Deborah L. Stanley

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.

A pilot interlaboratory comparison of protocols that simulate aging of nanocomposites and detect released fragments

Environmental context Nanoparticles are contained in many commercialised products, but the lack of validated methods to assess their potential release into the environment hampers our ability to perform a reliable risk assessment. Equipment to simulate aging is available, but the challenge is to sample released entities, and to analyse those fragments with suitable nano-analytics. We describe methods to characterise the degradation and surface accumulation of nanoparticles, and to quantify fragments released during UV irradiation of polymer nanocomposites. Abstract The safe use of nanoparticles as fillers in nanocomposite materials depends, in part, on a good understanding of what is released from aging nanocomposites, and at which rate. Here we investigated the critical parameters of the nanoparticle release phenomenon by a pilot inter-laboratory study of a polyamide containing 4mass% of silica nanoparticles (nanosilica). The main focus is on the validity range of the aging and release protocols. Both induced release by mechanical shear after dry weathering at different UV intensities and spontaneous release during wet weathering were investigated. We propose a combined protocol based on the finding that the characteristics of released fragments – which are the essential input for fate, transport and (eco-)toxicological testing – were reproducible between laboratories and between different aging, sampling and analysis protocols: the released fragments were a polydisperse mixture of predominantly composite fragments from the nanometre up to several micrometre diameter, and of clustered or individual nanosilica unbound to polymer. The unbound fraction was microscopically observed but could not be quantified. We found that aging conditions are very critical for the release rates, not for release characteristics. The sampling protocol tolerates some differences. Simplified aging + immersion protocols can at least partially replace, complement and extend dedicated weathering apparatus with run-off collection.

Statistical Methods for Degradation Data With Dynamic Covariates Information and an Application to Outdoor Weathering Data

Degradation data provide a useful resource for obtaining reliability information for some highly reliable products and systems. In addition to product/system degradation measurements, it is common nowadays to dynamically record product/system usage as well as other life-affecting environmental variables, such as load, amount of use, temperature, and humidity. We refer to these variables as dynamic covariate information. In this article, we introduce a class of models for analyzing degradation data with dynamic covariate information. We use a general path model with individual random effects to describe degradation paths and a vector time series model to describe the covariate process. Shape-restricted splines are used to estimate the effects of dynamic covariates on the degradation process. The unknown parameters in the degradation data model and the covariate process model are estimated by using maximum likelihood. We also describe algorithms for computing an estimate of the lifetime distribution induced by the proposed degradation path model. The proposed methods are illustrated with an application for predicting the life of an organic coating in a complicated dynamic environment (i.e., changing UV spectrum and intensity, temperature, and humidity). This article has supplementary material online.

DNA damaging potential of photoactivated p25 titanium dioxide nanoparticles.

Titanium dioxide nanoparticles (TiO2 NPs) are found in numerous commercial and personal care products. Thus, it is necessary to understand and characterize their potential environmental health and safety risks. It is well-known that photoactivated TiO2 NPs in aerated aqueous solutions can generate highly reactive hydroxyl radicals ((•)OH), which can damage DNA. Surprisingly, recent in vitro studies utilizing the comet assay have shown that nonphotoactivated TiO2 NPs kept in the dark can also induce DNA damage. In this work, we utilize stable isotope-dilution gas chromatography/tandem mass spectrometry to quantitatively characterize the levels and types of oxidatively generated base lesions in genomic DNA exposed to NIST Standard Reference Material TiO2 NPs (Degussa P25) under precisely controlled illumination conditions. We show that DNA samples incubated in the dark for 24 h with TiO2 NPs (0.5-50 μg/mL) do not lead to the formation of base lesions. However, when the same DNA is exposed to either visible light from 400 to 800 nm (energy dose of ∼14.5 kJ/m(2)) for 24 h or UVA light at 370 nm for 30 min (energy dose of ∼10 kJ/m(2)), there is a significant formation of lesions at the 50 μg/mL dose for the visible light exposure and a significant formation of lesions at the 5 and 50 μg/mL doses for the UVA light exposure. These findings suggest that commercial P25 TiO2 NPs do not have an inherent capacity to oxidatively damage DNA bases in the absence of sufficient photoactivation; however, TiO2 NPs exposed to electromagnetic radiation within the visible portion of the light spectrum can induce the formation of DNA lesions. On the basis of these findings, comet assay processing of cells exposed to TiO2 should be performed in the dark to minimize potential artifacts from laboratory light.

Cracking and delamination behaviors of photovoltaic backsheet after accelerated laboratory weathering

The channel crack and delamination phenomena that occurred during tensile tests were utilized to study surface cracking and delamination properties of a multilayered backsheet. A model sample of commercial PPE (polyethylene terephthalate (PET)/PET/ethylene vinyl acetate (EVA)) backsheet was studied. Fragmentation testing was performed after accelerated aging with and without ultraviolet (UV) irradiation in two relative humidity (RH) levels (5 % RH and 60 % RH) at elevated temperature (85 °C) conditions for 11 days and 22 days. Results suggest that the embrittled surface layer resulting from the UV photo-degradation is responsible for surface cracking when the strain applied on the sample is far below the yielding strain (2.2 %) of the PPE sample. There was no surface cracking observed on the un-aged sample and samples aged without UV irradiation. According to the fragmentation testing results, the calculated fracture toughness (KIC) values of the embrittled surface layer are as low as 0.027 MPa·m1/2 to 0.104 MPa·m1/2, depending on the humidity levels and aging times. Surface analysis using attenuated total reflectance Fourier transform infrared and atomic force microscopy shows the degradation mechanism of the embrittled surface layer is a combination of the photodegradation within a certain degradation depth and the moisture erosion effect depending on the moisture levels. Specifically, UV irradiation provides a chemical degradation effect while moisture plays a synergistic effect on surface erosion, which influences surface roughness after aging. Finally, there was no delamination observed during tensile testing in this study, suggesting the surface cracking problem is more significant than the delamination for the PPE backsheet material and conditions tested here.

Photodegradation modeling based on laboratory accelerated test data and predictions under outdoor weathering for polymeric materials

Photodegradation, driven primarily by ultraviolet (UV) radiation, is the primary cause of failure for organic paints and coatings, as well as many other products made from polymeric materials exposed to sunlight. Traditional methods of service life prediction involve the use of outdoor exposure in harsh UV environments (e.g., Florida and Arizona). Such tests, however, require too much time (generally many years) to do an evaluation. To overcome the shortcomings of traditional methods, scientists at the U.S. National Institute of Standards and Technology (NIST) conducted a multi-year research program to collect necessary data via scientifically-based laboratory accelerated tests. This paper presents the statistical modeling and analysis of the photodegradation data collected at NIST, and predictions of degradation for outdoor specimens that are subjected to weathering. The analysis involves identifying a physics/chemistry-motivated model that will adequately describe photodegradation paths. The model incorporates the effects of explanatory variables which are UV spectrum, UV intensity, temperature, and relative humidity. We use a nonlinear mixed-effects model to describe the sample paths. We extend the model to allow for dynamic covariates and compare predictions with specimens that were exposed in an outdoor environment where the explanatory variables are uncontrolled but recorded. We also discuss the findings from the analysis of the NIST data and some areas for future research.

Impact of Environmental Factors on Polymeric Films Used in Protective Glazing Systems

Accelerated and natural aging of safety films used in protective glazing systems was investigated by the use of Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy, and tensile tests. Accelerated conditions involved simultaneous exposure of specimens to ultraviolet (UV) radiation between 295 and 450 nm and each of four temperature/relative humidity (RH) environments, i.e., (a) 30 °C at <1 % RH, (b) 30 °C at 80 % RH, (c) 55 °C at <1 % RH, and (d) 55 °C at 80 % RH. Outdoor weathering was performed in Gaithersburg, MD, in two different time periods. FTIR spectra indicate that different exposure conditions have no consequence on the nature and the proportions of the oxidation products, suggesting that similar degradation mechanisms were operative under all outdoor and indoor conditions. In the accelerated exposure, the rate of degradation is found to be influenced dominantly by UV radiation. The combination of UV radiation and temperature results in a cumulative effect, producing more rapid degradation. Analogous to the chemical changes, post-yield mechanical behaviors (such as strain hardening modulus and elongation to break) are markedly reduced, while the Young’s modulus is minimally affected. Photodegradation leads finally to instability in the polymer’s necking behavior and embrittlement, which is explained in terms of chain scissions of the tie molecules in the amorphous region. Samples subjected to outdoor weathering exhibit significantly slower photodegradation, but the degradation mechanism is the same so higher doses of environmental factors can be used to provide reliable acceleration in short-term aging tests.