydrocarbons,
esters of acetic acid, glycol ethers, alcohols, and some ketones. Solvents dissolve
solid and highly viscous binder components. They enable incompatibility between
paint components to be overcome, improve pigment wetting and dispersion,
and control storage stability and viscosity of the coating. They promote the release
of included air from the liquid coating film, control the drying behavior of the
coating, and optimize flow properties and luster. Organic solvents are used in most
liquid coatings systems, including, waterborne coatings, in which they perform important
fluctions.
After paint application, the solvents should evaporate as quickly as possible,
leaving the film. If no special precautions are taken, the solvents enter the atmosphere
as pollutants. To protect operating personnel from the toxic effects of evaporating
solvents, safety measures such as ventilation and air exhaust are necessary.
To protect the environment, incineration and sometimes solvent-recovery plant is
installed to prevent solvents entering the atmosphere. Other measures for the protection
of the workplace and the environment from solvent vapors include the development
and use of new low-solvent or solvent-free coatings, e.g., high-solids paints,
waterborne coatings, and powder coatings.