ASPHALT
FUME EXPOSURES DURING THE
MANUFACTURE OF ASPHALT ROOFING PRODUCTS
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Current
Practices for Reducing Exposures
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Previous
Section |
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1.
Introduction
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The primary
purpose of this document is to increase awareness among plant managers,
safety and health professionals, and engineers of the potential for
occupational exposure to asphalt and asphalt
fumes during the manufacture of asphalt roofing products. The document
represents a collaborative effort of industry, labor, and government.
During public meetings held in Cincinnati, Ohio (July 2223, 1996),
several groups agreed to develop a series of documents that (1) describe
the extent of asphalt exposure during the production of asphalt roofing
products and during asphalt roofing operations, and (2) provide information
about measures to reduce exposures. These groups included the National
Roofing Contractors Association (NRCA);
the Asphalt Roofing Manufacturers' Association (ARMA);
the Asphalt Institute (AI); the United
Union of Roofers, Waterproofers, and Allied Workers; and the National
Institute for Occupational Safety and Health (NIOSH).
This document identifies work practices and other control measures that,
when available, may be effective in reducing worker exposures to asphalt
fumes during the manufacture of asphalt roofing products.
In 2000,
NIOSH conducted a review of the health effects data on asphalt that
had become available since the publication of the 1977 criteria document
on asphalt [NIOSH 2000]. This review addresses acute and chronic effects
and is available at the NIOSH Web site (www.cdc.gov/niosh)
for readers interested in additional information.
NIOSH,
labor, and industry are working together to better characterize and
quantitate the health risks from asphalt exposure. Representatives of
industry, labor, government, and academia met in Cincinnati, Ohio, on
September 11 and 12, 2000, and identified research to assess completely
the health risks associated with exposure to asphalt. Through these
and other efforts of this partnership, effective workplace measures
can be implemented to reduce worker exposure to asphalt fumes.
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2.1
Composition and Uses of Asphalt |
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Asphalt
(CAS* No. 8052424) is a dark brown to black, cementitious,
thermoplastic material found in a natural state or (more commonly) manufactured
in petroleum refineries by atmospheric or vacuum distillation; it may
also be left as residue after evaporating or otherwise processing crude
oil or petroleum. Asphalt is solid or highly viscous at ambient temperatures.
This material is an extremely complex mixture containing a large number
of high molecular weight organic compounds [King et al. 1984]. Asphalt
is now the dominant material in roofing in the United States. However,
coal tar is still used in some roofing work, usually to conform to government
building specifications that require the use of coal tar [Freese and
Nichols, Inc. 1994].
Most of
the asphalt used in the United States is used in paving (87%) and roofing
(11%) operations. Only about 1% is used for waterproofing, dampproofing,
insulation, paints, and other activities [AI 1990a]. Asphalt roofing
products and systems include shingles and roll roofing, ply felt, built-up
roofing (BUR) systems, saturated felt used as underlay for shingles,
and modified bitumen systems. These products
and systems are described in Section 2.4.
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*Chemical Abstracts Service.
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2.2
Manufacture of Asphalt Roofing Products |
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Ninety-seven
asphalt roofing manufacturing plants currently operate in the United
States. (The North American Industry Classification System [NAICS]
code is 324122 for asphalt shingle and coating materials manufacturing.)
These plants are owned by 27 companies and are located in 28 States,
generally within 500 to 800 miles of their primary markets. The plants
vary greatly in size, number of workers, and products manufactured.
A typical asphalt roofing plant manufactures several products, including
shingles, roll roofing (smooth or mineral surfaced), ply felt for use
in BUR, and saturated felt used as underlay
for shingles. Modified bitumen products are manufactured on machines
designed for polymer-modified materials.
Approximately
3,000 to 4,000 workers are exposed to asphalt fumes in approximately
100 roofing manufacturing plants [ARMA 2001].
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2.3
Manufacture of Roofing Asphalts |
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The three
basic grades of roofing asphalt are (1) saturant-grade
asphalt, a nonoxidized or oxidized asphalt
used to manufacture saturated felt plies used in the construction of
BUR systems, organic felt shingles, and
other roofing materials such as roll roofing; (2) coating-grade asphalt,
an oxidized asphalt used to manufacture roofing materials for a variety
of roofing systems such as asphalt shingles, polymer-modified bitumen
roofing, reinforcing and underlayment felts, and roll roofing products;
and (3) mopping-grade asphalt, an oxidized
asphalt that is melted and used in the construction of BUR and modified
bitumen systems. Each grade of asphalt is also used to manufacture a
variety of asphalt coating and sealant products.
The principal
differences between saturant and coating grade asphalts are viscosity
and softening point. Saturant asphalts typically have a softening point
of about 120 to 140 °F (50 to 60 °C), making them less viscous
than coating asphalts, which have a softening
point of approximately 200 to 225 °F (95 to 105 °C). Despite
their lower viscosity, saturant asphalts are processed at significantly
higher temperatures (about 425 to 475 °F [218 to 245 °C]) than
coating asphalts (about 380 to 460 °F [190 to 238 °C]) because
of the need to ensure adequate impregnation of the organic felts that
use saturant asphalts [ASTM 1995].
The four
types of mopping-grade asphalt are described in Table
21. The viscosity of mopping-grade asphalts depends on which
of the four types is being manufactured. Type I is the softest (least
viscous) grade and is used on very-low-slope roofs. Type IV is the hardest
(most viscous) grade and is used on the highest slope roofs suitable
for BUR systems.
Petroleum
refineries and independent asphalt manufacturers produce oxidized roofing
asphalt by air-blowing the residuum of
refinery atmospheric or vacuum distillation processes. This starting
material, termed "asphalt flux,"
also may be a blend of residua from different sources. In the air-blowing
or oxidation process, heated asphalt flux is placed into a tank known
as a blowing still, and air is blown
through it. The reactions that take place are exothermic, so the temperature
is controlled within the range of 400 to 550 °F (204 to 288 °C).
The temperature and the amount of air are varied by the manufacturer
depending on the nature of the asphalt flux and the intended characteristics
of the oxidized roofing asphalt being produced. This process raises
the softening point and viscosity and lowers the penetration and ductility
of the asphalt [King et al. 1984; IARC 1985; Corbett 1979].
At the
temperatures of the air-blowing process, the oxidations and subsequent
reactions ultimately yield compounds of increased polarity and higher
apparent molecular weight [Boduszynski 1981; Corbett 1975; Goppel and
Knotnerus 1955]. Compared with the asphalt flux, the air-blown asphalts
contain an increased proportion of asphaltenes, decreased proportions
of naphthene-aromatics and polar aromatics, and about the same proportion
of saturates [Corbett 1975; Boduszynski 1981; Moschopedis
and Speight 1973]. The process effluent contains water, carbon dioxide,
and other reaction products and small amounts of relatively volatile
components of the asphalt [Corbett 1975; Goppel and Knotnerus 1955].
The oxygen added to asphalt in the air-blowing process appears to reside
in hydroxyl, peroxide, and carbonyl functional groups, the latter including
ketones, acids, acid anhydrides, and esters [Campbell and Wright 1966;
Petersen et al. 1975; Goppel and Knotnerus 1955].
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For
determination of gross composition, asphalt is frequently fractionated
by treatment with heptane or a similar hydrocarbon solvent to precipitate
the asphaltenes; fractionation is followed by chromatography of the
maltenes (soluble portion) into three fractions, which are (in order
of increasing polarity) the saturates, naphthene-aromatics, and polar
aromatics [Corbett 1975; Boduszynski 1981].
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2.4
Asphalt Roofing Products and Systems |
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Today,
three commercially popular roofing products or systems are made from
roofing asphalt, each with different characteristics and applications:
- Asphalt
shingles and roll roofing are used in residential and steep-slope
commercial roofing.
- BUR
systems are asphalt-impregnated felt pieces sealed and surfaced
with hot mopping asphalt; the systems are used in low-slope commercial
roofing.
- Modified
bitumen systems are also a low-slope commercial product using
polymer-modified roofing asphalts to impregnate and coat one or more
fabric plies.
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2.4.1
Asphalt Shingles and Roll Roofing |
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Asphalt
shingles introduced in the early 1900s account today for about 75% of
new construction and reroofing in steep-slope residential and some commercial
roofing applications [NRCA 1995]. Roll
roofing was the mainstay of the steep-slope market until it was largely
displaced by shingle products. Today, roll roofing is used mainly in
BUR systems on low-slope roofs. With low-slope roofing, smooth-surface
roll roofing can be used in building the BUR membrane, and mineral-surfaced
roll roofing is used as a cap or "top sheet" [NRCA 1996; AI
1990a].
Asphalt
shingles and roll roofing both consist of a reinforcing felt covered
with coating asphalt; organic felts are impregnated with a saturant
asphalt. In most cases, asphalt shingles and roll roofing contain a
surfacing material, usually coarse or fine mineral. Asphalt shingles
and roll roofing are installed using mechanical fasteners or cold-applied
adhesives; they do not require hot mopping asphalt. In addition, they
are typically installed over an underlayment felt that has been impregnated
with coating asphalt during manufacture, and both are affixed to the
roof substrate by mechanical means or cold adhesives [NRCA 1996].
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2.4.2
BUR Systems |
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BUR systems
were introduced in the late 1800s and remain the most popular roofing
system for commercial and industrial buildings, accounting for about
30% of the new and retrofit market for low-slope roofs [NRCA 1995].
The BUR membrane is composed of layers (or "moppings") of
mopping asphalt between felt plies of saturant asphalt or coating asphalt
reinforcing fabric, such as organic felts (e.g., cellulose), fiberglass
scrim or mat, or polyester fabric. BUR membranes are installed in multiple-ply
configurations that typically involve three to six interply moppings
of mopping asphalt. In addition, roll roofing made from organic or inorganic
materials, or a flood coat of mopping asphalt (usually
Type I) is applied as a weatherproofing top layer.
All three
grades of roofing asphalt (coating, saturant, and mopping) may be used
in the manufacture or construction of BUR systems: saturant asphalts
are used to manufacture organic felts and roll roofing; coating asphalts
are used for virtually all felt ply and roll goods; and heated mopping
asphalts are used for the interply moppings and, in some cases, the
flood coats applied in constructing the BUR membrane [NRCA 1996].
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2.4.3
Modified Bitumen Roofing Systems |
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Polymer-modified
bitumen roofing systems were introduced in the 1970s and today account
for about 23% of the new and retrofit market for low-slope (i.e., primarily
commercial and industrial) roofs [NRCA 1995]. Modified bitumen products
are of two types: (1) those made primarily with atactic polypropylene
(APP), and (2) those made primarily with styrene-butadiene-styrene (SBS)
as the polymer modifier.
APP
membranes are primarily torch-appliedthat is, they are adhered
to an underlying base sheet or to the manufacturer's approved substrate
by heating the back side of the APP membrane and the substrate with
high-intensity, propane-fired torches or specially designed hot-air
welders. The heat is applied only as needed to soften the asphalt and
make the modified bitumen membrane adhere to the substrate; these products
can be cold-applied with adhesives. SBS membranes may be applied by
adhesion in hot asphalt or in a cold-applied, solvent-based asphalt
adhesive; or they may be torch-applied [NRCA 1996].
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Flood
coat is the surfacing layer of asphalt into which surfacing aggregate
is embedded on an aggregate-surfaced, built-up roof. A flood coat
is generally thicker and heavier than a glaze coat and is applied
at approximately 45 to 60 lb/100 ft2 (a square) (2 to 3
kg/m2).
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