Binders are substances that create or promote chemical bonds at the phase boundaries of other substances or trigger or increase effects such as cohesion , adsorption and adhesion or friction . They connect substances by absorbing them, attaching them, holding them together, linking them or sticking them together.
Depending on the subject and context, the definition and type of binders used differ greatly from one another.
Binding agents for solids are mostly used in liquid form in order to encase substances in a fine degree of division (e.g. bulk material or powder ) and to connect them to one another and, if necessary, to a substrate . If only a few workpieces are connected, one speaks more of glue or potting compound .
In contrast, solids are predominantly used as binders for liquids (e.g. oil and chemical binders , drying agents , thickeners ). The liquid is either adsorbed by the binding agent or converted into a solid or plastic state. Alternatively, only the internal friction, i. H. the viscosity of a liquid can be increased (e.g. by using many thickening agents ) or the mobility of the liquid can be decreased by adhesion .
In some contexts, binders are also called film formers ( pharmaceuticals , colorant technology , construction chemistry ), thickeners ( food technology and chemical industry ) or painting media (art painting ). Natural and synthetic resins are the most common binders in handicrafts and households. However, there are a host of others as well.
Depending on the mode of operation and the desired effect, certain binders are mainly or exclusively used to bind solids, liquids or gases.
Typical areas of application for binders are:
- the construction industry
- the coating , molding and sintering
- cooking and food technology
- the chemistry , medical technology , pharmaceuticals and cosmetics
- the paper, wood-based materials and ceramics industries
Binders based on polymers (such as resins , oils and dispersions , as well as cellulose-based ), starch , coal , silicates ( zeolites , silica gel ), calcium carbonate (lime, cement ) and proteins (e.g. gelatine ) are widespread .
Solidification and film formation
Synthetic resin dispersion paints can harden both physically through the interaction between the polymers they contain (entanglement of the polymer chains ) and chemically through the formation of new polymers.
Physical solidification occurs through evaporation or drying of the solvent. The active components of the binder approach each other and develop an even stronger mutual cohesion .
Binders that can bind liquids are also known as thickeners . They swell or act as fibers or simply as finely divided fillers inhibiting the movement of the liquid. Fibers can intertwine with each other and with the environment. You can also act as a binder for finely divided solids by holding them in the fiber matrix, such. B. the paper mache .
If a solvent is included, the weak bond between the molecules of the solvent and the molecules of the binder initially hinders the crosslinking of the binder. Only when the solvent has evaporated does the binding agent solidify.
In some cases the binder will solidify without any significant reduction in volume or weight, e.g. B. with two-component resins.
Chemical solidification can be done in different ways:
- through oxidation , i.e. absorption of atmospheric oxygen and subsequent crosslinking (e.g. in the case of drying oils , natural resin and some synthetic resins such as alkyd resin and epoxy ester)
- through chain polymerization (e.g. with unsaturated polyester resin )
- by polycondensation (e.g. with condensation resins such as phenolic resin , urea resin and melamine resin ), in this case water or another substance is released and evaporated during crosslinking
- through polyaddition (e.g. with reaction resins such as polyurethane and epoxy resin )
In the case of some substances, crosslinking only comes about through the addition of a hardener (e.g. epoxy and polyester resin ), a crosslinking agent ( sulfur , peroxides or metal oxides in the vulcanization of rubber ), a catalyst (e.g. acid or siccatives ) or by supplying heat or UV radiation .
The chemical bond is usually irreversible.
In the manufacture of paint, color pigments or dyes , fillers and binders are mixed with one another in order to achieve uniform distribution and wetting of the particles. Universally applicable binders should be neutral in color so as not to impair the color effect of the pigment. The type of binding agent should depend on the painting technique , painting surface and the desired properties of the paint (drying, gloss, opacity).
Commonly used as color binders are:
- Alkyd resins for printing inks and lacquers
- Emulsions in the temperature painting, with the addition of mastic or dammar .
- Epoxy resins in a wide range of applications
- Quicklime , cement , clay and potassium silicate in construction and wall painting
- Gum arabic (soluble in water) in watercolor painting
- Resins in oil painting and in industrial paints and varnishes
- Casein in wall and panel painting
- Plastic dispersions in acrylic painting and in industrial paints and varnishes
- drying vegetable oils in lacquers , wood preservatives and in oil painting
- Polyurethanes in a wide range of applications
- Silicone resin (emulsion) in paints
- Wax in the encaustic
- Cellulose glue ( paste ) in glue paints and paper mache
Binders in building materials are mineral substances that achieve high strength through crystallization, or organic substances (e.g. synthetic resin dispersions or 2-component reaction resins ) that harden through polymerization . Binders based on renewable raw materials such as starch and sugar are also used, for example, in plasterboard , in mineral fiber boards or as wallpaper paste .
In the construction industry, a distinction is made between hydraulic binders that harden both in the air and under water (e.g. cement , mixed binders , hydraulic lime ( trass ), plaster and wall binders based on cement / acrylic ) and non- hydraulic binders (also air binders) that harden only in the air (e.g. air lime , plaster , magnesia binders ) or only through drying out, such as clay . Non-hydraulic binders are not water-resistant when hardened.
Common binders in building materials are:
- Bitumen (roof waterproofing, road construction)
- Dispersion paints ( emulsion paint , dispersion plasters )
- Slaked lime ( mortar , plaster , paint )
- Hemihydrate , anhydrite (gypsum products such as building boards, plaster, screed )
- Clay (earth building, earth plaster, coarse and fine ceramics)
- Cement (mortar, concrete , screed)
In the case of two-component adhesives , in the broader sense of all two-phase polymers , the binder is the substance that causes the frictional connection and the hardener is the substance that serves as a catalyst for the polymerization.
In the case of materials for or made of ceramics, the actual ceramic component, which is solidified by thermal sintering (“baking” at high temperature) or other chemical processes (such as pyrolysis of polymers ), serves as a binding agent. Due to the abnormally high shrinkage when burning, a mixture of raw materials that is too rich is extremely unfavorable. Traditional pottery - or Hafner ceramics are leaner with sands or fireclay ( temper ) .
In the manufacture of wood-based materials , such as chipboard or MDF boards , binders are used to glue the wood particles (wood chips or wood fibers). In terms of quantity, aminoplasts and PMDI are the most important. Binders made from renewable raw materials are in development, but still play practically no role in the wood-based materials industry.
Binders make the paper writable because it becomes less absorbent and less hygroscopic . In papermaking, the hydrophobization of the fiber is called sizing . Glues are often chemically modified (saponified) tree resins in combination with potassium alum or aluminum sulfate or polymers based on acrylates or polyurethanes. Starches and sugars are used in the paper industry to increase the tear resistance and better printability of paper and cardboard, proteins from legumes to increase the mechanical strength and better adhesion of water-soluble printing inks.
In the case of printing inks for mass print products, immediate drying (e.g. by so-called absorption on absorbent paper) is of decisive importance so that the ink does not transfer to conveyor rollers or adjacent paper layers or even stick them together ( block-free ).
Binders for liquids
The task of binding agents in food is to give food the desired consistency . Most swellable proteins and carbohydrates are used. They should have a neutral taste or be adapted to the food to be bound.
Examples of binders commonly used in the kitchen:
- egg yolk
- Flour butter
- Breading - as a binding agent for farces and fillings
- Arrowroot meal
Binders used in the food industry (also thickeners , thickeners):
In addition to the above-mentioned means of food technology, other substances can also be used as thickeners.
In general, solid or powdery fillers have a thickening effect on liquids. Fibers, in particular, thicken liquids because they intertwine with one another or with other fillers and can lie crosswise in the medium and thus hinder the flow.
Binding agent (hazard prevention)
Binders for gases
Getters react with gases or adsorb gaseous substances to create or maintain a vacuum.
Oxygen Zehrer (engl. Oxygen scavengers ) such as (food) packaging be attached to slow down oxidation processes.
- Synthetic binders - an introduction , p. 23ff, Vincentz Network GmbH & Co. KG, Hanover; on FarbeUndLack.de
- Helmuth Heid, Wolfgang Imhof, Emil Jakubowski, Jürgen Reith: Malerfachkunde , p. 161, Springer Verlag
- Association of the German wood-based materials industry (VHI) : binders in wood-based materials.
- Agency for Renewable Raw Materials: Plants for Industry (PDF; 1.6 MB), 2005.
- Lebensmittellexikon.de: Gelling agents, binders, thickeners .
- Bockwurst with phosphate , article in the magazine Der Spiegel, 52/1959.