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Binders and their tasks

Production, processing and use of binders

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).

Synthetic binders

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.

Inorganic 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.

Special features

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.

Historical binders

For several decades now, the proportion of synthetic binders in arts and crafts has been increasing. In past centuries, people made do with natural alternatives whose durability has been proven many times over.

Proteins

Animal proteins - obtained from milk, egg, hides and bones - were used as binders very early on because they were relatively readily available. However, disadvantages include the easy perishability of the ready-to-process material and the sensitivity of proteins to acids, bases, alcohol and salts.

Animal glues are obtained by boiling skin, bone, or cartilage for a very long time and allowing the collagen they contain to dissolve. This water-collagen solution can then be used as a binder - either directly, or the collagen is first made storable by drying and only dissolved in water again when needed. The individual names (rabbit glue, bone glue, etc.) refer to the origin of the material. Fish glue, obtained from the swim bladder of sturgeons, is the purest type.

Casein is produced during the processing of milk into butter and was mainly used in wall paints with lime. It must first be broken down with an alkaline solution and then dries to become waterproof and very durable.

Egg white was used, for example, in medieval book illumination. Unlike glue, it binds not only physically (by evaporation of the water it contains), but also chemically by polymerization after some time when exposed to light, which is why, unlike the latter, it is no longer swellable or water-soluble after drying.

Oils and fats

In Stone Age cave paintings, the pigments used, e.g. coal dust, were already partly mixed with animal fat. Vegetable fats from pressed seeds (e.g. linseed oil or olive oil) had been known to man since antiquity at the latest; since the Middle Ages, drying oils in combination with other materials - mostly egg - were finally used in painting. Finally, around 1450, painting with pure oil paints developed. Because of its light color and relatively easy availability, linseed oil became the oil mainly used, at least in Europe; for pure whites or particularly light blues, people also resorted to even lighter poppy seed or walnut oil.

Unlike aqueous binders, oils do not "dry" in the physical sense, but harden when the fatty acids they contain polymerize in combination with atmospheric oxygen to form long-chain molecules. This process can take years or even decades.

Resins

Like rubber, resins are the sap of trees, which is brittle and translucent in its undissolved, dried state. Unlike rubber, however, resin is not soluble in water, but only in alcohol, turpentine oil or other organic solvents. In painting, coatings of dissolved resins often served as intermediate or final varnish, but were probably also added to oil paints as a painting medium. The most popular was dammar, which comes from a family of winged fruit plants and was imported to Europe from the 16th century, mainly from India and Southeast Asia. It provides a special depth as a final varnish in oil paintings. Turpentine resin, mastic and fossil resins (amber and copal) have been known in Europe for some time.

Plant gum

Similar to resins, plant gums are obtained from the secretions of trees, but unlike these they are water-soluble. The most widely used is gum arabic, which since ancient times has been extracted from the bark of several species of acacia, mainly native to Africa, and imported to Europe and elsewhere. In ancient China and Egypt, it was used to make ink; in medieval book painting, it served as a binder and is still used today in watercolor paint. Cherry gum, obtained from cherry and plum trees, is similar but of a darker color. Traganth, another plant gum, serves only as a thickener, since it does not dissolve completely but only swells.

Wax

In ancient times, for example, the famous mummy portraits were made with beeswax using the encaustic technique on wood. Later, it was sometimes used as an additive in varnishes to give a matte finish. Carnauba wax has been extracted since the 17th century from the Brazilian carnauba palm, which forms it as a coating for its leaves. Since it has a comparatively very high melting temperature of about 85°C and is the hardest wax available, it is also sometimes used in painting varnishes.

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