This issue’s Technology Interchange topic helps you understand the critical relationship between materials and processes, specifically how your coating chemistry behaves in your curing oven. Most important is the long term durability of the coating, but we’ll also examine how appearance properties such as color and gloss are affected by the intersection of product and bake conditions.
Oven Conditions and How to Control Them
Let’s first talk about oven conditions. It is paramount to understand that powder coatings respond best when the part temperature coincides with the recommended bake provided by your powder coating supplier. Your oven needs to be the right temperature for the coated parts to achieve the proper time at or above the temperature stated on the powder’s Technical Data Sheet (aka Product Data Sheet). Some suppliers will provide a range of time/temperature combinations which gives you an even better understanding on how a product cures in an oven.
So when a TDS recommends 15 minutes at 375°F, you must ensure that all your coated parts reach at least 375°F and stay at or above that temperature for 15 minutes. Some coaters will load coated parts into an oven and wait until the oven temperature readout recovers to the recommended temperature before they start their timer for the recommended bake time. This practice doesn’t ensure adequate cure and performance of the powder. Recovery of temperature from loading parts is a good indicator that your oven has stabilized, however it does not accurately indicate the part temperature.
It is much wiser to measure the actual temperature of the part(s). For a small batch oven operation, a simple non-contact infrared thermometer works well. These have come down in price over the years. A reliable one can be purchased for under $40.
Another issue has to do with curing the coating on parts of differing metal mass. It is important to be aware that metal mass affects the heat-up rate of a part. For example, if you are coating parts for a motorcycle and you place fenders into an oven along with wheel rims, the denser, heavy rims will take longer to heat up than the thin gauge metal of the fenders. It is critical to ensure that the heaviest parts see the minimum bake time at temperature. Hence when measuring part temperature, scan a variety of parts to make sure all of them achieve adequate cure.
If your finishing line is conveyorized, your parts probably run through a tunnel oven and it will be a little more of a challenge to measure part temperature. You may have openings or doors on the oven walls that provide access for an IR thermometer temperature readout. However, the best solution is to use a data logging temperature profiler. These are comprised of a solid state data logger that is encased in a thermally protected box. The logger has six or more high temp wire leads that can be attached to the surface of parts to collect and send part temperature to the data logger. The data logger is attached to the parts hanger and follows the parts into and through the oven. After the hardware exits the oven and has some time to cool the logger can be extracted from the thermally protected case and plugged into a laptop to download and analyze the data.
The data analysis varies by manufacturer but will include time-temperature scans of each measurement location and also an analysis of time above predetermined temperatures. The beauty of using one of these instruments is the ability to determine the temperature profile of numerous parts and/or part locations under the same conditions. This allows you to adjust your oven settings to compensate for the differential in part density and loading. After establishing your standard oven conditions, it’s a good idea to periodically run a time-temperature scan with representative parts. Some plants do this on a monthly basis; I recommend that you do this at least that often and any time you make a significant change in the number or density of parts run through your oven.
If your oven is heated by natural gas, another issue that affects your powder’s performance is the efficiency of the oven’s burners. Incomplete combustion will generate an excess of carbon and noxious carbon monoxide. Excess carbon will adversely affect the color of your finish, typically resulting in an unexpected increase in yellowing. The flame of your burner should be blue. Indirect heating designs will minimize the tendency to yellow as an intermediate heat exchanger transfers the heat to the oven; however, it is always wise to keep your burners running efficiently.
Oven exhaust also impacts coating quality. Adequate amounts of fresh air need to be supplied to the oven to minimize the accumulation of volatiles which, in turn, influence the ultimate finish of a powder coating. All powder coatings possess at least a small percentage of volatiles that are expelled during the curing process. These volatiles can accumulate in the oven and cause discoloration of the final finish. Oven experts recommend a minimum of three oven exhaust turnovers per hour, but ideally seven or eight should be used to ensure a clean oven and a consistent, good looking finish.
Powder Coating Performance
The majority of powder coatings are thermosetting in nature, which means an irreversible chemical reaction occurs during curing in the oven. The degree of cure has a significant impact on the ultimate performance of the finish. Appearance features and coating durability are affected by the degree of cure of the powder.
Low gloss and matte finish powder coatings are often formulated by manipulating the cure response of various components in the formula. It’s a common trick we formulators use to create a very smooth, but low gloss, finish. Hence inadequate cure can lead to variations in gloss. This is most common in hybrid and epoxy powders but can also be observed in some polyester and polyurethane formulations. Therefore, management of oven conditions is critical when curing matte and low gloss powders. If the gloss on parts is higher than expected, you are probably undercuring the powder coating. In most cases you can re-bake the part and the gloss will drop to the specified range.
One of the unique aspects of thermosetting powder coatings is that the coating can “look” cured but, in fact, be undercured. Liquid paints inherently are easier to recognize undercure as they can be sticky or soft to the touch if they haven’t seen adequate time and/or temperature. Powder coating finishes can still feel hard to the touch even when undercured. Incomplete cure usually produces a finish with poor mechanical properties. The coating will be brittle; consequently, adhesion and chip resistance can be compromised. Often this is discovered when parts are assembled and the finish cracks or chips when fastening components. Worse yet is if the parts leave your facility and are either assembled at a different location or placed in the field. A myriad of mechanical coating failures are possible that could lead to expensive remediation and/or product liability claims.
Undercuring a powder coating not only compromises mechanical performance, but also the ultimate durability of the coating. Incomplete crosslinking can lead to a decrease in corrosion resistance, lowered resistance to chemicals and solvents and poor outdoor durability. It is therefore crucial to ensure complete cure.
One of the most common tests involves evaluating the coating’s solvent resistance. ASTM D5402 describes the test. Basically you can use either a shop rag or cotton swab soaked with acetone (or methyl ethyl ketone) and rub the surface 25 to 50 times. Your powder coating supplier will be glad to recommend how many rubs are appropriate for your coating. In some cases a less aggressive solvent blend, such as 90% xylene/10% MEK, is recommended for this test. Depending on the coating chemistry, a fully cured powder will soften slightly with a solvent rub test, but not dissolve and transfer coating to the shop rag or cotton swab.
Oven Condition Effects on Powder Chemistries
Coating formulation has a significant influence on how a product behaves in an oven. A summary of the properties most affected by degree of cure ensues.
The volatile content of thermosetting powder coatings varies depending on the formulation and the chemistry. All powder coatings emit approximately 0.5 to 1.0% water during the bake. Another small percentage of organic volatiles are expelled from the coating as well. These are typically additives and low molecular weight fragments of resin(s). Epoxies, hybrids, TGIC polyesters and most acrylics emit only a nominal amount of volatiles and therefore do not pose significant oven exhaust or fouling issues.
A few chemistries emit what are known as volatiles of cure during the crosslinking process in the oven. Most notable are most polyurethanes and HAA (hydroxyl alkyl amide) polyesters (aka non-TGIC or Primid™ polyesters). In the case of polyurethanes a common blocking agent, ε-caprolactam, is emitted during cure in the oven. This is exhibited by a smoky haze in the oven and emissions can be up to 5% of the coating by weight. Appropriate oven exhaust is necessary to minimize accumulation of this volatile in the oven. Inadequate exhaust results in fouling of the oven which can cause color instability of the powder finish. In addition, the ε-cap will condense on the interior surfaces of the oven and exhaust ductwork causing further issues with cleanliness and possible flammability.
HAA polyesters emit water as a volatile of cure in addition to the ambient moisture present in a typical powder coating. This emission is relatively innocuous and is easily exhausted, though thicker than normal films can exhibit pinholes from the escaping water molecules.
While it is critical to ensure that your parts experience enough time and temperature per the recommendation on a product’s data sheet, some chemistry’s cure behavior is significantly different than others. Epoxies, hybrids and TGIC polyesters are the least sensitive to variations in cure response. A long duration in the oven at slightly lower temperatures than specified is usually acceptable to achieve adequate cure with these chemistries. Polyurethanes respond quite differently however. With these products, a minimum temperature is requisite to initiate any cure and hence coating performance. A typical polyurethane needs at least 350°F for the curing reaction in the coating to begin. Oven conditions below 350°F will allow the coating to melt and flow, but not develop any meaningful cure, consequently resulting in poor coating performance.
HAA polyesters are also more sensitive to cure conditions than TGIC polyesters, epoxies and hybrids, but not to the extent of polyurethanes. Typical HAA polyesters require a minimum cure temperature of approximately 325°F or higher to initiate cure. Hence it is more critical to monitor part temperature with these products.
The beauty of a data logger is the ability to determine the temperature profile of numerous parts and/or part locations under the same conditions.
Epoxies tend to exhibit the least color and overbake stability. This is especially evident with whites and light colors. Hybrids are somewhat better and polyurethanes and TGIC polyesters have the best color stability across a range of temperatures. Low temperature cure powders tend to have less color stability than their standard bake counterparts because they contain catalysts to augment accelerated cure. HAA polyesters tend to overbake yellow more so than TGIC polyesters.
As you can see, not all powder coatings are created equal and each responds to oven conditions in their own characteristic manner. Oven settings including part density and load are critical to obtaining adequate cure and acceptable coating performance. It is therefore requisite to understand your powder coating products and to set and monitor your oven parameters to ensure consistent quality and coating performance.
Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.