Reactive Coatings Containing GrafGuard® Expandable Graphite Provide Fire Protection
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Coatings are being required to do a lot more than to simply protect the substrates they cover. Coating formulators constantly strive to enhance value by creating multifunctional products incorporating materials that form reactive coatings which transform when activated by specific stimuli.
For fire suppression, the premier intumescent reactive material is GrafGuard® Expandable Graphite from NeoGraf Solutions™, LLC in Lakewood, Ohio. GrafGuard® has been used as a reliable and high performing intumescent material for flame retardant applications for more than twenty-five years.
Fire suppression coatings are very diverse, they vary based on application; from structural steel support members to engineered wood beams to polymer water supply tubing to drop down ceiling tiles. The substrates that need to be protected are as varied as the applications, therefore the reactive coatings need to be compatible with many polymeric coating systems. The end result of these coatings are to maximize the safe-exit-time for the affected people in the path of a fire, while adhering to all the regulations and requirements of the geographical region. The reactive coatings can also reduce loss of property due to fire and smoke damage.
Reactive coatings are often a composite of materials that have very specific functions individually and in most instances also provide functionality due to their interactions. GrafGuard® can be used in multiple layers of the composite, or simply as the reactive substance in the fire protective layer. The versatility of GrafGuard® is attested by the various polymeric coating systems that it has been incorporated, nearly all existing premier coatings solutions have used this graphite based intumescent coating including epoxies, latex, silicones, siloxanes, and polyurethanes.
A typical fire protection reactive coating may consist of a single component or the coating may be more efficient with multiple different fire retardant materials. The synergistic effects of GrafGuard® combined with metal hydroxides, phosphorous compounds, and zinc borate have been observed.
The width and breadth of applications for GrafGuard® is not just limited to coatings. GrafGuard® has been incorporated into polyurethane, latex (natural and synthetic), ethylene propylene diene monomer rubber, polystyrene, and high impact polystyrene in the forms of sheets and molded foams. The use is not limited to coatings and foams as GrafGuard® can be used in mastic adhesive materials, bituminous roofing sealing products, epoxies, polypropylene (low, medium, and high density), polyethylene, silicones, siloxanes, thermoplastic polyurethanes, and polyvinyl chloride. The GrafGuard® products may also be introduced directly into some building materials such as oriented strand-board, particle board, sheet rock, and cement board.
Upon exposure to high temperatures, GrafGuard® expands and forms a graphite char that is more fire resistant than other carbon chars that are formed from typical chemical intumescent materials. A carbon char is a protective carbon layer that effectively shields the “fuel” from the fire. GrafGuard® contains no halogenated additives and is manufactured without hazardous heavy metals like lead or chromium, which may be found in other expandable graphite flakes.Through a proprietary processing technique, GrafGuard® is manufactured by placing an intercalant between the layers of the graphite flake. When exposed to elevated temperatures as low as 160°C, the intercalant decomposes to a gas which forcefully expands the graphite flake in the “Z” direction separating the graphite layer planes apart forming a highly effective carbon char layer. This resulting char protects the reactive coated substrate from the heat of the fire and minimizes the smoke generated from the burning of the substrate.
As a rough rule of thumb, it is recommended that the minimum thickness for the GrafGuard® containing layer be around 200 µm. This is due to the fact that the graphite flakes have a nominal thickness of around 35 µm and to be most effective, multiple flakes need to be incorporated into the graphite char layer that is formed. This will allow the expanded graphite to knit together as the char layer is formed and generate a barrier-effect that keeps the heat away from the rest of the system and the substrate. This process is demonstrated by Figure 2.
The nomenclature for the GrafGuard® expandable graphite products is: GG 160-50N. This is a representative name describing the on-set temperature (°C) – particle size (US mesh) – and surface chemistry (A, N, or B, corresponding to Acidic, Neutral, or Basic).
On-set temperature is defined as the temperature at which the graphite material begins to expand in an irreversible form. All aspects of processing including curing or post-curing of the coating must be considered when choosing the appropriate grade of GrafGuard® for the application, to prevent expansion of the graphite flake during the processing. The on-set temperatures can be controlled from 160°C to around 250°C, depending upon the grade of the GrafGuard® material. To measure the on-set temperature a TA Instruments Q400 TMA (Thermal Mechanical Analyzer) is equipped with the expansion probe which measures the change in thickness of a known volume of material within the fixture. The fixture comprises of a 3.0 mm diameter and 26 mm deep cylinder, and is capped with a 2.8 mm diameter plunger that rides on top of the sample.
The fixture is then automatically raised into the heating chamber, to precisely heat the sample at a fixed rate. For the on-set temperature test, a ramp rate of 10°C/minute up to 400°C is used. The displacement of the expansion probe is measured in microns and recorded reflecting the volumetric increase as a response to the temperature in the heating chamber. In this test, 25 mg of the graphite flake was compacted into the testing fixture by the graphite plunger. Machined fine-grain graphite is used for the fixture and plunger to minimize any CTE effects that could cause additional sources for variability in expansion rates. Figure 3 displays the TMA curves of three different grades of GrafGuard® materials (GG 160-50N, 180-60N, and 200-100N). Notice how the shapes of the curve are similar, but the on-set temperatures are different.
Flake size is the next specified feature of the GrafGuard®. In general, the larger the flake the more expansion can be expected. Flake size is an important characteristic to consider when making a coating since the flakes may be visible and possibly alter the texture of the coating. Application techniques will also effect the texture of the coating by aligning the flakes to the substrate allowing for the thinnest possible coating.
Surface treatment of the GrafGuard® flakes is used to make the flakes more miscible in the polymer system. Depending upon the system, the acidic active sites on the surface of the GrafGuard® may allow for ionic suspension of the flakes, while the same groups could cause the polymer to aggregate around the flakes causing a non-homogeneous system. The flakes are typically neutralized to a pH around seven, but can be altered to have a lower or higher pH depending upon the sensitivity and characteristics of the coating system.
By David Stuart