Primer surface protection

Currently, the structural adhesive market essentially relies on two classes of primer surface protection primers and corrosion inhibition primers. There is a further class of materials which are said to act as both surface pretreatment agents in their own right as well as surface protection and, it is claimed, bond enhancing... 

Function of primers - Protect treated surface - Protect treated surface... 

As indicated in Table 10.18, a zinc-rich primer is often recommended. It can be an organic zinc-epoxy or an inorganic zinc-ethyl-silicate primer. Zinc-rich primers are also used as so-called shop primers, or prefabrication primers, for temporary protection of semi-manufactured steel goods. After fabrication, e.g. of welded steel structures, the shop primer surface must be cleaned (degreased), and possible shop primer defects and weld joints have to be blast cleaned and coated with a primer before the whole structure is painted. Iron oxide is also used as a pigment in some shop primers. These must not be overpainted with a zinc-rich paint. 

Three classes of primer are in current use surface protection primers , corrosion inhibition primers and organosilanes, (which are claimed both to protect the surface and enhance the adhesion). By far, the greatest numbers of commercial products fall into the second class. 

Surface protection primers rarely enhance the mechanical performance of the adhesive indeed, in a few cases, a reduction in bonding strength occurs. 

In addition to surface protection, these impart some corrosion resistance, protecting the joint against attack of moisture and electrolytic corrosion. Again, the primer may be simply dried or dried and cured before bonding. 

Surfaces exposed to solutions in the containment may be painted or bare steel. Most painted surfaces or clean steel surfaces in contact with water are not good iodine absorbers. Several studies have shown, however, that the zinc primer used to protect carbon steel in containment from corrosion can absorb iodine effectively from solutions at high pH (pH 9-10). The detailed mechanism of iodine absorption on zinc primer surfaces has not been identified. It has been proposed that the metallic zinc in the coating is oxidised by dissolved oxygen in water to form various divalent, insoluble complexes, and that iodide ion, I, may be incorporated in these complexes. The solubilities of the divalent zine complexes depend on pH and the minimum solubilities are in the range of pH = 9.3 to 9.7. At higher and lower values of pH, reactions of iodide ion with zinc primers would be less effective at mitigating the production of molecular iodine. 

Zinc primer provides outstanding corrosion resistance and undercutting resistance. TTie zinc primer is protected by a barrier coating of epoxy primer, while the finish coat of polyurethane provides color and gloss retention. TTiis is a premium industrial finish for steel surfaces. Can only be used on carbon steel. 

If prolonged storage is necessary, a compatible primer may be used to coat the treated substrates after surface preparation. The primer wiU protect the surface during storage and interact with the adhesive during bonding. Many primer systems are sold with adhesives for this purpose. 

Immediately after blasting, the freshly exposed metal will rapidly oxidize and begin to adsorb contaminants from the air. For this reason, it is often desirable, especially with sensitive metals like steel and copper, to coat the adherend immediately with a primer. This protects the surface and allows the item to be stored for a period of time before bonding, without sacrificing bond quality. Additionally,... 

The process is a two-step one where the initial stage is to prime and then dry/stove the substrate to be bonded. Although the literature does not give examples of the primer used, it is fairly clear that a suitable surface protection or corrosion protection primer could be used by inference, this could be extended to a silane-based primer. 

The perceived drawback with primers, however, is that both the surface protection and, particularly, the corrosion inhibiting systems can be very sensitive to coating thickness. It is possible almost to halve the peel performance of some adhesives when going from primer coating thicknesses in the region of 2-5 xm to thicknesses above 8-10 (xm. 

This means, therefore, that the chemistries associated with the primers could be as varied as the adhesives used with them. In aerospace applications, however, the predominant surface protection primer chemistries, for structural bonding, are those based on epoxy phenolic-, polyimide- and polyurethane-based primers are also encountered. 

It should, however, be remembered that surface protection primers essentially donate nothing to the bonded joint apart from the protection of the pretreatment applied to the substrate and acting as an adequate key between adhesive and adherend. Enhancement of the mechanical performance of the adhesive being used is rarely seen. Indeed, in a few cases a reduction in bonding strength occurs. 

These do the same job as the surface protection primers but also impart a degree of corrosion resistance into the bonded joint protecting it against the attack of moisture and electrolytic corrosion. As for the surface protection systems, the primer can be dried or dried and cured following application. 

The first of these supporting adhesives, the surface protection primers and/or the corrosion inhibiting primers, have been dealt with in Section 5.5.3. 

Although many non-structural adhesives ean be solvent or water based, in aerospace applications, this format is usually the preserve of the surface protection and corrosion inhibiting primers. However, as indicated, there are exceptions and these are often those adhesives which are based on phenolic resoles (for example, Redux 775 Liquid) or on polyimides (for example, PMR -2). 

The perforated skins are usually pretreated by grit abrasion and then, occasionally, primed with a surface protection primer such as Redux 122 or with an adhesion-promotion primer. The adhesive is epoxy based and can be in film or in paste form. The acoustic panel so formed is either bonded directly to the engine casing or is first closed with a glass-reinforced epoxy laminate to promote stiffness and this GRP skin is then bonded directly to the casing. 

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