Pumps & Systems, August 2008
Epoxy based polymeric coatings and repair materials provide cost effective, prompt solutions for rebuilding, repairing and protecting industrial equipment from corrosion, erosion and chemical attack. A critical aspect of any coating or repair material is the ability to adhere to the substrate. Bonding of a polymeric material to the substrate can be separated into three general modes-dispersative bonding, chemical bonding and mechanical bonding. Dispersative bonding is the result of weak intermolecular dispersion forces between the polymer molecules and the underlying substrate atoms. It is also the weakest of the three modes and chiefly used by most retail glues.
Chemical bonding occurs when the substrate and the polymer chemically react with each other and form a direct chemical bond. Chemical bonding is much stronger than dispersative bonding but can only occur when the substrate and polymer are of compatible chemical composition to allow a reaction to occur.
Mechanical bonding provides the strongest bond force by far and does not require the substrate and polymer to be of similar composition. A good mechanical bond is achieved by providing a rough angular profile on the substrate to allow the polymeric material to physically grip the substrate. Roughening the surface also increases the contact area of the bond. Common standards for coatings and repair materials require at least a 3.0-mil angular profile and NACE No. 2 /SSPC-SP 10 surface cleanliness. In addition, it is critical that the substrate be clean, dry and free of any contaminants.
Unfortunately, meeting all these requirements in the field can be difficult, if not impossible. New technology in polymeric materials has led to the development of epoxy-based products that can be applied onto wet and oily surfaces, which eases some of the typical surface preparation requirements. Paste grade repair material and coating grade material are both capable of being applied onto wet or oily surfaces. These materials are effective at displacing contaminants from the substrate via a strong electronic affinity with the metallic substrate, which allows the epoxy to penetrate the profile and create a strong mechanical bond.
Developments such as these allow for emergency applications in situations where ideal substrate cleaning is not possible. If a live oil leak cannot be completely stemmed or effectively degreased, a fast curing epoxy can be used to momentarily stop the leak. A reinforcement cloth saturated with paste grade repair material can then be wrapped tightly around the pipe. Alternatively, a metal reinforcement plate can be bonded with the same material over the area and wrapped as described to produce a high strength repair.
Preliminary lab tests demonstrate the abilities of these products to adhere to contaminated substrates. Figure 1 compares the tensile adhesion values as per ASTM D4541 standards. Error bars represent the 95 percent confidence interval computed from various repetitions.
Figure 1. Tensile adhesion variation as a function of surface contamination and surface preparation method
The data indicates the material can adhere strongly to contaminated surfaces. The plasticizers are also more effective when adhering to a wet surface than to an oily surface. The combination of oil and water produces an interesting effect (compared to oil) that is still not understood and will be investigated in future testing.
In addition to live leak repairs, a coating grade surface tolerant barrier can be used for coating applications where the surface cannot be dried or degreased.
Such materials can provide an excellent solution for coating areas where surface contamination cannot be completely eliminated. These coatings were developed particularly for offshore drilling where corrosion is a serious problem in the splash zones. Surface tolerant coatings can coat jetty legs, pylons and other splash zone surfaces. In these areas, the only other solutions are removing the equipment from the water, which is not always possible, or constructing coffer dams, which are not economically attractive.
These products are bringing new life and practicality to the coating industry with fast, effective solutions that remove some impractical steps required by traditional coatings and repair polymeric materials.
