This study investigated the mechanical behaviors of injection molded polylactic acid (PLA) composites reinforced with carbon fiber (CF) at different fiber loading levels (5 wt%, 10 wt%, 15 wt% & 20 wt%). PLA, a biodegradable thermoplastic derived from renewable resources, has been replacing petroleum-based plastics in many applications due to its sustainability and low environmental impact. However, the low mechanical strength limits its wide structural applications. The addition of small amount of CF significantly increased the tensile strength and modulus while leading to reducedductility. Compared to pure PLA, the composites with 5 wt% CF content had a 40% increase of tensile modulus and a 63% decrease of elongation-at-break. The effects of water absorption on the mechanical properties of PLA/CF composites were also studied.
By Muralidhar Reddy Lingam, Gangjian Guo, John Jung-Woon Yoo and Victoria L. Finkenstadt * Industrial & Manufacturing Engineering & Technology Department, Bradley University, IL, USA
*National Center for Agricultural Utilization Research, Agricultural Research Service, US Department of Agriculture, 1815 North University Street, Peoria, IL 61604, USA
PLA is a biodegradable plastic material derived from renewable sources. The demand for ecologically friendly material makes PLA a promising material. However, the relatively weak mechanical properties of PLA limit its wide structural applications. Incorporating fibers into the PLA matrix is one of effective ways to tailor and enhance the mechanical properties. To create green composites, natural fibers such as kenaf fiber, hemp fiber, flax fiber, have been added to PLA to tailor its properties. Ochi et al 1 studied mechanical properties of kenaf/PLA composites, and reported that the tensile strength and flexural strength of the composites increased linearly up to a fiber loading of 50 vol.%. The kenaf/PLA composite specimens were produced by heating prepregs in metallic mold and then hot pressing. Oksman et al 2]conducted a study on PLA/flax composites and PP/flax composites. It was reported that flax can be used as a reinforcing material with the PLA and the strength of PLA/flax composites are about 50% better compared to similar PP/flax composites. The test samples were produced in a two-step process. The composite materials were extruded with a twin-screw extruder and then compression molded. Huda et al 3 conducted a study on PLA composites reinforced with recycled newspaper cellulose fiber (RNCF) and chopped glass fibers through a twin-screw extruder and then an injection molding machine. It was reported that mechanical properties of the PLA composites were favorable compared to the properties of PP composites. Guo et al4 studied statistically the effects of molding conditions on the mechanical behavior of PLA/wood-fiber composites with response surface methodology.
Use of natural fibers to reinforce PLA is not sufficient for structural applications. Conventional fibers, glass fiber (GF) and carbon fiber (CF), are excellent reinforcements for enhancing mechanical properties. CF has a density of about 1.6 g/cc, and is lighter than GF which has a density of 2.1-2.7 g/cc. Developing lightweight structural composites is important in automotive engineering to improve fuel efficiency. In literature, a few studies were conducted on PLA/CF composites. Shen et al 5 studied the mechanical properties of hydroxyapatite (HA)/PLA composite reinforced with 20 vol.% CF. The CF/PLA/HA composite was prepared by hot pressing a prepreg which consisting of PLA, HA and CF. Wan et al 6 studied the influence of surface treatment on the mechanical properties of PLA/CF composites. It was reported that surface treatment of oxidized CF with nitric acid improved mechanical properties of the CF/PLA composites. PLA/CF composites were produced by compression molding. Another paper 7 reported the mechanical and thermal properties of PLA and recycled carbon fiber composites. The composites studied were produced by compounding PLA and recycled CF in micro-compounder and then processing the composite material in injection molding machine.
The conventional injection molded composites are typically produced by a two-step process, which includes pre-compounding of polymer with additives and then processing with an injection molding machine. There were studies conducted on a one-step injection molding 8,9 process, where composite parts are molded directly from a dry blend of the resin, reinforcement, and fillers. Moriwaki et al [8] reported that the breakage of GFs is smaller in a one-step injection molded GF/PA composite. The advantages for one-step injection molding are the reduced operation cost and the reduced fiber breakage occurred in high-shear compounding process. Nakao et al 10 studied direct fiber feeding injection molding (DFFIM) by using CF and commingle yarn as reinforcing fibers with PA6 and PA66. The fiber feeding is done through a vent opening on the resin metering equipment.
It was reported that the fiber was uniformly dispersed in the polymer matrix with both CF and commingle yarn. It has not been reported in the literature about the single-step injection molding of PLA/CF composites with a low content of CF. PLA is naturally hydrophilic due to its polar oxygen linkages and is prone to hydrolytic degradation in the presence of water. Studies were conducted to investigate the effects of water absorption on PLA composites. Tham et al 11 studied the effects of water absorption on the thermal and impact properties of PLA/halloysite nanotube (HNT) composites at three different temperatures. It was reported that the water uptake increased with an increase in water temperature, and the diffusion coefficient of pure PLA is higher than that of the PLA/HNT composites due to the presence of HNT in the composites. PLA/HNT composites were prepared with melt compounding followed by compression molding. Ndazi et al 12 reported that the water uptake of PLA/rice hulls composites increased with an increase in water temperature when subjected to water absorption test. Also, the increase of rice hulls content increased the diffusion of water into thecomposites. There were no studies to understand the effects of water absorption on the PLA/CF composites. This paper investigates the effects of water absorption on the mechanical properties of PLA/CF composites and the effect of CF on the water absorption behavior of PLA/CF composites.
Experimental
Materials
The plastic material used in this study was PLA (3052D Ingeo grade, with MFR = 14 g/10min, Specific Gravity = 1.24), The CF used was Panex® 35 chopped fiber (Type-65) from Zoltek Corp., the normal fiber length is 6 mm. All the materials were used as received. The material formulation for the injection molding experiments is shown in Table 1.
Mold Design and Fabrication
A two-part family mold was designed according to the ASTM D638 13, as shown in Figure 1. The fan gate wasselected to achieve a uniform material flow, minimizebackfilling and part warpage, and keep the cross sectional area constant. It is also suitable for rapid filling of large parts or fragile section mold area with large entry area. The steel insert mold base from DME (Model 08/09 U Style Frame) was cut with a CNC machining center.
Injection Molding Machine
The injection machine used for making the composite specimens was Engel E-victory 30 with a 30-ton clamping force. The screw diameter is 22 mm with L/D ratio of 30. The injection process parameters, shown in Table 2, were used for all the experiments.
Experimental Design
The PLA and CF were dry blended according to the material formulation in Table 1. This blended material was directly put into the hopper of injection molding machine to process composite specimens. For all the experiments, the injection processing parameters were kept the same. Before switching from one experiment to another, a sufficient amount of pure PLA was used to purge the system. Specimens were collected for each material formulation according to ASTM D638.
Tensile tests were performed on an MTS tensile testing machine with the load cell capacity of 250 kN. The grip distance was 115 mm and the crosshead speeds of 5 mm/min. 10 specimens were tested for each material formulation. The data reported were the average values of the measurements. A water absorption test was conducted by placing the specimens in water for 24 hours at room temperature. Immediately, after the water absorption test, specimens were weighed and subjected to a tensile test. 10 specimens were tested for each material formulation and the data reported were the average values of the measurements.
Results and Discussion
Tensile Load vs. Extension
Figure 2 shows the tensile test curves for the pure PLA and PLA/CF composites. The tensile strength and modulus increased with an increase in CF content from 5 wt% to 20 wt% when compared to those of pure PLA. The ductility of the composite materials decreased with the increase in CF content from 5 wt% to 20 wt%. Figure 3 shows the fractured specimens after the tensile test, for all the formulations including pure PLA and PLA/CF composites.
Density of Injection Molded Composites
Figure 4 shows the density of the PLA and PLA/CF composites. Compared to pure PLA, the density of composites increased gradually with the increase in CF content from 5 wt% to 20 wt%. It was also observed that the variability or standard deviation increased for the composites with 15 wt% and 20 wt% CF, which indicates the mixing and distribution of CF would become difficult when the CF content increased to 20 wt% in this single step injection molding process without pre-compounding.
Tensile Strength and Tensile Modulus
Figure 5 shows the tensile strength of all the materials before and after they were subjected to the water absorption test. As the CF content increases, the tensile strength of the composites increases almost linearly. Compared with pure PLA, the composites with 20 wt% CF has about 54% increase in tensile strength. However, between the composites with 15 wt% and 20 wt%, the strength increase is not significant. This is probably due to the poor mixing at a relatively high CF content. The composites after water absorption show a slight decrease in tensile strength, it could be because the penetrated water would weaken the interfacial bonding between the fiber and the PLA matrix. Another contributor could be a slight degradation of the PLA from
hydrolysis as water penetrates along the interfaces.
Figure 6 shows the tensile modulus of all the materials before and after they were subjected to the water absorption test. Compared with pure PLA, the composites had a significant increase in tensile modulus with the increase of CF content from 5 wt% to 20 wt%. The addition of 5 wt% CF leads to an increase of 40% in tensile modulus. As the CF content increases to 20 wt%, the tensile modulus significantly increases to 2.4 times that of pure PLA.
before and after
water absorption.
Elongation at Break
Elongation at break, also known as fracture strain, is the ratio between the changed length and the initial length after breakage of the test specimen. It is also a measure of the ductility of the composites. Figure 7 shows the elongation at break for all materials before and after they were subjected to the water absorption test. The elongation at break decreased significantly when the 5 wt% CF was incorporated in the PLA matrix. As the CF content increases, the elongation at break slightly decreases. After the water absorption test, the pure PLA shows a slight increase in elongation at break.
Water Absorption
Figure 8 shows the water uptake of pure PLA and PLA/CF composites after they were subjected to water absorption test. The water uptake of the composites increased as the CF content increases. This could be because the increase of interfacial voids between the CF and the PLA matrix, as well as the increase of porous CF content. However, compared with pure PLA, the composites with 5 wt% CF has a decrease of 51% in water uptake. There could be two possible explanations. First, there could be micro cracks on the PLA surface adsorb water. Second, the addition of CF increases the crystallinity of PLA/CF composites, because it acts as a nucleating agent and promotes a faster formation of crystalline domains. As a result, higher crystallinity is considered as a barrier against the advance of penetrant and a reduction in the diffusion coefficient 14. Further investigation is needed to clarify.
Conclusions
This paper investigated the mechanical characteristics of PLA and PLA/CF composites produced from a single injection molding process. The effects of water absorption on the mechanical characteristics of PLA/CF composites were studied.
Experimental results show an increase in tensile strength and tensile modulus of the composites, as the CF content increases. The addition of 5 wt% CF increased the tensile strength and tensile modulus by 19% and 40%, respectively. As the CF content increased to 20 wt%, the tensile strength and tensile modulus significantly increased by 54% and 243% respectively, compared to those of pure PLA. However, the increase in CF content led to the decrease of the elongation at break of the composites and the loss of ductility.
Water absorption has a larger influence on the tensile strength than the tensile modulus and the elongation at break of the composites. This paper suggests that a single step injection molding would be proper for making the composites with a CF loading less than 20 wt%. Moreover, the addition of small amount of CF (i.e., 5 wt%) would significant improve the tensile modulus and tensile strength.
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