Biomedical Engineering Reference
In-Depth Information
The rubber was identified from simple swelling measurements, where
cut sections of hose are dipped into several different organic solvents: the
degree of swelling shows the type of elastomer present. Identification can
be achieved somewhat more rapidly and more exactly using attenuated
total reflectance (ATR) IR spectroscopy by pressing a free surface against a
single germanium crystal and obtaining the absorption spectrum on an FTiR
spectrometer. Because germanium is transparent to iR radiation, the beam
strikes the surface of the polymer from within the crystal and is reflected
back for analysis.
10.7.2 Presence of ozone
Ozone gas is produced near electrical circuits, especially where either
sparking or silent discharge of current occurs. Rubbers with double bonds
in their chains, natural rubber (NR), styrene-butadiene rubber (sBR) and
NBR in particular are susceptible to cracking because the gas attacks the
bonds and cleaves them very quickly. Only very low gas concentrations are
needed (parts per billion in air), and so high concentrations of the gas may
be expected near to the electrical distribution system. Thus, if the damage
was produced by about 20 000 miles car usage, then failure (deepest crack
reaches tube bore) would be expected between 30 000 and 40 000 miles,
assuming constant ozone production and a constant growth rate.
10.7.3 Mechanism of attack
Ozone is an allotrope of oxygen (O
2
) and is highly reactive with many
organic materials.
23-28
It reacts directly with double (olefinic) bonds and,
because many rubbers exhibit such bonds in their main chains, it reacts with
them and splits the chains:
-CH==CH- + O
3
Æ Æ -CHO + CO
2
H-
The end groups are aldehydes, alcohols or carboxylic acids, which are
also reactive, and aldehydes and alcohols oxidise further to form acids.
Rubbers with double bonds include NR, NBR and butadiene rubber (BR)
plus copolymers, which have repeat units:
NR
-[CH
2
-CH==CHCH
3
-CH
2
]- or polyisoprene
NBR
copolymer of isoprene and acrylonitrile repeat units
-[CH
2
-CH==CHCH
3
-CH
2
]- and -[CH
2
-CHCN]-
BR
-[CH
2
-CH==CH-CH
2
]-
sBR
copolymer of butadiene and styrene units
-[CH
2
-CH==CH-CH
2
]- and -[CH
2
-CH-C
6
H
5
]-
Search WWH ::
Custom Search