Page 352 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Plants 315

The concentration of melanine also contributes to the color of our hair (except for redheads where
the iron-rich pigment trichosiderin dominates). The bleaching of hair, generally achieved through
exposure to hydrogen peroxide, is a partial result of the oxidation of the melanine. A side reaction
of bleaching is the formation of more sulfur cross-links leading to bleached hair being more brittle
because of the increased cross-linking leading to a decrease in hair fl exibility.
Melanine also provides a dark background in our eye’s iris, is involved in animal color changes
(such as the octopus and chameleon), is formed when fruit is bruised, and is partially responsible
for the coloration of tea.

9.15 ASPHALT
Nearly 100 billion pounds of asphalt are used annually in the United States. In crude oil, it is the
most viscous component that is not easily distilled as are move volatile fractions such as gasoline
and kerosene. They are also found in natural deposits such as the La Bera Tar Pits in California. It is
sometimes referred to as bitumen, covered in Section 12.18. Here, we will cover additional material
related to asphalt.
About 80% of asphalt is employed along with rocks to form our roadways. The asphalt acts as the
binder for the rock aggregates similar to concrete. Asphalt concrete is the most widely used recycled
material, on a weight and percentage basis, in the United States. More than 80% of asphalt from
road surfaces removed from resurfacing projects is reused as a component of the new construction.

Roofing shingles are also a major use of asphalt.
Asphalt is a viscoelastic material and this viscoelasticity must be present over the temperature
range where the material is being used. Unfortunately, asphalt often softens during summer days,
allowing rutting or permanent deformation to occur. During the winter, asphalt chains tend to
become more organized, resulting in a more brittle material that can undergo thermal and fatigue
cracking. With time, more volatile, shorter chains leave the mixture causing the material to lose

some of its flexibility. This process is called aging.
Asphalt emulsions are made by mixing the asphalt, generally with petroleum solvents, but
some formulations include the more environmentally acceptable water. These emulsions are often
employed to resurface concrete and asphalt surfaces such as driveways.
Along with short-chained polymers with up to 150 carbons, asphalt contains a variety of other
elements. The composition varies as to the source and treatment of the crude oils. Crude oil from
Venezuela contains particularly high amount of metals, such as iron, nickel, and vanadium, in com-
parison to crude oil from the Middle East. Some of the asphalts contain polar functional groups
such as alcohols, amines, thiols, and carboxylic-containing units. This allows the asphalt chains to
aggregate similar to many detergents with the aliphatic units reaching outward from a polar core.
For rocks with a mildly polar surface, this results in some binding between the rock surfaces and
the asphalt, resulting in a more secure material.

9.16 SUMMARY

1. Polysaccharides are the most abundant, on a weight basis, naturally occurring organic poly-
mer. They are truly complex molecules with most structures representable on only an “aver-
age” basis. They are diverse with respect to both occurrence and size. The most important, on
a knowledge, weight, and use basis are cellulose and starch. Cellulose is composed largely of
d-glucose units that are associated with other materials such as lignin. Cotton is one of the pur-
est forms of cellulose, though we get most of our cellulose from wood pulp. Cellulose becomes
soluble in aqueous solutions only when the hydrogen bonding within the cellulose is broken.
2. While cellulose is used extensively in the form of paper and paper-based products, it also
forms the basis of a number of synthetic, including a variety of acetylated products such as
rayon.

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