Evaluation of Surface Properties in Biosilica-Reinforced Biobased Polyester Nanocomposites

Abstract

This study investigates the surface properties of bio-based unsaturated polyester resin (b-UPR) nanocomposites reinforced with biosilica nanoparticles derived from rice husk. The b-UPR matrix was synthesized from recycled polyethylene terephthalate (PET) and renewable monomers, providing a sustainable alternative to conventional polyester resins. Unmodified and modified biosilica particles with silanes: (3-trimethoxysilylpropyl methacrylate—MEMO, trimethoxyvinylsilane—VYNIL, and 3-aminopropyltrimethoxysilane with biodiesel—AMBD) were incorporated in different amounts to evaluate their influence on the wettability, topography, and viscoelastic behavior of the composites. Contact angle measurements revealed that the addition of modified biosilica significantly improved the hydrophobicity of the b-UPR surface. The greatest increase in the wetting angle, amounting to 79.9% compared to composites with unmodified silica, was observed in the composites containing 5 wt.% SiO₂-AMBD. Atomic force microscopy (AFM) analysis indicated enhanced surface roughness and uniform dispersion of the nanoparticles. For the composite containing 1 wt.% of silica particles, the surface roughness increased by 25.5% with the AMBD modification and by 84.2% with the MEMO modification, compared to the unmodified system. Creep testing demonstrated that the reinforced nanocomposites exhibited improved dimensional stability under sustained load compared to the neat resin. These findings confirm that the integration of surface-modified biosilica not only enhances the mechanical properties but also optimizes the surface characteristics of bio-based polyester composites, broadening their potential for high-performance and sustainable applications.

1. Introduction

The increasing awareness of environmental issues has spurred interest in bioplastics and prompted the exploration of innovative synthetic methods for producing them from renewable sources. As a result, stricter environmental regulations are being implemented, compelling industries to adopt cleaner and more sustainable production processes. These changes are largely driven by concerns over greenhouse gas emissions and environmental pollution [1]. Plastic waste has been linked to adverse effects such as global warming and biodiversity loss. In contrast, biobased materials offer a promising solution, as they tend to decompose more quickly and can often be repurposed as compostable organic matter in agricultural settings [2]. As the demand for sustainable materials grows, natural substances are increasingly being used as reinforcements in composite materials. Fibers such as sisal and jute are gaining attention due to their high specific strength, low weight, and biodegradability. Rice husk (RH), often considered agricultural waste, contains a high amount of cellulose and silica, making it a valuable and eco-friendly filler for industrial applications. Its use in composites not only supports waste reduction but also lowers manufacturing costs [3,4].

Unsaturated polyester (UPR) is an affordable and widely used thermosetting resin, typically produced by reacting polyols with unsaturated dibasic acids—a process developed in the 1930s [5]. These resins are low-molecular-weight, pale-yellow oligomers with varying viscosity based on their composition. As members of the ester family, USP resins offer excellent corrosion, chemical, and fire resistance, along with fast curing, dimensional stability, and design versatility [6,7]. They are commonly used in sanitary products, composites, storage tanks, pipes, and marine or transport components.

However, like other thermosets (e.g., epoxy and vinyl esters), UPRs are inherently brittle and have limited thermal and mechanical performance [8,9]. Improvements can be achieved through chemical structure modifications or by incorporating nanofillers with tailored properties. These fillers enhance strength, heat resistance, fire retardancy, and reduce shrinkage during curing [10,11,12]. For example, Embirsh et al. [13] used bio-based UPR from recycled PET and maleic anhydride, reinforced with vinyl-modified biosilica from rice husk, achieving an 88% increase in tensile strength. Numerous studies have shown that incorporating nanosilica into polymers significantly enhances properties such as hardness, abrasion resistance, mechanical strength (both static and dynamic), scratch resistance, adhesive strength, and UV stability [14,15,16]. These improvements make nanosilica a valuable additive in a variety of applications, including coatings, rubber, plastics, sealants, and fibers [17]. To date, most silica-modified polymers have been developed using silane coupling agents or functionalized components (e.g., –COOH, –OH, –CHCH₂O, –NCO) to enhance compatibility between hydrophilic silica and hydrophobic organic matrices, often based on acrylic or siloxane polymers [18,19,20]. For instance, Hu et al. [21] used hydrophobically treated silica to improve the mechanical moduli of polymethyl methacrylate—polymethyl methacrylate (PMMA)/silica nanocomposites. Liu et al. [22] synthesized PMMA/silica films through copolymerization with allyl glycidyl ether-functionalized silica. Chan and Chu [23] created nanocomposites by polymerizing n-butyl methacrylate with methacryloxypropyl trimethoxysilane and blending the resulting copolymer with silica sol. Additionally, nanosilica modified with trimethoxysilanes has been used to reinforce transparent polyacrylates in UV-curable systems [20].

This study has multiple (eco-friendly, scientific, industrial, economic, and environmental, social and governance (ESG)) impacts which can be viewed with multiple contributions:

• Sustainability performance and environmental impact (recycled PET as matrix and a renewable source as reinforcement); it means eco-conscious innovation with added agricultural waste as a solution for biomass utilization,
• Scientific advancement (surface optimization and mechanical reinforcement effect). It demonstrates how chemical surface modifications of nanosilica particles with three silanes (MEMO, VINYL, AMBD) dramatically enhance the wettability and roughness properties of the designed composite, which are critical for adhesion and mechanical performance. Through microhardness and creep tests, it proves how biosilica reinforcement improves long-term stability under load (creep) and how a small part of a second hard segment into the main soft segment of the matrix improves microhardness.
• Industrial aspect—this composite is a dimensional stable material with tailored wettability and durability, and with excellent mechanical properties, suitable for smart packaging or construction materials, but not ideal for moisture-resistant applications. This supports local economies and creates opportunities for green jobs in recycling and bio-refining.
• Economic aspect—this study used low-cost starting material. From the perspective of local economic development, the cost of transporting raw materials for production is minimal.
• ESG criteria—incorporating recycled PET diverts plastic waste from landfills and oceans. Companies adopting these composites can leverage ESG credentials to attract investment and meet regulatory incentives. Life-cycle assessments of the UPR-based composite show a positive reduction in CO₂ emissions, compared to fossil-based resins. Another advantage of bio-based thermoset composites such as UPR silica is its end-of-life recyclability and small negative impact on climate change (reduction in CO₂). Its social impact is reflected in green job creation with health and safety goals for employees and such solutions are encouraged by the government in many countries.

This research represents a continuation of our previous research [13], and compared to previous publications, we have focused on surface methods for sample characterization, wetting resistance and mechanical properties (microhardness, creep and strength). In short, the novelty lies in the combination of sustainable synthesis, innovative surface modification of biosilica and comprehensive evaluation of surface and mechanical properties, paving the way for environmentally friendly, high-performance nanocomposites.

The aim of this work is to investigate composite materials based on an unsaturated polyester matrix reinforced with unmodified and surface-modified silica particles. The study investigates how the incorporation of these particles affects important surface properties, including hardness, wettability and roughness, with the aim of improving the overall performance and applicability of the resulting composites in various industrial areas.

2. Materials and Methods

2.1. Materials

2.1.1. Preparation and Modification of Biosilica Particles

Rice husk obtained from Levidiagro (Kočani, Skopje, North Macedonia) was thoroughly washed, dried, and ground into powder. It was then treated with 10% sulfuric acid at 80 °C for 3 h to remove impurities. After washing until neutral pH (~7) was achieved, the material was dried at 105 °C and subsequently combusted, resulting in the formation of white biosilica particles. More detailed preparation steps are available in the literature [24].

The biosilica particles were surface-modified using three different silane coupling agents: 3-trimethoxysilylpropyl methacrylate (MEMO), trimethoxyvinylsilane (VYNIL), and 3-aminopropyltrimethoxysilane (APTES), all sourced from Sigma-Aldrich (Schnelldorf, Germany). Various analytical-grade chemicals were also employed during modification.

For MEMO and VYNIL modifications, 6 g of biosilica was dispersed in 20 mL of toluene and ultrasonicated to ensure uniform dispersion. Then, 2.5 mL of the respective silane was added, and the mixture was stirred gently at 70 °C for 24 h under nitrogen atmosphere. The modified particles were filtered, washed with toluene and ethanol to remove unreacted silane, and dried at 60 °C. The APTES modification followed a similar procedure. The amino-functionalized biosilica, with an amino content of approximately 280 μmol/g, was then reacted with biodiesel synthesized from soybean oil (according to Rusmirović et al. [25]) in tetrahydrofuran (THF) and the particle designation is AMBD. This reaction was conducted at 25 °C for 12 h, followed by heating at 60 °C for 2 h. The final product was isolated by filtration, washed, and dried at 40 °C. More detailed descriptions are also given in the previous work [13].

The thermal stability of biosilica nanoparticles was investigated in a previous work [13] and is shown in the Supplementary Material Section S1.1.1 in Figure S1.

2.1.2. Preparation of b-UPR Composites

The synthesis of bio-based unsaturated polyester resin (b-UPR) involved the use of glycolyzed waste polyethylene terephthalate (PET) along with renewable-source maleic anhydride (MAnh) and propylene glycol (PG). A comprehensive characterization of the resulting resin was reported in an earlier study [13]. The results of 1H and 13C NMR analyses for b-UPR were detailed previously in Figures S2 and S3 in Supplementary Material Section S1.2. FTIR spectra of b-UPR resin are given in Figure S4.

Biobased unsaturated polyester resin (b-UPR) was used as the matrix, with unmodified or modified biosilica particles (1, 2.5, and 5 wt.%) as reinforcement. The components were mixed using a vacuum planetary mixer for 10 min to ensure uniform dispersion and remove air bubbles. Afterward, 1 wt.% methyl ethyl ketone peroxide (MEKP) and 0.5 wt.% cobalt octoate were added, followed by a 2 min vacuum homogenization. The resulting paste was poured into molds and cured at room temperature for 24 h, then post-cured at 65 °C for 4 h and 80 °C for 5 h [13]. Depending on the type of modification, the obtained composites are labeled as follows: for unmodified silica, the label b-UPR/SiO₂ is used, for AMBD modification, it is b-UPR/SiO₂-AMBD, for vinyl modification, it is b-UPR/SiO₂-V and for MEMO modification b-UPR/SiO₂-M

2.2. Methods of Characterization of the Specimens

The morphological properties of the synthesized silica particles were examined using a Mira3 Tescan field emission scanning electron microscope (FE-SEM) operated at 20 kV and a Philips CM12 transmission electron microscope (TEM) equipped with an SIS MegaView III digital camera. Prior to FESEM analysis, the samples were coated with a thin layer of gold to increase their electrical conductivity and ensure higher-quality imaging during the scanning process. Prior to TEM analysis, samples were cut in 80 nm thick sections using a Leica UC6 ultramicrotome (Leica Microsystems, Wetzlar, Germany), and mounted on carbon coated grids (Agar Sci, Essex, Rotherham, UK).

The microhardness of the composite materials was assessed using a Vickers microhardness tester (Leitz Kleinert Prüfer DURIMET I, Leitz, Oberkochen, Germany). Throughout the testing procedure, a 100 gf (0.98 N) load was maintained for a period of 25 s. To determine the microhardness, three indentations were made, and the average length of their diagonals was used in the calculation. Indentation creep was tested by variations in an indentation dwell time (5–50 s) at a constantly applied indentation load of 0.98 N. All micro-indentation and indentation creep tests were performed at room temperature. Based on the microhardness values, creep parameter, tensile and yield strengths were estimated according to mathematical relations.

The three-dimensional topography of the composite materials was examined using an atomic force microscope (NTEGRA AFM, NT-MTD, Moscow, Russia) operated in tapping mode. Scans were conducted over a 25 × 25 µm² area with 256 points per line and 256 lines in total. The cantilever resonant frequency was 150 kHz. The NSG01 cantilever was manufactured from single crystal silicon (n-type, 0.01–0.25 Ω·cm), doped with antimony, and coated with a reflective layer of Au. The set point for AFM measurements was 60% of the free oscillation amplitude. Surface roughness parameters, including root mean square (Sq), arithmetic mean roughness (Sa), maximal peak height (Sp), maximal pit depth (Sv), and maximal height (Sz) were calculated using free software Gwyddion 2.61 (Open-Source software, Czech Metrology Institute, Jihlava, Czechia) from mid-plane AFM image data.

The wettability of the prepared composites was evaluated by measuring the static contact angle using the sessile drop method. A 5 μL droplet of deionized water (18 MΩ·cm) and glycerin was placed at five randomly selected locations on each sample. Images were captured five seconds after drop placement using an optical microscope (Delta Optical Smart 5.0 MP Pro, Minsk Mazowiecki, Poland) equipped with a high-resolution camera. Contact angles (WCA and GCA) were analyzed using Image-Pro Plus 6.0 software. All measurements were conducted under ambient conditions (25 °C, 50% relative humidity).

3. Results and Discussion

3.1. Morphology and Topography of Composite Samples

Figure 1 was obtained from a transmission electron microscope (TEM) and shows four micrographs (a–d), each of which represents a nanocomposite containing 2.5 wt.% biosilica particles with different modifications.

The size of SiO₂ particles was calculated using image analysis software and the mean diameter was found to be in the range between 5 and 25 nm, as reported in our previous work [13]. After the modification of SiO₂-V and SiO₂-M particles with silane, the largest number of particles had a diameter of about 10 nm, while for the unmodified image and SiO₂-AMBD, that distribution was about 15 nm. The unmodified biosilica particles appear slightly agglomerated, forming irregular and branched clusters (Figure 1a). The contrast between the particles and the matrix is moderate, suggesting a relatively smaller particle size. Large voids or pores are visible in the matrix, indicating poor interaction between the biosilica and the matrix. In contrast, the SiO₂-AMBD particles are more densely packed and uniformly embedded as aggregate within the matrix, compared to those in Figure 1a. The dispersion is noticeably improved, with significantly fewer large agglomerates observed. The matrix exhibits fewer pores, suggesting better compatibility and possible chemical interaction due to amino functionalization (Figure 1b). The SiO₂-M particles are highly dispersed and appear as distinct black dots evenly distributed throughout the matrix. The surrounding matrix forms a network-like structure, likely resulting from strong interactions between the filler and the polymer. This micrograph demonstrates the best dispersion observed so far, indicating effective surface modification (Figure 1c). The SiO₂-V nanoparticles are uniformly distributed and appear rounder and more well-defined. The higher contrast suggests larger or denser particles. Minimal agglomeration is observed, and the particles are homogeneously embedded within the matrix, indicating excellent dispersion and compatibility (Figure 1d).

The distribution of the silica, the topography and the roughness of the nanocomposites, based on a b-UPR matrix with 1.0 wt.% and 5.0 wt.% biosilica nanoparticles, were estimated using AFM. The 3D AFM images for all samples in tapping mode are displayed in Figure 2 and Figure 3, which show the surface of the b-UPR/SiO₂, b-UPR/SiO₂-AMBD, b-UPR/SiO₂-M, and b-UPR/SiO₂-V hybrid composites. The 2D images from AFM are provided as Figure S7 in the Supplementary Material Section S2.

The b-UPR/SiO₂-AMBD and b-UPR/SiO₂-V samples exhibit higher surface roughness values. However, in general, the addition of biosilica reduces the surface roughness of the hybrid samples. The roughness of the b-UPR/SiO₂ and b-UPR/SiO₂-AMBD sample is 121.3 and 223.4 nm, respectively (see Figure 2a,b). The roughness of the hybrid composite b-UPR/SiO₂-V is significantly reduced as shown in Figure 2d, with a minimum roughness of 120.9 nm. The roughness of the b-UPR/SiO₂-MEMO hybrid nanocomposites then increase, and the roughness parameter is 152.3 nm (Figure 2c).

All figures show several wide and lofty peaks, indicating the aggregation of large grains of nano-biosilica particles. Measuring the length and diameter of unmodified silica particles and those directly modified particles by AMBD, M, and V silanes in b-UPR matrix [26] is not relevant due to non-uniform dispersion and the presence of very rough regions partially covering the sample surfaces.

The roughness parameters—Sq (RMS roughness), Sa (arithmetic averages of the absolute roughness), Sp (maximal peak height), Sv (maximal pit depth), and Sz (maximal height) are given in

Table 1. Values of characteristic roughness parameters obtained by applying AFM software (Gwyddion, Ver. 2.53) on a 25 × 25 µm² scan area for the b-UPR/silica hybrid materials obtained without/with chemical modification of nano-biosilica particles. The concentration of silica particles in the b-UPR matrix was 1.0 wt.%.

No.	Sq	Sa	Sp	Sv	Sz
b-UPR/SiO2	121.3	98.1	313	380	693
b-UPR/SiO2-AMBD	223.4	159.8	1101	878	1979
b-UPR/SiO2-M	152.3	125.6	609	364	974
b-UPR/SiO2-V	120.9	83.7	909	257	1165

for the b-UPR/1.0 wt.% SiO₂ and

Table 2. Values of characteristic roughness parameters obtained by applying AFM software (Gwyddion, Ver. 2.53) on a 25 × 25 µm² scan area for the b-UPR/silica hybrid materials obtained without/with chemical modification of nano-biosilica particles. The concentration of silica particles in the UPR matrix was 5.0 wt.%.

No.	Sq	Sa	Sp	Sv	Sz
b-UPR/SiO2	198.4	169.1	444	747	1192
b-UPR/SiO2-AMBD	235.1	215.4	415	338	753
b-UPR/SiO2-M	207.2	164.7	782	449	1231
b-UPR/SiO2-V	157.2	120.6	372	706	1077

for the b-UPR/5.0 wt.% SiO₂ hybrid materials.

The Sq and Sa roughness parameters of the b-UPR hybrid samples increased with the rise in the concentration of nano-biosilica particles (Table 1 and Table 2). This is due to the incorporation of silica particles into the b-UPR matrix; a higher concentration of biosilica particles results in greater integration within the polymer surface, and consequently, higher material roughness.

As a general rule, in the 3D AFM images, the surface profile oscillations are indicated by color depth [27]. Indentations are shown in red, while bulges are represented in blue. From the images, it can be seen that the surface topography depth of b-UPR/SiO₂ with 1.0 wt.% silica ranges from −3.84 to 4.47 μm. For b-UPR/SiO₂ with 5.0 wt.% silica, the surface topography depth ranges from −3.10 to 4.30 nm, which confirms that the UPR resin surface becomes rougher with increased silica content.

The reduction in void size and channel depth in the b-UPR nanocomposite samples represents a more uniform dispersion of silica particles [28]. The best dispersion of silica particles in the b-UPR matrix was observed for sample b-UPR/SiO₂-V (see Figure 3d and Table 2).

An SEM device was used to examine the structure of the b-UPR resin containing 2.5% silica particles with all modifications. Figure 4a–e were generated as 3D topography using image processing software (Image Pro Plus 6.0). The goal was to observe a larger sample surface area, a few times larger than on AFM. The 3D reconstruction of 2D-SEM images helps visualize features like cracks, pores, grains, and rough surface in a more intuitive way than AFM analyses [29].

FESEM images of polyester matrix samples as well as nanocomposites with 2.5 wt.% biosilica nanoparticles are shown in the Supplementary Material Section S2 in Figure S8. The surface roughness analysis of SEM line profiles revealed distinct topographical differences across the various composite formulations, highlighting how different treatments and filler modifications influence the surface morphology [30]. These variations in roughness not only affect the physical appearance of the composites but also have significant implications for their mechanical properties, adhesion behavior, and overall performance in practical applications. By examining parameters such as average roughness (Ra), root mean square roughness (Rq), and peak-to-valley height (Rz), the study was able to correlate specific surface features with the composite’s microstructural characteristics [31].

Surface roughness is known to influence interfacial bonding strength, as rougher surfaces typically provide increased mechanical interlocking sites, enhancing adhesion. Moreover, controlled modification of filler particles can lead to more uniform dispersion within the matrix, resulting in smoother topographies and improved material properties. The results of roughness parameters obtained by applying SEM imaging are given in

Table 3. Values of characteristic roughness parameters obtained applying Image Pro Plus software.

No.	Rq	Ra	Rp	Rz
b-UPR	82.1	79.6	100	96.4
b-UPR/SiO2	86.3	88.7	119.0	113.4
b-UPR/SiO2-AMBD	84.2	87.0	169.0	157.2
b-UPR/SiO2-M	83.5	86.2	181.0	165.4
b-UPR/SiO2-V	82.7	85.6	163.0	149.8

.

Based on the topographical analysis of the samples, the vinyl-modified silica exhibited the most uniform surface with minimal roughness, indicating improved dispersion within the b-UPR matrix. In contrast, the unmodified silica sample showed the poorest dispersion, contributing to increased roughness and uneven surface features. Although the MEMO-modified silica displayed the highest Rt and Rz values—reflecting pronounced surface irregularities likely caused by localized agglomeration or uneven surface energy distribution—its Ra and Rq values were slightly lower than those of unmodified silica, suggesting better average smoothness.

All surface modifications increased the roughness relative to the pure matrix; however, the nature of this roughness varied. Unmodified silica, despite raising roughness, lacks chemical bonding capability, which may promote brittle failure. On the other hand, AMBD and MEMO treatments improved chemical compatibility, although their pronounced surface features could introduce stress concentrations if not well dispersed. VYNIL-modified silica strikes a balance by achieving moderate roughness with smoother transitions, potentially enabling effective mechanical interlocking and more uniform stress distribution. The relationships between structure, roughness parameters, microhardness, creep resistance, contact angles with two different fluids, and free surface energy will be further examined, alongside the effects of individual chemical modifications of the nano-biosilica particles.

The extracted line edge profiles were superimposed onto the SEM image to visually correlate the surface roughness measurements with the actual topographical features observed, enabling a comprehensive analysis of the composite’s surface morphology (Figure 4f) [32].

3.2. Microhardness

Figure 5 shows the appearance of Vicker’s diagonals for different applied indentation loads and of sample types (matrix, composite without/with silica modification). Figure 5a–e show indents made with smaller loads and standard dwell times (25 p and 20 s). Silica modification with vinyl (see Figure 5e) resulted in the best dispersion of silica particles, whereas modification with MEMO was less effective (Figure 5d); large agglomerates of silica particles were observed around the impressions. The indentation test is a destructive method that causes deformation of the neat b-UPR matrix and the b-UPR/silica-based composites, leading to permanent plastic deformation. For smaller applied loads (25 p and less) and shorter dwell times (20 s and less), rapid elastic recovery can occur, resulting in an artificially smaller impression.

The microhardness results for the b-UPR/silica-based composites with varying silica particle contents (1, 2.5, and 5 wt.%) are presented in

Table 4. Microhardness data and standard errors for UPR-based composites.

No./C (wt.%)	1.0 wt.%	2.5 wt.%	5.0 wt.%
Microhardness, H (MPa)
b-UPR/SiO2	217.1 ± 4.1	242.0 ± 7.3	290.4 ± 10.8
b-UPR/SiO2-AMBD	236.3 ± 5.8	252.6 ± 8.2	299.5 ± 11.8
b-UPR/SiO2-M	246.1 ± 6.1	280.1 ± 9.4	321.6 ± 12.8
b-UPR/SiO2-V	355.2 ± 6.3	383.7 ± 10.1	359.7 ± 12.8

. The standard deviation for each sample is given, and measurements were performed at three different locations, three times. The microhardness of the neat b-UPR was 66.8 MPa [13].

A comparison of the microhardness values for the neat UPR and the composites with the different silica contents (wt.%) clearly shows the reinforcing effect of the SiO₂ particles and their surface modifications (Table 4). Improved particle dispersion within the UPR matrix enhances the material’s resistance to indentation, resulting in higher microhardness values. Obviously, the microhardness of b-UPR/SiO₂ with 1.0 wt.% silica particles significantly increased for all modified samples. However, at 5.0 wt.% of silica, particle agglomeration became prominent. The optimal filler content was determined to be 2.5 wt.% to minimize the formation of agglomerates and micropores at higher loadings. The results show that silica reinforcement increased hardness from 66.8 MPa for the neat b-UPR to 383.7 MPa for the composite containing 2.5 wt.% SiO₂-V. This increase in microhardness is attributed to microstructural changes in the composite during curing and the increased reactivity of the modified SiO₂ particles [33]. The chemical surface modification of the silica improves intermolecular covalent bonding between the organic matrix and the modified SiO₂ particles and also alters the polymer’s surface topography after curing, as indicated by the oscillations in roughness parameters.

By comparing the microhardness results obtained at higher loads (500 p) in our earlier publications [13,34], it can be observed that lower loads yield higher microhardness values, but the indentation size effect (ISE) becomes more pronounced [35]. The ISE in unsaturated polyester resin (UPR) composites refers to the phenomenon where the measured hardness varies with the applied indentation load—typically, the apparent hardness increases as the load decreases. At low loads, the smaller indentations are more susceptible to optical measurement errors, which can falsely elevate hardness values. In contrast, higher loads may penetrate beyond surface anomalies, offering more representative bulk properties [36].

The standard method for determining the microhardness of plastics is generally the Shore method D. However, due to the use of standard units (MPa), the Vickers method is increasingly used. In our case, the optimum sample is the one that has a satisfactory surface area, the best dispersion of particles and the highest microhardness value, i.e., the sample with 2.5 by weight of silica particles modified with VYNIL. Knowledge of the microhardness properties enables the evaluation of the creep behavior during indentation, the adhesive properties of the material over the critical reduction depth, the indentation toughness and the strength of the nanocomposite. Particle treatments that increase the hardness of UPR-based composites (through cross-linking or filler interaction) can also change the surface energy and thus indirectly influence wettability.

The issue of deviations in microhardness data can be solved by polishing the sample surface. Polishing the surface of the b-UPR composite before testing reduces variability in hardness. Unpolished surfaces may retain resin-rich layers or oxygen-inhibited zones that skew results, and this is clearly visible in the AFM images, where terraces are formed (Figure 2 and Figure 3). Additionally, the presence of silica particles, the size of agglomerates, and their distribution within the b-UPR matrix significantly affect the indentation size effect (ISE). A higher filler content (5.0 wt.%) hardness is generally increased (see Table 4) but can also amplify the ISE due to heterogeneous stress distribution and the influence of chemical modification on stress distribution (see Figure 5e). Chemical functionalization of the biosilica fillers (AMBD, MEMO, V) improves their compatibility with the UPR matrix. This stronger matrix–filler adhesion reduces micro voids and stress concentrations, leading to more consistent hardness values across different loads—thus mitigating the ISE [37].

3.3. Indentation Creep

The indentation creep in bio-based unsaturated polyester resin (b-UPR) composites refers to the time-dependent increase in the penetration depth under constant stress. This behavior is particularly important for polymers like b-UPR, which exhibit viscoelastic and viscoplastic properties [38]. It is important to clearly distinguish between tensile creep (macroscale) and indentation creep (microscale) measurements. This study presents a microscale characterization of the creep properties of UPR-based composites and investigates the influence of chemical functionalization of silica particles on the stress exponent parameter [39,40,41,42]. Hardness measurements were used to identify the creep mechanism or creep resistance of the UPR composites by calculating the stress exponent μ for composites containing optimally 2.5 wt.% nanosilica particles with different surface modifications. The Sargent–Ashby model [40] was applied, and Equation (1) relates the time-dependent composite hardness Hc to the stress exponent μ as follows:

$$H_c={\displaystyle \frac{{\epsilon }_0}{c·\mu ·t}},$$

(1)

where ε₀ is the strain rate at reference stress σ₀, c is constant, t is dwelling time and μ is the stress exponent.

If ln Hc is plotted against ln t (see Figure 6), straight lines with slopes corresponding to the negative inverse stress exponent (−1/μ) are obtained. The value of the exponent μ can be used to predict the material creep mechanism using the theoretical model S-A and Equation (1).

The results obtained using the Sargent–Ashby (SA) model are given in

Table 5. Fitting results obtained according to SA model. The values of stress exponents for the UPR/SiO₂ composite obtained with optimal concentration of silica particles and with various modifications of silica particles in b-UPR matrix, at a constant indentation load 100 p, are given.

No.	Slope (k)	Intercept (n)	Regression Coefficient (R2)	Stress Exponent (m)	Modulus of Elasticity (E/MPa)
b-UPR	−0.3805	−0.7882	0.9846	2.2681	276
b-UPR/SiO2	−0.3029	−0.7936	0.9983	3.3014	412
b-UPR/SiO2-AMBD	−0.3018	−0.5833	0.9952	3.3134	490
b-UPR/SiO2-M	−0.2915	−0.5189	0.9932	3.4305	558
b-UPR/SiO2-V	−0.2665	−0.3308	0.9888	3.7523	607

. Creep is a time-dependent deformation that occurs under sustained stress, and materials with a lower modulus of elasticity tend to exhibit more creep. For this reason, the modulus of elasticity measured in an earlier publication using a tensile test [13] is compared here with the stress exponent values determined by micro-indentation in this study.

A higher stress exponent value indicates higher creep resistance of the sample [39,40,41,42]. The values in Table 4 show that the b-UPR composite with vinyl-functionalized silica particles has the highest creep resistance, while the pure UPR matrix has the lowest. The order of creep resistance is as follows: b-UPR/SiO₂-V > b-UPR/SiO₂-M > b-UPR/SiO₂-AMBD > b-UPR/SiO₂ > b-UPR. This behavior can be explained by the “sliding mechanism,” where each silica particle boundary acts as a barrier to polymer creep within the UPR matrix during indentation.

When silica particles are incorporated into a b-UPR matrix, the particle–matrix interface acts as a micro-barrier to the movement of polymer chains [39]. During indentation creep, polymer chains attempt to deform under sustained load, but their mobility is restricted upon encountering silica particles. The build-up of local stresses causes the chains to either slide around the particle (shear at the interface) or take a detour through the matrix (longer path length and higher resistance) [43]. Figure 6 shows that all UPR/silica-based composites exhibit nearly parallel curves, indicating the reinforcing effect of nanosilica in the polymer matrix. The slopes in the diagram represent the creep rate, from which it can be concluded that the b-UPR/SiO₂-V sample has the lowest creep rate. Conversely, lower E values (poor filler dispersion, due to lower crosslink density, or incomplete cure) often result in higher creep rates, as observed in the b-UPR/SiO₂ sample. The higher crosslinking density is shown by samples of b-UPR/SiO₂-M and b-UPR/SiO₂-V composites [13], so it was expected that these samples would have the highest creep resistance. The opposite trend is true for b-UPR/SiO₂ and b-UPR/SiO₂-AMBD samples, which is due to the considerable amount of surface-OH groups in unmodified SiO₂ particles and the long fatty acid chains in the BD residue, which have such an orientation that they prevent interactions with b-UPR chains.

3.4. Correlations of Yield Strength and Tensile Strength with Indentation Dwell Time for UPR/Silica-Based Composite

Tensile and yield strengths were computed from the microhardness data, following relations 2, and 3 [28,44,45]:

$$Ts=-99.8+3.734Hc,$$

(2)

$$Ys=-90.7+2.876Hc,$$

(3)

where Ts is tensile strength, Ys is yielding strength, and Hc is hardness of b-UPR/silica composite. The idea is based on monitoring the change in material strength (Ts and Ys) with the change in dwell time, for all composites with and without modification of silica particles. Figure 7 and Figure 8 showed the changes in tensile strength depending on the microhardness measurement conditions (dwell time).

This dependence Ts vs. t and Ys vs. t can be described by a power law function. With increasing indentation time, a decrease in tensile strength is evident for all samples. The influence of silica modification is also evident. The sample with the vinyl modification of silica particles shows the highest strength, while the lowest strength is recorded for the pure b-UPR matrix. A review of the literature data shows that the tensile strength evaluated using the tensile test machine for this class of hybrid composites is drastically lower. Sudirman, et al. showed that the addition of silica particles in a b-UPR matrix increases the tensile strength from 54 MPa (pure b-UPR) to 64 MPa for b-UPR/SiO₂ composite with 1 wt.%, and then decreases to 42 MPa for a composite with 2.5 wt.% silica particles [46]. Nasution et al. showed oscillation values of the tensile strength for UPR/silica composites; the results were: 40 MPa (for clear UPR), 22.7 MPa (UPR + 10 wt.% silica), 26.6 MPa (UPR + 20 wt.% silica), 33.4 MPa (UPR + 30% silica), and 27.6 MPa (UPR + 40 wt.% silica) [47]. The strength values obtained through the microhardness test are incomparable with the values obtained through the tensile test. One explanation may be the difference in scales, i.e., testing the sample on the macro- and microscale. Also, silica particles can absorb the matrix due to their large surface area and pore volume. In view of this absorption process, it is possible to bind the matrix trapped in the silica pores to improve the interaction between the filler and the matrix [47].

The indentation creep of a hybrid material depends on the ratio of soft/hard segments; filler type (i.e., modification type), filler grain size, and surface roughness. The AFM images are characterized by separate surfaces that have different sizes of agglomerates, which affects the creep resistance.

3.5. Contact Angles and Wetting Properties of b-UPR/Silica Composite

The contact angle results quantify the changes in wettability caused by different fluid exposure. Static contact angles provide a direct insight into the wettability of the surface, the free surface energy, and the work of adhesion and reflect its interaction with water and glycerin. Figure 9a illustrates the water drops for measuring the water contact angle and Figure 9b shows the glycerin drops for measuring the glycerin contact angle. The static contact angle was determined at each end tip of the drop where it touches the surface in a dome shape.

From Figure 9, it is observed that the contact angle values of water (Figure 9a) are higher than the contact angles of glycerin (Figure 9b) on polar b-UPR-based surface samples. Water has a very high surface tension (~72.2 mN/m at 25 °C), which drives it to spread more readily on high-energy (polar) surfaces, but glycerin has a lower surface tension (~64.0 mN/m), so it does not spread as aggressively—even though it is also polar [48]. From a molecular point of view, water molecules are small and highly cohesive due to strong hydrogen bonding, but glycerin molecules are larger and more viscous, with multiple OH groups, but their internal cohesion resists spreading more than water does. Photographs of droplets for the two different fluids are shown in Figure 10.

Water contact angle (WCA), glycerin contact angle (GCA), work of adhesion (WA), and surface energy (γs) for the b-UPR matrix, and b-UPR/silica composites before and after modifications of particles are collected in

Table 6. Values of contact angles (WCA and GCA), work of adhesion (WA) and surface energy (γ_s) of b-UPR and hybrid UPR/silica composites.

Sample	Water WCA (°)	Glycerin GCA (°)	${\mathit{\gamma }}_{\mathit{s}}^{\mathit{d}}$ (mN/m)	${\mathit{\gamma }}_{\mathit{s}}^{\mathit{p}}$ (mN/m)	γs (mN/m)	WAwater (mN/m2)	WAglycerin (mN/m2)
b-UPR	72.20	75.40	2.58	31.44	34.02	95.09	80.17
b-UPR + 1 wt.% SiO2	45.82	61.96	0.01	75.80	75.80	123.5	94.11
b-UPR + 2.5 wt.% SiO2	42.55	59.70	0.02	80.09	80.11	126.4	96.31
b-UPR + 5 wt.% SiO2	34.51	57.48	0.58	95.94	96.52	132.8	98.43
b-UPR + 1 wt.% SiO2-M	47.65	61.78	0.03	73.89	73.92	121.9	94.29
b-UPR + 2.5 wt.% SiO2-M	48.65	63.25	0.00	71.43	71.43	120.9	92.83
b-UPR + 5 wt.% SiO2-M	51.80	68.00	0.28	70.76	71.07	117.8	88.01
b-UPR + 1 wt.%SiO2-AMBD	58.40	67.5	0.57	52.92	53.49	110.9	88.52
b-UPR + 2.5 wt.%SiO2AMBD	60.59	68.17	1.02	48.17	49.19	108.6	87.83
b-UPR + 5 wt.% SiO2-AMBD	62.08	69.84	0.82	47.51	48.32	106.9	86.09
b-UPR + 1 wt.% SiO2-V	49.54	64.73	0.03	72.46	72.49	120.1	91.35
b-UPR + 2.5 wt.% SiO2-V	56.39	66.66	0.34	56.84	57.17	113.1	89.39
b-UPR + 5 wt.% SiO2-V	59.46	69.29	0.29	53.81	54.09	109.8	86.66

.

The equation by Owens and Wendt [28,49,50,51,52] was used to calculate the surface energy of the solid (4):

$$\mathit{cos}\theta =\left[2\sqrt{{\gamma }_s^d}\left({\displaystyle \frac{\sqrt{{\gamma }_L^d}}{{\gamma }_L}}\right)+2\sqrt{{\gamma }_s^p}\left({\displaystyle \frac{\sqrt{{\gamma }_L^p}}{{\gamma }_L}}\right)-1\right],$$

(4)

where θ is contact angle, γs and γ_L are the solid and liquid surface energy, ${\gamma }_s^d$ and ${\gamma }_L^p$ are dispersion and polar forces of total surface energy [28]. The equation for estimated work of adhesion (W_A) for water (WA_water) and for glycerin (WA_glycerin) is (5) as follows:

$$WA={\gamma }_L·\left(1+\mathit{cos}\theta \right),$$

(5)

where WA is work of adhesion between the UPR-silica composite and liquids (WA_water or WA_glicerin) in (mN/m²). Water contact angle (WCA) or glycerin contact angle (GCA) are contact angle θ in radians. The liquid surface energy, γ_L, is the sum of polar and disperse parts.

The calculated value of surface energy (dispersive, polar parts and total) for b-UPR matrix and based nanocomposites samples with different concentration silica fillers and modifications is presented in Table 6. The results show that filler chemistry and concentration significantly affect both the dispersive and polar contributions to surface energy. Low values of dispersive components mean higher polarity of all samples shifting the surface toward hydrophilicity. Notably, unmodified silica maximizes the polar character, while AMBD-functionalized silica enhances dispersive properties.

The total surface energy of the b-UPR composite is increased from 34.02 to 96.2 mN/m when silica particles (non-modified) are added. However, the hydrophilicity of the surface of the b-UPR/SiO₂ nanocomposite is noticed for all concentrations of silica and for all types of modifications in comparison to b-UPR. Silica particles can also increase surface roughness [53], which—depending on the wetting regime—can amplify the apparent surface energy. Silica (SiO₂) has an inherently high surface energy due to its polar surface groups (e.g., silanol –SiOH) [54]. These groups can form hydrogen bonds or interact strongly with polar liquids/groups. When dispersed in a polymer matrix, silica introduces polar sites, making the surface more hydrophilic and energetically favorable for interactions with polar substances. It can also be observed that the addition of silica nanoparticles to the b-UPR polymer matrix increases the total free surface energy (γ_s = ${\gamma }_s^d+{\gamma }_s^p$) related to neat UPR; it means that liquids spread harder on low-energy surfaces. On the other hand, higher surface energy enhances adhesion to other materials, so it can be concluded that samples with maximum unmodified silica content in the matrix have the best adhesion properties. The low total surface energy (samples with modified silica) can reduce liquid spreading, it may also hinder adhesion, because the silanization process introduces nonpolar moieties. Even with functionalization, the sheer increase in surface area due to the high filler content could improve the mechanical interlocking and compensate somewhat for the energetic disadvantages. A high WA value indicates good adhesion properties, i.e., strong molecular interaction and bonding with other materials. It is typical for surfaces that are rich in polar groups or have a roughness that promotes mechanical anchoring. Modifications and the introduction of functional groups, which have a smoothing effect on the surface, can worsen the adhesive properties as they reduce the roughness.

The addition of biosilica exhibits the highest contributions to electrostatic component increase across all nanofiller loadings, indicating mainly polar/dipolar (hydroxyl-oxygen keto ester and hydroxyl-oxygen ester intermolecular interactions). Due to the hydrophilic nature of the biosilica surface incorporated into the UPR matrix increasing surface wettability, i.e., hydrogen bonding interaction decreases surface tension while increasing surface energy. The impact of individual modifications is explained in the following discussion.

Opposite trends for contact angle changes for MEMO-modified silica/UPR and biosilica/UPR composites versus filler addition were observed. Somewhat higher values were found for the MEMO-modified silica/UPR composite reflecting improved compatibility with polar UPR matrix. MEMO-functionalized silica exerts a balanced contribution of both polar and dispersive contributions, making it versatile for applications requiring moderate adhesion and wettability. Modification of the biosilica surface with the methacryloyl group introduces polar functionalities which participate in the cross-linking process [5]. The polar component, i.e., keto ester group, helps with spreading water droplets, while on the other hand, aliphatic moiety leads to increasing the surface tension [55].

Similar to the MEMO-modified silica/UPR composites’ change in the wetting angles, the vinyl counterpart showed an analogous trend but at somewhat higher values for wetting angles [56]. The introduction of the vinyl modification led to participation in the cross-linking reaction [57] which provides hydrophobic covalently bonded surface groups, in contrast to the hydrophilic non-modified biosilica surface. In this way, the vinyl modification offers a balanced profile, with moderate increases in dispersive components as loading rises versus hydrogen bonding decreases contribute to surface energy decreases (Table 6) and mechanical property enhancement [58].

The modification of silica particles with AMBD demonstrates the highest apolar component [59] among all fillers due to the presence of unsaturated fatty acid residue, emphasizing its suitability for apolar matrix systems or hydrophobic interfaces. Regular differences and higher values of the wetting angles for AMBD-functionalized silica/UPR versus MEMO-functionalized/UPR composites reflects the significance of apolar fatty acid residue and structural arrangement around the silica surface and the wetting angle increase. The hydrophilic nature of the biosilica surface has a lower contribution with higher AMBD-functionalized silica loading. The effect of keto ester group polarity is suppressed by the presence of apolar unsaturated acid residues. APTES helps in the formation of a hydrophobic layer with hydrophilic reactive amino groups. Because of this, the subsequent reaction with soy biodiesel transforms it to an amide bond having terminal hydrophobic and reactive soy fatty acid residues. Hydrophilicity occurs due to the presence of silica particles, which are hydrophilic, and particle modification was also carried out to increase both the particle reactivity in the course of b-UPR cross-linking and the hydrophobicity of the composite. The presented results should be considered in a dual sense: the addition of silica nanoparticles was used as composite reinforcement (silica—physical and modified silica—vinyl reactive reinforcement) and as a surface modifier/wettability change. Balancing these properties is important from the point of view of the composite’s application (mechanical properties and water resistance) and, after end of life, it determines the design of the treatment technology. Higher mechanic properties and low water wettability provide a wide range of applicability for the UPR-based composites, while improved water wettability could help in the development of future strategies for biodegradation technology of discarded materials (current study). The modification of this type of composite was monitored by FTIR analysis in a previously published paper; this is just a continuation of the research [13].

Also, the contact angle and adhesion work (WA) for b-UPR-based nanocomposites are affected by the water. The surface roughness and molecular interactions are the main reasons for these results.

According to the Wenzel model [60], the increased roughness increases the intrinsic wettability of a surface, but for hydrophilic surfaces, the roughness decreases the contact angle (attracting more water). In this study, modified biosilica (e.g., AMBD, MEMO) led to an increase in roughness and thus to higher contact angles, confirming improved hydrophobicity.

An optimal modification with good roughness parameters may reflect a better dispersion of the nanoparticles and a stronger interfacial bond, which would contribute to increased hardness. In this case, the VYNIL modification makes the greatest contribution to the hardness of the nanocomposite. The load-bearing capacity of the nanocomposite is increased in this way, as high hardness also indicates an improvement in the performance of the nanocomposite.

4. Conclusions

In this study, we focused on examining the surface properties of nanocomposite materials that were developed using a bio-sourced polyester resin matrix reinforced with silica particles derived from rice husk waste, an abundant agricultural by-product. To enhance the compatibility and interfacial bonding between the silica particles and the polyester resin, the silica fillers were chemically modified using three different types of silane-coupling agents. These silane treatments aimed to improve the dispersion of the silica particles within the polymer matrix and optimize the overall performance of the resulting nanocomposites. The research included detailed analyses of how these modifications affected the surface properties of the nanocomposites.

• As silica concentration rises, particles can disrupt the matrix continuity, leading to microscale texture and increased roughness parameters (Sa, Sq, Sz). Nonmodified silica tends to agglomerate, creating peaks and valleys that elevate surface roughness. Vinyl groups improve compatibility with b-UPR, leading to more uniform distribution and reduced roughness, while AMBD increases roughness. If the surface roughness of the UPR/silica is observed on a macroscale, roughness parameters increase with filler loading, but on the microscale, chemical modification of silica particles (like vinyl) can locally reduce roughness by minimizing agglomerates.
• Higher concentrations of silica particles in b-UPR matrix improve microhardness, because silica particles are inherently hard and adding it to the b-UPR matrix creates a rigid filler network that resists indentation. Modified silica particles with vinyl bond more effectively with the b-UPR matrix, reducing interfacial slippage under load and have maximal microhardness (383.7 ± 10.1 MPa with optimal concentration at 2.5 wt.%).
• For each sample, the tensile strength decreases with increasing indentation time. It is also clear that the silica change has an influence. The pure b-UPR matrix has the lowest strength, while the sample with the vinyl modification of silica particles has the highest strength.
• The creep resistance follows this trend: b-UPR/SiO₂-V > b-UPR/SiO₂-M > b-UPR/SiO₂-AMBD > b-UPR/SiO₂ > b-UPR.
• The addition and surface modification of silica particles significantly increase the free surface energy and adhesion of b-UPR composites due to the introduction of silica functionalities and adjusting the surface roughness.
• By selecting appropriate types and amounts of modification (e.g., MEMO, VYNIL, AMBD), it is possible to balance hydrophilicity and hydrophobicity, optimize wettability, and enhance the mechanical properties of the final material.