Adhesive joints surface protection primers

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. 

To be effective, there must be a certain minimum concentration of inhibitor at the interface to be protected. Therefore, there must be sufficient inhibitor in the primer, and these inhibitors need to be soluble enough in water to enable transport of inhibitor to the oxide surface as water permeates the adhesive joint. However, too high of a solubility will rapidly deplete the primer layer of inhibitor resulting in a loss of protection. One of the fortuitous properties of zinc and strontium chromates is the limited solubility of these compounds in water (about 1.2 g/1 at 15°C [33]). 

Some primers will improve the durability of the joint by protecting the substrate surface area from hydration and corrosion. These primers suppress the formation of weak boundary layers that could develop during exposure to wet environments. Primers that contain film-forming resins are sometimes considered interfacial water barriers. They keep water out of the joint interface area and prevent corrosion of the metal surfaces. By establishing a strong, moisture-resistant bond, the primer protects the adhesive-adherend interface and lengthens the service life of the bonded joint. 

While this book was in its final preparation stage, two additional types of adhered sheet linings for chemical exposures have come to our attention. One is a sheet with, as the exposed face, a Tedlar(sf film adhered to a layer of rubber-asphalt blend. The sheet is applied with rollers to a substrate surface that has been coated with an adhesive primer. Sheet edges are butted with an adhesive lap strip of Tedlar applied over the joint to protect it. This is a proprietary product presently available from only one source, which will supply particulars of the application procedure and chemical resistance tables upon request. 

The application of corrosion-resistant primers has become standard practice for the structural bonding of aluminum in the automotive and aerospace industries. The adhesive-primer combinations are chosen to provide both maximum durability in severe environments and higher initial joint strength. Improved service life is typically achieved by establishing strong and moisture-resistant interfacial bonds and protecting the substrates surface region from hydration and corrosion. 

The purpose of this section is to outline some of the complexities of the problem. To start with, the joint does not consist of simple, separate, elastic materials with a clear, mathematical geometry. The surface of the adherend is rough and usually has an oxide or similar layer. The thickness and elastic modulus of this layer are often indeterminable. In addition, a primer is sometimes applied to the surface of the adherend for reasons of chemical compatibility, protection during storage, handling, and so on. The thickness and elastic modulus of the primer are again variables to be allowed for before the adhesive proper is reached. If the primer is similar to the adhesive, then a uniform state is reached more quickly. 

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