Fabrication and Properties of Pine Fiber-Reinforced Polymer Composite Incorporating Suberinic Acids Extracted Under Different Conditions

TL;DR

This study explores how adding suberinic acids extracted from birch bark improves the processing and properties of pine fiber-reinforced polymer composites.

Contribution

The novel use of suberinic acids extracted under different conditions to enhance the processability and flexibility of wood–plastic composites.

Findings

Suberinic acids reduced extruder rotor torques, indicating better processability of the composite.

SAs increased elongation at break and bending deformation by up to 51.6% and 17.5%, respectively.

Maleic anhydride-grafted polypropylene improved mechanical properties and reduced water sorption.

Abstract

To improve the extrusion processing of wood–plastic composites (WPCs), functional additives known as internal lubricants are incorporated into the composite formulations. The lubricants play a crucial role in decreasing the melt viscosity of WPCs, which in turn has a positive impact on energy consumption, productivity, and overall composite performance. This study shows the effect of suberinic acids (SAs), extracted from birch outer bark via alkaline water and water–ethanol hydrolysis at different pH values, on the processing behavior and properties of a recycled polypropylene-based composite filled with pine microfibers. The extracted SAs were characterized by gas chromatography–mass spectrometry, Fourier transform infrared spectroscopy, gel permeation chromatography, thermogravimetric analysis, and differential scanning calorimetry. The conducted analyses revealed notable differences in the chemical composition, molecular weight, and molecular polydispersity of the SAs. Betulin was identified as the dominant component (49–86%). The pine sawdust was treated with 2% NaOH at 90 °C for 90 min prior to composite fabrication. The incorporation of 4.0 wt% SAs into the WPC formulations reduced the extruder rotor’s maximum and minimum torques torque, indicating improved processability of the composite. Mechanical and wetting properties of the WPC samples were evaluated. The samples containing SAs exhibited an increased elongation at break by 37.9–51.6% and bending deformation by 12.8–17.5%, depending on the extraction conditions of SAs, accompanied by a slight reduction in the mechanical properties and slight increase in water sorption compared with the composite filled with the alkaline-treated pine microfibers. The results showed enhanced flexibility and ductility in the SAs-containing WPCs. The presence of a 1.0 wt% maleic anhydride-grafted polypropylene in the samples led to an increase their mechanical properties, along with the reduced water sorption.

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1. Introduction

Ranking among Europe’s most forested nations, Latvia has more than 50% of its territory under forest cover. The forest and wood processing sector is a fundamental pillar of Latvia’s economy and its most significant industrial branch, contributing a substantial portion to the country’s gross domestic product. Sawdust, shavings, sanding dust, wood chips, and bark are common waste products of the wood processing industry. Sawn wood has the biggest share of export value, totaling EUR 703.5 million. Pine wood is a versatile and popular material in construction and interior design due to its properties and availability. It is a wood that is easy to work and shape, resistant to decay and rot, and has a clear and uniform appearance. Pine wood is a popular option for outdoor applications and interior design, from fence posts to hardwood floors. Scots pine (Pinus sylvestris) is the predominant species in Latvia, found in nearly 39% of the forest area [1].

Wood–plastic composite (WPC) is a sustainable material made by combining polymers with wood fibers, making it an eco-friendly composite [2,3]. With growing global concern over plastic waste, using recycled polymers as a matrix in WPCs has become increasingly appealing [4,5]. By combining recycled plastics with wood waste products, manufacturers can design WPCs with tailored properties, providing an effective way to reduce the environmental impact and improve the eco-efficiency of composite materials. Sometimes, WPCs using recycled polymers as a polymer matrix exhibit significantly enhanced properties compared to the composites based on virgin polymers [4,6]. Additionally, replacing traditional wood timbers with wood wastes for obtaining a filler for WPC formulations offers significant environmental benefits, saving forest resources [7,8,9].

The manufacturing of WPCs depends on many factors, such as polymer origin, its content, processing type, price, etc. The main conventional processes used in WPC manufacturing are extrusion [10,11], injection molding [12], and compression molding [13]. In addition, new advanced additive manufacturing processes using laser sintering [4] and fused layer mode [14,15,16] have also been applied. However, traditional WPC processing methods, such as extrusion and injection molding, are preferred for commercial production.

It is known that compatibility between a hydrophobic polymer matrix and a hydrophilic lignocellulosic filler significantly affects the properties of WPCs [17,18]. Most of the proposed methods for regulating the interaction at the polymer/lignocellulosic filler interface are based on a purposeful change in the functional composition of a wood filler, as well as on the application of functional additives such as compatibilizers, coupling agents, adhesion enhancers, surfactants, bonding agents, and dispersing agents that increase the adhesive affinity between the components in WPCs [19,20]. Several chemical methods can be employed to activate and functionalize lignocellulosic fibers, including alkalization, amination, ammoxidation, acetylation, benzylation, graft copolymerization, treatment with acids, peroxide, various anhydrides, permanganate, silane, etc., aimed at enhancing the adhesion on the polymer matrix/wood filler interface in the composites [21,22,23,24].

The effect of chemical treatments such as acetylation, mercerization, silane, maleic anhydride-grafted polypropylene (MAPP) treatment and isocyanate on the strength of the fiber and composites made from these fibers is reviewed.

The treatment of lignocellulosic fibers with an alkaline water solution is one of the most used chemical methods for their activation [25,26,27,28]. It is favored due to its ease of implementation and positive results achieved in terms of improved interfacial adhesion and overall composite performance. During alkalization, the hydrogen bonding within the wood network is disrupted or collapsed. The alkalization leads to the activation of the lignocellulosic surface and an increase in the availability of reaction sites as a result of the isolation of hemicelluloses due to the breaking of ester bonds, as well as the release of extractives under the action of an enhanced temperature. All the factors positively affect adhesive affinity between the wood filler and the polymer matrix in WPCs.

A wood biorefinery is a facility that uses wood as a raw material to produce a variety of bio-based products, including biofuels, biochemicals, and bioproducts, in addition to traditional paper and wood products. The concept is based on the idea of using renewable biomass resources to replace fossil-based resources. Wood biorefineries today use existing infrastructure (pulp/paper) to create basic chemicals from residues, but the future involves advanced technologies, shifting to a fully circular, product-focused bioeconomy with less fossil reliance [29,30,31].

A kraft pulp mill with an annual production of 400,000 tons and a plywood mill with an annual production of 252,000 m^3^ contribute significantly to the by-product bark stream. Specifically, these facilities produce approximately 28,000 and 16,000 tons of outer bark, respectively. It is worth noting that the calorific value of this outer bark exceeds 30 MJ/kg, making it a strong energy source. In addition to other residual materials such as veneer chips, wood chips, and sawdust, outer bark is used in power plants of pulp and plywood factories where it is burned efficiently [32,33,34]. On the other hand, outer bark is a sustainable source for replacing fossil fuel-based substances, considering the birch outer bark contains suberin. Suberin is a natural aliphatic-aromatic cross-linked polyester present in the cell walls of both normal and damaged external tissues. Due to the diverse chemical composition of suberin, it is an attractive alternative to hydrocarbon-based materials. Although its potential is recognized, it is not widely used in the polymer industry.

Depolymerization of suberin is the most widely exploited mode for obtaining suberinic acids (SAs) [35,36], which represent a complex of fatty acids (long-chain aliphatic saturated, unsaturated and dicarboxylic acids) covalently bonded with aromatic structures [37]. Due to the peculiarities of the chemical composition, SAs can be used as a formaldehyde-free adhesive for plywood [38,39], particleboards [40,41,42] and as a raw material for obtaining polyols for polyurethanes [37].

The functional additives that are traditionally used in manufacturing WPCs are essential to improve their processing, performance, durability, and appearance. The additives which are used to increase the interactions between polymers and wood fibers, and consequently the bonding between each other by treating their surfaces, are coupling agents and compatibilizers [43,44,45,46,47,48]. For improving the processing, standard lubricants are often used, such as glycerin monostearate, ethylene bis-stearamide, stearic acid, zinc stearate, paraffin waxes, oxidized PE, etc. [49,50]. Adding lubricants to WPC formulations decreases friction between the extrudate and the extruder barrel by forming a protective layer at the interface, reducing its viscosity, as well as increasing the productivity of the extruder and reducing energy consumption [51].

Despite their effectiveness, these synthetic lubricants are not biodegradable. Recently, an article [52] was published that showed that the SAs extracted from birch outer bark in water medium at pH 2 can be used as an internal lubricant, which positively affects the processing parameters and the properties of the developed wood–plastic composite reinforced with birch wood microparticles.

This research aim to study the effect of the chemical composition and thermal characteristics of three SAs extracted under different conditions on the processing and properties of a recycled polypropylene-based composite reinforced with pine waste microfibers.

2. Materials and Methods

2.1. Materials

Pine wood sawdust, a by-product of wood processing, was sourced from a Latvian private industry. The wood sawdust was subjected to elemental analysis (Elementar Analysensysteme GmbH, Langenselbold, Germany), and the chemical composition was determined using the Klason method for lignin (TAPPI 2002–2003) [53] and the Kürschner method for cellulose (TAPPI 1999) [54] content in pine wood. Hemicelluloses were determined according to TAPPI 1997 [55] and extractives according to [56]. For the alkaline treatment, the pine wood sawdust was milled and fractioned to obtain fiber particles ≤ 250 μm.

2.1.1. Milling and Sieving of Pine Sawdust

The pine wood sawdust was milled using an SM 100 cutting mill (Retsch GmbH, Haan, Germany). To separate the milled wood shavings into fractions, a vibratory sieve shaker AS 200 Basic (Retsch GmbH, Haan, Germany) equipped with a set of sieves was employed.

2.1.2. Alkaline Treatment of Milled Pine Wood Sawdust

The milled fraction was subjected to alkaline treatment in a 5 L three neck reactor equipped with mechanical stirring, a condenser, and temperature control. Two treatment regimes were compared: first, utilizing a 1% NaOH solution at 90 °C for a duration of 5 h; and second, applying a 2% NaOH solution, also at 90 °C, but for a shorter period of 1.5 h. A solid-to-liquid ratio of 1:20 was maintained in both cases. After treatment, the fibers were washed to neutral pH and dried. This procedure was used to enhance fiber accessibility and reduce extractive content prior to composite fabrication.

2.1.3. Preparation of Suberinic Acids SAs

Suberinic acids SAs were isolated from birch outer bark supplied by Latvijas Finieris (Riga, Latvia). The bark was air dried (moisture content of 4–5 wt%), milled, and fractionated (1–4 mm). Extractives were removed using ethanol according to an established protocol [57], and the remaining bark served as feedstock for depolymerization.

Approximately 2.0 kg of birch outer bark underwent depolymerization, which was conducted using two separate methods that differing in the medium—water or ethanol:

Alkaline hydrolysis in water: The alkaline suspension resulting from suberin depolymerization was acidified to pH 2 (SAs pH 2) and 5 (SAs pH 5) using HNO_3_, followed by sequential filtration to obtain SA pastes. The SAs were subsequently suspended in water (13 litres) and subjected to further filtration to eliminate particles above 1.5 mm before a second filtration was performed to remove excess water.

Alkaline hydrolysis in ethanol: 2.5 kg of extracted birch outer bark (1 ≤ d < 2 mm) was treated in a 30 L reactor using 4.3 wt% KOH in ethanol (1:8 bark-to-liquid ratio) at 78 °C for 30 min. After filtration, most of the ethanol (70 wt%) was recovered, and the concentrate was diluted with water and acidified to pH 2 using HNO_3_. Finally, the suspension was filtered under vacuum using a 4 L Büchner funnel and Grade 3 filter paper to separate KNO_3_ salts and other water-soluble compounds from the obtained SAs. The SAs were then washed with water and filtered again to obtain the final product—SAs Et-OH.

All SA samples contained 16–26% moisture and were used without further drying (Figure 1).

2.1.4. Incorporation of SAs into Fiber Formulations

Volumes of 100 mL of aqueous SA suspensions (14 wt% solids) were mixed with 100 g of alkaline-treated pine fibers to achieve a final SA content of 4 wt% in the composite, which corresponds to their optimal content examined by us earlier [34]. Each mixture was homogenized manually (3–5 min) and dried first at ambient conditions and then at 60 °C.

2.1.5. Composite Processing

Recycled polypropylene (rPP) from a Latvian recycling facility (Nordic Plast Ltd., Olaine, Latvia) served as the polymer matrix. A maleic anhydride-grafted polypropylene (Licocene PP MA 7452, Clariant Company, Muttenz, Switzerland) with ~7% graft content and a melt viscosity of 1100 mPa·s at 170 °C was used as a compatibilizer (≤1 wt%). Before compounding, all components were premixed and dried at 60 °C for 48 h. Composites were produced using a HAAKE MiniLab II twin screw extruder (Thermo Fisher Scientific, Karlsruhe, Germany) at 175 °C and 130 rpm with a 5 min residence time. Test specimens were molded using a MiniJet II injection system at 120 °C and 60 MPa. Dog bone samples for tensile testing and rectangular specimens for bending tests were prepared according to the methodology described before [34].

2.2. Characterization Methods

2.2.1. Elemental Analysis

The elemental analyzer Vario MACRO CHNS (Elementar Analysensysteme GmbH, Langenselbold, Germany) with a heat conduction detector was used to determine the elemental composition of pine wood shavings before and after the alkali treatment.

2.2.2. Torque

Rotor torque and the corresponding apparent melt viscosity of the rPP were evaluated using a HAAKE MiniLab micro compounder, which is designed for small-scale blending and rheological assessment of polymer composites. Measurements were performed at 175 °C and a screw speed of 130 rpm with a 5 g feed. Each formulation was tested in at least five independent runs to ensure reproducibility.

2.2.3. Mechanical Tests

Mechanical testing was performed on a Zwick/Roell universal testing machine (Ulm, Germany) equipped with a 0.5 kN load cell. Tensile measurements were carried out at 50 mm min^−1^, while flexural tests were conducted at 2 mm min^−1^. The procedures followed ASTM D638 (2007) [58] for tensile evaluation and ISO 178 (2010) [59] for bending. Data acquisition and processing were performed using the TestXpert software package. Prior to testing, specimens were conditioned at 60 °C for 24 h. Each property was determined from five replicate samples, and standard deviations were calculated accordingly.

2.2.4. Impact Strength

Impact resistance was evaluated using the Charpy method in accordance with ISO 179 2 [60]. Tests were performed on a CEAST 9050 pendulum impact tester (Instron, Norwood, MA, USA) equipped with a 5 J hammer operating at 2.9 m s^−1^. A minimum of five specimens was tested for each formulation to ensure reliable averages.

2.2.5. Water Uptake Test

Water absorption (WA) was assessed in accordance with ASTM D570 [61]. Prior to immersion, the composite specimens were dried at 60 °C for 24 h. The conditioned samples were then placed in distilled water and kept submerged for 24 days. After removal, excess surface moisture was gently wiped off and the samples were weighed. The WA was calculated using Equation (1)

$$[eqn]$$

where WA_t_ is the mass after 24 days of immersion and WA_0_ is the initial dry mass. Each formulation was evaluated using five replicates, and standard deviations were determined for all measurements.

2.2.6. Gel Permeation Chromatography (GPC)

GPC was carried out on an Agilent Infinity 1260 system equipped with a degasser, autosampler, refractive index detector, viscometer (ViscoStar, Goleta, CA, USA), and multi-angle light scattering detector (miniDAWN, Wyatt Technology Corporation, Santa Barbara, CA, USA). Separation was performed using two PLgel Mixed E columns (3 µm, 300 × 7.5 mm). Samples were dissolved in THF, and 100 µL injections were analyzed at a flow rate of 1 mL min^−1^ with the refractive index detector maintained at 35 °C. Calibration was based on polystyrene standards (500–30,000 Da) prepared at 2 mg mL^−1^ in THF. From the chromatograms, the weight average molecular weight (Mw), number average molecular weight (Mn), and polydispersity index (Pd = Mw/Mn) were determined.

2.2.7. Gas Chromatography–Mass Spectrometry (GC–MS)

GC–MS analysis was performed using a Thermo Scientific (Waltham, MA, USA) TRACE 1300 gas chromatograph coupled to an ISQ quadrupole mass spectrometer. Separation was achieved on a TG 5MS capillary column (30 m × 0.25 mm × 0.25 µm). Samples (1 µL) were injected in splitless mode at 250 °C, with helium as the carrier gas at 1.20 mL min^−1^. The oven program consisted of an initial hold at 150 °C for 5 min, followed by heating to 151 °C at 10 °C min^−1^, then to 300 °C at 2 °C min^−1^ with a final 15 min hold. The transfer line and ion source were maintained at 300 °C and 200 °C, respectively, and spectra were collected over 45–700 m/z.

Prior to analysis, suberinic acid samples were converted to their trimethylsilyl derivatives. Approximately 5 mg of material was dissolved in 100 µL of pyridine, mixed with 200 µL of a silylation reagent blend ((trimethylsilyl)imidazole/BSTFA/TMCS, 3:3:2 v/v/v), and heated at 70 °C for 20 min. Compound identification was based on comparison with the NIST mass spectral library.

2.2.8. Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectra were recorded using a Nicolet iS50 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a diamond ATR crystal. Each spectrum was collected at a resolution of 0.2 cm^−1^ by averaging 32 consecutive scans.

2.2.9. Thermogravimetric (TGA) Analysis

TGA was carried out on a Discovery TGA 5500 instrument (TA Instruments, New Castle, DE, USA). Samples were placed in platinum crucibles and heated under a nitrogen atmosphere (50 mL min^−1^). The temperature was increased from 30 °C to 700 °C at a constant rate of 10 °C min^−1^, and mass loss was recorded throughout the run.

2.2.10. Differential Scanning Calorimetry (DSC)

Thermal transitions of the SA samples were examined using a DSC822 differential scanning calorimeter (Mettler Toledo, Greifensee, Switzerland). Measurements were carried out on aluminum sealed pans under a nitrogen purge, with samples heated from 25 °C to 250 °C at a rate of 10 °C min^−1^.

3. Results and Discussion

3.1. Characterization of SAs Suberinic Acids

To investigate how various extraction conditions of SAs influence the performance as internal lubricants in a pine fiber-reinforced polymer composite, the chemical composition, and molecular weight distribution and thermal properties of the extracted SAs were analyzed.

3.1.1. Gel Permeation Chromatography Analysis

The results of GPC revealed distinct differences in the values of molecular weight and polydispersity index of SAs depending on their extraction conditions (Figure 2). SAs extracted in a water medium and precipitated at pH 2 have high values of Mw ≈ 63.1 kDa and Mn ≈ 5.17 kDa, and the highest polydispersity index (Pd ≈ 12.2).

These findings testify to the presence of mixtures containing both small oligomers and large aggregates formed by co-extracted polyphenolics, polymeric residues and triterpenoids. The sample of SAs extracted at pH 5 was characterized by the highest values of Mw ≈ 79.6 kDa and Mn ≈ 11.5 kDa, but had a narrower polydispersity (Pd ≈ 6.9) than the sample extracted at pH 2. It was proposed that this sample enriched both oligomers and mid-to-high-MW aggregates having a higher average molecular weight than the suberinic acid aggregates extracted at pH 2. The lowest values of average molecular weights (Mw ≈ 3.12 kDa and Mn ≈ 1.48 kDa) and the index of polydispersity (Pd ~2.1) were revealed for the sample extracted from the bark in ethanol media and then precipitated at pH 2, indicating the prevalence of oligomeric/monomeric fractions in its content.

3.1.2. Gas Chromatography–Mass Spectrometric Analysis

The GC–MS analysis was conducted to identify the chemical composition of suberinic acid samples extracted in water and ethanol media and precipitated at different pH values. The results indicate that the extraction conditions applied to the birch outer bark resulted in differences in the content of diols, fatty acids and their esters, hydroxy acids and their esters, diacids and their esters, as well as the presence of betulin and lupeol, as shown in Table 1.

GC–MS analysis revealed significant compositional differences among the SA samples (Table 1). SAs pH 5 contains the highest proportion of betulin (~86%), a rigid triterpenoid with limited mobility, which likely explains its weaker lubricating efficiency and lower impact on torque reduction. In contrast, SAs Et-OH is rich in methyl and ethyl esters of long-chain fatty acids, which are more fluid and thermally stable at processing temperatures, resulting in improved lubrication and reduced melt viscosity. SAs pH 2 contains a broader mixture of esters, diols, and carboxylic acids, providing a balance between lubrication and interfacial interaction. The higher content of polar functional groups in SAs pH 2 enhances compatibility with MAgPP, leading to improved mechanical performance.

At the same time, the sample extracted in water and separated at pH 5 does not have hexadecanoic acid methyl ester and ethyl stearate in its chemical composition, but contains 1,12-dodecanediol, 2-oleoylglycerol and cis-13-eicosenoic acid. The main component of its composition is betulin (≈86% area). This indicates that acidifying at pH 5 causes preferential recovery or co-precipitation of hydrophobic triterpenes and perhaps higher-MW oligomers.

The chemical composition of SAs extracted from bark in ethanol is very similar to the composition of the sample extracted in water and precipitated at pH 2. It contains hexadecanoic acid methyl ester and ethyl stearate, as well as aliphatic esters betulin and betulinic acid, cis-13-eicosenoic acid, various diols and long-chain esters. On the other hand, the features of the extraction of SAs from bark in water–ethanol media are conditioned by a higher relative share of lower-MW esters (C16 methyl ester ~28% area) and a narrower GPC distribution. The low index of polydispersity (Pd ≈ 2.1) testifies to this suggestion.

3.1.3. Fourier Transform Infrared (FTIR) Spectroscopy Analysis

The FTIR spectra of SAs extracted and precipitated under different conditions are shown in Figure 3.

The FTIR spectra of the three suberinic acid (SA) samples reveal clear compositional differences. Based on standard band assignments for SAs, several common features can be identified in all spectra: a broad O–H band at approximately 3350 cm^−1^, indicating the presence of free hydroxyl groups that may participate in interactions with MAgPP; strong C–H stretching bands at 2920 and 2850 cm^−1^, confirming the predominance of long aliphatic chains; and an intense absorption band at around 1730 cm^−1^, corresponding to ester carbonyl groups. Additionally, the band at approximately 1710 cm^−1^ is attributed to free carboxylic acids. The aromatic C=C vibration near 1600 cm^−1^ suggests the presence of phenolic and triterpenoid structures, particularly in the SAs sample separated at pH 2. The pronounced C–O stretching region (1250–1050 cm^−1^) supports the presence of ester linkages, which are most prominent in the SAs extracted in ethanol (SAs Et–OH). These functional groups are expected to play a key role in both the lubrication behavior and interfacial interactions with the polymer matrix. Overall, the FTIR data suggest that SAs extracted in ethanol are more chemically depolymerized, exhibit lower polydispersity, contain a higher proportion of ester functionalities, and possess fewer strongly hydrogen-bonding phenolic groups.

The FTIR spectrum of the sample extracted in water and separated at pH 2 also shows clear carbonyl and ester bands; however, it exhibits a more pronounced broad O–H absorption and enhanced aromatic/phenolic signals around 1600 cm^−1^ compared to the samples obtained from a water–ethanol mixture. This indicates a more heterogeneous composition, likely comprising a mixture of lower-molecular-weight esters and alkyl fragments together with a larger proportion of polyphenolic and extractive components (such as triterpenes and phenolics), which contribute to the additional hydroxyl and aromatic absorptions.

In contrast, the FTIR spectrum of the sample extracted in water and precipitated at pH 5 is characterized by strong aliphatic and carbonyl group absorption, a reduced intensity of aromatic bands, and a narrower O–H absorption region compared to the pH 2 water-extracted sample. This suggests that this fraction contains fewer phenolic components and is dominated by more hydrophobic, higher-molecular-weight or oligomeric suberinic structures together with a triterpene fraction. According to Figure 3, the FTIR spectrum of SAs extracted in ethanol is the richest in esterified, lower-molecular-weight suberin fragments.

Overall, by integrating the FTIR, GPC, and GC–MS data, it can be concluded that SAs derived in ethanol are chemically more homogeneous and enriched in esterified oligomers, whereas the other samples retain a broader chemical diversity, including polyphenolic compounds and higher-molecular-weight triterpenoids.

3.1.4. Thermogravimetric Analysis of SAs Suberinic Acids

Thermogravimetric analysis (TGA) was performed to assess the thermal stability of SAs obtained via alkaline extraction using water and ethanol as solvents at different pH values. The TGA profiles of the three samples of SAs extracted under different revealed distinct thermal degradation behaviors, reflecting the influence of solvent polarity and a precipitation pH on the composition and thermal stability of the SAs (Figure 4).

According to Figure 4, all SAs samples exhibited a minor weight loss in the low-temperature region up to ~150 °C, attributed to the evaporation of residual moisture and volatile compounds. The SAs Et-OH sample showed slightly lower initial loss, suggesting reduced hydrophilicity and fewer entrapped volatiles. At the main decomposition phase (~200–400 °C) it demonstrated the highest thermal stability, with onset of major degradation occurring above 250 °C and peak decomposition around 430 °C. This behavior indicates a more cross-linked or condensed structure, likely due to the selective solubilization of phenolic and aliphatic components in ethanol. The SAs pH 2 sample showed earlier degradation onset (~220 °C) with a broader decomposition range, suggesting a more heterogeneous composition with thermally labile fractions. Meanwhile, the SAs pH 5 sample exhibited intermediate stability, with degradation onset near 240 °C and a smoother decomposition curve, possibly due to reduced hydrolysis and better preservation of ester linkages during extraction. The char yield (at 600 °C) was highest for the SAs pH 5 sample, indicating a high content of thermally stable aromatic structures. According to the obtained results, despite of the difference in the extracted conditions, the obtained SAs are thermally stable up to 200 °C, which indicate the low weight loss.

3.1.5. Differential Scanning Calorimetry

DSC analysis was conducted to evaluate the thermal behavior of SAs obtained through alkaline extraction using water and ethanol as solvents at varying pH values. The DSC thermograms revealed distinct thermal transitions across the temperature range of 0–100 °C, indicating solvent- and pH-dependent variations in the molecular structure and thermal stability of the SAs. According to Figure 5, the SAs obtained at pH 2 and pH 5 exhibit broad endothermic transitions between 30 °C and 70 °C, suggesting the presence of loosely bound water and low-molecular-weight fractions. The sample extracted at pH 5 showed a slightly shifted and more pronounced endothermic peak, indicating increased thermal stability likely due to reduced hydrolysis and better preservation of ester linkages.

While the SAs Et-OH demonstrate a sharper endothermic peak around 60 °C, followed by a subtle exothermic event. This behavior may be attributed to the presence of ethanol-soluble phenolic compounds and a higher degree of cross-linking, which enhances thermal resistance and alters the decomposition profile. The SAs Et-OH exhibited the lowest temperature endothermic transitions and a relatively sharp single melting region, consistent with its lower molecular weight (Mn ≈ 1.5 kDa, Pd ≈ 2.1) and higher ester content observed by GC–MS and FTIR. In contrast, the SAs pH 2 showed a broad, multimodal thermal response with overlapping endothermic regions, reflecting its highly polydisperse composition (Mn ≈ 5 kDa, Pd ≈ 12). The SAs pH 5 displayed the highest softening temperatures and strongest high-temperature endothermic peaks, in agreement with its enrichment in high-molecular-weight triterpenoid and hydrophobic components.

3.2. Fabrication and Properties of WPC Samples

3.2.1. Alkaline Treatment of Pine Sawdust

The key challenge for wood–plastic composites (WPCs) is the weak adhesion between the polymer matrix and wood filler, which reduces their performance. Strong interfacial bonding at the polymer matrix–wood filler interface is essential for effective stress transfer, proper load distribution, and preventing fiber–fiber interactions that cause the filler agglomeration. Thus, developing efficient wood fiber functionalization methods is crucial. Numerous physical and chemical treatments of wood are known for enhancing adhesion at the polymer–wood interface. Among them, alkaline treatment is one of the most effective, widely used, and inexpensive methods to improve compatibility between lignocellulosic fibers and polymer matrices. An alkaline medium disrupts hydrogen bonds in the fiber structure, increasing amorphous cellulose content, surface roughness, and fiber separation from the bundles, while also removing water-soluble and partially water-insoluble extractives. As a result, the wood surface becomes more accessible for interfacial interaction with the polymer during processing.

In Figure 6, the changes in the composition of the pine sawdust after its treatment with different concentrations of NaOH solution and duration are shown. The difference in the composition of the pine sawdust after treatment under both regimes is minimal. The difference in the cellulose content is no more than 2.4%, while the differences in hemicellulose and lignin content are 3.1% and 1.4%, respectively. Additionally, the difference in the extractives content is limited to 2.7%.

Due to the weak dependence of the main wood component content in the pine sawdust after its treatment using two treatment regimes, for diminishing the treatment time to preserve the alkali treatment’s efficiency, the second regime for the modification of the pine sawdust was chosen.

3.2.2. Formulating and Processing WPC Samples

The composite samples reinforced with the initial and the alkali-treated pine microfibers (<250 μm) (TPS), containing SAs derived under different conditions, were prepared by the extrusion and molding method. To increase interface adhesion between rPP and microfibers, maleic anhydride-grafted polypropylene (MAgPP) was used as a compatibilizer. According to the formulations of the obtained WPCs samples given in Table 2, the contents of SAs and MAgPP in all composite samples are 4.0 wt% and 1.0 wt%, respectively.

It is known that the presence of a lignocellulosic filler in WPC increases its melt viscosity [62,63]. This is associated with the limited mobility of polymer chains in the presence of lignocellulosic particles as well as with filler particle cohesive interaction that creates defined difficulties in composite processing. Their main benefit is enhanced extruder rotor torque. The addition of a lubricant to a composite formulation can optimize the processing process by lowering the melt viscosity of the composite, thereby reducing the rotor torque and contributing to energy savings. Earlier, it was shown [52] that SAs extracted in water media and precipitated at pH 2 can be used as a potential internal lubricant to improve the processability of a developed WPC containing birch sawdust microparticles < 100 mk.

From the torque curves corresponding to the typical rheological behavior of a WPC sample during the extrusion process, the maximal and minimal torque values for the composite samples were obtained. The start of the sample melting and compounding related to the first torque peak; the minimal torque value after 5 min was steady-state and coincided with the end of the compounding. The values of maximum and minimum torques during the processing of the composite samples with different formulations are given in Figure 7.

It is evident that the treatment of the pine microfibers with the SAs leads to a decrease in both the maximum and minimum torque values compared to the torque values of the samples that do not contain these acids. The decrease in the minimum torque values is 18.2–27.3%, while the maximum torque decreases by 16.7–20.1%, depending on the SAs extraction conditions, in comparison with the sample filled with only alkaline-treated fibers. The obtained results testify the action of SAs as an internal lubricant in wood–plastic composites. On the other hand, it can be noted that the impact of the extraction conditions of SAs on their ability to reduce both torques is not significant. SAs obtained in ethanol show a pronounced tendency to reduce both torques, while the lowest reduction of both torques is observed with SAs pH 5. Conversely, the addition of a compatibilizer, MAgPP, to the sample formulations leads to the opposite effect, causing a slight increase in both torques. The observed increase in maximum and minimum torque ranges from 5.6 to 6.3% compared to their previous values. Since MAgPP in wood-plastic compositesWPCs forms covalent and hydrogen bonds with hydroxyl and carboxyl groups on the surface of the wood fibers, the negative effect on the torque values can be attributed to the partial cross-linking of the composite samples. As a result, this cross-linking enhances the melt viscosity of the material.

3.2.3. Mechanical Properties

Figure 8 and Figure 9 show the results of the mechanical tests on the obtained WPC samples containing the initial, the alkaline-treated pine sawdust, as well as the alkaline-treated pine sawdust containing different SAs. It can be seen that the alkaline treatment of the pine sawdust increases tensile strength from 16.9 MPa to 18.3 MPa (by 7.1%) and bending strength from 17.9 to 18.7 MPa (by 4.5%); tensile and bending moduli increase from 749 MPa to 802 MPa (by 6.9%) and from 1545 MPa to 1605 MPa (by 3.7%), respectively; and elongation and bending deformation decrease from 12 to 9.5% (by 20.9%) and from 4.3 to 3.8 mm (by 9.3%), respectively, relative to the mechanical indexes of the composite filled with the initial sawdust.

The increased mechanical strength, modulus and the decreased deformation indicate the improved interfacial adhesion and the enhanced stiffness of the composite sample filled with the alkali-treated filler as a result of the disruption of hydrogen bonds between the cellulose fibers and the increase in the cellulose content and the partial removal of hemicellulose and extractives, making the wood surface more accessible for interaction with the recycled polypropylene matrix.

The treatment of wood fibers with natural compounds extracted from lignocellulosic materials has become a more attractive method for wood modification [64,65]. Compounds derived from lignocellulosic materials such as oils, extractives, resins and waxes can modify the wood surface and change its properties. In this study, for the following treatment of the alkaline-treated pine microfibers, SAs extracted under different conditions from birch outer bark were used. According to Figure 8 and Figure 9, the treatment of the alkaline-treated sawdust with SAs results in a slight decrease in both tensile (by 7.2–11.0%) and flexural (by 4.8–7.1%) mechanical strengths, as well as decreases in tensile and bending moduli (by 3.3–4.6% and 11.2–12.9%, respectively) relative to the mechanical indices of the composite sample filled with the alkaline-treated pine microfibers.

The significant increase in elongation (by 37.9–51.6%) and bending deformation (by 12.8–17.5%) for the samples containing Sas, against the background of a slight drop in the values of their mechanical properties in comparison with the sample filled with the initial sawdust, indicates that the obtained composite becomes more flexible and ductile. Such behavior of the composite samples in the presence of SAs indicates weakening of the interactions between the polymer matrix and the wood filler. Earlier, it was shown [52] that SAs precipitated at pH 2 can perform combined functions as both an internal lubricant and a compatibilizer. However, the SAs adsorbed at the surface of pine sawdust were not able to act as a compatibilizer in the composite filled with the softwood microfibers. This can be explained by the differential content of water-insoluble extractives in softwoods and hardwoods such as resins, fats, waxes, terpenes and some phenolic compounds that do not dissolve in water. The total amount of the water-insoluble substances is greater in softwoods (2–8% on dry weight) than in hardwoods (1–5% on dry weight). It is known that the alkaline treatment removes some water-insoluble extractives from pine, such as resin acids and fatty acids, by converting them to soluble forms, but it cannot remove waxes and terpenes [66,67]. This confirms the data presented in Figure 6, according to which the alkaline treatment does not fully remove the hydrophobic extractives from the pine sawdust. It can be suggested that the adsorption of SAs on the pine sawdust proceeds due to hydrophobic interactions between the hydrophobic extractives remaining on the pine fiber surface and hydrophobic compounds present in SAs; meanwhile, their hydrophilic species in the form of hydroxylic and carboxylic groups are oriented towards the polymer matrix, which can hinder the interfacial interaction between the polymer matrix and pine fibers, resulting in an insufficient decrease in mechanical strength.

According to Figure 8 and Figure 9, the difference in the effect of the addition of SAs extracted under different conditions on the mechanical properties of the composite samples is not pronounced. The composite sample containing SAs pH 5 shows a slightly higher value of tensile strength, tensile modulus and a comparatively lower elongation. At the same time, the samples containing SAs pH 2 and SAs Et-OH are slightly more flexible in comparison with the sample containing SAs pH 5. The difference in the bending properties of the composite samples with the SAs derived under different conditions was minimal. It was shown that the tensile properties are more sensitive to the action of SAs than the bending properties, because the softening of the polymer matrix under the action of the SAs as an internal lubricant has less effect on the ability of the composite to bend.

It is known that coupling agents and compatibilizers are often used for enhancing interfacial bonding in wood–plastic composites. Polypropylene grafted with maleic anhydride (MAgPP) is often used for this purpose. This additive binds a wood filler and a polymer matrix by forming hydrogen or covalent bonds between the hydroxyl groups at the wood fiber surface and MAgPP. In addition, chains of the polypropylene matrix can form entanglements with the MAgPP polymer chains, which result in higher interfacial adhesion. The impact of the incorporation of 1% MAgPP in the formulation of the composite samples on its mechanical properties is reflected in Figure 8 and Figure 9. According to the obtained results, the presence of the compatibilizer causes a significant improvement in the mechanical properties of all samples, including the sample filled with the alkaline-treated pine microfibers, relative to their values of mechanical testing of the composite samples without MAgPP. It can be seen that the increase in tensile strength for the samples containing SAs is by 40.5–46.6%, bending strength by 56.3–66.8%, and for the sample containing the alkali-treated sawdust without MagPP by 33.1 and 66.3%, respectively. The increase in the tensile and bending moduli for the samples containing SAs varies from 6.1 to 14.6% and 14.1 to 18.5%, respectively. The increase in the mechanical properties is accompanied by a sharp drop in elongation deformation by 41.0–61.5% and, to a lesser extent, in the bending deformation by 2.3–4.4%. While the differences in mechanical properties among the samples with varying SAs are not highly pronounced, the results indicate that the sample containing SAs at pH 2 exhibits the highest mechanical properties. This sample demonstrates a tensile strength of 23.9 MPa, tensile modulus of 899 MPa, bending strength of 29.7 MPa, and bending modulus of 1688 MPa. In contrast, the sample with SAs Et-OH shows relatively lower properties, with a tensile strength of 23.2 MPa, tensile modulus of 812 MPa, bending strength of 27.5 MPa, and bending modulus of 1595 MPa. Notably, this sample also exhibits a reduction in tensile strain. These findings support the suggestion of a preferred orientation of SAs at the adsorption on the surface of pine microfibers. Since MAgPP in the wood-plastic composite samples primarily reacts with hydroxyl and carboxyl groups on the fiber surface to form covalent and hydrogen bonds, the composite sample containing SAs at pH 2 is expected to show the highest increase in mechanical properties. This is because, according to the results of conductometric titration presented in Table 3, SAs extracted at pH 2 have the highest concentration of hydroxyl and carboxyl groups among the other acids. The GC–MS and FTIR data support the obtained results.

In contrast, SAs derived from ethanol have the lowest molecular weight and have more esterified fatty acids, hydroxy acids, diacids, and other hydrophobic compounds, along with fewer hydroxyl groups. The composite containing SAs at pH 5 exhibits intermediate properties.

3.2.4. Impact Strength Analysis

Impact resistance is an important factor in assessing composite materials in different applications. These materials usually absorb energy from applied forces quickly, which can lead in some cases to mechanical failure in defined situations, namely, a composite material is more likely to break under an impact force than under a gradually applied force. Figure 10 presents the results of impact strength tests depending on the formulations of the samples. It is evident that both alkaline treatment of the filler and the inclusion of SAs enhance the samples’ ability to absorb impact energy. Alkaline treatment is known to increase the surface area of the wood filler, improving the interfacial adhesion between the filler and the polymer matrix, which in turn positively impacts the material’s impact strength. The increase in impact strength for the sample containing the alkali-treated filler, compared to the sample with untreated sawdust, is limited to 5%. For the samples containing SAs, the improvement in impact strength relative to the alkali-treated filler sample depends on the extraction conditions of the SAs. The most significant increase, 11.2%, is observed for the sample with SAs Et-OH. The impact strength of samples with SAs extracted at pH 2 increases by up to 5.2%, while the sample with SAs extracted at pH 5 shows a change of less than 1%. The increase in impact strength for the samples containing SAs can be attributed to both enhanced mobility of the matrix polymer chains and a reduction in the stiffness of the samples. This is supported by the high tensile deformation values observed for the samples containing the SAs compared with the samples filled with the initial and the alkali-treated pine fibers.

The addition of the compatibilizer to the sample formulations resulted in a significant decrease in impact strength. The largest reduction in impact strength was observed in the sample with SAs at pH 2 (12.4%), while the smallest decrease occurred in the sample with SAs Et-OH (11%). Despite the reduction in impact strength, the samples containing SAs demonstrated a greater ability to absorb impact energy compared to the sample with only alkali-treated filler. Additionally, the sample with SAs Et-OH exhibited the highest impact strength value, measuring 12.0 kJ/m^2^. The improvement in the mechanical properties of the samples with MAPP, as previously mentioned, is due to the enhanced interfacial adhesion. This strengthening occurs through the cross-linking of the composite system, which results from the interaction between the hydroxyl groups of the acids and the reactive groups of MAPP during processing, leading to the formation of ether bonds. It is obvious that decreasing flexibility and increasing stiffness lead to brittleness, decreasing the ability of the samples to absorb impact energy.

3.2.5. Water Orption

It is known that the behavior of wood-plastic composites WPCs when exposed to water is significantly influenced by the design of the formulation [68,69]. Water sorption can lead to dimensional instability, a loss of mechanical properties, and increased vulnerability to microbial attack. When moisture from the environment reaches the fiber/matrix interface, the wood fibers absorb water and expand. This swelling generates shear stress at the interface, eventually causing the interface to debond, resulting in a greater loss of mechanical strength after water exposure. Figure 11 presents the water sorption values of samples filled with alkaline-treated sawdust, as well as those containing various SAs with and without MAgPP. The results show that the inclusion of SAs extracted at pH 2 and pH 5 increases water uptake by 3–10% compared to the sample filled with the alkaline-treated pine fibers without the lubricant. Additionally, the incorporation of SAs Et-OH into the composite formulation results in a 10% reduction in water sorption.

The variation in the behavior of the samples can be attributed to the increased presence of polar functional groups, such as hydroxyls and carboxyls, in the chemical composition of the SAs extracted at these specific pH values. These functional groups are capable of attracting and binding water molecules through hydrogen bonding, which results in higher water uptake. In contrast, the sample containing SAs Et-OH shows lower hydrophilicity, which is consistent with the chemical composition of SAs Et-OH. The inclusion of MAgPP in the sample formulations significantly enhances their hydrophobicity by reducing the content of free hydrophilic groups, primarily through etherification and hydrogen bonding. The greatest reduction in water uptake (16%) was observed in the sample containing SAs at pH 2, while the smallest decrease (11%) occurred in the sample with SAs Et-OH. Notably, the sample containing SAs at pH 5 exhibited higher water sorption values than the other samples, both before and after the addition of MAgPP.

4. Conclusions

Three samples of suberinic acids SAs were extracted under different conditions: in an aqueous medium at pH 2 and pH 5, and in an ethanol medium. The samples differed in chemical composition, molecular weight, and polydispersity index. The acids extracted at pH 2 contained a broader mixture of esters, diols, and carboxylic acids and exhibited a weight-average molecular weight of 63.1 kDa with the highest polydispersity index. In contrast, the SAs extracted at pH 5 were enriched in hydrophobic triterpenes and higher-molecular-weight oligomers, resulting in the highest molecular weight of 79.6 kDa, but a narrower molecular weight distribution. The acids extracted in ethanol exhibited the lowest molecular weight of 3.12 kDa and the lowest polydispersity index. The SAs extracted at pH 5 contained the largest amount of betulin (~86%), which likely accounted for their weaker lubricating efficiency and reduced effect on torque reduction. The ethanol-extracted SAs were rich in methyl and ethyl esters of long-chain fatty acids. These compounds were more fluid and thermally stable at processing temperatures, leading to improved lubrication and reduced melt viscosity. The incorporation of 4.0 wt% SAs into the composite samples resulted in improved extrusion conditions, as evidenced by reductions in both maximum and minimum torques of 16.7–20.1% and 18.2–27.3%, respectively, confirming the lubricating function of these acids in the composites. The SAs extracted in ethanol produced a greater reduction in the torques than those obtained in an aqueous medium, whereas the composite containing the acids extracted at pH 5 exhibited the lowest torques reduction. The incorporation of the SAs in the composite samples led to decreases in the tensile strength (7.2–11.0%), flexural strength (4.8–7.1%), and tensile (3.3–4.6%) and flexural moduli (11.2–12.9%). At the same time, water sorption increased by 3–10% relative to the composite filled with alkali-treated pine fibers. The observed improvements in impact strength of the obtained compositesamples depended on the extraction conditions of the acids. The largest increase in the impact strength by 11.2% was observed for the composite containing the SAs extracted in ethanol, while the impact strength increased by up to 5.2% for the acids extracted at pH 2 and by less than 1% for those extracted at pH 5. The relatively small differences in the properties of the composite samples containing the SAs with varying chemical compositions, molecular weights, and molecular polydispersity may be attributed to the specifics of the adsorption of the compounds included in the SAs onto the surface of the alkali-treated pine microfibers. According to the assumption, the adsorption of the SAs on the surface of the alkaline-treated pine microfibers may occur via hydrophobic interactions, while the hydrophilic groups of the SAs are oriented toward the polymer matrix. The validity of this assumption was confirmed by the fact that the incorporation of only 1.0 wt% maleic anhydride-grafted polypropylene into the samples led to a remarkable increase in their mechanical properties—by 40.5–46.6% and 56.3–66.8% in tensile and bending strengths, respectively—which overruled the effect of this compatibilizer on the mechanical properties of the composite reinforced with the alkaline-treated fibers. The results indicate that the composite WPC sample containing the acids extracted at pH 2, which has the highest content of hydroxyl groups, resulted in the greatest enhancement in mechanical properties. Despite the increase in the stiffness of the composite samples in the presence of the compatibilizer, the increase in both torques did not exceed 5.6–6.3% compared to their previous values, confirming the role of the SAs as an internal lubricant.