Лако-красочные материалы — производство

Adhesives are defined as nonmetallic substances capable of joining materials by surface bonding (adhesion), the bond itself possessing adequate internal strength (cohesion). Adhesive is a generic term and covers other common terms, such as glue, paste, gums, adhesive cement, and bonding agent.

History. Adhesion is among the oldest technologies of mankind [129]. As early as the Stone Age, our ancestors made stone axes and other tools using mineral pitch or wood rosin. Asphalt was used in building the Tower of Babel. In ancient Egypt, it was already customary to veneer furniture with the aid of animal glues. Flour — and casein-based adhesives were applied in ancient Rome. Adhesives of plant origin such as wood rosin were as well-known in China, as was the gum arabic and the caoutchouc (“weeping wood”) from the tropical regions of South America and Asia.

In the Middle Ages, the first glue-boiling plants came into being and produced protein glues from animal raw materials (glutin from hides and bones, blood albumin, casein from milk) or starch paste from plants.

With the developments in chemistry in the early 1900s, the technology of adhesives began to boom. Phenolic resins (ca. 1900), melamine resins, and urea resins (ca. 1930), polymer dispersions, epoxy resins (1938), and cyanoacrylates (1957) largely superseded the classic natural adhesives and, with a multitude of bindings agents, form the basis for modern adhesive technology, which is one of the most advanced joining processes.

Composition. An adhesive is composed of basic raw materials, which are called binders [1] and which determine its adhesiveness (adhesion) and its internal strength (cohesion), and of frequently necessary auxiliaries, which establish particular end-use and processing characteristics. The adhesiveness of an adhesive, its internal strength after setting, and its processing characteristics are the fundamental properties that determine its suitability for use in forming adhesive joints. Adhesive joints are the joints formed between substrates and adherents using adhesives.

The binders used for adhesives are primarily high polymers having optimal strength properties. High internal strength (cohesion) is essential if the adhesive in an adhesive joint is to be able to transmit forces from one adherent to the other. Most adhesives contain high molecular mass organic substances as their basic raw materials or reactive organic compounds that are preliminary stages of polymers and that react during the bonding process to form polymers. Inorganic polymers, such as the various types of waterglass, are used only to a very limited extent.

Virtually any standard polycondensate, homopolymer, and copolymer and also polyadducts may be used, provided they can be applied as solutions, dispersions, emulsions, or melts. In addition to these raw materials, auxiliaries such as resins, plasticizers, fillers, thickeners, solvents, antiagers, preservatives, hardeners, or setting retarders, are required, depending on the end use. Their function is inter alia to adjust tack, to improve adhesion, to make flexible, to regulate viscosity, to stabilize, and to influence setting or hardening.

Adhesive Joints. In many cases, adhesion is as effective as other joining techniques, such as riveting, welding, soldering, and screwing; in some cases it complements those techniques; and frequently it affords numerous advantages. On the other hand, certain processing requirements and material properties related to chemical structure can restrict the use of adhesives.

Bonding has the advantage that the joining of adherents can be carried out very rationally, often extremely quickly, and particularly economically [2]. One important feature common to all adhesive joints is the highly uniform distribution of forces over the entire joint area by comparison with rivet and screw joints. The uniform stress levels frequently provide for more optimal utilization of material strength. Accordingly, thinner and hence lighter sections of material may be used. This has resulted in the saving of material and, for example, in the automotive, aircraft, manufacturing, and furniture industries, in totally new structural elements (sandwich elements).

The use of adhesives enables numerous materials, even those differing widely in type, to be joined to one another. This applies above all to materials that cannot be joined to one another by other techniques. Because some adhesives set even in relatively thick layers, any unevenness in the constituent material of the adherents may be smoothed out during bonding, or significant dimensional tolerances may even be bridged by the gap-filling principle. The adhesive layer of an adhesive joint even may have a vibration — dampening effect and, by virtue of its insulating properties, can prevent contact corrosion in adhesive joints involving metals of different normal potential. Another advantage is the fact that many adhesives can be applied at room temperature or, when heat must be used for application, it is sufficient to apply temperatures at which the constituent materials of the adherents are not affected, as can happen in the welding of metals and plastics.

The usability of adhesives can be restricted when stringent requirements are imposed on the thermal stability of an adhesive joint [2]. Like all plastics, adhesives based on organic polymers also show a marked dependence on temperature in their strength properties. The strength properties of an adhesive joint can be kept constant and sufficiently high in only a relatively narrow temperature range. Adhesives cured by thermal cross-linking which are still relatively temperature resistant are generally capable of withstanding temperatures of up to about 150 °С. Using new, but more difficult to apply adhesives based on polyimides [3], it is possible to form adhesive joints which are capable of withstanding temperatures of the order of 250 °С for long periods. On the other hand, some adhesives, when subjected to heavy static stress for long periods, show a tendency even at room temperature to creep, a plastic deformation that can result in separation of the adhesive joint.