18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS EFFECT OF MOLDING CONDITION ON THE MECHANICAL PROPERTIES OF HEMP FIBER/POLY(LACTIC ACID) COMPOSITES. K. Takada 1 , S. Kobayashi 1 1 Dept. of Mechanical Engineering, Tokyo Metropolitan Univ., Tokyo, Japan, * Corresponding author (koba@tmu.ac.jp) Keywords : hemp fiber, PLA, micro-braiding 1 Introduction expected since the reinforcing fibers and the matrix fibers can be combined as one braided yarn. Advances in science and technology pose new It is well known that the mechanical properties of a challenges on the materials in relation to certain continuous fiber reinforced thermoplastic composite environmental issues, such as biodegradability, are affected by its processing conditions such as recyclability, eco-friendliness etc., that need to be pressure, temperature, holding time, and so on. In addressed to help preserve and protect our this study, Hemp fiber/PLA composites fabricated at environment. Composite materials from the different conditions, those composites were renewable natural fibers and biodegradable matrices performed tensile tests. This study investigates the have been developed in the past decade in an attempt effect of molding condition on the mechanical to find alternatives to the fossil fuel-based polymeric properties of hemp fiber/PLA composites. materials in the automotive and packaging industries. For an example, Khondker et al [1] investigate jute fiber reinforced poly(lactic acid) (PLA) composites. 2 Experimental In their study, jute fiber and PLA are natural and biodegradable materials, respectively. 2. 1 Molding Methods Composites developed based on thermoplastic matrix materials have several advantages over those based on thermoset matrix materials, such as storage The reinforce fiber used in this study was hemp stability of intermediate materials without freezing fiber (590 tex).The matrix material used in this study device, higher stability in mechanical performance, was biodegradable PLA fiber (55 tex) from cycling usage ability, in situ adaptability (e.g., TORAY., Japan. An intermediate material fabricated through a hot treatment), and so on, and hence have ising a micro-braiding technique. Continuous hemp received more attention in recent years. Unlike fibers were used as the straightly inserted axial thermoset polymers, thermoplastic polymers have fibers, and matrix fibers was braided around the relatively high viscosity. Therefore, it is more reinforcing hemp fibers (Fig. 1). difficult to impregnate thermoplastic resin into continuous fiber yarns. A lot of studies for the short Reinforcing Fiber Reinforcing Fiber Matrix Resin Fiber Matrix Resin Fiber Reinforcing Fiber Matrix Resin Fiber fibers reinforced thermoplastic composites are Matrix Resin Fiber Matrix Resin Fiber Matrix performed, for example, jute fiber composites by A. Resin Fiber Spindle Spindle K Bledzki, et al [2], flax fiber composites by Spindle Benjamin Bax et al [3] and hemp fiber composites by Nina Graupner et al [4]. However, there are few Matrix Fiber studies about continuous natural fiber reinforced Orbit Orbit composites. Braiding Machine Braiding Machine Orbit In order to improve the impregnation of Braiding Machine thermoplastic resin into fiber yarns, Sakaguchi et al. Fig. 1 Fabrication of Micro-Braiding. developed a micro-braiding method to supply flexible material design on continuous fiber Fabrication of continuous hemp fiber reinforced reinforced thermoplastic composites [5]. The PLA thermoplastic composites consisted of two-fold developed method allows many design choices of process. The first step included 20 times winding the thermoplastic matrices. Furthermore, high matrix intermediate material in parallel onto a metallic impregnation and good fiber dispersion can be
range from 400 to 4000 cm -1 and cumulated number frame. The second step involved placement of the metallic frame with intermediate materials in a pre- of 20. heated molding die for consolidation by compression molding to produce composite 2. 5 Calculation of fiber volume fraction and specimens (Fig. 2). The effects of molding porosity temperature (170 ºC, 190 ºC, 210 ºC and 230 ºC) and pressure (1 and 3 MPa), time (4 and 8 min) were investigated. Twelve types of hemp/PLA composite The calculation fiber volume fraction V f and porosity are based on the two weight and volume specimens were fabricated. relationships that are general for all composite materials: Metaric Mold w w f m w w w 1 W W (1) = + ⇒ = + = + c f m f m w w c c v v v f p m Hemp Fiber v v v v 1 = + + ⇒ = + + c f m p v v v (2) c c c Metaric Frame V V V = + + Fig. 2 Alignment of Micro-Braiding for compression f m p molding. where w and v are weight and volume, respectively, W and V are weight fraction and volume fraction, 2.2 Tensile testing respectively, and the subscripts c , f , m and p are composite, fibers, matrix and porosity, respectively. Based on the density of fibers and matrix, Tensile specimens of 180 mm × 20 mm in expressions for V f and V m , V p can be formed: nominal dimensions, with the fiber axis along the ρ loading direction were used in this study. Specimens c V W (3) = f f were clamped over an area of 30 mm × 20 mm at ρ f each end, which results in a gauge length of 120 mm. ρ Aluminum tabs were glued on the clamped area. c V W (4) = m m Strain was measured using a strain gauge, which ρ m were bonded onto the central surface of the V 1 ( V V ) (5) = − + p f m specimen. Tensile test were conducted using an AGS-1000A with a cross-head speed of 1.0 mm/min. where ρ is density. Density is determined based on the principle of Archimedes. 2. 3 Thermal Gravimetric Analysis 3 Results and discussion Thermal gravimetric analysis (TGA) was used to determine the thermal degradation temperature of The stress-strain curve of the composites molded at hemp fiber. The measurements were conducted from 170 º C, 190 º C and over 210 º C are shown in Fig. 3 room temperature to 170, 190, 210 and 230 º C, at a and 4, 5, respectively. Tensile tests could not be rate of 5 º C/min, at hold time of 30 min under air conducted on the specimen molded at 230 º C - 3 atmosphere. MPa – 8 min, because the composites was severely degraded during molding. In case of molding 2. 4 FTIR measurement temperature of 170 º C, elastic moduli were clearly changed with molding pressure and time.. In contrast, FTIR measurement was used to investigate in case of molding temperature of over 190 º C, thermal degradation of hemp fiber during molding. elastic moduli were kept almost constant. In order to Hemp fibers were extracted from the composites evaluate quantitatively, the elastic modulus of the using chloroform. FTIR spectra were obtained by composites molded at different conditions are shown ATR method. The test conditions were measurement in Fig. 6. When the process temperature increased
from 170 º C to 190 º C, elastic modulus increased. The fracture strain of the composites molded at different conditions are shown in Fig. 8. Fracture When the process temperature increased over 190 º C, strain also decreased with increasing molding elastic moduli were kept constant. temperature. 250 3MPa-4min 1MPa-8min 200 Stress (MPa) 3MPa-8min 150 100 50 1MPa-4min 0 4min 8min 4min 8min 0 1 2 3 4 3MPa 1MPa Strain (%) Fig. 6 Elastic modulus of hemp/PLA composites molded at different conditions. Fig. 3 Stress-Strain curve of hemp/PLA composites molded at 170 º C. 250 3MPa-4min 3MPa-8min Stress (MPa) 200 150 1MPa-8min 100 8min 4min 4min 8min 1MPa-4min 1MPa 3MPa 50 Fig. 7 Tensile strength of hemp/PLA composites 0 molded at different conditions. 0 1 2 3 4 Strain (%) Fig. 4 Stress-Strain curve of hemp/PLA composites molded at 190 º C. 250 Stress (MPa) 200 3MPa-8min 150 8min 4min 4min 8min 1MPa-4min 1MPa 3MPa 100 230ºC-1MPa-4min Fig. 8 Fracture strain of hemp/PLA composites 50 molded at different conditions. 0 0 1 2 3 4 As mentioned above, when the process Strain (%) temperature increased from 170 º C to 190 º C, elastic modulus and tensile strength increased and fracture Fig. 5 Stress-Strain curve of hemp/PLA composites strain decreased. In general, increasing molding molded at over 210 º C. temperature reduces viscosity of resin, which result in the good impregnation. At the same time, the The tensile strength of the composites molded at melt overflowing from a molding die makes fiber different conditions are shown in Fig. 7. When the volume fraction of a composite higher. Increasing process temperature increased from 170 º C to 190 moduli and strength with temperature may be º C, tensile strength also increased clearly, whereas attributed to increasing volume fraction. Higher the process temperature increased over 210 º C, fracture strain at molding temperature 170 º C tensile strength decreased. In addition, molding indicated poorer impregnation and interfacial pressure and time hardly affected tensile strength. bonding between fiber and matrix. That is, bonding
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