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Mechanical behaviour of polypropylene reinforced sugarcane bagasse fibers composites

E. F. Cerqueiraa*, C. A. R. P. Baptistab, D. R. MulinariaaUniFoa, Av. Paulo Erlei Alves Abrantes 1325 Trs Poos, Volta Redonda, Brazil

bDepartment of Materials Engineering, EEL/USP - University of So Paulo, Lorena/SP, Brazil

Abstract

Recently the interest in composite materials reinforced with natural fibers has considerably increased due to the new environmental legislation as well as consumer pressure that forced manufacturing industries to search substitutes for the conventional materials, e.g. glass fibers. This way, the objective of paper was evaluate the effect of chemical modification on mechanical properties of sugarcane bagasse fiber/PP composites. Fibers were pretreated with 10% sulfuric acid solution, followed by delignification with 1% sodium hydroxide solution. These fibers were mixed with the polypropylene in a thermokinetic mixer, and compositions with 5 to 20 wt% of fibers were obtained. The mechanical properties were evaluated by means of tensile, 3-point bending and impact tests. In addition, fracture analysis via SEM (secondary electrons mode) was performed. Results showed improve the tensile, flexural and impact strength of the composites in comparison to the polymer pure.

Keywords: Sugarcane bagasse; Polypropylene; Mechanical properties.

1. Introduction

The use of natural fibers as reinforcements for composite has attracting more interest of industries [1]. Fibers reinforced polymer composites have many applications as class of structural materials because of their ease of fabrication, relatively low cost and superior mechanical properties compared to polymer resins [2]. For example in the automotive industry, the effort to reduce weight in order to improve fuel

* Corresponding author. Tel.: +55-24-33408400; fax: +55-24-33408404. E-mail address: dudafercer@hotmail.com

doi:10.1016/j.proeng.2011.04.339

Procedia Engineering 10 (2011) 20462051

1877-7058 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11

Open access under CC BY-NC-ND license.

2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11

Open access under CC BY-NC-ND license.

E. F. Cerqueira et al. / Procedia Engineering 10 (2011) 20462051 2047

economy and to comply with tighter governmental regulations on safety and emission has led to the introduction of increasing amounts of plastics and composites materials in place of the traditionally used steels [3].

Natural fibers have different origins such as wood, pulp, cotton, bark, bagasse, bamboo, cereal straw, and vegetable (e.g., flax, jute, hemp, sisal, and ramie) [4-7]. These fibers are mainly made of cellulose, hemicelluloses, lignin and pectins, with a small quantity of extractives. Compared to glass fiber and carbon fibers, natural fibers provide many advantages, such as, abundance and low cost, biodegradability, flexibility during processing and less resulting machine wear, minimal health hazards, low density, desirable fiber aspect ratio, and relatively high tensile and flexural modulus. Incorporating the tough and light-weight natural fibers into polymer (thermoplastic and thermosetting) matrices produces composites with a high specific stiffness and strength [8]. However, the processes involved in using natural fibers as reinforcement are different from those using industrial products such as a glass and carbon fibers [9].

Also the natural fibers present some drawbacks, such as the incompatibility between fibers and polymer matrices, the tendency to form aggregates during processing and the poor resistance to moisture, reduce the use of natural fibers as reinforcements in polymers [10]. On the other hand, there are several methods of surface modifications to improve fibers and polymer matrices compatibility, which can be physical or chemical according to modification technique to reduce the hydrophilic character. Frequently treatments are bleaching, estherification, silane treatment, use of compatibilizer, plasma treatment, acetylation, alkali treatment and treatment with other chemicals [11-15].

Mulinari et al. [16] studied bleaching of cellulose fibers from sugarcane bagasse and evidenced an improve interfacial of the fibers and matrix.

The objective of this paper was to evaluate the effect of chemical modification on mechanical properties of cellulose pre-treated sugarcane bagasse fibers/PP composites. The modification the fibers were evaluated by techniques scanning electron microscope (SEM), X-ray diffractometry (XRD) and Fourier Transformed Infrared (FTIR). These fibers were mixed with the polypropylene in a thermokinetic mixer, and compositions with 5 to 20 wt% of fibers were obtained.

2. Experimental

2.1. Materials description

The fibers used in this study were manufactured by Edras Ecosistema. In nature sugarcane bagasse was pretreated with 10% sulfuric acid solution (reactor of 350 L at 120C, 10 min), followed by centrifugation with the purpose to separate the rich pentosanes solution. Extracted lignocellulosic fraction was delignified with 1% NaOH solution (reactor of 350L at 100C, 1 h) and as a result the crude pulp (cellulose fibers) was obtained being obtained. Polypropylene (PP) was provided, as pellets, by BRASKEM (Triunfo, Brazil). Its melting temperature, degree of isotacticity and density at room temperature were 173 0C, 9398% and 0.905 g.cm-3.

2.2. Chemical characterization

FTIR allows the measurements of variations in the fibers composition after chemical treatments. The chemical structure of in nature and pre-treated sugarcane bagasse was evaluated by FTIR spectrophotometer (Perkin Elmer). The samples were prepared by mixing the materials and KBr in a proportion 1:200 (w/w). For all spectra, 16 scans were accumulated with a 4 cm-1 resolution.

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2.3. Morphological characterization

Prior to SEM evaluation, the specimen was coated using gold sputtering. SEM micrographs of in nature and pre-treated sugarcane bagasse fibers were obtained using a JEOL JSM5310 field emission scanning electron microscope, operated at 15 kV.

2.4. Composites preparation

Pre-treated sugarcane bagasse fibers were mixed with the PP in a thermokinetic mixer, model MH-50H, with speed rate maintained at 5250 rpm, in which fibers were responsible for 5 to 20 wt% in the composition. After the mixture, composites were dried and ground in mill. Composites and pure PP were placed in an injector camera at 165 oC and 2 C min-1 heating rate in a required dimension pre-warm mold with specific dimensions for tensile, flexural and impact specimens.

2.5. Mechanical properties

Composites were analyzed in an EMIC testing machine (model DL2000), equipped with pneumatic claws. In the tensile tests, five specimens were analyzed, with dimensions in agreement with the ASTM D 638 standard: 19 mm width, 165 mm length and 3.2 mm thickness and 10 mm min-1 crosshead speed. In the flexural tests, a load was applied on the specimen at 2.8 mm min-1 crosshead motion rate. Five specimens were analyzed with dimensions in agreement with the ASTM D 790 standard: 25 mm width, 76 mm length and 3.2 mm thickness. The adopted flexural test was the 3-point at 1/3 points method. In the impact tests, five specimens were analyzed, with dimensions in agreement with the ASTM D 6110 standard: 12 mm with, 63.5 mm length and 12 mm thickness. It was evaluate impact strength.

3. Results

3.1. FTIR characterization

Infrared spectra of in nature and pre-treated sugarcane bagasse fibers are displayed in Figure 1.

4000 3000 2000 1000 0

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

in nature sugarcane bagasse fibers pre-treated sugarcane bagasse fibers

Abs

orba

nce (

a.u.)

Wavenumber (cm-1)

Figure 1. FTIR spectra of the in nature and pre-treated sugarcane bagasse fibers (wavenumber 400 to 4000 cm-1).

E. F. Cerqueira et al. / Procedia Engineering 10 (2011) 20462051 2049

The most visible differences between are the modifications of the signal at 2symmetrical CH groups and stretching ofConsidering the first region, the ratio betin the spectrum of the pre-treated sugarbagasse fibers. On the other hand, at the sratio between the intensities of the C=O s

3.2 Morphological characterization fibers

The SEM evaluations were appliecharacterizations. In nature sugarcane superficial layer with high percentage morphological structure of the in naturesulfuric acid solution and delignificatioextractives on surface fibers as observedsuperficial layer can improve increased th

3.4. Mechanical properties

Mechanical properties of studied commechanical properties, indicating that thepure polymer (PP). However, the amounand flexural modulus. Composites showtensile modulus compared to the PP. In theffectively contributed to mechanical promatrix interfacial bond. It is interesting tobehaviour of the composites, flexural prothose of PP. This result may reflect a developed in part of the transverse sectcondition of the fibers.

(A) Figure 2. SEM of the sugarcane bagasse fibers:

70

the spectra of in nature and pre-treated sugarcane bag2885 cm-1 and 1732 cm-1, characteristics of the strf unconjugated CO groups present in polysaccharides aween intensity of the C-H stretching band (~2900 cmrcane bagasse fibers than observed for the in naturesecond region it may be observed modifications, espectretching band (~1730 cm-1).

s

ed for in nature and pre-treated sugarcane bagabagasse fibers are shown in Figure 2(a), which eof extractives on fibers. On the other hand, a chae sugarcane bagasse occurred after the pretreatment on with NaOH solution. It was observed the remo in the Figure 2(b). It was verified also that with elim

he contact area because of the exposition of fibrils (ree

mposites are summarized in Table 2. Composites showe treatment affects the fibers-matrix interaction compnt of added reinforcement contributes to variation of

wed an increase of 16% in the tensile strength and 5his case, the higher strength and Youngs modulus of operties improvement of the composite, due to a goodo notice that, despite of the considerable discrepanciesoperties are slightly different, which means higher cobetter fibers-matrix interaction under the compressi

tion of the specimens during bending for whatever t

(B) (a) in nature; (b) pre-treated.

0 m 70

gasse fibers retching of and xylans.

m-1) is lower e sugarcane cially in the

asse fibers evidence a

ange in the with 10%

oval of the mination of entrance).

wed distinct pared to the

the tensile51% in the fibers have

d fibers and s on tensile ompared to ve stresses the surface

0 m

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Table 2. Mechanical properties of the materials

Samples Tensile strength (MPa)

Tensile Modulus (MPa)

Flexural strength (MPa)

Flexural Modulus

(MPa) PP 19.3 1.1 955.1 93.3 27.5 0.9 906 35.8

PP/FSB5% 22.9 1.4 1105.5 22.6 34.8 2.9 1047.3 234.5

PP/FSB10% 23.0 0.6 1027.1 82.9 35.5 3.6 960.7 139.2

PP/FSB20% 22.3 0.8 1442.5 68.7 37.2 2.1 1200.8 112.9

(Reinforcement in wt%)

Impact strength of composites depends of fibers, matrix, fiber/matrix interface and the test conditions. Experimental results in Table 3 may be explained by the interaction observed between fiber and matrix during the mixture process. Composites (PP/FSG10% and PP/FSG20%) presented high average values impact strength when compared to polypropylene. It was observed an increase of 45% impact strength. This fact can be explained by good interface between fibersmatrix. These interactions between fiber and matrix can be observed in the fractured composites after impact test.

Table 3. Results of materials obtained by impact test

Samples Impact Strenght (J.m-2) PP 36,1 1,1 PP/FSB5% 32,7 6,0 PP/FSB10%

PP/FSB20%

45,0 0,1 52,5 0,6

Figure 3 presets the fractured region after tests, where it was verified fibers distribution in the matrix, fibers fractured in the matrix and pull out fibers, characterizing mechanism of fragile fracture. It was also observed energy dissipation during the frictional process mechanics.

Figure 3. SEM of fracture surface of PP/F SB20% composites.

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Thus agro-residue modification was adequate for thermoplastics composites preparation.

4. Conclusions

The feasibility of utilizing the agro-residue as alternative reinforcement in thermoplastics was studied. Chemical modification of cellulose fibers from sugarcane bagasse was studied to demonstrate the effect of modification on the mechanical properties of the composites and to study the practicability of processing these agro-residue with thermoplastics. The modification of fibers from sugarcane bagasse was successfully accomplished and it was verified that effectively improves the tensile, flexural and impact strength in comparison to the polymer pure.

Acknowledgements

Authors are grateful for the research support by UniFOA.

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