Tung Oil Wood Finishes with Improved Weathering, Durability, and

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Tung Oil Wood Finishes with Improved Weathering, Durability, and Scratch Performance by Addition of Cellulose Nanocrystals Youngman Yoo, and Jeffrey Paul Youngblood ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04931 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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ACS Applied Materials & Interfaces

Tung Oil Wood Finishes with Improved Weathering, Durability, and Scratch Performance by Addition of Cellulose Nanocrystals

Youngman Yoo and Jeffrey P. Youngblood *. School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States * J. P. Youngblood. Email: [email protected]. Phone: +1 765-496-2294. Fax: +1 765 494-1204.

Keywords: wood coating; cellulose nanocrystals; tung oil; composite; weathering; durability, varnish or finish.

Abstract The main aim of this study is to verify whether cellulose nanocrystal (CNCs)-reinforced tung oil composites are effective for wood finishes and offer enhanced mechanical and weathering performance owing to the high strength, stiffness, and barrier properties of CNCs. To achieve even dispersion of cellulose nanocrystal (CNC) particles in a polymeric coating film, surface hydrophobization of the CNCs was carried out by grafting poly (lactic acid) oligomers and oleic acid. These new tung oil (TO) coating formulations contain 0 (controlled sample) to 10 wt% of hydrophobized cellulose nanocrystals (hCNCs). The coating performance (degree of wrinkle, leveling, and instantaneous filling) of the hCNC-TO finishes, as well as their coating properties (topography, optical properties, mechanical properties, and gas permeability) was investigated in this study. The influence of the hCNC content in the tung oil composite coatings was examined using scratch/impact resistance tests and oxygen transmission rate (OTR) measurements. An increase in the hCNC content led to an increase in scratch/impact resistance as well as a slight decrease in the color-b change, gloss, surface roughness and OTR value of their film coatings.

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The hCNC-TO composites for wood coatings presented here showed enhanced performance for utilization in wood-working processes in terms of desired mechanical properties (scratch and impact resistance), weathering performance (color stability), and easy production without any deterioration in surface gloss and roughness after the addition of hCNC to a TO matrix. In conclusion, the hCNC enhanced coating system is a promising candidate for substantial protection of wood surfaces in demanding settings.

Introduction Color change in wood, paper, or plastic products made of organic polymers is unavoidable because of their sensitivity to weathering by light and aerobic oxidation1-2. Opaque pigmented coatings can be utilized as a very effective UV light/oxygen screen in wood protection due to absorption and scattering of light by pigments and their lower oxygen permeability; yet most people prefer varnishes to paints for preserving the appearance of wood. Likewise, wood requires special varnishes or drying oil coatings to retain its natural look and functionality3-4. Tung and linseed oil are the most commonly used drying oils in woodworking, which is attributed to their durability and appearance. Drying oils are a natural product which cure slowly by oxidation to a soft, wrinkled film if applied thickly. On the other hand, varnish is a synthetic product made by formulating a plant oil with a resin, such as polyurethane, acrylics, alkyd or phenolic resins3,

5-10

. Varnish cures

relatively rapidly to a hard, smooth film if it is applied thickly. However, the varnishes do not have the mystique or aesthetics of drying oils, and they require a lot of brushing or spraying, which makes them more difficult to apply than drying oils. In addition, in the case of phenolic or acrylic resins, any remaining volatile residues (i.e. formaldehyde or acrylic monomers) in the


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coating provide unpleasant odors and may be a threat to human health. Thus, plant oils are preferable for making wood finishes which meet both the environmental requirements and compelling aesthetic values. Of the natural drying oils, there is growing interest in tung oil (TO), also known as china wood oil. TO has major components of α-eleostearic acid (77–82%) with three conjugated double bonds (at carbons 9 cis, 11 trans, 13 trans), oleic acid (3.5–12.7%) with one double bond and linoleic acid (8–10%) with two non-conjugated double bonds11-12. These components make TO excellent for use as a drying oil in the wood finish industry because no other drying oil has the same resistance to water, mold, bacteria, yellowing, and darkening that also offers strength and flexibility. While an excellent natural oil coating, the coating properties of TO, such as surface hardness (scratch resistance), deformability, and anti-weathering performance, must be improved to be comparable to synthetic varnish resins. Furthermore, an inherent problem with TO as a wood finish is its inefficient application process. For example, it takes longer to cure than other drying oils and, typically, several coatings are needed. Every layer, except the last, must be sanded to remove all excess finish and create a stronger bonding surface for successive coatings. On the other hand, the straightforward way to improve the poor weathering characteristics and mechanical properties of the TO coatings is to introduce inorganic fillers. The use of various nanoparticles, including common inorganic metal oxides (ZnO, Al2O3, TiO2, CeO2, etc.) as mechanical reinforcement additives in coatings, represents a significant improvement to reaching this end13-17. However, the design and fabrication of robust, lightweight, and sustainable wood coatings having a well dispersed inorganic-filler composite remains challenging and thus suggests that organic particles, such as cellulose micro/nanocrystalline, wood flour, and chitin can be utilized18-23.



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Previously, we demonstrated that hydrophobized cellulose nanocrystals (hCNCs) could be utilized in polymeric nanocomposite applications because of their unique compatibility with hydrophobic polymeric matrices.24 Those CNCs were surface-grafted using lactic acid as a reactive solvent for the esterification of CNCs with acids or esters having a long hydrocarbon chain (fatty acid). In addition, the hCNC enhanced polymeric composites were shown to have a strong, dense composite structure which serves as an excellent barrier against leakage due to its lengthened diffusion path, and guarantees enough mechanical strength from rupture for handling or postprocessing.25-26 In this study, we also made use of the advantageous properties of CNCs (high stiffness/resilience, low coefficient of thermal expansion, and high gas barrier performance with a high aspect ratio) to produce a nanocomposite for wood coatings by dispersing hCNC into TO. 27-31 The TO was chosen as a binder material because of its desirable coating properties in demanding situations, and hCNC as a reinforcing material was used to enhance mechanical and weathering features of the nanocomposite wood coatings. The hCNC-reinforced coating resins effectively protect wood coatings against oxidation by air and moisture, while simultaneously making them durable (anti-scratch and wear resistant) and enhanced UV resistance (nonyellowing) without changing other desirable properties such as optical transparency, color, and gloss. In addition, hCNC allows oil finishes to build a thicker coating without wrinkling and reduces the number of layers needed, improving efficiency of coating. The morphologies and optical properties of wood finish having an hCNC enhanced tung oil coating were examined by 3D surface profilometer and colorimeter/glossmeter. The mechanical characterization of the wood coatings was investigated by pencil scratch tests for hardness and falling weight impact tests for strength and durability. The weathering features of the obtained wood coatings were examined by accelerating weathering tests and oxygen permeation measurements.


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Experimental Section Materials. Zinc acetate dihydrate at 98 %, dibutyltin dilaurate at 95 %, DL-lactic acid (LA) at 85 % syrup, oleic acid at 90 %, tung oil (ester of α-eleostearic acid at 80% and esters of linolenic, 9,12-linoleic, oleic, stearic and palmitic acids at 20%), and orange oil (natural, cold-compressed, California origin, FG) were purchased from Sigma Aldrich, St. Louis, MO, USA. Ethanol (200 proof) and acetone (ACS grade) were purchased from VWR, West Chester, PA, USA. All reagents were used as received without further purification. 11.9 wt % never-dried CNC (Lot# 2016-FPL-CNC-065, dimension: 64±5 nm (L) × 7±1 nm (d), aspect ratio = 9.5, and crystallinity index 72 %)24 suspension in water, which is in sulfate half-ester form with 1 wt % sulfur and a sodium counterion, was manufactured by USDA Forest Service-Forest Products Laboratory (FPL), Madison, WI, USA and distributed by University of Maine, Orono, ME, USA. Metering rod (316 stainless steel fully threaded rod, #4-40 thread size, 12" length) was purchased from Amazon. Chemical Hydrophobization of Cellulose Nanocrystals. CNCs were surface-modified to graft poly (lactic acid) oligomers (PLA) and oleic acid as side chains (following our previous publication24, which can be considered the primary reference for detailed explanations of the fabrication procedures). Briefly, an aqueous CNC suspension at 11.9 wt% and additional deionized (DI) water were mixed to prepare a 5 wt% CNC water suspension. An excess of 85 wt% DL-lactic acid syrup (the ratio of the equivalent COOH of the lactic acid and OH of the dried CNC = 10) was introduced into the aqueous CNC suspension and ultra-sonicated for 1 min. A 500 mL 3-necked round flask equipped with a condenser and a mixer was charged with the suspension mixture and heated to reach 150 °C. Then, a zinc acetate dihydrate catalyst (150 ppm



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based on the DL-lactic acid (LA) weight) was further inserted thereto at 150 ℃, and polyesterification was implemented by water distillation at 170 - 180 ℃. After 70 to 99 % of the water based on the theoretical byproduct amount was collected into a recovery flask, an excess of oleic acid (the ratio of the equivalent COOH of the oleic acid and OH of the dried CNC = 2.8) and a dibutyltin dilaurate (DBTDL) catalyst (200 ppm based on the fatty acid weight) were added to the intermediate products which consist of PLA oligomer and PLA oligomer grafted CNC (CNC-g-PLA). The reaction pressure was gradually decreased to 100 mmHg for 30 min, and then an excess of lactic acid was eliminated by performing the reaction under reduced pressure. Then, the reaction pressure was further reduced to 10 mmHg and was kept until the distillation column top temperature fell below 35 ℃, thereby fabricating an oleic acid and PLA grafted CNC. Lastly, the purification of the hCNCs from the remaining free fatty acids and homo-PLA oligomers was carried out using dispersion-centrifugation (6,000 rpm at 25 ℃ for 30 min.) four times with an excess of ethanol. Finally, the hCNC was suspended in ethanol and freeze-dried using a lyophilizer (Labconco FreeZone Plus 4.5 L). According to

13

C CP/MAS

solid-state NMR and 1H NMR analysis results, the number of repeating lactic acid units (grafted PLA chain length) is about three and the degree of substitution of grafted PLA and oleic acid are about 30 % and 20 %, respectively. Preparation of hCNC-TO Formulations and Nanocomposite Coatings. An hCNC that was centrifuged and thus kept “wet” and “concentrated” in its washing solvent (ethanol) was utilized to ease dispersion in the TO formulation. The percentage (w/w) of the hCNC in the "wet” dispersions was determined to be approximately 18 wt%. An appropriate amount of “wet” sample was used to obtain 5 g of hCNC. TO was introduced so that the final concentration of hCNC was kept at 10 wt%. The “wet” samples were redispersed in the TO using an ultra-


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sonicator, and then its dispersing solvent (ethanol) was removed under reduced pressure by a rotary evaporator. According to typical wood finishing instructions, the 1:1 (w/w) mixing ratio of orange oil citrus solvent to TO works well on the porous surfaces of various raw woods. For the preparation of hCNC-TO solution in orange oil, 10 % hCNC-TO solution was initially diluted with an appropriate amount of TO (to make various contents of hCNC: 0, 5, and 10 %) and orange oil (to make various concentrations of hCNC-TO solution: 20 - 50 wt%), and ultrasonicated for 1 min in order to avoid the coagulation of the hCNC particles. Using a flat paint brush, maple wood plank specimens (60×90×6mm) were coated for the measurement of coating roughness and optical properties (multilayer coating thickness: 100 µm). Glass plates (25×76×1mm) and steel panels (60×90×0.1mm), which were swept with emery paper (No. 60) followed by rinsing with acetone and toluene solvents, were coated for the measurement of hardness and impact strength, respectively, with hCNC-TO nanocomposite coatings (multilayer coating thickness: 100 µm). Furthermore, film coatings (one-layer coating thickness: 4-6 µm) were cast on plain biaxially-oriented PLA (polylactide) films (20 µm thickness, NatureWorks, 4032D, discharge-treatment inside) using a metering rod to evaluate the dispersion of hCNC particles in dried/UV-cured coatings and to characterize surface roughness properties. First, specimens were dried for 2 h at 55 ℃ in a convection oven to remove orange oil. Finally, all specimens were cured by illuminating a 20×20cm cured area with a curing dose of 80 mW/cm2 long wave, ultraviolet (UVA) energy. In addition, UV-induced polymerization was carried out with the aid of a conventional metal halide lamp (400 W·cm-1). Physical Characterization of the Materials: Rheology of hCNC-TO solutions. The rheological characterization of individual formulations prepared was accomplished in a strain controlled Rheometer (Bohlin Gemini HR Nano Rheometer). The measurements were carried



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out with a cup and bob flow geometry (coaxial cylinder No. C25) in the steady state mode, at room temperature (25 ℃) using a shear rate ranging from 0.05 s-1 to 100 s-1. In order to avoid any aggregation of the hCNC particles, a first sweep experiment was typically followed by a second one performed on the same sample and using the same conditions. Reproducibility was checked by performing three measurements. Color and Gloss Measurements. Color and gloss were measured at three locations on each TO-coated maple specimen. The variation in gloss of wood finish coatings was analyzed using a gloss meter (BYK-Gardner, micro-TRI-gloss 4520). The measurements were taken at 20˚ (for matt surface), 60˚ (for both surfaces), and 85˚ (for shiny surface) angles following ISO 2813 (1994). The amount of reflected light at an equal but opposite angle onto a surface was measured in determining the gloss of the finish layers. Wood finish color was measured by a Chroma meter CR-410 (Minolta, Suita-shi, Japan). Color was reported with L*, a*, and b* values (trichromatic opponent color classification; L* measures relative lightness, a* relative redness, and b* relative yellowness). These values were utilized to determine the overall color changes ∆E* using the following eq 1: ∗ = (∆ ∗ + ∆ ∗ + ∆ ∗ )/ (1) where ∆L*, ∆a* and ∆b* are the difference between two chromatic values before and after UV irradiation. A low ∆E* value corresponds to anti-weathering performance with a small color change. Anti-scratch tests. Pencil scratch tests have been performed to measure the hardness of 100 µm thickness coating films, according to ASTM D 3633. The method contains a process for effective, easy measurement of the film hardness of a coating on a substrate by means of drawing pencil leads of known hardness across the surface of the coatings. A coated glass plate was


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arranged on a firm horizontal surface. The pencil was held at a 90˚ angle to the abrasive paper and the lead was run over it until a flat, smooth, and circular cross-section was obtained. Then, the pencil was held firmly with the setting block of the pencil tester (SSEYL Lantek HT-6510P Pencil Hardness Tester, 1000 g load) against the film at a 45˚ angle and pushed away from the operator in an approximately 6 mm stroke. The speed of the pencil tester was between 0.5 mm/s to 1 mm/s. The process began with the softest pencil and continued up the scale of hardness to either of two end points: one, the pencil will not cut into or gouge the film (hardness); or two, the pencil will not scratch the film (scratch hardness) determined by a close visual inspection using a microscope with between x6 and x10 magnifications. Mechanical Testing. Impact resistance was examined through a dart drop impact tester using 80 µm-thick film coated steel panels (1 mm thickness) according to ASTM D3029 or ASTM D3170. Individual plate specimens were fixed on a base plate with a hole of 40 mm diameter. A hemispherical tipped dart was dropped onto the specimens at a drop speed of 2 m/s using an impactor weight of 1 lb (0.454 kg). The dropping height was sufficient to fracture each sample. Four tests of each sample were conducted. Surface Profilometer. For a noncontact surface topography of the film coatings on the wood specimens, an NV-1800 surface profiling analyzer (NanoSystem Co.) was utilized to measure roughness and to provide a closer visual inspection in terms of surface quality of the specimens. The analyzer has a non-contact light source and a contact inductive gauge providing 1D, 2D, and 3D surface images of the specimen. The non-contact LED illuminator utilizes white-light scanning interferometry (WSI) and phase shift interferometry (PSI) with a vertical resolution of WSI: < 0.5 nm / PSI: < 0.1 nm, scanning range of a maximum 270 µm, Z stroke of 30 mm and X, Y stroke of 50 mm by 50 mm on the specimens. Three measurements were also



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taken for the surface of individual specimens at a scanning velocity of 7.2 µm/s by using the NV1800. Oxygen Permeability. An OX-TRAN Model 2/61 (Mocon, Minneapolis, MN) was utilized in measuring O2 transmission rate (OTR) of individual TO coatings. The OTR of the coating cannot be performed after being applied on a wood substrate and a free-standing flat film could not be fabricated. Therefore, PLA film was used as a substrate for the TO and coatings were prepared by the method described in Preparation of hCNC-TO Nanocomposite Coatings section. Oxygen gas (100%) was introduced on the coated surface of the film. Nitrogen gas (98% nitrogen, 2% hydrogen) was provided on the opposite side. Measurement was carried out at 23 ℃ and a relative humidity of 0% (ASTM D 1434). The average data was calculated by conducting two measurements. The total transmission rate (Ttot) of oxygen was normalized by using the total thickness (ttot), and the transmission rate (T1) through TO nanocomposite coatings was calculated as follows32:

= + (2)

where t1 and t2 are the thickness of the coated film and the neat PLA film, and T2 is the transmission rate of the neat PLA film. Oxygen permeability (OP) (cc m/m2 day atm) of the TO coating was obtained by multiplying the OTR by the coated film thickness.

Results Coating Performance. In general, drying oils are too thick to penetrate or impregnate far into most solid wood surfaces. Thinning with a dilution solvent (desirably, a natural solvent for ecofriendly process) allows the oil to penetrate deeper and faster into the substrate. Using a flat


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paint brush, steel shims (60×90×6mm) were coated with TO solutions (50 wt% orange oil) containing various hCNC contents (up to 3 wt%) for coating appearance assessment in Figure S1. Figures S1-c and d illustrate that there are coating stripes (brush marks) on the coatings (2 and 3 wt% hCNC content), which is attributed to the poor leveling property of their viscous fluids. Rheological analyses were conducted to examine the flow characteristics of individual hCNC-TO formulations (containing orange oil). Rheology is a significant factor in the development of finished products. Typically, the shear rate generated by brushing paints on a substrate is expected to range from 130 s-1 to 260 s-1; thus, paint formulations need to be adjusted to have a viscosity from 0.2 to 0.5 Pa s and a yield stress from 40 to 140 Pa in order to stick to a brush and a wall with low brush drag and without dripping off.33 Viscosity at low shear rates (under 50 s-1) is associated with coating solution properties such as sagging, settling, or leveling.33 Leveling of a smooth coating takes more time with higher viscosity paints and with thicker coatings. In this system, two different fluid behaviors were observed: Newtonian and non-Newtonian. As expected of a Newtonian fluid, no change in the viscosity of pure TO (Figure 1-a) was observed. However, the 5 wt% hCNC-TO exhibits a high viscosity (Figure 1-b) and followed non-Newtonian behavior.34 As a typical example, Figure 1-b shows the pseudo-plastic flow behavior (exhibited by shear-thinning behavior) often observed of a dilute liquid suspension of non-spherical nanoparticles35 up to a shear rate of 300 s-1. Compared with the Newtonian behavior of the TO, the pseudo-plastic flow behavior of the hCNC-TO has a yield point of < 50 Pa, which is expected to have a kink in the shear rate-stress diagram (Figure 1-b), and has advantages in preventing sagging, settling, and dripping for mixing and finishing.34 However, there are drawbacks of the hCNC-TO, specifically: a significantly high viscosity and a yield



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point. These can be adjusted by means of dilution with thinner at different proportions (25 or 50 wt% TO solution in orange oil). Therefore, the hCNC-TO formulations should be diluted by mixing an orange oil or mineral spirit solvent with a TO/solvent ratio: 1/2 or 1/3 (w/w) for the instantaneous impregnation of porous and rough wood surfaces.

Figure 1. Rheological behaviors: shear rate vs. a) viscosity and b) shear stress diagrams of pure TO and its solution diluted with orange oil (1:1 w/w) and 5% hCNC-TO and its dilution solutions with orange oil (1:1 and 1:3 w/w).

During metered coating of hCNC-TO solution, a wet film thickness is determined by coating solution characteristics such as density and viscosity and by the metering speed of the coating solution (Figure S2-a).36 When the wet film dries, the dry film thickness is adjusted by the same solution features and by the solid content of the applied solution. Several TO coating layers (typically six to seven layers) need to be applied to obtain a good-looking finish for a wood product (Figure S2-b). Furthermore, each layer, but the last, must be buffed to remove all excess finish and create a stronger bonding surface for successive coatings using a fine-grade steel wool or fine-grit sandpaper. In addition, film formation by metering can be substantially affected by


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the porosity of the substrate. A considerable contribution to coating thickness occurs through the instantaneous filling of the porous and rough surface.37 Capillary and hydrodynamic pressure promote liquid penetration. When a suspension having a nanofiller is coated on a web, the suspended liquid penetrates the web, but the nanoparticles can be left on its surface, like filter caking. Therefore, the concentrated suspension on the substrate has a much higher viscosity than that of the original suspension and can be considered less flowable. This problem can be reduced by controlling coating speed and solids content and by minimizing colloidal instabilities and shear-induced aggregation. Initial trial and error to optimize coatings for the nonporous glass substrate is seen in Table 1. In order to obtain the uniform, almost identical coating thickness (about 4-6 µm) with pure TO and hCNC(5 and 10 wt%)-TO formulations using a metering applicator, their appropriate dilution ratios (from 3:7 to 1:4 w/w) with orange oil were determined by feedback of the aforementioned trial and error procedure.



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Table 1. Coating thickness of TO film on a non-porous glass substrate as a function of hCNC content. coating methods

brushinga)

meteringb)

formulations

no of layers

final coat thickness (µm)

dry coat wet coat thickness/layer thickness/layer (µm) (µm)

pure TO:orange oil =1:3

6

63.6 ± 7.2

10.6

42.4

5%hCNC-TO:orange oil =1:3

6

95.4 ± 3.6

15.9

63.6


6

130.8 ± 9.0

21.8

87.2

pure TO:orange oil =3:7

1

-

6.0 ± 0.8 (3.3 ± 0.5)c)

20.1 (11.1)


1

-

5.1 ± 0.6 (2.7 ± 0.5)

20.5 (10.7)


1

-

4.2 ± 0.6 (2.0 ± 0.6)

20.9 (10.0)

a) 2-inch-wide flat paint brush, b) 316 stainless steel fully #4-40 threaded, 12" length rod delivers the wetting speed of 6 ft/min. c) coating thickness of TO film on a porous PLA film substrate

A 24-well clear polystyrene (PS) cluster plate was used to produce hCNC-TO coating films with different thicknesses (Figure 2). The coating thickness of the film was controlled by the solid content in the coating solution; for example, an average wet film and dry film thickness of the TO coatings were calculated using an applied coating volume. Coating thicknesses between 12 and 65 µm were successfully fabricated on the PS substrate after thermal curing for 24 h at 50 ℃. In the measurement range of coatings applied, the coating performance (degree of wrinkle) was significantly dependent on the coating thickness and content of hCNC applied; thinner and higher content of hCNC applied coating films showed less degree of wrinkle and matting (i.e.


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they have less background image opacity). This may occur because of a tendency of drying oils to make a skin during curing, which obstructs both oxygen and dilution solvent molecules from diffusing rapidly in and out through the film during the curing procedure. Oxidative curing is the main polymerization mechanism of drying oils (tung, linseed, soybean, safflower, and castor oil). As is known, the oxidative drying rate is increased with high content of polyunsaturated fatty acids (linolenic or α-eleostearic acid) in the oil. The fatty acids react more rapidly and show shorter induction times compared to oils rich in less unsaturated fatty acids.38-39 Among the drying oils, TO contains over 80 % of α-eleostearic acid having three double bonds. This characteristic results in much faster drying than other oils, generating high resistance against mixture penetration and hydrolysis. Therefore, for thick coatings, this fastoxidative curing leads to the formation of a solid skin layer (acting as a diffusion barrier to atmospheric oxygen) while TO is still viscous under this thin skin.40 Consequently, poor through-drying is one of the major drawbacks of drying oils since the following curing of oil under the skin layer will generate tension at the interface, which may bring about a characteristic “frosted” or "blistering" appearance (having a skinning tendency and wrinkled finish). This property may be soothed by adding hCNC to TO as an oxygen gas barrier, which would act to control the drying rate of the nanocomposite coatings. Alternatively, the reduced wrinkling and frosting may be due to reduced shrinkage attributable to the stiff nanoreinforcement. Nevertheless, if this influence is not desired, the hCNC-TO can be subjected to heat bodying, similar to boiled linseed oil. To reduce a curing time of the modified TO, a slightly polymerized TO can be prepared by heating the oil to 280°C or some oxidative drier (zirconium, manganese or cobalt), which will promote the through-drying or polymerization of the film, can be added to the hCNC-TO formulations. The modified oils may have much better through drying



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performance even when curing is accelerated by both the use of a metal drier or high temperature.38, 41-42 However, in comparison with linseed oil, an excessive growth in viscosity of the oil, which is caused by an irreversible “gelling” of the oil, can be controlled by a careful heating rate/time and by blending with other drying oils or dilution oils during the boiling process.42

Figure 2. Background image opacity of casted TO films with different thicknesses and hCNC content.

Mechanical Properties. The hCNC-TO nanocomposite films (multi-layered to be 80-100 µm thickness) on glass substrates were fabricated using formulations containing various contents of hCNC (0 - 10 wt%). All films had great adhesion and excellent appearance. The increase of stiff hCNC in the TO formulations caused the film surface hardness to increase markedly; thus, the scratch hardness of the films increased from ‘2B’ to ‘F’ with the increasing hCNC content. In comparison with pencil hardness of common coatings, the ‘F’ hardness of the TO nanocomposite coating is higher than the ‘3B’ hardness of a water clear acrylic aerosol and comparable to that of


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a urethane/nitrocellulose lacquer, though not the ‘2H’ hardness of a catalyzed acrylic polyurethane coating. Other researchers have previously reported a common scratch failure pattern of a coating material during pencil testing.43-44 This scratch damage is caused by film cracking under tensile stress and substrate plastic deformation, relating to the hardness of the coating and the yield strength of the substrate. During the formation of a gouge and scratch mark, the coating behind the pencil lead (within the scratching mark) and the coating layer outside the scratch mark undergo tensile stress due to the friction of the pencil tip and the plastic deformation of the substrate, respectively, causing cracks. Furthermore, when a dart drop impact is applied, the fracture toughness (the critical energy release rate) of the coating matrix corresponds to the coating resistance in cracking and scratch failure. In particular, a gouge failure is generally followed by a crack failure. This may be due to the fact that the pencil tip penetrates into the gouged area; thus, the coating at the edge of the gouged area is typically subjected to an additional shear stress, drastically increasing the frictional force. This result confirms, therefore, that avoiding gouge failure is an effective way of improving the pencil hardness grade or impact toughness against a falling object. To prevent the gouge failure, an increase in coating thickness or filler content allows a plastic zone to remain and expand within the coating material. Likewise, no failure would occur since an applied stress would not exceed the increased yield strength (which is much higher than the substrate) of the coating matrix. Since the relatively high scratch resistance of the TO nanocomposites was dependent on the amount of hCNC loadings, it may occur due to the stiffening of the matrix by the hCNC particles and larger plastic zone via nanofiller/matrix interface debonding.45

An impact test can be a fundamental tool for

evaluating the mechanical strength of new composite materials since the absorption ability of



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kinetic energy is highly dependent on the interface adhesion strength and filler content. Figure 3 exhibits the impact strength of the TO coatings reinforced with 0, 1, 2, 3, 5, and 10 wt% of hCNC on the steel substrate. The impact resistance (chip resistance) shows a marked increase with hCNC content (in particular, 5 and 10 wt% of hCNC). This marked improvement is attributed to the significantly higher resistance to fracture of the hCNC, which may allow enhanced energy dissipation by a decreased Young’s modulus at the hCNC particle-TO matrix interface, the so-called “inter-phase”.46-47 It is very interesting that the impact strength, as well as hardness increases with the hCNC content in the TO matrix, since in traditional nanocomposites higher stiffness is typically achieved at the cost of increased brittleness. The higher impact strength up to 10 wt% hCNC content may be due to the high bond strength (well-dispersed hCNC) and Young’s modulus (larger plastic zones) of the exfoliated structure. An increase in the mechanical strength can be attributed mainly to appropriate filler-matrix physical binding forces (mechanical interlocking, electrostatic or van der Waals interactions, etc.). Likewise, nanofiller debonding and plastic void growth are regarded to be major toughening mechanisms for nanocomposites. If their interactions were chemical forces (covalent, ionic or metallic, etc.), the addition of the rigid hCNC would lead to embrittlement of a material.25,

48

The synergistic

toughening effect of the well-dispersed hCNC and ductile TO matrix obstructs from initiating and propagating cracks by trapping/pinning toughening mechanisms, which are promoted by optimal interface binding force (excellent compatibility of hCNC with TO binder).49-50 However, while an appropriate addition of hCNC in the TO can improve the film performance, it is expected that a further increasing of hCNC in the TO will bring about fabrication difficulty due to the high viscosity of its formulation. This drawback may result in poor film characteristics: low gloss, haziness, or deteriorating mechanical properties.


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Figure 3. Impact strength and pencil hardness of TO coatings containing 0, 1, 2, 3, 5, and 10 wt % hCNC.

Optical properties. A relatively high gloss value was achieved by using tung oil on a maple wood substrate (Figure 4). According to previous publications5-6, 51-52, this behavior is attributed to good flow properties of the drying oils before curing, as long as the unsaturation induced film shrinkage of tung oil coatings remains controlled. TO formulations (25% solid content in orange oil) are thin enough to flow and not to leave brush marks, avoiding any coating defects (cracking, wrinkling, blistering, foam, flooding etc.). The average of the obtained gloss values of individual samples with different content of hCNC (0, 5, and 10 wt%) were calculated in Table 2. The gloss 55.7 and 59.2 of TO composite coatings (containing 5 and 10 wt% hCNC respectively) at a 60˚ angle were found to be slightly lower than that of a 63.9 neat coating.



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Theoretically, the addition of nanoparticles to a polymeric coating results in lowering of the gloss which is typically dependent on the roughness of the substrate surface. However, no markedly reduced gloss (caused by the coagulation of the hCNC particles) was shown in this study. Therefore, these results will correspond to the following pencil hardness and roughness data.


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Figure 4. Images of TO coated (multi-layers applied to be about 100 µm thickness as seen in Figure S2-b) maple wood specimens with (a) bare wood, (b) 0 wt%, (c) 5 wt%, and (d) 10 wt% hCNC using a 2-inch-wide flat paint brush.



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Table 2. Gloss and roughness values of bare wood, pure TO coated, and hCNC-TO coated wood substrates. roughnessa)

gloss

case 20o

60o

85o

Ra

Rq

Rt

Rz

bare wood

0.7±0.08

4.9±0.6

32.7±3.4

0.64 ± 0.08

0.97 ± 0.08

13.74 ± 1.66

10.67 ± 0.43

pure Tung oil

25.3±3.9

63.9±3.8

77.0±3.5

0.54 ± 0.20

0.71 ± 0.24

10.58 ± 5.95 ± 1.65 4.38

5% hCNCTung oil

13.4±1.0

55.7±2.3

72.5±2.5

0.48 ± 0.02

0.62 ± 0.05

6.89 ± 2.91

4.70 ± 1.07

10% hCNCTung oil

14.3±1.1

59.2±1.8

77.6±1.4

0.37 ± 0.09

0.49 ± 0.12

8.27 ± 2.18

4.64 ± 0.97

a) average roughness Ra, rms roughness Rq, maximum peak to valley roughness height Rt, and ten-point height Rz values obtained from the 2D roughness profiles of the TO coatings.

Surface roughness measurement. The effect of the hCNC content on the film was also studied by 3D profilometry. The data showed all TO coated surfaces to be relatively smooth, but a roughness analysis showed small but apparent differences in arithmetic average roughness (Ra), root-mean-square roughness (Rq), maximum peak to valley roughness height (Rt), and ten-point height (Rz) as shown in Table 2. The 5 and 10 wt% hCNC-TO coated specimens had more smooth surface features across the chosen 2D profiles with Ra values of 0.48 and 0.37 µm and Rq values of 0.62 and 0.49 µm respectively, while bare wood and pure TO coated samples had more rough Ra values of 0.64 and 0.54 µm and Rq values of 0.97 and 0.71 µm in Table 2. This trend of Ra and Rq values for all specimens is similar to the other surface parameters of mean peak-tovalley height (Rt) and ten-point height (Rz). Likewise, the surface of nanocomposite coatings in


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this study were relatively smooth at surface roughness below 1 µm, and unexpectedly, the roughness of nanocomposite coatings gradually decreased with increasing hCNC content. Typically, the poor shelf life (due to low oxidative stability) of drying oils leads to solidification of coating residues on the container walls. The exceptionally large solid particles (gel particles) could be introduced into the bulk solution resulting in film contamination. During the filming and curing process, the solid particles (which typically result in deterioration of the film's clarity, gloss, and roughness) can be observed on the coating surfaces as shown in Figure S5-b. In addition, some studies anticipated that the presence of nanoparticles in coating surfaces (these particles have a tendency to migrate on the coating surface) is attributed to the growth of roughness.13, 17, 19-20 The systems with low viscosity and small size fillers theoretically show a smoother surface of their nanocomposite coatings due to better leveling and dispersion performance in comparison with those of high viscosity and large size nanofillers. Highly viscous solutions will bring about brush texture or marks with a relatively high roughness value, whereas for low viscosity solutions the opposite will happen. In addition, the viscosity of the solutions will also affect the evaporation rate of solvent and the penetration/spreading rate on the substrate. Therefore, a coating with an appropriately viscous solution produces the optimum film. In this system, although the addition of hCNC to TO solution led to an increase in the solution viscosity, an increase in the coating surface roughness could not be observed. This may be related to the achievements of the proper viscosity, shear thinning behavior, and welldispersed nanoparticles of orange oil-diluted hCNC-TO suspensions at a high shear rate during its film forming process. The presence of hCNC coagulates and TO gel particles in low viscosity suspension may result in a marked increase in surface roughness of the film, whereas there may



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be a marginal increase in the case of high viscosity suspension. This can be explained by the fact that as long as the ability of the suspension to completely cover the particles and suppress the migration of the particles toward the web surface before curing is not exceeded, the particles do not protrude through the film. This finding could also correspond to an insignificant gloss change of the hCNC-TO coated films (this coating is still a gloss finish, not a matte finish) from a high gloss finish of pure TO coated film. Typically, the loss of gloss is attributed to an increase in surface roughness due to the existence of the particles and aggregates. Such surface roughness causes a decrease in the diffuse reflection of visible light.53-54 However, the TO coatings (having 5 or 10 wt% hCNC) did not show a markedly lower gloss compared to the pure TO coating despite a slight difference between the refractive indexes of CNC (1.54) and TO film (1.49-1.52). This may correlate to excellent compatibility of hCNC with the TO film and a lower Ra of the hCNC-TO films. These results indicate that adding an appropriate amount of hCNC in the TO does not deteriorate the film’s gloss and roughness.


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Figure 5. 3D surface topography of a) bare wood, b) pure TO coated, c) 5wt% hCNC-TO coated, and d) 10wt% hCNC-TO coated wood specimens.

To obtain knowledge of the surface roughness, we have generated 3D topography images of the observed and measured samples (Figure 5). It is clear from the smooth surface coatings (Figures 5-c and d) that the hCNCs are well-dispersed, complying with the previous pencil hardness data. The existence of coagulated nanoparticles or TO gel particles would increase the roughness of the coating and deteriorate the scratch resistance. The results are probably attributed to the fact that the surface hardness and roughness of the nanostructured hCNC-TO coatings lead to the following trends: less roughness and higher surface hardness (excellent resistance to deformation, scratching, penetration, and erosion). This finding also indicates that the roughness reflects the microstructure state of the coatings as an indicator of the coating performance.



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Accelerated Weathering Tests.

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Photo induced/oxidative degradation, which reduces

effective durability or period of use, is one of the critical issues of functional materials for outdoor applications. UV light can decompose and oxidize polymeric coatings, especially in the presence of air. Organic coatings exposed to UV light and oxygen lose their mechanical/cohesive strength and integrity, develop defects such as cracking/skinning/blistering, and show color changes. The influence of UV light/oxygen exposure on surfaces of pure TO and hCNC-TO coated wood is shown in Table 3 and Figure 6. Using a flat paint brush, maple wood plank specimens (60×90×6mm) were coated (multilayer coating thickness: 100 µm) for the photo induced/oxidative degradation tests of the TO coated films and bare wood substrate. Accelerated weathering of the TO coated wood plate masked with a screen was performed with a 24 h highintensity light exposure (0.69 J/m2 at 340 nm) using a 400watt metal halide lamp QUV device as shown in Figure 6-a. Added hCNC did not negatively affect the original color of the TO coatings. The chromatic parameters (lightness, redness, and yellowness coordinates: L*, a*, and b*) of hCNC-TO coated wood are compared with bare wood and pure TO coated wood. The color of both specimens (with and without hCNC) darkened when exposed to UV radiation. The quicker color changes at unscreened areas on the bare wood and pure TO coated wood specimens upon irradiation were induced by lowering lightness values (ΔL* 15.8 and 15.4, respectively) and by increasing redness and yellowness value (Δa* 5.9 and 11.2 and Δb* 17.2 and 3.5, respectively) as shown in Table 3. The hCNC(5 and 10 wt%)-TO coated wood surfaces exhibited no marked reduction in photo-yellowing as compared to the bare wood and pure TO coated wood, which indicates that the hCNC composite coatings were slowly degraded by irradiation (ΔL* 7.5 and


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7.5, Δa* 7.7 and 7.3, and Δb* 2.2 and 2.4). The overall color change (ΔE*) in individual specimens after 24 h (UV dose amount = 0.69 J/m2) are compared in Table 3. The ΔE* variation is highest for the bare wood coating and pure TO coated wood, and the lowest for the hCNC-TO coated wood with about the same pattern for the individual color parameters, ΔL*, Δa*, and Δb* (Figure 6). Consequently, these results indicate that incorporation of hCNC in TO is effective in blocking UV induced degradation of the TO film. However, this decrease in color change of the TO coating with hCNC was not expected because the size of hCNC particles is too small to block UV light.14, 20 An explanation may be gleaned from color changes due to lignin photooxidation in wood, where the role of oxygen has been proven in several studies.55-57 Therefore, the superior UV resistance of the hCNC-TO coated wood could be expected to be due to the oxygen barrier enhancement of hCNC.

Table 3. Overall color change (∆E*) in TO coated films after 24 h light exposure. samples

△L*

△a*

△b*

△E*a)

bare wood

15.8

5.9

17.2

24.1

pure Tung oil

15.4

11.2

3.5

19.4

5% CNC-Tung oil

7.5

7.7

2.2

11.0

10% CNC-Tung oil

7.5

7.3

2.4

10.7

a) UV dose amount = 0.69 J/m2



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Figure 6. (a) Image of UV light irradiated surface of bare, pure TO, and hCNC-TO coated wood plate and changes in (b) lightness L*, (c) redness a*, and (d) yellowness b* color coordinates over exposure time.

Oxygen barrier properties. Due to the superior weathering performance of the modified TO coatings and as studies have reported that CNC coatings improve oxygen barrier properties of conventional flexible food packaging materials58-59, the influence of hCNC on the oxygen permeability (OP) of the TO films was studied. The OTR values of coated PLA (Figure S6) shows that TO coating decreases oxygen transmission and that hCNC decreases the OTR, even


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though the coatings are thinner. To compare materials, OP of the individual materials was calculated and presented in Figure 7. OP values of pure TO, hCNC(5%)-TO, and hCNC(10%)TO films were all significantly different with the OP of hCNC(5%)-TO coating film being 50 % lower than that of pure TO coating film and hCNC(10%)-TO coating film being even lower. This high O2 barrier feature makes hCNC-TO coatings desirable for wood finishes. Many previous studies reported that the insufficient dispersion of nanoparticles into a polymer matrix (caused by the incompatibility of a nanofiller with a polymer) deteriorates gas-barrier performance.60-61 However, in this system, a tortuous pathway for diffusion (created by the impermeable hCNC particles) leads to improved gas barrier performance.62-63 Tung oil is a valuable option used in wood finish, but O2 easily diffuses into the inner surface of wood products, thereby leading to potential oxidative degradation. Therefore, a TO coating with hCNC is promising since it can reduce weathering rates in wood products.

Figure 7. OP (oxygen permeability) of neat PLA, pure TO, 5wt%hCNC-TO coating, and 10wt%hCNC-TO coating films.



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Conclusion Tung oil (TO) wood nanocomposite coatings were successfully prepared by adding hydrophobized cellulose nanocrystals (hCNCs), which are created by grafting poly (lactic acid) oligomers and oleic acid to improve their dispersability in the polymeric matrices. The influence of hCNC on mechanical, optical, topographic, and permeation properties of TO nanocomposite coatings was examined. In comparison to neat coatings, the nanocomposite coatings presented here have significantly higher scratch/impact resistance and provide marked barrier enhancement against O2 gas permeation through the TO film membrane. Despite the addition of nanoparticles, the gloss and color of the nanocomposite coatings was adequately maintained. These results indicate the hCNC particles were well dispersed in the polymeric matrices. Furthermore, the nano-reinforcement of the hCNC was shown to reduce surface roughness by suppressing the immigration of nanoparticles or gel particles toward the web surface due to the increased viscosity of the wet film coatings. The hCNC can be added to the TO formulations up to 10 wt%; yet, the formulations must be diluted by mixing an orange oil or mineral spirit with a TO/solvent at a ratio of 1/2 or 1/3 (w/w) for the instantaneous filling of porous and rough wood surfaces. This hCNC can provide enhanced mechanical and weathering properties as a reinforcing agent, without any loss in optical properties (gloss and color-b), while maintaining well-matched surface coating applications to effectively protect the surface of the wood substrates. The hCNCTO composite coatings show excellent anti-resistance, hardness, impact strength, gloss/color, gas barrier properties, and anti-weathering performance which are of great importance in the wooden furniture and musical instruments manufacturing industries where coating surfaces must maintain great properties for many years.


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Supplementary Information Supporting information is available free of charge on the ACS Publications website at DOI: Particle size distribution of encapsulated phase change materials used in this study (PDF). Acknowledgements The authors would like to thank Jong-Ki Sim from SK Chemicals Corp. for obtaining the wood surface images and Yun-Suk Choi from SKC Corp. for obtaining the OTR data. The authors acknowledge

the

financial

support

of

the

National

Science

Foundation

Scalable

Nanomanufacturing program under award CMMI-1449358, the Forest Products Laboratory under awards 11-JV-11111129-118 and 11-CR-11111129-109. The authors declare no competing financial interests.

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For Table of Contents Use Only

Tung Oil Wood Finishes with Improved Weathering, Durability, and Scratch Performance by Addition of Cellulose Nanocrystals

Youngman Yoo and Jeffrey P. Youngblood *.

Synopsis New oxygen-permeability controlled and nanoreinforced wood finishes through the incorporation of hydrophobized CNCs into tung oil are discussed.



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Figure 1. Rheological behaviors: shear rate vs. a) viscosity and b) shear stress diagrams of pure TO and its solution diluted with orange oil (1:1 w/w) and 5% hCNC-TO and its dilution solutions with orange oil (1:1 and 1:3 w/w). 532x247mm (96 x 96 DPI)


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Figure 2. Background image opacity of casted TO films with different thicknesses and hCNC content. 516x257mm (96 x 96 DPI)



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Figure 3. Impact strength and pencil hardness of TO coatings containing 0, 1, 2, 3, 5, and 10 wt % hCNC. 404x310mm (96 x 96 DPI)


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Figure 4. Images of TO coated (multi-layers applied to be about 100 µm thickness as seen in Figure S2-b) maple wood specimens with (a) bare wood, (b) 0 wt%, (c) 5 wt%, and (d) 10 wt% hCNC using a 2-inchwide flat paint brush. 464x313mm (96 x 96 DPI)



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Figure 5. 3D surface topography of a) bare wood, b) pure TO coated, c) 5wt% hCNC-TO coated, and d) 10wt% hCNC-TO coated wood specimens. 538x311mm (96 x 96 DPI)


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Figure 6. (a) Image of UV light irradiated surface of bare, pure TO, and hCNC-TO coated wood plate and changes in (b) lightness L*, (c) redness a*, and (d) yellowness b* color coordinates over exposure time. 563x436mm (96 x 96 DPI)



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Figure 7. OP (oxygen permeability) of neat PLA, pure TO, 5wt%hCNC-TO coating, and 10wt%hCNC-TO coating films. 292x224mm (96 x 96 DPI)


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Tung Oil Wood Finishes with Improved Weathering, Durability, and

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