Chemical changes, measured using spectroscopy, and crosslink density, measured by mechanical thermal analysis, were determined during accelerated weathering on a model polyester-urethane coating of known composition. The tensile modulus, measured above the glass transition temperature, and thus the crosslink density, decreased with exposure, as expected from the chemical changes. However, the tensile modulus, measured at room temperature, increased with exposure. Physical aging of the polymer network was found to occur concurrently with photodegradation and accounts for much of the increase in room temperature modulus. Increased hydrogen bonding in the increasingly oxidized polyesterurethane may also contribute to the increase in modulus at room temperature. Both physical and chemical changes must be determined if changes, and rates of change, in performance due to weathering are to be understood. C oatings are used in many applications and environments where high levels of durability are required. Often, the requirements include maintenance of appearance, barrier properties, toughness, etc. during exposure to natural weathering, which includes solar ultraviolet radiation, heat, moisture, and pollutants. In many cases, the lifetime of the coating is determined by a change in appearance, but crucial equipment may rely on the protective properties of the coating. However, understanding the relationship between coating composition and how it changes during exposure and how that is related to macroscopic end-use properties remains a fertile field of research. Many protective attributes of a coating can be related to its mechanical integrity, e.g., chemical and corrosion protection depend crucially on whether a coating remains intact and continuous during service. Cracks or adhesion failures due to in-service stresses immediately define the useful service life of a coating. These macroscopic failures are the accumulations of, or initiated by, the erosion processes that result from chemical changes during degradation.Understanding how the mechanical integrity of a coating, or any construction material, changes under exposure is very important. However, mechanical measurements, while relating closely to end-use and service life, are not directly linked to the chemical changes in the material and, thus, how the environment affects a polymer. It is necessary to examine the chemical changes, relate those to changes in the polymer network, and include any other physical changes that occur before understanding the important factors that determine the macroscopic mechanical properties of a coating.There has been a great deal of excellent basic work done in determining the chemical changes that occur during degradation of a particular coating's chemistry. Unfortunately, further progress towards connecting that information with macroscopic properties has often been hindered by examining commercial polymers that are undisclosed complex mixtures of functionality and composition. This work examines model polyester-urethane systems...
American football helmets are subjected seasonally to a myriad of environmental conditions from expected use and storage and yet are reused without a relational understanding between service life degradation and changes in impact performance. Comprehensive investigations could link rates and degrees of material degradation to scientifically and clinically meaningful changes in helmet performance. Therefore, the purpose of this research was to preliminarily quantify the effects of accelerated weathering on (1) colorimetric, chemical, fluorescent, and thermal properties; (2) surface and bulk mechanical properties; and (3) impact performance of an American football helmet outer shell material. Helmet-grade plaques were exposed to 480 h of accelerated weathering. Surface-specific shifts (p < 0.05) in colorimetric, chemical, fluorescent, thermal, and mechanical properties were observed at the plaque surface. Plaque-derived tensile specimens underwent monotonic tensile testing, and the photodegraded ∼1% of the Weathered plaque surface thickness led to 10%, 12%, and 9% increases (p < 0.05) in Young’s modulus, yield stress, and ultimate tensile stress, respectively. Impact performance was analyzed with a protocol attempting to employ expected on-field impact conditions. Weathered and Non-weathered helmet surrogate systems managed impact energy progressively less effectively across five repetitive trials (p < 0.05); yet the absence of significant Weathered differences demonstrated that the plaque–foam systems performed similarly. Results identified a battery of diagnostic tools to characterize the degradation of outer shell material exposed to accelerated weathering. Thus, the comprehensive approach herein may be used toward the evaluation of additional service life exposures, as well as examine on-field deterioration of full helmet outer shells.
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