D.G. Goodwin Jr., C. Bernard, D. Stanley, T. Nguyen, L. Sung
National Institute of Standards and Technology,
Keywords: polymer nanocomposites, graphene oxide, UV aging, weathering, coatings
Summary:Graphene oxide (GO) can be used to enhance the properties and extend the service life of polymer nanocomposites (PNCs). GO is a good nanofiller candidate for water-borne polyurethane (WBPU) coatings since GO has outstanding physicochemical and mechanical properties, disperses in water, and is relatively low in cost. GO can also potentially enhance the anti-corrosion properties of WBPU. For this reason, well-dispersed GO/WBPU nanocomposites were prepared at five different GO loadings and characterized with laser scanning confocal microscopy (LCSM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The performance properties, including adhesion strength, mechanical strength, oxygen permeability, flammability, thermal stability, thermal conductivity, and water vapor sorption of GO/WBPU and unfilled WBPU coating were measured and compared. In terms of mechanical strength, 2% w/w GO increased the modulus and yield strength of WBPU by 300% and 200%, respectively. At 1.2% w/w loading, GO reduced the burning heat release rate (flammability) by 43% and increased the thermal conductivity by 2.2%, respectively. With respect to barrier properties, a 7-fold decrease in oxygen permeability was observed while the water uptake at 90% RH increased by 38%. Relative to unfilled WBPU, GO/WBPU nanocomposites were shown to have superior performance properties, with the exception of water uptake at high humidity. In combination with simple processing methods, this suggests that GO is an economically viable nanofiller for WBPU coatings. During the life cycle of GO/PNCs, degradation of the nanocomposite can occur and have an impact on material durability and environmental health and safety. Previously, when exposed to UV radiation having wavelengths similar to those of sunlight, the WBPU matrix of GO/WBPU nanocomposites underwent photodegradation. Subsequent mass loss and accumulation of a large GO concentration at the nanocomposite surface was observed. In terms of environmental health and safety, this indicates that exposure of GO at the material surface occurs and can potentially lead to GO release from the WBPU matrix with unknown effects on the surrounding environment. Loss of GO via release would also have a negative impact on the service life of the GO/PNC nanocomposite. For this reason, GO release from aged PNCs was evaluated in this research. Simulated aging of GO/PNCs in the environment was achieved by GO/PNC exposure to a highly collimated, uniform flux of ultraviolet (UV) light under controlled temperature and humidity conditions (NIST SPHERE). Released materials were collected from the surface using simulated rain and analyzed with UV-Visible spectroscopy, Raman spectroscopy, and thermal techniques. For the first time, insight into the concentration of released graphene-family nanomaterials from aged PNCs will be described along with the challenges associated with these measurements. Specifically, the interference of released polymer fragments, which are also carbonaceous materials, will be discussed. In summary, the performance and aging characteristics of GO/WBPU nanocomposites will be evaluated to determine the benefits and limitations of using GO nanofillers, with a focus on coatings applications.