Abstract
Susceptibility of wood to UV degradation decreases the service life of wood products outdoors. Organic UV absorbers (UVAs) and hindered amine light stabilizers (HALSs), as well as inorganic UVAs, are added to coatings to improve the UV stability of coated-wood products. Although about 85% of UV radiation is absorbed by lignin in the wood, it is unclear which UV stabilizers can minimize lignin degradation. In this study, the photodegradation of softwood organosolv lignin was monitored over 35 days of UV exposure. Changes in lignin properties were assessed using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR). It was found that the aromatic rings of lignin underwent significant degradation, resulting in increased glass transition temperature and molecular weight of lignin. Subsequently, 18 different additives were mixed with lignin and exposed to UV irradiation. The analysis of samples before and after UV exposure with FTIR revealed that inorganic UVAs (cerium oxide and zinc oxide) and a mixture of organic UVAs and HALSs (T-479/T-292, T-5248, and T-5333) were the most effective additives in reducing lignin degradation. This study can help coating scientists to formulate more durable transparent exterior wood coatings.
1 Introduction
Wood products used outdoors are susceptible to photodegradation (Evans 2012), which usually causes yellowing, discoloration, loss of gloss, increased roughness, and diminished mechanical and physical properties (Hon 1984; Nejad and Cooper 2011; Turkulin and Sell 1997). Wood is mainly composed of three compounds: cellulose, hemicellulose, and lignin; these compounds have different sensitivities to UV light during photodegradation. Lignin is a macromolecule in wood that acts as a binder in holding cellulosic fibers (Hayoz et al. 2003); it is composed of various arrangements of three monolignols: guaiacyl, syringyl, and p-hydroxyphenyl units (Sakakibara and Sano 2000).
Cellulose and hemicellulose only absorb 5–20% of UV light, while lignin absorbs about 80–95% of the UV light due to the presence of chromophores and aromatic rings, making it more prone to decomposition by photooxidation reactions (Davidson 1996; Hayoz et al. 2003). During UV irradiation, three chemical reactions occur in lignin, 1) dehydrogenation, 2) dehydroxymethylation, and 3) demethoxylation (Kutz 2018). The formation of free radicals triggers the UV degradation process, followed by the oxidation of phenolic hydroxyl groups in lignin (Müller et al. 2003; Williams 2005). Free phenolic radicals are generated immediately under UV irradiation. This way, the radicals delocalization favors the formation of o- and p-quinonoid structures after demethylation and cleavage of the side chain (Hon and Shiraishi 2000). As shown in Scheme 1, the newly formed carbonyl groups in o- and p-quinonoid are considered chromophoric groups that cause significant color changes on the wood surface (Hon and Shiraishi 2000).
Formation of o- and p-quinonoid structures resulting from UV degradation of lignin (Hon 2001).
In addition to lignin, extractives are also susceptible to UV degradation. Extractives types and contents significantly affect the color, odor, and biological durability of the wood (Umezawa 2000). Similar to lignin, extractives will also undergo structural changes after UV exposure contributing to wood discoloration (Abe 1994; Chang et al. 1999; Funaoka et al. 1963; Pandey 2005a,b).
UV radiation (295–400 nm) provides sufficient energy to dissociate lignin moieties that have carbonyl, biphenyl, or ring-conjugated structures. It was shown that violet light (380–430 nm) extends photodegradation into the wood beyond the area affected by UV light since larger wavelengths penetrate deeper into the wood than UV light (Kataoka et al. 2006).
In addition to UV light, water also plays a vital role during the photodegradation of wood, including carrying the radicals formed on the wood surface to deeper layers in the wood, forming hydroperoxide that can initiate chain scission reactions in polymeric wood compounds. Kalnins (1966) reported that oxygen is necessary for free radical initiation, and phenoxy radicals are formed during lignin photodegradation can react with oxygen to form O-quinoid structures after demethylation. Hon also stated that the reaction of oxygen to form hydroperoxide is an essential part of the photodegradation process (Hon et al. 1982).
Researchers have explored many ways to improve weathering performance of wood. Treating wood surfaces with chromic acid is a well-known method that stabilizes wood by oxidizing phenolic sub-units at the surface, making them more resistant to photodegradation (Chang et al. 2007; Evans and Schmalzl 1989). However, concern about chromium compounds’ potential adverse health effects limited the commercialization of chromic acid treatment for wood (Schmalzl and Evans 2003). Chemical modification has also been widely used to enhance the weathering stability of wood. Chemically treated wood with butylenes, butylene oxide, and methyl isocyanate, benzoyl chloride, as well as acetylation showed better weathering performance (Evans et al. 2002; Feist et al. 2007; Rowell et al. 1981). Additionally, 1,3-dimethylol-4,5-dihydroxyethyleneurea (mDMDHEU) was used to treat scots pine wood and reported to significantly improved the weathering stability of the wood (Xie et al. 2008). One of the most common ways to increase the photostability of wood while maintaining its esthetic appeal is to use transparent coatings such as epoxy, polyurethane, alkyd, and acrylic, containing UV stabilizers and/or nano-pigments (Forsthuber and Grüll 2010; Nikafshar et al. 2017; Saha et al. 2013; Saiter et al. 1995; Van den Bulcke et al. 2006).
Organic UV stabilizers are categorized into two groups: 1) UV absorbers (UVAs) and 2) hindered amine light stabilizers (HALSs). UVAs like 2-(2-hydroxyphenyl)-benzotriazoles (BTZ) filter out the vulnerable wavelengths of the light before they reach the wood surface; therefore, decreasing the rate of radical formation due to their high absorbance profile in the UV region (typically 300–350 nm) (Schaller et al. 2008). The most common UVAs have primary photophysical properties, including a high absorbance profile in the UV range and high photochemical stability (Decker et al. 1995). HALS (a common derivative of 2,2,6,6-tetramethylpiperidine), also known as a radical scavenger, inhibiting the photo-oxidative degradation of polymers (Klemchuk et al. 1990). It was shown that a combination of organic UVA and HALS improved the transparent wood coating performance (Morris and McFarling 2006).
On the other hand, inorganic UVAs are based on metal oxide particles like ZnO or TiO2, which are applied to scatter or absorb light. Using nanosized pigments can protect both coating and the substrate (wood) while preserving transparency in the visible spectrum (Nkeuwa et al. 2014; Salla et al. 2012). Pretreatment of wood for dimensional stability and applying the flexible and photostable coating is another way to achieve durable exterior coated wood products (de Meijer and Nienhuis 2009; Evans et al. 2015). Also, it has been shown that phenol-formaldehyde resins can improve weathering stability of plywood samples (Passauer et al. 2021; Williams 2005).
Although several studies have evaluated the photostability performance of different wood coatings, they were mainly focused on assessing color change as a visual indicator (Tolvaj and Mitsui 2010), to the best of our knowledge, there is no published work on examining the interaction between lignin, as the main UV susceptible component of the wood, with a wide range of UV stabilizers. The aim of the present research was to find the most effective additives that can reduce lignin degradation. The results of this study can help coating formulators or preservative producers choose additives proven to minimize lignin degradation, thus improving the UV performance of wood products in exterior applications.
2 Materials and methods
Since most exterior wood products (fences and decks) are made of softwood in North America, a high purity softwood organosolv lignin provided by Lignol (Current Suzano) was used. An organosolv lignin was chosen because, among technical (commercially available) lignins, organosolv lignin has the closest structure to the structure of native lignin in the wood due to the mild ethanol biorefinery isolation process (Mandlekar et al. 2018). Eighteen different additives (Table 1), including organic UVAs, organic HALSs, and inorganic UVAs, were kindly supplied by chemical industries. All other chemicals were purchased from Fisher Scientific Co and Sigma Aldrich and used as received.
Composition of prepared samples.
| ID | Sample name | Role | Supplier |
|---|---|---|---|
| 1 | Tinuvin-1130 | Organic UV absorber | BASF |
| 2 | Tinuvin-400 | Organic UV absorber | BASF |
| 3 | Tinuvin-479 | Organic UV absorber | BASF |
| 4 | Tinuvin-384 | Organic UV absorber | BASF |
| 5 | Chiguard 5330 | Organic UV absorber | Chitec Technology |
| 6 | Tinuvin-292 | Organic HALSa | BASF |
| 7 | Tinuvin-123 | Organic HALS | BASF |
| 8 | Chiguard 101 | Organic HALS | Chitec Technology |
| 9 | Tinuvin-5333 | Organic UV absorber & HALS | BASF |
| 10 | Tinuvin-5248 | Organic UV absorber & HALS | BASF |
| 11 | Tinuvin-479-123 | Organic UV absorber & HALS | BASF |
| 12 | Tinuvin-292-1130 | Organic UV absorber & HALS | BASF |
| 13 | Tinuvin-384-292 | Organic UV absorber & HALS | BASF |
| 14 | Tinuvin-400-123 | Organic UV absorber & HALS | BASF |
| 15 | Tinuvin-479-292 | Organic UV absorber & HALS | BASF |
| 16 | Fe3O4 b | Inorganic UV absorber | US Research Nanomaterials |
| 17 | Cerium oxideb | Inorganic UV absorber | Strem Chemical, Inc. |
| 18 | Zirconium oxideb | Inorganic UV absorber | US Research Nanomaterials |
| 19 | Zinc Oxideb,c | Inorganic UV absorber | Zochem |
| 20 | Titanium oxideb,c | Inorganic UV absorber | NYACOL Nano Technologies Inc. |
| 21 | Minexc | UV and binder stabilizer | Covia Canada Ltd. |
| 22 | Control (pure lignin) | – | Lignol (Current Suzano) |
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aHALS, hindered amine light stabilizer. bNanosized pigments. cProvided by the third-party supplier, and it is assumed were obtained from listed producers.
2.1 Sample preparation
First 4 g of organosolv softwood lignin was dissolved in 15 g of tetrahydrofuran (THF). Next 2 wt.% of individual additives were added to the mixture (lignin and THF). The solution was mixed for 15 min at room temperature (200 rpm) and then was poured into aluminum pans (40 mm diameter, 10 mm height). For some samples, the mixture of both organic UVA and HALS was used, as recommended by the additives manufacturers. The mixtures of inorganic UVAs in lignin-THF were placed in a sonication bath for 10 min to ensure their homogenous dispersions. Minex, a functional filler made from nepheline syenite, that is used as a performance enhancer for brightness, weathering stability, abrasion, and burnish resistance in coating formulations, was also added to the lignin as an additive. The control sample (lignin, without any additives) was also dissolved in THF to obtain a uniform and smooth surface similar to other samples. Later, all samples were placed under a fume hood (with lights off for 24 h) to let the solvent evaporate (THF). Then the samples were kept in the dark place for one week at room temperature to ensure complete removal of solvent while avoiding any potential exposure to light that might trigger undesirable reactions of photoactive compounds. The names and compositions of all samples are shown in Table 1.
2.2 UV exposure
Photodegradation of the lignin mixtures was studied following the EN 927-6 Standard procedure (EN 927-6 2006). The UVA-340 lamp (recommended by EN 927-6 Standard) manufactured by Q-Lab Corporation (Canada) was chosen for this study due to its similar spectrum to sunlight (Table 2). All samples were placed under UV light irradiation at a 5 cm distance from the lamp for 35 days. UV exposure was run in ambient conditions (24 °C, 70–80 RH%, and air atmosphere). There were not any dark periods during the UV exposure, except for brief sample collections. Because powdered lignin was used during the test, the samples were not exposed to water during the photodegradation test.
Relative spectral irradiance of UV-A 340 lamp.
| Wavelength (nm) | Relative spectra irradiance (%) |
|---|---|
| 290 < λ ≤ 400 | 100 |
| λ ≤ 290 | 0.0 |
| 290 < λ ≤ 300 | 0.2 |
| 300 < λ ≤ 320 | 6.2–8.6 |
| 320 < λ ≤ 340 | 27.1–30.7 |
| 340 < λ ≤ 360 | 34.2–35.4 |
| 360 < λ ≤ 380 | 19.5–23.7 |
| 380 < λ ≤ 400 | 6.6–7.8 |
2.3 Lignin characterization
The pure lignin and mixture of additives and lignin samples were analyzed with Fourier-transform infrared spectroscopy (FTIR) before UV irradiation and then every week during 35 days of UV irradiation. A Jasco FTIR-ATR-6600 equipped with an attenuated total reflection accessory (ATR) was used to monitor potential chemical changes of samples because of the photodegradation reaction. Spectra were recorded in a wavenumber range between 500 and 4000 cm−1 at a resolution of 4 cm−1 with 64 scans. All spectra were normalized and baseline-corrected before quantitative analysis using OriginPro 2015 software (version 2.214). The spectra were normalized by selecting the C–H deformation band at 2921 cm−1 as a reference since its intensity remained the same during UV exposure. Linear background correction in the absorbance mode was applied to remove any potent background compound. The same procedure was applied for selected bands (aromatic skeleton or carbonyl), which are expected to change during the UV exposure. The lignin and carbonyl indexes were calculated by dividing the absorbance of a specific band (after background correction) by the absorbance of the C–H band (the unchanged band used as reference).
A Q100 differential scanning calorimeter (DSC) by TA Instrument, equipped with refrigerated cooling, was used to analyze the thermal property of the lignin sample. About 5–10 mg of lignin was placed in an aluminum pan, and a non-isothermal test with a heating rate of 10 °C/min under a nitrogen flow of 70 mL/min was carried out at a temperature range of 25–200 °C in a heat-cool-heat cycle. The second heating cycle was used to calculate the glass transition temperature (T g ) before and after UV-irradiation.
Gel permeation chromatography (GPC) was used to determine the molar mass distribution of the lignin sample. The samples were dissolved in THF (HPLC grade) at a 5 mg/mL concentration and were filtered using a syringe filter (PTFE, 0.45 μm); the filtrate samples were used for GPC analysis. A Waters GPC system was then used to analyze the filtrate at a flow rate of 1 mL/min, using a 300 mm × 7.8 mm column, Styragel HR 3 THF (500–30 k Å). Polystyrene standards of specific molecular weight (162, 370, 580, 945, 1440, 1920, 3090, 4730, 6320, 9590 Da) were used as calibration standards.
The hydroxyl contents of the lignin samples were measured using phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR) (Asgari and Argyropoulos 1998). About 40 mg oven-dried lignin was dissolved in 500 μL of a mixture of anhydrous pyridine/deuterated chloroform (1.6:1, v/v). Next, 100 μL cyclohexanol solution (22 mg/mL in anhydrous pyridine and deuterated chloroform [1.6:1, v/v]), was added as an internal standard, and 50 μL of chromium (III) acetylacetonate solution (5.6 mg/mL in anhydrous pyridine and deuterated chloroform [1.6:1, v/v]) was used as a relaxation reagent. Finally, 100 μL phosphitylating reagent (2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane [TMDP]) was added to the mixture and mixed for 30 s. Subsequently, 600 μL of the mixture was transferred to a 5 mm NMR tube, and NMR analyses were performed using an Agilent DDR2 500 MHz NMR spectrometer equipped with 7600AS, running VnmrJ 3.2A, with a relaxation delay of 5 s, and 128 scans. To avoid any decomposition, prepared samples were tested within 1 h of sample preparation.
The hydroxyl content of each lignin sample was calculated based on the ratio of the internal standard peak area (cyclohexanol) to integrated areas over the following spectral regions: aliphatic hydroxyls (149.1–145.4 ppm), cyclohexanol (145.3–144.9 ppm), condensed phenolic units (144.6–143.3; and 142.0–141.2 ppm), syringyl phenolic units (143.3–142.0 ppm), guaiacyl phenolic hydroxyls (140.5–138.6 ppm), p-hydroxyphenyl phenolic units (138.5–137.3 ppm), and carboxylic acids (135.9–134.0 ppm). The entire region of (144.6–138.6) was considered as condensed phenolic since softwoods do not have any syringyl units (Asgari and Argyropoulos 1998).
3 Results
3.1 Lignin degradation
To confirm that the lignin structure was not changed after dissolving it in THF and solvent was entirely removed, the sample was analyzed with 31P NMR. The analysis showed that the drying method was effective with no residual THF in lignin (based on quantitative data). No change in lignin structure was observed compared with spectra of original lignin before dissolving it in THF. FTIR spectroscopy was used to monitor chemical changes of lignin (with and without additives) before UV irradiation and at weekly intervals over 35 days of UV irradiation. Table 3 shows different assignments of FTIR bands in lignin. The C=C stretching vibrations of aromatic rings, which have an absorption band at 1508 cm−1, were considered as the lignin bands (Anderson et al. 1991). The carbonyl groups showed an absorption band at 1714 cm−1, which moved to 1735 cm−1 after UV exposure that was assigned to formed quinone groups (Pandey 2005a,b). The wide absorption band around 3400 cm−1 was assigned to hydroxyl groups (Kline et al. 2010). The FTIR spectra of pure lignin before and after each week of UV irradiation are shown in Figure 1.
Summary of the major Fourier transform infrared spectroscopy (FTIR) peaks of the organosolv lignin.
| Wavenumber (cm−1) | Band assignment |
|---|---|
| 3414 | O–H stretching |
| 2925 | C–H stretching |
| 2842 | C–H stretching |
| 1714 | C=O stretching |
| 1508 | Aromatic skeletal vibration |
| 1182 | Guaiacyl C–H |
Fourier transform infrared spectroscopy (FTIR) spectra of pure lignin sample (control) before and after 35 days of UV-irradiation, 1508 cm−1, vibrations of aromatic rings; 1735 cm−1, vibration of carbonyl groups.
All spectra were baseline corrected and normalized using the band at 2921 cm−1 as reference band, which was not affected by photodegradation (Stark and Matuana 2004). Two parameters were used to monitor potential photodegradation of lignin: I 1508/2921 and I 1735/2921, corresponding to lignin and carbonyl indices, respectively, where “I” represents the measured intensity from the top band to the baseline (Stark and Matuana 2004).
Different indices of pure lignin at different UV irradiation times are displayed in Figure 2. The lignin content decreased rapidly as a result of photodegradation, which was accompanied by the formation of carbonyl groups (Popescu et al. 2011). It can be seen that with the increase in exposure time, lignin index (I 1508/2921) decreased as a result of the decreased amount of aromatic rings during the photodegradation phenomenon, as also confirmed by previous studies (Ganne-Chédeville et al. 2012; Pandey 2005a,b). On the contrary, carbonyl index (I 1735/2921) increased due to ortho and para-quinoid formation forming as free radicals react with oxygen forming carbonyl and carboxylic groups (Feist and Hon 1984; Hon and Shiraishi 2000). The results showed that the rate of carbonyl formation significantly increased after 21 days of UV irradiation.
Lignin and carbonyl indices of pure lignin at different UV irradiation times.
Table 4 shows the properties of pure lignin before and after 35 days of UV irradiation. The molecular weight of lignin increased after UV irradiation. This enhancement may be related to radical coupling reactions, causing the formation of higher molecular weight compounds such as condensed phenyl propane structures like 5-5, 4-O-5, and β-5 (Argyropoulos and Sun 1996; Castellan et al. 1991). Also, the polydispersity index (PDI) of lignin was increased significantly, which is probably due to depolymerization and repolymerization of lignin polymeric chains under UV light (Azadfalah et al. 2008; Holmbom 1991). The T g s of lignin increased from 93 °C to 122 °C after UV exposure. The increase in T g is reported to be due to the formation of polar groups, such as carbonyl during UV irradiation, confirmed by FTIR data (Aloui et al. 2007; Hill et al. 1994).
Summary of the measured lignin properties before and after 35 days of UV irradiation.
| Lignin properties | Before | After 35 days of UV irradiation |
|---|---|---|
| Mn (Da) | 1190 | 1290 |
| Mw (Da) | 5250 | 7050 |
| PDI | 4.4 | 5.5 |
| T g (°C) | 93 ± 2 | 122 ± 3 |
| Aliphatic hydroxyl (mmol/g) | 1.31 | 1.04 |
| Phenolic hydroxyl (mmol/g) | 2.39 | 1.99 |
| Carboxylic acid (mmol/g) | 0.42 | 0.68 |
Figure 3 illustrates the 31P NMR spectra of lignin before and after UV irradiation. The aliphatic and phenolic hydroxyl functional groups of lignin decreased by 21% and 17%, respectively, after UV exposure, while the carboxylic groups increased by 62%. The decrease in aliphatic and phenolic hydroxyl groups could be related to the formation of quinone structures, whereas an increase in carboxylic acid functional groups could be due to the ring-opening of quinone compounds (Wang et al. 2016).
Phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR) spectra of lignin before (red) and after 35 days (blue) of UV irradiation.
3.2 Efficacy of different additives on improving UV stability of lignin
Figure 4 shows the percent decrease of the lignin index
Decrease in lignin index (%)
The average lignin loss of organic UVAs (65%) was significantly lower than the average lignin loss of HALSs (74%), and other additives (78%). The combination of organic UVAs and HALSs showed 58% lignin loss, which is remarkably lower than using them individually. Overall, inorganic UVAs (56%) were more effective in reducing lignin loss than a mixture of organic UVAs and HALSs (UVAs 65% and HALSs 74%) and other additives (78%). Also, using a mixture of both organic UVAs and HALSs were more effective in reducing the photodegradation of lignin (58% lignin loss) than using individual components. When they are used together, UVA helps to absorb UV light, and HALS captures any free radicals that form during exposure resulting in reducing the UV degradation of lignin (Jia et al. 2007).
When lignin undergoes photodegradation, quinone groups, which have two carbonyl groups, are formed (Scheme 1). Therefore, the photodegradation of lignin will increase the number of carbonyl groups in the lignin. As such, the carbonyl index of all samples increased after 35 days of UV irradiation (Figure 5). Among tested additives, the organic UVA (T-479), zinc oxide, and to some extent, cerium oxide proved to be the most effective additives in protecting lignin, which resulted in lower carbonyl group formation. It was shown that zinc oxide could quench free radicals and acts as a radical scavenger (Kumar et al. 2014; Soren et al. 2018). UVAs filter the high-energy UV spectrum while radical scavengers neutralize high-energy and destructive free radicals. Therefore, when organic UVAs and HALSs were used together, the UV protection was significantly improved compared to using each additive individually.
Increase in carbonyl index (%)
4 Discussion
In this study, lignin was used as a model compound to evaluate the efficacy of a wide range of light stabilizer additives used in coatings. It was observed that zinc oxide and a mixture of organic UVA/HALS were the most effective additives for increasing the photostability of lignin. It was assumed that these additives can potentially increase the UV stability of wood when added to coating formulations designed for exterior applications. Although other wood components, especially extractives, can also play a role in photodegradation of wood, reducing structural degradation of lignin which absorbs 80–95% of UV-light (Hon and Shiraishi 2000), is a good starting point in choosing the right additives for formulating clear wood coatings. It is important to point out that the organosolv lignin used in this study has a different structure than native lignin in the wood. It is indisputable that any isolation process will change the structure of lignin; therefore, the additives that have worked in this study might not have the same interaction effect with the lignin on the surface of the wood. That is why the effectiveness of these best-performing additives are being studied (by authors) when added into different resins (alkyd, acrylic, and polyurethane). The performance of coated-wood samples will be monitored during exposure to a combination UV and rain (accelerated weathering).
Some of these additives can still protect the wood surface without chemically bonding with lignin. They can reduce the UV degradation of wood by shielding (dispersion or absorption) the UV light or acting as radical scavengers to capture the formed radicals. Pánek et al. (2018) reported that the combination of UVA and HALS was the most effective treatment for color stabilization of wood in exterior applications. Since similar results were found, studying lignin degradation as a simpler structure than wood seems to be a reliable method to evaluate the performance of newly developed light stabilizers.
5 Conclusions
The main goal of this work was to investigate the photodegradation phenomena of lignin and study the efficacy of different additives in improving the UV stability of lignin. The analysis of samples with FTIR before and after 35 days of UV exposure showed that the aromatic rings in lignin (lignin index) decreased (85%), while the carbonyl index increased (94%). Also, the glass transition temperature, molecular weight, and carboxylic acid content of lignin increased after UV exposure. The addition of nanosized metal oxides like cerium oxide significantly reduced the photodegradation of lignin. Moreover, it was observed that using a combination of organic UVAs and HALS (such as T-292/T-1130, T-479/T-123, T-400/T-123, and T-5333) were very effective in improving the overall UV stability of lignin, compared to using each additive individually. The results of this study can serve as the first step in choosing the most effective additives for formulating high-performance, durable transparent/clear wood coatings. Keeping lignin intact on the wood surface as a hydrophobic compound reduces wood discoloration and fiber delamination and improves the dimensional stability of wood by improving its water-resistance properties.
Funding source: U.S. Department of Energy
Award Identifier / Grant number: DE-EE0008148
Funding source: Wood-Based Composites Center, a National Science Foundation Industry/University Cooperative Research Center
Award Identifier / Grant number: 1624536-IIP
Funding source: National Institute of Food and Agriculture
Award Identifier / Grant number: 1021850
Acknowledgments
The authors would like to thank technical advisors from Willamette Valley, Oxiquim, Arauco, Louisiana Pacific (LP), Bosie Cascade, and Timber Specialties for their invaluable advice throughout the project.
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: The authors appreciate the funding supports from the U.S. Department of Energy (DOE) EERE under contract no. DE-EE0008148, the Wood-Based Composites Center, a National Science Foundation Industry/University Cooperative Research Center (Award 1624536-IIP), and the USDA National Institute of Food and Agriculture, McIntire Stennis, 1021850.
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Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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