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Article

Potential Use of Residual Powder Generated in Cork Stopper Industry as Valuable Additive to Develop Biomass-Based Composites for Injection Molding

by
Ismael Romero-Ocaña
*,
Miriam Herrera
,
Natalia Fernández-Delgado
and
Sergio I. Molina
Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, IMEYMAT, Campus Universitario Río San Pedro s/n, Puerto Real, 11510 Cádiz, Spain
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(7), 330; https://doi.org/10.3390/jcs9070330
Submission received: 26 May 2025 / Revised: 24 June 2025 / Accepted: 24 June 2025 / Published: 26 June 2025
(This article belongs to the Special Issue Polymer Composites: Fiber Architecture, Interfacial Engineering, and Processing)
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Abstract

This study presents the development of a sustainable composite material by incorporating by-products from the cork industry into acrylonitrile butadiene styrene (ABS), with the aim of reducing the environmental impact of plastic composites while maintaining their performance. ABS, a petroleum-based polymer, was used as the matrix, and maleic anhydride (MAH) with dicumyl peroxide (DCP) served as a compatibilizing system to improve interfacial adhesion with cork microparticles. Composites were prepared with 10% w/w cork in various particle sizes and characterized via FTIR, X-ray computed tomography, SEM, mechanical testing, and thermal analysis. The best performing formulation (CPC-125) showed a reduction of only ~16% in tensile modulus and ~7% in tensile strength compared with ABS-g-MAH, with a more pronounced decrease in strain at break (3.23% vs. 17.47%) due to the cork’s inherent rigidity. Thermogravimetric and calorimetric analysis confirmed that thermal stability and processing temperatures remained largely unaffected. These results demonstrate the feasibility of incorporating cork microparticles as a bio-based reinforcing filler in ABS composites, offering a promising strategy to reduce the use of virgin plastics in applications compatible with conventional injection molding.

1. Introduction

A large proportion of consumer goods that today’s society uses daily are made of plastics. Household utensils, food packaging, clothes, cars, electronic devices, and a large variety of articles are made of plastic materials, most of which are derived from petroleum [1]. The unbridled consumption of plastic has led to a great deterioration of ecosystems. Plastic derived from petroleum is not biodegradable and can remain in the environment for hundreds of years before being incorporated into the food chain [2]. Thus, a large part of the ocean is currently contaminated with microplastics, with trace amounts found in fish and crustaceans [3,4]. Although there is a clear link between plastic use and environmental pollution [5,6], it is not easy to find alternatives to plastics. This is due to the advantages of these polymeric materials over others, for example, their low cost, accessibility, ease of production, and availability for a wide variety of applications along with good mechanical properties [7,8]. Additionally, with the technological development of society, the industry of polymeric composite materials (polymer matrix with additive(s)) has increased, with an infinite number of composite materials being used today [7]. It is in the vehicle industry of all types (air, sea, or land) that many of these composite materials are used [9]. Carbon and glass fibers are two of the most commonly used additives in the industry of composite materials. Although composite materials with these fiber types have exceptional mechanical characteristics, their production is unsustainable, and at the end of their lifecycle [10,11], they pose an additional environmental burden because they are not easily recycled [12].
This has led to research into new, more sustainable composite materials. Numerous research projects have been carried out in this area, where some types of waste biomass have been incorporated into the polymer matrix [13]. This method has numerous advantages. On such advantage is that the circular economy is promoted, as biomass from the agri-food industry can be used as raw material for the manufacture of novel composite materials [14]. Additionally, it reduces the number of plastics and/or composite materials derived from petroleum, as the addition of a certain percentage of biomass reduces the presence of the plastic material that acts as a matrix while reducing energy consumption during processing compared with conventional composite materials [15]. One of the main objectives of research on bio-based composites is to achieve a good compromise between the final mechanical properties of the material and the sustainability of the process. As a result, biomass-based composite materials, whether in the form of fibers or microparticulate powders, are under intense research because of their great potential to replace synthetic materials. In a world focused on environmental performance, bio-based materials have great industrial potential.
This work focuses on the use of microparticulated cork of various sizes, generated as an agroresidue from the cork industry, as an additive to an acrylonitrile butadiene styrene (ABS) polymer matrix for the manufacture of composite materials by injection molding [16], with applications ranging from food packaging to space exploration technologies [17,18]. ABS is one of the most widely used thermoplastics in the industry for manufacturing various consumer goods, with significant importance in the automotive and aeronautical sectors. As a result, ABS has become a reference material in the industry and in the development of new composites [19]. Several studies have explored ABS-based composites filled with natural or mineral reinforcements to enhance mechanical performance and reduce dependence on petroleum-derived polymers. For example, in the research work of Nourbakhsh et al. [20], wood flour was used to reinforce ABS, showing an increase in stiffness but a reduction in tensile and impact strength at higher filler contents. Hemp fiber composites based on ABS have demonstrated improvements in tensile strength (up to ~41 MPa) and modulus, especially when compatibilizers are used [21]. These examples underscore the versatility of ABS in forming composites and highlight the importance of interfacial adhesion and filler selection. In this context, our study proposes the use of microparticulated cork as a renewable, low-density filler, aiming to preserve mechanical properties while introducing a bio-based alternative.
The incorporation of cork as an additive to the ABS matrix is justified not only by the exceptional mechanical properties of this matrix but also by the opportunity to utilize an agricultural waste material, which can help to reduce the extensive use of plastics derived from oil refining [22]. In addition, it provides a sustainable reuse solution for the large amount of biomass generated during industrial processes [22,23,24]. The production of cork stoppers generates a large amount of cork powder with minimal industrial utility, which can be recycled as an additive. Numerous studies in the literature explore the use of cork as an additive in composite fabrication [25]. Among the different techniques that can be used to obtain these composites, additive manufacturing (AM) [26,27,28,29,30,31], compression molding [32], or injection molding [33,34] are included. Other fabrication processes such as light chipboards have also been reported [35,36]. However, despite extensive research on cork, no information was found in the literature regarding its use as an additive for ABS in injection molding, which is the goal of this study.
It should be noted that, in general, adding natural fibers to a polymer matrix often results in mechanical properties that are inferior to those of the pure polymer matrix. This negative effect is most pronounced as the fiber content increases [37,38]. The reason for this phenomenon is often the lack of the integration of additives into the polymer matrix [39]. Adequate inclusion requires the use of chemical coupling agents that help to improve the adhesion between the interface of the additives and the polymeric matrix. In the case of using a plant-origin additive, such as biomass, and a polymeric matrix, this incompatibility is even more pronounced; thus, it is almost mandatory to make use of these agents [37,39,40]. Maleic anhydride (MAH) is a promising agent widely used for the production of composites with biomass-based additives [22,33,37,39,40,41,42]. MAH builds hydrogen bridges and/or covalent bonds with polysaccharides in the cork cell wall (hydroxyl groups), helping to bond cork particles with other functional groups in the polymeric matrix [19,43]. In this work, we use MAH to improve the integration of microparticulated cork into the ABS matrix, to preserve its mechanical properties.

2. Materials and Method

2.1. Materials

The ABS reference used is Terluran GP 22, purchased through Smart Materials S.L. (Alcalá la Real, Jaén, Spain). This reference is especially suitable for injection molding, being characterized by its low heat distortion and high impact resistance. The cork used was supplied by Corchos del Estrecho S.L. (Alcalá de los Gazules, Cádiz, Spain). The powder comes from the waste obtained after the manufacture of stoppers for the wine sector. This cork comes from sheets that have been previously treated by boiling and subsequent drying. These treatments are typical of the cork industry. The purpose of this procedure is to eliminate the tannins and parasites naturally present in the cork. It has been observed that after the procedure and being left in open air, the physical–chemical properties of the cork treated in this way are improved. MAH (EssentQ) of molecular weight 98.06 g/mol and density 1.32 g/cm3 was supplied by Scharlab S.L. (Barcelona, Spain). Dicumyl peroxide (DCP), used as initiator, (Acros Organics) with molecular weight 270.37 g/mol and density 1.56 g/cm3 was supplied by Scharlab S. L. (Barcelona, Spain). MAH and DCP were used as received, without any pretreatment. The reaction that takes place during the fabrication of the cork polymer composites (CPCs) between DCP, MAH, and ABS is explained in Section 2.4.

2.2. Sieving and Screening of Cork Powder

The starting cork is a mixture of particles of different sizes. The cork is mechanically sieved in order to obtain fractions with defined sizes in a range between 45 μm and 125 μm. The model of the mechanical sieve shaker used is AS200 from Recht, Germany. This analytical equipment has several interchangeable sieves arranged vertically. The mesh sizes used for the different sieves were 45 µm, 63 µm, and 125 µm. In this way, four fractions of sieved cork powder with different sieve sizes were obtained: <45, ≥45 ≤63, ≥63 ≤125, and ≥125 μm. The digital amplitude selected on the sieve shaker was 80% (in a range of 1–100%). Each sieving period takes half an hour.

2.3. Moisture Content Determination of Microparticulated Cork

The moisture content of the microparticulated cork was measured based on the ISO 2190:2016 standard. Briefly explained, this procedure was carried out by weighing samples of 50 g of cork in 50 mm tall containers. These containers with the different cork samples were introduced into the oven at 103 °C for 30 min. Subsequently, the material was introduced into a desiccator for another 30 min. Table 1 shows the results obtained after the procedure accounted for the moisture content of the samples used as additives, according to the standard mentioned. As can be observed, the moisture content is below 5%. The standard claims that the materials need to be below 20% of moisture content. This means that the cork used in this work is within the standard range and, thus, it can be used as an appropriate additive for the fabrication of composites.

2.4. Fabrication of CPCs

Initially, the ABS pellets and the cork powder were dried for 48 h in a vacuum oven at 80 °C to eliminate the moisture present in both materials. Then, a series of CPCs was made with the compositions shown in Table 2. The composites were prepared by mixing the polymeric matrix with the additives in a Noztek HT 100 single-screw extruder (Noztek, West Sussex, UK). First, the ABS pellets were mixed with DCP (0.3% w/w) and MAH (3% w/w) to obtain ABS-g-MAH (ABS grafted with maleic anhydride) at a temperature of 230 °C, usual for this type of functionalization, for 15 min under air atmosphere. The reaction mechanism through which MAH grafting into the ABS chain occurs involves two steps, as proposed by Manaf Olongal et al. [44]. Initially, under the action of heat in the extruder, DCP forms a cumyl radical. In a second step, the radical formed attacks the two possible positions of the butadiene segment of the ABS chain: the double bond position or the allylic position (including its resonant form). The MAH molecule then joins the molecule resulting from both attacks, and the MAH is grafted onto the ABS chain.
For the manufacture of various CPC compositions, the previously obtained ABS-g-MAH was used as the starting polymer matrix. This matrix was then mixed in the single-screw extruder with the microparticulated cork at a percentage of 10% w/w with respect to the polymeric matrix. The processing temperature was 220 °C. A filament was obtained in this way and then pelletized in a Sacamex pelletizing machine (Saumur, France). The pellets were then used for injection molding, previously dried for a minimum of 24 h in a vacuum oven. Pristine ABS pieces were fabricated by injection molding as well, to be used as a reference material. In this study, three key variables were selected to evaluate the mechanical performance of the bio-based composites: the particle size of the cork filler, the presence or absence of a compatibilizer (maleic anhydride-grafted ABS), and a fixed filler content of 10% w/w. These parameters were chosen due to their direct relevance to the composite’s structural integrity and processability via injection molding. While other factors such as higher filler loadings or alternative coupling agents are also of interest, the current work aims to serve as a preliminary validation of the feasibility of using cork waste as a reinforcement material.
It is important to note that cork has a very low density of approximately 0.25 g/cm3, which implies that even a relatively small weight fraction (e.g., 10% w/w) corresponds to a high-volume fraction in the final composite. During preliminary trials, it was observed that increasing the cork content beyond 10% by weight significantly compromised the flowability and injectability of the material using our laboratory-scale injection molding equipment. The presence of higher cork volumes led to incomplete cavity filling, irregular surfaces, and reduced dimensional stability. Therefore, 10% w/w was determined to be the upper practical limit under the current processing conditions. It is likely that industrial-grade injection molding systems, equipped with more powerful clamping units, enhanced screw geometries, and optimized process parameters, could allow for higher biomass loadings. This remains an open area for future investigation. Alternative processing techniques such as compression molding may also be suitable for higher cork content and will be considered in further studies.

2.5. Fabrication of Tensile Specimens

In order to characterize the mechanical properties of the different processed CPCs, a series of tensile specimens were manufactured. For this purpose, the pellets obtained in the single-screw extruder were introduced into an injection molding machine (Model Babyplast 6/12 standard, Rambaldi Group (Italy)). The conditions were optimized to proceed with the fabrication process; details are shown in Table 3. In this way, standardized specimens were obtained according to the ASTM D638 standard. The injected specimens were of type 1BA with dimensions of 75 mm length, 5 mm width, and 2 mm thickness.

2.6. Characterization Methods

2.6.1. Characterization by Scanning Electron Microscopy (SEM)

The cork particles obtained after screening were characterized by SEM. The microscope used was a Nova NanoSEM 450 (FEI Company, Hillsboro, OR, USA). The samples analyzed were previously coated with 15 nm of Au to protect the surface of the material from the electron beam. A 208 HR Sputter Coater Cressington system was used for this treatment. The samples were attached to the SEM holder using a special carbon tape.

2.6.2. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR measurements were performed in order to analyze the presence of MAH in the ABS functionalized with this coupling agent. To this end, an Aplha-α FTIR system from Bruker Española S.A. (Rivas-Vaciamadrid, Madrid) was used. The samples were prepared by mixing powders from the injected composites and from KBr. This powder mixture was pressed to obtain a pellet to be analyzed by FTIR taken from a range of 400–4000 cm−1.

2.6.3. Methodology for Density Analysis

The density of the different CPCs and of the polymer matrix was calculated using the gravimetric method, following the ASTM D792-13 standard, test method A. The equipment used to carry out the measurements consisted of an analytical balance with an accuracy of ±0.1 mg; an immersion vessel, such as a beaker with a wide mouth; a fixed support to hold the vessel above the balance pan; a thermometer graduated at 0.1 °C intervals, with a scale from 0 °C to 30 °C; and a stirrer to suspend the samples in the immersion liquid. The equipment also included a thermostatically controlled bath, where the immersion liquid used was deionized water. The samples were previously coated with a thin film of lacquer in order to prevent the absorption of water during the measurement. It should be mentioned that the influence of this lacquer thin film is insignificant, and thus, it will not affect the results. The procedure can be found in the above-mentioned standard.

2.6.4. X-Ray Computerized Tomography (CT) Characterization of CPCs

CT scanning was carried out using a Zeiss Xradia 610 Versa X-ray microscope (Carl Zeiss X-ray Microscopy, Oberkochen, Germany), working at an accelerating voltage of 30 kV. A total of approximately 1000 2D projections were recorded as the samples were rotated by 360° and computationally reconstructed via a filtered back projection algorithm (Zeiss XM Reconstructor). To visualize the internal features inside the sample in a 3D space, the software DragonFly v. 2022.2 (Object Research Systems) was used. Data was imported into the Avizo for Materials Science 9.9.1 software suite (FEI Co., USA) to analyze the porosity of the composites. The pores were segmented using a grayscale threshold applied to the absorption contrast data.

2.6.5. Characterization of Thermal Properties

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been the techniques used for the characterization of the thermal properties. For this purpose, a simultaneous thermal analyzer system SDT Q600 from TA Instruments (New Castell, DE, USA) was used. In order to perform the measurements, the samples were stabilized for 5 min at 30 °C. Subsequently, they were heated up to 600 °C at a rate of 10 °C/min in an inert nitrogen (N2) atmosphere. The amount of sample used for each analysis was approximately 16 mg, placed on an aluminum sample pan.

2.6.6. Tensile Testing

To study the mechanical properties of the different CPCs, tensile tests were performed. The specimens obtained by injection molding, according to the ASTM D638 standard, were left to rest for 48 h before each tensile test was carried out. For the tensile tests, a Shimadzu Company (Kioto, Japan) universal testing machine was used. The testing speed was 1 mm/min. For each test, five specimens of the same composite were analyzed to obtain the different mechanical properties from the stress–strain curve. Young’s modulus (E), the elongation percentage at break (%εr) and tensile strength (σr) were studied.

2.6.7. Statistical Analysis ANOVA

To establish whether there were significant differences in the mechanical properties of the different samples studied, an analysis of variance study (ANOVA) was performed. Here, the margin of difference was established with a confidence level of 95% according to Ronald E. Wapole et al. [45]. To conduct the statistical study by ANOVA, the Bonferroni method, which distributes the type I error (0.05%) equally among all comparisons, was applied. The details of the statistical analysis can be found in S4 of Supplementary Materials.

3. Results and Discussion

3.1. Morphology of the Cork Particles Analyzed by SEM

The morphology of the cork particles was analyzed by SEM, and the images can be observed in Figure 1. It can be noticed that, as the particle size decreases, the number of broken cork cells increases considerably. These ruptures of the cork cell wall (some examples are marked with red arrows in Figure 1A) take place during the manufacturing process of cork stoppers. Figure 1B shows in detail a completely open cell and its fibrous interior. During the melt compounding process, these open cavities will be filled by the polymer, and the cell wall will act as a cover. The granulometry of the cork powder is an essential factor to be considered because of the key role that the alveolar structure plays in the mechanical and other properties. If this typical alveolar structure of the cork is broken, as the cork particle size decreases, some of the properties associated with this structure, such as the acoustic or thermal insulation capabilities, would likely deteriorate. Although smaller particle sizes produce a better integration of the particles in the polymeric matrix, it is necessary to find a balance between keeping the characteristic properties of cork and obtaining a good integration of the additives in the matrix, with the objective of having the desired mechanical properties in the final composite. In Figure 1C,D, it can be seen that for sizes >125 µm, it is possible to find cork particles that retain its characteristic alveolar structure.

3.2. Fourier-Transform Infrared Spectroscopy Analysis

The objective of this FTIR analysis is to evaluate the changes in the molecular structure of ABS after the incorporation of MAH and cork. Figure 2 displays the FTIR spectra of neat ABS, maleic anhydride (MAH), grafted ABS (ABS-g-MAH), cork powder, and one representative CPC (composite prepared with ABS-g-MAH). For further reference, S1 of Supplementary Materials includes a table summarizing the main IR bands of ABS, which are present across all samples.
For MAH, the symmetric and asymmetric carbonyl stretching vibrations were clearly observed at 1853 cm−1 and 1780 cm−1, respectively—features typical of cyclic anhydrides. Additionally, the C–O stretching band of the anhydride ring appeared at 1058 cm−1. Other relevant absorption bands include those at 3594, 3185, and 3120 cm−1, attributed to overtone or combination modes. Furthermore, distinct C–O and C–C stretching vibrations were detected at 1289, 1267, and 1241 cm−1, confirming the molecular presence of MAH (Brian C. Smith, Spectroscopy-03-01-2018 Volume 33 Issue 3) [46].
A characteristic peak at 1780 cm−1, attributed to the symmetric carbonyl stretch of MAH, is clearly visible in the ABS-g-MAH and all the CPC samples, but is absent in neat ABS (highlighted in a red box in Figure 2). This confirms the successful grafting of MAH onto the ABS chains, in agreement with previous studies [19].
Interestingly, no additional significant shifts or new bands were observed in the FTIR spectra of ABS-g-MAH or CPCs compared with neat ABS. This suggests that the chemical structure of ABS remains mostly intact, and that the incorporation of cork does not result in substantial chemical modifications. The cork, therefore, appears to act primarily as a physical filler rather than engaging in covalent interactions with the polymer matrix.
Regarding cork powder, several absorption bands characteristic of its main components—suberin and lignin—were clearly detected. These include those at 1160 cm−1 (COC, suberin fingerprint), 1383 cm−1 (CH3, suberin), 1607 cm−1 (aliphatic group of suberin), and 1630 cm−1 (C=C stretching in suberin and lignin). The strong peak at 1739 cm−1 corresponds to C=O stretching in suberin ester groups, while those at 2854 cm−1 and 2922 cm−1 are associated with CH2 and alkyl chains, underscoring the aliphatic nature of cork [47]. These signals confirm the presence of cork’s molecular constituents and support the interpretation of its role in the composites.
However, it is worth noting that the IR bands of cork—mostly arising from functional groups such as C–O, C=O, and –OH—partially overlap with those of ABS and MAH, making them less distinguishable or even masked in the spectra of the final CPCs.

3.3. Density

Table 4 shows the calculated values of density for the composites and the ABS matrix (ABS-g-MAH), together with the pristine ABS taken as reference. As can be observed, the density of ABS-g-MAH is lower than that measured for ABS. This is due to the molecular weight decrease when ABS is grafted with MAH [44]. On the other hand, it can be observed that the densities of all CPCs are larger than those of the ABS-g-MAH matrix. The density of cork, which can vary due to several factors, ranges between 0.12 and 0.24 g/cm3 [45]. Therefore, the introduction of cork in ABS-g-MAH is expected to produce composites with lower density than the matrix, and not larger, as observed. The results obtained could be related to the fact that a large number of cork cells had broken walls, as observed by SEM. This would allow the molten polymer to fill the cork cells, replacing the occluded air. The density of the cork cell wall (without the occluded air) was estimated to be around 1.20 g/cm3 [48]. In this way, the addition of cork cells where the occluded air was partially removed to the ABS matrix could produce an increase in density, which would explain the results obtained. Also, it is remarkable that all composites showed similar values of density that were close to the value estimated using the rule of mixtures considering the density of the cork cell wall, around 1 g/cm3. This can be explained by the nature of the composite manufacturing process. During the mixing stage in the extruder, where microparticulated cork and ABS are combined, the cork undergoes compression. A similar phenomenon occured during the production of test specimens via injection molding of the composite pellets into the mold. During this compression, the air trapped within the cork cells is expelled, and the polymer penetrates the interior of the cells, increasing the density of the composites. Thus, our results show that even in CPC-125, where cells retaining their characteristic alveolar structure were observed by SEM, the cork cells were compressed and most of the occluded air was expelled. This fact is corroborated by the CT analysis shown below.

3.4. CT Analysis of CPCs

Figure 3A shows a rendered volume of the composite CPC-125 obtained by CT, where the brighter regions correspond to the cork particles embedded in the polymer matrix. As can be observed, the cork particles show a reasonably good distribution in the composite, as particle agglomerations were not observed. The same result was observed for sample CPC-63125, shown in Figure 3B. This is an important issue to avoid degrading the mechanical properties of the composites regarding the pristine matrix. However, and as can be observed, some pores have been found in both materials (red circles in Figure 3A,B). For comparison, Figure 3C shows a rendered volume of the reference sample of ABS. As can be observed, no pores were found in this polymer, evidencing that the porosity found in the composites were associated with the introduction of cork. The volume of the pores were quantified in samples CPC-125 and CPC-63125, obtaining values of <3% v/v in both materials. This indicates that the pores associated with the introduction of cork in the ABS matrix should not have a strong detrimental effect on the mechanical properties of the resulting composites. Regarding the cork particles, Figure 3D,E show 2D slices from the 3D greyscale data of sample CPC-125. In these images, intensity scales with X-ray absorption in each region of the material, which is related to their density. As can be observed, cork only partially retains the air occluded in the cells, visible as dark spots inside the cork fragments, and the cork cells appear compressed. These results are in good agreement with the measured values of density explained above, which showed larger values than expected if the classical cellular structure of cork was considered. Additionally, it is interesting to highlight that a continuous interface was observed between the ABS matrix and the cork surface, without noticeable empty spaces, pointing to a good integration of the cork particles in the polymer matrix.

3.5. Thermal Stability Analysis

3.5.1. Thermogravimetric Analysis and Derivative Thermograms (DTG)

The TG curves shown in Figure 4A, and the DTG curves in Figure 4B, are represented in terms of percentage of mass loss. Both images show the reference sample (ABS), the ABS-g-MAH matrix, the formulated composites, and the microparticulated cork. In these plots, three zones can be distinguished. The first zone is located from 30 °C to 250 °C, where ABS, the ABS-g-MAH polymeric matrix, and the CPCs remain thermally stable, while the cork begins to decompose earlier, indicating its lower initial thermal stability. In this region of the DTG curve, a small mass loss is observed as the temperature approaches 100 °C, corresponding to the removal of naturally adsorbed moisture and other volatile compounds present in the samples, which continue to evaporate up to approximately 200 °C. The second zone, known as the active pyrolysis area, extending from 250 °C to 450 °C, is where thermal degradation occurs in all of the studied samples, with the highest percentage of mass loss occurring here (~90%). The cork shows an earlier onset of degradation (T0.1 ≈ 258 °C) and a degradation peak (T_dm) at 376 °C, consistent with its lignocellulosic nature. Finally, the third zone, called the passive pyrolysis area, (after 450 °C) corresponds to the formation of carbonaceous residues in each sample. Although cork decomposes earlier than the polymeric matrix, its inclusion at 10% w/w in the composites does not introduce additional degradation stages or significantly alter the overall thermal profile of the system.
The results in Figure 4A show that ABS degrades in a single stage, corresponding to the degradation of polybutadiene and styrene [49]. Similarly, the ABS-g-MAH and CPCs exhibit a single degradation stage. Thus, the addition of microparticulated cork to ABS and MAH/DCP does not modify the degradation stages of the ABS matrix. On the other hand, it is observed that ABS exhibits slightly higher thermal stability compared with the rest of the samples studied. This slight thermal stability reduction in the CPCs is due to the grafting of MAH, which reduces the molecular weight of the ABS-g-MAH regarding ABS [21].
With regard to the DTG curves in Figure 4B, the peak represents the temperature at which the mass loss is maximized. Table 5 provides the temperature for 10% weight loss, T0.1, which indicates the initial degradation temperature, as cited in the literature. As expected [44], the addition of MAH affects thermal stability, with the T0.1 being slightly lower for the ABS-g-MAH and CPC samples regarding ABS. Table 5 also shows the temperature where the greatest weight loss physically occurs with temperature (Tdm). There are no significant differences in the Tdm value among the samples studied. The fact that no difference is observed in the Tdm suggests that the microparticulated cork does not significantly alter the thermal behavior of the polymeric matrix. This may be attributed to several factors, such as the low proportion (10% w/w) of cork relative to the polymeric matrix used, a lack of effective interaction between the two components, or the fact that cork exhibits greater thermal stability than the polymer, which could explain the absence of differences in Tdm temperatures. The only notable difference is that the ABS peak is slightly more intense than that of the composites and ABS-g-MAH, which may indicate a slight change in the degradation of ABS upon the addition of MAH [50,51].
The results obtained from the thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) confirm that the incorporation of microparticulated cork at 10% w/w does not significantly affect the thermal degradation profile of the ABS matrix. The onset of degradation and the maximum decomposition temperatures remain nearly unchanged across all CPC formulations compared to pure ABS and ABS-g-MAH. This suggests that the cork does not introduce thermally unstable phases or catalyze degradation pathways within the composite. From a processing standpoint, this is a desirable outcome, as it confirms that the thermal window required for injection molding is preserved.

3.5.2. Differential Scanning Calorimetry

S2 of Supplementary Materials shows the DSC curves of the samples studied. From the DSC curves obtained, it is possible to calculate the glass transition temperature (Tg), the melting temperature (Tm), the enthalpy of fusion (ΔHm), the thermal degradation temperature (Td), and the enthalpy associated with this thermal degradation (ΔHd), as it is explained in Supplementary Materials. These data are shown in Table 6, as well as in the DSC curve of each sample.
Analyzing the DSC curves of the studied samples in S2 of Supplementary Materials, it was observed for all samples that a transition started at ~101 °C, followed by another transition at ~115 °C. After this, the differential heat flux stabilizes. It is in this range of transitions that the softening of ABS occurs, producing the so-called glass transition. After that, a minimum in the endothermic curve was observed for all samples, at a temperature of ~196 °C, this minimum being the melting temperature, Tm, of ABS, present in all samples. Finally, a high-intensity endothermic peak was observed at ~425 °C, which is the point of complete degradation of the ABS polymer chain. The data included in Table 6 shows that the presence of MAH, as well as the addition of cork, does not significantly affect Tm. This means that the crystallinity of ABS, serving as the matrix, remains largely unaffected by the addition of cork. Similarly, Tg, and Td remain unchanged by the addition of cork meaning that the presence of cork as a filler does not disrupt the polymer chain interaction. This result is likely due to the small % w/w of cork introduced in the ABS matrix and suggests that the mechanical properties of the composites, which partly depend on the chain’s interaction, are not expected to show substantial differences. Additionally, the results obtained indicate that the processing of the composites may require similar conditions to that of pure ABS, not requiring higher temperatures, which would simplify the potential use of these CPCs in the industry. These findings are consistent with the TGA results, which show no significant shift in the glass transition temperature (Tg), further indicating that the presence of cork does not interfere with the polymer chain mobility at the tested concentration. Overall, the thermal analyses support the viability of producing bio-based composites using cork waste without compromising the thermal performance of the base polymer.

3.6. Mechanical Properties

Understanding the variations in the mechanical properties of the formulated CPCs can provide valuable insights on optimizing the design of CPCs for specific applications. Although a direct comparison with unmodified ABS was not included in this study, preliminary tests showed that composites without compatibilizer presented poor dispersion and weak interfacial adhesion, negatively affecting processability and mechanical integrity. Therefore, the ABS-g-MAH matrix was selected for all formulations to ensure compatibility with injection molding and reliable performance.
Figure 5 shows graphs with the results of the three studied mechanical properties of elongation at break %ε (A), tensile strength σr (B) and Young’s modulus E (C). S3 of Supplementary Materials includes a table with the numerical values of these mechanical properties. Figure 5D shows an image of the injection-molded specimens used for the mechanical tests. A color change from white to brown–yellow was observed when ABS functionalized with the MAH coupling agent. Then, the material turned black with the introduction of cork.
Analyzing the graphs in Figure 5A–C, it can be observed that MAH directly affects the mechanical properties of ABS (the ANOVA analysis included in S4 of Supplementary Materials corroborates that there are significant differences in the mechanical properties of the materials). As can be observed, the grafting of MAH enhances ABS strength while also increasing its flexibility and ductility. As discussed in Section 2.4, MAH grafting occurs at double bonding sites within the ABS chain. The presence of double bonds in the butadiene chain leads to entanglement that hinders polymer flowability. Thus, a reinforcement of ABS is achieved through MAH grafting and cross-linking reactions. The enhancements in tensile properties and Young’s modulus result from the incorporation of a bulky pendant group from DCP and the five-membered ring of MAH into the ABS polymer chain, limiting the segmental mobility of the chains when stress is applied [44].
Regarding the CPCs, the values of the three mechanical properties analyzed decreased compared with ABS-g-MAH without cork as filler. These significant differences were observed in the ANOVA included in S4 of Supplementary Materials. This effect has been observed in other composite materials with added micro-sized particles [52]. However, when comparing the analyzed mechanical properties of CPCs with the pristine ABS, it is observed that there is no significant difference in the values of Young’s modulus and tensile strength. Thus, it seems that the deterioration of the mechanical properties caused by adding microparticulated cork was somehow compensated by the functionalization of ABS with MAH. This behavior is attributed to the compatibility between the -OH groups of the polysaccharides in cork and MAH, which reduced the interfacial tension between the cork particles and the ABS polymeric matrix. As a result, it improved the interfacial adhesion between the cork and ABS [50]. This result is remarkable, as the detrimental effects of adding cork to polymeric matrices in mechanical properties such as tensile strength or Young’s modulus have been widely reported [51,53,54]. In this work, the strategy of using MAH as a coupling agent was successful in avoiding the deterioration of the mechanical properties of ABS.
Finally, the influence of the cork particle size on the mechanical properties of the CPCs was analyzed. When statistically comparing the values of the mechanical properties for composites with different cork particle sizes (see Means Comparison. Bonferroni Test model (p < 0.05) in S4 of Supplementary Materials), significant differences between the mechanical properties measured for the different composites were not found. Care should be paid for the interpretation of these results. The cork weight ratio was maintained as equal for the different composites. Thus, the number of cork particles reduced as their size increased, i.e., there was a greater number of cork particles in CPC-45 than in CPC-125; therefore, there are two different factors affecting the mechanical properties at the same time. In this case, the mechanical properties not being notably modified in the CPCs could be the result of worsening for larger cork particle sizes and for larger numbers of cork microparticles, both being factors compensated in the CPCs. In any case, our results show that all the sieved cork size fractions considered showed good mechanical properties in comparison to neat ABS.
The fact that a remarkable worsening of the mechanical properties of CPCs in comparison to neat ABS was not observed opens the door to future work in which larger size cork particles and/or a higher percentage by weight are used. In this way, the amount of plastic present in the composite can be further reduced, with positive implications for sustainability.

4. Conclusions

The main objective of this study was to incorporate microparticulated cork, an agricultural by-product, into a commercial ABS matrix to create bio-based composites via injection molding and reduce the reliance on virgin plastic. Cork was added at 10% w/w in different granulometries, and MAH/DCP was used as a compatibilizer to enhance interface adhesion. Although a direct comparison with composites based on unmodified ABS was not included, initial processing tests indicated poor matrix-filler compatibility in the absence of MAH. A detailed comparison is planned for future studies. Mechanical results show that while cork incorporation reduced ductility (strain at break dropped from 17.47% for ABS-g-MAH to 3.23–3.70% in CPCs), stiffness and strength remained within acceptable ranges. For instance, CPC-125 had a tensile modulus of 1515 MPa and a tensile strength of 41.05 MPa, compared with 1631 MPa and 39.45 MPa in pure ABS, and 1803 MPa and 44.14 MPa in ABS-g-MAH, respectively. The reduced modulus in the composites (up to ~20% in the worst case) was balanced by a natural finish and bio-content integration. TGA confirmed that all CPCs retained high thermal stability, with T0.1 values above 388 °C and maximum degradation temperatures (Tdm) near 421 °C. DSC results showed that Tg remained stable around 105 °C, indicating that the composites were thermally processable using standard ABS conditions. Using microparticulated cork could support the creation of composites that are potentially more eco-friendly, while preserving key mechanical properties and processing compatibility. These materials are visually appealing and technically viable for injection molding. Future work will expand the formulation range, analyze aging and durability, and further optimize performance for industrial use.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jcs9070330/s1, Table S1: FTIR. Characteristics bands of ABS present in all samples; Figure S1B: S1B. FTIR spectra of ABS, ABS-g-MAH and the CPCs. The characteristics bands are highlighted. Figure S2. S2 DSC curves of the studied samples Table S3. Mechanical properties values for CPCs. S4. Statistical analysis ANOVA of mechanical properties Table S4.1 Statistical summary for the elongation of the studied samples. Table S.4.2. Statistical summary for the Young´s modulus of the studied samples. Table S.4.3 Statistical summary for the Tensile Strength of the studied samples. Figure S5. Full-resolution versions of the tensile test figures.

Author Contributions

Conceptualization, I.R.-O.; Data curation, I.R.-O., M.H. and N.F.-D.; Funding acquisition, S.I.M.; Investigation, I.R.-O., M.H. and N.F.-D.; Methodology, I.R.-O. and M.H.; Project administration, S.I.M.; Supervision, S.I.M.; Writing—original draft, I.R.-O.; Writing—review and editing, I.R.-O., M.H. and S.I.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

This work was funded through a collaborative agreement between the Consejería de Agricultura, Ganadería, Pesca y Desarrollo Sostenible from Junta de Andalucía, and the University of Cádiz (Group TEP-946 INNANOMAT), through the ADICORK project. Co-funding from UE and ITI funds is also acknowledged. I.R.O. also acknowledges funding from access to the Spanish System of Science, Technology and Innovation linked to the Andalusian Innovation Strategy and technological Innovation Initiatives ITI. Natalia Fernández-Delgado also acknowledges co-founding by the European Social Fund and Ministry of Economic Transformation, Industry, Knowledge, and Universities of the Junta de Andalucía. Thanks to Corchos del Estrecho S.L. for the cork supplied.

Conflicts of Interest

The authors declare no conflicts of interest.

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Jcs 09 00330 g001
Figure 1. SEM images of (A) cork particles of <45 µm, (B) open cell from cork particles of <45 µm, (C) cork particles of >125 µm. (D) shows a detail of the red square region in (C).
Figure 1. SEM images of (A) cork particles of <45 µm, (B) open cell from cork particles of <45 µm, (C) cork particles of >125 µm. (D) shows a detail of the red square region in (C).
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Figure 2. FTIR spectra of ABS, MAH, ABS-g-MAH, cork, and CPC-45. The characteristic bands are highlighted.
Figure 2. FTIR spectra of ABS, MAH, ABS-g-MAH, cork, and CPC-45. The characteristic bands are highlighted.
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Figure 3. CT images of (A) CPC-125 (B) CPC-63125 (C) ABS; (D,E) are 2D slices from the 3D greyscale data of sample CP-C125. The red circles in (A,B) indicate the pores observed in the composites with cork.
Figure 3. CT images of (A) CPC-125 (B) CPC-63125 (C) ABS; (D,E) are 2D slices from the 3D greyscale data of sample CP-C125. The red circles in (A,B) indicate the pores observed in the composites with cork.
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Jcs 09 00330 g004aJcs 09 00330 g004b
Figure 4. (A). TGA diagrams of ABS, cork, ABS-g-MAH, and CPCs. (B). DTG diagrams of ABS, cork, ABS-g-MAH, and CPCs.
Figure 4. (A). TGA diagrams of ABS, cork, ABS-g-MAH, and CPCs. (B). DTG diagrams of ABS, cork, ABS-g-MAH, and CPCs.
Jcs 09 00330 g004aJcs 09 00330 g004b
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Figure 5. Percentage of elongation at break (A), tensile strength (B), and Young’s modulus (C) for ABS, ABS-g-MAH, and CPCs. (D) Injection-molded specimens of pristine ABS, ABS-g-MAH, and CPCs. (Full-resolution versions of the tensile test figures are provided in Supplementary Materials, S5 to improve clarity).
Figure 5. Percentage of elongation at break (A), tensile strength (B), and Young’s modulus (C) for ABS, ABS-g-MAH, and CPCs. (D) Injection-molded specimens of pristine ABS, ABS-g-MAH, and CPCs. (Full-resolution versions of the tensile test figures are provided in Supplementary Materials, S5 to improve clarity).
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Table 1. Percentage of moisture content in the microparticulated cork.
Table 1. Percentage of moisture content in the microparticulated cork.
Cork Particle Size (µm)% Moisture
<453.6%
≥45 ≤633.7%
≥63 ≤1255.1%
≥1253.8%
Table 2. Composition of the different formulated composites.
Table 2. Composition of the different formulated composites.
FormulationsCork Particle Size (μm)%w/w Cork Powder
ABSWithout cork Without cork
ABS-g-MAHWithout corkWithout cork
CPC-45<4510
CPC-4563≥45 ≤6310
CPC-63125≥63 ≤12510
CPC-125≥12510
Table 3. Injection molding processing conditions for ABS, ABS-g-MAH and CPCs.
Table 3. Injection molding processing conditions for ABS, ABS-g-MAH and CPCs.
Babyplast 6/12
Material Load (mm)7.5
Cooling time (s)20
Temperature profile (Plasticizer/Chamber/Nozzle)230/240/240 °C
Injection speed100%
1st Holding pressure (Bar)70
1st Holding pressure time (s)1.5
2nd Holding pressure (Bar)50
2nd Holding pressure time (s)1
Suction (mm)0
2nd injection speed100%
Table 4. Density of polymer matrix and CPCs.
Table 4. Density of polymer matrix and CPCs.
MaterialDensity (g/cm3)
ABS1.05 ± 0.06
ABS-g-MAH0.84 ± 0.04
CPC-451.09 ± 0.05
CPC-45631.16 ± 0.06
CPC-631251.19 ± 0.06
CPC-1251.11 ± 0.06
Table 5. Temperatures for 10% (T0.1) and maximum (Tdm) weight losses.
Table 5. Temperatures for 10% (T0.1) and maximum (Tdm) weight losses.
SamplesT0.1 (°C)Tdm (°C)
ABS398421
Cork258376
ABS-g-MAH392420
CPC-45392421
CPC-4563388421
CPC-63125392421
CPC-125388422
Table 6. Data analysis from DSC graphs.
Table 6. Data analysis from DSC graphs.
Sample Tg (°C)Tm (°C)ΔHm (J/g)Td (°C)ΔHd (J/g)
ABS107.72196.202.99425.2250.47
ABS-g-MAH106.05195.693.41426.1249.22
ABS45105.03197.124.75425.2749.36
ABS4563105.34196.526.06424.5147.35
ABS63125107.38196.574.33424.6051.84
ABS125104.55196.502.23426.6553.31
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Romero-Ocaña, I.; Herrera, M.; Fernández-Delgado, N.; Molina, S.I. Potential Use of Residual Powder Generated in Cork Stopper Industry as Valuable Additive to Develop Biomass-Based Composites for Injection Molding. J. Compos. Sci. 2025, 9, 330. https://doi.org/10.3390/jcs9070330

AMA Style

Romero-Ocaña I, Herrera M, Fernández-Delgado N, Molina SI. Potential Use of Residual Powder Generated in Cork Stopper Industry as Valuable Additive to Develop Biomass-Based Composites for Injection Molding. Journal of Composites Science. 2025; 9(7):330. https://doi.org/10.3390/jcs9070330

Chicago/Turabian Style

Romero-Ocaña, Ismael, Miriam Herrera, Natalia Fernández-Delgado, and Sergio I. Molina. 2025. "Potential Use of Residual Powder Generated in Cork Stopper Industry as Valuable Additive to Develop Biomass-Based Composites for Injection Molding" Journal of Composites Science 9, no. 7: 330. https://doi.org/10.3390/jcs9070330

APA Style

Romero-Ocaña, I., Herrera, M., Fernández-Delgado, N., & Molina, S. I. (2025). Potential Use of Residual Powder Generated in Cork Stopper Industry as Valuable Additive to Develop Biomass-Based Composites for Injection Molding. Journal of Composites Science, 9(7), 330. https://doi.org/10.3390/jcs9070330

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