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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 685104, 6 pages
http://dx.doi.org/10.1155/2013/685104
Research Article

Continuous Natural Fiber Reinforced Thermoplastic Composites by Fiber Surface Modification

1Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
2Komatsu Seiren Co., Ltd., Nu-167 Hama-machi, Nom, Ishikawa Prefecture 929-0124, Japan
3Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan

Received 1 June 2012; Accepted 1 March 2013

Academic Editor: Johanne Denault

Copyright © 2013 Patcharat Wongsriraksa et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Continuous natural fiber reinforced thermoplastic materials are expected to replace inorganic fiber reinforced thermosetting materials. However, in the process of fabricating the composite, it is difficult to impregnate the thermoplastic resin into reinforcement fiber because of the high melt viscosity. Therefore, intermediate material, which allows high impregnation during molding, has been investigated for fabricating continuous fiber reinforced thermoplastic composite by aligning resin fiber alongside reinforcing fiber with braiding technique. This intermediate material has been called “microbraid yarn (MBY.)” Moreover, it is well known that the interfacial properties between natural fiber and resin are low; therefore, surface treatment on continuous natural fiber was performed by using polyurethane (PU) and flexible epoxy (FLEX) to improve the interfacial properties. The effect of surface treatment on the mechanical properties of continuous natural fiber reinforced thermoplastic composites was examined. From these results, it was suggested that surface treatment by PU with low content could produce composites with better mechanical properties.

1. Introduction

The current emphasis is to develop materials that are renewable, degradable, ecofriendly, and recyclable, better known as “green materials,” as alternative to the petroleum or mineral-based material [1, 2]. Natural fiber-reinforced thermoplastic composites are becoming more and more commonplace by the development of new production techniques and processing equipment. Compared with the traditional synthetic fibers, natural fibers present lower density, lower cost, and they are renewable and biodegradable [3].

The continuous natural yarns were used as reinforcement to enhance the mechanical properties of thermoplastic composites. The problem with producing continuous natural fiber-reinforced thermoplastic composite, however, is the difficulty to impregnate the thermoplastic resin into the fibers due to the high melt viscosity. In these circumstances, intermediate material, which allows high impregnation during molding, has been developed for fabricating continuous fiber-reinforced thermoplastic composite by aligning resin fiber alongside reinforcing fiber with braiding technique [4, 5]. This intermediate material has been called “microbraided yarn (MBY)” as shown in Figure 1. Since resin fibers are located close to the reinforcement fiber bundle, the impregnation state is excellent [6].

685104.fig.001
Figure 1: Microbraided yarn (MBY).

Since the interfacial properties between natural fiber and polymer matrix are low, surface treatment on jute fibers was required. In this study, polyurethane (PU) and flexible epoxy (FLEX) were used to improve the thermal resistance and interfacial properties of jute fiber. Polyurethane (PU) is one of the most rapidly developing branches in polymer technology with a wide variety of applications, such as foams, coatings, elastomers, resins, and medicine. PU was used for the improvement of the wear resistance and mechanical properties. Epoxy resins are a class of high performance thermosetting polymers widely used as matrix resins for advanced composite materials for application in the automotive, construction, and aerospace industries. Flexible epoxy resin (FLEX) was used to create flexible interphase and improve thermal degradation resistance. Smith Thitithanasarn, et al. shows that thermal resistance of jute fiber was improved by coating with flexible epoxy resin [7]. The objective of this study was to investigate the effect of surface treatment on the mechanical properties of continuous natural fiber-reinforced thermoplastic composites.

2. Experimental Details

2.1. Materials

The materials used in this study were jute spun yarn as the reinforcement and biodegradable polylactic acid (PLA) fibers that will become the matrix after compression molding. Jute yarns of 400 tex fineness were supplied by Bangladesh Janata Jute Mills Ltd., while PLA fibers (Lactron) were supplied by Unitika Ltd. Polyurethane (PU) and flexible epoxy (FLEX) were used as a surface treatment for the jute spun yarn to improve interfacial adhesion between jute and PLA matrix. PU was supplied by Nicca Chemical Co., Ltd., and FLEX was supplied by Daicel Chemical. Co., Ltd.

2.2. Sample Preparation

Figure 2 shows the processing flow chart of jute spun yarn untreated and treated with PU or FLEX/PLA composite. The method for PU or FLEX treatment is shown in Figure 3; the jute spun yarn was treated by using PU and FLEX with various contestations at 0, 5, 10, 15, and 20 wt%. The jute spun yarns were immersed into PU or FLEX bath for 10 minutes and then dried in an oven at 80°C for 2 hours. Untreated and PU or FLEX treated jute yarns were braided with PLA fibers to yield jute yarn and PLA MBY, which was later used to prepare composites. A tubular braiding machine was used for fabricating MBY as an intermediated material for producing unidirectional continuous natural fiber-reinforced plastic composites [8]. Jute yarns as the reinforcement fibers were aligned vertically and braided with PLA fibers to yield jute/PLA MBY by using a technique known as microbraiding. The MBYs were wound onto a steel frame to obtain an unidirectional alignment of 18 yarns side by side. The steel frame had a spring mechanism to adjust the tension caused by the thermal shrinkage of fiber during processing shown in Figure 4. The wound frame was then dried in the oven at 80°C with drying times of 2 hours. The frame was then placed into a heated mold with size of 20 × 200 mm concave geometry and compressed at 200°C with a molding pressure of 1.33 MPa for 8 minutes. Cooling was subsequently performed by running water through the mold while keeping the specimens under constant pressure. The time taken to transfer the steel frame from the oven to the compression molding machine was approximately 1-2 minutes, because the natural fiber itself is “hygroscopic” and easily absorbs moisture. From previous study, the moisture absorption rate was increased with increasing exposure time, and the percentage of moisture absorption at 1-2 min was about 0.02%.

685104.fig.002
Figure 2: Processing flow chart of jute spun yarn untreated and treated with PU or FLEX/PLA composite.
685104.fig.003
Figure 3: Method for PU and FLEX treatment.
685104.fig.004
Figure 4: Fabrication method of composites.
2.3. Testing Method

Thermal resistance of jute spun yarn and composites was investigated by thermogravimetric analyzer (TGA2950, TA Instruments) at heating rate of 20°C/min from 30°C to 600°C in air atmosphere. The weight change in a material was determined by TGA as a function of temperature (or time) under a controlled atmosphere. Measurements are used primarily to determine the composition of materials and to predict their thermal stability at temperatures up to 600°C. The technique can characterize materials that exhibit weight loss or gain due to decomposition, oxidation, or dehydration.

Tensile properties of the composite (JIS, K 7165) was tested on the INSTRON universal testing machine (model 4206). The specimen size was 200 mm in length, 20 mm in width, and 0.8~1 mm in thickness, and aluminum tabs (thickness 0.5 mm, width 20 mm, and length 50 mm) were put on both ends. The span length was 100 mm and the crosshead speed was 1 mm/min.

For interfacial properties (JIS, K 7017), 3-point bending test of unidirectional composites in 90 and 0 degree direction was performed by using an INSTRON universal testing machine (model 4206). The specimen size was 50 mm in length, 15 mm in width, and 1.5~1.8 mm in thickness. The span length was 25 mm, and the test speed was 1 mm/min, and the reinforcing fibers were oriented parallel to the loading bars.

For the cross-sectional observation, the cross-section of jute/PLA composites were polished and observed by using an optical microscope OLYMPUS-PME3 (IC5).

3. Results and Discussion

Figure 5 shows the relationship between thermal resistances of jute spun yarn treated with PU and FLEX and content of surface treatment. In the case of jute spun yarn treated with PU, thermal resistance was increased at PU 5 wt% and slightly decreased with increase in surface treatment content. Meanwhile, in the case of jute spun yarn treated with FLEX, thermal resistance was increased with increasing surface treatment content. The jute spun yarn treated with FLEX exhibited higher thermal resistance as compared to the PU.

685104.fig.005
Figure 5: Relationship between thermal resistances of jute spun yarn treated with PU and FLEX and content of surface treatment.

Figures 6 and 7 show the impregnation state by cross-sectional photographs of composites by varying surface treatment content of PU and FLEX. Specimens untreated and treated with 5 wt% of PU and FLEX provided more impregnated amount of PLA resin into the jute fiber bundle than the specimens treated with 10, 15, and 20 wt%. Meanwhile, at the specimens treated with PU, the PLA resin appeared in the jute fiber bundle more than in specimens treated with FLEX because the specimens treated with FLEX inhibited the impregnation of matrix resin. Therefore, it is suggested that PLA was well impregnated into jute fiber bundle with low content surface treatments of PU.

685104.fig.006
Figure 6: Cross-sectional photographs of composites by varying surface treatment content of PU.
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Figure 7: Cross-sectional photographs of composites by varying surface treatment content of FLEX.

The interfacial properties of the composites were examined by 90 degree bending test conducted parallel to the alignment of jute fibers, and the results are shown in Figure 8. The bending strength of jute spun yarn/PLA composites was measured with varying the content of surface treatment of PU and FLEX; the bending strength of jute spun yarn/PLA composites showed the highest value at surface treatment content of 5 wt% both for PU and FLEX and slightly decreased with increase in surface treatment content. Specimen treated with PU showed higher strength than the specimens treated with FLEX. This indicates that FLEX was able to improve the interfacial between fiber-matrix interfacial adhesions.

685104.fig.008
Figure 8: 90 degree bending strength (MPa) of composites varying surface treatment content of PU and FLEX.

Figure 9 shows the relationship between thermal resistances of composites treated with PU and FLEX and content of surface treatment. In the case of jute spun yarn/PLA composites treated with PU, thermal resistance was increased at PU 5 wt% and slightly decreased with increasing surface treatment content. Meanwhile in the case of jute spun yarn/PLA composites treated with FLEX, thermal resistance was slightly increased with increasing surface treatment content. The jute spun yarn/PLA composites treated with FLEX exhibited higher thermal resistance as compared to the PU.

685104.fig.009
Figure 9: Relationship between thermal resistances of composites by treated with PU and FLEX and content of surface treatment.

Figures 10 and 11 show tensile modulus and strength of jute spun yarn/PLA composites with varying the content of surface treatment of PU and FLEX. The tensile modulus and strength of jute spun yarn/PLA composites showed the highest value at surface treatment content of 5 wt% both for PU and FLEX and slightly decreased with increase in surface treatment content. Specimen treated with PU showed higher tensile modulus and strength than the specimens treated with FLEX, because it had good impregnation of PLA into the jute fibers bundle.

685104.fig.0010
Figure 10: Tensile modulus (GPa) of composites by varying surface treatment content of PU and FLEX.
685104.fig.0011
Figure 11: Tensile strength (MPa) of composites by varying surface treatment content of PU and FLEX.

From these results, impregnation state was decreased with increase in the surface treatment content, especially for FLEX. On the other hand, it is considered that the interfacial properties were improved by surface treatment especially with PU. Therefore, it was found that the optimum content of surface treatment was 5 wt% with PU to achieve higher mechanical properties.

4. Conclusion

It was established that surface treatment by PU with low content could produce composites with better mechanical properties. This indicates that surface treatment by PU was effective in the impregnation state and the interfacial properties.

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