*Article* **Weathering Stability and Durability of Birch Plywood Modified with Different Molecular Weight Phenol-Formaldehyde Oligomers**

**Juris Grinins 1,\* , Vladimirs Biziks <sup>2</sup> , Brendan Nicholas Marais <sup>2</sup> , Janis Rizikovs <sup>1</sup> and Holger Militz <sup>2</sup>**


**Abstract:** This study investigated the effect of phenol-formaldehyde (PF) resin treatment on the weathering stability and biological durability of birch plywood. Silver birch (*Betula pendula*) veneers were vacuum-pressure impregnated with four different PF resins with average molecular weights (Mw) of 292 (resin A), 528 (resin B), 703 (resin C), and 884 g/mol (resin D). The aging properties of PF resin modified birch plywood were analyzed using artificial weathering with ultraviolet (UV) light, UV and water spray, and weathering under outdoor conditions. The same combinations of PF-treated plywood specimens were then tested in soil-bed tests to determine their resistance against soft-rot wood decay. It was not possible to compare weathering processes under artificial conditions to processes under outdoor conditions. However, the weathering stability of birch plywood treated with PF resins A, B, and C, scored better than plywood treated with commercial resin D (regardless of solid content concentration [%]). Results from unsterile soil bed tests showed improvements in resistance to soft-rot wood decay compared to untreated plywood and solid wood. Mass loss [%] was lowest for birch plywood specimens treated with resin of highest solid content concentration (resin D, 20%). Provisional durability ratings delivered durability class (DC) ratings of 2–3, considerably improved over untreated solid wood and untreated birch plywood (DC 5).

**Keywords:** birch plywood; molecular weight; phenol-formaldehyde resin; soft-rot; weathering stability

#### **1. Introduction**

Wood is increasingly being used for outdoor applications, yet it is still limited by complex wood-water-ultraviolet (UV) light interactions. During weathering, wood is cycling through wet and dry states, thus inducing repeated swelling and shrinkage and generating tension stresses. Wood responds to these wetting-drying stresses through creeping and surface cracking. Once the stresses exceed the fracture strength of wood, it has a tendency to develop longer and deeper cracks at later stages [1]. These cyclic changes in moisture content and dimensions are most pronounced at the wood surface, which is directly exposed to rain, humidity and sunlight (UV and visible light). Moreover, UV light exposure intensifies crack formation because photodegradation weakens the wood surface and degrades its microstructure [2]. The presence of cracks and other defects is thus a major drawback and may reduce its service life, market value and mechanical strength. These defects also lead to increased water uptake, thus producing optimal moisture conditions for wood-decay fungi to attack [3].

Silver birch (*Betula pendula*) plywood is widely used in construction, interior and exterior decoration, vehicle construction, sports equipment, furniture, packaging materials, and toy production. However, its application in outdoor, high humidity conditions is limited due to poor biological durability (durability class 5 according to EN 350-1:2016 [4]).

**Citation:** Grinins, J.; Biziks, V.; Marais, B.N.; Rizikovs, J.; Militz, H. Weathering Stability and Durability of Birch Plywood Modified with Different Molecular Weight Phenol-Formaldehyde Oligomers. *Polymers* **2021**, *13*, 175. https://doi.org/10.3390/ polym13020175

Received: 11 December 2020 Accepted: 30 December 2020 Published: 6 January 2021

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The effect of wood degrading fungi, humidity and UV light, essentially impair the technical properties of plywood through weakening of the bonded veneers, which in-turn weaken mechanical strength and deteriorate the surface finish. Moisture easily penetrates into veneer layers, which causes swelling and characteristic waves on the plywood surface. Water uptake potential can be decreased by covering the plywood with a hydrophobic (water repelling) laminate. Melamine and phenolic resins are mostly used for the production of resin laminates, which are subsequently hot-pressed onto one or both sides of the finished plywood board surfaces. Coating plywood protects the top veneer layer from direct contact with water and UV light, but when the upper laminate coating is damaged, the inner veneer layers can swell, provoking noticeable surface failure and promoting attack by biodegradation agents. Most wood species used for plywood production have poor resistance to swelling, biodegradation and UV weathering under outdoor and high humidity conditions. Therefore, when using plywood in conditions with high humidity, where regular wetting is possible, it is necessary not only to cover it from the outside with a hydrophobic laminate, but also to treat the single veneers constituting the entire plywood board. Prolonging the service life of wood and wood-based products results in a positive effect on greenhouse gas emissions by storing biomass carbon for longer periods [5].

Wood modification alters the material properties to a greater extent than preservative treatment, and the magnitude of changes depends on the applied modification method [6]. Wood modification can simultaneously overcome several weaknesses of wood, such as poor dimensional stability, low decay resistance, high equilibrium moisture content, and aesthetical issues such as optical appearance can be diversified and enhanced. Wood impregnated with most thermosetting resins causes changes in colour [7]. Compared to untreated wood, acetylated wood [8,9], glutaraldehyde treated wood [10], DMDHEUand melamine- treated wood [7] exposed to accelerated or outdoor weathering develops fewer cracks because of its improved dimensional stability. Surface discoloration (graying) and crack development during longer exposure times is reduced, whereas in the case of thermal treatment, the rate of graying and crack development is the same or even faster than that of untreated wood [3,11]. In contrast, the phenol formaldehyde (PF) treated boards remained darker, ranging from light brown to dark brown. PF resin turns wood red-brown, due to differences in pH, because wood is acidic and resole PF resins are alkaline [12]. This change in optical appearance depends on both the resin type and the average molecular size of the PF resin oligomers used for treatment. PF resin acts as a UV absorber for the photostabilization of wood [13]. PF resin is also an antioxidant, which may also impact its ability to photostabilize wood [14]. Kielmann and Mai [15,16] have concluded that the surface of PF-treated wood without a coating has improved resistance against photodegradation compared to the surface of N-methylol melamine (NMM)-treated wood because PF inhibits lignin degradation. The resistance of wood treated with low molecular weight PF resin to weathering can be improved by increasing the concentration of PF resin and by combining it with a water soluble hindered amine light stabilizer [17]. PF resins could be modified with ferric chloride and a mixture of ferric sulphate and hydrolysable polyphenols to darken the colour of European beech wood (*Fagus sylvatica* L.) and enhance colour stability [18]. Although a proper improvement in dimensional stability and biological durability and a considerable reduction in water sorption are attained, the appearance of the treated wood still undergoes considerable changes during weathering. Therefore, over the past decade, the combined approach of coating chemically treated wood has become an increasing point of interest as a way of increasing wood service life.

As mentioned, outdoor wooden components are subjected to a variety of biotic and abiotic degradation factors. Additionally, wood used in soil contact is of particular risk due to the permanent to semi-permanent presence of moisture [19]. Important considerations for the successful proliferation of wood-decaying fungi include a carbon substrate, moisture, temperature, and oxygen [20]. Various wood decaying fungi; brown-, white-, and softrot fungi, can all be found on wood utilized in-ground, but these decay types can vary significantly, not only in frequency and spatial distribution, but also in combinations from

one site to the next [21,22], and with decay progress [23]. Soft-rot seems to be able to cope with high soil moisture content (MCsoil) better than brown- and white-rot fungi, and continues to remain active over a broader temperature range (Tsoil) compared to brownand white-rot fungi [21,24,25].

In the pursuit of new techniques for improved wood protection, a rapid assessment of the technique's effectiveness can be attained through laboratory tests. Such tests deliver results quickly and thus form the basis for the decision for further test steps. It is of the greatest interest that the predictions obtained from these tests can be transferred to practice (field tests) with a high degree of certainty. One possibility to better assess the risk factors that determine wood degradation when used in contact with soil is to test the wood in-field and in contact with soil. This test in an option when assessing the behavior of wood preservatives by DIN EN 252:2015 [26]. However, depending on the duration and characteristics of the vegetation periods, it may take several years before results from this type of test are available. Since this method requires a lot of time, the use of pure-culture basidiomycete tests such as CEN/TS 15083-1:2005 [27] and unsterile soil bed tests such as CEN/TS 15083-2:2005 [28] under controlled laboratory conditions are often employed. The results from these tests are designed to complement each other in combination with longer-term field tests using specimens of larger dimension. Newly developed wood preservative and modification techniques can therefore be tested to deliver preliminary durability ratings.

This study compared the color changes of a developed birch plywood modified with different PF resins after weathering under artificial conditions with UV light only, and UV light and water spraying, and under real outdoor conditions. Additionally, so called terrestrial microcosms (TMC) consisting of unsterile organic soil were used to test the resistance of the developed birch plywood against wood decay by soft-rot fungi. This study impregnated birch wood veneers with PF resin solutions of different solid content concentrations and PF oligomer sizes, to understand the effect on dimensional stability, weathering performance, and biological durability. Theoretically, the photostability and resistance to wood decaying fungi of the wood material treated with low molecular weight PF resins should be improved. Also, increasing the concentration of PF resin should improve the weathering and biological durability of the treated wooden material. Criteria for birch veneer treatment was based on minimum WPG requirements to achieve the maximum improvement in properties, therefore to limit PF resin load in the wood material.

#### **2. Materials and Methods**

*2.1. Weathering Stability Test*
