PC Summitry: Achieving Powder Cure and Avoiding Craters

In a perfect world, troubleshooting would never be an issue. Products would arrive before they were ever needed, application performance would work universally across all types of spray equipment, and coatings would perform flawlessly over a wide range of handling and baking conditions. As we all know, life in the finishing world is seldom perfect. Unforeseen daily challenges erupt when we least expect them, and our wits are put to the test on an unrelenting pace. This article will touch on a couple of the more common problems encountered with a powder coating finishing operation and the remedies to fix them. In addition, some strategies to avoid these pitfalls will be offered.

Achieving Acceptable Cure Under-cured powder coatings can be difficult to detect but will cause a whole host of problems including chipping, loss of adhesion, poor durability and scratch resistance. One of the problems with recognizing under-cure is that a poorly cured powder looks just like a fully cured one. Powder coaters don’t have the luxury of a “wet paint” condition to assess completeness of cure.

The first place to start is the vendor supplied Product Data Sheet (aka Technical Data Sheet). This document will describe the time and temperature required for the coated part to achieve acceptable cross-linking to ensure film performance. It is very important to note that the time and temperature stated refers to the part temperature and not the oven air temperature. Consequently it is incumbent upon you to know the part temperature and how long it maintains or exceeds this minimum temperature. This can be done using an industry standard time/ temperature logger such as a DatPaq™ or Curvex-2™. Otherwise an infrared pyrometer can help however it requires a line of sight to the surface of the coating and can be cumbersome to use with a batch oven.

The flip side of under-cure is over bake issues. Most polyester type powder coatings provide a fairly wide bake condition window and perform well with longer bake times and/ or higher temperatures. Epoxy and hybrid powders tend to discolor with excessive heat especially in whites and light colors. In addition semi-gloss and matte powders typically slide out of gloss specification when over baked or under-cured.

Ensuring acceptable powder bake conditions requires the control of the following:

• Metal Mass in the Oven. Uneven and inconsistent loading of parts skews the metal temperature and may cause significant over bake or under-curing of the coating.

• Conveyor Line Speed. Line stoppages can create over bake conditions whereas increased line speed may result in under-cure.

• Mixing Dense and Thin Gauge Parts. Dense parts such as castings require significantly longer oven dwell times to achieve the same temperature curve as light gauge sheet metal parts.

In addition it is wise to select a powder coating with a wide recommended bake range. Table 1 above depicts the typical bake/curing performance of generic powder coating chemistries.

The best way to verify powder cure on an operating finishing line is to perform solvent rubs in an inconspicuous area of the coated part. A shop cloth or Q-Tip saturated with acetone is a common method. It is best to compare the solvent resistance of a coated part with that of a known control that has been confirmed as fully cured. This can be a coated test panel from your powder vendor or a self-generated one. Typically cured polyesters can withstand 25 acetone double rubs with a minimal of color transfer and softening on the coating. Epoxies, hybrids and polyurethanes usually transfer no color with negligible softening of the film. Gloss and color measurement can also be instructive of cure behavior. Discoloration usually indicates over bake and higher gloss typically points to under-cure.

Another question is how often to check completeness of cure. I recommend checking with every product change and certainly with any process change (e.g., line speed, oven load, different parts mix, etc.). Intuitively it makes sense to run a test at least every shift regardless.

Avoidance of Cratering Indeed this is the bane of every finishing shop—the elusive and often sporadic incidence of craters. Craters can be defined as a circular interruption in a coating surface that is a result of a contaminant that has a significantly different surface tension. Severe craters are also known as “fisheyes” for their obvious resemblance to the piscean ocular feature.

What are common sources of craters? The most prominent cause is unclean compressed air in your spray equipment. Most air compressors rely on lubrication of the pistons or screws that create the compressed air. This lubrication must be captured before the air reaches the spray equipment. A couple measures are essential for oil-free application air. The supply line from the compressor should employ an air cooler to condense and remove ambient moisture that gets entrained in the air compressor. Some oil removal is also accomplished at this stage. A two-stage filter system should be installed closely downstream from the air cooler. This system is comprised of an air filter (5 micron is good) that removes particulates and some moisture followed by a coalescing filter that traps oil mist particles. Finally another air filter should be present just before your air supply to your application equipment. This catches any particulates that may have come from your supply lines.

Having installed a high quality cooling and filtration system on your compressed air line isn’t good enough to ensure crater-free coatings. It is incumbent upon you to perform regular inspections and maintenance on your filters and air cooler to keep the air pristine.

If you know you have clean compressed air and still experience craters the source is probably due to either a material or environmental contaminant. From a material standpoint the source of craters includes: substrate contaminants such as forming oils, drawing compounds and press lubricants. Intermixing of incompatible powders can result in cratering. Most egregious of these are silicones (typically high heat formulas) and acrylics (mainly automotive grades). Miniscule contamination of "standard" Powder coatings (polyester, epoxy and hybrids)  by acrylic or silicone powders will result in a cratering problem that may take herculean effort to correct. (See Table 2 above)

Environmental contaminants emanate from a wide variety of sources and include lubricants such as WD-40™and other rust preventatives, hand lotions and creams, cosmetics, silicone and petroleum based greases and silicone paints. Awareness of potential sources of craters and personal protective measures to isolate skin care products from the application area can go a long way in avoiding these types of defects.

Preventing material crater contamination requires thorough cleaning of substrates to eliminate any residual forming compounds or lubricants.  Avoiding inter-powder contamination requires strict cleaning protocols between product changes on your finishing line. If the compatibility of two powders is in question it is advisable to test the compatibility by deliberately spiking a powder with a small amount (0.1%) of the previous powder sprayed and observing the film appearance.

Avoiding environmental cratering sources requires vigilance in keeping your compressed air contaminant-free and your application area clean and isolated from the rest of your plant. Pay careful attention to maintenance procedures and materials used to lubricate mechanical equipment and conveyors. Also avoid silicone greases and traditional rust preventatives.

Running a powder finishing is always a challenge. Achieving adequate cure and avoiding craters in your finished product are but two of the many requirements for a successful operation. Careful process control, procedures, and training of personnel will provide some of the tools to keep your finishing system out of trouble.

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