Metallic basecoats

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 solvent-borne basecoat, consisting of polyester or acrylic binder to be cross-lined with melamine or isocyanate, is applied at a dry film thickness of between 0.6 and 1.5 mils, depending upon the color. Poorer-hiding colors, such as red and white straight shades, require the higher film builds, whereas metal lies and darker colors are applied at the lower film builds. The solvent-borne basecoat is allowed to flash-ofif solvent for approximately 3-5 min at ambient conditions prior to application or the solvent-borne clear-coat. This process is referred to as a wet-on-wet application because the basecoat is not in its cured or cross-linked state. 

Optionally, a waterborne basecoat can be utilized to achieve better aluminum orientation in metallic films. Several papers910 have described metallic orientation and the advantages of waterborne basecoats, so this concept will not be thoroughly discussed here. 

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. 

Today, a large and increasing proportion of the automotive OEM metallic basecoats in Europe and North America are already waterborne and refrnish basecoats are also available in waterborne versions. Other paint applications have followed and the significant growth of waterborne systems can easily be seen in the statistics of the paint associations. Also in printing inks, the use of waterborne inks is steadily growing. 

Automotive Finishes. Important industrial automotive topcoats are produced from alkyd resins. Two-coat metallic finishes consist of an acrylic clearcoat and a metallic basecoat. Solventborne basecoats are generally composed of cellulose acetobutyrate and a low molecular mass, branched polyester resin which exhibits high reactivity with precondensed melamine cross-linking resins. 

High-speed rotating atomizers and electrostatic spraying Metallic basecoat 13 670 75 8.93... 

Clearcoat, acrylic-melamine basecoat, polyester-cellulose, acetobutyrate-melamine, silver metallic bake 130"C, 30 min exposure Florida, 5 South, black box unheated percentage of light stabilizer relative to binder solids benzotriazole I, 2-(2//-benzotriazole-2-yl)-4,6-bis(l-methyl-1-phenylethyOphenol [70321-86-7], Cracking after 2.25 years. Cracking after 5.5 years. 

Clearcoat high-solids acrylic melamine Basecoat high-solids acrylic - melamine, silver metallic Bake 120 C, 30 min Benzotriazole II 3-(2//-benzotriazole-2-yl)-5-(l.l-dimethylethyl)-4-hy-droxyoctyl benzenepropanoate [84268-23-5]... 

The basecoat-clearcoat system is presently the most commonly used type of topcoat for cars because it is the standard application system for metallic colors. Today, about 70 % of all cars have metallic topcoats. The basecoat-clearcoat system consists of a colored layer (basecoat) which is overcoated after a short flash-oflf time with a protective layer of clearcoat. Both coats are cured together at 120-140 "C. The basecoat contains pigments which provide two types of finish solid (straight) colors or metallic. 

Solventborne metallic basecoats contain 15 to 30% solids and 85 to 70% volatile organic solvents. These solvents are not released into the atmosphere, but are converted to combustion gases in afterburners. To reduce emission of organic solvents from this source, waterborne basecoats have been developed. 

Waterborne basecoats with higher solids contents are now available metallic basecoats contain about 18wt% solids and solid (straight) color basecoats 25-40 wt%. The solvent in waterborne paints is not pure water about 15% of organic solvents is still needed as a cosolvent for proper film formation. Metallic basecoats are applied at a DFT of 15 pm, solid color basecoats at a DFT of 20-25 pm. 

Basecoats are sprayed in two layers. The first layer is sprayed electrostatically with high-speed rotation bells, the second layer is sprayed with compressed air to achieve proper orientation of the aluminum particles in metallic paints. The basecoat is then dried for 3-5 min in a warm air zone at 40 - 60°C. 

Uses Abrasion resist, aid, toughener, flexibilizer, hardener, gloss aid in water-based coatings (automotive OEM, refinish, primers, basecoats, topcoats), adhesives for wood/plastic/metal substrates, inks binder in waterborne inks, bonding and laminating applies. 

Uses Abrasion resist, aid, toughener, flexibilizer, hardener, gloss aid in water-based coatings (automotive OEM, refinish, primers, basecoats, topcoats), adhesives for wood/plastic/metal substrates, inks Features Low vise. exc. adhesion to substrates crosslinkable thru carboxyl groups provides improved soK/ychem. resist, when crosslinked Properties Vise. 50-100 cps pH 9.5 VOC 0.3 Ib/gal film props, after 30 min 120 C hardness ( ard) 55 tens. str. 5500 psi elong. 260% 100% mod. 3400 psi 35 A solids... 

Uses Detoamer, antipopping agent for water-reducible and emulsion systems, metallic basecoats, and topcoats Features Esp. effective tor higher solids systems with high surf. tens. 

Features Good recoatability smooth feel stabilizes and orients metal flakes in basecoats can be post-added Properties Fine particle size disp. 

Chem. Descrip. Chain stopped aikyd resin in xylene Uses Aikyd for maintenance enamels, implement finishes, auto-refinish basecoats, general purpose air dry or force dry coatings, metal protective topcoats... 

Chem. Descrip Acrylic resin in methyl n-amyl ketone Uses Acrylic for plastic coatings/basecoats, vehicle refinish basecoats/ monocoats, metal protective coatings Featiaes Exc. applic. and appearance durable glossy low VOC zero HAPs... 

Uses Resin for plastic coatings/basecoats, vehicle refinish primers eal-ers, metal protective coatings... 

Uses Acrylic for prod, of unicolored automobile finishes and isocyanate systems, with light stabilizer for clear coats over metallic basecoats Features Heat-reactive with amino resins superior radiance, high surf, hardness, exc. weather resist. 

The use of microgels has been particularly successful in solvent-borne and water-borne metallic basecoats used in motor car production. Here the metallic visual effect requires viscosity control during the high film shrinkage that occurs on solvent evaporation, so that the metallic aluminium flakes align parallel to the substrate in the dry film. 

Table 13.2 Formulations for a green metallic basecoat for motor cars (solvent-borne)...

---