Baked Enamels

Amino resins are also often used for the cure of other resins such as alkyds and reactive acryUc polymers. These polymer systems may contain 5—50% of the amino resin and are commonly used in the flexible backings found on carpets and draperies, as well as in protective surface coatings, particularly the durable baked enamels of appHances, automobiles, etc. 

In counterflow movement, heat from the outgoing sohds is transferred directly to cold incoming solids, reducing heat losses and fuel requirements. Continuous conveyor ovens are employed also for diy-ing refractoiy shapes and for drying and baking enameled pieces. In many of these latter, the parts are suspended from overhead chain conveyors. 

Fig. 36 Dispersion of Pigment Violet 19, (l-modification, in an alkyd-melamine resin baking enamel with a paint shaker. Reduction 1 2 with Ti02. Tinctorial strength-time curve.

Fig. 38 Dispersion of Pigment Blue 15 6 in an alkyd-melamine resin baking enamel. Effect of the dispersion time and the dispersion temperature on the coloristic properties (depth of shade). White reduction 1 50 Ti02.

Fig. 48 Electron micrographs of ultramicrotome-cut thin sections of a nonirradiated and an irradiated layer (alkyd-melamine resin baking enamel) containing Pigment Violet 19, y-modification. Above irradiated, below nonirradiated.

Fig. 67 Influence of the particle size of P.Y.83 on the shade of alkyd-melamine resin baking enamels (white reductions) demonstrated by means of reflectance...

Although inferior fastness to organic solvents, involving unsatisfactory migration properties, excludes P.Y.l from important areas of application, such as baking enamels, there are certain conditions under which it maybe applied in such media. The user must in this case observe a certain concentration limit beyond which the pigment may bloom. The pigment is stable up to 140°C. 

The paint industry shows little interest in P.Y.12, since it is not sufficiently fast to overcoating for use in baking enamels. In air drying paints, lightfastness of P.Y.12 in white reduction (with 1 5 TiOz) only equals step 2 on the Blue Scale. 

In the paint field P.Y.16 is primarily used in industrial finishes. The similarity to P.Y.l is limited to a likeness in shade but P.Y.16 is fast to overcoating and shows no tendency to bleed in baking enamels. Although its full shade lightfastness equals step 7-8 on the Blue Scale, types that are only slightly reduced with TiOz (1 5) reach only step 5. Only full shades provide good weatherfastness. 

P.O.13 shows less stability in paints than P.O.34 types of similar particle size. This includes both fastness to overpainting in baking enamels and lightfastness in air drying paints. The volume of trade sales for this purpose is accordingly limited. 

In the paint industry, transparent types are used only to a limited extent. In air drying systems, P.O.34 equals step 6-7 on the Blue Scale in full shades, while opaque colorations with TiOz (1 5) only reach step 3 for lightfastness. In baking enamels, the pigment is not fast to overpainting. 

P.R.3 is very likely to bloom in baking enamels. At 120°C, the concentration limit for blooming is 2.5% beyond about 140°C, blooming occurs with certainty, irrespective of the concentration. Therefore, only full shades of P.R.3 are used to color baking enamels, and only where application temperatures are low. The pigment is used extensively in combination with Molybdate Red. 

In the print field the pigment is almost exclusively used in air drying systems. P.R.4 is very likely to bloom in stoving enamels. At only 120°C, the concentration limit for blooming is as low as 2.5%, and at 140°C the limit is at 5%, which makes it necessary to carry out test experiments. Application is consequently restricted to baking enamels targeted for low temperature purposes. In epoxy resins, the pigment turns brown, as does P.O.5, and is therefore unsuitable for use in these media. 

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