Developing a Coating Performance Specification

Written by: Nick Liberto P.E., Powder Coating Consultants, Division of Ninan, Inc.

Coating performance is often overlooked by the product designer, even though coating performance is the product feature that the customer uses to establish the value of the product. How a product looks, feels, and survives its “use” environment is as important to customers as how well the product functions. This fact is proven in many industries where customer complaints about “the paint” on the product usually exceed all other comments or complaints. Nothing proves this point more than the automotive industry where past customer complaints about paint and premature corrosion led to the “6-year/60,000 mile no rust through warranty” used by most manufacturers today.

Coating performance must be closely tied to customer expectations. Unfortunately, customers do not use the same “coating-term” vocabulary that suppliers and manufacturers use. This requires the product designer to translate customer “druthers” into coating performance requirements. This task is never easy, as the designer not only has to determine the specific coating properties desired by the customer but they also need to establish their performance values (significance), as well. For instance, a customer may say they do not want to see rusting on the product surface during the product life cycle, but the designer must establish the performance value for corrosion resistance that will determine how long this product will remain rust-free, in what environment, and with what maintenance intervention provided by the customer during the estimated product life cycle. This article will present the designer with information to help them in developing a coating performance specification that meets their customer’s expectations.

Collect The Coating Performance Information:

If you agree that your customer is the final arbiter of your product’s quality, this presents the first dilemma; “How do you identify what your customer is expecting for product finish quality?” The simple answer is: “Just ask them.” This has become especially easy in our “connected world” where Facebook, internet questionnaires, customer feedback forums, etc. abound. Of course, you need to carefully craft the information gathering questions, or you may end up getting poor data that is unusable. For instance, every customer wants their purchased products to be the made from the best materials and craftsmanship possible, but they are rarely willing to pay a premium for these high standards. So it really comes down to; “What finish performance is the customer willing to pay for?”

Translate the Information into Coating Requirements:

Customers may talk about the finish on their product being smooth and easy to clean, scratch resistant, not rust quickly, be long-lasting, not get dull, and use many other subjective descriptors. As stated previously, these customer driven descriptors need to be “translated” into actual product design and finishing performance requirements. These finishing performance requirements are then used to select the substrate materials, pretreatment chemicals, primer and topcoat finishes, and the process equipment and workmanship standards necessary to meet these customer driven goals.

The following are some terms and definitions that will help the product designer translate customer “druthers” into coating performance criteria (the customer “druther” is on left and the coating performance parameter is on the left):

  • Color/Hue = Color, Contract Ratio
  • Gloss = Gloss
  • Look & Feel = Smoothness, Texture, Slip, DOI, etc.
  • Fading & Chalking = Weathering (UV Resistance)
  • Paint Life = Weathering, Humidity Resistance
  • No Rusting = Corrosion (Salt Spray, Humidity) Resistance, Edge Coverage
  • Dent Resistance = Impact Resistance, Flexibility
  • Post Formability = Flexibility
  • Scratch Resistance = Hardness (Mar) Resistance
  • No Chipping = Chip Resistance, Adhesion
  • Coating Wears Well = Abrasion Resistance
  • No Staining = Stain Resistance
  • Cleanability = Solvent Resistance

Definitions of these coating performance parameters are as follows:

  • Color is defined as the hue, brightness, and saturation for objects and light sources. (i.e. red, blue, yellow, etc.)
  • Gloss is defined as the shininess of a given surface. (i.e. flat, matte, high gloss, etc.)
  • Smoothness is defined as the coating surface texture or the lack thereof.
  • DOI of a coated surface relates to both gloss and smoothness.
  • Contrast ratio describes the hiding power of a given coating.
  • Film thickness is the measurement of the cured coating thickness on a given substrate.
  • Impact resistance is measured by the rapid deformation of the coated surface in both the front (direct) and back (indirect) of the deformed area.
  • Flexibility is defined as a coating’s ability to resist the bending of the substrate.
  • Adhesion is defined as the coating ability to adhere to the substrate surface. Poor adhesion will effect other tested properties such as impact and corrosion resistance.
  • The hardness of a coating effects its ability to resist deformation, marring, scratching, etc.
  • Abrasion resistance determines a coating’s ability to resist mechanical degradation.
  • Edge coverage is defined as the thickness, or lack thereof, of the coating along the edge of the part. Edge coverage will effect corrosion resistance when it is too low and assembly when it is too high.
  • The chip resistance of a coating is its ability to resist cracking or chipping when the coating is hit by another object. Chip resistance is related to both the impact and adhesion properties of the coating.
  • Solvent resistance is the coating’s ability to resist softening or dissolution when immersed in a given solvent(s).
  • Stain resistance is defined as a coating’s ability to resist discoloration when contacted with various substances.
  • Humidity resistance is defined as a coating’s ability to resist the effects of exposure to pure water (100% humidity).
  • Salt spray or salt fog testing evaluates a coating’s resistance to one type of corrosive environment.
  • Weathering is measured as a coating’s ability to resist the effects of outdoor exposure (both UV light and moisture).

Segregate the Requirements Into Coating Performance Categories:

Once the designer has the customer “druthers” translated into coating performance criteria, it makes sense to segregate these issues into coating performance categories. This will make writing a coating performance specification much easier. The following coating performance categories are universally accepted in our industry:

1. Appearance Properties:

  • Color
  • Gloss
  • Smoothness
  • Distinctness of Image (DOI)
  • Contrast Ratio

2. Functional Properties:

  • Film Thickness
  • Impact Resistance (direct/indirect)
  • Flexibility
  • Adhesion
  • Hardness
  • Abrasion Resistance
  • Edge Coverage
  • Chip Resistance

3. Environmental Properties:

  • Solvent Resistance
  • Stain Resistance
  • Humidity Resistance
  • Salt Spray (Corrosion) Resistance
  • Weathering (cyclical testing & UV resistance)

Establish Performance Values (Significance) to the Coating Requirements:

Now that you have a list of coating parameters, you need to establish performance values to each of these to provide significance. As each of these parameters has specific relevance to your product, the performance values must reflect this significance. For instance, if your product is designed to provide high functionality in severe environments (i.e. pumps used on off-shore drill rigs), then the functional and environmental properties of the coating have the most significance and the appearance properties have less significance. Conversely, if your product is designed to look great and is used in mild environments (i.e. indoor products like computers), then the appearance properties are more significant than the functional properties.

Additionally, some properties conflict with other properties and you must choose which are more important or make design compromises to reach balance. For instance coatings with high hardness and abrasion resistance typically have poor flexibility and impact resistance.

Coating Process Limitations and Other Concerns:

In addition to customer driven issues, there are numerous process limitations that need to be described to the coating suppliers to ensure the coatings provided can be sprayed and cured using your existing process equipment. These process limitations include powder feed systems (fluidized or box feeders), reclaim or spray-to-waste systems, cure oven technology (convection, IR, or UV), cure oven profiles (time @ cure temperature limitations), gun charging technology (corona or tribo), reclaim sieve screen size, etc.

Product design-specific issues also need to be discussed in the coating specification, as well. These include film thickness, complete/consistent coverage, and temperature sensitive substrates, among others. Incorporating these product design constraints into a comprehensive coating performance specification is extremely important, as well.

Drivers for product cost may also need to be identified and described in this document. These include min.max film thickness and specific gravity values, since they most affect product coating costs.

Finally, rework and touch-up must be discussed in this document. The coating must allow for repainting of reworked parts and liquid touch-up paint must be compatible with the coating for easy field and assembly repairs. The coating supplier must provide procedures to rework parts and recommend liquid touch-up formulas that are compatible with the coating provided.

Write a Coating Performance Specification:

What you should end up with at this point is a complete list of coating performance standards that need to be verified by testing to ensure that customer driven requirements are met using your existing coating process. Add to these customer driven requirements form and fit requirements defined by the product’s assembly and end-use and you have the makings of a detailed and appropriate coating performance specification.

Organize these coating performance objectives into a document that is included in your engineering document control system for easy reference and revision control.

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