Drawdown charts are rectangular pieces of non-fluorescent

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distrib ...

which are used to test a variety of

coating A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Pow ...

properties. These properties include opacity, spreading rate, penetration, and flow & leveling behavior. This non-fluorescent material has to be especially rugged in order to maintain its structure and give reliable readings, as the coatings tested are often

corrosive A corrosive substance is one that will damage or destroy other substances with which it comes into contact by means of a chemical reaction. Etymology The word ''corrosive'' is derived from the Latin verb ''corrodere'', which means ''to gnaw'', ...

abrasive An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflec ...

. These charts are necessary in the testing of any coating as they give reliable and accurate readings for any type of coating before the coating is applied to the intended material.

Drawdown bars

Drawdown bars are used in collaboration with drawdown charts. The bars are generally made of stainless steel or

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

and touch the chart at each end, while the center is slightly raised to a desired height in order to obtain the desired film thickness. As with the charts, the bars have to be especially resistant to

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

because of the corrosive nature of some coatings. There are a multiplicity of more complicated designs including some in which the paint is already in the chart apparatus, but the bar method described above is the most commonly used.

Procedure

Initially the desired coating is applied in a puddle on the drawdown chart. Then the bar is placed at the top of the chart, and drawn down over the paint in order to spread it evenly along the length of the chart. Once this evenly spread paint dries, it can be tested for a variety of properties.Koleske, ''Paint and Coating Testing Manual'', p. 419,

References

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