Paint and pigment industry : An Overview

 All paints are basically similar in composition in that they contain a suspension of finely ground solids (pigments) in a liquid medium (vehicle) consisting of a polymeric or resinous material (binder) and a volatile solvent.

With a brush, a roller, or a spray gun, paint is applied in a thin coat to various surfaces such as wood, metal, or stone. Paint is used to decorate, protect and prolong the life of natural and synthetic materials, and acts as a barrier against environmental conditions. Paints may be broadly classified into Decorative paints, applied on site to decorate and protect buildings and other objects, and Industrial coatings which are applied in factories to finish manufactured goods such as cars.[1]

The constituents of paint:

Paints contain:

  • Pigment(s) - prime pigments to impart colour and opacity
  • Binder (resin) - a polymer, often referred to as resin, forming a matrix to hold the pigment in place
  • Extender - larger pigment particles added to improve adhesion, strengthen the film and save binder
  • Solvent (sometimes called a thinner) - either an organic solvent or water is used to reduce the viscosity of the paint for better application. Water-borne paints are replacing some paints that use volatile organic compounds such as the hydrocarbons which are harmful to the atmosphere.
  • Additives - used to modify the properties of the liquid paint or dry film

The binder (resin) and solvent together are sometimes known as the vehicle. The binder may be dissolved as a solution or carried as a dispersion of microscopically small particles in a liquid.

Depending on the type of paint and intended use, additives may include:

  • dispersants - to separate and stabilise pigment particles
  • silicones - to improve weather resistance
  • thixotropic agents - to give paints a jelly-like consistency that breaks down to a liquid when stirred or when a brush is dipped into it
  • driers - to accelerate drying time
  • anti-settling agents - to prevent pigment settling
  • bactericides - to preserve water based paints in the can
  • fungicides and algaecides - to protect exterior paint films against disfigurement from moulds, algae and lichen

Paints are formulated according to their proposed use - primer, undercoat, special finishes (matt, gloss, heat resistance, anti-corrosion, abrasion resistance).[2]

    Contents of a white gloss (alkyd) paint and a white matt emulsion (acrylic) paint.                     

         Figure 1: Contents of a white gloss (alkyd) paint and a white matt emulsion (acrylic) paint.

Properties of an ideal paint:

These vary greatly according to the particular end use. The requirements for an automotive topcoat, for example, will be very different to those for a decorative ceiling paint.

Some of the typical attributes required can include:

  • ease of application
  • good flow out of application marks (e.g. brush-marking)
  • forming a continuous protective film
  • high opacity
  • quick drying
  • corrosion resistance
  • water resistance
  • heat resistance
  • color stability (i.e. against visible and ultraviolet radiation) 
  • abrasion and scratch resistance 
  • durability 
  • flexibility 
  • easily cleaned

Manufacturing:

Raw Materials:

A paint is composed of

Ø  pigments (Hundreds of different pigments, both natural and synthetic, exist. The basic white pigment is titanium dioxide, selected for its excellent concealing properties, and black pigment is commonly made from carbon black. Other pigments used to make paint include iron oxide and cadmium sulfide for reds, metallic salts for yellows and oranges, and iron blue and chrome yellows for blues and greens.)

Ø  solvents (They include petroleum mineral spirits and aromatic solvents such as benzol, alcohols, esters, ketones, and acetone)

Ø  resins (The natural resins most commonly used are lin-seed, coconut, and soybean oil, while alkyds, acrylics, epoxies, and polyurethanes number among the most popular synthetic resins.)

Ø  various additives. (Some, like calcium carbonate and aluminum silicate) [1]

Process:

Making the paste

  • Pigment manufacturers send bags of fine grain pigments to paint plants. There, the pigment is premixed with resin (a wetting agent that assists in moistening the pigment), one or more solvents, and additives to form a paste.

Dispersing the pigment

  • The paste mixture for most industrial and some consumer paints is now routed into a sand mill, a large cylinder that agitates tiny particles of sand or silica to grind the pigment particles, making them smaller and dispersing them throughout the mixture. The mixture is then filtered to remove the sand particles.
  • Instead of being processed in sand mills, up to 90 percent of the water-based latex paints designed for use by individual homeowners are instead processed in a high-speed dispersion tank. There, the premixed paste is subjected to high-speed agitation by a circular, toothed blade attached to a rotating shaft. This process blends the pigment into the solvent.
Manufacturing process of paint

Figure 2: Manufacturing process of paint

Thinning the paste

  • Whether created by a sand mill or a dispersion tank, the paste must now be thinned to produce the final product. Transferred to large kettles, it is agitated with the proper amount of solvent for the type of paint desired.

Canning the paint

  • The finished paint product is then pumped into the canning room. For the standard 8 pint (3.78 liter) paint can available to consumers, empty cans are first rolled horizontally onto labels, then set upright so that the paint can be pumped into them. A machine places lids onto the filled cans, and a second machine presses on the lids to seal them. From wire that is fed into it from coils, a bailometer cuts and shapes the handles before hooking them into holes precut in the cans. A certain number of cans (usually four) are then boxed and stacked before being sent to the warehouse.[1]

Problems and solution in technical and engineering way:

A recent regulation (California Rule 66) concerning the emission of volatile organic compounds (VOCs) affects the paint industry, especially manufacturers of industrial oil-based paints. It is estimated that all coatings, including stains and varnishes, are responsible for 1.8 percent of the 2.3 million metric tons of VOCs released per year. The new regulation permits each liter of paint to contain no more than 250 grams (8.75 ounces) of solvent. Paint manufacturers can replace the solvents with pigment, fillers, or other solids inherent to the basic paint formula. This method produces thicker paints that are harder to apply, and it is not yet known if such paints are long lasting. Other solutions include using paint powder coatings that use no solvents, applying paint in closed systems from which VOCs can be retrieved, using water as a solvent, or using acrylics that dry under ultraviolet light or heat. A consumer with some unused paint on hand can return it to the point of purchase for proper treatment.

A large paint manufacturer will have an in-house wastewater treatment facility that treats all liquids generated on-site, even storm water run-off. The facility is monitored 24 hours a day, and the Environmental Protection Agency (EPA) does a periodic records and systems check of all paint facilities. The liquid portion of the waste is treated on-site to the standards of the local publicly owned wastewater treatment facility; it can be used to make low-quality paint. Latex sludge can be retrieved and used as fillers in other industrial products. Waste solvents can be recovered and used as fuels for other industries. A clean paint container can be reused or sent to the local landfill.[2]

Pigments:

Pigments are finely ground natural or synthetic, insoluble particles used to impart color when added to paints and coatings formulations. They are also used to impart bulk or a desired physical and chemical property to the wet or dry film. [3]

Classifications:  Some of the main pigment classes include:

Ø  Organic pigments

Ø  Inorganic pigments

Ø  Functional pigments

Ø  Special effect pigments 

While organic pigments do not disperse easily and form agglomerates (clumps of pigment particles), inorganic pigments get more easily dispersed in the resin. Functional fillers impart a desired property to the coating like corrosion inhibition and special effect pigments create optical effects like metallic, hammer finish and diverse color perceptions depending on the angle.[3]


Manufacturing:

Sulfuric acid method

Raw materials:

Ø  Ilmenite 

Ø  H2SO4

Process:

In this method ilmenite was treated with concentrated H2SO4 at 110–120°C to form ferrous and titanyl sulfates:

                                     FeTiO3 + 4H+ → Fe2+ + TiO2+ + 2H2O

Production of TiO2 pigment by the sulfuric acid process

Figure 3:  Production of TiO2 pigment by the sulfuric acid process

The reaction is conducted in large concrete tanks lined with acid resisting brick, heated by direct injection of high pressure steam or in a pug mill. The solidified mass produced in the reactor at the end of the reaction was then discharged from the reactor by dissolution in water or dilute acid. After removing the insoluble residue by filtration, the solution containing 120–130 g/L TiO2 and 250–300 g/L FeSO4 was concentrated under vacuum at 10°C to crystallize FeSO4·7H2O which was then centrifuged. Titanium oxide is then precipitated from solution by dilution and seeding resulting in the formation of dilute H2SO4 for disposal.[5,6]

Chlorination method

In this method pigments are produced by direct chlorination of ilmenite ore, separation of products by fractional distillation, then oxidation of TiCl4.

Simplified Du Pont process for pigment production from ilmenite

Figure 4: Simplified Du Pont process for pigment production from ilmenite

           2FeTiO3 + 7Cl2 + 3C → 2TiCl4 + 2FeCl3 + 3CO2

           TiCl4 + O2 → TiO2 + 2Cl2

The problem of this process is recovery of chlorine from ferric chloride or marketing the large amounts of this co-product.[6]

Electric furnace process

The ore was mixed with a certain amount of anthracite which was just enough to reduce the iron oxide component of the ore, then charged in an electric furnace at 1 650°C where iron oxide is reduced to metal while titanium is separated as a slag. The reactions taking place during reduction are the following:

Electric furnace process for iron separation

Figure 5: Electric furnace process for iron separation

               FeTiO3 + C → Fe + CO + TiO2(slag)

               Fe2O3 + 3C → 2Fe + 3CO

The reduction of the iron oxides is not taken to completion so that some iron oxide is left in the slag to decrease its melting point. Melting point of TiO2 1840°C and ilmenite 1435°C.

Titanium slag was used only for making pigment by the sulfuric acid process. The slag was treated in the same way as ilmenite with the exception that no separation of ferrous sulfate was necessary because the bulk of iron was already separated by reduction in the earlier step. The sulfuric acid treatment process of the slag, however, still suffered from the disposal problem of the waste acid and as a result it was abandoned in the 1980s and replaced by a new technology based on upgrading the slag to 94.5% TiO2 by leaching away most of the impurities by HCl under pressure to render it suitable for chlorination.[6]

Leaching of titanium slag for production of TiO2 pigment, now obsolete

Figure 6: Leaching of titanium slag for production of TiO2 pigment, now obsolete

Pigment Dispersion:


High quality coatings of high brilliance and color strength are characterized by:

  • A perfect pigment dispersion
  • Optimal pigment particle size
  • Long-term stabilization of the dispersed particle in the formulation.

The dispersion process consists of the permanent breaking down of agglomerates into, as far as

possible, primary particles. There are four aspects to the dispersion process:

Pigment Dispersion

Figure 7: Pigment Dispersion

  • Deagglomeration – The Breaking down of the agglomerates and aggregates by the mixture of crushing action and mechanical shearing force.
  • Wetting – It occurs at the surface of a pigment when a surface active agent sticks to the pigment's surface and acts as a connection between the pigment and the binder.
    Wetting out time depends on the viscosity. Heat produced by the mechanical shearing process causes the temperature of the mixture, hence reduces the viscosity, thus helping the wetting out process.
  • Distribution – It demands the pigment to be equally dispersed throughout the binder system. A lower viscosity tends to lead to a more even pigment distribution.
  • Stabilization – It prevents the pigments from re-agglomerating. The pigment dispersion is stabilized by dispersing agents in order to prevent the formation of uncontrolled flocculates. The resultant suspension is stabilized due to the adsorption of binder species or molecules at the pigment surface.[3,4]

Factors influencing pigments performance:

Performance of a pigment can be measured by the following properties:[3]

Color of Pigment
The color of a pigment is mainly dependent on its chemical structure, which is determined by the selective absorption and reflection of various wavelengths of light at the surface of the pigment.
Colored pigments absorb part of all the wavelengths of light. For example:

  • Blue pigment reflects the blue wavelengths of the incident white light and absorbs all of the other wavelengths. Hence, a blue car in orange sodium light looks black, because sodium light contains virtually no blue component.
  • Black pigments absorb almost all the light.
  • White pigments reflect virtually all the visible light falling on their surfaces.
  • Fluorescent pigments have an interesting characteristic. As well as having high reflection in specific areas of the visible spectrum, they also absorb light in areas outside the visible spectrum (ultra-violets that human eye cannot detect), splitting the energy up, and re-emitting it in the visible spectrum.
    Hence, they appear to emit more light than actually falls upon them, producing their brilliant color.

Color Strength


Color strength (or tinctorial strength) must be considered when choosing a pigment. Color strength is the facility with which a colored pigment maintains its characteristic color when mixed with another pigment. The higher the color strength, the less pigment is required to achieve a standard depth of shade.

Chemical Structure

                                It is one of the factors that influence the color strength of a pigment.

  • In organic pigments, color strength depends on the ability to absorb certain wavelengths of light. Highly conjugated molecules and highly aromatic ones show increased color strength.
  • Inorganic pigments that are colored due to having metals in two valency states, show high color strength. In contrast, those that have a cation trapped in a crystal lattice are weakly colored.

Particle size

Particle size also influences the color strength of a pigment. Higher color strength is obtained with smaller particles. Manufacturing conditions are the main factor that influences the particle size of pigment crystals. Pigment manufacturers play a crucial role. They can:

  • Reduce the size of the particles by preventing the growth of crystals during synthesis
  • Increase color strength by efficient dispersion

Pigment dispersion also plays a major role in the color strength of the paint. Indeed, it imparts colloidal stability to the finer particles, avoiding their flocculation and using their full intrinsic color strength.

Heat Resistance


Few pigments degrade at temperatures normally associated with coatings. However, at higher temperatures, pigments become more soluble and shading can occur. Thus, for organic pigments, heat stability is closely related to solvent resistance.

Light Fastness


Light fastness is evaluated in relation to the whole pigmented system, not just the pigment. The binder imparts a varying degree of protection to the pigment, so the same pigment will tend to have better light fastness in a polymer than it will in paint.
Other pigments that may influence light fastness in a pigmented system. These include:

  • Titanium dioxide promotes the photodegradation of most organic pigments. Therefore, high ratios of titanium dioxide lead to poorer levels of light fastness.
  • Iron oxide can improve the light fastness of organic pigments, due to the fact that it is an effective absorber of UV light.

When the association of two pigments gives a better light fastness, it is called a synergistic effect and when the light fastness obtained is lower, it is called an antagonistic effect.

Weather Stability

For outdoor applications, pigments used for coloring should be selected for their weather resistance characteristics. The selection of pigments for outdoor use depends on:

  • Outdoor performance required (life time, climatic region/ Kilo Langley)
  • Binder type
  • Concentration of the pigment
  • Presence of titanium dioxide (which typically accelerates fading)
  • Concentration and type of light stabilizers used

Insolubility
A pigment must be insoluble in the vehicle (the medium in which it is dispersed), and it must not react with any of the components of the paint, such as crosslinking agents.
Pigments are required to retain these properties even when the paint is being dried, which is frequently carried out at elevated temperatures. Once in the dried film, the pigment must also remain unaffected by the substrate and to agents with which it comes into contact, including water, which may simply be in the form of condensation, or acidic industrial atmospheres.

Opacity/ Hiding Power


Hiding power is the ability of a pigmented coating to obliterate the surface. It is dependent on the ability of the film to absorb and scatter light. Naturally, the thickness of the film and the concentration of the pigment play a fundamental role. The color is also important.

Chemical Stability
Resin, crosslinking agents, UV-initiators, and any other additive may react with the pigment and alter its performance.

Problems and solution in technical and engineering way:

Solubility of a pigment generates the following problems:

  1. Blooming - If the pigment dissolves in the solvent, as the paint dries, the solvent comes to the surface and evaporates, leaving crystals of the pigment on the surface in the form of a fine powder. As solubility increases with temperature, this phenomenon is made worse at elevated temperatures.
  2. Plate out - The effect of plate out looks similar to blooming, but occurs in plastics and powder coatings. However, it is not due to the pigment dissolving, but rather to the surface of the pigment not being properly wetted out. It usually occurs mainly with complex pigments and once wiped from the surface does not reappear.
  3. Bleeding - Pigments in a dried paint film may dissolve in the solvent contained in a new coat of paint applied on top of the original film. If the topcoat is a different color, particularly a white or pale color, the result can be disastrous. Again elevated temperatures exacerbate the problem.
  4. Recrystallization - This phenomenon was almost unknown until the introduction of beadmills. During the milling stage, heat is generated, which dissolves a portion of the pigment. Over a period of time, the dissolved "pigment" starts to precipitate out, loses brilliance and color strength. This becomes especially noticeable in the case of paints containing two differently colored pigments that have different solubility characteristics. The more soluble pigment dissolves and then as it comes out of solution and precipitates, the paint will take the shade of the second pigment. Recrystallization can even take place in aqueous systems. It can be avoided by using less soluble pigments and/or by controlling the temperature during the dispersion process.[3]

Another adverse effect can come from:

Chemicals that the coating gets in contact with. Water, in the form of condensation, can seriously affect a paint film, particularly in bathrooms and kitchens. Many of the detergents used for cleaning paintwork are harsh and have an abrasive affect upon the pigment. Should the coating come into contact with food, it is essential firstly, that the coating is unaffected and secondly, that the food remains unchanged.[5]

References:

1. http://www.madehow.com/Volume-1/Paint.html

2. https://www.essentialchemicalindustry.org/materials-and-applications/paints.html

3. https://coatings.specialchem.com/selection-guide/pigments

4. Habashi F (1993) A Textbook of Hydrometallurgy (2ndedtn). Métallurgie Extractive Québec, Québec City, Canada

5. Habashi F (1996) Pollution Problems in the Mineral and Metallurgical Industries, Metallurgy Extractive Quebec, Quebec City.

6. https://www.oatext.com/Ilmenite-for-pigment-and-metal-production.php

7. https://en.wikipedia.org/wiki/Ceramics_industry_in_Bangladesh

8. Ceramic Industry of Bangladesh: A Perspective from Porter’s Five Forces Model-Nusrat Jahan Volume– V, Issue– 02, July-December, 2010

9. https://solutionsinmotion.clevelandvibrator.com/eight-steps-of-ceramics-processing-and-industrial-vibration/

10. https://www.slideshare.net/hzharraz/ceramics-32994025


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