Procedures for testing powder coating

There are certain specific tests for powder and for the cured coating. Once powder is delivered to the end user, it can’t be adjusted like a solvent based paint. Therefore extremely strict quality control must be exercised by the powder manufacturer during all stages of the manufacturing process. This will ensure that all powders are of a consistent high quality. There are a number of general test procedures on powders and coating performance, which are applicable to all powders, in addition to any special tests required for specific customers.

Particle size distribution

There are a number of techniques which can be utilized to determine particle size distribution. These vary widely and because thermosetting powder particles are in irregular shape and size, different results between the various techniques arise.

Sieve analysis

This is the simplest, most versatile and fastest technique and does not require highly skilled personnel or calculations. It can separate broad fractions quickly, and for thermosetting powders, appears to adequately provide the powder in ‘cuts’ appropriate for the application method.

In the simplest form a series of sieves, of different sieve meshes, set in an automatic shaker can be used. However, this has limitations especially with fine powders, in that the fine meshes tend to become blocked extremely quickly.

Angle of repose

This method entails allowing a quantity of powder to fall on to a horizontal collecting plate to form a cone. The angle assumed by the side of the cone is an indication of the dry flow characteristics of the powder.

SAMES flowmeter (method Afnor)

This consists of a fluidising bed in the form of a vertical transparent plastic cylinder. In the side of the cylinder is a small outlet which can be closed by means of a plug. Dry compressed air can be fed through the fluidising plate at a controlled rate and pressure. A sample of the test powder (250g) is introduced into the cylinder and the air supply switched on. The height to which the powder rises is measured; the air supply is switched off and the powder allowed to settle and stabilise and the height is again measured. The air is again switched on and during the fluidisation the plug removed for 30 seconds; the powder escaping through this orifice in this time is collected and weighed.

If   h1 = Fluidised height of powder
h0 = Settled powder height
m = Mass of powder collected in 30 seconds

then the index of fluidisation,

r, is given by: r = (h1  h0) x m

The index of fluidisation can give a guide to the dry flow characteristics of a powder but the results should be treated with caution.

Powder storage

It is important that the powder should not cake or form lumps during bulk storage especially when subjected to warm conditions. Nor should chemical reaction between resin and curing agents proceed during storage otherwise the application properties, flow and gloss can be affected.

Storage tests are usually carried out by placing a known amount of powder in a container and storing containers over a given period in an oven maintained at a constant temperature. It is usual to apply a weighted disc, eg. 100g, to the surface of the powder during the test. No blocking, caking or change in reactivity of the powder should be in evidence after 1 month’s storage under these conditions.

Moisture content

This can influence the blocking and dry flow characteristics of the powder.

  1. The simple direct method of determining moisture content is to heat a small, weighed quantity of powder in an oven at 105oC to constant weight. Unless the powder is spread in a thin uniform layer, this method can give very inaccurate results owing to entrapment of volatiles.
  2. An alternative method is to dry a weighed amount of the powder for 8 days in a desiccator over phosphorus pentoxide, then re-weigh.

Mass loss on heating

A known weight (ca. 0.5-1.0g) of powder is heated at 200oC for 15 minutes and allowed to cool in a desiccator. The weight loss is calculated as a percentage of initial powder weight. This mass loss may be significant in the case of powders where volatile materials are evolved, eg. caprolactam in the case of polyurethanes.

Specific gravity

This is vital in order to calculate the covering power of the powder and hence the actual coating cost per unit area covered at a given film thickness.

Two methods for its determination are available:

  • Displacement of a non-solvent liquid of known specific gravity.
  • Gas pycnometer.

Gas pycnometer

This is a much more accurate and rapid method. This is a specialised piece of apparatus which can operate with either air or helium although the air-operated version is much cheaper and adequately accurate. The apparatus measures directly the volume of air displaced by a known weight of powder and the test takes only 2-3 minutes to perform.

Gel time

The gel time of a powder is useful as an indication of:

  • Curing speed.
  • Consistency of powder quality.
  • Chemical instability on storage.

A marked reduction in gel time after the storage test outlined above would indicate that reaction in the powder had occurred, which would have an adverse effect on the applied powder film.

The apparatus used for determining the gel time consists of a heating block which can be maintained at the required temperature (usually 180-200oC) to an accuracy of ±1oC. A small quantity (ca.0.25g) of powder is placed in the centre of the heated plate and a stop-clock started. The molten mass is manipulated with a small wooden spatula. When threads can no longer be pulled from the mass with the spatula, the elapsed time is recorded as the gel time.

Other quality control procedures

These include infrared spectroscopy and differential scanning calorimetry. Both these methods are valuable in monitoring powder quality.