Page 165 - Fundamentals of Air Pollution

132 9. Effects on Materials and Structures

bient air levels of nitrogen dioxide and humidity reproduced these ef-
fects (8).
Ozone has also been found to cause fading of material. This was discov-
ered when white fabrics developed a yellow discoloration (9, 10), leading
researchers to investigate the effects of ozone on other chemicals added
to the material, including optical brighteners, antistatic and soil-release
finishes, and softeners. A very complex process was occurring where the
dyes were migrating to the permanent-press-finish materials, e.g., soften-
ers. Softeners have been found to be good absorbers of gases. Fading results
from the combination of dye and absorbed nitrogen dioxide and ozone.
This combination with high relative humidities has caused color fading in
numerous types of material and dye combinations. Although some progress
has been made in the development of fade-resistant dyes, they are more
expensive and have poorer dyeing properties.

IV. EFFECTS ON LEATHER, PAPER, PAINT, AND GLASS

Sulfur dioxide affects the composition of leather and paper, causing
significant deterioration. The major concern is the destruction of leath-
erbound books in the libraries of the world. SO 2 is absorbed by leather
and converted to sulfuric acid, which attacks the structure of the leather.
Initially, the edges of the exposed back of the book begin to crack at the
hinges. As the cracks expand, more leather is exposed and the cracks
widen, with the back eventually falling off the book. Preventive measures
now include storage in sulfur dioxide-free air.
The cellulose fiber in paper is attacked and weakened by sulfur diox-
ide. Paper made before about 1750 is not significantly affected by sulfur
dioxide (11). At about that time, the manufacture of paper changed to a
chemical treatment process that broke down the wood fiber more rap-
idly. It is thought that this process introduces trace quantities of metals,
which catalyze the conversion of sulfur dioxide to sulfuric acid. Sulfuric
acid causes the paper to become brittle and more subject to cracking and
tearing. New papers have become available to minimize the interaction
with SO 2.
Paints are designed to decorate and protect surfaces. During normal wear,
paint chalks moderately to clean the surface continuously. A hardened paint
surface resists sorption by gases, although the presence of relatively high
3
concentrations of 2620 to 5240 /ag/m SO 2 (1-2 ppm) increases the drying
time of newly painted surfaces. Hydrogen sulfide reacts with lead base
pigments