Bark based porous materials obtained with a simple mechanical foaming procedure

Bark is the outer layer of trees and corresponds to between 9 and 24% of the tree stem’s volume depending on the tree species (Smith and Kozak 1967). It is mainly responsible for the transport of nutrients, but also for ensuring the protection of the cambium from any outer threats as fungi, extreme temperatures and mechanical impacts (Fengel and Wegener 1989). Despite its relevance for the plant, bark is still treated as a low value by-product of the timber industry and accumulates in rough quantities (Wagner et al. 2019). As a result, further valorization of bark pieces, which are detached from the tree when cut, is needed. Every year, millions of cubic meters are predominately burned (Feng et al. 2013), which creates emissions that are declared as CO2 neutral, as the released carbon dioxide was captured during the tree’s growth. From this point of view, biomass that is directly fueled after harvesting does not contribute to reduce the overall CO2 concentration in the atmosphere. Hence, the nature inspired use of renewable resources in materials that also contribute to a reduction in CO2 emissions through their function seems to be a better approach than instantly fueling the raw materials.

Currently households, manufacturing, electricity supply, agriculture, transportation and storage are quite resource-consuming sectors and major contributors to CO₂ emissions in the European Union (European Commission 2018). For this reason, developing materials inspired by nature could both enhance the life cycle of bark in the wood industry and help the current environmental situation by designing new materials for the packaging or building construction industries.

Regarding bark, one of its main drawbacks is its chemical heterogeneity (Kuo et al. 2018; Wenig et al. 2021), which makes it difficult to introduce it in large industries. This is why studying bark in granular form can be interesting and a helpful strategy. Bark has been investigated for years to be used for panel production or as part of wood composites. In addition, one of the most studied bonding agents has been urea formaldehyde resin (UF), investigated since the 1970s (Feng et al. 2013; Muszynski and McNatt 1984) examined the gradual addition (from 0 to 100%) of bark in wood panels using urea formaldehyde resin as the adhesive. The strength properties decrease with the addition of bark, but even with a 30% addition of bark the mechanical properties were appropriate for furniture production (Muszynski and McNatt 1984). Similar results were obtained by Xing et al. (2006) who produced medium density fiber boards with refined bark fibers of several species varying its content from 0 to 40% and urea formaldehyde resin. No negative effect on thickness swelling was reported here (Xing et al. 2006). Nemli and Colakoglu (2005) proved that an addition of more than 12.5% of bark reduces significantly the mechanical properties and is therefore not recommended, even though the formaldehyde emissions are diminished. Pressing temperature and particle size of bark particles have also been the subject of other studies, concluding that they have a great influence on the final density and properties. Mechanical and physical properties are shown to improve with the pressing temperature and the use of fine particles, of approximately 1 mm, compared to bigger or mixed particle sizes (Gupta et al. 2011). Kain et al. (2012, 2018) produced different thermal insulation panels (200 and 500 kg/m³) using bark particles and urea formaldehyde as a binder. Additionally, a resin formulation based on tannin was used for bark based insulation panels, that acquired excellent thermal conductivity values of 69 mW/(mK) with densities of 250 kg/m³ (Kain et al. 2016). Pásztory et al. (2019) also reported data for bark insulating panels made out of different species and particle sizes. Boards made from particles with sizes between 1 and 8 mm showed a thermal conductivity of 64.5 mW/mK, and lower formaldehyde emission than boards produced with bigger particle sizes (Pásztory et al. 2017). The same group also investigated the addition of fiber glass and paper sheets to bark panels with urea formaldehyde in order to improve their properties and develop a thermal insulation material. Thermal conductivity values ranged between 67 and 74 mW/mK (Tsalagkas et al. 2019).

Besides investigations as potential raw material for the wood panel production, tree bark was also examined as a source to obtain chemicals and produce adhesives, mainly with its main extractive: tannins (Ogunwusi 2013). Studies on tannin-based resins have been performed since the 1950s and condensed tannins are preferred compared to hydrolysable tannins for this specific application (Feng et al. 2013). In addition, tannin foams are another product that is being studied. Tannin is a polyol both with aliphatic and aromatic hydroxyl groups. As a consequence it can be used to produce tannin-based polyurethane foams, tannin rigid foams and tannin formaldehyde furfuryl foams (Tondi and Pizzi 2009; Feng et al. 2013). There are several production methods using different formulations that create different types of porosity and therefore properties. One simple method to outturn foams is mechanical frothing. It has been used both for tannin-based foams (Szczurek et al. 2014) and with other by-products of the wood and pulp industry like black liquor (Jalalian et al. 2018). Industrially, mechanical foaming is not considered a major technology to produce polymeric foams (Lee et al. 1998). Other methods like soluble or reactive foaming are more common and well-developed techniques. However, mechanical foaming is still used for some batch process, like in the production of latex foams (Eaves 2004). Additionally, it has the advantage of being an inexpensive and simple manufacturing technique. Another important parameter to take into account is the type of bark used. In this study spruce bark was selected. Spruce is a coniferous tree whose bark is formed by sieve cells, the walls of which are made up of small pores, making the bark a porous structure itself (Fengel and Wegener 1989). This could be beneficial since small pores within the structure can have an impact on lowering the thermal conductivity of the material (Kain et al. 2013).

In the present study, a mechanical foaming method was applied to obtain several types of open porous materials, using bark as the main component. In addition, PVAc-D3, a common adhesive in the wood industry, and a surfactant (sodium dodecyl sulfate) were added to maintain the porous structure. PVAc is a thermoplastic used as an emulsion. In its structure, there are free hydroxyl groups that can crosslink with other compounds (Conner 2001). Other common adhesives like urea-formaldehyde have been studied to produce foams using the multifold reactions of urea and formaldehyde. Unfortunately, these materials tend to be brittle and highly negatively influenced by water absorption (Liu et al. 2019; Wu et al. 2020).

It is the aim of the present study to investigate the influence of different types of particle sizes and morphologies of bark as well as different amounts of surfactant on the properties of the materials produced. All the materials obtained showed an open macroporous and heterogeneous structure. Properties like density, thermal conductivity, water immersion and mechanical properties were studied, and some of them varied greatly as a function of the surfactant amount and the type of particles used.

Porous materials obtained with a foaming procedure and granulated bark as a raw material were successfully produced and studied.

The thermal conductivity and density results indicate a general trend of decreasing with the increase in surfactant. Correlation between porous characteristics and surfactant concentration has been described in other studies, where optimum pore characteristics were obtained with surfactant concentration close to the critical micellar concentration (Mohammad et al. 2018). Critical micellar concentration (cmc) is often used as a concentration criterion for foamability (Saint-Jalmes et al. 2005). SDS, the surfactant used in this study, has a defined cmc of 8.1–8.2 mmol/l at 25 °C. (Al-Soufi et al. 2012; Zhang and Meng 2014).

This matches well with the findings in this study, where the critical micelle concentration corresponds to the surfactant amount of 8 mmol/l. If higher values are employed, the results of density, thermal conductivity, maximum stress and change in mass due to immersion of the samples in water remain relatively constant. Lowest thermal conductivity recorded was approximately 0.06 W/mK for samples made with the refined particles and a density of around 0.2 g/cm³. Other thermal insulation materials made with bark constituent, like lignin, present similar results. Tondi et al. (2016) produced lignin-based foams, which had densities of 0.185 to 0.407 g cm^− 3 as well as thermal conductivities between 0.075 and 0.095 W m^− 1 K^− 1. Panels made directly with bark particles also present similar results. As an example, Kain et al. (2016) produced panels with thermal conductivities of 0.069 m^− 1 K^− 1 and densities of 0.25 g/cm³.

Mechanically disintegrating the raw material with the dry milling method using an ultracentrifuge (milled particles − 2 and – 0.2) produced particles with higher amounts of dust, particles smaller than 0.2 mm, as well as less variation in particle size range than with the wet refining procedure studied. As a result, when producing the materials viscosity is increased and agglomerations can be formed. This has a direct impact on several properties like density, thermal conductivity and compression resistance. Higher amount of bark dust enhances the particles interaction and produces agglomeration, increasing viscosity and therefore producing denser materials. Similar behavior is observed with other lignocellulosic materials, like wood (Yang et al. 2010). In regards to the refined particles, the amount of dust is considerably reduced and even if the particle size distribution is very similar to the milled particles − 2, their length to thickness ratio is higher. This particle’s characteristic produces bundles of elongated particles and is something that needs to be considered when analyzing the raw material (Stark and Rowlands 2003). Because of this, particle size distribution analysis needs to be studied with other characterization methods like, for example, microscopy. Aspect ratio is an important parameter that influenced the final properties, being in some studies even more critical than particle size distribution (Stark and Rowlands 2003). However, regardless of which disintegration procedure was used, all particles and fibres show irregularity in particle shapes, which is considered a positive influence on foam and emulsion stability (Lam et al. 2014). Actually, even if these two types, i.e. the refined and milled particles − 2, have similar particle size distribution, the results of density and thermal conductivity are different, demonstrating once again that not only the bark particle size distribution influences the final materials but also their morphology, and implementing a fibrous raw material shows an advantage in relation to the characterization performed. Applying wet thermo-mechanical disintegrating procedures to lignocellulosic materials is believed to enhance the surface morphology and chemistry of the resulting fibres, and consequently the adhesion and reaction activity to form materials is improved (Gao et al. 2011), which may be the reason why the porous materials obtained with the refined fibres show better results in comparison to the other particles studied.

The stress-strain curves exhibit a linear region when using low amount of surfactant (none at all or 2 mmol/l), and in these cases, the curves present the typical shape of cellular materials, and moduli values between 10 and 30 MPa, which are comparable to other foam structures (Szczurek et al. 2015). On the other hand, samples produced with a surfactant content higher than 2 mmol/l seem to produce too fragile materials. Even if their density and thermal conductivity are favourably lower, the mechanical properties are not comparable to other available materials. It seems that there is a turning point at which the mechanical properties start to decrease drastically, and this varies depending on the type of particle used. In the case of small particles, the limit is between 4 and 8 mmol/l and in the case of refined and medium particles, it is between 2 and 4 mmol/l. From this point where the mechanical properties decrease drastically, it may be because the surfactant content is too high, which creates a very porous, fragile and mechanically less stable structure (Gibson and Ashby 1999).

In other studies, tannin-based foams showed higher vapour sorption values, up to 41% at 90% moisture content (Jalalian et al. 2018) or in the case of tannin foams, water uptake values go up to 23% (Delgado-Sánchez et al. 2016). Bark contains a higher amount of fatty acids, compared with wood, like for example suberin, which is expected to limit moisture uptake (Heinämäki et al. 2015). This can contribute to the reduced results when comparing other foams or wood plastic composites. Regarding the biggest set of particles (milled particles − 2), they show moisture uptakes similar to non-treated spruce bark (Holmberg et al. 2016), which can be explained by the big size of the particles which is up to 3.15 mm. Additionally water absorption seems to be slightly increased with the increase in the surfactant amount. This result is in accordance with the study of Tondi and Pizzi (2009), where they studied the water absorption of tannin foams and concluded that increasing the surfactant concentration can cause an increase in water absorption.

Regarding the water uptake, samples with higher porosity, like the ones produced with the refined particles and high amounts of surfactant show higher water uptake values. However, it must be taken into account that when the samples are immersed in water, a certain amount may be trapped in the open porous structure, and as a consequence give higher water uptake values. Reduction was attempted after removing the samples and draining them for 30 min, after removing them from the water and before weighing them.

In the present work, four different batches of spruce bark were used to create different porous structures. All materials were produced by a simple mechanical foaming method using SDS as a surfactant and PVAc as an adhesive. Properties of the materials vary greatly, due to the differences in characteristics, especially particle morphology and size, of the initial bark particles and fibres, which can be an advantage if the objective is to tailor natural materials.

The materials present an open heterogeneous porosity with density results that go from 0.15 g/cm³ to up to 0.54 g/cm³ and thermal conductivity values of minimum 0.06 W/mK. All materials are stable when immersed in water for longer periods, absorbing up to 300% of their mass. Compared with results reported in the literature, the mechanical properties are similar to other natural and foamy structures; however, only when using low amounts of surfactant. On the other hand, using concentrations of surfactant similar to the critical micelle concentration presented the best results in terms of density and thermal conductivity.