Transformation of the released asbestos, carbon fibers and carbon nanotubes from composite materials and the changes of their potential health impacts

Abstract

Composite materials with fibrous reinforcement often provide superior mechanical, thermal, electrical and optical properties than the matrix. Asbestos, carbon fibers and carbon nanotubes (CNTs) have been widely used in composites with profound impacts not only on technology and economy but also on human health and environment. A large number of studies have been dedicated to the release of fibrous particles from composites. Here we focus on the transformation of the fibrous fillers after their release, especially the change of the properties essential for the health impacts. Asbestos fibers exist in a large number of products and the end-of-the-life treatment of asbestos-containing materials poses potential risks. Thermal treatment can transform asbestos to non-hazardous phase which provides opportunities of safe disposal of asbestos-containing materials by incineration, but challenges still exist. Carbon fibers with diameters in the range of 5-10 μm are not considered to be respirable, however, during the release process from composites, the carbon fibers may be split along the fiber axis, generating smaller and respirable fibers. CNTs may be exposed on the surface of the composites or released as free standing fibers, which have lengths shorter than the original ones. CNTs have high thermal stability and may be exposed after thermal treatment of the composites and still keep their structural integrity. Due to the transformation of the fibrous fillers during the release process, their toxicity may be significantly different from the virgin fibers, which should be taken into account in the risk assessment of fiber-containing composites.

Fig. 1

SEM images of a the initial sample of a pure amphibole asbestos; b the pure amphibole asbestos sample after thermal treatment; c the initial sample of a commercial asbestos-cement for external roofs pipes; d the asbestos-cement sample after thermal treatment. (Adapted from [20], with the permission of Elsevier)

Fig. 2

Example SEM and TEM images of the released particles following the rupture of CFRP cables in the tensile strength test. (Partially adapted from [41]

Fig. 3

TEM images of abraded particles from a CNT/Epoxy composite by the Taber Abraser. a, b Protruding CNTs from abraded particles of the 1 w% CNT composite; c–e free-standing individual CNTs; f an agglomerate of CNTs with a couple of individual CNTs scattered nearby. (Partially adapted from [80]