Oil Palm Empty Fruit Bunch (OPEFB) Fiber-Reinforced Acrylic Thermoplastic Composites: Effect of Salt Fog Aging on Tensile, Spectrophotometric, and Thermogravimetric Properties

Valle, Vladimir; Aguilar, Alex; Kreiker, Jeronimo; Raggiotti, Belén; Cadena, Francisco

doi:https://doi.org/10.1155/2022/6372264

International Journal of Polymer Science

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2022 | Article ID 6372264 | https://doi.org/10.1155/2022/6372264

Oil Palm Empty Fruit Bunch (OPEFB) Fiber-Reinforced Acrylic Thermoplastic Composites: Effect of Salt Fog Aging on Tensile, Spectrophotometric, and Thermogravimetric Properties

Vladimir Valle,¹Alex Aguilar,¹Jeronimo Kreiker,²Belén Raggiotti,³and Francisco Cadena¹

Academic Editor: Marta Fern ndez Garc a

Received06 Feb 2022

Accepted15 Mar 2022

Published15 Apr 2022

Abstract

The prioritization of agroindustry fiber wastes as raw materials in development of composites has become a challenge to obtain higher value-added products with targeted applications. In this study, natural fiber-reinforced polymer matrix composites were elaborated using two fiber sizes (605 μm and 633 μm) of oil palm empty fruit bunch (OPEFB) and acrylic thermoplastic resin. In doing so, resin and fibers were mixed at room temperature by maintaining filler content of 42 wt. % for all formulations. In addition, thermomechanical compression moulding was used as composite manufacturing process at four processing temperatures (80, 100, 120, and 140°C). All formulations were subsequently exposed to salt fog spray aging for 330 hours. The effects of accelerated aging process on mechanical, spectrophotometric, and thermogravimetric characteristics were studied. On the whole, results have shown feasibility to use a facile method to elaborate composites based on waterborne acrylic matrix and OPEFB fibers. After salt spray testing, it was observed detectable levels of Aspergillus spp. of fungi in all samples, as a result of phylogenetic organization of microbial activity. Tensile behavior of composites was significantly influenced by processing temperature and fiber size. In broad terms, their overall mechanical properties were improved by the increase of temperature. Additionally, infrared spectroscopy results showed important bands mainly associated to biodegradation of cellulose, hemicellulose, and lignin. On the other hand, two degradation stages were mainly identified in thermogravimetric evaluation. Noteworthy, aging had no significant effect on the thermal properties of composites.

1. Introduction

The application of natural fibers as reinforcement in polymer matrices has expanded during the last decade in several industry sectors. Looking towards a sustainable development, it is desirable the prioritization of agroindustry wastes as raw materials in development of eco-friendly, high-productivity, and cost-effective composites [1, 2]. Encouraged by circular economy approach, the use of oil palm empty fruit bunch (OPEFB) wastes constitutes an interesting challenge to obtain higher value-added products with specific applications. In this respect, several researchers have attempted to improve physical and chemical properties of composites based on OPEFB fibers in various polymer matrices [1, 3–10]. Even though many studies regarding the effects of lignocellulosic fiber characteristics on the properties of polymer matrix composites have been conducted, investigations based on different OPEFB fiber sizes and processing temperatures have not been broadly described in the literature.

Composite materials reinforced with natural fibers are usually affected by their susceptibility to environmental degradation. Both polymers and natural fibers react differently to changes in environmental factors and mechanical solicitations. Particularly, an important setback of natural fibers is relatively poor in its mechanical integrity due to vulnerability to moisture absorption. Furthermore, it is often noted that chloride-containing salt of coastal climates is an important parameter affecting properties of these composite materials. Thus, the performance of fiber reinforced polymer matrix composites—under extreme salty conditions—becomes highly important because chloride levels as well as rate of absorption and desorption of natural fibers in wet-dry cycles have significant influence on the durability of the composites [11, 12].

There are many polymeric matrices extensively used in the industry to meet high performance requirements; in particular, acrylic is an exceptionally versatile polymer due to a wide range of properties, which can be achieved by carefully selecting appropriate combinations of monomers. Copolymers of acrylate and methacrylate exhibit desired balanced between hardness and flexibility. Toughness, hydrolysis resistance, and low absorption of high-energy ultraviolet portion make acrylic polymers suitable for applications requiring good weathering resistance [13]. In recent years, numerous studies have employed acrylic matrix in composite fabrication with glass fibers [14–17], carbon fibers [18–20], metal fiber [21, 22], polyethylene fibers [23, 24], rayon fibers [25], and hybrid fibers [26]. Other researches have narrowly dealt with natural fibers like flax [12, 27], jute [28, 29], ramie [30], hemp [31], and wood [32]. Notwithstanding, there is a research gap on the use of OPEFB fibers as reinforcement in acrylic matrix composite development.

Acrylic emulsion polymers have traditionally been used as binders for general purpose in architectural and industrial coatings due to color stability and transparency [33]. The elaboration of acrylic latexes involves monomer emulsification following by water phase polymerization. Unlike in thermosetting acrylic resins, which involve oxidizing or cross-linking reactions, acrylic emulsions were devised not only to form films with solvent evaporation but also to be fast-drying [13]. Low-cost and relatively simple technologies are of paramount significance in industrial sectors that produce mature and conventional standard products. In constructive applications, for example, this requirement can be fulfilled by waterborne polymers.

From the perspective of fiber-reinforced composite manufacturing, literature has reported several fabrication techniques, namely, injection moulding, compression moulding, resin transfer moulding, extrusion, melt electrospinning, filament winding, and vacuum infusion. The latter provides a high-quality alternative to develop composites at room temperature by using liquid thermoplastic resins [2]. Nevertheless, vacuum infusion has some limitations in terms of application range. Aside from a successful vacuum bagging system requirement, this technique implicates higher consumable costs and relatively slower cycle times [34, 35]. Systematic investigations have also revealed that processing temperature shows critical effects on final properties of composite materials. In this regard, the influence of temperature on the viscosity of the polymeric matrix must be considered as one of the main processing parameters that modify the interfacial interactions between fibers and matrix [27–29].

On the other hand, aging evaluation has been frequently used in paint and coating development to mainly simulate outdoor exposure degradation. There are several highly specialized methods including elements like sunlight, moisture, temperature, oxygen, and chemical exposure. However, salt spray test plays a very major role to mimic saline habitats of coastal regions, hypersaline lakes, solar salterns, and other salinity stress environments produced by anthropogenic activities [13, 36]. Therefore, accelerated testing methods can be also useful options to simulate, as closely as possible, the aging of composites in specific environments in a much shorter time. Herein, the study is aimed at elaborating OPEFB fiber-reinforced acrylic thermoplastic composites by means of a facile method and subsequently studying the effect of salt fog atmosphere on their mechanical, spectrophotometric, and thermogravimetric characteristics. The influence of both fiber size and processing temperature over composite behavior under accelerating test was particularly investigated.

2. Materials and Methods

2.1. Materials

Liquid acrylic thermoplastic resin SINTACRIL A-292® was purchased from Poliacrilart (Quito, Ecuador). The resin was waterborne formulated by copolymers of acrylate and methacrylate with 42 wt. % solid content. Its Brookfield viscosity (SP1, 12 rpm) and density (25°C) were 0.07 Pa·s and g/cm³, respectively. OPEFB wastes used in this study and consisting of about 46% of cellulose, 24% of lignin, and 3% of extractive [37] were supplied by a local company specializing in oil palm production.

2.2. Elaboration of OPEFB/Acrylic Thermoplastic Composites

2.2.1. Preparation of Composites

OPEFB wastes were initially dried under sunlight for 24 h and then milled through a blade milling machine SHINI, model SG-2348E (Ningbo, China). The resulting OPEFB fibers were sieved to obtain two fiber sizes. In order to eliminate the influence of outliers on the tails of fiber size distribution, the actual average sizes were obtained from a robust statistical method of central tendency by means of trimmed-mean estimator. To do so, two hundred fibers per size were measured using a stereomicroscope MEIJI TECHNO, model EMZ-13TR (Saitama, Japan). The acquisition and processing of the images were performed with the TCapture software. The trimmed-mean values of fibers sizes were 605 μm and 633 μm.

OPEFB/acrylic thermoplastic composites were separately formulated with the two fiber sizes obtained in OPEFB conditioning. An appropriate proportion of resin and fibers were mixed at room temperature, using mechanical stirrer at 500 rpm for 30 min. After, specimens were processed using a compression moulding machine LAB TECH, model LP-S-50 (Mueang Samut Prakan, Thailand), at four temperatures: 80, 100, 120, and 140°C, under a pressure of 150 bar for 40 min. The processing temperatures were selected based on the thermal stability of raw materials. It should be also indicated that composite was performed with the highest content of OPEFB fiber obtaining through the elaboration method aforementioned. As such, all samples were prepared by maintaining 42 wt. % and 58 wt. % of filler content and matrix, respectively. Figure 1 shows the schematic representation of the experimental procedure.

2.2.2. Characterization

Mechanical evaluation, regarding tensile modulus, tensile strength, elongation at break, and toughness, was accomplished by the use of a universal testing machine INSTRON, model 3365 (Norwood, USA), in accordance with standard ASTM D 638, with load cell of 500 N and crosshead speed of 20 mm/min. The mean values of tensile results were determined from ten test specimens for each composite formulation. Moreover, Fourier transform infrared spectroscopy (FTIR) in attenuated total reflection mode was performed by using a spectrometer JASCO, model FT/IR-C800 (Tokyo, Japan), from 4000 to 600 cm^-1 with a resolution of 4 cm⁻¹ and 20 scans. In addition, thermogravimetric analysis (TGA) was carried out using a thermobalance SHIMADZU, model TGA-50 (Kyoto, Japan), from 20°C to 600°C with heating rate and nitrogen flow of 10°C/min and 50 mL/min, respectively. The morphology of composites was complementarily evaluated by scanning electron microscopy (SEM) with electron microscope ASPEX, model PSEM eXpress (Billerica, USA), under 20 kV of accelerating voltage.

2.3. Salt Spray Testing

Aiming towards simulating critical salty environmental circumstances, OPEFB/acrylic thermoplastic composite samples measuring were exposed to a controlled salt spray atmosphere. The test was performed in a chamber Q-FOG, model CCT600 (Cleveland, USA), based on the protocol of ASTM B117. Samples were exposed to a direct and continued salt fog of 6 wt. % sodium chloride aqueous solution at °C. After 330 hours of exposition, when a notable deterioration of the material surface was observed, specimens were removed from the testing chamber. To further understand the influence of salt fog spray aging over composite performance, exposed samples were analyzed in terms of their mechanical, infrared, thermal, and morphological characteristics according to the set of conditions described in the previous section.

After aging evaluation, a qualitative microbiological analysis was performed. In doing so, fungi were isolated from different areas of the composite samples. Two-gram sample was dissolved in 100 mL of sterilized distilled water. After three serial dilutions, 100 μL of this diluted sample was then transferred to petri plates containing potato dextrose agar (PDA). Afterward, petri plates were then placed at 25°C for 3–10 days. Fungal colonies were transferred into PDA slants and incubated for 7–30 days, after which their genera were determined on macromorphology criteria.

3. Results and Discussion

Overall properties of composites depend on compatibility between fiber and matrix [38, 39]. Adhesion and nature of bonding of composite elements are determined by various aspects, chiefly of diffusivity of element materials and morphological properties of natural fibers [40]. From the imbibing process, OPEFB fibers were completely covered by the acrylic thermoplastic matrix, as shown in Figures 2(a) and 2(b). Taking into account porous surface of OPEFB fibers, it can be seen that the polymer matrix was also covering superficial parts of micropores. As a result of processing molding, there were obtained composites with relatively homogeneous surface. Figures 2(c) and 2(d) present images of composites before salt spray testing.

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From Figure 3(a), it could be seen that after 330 hours of exposition to controlled salt fog atmosphere, the surfaces of the composites showed fungal presence. In the light of the results presented in Figure 3(b), morphological identification evidenced detectable levels of Aspergillus spp. On review of different researches related to environmental conditions for fungi growth, some genera are easy adaptable to humid climate and can survive at hypersaline environment of sodium chloride. It is however worthy of note that edaphoclimatic conditions of OPEFB wastes could generate several microbial diversities, including fungi and bacteria. Numerous phylogenetically unrelated fungi have been reported to grow in salty water with high concentration of sodium chloride [41–43]. From the perspective of lignocellulose biodegradation, fungi are the predominant decomposers of cellulose and hemicellulose by means of hydrolysis reactions to form fermentable sugars whilst lignin is a macromolecule with nonfermentable phenylpropane units linked C–C and C–O–C bonds requiring high energy to break down. Nevertheless, a number of fungi usually produce lignin peroxidases, manganese peroxidases, laccase-like multicopper oxidases, and so forth [41].

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Figure 4 depicts SEM micrographs of worn surfaces after salt fog atmosphere. Expectedly, during the test water and salt were absorbed by the matrix and OPEFB fibers. The absorbed moisture caused increase in microbial activity which in turn resulted in random orientation microstructural changes. In particular, biodegradation process produced significative defects like voids and microcracks in the composites; these tend to decrease with processing temperature.

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3.1. Mechanical Characterization

Polymeric composites undergo a variety of stresses when being in service; therefore, inspecting the response to tensile solicitations is one of the most relevant evaluation criteria for application purposes [44]. Mechanical properties of fiber-reinforced polymer composites depend on several factors. The fiber type is, at least in principle, a critical aspect when considering tensile mechanical performance [38, 39]. In this context, the studies regarding the use of acrylic thermoplastic resins with different synthetic fibers, in particular with glass fiber and carbon fiber [45], have revealed notable improvements, both in tensile strength and modulus, compared to those obtained when the reinforcement is natural, such as flax [27, 29, 46]. The reported literature on tensile stress-strain behavior of natural fiber-reinforced acrylic thermoplastic composites has identified three distinct regions [27, 46]. The first was associated to the initial linear elastic behavior, the second to the evolution of damage with loss of stiffness (slight nonlinearity), whereas the third to the intensification of damage leading up to ultimate failure. In the study reported herein, overall results of tensile stress-strain showed ductile-like behavior. In all cases, noticeable plastic deformation was observed; however, the strain hardening zone was clearly higher than the necking region. Tensile stress-strain behavior of raw materials and composite is depicted in Figure 5.

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Fiber loading is another relevant factor that influences tensile mechanical properties of natural fiber-reinforced polymer composites [38, 40]. A variety of polymers, modified by OPEFB fibers, have revealed that fiber content considerably affects properties of composites [47, 48]. Due to the fact that the reinforcement generally plays a dominant role over the matrix, the composite strength is proportional to the fiber strength because natural fiber is often stronger than polymer matrix [38]. The plot of Figure 5(c) shows three distinct regions in the stress-strain curve, which are in concordance with similar literature reports about natural fiber-reinforced acrylic thermoplastic composites [27, 46]. Nonetheless, it should be noted that, in those studies, the matrix used (Elium®) had different mechanical characteristics than the polymer used for our composite formulations, despite both being acrylic thermoplastic polymers. Comparing Figures 5(a)–5(c), it could be seen that the initial part of the stress-strain curve shows improvement of stiffness and strength that is probably associated to unlinear performance of OPEFB fiber reinforcement, as it has been postulated in the literature [38, 40, 49–52]. This behavior predominated in the composite due to the high load of added fibers. The second part of the stress-strain curve, Figure 5(c), P2, acquainted as a transition zone, could be associated to the beginning and propagation of failure mechanism owing to the deviation and fall of the stress-strain curve. Lastly, the third part of curve might be assigned as an intensification region of the failure mechanism during the deformation of the polymer matrix. Section marked by a decrease in composite stiffness leading up to ultimate failure [27, 38, 46].

On the other hand, Figure 6 illustrates the influence of fiber size and processing temperature over tensile behavior. It was initially observed uneven statistic dispersion among formulations; nonetheless, mechanical response of natural fiber composites is largely dependent on fiber properties, which in turn are influenced by a number of factors like size, geometry, water absorption, tendency to form aggregates, chemical composition, organic extractives, inorganic (ash) components, and different types of waxes [40, 53]. In particular, macrostructure of OPEFB fibers involves nonuniform length-to-diameter ratio, irregular shape, rugose surface, and so forth. Additionally, main morphology features include different microfibril angles, a central empty section named as lacuna as well as randomly and loosely organized network of microfibrils [47, 54]. Aforementioned characteristics join with impurity presence could be considered as natural defects leading to a distribution widely spread out from the mean. It is thus of practical significance to understand that it is neither possible to avoid relatively high dispersion in the response of OPEFB/acrylic thermoplastic composites under tensile loading.

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Tensile mechanical properties of composites also depend on adhesion in terms of better compatibility between fiber and matrix [28, 38]. Even though OPEFB fibers were initially imbibed by the matrix, as shown in Figures 2(a) and 2(b), acrylic microstructure typically shows porous that allows both liquids and gases diffuse through the polymer [55]. Thus, during aging test, water diffusion and further sorption by OPEFB fibers induced both differential swelling of natural fiber [12] and microorganism growth, which can be observed in the images of Figure 3(a). Aspergillus spp. have been reported as one of the microorganisms responsible for biodeterioration of acrylic-based polymers such as in glass fiber composites. The changes observed in the acrylic matrix due to microbiological attack were disfigurement, erosion, and loss of mechanical properties; the latter is a consequence of microcracking on the polymeric surface [56]. These structural defects were found on our composite surface after salt fog aging, as illustrated in Figure 4. In addition, stresses generated by fiber swelling at the interface level contributed to the damage and microcracking of the matrix, which intensified water absorption. Because of this fact, further degradation was probably produced by the access of free oxygen, enzymes, and free radicals which lead to weaken fiber–matrix interfacial adhesion [12]. Given these points and based on Figure 6, tensile mechanical properties were somewhat lower after salt fog spray aging except for tensile modulus, whose increment after salt fog aging demonstrates stiffness boost of the acrylic matrix.

Tensile modulus and tensile strength results are depicted in Figures 6(a) and 6(b). No specific tendencies of these properties with temperature were identified. These behaviors are possibly caused not only by random orientation and dispersion of fibers but also by some changes in mechanical response of the matrix [14, 27, 29]. Nonetheless, there was a notable decrease in both tensile parameters at 140°C that was linked to the influence of temperature on the viscosity of the polymeric matrix [29]. Indeed, results showed strengthening of modulus and slight enhancing of stress resistance along composite with increasing fiber size. In other words, longer fibers provided greater stiffness to the composite and were able to withstand stresses a little more easily. Additionally, due to the fact that composites were made with the same fiber load, there were observed slight changes in tensile strength between formulations of different fiber sizes processed at the same temperature. Tensile mechanical properties of natural fiber-reinforced polymer composites especially the tensile strength have strong relationship with fiber volume fraction, because fibers serve as the principal load-bearing members in a composite [38–40, 49].

As can be seen in Figure 6(c), elongation at break was clearly influenced by fiber size. Considering obtained results, the increasing of fiber size weakened ultimate elongation of composites, regardless of processing temperature. Additionally, it is important to consider that the larger the fiber size, the worse the mechanical performance of natural fiber-reinforced composites [38]. This drawback was probably intensified by deficiencies in interfacial adhesion between larger OPEFB fibers and acrylic thermoplastic matrix [28, 40]. Aside from fiber size, changes on tensile characteristics were noticed as a result of processing temperature. It should be stated that the increase of processing temperature produced higher ultimate failure. Data suggest that water and organic solvents were gradually evaporated from the composite. Indeed, the temperature increase leads polymer to cover porous, lacunas, and microfibrils [27, 29]; therefore, a better compaction of the final composite at 140°C was distinguished. According to the literature, this behavior can be linked to better fiber–matrix interactions due to the fact that manufacturing parameters are main aspects to be considered in tensile performance [44, 53, 57]. Nevertheless, the only exception to the above arguments was the composite with the smallest fiber size processed at 80°C. This formulation exhibited a fracture strain similar to the highest elongation at break of all formulations, elongation showed by the composite with the same fiber size but processed at 140°C. Hence, it is possible to infer that the effect of smaller fiber size, prior to the contribution of the increase in the processing temperature to strengthen the fiber–matrix interactions [27, 29], equated the best performance of the matrix without using a higher energy expenditure in the process.

Unlike fibers with a uniform cross section, the irregular shape and nonuniform length-to-diameter ratio of OPEFB fibers did not allow efficient stress transmission from acrylic matrix to the filler [40, 54]. This effect is higher increasing fiber size from 605 μm to 633 μm, as shown in Figure 6(d). Fibers are usually the principal load-carrying element in a composite; however, matrix also serves not only to keep fillers randomly or specific orientated but also to be a load transfer medium between them [44]. On the view of matrix response, methyl-methacrylate and acrylate proportion allowed the matrix somewhat desired balance between flexibility and toughness.

3.2. Infrared Evaluation

Once infrared spectroscopy is based on vibration modes of functional groups of molecules, the analysis of obtained results was focused mainly on spectral differences of aliphatic hydrocarbons, oxygen-containing compounds, aromatic compounds, heterocyclic compounds, and nitrogen-containing compounds molecules [58]. Figure 7 illustrates the FTIR results of raw materials, i.e., acrylic resin and OPEFB fiber. The acrylic resin spectrum resulted from the frequencies of several groups. As such, it was observed the presence of a broad band at 3360 cm^-1 attributed to the absorption vibration of O–H group, due to waterborne nature of resin. The band of C–H stretching, associated not only to backbone chain but also to methyl substituent, was identified at 2956 cm^-1. The band at 1729 cm^-1 corresponded to C=O stretching of the ester group, whereas the band at 1451 cm^-1 was linked to the bending of O–CH₃. In addition, bands at 1386 cm^-1 and 1238 cm^-1 were ascribed to C–CH₃ bending and C–C–O stretching, respectively. Besides, the band at 1164 cm^-1 corresponded to C–O–C bending [58].

As regards OPEFB, the spectrum evidenced a broad band at 3328 cm^-1 corresponding to vibration of O–H group which was ascribed to water, alcohols, and phenol components [59–63]. The bands at 2921 cm^-1 and 2854 cm^-1 were associated to methylene asymmetric C–H stretching and methylene symmetric C–H stretching, respectively [63, 64]. The band at 2854 cm^-1 was related to CH and CH₂ groups from cellulose and hemicellulose. Ester group of hemicellulose and waxes was confirmed through the band at 1735 cm^-1. Additionally, the presence of C–O–C stretching vibration of pyranose ring in polysaccharides was identified at 1158 cm^-1. Moreover, glycosidic bond was corroborated by transmittance band at 898 cm^-1 [62]. In general, cellulose was confirmed by bands at 898, 1029, and 1158 cm^-1, whereas hemicellulose was identified by the presence of bands at 1029, 1234, 1319, 1371, 1421, 1735, 2921, and 3328 cm^-1. Furthermore, aromatic ring corresponding to lignin was observed at 1455 and 1511 cm^-1 [60, 65].

Infrared spectra of OPEFB/acrylic thermoplastic composites are presented in Figure 8. Before salt fog test, the results showed minimal differences in infrared absorption of composites elaborated with 633 μm and 605 μm. The fiber size used in the current study did not produce significant changes in the modes of vibration of the composite molecules. On the other hand, it should be note that the increase of processing temperature not only led to a thermodynamically fast evaporation of water and solvents but also helped to coalescence of dispersed phase. However, this effect was not clearly observed in the composites before aging due to overtone bands.

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According to the results presented in Figures 3 and 4, the salt fog atmosphere produced some deterioration patterns probably dominated by different biodegradation mechanisms. However, spectra could not evidence all of them, due to some drawbacks like overtone bands, Fermi resonance, and coupling. Comparing FTIR curves before and after aging, the increase of the band in the range of 3700–3000 cm^-1 could be produced not only by the presence of O–H stretching of carboxylic acids but also to O–H from a range of phenolic compounds resulting of fungal growth [66]. However, the absorption of this group was dominated by processing temperature; specifically, the highest temperature, produced less O–H infrared absorption, due to better coalescence of the matrix. The set of the bands between 3000 cm^-1 and 2600 cm^-1 has been decreased due to noticeable wear of acrylic matrix as well as to the degradation of cellulose and hemicellulose. In addition, it was clearly distinguishable new bands in the range of 2500-2000 cm^-1, which were assigned to fungi presence. The new band at 1640 cm^-1 was ascribed to protein C=O stretching of amide I, whereas the band at 1540 cm^-1 was associated to C–N stretch and protein in N–H bend; according to the literature, the former bands are assigned to fungi mycotoxins [67].

3.3. Thermal Stability

Thermogravimetric behavior of acrylic resin and OPEFB fibers is shown in Table 1. The onset degradation for acrylic resin, defined by 50% weight loss, was linked to water evaporation followed by solvents and additives losing within the range of 40°C to 150°C. It was also observed the degradation of methylmethacrylate polymer chains due to unzipping reactions. This step involved nearly a 48% weight loss, and it was produced over 150°C. As regards OPEFB fibers, losing of free water and volatiles were produced in the range of 20°C to 150°C after which, it was evidenced the highest degradation process. This stage was observed from 150°C to 460°C and corresponded to the degradation of pectin, hemicellulose, cellulose, and lignin as well as to organic extractives decompose [68].

Thermal degradation mechanisms are narrowly related with the microstructure of lignocellulosic fibers. In a broad sense, cellulose comprises sequences of both crystalline and amorphous regions. Cellulose structure is composed with β (1-4)-linked D-glucose units that are intimately connected and periodically ordered in three-dimensional disposition. On the contrary, hemicellulose contains non-crystalline regions which are easily hydrolysable. Although cellulose and hemicellulose show significative differences, important interactions are produced between them. It has been proposed that shear forces between cellulose fibril surface and hemicellulose are generated through the interactions of hemicellulose side groups with the free hydroxyl groups of the cellulose [69, 70]. Initial thermal decomposition takes place in amorphous structure of cellulose, hemicellulose, and lignin, followed by degradation of cellulose crystalline segments [71]. Intermolecular hydrogen bonds of cellulose generate tertiary fibrillar structures of high crystallinity requiring more heat for thermal degradation [72]. In this regard, thermal results of OPEFB were found to be in good agreement with a previous study [37].

As can be seen in Table 2, thermogravimetric results of composites showed different degradation patterns to the pristine materials. Considering morphologic aspects of imbibed fibers presented in Figures 2(a) and 2(b) and overall thermal results, it is noticed that the incorporation of OPEFB into acrylic matrix increased thermal stability of the system. Specifically, results revealed two weight loss zones between 30°C and 600°C. The first step, with approximately 3% weight loss, was ascribed to the evaporation of solvents and residual water [73]. The second stage exhibited the highest weight loss (85%) due to a number of different degradation mechanisms associated, as aforementioned, to decomposition stages of cellulose, hemicellulose, and lignin. In addition, processes with greater activation energy like main-chain and side-group scissions as well as depolymerization were produced in the second step [74]; typically, carbon-hydrogen, carbon-hydrogen, carbon-oxygen, carbon-carbon, and hydrogen-oxygen bonds are broken with energy requirements ranging from 340 to 460 kJ/mol [73].

Regarding fiber size and processing temperature, there were relatively minor differences in composites elaborated with fibers sized 605 and 633 μm. This behavior was similar in the case of processing temperatures. Literature surveys focused on TGA of natural fiber-reinforced thermoplastic polymer composites showed somewhat similar results to the present study [75, 76]; nevertheless, those studies are not conducted with OPEFB fibers and acrylic thermoplastic polymer as matrix.

Based on the results after aging, it is also evident minor quantitative differences in thermal degradation of OPEFB/acrylic thermoplastic composites. Even though SEM micrographs of Figure 4 evidenced some biodegradation signals, they seem not to have important effects on thermogravimetric behavior of the composites. At this stage of the investigation, physicochemical characteristics of OPEFB fibers, fungal, voids, and microcracks produced weight loss differences among formulations, which are considered to be within the variation expected experimentally. On the other hand, the degradation of raw materials can produce undesirable effects on the thermal stability; however, the manufacture of studied composites requires the mixing of fibers and matrix at temperatures under 150°C. Notice that extensive degradation of composite took place before and after aging process, over 200°C. Therefore, these differences are not considered, from a practical significance, as important decomposition patterns.

4. Conclusions

The salt fog spray aging of acrylic thermoplastic composites reinforced with OPEFB fiber was investigated at different processing temperatures and fiber sizes to better understand potentiality of both waterborne acrylic matrix and OPEFB wastes. For all composites, it was observed detectable levels of Aspergillus spp. of fungi after salt spray testing as a result of biodegradation of lignin, cellulose, and hemicellulose. SEM analysis showed random orientation microstructural defects like voids and microcracks in the composites. Mechanical behavior of composites turned out to be dominated by the processing temperature and fiber size. Although some nonhomogeneous variations in tensile properties at different fiber sizes and processing temperatures were observed, overall results evidenced a positive balance of tensile performance with the processing temperature increase. On the other hand, FTIR results showed important bands associated to carboxylic acids, phenolic compounds, carbon dioxide, and amides that confirmed fungal growth. TGA results showed two degradation stages almost constant for all specimens, and no significant influence of processing temperature and fiber size was observed. Furthermore, aging with salt spray atmosphere did not affect thermal stability of composites at this research stage. The findings and observations of this research lead to further understanding of OPEFB fiber-reinforced acrylic matrix composites. Such knowledge is essential for highlighting applications where obtained composite would most effectively be employed and may provide guidance on necessary performance enhancing modification for future industrial scale up.

Data Availability

There is no extra data supporting the results.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors gratefully acknowledge the financial support provided by the Escuela Politécnica Nacional, for the development of the Project PIGR-19-10: “Aprovechamiento de desechos industriales de aceite de palma africana en el desarrollo de polímeros compostables, composites y sistemas de biofiltración.”

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