The high temperature components of a motorized vehicle are frequently made of metals that corrode if unprotected. Corrosion can weaken a part to the point of failure and shorten the lifespan of the product. A failure can cause a dangerous situation if it happens while a vehicle is in motion. A protective coating shields the part from corrosion and adds aesthetic value. The coating is considered high temperature if it is proven to withstand the heat, expansion, and contraction with the substrate as the temperature changes.
The extra challenge in extreme machines is the multiple environmental factors a machine could encounter when it leaves the garage: UV light, water, salts, snow, ice, hail, mud, gravel, oils, and grime, among other hazards. Take, for example, a day in the life of a dirt bike. It might be exposed to UV light, be pummeled by abrasive mud and rock, and reach peak temperatures during operation while being simultaneously submerged in glacier-fed mountain streams. It goes through multiple heat-and-quench cycles and has potential impact damage waiting around every curve.
This is what these machines are designed to do, and as such, their coatings are expected to match the performance demands. Extend the environmental factors to aerospace rocket boosters—one of the most extreme uses for high temperature powder coatings—and one can imagine the unpredictable environmental conditions for which a research chemist is asked to engineer.
Not all coatings must pass every test or survive every possible temperature. The coating formulation diverges based on the specific parts for which the coating is intended. For example, parts hidden under the hood won’t require UV light resistance but may need extra solvent and chemical resistance because of the cleaning and maintenance products used on them in the shop. In contrast, parts near or on the muffler exposed to the outside elements will need UV protection as well as salt and impact resistance. Some requirements can be considered universal. All parts on a motorized vehicle have the potential to be splashed with water while hot (“quenched”). Therefore, a high temperature coating must maintain its integrity while the temperature of both the coating and substrate changes dramatically and abruptly during quenching.
For visible parts, vehicle designers may also add aesthetic requirements. Appearance on the showroom floor helps bolster the notion of quality in the consumer’s mind. While most consumers would not give much thought to the color of their vehicle’s functional parts such as its muffler, if the appearance of the part changes noticeably with normal use, the consumer may become concerned. Even if a coating’s function and performance is not compromised by fluctuations in gloss or color, the consumer may think these aesthetic changes indicate compromised performance. Consistency of appearance throughout a product’s lifespan helps boost consumer confidence in the product and brand.
How Do High Temperature Coatings Work?
Silicone resins are the primary technology used to maintain film integrity of a coating even at very high operating temperatures. Other key chemicals are robust pigments which do not change their color or burn off during heating, appropriate additives, catalysts, and anti-corrosive pigments where necessary. The silicon resin’s superior performance at high temperatures is due to the silicon-oxygen bond—known as a siloxane bond—which is extremely strong relative to other bonds. The heat required to degrade the bond is far in excess of most coating operating temperatures. The siloxane bond is about 25 percent stronger than a carbon-carbon bond found in organic resin structures. This is why silicone-based coatings can be used at much higher temperatures than typical coating types.
When silicone-based coatings cure, they tend to form very highly cross-linked structures. These types of structures are generally very hard, chemical resistant, heat resistant, weathering resistant, and light-degradation resistant. This leads to better maintenance of gloss with light exposure than most alternative organic coatings. Electrical insulative properties are another aspect of silicone-based coatings.
A trade-off with high-hardness coatings is brittleness, especially when shocked with an impact or very rapid temperature change. Those cured coatings with the highest cross-link density are the most sensitive to thermal shock and are the least flexible. Rapid changes in temperature and traveling in highly abrasive landscapes is to be expected for motorized vehicles. As they were designed for this purpose, the manufacturers and users of these vehicles expect all parts to be up to the challenge. Since the cross-linking structure of high temperature resins tends towards brittleness, the coatings chemist must formulate to minimize undesired effects. Very high molecular weight resins can provide a very hard yet tough coating that is crack resistant. However, this new technology is just beginning to enter the market.
Like all coating resin families, silicone resins come in a variety of “flavors.” There are different types of functional groups, which are small organic molecule sections attached to a silicone atom somewhere along the resin backbone structure. In powder coatings the options are a little more staid than in liquids since the resins must be solid at room temperature due to the nature of powder coatings. They can be “methyl type,” in which the functional group consists of a carbon bonded with three hydrogens, or “phenyl type,” in which the functional group is a ring of six carbon atoms, five of which are attached to a hydrogen atom each. These two functional groups lead to different burn characteristics. Mixed type with both methyl and phenyl groups are not uncommon. In addition, some variation is available in backbone length (associated with flexibility), alternate functional groups, and functionality (the number of functional groups per molecule available to bond with nearby resin molecules during the curing process).
When selecting which resin to use, a formulator considers the temperature requirement of the final application, the substrate and the preparation it is expected to undergo, coating property requirements along the lines of gloss, texture, impact resistance, chemical resistance, and other key metrics, and then researches a resin or blend of resins that balances all the requirements.
When heated, methyl groups tend to leave the cured resin structure between about 400 and 550 degrees Celsius (750 and 1020 degrees Fahrenheit). Phenyl groups tend to leave between about 550 and 700 degrees Celsius (1020 and 1290 degrees Fahrenheit). Interestingly, phenyl type resins are typically used for medium temperature coatings; not only because they are more compatible with organic resins that are likely co-ingredients for lower temperature applications —around 300 degrees Celsius (570 degrees Fahrenheit) and below—but because they last ten times longer than methyl groups at 250 degrees Celsius (480 degrees Fahrenheit). However, at higher temperatures they actually burn off more. That is, there is a greater mass loss of phenyl type resins at higher temperatures than methyl types, and the coatings have poorer long-term high temperature performance. Methyl type resins leave behind a residual ash of higher SiO2 (silicon dioxide) amount after “burning off” for some time at 500 degrees Celsius (930 degrees Fahrenheit). Because of higher residual ash amount, methyl type resins have higher film integrity than phenyl types at very high temperatures. They are therefore most desirable as the majority resin type for high temperature applications. Methyl types also tend to have a higher hardness value than methyl/phenyl or phenyl types.
Substrate condition plays a role in many of the properties of these coatings. Abrasive-blasted substrate increases post burn adhesion and cleans a surface of greases admirably. If substrate preparation instead consists of washing and pretreatment application, it is important to check the high temperature performance of the coating over the pretreatment selected. Pretreatments inappropriate for high temperature applications will fail and take the coating with them. Testing the actual system to be used is key. High temperature coatings tend to be sensitive to surface soils primarily due to the effects of high temperatures themselves. Adhesion in soiled areas will always be suspect if patchy failures are noted in coatings of this type.
The Powder Coater’s Cheat Sheet for Success with High Temperature Powder Coatings
High temperature powder coatings have a few peculiarities applicators should keep in mind.
• Storage temperature matters: High temperature coatings tend to be more reactive than standard coatings. Some are very stable against a few hot days, while others will be adversely affected. Storage at elevated temperatures can make them sinter (stick together in persistent clumps even after vibration). Once the powder has sintered there is no reversing the damage. If the sintered clumps are sifted out, the remaining powder may still be used, but one must conduct full temperature testing of the resulting cured coating on test pieces. Post-burn adhesion failure (following the peak burn schedule listed by the manufacturer) would indicate the powder is unusable.
• This goes for shipping too: If ordering and receiving high temperature powder coatings during the summer, it is a good idea to ask about the supplier’s special shipping arrangements. Also ensure that one’s own receiving department can immediately store the powder in an appropriately cool space. An air conditioner in a plastic-sheeted rack can be enough protection in many cases.
• Test the coating: For a new high temperature coating job, working with your supplier can save costly mistakes. Coat a few pieces or send the substrate to the supplier and arrange to have them test the pieces under the actual application conditions. Testing at the beginning of the process can clarify the cure conditions needed, the minimum required surface prep for the job, and other factors.
• A little goes a long way: For best results, apply high temperature powder coatings using the film build recommended by the supplier. Applying too thick can result in post-burn adhesion failures, mud-cracking, or blisters due to the high cross-link density and resulting lack of flexibility.
• One coat will do it: High temperature coatings do best when only one coat is applied.
High temperature powder coatings open up a wide swath of applications for coating applicators and end users. Their benefits include the ability to maintain protection for valuable assets even under high temperatures, thermal cycling, sunlight, humidity, and other degrading effects. These properties make silicone resin-based high temperature coatings a valuable component of the automotive industry’s suite of tools for providing value far into the future.
Sasha Tavenner Kruger, Ph.D., is a research chemist at Forrest Technical Coatings.