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Liquid Organic Coatings(2)
- Jul 30, 2018 -

3.3 FILM FORMATION

The resin is the primary ingredient of an organic coating, the part of the coating

that forms a film when it dries. The resin is mixed with other ingredients to create an

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FIGURE 3.1 Paint viscosity at different temperatures.

engineered product for application. As the coating is applied, it will flow and stabilize into a relatively uniform film. This stage is referred to as fixation. After the coating has stabilized, it will be cured by time, temperature, or some other means.


3.3.1 APPLICATION

Application is the spreading of the composition into a thin layer over the surface of a product. Many methods of application can be used including a brush, a roller, a spray gun, curtain coating processor, or electrodeposition system.

The method of application and effective process control will determine the thickness of the coating, uniformity, and efficiency. The application equipment and set-up must be compatible with the particular viscosity and flow characteristics of the coating to provide uniform and consistent results.

3.3.2 FIXATION

During fixation, the coating is stabilized so that it will not run off or form an uneven layer on the painted surface. Fixation usually occurs by evaporation of volatile solvent from an organic solvent system or of water from a latex system. Rheological resistance to flow grows within the film as the nonvolatiles concentrate. Fixation may also result from thixotropy built into the paint by the formulator.

An ideal liquid paint would be free flowing during application, flow evenly over the work surface, stabilize quickly, and remain in place. In reality, there is a time interval between the flowing and the nonflowing states. A paint material may never actually be completely free flowing. In many cases, some resistance to flow has to be built into the paint to avoid sagging on the work surface.

Several conditions affect the formulation for flow.

• The solvents used must have sufficient dissolving power for the resin in

the paint.

• The desired final dry-film thickness and the number of coats.

• The amount of surface texture that can be tolerated.

The behavior of the film is affected by the rate of evaporation of the solvent mix and the rheology built into the paint.

3.3.2.1 The Evaporation Factor

Some house paints that are formulated with raw and modified vegetable-type oils arelow enough in viscosity to be free flowing without solvent addition. However, most other coating compositions need solvent to fluidize the solid resins in the binder. The solvents used to blend the coating evaporate at different rates during application and drying.

In the simplest model, evaporation takes place through at least two mechanisms. One is solvent evaporation from the surface of the film that is similar to the same factors as evaporation from the surface of the pure solvent by itself. The other is solvent diffusion from within the body of the film from the lower depths to the surface. The rate of solvent evaporation can affect fixation.

Some problems can occur during solvent evaporation. The rapid cooling effect that occurs as solvent evaporates from the applied film will also cool water vapor in the atmosphere around the paint film. If its vapor pressure is higher than thesaturated vapor pressure of water at the surrounding ambient temperature, it will cause condensation on the paint film. The condensed moisture is trapped in the coating where it can cause a white discoloration called solvent-blush.

Another problem sometimes experienced during spray application is dry spray.

If a particular solvent evaporates in the atomized paint stream before it reaches the surface of the part, it may affect the flow-out of the coating and create a grainy appearance. Dry spray is often caused by problems with spray technique (gun too far from target), but it can also be a result of using the wrong solvent or the incorrect volume of a particular solvent.

3.3.2.2 Rheology

Even a material with high viscosity will tend to flow downward on a vertical surface. A paint material needs some physical structure to prevent sagging. Finely divided powders that make up the pigments and extenders in the paint can provide physical structure through thixotropy.

When stirring causes a temporary decrease in viscosity, the paint is said to be thixotropic. The more vigorously a thixotropic paint is stirred, the less viscous it becomes. The viscosity will go up again if the stirring is stopped.

Thixotropy is often due to reversible flocculation of pigment. If the pigment flocculates without settling, the network of loosely associated particles resists the movement of a brush or stirring paddle. As the brush or paddle is forced through the liquid, the flocculated structure breaks down and allows the liquid to flow more easily. Once the stirring has ceased, the particles again form a network in the container. The network restores “body” to the paint and the viscosity increases.

When mechanically disturbed, such as agitation during application, a thixotropic composition thins out to become comparatively free flowing. Left in an undisturbed state, it will build up structure over time. The time, depending on the actual composition, may be a fraction of a second, minutes, or even hours, and the ultimate level of structure may be low or high. Thixotropy may therefore be harnessed to provide a nearly ideal coating, free flowing during application but rigid immediately

thereafter.

A coating needs to be uniformly mixed to perform properly. There are other conditions commonly encountered in a coating where ingredients separate due to flocculation, settling, or kick-out.

3.3.2.3 Flocculation and Settling

Flocculation is the formation of loose clusters of pigment particles in a liquid paint.

A certain amount of flocculation is desirable in a paint, because it gives the liquid more “body” than what it has with isolated pigment particles and helps resist sagging.

Flocculated pigments that have settled to the bottom of the paint container may sometimes

be successfully blended back into the material unless they form hard, compact layers.

3.3.2.4 Kick-Out

The term “kick-out” refers to the binder coming out of solution as small lumps of soft or hard material. Sometimes the process is reversible, but more often the paint is ruined. Kick-out results when the binder and the solvent are no longer compatible. The binder may kick-out if the wrong thinner is used. Kick-out usually occurs because a paint is too old and the binder has chemically changed to the point where it is incompatible with the original solvent blend. This is one reason why the oldest paint in stock should be used first.

3.3.3 CURE

Fixation is followed by cure. Fixation and curing may be combined or overlapped. Common lacquers combine fixation and cure in the evaporation of solvents during and after application. A catalyzed paint material begins to combine fixation with curing.

Somecoatings are in or near their final state of cure when they complete the fixation period. Lacquers are a broad group of coatings that cure by solvent evaporation alone.

Many coatings need thermal energy or light energy to cure. Thermosetting products are very common for industrial applications.

3.3.3.1 Air Dry

In an air-dry coating, the formulator sets the period of cure with the proper blend of drier catalysts. As a rule, the oxidation process does not start as soon as the paint is spread and exposed to the air. An introduction period is involved, after which the unsaturated vegetable-type acid esters in the paint start to absorb oxygen to create chemically reactive sites in the constituent molecules. These molecules combine to form larger molecules, which result in a paint film.

Actually, the drying process never stops. It slows down greatly and must be controlled to avoid the ultimate formation of an excessively brittle film.

An increase in temperature will increase the rate of the chemical reaction. Curing processes that take place at normal temperatures do so much faster under the influence of heat.

3.3.3.2 Catalyzed Materials

Catalyzed materials have two major or more constituents that will begin a curing reaction when they are mixed together. The cure time varies and temperature may vary, but is possible to reach full film properties, even at room temperature. Application of heat will accelerate the cure cycle.

Obviously, the coating components are separately packaged until they are blended for application. A catalyst may be added, if it cannot be included in one of the components. Catalyzed materials are typically very durable coatings.

One example of a two-component coating is the epoxy-polyamide system. Epoxy resins are chemically reactive oxygen atoms that are bound in the molecule to two adjacent carbon atoms to form a three-part ring. Such rings are sensitive to a number of other reactive groups. The polyamide resins are low-molecular-weight relatives of the nylons. Their amide groups can tie up rapidly at room temperature with epoxy groups to form large, bulky molecules that are excellent binders in coatings.

3.3.3.3 Baking Enamels

Some coatings do not cure at room temperature and fortunately remain stable in the paint container for extended periods. They convert rapidly (in a fraction of an hour) when exposed to heat. Typical temperatures for baking enamels range from 280 to 350◦F (135–175◦C) for 10–30 min.

Amine resins are one of the many convertible coatings of this type. Broadly speaking,they divide into urea–formaldehyde derivatives and melamine–formaldehyde derivatives. When heated, these products can become brittle, so many common baking enamels are formulated with plasticizers, such as alkyd resins.

High molecular weight variations of certain resins are tougher and more durable

but unfortunately much less soluble than their chemical counterparts of lower

molecular weight. Therefore, even though they would provide excellent finished

films, these high molecular weight materials cannot be dissolved in solvents.

These high-molecular-weight resins in fine powder form can be mixed with pigment

and with a nonvolatile liquid to yield a tough, durable coating. The nonvolatile

liquid serves as a plasticizer for the resin. When it is heated, the resin will soften

and create a matrix that includes the plasticizer and the pigment. These materials

are called plastisols and are commonly based on high-molecular-weight polyvinyl

chloride and related polymers. This is cure by fusion.

If the plastisol mixture is too high in viscosity to be properly applied by the

selected method, it may be thinned with a minor amount of solvent for the plasticizer.

Liquid Organic Coatings 87

These variations are called organosols. After the solvent evaporates from the film, the

cure is also by fusion.

A coat of latex paint dries in two stages. First, the water evaporates and all the

ingredients, other than the latex globules, are concentrated in the space between the

globules. These become more closely packed in a geometric array and ultimately

touch each other. The film at this juncture has no particular strength and may be

easily wiped away.

Second, if a latex film is warmed sufficiently, the globules will sinter at their

points of contact and form a hardened coating. Of course, it is not practical to apply

heat to paint on a wall. Fortunately, it is possible to design latex compositions that

coalesce at ordinary ambient temperatures. It is possible to create latex-based systems

in which coalescence does not take place in the bulk package in storage but proceeds

smoothly once the coating has been applied as film.

Cure is a relative term. What may be an adequately cured film for gentle handling

may still be raw in respect to solvent attack. Many methods are available for

testing the degree of cure. The individual end use will dictate the most appropriate

method.

3.3.4 DRY FILM PROPERTIES

Once the paint is cured, the concern switches to dry film properties. In some

respects, the properties of a dry paint film are the most important of all since it

is the dry film, which is exposed to the service environment and to the customer’s

inspection.

3.3.4.1 Gloss

Ahigh-gloss surface reflects nearly all of the light that falls upon it. Reflection of more

than 90% of the incident light is possible with modern industrial finishes. High gloss

requires a smooth surface. Surface roughness results in scattering of incident light and

low gloss. Smoothness can be obtained by polishing or by using processes that allow

the paint to dry smoothly without cracks, wrinkles, pinholes, or protrusions. Highly

pigmented paints often have low gloss, because pigment particles extend through the

surface and reduce smoothness.

Gloss can be manipulated by addition of ingredients that provide hazing to reduce

reflection or smoothness to increase reflection.

3.3.4.2 Hiding Power (Opacity)

The ability of a paint to cover a surface and mask it from view is referred to as hiding

power. Since most paint binders are transparent, the job of hiding the surface falls

primarily to the pigment. Pigments contribute to hiding power in three ways. They

may reflect, refract, or absorb the light that enters the paint film.

With reflection and refraction, the light is turned back out of the film before it

reaches the painted surface. Figure 3.2 illustrates reflection, refraction, and absorption

by a paint film.

88 Paint Technology Handbook

A B C D E

A: Reflected at the surface; B: Reflected by the pigment; C: Absorbed by the pigment;

D: Refracted by the pigment; E: Reflected by the substrate

FIGURE 3.2 Light behavior on a painted surface.

3.3.4.3 Color

The color of a paint material is primarily due to the pigment interaction with light.

Ordinarily, white light is composed of all the colors visible to the human eye. Pigments

have the ability to absorb some of these colors and reflect or transmit others. The paint

film will be the color of the reflected or transmitted light. Therefore, the pigment in

a red paint absorbs all colors except red.

Pigments can be mixed to give colors different from the pigments themselves.

Awhite pigment is often added to a paint to give a lighter color. The difference between

a light and a dark blue is often the presence of white titanium dioxide particles in the

film with dark blue pigment.

3.3.4.4 Strength, Hardness, and Brittleness

These terms describe important properties of a dry paint film. They depend on how

effectively the binder molecules, pigment particles, and additive materials attract one

another. When there is high attraction between components of a paint material, the

film will be strong, hard, and able to resist bending and stretching. This is not always

desirable. Such strong, hard films may not be able to bend or stretch and recover

under ordinary use conditions. The paint must have some resiliency or stretchiness,

because the solid surface under the film may shrink and expand due to temperature

changes in the surrounding atmosphere.

Brittleness is the tendency to crack under impact. Paint is brittle when it cannot

deform in response to a mechanical stress. This inability to deform, results from

excessively high attractions between the paint components.

The degree of attraction between the binder and the pigment can be controlled by

the nature of the materials themselves and by the use of additives.

Paint binders may be linear or cross-linked. Linear binder molecules are like randomly

intertwined strands of spaghetti. They are generally softer, more flexible, less

brittle, more soluble, more water permeable, and less heat resistant than cross-linked

binders. Cross-linking means that the linear, threadlike binder molecules have become

laterally bonded together at various positions along their length. The resulting 3-D

networks are usually stiffer, harder, more brittle, less soluble, less water permeable,

and more heat resistant than linear binders are.

Binders are often classified on the basis of their heat behavior. Linear binders

that can be melted are called thermoplastic. Cross-linked binders that do not melt are

Liquid Organic Coatings 89

called thermosetting. Most primers and many industrial topcoats have thermosetting

binders while sealers and some topcoats are linear binders.

A high degree of pigmentation gives strength and stiffness to a paint film. This

is because the pigment particles can act as a load-bearing part of the film and also

because the particles resist movement within the film.

Another means of controlling strength, hardness, and brittleness is by the use of

additives. Plasticizers weaken, soften, and decrease the brittleness of a paint film.

They are small molecules that are thoroughly mixed throughout the film and separate

the binder molecules and pigment particles from themselves and from each other. This

separation reduces the attractive forces between the paint components and provides

a more flexible, softer, less brittle film. Because plasticizers are small molecules,

they have a tendency to slowly evaporate from the film over a period of months

or years. The result is that many paints tend to become harder and more brittle

with age.

3.3.4.5 Depth of Color (Metal-Flake Paints)

Some industrial paints are formulated to give an appearance of depth. Automobiles

are usually finished with metallic coatings designed for depth of finish. The depthof-

color look comes from small metal flakes scattered throughout the paint. These

flakes, which are usually aluminum, reflect light back to the surface and out of the

film. Although the flakes are too small to see individually, they give the effect of

light coming from various depths within the film. The paint will have a dazzling or

sparkling appearance.

3.3.5 HOW PAINT WEARS OUT

All paint films are semipermeable layers that are susceptive to wear. Exposure to

the atmosphere, physical abuse, sunlight, water, corrosives, and general wear will

cause discoloration, loss of gloss, cracking, chipping, or peeling from their substrate.

The three greatest natural enemies of a paint film are sunlight, moisture, and

temperature changes.

1. Ultraviolet radiation degrades the binder molecules by breaking bonds

between atoms.

2. As binder molecules shrink, film strength declines and moisture penetration

increases.

3. With temperature cycling, the moist paint film is first heated and then

cooled. The accompanying expansions and contractions put a strain on the

film that is poorly resisted, because the binder molecules are now smaller

and mechanically weakened.

4. The weakened film surface is now subject to mechanical rub-off and water

wash-off.

5. As the unpigmented surface film wears away, the pigment particles become

exposed to the elements. Continued degradation of the binder may actually

set the pigment mechanically free, at which point the paint begins to chalk.

6. Once exposed to sunlight and moisture, many pigments begin to fade and lose gloss.

7. These changes occur because of a gradual loss of plasticizer from the film

that makes it more brittle. The stiffening film is less able to resist the

strain of thermal expansion and contraction. Microcracks form and allow

moisture to penetrate. This exerts a lifting pressure when warmed by an

increase in the surrounding air temperature.

Water can more easily penetrate a weakened film. The overall result is that the paint

film gradually erodes away.