Testing Methods & Standards for Measuring Powder Coatings

What are Powder Coatings?

Powder coatings can be divided into two types:

Thermoplastic and
Thermoset

For both types, a free-flowing, dry powder is applied to a substrate (usually metal), which is then heated to a temperature that allows the powder to melt, flow and form a coating.

For thermoplastic systems, once the powder flows out and cools, the coating process is complete. This coating can be re-melted with the application of heat.
In thermoset systems, as the powder melts and flows, curing reactions take place. This results in a tough, crosslinked coating that is “set” i.e. will not re-flow when heat is applied.

For both types, the procedures for evaluating characteristics such as color, gloss, impact, adhesion, weathering and corrosion resistance are the same as for cured liquid coatings.

The testing of powder coatings differs from their liquid counterparts in the characterization of the powder itself. This includes tests such as:

Melt flow
Ease of dry powder flow, and
Particle size

Let’s explore how to make powder coatings, the different test methods used during manufacturing to evaluate powder coatings...

How to Make Powder Coatings

First, to make a powder coating, the raw ingredients in the formulation are weighed out. These raw ingredients are usually all solids and include:

After weighing, the ingredients are blended together then extruded.
For thermoset powder coatings, extrusion is done at a relatively low temperature, just high enough to melt mix the ingredients together without causing what is referred to as B-staging or cure reactions taking place in the extruder.
The extrudate is cooled, ground into a powder that is then classified to the correct particle size distribution for that specific product.

For a basic overview of how powder coatings are manufactured, see this video:

(Source: AkzoNobel Powder Coatings)

Tests to Characterize Coating Powders

The table below lists the main tests that are performed on coating powders.

The reactivity of a thermoset coating powder is evaluated by gel time and inclined plate flow.
How well the coating powder stands up under storage conditions can be inferred using a simple test called blocking or sintering. Blocking is usually is a challenge with products having a low glass transition temperature during shipping.
Glass transition of the coating powder can be measured by differential scanning calorimeter.
The physical nature of the powder particles (thermoset or thermoplastic) can be characterized by specific gravity, particle size distribution and various dry flow tests.

Test	Measurement	Need	Standards
Gel time	Gel time (seconds)	Hot plate, timer	ASTM D4217-07(2017) ISO 8130-6:1992
Inclined plate (plane) flow or Hot plate melt flow	Melt flow (mm)	Balance, pill press, oven, metal or glass plates in holder or hot plate at an angle	ASTM D4242-07(2017) ISO 8130-11:2019
Particle size	Particle size distribution	Standard sieves or laser diffraction particle sizer	ASTM D1921-18 ASTM D5861-07(2017) ISO 8130-13:2019
Blocking or Sintering	Storage stability	Oven, weights	ISO 8130-8:1994
Dry flow (by fluidization)	Fluidization factor	Fluid bed, ruler, timer	ISO 8130-5:1992
Dry flow (by angle of repose)	Height of cone or Angle of repose	Funnel, protractor	ASTM D6393-14 ISO 4324
Specific gravity (by calculation)	Ratio of material weight to volume	Densities and amounts of raw materials used in formulation, calculator	ASTM D5965-19
Specific gravity (by gas displacement)	Ratio of material weight to volume	Gas pyknometer, balance	ASTM D5965-19 ISO 8130-2: 1992
Glass transition temperature	Tg, °C	Differential scanning calorimeter	ASTM E1356-08(2014) ISO 16805:2003

Gel Time

One of the first tests that will be performed on a thermoset coating powder is gel time. This test measures the amount of time needed for a thermosetting powder to melt, flow and crosslink.

In this simple test, a small amount of powder (approx. 0.25g) is placed on a polished hot plate top that is at a temperature high enough to cause the cure reaction (usually 180-200°C).
A timer is started, and the powder is stirred with a wooden spatula (tongue depressor) until it cures up into a solid, which can no longer be stirred.
The time is noted along with the temperature of the hot plate.

This test is quite dependent on the operator. How fast or slow a person stirs the powder on the hot plate can affect the measured gel time. However, this test is a quick way to make sure that all the ingredients are present in the batch.

If the gel time is too long or the powder never “sets” this could indicate that ingredients are missing or present in the wrong amounts in the batch.

The standards used for gel time are:

Measurement	Standards
Gel time	ASTM D4217-07(2017) Standard Test Method for Gel Time of Thermosetting Coating Powder
Gel time	ISO 8130-6:1992 Coating powders Part 6: Determination of gel time of thermosetting coating powder at a given temperature

Inclined Plate Flow, Inclined Plane Flow or Hot Plate Melt Flow

How well the powder melts and flows out on the substrate is a very important aspect for the final coating step in coating formation. Inclined plate flow is a simple yet effective way to evaluate the melt viscosity of the coating powder as the coating melts and crosslinks.

A given amount of powder is pressed into a pill and placed on a surface that is inclined at a certain angle (generally 65° or 35°) and held at a given temperature (usually at the temperature specified to cure the coating).
The pill will melt and flow down the heated, inclined surface. The distance covered by the melted powder is characteristic of the specific formulation.
The inclined surface is either a plate which is then placed in a heated oven or it the polished metal surface of a hot plate that has been fixed at a specific angle.

Formulating Powder Coatings for Temperature-sensitive Substrates

In general, textured, as well as fast-cure formulations, will have very short melt flows. In both cases, the melt flow is inhibited. Texturing agents tend to reduce the melt flow. For fast-cure systems, the powder crosslinks before it has time to flow out.

Gel time and inclined plate flow are used as first checks of the thermoset coating powder’s reactivity.

If the gel time or melt flow is way out of specification, it can mean that something is wrong with the raw material, or the stoichiometry of the formulation is off.
Both tests are mainly used to compare one batch of coating powder to another for consistency in production.

Gel times and melt flows are dependent on the chemistry of the formulation.

Reference standards for inclined plate (plane) flow are:

Measurement	Reference standards
Inclined plate (plane) flow	ASTM D4242-07(2017) Standard Test Method for Inclined Plate Flow for Thermosetting Coating Powders
Inclined plate (plane) flow	ISO 8130-11: 2019 Coating powders - Part 11: Inclined-plane flow test

Dry Flow and Fluidization

The key to the application of powder coatings is how well the powder fluidizes. Powder can be applied via fluid bed dip or electrostatic spray. For both, the powder is mixed with compressed air in a fluidized bed. This powder-air mixture acts like a fluid.

For electrostatic spray application, this mixture is pumped from the bed though hoses to the spray guns.
Continuous and consistent delivery of powder to the spray equipment is important to accomplishing uniform coatings. This depends on how well the powder fluidizes.
Factors that influence how well the powder fluidizes are:
- The shape of the particles
- Particle size distribution
- Chemical composition
- Moisture content, and
- Tendency for the powder to agglomerate

To get an idea of how the powder by itself flows (without air), angle of repose, or height of cone is measured.

In this test, a quantity of powder poured onto a horizontal surface to form a cone (think of it as a small mountain or hill).
The angle formed by the side of the cone is measured and gives an indication of the dry flow of the powder.
This test is mostly used to compare one batch of powder to another.

Angle of repose does not directly test the ability of the powder to fluidize.

To directly measure how well the powder fluidizes:

A given amount of powder is put into a small fluidized bed or fluid meter.
The height of the powder during and after fluidization is measured as well as the rate at which the fluidized powder flows through a specified orifice.
From these measurements, a fluidization factor is calculated.

Reference standards for height of cone and fluidization are:

Measurement	Reference standards
Angle of repose	ISO 4324: Surface active agents – Powders and granules – Measurement of the angle of repose
Fluidization	ISO 8130-5:1992 Coating powders – Part 5: Determination of flow properties of a powder/air mixture

Particle Size Distribution

Methods to Measure PSD in Powder Coatings

The particle size and particle size distribution of the powder have a major impact on how well the powder fluidizes, charges and applies. In general, thermoplastic coating powders have larger particles than thermosets. For example, thermoplastic powders have average particle sizes that are over 100 µm while typical thermoset powders have an average particle size in the range of 50 µm.

The level of fines or the portion of the particle size distribution that is below 10 µm is what causes the most problems with application in thermosets. Small particles tend to agglomerate, which leads to all types of issues in fluidization and spray application. It is important to understand what the complete particle size distribution of the powder is, not just the median or average particle size.

There are various ways to measure particle size distribution. Sieve analysis and laser diffraction are most commonly used in powder coatings.

In sieve analysis, a given amount of powder is put through a calibrated screen.
The amount of powder that goes through (or is retained by) by two or three calibrated screens gives a rather rough idea of the particle size distribution.

A more complete picture of the particle size distribution can be obtained by laser diffraction methods.

In these methods, the powder particles pass through a laser light source.
The particle diameter is determined by how much light it diffracts.
The analyzer will output a graphical representation of the particle size distribution along with other calculated values.
The laser diffraction method works well for particles in the size range of 1-300 µm.
For some thermoplastic powders, the particle size distribution may contain a portion that is above 300 µm, and a different optical characterization method should be used.

Due to the irregular shape and size of the particles, the particle size distribution determined using sieve analysis may dramatically differ from that determined by laser diffraction.

It is important to specify which method was used to determine the particle size distribution as the results from different methods cannot be compared.

For more information on sieve analysis and laser diffraction techniques, check:

Test methods for particle size distribution are:

Measurement	Test Methods
Particle size distribution	ASTM D1921-18, Standard Test Methods for Particle Size (Sieve Analysis) of Plastic Materials
	ASTM D5861-07(2017), Standard Guide for Significance of Particle Size Measurements of Coating Powders
	ISO 8130-13:2019 Particle size analysis by laser diffraction

Blocking, Sintering or Storage Stability

A coating powder can fluidize and spray out perfectly in a controlled lab environment, but after it sits in a box in a hot warehouse, it may not. The storage or blocking test is meant to simulate what happens to the powder after it is packaged, transported and stored.

Storage tests are carried out by placing a known amount of powder in a container and placing the container in an oven maintained at a constant temperature for a period of time.
A weight is placed on top of the powder during the test.
After a set amount of time (week to a month), the powder is removed and evaluated for physical and chemical changes.
If the powder has fused into a mass that cannot be broken down, it can pose problems in the field.
If the powder physically looks fine, the gel and flow should be measured and compared to what they were before the storage test.

It is also a good idea to spray out a panel to make sure the powder’s behavior was not affected by the storage conditions. This will help us to understand the chemical stability of the powder coating.

If powder coating has physical agglomeration and can be broken down to powder again, it is usable. But if it has chemically changed, then it is not usable.

Reference standard:

Measurement	Reference Standard
Storage Stability	ISO 8130-8:1994 Assessment of the storage stability of thermosetting powders

Storage Stability in Paints and Coatings

Glass Transition Temperature

Glass transition temperature is a second-order thermal transition that is characteristic of amorphous and semi-crystalline polymers. It is the temperature at which the glassy, brittle resin becomes a viscous taffy-like material. It is not a melting transition point but involves the material becoming “softer”. A good description of Tg can be found here.

Most of the resins used in thermoset powder coatings have Tg in the range of 50 to 70°C. If the Tg of the coating powder is near room temperature (i.e., below 50°C), the powder particles will tend to fuse together, forming a mass that is not sprayable.

The Tg of the formulated coating powder is not just dependent on the resin as crosslinkers and additives can modify the glass transition temperature.

Knowing the Tg of the uncured powder is especially important for low-temperature cure or fast-cure systems as these formulations tend to have a significant amount of catalyst, crosslinker, and/or use lower Tg resins to start with. If the Tg it low, special handling (air-conditioned transportation and storage) will be required.

Differential scanning calorimetry (DSC) is the technique of choice for measuring Tg and other thermal transitions of coating powders. DSC is a thermal analysis technique that measures how a material’s heat capacity changes with temperature.

For more information on how differential scanning calorimetry can be used to characterize coating powders, see:

Standards for determining glass transition temperature using DSC are:

Measurement	Standards
Glass transition temperature using DSC	ASTM E1356-08(2014) Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry
Glass transition temperature using DSC	ISO 16805:2003 Binders for paints and varnishes – Determination of glass transition temperature

Explore the basics of glass transition temperature.

Glass Transition Temperature of Coatings

Specific Gravity

Density is defined as mass per unit volume. Specific gravity is the density of a material at a certain temperature divided by the density of water at a certain temperature. The reference temperature is usually 20°C. Density is converted to specific gravity by dividing it by the density of water at a given temperature (0.99823 g/cc at 20°C).

The specific gravity of the powder is needed in order to figure out how much area a given amount of powder will cover. This number is needed to calculate the very important actual cost per unit area covered at a given film thickness.

There are three main ways to get at the specific gravity of a coating powder:

Displacement – a known amount of powder is placed in a non-solvent and the volume change is measured. Now know the mass and volume of the sample and can calculate the specific gravity.
Calculated – use the specific gravity of each ingredient and amount of each in the formulation to calculate the specific gravity of the coating powder.
Gas pyknometer – can operate with either air or helium. The apparatus measures directly the volume of air displaced by a known weight of powder.

The specific gravity is key to figuring out how much powder is needed to cover a given amount of substrate. It is also used to calculate the cost per unit area coated.

The formula for calculating powder coating coverage (in ft²/lb):

Actual Coverage Rate = 192.3/Specific gravity/Dry film thickness × Transfer efficiency

Where, 192.3 ft²/lb is the theoretical, best coverage possible with a pound of powder that has a 1.0 specific gravity, applied at thickness of 1.0 mil with 100% transfer efficiency.

Transfer efficiency is the percentage of powder that is applied to the part and not wasted as overspray. Transfer efficiency is highly dependent on how the operator is applying the powder, how well the part is grounded, spray gun settings, and the powder itself. It can be as low as 25% and as high as 85%.

The dry film thickness is the thickness of powder on the part in mils (1 mil = 1/1000^th of an inch).

Example calculation:

Powder X has specific gravity of 1.6

Dry film thickness required is 2.0 mils

Transfer efficiency of 60% (0.60)

Cost of the powder is $5.00/lb

Actual Coverage Rate = 192.3/1.6/2.0 × 0.60 = 36.05 ft²/lb

Cost per square foot = $5.00/36.05 = $0.14 per square foot of coverage by Powder X.

Most powder coating suppliers have online calculators that will give results in either ft²/lb or m²/kg, for example:

Reference standards for determining specific gravity are:

Measurement	Reference Standards
Specific gravity	ASTM D5965-19 Standard Test Method for Density of Coating Powder
Specific gravity	ISO 8130-2:1992 Determination of density by gas comparison pyknometer

General information on powder coating technology:

Powder Coated Tough, the official publication of the Powder Coating Institute
Powder Coating Institute
“Powder Coatings: Chemistry and Technology” by Emmanouil Spyrou, 3rd Edition, 2012, Vincentz Network, ISBN 3-86630-884-1
“Selection Guide: Powder Coatings and Formulation Tips” by Rogier van Duin

Equipment to evaluate different test measurements:

Tests	Equipment
Hot plates for gel time and hot plate melt flow	STI
Angle of repose	Qualtech Products Industry
Particle size analyzers	Horiba Scientific Shimadzu Malvern Panalytical
Differential scanning calorimetry	Netzch TA Instruments Mettler Toledo PerkinElmer
Gas pyknometry	Micromeritics
Powder coating equipment (spray guns, fluidized beds, booths)	Nordson Gema

Additives, Pigments and Polymers for Powder Coatings

View a wide range of raw materials available today used for formulating powder coatings, analyze technical data of each product, get technical assistance or request samples.

References:

ASTM: www.astm.org
ISO: www.iso.org

About Veronica Reichert

Veronica Reichert has more than 20 years of experience in the manufacturing sector with a strong focus on polymers and coatings. She currently is an Independent Consultant. As a consultant, Veronica helps her clients in all phases of product development from patent searches to scale-up.

Prior to becoming a consultant, Veronica held various technical positions at Motorola, Rohm and Haas (Powder Coatings), ITW and Valspar. She has experience in the formulation, application and testing of coating systems as well as in polymer characterization and processing.

Veronica earned a BS in Chemistry from the University of Wisconsin-Stevens Point and a PhD in Polymer Science from the University of Southern Mississippi. She is co-inventor on 10 US patents. She lives near Chicago, IL, USA.

All You Need to Know About Testing of Powder Coatings