The birch (Betula spp.) outer bark is a valuable product rich in betulin. After removal of betulin extractives, suberin containing tissues are left. Suberin is a biopolyester built from α,ω-bifunctional fatty acids (suberinic acids), which after depolymerization together with lignocarbohydrate complex is a potential adhesive as a side-stream product (residue) from obtaining suberinic acids for polyol synthesis. In this work, we studied the utilization possibilities in particleboards of the said residue obtained by depolymerization in four different solvents (methanol, ethanol, isopropanol and 1-butanol). The adhesives were characterised by chemical (acid number, solubility in tetrahydrofuran, epoxy and ash content) and instrumental analytical methods (SEC-RID, DSC, TGA and FTIR). Based on the results of mechanical characteristics, ethanol was chosen as the most suitable depolymerization medium. The optimal hot-pressing parameters for particleboards were determined using the design of experiments approach: adhesive content 20 wt%; hot-pressing temperature 248 °C, and hot-pressing time 6.55 min.

Developing polyols from biomass sources contributes to a more circular economy by replacing petroleum-based polyols in the vast production of polyurethanes (PUR). One such potential biomass source could be leftover birch bark from which suberinic acids (SA) can be obtained. The purpose of this study was to identify the best synthesis routes for novel SA-based polyols, obtain rigid PUR foams, and evaluate their competitiveness and potential suitability as thermal insulation material. Novel polyols were synthesized from depolymerized SA by esterification with various functionality and molecular weight alcohols in several molar ratios. The moisture content, hydroxyl and acid values, and apparent viscosity were tested. Free-rise rigid PUR foams from the most suitable SA-based polyol and tall oil-based polyol were successfully prepared, reaching ~20 wt.% total renewable material content in the foam. The obtained rigid PUR foams’ morphological, mechanical, and thermal properties were investigated and compared to present foam materials, including commercial foams. The apparent density (~33 kg/m³), as well as the closed cell content (~94%), compression strength (0.25 MPa, parallel to the foaming direction), and thermal conductivity (~0.019 W/(m·K)), approved the competitiveness and potential ability of SA-based rigid PUR foam production as thermal insulation material.

Global sustainability challenges prompt the world to modify its strategies and shift from a fossil-fuel-based economy to a bio-resources-based one and to the production of renewable biomass chemicals. Depolymerized suberinic acids (SA) were considered as an alternative resource to develop bio-polyols that can be further used in polyurethane (PU) material production. Birch (Betula pendula) outer bark was used as a raw material to obtain the SA, extracted with ethanol, and depolymerized with potassium hydroxide ethanol solution. By acidifying the filtrate to pH 5.0, 3.0, and 1.0 and drying it at 50 °C and 130 °C, 12 different SA potential feedstocks were obtained and characterized using chemical (total phenolics content, solubility in DMSO, acid, hydroxyl, and saponification number) and instrumental analytical methods (GC-MS, SEC-RID, DSC, and FTIR). Several bio-polyols were synthesized from the SA sample acidified to pH 1 and dried at 130 °C. Acid number and hydroxyl number values, the apparent viscosity and moisture content were measured. It was concluded that SA have a high enough saponification and acid value to investigate the polyol synthesis route via the esterification reaction. Moreover, SA had OH groups in their structure, which can be exploited for PU material development. The majority of SA compounds had relatively low molecular weight with <1300 Da that are suited for bio-polyol synthesis applied for rigid PU foam development. The synthesized bio-polyols had high hydroxyl number values necessary for bio-polyols to be used for rigid PU foam production.

Birch outer bark is a byproduct accumulated from birch tree (Betula pendula Roth. and Betula pubescens Ehrh.) processing and contains up to 50 wt% suberin. Suberin is a biopolyester built mostly from α,ω-bifunctional fatty acids (suberinic acids) which after depolymerization together with lignocarbohydrate complex is a potential adhesive for obtaining particleboard composites. These composites can be made in hot-pressing process at around 225 °C temperature. The high hot-pressing temperature for suberinic acid-based adhesive is a disadvantage compared to hot-pressing temperatures for conventional resins such as urea-formaldehyde and phenol-formaldehyde (120–130 °C and 170–180 °C respectively). In this study we looked into possible ways to improve the properties (modulus of elasticity (MOE), bending strength (MOR) of suberinic acid-based particleboards by utilization of catalysts (additives) for SA-based adhesive polymerization, thus reducing hot-pressing temperature by potentially achieving the same mechanical properties as for the catalyst-untreated boards hot-pressed at higher temperature. The suitability of potential protonic acid, Lewis acid, as well as tin-based and basic catalyst was studied with differential scanning calorimetry (DSC) by quantifying specific enthalpy of the exothermic peak with an onset temperature of ≈185 °C. Based on DSC results, six catalysts (mixtures) were chosen (sulfanilic acid, p-toluenesulfonic acid, Al₂(SO₄)₃, SnCl₂, SnCl₂/p-toluenesulfonic acid, dibutyltin oxide) for further studies on the effects of mechanical properties of particleboards. Treatment with 5 wt% sulfanilic acid, 5 wt% p-toluenesulfonic acid (free suberinic acid basis) significantly improved both MOE and MOR values. 6 wt% Al₂(SO₄)₃ significantly improved MOE value.

Suberin is a complex polyester biopolymer, and it is practically impossible to estimate the real content of suberin in suberised plant tissues. This indicates the importance of the development of instrumental analytical methods for the comprehensive characterisation of suberin derived from plant biomass for the successful integration of suberinic products into biorefinery production chains. In this study, we optimised two GC-MS methods—one with direct sylilation, and the second with additional depolymerisation, using GPC methods with RI detector and polystyrene calibration and with a three-angle light scattering detector and an eighteen-angle light scattering detector. We also performed MALDI-Tof analysis for non-degraded suberin structure determination. We characterised suberinic acid (SA) samples obtained from birch outer bark after alkaline depolymerisation. The samples were particularly rich in diols, fatty acids and their esters, hydroxyacids and their corresponding esters, diacids and their corresponding esters, as well as extracts (mainly betulin and lupeol) and carbohydrates. To remove phenolic-type admixtures, treatment with ferric chloride (FeCl₃) was used. The SA treatment with FeCl₃ allows the possibility to obtain a sample that has a lower content of phenolic-type compounds and a lower molecular weight than an untreated sample. It was possible to identify the main free monomeric units of SA samples by GC-MS system using direct silylation. By performing an additional depolymerisation step before silylation, it was possible to characterise the complete potential monomeric unit composition in the suberin sample. For the molar mass distribution determination, it is important to perform GPC analysis. Even though chromatographic results can be obtained using a three- laser MALS detector, they are not fully correct because of the fluorescence of the SA samples. Therefore an 18-angle MALS detector with filters was more suitable for SA analysis. MALDI-Tof analysis is a great tool for the polymeric compound structural identification, which cannot be done using GC-MS. Using the MALDI data, we discovered that the main monomeric units that makes up the SA macromolecular structure are octadecanedioic acid and 2-(1,3-dihydroxyprop-2-oxy)decanedioic acid. This corresponds with GC-MS results, showing that after depolymerisation hydroxyacids and diacids were the dominant type of compounds found in the sample.

The effects on birch outer bark (BOB) ethanolic extractives’ chemical composition after recrystallization with C2–C5 alkanols were studied in this paper. Ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 1-pentanol were used as solvents. The solubility of BOB extractives at the solvents boiling point was determined empirically. It was found that, with an increase of the boiling point of the solvents, the solubility of the extractives increased, reaching up to 486 g/L in 1pentanol at 142.2 °C. Recrystallization yields reached up to 67 wt% using 2-butanol and the purity of betulin up to 96 wt% using 2‐propanol as a solvent. Considering the yield of recrystallization, betulin content, boiling point of the solvent, solvent loss, toxicity and eco-friendliness of the solvent, ethanol was chosen to be the most suitable solvent for industrial scale purification of betulin in the BOB extractives. Using only one recrystallization step the content of betulin was increased by 20% — from 75 to 95 wt% and the yield of recrystallization was 32.1 g per liter of solvent.