The Life Cycle Assessment for Polylactic Acid (PLA) to Make It a Low-Carbon Material

The massive plastic production worldwide leads to a global concern for the pollution made by the plastic wastes and the environmental issues associated with them. One of the best solutions is replacing the fossil-based plastics with bioplastics. Bioplastics such as polylactic acid (PLA) are biodegradable materials with less greenhouse gas (GHG) emissions. PLA is a biopolymer produced from natural resources with good mechanical and chemical properties, therefore, it is used widely in packaging, agriculture, and biomedical industries. PLA products mostly end up in landfills or composting. In this review paper, the existing life cycle assessments (LCA) for PLA were comprehensively reviewed and classified. According to the LCAs, the energy and materials used in the whole life cycle of PLA were reported. Finally, the GHG emissions of PLA in each stage of its life cycle, including feedstock acquisition and conversion, manufacturing of PLA products, the PLA applications, and the end of life (EoL) options, were described. The most energy-intensive stage in the life cycle of PLA is its conversion. By optimizing the conversion process of PLA, it is possible to make it a low-carbon material with less dependence on energy sources.


Introduction
Nowadays, plastics are employed widely in different industries, such as construction, packaging, electronics, clothing, healthcare, and so on, due to their excellent physical, chemical, and mechanical properties, and economic viabilities compared to traditional materials [1][2][3][4]. The global plastics manufacturing started from 1.5 million tons in 1950, and reached 322 million tons in 2017, and is predicted to increase to 1.63 billion tons in 2050 [5,6]. This huge amount of plastics production worldwide has made its disposal a considerable global concern, with a great potential to harm the environment, humans, and animals. Plastics are found in seawater, jungles, or municipal solid wastes. More than 8.3 billion tons of plastics were produced in the span of 1950 to 2015, in which less than 20% were recycled or incinerated, and the rest were left in the environment or were landfilled [7,8]. The environmental issues and ecological impact associated with plastics have led to more studies and research into developing more sustainable materials. Currently, new factors such as recyclability and biodegradability are taken into account when developing new

The Life Cycle of PLA
The lifecycle of PLA is discussed through its waste management scenarios to determine the main drivers influencing its environmental aspects. Figure 1 depicts the system boundary of PLA production direction regarding energy, materials, and emissions flow. The pathway is divided into five stages, including (1) feedstock collection and its conversion, (2) processing, (3) use, and (4) EoL.

PLA Feedstock Collection and Its Conversion
The first step of the lifecycle of PLA is feedstock collection and conversion, according to Figure 1. PLA manufacturing consists of three main steps. Firstly, the bio-based sources such as corn or sugarcane should be collected and transported to a plant. Then, the feedstock is converted to lactic acid through the fermentation process of starch or sugar. This is the most common manufacturing method of lactic acid as it is chemical-and cost-efficient and leads to the fabrication of pure lactic acid. One of the main factors affecting the crystallinity and biological degradation of PLA is the optical purity of the lactic acid, and hence even a low concentration of impurities matters. Therefore, enough attention should be paid to the downstream processing of lactic acid, as the fermentation broth contains considerable amounts of impurities. The identification and separation of the impurities is a crucial step that determines the final properties of PLA. Finally, the as-synthesized lactide, the dimer of lactic acid, is polymerized to produce PLA via ring-opening polymerization. Due to the chiral characteristic of lactic acid, there are three forms of lactide, including L-lactide, D-lactide, and D, L-lactide. The production of optically pure high molecular weight PLA significantly depends on the line stream monitoring in the whole process.
The amount of different materials and energy used along the pathway of producing one kilogram of PLA from corn is summarized in Figure 2 [2]. According to Figure 2, natural gas and electricity take up most of the total energy used in the process, with 65% and 22%, respectively. It should be noted that there are some other materials or energy parameters that were not included in Figure 2 due to their negligible role.

PLA Processing
The second step toward the life cycle of PLA is the manufacturing of PLA products. It is worth mentioning the physicochemical and mechanical properties of PLA in this section to help with a better understanding of its applicable manufacturing methods. PLA is one of the biodegradable thermoplastics, with similar properties to polystyrene (PS) and polyethylene terephthalate (PET) [31,32]. Table 1 summarizes the physical and mechanical properties of PLA. As can be seen in Table 1, there is a range for each property due to different types of PLA isomers, different used natural sources, and different production procedures [33].
Based on the thermal history and stereochemistry of PLA, in the solid-state, there are both types, semi-crystalline and amorphous [34]. Regarding semi-crystalline PLAs, both Tm and T g are important values to predict its behavior for different applications [35]. On the other hand, transmission from a glassy structure to rubbery can occur above T g for transparent amorphous PLAs [36]. Below T g , the glassy structure of PLA with creep behavior will be formed until cooling to its β-transition at 60 • C. In addition, there are numerous solutions for PLA products, such as dioxane, acetonitrile, chloroform, methylene chloride, 1,1,2-trichloroethane, and dichloroacetic acid [37][38][39]. At boiling temperature, PLAs are soluble in ethylbenzene, toluene, acetone, and tetrahydrofuran. It should be noted that PLA is insoluble in water. Polymers based on lactic acid have a wide range of mechanical properties, varied from elastic plastics to high-strength polymers, depending on the semi-crystalline and amorphous structure and the degree of crystallinity.
Semi-crystalline PLAs indicate higher mechanical properties compared to amorphous PLAs. The mechanical properties of PLA are strongly affected by molecular weight (Mw), as by increasing the Mw from 50 to 100 kDa, the tensile strength and modulus of PLA were doubled [40]. Furthermore, controlling the stereochemical architecture of polymers based on lactic acid by polymerization with L-lactide, D-lactide, D, L-lactide, and meso-lactide leads to control the rate and speed of crystallinity, which have a significant impact on the quality of mechanical properties [41][42][43]. Considering the above-mentioned properties of PLA, there is a wide range of applicable manufacturing methods for PLA products in the forms of fibers, films, parts, and so on. PLA products can be fabricated on a large scale using blow molding, blending, compounding, electrospinning, injection molding, casting, thermoforming, foaming, extrusion, and additive manufacturing [33]. Among all the applicable manufacturing methods for PLA, extrusion and injection molding are the most used [52]. There is energy balance information for both manufacturing methods for manufacturing one kilogram of plastic, including PLA, according to Keoleian et al.'s study [53]. The extrusion-made PLA products use 2 MJ/kg electricity with 1.01 mass input factor (MIF), whereas injection molded ones use 7.2 MJ/kg with 0.95 MIF.

Photodegradation of the Samples
The third step in the life cycle of PLA is its use. Therefore, the applications of PLA are discussed here, as shown in Figure 3. PLA was firstly used for only medical purposes as it was rare and expensive. At present, since the availability of high molecular weight PLA, its products can be fabricated via all the aforementioned manufacturing methods, especially extrusion and injection molding. Owing to the comparable properties of PLA with PS and PET, PLA covers a wide range of applications [54]. The major use of PLA is in the packaging industry [56,57]. PLA use is rapidly growing as "green" food packaging, which is widely considered in the fresh products field and has become the best option for fruit, vegetables, and salad containers in retail markets [58][59][60]. On the other hand, delicatessen and fast-food restaurants use disposable cutlery, drinking and salad cups, plates, and containers which are manufactured by biodegradable polymers for serving foods [24]. These types of productions are in contact with various acidic and high-cholesterol foods with different storage temperatures, varying between below 25 and above 60 • C. Thus, the mechanical, physical, and optical properties of PLA must be tailored according to the packaging applications.
PLA has been widely used in various biomedical applications such as stents, plates, and screws for craniomaxillofacial bone fixation, interference screws in the ankle, spinal cages, soft-tissue implants, tissue engineering scaffolds, tissue cultures, and drug delivery devices, due to its biocompatibility [61,62]. PLA is considerably utilized in vascular stent applications due to its bioabsorbable property and favorable degradation behavior, and has the potential to be an appropriate replacement for metallic stents. Among PLA isomers, poly (L-lactic acid) (PLLA) is the most common biopolymer used for stent applications. PLA is a potential candidate for drug delivery systems due to its wide drug-releasing options. The PLA drug release occurs in several steps, including the breakage of ester bonds through hydrolytic cleavage, the transformation of hydrolytic products into nontoxic sub-products, exiting non-toxic products through natural cellular activities, and urine. In addition to the above applications, nanoparticles of PLA are used to encapsulate various drugs such as restenosis, oridonin, and so on. In addition, PLA is used for orthopedic devices. The most important reason for using biopolymers, e.g., PLA rather than metallic structures, is avoiding second surgery to remove the orthopedic devices, which reduces the costs and makes for a more facile recovery. The main uses of PLA in orthopedic devices are screws, fixation pins, plates, and suture anchor. Another application of PLA in the biomedical section is for tissue engineering (TE). One of the most applicable PLA forms in TE is three-dimensional (3D) porous scaffolds used for cell culturing applications, such as cardiovascular diseases [63]. The 3D PLA structure is affordable by electrospinning and 3D printing for patient-customized products [64]. In addition, PLA is also used in textile, plasticulture, service-ware, and environmental remediation films [52].

PLA EoL Options
Commonly, the EoL options include landfilling, composting, anaerobic digestion, incineration or thermal treatment, and recycling [65]. In this paper, the most common EoL options for PLA are considered, including landfilling, composting, and recycling. PLA and its products are biodegradable, but it does not allow for littering them in the environment or self-composting. PLA products are still stable in soil and their landfilling merely affects the environment because only one percent will be degraded after 100 years [66]. PLA, as a promising synthesis biopolymer, is difficult to degrade at natural environment temperature. The inherent slow crystallization kinetics of PLA lead to the slow degradation rate of PLA which occurs under anaerobic thermal conditions [67,68]. The CH 4 production of PLA landfilling at ambient temperature is below 0.1%, with insignificant CO 2 emissions. Although PLA products are generally produced for a short life span, their reuse is also achievable. Composting is generally considered as one of the worst EoL options due to no energy recovery and low compost quality. Figure 4 illustrates the different stages of PLA composting. The PLA composting process includes three steps, separation, grinding, and compost degradation. The organic compound, or the source of the nutrients, is the result of the composting process. It is noticeable that the energy resources have not been used during the composting process [69].
In addition, mechanical and chemical recycling are also two EoL options for PLA [70]. By avoiding using virgin PLA, a significant reduction in GHE and environmental issues is witnessed. Figure 5 presents the mechanical recycling of PLA. According to Figure 5, the process of PLA mechanical recycling contains eight main steps, including separation, grinding, washing, drying, extrusion, cooling, granulation, and sieving. The incorporation of a chain extender during the extrusion process can enhance the mechanical properties of the recycled PLA [71][72][73].  The chemical recycling process is categorized into hydrolysis and polymerization stages. As can be observed in Figure 6, the hydrolysis stage consists of separation, grinding, washing, reactor sector, cooling, decantation and filtration, and evaporation. Based on Marina et al.'s [69] study, the reactor ought to be immersed in insulating oil and the process temperature remained unchanged at 180 • C for 2 h. Moreover, the impurities added before the cooling step were removed in the decantation and filtration step. Ultimately, the concentrated lactic acid is produced by water evaporation for polymerization. The four main steps in polymerization are prepolymer production, lactide production, ring-opening polymerization, and extrusion. Although there are considerable attempts to develop recycling of the materials, PLA recycling is limited due to the poor available infrastructures. Considering the high cost of separation and the poor quality of the recycled PLA, the only applicable recycling method for PLA is "mixed" recycling [52]. Hence, the most plausible EoL option for PLA is landfilling. As mentioned earlier, PLA is biodegradable and will be degraded to H 2 O, CH 4 , and CO 2 . In other words, PLA biodegradation participates in the total amount of GHE in the life cycle of PLA [74].

Summary of the Existing LCAs of PLA
A summary of the existing life cycle studies on PLA and its products was extracted from the literature and is presented in Table 2. Comparing the characteristics, objectives, assumptions, data sources, and major findings of these studies will provide insights into better PLA LCA and address the environmental issues.
One of the pioneering studies on LCA of PLA dates back to 2003, on the NatureWorks™ PLA [75]. According to this study, the total required fossil energy for PLA was less than fossil-based polymers which can be used in other sections of the PLA production procedure. Later, in 2009, Madival et al. investigated the LCA of PLA clamshell containers in comparison with PET and PS clamshell containers [76]. According to the results, the PLA containers could be 100% recyclable and/or compostable. Moreover, PLA had less GHE (~28 kg CO 2 ) compared to PET (~830 kg CO 2 ) distributed by 16-ton trucks [76]. Then, Piemonte examined the PLA total energy demand and environmental impact in comparison with PE and PET in 2011 [77]. They found that bio-plastics usage instead of fossil-based plastics can lead to considerable energy and GHE savings [77]. Subsequently, in 2014, Papong et al. carried out a comparative investigation on the environmental impact of PET and PLA drinking water bottles from a life cycle outlook. The results showed that the production of PLA bottles can lead to a reduction in CO 2 emissions, lower toxicity, and less demand for non-renewable energy [78,79]. In the same year, Mahalle et al. studied a cradle-to-gate LCA of polylactic acid/thermoplastic starch (PLA/TPS) and wood fiberreinforced PLA bio-composites [80]. According to the results, bio-composites are able to perform in a more environmentally friendly manner in comparison with PP [80]. In 2015, Benetto et al. examined the LCA of PLA and TPS multilayer film designed by atmospheric plasma usage. Two system boundaries and two EoL were carried out, namely, cut-off, expansion, recycling, and incineration, respectively. Cut-off had a higher impact in comparison with expansion. In disposal, incineration and recycling had negative values for one kg of multilayer in I2002 [81]. Later, in 2017, Hottle et al. investigated the production of biopolymers and EoL comparisons through LCA [13]. Based on the results, recycling is able to reduce 40% to 60% of environmental impacts in fossil fuel depletion for petrochemical polymers [13]. In addition, Maga et al. studied the LCA of the PLA and its recycling options in 2019 [70]. They examined mechanical, solvent-based, and chemical recycling of the waste PLA. Based on the results, recycling PLA led to higher savings (0.3-1.2 times higher) in GHE compared to the PLA incineration. Furthermore, recycling had less cumulative energy demand (CED) compared to incineration [70]. In the same year, Morão et al. investigated the PLA's life cycle impact (LCI) produced by sugarcane in Thailand [15]. According to the results, several approaches were introduced to improve the PLA environmental impact, such as enhancement in the farming practice of sugarcane, exploitation of bagasse boilers with higher efficiencies at sugarmill, consumption reduction in auxiliary chemicals, and renewable energy usage enhancement in the sugar conversion process to PLA [15]. One of the most recent available LCAs of PLA was conducted by Bałdowska-Witos et al. for the PLA bottle shaping's environmental impact assessment in 2020 [82]. The results demonstrated that the GHE in the environment was affected by water, electrical energy, and raw materials usage during the bottle shaping process [82].  (1) Beverage bottling, labeling, storage, and distribution were excluded from the production process (2) Storage and transportation of raw materials were excluded -(1) Emission of nitric oxides, carbon dioxides, and sulfur oxides into the natural environment affected by electrical energy, water, and raw materials utilization during the bottle shaping process (2) End product degasification and cooling have the most important role in the emissions and fine particles' formation (3) Major impacts: global warming, water resources' usage, fine particles' formation, water acidification, and land use [82]

Summary of the Existing LCAs of PLA
Investigation of the GHE in the life cycle of a material can help with the best suggestions for making it a low-carbon material. As mentioned in the previous sections, most of the PLA products end up in landfilling or composting. In this section, the CO 2 emission of PLA in three different EoL options including landfilling without biodegradation and landfilling or composting with 60% biodegradation is evaluated. The PLA CO 2 emission is compared with PE products. GHE of all these materials is summarized in Figure 7. As PE is not considered as a biodegradable material, there are no EoL emissions displayed for it in Figure 7. It can be observed in Figure 7 that carbon uptake is considered only for biopolymers, which is their advantage in terms of environmental aspects compared to fossil-based plastics [83]. One kg of PLA is calculated to be able to uptake around 1.8 kg of CO 2 . Regarding the total GHE of PLA landfill with no biodegradation, it can be concluded that it releases 1.2 and 0.9 kg of CO 2 per kg of PLA less than LDPE and HDPE, respectively. Opposed to this, in the cases that the biodegradability of PLA is taken into account, the total GHE of PLA will enhance greatly, more so than HDPE and LDPE.
It should be noted that PLA is in the early steps of its progress and its production and conversion processes are not optimized compared to PE, which owns the first rank in terms of production worldwide among plastics [84]. By optimizing the conversion process of PLA, it is possible to reduce the energy demand and GHE of the procedure. For example, NatureWorks has been producing PLA for more than 15 years and is optimizing the processing of PLA. Therefore, it seems that one of the best suggestions for making PLA a low-carbon material is optimizing its conversion process, as it consists of more than 50% of PLA GHE in both landfilling and composting. In fact, PLA conversion releases about 2.9 kg of CO 2 per kg of PLA. The NatureWorks optimization shows that they were able to develop the production of PLA and could reach only 0.6 kg of CO 2 emission per kg of PLA. However, that data is not available to the public. This clearly shows the high potential of optimization of the PLA processing in reducing the GHE and coming up with more environmentally friendly PLA. Another suggestion to make PLA a low-carbon material is to develop recycling facilities to obtain new PLA products from the recycled PLA, of good quality and acceptable properties. By recycling, the EoL emissions, which are of considerable amounts, will be removed from the calculations.

Conclusions
Unlike fossil-based polymers, bio-based polymers derived from renewable origins offer more CO 2 absorption during their production process. However, the production of bio-based polymers is still dependent on fossil fuels as the source of energy in their fabrication process. PLA is considered as one of the most prevalent and commercial biobased polymers for numerous applications, with several EoL options, including mechanical recycling, chemical recycling, landfilling, and industrial composting. However, when the lifetime of PLA-based products is over, they will be mostly landfilled or composted. The lack of proper infrastructures for PLA processing leads to limitations to recycling them. There are several LCAs of PLA or comparing different plastics with PLA in terms of environmental aspects, energy demand, and GHE. By exploiting the LCAs of PLA, it can be optimized to be a more environmentally friendly material. The GHE attributed to the life cycle of PLA shows that the conversion of the bio-sources to lactic acid and then PLA is an energy-intensive process that releases a huge amount of CO 2 to the atmosphere. According to the available data, more than 50% (2.8 kg CO 2 /kg PLA) of the released CO 2 in the PLA life cycle belongs to its conversion. By optimizing the conversion process of PLA, there will be a high potential to make PLA a low-carbon material.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.