During the curing cycle, volatile ingredients may be emitted from the surface which can cause surface contaminants and imperfections in the finish coat. Potential sources of volatiles other than the powder ingredients include primer coatings that may be underneath the powder coating, metal pretreatment conversion coatings, and oils, moisture, or other contaminants that are on or in the substrate prior to coating. If excessive amounts of volatile substances are generated late in the cure cycle, they may create defects in the forming film.
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Preheating of a part prior to painting may eliminate enough volatile substances from the part to permit it to be painted without the occurrence of defects from outgassing. The time and temperature of preheating required to minimize outgassing must be determined by trials. Generally painting needs to be done shortly after preheating is carried out, since volatiles substances may recur, either by absorption from the environment or migration for the substrate interior.
While there is no powder coating product that can eliminate all occurrences of outgassing in severe cases, there are ways to reduce outgassing occurrences during the application process, and there are products PPG makes that are designed to reduce the chances outgassing occurs. We call these Anti-Outgassing, Gas-Free or Anti-Gas powder coatings.
Optimum conditions should be defined on a product-specific basis. Every powder product has an individually defined shelf life rating that is related to its storage temperature. You’ll find this information on the product data sheet.
Typically, low temperature has no harmful effect on powder performance properties. In fact, refrigerated storage of powders is a recommended method for maximizing the physical and chemical shelf life of a powder. However, cold powder acts as a condensation site when exposed to higher temperature, humid air. For this reason it is recommended that bulk containers of powder coating be allowed to equilibrate to application room temperature for at least several hours prior to opening of the container. This precaution is especially important during winter in northern climates when powder may have been transported or stored at very low temperatures, below 32°F (0°C).
Powder may form lumps in bulk containers, possibly caking together to form substantial agglomerates that cannot be pumped or easily broken by powder handling equipment. Heat may also be detrimental to the chemical stability of many powder products.
A cure curve is a graphical representation of the thermal conditions required to insure that an applied thermoset powder coating is completely cured, i.e. essentially all available resin and curing agent reactive groups are chemically reacted together.
No, each powder product manufactured by PPG has a cure curve associated with it, the specification of which is primarily determined by the formulation of its resin and curing agent system. However, there are commonalities between technologies, so it’s not unusual to see a family of related powder products sharing a single cure curve.
Yes. The simplest cure curve is one that specifies the minimum thermal conditions that must be provided to cure a powder during its baking process. Graphically, it is a single line or curve. Conditions of shorter time or lower temperature, i.e. those which fall below the cure curve line, are not recommended due to the likelihood of less than optimal cured film performance.
The complex (area) cure curve below is for the same low temperature cure polyester TGIC product line as shown above, and has identical minimum cure data points (blue line), but with the addition of a maximum cure conditions line (red).
Annotations can be added to complex cure curves indicating details of how they were determined or displaying the powder film properties or potential failure modes that are associated with various cure chart areas.
In the case of multi-film coating systems that undergo two or more heat-curing processes, cure curves for each applied coating layer need to be cross-referenced to account for the cumulative thermal experience of the system.
Historically, the laboratory determination of minimum bake time and temperature using standard pretreated metal test panels has been found to correlate well with customer experience. Often film flexibility or chemical resistance falls off dramatically once baking conditions fall below a certain point.
Unfortunately, the determination of maximum time and temperature bake schedules for curing powder coating products does not always correlate well between PPG's laboratory experiments and customer experience due to the fact that:
1. Powder coatings are remarkably heat resistant at common baking temperatures. They do not typically undergo embrittlement or chemical decomposition when exposed to 100% or even 200% of their standard baking times at a recommended temperature. Those properties of a powder coating that are significant for defining an overbake condition can be highly application specific (e.g. recoat, touch-up repair, silk screening, pinstriping, decal application, etc.), and different customers will evaluate the various properties in different ways (e.g. color and gloss tolerances vary widely across applications and industries).
2. Many application factors can determine the exact point at which the product begins to lose some of its performance properties because of excess heat exposure. Among these factors, which are specific to each customer's application and production environment, are:
Fortunately, although each customer has their own unique application and baking equipment situation, making it difficult for PPG to determine a maximum time-temperature cure chart line with great precision, most customers have relatively tight process controls and do not typically experience wide variation in bake times or temperatures.
Often it is sufficient for a customer to simulate, perhaps in a validation or PPAP trial, just a few of the "worst case" combinations of time and temperature their "worst case" parts are likely to experience.
Link to YD Powder Coating
A maximum cure curve line constructed from such data points is likely to be conservative relative to the product’s inherent capability, but it is often sufficient for practical, day-to-day process control purposes. Some customers utilize thermal data acquisition hardware ("oven logger") and its associated software (e.g. Datapaq® & Insight; Computer Aided Solutions Grant® & PaintView, etc.) to generate numerical analyses of thermal history for a part, including one or more "%-ofcure" parameters. Such theoretical indices may provide reassurance to a customer who is working only with a minimum recommended bake time/temperature cure curve that a detrimental overbake condition has not been experienced.
No. The customer must assume responsibility for verifying that proper cure is applied to the powder product under production conditions. PPG is unable to assess the degree of risk that a customer should assume, in light of the variability that characterizes its baking process. For most powders, there is seldom a problem with baking "above" the cure curve minimum. If anything goes slightly wrong, like cold spots in the oven, racking issues or variations in metal thickness, some parts will have an under cured coating.
Many PPG powder customers deliberately set oven conditions so that there is a comfortable safety margin between the minimum conditions specified in the cure curve and actual. For example, with a specified minimum of 15 minutes at 375°F (191°C), a customer might choose to adjust the oven burners settings so that every part’s temperature profile, measured with an oven recorder, at all positions on the racking system shows at least 21 minutes at 380°F (193°C).
To save time, and thus maximize productivity, many customers set their ovens higher than the maximum peak metal temperature achieved by the parts. For example, a set point of 425°F (218°C) may result in parts reaching a peak metal temperature of 395°F (202°C) just before they come out of the oven. But this approach carries the risk that if there is a conveyor line stoppage, unusually thin gauge metal, or oven "hot spots," some parts might see the temperature as high as the oven set point.
Anti-graffiti coatings resist or minimize the damage associated with application of unauthorized inscriptions, slogans, drawings, or other "artwork" that are scratched, scribbled or painted on a structure or surface, generally public in nature. A typical graffiti material is air-dry paint in an aerosol dispenser. An anti-graffiti coating may prevent the application of such material to its surface or permit easy removal of the material sometime after its application.
PPG has the ability to make anti-graffiti powder coatings in a wide range of colors. Not only does this allow the user the flexibility to select the color they need, but it allows the applicator to get the effects of the anti-graffiti performance, without needing the clear coat—eliminating a step in application.
Given the significant time and expense involved in qualifying a product to UL requirements, Underwriters Laboratories established a "product line" evaluation process that permits a family of similarly formulated products to receive recognition as a group. The products must utilize the same "organic" components (resins, curing agents, additives), as determined by Infrared (IR) spectroscopy. However, metallics and clear coats must be tested independently.
Underwriters Laboratories' website displays basic information about recognitions, by company to whom the recognition was granted. PPG’s UL file number is MH for Greensboro and Strongsville, MH for Gainesville and MH for Brazil. Click on the link to PPG Industries' UL page for most industrial coatings.
An online cure test or end-of-line testing, is a real-time assessment method for monitoring the curing process of powder coatings during production and helps to ensure that the coatings achieve the desired properties and performance characteristics.
Film performance tests generally include physical tests, resistance tests and field tests. Specifically for thermoset powder coatings, on-line cure tests provide quick assessments that correlate the degree of chemical cure shortly after baking, indicating long-term performance. However, on-line testing should not replace initial product qualification (verification) or ongoing quality assurance processes, such as Production Part Approval Process (PPAP), daily quality control and periodic product/process validation).
Adhesion and solvent rub testing are commonly used to detect undercured powder coating films or to confirm adequate curing. Adhesion testing is always destructive, while solvent rub testing can often be non-destructive, which is particularly important depending on the economic value of the coated part. Other less commonly used tests include impact resistance, flexibility and pencil hardness.
*ASTM D (standard practice for assessing the solvent resistance of organic coatings using solvent rubs), along with other global standard test procedures, outlines methods for applying organic solvents to coating films to evaluate their resistance to exposure. Various application methods exist, including finger-applied cloth and cotton-tipped swabs, and multiple organic solvents can be used for testing.
Evaluation criteria determine whether solvent exposure significantly impacts the film’s aesthetic or functional properties.
Given the multitude of powder product formulas and rub test variables, yes. A good starting point is to perform twenty double rubs using a 75/25 volume blend of 99% Isopropanol and Xylene. The primary failure criterion is color removal on the rubbing material. Note 99% technical grade isopropanol may also be labeled as 2-propanol.
Methyl Ethyl Ketone (MEK) has been a common solvent for liquid paints, and online cure evaluations are sometimes
referred to as MEK Double Rubs testing. However, MEK is aggressive toward most powder coating films. Acetone is
similarly aggressive and can cause rapid film softening, even in fully cured films based on other physical properties. In
contrast, straight 99% isopropanol is typically not aggressive enough to differentiate between films identified by other
analytical tests as undercured or fully cured.
Thermoset powder coatings that have advanced in their chemical reactions during storage typically show changes in the cured film, including:
• Reduced flow resulting in loss of film smoothness, characterized by:
- Increased orange peel texture at standard applied film thickness
- A fine sandpaper-like texture
• Gloss reduction especially at a 20-degree angle
• Decreased adhesion to the substrate
Product quality can be effectively assessed by testing key characteristics of an aged powder sample and comparing them to the values recorded at the time of manufacture.
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Answers about Powder Coating
Ok since we have two questions about the Eastwood powder coating system, I will combine them in one answer.
The first question is about "starter sets for powder coating"...The only company that I am aware of that makes complete kits for hobbyist (homeowner) powder coating is Eastwood. I had a hand in helping them select the ovens for their program and a few other technical issues over the years. They started out buying a "powder coating gun" from a China supplier that is pretty low-tech, but gets the job done. Sears and Summitt both have homeowner guns as well, but no ovens, etc. This gun uses a TV "fly-back" transformer to charge the powder during application, a far cry from industrial grade equipment. But it does not have the $4k price tag as the industrial equipment either. Once they had a gun, they needed all the other stuff to execute the powder process. They came up with a line of cleaning chemcials, cure ovens, booths, etc. to support the hobbyist and haven't looked back. Their real money maker is the powder they sell. Average powder coatings cost between $2.50 to $5.00 per pound (except the real sexy stuff) but come in one pound to 300 pound containers (automotive and appliance manufacturers buy their powder in pound totes!). Since the average homeowner does not need that much powder, Eastwood buys regular powder in 50 pound quantities and re-packages it into small containers. The price ends up at around $50.00 per pound. Nice business model!
To perform powder coating safely you need a spray gun, powder spray booth (designed for powder), cleaning method, and cure oven. The most expensive piece is the cure oven. You need the gun to charge and atomize the powder to get it to coat and stick to the part before placing it into the oven. You need the spray booth to contain the overspray and collect it for disposal (industrial users often re-use the overspray). Without a booth, you could have an explosion. You need a cleaning system to clean the parts before your apply the powder, because any paint job is only as good as the surface you apply it to. You need the cure oven to heat the part above 250 degrees F to melt and cure the powder. Do not use your wife's cooking oven, as the next meal may not taste like you expect! Check the Eastwood website for current prices of these equipment components.
As for the question about clear coating over the silver (chrome) powder...Silver (chrome look-a-like) powders look great but have insufficient mechanical properties. They will scratch and mar easily. This is why they reccomend applying the clear over the top of the chrome powder. The clear can get cloudy, as seen in your picture, due to several issues. First, the quality of the clear powder may not be that great. Second, the thickness of the clear can affect the clarity of the coating (too much clear will get cloudy). Third, the clear can cloud up if it is under or over cured (under baked or over baked). Fourth the clear can cloud up if the oven does not have sufficient exhaust (oven contaminants).
Clears and chromes are "high risk" powder coatings, as they are very finicky to apply and still look good. Start out by buying a good clear powder. Apply it at less than 3 mils (0.003 inches) thick. Check cure by dipping a q-tip in MEK and rubbing it on the surface. The clear coating should not become sticky or tacky after 25 double rubs (one double rub is one single back & forth motion). If it does get soft, it is under cured. Finially increase your oven exhaust, if you have one, to improve the quality of the cured clear coating.
Sorry for the lengthy reply. It is just the Engineer in me comming out!
Sorry folks, but since the weather got so nice in the Northeast the last couple of days I was out in the garage working on my 89 SC and did not check the posts for this thread lately. I check my emails frequently, but did not see anything from the SCCoA forum letting me know there were a few more posts!
Following are the next group of answers to the recently posted questions:
"I'm just using a kitchen oven (not the one we cook food in) I guess doesn't have much exhaust. A fan does kick on on it sometimes. Maybe I should figure a manual override for the fan?"
A kitchen oven does not have much exhaust at all, as it is designed to keep heat in and smoke out of your kitchen! Paint curing ovens on the otherhand have a heat source (gas burner or electric element), a circulation fan (convection heat type oven), and an exhaust fan. The heat source provides the heat energy, the circulation fan delivers the heat energy to the part, and the exhaust fan removes the by-product of combustion (natural gas only) and the by-products of powder cure. Having an oven with an inadequate heat source, no circulation fan, and inadequate exhaust will combine to cause numerous problems.
I recently answered a similar question for my Powder Coating magazine column last week:
Question: " i have been trying to powder coat a set of 350 chevy heads and when i shoot the heads with wet black powder and then cook them the powder is not flowing out it comes out as if it was never cooked. how can i get the parts to flow out. is it that the heads are soaking up the powder. please help me with this. i am baking at 500 degrees thank you for your help chris"
Answer: "Chris:
The 350 C.I.D. Chevy motor is very reliable. I had one in several vehicles over the year. However, I prefer the horsepower or the higher revving 327 C.I.D.
The description of the problem you are having tell me that you are not heating the cylinder heads to a point where the powder coating even melts, yet alone cures. If the powder on the part looks the same when you take it from the oven as when you put it in the oven (still a powder and not a coating), then it is time to evaluate what you are using to heat your part.
This problem has to do with the weight (mass) of the cylinder heads and the energy capacity of your heat source. I have lifted my share of Chevy heads over the years and from my recollection they weigh about 100 pounds. Considering that the cylinder head is made from steel and has a specific heat of 0.125 BTU per pound it will take 4,375 BTUs to head one cylinder head to 350 degrees F (the average cure temperature for powder coating). If you put this cylinder head into an oven whose heat source has an energy capacity of 1,000 BTUs per hour, it will take 4.375 hours to get the cylinder head up to temperature and an additional 25 minutes to fully cure the powder coating. That is almost five hours of “baking time”! However, if you use an oven with a heat source that has an energy capacity of 10,000 BTUs per hour it will take less than an hour. This relationship between the energy capacity of heat sources and cure time can easily be related to horsepower and speed; “The more you have the faster you will go!”.
I frequently tell my clients that the size of their oven burners determines how fast their products will achieve the desired powder cure temperature. I often tell them that you can cure a 10,000 pound part using a cigarette lighter, it will just take a couple of hundred years. The same goes for you. If you are trying to cure your cylinder heads in a toaster oven, then I hope you are very patient, as it will take quite a while to melt, flow, and cure the powder. Remember the old racer’s adage: “Go big or go home”. Of course, they were talking about engines (horsepower), but you get the point."
Powder Coating Safety
Makes sense.
Oh yeah, and whats gonna blow up if you don't have a booth?
Powder coatings are "organic" in nature and, as susch, will combust if mixed with the right amount of air. The same thing applies to any organic dust, such as baking flour, sugar, etc. I am sure you have heard of tragic cases where a sugar mill or flour mill had an explosion. Well powder coating materials are just as dangerous! Believe me, as I have investgated numerous powder coating lines that had fires and explosions, some of which had fatalities!
The risk with powder coating is dramitically lower than using solvent type liquid paints (i.e. laquers, etc.). However, that does not mean that they are not dangerous in their own right.
Powder coatings will not combust (burn) in the container as there is too much powder (fuel) and not enough air. Same goes for the opposite condition, where a small amount of powder is mixed with a lot of air, the powder will not combust either. The problem occurs when powder coatings are atomized with just the right amount of air. This "just-right" mixture is between the "lower explosion limit" (LEL) and the "maximum explosion limit" (MEL) and is often the exact mixture of powder and air at the gun applicator tip. If a source of ignition is present with this "perfect mixture" the powder cloud will ignite! The result is a fireball that releases a tremendous amount of energy. If this energy is expended in a small area (what is called containment) the result is an explosion.
It is just like your SC engine works: gasoline is mixed with air and ignited in a contained area (the combustion chamber) and the resultant explosion sends the piston in the opposite direction. Ignite the same gasoline/air mixture in an open cup, and a fireball will happen, but no explosion since there is insufficient containment .
Most explosions happen when powder coating ungrounded (or improperly grounded) parts. The part will absorb some of the electrostatic energy used to charge the powder (so that it sticks to the part before curing). If the part is improperly grounded (more than one megohm = one million ohms resistance), the part will eventually become saturated with electrostatic energy and discharge to the closest ground (typically the gun tip). This results in an arc (ignition source) much like when you discharge static electricity in the wintertime after shuffleing your feet on a carpet and touch the light switch. The ignition source and the powder/air cloud cause a fireball at the end of the gun, the person spraying the powder usually screams and drops the gun, and the fire goes out harmlessly as the gun trigger is released stopping the power/air from fueling the fire. This event usually requires a change of clothes for the powder sprayer, at least their pants anyways!!!
However, if the person has been spraying a lot of powder in an enclosed garage, without ventilation, and there is a large cloud of power, then the next sound they hear is St. Peter asking them what they did back on earth to get into Heaven! The large powder cloud provides a significant energy source or fuel and air and the garage provides the containment...a very bad combination. The energy released from such an explosion is awesome. I have seen 40 feet of cinderblock wall moved 20 feet in a powder explosion!
Now that I scared the Sh-t out of you, it is time to bring you back to reality. Powder coating is the safest method of painting, except for using laytex paint and a brush. Follow these important rules for safe powder coating:
1. Spray powder coatings in a powder coating booth that is designed with proper airflow. This ensures that there is not enough powder and too much air to have a combustion fireball (except right at the gun tip).
2. Always coat parts that are properly grounded. Use a ground wire attached to an electrical ground or cold water pipe at one end and attached to the part at the other end.
3. Eliminate all sources of ignition during spray operations. No smoking, no welding, no grinding, etc.
4. Cover all electrical devices within five feet of the spray area with air tight bags. Actually code requirements call for "dust tight explosion proof" electrical devices in this area, but they are very expensive.
5. Always wear a dust mask and safety glasses to protect your health when powder coating.
Powder Coating Wheels
If the center caps are the aluminum ones you can powder them. The earlier wheels have metal centers, right?
Yeah you will want to strip the wheels to bare metal.
Here are my recommendations for having your Aluminum wheels powder coated:
1. Remove the existing coating by either chemical stripping or media blasting. Do not use thermal stripping methods (burn-off oven) as the + degree heat will anneal (soften) the aluminum. NEVER USE THERMAL STRIPPING METHODS ON MAGNESIUM WHEELS, AS A FIRE WILL OCCUR THAT THE FIRE DEPARTMENT WILL HAVE GREAT DIFFICULTY EXTINGUISHING (AS WATER APPLIED TO BURNING MAGNESIUM WILL EXPLODE)! Acceptable blast media is Aluminum Oxide, CO2, or Plastic. Do not use steel based media, as it will start corrosion sites under the coating (galvanic reaction with the aluminum). Sand as a blast media has all but been outlawed (silicosis health problems).
2. Ask your powder coater if they can apply a chromate conversion coating to the aluminum wheels before powder coating. Sometimes reffered to as Alodine, this conversion coating will increase the service life of the coating by a factor of 5, or more.
3. Select either a TGIC polyester or Acrylic powder for your color, as these are the formulations that the original manufacturer used and are best suited for this application.
4. Consider using a clear coating over the color, for a better "depth of finish" and better wear life.
5. Instruct your powder coater to cure the coatings on the wheels at a temperature below 325 degrees F. This will ensure that you do not anneal the temper (hardness) of the aluminum. They will have to cure the wheels longer at this lower temperature, but it is the safe way to go!
This recipe will provide wheels that look great, will last a long time, and will not change the metalurgical charateristics of the wheel.