Release from Composites by Mechanical and Thermal Treatment - Safety of Nanomaterials along Their Lifecycle - page 266

In no case mechanistic studies investigating the influence of the variables on release
were conducted. Overall, the studies show that nanoscale particles can be released
by mechanical shock and that the release rate most likely depends on the energy
input. Anyhow detailed studies still have to be conducted and release scenarios to be
defined before pursuing international harmonization efforts.

12.3 RELEASE FROM THERMAL PROCESSES

12.3.1 t hermal d egradation
Thermal degradation is another mechanism selectively removing the matrix from a
nano-object containing polymer. The mechanisms involved here are evaporation of
matrix components as well as changes in the chemical composition by elimination
and cracking, as well as some oxidation reactions. All studies investigating thermal
degradation in the following presentation used commercially available thermogravi-
metric analyzers.
Orhan et al. (2012) examined thermal stability of nanocomposites containing
nanoclay (NC) and MWCNT using a thermogravimetric analyzer (TGA). They
found that the onset of degradation was delayed by the presence of the nanomate-
rials. In the presence of NC better flame retarding characteristics were observed,
explained by anhydride formation.
Peng et al. (2007) investigated the thermal degradation mechanism of polyvinyl
alcohol (PVA) with SiO 2 nanocomposites and found that the PVA/SiO 2 nanocompos-
ite showed a significantly improved thermal resistance. The degradation products
identified by Fourier transform infrared/thermogravimetric analysis (FTIR/TGA)
and pyrolysis-gas chromatography/mass spectrometric analysis (Py-GC/MS) sug-
gest that the first degradation step (ca. 350°C) of the nanocomposite mainly involves
the elimination reactions of H 2 O and residual acetate groups, as well as quite a
few chain-scission reactions. The second degradation step (>450°C) is dominated
by chain-scission reactions and cyclization reactions, and continual elimination of
residual acetate groups is also found in this step.
Costache et al. (2006) studied the thermal degradation of polymethyl methacry-
late (PMMA) and its nanocomposite to determine if the presence of clays (anionic
and cationic) or CNT has an effect on the degradation pathway. The thermal deg-
radation has been investigated by cone calorimetry and TGA, and the products of
degradation have been studied with FTIR/TGA and gas chromatography/mass spec-
trometry (GC/MS). They found that the presence of clay or CNTs has no qualitative
effect on the degradation mechanisms of PMMA, but the degradation of the nano-
composite occurs at higher temperatures. Since PMMA undergoes thermal degrada-
tion by a single process, the presence of filler cannot change its degradation pathway,
in contrast to other systems where fillers can promote one thermal decomposition
pathway at the expense of another.
Pramoda et al. (2003) focused on the study of thermal degradation and evolved
gas analysis using TGA coupled to FTIR spectroscopy to study PA6 with and without
clay nanocomposites prepared by melt compounding. PA6 with 2.5 mass% of clay