Characterization and modification of pore size distributions and defects in epoxy intumescent coating chars

Intumescent coatings are widely used to protect steel structures from fire by forming a thermally insulating char layer when exposed to high temperatures. When these coating are exposed to high temperatures, they expand and generate a porous residue whose protective performance is strongly influenced by its internal pore morphology. This thesis focuses on the structural analysis of the resulting char, aiming to develop a method for the quantitative characterization of its internal morphology, to evaluate the coating’s behaviour under varying conditions, and to investigate the mechanisms governing its growth during fire exposure.

The project began with a literature review highlighting the importance of pore morphology in the insulation performance of porous media such as intumescent chars. Findings from related fields such as porous ceramics, metallic foams, and polymer foams suggest that, for an optimal thermal insulation against fire, porosities above 90% are required, and pore sizes should range from 0.1 to 5 μm, or at least not exceeding 4 mm. Additionally, features such as crack-free surfaces, elliptical pores, and highly tortuous structures can further enhance the thermal protection provided by the char.

The next stage of the project involved the development of characterization methods that can reliably quantify the pore structure of intumescent chars. For this purpose, a new method named SNOWPOROS was developed. Built upon existing pore-identification algorithms, this approach enabled an automatic identification of pores, which allowed an objective comparison across different samples. SNOWPOROS proved to be particularly suitable for elliptical pores, commonly found in intumescent chars.

The method was used to study how heating conditions affect char morphology in two commercial epoxy-based intumescent coatings. The fire residues showed a stratified structure, with average pore sizes ranging from 3.7 to 1.8 mm in the upper layers and 0.3 to 0.2 mm in the lower layers. When the coatings were exposed to fast heating rates, the pores in the upper layers tended to grow less. An opposite phenomenon was found at the lower layers, underscoring the complexity of the materials and their sensitivity to external thermal conditions.

To further examine the influence of external conditions on char morphology, the effect of the testing methods was investigated. Two lab-scale setups and one industrial-scale furnace were used. In the latter equipment, different steel shapes were tested, including plates and H-columns. Chars formed on the outer flanges of the H-column exhibited similar porosities and pore sizes (92.7–95.1% and 2.0–2.9 mm, respectively) to those obtained in lab-scale setups. However, chars in the web areas, which were partially shielded from radiation, showed greater expansion and larger pores, with an average diameter of 4.2 mm. The analysis also considered atmospheric conditions: samples exposed to oxygen-rich environments developed larger pores and surface cracks compared to those formed under nitrogen.

Char development and pore growth during a fire test were studied. A boron-free epoxy coating was tested for this study. It was observed that, during the first 4 minutes of the heat exposure test, relatively large pores appeared, including a large internal void with an average size of 34 and 19 mm. Nevertheless, after 6 minutes, a compact layer with pore sizes between 100 and 58 μm began to emerge. The results revealed evidence suggesting that a possible disruption in the intumescent reaction sequence contributed to the observed variations in the foaming. Finally, the distribution of fillers in the final char was evaluated with SEM-EDS, revealing a uniform distribution of fillers across all the layers that constitute the char.

In summary, this thesis introduces a new visualization tool for mapping the pore structure of intumescent coatings, enabling the study of their morphology under varying environmental conditions and at different stages of the expansion process. The findings are expected to enhance understanding of the swelling process and char formation in intumescent coatings, contributing to the development of better fire-protection materials.