**3. Historical wood artifacts conservation**

The historical wood artifacts show different degrees of chemical and biological degradation that weakens the material resistance, and their physical and structural consolidation is essential in preserving these objects. The use of polymeric resins, which must have a good compatibility with the wood material and a high stability to environmental degradation, has an important contribution in the old wood objects consolidation [33–36]. The consolidation effect of the polymer resins is significantly improved by the nanodispersion in these solutions of mineral materials with reinforcing effect (especially metal oxides) [37]. The nanoinsertions of these nanocomposites into the wood support, besides the role of physical consolidation, also provide an important increase in resistance to oxidative and biological degradation (fungus mildew) and increased flame retardancy [38–41]. The most suitable polymers for wood preservation are aliphatic and aliphatic epoxy acrylic resins due to their stability to oxidative degradation, their adhesion, and processability. The most intensely used polymers for the preservation of modern and archaic wood articles are the polyacrylates, the low-molecular weight ethyl acrylate (Paraloid B 72) a metallic copolymer, recently used as composites with nanometric materials, being representative [42, 43]. The most used nanocomposites of polymeric resins are obtained with metal oxide nanoinsertions mainly ZnO, MgO, TiO<sup>2</sup> and metals Cu, Au and Ag [44–47].

### **4. Study case**

Under such context, poly(styrene-ethylene-butylene-styrene) has the advantage of being able to be used as the base material for polymer films on the one hand and the advantage of high stability, good mechanical properties, and resistance to biological attack. Also, its composite with ZnO increases the efficacy of this polymer. ZnO has been reported as a substance that provides an increased wood stability against degradation due to UV radiation because ZnO has the ability to block UV rays, both UVA and UVB, acting as physical filters that reflect or disperse UV radiation.

The positive values of Δ*a\** after 120 h of irradiation indicate a tendency of both wood surfaces to become reddish. But when the exposure time increases, the values of Δ*a\** become negative, which is associated with a tendency of both wood surfaces to become greenish. Lignin degradation leads to chromophoric groups formation, carbonyl, and carboxyl groups, which affect the color change mechanism [55]. The process of lignin degradation is accentuated by oxygen and moisture presence, decreasing the coating adhesion due to low-molecular degradation products. In these conditions, an efficient coating must respect more requirements, namely enough filter efficiency until 440 nm, oxygen barrier, water vapor permeation, and abrasion,

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The color changes in time can be highlighted by the chromatic variations, namely Δ*L*\*—variation of lightness and Δ*b*\*—variation of the blue-yellow chromatic coordinates. In [56], the authors presented these variations of acrylic-ZnO coating for wood. Δ*L*\* increase from −20.09 for the wood sample to −5.66 for the sample of impregnated wood with 4% ZnO, Δ*b*\*, the coordinate that marks the wood yellowness after UV exposure, a decrease from 15.27 until

Under such context, the gloss parameters could reflect the polymer quality, by irradiation, a small decrease is observed, most probably due to the polymer degradation, faster for

Auclair et al. concluded during their study [57] that ZnO is a more efficient photo-protector for wood than CuO. As polymer matrix, urethane-acrylate systems were used. In case of polymer-ZnO nanocomposites, the discoloration of clear-coated wood exposed outdoors was

The photo-yellowing and UV degradation of wood surfaces were overcome by coating with ZnO-maleic anhydride-modified polypropylene (MAPP)-polyurethane transparent

scratch and impact resistance [56].

0.99 after impregnation with the UV absorber (**Figure 1**).

reduced and the higher increase in gloss was obtained.

**Figure 1.** Chromatic parameters of fir wood samples after irradiation (60 min).

SEBS-MA than SEBS-MA + ZnO (**Figure 2**).

For the polymeric composition, poly(styrene-ethylene-butylene-styrene) block copolymer (SEBS) grafted with maleic anhydride (MA), mixed with ZnO, has been used for the preservation of wood surfaces by spraying the pretreated wood surfaces. A slight color change could be observed at the fir wood treated with SEBS-MA sample, because the consolidant retention, the penetration depth, and the uniformity of the consolidant distribution into the material are parameters that influence the consolidation effectiveness.

The protective behavior of these polymers on these samples was put into evidence by specific analytical techniques: Fourier-transform infrared spectroscopy (FTIR), chromatic analysis, and gloss index analysis [48–51].

The first change that indicates the wood degradation is identified by lignin degradation, through quinone compounds formed responsible for a yellowed surface. These compounds increase the surface roughness, the chemical bonds are weaker, and macroscopic cracks are formed [52]. The treatment of wood with different consolidants causes the alteration of the spectra aspect by the appearance or intensification of some characteristic absorption bands.

The temperature aging is characteristic to a reduction in hydroxyl groups, an increase of the unconjugated carbonyl groups, and an apparent slight increase of lignin. The different behavior of the studied wood species may be explained by their different chemical composition, especially hemicelluloses, lignin, and extractives content.

The color parameters that can indicate the wood change are *L*\* (degree of color lightness), *a*\* (green-red chromatic coordinate), *b*\* (blue-yellow chromatic coordinate), and Δ*E* (color variation and stability) that can be calculated using Eq. (1) (<sup>Δ</sup> *<sup>L</sup>*∗)<sup>2</sup> <sup>+</sup> (<sup>Δ</sup> *<sup>a</sup>*∗)<sup>2</sup> <sup>+</sup> (<sup>Δ</sup> *<sup>b</sup>*∗)2 (1)

$$
\Delta E = \sqrt{(\Delta L^\*)^2 + (\Delta a^\*)^2 + (\Delta b^\*)^2} \tag{1}
$$

where Δ*E*\*, Δ*a*\*, and Δ*b*\* are the differences between the sample specimens and the reference specimen; Δ*L*\* is the change of the light in the point, on different time intervals, compared with the initial value: Δ*L*\* = *L*<sup>1</sup> *\** − *L*initial*\**; Δ*a\** is the chromatic deviation of the *a*\* coordinates (red and green colors) of the same point, on different time intervals, compared to the initial value: Δ*a*\* = Δ*L*\* = *a*<sup>1</sup> \* − *a*initial\*; Δ*b\** is the chromatic deviation of the *b*\* coordinates (yellow and blue colors), respecting the same mathematic formula: Δ*b*\* = *b*<sup>1</sup> \* − *b*initial\* [53].

Δ*E*\* value is an evaluation criterion of the overall change color. If the value is smaller than 0.2, the difference is not visible. A small difference in color is given by a value between 0.2 and 2. Between 2 and 3, respectively, between 3 and 6 highlight a color change visible with high-quality, respectively, a medium-quality filter. At a value over 6 of Δ*E*\*, the color is highly changed or even different [54].

The positive values of Δ*a\** after 120 h of irradiation indicate a tendency of both wood surfaces to become reddish. But when the exposure time increases, the values of Δ*a\** become negative, which is associated with a tendency of both wood surfaces to become greenish. Lignin degradation leads to chromophoric groups formation, carbonyl, and carboxyl groups, which affect the color change mechanism [55]. The process of lignin degradation is accentuated by oxygen and moisture presence, decreasing the coating adhesion due to low-molecular degradation products. In these conditions, an efficient coating must respect more requirements, namely enough filter efficiency until 440 nm, oxygen barrier, water vapor permeation, and abrasion, scratch and impact resistance [56].

stability, good mechanical properties, and resistance to biological attack. Also, its composite with ZnO increases the efficacy of this polymer. ZnO has been reported as a substance that provides an increased wood stability against degradation due to UV radiation because ZnO has the ability to block UV rays, both UVA and UVB, acting as physical filters that reflect or

For the polymeric composition, poly(styrene-ethylene-butylene-styrene) block copolymer (SEBS) grafted with maleic anhydride (MA), mixed with ZnO, has been used for the preservation of wood surfaces by spraying the pretreated wood surfaces. A slight color change could be observed at the fir wood treated with SEBS-MA sample, because the consolidant retention, the penetration depth, and the uniformity of the consolidant distribution into the material are

The protective behavior of these polymers on these samples was put into evidence by specific analytical techniques: Fourier-transform infrared spectroscopy (FTIR), chromatic analysis,

The first change that indicates the wood degradation is identified by lignin degradation, through quinone compounds formed responsible for a yellowed surface. These compounds increase the surface roughness, the chemical bonds are weaker, and macroscopic cracks are formed [52]. The treatment of wood with different consolidants causes the alteration of the spectra aspect by the appearance or intensification of some characteristic absorption

The temperature aging is characteristic to a reduction in hydroxyl groups, an increase of the unconjugated carbonyl groups, and an apparent slight increase of lignin. The different behavior of the studied wood species may be explained by their different chemical composition,

The color parameters that can indicate the wood change are *L*\* (degree of color lightness), *a*\* (green-red chromatic coordinate), *b*\* (blue-yellow chromatic coordinate), and Δ*E* (color varia-

where Δ*E*\*, Δ*a*\*, and Δ*b*\* are the differences between the sample specimens and the reference specimen; Δ*L*\* is the change of the light in the point, on different time intervals, compared

(red and green colors) of the same point, on different time intervals, compared to the initial

Δ*E*\* value is an evaluation criterion of the overall change color. If the value is smaller than 0.2, the difference is not visible. A small difference in color is given by a value between 0.2 and 2. Between 2 and 3, respectively, between 3 and 6 highlight a color change visible with high-quality, respectively, a medium-quality filter. At a value over 6 of Δ*E*\*, the color is highly

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

(<sup>Δ</sup> *<sup>L</sup>*∗)<sup>2</sup> <sup>+</sup> (<sup>Δ</sup> *<sup>a</sup>*∗)<sup>2</sup> <sup>+</sup> (<sup>Δ</sup> *<sup>b</sup>*∗)2 (1)

\* − *b*initial\* [53].

*\** − *L*initial*\**; Δ*a\** is the chromatic deviation of the *a*\* coordinates

\* − *a*initial\*; Δ*b\** is the chromatic deviation of the *b*\* coordinates (yellow and

parameters that influence the consolidation effectiveness.

especially hemicelluloses, lignin, and extractives content.

tion and stability) that can be calculated using Eq. (1)

blue colors), respecting the same mathematic formula: Δ*b*\* = *b*<sup>1</sup>

Δ*E* = √

with the initial value: Δ*L*\* = *L*<sup>1</sup>

changed or even different [54].

value: Δ*a*\* = Δ*L*\* = *a*<sup>1</sup>

disperse UV radiation.

76 New Uses of Micro and Nanomaterials

and gloss index analysis [48–51].

bands.

The color changes in time can be highlighted by the chromatic variations, namely Δ*L*\*—variation of lightness and Δ*b*\*—variation of the blue-yellow chromatic coordinates. In [56], the authors presented these variations of acrylic-ZnO coating for wood. Δ*L*\* increase from −20.09 for the wood sample to −5.66 for the sample of impregnated wood with 4% ZnO, Δ*b*\*, the coordinate that marks the wood yellowness after UV exposure, a decrease from 15.27 until 0.99 after impregnation with the UV absorber (**Figure 1**).

Under such context, the gloss parameters could reflect the polymer quality, by irradiation, a small decrease is observed, most probably due to the polymer degradation, faster for SEBS-MA than SEBS-MA + ZnO (**Figure 2**).

Auclair et al. concluded during their study [57] that ZnO is a more efficient photo-protector for wood than CuO. As polymer matrix, urethane-acrylate systems were used. In case of polymer-ZnO nanocomposites, the discoloration of clear-coated wood exposed outdoors was reduced and the higher increase in gloss was obtained.

The photo-yellowing and UV degradation of wood surfaces were overcome by coating with ZnO-maleic anhydride-modified polypropylene (MAPP)-polyurethane transparent

**Figure 1.** Chromatic parameters of fir wood samples after irradiation (60 min).

**Figure 2.** Gloss parameter of fir wood samples under different conditions.

nanosystems [14]. The acid anhydride groups of MAPP ensure the compatibility with OH groups from wood.

FTIR analysis can be used for solid wood samples being a fast-spectroscopic method and requiring an easy sample preparation. Information regarding the wood degradation can be obtained based on composition, functional groups, and molecular structure [52, 58, 59]. The degradation mechanism depending on the wood chemical composition (hemicellulose, lignin, and extractives content) is reflected in the chemical changes regarding the reduction of OH groups, increasing of unconjugated carbonyl groups, and formation of aromatic carbonyl conjugated groups as quinoid structures [60] (**Table 1**) (**Figure 3**).

Using polymeric micro- or nanosystems, the moisture content decreases. Humidity along with density is the physical factor that influences the physical, mechanical, and dimensional properties of wood and also influences the wood structure degradation. According to ISO 13061-1 [61] and Eq. (2), the moisture content is calculated for both native and treated wood:

$$w = \frac{m\_w - m\_0}{m\_0}^\* \mathbf{100} \tag{2}$$

ZnO is a suitable component for UV protection in coatings. Dispersion in acrylic polymers leads to a reduced yellowing and improved optical properties after artificially weathering for up to 1500 h [56]. Shellac nanosystems were studied in [66] as wood coating and different property modifications were observed using various nanofillers. For shellac-ZnO systems, the inhibition or

1364–1375 Symmetric bending of methyl groups in lignin and hemicellulose, bending of hydroxyl

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ness. In both cases, the color, film-forming ability, water repellence, and adhesion were preserved. UV-waterborne polyurethane containing different nanoparticles in order to increase the wood coating was studied. Thus, alumina, silica, ZnO, and CuO were used [7, 67]. The PU coating glass transition temperature is enhanced using alumina and silica that decrease the chains mobility at nanoparticles interface. The water and UV barrier properties are also increased.

ensures an increased coating hard-

the slowing down of the UV degradation was obtained. ZrO<sup>2</sup>

**Figure 3.** FTIR spectra of fir wood samples.

**Main absorption bands, cm−1**

**Peak assignment [58]**

3300 Stretching of ▬OH groups

1730–1740 Stretching of carbonyl group

1423 Asymmetric C▬H deformation

1025–1029 C▬O in cellulose pyranose ring

809 and 670 Aromatic C▬H from lignin

**Table 1.** Main FTIR absorption bands of wood.

1638 Bending of water (the moisture content)

1157 Symmetric stretching of bridging oxygen

groups of polysaccharides

2895 C▬H aliphatic

898 Cellulose

1596 and 1512 Lignin aromatic ring

ZnO ensures a good photo-protection and clear-coated wood.

where *w* is the moisture content [%], *mw* is the sample weight measured at a certain moisture [kg], and m<sup>0</sup> is the weight of the sample oven-dried [kg].

As the humidity increases in the wood cell membranes, most of the mechanical properties of wood decrease, except for elasticity which increases. In the saturated air, the steadystate humidity will reach a maximum, which is precisely the same saturation humidity (the saturation point of the fiber). At this point, the sorption stops as a phenomenon of wood hygroscopicity, and the desorption begins if the external environment conditions change. Because relative air humidity is a function of temperature and humidity pressure, it results that the equilibrium humidity is directly dependent on relative humidity and temperature and, depending on them, there is a whole range of equilibrium humidities. Curves describing the evolution of sorption and desorption processes of water are not overlap, leading to the hysteresis area. Hysteresis indicates less water retention of dried capillaries as compared to those of the membrane in the wet state, due to the fact that the cell membranes suffer some deformations remaining during sorption. Hysteresis has a special role in drying and steaming wood as well as explaining the internal tensions. Water repellency can be improved by coating with natural wax [62], paraffin wax [63], palm oil [64], and esterified organosolv lignin [65].


**Table 1.** Main FTIR absorption bands of wood.

nanosystems [14]. The acid anhydride groups of MAPP ensure the compatibility with OH

FTIR analysis can be used for solid wood samples being a fast-spectroscopic method and requiring an easy sample preparation. Information regarding the wood degradation can be obtained based on composition, functional groups, and molecular structure [52, 58, 59]. The degradation mechanism depending on the wood chemical composition (hemicellulose, lignin, and extractives content) is reflected in the chemical changes regarding the reduction of OH groups, increasing of unconjugated carbonyl groups, and formation of aromatic carbonyl

Using polymeric micro- or nanosystems, the moisture content decreases. Humidity along with density is the physical factor that influences the physical, mechanical, and dimensional properties of wood and also influences the wood structure degradation. According to ISO 13061-1 [61] and Eq. (2), the moisture content is calculated for both native and treated wood:

> *m*0 ∗

where *w* is the moisture content [%], *mw* is the sample weight measured at a certain moisture

As the humidity increases in the wood cell membranes, most of the mechanical properties of wood decrease, except for elasticity which increases. In the saturated air, the steadystate humidity will reach a maximum, which is precisely the same saturation humidity (the saturation point of the fiber). At this point, the sorption stops as a phenomenon of wood hygroscopicity, and the desorption begins if the external environment conditions change. Because relative air humidity is a function of temperature and humidity pressure, it results that the equilibrium humidity is directly dependent on relative humidity and temperature and, depending on them, there is a whole range of equilibrium humidities. Curves describing the evolution of sorption and desorption processes of water are not overlap, leading to the hysteresis area. Hysteresis indicates less water retention of dried capillaries as compared to those of the membrane in the wet state, due to the fact that the cell membranes suffer some deformations remaining during sorption. Hysteresis has a special role in drying and steaming wood as well as explaining the internal tensions. Water repellency can be improved by coating with natural wax [62], paraffin wax [63], palm oil [64], and esterified

100 (2)

conjugated groups as quinoid structures [60] (**Table 1**) (**Figure 3**).

**Figure 2.** Gloss parameter of fir wood samples under different conditions.

is the weight of the sample oven-dried [kg].

*<sup>w</sup>* <sup>=</sup> *mw* \_\_\_\_\_\_ <sup>−</sup> *<sup>m</sup>*<sup>0</sup>

groups from wood.

78 New Uses of Micro and Nanomaterials

[kg], and m<sup>0</sup>

organosolv lignin [65].

**Figure 3.** FTIR spectra of fir wood samples.

ZnO is a suitable component for UV protection in coatings. Dispersion in acrylic polymers leads to a reduced yellowing and improved optical properties after artificially weathering for up to 1500 h [56]. Shellac nanosystems were studied in [66] as wood coating and different property modifications were observed using various nanofillers. For shellac-ZnO systems, the inhibition or the slowing down of the UV degradation was obtained. ZrO<sup>2</sup> ensures an increased coating hardness. In both cases, the color, film-forming ability, water repellence, and adhesion were preserved.

UV-waterborne polyurethane containing different nanoparticles in order to increase the wood coating was studied. Thus, alumina, silica, ZnO, and CuO were used [7, 67]. The PU coating glass transition temperature is enhanced using alumina and silica that decrease the chains mobility at nanoparticles interface. The water and UV barrier properties are also increased. ZnO ensures a good photo-protection and clear-coated wood.

The coatings without nanoparticles in their composition present photo-yellowing and wood surface degradation when exposed to UV light. Dispersion of ZnO nanoparticles in MAPP and PU coatings restricted the color changes and photodegradation of wood polymers [14].

TiO2 nanoparticles can impart hydrophobic or hydrophilic properties to the material, fungicidal, and bactericidal protection and present photo-catalytic activity. Using TiO<sup>2</sup> as coating for wood items ensures anisotropy, wettability, and UV protection [46, 68–71].

Zanatta et al. obtained the nanoparticles by a hydrothermal method using microwaves, and then the wood pretreated with chromated copper borate was coated with TiO<sup>2</sup> . The wood maintained the natural color and the fungi resistance was improved [15].

The improvement of UV resistance using anatase TiO<sup>2</sup> was demonstrated in [17] when microand nanoparticles were impregnated on acacia hybrid wood. The exposure of the treated wood at UV radiation 960 h did not lead to color changes.

Obtaining stable and uniform dispersed coating is an effectiveness indicator. Polypropylene glycol can be considered a good solvent for metal oxides (ZnO, CeO<sup>2</sup> and TiO2 ) nanoparticles films. An increased UV resistance and thus a decrease in lignin degradation were observed [72] (**Figures 4**–**6**).

TiO2 coatings are a good moisture barrier for wood. If the treated wood is exposed to a 20–60% relative humidity (RH), the weight maintained constant, and at an RH between 60 and 90%, the mass increased with only 6%. TiO<sup>2</sup> coatings drastically decreased the change in anisotropic swelling of wood [19]. Obtaining films with controlled physico-chemical characteristics broadens their field of application. TiO<sup>2</sup> films may have different morphologies and wettability depending on the precursor pH (1–14). This characteristic may be a great advantage in using the nanoparticles coatings in various humidity environments [16].

Impregnation with micronized copper quaternary (MCQ) and UV absorbing acrylic resin (UVA-acrylic) was found to be a good method to increase the wood resistance to weathering conditions, and good visual, physical, and chemical properties were obtained [73].

> Nkeuwa et al. [7] studied the behavior of UV-cured multilayer coatings where only the top coat consists of a nanoparticle—nanoclay with the RH variation. The color and gloss of initial, during, and after accelerated aging coatings were investigated. The color of coated samples does not present visual changes during and after aging. The studied color parameters (Δ*L*\*, Δ*a*\*, Δ*b*\*, and Δ*E*\*) increase with increasing RH while the gloss retention is lowered. At high

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Other additives used for wood protection are the fire retardants (FR). A good flame retardancy can be obtained using phosphorous, boron, and silicone. Phosphorous FR can exhibit both condensed and/or gas phase action and can generate less toxic gases and smoke during combustion [74]. Systems containing both phosphorous and nitrogen components are good substitutes for halogenated FR due to their synergistic effect [38, 39]. In [40], nitrogen-phosphorous FR dispersed in poly(sodium silicate-aluminum dihydrogen phosphate) (PSADP) was used to reduce poplar wood hygroscopicity and to improve its fire resistance. These two components present a synergic effect on the two properties being distributed over the inner surface and

RH, significant changes are observed.

**Figure 6.** The fracture of the polymeric film from fir wood + SEBS-MA + ZnO.

**Figure 5.** The fracture of the polymeric film from fir wood + SEBS-MA.

penetrate the cell cavities of wood.

**Figure 4.** The fracture of the polymeric film from fir wood.

Polymeric Micro- and Nanosystems for Wood Artifacts Preservation http://dx.doi.org/10.5772/intechopen.79135 81

**Figure 5.** The fracture of the polymeric film from fir wood + SEBS-MA.

The coatings without nanoparticles in their composition present photo-yellowing and wood surface degradation when exposed to UV light. Dispersion of ZnO nanoparticles in MAPP and PU coatings restricted the color changes and photodegradation of wood polymers [14].

nanoparticles can impart hydrophobic or hydrophilic properties to the material, fungi-

Zanatta et al. obtained the nanoparticles by a hydrothermal method using microwaves, and

and nanoparticles were impregnated on acacia hybrid wood. The exposure of the treated

Obtaining stable and uniform dispersed coating is an effectiveness indicator. Polypropylene

ticles films. An increased UV resistance and thus a decrease in lignin degradation were

in anisotropic swelling of wood [19]. Obtaining films with controlled physico-chemical

gies and wettability depending on the precursor pH (1–14). This characteristic may be a great advantage in using the nanoparticles coatings in various humidity environments [16]. Impregnation with micronized copper quaternary (MCQ) and UV absorbing acrylic resin (UVA-acrylic) was found to be a good method to increase the wood resistance to weathering

conditions, and good visual, physical, and chemical properties were obtained [73].

 coatings are a good moisture barrier for wood. If the treated wood is exposed to a 20–60% relative humidity (RH), the weight maintained constant, and at an RH between 60

as coating

. The wood

) nanopar-

was demonstrated in [17] when micro-

coatings drastically decreased the change

films may have different morpholo-

and TiO2

cidal, and bactericidal protection and present photo-catalytic activity. Using TiO<sup>2</sup>

then the wood pretreated with chromated copper borate was coated with TiO<sup>2</sup>

maintained the natural color and the fungi resistance was improved [15].

glycol can be considered a good solvent for metal oxides (ZnO, CeO<sup>2</sup>

The improvement of UV resistance using anatase TiO<sup>2</sup>

and 90%, the mass increased with only 6%. TiO<sup>2</sup>

**Figure 4.** The fracture of the polymeric film from fir wood.

characteristics broadens their field of application. TiO<sup>2</sup>

observed [72] (**Figures 4**–**6**).

80 New Uses of Micro and Nanomaterials

wood at UV radiation 960 h did not lead to color changes.

for wood items ensures anisotropy, wettability, and UV protection [46, 68–71].

TiO2

TiO2

**Figure 6.** The fracture of the polymeric film from fir wood + SEBS-MA + ZnO.

Nkeuwa et al. [7] studied the behavior of UV-cured multilayer coatings where only the top coat consists of a nanoparticle—nanoclay with the RH variation. The color and gloss of initial, during, and after accelerated aging coatings were investigated. The color of coated samples does not present visual changes during and after aging. The studied color parameters (Δ*L*\*, Δ*a*\*, Δ*b*\*, and Δ*E*\*) increase with increasing RH while the gloss retention is lowered. At high RH, significant changes are observed.

Other additives used for wood protection are the fire retardants (FR). A good flame retardancy can be obtained using phosphorous, boron, and silicone. Phosphorous FR can exhibit both condensed and/or gas phase action and can generate less toxic gases and smoke during combustion [74]. Systems containing both phosphorous and nitrogen components are good substitutes for halogenated FR due to their synergistic effect [38, 39]. In [40], nitrogen-phosphorous FR dispersed in poly(sodium silicate-aluminum dihydrogen phosphate) (PSADP) was used to reduce poplar wood hygroscopicity and to improve its fire resistance. These two components present a synergic effect on the two properties being distributed over the inner surface and penetrate the cell cavities of wood.
