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.	Cena and Peters [69], Huang et al. [56]
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	Kashiwagi et al. [57, 93, 94]
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	Sotiriou et al. [85, 86]
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]

ABS acrylonitrile butadiene styrene, FT-IR Fourier transform infrared spectroscopy, PA6 polyamide 6, PMMA poly(methylmethacrylate), POM polyoxymethlene, PP polypropylene, PU polyurethane, TGA thermal gravimetric analysis, TPU thermoplastic polyurethane