Investigating Degradation in Extrusion-Processed Bio-Based Composites Enhanced with Clay Nanofillers

Abstract

This research investigates the extrusion-based fabrication and characterization of nanocomposites derived from bio-sourced polypropylene (PP) and poly(butylene succinate) (PBS: a biodegradable polymer derived from renewable biomass sources such as corn or sugarcane), incorporating Cloisite 20 (C20) clay nanofillers, with a specific focus on their suitability for electrical insulation applications. The research includes biodegradation tests employing the fungus Phanerochaete chrysosporium to evaluate the impact of composition and extrusion conditions. These tests yield satisfactory results, revealing a progressive disappearance of the PBS phase, as corroborated by scanning electron microscopy (SEM) observations and a reduction in the intensity of Fourier transform infrared spectroscopy (FTIR) peaks associated with C-OH and C-O-C bonds in PBS. Despite positive effects on various properties (i.e., barrier, thermal, electrical, and mechanical properties, etc.), a high clay content (5 wt%) does not seem to enhance biodegradability significantly, highlighting the specific sensitivity of the PBS phase to the addition of clay during this process. This study provides valuable insights into the complex interplay of factors conditioning nanocomposite biodegradation processes and highlights the need for an integrated approach to understanding these processes. This is the first time that research has focused on studying the degradation of nanocomposites for electrical insulation, utilizing partially bio-sourced materials that contain PBS.

1. Introduction

Nanocomposites, encompassing various families of organic or inorganic matrices laden with micro- to nanoscale particles, stand at the forefront of contemporary scientific research. Defined by the achievement of particles at the nanoscale, nanocomposites emerge as innovative entities in the realm of nanoscience, offering promising prospects, particularly in the context of polymer matrix nanocomposites reinforced with clay [1].

The significant upsurge in the use of nanoscale reinforcements for polymer modification has proven to be an effective intervention strategy [2,3,4]. However, achieving the expected performances remains a major scientific challenge, primarily due to the complex synergistic interactions inherent in these systems.

In this contribution, the focus is particularly on the development of partially bio-sourced formulations to elaborate clay-based nanocomposites that address environmental imperatives and offer sustainable solutions to emerging ecological challenges. The resulting nanocomposites will then be subjected to biodegradability tests involving the influence of constituents, reinforcements, and host matrices.

This approach began with the extrusion-based elaboration of nanocomposites using organo-modified Montmorillonite clay (OMMT: Cloisite 20 (C20)). This process cleverly combines a blend of polypropylene (PP) and poly(butylene succinate) (PBS) [5,6,7], aiming to refine their electrical, physicochemical, and mechanical properties. Considerable attention has been focused on the content of clay nanofillers (C20) due to their notable enhancements in barrier, thermal, electrical, and mechanical properties when incorporated in small amounts (0.5 to 5 wt%) into polymeric matrices [8]. Mechanical [9] and electrical [10] improvements result from favorable interactions between OMMT and polymeric molecules, providing nanocomposites with superior discharge endurance, improved thermal and mechanical properties, reduced space charge accumulation, and extended lifespan. To address the compatibility issue and ensure adhesion between phases, polypropylene grafted with maleic anhydride (PP*: PP-g-MA+PP) was added [11]. This widely used compatibilizer induces favorable enthalpic attraction with the clay surface as well as with the polymer chains, mitigating the challenges associated with the poor thermodynamics of mixing (i.e., immiscibility).

The present work is therefore in line with the previous study [12], which focused on optimizing the formulation of the PP/PBS blend to produce a high performance nanocomposite with the desired properties. This study investigates the use of organically modified montmorillonite (OMMT), specifically Cloisite 20, as a reinforcing agent in nanocomposites of polypropylene (PP) and polybutylene succinate (PBS) blends. Rheological measurements reveal increased viscosity with OMMT, especially in the 70/30 blend. X-ray diffraction confirms an intercalated structure in all nanocomposites. Nanocomposites exhibit a higher crystallite size than pure C20, emphasizing the clay dispersion’s crucial impact. The 70/30 blend with 1% C20 is the optimal nanocomposite, enhancing mechanical, thermal, and electrical properties.

Polypropylene (PP) is generally not biodegradable. It is a thermoplastic polymer widely used in many applications due to its strength, durability, and low cost. Its chemical structure typically makes it resistant to natural biodegradation in the environment. However, PBS is of interest for its biodegradation properties in composites developed with PP. The biodegradability study concerns the 70/30 blends (PP*/PBS) with various levels of clay compared to pure PBS without clay (denoted 100PBS-0C). This biodegradability was evaluated in the presence of the fungus P. chrysosporium [13,14,15,16]. This basidiomycete fungus is renowned for its ability to secrete a large panel of enzymes. Specifically, it is well-known to decompose lignin through the excretion of non-selective oxidizing enzymes, endowing it with the capability to degrade various pollutants and toxic wastes. Thriving at an optimal growth temperature of 40 °C [17], P. chrysosporium flourishes in decomposition niches, such as compost or wood chips colonized by other microorganisms.

These previously cited bibliographic studies have focused on the degradation of bio-based composites, but there is no known research investigating the degradation of bio-based materials containing PBS for use in electrical insulation. The objective is to strike a delicate balance to achieve degradability without compromising the mechanical, electrical, rheological, and other properties of these nanomaterials processed by extrusion.

2. Materials and Methods

2.1. Materials and Twin-Screw Extrusion Process

The homopolymer serving as the matrix for the nanocomposites is PP, referenced as ISPLEN PP 050Y1E and supplied by Repsol (Madrid, Spain). Another polypropylene modified with maleic anhydride (PP-g-MA 10% by weight), known as Orevac CA100 and provided by Arkema (Villers-Saint-Paul, France), was introduced as a compatibilizing agent. The melt flow indices (MFIs) of PP and PP-g-MA were 5.8 g/10 min (230 °C/2.16 kg) and 10 g/10 min (190 °C/0.325 kg), respectively. These polymers had a melting temperature of around 164 °C. The PP/PP-g-MA blend was designated as PP* to distinguish it from pure PP. The PBS, referenced as PBE003 and obtained from Natureplast (Ifs, France), exhibited an MFI of 4–6 g/10 min (190 °C/2.16 kg) and a melting temperature of approximately 115 °C. The organically modified clay, named Cloisite 20 (C20) and supplied by BYK additives (Wesel, Germany), was used as a nanofiller. Modified with bis(hydrogenated tallow alkyl) dimethyl ammonium, the C20 nanoclay platelets had a thickness of 1 nm, a width of 200 nm, and a length of 400 nm, with an aspect ratio (i.e., width-to-thickness ratio) of about 200.

The sample preparation process involved dry mixing of PP, PP-g-MA, PBS (in pellet form), and C20 (in powder form). These components were then simultaneously introduced into the hopper and extruded using a laboratory-scale twin-screw extruder, Leistritz ZSE 27 MAXX (Nuremberg, Germany) (Figure 1). The blend was operated at specific temperatures for PBS- and PP-based materials. C20 was incorporated at rates of 1, 2, and 5 wt% relative to the matrix. The nanostructure of the nanocomposite presented in this figure is crucial for its performance and degradation. The orientation of the clay (C20), the function of the compatibilizing agent (PP-g-MA), and the phase separation between PP and PBS play crucial roles in determining the overall properties of the material. Clay nanoplatelets tend to align parallel to the polymer matrix (PP), thus improving the mechanical properties of the blend (PP-PBS) but potentially affecting degradation behavior. Additionally, the compatibilizing agent (PP-g-MA) facilitates the dispersion of clay within the polymer matrix, contributing to enhanced interfacial adhesion and overall composite performance. Phase separation phenomena between PP and PBS will be presented in the results section to study the blend morphology and its impact on degradation mechanisms.

2.2. Testing Sample Biodegradation

The samples made with the 70PP*/30PBS blend under optimal extrusion conditions (screw speed: 200 rpm, feed rate: 3 kg/h, barrel temperature control: 180 °C), representing the best electrical, mechanical, and thermal properties [12], underwent biodegradation tests with the fungus P. chrysosporium. The initial spore concentration was 3.65 × 10⁵ spores/mL, and the tests were conducted in flasks containing 200 mL of osmosis water, 2% malt extract, and 1 mL of the fungal suspension. The samples were stored in an incubator at 25 °C with agitation. Observations after 4 and 11 weeks revealed changes in the chemical composition of the materials, which were confirmed through subsequent characterizations.

The samples designated for degradation (100PBS/0C, 70PP*/30PBS/0C, 70PP*/30PBS/1C, 70PP*/30PBS/3C, and 70PP*/30PBS/5C) had a rectangular shape with the following dimensions: thickness, t = 2 mm; length, L = 2 cm; and width, w = 1.3 cm. These samples were carefully cleaned in an ultrasonic bath and dried to eliminate any impurities that may contaminate the biodegradation environment. Two samples of each material were placed in separate vials containing the fungal suspension (3.65 × 10⁵ spores/mL) and sealed with vented caps to allow exchanges with the environment (air) and aerobic fungi. The vials were stored in an incubator at 25 °C and agitated at a speed of 100 rpm. Figure 2 illustrates the samples and the suspension at the beginning of the tests, after 4 weeks, and after 11 weeks of incubation.

There was a change in the color of the fungal suspensions over time, suggesting the growth of P. chrysosporium mycelium. Initially almost transparent at t = 0, the solution became cloudy as the fungal pellets developed. Spore germination produced hyphae that surrounded the samples, indicating a conducive environment to microbial growth. After the incubation period, the samples underwent a preliminary cleaning before characterization. Figure 3 shows an image of the samples before degradation initiation and the same set after 11 weeks of incubation. A slight tint was observed on the 70PP*/30PBS samples, while a more pronounced coloration was noticed on the pure PBS. This change in color suggests that the exposure of materials to an aqueous environment in the presence of fungi leads to alterations in their chemical composition. This observation is supported by infrared spectroscopy and scanning electron microscopy.

2.3. FTIR Analysis

Fourier transform infrared spectroscopy (FTIR) is a non-destructive characterization method used to determine the nature of chemical bonds present in a molecule. It is one of the spectroscopic techniques employed for the identification of compounds and their structure, or for determining the chemical composition of a sample.

The analyses were conducted using an “IRAffinity-1S” SHIMADZU Fourier Transform Infrared Spectrometer (Marne la Vallée, France), using the attenuated total reflection (ATR) mode, within a spectral range of 600 to 4000 cm⁻¹. This spectrometer has an optimal resolution of 0.5 cm⁻¹, and the studies were carried out at a resolution of 4 cm⁻¹. Each analyzed sample underwent an average of ten scans.

2.4. SEM Observations

A scanning electron microscope (JEOL Ltd. 6460LV, Akishima, Japan) was used to study the topography of the nanocomposites’ surface, which were previously coated with a thin layer of carbon. The primary beam energy used for these observations was 20 keV. The morphology of the studied samples was observed with a variable magnification, including ×100 and ×3000.

3. Results

3.1. FTIR Analysis of Biodegraded Samples

The FTIR analysis conducted on the biodegraded samples, referenced from

Table 1. Wavenumber, functional groups, and molecular vibration modes of PBS [18].

Wavenumber (cm−1)	Functional Groups	Molecular Vibration Modes
802, 918 and 959	C-OH	Shearing
1046	O-C-C	Stretching
1053	C-O-C	Asymmetric stretching
1310	O=C-O-	Stretching
1716	C=O	Stretching
2850	CH2	Stretching

[18],

Table 2. Wavenumber, functional groups, and molecular vibration modes of PP* [19].

Wavenumber (cm−1)	Functional Groups	Molecular Vibration Modes
808 and 840	CH2	Wagging
898	CH3	Asymmetric rocking
972	C-C	Asymmetric and symmetric stretching
998	CH3	Asymmetric rocking
	CH	Shearing
	CH2	Scissoring
1160	C-C	Asymmetric stretching
	CH3	Asymmetric rocking
	CH	Shearing
1375	CH3	Symmetric shearing
	CH2	Wagging
	C-H	Shearing
	C-C	Stretching
1456	CH3	Asymmetric shearing
2839	CH2	Shearing
	CH2	Symmetric torsion
2872	CH3	Symmetric torsion
2920	CH2	Asymmetric torsion
2952	CH3	Asymmetric torsion

[19], and

Table 3. Wavenumber, functional groups, and molecular vibration modes of Cloisite 20 [20].

Wavenumber (cm−1)	Functional Groups	Molecular Vibration Modes
912	Al-O and Mg-O	Stretching
1010 and 1110	O-Si-O	Stretching
1467	CH2	Shearing
2848	CH2	Symmetric stretching
2929	CH2	Asymmetric stretching
3636	OH of silicate	Stretching

[20], provided valuable insights into the molecular changes occurring in the polymer blends. As shown in Table 1, Table 2 and Table 3, the FTIR spectra revealed distinctive peaks corresponding to various functional groups in the PBS, PP*, and Cloisite 20. Figure 4a displays the FTIR spectrum of Cloisite 20. The stacked structure of the clay layers results in specific bonds, as evidenced by a sharp peak on the spectrum defining the Si-O-Si bonds within the silica tetrahedra. Since the clay used is organically modified, characteristic peaks of the surfactant were also identified at 1467, 2848, and 2929 cm⁻¹. The wavenumbers and molecular vibration modes of these peaks were characterized. FTIR spectra normalized to the characteristic peak of the -CH₃ functional group of polypropylene, whose intensity is highest in the spectrum of undegraded samples, are shown in Figure 4c–f. The spectrum of the undegraded sample, which does not depend on the amount of clay added, was taken as a reference. It should be noted that characteristic cloisite peaks are absent in the FTIR spectra of the nanocomposites, probably due to the dispersion of the clay on the one hand, and the FTIR analysis confined to the surface on the other. Upon biodegradation, a careful examination of these spectra demonstrated significant alterations (Figure 4b–f). The most noticeable changes occurred in the PBS phase, with a reduction in the intensity of the peaks associated with carbonyl groups C=O (1716 cm⁻¹), C-OH bonds, and C-O-C bonds (900–1100 cm⁻¹) of ester functions. The decrease in peak intensity in the 2800–3000 cm⁻¹ range can be attributed mainly to stretching of the PBS CH₂ bonds, and not to symmetrical torsion of the polypropylene CH₃ bonds, as the latter is non-biodegradable. The decrease in peak intensity in the 2800–3000 cm⁻¹ range can be attributed to the PBS’ CH₂ bond stretching and CH₃ bonds symmetrical torsion. However, the results suggest a specific impact on the PBS (spectrum (a)) rather than the PP*. These modifications are consistent with the enzymatic degradation process induced by fungal activity [21].

3.2. Morphological Study by SEM

The scanning electron microscopy (SEM) analysis provided a detailed examination of the surface morphology of the materials before and after biodegradation. Micrographs captured at various magnifications (×100 and ×3000) depicted the evolution of the surface topography over time (Figure 5).

After 4 weeks of exposure to fungal activity, SEM images displayed a roughened surface, indicative of fungal penetration and hole formation. Remarkably, after 11 weeks, the surface morphology exhibited filamentous structures and cavities, especially pronounced in 100PBS, in the raw blend 70/30/0C, and in 70/30/1C. These observations align with similar studies on polymer nanocomposites [16] and poly(ester amide) degradation [22,23].

4. Discussion

The interpretation of the FTIR results indicates a reduction in absorbance, particularly in carboxyl groups within ester functions of PBS. This observation, in agreement with the work of Chonde et al. [21] on the degradation of nylon 6 by the fungus P. chrysosporium for 75 days, can be attributed to the hydrolysis process in the polymer. The decomposition of a substance by water, known as hydrolysis, involves water acting as a nucleophile, attacking the unsaturated carbon of the carboxyl group. Polyesters such as PBS can degrade via hydrolysis due to the susceptibility of ester bonds to water attack. As shown in Figure 6, the first step of the reaction mechanism involves the nucleophilic attack of water on the electrophilic site of the polymer, the ester bond, forming a tetrahedral intermediate. The molecular fragmentation of this intermediate leads to the cleavage of the polymer chains at the ester functions, resulting in the formation of two smaller macromolecular chains: one ending with a carboxyl function and the other ending with a hydroxyl group. The works of Corti et al. [22] on linear low-density polyethylene films used for greenhouses exposed to sunlight also show a decrease in the amount of oxidation-generated carbonyl groups, explained by fungal uptake of low-molecular-weight groups in the amorphous region of the polymer. These findings underscore the importance of understanding the hydrolysis process in polymer degradation, highlighting its role in altering their structure and properties over time.

SEM analysis revealed distinct morphological changes induced by biodegradation. The rough surface observed after 4 weeks and the filamentous structures with cavities after 11 weeks signify the progressive damage of the material. Indeed, phase separation between PP and PBS has a significant impact on nanocomposite biodegradation. Phase separation is manifested by the formation of PBS nodules dispersed in a continuous PP matrix (Figure 7). It promotes selective degradation of the PBS, resulting in morphological changes. As PBS degrades, it leaves cavities or holes in the PP matrix. These cavities increase the specific surface area of the nanocomposite, which can accelerate the biodegradability of the remaining PBS, but the intact PP matrix continues to preserve the overall structure of the nanocomposite to some degree. The size and distribution of PBS nodules directly influence roughness and cavity formation after degradation. Larger or more concentrated nodules can lead to larger cavities.

Our results agree with the findings of Shimpi et al. [16] on PP/PLA/Ca carbonate nanocomposites wherein observed cavities on materials surfaces are attributed to PLA degradation by the same fungus. Sasek et al. [24] conducted experiments on the biodegradation of two polyester-amides with different fungi and obtained the best results with P. chrysosporium, reinforcing the role of this fungus in surface degradation. FTIR spectra of the 70/30 blend and its nanocomposites underscore the sensitivity of the PBS phase peaks to biodegradation. Voids observed in SEM micrographs therefore correlate with the disappearance of the PBS phase. Furthermore, the impediment to biodegradation posed by clay nanofillers, as evidenced by the SEM analysis, emphasizes the importance of compatibilization in facilitating fungal access.

While PP* is traditionally considered non-biodegradable, insights from studies [25] on high-density polyethylene (a similar, non-biodegradable polymer) and the potential of P. chrysosporium suggest that biodegradation may be feasible. The proposed initiation through oxo-biodegradation aligns with the literature [22], displaying the complex interplay of environmental factors influencing the biodegradation of polypropylene.

The combined FTIR and SEM analyses provide a comprehensive understanding of the chemical and morphological changes during biodegradation, shedding light on the intricate interplay between polymer composition, nanofillers, and fungal activity, while also yielding valuable insights into the surface properties of the samples.

These findings contribute valuable insights for further research in developing sustainable polymer blends. The SEM results further complement the FTIR findings, providing a visual representation of the morphological transformations induced by fungal biodegradation. The presence of voids and filamentous structures is consistent with the literature, indicating the role of fungi in polymer degradation [16,24]. The lack of substantial improvement in biodegradation with higher clay concentrations in nanocomposites suggests that the clay layers act as barriers, hindering fungal access to the PBS phase. This is substantiated by the intercalation of PBS chains between clay layers and the entanglement with PP*, facilitated by clay as a compatibilizer. The potential biodegradation of PP*, as indicated by previous studies [24], introduces intriguing possibilities. While the study did not focus on PP* degradation, future research could explore its enzymatic degradation potential, especially under conditions optimized for fungal activity. Additionally, the initiation of oxo-biodegradation presents a pathway for further investigation, requiring the addition of pro-degrading additives, to accelerate the reaction between carbon in the polymer chains and oxygen in the atmosphere. With the generated oxidized groups, the polymer becomes hydrophilic facilitating water absorption and, then, the proliferation of fungal microorganisms [26,27,28].

5. Conclusions

The objective of this study was to investigate the degradability of bio-based nanocomposites comprising a blend of polypropylene (PP, non-biodegradable) and poly(butylene succinate) (PBS, biodegradable), incorporating nanoscale clay fillers, specifically Cloisite 20 (C20). PP is known for its mechanical and electrical properties, PBS contributes to the biodegradation of the composite material, PP-g-MA acts as a compatibilizing agent, and the clay nanofillers serve as both mechanical reinforcement and compatibilizer.

The focus was on finding a new nanocomposite that maintains the mechanical and electrical properties of PP while having a degradable matrix based on PBS. Specifically, we examined the 70PP/30PBS formulation with clay contents ranging from 1 to 5%, aiming to strike a balance between maintaining the mechanical and electrical properties of PP and achieving a degradable matrix based on PBS. The results revealed significant alterations in the chemical structure of the 70PP/30PBS nanocomposites, as evidenced by the reduction in the intensity of the FTIR peaks associated with characteristic bonds of the PBS phase. Scanning electron microscopy observations confirmed these changes, showing the appearance of cavities on the surface of the nanocomposites, indicative of the progressive disappearance of the PBS phase. Surprisingly, the addition of clay did not significantly improve the biodegradability of the nanocomposites, particularly at higher clay concentrations, as evidenced by the low detectability of degradation with 5% clay by FTIR analysis.