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Common Test Methods and Acceptance Criteria

You might think that it’s common sense that a manufacturer using industrial coatings would have a comprehensive performance specification. That specification after all is the road map to their coating’s success. However, we have encountered clients that had some of the test essentials in place for their end purpose, and even identified areas for improvement, but struggled to determine the appropriate test measures to finalize their complete specification. 

To assist, I’ve provided the “most common” test methods and acceptance criteria to assist if you too are rounding out your specification needs.    

Basic questions to determine specific testing requirements.

  • Does coated product have exterior exposure? QUV, salt spray, and humidity resistance?
  • Gloss, sheen?
  • Will the product be subjected to impact, abrasion, or flexibility?
  • How hard does the product’s film need to be? 
  • How is it applied? 
  • Thickness, viscosity?

General ASTM Testing Methods:

  • Gloss ASTM D523
  • Viscosity ASTM D4212
  • Color ΔE
  • Pencil Hardness ASTM D3363
  • Adhesion ASTM D3359-02 Method “B”
  • Impact Direct and Indirect ASTM D2794
  • Flexibility ASTM D522
  • Salt Spray Scribed / Un-scribed ASTM B117 ASTM D1654
  • Humidity Scribed / Un-scribed ASTM D2247 ASTM D714
  • UV (Weatherometer) ASTM D6695-03B
  • Taber Abrasion ASTM D4060
  • T-Bend ASTM D4145

Gloss ASTM D523

Measurements by this test method correlate with visual observations of surface shininess.  Generally, gloss is measured at 60 degrees but can be tested at other angles (i.e., 85 or 20).

Viscosity ASTM D4212

Viscosity is a measure of the resistance of a fluid that is being deformed by either shear or tensile stress. In everyday terms (and for fluids only), viscosity is “thickness” or “internal friction”. Thus, water is “thin”, having a lower viscosity, while honey is “thick”, having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).

Color ΔE

Delta-E is a measurement used to indicate how much a color deviates from an accepted standard. The higher the ΔE, the more inaccurate the color.  A perfect color match has a ΔE of zero. The human eye is only capable of detecting color differences at certain thresholds. The minimal detectable difference is about 1 ΔE. 

Typical Light Sources: 
D65 –   Daylight
A –   Incandescent light – A
CWF – Department store light (Cool White Florescent)

Pencil Hardness ASTM D3363

Hardness pencils come in an assortment of both hard and soft.  The hardest is a 9H, followed by 8H, 7H, 6H, 5H, 4H, 3H, 2H, and H. F is the middle of the hardness scale; then comes HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B, and 9B, which is the softest.

Crosshatch Adhesion ASTM D3359-02

Crosshatch adhesion is used to establish whether the adhesion of a coating to a substrate is at a generally adequate level for the coated products working environment.  It involves scribing a grid pattern (method B)on the coated substrate, applying pressure-sensitive tape, pulling it off, and observing the amount of coating removed. The ratings range from 0B to 5B, 5B being the best performance.

Direct/Indirect Impact ASTM D2794

Coatings are subjected to damaging impacts during the manufacture of articles and their use in service. In its use over many years, this test method for impact resistance has been found to be useful in predicting the performance of coatings for their ability to resist cracking and rapid deformation when impacted. The values stated in inch-pound units are to be regarded as standard. A 4 lb. weight is dropped a distance to strike an “indenter” that deforms the substrate, hence the applied coating. The indentation can be either intrusion or extrusion (direct or indirect) depending on what’s needed for a particular application or performance criteria. 

Salt Spray ASTM B117

Salt Spray testing is an accelerated corrosion test that produces a corrosive attack on the coated samples to predict their suitability for use as a protective finish by introducing test subjects to atomized salt water (5% NaCl). The appearance of corrosion  (oxides) is evaluated after a period of time. Test duration depends on the corrosion resistance of the coating; the more corrosion resistant the coating is, the longer the period in testing without showing signs of corrosion.

Salt spray testing is popular because it is well standardized and repeatable. There is, however, a weak correlation between the duration of the salt spray test and the expected life of a coating, since corrosion is a complicated reaction and can be influenced by many external factors. Nevertheless, salt spray testing is widely used in the industrial sector for side-by-side, controlled, and relative evaluations of corrosion resistance of finished surfaces or parts against a known standard.

Humidity Resistance ASTM D2247-Standard Practice for Testing Water Resistance of Coatings

Tests at 100% relative humidity and 100 degrees F. (38 Degree C.) are used for specification acceptance, quality control, and research and development for coatings and substrate pre-treatment. Some tests are used for a pass or fail determination at an arbitrary time. A coating system is considered to pass if there is no evidence of water-related failure after a required period of time.

Taber Abrasion ASTM D4060

A coating on a substrate can be damaged by abrasion during manufacturing and service. This test method covers the determination of the resistance of coatings to abrasion.  The tester resembles a turntable with an abrasive wheel riding on the panel as it turns a predetermined number of cycles.  Once the desired cycles are complete the substrate is weighed and the amount of coating that has been abraded is measured.  The greater the removal the less abrasion resistant the coating.

T-Bend ASTM D4145

This test is a means of evaluating the ability of a coating system to withstand the stresses of fabrication.

The test involves taking a cured panel and folding it back onto itself and inspecting it for cracks in the film.. The bend direction, whether the axis of bend is transverse to, or along the rolling direction of the metal, and the temperature of the specimen when it is bent can affect the results of this test and should be agreed upon between the supplier and the end-user.  Rating values would be values of 0T, 1T, 2T, etc. with 0T being best.

See more on this subject at https://marcuspaint.com/no-specification-could-mean-inconsistent-results/.

If you’re developing specification criteria for an existing application or exploring new opportunities, please contact the Marcus Paint team to learn more. There’s a vast amount of testing criteria available and this blog highlights the basics as a starting point.

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