Abstract
Prevalently, the light has been shed on the green composite from the viewpoint of environmental protection. Jute fibers are natural fibers superior due to light weight, low cost, and being environmentally friendly corresponding to the green composite materials. Meticulously, fibers of polylactic acid (PLA) thermoplastic biopolymer were used as the resin fibers. In this study, the fabrication of tubular jute spun yarn/PLA braided composite by pultrusion molding was presented. The intermediate materials were prepared by commingled technique. The braiding technique manufactured preform which had jute fiber diagonally oriented at certain angles with the glass fiber inserted into the braiding yarns along the longitudinal direction. The braided preforms were pulled through a heated die where the consolidation flow took place due to reduced matrix viscosity and pressure. The pultrusion experiments were done with jute/PLA commingled yarns and combined with glass fiber yarns to fabricate the tubular composite. Impregnation quality was evaluated by microscope observation of the pultruded cross-sections. The flexural mechanical properties of the pultruded were measured by four-point bending test.
1. Introduction
Many of common composite production methods are unsuitable for mass production. One exception is the pultrusion process; it is possible to maintain a continuous production of straight profile with constant cross-sections. Pultrusion is a manufacturing process in which reinforcing fibers impregnation with matrix is pulled through a die to form composites of a constant cross-section. Generally, the unidirectional fibers are impregnated with low viscosity thermosetting resins before passing through a series of dies for shaping and curing during pultrusion process [1–3]. While thermoset pultrusion is a well-known and commercially established manufacturing method, there is less knowledge about the thermoplastic pultrusion. In contrast to thermosets, thermoplastic matrix is generally polymerized, no further chemical reaction is necessary; therefore the processing is reduced to first melting the matrix, then shaping the composite under pressure, and finally cooling it to preserve the new shape. Thermoplastic resins contain very high melt viscosities which difficultly make the melt matrix resin impregnated into the reinforcement fiber. For this reason, various intermediate materials have been developed to overcome these problems such as microbraided yarn, commingled yarn, and parallel configuration yarn as shown in Figure 1. The schematic of tubular braiding fabric was showed in Figure 2, all fiber bundles are diagonally oriented, and the angle (θ) of the fiber bundle to the longitudinal direction can be adjusted freely. Also, the fiber bundle called the middle end yarn (MEY) can be inserted into the braiding yarns (BY) along the longitudinal direction. For this reason, the braiding technique can control the anisotropic of pultrusion molding.
The schematic of intermediate material.
The schematic of tubular braiding fabric.
In the early stage, jute fiber reinforced PLA resin has been made with good impregnation quality and mechanical properties using compression of microbraided yarns [4, 5]. The thermoplastic braided composite by pultrusion molding with various forms of materials has been widely studied. The method combines the braiding technique of the pultrusion process to produce multiaxially reinforced continuous beam were successful with using PP/Glass fiber [6]. The braiding technique was developed for pultrusion of the continuous fiber reinforced thermoplastic tube which commingled yarn carbon fibers and PA66 which were used as intermediate materials [7]. The pultrusion technique was used to manufacture the continuous composite using flax fiber reinforced PP which offers good opportunities as the reinforcement material for composite, good mechanical properties and the ecological and environment advantages [8].
The use of natural fibers derived from annually renewable resource as reinforcement in thermoplastic matrix composite provides the positive environment benefit with respect to ultimate disposability and raw materials utilization [9–11]. The biocomposites, using natural fibers compounded with natural matrices, could diminish the impact of plastic waste on the environment. Jute fibers are natural fibers superior due to light weight, low cost, and being environmentally friendly corresponding to the green composite materials. PLA is biodegradable aliphatic polyester derived from renewable resources, such as cornstarch, tapioca, or sugarcanes. PLA can be processed like most thermoplastics into fiber or film. Previously, jute fiber reinforced PLA resin has been made with good impregnation quality and mechanical properties using compression of microbraided yarns. In this study, the commingled yarus were used as intermediate materials (as shown in Figure 2 (b)).
The advantages of pultrusion process are threefold. Firstly, it is suitable to produce the continuous composite with uniform cross-sections. Secondly, it also has mass production and low cost. Finally, the composite contains high mechanical properties due to the continuous fiber. The objective of this investigation was to demonstrate the fabrication of the tubular jute spun yarn/PLA braided composite by pultrusion molding. The jute spun yarn was used as reinforcement, and PLA was used as the matrix resin corresponding to the green composite materials. Figure 3 shows the schematic of pultrusion process of braided composite in this study. The commingled yarns were used as intermediate materials to prepare the pultrusion preform by braiding technique. The fabrication quality of pultrusion process was evaluated by cross-section observation and mechanical properties evaluated by four-point bending test.
The schematic of pultrusion process of braided composite.
2. Materials and Experiment
2.1. Design Concept
In this paper, the designed concept of braided composite by pultrusion molding is described. The designed concept involves the materials design, structure design, and processing design as shown in Figure 4. Material designs consisted of interface such as surface treatment on reinforcement fiber, volume fraction, and configuration of yarn on intermediate materials. Braiding angle, gap between braiding yarns, and filling ratio are the important parameters of the structure design. Meanwhile, the processing designs consisted of pultrusion temperature, pulling speed, and pulling force.
Design concept of braided composite by pultrusion molding.
2.2. Materials
In the previous study, the intermediate materials were prepared by microbraided yarn and parallel configuration yarn. Successful tubular braided composite was realized using glass fiber for middle end yarns in the braided structure. It was found that successful tubular braided composite was realized using glass fiber 1,150 tex for middle end yarns in the braiding structure. The highest bending strength and modulus were found in the specimen using parallel yarn configuration. Meanwhile, the region of unimpregnation areas is seen inside reinforced fiber, and macrovoids are seen outside the fiber [12].
Following the designed concept in this study, jute fiber tows were commingled with PLA fiber. Figure 5 showed the commingled technique for mixing the resin fiber with reinforced fiber. The jute fibers tow having a fineness of ~400 tex were used as reinforcement fibers. The continuous PLA fibers in a tow configuration were used as resin fibers, having a fineness of ~56 tex. Glass fiber (GF) yarns having a fineness of 1150, 720, 600, and 520 tex were used as MEYs. They were used to enhance the strength of braided fabric. The braided fabric preforms for pultrusion were fabricated using 48 BY and 24 MEYs in a tubular braiding machine with 48 carriers (Murata Machinery). The braided preforms were done using braiding ring with diameter of 30 mm and mandrel with diameter of 20 mm, and the braiding angle was 30–38 degrees. The layer of braided fabric was 2 layers. Table 1 lists the four tubular braided preforms with different GFs as MEYs.
Lists of the four different tubular braided preforms.
Commingled technique.
Generally, glass fiber had strength around 1.5 GPa, jute fiber had strength around 207 MPa, and PLA fibers had strength around 48 MPa. The strength of materials was used to estimate the limited force for pultrusion molding. In the estimation, the fibers in braided preform were assumed in parallel configuration (braiding angle equal to 0 degree). The estimated limited pulling forces of preforms were 20.52, 22.82, 26.28, and 38.66 kN, respectively.
2.3. Molding
Figure 6 shows the schematic of the tubular pultrusion molding assembly, consisting of the preheater and the pultrusion die. The tubular molding die had outside diameter of 23 mm and inside diameter of 20 mm (mandrel). The length of the molding die was 270 mm. The preheater had length 500 mm, and the entrance side of the die was large and gradually reduced in taper region 50 mm until a constant cross-section of the die. According to the TGA data of jute and PLA (Figure 7), the thermal resistance of the jute degrades at ~240°C and PLA degrades at ~320°C. Meanwhile the melting temperature of PLA was ~175°C. Therefore, the processing window of pultrusion temperature could be ~175–240°C. In this study, the pultrusion temperature was designed at 195–235°C. Prior to pultrusion, the preforms were dried at 80°C in convection oven for 2 hours. The preheater was set to 100°C. The molding die had four separate heating zones, and the temperature at each zone was set, respectively, at 195, 195, 185, and 165°C from entrance side of the die. The temperature of mandrel inside the molding die was set to 165°C. The braided preforms were pulled through the molding die by a pulling mechanic at a speed of 18 mm/min. Generally, GF was used as MEY to enhance the strength of tubular braided preform for pultrusion molding. This study will select the preform which uses the small amount of glass fibers in braided preform to examine the effect of molding temperature. After that the five specimens were produced by changing the pultrusion temperature in molding die zone 1 and zone 2 with constant molding speed. The pultrusion temperature is shown in Table 2. The pultruded specimens were cut and polished in a direction perpendicular to longitudinal direction for the cross-section observation in order to investigate the internal state of the molding by using optical microscope.
Pultrusion temperature.
The temperature is changed in molding die zones 1 and 2.
Schematic of the pultrusion system.
The TGA data of jute and PLA.
2.4. Experiments
The mechanical properties were performed by four-point bending test. It was performed by using the pulley unit and the metal solid bar as shown in Figure 8. The pulley unit and steel bar are capable of decreasing the stress concentration generated at the point for support and loading nose. The bending test was performed by using an INSTRON universal testing machine with a span length of 300 mm and cross-head speed of 1 mm/min.
The schematic of four-point bending test.
3. Result and Discussion
The pultrusion of preform with GF520 and GF600 was interrupted due to the MEY breakage inside the pultrusion die. The preform with GF720 and GF1150 was fabricated without problem because it had enough strength for pultrusion. The tubular composite jute/PLA with GF1150 had rougher surface than using GF720. From these results, the preforms which had limited pulling force lower than 26.28 kN were unsuccessful for pultrusion molding. The preform with GF720 was successfully pultruded and had minimized usage of glass fiber and good surface quality as shown in Figure 9. Therefore, it was selected for the experiment with the different pultrusion temperature.
The tubular braided composite.
From Table 3, specimens no. 5 could not be successfully fabricated because the jute fibers of specimens were burned due to high temperature. The specimens with molding temperature of 195, 205, 215, and 225°C were successfully pultruded. The result from four-point bending test was shown in Table 3. The highest bending modulus of 9.64 GPa and strength of 29.96 MPa were obtained in specimens no. 2. Meanwhile, specimens no. 4 had the lowest modulus and strength because the temperature was higher than others.
The result from four-point bending test.
The characteristic of commingled yarn in this study shows the higher mechanical properties compares to the previous study with the similar filling ratio because the resin fibers were mixed with jute spun yarns using blow air. In the previous study, microbraiding yarn and parallel yarn configuration were used as braiding yarns for fabricating the preforms. The cross-section photographs of specimens are shown in Figure 10. From the photograph, the dark regions between the fiber bundles indicate macrovoid and the dark regions inside of fiber bundles indicate unimpregnation area. The relationship between void, unimpregnation, and molding temperature is shown in Figure 11. Consequently, it was clarified that void and unimpregnation area was decreased with increasing the molding temperature. Moreover, it was found that void and unimpregnation area was decreased with decreasing the molding speed. Figure 12 shows the cross-section photographs of specimens with temperature 205°C, and pulling speed was decreased to 10 mm/min. The void area was 7.47% and un-impregnation area was 32.36% (as shown in the comparison in Figure 11).
The cross-section observation of specimens.
Relationship between void, unimpregnation, and molding temperatures.
The cross-section observation of specimens with pulling speed 10 mm/min.
From these results, the impregnation quality was increased when the molding temperature increased because the matrix viscosity was reduced at higher temperature. The matrix resin was easily impregnated into the jute spun yarn and GF. Meanwhile, with increasing molding temperature the modulus and strength were decreased due to the high temperature affecting the degradation of jute spun yarns. Therefore, the molding temperature of 205°C is the optimum temperature for fabrication of the tubular jute spun yarn/PLA braided composite. Moreover the impregnation quality was improved by decreasing the pulling speed.
4. Conclusion
Pultrusion molding is one technique for manufacturing the continuous composite with uniform cross-sections. In this study, the processing design of tubular braided composite using jute spun yarns reinforced PLA by pultrusion molding was performed. The commingled technique mixed the resin fiber and jute spun yarn which were used as intermediate materials. The effects of processing design such as the molding temperature and the molding speed are effects on the impregnation quality and mechanical properties of tubular braided composite. It was clarified that the impregnation quality increased with increasing the molding temperature and decreasing the molding speed. While the temperature increased, the mechanical properties decreased. The pultrusion of jute spun yarn/PLA tubular braided composite in this study is an important step towards the economically viable production of high performance of the biocomposite products.