Chemistry Reference
In-Depth Information
is then oxidized by the introduction of air followed by controlled cooling of
the gas.
2 Sb
2
S
3
+ 9 O
2
2 Sb
2
O
3
+ 6 O
2
Antimony oxide can also be prepared from the hydrolysis of antimony
trichloride. Antimony trichloride is prepared by the reaction of calcium chlo-
ride with stibnite and oxygen.
9.8 HEAT STABILIZERS
PVC is an important and widely used polymer but it has poor thermal stabil-
ity. With heat or light, it can undergo dehydrochlorination reactions, forming
alkenes.
H
H
H
H
H
+ m HCl
H
Cl
H
H
n
m
n-m
This reaction is sometimes called unzippering and is thought to originate
at certain structural defects within the polymer [23].
Nine structural defects have been shown to occur in all samples of commer-
cial PVC and they include some structures which contain tertiary chlorines
as seen in two of the identified defect structures.
Cl
CH
2
Cl
CH
2
CH
2
Cl
CH
2
CH
2
CH
2
Cl
CH
2
H
2
C
H
2
C
CH
2
Cl
Cl
Cl
Defect structures found in PVC
When the PVC unzippers, it discolors and becomes brittle. To prevent this
dehydrochlorination reaction, heat stabilizers are added. Lead stabilizers are
effective and a major type of heat stabilizer. Data from 2003 [24] shows lead
stabilizers to represent more than 50% of the world PVC stabilizers, but less
than 20% in the U.S. where tin stabilizers predominate. The use of lead stabi-
lizers is on the decline and being phased out due to concerns about the health
effects of lead. Nonetheless, the news occasionally reports that lead has been
found in a toy or other PVC article imported from a country such as China.
The source of lead is likely due to the stabilizer. Another type of heat stabiliz-
ers is the mixed metal packages which are based upon mixtures of different
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