Page 166 - Fundamentals of Air Pollution

V. Effects on Rubber 133

to blacken white and light-tinted paints. Wohlers and Feldstein (12) con-
cluded that lead base paints could discolor surfaces in several hours at a
3
concentration of 70 ftg/m H 2S (0.05 ppm). In time the black lead sulfide
oxidizes to the original color. Paints pigmented with titanium or zinc do
not form a black precipitate. Alkyd or vinyl vehicles and pigments contain
no heavy metal salts for reaction with H 2S. Painted surfaces are also dirtied
by participate matter. Contaminating dirt can readily become attached to
wet or tacky paint, where it is held tenaciously and forms focal points for
gaseous sorption for further attack. Dirt that collects on roofs or in gutters,
blinds, screens, windowsills, or other protuberances is eventually washed
over external surfaces to mar decorative effects.
Paints and coatings for automobiles have not been immune to damage
by air pollution. Wolff and co-workers (13) found that damage to automobile
finishes was the result of scarring by calcium sulfate crystals formed when
sulfuric acid in rain or dew reacted with dry deposited calcium.
Glass is normally considered a very stable material. However, there is
growing evidence that SO 2 air pollution may be accelerating the deteriora-
tion of medieval glass. A corrosion surface forms on these glass surfaces
and the sulfate present helps prolong surface wetness. This condition is
conducive to further attack and degradation of the glass surface (14).

V. EFFECTS ON RUBBER

Although it was known for some time that ozone cracks rubber products
under tension, the problem was not related to air pollution. During the
early 1940s, it was discovered that rubber tires stored in warehouses in Los
Angeles, California, developed serious cracks. Intensified research soon
identified the causative agent as ozone that resulted from atmospheric
reaction between sunlight (3000-4600 A), oxides of nitrogen, and specific
types of organic compounds, i.e., photochemical air pollution.
Natural rubber is composed of polymerized isoprene units. When rubber
is under tension, ozone attacks the carbon-carbon double bond, breaking
the bond, The broken bond leaves adjacent C = C bonds under additional
stress, eventually breaking and placing still more stress on surrounding
C = C bonds. This "domino" effect can be discerned from the structural
formulas in Fig. 9-4. The number of cracks and the depth of the cracks in
rubber under tension are related to ambient concentrations of ozone.
Rubber products may be protected against ozone attack by the use of a
highly saturated rubber molecule, the use of a wax inhibitor which will
"bloom" to the surface, and the use of paper or plastic wrappings to protect
the surface. Despite these efforts, rubber products still crack more on the
West Coast than on the East Coast of the United States.