.- $i’ - FOCUS:Pretreatment for Painting/Powder Coating 54 PRODUCTS FINISHING T he classic definition of quality, as it pertains to manufacturing systems, is conformance to specification. Paint systems are mar- riages of pretreatment processes and paint processes. Different combina- tions of pretreatment and paint pro- cesses will meet varying degrees of quality, depending on specification needs. In the manufacturing system, the entire process must be taken into account. Each step has an impact on al1 subsequent processing steps, start- ing with raw material selection. The quality of the incoming substrate is the first step. It will affect the perfor- mance in each subsequent manufac- turing step. The most common sub- strates in paint processes are: cold- rolled steel; hot-dip-applied alloy over cold-rolled steel (galvanized, Galfan, Galvalume); annealed zinc- iron alloy (Galvaneal); electrodepos- ited alloy over cold-rolled steel (electrogalvanized, electroplated nickel, electroplated zinc-iron); hot- rolled steel; sheet aluminum; iron, aluminum and zinc die cast. Some substrates are chosen for low cost, some are selected for reduced weight and others for enhanced per- formance when exposed to harsh en- vironmental conditions. During manufacturing incoming substrates are altered either by bend- ing, drawing, drilling or machining. In many cases, an externa1 lubrica- tion source is required, such as draw- ing or machining fluids. Different types of fluids can be used, such as straight oils that may contain chlori- JANUARY, 1995
Transcript
PJ 8 SJI; .- $i’- FOCUS:Pretreatment for Painting/Powder Coating
54 PRODUCTS FINISHING
T he classic definition of quality, as it pertains to manufacturing systems, is conformance to
specification. Paint systems are mar- riages of pretreatment processes and paint processes. Different combina- tions of pretreatment and paint pro- cesses will meet varying degrees of quality, depending on specification needs.
In the manufacturing system, the entire process must be taken into account. Each step has an impact on al1 subsequent processing steps, start- ing with raw material selection. The quality of the incoming substrate is the first step. It will affect the perfor- mance in each subsequent manufac- turing step. The most common sub- strates in paint processes are: cold- rolled steel; hot-dip-applied alloy over cold-rolled steel (galvanized, Galfan, Galvalume); annealed zinc- iron alloy (Galvaneal); electrodepos- ited alloy over cold-rolled steel (electrogalvanized, electroplated nickel, electroplated zinc-iron); hot- rolled steel; sheet aluminum; iron, aluminum and zinc die cast.
Some substrates are chosen for low cost, some are selected for reduced weight and others for enhanced per- formance when exposed to harsh en- vironmental conditions.
During manufacturing incoming substrates are altered either by bend- ing, drawing, drilling or machining. In many cases, an externa1 lubrica- tion source is required, such as draw- ing or machining fluids. Different types of fluids can be used, such as straight oils that may contain chlori-
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,
nated naraffin oils. or sulfur com- pounds for extreme’pressure lubrica- tion; water-emulsifiable oils that may contain chlorinated paraffin oils or sulfur compounds; water-emulsifi- able vegetable oils; water-soluble synthetic lubricants and coolants; or water soluble soap-based lubricants and coolants.
The first job of these lubricants or coolants is to make or form the part to specification. Equal care must be taken in the selection process to en- sure compatibility with the subse- quent pretreatment step. Often a paint performance problem can be traced to an incompatibility between the fabrication and pretreatment.
For example, a soap-based syn- thetic lubricant cannot be used in a system employing a detergent iron phosphate. Soaps are neutralized fatty acids and slightly alkaline. When the soap reacts with the detergent iron phosphate, which is mildly acidic, it is converted back to the fatty acid. This residue will sti‘ck to the substrate, and it is difficult to rinse off. The residues may cause paint adhesion and performance problems.
Fabricated parts proceed to finish- ing where the first step is a multi- stage pretreatment system. This pro- cess can be either spray, immersion or a combination of the two. The pretreatment portion can be generi- cally broken into three steps: 1. Surface preparation.
l Cleaning: Removing organic con- taminants from the metal surface such as lubricants, coolants, and mil1 oil.
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This can be accomplished with an alkaline cleaner, or the surfactant portion of a detergent iron phosphate.
l Rinsing: Diluting cleaner resi- dues to a point where no contamina- tion of subsequent stages occurs.
l Descaling: Removing inorganic contaminants from the metal surface, such as rust, weld scale, smuts. This can be accomplished with dilute ac- ids or chelated alkaline descaler.
l Rinsing: Diluting descaler resi- dues to a point where no contamina- tion of subsequent stages occurs.
l Grain refiner: Usually a disper- sion of titanium salts on the surface of the metal to act as nucleating sites for zinc phosphate crystals. This also works with the phosphate chemistry to produce small, dense, uniform crys- tals over the surface. 2. Surface pretreatment is a chemical reaction with the substrate, where a coating is deposited and chemically bonded to the metal surface. This coating acts to increase the surface area available for paint bonding sites. The different types of coatings used include the following:
l Iron phosphate: an amorphous, non-crystalline coating of a matrix of iron oxide and iron phosphate.
l Chromate: An amorphous chro- mate conversion coating for ferrous and non-ferrous metals. Chrome phos- phate and chrome-free conversion coatings are in this category.
l Zinc phosphate: a crystalline coat- ing of zinc phosphate usually com- bined with other metals, such as nickel, cobalt, calcium and manga- nese, to help refine the crystal.
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FOCKPretreatment for Painting/Powder Coating
3. Surface purification removes any surface impurities, soluble and in- soluble, that would create a physical paint defect or contribute to a paint performance failure.
l Rinsing: Diluting zinc phosphate salts from the surface.
l Seal: a) Chrome precipitates re- sidual zinc phosphate salts as chro- mic phosphate andlor zinc (nickel, cobalt, calcium or manganese) chro- mate. b) Chrome-free puts an organic or inorganic matrix on the surface to further promote paint adhesion.
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l Rinsing: Dilutes any final resi- dues on the surface. This can be a recirculating tap water rinse, but is usually a recirculating DI rinse fol- lowed by a halo of fresh DI water. The DI water creates the purest sur- face possible, reducing chances of paint failures (See Table 1).
There are various pretreatment washer configurations, ranging from a three-stage process to as many as eight stages, depending upon avail- able floor space, finish quality and capital available.
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After dry-off, the parts are then painted using one or more of the following paint processes: dipldrain - solvent or water-borne; flow coat - water-borne; electrocoat 2 wa- ter-borne anodic and cathodic; elec- trostatic liquid spray - solvent or water-borne; electrostatic powder spray; fluidized bed powder.
The parts are then solvent flashed in air, if necessary, then baked per the paint manufacturers recom- mended cure cycle.
The most common paint systems in use are electrostatic spray, electrocoat and electrostatic powder coat. The more common pretreatmentl paint pairs are listed below.
Three- to five-stage ironphosphate/ electrostatic spray paint;
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Five- to six-stage zinc phosphatel electrostatic spray paint;
Three- to five-stage iron phosphate/ electrostatic powder paint;
Five- to six-stage zinc phosphate/ electrostatic powder paint;
Seven- to eight-stage zinc phos- phate/electrocoat paint.
Finishedparts are subjected to vari- ous types of corrosion and physical adhesion tests. The most common tests include the following.
l Salt spray test. This is a neutral salt (sodium chloride) fog exposure test. A panel is scribed down to the substrate and exposed to the salt fog atmosphere for a prescribed amount of hours. This test measures the re- sistance of ihe phosphate coating to undercut corrosion.
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FOCKPretreatment for Painting/Powder Coating
Note: For vary kigh perfofmance, mt~ltlple paint coats are usad utílizing an epotry primer with a polyester or
The phosphate coating acts as an insulator against corrosion at the metal surface. A crystalline coating will outperform a non-crystalline coating so a zinc phosphate will out- perform an iron phosphate coating. This only tests relative differences
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between process variables and should not be used as a predictive test of field use.
l Humidity test. This is a neutral fog test without the Salt. Performance in this test indicates the effective- ness of the surface purification. If
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4
~250 Hours
Modtfied Alkyd
2w 1,000 Hours
Pretnat f poxy Polyester Mybrid
Poiyester Urethane
>l,OOO Related Acrylic Hours
1
AC@iC Urethane
rethane topcoat.
any soluble salts are left on the part prior to dry-off (either from poor rinsing, or hard water salts), they will dehydrate in the dry-off oven.
When the part is painted and sub- jected to humidity, the condensed water, or the water vapor, will per-
l Impact. Reverse and direct. This is another deformation test that will measure the ability of the painted surface to maintain good adhesion even when the surface is deformed by the impact of a weighted ball.
l Pencil Hardness tests the paint cure by measuring the dry film’s abil- ity to withstand the marring effects of various pencil lead hardnesses.
l Crosshatch uses a grid of 100
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meate through the pores in the paint. Eventually, any salts on the surface will rehydrate. Since the paint is bonded to these salts, and not to the substrate, the paint will lose adhe- sion at this area and lift from the surface, creating a bubble or blister.
l Detergent. This test, used mostly in appliance manufacturing, tests the alkali resistance of the phosphatei paint marriage.
l Mandrel. Cylindrical, or coni- cal mandrel. This tests the ability of the painted panel to resist adhesion loss when the substrate is severely bent.
The most severe mandrel test is the zero T-bend where the panel is bent back upon itself. Variables that could lead to failure are large phos- phate crystals that actually fracture upon bending, and the paint adhering to the fractured crystal will pu11 away from the substrate. Other variables include brittle paint from too much cure or incompatible paint chemis- try; and adhesion loss due to poor pretreatment or oxide formation on the phosphate surface in the dry-off oven due to high levels ofunexhausted NO, and CO, combustion gases.
FOCUkPretreatment for Painting/Powder Coating squares scribed through the paint to the substrate. When the grid is taped and pulled, the result measures the paint adhesion.
The fabricated part must be evalu- ated according to its end use, such as exposure to the elements, including corrosion resistance, flexibility and adhesion, alkali resistance, and UV (ultraviolet) weatherability. A sum- mary of the relative properties of phosphate coatings and common paint resin systems is in Table II.
When these properties are ana- lyzed, a paint system can be devel- oped so that the optimum marriage between pretreatment and paint is made. Some examples of the varying degrees of these properties are: Corrosion resistance measured by salt spray Low: Indoor exposure, non-
UV weatherability measured by UV exposure test Low: Indoor storage, no
contact with sunlight Medium: Indoor storage, some
exposure to sunlight; Office furniture, toys, appliances
High: Outdoor exposure, automobiles, air condi- tioners, patio furniture
Table III is a summary of the dif- ferent properties combining the dif- ferent pretreatment configurations with different paint chemistries. A column is included with test expo- sures as a guideline for pretreatment paint combinations to satisfy differ- ing quality needs.
Too often, the marriage between the pretreatment and paint system is a shotgun wedding. Often a system is forced to operate beyond the design limits. And this may compromise quality. The most frequent compro- mise is increasing line speed to handle increased production demands. This can compromise quality. Contact
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times in urocess tanks are shortened, so there is less cleaning and phosphating. Shorter drain time be- tween stages creates higher drag-out so that contamination levels climb. Shorter times in paint bake ovens can undercure paint, leading to increased failures in corrosion testing.
Any time system changes are made, the entire process must be evaluated, and engineering modifications must be made to ensure that the quality of the system is not compromised.
Finally, normal process controls should be made on a regular basis. A log book should be maintained and include information on pretreatment: concentration, temperatures, spray préssures of al1 process tanks, line speed and dry-off oven temperature; and on paint: concentration (vis- cosity, solids, specific gravity, etc.), temperature, relative humidity, volt- age, bake oven temperature, and line speed, if different from pretreatment.
Keeping the above variables in control will keep the overa11 process under control. The important vari- ables that will ensure paint perfor- mance are pretreatment, including coating weight, crystal size and “drip- pings,” which is the conductivity of a collection of the water on the parts after the final process tank. This is a measure of the surface purity, and the amount of salts left on the surface under paint.
The quality of the pretreatment system needed is dependent upon what performance specifications are to be met by the entire paint process, so that the proper paint can be JANUARY. 1995
matched. The quality of the system is determined by the optimum pairing of pretreatment and paint. The over- al1 quality of the system is enhanced when the proper process controls are maintained, and the design limits of the process are not exceeded. PF
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