Pigments
Pigments are the fine solid powder which are not soluble in the medium or career in which they are incorporated and provide colours (black, white, red, yellow, blue, green etc). There are two types of pigments-organic and inorganic. Both the pigments are used to impart colour in paints, printing inks, rubbers, plastics, papers etc. In addition to colour they also provide other optical properties e.g gloss, opacity etc and functional properties (mechanical and chemical)e.g. abrasion resistance, hardness, UV resistance, acid and alkali resistance, heat resistance etc.
A pigment – physically or chemically is not an inert component in a paint they interact both chemically (including photochemically) and physically with other paint components. Such factors as texture, particle size/shape, tinting strength, reducing power, volatile matter, matter soluble in water, bulking value, form and crystal habit, particle size distribution, oil absorption, surface character, solubility and density significantly affect tinctorial effects, working properties, durability, and stability to moisture, chemicals and light. transparency, hiding power, strength, gloss and gloss retention, dispersibility, chalk resistance, masstone color and depth. Compatibility with vehicles, effect on viscosity, flow and levelling are some of the properties dependent not only on the vehicle system but on the physical and chemical properties of pigments in the finish
Classification of pigments
Wetting of Pigments
Why Wetting/Soaking is so important- Wetting control so many film properties for example a properly wetted and dispersed pigments provide:
- Better finish to the paint film
- Better gloss to the paint film
- Increases depth of the colour hence Improve opacity.
- Improve hardness of the coating
- Coating adhesion will be better.
- Abrasion resistance of the coating will improve.
- Chemical resistance of coating will be better.
- Settling of pigment will reduce or eliminated.
- Floating and flooding will be arrested.
Paint is a two-phase system.
- Solid Phase: Pigments and extender
- Liquid Phase (Medium): Solution of oils/resins/polymers and additives in solvent
Wetting of solid in liquid is not so easy specially when the two phases have different nature (Hydrophilic or hydrophobic), for example a face powder is not wetted by water but wetted by oil.- Why?
Face powder is a type of extender whose surface is hydrophobic (or lipophilic or non-polar) whereas the water is polar (lipophobic), so powder is not wetted with water. On the other hand, oil is hydrophobic or lipophilic, so powder is wetted by oil, as like dissolves like. Majority of inorganic pigments are polar (hydrophilic), and majority of organic pigments are non-polar (hydrophobic).
Wetting and dispersion are interlinked, if a pigment is wetted it will disperse properly, like titanium dioxide, calcium carbonate, ultramarine blue, iron oxide red or yellow are inorganic pigments and there surfaces are hydrophilic (polar) they can easily be wetted by water on the other hand the organic pigment like Hansa yellow, benzidine yellow are hydrophobic (lipophilic) will not be wetted by water but they will be wetted by oil based medium. Oil based paint means a paint which can be thinned with solvent.
Emulsion paints, which contain water are hydrophilic in nature, so the pigments whose surfaces are polar (hydrophilic in nature) will easily disperse in the emulsion, so if you are manufacturing emulsion paint and using the inorganic pigments then you may not require wetting agent in some cases. But if you are using inorganic pigments in oil-based paints then the wetting of pigment will be difficult. In such cases we will require some surfactant that is called additive also.
Similarly, if hydrophobic pigments like organic pigments are used in water based medium like emulsion, then also you will require surfactant.
Surfactant is a chemical which is having polar and nonpolar both the group the nonpolar group will go with the nonpolar part of your medium and the polar group of surfactants will attach with the pigment particles so the pigment particle will remain in suspension and will not coagulate are form agglomerates or flocculates. They will get stabilized also if you are using surfactant. So, on the other hand we can say surfactant in general will have three activity pigment wetting, pigment dispersion and pigment stabilization. If a surfactant is not taken the dispersion of nonpolar pigment in the polar medium will be difficult vice versa dispersion of polar pigment in nonpolar medium will be difficult but in general even the nonpolar pigment if dispersed into the nonpolar medium, we are using some additive (wetting agent) because in general the pigments are manufactured mostly by precipitation process are they are washed with water and then dried so because of this process as they are in contact with the water, there are chances that water along with air or gases are trapped in the pores or on the surface.
The pigment- air and moisture interface is to be replaced with pigment-medium interface for better wetting and dispersion. So, for replacement of pigment- air/moisture interface to pigment- medium interface we require surfactant.
Dispersion of Pigments-
Dispersion of pigment & extender is a phenomenon in which these solid fine powders are uniformly distributed in the medium (solution of resin or polymer or oil plus solvent or thinner and additives). This mass is known as paint, Hence the Paint is a material of fine dispersion of colouring matter (pigments) in a medium or carrier which when applied in a thin film on a surface gives a dry film. In fact, paint contains 4 main ingredients:
- Pigment(s) with and without extender
- Resin/polymer/oils- soluble in solvents or thinners.
- Solvents or thinners
- Additives soluble or miscible with above 2 and or 3.
Proper dispersion provides number of properties to the paint film like- good gloss, good hiding, good protection, good finish, bright color, good mechanical, and chemical properties etc.
Dispersion of pigments or extenders is achieved by grinding and mixing them with medium in a mill.
To understand the mechanism of dispersion, knowing their particle size is important. Pigments or extenders have following four stages of particle size:
- PRIMARY PARTICLES: Primary particles are very fine size of particle or initial stage of particle formation during synthesis of pigments or extenders. This size can be as small as 0.2 micron. In paint we don’t need this much fine size of Pigments or extenders, due to their draw backs in the coating.
- AGGREGATES: This stage of the pigments or extenders are obtained by cementing of primary particle on their surface. Here the primary particles join on the surface with no medium (air, moisture etc.) between the surfaces of the pigments or extenders. This provides very strong bond among the primary particles. There can be hundreds or thousands or primary particle joining to form one aggregate. The breaking of aggregates needs high energy.
- AGGLOMERATES: This is the stage where aggregates and primary particles can join in presence of air or moisture, so the bonds are not as strong as in aggregates. The aggregates can be broken with some force.
- FLOCCULATES: In this stage of particles, the ground or broken particles rejoin in presence of medium (solution of resin, solvent and additive).
Therefore, in dispersion we grind the pigments or extenders in a mill to break agglomerates in majority and to little extent aggregates also. The extent of grinding will depend upon the requirement of the paint for example in primer the ground pigments or extenders are coarse whereas in high gloss paint or enamel the grinding is done to get fine pigments or extenders.
This is also important to note that the ground material should be stabilized otherwise due to their surface energy, they will rejoin to give flocculates.
Grinding or breakdown of particle clusters–
Grinding refers to mechanical breakdown and separation of particle clusters to pigment particles. The effectiveness of grinding depends on the magnitude of forces holding the individual particles together in clusters. The breakdown mechanisms are generally either by smearing (shearing) and/or smashing (impact) types of forces which depend upon types of dispersion equipment.
Various types of grinding media (beads/balls e.g., steal, ceramics, zirconium etc. in different size e.g., 2mm to 70mm) are used. The selection of grinding media depends upon type of mill.
Pigment particles as we discussed above are clusters of aggregates and agglomerates. Reduction of these clusters is necessary to obtain optimum visual, economic and performance properties of pigments.
Importance of Particle shape-
The particle shape is also very important property of pigments. There are in general 5 shapes of pigments or extenders available
- Needle
- acicular/thread like
- spherical/nodular
- cubical
- lamellar/plate like/leafing type.
Each shape has its own importance. One can hardly control the shape. It is the basic characteristic of pigment or extender to develop in a particular shape,
Yellow oxide of iron is having acicular shape if we wish to have nodular or cubical shape from yellow oxide it is not possible the colour and other properties and even the chemical composition will change and it will convert to red oxide.
Different shape has different role in coating
- Plate like shape will provide better water or chemical resistance properties
- Nodular of spherical shape will give better abrasion resistance and adhesion property to the coating
- Needle like pigment will reinforce the film and so on.
Testing & evaluation (PIGMENTS)
Texture
Pigments/extenders can be fine and coarse in particle size and can also be soft and hard in texture. For example-
- carbon black pigment is fine and natural red oxide is coarse on the other hand calcium carbonate is soft and silica is hard.
Normally organic pigments are fine and soft in texture and most of the inorganic pigments are coarse and hard in texture.
Texture of pigment also control its end application for example the hard texture pigments are rarely used in printing inks.
The test method discussed in this video- play from start to 9:15 min.
Colour
This is the primary property of any pigment.
The colour of any material is the wavelength of visible range of light reflected by the material which falls on our eyes.
- White light (sun light) is made up of VIBGYOR (violet, indigo, blue, green, yellow, orange and red) lights.
- Wavelength of Violet light is shortest and that of red is longest.
A pigment can have variation in colour in different binder, The test of colour is relative test.
The test method discussed in this video- play from 9:20 min to 17:40 min.
Oil absorption
This property helps in paint manufacturing (grinding of pigments) and control the finish (gloss/matt) of paint.
- Two pigments having same particle size but due to their surface profile- The one having pores will have higher surface area and will have higher Oil Absorption Value.
- This property of pigment gives the idea of surface characteristic i.e., the surface area of pigment.
- Higher the oil absorption value higher will be the surface area.
The test method discussed in this video- play from 17:40 min to end.
Volatile matter
It is defined as the percent of water present in pigments. As normally water is present as volatile matter hence we test the percent of water. The source of water can be the humidity in the atmosphere and/or the improper drying of pigment during their processing. The presence of moisture can cause several defects in paint film and can result failure of coating. For example if the moisture is trapped in the coating the film can have pin holes or the bubbles or it can develop corrosion on ferrous surfaces. The maximum allowable moisture is 0.75 to 1.00 percent.
The test method discussed in this video- play from 18:00min to 28:10min.
Bulk density
The bulking value is defined as the ratio of volume of the pigment to weight of the pigment. Raw materials of paints including pigments are purchased on weight basis while paints are sold on volume basis. Therefore the pigments which have high bulking value would reduce the cost of paint.
The test method discussed in this video- play from 28:10min to end.
Tinting strength
It is the power of a pigment to impart its colour to the paint specifically tested with white pigment. We avoid the use of pigment in paint due to loss of gloss, therefore only optimum quantity of pigments are used to have desired colour and gloss. A colour pigment imparting more colour value over the other colour pigment of same class are preferred.
The test method discussed in this video- play from start to 13:40 min.
Reducing power
This is the power or strength of white pigments to reduce the colour of colour pigment. A white pigment if lighten the colour of colour pigment with lesser quantity, is preferred.
The test method discussed in this video- play from 13:40min to 18:00min.