coating materials and application methods

AUTODEPOSITION OF ORGANIC COATINGS

BY THOMAS C. JONES

HENKEL SURFACE TECHNOLOGIES, MADISON HEIGHTS, MICH.

Auto deposition of organic coatings refers to a chemical process for depositingpaint coatings onto metal surfaces. This process resembles electrodeposition inthat the objects to be coated are submerged in an aqueous bath containing ionizedorganic materials; however, in autodeposition, the coating is produced by achemical reaction between the metal surface and the bath constituents as opposedto electrodeposition where the coating is produced by the electrolysis of water dueto an imposed electric current. After autodeposition, the wet film is water rinsedand cured by application of heat to produce an inert, corrosion-resistant coating.

COATING MECHANISM

The first step in the coating mechanism for the autodeposition process is thechemical reaction between the metallic surface and the inorganic constituentsof the coating bath. The coating deposition step of the autodeposition processinvolves the controlled destabilization of an aqueous polymer latex dispersion,which is negatively charged, by the positive ions generated at the surface of themetal by the inorganic chemical reaction.The components of an autodeposition coating bath include a weak acid(hydrofluoric acid, HF) in the range of 0.2% to 0.3% by volume, an anionicallystabilized latex and pigment dispersion (latex/pigment), and chelated ferric ionin solution (FeF3). The total solids content of the bath is less than 10%, and thecoating solution has the viscosity of water.The chemical reactions that result in an auto deposition coating are as follows.For iron dissolution, the major contributor is

2 Fe F3 + Fe 3 Fe+2 + 6 F-1,

and a minor contributor is

2 HF + Fe Fe+2 + H2(g) +2 F-1;

deposition occurs whenFe+2 + (latex/pigment) Fe(latex/pigment).Iron that is not entrapped in the wet film is converted to FeF4-1 by the additionof an oxidizing agent. Since FeF4-1 does not react with the anionically stabilizedlatex/pigment dispersion, the process is sludge-free.As the autodeposited film builds, the diffusion of reactants to the surface isslowed, and the rate of film deposition decreases. This self-limiting mechanismresults in a final coating that is extremely uniform and conforms to the underlyingsurface. All areas exposed to the coating bath become coated. This featureof the autodeposition process is important since even enclosed areas will beprotected against corrosion, as long as the solution has wet the surface. Typicalcoating thicknesses are about 15 to 30 mm (0.6 to 1.2 mils).

FILM PROPERTIES

Analysis of a cured autodeposited film shows the presence of iron throughoutthe organic layer. It is believed that the positively charged iron reacts with anionicsites (e.g., sulfonate groups and carboxyl groups), formulated into the polymerbackbone, to effect the deposition. No additional cross-linking agents are required.Although many organic emulsion polymers can be autodeposited, thereactions are the same; however, the properties of the cured film depend on thechemical nature of the polymer. For example, polyvinylidene chloride (PVDC)latex emulsions are the most widely used at present in the commercial practiceof autodeposition. Films deposited show excellent resistance to the penetrationof moisture and oxygen to the base metal and, hence, offer superior corrosionresistance, as well as the excellent hardness, formability, and adhesion characteristicof paint films using this type of resin.Another commercial autodeposition process utilizes acrylic resin polymers toproduce films that are resistant to high temperatures (>400°F) in the presenceof aggressive fluids (e.g., alkaline polyglycols).Carbon black pigments, which are effectively encapsulated by the polymers andthus deposit simultaneously with the resin, are highly effective. A nonpigmentedversion of the polyvinylidene chloride latex is commercially available as a primerfor subsequent top coating. While other colored pigments have been successfullyevaluated on a laboratory basis, none are at present commercially available.Zinc and zinc-alloy coated steels can be effectively painted by autodepositionby varying the process chemistry.

PROCESS SEQUENCE

Commercial autodeposition systems employ movement/transfer of the workpackage from stage to stage by either a continuous conveyor or by an indexinghoist. Conveyorized systems offer the advantage of assured agitation (to bringfresh reactants to the metal surfaces) due to the movement of the work throughthe coating tank (as well as the cleaning and rinsing tanks). Hoist-operatedsystems conserve space due to decreased transfer distance.A typical process sequence is shown below. (Contact times in each stage varyfrom 30 seconds to 2 minutes, with the exception of the oven where 15 to 30minutes is common).

Stage 1: Spray alkaline cleaning

Stage 2: Immersion alkaline cleaning

Stage 3: Immersion water rinse (plant water)

Stage 4: Immersion (or spray) deionized water rinse

Stage 5: Autodeposition (immersion)

Stage 6: Immersion water rinse (plant water)

Stage 7: Immersion sealing rinse

Stage 8: Cure

Cleaning

Good cleaning is essential to successful autodeposition. Any residual soils, whichhinder the solubilization of metal ions, can prevent or reduce coating formation.Although most organic soils (e.g., drawing compounds and rust preventive oils)are readily removable by alkaline cleaners, inorganic soils (e.g., weld spatter, scale,and rust) often require cleaning in an acidic material.Immersion cleaning is usually required to ensure adequate soil removal fromrecessed areas such as tube interiors and box sections, which are inaccessible byspray. To protect the chemicals in the tank from excessive buildup of soils, a (smaller) spray cleaner stage is used beforehand. A slight overflow of this tankresults in decreased cleaner loss.

Rinsing

Plant water rinsing is employed to remove residual cleaner carried through onthe workpieces (and racks). This is followed by a rinse with deionized water toremove any hard water salts. The effect of salt buildup over time results in graduallydecreasing coating film thickness per unit of immersion time.

Autodeposition

Autodeposition occurs by the reactions given above. The control parameters arepaint solids (gravimetric determination), acid level (free fluoride concentration),and oxidation/ reduction potential (in millivolts), which is proportional to theratio of FeF3 to Fe+2.

Postcoating Rinsing

A plant water rinse (usually immersion, but low-pressure spray applicationshave been used) removes traces of unreacted latex. A final sealing rinse containschemicals that react with any soluble iron in the wet film to eliminate porosityafter the film is cured.

Cure

The drying of a polyvinylidene chloride autodeposited coating is simply theremoval of water from a coalesced wet film. No solvents are present. Commonlypracticed parameters are 15 to 30 minutes at convection oven temperaturesof 210 to 230°F. Shorter times (5 to 10 minutes) may be achieved on simple(i.e., line-of-sight to all surfaces) parts by the use of medium-intensity infraredradiation.Acrylic coatings require a higher temperature (320–350°F) range for completecross-linking.

FEATURES OF AUTODEPOSITION

A phosphate pretreatment process is not required for autodeposition, minimizingrequirements for capital and floor space. Dragout is also minimized becauseof low paint bath viscosity. There is no coating buildup on hangers because acured autodeposited film is inert to further reaction. Furthermore, since thecoating process relies on chemical reaction, coating of all hidden or recessedareas occurs with even coverage. The coating does not pull away from sharpedges, coats evenly over machined surfaces (e.g., threaded fasteners), and is freefrom runs, sags, orange peel, and similar defects. This effect is enhanced by thelow redispersibility of the wet film, which allows water rinsing to remove excesssupernatant prior to oven cure. Very low maintenance is required and energy useis reduced because of the elimination of the phosphate pretreatment process.Finally, autodeposition is environmentally benign with low or no VOC emissionor heavy metal effluent. No fire hazards are present.Since the autodeposition reaction is diffusion controlled, the supernatant filmon freshly coated pieces lifted from the bath continues to deposit paint solids.As a result, there is minimal loss (i.e., transfer efficiency averaging 95%) of solidsto the water rinse following the coating bath. This effect is further enhanced bythe absence of any external force (e.g., electric current), which would increase theconcentration of solids at the immersed surface of the work. Since bath solids aremaintained at only 5% to 7% by weight, any dragout effects are minimal.Autodeposition is a versatile means of coating complex parts and assem-blies. For example, in the automotive industry, the following parts are coatedby autodeposition: jacks, brake backing plates, fan blower housings, suspensioncomponents, headlamp mounting assemblies, intermediate steering shafts,leaf springs (individual and as assemblies), power brake booster housings, seatframes, seat tracks, and miscellaneous brackets, connectors, and fasteners. Somenonautomotive examples include office furniture components (e.g., drawer slides,file frames), appliance accessories, hand tools, exercise equipment, and patiofurniture.

EQUIPMENT CONSIDERATIONS

All of the stages for an autodeposition process, with the exception of the coatingtank, are identical to those employed in other finishing processes and arenot discussed here. For this reason, retrofit of an existing coating system toautodeposition can be relatively simple. The following comments pertain tothe coating tank alone:Materials of Construction: The coating tank is a mild steel tank lined with anacid-resistant material. To avoid damage to the liner by parts or racks fallinginto the tank, it is advisable to use materials with high impact resistance.Traditionally, three-ply (soft, hard, soft) rubbers have been employed.Agitation: A gentle agitation of the coating bath is provided by properly spacedmixers with AC variable frequency drive (0.5-hp motors typical). Agitation isrequired only when workpieces are in the paint tank and may be decreased orstopped during downtime since no paint settling will occur in short periods (e.g.,2–3 weeks).Cooling Equipment: The temperature of the bath should be controlled in the rangeof 68 to 72°F. Heating/cooling coils in the bath are required (1) as a safeguardagainst accidental heat/cold carry-in; or (2) if ambient plant conditions warrant(heating or cooling). Even when a bath is used to process metal at a high rate, there is no measurable temperature increase due to exothermic chemical reactions.