Components
All coatings and paints are composed of the four basic ingredients including solvents, resins, pigments, and additives. The paints are also known as a combination of pigments and a vehicle. The vehicle consists of the resin or binder and a suitable solvent or water. The paint will be ready after adding and mixing the pigment particles and any additives into the vehicle. Various paints and coatings categories such as paints, stains, clear coatings, and special coatings are produced with different combination of these four basic ingredients.
The paints are grouped in two types of water-based and oil-based paints. In the water-based paints, also known as latex paints, the vehicle is an emulsion of water and suitable resins. In the oil-based paints, or alkyd paints, a natural or synthetic (alkyd) oil resins is dissolved or dispersed in a solvent. Some water-based paints have small amount of solvent in the vehicle. Water-based paints are easy to apply, have little odor, are easily removed with water, dry quickly, have good resistance against cracks, and do not lose color over time. However, oil-based paints have an attractive gloss, create a smooth finish, tolerate poor surface preparation, perform well at lower temperatures, and have a more durable finish.
Nowadays, there is a strong shift away from oil-based paints in the industry because of the potential environmental and health problems caused by toxic substances in oil-paints. The reason is to eliminate or reduce the use of volatile organic compounds (VOCs) and lead. Most paint solvents are VOCs which evaporate after paint application. VOCs evaporation forms a chemical reaction with nitrous oxides and sunlight to form ozone and air pollutants. Besides, VOCs have negative impact on indoor air quality of the buildings. As a result, there are many restrictive VOCs legislations, and paint manufacturers are aiming to reduce or eliminate the amount of VOCs solvents in their products.
Pigments consist of two or more elements combined to form an organic or inorganic compound. They give the paint its color, level of sheen or gloss, and opacity. Opacity is the ability of paint to cover or hide the underlying surface. Adding more pigment in the coating results in more opacity. Finer ground particles provide better opacity and smoothness. Inorganic pigments are usually made from simple oxides of metal such as iron, zinc, or chromium. Titanium dioxide is the most common inorganic white pigment. Organic pigments are carbon-based chemical compounds. They are typically derived from coal tar and petroleum. Pigments are available for various functions including color pigments, hiding pigments, extender pigments, barrier pigments, inhibitive pigments, and sacrificial pigments.
Additives are usually added in small quantities to the paints for specific purposes and to improve its performance. Common types of additives are driers, anti-skinning agents, fungicides, thixotropes, flow agents, emulsion aids, UV absorbers, and plasticizers.
Resins or binders are used to keep pigment and additives together and attach them to the substrate. The type of resin determines durability and lots of other properties of the paint. Resins may be natural or synthetic. Natural resins are thermoplastic, so heating will make them soft and deformed. Some common natural resins are copal, lac, rosin, dammon, and manila. Synthetic resins are produced in manufacturing processes. They have good resistance to moisture, chemicals, and UV light. So, they are more widely used in both water-based and oil-based paints. Acrylic, alkyd, latex, and vinyl resin are some of the common synthetic resins.
Solvents are liquids used in oil-based coating to dissolve the pigments and additive particles. They reduce paint viscosity for easier application. After applying the paints to a surface, the solvent is evaporated and film formation process will start. In this process, paint becomes a dry, stable, and solid surface. Solvents are grouped into five types including hydrocarbon solvents, turpentines, ketones, esters, and alcohols. It is common to use a combination of solvents in a paint. The solvent combination is used in a paint to reach desired properties including consistency, leveling, drying, adhesion, and durability.
The film formation process by which a paint dries and cures is determined mainly by the characteristics of its resins. Film forming may happen in three processes including oxidation, evaporation, or chemical crosslinking. Oxidation is more common for oil-based paints in which after evaporation of the solvent, surface of resin will form a chemical reaction with the oxygen in the air and make a dry and stable surface. In case of water-based paints, water acts to disperse the pigments and additive particles to make an emulsion. The evaporation is more common for water-based paints in which the coating is converted to a solid film after water evaporation. No chemical reaction is involved in this process. In chemical crosslinking a chemical reaction happens to provide the final surface.
