Efficient Wood Hydrophobization Exploiting Natural Roughness Using Minimum Amounts of Surfactant-Free Plant Oil Emulsions

Abstract

Wood in service requires protection from excessive moisture. Herein, we demonstrate that efficient surface hydrophobization can be provided with small amounts of biobased oils, benefitting from the hierarchical roughness inherent to wood surfaces. The developed technique involves coating spruce wood with surfactant-free emulsions based on tung oil, linseed oil, or a linseed oil-based long oil alkyd resin. The ζ-potential of the emulsions was determined by electrophoretic mobility measurements. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and spectrophotometry were used to study coated surfaces. XPS measurements confirmed the presence of the tung oil coatings. Tung oil emulsions were effective at concentration levels as low as 0.04 wt % oil content, roughly equivalent to 0.04 g m^-2 and led to static water contact angles reaching up to >130°. SEM imaging and AFM measurements provide evidence that the micro- and nanostructures inherent to wood enhance the hydrophobization effect of the obtained coatings. A further benefit of the method lies in only minimal effects of the coating on the surface color and gloss. Thus, the mass-efficient process following several of the principles of green engineering led to improved water repellency while not affecting the visual appearance of the coated wood.

Figure 1

Chemical structures of α-linolenic acid (a) and α-eleostearic acid (b). Synthesis of alkyd resin by the reaction of triglycerides, glycerol, and phthalic anhydride in the monoglyceride process (c).

Figure 2

(a) ζ-Potential values measured for the three oil emulsions. Points and error bars are the arithmetic mean and standard deviation of triplicate measurements. (b) Pictures of emulsions of 1 g of oil in 99 g of water taken at different time intervals. Red ellipses highlight creamed tung oil and linseed oil and sedimented alkyd resin.

Figure 3

Deconvolution of the C 1s spectra obtained by X-ray photoelectron spectroscopy of uncoated spruce wood (a) and spruce wood coated with tung oil Emulsion-C (b) or pure tung oil (c).

Figure 4

Specimen wettability with water determined by static contact angle measurement. (a) Static water contact angle of coated and uncoated spruce surfaces as a function of oil content. The x-axis represents different oil contents of coatings as determined by gravimetric analysis. The arithmetic mean and 95% confidence interval of measurements of seven random locations per specimen is shown for samples cut from two different logs, distinguished by squares and triangles. The red curve shows a second-order polynomial fit calculated from all samples except Emulsion-A and uncoated samples. (b) Exemplary curves showing the stability of the contact angle of a droplet monitored for 120 s.

Figure 5

(a) Static WCA of planed vs sanded wood coated with Emulsion-D (from 1 g of an oil mixture in 99 g of water) and Emulsion-C, obtained by 5-fold dilution of Emulsion-D. (b) Static WCA on spruce wood coated with Emulsion-D of tung oil, linseed oil, and alkyd resin. (c) Static WCA of droplets of ∼8 μL as a function of the oil content after spray coating of spruce wood. Points represent the mean of seven measurements on individual spruce specimens. Error bars show the 95% confidence interval.

Figure 6

Scanning electron microscopy images of uncoated planed wood (a), uncoated sanded wood (b), and wood coated with low solid-content Emulsion-C (c) or pure tung oil (d) at three different magnifications (1–3).

Figure 7

Exemplary AFM images (top) and surfaces profiles (bottom) along a diagonal line for native spruce wood (a) and spruce coated with Emulsion-C (b), Emulsion-E (c), and pure tung oil (d).

Figure 8

Roughness factor R_f for uncoated spruce wood and for spruce wood coated with tung oil emulsions and pure tung oil. Emulsion-E was prepared by dispersing 5 g of the binder mixture in deionized water. The oil content of Emulsion-C is about 25 times less than in Emulsion-E. Different wood samples within one group are distinguished by different symbols.

Figure 9

(a) Color change due to coating visualized in the 3d CIElab color space of control samples immersed into deionized water (orange) and samples coated with dilute Emulsion-C (blue) and pure tung oil (black). (b) Scans of spruce samples before coating (top), after coating with Emulsion-C (bottom left), and after coating with pure tung oil (bottom right).

Figure 10

Graphical description of the emulsion preparation and the wood coating process.