Abstract
This paper deals with long fiber-reinforced composite strands using continuous ramie yarns, in which polypropylene including MA-PP (maleic anhydride-grafted polypropylene) was used as a matrix material. The composite strands were fabricated by a new method called multi-pin-assisted resin impregnation (M-PaRI) process, for which the equipment was newly applied after conventional extrusion process. The composite strands were then pelletized and injection molded. Tensile strength and Young's modulus of the resultant short ramie/PP reinforced composites were investigated. Results show that this new process improved the mechanical properties of injection-molded specimens.
1. Introduction
Glass fiber, a representative reinforcement for polymer matrix composites, has many excellent physical and mechanical properties such as low density, heat resistance, wear resistance, and high specific strength, and stiffness. Glass fiber-reinforced composites have thus been investigated for many years because of huge demand for their use in industry. They also have disadvantage such as difficulty in disposal after their lifetime. Important recent concerns are environmental problems such as global warming caused by petroleum-based materials and energy. These concerns have shifted the focus to producing alternative materials and energy from biomass resources commonly throughout the world. In December, 2010, the Japanese government made a cabinet decision to administer a plan called The Master Plan for the Promotion of Biomass Utilization [1]. By this plan, additional research and development of the utilization of herbaceous plants and woods were demanded.
Plant-based natural fibers are abundant, biodegradable, and renewable. Moreover, they have similar specific strength and stiffness to those of glass fiber [2–5]. On the other hand, plant-based natural fibers also have disadvantages such as poor compatibility with hydrophobic polymer matrices [6–8], flammability, and thermal instability [9–11]. Despite those drawbacks, increasing attention has been devoted to improvement of natural fibers as reinforcement of polymer matrix composites that can be substituted for petroleum-based fibers [12, 13]. Ramie, a well-known plant-based natural fiber, can be used as a textile fiber because of low lignin content [14]. It has advantages of high tenacity, silk-like luster, and resistance to bacteria. Ramie is also a popular reinforcement material used for polymer matrix composites [15].
During the last two decades, various attempts have been made to develop various methods for producing complete composites using reinforcements such as glass/natural fibers and thermoplastic resins. The main problem is that the fibers often break, resulting in shorter fiber length than the critical one [16] because the fibers were inserted directly into the extrusion machine. In order to maintain appropriate mechanical properties of the composites, the fiber length must be longer than the critical value. One method is to use pultrusion technique developed by ICI for the manufacture of Verton long fiber molding materials [17, 18]. The impregnation devices applied in the past are simple in form, as used in the processing of thermoset resin. Thomason [19] also introduced a coating process using jute yarns to produce thermoplastic resin matrix composite strands. However, poor performance of tensile properties on injection-molded composite materials was reported. Bledzki et al. [20] used a two-step extrusion process for the combination of natural fiber yarns and polylactic acid (PLA) to improve impregnation into interfibers. Recently, a new technology for production of jute-twisted yarn-reinforced composites was reported by Tanaka and Hirano [21], in which both complete impregnation and long fiber length were achieved. Results showed that the resultant composites yielded much better mechanical properties than those of short fiber-reinforced composites, while the disadvantage of this technology is that the fabrication procedures are complicated.
In this study, thus, a continuous ramie single yarn reinforced polypropylene (PP) composite strand was developed using a new and simple combined technique. The strands were chopped to pellets for injection-molding, and tensile properties of the molded specimens were investigated.
2. Experimental
2.1. Materials
Continuous ramie single yarns, having fineness of 95 tex, Type no. 16 (Tosco Co., Ltd., Japan) and polypropylene (Prime Polymer Co. Ltd., Japan) were used as reinforcement and matrix material, respectively. Physical and chemical properties of ramie fibers are listed in Table 1 [22, 23]. According to [22, 23], although cellulose contents vary widely from 65% to 85%, both data are in an agreement in terms of low lignin contents. The cross-sectional area of ramie fibers is not circular, but rather elliptical as shown later. The major axis is approximately 30 μm. The degree of microfibrillar angle of ramie fibers is one of the smallest classes among plant-based natural fibers.
Physical and chemical properties of ramie fibers.
It is known that high adhesion between hydrophilic fibers and hydrophobic resin by chemical bonding is induced by available OH groups on the fiber surface. The resin adheres to the fiber surface through molecular chain entanglement. During this reaction, maleic anhydride-grafted polypropylene (MA-PP) works as a coupling agent to realize such chemical bonding [24]. In this study also, MA-PP (Kayaku Akzo Co., Ltd., Japan) was used to promote a chemical interaction between the fiber and matrix.
2.2. Fabrication Procedures
Figure 1 shows a schematic of the present fabrication system. Continuous ramie single yarn/PP composites were produced through a new combined technique proposed in this study, which consists of resin coating and multi-pin-assisted resin impregnation (M-PaRI) processes, as shown in Processes A and B of Figure 1. The continuous ramie single yarns were first delivered via preheating process into a cross-head die attached to a ϕ 15 mm single screw extruder (Musashino Kikai Co., Ltd., Japan), into which PP pellets and MA-PP powders were fed at the same time. The mixed resin was coated onto the ramie yarns in the die at Process A (resin-coating process). Subsequently, it was impregnated into interfibers through the multi-pin system, as shown in Process B (M-PaRI) of Figure 1. The number and diameter of pins used here were 22 and 5 mm, respectively. Temperatures of the single screw extruder were all set at 190°C with a screw speed of 7.0 rpm. A motor was set to draw the composite strand with a screw speed of 45.0 rpm.
Schematic view of the present fabrication system. (Process A: resin-coating process, Process B: multi-pin-assisted resin impregnation (M-PaRI) process).
The continuous composite strands containing six ramie yarns were chopped to pellets of 2 mm length. The set temperature (s) was 190°C at Process A, and was changed in the 160–225°C range for Process B. The pellets were molded into specimen dies of two types on an injection molding machine (Shinko Sellbic Co., Ltd.). Then small- (Japanese Industrial Standards, JIS K 7162, Type 5B) and medium-sized (JIS K 7162, Type 1BA) tensile specimens were obtained, as shown in Figures 2 and 3, respectively. The temperatures were set at 180–185°C for injection molding. Fiber contents of small- and medium-sized specimens were adjusted to 30 wt% and 10–50 wt%, respectively.
Shape and dimensions of small-sized tensile specimen.
Shape and dimensions of medium-sized tensile specimens.
2.3. Tensile Test
Tensile tests were conducted using a universal testing machine (Desktop type universal testing machine, LSC-1/30, JT Toshi Co., Ltd.) for small-sized specimens, and tensile and compression testing machine (Minebea Co., Ltd.) for medium-sized specimens at a crosshead speed of 10 mm/min and 17 mm/min, respectively. The mean cross-section area of small- and medium-sized specimens was measured on three locations along the longitudinal direction, using a micrometer and then taking an average. Five specimens were evaluated to obtain an average value. Ultimate stress (MPa), stiffness (GPa, linear region between strain 0.05 and 0.25), and strain (%) were determined from the stress-strain curves for each test.
3. Results and Discussion
3.1. Degree of Continuous Strand Impregnation by the Proposed Combine Technique
Microscopic cross-section images of ramie/PP composite strands are shown in Figure 4, in which the temperature of Process B was set at 195°C. The strand was taken out of the fabrication system after it was stopped intentionally. Its several cross-sections before or inside Process B were observed. It is apparent from Figure 4 (a) that many voids exist among fibers before Process B, although resin is locally infiltrated. Bledzki et al. [20] carried out a two-step extrusion process, in which abaca/PLA strands obtained firstly by yarn coating process were pelletized and secondly inserted into a single extruder. It is guessed that the second process was conducted because such voids might not be removed in the first-step process. Figures 4 (b) and 4 (c) show images after the 11th and 15th pins, respectively. They show almost no void. The final product in Figure 4 (d) shows that the cross-section contains the resin completely infiltrated among fibers with no void. It is well verified that the attached multi-pin system assists in impregnating the resin into interfibers. This system does not need any additional extrusion process. The mechanism of this impregnation is estimated such that continuous contact and rubbing between resin-coating yarns and pins flattens the yarns and widens the interfiber spaces. Consequently, the resin can be impregnated easily among fibers.
Cross-sectional images of ramie/PP composite strands—(a) before M-PaRI process, (b) after 11th pin, (c) after 15th pin, and (d) final product. Images in (a) and (d) are obtained using 3D laser measuring microscopy. Images in (b) and (c) were obtained using scanning electron microscopy.
Tanaka and Hirano [21] achieved complete resin impregnation for jute-twisted yarn. They also widened the interfiber spaces through an untwisted yarn process. Although this process can control the magnitude of the spaces by changing the number of untwists, it has to be prepared for each yarn. It demands complicated and expensive facilities. On the other hand, the composite process proposed herein can be widened among fibers by a simple process, as mentioned previously. It is concluded that, thereby, the composite strands can be produced successfully and more easily through such a combined technique: a conventional resin-coating process (Process A) followed by M-PaRI process (Process B).
3.2. Tensile Properties
3.2.1. Stress-Strain Behavior
Figure 5 shows the effect of temperature during Process B on the tensile stress-strain behavior of the composites specimens containing 30 wt% fibers. The size of specimen was a small type in Figure 2. All of the stress-strain curves show nonlinearity at around 2–3%, but the stress levels during deformation of composite specimens are much higher than that of PP. The level of composite stress also depends on the temperature at Process B. Composite specimens produced at low temperatures during Process B, that is, 160°C and 185°C, exhibit less stress, while composite specimens produced at 195°C, 205°C, and 225°C show higher stress and almost identical stress-strain behavior, especially at the initial stage. However, composite specimens produced at 160°C, 185°C, and 195°C exhibit a similar strain at break, whereas those produced at 205°C and 225°C show less fracture strain. In other words, less fracture energy is brought from thermal exposure higher than 195°C during Process B.
Typical stress-strain curves of short ramie/PP reinforced composites produced at different temperatures during Process B.
3.2.2. Tensile Strength and Young's Modulus
Table 2 shows averages and coefficients of variation of the tensile properties of the small-sized PP and composite specimens. It can be seen that tensile strength and Young's modulus increase with increasing temperature over the whole 160–195°C range. Eventually, in the composites specimens of 195°C, not only did tensile strength improved 1.63 times higher than that of PP specimens, but also Young's modulus increased 1.87 times. However, a slight decrease in tensile strength in the specimens of 205°C is found with a subsequent dramatic decrease of 225°C. From the above, the maximum properties of composites could be obtained at 195–205°C in the present fabrication system.
Tensile properties of short ramie/PP composites.
Trip: temperature at multi-pin-assisted resin impregnation (M-PaRI) process.
Numbers in parentheses denote coefficients of variation.
It was observed that voids did not disappear, similar to Figure 4 (a), when the composite strands were produced at 160°C and 185°C. That is to say, the resin was not impregnated completely into interfibers even after Process B. The remaining voids decreased tensile strength and Young's modulus, as well as the whole stress level. It is inferred that, for the composite specimens of 205°C and 225°C, overheating causes degradation of ramie fibers during Process B. It is considered that such a thermal degradation of fibers induces premature fracture of the composite specimens.
3.2.3. Effects of Fiber Content
Figure 6 presents effects of fiber content on tensile properties of composite specimens. The size of the specimens was medium size of a type in Figure 2. For the specimens, the pellets produced at 195°C during Process B were used. It is apparent in Figure 6 (a) that the tensile strength increases with increasing fiber content. This trend reaches a maximum level at 40 wt%. Tensile strength of 40 wt% increased 1.73 times higher than PP specimens. Above this level of fiber content, the tensile strength starts to decrease. As can be seen in Figure 6 (b), a continuous increase in Young's modulus occurs as the fiber content increases. That of 50 wt% specimens increased 2.17 times higher than the PP specimen. Results for fracture strain are shown in Figure 6 (c). It is seen that fracture strain is decreased almost linearly between 4.0 and 7.0%. Consequently, although it exists between 30–50 wt%, it can be said that the weight fraction of fiber content giving the best tensile properties is about 40 wt%.
Effect of fiber content on tensile properties of short ramie/PP reinforced composites produced at 195°C during Process B—(a) tensile strength, (b) Young's modulus, and (c) fracture strain.
3.3. Fiber Dispersion and Fiber Length Distribution
Figure 7 shows a polished view of transverse section in a small-sized specimen produced at 195°C during Process B. The fiber content in Figure 7 is 30 wt%. The black bar-like areas show marks of ramie fibers separated from matrix during the polishing process. White dots denote cross-sectional area of ramie fibers. As presented from Figure 7, a higher proportion of white dots is visible. Such a phenomenon can also be observed on glass fiber-reinforced composite specimens [25, 26]. This means that short ramie/PP reinforced composites have a high degree of fiber orientation to the flow direction.
Laser micrograph of transverse section of short ramie/PP reinforced composites produced at 195°C during Process B.
Figure 8 shows a magnified view of Figure 7. Results show that individual fibers after injection molding are well dispersed, despite the fact that high-density fiber bundles were obtained after M-PaRI process, as shown in Figure 4 (d). When being strongly bonded, it is known that the bundles act as a reinforcing unit, which means that pull-out of bundles occurs at much lower stresses [26]. Consequently, it should be noted that much higher interfacial stress can be yielded on the surface of the fibers.
Laser micrograph of transverse section of short ramie/PP reinforced composites produced at 195°C during Process B.
To confirm the change in fiber length after the applied fabrication processes, small-sized composite specimens produced at 195°C during Process B were dissolved in boiling xylene for 24 hr to remove PP resin. Then, extracted ramie fibers were dried at 100°C for 2 hr. Figure 9 shows an SEM micrograph of ramie fibers. Although partial microfibril detachment is observed, the single fibers maintain their original structure even after Processes A and B with subsequent injection molding processing applied.
SEM micrograph of extracted fibers from short ramie/PP reinforced composites produced at 195°C during Process B.
Fiber length distribution of the specimens dissolved above is presented in Figure 10. The total number of measured fibers was 780. Average fiber length was 1.56 mm and the standard deviation was 0.61 mm. It is necessary for short fiber-reinforced composites to be longer than the critical fiber length. According to an earlier report [27], the critical fiber length of ramie fiber is 0.47 mm. It can be clearly seen that over 98% of ramie fibers are longer than the critical length in this study. This brings sufficiently high stress to composite specimens because the matrix transfer mechanism does not cause low fiber stress.
Fiber length distribution of extracted fibers from short ramie/PP reinforced composites produced at 195°C during Process B.
4. Conclusions
A new combination technique of resin-coating and multi-pin-assisted resin impregnation (M-PaRI) processes was introduced to produce a continuous ramie single yarn/polypropylene (PP) reinforced composite strand. By addition of M-PaRI process, we found that the resin can be impregnated completely into the yarn interfibers. Furthermore, tensile tests of injection-molded composites were conducted using strands produced at different temperatures of M-PaRI process. Results show that the maximum mechanical properties of composites could be obtained between 195 and 205°C. The fiber content giving the best tensile properties was about 40 wt%.
The new process presented in this study demonstrates marked improvements of mechanical properties on composites in comparison with conventional methods, in which fibers are inserted directly during the extrusion process. A fascinating point is the simplification of production for long natural fiber-reinforced composites. The composite strands obtained here are expected for use as a semifinished material for injection and compression molding products.