Raimonds Makars

Birch (Betula spp.) outer bark is a renewable side stream rich in suberin, a natural polyester that can be depolymerized into suberinic acids (SA) containing hydroxyl, carboxyl, epoxy, and other functionalities relevant for bio-polyol synthesis. A two-level fractional factorial design (2⁵⁻¹), combined with regression modelling and response-surface visualization, was applied to evaluate the effects of alkali type, alkali concentration, temperature, reaction time, and acidification pH on SA composition and properties. Sixteen depolymerization experiments were performed, and the resulting SAs were characterized by epoxy content, acid and hydroxyl numbers, saponification number, total phenolic content, molecular weight (MW) distribution, extractives fraction, ash content, and GC–MS-detectable low-MW compounds. Most statistical models showed satisfactory explanatory performance within the investigated experimental domain (R² = 0.77–0.93), while the model describing hydroxy acids and corresponding esters exhibited lower explanatory power (R² = 0.49) and was excluded from the response-surface optimization. Among the investigated factors, acidification pH exerted the strongest influence on acid number, MW distribution, and recovery of depolymerization products. Alkali concentration generally increased depolymerization severity, resulting in higher proportions of low-MW compounds and increased functionality. Differences between KOH and NaOH were observed in several compositional parameters, suggesting differences in depolymerization and recovery behavior. The results identify experimentally relevant regions within the investigated factor space that may be suitable for producing SAs with different compositional profiles depending on subsequent purification requirements. These findings provide an initial quantitative description of the relationships between depolymerization conditions and SA characteristics and support further development of birch-bark-derived SAs as renewable feedstocks for bio-based polyol production.

In the last two decades, there has been increased interest in research focused on developing innovative polymer composite materials for food packaging, obtained by compounding polymers with organic fillers, intended for the manufacture of food storage containers. Woodplastic composites (WPCs), due to high content of lignocellulosic filler, are consideredbiocomposites and can be used for the fabrication of such types of containers. Their formulations include, along with the matrix and the filler, functional additives. One of the most important considerations while developing a food packaging material is to choose eco-friendly additives. The suberinic acids (SAs), extracted from birch outer bark by hydrolysis in KOH water solution, were examined as lubricants in the formulation of recycled polypropylene/polylactic acid (rPP/PLA) composite filled with the alkali-treated milled birch shaving microparticles and proceeded by extrusion and injection molding. The incorporation of Sas in a birch wood-rPP/PLA composite was performed by treaing the wood microparticles with the SAs water suspension at the defined concentration. Their presence at the optimal content in the composite improved its processing by reducing the extruder rotor torque and injection pressure, which increased the mechanical properties and decreased the wettability of the composite.

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.

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.

Global sustainability challenges prompt the world to modify their strategies and shift from a fossil-fuel-based economy to a bio-resources-based one and to the production of renewable biomass-based chemicals. Different processes exist that allow the transformation of raw biomass into desirable bio-based products and/or energy. Depolymerized suberinic acids (SA) can be considered as an alternative resource to different feedstocks for value-added product obtaining, for example to develop bio-polyols that can be further used in polymeric material production. Birch outer bark was used as a raw material for obtaining SA and was extracted with ethanol to remove extractives. Further extracted birch outer bark was depolymerized with potassium hydroxide ethanol/methanol/n-butanol/2-propanol solutions. Obtained SA suspension was dried by performing lyophilization or by drying at 100 °C. As a result, we obtained 8 SA samples, which were characterized using chemical (total phenolics content, solubility in DMSO, acid number, hydroxyl number, and saponification number) and instrumental analytical methods (GC-MS, SEC-RID, FTIR). The most suitable SA for obtaining value-added product, such as bio-polyol, were obtained by using ethanol as a solvent for depolymerization process. The corresponding SA had the best results in terms of dry matter content, yield, solubility in DMSO, epoxy groups, acid number and ash content. After GC-MS results it was concluded that the amount of diacids, hydroxyacids and extractives are dependent of the used alkanol for SA depolymerization as well as the drying method.

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.

Application of the adapted methylene blue (MB) adsorption method in determining specific surface area was investigated on particles containing hydrogels obtained from purified (99 wt%) and unpurified (60 wt%) betulin samples. In order to study the adsorption isotherms of MB on betulin particles in aqueous dispersions, initial concentration of MB was varied. An analysis of the adsorption process with the Langmuir adsorption model was performed. The results indicate that the experimental data fits very well with the Langmuir adsorption model. Straight isotherms are obtained with very good determination coefficients (R²=0.99; R²=1.00), which means that the surface of purified and unpurified betulin particles is homogeneous energetically and a monomolecular coverage forms during adsorption. The MB adsorption method adapted for betulin particles is simple and requires less complex apparatus and time than other methods. The determined specific surface area can be used to characterize the surface properties of betulin particles.

The aim of the study was to investigate the effect of process parameters on the production of hydrogels with antisolvent precipitation using unpurified (UB) and purified (PB) birch outer bark betulin powder samples. Experimental activities are focused on the production of hydrogels using concentration of colloidal dispersions with filtration of UB or PB sample particles obtained by antisolvent precipitation method varying the dilution of saturated (at boiling temperature) ethanol solutions with different water content. During the study the maximum solubility in ethanol at boiling point of PB (22.0 g/L) and UB (55.0 g/L) was determined. For obtaining gel with the highest liquid content (absorption capacity 37.45 g per 1 g of dry matter) from PB by antisolvent precipitation the optimal saturated ethanol solution dilution with water was 12.5 vol%. In the case of UB hydrogel, by dilution of ethanol from 25 to 10 vol%, a gradual decrease of hydrogel dry matter yield and a simultaneous increase of the filtrate dry matter yield can be observed, which is related with leaching of the particles through the filter paper together with a solution and form a colloidal suspension of particles. For obtaining gels with high liquid content from UB the optimal saturated ethanol solution dilution with water was in the range from 20.0 to 12.5 vol% (absorption capacity in average 18 g per 1 g of dry matter). Comparing the chemical compositions of the raw materials and dry matter of hydrogels in the case of PB composition remains quite similar while in UB dry matter composition the betulin content increased from 52 to even 69 wt%. PB at the optimal dilution of 12.5 vol% of ethanol managed to achieve average particle size - 231.7 nm and with UB in the optimal dilution range from 20 to 12.5 vol% the range of average particle size was from 304.7 to 189.8 nm.

Dynamic light scattering (DLS) is a well-established technique to analyze particle size in a liquid medium and sample preparation is an essential step of the analysis to obtain reliable data. In this study, the pretreatment of an aqueous suspension containing betulin particles was studied to characterize the colloidal betulin particles in supramolecular hydrogels obtained by the liquid antisolvent precipitation from purified and unpurified betulin samples. Parameters of two-step homogenization process using rotor and ultrasonic homogenizer, particle concentration, and pH were systematically varied to obtain stable aqueous suspensions that meet the requirements of DLS technique for good particle size analysis. It was found that the purified betulin particles have a higher tendency to agglomerate and their suspension is stable in a narrower concentration and pH range than unpurified. Particle size analysis in suspensions prepared independently shows good reproducibility of sample preparation.

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.