PC Summitry:Quality Control in the Powder Industry

Posted on Friday, November 1, 2013

Because powder coatings are difficult to adjust toward the end of manufacture, the utmost care must be taken to ensure the batch is correct right from the start.

By Kevin Biller

The purpose of a quality control is two-fold. First of all, it provides a check on performance, ensuring that the product shipped to your customer meets their requirements, and secondly, it is a means to ensure that you are producing a consistent powder, batch to batch.

In powder coating manufacture, raw materials are dedicated and subsequently processed to meet a customer’s requirement. Unlike paint manufacture where the batch is processed and eventually adjusted near the end to meet quality requirements (i.e., shade, viscosity, etc.), powder coatings are very difficult to adjust toward the end of manufacture. The utmost in care must be taken to make sure the batch is correct right from the start. Consequently quality control really should emphasize raw material and process control rather than assessing properties after the batch is made.

Control of Incoming Raw Materials

This can be accomplished by guarantying that your raw materials are consistent and within specification. Reputable raw material suppliers will gladly furnish a certificate of analysis (COA) for the products that they sell to you. The COA should demonstrate performance for the critical features of the product. A basic compendium is listed below.

Resins and Crosslinkers

• Functionality: Acid Value, Hydroxyl Number or Epoxide Number.

• Melt Viscosity (ASTM 4287)

• Color (ASTM D1544)

• Cleanliness


• Color (ASTM D2244)

• Oil Absorption

• % Water

Extender Pigments

• Oil Absorption

• Particle Size

• Color


8 Non-volatile Content

• Viscosity

• Melt Point or Softening Point

Larger powder manufacturers may have the luxury, if they choose, to staff a raw material quality assurance laboratory that can evaluate the properties of incoming raw materials. However, most powder makers must rely upon the conscientiousness and integrity of their suppliers. It pays to be aware of how tight a raw material specification can be and if you, as a powder producer, can specify how tightly your supplier can control their product.

Resins and Crosslinkers

In general, resins and crosslinkers COAs should report a measure of functionality of their product. Acid value is the preferred assessment of the functionality of a carboxyl polyester or carboxyl acrylic resin. Accordingly hydroxyl number is a good indicator of functionality for hydroxyl polymers. Epoxide value or epoxide equivalent weight (EEW) is the best means to represent the functionality of an epoxide containing resin. Crosslinker quality needs to be cited specific to the functional group on the product. Blocked isocyanates are characterized by % NCO (isocyanate), whereas TGIC (triglycidyl isocyanurate) is controlled by EEW. The quality of amine functional crosslinkers is measured by amine equivalent weight whereas phenolic curing agents are evaluated by phenolic hydroxyl number.

The molecular weight of resins and most crosslinkers is related to melt viscosity. Viscosity measurement is dependent upon the instruments and techniques developed by your supplier. Regardless of specific technique, it entails assessing the stress caused to a sample when shear is applied at a temperature that allows the material to be fluid. Most common in the industry is the use of a cone and plate roto-viscometer. The sample is placed on the heated platen of the roto-viscometer. The platen is heated to a prescribed temperature typically between 150 and 200°C. The resistance to rotation is measured and converted into a viscosity unit.

The color of a resin or crosslinker is important for two reasons. First it will affect the final color of a clear coat or light colored powder coating, and second it is an indication of how careful a batch was made. Most polymers tend to yellow when oxygen is introduced during polymerization. This is an indication of lack of tight process control. Your incoming resin or crosslinker should be very nearly water white. Historically resin suppliers used the Gardner Color scale and specified a “2” maximum. This equates to a very slight yellow cast. Improved process control has compelled suppliers to graduate to an APHA scale of color. This scale is much more sensitive than the Gardner scale and is now employed by most raw material suppliers.

It is incumbent for a resin manufacturer to produce clean, bit-free product. Resin makers can ensure the cleanliness of their products by filtration as the material is discharged from their reaction vessels. You know what size screens your suppliers use when making their resins. Some resin manufacturers filter product through screens as fine as 10 microns. This provides the cleanest resins available.


Pigments are controlled by their color. This is typically reported in the coloristic values of “L” (lightness-darkness), Manufacturing powder coatings of consistent quality requires diligent incoming raw material control and conscientious process control. “a” (red-green), and “b” (yellow-blue). These aspects are usually stated as a delta value (i.e., ΔL, Δa and Δb), which denotes the divergence versus a targeted value. The oil absorption of a pigment is also important as it influences the melt viscosity of the finished powder. It is defined as how many grams of oil (similar in nature to resin) can be absorbed by 100 grams of the pigment. Details can be found in ASTM D285 or ASTM D1483 test methods.

Extender Pigments

Extender pigments are meant to be incorporated as a means to lower overall raw material cost while causing no or minimal impact on other properties. Critical to an extender’s quality is particle size, color and oil absorption. The particle size is usually quoted per sieve analysis. Crucial to the powder coating manufacturer is not only the particle size distribution but also the top end particle size. Large particles (e.g. greater than 40 microns) will protrude through the powder finish if applied at a thin film thickness. Most extender pigment suppliers will state % retained on a 325 mesh (44 micron) sieve. Greater detail is needed to properly assess an extender’s particle size. It is recommended to require the median particle size and the absolute top end particle size. Oil absorption is also important for the same reason expressed above.


Qualifying the suitability and consistency of additives is very specific to each material. Viscosity, melt point and/ or softening point are all good places to start. Some additives are relatively low in molecular weight and therefore your supplier should provide a statement of a minimum of nonvolatile content. The reactive additives should also be controlled by some characteristic chemical measurement related to functionality (e.g., equivalent weight) or reactivity.

It is important to establish a good relationship with your raw material suppliers. The success of your operation and the quality of your products is contingent upon the quality of your incoming raw materials. When choosing a raw material supplier, economics are certainly crucial, however value is also measured in the quality and consistency of their products and most important the confidence and trust you have in the way they do business with you.

Process Control Rather than Quality Control

After establishing consistent, trusted sources of raw materials it is critical to institute strict process control of your manufacturing functions. It is very difficult to “fix” a poorly manufactured powder coating. After extrusion your powder coating formula is “locked-in.” The components, their concentrations and their ratios cannot be adjusted. It is therefore paramount to ensure that your processes are in control.

The Formula. This starts with guarantying that your formulas are correct before they are introduced to your plant. It is best to have an independent party review formulas as they are inputted into your production system. This should be someone knowledgeable in formulating, but not necessarily one of your chemists. The quality control supervisor may fit this role. This review should make sure that only preferred raw materials are used. Additionally ratios and concentrations of raw materials should be scrutinized. Important analysis should include: Are the resin to crosslinker ratios at or near stoichiometric? Is the pigment concentration high enough to provide adequate hiding? Are the additives incorporated at reasonable levels?

Weigh-Up. A well-designed production weigh-up system helps to minimize errors that can occur during weigh-up. Small quantities of critical raw materials (i.e., less than 100 grams) should be weighed on appropriately sensitive balances. These should be at least accurate to 0.10 grams. On the other hand, large quantities of less critical raw materials should not be weighed to inordinately precise levels.Extremely small quantities can be better incorporated by using a master batch technique. This entails preparing an “intermediate” comprised of a higher concentration of additive in a common raw material such a filler pigment or resin. This allows the manufacturer to weigh a more easily managed quantity of material.

The weighing of a batch should be endeavored by a welltrained operator; an individual with a good sense of numbers and familiarity of the scales used. Scales should be well maintained and regularly calibrated by an outside party on at least a quarterly basis. The weighing process should include the reporting of the batch or lot number of each raw material. The weigh-up record should have a placeholder for not only the operator to initial as each material is weighed, but also a space for another individual to double check the weighing operation.

Premixing. The order of addition of raw materials into a premixing vessel is an important aspect of process control. Minor additions of raw materials should be “sandwiched” between bulk materials to ensure best distribution during mixing. Taking a small sample of premixed material and melting it on a hot plate allows you to make a quick qualitative assessment of viscosity and color.

Extrusion. After premixing is complete a small amount of material should be processed through the extruder before sampling this process. Extruded material should be collected and made into a powder in your laboratory. It should then sprayed to a relevant film thickness onto suitable substrate and baked according to conditions representative of your customer’s process. This extruder check should be evaluated for color, gloss, mechanical performance and cleanliness. If the extruder check passes these tests, then the extrusion process can recommence. This extruder check should be made for every distinct mixture that is introduced to the extruder. For example, if your mixing operation utilizes 500 lbs. mixes then every new 500 lbs. mix should be checked at the extruder.

Grinding and Classification. Extruded material is cooled then comminuted into flake. This flake is subsequently introduced into the grinding/sifting process and the finished powder is packaged. The finished powder should be checked for particle size distribution and rheology. It should also be applied to a suitable substrate at a specified film thickness and then tested for color, gloss, cleanliness, and mechanical performance. It is important to evaluate your powder at a meaningful film thickness. Too thin a film build can introduce texture (which affects gloss) and can create inadequate hiding (which affects color). Too thick a coating minimizes texture and can erroneously provide hiding. Additionally, film thickness sometimes affects mechanical performance. Thicker films tend to be less flexible. The film thickness specified should correlate to the minimum expected on your customer’s finishing line.

Particle size distribution can be measured by either sieve analysis or laser particle diffraction techniques. Sieve analysis involves passing a sample through sieves of prescribed openings. Fractions are weighed before and after sieving. Results are quoted as percent retained on a given mesh. Analysis should include measurement of the large particle size and small particle size components of a sample. Percent retained on a 90 micron mesh sieve is a good means to evaluate the concentration of large particles. This fraction should be minimized mainly through efficient pulverization during manufacture. A level of <0.5% is a good standard for a typical industrial grade powder coating. Sample retention on a finer mesh sieve such as a 45 micron screen provides a good assessment of the smaller particle size portion of your material. This fraction should range from 22% to 28%. Higher levels of fine particles cause handling and application problems in the field due to excessive agglomeration and poor electrostatic charging.

Particle size measurements using laser diffraction are much more sophisticated than sieve analysis. Instruments using this technique can be quite expensive. These devices delineate the particle size distribution into a large number of narrow fractions. Moreover they can provide important statistical analysis of the particle size distribution such as: particle size average, median, mode, a variety of percentile data and analysis of the particle size distribution curve. These instruments are quite useful when producing high quality powder coatings for very demanding applications such as the automotive industry.

Finished Powder Testing

The specification should embody critical tests that correspond to the performance needed by your customer. Sometimes specification authors include exhaustive batteries of tests that exceed any practical correlation to what is needed by the manufacturer. This is to be avoided. It is important for you to ascertain the performance your customers need, and then set out to create a quality specification for the coatings. Here are a few tests to consider.

The three key performance areas to be regularly ascertained are appearance, cure and application.


Appearance requirements involve color (both visual and instrumental), gloss, smoothness and cleanliness. Color should be measured with an instrument capable of delineating lightness-darkness (“L” value), red-green (“a” value) and blue-yellow (“b” value). Your instrument should also be capable to compare one color to another. This comparison is typically expressed as “ΔE”. “ΔE” is described as the mathematical relationship of the square root of (ΔL2 + Δa2 + Δb2). Color tolerance should be dictated by your customer’s requirements. Generally the lighter the color, the lower the tolerance. The color of whites and pastel shades should within a ΔE of 1.0 or less. On the other hand, bright reds, yellow and oranges can sometimes be visually acceptable even though they may far off as measured instrumentally. Additionally, powders that contain metallic, pearlescent or bronze pigments are not easily measured using most conventional color instrumentation. The color of these types of products must usually be assessed visually.

Gloss is simply measured by an instrument that projects a beam of light onto the coating surface. A sensor located in the instrument measures the amount of incident light reflected off the coated surface and compares it to a black glass standard. The amount of light is reported as a percentage of incident light reflected at a given angle from the perpendicular. The angle of illumination has an influence in comparing surfaces of differing glosses. Very high gloss coatings should be compared at an illumination angle of 20° whereas low to medium gloss coatings differentiate better at an illumination angle of 60°.


The cure of a finished powder can be measured by means of a simple gelation test. This test measures the time it takes for a sample to gel or crosslink at a prescribed temperature. The temperature should somewhat coincide with the recommended baking temperature. The sample is placed on a heated surface and manually agitated until it hardens. The end point is relatively subjective and has to be fairly well defined in the test method.

A very simple yet effective melt viscosity test measures involves measuring the distance a compressed pellet of powder travels down an inclined plate at an elevated temperature. This test known as the pellet flow or inclined plate flow test uses a small sample of powder formed into a pill or pellet. The powder can be compressed into a pellet using a commercially available hand press. The pellet is then placed on a horizontal preheated surface (can be metal or glass) and allowed to melt for a minute or less to provide adhesion to the surface. The surface is then tilted to a prescribed angle and the pellet is allowed to flow down the surface. 65 degrees from the horizontal is a good angle for this test. The apparatus is retrieved after a prescribed duration that should somewhat coincide with the recommended bake time and the temperature of the powder coating. The distance of flow is measured after the plate has cooled. Consistently formulated and processed powders should perform within a range of +/- 10%.

Mechanical Performance

Powder coated test panels should be prepared and assessed for mechanical performance. Quick tests include crosshatch adhesion, impact resistance and mandrel bending. Crosshatch adhesion involves cutting into the cured coating film with a suitable blade such as a razor or craft knife. A number of parallel incisions are made to the metal. These are then intersected by perpendicular incisions. Distance between parallel cuts should be consistent. A distance of 1.0 to 2.0 mm is acceptable. The adhesion is then evaluated by placing a suitable strong tape to the crosshatch and rapidly pulling it away from the coating. Adhesion to the substrate should 95% or greater.

Impact resistance involves rapid deformation caused by dropping a weighted ball onto the coated surface at a prescribed film thickness. The diameter of the ball is critical. The impact resistance of the coating should be known through laboratory testing prior to introducing the formula to production. The results of this test are quoted as weight times distance. Inch pounds or centimeter kilograms are typically the units quoted. Impact resistance is affected not only coating type and degree of cure but also by coating and substrate thickness. Undercured powders possess inferior impact resistance. Thicker coatings tend to be less flexible. Thinner substrate deforms more than thicker substrate. It is important to measure impact resistance at a consistent film thickness and over a standardized substrate.

Mandrel bending quantifies slow deformation of a powder coating. This test can be as simple as manually bending a coated test panel. Apparatus is available that will allow the tester to deform the coated panel in a tightly controlled orientation. This test can be conducted with a conical mandrel, which provides deformation over an ever-increasing mandrel. The performance is measured as the smallest diameter bend that the coating survives. Film thickness and substrate thickness both affect mandrel flexibility. Thicker films tend to be less flexible. Thicker substrates tend to provide more coating flexibility.

Application Performance

Application performance is dominated by particle size distribution. A tightly controlled particle size distribution will usually yield a powder coating with sufficient application performance. Particle size can be measured using laser diffraction with a sophisticated instrument. These instruments measure the diffraction of light as particles are passes through a laser. The degree of diffraction coincides with particle diameter. The particle size distribution is reported in a series of channels representing small particle size ranges. Mean, median and mode are all calculated as are other descriptions of the distribution (percentiles, etc.).

Powder fluidity can be measured if a manufacturer feels this is a necessary test. Fluidity can be assessed by placing a suitably large sample of powder (50 to 100 grams) into a lab size fluidization hopper and measuring the expansion of the sample after a prescribed amount of fluidizing air is introduced to the hopper. Fluidity can also be measured by placing a sample of powder onto a surface and measuring how well it flows peripherally. This is quantified by measuring the angle of repose of the powder sample. Good fluidization id earmarked by a low angle of repose.

Chemical Resistance

The cure of a powder coating can be measured by evaluating its resistance to a strong solvent. Usually a solvent rub test is sufficient. This can be accomplished by rubbing a saturated soft cloth or cotton swab against the surface of the coating. Undercured powders will soften or solvate. Methyl ethyl ketone or acetone are commonly used. More exotic chemical resistance testing is usually outside the realm of quality control testing unless specifically required by your customer.

In summary, manufacturing powder coatings of consistent quality requires diligent incoming raw material control and conscientious process control. Eliminating problems before they appear is the mantra of a good powder manufacturer. The focus of the quality team must always be consistency in processing, not finding problems after large amounts of time and money are committed. By paying most attention to the front end of the process you will avoid costly errors and the generation of non-conforming powder coatings.

Kevin Biller is technical editor of Powder Coated Tough and the president of The Powder Coating Research Group. He can be reached at: kevinbiller@yahoo.com.