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Article

Influence of Phenol–Formaldehyde Resin Oligomer Molecular Weight on the Strength Properties of Beech Wood

by
Qian Lang
1,*,
Vladimirs Biziks
2 and
Holger Militz
2
1
Faculty of Engineering, Northeast Agricultural University, Haerbin 150030, China
2
Wood Biology and Wood Product, Burckhardt-Institute, Georg-August University of Göttingen, Büsgenweg 4, 37077 Göttingen, Germany
*
Author to whom correspondence should be addressed.
Forests 2022, 13(12), 1980; https://doi.org/10.3390/f13121980
Submission received: 16 August 2022 / Revised: 21 October 2022 / Accepted: 12 November 2022 / Published: 23 November 2022
(This article belongs to the Special Issue Advanced Technologies in Physical and Mechanical Wood Modification)
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Abstract

:
The objective of this study was to determine the effects of four phenol–formaldehyde (PF) resin treatments with different molecular weights at four different concentrations (5, 10, 15, and 20%) in treated beech wood. The mechanical properties of untreated and treated beech wood were evaluated. After impregnation with PF resin, all modified beech wood at all PF resin concentrations exhibited an increase in weight percent gain compared with that in untreated beech samples. PF resins with lower molecular weights more easily penetrate the wood cell wall, leading to increased bulking of the wood structure, which in turn improves the dimensional stability of the wood. The PF resin treatment with a molecular weight of 305 g/mol showed better impregnation ability than that of the other PF resins. The impact bending strength of PF-treated wood was considerably reduced because PF-cured resins formed inside the wood and are rigid and brittle. Additionally, PF resin treatments at all concentrations decreased the modulus of elasticity of the wood. Scanning electron microscopy and light microscopy revealed that the PF resins were comparatively well fixed in the wood samples. The results indicate that the large molecular weight PF resins are more uniformly distributed in the fiber lumens.

1. Introduction

Wood is a hygroscopic and polymeric material due to its abundance of hydroxyl groups associated with cell wall polymers. Wood also has two types of internal voids: relatively large voids, such as cell lumina and pit openings, and cell wall microvoids [1,2]. The chemical modification of wood involves the use of chemical and/or chemi-physical techniques to improve its dimensional stability, which occurs mainly through the covalent bonding of chemicals with the OH groups of cell wall polymers or the deposition of sterically fixed compounds in the cell wall [3]. With chemical modification, hygroscopic hydroxyl groups of the cell walls become partly blocked, and the nanopores (nanocapillaries) of the cell walls are filled with the chemicals. The water sorption sites decrease when the hydroxyl groups are blocked. Additionally, the deposition of chemicals in the cell walls reduces the space available for moisture from the environment, thereby making them more dimensionally stable [4,5].
To enhance the dimensional stability of solid wood, the modification of wood with water soluble and low molecular weight (Mw) phenol–formaldehyde (PF) resins has been practiced since the 1930s. The mechanical properties of commercial plywood composites, i.e., impreg and compreg plywood, are also enhanced by PF modification [6]. PF resins are known to penetrate and bulk the cell walls of solid wood. Additionally, PF resin modification has been shown to enhance the resistance of wood to white rot and brown rot fungi.
The use of water soluble and low Mw PF resin groups to enhance the dimensional stability and physical mechanical performance of wood has been intensively studied as a means of chemical modification [7,8,9]. PF resin is an aqueous mixture of oligomers that differ in terms of their Mw and shape. These PF resins can increase the dimensional stability of wood because they penetrate and swell the cell wall, thereby reducing hygroscopicity and forming a rigid crosslinked network upon curing. Furthermore, PF resins have a softening effect, similar to that of steam or heat, which plasticizes the cell walls of wood.
The reticular structure of cellulose fibrils, hemicelluloses, and lignin that comprises the cell walls of wood fibers form a three-dimensional structure in which the spacing between the various components defines the porous structure. Moreover, the porous structure determines the outside accessible surface area and upper size limit of molecules at which the cell wall can exchange with an external medium [10]. Accordingly, the particle size of the chemicals used for modification must be smaller than the minute openings of the small pores in the polymeric network structure of the cell walls.
The aim of the present study was to evaluate the physical properties of beech wood samples treated with four PF resins with different Mw values at different concentrations. The properties studied in the wood included the modulus of elasticity (MOE) and impact bending strength (IBS), which were chosen to characterize the application of wood in outdoor structures.

2. Materials and Methods

2.1. Wood Samples

To evaluate the effectiveness of impregnation of PF resins in the inner structure of the wood, two sample types with different dimensions, i.e., 25 × 25 × 10 mm3 and 10 × 10 × 150 mm3 (R × T × L), were prepared from beech (Fagus sylvatica) wood and used to investigate the bulking coefficient and mechanical properties. In total, 629 beech wood samples were selected for use in the study. The variance in density between the samples was <10%. As shown in Figure 1, the density was distributed mainly from 620 to 660 kg/m3. In total, 17 groups of beech wood samples (16 treated wood sample groups and 1 untreated wood sample group) were selected. The oven dry weights of samples were recorded after drying at 103 °C ± 2 °C for 28 h. The dried wood specimens were then impregnated using a vacuum on the same day. Ten blocks per group and treatment were used.

2.2. PF Resins

Four different types of PF (Hexion Oy, Puhos, Finland) resin were used to prepare aqueous solutions and impregnate the beech wood samples. For each of the four types of resin assessed, 10 samples were impregnated with an aqueous solution containing 5, 10, 15, and 20% (w/w) PF resin solid content. The physical–chemical properties of the four types of PF resin are listed in Table 1. The purity grades of the four PF resins and catalyst were 99%.

2.3. Wood Treatment

Oven-dried wood samples with the aforementioned dimensions of 25 × 25 × 10 mm3 and 10 × 10 × 150 mm3 (R × T × L) were impregnated in the above-listed aqueous solutions in an autoclave under 0.008 kPa gage pressure in a vacuum for 45 min using a two-step vacuum impregnation method. During impregnation, the samples were subjected to room temperature (20 °C) followed by 101.32 kPa gage pressure for 60 min. They were removed from the modified solution after vacuum-pressure impregnation. Control samples were not modified with PF and were VTC (viscoelastic thermal compressed)-processed directly from their conditioned state.

2.4. Weight Percent Gain

Weight percent gain (WPG) was characterized using 10 replicates of treated beech wood with a dimension of 25 × 25 × 10 mm3 (R × T × L). The average WPG of untreated and treated beech wood samples was estimated using the dry mass according to the following equation:
WPG = ((mt − mu)/mu) × 100
where mtreated is the oven-dried mass of the treated wood samples and muntreated is the oven-dried mass of the untreated wood samples.

2.5. Bulking Coefficient

The volume difference (bulking coefficient (BC)) due to impregnation was determined by comparing the dry specimen volumes before and after modification. Wood samples with dimensions of 25 × 25 × 10 mm3 (R × T × L) were used to determine cell wall bulking. The BC was calculated using the following equation:
BC = ((Vt − Vu)/Vu) × 100
where Vt is the oven-dried volume of the treated wood samples and Vu is the oven-dried volume of the untreated wood samples.

2.6. MOE

The MOE was determined following EN 408 (2011) in a conditioned atmosphere (50% relative humidity and 20 °C). Ten specimens from each formula were analyzed using a universal testing machine (Zwick GmbH & Co., KG, Ulm, Germany) equipped with a 10 kN load head and testing software Xpert II. The loading direction was perpendicular to the grain with a loading speed of 10 mm/min. The distance between the supporters was 100 mm with 0.5% accuracy, and the rate of loading was adjusted to ensure the maximum load was reached within 60 ± 30 s. Specimens were kept in sealed plastic bags before the experiment.

2.7. Bending and Impact Bending Tests

IBS was assessed according to DIN 52,189 (1981) with modified specimen dimensions of 10 × 10 × 150 mm3 (Resil Impactor, Instron, Norwood, MA, USA). The capacity of the machine used was 30 kgw/cm and bearing width was 120 mm. The machine was equipped with a 25 J hammer and an integrated force-measuring device. To calculate energy (Aw in kJ.m−2), the following equation was used:
Aw = 1000·Q/b·h
where Q is the energy (J) required to fracture the test piece, and b and h are the dimensions of the test samples in the radial and tangential directions (mm) [11].

2.8. Morphology Analysis

Scanning electron microscopy (SEM; Supra 45, Leo Elektronenmikroskopie GmbH, Oberkochen, Germany) was used to observe the untreated and treated beech wood samples, which were first sputter-coated with carbon. The observations were conducted with an accelerating voltage of 15 kV and a 70 Pa vacuum.
Light microscopy was performed using an Ellipse E600 microscope (Nikon, Tokyo, Japan) equipped with a digital camera (Nikon DS-Fi1c). The transverse section of the untreated and treated beech wood samples was cut using Leica microtome blades (Leica DB80 HS (Leica, Wetzlar, Germany). The samples were then stained with 0.09% safranin solution for 6 min, after which they were dehydrated using two ethanol solutions (50 and 99%). All specimens were dried at 42 °C for 25 min before they were embedded in glass.

3. Results

3.1. WPG

Figure 2 demonstrates the average WPG of PF resin-treated beech wood samples (treated with PF resin modifiers with four different Mw values at four different concentrations, i.e., 5, 10, 15, and 20%). As expected, the average WPG increased linearly as the concentration of the PF resin modifier increased, confirming that the PF resin was able to impregnate the wood samples at all concentrations. The same PF resins have the same distances between the points, which also leads to a linear increase in WPG. However, significant differences in WPG were not detected between the untreated and PF resin-treated beech wood samples. The samples treated with oligomers with a Mw of 237 g/mol exhibited higher WPGs compared with those of samples treated with oligomers with Mw values of 305, 403, and 520 g/mol. For the PF resin treatment with a Mw of 237 g/mol, the WPG of the wood was 6.7, 12.3, 18.3, and 24.7% with the 5, 10, 15, and 20% resin concentration (w/w) treatments. For the PF resin treatment with the highest Mw (520 g/mol), the WPGs of wood were lower, indicating that larger oligomers could not easily penetrate the cell wall of beech wood because less PF resin was found in the cell wall. Lower WPG might also be due to the macroporosity of the beech wood (influenced by the early and late wood within the year ring, number of year rings, and quantity of rays, vessels, and fibers) [12].

3.2. Bulking Effect

The bulking effect also tended to increase with the concentration of the PF resin treatment. Figure 3 shows average BC results of beech wood samples.
Comparing the four resin types at similar concentration levels, the bulking effect of the PF resin with a Mw of 305 g/mol was higher than that of the PF resins with Mw values of 237, 403, and 520 g/mol. For example, at the 20% concentration level, PF resins with Mw values of 237, 305, 403, and 520 g/mol led to bulking percentages of 12.2, 12.8, 11.0, and 7.9%, respectively, in treated wood. PF resins with low Mw values can more easily penetrate the cell walls of wood and have a higher bulking effect on the inner structure of the wood, which improves the wood’s dimensional stability [13]. The increased volume of the wood specimens after PF resin treatment implies the presence of cell wall bulking due to the incorporation of PF resin polymers into the cell wall [2]. The step-wise increase in concentration led to an unequal increase in bulking, and an increase in PF resin Mw markedly reduced the BC [14,15]. Bulking is known to reduce the ability of the cell wall to shrink and swell; thus, bulking enhances the dimensional stability of beech wood [11]. The present BC results also demonstrate that the Mw of a resin is important for its ability to impregnate resin polymers into the cell walls of beech wood.

3.3. MOE

Figure 4 shows the MOE of beech wood samples before and after treatments with four PF resin types with different Mw values at four different concentrations (5, 10, 15, and 20%). At all concentrations, PF resin treatments led to slightly lower MOE values compared with those in the control samples. The MOE was reduced by approximately 7.9% when the PF resin treatment had Mw of 403 g/mol. However, the MOE did not differ markedly among sample groups modified with PF resins with different Mw values at the same concentration. Thermoset resins are usually used to modify wood. The impregnated wood then forms a rigid resin cell wall network in the wood structure after it is pressed in a thermal mold. A reduced MOE value can be attributed to the fracturing of the rigid network of the PF resin, which was precured in a thermal press mold [16]. The decrease in MOE could also be due to the thermal degradation of wood cell wall polymers, especially hemicelluloses, which are affected by high temperature. High temperature exposure can change the structure of fibers under thermal treatment [17,18]. The present results indicated that PF resin with a low Mw in the wood structure serves as a plasticizer and leads to substantial softening of the cell wall, inducing a reduction in the MOE of the cell wall in the cross fiber direction [19].

3.4. IBS

Impact strength indicates the ability of a material to achieve a rapid transfer of stress/strain into a bulk material without critical stress peaks [5]. The impact strengths of PF resin-modified and -unmodified wood samples are presented in Figure 5. The IBS of the treated wood samples was lower than that of the unmodified specimens. However, there were no differences in IBS among the modified wood samples at all concentration levels. The loss in IBS was approximately equivalent among concentrations and ranged from 59.62 to 63.9% compared with that in the untreated samples. IBS can be reduced following chemical modification for several reasons. The IBS of the modified beech wood samples may have been reduced because PF resin impregnation reduced the pliability of the wood. It is also possible that the IBS was affected by the acidic hydrolysis of polysaccharides due to the use of magnesium chloride as a catalyst. The movement limitation of the cell wall due to the crosslinking with the PF resin could also reduce the IBS value [20,21,22,23,24]. The cell wall polymers in the inner wood contribute to IBS. The large decrease in IBS in the modified wood samples could also be attributed to reduced pliability due to a rigid, tree-dimensional, corset of PF resin in the beech wood matrix [25].

3.5. Morphology Analysis

The distribution of PF resin in untreated and PF resin-treated beech wood samples was characterized using SEM and light microscopy (Figure 6). As shown in the SEM images, the unmodified beech wood samples exhibited an obvious pore structure. After impregnation with the PF resin, the transverse section of the modified beech samples contained a remarkable amount of PF resin. Moreover, the unmodified beech wood sample had a red color from the adsorbed safranin in the wood. In the modified beech wood samples, this red color became pale yellow. This color change can be attributed to the impregnation of PF resin into the beech wood structure [13,26]. In the modified wood samples, wood cells near the vessel were filled with resin, and the PF resin was located in the lumen of vessels [27]. More bubbles in the resin films were formed in the radical section of the treated wood samples (Figure 6d,e). Additionally, the PF resins were uniformly distributed in the wood samples and existed at high levels in the fiber lumens.

3.6. Mechanism Underlying the Modification of Beech Wood via PF Resin

The structures in beech wood that favor resin impregnation are the lumen and cell wall [28]. When a PF resin with a low Mw is impregnated into the wood, the resin is mainly distributed in small amounts in the lumen through the cell wall. Moreover, the molecule size of the PF resin is small, illustrating that a small molecule resin has good permeability, which easily penetrates the cell wall. When a PF resin with a high Mw was impregnated into beech wood, the resin was mainly distributed in the lumen [29,30,31], (see Figure 7).
Beech wood has many porous structures. The major types of internal void are large voids and cell wall microvoids [32,33]. The porous structure of wood provides many places into which PF resin can impregnate. As a water solution with polymer molecules, no chemical reaction usually takes place between the PF resin and functional groups in the wood structure. Therefore, resin and wood are mutually selective in the modification process. PF resins have different properties based on their Mw and shape. As passive impregnation agents, it is important to evaluate the relationships among the penetration depths of PF resins, capacity of PF resins, and chemical and physical characteristics of the wood structure [34,35,36].

4. Conclusions

PF resins with the four different Mw values at four concentrations (5, 10, 15, and 25%) in water were impregnated into beech wood. The average WPG of the modified beech wood increased linearly as the concentration of PF resin increased. Beech wood treated with a PF resin with a low Mw (237 g/mol) exhibited a higher WPG. The bulking effect values following treatment with PF resin with a Mw of 305 g/mol were higher than those following treatments with PF resins with Mw values of 237, 403, and 520 g/mol. The MOE and IBS results showed that the Mw of a resin is important for the impregnation of resin oligomers into the beech wood cell walls. The relationship between the PF resin and beech wood cell structure was investigated using SEM and light microscopy, and the PF resins were more uniformly distributed in the modified beech wood samples and mainly existed in the fiber lumens. The physical properties of modified beech wood were improved by PF resin treatments with different Mw values. A low average Mw oligomer and small polydispersity index are conductive to the penetration of a PF resin.

Author Contributions

Conceptualization, Q.L. and V.B.; methodology, Q.L. and V.B.; software, Q.L. and V.B.; validation, H.M.; formal analysis, H.M; investigation, Q.L. and V.B.; resources, H.M.; data curation, H.M.; writing—original draft preparation, Q.L.; writing—review and editing, Q.L.; visualization, Q.L. and V.B.; supervision, H.M.; project administration, Q.L.; funding acquisition, Q.L. and V.B. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the China Postdoctoral Science Foundation (2017M621234), Heilongjiang Province Natural Science Foundation (LH2020C002), Heilongjiang Province Postdoctoral Science Foundation (LBH-Z17029) and High-end Foreign Experts Introduction Program(G2022011017L).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

All data included in this study are available upon request by contact with the corresponding author.

Acknowledgments

The authors thank colleagues in Northeast Agricultural University for giving good suggestions regarding experiments and the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Density distribution of each group.
Figure 1. Density distribution of each group.
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Figure 2. The weight percent gain (WPG) of wood samples treated with PF resins.
Figure 2. The weight percent gain (WPG) of wood samples treated with PF resins.
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Figure 3. Bulking effect in wood samples treated with PF resins.
Figure 3. Bulking effect in wood samples treated with PF resins.
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Figure 4. MOE of untreated and PF resin-treated beech wood samples. (A) Mw values of 237 g/mol, (B) Mw values of 305 g/mol, (C) Mw values of 403 g/mol, (D) Mw values of 520 g/mol.
Figure 4. MOE of untreated and PF resin-treated beech wood samples. (A) Mw values of 237 g/mol, (B) Mw values of 305 g/mol, (C) Mw values of 403 g/mol, (D) Mw values of 520 g/mol.
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Figure 5. Impact bending strength of untreated and PF resin-treated wood samples. (a) Untreated control, and wood treated with PF resins with Mw values of (b) 237, (c) 305, (d) 403, and (e) 520 g/mol.
Figure 5. Impact bending strength of untreated and PF resin-treated wood samples. (a) Untreated control, and wood treated with PF resins with Mw values of (b) 237, (c) 305, (d) 403, and (e) 520 g/mol.
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Figure 6. Morphology of untreated and PF resin-treated beech wood. SEM images of (a) untreated wood samples and (b) PF resin-treated wood samples. Light microscopy images of the transverse sections of (c) untreated wood samples and (d) PF resin-treated wood samples. Light microscopy images of the radical sections of (e) untreated wood samples and (f) PF resin-treated wood samples.
Figure 6. Morphology of untreated and PF resin-treated beech wood. SEM images of (a) untreated wood samples and (b) PF resin-treated wood samples. Light microscopy images of the transverse sections of (c) untreated wood samples and (d) PF resin-treated wood samples. Light microscopy images of the radical sections of (e) untreated wood samples and (f) PF resin-treated wood samples.
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Figure 7. Mechanism underlying the modification of PF resin.
Figure 7. Mechanism underlying the modification of PF resin.
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Table
Table 1. Properties of the PF resins used for beech wood modification.
Table 1. Properties of the PF resins used for beech wood modification.
ResinMolecular Weight Mn (g/mol)Solid Content (%)CatalystAmount of Formaldehyde (%)Free Phenol (%)
A23744.1NaOH<1<4
B30545.8NaOH<1<4
C40347.02NaOH<1<4
D52046.6NaOH1.880.24
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Lang, Q.; Biziks, V.; Militz, H. Influence of Phenol–Formaldehyde Resin Oligomer Molecular Weight on the Strength Properties of Beech Wood. Forests 2022, 13, 1980. https://doi.org/10.3390/f13121980

AMA Style

Lang Q, Biziks V, Militz H. Influence of Phenol–Formaldehyde Resin Oligomer Molecular Weight on the Strength Properties of Beech Wood. Forests. 2022; 13(12):1980. https://doi.org/10.3390/f13121980

Chicago/Turabian Style

Lang, Qian, Vladimirs Biziks, and Holger Militz. 2022. "Influence of Phenol–Formaldehyde Resin Oligomer Molecular Weight on the Strength Properties of Beech Wood" Forests 13, no. 12: 1980. https://doi.org/10.3390/f13121980

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