Fire and heat can severely damage materials and structures. Specially formulated coatings can moderate fire damage in two ways, fire resistance and fire retardancy. The difference between these two is sometimes difficult to distinguish, and a single coating may have both properties in varying degrees. A crucial distinction comes down to whether or not the object still serves its intended purpose after fire exposure. An example of a fire-resistant coating would be on the inside of a grill or oven, which is exposed to heat or flames at regular intervals and must remain functional. Coatings used to protect structural steel in a building from fire damage during a one-off event would be considered fire retardants.
Since fire retardant coatings are designed to protect the substrate through a single event, the structure or object would need to be rebuilt or refurbished before returning to service. Properly formulated and applied fire-resistant coatings allow for routine fire exposure within design limits. These design limits are determined by formulation factors that are tailored for the expected service conditions. The proper combinations of polymer binder, extender and pigmentation are required to achieve fire resistance in a coating.
Before discussing formulation, we need to understand the customers’ expected service conditions. Obviously, the degree of fire resistance required for a consumer-oriented barbecue grill can be very different from those for an industrial heat-treating oven. Service conditions, including temperature, exposure duration, fuel source and ambient atmosphere, all factor into the formulation equation. The coating must also stand up to any corrosive combustion products generated during use. Accurate information describing the service conditions and customers’ expectations of performance under those conditions are required before formulating any prototype coatings. Once these parameters are confirmed, the formulation process can begin.
As with most formulation development work, the process begins with selection of candidate polymeric binder systems. The maximum service temperature and duration of exposure are the primary determinant factors when considering possible base polymers. Binder systems must be able to maintain adhesion to the substrate and film integrity under the expected conditions encountered during use. Resistance to blistering and charring are also essential. A variety of organic, inorganic and hybrid binder systems are appropriate as the basis of fire-resistant coatings. Technical data sheets for candidates should clearly state the performance limits of the resin.
Silicone modified resins of various types are common starting points to formulate heat-resistant coatings. Alkyd resins containing silicone functional groups were one of the earliest fire-resistant coatings binders. Where allowed by VOC regulations, they are still excellent for many lighter-duty applications. These polymers may be suitable for applications where service temperatures reach up to 370 degrees Fahrenheit. Alkyd resins may also have the ability to cure at ambient temperatures; this facilitates the coating of objects too large to bake. Resistance to higher temperatures can be achieved using binders such as phenyl polysiloxane resins.
The choice of pigments and extenders is also critical to performance and will be dependent on the service conditions. Proper selections can enhance the performance properties of the base resin system, allowing use under more severe service conditions. Inorganic pigments are generally preferred since the heat stability of most organic pigments is limited. Inorganic pigments vary in heat resistance but ranges of 300 to 800 degrees Celsius are common. Hydrated extenders should be avoided since they can expel the water of hydration under high temperatures and cause film defects such as pinholes or blisters. Coatings for very high service temperatures are sometimes formulated with extenders that can sinter or fuse together to retain a ceramic coating even if the polymer binder system is completely removed.
Since fire-resistant coatings are for specialized applications, they may be regulated differently than products for other markets such as architectural coatings. Generally, higher VOC limits are allowed for high temperature-resistant coatings. For example, under the aerospace coating rules in the South Coast Air Quality Management District, high-temperature coatings are allowed to contain up to 850 grams per liter of Volatile Organic Compounds. But the currently lenient regulatory environment cannot predict what the future might hold. In anticipation of tougher future VOC regulations, coatings material suppliers are beginning to offer waterborne binder systems capable of performing under high-temperature conditions.
High temperature and fire-resistant coatings provide protection to substrates under demanding conditions. This allows our barbecue grills, appliances and vehicles to operate at high temperatures without damage from corrosion and fire. Formulating these products is a specialized skill that is hard to discuss in-depth in a short article. Material databases such as UL Prospector® or your suppliers’ technical sales representatives are an important resource if your goal is to develop successful fire-resistant formulations.
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