Corrosion Policy Decision Making. Группа авторов

Чтение книги онлайн.

Figure 2.11 Lack of curing of ethyl silicate primer used in dry area.

The four main components of industrial paints, including pigments, binders, solvents, and additives play their own special role in paint performance. In addition, paint systems consist of appling industrial paints in continuous and sequential layers, each layer having certain properties.

Pigments play an important role in improving the quality of paint. Although some pigments have decorative properties, they also play an important role in optimizing the texotropic properties and rheology of the paints, such as reducing sagging and dripping problems. The strength of the paint and the gloss and hiding properties on the previous layer are other characteristics that are enhanced by the pigments. They also play an important role as corrosion inhibitors in the primer.

      In the simplest classification, pigments are divided into two main groups; pigments and extenders [7]. Pigments include zinc powder and zinc oxide, iron oxides, titanium dioxide, lead chromate, basic lead‐silico chromate, molybdate, zinc chromate, lead cyanimides, zinc powder, blue iron (Prussian blue), aluminum pigments (powder and paste), bronze, mercury oxide, etc. and their chemical properties are explained in handbooks and factory catalogs.

Zinc powder is one of the most common primer pigments. It is sometimes packaged as a third part for primers. Often, not observing the exact mixing ratio or insufficient stirring leads to poor wetting and causes a complication in the dry film (Figure 2.12).

Titanium dioxide is one of the most common pigments used in paints. Titanium is present in the structure of anatase and rutile. The rutile type consists of a denser crystal lattice than anatase, and it has a higher refractive index, and this type has good resistance to deterioration. The use of anatase as pigments of the top coat paints leads to severe chalking when the system is expose to sunlight (Figure 2.13).

      Extenders are solid minerals that may added to paint formulation along with the pigments to modify some of the properties of the paints as well as to reduce the cost of production. They are also used to improve the following properties, and optimizing quality of paints.

      Some extenders include calcium carbonate, white carbonate, aluminum silicate, magnesium silicate (talc), mica, silica, calcium sulfate, asbestos, barium sulphate, and zinc sulphate. Their chemical properties described in the reference [7].

      Organic pigments and metal pigments also used in a few applications whose properties are described in some sources [7, 15]. For example, the use of the pigments of aluminum, copper, nickel, iron, stainless steel, zinc, tin, etc. in paints were described [7].

      The resin is most important to form a uniform and continuous film on the metal surface. This component used in the paint formulation allows for adherance to the substrate and allows the continuity of paint components to bond in the dry film. It should also create a dry film that prevents the diffusion of water vapor and various corrosive contaminants from the environment to the subsurface.

Various polymers are used as binders in paints. Their properties, such as the molecular weight of the polymers, the mechanical strength of the bonds formed in film, the permeability of the film, thermal resistance, etc., are effective in the paint and coating capabilities. The paint curing mechanism is the most important method for classification of binders.

      There is a group of binders in which the polymers react with air humidity or oxygen of the air for curing; they are known as air‐dry binders. Drying oils and phenolic varnish castor oil (polished oil) are natural air‐dry and alkyd binders; vinyl alkyd, epoxy ester, silicon alkyd, etc. are synthetic air‐dry binders [7]. This group of binders is used in single‐component paints. They begin to form a skin during shelf time, especially when the ambient temperature rises or the can or container lid is left open or the container damaged. It is also important to adjust the ratio of surface drying and deep drying additives in these paint formulations.

      There is another group of binders where the polymerization reaction of the binder is completed during fabrication at the factory. When they apply to the substrate, the paint sticks to the substrate and forms a dry film only by the evaporation of solvent. Nitrocellulose binders, vinyl resins, and chlorinated rubbers are of this type. These paints are a single part similar to the previous group. Paint solvents play an essential role in the drying speed and only the use of suitable solvents made by the same factory is recommended for them to reduce the paint defects.

      Another group of paints is known as co‐reactive, and is packaged in two or three separate containers during storage. In order to start the polymerization reaction, they have to be mixed in right amounts and the mixture needs to rest for some minutes before application. The main binder is a pre‐polymer and includes some other paint constituents packaged separately. Hardeners are other chemical compounds that are needed for initiation and to continue polymerization reactions and which are stored in separate container. The contents of the containers are added to each other before application. In this way, complementary polymerization reactions of the paint take place after mixing and application. Epoxy, polyurethane, polyester, and coal tar epoxy paints are of this type. To cut the defects in this group of binders, it is necessary to obey the exact ratio of mixing of the paint and hardener. It is often suggested to wait half an hour after mixing to start the pre‐reactions and then start diluting and applying. It often takes more than a few days for full curing of these binders. Therefore, in the primer and middle layers, we have to wait at least 24 hours before the next layer and for the topcoat paint, and at least one week before service operation.

      The polymerization of another group of binders can be completed by increasing the temperature to a certain extent; paints containing these binders are known as thermal curing. Pure phenolic paints and phenolic epoxy are two well‐known categories of thermal curing [7]. It is possible to meet their last curing at the temperature of 230 °C.

Silicon binders are another heat‐curing polymer. It is a mineral‐based heat‐bonding binder with the main constituents of silicon and oxygen. Polysiloxane‐type silicone binders are also used in paints; this binder has a very good resistance to chemical agents due to having silica in the formula. Silicone binders are suitable for use at high temperatures (540–640 °C) and may be used to make paints resistant to the same temperature. For example, when using ceramic pigment particles, using them at up to 760 °C is recommended. This binder may also modify with alkyd for use at lower temperatures.

      This group of paints has a very low mechanical strength before the final СКАЧАТЬ