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. Author manuscript; available in PMC: 2020 Oct 6.
Published in final edited form as: NanoImpact. 2020;17:https://doi.org/10.1016/j.impact.2019.100199.

Evaluating performance, degradation, and release behavior of a nanoform pigmented coating after natural and accelerated weathering

Ronald S Lankone a,1, Emmanuel Ruggiero b,1, David G Goodwin Jr a, Klaus Vilsmeier b, Philipp Mueller b, Sorin Pulbere b, Katie Challis c, Yuqiang Bi d, Paul Westerhoff d, James Ranville c, D Howard Fairbrother e, Li-Piin Sung a, Wendel Wohlleben b,*
PMCID: PMC7537477  NIHMSID: NIHMS1588431  PMID: 33029568

Abstract

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.

Keywords: Weathering, Release, Coating photodegradation, NanoRelease protocol

1. Introduction

Engineered nanomaterials (ENMs) are widely incorporated in polymeric materials to improve their performance properties which can include mechanical, electrical, gas barrier, and ultraviolet (UV) resistance (Mohanty et al., 2015; Althues et al., 2007; McNally and Potschke, 2011). These high performance nano-enabled products (NEPs) are regularly used in outdoor environments. As a result, NEPs are subjected to natural degradation processes initiated by ultraviolet (UV) radiation and/or mechanical stresses (e.g., scratch and abrasion). The increasing manufacture and use of such commercial NEPs has motivated research efforts to improve understanding of the release, fate, and effects of ENMs in the environment. Consequently, many studies have focused on the degradation of NEPs and release of ENMs after laboratory based artificial aging and outdoor natural aging (Koivisto et al., 2017; Harper et al., 2015; Mitrano et al., 2015).

In the past decade, weathering chambers have been employed to accelerate UV–visible irradiation (often combined with water-spraying) to investigate the release of different classes of ENMs embedded in commercial polymeric matrices (paint, coating, plastic). Release of TiO2, SiO2, CeO2, ZnO in nanoparticle form as well as graphene and carbon nanotubes (CNTs) from paints, coatings, and thermoplastics have been studied, often showing the release of free or matrix-embedded NPs (Al-Kattan et al., 2013; Hsu and Chein, 2007; Sung et al., 2015; Al-Kattan et al., 2015; Hsueh et al., 2017; Scifo et al., 2017; Auffan et al., 2014; Klingshirn et al., 2010; Jacobs et al., 2016; Fiorentino et al., 2015; Arvidsson et al., 2013; Pacurari et al., 2016; Wohlleben et al., 2017a; Lankone et al., 2017a; Nguyen et al., 2017; Han et al., 2018; Hirth et al., 2013; Bernard et al., 2011; Goodwin et al., 2018).

Outdoor weathering conditions have also been used to investigate the degradation of coatings, paints, and thermoplastics. However, because outdoor exposure is usually time-consuming, laborious, and expensive, only a small number of studies have been performed to date, and have focused on nanoparticles such as TiO2, AgNP, ZnO, and nanoclays (Cloisite 30B) (Kaegi et al., 2008; Kaegi et al., 2010; Miklečić et al., 2015; Zaidi et al., 2010; Lankone et al., 2017b). Recently, Lankone et al. developed a novel methodology to simultaneously quantify the emission of ENMs during natural weathering across multiple outdoor weathering locations (Lankone et al., 2017b). The study investigated a range of polymer composite types embedded with either AgNPs or CNTs and found that after a year of natural weathering, < 5% of initially embedded nanomaterial released from the composite materials.

The extensive time required for natural weathering studies (i.e., > 1 year) has been a persistent motivation to develop simulated weathering protocols that accurately reproduce and accelerate natural weathering conditions so that material degradation may be investigated in an experimentally tractable timeframe. Aging phenomena at specific local weather conditions can be similarly reproduced, with different acceleration factors, by optimizing UV irradiation, intensity, and rain cycles (Nichols et al., 2013). However, it is also important to determine if the degradation mechanisms are the same across natural and accelerated weathering studies (Diepens and Gijsman, 2011; Lv et al., 2015; Santos et al., 2013).

One important class of ENMs employed worldwide is nanoforms of organic and metallo-organic pigments such as diketopyrrolopyrrole (DPP) and Cu-phthalocyanine (CuPhthalo). These ENMs are of interest to the coatings industry, and specifically automotive manufactures, for their use in improving color and gloss properties of coatings. These pigments are today considered an ENM (Babick et al., 2016; Ministère de l’Environnement, de l’Énergie et de la Mer, 2015; Wigger et al., 2018) and in Europe need to be registered by 2020 as nanoforms (Amendments of the Annexes to REACH for Registration of Nanomaterials, 2018; ECHA, 2017). The consumption of CuPhthalo in Europe was approximately 21 thousand metric tons in 2010 as both nano and non-nanoforms (SRI_consulting, 2011), with the automotive industry contributing to the greatest extent to this market. Despite the increased interest in pigmented coating technologies, few studies have been published regarding the aging of coatings pigmented by either nanoforms or non-nanoforms (Ruggiero et al., 2019; Zepp et al., 2019).

In this work, an acrylic-based automotive coating was pigmented with CuPhthalo nanoform pigment and subjected to natural and accelerated weathering. The goal of this study was to investigate and compare surface degradation and the likelihood of ENM release from this matrix after natural and artificial aging. Artificial weathering was performed following an accelerated aging procedure adapted from ISO 11341/4892-2 and referred to in the automotive industry as the Kalahari protocol (TPE 2003: The 6th International Conference on New Opportunities for Thermoplastic Elastomers; Brussels, Belgium, 16–17 September 2003, TPE 2003; 6; 2003; Brussels, Brussels, 2003; Lederer et al., 2014; Riedl, 2006), while outdoor aging was conducted for 15 months in three locations with diverse climates across the continental United States, following a previously developed methodology (Lankone et al., 2017b). Surface transformations and changes in coating performance were characterized after both exposure strategies and nanoparticles and polymer fragment release from both natural and accelerated weathering samples were also quantified with a robust methodology, named NanoRelease (Wohlleben et al., 2017a). This protocol was utilized for its consistency, sensitivity, and wide applicability towards different materials potentially released, as evidenced by its recent application to quantify release from NEPs such as polymeric plates and coating (Ruggiero et al., 2019; Zepp et al., 2019; Wohlleben et al., 2017a; Neubauer et al., 2017). Collectively, findings from this study provide knowledge on both the weathering behavior of this specific nanoform pigmented coating as well as the validity of this specific accelerated weathering strategy to study the class of coating materials that contain nanoform pigments.

2. Experimental

2.1. Materials

2.1.1. Metal-organic nanoparticles

The nanoform of Cu-Phthalocyanine used in this work has a planar nanostructure with platelet shape with minimum external dimensions of 20 nm. The Phthalocyanine macrocycle provides strong π-π interactions, generating very stable ENMs with low solubility in most organic solvents and in water. Fig. SI 1 displays representative transmission electron microscopy (TEM) scans of the nanoform of the CuPhthalo pigment, Table S1 in the supporting information (SI) describes the main physico-chemical properties, and further characterization data may be found in previous investigations (Wohlleben et al., 2019).

2.1.2. CuPhthalo nanoform pigmented coatings

Metal panels coated with a CuPhthalo pigmented coating were prepared by a proprietary industrial automotive standard for the evaluation of additives (such as the pigment) at BASF in Germany. CuPhthalo nanoform pigments were mixed with a disperser in a typical solvent-borne acrylic binder. The coating was applied to a basecoat, on steel plates, 18 cm × 10 cm in size, by spray application. For post-cure, the coating plates were heated up in an oven following the specific parameters of the materials. CuPhthalo nanoform pigmented coating samples (i.e., CuPhthalo sample plates) and Reference sample plates (prepared pigment-free) were produced identically. The sample plates were cut to sizes of 10 cm × 9 cm and 5 cm × 4.9 cm to fit in sample holders for accelerated and natural weathering, respectively. The CuPhthalo pigment is the only difference between the Reference plates and the CuPhthalo plates, thus any modulation of durability or release is attributed to the pigment.

2.2. Accelerated weathering

Degradation of the acrylic coating was simulated using a Suntest XLS+ climate chamber (Atlas-MTS, Mount Prospect, IL). The CuPhthalo sample plates and Reference sample plates were placed inside the chamber and aged following procedures modified from ISO 11341/4892-2 (Fig. SI 2a). This modified procedure is known as the Kalahari protocol in the automotive industry and is a standard quality control method that employs aggressive conditions which simulate a hot and dry climate (i.e., The Kalahari Desert). The specific conditions are 65 W/m2, wavelength 300 nm to 400 nm, at 90 °C with an exposure time series of (1, 2, and 3) months (157 MJ m−2, 314 MJ m−2, 472 MJ m−2). Two CuPhthalo sample plates and one Reference sample plate were removed sacrificially after each period of exposure for characterization. The protocol contains no rain events and is referred to in the text as KalExp.

2.3. Outdoor weathering

CuPhthalo sample plates and Reference sample plates were distributed to Baltimore, MD, Golden, CO, and Tempe, AZ for natural weathering. These locations were selected for the diversity of their local climates, as described by the Koppen classification: Cfa (warm temperate, fully humid, hot summer), Dfc (snow, fully humid, cool summer), Bwh (arid, desert, hot arid), respectively. Individual sample plates were suspended above a sample collection jar with a Teflon strap such that incident rainwater would accumulate below but not submerge the sample itself (Fig. SI 2b). In total, six CuPhthalo sample plates, six Reference sample plates, and three empty Blank (plate free) collection jars were deployed at each location. All samples were weathered continuously, for up to 15 months, following a previously developed methodology (Lankone et al., 2017b). Each location was also equipped with a Wireless Vantage Pro Plus (Davis Instruments, Hayward, California) including UV & Solar Radiation Sensors weather station to continuously record local weather conditions (i.e. solar fluence, temperature, precipitation), and the recorded data (Fig. S3) was retrieved monthly for comparison to CuPhthalo NEP release data. We do note that release sampling was paused for the months of January and February at each location, but the CuPhthalo samples continued to weather. For comparison, weather data collected during the same period of natural weathering at the three nearest USDA UV-B Monitoring and Research Program sites is also included in Table S2 (Bigelow et al., 1998).

2.4. Nanorelease protocol

After natural and accelerated weathering, both CuPhthalo sample plates and Reference sample plates underwent a release protocol named NanoRelease (Wohlleben et al., 2017a). Briefly, sample plates were first immersed in nanopure H2O and subjected to mechanical stimulation using an ultrasonication bath (Bandelin Sonorex Digital 10P at 720 W and 35 kHz) for 1 h for sampling of detachable fragments. Small (24.5 cm2) and large (90 cm2) plates were immersed respectively in 3 mL and 12 mL of water. Afterwards, 1 mL of the immersion fluid was used for elemental analysis by ICP-MS and 0.5 mL for TEM. Sodium dodecyl sulfate (SDS) was added to the remaining fluid to help disperse any released material from the plates. The final concentration of SDS in the fluid was 0.9 g L−1 for small plates and 0.5 g L−1 for large plates. The SDS immersion fluid was then sonicated with an ultrasonic probe (Branson SFX 550 Digitaler Sonifier at 550 W and 2 kHz) for 20 s. The immersion fluid was analyzed by UV–Visible (UV–Vis) spectroscopy using a PerkinElmer Lambda 35 UV/Vis Spectrometer and Analytical Ultracentrifuge (AUC, Beckman Ultracentrifuge type XLA with integrated absorption optics) to obtain an absorption profile indicative of the relative mass concentration of all material released from the sample plates into solution. It is important to note that concentration measurements from UV–Vis spectroscopy were indicative of the concentration of all released material present in the immersion fluid, while AUC measurements were indicative of the concentration of released particulate material 10 nm to 1000 nm in size present in the immersion fluid.

2.5. Material characterization

Instrumental and methodological details regarding the characterization of coated sample plates (Color and gloss measurements, Attenuated Total Internal Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), Laser scanning confocal microscopy (LSCM)) and released material after natural weathering using Inductively coupled plasma mass spectrometry (ICP-MS) are provided in the SI.

3. Results and discussion

3.1. Changes in coating performance after accelerated weathering

CuPhthalo sample plates and References sample plates were subjected to three months of accelerated weathering (KalExp), in one-month intervals, with increasing UV doses totaling 157 MJ m−2, 314 MJ m−2, and 472 MJ m−2, respectively. This weathering method is particularly aggressive, intended to accelerate the aging process, and irradiated samples with a factor of > 5 times greater UV light intensity than they would have encountered naturally (depending on weathering location) (Pickett et al., 2005; Crewdson, 2016). As seen in Fig. 1, this UV exposure strategy rapidly deteriorated the coating on the Reference sample plates and ultimately resulted in complete top coating failure. Reference sample degradation appeared to increase systematically with increasing UV dose: after one month of KalExp, the Reference sample showed signs of discoloration and slight blistering of the top coat; after two months, a layer of degraded coating was completely exfoliated; and after three months of exposure, the exfoliated layer continued to deteriorate and the base-coat layer remaining on the substrate showed evidence of degradation (e.g., discoloration and no gloss) as well. This aggressive degradation behavior contrasted with the apparent coating resiliency against deterioration of the CuPhthalo sample plates, which were observed to remain free from obvious signs of discoloration or blistering after three months of KalExp (Fig. 1). These qualitative observations indicated the nanoform CuPhthalo pigment (originally added to impart color to the coating) conferred improved resistance against photodegradation of the coating. Quantitative measurements were performed to further investigate this behavior.

Fig. 1.

Fig. 1.

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CuPhthalo sample plates and Reference sample plates before and after (1, 2, and 3) months of KalExp.

Color and gloss measurements of the CuPhthalo sample plates and Reference sample plates before and after (1, 2, and 3) months of KalExp are shown in Fig. 2. Color and gloss results show that CuPhthalo samples exhibited an increasing change in color performance (i.e., color shift (ΔE > 1) and ΔYellowing index (ΔYI > 10)) after one and two months of exposure, but changes in color performance plateaued from two to three months of exposure. Change in lightness (ΔL, Fig. SI 4) data of the CuPhthalo samples showed a similar trend, with an initial rapid change after the first month of exposure followed by stabilization as exposure continued. Regarding sample gloss (measured at 20°), results indicated an increase in gloss loss of CuPhthalo samples with increasing periods of UV exposure, and after three months of UV exposure, gloss retention remained just above 60%. These gloss results are indicative of an increase in surface roughness, later confirmed by LSCM measurements.

Fig. 2.

Fig. 2.

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Color and gloss results of CuPhthalo sample plates and Reference samples plates after one, two, and three months of KalExp. Note, three months data for the Reference sample is not included as the coating exfoliated and demonstrated complete failure. Error bars reflect one standard deviation of the average calculated from 24 measurements per sample plate (with one sample plate analyzed per time point).

As observed in Fig. 1, the qualitative appearance of the Reference samples showed no evidence of abatement in degradation, with ΔE, ΔYI, and gloss loss all increasing rapidly over the first two months of exposure. It is noted that color and gloss measurements for the three-month exposed Reference sample are not shown since the Reference sample coating fully exfoliated after two months of UV exposure, demonstrating complete coating failure, and therefore nullified the opportunity to perform quantitative color performance measurements. After two months of exposure, the Reference sample color shift (ΔE) was five times greater compared to the CuPhthalo sample. Additionally, after two months of exposure, over 99% of the initial gloss was lost from the Reference sample. These measurements, along with visual inspection, provide quantitative evidence that the color, gloss, and overall appearance of the coating after accelerated weathering was better preserved by the addition of the nanoform CuPhthalo pigment, compared to Reference samples.

3.2. Changes in coating performance after natural weathering

CuPhthalo samples and Reference samples were also naturally weathered. After 15 months of continuous outdoor natural weathering in three different climates, CuPhthalo samples and Reference samples were characterized for color and gloss performance. It is important to note that CuPhthalo samples were characterized both as weathered (i.e., uncleaned) and after a washing procedure adopted from Platten et al. (2016) to gently remove accumulated environmental deposits (e.g., dirt) while leaving the coating surface intact, by minimizing the sheer forces applied during washing. We As seen in Fig. 3, both unwashed CuPhthalo samples and unwashed Reference samples measurements exhibited large changes color and gloss performance after weathering, with increased ΔE, ΔYI, and a loss of gloss retention. However, after washing, it is observed that the color of CuPhthalo samples weathered at all three locations returned to near non-weathered values, with ΔE values returning to < 1 (the human eye can clearly distinguish color changes once ΔE exceeds a value of about 3) and ΔYI values measured to be within measurement uncertainty (one standard deviation) of zero. Measured gloss retention after washing also increased. However, measurements indicated that samples weathered in Tempe, AZ retained lower amounts of gloss than those weathered in Golden, CO and Baltimore, MD. The elevated gloss loss for the washed CuPhthalo samples weathered in Tempe, AZ, (contrasted to low measured values of ΔE and ΔYI) is in good agreement with the sustained gloss loss measured for CuPhthalo samples after three of months of KalExp. This similarity in gloss loss behavior suggests that gloss is more sensitive to the hot and dry irradiation conditions found in both Tempe, AZ (Fig. SI 3) and during KalExp. Overall, these findings indicated that after 15 months of natural weathering, accumulated dirt led to the greatest change in color and loss of gloss performance (i.e., appearance) but these effects were easily remedied by sample washing. Additionally, 15 months of natural weathering of CuPhthalo samples resulted in significantly less deterioration of the objective appearance of the samples, compared to three months of KalExp. However, both natural and artificial hot and dry conditions did impact gloss retention most extensively of all three appearance metrics.

Fig. 3.

Fig. 3.

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Color and gloss results following 15 months of natural weathering in three locations across the United States. Results of “washed” CuPhthalo samples are also plotted for comparison. Error bars reflect one standard deviation of the average calculated from 72 measurements, resultant from 24 measurements collected from each of the three replicates per sample.

3.3. Spectroscopic investigation of coating degradation

ATR-FTIR was performed to provide chemical insight into the improved color performance and retained structural integrity exhibited by CuPhthalo pigmented samples, compared to Reference samples. FTIR spectra were collected from 4000 cm−1 to 500 cm−1 with the increase in intensity of the peak at 1685 cm−1 (spectra shown in Fig. SI 5) providing a diagnostic for assessing the extent of sample degradation as it is indicative of the degree of coating oxidation (i.e., photodegradation) (Rabek, 1995; Rabek, 1996). After 15 months of natural weathering, samples at all three locations experienced small and similar amounts of coating oxidation (Δ A.U. < 0.2) and CuPhthalo samples (unwashed and washed) and Reference samples experienced a similar extent of oxidation within measurement uncertainties (Fig. 4). The observation that no one location led to more extensive coating oxidation agreed with appearance measurements (ΔE and ΔYI, Fig. 3), however, these results do indicate that changes in coating oxidation do not correspond as well with measured changes in sample gloss. Furthermore, FTIR measurements confirmed oxidation driven by natural weathering is a slow process that does not lead to extensive coating degradation during the 15 months of exposure.

Fig. 4.

Fig. 4.

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ATR-FTIR measurements of changes in band intensity (Δ A.U.) at 1685 cm−1 are indicative of coating oxidation, following 15 months of natural weathering and three months of KalExp for both CuPhthalo samples and References samples. Error bars reflect one standard deviation calculated from 6 measurements per sample replicate (with three sample replicates analyzed for each natural weathering location and one sample replicate from KalExp).

ATR-FTIR measurements of KalExp samples showed a greater extent of sample oxidation than naturally weathered samples and a clear, systematic increase in sample oxidation with increasing exposure (Fig. SI 6). While results indicate both CuPhthalo samples and Reference samples experienced increasing oxidation with increasing exposure, the Reference samples oxidized more, compared to the pigmented samples, as seen by the greater increase in absorbance at 1685 cm−1 compared to CuPhthalo samples (Δ A.U. 0.09 vs. Δ A.U. 0.06, respectively). These results suggest that the improved color performance demonstrated by the CuPhthalo samples following KalExp (Fig. 2) was due to the nanoform CuPhthalo pigment inhibiting extensive coating oxidation. The UV–Vis absorption profile of the CuPhthalo nanoform pigment, Fig. SI 7, supports this assertion and indicates that the pigment absorbs strongly in the UV region (< 400 nm). This UV-absorbing property of the pigment is believed to be responsible for resistance against extensive photodegradation exhibited by CuPhthalo samples, by effectively reducing the dose of UV light available to degrade the polymeric coating. It is important to note that UV absorption is governed by the chemical composition and is not unique to either the nanoform or the non-nanoform of pigments. ATR-FTIR was also used to probe both the chemical nature of the remaining surface after top coating exfoliation in the Reference samples and the mechanism of coating failure (Fig. SI 8) and is discussed further in the SI.

3.4. Morphological investigation of coating degradation

To compliment spectroscopic characterization of the CuPhthalo sample plates and Reference sample plates before and after KalExp, LSCM was used to directly measure changes in surface roughness and quantify the extent to which the addition of the nanoform CuPhthalo pigment inhibited roughening (i.e., deterioration) of the coating. As shown in Fig. 5, the initial surfaces of both CuPhthalo samples and Reference samples were very smooth and free of any signs of cracking or scratches. After three months of UV exposure, small pockmark-like features formed on the CuPhthalo samples while the Reference samples showed extensive roughening and direct evidence of top-coat loss from the base-coat, via the presence of large cavities and pits on the remaining surface. Analysis of the LSCM images for surface roughness (via ISO 25178) quantified these observations. LSCM image analysis of CuPhthalo samples and Reference samples yielded an initial surface roughness (Sq) of approximately 0.08 μm ± 0.04 μm (one standard deviation) and 0.05 μm ± 0.01 μm (one standard deviation), respectively. After three months of KalExp, LSCM image analysis of the measured surface roughness of the CuPhthalo samples increased to 0.12 μm ± 0.02 μm while the roughness of the Reference samples was measured to increase by over a factor of 20 to 1.08 μm ± 0.13 μm. These LSCM measurements (images and RMS roughness values) provided further evidence that the addition of the CuPhthalo nanoform pigment to the coating reduced the extent of coating oxidation and degradation, compared to Reference samples.

Fig. 5.

Fig. 5.

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LSCM images and surface roughness measurements (RMS) of CuPhthalo samples and Reference samples before and after three months of KalExp. Error bars reflect one standard deviation of the average calculated from three locations measured, from one sample replicate per exposure duration. Scale bar is 100 μm.

3.5. Characterization of released material

In addition to characterization of changes to the chemical structure and morphology of the surface of CuPhthalo samples and Reference samples after weathering, measurements were also performed to investigate the release of the nanoform of CuPhthalo pigment during natural weathering as well as the release of overall coating fragments (i.e., CuPhthalo and polymeric material) after both natural and accelerated weathering. Release measurements during natural weathering were conducted such that samples weathered outdoors were suspended above jars so that rainwater incident to the sample could freely rinse off and accumulate below. Accumulated rainwater was measured for copper content to quantify monthly copper release from each sample. The release of copper from CuPhthalo samples naturally weathered in Tempe, AZ, Golden, CO, and Baltimore, MD is shown in Fig. 6. Also shown in each plot are copper values measured from Reference samples as well as sample-free control jars (i.e., Blank jars). As shown in Fig. 6, there was never a period of weathering in any location that led to copper release above background copper measurements. The spikes in copper release observed for samples weathered in Tempe, AZ are in fact from the deposition of environmentally sourced copper in to the collection jars (Upadhyay et al., 2011), as evidenced by concomitant spikes in copper measured from Blank and/or Reference jars. These data, when considered with limited measured coating oxidation (Fig. 5), indicate that copper release from the CuPhthalo samples is a degradation-driven process that is unlikely to occur to a significant extent (above ambient environmental copper levels) without extensive coating degradation extending beyond the conditions tested in this study.

Fig. 6.

Fig. 6.

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Monthly copper release values measured via ICP-MS from CuPhthalo samples, References, and Blank jars weathered naturally in Tempe, AZ, Golden, CO, Baltimore, MD. Error bars were calculated from one standard deviation of three replicate rainwater collection jars. Note, sampling was paused for January and February at each location.

Naturally weathered and KalExp samples also underwent a well-controlled lab-based release protocol, titled NanoRelease (Wohlleben et al., 2017a). Immersion fluids prepared following the NanoRelease protocol were analyzed with UV–Vis spectroscopy and analytical ultracentrifuge coupled to a UV–Vis spectrometer detector (AUC-UV) to evaluate the release of all material and particulate fragments of a specific size range (10 nm to 1000 nm in diameter), respectively.

UV–Vis and AUC-UV measurements of immersion fluids showed that washed naturally weathered samples released less material than unwashed samples. This result is unsurprising as unwashed samples likely released accumulated dirt during the NanoRelease protocol, in addition to potential coating material. UV–Vis and AUC-UV measurements of immersion fluids also showed that KalExp samples released substantially more coating fragments than naturally weathered samples (Fig. 7). This is consistent with all previous observations, in that the KalExp samples experienced increased degradation, which resulted in increased release of fragments, likely photodegradation by-products of the polymer matrix. Moreover, results from both UV–Vis and AUC-UV measurements indicated that the increased degradation experienced by the Reference samples with increasing UV exposure time, compared to CuPhthalo samples, produced more readily releasable material steadily with increasing exposure (Fig. SI 9).

Fig. 7.

Fig. 7.

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Optical characterization of laboratory prepared release solutions collected from samples weathered naturally (in Tempe, AZ and Golden, CO) and after KalExp (3 months of exposure) to assess the release of all material (UV–Vis) and material 10 nm to 1000 nm in size (AUC). Error bars for UV–Vis measurements were calculated from one standard deviation of three immersion fluids collected per sample.

While both techniques indicated that CuPhthalo samples emitted less material into the immersion fluid compared to Reference samples, comparison of the two methods suggests that material released from CuPhthalo samples was primarily not nanoparticulate. This is observed most readily for KalExp samples: UV–Vis results indicated three month KalExp CuPhthalo samples released approximately three times more overall material than unexposed samples, as evinced by an increase in the absorbance measured at 280 nm (normalized to immersion fluid volume and sample surface area, i.e. Abs.λ=280nm·mL·cm−2) from 0.06 ± 0.02 to 0.19 ± 0.02, after exposure. Alternatively, AUC-UV results indicated approximately four times less material 10 nm to 1000 nm in size was released from the same sample after three months of KalExp (decrease from 172 mg m−2 to 46 mg m−2). This release behavior is observed for naturally weathered samples as well, albeit to a lesser extent, and suggests that in the natural environment, the improved resistance against photodegradation conferred by the nanoform CuPhthalo pigment to the coating will also result in a decreased release of nano-particulate fragments, compared to unpigmented coatings. Additionally, ICP-MS measurements of the immersion fluids did not detect the release of Cu from CuPhthalo samples greater than the detection limit of the instrument (< 0.1 mg L−1) before or after KalExp; when considered with UV–Vis and AUC-UV measurements, this strongly suggests that the released material from CuPhthalo samples, before and after exposure, was entirely polymeric in nature. It is important to note that the NanoRelease protocol can be destructive, due to its use of ultrasonication. After exposure to this energetic event, however, both KalExp and naturally weathered CuPhthalo samples did not show any evidence of coating pealing, unlike naturally aged Reference samples which showed some evidence of exfoliation after sonication (Fig. SI 10). This observation further highlighted the protective effect of the nanoform CuPhthalo pigment and its ability to preserve the structural and chemical integrity of the top-coat sufficiently that it can withstand an energetic even such as sonication after weathering.

3.6. Broader impacts on future studies

Collectively, all findings in this study indicated that KalExp increased the rate of CuPhthalo coating degradation without altering the degradation pathway. To explicitly investigate this behavior, ATR-FTIR was performed and spectroscopic evidence of both exposure conditions leading to similar degradation behavior is shown in Fig. 8. Crucially, both naturally weathered samples (after washing) and KalExp samples exhibited similar FTIR spectra. More specifically, the spectra collected from KalExp samples, compared to spectra collected from naturally weathered samples, only differed in spectral intensities and did not exhibit loss or formation of any new spectroscopic features. This indicates that KalExp did not lead to the formation of any new measurable chemical species in the coating that would have otherwise not formed during natural weathering. We do note KalExp may have led to the formation of chemical species with low cross-sections of absorbance; however, spectra collected of the hydroxyl region, C-H region, and carbonyl region for both KalExp samples and naturally weathered samples only indicate difference in the extent of degradation of these chemical bonding environments. Overall, these FTIR results strongly suggest that KalExp accelerated the rate of coating oxidation but did not fundamentally alter the degradation pathway the coating experienced during natural weathering. This finding provides further evidence that simulated Kalahari exposure is a viable approach to investigate the long-term degradation and performance of nanoform pigmented coatings on experimentally accessible timescales. Additionally, similarities in release behavior after natural weathering and simulated Kalahari exposure, albeit to different total magnitudes, suggest KalExp is not limited to examining changes on the surface nanoform pigmented coating, but rather, may also be used to accelerate exposure for the examination of release behavior of such coating materials in the natural environment.

Fig. 8.

Fig. 8.

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ATR-FTIR spectra of naturally weathered and washed CuPhthalo samples and KalExp CuPhthalo samples from (left) 4000 cm−1 to 2500 cm−1 and (right) 2000 cm−1 to 500 cm−1.

4. Conclusions

A nanoform of CuPhthalocyanine, a metal-organic pigment, was added to a top-coat formulation to confer a blue appearance in a coating intended for use in exterior automotive applications. The color performance, degradation, and release of degraded material from this top-coat were investigated after both natural weathering and simulated weathering conditions. Color and gloss measurements indicated that after 15 months of natural weathering, the objective appearance of CuPhthalo samples was largely unchanged, while two months of simulated weathering lead to a measurable changes in coating appearance (e.g., ΔE > 1). Sample characterization with ATR-FTIR indicated that CuPhthalo samples and Reference samples followed the same degradation pathway (i.e., photooxidation) irrespective of exposure condition; however, the increased rate of sample degradation driven by simulated weathering revealed distinct differences in the degradation of CuPhthalo samples and Reference samples. Specifically, characterization with ATR-FTIR, color and gloss measurements, and LSCM indicated that CuPhthalo samples experienced less extensive coating oxidation than Reference samples, which resulted in improved color performance and appearance. The resiliency of the CuPhthalo samples was found to be the result of the pigment strongly absorbing UV light, thereby inhibiting UV driven photodegradation of the overall coating. Release measurements during natural weathering indicated that the slow rate of coating oxidation in the natural environment resulted in little to no measurable copper release from pigmented samples over one year of natural weathering. NanoRelease protocol on samples after both weathering strategies found that plates pigmented with the nanoform of CuPhthalo released less nanoscale fragments than Reference samples and results indicated that material released from CuPhthalo samples was mostly non-particulate and not copper in its form and composition, respectively. Findings from this study suggest that pigments with strong UV absorbing properties will mitigate the extent of overall coating oxidation, which in turn will reduce the total release of particulate material – both embedded nanomaterial and the surrounding polymer matrix – into the environment.

Supplementary Material

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Acknowledgement

The authors would like to thank the LCnano Research Center for providing access to their outdoor weathering network, which was partially supported by provided from the US Environmental Protection Agency through the STAR program (RD83558001). For ER, WW, KV, PM, SP, this work was partially funded by the BMBF (German Federal Ministry of Education and Research) project nanoGRAVUR - Nanostructured materials – Grouping in view of worker, consumer and environmental safety and risk minimization (FKZ 03XP0002X). ER, WW, KV, PM, SP thank Petra Herrmann (BASF Coatings, Münster) for the selection of suitable specimens and testing standards. RL, DG, LS thank Chen Yuan for color measurements.

Footnotes

NIST disclaimer

Certain instruments or materials are identified in this paper in order to adequately specify experimental details. In no case does it imply endorsement by NIST or imply that it is necessarily the best product for the experimental procedure.

BASF disclaimer

ER, WW, KV, PM, SP are employees of BASF, a company that produces and markets nanomaterials.

Declaration of competing interest

The authors declare the following financial interest/personal relationship which may be considered as potential competing interest: ER, WW, KB, PM, SP, are employees of BASF, a company that produces and markets nanomaterials.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.impact.2019.100199.

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