A-level Applied Science/Colour Chemistry/Paint
Paint is the general term for a family of products used to protect and add colour to an object or surface by covering it with a pigmented coating.
Paint can be applied to almost any kind of object. It is used, among many other uses, in the production of art, in industrial coating, as a driving aid (lane markings), or as a preservative (to prevent corrosion or water damage). Paint is a semifinished product, as the final product is the painted article itself.
There are generally four components to a paint: binder, solvent (also known as 'thinner' or 'diluent'), filler, and additives.
Only the binder is absolutely required. The binder is the part which eventually solidifies to form the dried paint film.
After application, the paint solidifies and becomes tack-free. Depending on the type of binder, this hardening may be a result of curing (resin polymerisation), evaporation, or even cooling.
Binders that dry form a solid film when the solvent evaporates.
Binders that include synthetic or natural resins polymerise into irreversibly bound networked structures, which will not redissolve in the solvent.
Films which are formed simply by cooling of the binder include encaustic or wax paints. They are liquid when warm, and harden upon cooling.
(Also known as 'extenders')
Fillers serve to thicken the film, support its structure and simply increase the volume of the paint. Not all paints include fillers. Pigments that also function as fillers are called simply "pigments".
Fillers are generally colour-neutral and opaque. It is necessary to adjust the resulting off-white colour of the filler with pigments to give the desired colour. Common fillers are cheap and inert, such as talc, lime, baryte, bentonite clay, etc. Depending on the paint, most of the paint film may consist of filler and binder, the rest being additives.
Typical additives include:
- driers - reduce drying time.
- stabilising agents.
- anti-settling agents - prevent the pigment settling out.
- emulsifiers/dispersants - ensure even distribution of pigment.
- bactericides and fungicides
- texturisers - add bulk or texture to paint which would otherwise form a thin smooth layer. e.g. To make acrylic paint dry like oil paint.
- adhesion promoters.
- flatteners (de-glossing agents).
- silicones - for weather resistance.
White gloss (alkyd) paint is typically:
- 55% vehicle (alkyd resin with 25% solvent)
- 25% pigment
- 15% solvent (white spirit)
- 5% additives
White matt (emulsion) paint is typically:
- 28% vehicle (polymer latex with 50% water)
- 25% pigment (titanium dioxide)
- 30% water (solvent)
- 5% additives
- 12% extender (clay)
Paint can be applied as a solid, a gaseous suspension (aerosol) or a liquid. Techniques vary depending on the practical or artistic results desired.
As a solid (usually in industrial and automotive applications), the paint is applied as a very fine powder, then baked at high temperature. This melts the powder and causes it to adhere (stick) to the surface. The reasons for doing this involve the chemistries of the paint, the surface itself, and perhaps even the chemistry of the substrate (the overall object being painted).
As a gas or as a gaseous suspension, the paint is suspended in solid or liquid form in a gas that is sprayed on an object. The paint sticks to the object. The reasons for doing this include:
- the application mechanism is air and thus no solid object ever touches the object being painted;
- the distribution of the paint is very uniform so there are no sharp lines
- it is possible to deliver very small amounts of paint or to paint very slowly;
- a chemical (typically a solvent) can sprayed along with the paint to dissolve together both the delivered paint and the chemicals on the surface of the object being painted;
- some chemical reactions in paint involve the orientation of the paint molecules.
In the liquid application, paint can be applied by direct application using brushes, paint rollers, blades, other instruments, or body parts.
Rollers generally have a handle that allows for different lengths of poles which can be attached to allow for painting at different heights. Generally, roller application takes two coats for even colour. A roller with a thicker nap is used to apply paint on uneven surfaces. Edges are often finished with an angled brush.
After liquid paint is applied, there is an interval during which it can be blended with additional painted regions (at the "wet edge") called "open time." The open time of an emulsion paint can be extended by adding white spirit, similar glycols such as Dowanol™ (propylene glycol ether) or commercial open time prolongers. This can also facilitate the mixing of different wet paint layers for aesthetic effect.
Paint may also be applied by flipping the paint, dripping, or by dipping an object in paint.
- Wood stain is a type of paint that is very "thin," that is, low in viscosity, and formulated so that the pigment penetrates the surface rather than remaining in a film on top of the surface. Stain is predominantly dye, pigments and solvent with little binder, designed primarily to add colour without providing a surface coating.
- Varnish and shellac provide a protective coating without changing the colour. They are paints without pigment.
- Lacquer is usually a fast-drying solvent-based paint or varnish that produces an especially hard, durable finish.
- An enamel paint is a paint that dries to an especially hard, usually glossy, finish. Enamel can be made by adding varnish to oil-based paint.
- Inks are similar to paints, except they are typically made using dyes exclusively (no pigments), and are designed so as not to leave a thick film of binder.
- Anti-Graffiti paints are used to defeat the marking of surfaces by vandals. There are two categories, sacrificial and non-bonding. Sacrificial coatings are clear coatings that allow the removal of graphiti, usually by pressure washing the surface with high-pressure water, removing the graphiti, and the coating (hence, sacrificed.) They must be re-applied afterward for continued protection. This is most commonly used on natual-looking masonry surfaces, such as statuary and marble walls, and on rougher surfaces that are difficult to clean. Non-bonding coatings are clear, high-performance coatings, usually catalyzed polyurethanes, that allow the graffiti very little to bond to. After the graffiti is discovered, it can be removed with the use of a solvent wash, without damaging the underlying substrate or protective coating. These work best when used on smoother surfaces, and especially over other painted surfaces, including murals.
- Anti-climb paint is a non-drying paint that appears normal whilst being extremely slippery. It is usually used on drainpipes and ledges to deter burglars and vandals from climbing them, and is found in many public places. When a person attempts to climb objects coated with the paint, it rubs off onto the climber, as well as making it hard for them to climb.
Hiding power can be defined as either:
- The Contrast Ratio at a given film thickness, or
- The Film thickness at 0.98 (98%) contrast ratio.
The contrast ratio of 0.98 is conventionally accepted as representing "complete" hiding, but visually, as well as photometrically, it is slightly less than complete.
- Painting with acrylics - h2g2 article.
- The Essential Chemical Industry (1985) Polytechnic of North London
- ANSI Standard Test Method for Measuring the Hiding Power of Powder Coatings
- History of Paint
- Glossary of painting terms
- 20 recipes for homemade paint (text)
- Homemade Paint Recipes for Children
- Comparing Commercial Paints FAQ
- The Quest of Purchasing Oil Paints
- School-based chemical synthesis of pigments for use in oil paint.
- How to mix paints (oil or acrylic).
- The composition and manufacture of commercial paints.