Table 1 Summary of the possible transformation of the released fibrous fillers from composites
Fibrous filler | Composite matrix | Release process | Transformation of released fibers | References |
|---|---|---|---|---|
Asbestos | Cement; other construction materials | Thermal treatment of about 1000 °C and above | Asbestos were transformed to non-hazardous silicate phase | Gualtieri and Tartaglia [20], Gualtieri et al. [3, 27], Kusiorowski et al [30], Yamamoto et al. [31] |
Different carbon fibers | Epoxy | Hammer mill, dry and wet cutting, grinding, drilling | Fibers split along the axis from the original fibers were released. They had smaller diameters and might be respirable | Holt and Horne [39], Mazumder et al. [40], Bello et al. [37, 46] |
Different carbon fibers | Epoxy | Heating to 400 and 850 °C | At 850 °C, fibers underwent fragmentation during oxidation, and lost crystalline property | Mazumder et al. [40] |
PAN-based carbon fibers | Polymer cable | Tensile stress test to cable failure | Respirable fibers split along the fiber axis from the original fibers were released | Schlagenhauf et al. [41] |
CNTs | Epoxy | Sanding | CNTs protruding from fragments of matrix material had similar diameters as the original ones. | |
CNTs | Epoxy | Abrasion | Free standing single and agglomerated CNTs were released and had average length (304 nm) shorter than the CNTs in the matrix (0.7 µm) | Schlagenhauf et al. [80] |
CNTs | Epoxy; POM; TPU | UV exposure | Surface of the sample was covered by a network of CNTs and their integrity was not damaged | Nguyen et al. [77], Petersen et al. [78], Wohlleben et al. [87, 88] |
CNTs | Epoxy | Combination of sanding and weathering | Protruding CNTs had the same diameter as the original ones and formed a surface layer with length around 0.3 µm, shorter than the original length of 1–20 µm | Hirth et al. [73] |
CNTs; carbon nanofibers | PP; PMMA | Fire test | A protective CNT network was formed in the combustion residuals. The tubes in the network were more ‘intertwined’ and larger than the original ones. The tubes were partially oxidized. Iron catalysts were also oxidized | |
CNTs | ABS | Combustion | Free isolated and bundled CNTs were released to the air with dimensions similar to the original MWCNTs | Bouillard et al. [68] |
CNTs | PU | Thermal decomposition | CNT protrusions were observed from the surface of the ash and CNTs were assumed to be intact | |
Pristine and compatibilized CNTs | PA6 | Calcination | Recovered CNTs showed no difference from the original ones by FT-IR or TGA analysis but showed a small amount of attached polymer in TEM | Vilar et al. [100] |