Performance studies and char characterizations of hydrocarbon intumescent coatings

Hydrocarbon intumescent coatings are an efficient way to protect steel structures during fires in high-risk environments such as offshore oil rigs. The fire-resistance performance evaluation of a hydrocarbon intumescent coating, according to standard hydrocarbon fire test methods (e.g. UL 1709), is typically conducted using industrial furnaces, because regular laboratory equipment, such as Bunsen burners, gas lamps, or cone calorimeters, cannot provide the required fast heating curves.

These constraints of experimental facilities have greatly limited the understanding of the behavior of hydrocarbon intumescent coatings at conditions of industrial relevance. Consequently, in the present PhD project it is has been one of the goals to develop a laboratory-scale setup that can simulate the standard fire test curve UL 1709.

Using this setup, a mapping of the performance of hydrocarbon intumescent coatings as a function of the concentration of coating ingredients has been undertaken. The first ingredient studied was zinc borate and the fire-resistance experiments showed that the best performance (i.e. longest critical times of coated steel plates to reach 400 and 550 oC) was obtained when using a hydrocarbon intumescent coating with 15 wt.% zinc borate.

Due to the strong influence of zinc borate on the rheological behavior of the coatings, increasing the level of zinc borate resulted in a more uniform, but less expanded char layer. The compositional profiles of the char layers indicated that the yield of phosphates (especially BPO4) was enhanced by raising the zinc borate content.

Analyses on thermal degradation of the coatings suggested that zinc borate can improve the thermal stability of the epoxy binder and the anti-oxidation properties of the char layer. Before proceeding the investigation further to other ingredients, the initial laboratory-scale setup was upgraded with better insulation to the backside of steel plates to give a more clear distinction between different performances of the coatings.

The reliability of the upgraded version was evaluated by comparing its performance in assessing five selected hydrocarbon intumescent coatings to those exposed to an industrial fire test furnace. A good agreement between these two setups was found for the temperature responses of the coated steel plates, and the physical appearance (relative expansion and morphological structure) and chemical composition (crystalline phases) of the char layers.

With the confirmed reliability, the upgraded laboratory-scale furnace was applied to study the effects of various ingredients (ammonium polyphosphate, melamine, titanium dioxide, calcium carbonate, and vitreous silicate fiber) on the performance of selected zinc borate (ZB)-containing and ZB-free intumescent coatings.

The results showed that increasing the content of ammonium polyphosphate (APP) or decreasing the content of melamine (MEL) in the ZB-containing coatings generally enhanced the critical times of the steel substrates and the physical appearance of the chars. Among the investigated formulations, the coatings with 25 wt.% APP or 5 wt.% MEL obtained the best performance.

With respect to titanium dioxide (TiO2), calcium carbonate (CaCO3), and vitreous silicate fiber, varying their levels barely had an effect on the performance of the ZB-containing coatings, but the critical times and char appearance were strongly affected for the ZB-free intumescent coatings. The ZB-free coating with 1.5 wt.% TiO2, 2.5 wt.% CaCO3, or 5 wt.% fiber had the longest critical times.

Taking advantage of a pool of experimental data from the ingredient studies, the relationship between the char properties and the critical times was further explored. A strong correlation was found between the critical time and a so-called integrated parameter, which combines the relative expansion and the insulation efficiencies of the three distinct char phases observed (i.e. sponge-like, macroporous and compact).

The dynamic viscosity minimums of the corresponding coatings were measured and showed an exponential relationship with the physical appearance of the chars. It suggests that the performance of a hydrocarbon intumescent coating may be controlled by manipulating the rheological behavior of the coating, owing to the correlations between these two properties with the integrated parameter.

In summary, the present thesis study has proposed a promising laboratory-scale for investigation of hydrocarbon intumescent coatings at conditions of industrial relevance. Moreover, the ingredient studies contribute to an understanding of the fundamental properties of hydrocarbon intumescent coatings and provide a scientific basis for formulation optimization or replacement of compounds with health risks.