Page 348 - Engineering Plastics Handbook

Polyarylethersulfones (PAES) 305

environmental stress cracking, and the ability to withstand various
environments is called environmental stress crack resistance (ESCR).
The amount of susceptibility that a plastic exhibits to an environment
depends on three important factors: temperature, amount of applied
stress, and the concentration of the chemically aggressive medium.
Because polysulfones are amorphous, they are vulnerable to some
organic environments. This is due to greater ease of diffusion of aggres-
sive solvents into the polymer, which in turn disrupts the electrostatic
and dipole forces between the chain segments and facilitates chain dis-
entanglement under applied stress. The potency of a stress cracking
agent is related to the match between the solubility parameter of the
solvent and that of the polymer. For example, PSF, which has a solubility
3 1/2
parameter of δ= 21.8 (J/cm ) , resists aliphatic hydrocarbons of lower
δ and most alcohols of higher δ, but readily undergoes stress cracking
by ketones that have δ values close to that of the polymer. For polysul-
fones, the most problematic solvents are aromatics, chlorinated hydro-
carbons, ketones, and esters. The resistance of the commercially
available polysulfones generally improves in the order PSF < PES <
PPSF, as shown in Table 13.8.
The exact mechanism of environmental stress cracking is not fully
understood, but it involves a weakening of secondary bonds between
neighboring polymer chains due to solubility and diffusion of a chemical

TABLE 13.8 Resistance* of Natural (Unreinforced) Polysulfone, Polyethersulfone,
and Polyphenylsulfone Resins to Various Chemical Environments †
Ranking ‡
Environment PSF PES PPSF
Aliphatic hydrocarbons G E E
Aromatic hydrocarbons P P F
Alcohols and glycols G E E
Esters P P F/P
Ketones P P P
Chlorinated hydrocarbons P P P
Aliphatic amines E E E
Aromatic amines P P F
Electrolyte solutions E E E
Acids § E E E
Bases E E E
Ionic surfactants E E E
Nonionic surfactants F G G
*
The above data are for comparative purposes only; actual resistance depends on many
factors including stress, temperature, concentration, and exposure duration.
†
Stress cracking resistance is substantially enhanced in the presence of reinforcement such
as glass fibers.
‡
Ranking codes: E = excellent; G = good; F = fair; P = poor.
§
Nonoxidizing.