Green Mirage: Exposing the True Carbon Footprint of PLA Plastics
Contemporary environmental challenges are underscored by the staggering daily consumption of approximately 400,000 barrels of oil for the production of conventional plastics, concurrently burdening ecosystems with billions of tonnes of persistent plastic waste (Yaradoddi et al., 2016). In response to this crisis, bioplastics have garnered considerable attention for their ability to biodegrade within a brief temporal span of 180 days under specific conditions (Shruti & Kutralam-Muniasamy, 2019). Positioned within this burgeoning discourse, polylactic acid (PLA), identified as a vanguard force (Song et al., 2011), emerges as a leader in the realm of sustainable bioplastics.
Our exploration now pivots after having meticulously elucidated the intricacies characterizing PLA’s production process and underscored its commendable biodegradability. Within the ambit of environmental assurances, a pivotal inquiry crystallizes: How does the global warming potential (GWP) of PLA fare in comparison to extant sustainable alternatives?
Prior to delving into the intricate examination of poly(lactic acid)’s (PLA) global warming potential (GWP), it is imperative to elucidate the fundamental concept of GWP, which serves as a pivotal metric in quantifying the impact of greenhouse gas emissions and forms an integral component in the comprehensive assessment of the environmental footprint of materials.
Global Warming Potential (GWP)
Greenhouse gases (GHGs) contribute to global warming by impeding energy escape from Earth, acting as an insulating blanket (United States Environmental Protection Agency, 2023). GHGs’ radiative efficiency and atmospheric lifetime differ in their impact on Earth’s warming. The Global Warming Potential (GWP) facilitates the comparison of these impacts, quantifying the energy absorption of 1 ton of a gas over a specified time relative to 1 ton of carbon dioxide (CO₂). A higher GWP indicates a more significant warming effect than CO₂ over the designated time, typically 100 years. This standardized unit allows analysts to aggregate emissions estimates and aids policymakers in evaluating reduction opportunities across sectors and gases.
Carbon dioxide, serving as the reference gas, has a GWP of 1 across all periods due to its extended atmospheric residence, lasting thousands of years. Methane (CH₄), with a GWP of 27–30 over 100 years, exhibits a shorter lifespan but higher energy absorption, considering factors like its role in ozone formation. Nitrous Oxide (N₂O) boasts a GWP 273 times that of CO₂ over a 100-year timescale, persisting in the atmosphere for more than a century on average. High-GWP gases such as CFCs, HFCs, HCFCs, PFCs, and SF6 trap significantly more heat than CO₂ for a given mass, with GWPs reaching into the thousands or tens of thousands.
Continuing our examination, let’s delve into the assessment of the Global Warming Potential (GWP) associated with the production of PLA.
Cradle-to-Gate Approach
In the study conducted by (Morão & de Bie, 2019), the Global Warming Potential (GWP) was computed for the Corbion and Total Corbion PLA plant situated in Thailand, utilizing sugarcane as the feedstock. Adopting a cradle-to-gate approach and accounting for the carbon dioxide uptake within the PLA molecule, the calculated GWP stands at 501 kg CO₂ eq/tonne PLA. Without considering the carbon dioxide uptake, the GWP is quantified at 2334 kg CO₂/tonne of PLA. The specific details of these calculations are outlined below.
Carbon Dioxide Uptake
The assessment of the GWP for PLA commenced with the quantification of carbon dioxide (CO₂) uptake from the atmosphere during the growth of sugarcane. Employing the biomaterial storage approach (Pawelzik et al., 2013), the CO₂ fixated within the PLA material was determined to be 1833 kg CO₂/ton PLA. In cradle-to-grave evaluations, this CO₂ must be incorporated into the overall footprint, as it is anticipated to be released at the end-of-life of the product.
Sugarcane Production
Nitrogen dioxide (N₂O) emissions from crop residues and nitrogen fertilizers accounted for 42% of greenhouse gas (GHG) emissions related to sugarcane production, whereas direct land-use change (DLUC) accounted for 30%. The remaining 28% of emissions were caused by a variety of agriculture-related activities, including fuel consumption, fertilizer production, and fuel use for agricultural machines. The degree of uncertainty around N₂O emissions, especially those resulting from fertilizer application, was considerable since agricultural emissions are difficult to precisely quantify or calculate.
Sugar Milling
The net impacts of the sugar mill process resulted in a negative GWP, primarily attributed to credits from by-products that surpassed the minor environmental footprint generated by chemicals and energy consumption for raw sugar production. Co-generated steam and electricity from bagasse, along with the utilization of by-products like electricity, molasses, and filter cake, played a crucial role in minimizing overall emissions.
Lactic Acid Production
The most substantial contributor to the GWP of PLA was identified in the lactic acid production phase. The associated greenhouse gas emissions were primarily linked to energy consumption, constituting 40% of the total emissions. Chemicals used during lactic acid production contributed as follows: lime (34%), sulfuric acid (6%), and other chemicals (9%). Notably, the noteworthy impacts of lime were attributed to direct CO₂ emissions during limestone calcination and kiln fuel combustion. Transportation of raw materials to the Corbion site accounted for 13% of emissions. The by-products generated, including gypsum, biomass, and stillage, were valorized and made a slightly negative contribution (-2%).
Lactic acid, a crucial intermediate for PLA production, demonstrated a cradle-to-gate GWP of -224 kg CO₂ eq/ton, encompassing all inputs and outputs from the farm to the final lactic acid product.
Lactide and PLA Production
The GHG emissions from the lactide and PLA production steps were predominantly associated with natural gas and electricity consumption within the plant.
Cradle-to-Grave Approach
In a comprehensive cradle-to-grave analysis, due consideration must be given to the biodegradation of Polylactic Acid (PLA) during its end-of-life (EoL) stages. Various EoL options exist, encompassing landfilling, composting, anaerobic digestion, incineration or thermal treatment, and recycling, with landfilling, composting, and recycling emerging as the predominant choices (Ghomi et al., 2021; Zhao et al., 2018). Despite its prevalence as an EoL option, composting is generally regarded unfavorably due to the absence of energy recovery and the resultant low-quality compost. Efforts to enhance recycling practices for PLA are notable; however, the limitations in existing infrastructures curtail its recycling potential, primarily attributed to the high cost of separation and the inferior quality of recycled PLA (Castro-Aguirre et al., 2016). Consequently, “mixed” recycling stands as the sole applicable recycling method for PLA.
Given these constraints, landfilling emerges as PLA’s most practical EoL option. It is imperative to underscore that the inherent biodegradability of PLA when landfilled, facilitates its degradation into water (H₂O), methane (CH₄), and carbon dioxide (CO₂). This biodegradation process actively contributes to the overall greenhouse gas emissions in the life cycle of PLA (Lyu et al., 2007).
As outlined in Ghomi et al.’s (2021) investigation, the total greenhouse gas emissions (GHE) of PLA landfill disposal, excluding biodegradation, indicate a release of 1.2 kg and 0.9 kg of CO₂ per kg of PLA less than LDPE and HDPE, respectively. However, when considering PLA’s biodegradability, the overall GHE of PLA significantly increases, surpassing both HDPE and LDPE. In instances of 0% biodegradation in landfill disposal, PLA releases 1.7 kg CO₂ equivalent per kg of PLA. This figure substantially rises to 3.7 kg CO₂/kg PLA for landfill disposal with 60% biodegradation and 3.3 kg CO₂/kg PLA for composting with 60% biodegradation.
An effective strategy to reduce PLA’s carbon footprint involves optimizing its conversion process, given that over 50% of PLA’s GHE occurs during landfilling and composting (Ghomi et al., 2021). Specifically, the conversion process contributes approximately 2.9 kg of CO₂ per kg of PLA. Notably, NatureWorks’ optimization efforts demonstrate progress, reducing to only 0.6 kg of CO₂ emissions per kg of PLA in their production.
In Conclusion…
Despite its reputation as a more environmentally friendly plastic, the carbon footprint of PLA presents a nuanced reality. The findings underscore the imperative for ongoing efforts to enhance the sustainability profile of PLA. To align with environmental goals, there is a crucial need for further improvements, aiming to render PLA either carbon-neutral or, ideally, carbon-negative. This can be achieved through the optimization of production processes, seeking increased efficiency, or exploring alternative feedstocks with better yields and superior carbon dioxide uptake capabilities. The quest for a truly eco-friendly PLA involves a commitment to continuous innovation and sustainable practices, steering the trajectory towards a greener and more environmentally responsible future.
References
Castro-Aguirre, E., Iñiguez-Franco, F., Samsudin, H., Fang, X., & Auras, R. (2016). Poly(lactic acid) — Mass production, processing, industrial applications, and end of life. In Advanced Drug Delivery Reviews (Vol. 107, pp. 333–366). Elsevier B.V. https://doi.org/10.1016/j.addr.2016.03.010
Lyu, S. P., Schley, J., Loy, B., Lind, D., Hobot, C., Sparer, R., & Untereker, D. (2007). Kinetics and time-temperature equivalence of polymer degradation. Biomacromolecules, 8(7), 2301–2310. https://doi.org/10.1021/bm070313n
Morão, A., & de Bie, F. (2019). Life Cycle Impact Assessment of Polylactic Acid (PLA) Produced from Sugarcane in Thailand. Journal of Polymers and the Environment, 27(11), 2523–2539. https://doi.org/10.1007/s10924-019-01525-9
Pawelzik, P., Carus, M., Hotchkiss, J., Narayan, R., Selke, S., Wellisch, M., Weiss, M., Wicke, B., & Patel, M. K. (2013). Critical aspects in the life cycle assessment (LCA) of bio-based materials — Reviewing methodologies and deriving recommendations. In Resources, Conservation and Recycling (Vol. 73, pp. 211–228). https://doi.org/10.1016/j.resconrec.2013.02.006
Ghomi, E. R., Khosravi, F., Ardahaei, A. S., Dai, Y., Neisiany, R. E., Foroughi, F., Wu, M., Das, O., & Ramakrishna, S. (2021). The life cycle assessment for polylactic acid (PLA) to make it a low-carbon material. In Polymers (Vol. 13, Issue 11). MDPI AG. https://doi.org/10.3390/polym13111854
Shruti, V. C., & Kutralam-Muniasamy, G. (2019). Bioplastics: Missing link in the era of Microplastics. In Science of the Total Environment (Vol. 697). Elsevier B.V. https://doi.org/10.1016/j.scitotenv.2019.134139
Song, J., Kay, M., & Coles, R. (2011). Bioplastics. www.european-bioplastics.org
United States Environmental Protection Agency. (2023). Understanding Global Warming Potentials. https://www.epa.gov/ghgemissions/understanding-global-warming-potentials
Yaradoddi, J., Patil, V., Ganachari, S., Banapurmath, N., Hunashyal, A., Shettar, A., & Yaradoddi, J. S. (2016). Biodegradable Plastic Production from Fruit Waste Material and Its Sustainable Use for Green Applications. Available Online Www.Ijpras.Com International Journal of Pharmaceutical Research & Allied Sciences, 5(4), 56–66. www.ijpras.com
Zhao, P., Rao, C., Gu, F., Sharmin, N., & Fu, J. (2018). Close-looped recycling of polylactic acid used in 3D printing: An experimental investigation and life cycle assessment. Journal of Cleaner Production, 197, 1046–1055. https://doi.org/10.1016/j.jclepro.2018.06.275
