Glass beads composed of soda lime glass are essential for providing retroreflectivity of road markings. Retroreflectivity occurs when incident light from vehicles is refracted within glass beads that are embedded in road surface markings and then reflected back into the driver's field of view. In North America, approximately 227 million kilograms (500 million lb) of glass beads were used for road surface markings annually in the 2010s. Roughly 520 kilograms (1,150 lb) of glass beads are used per mile during remarking of a five-lane highway system; typically in Europe, the glass beads are spread at 0.4 kg/m² of marked surface. The massive demand for glass beads has led to importing from countries that used outdated manufacturing regulations and techniques. Consequently, glass beads contaminated with toxic elements found their way to the markets. In the past, heavy metals such as arsenic, antimony, and lead were added during the manufacturing process as decolourizers and refining agents. It has been found that these toxic elements incorporated into the glass matrix may leach to the environment. To ensure that glass beads used for road marking (pavement markings/traffic paints) remain free from harmful elements, regulations were set in many countries. Consequently, contaminated glass beads were eliminated from the market. Recent analyses performed in Europe showed that glass beads used for road markings, from several manufacturers worldwide, were depleted of the harmful elements such as lead, arsenic, antimony, cadmium, chromium, and mercury., Glass beads collected from the environment were not contaminated. >,
Composition and manufacturing
The majority of glass beads for road markings and other industrial usage (such as blasting, peening, filtration, and filling of plastic composites) are made from crushed recycled float glass in special vertical furnaces, where at about 1300 °C the irregular shards melt and within milliseconds acquire a round shape. The preparation of such glass beads can also be done using virgin glass melts. In such case, the synthesis begins when calcium carbonate is heated to 800–1300 ∘ {\displaystyle ^{\circ }} C. This heating causes a decomposition reaction which forms solid calcium oxide and releases carbon dioxide gas.
Similarly, sodium carbonate decomposes to sodium oxide and releases carbon dioxide gas.
Sodium oxide is then reacted with silica to produce sodium silicate liquid glass.
Lastly, to complete the general structure of the soda-lime glass, calcium oxide is dissolved in solution with sodium silicate glass, which ultimately reduces the softening temperature of the glass. Additional metals and ions are added to this melted glass to improve its properties, and the compound is then sprayed and formed into beads using either the direct or indirect method.
Overall, the percent composition of major compounds found in the final glass beads with a refractive index of 1.5 made from virgin raw materials is shown below. Essentially the same composition has glass beads prepared from recycled float glass.
In addition to these primary components of soda-lime glass, manufacturers used to include, before the standards and regulations were imposed and enforced, the heavy metals arsenic, antimony, and lead to refine and improve the properties. Lead in the form of PbO is added to increase the durability of the glass to withstand harsh road conditions. Arsenic and antimony are used as fining agents that facilitate the removal of gas bubbles from the molten mixture. Carbon dioxide produced by the decomposition of calcium carbonate and sodium carbonate is removed to obtain the required retroreflective properties of the glass. In addition, both arsenic and antimony are used as decolorizers. Having a colorless glass is crucial to maximizing retroreflectivity. Arsenic in its inorganic form assists in the decolorization of the glass by controlling iron's oxidation state. Arsenic oxidizes ferrous oxide to its less colorful counterpart, ferric oxide. Antimony in the form of Sb2O5 performs a similar reaction as arsenic, oxidizing ferrous oxide to ferric oxide.
According to the US Environmental Protection Agency, the Resource Conservation and Recovery Act limits the levels of heavy metal content in accordance with their toxicity. It was reported that between 2008 and 2015 these three heavy metals were found in glass beads imported to the United States and to Brazil from countries with little to no regulation on heavy metal content, but also were identified in domestic production in varying concentration. For example, beads obtained from North America were reported to contain approximately 15 mg of arsenic per kg of beads, while some from China had concentrations of up to 1000 mg/kg. Concentrations of each of these metals and the comparison between the old and new reports are listed in the table below.
(a) Standard deviations from three determinations are provided in parentheses. (b) The analysis for content of Cr(VI) resulted in no detection above 0.1 mg/kg. (c) Sample could not be fully digested. (d)Analysis for Cr(VI) was not done and is not required for this type of GB.
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