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.
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.
An air-jet sieve in which an air-jet is used to ‘blow’ the powder on the sieve mesh, can separate the powder particles rapidly and keep the mesh from blinding up (becoming blocked).
The Alpine air-jet sieve can be used to determine the particle size distribution using sieves which can give particle size cuts from 125µ to 10µ.
These include the Andreasen pipette, Backman and Shimadzu sedimentation balance and photosedimento-meter. All these techniques involve dispersion of powder in a liquid, usually water containing a percentage of a powder wetting or dispersing agent and allowing the powder particles to settle out over a time period.
As the powder particles settle out a trace records the weight over this time. From the trace recorded on a graph as the particles settle out, the particle size distribution can be calculated.
In this method a dilute dispersion of powder particles in an electrolyte is made to pass through a narrow orifice. The change in electrical resistance of the electrolyte as the particle passes, is used to estimate the volume of the particle.
The figure derived is the equivalent spherical volume and the method gives no guidance as to the shape of the particle.
Laser light diffraction techniques
In this method a sample of powder suspended in an air stream is passed across the path of a laser light beam. The angle(s) at which the beam is diffracted from a particle is determined by its diameter. The diffracted beams impinge on a series of concentric sensors. The energy absorbed by each of these sensors is fed to a computer which gives a print-out of particle size distribution.
Powder dry flow
This, as stated previously, is important in the handling and transport of the material through the powder feed lines during application and recovery. To a certain extent this is dependent on the particle size distribution of the powder but other factors such as the resin characteristics, pigment & extender loadings and other additives can have a pronounced effect. There are several methods of determining the flow characteristics of a powder:
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.
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 (in the range 30-40oC). 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.
This can influence the blocking and dry flow characteristics of the powder.
- 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.
- 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.
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.
An analytical balance, 500cm3 measuring flask, petroleum ether and a sample of powder are required to carry this out and the following procedure should be used: The sample of powder, say 50g, is weighed out into a 500cm3 measuring flask of known weight and the volume made up to the mark with petroleum ether. If the contents of the full flask weigh, say 378g, the weight of the petroleum ether is calculated by deducting the weight of the powder, ie. 378g less 50g = 328g. If the specific gravity of the petroleum ether is measured as 0.7, its volume can now be calculated as follows:
Volume = weight / specific gravity
Therefore : Volume = 328 / 0.7 = 468.8cm3
It then follows that the powder in the slurry has a volume of 31.2cm3 (ie. 500cm3 less 468.8cm3). The specific gravity of the powder is therefore given by:
SG = 50 / 31.2 = 1.6
However, the above method is not wholly satisfactory. For many modern powders it is very difficult to find a liquid which will sufficiently displace the air from the powder without exerting some solvent effect.
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.
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.
A sample of powder is weighed out into a porcelain dish which is placed into a cold furnace which is gradually heated to calcining temperature. (Heating should be slow so that no powder escapes at the beginning with the entrapped air). After cooling, the dish is re-weighed.
Other quality control procedures
These include infrared spectroscopy and differential scanning calorimetry. Both these methods are valuable in monitoring powder quality.
Cured coating procedures
As with solvent-based industrial paints, the cured coating of each production batch must be checked for a wide range of characteristics before despatch to end user.
For thermosetting powders, three types of substrates are generally employed, depending on the test requirements:
- Untreated steel
- Lightweight iron phosphated steel
- Lightweight zinc phosphated steel
- (Panels are available from proprietary suppliers)
The powder is spray applied electrostatically and cured at the appropriate temperature for the specified time at a controlled film thickness.
The usual tests conform to BS3900. For specific applications, eg. architectural aluminium extrusion, tests must conform to BS4842. Very often end users issue their own test specifications.
Information relating to methods of determination of particle size of powders can be obtained from:
- BS410 Test sieves
- BS2955 Glossary of terms relating to powders
- BS3483 Methods for testing pigments for paints
Methods for paint testing can be obtained from BS3900, ‘Methods of Test for Paint’.