mechanical behavior of sisal/epoxy composite ...

2ND BRAZILIAN CONFERENCE ON COMPOSITE MATERIALS – BCCM2 São José dos Campos-SP, September 15-18, 2014

MECHANICAL BEHAVIOR OF SISAL/EPOXY COMPOSITE PROCESSED BY RTM Andressa C Milanese1*, Thatiane Brocks2, Sergio R. Montoro2, Maria Odila H. Cioffi2 and Herman J. C. Voorwald2 1 Department of Civil Engineering, UNESP – Univ Estadual Paulista, FEG - Fac de Eng de Guaratinguetá, Fatigue and Aeronautical Materials Research Group, Guaratinguetá/SP, Brazil, 2 Department of Materials and Technology, UNESP – Univ Estadual Paulista, FEG - Fac de Eng de Guaratinguetá, Fatigue and Aeronautical Materials Research Group, Guaratinguetá/SP, Brazil * Corresponding author ([email protected], [email protected])

Abstract: The study of natural fibers as reinforcement in polymeric composites for technical applications on the civil and automotive areas has been a growing research subject of scientists. In these areas, there are great interests in the application of sisal fiber as substitutes for glass fibers, motivated by potential advantages of weight saving, lower raw material price, and ecological advantages of using green resources which are renewable and biodegradable. The use of epoxy resin reinforced with sisal fabric is being considered to application as reinforce of degraded timber structures. The composite was processed by resin transfer molding (RTM) at room temperature and sisal fibers were thermally treated in an oven at 60ºC for 72h. This research covers the mechanical properties by tensile test and flexural test that uses three point loading system. Experimental results showed a tensile strength at maximum load of 36.1 ± 1.2 MPa with an elongation corresponding to 2.3% and elastic modulus of 2.19 GPa while flexural strength at maximum load of sisal/epoxy composite is 74.5 ± 6.8 MPa with an flexural strain corresponding to 3.5% and an elastic modulus in bending of 3.86 GPa.

Keywords: sisal/epoxy composite, resin transfer molding (RTM), natural fiber, tensile strength, flexural test.

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1 Introduction Composites were first employed of structural projects in the aeronautical industry, but with the advances in processing techniques and the necessity of reduce the materials costs, composites began to be used on the nautical, automobile, construction sectors and medical area [1]. However, the high demand of these materials and the creation of environmental laws in regards to the use and disposal of synthetic resins and fibers, from the 90s, became indispensable the enhancement and development of less pollutant processing techniques that uses raw materials from natural sources and, consequently, with lower cost involved. This research aims to develop a polymeric composite formed by epoxy matrix and sisal fibers via resin transfer molding (RTM) process, and study their mechanical behavior. The study of sisal fiber as raw materials in the production of composites contributes to environmental preservation, once this material is derived from natural, biodegradable and renewable source and the purpose of using the RTM process is to minimize voids commonly found in composites processed by compression, with the control of temperature and flow rate of the injected resin, emission of volatile, as well as, the curing temperature. Natural fibers, classified as lignocellulosic fibers, like curauá, coir, jute, sisal, ramie, bagasse from sugarcane, hemp, flax, banana and pineapple have been studied as reinforcement material in composites. The natural fibers present innumerable advantages, such as low specific mass, low superficial damage in equipment as extrusions, easy handling, biodegradability having its origin from a renewable source, being a thermal, electric and acoustic insulator, aesthetic aspects, non-toxicity, and besides having a low cost when compared with synthetic fibers [2-4]. Brazil has a great production of sisal and it is exported to the entire world, with a production of 283.141 tons, in 2011. It is also responsible for 66% of the worldwide production, exporting the majority of its production, about 70% in 2011, for China, Europe and United States of America. The main producing states in Brazil are Bahia – 96.8%, Paraíba – 2.65%, Ceará – 0.35% and Rio Grande do Norte – 0.20%, the semiarid region [5,6]. Sisal fiber is a vegetal fiber that is extracted from Agave sisalana perrine leaves, monocotyledon originated from Mexico. There are now 57 species of sisal catalogued [2,7]. Each leaf of sisal provides,

in weight, 4% of staple fibers, 0.75% cuticle, 8% dry matter and 87.25% water [2,8]. The fibers are removed for mechanical process after the leaves are cut because dry fibers adhere to the pulp [7]. The microstructure of the natural fibers is constituted from cellulose fiber, reinforcing the amorphous hemicellulose and lignin matrices. These celluloses consist of microfibrils dispersed all along the length of the sisal fiber. The sisal fibers generally contain 60-80% cellulose, 5-20% lignin and 5-20% moisture [2] and their characteristics depend on the properties of each constituent besides their extraction source, age, etc [8,9]. The resin transfer molding (RTM), according Alves [1], is performed for the composites processing of polymer matrix reinforced with fibers, for highperformance applications in aeronautics, naval, automotive, civil, biomedical and sports industries. This process, consists of introduce under pressure the liquid resin into a rigid closed mold that is made of two parts and contains the dry fibers, in form of mat or fabric [10-12]. There are many reports about sisal fibers composites, Li [13] compared the mechanical strength of vinylester matrix reinforced with sisal fabric treated by permanganate, molded via compression and RTM. Microstructures analyzes revealed a lot of voids on the composites molded by compression instead of RTM, explaining the smaller tensile, flexural and impact strength when compared to the RTM process. Composites of epoxy matrix and unidirectional sisal fibers processed by RTM were studied by Oksman et al. [10]. Comparing the mechanical properties of this natural composite with epoxy/glass fibers composites, it was found that the specific modules have similar values, but tensile strength is equivalent to 27% of the tensile for composites reinforced with glass fibers. The researchers Sreekumar et al. [14] produced composites with short sisal fibers and polyester via RTM and analyzed their dynamic-mechanical behavior. Experimental values showed better interaction between fiber and matrix for a fiber volume of 40% when compared with 20% and 30%. Pothan et al. [15] compared the mechanical strength of sisal fabric/polyester composite by RTM when the fiber treatment were varied. The higher tensile strength was to the composite with untreated fiber, followed by fabrics treated with silane, thermally and NaOH, respectively. So, the main objective of this paper is to characterize mechanical behavior of sisal/epoxy composite manufactured via RTM, by tensile and flexural tests. 2


2 Materials and Methods 2.1 Fiber Woven sisal fabric used as reinforcement was obtained from the Northeast region of Brazil, shown in Fig. 1, and received in form of plain weave. The fabric presents 2.4 millimeters of thickness, with a fiber diameter about 100 - 200 μm and a mesh of 947.3 ± 74.0 g/m2. To modify the fiber surface structure in order to enhance the bond strength between fiber and matrix and to reduce the water absorption of sisal fiber, thermal or hydrothermal treatment was used. The fabrics were washed in boiling water and thermally treated in the oven at 60oC for 72 hours before molding.

the process ranged 62 to 124 kPa with an injection time at about 30 min. 2.5 Tensile test Tensile specimens of composites were prepared in agreement to the ASTM D3039 [16] while resin specimens were prepared according to ASTM D638 [17]. Tensile tests as well as flexural tests of resins and composites were performed using a universal machine, INSTRON, model AG-X at a load-cell of 500 Kgf and samples were carried out at room temperature. Tensile specimens of epoxy resin and sisal/epoxy composite were tested at a rate of 5 mm/min and 2 mm/min, respectively. Minimum of five specimens per test condition were tested. Elongation was calculated by the following Eq. (1):  (%) 

Fig.1. Plain weave sisal fabric

L  L0 L  100   100 L0 L0

(1)

Where  is the elongation (%), L is the final length and L0 is the original length.

2.2 Matrix

2.5 Flexural test

The bi-component epoxy resin used is a low viscosity system that cures at room temperature formed by the reagents RL3135-MV and EL3135-M (cycloaliphatic amine adduct hardener), manufactured by Polipox – Indústria e Comércio Ltda. After mixing, the components present a pot life of 110 min at 25ºC and a density of 1.22 g/cm3 after the cure. Neat epoxy resin specimens was molded between two glass plates with internal silicon frame of 3.2 mm in thickness and Frekote 700NC product was used as a demolding agent. The polymerization method was similar to the applied with the composite.

Three point bending tests of resin and composite were performed according to the ASTM D790 standard [18] and a minimum of five specimens per test condition was tested. Sisal/epoxy laminates used a crosshead speed of 1.8 mm/min, while epoxy resin used a crosshead speed of 1.6 mm/min. The ratio used between the supports distance and the thickness of samples in the test was 16. Flexural stresses tested are calculated following Eq. (2):

2.3 Composite Sisal/epoxy composites were prepared with 39.4% of reinforcement by RTM at room temperature. Laminate thickness is the equivalent of 4.4 mm and composed by two fabric layers. The composite was demolded after 24 hours and submitted at 100 oC for 4 hours to accelerate the cure. 2.4 RTM process Composites were produced into a metallic mold via RTM, without vacuum, using the equipment Radius 2100cc RTM Injector. The injection pressure during

f 

3 P  L 2bd2

(2)

Where f is the stress in the outer fibers at midpoint (MPa), P is the load (N), L is the support span (mm), b is the width of beam tested (mm) and d is the depth of beam (mm). While, the deformation, in percentage, is calculated according to Eq. (3):

f 

6 Dd 100 2 L

(3)

Where f is the strain in the outer surface (mm/mm) and D is the maximum deflection of the center of the beam (mm). 3


3 Results and Discussion 3.1 Tensile behavior Table 1 presents tensile data of epoxy resin and sisal/epoxy composite. Average of tensile strength at yield to the epoxy resin is 39.8 MPa, while the elongation is 2.33% and an elastic modulus of 2.13 GPa. The values of tensile strength at yield vary between 39.8 and 42.5 MPa presenting a 12.1% of variation. It was observed that none of the specimens showed flow before rupture, so the epoxy resin behaves as a brittle material that breaks without plastic deformation. Compared with the values of the epoxy resin formulated with 62% diglycidyl ether bisphenol-A and 38% of ancamine 2143, presented by ASM Handbook [19], that presents a tensile strength of 51 MPa, elongation of 2.8% and elastic modulus in tension of 2.98 GPa, the epoxy resin used has a lower strength, elongation and tensile elastic modulus. Table 1. Tensile data of resin and composite Material Properties Tensile strength at yield (MPa) ± SD CV (%) Elongation at yield (%) ± SD CV (%) Elongation at break (%) ± SD CV (%) Elastic Modulus (MPA) ± SD CV (%)

Resin

Composite

39.8 ± 4.8

36.1 ± 1.18

12.1

3.28

2.33 ± 0.33

2.32 ± 0.14

14.2

6.16

2.33 ± 0.33

2.41 ± 0.14

14.2

5.89

2132.8 ± 45.2

2189.7 ± 93.2

2.12

4.26

SD – Standard deviation CV – Coefficient of variation

Tensile strength at yield to sisal/epoxy composite is 36.1 MPa, while the elongation at yield and at break is 2.32% and 2.41%, respectively. The composites presents an elastic modulus of 2.19 GPa and the values of tensile strength at yield vary between 34.1 and 37.4 MPa presenting only 3.3% of variation. It was observed that, in most cases, the composite samples showed the value of tensile strength at yield followed by a reduction of the same. After that, a short recovery was observed until to the reach tensile at break. This reduction is due to rupture of the matrix and some fibers without complete breakage of the fibers, and these still suffer from a small deformation before the final rupture.

Analyzing the tensile strength at yield values of resin and composite (Table 1) and considering the standard deviation, the composite presents the same value of tensile strength than neat resin, therefore, there was not found an increase of the tensile strength with the addition of sisal fiber into resin, but there was a decrease in the total amount of plastic material used, which can be considered as an environmental contribution, considering the long time required for the process of synthetic materials (matrix) degradation. The main differences with the addition of sisal fiber into the epoxy matrix was a small increase in the elongation before rupture, from 2.33% to 2.41%, and the reduction in the value of the standard deviation of tensile strength. Analyzing research that employ sisal fiber as reinforcement of the epoxy matrix, Patra e Bisoyi [20] used epoxy resin with random short sisal untreated and treated with ethanol and benzene, molded by compression, and they observed a tensile strength of 25 MPa and 33 MPa, respectively. Song, Mun and Kim [21] studied composites via RTM, formed with epoxy/plain-weave sisal untreated, treated with permanganate and treated with silane, found a tensile strength of 46 MPa, 52 MPa and 31 MPa, respectively. Boopalan, Umapathy and Jenyfer [22] researchers studied composites molded via compression of epoxy/mat sisal untreated and with alkali treatment (20% NaOH) whose tensile strength is 46 MPa and 76.4 MPa, respectively. Kim and Seo [23] studied composites of epoxy/plain-weave sisal treated with silane via RTM, they found a tensile strength around 44 MPa. Therefore, the composite produced in this work showed a value of tensile strength close to the values of composites that employ the sisal fiber with epoxy matrix, studied by other researchers. 3.2 Flexural behavior Table 2 presents flexural data of epoxy resin and sisal/epoxy composite. Flexural strength to the epoxy resin is 89 MPa, while the strain at maximum load and at break is 4.23% and 5.56%, respectively. All epoxy resin specimens show fracture after to suffer plastic deformation and to reach failure stress. It was observed that during the bending tests the fracture of the samples started with the development of cracks from the part that is under traction (bottom sample) and propagated through the sample thickness, with complete rupture.

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Table 2. Flexural data of resin and composite Material Properties Flexural strength (MPa) ± SD CV (%) Flexural strain at yield (%) ± SD CV (%) Flexural strain at break (%) ± SD CV (%) Elastic Modulus (MPA) ± SD CV (%)

Resin

Composite

89.0 ± 1.77

74.5 ± 6.80

1.99

9.13

4.23 ± 0.17

3.53 ± 0.86

4.06

24.4

5.56 ± 0.57

-

10.2

-

2846.7 ± 223.0 3864.8 ± 118.8 7.83

3.07

SD – Standard deviation CV – Coefficient of variation

Flexural strength to the composite is 74.5 MPa, while its deformation at yield is 3.53% and modulus of elasticity in bending of 3.86 GPa. Flexural strength values range from 65.4 MPa to 83.2 MPa with a coefficient of variation of 9.13%. It was observed during the flexural tests that all the composite specimens did not break at strains of up to 5 % (Fig. 2).

Fig.2. Side sample image of sisal/epoxy composite after 6% of deformation From Fig. 2, it is noted that the rupture of the sample starts with the development of cracks in the tensioned part (bottom of sample) and propagates through the thickness of it, but neither of the samples presented a total rupture. Analyzing the flexural strength of epoxy resin with sisal/epoxy composite, Table 2, there was a reduction of 14.5 MPa in flexural strength with the addition of two layers of sisal fabric into neat resin. Otherwise, there was a 39% increase in flexural elastic modulus, besides the increase of deflection, both favorable factors to application of the sisal/epoxy composite as reinforcement in timber structures. Observing studies that employ sisal fibers as reinforcement of the epoxy matrix, Patra and Bisoyi [20] use epoxy/random short sisal untreated and treated with ethanol and benzene, via compression, and the researchers found a flexural strength of 55 MPa and 60 MPa, respectively.

In Pothan et al. article [24] who studied polyester/plain-weave sisal untreated composites via RTM, varying the viscosity of the resin 490 mPas, 420 mPa.s and 140 mPa.s, the flexural strength found were 48 MPa, 49 MPa and 50 MPa, respectively. Importantly to emphasize that for both polyester resins of 420 mPa.s and 140 mPa.s, the introduction of sisal reinforcement in the form of plain weave decreased the flexural strength, but increased the modulus of elasticity in bending. 4 Conclusions The application of sisal/epoxy composite as reinforcement of timber structures has shown viable, when considering its mechanical resistance. The composite showed superior properties and benefits to be used as reinforcement materials in structures when compared to the epoxy matrix, because with the incorporation of sisal fibers into the matrix, the fracture mode is no longer brittle, becoming ductile with plastic deformation. The addition of fibers did not change the tensile strength or tensile elastic modulus and, although decreasing the flexural strength, the presence of fibers greatly increased its deformation (deflection), showing no rupture after the maximum load. Another advantage of introduce natural fiber into resin is decreased amount of used plastic (synthetic resin), contributing to the environmental control. As Brazil is the largest producer of sisal fibers, and producer states belong to the semiarid region, the use of these fibers will collaborate with the local economy and cooperate with the generation of new jobs. As specific conclusions, were observed that: 1) Tensile strength at yield of sisal epoxy composite is 36.1 ± 1.2 MPa with an elongation corresponding to 2.3% and elastic modulus of 2.19 GPa; 2) Flexural strength at maximum load of composite was 74.5 ± 6.8 MPa with a deformation corresponding to 3.5%, and elastic modulus in bending of 3.86 GPa; 3) The epoxy resin showed higher stiffness than the sisal/epoxy composite, but composite showed higher deformation after maximum load, and higher modulus in bending. Acknowledgements The authors express their acknowledgements for the financial support provided by CAPES/PDEE process no. 4833/10-4 and CAPES/PNPD. 5


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