Лако-красочные материалы — производство

Contact adhesives are made from polymer components which are already in a high — molecular state, but which are not yet chemically crosslinked. By means of the addition of solvents, a low-viscosity state is obtained which allows the adhesive to wet the surface of the solid material. Contact adhesives must be applied to both adherents, and solidification takes place by a drying process before the materials are joined. The adherents are pressed together as soon as the adhesive layers are dry to the touch.

During the application of pressure, and even afterwards, a diffusion process takes place between the adhesive molecules of both coats, firmly joining the adherents together. The strength of the bond can increase, often within hours, after the application of pressure. If at least one adherent is porous, the adherents can be joined before the drying process has taken place at all or only partly, because the remaining solvent is capable of evaporating through the porous adherent. In this case, it is also possible to reposition the adherents after having joined them because, in the semi-dry condition, the adhesive still largely presents the properties of a liquid.

In the dried or cured condition, the adhesive coat is generally in the thermoplastic state — that is, the molecules are not three-dimensionally crosslinked. As a conse­quence, the adhesive coats applied do not present the same resistance to heat and creep as chemically cured adhesives. It is possible, however, to add slowly acting hardeners to these polymer systems; this induces a three-dimensionally wide — meshed crosslinking of the systems over the course of several days. While the addition of hardeners does not alter the processing properties of the contact adhesives, the durability and strength of the cured system is considerably improved. One disadvantage of contact adhesives is an occasional very high solvent content (up to 80% by weight), so that appropriate protective measures must be taken during processing.

As a consequence, solvent-containing contact adhesives are now being replaced, when possible, by so-called ‘dispersion adhesives’. These consist of a dispersion of

the adhesives in water, during the evaporation of which the dispersed polymer particles combine by diffusion (see Section 3.2.2). Unfortunately, dispersion adhe­sives have the disadvantage that the drying process of water takes considerably longer than that of solvents, and an acceleration can only be obtained by the application of heat, which requires additional energy input. Furthermore, during the drying process, dispersion adhesives develop inadequate tack, a property which is desirable in many cases. A drawback of this approach is that the strength and durability of dried adhesive coats are generally poorer than those of solvent-containing contact adhe­sives; this occurs because the polymer layer created is not absolutely homogeneous and has a tendency to redisperse under the influence of moisture.

4.2.2