Page 454 - Handbook of Materials Failure Analysis

452 CHAPTER 17 Application of pyrolysis

good low-temperature and viscosity-temperature properties, physical and chemical
stability, protection of metals from corrosion, inactivity with respect to mechanical
rubber articles, and lubricating effect [6]. Synthetic brake fluid bases on mixtures of
diethylene glycol and di-, tri-, along with tetraethylene glycol monoalkyl ethers.
They contain inhibitors to prevent the corrosion of metallic brake components
and to reduce oxidation at increased temperatures. To increase the boiling point,
tris(methyl glycol) borates are introduced in brake fluids [6].
In this case study, in order to find the difference between the chemical compo-
sition of the good functioning and the claimed motor vehicle brake fluids, both
samples were silylated and analyzed by direct GC/MS without previous pyrolysis.
Silylation is the introduction of a silyl group into a chemical molecule, usually in
substitution for active hydrogen in the hydroxyl group (dOH) in alcohols, phenols,
carboxylic acids, oximes, sulpho-acids, boric acids, phosphorous acids, in the
dNH groupinamines, amides, imines, andinthe SH group inthiolsandthiolcar-
boxylic acids by a silyl group. Replacement of active hydrogen by a silyl group
reduces the polarity of the compound and reduces hydrogen bonding [7,8]. The
silylated derivatives are more volatile and thermally stable. The detection of
compounds is enhanced. The trimethylsilyl (TMS) group, dSi(CH 3 ) 3 ,isthe
most popular and versatile silyl group for GC and GC/MS analysis. Nonpolar
polymethylphenylsiloxane GC stationary phases, such as the DB-5ms phase used
in this work, combine inertness and stability with excellent separating character-
istics for these derivatives. Figure 17.6 shows the obtained TIC of both investigated
brake fluids after trimethylsilylation reaction with N-methyl-N-(trimethylsilyl)-
trifluoroacetamide at 70 °C. The silylated compounds of both brake fluids identi-
fied by interpretation of the obtained mass spectra and by using the NIST 05 mass
spectral library are summarized in Table 17.2. The mass spectrum of the trimethyl-
silylated derivative of triethylene glycol monomethyl ether (main ingredient of
the investigated brake fluids) is shown in Figure 17.7. The weak signal at m/z
+
237 in Figure 17.7 shows the quasi-molecular ion [M+H] of trimethylsilylated
triethylene glycol monomethyl ether (see the structural formula in Figure 17.7).
+
The fragment at m/z 221 corresponds to the ion [M CH 3 ] .The fragmentsat
+
m/z 31, 45, 59, 75, 89, 103, 119, 133, and 147 correspond to the ions [CH 3 O] ,
+
+
+
+
[CH 3 OCH 2 ] ,[CH 3 OCH 2 CH 2 ] ,[CH 3 OCH 2 CH 2 O] ,[CH 3 OCH 2 CH 2 OCH 2 ] ,
+
+
[CH 3 OCH 2 CH 2 OCH 2 CH 2 ] , [CH 3 OCH 2 CH 2 OCH 2 CH 2 O] , [CH 3 OCH 2
+
+
CH 2 OCH 2 CH 2 OCH 2 ] ,and[CH 3 OCH 2 CH 2 OCH 2 CH 2 OCH 2 CH 2 ] , respectively.
They occur through a cleavage mechanism of the alkoxy chain in the structure
of the identified trimethylsilylated triethylene glycol monomethyl ether molecule.
The peaks in the mass spectrum in Figure 17.7 at m/z 73, 89, 103, and 117 are
+
+
+
formed from the [Si(CH 3 ) 3 ] , [OSi(CH 3 ) 3 ] , [CH 2 OSi(CH 3 ) 3 ] , and
+
[CH 2 CH 2 OSi(CH 3 ) 3 ] ions, respectively.
As can be seen from Table 17.2, the chemical composition of the claimed brake
fluid differs from the composition of the good functioning brake fluid. Although the
content of the main component triethylene glycol monomethyl ether in both samples
is approximately equal to 46.5%, the claimed brake fluid contains additional