Search

The past decade has seen increased research interest in studying bicontinuous structures formed via colloidal self-assembly due to their many useful applications. A new type of colloidal gel, solvent segregation-driven gel (SeedGel), has been recently demonstrated as an effective approach to arrest bicontinuous structures with unique and intriguing properties, such as thermoreversibility, structural reproducibility, and sensitive temperature response. Here, using a model system with silica particles in the 2,6-lutidine/water binary solvent, we investigate the factors controlling the domain size of a SeedGel system by varying the particle concentration, solvent ratio, and quenching protocol. A phase diagram is identified to produce SeedGels for this model system. Our results indicate that by adjusting the sample composition, it is possible to realize bicontinuous domains with well-controlled repeating distances (periodicities). In addition, the effect of quenching rate on the domain size is systematically investigated, showing that it is a very sensitive parameter to control domain sizes. By further heating SeedGel up into the spinodal region, the structure evolution under high temperatures is also investigated and discussed. These results provide important insights into how to control bicontinuous structures in SeedGel systems.

Graphene oxide (GO) is a good nanofiller candidate for waterborne coatings because of its outstanding physical and mechanical properties, good dispersibility in water, and low cost relative to graphene. Here, we report on the performance of a one-part, waterborne polyurethane (WPU) nanocoating formulated with four different GO loadings ([0.4% to 2.0%] by mass). The degree of GO dispersion/adhesion was evaluated using scanning electron microscopy, laser scanning confocal microscopy, and Raman microscopy. Nanocoating performance was evaluated using a dynamic mechanical thermal analyzer for mechanical properties, a customized coulometric permeation apparatus for oxygen barrier properties, a combustion microcalorimeter for flammability, a hot disk analyzer for thermal conductivity, thermogravimetric analysis for thermal stability, and a moisture sorption analyzer for water uptake. The results show that GO sheets were well dispersed in, and have good adhesion to, WPU. At the higher mass loadings ([1.2% or 2%] by mass), GO increased the modulus and yield strength of WPU by 300% and 200%, respectively, increased the thermal conductivity by 38%, reduced the burning heat release rate (flammability) by 43%, and reduced the oxygen permeability by up to sevenfold. The presence of GO, however, increased water vapor uptake at high humidity; the moisture content of 2% mass loading GO/WPU nanocoatings at 90% RH was almost twice that of the moisture content for unfilled WPU. Overall, with the exception of water uptake at very high humidity (> 70% RH), the observed improvements in physical and mechanical properties combined with the ease of processing suggest that GO is a viable nanofiller for WPU coatings.

Potential consumer exposure to nanoparticles (NPs) from nanoenabled food contact materials (FCMs) has been a driving force for migration studies of NPs from FCMs. Although NP migration from fresh, unused FCMs was not previously observed, conditions that result in significant changes to the surface of FCMs have not been investigated for NP migration into food. Therefore, a quantitative assessment of nanoparticle release from commercially available nanosilver-enabled FCMs was performed using an abrasion protocol to simulate cleaning, cutting, scraping and other stressful use conditions. Laser scanning confocal microscopy (LSCM) analysis showed a general increase in root mean square (RMS) roughness after FCM abrasion, and particle count (for particle sizes from 80 nm to 960 nm) at the surface was 4 orders of magnitude higher for the abraded FCMs. Migration was evaluated using both water and 3% (v/v, volume fraction) acetic acid as food simulants. Low concentrations of total Ag were detected in water simulants with a small portion (−2) in the form of silver nanoparticles (AgNPs). Median particle diameter ranged from 39 nm to 50 nm with particle number concentrations on the order of 106 particles dm− 2. Total Ag migration into 3% (v/v) acetic acid was significantly higher than in water; however, 3% (v/v) acetic acid was not suitable for evaluation of NP release due to dissolution of AgNPs to Ag+ under acidic solution chemistries.

Bifacial PV modules are promising to achieve a lower levelized cost of electricity (LCOE) compared to monofacial modules because light absorption occurs from both sides of the devices. The glass/transparent backsheet structure offers many advantages over the glass/glass structure for bifacial modules, such as lighter weight and higher hail resistance. However, research on the weatherability and long-term reliability of transparent backsheet materials and bifacial modules under their service environments is lacking. In this study, the material property and the durability of three fluoropolymer-based transparent backsheets and their laminated coupons have been investigated. The accelerated laboratory tests were carried out on the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) under UV/75 °C/50 % RH and UV/65 °C/50 % RH for transparent films and coupons, respectively. Optical and chemical properties of specimens were characterized by UV-visible spectroscopy, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and Raman microscopy. Nano-indentation was used to evaluate changes in surface modulus and scratch-resistance of the transparent backsheets. Preliminary results indicate that different backsheets perform differently after aging, and potential cracking could occur once films become brittle. The cracking of polyethylene terephthalate (PET) core layer was observed, as well as the cracking of the ethylene vinyl-acetate (EVA) inner layer. This study will provide a scientific basis for material choice and product development for a more reliable bifacial PV technology.

Understanding the responses of materials to environmental variables is essential for performing meaningful accelerated weathering and service life prediction. Samples of polycarbonate-b-resorcinol polyarylate copolymer (RPA), poly(acrylonitrile-co-butadiene-co-styrene) (ABS), and two polycarbonate copolymers with silicone or aliphatic diacids were exposed in the NIST (National Institute of Standards and Technology) SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) to determine the effects of ultraviolet intensity (UV irradiance), temperature, relative humidity (RH), and UV wavelength on yellowing and gloss loss and were compared to other aromatic polymers. All showed proportional response to irradiance (i.e., reciprocity) except ABS, which deviated notably at elevated temperatures. The activation energy for ABS yellowing was higher than other aromatic polymers (31 kJ mol(−1) ± 2 kJ mol(−1)) while RPA had a slightly negative activation energy (−5 kJ mol(−1) ± 3 kJ mol(−1)), reflecting differences in their photodegradation mechanisms. These two polymers also exhibited faster degradation when the RH was ≤ 10 % compared to ≥ 50 % RH. Wavelength effects varied among the polymers. The results indicate that predictive accelerated weathering should be performed with UV sources that accurately reproduce sunlight, at temperatures as close as possible to use conditions, and with RH > 10 %.

The ultraviolet (UV)-induced degradation of graphene/polymer nanocomposites was investigated in this study. Specifically, the effect of few-layer graphene nanofillers on the degradation of a thermoplastic polyurethane (TPU) and the release potential of graphene from the degraded nanocomposite surfaces were assessed. Graphene/TPU (G/TPU) nanocomposites and neat TPU were UV-exposed under both dry and humid conditions in the NIST SPHERE, a precisely controlled, high intensity UV-weathering device. Neat TPU and G/TPU were characterized over the time course of UV exposure using color measurements and infrared spectroscopy, for appearance and chemical changes, respectively. Changes in thickness and surface morphology were obtained with scanning electron microscopy. A new fluorescence quenching measurement approach was developed to identify graphene sheets at the nanocomposite surface, which was supported by contact angle measurements. The potential for graphene release from the nanocomposite surface was evaluated using a tape-lift method followed by microscopy of any particles present on the tape. The findings suggest that graphene improves the service life of TPU with respect to UV exposure, but that graphene becomes exposed at the nanocomposite surface over time, which may potentially lead to its release when exposed to small mechanical forces or upon contact with other materials.

Pigments with nanoscale dimensions are added to exterior coatings to achieve desirable color and gloss properties. The present study compared the performance, degradation, and release behavior of an acrylic coating that was pigmented by a nanoform of Cu-phthalocyanine after both natural (i.e., outdoor) and accelerated weathering. Samples were weathered outdoors in three geographically distinct locations across the United States (Arizona, Colorado, Maryland) continuously for 15 months. Identically prepared samples were also artificially weathered under accelerated conditions (increased ultraviolet (UV) light intensity and elevated temperatures) for three months, in one-month increments. After exposure, both sets of samples were characterized with color, gloss, and infrared spectroscopy measurements, and selectively with surface roughness measurements. Results indicated that UV-driven coating oxidation was the principal degradation pathway for both natural and accelerated weathering samples, with accelerated weathering leading to an increased rate of oxidation without altering the fundamental degradation pathway. The inclusion of the nanoform pigment reduced the rate of coating oxidation, via UV absorption by the pigment, leading to improved coating integrity compared to non-pigmented samples. Release measurements collected during natural weathering studies indicated there was never a period of weathering, in any location, that led to copper material release above background copper measurements. Lab-based release experiments performed on samples weathered naturally and under accelerated conditions found that the release of degraded coating material after each type of exposure was diminished by the inclusion of the nanoform pigment. Release measurements also indicated that the nanoform pigment remained embedded within the coating and did not release after weathering.

The macroscopic mechanical behavior of high-density polyethylene (HDPE) during photodegradation is characterized by decreases of tensile elongation-at-failure. An apparent linear relation between the elongation-at-failure and the molar mass indicates that the decrease of the elongation of HDPE over time is highly dependent on the decrease of the average molar mass. Possible preferential scission of the high molar mass chains was observed for HDPE exposed to ultraviolet (UV) irradiance higher than 40% (61 W/m2) of the full intensity at 50 °C. Tensile modulus of HDPE exposed at 50 °C increased with the exposure time until reaching the complete loss of ductility except the 5% UV. For 40% UV/30 °C as well as for 5% UV/50 °C, the young modulus trend cannot be evaluated with performed (small) duration. Nanomechanical test results suggest that the increased tensile modulus is due to stiffening of the entire cross-section. Furthermore, HDPE showing the complete loss of ductility exhibited significantly higher modulus in the surface regions than the core regions particularly for the UV intensity higher than 40% (61 W/m2), which increased crack sensitivity to cause embrittlement of the entire specimens.

As original equipment manufacturers (OEMs) strive to deliver improved coating performance with a sustainable footprint, opportunities for innovation are emerging, particularly on improving mechanical properties, appearance, and solids content. Resistance to scratch and mar damage is one of the key performance attributes that has been emphasized by both OEMs and consumers to maintain a vehicle's appearance and corrosion resistance over its service lifetime. Fundamental methodologies including instrumented scratch measurements at multiple size scales are used in this work as part of a product development strategy to better understand the scratch and mar behavior of automotive topcoats. This study compares physical properties of several melamin-formaldehyde and isocyanate cured clearcoats over the appropriate basecoats. Micro- and nano-scratch techniques were employed in combination with industry standard method, Amtec-Kistler carwash to identify performance differences under different scratch conditions. Mechanical and viscoelastic properties of the coatings were studied using tensile tests and dynamic mechanical thermal analysis (DMTA) to better understand the failure mechanisms associated with plastic deformation and fracture at different scratch scales. The information gathered from the above testing protocols is used to analyze coating performance in terms of the contact strain, transitions between elastic - plastic behavior, coefficient of friction and stress localization.

Migration evaluation involving nano-enabled food contact materials (FCMs) mostly focuses on potential nanoparticle release from new unused products. This may not represent consumer use practices encountered by the FCMs in their lifecycle. In order to determine if product use impacts the release of nanoparticles or other FCM components, it is necessary to perform migration evaluations under typical consumer use scenarios. A quantitative assessment of nanoparticle release from a commercially available nanosilver-enabled cutting board was performed under five conditions intended to simulate consumer use. Knife motion, washing and scratching scenarios were simulated by linear abrasion using knife blades, scrubbing pads and tungsten carbide burr attachments, respectively. Migration was evaluated using water and 3% acetic acid as food simulants. Low concentrations of silver (Ag) were detected in water simulants, a small portion (−2) in the form of silver nanoparticles (AgNPs) with particle number concentrations on the order of 106 particles dm−2. Median particle diameter was 40 nm. Nanoparticle release into water was observed under all five consumer use scenarios studied, however there was no correlation with the different levels of stress simulated.

The surface of coatings and plastics is the first target in any degradation process initiated by ultraviolet (UV) radiation or mechanical stress (via scratch and abrasion). Surface damage can lead to changes in optical, morphological, and mechanical properties and can result in pathways for ingress of moisture and corrosive agents. Current test methods for monitoring performance of protective coatings focus on chemical properties and optical properties, such as color and gloss measurements, or invasive tests such as abrasion and cross-cut adhesion. In this study, a macroscopically nondestructive performance protocol using nanoindentation metrology via a well-controlled scratch test was applied to evaluate the scratch resistance and monitor the surface mechanical property changes in a protective coating under accelerated weathering. Polyurethane (PU) coatings with different polyol compositions were chosen for this study. Coating specimens were exposed to high-intensity UV radiation at 55°C and 75% RH conditions. Exposed specimens were removed at specified UV exposure times for surface modulus/hardness and scratch resistance characterization via nanoindentation and scratch test. The effect of polyol type and UV radiation dose on the scratch damage (scratch morphology) was investigated and correlated with the surface hardness and modulus of the materials.

Multiwall carbon nanotubes (MWCNTs) are nanofillers used in consumer and structural polymeric products to enhance a variety of properties. Under weathering, the polymer matrix will degrade and the nanofillers may be released from the products potentially impacting ecological or human health. In this study, we investigated the degradation of a 0.72% (by mass) MWCNT/amine-cured epoxy nanocomposite irradiated with high intensity ultraviolet (UV) light at various doses, the effects of UV exposure on the surface accumulation and potential release of MWCNTs, and possible mechanisms for the release resistance of the MWCNT surface layer formed on nanocomposites by UV irradiation. Irradiated samples were characterized for chemical degradation, mass loss, surface morphological changes, and MWCNT release using a variety of analytical techniques. Under 295 nm–400 nm UV radiation up to a dose of 4865 MJ/m 2 , the nanocomposite matrix underwent photodegradation, resulting in formation of a dense, entangled MWCNT network structure on the surface. However, no MWCNT release was detected, even at very high UV doses, suggesting that the MWCNT surface layer formed from UV irradiation of polymer nanocomposites resist release. Four possible release resistance mechanisms of the UV-induced MWCNT surface layer are presented and discussed.

Temperature effect on the kinetics of photodegradation and surface accumulation of nanoparticles in an epoxy nanocoating exposed to ultraviolet light (UV) was investigated. A model epoxy coating containing 5% untreated nanosilica was selected. Exposed film specimens were removed at specified UV dose intervals for measurements of chemical degradation of the epoxy component, and nanosilica accumulation on specimen surface release as a function of UV dose for four temperatures. The chemical degradation was measured using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV–visible spectroscopy. Atomic force microscopy was employed to determine the kinetics of nanosilica accumulation on the nanocoating surface during UV exposure. The temperature dependence behaviors of kinetic parameters obtained by various measurement techniques will be used to better understand the degradation mechanism and surface accumulation of nanoparticles in exterior nanocoatings.

Service-life prediction for polymers remains an elusive goal. In this chapter, we describe efforts toward it for two polymers, polyethylene (PE) and a sealant. For both polymers, samples were weathered under accelerated laboratory conditions to characterize degradation as a function of ultraviolet radiation dose in MJ m−2. Samples were also weathered outdoors in Homestead Florida (FL), United States. Our intent was to predict the outdoor degradation using only the laboratory measurements. A Bayesian hierarchical model was used as a generative model for the laboratory measurements of degradation. It enabled predictions of outdoor degradation and the propagation of uncertainties associated with those predictions. The results of the exercises were mixed. The predictions were accurate for PE, including predictions of the high variability in the outdoor degradation measurements. The predictions were not accurate for the sealant. Potential reasons for this are discussed, and changes to our approach that improve the agreement between the outdoor predictions and the measurements of degradation are contemplated.

The NIST (National Institute of Standards and Technology) SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used to obtain acceleration parameters, such as activation energies, for bisphenol A polycarbonate (PC), poly(butylene terephthalate) (PBT), a PC/PBT blend, and poly(styrene-co-acrylonitrile) (SAN) containing 3% rutile titanium oxide white pigment. The responses were color shift and 60 gloss loss. The materials exhibited excellent reciprocity over the irradiance range of the SPHERE for both properties. Activation energies for color change were 16–21 and 9–16 kJ/mol for gloss loss. Relative humidity (RH) was varied over the range of

The extent to which hydrophilic GO nanofillers regulate polymer degradation during exposure to a combination of ultraviolet (UV) radiation and moisture is presently unknown. Accordingly, this study systematically evaluated the effect of GO on polymer degradability under both humid UV and dry UV conditions. Both GO accumulation at the polymer nanocomposite (PNC) surface and GO release following degradation were also investigated. Different mass loadings of GO were incorporated into waterborne polyurethane (WBPU), a commonly used exterior coating, and the resulting GO/WBPU nanocomposites were exposed to precisely controlled accelerated weathering conditions using the NIST Simulated Photodegradation via High Energy Radiant Exposure (SPHERE) device. Thickness loss and infrared spectroscopy measurements indicated GO slightly improved the durability of WBPU under dry UV conditions but not under humid UV conditions. Raman spectroscopy, scanning electron microscopy, and atomic force microscopy modulus measurements indicated that GO accumulation occurred at and near the PNC surface under both conditions but to a more rapid extent under humid UV conditions. Minimal GO release occurred under dry UV conditions as measured with Raman spectroscopy of aqueous run-off from a simulated rain spray applied to degraded PNCs. In contrast, PNC surface transformations under humid UV conditions suggested that GO release occurred.

Samples of polycarbonate (PC), poly(butylene terephthalate) (PBT), a PC/PBT blend, and poly(styrene-co-acrylonitrile) (SAN), all containing 3% TiO(2) (by mass), were exposed in the NIST (National Institutes of Standards and Technology) SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) to determine the effects of UV intensity (irradiance), temperature, relative humidity (RH), and UV wavelength on yellowing and gloss loss. There was no effect of irradiance, such that the samples obeyed reciprocity and doubling the irradiance doubled the rate of degradation. The activation energy for yellowing was determined to be ≈ 20 kJ/mol for PC, PC/PBT, and SAN and ≈ 16 kJ/mol for PBT. The activation energy for gloss loss was determined to be 9–16 kJ/mol. Thus, a 10 °C increase in temperature results in a 20%–30% increase in degradation rate. There was no consistent effect of RH on PC or PC/PBT yellowing or gloss loss. SAN degraded rapidly under dry conditions but showed little effect for RH > 10%. PBT lost gloss more slowly under dry conditions but displayed no RH effect with yellowing. Shorter wavelength UV had a greater effect on PC/PBT compared to PC or PBT.

An increase in production of commercial products containing graphene-family nanomaterials (GFNs) has led to concern over their release into the environment. The fate and potential ecotoxicological effects of GFNs in the environment are currently unclear, partially due to the limited analytical methods for GFN measurements. In this review, the unique properties of GFNs that are useful for their detection and quantification are discussed. The capacity of several classes of techniques to identify and/or quantify GFNs in different environmental matrices (water, soil, sediment, and organisms), after environmental transformations, and after release from a polymer matrix of a product is evaluated. Extraction and strategies to combine methods for more accurate discrimination of GFNs from environmental interferences as well as from other carbonaceous nanomaterials are recommended. Overall, a comprehensive review of the techniques available to detect and quantify GFNs are systematically presented to inform the state of the science, guide researchers in their selection of the best technique for the system under investigation, and enable further development of GFN metrology in environmental matrices. Two case studies are described to provide practical examples of choosing which techniques to utilize for detection or quantification of GFNs in specific scenarios. Because the available quantitative techniques are somewhat limited, more research is required to distinguish GFNs from other carbonaceous materials and improve the accuracy and detection limits of GFNs at more environmentally relevant concentrations.

Concomitant with the development of polymer nanocomposite (PNC) technologies across numerous industries is an expanding awareness of the uncertainty with which engineered nanoparticles embedded within these materials may be released into the external environment, particularly liquid media. Recently there has been an interest in evaluating potential exposure to nanoscale fillers from PNCs, but existing studies often rely upon uncharacterized, poor quality, or proprietary materials, creating a barrier to making general mechanistic conclusions about release phenomena. In this study we employed semiconductor nanoparticles (quantum dots, QDs) as model nanofillers to quantify potential release into liquid media under specific environmental conditions. QDs of two sizes were incorporated into low-density polyethylene by melt compounding and the mixtures were extruded as free-standing fluorescent films. These films were subjected to tests under conditions intended to accelerate potential release of embedded particles or dissolved residuals into liquid environments. Using inductively-coupled plasma mass spectrometry and laser scanning confocal microscopy, it was found that the acidity of the external medium, exposure time, and small differences in particle size (on the order of a few nm) all play pivotal roles in release kinetics. Particle dissolution was found to play a major if not dominant role in the release process. This paper also presents the first evidence that internally embedded nanoparticles contribute to the mass transfer, an observation made possible via the use of a model system that was deliberately designed to probe the complex relationships between nanoparticle-enabled plastics and the environment.

Nanoindentation was used to monitor the changes in surface mechanical property of nanotitanium dioxide (nanoTiO 2 )/acrylic urethane (AU) coatings exposed to high-intensity UV radiation at 55°C and dry conditions. Five TiO 2 nanoparticles of similar size but different crystal phases and surface treatments were selected in this study. Preliminary results indicate that two competing behaviors (hardening vs. softening) were found in surface modulus and hardness changes, as UV exposure time was increased. The relationship between photodegradation-induced surface mechanical behavior and the characteristics of nanoTiO 2 (crystal phases and surface treatments) was examined. It appears that AU coatings containing rutile nanoTiO 2 , regardless of surface treatment, exhibit the typical hardening behavior. However, the AU coatings containing anatase or anatase/rutile mix (high anatase ratio) nanoTiO 2 show a softening behavior. It implies that photoreactivity of nanoTiO 2 plays a major role in the photodegradation-induced surface mechanical behavior. Other key physical parameters, such as surface morphology and filler dispersion measured using laser scanning confocal microscope, and bulk mechanical properties measured by tensile testing were obtained. The correlation between these physical quantities and the surface mechanical properties may provide a fundamental understanding of the photodegradation-induced surface mechanical behavior for different nanoTiO 2 /AU coatings.

The objective of this study was to investigate the effect of nanoparticle dispersion on surface morphological changes and degradation process in polymeric coatings during exposure to ultraviolet (UV) radiation. Three types of nano-titanium dioxide (nano-TiO2) were selected and dispersed into acrylic urethane (AU) coating to generate degrees of nanoparticle dispersion states. Two accelerated exposure conditions: wet (30 degrees C and 75% relative humidity (RH)) and dry (30 degrees C and 0% RH), were selected. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was used to monitor surface chemical degradation. Laser scanning confocal microscopy (LSCM) was used to characterize nanoparticle dispersion and surface/subsurface morphological changes in the AU coatings during UV exposure. For a given nanoparticle, similar surface morphological changes of the coatings indicated the similar degradation processes under the wet and dry conditions, but the degradation was faster under the wet condition. Surface morphological changes were closely related to the nanoparticle dispersion in three coatings, and the heterogeneity in nanoparticle dispersion significantly affects the degradation process and dominates the degradation patterns. (C) 2014 Elsevier Ltd. All rights reserved.

Nanoparticles are increasingly used in polymer coatings (i.e., nanocoatings) to improve multiple properties including the mechanical, electrical, gas barrier, and ultraviolet (UV) resistance of traditional coatings. These high performance nanocoatings are often used in outdoor environments. However, because polymers are susceptible to degradation by weathering elements, nanoparticles in a nanocoating may be released into the environment during its life cycle, which potentially poses an environmental health and safety concern and may hinder application of these advanced coatings. This study presents protocols and experimental technique to quantify the release of nanosilica from epoxy nanocoating as a function of UV exposure. Specimens of an epoxy coating containing 5% untreated nanosilica in specially designed holders were exposed to UV radiation (295–400 nm) in a well-controlled high-intensity UV chamber. Exposed specimens were removed at specified UV dose intervals for measurements of coating chemical degradation, mass loss, nanosilica accumulation on specimen surface, and nanosilica release as a function of UV dose. Measurement of nanosilica release was accomplished by (a) periodically spraying UV-exposed specimens with water, (b) collecting runoff water/released particles, and (c) analyzing collected solutions by inductively coupled plasma-optical emission spectrometry using a National Institute of Standards and Technology (NIST)-developed protocol. Results demonstrated that the amount of nanosilica release was substantial and increased rapidly with UV dose. Mass loss, chemical degradation, and silica accumulation on specimen surface also increased with UV dose.

Many coatings properties such as mechanical, electrical, and ultra violet (UV) resistance are greatly enhanced by the addition of nanoparticles, which can potentially increase the use of nanocoatings for many outdoor applications. However, because polymers used in all coatings are susceptible to degradation by weathering, nanoparticles in a coating may be brought to the surface and released into the environment during the life cycle of a nanocoating. Therefore, the goal of this study is to investigate the process and mechanism of surface degradation and potential particle release from a commercial nanosilica/polyurethane coating under accelerated UV exposure. Recent research at the National Institute of Standards and Technology (NIST) has shown that the matrix in an epoxy nanocomposite undergoes photodegradation during exposure to UV radiation, resulting in surface accumulation of nanoparticles and subsequent release from the composite. In this study, specimens of a commercial polyurethane (PU) coating, to which a 5 mass % surface treated silica nanoparticles solution was added, were exposed to well-controlled, accelerated UV environments. The nanocoating surface morphological changes and surface accumulation of nanoparticles as a function of UV exposure were measured, along with chemical change and mass loss using a variety of techniques. Particles from the surface of the coating were collected using a simulated rain process developed at NIST, and the collected runoff specimens were measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES) to determine the amount of silicon released from the nanocoatings. The results demonstrated that the added silica nanoparticle solution decreased the photodegradation rate (i.e., stabilization) of the commercial PU nanocoating. Although the degradation was slower than the previous nanosilica epoxy model system, the degradation of the PU matrix resulted in accumulation of silica nanoparticles on the nanocoating surface and release to the environment by simulated rain. These experimental data are valuable for developing models to predict the long-term release of nanosilica from commercial PU nanocoatings used outdoors and, therefore, are essential for assessing the health and environmental risks during the service life of exterior PU nanocoatings.

The effect of biaxial-residual–compressive surface stress on the surface modulation of solid films is analyzed. The results show that the biaxial compressive stress can create surface modulation with the orientation being 45° (135°) to the principal directions of the stresses. Using uniaxial tension, a complementary set of nonsymmetrical polymer ripple gratings is formed on the fracture surface of a polymer film that is sandwiched between two relatively glass slides. The effect of geometrical shape and boundary confinement is examined on the surface modulation. The orientation of the polymer ripple gratings is found to be 45° to the longitudinal direction of the films for long-rectangular polymer thin films in accord with the analysis. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Two kinds of TiO2 filled epoxy coatings were designed and prepared to obtain pigments with different dispersion degrees of TiO2 particles. Laser scanning confocal microscope (LSCM) was used to investigate both the horizontal and vertical distributions of TiO2 particles in the coatings. The results indicated that TiO2 in the two samples shared considerable similarity in horizental dispersion, but exhibited great difference in vertical dispersion. TiO2 showed uniform vertical distribution in disp coating, wheras a gap about 1.1 μm was found in the non-disp coating, which significantly influenced the surface optical properties of the coatings during weathering. Based on the confocal data, the model of dispersion of pigments in the coatings was proposed and the change of surface properties during weathering was predicted: the surface optical properties showed an initial decrease followed by a subsequent increase, which was in good agreement with the weathering data.

Metal oxide nanoparticles are small but easily form agglomerates in suspension, depending on the strength of particle–particle and particle–media interactions. To understand the agglomeration behavior of nanoparticles in media and relate to it to product performance testing, measurement methods are desired to characterize highly scattering metal oxide nanoparticle suspensions without dilution. In this article, we describe the advantages of using photocorrelation spectroscopy (PCS) in a backscattering detection configuration to carry out a realistic agglomerate size measurement in multiple scattering media found in most metal oxide nanoparticle suspensions. The dynamic behavior of nano-titanium dioxide (TiO2) particles in buffer solutions of different chemical composition and pH values was investigated as a sample system using PCS. The resulting autocorrelation functions (AFs) at different time intervals, particle concentrations, and pH values were measured at several detection angles. The AF exhibits a multi-mode relaxation time feature and the calculated hydrodynamic diameters strongly depended on media composition and detection angle. This result indicates that the size and dispersion of nano-TiO2 agglomerates are significantly affected by solution media. A measurement protocol for determining size and dispersion of metal oxide particles in media is proposed and related to a performance test found in industry.

The modification of microscopically rough surfaces by commonly used varnishes and resultant changes in light reflection from these surfaces were studied using laser scanning confocal microscopy, stylus profilometry and specular gloss and distinctness-of-image gloss measurements. Natural resins as well as synthetic low molecular weight and polymeric resins were studied. The molecular weights of the resins were determined using size-exclusion chromatography. All the low molecular weight resins, with weight-average molecular weights in the range 800–6500, including the natural resins, essentially produced smooth surfaces and eliminated high-frequency as well as low-frequency roughness, while polymeric resins, with weight-average molecular weights between about 25000 and 210000, produced rougher surfaces, eliminating high-frequency roughness but not all low-frequency roughness. The spatial frequency range of the roughness of the polymeric coatings decreased with increasing molecular weight. As a resul...

The objective of this study was to investigate the effect of the photoreactivity of titanium dioxide (TiO2) pigments on the photodegradation of polymeric coatings used in exterior applications. Two polymer matrices, an amine-cured epoxy (EP) and an acrylic urethane (AU), containing three types of TiO2 pigments, classified by different levels of photoreactivity, were studied. Specimens were exposed on an ultraviolet (UV) weathering chamber, the Simulated Photodegradation by High Energy Radiant Exposure device at the National Institute of Standards and Technology. Two exposure conditions were used: ambient, dry condition [25°C and 0% relative humidity (RH)] and high temperature, wet condition (55°C and 75% RH), which is similar to more severe outdoor exposures. The physical and chemical degradations of the filled coatings were monitored at periodic intervals using a combination of laser scanning confocal microscopy (LSCM) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Progression of degradation on the coating surfaces was characterized by LSCM in terms of changes in surface roughness and morphology, pigment agglomerate size, and the occurrence of pits or holes in the coatings. The observed physical changes were correlated to the chemical changes measured by ATR-FTIR as a function of UV exposure time. Both EP and AU systems showed less degradation in terms of surface roughness and morphological changes under the dry conditions compared to the wet exposure conditions. It was observed that both the pigment type (and hence photoreactivity) and particle dispersion strongly affected the degradation of both EP and AU systems.

Surface topography and gloss are two related properties affecting the appearance of a polymeric coating system. Upon exposure to ultraviolet (UV) radiation, the surface topography of a coating becomes more pronounced and, correspondingly, its gloss generally decreases. However, the surface factors affecting gloss and appearance are difficult to ascertain. In this article, atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM) measurements have been performed on an amine-cured epoxy coating system exposed to outdoor environments in Gaithersburg, Maryland. The formation of the protuberances is observed at the early degradation stages, followed by the appearance of circular pits as exposure continues. At long exposure times, the circular features enlarge and deepen, resulting in a rough surface topography and crack formation. Fourier Transform Infrared Spectroscopy (FTIR) study indicates that the oxidation and chain scission reactions are likely the origins of the surface morphological changes. The relationship between changes in surface roughness and gloss has been analyzed. The root mean square (RMS) roughness of the coating is related to nanoscale and microscale morphological changes in the surface of the coating as well as to the gloss retention. A near-linear dependence of RMS roughness with the measurement length scale (L) is found on a double logarithmic scale, i.e., RMS ∼ L f. The scaling factor, f, decreases with exposure time. The relationship between surface topography, on nano- to microscales, and the macroscale optical properties such as gloss retention is discussed. Moreover, a recent development in using an angle-resolved light scattering technique for the measurement of the specular and off-specular reflectance of the UV-exposed specimens is also demonstrated, and the optical scattering data are compared to the gloss and the roughness results.

Incorporating metal-oxide nanoparticles such as nano-alumina and nano-silica into polymer coatings to enhance mechanical durability is widely utilized in the current antiscratch and mar technologies. In this article, a quantitative study of the effect of a nano-alumina additive on the surface mechanical properties and scratch behavior of a two-part polyurethane coating is reported. An instrumented indenter with a conical diamond tip is used to measure surface mechanical properties (modulus and hardness) and to perform scratch tests, over a wide range of scratch loads. The scratch behavior in terms of the onset of elastic–plastic transition and scratch morphology were characterized by laser scanning confocal microscopy. The scratch results were correlated to the surface mechanical properties and relevant bulk material properties to understand the overall scratch behavior of the coatings. The results show that the scratch behavior of the coatings depends strongly on the concentration of nano-alumina.

Interest in the use of nanoparticles of iron, titanium, aluminum, and zinc oxides in transparent coatings for wood is increasing. Such nano-composite coatings have the potential of not only preserving the natural color of the wood, but also stabilizing the wood surface against the combined degradative effects of sunlight and moisture. The nanoparticles can be used as additives to coating formulations or deposited directly as thin films on a substrate. Thin film deposition can be accomplished by plasma-enhanced chemical vapor or by sol-gel deposition. This paper describes sol-gel deposition of a hybrid inorganic-organic thin film on wood using a mixture of metal-organic precursors and its effect on weathering properties of the wood surface.

In recent years, poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers have become prominent in high strength applications such as body armor, ropes and cables, and recreational equipment. The objectives of this study were to expose woven PBO body armor panels to elevated temperature and moisture, and to analyze the chemical, morphological and mechanical changes in PBO yarns extracted from the panels. A 30% decrease in yarn tensile strength, which was correlated to changes in the infrared peak absorbance of key functional groups in the PBO structure, was observed during the 26 week elevated temperature/elevated moisture aging period. Substantial changes in chemical structure were observed via infrared spectroscopy, as well as changes in polymer morphology using microscopy and neutron scattering. When the panels were removed to an ultra-dry environment for storage for 47 weeks, no further decreases in tensile strength degradation were observed. In a follow-on study, fibers were sealed in argon-filled glass tubes and exposed to elevated temperature; less than a 4% decrease in tensile strength was observed after 30 weeks, demonstrating that moisture is a key factor in the degradation of these fibers.

The relationship between chemical degradation and thickness loss of an unpigmented, non UV-stabilized, crosslinked amine-cured epoxy coating exposed to three UV conditions was investigated. Spin-coated samples having a thickness of approximately 7 μm on an Si substrate were prepared from a stochiometric mixture of a bisphenol A epoxy resin and a tetra-functional amine curing agent. Samples were exposed outdoors and to two accelerated laboratory UV environments. Chemical degradation and thickness loss were measured by transmission Fourier transform infrared spectroscopy (FTIRS) and laser scanning confocal microscopy (LSCM), respectively. In addition, surface roughness and morphological changes were measured by atomic forcemicrosocopy (AFM) and LSCM. Substantial chemical degradation, thickness loss, and morpholocal changes occurred in the exposed films, and the rate of chemical degradation was greater than that due to the thickness loss. This additional chemical loss was attributed to an inhomogeneous degradation process in which nanoscale localized depressions initiate at certain sites on the surface, which then enlarge and deepen with exposure time. The results of this study provide a better understanding of the degradation mechanism and should lead to the development of scientific-based models for predicting the service life of crosslinked amine-cured epoxy coatings.

Semi-gloss commerical poly(vinylidene fluoride) (PVDF) coatings typically have 60° gloss values between 20 and 50. Gloss is affected by PVDF crystallite structures and by the pigmentation. In this article, we have demonstrated that for some pigmented PVDF coatings, after 10 years of Florida exposure, the principal proximal cause of gloss changes is the formation of micron-scale pits, rather than the emergence of pigment particles at the coating surface. We have used laser scanning confocal microscopy (LSCM) and light scattering to characterize the surface topography and near-surface structure of weathered and unweathered PVDF coatings. Florida-weathered PVDF coatings show only a modest increase in the root mean square (RMS) roughness of the surface, even when oticeable gloss loss has occurred. Changes in gloss can be correlated with surface roughness and other topographical, features, including the formation of pits and the emergence of pigments.

Current methods for scratch resistance assessment are often based on “relative but not quantitative” types of measurements, such as visual inspection, gloss changes, and changes in gray scale level or lightness. Most results are used for qualitative assessment purposes, which result in the lack of a repeatable and reliable standardized test method for the polymer materials community. To implement a scientifically based standardized test method for quantifying scratch resistance, it is vital to understand the relationships between material mechanical properties, morphology, and appearance (optical properties) of surface and subsurface deformation. In this article, preliminary results from a scratch testing protocol to identify the “onset” of plastic deformation in poly(methyl methacrylate) and poly(propylene) commercial samples are presented. Recent advances in optical scattering measurements to identify the onset of plastic deformation by analyzing specular and off-specular intensities are also presented.