Plasma Pre-treatments to Improve the Weather Resistance of Polyurethane Coatings on Black Spruce Wood

Abstract

We hypothesize that plasma treatments that increase the adhesion and penetration of transparent water, and solvent-borne polyurethane coatings into black spruce wood will improve the performance of coated wood exposed to accelerated weathering. We tested this hypothesis by modifying wood samples with plasma for 30, 180, 600, and 1200 s, and measuring coating penetration and adhesion using light microscopy and a mechanical pull-off test, respectively. Plasma treatment did not improve coating adhesion, but the solvent-borne coating showed deeper penetration into plasma-modified wood, and its resistance to accelerated weathering was better on plasma-modified wood than on untreated controls. Plasma treatments enhanced the penetration of water-borne polyurethane into wood, but the treatments did not improve weather-resistance of the coating. Plasma treatment increased the wettability of wood surfaces, and prolonged plasma treatment etched cell walls, increasing their porosity. These effects may explain the positive effect of plasma treatment on coating penetration, and the increased weather-resistance of the solvent-borne polyurethane on plasma-modified wood. In conclusion, our results indicate that the ability of plasma treatment to improve coating performance on black spruce depends on the coating type, and the effects of the treatment on the surface microstructure of wood.

Plasma Treatment Reduced the Discoloration of an Acrylic Coating on Hot-Oil Modified Wood Exposed to Natural Weathering

“…The oil did not completely cover the surface of the wood, but occluded some tracheids (arrowed in Figure 2c), while others were free of oil ( Figure 2c). Plasma treatment of unmodified wood removed (etched) the raised border and the internal membrane (margo and torus) of bordered pits creating large circular voids at the surface of the wood (arrowed in Figure 2d), as we have observed previously with other wood species [27,28].…”

“…They contain a cellulosic membrane (the margo) and a centrally thickened torus. Pit membranes in untreated wood are rapidly etched by plasma [27,28], and they are also highly susceptible to microbial degradation [44]. However, remnants of pit membranes were commonly observed at the surface of hot-oil modified and plasma treated wood, whereas they were absent from the surface of unmodified plasma treated controls.…”

“…Wood was thermally modified by placing pre-weighed and conditioned samples in a circulating oil-bath (Model HTB, Thermoline Scientific Equipment Pty Ltd., Wetherill Park, Sydney, Australia) containing soya-bean oil at 220 • C. Modified wood was removed from the oil bath after 2 h and blotted on paper towels to remove excess oil. Hot-oil modified blue-stained wood and unmodified wood samples (described above) were plasma-treated in a reactor designed to clean silicon wafers to produce high-energy surfaces [27,28]. One wood sample was placed in the chamber of the plasma reactor at a time, and a vacuum of 0.15 ± 0.01 Torr was drawn.…”

“…The wettability of the hot-oil modified and plasma treated wood was assessed by measuring the contact angle of 5 µL droplets of distilled water on three spots on wood surfaces. A 5 µL droplet of distilled water was placed on the wood surface using a syringe, after adjusting the needle of the syringe so it produced a sessile drop [28]. The static contact angle of the droplet on the sample was measured using an optical tensiometer (KSV CAM 101, KSV Instruments Ltd., Helsinki, Finland).…”

Preparation and Performance Optimization of Two-Component Waterborne Polyurethane Locomotive Coating

“…Xia et al [19] prepared a polyurethane coating by applying ultraviolet light curing, added fluoro-acrylic monomer and mercaptan, and achieved a luster of 86.9 ± 0.9, which improved the hydrophobicity, mechanical properties, and yellowing properties of the coating. Haase et al [20] found that the water-based coating turned yellow at 3024 h with fewer defects after plasma modified coating, while the solvent-based coating began to show defects at 1008 h after accelerated weathering.However, water as a solvent shows several disadvantages, including poor dispersion ability, high surface tension, foaming, poor light and color retention, poor weather resistance, unstable performance of large areas of coating, easy to flow hanging, and other defects [21]. In order to obtain a reliable and stable coating, we refer to the tests performed by relevant experts and compare the tests with different formulas, focusing on the comparison of QUV aging reliability properties.…”