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Sustainability, compostability and specific microbial activity of agricultural mulch films prepared from poly (lactic acid) Ponnoth Janardhanan Jandas, Smita Mohanty, and Sanjay Kumar Nayak Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 22 Nov 2013 Downloaded from http://pubs.acs.org on November 25, 2013

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Industrial & Engineering Chemistry Research

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Sustainability, compostability and specific microbial activity on agricultural mulch films prepared from poly(lactic acid) P. J. Jandas, S. Mohanty*, S. K. Nayak Laboratory for Advanced Research in Polymeric Materials (LARPM) Central Institute of Plastics Engineering and Technology (CIPET), Bhubenaswer, India

Dr. Ponnoth Janardhanan Jandas, Dr. Smita Mohanty and Dr. Sanjay Kumar Nayak Laboratory for Advanced Research in Polymeric Materials (LARPM) Central Institute of Plastics Engineering and Technology (CIPET), Bhubaneswar-751024 Orissa, India. Email: [email protected] Phone No: 0674 2742852 Fax No: 0674 2743863

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Abstract Agricultural mulch film of poly (lactic acid) (PLA) has been prepared under industrial conditions by extrusion blown film method with modified properties using poly (hydroxibutyrate) (PHB) and reactive compatibilizer maleic anhydride (MA). Processing parameters and blend composition have been optimized based up on processability and mechanical properties of the final materials. Since, PLA is a biopolymer, evaluation of service life period of the mulch film is very much important. Sustainability of the film has been analysed by keeping the films in a weatherometer, which can create accelerated weather conditions, followed by mechanical testing at each regular intervals. Similarly, variation in compostability has been analysed as per American standard for test method, ASTM D 5988 using vermi-compost. In addition, specific microbial action on the mulch films also has been analyzed using bacteria Berkholdaria Cepacia (B. Cepacia), which is selective in particular towards PLA degradation and in mixed fungal inoculums. Key words: PLA, Mulch film, B. Cepacia, Fungal inoculums.


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Introduction Poly (lactic acid) (PLA) has inherent brittleness characteristics which need to be modified to ensure its potentiality as a substitute for petroleum based polymers in the mulch film industry1. Most of the literatures in this area focussed primarily on plasticization technique, copolymerization and melt blending with flexible polymers. PLA toughening, particularly modification of impact toughness through melt blending of PLA has been emphasized by many researchers2. With the help of advanced technologies agriculture field is also consistently upgrading its potentiality in terms of maximum production from minimum resources. Agricultural mulch films are widespread today to minimize the usage of water consumption especially for short term crops. Also, contemporary material research has special interest on eco-friendly materials due to the environmental hazards created by petroleum based plastics. In this view, current study proposes a possible solution for it by introducing a polymer film for agricultural mulch applications prepared from completely biodegradable and renewable resources. In the present investigation poly(hydroxybutyrate) (PHB) has been utilized to enhance the flexibility of PLA to prepare film for mulch application. However, properties of polymer blends generally depend upon the thermodynamic miscibility in between the individual polymers. PLA and poly (hydroxybutyrate) (PHB) are biodegradable polyesters with comparable thermal and mechanical properties and are extensively used for several end use applications due to their biocompatibity, biodegradability and sustainability3. Even though, according to the thermodynamic miscibility constant values both are partially miscible in nature, so that a reactive compatibilizer maleic anhydride (MA) was used to enhance the interaction in between the polymers. PLA/PHB blend prepared through melt blending method has been reported by Zhang



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et al4. The authors suggested a partially miscible blend in between the polymers, which requires interfacial compatibilization to enhance the blend properties. Further, Phuong et al have reported PLA/PHB blends prepared using melt blending technique to improve the heat deflection temperature (HDT) and crystallization rate of PLA5. Also, the present investigation has been utilized organically modified nanoclays like hexadecyl trimethylammonium bromide modified natural montmorillonite (OMMT) and cloisite 30B (C30B) to modify the performance characteristics of compatibilized blend. The optimum compositions have been prepared in bulk under same optimized processing conditions and films have been prepared under industrial conditions. Since both the materials are biopolymers, evaluation of environmental sustainability is very much important and it has been studied by keeping the film in a weatherometer followed by mechanical property evaluation. On the other hand, the changes in biodegradability have been evaluated according to American standard for test method ASTM D 5988 in a vermi-compost medium. Further, specific microbial activity on the developed materials in comparison with virgin PLA (V-PLA) has been studied in Berkholdaria Cepacia (B. Cepacia) media and mixed fungal media under the laboratory conditions. Materials PLA (Grade 4042D) with a density of 1.24 g/cc (Mw = 211,332 g/mol and Mw/Mn=2), L-lactide and D-lactide ratio 92:8, was purchased from manufacturer & supplier (M/s) Natureworks, USA. PHB Plasticized grade (P226) with density of 1.25 g/cc (Mw = 426, 000 g/mol) was purchased from M/s Biomer, Germany. Natural montmorillonite (NaMMT), and commercially modified montmorillonite Cloiste 30B [(C30B) with modifier MT2EtOT: methyl tallow bis-2-hydroxy ethyl quaternary ammonium salt and cation exchange capacity (CEC) of 90


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meq/100g clay] were obtained from M/s Southern clay products, USA. Compatibilizer maleic anhydride (MA), initiator benzoyl peroxide (BPO) and surface modifier for nanoclay, Hexadecyltrimethyl ammonium bromide (HTAB) was procured from M/s Sigma Aldrich.Co. Germany and used without any modification. Methods Surface modification of NaMMT Hydrophilicity of NaMMT has been modified by cation exchange reaction using HTAB. 15g of NaMMT was dispersed in 500 ml of distilled water by emplying mechanical stirring process. Further, the surfactant HTAB (0.75g of HTAB in 10ml distilled water) was added to the acidified nanoclay dispersion. Stirring was continued for 2 hours, followed by centrifugation and filtration till the complete removal of bromide group from the filtrate. Removal of bromide was confirmed using AgNO3 test. The precipitate of organically modified nanoclay (OMMT) was dried at 80oC for 12 hours and cryomilled to fine powders. Preparation of blends and blend nanocomposites Preparation of blend nanocomposites: Prior to compounding, PLA and PHB were pre-dried at 80°C in a vacuum oven for 12 hrs. Subsequently, the blends were prepared in a co-rotating twinscrew extruder (Haake Rheomex OS, Germany) attached with blown film setup. Blends of different compositions were prepared with variable PHB content ranging from 10 to 40 percentages. Processing temperatures were optimized at 170, 175, 175, 180 and 175oC for the five successive zones of the extruder with a screw speed of 60 rpm. The pellets of blends and blend nanocomposites have been prepared and the pellets have been further extruded in a blown film setup after proper preheating at 50°C for 4 hours in a vacuum oven to obtain films of respective compositions. Further, the compatibilzer and the nanoclays (C30B and OMMT) have



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been incorporated to the optimized composition of blends under the above mentioned processing parameters for preparing compatibilized blends and blend nanocomposites films. Master batches of optimized compatibilized blend and blend nanocomposites have been opted as per the mechanical performances and prepared in bulk for industrial viability study. The pellets were sent to ‘Polyfilms, Chennai, India’ (a company which is famous for plastics film production) to prepare agricultural mulch films under industrial conditions. The processing parameters for the blown film process were kept under same range of 170-180°C and rpm at 60. Characterization Mechanical Testing Tensile measurements of virgin PLA (V-PLA), virgin PHB (V-PHB), blends and blend nanocomposites were carried out using Universal Testing Machine (UTM, Instron 3386 UK). In all the cases, rectangular shaped specimens of dimension (100×20×0.03) mm as per ASTM D 882 were used. Gauge length was kept fixed at 50mm with a cross head speed of 5mm/min for the test. Sustainability analysis Rectangular film samples of dimension (100×20×0.03) mm were kept in Atlas Weatherometer Ci5000, USA, under accelerated weathering conditions for various fixed time intervals of 10, 20, 30, 40 and 50 days. Further, the samples were tested for mechanical properties to evaluate the extent of sustainability of the film under accelerated weathering conditions. Biodegradation study Biodegradability of the agricultural mulch film in relation with the biodegradability of V-PLA has been evaluated in three different ways as follows


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Compostability in vermi-compost media according to ASTM D 5988-03 Degradability in specific bacterial condition of B.Cepacia Degradability in mixed fungal media Compostability Compostability under controlled composting conditions has been conducted using glass desiccators of 2 L capacity according to the standard test method ASTM D 5988-03. The vermicompost had a total nonvolatile solid content of about 61.3% and volatile solid content of about 24.7%. Water content has been raised further in to 50% before the testing started. pH of the compost has been reported as 7.6. Cellulose film (filter paper) and high density polyethylene (HDPE) film has been used as positive and negative controls respectively. The organic carbon content in the cellulose and PLA has been theoretically calculated as 39.4% and 43.2% respectively from the average molecular weight (Mw) values. A mixture of vermi-compost (250g by dry weight) and sample (10g by dry weight) is kept in the glass desiccators and incubated at room temperature (27°C). In this study total six reactors have been utilized for testing, one blank, two controls (positive and negative) and three sample vessels containing V-PLA, PLA/PHB/MA (70/30/7) and its nanocomposite with 3 weight% OMMT. CO2 evolved as a result of biodegradation was trapped in each vessel by means of 50 ml of 0.05 N KOH solution kept in a 100 ml beaker. The absorbing solutions were back titrated with 0.1 N HCl within regular 5 days interval up to 100 days. 50 ml distilled water was kept as reference solution in the each reactor. The percentage of biodegradation has been calculated according the steps mentioned in the standard. In addition, the extent of biodegradation also has been evaluated through thermogravimetric analysis (TGA), scanning electron microscopy (SEM), pH analysis of



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composting media and visual study. TGA analysis of the composted samples was conducted using Thermogravimetric analyzer (TGA, Q50, TA Instruments USA). Samples of about 7 mg were heated from 50 to 600°C at a rate of 10°C/min under N2 flow (60ml/min). Corresponding weight loss vis-a vis temperature was noted. Morphological changes on the film samples as a result of composting have been studied through SEM analysis by using Zeiss EVO MA, UK instrument. The samples were coated with gold using a vacuum sputter coater prior to test to improve the surface conductivity. Biodegradation under B.cepacia inoculums The specific microbial activity on the mulch film has been conducted in a bacteria media B. Cepacia media. It is a rod shaped bacterium with approximate dimensions of 1.3-2.3 µm x 0.7-1.2 µm. Bacteria was cultivated in a petridish having potato dextrose broth (PDB) in isolated conditions. For the preparation of basal PD media, 3.6 gm (2-4%) potato dextrose was dissolved in 150 ml of distilled water and mixed with 2.7 gm (1.8%) of agar-agar in a sterilized conical flask. The solution was thoroughly mixed by continuous shaking over a bunsen flame up to the formation of clear solution and poured into petridishes. The solution was cooled further and spread the bacteria sample under sterilized conditions. The conditioned samples at room temperature for 24 hours were kept in the petridishes containing PDB and B. Cepacia and the samples were incubated at 25 ±2°C and 50 ±5% relative humidity. After the regular intervals of incubation, films were washed with gluteraldehyde followed by deionized water, dried and weighed. The percentage of weight loss has been calculated as below, Initial Weight = [(Initial Weight – Final Weight)/% of Weight loss] X 100


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Biodegradation under mixed fungal inoculums Basal PD media for the fungal inoculation has been prepared as discussed above for the bacterial study. Further, soil sample was collected from a continuously moistened area of CIPET campus Bhubaneswar. 1N water solution of the soil was prepared in distilled water and the solution further diluted into a normality of 10-3 than that of its initial strength. Further, the solution has been spread over the solution on the PD media taken in a petridish. The mixture was incubated at 25 ±2°C and 50 ±5% relative humidity for seven days. Thereafter the selective fungal spores have been transferred to 250 ml conical flasks filled with 100 ml PDB solution and sample films. After each 5 days of intervals, the samples were collected from the fungal inoculums and washed with gluteraldehyde followed by deionized water. Further the films were dried and weighed. The percentage of weight loss has been calculated as mentioned above in case of B. Cepacia study. Results and Discussions Mechanical Properties Mechanical Properties of PLA/PHB Blends with variable PHB loading Apart from the elongation property tensile strength and modulus has been showed a decreasing tendency as what observed in case of a soft polymer incorporated PLA matrix6. However, the increase in elongation suggests some degree of molecular interaction at the interface of PLA and PHB in the blend. Zhang et al suggested that PHB material can act like finely dispersed crystals containing polar C=O groups which can induce dipole-dipole interaction or hydrogen bonding with PLA macromolecules4. This may tend to enhance the ductility of the blend, which provides better energy absorbing capability to the PLA matrix through a change in mechanical deformation process either through the promotion of extensive shear yielding or



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crazes formation or through a combination of both. In addition, according to Hildebrand solubility theory PLA and PHB have solubility parameters (δp) of 23.5 J1/2/cm J1/2/cm

3/2

3/2

and 19.8

, which are closer values and point to a partially miscible blend in between the two

polymers7.

However,

the

miscibility has

been

enhanced

significantly

by reactive

compatibilization using MA. A composition of PLA/PHB (70/30) with optimum elongation properties has been melt mixed with MA with different ratios. The results are depicted in table S1. Compatibilization process follows a grafting mechanism of MA on the α- carbon atom of the carbonyl group on the PLA and PHB macromolecules as depicted in the scheme1a. The proposed interaction between MA side group and the other macromolecules within the blend is depicted in scheme 1b. MA acts like bridging unit among PLA and PHB macromolecules through dipole-dipole or inter molecular hydrogen bonding. This may result in improved interaction between the two partially miscible polymers in presence of MA. In the current study, presence of additional physical interactions results a transition of fracture behaviour of the specimens during the tensile test from brittle characteristic of V-PLA to ductile fracture of PLA/PHB/MA blends. A continuous increase in flexibility has been observed with increasing amount of MA and 7% MA within PLA/PHB blend at 70/30 ratio exhibited optimum elongation at break at 540.17% (table S1). A comparative demonstration of variation in ductility as a function of compatibilization is depicted through stress-strain curve in figure 1. As observed from the figure, V-PLA was showed very rigid and brittle nature with a distinct yield point with subsequent failure immediately upon the tensile load. PLA/PHB (70/30) showed a relatively continuous strain after distinct yielding with stress remaining almost constant. Whitening phenomenon induced large amount of craze during the tensile test of the blends with and without MA. However, compatibilized blend has


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been given intense whitening phenomenon due to crazing. Crazing is a dilative process which involves localized plastic deformation. Extensive crazing of the blend resulted in larger strain at break and higher toughness than that of V-PLA. PLA/PHB/MA (70/30/7) showed initial strain softening after yielding and then underwent continuously cold drawing which meant the necked down region prolonging under stress. The stress-strain curve beyond yield point showed a combination of strain softening and cold drawing. This indicates that there was a competition between PLA chain orientation and crack formation. Hence a drop in stress with increasing strain has been observed. Beyond 100% of strain, a necking phenomenon appeared and only cold drawing dominated at relatively constant stress. This is suggested that the main part of fracture energy consumption was due to making plastic zone or stress whitening zone in front of precrack8. However, beyond 7 weight% of MA concentration, the blend matrix did not show any appreciable increase in elongation at break, which is probably due to presence of excess amount of MA, that contribute in slippage of chains, thereby reducing the properties. On the other hand, enhanced ductility results in further reduction in stiffness of PLA/PHB blends due to the MA compatibilization. With the incorporation of MA, the blend displayed a consistent decrease in tensile strength and modulus. The compatibilized blend at 7 weight% MA concentration within PLA/PHB blend at 70/30 ratio was taken for blend nanocomposite preparation and industrial trials. Mechanical Properties of PLA/PHB Blend Nanocomposites Reinforcing effect of nanoclays within the compatibilized blend resulted significant improvement in the tensile modulus and tensile strength without compromising its higher ductility. Both the blend nanocomposites showed similar range of increment in tensile properties



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as compared with V-PLA. Nanoclay loading up to 3 weight% has been showed a continuous increase in tensile strength and tensile modulus within the compatibilized blend matrix. An optimum value of tensile strength at 33.64 MPa and tensile modulus at 4222.64 MPa was observed in case of PLA/PHB/MA/C30B blend nanocomposite with 3 wt% nanoclay loading (table S1). On the other hand, blend nanocomposite prepared using OMMT nanoclay, exhibited better results with optimum values of tensile strength of 48.23 MPa and tensile modulus of 4332.56 MPa at 3 wt% nanoclay content. The increment in tensile modulus and tensile strength is mainly attributed to the nucleating characteristics of well dispersed nanoclay layers which enhances the percentage of crystallinity within the blend matrix (table S1). The increased crystallinity enhances the stiffness of the matrix thereby increasing the tensile strength and tensile modulus. HTAB modification of natural montmorillonite results in expansion of its basal spacing. This allows easy penetration of polymer macromolecules into the interstitial spaces of silicate layers which leads to a well intercalated/exfoliated nanoclays within the matrix. On the other hand, hydroxylated organomodifier end on C30B layers, establishes strong interfacial interactions with the carbonyl groups present in both the PLA and PHB macromolecules, results in better mechanical properties for the blend nanocomposite9. However, beyond 3 wt% of nanoclay loading both the blend nanocomposites have been showed decrease in mechanical properties. However, percentage of elongation of the blend nanocomposites has been decreased marginally as compared to the compatibilized blends with increased stiffness by the exfoliation/intercalation of nanoclays. 3 weight% loading of C30B and OMMT within the compatibilized blend has been reported 488 % and 457% elongation at break respectively (table S1). Based on the overall mechanical properties of the blend nanocomposites the composition


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with 3 weight% loading of both the nanoclays has been opted for further characterization and industrial scale trials. Observation from the industrial trials conducted for agricultural mulch films preparation Industrial trails have been conducted for five different samples including V-PLA, PLA/PHB(70/30), PLA/PHB/MA(70/30/7) and its two blend nanocomposites reinforced with C30B and OMMT at 3 weight%. V-PLA has been completely failed to blow due to its brittle characteristics whereas, PLA/PHB blend showed slight improvement in processability. On the other hand, the compatibilized blend has been shown significant improvement in processabilty due to the stronger interface by the bridging of MA units. Even though, consistency of the flow and thereby the thickness of film was not up to the mark in this case. However, the blend nanocomposite films prepared from both the nanoclays were showed much better melt strength and consistency in film thickness as a result of the reinforcing effect of intercalated/exfoliated nanoclays (figure 2). Environmental sustainability Environmental sustainability has been studied by keeping the film samples in a weatherometer for fixed time intervals followed by mechanical testing. The results are depicted in table S2. V-PLA has been showed a gradual deterioration in properties from 14th day of exposing. The sample after 14 day of exposure under accelerated conditions has been showed 5% decrease in mechanical modulus and 11% decrease in mechanical strength. The strength has been further reduced by 37% and modulus by 21% after 10 weeks of exposure. The results are good agreement with the study conducted by Yew et al10. Further, the blend has been showed deterioration in properties comparably faster way in which the film has been lost 48% tensile strength and 12.5% of modulus within a time period of 10 weeks. However, the blend



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nanocomposite has been showed better sustainability under the weathering conditions than that of PLA/PHB/MA blend. Tensile strength and modulus has been showed lesser deterioration around 41.4% and 23% lower than that of the initial properties. This may be due to the extra strength provided to the PLA film by the reinforced nanoclay layers. Water vapour permeability rate (WVTR) WVTR of V-PLA has been reported around 6.8 g/m2/day, where as it has been considerably increased in case of blend and blend nanocomposite systems. PLA/PHB/MA has been recorded WVTR value around 8.1 g/m2/day, on the other hand corresponding nanocomposite prepared from OMMT has been reported a marginally lesser value at 7.1 g/m2/day. Gerlowski11 suggested a general principle for decreased transportation of water vapour in a nanocomposite system. Water penetrates the galleries between the montmorillonite layers, forming water clusters in the nanocomposites, and does not allow the water to pass through quickly, and thus the diﬀusivity and the overall transport of water across the ﬁlms is decreased. As a result, even though the water absorption may enhanced due the presence of nanoclay, the transportation showed decreasing tendency than that of the blend matrix. Biodegradation study Based up on the mechanical properties and application point of view the optimized blend (PLA/PHB/MA) and blend nanocomposites (PLA/PHB/MA/OMMT) have been used to evaluate the biodegradability in comparison with V-PLA. Biodegradation by compostability Weight loss study As evident from the figure 3a and table S3, the percentage of biodegradation has been increased linearly with the increase in incubation time. Cellulose film has been degraded


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completely by showing within an incubation time period of 40 days. This confirms that the vermi-compost used in the present investigation is an active system for polymer degradation. On the other hand, the negative control, HDPE film does not reveal any considerable change in slope of the curve within the investigated time period of 100 days. The amount of CO2 produced by cellulose during composting, increased abruptly in the first 20 days of incubation at a degradation rate of 4.9 (slope). However, this was substantially decreased and attained a plateau phase beyond 40 days of composting. No lag phase was observed and the slope was decreased to 3 during a period of 30 to 40 days. However, V-PLA displayed a different biodegradation pattern with an initial lag phase in the first 10 days of composting followed by a linear phase till 75 days and plateau phase after 80 days. The slopes corresponding to the three different phases in PLA matrix is depicted in the table S3, which confirms that the microbial degradation reaches an optimum level between 20 to 80 days of incubation. It is evident that the degradation pattern of V-PLA changed considerably with the incorporation of PHB and nanoclay. As observed from the figure 3a and table S3, the initial lag phase present in the virgin material is absent in both the blend and blend nanocomposites due to the enhanced biodegradability in comparison with V-PLA. The enhanced biodegradability may be due the many factors, like molecular weight reduction, presence of microbial based PHB within the matrix, enhanced moisture absorption due to the presence of PHB and nanoclay, variation in optical purity etc. Absence of initial lag phase has been reported for both the samples which indicate an early ingress of microorganisms to the bulk regions. Reduction in molecular weight is commonly applicable for V-PLA, the blend and blend nanocomposite. However, in the blend nanocomposite the effect will be higher due to the extra



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shearing effect produced by the nanoclay reinforcement. This may be the reason for better biodegradation rate showed by the blend nanocomposite among the all samples. Kawai et al have reported the fact that relatively lower molecular weight PLA exhibits better degradation rate because of high concentration of accessible chain end groups12. This will in tune enhance the selectivity and affinity of microorganisms on PLA material which would enhance the biodegradability of the matrix in presence of nanoclays when subjected to composting. Moisture absorption may be another reason for the enhanced biodegradability in the blend and blend nanocomposite. The additional voids generated by the PHB and nanoclay reinforcement and hydrophilicity of the nanoclay in particular in the blend nanocomposite enhance the moisture absorption capability in the film. Since moisture absorption is one of the prime factor which can alter the rate of biodegradation, the increased water content in the nanocomposite samples may be another reason for enhanced biodegradation rate for these materials. In addition, the decreased crystallinity may be another major factor here for increased biodegradability for PLA/PHB/MA and PLA/PHB/MA/OMMT. As suggested by Ray et al, the preliminary attack of the microorganisms affects primarily on the amorphous domains of the semicrystalline PLA13. As a result, the consumption of polymer by the bioorganisms may become easier in both the cases. The slope values obtained were further confirming the above observations. Significantly higher slope values were observed for the blend and blend nanocomposite than that of V-PLA from the initial stages of incubation. According to El-Hadi et al, the rate of biodegradation is inversely proportional to the crystallinity of the sample14. Mechanical tests in the earlier sections suggest decrease in strength and modulus which indicates a decrease in total crystallinity of PLA in PLA/PHB blends and its blend nanocomposites. The reduction in crystalline domain in the matrix may be another major reason for the increased biodegradability for both the samples.


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Both the blend and blend nanocomposite has been attained a stable plateau of maximum degradation within a shorter time period of around 45-50 days in comparison with V-PLA15. During this period, PLA/PHB/MA blend has showed a maximum biodegradation of 86% and PLA/PHB/MA/OMMT bionanocomposite showed slightly lower value at 84.5%. TGA analysis of composted samples Since, the total crystallinity of the blend and blend nanocomposite is found to be decreased in comparison with V-PLA as a function of blending with PHB, ultimately tend to reduce the thermal stability of the PLA/PHB/MA blend. Further, the incorporation of nanoclay marginally modified the initial degradation temperature in comparison of compatibilized blend which is still observed to be lower than that of V-PLA. In case of composted there was a marginal increase in the initial degradation temperature of V-PLA after 10 and 20 days of composting, figure 4a. Single-step degradation has been observed for V-PLA with an initial degradation temperature of 261.3°C and a final degradation of 349.4°C. Initial degradation temperature increased to 269 after 10 days of incubation period. Since microorganisms attack preliminarily on the amorphous domain of the polymer matrix, may tends to enhance the crystallinity of the total system. This might have resulted increase in the thermal stability of the material. However, the maximum and final degradation temperatures for 10 days of composted samples have been decreased significantly as compared with the unexposed PLA. Maximum degradation temperatures have been decreased to 341 in similar fashion final degradation temperatures also deceased to 370 for 10th day composted sample. Subsequently, 20th and 30th day incubated samples have been exhibited a rapid decrease in the thermal stability. This indicates the initiated microbial attack within the crystalline domain of the PLA matrix during this time period. As result of the fragmentation process in the crystalline



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domain, the varied crystalline pattern and the oligomers with different molecular weight tend to follow a multi step degradation process instead of a single step degradation observed before composting. Similarly in case of 40 and 50 days of incubated samples the thermal degradation of V-PLA has been initiated at very low temperatures of 164 and 155°C respectively. The maximum and final degradation temperatures in these samples also were decreased to very lower temperatures as a result of rapid growth of microorganisms consuming the PLA matrix. Unlike V-PLA, the initial increase in thermal stability due to the early consumption of amorphous domain by the microorganisms is absent in case of PLA/PHB/MA blend and PLA/PHB/MA/OMMT blend nanocomposite (figure 4b & c). Since the total crystallinity of the blends and blend nanocomposites are much lesser than that of V-PLA, the reduced amorphous domain by biodegradation in the initial stages might have not altered the total crystallinity considerably. As a result the expected increase in thermal degradation temperature during initial days was not observed in this case of PLA/PHB/MA blend and PLA/PHB/MA/OMMT reinforced blend nanocomposite. The thermal stability of the blend and blend nanocomposite decreases continuously with the increase in the composting time period. As evident from the figures both the samples exhibits similar degradation pattern with multiple step weight loss due to the variation in molecular weight, optical purity and crystallinity. The initial degradation temperature of the blend and blend nanocomposite reduced by 15-20% within 10 days of incubation which is further decreased to 55% after 30 days of composting in comparison with the unexposed samples. Further, it was difficult to separate the fragmented samples from the soil after 30th day of incubation.


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Morphological observations of the composted samples The morphology of the composted samples of after 10 and 15 days has been evaluated employing SEM analysis. The samples of particular time period of incubation have been selected based on the fact that, after the above mentioned period the nanocomposite samples became too weak to be handled and remove from the soil. Also, even the 15th day sample, gets easily charged during the SEM analysis. The SEM of V-PLA and other samples after various incubation times have been depicted as figure 5 and 6. Comparing the 10th and 15th day micrographs with the initial sample morphology all the three samples were showed increased density of microorganisms on the surface. This suggests that PLA matrix offers a favorable media for microbial growth. Also, with the increase in incubation time, the surface imperfections through pits and cracks on the sample by the penetration of microorganisms have been increased considerably. SEM micrographs of initial, 10th and 15th day composted samples of PLA/PHB/MA blend and PLA/PHB/MA/OMMT blend nanocomposite are depicted in figure 5 b&c and 6 b&c respectively. As evident from the micrograph there have been substantial generation microorganisms over the 10th day samples. This may be because of the increased affinity of microorganisms on PLA after blending with PHB as well as incorporation of OMMT. It is also observed that the blend and the blend nanocomposite showed porous morphology after 10th day of incubation. Since the initiation process of biodegradation involves water absorption, large numbers of pores can accelerate the absorption of water. This might be one of the reasons for increased biodegradability in the PLA/PHB/MA blend and its blend nanocomposite as discussed earlier. The increased amount of microorganisms and pores on the 15th day sample further suggests the accelerated affinity of microorganisms with time on both the samples. However, in



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case of the blend nanocomposites, the SEM micrographs indicated higher biodegradation tendency, which further suggested increased affinity of microorganisms in nano filled matrix as elaborated in earlier. pH measurements Study of variation in pH is vital in case of biodegradation study because favorable microbial activity happens only if the physical parameters like temperature, humidity, pH etc are in an optimum range. In accordance with ASTM D 5988-03 pH range of 6-8 provides friendly condition for the growth of microorganisms. During the biodeterioration and assimilation, formation of more acidic end groups can reduce the pH according to the rate of biodegradation. Similarly production of CO2 and presence of moisture also may reduce the pH value, as CO2 can form weak carbonic acid in water medium. This in fact depends on the rate of biodegradation during the evaluating time period. The pH of the soil is determined using a 5:1 (distilled water: soil) slurry after each particular intervals. In the beginning pH has been maintained around 7.5 for all the samples including both the positive and negative controls. As evident from the figure 7, in case of the negative control of PE sample, the pH has been maintained in a marginally basic region of 7.5 to 7.7, till completion of 100 days of study. On other hand, the positive control cellulose has shown a sudden drop in pH to the acidic region of 6.5 after 5 days and maintained the value, till incubation time period of 25 days was achieved which further started to increase slowly to 7.2 at the 100th day of incubation. This behavior indicates the rapid degradation of cellulose within a time period of 25 days. Thereafter as observed from the biodegradation studies, a stable plateau region during 25 to 100 days has been observed here too.


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Similar trend was also observed for V-PLA as well as both filled samples, but with different intensity of variation in pH according to their nature of biodegradation. V-PLA exhibited a slow decrease in pH till 30 days of incubation and thereafter the value rapidly decreased to 5.6 during a period of 40 to 80 days. The rapid decrease in pH in the region of 4080 days indicates that the region involves major degradation, which is predominantly due to hydrolysis of PLA macromolecules. This underlines the suggestions from the composting study. Formation of CO2 and more carboxylic end groups during the degradation process enhances the acidity of the soil, thereby the observed decrease in pH. On the other hand, in case of PLA/PHB/MA blend and its blend nanocomposite pH of the soil dropped at the early stages of biodegradation which is further confirmed the observations of the weight loss study. The pH of the soil showed a sudden drop from 7.5 to 6.1 for both the samples within a short time period of 10 days of incubation. Further, it maintained in a region between 6.3-6.1 till 60th day for both the PLA/PHB/MA blend and PLA/PHB/MA/OMMT blend nanocomposite respectively. This further confirms the enhanced rate of biodegradation of PLA in the blend and blend nanocomposites. Degradation in B. Cepacia Medium The biodegradability of PLA and its nanocomposites also has been investigated using specific bacterial medium of B. cepacia inoculums. SEM of 10 and 15 days of inoculated samples are depicted in figure 7. In comparison with the initial day morphology (figure 5), after 10 days of inoculation the initiation of microbial attack can observable through the small cracks and holes created on the V-PLA surface. Further, 15th day V-PLA sample has been given more depleted morphology with the presence of the bacteria due to the enhanced rate of degradation with time. The increased affinity of B.Cepacia on the PLA/PHB/MA blend and PLA/PHB/MA/OMMT



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blend nanocomposites can see through the denser bacteria colonies on the samples than V-PLA surface. Increased number of pores and cracks on the 15th day sample surface as compared to that of 10th day indicates the increase in biodegradation rate by B.Cepacia with time also. Figure 3b represents the extent of B.Cepacia attack as a function of percentage weight loss with incubation time. It is evident that the weight loss curves follows a general trend of slow initiation in the early days followed by rapid increase in weight loss and a final stable plateau. V-PLA exhibit a weight loss of 8% within an incubation time period 10 days which subsequently increased steadily up to 60 days. The sample has been showed at this stage 91% weight loss. In comparison with the composting analysis, V-PLA has been shown enhanced degradation rate in B.Cepacia medium. This further confirms that inoculums provide a friendly atmosphere for the easy growth of B.Cepacia. As a result the enhanced consumption of polymer macromolecules has been observed in the more specific and denser microbial conditions. This leads to the increased rate of degradation in the bacterial medium than under controlled composted conditions. The weight loss study also suggests that both the blend and blend nanocomposites in B.Cepacia media degraded at a faster rate than V-PLA. Both the samples exhibited an accelerated rate of biodegradation from the initial period of incubation without any initial lag phase. The samples has been degraded by ≈60% weight loss within a time period of 30 days, thereafter the rate of weight loss became slower and complete degradation was attained from the plateau region in around 50-55 days of inoculation. This phenomenon is possibly due to the presence of two different matrices, i.e; PLA and PHB, for which the microorganisms takes additional time period for assimilation.


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Degradation in mixed fungal medium As observed in case of B. Cepacia the mixed fungal inoculums also provided good degradation media for the PLA/PHB/MA blend as well as PLA/PHB/MA/OMMT blend nanocomposites than that for V-PLA. This can be observed from the SEM micrographs of initial & degraded samples after 10 and 15 days of inoculation (figure 5 & 8). It is evident from the figures that the increased affinity of fungal species on PLA samples with time, from the denser fungal strands on the sample surface after 15 days of inoculation than that of 10th day samples. This is because once there are the structured communities of microorganisms interacting to produce schisms in the long hydrocarbon chains of the polymers, the process continues until all the hydrocarbons are eventually transformed into the carbon dioxide and water in aerobic biodegradation. As compared with V-PLA, denser fungal spores on the sample surfaces confirm the enhanced biodegradability of the blend and blend nanocomposite. A better view on this can be observed from the weight loss study depicted in figure 3c. Both the materials showed stable plateau of maximum degradation by weight loss within 25-30 days of time period in the fungal inoculums. The rate is around 50% higher than that of V-PLA’s degradation rate which has taken around 60 days for attaining maximum degradation plateau. Conclusions Completely biodegradable agricultural mulch film from modified PLA has been successfully prepared under industrial blown film conditions. The study was effective to suggest a novel method of processing conditions and a novel biodegradable composition for the anticipated application from renewable raw materials. PHB addition and MA compatibilization tend to enhance the flexibility and impact modification considerably in comparison with the properties of V-PLA. Further, the reduced crystallinity has been retained by the addition of nanoclays like



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OMMT and C30B. The environmental sustainability study using wetherometer followed by mechanical testing suggests the film is applicable for short terms crops which can complete within around 100-150 days. Similarly, the biodegradation study according to ASTM D 5988 has been suggested that enhanced rate of biodegradation for PLA by PHB and nanoclay reinforcement. This has been confirmed by weight loss study which has been suggested more than 45% increase in rate of biodegradation for PLA/PHB/MA blend and PLA/PHB/MA/OMMT blend nanocomposite than that of V-PLA. TGA and SEM analysis of the composted samples supports further the above observations. Specific microbial analysis in B.Cepacia and mixed fungal inoculums suggests newer mediums to degrade PLA in shorter time period than that in a composting medium. V-PLA has been degraded within a time period of 60 days in both the specific inoculums, however, it has been reduced by more than 50% in case of the blend and blend nanocomposite. Acknowledgement The authors gratefully acknowledge the support from Regional Plant Resource Center (RPRC), Bhubaneswar, for providing vermi-compost for biodegradation study and MITS institute of Biotechnology, Bhubaneswar, for allowing us to access their lab facilities for specific microbial studies. Supporting Information Available Mechanical properties of V-PLA, V-PHB, PLA/PHB blends and blend nanocomposites. Effect of accelerated weathering on the mechanical properties and data corresponding to biodegradation of mulch films in comparison with V-PLA. This material is available free of charge via the internet at http://pubs.acs.org."


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References (1) Hongzhi, L.; Wenjia, S.; Feng, C.; Li G., Jinwen, Z. Interaction of Microstructure and Interfacial Adhesion on Impact Performance of Polylactide (PLA) Ternary Blends. Macromol. 2011, 44, 1513. (2) Ohkoshi, I.; Abe, H.; Doi, Y. Miscibility and solid-state structures for blends of poly[(S)lactide] with atactic poly[(R,S)-3-hydroxybutyrate]. Polym. 2000, 41, 5985. (3) Koyama, N.; Doi, Y. Miscibility of binary blends of poly[(R)-3-hydroxybutyric acid] and poly[(S)-lactic acid]. Polym. 1997, 38, 1589. (4) Zhang, L.; Xiong, C.; Deng, X. Miscibility, crystallization and morphology of poly(βhydroxybutyrate)/poly(d,l-lactide) blends. Polym. 1996, 37, 235. (5) Zhang, L.; Xiong, C.; Deng, X. Biodegradable polyester blends for biomedical application. J. Appl. Polym Scie. 2003. 56, 103. (6) Kumar, M.; Mohanty, S.; Nayak, S. K.; Parvaiz, M. R. Effect of glycidyl methacrylate (GMA) on the thermal, mechanical and morphological property of biodegradable PLA/PBAT blend and its nanocomposites. Biores. Tech. 2010, 101, 8406. (7) Gedde, U. L. F. Polymer Physics; Kluwer Acadamic Publishers: The Netherlands, 2001. (8) Mohanty, S.; Nayak, S. K. Biodegradable nanocomposites of poly (butylenes adipate-coterephthalate) (PBAT) with organically modified nanoclays. Inter. J. plast. Techn. 2010, 14, 192. (9) Jandas, P. J.; Mohanty, S.; Nayak, S. K. Surface treated banana fiber reinforced poly (lactic acid) nanocomposites for disposable applications. J. Clean. Prod. 2013, 52, 392. (10) Yew, G. H.; Mohd, Y. A. M.; Mohd, I. Z. A.; Ishiaku, U. S. Natural weathering effects on the mechanical properties of Polylactic Acid/Rice Starch Composites, Proceeding of the 8th Polymers for Advanced Technologies International Symposium, Hungary. 2005, 13.



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(11) Gerlowski, L. Barrier Polymers and Structures. ACS Symposium Series, USA, 1989. (12) Kawai, H.; Ogata, N.; Jimenez, G.; Ogihara, T. Structure and thermal/mechanical properties of poly(l-lactide)-clay blend. J. Polym. Scie. Polym. Physi. 1997, 35, 389. (13) Ray, S. S.; Okamoto, K.; Okamoto, M. Structure-property relationship in biodegradable poly(butylene succinate)/layered silicate nanocomposites. Macromol. 2003, 36, 2355–2367. (14) El-Hadi, A.; Schnabel, R.; Straube, E.; Muller, G.; Henning, S. Correlation between degree of crystallinity, morphology, glass temperature, mechanical properties and biodegradation of poly (3-hydroxyalkanoate) PHAs and their blends. Polym. Test. 2002. 21, 665–674. (15) Mohanty, S.; Nayak, S. K. Starch based biodegradable PBAT nanocomposites: Effect of starch modification on mechanical, thermal, morphological and biodegradability behaviour. Inter. J. plast. Techn. 2009, 13, 163.


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Figure 1: Stress-strain curve of the blends and blend nanocomposites in comparison with virgin materials 254x191mm (150 x 150 DPI)



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Figure 2: PLA/PHB/MA/OMMT mulch film prepared by blown film method under industrial conditions 255x191mm (150 x 150 DPI)


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Figure 3: Weight loss pattern of the samples under (a) composting media, (b) B. Cepacia media and (c) Mixed fungal media 255x191mm (150 x 150 DPI)



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Figure 4: TGA analysis of composted samples (a) V-PLA, (b) PLA/PHB/MA and PLA/PHB/MA/OMMT 279x215mm (150 x 150 DPI)


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Figre 5: SEM of unexposed samples (a) V-PLA, (b) PLA/PHB/MA and (c) PLA/PHB/MA/OMMT 255x191mm (150 x 150 DPI)



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Figure 6: SEM of samples after 10th and 20th day of composting (a) V-PLA, (b) PLA/PHB/MA and (c) PLA/PHB/MA/OMMT 255x191mm (150 x 150 DPI)


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Figure 7: pH analysis of composting media within the composting time period 255x193mm (150 x 150 DPI)



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Figure 8: SEM of samples after 10 and 15 day of inoculation in B.Cepacia media 255x191mm (150 x 150 DPI)


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Figure 9: SEM of samples after 10 and 15 days of inoculation in mixed fugal media 255x191mm (150 x 150 DPI)



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Scheme 1: Reaction and interaction mechanism of MA within the PLA/PHB blend system 210x186mm (150 x 150 DPI)


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For TOC only 255x191mm (150 x 150 DPI)


Sustainability, Compostability, and Specific ... - ACS Publications

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