By Mwnciboo
The Chemistry of Paints, Inks and Dyes
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. It is most commonly used to protect, color or add texture to objects. Okay thats a bit heavy, so lets simplify it. It’s liquid, occasionally a solid and is made up of several items.
Pigment
Pigments are granular solids incorporated into the paint to contribute color, toughness, texture or simply to reduce the cost of the paint. Alternatively, some paints contain dyes instead of or in combination with pigments.
Pigments can be classified as either natural or synthetic types. Natural pigments include various clays, calcium carbonate, mica, silicas, and talcs. Synthetics would include engineered molecules,calcined clays, blanc fixe, precipitated calcium carbonate, and synthetic pyrogenic silicas.
Hiding pigments, in making paint opaque, also protect the substrate from the harmful effects of ultraviolet light. Hiding pigments include titanium dioxide, phthalo blue, red iron oxide, and many others.
Fillers are a special type of pigment that serve to thicken the film, support its structure and simply increase the volume of the paint. Fillers are usually made of cheap and inert materials, such asdiatomaceous earth, talc, lime, barytes, clay, etc. Floor paints that will be subjected to abrasion may even contain fine quartz sand as a filler. Not all paints include fillers. On the other hand some paints contain very large proportions of pigment/filler and binder.
Some pigments are toxic, such as the lead pigments that are used in lead paint. Paint manufacturers began replacing white lead pigments with the less toxic substitute, titanium white (titanium dioxide), even before lead was functionally banned in paint for residential use in 1978 by the U.S. Consumer Product Safety Commission. The titanium dioxide used in most paints today is often coated with silica or alumina for various reasons such as better exterior durability, or better hiding performance (opacity) via better efficiency promoted by more optimal spacing within the paint film.
Binders and Resins
The binder, commonly referred to as the vehicle, is the actual film forming component of paint. It is the only component that must be present; other components listed below are included optionally, depending on the desired properties of the cured film.
The binder imparts adhesion, binds the pigments together, and strongly influences such properties as gloss potential, exterior durability, flexibility, and toughness. Binders can be categorized according to drying, or curing mechanism. The four most common are simple solvent evaporation, oxidative crosslinking, catalyzed/cross linked polymerization, andcoalescence. There are others.
Note that drying and curing are two different processes. Drying generally refers to evaporation of the solvent or thinner whereas curing refers to polymerization of the binder. Depending on chemistry and composition, any particular paint may undergo either, or both processes. Thus, there are paints that dry only, those that dry then cure, and those that do not depend on drying for curing.
Paints that dry by simple solvent evaporation and contain a solid binder dissolved in a solvent are known as lacquers. A solid film forms when the solvent evaporates, and because the film can re-dissolve in solvent, lacquers are not suitable for applications where chemical resistance is important. Classic nitrocellulose lacquers fall into this category, as do non-grain raising stains composed ofdyes dissolved in solvent and more modern acrylic based coatings such as 5-ball Krylon aerosol. Performance varies by formulation, but lacquers generally tend to have better UV resistance and lower corrosion resistance than comparable systems that cure by polymerization or coalescence.
Latex paint is a water-borne dispersion of sub-micrometre polymer particles. The term "latex" in the context of paint simply means an aqueous dispersion; latex rubber (the sap of the rubber tree that has historically been called latex) is not an ingredient. These dispersions are prepared by emulsion polymerization. Latex paints cure by a process called coalescence where first the water, and then the trace, or coalescing, solvent, evaporate and draw together and soften the latex binder particles and fuse them together into irreversibly bound networked structures, so that the paint will not redissolve in the solvent/water that originally carried it. Residual surfactants in the paint as well as hydrolytic effects with some polymers cause the paint to remain susceptible to softening and, over time, degradation by water.
Paints that cure by oxidative crosslinking are generally single package coatings. When applied, the exposure to oxygen in the air starts a process that crosslinks and polymerizes the binder component. Classic alkyd enamels would fall into this category. Oxidative cure coatings are catalyzed by metal complex driers such as cobalt naphthenate.
Paints that cure by "catalyzed" polymerization are generally two package coatings that polymerize by way of a chemical reaction initiated by mixing resin and curing agent/hardener, and which cure by forming a hard plastic structure. Depending on composition they may need to dry first, by evaporation of solvent. Classic two package epoxies or polyurethanes would fall into this category. The word catalyst is a misnomer as catalyst should not be part of the polymer. Cobalt driers are catalysts, iso cyanates and epoxy adducts are not. There are paints called plastisols/organosols, which are made by blending PVC granules with a plasticiser. These are stoved and the mix coalesceses.
Still other films are formed by cooling of the binder. For example, encaustic or wax paints are liquid when warm, and harden upon cooling. In many cases, they will resoften or liquify if reheated.
Recent environmental requirements restrict the use of Volatile Organic Compounds (VOCs), and alternative means of curing have been developed, particularly for industrial purposes. In UV curing paints, the solvent is evaporated first, and hardening is then initiated by ultraviolet light. In powder coatings there is little or no solvent, and flow and cure are produced by heating of the substrate after electrostatic application of the dry powder.
Solvent
The main purposes of the solvent are to adjust the curing properties and viscosity of the paint. It is volatile and does not become part of the paint film. It also controls flow and application properties, and affects the stability of the paint while in liquid state. Its main function is as the carrier for the non volatile components. In order to spread heavier oils (i.e. linseed) as in oil-based interior housepaint, a thinner oil is required. These volatile substances impart their properties temporarily—once the solvent has evaporated or disintegrated, the remaining paint is fixed to the surface. This component is optional: some paints have no diluent.
Water is the main diluent for water-borne paints, even the co-solvent types.
Solvent-borne, also called oil-based, paints can have various combinations of solvents as the diluent, including aliphatics, aromatics, alcohols, ketones and white spirit. These include organic solvents such as petroleum distillate, esters, glycol ethers, and the like. Sometimes volatile low-molecular weight synthetic resins also serve as diluents. Such solvents are used when water resistance, grease resistance, or similar properties are desired.
Additives
Besides the three main categories of ingredients, paint can have a wide variety of miscellaneous additives, which are usually added in very small amounts and yet give a very significant effect on the product. Some examples include additives to modify surface tension, improve flow properties, improve the finished appearance, increase wet edge, improve pigment stability, impart antifreeze properties, control foaming, control skinning, etc. Other types of additives include catalysts, thickeners, stabilizers, emulsifiers, texturizers, adhesion promoters, UV stabilizers, flatteners (de-glossing agents), biocides to fight bacterial growth, and the like.
Additives normally do not significantly alter the percentages of individual components in a formulation.
Miniature Paints
So what does that all mean to Miniature Painting?
Given the above definitions and the wide variety of paints and their composition we can draw some general points to help us with our miniature painting. As miniature painters we mainly work with Acrylics, although there is scope to use oil and inks as well. Here are some rules of thumb to help you on your way.
1. Acrylic paints for miniature painting have a high pigment to Binder Ratio and therefore give strong vibrant colours but also allow us to thin the paints without the colour watering down.
2. Most Acrylic paints can be thinned with a flow medium or glaze, this allows for a smoother application of paint (e.g the brush marks wills not remain and it will cover the surface of the model more evenly). This is equally important when combining colours to make tints, shades and different hues.
3. Most Acrylic paints can be mixed with a small amount of water in order to be thinned.
4. With Acrylic paints it is possible to make your own Washes using a combination or Glaze Medium/ Flow Medium and water to give Shading effects to your miniatures.
5. Acrylics are quite flexible and versatile and are more forgiving to work with than Oil paints or inks.
6. They are easy to clean off brushes and do not require solvents and other chemicals in order to clean your tools or materials after you finish work.
7. Acrylic Air Paints can be used directly in Airbrushes and usually contain a very high pigment count and are generally pre-thinned for ease of use with your Airbrush. They can be further thinned with water or sometimes alcohol.
8. Some Metallic Acrylic paints contain small amounts of very fine metal, this can contaminate your water and brushes and therefore you should use separate brushes and water when using Metallic Paints (this should not be confused with Non Metallic Metal painting where the effect of metal is mimicked using a clever use of colour, light and shade to give the impression of a metallic effect).
9. Acrylics are much safer to use, than other paints which may contain chemicals not suitable for children and possible toxic elements, modern legislation in most countries has eliminated most of the risk. But be aware of buying cheap paints from abroad and the far east where laws on the use of chemical elements maybe more lax.
OIL PAINTS
SPECIALIST PAINTS
1. There are some specialist model paints (Vallejo Old Gold Series for example) where the metallic pigment is in Alcohol (the Alcohol being the vehicle). The alcohol evaporates and leaves the metallic element on the miniature. This results (after several coatings) of an impressive even metal finish. However Water must not be used with these alcohol based metallic paints, only alcohol and therefore the brush must be totally dry and cleaned in Alcohol (Methylated spirit, or Ethanol or any other pure chemical alcohol, not to be confused with consumable alcohol like Whiskey, Vodka etc which are not of high enough concentration and also contain water and other impurities.
AIRBRUSH PAINTS
Usually a high density, ultra finely milled pigment in a binder but is deliberately very viscous and runny. This has many advantages, the paint maybe further thinned because of the high pigament density
Many Airbrush paint lines are extremely thin, they are supposed to be thinned further before application. In a few cases, some Airbrush paint manufacturers produce ready to use paints that require no additional thinning and can be used directly into an airbrush.
PASTELS
Crush up to form Weathering powders some miniature painters use these powders to weather tanks and terrain. Pastels are essentially compressed pigment
INK
Ink is a semi liquid solid/paste that contains pigments and/or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing and/or writing with a pen, brush, or quill. Thicker inks, in paste form, are used extensively in letterpress and lithographic printing. Ink can be a complex medium, composed of solvents, pigments, dyes, resins, lubricants, solubilizers,surfactants, particulate matter, fluorescers, and other materials. The components of inks serve many purposes; the ink’s carrier, colorants, and other additives control flow and thickness of the ink and its appearance when dry.
So thats the science of Paint.....Now onto Priming Miniatures.
