Impregnation of pinewood with softwood Kraft lignin

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Impregnation of pinewood with softwood Kraft lignin

https://doi.org/10.21203/rs.3.rs-2305655/v1

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Scots pine wood was impregnated under vacuum with an aqueous 60% acetone solution containing about 10% softwood Kraft lignin. The impregnation treatment incorporated 6–8% of lignin (on wood dry mass) to the wood samples and caused a noticeable change in color from pale yellow to dark brown, but microscopy images showed that the lignin remained mostly within 200 µm from the wood surface. Pinewood samples impregnated with lignin and subjected to water leaching trials had a mass loss of only about 1%, in accordance with the insolubility of Kraft lignin in water. The impregnated wood exhibited lower hygroscopicity and considerably lower surface wettability, as indicated by an increase of more than 100% in water contact angle. Lignin impregnation also improved the fungal decay resistance of the wood against the brown rot fungus Rhodonia placenta, but had no effect against Coniophora puteana. On the other hand, lignin impregnation worsened the flammability properties of the pinewood as determined by cone calorimetry, especially in relation to smoke formation. Enhancing the penetration of lignin into the wood structure may help improving further the resistance to fungal decay, while the combination of flame retardant additives with the lignin may be required in order to improve the flammability properties of pinewood.

fire resistance

fungal decay

kraft lignin

pinewood

wettability

In recent years, a growing social demand to utilize renewable and sustainable materials and lower CO₂ emissions has intensified the use of wood in the construction and building sector. The wood products market in 2021 was valued at $631 billions, and is expected to reach over $900 billions in 2026, with an annual growth rate of 7.4% (Business Research Company 2022). In many cases, and particularly when placed outdoors, the properties of commonly used wood species need improvement to enhance dimensional stability as well as to provide protection against weathering and/or biological decay. Several chemical (i.e. acetylation, furfurylation) and thermal modification methods have been developed in the last decades to improve wood performance and avoid impregnation with toxic compounds, thus reaching the safety and sustainability standards required by industry and consumers (Sandberg et al. 2017). Unfortunately, current wood modification methods are not effective enough to improve its fire resistance properties, which are extremely important when considering the use of wood in building elements (Popescu and Pfriem 2020).

Wood is mainly composed of cellulose, hemicelluloses, and lignin, which are the three structural components of the wood cell walls. Cellulose and hemicelluloses are carbohydrates while lignin is a complex polyphenolic macromolecule. In addition to providing mechanical rigidity, lignin protects the plant against pests and pathogens (Bhuiyan et al. 2009; Gibson 2012). Lignin is also a major and abundant side-stream of the wood pulping and biorefining industries; more than 50 million tons of lignin are extracted every year, mostly from Kraft wood pulping processes (Sethupathy et al. 2022). Currently the extracted lignin is combusted at the mills to produce energy, but higher value applications of lignin are intensively being developed.

Inspired by the bio-protective effect of lignin in plants, and coupled with the abundance of lignin side-streams from the pulping industries, a few studies have investigated the use of lignin to enhance the fungal decay resistance in wood. Chirkova et al. (2011) studied the impregnation of pine wood with several lignins and found out that a remarkable increase in durability against brown and white rot fungi could be achieved, particularly with alkali and kraft lignins. However, the antifungal effect of kraft lignin could not be fully replicated by other authors after laccase-assisted grafting of lignin in wood (Fernández-Costas et al. 2017a; Bolaño et al. 2021). Lignin-rich bio-oils obtained by fast pyrolysis of wood, bark, or even lignin have been investigated as wood protection formulations with generally positive results, but the chemical complexity of the bio-oils hinder the identification of individual compounds with biocidal properties (Mohan et al. 2008; Dos Santos et al. 2016; Mattos et al. 2019). On the other hand, lignin has shown good performance as fire retardant agent when incorporated to several polymers such as polypropylene, polyamide, or polylactic acid, especially after functionalizing the lignin with nitrogen or phosphorous groups (Costes et al. 2017; Widsten et al. 2021). The use of lignin as fire protection agent in wood has not yet been explored.

Scots pine (Pinus sylvestris) is the most abundant tree species in Finland and one of the main softwoods used for construction purposes in the Nordic countries. Pinewood is also extensively exploited in pulping for the production of softwood kraft pulps. Therefore, in this study, pinewood was selected for vacuum impregnation with softwood kraft lignin and the retention, penetration, and leaching of the lignin was determined to assess the effectiveness of the impregnation process. The hygroscopicity, surface wettability, and decay resistance of the impregnated wood to the brown rot fungi Rhodonia placenta and Coniophora puteana was evaluated. Finally, the flammability properties of the impregnated wood were determined in a cone calorimeter and compared to those of untreated pinewood.

2.1. Wood and lignin material

Untreated square pinewood (P. sylvestris) dowels with dimensions of 20 x 20 mm² or 20 x 90 mm² in cross section and 1 m in length were purchased from a retail chain store in Finland. The dowels, which were planed and had a smooth surface, were sawn to prepare wood specimens with 50, 100 or 150 mm in length.

Softwood kraft lignin provided by a Finnish pulp mill was used for the wood impregnation trials. Details on the chemical and structural composition of the lignin can be found in Jääskeläinen et al. (2017). The lignin was added to an aqueous solution containing 60% (by volume) acetone to reach a 10% concentration, that is, 100 g (dry mass) of lignin per liter of acetone solution. After mixing for 20 min with a magnetic stirrer, the lignin-containing solution was centrifuged at 3800 rpm for 30 min, and the insoluble lignin fraction was removed and oven-dried. The final acetone solution containing about 9.7% of soluble lignin was stored in a refrigerator until further use.

2.2. Vacuum impregnation of wood

Pinewood specimens at ambient room conditions were placed inside a 12 L vacuum chamber (BacoEng) equipped with a vacuum pump. The specimens were stacked using thin wooden sticks as separators and a weight was placed on top to keep the wood submerged after addition of the lignin-containing acetone solution. The vacuum impregnation treatment was conducted at room temperature for 120 min in three steps: 30 min at 40 kPa, 30 min at 80 kPa, and 60 min at ambient pressure (~ 100 kPa). After the treatment time, the impregnated specimens were removed from the chamber, air-dried, and subsequently oven-dried at 103 ˚C overnight. Two impregnation trials were conducted with 16–20 wood specimens and 2-2.5 L of lignin solution in each trial.

2.3. Lignin retention, penetration, and leaching

The retention of lignin in the wood specimens was calculated on a dry mass basis before and after impregnation. Furthermore, the penetration of lignin into the wood structure was assessed by optical microscopy using a thin wood slice taken from the middle section of a specimen. Untreated and impregnated wood samples were imaged based on autofluorescence without staining by fluorescence microscopy (excitation 335–383 nm, emission > 420 nm) with a Zeiss AxioImager M2 microscope (Carl Zeiss GmbH, Göttingen, Germany). Micrographs were obtained using a Zeiss Axiocam 506 CCD color camera (Zeiss) and the Zen imaging software (Zeiss).

Leaching of the lignin was determined by submerging 6 impregnated (and oven-dried) wood specimens, with dimensions 20 x 20 x 50 mm³, in 5 volumes of distilled water for two weeks, with fresh water being replaced every 1 to 3 days (total 6 changes of water). At the end of the 2 weeks, the specimens were oven-dried and the degree of leaching was determined based on a dry mass basis.

2.4. Equilibrium moisture content and water contact angle

At least 5 untreated and impregnated wood specimens with dimensions 20 x 20 x 50 mm³ were conditioned at 23 ˚C and 50% or 80% relative humidity (RH). The specimens were weighed at regular intervals until no increase in mass was observed, which indicated that the wood had reached the equilibrium moisture content (EMC).

The wettability of the untreated and impregnated wood was determined by the static water contact angle (WCA), using the sessile drop method in a CAM 200 (KSV Instruments Ltd, Helsinki, Finland) goniometer equipped with video camera and placed in a room conditioned at 23°C and 50% RH. The WCA was recorded (several times per second) within 60 s immediately after placing the droplet (1 − 4 µL) on the surface of the wood specimen, and was measured from the droplet shape using the Young − Laplace equation. Two untreated and impregnated specimens (dimensions 20 x 20 x 50 mm³) were used for the WCA measurements, and the WCA was determined from 2–3 different locations in each of the 4 sides of the specimens along the longitudinal axis. A higher WCA indicates a lower wettability.

2.5. Fungal decay tests

Untreated and impregnated wood specimens with dimensions 20 x 20 x 50 mm³ were tested against two brown rot fungal strains, D-90413 R. placenta and D-88343 C. puteana, using the standard method EN113 (1996) as reference. One untreated and one impregnated specimen, 5 replicates in total, were placed in the same vessel containing the wood destroying fungus, while two control specimens were placed in uninoculated vessels. The cultivation media was malt extract agar, and the culture vessels were inoculated with actively growing fungus. The inoculation was performed by placing one piece of agar (d = 0.5 cm) on top of the culture medium. The wood specimens were exposed to the fungus as soon as the mycelium covered the media surface completely (after 4 weeks). The vessels were incubated in darkness at 22°C and 70% RH for 16 weeks, after which the fungus was brushed away and the specimens were oven-dried to determine their mass loss.

2.6. Fire resistance by cone calorimetry

The flammability properties of 5 untreated and impregnated wood specimens with dimensions 90 x 100 mm² and 20 mm thickness were determined in a cone calorimeter. The wood specimens were conditioned at 23 ˚C and 50% RH before testing. The tests were performed at Eurofins (Finland) according to the standard ISO 5660-1. The heat flux was set to 50 kW/m², and the burning time was 30 min.

3.1. Wood impregnation with lignin

The impregnation of pinewood with kraft lignin clearly changed the visual appearance of the wood from a pale yellow to a dark brown color reminiscent of thermally treated wood or some tropical wood species (Fig. 1). The retention of lignin in the wood, calculated on a dry mass basis before and after impregnation, ranged from 29 to 42 kg/m³, corresponding to a mass gain of roughly 6–8% (Table 1). These lignin retention values were higher than the ~ 4% retention achieved in spruce wood after vacuum impregnation with 10% coniferyl alcohol, one of the three main monolignols present in lignin (Raiskila et al. 2006), and also higher than the 1–2% retention in pine sapwood after impregnation with 1% lignin in water (Chirkova et al. 2011). On the other hand, similar levels of retention were reported by Tomak and Gonultas (2018) after vacuum impregnation of pinewood with 5–10% tannin solutions. The comparison of retention values, however, should only be taken as indicative because the retention of preservative substances impregnated into wood is strongly dependent on wood species, sapwood/heartwood ratio, drying history, wood pre-treatment, preservative formulation, and impregnation conditions (Tarmian et al. 2020).

Table 1

Lignin retention and mass gain in pine wood samples after vacuum impregnation.
Impregnation trial	Number of specimens	Wood dimensions (mm³)	Dry mass before impregnation (g)	Dry mass after impregnation (g)	Mass gain (%)	Lignin retention (kg/m³)
1	20	20x20x50	10.1 ± 0.2	10.7 ± 0.2	5.8 ± 0.3	29.4 ± 1.3
2	6	20x20x50	9.5 ± 0.7	10.2 ± 0.6	7.5 ± 1.4	35.2 ± 4.5
2	5	20x20x150	27.5 ± 1.2	29.7 ± 1.1	8.1 ± 0.7	37.1 ± 2.2
2	5	90x100x150	83.0 ± 2.3	90.0 ± 2.4	8.3 ± 0.2	41.9 ± 1.0

The actual penetration of lignin into the wood structure through the radial and tangential directions was evaluated by optical microscopy, using wood slices taken from the middle section of the specimens. As shown in Fig. 2, and compared to untreated wood, the wood samples impregnated with lignin clearly had a darker area around the edges, but the lignin appeared to penetrate only less than 200 µm from the surface. Some dark areas were also observed scattered throughout the cross section of the impregnated specimens, and the latewood annual rings were markedly darker than in the untreated pinewood. Overall, however, the permeability of wood to lignin appeared to be rather limited, which may be explained by the fact that Scots pine is generally considered a refractory species difficult to treat by impregnation, especially in the absence of a pressure treatment (Tarmian et al. 2020). It is possible that applying pressure after the vacuum treatment would improve the penetration of the lignin into the wood structure. Further improvement in permeability may also be achieved by enzymatic pre-treatment of the wood in order to destroy the torus in the aspirated tracheid pits, thus increasing liquid flow between adjacent tracheids (Durmaz et al. 2015; Widsten et al. 2022).

In impregnation processes, the permeability of wood to preservative substances is as important as the capacity of these substances to remain within the wood structure and do not leach out when in contact with liquid water. Here, leaching of the lignin was studied by determining the change in mass after submerging impregnated wood specimens in water for 2 weeks, replacing the water every 1 to 3 days. The mass loss due to leaching after the 2 weeks period was only 0.9 ± 0.1%, indicating that most of the lignin remained in the impregnated wood. This result was somewhat expected because kraft lignin is largely insoluble in water. Nonetheless, the fact that the wood samples were oven-dried at 103 ˚C after impregnation, and before the leaching tests, might have also induced some cross-linking of the lignin by thermal curing, consequently improving resistance to leaching (Zhou et al. 2018).

3.2. Hygroscopicity and wettability of wood

The hygroscopicity of the untreated and impregnated pinewood was determined by the EMC at 23 ˚C and 50% or 80% RH conditions. As shown in Table 2, the EMC of the impregnated wood samples was about 0.5% units lower than the EMC of untreated pinewood at both 50% and 80% RH. A decrease in hygroscopicity of poplar wood after impregnation with alkali lignin was also reported by Zhou et al. (2018). The reduction in EMC was most likely the consequence of the total mass of wood increasing by 6–8% by the addition of kraft lignin. In other words, the impregnation of lignin did not probably reduce the sorption of water molecules into the wood structure, but simply lowered the ratio of adsorbed water in the sample with respect to the sample mass. As the reduction in EMC was about 0.5%, much lower than the 6–8% mass gain after impregnation with lignin, it can be concluded that kraft lignin also contributed to water sorption (Volkova et al. 2012).

Table 2

Equilibrium moisture content (EMC), water contact angle (WCA), and mass loss after exposure to brown rot fungi of untreated and lignin-impregnated pinewood samples.
	EMC^a		WCA^b (°)	Mass loss from exposure to
	50% RH (%)	80% RH (%)	WCA^b (°)	R. placenta (%)	C. puteana (%)
Untreated pinewood	8.3 ± 0.1	11.0 ± 0.1	30.1 ± 7.4	24.7 ± 3.5	27.8 ± 5.4
Impregnated pinewood	7.8 ± 0.1	10.4 ± 0.2	71.5 ± 13.8	15.8 ± 6.6	29.8 ± 6.4
^a EMC determined at 23 ˚C
^a WCA determined at 23 ˚C and 50% RH

In addition to the hygroscopicity, the wettability of the wood samples was determined by the water contact angle (WCA) at 23 ˚C and 50% RH. The WCA increased more than 100% after lignin impregnation, from 30.1 ± 7.4 in untreated pinewood to 71.5 ± 13.8 in impregnated wood (Table 2). A similar decrease in wettability of wood surfaces after lignin impregnation or application of lignin-based coatings has previously been reported by other authors (Zhou et al. 2018; Hua et al. 2019). The WCA values measured from different sides of the same wood specimen showed a relatively large variability, which may be explained by morphological differences between the radial and tangential planes as well as by different proportions of earlywood and latewood (Nussbaum 1999; Hubbe et al. 2015). A decreased wettability of wood surfaces is a desired property especially for outdoor and construction applications where wood is more susceptible to fungal attack.

3.3. Fungal decay resistance

The exposure of untreated pinewood to the brown rot fungi R. placenta and C. puteana, which are among the most harmful wood-decaying basidiomycetes in the Nordics, caused a mass loss between 25–30% (Table 2). Two of the untreated wood specimens exposed to R. placenta and one specimen exposed to C. puteana were excluded from the analyses because their mass loss was lower than 20%, which is the lower mass loss accepted in the EN113 standard for untreated wood samples. Compared to untreated wood, impregnation with lignin slightly improved the decay resistance of pinewood against R. placenta, with an average mass loss of 15%, but had no effect on the resistance against C. puteana (Table 2). These results did not agree with previous findings that impregnation of pinewood with lignin could significantly decrease mass loss after exposure to the same brown rot fungi (Chirkova et al. 2011). On the other hand, Fernández-Costas et al. (2017a) also reported that Kraft lignin grafted enzymatically into pinewood did not exhibit biocidal properties against C. puteana. The same authors also showed that inhibition of fungal growth by kraft lignin was much more pronounced on R. placenta than on C. puteana, and in general, the biocidal effects were greater when lignin concentration was increased (Fernández-Costas et al. 2017b). It might be possible that, in this study, the predominant presence of Kraft lignin around the wood surface somewhat slowed down the fungal growth, but the rather poor penetration of lignin into the wood was insufficient to prevent extensive fungal colonization after 16 weeks of incubation.

3.4. Fire resistance

The flammability properties of the untreated and impregnated pinewood samples were tested in a cone calorimeter. The most important parameter measured by cone calorimetry is the heat release rate (HHR) during combustion of the sample exposed to constant heat flux over the duration of the test. As shown in Fig. 3, the HHR of both untreated and impregnated wood samples represented the typical curve for wood, characterized by an initial sharp peak followed by a drop in heat release rate to a rather steady state before the appearance of a second wider peak. The first peak in HHR occurs before the first layer of char is formed, the middle part of the curve corresponds to the burning through the sample thickness, and the second peak typically corresponds to the increased formation of volatiles in the remaining thin and unburned section of the sample (Grexa and Lübke 2001). The first part of the HRR curves from both untreated and impregnated wood samples looked rather similar, while the presence of lignin appeared to somewhat increased the width of the second peak and extend the burning time (Fig. 3). Nonetheless, the flammability parameters from the cone calorimetry tests shown in Table 3 indicated that the impregnation of wood with lignin did not bring any benefits in terms of fire resistance, rather the opposite, as the parameters related to heat release (i.e. THR, HRR, and PHRR) and especially the formation of smoke (TSR) were higher for the impregnated wood samples. An increase in HRR and THR by incorporation of Kraft lignin to polyamide was also observed by Mandlekar et al. (2017), while a considerable increase in smoke formation by addition of 2.5% wt. alkali lignin to polybutylene succinate (PBS) was reported by Liu et al. (2016). However, incorporation of lignin into various polymeric materials has often shown an improvement of the materials’ performance against fire, generally ascribed to increased char formation which acts as an insulation layer and reduces the diffusion of oxygen and heat into the material (Costes et al. 2017; Widsten et al. 2021). Based on these results, improving the flammability properties of pinewood by impregnation with lignin might require higher amounts of lignin retention with deeper penetration into the wood structure, as well as chemical modification of the lignin to bind nitrogen or phosphorous functional groups (Costes et al. 2017; Widsten et al. 2021).

Table 3

Flammability parameters from cone calorimetry tests of untreated and lignin-impregnated pinewood samples. Ti, time to ignition; Tt, total combustion time; THR, total heat release; HRR, heat release rate; PHRR, peak heat release rate; TSR, total smoke release; MLR, mass loss rate.
	Density (kg/m3)	Ti (s)	Tt (s)	THR (MJ/m2)	HRR (kW/m2)	PHRR (kW/m2)	TSR (m2/m2)	MLR (g/(s m2))
Untreated pine wood	512 ± 26	23 ± 2	1078 ± 13	137 ± 10	93 ± 11	199 ± 6	381 ± 30	7.2 ± 1.0
Impregnated pine wood	557 ± 6	24 ± 2	1125 ± 23	156 ± 18	106 ± 17	225 ± 16	571 ± 88	7.5 ± 1.2

Vacuum impregnation of pinewood with softwood kraft lignin resulted in the incorporation of 6–8% lignin mostly around the wood surface; the actual penetration of lignin into the wood structure was rather limited. The impregnated wood had a clear darker color, and exhibited a slight reduction in hygroscopicity and a significant decrease in wettability. The impregnation with lignin also improved the fungal decay resistance of the wood against the brown rot fungus R. placenta, but had no effect against C. puteana. Unfortunately, the flammability properties of pinewood were worsened by the presence of lignin, especially in relation to smoke formation. Further reduction in hygroscopicity and fungal decay may be achieved by improving the penetration of lignin into the wood, for instance by applying pressure. Improving the resistance of the wood against fire may also require chemical functionalization of the lignin prior to impregnation.

Acknowledgements

The author gratefully acknowledges the skillful technical assistance by Liisa Änäkäinen and Ulla Holopainen-Mantila with optical microscopy imaging, Vuokko Liukkonen with contact angle measurements, and Anniina Suhonen and Satu Salo with the fungal decay tests. Petri Widsten is also thanked for valuable discussions.

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Impregnation of pinewood with softwood Kraft lignin

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1. Wood and lignin material

2.2. Vacuum impregnation of wood

2.3. Lignin retention, penetration, and leaching

2.4. Equilibrium moisture content and water contact angle

2.5. Fungal decay tests

2.6. Fire resistance by cone calorimetry

3. Results And Discussion

3.1. Wood impregnation with lignin

3.2. Hygroscopicity and wettability of wood

3.3. Fungal decay resistance

3.4. Fire resistance

4. Conclusions

Declarations

Acknowledgements

References

Additional Declarations

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