Page 322 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.2 CHAPTER 5

5.1 STABILIZERS

The primary covalent bonds that hold polymer molecules together are generally quite sta-
ble, permitting their wide use in plastics, rubber, fiber, coatings, and adhesives. They are
not perfectly stable, however (Table 5.1). To a slight extent, they are in equilibrium with
radical and ionic states,
. . + –
A:B → A B or A B
which would be unstable and reactive. Input of energy via the following processes:

• Mechanical (especially shear and dynamic)
• Thermal (in processing and in use)
• Electrical (especially high voltage and high frequency)
• Optical (especially UV and IR)
• Chemical (especially oxygen, ozone, and moisture)
• Biological (especially microorganisms)
increases this dissociation, resulting in cleavage or cross-linking, structural changes, and
reactions with atmospheric oxygen, moisture, and chemical environments in general.
These reactions can be used to modify polymers and perhaps to recycle them but, more of-
ten, they cause degradation and loss of properties in aging.

TABLE 5.1 Stability of Chemical Bonds

Chemical bond Bond energy, kcal/mol Chemical bond Bond energy, kcal/mol
C-F 116 (CH ) CH-H 95
3 2
C H -N 110 C-O 93
6 5
C H -O 107 CH COCH -H 92
6 5
3
2
Si-O 106 (CH ) C-H 91
3 3
CH -H 104 CH =CHCH -H 85
2
3
2
C H -H 104 C-N 82
6 5
C-C 101 N-N 37
C H -C H 100 O-O 34
6 5
6 5
We can modify polymer structure to make it stabler and more resistant to degradation.
Generally, perfluorination, aromatic and heterocyclic resonance, and polysiloxane back-
bone produce great stability, most dramatically in our ultra-high-temperature plastics, but
involve high cost and difficult processability. Conversely, groups that can cause instability
include tertiary branch points, C=C, aliphatic nitrogen and oxygen atoms, and hydrolyz-
able groups such as ester, urethane, and amide; so avoiding such groups can improve sta-
bility.

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