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Physical-mechanical properties of bamboo fibers-reinforced biocomposites: Influence of surface treatment of fibers
Journal of Building Engineering 28 (2020) 101058
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Physical-mechanical properties of bamboo fibers-reinforced biocomposites: Influence of surface treatment of fibers ~ o, Jhon Ca �rdenas Martha L. S� anchez *, William Patin Universidad Militar Nueva Granada Carrera, Facultad de Ingeniería, Programa de Ingeniería Civil Carrera 11, N. 101 80, Bogot� a, D.C, Colombia
A R T I C L E I N F O
A B S T R A C T
Keywords: Biocomposites Bamboo fibers Orientation Surface modifications Physical properties Mechanical properties
The composition and the morphology of most plant fibers allow their use as reinforcement of biocomposites. In this study, the effect of the modification of the surface of the fibers on the physical and mechanical properties of biocomposites reinforced with bamboo fibers is analyzed. For the experimental procedure, three treatments were carried out: mercerization, ozone, and plasma. The influence of the treatments on the modification of the surface of the fibers was evaluated using X-ray diffraction (XRD), infrared spectroscopy (FTIR), atomic force microscopy (AFM), and scanning electron microscopy (SEM-EDS). The analysis of the properties of the fibers was focused on the determination of the tensile strength, modulus of elasticity, density, moisture content, and absorption ca pacity. The evaluation of physical properties of biocomposites consisted in the determination of the density, effective absorption, and dimensional stability. The determination of the mechanical properties focused in the evaluation of their behavior under compression and static bending. The results obtained were compared with the results for biocomposites made with untreated fibers, and they demonstrate that by using treated fiber it is possible to increase both the dimensional stability of the panels and their mechanical strength by more than 50%.
1. Introduction The implementation of methods of design, manufacture, and char acterization of biocomposites reinforced with vegetable fibers has become a topic of growing interest in the development of composite materials [1–26]. The concern for contributing to the reduction of the environmental impact caused by the production and use of synthetic fibers has spurred the promotion of the use of sustainable materials made from fibers of vegetable origin [7,25]. These materials are distinguished by their excellent mechanical properties, such as high tensile strength, good stiffness, high degree of toughness, low damage capacity, good thermal insulation, and low cost [26]. However, some factors, such as the hydrophilic nature of natural fibers and their high polysaccharide content, can affect their performance as reinforcement for hydrophobic polymeric resins [27]. This incompatibility between the fibers and the matrix can cause a weak interfacial adhesion, affecting the stress transfer of the matrix to the fibers, which harms the mechanical performance of the composite [27–30]. Recent studies have shown that for the efficient use of vegetable fi bers as reinforcement for biocomposites, preliminary treatments should be used to modify the surface of the fibers. These treatments allow the
elimination of impurities that remain adhered to the wall of the fibers after the mechanical extraction process is finished. Additionally, with these surface modification methods, it is possible to remove the lignin, hemicellulose, wax, and other extractable substances that affect the adhesion of the fibers to the matrix of composites [26–30]. According to the mechanism of action, surface modification treatments can be clas sified into three categories: chemical, physical, and biological modifi cation [31–54]. One of the chemical methods most used for the modification of vegetable fibers is mercerization. This method involves the modification of the structure and chemical composition of plant fibers using an aqueous solution of sodium hydroxide (NaOH). The concentration, temperature, and time of action of the solution will depend on the characteristics of the fibers and their application [31–39]. With this treatment, the removal of surface impurities, as well as the occurrence of fibrillation, is possible, obtaining a fibrous material with greater surface area and smaller diameter, which produces an increase in the tensile strength of the fibers and in the mechanical properties of the resulting composite [40–42]. Currently, the methods of physical modification have become important in the treatment of vegetal fibers. These methods cause
* Corresponding author. E-mail address: [email protected] (M.L. S� anchez). https://doi.org/10.1016/j.jobe.2019.101058 Received 10 May 2019; Received in revised form 4 November 2019; Accepted 6 November 2019 Available online 7 November 2019 2352-7102/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Building Engineering 28 (2020) 101058
fibrillation of the material by electrical discharge [43–51]. One of the best-known physical processes is the application of a cold plasma of methane, argon, or helium. This method allows the formation of free radicals and the polymerization of the material. Plasma treatment acts on the chemical structure of the fibers and their crystallinity index. Recently, ozone treatment has emerged as an eco-friendly method of surface modification [52,53]. According to Ai et al. (2019), ozone can oxidize the lignin and the hemicelluloses, producing soluble compounds of low molecular weight [52]. The purpose of this research is to evaluate the effect of surface treatments on the properties of composites made with bamboo fibers and vegetal resin. For this objective, three different treatments were applied: mercerization, ozone, and cold plasma. The biocomposite was fabricated via the compression method at room temperature. The results are compared with results obtained for biocomposites reinforced with untreated fibers and with wood agglomerates produced under the same laboratory conditions.
deposition (DC PACVD) system, powered by a bipolar and asymmetric pulsed DC current source, was used in the treatment of the plant fibers. The shape of the pulses emitted by the current source consisted of a negative pulse variable in amplitude (adjustable between 0 and -1000 V), followed by a constant positive pulse of 40 V. This pulse configuration contributes to the neutralization of the plasma, obtaining a plasma enriched in positive ions. The fibers were submitted to the action of a combination of 50% methane and 50% argon cold plasma for 10 min, using a working pressure of about 27 Pa and a DC potential of 700 V, with a gas flow of 10 sccm (standard cubic centimeters per minute) and a temperature between 18 � C and 26 � C. Ozone treatment: The procedure was carried out using a cylindrical tube of 1100 mm in height, batch type. The ozone was distributed by means of a porous plate of average pore size equal to 44 μm located in the back of the reactor. Ozone was added to the system using a Clear water Tech LLC 300P 120 V/60HZ Microzone generator with an ozone production of 230 mg O3/h. The fibers were treated for 120 min. After treatment, the fibers were dried to constant mass.
2. Materials and methods
2.3. Evaluation of the physical and mechanical properties of the fibers
2.1. Materials
The determination of the moisture content and the water absorption of the fibers was carried out according to the specifications of the standards ASTM D 4442–16 and ASTM D5229-14. The density of the fibers was determined by means of the pycnometer method at a tem perature of 18 � C, according to the specifications of ASTM D8171. To determine the tensile strength and the modulus of elasticity, fiber bundles of 50 mm in length were used. The test was performed on a Landmark MTS 370.10, at a loading speed of 1 mm/min. In order to guarantee the adequate fastening of the fibers to the testing machine, they were previously glued to a rectangular mold of 25 mm in width and 70 mm in length, according to procedures described by Okubo et al., 2004 [54]. The experimental assembly is shown in Fig. 1.
For the manufacture of the biocomposite, fibers extracted from the upper region of Macana bamboo were used. This type of bamboo is characterized by short internodes with regular nodal distances, thick walls, and uniform diameters. Since the material to be used is previously stored in the laboratory until it is crushed, all the material is immunized before being acquired by immersion in commercial borax and salts of boric acid at a concentration of 3%. A bi-component vegetable-based polyurethane (extracted from castor oil), AGT-1315, formulated by the cold mixing of a pre-polymer and a polyol component, was used as matrix of composite. The properties of the resin are shown in Table 1. 2.2. Separation and treatment of fibers
2.4. Evaluation of the effects of treatment on the properties of the fibers
For the extraction of the fibers, a conventional mechanical crusher was used. The extraction process was carried out with the material in the green state. The crushed material was classified according to size, using mechanical sieving. Fibers with a diameter between 0.6 and 0.9 mm were obtained. In order to guarantee an adequate adherence between fibers and composite matrix a surface modification of fibers was realized. The modification consists of the partial removal of lignin, hemicellulose, pectins and waxes present in plant fibers. Surface treatment of the crushed material was carried out in order to remove waxes, oils, lignin, and other substances that can affect the adhesion of the fibers to the matrix of the composite. For the surface treatment, three procedures were employed: mercerization, ozone, and cold plasma. All the treatments were carried out on untreated fibers. Details of procedures are described below. Mercerization treatment: This treatment consisted of the immersion of the crushed material in a 10% sodium hydroxide solution for a period of 48 h at room temperature. The concentration of sodium hydroxide solution was selected based on previous results and the technical liter ature consulted. This process was followed by washing with distilled water and a drying process until reaching a constant mass value. Cold plasma treatment: A pulsed plasma assisted chemical vapor
Infrared spectroscopy-attenuated total reflectance (IT-ATR) and Xray diffraction (XRD) techniques were applied in order to evaluate the influence of the treatments on the chemical properties of fibers. Microscopic studies such as scanning electron microscopy (SEM), transmission microscopy, and atomic force microscopy (AFM) were used to analyze the effect of the surface treatments on the morphology and roughness of the fibers. Details of the procedures are described below. Infrared spectroscopy-attenuated total reflectance (IT-ATR): For the determination of the characteristic absorption bands for the most com mon functional groups in untreated and treated fibers, the infrared spectroscopy-attenuated total reflectance (IT-ATR) technique was employed. The use of this technique allows evaluating the partial removal of lignin in the fibers by detecting the typical functional groups of this polysaccharide and its typical absorption bands. For the analysis,
Table 1 Properties of matrix. Properties
Component “A00
Component “B00
Boiling point Melting point Flash point Density (at 25 � C)
190 C 14 � C 200� C 1.22 g=cm3
313 � C 10 � C 229� C 1.98 g=cm3
�
Fig. 1. Tensile test: (a) Fastening system, (b) Assembly. 2
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Journal of Building Engineering 28 (2020) 101058
spectra were taken with a Shimadzu IR prestige-21 spectrophotometer with ATR module, performing 40 scans with a resolution of 4 cm 1. X-ray diffraction (XRD): A PANalytical XPert PRO MRD X-ray diffraction system was used. This equipment has a copper anode tube, a standard resolution goniometer containing the geometry (0θ–2θ), a minimum step size of 0.002� , and an X-ray proportional counter. For the XRD analysis, difractograms of the untreated and treated fibers were obtained and the crystallinity index (IC) was determined. Atomic force microscopy (AFM): The roughness of the fibers was studied via atomic force microscopy, using a Nanosurf Atomic Force Microscope, on an area of 2500 μm2. The average roughness was calculated using the average height of the irregularities observed in the direction perpendicular to the surface of thirty samples for each type of treatment. Scanning electron microscopy (SEM): Micrographs were obtained using a HITACHI S-570 device (at an acceleration voltage of 40 kV). Images taken with secondary electrons allowed determining the chem ical composition, differentiated by grayscale. Images taken with back scattered electrons allowed evaluating the morphology of the fibers.
movable crosshead of the testing machine of 0.5 mm/min. Static bending: The three-point static bending tests determined the flexural properties, such as the modulus of rupture, the apparent modulus of elasticity, the stress at the proportional limit, and the work at maximum load. The long axis of the panel was used. Thirty specimens of 70 mm in width, 7 mm in thickness, and 170 mm in length were tested for each of the treatments evaluated. The load was applied continuously throughout the test at a uniform rate of motion of the movable crosshead of the testing machine to achieve an outer fiber strain rate of 3 mm/min. 3. Results and discussion This section presents the effect of the surface treatment on the physical and mechanical properties of biocomposites reinforced with Macana bamboo fibers. 3.1. Effect of surface treatment on the physical properties of the fibers The influence of the treatment on the density of the bamboo fibers with moisture content between 9 and 11% can be seen in Fig. 2. According to the results presented in Fig. 2, it is possible to observe an increase in the absolute density value for treated fiber (regardless of the type of treatment). This increase in density is more noticeable in fibers treated with the mercerization method (approximately 60%). A similar behavior has recently been reported for the treatment of other vegetable fibers [56–59]. According Cai et al. [59], non-cellulosic sub stances present in fibers (hemicellulose, lignin, pectins, and liposoluble compounds) are characterized by having low-molecular-weight poly saccharides that form amorphous, random, and branched structures, whose removal promotes an increase in fiber density. On analyzing the effect of the treatment method, it can be seen that for fibers treated with plasma there were no significant variations in their density value. Ac cording to Minati et al. [60], this type of treatment allows the removal of impurities on the surface of the material, modifying its chemical struc ture by grafting specific functional groups, without appreciable loss of mass of the vegetable fiber. The samples were tested for water absorption after surface treat ment. The results for untreated and treated fibers are shown in Fig. 3. From the results, it is possible to observe an increase in the capacity of water absorption in the fibers whose surface was modified. According to Praveen et al. [61], some untreated vegetal fibers may have a hydro phobic character. This property could be associated with their basic composition (lignin, hemicellulose, and pectins). The application of a surface treatment on lignocellulosic materials modifies the water ab sorption capacity, increasing the hydrophilicity of the fibers. This
2.5. Manufacture of biocomposites The mixture of constituent materials was performed in a steel mold of 300 � 300 mm2. A thickness of 6 mm was previously established. At the suggestion of the supplier of the vegetable resin, a ratio of 1:1.5 (A:B) was used in the preparation of the resin that acts as the matrix of the composite. This mixture allows a workability time of approximately 15 min before starting the drying process. The constituent materials were mixed in a proportion of 70–30 with respect to the volume of the composite. The fabrication of the composite was carried out using the compression method. For the manufacture of the panels, a hydraulic press with loading capacity of up to 1000 kN was employed [55,56]. The compression load was 180 kN, and the compaction time was 24 h. The panels were cured for a week at room temperature. For the analysis of the results, the nomenclature presented in Table 2 was adopted. 2.6. Determination of the physical and mechanical properties of the composites The determination of the physical properties consisted of the calcu lation of the density, effective absorption, and percentage of swelling (at 2 h and 24 h), according the specifications of ASTM D-2395. The per centage of swelling was determined from the variation of thickness of square specimens of 50 mm in width after their immersion in water at room temperature for periods of 2 h and 24 h. The determination of the mechanical properties focused on compression and static bending tests. For the tests, the recommenda tions of ASTM 1037–12 were considered. The mechanical tests were performed on an MTS Landmark 370.10, using an operating range of up to 100 kN. Details of the procedures are presented below. Compression tests: The compressive strength, the modulus of elas ticity, and the stress at the proportional limit was carried out. Thirty specimens of 50 mm in width, 7 mm in thickness, and 100 mm in length were tested for each of the treatments evaluated. The load was applied continuously throughout the test at a uniform rate of motion of the Table 2 Nomenclature according type of treatment. Nomenclature
Description
UT MT PT OT
Untreated Mercerization treatment Plasma treatment Ozone treatment
Fig. 2. Effect of the treatments on the density of the bamboo fibers. 3
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Journal of Building Engineering 28 (2020) 101058
covered with waxes and polysaccharides such as lignin, hemicelluloses, or pectins, which give the fibers a rough and uneven surface appearance. On the other hand, the treated fibers have a lower surface roughness than the untreated fibers. According to Sair et al. [62], modifications of the surface texture of fibers can be associated with the partial dissolution of their amorphous portions. In the micrographs, it is possible to observe the separation of individual fibers from the surface of the fiber bundle. This phenomenon characterizes the onset of a fibrillation process and can be attributed to the removal of non-cellulosic compounds present in the middle lamella that bind the elementary fibers [62]. The creation of interfibral spaces favors the mechanism of anchoring the resin to the fibers, benefiting the charge transfer between the matrix and the rein forcement during the application of loads. A semiquantitative chemical analysis of the fibers was carried out using the energy-dispersive X-ray spectroscopy (EDS) The elementary composition of each sample was normalized with respect to the content of the main elements present in the fibers, that is, C (%), O (%), Si (%) and Na (%). The results are shown in Fig. 5. The resulting C/O and Si/C ratios are shown in Table 3. According to the elemental composition shown in Table 3, carbon (C), oxygen (O), silicon (Si), and sodium traces (Na) were found in the composition of the fibers. The presence of Na in the fibers is a conse quence of the mercerization treatment with a solution of NaOH. The treated fibers were characterized by a significant reduction in the C/O and Si/O ratios. The removal of the waxy and hemicellulose content may cause exposure to lignin. This statement can be supported by the reduction in the C/O ratio. According to Fig. 5 and Table 3, the initial C/ O ratio for the untreated fiber was 1:20 and decreased with the appli cation of the surface treatments evaluated, and this decrease was even more pronounced for the plasma and ozone treatments (approximately 17%). A reduction in the Si/C ratio also indicates that the applied treatments allow extracting the silica from the fibers without degrading
Fig. 3. Effect of treatment on the water absorption of bamboo fibers.
increase can be attributed to the chemical and physical changes induced by the surface treatments. 3.2. Effect of surface treatment on the physical and structural properties of the fibers Micrographs for untreated and treated fibers were obtained by means of HITACHI S-570 equipment (at an acceleration voltage of 40 kV) in order to observe the effect of the treatment on its structure and morphology. The results are shown in Fig. 4. In Fig. 4, it can be seen that the surface of the untreated fibers is
Fig. 4. SEM micrographs for untreated and treated fibers. 4
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Journal of Building Engineering 28 (2020) 101058
Fig. 5. Results of EDS semi-quantitative elementary analysis for a) untreated fibers, b) mercerization-treated fibers, c) plasma-treated fibers, and d) ozonetreated fibers.
5
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Journal of Building Engineering 28 (2020) 101058
Table 3 Semi-quantitative elemental analysis results for the untreated and treated fibers. Type of fiber
C (%)
O (%)
Si (%)
Na (%)
C/O
Si/C
UT MT PT OT
48.76 50.93 48.54 47.45
40.56 46.05 48.50 47.35
11.04 – 1.09 5.20
– 3.02 – –
1.20 1.11 1.00 1.00
0.23 – 0.02 0.11
the cellulose [63]. The results of the elemental composition of the fibers obtained via EDS analysis similar as reported by George et al. [63] and are coherents with the morphology of the fibers obtained by means of the SEM technique (Fig. 4) showing the efficiency of the surface treat ments for the removal of non-cellulosic compounds. The surface of the fibers was analyzed using atomic force microscopy (AFM). The average roughness (Ra) was quantified using the average height of the irregularities observed in the direction perpendicular to the surface of the samples. The results are shown in Table 4. From the results obtained and presented in Table 4, it is possible to see that the treatment affects the surface roughness. A significant reduction in the value of the average surface roughness can be seen (almost 30% for PT fibers and 80% for MT and OT fibers). Barra et al. [64] reported similar results. In the case of plasma-treated fibers, the reduction in roughness could be associated with the deposition of a thin hydrogenated amorphous carbon film on the surface of the material. For the case of fibers treated with sodium hydroxide solution, recent studies have shown that factors such as the concentration of the solution, the temperature, and the time of treatment influence the effect of the treatment on the roughness of the fibers. Recent reports have shown that treatments carried out with concentrations greater than 7% can cause a greater effect in the removal of impurities, which is reflected in the value of the average surface roughness of the fibers [65]. The results obtained for fibers treated with ozone are consistent with those presented by Demir et al. [66] and confirm the efficiency of this method for exfoli ating the surface of the fibers in order to increase their roughness. For the analysis of the crystallinity, diffratograms of the fibers were obtained (see Fig. 6). The crystallinity index (I.C) using the Seagal method was calculated. The results are presented in Table 5. The spectra presented in Fig. 6 show the existence of two peaks located at the Bragg 15� and 22.5� angles. These peaks are associated with the crystallographic planes (101) and (002) and show the presence of the typical crystalline structure of cellulose (cellulose I). The low intensity of the peaks identified in the diffractogram of the untreated fibers could be associated with the non-crystalline fraction of the cel lulose as well as with the presence of amorphous compounds in the fiber (lignins, pectins, and hemicelluloses) [69]. In Fig. 6, the presence of another typical peak of cellulose close to 34.5� in plane (040) can be seen. On analyzing the constant location of the peaks, it is possible to conclude that the treatments carried out did not modify the structure the crystalline structure of cellulose. The results presented in Table 5 reveal an increase in the value of the crystallinity index of the treated fibers. This increase suggests the removal of the non-crystalline portion of the fibers and an increase in their percentage of cellulose. The results indicate an increase of approximately 15% for fibers treated with ozone and approximately 30% for fibers treated with plasma. According to Joonobi (2010) [67], the increase in the crystallinity index can be attributed to the partial
Fig. 6. Diffractograms for untreated and treated fibers. Table 5 Crystallinity index for the untreated and treated fibers. Type of fiber
I.C (%)
UT MT PT OT
57.51 71.11 73.88 65.60
reduction of amorphous fiber components, which mainly consist of impurities and non-cellulosic substances, during the fiber surface modification processes. Fibers treated with a relatively high crystallinity are beneficial for the manufacture of biocomposites and can improve their mechanical strength (Rosa et al., 2012) [68]. The results are consistent with those reported by Sair et al. [62] and can positively affect the performance of the fibers as reinforcement for the composite. For the determination of the principal functional groups of untreated and treated fibers, an attenuated total reflection Fourier transform IRATR technique was employed. The spectra are shown in Fig. 7. The existence of an absorption band very close to a wavelength of 3400 cm 1 can be seen in all the fibers. This absorption band can be attributed to different modes of stretching of the O–H bond, typical of the cellulose structure [70]. The presence of two absorption bands around 2920 and
Table 4 Surface roughness for the untreated and treated fibers. Type of fiber
Ra (nm)
UT MT PT OT
67.30 80.40 44.20 77.60
Fig. 7. Typical absorption bands. 6
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Journal of Building Engineering 28 (2020) 101058
2850 cm 1 in untreated and mercerized fibers could be caused by the presence of pectins, waxes, and esters containing methyl and methylene groups. A peak near 1635 cm 1 in fibers treated through mercerization and ozone could be related to their water absorption. The localized peaks close to the wavelengths 1595 and 1510 cm 1 in treated fibers can be attributed to the stretching or bending vibrations of different groups – C, C–O) [71]. A band close to 1460 cm 1 in fibers present in lignin (C– treated with mercerization is characteristic of deformations of C–H and C–O through the bending or stretching of many groups present in lignin and carbohydrates. The bands at 1240 and 1165 cm 1 present in fibers treated with plasma, as well as the bands located at 1060 and 1030 cm 1, present in treated and untreated fibers, can be assigned to – O, C–H, C–O–C, and C–O deformation or the stretching of the vi C– brations of different groups in the carbohydrates. A peak near 1100 cm 1 in-untreated fibers corresponds to C–O–C stretching vibra tions of cellulose [72].
density of the composites. The dimensional stability of the panels manufactured with modified fibers contributes to improving their me chanical performance, mainly in those applications where the composite is exposed to wet environments. For the evaluation of the flexural properties, a three-point static bending test was carried out. The modulus of rupture (MOR), apparent modulus of elasticity (MOE), stress at proportional limit (Spl), and work at maximum load (Wml) were calculated. The results are shown in Table 8. From the results presented in Table 8, it is possible to verify the effect of the treatments on the flexural properties of the biocomposites. The modulus of rupture, the modulus of elasticity, and the proportional limit of the composites increase significantly in the panels made with treated fibers. This increase is more noticeable for panels made with plasma fibers (more than 50%) and may be due to the union of the fiber with the vegetable resin, thus improving fiber-matrix interactions. For the evaluation of the compressive properties, the maximum stress (Rc), modulus of modulus of elasticity (E), and the stress at pro portional limit (Spl) were calculated. The results are shown in Table 9. From the results presented in Table 9, it can be seen that the use of treated fibers as reinforcement causes an increase both in the maximum compressive stress and in the stiffness of the composite material. In panels reinforced with OT fibers, an increase of approximately 50% in the value of maximum stress is seen. Furthermore, this type of panel exhibits a significant increase in the values of the modulus of elasticity and stress at the proportional limit (more than 50%). According to the SEM results (Section 3.2), surface treatments promote the creation of interfibral spaces and the anchoring mechanism of the resin to the fibers, increasing the load transfer between the matrix and the reinforcement and the mechanical strength of the panels during the application of external loads (i.e, modulus of rupture and compressive strength).
3.3. Effect of surface treatment on the mechanical properties of the fibers For the evaluation of the mechanical properties of the fibers, a ten sion test was carried out, following the procedure described in section 2.3. The results are shown in Table 6. From the results presented in Table 6, it is possible to see that the application of a surface treatment does not produce significant modifi cations in the value of the mechanical strength. However, a reduction of approximately 20% in the value of the strain at the break point and a reduction of approximately 75% in the value of the toughness was observed for fibers treated with ozone and with the mercerization method. In agreement with these results, an increase of between 8 and 26% in the values of the modulus of elasticity can be seen. The increase of the stiffness of the fibers, as well as a reduction in their toughness, could be associated with the effect of the treatments applied on the crystallinity of the fibers [59]. Similar behavior can be observed in fibers treated with plasma; however, differences of less than 10% were found when comparing their results with those obtained for the untreated fibers.
4. Conclusions The influence of methods of surface treatment on the behavior of the physical and mechanical properties of biocomposites was evaluated. The results obtained with the X-ray diffraction, attenuated total reflection Fourier transform, and atomic force microscopy techniques allow demonstrating that when applying a treatment to the surface of the fibers, the index of crystallinity, average surface roughness, and intensity of typical absorption bands can be modified. These modifica tions contribute to reducing the absorption capacity and the percentage of swelling of the panels manufactured with this type of reinforcement and therefore to improving the performance of the composite material during its service life. The results obtained in the physical and mechanical tests of panels made with treated fibers allow verifying that by modifying the surface of the fibers it is possible to significantly increase the strength and stiffness of the material. Surface treatments promote the creation of interfibral spaces and the anchoring mechanism of the resin to the fibers, increasing the load transfer between matrix and reinforcement. As a consequence, the modulus of rupture and the compressive strength of the fiber-treated panels increased by more than 50%. The results of this study prove the efficiency of the mercerization, plasma, and ozone treatments for cleaning the surface and the removal of non-cellulosic materials in bamboo fibers. The surface treatments turn the bamboo fibers into promising raw materials for the manufacture of multi-benefit compounds and matrices commonly used in the industrial sector.
3.4. Effect of surface treatment on the physical and mechanical properties of the panels The physical properties of the biocomposites were determined ac cording to the procedures described in Section 2.6. The characterization was focused on the determination of the relative density (D), absorption (A), and percentage of swelling (SW) at 2 h and 24 h. The results are shown in Table 7. From the results presented in Table 7, a reduction in absorption (between 45 and 65%) and percentage of swelling values for treated panels (between 20 and 40%) with respect to the values obtained for untreated panels is evident. According to Trujillo (2014), the surface modification of the fibers partially removes their content of hemicellu lose, lignin, pectins, waxes, and other impurities. The elimination of these compounds promotes the compatibility of the fibers with the resin that acts as the matrix of the composite. By improving the adhesion between the fibers and the matrix, the empty content in the composites can be reduced, resulting in a lower chance of moisture entering the material [73]. In Table 7, no significant differences were found in the Table 6 Mechanical properties of fibers. ID
Tensile stress (MPa)
Strain at failure (%)
Thougness (MJ)
Elastic modulus (MPa)
UT MT PT OT
583 � 48 510 � 67 616 � 74 579 � 40
2.099 � 0.19 1.689 � 0.22 2.025 � 0.23 1.668 � 0.18
41.25 � 6.73 8.23 � 0.76 30.64 � 4.28 12.54 � 5.92
25513 � 3760 27707 � 1662 25038 � 1525 32165 � 3790
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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Journal of Building Engineering 28 (2020) 101058
Table 7 Physical properties of panels. Panel
D (g/cm3)
SW 2 h (%)
SW 24 h (%)
A (%) 2 h
A (%) 24 h
UT panels MT panels PT panels OT panels
1.10 � 0.03 1.12 � 0.03 0.97 � 0.02 1.14 � 0.04
4.50 � 0.03 2.64 � 0.05 2.42 � 0.05 2.28 � 0.07
7.50 � 0.04 6.02 � 0.06 4.15 � 0.07 4.11 � 0.06
6.08 � 0.05 2.06 � 0.04 3.59 � 0.03 2.93 � 0.05
8.15 � 0.06 2.70 � 0.08 4.64 � 0.04 3.27 � 0.06
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Table 8 Flexural properties of panels. Panel
MOR (MPa)
MOE (MPa)
Spl (MPa)
Wml (N⋅mm ⁄mm3)
UT panels MT panels PT panels OT panels
43.87 � 9.91 56.76 � 5.28 70.94 � 8.35 62.04 � 9.22
3774 � 679 5796 � 927 9166 � 125 5675 � 795
24.01 � 3.84 27.04 � 4.87 46.83 � 0.93 35.29 � 3.53
0.06 � 0.02 0.08 � 0.02 0.06 � 0.01 0.06 � 0.01
Table 9 Compressive properties of panels. Panel
Rc (MPa)
E (MPa)
Spl (MPa)
UT panels MT panels PT panels OT panels
21.39 � 4.06 28.50 � 2.85 24.82 � 2.48 32.20 � 1.50
2273 � 340 2988 � 329 2554 � 256 3493 � 105
16.86 � 3.37 23.72 � 4.74 22.52 � 2.93 25.43 � 2.03
Acknowledgment This paper is a derivative product of the project (INV-ING-2623) financed by the Vicerectory of Research of Universidad Militar Nueva Granada-validity (2018)”. References [1] A. Mancino, G. Marannano, B. Zuccarello, Implementation of eco-sustainable biocomposite materials reinforced by optimized agave fibers, Procedia Struct Integrity 8 (2018) 526–538, https://doi.org/10.1016/j.prostr.2017.12.052. [2] M. Sood, G. Dwivedi, Effect of fiber treatment on flexural properties of natural fiber reinforced composites: a review, Egypt. J. Petrol. 27 (2018) 775–783, https://doi. org/10.1016/j.ejpe.2017.11.005. [3] E. Uitterhaegen, P. Parinet, L.T. Labonne, S. Ballas, T. V� eron� ese, O. Merah, T. Talou, C.V. Stevens, F. Chabert, Ph. Evon, Performance, durability and recycling of thermoplastic biocomposites reinforced with coriander straw, Composites Part A 113 (2018) 254–263, https://doi.org/10.1016/j.compositesa.2018.07.038. [4] B. Zuccarello, G. Marannano, Random short sisal fiber biocomposites: optimal manufacturing process and reliable theoretical models, Mater. Des. 149 (2018) 87–100, https://doi.org/10.1016/j.matdes.2018.03.070. [5] B. Zuccarello, M. Zingales, Toward high performance renewable agave reinforced biocomposites: optimization of fiber performance and fiber-matrix adhesion analysis, Composites Part B 122 (2017) 109–120, https://doi.org/10.1016/j. compositesb.2017.04.011. [6] H. Abrala, A. Hartono, F. Hafizulhaq, D. Handayani, E. Sugiarti, O. Pradipt, Characterization of PVA/cassava starch biocomposites fabricated with and without sonication using bacterial cellulose fiber loadings, Carbohydr. Polym. 206 (2019) 593–601, https://doi.org/10.1016/j.carbpol.2018.11.054. [7] A. Pantano, B. Zuccarello, Numerical model for the Characterization of biocomposites reinforced by sisal fibers, Procedia Struct. Integr. 8 (2018) 517–525, https://doi.org/10.1016/j.prostr.2017.12.051. [8] H. Yaghoobi, F. Fereidoon, Preparation and characterization of short kenaf fiberbased biocomposites reinforced with multi-walled carbon nanotubes, Composites Part B 162 (2019) 314–322, https://doi.org/10.1016/j.compositesb.2018.11.015. [9] B. Zuccarello, R. Scaffaro, Experimental analysis and micromechanical models of high performance renewable agave reinforced biocomposites, Composites Part B 119 (2017) 141–152, https://doi.org/10.1016/j.compositesb.2017.03.056. [10] C. Badji, J. Beigbeder, H. Garay, A. Bergeret, J.C. B� en�ezet, V. Desauziers, Exterior and under glass natural weathering of hemp fibers reinforced polypropylene biocomposites: impact on mechanical, chemical, microstructural and visual aspect properties, Polym. Degrad. Stab. 148 (2018) 104–116, https://doi.org/10.1016/j. polymdegradstab.2017.12.015. [11] R. Scaffaro, F. Lopresti, L. Botta, PLA based biocomposites reinforced with Posidonia oceanica leaves, Composites Part B 139 (2018) 1–11, https://doi.org/ 10.1016/j.compositesb.2017.11.048. [12] N. Kumar, D. Das, Fibrous biocomposites from nettle (Girardinia diversifolia) and poly(lactic acid) fibers for automotive dashboard panel application, Composites Part B 130 (2017) 54–63, https://doi.org/10.1016/j.compositesb.2017.07.059.
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Journal of Building Engineering journal homepage: http://www.elsevier.com/locate/jobe
Physical-mechanical properties of bamboo fibers-reinforced biocomposites: Influence of surface treatment of fibers ~ o, Jhon Ca �rdenas Martha L. S� anchez *, William Patin Universidad Militar Nueva Granada Carrera, Facultad de Ingeniería, Programa de Ingeniería Civil Carrera 11, N. 101 80, Bogot� a, D.C, Colombia
A R T I C L E I N F O
A B S T R A C T
Keywords: Biocomposites Bamboo fibers Orientation Surface modifications Physical properties Mechanical properties
The composition and the morphology of most plant fibers allow their use as reinforcement of biocomposites. In this study, the effect of the modification of the surface of the fibers on the physical and mechanical properties of biocomposites reinforced with bamboo fibers is analyzed. For the experimental procedure, three treatments were carried out: mercerization, ozone, and plasma. The influence of the treatments on the modification of the surface of the fibers was evaluated using X-ray diffraction (XRD), infrared spectroscopy (FTIR), atomic force microscopy (AFM), and scanning electron microscopy (SEM-EDS). The analysis of the properties of the fibers was focused on the determination of the tensile strength, modulus of elasticity, density, moisture content, and absorption ca pacity. The evaluation of physical properties of biocomposites consisted in the determination of the density, effective absorption, and dimensional stability. The determination of the mechanical properties focused in the evaluation of their behavior under compression and static bending. The results obtained were compared with the results for biocomposites made with untreated fibers, and they demonstrate that by using treated fiber it is possible to increase both the dimensional stability of the panels and their mechanical strength by more than 50%.
1. Introduction The implementation of methods of design, manufacture, and char acterization of biocomposites reinforced with vegetable fibers has become a topic of growing interest in the development of composite materials [1–26]. The concern for contributing to the reduction of the environmental impact caused by the production and use of synthetic fibers has spurred the promotion of the use of sustainable materials made from fibers of vegetable origin [7,25]. These materials are distinguished by their excellent mechanical properties, such as high tensile strength, good stiffness, high degree of toughness, low damage capacity, good thermal insulation, and low cost [26]. However, some factors, such as the hydrophilic nature of natural fibers and their high polysaccharide content, can affect their performance as reinforcement for hydrophobic polymeric resins [27]. This incompatibility between the fibers and the matrix can cause a weak interfacial adhesion, affecting the stress transfer of the matrix to the fibers, which harms the mechanical performance of the composite [27–30]. Recent studies have shown that for the efficient use of vegetable fi bers as reinforcement for biocomposites, preliminary treatments should be used to modify the surface of the fibers. These treatments allow the
elimination of impurities that remain adhered to the wall of the fibers after the mechanical extraction process is finished. Additionally, with these surface modification methods, it is possible to remove the lignin, hemicellulose, wax, and other extractable substances that affect the adhesion of the fibers to the matrix of composites [26–30]. According to the mechanism of action, surface modification treatments can be clas sified into three categories: chemical, physical, and biological modifi cation [31–54]. One of the chemical methods most used for the modification of vegetable fibers is mercerization. This method involves the modification of the structure and chemical composition of plant fibers using an aqueous solution of sodium hydroxide (NaOH). The concentration, temperature, and time of action of the solution will depend on the characteristics of the fibers and their application [31–39]. With this treatment, the removal of surface impurities, as well as the occurrence of fibrillation, is possible, obtaining a fibrous material with greater surface area and smaller diameter, which produces an increase in the tensile strength of the fibers and in the mechanical properties of the resulting composite [40–42]. Currently, the methods of physical modification have become important in the treatment of vegetal fibers. These methods cause
* Corresponding author. E-mail address: [email protected] (M.L. S� anchez). https://doi.org/10.1016/j.jobe.2019.101058 Received 10 May 2019; Received in revised form 4 November 2019; Accepted 6 November 2019 Available online 7 November 2019 2352-7102/© 2019 Elsevier Ltd. All rights reserved.
M.L. S� anchez et al.
Journal of Building Engineering 28 (2020) 101058
fibrillation of the material by electrical discharge [43–51]. One of the best-known physical processes is the application of a cold plasma of methane, argon, or helium. This method allows the formation of free radicals and the polymerization of the material. Plasma treatment acts on the chemical structure of the fibers and their crystallinity index. Recently, ozone treatment has emerged as an eco-friendly method of surface modification [52,53]. According to Ai et al. (2019), ozone can oxidize the lignin and the hemicelluloses, producing soluble compounds of low molecular weight [52]. The purpose of this research is to evaluate the effect of surface treatments on the properties of composites made with bamboo fibers and vegetal resin. For this objective, three different treatments were applied: mercerization, ozone, and cold plasma. The biocomposite was fabricated via the compression method at room temperature. The results are compared with results obtained for biocomposites reinforced with untreated fibers and with wood agglomerates produced under the same laboratory conditions.
deposition (DC PACVD) system, powered by a bipolar and asymmetric pulsed DC current source, was used in the treatment of the plant fibers. The shape of the pulses emitted by the current source consisted of a negative pulse variable in amplitude (adjustable between 0 and -1000 V), followed by a constant positive pulse of 40 V. This pulse configuration contributes to the neutralization of the plasma, obtaining a plasma enriched in positive ions. The fibers were submitted to the action of a combination of 50% methane and 50% argon cold plasma for 10 min, using a working pressure of about 27 Pa and a DC potential of 700 V, with a gas flow of 10 sccm (standard cubic centimeters per minute) and a temperature between 18 � C and 26 � C. Ozone treatment: The procedure was carried out using a cylindrical tube of 1100 mm in height, batch type. The ozone was distributed by means of a porous plate of average pore size equal to 44 μm located in the back of the reactor. Ozone was added to the system using a Clear water Tech LLC 300P 120 V/60HZ Microzone generator with an ozone production of 230 mg O3/h. The fibers were treated for 120 min. After treatment, the fibers were dried to constant mass.
2. Materials and methods
2.3. Evaluation of the physical and mechanical properties of the fibers
2.1. Materials
The determination of the moisture content and the water absorption of the fibers was carried out according to the specifications of the standards ASTM D 4442–16 and ASTM D5229-14. The density of the fibers was determined by means of the pycnometer method at a tem perature of 18 � C, according to the specifications of ASTM D8171. To determine the tensile strength and the modulus of elasticity, fiber bundles of 50 mm in length were used. The test was performed on a Landmark MTS 370.10, at a loading speed of 1 mm/min. In order to guarantee the adequate fastening of the fibers to the testing machine, they were previously glued to a rectangular mold of 25 mm in width and 70 mm in length, according to procedures described by Okubo et al., 2004 [54]. The experimental assembly is shown in Fig. 1.
For the manufacture of the biocomposite, fibers extracted from the upper region of Macana bamboo were used. This type of bamboo is characterized by short internodes with regular nodal distances, thick walls, and uniform diameters. Since the material to be used is previously stored in the laboratory until it is crushed, all the material is immunized before being acquired by immersion in commercial borax and salts of boric acid at a concentration of 3%. A bi-component vegetable-based polyurethane (extracted from castor oil), AGT-1315, formulated by the cold mixing of a pre-polymer and a polyol component, was used as matrix of composite. The properties of the resin are shown in Table 1. 2.2. Separation and treatment of fibers
2.4. Evaluation of the effects of treatment on the properties of the fibers
For the extraction of the fibers, a conventional mechanical crusher was used. The extraction process was carried out with the material in the green state. The crushed material was classified according to size, using mechanical sieving. Fibers with a diameter between 0.6 and 0.9 mm were obtained. In order to guarantee an adequate adherence between fibers and composite matrix a surface modification of fibers was realized. The modification consists of the partial removal of lignin, hemicellulose, pectins and waxes present in plant fibers. Surface treatment of the crushed material was carried out in order to remove waxes, oils, lignin, and other substances that can affect the adhesion of the fibers to the matrix of the composite. For the surface treatment, three procedures were employed: mercerization, ozone, and cold plasma. All the treatments were carried out on untreated fibers. Details of procedures are described below. Mercerization treatment: This treatment consisted of the immersion of the crushed material in a 10% sodium hydroxide solution for a period of 48 h at room temperature. The concentration of sodium hydroxide solution was selected based on previous results and the technical liter ature consulted. This process was followed by washing with distilled water and a drying process until reaching a constant mass value. Cold plasma treatment: A pulsed plasma assisted chemical vapor
Infrared spectroscopy-attenuated total reflectance (IT-ATR) and Xray diffraction (XRD) techniques were applied in order to evaluate the influence of the treatments on the chemical properties of fibers. Microscopic studies such as scanning electron microscopy (SEM), transmission microscopy, and atomic force microscopy (AFM) were used to analyze the effect of the surface treatments on the morphology and roughness of the fibers. Details of the procedures are described below. Infrared spectroscopy-attenuated total reflectance (IT-ATR): For the determination of the characteristic absorption bands for the most com mon functional groups in untreated and treated fibers, the infrared spectroscopy-attenuated total reflectance (IT-ATR) technique was employed. The use of this technique allows evaluating the partial removal of lignin in the fibers by detecting the typical functional groups of this polysaccharide and its typical absorption bands. For the analysis,
Table 1 Properties of matrix. Properties
Component “A00
Component “B00
Boiling point Melting point Flash point Density (at 25 � C)
190 C 14 � C 200� C 1.22 g=cm3
313 � C 10 � C 229� C 1.98 g=cm3
�
Fig. 1. Tensile test: (a) Fastening system, (b) Assembly. 2
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spectra were taken with a Shimadzu IR prestige-21 spectrophotometer with ATR module, performing 40 scans with a resolution of 4 cm 1. X-ray diffraction (XRD): A PANalytical XPert PRO MRD X-ray diffraction system was used. This equipment has a copper anode tube, a standard resolution goniometer containing the geometry (0θ–2θ), a minimum step size of 0.002� , and an X-ray proportional counter. For the XRD analysis, difractograms of the untreated and treated fibers were obtained and the crystallinity index (IC) was determined. Atomic force microscopy (AFM): The roughness of the fibers was studied via atomic force microscopy, using a Nanosurf Atomic Force Microscope, on an area of 2500 μm2. The average roughness was calculated using the average height of the irregularities observed in the direction perpendicular to the surface of thirty samples for each type of treatment. Scanning electron microscopy (SEM): Micrographs were obtained using a HITACHI S-570 device (at an acceleration voltage of 40 kV). Images taken with secondary electrons allowed determining the chem ical composition, differentiated by grayscale. Images taken with back scattered electrons allowed evaluating the morphology of the fibers.
movable crosshead of the testing machine of 0.5 mm/min. Static bending: The three-point static bending tests determined the flexural properties, such as the modulus of rupture, the apparent modulus of elasticity, the stress at the proportional limit, and the work at maximum load. The long axis of the panel was used. Thirty specimens of 70 mm in width, 7 mm in thickness, and 170 mm in length were tested for each of the treatments evaluated. The load was applied continuously throughout the test at a uniform rate of motion of the movable crosshead of the testing machine to achieve an outer fiber strain rate of 3 mm/min. 3. Results and discussion This section presents the effect of the surface treatment on the physical and mechanical properties of biocomposites reinforced with Macana bamboo fibers. 3.1. Effect of surface treatment on the physical properties of the fibers The influence of the treatment on the density of the bamboo fibers with moisture content between 9 and 11% can be seen in Fig. 2. According to the results presented in Fig. 2, it is possible to observe an increase in the absolute density value for treated fiber (regardless of the type of treatment). This increase in density is more noticeable in fibers treated with the mercerization method (approximately 60%). A similar behavior has recently been reported for the treatment of other vegetable fibers [56–59]. According Cai et al. [59], non-cellulosic sub stances present in fibers (hemicellulose, lignin, pectins, and liposoluble compounds) are characterized by having low-molecular-weight poly saccharides that form amorphous, random, and branched structures, whose removal promotes an increase in fiber density. On analyzing the effect of the treatment method, it can be seen that for fibers treated with plasma there were no significant variations in their density value. Ac cording to Minati et al. [60], this type of treatment allows the removal of impurities on the surface of the material, modifying its chemical struc ture by grafting specific functional groups, without appreciable loss of mass of the vegetable fiber. The samples were tested for water absorption after surface treat ment. The results for untreated and treated fibers are shown in Fig. 3. From the results, it is possible to observe an increase in the capacity of water absorption in the fibers whose surface was modified. According to Praveen et al. [61], some untreated vegetal fibers may have a hydro phobic character. This property could be associated with their basic composition (lignin, hemicellulose, and pectins). The application of a surface treatment on lignocellulosic materials modifies the water ab sorption capacity, increasing the hydrophilicity of the fibers. This
2.5. Manufacture of biocomposites The mixture of constituent materials was performed in a steel mold of 300 � 300 mm2. A thickness of 6 mm was previously established. At the suggestion of the supplier of the vegetable resin, a ratio of 1:1.5 (A:B) was used in the preparation of the resin that acts as the matrix of the composite. This mixture allows a workability time of approximately 15 min before starting the drying process. The constituent materials were mixed in a proportion of 70–30 with respect to the volume of the composite. The fabrication of the composite was carried out using the compression method. For the manufacture of the panels, a hydraulic press with loading capacity of up to 1000 kN was employed [55,56]. The compression load was 180 kN, and the compaction time was 24 h. The panels were cured for a week at room temperature. For the analysis of the results, the nomenclature presented in Table 2 was adopted. 2.6. Determination of the physical and mechanical properties of the composites The determination of the physical properties consisted of the calcu lation of the density, effective absorption, and percentage of swelling (at 2 h and 24 h), according the specifications of ASTM D-2395. The per centage of swelling was determined from the variation of thickness of square specimens of 50 mm in width after their immersion in water at room temperature for periods of 2 h and 24 h. The determination of the mechanical properties focused on compression and static bending tests. For the tests, the recommenda tions of ASTM 1037–12 were considered. The mechanical tests were performed on an MTS Landmark 370.10, using an operating range of up to 100 kN. Details of the procedures are presented below. Compression tests: The compressive strength, the modulus of elas ticity, and the stress at the proportional limit was carried out. Thirty specimens of 50 mm in width, 7 mm in thickness, and 100 mm in length were tested for each of the treatments evaluated. The load was applied continuously throughout the test at a uniform rate of motion of the Table 2 Nomenclature according type of treatment. Nomenclature
Description
UT MT PT OT
Untreated Mercerization treatment Plasma treatment Ozone treatment
Fig. 2. Effect of the treatments on the density of the bamboo fibers. 3
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covered with waxes and polysaccharides such as lignin, hemicelluloses, or pectins, which give the fibers a rough and uneven surface appearance. On the other hand, the treated fibers have a lower surface roughness than the untreated fibers. According to Sair et al. [62], modifications of the surface texture of fibers can be associated with the partial dissolution of their amorphous portions. In the micrographs, it is possible to observe the separation of individual fibers from the surface of the fiber bundle. This phenomenon characterizes the onset of a fibrillation process and can be attributed to the removal of non-cellulosic compounds present in the middle lamella that bind the elementary fibers [62]. The creation of interfibral spaces favors the mechanism of anchoring the resin to the fibers, benefiting the charge transfer between the matrix and the rein forcement during the application of loads. A semiquantitative chemical analysis of the fibers was carried out using the energy-dispersive X-ray spectroscopy (EDS) The elementary composition of each sample was normalized with respect to the content of the main elements present in the fibers, that is, C (%), O (%), Si (%) and Na (%). The results are shown in Fig. 5. The resulting C/O and Si/C ratios are shown in Table 3. According to the elemental composition shown in Table 3, carbon (C), oxygen (O), silicon (Si), and sodium traces (Na) were found in the composition of the fibers. The presence of Na in the fibers is a conse quence of the mercerization treatment with a solution of NaOH. The treated fibers were characterized by a significant reduction in the C/O and Si/O ratios. The removal of the waxy and hemicellulose content may cause exposure to lignin. This statement can be supported by the reduction in the C/O ratio. According to Fig. 5 and Table 3, the initial C/ O ratio for the untreated fiber was 1:20 and decreased with the appli cation of the surface treatments evaluated, and this decrease was even more pronounced for the plasma and ozone treatments (approximately 17%). A reduction in the Si/C ratio also indicates that the applied treatments allow extracting the silica from the fibers without degrading
Fig. 3. Effect of treatment on the water absorption of bamboo fibers.
increase can be attributed to the chemical and physical changes induced by the surface treatments. 3.2. Effect of surface treatment on the physical and structural properties of the fibers Micrographs for untreated and treated fibers were obtained by means of HITACHI S-570 equipment (at an acceleration voltage of 40 kV) in order to observe the effect of the treatment on its structure and morphology. The results are shown in Fig. 4. In Fig. 4, it can be seen that the surface of the untreated fibers is
Fig. 4. SEM micrographs for untreated and treated fibers. 4
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Journal of Building Engineering 28 (2020) 101058
Fig. 5. Results of EDS semi-quantitative elementary analysis for a) untreated fibers, b) mercerization-treated fibers, c) plasma-treated fibers, and d) ozonetreated fibers.
5
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Table 3 Semi-quantitative elemental analysis results for the untreated and treated fibers. Type of fiber
C (%)
O (%)
Si (%)
Na (%)
C/O
Si/C
UT MT PT OT
48.76 50.93 48.54 47.45
40.56 46.05 48.50 47.35
11.04 – 1.09 5.20
– 3.02 – –
1.20 1.11 1.00 1.00
0.23 – 0.02 0.11
the cellulose [63]. The results of the elemental composition of the fibers obtained via EDS analysis similar as reported by George et al. [63] and are coherents with the morphology of the fibers obtained by means of the SEM technique (Fig. 4) showing the efficiency of the surface treat ments for the removal of non-cellulosic compounds. The surface of the fibers was analyzed using atomic force microscopy (AFM). The average roughness (Ra) was quantified using the average height of the irregularities observed in the direction perpendicular to the surface of the samples. The results are shown in Table 4. From the results obtained and presented in Table 4, it is possible to see that the treatment affects the surface roughness. A significant reduction in the value of the average surface roughness can be seen (almost 30% for PT fibers and 80% for MT and OT fibers). Barra et al. [64] reported similar results. In the case of plasma-treated fibers, the reduction in roughness could be associated with the deposition of a thin hydrogenated amorphous carbon film on the surface of the material. For the case of fibers treated with sodium hydroxide solution, recent studies have shown that factors such as the concentration of the solution, the temperature, and the time of treatment influence the effect of the treatment on the roughness of the fibers. Recent reports have shown that treatments carried out with concentrations greater than 7% can cause a greater effect in the removal of impurities, which is reflected in the value of the average surface roughness of the fibers [65]. The results obtained for fibers treated with ozone are consistent with those presented by Demir et al. [66] and confirm the efficiency of this method for exfoli ating the surface of the fibers in order to increase their roughness. For the analysis of the crystallinity, diffratograms of the fibers were obtained (see Fig. 6). The crystallinity index (I.C) using the Seagal method was calculated. The results are presented in Table 5. The spectra presented in Fig. 6 show the existence of two peaks located at the Bragg 15� and 22.5� angles. These peaks are associated with the crystallographic planes (101) and (002) and show the presence of the typical crystalline structure of cellulose (cellulose I). The low intensity of the peaks identified in the diffractogram of the untreated fibers could be associated with the non-crystalline fraction of the cel lulose as well as with the presence of amorphous compounds in the fiber (lignins, pectins, and hemicelluloses) [69]. In Fig. 6, the presence of another typical peak of cellulose close to 34.5� in plane (040) can be seen. On analyzing the constant location of the peaks, it is possible to conclude that the treatments carried out did not modify the structure the crystalline structure of cellulose. The results presented in Table 5 reveal an increase in the value of the crystallinity index of the treated fibers. This increase suggests the removal of the non-crystalline portion of the fibers and an increase in their percentage of cellulose. The results indicate an increase of approximately 15% for fibers treated with ozone and approximately 30% for fibers treated with plasma. According to Joonobi (2010) [67], the increase in the crystallinity index can be attributed to the partial
Fig. 6. Diffractograms for untreated and treated fibers. Table 5 Crystallinity index for the untreated and treated fibers. Type of fiber
I.C (%)
UT MT PT OT
57.51 71.11 73.88 65.60
reduction of amorphous fiber components, which mainly consist of impurities and non-cellulosic substances, during the fiber surface modification processes. Fibers treated with a relatively high crystallinity are beneficial for the manufacture of biocomposites and can improve their mechanical strength (Rosa et al., 2012) [68]. The results are consistent with those reported by Sair et al. [62] and can positively affect the performance of the fibers as reinforcement for the composite. For the determination of the principal functional groups of untreated and treated fibers, an attenuated total reflection Fourier transform IRATR technique was employed. The spectra are shown in Fig. 7. The existence of an absorption band very close to a wavelength of 3400 cm 1 can be seen in all the fibers. This absorption band can be attributed to different modes of stretching of the O–H bond, typical of the cellulose structure [70]. The presence of two absorption bands around 2920 and
Table 4 Surface roughness for the untreated and treated fibers. Type of fiber
Ra (nm)
UT MT PT OT
67.30 80.40 44.20 77.60
Fig. 7. Typical absorption bands. 6
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2850 cm 1 in untreated and mercerized fibers could be caused by the presence of pectins, waxes, and esters containing methyl and methylene groups. A peak near 1635 cm 1 in fibers treated through mercerization and ozone could be related to their water absorption. The localized peaks close to the wavelengths 1595 and 1510 cm 1 in treated fibers can be attributed to the stretching or bending vibrations of different groups – C, C–O) [71]. A band close to 1460 cm 1 in fibers present in lignin (C– treated with mercerization is characteristic of deformations of C–H and C–O through the bending or stretching of many groups present in lignin and carbohydrates. The bands at 1240 and 1165 cm 1 present in fibers treated with plasma, as well as the bands located at 1060 and 1030 cm 1, present in treated and untreated fibers, can be assigned to – O, C–H, C–O–C, and C–O deformation or the stretching of the vi C– brations of different groups in the carbohydrates. A peak near 1100 cm 1 in-untreated fibers corresponds to C–O–C stretching vibra tions of cellulose [72].
density of the composites. The dimensional stability of the panels manufactured with modified fibers contributes to improving their me chanical performance, mainly in those applications where the composite is exposed to wet environments. For the evaluation of the flexural properties, a three-point static bending test was carried out. The modulus of rupture (MOR), apparent modulus of elasticity (MOE), stress at proportional limit (Spl), and work at maximum load (Wml) were calculated. The results are shown in Table 8. From the results presented in Table 8, it is possible to verify the effect of the treatments on the flexural properties of the biocomposites. The modulus of rupture, the modulus of elasticity, and the proportional limit of the composites increase significantly in the panels made with treated fibers. This increase is more noticeable for panels made with plasma fibers (more than 50%) and may be due to the union of the fiber with the vegetable resin, thus improving fiber-matrix interactions. For the evaluation of the compressive properties, the maximum stress (Rc), modulus of modulus of elasticity (E), and the stress at pro portional limit (Spl) were calculated. The results are shown in Table 9. From the results presented in Table 9, it can be seen that the use of treated fibers as reinforcement causes an increase both in the maximum compressive stress and in the stiffness of the composite material. In panels reinforced with OT fibers, an increase of approximately 50% in the value of maximum stress is seen. Furthermore, this type of panel exhibits a significant increase in the values of the modulus of elasticity and stress at the proportional limit (more than 50%). According to the SEM results (Section 3.2), surface treatments promote the creation of interfibral spaces and the anchoring mechanism of the resin to the fibers, increasing the load transfer between the matrix and the reinforcement and the mechanical strength of the panels during the application of external loads (i.e, modulus of rupture and compressive strength).
3.3. Effect of surface treatment on the mechanical properties of the fibers For the evaluation of the mechanical properties of the fibers, a ten sion test was carried out, following the procedure described in section 2.3. The results are shown in Table 6. From the results presented in Table 6, it is possible to see that the application of a surface treatment does not produce significant modifi cations in the value of the mechanical strength. However, a reduction of approximately 20% in the value of the strain at the break point and a reduction of approximately 75% in the value of the toughness was observed for fibers treated with ozone and with the mercerization method. In agreement with these results, an increase of between 8 and 26% in the values of the modulus of elasticity can be seen. The increase of the stiffness of the fibers, as well as a reduction in their toughness, could be associated with the effect of the treatments applied on the crystallinity of the fibers [59]. Similar behavior can be observed in fibers treated with plasma; however, differences of less than 10% were found when comparing their results with those obtained for the untreated fibers.
4. Conclusions The influence of methods of surface treatment on the behavior of the physical and mechanical properties of biocomposites was evaluated. The results obtained with the X-ray diffraction, attenuated total reflection Fourier transform, and atomic force microscopy techniques allow demonstrating that when applying a treatment to the surface of the fibers, the index of crystallinity, average surface roughness, and intensity of typical absorption bands can be modified. These modifica tions contribute to reducing the absorption capacity and the percentage of swelling of the panels manufactured with this type of reinforcement and therefore to improving the performance of the composite material during its service life. The results obtained in the physical and mechanical tests of panels made with treated fibers allow verifying that by modifying the surface of the fibers it is possible to significantly increase the strength and stiffness of the material. Surface treatments promote the creation of interfibral spaces and the anchoring mechanism of the resin to the fibers, increasing the load transfer between matrix and reinforcement. As a consequence, the modulus of rupture and the compressive strength of the fiber-treated panels increased by more than 50%. The results of this study prove the efficiency of the mercerization, plasma, and ozone treatments for cleaning the surface and the removal of non-cellulosic materials in bamboo fibers. The surface treatments turn the bamboo fibers into promising raw materials for the manufacture of multi-benefit compounds and matrices commonly used in the industrial sector.
3.4. Effect of surface treatment on the physical and mechanical properties of the panels The physical properties of the biocomposites were determined ac cording to the procedures described in Section 2.6. The characterization was focused on the determination of the relative density (D), absorption (A), and percentage of swelling (SW) at 2 h and 24 h. The results are shown in Table 7. From the results presented in Table 7, a reduction in absorption (between 45 and 65%) and percentage of swelling values for treated panels (between 20 and 40%) with respect to the values obtained for untreated panels is evident. According to Trujillo (2014), the surface modification of the fibers partially removes their content of hemicellu lose, lignin, pectins, waxes, and other impurities. The elimination of these compounds promotes the compatibility of the fibers with the resin that acts as the matrix of the composite. By improving the adhesion between the fibers and the matrix, the empty content in the composites can be reduced, resulting in a lower chance of moisture entering the material [73]. In Table 7, no significant differences were found in the Table 6 Mechanical properties of fibers. ID
Tensile stress (MPa)
Strain at failure (%)
Thougness (MJ)
Elastic modulus (MPa)
UT MT PT OT
583 � 48 510 � 67 616 � 74 579 � 40
2.099 � 0.19 1.689 � 0.22 2.025 � 0.23 1.668 � 0.18
41.25 � 6.73 8.23 � 0.76 30.64 � 4.28 12.54 � 5.92
25513 � 3760 27707 � 1662 25038 � 1525 32165 � 3790
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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Table 7 Physical properties of panels. Panel
D (g/cm3)
SW 2 h (%)
SW 24 h (%)
A (%) 2 h
A (%) 24 h
UT panels MT panels PT panels OT panels
1.10 � 0.03 1.12 � 0.03 0.97 � 0.02 1.14 � 0.04
4.50 � 0.03 2.64 � 0.05 2.42 � 0.05 2.28 � 0.07
7.50 � 0.04 6.02 � 0.06 4.15 � 0.07 4.11 � 0.06
6.08 � 0.05 2.06 � 0.04 3.59 � 0.03 2.93 � 0.05
8.15 � 0.06 2.70 � 0.08 4.64 � 0.04 3.27 � 0.06
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Table 8 Flexural properties of panels. Panel
MOR (MPa)
MOE (MPa)
Spl (MPa)
Wml (N⋅mm ⁄mm3)
UT panels MT panels PT panels OT panels
43.87 � 9.91 56.76 � 5.28 70.94 � 8.35 62.04 � 9.22
3774 � 679 5796 � 927 9166 � 125 5675 � 795
24.01 � 3.84 27.04 � 4.87 46.83 � 0.93 35.29 � 3.53
0.06 � 0.02 0.08 � 0.02 0.06 � 0.01 0.06 � 0.01
Table 9 Compressive properties of panels. Panel
Rc (MPa)
E (MPa)
Spl (MPa)
UT panels MT panels PT panels OT panels
21.39 � 4.06 28.50 � 2.85 24.82 � 2.48 32.20 � 1.50
2273 � 340 2988 � 329 2554 � 256 3493 � 105
16.86 � 3.37 23.72 � 4.74 22.52 � 2.93 25.43 � 2.03
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