Bark from Nordic tree species – a sustainable source for amphiphilic polymers and surfactants

Introduction

The United Nations (UN) has set up the 17 Sustainable Development Goals (United Nations 2022) where several goals aim to develop a more sustainable society by replacing fossil-based or environmentally hazardous products with bio-based materials (Government Offices of Sweden 2018). The circular bioeconomy is focused on achieving a sustainable and resource-efficient upgrading of biomass in integrated, multi-output production chains, making use of residues and wastes, and optimizing the value of biomass over time via cascading (Stegmann et al. 2020).

The forestry resources available allow a large amount of wood-based products to be produced and exported (Convention on Biological Diversity), which means that by-products such as bark are produced in large quantities. Barks are non-wood lignocellulosic substances and in recent years, the concept of exploiting bark in value-added applications has gained attention to their conventional incineration as solid fuel (Neiva et al. 2018). The extractives in barks have a potential for use in many applications (Krasutsky 2006, Almeida et al. 2019) such as surface coatings, textiles, and food. According to Food and Agriculture Organization of the United Nations (FAO), the annual global amount of bark is approximately 359 million m³ (Food and Agriculture Organization of the United Nations 2015). In comparison, the global surfactant market demand was 15.6 million tons in 2014 and most of the surfactants were prepared from fossil-based chemicals (Alwadani and Fatehi 2018, Rojas et al. 2009, Grand View Research 2015). This high demand of surfactants in combination with the high availability of industrial bark make it a potentially cost-effective and an environmentally friendly solution to utilize bark as a raw material source when producing surfactants (Isikgor and Becer 2015, Hazarika and Gogoi 2014).

Amphiphilic molecules, comprises both a hydrophilic head group, which can be either charged or uncharged, and a hydrophobic end, i. e., tail, group. Due to their unique molecule assembly, amphiphiles are usually seen as surfactants. For example, detergents, emulsifiers, and foaming agents are surfactants which means that they have the ability to decrease the interfacial surface tension of the liquid. There are different types of amphiphilic molecules depending on the head group, as aforementioned. When the head group is charged, the surfactant is classified according; non-ionic, ionic (anionic or cationic), zwitterionic meaning both negatively and positively charged, and amphoteric meaning it behaves differently depending on pH (cationic at low pH and anionic at high pH) (Hamley 2007, Barnes and Gentle 2011). Another unique property of amphiphiles is their ability to self-assembly, and there are many different interactions involved, such as hydrogen bonds, the hydrophobic effect, van der Waals interactions, double layer interaction, and Derjaguin, Landau, Vervey, and Overbeek (DVLO). When amphiphilic surfactants are in contact with water, the molecules organize themselves in various assemblies where the hydrophobic tails aggregates and get shielded from the water by the headgroups. Forms like micelles, vesicles, liquid crystals, and lamellar are typical assemblies of amphiphilic molecules and which form that takes place depends on concentration, temperature, and pH (Lombardo et al. 2015, Sorrenti et al. 2013). Many of the surfactants used today are produced from fossil-based raw material. These are not only toxic for both humans and animals but also hard to degrade. Alkylbenzene sulphonate and sodium dodecyl sulphate (SDS) are commonly used surfactants in industries and cleaning products (Rocha e Silva et al. 2018). Customer concerns and requirements and new government environmental regulations have pushed the research towards more sustainable raw materials and solutions.

This review brings up the chemical composition in the bark from the most common Nordic tree species. We will discuss the applicability of these compounds, with the focus on betulin, condensed tannin and suberin, into high-value products such as surfactants. To summarize, life-cycle assessments and techno-economic analysis have been made on these compounds.

Nordic tree species

The Nordic countries are usually known for their forest-rich landscapes. Especially in Finland and Sweden whereas Norway, Iceland and Denmark have less. More than 70 % of Swedish and Finnish territory is covered by forests, but the proportions of forest in Norway, Denmark and Iceland are 40 %, 15 % and 0.5 %, respectively.

Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) are the main softwoods in Sweden, and they cover approximately 44 % and 33 %, respectively, of the total Swedish forestry area (Lindbladh et al. 2014). Scots pine, the most common species in Finland, covers about 50 % of the total Finnish forestry area (Repola 2009, Henttonen et al. 2020, Korhonen et al. 2017). In Norway, spruce trees dominate and occupy as much as 42 % of the forestry area (Breidenbach et al. 2021). Table 1 shows the most common tree species in these Nordic countries and their proportions of the total forestry area. Other tree species include among other goat willow, European beech, and oak.

Bark properties and chemical composition

All tree species are different, and likely so also the bark properties and chemical composition. Figure 1 shows bark of Norway spruce and Scots pine, and Figure 2 shows bark from birch.

Figure 1

Bark of a) Norway spruce and b) Scots pine. (Pictures approved for use by Holmen AB).

Figure 2

Picture of birch bark (Picture taken and approved for use by Barbara Rietzler).

The bark thickness varies not only of the tree species but also geographical location of the tree and the season. Musić et al. (2019) reported that the thickness of Norway spruce bark depends largely on the size of the stem, from 9 to 32 mm approximately, and that the volume proportion of bark in the whole cross section varies from 7 to 14 %. Viherä-Aarnio and Velling (2017) reported that the thickness of silver birch bark also depends on factors such as geographical location and height of the stem, and that the thickness of birch bark varies from 2 to 11 mm. Wilms et al. (2021) measured bark thickness of Scots pine, and reported that it varied across the whole stem, from approximately 1 to 2 cm, and that the increasing age of the tree decreased the bark proportion, from 13 to 12 %.

There are many different compounds found in bark which can be utilized to produce value-added products. Pásztory et al. (2016) has summarized studies that investigated various tree species, extraction methods, and the utilization of the components from the trees’ bark. Yazaki Y. (2015) discusses the use of tannins from different tree species in adhesives and health/food applications. Pizzi A. (2008) discusses the applications of tannins, among others leather manufacture, food industry, adhesives, and industry of pharmaceutics and medicine. Liimatainen et al. (2008) used methanol as extraction solution for inner bark of Betula pendula and found two, at that time, new phenylbutanoids. The same group later also performed an in-depth characterization of the phenolic compounds in the silver birch inner bark and found 30 various phenolic glycosides and aglycones, whereas two of them had not been reported before (Liimatainen et al. 2012). Karonen and co-workers (Karonen et al. 2011) used hydrophilic interaction chromatography (HILIC) to study the polycyanidins mixture from birch inner bark. Betulin from birch bark has been studied in numerous medical application articles where they have used betulin for among others cancer therapy (So et al. 2018), wound healing (Scheffler 2019), and skin care (Metelmann et al. 2013). Huang et al. (2019) used birch bark betulin to surface modify textiles. Li et al. used suberin extracted from birch bark to modify the surface of cellulose fiber (Li et al. 2015b, 2015a).

Indeed, all bark contains cellulose, hemicellulose and lignin which are the most abundant components but in different amounts (Isikgor and Becer 2015, Pásztory et al. 2016). However, there are also a large amount of various extractives in the bark. Table 2 shows the total amount of extractives found in the bark of the specific Nordic tree species, and highlighted the extractive components we have chosen to focus on in this review, i. e., suberin, tannins and betulin, according to different studies. It is known that different tree species have different type of extractives and that the amount vary from species to species and location of the bark. The data shown do not take into consideration the location in the bark the component is found and is presented regardless of solvent or extraction method used.

Betulin

Betulin is a triterpene and is present in the outer bark of birch (Huang et al. 2018), and the chemical structure is shown in Figure 3. It is insoluble in water, but it is soluble in common organic solvents (acetone, ethanol and ethyl acetate) and can be obtained by solvent extraction (Cao et al. 2007). Betulin can be modified at the primary and secondary hydroxyl groups or at the double bond to meet specific requirements, such as the preparation of amphiphiles. Zhao et al. (2012) reported that, by using phosphazene-promoted metal-free anionic polymerization, an amphiphilic polymer based on poly(ethylene oxide) (PEO) and betulin can be prepared. With a trace amount of t-BuP₄ used as catalyst, betulin was polymerized with PEO at 45 °C to prepare an amphiphilic biohybrid polymer. The betulin may be replaced by other hydroxyl-containing terpenes such as menthol, retinol and cholesterol to produce a biohybrid polymer containing a hydrophobic terpene moiety and a hydrophilic PEO chain. Similarly, Chen et al. (2018) reported that, by taking advantage of a one-pot organocatalytic route involving chain-growth and step-growth polymerizations, PEO and betulin can be used to synthesize multiblock-like amphiphiles, e. g., polyurethane derivatives. Such amphiphilic polymers are believed to have a potential in antifouling applications. Krasutsky et al. (2008) have patented a process for preparing a betulin-based surfactant by synthesizing quaternary amine derivatives of betulin giving an amphiphilic compound that can bind hydrophobic molecules and is therefore a possible candidate for adsorbing on-cytoplasmic membranes.

Figure 3

Chemical structure of betulin.

Tannin

Tannins can be found in the bark of several tree species (Matthews et al. 1997, Zhang and Gellerstedt 2009, Mannila and Talvitie 1992, Norin and Winell 1972). They have a complex chemical structure consisting of several basic units, the most common being gallic acid, phloroglucinol and flavanols. Depending on the base units, tannins can be divided into condensed tannins (flavanol as base unit) and hydrolyzable tannins (gallic acid as base unit), whereas condensed tannins are the most common (Figure 4) (Raitanen et al. 2020). Procyanidins (PCs) and prodelphinidins (PDs) are two of the most abundant types of condensed tannins. The former exists in the barks of Norway spruce, Scots pine and silver birch but the latter exist only in Norway spruce (Matthews et al. 1997). Tannins can be extracted via hot water extraction, and the yield depends both on the bark region and the tree species (Table 3) (Raitanen et al. 2020).

Table 3

Proportions of tannins (%) in the barks of Norway spruce, Scots pine and silver birch (Raitanen et al. 2020, Matthews et al. 1997).

	Norway spruce	Scots pine	Silver birch
Whole bark	8.1	3.6	0.6–3.0
Inner bark	9.2	4.1	–
Outer bark	4.8	1.1	–

1. – data not available

Figure 4

Proposed chemical structure of condensed tannin.

Due to their amphiphilicity, tannins can be used in fields where certain surfactant properties are needed. Figueroa-Espinoza et al. (2015) reported that tannins can be used as emulsifiers with antioxidative functions, thus, three types of condensed tannins (catechin, grape seed and apple tannins) being utilized to stabilize oil-in-water emulsions. Methyl oleate was added to a mixed aqueous phase of tannins and phosphate to prepare the emulsion by a sonicating technique, and it was shown that oxidized grape seed tannins, with a relatively high molecular weight can stabilize oil-in-water emulsion, and such by-products from the forestry or winery industries can thus be used in value-added applications as emulsifiers. Gallic acid is one of the base units of hydrolysable tannins and it can be used as a starting material to synthesize non-ionic surfactants. Negm et al. (2014) in 2014 reported that non-ionic surfactants were easily prepared by esterifying gallic acid with polyethylene glycol (PEG) of different molecular weights. The longer the PEG chain, the greater was the surface activity in solution.

Figure 5

Possible chemical structure of suberin.

Suberin

Suberin (Figure 5) can be found in the birch outer bark and can constitute up to 50 % of the dry weight (Ferreira et al. 2013). After alkaline hydrolysis, acidification and precipitation, suberin monomers can be separated from the bark (Iversen et al. 2010). Suberin plays a role as a hydrophobic barrier to prevent water from penetrating into cell walls during the tree’s development (Vishwanath et al. 2015). It can be subdivided into two ester-bonded aromatic and aliphatic domains, and glycerol moieties can also be found in the structure. The aromatic domains are covalently linked to both carbohydrates in the primary cell and to adjacent aliphatic domains (Vishwanath et al. 2015). A variety of fatty acids are included in the aliphatic domain, such as cis-9,10-epoxy-18-hydroxyoctadecanoic acid (Figure 6), which is one of the most abundant components, constituting ca. 10 % of the dry outer bark of birch (Ekman 1983). To the best of the authors knowledge, there has not been any studies on utilizing suberin as a surface-active component.

Figure 6

Chemical structure of cis-9, 10-epoxy-18-hydroxyoxtadecanoic acid.

Amphiphilic surfactants from bark constituents

Amphiphiles from the forestry are gaining more and more of researcher’s attention. Examples of naturally occurring amphiphile is saponin which can be found in the bark of soapbark tree (Quillaja saponaria) (Williams and Gong 2007), and coco betaine which may be derived from coconut oil (Guzmán et al. 2020). From our Nordic trees, i. e., Norway spruce, Scots pine and silver birch, also have interesting amphiphilic compounds that can be used as surfactants. Betulin, suberin and condensed tannins are attractive starting compounds because they are abundant and easy to obtain by extraction or hydrolysis. Additionally, these substances have antiviral properties which adds to the beneficial properties of utilizing them as amphiphilic molecules. Table 4 shows the functional groups and properties of these compounds.

Life-cycle assessment (LCA) and techno-economic analysis

Tannin extraction producers use the solid-liquid extraction process, employing large amounts of hot water extractions followed by evaporation under reduced pressure, although numerous alternative processes from microwave- to ultrasound-assisted extraction on a laboratory scale have shown to generate higher tannin yield and reduced extraction times. Ding et al. (2017) have studied the environmental impact of the condensed tannins product. In their study, a life-cycle assessment (LCA) method was used to compare different tannin extraction scenarios, and they showed that among all the process steps which include initial cold-water extraction, extract drying and ultrafiltration, the extract drying (evaporation) is the process that has the highest impact on the environment. The initial cold-water extraction and ultrafiltration gave fewer non-tannin compounds and were, thus, less harmful to the environment. The LCA study points towards a reduction in water consumption in the extraction as a way to reduce the environmental impact as also shown by Carlqvist et al. (2020).

Hot water extractions are appealing because water is the only solvent used, but the tannin yield is generally limited to less than 10 % of the bark dry mass (Kemppainen et al. 2014). Borrega et al. (2022) have demonstrated on pilot scale that alkaline extraction of industrial spruce bark is an effective and scalable technology to extract polyphenols in high yield, from 20 % to 27 %, respectively, of the dry mass of the bark. The extraction includes the co-products lignin and carbohydrates. The required purity of the extracted target compound depends on the intended use of the final product.

In the alkaline extraction process, the extraction liquid is the main product, and the extracted bark residue is the co-product which can be utilized as pulp. The extraction of spruce bark in an industrial process is shown in Figure 7. For 1 ton of spruce bark, 150 kg of sodium hydroxide (NaOH) and approximately the same amount of sulfuric acid (H₂SO₄) is needed to obtain 300–400 kg of pulp and ca. 250 kg of polyphenols as product.

Figure 7

Rough estimation of input and output of material in the industrial extraction of polyphenols.

A pulp mill based on birch wood produces approximately 10,000 tons of birch bark annually, from which it would be possible to produce 1,800 tons of betulin (Krasutsky 2006). Production of betulin from birch bark with an appropriate purity and quality while considering environmental aspects such as solvent and energy consumption has been investigated (Fridén et al. 2016). The optimized reflux boiling (RB) process was selected as a model for industrial calculations for 50,000 tons of birch bark (Gruvön pulp mill). Among others, it could be demonstrated that if the ethanol concentration from the distillation column is lower than 90 % results in drastically decreasing energy consumption.

Betulin production on a commercial scale has currently been started. Thus, the betulin extraction pilot plant Betulin Lab of Latvijas Finieris was officially opened in May 2022 (Latvijas Finieris 2022). Innomost aims to build a production facility capable of producing 20 tons of birch bark products per year by 2023 (Innomost 2021).

The most optimal method for extracting suberin from birch bark has been found to be alkali hydrolysis in the presence of alcohol, and the results were evaluated based on the yield and purity of the product, the preservation of epoxy groups and the cost (Lepistö 2021). For the process scale-up, the flow chart of the process was drawn, mass and energy balances were calculated, and the necessary process equipment was determined. In addition, the evaluation of the investment and the opening costs were defined. Results of the evaluated profitability showed that 150 tons of suberin can be extracted per year from birch bark by alkali hydrolysis, and the profit of the process could go up to even €1.4 million annually (Lepistö 2021).

Conclusions

In this review, we have discussed the availability of industrial bark and the use of bark extractives as amphiphilic compounds for surfactant usage. Other than cellulose, hemicellulose and lignin, we have chosen to focus on other compounds, i. e., betulin, condensed tannins and suberin. As there is a high demand on surfactants and a high availability of bark, it is of great potential and environmental benefit to utilize bark as a raw material for amphiphilic molecules and polymers. For example, betulin-based surfactants can be obtained by synthesizing quaternary amine derivatives of betulin. Condensed tannins have been used as emulsifiers with antioxidative functions, and gallic acid has been used to synthesize non-ionic surfactants. Lastly, suberin has, to the best of our knowledge, not been used to produce surfactants.

Future research is needed in order to increase the knowledge of starting molecules, preparation methods and target products. The life-cycle assessment and techno-economic analysis indicates that betulin, tannins and suberin could be sustainable backbones for amphiphilic polymers and surfactants.