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Auto Paint Component (Part.3) Additives
- Apr 09, 2018 -


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Extenders are paint additives that are insoluble in the binder and solvents of the formula and have little or no opacity or color effect on the film. They are added to modify the flow and mechanical properties of the paint as well as the, permeability, gloss, and leveling characteristics of the paint film.

BARITES

Ahard, dense, naturally occurring inorganic mineral that is very resistant to the action of acids and alkalis. It is transparent in oil and resinous media so that it can be used in solvent based paints without affecting the color or opacity of the film. The high specific gravity of barites can result in settling problems in stored paint. It is used as chemically inert filler that reinforces the mechanical properties of the film. It will reduce the durability of the film, if used at high levels in a coating. Asynthetic form of barium sulfate is also used as an extender and has properties similar to those of barites.

KAOLIN CLAY

Anaturally occurring hydrated aluminum silicate used in a fine particulate form. It is used as filler in solvent-based paints but in small amounts, because the small particle size can adversely affect the flow characteristics of the paint. The fine particle size does help to prevent settling in storage. Kaolin clay is used as a flattening agent (gloss reducing) in undercoats and eggshell finish paints and widely used in emulsion paints.

TALC

Anaturally, occurring hydrated magnesium silicate mineral. It occurs as a mixture of lamellar and fibrous particles, the latter providing a reinforcing effect while improving film flexibility. The platelets have an effect similar to mica in reducing water permeability. As a result, talc is often used in protective paints where high durability is required. The exact nature of how talc and mica improve corrosion resistance is not fully understood.

CALCIUM CARBONATE (LIMESTONE)

Naturally occurring limestone that is (also produced synthetically) widely used in interior and exterior emulsion paints and undercoats. Acids will attack limestone, so it cannot be used in aggressive environments. Nevertheless, it has wide usage in both aqueous and solvent-based finishes.

There are many other pigments that will provide corrosion resistance and other properties such as improved adhesion, mechanical properties, and viscosity control.

The precise blend selected for a coating is related to the needs of the end user. Color, performance, cost, and environmental concerns affect the particular blend of pigments used. Figure 2.1 is a pictorial representation of the optimum pigment loading that canbe calculated. The common term for this is the pigment to binder ratio. Although everyone talks about pigment to binder ratios, experience has shown that very few paint formulators can actually determine the pigment-to-binder ratio. The optimumpigment loading is generally based on empirical observations, trial and error, and history.


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Figure 2.1 shows three different pigment-to-binder ratios. The pigment volume concentration (PVC) at which the pigment particles just touch each other with binder filling the space between them is called the critical pigment volume concentration (CPVC). The picture A in the figure has too little pigment, B is the correct CPVC, and C has too much pigment. The percentage of pigment required to achieve the CPVC depends on the size and shape of the particles. Smaller particles can be packed together with greater efficiency, resulting in higher CPVC.

The significance of PVC is illustrated in Table 2.3. It shows how a number of important film properties vary with pigment level. The amount of pigment in paint is often a compromise between several of the properties listed in Table 2.3.