Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation

Abstract

Photodegradation of polymer coatings generally involves photooxidation, resulting in the formation of oxidized products, chain scission, and crosslinking. On severe exposure to ultraviolet (UV) light in the presence of air, chemical degradation transforms into substantial changes in the physical and mechanical properties, leading to failures of the coatings. Systematic research by NIST on service life prediction of polymeric coatings indicates that the degradation of polymer coatings starts from the submicrometer degradation-susceptible regions at the surface and then grows in width and depth. Additionally, due to the oxygen diffusion effect and the attenuation of the UV light passing through the polymer, the degradation can be spatially heterogeneous. In this study, the changes with depth of the mechanical and chemical properties of a UV-exposed epoxy/polyurethane system were measured by nanoindentation and Fourier transform infrared spectroscopy (FTIR) microscopy using cross-sectioned specimens. Multilayers of epoxy/polyurethane samples were prepared by a draw-down technique. After curing, samples were exposed to the outdoors in Gaithersburg, MD, for four months. Crosssectioned slices of the exposed and unexposed samples, approximately 500 nm thick as-prepared by microtoming, were used for micro-FTIR imaging. Samples for nanoindentation were prepared by embedding the epoxy/polyurethane multilayers (both exposed and unexposed) in a molding compound, followed by microtoming and polishing the embedded films in the thickness direction. Micro-FTIR images clearly show that, for the outdoor exposed samples, substantial amounts of oxidation products are distributed in the 60 lm deep region from the surface to the epoxy bulk, decreasing in the center of epoxy region and increasing again toward the epoxy/urethane interface. Nanoindentation results also show that the modulus significantly increases in the first 60 lm region after UV degradation, and then decreases gradually with depth until a value slightly higher than the modulus of the undegraded epoxy is reached. The modulus rises again in the region near the epoxy/urethane interface. These similarities in the depth profiles of the properties indicate the linkage between the chemical degradation and the mechanical degradation. The study clearly shows that the spatial distribution of chemical species and mechanical properties is heterogeneous in the thickness direction for polymer coatings after UV degradation. It also demonstrates that cross-sectional analysis using nanoindentation and micro-FTIR imaging techniques is a useful method to characterize the mechanical and chemical depth profiles of polymer coating degradation.

Effect of Hexafluorozirconic Acid Pretreatment on Cathodic Delamination of Epoxy Coatings from Steel Substrates

“…When an organic coating is physically damaged by impact, scratching, or wear and subsequently is exposed to an electrolyte, the electrolyte's access to the underlying steel substrate allows electrochemical reactions to take place at the coating/substrate interface. [1][2][3] As a result, oxygen reduction occurs with electrons released by anodic dissolution of the steel substrates in defect areas, whch results in a buildup of hydroxide ions under the coatings, which in turn degrades the adhesion between coatings and substrates through alkalization. [4][5][6] This process, called cathodic delamination, is recognized as one of the primary mechanisms of coating failures on steel.…”

Challenges and Progress in High‐Throughput Screening of Polymer Mechanical Properties by Indentation

“…Mechanical properties of thin polymeric or hybrid films deposited on a substrate, as well as mechanical property gradients within the film, can be investigated by indenting either on the surface of the film or along cross sections of the film. [9,10] DSI is the preferred technique if the amount of material available for testing is limited, owing to either limited material supply or prohibitive material cost. Such limited Depth-sensing or instrumented indentation is an experimental characterization approach well-suited for high-throughput investigation of mechanical properties of polymeric materials.…”

Surface degradation process affected by heterogeneity in nano-titanium dioxide filled acrylic urethane coatings under accelerated UV exposure

“…The preparation of nano-TiO 2 /AU films is as follows: (1) Nanoparticles were added slowly into acrylic resin (Joncryl 588, BASF) and stirred using a high speed mechanical mixer (Dispermat -BYK Gardner) at 52.4 rad/s for 10 min; followed by continuous mixing at 366.5 rad/s for 30 min; (2) isocyanate (Desmdur N3200, Bayer) was added to the mixture at a 65/35 acrylic / isocyanate mass ratio and mixed at 366.5 rad/s for an additional 10 min; (3) the mixture was degassed for 2 h in a vacuum oven; (4) the mixture was applied to release paper using a drawdown technique; (5) film samples were cured at room temperature overnight, and then post-cured at 130 o C for 30 min. The thickness of dry film was approximately 110 µm measured by micrometer.…”