Journal of Environmental Management 261 (2020) 110211 Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: http://www.elsevier.com/locate/jenvman Research article Biodegradable kinetics and behavior of bio-based polyblends under simulated aerobic composting conditions Naba Kumar Kalita a, Siddharth Mohan Bhasney a, Ajay Kalamdhad b, Vimal Katiyar a, * a b Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Assam, India Department of Civil Engineering, Indian Institute of Technology, Guwahati, Assam, India A R T I C L E I N F O A B S T R A C T Keywords: Biodegradation Blends Composting Poly(lactic acid) Linear low density polyethylene Microcrystalline cellulose crystal The current study evaluates aerobic biodegradation of melt extruded poly(lactic acid) PLA based blends under composting conditions. Samples of neat PLA (NPLA) and bio-based polyblend composites of PLA/LLDPE (linear low-density polyethylene) having different concentration of MCC (microcrystalline cellulose crystal) were analyzed to understand the biodegradation behavior of these blends under simulated composting conditions. Biodegradation kinetics revealed that higher content of MCC and PLA accelerated the biodegradation process of the polymeric blends. Increase in the spherulite growth size and decrease in the spherulite density of the biodegraded samples confirmed the decline in amorphous portion of the test samples due to microbial assimilation, leaving behind the crystalline portion. Surface morphological analysis revealed that the samples of PLA/LLDPE/ MCC blends underwent surface erosion prior to bulk biodegradation (50–80%) until the 90th day and the PLA formed fibril-like structures after degradation. This study would help in the design and preparation of biodegradable bio-based commercial blends in the future. 1. Introduction Biodegradability is steadily gaining importance in the plastic industry due to the ever increasing environmental concerns related to nonbiodegradable products and their ecological impacts. Conventional polymers like linear low-density polyethylene (LLDPE), polyethylene (PE), polypropylene (PP) are very cheap and therefore used extensively in packaging industries as well as in the manufacture of daily household goods. Alternatives to conventional plastics are required in order to reduce their overuse. The cost of biodegradable plastics is high which has limited its use (Arrieta et al., 2017; Bhasney et al., 2018). Blending is � et al., one of the options of reducing the cost of bioplastics (Peka�rova 2018). Blending of conventional polymers with bioplastics might help in the improvement of certain properties of these bio-based blends (Dubey et al., 2016; Bhasney et al., 2018). PLA, a biodegradable polymer, is comparable to its conventional counterparts due to its thermoplastic nature, and can be used in these blends (Dhar et al., 2016; Gupta et al., 2018). Recent research studies suggest that blending of bioplastics like PLA with conventional plastics like LLDPE is a growing trend, although their biodegradability remains a key issue (Siracusa et al., 2008; Trongsatitkul and Chaiwong, 2017; Bhasney et al., 2018). Biodegradable polymers are either partially or completely degradable under certain controlled conditions and environments (Campos et al., 2011; Weng et al., 2013; Andrade et al., 2018; Narancic et al., 2018). According to the ASTM International D-5338-15 standard, biodegradation of plastics is a process where 90 percent carbon content of the individual plastic material is converted into carbon dioxide (CO2), water and heat (ASTM Standard D 5338, 2003; Leejarkpai et al., 2011). Composting of bioplastics is the study of biodegradation of biopolymers and its blends under certain controlled conditions (Pradhan et al., 2010b; Leejarkpai et al., 2011; Musioł et al., 2016; Kalita et al., 2019). Aerobic composting of bioplastics involves the use of compost as the substrate and biopolymers as the carbon source, put in composting vessels under controlled conditions. The compost contains microbial flora, which function at thermophilic temperature, adequate moisture content and 7–7.8 soil pH and perform microbial assimilation of the biopolymer intermediates formed by the hydrolysis process (Kale et al., 2007a, 2007b; Cheung et al., 2010; Pradhan et al., 2010a; Husorova et al., 2014; Boonmee et al., 2016; Qi et al., 2017; Kalita et al., 2019). Various parameters like constant thermophilic temperature, relative humidity, pH and moisture content are maintained throughout the degradation process (Leejarkpai et al., 2011; Stloukal et al., 2015; Kalita * Corresponding author. E-mail address: vkatiyar@iitg.ac.in (V. Katiyar). https://doi.org/10.1016/j.jenvman.2020.110211 Received 24 October 2019; Received in revised form 2 January 2020; Accepted 27 January 2020 Available online 2 March 2020 0301-4797/© 2020 Elsevier Ltd. All rights reserved. N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 Various studies related to the biodegradation of PLA and other biopolymers under composting conditions have been done till date (Modelli et al., 1999; Hoshino et al., 2002; Schumacher, 2002; Leja and Lewandowicz, 2010; Pradhan et al., 2010b; Alshehrei, 2017; Qi et al., 2017; Andrade et al., 2018; Kalita et al., 2019). However, very limited data is available on biodegradation of polymer blends made up of biopolymers and conventional plastic under aerobic composting conditions. The objective of the present study is to study a new class of melt extruded PLA/LLDPE based blends prepared according to Bhasney et al. (2018), whose biodegradation activity has never been reported under aerobic thermophilic phase of the composting process and compare its activity with that of NPLA. Different concentrations of microcrystalline cellulose crystal (MCC) was added to the PLA/LLDPE blends to improve its biodegradability and the degradation kinetics of these blends was studied under simulated composting conditions. Table 1 Composition of the polyblend composites. Sample designation PLA (g) LLDPE (g) MCC (g) MCC (wt% of polymer) NPLA LLDPE PLA/LLDPE PLA/LLDPE/MCC 1% PLA/LLDPE/MCC 3% PLA/LLDPE/MCC 5% 10 0 8 7.92 0 10 2 1.98 0 0 0 0.1 0 0 0 1 7.76 1.94 0.3 3 7.6 1.9 0.5 5 et al., 2019). Even though composting is an end-use technology, it has only been used for producing organic manure. Good quality organic compost has been obtained by maintaining various physical parameters like pH, moisture content, C/N ratio, microflora, etc. In drum composters (Jiang et al., 2018; Jain and Kalamdhad, 2019). All carbon-rich agents have been found to be suitable for the production of compost with the nutritional concentration suitable for agricultural proposes. This signifies the importance of controlling or maintaining physical parameters during composting for the yield of good quality end-product i.e., compost (Varma and Kalamdhad, 2014; Jain et al., 2019). Similar conditions were maintained during the biodegradation of bioplastics under compost environment under controlled physical parameters like temperature, moisture content, volatile solids and carbon content, which plays a vital role in the degradation of plastics under simulated composting environment (Leejarkpai et al., 2011). Biodegradation of biopolymers has been demonstrated via composting under controlled conditions by various international standards such as ASTM D-6400 (ASTM International) and ISO 14855–2 (International Organization for Standardization). These organizations have maintained certain basic protocols for terming plastics as biodegradable, which are followed by researchers as well as industries. According to these standards, biodegradation experiments are conducted under various environments and composting conditions. Parameters such as moisture, heat, humidity, volatile solid content and carbon content of the substrate are necessarily controlled during the composting process (Astm, 2014; Kalita et al., 2019). Biodegradation of PLA follows two sequential steps: hydrolysis followed by biological degradation. Hydrolytic degradation of PLA occurs during the primary phase of the biodegradation process triggered by humidity and thermophilic temperature maintained throughout the degradation process. Due to action of water molecules by random chain scission, molecular weight (Mn) tends to decrease down (to as low as <20000 Da). These low molecular weight chains of oligomers and lactic acids are digested my microbes present in the compost, leading to cara et al., 2018). It is noteworthy that low bon mineralization (Peka�rov� molecular weight to middle molecular weight polymer chains are required for microbial assimilation into CO2 and water (Pradhan et al., 2010a; Valapa et al., 2016). Many composting studies on various biocomposites have been done to assess the percentage of biodegradation of PLA and its derivatives materials by diverse analytical techniques (Rudeekit et al., 2008; Weng et al., 2010). Among them, CO2 evolution measurement is a very distinct technique used for measuring biodegradability. Different methods like direct measurement respirometry (DMR), non-dispersive infrared (NDIR), cumulative measurement respirometry (CMR) titration have been used to measure the evolution of CO2 under different environments (Kale et al., 2007c; Leejarkpai et al., 2011; Stloukal et al., 2015). In a previous work by our group, gas chromatography (GC) was used for measuring the evolved CO2 value to predict the percentage biodegradation. This technique was found to be very precise in measuring aerobic biodegradation of test materials for various PLA based biocomposites. 2. Materials and methods 2.1. Materials Poly(lactic acid) polymer 2003D (number average molecular weight ¼ 140 kDa) was obtained from Nature Works®, USA (https://www. natureworksllc.com) (Latitude 41� 320 4400 N, Longitude 96� 80 400 W). LLDPE was purchased from Haldia Petrochemicals Limited, India (http://www.haldiapetrochemicals.com) (Latitude 22.0667� N, Longitude 88.0698� E) and MCC was purchased from Sigma Aldrich, India (https://www.sigmaaldrich.com) (Latitude 12� 590 N, Longitude 77� 350 E). 2.2. Preparation of blends PLA based biocomposite strips were fabricated in a twin-screw extruder (Model-Haake MiniLab II, Make- Thermo Fisher Scientific, Germany. 2015). Processing temperature was maintained at 180 � C with ~2 min residence time. All the parameters were kept same for the fabrication of different polyblends of PLA, namely, neat PLA (NPLA), PLA/LLDPE, PLA/LLDPE/1% MCC, PLA/LLDPE/3% MCC and PLA/ LLDPE/5% MCC (Bhasney et al., 2018). The thickness of the entire sample strips were maintained as 0.45 mm on an average and their composition is presented in Table 1. 2.3. Preparation of compost Paper mill sludge waste was collected in the month of October 2017 from Jagiroad Paper Mill (Morigaon, Assam, India, Latitude 26.253317� N, Longitude 92.342405� E) and dry leaves were collected from the campus of Indian Institute of Technology (IIT) Guwahati (Guwahati, Assam, India, Latitude 26� 100 2000 N, Longitude 91� 440 4500 E). Cow dung and saw dust were obtained from Amingaon village (North Guwahati, Assam, India, Latitude 26.18� N, Longitude 91.72� E). The compost was prepared as described in an earlier investigation (Varma and Kalamdhad, 2014). 2.4. Composting process The composting set-up consisted of a number of composting vessels (CVs) made of Borosil glass, having a volume of 2 L each, connected to a gas chromatography (GC) system through a 16 port gas analyzing valve. Concentrated sulfuric acid kept in a cylindrical glass vessel, placed in between the composting vessel and the gas valve connected to the GC, was used to trap moisture. CO2 free air compressor was used to create aerobic condition inside the CVs. A locally purchased rotameter was used to control the air flow. GC (Model: Trace 1310, Make: Thermo Fisher Scientific, Germany, 2015) was used to measure the evolved CO2. Helium, hydrogen and zero air with 99.999% purity were used as the mobile phase for GC analysis. 2 N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 Fig. 1. Manual observation of all the test samples before and after biodegradation. Fig. 2. Biodegradation kinetics of PLA and its blends: (a) NPLA, (b) PLA/LLDPE/5% MCC (c) PLA/LLDPE/3% MCC, (d) PLA/LLDPE/1% MCC, (e) PLA/LLDPE. Error bars represent standard deviation. 3 N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 Table 2 Kinetic model parameters and coefficient of determination of the biodegradation study. Parameters NPLA kaq kr km ks Cr0 Cm0 Cs0 Caq0 Lag Phase R2 1 0.057 0.049 0 42.790 48.380 5.882 2.946 5 99 PLA/ LLDPE 0.99 0.007 0.006 0 31.525 31.978 24.044 12.450 88 PLA/ LLDPE/1% MCC PLA/ LLDPE/3% MCC PLA/ LLDPE/5% MCC 1 0.017 0.017 0 35.729 37.064 18.011 9.194 4 96 0.99 0.0207 0.020 0 36.786 38.424 16.432 8.355 4 97 1 0.046 0.046 0 40.178 44.493 10.186 5.141 8 98 Fig. 4. Polarizing optical micrographs of the samples before biodegradation (Initial) and after biodegradation (90 Days). 2.5. Biodegradation study In this test, all the prepared blends were subjected to biodegradation under simulated aerobic composting conditions in accordance to ASTM D5338-15 standard (ASTM Standard D 5338, 2003). 50 g of test material strips was buried under 500 g of compost in the CVs and the experiments were done in triplicate. Thorough mixing of the compost and sample was done, followed by incubation for a time period of 140 days at a temperature of 58 � 5 � C and relative humidity (RH) of 60%. Routine observations were made to ensure a constant oxygenated environment of more than 15% to maintain a proper aerobic condition during the composting process at a constant temperature. At an interval of every 7 days, any moisture loss was replenished with distilled water and the compost mixes were returned to the CVs (Pradhan et al., 2010b). Chemical properties of the test were monitored according to ASTM D5338-15 standard (ASTM Standard D 5338, 2003). Cumulative evolution of CO2 and net cumulative CO2 production were determined according to ASTM D5338-15 standard [17]: Fig. 3. Number average molecular weight analysis of the samples: (a) before biodegradation, (b) after biodegradation. Table 3 Number average molecular weight analysis of the samples before and after biodegradation. %Biodegrdation ¼ Number average molecular weight (kDa) Sample Before biodegradation After biodegradation NPLA PLA/LLDPE PLA/LLDPE/1% MCC PLA/LLDPE/3% MCC PLA/LLDPE/5% MCC 104 74 73 87 75 32 45 41 38 21 ðCO2 ÞT ðCO2 ÞB � 100 ThCO2 (1) where, (CO2)T was the cumulative amount of CO2 evolved in each composting flask containing the polymer sample, unit-g/flask; (CO2)B was the mean cumulative amount of CO2 evolved in blank flask, unit-g/flask; ThCO2 was the theoretical amount of CO2 evolved from the test 4 N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 Fig. 5. Spherulite diameter growth of the samples: (a) before biodegradation, (b) after biodegradation. respectively. Caq was the percentage of intermediate water soluble carbon, and CT depicted the percentage of mineralization, i.e., the cumulative C–CO2 production of the polymer sample. khr, khm & khs were the hydrolysis rate constants of rapidly, moderately and slowly hydrolysable carbon fractions, respectively. kaq was the ultimate mineralization rate constant, describing the formation of CO2 from the water soluble intermediate. The analytical solutions for the biodegradation kinetics were modified according to our experimental observations during the lag phase and were found to be more accurate than the solutions given by other researchers wherein lag phase was considered to be a period of negligible carbon mineralization (Leejarkpai et al., 2011; Stloukal et al., 2015). Cr t ¼ Cro :e Cm t ¼ Cmo :e Cs t ¼ Cso :e Fig. 6. Spherulite density of the samples before biodegradation (initial density) and after biodegradation (final density). þCmo :khm : 2.6. Biodegradation mechanism and kinetics dCr ¼ dt khr :Cr (2) dCm ¼ dt khm :Cm (3) dCs ¼ dt khs :Cs (4) dCT ¼ kaq :Caq dt � ðe (8) for t > c; (9) for t > c; kaq :ðt cÞ khm :ðt cÞ kaq ðe þ Cro :khr : kaq :ðt cÞ e khm � khr :ðt cÞ kaq þ Cso :khs : e kaq :ðt khr ðe � cÞ khs :ðt cÞ kaq e kaq :ðt khs cÞ � for t > c; (10) 0 Biodegradation of polymers is a process comprising of a series of intricate steps (Leejarkpai et al., 2011). The rate equations depicting the change of each component involved in the entire mechanism are as follows: kaq :Caq khs ðt cÞ (7) for t > c; khm ðt cÞ Caq t ¼ Caqo :e materials assuming that all the carbon of the test material was transformed into CO2. dCaq ¼ ðkhr :Cr þ khm :Cm þ khs :Cs Þ dt khr ðt cÞ CT t 1 � Caqo : 1 e kaq :ðt cÞ B C B � �� C � B C k k aq hr Bþ C : 1 C :e khr :ðt cÞ þ :e kaq :ðt cÞ C B ro kaq khr kaq khr B C B C B � C � �� ¼B C kaq khm B C khm :ðt cÞ kaq :ðt cÞ B þ Cmo : 1 C :e þ :e B C k k k k aq hm aq hm B C B C �� C � B � A @ kaq khs þ Cso : 1 :e khs :ðt cÞ þ :e kaq :ðt cÞ kaq khs kaq khs (11) for t > c; or ðCc =cÞ:t for t � c where, Cro, Cmo & Cso referred to the initial percentages of rapidly, moderately and slowly hydrolysable carbon fractions, respectively, while Cr_t, Cm_t & Cs_t were the percentages of the same fractions at time t, respectively. Caqo was the initially present percentage of the intermediate water-soluble carbon, and Caq_t denoted the same at time t. CT_t gave the cumulative percentage of C–CO2 production at time t. Cc denoted the C–CO2 percentage. The mathematics involved in this particular scenario was a first order kinetic equation with subsequent product formation, along with the existence of a lag phase, c. Hence, to further upgrade the existing model developed by previous authors, (5) (6) where, Cr, Cm & Cs were the percentages of rapidly, moderately and slowly hydrolysable solid carbon in the initial polymer chain, 5 N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 Fig. 7. a)DSC thermograms of the samples before biodegradation. b)DSC thermograms of the samples after biodegradation. mineralization was assumed to occur linearly before the onset of first order mechanism during the lag phase for the sake of uniformity. Unique duration and percentage conversion was considered for the lag phase of each sample and experimentally determined. Generalized reduced gradient (GRG) non-linear optimization method, an extension of Excel Solver, was adopted to minimize the objective function. The fundamental idea behind this algorithm was to express a particular set of variables called the basic variables, in terms of the remaining non-basic variables. The numeric values of Cro, Cmo, Cso, kaq, khr, khm and khs were thus calculated. A set of constraints, validating both the theoretical and mathematical sections of the problem, were employed to facilitate the Excel Solver in finding the optimum solution. Initially, the entire solid carbon fraction (CC0) was assumed to be comprised of portions of rapidly, moderately and slowly hydrolysable carbon fractions, along with a certain amount of the water soluble intermediate. At time t, the remaining solid carbon (CFC) comprised of the same whereas the rest was converted to CO2. As far as the rate constants were concerned, the mineralization rate constant was taken to be the highest, followed by the rapid and moderate hydrolysis rate constants, respectively. The slow hydrolysis rate constant was assumed to be zero. All the kinetic parameters were constrained to be positive. Mathematically, the constraints were as follows: � � CC0 ¼ Cro þC moþCso � þ Caqo (13) 0 � khm< khr< kaq, khs¼ 0 (14) CT_t ¼ (Cc /c).t, for t � c (15) � � � An elemental analyzer (Model: E 3000: Make: Euro Vector, Italy, 2015) was used for elemental analysis of the blend samples following the ASTM International D5373-16 standard. Carbon percentage of the samples were analyzed using the same instrument. 2.8. Molecular weight analysis The number average molecular weight (Mn) of the samples was determined using a gel permeation chromatography (GPC) system (Model: UFLC, Make: Shimadzu, Japan, 2014) equipped with a refractive index detector (RID-10 A) and calibrated using polystyrene standards. Sample preparation was done by using 0.25 μm syringe filters for filtering out the solution, which was dipped overnight in chloroform (Tesfaye et al., 2017). Analysis was done before and after biodegradation for all the samples. 2.9. Polarizing optical microscopic (POM) analysis for observing spherulite growth POM was used to calculate the spherulite density and spherulite size of the test samples both before and after biodegradation. Analysis was done at 120 � C before biodegradation and at 100 � C after biodegradation since the recrystallization temperature of the samples decreased to almost or little less than 100 � C after biodegradation. POM was done using (Model: Nikon H600L microscope, Make: Nikon, Japan, 2014) mounted on a hot stage (Model: Linkam TST350, Make: Linkam Scientific Instruments, United Kingdom, 2014). At first, the samples were mounted on a glass slide cover and heated to 200 � C at the rate of 50 � C/ (12) � CFC ¼ Cr,final þ Cm,final þ Cs,final þ Caq,final � 2.7. Elemental analysis � � 6 N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 Fig. 7. (continued). 2.11. Morphological analysis Table 4 Percentage crystallization of the test samples before and after biodegradation. Sample Before biodegradation (XC) After biodegradation (XC) NPLA PLA/LLDPE PLA/LLDPE/1% MCC PLA/LLDPE/3% MCC PLA/LLDPE/5% MCC ~7 ~20 ~23 ~16 ~14 ~27 ~30 ~40 ~31 ~38 Field emission scanning electron microscopy (FESEM) (Model: Sigma 300, Make: ZEISS, USA, 2017) was used to study the morphology of the test materials. This study was used to understand the action of biodegradation on the blends. Similarly, transmission electron microscopic (TEM) (Model: 2100, Make: JEOL, Japan, 2016) images were captured at an accelerating voltage of 200 kV, to study the samples before and after biodegradation. min. The samples were then cooled to 100 � C at the rate of 50 � C/min. Next, the samples were kept for 30 min in isothermal conditions and their spherulite growth and size were measured (Pal and Katiyar, 2016). 2.12. Water contact angle analysis The water contact angle of the test samples was measured using a goniometer (Model: DSA-25 Expert, Make: Kruss, Germany, 2015). Analyis was done according to Kalita et al. (2019) with water as the solvent. The test samples were rinsed with distilled water before and after biodegradation. 2.10. Differential scanning calorimetric (DSC) analysis The DSC instrument (Model: 204 F1 Phoenix, Make: Netzsch, Germany, 2004) was pre-calibrated using indium standards. ~5–6 mg of the test samples were used for this analysis. Second heating cycle was programmed to measure the crystallinity percentage as mentioned elsewhere (Kalita et al., 2019). Melting enthalpy of 100% crystalline PLA was taken as 93.1 J/g for the determination of percentage crystallinity as follows: � � ΔHm ΔHcc Xcð%Þ ¼ � 100 (16) ΔH0 3. Results and discussion 3.1. Biodegradation study Physio-chemical characteristics of the compost used for the biodegradation study were as follows: pH- 7.78, MC- 45%, VS content- 58%, OUR- 5.12 mg g 1 VS day 1, C/N ratio- 21 and TOC- 40%. Elemental analysis showed that the organic carbon (OC) content of the compost and the test samples was almost 40% and 50%, respectively. Standard cellulose strips showed degradation of more than 90% within 37 days and this confirmed the similarity of our composting set-up to that described in ASTM International D5338 standard protocol. Manual where, ΔHm was the melt enthalpy, ΔHcc was the cold crystallization enthalpy, and ΔH0 was the enthalpy of melting of 100% crystalline PLA samples (Pantani and Sorrentino, 2013). 7 N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 solid carbon, which are rapidly, moderately and slowly hydrolysable. Data analysis of CO2 evolution and biodegradation percentages of the present study showed a faster rate of biodegradation of the NPLA biocomposite strips as compared to the other test samples. The percentage of CO2 evolution of the test samples namely NPLA, PLA/LLDPE/5% MCC and PLA/LLDPE/3% MCC ranged from ~70% to 90% in 140 days, signifying their suitability for industrial composting due to their fast decomposition rates. Quick biodegradation of the NPLA and PLA/ LLDPE/5% MCC samples might be due to the initial amount of Cr0 leading to a faster rate of biodegradation (Table 2). Leejarkpai et al. (2011) showed that degradation of MCC and PLA resulted in high amounts of C–CO2 evolution, which gave readily hydrolysable carbon values of 55.49% and 40.17%, respectively, with hydrolysis rates of 0.338 day 1 and 0.025 day 1, respectively. The value of Cr0 for the test samples PLA/LLDPE and PLA/LLDPE/ 1% MCC was very low due to which their biodegradation rate was also low, although the ultimate mineralization rate was almost same for all the test samples (Table 2). Higher amount of Cm0 & Cs0 hindered biodegradation. Lower rate of biodegradation could also be attributed to the lower rate constant value kr for the test samples PLA/LLDPE and PLA/LLDPE/1% MCC. From the modelled data analysis, it was observed that addition of higher percentages of MCC increased the mineralization rate of the test samples. Similar study done on neat PLA and organomodified montmorillonites (MMT) showed the acceleration of chain scission in PLA blended with nanoclays, which was confirmed by determining the resultant rate constants of the hydrolytical chain scission, resulting in further carbon mineralization (Stloukal et al., 2015). Therefore, it could be concluded that the addition of MCC in the polymer blends could enhance the rate of biodegradation and thus, the biodegradability of the materials, making the manufacture of bio-based polymeric blends a cost-effective strategy. 3.2. Molecular weight analysis Fig. 8. FESEM images of the test samples before and after biodegradation (90th day). Decrease in molecular weight is one of the most reliable indication of biodegradation in biopolymers under various environments (Rudeekit et al., 2008; Peka�rov� a et al., 2018; Kalita et al., 2019). From the molecular weight data, it was clear that the PLA matrix was degraded the most under composting conditions. It was also seen that blending of PLA with conventional polymers like LLDPE did not affect its biodegradability. Since, LLDPE was insoluble in chloroform, it remained in the form of microparticles in the solution, which were filtered out before analysis. Significant loss in molecular weight during the degradation period was evident from Fig. 3 which depicted that the peaks became broad and less intense with a decrease in retention time of the samples, suggesting degradation of the PLA matrix due to chain scission mechanism (Musioł et al., 2016). Molecular weight of the samples before and after degradation are shown in Table 3. The critical molecular weight for the hydrolysis of PLA was observed to be higher than the critical molecular weight for the onset of PLA mineralization, suggesting that PLA chains must be further shortened to be assimilated by microorganisms. This chain scission leads to carbon mineralization of the polymers (Stloukal et al., 2015; Kalita et al., 2019). observation of samples before and after 90th day of degradation is presented in Fig. 1. Percentage biodegradation of the samples in terms of CO2 evolution was measured at regular intervals until 140 days (Fig. 2). In this study, only percentage mineralization was studied until 140th day and rest of the characterization of the degraded films was done till the 90th day, since after the 90th day, the samples were difficult to characterize except for the determination of carbon mineralization. It was observed that NPLA got more than 90% biodegraded under the paper industry sludge composting conditions whereas PLA/LLDPE showed only 50% degradation. PLA/LLDPE/1% MCC, PLA/LLDPE/3% MCC and PLA/LLDPE/5% MCC showed almost 70%, 75% and 80% biodegradation, respectively. This clearly indicated that an increase in the MCC content of the test samples accelerated biodegradation. This might be due to the presence of high amount of –OH end groups in MCC, which were more susceptible towards hydrolysis (Aouat et al., 2018). It was also observed that only the PLA and MCC parts of the polyblends degraded whereas LLDPE showed almost negligible amount of carbon mineralization. The test samples showed the following trend of biodegradation: NPLA > PLA/LLDPE/5% MCC > PLA/LLDPE/3% MCC > PLA/LLDPE/1% MCC > PLA/LLDPE. Various studies have reported 90% biodegradation of PLA under aerobic thermophilic composting conditions and that the biodegradation accelerated when PLA was mixed will fillers like cellulose, wheat straw, etc (Cheung et al., 2010; Pradhan et al., 2010a; Nakayama et al., 2018). Thus, it was clear that the bio-based PLA blends could attain almost 50–60% biodegradability if mixed with conventional polymers, which would reduce its carbon footprint in the environment. According to the biodegradation kinetics equation, as the entire chain of a polymer is not identical in its affinity towards hydrolysis, the primary chain is assumed to composed of three different kinetic forms of 3.3. Spherulite growth analysis using POM POM images of the samples before and after biodegradation are shown in Fig. 4. There was a significant increase in the spherulite size in case of NPLA after biodegradation. Spherulite size of NPLA observed during the 30-min study increased from ~30 μm to ~150 μm (Fig. 5). This increase in spherulite size in degraded samples might be attributed to the space between the spherulites that contained amorphous material and the attached compost within the polymer that were unable to crystallize. After biodegradation of the amorphous portion, crystalline region got more space for chain folding, chain mobility and spherulite formation of bigger diameter than the initial samples. Formation of large 8 Journal of Environmental Management 261 (2020) 110211 N.K. Kalita et al. Fig. 9. TEM images of the test samples: (a) PLA/LLDPE/1% MCC before biodegradation, (b) PLA/LLDPE/5% MCC before biodegradation, (c) PLA/LLDPE/1% MCC after biodegradation, (d) PLA/LLDPE/5% MCC after biodegradation. spherulites after biodegradation is due to impute of longer chains of higher molecular weight left behind as crystalline portion because of the microbial assimilation of medium and shorter chains. Crystallization temperature (Tc) also plays a crucial role in spherulite growth and density change. The spherulite growth rate increases significantly until a reduced temperature peak is reached, and after this point, as the crystallization temperature decreases further, the growth rate also decreases due to an increase in the viscosity of the polymer making it difficult for chain motion which in turn affects alignment and ordering (Owen, 2013). This phenomenon could be well understood from the DSC data where Tc of all the biodegraded samples decreased, leading to a reduction in the spherulite density as shown in Fig. 6. This analysis helped in understanding that the microbes were readily degrading the amorphous region leaving behind the crystalline region, which led to an increase in crystal size and decrease in crystal density. Here, lamella formation occurred due to the joining of the molecular chains to the nuclei, which changed the dimensions of the lamella causing an increase in the spherulite diameter. Similar observations were noted for all the test samples. Fig. 10. Water contact angle analysis of the test samples: (a) NPLA, (b) PLA/ LLDPE/5% MCC, (c) PLA/LLDPE (d) PLA/LLDPE3% MCC (e) PLA/LLDPE/1% MCC before biodegradation, (f) NPLA, (g) PLA/LLDPE/5% MCC, (h) PLA/ LLDPE, (i) PLA/LLDPE/3% MCC, (j) PLA/LLDPE/1% MCC after biodegradation. Table 5 Water contact angle of the test samples before and after biodegradation. Sample Water contact angle (� ) before biodegradation Water contact angle (� ) after biodegradation NPLA PLA/LLDPE/ 5% MCC, PLA/LLDPE/ 3% MCC PLA/LLDPE/ 1% MCC PLA/LLDPE 78 � 5 66 � 4 50 � 2 54 � 7 69 � 3 55 � 5 70 � 2 39 � 4 81 � 3 69 � 5 3.4. DSC analysis Comparison of the thermographs of the test samples is shown in Fig. 7. With an increase in the biodegradation time, percentage crystallinity (Xc) also increased (Table 4) which signified biodegradation of the amorphous portions of the test samples, leaving behind only the crystalline portions (Pantani and Sorrentino, 2013; Kalita et al., 2019) as also shown in the POM analysis. 9 N.K. Kalita et al. Journal of Environmental Management 261 (2020) 110211 Samples having higher percentage of PLA and MCC had a tendency to degrade faster as seen from the kinetic results and further confirmed by the percentage crystallization values (Table 4). It was observed from the DSC thermograms that the melting point (Tm) of the samples decreased gradually, signifying biodegradation. Tm peaks showed double melting point in PLA/LLDPE/5% MCC which indicated formation of crystallites a et al., 2018), as observed in the of different lamella thickness (Peka�rov� POM studies (Figs. 4–6). This results are in line with the study done by � et al. (2018), where they also found two melting points in the Peka�rova PLA based samples after biodegradation under compost environment. Studies revealed that low molecular weight intermediates/oligomers formed during biodegradation diffused away from the sample surface (Leejarkpai et al., 2011; Kalita et al., 2019) and became available to the enzymatic attack by the microbes (Pantani and Sorrentino, 2013; � et al., 2018). Peka�rova It was also noticed that the melting peak of LLDPE in the PLA/ LLDPE/5% MCC, PLA/LLDPE/3% MCC, PLA/LLDPE/1% MCC and PLA/ LLDPE sample blends did not show any significant decrease in Tm as the biodegradation progressed. This indicated that LLDPE did not break down during the degradation. With a decrease in molecular weight, the melting temperature of the PLA component in the polyblend also decreased significantly, which again suggested biodegradation of the test samples (Stloukal et al., 2015). Similarly, recrystallization temperature of all the test samples decreased significantly during the course of biodegradation. Kalita et al. (2019) had also reported that water contact angle of neat PLA sample decreased from ~78� to ~40� as ester bonds hydrolysis created new end groups, which increased the hydrophilicity of the PLA microsphere. This might be due to the formation of oligomeric components attached to the surface and also due to surface degradation, leading to the formation of carboxyl functional groups and hydroxyl groups on the surface bonded by a water bridge known as solvation (Alex et al., 2018). 4. Conclusion NPLA and its extruded bio-based polyblends like PLA/LLDPE/5% MCC, PLA/LLDPE/3% MCC, PLA/LLDPE/1% MCC and PLA/LLDPE were analyzed in the present biodegradation study. The results of biodegradation percentage showed that with higher MCC component in the test samples, biodegradation was much faster as compared to lower percentage of MCC in the blended samples. Kinetic data revealed that higher amount of Cr0 hindered biodegradation and lower rate of biodegradation was due to the lower rate constant value kr for the test samples, PLA/LLDPE/1% MCC and PLA/LLDPE. It was also observed that with an increase in the MCC content, the mineralization rate of the test samples also increased, which signified that the addition of MCC accelerated the rate of biodegradation in the prepared blends. Decrease in the molecular weight of the test samples proved the occurrence of chain scission mechanism, leading to the formation of intermediates, which resulted in microbial assimilation. Microbes attacked the amorphous regions of the samples leaving behind the crystalline portions as proved by the POM analysis, which showed an increase in spherulite size and spherulite density due to spaces formed after erosion of the amorphous regions from the surface of the blended samples. Formations of cavities were also observed by FESEM study, which revealed PLA forming fibril-like structures due to degradation under composting conditions. Furthermore, it was clear from the TEM study that LLDPE remained with the remaining PLA matrix after the biodegradation of the bio-based blends, which suggested that these kinds of blends were compostable under simulated environmental conditions. Water contact angle study showed increased hydrophilicity of the test sample surface, which facilitated biodegradation. The predicted results were in line with our experimental datasets. The results obtained signified that PLA based bio-blends were almost 50–80% biodegradable and it might reduce the cost of its production and its carbon footprint in the environment. This study highlighted the high biodegradability of the PLA/LLDPE/MCC blend samples and would provide guidance and characterization for the preparation of these blends keeping in mind their biodegradability. 3.5. Morphological analysis Surface morphology of the test samples are shown in Fig. 8. The morphological changes were observed on the 90th day of the composting process. Large holes and cracks formation were observed on the polymer surface after biodegradation, which could be attributed to the action of microbes or biofilm formation on the surface of the polymers and diffusion of the cells inside the polymer (Sedlarik et al., 2008). This surface erosion of the PLA based biocomposites could be characterized by the presence of cavities (Leejarkpai et al., 2011; Arrieta et al., 2014). It happened as water (moisture inside the CV) diffused into the amorphous domains that initiated hydrolytic degradation. With progress in the hydrolytic degradation process, the fillers tried to move away from the matrix towards the compost, resulting in holes, cavities and cracks which were observed on the 90th day. Fig. 8 reveals how PLA was getting degraded while LLPDE was attached in the cavities, after the PLA formed fibril-like structures in the sample. This suggested biodegradation through surface erosion. Similar behavior was also observed for all the test samples except neat PLA samples, which indicated formation of fibrils during the degradation process. From the TEM analysis (Fig. 9) of PLA/LLDPE/1% MCC and PLA/ LLDPE/5% MCC samples, it could be observed that PLA got degraded rapidly forming voids after biodegradation, leaving behind LLDPE which was embedded in the PLA matrix (Bhasney et al., 2018). It was clear that after biodegradation, LLDPE remained behind with the remaining PLA matrix, which suggested that these bio-based blends were compostable under certain environmental conditions. CRediT authorship contribution statement Naba Kumar Kalita: Writing - original draft, Formal analysis, Data curation. Siddharth Mohan Bhasney: Formal analysis. Ajay Kalamdhad: Supervision. Vimal Katiyar: Supervision, Methodology, Project administration. Acknowledgement The authors sincerely thank the Centre of Excellence for Sustainable Polymers at IIT Guwahati funded by the Department of Chemicals and Petrochemicals, Ministry of Chemicals and Fertilizers, Government of India for the facilities to perform this research work. Authors also acknowledge Department of Chemical Engineering and Central Instrument Facility, IIT Guwahati for providing the necessary research facilities. 3.6. Water contact angle analysis The blend matrix was observed both before and after biodegradation for wettability changes occurring on the surface of the polyblends. It was found that in all the test samples, hydrophilicity decreased with an increase in degradation time. This behavior was attributed to the formation of carboxylic and hydroxyl groups on the blend surface as well as the presence of cavities on the PLA matrix leading to hydrophilicity and higher chain mobility, which resulted in microbial assimilation on the surface. 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