Spraying viscosity

Primer Formulations. Coatings were formulated using standard techniques. Mill bases were prepared by dispersing the oligomer solution with pigments (silica, carbon black, titanium dioxide and barium sulfate in a 1 1 1 10 ratio). The viscosity of the formulation was reduced to spray viscosity by addition of solvent. 

Coating To Replace Porcelain. Porcelain is still widely used in appliances and is accepted as a quality finish. Porcelain frits are generally ground on site in large ball mills and let down to spray viscosity. Porcelain also requires zero carbon steel and a pickling process for proper adhesion. 

Nitrocellulose lacquers were used very early as automobile topcoats. They produced very attractive high-gloss finishes. Since the nitrocellulose molecular weight is high, a very high level of solvent was needed to reach spray viscosity. This resulted in the need for many coats to reach adequate film thickness. Both the high solvent levels and the use of multiple coats contributed to extremely high volatile losses from nitrocellulose lacquers. 

Unlike liquid solvents, supercritical carbon dioxide does much more than dilute the coating to a low spray viscosity - it changes the nature of the atomization mechanism. Thus, low spray viscosity is less important than with conventional spray methods, and many new factors contribute to the spray-ability of the coating. 

Blends of alkyd resins, cellulose nitrate, and catalyst-convertible butylated urea-formaldehyde resins are another development in wood-furniture lacquers. In the presence of an appropriate catalyst such as an alkyl acid phosphate, these components react under air-drying conditions, as well as at the higher temperature (150°F), to form an insoluble film. These lacquers show improvements over the conventional alkyd-cellulose nitrate blends in the alcohol resistance of the film and in having higher solids at spraying viscosities. 

Nitrocellulose, also known as cellulose nitrate, was the workhorse thermoplastic binder to produce fast-drying lacquers for many decades. However, its very low solids ( 20 %) at spraying viscosity was the main reason for the steep decline of its market share with inception of legislation pertaining to the reduction in VOC emissions. 

The replacement of 30% hydroxy acrylic by the caprolactone based polyols in one system gave an increase in volume solids content at spray viscosity of 5%, better impact resistance and greatly improved gloss after 1000 hours exposure to the QUV Weatherometer. 

The move from thermoplastic topcoats (NC or TPA s) was a big step forward in reducing solvent levels. Typically, the volume solids content at spray qyplication viscosity for a thermoplastic acrylic is less than 15%. This means that if one used 10 kgs of a thermoplastic topcoat lacquer at spraying viscosity to coat a car 8.5 kgs of solvent will be released to the atmosphere. In the U.S., one objective has been to reduce solvent emissions resulting from the application of a topcoat down to 1.6 kg per vehicle. With standard thermoset acrylics it is possible to more than double the volume solids at application, thus halving the solvent emission. 

The basecoat is thirmed to a spray viscosity of 15-20 secs Ford No. 4 Cup, with fast evaporating solvents and this is applied in 2 thin coats, and allowed to dry for at least 15 minutes prior to application of the thermosetting clear overlacquer. 

Thin to spray viscosity (20-25 secs Ford No 4 Cup) with xylene, butanol, butyl acetate solvent mixture... 

Mix Parts I II and thin to spray viscosity in a 60 25 15 blend of butyl acetate/xylene/Solvesso 100. Solvesso 100 and methoxy propyl acetate help to adjust levelling. Again, the use of UV absorbers is essential. The silicone oil improves wetting over the waxy base basecoat. Use of catalyst is optional, but its presence can reduce pot life. 

Mix parts A and B. Thin to spray viscosity with 9 3 1 butyl acetate methoxy propyl acetate ethyl acetate. 

A suitable acrylic is Synocure 865SD, 70 %nvc in EEP with an OH content of 1.09%. In combination with a suitable isocyanate, such as Desmodur N3390, lacquers with a non volatile content greater than 45% at spray viscosity are possible. 

The amine fimctional aciylic is used to disperse the pigment. Levelling and slip additives are also added to the grind resin. Thin to spray viscosity in a 4 3 2 3 blend of xylene di-acetone-alcohol isobutanol methoxy propyl acetate. 

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