Chemical Wood Surface Improvements: Comparison
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Increasing the use of wood in buildings is regarded by many as a key solution to tackle climate change. For this reason, a lot of research is carried out to develop new and innovative wood surface improvements and make wood more appealing through features such as increased durability, fire-retardancy, superhydrophobicity, and self-healing.

  • wood
  • Surface Improvements
  • Modifications
  • Coatings

1. Coatings

Coatings were the most intensively studied methods for the protection of wood surfaces during the last five years. A total of 144 papers were found on this subject, using diverse materials and application methods to improve surface properties, such as hydrophobicity, hardness, abrasion resistance, photostability, thermal stability, self-healing, self-cleaning and more.

1.1. Organic Coatings

Organic coatings are very common in wood protection, including waxes, oils, and film-forming resins such as acrylics, alkyds and polyurethanes. While they can be improved with additives, a popular approach found in the latest literature to enhance the properties of film-forming organic coatings was to use strategic reactives to change key properties of the resin itself. Different fire-retardant coatings were reported using phosphorus containing monomers or reactive diluents [30,31,32,33][1][2][3][4]. Lokhande et al. used glycidyl methacrylate, piperazine, and cyclic ethylene chlorophosphite to develop a diacrylate reactive diluent yielding UV-cured coatings with increased thermal resistance, hardness, hydrophobicity and stain resistance [34][5]. They found that the thermal properties of the coating would improve with the content of their reactive diluent, a concentration of 25% increasing the weight-loss temperatures, the heat-index resistance (from 149.7 °C to 184.0 °C), the practical char yield at 600 °C (7.12% to 22.21%), and the limiting-oxygen index (23% to 33%) when compared to the same coating without the reactive diluent. A similar coating was developed by Mulge et al. using epoxy acrylate oligomers and phenylphosphonic dichloride [35][6]. However, it was found that while a higher P content would improve the thermal stability of the coating, it would eventually impair its physical properties. In a similar way, Paquet et al. created self-healing, film-forming coatings by using acrylic monomers and oligomers containing many hydroxyl groups [36][7]. By using 2-hydroxyethyl methacrylate and an aliphatic urethane acrylate oligomer (Ebecryl 4738), the obtained polymer that could completely heal a 5 μm deep scratch or regain 83% of the gloss lost to abrasion after being heated to 80 °C for two hours. Another use for this practice was to build fast UV-curing acylic [37][8] and polyurethane-acrylate [38][9] formulations.
The same strategy was used to prepare more environmentally friendly coatings by using bio-based materials as a reactive. Raychura et al. prepared polyurethane coatings by reacting diisocyanates with fatty amides of mahua [39][10] and peanut [40][11] oils. The obtained wood coatings scored 100% on a cross-cut adhesion test, 1H or 2H on pencil hardness tests, and had a good stability to weak acids, water and NaCl solutions. Interestingly, wood coatings with good resistance to termites and/or white rot fungi could be obtained by reacting the starch from a yam (Dioscorea hispida sp.) with polyvinyl alcohol [41][12] or polyacrylamide [42][13]. Another antiseptic coating was prepared by Dixit et al. with citric acid and glycidyl methacrylate [43][14], which had high adhesion (5B), pencil hardness (6H), solvent resistance and a 15 mm zone of inhibition against the bacteria Staphylococcus aureus. It will be shown later in the review that cellulose nanocrystals (CNCs) were extensively studied as additives for organic coatings; Kong et al. [44][15], however, innovated by using CNCs as a reactive to imbue a waterborne polyurethane coating with a higher hardness and resistance to abrasion. They found that using only 0.1% of CNCs modified with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in their coating would increase its tensile strength and tensile elongation by 59% and 55%, respectively. More bio-based chemicals were studied by other workers, including soybean oil [45][16], rapeseed oil [46][17], and biosourced alcohols and acids [47][18].
Some organic coatings were prepared using very distinctive methods and could not be categorized into a specific trend, but still deserve some attention [48,49,50][19][20][21]. Janesch et al. dip-coated spruce in tung oil, bee wax, or a mix of both, before sifting sodium chloride (NaCl) on the freshly coated wood [51][22]. The NaCl, which created a micro-/nanoscale architecture into the coating, was removed one week later by rinsing with distilled water. The resulting wood surface was 100% natural, food safe and had a contact angle with water of 161°, but was not considered superhydrophobic as its roll-off angle was extremely high. Zhang et al. designed a biogel coating based on chitosan, gelatin and glycerol that had quite a low adhesion (1.4 MPa), but some very interesting features [52][23]. It could completely heal medium damages under heating, be reused after being scrapped from the wood and dissolved in water, and be colored with water-soluble dyes. A 3-layers coating was produced on beech wood with a polydopamine primer, an hydroxyapatite second layer, and a chitosan topcoat made from shrimp wastes [53][24]. The composite coating showed good hydrophobicity (contact angle = 130°), photostability and resistance to seawater. After 6 months of immersion in the sea, the treated samples showed lower chemical degradation, color changes (E* = 12.68 vs. 22.24) and damages from barnacles than the controls samples. Liu and Hu prepared polystyrene colloidal microspheres with different acrylate-based copolymers [54][25]. Once casted on aspen, the very densely arranged microsphere exhibited different colors, such as green, orange and red. Other unique organic coatings were developed by using materials, such as chitosan oligomers, vegetable oil, castor oil and lignin to protect wood from decay fungi [55][26], fire [56][27], and photodegradation [57,58][28][29].

1.2. Additives in Organic Coatings

While it was shown in the previous section that organic wood coatings could be improved by using the appropriate reactives, another great way to obtain performant coatings is through the inclusion of functional additives. Organic, bio-based materials have been the subject of much research over the last 5 years. They are very interesting substances for wood protection, as they biodegrade upon leaching. Cellulose nanocrystals and nanofibrils received a lot of attention due to their potential to improve the mechanical properties of the softer oil- or resin-based organic coatings [59,60,61,62,63][30][31][32][33][34]. Tian et al. prepared a renewable UV-cured polyester methacrylate coating based on L-lactide and ε-caprolactone, containing 0% to 7.5% of cellulose nanocrystals (CNCs) [64][35]. The properties of the resulting composite coating changed proportionally to the concentration of CNCs, with an increase in bending strength, bending modulus, hardness and water contact angle, but a decreased tensile strength and elongation at break. At a 7.5% content of CNCs, the coating had a grade 3 adhesion, a 5H pencil hardness and a 103° contact angle. Veigel et al. incorporated 1% of cellulose nanofribrils (CNFs) to linseed oil after modification with acetic anhydride and (2-dodecen-1-yl)succinic anhydride to increase their solubility [65][36]. While the beech substrate coated with this varnish had the same initial hydrophobicity as the samples treated without the CNFs, the reduction of the hydrophobicity caused by multiple cycles of abrasion with a Taber Abraser was much slower for the CNFs containing formulations, showing a greatly reduced loss of oil. Kaboorani et al. modified cellulose nanocrystals (CNCs) with hexadecyltrimethylammonium bromide to improve their compatibility with a UV-cured acrylic resin [66,67,68][37][38][39]. They found that a loading of 3% of CNCs significantly enhanced the pencil hardness, tensile strength, modulus of elasticity, and thermal stability of the coating, while reducing its mass loss following abrasion and water vapor uptake and transmission rate. Cheng et al. found that adding CNCs and silver nanoparticles to a polyurethane coating exhibited a synergistic effect toward the antimicrobial properties of the coating, while also improving its adhesion [69][40]. Tree extracts were another type of additives that received a lot of attention, this time to imbue wood with better photostability [70,71][41][42]. Acrylic coatings containing condensed tannins and modified tannins were prepared by Grigsby to protect radiata pine [72][43] before exposition to natural and accelerated weathering. A loading of less than 0.5% of tannins was sufficient to extend the coating’s life up to 20% more than commercially available hindered amine light stabilizers (HALS) and phenolic stabilizers could. Tomak et al. coated Scots pine with water-based acrylics containing tannins from different woods species in the presence [73][44] or absence [74][45] of metallic oxides. They found that after 1512 h of artificial weathering, their coatings could outperform the commercial reference coating in terms of color changes and chemical degradation. It appeared that the lower concentrations of extractives were more effective against UV degradation, and that the interactions between the different tannin and oxides types were completely random. Waterborne acrylic containing CNF and bark extractives was prepared by Huang et al., which yielded both photostability and better mechanical properties (hardness and abrasion resistance) [75][46]. Finally, Yan et al. prepared acrylic wood coatings containing delignified wheat-straw powder, either raw or after calcination [76][47]. The resulting coatings showed good resistance to molds, especially when the wheat straw was calcined.
Some non-bio-based, organic wood coating additives were also studied over the last five years, sometimes under the form of microcapsules. Zhu et al. prepared urea-formaldehyde microcapsules loaded with thermochromic material to make color-shifting wood under thermal stimuli [77][48]. A waterborne varnish containing 20% of the microcapsules showed an important color change toward the red and yellow, according to the CIELAB analysis, when heated between 31 °C and 37 °C. The color change was then perfectly reversible between 34 °C and 26 °C. Other workers studied the development of heat sensitive wood, either with microcapsules [78][49] or not [79,80][50][51]. A self-healing acrylic coating was prepared by Yan and Peng by encapsulating resin in urea-formaldehyde microcapsules [81][52]. They found that a loading of 4% of microcapsules was enough to imbue the coating with good self-healing capacities without affecting its mechanical properties. Similarly, Queant et al. encapsulated organic UV absorbers in calcium carbonate microspheres in order to protect them from degradation [82][53]. A 2500 h accelerated weathering test showed that wood coated with a transparent waterborne latex would suffer less color changes when the UV absorbers were encapsulated. Other uses for organic additives in wood coatings during the last five years included the enhancement of their mechanical properties [83][54] and the reduction of the oxygen inhibition [84][55].
Nanoparticles of metal oxides and silica as wood coating additives allow the preparation of surfaces with a wide array of functionalities. An interesting method to achieve superhydrophobicity was to modify the nanoparticles with a low surface free energy chemical before their incorporation into the coating [85,86,87,88][56][57][58][59]. The low surface free energy of both the modified nanoparticles and the resin would imbue water repellency, and the nanoparticles brought an appropriate micro-/nanoscale architecture to the coating, joining together the two requirements to achieve superhydrophobicity. Sevda et al. experimented the addition of SbO3 and TiO2 to an intumescent paint [89][60]. They noted that a loading of 2% of nanoparticles increased of LOI and greatly decreased the weight loss and smoke generation in comparison to the paint alone. Guo et al. prepared bio-sourced silica particles through the calcination of rice husk [90][61]. After modification with a silane coupling agent (KH-570), a 2% loading of the silica in a waterborne acrylic coating improved the elongation at break (244.72% to 303.06%), tensile strength (32.509 MPa to 48.673 MPa), modulus of elasticity (3.010 MPa to 6.672 MPa), and pencil hardness (1H to 2H) of the resulting coating. Other workers explored the possibilities of these compounds to improve the resistance of wood to decay fungi [91[62][63],92], black-stain fungi [93][64], and photodegradation [94][65], as well as to improve its mechanical properties [95,96][66][67].
In addition to metallic oxides and silica, inorganic compounds of mineral origin were used as additives to improve the properties of organic wood coatings. Atienza et al. used oyster shell powder to make a thermally stable acrylic coating [97][68]. Because the shells are made of 95%–98% of incombustible calcium carbonate, the addition of 75% of oyster shell powder to the coating increased the time of burning of the wood samples from 18.00 min to 29.67 min. Zeolites were also considered as potential fire retardants in a melamine-urea-formaldehyde resin containing ammonium polyphosphate [98][69]. Due to their very porous nature, most of the zeolites studied showed an appreciable decrease in CO2 production. The ignition time was also greatly delayed, from 138 s for the resin containing only ammonium polyphosphate to 279 s with 3A zeolites. Although different zeolites performed the best on the different aspects of fire protection, 3A zeolites were, overall, the most performant. Other compounds studied as fire retardants were graphene [99][70] and sepiolites [100][71], which could also be used to make hydrophobic coatings. Kolya and Kang coated various species of hardwood with polyvinyl acetate coatings containing modified graphene oxide [101][72]. The graphene oxide, which had been reduced with NaBH4 in presence of urotropin and further functionalized with poly (diallyl dimethylammonium chloride), slightly increased the water contact angle of the coating on most wood species, with average angles of 91.5° and 92.7° on the radial and cross-sectional face, respectively, as compared to 72.5° and 80.1° for the polyvinyl acetate coating alone. Similarly, Chen et al. functionalized sepiolite with polysiloxane and mixed them with an epoxy [102][73]. They found that the hydrophobicity of the coating increased rapidly with higher sepiolite:epoxy ratios, with highly superhydrophobic (water contact angle = 166° and roll-off angle = 5°) at 7:5. The wood surface also exhibited good self-cleaning properties and the ability to separate water and oil.

1.3. Organic-Inorganic Composite Coatings

An extremely large share of the research into wood coatings, over the last five years, focused on the organic-inorganic composite coatings. This section differs from the additives in organic coatings for a few reasons: 1- the organic and inorganic parts, in this section, are not always blended together, 2- the organic part is often not a resin, and 3- the inorganic part is frequently the main component of the coating. Also, organosilicons are an important element of these coatings.
A method frequently encountered to improve the hydrophobicity of the wood surface, and often reach superhydrophobicity, was to use nanoparticles to form a proper micro-/nanoscale architecture and thereafter reduce its surface free energy. For this matter, an organic coating could be used to reach the desired low surface free energy [103,104][74][75]. A superhydrophobic wood surface was prepared by Lozhechnikova et al. after applying positively charged ZnO nanoparticles and a negatively charged carnauba wax on Norway spruce through layer-by-layer deposition [105][76]. Not only did the coating reach a 155° water contact angle (WCA), but it also displayed higher UV stability and moisture buffering. Lu et al. pretreated rubberwood with IPBC, an organic biocide, before dipping it in polystyrene and SiO2 solutions [106][77]. At higher SiO2 concentrations (2%), the WCA was 155.6° and the antiseptic performances of the IPBC were preserved by the coating, which reduced its leaching. A superhydrophobic coating with a high thermal energy storage capacity was designed by Kong et al. through spraying with mesoporous polydivinylbenzene nanotubes, fluorine-containing SiO2 nanoparticles and paraffin wax [107][78]. Upon exposition to excessive heat, the wax trapped in the nanotube would melt to store thermal energy and later release this energy through crystallization. This kind of wood surface improvement strategy was also used by other workers to imbue wood photostability [108][79], molds resistance [109][80], self-healing [110][81], thermal stability [111][82], and improved adhesion of UV-curing coatings [112][83].
The hydrophobization of the micro-/nanoscale architecture could also be achieved by replacing the resins with low surface free energy components [113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103]. As a general rule, these coatings showed superhydrophobicity (contact angle > 150°, roll-off angle < 10°), high resistance to mechanical wear (abrasion, cutting, etc.), and sometimes properties such as chemical resistance and self-healing. Wang et al. developed an interesting method to grant superhydrophobicity to Chinese fir, where they obtained the desired micro-/nanoscale architecture simply by sanding the wood surfaces with a 240-grit sandpaper [133][104]. The surface free energy of the produced micro/nanoscale architecture was subsequently reduced by deposition of a fluoroalkylsilane/silica composite suspension to obtain a superhydrophobic surface with good abrasion resistance and self-healing capabilities. Guo et al. created a Mg-Al-layered double-hydroxide coating to improve the fire safety of birch wood via thermal deposition followed by hydrophobization with trimethoxy(1H,1H,2H,2H)heptadecafluorodecyl)silane [134][105]. The limiting oxygen index (LOI) of the coated wood increased from 18.9% to 39.1%, and its total smoke generation and total heat release decreased by 58% and 40%, respectively. Wang et al. coated poplar wood by dipping in polydopamine for 24 h, electroless Cu deposition for 12 h, and dipping in octadecylamine for 24 h [135][106]. Although the process was quite tedious, the resulting coating was extremely durable, keeping its superhydrophobicity even after degradation by UV light, acids, bases, organic solvents (n-hexane, acetone, ethanol, and DMF), and boiling water. Huang et al. modified nanofibrillated cellulose [136][107] and lignin-coated cellulose nanocrystals [137,138][108][109] coated wood with 1H,1H,2H,2H-perfluorooctyltrichlorosilane through chemical vapor deposition. The resulting wood surface showed high sandpaper abrasion and UV resistance, as well as superhydrophobic and self-cleaning behaviors. The naturally hydrophobic micro/nanoscale structure of canna leaves [139][110] and rose petals [140][111] was re-created by Yang et al. through nanoimprint lithography. They first created a PDMS template of the canna leaves and rose petals, which was use to make a perfect copy of the said structure with SiO2 and polyvinyl butyral. The copy could then be peeled from the template and stuck to the wood surface to reach superhydrophobicity. A similar strategy was used by Chen et al. to create superhydrophobic and magnetic wood surfaces based on the structure of taro leaves with F3O4 and PDMS [141][112]. Gan et al. also prepared superhydrophobic wood surfaces with a ferromagnetic behavior by dipping poplar samples in a solution of hydrophobized CoFe2O4 nanoparticles, which had a contact angle of 158°, high resistance to sandpaper abrasion, and improved microwave absorption properties [142][113].
Some organic-inorganic composite coatings were prepared simply by adding organic and inorganic moieties together on the wood surface. Wang et al. used tannic acid-Fe3+ complexes in combination with silver nanoparticles to create a superhydrophobic coating [143][114]. The developed coating was highly durable, keeping a contact angle higher than 150° after UV exposition and degradation by HCl, NaOH, n-hexane, acetone, ethanol, DMF and boiling water. A magnetic wood coating based on chitosan, sodium phytate and nano-Fe3O4 was prepared by Tang and Fu through layer-by-layer deposition [144][115]. They found that paramagnetic wood with narrow and long magnetic hysteresis loops could be created with this method, the magnetic properties of the treated wood being directly related to the number of layers in the coating. Uddin et al. prepared a paste with Mg(OH)2 and casein to improve Scots pine’s resistance to fire [145][116]. The prepared wood surface had a delayed time to ignition (12.1 s to 30.4 s), a lower peak heat release (216 kW/m2 to 119 kW/m2) and a lower total heat release (79.5 MJ/m2 to 53.3 MJ/m2), as well as decreased smoke production and mass loss. Another fire-retardant coating was prepared by Xie et al. through dipping in different solutions containing graphene oxide and functional cellulose [146][117]. These coatings scored a V-0 rating in a vertical burning test, FH-1 or FH-2 rating in a horizontal burning test, had a greatly increased LOI, and could self-extinguish when removed from the flame source. In presence of moisture, they could even self-heal incisions with widths up to 320 μm.

1.4. Inorganic Coatings

Metallic oxides and silica played an important role in the last five years of research in the domain of wood coatings [147,148,149][118][119][120]. They were layered on wood surfaces with a variety of methods, including sol-gels [150,151,152][121][122][123]. Sol-gels with SiO2, TiO2, and Fe3+ [153][124] or Zr4+ [154][125] were used to make photostable wood surfaces with photocatalytic activity, granting them self-cleaning capabilities through the photodecomposition of organic pollutants. Qian et al. also used sol-gels to develop a coating based on microcapsules with a Fe3O4/SiO2 shell and a phase changing material core to imbue energy storage and magnetism to poplar wood [155][126]. The hydrothermal growth and deposition of metallic oxides were also the subject of many publications [156,157,158,159][127][128][129][130]. Sun and Song casted WO3 on poplar wood through hydrothermal in situ synthesis [160][131] or nanosheet deposition [161][132] to build photochromic wood. The resulting wood surfaces could reversibly change color after exposition to UV radiations, had better photostability and could be hydrophobized with 1H,1H,2H,2H-heptadecafluorodecyl)silane. Similarly, MoO3 was hydrothermally grown or deposited on birch to yield photo-responsive wood with a blue shift when exposed to UV light [162,163][133][134]. Wang et al. prepared magnetic wood with fire-retardancy through the hydrothermal deposition of MnFe2O4 [164][135]. The initial burning time of the coated wood was delayed from 6 s to 20 s, its electromagnetic waves absorption capacity was improved, and it could additionally be hydrophobized with fluoroalkylsilanes [165][136]. A very interesting silica coating was prepared by Belykh et al. by mixing sodium liquid glass and black shale, which are byproducts from the fabrication of ferrosilicon and gold mining activities, respectively [166][137]. They found that good adhesion could be achieved by using 20%–35% of black shale, while 10%–25% yielded a good reduction of the mass loss when exposed to fire. They also found that adding 1% of a synthetic foaming agent (PO-6) improved both the adhesion and fire resistance of the coating.
Among the other approaches to coat wood with inorganic materials, Pan et al. used an electroless plating method to coat poplar disks with nickel (Ni) [167][138]. They found that the disks’ resistance would decrease from 12 Ω to 1 Ω within the first 5 minutes of electroless plating, that the hydrophobicity of the coated wood slightly increased, and that an electromagnetic shielding effectiveness between 55 Db and 65 Db could be achieved. Similar methods were also studied to coat poplar with Cu-Ni [168][139] and Ni-P [169][140] composites. A superhydrophobic wood surface was created by Wang et al. by deposing copper on a wood substrate with a vacuum evaporator, followed by the growth of a silver layer through immersion in a AgNO3 solution [170][141]. The prepared wood surface, which had a contact angle of 160.5° and a roll-off angle near 0°, could keep its superhydrophobicity after 100 cycles of tape abrasion or 200 cm of sandpaper abrasion with a 50 g weight. Hydrophobic wood surfaces were prepared by Łukawski et al. via drop casting and dipping with different solutions of carbon black, graphene, and carbon nanotubes [171][142]. Concentrations as low as 0.05 g/m2 of nanomaterials were sufficient to reach very high water contact angles (up to 143°), although superhydrophobicity was not achieved. They also found that, while carbon nanomaterials do not make covalent bounds with wood, the coatings’ resistance to sandpaper was quite good. Yuan et al. used hydrothermal deposition of graphitic carbon nitride nanosheets to improve the photostability of poplar wood [172][143]. They found that the nanosheets could absorb 90% of the UVA and UVB, substantially reducing the color changes after accelerated weathering. Furthermore, TG and DTG showed an improvement of the thermal stability of the coated wood. Another use of inorganic nanosheets as wood coatings was explored by Liu et al. who coated cedar wood with boron nitride nanosheet to improve its performance against fire [173][144]. The obtained wood surface showed a good thermal stability after 60 s of exposition to a lighter and an improved resistance to oxidation.

2. Wood Surface Modification

Instead of improving wood surfaces through the addition of an outer layer, different modification methods allow for the enhancement of the wood’s properties directly. These methods involve the chemical modification of the wood surfaces by different means to improve their wettability, decay-fungi resistance, photostability, and more.

2.1. Plasma Modification

An environmentally-friendly way to modify the surface of wood is through plasma treatments. Plasmas are highly reactive chemical environments with interesting features such as scalability and the absence of solvents [174][145]. They exist under various forms, which can be thermal or non-thermal [175][146]. In the case of wood protection, however, non-thermal plasmas are preferred to avoid its thermal degradation. An interesting use of plasmas in wood surface modification is to oxidize its polymeric constituents to increase its surface free energy and wettability. Over the last 5 years, many workers explored this application to increase the interactions between wood and adhesives or coatings [176,177,178,179,180,181,182,183,184,185,186,187][147][148][149][150][151][152][153][154][155][156][157][158]. They found that using reactive carrier gases such as air and O2 would lead to the creation of polar groups such as carboxyl and carbonyl at the surface of the treated wood, increasing its hydrophilicity and surface free energy. The plasma-treated wood would therefore have much lower contact angles with water and water-based coating solutions, showing an enhanced wettability, as well as a faster and deeper absorption of those liquids. As a result, the adhesion of coatings and adhesives would often be greater on treated wood than on its untreated counterpart.
Žigon et al. [188][159] and Žigonand Dahle [189][160] used a floating electrode dielectric barrier discharge (FE-DBD) plasma to treat Norway spruce following a short dip in NaCl solutions. They found that the electric conductivity and the intensity of the discharge were both increased by the NaCl, which led to an enhanced wetting of the wood following the plasma treatment. As a result, the contact angle with water and a water-based coating were lower and the tensile strength of the coating was improved. In another study, Žigon et al. used different adhesives to bind beech to aluminum and steel after treating both the wood and metals with a FE-DBD plasma [180][151]. The surface free energy of all the substrates increased following the plasma treatment, which led to higher tensile shear strength in most of the studied scenarios. After noting the poor adhesion of a water-based primer to beech veneers and of a water-based topcoat to an oily UV-cured primer, Peng and Zhang treated both the wood veneers and the UV-cured primer with a DBD plasma before laying the subsequent coat [190][161]. They found that the wettability of the beech veneers and the primer increased following the plasma treatment, which led to a large enhancement of their adhesion. Similarly, Dahle et al. dip-coated pine samples with polystyrene microspheres before proceeding to plasma modification of the coating [191][162]. As hydroxyl and carbonyl groups were created on the polystyrene layer, the wood surface became superhydrophilic and may eventually be used as a primer for further plasma polymerization. In an effort to improve the fireproofing of wood, Gospodinova and Dineff studied the effect of a plasma treatment on the absorption of fire-retardant solutions [192][163]. They found a substantial increase in the surface free energy and of the penetration-spreading parameter after the plasma treatment, but also a rapid decrease in these two variables during a post-treatment storage. Finally, Volokitin et al. used a thermal plasma treatment to mimic a thermal treatment on the surface of pine and birch [193][164]. Similar to typical thermal treatments, the modified samples were darker, their water contact angle increased, and their water absorption declined.
Other uses for plasma treatments in wood-surface protection are the grafting of chemical components and the creation of thin coatings. Due to their highly energetic and reactive nature, plasmas can melt metallic particles or break down organic molecules into reactive moieties while creating radicals on the surface of wood, which allows the growth of thin coating layers or the functionalization of wood surfaces [194,195][165][166]. Thereby, a Zn/ZnO thin coating was deposited on beech wood by Wallenhorst et al. through the cold plasma spraying of Zn microparticles with air as the process gas [195][166]. After 50 h of exposition to UV radiations, the thickest coating almost completely inhibited to color changes (E* ≈ 0), while uncoated wood had a E* of 10. They also found that the coating could protect a polyurethane topcoat from photodegradation. Similarly, Profili et al. prepared a hydrophobic ZnO/SiO2 composite coating on sugar maple with a DBD plasma [196][167]. The coating, formed by the embedding of ZnO particles in a SiO2 layer, displayed a static water contact angle of 100°, while the untreated samples quickly absorbed the water droplets. A superamphiphobic coating was casted on birch wood by Tuominen et al. by depositing titanium nanoparticles with a liquid flame spray followed by the plasma polymerization of perfluorohexane [197][168]. The coating, which displayed contact angles > 160° with water, ethylene glycol, diiodomethane, and olive oil, was also highly resistant as its wetting properties were still intact after a 500,000 water drops impact test. Furthermore, good self-cleaning properties were noted with both water and oil.
The hydrophobization of wood surfaces through plasma treatments with fluoroalkanes and organosilicons also received some attention over the last five years. Notably, de Cademartori et al. created a fluorocarbon film on white spruce and Brazilian cedar by polymerizing octofluoropropane with a DBD plasma [198][169]. They found that longer treatment periods would lead to higher hydrophobicity, with optimal water contact angles of 135.2° and 129.8° on the spruce and cedar, respectively. Levasseur et al. also used C3F8 to improve the hydrophobicity of sugar maple wood by DBD plasma with inert gases (Ar and N2) [199][170]. They noted that the hydrophobicity of the obtained surfaces was directly linked to the voltage of the plasma, which could yield a 140° static water contact angle at the highest voltage (10 kV). After letting the coating age for 125 days under uncontrolled conditions, the wetting properties of the wood surface remained unchanged. The possibility to increase or decrease the wettability of wood through cold remote (N2 +O2) plasma was studied by Bigan and Mutel [200][171]. They noted that the water absorption of different plasma-treated wood species could significantly increase (up to 5.5 timesin the case of beech) by using the plasma treatment alone, but that adding 1,1,3,3-tetramethyldisiloxane could make the wood superhydrophobic. Wood coatings with both hydrophobicity and good thermal stability were prepared by Sohbatzadeh et al. [201][172] and Chen et al. [194,202][165][173] through the plasma polymerization of hexamethyldisiloxane. Wood surfaces with lower surface free energy and increased roughness were obtained, yielding water contact angles as high as 138°. Many other studies were conducted over the last five years to improve the properties of wood surface through plasma treatments, including polyester powder [203][174], polyester with aluminium coated silver and bismuth oxide [204][175] or TiO2 [205][176], copper and aluminium microparticles with an acrylic binder [206][177], ZnO [207][178], TiO2 [208][179], and various biocidal precursors [209][180].

2.2. Other Surface Modification Methods

Beside plasma treatments, many methods were used, over the last years, to improve the properties of wood surfaces through modification. Herein, those methods are classified into two categories: chemical methods and carbonization methods. The chemical methods relied on chemical reactions or interactions to tone the properties of the outmost surface of the treated wood; a slight penetration of the chemicals into the wood was considered a surface impregnation, which will be reviewed in the next section. The chemical grafting of chemicals on the surface of wood involves the creation of a covalent bond between the wood cell wall and the modifying agent. This method allows to improve the surface properties of wood while reducing the leaching of the chemicals [210][181]. Wang et al. grafted poly(2- (perfluorooctyl)ethyl methacrylate) on the surface of Chinese fir by atom transfer radical polymerization [211][182]. The modified wood had a strong superhydrophobic behavior, which was only slightly affected by finger-wiping and tape-adhesion abrasion tests. Furthermore, the treated wood showed excellent resistance to the mold Aspergillus niger and self-cleaning properties. A similar method was used by Sharma et al. to graft acetonitrile and ethyl acrylate on pine wood [212][183]. Under optimal conditions, they obtained a percentage grafting of 85.34%, which greatly reduced the swelling of the treated wood in different solvents and solutions, as well as its weight loss when dipped in strong bases and acids. An environmentally-friendly treated was developed by Filgueira et al. as they grafted modified Pinus radiata tannins and condensed tannins on beech wood through the action of a laccase enzyme [213][184]. They found that treating wood this way, under an alkaline medium (pH = 10), would reduce its water absorption over 72 h by 20% and reduce the leaching of the treatment by 76%. Song et al. modified the surface of balsa wood by dipping samples for 60 s in aqueous solutions containing 0.75% of different salts [214][185]. They found that the metal ions, particularly Zr4+, could attain a water contact angle up to 145° through the creation of a microstructure and crosslinking. However, the durability of such treatment seems rather low, as the contact angle dropped to 138° after 14 days ambient conditions. The fluorination of silver fir and Douglas fir with gaseous F2 was studied by Pouzet et al. [215,216][186][187]. They found that the treatment would substitute hydroxyl groups from the cell wall polymers for fluorine, reducing the surface free energy of the treated wood. As a result, the water contact angle increased up to 120°, the water absorption decreased drastically, and the treatment was still as hydrophobic two years later. However, while short treatment times had only a low effect on the integrity and color of the treated wood, treatments of 20 min led to a severe degradation of the tracheids and browning of the wood surfaces. In another study, they also noted that a torrefaction post-treatment would help to purge the HF produced by the reaction of F2 with wood and further slow down the ingress of water without causing any defluorination of the treated wood [217][188]. The combination of a laccase enzyme surface treatment and pressure impregnation of copper(II) sulfate pentahydrate was explored by Gabrič et al. [218][189]. They found that a laccase pre-treatment would make the cell walls of the wood swell, preventing its impregnation; however, using the laccase as a post-treatment would greatly reduce the leaching of the copper. Furthermore, the laccase treatment alone could reduce the mass loss due to the degradation by brown- and white-rot fungi by roughly a third. More chemicals were studied over the course of the last five years to modify the surface of wood, including aminoborates [219][190], methanol [220][191], cellulase [221,222][192][193], poly(methylhydrogen)siloxane [223][194], and chitosan [224][195].
As the name implies, the carbonization methods revolve around charring the surface of wood to modify its properties. The use of CO2 lasers with radiation doses up to 75 J/cm2 was studied in several publications, as well as its effect on the tensile strength of adhesives, the resistance to molds of the treated wood and its surface free energy [225,226,227][196][197][198]. The authors found that high radiation doses would increase the wood surface blackening, as a result of the carbonization, as well as its resistance to molds. However, they noted that the treatment was only effective against Aspergillus niger. Studies of the surface properties showed that the loss of hydroxyl groups reduced the surface free energy of the treated wood, which decreased the tensile strength of polyurethane and polyvinyl acetate adhesives. Other authors explored the carbonization of wood by pressing a single surface of the treated wood with a hot metal plate at different temperatures (220 °C to 400 °C) for different periods (30 s to 2 h) [228,229,230][199][200][201]. They observed highly modified moisture-related behaviors, with higher water contact angles, lower water absorption, and lower equilibrium moisture contents (EMC). As a result of the reduced EMC, the modulus of rupture of the charred wood was also slightly higher. A similar study was conducted by Akçay et al., wherein, pine and beech surfaces were carbonized with a blow torch to improve the wood’s resistance to white- and brown-rot fungi [231][202].

2.3. Wood Surface Impregnation

The properties of the wood surfaces can be improved with a shallow impregnation of chemicals. It can be achieved by different means, from brushing and very short dippings (few seconds) to longer dippings (few hours) and single face vacuum impregnation. Very few publications on the subject of impregnation presented details about the impregnation depth of the chemicals or their distribution into the wood; consequently, different parameters were taken into consideration to decide if a method would be considered as a surface impregnation: the use or not of pressure/vacuum, the duration of the treatment, the size of the samples, whether the samples were completely or partially covered by the treatment, and the weight gain. Petrič described surface impregnation as the impregnation of the first few millimeters of the cross-section of wood [175][146]. While this definition was used as the basis to classify the treatments as surface impregnation, it seemed rather ambiguous, as some hard-to-treat species can only be treated by a few millimeters in the cross-section, even with a vacuum/pressure process. Accordingly, treatments that would allow high longitudinal penetration were also rejected.
A primary way to treat wood by surface impregnation was through the insertion reactive material. Triquet et al. chemically increased the surface density of various hardwood species by in situ polymerization of acrylate monomers [232][203]. The monomers were vacuum impregnated for 150 s after being dropped on a single surface of wood, which was followed by electron-beam polymerization. The density of the treated wood increased by nearly 200 kg/m3 near the surface, which lead to an augmentation of the Brinell hardness. Different workers studied the possibility of reducing the set-recovery of unilaterally compressed wood with the impregnation of chemical agents. While the compression of wood itself is not a subject of this review, the effect of a surface pre-treatment on its durability was deemed appropriate. Wu et al. impregnated poplar wood with a reactive waterborne acrylic resin by immerging a quarter of the wood blocks into the resin solution and applying a vacuum, leading to weight percent gains ranging from 1.1% to 4.7% [233][204]. Afterward, the impregnated surface was densified with a hot press under different temperatures ranging from 150 °C to 180 °C. At the highest loading of resin, the set-recovery of the impregnated wood was only 1.8%, while the control samples reached 73.0%. Similarly, Han partially soaked Scots pine for different durations in a furfuryl alcohol solution containing a maleic anhydride catalyst before pressing a single surface with a hot metal plate [234][205]. Under optimized conditions, the set-recovery of the densified wood decreased from 60% to 14%. Various impregnation agents were studied by Neyses et al. to achieve the same goal, although the results were not as satisfying [235][206]. Lafond et al. improved the embedment capacity of black spruce connectors through the impregnation of acrylates [236][207]. They found that a chemical retention of 7% could improve the bearing strength of the connector by 48% and their stiffness by 27%. Finally, the development of colored wood surfaces through the creation of complexes between phenolic extractives and metal ions was explored by Dagher et al. [237][208]. After simply applying a 1% ferric sulfate solution on the surface of different hardwoods with a foam roller applicator, different colors were developed for each species according to their phenolic extractives content.
Another way to protect wood by surface impregnation is to simply insert protective agents slightly under its surface. Harandi et al. brushed 5% and 10% solutions of poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVBVA) on silver fir to improve its mechanical properties [238][209]. They found that both solutions increased the water contact angle to 90%, reaching hydrophobicity, and improved the modulus of rupture (MOR), modulus of elasticity (MOE), plastic hardness and Martens hardness of the treated specimens. Although the more viscous 10% solution took more time to be absorb, it yielded equivalent MOR and MOE, as well as higher plastic hardness and Martens hardness than the 5% solution. Kumar et al. dipped Norway spruce blocks in a 1% solution of octadecyltrichlorosilane for 30 min to 120 min, yielding from 0.7% to 2.4% weight percentage gains [239,240][210][211]. They found that the treated specimens had very high static water contact angles (140–150°), a negligible water absorption through immersion, a lowered equilibrium moisture content when exposed to a high relative humidity (95%), an increased dimensional stability, and a reduced mass loss when exposed to the brown-rot fungi Coniophora puteana. The impregnation of poplar wood with K2CO3 and SiO2 solutions to improve its fire-retardancy was studied by He et al. [241][212]. They found that the limiting oxygen index (LOI) increased from 20.5% to 33.5% prior to the treatment, and only decreased to 30.5% after leaching. Thermogravimetric measurements showed that the mass loss decreased during the charring phase (63.2% to 47.4%) and the calcining stage (34.8% to 24.3%), while the char generation increased. Thermochromic wood veneers were prepared by Zhu et al. through the ultrasonic impregnation of a thermochromic dye and a color developer [242][213]. The treated wood, which was very dark, could return to its original color between 28 °C and 38 °C, while the discoloration was reversible between 34 °C and 22 °C. Other publications reported the surface impregnation of chemicals to decrease the wettability [57][28], flammability [243][214], mold [244,245][215][216] and mildew [246][217] degradation, and dimensional instability [247][218] of various wood substrates.

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