Biodegradation Assessment of Poly (Lactic Acid) Filled with Functionalized Titania Nanoparticles (PLA/TiO2) under Compost Conditions
This paper presents a biodegradation study conducted for 90 days under standardized controlled composting conditions of poly (lactic acid) (PLA) filled with functionalized anatase-titania nanofiller (PLA/TiO2 nanocomposites). The surface morphology, thermal properties, percentage of biodegradation, and molecular weight changes at different incubation times were evaluated via visual inspection, scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and gel permeation chromatography (GPC) by taking degraded samples from compost at the end of target biodegradation time interval. The rapid increase of crystallinity indicated that the PLA and PLA/TiO2 nanocomposites had heterogeneous degradation mechanisms under controlled composting conditions. The biodegradation rate of PLA/TiO2 nanocomposites was higher than that of pure PLA because water molecules easily penetrated the nanocomposites. The dispersion of the nanoparticles in the PLA/TiO2 nanocomposites affected the biodegradation rate of PLA. Moreover, the biodegradation of PLA could be controlled by adding an amount of dispersed TiO2 nanofillers under controlled composting conditions.
KeywordsBiodegradation PLA Functionalized TiO2 Compost
Differential scanning calorimetry
Percent of biodegradation
Gel permeation chromatography
Number-average molecular weight
Weight-average molecular weight
Poly (lactic acid)
Scanning electron microscopy
Cold crystallization peak
Glass transition temperature
Poly (lactic acid) (PLA), a synthetic biodegradable polymer, is investigated worldwide for biomedical and consumer applications because of the increasing need for renewable materials that are sustainable alternatives to petrochemical-derived products [1, 2, 3, 4]. PLA is the product that results from the polymerization of lactide or lactic acid, which is the most extensively produced carboxylic acid in nature by microbial fermentation of carbohydrates . However, the applicability of PLA has been relatively limited because its heat distortion temperature, toughness, and degradation rate are unsatisfactory [6, 7]. One of the methods to resolve these drawbacks is to modify PLA by adding inorganic nanoparticles, including typical nanoclay, carbon nanotubes, zinc oxide, and anatase (A-TiO2) [8, 9, 10, 11, 12, 13, 14, 15]. Recently, the PLA/TiO2 nanocomposites were prepared by us via melting blending PLA with chemically modified TiO2 (solution lactic acid grafted TiO2, hereafter referred to as g-TiO2) . Results showed that TiO2 nanoparticles had a significant effect on the improvement of the mechanical properties of the PLA/TiO2 blends, such as strain at break and elasticity, compared with pure PLA. At the same time, g-TiO2 nanoparticles had a strong influence on hydrolytic degradation and photodegradation of PLA [17, 18].
The study of biodegradability and biodegradation mechanism of biodegradable materials using laboratory-scale test is an extremely important method from industrial and scientific point of view which provides understanding of the service life of these materials . There are several methods currently available to assess the biodegradability of biodegradable materials, which are in general based on an indirect measurement, such as carbon dioxide production, biogas generation, or oxygen consumption [19, 20].
Biodegradation characteristics of PLA in compost have been studied and reported [21, 22, 23]. Composting is an accelerated biodegradation of organic materials in a warm, moist, and aerobic environment under a combination of microbial population and controlled composting conditions [24, 25]. Moreover, the biodegradation of PLA in composting conditions, a temperature- and humidity-dependent process, involves several processes, namely, water uptake, ester cleavage, and formation and dissolution of oligomer fragments . The most accepted mechanism of the PLA biodegradation involves a two-step degradation process. Initially, the heat and moisture in the compost attack the PLA chains and split them apart, thereby producing small Mw polymers and, eventually, lactic acid. Thereafter, the microorganisms in the compost and soil mineralize the oligomer fragments and lactic acid to generate methane and carbon dioxide (CO2) under anaerobic and aerobic conditions, respectively [27, 28, 29].
Recently, the effect of fillers on the biodegradation of PLA has attracted great attention and particular attention has been focused on nanofillers, such as nanoclays, carbon nanotubes, and hydroxyapatite [23, 30, 31, 32, 33, 34, 35, 36, 37, 38]. Some authors [32, 33, 34] found out that adding nanoparticles could accelerate biodegradation of PLA, which was attributed to the high relative hydrophilicity of the nanoparticles, thereby enabling the easy permeability of water into the polymer matrix and triggering hydrolytic degradation. However, other studies [35, 36, 37, 38] reported that biodegradation was retarded because of the enhanced barrier properties of the nanocomposites.
Although there have been some literatures focusing on the biodegradation of PLA materials, the role that TiO2 plays in PLA degradation remains controversial. How did the TiO2 nanoparticles affect the biodegradation of PLA was not clear. So, a study of the biodegradation of PLA, modified by TiO2 nanofillers under compost condition, is still needed. The current study, based on the estimation of the evolving CO2, assessed the biodegradation of PLA/TiO2 nanocomposites extensively under controlled laboratory compost conditions, a complement of degradability of the PLA/TiO2 nanocomposites under different degradation conditions, could extend PLA’s use in various end-use applications in the future.
PLA (manufactured by Natureworks@ (4032D)) exhibited a weight-average molecular weight (Mw) of 19,600 kDa and polydispersity of 1.89 as determined through gel permeation chromatography (GPC). PLA dried at 65 °C for 24 h under reduced pressure and stored in vacuum with humidity absorber before use. Lactic acid (88%, Guangshui National Chemical Co.) was distilled at 80 °C to remove water before use. The anatase titania nanoparticles, with an average primary particle size of ca. 20 nm, were supplied by Pangang Co., Ltd. Toluene and chloroform were used as received. Chromatographic grade microcrystalline cellulose was supplied by Shanghai Chemical Reagent Co., Ltd. The composting inoculums, which were obtained from an organic fraction of municipal solid waste (MSW), were supplied by the Degradable Plastics Professional Committee of the China Plastics Processing Industry Association (CPPIA).
Detailed information on the functionalization of the TiO2 nanoparticles and preparation of the PLA/TiO2 nanocomposites has been reported . G-TiO2 nanofillers were prepared by grafting lactic acid oligomer onto anatase surface. PLA/TiO2 nanocomposites were prepared by melt blending via a corotating twin-screw extruder. Pure PLA was subjected to same mixing treatment so as to have the same thermal history as nanocomposites. The samples with 0, 0.5, 1.0, 2.0, 5.0, 8.0, and 15.0 wt% g-TiO2 were prepared and labeled as PLA, PLA/TiO2–0.5, PLA/TiO2–1, PLA/TiO2–2, PLA/TiO2–5, PLA/TiO2–8, and PLA/TiO2–15 nanocomposites.
Small chip specimens of PLA and g-TiO2 at different ratios were converted into sheets of approximately 0.5 mm in thickness by pressing at 190 °C for 4 min under 10 MPa followed by cooling at room temperature for 5 min under 5 MPa. Thereafter, the compression molded samples were cut into 5 mm × 5 mm size and weighed.
Physicochemical properties of inoculums
GB/T 19277–2003 ISO 14855-2005
Total dry solid (TS) %
Volatile solids (VS, % on TS)
Residual ash content (RAC, % on TS)
The CO2 that evolved during the biodegradation process was trapped in NaOH solutions and measured at regular intervals using titration method. The NaOH was titrated with standard HCl solution to the phenolphthalein endpoint. The total CO2 evolved during biodegradation was calculated with reference to the control flask. The data reported for each sample was the mean value obtained from three samples.
Scanning electron microscopy (SEM) images were obtained using a Philips FEI INSPECT F instrument operated at 5 kV. All specimens were sputter coated with gold prior to analysis.
Thermal properties of samples were studied by differential scanning calorimetry (DSC) (TA Q20, TA Instruments). Thermograms were obtained under nitrogen flow (50 mL/min) at a heating and cooling rates of 10 °C/min in the temperature range from room temperature to 200 °C and from 200 to − 50 °C, respectively.
X-ray diffraction (XRD) analyses were performed using a DX-1000 X-ray diffractometer (Dandong Fanyuan Instrument Co. LTD. China) equipped with a Cu Kα (λ = 0.154 nm) source. The generator was operated at 25 mA and 40 kV. Samples were scanned at different angles (i.e., from 2 to 70°) at a scanning rate of 6°/min.
Determination of the Percent of Biodegradation (D t, %)
Molecular Weight Measurement
The molecular weights of the PLA nanocomposites before and after composting were determined through GPC. The GPC system was equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and Waters 717 plus autosampler. Chloroform was used as eluent at 0.8 mL/min flow rate at 30 °C. Calibration was accomplished with polystyrene standards.
Results and Discussion
Under composting conditions, the factors that affect the biodegradation tendency of PLA would control the degradation of the PLA/TiO2 nanocomposites. When an amount of g-TiO2 was homogeneously dispersed in the PLA matrix, the water molecules penetrated easily within the samples to trigger the degradation process . Consequently, Mn decreased substantially in the first phase. The evolution of the lag phase of CO2 for PLA and its nanocomposites during this period indicated that microorganisms need suitable polymer chains to mineralize. With increased incubation time, the polymer chains in amorphous regions degraded and the number of amorphous regions decreased; thus, the percentage of crystalline to the amorphous region (i.e., χc) increased , thereby leading to the decrease of k in the second phase. However, the oligomer fragments began to be mineralized by microorganisms in this stage, thereby indicating that the productive phase for the PLA mineralization occurred. With the decrease of the remaining oligomer fragments and the increase of χc, k and Dt decreased and a nearly long plateau phase was observed for k and Dt in the third stage. In our previous study , the morphology of each nanocomposite was reported and determined through SEM and TEM; the results showed that the dispersion of g-TiO2 with under 5 wt% in the PLA/TiO2 nanocomposites was better than that obtained with a high concentration of nanofillers. In terms of the dispersion and content of TiO2, PLA/TiO2–5 had the largest k and Dt compared with the other nanocomposites in our experiment.
PLA/TiO2 nanocomposites were prepared (based on PLA and functionalized g-TiO2) and subjected to biodegradation under controlled composting conditions. Using such a standard, the information of patterns on the surface of the samples and the rapid increase of crystallinity indicated that the PLA and PLA/TiO2 nanocomposites had heterogeneous biodegradation mechanisms. The degradation study of nanocomposites under composting conditions showed that the inherent degradable character of PLA remained after the incorporation of functionalized titania nanoparticles (PLA/TiO2). The addition of the TiO2 nanoparticles increased the degradation rate of the PLA matrix because the water molecules easily penetrated the PLA/TiO2 nanocomposites, thereby activating the degradation process. This phenomenon was particularly evident for PLA/TiO2–5 because of its high TiO2 content and good dispersion of TiO2 nanofillers in the PLA matrix compared with other nanocomposites.
This work was supported by the Key projects of social science planning in Sichuan Province SC18A013 and Sichuan University Research Cluster for Regional History and Frontier Studies.
The authors gratefully acknowledge the financial support from High Level Research Team Building Plan of Social Sciences in Sichuan Province (Sichuan Federation of Social Science Association  43–2) and Advanced Interdisciplinary Innovation Research Project of Sichuan University (skqy201216).
Availability of data and materials
All the data are fully available without restriction.
LYB contributed to the experimental work, data analysis, and was a major contributor in writing the manuscript. LZC contributed to the experimental work and data analysis. GG analyzed and interpreted the data. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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