FUNCTIONS OF PAINT COATINGS
A paint coating imparts two important functions to the underlying substrate:
However, due to a myriad of reasons, paint coatings fail in service, causing the paint coating to lose its aesthetic and protective functions. Paint failure is a ubiquitous and on-going problem for the general public globally. It causes hundreds of millions of dollars in damage annually and compromises the safety, environmental, and appearance characteristics of the affected items. When that occurs, I am often contacted by attorneys representing plaintiffs and defendants whose paint coatings have failed in service to act as expert witness and determine why the paint coating failed.
CASE EXAMPLES ON WHICH I SERVED AS AN EXPERT WITNESS
Case Example 1. This case involved corrosion of painted aluminum extrusion door and window frames. Dozens of residential buildings in coastal areas showed excessive and premature paint blistering and corrosion, and home owners filed suit. My role as an expert witness was to determine why they failed so quickly – usually within a few years. Using sophisticated analytical techniques, I determined the root cause of the premature paint failure was that the paint primer and topcoat were lifting off the pretreatment layer due to poor manufacturing practices by the company who pretreated and painted the extrusions. The failure was exacerbated by un-coated extrusion edges, and use in severe marine environments.
Case Example 2. This case involved premature paint blistering and delamination of paint coatings applied to exterior wood window and door frames. The root cause of this paint failure was poor formulation. The paint manufacturer substituted a lower cost resin raw material for one they had been using for years. The lower cost resin had much poorer properties that the original one, counter to the claims of the resin supplier.
Case Example 3. This case involved the use of an antimicrobial paint coating on galvanized steel for use in hospitals. The paint delaminated from the galvanized surface prematurely, causing the galvanized to be replaced shortly after installation. The root cause for the premature paint delamination was inadequate surface preparation (i.e., cleaning and pretreating) of the galvanized surface prior to applying the paint.
TYPICAL REASONS FOR PAINT FAILURE
There are a huge number of reasons why paint coatings fail in service. The service life of all painted products is affected by many variables, including weathering variables and variables associated with the materials, manufacturing, and design of the product. Failure of a paint coating can normally be attributed to a number of root faults which are associated with the (1) formulation of the paint, (2) application of the paint (e.g., spraying, brushing, rolling, dipping, environmental conditions during application, etc.) (3) design of the structure to be coated, (4) manufacturing of the paint, (5) properties of the materials comprising the coating system, and (6) exposure environment.
All of the possible reasons for paint coating failure cannot possibly be discussed in this brief treatise. However, I will discuss a few of the primary reasons in the following, in approximate order of importance.
Poor Formulation and Choice of Raw Materials
There are typically several raw materials that make up a paint formulation. These include major components such as the binder (i.e., the main resin like acrylic, urethane, polyester, etc.), pigments, and solvents. Minor, though still critical, components include additives like rheology modifiers, surfactants, defoamers, coalescents, drying agents, ultraviolet light absorbers, corrosion inhibitors, and biocides. All of these component determine the properties of the paint coating. They must be chosen carefully, both in quality and proportion of each in the formulation. Poor choices almost always result in poor paint properties. To emphasize, the book Paint Formulation – Principles and Practice explains “The absolute amounts and relative proportions of the various ingredients play a major role in determining the performance of the resultant coating formulations although, obviously, the characteristics of the selected components must also be suitable for the coating in its service environment. In fact, the incorrect choice of only one component can reduce a paint’s performance even if the other additions and general formulation requirements are otherwise satisfactory. This is a frequent cause of premature paint failure in practice.” (Boxall, J., & J.A. Von Fraunhofer, Paint Formulation – Principles and Practice, Industrial Press, Inc., 1981, p. 35.)
Inadequate Surface Preparation
Inadequate cleaning and chemical pretreatment steps prior to painting are one of the primary reasons for paint failure in service. The cleaning and chemical pretreatment steps are critical to obtaining a paint coating that will perform its intended functions for the long-term. If not done properly,
cleaning and pretreatment steps can leave residual soils on the substrate surface, can leave hard water salts from rinse or process water, and/or chemical contaminants from unreacted pretreatment chemicals that are not adequately removed by rinsing. These soils and chemical impurities can reduce paint adhesion, and can attract moisture permeating through the paint that can lead to blistering and other defects in the paint.
Improper Curing of the Paint Coating
Incomplete curing/baking the paint can result in residual solvents or water remaining in the paint that can cause poor adhesion, blistering, too soft a coating, and other defects. Over-baking the paint can affect the appearance of the paint, and the paint will likely be very brittle. Paint brittleness is a particular issue if the metal is to be formed after painting. Curing the coating too fast or too slowly can result in blisters or other defects in the paint coating.
Poor Design/Misuse of Product
Poor design or misuse of the product can lead to premature paint failure. Examples of poor design include non-coated edges of painted metals used severe service environments, tight forming bends that crack the paint, small crevices that are prone to retention of moisture and other corrodents which accelerate corrosion, and connection to unpainted dissimilar metals. Examples of misuse include placing the product in a severe environment for which the paint formulation is not suitable, such as near or in salt water, and prolonged exposure to extreme heat, cold, humidity, chemicals, pollution, or intense ultraviolet light.
ABOUT THE AUTHOR
Robert A. Iezzi, Ph.D., is founder of RAI Technical Solutions®, Inc. (www.rai-technical-solutions.com), a technology company that provides expert witness and consulting services on paint coatings, metal coatings, corrosion, surface preparation/pretreatments for metals and plastics prior to painting, and plastics.
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