Abstract
Luffa fibers were evaluated as a reinforcement material in poly-hydroxy-butyrate matrix composites. The treatments consisted of varying the incorporation percentage of mercerized and non-mercerized luffa fibers in a poly-hydroxybutyrate (PHB) matrix (5%, 10%, and 20% w/v). Composites made with PHB and reinforced with luffa fibers (treated and non-treated) were mechanically evaluated (tensile strength, Young’s modulus, and percentage of elongation at break), the surface morphology was described by using scanning electronic microscopy, and the degradability behavior of composites was obtained. According to the results, mechanical properties decreased when the percentage of fibers increased and no significant effects were observed when compared with mercerized fiber composites. Degradability tests demonstrated that the weight loss increased with increased fiber content in composites, independent of the applied pretreatments. Microscopy images exhibited that mercerization improved the fiber incorporation into the polymeric matrix, diminishing the “pull out” effect; the above-mentioned result was supported by using the Fourier-transform infrared spectroscopy technique, observing the reduction of lignin and hemicellulose peaks in mercerized fibers. Based on the composite mechanical performance and degradability behavior, it was concluded that this material could be used in the packaging sector as biodegradable secondary packaging material.
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Characterization of Poly-hydroxybutyrate/Luffa Fibers Composite Material
Andrea Melina Avecilla-Ramírez,a Ma. del Rocío López-Cuellar,b Berenice Vergara-Porras,c,d Adriana I. Rodríguez-Hernández,b and Edgar Vázquez-Núñez a,*
Luffa fibers were evaluated as a reinforcement material in poly-hydroxy-butyrate matrix composites. The treatments consisted of varying the incorporation percentage of mercerized and non-mercerized luffa fibers in a poly-hydroxybutyrate (PHB) matrix (5%, 10%, and 20% w/v). Composites made with PHB and reinforced with luffa fibers (treated and non-treated) were mechanically evaluated (tensile strength, Young’s modulus, and percentage of elongation at break), the surface morphology was described by using scanning electronic microscopy, and the degradability behavior of composites was obtained. According to the results, mechanical properties decreased when the percentage of fibers increased and no significant effects were observed when compared with mercerized fiber composites. Degradability tests demonstrated that the weight loss increased with increased fiber content in composites, independent of the applied pretreatments. Microscopy images exhibited that mercerization improved the fiber incorporation into the polymeric matrix, diminishing the “pull out” effect; the above-mentioned result was supported by using the Fourier-transform infrared spectroscopy technique, observing the reduction of lignin and hemicellulose peaks in mercerized fibers. Based on the composite mechanical performance and degradability behavior, it was concluded that this material could be used in the packaging sector as biodegradable secondary packaging material.
Keywords: Alkali treatment; Biodegradation; PHB biocomposites; Natural fiber; Sustainable biomaterials
Contact information: a: Departamento de Ingenierías Química, Electrónica y Biomédica, División de Ciencias e Ingenierías, Campus León, Universidad de Guanajuato, León, Guanajuato, México, Lomas del Bosque 103, Lomas del Campestre, C.P. 37150; b: Cuerpo Académico de Biotecnología Agroalimentaria, Instituto de Ciencias Agropecuarias, Universidad Autónoma del Estado de Hidalgo, Avenida Universidad Km 1.0, Tulancingo 43600, Hidalgo, México; c: Escuela de Ingeniería y Ciencias, Tecnológico de Monterrey, Campus Estado de México, Carretera Lago de Guadalupe Km 3.5, Atizapán de Zaragoza 52926, Estado de México, México; d: Biology Department, Massachusetts Institute of Technology, Cambridge, MA 02142, USA; *Corresponding author: edgar.vazquez@ugto.mx
INTRODUCTION
In recent years, interest in the utilization of new renewable materials has rampantly increased, especially interest in those coming from non-valorized vegetable biomass (Popa 2018). These new green materials offer competitive advantages compared with oil-based polymers, such as their biodegradability and low cost (Muniyasamy et al. 2019), rendering them a sustainable alternative that can reduce negative environmental impacts (Varma 2019). The gradual substitution of petroleum-based materials with bio-based materials in diverse sectors, such as packaging, biomedicine, and construction, among others, has reduced the dependency on fossil fuel-based materials and decreased plastic solid wastes in landfills (Koronis et al. 2013; Luzi et al. 2019). Around the world, many research groups have centered their interest around developing new material using nature-based raw materials.
The introduction of natural fibers to reinforce bio-based polymers has attracted international interest because it was demonstrated that these blended materials show good physical, chemical, and mechanical properties, making them ideal for multiple applications (Peças et al. 2018). Among the most important advantages are the availability of their raw materials, their high tensile strength and relatively high modulus of elasticity, and the well-developed technology for manufacturing these blended composites (Duval and Lawoko 2014; Narayanan et al. 2015; Goh et al. 2018). Currently, the lignocellulosic residues have become relevant in the development of green materials due to their versatility and availability. Diverse studies have been published recently, reporting that these biopolymers have modifiable properties, and trying to determine why. One important factor in developing this mechanical performance lies in the chemical interactions between the cellulosic fibers and the polymeric matrix, which could be not only chemical in nature but also physical. In most of the reported cases, the interactions were the combination of both, due to the high hydrophobicity of lignin (Ali et al. 2018; Yang et al. 2019). There have been developed pretreatments, which are mainly chemical, such as mercerization (alkaline solution treatment of fibers), acetylation (treatment of fibers with acetic anhydride and sulfuric acid solutions), etc., in order to remove the lignin and facilitate its incorporation into the polymeric matrix and positively impact the properties mentioned above.
Luffa is a genus of subtropical and tropical vines in the cucumber family (Mohana Krishnudu et al. 2018; Ittah and Kwon-Ndung 2019). The term “luffa” refers specifically to the fruit of the two species Luffa aegyptiaca and Luffa acutangular. Luffa is distributed in tropical countries (Fig. 1) and, due to its high fiber content, it has been used as a sponge for cleaning and grooming services. It is produced in a non-industrialized manner, in many cases, by indigenous communities. Therefore, it is important to carry out studies that demonstrate its viability in the market, as it could represent an opportunity to improve the living standards of the producing communities.
Fig. 1. Geographical distribution of Luffa spp. members
Recently, diverse authors have executed novel research related to the uses of luffa, including, among other studies, Yang et al. (2018), who evaluated the photoactivity of Fe3O4/Pr-BiOC/luffa composites, and Tanobe et al. (2014), who described the process for incorporating sponge gourd into polyester composites and its effect on mechanical properties.
Guo et al. (2019) reported the characterization of treated and untreated luffa/poly(hydroxybutyrate-co-valerate) (PHBV) composites. The author observed that the chemical pretreatment (immersion in NaOH and H2O2 solutions) of the fibers resulted in a better moisture resistance of composites.
The luffa fiber has a high content of cellulose (60%), hemicellulose (15%), lignin (10%), and other components (Mazali and Alves 2005). Its density is between 0.82 and 0.92 g cm-3 (Dairo et al. 2007), which is lower than other studied biomass, including sisal (1.26 g cm-3) (Dun et al. 2019), hemp (1.48 g cm-3) (Battegazzore et al. 2018), and cotton (1.51 g cm-3) (Abbott et al. 2010).
In the present study, the effects of carrying out a delignification pretreatment and the incorporation percentage of luffa fibers used as reinforcing material in a polyhydroxybutyrate matrix were investigated. The treated and untreated composites were examined to determine their mechanical performance and biodegradability behavior. Scanning electron microscopy (SEM) was undertaken to characterize the surface topology. To the best of our knowledge, this is the first attempt to develop a composite material based on PHB and luffa fibers, both treated and untreated. Finally, this new biobased composite could be a new class of sustainable material for the packaging industry.
EXPERIMENTAL
Chemical Reagents and Luffa Fibers
Polyhydroxybutyrate (PHB) (Mw = 430 KDa) was supplied by Goodfellow Cambridge Ltd. (#BU396311, Huntingdon, England). According to the provider, the polymer had a density of 1.25 g cm-3, 6% of elongation at break, a tensile modulus of 3.5 GPa, and a tensile strength of 40 MPa. The luffa was obtained at local markets in León, Guanajuato, Mexico. The ripe and dried fruits were superficially cleaned with distilled water. The outer layer was removed, the sponge was cut, and the seeds removed. The sponge was size reduced using physical procedures and the pieces were stored in plastic bags at room temperature until their use.
Pretreatment of Luffa Fibers
The sliced pieces of luffa were washed thoroughly with distilled water; all impurities were removed and then dried in an oven at 70 °C for 24 h.
Luffa fibers were mercerized (immersion in 5% w/v NaOH solution) at constant stirring for 1 h at room temperature. Fibers were then washed repeatedly with distilled water to remove excess NaOH and oven-dried at 70 °C for 24 h. Untreated fibers were immersed in distilled water following the same procedure. Both treated and untreated fibers were ground and sieved; the average size of fibers was between 0.4 and 0.7 mm.
Composite Fabrication
The PHB/luffa fibers composites containing 0%, 5%, 10%, and 20% w/w of treated and untreated fiber were prepared by the solvent casting method according to Pérez-Arauz et al. (2019). Neat PHB was mixed with chloroform (Sigma Aldrich, St. Louis, MO) at 10% w/v at 150 °C for 2 h. Fibers were then added and mixed for 10 min at 60 °C and 150 rpm to allow the efficient dispersion of fibers in the matrix; the mix was executed in a 50 mL-glass bottle with a polypropylene cap (Pyrex®, Shanghai, China). The dissolved solution was poured into a glass container that had been previously heated to 60 °C and allowed to dry covered at room temperature for 24 h.
Fiber and Composite Characterization
FTIR
The infrared spectrum of untreated and treated composites, as well as for all prepared composites, was recorded using Fourier transform infrared (FTIR) spectroscopy (Agilent Cary 630 FTIR, Santa Clara, CA, USA). Tests were performed at wavenumbers between 4000 cm-1 and 700 cm-1.
Surface topology analysis
The morphology of treated and untreated fibers, as well as composites, was analyzed by SEM in a JSM-6510LV microscope (JEOL, Tokyo, Japan). For better resolution, the samples were first covered with gold in a Desk II, Denton Vacuum (Denton Vacuum, Cherry Hill, NJ, USA).
Mechanical characterization
The mechanical characteristics in terms of tensile strength, percentage of elongation, and Young’s modulus were evaluated in treated and untreated luffa fibers blended with PHB. The composites were conditioned in a chamber with controlled relative humidity (50 ± 10%), and an average temperature of 25 °C over 48 h according to Kitic et al. (1986) and the corresponding ASTM standard (ASTM E104-02 2002). Composite samples (10 cm x 1 cm x 0.6 mm) were cut according to the specifications of the standard ASTM D882-02 (2002). The Young’s modulus data were obtained from the force versus deformation curves using a universal testing machine (Instron, Model 4500, Canton, MA, USA). Tensile tests were performed at a deformation rate of 12.5 mm/min. For each test, nine specimens per treatment were tested and the average value was reported.
Biodegradation
Biodegradability behavior was determined by measuring weight loss in soil burial according to ASTM G160-03 (2003). Composite samples were cut into 3 x 2 cm pieces and buried at a depth of 10 cm in boxes containing agricultural soil. Soil moisture was monitored weekly and maintained at around 30%. Samples were retrieved at 15, 30, 60, 90, and 120 days. Five specimens were measured at each time period for each sample.
In order to minimize the possibility of any contamination or matrix loss sample, retrieving and washing was done with utmost care.
Statistical Analysis
The statistical analysis was executed using the software IBM® SPSS® Statistic version 21 (IBM Corp., Redmond, WA, USA). The comparison between treatments was executed applying the Welch’s ANOVA and Brown-Forsythe (BF) tests, since data did not satisfy variance homogeneity. Weight loss data per day was analyzed by one-way ANOVA.
RESULTS AND DISCUSSION
FTIR Spectroscopy
The FTIR spectra of both treated and untreated L. cylindrica fibers are presented in Fig. 2. In the untreated fiber spectrum, typical cellulose peaks can be observed between 4000 and 600 cm-1. The O–H stretching vibrations for cellulose, hemicellulose, and lignin can be assigned at 3335 cm-1 (Chen et al. 2017), while the peak at 2900 cm-1 corresponds to C–H stretching vibrations in both cellulose and hemicellulose (Fiore et al. 2014; Chen et al. 2017). The absorbance peaks at 1415 and 1033 cm-1 are attributed to O–H in-plane bending vibrations and C–O stretching vibrations, respectively (NagarajaGanesh and Muralikannan 2016; Molina-Guerrero et al. 2018). The band at 895 cm-1 features the characteristic β-glycosidic linkage between anhydroglucose units (Oh et al. 2005; Molina-Guerrero et al. 2018).
Fig. 2. Infrared spectrum for untreated (a) and treated (b) luffa fibers
Lignin in fibers can be correlated with different bands. Bands at 1655 cm-1 were due to stretching of the conjugated carbonyl group (Chen et al. 2018). Bands at 1592 cm-1 were indicative of the C=C stretching of the aromatic ring (NagarajaGanesh and Muralikannan 2016). Those at 1454 cm-1 could be attributed to C–H deformation stretching in either lignin or xylan molecules, the main component of hemicelluloses (Chen et al. 2018; Molina-Guerrero et al. 2018). At 1372 cm-1, bands indicated C–H bending (Seki et al. 2013), and at 1157 cm-1 bands corresponded to C–O–C ring vibrational stretching (Kesraoui et al. 2016).
Absorbance peaks found at 1735 and 1238 cm-1 are associated with C=O and C–O stretching vibrations of the acetyl group in hemicellulose (Wang and Shen 2012; Chen et al. 2018). Both peaks were strongly reduced or almost absent after mercerization treatment. Infrared spectra of alkali-treated luffa in several studies have shown clear dampening of these bands (Wang and Shen 2012; Saw et al. 2013; Seki et al. 2013; Chen et al. 2017) because alkali treatment has been proven to accelerate the dissolution of acetyl groups in hemicelluloses (Chen et al. 2017). Furthermore, the xylan peak at 1454 cm-1 weakened. This value indicated that hemicelluloses in luffa fibers were partly removed due to alkali treatment.
Absorbance bands corresponding to lignin at 1655, 1454, and 1506 cm-1 also weakened to some extent, suggesting a partial removal of lignin in mercerized fibers. The spectra of PHB/luffa composites did not show differences between mercerized and raw fiber composites because the spectrum obtained was primarily that of PHB.
Fig. 3. The infrared spectrum for PHB composites reinforced with untreated (UL) (a) and treated (TL) (b) luffa fibers at different percentages of fiber content
Changes in the intensity of characteristic PHB peaks were evident when comparing fiber content; intensity decreased with increasing fiber content as a result of reduced PHB concentration in the samples. It is relevant to mention that both sides of the films were analyzed, and the resulting spectra presented slight variances on each side, which could be expected as fibers were more exposed on one side of the films.
Hydrogen bonding between the matrix and the fiber is greatly desired because it can allow the transfer of stress to the fiber. The bonding can occur on hydroxyl groups present on the fiber surface, represented by peaks at approximately between 3600 and 3000 cm-1. Changes in these characteristic peaks reveal the formation of hydrogen bonds at the fiber-matrix interface. Increased pronunciation and widening of the bands indicates the formation of new hydrogen bonds from the free hydroxyl groups; if there is a good interaction between the fiber and the matrix, this effect would increase with the increase in the amount of fiber. Furthermore, the displacement of the band at a lower frequency indicates a higher level of hydrogen bonding; while a shift to a higher frequency points to little or no bonding (Mofokeng 2012; Gunning et al. 2013). The small peak found at approximately 3400 cm-1 for all composites (Fig. 3) suggested that hydrogen bonding was infrequent or nonexistent in the composites analyzed possibly due to a poor fiber-matrix interaction, which could affect mechanical properties.
Surface Topology Analysis
Figure 4 shows the SEM micrographs of raw and mercerized luffa fibers. It can be seen in Fig. 4a that impurities and small particles covering the surface of untreated fibers—expected to be mainly lignin, hemicelluloses, and waxy substances—had been removed due to alkaline treatment (see Fig. 4b).
Fig. 4. Micrographs of untreated (a, c) and treated (b, d) luffa fibers
A size estimation based on SEM images revealed a reduction in single fiber diameter of approximately 22% for treated fibers (images not shown). Additionally, Figs. 4c and 4d show some microfibrils detached from the main body, which could have acted as mechanical anchors to the PHB matrix and therefore improve the fiber-matrix interfacial adhesion. However, contrary to the expected performance, the results of the mechanical tests did not show a positive effect. Similar effects of alkalization were previously reported by Russo et al. and Ibrahim et al. (Russo et al. 2013; Ibrahim et al. 2018).
The surface morphology of composite samples is shown in the SEM micrographs of Fig. 5. Poor fiber-matrix adhesion was observed in untreated fiber composites, along with the presence of voids and gaps, indicating low compatibility between the fiber and matrix (Beber et al. 2018). Mercerized fibers were better embedded in the matrix (Fig. 5b), and even showed residual resin on the luffa surface. This is shown in Fig. 5d, but it was not observed in the case of untreated fibers. This reflects an improved adhesion between the two materials; however, the composite surface still showed some voids, particularly closer to the fiber.
* Red arrows point to voids in the PHB matrix
Fig. 5. Surface micrographs of PHB composites with untreated (a, c) and treated (b, d) luffa fibers
Cryofractured samples were also observed by SEM. Figure 6 presents fractured surface micrographs of mercerized and non-mercerized composites with 10% fiber content. Untreated fiber composites exhibited several voids created by fiber pullouts (see Fig. 6a), as well as a clear interface gap between fiber and matrix (indicated in Fig. 6c). Literature shows that these characteristics are evidence of weak interfacial interaction between the fiber and the matrix, resulting in reduced performance of the composite material (Muthuraj et al. 2017).
After alkali treatment, composites showed signs of improved adhesion, namely, fiber pullouts were markedly less, and the gap between fiber and matrix was visibly reduced, as portrayed in Figs. 6b and 6d. However, the existence of both indicated that the interfacial adhesion was still not the strongest, which could limit the mechanical properties of the composite.