Joannie W. Chin

Sorption and Diffusion of Water, Salt Water and Concrete Pore Solution in Composite Matrices

In recent years, the use of fiber-reinforced polymer composites in civil infrastructure has been promoted as a solution to the deterioration of bridges, buildings, and other structures composed of traditional materials, such as steel, concrete, and wood. Any application of a polymer composite in an outdoor environment invariably involves exposure to moisture. There is also potential for exposure to saline conditions in waterfront or offshore structures, and alkaline environments, as would be encountered by a reinforcing bar in a cementitious material. This study characterizes the sorption and transport of distilled water, salt solution, and a simulated concrete pore solution in free films of vinyl ester, isophthalic polyester (isopolyester) and epoxy resins, all commercially important materials for use in structural composites. Diffusion of all three liquids in each of the three materials was observed to follow a Fickian process. Mass loss was observed for the isopolyester in salt water and concrete pore solution at 60°C, suggesting hydrolysis that was accelerated by the high temperature exposure. Both the rate of uptake, as well as the equilibrium uptake, were greater at 60°C, compared with ambient conditions. Diffusion coefficients calculated from the mass uptake data revealed that, although the epoxy resin had the highest equilibrium uptake, it had the lowest diffusion coefficient.

Fate of nanoparticles during life cycle of polymer nanocomposites

Nanoparticles are increasingly used in consumer and structural polymeric products to enhance a variety of properties. Under the influence of environmental factors (e.g., ultraviolet, moisture, temperature) and mechanical actions (e.g., scratching, vibrations, abrasion), nanoparticles could potentially release from the products and thus have negative effects on the environment, health and safety. The fate of nanoparticles in polymer nanocomposites during their exposure to UV environment has been investigated. Epoxy polymer containing multi-walled carbon nanotubes (MWCNTs) and silica nanoparticles were studied. Specially-designed cells containing nanocomposite specimens were irradiated with UV radiation between 295 nm and 400 nm. Chemical degradation, mass loss and surface morphology of the epoxy nanocomposites, and release of nanoparticles were measured. Epoxy containing MWCNTs exposed to UV radiation degraded at a much slower rate than the unfilled epoxy or the epoxy/nanosilica composite. Photodegradation of the matrix resulted in substantial accumulation of nanoparticles on the composite surfaces. Silica nanoparticles were found to release into the environment, but MWCNTs formed a dense network on the composite surface, with no evidence of release even after prolonged exposure. Conceptual models for silica nanoparticle release and MWCNT retention on the surface during UV exposure of nanocomposites are presented.

Linking Accelerated Laboratory Test with Outdoor Performance Results for a Model Epoxy Coating System

Laboratory and outdoor exposure results have been mathematically linked for a model epoxy coating system using a reliability-based methodology. Accurate and timebased measurements on both exposure environments and degradation properties for epoxy specimens exposed to accelerated laboratory weathering device and outdoor environments were performed. Laboratory weathering tests were conducted on the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure), a device in which spectral ultraviolet (UV) wavelength, spectral intensity, temperature, and relative humidity (RH) can be precisely and accurately controlled over time. A factorial design consisting of 4 temperatures, 4 RH levels, 4 UV spectral wavelengths, and 4 UV spectral intensities was used in exposing the epoxy samples on the SPHERE to assess the effects of critical environmental factors on chemical degradation of this material. Outdoor exposure experiments were carried out on the roof of a NIST laboratory located in Gaithersburg, MD. Panel temperature and ambient RH of the outdoor exposure and the solar spectrum were used to characterize the roof environment at 12 minute intervals. The chemical degradation for specimens exposed on the SPHERE and in the outdoor environments was quantified by transmission FTIR and UV-visible spectroscopies. Tests using FTIR absorbance ratios showed that the mechanisms of chemical degradation for samples exposed outdoors and in the laboratory were similar. Two approaches, a modelfree heuristic approach and a mathematical predictive model, were used in linking field and laboratory exposure results. Successful linkages have been made using both approaches. The study strongly demonstrated that the reliability-based methodology is capable of linking laboratory and field exposure data and predicting the service life of this type of polymeric material.

Fate of graphene in polymer nanocomposite exposed to UV radiation

Graphene is increasingly incorporated in polymers to enhance their mechanical, thermal and electrical properties. However, organic polymers are prone to degrade when exposed to UV radiation. Therefore, graphene in polymer nanocomposites could eventually be released into the environment during their life cycle, which might have a negative impact on the environment and thus presents a roadblock to their use. This study investigates the degradation of a graphene/polyurethane composite and characterizes the graphene concentration at the nanocomposite surfaces during exposure to UV radiation. The polyurethane was a one-component, water-borne polyurethane and graphene material was graphene oxide (GO) sheets. GO/WBPU composites having a thickness between 105 μm and 150 μm were exposed to 75% RH, 50°C, and UV radiation between 290 nm and 400 nm in a NIST-developed UV chamber. Chemical degradation, mass loss, and surface morphology were measured at specified exposure time using FTIR, gravimetry, SEM, AFM and LCSM techniques. Results showed that, when exposed to UV radiation having wavelengths similar to those of the sunlight, the polyurethane matrix underwent photodegradation, subsequent mass loss and accumulation of a large amount of graphene on the composite surface.

Critical role of particle/polymer interface in photostability of nano-filled polymeric coatings

Nanoparticle-filled polymeric coatings have attracted great interest in recent years because the incorporation of nanofillers can significantly enhance the mechanical, electrical, optical, thermal, and antimicrobial properties of coatings. Due to the small size of the fillers, the volume fraction of the nanoparticle/polymer interfacial area in nano-filled systems is drastically increased, and the interfacial region becomes important in the performance of the nano-filled system. However, techniques used for characterizing nanoparticle/polymer interfaces are limited, and thus, the mechanism by which interfacial properties affect the photostability and the long-term performance of nano-filled polymeric coatings is not well understood. In this study, the role of the nanoparticle/polymer interface on the ultraviolet (UV) stability of a nano-ZnO-filled polyurethane (PU) coating system was investigated. The effects of parameters influencing the particle/polymer interfacial properties, such as size, loading, surface modification of the nanoparticles, on photodegradation of ZnO/PU films were evaluated. The nature of the interfacial regions before and after UV exposures were characterized by atomic force microscopy (AFM)-based techniques. Results have shown that the interfacial properties strongly affect chemical, thermo-mechanical, and morphological properties of the UV-exposed ZnO/PU films. By combining tapping mode AFM and novel electric force microscopy (EFM), the particle/polymer interfacial regions have been successfully detected directly from the surface of the ZnO/PU films. Further, our results indicate that ZnO nanoparticles can function as a photocatalyst or a photostabilizer, depending on the UV exposure conditions. A hypothesis is proposed that the polymers in the vicinity of the ZnO/PU interface are preferentially degraded or protected, depending on whether ZnO nanoparticles act as a photocatalyst or a photostabilizer in the polymers. This study clearly demonstrates that the particle/polymer interface plays a critical role in the photostability of nano-filled polymeric coatings.

Analysis of the single-fiber fragmentation test

An analysis of the single-fiber fragmentation test was investigated.An approximate solution for the stress fields of a fiber embedded in a polymer matrix of different elastic moduli was obtained by the Eshelby method. The fiber was modeled as a prolate spheroid. The axial stress of the fiber increases with increasing aspect ratio and fiber-matrix shear modulus ratio and decreases with increasing matrix and fiber Poissons ratios. Using this analysis, the fracture stress of a single-fiber fragmentation specimen was derived. The applied stress at fiber fracture decreases monotonically with increasing aspect ratio of the fragmented fiber and increases with increasing fiber and matrix Poissons ratios. This model is in qualitative agreement with published experimental data.

More durable or more vulnerable? – Effect of nanoparticles on long-term performance of polymeric nanocomposites during UV exposure

ZnO nanoparticle is being used as an inorganic UV absorber for polymers. However, the mechanism of how ZnO nanoparticles influence the photo degradation of polymersis is not well understood. This study has investigated the role of ZnO nanoparticles in the long-term performance of a polyurethane (PU) nanocomposite subject to UV radiation. PU samples containing different levels of ZnO nanoparticles were exposed to the NIST Simulated Photodegradation via High Energy Radiant Exposure (SPHERE) UV chamber under both dry (0% RH) and moist (75% RH) conditions at 45°C. Chemical and physical properties with exposure times were characterised using multiple spectroscopic and microscopic techniques. The results indicated that the studied ZnO nanoparticles acted as a catalyst and accelerated the photodegradation of PU. The photo-catalytic effect was dependent on ZnO concentration and RH. It is suggested that systematical long-term performance study under different exposure environments is important for correctly evaluating the role of nanoparticles on durability and sustainability of polymer nanocomposites.