Page 476 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 439

30 to about 400. Thus, they are generally oligomeric to short polymers. Asphalt has a C/H ratio of
about 0.7 while coal tar has a C/H ratio of about 1.5 approaching that of a typical hydrocarbon where
the C/H ratio is about 2.
As with most nonpolar hydrocarbon-intense polymers, bitumens exhibit good resistance to attack
by inorganic salts and weak acids. They are dark, generally brown to black, with their color diffi cult
to mask with pigments. They are thermoplastic materials with a narrow service temperature range

unless modified with fi brous fillers and/or synthetic resins. They are abundant materials that are

relatively inexpensive, thus their use in many bulk applications.
At temperatures above the T , bitumens generally show Newtonian behavior. Below the T bitu-
g g
mens have rheological properties similar to elastomers.
Bitumens are consumed at an annual rate in excess of 75 billion pounds in the United States.
Bitumens are generally used in bulk such as pavements (about 75%), and in coatings for roofs (15%),
driveways, adhesive applications, construction, metal protection, and so on where the bitumen acts as
a weather barrier. Bituminous coatings are generally applied either hot or cold. Hot-applied coatings

are generally either filled or nonfilled. Cold-applied coatings are generally either nonwater or water

containing. In the hot-applied coatings, the solid is obtained through a combination of cooling and liq-
uid evaporation while in the cold-applied coatings the solid material is arrived at through liquid evap-
oration. One often used coating employs aluminum pigments compounded along with solvents. These

coatings are heat reflective and decrease the energy needs of building using them. The aluminum-
metallic appearance is generally more desirable than black, and the reflective nature of the aluminum

reflects light that may damage the bitumen coating allowing the coating a longer useful life. Today,
many of the bitumen coatings contain epoxy resins, various rubbers, and urethane polymers.
12.21 CARBON BLACK

Carbon Black is another of the carbon-intensive materials. It is formed from the burning of gaseous
or liquid hydrocarbons under conditions where the amount of air is limited. Such burning favors
“soot” formation, that is, carbon black formation. It was produced by the Chinese more than 1,000
years ago. Today, it is produced in excess of 1.5 million tons annually in the United States. The most
widely used carbon black is furnace carbon black. The particle size of this raw material is relatively
large, about 0.08 mm. It is soft with a Mohs scale hardness of less than one.
In addition to carbon, carbon black also contains varying amounts of oxygen, hydrogen, nitrogen,
and sulfur. A typical carbon black contains about 98% carbon, 0.3%–0.8% hydrogen, 0.3%–1.2%
oxygen, 0.0%–0.3% nitrogen, and 0.3%–1% sulfur. The impurities in the water employed to quench
the burning carbon is mainly responsible for the noncarbon and hydrogen content and the sulfur
comes mainly from the feedstock.
While some describe the structure of carbon black as being small chain-like structures, recent
information shows it as being somewhat graphite like in structure where the parallel planes are sep-
arated by about 0.35–0.38 nm, always greater than the interlayer distance for graphite of 0.335 nm
(Figure 12.12). The microstructure of carbon black has been studied extensively employing varying
spectroscopies, including ATR (attenuated total refl ectance infrared), high-resolution transmission
electron microscopy (TEM), and X-ray diffraction. Various models have been developed to describe
the average structure in greater detail. These models are related to the structure of graphite.
As with other polymeric materials, the surface structure is different from the bulk and is related
to the conditions under which the material was manufactured. Since the material is formed in air,
the surface is rich in oxygen. These oxygen atoms play an important role in the resulting carbon

black properties. The surface can be modified using a variety of treatments. Heat treatments above
o
800 C act to increase the amount of crystallinity in the overall structure. Under inert conditions sur-
o
o
face groups are modified and the amount of oxygen decreased with heating from 200 C to 1,200 C.

Plasma treatments are employed to modify the carbon black surface creating and destroying var-
ious functional groups in the presence of other reactants. Chemical oxidation is also employed to

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