A binder is the component of the paint that physically holds the other components together. Along with pigments, it is one of the two absolutely necessary components of any paint; depending on the type, solvents, fillers or others may also be added.
Often a certain category of paint is named after the only or mainly contained binder (e.g. oil paint, acrylic paint).
Binders can be divided according to their material type (water-soluble / fat-soluble - organic / inorganic - natural / synthetic) or according to their drying behavior: Physical drying means that a solvent evaporates, leaving behind the solid components of the binder; chemical drying, that smaller molecules combine (for example, in reaction with atmospheric oxygen) to form larger ones, making the mass as a whole more solid.
A mixed form between the two is represented by egg tempera and other emulsions, in which both processes take place simultaneously. In addition, there is also the case where the binder is heated for processing and thus brought into a liquid aggregate state; after cooling to room temperature, it hardens again.
History, origin and background of binders
Since the first production of paint in the Stone Age, pigments have been mixed with various types of binding substances. Initially, these were immediately available substances with a low degree of processing, such as animal fat or glue. Resins and gums obtained from trees, wax and oil have been known since ancient times; in ancient Egypt, the fresco technique was developed, in which the freshly applied plaster on a wall serves as a binder, then as now one of the most durable techniques. The tempera technique, in which aqueous and fatty binders are emulsified, also existed at the turn of the last century. At the same time, techniques for the production of varnishes and porcelain glazes were being developed in ancient China.
In the late Middle Ages in Europe, the technique of painting with drying oils emerged; at the same time, trade and military expansion made new substances accessible, such as dammar resin and carnauba wax. Finally, since the middle of the 20th century, synthetic binders have been on the market and are becoming increasingly popular.
Production - Typical processing
Some binders are available ready for use, others (such as glue, gum or resins) must first be dissolved in a suitable solvent. Typically - in the case of water-soluble binders - the pigment is soaked in water and then the finished binder is added to prevent the pigments from clumping together ("pigment nests"). Oil paints, on the other hand, can be mixed on a glass plate with a runner or directly in a container. After mixing, other staining agents are also added if required - these may also have to be sumped in separately first (such as marble powder or champagne chalk in water-based stain).
Purely synthetic binders based on petroleum have existed since the 20th century, for example in acrylic paints and synthetic varnishes. Their durability has yet to be proven; in processing, they have many favorable properties: they are water-dilutable, elastic, compatible with most pigments, relatively inexpensive, almost never crack when drying, do not require potentially harmful solvents and are waterproof after drying. Disposal is problematic because they are not biodegradable. Since they are typically derived from petroleum or other fossil materials and their chemical structure is thus based on organic compounds, they are also referred to as organic binders.
However, there are a large number of binders that are actually of inorganic origin: traditionally, for example, lime putty, which acts both as a binder and as a pigment in lime coatings. Mineral binders in building materials such as cement, lime and gypsum are inorganic substances that achieve high strength through the process of crystallization. The so-called Keim paints (named after their inventor Adolf Wilhelm Keim) are based on silicate / water glass, a solution of the naturally occurring mineral potassium silicate in water.
In the case of fresco and lime paints, the prepared milk of lime, a mixture of lime and water, reacts with CO2 contained in the air to form calcium carbonate, a stable compound; any pigments contained in the paint are firmly enclosed in it (fresco technique). The situation is similar with plaster.
Porcelain paint contains two binders that are active in different steps: First, pigments are melted into glass, which is then ground. The flour is mixed with linseed oil (first binder) so that it can be applied to the piece with a brush. The piece is then fired; the linseed oil burns without residue, the glass melts and permanently encloses the pigment as the second, final binder.
Due to their very different chemical structures, the various binders differ immensely in terms of their properties, especially their resistance to various environmental influences. Different binders are therefore suitable for different purposes. Most are light-resistant or even increase their binding power when exposed to light (like egg white); however, only porcelain paint and, to a certain extent, lime and frescoes are resistant to intense heat. Water-soluble binders are very sensitive to moisture; protein-containing agents are also sensitive to acids and alkaline solutions.
From a toxicological point of view, most binders are harmless; however, some solvents used with them, such as turpentine oil, can cause headaches and dizziness due to their vapors. Inhalation of the smallest dust particles, such as when working with lime, can be prevented by using a cloth made of light fabric or mouth/nose protection.