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Sisal fiber biocomposites
Composites Part B 173 (2019) 106895
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Recyclability analysis of PLA/Sisal fiber biocomposites Saurabh Chaitanya a, Inderdeep Singh b, Jung Il Song a, * a b
Department of Mechanical Engineering, Changwon National University, Changwon, 51140, South Korea Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Uttarakhand, 247667, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Biocomposites PLA Sisal fiber Recycling Extrusion Injection molding DMA
This study presents the recyclability analysis of PLA/Sisal biocomposites comprising sodium bicarbonate treated Sisal fibers (30 wt%). The biocomposites were recycled (8 times) using the extrusion process. The tensile strength of injection molded biocomposites declined by 20.9% up to the third recycle. The dynamic mechanical analysis revealed a severe reduction in storage and loss modulus beyond third recycle. The morphological and thermal characterization of recycled biocomposites also revealed severe fiber and matrix degradation. The infrared spectroscopy exhibited hydrolysis as one of the causes of PLA degradation after recycling. The recycling of PLA/ Sisal biocomposites beyond third recycle is not recommended. However, the biocomposites recycled up to third recycle can be used for making products for low to medium strength non-structural applications.
1. Introduction The application spectrum of fiber reinforced plastics has increased exponentially during the last few decades. This has led to the generation of a huge amount of non-degradable plastic waste throughout the globe [1]. Petroleum-derived polymers and synthetic fibers (such as glass, carbon, etc.) exhibit extremely slow degradation behavior and are hence termed as non-degradable polymer composites. To tackle this global environmental concern, the focus of material scientists is shifting to wards the development of eco-sustainable materials [2–4]. Another reason for this shift can be attributed to the environmental regulations and directives issued by governments of various countries. For example, in order to curb the increasing pollution, the European government has issued a directive that allows an incineration quota of only 5% for dis carded cars [5]. The Japanese government also issued a directive to use 20% of bioderived plastics by 2020 [5]. Eco-sustainable materials can be defined as materials derived from renewable resources which can be recycled and are triggered biode gradable [5]. Polylactic Acid (PLA) has been identified as the most promising biopolymer having the potential to replace traditional engi neering and commodity plastics in various applications [6,7]. To reduce the cost of PLA and improve its applicability, the incorporation of nat ural lignocellulosic fibers as reinforcement into PLA has been explored by several researchers in the past decade. The lignocellulosic fibers mainly comprise of lignin, cellulose, pectin, and hemicellulose [8,9]. The composition of these constituents defines the fiber characteristics as
well as the interfacial bonding between the matrix and the fibers. This composition can be controlled by modifying the fiber surface using different techniques reported in the literature [10,11]. The bio composites incorporating lignocellulosic fibers have been reported to exhibit the potential to be used in a variety of applications. This increase in application potential of biocomposites has warranted the need to explore their commercial processability [12]. The PLA based biocomposites can be easily processed via commer cially available processing routes used for thermoplastics such as extrusion, injection molding and compression molding [12,13]. The commercial processing of biocomposites using rapid molding processes like injection molding leads to the generation of inevitable industrial waste in the form of sprue, runners, gates, etc., which must be trimmed off after injection molding [14]. Hence, the waste generated after the end of useful product life as well as industrial waste generated during processing of biocomposites is a serious concern for the biocomposite industry. PLA and PLA based biocomposites are triggered degradable, which can biodegrade under composting or specific environmental conditions [15]. Apart from biodegradation, PLA based biocomposites have an added advantage of being recyclable. The recyclability of bio composites is a lucrative option which is yet to be thoroughly explored. Recyclability reduces the cost of the product as well as the demand for landfills. Recycling or reuse of the material after the end of the useful life cycle of a product is highly encouraged. Moreover, strict environmental regulations have been proposed by several countries encouraging the researchers and designers to use recyclable materials [16]. Hence, the
* Corresponding author. E-mail address: [email protected] (J.I. Song). https://doi.org/10.1016/j.compositesb.2019.05.106 Received 19 March 2019; Received in revised form 27 April 2019; Accepted 12 May 2019 Available online 16 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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industrial waste generated during fabrication of biocomposite product in terms of trims and the excess material removed should be recycled to limit the industrial waste and reduce the production costs. The recycling of biocomposites is challenging as composite materials are heterogeneous in nature [17]. The recycling methods for bio composites can be classified into three categories, mechanical recycling, chemical recycling and thermal processing [18]. Mechanical recycling involves remelting and remolding of biocomposites using multiple extrusion and/or injection cycles. Chemical recycling involves the dissolution of the polymer matrix and separation of fibers from the matrix, while thermal processing involves energy recovery through incineration or combustion. Mechanical recycling is considered as one of the most successful material recovery methods due to its ease of pro cessing, relatively low investment and ability to control technological parameters [19]. Mechanical recycling of thermoplastic materials like PLA is hence a desirable option given its ease of processing and para metric control [19,20]. During the last decade, few studies exploring the recyclability of the PLA matrix have been reported. The behavioral analysis of up to 10 recycles of PLA was performed using multiple extrusion and subsequent injection molding process by Zenkiewicz et al. [21]. The authors reported a marginal decrease in the tensile strength while the impact resistance declined with increasing recycling number. In a different study by Taubner et al. [22], it was reported that the multiple extrusions of PLLA at higher processing temperatures can lead to a severe reduction in its molecular weight. Badia et al. [19] explored the recycling behavior of PLA for up to five extrusion cycles. The authors also reported severe loss of molecular weight of PLA after 5 recycles. However, when reinforced with short lignocellulosic fibers, the recycling behavior of PLA based biocomposites becomes complex and is scarcely reported in the literature [5]. In the previous studies, it has been concluded that PLA/Sisal biocomposites have the potential to replace conventional plastics used in several commercial applications like automotive interiors, furniture, toys, disposable cutlery, electronic equipment, etc. [23,24]. Hence, it becomes imperative to study and report the effect of mechanical recycling of PLA/Sisal biocomposites on their thermal and mechanical behavior along with the determination of number of recycles that PLA/Sisal biocomposites can undergo within the acceptable behavioral range. This recyclability study is performed on PLA/Sisal biocomposites made by incorporating Sisal fibers treated with a novel eco-friendly sodium bicarbonate treatment as reported earlier by authors [24].
2.2. Methodology 2.2.1. Fiber surface treatment The Sisal fibers were treated by soaking them in an aqueous solution of sodium bicarbonate (NaHCO3) and distilled water, having NaHCO3 concentration of 10% (w:v). The Sisal fibers were soaked in the NaHCO3 solution for an optimum duration of 72 h and stirred at regular intervals [24]. The treated fibers were then washed with distilled water and subsequently dried in an air oven at a temperature of 100 � C for 8 h. The characteristics of raw and treated Sisal fibers are given in Table 2. 2.2.2. Recycling of biocomposites The recycling of biocomposites incorporating treated Sisal fibers is simulated experimentally by using multiple extrusion route. In this technique, the PLA biocomposites reinforced with Sisal fibers having fiber weight fraction of 30%, were recycled using a single screw extruder (Sai Extrumech, India). Based on the previously reported studies, the screw speed of 60 rpm and a temperature profile of 150–175-180-185 � C (feed zone to die) were selected during processing [24]. The extrudate in the form of 4 mm strand was quenched in water and subsequently pelletized to 5 mm pellets using a mechanical pelletizer. The extruded biocomposite pellets were then dried in an air oven at 100 � C for 8 h after each cycle. The biocomposite pellets were extruded for nine times, as further recycling caused severe degradation of PLA-Sisal melt blend, making it difficult to process. A small batch of biocomposite pellets from each extrusion cycle was used for the development of test specimens using a commercial injection molder (Electronica Endura-60, India). The injection molding processing parameters used to develop recycled bio composite test specimens have been listed in Table 3. A total of nine batches of biocomposite pellets were prepared leading to the develop ment of eight recycled biocomposites. The nomenclature of the recycled biocomposites is given in Table 4. 2.2.3. Mechanical behavior To explore the recyclability of PLA/Sisal biocomposites, their me chanical behavior in terms of tensile, flexural and impact properties, were determined in accordance with ASTM 3039M-14, ASTM-D790-10, and ASTM-D256-10, respectively. Tensile and flexural tests were per formed using an Instron-5982, universal testing machine, while IzodImpact tests were performed employing (IT504, Tinius Olsen) low en ergy impact tester. The gauge length and a crosshead speed of 50 mm and 1.5 mm/min were selected during tensile tests. The flexural tests were conducted at a crosshead speed of 2 mm/min and a gauge length of 64 mm. Impact tests were conducted using a notched impact test (Izod) specimens. A total of 4 specimens were examined during each test and
2. Materials and method 2.1. Materials High-performance PLA biopolymer (Grade: Ingeo 3260HP) in the form of pellets was sourced from NatureWorks LLC, USA. The properties of PLA used in this experimental investigation are given in Table 1. Sisal fibers chopped to 3 mm in length were sourced from Women’s Devel opment Organization, Dehradun, India. The image of Sisal fibers is given in supplementary file: Fig. S1. In order to treat the surface of Sisal fibers prior to the development of biocomposites, sodium bicarbonate (extrapure AR, 99.5%) was sourced from SRL Pvt. Ltd., India.
Table 2 Properties of raw and treated Sisal fibers. Material
Property
Raw Sisal Fiber Treated Sisal Fiber
Single Fiber Strength (MPa)
Cellulose (%)
Lignin (%)
Hemicellulose (%)
Reference
337.6 (�79) 669.7 (�62)
64.9
9.2
24.6
[24]
84.9
8.8
6.2
[24]
Table 1 Properties of PLA. Material
PLA
Property Melt Flow Rate (g/10 min) (210C, 2.16 Kg)
Specific gravity
Tensile strength (MPa)
Tensile modulus (MPa)
Flexural Strength (MPa)
Flexural Modulus (MPa)
Impact Strength (J/m)
Reference
65
1.24
43.9 (�0.8)
1997.4 (�133.1)
60.2 (�1.4)
3395.7 (�197.4)
16.3 (�0.5)
[24]
2
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cooling rate, this study only focuses on the first heat scan results to determine the onset glass transition temperature (TG) (mid-point method), glass transition temperature (TOG), crystallization temperature (TC), melting temperature (TM), onset melting temperature (TOM), enthalpy of melting (ΔHM) and enthalpy of crystallization (ΔHC). The crystallinity (XC) of the recycled biocomposites was evaluated using the relation given in equation (1).
Table 3 Injection molding processing parameters [24]. Parameter
Value
Holding Pressure Holding time Screw speed Barrel temperature Mold temperature Back pressure Injection time Injection pressure
55 MPa 8s 120 rpm (feed-to-nozzle) 160–175–185–195 � C 30 � C 5 MPa 1.8 s 60 MPa
XC ð% crystallinityÞ ¼
Recycling Order
Designation of Biocomposites
1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8
PL/TS PL/TS-1 PL/TS-2 PL/TS-3 PL/TS-4 PL/TS-5 PL/TS-6 PL/TS-7 PL/TS-8
(1)
where ‘w’ represents the weight fraction of PLA in the sample and ΔHom (93 J/g) is the crystallinity of 100% crystalline PLA [25–27].
Table 4 Nomenclature of recycled biocomposites. Extrusion Cycle Number
ΔHM ΔHC 100 � w ΔH oM
2.2.8. Fourier transform infrared spectroscopy (FTIR) FTIR spectroscopy technique was used to ascertain any changes occurring in the chemical composition of the biocomposites after recy cling. FTIR spectroscopy of the recycled biocomposites was performed using FTIR spectrometer (Magna 760, Thermo Nicollet). The bio composite specimens were grounded and mixed with KBr and then pressed to make thin discs of 10 mm diameter. The FTIR spectra were recorded in the transmittance mode between 400 and 4000 cm 1. 2.2.9. Surface roughness In order to measure the surface imperfections of the injection molded biocomposites, their average surface roughness value (Ra) was measured using a surface roughness tester (Mitutoyo SJ-201).
their average value along with error bars representing standard devia tion have been reported.
3. Results and discussion
2.2.4. Morphological evaluation Morphological evaluation of the fractured tensile test samples and surface of recycled biocomposites was done using scanning electron microscope (LEO 1550). To study the fiber morphology after the pro cessing of biocomposites, Sisal fibers were extracted from recycled biocomposites by dissolving the matrix into chloroform (SRL Pvt. Ltd., India). The extracted fibers were thus spread on a glass slide and viewed under a stereo microscope (Nikon SMZ-745T). Extracted fiber lengths of approximately 100 fibers for each specimen were measured and their mean along with standard deviation has been reported.
3.1. Dynamic mechanical analysis Storage Modulus (E0 ) is indicative of the energy stored in a substance, during deformation and is often linked with the stiffness of the material [28]. In comparison to neat PLA (2526.14 MPa), the storage modulus of PL/TS biocomposites significantly improved after incorporation of treated Sisal fibers (Fig. 1). At room temperature, storage modulus was observed to lie within the range of 3400–4200 MPa for all the reproc essed biocomposites. Compared to storage modulus of PL/TS bio composite an imperceptible increase in the E0 values of recycled biocomposites was observed after first (PL/TS-1) and second (PL/TS-2) recycle. This is indicative of enhanced interfacial bonding between the PLA and Sisal fibers due to thermo-mechanical reprocessing. However, beyond the third recycle, a gradual decline in the storage modulus was observed. A steep decline in E0 value was observed between 55 and 65 � C for all the developed biocomposites. This decline in the E’ value between the temperature range of 55–65 � C is attributed to the loss of stiffness around glass transition temperature (TG) [29]. A gradual reduction in the onset temperature of this decline beyond third recycle was observed, indicating a decline in TG. The loss modulus of a material is related to its molecular chain movements [30,31]. Fig. 2 depicts the Loss Modulus (E00 ) of the recycled biocomposites as a function of temperature. It can be observed from Fig. 2 that the E00 peak of neat PLA rises significantly after the incor poration of Sisal fibers (PL/TS). Similar to the trend observed in storage modulus curves, the E00 peaks of PL/TS-1, PL/TS-2 and PL/TS-3 were observed to be higher as compared to PL/TS biocomposites. The increase in the E00 peak signifies the increased restrictions in the mobility of the polymer chains due to good fiber-matrix interaction [28]. However, beyond the third recycle, the E00 peaks were observed to gradually decline as well as a shift towards lower temperature. This declining trend in the E00 peaks can be attributed to rise in the mobility of the polymer chains. The increase in mobility of the polymer chains can be due to the fiber attrition as well as polymer chain scission taking place during reprocessing. The shifting of E00 peak towards lower temperature signifies the reduction in the TG of recycled biocomposites. Moreover, in
2.2.5. Dynamic mechanical analysis (DMA) Thermo-mechanical behavior of the recycled biocomposites was examined employing a Dynamic Mechanical Analyzer (TA-DMA-Q800, TA Instruments) equipped with a dual-cantilever fixture in accordance with ASTM D4065-12. The DMA tests were conducted at an operating sinusoidal frequency of 1 Hz, between the temperature range of 35 � C to 90 � C with a temperature ramp of 3 � C/min. The dynamic mechanical behavior of the developed biocomposites was assessed in terms of storage modulus (E0 ), loss modulus (E00 ), and damping coefficient (tan δ). 2.2.6. Thermogravimetric analysis Thermogravimetric analysis (TGA) of the recycled biocomposites was conducted using Thermogravimetric Analyzer (EXSTAR 6300, Seiko Instruments). During this analysis, a small quantity (10 mg) of recycled biocomposite samples were examined at a temperature ramp rate of 10 � C/min from ambient temperature to 550 � C, in the presence of ni trogen gas. The weight loss of the biocomposites with respect to tem perature is recorded to ascertain their thermal degradation behavior. 2.2.7. Differential scanning calorimetry (DSC) In order to thoroughly examine the effect of recyclability on the thermal properties of recycled biocomposites, the DSC analysis of bio composite specimens was performed using Simultaneous Thermal Analyzer (STA 6000, PerkinElmer). During this analysis, a small quan tity (10–15 mg) of recycled biocomposite samples were heated in a crucible at a temperature ramp rate of 10 � C/min from 35 � C to 200 � C, in the presence of nitrogen. As crystallization of PLA is sensitive to 3
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Fig. 1. Storage modulus curves of recycled biocomposites.
Fig. 2. Loss modulus curves of recycled biocomposites.
comparison to neat PLA, the E00 peaks of recycled biocomposites were observed to widen, resulting in broadening of glass transition range. The curves of Tan δ as a function of temperature are depicted in Fig. 3. Tan δ is defined as the ratio of E00 to E0 . It is evident from Fig. 3 that, the peak of Tan δ significantly decreases after the addition of Sisal fibers into neat PLA. The decrease in Tan δ peaks after the incorporation of Sisal fibers signifies restricted polymer chain mobility [28,32]. However, the Tan δ peak was observed to gradually rise with subsequent reprocessing cycles. This increase may be attributed to the reduced fiber size and PLA chain scission during repeated thermo-mechanical pro cessing, both resulting in higher molecular chain mobility.
3.2. Thermogravimetric analysis The thermogravimetric curves of the recycled biocomposites are represented in Fig. 4. The thermal degradation of PL/TS biocomposites was observed to take place in three degradation steps. First weight reduction takes place between 50 � C and 250 � C due to the loss of moisture and pyrolysis of hemicellulose present in Sisal fibers. The second step of thermal degradation takes place between 260 � C and 365 � C due to the pyrolysis of PLA, cellulose and some part of lignin. The third step of degradation observed between 360 � C and 465 � C occurred due to the pyrolysis of residual lignin and PLA [33]. It can be clearly observed from second and third degradation steps, that the thermal
Fig. 3. Tan Delta curves of the developed biocomposites. 4
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Fig. 4. Thermogravimetric thermographs of the recycled biocomposites.
stability of the recycled biocomposites gradually declined after each recycle. The thermal stability of recycled biocomposites was observed to imperceptibly decline up to third recycle (PL/TS-3). However, beyond the third recycle, the resistance to thermal degradation of recycled biocomposites was observed to reduce significantly. This decrease in thermal stability can be attributed to the thermo-mechanical degrada tion of biocomposites which causes both fibers and the PLA matrix to degrade during reprocessing [34]. Lopez et al. [35] also reported a progressive decline in the thermal stability of PLA after successive thermomechanical recycles, indicating the degradation of the PLA matrix.
Table 5 Thermal properties biocomposites.
3.3. Differential scanning calorimetry The DSC curves obtained from the first heat scan of PLA/Sisal bio composites are depicted in Fig. 5. The values of TOG, TG, TC, TOM, TM, ΔHC and ΔHM obtained from DSC analysis are given in Table 5. It can be observed that as compared to neat PLA, the values of TOG and TG increased after the addition of Sisal fibers into the PLA matrix. However, the values of TOG and TG of the recycled biocomposites declined with subsequent recycle. The values of glass transition temperature margin ally declined up to the third recycle while after the fourth cycle, a gradual decline was observed. This phenomenon can be attributed to the increased molecular chain mobility of the PLA/Sisal biocomposites after
determined
from
DSC
thermograms
of
recycled
Sample
TOG [� C]
TG [� C]
TC [� C]
TOM [� C]
TM [� C]
HC [J/g]
HM [J/g]
XC [%]
Neat PLA PL/TS PL/TS1 PL/TS2 PL/TS3 PL/TS4 PL/TS5 PL/TS6 PL/TS7 PL/TS8
64.8
66.7
94.8
169.4
173.8
30.5
53.1
24.2
66.8 66.6
68.9 68.8
91.5 91.2
168.1 166.5
178.7 177.2
15.6 16.9
28.4 29.5
19.6 20.5
66.2
68.3
91.1
166.6
175.2
18.1
31.8
21.0
66.1
67.9
89.9
165.6
174.5
20.1
32.1
18.4
64.1
66.6
88.2
164.2
173.7
19.6
31.5
18.2
64.8
65.3
87.8
160.4
169.3
18.1
29.8
17.9
63.5
65.6
86.7
164.6
170.7
20.3
31.8
17.6
60.1
62.8
85.1
154.4
167.3
17.6
29.2
17.8
58.2
61.8
84.1
155.0
166.9
16.8
28.3
17.6
recycling [36]. During the recyclability study of high cellulose filled PLA biocomposites, Åkesson et al. [37] reported a similar behavior and attributed the decline in TG to a significant amount of degradation. In comparison with neat PLA, the crystallization temperature (TC) of PLA/TS biocomposites was observed to decline after the incorporation of Sisal fibers. The crystallization temperature was further observed to decline for every subsequent recycle. The TC for biocomposites dropped from 91.5 � C to 84.1 � C after recycling. This decline in the TC can be attributed to shorter molecular chains and increased number of fibers ends within the PLA matrix which are expected to enhance the crystal lization rate of the biocomposite, enabling it to crystallize at lower temperatures. Similar observations were also reported during the recy clability study of PLA and PLA based composites by other researchers [21,36,38]. The melting temperature (TM) of PLA/TS biocomposites was observed to significantly improve compared to that of neat PLA, while onset temperature (TOM) did not exhibit any significant change. The TOM and TM values declined with subsequent recycle. A sharp decline in the TOM and TM values was observed after the fourth recycle. This decline in the melting temperature after recycling can be attributed to the degra dation of PLA [35]. The crystallization enthalpy and melting enthalpy values of recycled biocomposites were observed to slightly increase up to the third recycle and then a marginal decline for the subsequent re cycles. The shorter chains are expected to facilitate the crystallization justifying the slight increase observed in the ΔHM. The crystallization (XC) of PLA is complex and is highly sensitive to cooling rate. As during
Fig. 5. DSC thermograms of the recycled biocomposites. 5
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the injection molding process, a high rate of cooling results in very low crystallinity, no conclusive remarks can be made. 3.4. Fourier transform infrared spectroscopy The FTIR spectra of recycled biocomposites depicted in Fig. 6, did not exhibit the formation of any new peak after recycling. The broad ening of the peak representing hydroxyl groups (-OH) around 3434 cm 1 was observed, indicating the degradation of biocomposites after recycling [37]. An increase in the intensity of C–H deformational peak of CH3 groups present in PLA around 1363 cm 1 and 1384 cm 1 was observed due to the degradation of PLA [39]. The intensity of the peaks representing C¼O stretching around 1758 cm 1 was observed to increase with successive recycles. The increase in the intensity of this peak is related to the hydrolytic degradation of PLA due to the rise in the number of carboxylic end groups within the polymer chain [39]. Mofokeng et al. [40] attributed this phenomenon to the degradation of fibers during the processing of PLA biocomposites at high temperature. The chemical structure of PLA is prone to hydrolytic degradation espe cially in the presence of hydrophilic lignocellulosic fibers and high temperature during processing [41]. During the recycling of bio composites using extrusion process, the extruded strands of recycled biocomposites were quenched in water and pelletized. Although these pellets were dried in an air oven before injection molding, this process may have contributed to the hydrolytic degradation of PLA as evident by FTIR spectra.
Fig. 7. Tensile properties of recycled biocomposites.
3.5. Mechanical characterization The tensile properties depicting tensile strength and modulus of the recycled biocomposites are shown in Fig. 7. The tensile strength of the recycled biocomposites declined with every subsequent recycle. Compared to PL/TS biocomposites (i.e. first extrusion-injection cycle), the tensile strength declined by 11.64% after the first recycle (PL/TS-1). A decline of 20.9% in the tensile strength of recycled biocomposites was observed until the third recycle (PL/TS-3). However, beyond the third recycle (PL/TS-3), a sharp decline in the tensile strength was observed and this decline continued up to the eighth recycle (PL/TS-8). The tensile strength of PL/TS-3 was found to be comparable to that of neat PLA (43.9 MPa) while the tensile strength of PL/TS-4 biocomposites was observed to be significantly lower than that of neat PLA. The flexural strength of the recycled biocomposites after multiple recycles depicted in Fig. 8 was observed to decline with every subse quent recycle. The flexural strength after the first recycle (PL/TS-1) was
Fig. 8. Flexural properties of recycled biocomposites.
observed to be lower by 8.84%, compared to PL/TS biocomposites. The further reduction of 13.1% and 21.2% in the flexural strength was observed after the second (PL/TS-2) and third (PL/TS-3) recycle, respectively. The flexural strength of the recycled biocomposites was also observed to severely decline beyond the fourth recycle. Lopez et al. [35] reported a 50% reduction in the flexural strength of chem ithermomechanical pulp (CTMP) fiber reinforced PLLA biocomposites after third recycle. The strength retention of PL/TS biocomposites after recycling was observed to be lower compared to the strength retention ability of neat PLA as reported in the literature [20]. This decrease in the mechanical behavior of biocomposites can be attributed to the presence of ligno cellulosic fibers in the PLA matrix. Reprocessing or recycling causes severe fiber attrition during the multiple extrusion cycles of bio composites as observed in Fig. 9. The decline in the strength of the biocomposites recycled up to 3 times can be attributed to the severe fiber attrition. The fiber attrition results in the generation of uneven fibers and reduction of fiber length beyond the critical length necessary to share the load with the matrix, resulting in loss of strength [42]. The significant decline in strength observed beyond third recycle can be attributed to the combined effect of fiber attrition and PLA degradation [42]. The presence of hydrophilic fibers within the biocomposites can enhance the hydrolytic degradation of the matrix [43]. Contrary to the tensile and flexural strength, the tensile and flexural modulus of the reprocessed biocomposites was however retained up to the fourth recycle (PL/TS-4), beyond which it was observed to gradually decline with subsequent reprocessing. The modulus of biocomposites
Fig. 6. FTIR spectra of recycled biocomposites. 6
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Fig. 9. Average fiber length of extracted fibers after every recycle.
strictly depends on the interfacial bonding or the load sharing capability between the fibers and the matrix [44]. The retention of modulus in dicates enhancement in the interfacial bonding between Sisal fibers and the PLA as a result of multiple extrusion-injection cycles. However, beyond the fourth recycle, the decline in the modulus can be attributed to the reduced load sharing between the matrix and the fibers due to the latter’s attrition and former’s degradation during thermo-mechanical recycling. These findings are in consonance with the observations made during the DMA analysis. The impact strength of the recycled PL/TS biocomposites depicted in Fig. 10, shows a decline in the impact strength with subsequent reprocessing. The impact strength was observed to imperceptibly decline after the first recycle. However, beyond the first recycle the impact strength was observed to gradually decline with each recycle. This decline in the impact strength can be attributed to the severe fiber attrition and the polymer degradation occurring during each thermomechanical recycle [42].
Fig. 11. Fractographs biocomposites.
of
failed
tensile
test
specimens
of
recycled
average surface roughness (Ra) (refer supplementary file for graphical representation: Fig. S2) is depicted in Fig. 12. It can be observed that the surface roughness of the injection molded biocomposite specimens gradually increases with successive recycles. The surface imperfections on the surface of recycled biocomposites significantly increased after the fourth recycle. This increase in the surface roughness can be directly attributed to the degradation of the PLA matrix. The repeated recycling of PLA results in its hydrolytic degradation which lowers its molecular weight and increases its melt flow rate [37]. These factors might have contributed to the surface imperfections observed in recycled bio composites. Hence, the PLA/Sisal biocomposites should not be recycled beyond the third recycle. The photographic image of the specimens after each recycle (Fig. 13) also indicates the thermo-mechanical degradation of PL/TS biocomposites with each recycle. It can be clearly observed that the color of the biocomposite specimens exhibits a darker shade after each subsequent processing, indicating towards thermal degradation of both Sisal fibers and PLA matrix during recycling.
3.6. Morphological characterization The fractured surface of failed tensile test specimens, shown in Fig. 11 also indicates the degradation of the matrix. Apart from improved interfacial adhesion between the Sisal fibers and the PLA matrix during initial recycles, the fractographs show an increase in the matrix cracking along with the reduction in fiber visibility at the frac tured surfaces with an increasing number of recycles. The increase in matrix cracking and lower number of fibers present on the fractured surface can be attributed to the PLA matrix degradation and severe fiber attrition, respectively. The surface morphology of recycled biocomposites along with their
4. Conclusions The present experimental investigation reveals the recycling behavior of PLA/Sisal biocomposites developed by incorporating 30 wt % of Sisal fibers treated with novel sodium bicarbonate treatment. The biocomposites were recycled 8 times using the extrusion process and subsequently, injection molded after each recycle. The dynamic me chanical analysis of recycled biocomposites exhibited a decrease in the storage and loss modulus beyond the third recycle indicating chain scission of PLA. The thermal stability of the recycled biocomposites assessed by TGA analysis was observed to gradually decline after every recycle. The glass transition temperature of recycled biocomposites determined from DSC analysis also reduced from 68.9 � C to 61.8 � C after
Fig. 10. Impact strength of recycled biocomposites. 7
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Fig. 12. Surface morphology of recycled biocomposites.
Fig. 13. Photographic image of the specimens after each recycle.
eight recycles due to severe degradation of PLA. The FTIR spectra indicate towards hydrolytic degradation of PLA during recycling. The tensile and flexural strength of the recycled biocomposites was observed to decline by 20.9% and 21.2% respectively, up to the third recycle, beyond which a significant reduction was observed. The Impact strength was observed to gradually decline after each recycle. The degradation of the PLA matrix along with degradation of fibers due to fiber attrition collectively contributed to the decline in mechanical properties. The surface imperfections of the injection molded recycled biocomposites observed using SEM and surface roughness analyzer, increased signifi cantly beyond third recycle. Hence, based on these findings, the me chanical recycling of PLA/Sisal biocomposites beyond third recycle is not recommended. It can be further concluded that the PLA/Sisal bio composites recycled up to third recycle can be effectively used for
making products commercially for low to medium strength nonstructural applications. Acknowledgment This research work was supported by the Basic Science Research Program, through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (MSIP), grant numbers (2018R1A6A1A03024509 and 2019R1A2B5B03004980). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.05.106. 8
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9
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Recyclability analysis of PLA/Sisal fiber biocomposites Saurabh Chaitanya a, Inderdeep Singh b, Jung Il Song a, * a b
Department of Mechanical Engineering, Changwon National University, Changwon, 51140, South Korea Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Uttarakhand, 247667, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Biocomposites PLA Sisal fiber Recycling Extrusion Injection molding DMA
This study presents the recyclability analysis of PLA/Sisal biocomposites comprising sodium bicarbonate treated Sisal fibers (30 wt%). The biocomposites were recycled (8 times) using the extrusion process. The tensile strength of injection molded biocomposites declined by 20.9% up to the third recycle. The dynamic mechanical analysis revealed a severe reduction in storage and loss modulus beyond third recycle. The morphological and thermal characterization of recycled biocomposites also revealed severe fiber and matrix degradation. The infrared spectroscopy exhibited hydrolysis as one of the causes of PLA degradation after recycling. The recycling of PLA/ Sisal biocomposites beyond third recycle is not recommended. However, the biocomposites recycled up to third recycle can be used for making products for low to medium strength non-structural applications.
1. Introduction The application spectrum of fiber reinforced plastics has increased exponentially during the last few decades. This has led to the generation of a huge amount of non-degradable plastic waste throughout the globe [1]. Petroleum-derived polymers and synthetic fibers (such as glass, carbon, etc.) exhibit extremely slow degradation behavior and are hence termed as non-degradable polymer composites. To tackle this global environmental concern, the focus of material scientists is shifting to wards the development of eco-sustainable materials [2–4]. Another reason for this shift can be attributed to the environmental regulations and directives issued by governments of various countries. For example, in order to curb the increasing pollution, the European government has issued a directive that allows an incineration quota of only 5% for dis carded cars [5]. The Japanese government also issued a directive to use 20% of bioderived plastics by 2020 [5]. Eco-sustainable materials can be defined as materials derived from renewable resources which can be recycled and are triggered biode gradable [5]. Polylactic Acid (PLA) has been identified as the most promising biopolymer having the potential to replace traditional engi neering and commodity plastics in various applications [6,7]. To reduce the cost of PLA and improve its applicability, the incorporation of nat ural lignocellulosic fibers as reinforcement into PLA has been explored by several researchers in the past decade. The lignocellulosic fibers mainly comprise of lignin, cellulose, pectin, and hemicellulose [8,9]. The composition of these constituents defines the fiber characteristics as
well as the interfacial bonding between the matrix and the fibers. This composition can be controlled by modifying the fiber surface using different techniques reported in the literature [10,11]. The bio composites incorporating lignocellulosic fibers have been reported to exhibit the potential to be used in a variety of applications. This increase in application potential of biocomposites has warranted the need to explore their commercial processability [12]. The PLA based biocomposites can be easily processed via commer cially available processing routes used for thermoplastics such as extrusion, injection molding and compression molding [12,13]. The commercial processing of biocomposites using rapid molding processes like injection molding leads to the generation of inevitable industrial waste in the form of sprue, runners, gates, etc., which must be trimmed off after injection molding [14]. Hence, the waste generated after the end of useful product life as well as industrial waste generated during processing of biocomposites is a serious concern for the biocomposite industry. PLA and PLA based biocomposites are triggered degradable, which can biodegrade under composting or specific environmental conditions [15]. Apart from biodegradation, PLA based biocomposites have an added advantage of being recyclable. The recyclability of bio composites is a lucrative option which is yet to be thoroughly explored. Recyclability reduces the cost of the product as well as the demand for landfills. Recycling or reuse of the material after the end of the useful life cycle of a product is highly encouraged. Moreover, strict environmental regulations have been proposed by several countries encouraging the researchers and designers to use recyclable materials [16]. Hence, the
* Corresponding author. E-mail address: [email protected] (J.I. Song). https://doi.org/10.1016/j.compositesb.2019.05.106 Received 19 March 2019; Received in revised form 27 April 2019; Accepted 12 May 2019 Available online 16 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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industrial waste generated during fabrication of biocomposite product in terms of trims and the excess material removed should be recycled to limit the industrial waste and reduce the production costs. The recycling of biocomposites is challenging as composite materials are heterogeneous in nature [17]. The recycling methods for bio composites can be classified into three categories, mechanical recycling, chemical recycling and thermal processing [18]. Mechanical recycling involves remelting and remolding of biocomposites using multiple extrusion and/or injection cycles. Chemical recycling involves the dissolution of the polymer matrix and separation of fibers from the matrix, while thermal processing involves energy recovery through incineration or combustion. Mechanical recycling is considered as one of the most successful material recovery methods due to its ease of pro cessing, relatively low investment and ability to control technological parameters [19]. Mechanical recycling of thermoplastic materials like PLA is hence a desirable option given its ease of processing and para metric control [19,20]. During the last decade, few studies exploring the recyclability of the PLA matrix have been reported. The behavioral analysis of up to 10 recycles of PLA was performed using multiple extrusion and subsequent injection molding process by Zenkiewicz et al. [21]. The authors reported a marginal decrease in the tensile strength while the impact resistance declined with increasing recycling number. In a different study by Taubner et al. [22], it was reported that the multiple extrusions of PLLA at higher processing temperatures can lead to a severe reduction in its molecular weight. Badia et al. [19] explored the recycling behavior of PLA for up to five extrusion cycles. The authors also reported severe loss of molecular weight of PLA after 5 recycles. However, when reinforced with short lignocellulosic fibers, the recycling behavior of PLA based biocomposites becomes complex and is scarcely reported in the literature [5]. In the previous studies, it has been concluded that PLA/Sisal biocomposites have the potential to replace conventional plastics used in several commercial applications like automotive interiors, furniture, toys, disposable cutlery, electronic equipment, etc. [23,24]. Hence, it becomes imperative to study and report the effect of mechanical recycling of PLA/Sisal biocomposites on their thermal and mechanical behavior along with the determination of number of recycles that PLA/Sisal biocomposites can undergo within the acceptable behavioral range. This recyclability study is performed on PLA/Sisal biocomposites made by incorporating Sisal fibers treated with a novel eco-friendly sodium bicarbonate treatment as reported earlier by authors [24].
2.2. Methodology 2.2.1. Fiber surface treatment The Sisal fibers were treated by soaking them in an aqueous solution of sodium bicarbonate (NaHCO3) and distilled water, having NaHCO3 concentration of 10% (w:v). The Sisal fibers were soaked in the NaHCO3 solution for an optimum duration of 72 h and stirred at regular intervals [24]. The treated fibers were then washed with distilled water and subsequently dried in an air oven at a temperature of 100 � C for 8 h. The characteristics of raw and treated Sisal fibers are given in Table 2. 2.2.2. Recycling of biocomposites The recycling of biocomposites incorporating treated Sisal fibers is simulated experimentally by using multiple extrusion route. In this technique, the PLA biocomposites reinforced with Sisal fibers having fiber weight fraction of 30%, were recycled using a single screw extruder (Sai Extrumech, India). Based on the previously reported studies, the screw speed of 60 rpm and a temperature profile of 150–175-180-185 � C (feed zone to die) were selected during processing [24]. The extrudate in the form of 4 mm strand was quenched in water and subsequently pelletized to 5 mm pellets using a mechanical pelletizer. The extruded biocomposite pellets were then dried in an air oven at 100 � C for 8 h after each cycle. The biocomposite pellets were extruded for nine times, as further recycling caused severe degradation of PLA-Sisal melt blend, making it difficult to process. A small batch of biocomposite pellets from each extrusion cycle was used for the development of test specimens using a commercial injection molder (Electronica Endura-60, India). The injection molding processing parameters used to develop recycled bio composite test specimens have been listed in Table 3. A total of nine batches of biocomposite pellets were prepared leading to the develop ment of eight recycled biocomposites. The nomenclature of the recycled biocomposites is given in Table 4. 2.2.3. Mechanical behavior To explore the recyclability of PLA/Sisal biocomposites, their me chanical behavior in terms of tensile, flexural and impact properties, were determined in accordance with ASTM 3039M-14, ASTM-D790-10, and ASTM-D256-10, respectively. Tensile and flexural tests were per formed using an Instron-5982, universal testing machine, while IzodImpact tests were performed employing (IT504, Tinius Olsen) low en ergy impact tester. The gauge length and a crosshead speed of 50 mm and 1.5 mm/min were selected during tensile tests. The flexural tests were conducted at a crosshead speed of 2 mm/min and a gauge length of 64 mm. Impact tests were conducted using a notched impact test (Izod) specimens. A total of 4 specimens were examined during each test and
2. Materials and method 2.1. Materials High-performance PLA biopolymer (Grade: Ingeo 3260HP) in the form of pellets was sourced from NatureWorks LLC, USA. The properties of PLA used in this experimental investigation are given in Table 1. Sisal fibers chopped to 3 mm in length were sourced from Women’s Devel opment Organization, Dehradun, India. The image of Sisal fibers is given in supplementary file: Fig. S1. In order to treat the surface of Sisal fibers prior to the development of biocomposites, sodium bicarbonate (extrapure AR, 99.5%) was sourced from SRL Pvt. Ltd., India.
Table 2 Properties of raw and treated Sisal fibers. Material
Property
Raw Sisal Fiber Treated Sisal Fiber
Single Fiber Strength (MPa)
Cellulose (%)
Lignin (%)
Hemicellulose (%)
Reference
337.6 (�79) 669.7 (�62)
64.9
9.2
24.6
[24]
84.9
8.8
6.2
[24]
Table 1 Properties of PLA. Material
PLA
Property Melt Flow Rate (g/10 min) (210C, 2.16 Kg)
Specific gravity
Tensile strength (MPa)
Tensile modulus (MPa)
Flexural Strength (MPa)
Flexural Modulus (MPa)
Impact Strength (J/m)
Reference
65
1.24
43.9 (�0.8)
1997.4 (�133.1)
60.2 (�1.4)
3395.7 (�197.4)
16.3 (�0.5)
[24]
2
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cooling rate, this study only focuses on the first heat scan results to determine the onset glass transition temperature (TG) (mid-point method), glass transition temperature (TOG), crystallization temperature (TC), melting temperature (TM), onset melting temperature (TOM), enthalpy of melting (ΔHM) and enthalpy of crystallization (ΔHC). The crystallinity (XC) of the recycled biocomposites was evaluated using the relation given in equation (1).
Table 3 Injection molding processing parameters [24]. Parameter
Value
Holding Pressure Holding time Screw speed Barrel temperature Mold temperature Back pressure Injection time Injection pressure
55 MPa 8s 120 rpm (feed-to-nozzle) 160–175–185–195 � C 30 � C 5 MPa 1.8 s 60 MPa
XC ð% crystallinityÞ ¼
Recycling Order
Designation of Biocomposites
1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8
PL/TS PL/TS-1 PL/TS-2 PL/TS-3 PL/TS-4 PL/TS-5 PL/TS-6 PL/TS-7 PL/TS-8
(1)
where ‘w’ represents the weight fraction of PLA in the sample and ΔHom (93 J/g) is the crystallinity of 100% crystalline PLA [25–27].
Table 4 Nomenclature of recycled biocomposites. Extrusion Cycle Number
ΔHM ΔHC 100 � w ΔH oM
2.2.8. Fourier transform infrared spectroscopy (FTIR) FTIR spectroscopy technique was used to ascertain any changes occurring in the chemical composition of the biocomposites after recy cling. FTIR spectroscopy of the recycled biocomposites was performed using FTIR spectrometer (Magna 760, Thermo Nicollet). The bio composite specimens were grounded and mixed with KBr and then pressed to make thin discs of 10 mm diameter. The FTIR spectra were recorded in the transmittance mode between 400 and 4000 cm 1. 2.2.9. Surface roughness In order to measure the surface imperfections of the injection molded biocomposites, their average surface roughness value (Ra) was measured using a surface roughness tester (Mitutoyo SJ-201).
their average value along with error bars representing standard devia tion have been reported.
3. Results and discussion
2.2.4. Morphological evaluation Morphological evaluation of the fractured tensile test samples and surface of recycled biocomposites was done using scanning electron microscope (LEO 1550). To study the fiber morphology after the pro cessing of biocomposites, Sisal fibers were extracted from recycled biocomposites by dissolving the matrix into chloroform (SRL Pvt. Ltd., India). The extracted fibers were thus spread on a glass slide and viewed under a stereo microscope (Nikon SMZ-745T). Extracted fiber lengths of approximately 100 fibers for each specimen were measured and their mean along with standard deviation has been reported.
3.1. Dynamic mechanical analysis Storage Modulus (E0 ) is indicative of the energy stored in a substance, during deformation and is often linked with the stiffness of the material [28]. In comparison to neat PLA (2526.14 MPa), the storage modulus of PL/TS biocomposites significantly improved after incorporation of treated Sisal fibers (Fig. 1). At room temperature, storage modulus was observed to lie within the range of 3400–4200 MPa for all the reproc essed biocomposites. Compared to storage modulus of PL/TS bio composite an imperceptible increase in the E0 values of recycled biocomposites was observed after first (PL/TS-1) and second (PL/TS-2) recycle. This is indicative of enhanced interfacial bonding between the PLA and Sisal fibers due to thermo-mechanical reprocessing. However, beyond the third recycle, a gradual decline in the storage modulus was observed. A steep decline in E0 value was observed between 55 and 65 � C for all the developed biocomposites. This decline in the E’ value between the temperature range of 55–65 � C is attributed to the loss of stiffness around glass transition temperature (TG) [29]. A gradual reduction in the onset temperature of this decline beyond third recycle was observed, indicating a decline in TG. The loss modulus of a material is related to its molecular chain movements [30,31]. Fig. 2 depicts the Loss Modulus (E00 ) of the recycled biocomposites as a function of temperature. It can be observed from Fig. 2 that the E00 peak of neat PLA rises significantly after the incor poration of Sisal fibers (PL/TS). Similar to the trend observed in storage modulus curves, the E00 peaks of PL/TS-1, PL/TS-2 and PL/TS-3 were observed to be higher as compared to PL/TS biocomposites. The increase in the E00 peak signifies the increased restrictions in the mobility of the polymer chains due to good fiber-matrix interaction [28]. However, beyond the third recycle, the E00 peaks were observed to gradually decline as well as a shift towards lower temperature. This declining trend in the E00 peaks can be attributed to rise in the mobility of the polymer chains. The increase in mobility of the polymer chains can be due to the fiber attrition as well as polymer chain scission taking place during reprocessing. The shifting of E00 peak towards lower temperature signifies the reduction in the TG of recycled biocomposites. Moreover, in
2.2.5. Dynamic mechanical analysis (DMA) Thermo-mechanical behavior of the recycled biocomposites was examined employing a Dynamic Mechanical Analyzer (TA-DMA-Q800, TA Instruments) equipped with a dual-cantilever fixture in accordance with ASTM D4065-12. The DMA tests were conducted at an operating sinusoidal frequency of 1 Hz, between the temperature range of 35 � C to 90 � C with a temperature ramp of 3 � C/min. The dynamic mechanical behavior of the developed biocomposites was assessed in terms of storage modulus (E0 ), loss modulus (E00 ), and damping coefficient (tan δ). 2.2.6. Thermogravimetric analysis Thermogravimetric analysis (TGA) of the recycled biocomposites was conducted using Thermogravimetric Analyzer (EXSTAR 6300, Seiko Instruments). During this analysis, a small quantity (10 mg) of recycled biocomposite samples were examined at a temperature ramp rate of 10 � C/min from ambient temperature to 550 � C, in the presence of ni trogen gas. The weight loss of the biocomposites with respect to tem perature is recorded to ascertain their thermal degradation behavior. 2.2.7. Differential scanning calorimetry (DSC) In order to thoroughly examine the effect of recyclability on the thermal properties of recycled biocomposites, the DSC analysis of bio composite specimens was performed using Simultaneous Thermal Analyzer (STA 6000, PerkinElmer). During this analysis, a small quan tity (10–15 mg) of recycled biocomposite samples were heated in a crucible at a temperature ramp rate of 10 � C/min from 35 � C to 200 � C, in the presence of nitrogen. As crystallization of PLA is sensitive to 3
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Fig. 1. Storage modulus curves of recycled biocomposites.
Fig. 2. Loss modulus curves of recycled biocomposites.
comparison to neat PLA, the E00 peaks of recycled biocomposites were observed to widen, resulting in broadening of glass transition range. The curves of Tan δ as a function of temperature are depicted in Fig. 3. Tan δ is defined as the ratio of E00 to E0 . It is evident from Fig. 3 that, the peak of Tan δ significantly decreases after the addition of Sisal fibers into neat PLA. The decrease in Tan δ peaks after the incorporation of Sisal fibers signifies restricted polymer chain mobility [28,32]. However, the Tan δ peak was observed to gradually rise with subsequent reprocessing cycles. This increase may be attributed to the reduced fiber size and PLA chain scission during repeated thermo-mechanical pro cessing, both resulting in higher molecular chain mobility.
3.2. Thermogravimetric analysis The thermogravimetric curves of the recycled biocomposites are represented in Fig. 4. The thermal degradation of PL/TS biocomposites was observed to take place in three degradation steps. First weight reduction takes place between 50 � C and 250 � C due to the loss of moisture and pyrolysis of hemicellulose present in Sisal fibers. The second step of thermal degradation takes place between 260 � C and 365 � C due to the pyrolysis of PLA, cellulose and some part of lignin. The third step of degradation observed between 360 � C and 465 � C occurred due to the pyrolysis of residual lignin and PLA [33]. It can be clearly observed from second and third degradation steps, that the thermal
Fig. 3. Tan Delta curves of the developed biocomposites. 4
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Fig. 4. Thermogravimetric thermographs of the recycled biocomposites.
stability of the recycled biocomposites gradually declined after each recycle. The thermal stability of recycled biocomposites was observed to imperceptibly decline up to third recycle (PL/TS-3). However, beyond the third recycle, the resistance to thermal degradation of recycled biocomposites was observed to reduce significantly. This decrease in thermal stability can be attributed to the thermo-mechanical degrada tion of biocomposites which causes both fibers and the PLA matrix to degrade during reprocessing [34]. Lopez et al. [35] also reported a progressive decline in the thermal stability of PLA after successive thermomechanical recycles, indicating the degradation of the PLA matrix.
Table 5 Thermal properties biocomposites.
3.3. Differential scanning calorimetry The DSC curves obtained from the first heat scan of PLA/Sisal bio composites are depicted in Fig. 5. The values of TOG, TG, TC, TOM, TM, ΔHC and ΔHM obtained from DSC analysis are given in Table 5. It can be observed that as compared to neat PLA, the values of TOG and TG increased after the addition of Sisal fibers into the PLA matrix. However, the values of TOG and TG of the recycled biocomposites declined with subsequent recycle. The values of glass transition temperature margin ally declined up to the third recycle while after the fourth cycle, a gradual decline was observed. This phenomenon can be attributed to the increased molecular chain mobility of the PLA/Sisal biocomposites after
determined
from
DSC
thermograms
of
recycled
Sample
TOG [� C]
TG [� C]
TC [� C]
TOM [� C]
TM [� C]
HC [J/g]
HM [J/g]
XC [%]
Neat PLA PL/TS PL/TS1 PL/TS2 PL/TS3 PL/TS4 PL/TS5 PL/TS6 PL/TS7 PL/TS8
64.8
66.7
94.8
169.4
173.8
30.5
53.1
24.2
66.8 66.6
68.9 68.8
91.5 91.2
168.1 166.5
178.7 177.2
15.6 16.9
28.4 29.5
19.6 20.5
66.2
68.3
91.1
166.6
175.2
18.1
31.8
21.0
66.1
67.9
89.9
165.6
174.5
20.1
32.1
18.4
64.1
66.6
88.2
164.2
173.7
19.6
31.5
18.2
64.8
65.3
87.8
160.4
169.3
18.1
29.8
17.9
63.5
65.6
86.7
164.6
170.7
20.3
31.8
17.6
60.1
62.8
85.1
154.4
167.3
17.6
29.2
17.8
58.2
61.8
84.1
155.0
166.9
16.8
28.3
17.6
recycling [36]. During the recyclability study of high cellulose filled PLA biocomposites, Åkesson et al. [37] reported a similar behavior and attributed the decline in TG to a significant amount of degradation. In comparison with neat PLA, the crystallization temperature (TC) of PLA/TS biocomposites was observed to decline after the incorporation of Sisal fibers. The crystallization temperature was further observed to decline for every subsequent recycle. The TC for biocomposites dropped from 91.5 � C to 84.1 � C after recycling. This decline in the TC can be attributed to shorter molecular chains and increased number of fibers ends within the PLA matrix which are expected to enhance the crystal lization rate of the biocomposite, enabling it to crystallize at lower temperatures. Similar observations were also reported during the recy clability study of PLA and PLA based composites by other researchers [21,36,38]. The melting temperature (TM) of PLA/TS biocomposites was observed to significantly improve compared to that of neat PLA, while onset temperature (TOM) did not exhibit any significant change. The TOM and TM values declined with subsequent recycle. A sharp decline in the TOM and TM values was observed after the fourth recycle. This decline in the melting temperature after recycling can be attributed to the degra dation of PLA [35]. The crystallization enthalpy and melting enthalpy values of recycled biocomposites were observed to slightly increase up to the third recycle and then a marginal decline for the subsequent re cycles. The shorter chains are expected to facilitate the crystallization justifying the slight increase observed in the ΔHM. The crystallization (XC) of PLA is complex and is highly sensitive to cooling rate. As during
Fig. 5. DSC thermograms of the recycled biocomposites. 5
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the injection molding process, a high rate of cooling results in very low crystallinity, no conclusive remarks can be made. 3.4. Fourier transform infrared spectroscopy The FTIR spectra of recycled biocomposites depicted in Fig. 6, did not exhibit the formation of any new peak after recycling. The broad ening of the peak representing hydroxyl groups (-OH) around 3434 cm 1 was observed, indicating the degradation of biocomposites after recycling [37]. An increase in the intensity of C–H deformational peak of CH3 groups present in PLA around 1363 cm 1 and 1384 cm 1 was observed due to the degradation of PLA [39]. The intensity of the peaks representing C¼O stretching around 1758 cm 1 was observed to increase with successive recycles. The increase in the intensity of this peak is related to the hydrolytic degradation of PLA due to the rise in the number of carboxylic end groups within the polymer chain [39]. Mofokeng et al. [40] attributed this phenomenon to the degradation of fibers during the processing of PLA biocomposites at high temperature. The chemical structure of PLA is prone to hydrolytic degradation espe cially in the presence of hydrophilic lignocellulosic fibers and high temperature during processing [41]. During the recycling of bio composites using extrusion process, the extruded strands of recycled biocomposites were quenched in water and pelletized. Although these pellets were dried in an air oven before injection molding, this process may have contributed to the hydrolytic degradation of PLA as evident by FTIR spectra.
Fig. 7. Tensile properties of recycled biocomposites.
3.5. Mechanical characterization The tensile properties depicting tensile strength and modulus of the recycled biocomposites are shown in Fig. 7. The tensile strength of the recycled biocomposites declined with every subsequent recycle. Compared to PL/TS biocomposites (i.e. first extrusion-injection cycle), the tensile strength declined by 11.64% after the first recycle (PL/TS-1). A decline of 20.9% in the tensile strength of recycled biocomposites was observed until the third recycle (PL/TS-3). However, beyond the third recycle (PL/TS-3), a sharp decline in the tensile strength was observed and this decline continued up to the eighth recycle (PL/TS-8). The tensile strength of PL/TS-3 was found to be comparable to that of neat PLA (43.9 MPa) while the tensile strength of PL/TS-4 biocomposites was observed to be significantly lower than that of neat PLA. The flexural strength of the recycled biocomposites after multiple recycles depicted in Fig. 8 was observed to decline with every subse quent recycle. The flexural strength after the first recycle (PL/TS-1) was
Fig. 8. Flexural properties of recycled biocomposites.
observed to be lower by 8.84%, compared to PL/TS biocomposites. The further reduction of 13.1% and 21.2% in the flexural strength was observed after the second (PL/TS-2) and third (PL/TS-3) recycle, respectively. The flexural strength of the recycled biocomposites was also observed to severely decline beyond the fourth recycle. Lopez et al. [35] reported a 50% reduction in the flexural strength of chem ithermomechanical pulp (CTMP) fiber reinforced PLLA biocomposites after third recycle. The strength retention of PL/TS biocomposites after recycling was observed to be lower compared to the strength retention ability of neat PLA as reported in the literature [20]. This decrease in the mechanical behavior of biocomposites can be attributed to the presence of ligno cellulosic fibers in the PLA matrix. Reprocessing or recycling causes severe fiber attrition during the multiple extrusion cycles of bio composites as observed in Fig. 9. The decline in the strength of the biocomposites recycled up to 3 times can be attributed to the severe fiber attrition. The fiber attrition results in the generation of uneven fibers and reduction of fiber length beyond the critical length necessary to share the load with the matrix, resulting in loss of strength [42]. The significant decline in strength observed beyond third recycle can be attributed to the combined effect of fiber attrition and PLA degradation [42]. The presence of hydrophilic fibers within the biocomposites can enhance the hydrolytic degradation of the matrix [43]. Contrary to the tensile and flexural strength, the tensile and flexural modulus of the reprocessed biocomposites was however retained up to the fourth recycle (PL/TS-4), beyond which it was observed to gradually decline with subsequent reprocessing. The modulus of biocomposites
Fig. 6. FTIR spectra of recycled biocomposites. 6
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Fig. 9. Average fiber length of extracted fibers after every recycle.
strictly depends on the interfacial bonding or the load sharing capability between the fibers and the matrix [44]. The retention of modulus in dicates enhancement in the interfacial bonding between Sisal fibers and the PLA as a result of multiple extrusion-injection cycles. However, beyond the fourth recycle, the decline in the modulus can be attributed to the reduced load sharing between the matrix and the fibers due to the latter’s attrition and former’s degradation during thermo-mechanical recycling. These findings are in consonance with the observations made during the DMA analysis. The impact strength of the recycled PL/TS biocomposites depicted in Fig. 10, shows a decline in the impact strength with subsequent reprocessing. The impact strength was observed to imperceptibly decline after the first recycle. However, beyond the first recycle the impact strength was observed to gradually decline with each recycle. This decline in the impact strength can be attributed to the severe fiber attrition and the polymer degradation occurring during each thermomechanical recycle [42].
Fig. 11. Fractographs biocomposites.
of
failed
tensile
test
specimens
of
recycled
average surface roughness (Ra) (refer supplementary file for graphical representation: Fig. S2) is depicted in Fig. 12. It can be observed that the surface roughness of the injection molded biocomposite specimens gradually increases with successive recycles. The surface imperfections on the surface of recycled biocomposites significantly increased after the fourth recycle. This increase in the surface roughness can be directly attributed to the degradation of the PLA matrix. The repeated recycling of PLA results in its hydrolytic degradation which lowers its molecular weight and increases its melt flow rate [37]. These factors might have contributed to the surface imperfections observed in recycled bio composites. Hence, the PLA/Sisal biocomposites should not be recycled beyond the third recycle. The photographic image of the specimens after each recycle (Fig. 13) also indicates the thermo-mechanical degradation of PL/TS biocomposites with each recycle. It can be clearly observed that the color of the biocomposite specimens exhibits a darker shade after each subsequent processing, indicating towards thermal degradation of both Sisal fibers and PLA matrix during recycling.
3.6. Morphological characterization The fractured surface of failed tensile test specimens, shown in Fig. 11 also indicates the degradation of the matrix. Apart from improved interfacial adhesion between the Sisal fibers and the PLA matrix during initial recycles, the fractographs show an increase in the matrix cracking along with the reduction in fiber visibility at the frac tured surfaces with an increasing number of recycles. The increase in matrix cracking and lower number of fibers present on the fractured surface can be attributed to the PLA matrix degradation and severe fiber attrition, respectively. The surface morphology of recycled biocomposites along with their
4. Conclusions The present experimental investigation reveals the recycling behavior of PLA/Sisal biocomposites developed by incorporating 30 wt % of Sisal fibers treated with novel sodium bicarbonate treatment. The biocomposites were recycled 8 times using the extrusion process and subsequently, injection molded after each recycle. The dynamic me chanical analysis of recycled biocomposites exhibited a decrease in the storage and loss modulus beyond the third recycle indicating chain scission of PLA. The thermal stability of the recycled biocomposites assessed by TGA analysis was observed to gradually decline after every recycle. The glass transition temperature of recycled biocomposites determined from DSC analysis also reduced from 68.9 � C to 61.8 � C after
Fig. 10. Impact strength of recycled biocomposites. 7
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Fig. 12. Surface morphology of recycled biocomposites.
Fig. 13. Photographic image of the specimens after each recycle.
eight recycles due to severe degradation of PLA. The FTIR spectra indicate towards hydrolytic degradation of PLA during recycling. The tensile and flexural strength of the recycled biocomposites was observed to decline by 20.9% and 21.2% respectively, up to the third recycle, beyond which a significant reduction was observed. The Impact strength was observed to gradually decline after each recycle. The degradation of the PLA matrix along with degradation of fibers due to fiber attrition collectively contributed to the decline in mechanical properties. The surface imperfections of the injection molded recycled biocomposites observed using SEM and surface roughness analyzer, increased signifi cantly beyond third recycle. Hence, based on these findings, the me chanical recycling of PLA/Sisal biocomposites beyond third recycle is not recommended. It can be further concluded that the PLA/Sisal bio composites recycled up to third recycle can be effectively used for
making products commercially for low to medium strength nonstructural applications. Acknowledgment This research work was supported by the Basic Science Research Program, through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (MSIP), grant numbers (2018R1A6A1A03024509 and 2019R1A2B5B03004980). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.05.106. 8
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