A Facile Technique to Extract the Cross-Sectional Structure of Brittle Porous Chars from Intumescent Coatings

Abstract

Intumescent coatings are part of passive fire protection systems. In case of fire, they expand under thermal stimuli and reduce heat transfer rates. Their expansion mechanisms are more or less recognized, but the fire testing data shall be interpreted as function of coating morphology. Expansion ratios are examined together with the inner structures of specimens submitted to fire. Bare cutting techniques damage the highly porous and fibrous specimens because they become very crumbly due to charring. So far, absorption contrasted X-ray computed microtomography (CT) was used as a non-destructive technique. Nevertheless, access to X-ray platforms can be relatively expensive and scarce for regular use. Also, it has some drawbacks for carbon rich specimens strongly adhering on steel substrates because it leads sometimes to noisy images and lost data due to resolution limits on specimens reaching ten of centimeters. Therefore, we propose an inexpensive and more accessible experimental approach to observe those specimens with minimized structural damage under visible lighting. To that end, charred specimens were casted into pigmented epoxy resin. After surface treatments, color contrasted cross-sections could be observed under optical digital microscopy thanks to high level of interconnectivity of pores. Subsequent image treatments confirmed that the structural integrity was kept when compared to previous CT data. The proposed method is practical, cheaper and more accessible for the quantitative assessment of inner structure of charred brittle specimens.

Quantitative Characterization of Highly Porous Structures with Fluorescence Microscopy and Microcomputed Tomography

“…Kang et al 25,26 calculated char porosities from their CT data. However, the calculated values were generally below 50% (similar values were obtained by others 20,21 ). It should be noted that the investigated chars expanded by a factor of approx.…”

“…Difficulties in analyzing intumescent char structures with CT were previously reported and explained by the low atomic number of the carbonaceous material and potential air voids below the CT resolution limit within the solid phase. 20,22,23,26 A common procedure to improve the noisy and low-contrast CT data would be to modify the brightness or apply some kind of contrast enhancement or noise reduction filter. 8,26 Subsequently, image thresholding would lead to the binary image.…”

“…17 Simple light microscopes or cameras were used as well to look at char cross sections but again only in a qualitative sense. 6,7,28 A notable development was introduced by Okyay et al, 20,21 who impregnated their chars with a pigmented epoxy resin. The advantages were an increased contrast between the colored epoxy and black char when viewed under a light microscope and that smooth cross sections even of fragile materials can be prepared.…”

“…The advantages were an increased contrast between the colored epoxy and black char when viewed under a light microscope and that smooth cross sections even of fragile materials can be prepared. 20,21 However, the reported method led to some large air bubbles in the structure and the contrast between the two phases (i.e., pores and solid) were not ideal (red pigment and black char), while scratches and reflections complicated image processing. 20,21 Notably, also other fields apply colored resins to enhance the contrast between voids (i.e., pores or cracks) and solid phases, for example, when looking at asphalts, rocks, or porous ceramics.…”

“…20,21 However, the reported method led to some large air bubbles in the structure and the contrast between the two phases (i.e., pores and solid) were not ideal (red pigment and black char), while scratches and reflections complicated image processing. 20,21 Notably, also other fields apply colored resins to enhance the contrast between voids (i.e., pores or cracks) and solid phases, for example, when looking at asphalts, rocks, or porous ceramics. 29−32 These studies employed fluorescent dyes rather than pigments, providing excellent contrast when viewed under a fluorescence microscope.…”