Although there is considerable published research pertaining to the usefulness of or problems associated with durability testing of polymers or paints and coatings, there is little published work specifically addressing durability testing for retroreflective sheetings. The simulation of exterior exposure stresses in artificial accelerated tests is discussed and measurements of the spectral power distribution of light sources commonly used in accelerated tests are compared with that for sunlight. Two exposure experiments are described that show how poor simulation of exterior stresses can lead to reversals between prdictions from artificial tests and exterior exposures. Variability in exterior exposures is illustrated by results from a single set of sheetings exposed at different times at the same site. Experiments using a single lot of retroreflective sheeting exposed in identical devices all operating the same test cycle are used toquantify the variability associated with artificial accelerated testing. Artificial accelerated and exterior exposures of a series of eight lots of retroreflective sheeting are compared and show that there are large differences in sheeting rank performance between the artificial accelerated tests and 5-year exterior exposures. An experiment comparing several exposure orientations for a model retroreflective sheeting indicated a 2:1 increase in degradation rate for a 45 degree angle or solar tracking exposures relative to the vertical. This paper appears in transportation research record no. 1230, Work-zone traffic control and tests of delineation material.
Abstract