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Wood Coatings

A special issue of Coatings (ISSN 2079-6412).

Deadline for manuscript submissions: closed (31 August 2017) | Viewed by 70040

Special Issue Editor


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Guest Editor
Department of Forestry, Michigan State University, East Lansing, MI 48824, USA
Interests: wood coatings; lignin-based bioproducts
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The growth of multi-story wooden building constructions in recent years, as well as the positive consumer and industry attitudes toward sustainable materials, has opened many more opportunities for wood products. These developments have led to a larger market for wood coatings. Throughout the last few decades, wood coating formulators have dealt with a few major changes in the industry, such as the transition from solvent-based to water-based or UV-Cure formulations, the development of coatings for Cu-based preservative treated wood upon the phase out of CCA, and the formulation of coatings for heat-modified or acetylated wood in exterior applications. Many researchers have also devoted their time to studying the applications of nanoparticles and renewable materials in wood coating formulations and their performance on wood.

This Special Issue of “Wood Coatings” is intended to invite researchers within the field from across the world to publish their latest work in an online coating journal. Since there is no journal exclusively dedicated to wood coatings, this issue will serve as a starting point for gathering all the researchers in wood coatings. As such, the Special Issue may also be seen as a comprehensive compendium for students interested in pursuing research in wood coatings, and as a resource for industries looking for experts in the field.

Dr. Mojgan Nejad
Guest Editor

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Keywords

  • exterior wood coatings
  • interior wood coatings
  • weathering performance
  • renewable materials
  • nanocoatings
  • treated wood
  • UV-Cure coatings
  • engineered wood products

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Published Papers (9 papers)

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2198 KiB  
Article
Synergistic Effect of Addition of Fillers on Properties of Interior Waterborne UV-Curing Wood Coatings
by Xiaoxing Yan, Xingyu Qian, Rong Lu and Tetsuo Miyakoshi
Coatings 2018, 8(1), 9; https://doi.org/10.3390/coatings8010009 - 23 Dec 2017
Cited by 26 | Viewed by 5812
Abstract
A waterborne ultraviolet (UV)-curing coating was prepared on the surface of wood materials with modification of talcum powder and calcium carbonate (CaCO3). When the waterborne UV-curing coatings on the surface of wood materials (WUVCW) was radiated for 1 min by UV [...] Read more.
A waterborne ultraviolet (UV)-curing coating was prepared on the surface of wood materials with modification of talcum powder and calcium carbonate (CaCO3). When the waterborne UV-curing coatings on the surface of wood materials (WUVCW) was radiated for 1 min by UV (λ = 365 nm) and dried at 40 °C for 10 min, it showed good hardness, adhesion, and impact strength, with controlling the talcum content of 2.0% and CaCO3 content of 1.0%, respectively. When the content of talcum powder was higher than 2%, the mechanical properties and gloss of the WUVCW decreased, and when the talcum powder of WUVCW increase to more than 5%, a matte surface appeared after curing. When CaCO3 and talcum powder were present at the same time, the mechanical properties of WUVCW were better than those of only CaCO3 or talcum powder. Full article
(This article belongs to the Special Issue Wood Coatings)
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Figure 1

Figure 1
<p>Main effects plot of CaCO<sub>3</sub>, talcum content and ultraviolet (UV) irradiation time (samples 1–4 in <a href="#coatings-08-00009-t002" class="html-table">Table 2</a>).</p>
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<p>Thermogravimetric curves of WUVCW with 0 (<b>A</b>, sample 5) and 2.0% (<b>B</b>, sample 6) talcum content.</p>
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<p>Scanning electron microscope (SEM) of WUVCW from sample 6 and sample 9: (<b>A</b>) 1.0% CaCO<sub>3</sub> and 2% talcum and (<b>B</b>) 1.0% CaCO<sub>3</sub> and 5% talcum.</p>
Full article ">Figure 4
<p>Infrared spectroscopy (FT-IR) of WUVCW with different CaCO<sub>3</sub> and talcum content (samples 11, 5, 23 and 6 in <a href="#coatings-08-00009-t001" class="html-table">Table 1</a>).</p>
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<p>UV-Vis spectrum of WUVCW after adding CaCO<sub>3</sub> and talcum (<b>A</b>) before curing, (<b>B</b>) UV radiation time of 30 s, (<b>C</b>) UV radiation time of 1.0 min (sample 6 in <a href="#coatings-08-00009-t001" class="html-table">Table 1</a>).</p>
Full article ">Figure 6
<p>Weight change of WUVCW with the drying time at 40 °C: 0 (<b>A</b>, sample 5) and 2.0% (<b>B</b>, sample 6) talcum content.</p>
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<p>Effect of single filler on hardness of coating (samples 5, 11–24 in <a href="#coatings-08-00009-t001" class="html-table">Table 1</a>).</p>
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<p>Effect of single filler on adhesion of coating (samples 5, 11–24 in <a href="#coatings-08-00009-t001" class="html-table">Table 1</a>).</p>
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<p>Effect of single filler on impact strength of coating (samples 5, 11–24 in <a href="#coatings-08-00009-t001" class="html-table">Table 1</a>).</p>
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<p>Effect of single filler on gloss of coating (samples 5, 11–24 in <a href="#coatings-08-00009-t001" class="html-table">Table 1</a>).</p>
Full article ">
470 KiB  
Communication
Evaluation of Selected Properties of Alder Wood as Functions of Sanding and Coating
by Emilia-Adela Salca, Tomasz Krystofiak and Barbara Lis
Coatings 2017, 7(10), 176; https://doi.org/10.3390/coatings7100176 - 21 Oct 2017
Cited by 27 | Viewed by 5726
Abstract
The objective of this study was to optimize the sanding and coating processes of black alder wood to promote and support its use in furniture manufacturing. Two criteria have been applied for process optimization, namely, the minimum surface roughness of the samples and [...] Read more.
The objective of this study was to optimize the sanding and coating processes of black alder wood to promote and support its use in furniture manufacturing. Two criteria have been applied for process optimization, namely, the minimum surface roughness of the samples and power consumption during sanding as a function of various sanding systems. The surface roughness of the sanded specimens and the power consumption during sanding strongly depends on the grit size used. Two eco-varnishes were applied to the samples by spraying. Moreover, the effect of the surface preparation and varnish type on the coating properties expressed by the varnish layer adherence to the substrate and surface glossiness was evaluated. For better glossiness values, the UV-cured varnish was preferred. The sanding with a grit sequence of 60, 120, and 150 grit size abrasives was found to be optimal when applied to black alder wood, and it is recommended to obtain performant UV-coated wood surfaces for furniture products. Full article
(This article belongs to the Special Issue Wood Coatings)
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Figure 1
<p>Variation of power consumption and processing roughness during sanding per each final grit size and total power per sanding system.</p>
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3300 KiB  
Article
Efficacy of Hydrophobic Coatings in Protecting Oak Wood Surfaces during Accelerated Weathering
by Miloš Pánek, Eliška Oberhofnerová, Aleš Zeidler and Přemysl Šedivka
Coatings 2017, 7(10), 172; https://doi.org/10.3390/coatings7100172 - 18 Oct 2017
Cited by 35 | Viewed by 6683
Abstract
The durability of transparent coatings applied to an oak wood exterior is relatively low due to its anatomic structure and chemical composition. Enhancement of the protection of oak wood against weathering using transparent hydrophobic coatings is presented in this study. Oak wood surfaces [...] Read more.
The durability of transparent coatings applied to an oak wood exterior is relatively low due to its anatomic structure and chemical composition. Enhancement of the protection of oak wood against weathering using transparent hydrophobic coatings is presented in this study. Oak wood surfaces were modified using UV-stabilizers, hindered amine light stabilizer (HALS), and ZnO and TiO2 nanoparticles before the application of a commercial hydrophobic topcoat. A transparent oil-based coating was used as a control coating system. The artificial weathering test lasted 6 weeks and colour, gloss, and contact angle changes were regularly evaluated during this period. The changes in the microscopic structure were studied with confocal laser scanning microscopy. The results proved limited durability against weathering of both tested hydrophobic coatings. The formation of micro-cracks causing the leaching of degraded wood compounds and discolouration of oak wood were observed after 1 or 3 weeks of the weathering test. Until then, an oil-based coating film had protected the wood sufficiently, but after 6 weeks the wood was fully defoliated to its non-homogenous thickness, which was caused by the presence of large oak vessels, and by the effects of specific oak tannins. Using transparent hydrophobic coatings can prolong the service life of the exteriors of wood products by decreasing their moisture content. Without proper construction protection against rainwater, the hydrophobic coating itself cannot guarantee the preservation of the natural appearance of wood exteriors. Full article
(This article belongs to the Special Issue Wood Coatings)
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Figure 1

Figure 1
<p>Scheme of colour and gloss measurements on the surfaces of tested oak wood samples.</p>
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<p>Colour changes to tested coatings during 6 weeks of artificial weathering. REF: native untreated oak wood; A: oak wood with the nano-based hydrophobic coating; B: oak wood with the hydrophobic coating with wax additives; 1, 2, 3, 4: the types of oak wood surface treatments; C: oil-based coating. (<b>a</b>) Increasing of <span class="html-italic">L*</span> values (lightness) of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4—see <a href="#coatings-07-00172-t001" class="html-table">Table 1</a> and <a href="#coatings-07-00172-t002" class="html-table">Table 2</a>); (<b>b</b>) Increasing of <span class="html-italic">L*</span> values (lightness) of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4); (<b>c</b>) Decreasing of <span class="html-italic">a*</span> values (red shades) of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4); (<b>d</b>) Decreasing of <span class="html-italic">a*</span> values (red shades) of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4); (<b>e</b>) Decreasing of <span class="html-italic">b*</span> values (yelow shades) of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4); (<b>f</b>) Decreasing of <span class="html-italic">b*</span> values (yelow shades) of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4); (<b>g</b>) Total colour changes ∆<span class="html-italic">E*</span> of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4); (<b>h</b>) Total colour changes ∆<span class="html-italic">E*</span> of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4). Comparison with untreated oak wood (REF—similar trends) and oil-based coating (C—initial higher increasing of <span class="html-italic">a*</span> and <span class="html-italic">b*</span> values and their final decreasing) is shown.</p>
Full article ">Figure 2 Cont.
<p>Colour changes to tested coatings during 6 weeks of artificial weathering. REF: native untreated oak wood; A: oak wood with the nano-based hydrophobic coating; B: oak wood with the hydrophobic coating with wax additives; 1, 2, 3, 4: the types of oak wood surface treatments; C: oil-based coating. (<b>a</b>) Increasing of <span class="html-italic">L*</span> values (lightness) of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4—see <a href="#coatings-07-00172-t001" class="html-table">Table 1</a> and <a href="#coatings-07-00172-t002" class="html-table">Table 2</a>); (<b>b</b>) Increasing of <span class="html-italic">L*</span> values (lightness) of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4); (<b>c</b>) Decreasing of <span class="html-italic">a*</span> values (red shades) of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4); (<b>d</b>) Decreasing of <span class="html-italic">a*</span> values (red shades) of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4); (<b>e</b>) Decreasing of <span class="html-italic">b*</span> values (yelow shades) of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4); (<b>f</b>) Decreasing of <span class="html-italic">b*</span> values (yelow shades) of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4); (<b>g</b>) Total colour changes ∆<span class="html-italic">E*</span> of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4); (<b>h</b>) Total colour changes ∆<span class="html-italic">E*</span> of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4). Comparison with untreated oak wood (REF—similar trends) and oil-based coating (C—initial higher increasing of <span class="html-italic">a*</span> and <span class="html-italic">b*</span> values and their final decreasing) is shown.</p>
Full article ">Figure 3
<p>Samples before artificial weathering (0 weeks); reference untreated oak (<b>1</b>); oak treated with hydrophobic coating A (<b>2</b>); oak treated with hydrophobic coating B (<b>3</b>); oak treated with oil based coating C (<b>4</b>); oak with surface modification No. 3 and hydrophobic coating A (<b>5</b>); oak with surface modification No. 4 and hydrophobic coating B (<b>6</b>).</p>
Full article ">Figure 4
<p>Samples after 6 weeks of artificial weathering (6 weeks); reference untreated oak (<b>7</b>); oak treated with hydrophobic coating A (<b>8</b>); oak treated with hydrophobic coating B (<b>9</b>); oak treated with oil based coating C (<b>10</b>); oak with surface modification No. 3 and hydrophobic coating A (<b>11</b>); oak with surface modification No. 4 and hydrophobic coating B (<b>12</b>). Visible degradation of tested coatings, opening of oak vessels, and creation of micro-cracks and cracks on all tested surfaces.</p>
Full article ">Figure 5
<p>Gloss changes of tested coatings during 6 weeks of artificial weathering. REF: native untreated oak wood; A: oak wood with the nano-based hydrophobic coating; B: oak wood with the hydrophobic coating with wax additives; 1, 2, 3, 4: the types of oak wood surface treatments; C: oil-based coating. (<b>a</b>) Relativelly small gloss changes of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4—see <a href="#coatings-07-00172-t001" class="html-table">Table 1</a> and <a href="#coatings-07-00172-t002" class="html-table">Table 2</a>); (<b>b</b>) Relativelly small gloss changes of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4). Comparison with untreated oak wood (REF—similar trends) and oil-based coating (C—high decrease of gloss after 1 week of accelerated weathering) is shown.</p>
Full article ">Figure 6
<p>Changes in the contact angle of tested coatings during 6 weeks of artificial weathering. REF: native untreated oak wood; A: oak wood with the nano-based hydrophobic coating; B: oak wood with the hydrophobic coating with wax additives; 1, 2, 3, 4: the types of oak wood surface treatments; C: oil-based coating. (<b>a</b>) Decreasing of hydrophobicity of hydrophobic coating (A) with different surface treatments of oak wood (No. 1–4—see <a href="#coatings-07-00172-t001" class="html-table">Table 1</a> and <a href="#coatings-07-00172-t002" class="html-table">Table 2</a>); (<b>b</b>) Decreasing of hydrophobicity of hydrophobic coating (B) with different surface treatments of oak wood (No. 1–4). Comparison with untreated oak wood (REF) and oil-based coating (C) is shown.</p>
Full article ">Figure 7
<p>Photo demonstrating colour changes of selected tested samples after 0, 1, 3, and 6 weeks (w.) of accelerated weathering. (<b>a</b>) native untreated oak wood; (<b>b</b>) treated (3)–coated (A); (<b>c</b>) treated (4)–coated (A); (<b>d</b>) coated (C). A: oak wood with the nano-based hydrophobic coating; 3, 4: the types of oak wood surface treatments; C: oil-based coating—see <a href="#coatings-07-00172-t001" class="html-table">Table 1</a> and <a href="#coatings-07-00172-t002" class="html-table">Table 2</a>. The similar trends of discolouration of hydrophobic coatings (B) and (A) were observed.</p>
Full article ">
2851 KiB  
Article
Atmospheric Pressure Plasma Coating of Wood and MDF with Polyester Powder
by Robert Köhler, Philipp Sauerbier, Holger Militz and Wolfgang Viöl
Coatings 2017, 7(10), 171; https://doi.org/10.3390/coatings7100171 - 17 Oct 2017
Cited by 25 | Viewed by 7050
Abstract
In this study, polyester powder based on iso- and teraphthalic acid was deposited with an atmospheric plasma jet. The powder was fed into the effluent plasma zone and deposited on European beech wood (Fagus sylvatica L.), Grand fir (Abies grandis Lindl.) [...] Read more.
In this study, polyester powder based on iso- and teraphthalic acid was deposited with an atmospheric plasma jet. The powder was fed into the effluent plasma zone and deposited on European beech wood (Fagus sylvatica L.), Grand fir (Abies grandis Lindl.) and medium density fiberboard (MDF). The substrates were annealed subsequent to the coating process. To exclude decomposition of the polyester layers by the plasma treatment, the surface chemistry of the layers has been examined by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) and compared with the polyester powder reference. Furthermore, topographical investigations were carried out using laser scanning microscopy (LSM). Adhesive strength of the layers was evaluated by dolly test and gloss measurements with a goniophotometer. The deposited layers showed no chemical changes compared to the reference. The adhesive strength of the layer met practical requirements of >1 MPa. It was demonstrated that the deposition of a macroscopic layer is possible without a pretreatment or the usage of additives. Therefore this coating process by atmospheric pressure plasma for wood and wood based materials could represent an environmental-friendly alternative to conventional coating methods. Full article
(This article belongs to the Special Issue Wood Coatings)
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Figure 1

Figure 1
<p>Cross-section of the electrode setup with particle inlet.</p>
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<p>XPS detail spectra of polyester reference: (<b>a</b>) C1<span class="html-italic">s</span> peak and (<b>b</b>) O1<span class="html-italic">s</span> peak.</p>
Full article ">Figure 3
<p>Comparison of XPS measurements of untreated reference, single-, double-coated and polyester reference: (<b>a</b>) C1<span class="html-italic">s</span> European beech; (<b>b</b>) C1<span class="html-italic">s</span> Grand fir; (<b>c</b>) C1<span class="html-italic">s</span> MDF; (<b>d</b>) O1<span class="html-italic">s</span> Beech; (<b>e</b>) O1<span class="html-italic">s</span> Grand fir; (<b>f</b>) O1<span class="html-italic">s</span> MDF.</p>
Full article ">Figure 4
<p>Comparison of FTIR spectra of single- and double-coated beech and the powder reference. All spectra are corrected by moving average baseline subtraction.</p>
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<p>FTIR spectra of uncoated European beech and the powder reference.</p>
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<p>LSM data showing the coating thickness of single- and double-coated European beech, Grand fir and MDF (<span class="html-italic">N</span> = 10).</p>
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<p>Pull-off strength of single and double-coated European beech, Grand fir and MDF (<span class="html-italic">N</span> = 10).</p>
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1888 KiB  
Article
Influence of Coating Formulation on Its Mechanical Properties and Cracking Resistance
by Laurence Podgorski, Mari De Meijer and Jean-Denis Lanvin
Coatings 2017, 7(10), 163; https://doi.org/10.3390/coatings7100163 - 30 Sep 2017
Cited by 9 | Viewed by 4873
Abstract
The mechanical properties of coatings strongly influence wood coatings’ performance, as coatings may be stressed by dimensional variations of wood when exposed outdoors. Within the European project SERVOWOOD (2014–2016), the influence of coating formulation on mechanical properties and cracking resistance has been studied. [...] Read more.
The mechanical properties of coatings strongly influence wood coatings’ performance, as coatings may be stressed by dimensional variations of wood when exposed outdoors. Within the European project SERVOWOOD (2014–2016), the influence of coating formulation on mechanical properties and cracking resistance has been studied. Several acrylic and alkyd formulations with different pigment volume concentrations (PVCs), with and without UV protection have been applied on pine samples and exposed to artificial weathering (EN 927-6) for 12 weeks. Persoz hardness of coatings applied on wood was assessed before and after weathering. Tensile tests on free films have been carried out at −10 °C, 20 °C, and 45 °C. For each formulation, elastic modulus, tensile strength, and strain at break have been determined for the three test temperatures. For each test temperature, there was no correlation between the elastic modulus and strain at break, nor between tensile strength and strain at break. The results showed a relation between Persoz hardness and elastic modulus. The best performing formulation had a mean elastic modulus at room temperature lower than 400 MPa and a mean strain at break higher than 30%. Full article
(This article belongs to the Special Issue Wood Coatings)
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Figure 1

Figure 1
<p>Mean Persoz hardness and confidence interval at 95% for the mean for the 24 coatings.</p>
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<p>Interaction plot for the mean Persoz hardness.</p>
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<p>Comparison of the tensile strength–strain curves of the different coatings (examples with coatings 06, 18, 30, and 42 made with low PVC).</p>
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<p>Influence of coating formulation on elastic modulus for the three test temperatures (in blue: clear PVC; in red: low PVC; in green: high PVC).</p>
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<p>Influence of coating formulation on strain at break for the three test temperatures (in blue: clear PVC; in red: low PVC; in green: high PVC).</p>
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<p>Influence of coating formulation on tensile strength for the three test temperatures (in blue: clear PVC; in red: low PVC; in green: high PVC).</p>
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<p>Main effects plot for the mean cracking.</p>
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<p>Main effects plot for the mean elastic modulus (room temperature).</p>
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<p>Main effects plot for the mean strain at break (room temperature).</p>
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<p>Relation between Persoz hardness and elastic modulus at room temperature.</p>
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7211 KiB  
Article
Microstructure and Mechanism of Grain Raising in Wood
by Philip D. Evans, Ian Cullis, Joseph Doh Wook Kim, Lukie H. Leung, Siti Hazneza and Roger D. Heady
Coatings 2017, 7(9), 135; https://doi.org/10.3390/coatings7090135 - 29 Aug 2017
Cited by 17 | Viewed by 8388
Abstract
Grain raising, the lifting of fibres when water is applied to wood surfaces, is a reason why some companies are reluctant to finish wood products with water-borne coatings. However, the elements that lift-up and cause grain raising have not been identified, and the [...] Read more.
Grain raising, the lifting of fibres when water is applied to wood surfaces, is a reason why some companies are reluctant to finish wood products with water-borne coatings. However, the elements that lift-up and cause grain raising have not been identified, and the relationship between wood density and grain raising has not been clarified. Our work sought answers to both questions. We planed or sanded different woods using aluminum oxide abrasive paper, and characterized surfaces using profilometry and SEM. Surfaces were re-characterized after wetting and drying. Grain raising is inversely related to wood density. In particular, very low-density woods are highly susceptible to grain raising, whereas grain raising does not occur in high-density woods or planed woods. In low-density woods, sanding tears cell walls creating loosely-bonded slivers of wood that project from surfaces, particularly after wetting and drying. This mechanism for grain raising was confirmed by modelling the action of abrasives on wood cell walls using an array of hollow tubes and a serrated tool. Less commonly, fibres and fibre-bundles project from surfaces. We observed that grain raising was correlated with the coarseness of the abrasive and conclude that it can be reduced in severity by tailoring sanding to account for the density and surface microstructure of wood. Full article
(This article belongs to the Special Issue Wood Coatings)
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Figure 1

Figure 1
<p>Relationship between density of woods and grain raising defined as the increase in surface roughness after sanding and wetting and air drying. Wood species can be identified using the abbreviations in <a href="#coatings-07-00135-t001" class="html-table">Table 1</a>. Values for roughness increases of some wood species were similar and their abbreviations overlapped. In these cases the precise numerical values for roughness increases are indicated by a period (.). In four cases an oblique line is drawn from the period to the abbreviated species name.</p>
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<p>Grain raising of sanded and planed wood surfaces. Differences in roughness that exceed the length of the error bar (LSD) are statistically significant (<span class="html-italic">p</span> &lt; 0.05).</p>
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<p>Confocal profilometry images of sanded surfaces before (<b>a</b>,<b>c</b>,<b>e</b>,<b>g</b>) and after grain raising (<b>b</b>,<b>d</b>,<b>f</b>,<b>h</b>): (<b>a</b>,<b>b</b>) balsa; (<b>c</b>,<b>d</b>) western red cedar; (<b>e</b>,<b>f</b>) black cherry; (<b>g</b>,<b>h</b>) sugar maple.</p>
Full article ">Figure 3 Cont.
<p>Confocal profilometry images of sanded surfaces before (<b>a</b>,<b>c</b>,<b>e</b>,<b>g</b>) and after grain raising (<b>b</b>,<b>d</b>,<b>f</b>,<b>h</b>): (<b>a</b>,<b>b</b>) balsa; (<b>c</b>,<b>d</b>) western red cedar; (<b>e</b>,<b>f</b>) black cherry; (<b>g</b>,<b>h</b>) sugar maple.</p>
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<p>Balsa wood surfaces: (<b>a</b>) surface after sanding with 120 and 180 grit aluminium oxide abrasive papers; (<b>b</b>) sanded surface after wetting and redrying; (<b>c</b>) planed surface after wetting/redrying; and (<b>d</b>) loose surface material removed from (<b>b</b>) using transparent tape. Scale bars = 200 μm.</p>
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<p>Western red cedar wood surfaces after: (<b>a</b>) sanding; (<b>b</b>) sanding and wetting/redrying; (<b>c</b>) planing/wetting/redrying; and (<b>d</b>) material removed from (b) using transparent tape. Scale bars = 100 μm.</p>
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<p>Western red cedar wood surfaces after: (<b>a</b>) sanding; (<b>b</b>) sanding and wetting/redrying; (<b>c</b>) planing/wetting/redrying; and (<b>d</b>) material removed from (b) using transparent tape. Scale bars = 100 μm.</p>
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<p>Sugar maple wood surfaces after: (<b>a</b>) sanding; (<b>b</b>) sanding/wetting/redrying; (<b>c</b>) planing/wetting/ redrying; and (<b>d</b>) material removed from (<b>b</b>) using transparent tape. Scale bars (a,b) = 50 μm; (c,d) = 100 μm.</p>
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<p>Lignum vitae wood surfaces after: (<b>a</b>) sanding; (<b>b</b>) sanding/wetting/redrying; (<b>c</b>) planing/wetting/ redrying; and (<b>d</b>) material removed from (<b>b</b>) using transparent tape. Scale bars (a,b) = 50 μm; (c,d) = 100 μm.</p>
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<p>Tubular plastic wood model after a serrated tool was drawn across its surface.</p>
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<p>Effect of grit size of abrasive belts on the grain raising of maple veneer-faced panels sequentially sanded using an industrial wide belt sander.</p>
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<p>Appearance of maple veneer surfaces after composite veneer-faced panels were sanded with an industrial wide belt sander: (<b>a</b>) surface sanded with 120 grit aluminum oxide belt; (<b>b</b>) surface sanded with 120 grit aluminum oxide belt and then subjected to grain raising procedure; (<b>c</b>) surface sanded with 120/150/180 grit aluminum oxide belts; and (<b>d</b>) surface sanded with 120/150/180 grit aluminum oxide belts and then subjected to the grain raising procedure. Scale bars = 250 μm.</p>
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Review

Jump to: Research

5350 KiB  
Review
Role of Moisture in the Failure of Coatings on Wood
by Roger Rowell and Ferry Bongers
Coatings 2017, 7(12), 219; https://doi.org/10.3390/coatings7120219 - 2 Dec 2017
Cited by 16 | Viewed by 4979
Abstract
Most wood coating tests are done either in a short term artificial weathering chamber or long term on an outdoor rack/fence. In both cases, the coatings are exposed to both ultraviolet radiation and water. This study is focused on the influence of moisture [...] Read more.
Most wood coating tests are done either in a short term artificial weathering chamber or long term on an outdoor rack/fence. In both cases, the coatings are exposed to both ultraviolet radiation and water. This study is focused on the influence of moisture alone on wood opaque film forming coating failures. As moisture is sorbed into the wood structure, the wood swells in proportion to the volume of water sorbed. As moisture is lost, the wood shrinks in proportion to the volume of the water lost. Moisture in the wood end grain is responsible for coating failure in, for example, window corners and end to end siding. The wood cell wall moisture can be greatly reduced by a process known as acetylation which not only reduces the moisture sorbed in the cell wall but results in high levels of dimensional stability. The reduced moisture uptake along with the stability results in less stress created between the coating wood surface interface improving the performance of the coating and increasing its useful lifetime. Full article
(This article belongs to the Special Issue Wood Coatings)
Show Figures

Figure 1

Figure 1
<p>Models of water added to the wood cell wall. (<b>a</b>) Water molecules entering the wood cell wall; (<b>b</b>) water molecules unzipping hydrophylic hemicellulose polymer chains.</p>
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<p>Sorption Isotherms for wood and cell wall polymers [<a href="#B5-coatings-07-00219" class="html-bibr">5</a>].</p>
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<p>Elements in the weathering process [<a href="#B8-coatings-07-00219" class="html-bibr">8</a>].</p>
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<p>Coating failure due to moisture build up in end grain.</p>
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<p>Buildup of moisture at the interface of wood and coating (red line) [<a href="#B1-coatings-07-00219" class="html-bibr">1</a>].</p>
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<p>Cracking, bubbling and loss of paint adhesion to the substrate caused by high levels of moisture [<a href="#B1-coatings-07-00219" class="html-bibr">1</a>].</p>
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<p>Cracking of the coating due to differential swelling of the wood and coating.</p>
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<p>Cross-section cut-through an exposed board, showing a small crack developing due to wood swelling as the coating fails [<a href="#B1-coatings-07-00219" class="html-bibr">1</a>].</p>
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<p>Reaction of wood with acetic anhydride [<a href="#B9-coatings-07-00219" class="html-bibr">9</a>].</p>
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<p>(<b>a</b>) Reaction of acetic anhydride with wood; (<b>b</b>) Reaction of acetic anhydride with isolated cell wall polymers [<a href="#B1-coatings-07-00219" class="html-bibr">1</a>,<a href="#B9-coatings-07-00219" class="html-bibr">9</a>].</p>
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<p>Control (larch, (<b>a</b>)) and acetylated (pine, (<b>b</b>)) samples after 7 years.</p>
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<p>(<b>a</b>) Control larch, (<b>b</b>) acetylated pine, and (<b>c</b>) western red cedar after 7 years of outdoor testing.</p>
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<p>Opaque white alkyd coated control (<b>a</b>) and acetylated (<b>b</b>) after 9.5 years [<a href="#B11-coatings-07-00219" class="html-bibr">11</a>,<a href="#B12-coatings-07-00219" class="html-bibr">12</a>].</p>
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3698 KiB  
Review
The Search for Durable Exterior Clear Coatings for Wood
by Philip D. Evans, Jonathan G. Haase, A. Shakri B.M. Seman and Makoto Kiguchi
Coatings 2015, 5(4), 830-864; https://doi.org/10.3390/coatings5040830 - 12 Nov 2015
Cited by 90 | Viewed by 17911
Abstract
The goal of a durable exterior clear coating has eluded generations of coatings technologists, despite long-standing desire amongst the public for such a coating. The journey towards this goal initially focused on modifications to coating formulation, but took a completely different direction when [...] Read more.
The goal of a durable exterior clear coating has eluded generations of coatings technologists, despite long-standing desire amongst the public for such a coating. The journey towards this goal initially focused on modifications to coating formulation, but took a completely different direction when it was found that a UV-transparent silicone clear coating on wood modified with chromic acid met consumer expectations of coating durability. This finding sparked world-wide interest in wood pre-treatments as a way of enhancing the durability of clear coatings. This interest initially focused on transition metal compounds, but has now shifted in the direction of organic and inorganic photostabilizers or even more drastic pre-treatments. Pre-treatments that dimensionally stabilize wood, protect it from microbial degradation and photostabilize lignin, when combined with flexible, photostable, coatings provide the next way-stop on the journey towards achieving the goal of durable exterior clear coatings for wood. This paper reviews this journey, drawing upon our research and that of other groups who have focused on this elusive goal. Full article
(This article belongs to the Special Issue Wood Coatings)
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Figure 1

Figure 1
<p>Summary of the results of the California Redwood Association’s testing of clear finishes on wood exposed outdoors (re-drawn using data from Estrada [<a href="#B3-coatings-05-00830" class="html-bibr">3</a>]): (<b>a</b>) Numbers of finishes tested versus those listed as being suitable for the finishing of redwood; (<b>b</b>) Average life-times of the better listed finishes versus the poorer systems.</p>
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<p>Formation of pit micro-checks and longitudinal separation of tracheids at wood surfaces exposed to weathering: (<b>a</b>) Pit micro-checking (arrowed left and right of centre) at the surface of western hemlock wood exposed to natural weathering for 62 weeks. Note the colonization of the surface by staining fungi; (<b>b</b>) Deep pit micro-checking (arrowed left and right of centre) at the surface of western hemlock wood exposed to artificial accelerated weathering for 4000 h. There is also separation of tracheids from the underlying wood due to degradation of middle lamellae.</p>
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<p>Surface of kempas wood beneath a clear coating exposed outdoors for 18 months: (<b>a</b>) Area beneath part of the coating that adhered well to the wood; (<b>b</b>) Area beneath part of the coating that adhered poorly to the wood. Note the colonization of the wood surface by staining fungi.</p>
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<p>Peeling and delamination of clear coatings on wood due to photodegradation of wood beneath the coating: (<b>a</b>) Clear coating peeling from a vertical window casing in a private house in southern France. Note the fungal staining of wood beneath the clear coating; (<b>b</b>) Fluorine resin-based clear coating peeling from glulam test specimens exposed to the weather in Japan.</p>
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<p>Chemical structures of typical (<b>a</b>) benzophenone and (<b>b</b>) benzotriazole UV absorbers used to photostabilize clear coatings and wood.</p>
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<p>Generalized structures of hindered amine light stabilizers (HALS): (<b>a</b>) 2,2,6,6-tetramethyl piperidine; (<b>b</b>) Low molecular weight HALS; (<b>c</b>) Oligomeric HALS.</p>
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<p>Effect of adding a UV absorber and hindered amine light stabilizer (HALS) to a clear coating on the performance of clear-coated kempas wood panels exposed to the weather for 18 months in Malaysia. From left to right: Unweathered control; weathered control; coating containing 2% benzotriazole UVA; coating containing 2% benzotriazole UVA and 2% HALS; coating containing 2% HALS.</p>
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<p>Effect of a polyethylene glycol (PEG) pre-treatment on the performance of a clear coating on Douglas fir plywood panels exposed outdoors for two years at six different sites in Japan: (<b>a</b>) Untreated clear-coated panels; (<b>b</b>) Clear-coated panels pre-treated with 10% PEG.</p>
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<p>Effect of chromium VI pre-treatment on the performance of clear finishes on wood: (<b>a</b>) Silicone clear coating adhering to pre-treated wood exposed to the weather for ~15 years (the finishes to the right and left of the silicone finish have been completely degraded and lost, photo c/o Sam Williams US Forest Products Laboratory; (<b>b</b>) Clear coating performing well on Cr VI pretreated wood after 28 years of outdoor exposure.</p>
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<p>Effect of pre-treating radiata pine with a combination of a UV absorber and a hindered amine light stabilizer (HALS) on the performance of a durable acrylic coating: (<b>a,b</b>) Untreated controls exposed to the weather for 28 months in Canberra, Australia. Note darkening of wood beneath the coating and failure of the coating; (<b>c</b>) Panel pre-treated with a UVA and low molecular weight HALS; (<b>d</b>) Panel pre-treated with a UVA and an oligomeric HALS (panels are 23 cm × 8 cm in size, length × width)</p>
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<p>Effects of grafting the epoxy-functionalized UV absorber, 2-hydroxy-4(2,3-epoxypropoxy)-benzophenone on the performance of an acrylic silicone clear coating on sugi wood: (<b>a</b>) Unmodified wood; (<b>b</b>) Wood treated to a weight gain of 5.6%. Unmodified and treated and coated panels were exposed to the weather for 18 months in Tsukuba, Japan.</p>
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<p>Effect of plasma treatment for 3 min on the performance on an oil-borne polyurethane coating on black spruce exposed to 2000 h of artificial accelerated weathering: (<b>a</b>) Untreated control; (<b>b</b>) Plasma treated. Scale bars = 15 mm.</p>
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<p>Appearance of an alkyd clear coating on western red cedar panels pre-treated with reactive titanium or zirconium compounds and exposed to the weather for 22 months in Australia. Note the positive effect of one of the compounds on the performance of the coating (arrowed top). Untreated controls are arrowed bottom (photo provided by Karl Schmalzl, formerly of CSIRO Division Forestry and Forest Products, Melbourne, Australia).</p>
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1693 KiB  
Review
Coating Acetylated Wood
by Roger Rowell and Ferry Bongers
Coatings 2015, 5(4), 792-801; https://doi.org/10.3390/coatings5040792 - 6 Nov 2015
Cited by 18 | Viewed by 7012
Abstract
Wood exposed to the outdoor environment is susceptible to weathering due to a series of chemical, biological and physical processes. Acetylation of wood is known to reduce cell wall moisture content, improve dimensional stability and durability against fungal decay. As a result of [...] Read more.
Wood exposed to the outdoor environment is susceptible to weathering due to a series of chemical, biological and physical processes. Acetylation of wood is known to reduce cell wall moisture content, improve dimensional stability and durability against fungal decay. As a result of these improvements, less stress is created between the coating and the wood surface improving the performance of the coating and increasing its useful lifetime. This paper is a review of research done on the chemistry of the acetylation process, the coating performance of acetylated wood and concentrates on the factors influencing coating performance. Full article
(This article belongs to the Special Issue Wood Coatings)
Show Figures

Figure 1

Figure 1
<p>Elements in the weathering process.</p>
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<p>Models of water added to the wood cell wall. (<b>A</b>) Water molecules entering the wood cell wall; (<b>B</b>) water molecules unzipping hydrophylic polymer chains; (<b>C</b>) water bonding to the cell wall either as primary water ● or secondary water ○; (<b>D</b>) fully hydrated cell wall at the fiber saturation point.</p>
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<p>Cracking of surface coating due to differential swelling of the coating and wood (<b>A</b>); micro checks occur in the wood due to moisture evasion and swelling (<b>B</b>); water collecting under coated wood (<b>C</b>); and coating failure due to moisture build up in end grain (<b>D</b>).</p>
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<p>Coating on the surface of the wood (<b>A</b>); moisture gradient at the interface of the coating and the wood (<b>B</b>); and coating delamination (<b>C</b>).</p>
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<p>Stile and rail door part showing swelling of the control (left) and no swelling in the acetylated door (right). No longitudinal swelling in either control or acetylated wood but radial and tangential swelling in the control but not in the acetylated wood.</p>
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<p>Corner window joint in an acetylated window.</p>
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<p>Scanning electron microscope of brown-rot fungal attack on wood. (<b>A)</b> control; (<b>B</b>,<b>C</b>) 51.1% weight loss after the 12 weeks in the ASTM soil block test; (<b>D)</b> acetylated pine after the soil block test.</p>
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<p>Decay in the corner of a control window after three years (<b>A</b>); no swelling or decay in the acetylated window after five years (<b>B</b>).</p>
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<p>Performance of acetylated coated panels after 13 years of exposure. The four panels on the right are acetylated and the others are unmodified controls. Prior to testing the panels were coated with alkyd primer followed by acrylic top coating [<a href="#B15-coatings-05-00792" class="html-bibr">15</a>].</p>
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<p>Photographic impression performance of opaque white (<b>A</b>,<b>B</b>) and black (<b>C</b>) coatings after 9½ years of outdoor exposure. (<b>A</b>) opaque white acrylic, left control, right acetylated; (<b>B</b>) opaque white alkyd, left control, right acetylated; (<b>C</b>) opaque black alkyd, left control, right acetylated. [<a href="#B16-coatings-05-00792" class="html-bibr">16</a>].</p>
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<p>Acetylated radiata pine door (<b>A</b>) and windows (<b>B</b>) that were installed in Japan 2008.</p>
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<p>Acetylated door at a summer home in Sweden.</p>
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