Release and transformation of ZnO nanoparticles used in outdoor surface coatings for UV protection

Abstract

A major area of growth for "nano-enabled" products has been the addition of nanoparticles (NPs) to surface coatings including paints, stains and sealants. Zinc oxide (ZnO) NPs, long used in sunscreens and sunblocks, have found growing use in surface coating formulations to increase their UV resistance, especially on outdoor products. In this work, ZnO NPs, marketed as an additive to paints and stains, were dispersed in Milli-Q water and a commercial deck stain. Resulting solutions were applied to either Micronized-Copper Azole (MCA) pressure treated lumber or a commercially available composite decking. A portion of coated surfaces were placed outdoors to undergo environmental weathering, while the remaining samples were stored indoors to function as experimental controls. Weathered and control treatments were subsequently sampled periodically for 6 months using a simulated dermal contact method developed by the Consumer Product Safety Commission (CPSC). The release of ZnO NPs, and their associated degradation products, was determined through sequential filtration, atomic spectroscopy, X-Ray Absorption Fine Structure Spectroscopy, and electron microscopy. Across all treatments, the percentage of applied zinc released through simulated dermal contact did not exceed 4%, although transformation and release of zinc was highly dependent on dispersion medium. For MCA samples weathered outdoors, water-based applications released significantly more zinc than stain-based, 180 ± 28, and 65 ± 9 mg/m² respectively. Moreover, results indicate that the number of contact events drives material release.

Keywords: Exposure; Nano-enabled; Nanomaterial; Surface coating; XAFS; Zinc.

Figure 1:

HAADF TEM images of ZnO NPs used in this study. Particle Size was calculated as 28 +/− 22 nm by manual analysis of 270 particles.

Figure 2:

Release of zinc from coated boards. a.) Aqueous application on composite decking, b.) Aqueous application on MCA lumber, c.) Stain application on MCA lumber. Data for both weathered boards (Outdoor) and controls (Indoor) are an average of at least three replicates. Time Zero corresponds with the first wipe event after application of surface coatings and dry time of at least 48 h. Data has been normalized based on the area wiped per sampling event. Notice the difference in zinc release (y-axis) for each experiment treatment. The data presented represent the average of four replicate measurements and the error bars the standard deviation of the replicate values.

Figure 3:

Release of zinc from MCA lumber that has undergone 1260 h of UV degradation before application of surface coatings. Data was collected in triplicate and has been normalized by the total area wiped during a sampling event. The data at Time Zero corresponds with the first wipe event after application of surface coatings and a dry time of at least 48 h. The data presented represent the average of four replicate measurements and the error bars the standard deviation of the replicate values.

Figure 4:

Release of zinc from pristine MCA lumber and UV degraded MCA lumber per wipe event. Data was collected in triplicate and has been normalized by the total area wiped during a sampling event. Notice the larger difference in zinc release (y-axis) for each experiment treatment. The data presented represent the average of four replicate measurements and the error bars the standard deviation of the replicate values.

Figure 5:

BSE FESEM image of particulate matter retained on a 0.45μm filter after release and extraction using the CPSC estimate of dermal transfer test method. Sample is from a micronized copper pressure treated lumber coated with ZnO NPs dispersed in water after a 72-h drying period. Zinc is heavily collocated with copper as shown in the EDS Micrograph.

Figure 6:

Normalized zinc XAFS data collected on 0.45μm filters collected from Water/Wood/Outdoor samples during simulated dermal contact experiments. Time Zero represents the first wipe event approximately 48 hours after coating application. The 20-week sample has undergone continuous outdoor weathering. Changes in spectra features highlight the degradation of pristine ZnO in the coating to amorphous absorbed zinc species.

Figure 7:

Results of Leaching Test performed on surface coupons that had been painted with different experimental treatments. Samples were mixed in SPLP (pH 4.2) solution for 72 h and then sequential filtered to determine size fractions of released material. Composite samples are coated only with ZnO dispersed in water as wood stain is not intended for application on composite surfaces. The lower-case letters indicate statistical differences in the mean concentration of Zn wiped from the surface over the six-month experiment as determined by a Fisher LSD means test (n= 4; α = 0.05). Identical letters indicate no statistical difference in the mean. The data presented represent the average of four replicate measurements and the error bars the standard deviation of the replicate values.