Fire Science
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For intumescent coatings durability is an important aspect. Due to ageing, which is caused by climatic conditions, the thermal protection performance of intumescent coatings can be reduced during the working life. The national German and the European procedures to assess the durability of intumescent coatings presume a working life of 10 years. For a longer period, additional investigations are required. However, the European assessment procedure does not provide specific information how to prove a durability of more than 10 years. In addition to this formal reason, also from a scientifical position, it is necessary to investigate the degradation of intumescent coatings in detail to propose a corresponding test concept for a durability of more than 10 years.
The presentation summarises the existing knowledge about the degradation behaviour and ageing of intumescent coatings. Furthermore, the results of different approaches from a recently conducted research project are presented. Starting from a water-based and an epoxy-based guide formulation, in a first approach a degradation of the intumescent coating was carried out directly during the production process, i.e. by reducing the concentration of functionally relevant chemical components. In another approach, the duration of the short-term weathering tests was significantly extended, i.e. by multiple repetition of the artificial weathering. The thermal protection performance of the intumescent coating was assessed by fire tests and tests with electrical heating source. In addition, small-scale tests were carried out to determine the thermal material properties of the intumescent coating and thermo-analytical methods were applied to characterise the degradation. Derived from this knowledge, possibilities to test and assess a working life for a period of more than 10 years are proposed.
Intumescent coatings for fire protection offer advantages over (non-intumescent) cementitious coatings and boards regarding speed of construction, architectural aesthetics, sometimes costs, and other features. However, conventional organic intumescent coatings as well as soluble silicate (waterglass) coatings form foams with low mechanical stability, and the latter coatings generally suffer from low resistance against humidity. Therefore, the search for novel intumescent coatings for more demanding conditions (e.g., abrasive environments) is a necessity in the context of steadily increasing requirements of society and industry.
In this contribution, we present results on intumescent aluminosilicate coatings for fire protection that form foams with significantly increased mechanical strength. Two base formulations, a metakaolin/silica-based mix, adapted from Krivenko et al., and a silica/corundum-based mix, developed at Curtin University, as well as formulations modified with additives (Al(OH)3, Mg(OH)2, B2O3, Na2B4O7), were applied to steel plates (75 mm × 75 mm) and exposed to simulated fire conditions (fire curve according to ISO 834-1:1999). Temperature-time curves were recorded to assess the degree at which the coatings insulated the substrate. In addition, XRD, TG, oscillatory rheometry, and SEM were employed to characterise the coatings.
The coatings were observed to partly expand during hardening due to H2 formation. When the hardened coatings were exposed to elevated temperatures they intumesced as expected, with the degree and nature of expansion dependent on the formulation. Oscillatory rheometry provided insights into the intumescent processes in an apparently brittle material. It revealed that the hardened aluminosilicate coatings became viscous (loss factor > 1) at 75–225 °C, in the temperature range of major water release, as opposed to a “standard” metakaolin-based geopolymer, which continued to behave as a solid. This explains the intumes¬cent behavior of the coatings, i.e. further expansion and foam formation. Microstructural analysis confirmed pore expansion and coalescence; XRD showed that the phases formed after heating (max. temperature ~840 °C) were of ceramic-type.
The fire protection (defined here as the time for the steel substrate to reach the critical temperature of 500 °C) depended mainly on the thick¬ness of the fully expanded coating, i.e. after intumescence. An alumino¬silicate coating free of additives with an original thickness of 12 mm was able to protect the steel for >30 min. The addition of 10 % anhydrous borax (Na2B4O7) caused a significant improvement, such that an original coating thickness of only 6 mm was sufficient to protect the steel for ~30 min. This was caused by the formation of sodium metaborate dihydrate (NaB(OH)4) in the coating that led to a significantly extended dehydration plateau in the temperature-time curve at ~100 °C during the fire exposure.