一周年庆典－－PAINT COATINGS—FUNCTIONING PRINCIPLES

BMA004

PAINT COATINGS—FUNCTIONING PRINCIPLES

Paint coatings offer corrosion protection through three main functioning principles:
---The barrier principle

The paint coating film acts as a barrier precluding ingress of (liquid) water and oxygen to the underlying steel substrate. It should be noted that paint-coating films couldn’t fully preclude ingress of water molecules if the coated object is immersed.

All organic paint coating films work on this principle. In addition to the impermeability of the binder(s), frequently special pigments like, e.g. brilliant leafing aluminum, micaceous iron oxide, are added to increase the film’s impermeability properties.
---The inhibitor principle

The paint coating contains corrosion inhibiting pigments like, e.g. zinc phosphate. Previously chromates were commonly used, however, these pigments have now been omitted from formulations due to their cancerogenic properties.

The inhibitors used are of such a nature that they will easily react with iron ions, and that iron ions will more readily react with the inhibitor than with f.i. hydroxyl ions.
---The galvanic principle

The paint coating contains active corrosion preventing pigments in high concentration. Metallic zinc is the most commonly used pigment in this class of materials.
AdhesionA major factor determining the performance of paint coating films is adhesion. The most fundamental requirement of any protective paint coating is that it must adhere to the substrate. But before we can consider the mechanisms by which the coating adheres to surfaces, we must ensure that the substrate is a sufficiently strong base for the coating, and we have to bring the paint coating close enough to the substrate for the bonding forces that hold the two together to attract. In the below we will discuss the principles behind these requirements.
Regardless of the nature of the bond, the required distance between attracting moieties on the metal (e.g. oxides and hydroxides) and on the coating binder (e.g. acidic groups, hydroxyls) is very small (<5A, or no more than 3 times the diameter of an oxygen atom). Moreover, as the distance between the attracting increases, the adhesive forces that pull the two materials together diminish very rapidly (falling off at the sixth power of the inter-atomic distance). On this scale, a speck of dust on the substrate is like a big rock, and a monomolecular layer of oil is like a slick, smooth sheet of ice.
The first requirement for good adhesion, therefore, is a sound base or substrate. This is met by removing from that substrate any material which is not sound, or which contaminates the surface, or which occupies areas on the substrate that could otherwise be occupied by the paint coating film to be applied.
The second requirement, getting the coating close enough to the substrate to bond, is met by making certain that the coating wets the substrate adequately.
The purposes of surface preparation:
Effective surface preparation accomplishes 4 principal tasks.
---First, it ensures that the substrate is uniform and resembles as closely as possible the model originally conceived by the engineer who designed the coating system.
---Second, it ensures that a sufficiently large number of potentially reactive sites on the steel will be available for reaction with the coating, whether the reaction is via chemical (primary valency) bonding or physiochemical (secondary valency) bonding. Surface preparation ensures, therefore, that all interfering material on the substrate is removed; whether the material be rust, rust scale, mill scale, oils, greases, or other organic or inorganic substances.
---Third, good preparatory techniques also ensure that the mechanism by which the paint system protects the steel is neither hindered nor prohibited by the presence of water-soluble organic or inorganic moieties beneath the film.
---Fourth, good surface preparation will ensure adhesion by increasing the number of reactive sites available on the metal for subsequent reaction with the coating. This is done by increasing the area of the substrate. Techniques that accomplish this fourth requirement are normally those that provide a mechanically or chemically scarified surface (abrasive blasting or pickling).
WettingAll the surface preparation in the world is futile, however, if the coating is subsequently unable to get close enough to the substrate to allow its own reactive groups to approach and react with the complementary groups on the substrate. The approach distance required per reaction is, as we have seen, unfortunately very close. Notwithstanding this, with most paint systems on clean steel, this distance is not difficult to achieve.
Clean metal (or metal oxide) surfaces are of high surface energy (several hundred dynes per centimeter) and are easily wetted by polymer solutions that have relatively low surface-free energy (surface tension). Difficulties will be experienced if the substrate has a lower surface energy than the coating, under which conditions the paint will simply not wet. Teflon, for example, with a surface-free energy of something like 17 dynes per centimeter, is not easily wetted by a coating having a surface-free energy of around 25 dynes per centimeter.
Problems with wetting may occur when steel is contaminated with lower energy materials such as oils and greases after surface preparation. These thin films of oil, with surface-free energy of less than 20 dynes per centimeter, have a strong affinity for a high-energy metal surface, and they can rapidly spread across it. In doing so, they reduce the high surface-free energy of the metal surface to approximately their own surface-free energy. Thus, a coating with a surface energy of about 30 dynes per centimeter will easily wet a clean metal with a surface energy of about 400 dynes per centimeter, but the coating will not wet the contaminated substrate with a surface energy reduced to 20 dynes per centimeter.
Hence, it is necessary to keep the substrate free of low energy contamination up to the time of coating application. It is also necessary to use suitable traps and filters on abrasive blast lines and to continually clean reusable abrasive in centrifugal blasting operation.
The nature of adhesion
To this point, we have been occupied almost exclusively with getting the coating onto the sound substrate. We have said nothing about what keeps the coating there. We will now consider the nature of adhesion itself. What is the nature of the line that holds our vessel to its mooring?
It used to be rather fashionable to categorize adhesion as either chemical or mechanical. In the first case, the coating was said to bond through actual chemical reactivity with the surface. In the second case, adhesion was thought to be obtained through a mechanical interlocking that resulted from the coating flowing into pores and cavities of the roughened steel and hooking and catching onto the irregular surface.
Modern thinking, however, discounts the mechanical methodology except on very porous substrates and describes the adhesive process as being due to force phenomena of the same type that cohesively holds the coating together.
We may, therefore, more accurately categorize the adhesive process as being 1 of 2 types, either purely chemical (primary valency) bonding or physiochemical (secondary valency) attraction. If the process is purely chemical, there is an irreversible chemical reaction between the molecules of the coating polymer and the molecules of the substrate. If the process is physiochemical, the molecules of the coating polymer and substrate are attracted by those same types of forces that might prevent two flat plate glass surfaces in contact with each other from being lifted apart.
As might be expected, true chemical or primary valency bonding provides much higher adhesive strength than does secondary valency attraction. Primary valency bonding is typified by the adhesion of an acid-based, wash-primer pretreatment to the metal surface, and probably by the adhesion of an inorganic zinc-rich primer to a steel surface. In this case, the adhesion is so strong that the interface may be regarded as almost disappearing entirely or advancing with the applied coating. The adhesion may be ionic in nature, as is the case with the wash primer, or covalent, such as when urethane linkages are formed as isocyanates in an applied coating and react with hydroxyls on a wooden substrate or on a hydroxylated polymer of a primer film.
While primary valency bonding is much preferred, the majority of coatings must rely on the physiochemical secondary valency attractions for adhesion. These attractions are reversible, and they are typified by hydrogen bonds and other weak secondary forces such as Van der Waal forces. Because there is no actual chemical linkage to be cleaved, they are all much weaker forces than the primary valency forces and much more easily broken. Hence, coatings adhering via secondary valency forces are not as resistant to subsequent delamination as are coatings that adhere via primary valency adhesion. Adhesion produced by the primary valency bond between a wash primer and steel has been measured at approximately 6 times that of a secondary valency bond between a vinyl acetate lacquer and steel.
Primary valency bonds often make use of acid groups on the coating; secondary valency bonds utilize other polar groups such as hydroxyls, ethers, amino compounds, etc. The adhesion of epoxy, for example, is directly related to the abundance of polar groups in the epoxy molecule (a polar hydroxyl and two ether linkage on each standard low molecular weight Bisphenol A type epoxy) and on the curing agent. (Amides have an advantage in addition to their carbonyl and carbon-nitrogen linkages, in that their long hydrocarbon chain greatly improves the ability of the coating to wet a substrate.) Other polymers such as alkyds owe their excellent adhesive properties to the many hydroxyl groups in their molecule. The modification of vinyl chloride/vinyl acetate polymers with small quantities of maleic acid is specifically designed to provide adhesion to metal through the carboxylic acid that attacks the substrate.