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Durability of waterborne road marking systems with various glass beads
Abstract
Horizontal road markings belong to ubiquitous safety features on all roads. Based on scientific evidence, markings with higher retroreflectivity lead to a meaningful drop in the number of accidents. Hence, the recently proposed by the European Road Federation 150×150 formula to maintain lines 150 mm wide and with retroreflectivity at all times above 150 mcd/m²/lx (35 mcd/m²/lx under wet conditions) on every European highway ought to increase road safety. To propose the optimum waterborne road marking system for the 150×150 formula, durability of systems consisting of three types of paints reflectorised with three types of glass beads was assessed.
After ±1.4 million vehicle passes (the period included winter road maintenance), drop in retroreflectivity was measured in all systems. The drop was highest in case of a quick-dry paint and lowest with high-performance paint. In all of the cases, selection of glass beads also played critical role and markings reflectorised with high-performance glass beads showed highest retroreflectivity under dry and wet conditions both initially and after the test – such system should be the first choice to maintain the 150×150 formula. The systems where other glass beads types were utilised performed satisfactorily or very good. Since failures after one year were measured with only a few systems, the testing continues with hopes to furnish a two-year paint-based road marking system.
Road markings are a niche category of industrial coatings with requirements related to both the coating itself and the layer of glass beads on top of it that furnish retroreflectivity (RL) – their key property, particularly needed during driving at night time and in rainy weather conditions. The most frequently used glass beads, with refractive index 1.5, are capable of delivering RL reaching only about 400 mcd/m²/lx in white and 200 mcd/m²/lx in yellow markings; very low retroreflectivity under wet conditions, seldom exceeding 50 mcd/m²/lx can be obtained. Recently introduced ‘premium’ glass beads, with refractive index marginally increased, to 1.6–1.7, can yield more than twice as high RL. It was envisaged that high RL in a yellow road marking could be achieved if a structure with high vertical profile were reflectorised with these premium glass beads. Because of strict compositional requirements for road markings used in North America, a demonstration trial was made possible due to a special specification that allowed some departure from the fixed standards, but within new high-level demands related to RL under both dry and wet conditions. The tested material was a thermoplastic mass applied as regular dots that, due to round surface, would facilitate rainwater drainage, thus permitting the glass beads surface to be exposed to deliver adequete RL also under wet conditions. The performance achieved in the field was excellent: after 30 days, RL 894 mcd/m²/lx under dry and 160 mcd/m²/lx under wet conditions were achieved. Moreover, the applied road marking system maintained the required performance for 12 months (RL 774 mcd/m²/lx), while the control failed (RL
European Union Green Public Procurement (GPP) criteria for road marking materials (comprising paints, plastic masses, and tapes) have been recently published. Among the criteria, the key is limitation in the contents of Volatile Organic Compounds (VOC); as a novelty in Europe, several solvents would be exempt from the VOC content calculations. Other parameters include exclusion of hazardous materials, heavy metals, and ingredients that would cause significant hazard warnings. In addition, contents of titanium dioxide and the quantity of recycled materials in glass beads are specified and durability of road marking systems is included. In this review, the key points of the GPP are assessed and uncertainties are pinpointed and discussed. Particular attention is given to solvents, which belong to inalienable ingredients in road marking paints. Amongst demonstrated weaknesses of the GPP, the use of nominal VOC contents instead of Ozone Formation Potential and downplaying the impact of service life on overall emissions and materials consumption appear to be the most important. Results from field testing of road marking paints are provided to exemplify this weakness.
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