Building materials that contain asbestos may at times be exposed to high temperatures or chemicals. Asbestos insulation samples applied to boilers, furnaces, steam pipes, etc., are exposed to very high temperatures for extended periods of times. All types of building materials may be exposed to very high temperatures in fires of all types (wildfires, building fires, etc.). This exposure can eventually alter the asbestos present, changing slightly or not so slightly, the crystal structure and chemical composition of the fibers. Other scenarios, such as exposure to acids, can also have similar effects on asbestos fibers, but by far, the most common example is the alteration of the asbestos by high temperatures.
Though amphiboles are common in these types of samples, it is typically chrysotile that shows the greatest degree of
alteration, because it can be altered at a lower temperature as compared to most amphiboles. For chrysotile, the optical properties and Selected Area Electron Diffraction (SAED) are the most affected, since this relates to the fiber’s crystal structure. Amphiboles also will exhibit a change in Refractive Index (RI), but the SAED, as measured in TEM, is not typically affected. For all asbestos types, heating fibers will eventually cause their RIs to increase. For amphiboles, crocidolite and amosite seem to show the earliest and most significant change in RI when heated, though other amphibole varieties do show more moderate changes. The loss of SAED in chrysotile may be related to the more delicate nature of the chrysotile scrolled structure, where bonds between the scrolled structures may be more stressed and likely to break than in the more durable amphibole crystal. More extreme alteration is needed to have a significant effect on amphibole SAED.
Morphological changes include change of color; chrysotile becomes a light-brownish color, amosite becomes rusty
brown, and crocidolite becomes reddish brown. The most notable change in these fibers is crocidolite loses its grey-blue appearance and changes to a positive sign of elongation. Crocidolite also is the first of these types to exhibit any change at all; a short duration at 375°C is enough to start causing these changes. Additionally, most asbestos types become less flexible after heating. Chemical alteration also effects RI, but instead of an increase, this normally lowers the refractive index.
In PLM, when the analyst sees suspect asbestos fibers that just don’t look quite right, morphology may be similar, but
some of the fibers look like they have been fused together into bundles with a “blocky” appearance. RIs may either be too high or low for asbestos for the suspect fiber type, and color of fibers in plane polarized light has a light brown, reddishbrown, or rust brownish color. If you see some of these ‘indicators,’ you will probably also find the refractive indices are no longer in range to identify as asbestos, and in the case of suspect crocidolite, the sign of elongation will be positive.
In TEM, when the chrysotile scroll looks to be damaged or fused with neighboring fibers, an analyst typically
finds SAED patterns for suspect chrysotile fibers will most likely not be attainable. In addition, the elemental identified as asbestos, even if the chemistry remains unchanged. It is also important to note that according to OSHA (and a number of its state counterparts), altered asbestos is considered to be asbestos, even if not considered countable asbestos by the method.
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