This is the area in which the user is most likely to come into contact with solvents.

The thinner is usually composed of a solvent blend similar to that used in the product. Any attempts to use lower grade solvents in order to cheapen the thinner can have serious effects on the application, drying and appearance of the coating.

With 2 pack products, the thinner is usually part of the package.

With cellulose products, the so-called "economy" thinners (sold in bulk) contain poor quality solvents and large amounts of diluent to reduce the cost.

The effects are:

(i)                 Poor cutting power, i.e. more thinly needed to application viscosity, resulting in lower solids and build. This is for undercoats as well as topcoats.

(ii)               Poor flow, due to lack of expensive high boilers.

(iii)             Poor overspray absorption, due to lack of high boilers.

(iv)             Low gloss, due to poor solvency for the binder.

(v)               Blushing, due to poor solvency for the binder.

BLUSHING

The fault which can show as haze or milkiness or loss of gloss during the drying of lacquers.

Due to a faulty solvent balance, there are two ways in which it occurs.

(i)                 Resin Blush - Caused by precipitation of the NC or the modifying resin and due entirely to an unbalanced solvent mixture. It is important that the least volatile solvent (or highest boiler) continues to act as a solvent for both the NC and the resin during the drying process.

(ii)               Water Blush - A more common fault is associated with the chilling of the lacquer. During evaporation of the solvents, the drop in temperature of the solvent cools the surrounding air, and if the humidity is sufficiently high, dew point may be reached, resulting in the deposition of water. The film of water penetrates the paint surface and causes incompatibility of the NC or resin, manifested by the haze of whitening described above.

The low boiling ketones (Acetone and MEX) tend to induce water blush. Medium and high boiling esters and alcohols impart good blush resistance.

THINNER COMPOSITION

1.      CELLULOSE SPRAY THINNER

2.       SYNTHETIC SPRAY THINNER

3.      2 PACK SPRAY THINNER

4.      FADE OUT THINNER

5.      ONE PACK POLYURETHANE SPRAY THINNER

TERM USED IN MODULE 4.

FUNCTIONAL GROUPS USED IN MODULE 4

Vẽ hình trang 4/14