2.2.4. Preparation of Soil Substrates to Reach Target Soil Moisture Content (MCsoil,target)

Once 6000 g of oven-dry soil was filled into each of the six TMC boxes, MCsoil for each TMC was set to equal 95% of the measured WHCsoil, expressed as 95%WHCsoil. In order to understand the quantity of distilled water required to reach the MCsoil equal to 95%WHCsoil, a target soil moisture metric MCsoil,target [%] was defined. Equation (5) below was used to calculate the mass [g] in distilled water required to add to the soil mixture to reach MCsoil,target. Distilled water was subsequently added to each of the TMCs to reach MCsoil,target. To account for losses in MCsoil resulting from fungal activity and evaporation, rewetting to MCsoil,target occurred once per week throughout the 24-week incubation period:

$$\mathbf{m}\_{\text{water}} = \left(\frac{\mathbf{M}\mathbf{C}\_{\text{soil,target}} - \mathbf{M}\mathbf{C}\_{\text{soil,current}}}{100}\right) \times \mathbf{m}\_{\text{total,dry}}\tag{5}$$

where:


#### 2.2.5. Preparation and Exposure of Wood Specimens

As already mentioned, some aspects of this TMC test against soft-rot wood decay deviated from the standard CEN/TS 15083-2:2005 [28]. One deviation was the inoculum aggressiveness of the soil material used, the other deviation, which also influenced the decision to use a more aggressive soil was the specimen dimension. Plywood boards of birch with nine veneer layers were prepared. The final height (thickness) of the 9-layer plywood boards amounted to 11.5 mm. Thereafter, individual specimens were prepared from the larger plywood boards to final dimensions of 10 <sup>×</sup> 11.5 <sup>×</sup> 100 mm<sup>3</sup> .

Before specimens were prepared from the larger plywood boards, the boards were conditioned to wood moisture content (MCwood) of 12 ± 2%. MCwood was tested using a 2-pronged electrical resistance moisture content measuring device. Specimens were then prepared from strips of the boards, with a cross-section of 10 ± 0.1 mm × 11.5 ± 0.1 mm (board thickness). Transverse cuts of the cross-section delivered sharp edges and a finesawn finish to the end-grain surface, with a final specimen length of 100 ± 1 mm. All specimens were free from obvious defects such as cracks, decay and discolouration.

After specimen preparation, all specimens were oven-dried at 103 ◦C for 24 h and weighed for oven-dry mass to the nearest 0.001 g. Prior to soil exposure, all specimens were again conditioned to MCwood of 12 ± 2% (confirmed by Equation (6) and buried 4/5 of their length into the soil substrate with 38 specimens per TMC box. For control (plywood, birch and beech solid wood) 30 replicate specimens were used with three specimen removal intervals (16, 20, 24 weeks). For each PF-treated plywood type 20 replicate specimens were used, with two specimen removal intervals (16 and 24 weeks). After soil exposure, specimens were removed, cleaned of remaining soil and again oven-dried at 103 ◦C for 24 h. Specimens were then weighed again to the nearest 0.001 g with oven-dry wood mass loss (MLwood) calculated according to Equation (7) below. Mean MLwood and standard deviation of mean MLwood was calculated according to Equation (8) and Equation (9) below:

$$\text{MC}\_{\text{wood}} = \left(\frac{\text{m}\_{\text{3}} - \text{m}\_{\text{2}}}{\text{m}\_{\text{2}}}\right) \times 100\tag{6}$$


• m<sup>2</sup> is the wood specimen's oven-dry mass after TMC exposure, [g]. Oven-dry mass loss (MLwood) of wood was calculated according to Equation (7) below:

$$\text{ML}\_{\text{wood}} = \left(\frac{\text{m}\_1 - \text{m}\_2}{\text{m}\_1}\right) \times 100\tag{7}$$

where:


$$\text{mean ML}\_{\text{wood}} = \frac{1}{\mathbf{n}} \sum\_{i=1}^{\mathbf{n}} \mathbf{x}\_{i} \tag{8}$$

where:


Standard deviation of mean MLwood was calculated according to Equation (9) below.

$$s = \sqrt{\frac{\sum\_{i=1}^{n} \left(\mathbf{x}\_{i} - \bar{\mathbf{x}}\right)}{n-1}} \tag{9}$$

where:


#### 2.2.6. Calculation of x-Value towards Provisional Durability Rating

According to CEN/TS 15083-2:2005 [28], the percentage oven-dry mass loss of the tested wood specimens is used to determine the resistance of hardwood test timbers to wood decay by soft-rotting fungi. The calculation of the x value based on the median oven-dry mass loss of the treated test specimens and the untreated reference specimens is used to define a provisional durability class (DC) rating. However, DC ratings do not equal use class ratings (like those covered in EN 335:2013 [35]. Use classes, and the combination of use classes and DC to assess wood suitability for a particular application are addressed in EN 460:1994 [36] and prEN 460:2019 [37], respectively. Equation (10) below was used to calculate the x value:

$$\mathbf{x} = \frac{\text{median ML}\_{\text{wood}} \text{ of the various treated test specimen group}}{\text{median ML}\_{\text{wood}} \text{ of reference test species}} \tag{10}$$


#### **3. Results and Discussion** For all PF treated specimens, the yellowness (Δb) parameter increased similarly to between 11 and 13 units after 360 h, while untreated plywood became less yellow after

**3. Results and Discussion**

*3.1. Artificial Weathering (UV Only)*

#### *3.1. Artificial Weathering (UV Only)* UV weathering reaching 9 units after 360 h of exposure (Figure 1c).

*Polymers* **2021**, *13*, x FOR PEER REVIEW 10 of 19

had the highest changes in redness parameter (Figure 1 b).

There is a demand among end users for new plywood products with predictable, long-term aesthetic properties. Plywood products with improved colour stability naturally have an advantage over competitors. In order to evaluate how UV radiation affects the PF resin treated plywood developed in this study, colour parameters were evaluated at different exposure intervals. The results presented in Figure 1 show the colour parameters of PF treated and untreated birch plywood after UV weathering of 360 h. Both the untreated controls and the plywood treated with PF resin became darker after UV weathering (decreasing ∆L\*), and this effect was more pronounced for plywood treated with commercial resin D (M<sup>w</sup> = 703 g/mol) at all tested concentrations (Figure 1a). This is also clearly seen in the specimen photos after UV weathering (Figure 4: UV only). The total colour change (Figure 1d: ΔEab) for all PF treated plywood specimens was between 12 and 18 units after 360 h of exposure. Resins A, B and C (M<sup>w</sup> = 292, 528 and 884 g/mol) had lower ΔEab values (12–13 units) compared to commercial resin D (M<sup>w</sup> = 703 g/mol), at all tested concentrations (15–18 units). Resin D-treated plywood did not show major differences in colour change between different concentrations, D15 showing the highest colour difference. For all PF-treated specimens, colour changes under the influence of UV light occured most rapidly in the first 24 h, after which the rate of change slowed down considerably. For untreated plywood, the change in colour parameters reached a maximum during the first 48 h and then decreased slightly further during the testing period.

There is a demand among end users for new plywood products with predictable, long-term aesthetic properties. Plywood products with improved colour stability naturally have an advantage over competitors. In order to evaluate how UV radiation affects the PF resin treated plywood developed in this study, colour parameters were evaluated at different exposure intervals. The results presented in Figure 1 show the colour parameters of PF treated and untreated birch plywood after UV weathering of 360 h. Both the untreated controls and the plywood treated with PF resin became darker after UV weathering (decreasing ∆L\*), and this effect was more pronounced for plywood treated with commercial resin D (M<sup>w</sup> = 703 g/mol) at all tested concentrations (Figure 1a). This is also

Untreated and PF resin treated plywood became redder during UV weathering (increasing Δa). The change in red colour parameter was minimal for resin B (M<sup>w</sup> = 528 g/mol) while resin A and B (M<sup>w</sup> = 292 and 884 g/mol) had similar changes compared to the untreated plywood. Resin D (M<sup>w</sup> = 703 g/mol) treated plywood at all tested concentrations

clearly seen in the specimen photos after UV weathering (Figure 4: UV only).

**Figure 1.** Changes in colour of untreated (UNT) and PF resin treated plywood after exposure to UV light during 360 h in QUV camera: (**a**) Changes in the CIE parameter ΔL\* (lightness); (**b**) Changes in the CIE parameter Δa (red-green); (**c**) Changes in the CIE parameter Δb (yellow-blue); (**d**) colour difference parameter ΔEab. **Figure 1.** Changes in colour of untreated (UNT) and PF resin treated plywood after exposure to UV light during 360 h in QUV camera: (**a**) Changes in the CIE parameter ∆L\* (lightness); (**b**) Changes in the CIE parameter ∆a (red-green); (**c**) Changes in the CIE parameter ∆b (yellow-blue); (**d**) colour difference parameter ∆Eab.

*3.2. Artificial Weathering (UV + Water Spray)* In order to recreate so called "real-life" conditions, artificial weathering using a combination of UV radiation and moisture spraying was used. Moisture spraying recreated the effect of rain, mist and dew. Results presented in Figure 2 show colour parameters of PF treated and untreated birch plywood after UV weathering and water spraying, after a Untreated and PF resin treated plywood became redder during UV weathering (increasing ∆a). The change in red colour parameter was minimal for resin B (M<sup>w</sup> = 528 g/mol) while resin A and B (M<sup>w</sup> = 292 and 884 g/mol) had similar changes compared to the untreated plywood. Resin D (M<sup>w</sup> = 703 g/mol) treated plywood at all tested concentrations had the highest changes in redness parameter (Figure 1b).

total test time of 1000 h. After 100 h of UV weathering and water spraying, plywood treated with PF resin as well as untreated plywood controls became darker (L\* decreased) compared to unweath-For all PF treated specimens, the yellowness (∆b) parameter increased similarly to between 11 and 13 units after 360 h, while untreated plywood became less yellow after UV weathering reaching 9 units after 360 h of exposure (Figure 1c).

ered reference specimens. For PF resin-treated specimens, lightness decreased by 15–27 units, but for untreated plywood by only 6 units (Figure 2a). For untreated plywood after

(b\*) by 10 units (Figure 2b,c). For plywood treated with resins A and B (M<sup>w</sup> = 292, 528 g/mol), the redness and yellowness parameters were affected negligibly, while for resin C (M<sup>w</sup> = 884 g/mol) only yellowness decreased. For plywood treated with resin D (M<sup>w</sup> = 703 g/mol), redness increased by 2–6 units at all concentrations, while yellowness was only affected for 10 and 15% solid content concentration treatments (b\* decreased by 5–6 units). The smallest changes in colour (ΔEab) after UV + water spray were observed for untreated plywood (13 units), for resins A, B and C, the changes reached 17–19 units, but the largest changes (23–28 units) were for resin D treated specimens at all tested concentrations (Figure 2d). A similar trend was observed for total colour change after only UV weathering. Visual assessment of the specimens showed that the untreated plywood turned gray with surface cracking after 1000 h of UV and water spraying. All PF resin treated specimens acquired an uneven brownish colour with some shades of grey that differed considerably from the original colour of the specimens prior to weathering (Figure 4: UV + water spray). As a result of UV light and water spraying, PF resin that was not fixed within the wood cell wall leached to the plywood surface, causing uneven discolouration patterns during simulated weathering. Kielmann and Mai [15] have tested PF treated (25% (w/w) aqueous solution) beech (*Fagus sylvatica* L.) wood boards with and without coatings. After UV and water spray PF treated beech surface became darker brown, however the total colour difference (ΔEab) for PF treated wood was similar to untreated wood reaching 25–

Our data after artificial weathering with only UV light and then with UV and water spraying contradicted general information available in literature that suggests treatment with PF resin significantly improves the colour stability of the wood material [9,15–18]. Data obtained in this study shows that the total colour difference of PF treated specimens is similar (in case of only UV weathering) or even greater than that of untreated plywood.

30 units.

The total colour change (Figure 1d: ∆Eab) for all PF treated plywood specimens was between 12 and 18 units after 360 h of exposure. Resins A, B and C (M<sup>w</sup> = 292, 528 and 884 g/mol) had lower ∆Eab values (12–13 units) compared to commercial resin D (M<sup>w</sup> = 703 g/mol), at all tested concentrations (15–18 units). Resin D-treated plywood did not show major differences in colour change between different concentrations, D15 showing the highest colour difference. For all PF-treated specimens, colour changes under the influence of UV light occured most rapidly in the first 24 h, after which the rate of change slowed down considerably. For untreated plywood, the change in colour parameters reached a maximum during the first 48 h and then decreased slightly further during the testing period.

#### *3.2. Artificial Weathering (UV + Water Spray)*

In order to recreate so called "real-life" conditions, artificial weathering using a combination of UV radiation and moisture spraying was used. Moisture spraying recreated the effect of rain, mist and dew. Results presented in Figure 2 show colour parameters of PF treated and untreated birch plywood after UV weathering and water spraying, after a total test time of 1000 h. *Polymers* **2021**, *13*, x FOR PEER REVIEW 12 of 19

**Figure 2.** Changes in colour of untreated (UNT) and PF resin treated plywood after exposure to UV light and water spray after 1000 h in QUV camera: (**a**) Changes in the CIE parameter L\* (lightness); (**b**) Changes in the CIE parameter a\* (redgreen); (**c**) Changes in the CIE parameter b\* (yellow-blue); (**d**) Colour difference parameter ΔEab. *3.3. Weathering under Outdoor Conditions* Real, outdoor conditions test the ability of a material to resist weathering and discol-**Figure 2.** Changes in colour of untreated (UNT) and PF resin treated plywood after exposure to UV light and water spray after 1000 h in QUV camera: (**a**) Changes in the CIE parameter L\* (lightness); (**b**) Changes in the CIE parameter a\* (red-green); (**c**) Changes in the CIE parameter b\* (yellow-blue); (**d**) Colour difference parameter ∆Eab.

ouration processes most accurately. Therefore, untreated and PF resin treated plywood was also tested outdoors. The results presented in Figure 3 show colour parameters of PF resin treated and untreated birch plywood after 3 months of weathering in real, outdoor conditions. Here, both untreated and PF resin-treated plywood became darker (L\* decreased) after weathering. For PF-treated specimens, lightness decreased by 5–17 units, while for untreated plywood, lightness decreased by 23 units (Figure 3a). For untreated plywood after outdoor exposure, the redness parameter (a\*) decreased by 4 units and yellowness (b\*) by 13 units, with an overall grey discolouration. For plywood specimens treated with PF resins A, B and C (M<sup>w</sup> = 292, 528 and 884 g/mol), redness decreased by 3– 5 units and yellowness by 5–6 units. For all specimens treated with resin D (M<sup>w</sup> = 703 g/mol), the opposite tendency was observed and redness increase was 1–3 units while After 100 h of UV weathering and water spraying, plywood treated with PF resin as well as untreated plywood controls became darker (L\* decreased) compared to unweathered reference specimens. For PF resin-treated specimens, lightness decreased by 15–27 units, but for untreated plywood by only 6 units (Figure 2a). For untreated plywood after UV and water spraying, the redness parameter (a\*) decreased by 3 units and yellowness (b\*) by 10 units (Figure 2b,c). For plywood treated with resins A and B (M<sup>w</sup> = 292, 528 g/mol), the redness and yellowness parameters were affected negligibly, while for resin C (M<sup>w</sup> = 884 g/mol) only yellowness decreased. For plywood treated with resin D(M<sup>w</sup> = 703 g/mol), redness increased by 2–6 units at all concentrations, while yellowness

yellowness 2–5 units (Figure 3b,c). As a result, the largest changes in colour (ΔEab) after

the colour changes reached approximately 10 units, but for specimens treated with resin D at all concentrations, the changes reached 12–18 units (Figure 3d). Comparable results were obtained by Evans et al. [17], who exposed PF treated radiata pine veneers in outdoor weathering for 2000 h. They concluded that higher resin loading in wood causes higher total colour changes after weathering. Only 10% of PF solution treated specimens showed lower total colour difference (ΔEab) compared to untreated wood while 20 and 30% treat-

ments had similar values.

was only affected for 10 and 15% solid content concentration treatments (b\* decreased by 5–6 units). The smallest changes in colour (∆Eab) after UV + water spray were observed for untreated plywood (13 units), for resins A, B and C, the changes reached 17–19 units, but the largest changes (23–28 units) were for resin D treated specimens at all tested concentrations (Figure 2d). A similar trend was observed for total colour change after only UV weathering.

Visual assessment of the specimens showed that the untreated plywood turned gray with surface cracking after 1000 h of UV and water spraying. All PF resin treated specimens acquired an uneven brownish colour with some shades of grey that differed considerably from the original colour of the specimens prior to weathering (Figure 4: UV + water spray). As a result of UV light and water spraying, PF resin that was not fixed within the wood cell wall leached to the plywood surface, causing uneven discolouration patterns during simulated weathering. Kielmann and Mai [15] have tested PF treated (25% (*w*/*w*) aqueous solution) beech (*Fagus sylvatica* L.) wood boards with and without coatings. After UV and water spray PF treated beech surface became darker brown, however the total colour difference (∆Eab) for PF treated wood was similar to untreated wood reaching 25–30 units.

Our data after artificial weathering with only UV light and then with UV and water spraying contradicted general information available in literature that suggests treatment with PF resin significantly improves the colour stability of the wood material [9,15–18]. Data obtained in this study shows that the total colour difference of PF treated specimens is similar (in case of only UV weathering) or even greater than that of untreated plywood.

## *3.3. Weathering under Outdoor Conditions*

Real, outdoor conditions test the ability of a material to resist weathering and discolouration processes most accurately. Therefore, untreated and PF resin treated plywood was also tested outdoors. The results presented in Figure 3 show colour parameters of PF resin treated and untreated birch plywood after 3 months of weathering in real, outdoor conditions. Here, both untreated and PF resin-treated plywood became darker (L\* decreased) after weathering. For PF-treated specimens, lightness decreased by 5–17 units, while for untreated plywood, lightness decreased by 23 units (Figure 3a). For untreated plywood after outdoor exposure, the redness parameter (a\*) decreased by 4 units and yellowness (b\*) by 13 units, with an overall grey discolouration. For plywood specimens treated with PF resins A, B and C (M<sup>w</sup> = 292, 528 and 884 g/mol), redness decreased by 3–5 units and yellowness by 5–6 units. For all specimens treated with resin D (M<sup>w</sup> = 703 g/mol), the opposite tendency was observed and redness increase was 1–3 units while yellowness 2–5 units (Figure 3b,c). As a result, the largest changes in colour (∆Eab) after outdoor weathering were found for untreated plywood (26 units). For resins A, B and C, the colour changes reached approximately 10 units, but for specimens treated with resin D at all concentrations, the changes reached 12–18 units (Figure 3d). Comparable results were obtained by Evans et al. [17], who exposed PF treated radiata pine veneers in outdoor weathering for 2000 h. They concluded that higher resin loading in wood causes higher total colour changes after weathering. Only 10% of PF solution treated specimens showed lower total colour difference (∆Eab) compared to untreated wood while 20 and 30% treatments had similar values.

Under outdoor weathering conditions, PF resin-treated plywood specimens showed considerably better colour stability than untreated plywood compared to artificial conditions. This suggests that artificial weathering data do not reflect the actual properties of the material in real-use conditions.

After three months of outdoor exposure, untreated plywood specimens turned grey with surface cracking as well as mould and blue stain fungal growth (Figure 4: Outdoor weathering). Specimens treated with PF resins A, B and C also show cracks, mould and blue stain on the surface after outdoor exposure. However, their intensity was lower and the total colour change was less pronounced. Resin D treated specimens (all solid content

concentrations) changed colour the most. Mould and blue stain fungal growth developed within surface cracks. the total colour change was less pronounced. Resin D treated specimens (all solid content concentrations) changed colour the most. Mould and blue stain fungal growth developed

Under outdoor weathering conditions, PF resin-treated plywood specimens showed considerably better colour stability than untreated plywood compared to artificial conditions. This suggests that artificial weathering data do not reflect the actual properties of

After three months of outdoor exposure, untreated plywood specimens turned grey with surface cracking as well as mould and blue stain fungal growth (Figure 4: Outdoor weathering). Specimens treated with PF resins A, B and C also show cracks, mould and blue stain on the surface after outdoor exposure. However, their intensity was lower and

*Polymers* **2021**, *13*, x FOR PEER REVIEW 13 of 19

the material in real-use conditions.

within surface cracks.

**Figure 3.** Changes in colour of untreated (UNT) and PF resin treated plywood after exposure to outdoor conditions for 3 months: (**a**) Changes in the CIE parameter L\* (lightness); (**b**) Changes in the CIE parameter a\* (red-green); (**c**) Changes in the CIE parameter b\* (yellow-blue); (**d**) Colour difference parameter ΔEab. **Figure 3.** Changes in colour of untreated (UNT) and PF resin treated plywood after exposure to outdoor conditions for 3 months: (**a**) Changes in the CIE parameter L\* (lightness); (**b**) Changes in the CIE parameter a\* (red-green); (**c**) Changes in the CIE parameter b\* (yellow-blue); (**d**) Colour difference parameter ∆Eab. *Polymers* **2021**, *13*, x FOR PEER REVIEW 14 of 19

**Figure 4.** Untreated and PF resin treated plywood photo fixation after weathering with UV only, UV + water spray and in outdoor conditions for 3 months. **Figure 4.** Untreated and PF resin treated plywood photo fixation after weathering with UV only, UV + water spray and in outdoor conditions for 3 months.

In outdoor aboveground conditions, wood materials are affected not only by abiotic natural weathering through UV radiation and moisture, but also though biotic damage

All PF-treated specimens reached a rating of 3–4 (medium growth 26–50% to severe growth >50%) after 3 months. However, the best results were shown by specimens impregnated with resins B and C (M<sup>w</sup> = 528 and 884 g/mol), for which the colour marks rated

Specimens treated with resin A, B and C (M<sup>w</sup> = 292, 528 and 884 g/mol) with coated edges showed greater mould and blue stain growth after 2 and 3 months. Such a tendency was not observed with the specimens treated with resin D at all tested concentrations. In our opinion, this is due to the fact that the specimens with coated edges after wetting under the influence of rain dried up much slower, thus creating a more favourable environment for the development of mould and blue stain. However, the moisture content in specimens during outdoor test was not measured and it is difficult to confirm this assertion. Our data testify that resins A, B and C are relatively well penetrated and fixed in the birch wood cell wall and WPG after leaching decreases by 1.5–2.0% (unpublished results). Maybe this might be attributed to different unfixed PF resin part leaching during interac-

can also deteriorate the surface appearance of wood. Table 4 below shows the growth rate of mould and blue stain fungi on PF resin treated plywood and untreated reference plywood throughout the 3-month outdoor exposure period. After one month, no fungal growth was observed on tested specimens. The first signs of fungal growth on PF treated plywood appeared after 2 months (rated 1–3) of outdoor exposure. After 2 months more than 50% of the untreated plywood surfaces were colonised by fungi (rated 4). After 3 months all PF resin D (M<sup>w</sup> = 703 g/mol) treated specimens reached rating 4 (severe growth >50%). Higher resin loading for 15 and 20% treated specimens seemed to show no protec-

*3.4. Growth of Wood Discolouring Fungi on PF Treated Plywood*

tive effect against mould and blue stain on the plywood surface.

3–3.7.

tion with water.

## *3.4. Growth of Wood Discolouring Fungi on PF Treated Plywood*

In outdoor aboveground conditions, wood materials are affected not only by abiotic natural weathering through UV radiation and moisture, but also though biotic damage caused by fungi and mould growth. Other anthropogenic sources, such as air pollution, can also deteriorate the surface appearance of wood. Table 4 below shows the growth rate of mould and blue stain fungi on PF resin treated plywood and untreated reference plywood throughout the 3-month outdoor exposure period. After one month, no fungal growth was observed on tested specimens. The first signs of fungal growth on PF treated plywood appeared after 2 months (rated 1–3) of outdoor exposure. After 2 months more than 50% of the untreated plywood surfaces were colonised by fungi (rated 4). After 3 months all PF resin D (M<sup>w</sup> = 703 g/mol) treated specimens reached rating 4 (severe growth >50%). Higher resin loading for 15 and 20% treated specimens seemed to show no protective effect against mould and blue stain on the plywood surface.

**Table 4.** Mould and blue stain growth marks on PF resin treated plywood in outdoor, aboveground conditions tested according to EN 152:2011 [33].


All PF-treated specimens reached a rating of 3–4 (medium growth 26–50% to severe growth >50%) after 3 months. However, the best results were shown by specimens impregnated with resins B and C (M<sup>w</sup> = 528 and 884 g/mol), for which the colour marks rated 3–3.7.

Specimens treated with resin A, B and C (M<sup>w</sup> = 292, 528 and 884 g/mol) with coated edges showed greater mould and blue stain growth after 2 and 3 months. Such a tendency was not observed with the specimens treated with resin D at all tested concentrations. In our opinion, this is due to the fact that the specimens with coated edges after wetting under the influence of rain dried up much slower, thus creating a more favourable environment for the development of mould and blue stain. However, the moisture content in specimens during outdoor test was not measured and it is difficult to confirm this assertion. Our data testify that resins A, B and C are relatively well penetrated and fixed in the birch wood cell wall and WPG after leaching decreases by 1.5–2.0% (unpublished results). Maybe this might be attributed to different unfixed PF resin part leaching during interaction with water.

PF resin-treated plywood did not considerably lower growth rate of mould and blue stain compared to untreated plywood. However, we observed that mould and blue stain on the surface of PF resin treated plywood show a weaker colouration, resulting in a lower total colour change compared to untreated plywood (Figure 4 above). Surprisingly, treatment with 15 and 20% resin D solutions did not provide better surface protection for plywood against mould and blue stain. This was most likely due to the number of surface cracks that developed during the test period. All PF resin D treated specimens were more cracked compared to PF resin A, B and C treated specimens.

#### *3.5. Resistance Against Soft-Rot in Unsterile Soil-Bed Test*

Figure 5 below shows the mean MLwood and standard deviation of the untreated plywood, birch and beech solid wood and PF-treated plywood specimens. For untreated birch plywood, beech solid wood and birch solid wood specimens, measurements of MLwood were performed after 16, 20 and 24 weeks of incubation. Due to limitations in the

number of treated test specimens, PF resin treated specimens were only removed after 16 and 24 weeks of incubation. For all control specimens, MLwood increased almost as a linear function within the test period. Among the control specimens, the highest MLwood range attained was for birch solid wood (26–38%), following with beech solid wood (22–34%), while the lowest was birch plywood (20–30%). *Polymers* **2021**, *13*, x FOR PEER REVIEW 16 of 19

**Figure 5.** Oven-dry mass loss (MLwood) [%] and standard deviation [%] of PF resin impregnated plywood material as well as commercially produced untreated plywood and solid wood after 16– 24 weeks of incubation in unsterile soil. **Figure 5.** Oven-dry mass loss (MLwood) [%] and standard deviation [%] of PF resin impregnated plywood material as well as commercially produced untreated plywood and solid wood after 16–24 weeks of incubation in unsterile soil.

x-Value towards Provisional Durability Rating After 16 weeks of incubation, sufficient mean MLwood (20%) of 10 reference specimens was reached for all reference specimens, exception for commercially sourced, untreated birch plywood. Subsequent incubation periods of 20 and 24 weeks also showed sufficient MLwood for all untreated reference specimens. Median MLwood was used to calculate an x value towards a provisional durability rating for the developed plywood material in accordance with CEN/TS 15083-2:2005 [28]. A provisional durability class (DC) rating was calculated after 16 and 24 weeks for all PF resin treated specimens against the various untreated reference control specimens (birch solid wood, beech solid wood and birch plywood). All PF treated plywood specimens could be classified as durable (DC 2) or moderately durable (DC 3). After treatment with 10% solid content concentration solutions, the best resistance to soft-rot was shown by resin A (M<sup>w</sup> = 292 g/mol) treated specimens with DC 2 against birch solid wood after 16 and 24 weeks (lowest x value: Table 5). Plywood specimens treated with resins B and C (Mw = 528 and 884 g/mol) showed DC 3 The PF resin-treated plywood specimens showed a considerably reduced MLwood compared to untreated references. The lowest MLwood attained was for specimens treated with 10% solid content concentration of PF resins A and D (M<sup>w</sup> = 292 and 703 g/mol), showing MLwood of 4.8–7.6% and 5.3–7.8%, respectively. Specimens treated with resins B and C showed MLwood of 6.0–9.4% and 7.6–11.4%, respectively. For laboratory prepared resin A, B, and C (M<sup>w</sup> = 292,528 and 884 g/mol), a clear trend could be identified—use of higher molecular mass resins for veneer treatment caused higher MLwood of plywood after exposure to unsterile soil. Specimens treated with commercially acquired resin D (M<sup>w</sup> = 703 g/mol) at 10% solid content concentration did not fit into this trend since it showed lower MLwood compared to PF resin B and C specimens treated at the same 10% solid content concentration. However, the assumption that an increase of resin loading in wood improves resistance against soft-rot was confirmed. The best results were shown by specimens treated with 15 and 20% solid content concentration solutions of PF resin D, with MLwood of 3.7–5.1% and 3.4–4.1%, respectively.

#### against all controls at 16 and 24 weeks. Resin D (Mw = 703 g/mol) showed DC 2 relative x-Value towards Provisional Durability Rating

to birch solid wood after 24 test weeks. Solid content concentration increases of resin D at 15 and 20% improved the resistance to soft-rotting fungi with specimens showing DC 2 against all reference materials after 16 and 24 weeks (Table 6). **Table 5.** x values calculated according to CEN/TS 15083-2 [28] for the developed plywood material against various untreated reference wood materials for 16 and 24 weeks of incubation in unsterile soil. **Wood Material Reference Material and Exposure Time [Weeks] Birch Solid Beech Solid Untreated Plywood Birch Solid Beech Solid Untreated Plywood 16 16 16 24 24 24** After 16 weeks of incubation, sufficient mean MLwood (20%) of 10 reference specimens was reached for all reference specimens, exception for commercially sourced, untreated birch plywood. Subsequent incubation periods of 20 and 24 weeks also showed sufficient MLwood for all untreated reference specimens. Median MLwood was used to calculate an x value towards a provisional durability rating for the developed plywood material in accordance with CEN/TS 15083-2:2005 [28]. A provisional durability class (DC) rating was calculated after 16 and 24 weeks for all PF resin treated specimens against the various untreated reference control specimens (birch solid wood, beech solid wood and birch plywood). All PF treated plywood specimens could be classified as durable (DC 2) or moderately durable (DC 3). After treatment with 10% solid content concentration solutions, the best resistance to soft-rot was shown by resin A (M<sup>w</sup> = 292 g/mol) treated specimens

C10 0.31 0.36 0.40 0.29 0.32 0.38 D10 0.22 0.25 0.27 0.20 0.22 0.26 D15 0.15 0.17 0.18 0.13 0.14 0.17 D20 0.14 0.15 0.17 0.11 0.12 0.14 with DC 2 against birch solid wood after 16 and 24 weeks (lowest x value: Table 5). Plywood specimens treated with resins B and C (Mw = 528 and 884 g/mol) showed DC 3 against all controls at 16 and 24 weeks. Resin D (Mw = 703 g/mol) showed DC 2 relative to birch solid wood after 24 test weeks. Solid content concentration increases of resin D at 15 and 20% improved the resistance to soft-rotting fungi with specimens showing DC 2 against all reference materials after 16 and 24 weeks (Table 6).

**Table 5.** x values calculated according to CEN/TS 15083-2 [28] for the developed plywood material against various untreated reference wood materials for 16 and 24 weeks of incubation in unsterile soil.


**Table 6.** Durability class ratings according to CEN/TS 15083-2:2005 [28] after 16 and 24 weeks of incubation in unsterile soil.


#### **4. Conclusions**

In the Introduction we assumed that the weathering stability and resistance to biotic degradation agents of wood treated with low molecular weight PF resin would be improved, especially by using lower molecular weight PF resins, which in-turn increase resin loading in the wood cell wall.

Our results show that it is not possible to compare weathering processes that take place under artificial conditions with processes that take place under real, outdoor conditions. Even the combination of UV and water spraying under artificial conditions was not able to simulate similar material weathering processes as those achieved outdoors.

Birch veneers that were treated with laboratory synthesized PF resins A, B, and C (M<sup>w</sup> = 292, 528 and 884 g/mol) prior to plywood production behaved better in both simulated and outdoor weathering conditions compared to plywood veneers treated with commercially sourced PF resin D. The assumption that increasing the concentration of resin in wood improves its weathering stability was not confirmed. Furthermore, it was also not possible to determine a clear trend of how mean M<sup>w</sup> of a particular resin affects UV stability. Theoretically, resins with a higher molecular weight penetrate the wood cell wall less, so their concentration on the wood surface is potentially higher. Therefore, weathering stability of specimens treated with higher molecular weight resins could be expected to be better. However, our results do not support this theoretical consideration.

Resins A and D (M<sup>w</sup> = 292 and 703 g/mol) seemed to be the most suitable for protection against soft-rot in the adapted unsterile soil bed test carried out in this study. For plywood treated with laboratory synthesized PF resins A, B, and C (Mw = 292, 528 and 884 g/mol), results suggest that the use of higher M<sup>w</sup> resin increase ML wood after incubation in unsterile soil under various incubation periods. However, no clear correlation between M<sup>w</sup> and MLwood could be established when comparing all four resins used in this study-plywood veneers treated with resins A, B, C and D, at 10% solid content concentration. Specimens treated with resin D showed better resistance against soft-rot decay compared to resin B and C. It was confirmed that an increase of resin D loading in wood improves resistance against soft-rot.

**Author Contributions:** Conceptualization, J.G., V.B. and B.N.M.; Methodology, J.G., V.B. and B.N.M.; Formal Analysis, J.R. and H.M.; Investigation, J.G., V.B. and B.N.M.; Writing—Original Draft Preparation, J.G., V.B. and B.N.M.; Writing—Review and Editing, B.N.M., J.R. and H.M.; supervision, V.B. and J.R.; Project administration, J.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Post-doctoral research project No.1.1.1.2/VIAA/1/16/210, agreement No. 1.1.1.2/16/I/001.

**Acknowledgments:** Special thanks to Prefere Resins Germany GmbH Erkner and personally Elke Fliedner, Klaus Dück and Christopher Knie for providing PF resins and for help with the GPC analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
