Mechanical property changes and degradation during accelerated weathering of polyester-urethane coatings

Abstract

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...

Application of augmented finite element and cohesive zone modelling to predict damage evolution in metal matrix composites and aircraft coatings

“…Hence, the predicted lifetimes presented below apply to coatings at high strain areas and therefore represent worst case estimations. c) Degraded material properties for PU matrix: Mechanical property changes during degradation of the PU matrix were taken from (Skaja et al, 2006). In their studies, unfilled coating samples were exposed to cycles of four hours light (0.55 W/m 2 at 340 nm) at 60°C and four hours dark with water spray at 25°C according to ASTM D 4587-91.…”

“…The tensile strain-at-break value decreases dramatically within the first two weeks from approx. 100% (the exact initial value is not given in (Skaja et al, 2006) to only 8%, followed by a moderate drop to 3.5% over the next 4 weeks. Reduced elongation-at-break is a common observation for polymers subjected to weathering, where brittle fracture initiates from flaws generated by photo oxidation (Skaja et al, 2006).…”

“…100% (the exact initial value is not given in (Skaja et al, 2006) to only 8%, followed by a moderate drop to 3.5% over the next 4 weeks. Reduced elongation-at-break is a common observation for polymers subjected to weathering, where brittle fracture initiates from flaws generated by photo oxidation (Skaja et al, 2006). Because of this embrittlement of the PU matrix during degradation CZM for the matrix material was not considered in the simulation.…”

“…d) Crack initiation locations: There were two types of possible crack initiation locations considered. The first was to initiate from flaws generated by photo oxidation at the coating surface since the surface suffers from earlier degradation than the bulk (Skaja et al, 2006). This type also includes mechanical damage such as scratches, and repair or refinishing that can introduce non-uniform external surface features which can act as crack initiation points (Jaya et al, 2012).…”

Effects of Environmental Conditions on Degradation of Automotive Coatings

“…Weathering factors are those that are applied to the coating by weathering (or climate), and cause alteration in chemical structure (Nguyen et al, 2002 a;, Bauer, 1982, affecting various aspects of the coating properties such as physical (Osterhold & Patrick, 2001), mechanical (Tahmassebi & Moradian,2004;Nichols et al, 1999;Gregorovich et al, 2001;Nichols & Darr, 1998;Nichols,2002;Skaja, 2006) and electromechanical (Tahmassebi et al, 2005) properties. The severity of degradation caused by weathering factors depends strongly on climatic condition.…”