Лако-красочные материалы — производство

The polymer chemistry of primers used in adhesive bonding technology is basically the same as the polymer chemistry of adhesives. A primer may simply be an adhesive solution that improves wetting and protects the surface in the dry or cured condition. Strictly speaking, the primer transforms the adherent surface into a polymer surface that, with regard to adhesive bonding technology, is more conve­nient than inorganic surfaces, particularly with regard to long-term durability, provided that the binding polymer contains at least polar groups or residual reactivities for good adherence of the adhesive. A standard example of this type of primer is a primer solution that has been used in aircraft manufacture for many years, containing resols, polyvinyl formal and ethanol and ethylene dichloride, for example, as solvents. In the dried condition, there is no chemical difference between this primer and the adhesive, except for the mixture of the components that is more homogeneous in the case of the primer (see Section 5.4); in this way the primer can be more brittle.

However, as the primer layer has only a minor thickness this is not critical. It is worthy of mention that, in the cured state, this phenolic resin primer also represents a surface with excellent adhesiveness for epoxy resin adhesives. Fokker advanced this concept for many years in the area of longitudinal joints in the fuselage of aircraft, where components primed with phenolic resin and bonded with cold-setting, two — part adhesives were riveted together to extend the service life; the adhesive was then post-cured at 60 °C using heating pads.

As phenolic resin adhesives were difficult to process and rather brittle, they were replaced — at least in aircraft manufacture — by epoxy resin systems with a lower curing temperature and higher plasticity (see Section 8.2.1). Unfortunately, however, these adhesives did not meet the high expectations of aircraft manufacturers, neither alone nor in combination with primers of a similar type. Therefore, anticorrosive primers were developed which had other formulations than the adhesives, and which are still in use today. These primers are composed of monuron-accelerated epoxy resins, resols once again, sometimes special hardening agents for both, and strontium chromate, for example, as functional component. Resols were added because experience had taught that resol-type phenolic resins, without other aids, provided good adhesion on aluminum surfaces. At present, this is explained with their capacity to form chelate complexes with the aluminum surfaces, and their weak acidity in the polymer condition (see Chapter 3). Strontium-chromate accelerates the curing process and improves the resistance to corrosion whenever the bonded joint comes into contact with water, for example at the edges or in the case of damage. It is

dissolved and, put simply, stabilizes the aluminum oxides against electrolytic alterations which might lead to bond-line corrosion (see Section 7.5).

As mentioned in Section 4.4, dissimilar-type primers, based on epoxy resins, for example, are also used with polyurethane adhesives to improve durability. Today, they contain chromate-free, anticorrosive pigments.

Primers may contain components which, if added directly to the adhesive, would interfere with its characteristics. This opens up a wide array of possibilities to optimize adhesion properties, even in the case of surfaces that are difficult to bond, such as nonpolar plastics. The primers used for this type of material contain components that swell or etch the adherent, and this results in molecular mixing — and thus good adhesion — without any special oxidative pretreatment. These components, for example in the form of reactive solvents, may also be added to the adhesive, but this is difficult to formulate and would involve poor adherence of the adhesive on other substrates.

Today, the adhesive manufacturers keep the chemistry of the primers as trade secrets, just as they do the chemistry of the adhesives. While further details will not be discussed at this point, it has become clear that the primer system offers many opportunities for development, especially if it becomes possible to eliminate those solvents that are still needed at present, and to overcome the problems associated with chromates.

5.10.2