Chemistry Reference
In-Depth Information
Unlike polytetrafluoroethylene
, polyvinyl fluoride, poly(vinylidine fluoride),
and
polytrifluor-
oethylene
yield primarily on heating HF [
457
]. Among these three, poly(vinylidine fluoride)
yields larger amounts of HF than do the other two polymers with an accompanying formation of
double bonds.
9.8.6 Thermal Degradation of Poly(Vinyl Acetate)
Thermal decomposition of poly(vinyl acetate) results in a loss of acetic acid. The reaction is typical of
thermal cleavages of esters. It is facilitated by formation of pseudo six-membered rings as a result of
interactions between the
b
-hydrogens of the alcohol residues and the carboxylic groups:
H
O
O
O
O
When double bonds form, adjacent methylene groups become activated. The loss of acetic acid is
the main product at temperatures up to 200-250
C. Beyond these temperatures, aromatic pyrolytic
compounds form.
Studies of thermal degradation of copolymers of vinyl chloride with vinyl acetate showed that the
copolymers are thermally less stable than the homopolymers [
502
,
503
]. The ratio of hydrochloric
acid to that of acetic acid that volatilize remains constant during the degradation. This indicates that
neither is evolved preferentially, once the reaction begins [
502
,
503
]. It is interesting to note that
degradation studies of a copolymer of vinyl chloride and styrene also demonstrated that the copoly-
mer is less stable than each of the homopolymers [
504
].
9.9 Thermal Degradation of Common Step-Growth Polymers
The thermal decomposition of step-growth polymers cannot take place by a chain reaction like that of
chain-growth polymers. As a result, these materials degrade in a random fashion, rupturing at the
weakest bonds first.
9.9.1 Thermal Degradation of Polyoxides
depolymerizes into formaldehyde at 220
C. This was found to be a first-order
reaction with the rate varying from 0.42 to 5.8%/min, depending upon conditions of polymer
preparation and the molecular weight of the polymer [
457
].
Poly(ethylene oxide)
Polyoxymethylene
decomposes upon heating at lower temperatures than does polyethylene.
Among the volatile products are found formaldehyde, ethanol, ethylene oxide, carbon dioxide, and
water.
is also less heat stable than polypropylene. Isotactic poly(propylene
oxide) is somewhat more stable than the atactic one.
Poly(propylene oxide)
