mechanical properties of epoxy banana fibre

BCCM-3 – Brazilian Conference on Composite Materials Gramado, RS - Brazil, August 28-31, 2016

MECHANICAL PROPERTIES OF EPOXY BANANA FIBRE COMPOSITE TRATED WITH SODIUM CARBONATE Adriana Caldas1, Júlio C. dos Santos1, Tulio H. Panzera1, Kurt Strecker1,* 1

Departamento De Engenharia Mecânica, Universidade Federal de São João Del-Rei, São João Del-Rei, MG, Brazil *Corresponding author: [email protected]

Abstract: The objective of this work was to produce and study the mechanical properties of a composite based on epoxy resin matrix with banana fibers as reinforcement. The extracted banana fibers were treated with sodium carbonate solution to improve their resistance to water absorption. The composites were manufactured with two fiber distributions: aligned and random. Furthermore, two volume fractions, 30 and 40%, of banana fibers were investigated. The composites were manufactured with the aid of a hot press at a temperature of 100 °C for 20 min. After curing specimens were prepared for determination of the tensile and flexural strength according to ASTM D3039 / D3039M and ASTM D790-10, respectively. With assistance from DOE experiment or planning for randomized levels, it was possible to evaluate the variables alignment of the fibers, processing and volume fraction and their effect on density, tensile and flexural strength, as well as elastic modulus. The incorporation of banana fibers resulted in a decrease in the density of the composites, following the rule of mixture. Regarding the mechanical properties, a significant increase of the tensile and flexure strength and the elastic modulus of the composites were determined when compared to the pure epoxy resin matrix material. As expected, tensile strength of composites reinforced with long, aligned fibers was superior compared to short, randomly oriented fibers. Keywords: Manufacturing process, composite, epoxy resin, banana fiber.

1. INTRODUCTION Natural fibers have received increasing attention in regard to synthetic fibers due to their biodegradability, lower density and low cost. A wide variety of natural fibers exist which can be used as reinforcement of thermoplastic or thermosetting polymers [1], such as sisal and banana fibers. Further motivation in studying natural fibers lies in their potential as a renewable source and their biodegradability, thus contributing to the preservation of the environment [2]. Mechanical properties of composites reinforced with natural fibers were found to compare with polypropylene based composites with glass fiber additions [3]. Wambua et al. [3] suggest that natural fiber composites may possibly replace glass fiber composites that do not require high load bearing capability. Cellulose fibers, of hydrophilic nature, are inherently incompatible with hydrophobic polymers. However, compatibility between the fibers and the polymer can be induced by mercerization or an alkali treatment of the fibers, thus modifying the surface of the natural fibers [4]. Specifically, an alkali treatment by sodium hydroxide, NaOH disrupts the hydrogen bonding in the network structure and removes lignin, waste and oil covering the fiber surface, thus resulting in increased surface roughness [5-7]. However these chemical treatments are more or less expensive and

Adriana Caldas, Júlio C. Santos, Tulio H. Panzera, Kurt Strecker

may be harmful to environment and manufactures [1]. Only a small number of common lignocellulose fibers are being considered for reinforcement for composites because of their availability and mechanical properties. Banana fibers have being studied as possible composite reinforcement [8-10], due to their elevated tensile strength of up to 1500 MPa [10]. Benítez et. al. [8] showed that banana fibers containing lignin, pectin and hemicellulose, must be treated with alkali solutions prior to its use as reinforcement of polymeric matrices. The present study investigated physical and mechanical properties of epoxy resin reinforced with unidirectional long fibers or randomly orientated short fibers. Furthermore, a new, eco-friendly fiber treatment with a 10% sodium bicarbonate solution to enhance the adhesion of the fibers to the matrix, was evaluated. The banana fibers were analyzed in their natural state and after treatment with the Na 2CO3 solution by scanning electron microscopy (SEM) and Fourier transformed infrared spectroscopy (FTIR). The tensile strength and density of single fibers has also been determined. The results showed that chemical treatment with sodium bicarbonate was effective to improve the mechanical properties of composites produced.

2. MATERIALS AND METHODS 2.1 FIBER PREPARATION The origin of banana fibers used in this work is from the species Musa balbisiana, also known as “silver banana”, from the Brazilian state of Minas Gerais. The fibers were obtained by scraping the outer part of the pseudo-stem, cleansing with water and drying. The obtained dry fibers were of approximately 30cm in length and were cut according to the desired function. Part of the banana fibers was treated with 10 wt-% sodium carbonate solution for 12h. After treatment the fibers were washed until neutral pH and dried for 24h at 60º.

2.2 COMPOSITE MANUFACTURE The composites were prepared manually with treated and untreated banana fibers with unidirectional (fibers of 200 mm length) or random orientation (fibers of 30 mm length). The matrix phase was composed of an epoxy resin (Renlam M) and hardener (Renlam HY 951) provided by Huntsman Brazil. The composites were manufactured using a metallic mold of 200 x 200 x 1mm in size, see Fig. 1a. The short and long fibers were put, Fig. 1b and c, and the epoxy resin added and distributed. The process was completed in a uniaxial press under a pressure of 1 MPa at a temperature of 100ºC for 40 min.

a.)

b.)

c.)

Figure 1: Composite manufacture; a.) mold, b.) random orientation of short fibers c.) alignment of long fibers.


2.2 DENSITY MEASUREMENT Archimedes’ principle was used to determine the apparent density of the fibers, of the epoxy resin matrix and of the composites produced thereof. For each sample an average of 10 measurements was carried out. The standard ASTM D792 [11] was used for fiber and composites, respectively. 2.3 TENSILE STRENGTH The tensile strength and Young’s modulus of the banana fibers and epoxy resin were conducted according to the standards ASTM D3822-07 [12] and ASTM D638-03 [13]. The fibers and the polymer were tested at a constant crosshead speed of 2 mm/min. The mechanical properties, tensile and flexural strength and young’s modulus, of the composites were determined following the recommendations of the ASTM D790 [14] and ASTM D3039/D3039M [15] standards. A constant crosshead velocity of 2 mm/min was adopted for both tests. All tests were performed using a Shimadzu Ag-X Plus testing machine with a load capacity of 100 kN.

2.4 MICROSTRUCTURAL ANALYSIS Fourier transformed infrared spectroscopy (FT-IR) was used to identify chemical groups of fibers, before and after alkaline treatment, by the characteristic absorption of wavelengths by the bonds, Perkin Elmer, model Spectrum GX/2000. The spectra were obtained in the transmission mode on KBr pellets with a resolution 4cm-1. The morphology was analyzed by scanning electron microscopy, SEM, Hitachi TM-3000 in backscattered mode. 3. RESULTS The physical and mechanical properties of the fibers, in their natural state and after treatment with Na2CO3 solution, of the pure epoxy resin material and of the composites produced thereof are shown in Table 1. Long fiber reinforced composites are abbreviated by LFC, whereas short fiber reinforced composites are named SFC, both with their respective fiber amount in weight percentage, i.e. LFC 40 represents a composite reinforced with 40 wt-% of long fibers. According to the results listed in Table 1 the treatment of the banana fibers with sodium carbonate resulted in an increase of the tensile strength of 79.61% from 281.39 to 505.41 MPa, and the elastic modulus increased 28,01% from 13.64 to 17.46 GPa, respectively. The fiber diameter was determined by SEM observations of 10 samples. The increase in tensile strength observed in this work by the Na2CO3 treatment was higher than that reported by Merlini et. al. [16] who used a 10 wt% NaOH solution, indicating that the treatment with Na2CO3 is less aggressive and improves fiber strength, see Fig. 2. The FTIR analysis of the fibers before and after treatment, Fig. 3, reveals that hemicelluloses has been removed as indicated by the absence of the band at 1726 cm-1, which is attributed to the vibrational stretching of unconjugated C=O groups of hemicellulose [17, 18]. Furthermore, a slight decrease of the band at 1274 cm-1, related to C-H bonds of lignin, has been observed, suggesting a reduction of the lignin content. These findings are in agreement with findings reported by Merlini et. al. [16]. According to Gassan and Bledzki [17], the removal of lignin and hemicelluloses facilitates the rearrangement of fibrils axis of tensile deformation, resulting in an increased tensile strength. The hemicelluloses removal turns the interfibrillar region less dense and less rigid. In consequence, the fibrils are capable of rearranging themselves along the direction of stress application resulting in an increased load capacity of the treated fibers.


Table 1 - Physical and mechanical properties of fiber, matrix and composites Elastic Modulus

Tensile Strength

Density

(GPa)

(MPa)

(g/cm3)

Raw fiber

13.64

281.39

0.18

Treated fiber

17.46

505.41

0.17

Matrix phase

1.41 ± 0.22

31.58 ± 4.19

1.10

LFC30

6.89 ± 0.40

65.20 ± 20.03

0.73

Untreated

LFC40

7.80 ± 1.33

51.98 ± 37.36

0.70

fibers

SFC30

2.47 ± 0.06

18.72 ± 5.19

0.77

SFC40

3.04 ± 0.18

29.71 ± 3.83

0.66

LFC30

7.52 ± 0.85

70.47 ± 11.96

0.66

Treated

LFC40

8.64 ± 1.46

84.97 ± 19.98

0.73

fibers

SFC30

3.91 ± 0.73

28.10 ± 20.03

0.82

SFC40

3.12 ± 0.30

25.68 ± 18.68

0.74

Setup

a.)

b.)

Figure 2 - Banana fibers: (a) untreated and (b) after treatment. 1370

Transmittance

Raw

1726

Treatment Na2CO3

1247

1646

1261

2900

4000

3500

3000

2500

2000

Wave number (cm

1500 -1

1000

500

)

Figure 3 - FTIR spectra of raw and treated banana fiber (Musa balbisiana). The density of the fibers varied from 0.17 to 0.18 g/cm3 and of the composites between 0.66 to 0.82g/cm3, depending on the amount of fibers added, that is higher amounts of low density fibers


result in lower densities of the composites. No significant variations depending on the fiber length and/or fiber treatment have been observed. The results presented in Table 1 reveal that only long and orientated fibers resulted in an improvement of the tensile strength of the epoxy resin polymer matrix, while the tensile strength of short fiber reinforced composites was lower than that of the pure matrix. In the case of 40% of long fibers the strength increased 269% from 31.58 to 84.97 MPa, while 30% of long fibers still resulted in an increase of 223% to 70.47 MPa. Long, but untreated fibers also increased the tensile strength, although in a less degree. In this case the strength improvement measured was 206 and 165% for additions of 30 and 40% of fibers, respectively. This behavior was expected considering the higher tensile strength of the treated fibers. On the other hand the strength of unaligned short fibers did not result in an improvement of the tensile strength, independent of using treated or untreated fibers and also independent of the amount added. This behavior can be explained by the increase in the composites’ porosity. Defects of the matrix by insufficient flow of the polymer around the fibers cause voids at the fiber-matrix interface leading to easy cracking even under low forces [18], see Fig. 4. The strength reduction for short, treated fiber composites is lower, 11%, indicating a better bounding of the fibers to the matrix.

a.)

b.)

Figure 4 – Failure modes of composites a.) short, randomly orientated fibers and b.) long, aligned fibers. The reinforcement of the epoxy resin matrix with banana fibers resulted in an increase of the elastic modulus. Short fibers were less effective as long fibers, for example 8.64 GPa of samples LFC40 and only 3.12 GPa of samples SFC40. Using treated fibers results in slightly higher modulus, compare the modulus of samples LFC40 with treated fiber of 8.64 GPa to 7.80 GPa of composite LFC40 with untreated fibers. Furthermore, a higher amount of fibers (30 or 40 wt-%) also reflected in a higher modulus, compare sample LFC40 with a modulus of 8.64 GPa to 7.52 GPa of sample LFC30.

4. CONCLUSIONS The treatment of the banana fibers with 10 wt-% sodium carbonate solution was efficient in removing lignin and hemicelluloses, resulting in an increased tensile strength. The strength of epoxy resin based composites reinforced with long aligned banana fibers resulted in a significantly increased tensile strength of up to 270% (40% of treated fibers), while the incorporation of short fibers reduced the strength due to the formation of more voids in the composite. The elastic modulus of all composites prepared increased with the incorporation of banana fibers. In this case, treated fibers also resulted in a higher elastic modulus, confirming the efficiency of the treatment.


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