Лако-красочные материалы — производство

The most important adhesives of this group are polyester, cold-setting epoxy resins, polyurethanes and acrylic adhesives. Common two-part adhesives consist of several components that are mixed in a specific ratio before application. For acrylic adhesives of the ‘second generation’, it is possible to apply each of the components separately to one of the adherent surfaces, and then to press the adherents together, thereby initiating the curing process.

In addition to the specific mixing ratio, the so-called ‘pot life’ must also be respected; this is the period of time during which the adhesive can be used (i. e. be applied) after mixing the components, and is dependent on the type of

adhesive used and the volume prepared. As the crosslinking process gradually starts during the pot life, and the viscosity of the adhesive gradually increases during this time, exceeding the pot life will cause a lack of wetting of the adherent surfaces by the adhesive and result in poor adhesion properties within the bond-line. The cross­linking oftwo-part or multi-part adhesives, respectively, almost always takes place as an exothermic reaction, so that the adhesive will be heated up during this process. When a small volume is prepared it will warm up more slowly than a larger volume, for which the ratio ‘volume to heat-emitting surface’ is less favorable. This effect represents a particular risk. It is difficult to recognize the moment when crosslinking begins by the degree of viscosity because the heat emitted will lead to a decrease in viscosity. This in turn will neutralize the effect of the crosslinking process, which increases the viscosity of the mixture. If such a warmed system is applied to cold adherents, the partly crosslinked system will solidify immediately, without adequately wetting the surfaces.

4.3.2