Screening Level Assessment of Arsenic and Lead Concentrations in Glass Beads Used in Pavement Markings

Bead Manufacturing

The bead manufacturing process is a multistep process. The process starts when recycled glass cullet is received by the manufacturer. The cullet arrives via truck or rail and is stored in piles on the property (often exposed to the ambient conditions) until the cullet is turned into beads. The cullet is moved from the storage pile to large hoppers using a pay loader. The cullet is then passed through a series of grinders that breaks the cullet down into pea-size pellets. During the grinding portion of the process, dust suppression controls are typically used.

In the primary manufacturing process used in industry today, the ground pellets are then introduced into an up-flow furnace. The pellets are melted in the lower portion of the furnace, and the molten glass rises due to temperature-driven convection and advective air flow. The flow rate through the furnace, combined with the temperature within the furnace, creates the spherical beads as the molten glass rises, cools, and re-solidifies within the furnace. The process to create the beads results in formation of beads across several particle size fractions. The formed beads are then passed to additional hoppers and are immediately sieved to isolate beads of the desired particle size, which are then stored in bins until formulated into products.

Products are formulated by combining beads of different particle sizes together to achieve the final blend. Beads from different bins are fed by gravity through a chute, and the resulting mix of various bead sizes are packaged into super-sacks, 50-lb bags, or other customer-defined packaging.

Worker exposure to the beads occurs at three main points:

1.      During formulation of the product as the blended beads are fed into their final packaging.

2.      During QA/QC testing.

3.      As a result of spilled beads.

During product formulation, workers aid in the process by manually aligning the chute to the packaging for the super-sacks. For the 50-lb bags, the workers hold the bags while they are filled. Samples of each formulated product are tested to ensure the correct particle size distribution and spherical shape. Workers within the quality control laboratory come into incidental contact with the beads during testing. Workers may also come into contact with the beads that are spilled during the process. However, once the beads are packaged, worker exposure to the beads is minimal. The packaged beads are transported in sealed packaging around the manufacturing floor using a palate and forklift. The beads are stored within the facility until they are loaded via forklift for transport to the consumer. Workers may also be exposed to heavy metals through contact with the glass cullet in addition to exposure to metals in the formulated bead products.

Bead Blending

At some locations within the United States, both domestic beads and foreign beads are blended into final products without onsite bead manufacturing. Worker exposure to the beads at these operations is similar to worker exposure to beads occurring during bead formulation described in the preceding subsection.

Bead Transport

The potential for bead release during transport from the manufacturer to the pavement-marking agency is minimal. Because the products are individually packaged in sacks and covered with a water-protective barrier, such as a tarp, during transport, the loss of beads and exposure to the driver are low. The beads are delivered to the consumer and off loaded into storage using a forklift.

Bead Storage and Transfer

The end user of the beads is either a public entity or a private contractor that marks roadways. The agency receives the palletized beads in either a super-sack or a 50-lb sack. The large sacks are either opened in the storage yard, where beads are transferred into the specially designed marking vehicles, or taken unopened into the field where they are staged for later use. The smaller bags are used where the markings are limited in size and/or where hand application of beads is the most practical approach.

Beads may be spilled during receipt and storage, but localized releases mainly occur during transfer of beads to equipment in the storage yard. The extent to which released beads migrate outside of the yard is a function of a number of factors, the two most important being yard construction (e.g., protective coverings, land slope, design features such as sumps) and climate (arid/dust, wet/leaching). Materials released within the storage yard are of particular interest because they may accumulate over time. Considering the durability of the beads and the low associated rate of break-down, a storage yard used over many years could accumulate a significant concentration of beads. The transfer of beads to equipment outside of the yard will also potentially result in releases to the environment; however, the beads would be distributed over a wide area reducing the likelihood of accumulation.

Bead Application

The potential exists for a significant amount of beads to be released into the environment while being applied to the base marking material. To ensure good coverage, excess beads are often applied and a level of overspill is accepted as industry practice. Methods to curtail bead loss are in limited use (e.g., vertical drops, zero relative velocity nozzles, bead sheaths). The low cost of the beads makes more precise application or the use of recovery devices relatively uneconomical.

The degree to which beads become mobilized is based on many factors (including weather, physical characteristics of the roadway, and street sweeping frequency), as well as the type of markings being applied. The general classes of roadway line markings are short (lines for intersections, traffic signals, and crosswalks) and long lines (lines applied over distance using specially designed vehicles). The application of beads to short lines is generally accomplished with small, single operator equipment or by hand spreading. The amount of bead loss is not quantified; however, an amount similar to that for long lines was observed in the field for an urban area with gutters (which tends to accumulate beads) along the roadside.

For long lines, bead application is accomplished through automated bead drop delivery systems that place beads onto the marking material in a continuous process figure 11 and figure 12). In general, beads are fed by gravity from a holding vessel to the bead drop nozzles. The amount of beads deposited and the direction of bead release is governed by manual adjustment of the nozzle aperture. In some cases, multiple nozzles may be used to provide additional bead loading for wide lines or to allow higher vehicle speeds.

Road Marking Wear and Degradation

A portion of the beads that were successfully applied to the base coat of marking material will be abrasively removed as a result of traffic wear and weathering. The majority of the detached beads will be intact, and little chipping/crushing of beads is expected to occur. The fraction of beads removed can be expressed relative to the loss of reflectivity of the lines. Where lines have lost 25 percent of the reflectivity, it can be inferred that 25 percent of the adhered bead load (assumed to be 85 percent of the applied bead load) has been removed by wear. At the point that re-marking is required, an additional layer of beads and base material is applied on top of the remaining line, or the remaining line/base material is removed. Application of several layers of lines prior to removal and re-application is typical.

Figure 11. Photo. Application of glass beads to long line markings.

Figure 12. Photo. Close-up of glass bead application to long lines.

Bead Removal/Disposal

The final disposition of the beads and base material used in roadway markings, owing to reaching the maximum feasible number of layers or to demolition and removal of the road itself, may result in release of beads to the environment. The line remnants are removed by a number of methods (including abrasives, water jets, and chemical removal), and the resulting material is either collected using a vacuum or released to the environment. When vacuum attachments are used, very little material is lost to the environment and releases are dependent on ultimate disposition of the vacuumed material in a landfill or at the equipment storage yard. In the case of mechanical removal without vacuum, the majority of remnant material (including beads or pulverized bead fractions) is likely to be lost to the environment, and a portion may become airborne. In addition, the beads would be concentrated because the remaining beads are held together in the base material that would also be removed. Because some base materials (such as epoxy and thermoplastic) would retain the beads in one area and prevent them from dispersing, the concentrations associated with removed marking material may be increased.

Human Exposures Associated With the Bead Workflow

Human exposure to glass beads using in pavement markings may occur at any phase in the workflow lifecycle of the beads. Field observations of manufacturing environments, storage facilities, product applications, and product removal were carried out to determine the individuals who are most likely to experience significant exposure to beads released to the environment. The results of the field observations are reported in the next section.