AIP Conference Proceedings 2233, 040002 (2020); https://doi.org/10.1063/5.0001597 2233, 040002
© 2020 Author(s).
Effect of empty fruit bunch fibre loading on properties of plasticised polylactic acid biocomposites
Cite as: AIP Conference Proceedings 2233, 040002 (2020); https://doi.org/10.1063/5.0001597 Published Online: 05 May 2020
Yan Ding Lee, Ming Meng Pang, Seong Chun Koay, Thai Kiat Ong, and Kim Yeow Tshai
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Effect of Empty Fruit Bunch Fibre Loading on Properties of Plasticised Polylactic Acid Biocomposites
Yan Ding Lee
,1, Ming Meng Pang
1, a), Seong Chun Koay
1, Thai Kiat Ong
2and Kim Yeow Tshai
31 School of Engineering, Taylor’s University Lakeside Campus, Selangor, Malaysia
2Faculty of Engineering, Tunku Abdul Rahman University College, 53300 Kuala Lumpur, Malaysia.
3Faculty of Engineering, University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia.
a) Corresponding author: [email protected]
Abstract. Polylactic acid (PLA) is an environmentally friendly bioplastic that is derived from renewable resources;
however, the inherent brittleness, poor mechanical properties and high material cost limited its usage in current market.
Hence, the addition of plasticiser and natural fibre can reduce the cost and improve the properties of the PLA biocomposite.
In this research, the alkaline treated and untreated empty fruit bunch (EFB) fibre at different loading, i.e., 5, 10, 20 and 30 wt. % were added into coconut oil (CO) plasticised PLA to produce biocomposites. The sample preparation was performed via melt blending method followed by compression molding to produce thin sheet for characterisation. The results showed treated fibre samples have better enhancement in terms of tensile properties, particularly in tensile modulus as compared to untreated fibre. The optimum loading of the fibre was identified as 5 wt.% and 10 wt.% depending on the focus of tensile strength or longer elongation. The 5 wt.% treated fibre biocomposite has a better elongation at break (>8%) while 10 wt.%
treated fibre biocomposite has the highest tensile strength (23.8 MPa) as compared to other fibre content. It is observed that high fibre loading can result in deterioration of the tensile properties. The density of biocomposites decreased significantly with the addition of CO but increased with the addition of EFB fibre. The SEM images of the untreated fibre biocomposites showed more fibres pull out, empty cavities and voids, this explained the poor performance in the tensile properties of the untreated fibre biocomposites.
INTRODUCTION
Plastic is a petrochemical product that are widely used in the past few decades in every aspect of human lifestyle.
However, the non-biodegradable properties of plastic had never been considered as an issue when Leo Hendrik Baekeland first invented plastic [1]. As one of the biggest waste management issue in the world, plastic had polluted the earth and affecting the life of all living creatures. Alternative products to replace non-biodegradable plastic were actively developed in the past decades to solve the disposal problem of non-biodegradable plastics. The invention of polylactic acid (PLA) as a biobased and biodegradable polymer has once caught huge attention of the world with the intention to replace the non-biodegradable plastic. However, the inherent brittleness and the poor mechanical properties of PLA are one of the drawbacks that restricted its potential to replace the non-biodegradable plastic in many applications. Several attempts such as adding plasticiser into the PLA to increase the mechanical properties have been carried out but the performance is still not satisfied [2].
PLA is a biodegradable polyester produced from renewable resources and has very high value on replacing the fossil based thermoplastics [3]. PLA mainly used to produce packaging, including bottles for drinks and food wrap.
Method to synthesis PLA are direct condensation of lactic acid and ring-opening polymerisation of the cyclic lactide dimer [4]. However, the high production cost and brittle characteristic of PLA are the main constrain to make it as a reliable product [5]. The brittleness of the PLA is solved by adding plasticiser and had been proven to increase the flexibility of the PLA but at the cost of lower tensile modulus. From the literature review [6], coconut oil (CO) was used as natural plasticiser and reported that CO has the potential to leach out from the product if it was added at high percentage. Coconut oil (CO) is a natural oil that can be easily found in Malaysia, and it has a high potential to act as a plasticiser to improve the properties of the polymer composite. CO contains high content of medium chain saturated fatty acids that present in the form of triglycerides. Due to the highly similar characteristic of CO compared to some commercial plasticisers such as poly(ethylene glycol), CO is believed to serve as a good plasticiser [6].
There are many studies [5–10] had been conducted to improve the mechanical properties such as tensile modulus by mixing the thermoplastics, e.g., PLA with natural fibre such as sugarcane, kenaf, corn leaves, etc. The addition of natural fibre as a reinforcing agent can enhance the mechanical properties of thermoplastic [11]. In Asia, palm oil tree plantation can be easily found as it is a very popular vegetable oil crop. The empty fruit brunch (EFB) of palm oil after the extraction of palm oil is a waste that can be utilised to maximise the economic value. Moreover, EFB has shown great potential to act as a natural reinforcement agent for polymer composite [12]. However, only using natural fibre as a filler in PLA will not solve the inherent brittleness of PLA/filler composite. Thus, in this study, fixed amount of CO (10 wt.%) was added to provide plasticising effect together with various dosage of EFB added into the polymer matrix to form PLA/CO/EFB biocomposites.
The mixing of PLA/plasticiser and PLA/filler were highly reviewed and experimented for the past few years, but the combination of PLA/plasticiser/filler had attained a low attention of research area. In this study, the focal point is on producing the CO plasticised PLA biocomposites reinforced with different loadings of EFB fibre. There is very limited research conducted on CO plasticised PLA, and the addition of EFB into the CO plasticised PLA is not reported elsewhere.
Two types of EFB fibre were used in this study, namely untreated and alkaline treated EFB fibre. According to Yousif et al [13], treating the fibre with 5-6% of sodium hydroxide (NaOH) can provide better modification of composite by improving the interfacial adhesion and subsequently improve the properties of the PLA composite. Both untreated and treated EFB fibres were added into the CO plasticised PLA, respectively and compounded into thin sheet. The characterisation tests including tensile, density and morphology to investigate the effect of the addition of untreated and treated EFB fibre in the CO plasticised PLA biocomposite.
RESEARCH METHODOLOGY
The initial stage of this research is to prepare the treated and untreated EFB fibre. The treated EFB fibre was obtained by soaking in NaOH solution. The compounding of sample was done by using internal mixer followed by compression molding to form thin sheet. The thin sheet samples were used for characterisation testing such as tensile test, density test, and scanning electron microscope (SEM).
Raw Materials
PLA resin grade 2003D with the melting point of 150°C was purchased from Nature Works. Palm oil EFB was supplied by C&C Harta Sdn.Bhd. Coconut oil (82.5% saturated fatty acids, 6% oleic acid) was purchased from IKO natural beauty company and sodium hydroxide (NaOH) was purchased from R&M Chemicals.
Alkaline Treatment
EFB immersed in 5% NaOH solution for 8 hours at room temperature. After 480 minutes, removed the fibre and rinse with distilled water until the pH reaches neutral. The pH of the fibre was measured using pH meter. Lastly, the fibre were air-dried for at least 2 days [10].
Sample Preparation
The melt blending of PLA/CO and PLA/CO/EFB were performed by using Haake Polylab Mixer at 180°C with rotor speed of 100 rpm for 10 minutes. The biocomposite was compressed into a 0.1 cm thickness sheet through hot press equipment (Moore E53) at 180°C and 150 bars of pressure. The samples were prepared as shown in Table 1 and Table 2. The amount of untreated (U) and treated (T) EFB were added in the range of 5, 10, 20 and 30 wt.% into the fixed amount of 10 wt.% CO plasticised PLA.
TABLE 1. Biocomposite content with untreated EFB fibre (UTF)
Sample PLA (wt.%) Coconut Oil (wt.%) Untreated EFB (wt.%)
Neat PLA 100 0 0
PLA/CO 90 10 0
5 UTF 85 10 5
10 UTF 80 10 10
20 UTF 70 10 20
30 UTF 60 10 30
TABLE 2. Biocomposite content with treated EFB fibre (TF)
Sample PLA (wt.%) Coconut Oil (wt.%) Treated EFB (wt.%)
5 TF 85 10 5
10 TF 80 10 10
20 TF 70 10 20
30 TF 60 10 30
Tensile Test
The tensile test was performed according to ASTM D638 with dumbbell specimen (Type IV) by using Instron Tensile Test Machine (model: Instron 3366). The crosshead speed of the testing was 2 mm per min with a load cell of 50kN, and the tests were conducted at room temperature. The tensile properties (tensile strength, Young’s modulus and elongation at break) of an average of 5 specimens were reported.
Density Test
Thin sheet was cut into size of 1 cm x 3 cm using laser cutting machine with the power of 80 and 60 rpm speed.
The density of each sample was measured using Mettler Toledo density weighing machine (model: ME204).
Scanning Electron Microscopy (SEM)
The fracture surface of the samples was examined through SEM, model FEI Quanta 400F Field Emission Scanning Electron Microscope (FESEM). The sample was coated with platinum (10-25 nm) to prevent electrical charging.
RESULTS AND DISCUSSION Tensile Properties
The specimen in dumbbell shape before and after the tensile testing is shown in Fig.1(a) and 1(b). The tensile strength result was shown in Fig.2. The neat PLA showed the highest tensile strength at 66 MPa, and the addition of 10 wt.% CO into PLA reduced the tensile strength to 22 MPa. The reduction in tensile strength is attributed to the plasticiser effect. Plasticiser CO reduced the intermolecular forces along the PLA polymer chains, substitute them with weak hydrogen bonds formed between plasticiser and PLA molecules, which caused reduction in rigidity and
tensile strength. Overall, the treated fibre biocomposites showed higher tensile strength as compared to untreated fibre biocomposites. The increase of treated fibre content enhanced the tensile strength of the biocomposite up to 10 wt.%
(23.8 MPa) and thereafter, no improvement was recorded. Similar tensile strength trend was observed in untreated fibre biocomposites where beyond the 10 wt.% of fibre content, the tensile strength started decreasing.
FIGURE 2. Tensile strength for biocomposites with treated and untreated EFB at different content.
The elongation at break of each biocomposites were illustrated in Fig.3. The addition of CO into the PLA showed improvement in the elongation at break, approximately double the elongation value of the neat PLA. The addition of EFB fibre as stiff filler restricted the movement of the polymer chains, thus, decreasing the flexibility of the biocomposites. This is reflected in lowest elongation at break was recorded at 30 wt.% fibre loading for both treated and untreated fibre biocomposites. The PLA matrix had reached the limit to form fibre-matrix interaction instead, fibre-fibre interaction increases once the amount of fibre loading exceeds the maximum allowable content [13].
Nonetheless, the elongation at break of treated fibre biocomposites is still better than untreated fibre, attributed to the better interfacial adhesion between the treated fibre and PLA matrix after the removal of impurities and lignin by NaOH solution.
0 10 20 30 40 50 60 70 80
Neat PLA PLA/CO 5 10 20 30
Tensile Strength (MPa)
Fibre Content (wt%)
Untreated Fibre Treate Fibre FIGURE 1(A). Before tensile test FIGURE 1(B). After tensile test
FIGURE 3. Elongation at break for biocomposites with treated and untreated EFB at different content.
The tensile modulus or Young’s modulus refers to the stiffness of the composite. Figure 4 shows the Young’s modulus of different EFB loading in CO plasticised PLA biocomposites. As mentioned earlier, the addition of CO plasticiser into PLA can improve the elongation at break but it also caused reduction in Young’s modulus. In this study, the addition of EFB fibre as a stiff filler has successfully improve the Young’s modulus of the PLA/CO biocomposites, by forming interlocking with PLA matrix [14,15]. The Young’s modulus for PLA/CO is reported as 3603 MPa and the addition of 20 wt.% treated fibre has increased the Young’s modulus of the biocomposite to 5529 MPa. The treated fibre biocomposites showed overall higher Young’s modulus than untreated fibre biocomposites, this is attributed to the better interfacial adhesion after alkaline treatment. The Young’s modulus dropped drastically at 30 wt.% since the fibre loading was too much and fibre-fibre interaction occurred instead of fibre matrix interlocking.
FIGURE 4. Young’s modulus of biocomposites with treated and untreated EFB at different content.
0 2 4 6 8 10 12
Neat PLA PLA/CO 5 10 20 30
Elongation at break (%)
Fibre Content (wt%)
Untreated Fibre Treated Fibre
0 1000 2000 3000 4000 5000 6000 7000
Neat PLA PLA/CO 5 10 20 30
Young's Modulus (MPa)
Fibre Content (wt%)
Untreated Fibre Treate Fibre
Density
Refer to Fig. 5, the density of CO plasticised PLA (1.201 g/cm3) was lower than neat PLA (1.243 g/cm3). This can be explained by the density of oil is much lower compared to PLA. Thus, the addition of CO will decrease the density of the PLA biocomposites. According to previous studies, the natural fibre density is ranged between 1.4 -1.6 g/cm3, while the density for coconut oil is reported as 0.912 g/cm3. [16, 17]. The addition of EFB fibre into PLA/CO can increase the density of the biocomposite since fibre density is higher than CO. As shown in Fig.6, the density of biocomposites increased with increasing of EFB fibre content. It is observed that the density of the treated fibre biocomposites is higher than untreated fibre, this could be due to the removal of impurities such as wax which is light in weight and subsequently increase the density of the treated fibre.
FIGURE 5 Density of PLA and PLA/CO
FIGURE 6. Density of different content of untreated and treated EFB fibre reinforced PLA/CO biocomposites 1.243
1.201
1.170 1.180 1.190 1.200 1.210 1.220 1.230 1.240 1.250
PLA PLA/CO
Density (g/cm3)
1.207 1.209
1.229
1.212 1.207
1.217
1.235 1.236
1.190 1.195 1.200 1.205 1.210 1.215 1.220 1.225 1.230 1.235 1.240
5 10 20 30
Density (g/cm^3) Untreated Fiber
Treated Fiber
Morphology
The fracture surface of PLA biocomposites can be observed via SEM. The addition of CO into PLA formed porous material (Fig. 7). The formation of pores might happen due to hot pressing the sample at high temperature (180°C), and this cause the coconut oil starts to evaporate (smoke point of 175°C), left behind large number of pores and voids [18]. Figure 7(A) and (B) shows the comparison between PLA and PLA/CO.
The addition of EFB filler in powder form into PLA/CO can fill up some of the pores in the PLA matrix. These changes can be observed from Fig. 8(A) and (B) with dispersion of 10 wt.% fibre as lesser pores/voids were present in matrix. The EFB fibres filled up some of the porous and voids in the PLA/CO matrix, and the rigid structure of natural fibre can aid the transfer of stress from PLA matrix to fibre as reflected in the enhancement of tensile properties such as higher Young’s modulus.
As the fibre content increased to 20 wt.%, more fibres pull-out and empty cavities can be seen from Fig. 9(A) and (B) as compared to 10 wt.% fibre in Fig. 8(A) and (B). Between the treated and untreated fibre biocomposites, more voids and pores can be viewed in untreated fibre sample as compared to same fibre content in treated fibre. Similar result was observed from 30 wt.% fibre content, where more voids and empty cavities appeared in untreated fibre matrix after the agglomerate of fibres pull out, attributed to the poor interfacial adhesion between the fibre-matrix as compared to treated fibre sample shown in Fig. 10 (A) and (B).
CONCLUSION
Treated EFB fibre is a better reinforcing agent as compared to untreated EFB fibre since treated sample shows a better tensile property in terms of tensile strength, tensile modulus and elongation at break. The optimum treated fibre loading lies between 5 wt.% and 10 wt.% where 5 wt.% have a higher elongation at break (>8%) while 10 wt.% have a higher tensile strength (23.8 MPa) but both biocomposites have similar Young’s modulus (5315 MPa and 5462 MPa). Fibre content exceeding 10 wt.% caused tensile properties reduction since the EFB fibres tend to agglomerate as it is hard to disperse in high content, it formed gap between fibre-matrix and led to poor interfacial adhesion.
PLA/CO showed the lowest density among all and as the loading of EFB increased, the density increased as well.
ACKNOWLEDGEMENT
The authors acknowledge the contribution of C&C Harta Sdn. Bhd. and Taylor’s University for the material and equipment.
FIGURE 7(B). PLA/CO FIGURE 7(A). Neat PLA
pores voids
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