|
|
|
|
|
|
|
|
|
August 2001 |
|
||
Contents
|
||||||||||||||||||||||||||||||||||||||||||||
| Main Page | |
 Manufacture of Asphalt Roofing Products |
|
| Occupational Exposure to Asphalt Fumes | |
| Guidelines for Protecting Workers | |
| References | |
| Abbreviations | |
| Glossary | |
| Appendix | |
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].
The three basic grades of roofing asphalt are
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 2–1. 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].
| Type | Susceptibility to flow at roof temperatures | Highest slope suitable for use (% slope) | Softening point (°F) |
|---|---|---|---|
| I, dead level | Relatively susceptible | 2 |
135–151 |
| II, flat | Moderately susceptible | 4 |
158–176 |
| III, steep | Relatively non susceptible | 25 |
185–205 |
| IV, special steep | Relatively non susceptible | 50 |
210–225 |
Today, three commercially popular roofing products or systems are made from roofing asphalt, each with different characteristics and applications:
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].
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].
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:
APP membranes are primarily torch applied— that 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].
The manufacture of asphalt roofing products consists of six major operations:
The six major production support operations are as follows:
Figure 3–1 shows a typical manufacturing line for an asphalt roofing product. The following sections give detailed descriptions of the major processes associated with the manufacture of asphalt roofing products.
Preparing the roofing asphalt is an integral part of manufacturing asphalt roofing products. This preparation, called blowing or air-blowing, involves bubbling air through liquid asphalt flux in blowing stills. Blowing stills are closed-process vessels located out-of-doors. Emissions from the vessels are vented to capture devices located away from work areas and to incinerators (see Figure 3–2). Blowing operations may be located at oil refineries, asphalt processing plants, or asphalt roofing products plants.
A blowing still is a tank fitted near its base with a sparger (air lines in a spider arrangement). The purpose of the sparger is to increase contact between the air and the asphalt. Air is forced through holes in the sparger into a tank of hot asphalt flux. The air rises through the asphalt, resulting in an exothermic oxidation reaction.
Sometimes a catalyst such as ferric chloride is added to assist in this transformation. Because of the exothermic nature of the reaction, the temperature may reach 500 °F (260 °C); the use of external water cooling systems or internal water sprays is required to maintain the desired temperature. Such temperature control also prevents thermal cracking of the chemical constituents of the asphalt.
Air-blowing the asphalt raises its softening temperature and reduces its penetration, resulting in a "harder", more viscous asphalt suitable for roofing applications.
Asphalt is normally delivered in bulk to the asphalt roofing plant by pipeline, tanker truck, or railcar. Bulk asphalts are delivered in liquid form at temperatures of 200 to 450 °F (93 to 232 °C), depending on the type of asphalt and local practice.
Several tanker unloading techniques are used. The most common method is to couple a flexible pipe to the tanker and pump the asphalt directly into the appropriate storage tanks. Another unloading procedure is to pump the hot asphalt into a large, open funnel connected to a surge tank. From there, asphalt is pumped into storage tanks.
Asphalt flux (see Section 2.3) is usually stored at 124 to 174 °F (51 to 79 °C), although storage temperatures of up to 450 °F (232 °C) have been noted. Lower temperatures of the asphalts are usually maintained with steam coils in the tanks. Saturant and coating asphalts are normally stored at 400 to 500 °F (204 to 260 °C). Temperatures are maintained by heating the tanks directly or by cycling the asphalt through external heat exchangers, usually of the closed-tube type. Asphalt is transferred within the plant by closed pipeline.
Figure 3-1. Diagram of a typical manufacturing line for an asphalt roofing product. (Source: EPA [1998].)
Figure 3-2. Preparation of roofing asphalt by air blowing. (Source: EPA[1998].)
The following sections provide detailed descriptions of the manufacturing processes for roofing asphalt products and product groups.
Saturant asphalt felt is the most basic asphalt roofing product. Figure 3–3 depicts a typical manufacturing line for asphalt-saturated felt. It consists of a dry felt feed roll, a dry looper section, a saturator spray section, a saturator dipping section, heated drying-in drums, a wet looper, a finish floating looper, and a roll winder.
Organic felt may weigh from approximately 20 to 55 lb/480 ft2 (a common unit in the paper industry), depending on the intended product. The felt is unrolled from the unwind stand onto the dry looper, which maintains a constant tension on the material. The dry floating looper provides a reservoir of web material to match the intermittent operation of the roller to the continuous operation of the line.
From the dry looper, the felt may pass into the spray section of the saturator (not used in all plants), where saturant asphalt at 400 to 480 °F (205 to 250 °C) is sprayed onto one side of the felt through several nozzles. In the saturator dip section, the saturated felt is drawn over a series of rollers, with the bottom rollers submerged in hot saturant asphalt at 400 to 480 °F (205 to 250 °C).
During the next step, the heated drying-in drums press saturant asphalt into the felt. Occasionally, additional saturant asphalt is added at this point. The wet looper increases absorption by providing sufficient time for the saturant asphalt to penetrate the felt.
The saturated felt then passes through water-cooled rolls onto the finish floating looper. These steps allow the felt to cool and serve as an accumulator to match the continuous operation of the line to the intermittent operation of the roll winder. Finally, the felt is rolled and cut to size on the roll winder. Common weights of saturant felt are approximately 15 to 50 lb/108 ft2 of saturant felt (which covers exactly 100 ft2 of roof).
Figure 3–4 illustrates the typical manufacturing process for producing organic asphalt roofing shingles, mineral-surfaced rolls, and smooth rolls. For organic products, the initial processing steps are similar to the saturant asphalt felt line just described and include the saturator and looper. Organic felt products pass to the saturator and then, in some plants, on to a coater.
The coater employs a roller-type system to apply a stabilized coating asphalt to the top and bottom surfaces of the webbing sheet. Stabilized coating asphalt is made up of harder, more viscous asphalt that has a higher softening point than saturant asphalt. The mineral stabilizer* may consist of finely divided lime, silica, slate dust, dolomite, or other mineral materials. The softening point of saturant asphalt varies from 104 to 165 °F (40 to 74 °C), whereas the softening point of coating asphalt varies from 210 to 240 °F (99 to 116 °C)
The "filled" or "stabilized" coating asphalt applied during the coater operation is made in the coater-mixer, which is usually positioned above the manufacturing line at the coater. Coating asphalt, typically at 400 to 520 °F (204 to 271 °C), is piped into the mixer, and the mineral stabilizer is delivered by screw conveyor. To eliminate moisture problems and to help maintain the temperature above 360 /F (182 /C) in the coater-mixer, the mineral stabilizer is dried and preheated before being fed into the coater-mixer.
The weight of the finished product is controlled by the amount of coating asphalt used. The coater rollers can be moved closer together to reduce the amount of coating asphalt applied to the felt, or they can be separated to increase it. Most modern plants are equipped with automatic scales or profile scanners that monitor the sheets during the manufacturing process and warn the coater operator when the product is running under or over specifications.
Colored granules are applied in a section of the manufacturing line that usually consists of a multi-compartmented granule hopper, two parting agent hoppers, and two large press rollers. The hoppers are fed through flexible hoses from one or more machine bins above the line. These machine bins (sometimes called surge bins) provide temporary storage. The granule hopper drops colored granules from its various compartments onto the top surface of the moving sheet of coated webbing in the sequence necessary to produce the desired color pattern on the roofing. Parting agents such as talc and sand are applied from parting agent hoppers to the back surface of the coated sheet. Smooth-surfaced products do not require the application of colored granules. Talc or sand is usually applied to both sides when smooth roll roofing is made.
The next step is product cooling and seal-down strip application. As in the case of the saturated felt line, the sheet is cooled rapidly by passing it around water-cooled rollers in an abbreviated looper arrangement. Usually, water is also sprayed on the surfaces of the sheet to speed this cooling process.
Subsequently, the asphalt seal-down strip is applied to self-sealing coated roofings by a roller partially submerged in a pan of hot sealant asphalt that is usually covered. Extrusion is another method of seal-down application. Some products are also texturized at this point by passing the sheet over an embossing roll that forms a pattern in the surface of the sheet. The final steps in the production of asphalt roofing shingles and roll roofing are the finish or cooling looper (which is essentially the same process used for saturated felts), cutting, and packaging. The sheet destined for roll roofing is wound on a mandrel, cut to the proper length, and packaged. When shingles are made, the material from the finish looper is fed into the shingle cut machine. After the shingles have been cut, they are usually moved by roller conveyor to automatic packaging equipment— although manual packaging is still used for some shingle products. The packaged shingles are then stacked on pallets and transferred by forklift to storage areas or waiting trucks.
For fiberglass products (which today make up about 85% of the asphalt roofing shingle market and more than 90% of the ply sheets used in BUR), the initial saturation operation is eliminated. As shown in Figure 3–5, the manufacture of fiberglass roofing products begins (as does the manufacture of organic felts) with the feeding of the mat through a dry looper; it then proceeds to a coater.
Figure 3-3. Typical flow diagram for the production saturant asphalt felt. (Source: EPA[1988].)
Figure 3-4. Typical flow diagram for the production organic asphalt shingles and rools. (Source: EPA[1988].)
The manufacturing line for modified bitumen products is similar to that used for organic asphalt roofing shingles and roll roofing. A diagram for the production of modified bitumen products appears in Figure 3–6.
The first step in the manufacture of modified bitumen products is mixing the asphalt with either atactic and isotactic polypropylene or styrene-butadiene-styrene and a mineral stabilizer in large heated tanks (356 °F [180 °C]). This mixture is then applied in an impregnation vat to a polyester and/or fiberglass, mat-base webbing. (The impregnation vat consists of a looper-type arrangement in a shallow vat of the asphalt mixture and a set of rollers that meter the thickness of the product.) During the cooling process, either a polypropylene backing or granules are applied to the top side of a sheet (which is still hot) floating on a water-filled tray. The sheet is then either completely submerged in another water-filled tray or sprayed with water to finish the cooling process. After the sheet is dried with a fan, talc or sand is applied to the under-surface side as a parting agent. The sheet then passes through a finish looper (accumulator) and is wound on rolls.
Figure 3-5. Typical flow diagram for the production of fiberless shingles and rolls. (Source: ARMA[1997].)
Figure 3-6. Diagram for the production of modified bitumen product. (Source: EPA[1988].)
†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].
‡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).
*The mineral stabilizer used in asphalt roofing applications is an inorganic material, typically a crushed rock. Because this material is inorganic, it is less susceptible to temperature change and fire. These properties are important for shingle manufacturing. Asphalt with stabilizer is uniform and consistent within the climatic temperature range. Essentially, the stabilizer reduces viscosity in colder weather (making the shingle less brittle) and increases viscosity in warmer weather (increasing the softening point). In addition, the mineral stabilizer decreases the flammability of asphalt, thus allowing a higher fire rating of the shingle.
| View
all Alerts 2001-127.pdf (Full Document) |
This
document is also available in PDF format.
