Valorization of poly(lactic acid) wastes via mechanical recycling: Improvement of the properties of the recycled polymer

Abstract

Poly(lactic acid) (PLA) is a biobased polymer that represents one of the most interesting alternatives to fossil-fuel based polymers in food packaging applications. Most of the PLA used in food packaging is used only once and then discarded, even though the PLA types used in packaging have good properties and stability. Therefore, it seems reasonable to consider the possibility of recycling the used polymer through a mechanical recycling process. The main aims of this work are to study the effect of the mechanical recycling on the properties of PLA and the usefulness of different upgrading methods to obtain recycled PLA with improved properties. A commercial type of PLA was subjected to accelerated thermal, photochemical and hydrolytic aging and then reprocessed. During reprocessing, aged PLA was blended with virgin PLA and a commercial chain extender was added. Results point out that recycling causes the degradation of PLA, and negatively affects the thermal stability and mechanical properties. However, addition of virgin PLA, and the chain extender, led to an increase of up to 9% in the intrinsic viscosity and 8% in the Vickers hardness of the recycled material. These results suggest that mechanically recycled PLA with improved performance can be obtained, a fact which might improve the recyclability of PLA and thus the environmental impact of this material.

Keywords

Poly(lactic acid)mechanical recycling packaging materials waste waste valorization thermal properties mechanical properties

Introduction

Global production of polymers reached 322 million tons in 2015, of which around 40% were destined to packaging applications (Hahladakis et al., 2018). Nowadays, the plastics manufacturing industry is very dependent on fossil-fuel based polymers, due to their low production costs, convenience, safety and good aesthetical properties. However, the massive use of petroleum-based polymers is related to the high consumption of non-renewable resources and the potential accumulation of non-degradable wastes in the environment (Sinha Ray and Bousmina, 2005). As a response to these problems, during recent years, industry and researchers have devoted a growing attention to biopolymers (Arrieta et al., 2017; Beltrán et al., 2016b).

Bioplastics represent a wide range of materials, which may be biobased and/or biodegradable, and that can be produced from renewable sources, petroleum or a mixture of both (Reddy et al., 2013). As it was previously stated, these materials are gathering a lot of interest during the past years, in fact, a 50% increase in the global production capacity of bioplastics is expected over the next four years (Aeschelmann and Carus, 2017).

Poly(lactic acid) (PLA), is one of the most interesting bioplastics. PLA is an aliphatic polyester produced by ring-opening polymerization of the cyclic dimer of lactic acid, which is obtained from the fermentation of the sugar present in agricultural feedstock. PLA is considered one of the best alternatives to fossil-fuel based polymers in packaging applications, especially for food products, due to its good mechanical and optical properties, along with its safety in food contact and good processability (Auras et al., 2010).

Despite the environmental advantages related to the replacement, in some applications, of fossil-fuel based polymers with biopolymers, the increasing production and use of PLA could generate some problems. On the one hand, there are social and moral concerns about the increasing amounts of cropland needed to produce PLA and other bioplastics. Food prices might increase as a result of the use of cropland in PLA production instead of food production, threatening the sustenance of poorer countries (Mülhaupt, 2013). On the other hand, the management of PLA wastes is not easy. Commercial types of PLA used in packaging applications degrade at a very slow rate (Niaounakis, 2013), which may lead to the accumulation of wastes (Leejarkpai et al., 2016).

These problems have brought to the fore the need to review the different valorization alternatives for PLA wastes coming from packaging applications, namely composting, energy recovery, and chemical and mechanical recycling (Badia et al., 2017; Hahladakis et al., 2018).

Composting could be considered as an interesting alternative for the valorization of contaminated PLA wastes; however, it presents several drawbacks: commercial types of PLA degrade at a very slow rate; many composting facilities do not accept bioplastics; and composting does not reduce the amount of raw materials used (Castro-Aguirre et al., 2016; Cosate de Andrade et al., 2016). Energy recovery consists of the thermal cracking of plastic wastes to produce energy. However, this method, like composting, does not reduce the demand for raw materials used in PLA production (Badia et al., 2017). Chemical recycling consists of the depolymerization of PLA to produce smaller molecules, which could be polymerized to obtain high molecular weight PLA, helping to reduce the consumption of the raw materials. This process still needs to be deeply studied, since nowadays it is more expensive than obtaining the monomers as raw materials (Soroudi and Jakubowicz, 2013).

Lastly, mechanical recycling consists of the recovery, sorting, regrinding, and reprocessing of PLA wastes. According to several authors, it is the valorization method with the lowest environmental impact (Cosate de Andrade et al., 2016; Piemonte, 2011; Rossi et al., 2015). Mechanical recycling also allows for the reduction in consumption of energy and raw materials. These advantages make mechanical recycling one of the most interesting alternatives for the valorization of PLA wastes coming from packaging applications. However, the feasibility of this process for PLA is still under discussion, since several issues need to be resolved first.

Firstly, the current plastics waste recovery infrastructure would probably need to be modified to create a separate recycling stream for PLA. According to Cornell (2007), a critical mass of 200,000 t/y would be needed for this separate stream to be viable. This threshold was surpassed in 2014 (Aeschelmann and Carus, 2017). Secondly, it is necessary to study the effect of the mechanical recycling on the performance of PLA, since if the properties of the recycled polymer are poor, no added value products could be obtained from the process (Hildebrandt et al., 2017; Soroudi and Jakubowicz, 2013). In this regard, several studies have been published over the last few years. Beltrán et al. (2016b) recycled PLA previously subjected to accelerated aging processes, to simulate the degradation of the polymer during service life, and to a demanding washing process to simulate the cleaning process usually employed with polymers used in food packaging. The results point out that the inclusion of the washing step led to a more severe degradation of PLA, along with a decrease of the mechanical and gas barrier properties of the material (Beltrán et al., 2018c).

The previously mentioned studies point out that mechanical recycling has a negative effect on the performance of PLA, limiting the profitability of the recycling process. Several strategies could be followed to improve the properties of mechanically recycled PLA: reactive extrusion; use of reinforcements; and blending with virgin polymers (Badia and Ribes-Greus, 2016).

Most of the above studies were performed using virgin or reprocessed PLA, that is, materials not subjected to the aging and washing processes to which the post-consumer polymer is subjected, so that the effects of the improvement processes on the structure and properties of recycled PLA coming from post-consumption are very scarcely known. Therefore, the main aim of this work is to study different alternatives for the improvement of the properties of PLA subjected to accelerated aging and mechanical recycling processes, including a demanding washing step. A commercial type of PLA was subjected to:

a first melt compounding and compression molding step;

an accelerated aging including photochemical, thermal and hygrothermal degradation;

a washing process similar to those applied to food packaging plastics; and

a second processing step, in which the recycled material was blended with virgin PLA and a chain extender.

The effect of the upgrading alternatives was characterized by intrinsic viscosity measurements, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), microhardness measurements, and ultraviolet–visible (UV-Vis) spectroscopy.

Materials and methods

Materials

A commercial type of PLA, Ingeo^TM 2003D, was supplied by Natureworks. It has a melt mass-flow rate of 6 g/10 minutes (2.16 kg at 210°C) and a specific gravity of 1.24.

A commercial chain extender, labeled as CESA^®-extend BLA0025041-ZN was kindly supplied by Clariant. This chain extender is a masterbatch based on PLA, containing reactive epoxy and dianhydride groups that allow the chain extension reactions.

Recycling process

Figure 1 schematically shows the mechanical recycling process used in this work. The original polymer was melt compounded in a Rondol Microlab twin-screw microcompounder, with L/D = 20 and a screw speed of 60 rpm. The temperature profile, from hopper to die, was 125, 160, 190, 190, and 180°C. Then, the resulting material was transformed into films with a thickness of 200 ± 20 µm in an IQAP-LAP hot-plate press, at 14 MPa and 190°C.

Part of these films was then subjected to an accelerated aging process, which included 40 hours of photochemical aging in an Atlas UVCON chamber with eight F40UVB lamps (with a power of 40 W), 468 hours of thermal degradation at 50°C, and 240 hours of a hydrolytic aging in deionized water at 25°C. The aged samples were then washed at 85°C in a solution of NaOH (1.0 wt.%) and a surfactant, Triton X, (0.3 wt.%) (Chariyachotilert et al., 2012). After the washing process, the material was grounded and then reprocessed, by extrusion and compression molding. Table 1 summarizes the obtained materials, according to the upgrading strategy used.

Table 1.

Materials obtained after the simulation of a mechanical recycling process.

Description	Abbreviated name
Poly(lactic acid) (PLA) subjected to the first processing step	PLA-V
PLA subjected to the simulated mechanical recycling process	PLA-R
Recycled PLA + 50 wt.% virgin PLA	PLA-RV
Recycled PLA + 50 wt.% virgin PLA + 1.5% chain extender	PLA-RVC
Aged and washed PLA, prior to the reprocessing step	PLA-AW

It is worth noting that prior to processing, all PLA was crystallized at 100°C for 20 minutes. Then, all the materials, including the chain extender, were dried in a vacuum oven at 85°C for two hours.

Characterization of the recycled materials

The films of the different samples were then characterized using the experimental techniques detailed below.

Intrinsic viscosity measurements were performed at 25.0 ± 0.5°C using an Ubbelohde viscosimeter and chloroform as a solvent. Intrinsic viscosity was obtained using the Kraemer equation (Kraemer, 1938), measuring four concentrations for each sample.

The DSC scans were performed on a TA Instruments Q20 calorimeter. Samples of 5 mg were placed in standard aluminum pans in nitrogen atmosphere. The thermal method consisted in a first heating scan, at 5°C/minute, from 30 to 180°C, an isothermal step for three minutes, a cooling scan, at 5°C/minute, until 0°C, an isothermal step for one minute, and a second heating scan, at 5°C/minute, until 180°C.

Thermogravimetric analysis was conducted in a TA Instruments TGA 2050 thermobalance. Samples of 12 mg were heated from room temperature to 800°C, at 10°C/minute, in nitrogen atmosphere.

The UV-Vis spectra of the samples were recorded at 200 nm/minute in a Shimadzu 2401 PV UV-Vis spectrophotometer, equipped with a Shimadzu integrating sphere. The overall transmittance in the visible light region was measured according to the ISO 13468 standard. Three measurements were taken for each sample.

Microhardness measurements of all the samples were performed using a Type M Shimadzu microhardness tester equipped with a Vickers pyramidal indenter. The applied load was 25 g, for 10 seconds. Each measurement was repeated six times.

Results and discussion

Effect of the upgrading methods on the intrinsic viscosity

Poly(lactic acid) is susceptible to degradation during the service life and the different stages of the mechanical recycling process. This degradation reduces the average molecular weight of the polymer, which leads to a worsening of the performance of the recycled material and a reduction of the intrinsic viscosity, a crucial parameter in the processing of the polymer. Therefore, it is important to evaluate the effect of the recycling and of the upgrading alternatives on the intrinsic viscosity of PLA.

Table 2 shows the intrinsic viscosity values of all the materials. Firstly, it can be seen that mechanical recycling caused a 17% decrease of the intrinsic viscosity of PLA. Similar results have been reported in previous studies conducted by our group (Beltrán et al., 2018a, 2018c), and could be explained by the chain scission reactions that take place because of the high temperatures and shear stresses to which PLA was subjected during the melt processing steps.

Table 2.

Intrinsic viscosity, light transmission, hardness, and temperature at which 10% of the mass is lost (T₁₀) values of the different samples.

Sample	Intrinsic viscosity (mL/g)	Light transmission (%)	Vickers hardness (MPa)	T₁₀ (°C)	Maximum decomposition temperature (°C)
PLA-V	132 ± 2	90.1 ± 0.3	174 ± 5	344.9	376.4
PLA-R	109 ± 4	90.5 ± 0.1	169 ± 7	337.9	372.3
PLA-RV	115 ± 8	91.4 ± 0.3	172 ± 5	338.2	375.7
PLA-RVC	119 ± 4	90.8 ± 0.3	182 ± 4	343.2	376.3

Note: for definition of sample abbreviations, see Table 1.

It is also worth noting that the accelerated aging and the washing process affected the molecular weight of PLA. Intrinsic viscosity measurements were performed on PLA after the washing process, obtaining a value of 129 ± 2 mL/g. This small decrease of the intrinsic viscosity suggests that PLA might degrade during the accelerated aging and washing processes. However, most of the degradation takes place during the reprocessing step. Similar results have been reported in a previous work (Beltrán et al., 2018b), and are attributed to the catalyzing effect of the carboxyl end groups, generated during the aging and washing steps, on the thermomechanical degradation of PLA.

Secondly, regarding the effects of the studied upgrading alternatives, Table 2 shows that both upgraded materials present a slightly higher intrinsic viscosity. The increase of the intrinsic viscosity when blending virgin and recycled PLA is not surprising, since the molecular weight of virgin PLA is higher than that of PLA-R. However, it is worth noting that the addition of 1.5% of chain extender led to a further increase (around 3%) of the intrinsic viscosity in comparison with the sample without chain extender. This behavior might be explained by the reactions between the functional groups of the chain extender and the -OH and -COOH groups present in degraded PLA, leading to chemical bonding of several PLA chains and to an increase of the intrinsic viscosity (Tuna and Ozkoc, 2017). The increase of 3% of the intrinsic viscosity allowed for almost reaching the viscosity values of PLAV. Although the increase is small, it is relevant because it opens the possibility of recovering the viscosity on more degraded wastes by adding small amounts of additives.

Structural changes in upgraded PLA

Beside the molecular weight, the structure of PLA could also play an important role in the gas barrier, optical, and mechanical properties of the resulting material. Therefore, the effect of the recycling and the different upgrading methods on the structure of PLA was studied by means of DSC. The scans are shown in Figure 2, while Table 3 summarizes the most important thermal transitions: cold crystallization (T_CC); melting (T_M); the cold crystallization and melting enthalpies calculated from the first heating scan (ΔH_CC and ΔH_M, respectively); and the crystallinity degree (X_C).

Table 3.

Differential scanning calorimetry results of all the samples, cold crystallization (T_CC), melting (T_M) correspond to the second heating scans, while cold crystallization and melting enthalpies calculated from the first heating scan (ΔH_CC and ΔH_M, respectively), and crystallinity degree (X_c) are calculated from the first heating scans.

Sample	(°C)	T_M (°C)	ΔH_CC (J/g)	ΔH_M (J/g)	X_c (%)
PLA-V	109.3	149.3–156.4	28.6	30.7	2
PLA-R	102.8	147.1–156.1	27.2	27.8	1
PLA-RV	103.7	147.2–155.3	28.5	29.9	1
PLA-RVC	102.6	147.5–155.6	26.4	27.5	1
PLA-AW	107.0	148.7–156.3	0.0	29.0	31

Note: for definition of sample abbreviations, see Table 1.

Figure 2 shows that the second heating scans of all the samples are very similar, presenting a glass transition around 60°C, a cold crystallization over 100°C, and melting endotherm over 140°C. All the samples show a double melting peak, which has been previously reported for PLA, and that has been attributed to a melt recrystallization mechanism. This process involves the melting at lower temperatures of the less perfect crystals, their rearrangement into more perfect structures during the heating, and their melting at higher temperatures (Di Lorenzo, 2006).

Regarding the effect of the mechanical recycling and upgrading methods on the thermal transitions, the only noticeable difference can be found in the cold crystallization temperature (T_CC) of the samples. Recycled materials show lower values of T_CC, which could be explained by the degradation of the polymer. The shorter polymer chains have increased mobility, which allow them to rearrange into crystalline structures at lower temperatures (Beltrán et al., 2018c). It could also be seen that the aged and washed sample presented a lower T_CC value than PLAV, but higher than those of the recycled materials. This result is in good agreement with those of intrinsic viscosity, which showed that most of the degradation took place during the reprocessing step.

The low T_CC value of PLA-RVC is quite surprising, since intrinsic viscosity values showed slight increases in the molecular weight due to the addition of the chain extender. These results might be explained by the branching of PLA chains (due to the chain extension reactions), which promote the formation of less perfect crystals, thus decreasing the cold crystallization temperature of PLA, as it was described by Yang et al. (2008a) and Zhang et al. (2014) in studies with crosslinked PLA samples.

Table 3 shows that the values of the ΔH_CC and ΔH_M are very close in the virgin and recycled samples, indicating that virgin and recycled materials are amorphous. This result is very important from the point of view of packaging applications, since crystallinity could cause the embrittlement of the material and also negatively affect the optical properties. It can also be seen that aged and washed PLA presents a crystallinity degree of 31%. This behavior is due to the effect of the relatively high temperatures used during the washing process. Water has a plasticizing effect on PLA (Beltrán et al., 2016a), which combined with the high temperatures led to an increase of the mobility of the molecular segments causing the crystallization of the polymer. The presence of these crystalline structures could affect the properties of the material. However, these crystals are removed during the reprocessing of the polymer, therefore, they do not pose any problem to recycled material performance.

Optical properties of upgraded PLA

Optical properties play a key role in the field of packaging applications, especially in the packaging of food. Therefore, it is important to determine if the mechanical recycling or the upgrading methods affect the optical properties of PLA. In this work, UV-Vis spectra of all the samples were recorded and the obtained results are presented in Figure 3 and Table 2.

Figure 3 shows that PLA-R and PLA-RV exhibit a small absorption band centered at 275 nm. The presence of this absorption band in mechanically recycled PLA-based materials has been reported in previous studies (Beltrán et al., 2018a, 2018c), and it has been attributed to the presence of carboxyl end groups, due to the degradation of PLA during the different recycling processes. However, it could also be seen from Figure 3 that the sample with 1.5% of chain extender presents an important absorption around 305 nm. This absorption could be related to the dianhydride groups of the chain extender (concretely pyromellitic dianhydride). Some studies point out that the aromatic moieties present in pyromellitic dianhydride present an absorption band centered between 295 and 320 nm (Oliveira et al., 2013). This result suggests that the addition of the chain extender might provide some protection against UV radiation, which is very interesting from the point of view of food packaging applications.

Secondly, Figure 3 and Table 2 show that virgin and recycled material have a very good light transmission in the visible region (400–800 nm), which could be attributed to the absence of crystalline structures (as it was seen via DSC) that may act as light scattering and reflecting centers. These results suggest that the recycling and upgrading processes do not affect the optical clarity of PLA. This behavior could be interesting for food packaging applications, in which a high optical clarity is usually demanded.

Figure 4 shows a photograph of the different samples. It can be seen that PLA-V, PLA-R, PLA-RV, and PLA-RVC have a very good optical clarity (as it was seen via UV-Vis spectroscopy), but also show very low haze levels, which is important from a packaging applications point of view. Regarding the behavior of PLA-AW, it can be seen that it is an opaque material. This is due to the presence of crystalline structures and microvoids which act as reflecting and scattering centers, reducing the light transmission. However, this does not represent a problem, since those crystalline structures and defects are eliminated during the reprocessing of the polymer.

Microhardness measurements of upgraded PLA

Mechanical properties are also important in the field of food packaging applications. In this work, mechanical properties were studied by means of microhardness measurements, whose reports are included in Table 2. It can be seen that mechanical recycling caused a slight decrease of the hardness of PLA, which is a consequence of the degradation of PLA during mechanical recycling.

Regarding the effect of the upgrading options, Table 2 shows that both alternatives caused a small increase of the hardness of PLA, especially the addition of the chain extender. This behavior could be related to the increase of the intrinsic viscosity as a result of the incorporation of the virgin polymer. Furthermore, the incorporation of the additive led to a hardness value 6% higher than the sample without the chain extender. This result might be explained by the crosslinking and chain extension reactions promoted by the chain extender. Some authors point out that branching and slight crosslinking could lead to increased mechanical properties (Yang et al., 2008b). This increase of the hardness, although small, is relevant because it points out that it is possible to obtain recycled PLA with mechanical properties similar, or even better, than those of the virgin material in a cost-effective and environmentally acceptable way.

Thermal stability of upgraded PLA

The TGA tests were conducted on all the samples in order to determine the effect of the recycling and upgrading processes on the thermal stability of PLA. One of the parameters most commonly used to evaluate the thermal stability is the T₁₀ value, which is the temperature at which 10% of the mass is lost. T₁₀ values of the different materials are reported in Table 2. Maximum decomposition temperature values (T_max) are also reported

On the one hand, it can be seen that mechanical recycling caused a small decrease of the thermal stability of PLA, because of the degradation of the polymer during the different stages of the recycling process. On the other hand, Table 2 shows that both upgraded samples, PLA-RV and PLA-RVC, present a higher thermal stability than PLA-R, which is in good agreement with the results of intrinsic viscosity measurements, where an increase of the average molecular weight was observed. Furthermore, it is worth noting that T₁₀ values of PLA-V and PLA-RVC are very close, indicating that the addition of the chain extender greatly improves the thermal stability of the recycled material.

Regarding the behavior of T_max, it can be seen from Table 2 that it follows a similar trend to that presented by T₁₀. The shorter polymer chains present in PLAR decompose slightly faster than those present in PLAV. However, the both upgraded materials present T_max values closer to that of PLAV, as a result of the increase of the molecular weight. It is also worth noting, that after the test, all the samples showed a residue below 0.20%, indicating that neither the recycling nor the upgrading processes caused change in the amount of decomposition residue of the samples.

The obtained results show that it is possible to improve the properties of recycled PLA using small amounts of additives, which could improve the recyclability of PLA; however, there are several challenges that need to be solved for mechanical recycling of PLA to become a reality. Firstly, the share of this polymer in the plastic waste needs to increase in order to make the process attractive for the recyclers. Secondly, it is necessary to develop better recovery and sorting facilities, which allow for efficiently separating PLA from other plastics (in this regard, near infrared sorting is an interesting technology) and other materials. Finally, the use of recycled PLA in food packaging materials brings additional issues such as the presence of coloring and odors, but above all, recycled PLA must be approved by the authorities as a safe food contact material. Preliminary studies show that there are not important changes in the hydrolytic degradation of recycled and virgin PLA (Beltrán et al., 2016b); however, it is necessary to analyze the migration of those degradation products into food products.

Conclusions

The effects of different upgrading options on the molecular weight, structure, optical, thermal, and mechanical properties of a mechanically recycled PLA were studied. The PLA films were previously subjected to an accelerated aging process and a demanding washing step, in order to simulate the degradation in the use of a post-consumer PLA. Intrinsic viscosity results indicate that mechanical recycling causes the degradation of PLA. This degradation was also observed by means of UV-Vis, in which an absorption band corresponding to PLA degradation products was observed, although the optical clarity was unaffected. The decrease of the molecular weight also caused a slight decrease in the thermal stability, Vickers hardness, and cold crystallization temperature of PLA. It is also important to point out that the amorphous nature of PLA remained unchanged after the mechanical recycling.

Regarding the effect of upgrading methods, the blending of recycled PLA with virgin PLA and the addition of 1.5% of a commercial chain extender led to an increase of the intrinsic viscosity values, thermal stability, and Vickers hardness values of PLA. Summarizing, it is possible to improve the properties of recycled PLA through its blending with virgin polymer, and with the addition of small amounts of a chain extender. These results might help to improve the recyclability of PLA, since recycled material with better properties could be obtained.

Despite the possibility of obtaining recycled PLA with improved properties, it is important to note that several issues need to be addressed to implement mechanical recycling of PLA, among which are the need for a higher share of PLA wastes, developing efficient recovery, and sorting systems and assessment of the safety of the recycled material.

Footnotes

Acknowledgements

A preliminary version of these results was presented at the ATHENS 2017 5th International Conference on Sustainable Solid Waste Management, 21 June 2017–24 June, 2017.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors acknowledge the financial support of the Ministry of Economy and Competitiveness – Spain (project CTM2017-88989-P) and Universidad Politécnica de Madrid (project UPM RP 160543006).

ORCID iD

F.R. Beltrán

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