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HDPE composites obtained by extrusion
Composites Science and Technology 69 (2009) 214–219
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Sugarcane bagasse cellulose/HDPE composites obtained by extrusion Daniella Regina Mulinari a,*, Herman J.C. Voorwald a, Maria Odila H. Cioffi a, Maria Lúcia C.P. da Silva b, Tessie Gouvêa da Cruz a, Clodoaldo Saron b a b
Faculdade de Engenharia de Guaratinguetá/UNESP, Av Dr. Ariberto Pereira da Cunha, 333, Pedregulho, CEP, 12516-410 Guaratinguetá/SP, Brazil Escola de Engenharia de Lorena, Universidade de São Paulo, P.O. Box 116, CEP, 12600-000 Lorena/SP, Brazil
a r t i c l e
i n f o
Article history: Received 11 April 2008 Received in revised form 8 October 2008 Accepted 8 October 2008 Available online 17 October 2008 Keywords: A. Fibers A. Polymer-matrix composites B. Mechanical properties D. Scanning electron microscopy (SEM) E. Extrusion
a b s t r a c t Natural fibers used in this study were both pre-treated and modified residues from sugarcane bagasse. Polymer of high density polyethylene (HDPE) was employed as matrix in to composites, which were produced by mixing high density polyethylene with cellulose (10%) and Cell/ZrO2nH2O (10%), using an extruder and hydraulic press. Tensile tests showed that the Cell/ZrO2nH2O (10%)/HDPE composites present better tensile strength than cellulose (10%)/HDPE composites. Cellulose agglomerations were responsible for poor adhesion between fiber and matrix in cellulose (10%)/HDPE composites. HDPE/natural fibers composites showed also lower tensile strength in comparison to the polymer. The increase in Young’s modulus is associated to fibers reinforcement. SEM analysis showed that the cellulose fibers insertion in the matrix caused an increase of defects, which were reduced when modified cellulose fibers were used. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In recent years natural fibers composites have found an increasing number of applications [1–4]. These composites have shown special interest in interior components for automotives such as seat frames, side panel and central consoles [5,6]. Advantages of natural fibers are low cost, low density, high specific properties, biodegradable and non-adhesive characteristics. The main disadvantages of natural fibers are low permissible processing temperatures, the tendency to form clumps, and the hydrophilic nature [7]. Some research works were developed using thermoplastic polymers/natural fibers showing excellent results for many applications, especially in interior components for automotives. Thermoplastics such as polyethylene (PE) [8,9], polypropylene (PP) [10,11], poly(lactic acid) (PLA) [12] and poly(vinyl chloride) (PVC) [13] have been compounded with natural fibers (such as sisal, jute, and sugarcane bagasse) to prepare composites. Sugarcane bagasse is a residue widely produced and contains cellulose (46.0%), hemicellulose (24.5%), lignin (19.95%), fat and waxes (3.5%), ash (2.4%), silica (2.0%) and other elements (1.7%) [14]. Bagasse is a vegetable fiber mainly constituted by cellulose, that is a glucose-polymer with relatively high modulus, often found as fibrillar component of many naturally occurring compos-
* Corresponding author. Tel.: +55 12 31232865; fax: +55 12 31232852. E-mail addresses: [email protected], [email protected] (D.R. Mulinari). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.10.006
ites (wood, sugarcane straw and bagasse) in association with lignin [15,16]. Methods as extrusion, compression and injection molding are used to place together fibers and thermoplastics matrix [17,18]. The high density polyethylene (HDPE) is an engineering thermoplastic used for several industrial applications due to low cost, desired mechanical properties and processing facility [8,9,19]. The combination of lignocellulosic material with thermoplastic matrix in general presents a considerable problem associated to incompatibility between the polar and hygroscopic fiber and the non-polar and hydrophobic matrix. Superficial treatments in natural fibers have been used to improve the matrix-reinforcement adhesion in composites [2]; however, it is a critical step considering that the process could degrade the strong interfiber hydrogen bonding, which holds the fibers together. Surface modification methods can be physical or chemical according to the fiber surface modification mechanism. Frequently, the methods used for surface modification are bleaching, acetylation and alkali treatment [20,21]. Studies using the presence of hydroxyl groups which are reactive and susceptible to chemical reactions have been developed to analyze weakness of the matrix–fiber interface. A chemical modification is carried out according to the insertion of non-polar groups on the fibers resulting in a hydrophobic surface compatible with thermoplastic matrices [22]. Several techniques have been used to determine possible modifications in lignocellulosic materials such as X-ray diffractometry (XRD) and thermal analysis (TG). Gomes et al. [23] studied the
D.R. Mulinari et al. / Composites Science and Technology 69 (2009) 214–219
development and effect of alkali treatment on tensile properties of curaua fiber green composites and evidenced that appropriate alkali treatment is a key technology for improving mechanical properties of cellulose-based fiber composites. Sisal fiber/high density polyethylene (HDPE) composites interface was studied and an improved method, which evaluates more accurately the natural fiber and polymeric matrices interfacial properties, was proposed. By this method two types of fiber surface modification were conducted: the chemical coupling by silane and the oxidation process using permanganate. It was observed, in both cases, an increase of fibers roughness introducing a mechanical interlocking with the matrix [9]. Coupling agents were used also by Keener et al. [24] to treat natural fiber polyolefin composites increasing significantly the interface strength, and consequently it was observed that mechanical properties as tensile strength and impact energy increased twice and three times, respectively, when compared to non-coupled blend of wood and polyethylene. Tserki et al. [25] studied the effect of acetylation and propionylation surface treatments on natural fibers and observed that the esterification phenomena decreased the hydrophilic characteristics of the natural fiber surface beyond the fiber crystalline as a result of the increase in the amorphous portion produced by the treatment. Shao et al. [26] studied the modification of carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) with molybdenum oxide and it was observed a formation of highly regularly packed polymer/molybdenum oxide. Marques et al. [27] studied titanium dioxide/cellulose nanocomposites prepared through the titanyl sulphate hydrolysis in acidic medium in the presence of cellulose fibers and it was observed that this material can be used as reinforced fibers in polymer matrix. In this research work, sugarcane bagasse cellulose/HDPE composite and sugarcane bagasse cellulose modified with zirconium oxychloride (ZrOCl28H2O)/HDPE composite were mixed by extrusion and compression molded. Mechanical and thermal characterization, X-ray diffraction and surface area measurement were obtained and compared. 2. Methods 2.1. Isolation crude cellulose from sugarcane bagasse The sugarcane bagasse was pre-treated with 10% sulfuric acid solution (reactor of 350 L at 120 °C, 10 min) to isolate the cellulose, followed by centrifugation to separate the rich pentosanes solution. Extracted lignocellulosic fraction was deslignificated with
215
1% sodium hydroxide solution (reactor of 350 L at 100 °C, 1 h) to obtain the crude pulp, which was bleached with sodium chloride. The bleached cellulose (Fig. 1b) was dried in a store at 50 °C for 12 h. 2.2. Chemical modification of sugarcane bagasse cellulose with zirconium oxychloride About 2 g of zirconium oxychloride (ZrOCl28H2O) were dissolved in 100 mL of aqueous hydrochloric acid solution (0.5 mol L1) and 5 g of cellulose were immersed in this solution. The material was precipitated with ammonium solution (1:3) at pH 10.0, under stirring, filtered under vacuum and exhaustively washed with distilled water for the complete removal of chloride ions (negative silver nitrate test). The product was dried at 50 °C for 24 h. The resulting material was designated as Cell/ZrO2nH2O [28]. 2.3. Materials Characterization Sugarcane bagasse cellulose, hydrous zirconium oxide and sugarcane bagasse cellulose modified with metallic oxide were characterized by X-ray diffractometry (XRD), Surface area measurements (BET) and Thermal analysis (TG). X-ray diffractograms were obtained in a Rich Seifert diffractometer model ISO-DEBYFEX1001. Conditions used were: radiation CuKa, tension of 30 kV, current of 40 mA and 0.05 (2h/5 s) scanning from values of 2h it enters 10–70° (2h). Surface area measurements were performed in a Quantachrome instrument model NOVA 1000 in nitrogen atmosphere. Materials were pre-treated at 50 °C for 3 h. TG curves were generated in a Shimadzu instrument model TGA-50. The experiments were carried out under continuous nitrogen flow and with a heating rate of 10 °C min1. 2.4. Preparation of composites Sugarcane bagasse cellulose and sugarcane bagasse cellulose modified with zirconium oxychloride (ZrOCl28H2O) were mixed with the polymeric matrix (HDPE) in an extruder screw, marks IMACOM, in which fibers were responsible for 10 wt% in the composition. Respective temperatures for the four different processing zones from the hopper to horizontal die of the extruder were set as 120/130/140/150 °C and the screw speed rate was maintained at 50 rpm. The extruded materials were subsequently compression molded into samples for mechanical testing. 2.5. Compression molding In an uniaxial press, containing heating elements (inferior and superior), a molding tool was placed, containing the materials for compression (cellulose (10%)/HDPE composite, Cell/ZrO2nH2O (10%)/HDPE composite and high density polyethylene (HDPE)). Both composites and HDPE plates were obtained at 150 °C during 5 min at 5.000 kgf. Cellulose (10%)/HDPE composite, Cell/ZrO2nH2O (10%)/HDPE composite and high density polyethylene (HDPE) plates were cut in the necessary dimensions for the mechanical tests. 2.6. Tensile tests
Fig. 1. X-ray: (a) pure cellulose; (b) Cell/ZrO2nH2O.
Mechanical tests were carried out according to ASTM D-638 specification. Five specimens from each composition were tested in an INSTRON universal-testing machine (model-8801), equipped with pneumatic claws at a cross-head speed of 4.5 mm min1.
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D.R. Mulinari et al. / Composites Science and Technology 69 (2009) 214–219
2.7. Scanning electron microscopy The specimens submitted to tensile tests were cut and the composite intact fracture surface was analyzed in LEO 1450 V scanning electron microscopy with tungsten filament operating at 20 kV, employing low vacuum technique and secondary electron detector. 3. results and discussion The incorporation of oxide on sugarcane bagasse cellulose surface can be described by the following reaction:
Cell þ ZrOCl2 8H2 O þ HCl þ NH4 OH ! Cell=ZrO2 nH2 O þ NH4 Cl
ð1Þ
Main differences between sugarcane bagasse cellulose and modified cellulose with zirconium oxychloride (ZrOCl28H2O) can be observed in the X-ray diffractogram technique presented in Fig. 1. It is possible to observe a major diffraction peak for 2h ranging between 22° and 23°, which corresponds to the cellulose (0 0 2) crystallographic planes. The spectrum corresponding to the non-modified sugarcane bagasse cellulose fibers shows diffraction peaks at the following 2h angles: 15.7° and 22.8°. For modified sugarcane bagasse cellulose the same peaks were observed at 15.9° and 22.8°. The presence of the peaks at 15° and 22° is an evidence of the modification on fiber. These peaks indicate an increase of the interplanar distance in relation to the modified fiber. This behavior occurs due to the generation of disorder when fibers are modified. The projection substituting groups along the axis is associated with an increase in the interfibrillar distance [29]. Patterns for both materials are similar; however, non-modified fiber is less crystalline than the modified one. Crystallinity index (CI), which is a measurement of the amount of crystalline cellulose with respect to the global amount of amorphous materials, was evaluated using Segal empirical method according to following Eq. (2):
CI ð%Þ ¼
I0 0 2 Iam 100 Iam
ð2Þ
where I0 0 2 is the maximum intensity of the 0 0 2 lattice reflection of the cellulose and Iam is the maximum intensity of X-ray scattering broad band due to the amorphous part of the sample. According to this method, non-modified and modified fibers presented 47% and 53% of crystallinity, respectively. The amount of oxide incorporated into cellulose was determined by calcining 0.3 g of sample (Cell/ZrO2nH2O) at 1073 K in air for 3 h and by weighting the residue. The quantity of oxide was calculated and results are presented in Table 1. These data were confirmed from surface area measurements. Comparing the specific superficial areas of the pure cellulose and Cell/ZrO2nH2O material, it was observed an increase. The oxide deposition is an effective phenomena associated to the increase of specific surface area, SBET which was from 0 m2 g1 (see Table 1) for pure cellulose to 36 m2 g1 for the Cell/ZrO2nH2O material [18]. Thermal analyses (TG) also confirm these results which are indicated in the Table 2, and presents values of mass loss and residue in the respective range temperature.
Table 2 Results of the thermogravimetric (TG) curves of the materials, with the mass losses (m), in the respective range of temperature (DT) and its respective residues (R). Material
m (%)
DT (°C)
Cellulose
4.7 83.9 8.6
40–200 200–500 500–800
2.8
Cell/ZrO2nH2O
7.1 77.7 2.9
40–200 200–500 500–800
12.3
Data (Table 2) indicate that the TG curve of the pure cellulose presents higher mass loss in the range temperature 200–500 °C. The amount of residue in the Cell/ZrO2nH2O material increased from 2.8% to 12.3% due to the presence oxide on the cellulose surface. TG curves are presented in Fig. 2. Curve A corresponds to sugarcane bagasse cellulose and curve B to sugarcane bagasse cellulose modified with metallic oxide (Cell/ ZrO2nH2O). Pure cellulose presents two decomposition phases: the first one at 300 °C corresponds to degradation temperature and the second one at 380 °C corresponds to complete temperature decomposition. Cell/ZrO2nH2O material shows two different phases compared to pure cellulose: at 260 and 338 °C. This decrease of the temperature compared to pure cellulose is attributed to the presence of oxide particles on the cellulose surface, indicating a strong interaction between hydrous zirconium oxide and cellulose fibers [28]. Cell/ZrO2nH2O material was obtained under determined condition, appropriated (acid medium at the beginning and basic medium at the end) to avoid the degradation of the cellulose fiber because it is known that the cellulose degradation occurs at pH 10 [30]. Therefore, during the process care was taken in order not to reach this pH. Toledo et al. [31] studied the antimony (III) oxide film on a cellulose fiber surface and observed similar results. Table 3 indicates the mechanical properties of these materials involved in this research, in special the effect of chemical modification of sugarcane bagasse cellulose with zirconium oxychloride (ZrOCl28H2O) reinforced HDPE. Composite reinforced with non-modified cellulose presents lower average values for tensile strength and elongation at break compared to high density polyethylene. On the other hand, composite reinforced with modified cellulose shows significant decrease in elongation at break compared to high density polyethylene and cellulose (10%)/HDPE composite. Cell/ZrO2nH2O/HDPE composite presented higher tensile strength compared to the cellulose/HDPE composite, but still lower than high density polyethylene. An interesting increase in tensile
Table 1 Quantity of ZrO2nH2O incorporated on cellulose surface. Samples
ZrO2nH2O incorporated (wt%)
SBET (m2 g1)
Cellulose Cell/ZrO2nH2O
– 3.66
0 36
R (%)
Fig. 2. TG curves: (A) pure cellulose; (B) Cell/ZrO2nH2O.
D.R. Mulinari et al. / Composites Science and Technology 69 (2009) 214–219 Table 3 Mechanical properties of the materials obtained by compression molding. Materials
Elongation at break (tensile) (%)
Tensile strength (MPa)
Tensile modulus (MPa)
High density polyethylene (HDPE) Cellulose/HDPE composite Cell/ZrO2nH2O/HDPE composite
1.96 ± 0.087
16.7 ± 0.15
850.9 ± 28.2
1.62 ± 0.097
14.4 ± 0.58
880.1 ± 63.5
1.2 ± 0.185
15.6 ± 1.11
1324.2 ± 211.0
Reinforcement in wt%.
modulus occurred as a consequence of the oxide particles on cellulose fibers. Experimental data in Table 3 also showed a poor interaction between fibers and matrix during the mixture process. Composites were obtained with homogeneous distribution, but with agglomerations in some points caused by inefficient fibers dispersion inside matrix. This agglomeration of reinforcement was responsible by decrease of the tensile strength compared to the high density polyethylene. Composites reinforced with modified cellulose presented better tensile strength and adhesion between fiber and matrix than nonmodified cellulose, due to the agglomerations decrease. Luz et al. [32] observed similar results by mechanical testing and microstructural analysis of sugarcane bagasse fibers reinforced polypropylene composites. Fig. 3 shows the presence of pull out in the cellulose/HDPE composite fracture surface by the SEM technique. The fracture of the specimen was caused by the presence of agglomerates, which is considered as defect in the material. Image
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analyses method was used in this research to quantify the reinforcement particles (or fibers). Analyzing Fig. 3a and b it was observed pull out caused by low adhesion, which acts as stress concentrator. However, it was not observed the same considering Fig. 3c and d. The amount of agglomerates in the composites reinforced with modified cellulose decreased, which resulted, as a consequence, the decrease in ductility. The reduction in the elongation at break for this condition compared to the high density polyethylene is associated to defects generated in the material after fibers insertion. However, the addition of modified cellulose in the matrix reduced these defects, confirming the interfacial improvement between fibers and matrix. The mechanical behavior of composites is strongly dependent on the microstructural distribution of the reinforced particles in matrix. Particle agglomerates act as defects or stress concentration sites for crack nucleation. The ability to quantify the size and distribution of particle agglomerates and their effect on crack growth behavior is extremely important [33]. Tang et al. [34] studied particle distribution characterization by multi-scalar analysis of area fractions (MSAAF) technique to analyze the particle spatial distribution of composite samples obtained by vacuum hot pressing (VHP). The basis principle of MSAAF technique is to characterize the spatial heterogeneity in composite microstructures by obtaining statistical information about the variability of reinforced particle area fractions over various length scales by quantitative image analyses methods. A gray-scale image often contains only two levels of significant information, namely the foreground level constituting objects of interest and the background level against which the foreground is discriminated [35].
Fig. 3. Scanning electron microscopy of fractured surfaces of composites: (a) cellulose/HDPE composite; (b) cellulose/HDPE composite with higher magnification of the limit region between fiber and matrix; (c) Cell/ZrO2nH2O/HDPE composite; (d) Cell/ZrO2nH2O/HDPE composite with higher magnification of the limit region between fiber and matrix.
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Table 4 Reinforcement particle area fractions obtained for statistical analyses.
References
Materials
Average
Cellulose/HDPE composite Cell/ZrO2nH2O/HDPE composite
46.6 ± 7.8 33.2 ± 5.5
In this article 26 images in random regions of each sample (Cellulose/HDPE composite and Cell/ZrO2nH2O/HDPE composite) were captured to establish the reinforcement particle area fractions obtained by SEM (Fig. 3) utilizing image analyses method. These area fractions were classified as clear regions (lesser values of gray levels) and less clear regions (higher values of gray levels). The clearest regions are associated to higher reinforcement particle and the least clear regions mentions to lesser reinforcement particle. Table 4 shows results obtained for statistical analyses of composites. The area fraction obtained for Cell/ZrO2nH2O/HDPE composite decreases 13.4%, indicating an increase of filled regions by reinforcement particle. Data of Table 4 indicate a tendency of non homogeneous distribution in the reinforcement particle. Therefore, these results showed the fulfilling regions, confirming that modification of the sugarcane bagasse cellulose with zirconium oxychloride improved the adhesion between fibers and matrix. This way, the modification of sugarcane bagasse cellulose and compression molding were adequate for thermoplastic composites preparation. 4. Conclusions The modification of sugarcane bagasse cellulose with zirconium oxychloride was successfully accomplished and it was verified that effectively improves the tensile strength compared to non-modified sugarcane bagasse cellulose. The preparation of composites using non-modified sugarcane bagasse cellulose presented agglomerations. The agglomerations and/or the non-homogeneity distribution of fibers are defects in the material that directly interfere in the mechanical properties, harming the production of a high resistance material. The modification of sugarcane bagasse cellulose reduced the composites elongation to 26% compared to non-modified sugarcane bagasse cellulose; on the other hand the tensile modulus increased 50%. Therefore the molding process using extrusion and hydraulic press was appropriate. Acknowledgements The authors express their acknowledgements to CAPES for the financial support. Appendix A BET Cell/ ZrO2nH2O Cellulose HDPE MSAAF SEM VHP XRD TG
surface area measurement sugarcane bagasse cellulose modified with hydrous zirconium oxide sugarcane bagasse cellulose non-modified high density polyethylene multi-scalar analysis of area fractions scanning electron microscopy vacuum hot pressing X-ray diffractometry thermal analysis
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Sugarcane bagasse cellulose/HDPE composites obtained by extrusion Daniella Regina Mulinari a,*, Herman J.C. Voorwald a, Maria Odila H. Cioffi a, Maria Lúcia C.P. da Silva b, Tessie Gouvêa da Cruz a, Clodoaldo Saron b a b
Faculdade de Engenharia de Guaratinguetá/UNESP, Av Dr. Ariberto Pereira da Cunha, 333, Pedregulho, CEP, 12516-410 Guaratinguetá/SP, Brazil Escola de Engenharia de Lorena, Universidade de São Paulo, P.O. Box 116, CEP, 12600-000 Lorena/SP, Brazil
a r t i c l e
i n f o
Article history: Received 11 April 2008 Received in revised form 8 October 2008 Accepted 8 October 2008 Available online 17 October 2008 Keywords: A. Fibers A. Polymer-matrix composites B. Mechanical properties D. Scanning electron microscopy (SEM) E. Extrusion
a b s t r a c t Natural fibers used in this study were both pre-treated and modified residues from sugarcane bagasse. Polymer of high density polyethylene (HDPE) was employed as matrix in to composites, which were produced by mixing high density polyethylene with cellulose (10%) and Cell/ZrO2nH2O (10%), using an extruder and hydraulic press. Tensile tests showed that the Cell/ZrO2nH2O (10%)/HDPE composites present better tensile strength than cellulose (10%)/HDPE composites. Cellulose agglomerations were responsible for poor adhesion between fiber and matrix in cellulose (10%)/HDPE composites. HDPE/natural fibers composites showed also lower tensile strength in comparison to the polymer. The increase in Young’s modulus is associated to fibers reinforcement. SEM analysis showed that the cellulose fibers insertion in the matrix caused an increase of defects, which were reduced when modified cellulose fibers were used. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In recent years natural fibers composites have found an increasing number of applications [1–4]. These composites have shown special interest in interior components for automotives such as seat frames, side panel and central consoles [5,6]. Advantages of natural fibers are low cost, low density, high specific properties, biodegradable and non-adhesive characteristics. The main disadvantages of natural fibers are low permissible processing temperatures, the tendency to form clumps, and the hydrophilic nature [7]. Some research works were developed using thermoplastic polymers/natural fibers showing excellent results for many applications, especially in interior components for automotives. Thermoplastics such as polyethylene (PE) [8,9], polypropylene (PP) [10,11], poly(lactic acid) (PLA) [12] and poly(vinyl chloride) (PVC) [13] have been compounded with natural fibers (such as sisal, jute, and sugarcane bagasse) to prepare composites. Sugarcane bagasse is a residue widely produced and contains cellulose (46.0%), hemicellulose (24.5%), lignin (19.95%), fat and waxes (3.5%), ash (2.4%), silica (2.0%) and other elements (1.7%) [14]. Bagasse is a vegetable fiber mainly constituted by cellulose, that is a glucose-polymer with relatively high modulus, often found as fibrillar component of many naturally occurring compos-
* Corresponding author. Tel.: +55 12 31232865; fax: +55 12 31232852. E-mail addresses: [email protected], [email protected] (D.R. Mulinari). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.10.006
ites (wood, sugarcane straw and bagasse) in association with lignin [15,16]. Methods as extrusion, compression and injection molding are used to place together fibers and thermoplastics matrix [17,18]. The high density polyethylene (HDPE) is an engineering thermoplastic used for several industrial applications due to low cost, desired mechanical properties and processing facility [8,9,19]. The combination of lignocellulosic material with thermoplastic matrix in general presents a considerable problem associated to incompatibility between the polar and hygroscopic fiber and the non-polar and hydrophobic matrix. Superficial treatments in natural fibers have been used to improve the matrix-reinforcement adhesion in composites [2]; however, it is a critical step considering that the process could degrade the strong interfiber hydrogen bonding, which holds the fibers together. Surface modification methods can be physical or chemical according to the fiber surface modification mechanism. Frequently, the methods used for surface modification are bleaching, acetylation and alkali treatment [20,21]. Studies using the presence of hydroxyl groups which are reactive and susceptible to chemical reactions have been developed to analyze weakness of the matrix–fiber interface. A chemical modification is carried out according to the insertion of non-polar groups on the fibers resulting in a hydrophobic surface compatible with thermoplastic matrices [22]. Several techniques have been used to determine possible modifications in lignocellulosic materials such as X-ray diffractometry (XRD) and thermal analysis (TG). Gomes et al. [23] studied the
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development and effect of alkali treatment on tensile properties of curaua fiber green composites and evidenced that appropriate alkali treatment is a key technology for improving mechanical properties of cellulose-based fiber composites. Sisal fiber/high density polyethylene (HDPE) composites interface was studied and an improved method, which evaluates more accurately the natural fiber and polymeric matrices interfacial properties, was proposed. By this method two types of fiber surface modification were conducted: the chemical coupling by silane and the oxidation process using permanganate. It was observed, in both cases, an increase of fibers roughness introducing a mechanical interlocking with the matrix [9]. Coupling agents were used also by Keener et al. [24] to treat natural fiber polyolefin composites increasing significantly the interface strength, and consequently it was observed that mechanical properties as tensile strength and impact energy increased twice and three times, respectively, when compared to non-coupled blend of wood and polyethylene. Tserki et al. [25] studied the effect of acetylation and propionylation surface treatments on natural fibers and observed that the esterification phenomena decreased the hydrophilic characteristics of the natural fiber surface beyond the fiber crystalline as a result of the increase in the amorphous portion produced by the treatment. Shao et al. [26] studied the modification of carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) with molybdenum oxide and it was observed a formation of highly regularly packed polymer/molybdenum oxide. Marques et al. [27] studied titanium dioxide/cellulose nanocomposites prepared through the titanyl sulphate hydrolysis in acidic medium in the presence of cellulose fibers and it was observed that this material can be used as reinforced fibers in polymer matrix. In this research work, sugarcane bagasse cellulose/HDPE composite and sugarcane bagasse cellulose modified with zirconium oxychloride (ZrOCl28H2O)/HDPE composite were mixed by extrusion and compression molded. Mechanical and thermal characterization, X-ray diffraction and surface area measurement were obtained and compared. 2. Methods 2.1. Isolation crude cellulose from sugarcane bagasse The sugarcane bagasse was pre-treated with 10% sulfuric acid solution (reactor of 350 L at 120 °C, 10 min) to isolate the cellulose, followed by centrifugation to separate the rich pentosanes solution. Extracted lignocellulosic fraction was deslignificated with
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1% sodium hydroxide solution (reactor of 350 L at 100 °C, 1 h) to obtain the crude pulp, which was bleached with sodium chloride. The bleached cellulose (Fig. 1b) was dried in a store at 50 °C for 12 h. 2.2. Chemical modification of sugarcane bagasse cellulose with zirconium oxychloride About 2 g of zirconium oxychloride (ZrOCl28H2O) were dissolved in 100 mL of aqueous hydrochloric acid solution (0.5 mol L1) and 5 g of cellulose were immersed in this solution. The material was precipitated with ammonium solution (1:3) at pH 10.0, under stirring, filtered under vacuum and exhaustively washed with distilled water for the complete removal of chloride ions (negative silver nitrate test). The product was dried at 50 °C for 24 h. The resulting material was designated as Cell/ZrO2nH2O [28]. 2.3. Materials Characterization Sugarcane bagasse cellulose, hydrous zirconium oxide and sugarcane bagasse cellulose modified with metallic oxide were characterized by X-ray diffractometry (XRD), Surface area measurements (BET) and Thermal analysis (TG). X-ray diffractograms were obtained in a Rich Seifert diffractometer model ISO-DEBYFEX1001. Conditions used were: radiation CuKa, tension of 30 kV, current of 40 mA and 0.05 (2h/5 s) scanning from values of 2h it enters 10–70° (2h). Surface area measurements were performed in a Quantachrome instrument model NOVA 1000 in nitrogen atmosphere. Materials were pre-treated at 50 °C for 3 h. TG curves were generated in a Shimadzu instrument model TGA-50. The experiments were carried out under continuous nitrogen flow and with a heating rate of 10 °C min1. 2.4. Preparation of composites Sugarcane bagasse cellulose and sugarcane bagasse cellulose modified with zirconium oxychloride (ZrOCl28H2O) were mixed with the polymeric matrix (HDPE) in an extruder screw, marks IMACOM, in which fibers were responsible for 10 wt% in the composition. Respective temperatures for the four different processing zones from the hopper to horizontal die of the extruder were set as 120/130/140/150 °C and the screw speed rate was maintained at 50 rpm. The extruded materials were subsequently compression molded into samples for mechanical testing. 2.5. Compression molding In an uniaxial press, containing heating elements (inferior and superior), a molding tool was placed, containing the materials for compression (cellulose (10%)/HDPE composite, Cell/ZrO2nH2O (10%)/HDPE composite and high density polyethylene (HDPE)). Both composites and HDPE plates were obtained at 150 °C during 5 min at 5.000 kgf. Cellulose (10%)/HDPE composite, Cell/ZrO2nH2O (10%)/HDPE composite and high density polyethylene (HDPE) plates were cut in the necessary dimensions for the mechanical tests. 2.6. Tensile tests
Fig. 1. X-ray: (a) pure cellulose; (b) Cell/ZrO2nH2O.
Mechanical tests were carried out according to ASTM D-638 specification. Five specimens from each composition were tested in an INSTRON universal-testing machine (model-8801), equipped with pneumatic claws at a cross-head speed of 4.5 mm min1.
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2.7. Scanning electron microscopy The specimens submitted to tensile tests were cut and the composite intact fracture surface was analyzed in LEO 1450 V scanning electron microscopy with tungsten filament operating at 20 kV, employing low vacuum technique and secondary electron detector. 3. results and discussion The incorporation of oxide on sugarcane bagasse cellulose surface can be described by the following reaction:
Cell þ ZrOCl2 8H2 O þ HCl þ NH4 OH ! Cell=ZrO2 nH2 O þ NH4 Cl
ð1Þ
Main differences between sugarcane bagasse cellulose and modified cellulose with zirconium oxychloride (ZrOCl28H2O) can be observed in the X-ray diffractogram technique presented in Fig. 1. It is possible to observe a major diffraction peak for 2h ranging between 22° and 23°, which corresponds to the cellulose (0 0 2) crystallographic planes. The spectrum corresponding to the non-modified sugarcane bagasse cellulose fibers shows diffraction peaks at the following 2h angles: 15.7° and 22.8°. For modified sugarcane bagasse cellulose the same peaks were observed at 15.9° and 22.8°. The presence of the peaks at 15° and 22° is an evidence of the modification on fiber. These peaks indicate an increase of the interplanar distance in relation to the modified fiber. This behavior occurs due to the generation of disorder when fibers are modified. The projection substituting groups along the axis is associated with an increase in the interfibrillar distance [29]. Patterns for both materials are similar; however, non-modified fiber is less crystalline than the modified one. Crystallinity index (CI), which is a measurement of the amount of crystalline cellulose with respect to the global amount of amorphous materials, was evaluated using Segal empirical method according to following Eq. (2):
CI ð%Þ ¼
I0 0 2 Iam 100 Iam
ð2Þ
where I0 0 2 is the maximum intensity of the 0 0 2 lattice reflection of the cellulose and Iam is the maximum intensity of X-ray scattering broad band due to the amorphous part of the sample. According to this method, non-modified and modified fibers presented 47% and 53% of crystallinity, respectively. The amount of oxide incorporated into cellulose was determined by calcining 0.3 g of sample (Cell/ZrO2nH2O) at 1073 K in air for 3 h and by weighting the residue. The quantity of oxide was calculated and results are presented in Table 1. These data were confirmed from surface area measurements. Comparing the specific superficial areas of the pure cellulose and Cell/ZrO2nH2O material, it was observed an increase. The oxide deposition is an effective phenomena associated to the increase of specific surface area, SBET which was from 0 m2 g1 (see Table 1) for pure cellulose to 36 m2 g1 for the Cell/ZrO2nH2O material [18]. Thermal analyses (TG) also confirm these results which are indicated in the Table 2, and presents values of mass loss and residue in the respective range temperature.
Table 2 Results of the thermogravimetric (TG) curves of the materials, with the mass losses (m), in the respective range of temperature (DT) and its respective residues (R). Material
m (%)
DT (°C)
Cellulose
4.7 83.9 8.6
40–200 200–500 500–800
2.8
Cell/ZrO2nH2O
7.1 77.7 2.9
40–200 200–500 500–800
12.3
Data (Table 2) indicate that the TG curve of the pure cellulose presents higher mass loss in the range temperature 200–500 °C. The amount of residue in the Cell/ZrO2nH2O material increased from 2.8% to 12.3% due to the presence oxide on the cellulose surface. TG curves are presented in Fig. 2. Curve A corresponds to sugarcane bagasse cellulose and curve B to sugarcane bagasse cellulose modified with metallic oxide (Cell/ ZrO2nH2O). Pure cellulose presents two decomposition phases: the first one at 300 °C corresponds to degradation temperature and the second one at 380 °C corresponds to complete temperature decomposition. Cell/ZrO2nH2O material shows two different phases compared to pure cellulose: at 260 and 338 °C. This decrease of the temperature compared to pure cellulose is attributed to the presence of oxide particles on the cellulose surface, indicating a strong interaction between hydrous zirconium oxide and cellulose fibers [28]. Cell/ZrO2nH2O material was obtained under determined condition, appropriated (acid medium at the beginning and basic medium at the end) to avoid the degradation of the cellulose fiber because it is known that the cellulose degradation occurs at pH 10 [30]. Therefore, during the process care was taken in order not to reach this pH. Toledo et al. [31] studied the antimony (III) oxide film on a cellulose fiber surface and observed similar results. Table 3 indicates the mechanical properties of these materials involved in this research, in special the effect of chemical modification of sugarcane bagasse cellulose with zirconium oxychloride (ZrOCl28H2O) reinforced HDPE. Composite reinforced with non-modified cellulose presents lower average values for tensile strength and elongation at break compared to high density polyethylene. On the other hand, composite reinforced with modified cellulose shows significant decrease in elongation at break compared to high density polyethylene and cellulose (10%)/HDPE composite. Cell/ZrO2nH2O/HDPE composite presented higher tensile strength compared to the cellulose/HDPE composite, but still lower than high density polyethylene. An interesting increase in tensile
Table 1 Quantity of ZrO2nH2O incorporated on cellulose surface. Samples
ZrO2nH2O incorporated (wt%)
SBET (m2 g1)
Cellulose Cell/ZrO2nH2O
– 3.66
0 36
R (%)
Fig. 2. TG curves: (A) pure cellulose; (B) Cell/ZrO2nH2O.
D.R. Mulinari et al. / Composites Science and Technology 69 (2009) 214–219 Table 3 Mechanical properties of the materials obtained by compression molding. Materials
Elongation at break (tensile) (%)
Tensile strength (MPa)
Tensile modulus (MPa)
High density polyethylene (HDPE) Cellulose/HDPE composite Cell/ZrO2nH2O/HDPE composite
1.96 ± 0.087
16.7 ± 0.15
850.9 ± 28.2
1.62 ± 0.097
14.4 ± 0.58
880.1 ± 63.5
1.2 ± 0.185
15.6 ± 1.11
1324.2 ± 211.0
Reinforcement in wt%.
modulus occurred as a consequence of the oxide particles on cellulose fibers. Experimental data in Table 3 also showed a poor interaction between fibers and matrix during the mixture process. Composites were obtained with homogeneous distribution, but with agglomerations in some points caused by inefficient fibers dispersion inside matrix. This agglomeration of reinforcement was responsible by decrease of the tensile strength compared to the high density polyethylene. Composites reinforced with modified cellulose presented better tensile strength and adhesion between fiber and matrix than nonmodified cellulose, due to the agglomerations decrease. Luz et al. [32] observed similar results by mechanical testing and microstructural analysis of sugarcane bagasse fibers reinforced polypropylene composites. Fig. 3 shows the presence of pull out in the cellulose/HDPE composite fracture surface by the SEM technique. The fracture of the specimen was caused by the presence of agglomerates, which is considered as defect in the material. Image
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analyses method was used in this research to quantify the reinforcement particles (or fibers). Analyzing Fig. 3a and b it was observed pull out caused by low adhesion, which acts as stress concentrator. However, it was not observed the same considering Fig. 3c and d. The amount of agglomerates in the composites reinforced with modified cellulose decreased, which resulted, as a consequence, the decrease in ductility. The reduction in the elongation at break for this condition compared to the high density polyethylene is associated to defects generated in the material after fibers insertion. However, the addition of modified cellulose in the matrix reduced these defects, confirming the interfacial improvement between fibers and matrix. The mechanical behavior of composites is strongly dependent on the microstructural distribution of the reinforced particles in matrix. Particle agglomerates act as defects or stress concentration sites for crack nucleation. The ability to quantify the size and distribution of particle agglomerates and their effect on crack growth behavior is extremely important [33]. Tang et al. [34] studied particle distribution characterization by multi-scalar analysis of area fractions (MSAAF) technique to analyze the particle spatial distribution of composite samples obtained by vacuum hot pressing (VHP). The basis principle of MSAAF technique is to characterize the spatial heterogeneity in composite microstructures by obtaining statistical information about the variability of reinforced particle area fractions over various length scales by quantitative image analyses methods. A gray-scale image often contains only two levels of significant information, namely the foreground level constituting objects of interest and the background level against which the foreground is discriminated [35].
Fig. 3. Scanning electron microscopy of fractured surfaces of composites: (a) cellulose/HDPE composite; (b) cellulose/HDPE composite with higher magnification of the limit region between fiber and matrix; (c) Cell/ZrO2nH2O/HDPE composite; (d) Cell/ZrO2nH2O/HDPE composite with higher magnification of the limit region between fiber and matrix.
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Table 4 Reinforcement particle area fractions obtained for statistical analyses.
References
Materials
Average
Cellulose/HDPE composite Cell/ZrO2nH2O/HDPE composite
46.6 ± 7.8 33.2 ± 5.5
In this article 26 images in random regions of each sample (Cellulose/HDPE composite and Cell/ZrO2nH2O/HDPE composite) were captured to establish the reinforcement particle area fractions obtained by SEM (Fig. 3) utilizing image analyses method. These area fractions were classified as clear regions (lesser values of gray levels) and less clear regions (higher values of gray levels). The clearest regions are associated to higher reinforcement particle and the least clear regions mentions to lesser reinforcement particle. Table 4 shows results obtained for statistical analyses of composites. The area fraction obtained for Cell/ZrO2nH2O/HDPE composite decreases 13.4%, indicating an increase of filled regions by reinforcement particle. Data of Table 4 indicate a tendency of non homogeneous distribution in the reinforcement particle. Therefore, these results showed the fulfilling regions, confirming that modification of the sugarcane bagasse cellulose with zirconium oxychloride improved the adhesion between fibers and matrix. This way, the modification of sugarcane bagasse cellulose and compression molding were adequate for thermoplastic composites preparation. 4. Conclusions The modification of sugarcane bagasse cellulose with zirconium oxychloride was successfully accomplished and it was verified that effectively improves the tensile strength compared to non-modified sugarcane bagasse cellulose. The preparation of composites using non-modified sugarcane bagasse cellulose presented agglomerations. The agglomerations and/or the non-homogeneity distribution of fibers are defects in the material that directly interfere in the mechanical properties, harming the production of a high resistance material. The modification of sugarcane bagasse cellulose reduced the composites elongation to 26% compared to non-modified sugarcane bagasse cellulose; on the other hand the tensile modulus increased 50%. Therefore the molding process using extrusion and hydraulic press was appropriate. Acknowledgements The authors express their acknowledgements to CAPES for the financial support. Appendix A BET Cell/ ZrO2nH2O Cellulose HDPE MSAAF SEM VHP XRD TG
surface area measurement sugarcane bagasse cellulose modified with hydrous zirconium oxide sugarcane bagasse cellulose non-modified high density polyethylene multi-scalar analysis of area fractions scanning electron microscopy vacuum hot pressing X-ray diffractometry thermal analysis
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