Potential of Biodegradable Polyhydroxyalkanoates for the Construction of Sustainable Polymer Composite Materials

TL;DR

This study explores biodegradable composites made from poly(3-hydroxybutyrate) and plant materials, showing their potential as sustainable materials with varying mechanical and degradability properties.

Contribution

The study introduces perspective biodegradable composites with plant-based fillers and provides new insights into their properties for sustainable material development.

Findings

Composites with 70% wood flour showed the highest degradation rate of 77.4% after 120 days in soil.

The Young’s modulus of the composites ranged from 2640 to 3715 MPa depending on filler composition.

Plant-based fillers significantly influenced mechanical and degradability properties of the composites.

Abstract

The article presents the results of a study of constructed composites based on degradable poly(3-hydroxybutyrate) (P(3HB)) filled with plant materials of 30, 50, and 70% of different origin—wood flour (WF) from birch (Betula pendula), hemp hurds (HH) or hemp fiber (HF) (Cannabis sativa). Composite bar samples were obtained by hot pressing homogeneous mixtures of polymer and fillers at 170 °C and a specific pressure of 6.13 MPa. The influence of the filler type and the polymer/filler ratio on the temperature characteristics of the samples, density, microstructure, surface properties, water absorption, physical and mechanical properties, and degradability in soil was determined. The Young’s modulus of the samples ranged from 2640 to 3715 MPa, depending on the composition. The maximum degradation of the composites after 120 days of exposure to soil was recorded at 70% WF, HH, or HF filling, amounting to 77.4, 63.5, and 38.6%, respectively. Perspective biodegradable composites based on P(3HB) filled with various plant-based fillers were obtained and characterized, along with new knowledge about their properties, the lack of which currently hinders the active development and commercialization of such in-demand materials.

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Full text

1. Introduction

Degradable polymers of hydroxy derivatives of alkanoic acids (polyhydroxyalkanoates, PHA) are a relevant and high-demand biotechnology product. Current PHA production levels and growth rates vary slightly across sources but demonstrate significant growth in production volumes. According to Market Reports [1], PHA production volumes in 2024 were estimated at 70,000 metric tons, or US$1.25 million. The polyhydroxyalkanoates (PHA) market is emerging as a key element in the global transition to sustainable and biodegradable materials and is expected to reach US$2.39 million by 2033, registering a CAGR of 8.45% from 2025 to 2033. In the study [2], the achieved PHA production volume is estimated at 0.1 million tons per year, which in monetary terms amounts to US$93.55 million, with a projected growth rate of US$244.34 million by 2030. This represents an average annual growth rate of 14.7%. The relevance of studying PHA, as well as degradable bioplastics in general [3], and increasing their production volumes is justified by global environmental problems caused by the accumulation of enormous amounts of waste from non-degradable polymeric materials in the biosphere, the production volumes of which have reached 450 million tons [4]. The global production of biodegradable plastics (CAGR) reached 1.18 million tons in 2023, and it is expected to increase to 7.43 million tons by 2028 [5]. This may seem insignificant compared to the production of synthetic plastics, but the biodegradable plastics market is continuously growing and diversifying at a CAGR of 16.3% and is projected to reach US17.13 billion by 2027 [[6](#B6-polymers-18-00569)]. The most developed degradable bioplastic is polylactide (PLA), which is a so-called “mature” market product [[7](#B7-polymers-18-00569)]. The global market price of PLA is more than twice that of polyolefins derived from depleted petroleum, amounting to approximately US1.0–2.0/kg [8]. Given the enormous production and consumption volumes of synthetic non-degradable plastics, it is impossible to resolve the problem of their gradual replacement with biodegradable polymer materials by focusing solely on PLA production. This requires not only a significant increase in production volumes (CAGR) but also an expansion of the product range.

Although the active development of PHA began relatively recently (in the late 1980s–early 1990s) and their cost is 2.0–2.5 times higher than the cost of polylactides (that is, 4–6 times higher than the cost of polyolefins), great expectations are associated with these polymers. This is due to the combination of high consumer properties of PHA and the possibility of reducing their cost by using various carbon substrates, including technosphere waste, for biosynthesis [9,10,11]. PHA can be produced using controlled biosynthesis, with resulting polymers having various chemical compositions and different properties, from rigid thermoplastics to rubber-like elastomers. PHA, unlike polylactide, are not hydrolyzed in liquid media and are thermoplastic, which allows them to be processed by various methods—hot pressing, extrusion, and FDM 3D printing [12,13,14,15,16]. In addition to the high cost, PHA have high crystallinity and very low thermal stability. The low crystallization rates of poly-3-hydroxybutyrate, the most widely used representative of commercially produced PHA, and short-chain-length copolymers of 3-hydroxybutyrate with 3-hydroxyvalerates complicate the processing of these polymers into products, which do not have high mechanical strength and age over time due to brittleness caused by the formation of large spherulites and secondary crystallization processes [17,18,19]. To overcome this problem, various approaches can be used, including the synthesis of less crystalline PHA copolymers, the use of plasticizers and nucleating agents that affect the crystallization processes of polymers, and the construction of PHA composites with other materials of various origins [20,21,22,23]. Thus, the fundamental possibility of improving the technological properties of PHA in combination with the possibility of their synthesis using available industrial and agricultural wastes are arguments confirming the considerable potential of these bioplastics, which today are rightly called the material of the 21st century. The current multidisciplinary research demonstrates the possibility of reducing the cost of PHA by utilizing waste. This is consistent with contemporary trends in the transition from a linear to a circular economy [10,24], opening up avenues for using these polymers not only in biomedicine and pharmacology, but also in the production of biodegradable packaging and containers (which require hundreds of thousands of tons of polymers), household items, and materials and preparations for agricultural and industrial applications [9].

An innovative application of PHA is the production of PHA-based composites with various natural filler materials. A rational combination of fillers and a polymer matrix will result in the production of effective structures with a high degree of perfection of physical and mechanical properties; and their combination provides a synergistic effect associated with the emergence of properties in the composites that are not characteristic of each separate component [25]. Estimates by the specialized consulting company Precedence showed that if in April 2024 the global biocomposites market amounted to US$30.86 billion, then by 2033 it will reach US$171.75 billion with an expected average annual growth rate of approximately 18.7% [26].

The current area of research is the creation of composites using renewable cellulose-containing materials formed during photosynthesis on a planetary scale as a filler [27] and polymeric materials as a matrix (binder). Composite polymeric materials filled with forest industry waste (wood chips, shavings, flour, etc.) are widely used [28]. Among them are environmentally friendly wood–plastic composite materials (WPC) for construction (wood fiber and wood chips, panels, window and door frames, and internal partitions), for the production of furniture, railway sleepers, etc. [29,30]. However, as reproduction of forest resources is a long process, waste from annual herbaceous plants, such as kenaf, agave, hemp, jute, etc., is more accessible for constructing composites [31,32,33,34], as well as lignocellulosic biowaste [35]. The herbaceous plant hemp (Cannabis sativa), characterized by rapid growth and high yields, has undoubted potential as a filler for composite materials. Hemp consists of highly durable and long-lasting fibers that are lighter and more flexible than wood, which makes it possible to create lighter, yet stronger products [34,36]. A by-product of hemp processing is hemp hurds, which constitute up to 70% of the mass of the stems. Hemp hurds are considered a bioenergy resource for the production of biofuels (biogas and pellets) [37] and other applications, including the creation of composite materials [38].

The demand for composite materials containing plant waste is currently growing worldwide. Examples of composites based on synthetic polymers filled with plant fillers, such as wheat straw [39], cotton [40], flour from sunflower stalk waste [41], hemp fiber [42], and various wood waste (from aspen, birch, spruce, pine, fir, poplar, and cedar) [43,44,45,46], have been described. The problem with modern polymer composite materials filled with plant waste is their non-degradability in the natural environment and, as a consequence, disposal-related issues, because they are produced using synthetic polymer resins, ranging from toxic phenol-formaldehyde resins to inert polyolefins (polyethylene and polypropylene) as binders for plant fillers. Polyolefins mitigate the negative aspects of using toxic resins, but do not solve the problem of non-degradability [47,48].

Currently, there is a high demand for so-called ecoplastics, produced using natural resources and biodegradable “green” bioplastics, which include microbial polyhydroxyalkanoates [49,50]. Filling degradable plastics with plant waste makes it possible to produce composite materials with high mechanical strength and a wide range of useful properties, including high air permeability, porosity and hygroscopicity, etc. The use of such composites reduces the accumulation of waste in the biosphere and will reduce water consumption and carbon dioxide emissions [16,51,52].

The use of degradable PHA opens up new possibilities for the creation of high-quality and functional composite materials and alleviates the problem of waste disposal. The basic physicochemical properties of PHA are similar to those of non-degradable polypropylene. Furthermore, PHA have a lower melt viscosity than traditional polyolefins, which may prove advantageous in composite production [53]. In a pioneering series of publications by colleagues from Australia, the properties of PHA-based composites with Australian pine flour, as well as their thermomechanical properties and degradability in the natural environment, were studied [54,55]. Similar mechanically strong and degradable composites based on PHA, filled with Siberian pine flour and nanocellulose [56], birch sawdust [57], poplar wood waste [58], oak [59], pine cones and walnut shells [60], etc., have been described. There are a few known studies using PHA filled with herbaceous fillers: fibers of nettle stems and leaves [60], kenaf [32], agave [33], jute [31], hemp [31,61]. A review of these publications shows a general trend in the influence of plant fillers on the properties of composites. The introduction of plant fillers can increase the stiffness and strength of composites, and, as a rule, it accelerates biodegradation compared to pure PHA. Depending on the PHA type, the type of wood filler, its content, and the method of manufacturing of the composites, diverse mechanical properties have been noted: flexural strength of approximately 10–25 MPa and Young’s modulus of approximately 1–3 GPa. Mass loss in soil or compost was approximately 40–60% over 6–12 months of exposure of the composites [56,57,58,59]. The degree of biodegradation of the composites was in the range of approximately 40–80% mass loss over 6–12 months of testing in soil or compost. With herbaceous fibrous fillers (nettle, kenaf, agave, jute), the flexural strength was in the range of approximately 15–20 MPa, i.e., comparable to PHA-based composites with wood fillers; Young’s modulus in bending was in the range of approximately 0.5–4 GPa. The introduction of short plant fibers in an amount of approximately 20–30 wt.% significantly increases the modulus and flexural strength, but may be accompanied by a decrease in impact strength due to a brittle interface with insufficient matrix-filler adhesion, which emphasizes the importance of selecting the content and methods of homogenization of the matrix with the filler [31,32,33,60,61].

However, there are very few available comparative data on the properties of PHA-based composites filled with woody and/or herbaceous plants. This determined the aim of the present study: to develop and comparatively study the properties of composites constructed using birch wood flour, hemp fibers and hemp hurds as fillers and biodegradable poly(3-hydroxybutyrate) (P(3HB)) as a binder.

2. Materials and Methods

2.1. Research Objects

The polymer—P(3HB) was synthesized in the Laboratory of Chemoautotrophic Biosynthesis of the Institute of Biophysics of the Siberian Branch of the Russian Academy of Sciences using the natural strain Cupriavidus necator B-10646 according to a patented technology [62]. Waste fish oil (WFO), obtained by the enzymatic method using the enzyme Alcalase^®^ 2.4 L (Novozymes, Bagsværd, Denmark) from the waste of Baltic sprat (Sprattus balticus) and kindly provided by the Department of Food Technology, Kaliningrad Technical University (Kaliningrad, Russia) [63], was used as the carbon substrate for polymer synthesis. Extraction and purification of polymer samples, as well as methods for analyzing physicochemical properties, were described in detail previously [64].

Wood and herbaceous fillers were used as plant fillers: wood flour (WF) from birch (Betula pendula) (WF) grade “180” (OOO “Wood flour”, Orichi, Russia); hurds (HH) and hemp fiber (HF) (Cannabis sativa) were obtained at the Department of Machines and Apparatus for Industrial Technologies of the Siberian Technological University named after M.F. Reshetnev from industrial hemp (“HEMP and KO”, Khakassia, Russia).

2.2. Production of Mixtures of Raw Materials and Composites

To form homogeneous mixtures, the components were pre-milled to improve miscibility. Hemp hurds and polymer were impact-milled using a ZM 200 ultracentrifugal mill (Retsch, Hann, Germany). To mill the hemp hurds, multi-stage milling was used, with a stepwise reduction in screen size from 2.0 to 0.5 mm at a speed of 12,000 revolutions/min. To mill the polymer, screens with a 2.0 mm aperture diameter were used at a rotor speed of 18,000 revolutions/min. An AS 200 control sifter (Retsch, Haan, Germany) separated the particles into fractions. Fractions ranging from 100 to 299 µm were used to produce composites. Hemp fiber was used without pre-milling to prevent loss of the material’s mechanical properties.

The particle sizes of the initial crushed materials were determined using an ECLIPSE Ci-POL optical microscope (Nikon, Tokyo, Japan), and the obtained images were analyzed using ImageJ v1.52u software. Electron microscopy (Hitachi S-5500 microscope, Tokyo, Japan) was used to study the microstructure of the materials with preliminary sputtering of samples with platinum using a low-vacuum benchtop analyzer SEM EM ACE200 (Leica, Vienna, Austria) in an argon atmosphere. The moisture content of the fillers was measured on a moisture analyzer (Mettler Toledo, Greifensee, Switzerland).

To obtain a homogeneous polymer/filler mixture, a 5% P(3HB) solution in chloroform containing 30, 50, or 70% filler was used. The resulting suspension was placed in a glass flask, which was placed on magnetic stirrers for 30 min at 50 °C and 150 rpm to ensure uniform distribution of the filler particles throughout the polymer solution. Afterwards, ethanol was added to the suspension to precipitate the polymer and filler mixture and it was kept at +5 °C for 24 h. The resulting suspension was then filtered, and the precipitates were dried at high temperatures for 24 h. The resulting dried polymer/filler mixtures were used to form composite samples by hot contact pressing on an Avto Plus series automatic press (Carver Inc., Wabash, IN, USA) at the melting point of poly(3-hydroxybutyrate) (168–174 °C) and a specific pressure of 6.13 MPa for 5 min. After preparation, the samples were cooled and stored in a desiccator.

2.3. Characterization of Composites

The physicochemical properties of the composites were studied using a set of standardized methods. Moisture sorption and water absorption of the samples were measured gravimetrically after drying and storing them in a room at a temperature of 20–22 °C and a relative humidity of 40–60%. Water absorption was measured gravimetrically by assessing the mass of the samples after soaking them in water for 24 h.

Thermal analysis of the initial materials and blends was performed using a DSC-1 differential scanning calorimeter (Mettler Toledo, Greifensee, Switzerland). A sample was placed in an aluminum crucible. Each sample was measured at least 3 times. The samples were heated to 200 °C at a rate of 10 °C/min, held for 2 min, then cooled to −20 °C, held for 2 min and reheated to 200 °C. The crystallization temperature (T_crystal_.) and melting temperature (T_melt_.) were recorded on the thermograms. Thermal stability of the samples was studied using TGA 2 (Mettler Toledo, Greifensee, Switzerland). The temperature at which the sample began to lose weight was taken as the thermal degradation temperature (T_degr_.). Thermal analysis results were processed using the StarE v11.0 software. The theoretical degree of crystallinity (C_x_, the content of the crystalline phase expressed as a percentage) was calculated using the formula:

$$[eqn]$$

where ∆H_i_ is the specific enthalpy of melting of the sample (J/g); ∆H_0_ is the specific enthalpy of melting of 100% crystallized P(3HB), 146 J/g; w is mass fraction of P(3HB) [65].

The density of pressed composite forms was determined as the ratio of the mass of the sample to its volume [g/cm^3^]. The physical and mechanical properties of the composites were studied using a universal electromechanical testing machine Instron 5565 (Instron, Buckinghamshire, UK) using a three-point bending device. The crosshead speed was 3 mm/min. The test was completed when the sample load decreased to 80% of the maximum. Using Bluehill 3 software (version 3.15.1343), the values of Young’s modulus [MPa], which is calculated from the initial slope of the stress–strain curve, flexural strength [MPa] and strain at failure of the sample [%] were calculated.

2.4. Surface Properties of Composites

The surface microstructure of the pressed composites was analyzed using SEM (Hitachi TM4000 Plus microscope, Tokyo, Japan) and image analysis (ImageJ v1.52u software).

The contact angle of the sample surfaces with water (an indicator of hydrophobic–hydrophilic balance) was measured using a DSA-25E drop shape analyzer (Krüss, Hamburg, Germany). Drop images were recorded at 0, 0.5, 1, 2, 5, and 10 s, measuring the liquid-surface contact angles at ten different locations on each sample.

The adhesive properties of the composite surfaces were studied with respect to the test microorganisms Micrococcus luteus and Bacillus subtilis, grown in test tubes on meat-peptone agar (MPA) slant (Nutrient agar, HiMedia, Mumbai, India) in a BD115 dry-air incubator (Binder, Tuttlingen, Germany) at 30 °C for 24 h. The composites were disinfected with 70% ethanol, rinsed in sterile water, and placed in test tubes containing the prepared bacterial suspension. The optical density of the suspension was determined using a DEN-1 densitometer (Biosan, Riga, Latvia) and adjusted to 0.5 units using the McFarland turbidity standard, corresponding to 1.5 × 10^8^ cells per 1 mL. After exposure (24 h at 30 °C), the samples were rinsed in sterile water to remove non-adherent cells and vortexed for 1 min using a Vortex V-1 plus (Biosan, Riga, Latvia) in a test tube with sterile water to separate adherent cells. Dilutions of up to 10^5^ were prepared from the resulting suspension of adherent bacterial cells. Bacteriological culture was performed using the Koch method on Petri dishes in triplicate. The dishes were incubated in a thermostat at 30 °C, and colonies were counted on day 3.

2.5. Composite Degradation in Soil

Composite degradation was studied in a laboratory soil microecosystem with a characterized microbiocenosis structure. Garden soil, a chernozem, was collected in the Eastern Siberian region (Krasnoyarsk Krai, Russia) (56° N, 92° E). Microbiological analysis was performed using standard methods [66]: copiotrophic bacteria were counted on nutrient agar, and mycelial soil fungi were counted on Sabouraud dextrose agar with chloramphenicol (HiMedia). Triplicate cultures were performed using soil suspension dilutions up to 10^6^. The species identity of the cultured bacteria was determined using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) on a Bio Typer Microflex LT/SH mass spectrometer (Bruker, Bremen, Germany).

Samples measuring 6 × 6 × 4 mm, packed in organza, were placed in containers with 150 g of soil at a depth of 10.0 mm. The containers were exposed to a Phytotron-LiA-2 climate chamber (Okabiolab, Moscow, Russia) at a stabilized temperature (25 °C) and soil moisture (50%). Some samples were removed every 30 days for analysis. Composite degradation was measured by determining sample mass loss using a class 4 laboratory scale (Ohaus PA-512C) and by analyzing microstructural changes (SEM images) using a Hitachi TM4000 Plus (Tokyo, Japan).

2.6. Statistics

Statistical processing of the results was performed by generally accepted methods using the standard Microsoft Excel software package (Ver. 2110). All experiments were carried out in five replicates (n = 5). Arithmetic means and standard deviations (SD) (mean ± SD) were calculated. Statistical analysis of the wettability of the sample surfaces was performed using the built-in methods of the DSA-4 program. Two-way analysis of variance (ANOVA) of the results was carried out to assess the influence of factors and their combination on the studied properties of the composites (significance level: p ≤ 0.05). To compare groups, the Mann–Whitney test was used at a significance level of p < 0.05. Some statistical calculations were performed in the STATISTICA Trial Ver. 13.0 program, using applied data analysis packages.

3. Results and Discussion

The success of producing high-quality and high-strength composites depends on the initial properties of the filler materials used, the binder polymer base, and the molding conditions. The creation of homogeneous mixtures is a prerequisite for the successful production of homogeneous mixtures for the manufacture of high-quality wood–plastic composites [67]. One of the most common methods for producing homogeneous mixtures is the mechanical mixing of components using drum or planetary mixers [67,68]. This method proved unsuitable for the manufacture of composites with hemp fiber and for obtaining homogeneous and uniformly dispersed polymer/filler components. Therefore, a method of homogenizing mixtures by adding filler powders to a polymer solution was used, which allows for control over the dispersion of the filler in the polymer base, but is more time-consuming [68,69].

3.1. Formation of Mixtures for Obtaining P(3HB) Composites with Plant-Based Fillers

The ground polymer and plant-based filler samples were dried, finely dispersed powders with different residual moisture contents and filler particle sizes (Figure 1). Wood flour had a residual moisture content of 4.93% and consisted of particles with average diameters and lengths of 115 and 196 μm, respectively. Hemp hurds had a residual moisture content of 6.02%, with average particle sizes of 125 × 327 μm. Ground hemp fiber, with a residual moisture content of 6.16%, consisted of fibers with an average diameter of 38 μm and a length of 783 μm. The hydrophobic polymer powder, with a residual moisture content of 0.15%, had an average particle size of 125 μm.

The resulting blended powders were used to form homogeneous polymer/filler mixtures with varying component ratios, in which the filler content varied from 30 to 70%. A very wide range of polymer/filler ratios was used to evaluate the influence of their ratios on the quality and characteristics of the resulting composites. The resulting homogeneous mixtures were subjected to hot contact pressing at 170 °C (Figure 2), resulting in dense composite bars measuring 40 × 20 × 4 mm (Figure 3a).

During hot contact pressing and melting of P(3HB), filler particles are distributed in the polymer melt. Filler particles and fibers distributed in the polymer adhesive matrix are visible on the surface of the composites (Figure 3b). SEM images showed the influence of the filler type on the surface microstructure. The surfaces of composites with wood flour and hemp hurds were more uniform. The area of defects in the form of microcracks and voids was low: 0.8 ± 0.1 and 0.9 ± 0.1% for composites containing 50% wood flour or hemp hurds, respectively. Composites with larger hemp fibers had a less uniform surface, with clusters of large fiber agglomerates visible on it. On the surface of these composites, the defect area was an order of magnitude larger and reached 9.1 ± 2.8%.

The starting materials differed not only in the geometric sizes of the particles, but also in their temperature characteristics, and this affected the thermal behavior and properties of the composites (Table 1, Figure 4).

Thermoplastic poly(3-hydroxybutyrate) had a gap of 121 °C between the melting onset temperature (164 °C) and the thermal degradation temperature (284 °C). Thermal degradation of wood flour was determined at 264 °C; hemp hurds and fibers, respectively, were 270 and 290 °C. The thermal degradation temperature for all obtained WPCs was lower than that of the original P(3HB) and was virtually independent of the polymer/filler ratio (Table 1). The greatest decrease in T_degr_. was observed for samples containing hemp fiber as a filler. Thermal degradation of these samples began at a temperature of 238–240 °C. A similar decrease in thermal stability is characteristic of WPC and was noted for polypropylene composites with hemp fiber in [42], and was also described by us earlier for composite samples of P(3HB) with Siberian pine wood flour [56].

The presence of heat-labile components, such as hemicelluloses and pectins, lowers the degradation temperature. Their breakdown products can catalyze the degradation of the polymer matrix. Additionally, poor adhesion at the phase interface can accelerate degradation by facilitating the rapid penetration of heat and moisture.

The DSC thermograms for all composite samples during the first heating showed a broad melting peak in the temperature range of 70–100 °C. The peak area increased with increasing filler content. A similar peak of moisture evaporation during the first heating was also recorded for polypropylene composites with hemp fiber [46] and P(3HB-*co-*3HH) copolymer with flax fiber [70]. The small peak on the right shoulder of this endothermic peak, observed at higher temperatures, likely relates to the sample’s history and corresponds to the melting or reorientation of crystals. This phenomenon is attributed to steric hindrance, stemming from the limitation of free volume imposed by the filler particles.

The melting temperature of P(3HB)/WF and P(3HB)/HH samples filled with wood flour and hemp hurds during the first and second heating did not differ significantly from the melting temperatures of pure P(3HB) and lay in the range of 173–175 °C and 170–171 °C, respectively. The amount of filler in these samples did not affect the melting temperature. A different picture was obtained for the samples filled with P(3HB)/HF hemp fiber: with an increase in the filler content from 30 to 50%, the melting temperature increased from 172 to 176 °C, but a further increase in the filler content led to a decrease in the melting temperature to 174 °C. Reheating of P(3HB)/HF samples containing 30 and 70% fiber was accompanied by a significant decrease in the melting temperature, to 154 °C and 168 °C, respectively. This is 4 and 2 °C lower than that of native P(3HB) (170 °C). However, the sample containing 50% hemp fiber showed an increase in melting temperature by 2 °C compared to native P(3HB).

At a fiber content of 30%, these components can be distributed discretely, acting as nucleation centers. However, their quantity is insufficient. Consequently, they are more likely to cause crystal deformation, leading to a decrease in melting temperature. At a fiber content of 50%, a skeletal framework begins to form. Under these conditions, more uniform and smaller crystals can form, resulting in the stabilization of the melting temperature. At a fiber content of 70%, there is insufficient polymer to form a continuous phase; the polymer chains become trapped in the narrow spaces between the hemp fibers. This confinement leads to constrained crystallization, forming a large number of defective crystals that melt at a lower temperature.

The literature data on WPC are mixed. There are reports of both an increase in melting point with the addition of plant fillers [42,70,71,72] and a decrease in it [73,74]. A decrease in the melting point of WPC may indicate the plasticizing properties of the filler. An increase in melting and crystallization temperatures suggests that the filler acts as a nucleating agent. In the present study, all samples, except for the P(3HB)/HF samples, showed an increase in crystallization temperature compared to the original P(3HB). This indicates the possible role of the studied plant fillers as nucleating agents. Hemp fiber, as the study showed, can exhibit a dual function, but this issue requires additional and specialized research.

The degree of crystallinity, calculated from the initial heating results for the P(3HB)/WF and P(3HB)/HF samples, decreases with increasing filler content from 30% to 70%. This may indicate that WF and HF act as nucleating agents at a filler concentration of 30%. However, at higher concentrations, they behave as inert fillers, creating steric hindrance and impeding the crystallization of the polymer matrix. Samples containing HH demonstrate an increase in the degree of crystallinity as the filler concentration reaches 50% to 71%, indicating the structure-forming properties of HH. HH particles are needle-shaped, smaller than those of HF but larger than those of WF. This specific geometry can create a template for epitaxial crystallization, with maximum nucleation observed at 50% HH. Further increases in the HH content lead to steric hindrance and a reduction in free volume. The obtained results correlate well with the mechanical properties of the samples. After the second heating cycle, all samples exhibited a lower degree of crystallinity than after the first heating. All samples also showed a tendency for crystallinity to decrease with increasing filler content. The difference in crystallinity between the first and second heating is attributed to the “technological memory” of the samples, as demonstrated by the results of the initial melt processing. High initial crystallinity is likely due to shear stress, which induces orientation crystallization, with fillers acting as templates for this orientation. Under DSC conditions, after the first heating, the polymer cools under steady-state conditions without shear stress, and the fillers subsequently act as barriers to crystallization, resulting in a decrease in crystallinity proportional to the volume occupied by the filler.

The conducted analysis of the thermal properties of composites revealed the following influence of plant fillers of various types. The amount of wood flour or hemp hurds filler did not affect the melting temperature, but for hemp fiber-filled samples, the melting temperature increased from 172 to 176 °C with an increase in the filler content from 30 to 50%, and at 70% HF, it led to a decrease in the melting temperature to 174 °C. The presence of plant-based fillers consistently lowers the degradation temperature of P(3HB). Hemp hurds exhibit a pronounced nucleating effect at a 50% content, leading to maximum crystallinity (71%). This phenomenon is attributed to epitaxial crystal growth on the developed surface of needle-shaped particles under industrial shear conditions. Wood flour acts as a passive filler: its increasing concentration results in a linear decrease in polymer crystallinity (from 62 to 58%), indicating that steric hindrance predominates over any nucleation effect. Hemp fiber (at 70% loading) induces a constrained crystallization effect. It limits the growth of P(3HB) lamellae within the narrow interfiber gaps, consequently reducing the degree of crystallinity to 43%.

3.2. Physical and Mechanical Properties of Composites

The resulting WPC samples, filled with various plant materials (sawdust, grass hurds, and hemp fibers) had varying densities. These densities were also affected by the filler content. For all tested samples, density increased with an increase in filler content from 30 to 70%. For samples filled with wood flour, the density increased from 1.240 ± 0.002 to 1.273 ± 0.001 g/cm^3^ with increasing filler content. A similar relationship was also observed for grass fillers. In this case, the composites with hemp hurds turned out to be less dense; with an increase in its content from 30 to 70%, the density changed from 1.236 ± 0.003 to 1.271 ± 0.002 g/cm^3^, which differs by 3.1–6.5% from the parameters of the samples filled with hemp fibers. The density of the obtained composites turned out to be generally higher compared to the previously obtained P(3HB) composites filled with Siberian pine flour or bacterial nanocellulose [56]. The obtained results are consistent with publications that show an increase in the density of composites with an increase in the proportion of filler. Thus, the density of WPC based on high-density polyethylene increased linearly depending on the amount (10–60%) of various wood waste (aspen, birch, spruce, pine, and fir) [44,45,46]. A similar pattern was recorded when polypropylene was filled with hemp fibers from 30 to 70% [75], as well as in copolymer composites poly(3-hydroxybutyrate-co-3-hydroxyvalerate) ((P(3HB-co-3HV)) filled with hemp fibers from 15 to 45% [61].

The physical and mechanical properties of WPCs are determined by Young’s modulus, fracture strength, and fracture strain, which determine their durability and performance characteristics. To characterize the resulting composites, a three-point bending method was used, creating multiple, non-uniform stress states in the sample, where the sample experiences both compressive and tensile forces. Therefore, the strength response of composites in bending tests is usually higher than in tensile tests. It is known that the results of bending tests of such composites depend not only on the nature of the filler, but also on the polymer/filler ratio, as well as the distribution and interaction of the filler fibers in the composite [76,77,78].

Various changes in the mechanical strength of the obtained composites were revealed depending on the type and content of the filler, as shown in Figure 5 and Table 2. For the P(3HB) composite with wood flour, with an increase in its content from 30 to 70%, the Young’s modulus decreased from 3659.06 ± 143.57 to 3027.99 ± 165.87 MPa, and the fracture strength from 36.11 ± 4.22 to 25.12 ± 1.09 MPa, respectively. The magnitude of the fracture strain of the samples also changed insignificantly, remaining within 1.96–1.87%. A similar effect was described for composites of the P(3HB-co-3HV) copolymer with oak wood flour [59]. Opposite results were obtained for composites of the same copolymer filled with P(3HB-co-3HV) fiber and bamboo, in which Young’s modulus increased with the introduction of fiber, while the flexural strength remained virtually unchanged [79].

A similar effect of filler on the reduction in Young’s modulus was recorded in a study of P(3HB) composites filled with hemp fibers. The index decreased from 3630.22 ± 139.45 to 2640.35 ± 153.69 MPa with an increase in the hemp fiber content from 30 to 70%. Fracture strength remained virtually unchanged, but fracture strain increased from 2.49 ± 0.19 to 4.11 ± 0.43%, suggesting a slight increase in the elasticity of P(3HB)/HF composites with an increase in filler content. It is recognized that an important factor determining the strength of WPC is the adhesion of particles to the binding polymer matrix, which is quite natural, since the strength of the material directly depends on how efficiently the stress is transferred from the filler to the matrix, namely, adhesion is the main mechanism controlling this process and influencing the strength and brittleness of the material [80]. Similar results of a decrease in mechanical strength depending on the proportion of plant filler were obtained in the work of Mazur et al. [60]. The authors, studying the mechanical behavior of P(3HB-*co-*3HV) composites with agricultural waste in the form of nettle fibers (stems and leaves), pine cone flour and walnut shell flour as reinforcing fillers, showed that an increase in plant fillers in the composites leads to a slight decrease in flexural strength.

A study of another type of composite filled with hemp hurds (P(3HB)/HH) revealed no significant relationship between mechanical strength and hurds content. Young’s modulus, fracture strength, and fracture strain remained unchanged at 3600, 40 MPa, and 1.65%, respectively, across all polymer-to-filler ratios. It is not possible to conduct a comparative analysis among the few publications on the mechanical properties of composites with hemp, as these studies focused on tensile mechanical properties [42,72], but did not examine the flexural strength of the composites.

Previously obtained P(3HB) composites, filled with Siberian pine wood flour, had lower mechanical strength values, which decreased with an increase in the filler content [56]. Thus, with a wood flour content from 30 to 90%, Young’s modulus decreased from 1783.7 to 1124.1 MPa, and the ultimate strength at fracture from 21.9 to 10.2 MPa. Thus, the use of birch or wood flour, or hemp fibers as fillers made it possible to obtain stronger composites. In general, the obtained WPC samples had mechanical strength comparable to commercial wood–plastic composites based on polyolefins [81,82]. For example, for polystyrene, when using maleic anhydride as a binding agent, Young’s modulus was 3380 MPa, and the fracture strength was 27.55 MPa, when using diammonium phosphate, these values were 3900 MPa and 52 MPa, respectively. For polyvinyl chloride, also when using maleic anhydride, Young’s modulus was 2990 MPa, and the fracture strength was 2.99 MPa [82]. In general, the strength properties of WPC varied widely: Young’s modulus ranged from 540 to 5800 MPa, and the fracture strength ranged from 8.5 to 84.3 MPa. These properties were influenced by the type of polyolefin used (high and low density polyethylene, polypropylene, polyvinyl chloride, polystyrene), the type of binding agent, as well as the selected production methods and conditions [81,82].

3.3. Water Absorption of Composites

Water absorption of WPC is an important indicator affecting the long-term performance, quality, and mechanical strength of wood–plastic composites. In the first stage, the hydrophobic–hydrophilic properties of the composite surfaces, which influence subsequent interaction with liquids, were determined. The dependence of the contact angles of the composite sample surfaces on their composition (type and content of plant fillers) was studied. Contact angles are the angles at the interface between a liquid, air, and a solid. High contact angles with water indicate the tendency of the surface to repel liquid; therefore, the smaller the angle, the greater the tendency of the liquid to spread over the solid surface. According to literature, surfaces are considered hydrophobic if the contact angle with water is greater than 90° [83]. The results of contact angle measurements with water on the surface of the composites after 0, 0.5, 1, 2, 5, and 10 s are given in Table S1. Figure 6 shows, as an example, a photograph of a drop lying on the surface of composite samples with a 50/50 component ratio at the beginning and end of the angle measurement, that is, how the value of the contact angles decreased over time from 0 to 10 s after the drop was applied to the surface.

The minimum angle value, i.e., the highest surface wettability after 10 s of applying a drop of water to the surface, was recorded for composite samples with hemp fiber. These samples were characterized by the highest number and area of surface defects (Figure 3b). In [41], it was also shown that surface defects of WPC composites improve wettability, since depressions and cracks create capillary forces that attract water. Composite samples with wood flour or hemp hurds, which have a denser and smoother surface, were more hydrophobic. Filling the polymer with plant particles increased the contact angle value compared to native P(3HB) (average 78.2°). The maximum contact angle was recorded for the sample with 30% wood flour (97.4 ± 2.3°), the minimum (69.2 ± 3.9°) was recorded for the sample with 70% hemp fiber (Table S1). Similar results were obtained in [84], where the inclusion of babassu fiber in the composites increased the contact angle compared to the P(3HB) control (76.2°). With 10 and 20% babassu fiber content, the surface hydrophobicity and the angle value increased, but with a higher filler content the effect was opposite: the angle value decreased. A number of studies also noted an increase in the wettability of the WPC surface with an increase in the filler proportion [85,86]. However, in a study [41], when polypropylene (58.1°) was filled with flour from sunflower stem waste from 30 to 60%, the contact angle, on the contrary, increased from 97.4 to 107.0°.

Non-uniform wetting of plant fibers with P(3HB) melts during hot pressing apparently impaired interfacial compatibility. It was shown that at a filler content of approximately 70%, the formation of fiber clusters and their partial encapsulation by the polymer matrix is likely, which leads to the formation of regions with a higher proportion of open lignocellulose and is accompanied by a decrease in the hydrophobicity of the surface of the composites. It was shown that an increase in the filler content increases the number of defects in the form of cracks and microcavities, disrupting the homogeneity of the surface. This alters the interaction of liquid droplets with the surface, affecting the wettability indices and the value of the contact angle [87]. To improve the interfacial interaction of components in composites, examples are known of preliminary esterification of wood fibers, the hydrophobicity of which increases compared to the original [88]. The authors of the study believe that esterification of plant fillers can promote more pronounced adhesion and compatibility of plant fibers with the polymer as a binding component.

Figure 7 also illustrates the change in water absorption and swelling of the samples (thickness increase) after 24 h of exposure to water. The sorption moisture content of native P(3HB) is low, approximately 0.1%; therefore, as the polymer content decreased and the plant component increased, the water absorption of the samples increased. For the WPC samples studied, an increase in sorption moisture content from 0.9 to 3.9% was observed with increasing fillers, with a direct correlation between the increase in hygroscopicity and the filler content.

Increasing the filler content from 30 to 70% resulted in an 8–10-fold increase in water absorption, depending on its type. Lower values of water absorption and swelling of the samples were typical for composites with wood flour; at the minimum WF content, they were 3.5 ± 0.3 and 1.4 ± 0.1%, respectively. The highest water absorption and swelling were recorded for the composite with hemp fibers, up to 37.9 ± 1.9% and 38.9 ± 1.7%, respectively, at a filler content of 70%. Similar dependencies were shown in the work of Butylina et al., who demonstrated the influence of wood fiber size on the water absorption capacity of a composite with polypropylene [89]. In a study [72], the water absorption of P(3HB-*co-*3HV)-based composites was significantly higher when flax fibers were used as the reinforcing material compared to hemp fibers. The influence of the type and amount of filler on the water absorption of composites was noted in [44]. A series of studies has shown that an increase in the water absorption of WPC increases the number and size of defects (cracks and voids), which reduces the performance characteristics of products [54,90,91].

It should be noted that the properties of the WPC surface are not determined only by the amount of filler, as they depend on a combination of factors, including fiber dispersion, interfacial interactions that affect the surface relief features, hydrophilicity, moisture capacity, etc. This is confirmed by studies that show that the wettability of the WPC surface is associated with the type of coating, the size of defects and the porosity of the surface [83,84].

3.4. Adhesive Properties of Composite Surfaces

An important property of wood–plastic composite materials used in the furniture and construction industries is their durability, which is influenced by abiotic and biological factors. The service life of these composites used in construction depends on weather and climate conditions (insolation, humidity, and temperature), which are particularly pronounced in tropical regions. Microorganisms, which rely on cellulose-containing fillers as a growth substrate, pose a significant threat to the quality and service life of WPC products [92]. Naturally, the composition of the composites, including the type of plant fillers used and the polymer/filler ratio, as well as the surface structure, density, moisture absorption capacity of the composite products, and their operating conditions (outdoors or indoors) influence the activity of microbial growth and subsequent degradation of WPC [93,94,95,96].

The adhesion properties of composite surfaces filled with 50% various plant materials were studied against two test microbial cultures: Bacillus subtilis and Micrococcus luteus. The choice of test microorganisms was determined by their common occurrence in both indoor and outdoor air [97]. M. luteus is found in soil, dust, the respiratory tract, and on human skin [98]. Bacillus species often predominate in aerosols, dust, surfaces, and equipment indoors. Furthermore, the ability of bacilli to form spores makes them resistant to dehydration and chemical disinfection. The broad enzymatic lability of these bacteria facilitates the biodegradation of organic materials [98,99,100]. Figure 8 illustrates the number of grown colonies of the test microorganisms adhered to the surface of the samples after 24 h of exposure to the bacterial suspension, compared to a control sample made of native polymer.

Microorganism colonization of the surfaces of the composite samples occurred with varying activity, in contrast to the sample made of hydrophobic P(3HB), on which the number of bacteria was insignificant and amounted to (0.39 ± 0.11) × 10^4^ and (0.08 ± 0.01) × 10^4^ CFU/mm^2^ for B. subtilis and M. luteus, respectively. This indicates the resistance of the polymer to microbial fouling of the surface compared to composites.

B. subtilis bacteria more actively colonized the composites with herbaceous fillers. The number of microorganisms on more hydrophilic samples with higher water absorption (Figure 6 and Figure 7) filled with hurds or hemp fibers was (44.97 ± 3.75) × 10^4^ and (27.57 ± 1.38) × 10^4^ CFU/mm^2^, respectively. This was significantly higher than the figure for the composite filled with birch wood flour—(15.54 ± 1.97) × 10^4^ CFU/mm^2^. The surface of the composites turned out to be less attractive to M. luteus, the number of which was highest (17.86 ± 1.39) × 10^4^ CFU/mm^2^) on the P(3HB)/WF composite. This is comparable to the value for B. subtilis, but significantly lower than that of samples with hemp hurds and fiber, respectively, (4.37 ± 1.81) × 10^4^ and (7.82 ± 0.74) × 10^4^ CFU/mm^2^. Statistical processing of the results using a two-way analysis of variance revealed a more significant effect (p < 0.05) of bacterial species on the microbial adhesion of the composite surface. The effect of filler type and the interaction of factors (filler type + bacterial species) also had a significant effect (p < 0.05) (Table S2).

The obtained results confirm that the choice of plant filler influences the adhesive properties of WPC. This observation is consistent with published data demonstrating how the composition of composites and their exposure conditions determine the degree of interaction of microorganisms with the surface [94,95,101,102]. In ref. [43], when studying polypropylene-based composites filled with rubber, white pine, bamboo, white poplar, cedar, or Chinese fir fibers, different microbial fouling activity of surfaces and different resistance to mold were demonstrated, which is directly related to the durability of these materials during operation.

Important results were obtained by colleagues from Australia, who studied the effect of the component ratio in a composite of P(3HB-*co-*3HV) copolymer with Australian pine flour and environmental conditions on microbial damage and changes in the properties of the composites. It was shown that the native P(3HB-*co-*3HV) copolymer and samples with 20% wood flour content in open air under natural weathering conditions showed minimal mold growth and an insignificant loss of mechanical properties over 12 months; in samples filled with 50% wood flour, slow microbial fouling was noted [54]. Similar samples in a room retained stability and their original appearance for 12 months [91]. However, when incubating the P(3HB-*co-*3HV) composite with 50% filler in soil, the authors observed active microbial fouling of the samples and a loss of mechanical properties of the samples during biodegradation [103].

Thus, microorganisms adhering to the surface of WPC products can have a negative impact, degrading their quality and shortening their service life. However, microorganisms can also have a positive impact. When microorganisms come into contact with waste from such products, which have reached the end of their lifespan and are sent to landfills, the microorganisms, by breaking down the waste, shorten its disposal time, which is important for reducing waste accumulation in the biosphere and promoting its greening.

3.5. Degradation of Composites in Soil

Currently, the number of studies devoted to degradable wood–plastic materials is limited. This is due to the narrow spectrum and small volumes of production and application of degradable plastics, including degradable polyhydroxyalkanoates. Degradability is one of the most attractive properties of PHA, which has received considerable attention, as confirmed by a huge number of publications. The biodegradation of PHA is a complex process dependent on the interaction of biotic and abiotic factors. Natural microorganisms play a dominant role, while the degradation activity and biodegradation patterns of PHA depend on environmental conditions and the basic properties of the polymers themselves; therefore, the indicators vary significantly. For example, the half-life of PHA can range from 30 to 60 days in chernozems of the Siberian region [104], in tropical mangrove ecosystems [105], in continental soils of Vietnam and on the sea coast [106]. A longer process of PHA biodegradation, with a half-life from 100–120 to 200–300 days has been recorded in compost [107], in arid red soils in southern India [104], in the Yellow Sea [108], and the brackish Lake Shira in Khakassia (southern Siberia) [109].

The degradability of the constructed composites was studied in a laboratory soil microecosystem with a characterized microbiocenosis structure. Microorganisms play a significant role in the biodegradation of polymers in soil. The most significant ecotrophic groups in this process are hydrolytics and copiotrophs: the former hydrolyze complex molecules to monomers, while the latter assimilate readily available monomeric compounds. Soil analysis before sample placement revealed a high abundance of copiotrophs and lower abundances of prototrophic and oligotrophic bacteria (Table 3). Low mineralization and oligotrophicity indices indicate the presence of available organic matter in the soil and ongoing mineralization processes, which is typical of agrocenotic soils. This is also confirmed by the high abundance of micromycetes, which thrive on substrates rich in organic matter. Low numbers of nitrogen-fixing bacteria indicate the presence of available forms of nitrogen in the soil.

Among the cultivated bacteria in the studied soil, the most prevalent were species of the genera Bacillus (B. pumilus, B. mycoides, B. simplex)—56%, Arthrobacter (A. polychromogenes, A. pascens)—17%, Pseudomonas (P. kilonensis, P. brassicacearum, P. mucidolens)—14%, and minor species of the genera Agromyces, Rhizobium, Sphingomonas, Kocuria. The dominant filamentous fungi were species of the genera Penicillium—64%, *Trichoderma—*16%, and Mortierella—11%.

During the degradation of the composites, changes in the appearance of the samples to a darker color were noted due to the absorption of moisture from the soil and swelling (Figure S1). This was accompanied by an increase in surface defects, such as voids and cracks, resulting in a looser structure of the specimens. Changes in the surface microstructure of the composites are shown in SEM images (Figure 9).

SEM images of the composite surfaces show numerous cracks and local voids (marked with arrows), the number of which has increased significantly compared to the original samples (Figure 3b). The total area of defects increased to 21.59, 25.36, and 63.27% for samples with WF, HH, and HF fillers, respectively. Similar changes in the surface structure during degradation were described in other studies in the study of the P(3HB)/birch sawdust composite [57] and the composite based on P(3HB-*co-*3HV) with pine wood flour [103].

The dynamics of sample mass decrease as an indicator of destruction is shown in Figure 10. Mass decrease depended on the filler type and the polymer/filler ratio, varying from 13.2 to 67.4% over 120 days of exposure in soil (Figure 10a). At the same time, the residual mass of the control polymer P(3HB) was insignificant (89.4%). This is comparable to the degradability of P(3HB)/WF samples filled with wood flour, in which the residual mass decreased from 86.8 to 77.4% with an increase in filler proportion from 30 to 70%. P(3HB)/HH composites showed a higher degree of degradation, their residual mass decreased from 85.9 to 63.5% (with an increase in the proportion of hemp hurds from 30 to 70%). Composites containing hemp fiber, which had the most defective surface and high water absorption, degraded faster than the others; their residual mass decreased from 75.2 to 38.6% with a similar increase in filler content. The half-life of the P(3HB)/HF sample with 70% filler was 90 days; with 50% filler, it was 120 days. Composites containing 70% hemp hurds degraded by 63.5% during this period, occupying an intermediate position in degradation between the samples containing wood flour and hemp fiber.

The degradability of the composites over the course of the experiment is shown in detail in Figure 10b using samples with a filler content of 50%. Note the presence of a lag phase and the absence of changes in the mass of the composites during the first 30 days. This is due to the fact that time is required for microorganisms to migrate and colonize the surface of the samples, adapt to the polymer as a growth substrate, and activate the synthesis of extracellular PHA depolymerases. Microbial depolymerases play a key role in the degradation of PHA, hydrolyzing the original high-molecular-weight polymer to form tetra-, di-, and monomers of butyric acid, which are available for transport into cells and metabolism [107].

An analysis of the few publications on the degradability of PHA composites with plant fillers has shown significant variability in the activity of this process, which is due to differences in the properties of the fillers and the methods of sample preparation. Composites of P(3HB), obtained by similar hot pressing and filled with bacterial nanocellulose or Siberian pine wood flour, degraded differently: the half-life varied from 180 to 240 days in the case of nanocellulose or wood flour, respectively [56]. However, previously obtained, less dense P(3HB) composites with birch sawdust, characterized by a large number of pores and cracks and obtained by cold pressing of powders, degraded in soil significantly faster, with a half-life of about 35 days [57]. Similar observations were obtained in studies where filling P(3HB), grafted with maleic anhydride, with wood flour led to accelerated biodegradation [110,111]. Composites with another type of PHA, P(3HB-*co-*3HV) copolymers and peach palm wood particles, also decomposed in soil faster than native P(3HB-*co-*3HV) [112]. An opposite trend was found in [113], where P(3HB-*co-*3HV)/radiata pine wood fiber composites obtained by hot pressing showed a decrease in degradation with an increase in the filler content from 15 to 25%. Similar results were obtained in a study of composting P(3HB-*co-*3HV) composites with 15% wood sawdust [114] and 10% wheat straw [73], where the filler content in the composite reduced degradability compared to the native copolymer. In the work of Chan et al., the degradation of P(3HB-*co-*3HV) and Australian pine wood flour composites obtained by melt pressing was studied over the course of one year. It was shown that the degradability of the composites exceeded the degradability of the copolymer by 2.5 to 5.0 times, depending on the filler content (20 and 50%, respectively). The mass loss was 6.7 and 12.7%, respectively, while the mass of the native copolymer decreased by only 2.6% [54]. Thus, the use of various plant fillers and varying the polymer/filler ratio makes it possible to control the degradability of WPC based on polyhydroxyalkanoates by stimulating or slowing down the biodegradation process.

A study of the resulting composites of degradable poly(3-hydroxybutyrate) with three different plant-based fillers revealed the influence of filler type and component ratio (polymer/filler) on the WPC characteristics, including density, water capacity, temperature, physical–mechanical properties, and degradability in soil, which were not always unidirectional and unambiguous. Processing the results using a two-way analysis of variance (ANOVA) allowed us to identify the factors causing the strongest and most significant effects of fillers and their content on the properties of the composites (Table S3). Thus, the factors filler type and amount had a reliable effect (p < 0.05) on all the studied properties of the composites. Filler type was the most significant factor (p < 0.05) influencing such properties as water contact angle, fracture strength, and degradation of composites in soil, compared with the amount of filler, which also showed a reliable value (p < 0.05). The filler content had a more significant effect on water absorption and through-thickness swelling of the samples, as well as on Young’s modulus (p < 0.05). The interaction of factors had a significant effect (p < 0.05) on the studied composite properties, with the exception of water contact angle, where the p-value was 0.21.

3.6. Preliminary Technical and Economic Analysis

In biotechnology, as in other industries, a feasibility study, at least a preliminary one, is essential for research aimed at developing new promising products. This allows one to assess the market potential of a new product and its niche. The polyhydroxyalkanoates studied in this study have a wide range of applications, from technical fields, agriculture, and public utilities to biomedicine and pharmacology. This allows for the planning of small-scale synthesis facilities or large-scale plants. Depending on the intended application of these polymers—surgical implants, pharmaceutical formulations, agricultural products, or degradable packaging—various raw material sources, varying in cost and purity, may be used. According to accepted estimates, the cost of carbon feedstock in PHA production can account for up to 45–50% of the total cost. It is accepted that the cost of PHA, depending on the production volumes and types of polymers produced (copolymers are more expensive than homogeneous poly-3-hydroxybutyrate), can vary from US$4.0 to 6.0/kg [115].

The results of a potential market assessment for wood–plastic composites based on degradable PHA are presented in the work of colleagues from Australia [116]. The authors reviewed the latest data on global PHA production capacity, estimated at 66,000 tons per year for the period under study (2016), and studied the implications of PHA use for WPC production for the market of such composites. The results showed that current global PHA production for wood–plastic composites could account for about 4.4% of the global WPC market, which currently demonstrates an average annual growth rate of 10.5%. The economic assessment showed that PHA-based wood–plastic composites, as an alternative to industrially produced composites, could cost only 37% of their pure PHA-based counterpart. The authors concluded that PHA/wood filler composites made from waste PHA are promising for PHA use, as this could reduce the cost of products made from these polymers by 21% compared to pure PHA. This is important from an environmental perspective for the market prospects of PHA itself and is positive for the rapidly growing wood–plastic composites market.

The presented manuscript, devoted to model laboratory studies of the characteristics of composites based on a degradable polymer and various fillers, does not allow for a technical and economic analysis of the potential cost of these products. In this work, to design composites, we used polymer samples synthesized using waste rather than pure sugars (as is typically the case when studying laboratory processes). Moreover, the possibility of increasing PHA production volumes and expanding their application areas directly depends on the cost of these polymers. Given the significant cost of the C-substrate, we estimated the potential cost of carbon feedstock in the case of using fatty waste extracted from non-recyclable fish processing waste. For example, smoked sprat heads from the production of canned sprats are transported to solid waste landfills for disposal due to the presence of smoking elements. Our studies demonstrated the high efficiency of PHA synthesis using fatty waste from fish processing [117]. The economic coefficient of the bacterial culture grown on the studied fat wastes was approximately 0.45–0.50 kg/kg, i.e., 2.0 kg of fat substrate is consumed to synthesize 1.0 kg of bacterial biomass. The obtained data allowed us to perform a preliminary estimate of the carbon raw material costs. With an intracellular PHA content of 75 ± 5%, the specific costs of PHA synthesis on the studied wastes do not exceed 2.5 kg/kg. This is significantly lower compared to the indicators obtained in processes on glucose and sugar-containing C-substrates, where the economic coefficient for the carbon substrate is 3.0–3.3 kg/kg, and up to 3.5–4.0 kg of sugar are consumed per 1.0 kg of biomass. The preliminary cost of the fat substrate obtained from the heads of smoked sprats is estimated at US$0.45/kg. Using the studied fat waste from smoked sprat heads, the substrate cost was estimated at US$1.13/kg. This is several times lower, from 2.5 to 3.5 times, than the cost of PHA on glucose or glycerin (biodiesel production waste). Thus, it was demonstrated that the cost of the C-substrate can be reduced by 3.0–3.5 times compared to sugars. This suggests an overall reduction in PHA production costs to a level comparable to that of polylactides (unpublished data from the authors). Overall, the potential for PHA use, including for the production of sought-after wood–plastic composites, depends on the existing cost reduction potential for these polymers and the active implementation of the trend toward greening the technosphere and the transition to a closed-loop economy, in this case focused on the utilization of plant waste to produce relevant and in-demand composites. Synthesizing PHA from waste and reducing their cost expands the scope of polymer applications, including the production of highly relevant, fully degradable composites filled with plant waste.

4. Conclusions

Given the urgent need to transition to environmentally friendly materials, this study constructed composites from degradable poly(3-hydroxybutyrate) (P(3HB)) and plant-based waste. The resulting P(3HB)-based composites were filled with birch (Betula pendula) wood flour (WF), hemp (Cannabis sativa) hurds (HH), or hemp fiber (HF), pressed from homogeneous mixtures at 170 °C and a specific pressure of 6.13 MPa.

The following key results were obtained:

• -The influence of the type and content of fillers in the composites on their surface, thermal, physicochemical, and mechanical properties, as well as degradability in laboratory soil microcosms, was determined;
• -It was established that filling P(3HB) with the studied wood or herbaceous plant materials allows for the production of mechanically strong and biodegradable wood-polymer composites;
• -It was determined that the type of plant filler and the polymer/filler ratio influence the composites’ performance and their biodegradability in soil.

Overall, environmentally friendly and fully degradable composites with promising applications were obtained, along with new insights into their performance and stability during use, a deficiency that currently hinders the active development and commercialization of these sought-after materials.