coatings technology: binders
Binders (polymers, resins)
Paints generally contain a binder or polymer that acts as the adhesive component for bonding to the surface. This can be an inorganic type such as the silicates used in inorganic zinc rich primers, but in most cases it is an organic type. There are many chemically different types of binders used in modern coatings, but the basic function is to cure or convert from a liquid state to a solid state after application. Binders can cure by oxidation, heat, chemical reaction or by the evaporation of solvent.
Binder Film-Forming Mechanisms
- Requires only the evaporation of solvent to cure - so binders of this type remain soluble in the same solvent(s) and may be dissolved by their original solvent even after application and drying.
- Requires heat to reach full cure. A good example is amino modified alkyd baking enamels used on metal furniture. Heat is required to initiate and speed up the chemical cross-linking between the amino reactive group and the free hydroxyls available on the alkyd.
- Requires a combination of metallic catalysts to effect curing; typical examples are alkyds and oil paints.
- A catalytic coating requires the addition of a curing agent or catalyst just prior to application. Catalytic paints tend to have a fixed amount of time to be used, or “pot-life.” Good examples are epoxies, two component polyurethanes, polyester - peroxides, etc. Moisture curing polyurethanes (ASTM Type II) could be considered reactive as free moisture in the air catalyzes the polymer into curing.
How Binders are Present in Paint
- A solution binder is a binder dissolved in a compatible solvent or solvents.
Most thermoplastic (and some thermoset) binders are available in a solid form dissolved in a compatible solvent or mixture of solvents to make a liquid coating.
- An emulsion is a stabilized aqueous dispersion of a polymer or co-polymer. In most commercial processes, the polymer or copolymer is formed during emulsification (emulsion polymerization). An emulsion seen close-up would appear as multitudes of spherical particles of polymer separated by water. These particles are not soluble in the water carrier but are stabilized by surfactants at the water/polymer interface that have an affinity for water and oil. The polymer particles in an emulsion can range in size from 1-2 microns to 0.05 microns. (1 micron = .0001 mm). The smaller sized emulsions appear clear in solution and the larger sized appear opaque or milky.
- A dispersion binder is a solubilized binder dispersed in a non-solvent. Generally a surfactant is not required if the polymer is soft enough to form a film at room temperature. With harder polymers, an active solvent may be added to assist in film formation. The dispersion can be made with polymers that are thermoplastic or reactive.
- A 100% solids paint certainly isn't solid, nor is it even necessarily thick in viscosity. 100% solids really means "100% non-volatile" - meaning that the raw materials used will all remain as part of the cured paint film, and in and of themselves possess the required properties (including fluidity) to apply and cure on the surface without the addition of a volatile solvent. 100% solids coatings are formulated with low molecular weight materials including specialty polymers that use blocked and chain stopped polymerization techniques. Epoxy, polymers that use blocked and vinyl plastisol binders are available as 100% solids.
- Powder coating polymers are solid polymers that have been reduced to a fine particle size. Technically, with powder, the entire coating formulation is contained within the polymer, as no solvent is needed for application.
Acrylic resins are used to a large extent in the coatings industry. Various combinations of acrylics can have physical properties ranging from rubbery elastomers to hard, glossy automotive coatings. Acrylics can also be co-polymerized with other types of monomers such as styrene, vinyl, and alkyd polymers to create a wide variety of coatings resins. One of the major uses for acrylic resin is in the residential and commercial painting environment. As a “waterborne” or water reducible latex, acrylic has replaced most of the alkyd coatings that were used in the past. Because of no-to-low VOCs, these acrylic coatings can be used in most populated environments with little problem. Every day paint manufacturers are finding new uses for acrylic, and they’re probably the most popular architectural coating system used today.