Basecoat dispersions

The total automotive coating composite, however, consists of more than just the basecoat/clear-coat. The metal sheet stock that is formed into the automotive car body is treated with a corrosion-resistant primer applied by cathodic electrophoresis. This coating, often referred to as ECOAT or uni-prime is a hydroxy functional epoxy/ blocked isocyanate dispersion that deposits onto the cathode of an electrolytic cell at a voltage of between 250 and 425 V. Film thickness, which is dependent upon the residence time, temperature, and coating voltage of the electro-coat bath (electrolytic cell), varies between 0.8 and 1.2 mils (1 mil = 25 xm). The curing temperature required to deblock the isocyanate and yield a cross-... 

The waterborne basecoat consists of a urethane or an acrylic dispersion with passivated aluminum for metallic glamour. The aluminum must be passivated to prevent reaction with water and the evolution of hydrogen gas. Passivating agents can include polymers, hexavalent chromium (now mandated for removal), or phosphate esters. The basecoat is pre-baked (with infrared or thermal ovens) to release the majority of the water prior to clear-coat application. 

Organosol PVC dispersed in an appropriate varnish and conventionally stabilised with a low molecular weight epoxy, resin or epoxidised bean/seed oils Very good Very good Drawn cans Easy-open ends Closures Often used over epoxy-phenolic basecoat... 

Disperse-Ayd W-22 wetting agent, pigments wood coating basecoats... 

The pigmented basecoat is formulated on either a highly durable solvent based polyester, or an aqueous acrylic dispersion. 

Critical to formulating a successful metallic basecoat is the selection of aluminium paste. In general the larger the particle size of the flake, the better the lustre. A polyethylene wax dispersion is also incorporated into the formulation to aid metal control and lustre. The binder in solvent based systems is a durable saturated polyester cured with a melamine formaldehyde resin and modified with CAB, to assist in film setting and hardness. 

A good basecoat is important in a COB system. Coloured metallic basecoats use strong, transparent, light fast pigments dispersed in an oil free polyester resin. As the film thickness is thin, relatively high levels of pigment are required to give deep metallic shades. 

A wax dispersion is preferred over powdered waxes because of the difficulty in obtaining the required degree of dispersion from the latter. The wax controls the aluminium lay-down and improves lustre. It also protects the basecoat from attack by solvents in the overlacquer. 

DWI external coatings are split between acrylic and polyester systems, both of which contain carboxy functionality, neutralised with an amine such as dimethylethanolamine. Current developments in DWI basecoat technology are towards hybrid acrylic polyester water systems. The acrylic portion provides good pigment dispersion properties, whilst the polyester will give improved performance during the spin necking operation. 

The water-based basecoat consists of a urethane or an acrylic dispersion that may or may not be crosslinked. Crosslinkers generally consist of blocked isocyanates or MF resins. The basecoat is spray applied and prebaked (with infrared, microwave, or thermal convection ovens) in what is generally referred to as a heated flash to remove a majority of the water (>90%) prior to clearcoat application. 

Water-borne emulsion polymers are used in formulating automotive coatings for electrocoat, primers and basecoats. Clearcoat polymers are primarily still solvent-borne. A powder dispersed in water (powder slurry) has been developed as a binder for clearcoats [7]. 

Polymer dispersions are applied as coating binder mainly in three of the four layers electrocoat, primer and basecoat. These layers are now described in more detail. 

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