Characterization of degradation behaviors of PLA biodegradable plastics by infrared spectroscopy

https://doi.org/10.1016/j.saa.2022.121376Get rights and content

Highlights

  • First, A method for calculating the effective area of THz characteristic peak was proposed.

  • Second, A new method, THz spectroscopy, was found to characterize the degradability.

  • Third, The deficiency of MIR spectra in characterizing degradable products.

  • Forth, The spectral differences reflected in the polymerization of lactic acid were found.

Abstract

In this paper, the degradation behavior of two kinds of polylactic acid (PLA) biodegradable material products (pure PLA cup cover and modified PLA straw) was studied. It was found that under the composting environment specified in the International Standard, in the first 35 days, the degradation rate of the straw (with 50%-60% poly butylenes succinate (PBS)) was faster than that of the pure PLA cup cover, but in the later stage, the PLA cup cover exceeded the straw and disintegrated preferentially, and both could be degraded in about 70 days. After further analyzing the far-infrared (FIR, can also be called THz) and mid infrared (MIR) spectra of cup cover and straw, we observed that the material structure had not changed until disintegration, only the ester bond was hydrolyzed, the polymers became oligomers, which could be reflected in the change of the effective area of the characteristic peak at 7.15 THz (cup cover, labeled 1921) and 6.99 THz (straw, labeled 4386) in the THz spectrum. With the degradation, the effective area decreased continuously. Due to the strong absorption of the material in MIR band, most characteristic peaks were flattened and lost analytical value. The bivariate correlation of degradation time, biodegradation rate, total carbon dioxide release and the effective area of the characteristic peak at 7.15 THz (1921) and 6.99 THz (4386) in THz spectrum was analyzed by SPSS software. We discovered that the degradation time was significantly positively correlated with biodegradation rate and carbon dioxide release at the level of 0.01 and negatively correlated with the effective area of characteristic peak at the level of 0.05. The biodegradation rate was significantly negatively correlated with the effective area of characteristic peak at the level of 0.01. Taking the degradation time as the independent variable and the biodegradation rate, carbon dioxide release and effective area of characteristic peak as the dependent variables, we got that the THz spectrum could be used to describe the degradation behavior of PLA products as long as appropriate coefficient correction was made. In this way, we could separate from the laboratory environment, study the impact of environmental diversification on material degradation performance, and reduce the cost of material degradation performance identification. Using density functional theory (DFT), reduced density gradient (RDG) method and visualization software, the changes of weak interaction position and intensity in the molecule during the polymerization of lactic acid into PLA were further analyzed. We found that the vibration of ester bond corresponded to the characteristic peak with weak intensity in the spectrum, and the peak with large intensity mainly originated from the out-of-plane swing of O-H bond in the molecule.

Graphical abstract

The degradation process of the material was characterized by the change of the effective area of some characteristic peaks in THz spectrum, and the change of some characteristic peaks in the process of lactic acid polymerization was studied theoretically.

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Introduction

On May 10, 2019, the Basel Convention on updating the control of hazardous wastes was reached at the meeting of the United Nations Environment Programme (UNEP). It was pointed out that in the future, packaging materials should develop towards more environmentally friendly, recyclable, and degradable. The traditional plastic products industry would face great changes, and many fields such as medical care, aerospace, fashion and food would be greatly affected. Plastic restriction and prohibition had become important measures in promoting sustainable development all over the world [1], [2], [3], [4], [5]. Degradable plastic products were mostly membrane bags or disposable tableware. The main component of membrane bag was the mixture of poly (butylene adipate-co-terephthalate) (PBAT) and polylactic acid (PLA), while the disposable tableware was mainly pure PLA or the mixture of PLA and poly (butylene succinate) (PBS). Therefore, the two plastic products studied in this paper (pure PLA cup cover and straw made of PLA and PBS) were very typical.

Degradable materials were in the early stage of vigorous development, lack of effective detection methods and standards. For example, there was a lack of research on the toxicity caused by modification, and lack of rapid and effective methods for qualitative and quantitative analysis of degradable plastic products. The detection of degradability of degradable plastic products was an important indicator of product sales. The existing detection methods under the International Standard ISO 14855–1 needed to collect and analyze the carbon dioxide emission in real time, which had the disadvantages of long detection time (about half a year), high cost, complex process, low efficiency (only one sample could be tested at a time), and unable to leave the laboratory environment, which limited the development of the industry. It was urgent to improve the detection efficiency.

As a rapid detection method, spectral detection technology was widely used in all walks of life [6]. In traditional MIR spectroscopy, the material needed to be mixed with KBr and diluted to obtain sharp characteristic information. When characterizing the degradation performance of materials, it was necessary to retain the original morphology of materials, and the advantages of MIR spectra were insufficient. Moreover, MIR spectroscopy could only reflect the change of molecular structure, and could not obtain the information of molecular configuration or crystal form change from spectral information [7], [8]. This study found that the compost degradation process was complex, and the change information of characteristic peaks in MIR spectra had little correlation with it. Far infrared (THz) spectroscopy could not only characterize the changes of molecular structure, but also the changes of molecular configuration and crystal form. The sharp characteristic peak information of degradable materials and products could be obtained without any pretreatment [9]. Taking the pure PLA cup cover and the modified PLA straw as the research objects, according to the International Standard, the degradation performance data, including degradation time, biodegradation rate and total carbon dioxide emission, were obtained by composting. Combined with FIR (THz) and MIR spectroscopy, the process of ester bond breaking and polymer changing into oligomer in the process of degradation could be well reflected in the change of spectrum. The significant correlation between the degradation performance data and the effective area of some specific characteristic peaks in THz spectrum was found, and then the linear regression equation was established. Finally, we discovered that THz spectroscopy could be used to efficiently analyze the degradation performance of materials instead of the International Standard real-time carbon dioxide collection method.

Section snippets

Procedure for experiment

Polybutylene succinate (PBS, white granular), degradable plastic cup cover (transparent) and straw (white) used in the experiment were provided by CSIC Pacli (Nanjing) Technology Co., Ltd. In Fig. 1, the samples labeled 1921 were pure polylactic acid (PLA) cup cover during different degradation times, and the samples labeled 4386 were modified PLA straw (about 50–60 % PBS was added). It could be seen from Fig. 1 that the pure PLA cup cover was transparent before degradation; After 10 days of

Calculation of biodegradation rate and carbon dioxide emission

International Standard ISO 14855–1 designated by the International Organization for Standardization, determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions-method by analysis of evolved carbon dioxide. According to the International Standard, the rate of bio-degradation was determined by measuring the amount of carbon dioxide released. After the experiment, the biomass and polymerization residues in the solid bed were measured

Compost degradation performance

According to the requirements of the International Standard, micro-crystalline cellulose was selected as the reference material, and CO2 production of the blank composting container was deducted. Fig. 6 was the decomposition rate of reference material (micro-crystalline cellulose) and test material (PLA-4386 and PLA-1921). Fig. 7 was the cumulative CO2 production of blank, reference material and test material. It could be seen from the Fig. 6 that the degradation rates of cup cover (PLA-1921)

Conclusion

Degradation performance was an important parameter to judge whether the material meets the degradation standard. The currently available standard was the compost degradation method specified by the International Standard ISO 14855–1. This method needed to collect the carbon dioxide emission in the degradation process of samples in real time, so only one sample could be tested at one time. The detection cycle of a single sample was up to 3–6 months, which had seriously failed to keep up with the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This study is funded by the National Natural Science Foundation of China under contracts 31170668 and 32101535, by Jiangsu Postdoctoral Research Foundation (2021K112B), by the Natural Science Foundation of Jiangsu Higher Education Institutions of China (18KJB140014), by 2021 High-end training of professional leaders of teachers in higher vocational colleges in Jiangsu Province under No. 2021GRGDYX009, and by 2021 “Qinglan Project” outstanding young backbone teacher training in Jiangsu Province.

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