2.1.1. PF Resin Synthesis

For the synthesis of resin A, B, and C, phenol was hydroxymethylated under alkaline reaction conditions, whereby the molar ratio of formaldehyde/phenol/sodium hydroxide was 2.0/1.0/0.2. During the synthesis of each resin, a measured amount of phenol and aqueous sodium hydroxide solution was weighed out in a 4-neck laboratory reactor (1 L) equipped with a thermometer, dropping funnel, reflux condenser and stirrer. Some ethanol was also added in order to maintain a homogeneous reaction. The 4-neck reactor was submerged in a thermostatic water-bath. As soon as the temperature in the flask reached the necessary synthesis temperature, the aqueous formaldehyde solution was added slowly via a drip over a 25–30 min period. The reaction temperature (65, 75, 85 ◦C) was kept constant during the entire reaction period (2 and 4 h). The resol synthesis was ended by cooling the reactor with cold running water and allowing the resol to cool down to 20 ± 3 ◦C. Resin D was provided by Prefere Resins Germany GmbH (Erkner, Germany). The different characteristic parameters of the synthesized PF resins are listed in Table 1.


**Table 1.** Characteristic parameters of PF resins used in the study.

#### 2.1.2. Resin Characterisation

The dynamic viscosity of liquid PF resins was determined by a Fungilab Viscolead Adv meter (Fungilab S.A., Barcelona, Spain) equipped with a suitable spindle. The non-volatiles content (solid content) was determined according to DIN EN ISO 3251:2019 [29]. The pH value was determined using a digital pH meter (GPH 114 Greisinger, Regenstauf, Germany) by inserting the pH meter electrode into the PF resins. The pH meter was calibrated with buffer solutions at pH 4.0 and 10.0 prior pH measurements. Free formaldehyde content was determined by the hydroxylamine hydrochloride method according to DIN EN ISO 9397:1997 [30].

For gel permeation chromatography (GPC) analysis, a 1260 Infinity system (degasser, isocratic pump, automatic liquid sampler, heatable column compartment, RID, MWD @ 280 nm, Agilent (Santa Clara, CA, USA) was used, where: column: 3 × PLgel 5 µ (50 Å, 100 Å, 1000 Å), 7.5 × 300 mm; solvent: tetrahydrofuran (THF); flow rate: 0.6 mL/min; flow rate marker: acetone; calibration: polystyrene standard.

Approximately 40 mg of resin was dissolved in 5 mL of THF. If the resin did not completely dissolve, it was sonicated with slow addition of H2SO<sup>4</sup> (5% in methanol) until neutral. If the resin was dissolved, but precipitate from additives (such as salts) remained, the mixture was filtered with a syringe filter.

## 2.1.3. Treatment of Veneer Material

Air-dried veneer sheets of silver birch wood (300 <sup>×</sup> <sup>300</sup> <sup>×</sup> 1.5 mm<sup>3</sup> and 400 <sup>×</sup> <sup>400</sup> <sup>×</sup> 1.5 mm<sup>3</sup> <sup>L</sup> <sup>×</sup> <sup>R</sup> <sup>×</sup> T) were prepared for impregnation. Oven-dry mass was determined after drying at 103 ± 2 ◦C for 24 h. Previous experiments suggested that treatment of birch wood with commercial PF resin solutions of 10% concentration delivered weight percentage gain (WPG) of 9–12%, which subsequently improved dimensional stability and allowed for a provisional durability class (DC) rating of 1 against decay fungi [31,32]. Veneers (300 <sup>×</sup> <sup>300</sup> <sup>×</sup> 1.5 mm<sup>3</sup> ) were conditioned to 5–6% moisture content and impregnated with 10% solid content concentration solutions of PF resins A, B, and C. Impregnation was carried in a 340-litre chamber produced by Wood Treatment Technology (WTT, Grindsted, Denmark). Veneers (n = 40) were placed in a tub filled with the resin solution. The veneers were prevented from floating using a mesh grid and heavy weight. Impregnation was carried out in two steps. The first, vacuum step (1 h, 0.1 bar of pressure), was used to ensure the free air was purged from the specimens. The chamber was then pressurized to ensure sufficient diffusion of the PF oligomers into the wood cell walls (1 h, 4 bars of pressure). The veneers were then removed from the impregnation chamber and measured for solution uptake. The remaining resin solution was drained from the tub and the veneers were positioned vertically to allow excess resin solution to drip off. After impregnation, veneers were oven dried to 4–6% moisture content, with moderate air circulation and air exchange for 72 h using incrementally rising temperature intervals from 30–50 ◦C. A subset of PF resin impregnated veneers from each resin impregnation treatment was measured for WPG. These veneers were cured at 140 ◦C for 1 h. The WPG was calculated to assess

the amount of PF resin in the veneers. The average WPG was calculated for each treatment according to Equation (1) below:

$$\text{WPG} = \frac{(\text{M}\_1 - \text{M}\_2)}{\text{M}\_2} \times 100 \tag{1}$$

where:


Commercial resin D was used to evaluate the behaviour of different loading of PF resin in veneers. Therefore, before being used for impregnation, the stock solution of the PF resins D was diluted with distilled water to 10, 15 and 20% solid content concentration. Veneers (400 <sup>×</sup> <sup>400</sup> <sup>×</sup> 1.5 mm<sup>3</sup> ) were conditioned to 5–6% moisture content and impregnated with PF resin solutions in a 1000-litre impregnation chamber at the University of Göttingen. The impregnation and drying parameters were set the same as for veneers of <sup>300</sup> <sup>×</sup> <sup>300</sup> <sup>×</sup> 1.5 mm<sup>3</sup> . The WPG was calculated to assess the amount of PF resin in the veneers. Veneer WPG after impregnation and curing for both veneer dimensions is shown in Table 2 below.

**Table 2.** Veneer WPG after treatment with PF resin solutions.


#### 2.1.4. Plywood Production

Standard PF adhesive (sourced from plywood industry partners) was applied to the veneer sheets in preparation for pressing into plywood (nine layers). PF adhesive viscosity was 380 mPas, solid content 44.5%, free formaldehyde content <1% and pH 12.6. A quantity of 150 g/m<sup>2</sup> was applied to one surface of eight of the nine plywood sheets constituting a 9-layer plywood board. After adhesive application, veneers were pre-dried at room temperature for 15 min (adhesive open time) before assembling nine individual veneer sheets in a crosswise, perpendicular fashion in preparation for pressing. Assembled sheets were then pressed in a hot press (Joos LAP 40, Gottfried Joos Maschinenfabrik GmbH & Co. KG, Pfalzgrafenweiler, Germany) at 140 ◦C and 1.5 N/mm<sup>2</sup> for 20 min (90 s/mm) to deliver a plywood board with thickness of approximately 11 mm. Thereafter specimens were prepared for artificial weathering, outdoor weathering and unsterile soil-bed tests with dimensions of 150 <sup>×</sup> <sup>70</sup> <sup>×</sup> 11 mm<sup>3</sup> , 110 <sup>×</sup> <sup>40</sup> <sup>×</sup> 11 mm<sup>3</sup> and 100 <sup>×</sup> <sup>10</sup> <sup>×</sup> 11 mm<sup>3</sup> , respectively.

#### 2.1.5. Artificial Weathering Tests

Artificial weathering tests were performed in a QUV accelerated weathering tester, (Q-Lab Europe, Ltd., Farnworth Bolton, England) equipped with UVA-340 type fluorescent lamps. Three plywood specimens (150 <sup>×</sup> <sup>70</sup> <sup>×</sup> 11 mm<sup>3</sup> ) were used for each resin treatment. The lamps provided a good simulation of sunlight in the short wavelength region; from 295 nm to 365 nm, with a peak emission at 340 nm. The UV radiation flux density at 340 nm was 0.89 W/m<sup>2</sup> and the chamber temperature throughout the test was kept constant at 60 ◦C. The intensity of the full UV spectrum's (290–400 nm) irradiation was 21.5 W/m<sup>2</sup> . In the study, two different artificial weathering tests were carried out. The first test involved only UV irradiation. This test was regularly suspended to measure the change in colour of the specimens. The total duration of the test was 360 h. The second artificial weathering test involved both UV irradiation and water spray. The test involved the following steps; 2.5 h of UV radiation at the same conditions as described earlier, followed by 30 min of water spray. In total, 60 cycles were preformed to reach an exposure time of 180 h from

which 150 h accounted for UV irradiation. Colour measurements after both weathering methods was performed.

#### 2.1.6. Surface Colour Measurements

Colour of the specimens was measured with a CM-2500d spectrophotometer (Konica Minolta, Ramsey, NJ 07446, USA) and expressed according to the CIELAB threedimensional colour system. On each of the specimen, five locations were randomly chosen and marked. For the marked locations, the colour was measured before and after the weathering tests as well as during the test after 2, 4, 8, 16, 24, 48, 120, 192, 264 and 360 h. The colour was measured to evaluate the discolouration caused by weathering. The total colour change ∆Eab was calculated according to the Equation (2) below. L\* is a lightness parameter, a\* is a chromaticity parameter which represents red-green coordinates and b\* is a chromaticity parameter which represents yellow-blue coordinates:

$$
\Delta \mathbf{Eab} = \sqrt{\left(\mathbf{L\_x^\*} - \mathbf{L\_o^\*}\right)^2 + \left(\mathbf{a\_x^\*} - \mathbf{a\_o^\*}\right)^2 + \left(\mathbf{b\_x^\*} - \mathbf{b\_o^\*}\right)^2} \tag{2}
$$

where:


#### 2.1.7. Outdoor Weathering and Fungal Tests

Six replicates of all PF resin treated plywood with dimensions of 110 <sup>×</sup> <sup>40</sup> <sup>×</sup> 11 mm<sup>3</sup> , along with untreated specimens, were used. For half of the specimens (3 from each impregnation treatment), the side edges were coated with urethane alkyd paint (brushed on application, three coats). According to EN 152:2011 [33], the samples were placed in an aluminium rack at 45◦ , one meter above the ground, facing the south direction with most severe weather conditions. Microorganisms were allowed to attack the specimens. The test site in the courtyard of Latvian State Institute of Wood Chemistry (Riga, Latvia) was free of vegetation, shade and extreme environmental conditions. The test lasted for 3 months, from 19 June to 21 September 2020 and weather data are listed in Table 3.

**Table 3.** Weather data for the outdoor, aboveground weathering test site for the 3-month test period obtained from ©weatheronline.co.uk.


Mould and blue stain growth was evaluated once per month by stereomicroscopy (M8, Leica, Wetzlar, Germany) and digital photography (2 MB) according to the rating scale 0–4: 0—clean, 0% attack; 1—trace, ≤5% growth; 2—slight, 6–25% growth; 3—medium, 26–50% growth; 4—severe, >50% growth. Colour change measurements were also performed once per month.

#### *2.2. Unsterile Soil-Bed Test: Resistance against Soft-Rot Wood Decay*

Terrestrial microcosms (TMCs) in accordance with CEN/TS 15083-2:2005 were utilised to test the resistance of the developed plywood material against soft-rot wood decay. The standard stipulates that a natural topsoil or a fertile loam-based horticultural soil substrate is used, with pH 6–8, and no additives. The soil should have a WHCsoil of 20–60%, MCsoil equal to 95%WHCsoil, and the test should be conducted in a dark, climate-controlled room set to a temperature of 27 ◦C and relative humidity of 65%.

#### 2.2.1. Soil Substrate

The soil substrate was a horticultural compost produced at the forest botanical garden at the University of Göttingen's North Campus. The compost comprised of fallen leaves and cuttings from grass and trees. Soil was passed through a sieve with nominal aperture size of 8.5 mm. WHCsoil was then determined according to the 'cylinder sand bath method' according to ISO 11268-2:2012 [34]. The test deviated from the standard in that no silica sand was added to the soil in order to reduce the soil's water-holding capacity. Silica sand acts to standardize and reduce the soil's inoculum potential to attain reproduceable wood decay results which serves as a provisional durability rating.
