Organic Coatings

Paint is applied to a steel sheet or a part for one or both of the following reasons: enhancement of the aesthetic value of the product and/or preservation of structural integrity. The former goal is a consideration for the designer and will receive no further discussion in this section. The latter goal, however, will be discussed as it applies to prepainted steel.

This section will describe how paints deter corrosion, the prepainting process, the primary differences between prepaint paint formulations and postpaint formulations, considerations about part design, and selection criteria for the appropriate paint system. More detailed information on organic coating materials can be found in Ref 48 and the articles "Painting" and "Elastomeric Coatings and Linings" in this Volume.

How Paint Films Deter Corrosion. In the presence of water and oxygen, iron corrodes to form iron oxides and hydroxides. The corrosion rate is accelerated when electrolytic solutes, such as the chloride or sulfate salts of alkali metals, are present. Of course, temperature also increases the corrosion rate, and where possible, decreasing the service temperature will increase the service life of the part. However, because little can usually be done to change service temperature, the exclusion of one or more of the principal reactants (oxygen, water, or electrolytes) from the steel surface will help deter corrosion. Such exclusion of reactants is the purpose of the paint film.

There are primarily three methods of protection: barrier, passivation, and galvanic. In barrier protection, the paint film retards the diffusion of water, oxygen, or salts to the steel substrate. It has been demonstrated that the flow of water and oxygen through the film is rapid; therefore, the contribution of the coating to lowering the corrosion rate is the addition of a high electrolytic resistance (Ref 49). Flake-shaped pigment particles that leaf (overlap) can increase the path length a reactant must traverse before reaching the surface; this increases the effectiveness of the barrier film. Some aluminum and stainless steel pigments protect in this fashion. The reactivity of the steel surface can be decreased when the paint film contains passivating pigments such as chromate salts. As described below in the section on "Painting With Zinc-Rich Paints," paints are also often pigmented with zinc for both barrier and galvanic protection. The zinc loading must be sufficiently high for interparticle contact, a condition that requires that the critical pigment volume be exceeded; that is, the pigment particles are not completely wetted by the paint vehicle. Although some galvanic protection is afforded, most of the protection is provided by the barrier formed by zinc corrosion products (Ref 50).

Prepaint Processing. Much of the painted steel used today is prepainted in coil form (coil coated) before shipment to fabricators. Modern, high-speed paint lines can process bare or coated steel strip, and can be used to apply a wide variety of organic coatings.

After decoiling, the first step in the prepaint process is to clean the steel strip with an alkaline detergent. The steel strip is then brushed with an abrasive roll to remove mill oils and grime and to reduce the level of an amorphous form of surface carbon indigenous to steel strip processing. High levels of surface carbon lower corrosion resistance (Ref 51). Cleaning is usually more effective on flat strip than on a formed part. Next, the strip is rinsed and pretreated to improve paint adhesion and to reduce corrosion. A prepaint treatment may consist of phosphate coating or an organic pretreatment known as wash primer or etch primer. Such pretreatments are described in the article "Painting" in this Volume. Following pretreatment, a paint is then applied and cured in an oven. Depending on the paint formulation and the paint line, the dwell time in the oven is generally between 20 and 50 s. A second coat may then be applied and cured.

Differences Between Prepaint and Postpaint. In formulating a paint designed for a prepaint application, the forming step must be considered. Aside from steel consideration, successful forming of the part will depend on the flexibility and the abrasion resistance of the coating. The paint film must be flexible enough to withstand the strains induced from bending without crazing, which may compromise corrosion resistance. In addition, in the forming stages, the bend radii are often more severe than in the final part.

The coating must also withstand the abrasive forces of handling and forming. For a given coating type, the harder the coating, the more abrasion resistant the coating will be. Unfortunately, flexibility and hardness are inversely related; that is, the more flexible the coating, the softer the coating.

A method for overcoming the problems associated with coating flexibility is covered in the discussion on part design considerations in this section. Flexibility and hardness are considerations only for the end use of postpainted parts, while forming and handling are also factors of concern in the formulation of a paint designed for precoating a steel strip.

The final dried paint thickness, or dry-film thickness, on a prepainted steel strip is usually no more than 0.25 mm (1 mil). Plastisols and organosols are the major exceptions. Therefore, the prepaint dry-film thickness is much less than the typical dry-film thickness on a postpainted part. However, because of the method of application, the film is more evenly distributed; this results in significantly fewer areas of low dry-film thickness and the elimination of many of the appearance defects observed on finished postpainted parts. Many areas on postpainted parts will receive little or no paint because of the part shape. The formulations for prepaints are engineered to account for the lower dry-film thicknesses.

Part Design Considerations. When designing a part to be fabricated from prepainted steel, the maximum bend radius, the forming equipment, and the joining method must be considered. As mentioned earlier, the maximum bend radius is often smaller than that specified on the blueprint of the part. This radius should be as generous as the structural and decorative criteria will allow. In considering the part shape, the avoidance of catchment areas, where possible, will decrease failures due to corrosion.

The forming equipment should be well maintained to avoid marring the surface. Where possible, roll forming is preferable to stamping. In cases in which hard finishes in conjunction with tight radii (high flexibility) are required, the prepainted strip can be warm formed. In warm forming, the paint is heated into or above its glass transition temperature range. In this temperature range, the paint is softer and more flexible, thus allowing tighter radii to be achieved during forming. After cooling, the paint becomes harder and more abrasion resistant.

Lastly, the part may require joining. Welding and mechanical fastening damage the paint film. Therefore, it is necessary to touch up the scars to restore corrosion resistance. Adhesive bonding eliminates the need for touch up of these damage areas. Taking these factors into account, prepainted steel has been successfully fabricated into finished or semifinished (requiring a post finish coat operation) parts in many automotive, appliances, or office furniture manufacturing plants. Prepainted parts have been produced on lines designed for their use and on existing lines, sometimes with no modification to the line.

Selection Guideline. As an aid to understanding the evaluation process, Table 29 compares various common coatings in several categories of performance. Changes in pigmentation and resin source for the vehicle can influence the rating by a factor of one or more. Table 29 is merely a guideline to the performance of these coatings. Comments from technical personnel at a paint company should be sought before making any decision on paint selection.

Table 29 Relative hardness rankings of various coatings in different performance categories

Category key: A, hardness; B, flexibility; C, humidity resistance; D, corrosion resistance to industrial atmospheres; E, salt spray; F, exterior durability, pigmented film; G, exterior durability, clear film; H, paint cure temperature, in °C (°F); I, cost guide. Ratings key: 1, excellent; 2, good; 3, fair; 4, poor; H, high cost; M, moderate cost; L, low cost

Table 29 Relative hardness rankings of various coatings in different performance categories

Category key: A, hardness; B, flexibility; C, humidity resistance; D, corrosion resistance to industrial atmospheres; E, salt spray; F, exterior durability, pigmented film; G, exterior durability, clear film; H, paint cure temperature, in °C (°F); I, cost guide. Ratings key: 1, excellent; 2, good; 3, fair; 4, poor; H, high cost; M, moderate cost; L, low cost

Type

A

B

C

D

E

F

G

H

I

Silicone acrylic

1

3

2

2

2

2

1

230 (450)

H

Thermoset acrylic

2

2

1

2

1

2

2

220 (430)

M

Amine-alkyd

2

3

2

2

3

2

3

170 (340)

L

Silicone alkyd

2

3

2

2

2

1

2

215 (420)

H

Vinyl-alkyd

2

2

1

2

2

3

3

170 (340)

M

Straight epoxy

1

2

1

1

1

4

4

205 (400)

H

Epoxy-ester

2

2

1

2

1

4

4

205 (400)

M

Organosol

2

1

1

1

1

2

3

175 (350)

L

Plastisol

3

1

1

1

1

2

3

175 (350)

L

Polyester (oil-free)

1

2

1

2

1

2

3

205 (400)

M

Silicone polyester

2

2

1

2

1

1

2

230 (450)

H

Polyvinyl fluoride

2

1

1

1

1

1

1

230 (450)

H

Polyvinyl idene fluoride

2

1

1

1

1

1

1

230 (450)

H

Solution vinyl

2

1

1

2

1

2

3

150 (300)

M

The columns in Table 29 are self-explanatory, with the exception of those involving exterior durability and salt spray. Exterior durability is the resistance to weathering, particularly the resistance to ultraviolet light. Ultraviolet light causes some coatings to chalk. Proper pigmentation will prevent this phenomenon for some coatings, and this can be determined by comparing the columns for pigmented and clear films.

Salt spray is not a predictor of service life and coatings cannot be compared for end use on this basis. However, salt spray does detect coating defects and can be put to good use for detecting induced flaws by comparing results for flat panels with those of panels with coating defects induced, for example, by forming or abrasion.

The first step in the evaluation is the selection of a steel mill and/or paint company that is willing and able to help evaluate the needs of the final product. These needs can be categorized as either preservice or service. The preservice conditions involve forming, handling, and joining. The service conditions are those to which the customer exposes the product: humidity, temperature, corrosive agents, sunlight, and abrasion. Of course, the preservice conditions can affect the service life of the final product, and these effects should be evaluated.

The next step is the experimental design. The test program compares candidate materials to the current product, if possible. Evaluation in actual service conditions is often not possible because of time limitations. Therefore, accelerated and laboratory tests are needed (see the article "Corrosion Testing" in this Volume). From these results, acceptable candidates are identified and are included in the next level of tests. A set of suitable parts is identified for testing the candidates. After the parts are fabricated, they are inspected to determine whether coating damage occurred and whether corrosion resistance was compromised. In general, one material will not be superior in all aspects. Therefore, the desirable properties must be prioritized.

Advantages of Prepainted Steels. Although the above evaluation sequence may seem formidable, many manufacturers have found the use of prepainted steel to be productive and economical. The use of prepainted steel reduces or eliminates the problems of the waste treatment of the emissions from paint lines. The postpainting line is often the slow step in the process, and using prepainted steel increases output. Although the material cost of prepainted steel is higher than the bare metal, the final part cost is lower because of increased productivity and the reduction of other costs, such as emissions control. Although prepainted steel cannot replace postpainted steel in every application, prepainted steel has demonstrated its productive and economic advantages.

0 0

Post a comment